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UBC Theses and Dissertations

CH₄ oxidation catalysts evaluated in a monolith reactor AlMohamadi, Hamad Hamoud 2019

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CH4 Oxidation Catalysts Evaluated in a Monolith Reactor  by  Hamad Hamoud AlMohamadi  B.Sc., King Saud University, 2008  M.Sc.  University of Maine, 2014  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY in The Faculty of Graduate and Postdoctoral Studies (Chemical and Biological Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)   June 2019  © Hamad AlMohamadi, 2019  ii    The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled: CH4 Oxidation Catalysts Evaluated in a Monolith Reactor  submitted by Hamad AlMohamadi in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemical and Biochemical Engineering  Examining Committee: Kevin J. Smith, Chemical and Biological Engineering Supervisor  Madjid Mohseni, Chemical and Biological Engineering Supervisory Committee Member  Mark MacLachlan, Department of Chemistry Supervisory Committee Member Mark Martinez, Department of Chemical Engineering University Examiner Marek Pawlik, Norman B. Keevil Institute of Mining Engineering University Examiner  Additional Supervisory Committee Members:  Supervisory Committee Member  Supervisory Committee Member  iii  Abstract  Unburned CH4 from natural gas vehicle (NGVs) exhausts limits the use of natural gas as a vehicle fuel. CH4 is a potent greenhouse gas with a high C-H bond strength (~435 kJ mol-1) making it difficult to oxidize in catalytic converters. This study is focused on the assessment of the activity and stability of selected catalysts placed in a monolith reactor with the goal of improving NGV emission control. Firstly, the washcoat formulation was investigated with the activity and stability of PdO/-AlOOH/-Al2O3, PdO/Ce/-AlOOH/-Al2O3 and Pt-PdO/Ce/-AlOOH/-Al2O3 monolith catalysts for CH4 oxidation in the presence of H2O, CO, CO2 and SO2 reported. Secondly, the effect of adding a washcoat overlayer to improve the performance of the monolith catalyst was investigated.  The monolith catalysts were prepared using a cordierite (2MgO.2Al2O3.5SiO2) mini-monolith (400 cells per square inch (CPI), 1 cm diameter x 2.5 cm length; ~52 cells) that was washcoated using a -Al2O3 suspension combined with boehmite (-AlOOH), followed by sequential deposition of Ce and Pd(Pt) by wet impregnation. The initial activity of the mini-monolith catalyst was measured by temperature-programmed CH4 oxidation (TPO) at a GHSV of 36000 h−1. Time-on-stream (TOS) tests were used to quantify the stability of the catalysts at 425 and 550oC using a feed gas with 10 vol % H2O. The results showed that the composition of the washcoat plays a major role in the stability of the catalysts, with both -AlOOH or CeO2 enhancing the stability of the PdO/-Al2O3 catalyst in the presence of H2O.  Moreover, a washcoat overlayer applied to the PdO(Pt/CeO2)/-AlOOH/-Al2O3 monolith catalysts, is shown iv  to enhance CH4 oxidation activity and stability at low temperature (< 500 C) in the presence of H2O and SO2.  Three recent kinetic models of CH4 oxidation reported in the literature and based on Langmuir-Hinshelwood kinetics have been applied to the data measured herein using the mini-monolith reactor. Data obtained in dry feed gas and with 2 and 5% H2O in the feed gas were analysed. The results of the models show that adding CeO2 or the washcoat overlayer decreases H2O adsorption, which leads to enhanced catalyst activity.    v  Lay Summary  This thesis studied CH4 oxidation catalysts to reduce the emissions from natural gas vehicles. The monolith-type catalysts with different washcoats and active metals were investigated. Catalyst deactivation in the presence of water and sulphur oxides is a major problem in NGV emission control. This thesis reports a new catalyst formulation and develops the use of a washcoat overlayer to minimize deactivation by water and sulphur oxides. A mathematical model of the monolith reactor accounting for CH4 oxidation kinetics in the presence of H2O, CO and CO2 at low temperature is also developed to quantify the effect of the washcoat overlayer.   vi  Preface  This thesis includes seven chapters. Versions of Chapter 3, 4 and 5 have been submitted to peer reviewed journals. Also, a version of Chapter 5 was submitted as an invention disclosure. The preparation of this thesis, the journal papers and the invention disclosure were all done by Hamad Almohamadi with the final approval of Professor Kevin J. Smith in the Department of Chemical and Biological Engineering at UBC.  The literature review, reactor set-up, catalyst preparation, catalyst characterization, catalyst testing, reaction modeling, kinetics study, and data analysis were done by Hamad Almohamadi under the direct supervision of Professor Kevin J. Smith.   The list of publications and conferences associated with this thesis are as follows:  H. Almohamadi, K.J. Smith, CH4 oxidation catalysts evaluated in a monolith reactor, 67th Canadian Chemical Engineering Conference, Edmonton, AB (2017).  H. Almohamadi, K.J. Smith, Washcoat overlayer for improved activity and stability of natural gas vehicle (NGV) monolith catalysts in the presence of water and SOx. Invention Disclosure No. 19-007, UBC, April, 2018.  vii  H. Almohamadi, K.J. Smith, Beneficial effect of adding -AlOOH to the -Al2O3 washcoat of a PdO catalyst for methane oxidation, The Canadian Journal of Chemical Engineering, Accepted, May, 2019.  H. Almohamadi, K.J. Smith, CeO2 reduces the impact of H2O and SO2 on CH4 oxidation over over PdO/CeO2/-AlOOH/-Al2O3 monolith catalysts, Catalysts, Accepted, June, 2019.  H. Almohamadi, K.J. Smith, Washcoat Overlayer for improved activity and stability of natural gas vehicle (NGV) monolith catalysts operating in the presence of water and SOx, to be submitted 2019.    viii  Table of Contents  Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ....................................................................................................................... viii List of Tables .............................................................................................................................. xiii List of Figures ............................................................................................................................ xvii List of Symbols ......................................................................................................................... xxvi List of Abbreviations .................................................................................................................xxx Acknowledgements ................................................................................................................ xxxiii Dedication .................................................................................................................................xxxv Chapter 1: Introduction ................................................................................................................1 1.1 Literature Review............................................................................................................ 8 1.1.1 Monolith Catalyst Preparation .................................................................................... 9 1.1.2 Effect of Support on the Catalyst CH4 Oxidation Activity ....................................... 16 1.1.3 The Use of Pd-bimetallic Catalysts for CH4 Oxidation ............................................ 25 1.1.4 Effect of SOx on CH4 Oxidation over Pd Catalysts .................................................. 28 1.1.5 Washcoat overlayer ................................................................................................... 30 1.1.6 CH4 Oxidation Mechanism and Kinetics .................................................................. 34 1.1.7 Summary of the Literature Review ........................................................................... 38 1.2 Thesis Statement and Objectives .................................................................................. 40 1.3 Outline of the Thesis ..................................................................................................... 41 ix  Chapter 2: Experimental .............................................................................................................43 2.1 Materials ....................................................................................................................... 43 2.2 Preparation of the Suspensions ..................................................................................... 44 2.3 Preparation of the Monolith Catalysts .......................................................................... 44 2.4 Characterization of the Monolith Catalysts .................................................................. 48 2.4.1 N2 Adsorption-desorption ......................................................................................... 48 2.4.2 Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy ........... 49 2.4.3 X-Ray Diffraction (XRD) ......................................................................................... 50 2.4.4 X-ray Photoelectron Spectroscopy (XPS) ................................................................ 50 2.4.5 CO Chemisorption .................................................................................................... 50 2.4.6 Adhesion and Thermal Stability ............................................................................... 51 2.5 Catalyst Testing ............................................................................................................ 52 2.5.1 Experimental Setup ................................................................................................... 52 2.5.2 Temperature Programmed Oxidation ....................................................................... 54 2.5.3 Time-on-Stream Experiments ................................................................................... 54 Chapter 3: Beneficial Effect of Adding -AlOOH to the -Al2O3 Washcoat of a PdO Catalyst for Methane Oxidation .................................................................................................56 3.1 Introduction ................................................................................................................... 56 3.2 Results ........................................................................................................................... 56 3.2.1 Catalyst Characterization .......................................................................................... 56 3.2.2 Monolith Catalyst Activity and Stability .................................................................. 68 3.3 Discussion ..................................................................................................................... 73 3.4 Conclusions ................................................................................................................... 78 x  Chapter 4: CeO2 Reduces the Impact of H2O and SO2 on CH4 Oxidation over PdO/-AlOOH/-Al2O3 Monolith Catalysts ...........................................................................................79 4.1 Introduction ................................................................................................................... 79 4.2 Results ........................................................................................................................... 79 4.2.1 Monolith Characterization ........................................................................................ 79 4.2.2 Catalysts Activity and Stability ................................................................................ 88 4.3 Discussion ..................................................................................................................... 93 4.4 Conclusions ................................................................................................................... 99 Chapter 5: Washcoat overlayer for improved activity and stability of natural gas vehicle (NGV) monolith catalysts operating in the presence of H2O and SO2 ..................................100 5.1 Introduction ................................................................................................................. 100 5.2 Results ......................................................................................................................... 101 5.2.1 Monolith characterization ....................................................................................... 101 5.2.2 Catalysts Activity and Stability .............................................................................. 113 5.3 Discussion ................................................................................................................... 120 5.4 Conclusions ................................................................................................................. 128 Chapter 6: Modeling of Monolith Catalytic Reactors ............................................................129 6.1 Introduction ................................................................................................................. 129 6.2 Mathematical Model ................................................................................................... 130 6.3 Determining the Reaction Kinetics of CH4 Oxidation:............................................... 135 6.3.1 Model Proposed by Alyani and Smith .................................................................... 136 6.3.2 Model Proposed by Habibi et al.............................................................................. 137 6.3.3 Model Proposed by Ciuparu et al............................................................................ 139 xi  6.3.4 CP Model with the Effect of the Washcoat Overlayer (Diffusion Barrier) ............ 142 6.4 Results ......................................................................................................................... 146 6.5 Discussion ................................................................................................................... 158 6.6 Conclusion .................................................................................................................. 163 Chapter 7: Conclusions and Recommendations .....................................................................164 7.1 Conclusions ................................................................................................................. 164 7.2 Recommendations ....................................................................................................... 166 7.2.1 Kinetic Modeling for a Wider Range of CH4 Concentrations ................................ 166 7.2.2 Study the Effect of the Support on Monolith Catalyst ............................................ 167 7.2.3 Scale-up and Engine Test........................................................................................ 167 Bibliography ...............................................................................................................................169 Appendices ..................................................................................................................................183 Appendix A Catalyst Characterization ................................................................................... 183 A.1 BET ......................................................................................................................... 183 A.2 XRD ........................................................................................................................ 186 A.3 CO Chemisorption .................................................................................................. 187 Appendix B MFC and MS Calibration ................................................................................... 188 B.1 MFC Calibration ..................................................................................................... 188 B.2 MS Calibration ........................................................................................................ 194 Appendix C Reaction System ................................................................................................. 196 C.1 Washcoat monolith without Pd (blank run) ............................................................ 196 C.2 Washcoat Loading .................................................................................................. 197 C.3 Effect of Calcination Duration ................................................................................ 198 xii  C.4 CH4 Conversion by Total Carbon Flow Rate ......................................................... 200 C.5 CH4 Conversion by CH4 Flow Rate ........................................................................ 202 C.6 TPO and TOS Reaction Repeatability .................................................................... 219 Appendix D Supporting calculations ...................................................................................... 221 D.1 Plug Flow Criterion: ............................................................................................... 222 D.2 External and Internal Mass Transfer Calculation ................................................... 225 D.3 Isothermal Conditions ............................................................................................. 227 Appendix E Scale up ............................................................................................................... 228 E.1 Preparation the 0.5 L Monolith Catalyst ................................................................. 228 E.2 Catalyst Testing ...................................................................................................... 230 Appendix F MATLAB M-files Code ..................................................................................... 233  xiii  List of Tables  Table 1.1 Natural gas compositions [4]. ......................................................................................... 1 Table 1.2 Properties of ceramic monoliths [56]............................................................................ 10 Table 1.3 Weight loss of washcoat/ceramic honeycomb treated by ultrasonic vibration and thermal shock (Adapted with permission from [66]). ................................................................... 13 Table 1.4 Light-off temperatures for 6.5Pd/𝛾-Al2O3 and 2.9Ce/6.5 Pd/Al2O3 catalysts [12]. ...... 22 Table 1.5 T50 for fresh and steam aged Pd and Pt-Pd catalysts operated in dry feed. Combustion conditions: 4067 vol. ppm CH4; total flow rate of 234.5 cm3.min-1; 500 mg catalyst (Adapted with permission from [94]). .......................................................................................................... 27 Table 1.6 Adsorption of SO2 at 320 °C desorption at 650 °C for 6% Pd on different supports [11]. ............................................................................................................................................... 30 Table 1.7 CH4 conversion and outlet temperature for monolith catalysts with different thickness of the washcoat overlayer and 7.6% Pd loading. The feed was 1% CH4 in air and the inlet temperature was 452 °C (Adapted with permission from [102]). ................................................. 32 Table 1.8 Apparent activation energy and order of CH4 combustion reaction over Pd catalysts (Adapted with permission from [6]). ............................................................................................ 35 Table 2.1 Nominal compositions of washcoat suspensions. ......................................................... 45 Table 2.2 Nominal composition of monolith catalysts with different loading of -AlOOH. ....... 45 Table 2.3 Nominal composition of monolith catalysts with different loading of CeO2. .............. 46 Table 2.4 Nominal composition of monolith catalysts with and without washcoat overlayer. .... 48 Table 2.5 Reaction conditions used in the present study for TOS tests. ....................................... 55 Table 3.1 Textural properties of washcoat precursors and monolith catalysts. ............................ 59 xiv  Table 3.2 EDX elemental analysis of the monolith catalysts. ...................................................... 63 Table 3.3 CO uptake and XPS analysis of fresh and used Pd0B and Pd5B catalysts. ................. 66 Table 3.4 T50 from TPO of methane in dry and wet feed gas. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5vol% H2O in N2 and He. ............................................................... 69 Table 4.1 Textural properties and CO uptake of PdO/CeO2/-AlOOH/-Al2O3 monolith catalysts with varying Ce content. ............................................................................................................... 81 Table 4.2 EDX elemental analysis of the monolith catalysts. ...................................................... 83 Table 4.3  XPS analysis of PdO/CeO2/-AlOOH/-Al2O3 monolith catalysts with varying Ce content. .......................................................................................................................................... 85 Table 4.4 Temperature-programmed oxidation T50 conversion for the catalysts in dry and wet feed gas. [Reaction conditions: GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol % O2, 0.06 vol % CO, 8 vol % CO2 and 0, 2 or 5 vol% H2O in N2 and He]..................................... 88 Table 5.1 Textural properties of washcoat monolith catalysts. .................................................. 104 Table 5.2 EDX elemental analysis of the monolith catalysts. .................................................... 107 Table 5.3 Summary of XPS analysis of monolith catalysts. ....................................................... 109 Table 5.4 T50 from TPO of methane in dry and wet feed gas. Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5 vol% H2O in N2 and He. ............................................................................................................................... 114 Table 6.1 Nominal composition of monolith catalysts used in kinetic model analysis. ............. 145 Table 6.2 Monolith properties..................................................................................................... 146 Table 6.3 Estimated values obtained from the AS Model for CH4 oxidation over Pd0Ce and Pd1Ce. ......................................................................................................................................... 147 xv  Table 6.4 Estimated values obtained from HSH Model for CH4 oxidation over Pd0Ce and Pd1Ce. ......................................................................................................................................... 149 Table 6.5 Estimated values of rate constant, equilibrium constant for H2O adsorption, and equilibrium constant for CH4 adsorption for Pd0Ce and Pd1Ce using HSH Model. ................. 150 Table 6.6 Estimated values obtained from the CP Model for CH4 oxidation over Pd0Ce and Pd1Ce. ......................................................................................................................................... 151 Table 6.7 Estimated values of rate constant, equilibrium constant for H2O adsorption, and equilibrium constant for CH4 adsorption for Pd0Ce and Pd1Ce using CP Model. .................... 151 Table 6.8 Estimated values obtained from the AS Model for CH4 oxidation over PdPtCe-WC.153 Table 6.9 Estimated values obtained from the CP Model for CH4 oxidation over PdPtCe-WC and O-PdPtCe-WC............................................................................................................................. 153 Table A.1 BET analysis repeatability. ........................................................................................ 186 Table B.1 MFC calibration using a bubble flow meter for 1vol% CH4/N2. ............................... 188 Table B.2 MFC calibration using a bubble flow meter for 5 vol% CO /N2. .............................. 189 Table B.3 MFC calibration using a bubble flow meter for 50 vol% CO2/N2. ............................ 190 Table B.4 MFC calibration using a bubble flow meter for Air. ................................................. 191 Table B.5 MFC calibration using a bubble flow meter for N2. ................................................... 192 Table B.6 MFC calibration using a bubble flow meter for He. .................................................. 193 Table B.7 MS calibration for CH4 using He. .............................................................................. 195 Table C.1 Nominal composition of monolith catalysts with different loading of washcoat. ..... 197 Table C.2 Calcination time. ........................................................................................................ 199 xvi  Table C.3 CH4 conversion calculation for Pd5B catalysts during TPO experiment. [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4 and 8.5 vol% O2 in N2 and He. .......................................................................... 204 Table C.4 CH4 conversion calculation for Pd5B catalysts during TPO experiment. [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 18000 h−1, Feed composition: 0.07 vol% CH4 and 8.5 vol% O2 in N2 and He. .......................................................................... 209 Table C.5 CH4 conversion calculation for Pd5B catalyst during TPO experiment. [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He. ............................ 214 Table C.6 TPO Reaction Repeatability. [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5 vol% H2O in N2 and He]. ................................................. 219 Table C.7 TOS Reaction Repeatability at 425 oC. [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 10 vol% H2O in N2 and He]. ........................................................... 220 Table D.1 Physical properties of catalyst Pd0Ce. ....................................................................... 221 Table D.2 Washcoat properties. .................................................................................................. 222 Table D.3 Calculations for verification of the plug flow assumption and absence of axial dispersion. ................................................................................................................................... 224 Table D.4 Details of calculations for Mears criterion factor and Weisz-Prater number for Pd0Ce at 523 K. ...................................................................................................................................... 226 Table D.5 Calculations the maximum temperature inside the washcoat. ................................... 227 Table E.1 Nominal composition of 0.5L monolith catalysts. ..................................................... 228 xvii  List of Figures  Figure 1.1 Average prices per Gasoline gallon equivalent (GGE) for gasoline, CNG, diesel from 2000 to 2018 (Adapted from [7]). ................................................................................................... 2 Figure 1.2 Catalytic combustion of CH4 over 1.1wt%Pd/Al2O3 with different amounts of H2O added (vol%). Reaction conditions: 1vol.%CH4, 20 vol%O2, 0-20 vol% H2O, balanced in N2.  GHSV=48,000h-1 [26]. (Copyright © 2002 Elsevier) .................................................................... 6 Figure 1.3 The catalytic converter [4]. ............................................................................................ 7 Figure 1.4 Schematic of monolithic catalyst from [57]. (Copyright © 2003, Elsevier) ............... 11 Figure 1.5 The relation of the solids content in the slurry gel to the loading of coating. The pH value of gel: (1) 2; (2), 4; (3), 5 [66]. (Copyright © 2005, Elsevier) ............................................ 14 Figure 1.6 Effect of calcination temperature on washcoat loading (Al2O3–ZrO2) of cordierite monolith [77]. (Copyright © 2010, Elsevier) ............................................................................... 16 Figure 1.7 CH4 conversion over catalysts: fresh, after the run and after 24 h of aging at 1060 oC. Catalysts: (a) Pd/Al2O3; (b) Pd/Al2O3 -ZrSiO4; (c) Pd/Al2O3-SiO2 [78]. (Copyright © 2004, Elsevier) ........................................................................................................................................ 17 Figure 1.8 Comparison of CH4 conversion over monolithic catalysts. (1) Fresh 1%Pd/Al2O3; (2) fresh 1%Pd/0.3%Co/Al2O3; (3) 1%Pd/0.3%Co/Al2O3 after 20 h on stream; (4) 1%Pd/0.3%Co/ Al2O3 after ageing for 24 h at 1000 oC [79]. (Copyright © 2008, Elsevier) ................................. 19 Figure 1.9 CH4 conversion as a function of temperature in an oxidizing feed stream containing 0.2 vol% CH4, 0.1 vol% CO, and 1 vol% O2 balanced by He (A) without adding CeO2 to Al2O3 and (B) with adding CeO2 to Al2O3 [89]. (Copyright © 1991, Elsevier) ...................................... 21 xviii  Figure 1.10 Steady-state oxidation of CH4 in the presence of H2O (5 vol% H2O), using CHC-f/Pd or 5.3 wt %Pd/Al2O3 as the catalyst [53]. (Copyright © 2015, American Chemical Society) ..... 24 Figure 1.11 Time-course of catalytic activities of supported PdO catalysts: (●) PdO/α-Al2O3 and (○) PdO-Pt/α-Al2O3 for CH4 oxidation at 350 oC for 6 h [93]. (Copyright ©1999, Elsevier) ...... 26 Figure 1.12 CH4 conversion vs time on stream during HTA. Total catalyst loading is 100 mg, corresponding to a GHSV of 133800 ( L(STP).h−1.kgcat -1) with 0.4 vol% CH4, 19 vol% O2 , 5 vol% H2O in N2 [112]. (Copyright © 2018, Elsevier) .................................................................. 33 Figure 1.13 Mechanism of CH4 dissociation on Pd/PdO site pair [36]. (Copyright © 1998, Academic Press)............................................................................................................................ 36 Figure 2.1 Schematic diagram of experimental setup. .................................................................. 53 Figure 3.1 SEM image of washcoat powders: (A) -Al2O3 calcined at 450 oC for 7 h; (B) -AlOOH calcined at 350 oC for 7 h and (C) 20%-AlOOH/80%-Al2O3 calcined at 450 oC for 7 h........................................................................................................................................................ 60 Figure 3.2 XRD patterns for washcoat powders: (A) -Al2O3 calcined at 450 oC for 7 h; (B) -AlOOH calcined at 350 oC for 7 h and (C) 20%-AlOOH/80%-Al2O3 calcined at 450 oC for 7 h........................................................................................................................................................ 60 Figure 3.3 Effect of adding -AlOOH to the washcoat: (A) on the amount of washcoat deposited; (B) Relative washcoat adhesion and thermal stability as measured by relative weight loss after sonication. ..................................................................................................................................... 62 Figure 3.4 SEM images of the monolith catalysts:(A) Cross-section of Pd0B; (B) Channel wall of Pd0B; (C) Cross -section of Pd5B;(D) Channel wall of Pd5B; (E) Cross -section of Pd8B; (F) Cross -section of Pd11B................................................................................................................ 64 xix  Figure 3.5 SEM-EDX analysis showing distribution of Al and Pd in the monolith channel for Pd5B: (A) Al (B) Pd. .................................................................................................................... 65 Figure 3.6 XPS Pd 3d spectra measured for monolith catalysts: (A) Fresh Pd0B, (B) Pd0B-Used; (C) Fresh Pd5B (D) Pd5B-Used. .................................................................................................. 67 Figure 3.7 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5 vol% H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:Pd5B. ................................................................................................ 70 Figure 3.8 TOS results after adding 10 vol % H2O to the dry feed gas.  Reacting conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 10 vol% H2O in N2 and He. The period of the experiment was 7h A: at 425 oC and B: at 550 oC. ............................................................. 71 Figure 3.9 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature after the TOS tests. The total feed gas flow is 1025 cm3(STP)·min−1, corresponding to a GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He. ............................................................................................................... 72 Figure 3.10 Temperature-programmed oxidation profile for CO conversion as function of temperature for the catalysts. The total feed gas flow is 1025 cm3(STP)·min−1, corresponding to a GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He. ................................................................................................................................................. 72 Figure 4.1 Effect of CeO2 loading on the adhesion properties and thermal stability of the washcoat. ....................................................................................................................................... 80 xx  Figure 4.2 SEM images of the monolith catalysts Pd0Ce (A, B) and Pd2Ce, (C, D)................... 83 Figure 4.3 SEM-EDX analysis of Pd1Ce catalyst: (A) Al, (B) Ce, (C) Pd, and (D)Pd-Ce-Al..... 84 Figure 4.4 XPS Pd 3d spectra measured for catalysts: A – Pd0Ce; B- Pd1Ce; C: Pd2Ce; D:Pd4Ce. ....................................................................................................................................... 86 Figure 4.5 XPS Pd 3d spectra measured for catalysts: A - Pd0Ce; B - Pd0Ce-used; C - Pd2Ce; D - Pd2Ce-used. ................................................................................................................................ 87 Figure 4.6 Temperature-programmed CH4 oxidation profile showing the initial activity of the catalysts as a function of temperature. Reaction conditions = GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5vol% H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:Pd2Ce. ....................................................................................................................................... 89 Figure 4.7 TOS results after adding: (A) 10 vol % H2O added to the dry feed gas at 425 °C, (B) : (A) 10 vol % H2O added to the dry feed gas at 550 °C and (C) 10 vol % H2O and 5 ppmv SO2 added to the dry feed gas at 500 °C.  Reaction conditions: GHSV = 36000 h-1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 in N2 and He............... 91 Figure 4.8 Temperature-programmed CH4 oxidation profile: the initial activity of the catalysts as a function of temperature after the catalysts were regenerated. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He. ........................................................ 92 Figure 4.9 Temperature-programmed oxidation profile for CO conversion as function of temperature for the catalysts. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, and 8 vol% CO2 in N2 and He. ................................................................................................................ 93 xxi  Figure 5.1 Configuration of monolith catalyst with washcoat overlayer.................................... 101 Figure 5.2 The effect of adding the washcoat overlayer on the adhesion properties and thermal stability of the fresh and used catalysts (PdPtCe-WC and O- PdPtCe-WC) operated for 200 h in the presence of 10 vol% H2O and 2 ppm SO2 at 500 oC. ............................................................ 102 Figure 5.3 SEM images of selected monolith catalyst cross-sections: A: PdPtCe-WC; B: PdPtCe-WC used; C: O-PdPtCe-WC;  and D:O-PdPtCe-WC used. ........................................................ 105 Figure 5.4 SEM/EDX mapping of sectioned PdPtCe-WC monolith catalyst after use: A: SEM of  the washcoat; B - Al; C - O; D - Ce; E – Pd and F – S. .............................................................. 106 Figure 5.5 SEM/EDX mapping of sectioned O-PdPtCe-WC monolith catalyst after use: A -SEM of the washcoat; B - Al, C - O; and D - S. .................................................................................. 107 Figure 5.6 XPS Pd 3d spectra measured for fresh (A) Pd-WC, (B) O-Pd-WC, (C) PdPt-WC. . 110 Figure 5.7 XPS Pd 3d spectra measured for used (A) Pd-WC, (B) O-Pd-WC, (C) PdPt-WC. .. 111 Figure 5.8 XPS S 2p spectra measured for fresh (A) PdWC, (B) O-PdWC, (C) PdCeWC and, (D) O-PdCeWC. ................................................................................................................................ 112 Figure 5.9 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature. Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2, 5 or 10 vol% H2O in N2 and He.  (A) Dry feed 0 vol% H2O, (B) Wet feed 2 vol% H2O, (C) Wet feed 5 vol% H2O and (D) Wet feed 10 vol% H2O. .. 115 Figure 5.10 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature. Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2, 5 or 10 vol% H2O in N2 and He.  (A) Dry feed 0 vol% H2O, (B) Wet feed 2 vol% H2O, (C) Wet feed 5 vol% H2O and (D) Wet feed 10 vol% H2O. .. 116 xxii  Figure 5.11 TOS results for adding 10 vol% H2O and 5 ppm SO2.  Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2, 10 vol% H2O and 0 or 5 ppm SOx in N2 and He. (A) 0 ppm SOx at 425 oC for 10h and (B) 5 ppm SO2 at 500 oC for 24h............................................................................................................................................... 118 Figure 5.12 TOS results for adding 10 vol% H2O and 5 ppm SO2.  Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2, 10 vol% H2O and 0 or 5 ppm SOx in N2 and He. (A) 0 ppm SO2 at 425 oC for 10h and (B) 5 ppm SO2 at 500 oC for 24h............................................................................................................................................... 119 Figure 5.13 TOS results for adding 10 vol% H2O to the feed at 500 oC.  Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 10 vol% H2O in N2 and He. ....................................................................................................................... 119 Figure 5.14 TOS results for adding 10 vol% H2O and 2 ppm SO2 to the feed at 500 oC.  Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2, 10 vol% H2O and 2 ppm SO2 in N2 and He. ............................................................................... 120 Figure 6.1 Scheme of square geometry of the single monolith channel. .................................... 131 Figure 6.2 Profile of the CH4 concentration through the washcoat overlayer (diffusion barrier - DB). ............................................................................................................................................. 143 Figure 6.3 Comparisons of modeled and observed CH4 conversions for Pd0Ce. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5vol % H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:Pd0Ce. ..................................................................................................................................... 147 xxiii  Figure 6.4 Comparisons of modeled and observed CH4 conversions for Pd1Ce. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5vol % H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:Pd1Ce initial activity............................................................................................................... 148 Figure 6.5 Comparisons of effectiveness factor η value of Pd1Ce for the three models. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He. ...... 152 Figure 6.6 Comparisons of modeled and observed CH4 conversions for PdPtCe-WC using AS and CP models. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5vol% H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:PdPtCe-WC initial activity. ............................................................................... 154 Figure 6.7 Comparisons of modeled and observed CH4 conversions for O-PdPtCe-WC using CP models. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5vol% H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:O-PdPtCe-WC initial activity. ....................................................................................... 155 Figure 6.8 Comparisons of effectiveness factor η value of PdPtCe-WC and O-PdPtCe-WC. Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5 vol% H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:O-PdPtCe-WC η. ............. 157 Figure A.1 Isotherm data for calcined AlOOH. .......................................................................... 184 xxiv  Figure A.2 Pore size distribution of the Al2O3, AlOOH and 20% AlOOH/80% Al2O3. ............ 185 Figure B.1 Calibration equation obtained for 1vol%CH4/N2...................................................... 189 Figure B.2 MFC Calibration equation obtained for 5 vol% CO/N2. .......................................... 190 Figure B.3 MFC Calibration equation obtained for 50vol%CO2/N2. ......................................... 191 Figure B.4 MFC Calibration equation obtained for Air. ............................................................ 192 Figure B.5 MFC Calibration equation obtained for N2............................................................... 193 Figure B.6 MFC Calibration equation obtained for He. ............................................................. 194 Figure B.7 MS Calibration equation for CH4. ............................................................................ 195 Figure C.1 Temperature-programmed oxidation profile: the initial activity of the washcoat 6%AlOOH/21%Al2O3/73% cordierite (no Pd) as a function of temperature. The total feed gas flow is 1025 cm3(STP)·min−1, corresponding to a GHSV of 36000 h−1 with 0. 07vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He. ............................................................ 196 Figure C.2 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 5 vol% H2O in N2 and He............................................................................................. 198 Figure C.3 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 5 vol% H2O in N2 and He............................................................................................. 200 Figure C.4 CH4 conversion calculation for Pd5B catalyst with GHSV =36000 h-1. .................. 213 Figure C.5 CH4 conversion calculation for Pd5B catalyst with GHSV =18000 h-1. .................. 213 xxv  Figure C.6 Comparison of TPO raw data and TPO average data using Pd5B catalyst ; [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He. ............................ 217 Figure C.7 Comparison of TPO results with and without CO and CO2 for PdCe-WC [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol% O2, 0 or 0.06 vol% CO and 0 or 8 vol% CO2 in N2 and He. ............. 217 Figure C.8 Comparison of TPO results with and without CO and CO2 for O-PdCe-WC [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol% O2, 0 or 0.06 vol% CO and 0 or 8 vol% CO2 in N2 and He. ............. 218 Figure E.1 0.5L monolith (A) after deposited the washcoat and (B) after deposited the active phase. .......................................................................................................................................... 229 Figure E.2 SEM images of the 0.5L monolith catalysts:(A) X-section of the catalyst; (B) Channel wall of the catalyst. ..................................................................................................................... 230 Figure E.3 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature. Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, and 8 vol% CO2 in N2 and He. (A) calcined in the furnace and (B) recalcined in the reactor. .............................................................................................................................. 232 Figure E.4 TOS results for adding 10 vol% H2O at 500 oC for 24h.  Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 10 vol% H2O in N2 and He. ................................................................................................................................... 232 xxvi  List of Symbols  *    An O vacancy on the PdO surface  Ac    External catalyst surface area per catalyst bed volume, m2 m-3  C    Constant number in BET equation  𝐶𝐶𝐻4   CH4 concentration, mol m-3  𝐶𝐶𝐻40    CH4 concentration at reactor inlet, mol m-3 (STP)  Cp    Heat capacity, kJ g-1 K-1  db    Catalyst bed diameter, m dh    Hydraulic diameter of the monolith channel, m  𝐷𝐶𝐻4,𝐻𝑒  Binary bulk diffusivity, m2 s-1  𝐷𝐶𝐻4−𝐻𝑒𝑒𝑓𝑓  Effective bulk diffusivity, m2 s-1  dcrystal    Crystallite size, m  Deff    Effective diffusivity, m2 s-1  DK    Knudsen diffusivity, m2 s-1  DK_eff    Effective Knudsen diffusivity, m2 s-1  dp    Particle diameter, m  dpore    Pore diameter, m  DPd    Pd dispersion, %  Ea    Activation energy, kJ mol-1 𝐹𝐶𝐻4𝑜     CH4 molar flow at reactor inlet, mol s-1  h    Heat transfer coefficient, kJ m-2 s-1 K-1  xxvii  Ii    Mass signal of component i = CH4, CO2 and He, torr  Ire    Relative mass signal (based on He)  jD   jD factor,  k    Rate constant, s-1  K    The Scherrer constant (often 1)  𝑘𝑟   First order rate constant, m3 min-1 gcat-1  Ki   Adsorption equilibrium constant for component i = CH4 and H2O, kPa-1  kc    External mass transfer coefficient, m s-1  kr    Rate constant, mol s-1 kgcat-1  Lch   Channel length, m Lc   thickness of the washcoat, m Ldb   thickness of diffusion barrier or washcoat overlayer(m). Mw    Molecular weight, g mol-1  Mwfeed    Average gas molecular weight at reactor inlet, g mol-1  n   Reaction order,  𝑁𝐶𝐻4    Flux of CH4 , mol m-2 s-1 P    Pressure, kPa  P°    Saturation vapor pressure, kPa  Pd-*    Pd vacancy site  Pi    Partial pressure of component i = CH4, H2O, CO2 and O2, kPa  R    Gas constant, 8.314 Pa m3 K-1 mol-1  r2    Coefficient of determination  rm    Reaction rate based on catalyst mass, mol s-1 gcat-1  xxviii  rt    Catalyst bed radius, m SBET    BET surface area, m2 g-1  S.D.    Standard deviation  t    Time, min  T    Temperature, K us    Superficial gas velocity, m s-1  V0    Average pore volume, m3 kg-1  Vm    Monolayer volume, m3 (STP) kg-1  Wwash    Washcoat mass, kg  X   CH4 conversion, mol.% 𝑋𝑐𝑎𝑙𝑖     Calculated CH4 conversion, %  𝑋𝑜𝑏𝑠𝑖   Observed CH4 conversion, %  𝑦𝐶𝐻4   CH4 volume fraction  𝑦𝐻𝑒   He volume fraction  𝑦𝐶𝑂2   CO2 volume fraction 𝑦𝐶𝑂   CO volume fraction  𝑦𝑂2   O2 volume fraction 𝑦𝐻2𝑂   H2O volume fraction Yre    Relative volume fraction (based on He) z    Length, m   xxix  Greek Letters  β    FWHM, Radians  ΔHi    Heat of adsorption for component i = CH4 and H2O, kJ mol-1  εb    Bed porosity  εp    Particle porosity  η    Internal effectiveness factor  θ    Angle of reflection, °  θi    Fractional coverage of component i or i-*  θv    Fraction of vacant sites or *  λ    X-ray wave length, nm  μ   Gas dynamic viscosity, kg m-1 min-1  τ    Tortuosity factor  ν    Total volumetric flow rate, m3 (STP) s-1  ν0    Total volumetric flow rate at reactor inlet, m3 (STP) s-1  νi    Volumetric flow rate of component i, m3 (STP) s-1  ρwash    Washcoat density, kg m-3  φ    Thiele modulus  ΣC    Carbon balance, m3 (STP) s-1  ΣC0   Carbon balance at reactor inlet, m3 (STP) s-1  ΩD    Collision integral σ   Constriction factor   xxx  List of Abbreviations  A/F    Air/fuel  B.E.    Binding energy  BET    Brunauer-Emmett-Teller  BJH    Barrett-Joyner-Halenda  Cal    Calculated  CNG    Compressed Natural Gas  Conc.    Concentration  f    function  FF    Frequency factor  g    Gram(s)  GGE    Gasoline gallon equivalent  GHG    Greenhouse gas  GHSV   Gas hourly space velocity, m3 (STP) g-1 h-1 or h-1  h    Hour(s)  H/C    Hydrogen/Carbon  HC    Hydrocarbon  HTA    Hydrothermal aging  LH    Langmuir-Hinshelwood LMA    Levenberg-Marquardt algorithm LNG   Liquefied Natural Gas  MFC    Mass flow controller  xxxi  mi    Mile  MVK    Mars van Krevelen  NG    Natural gas  NGV    Natural gas vehicle  NOx    Nitrogen oxides  Obs    Observed  ODE    Ordinary differential equation  PDE    Partial differential equation  PGM    Platinum Group Metal  QMS    Quadropole mass spectrometer  RDS    Rate-determining step  Ref    Reference  RK4    Runge-Kutta 4th order  RSS    Residual sum of squares  s    Second(s)  SOx    Sulfur oxides TC    Thermocouple  TCD    Thermal conductivity detector  TEM    Transmission electron microscopy  Temp    Temperature  TGA    Thermal gravimetric analysis  TOF    Turnover frequency  TOS    Time on stream  xxxii  TPO    Temperature-programmed oxidation  TPR    Temperature-programmed reduction  TWC    Three way catalytic  vol%    Volume basis  wt%    Weight basis  XPS    X-ray photoelectron spectroscopy  XRD    X-ray diffraction  yr    Year  xxxiii  Acknowledgements  I would like to thank my doctoral supervisor, Professor Kevin J. Smith from the Chemical and Biological Engineering Department at the UBC for giving me the opportunity to do my PhD program under his supervision. His wide knowledge, patience and encouragement enlarged my vision of science and provided coherent answers to my questions. I have learned so much from him both academically and in general during my program at University of British Columbia.  I would like to thank my PhD committee, Professor Madjid Mohseni and Professor Mark MacLachlan for their helpful comments and suggestions throughout my PhD program.  I would like to thank the Ministry of Education in Saudi Arabia for giving me the scholarship to continue my education at UBC. The financial support of Westport Innovations Inc. and the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. Also, I would like to thank Prof. Jimmie L. Williams from Corning Incorporated for supporting me and providing the cordierite monoliths for this study  I would also like to thank all of the UBC Catalysis group members for lab assistance and equipment training: Rahman Gholami Shahrestani, Pooneh Ghasvareh, Ali Alzaid, Lucie Solnickova, Chujie Zhu, Majed Alamoudi, Shida Liu, and Haiyan Wang and from the Chemistry Department Yiling Dai and Dr. Vanama Pavan Kumar.  xxxiv  I would also like to thank my parents for advising and supporting me throughout my academic life. Also, I would like to thank my brother and sister. I would like to thank my beautiful daughters Leen, Tala and Lara for inspiring and motivating me every single day.  Finally, I would like to thank my loving, and adoring wife, Faizah, for supporting and motivating me during my PhD program.   xxxv  Dedication     To those who I love the most,  My parents, Hamoud and Saida  My daughters, Leen, Tala and Lara  My loving wife, Faizah1  Chapter 1: Introduction  Combustion of gasoline and diesel from land vehicles is a major contributor to air pollution. In Canada, 60 % of CO2 emissions and 50 % of oxides of nitrogen (NOx) emissions are produced from land vehicles [1]. Natural gas, which is cleaner burning than gasoline or diesel, has the potential to substantially reduce emissions of CO2 and other greenhouse gases from land vehicles [2]. Natural gas (NG) is an abundant source of energy that is expected to be used widely in the coming decades, especially with a limited supply of petroleum fuels. Natural gas has been used as a fuel for electricity generation, heating and currently, more than 19 million vehicles world-wide use natural gas as a transportation fuel [3]. The composition of natural gas varies, but CH4 is the primary component, representing almost 90% of natural gas.  A typical NG composition is presented in Table 1.1 [4].  Table 1.1 Natural gas compositions [4]. Component Vol% CH4 70-90 C2H6, C3H8, C4H10 0-20 CO2 0-8 O2 0-0.2 N2 0-5 H2S 0-5 He, Xe trace  2   Figure 1.1 shows that NG is cheaper than diesel and gasoline on an energy equivalent basis, and the price of NG has been relatively stable over the past decade. The prices in Figure 1.1 are compared based on the GGE, defined as the amount of alternative fuel that equals the energy content of one gallon of gasoline. The GGE allows one to compare the price of other fuels on an equivalent energy basis. In North America the total NG reserves are more than 11 trillion cubic meters of NG [5].      Figure 1.1 Average prices per Gasoline gallon equivalent (GGE) for gasoline, CNG, diesel from 2000 to 2018 (Adapted from [7]). 3  More than 19 million natural gas vehicles (NGVs) are being used worldwide and this number will increase by about 20% annually [6]. CH4 produces the lowest amount of CO2 per unit of energy among all fossil fuels [6]. CH4 is stored on-board vehicles as compressed natural gas (CNG) or liquefied natural gas (LNG). However, unburned CH4 emitted from the NGV exhaust is a big obstacle to using NGVs and has limited their use around the world. In the past decade, lower CH4 emission standards for NGVs were set by environmental agencies to decrease greenhouse gas (GHG) emissions. For example, in Europe CH4 emissions from heavy duty vehicles must be less than 0.5 g/kWh starting from 2014 (Euro VI) [7]. Consequently, unburned CH4 from natural gas vehicle exhausts limits the use of NG in the transportation sector and numerous studies have focused on developing catalysts and related technology to control CH4 emissions from NGVs [3, 6, 8-14]. The NGV engine can operate under stoichiometric or lean-burn conditions [3, 11, 14]. For stoichiometric operation the air to fuel mass ratio in the combustion chamber is close to stoichiometric (mass ratio of 14.7) which leads to lower fuel efficiency and higher exhaust gas temperatures [3, 11, 14]. Lean-burn NGVs, the focus of this thesis, operate with high air to fuel mass ratio which increases the fuel efficiency and decreases the exhaust gas temperature [3, 11, 14], making CH4 oxidation in a catalytic converter a challenge.   A high C-H bond strength (~435 kJ mol-1) makes CH4 difficult to oxidize and compared to other hydrocarbons, higher oxidation temperatures are required to convert CH4 to CO2 and H2O [9]. CH4 combustion produces the lowest amount of CO2 per unit of energy compared to other hydrocarbons [6]. High temperatures (> 600 °C) are required to reach optimum conversion of CH4. At lower temperature (350 to 450 °C) and near the exhaust gas temperature of lean-burn 4  NGVs (450–550 °C), it is difficult to control exhaust gas emissions from NGVs due the low concentrations of CH4 (400–1500 ppm), the low temperatures and the presence of high concentrations of H2O (5-15 vol.%) and CO2 (10 vol.%) [15].  There are two groups of catalysts described in the literature for CH4 oxidation - noble metals (e.g. Pd, Pt, Rh, Au) and transition metals (e.g. Co, Cu, Cr). The noble metal catalysts are more active than the metal oxides at low temperature and the most active metals are Pd and Pt supported on high surface area metal oxides (e.g. silica or alumina) [11, 16, 17]. The purpose of depositing the catalysts on a high surface area support is to increase metal dispersion which leads to an increase in the activity of the catalysts per unit mass. The supports also provide increased catalyst stability, especially in regard to sintering of the metal catalyst particles [17, 18]. Pt catalysts are not as active as Pd catalysts at low temperature for CH4 oxidation due to oxygen self-poisoning [3, 6, 11, 14]; however, they are less sensitive to H2O [3, 6, 19, 20] and sulfur compared to Pd [11, 21, 22].  Palladium is known to be the most active among all metals for CH4 oxidation at low temperature [6, 15, 23]. Recent studies of CH4 oxidation at low temperatures have also shown that PdO sites on Pd-O-Ce surfaces have high activity [8, 24]. Although Pd catalysts have high initial activity, they deactivate upon exposure to the exhaust gas for long periods [14, 25]. Also, Pd is very sensitive to sulfur and H2O which play a major role in inhibiting the catalyst activity [3, 16]. A comparison between Pd/𝛾-Al2O3 and Pd/Ce/Al2O3 powder catalysts [12] reported that Pd/𝛾-Al2O3 is more active than Pd/Ce/Al2O3 in a dry CH4/O2 gas environment; however, Pd/Ce/Al2O3 is more stable in the presence of H2O [12]. The advantage of Ce needs to be confirmed in a 5  monolith reactor operated at high space velocity and in the presence of CO, CO2 and H2O, all of which are present in the NGV exhaust gas.   Pd catalysts used for CH4 oxidation are known to deactivate in the presence of H2O and the effect of H2O decreases with increased temperature [17]. Figure 1.2 shows the decrease of the catalysts activity with increasing H2O concentration in the feed [26]. At low temperature, the deactivation of the catalyst by H2O is partly reversible due to the reversible adsorption of H2O [12]. However, the deactivation of the catalysts by H2O is irreversible for longer time exposure at higher temperature [13]. The rate of CH4 oxidation over Pd catalysts is reduced in the presence of H2O as reflected by the reaction order for H2O that varies from -1 to 0 [27]. The activation energy of CH4 oxidation over PdO catalysts is also changed significantly at temperatures below 600 °C in the presence of H2O [27, 28]. The structural collapse of the catalyst support [15, 29], sintering [30-33] and PdO conversion to Pd [15, 34, 35] have all been stated as the reason for the deactivation of Pd catalysts [6, 13]. The reversible inhibition occurs due to the adsorption of H2O on the active sites of PdO [15, 36]. Also, PdO conversion to Pd(OH)2 has been reported as another cause of the inhibition [17]. Previous studies that investigated the effect of H2O on CH4 oxidation over Pd catalysts at temperatures below 450 °C, reported that catalyst activity is limited by accumulation of hydroxyls on the Pd catalyst surface and that this impedes the O exchange between the gas phase, PdO and the support [37, 38]. 6   Figure 1.2 Catalytic combustion of CH4 over 1.1wt%Pd/Al2O3 with different amounts of H2O added (vol%). Reaction conditions: 1vol.%CH4, 20 vol%O2, 0-20 vol% H2O, balanced in N2.  GHSV=48,000h-1 [26]. (Copyright © 2002 Elsevier)  Monolith reactors were developed for Three-way-catalytic (TWC) converter technology, used to control emissions from gasoline powered vehicles. Similar technology is applied in NGV exhaust gas converters. A cordierite ceramic monolith is used in modern TWC converters which contain noble metals (Pt, Rh and Pd) dispersed on the monolith’s cell walls within a high surface area washcoat [39]. Usually the monolith has a honeycomb structure with a cell density of 400 cells per square inch (CPI). The monolith can handle the high gas flow with low pressure drop. 7  Conventional catalytic converters have not met CH4 emission standards due to the high concentrations of H2O and the low temperature (150 - 550 °C) of the exhaust from lean-burn NGVs [6, 17, 40]. Current commercial catalysts for CH4 oxidation show a high activity at low temperature; however, they are unstable in the presence of H2O [6, 17, 40]. Many studies have been reported on the effect of H2O on NGVs' exhaust catalysts [6, 15, 17, 41-44]. However, studies with a detailed analysis of the deactivation mechanisms of Pd catalysts deposited on a monolith and used for the CH4 oxidation in conditions close to real operating conditions (in the presence of H2O, CO and SO2 at low temperatures) are few in the literature.    Figure 1.3 The catalytic converter [4]. 8  1.1 Literature Review  Catalysts for CH4 oxidation over a range of operating conditions have been extensively studied in the literature and several review articles have summarized and compared the activities of these catalysts [6, 14, 15, 17, 26, 45, 46]. There are several factors that determine the catalysts’ activity and stability, such as preparation method, support type, composition, component precursors and the operating conditions. Most studies of catalytic CH4 oxidation focus on noble metal catalysts because they have higher activity per site and better resistance to sulfur at low temperature compared to metal-oxide catalysts [14]. Pd, Pt and Pd-Pt bimetallic catalysts are known to have the highest activity for CH4 oxidation at low temperature and consequently are the focus of most studies.  The activity of a series of supported noble metal catalysts (Pd, Pt, Ag) was investigated by Anderson et al. [47]. They stated that Pd was the most active metal for CH4 oxidation per gram of active metal, and the activation energy over Pd was lower than other noble metals. The activity of Pd and Rh on Al2O3 was investigated by Deng et al. [48] who also stated that Pd was the most active for CH4 oxidation. Moreover, Machid et al. [49] studied Pd, Rh and Pt supported on Al2O3, and stated that the order of activities for CH4 oxidation was Pd > Rh > Pt. The consensus from the literature is that Pd is the most active metal for CH4 oxidation.   The support of Pd catalysts can play an important role in determining catalyst activity. Hopos et al. [10] reported that the activity of a Pd/Al2O3 catalyst increased when Pd/Al2O3 was treated in a CH4-O2-N2 mixture at 600°C. However, for a similar treatment of Pd/SiO2 the activity did not change. Pd and Pt catalysts supported on different metal oxides (Al2O3, SiO2, TiO2) were investigated by Cullis and Willatt [50]. They reported that the support influenced the ability of 9  Pd to adsorb oxygen and there was an association between the adsorption capacity of the supported precious metal and its activity for CH4 oxidation. Haneda et al. [51] investigated the effect of support on the activity of Pd catalysts for CH4 combustion and tested two catalysts: a Pd/CeO2-Al2O3 that had been reduced in flowing H2 at 900°C prior to impregnation with Pd nitrate and a Pd/Al2O3 catalyst. They stated that the Pd/CeO2-Al2O3 was more active than the Pd/Al2O3 at 350°C since CeO2 accelerated Pd reoxidation which enhanced the catalyst activity [51]. Park et al. [52] have also stated that Pd/ZrO2 is more active and more stable than the Pd/𝛾-Al2O3 when hydrothermally aged at 600 °C for 100 h due to ZrO2 enhancing the stability of PdO species, since ZrO2 re-oxidized Pd0 to PdO faster than 𝛾-Al2O3. Since the present study will build on Pd catalysts studied and developed previously by researchers at  the University of British Columbia [12, 13, 53], the literature review that follows is focused on Pd, Pd-Ce and Pt-Pd-Ce based catalysts.  1.1.1 Monolith Catalyst Preparation   Monolith use is progressively expanding for many chemical applications, such as in petroleum industries and catalytic combustion [54]. Ceramic monoliths are prepared from synthetic cordierite (2MgO.2Al2O3.5SiO2) or γ-Al2O3 and are mostly used for automotive emission control applications [55]. The critical properties of ceramic monoliths are: (1) thermal shock resistance, (2) high melting point that exceeds 1450 °C, (3) compatibility with washcoat and catalysts, (4) reduction of fouling and plugging leads to extended catalyst lifetime, and (5) ease of scale-up [55]. The advantages of using monolith reactors are low-pressure drop, good mass transfer, and ease of product separation [54]. Table 1.2 shows the properties of the ceramic monolith [56]. For 10  the three way-converter (TWC), ceramic monoliths with 400 CPI are commonly used. A schematic of a ceramic monolithic catalyst is shown in Figure 1.4 [57]. A washcoat method is used to deposit a catalyst onto the walls of the monolith. First, a support layer (washcoat) of high surface area γ-Al2O3, SiO2, or ZrO2 is deposited on the monolith wall [58]. Then, an active phase such as Pt, Pd or Rh, is dispersed on the washcoat [58]. Alternatively, the support and the active phase can be deposited in a single step [57].   Table 1.2 Properties of ceramic monoliths [56]. Monolith  200 CPI 300 CPI 400 CPI 600 CPI Cell density per inch2 200 300 400 600 Wall thickness (mm) 0.3 0.3 0.15 0.15 Channel size (mm) 1.5 1.14 1.14 0.9 Open area (%) 69 63 77 73   11   Figure 1.4 Schematic of monolithic catalyst from [57]. (Copyright © 2003, Elsevier)  Typically, preparation of the catalytic monolith is done by a two-step coating of the monolith with a catalyst support material or washcoat, followed by deposition of the active phase on the washcoat [59]. The washcoat is commonly applied by dip-coating using a colloidal solution to deposit the washcoat on the monolith [60]. The usual procedure followed to apply the washoat is to firstly dry the monolith and then dip-coat the monolith by placing the monolith into the colloidal washcoat suspension for a brief period of time (~5 mins) [60]. Upon removal of the monolith from the washcoat suspension, pressurized air is used to remove excess liquid remaining in the monolith channels [60]. Finally, the monolith is calcined using air at 450 °C for a specific time period [60].     12  The impregnation method is commonly used to deposit the active phase on the washcoat. The concentration of metal in the impregnating solution is determined based on the amount of metal that is required to be deposited on the washcoat and the washcoat porosity [61]. The monolith is dipped into the solution containing the metal salt precursor for a specific time period [61]. Upon removal of the monolith from the solution, the excess solution is again blown out of the channels using pressurized air [61]. To avoid maldistribution of the metal in the washcoat, the dipping time should be short if the metal precursor interacts strongly with the support to control adsorption of the metal [61].  Drying the monolith is an important step which must be started after dipping the monolith to avoid maldistribution of the metal [61].  Several studies have investigated washcoat preparation using Al2O3 [62-67] because it has good porosity, low manufacturing costs and high surface area [68-70]. Depositing -Al2O3 as a washcoat on a cordierite monolith containing SiO2, MgO and Al2O3, makes the -Al2O3 the  primary support of the active phase [71, 72]. Although -Al2O3 has a high surface area, it is not stable because at high temperature (600 - 900 oC) in the presence of water, -Al2O3 transitions to α-Al2O3 which leads to a decrease in the activity of the catalysts [64, 70, 73]. The solid content, particle size of the Al2O3 and the viscosity of the suspension, all have a major impact on the adherence and homogeneity of the washcoat [62, 66, 74]. Blachou et al. [75] calcined gibbsite (Al(OH)3) at 600 oC for 3 hours to produce a -Al2O3  suspension and reported that the suspension viscosity was dependent on the solid content and pH of the suspension. They recommended a suspension with 42 wt% solids content and a pH of 3.7 to prepare a monolith with 18–22 wt% washcoat, obtained after two dip coatings of the monolith [75].  13  Pingping et al.[66] studied the effect of the solid content of the washcoat suspension on the loading of the washcoat and reported that as the solid content increased the loading of the washcoat increased, as shown in Figure 1.5. They prepared a washcoat suspension by mixing AlOOH and CeO2–ZrO2–La2O3 in citric acid and H2O and investigated the effect of the particle size on the adhesion properties by preparing three suspensions with the same solid content but different particle size [66]. Ultrasonic vibration was used to evaluate the mechanical strength of the bond between the washcoat and the cordierite monolith by placing the monolith in an ultrasonic water bath for 1 h. To assess the thermal stability of the washcoat, the same monolith was placed in an oven in air while heating to 1000 oC at 10 oC/min and holding the final temperature for 7 h. The weight loss of the monolith before and after the ultrasonic test and the thermal shock was taken as a measure of the washcoat adhesion. Table 1.3 summarizes the results of the adhesion tests [66]. The authors concluded that preparing the washcoat of the monolith from a suspension that has small particle size (<3 µm) increases the adhesion and thermal stability of the washcoat [66].   Table 1.3 Weight loss of washcoat/ceramic honeycomb treated by ultrasonic vibration and thermal shock (Adapted with permission from [66]). Washcoat Particle diameter (µm)   SBET  (m2 g-1) Weight loss after ultrasonic vibration (%) Weight loss thermal shock (%) Total Weight loss (%) I 0.04-2.9 50 0.6 6.4 6.9 II 06-34 41 3.7 6.3 10.0 III 14-123 26 14.6 9.1 23.7  14   Figure 1.5 The relation of the solids content in the slurry gel to the loading of coating. The pH value of gel: (1) 2; (2), 4; (3), 5 [66]. (Copyright © 2005, Elsevier)  Shimrock et al. [76] studied the effect of the suspension viscosity on the washcoat preparation and reported that a suspension with 35–52 wt% solid content and viscosity between 15–300 mPa.s was preferred to increase the loading and homogeneity of the washcoat. Agrafotis and Tsetsskou [62] studied the effect of particle size on the adhesion of the washcoat by using -Al2O3 suspensions with different -Al2O3 particle size distributions. They observed that a washcoat deposited from a suspension with particles with average particle size of 52 µm showed significant weight loss (~16 wt%) after hot air treatment for 16 h. For the washcoat obtained from a suspension with smaller particles (90% of particles < 6 µm) the weight loss was limited to 15  4 wt% [62]. They concluded that a suspension with smaller particles (90% of particles < 6 µm) is recommended to deposit the washcoat with good adhesion properties [62].  Zhou et al.[77] studied the effect of adding Al2O3, SiO2 or TiO2 as secondary support to ZrO2 by preparing three different suspensions ZrO2-Al2O3, ZrO2-SiO2, and ZrO2-TiO2 as the washcoats for a cordierite monolith. The monoliths were prepared by dip-coating twice each, to yield 25.1%, 18.7% and 17.4% washcoat loading of the ZrO2-Al2O3 , ZrO2-TiO2 and ZrO2-SiO2 suspensions, respectively [77]. They investigated the effect of calcination temperature on the washcoat loading, as reported in Figure 1.6, and concluded that calcination temperature from 350 – 950 °C did not have a major impact on the washcoat loading [77]. Rh and Pd were added to the washcoat and the activity of the catalysts was measured by temperature-programmed oxidation (TPO) which measured the conversion of C6H6 as function of temperature, using a total feed gas flow of 100 cm3(STP)·min−1, corresponding to a GHSV of 15000 h−1. The feed gas contained 1000 ppm of C6H6 in air. The TPO results showed that Pd/SiO2–ZrO2/cordierite was the most active catalyst and the Pd/Al2O3–ZrO2/cordierite was the least active for C6H6 oxidation since SiO2–ZrO2 improved the stabilization of Pd against sintering [77].  16   Figure 1.6 Effect of calcination temperature on washcoat loading (Al2O3–ZrO2) of cordierite monolith [77]. (Copyright © 2010, Elsevier)  1.1.2 Effect of Support on the Catalyst CH4 Oxidation Activity  Kucharczyk et al. [78] studied the effect of adding La2O3 and SiO2 or ZrSiO4 to the washcoat of a PdO/-Al2O3 catalyst and reported that these promoter metals improved the activity and stability and enhanced the thermal resistance of PdO/-Al2O3.  From XPS analysis they found that the dispersion of Pd on the surface of PdO/-Al2O3 after aging at 1060 oC for 24h, decreased from 25% to 1% [78].  The Pd dispersion decreased from 12 % to 1 % for the for PdO/-Al2O3-SiO2 and from 13 % to 2 % for PdO/-Al2O3 -ZrSiO4 after the same thermal treatment. They concluded that the decreased dispersion of Pd leads to a decrease in the activity of the catalyst, and adding ZrSiO4 and SiO2 to PdO/-Al2O3 improved the activity and the stability of the 17  catalysts by improving the thermal resistance of PdO/-Al2O3, as demonstrated by the data of Figure 1.7 [78].  Figure 1.7 CH4 conversion over catalysts: fresh, after the run and after 24 h of aging at 1060 oC. Catalysts: (a) Pd/Al2O3; (b) Pd/Al2O3 -ZrSiO4; (c) Pd/Al2O3-SiO2 [78]. (Copyright © 2004, Elsevier)  Kucharczyk and Tylus [79] studied the effect of adding cobalt oxide (Co3O4) to the washcoat on the activity of a Pd monolith catalyst for CH4 oxidation. The monolith (112 CPI, 2.6cm diameter x 7 cm length) was washcoated using two different suspensions, one with 25wt% Al2O3  and the second with 22.5wt% Al2O3 plus 2.5 wt%Co3O4 [79]. After dip-coating, the monolith was dried 18  at room temperature for 4 h and there after at 110 oC for 3 h and then calcined at 400 oC for 3 h. The calcined, washcoated monolith was wet impregnated with a Pd solution followed by calcination at 500 oC for 3 h to deposit 0.5% Pd or 1% Pd (in relation to the washcoat mass) on the monolith [79]. The authors reported that adding 0.3wt% Co to the 1%Pd/Al2O3 increased the Pd dispersion from 4% to 20%, which led to increased catalyst activity. Moreover, the activity of the catalysts decreased significantly after aging in air for 24 h at 1000 oC, and it was found that the Pd existed as Pd0 in the aged catalyst. Figure 1.8 reports the CH4 conversion of the Pd catalyst with and without Co3O4 in the washcoat, showing the higher conversion obtained in the presence of the Co due to increased Pd dispersion [79].  19   Figure 1.8 Comparison of CH4 conversion over monolithic catalysts. (1) Fresh 1%Pd/Al2O3; (2) fresh 1%Pd/0.3%Co/Al2O3; (3) 1%Pd/0.3%Co/Al2O3 after 20 h on stream; (4) 1%Pd/0.3%Co/ Al2O3 after ageing for 24 h at 1000 oC [79]. (Copyright © 2008, Elsevier)   𝛾-Al2O3 is commonly used as the washcoat of ceramic monoliths to support the active phase of TWCs, [80] since the ceramic monolith has low surface area [81, 82]. The washcoat provides a high surface area support on which to disperse the active phase [81]. One of the drawbacks of 𝛾-Al2O3 is a low thermal stability at high temperature in the presence of water and the resulting transition to low surface area α -Al2O3 that also decreases the activity of the catalyst [71]. Adding CeO2 to 𝛾-Al2O3 is one option to improve the thermal stability of the washcoat [83]. CeO2 has been extensively used in TWCs because of its ability to store O2 under lean burn operating 20  conditions and release O2 under fuel rich operating conditions [83-85]. Ozawa and Kimura [86] studied the effect of adding CeO2 to Al2O3 aged at high temperature (1000–1200◦C) in air for 5 h. They reported that the thermal stability of the 𝛾-Al2O3 was enhanced by CeO2 addition. Piras et al. [87] proposed that the formation of CeAlO3 was the reason for improved thermal stability of 𝛾-Al2O3 because CeAlO3 impedes the growth of 𝛾-Al2O3 crystals. Furthermore, Pingping et al. [66] studied the effect of adding CeO2–ZrO2–La2O3 to the 𝛾-Al2O3 washcoat for monolith catalysts.  They prepared a washcoat suspension by mixing 𝛾-Al2O3 and CeO2–ZrO2–La2O3 with H2O and citric acid and reported that adding CeO2–ZrO2–La2O3 enhanced the washcoat adherence to the monolith, increased washcoat homogeneity and improved the thermal stability of the washcoat [66].  Fan et al. [88] studied the effect of adding Ce to Pt-Pd/Al2O3 catalysts for CH4 oxidation. They prepared two samples using the wet impregnation method: 1%Pd-0.2%Pt/Al2O3 and 1%Pd-0.2%Pt/0.6%Ce/Al2O3. They reported that adding the Ce improved the activity and the stability of the Pt-Pd/Al2O3 [88]. Also, the stability of the catalyst was determined using a time–on-stream (TOS) test at a GHSV of 80 000 h−1 with a feed gas of 1.5 vol% CH4 in dry air [88]. The TOS data showed that adding Ce improved the stability of the Pt-Pd/Al2O3 catalysts [88].  Moreover, they reported that the Pt-Pd average particle size of the Pt-Pd/Al2O3 catalyst increased from 11 to 16 nm after 10 h reaction at 400 oC; whereas, the average particle size for the Pt-Pd/Ce/Al2O3 catalyst increased marginally from 9.5 to 9.8 nm. The authors concluded that adding the Ce improved the activity and the stability of the catalyst by preventing the growth of PdO particles [88].  21  Oh et al.[89] evaluated the CH4 oxidation activity of noble metals catalysts (Pd, Pt and Rh) supported on Al2O3 with and without CeO2 added to the support. The activity of the catalysts was measured by TPO with 0.2 vol% CH4, 0.1 vol% CO, and 1 vol% O2 balanced by He as the feed gas. The authors reported that without CeO2 added to the support, the activity of the catalysts ranked as Pd > Rh > Pt and as Rh > Pd ~ Pt with CeO2 added to the support, as shown in Figure 1.9 [89]. The authors proposed that the activity decrease, observed in the presence of CeO2, was because of increased Pd and Pt oxidation, which made the catalysts less active for CH4 oxidation. Adding CeO2 did not have any impact on the Rh catalyst activity due to the fact that the oxidation state of the Rh/Al2O3 is not affected by the added CeO2 [89]. The author concluded that adding CeO2 has a major impact on the activity of the Pd and Pt catalysts but not Rh due to the extent of oxidation of the metal by the CeO2.  Figure 1.9 CH4 conversion as a function of temperature in an oxidizing feed stream containing 0.2 vol% CH4, 0.1 vol% CO, and 1 vol% O2 balanced by He (A) without adding CeO2 to Al2O3 and (B) with adding CeO2 to Al2O3 [89]. (Copyright © 1991, Elsevier) 22  Recent studies of CH4 oxidation at low temperatures have also shown that PdO sites on Pd-O-Ce surfaces have high activity [8, 90]. In a previous study of PdO/Ce/𝛾-Al2O3 catalysts by Alyani and Smith, TPO tests showed that the activity decreased with increased Ce loading of the catalysts [12]. The T10, T50, and T90 temperatures of 6.5Pd/Al2O3 and 2.9Ce/6.5Pd/Al2O3 are presented in Table 1.4, measured in a feed gas of 1000 ppm of CH4, 20 vol% O2, the balance Ar and He, at a flow rate corresponding to a GHSV of 180 000 mL·gcat−1·h−1. Catalyst stability was determined by TOS tests conducted with 5000 ppm of CH4, 20 vol% O2, 5 vol% H2O, and the balance He corresponding to a GHSV of 180 000 mL·gcat−1·h−1 at 330 oC for 24h and 350oC for 24 h. Both catalysts showed a loss in activity in the presence of H2O but the 6.5Pd/Al2O3 had higher activity than the 2.9Ce/6.5Pd/Al2O3 catalyst. At 330°C both catalysts showed a high loss of activity compared to the results at 350°C due to an increase in H2O adsorption with decreased temperature [12]. A kinetic model of CH4 oxidation in the presence of H2O was developed to recognize the effect of adding Ce to a 6.5Pd/ Al2O3 catalyst. From the results, the equilibrium constant for H2O adsorption on the 6.5Pd/Al2O3 catalyst was shown to be higher than on the 2.9Ce/6.5Pd/Al2O3 catalyst, and the rate of H2O desorption was also higher on the Ce-promoted catalyst [12]. Therefore, adding Ce to a 6.5Pd/ Al2O3 catalyst led to a reduced inhibition of the catalyst activity following the addition of H2O [12].   Table 1.4 Light-off temperatures for 6.5Pd/𝛾-Al2O3 and 2.9Ce/6.5 Pd/Al2O3 catalysts [12]. Catalyst T10 (°C) T50 (°C) T90 (°C) 6.5 Pd/Al2O3 142 251 285 2.9Ce/6.5 Pd/Al2O3 193 253 290 23  A new method for constructing PdO/CeO2 nanostructured materials with high activity has been discovered by Gomathi et al. [53]. TPO experiments for CH4 oxidation were conducted on two catalysts prepared by surface assisted reduction CHC-f/Pd (1 mM) and CF/Pd (0.3 mM)) and a Ce/1% Pd catalyst prepared by a modified incipient wetness impregnation (MIWI) method [53]. CHC-f/Pd (1 mM) and CF/Pd (0.3 mM) samples were prepared by in situ reduction of 1 mM and 0.3 mM palladium nitrate on CHC-f (cerium hydroxycarbonate), respectively. Then, the materials were calcined at 400 °C to obtain 1 wt % Pd-CeO2 and 0.3 wt % Pd-CeO2 catalysts. The CHC-f/Pd (1 mM) and the CF/Pd (0.3 mM) had high activity with T50 < 300 °C and T100 approximately 400 °C. On the other hand, the 1% Pd/Ce had lower activity with T50 > 400 °C [53]. Time-on-stream (TOS) tests in which the CH4 conversion was monitored for extended periods of time were used to determine the stability of the catalysts in the presence of 5 vol.% H2O at 380 °C, and the results are shown in Figure 1.10 for the CHC-f/Pd (2 mM) catalyst containing 1 wt% Pd and the 5.3%Pd/Al2O3 catalyst which was prepared by conventional MIWI method [53]. From the results, CHC-f/Pd (2 mM) showed high stability after 24 h [53] and it can be concluded that the CHC-f/Pd catalysts are highly active and show very good hydrothermal stability [53]. The excellent Pd dispersion on the ceria-precursor nanostructures accounted for the high catalytic activity [53]. 24   Figure 1.10 Steady-state oxidation of CH4 in the presence of H2O (5 vol% H2O), using CHC-f/Pd or 5.3 wt %Pd/Al2O3 as the catalyst [53]. (Copyright © 2015, American Chemical Society)  Persson et al. [19]  studied the effect of Al2O3, ZrO2, LaMnAl11O19, and Ce-ZrO2 washcoats on Pd-Pt catalyst activity for CH4 oxidation. The initial CH4 oxidation activity was measured at atmospheric pressure in 1.5 vol% CH4 in dry air and a space velocity of 250,000 h-1. They reported that at low temperature (470 °C) Pd-Pt/Al2O3 was the most active catalyst while the Pd-Pt/Ce-ZrO2 catalyst was the most active at high temperatures (620°C - 800°C). The high surface area of the Al2O3 (90 m2/g) compared to the Ce-ZrO2 (10 m2/g) accounted for the higher activity at low temperature [19, 91].  Also, the authors stated that Ce-ZrO2 improved the stability of the PdO because Ce-ZrO2 re-oxidized Pd0 to PdO faster than Al2O3 [19].   25  1.1.3 The Use of Pd-bimetallic Catalysts for CH4 Oxidation  Pd-bimetallic catalysts have been investigated to enhance the stability of Pd catalysts for CH4 oxidation [92-95]. Although Pd is reported as the most active metal for CH4 oxidation, it is not stable in the presence of SOx and H2O in the feed and deactivates significantly during reaction.  Some studies report that Pd-bimetallic catalysts are more active than Pd alone [92, 93, 96, 97]. These studies suggest that adding a second metal to Pd dissociates O2 and the adsorbed O atoms sustain the PdO active sites. For instance, Ishihara et al. [97] reported that T50 (the temperature at which 50% CH4 conversion is achieved during light-off) for a 1 wt% Pd/Al2O3 catalyst was 533 oC but decreased to 380 °C for a Pd-Ni/Al2O3 catalyst (Pd:Ni = 9:1). Adding Pt to Pd catalysts has also been investigated to enhance the stability of Pd catalysts for CH4 oxidation [92-95]. Pt-Pd bimetallic catalysts are less sensitive to H2O [15] and sulfur compared to Pd [11]. Strobel et al. [98]  prepared monometallic Pd and a bimetallic Pt-Pd  supported on Al2O3 for CH4 oxidation, and the activity of the catalysts was measured by TPO  by heating from 200 to 1000 oC  at 10 °C/min with a feed gas composition of  1 vol% CH4 and 4 vol% O2 in He. They reported that the Pt-Pd was more active than Pd due to Pt enhancing the sintering resistance of Pd particles [98]. Similarly, Ozawa et al. [99] reported that adding Pt to PdO/Al2O3 improved the stability of the catalysts by suppressing the growth of PdO and Pd–Pt particles during CH4 combustion at high temperature (800 °C) [99]. On the other hand, Persson et al. [19] stated that Pd has higher catalytic activity than Pt-Pd for the same loading of the metal. Alloy formation between Pd and Pt is one reason for reduced activity of the bimetallic compared to the Pd alone [19], and the decomposition of PdO to Pd0 leads to decreased activity of the bimetallic catalysts, as reported by Kuper et al [100]. 26  Narui et al. [93] reported that the PdO-Pt/α-Al2O3 catalyst had a higher initial activity and higher stability compared to PdO/α-Al2O3 as reported in Figure 1.11 for methane oxidation at 350 oC with a feed gas of 0.5 vol% CH4 in dry air at a GHSV 18000 h-1 [93]. The data show higher stability of the PdO-Pt/α-Al2O3 because the Pt increased the dispersion of Pd from 14% for the Pd monometallic catalyst to 27% for the bimetallic catalyst [93]. The authors reported that the average PdO particle size for the PdO/α-Al2O3 catalyst increased from 8 to 11 nm after the catalyst was exposed to the reactant feed gas for 6 h at 350 °C. For the PdO-Pt/α- Al2O3 catalyst the average particle size did not change significantly [93]. The authors concluded that adding Pt to PdO/α-Al2O3 increased the dispersion of Pd and decreased the rate of PdO sintering which led to increased stability of the catalyst [93].  Figure 1.11 Time-course of catalytic activities of supported PdO catalysts: (●) PdO/α-Al2O3 and (○) PdO-Pt/α-Al2O3 for CH4 oxidation at 350 oC for 6 h [93]. (Copyright ©1999, Elsevier) 27  Abbasi et al. [94] studied the stability of Pt and Pt-Pd catalysts prepared on a cordierite monolith. One catalyst used monometallic Pt as the active phase and other used Pt-Pd bimetallic in a 4:1 ratio. The loading of the active phase for both monoliths was 3300 g/m3, based on the volume of the monolith. The feed contained 4067 ppm CH4 in air and the temperature of the reaction increased from 300°C to 700°C stepwise in 50°C increments [94]. For each step the conversion of CH4 was measured for 1h. Then the temperature of the reaction was decreased to 300°C stepwise in 50°C increments, and the conversion was measured for each step in the cooling cycle [94]. The activity for both catalysts was compared for both heating and cooling cycles. The authors reported that the Pt-Pd bimetallic was more active than the monometallic Pt. Moreover, the catalysts were aged at 650 oC in 4067 ppm CH4 and 5vol.% H2O in air for 20h. The activity of the catalysts was then measured for both heating and cooling cycles, and it was found the Pt-Pd was more active than Pt after being aged [94]. Table 1.5 summarizes the TPO results for both catalysts [94].  Table 1.5 T50 for fresh and steam aged Pd and Pt-Pd catalysts operated in dry feed. Combustion conditions: 4067 vol. ppm CH4; total flow rate of 234.5 cm3.min-1; 500 mg catalyst (Adapted with permission from [94]). Catalyst Fresh  Dry feed  T50 (°C) Steam-aged  Dry feed  T50 (°C) Pt 540 610 4:1 Pt-Pd 400 470   28  1.1.4 Effect of SOx on CH4 Oxidation over Pd Catalysts  Wilburn and Epling [21] studied the effect of the Pd-Pt ratio on the activity and stability of Pd-Pt supported on 𝛾-Al2O3. They varied the Pd-Pt ratio (Pd1.0 Pt0.0/Al2O3, Pd0.9 Pt0.1/Al2O3, Pd0.7 Pt0.3/Al2O3, Pd0.3 Pt0.7/Al2O3, and Pd0.0 Pt1.0/Al2O3) while the total noble metal loading remained constant [21]. The authors used TPO to determine the activity of the catalysts, using 29.3 mg of catalyst and a total feed gas flow rate of 200 cm3(STP)·min−1 corresponding to a GHSV of 50000h−1. The feed gas composition was 0.2 vol% CH4, 10 vol% O2 in N2 [21]. The authors reported a ranked catalyst activity in decreasing order: Pd0.9 Pt0.1 > Pd1.0 Pt0.0 > Pd0.7 Pt0.3> Pd0.3 Pt0.7> Pd0.0 Pt1.0. The catalyst stability was also investigated by exposing the catalysts to 30 ppm SOx and 10 vol% O2 in N2 at 100 ◦C until the sample reached SOx saturation. Then, the samples were purged using N2 at 100 oC to remove the residual SOx. After that the TPO was conducted on the regenerated catalysts at the same reactions conditions as was used for the fresh sample. The authors reported that the T50 for the Pd0.0 Pt1.0/Al2O3 (~600 oC) had not changed for the fresh and regenerated catalysts [21]. For the Pd1.0 Pt0.0/Al2O3 and Pd0.9 Pt0.1/Al2O3 catalysts the T50 increased by 18 and 14 oC, respectively. They concluded that adding Pt to Pd in a low ratio increased the activity of the catalyst and enhanced the stability of the catalyst in the presence of SOx since adding Pt to Pd provided some sinter resistance to the Pd catalyst [21].  Moreover, Gremminger at al.[101] investigated the effect of SOx and H2O on the activity and the stability of Pd-Pt/Al2O3. The authors reported that the surface area of a Pd-Pt/Al2O3 catalyst decreased from 145 to 130 m2/g after TPO in 3230 ppm CH4, 10 vol% O2, 2.5 ppm SOx and 12 vol% H2O balanced by N2 at a GHSV of 30,000 h−1. Also, they investigated the stability of Pt-29  Pd/Al2O3 catalysts by conducting TOS experiments in 3200 ppm CH4, 10 vol% O2, 12 vol% H2O 2.5 or 5 ppm SOx in N2 at 400 oC for 24h and at 450 oC for 24h. They reported that the deactivation of the catalysts by SOx depended on the SOx concentration and temperature of the reaction, the deactivation increasing with increased SOx concentration in the feed and decreasing with increased temperature. They concluded the main reason for deactivation of the catalysts by SOx  is that the SOx was adsorbed by the active phase and the support [101].  Lampert et al. [11] studied the effect of SOx on the activity and the stability of Pt and Pd monolith supported catalysts for CH4 oxidation. They investigated the effect of the support on the stability of the catalyst in the presence of the SOx and prepared the catalysts with 𝛾-Al2O3 (with different surface areas),α-Al2O3, ZrO-SiO2 and SiO2 for both Pt and Pd [11]. They tested the stability of the catalysts by conducting TOS tests with a feed gas of 800 ppm CH4, 8 vol% O2 and 0.9 ppm SO2 in N2, reacted at 320 oC for 8h and GHSV is 200000 h-1. The authors reported that Pd was more active than Pt; however, Pt was more resistant to deactivation by SOx [11]. Moreover, for Pd catalysts the authors found that the catalysts with non-sulfating supports (ZrO-SiO2, SiO2) deactivated very rapidly in the presence of SOx. For the catalysts with a sulfating support (Al2O3) that has the ability to adsorb SOx, the deactivation in the presence of SOx is slower since SOx was adsorbed onto the support. SOx adsorption by the Pd catalysts was measured by a TGA balance using 2% SO2 in air at 320 oC and the desorption in air at 650 oC. As reported in Table 1.6, the Al2O3 adsorbed more SOx than other supports and the adsorption increased with increased surface area of the Al2O3 [11]. From these data it can be concluded that the catalysts supported on (Al2O3) showed better resistance to SOx deactivation compared to the 30  catalysts supported on ZrO-SiO2 or SiO2 since the SOx absorbed on Al2O3 more than ZrO-SiO2 or SiO2 which reduced the adsorption of SOx on the active phase (Pd or Pt).  Table 1.6 Adsorption of SO2 at 320 °C desorption at 650 °C for 6% Pd on different supports [11]. Pd catalyst support Surface area (m2/g) SO2 adsorbed  at 320 °C (wt %) SO2 retained at 650°C (wt %) 𝛾-Al2O3 375 6.7 5.8 𝛾-Al2O3 150 5.3 3.9 Θ + α-Al2O3 50 0.9 0.2 ZrO-SiO2 240 0.7 0 SiO2 300 0.2 0  1.1.5 Washcoat overlayer  Since CH4 oxidation is an exothermic reaction, increasing temperature at the reaction surface must be avoided to limit catalyst sintering [102]. A patent application by Dalla Betta, Tsurumi et al. [102] described a method to control the rate of a catalytic combustion using a diffusion barrier or washcoat overlayer, so as to reduce the reactant concentration in contact with the catalyst and thereby reduce the rate of combustion and hence heat evolution. Monoliths made from cordierite, steel or Fe-Cr-Al composites were used in the study, with Al2O3 and ZrO2 used as washcoat materials and Pd the active phase of the catalysts. An inactive, porous diffusion barrier layer using SiO2 and ZrO2 was placed on top of the active phase and the washcoat [102]. The thickness of the diffusion barrier layer was between 10% and 100% the thickness of the washcoat. The 31  authors stated that damage to the catalysts at high temperature could be avoided by adding the diffusion barrier which limited the reaction rate and controlled the wall temperature [102]. A theoretical study, reported by Leung et al. [103] in which the effect of the diffusion in the washcoat on the rate of catalytic combustion reaction was assessed, demonstrated that the reaction rate decreased when the diffusion barrier was applied, and the wall temperature could be controlled. [103]   Hayes et al. [102] also studied the effect of a diffusion barrier or washcoat overlayer on Pd catalysts for CH4 oxidation. These authors conducted experiments using monolith catalysts with and without a diffusion barrier and developed a mathematical model of the monolith reactor, accounting for mass and heat transfer in both the washcoat layer and the diffusion barrier. The catalysts were prepared by depositing the Al2O3 as washcoat on the monolith and Pd as the active phase of the catalyst. The diffusion barrier layer was applied by adding a layer of Al2O3 on the top of the active phase [102]. The experiments were conducted at 101 kPa with a reactor inlet temperature of 350 oC which increased by 3-6 oC/min to 720 oC. The feed gas consisted of 0.4 - 1.4 % CH4, 0 - 3.2 % H2O, 0 - 3.2 % CO2 and 20.8 % O2. Table 1.7 reports the results for the effect of the washcoat overlayer thickness on the CH4 conversion and the outlet temperatures of gas. The results showed that all the catalyst deactivated in the presence of H2O and that CO2 did not have any impact on the catalyst performance. For all reaction conditions the results showed that adding the diffusion barrier decreased the reaction rate and the wall temperature could be controlled by adding the diffusion barrier. [102] The authors reported that as the thickness of the washcoat overlayer increased, CH4 conversion decreased as did the reactor outlet gas temperature [102]. 32  Table 1.7 CH4 conversion and outlet temperature for monolith catalysts with different thickness of the washcoat overlayer and 7.6% Pd loading. The feed was 1% CH4 in air and the inlet temperature was 452 °C (Adapted with permission from [102]).  Washcoat overlayer  thickness (µm) CH4 Conversion (%) Outlet temp of gas (°C) 0 78 648 1 56 592 2 41 555 5 27 519 10 16 492  The sintering of Pd particles is one of the reasons for Pd catalyst deactivation during CH4 oxidation [15, 37, 104]. At high temperature (> 500 oC) and in the presence of H2O, the Pd nanoparticles (NPs) agglomerate which leads to a decrease in the catalyst surface area available for reaction [25, 31]. Also, at low temperature the thermal sintering of Pd particles has been observed [105]. In the last decade, the encapsulation of metal NPs in various oxides has been studied in order to improve the thermal stability of NPs and to prevent or limit NPs sintering [106-108]. Pi et al. [109] investigated the catalytic oxidation of CH4 over Pd@SiO2 in dry/wet conditions and reported that encapsulation of Pd in SiO2 prevented sintering of the Pd NPs [110, 111].  Habibi et al. [112] recently studied the effect of encapsulation of Pd-Pt in SiO2 on CH4 oxidation catalyst stability, under reaction conditions that included water in the feed gas (wet conditions). The authors prepared PdPt@SiO2, PdPt/SiO2 and PdPt/Al2O3 catalysts, with the Pd and Pt 33  loading fixed at 4.2 wt% Pd and 6.98 wt% Pt. The time-on-stream tests were done at 385 and 550 oC in a feed gas with 0.4 vol% CH4, 19 vol% O2, 5 vol% H2O in N2 at a GHSV of 133,800 L(STP)/(h.kgcat). At 550 oC, PdPt@SiO2 and PdPt/Al2O3 showed the same stability and at 385 oC PdPt@SiO2 was more stable than the other catalysts as presented in Figure 1.12 [112]. Also, the authors reported that the aged PdPt@SiO2 had the highest activity among the catalysts tested. For PdPt@SiO2 no change in the catalyst morphology occurred after being exposed for 170 h to 5 vol% H2O. Pd particle sintering was observed for PdPt/SiO2 and PdPt/Al2O3 during the calcination and after exposure to 5 vol% H2O at 385 and 550 oC and formed large agglomerates [112].  Figure 1.12 CH4 conversion vs time on stream during HTA. Total catalyst loading is 100 mg, corresponding to a GHSV of 133800 ( L(STP).h−1.kgcat -1) with 0.4 vol% CH4, 19 vol% O2 , 5 vol% H2O in N2 [112]. (Copyright © 2018, Elsevier) 34  1.1.6 CH4 Oxidation Mechanism and Kinetics  The kinetics of CH4 oxidation by Pd based catalysts are influenced by H2O. Several studies have been published on the kinetics of CH4 oxidation for different catalysts and operating conditions [12, 42, 54, 103, 113-126]. The reaction orders are usually one and zero with respect to CH4 and O2 respectively [127]. H2O is known to inhibit CH4 oxidation; thus, the reaction order with respect of H2O varies from -1 to 0 as temperature increases from 320 to 600 °C [49, 50]. The inhibition of catalyst activity by H2O decreases with increasing temperature since the desorption rate of hydroxyl/H2O increases [8, 49] which leads to enhanced CH4-O2 reaction rates. The rate-determining step (RDS) of CH4 oxidation over Pd catalysts is commonly assumed to be the C-H bond activation of CH4 which is affected by H2O adsorption on the catalysts surface [34, 66]. Also, PdO conversion to Pd(OH)2 has been reported as another cause of the inhibition [17]. Furthermore, hydroxyl/H2O accumulation on the support that suppresses the oxygen exchange between the support and Pd has been proposed as one of the mechanisms of catalyst inhibition by H2O [38]. Van Giezen et al.[114] reported an activation energy of 86 kJ.mol-1 for CH4 oxidation over a 7.3%Pd/Al2O3 catalyst in dry feed gas, but this increased to 151 kJ.mol-1 for wet feed gas [114]. Ribeiro et al.[115] reported on the effect of adding H2O to the order of the CH4 oxidation reaction. At low temperature (300 °C) the reaction order with respect to H2O was -1.3 while for higher temperature (350 °C) the order was -0.9 [115]. Table 1.8 reports the activation energy and reaction orders over Pd catalyst obtained in different studies. The results agree on the reaction orders with respect to CH4, O2 and H2O as 1,0 and -1, respectively.    35  Table 1.8 Apparent activation energy and order of CH4 combustion reaction over Pd catalysts (Adapted with permission from [6]). Catalyst   Ea   kJ mol-1 Reaction Order Temperature Range oC Refs CH4 O2 H2O Supported Catalysts 10%Pd/ZrO2 174 1 0 -1 232-360 [115] 1.1%Pd/Al2O3 81 1 0 -1 290-500 [26] 1%Pd/ZrO2 170 1 0 -1 227-441 [128] 7.3%Pd/ Al2O3 86 1 0.1 -0.8 253-315 [114] 0.5%Pd/ Al2O3 60 0.9 0.08 -1.3 to- 0.9 240-400 [124]  Fujimoto et. al. [36] proposed a reaction mechanism for CH4 oxidation over a PdOx/ZrO2 catalyst which assumes CH4 dissociation on a Pd-PdO site pair, as illustrated in Figure 1.12. The CH4 oxidation occurs in three steps as the scheme shows [36]. In the first step, vacant Pd sites adsorb CH4. Secondly the PdO abstracts H atoms from the adsorbed CH4 (C-H bond activation). The final step is production of Pd-OH (surface hydroxyl groups). They proposed that the RDS is the H-abstraction step(C-H bond activation) - Step 2 [36]. In another study by Burch et al. [104], the decomposition of Pd−OH in step 3 was proposed as the RDS.  36   Figure 1.13 Mechanism of CH4 dissociation on Pd/PdO site pair [36]. (Copyright © 1998, Academic Press)  Recently, Stotz et al. [129] investigated the surface reaction kinetics of CH4 oxidation over a PdO/Al2O3 catalyst by conducting TPO experiments in the presence of H2O at temperatures below 650 oC and by density functional theory (DFT ) analysis. They proposed that CH4 was oxidized via a pyrolytic decomposition and formed formate (CHxOy) species.  They reported that adding H2O in the feed suppressed the formation of formate species that led to an inhibition of the catalyst activity. They concluded that adding H2O to the feed inhibited the catalyst by adsorption of H2O/OH on the surface of the PdO [129].   Kikuchi et al. [26] investigated the activity of 1.1wt%Pd/Al2O3 and 1.1wt%Pd/Al2O3-36NiO for CH4 oxidation in wet and dry feed gas. The activity of the catalysts was determined by TPO at reaction conditions: 1 vol% CH4, 20 vol% O2, 0-20 vol% H2O, balanced in N2 and a GHSV of 48,000h-1 [26].  They reported that the T30 for 1.1wt%Pd/Al2O3 increased from 345 oC for dry feed to 510 oC for wet feed (20 vol% H2O); whereas, for 1.1wt%Pd/Al2O3-36NiO T30 increased from 372 oC for dry to 445 oC for wet feed (20 vol% H2O) [26]. They investigated the performance of the catalyst using a kinetic model assuming that CH4 adsorption on PdO is the 37  rate-determining step (RDS) and H2O adsorption on PdO surface is the main cause of the catalyst inhibition. The rate equation is as follows: −𝑟𝐶𝐻4𝑚 = 𝑘𝑟 𝑃𝐶𝐻4𝜃𝑣                                                                                                                                        1.1 𝜃𝑣 = 1 − 𝜃𝐻2𝑂 =11 + 𝐾𝐻2𝑂𝑃𝐻2𝑂                                                                                                              1.2 𝑘𝑟 = 𝑘𝑟0exp [−𝐸𝑎𝑅𝑇]                                                                                                                                        1.3 where, 𝐸𝑎 is the activation energy (kJ.mol-1), Δ𝐻𝐻2𝑂 is the enthalpy of H2O adsorption (kJ.mol-1) 𝑘𝑟 is the rate constant (mol·gcat-1 s−1·Pa−1), 𝑃𝐶𝐻4  is the partial pressure of CH4 (Pa), 𝜃𝑣 is the fraction of vacant site pairs (both PdO and Pd-*),   𝐾𝐻2𝑂 is the equilibrium constant for H2O adsorption (Pa−1) and 𝑃𝐻2𝑂 is partial pressure of H2O (Pa) . They reported that adding NiO to Pd/Al2O3 decreased the strength of the H2O adsorption on the PdO surface with the heat of adsorption decreasing from -49 to -30 kJ.mol-1 [26]. They concluded that Pd/Al2O3 has the lowest activity in the presence of H2O in the feed due to the H2O covering the active site and limiting the CH4 adsorption on PdO. Addition of NiO decreased the H2O adsorption on the PdO surface and enhanced the activity for the catalysts in the presence of H2O. Many other studies have developed kinetic models of CH4 oxidation that account for the effects of H2O inhibition [12, 14, 26, 113] and deactivation by SO2 [130, 131], mostly using Langmuir-Hinshelwood kinetics.  Most consider similar mechanisms, with CH4 first activated on a Pd/PdO site pair, with re-oxidation of Pd species and hydroxyl removal as key steps. Three of these kinetic models are examined in Chapter 6 of this study and are used to interpret the effects of H2O inhibition and the impact of the washcoat overlayer.  38  1.1.7 Summary of the Literature Review  The most common method of monolith catalyst preparation is with the washcoat first applied by dip-coating followed by addition of the active phase by wet impregnation. The washcoat deposition is influenced by several factors, including the solid content of the suspension, the particle size and surface area of the solid. The solid content of the suspension plays a major role in the washcoat loading and washcoat quality, and as the solid content increases, the loading of the washcoat increases.  However, the washcoat adhesion and thermal stability decrease with increased washcoat mass. The calcination temperature of the washcoat does not have any impact on the washcoat loading. As noted above, most studies of monolith catalysts for NGVs have focused on the preparation of the washcoat and the effect of the suspension properties, with few studies reporting on the stability and activity of these catalysts for low temperature CH4 oxidation in the presence of H2O, CO, CO2 and SO2.  Adding CeO2 to Pd/Al2O3 influenced the initial activity of the catalyst and decreased the activity in dry feed for CH4 oxidation, even though adding CeO2 increased the Pd dispersion. In the presence of H2O in the feed, adding CeO2 improved the stability of the Pd/Al2O3 catalysts for CH4 oxidation since the CeO2 increased the O2 exchange between the Pd and the support. The advantage of adding CeO2 to Pd /𝛾-Al2O3 needs to be confirmed in a monolith reactor operated at high space velocity and at temperatures below 550 oC in the presence of CO, CO2 and H2O, all of which are present in a NGV exhaust gas.   39   The literature provides significant evidence that Pd is more active than Pt for CH4 oxidation, and some studies reported that adding Pt to Pd improved the activity of the catalysts for CH4 oxidation. Also, adding Pt to Pd enhanced the stability of the catalysts in the presence of H2O and SO2 since Pt is less sensitive to H2O and SOx. The CH4 oxidation data using catalyst powders show that encapsulation of metal NPs improves their stability during CH4 oxidation and prevents Pd particle sintering. Moreover, adding a washcoat overlayer to monolith catalysts enhances the control of the wall temperature of the monolith. However, the role of adding a washcoat overlayer on top of the active phase of the catalyst for the CH4 oxidation has not been reported, especially at the low temperature (200 – 550 °C) operating conditions relevant to NGVs in the presence of CO, CO2, H2O and SOx for Pd and Pt-Pd catalysts.  Most studies using monolith catalysts have focused on the effect of the support at higher temperatures in dry feed gas.  However, a comparison between different catalyst supports such as Al2O3, AlOOH and CeO2 assessed for low temperature CH4 oxidation in the presence of H2O, CO, CO2 and SOx that are also present in the NGV exhaust gas, is needed. Moreover, modeling the monolith reactor accounting for the washcoat layer using these supports and studying the kinetics of the CH4 oxidation reaction in in the presence of H2O on these supports in a monolith reactor, would provide the information needed to improve monolith converters for NGVs and address the deficiencies in catalyst activity and stability in the presence of high water concentrations and low temperatures of NGVs exhaust gas.  Consequently in this study, the preparation of monolith catalysts with different compositions will be investigated with the goal of obtaining an improved formulation of the washcoat and the active phase, such that improved catalyst activity and stability result. The study will focus on the catalyst inhibition that occurs in 40  the presence of H2O and SO2.  The use of a washcoat overlayer to mitigate these inhibition effects will also be investigated.    1.2 Thesis Statement and Objectives   Based on the learnings from the literature review, the following scientific questions are posed: 1. Can the the impact of H2O and SOX on the CH4 oxidation catalyst activity be reduced by improving the washcoat formulation and the washcoat preparation methodology? 2. Can the performance of the CH4 oxidation catalyst be improved by adding a washcoat overlayer that adsorbes H2O and SOx, thereby protecting the catalyst? 3. Can the beneficial effects of catalyst promoters such as CeO2, that have proven to be effective when using powdered catalysts, be scaled up from powder form to mini monoliths?  In this study, methane oxidation catalysts based on PdO for low temperature (200 – 550 °C) CH4 oxidation will be assessed using a monolith reactor configuration. The assessments will be done using a ceramic cordierite monolith 400 CPI, (length: 2.54 cm; inside diameter: 0.90 cm), washcoated with different supports. To address the above scientific questions, the thesis objectives are the following: • To determine the activity and stability of PdO/-AlOOH/-Al2O3, PdO/Ce/-AlOOH/-Al2O3 and Pt-Pd/ Ce/-AlOOH/-Al2O3 catalysts in a monolith reactor operated in the presence of H2O, CO, CO2 and SO2 • To determine the activity and stability of these same catalysts when adding a washcoat overlayer to the monolith 41  • To develop a better understanding of the impact of both the monolith catalyst composition and the washcoat overlayer on the monolith catalyst activity and stability • To develop a mathematical model of the monolith reactor accounting for the CH4 oxidation kinetics and diffusion in the presence of H2O at low temperature.  1.3 Outline of the Thesis  In Chapter 1, a general introduction of the effect of unburned CH4 on the environment and the need for active and stable catalysts to oxidize unburned CH4 from NGVs at low temperature (< 550°C) in the presence of H2O and CO2 is reported. Also, this chapter reviews the studies relevant to CH4 oxidation over Pd catalysts in the presence of H2O for powder and monolith catalysts.  In Chapter 2, catalyst preparation and catalyst characterization techniques are reported. Also, details of the experimental procedures and equipment that were used to measure the activity and stability of the catalysts is included in this chapter.  In Chapter 3, the beneficial effect of adding -AlOOH to the -Al2O3 washcoat of a ceramic cordierite (2MgO.2Al2O3.5SiO2) monolith, used to support a PdO catalyst, is reported for methane oxidation in the presence of water at low temperature (< 550 C).   42  In Chapter 4, the effect of varying Ce loading with a constant 0.5 wt% of Pd loading on the activity and the stability of the monolith catalysts is reported. The beneficial effect of adding CeO2 to Pd//-AlOOH/γ-Al2O3 in suppressing the negative effect of H2O is also described.    In Chapter 5, the beneficial effect of adding a washcoat overlayer on the activity and stability of Pd catalysts is investigated and the role of the washcoat overlayer in suppressing the negative effects of H2O and SO2 on catalyst activity and stability is demonstrated. The beneficial effect of adding Pt to Pd/CeO2/-AlOOH/γ-Al2O3 in improving the stability of the catalysts in the presence of H2O and SOx is also described.    In Chapter 6 the effect of H2O concentration on CH4 oxidation over Pd catalysts is reported and these data are used to analyse three CH4 oxidation kinetic models at temperatures relevant to NGVs that account for reaction inhibition by H2O. The models are then used to describe the impact of the washcoat overlayer on the catalyst performance.  In Chapter 7, the conclusions of this study, with recommendations for future work are reported. The appendices provide supplementary information for characterization techniques, the unit calibration, CH4 conversion calculation and kinetic model MATLAB codes.     43  Chapter 2: Experimental  An experimental and modelling approach will be used to meet the thesis objectives. The 3 target catalysts (Pd, Pd/Ce and PdPt/Ce) were prepared on mini-monoliths. The initial activities of the prepared monoliths were assessed by TPO and the stability of the catalysts was examined by time-on-stream experiments with H2O and SO2 added to the feed gas. The component concentrations in the feed gas were varied as was the temperature in the case of the TOS experiments. The TPO data were used to extract kinetic parameters for each of the catalysts, using a modeling method and accounting for the monolith reactor configuration.   2.1 Materials  A cordierite (2MgO.2Al2O3.5SiO2) monolith from Corning with 400 CPI was cut to obtain a mini-monolith (400 CPI, 1 cm diameter x 2.54 cm length; ~52 cells) with a mass of ~0.75g. A colloidal suspension of γ-Al2O3 (25 wt% Al2O3; average particle size 50 nm) provided by ULTRA TEC Manufacturing, Inc. was used to prepare the washcoat. Pseudo boehmite, (γ-AlOOH, Sasol North America, product number is 23N4-80) with an average particle size of 90 nm was used as binding agent. Ce(NO3)3.6H2O (Aldrich 99% purity) was used to prepare an aqueous solution of Ce (2.5wt%). Pd(NO3)2.xH2O (Aldrich ≥ 99% purity) was used to prepare the aqueous solution of Pd (0.27 wt%) for impregnation of the washcoated monolith. [Pt(NH3)4](NO3)2 (Aldrich ≥ 99% purity) was used with the Pd(NO3)2.xH2O to prepare an aqueous solution of Pt-Pd (0.03 wt% Pt – 0.24 wt% Pd).  44  2.2 Preparation of the Suspensions  Four different suspensions were used for the washcoat, prepared from the colloidal γ-Al2O3 suspension to which varying amounts of water and -AlOOH were added, as detailed in Table 2.1. To determine the effect of the binding agent -AlOOH, the solids content of all the suspensions was maintained at 25 wt% while varying the concentrations of γ-Al2O3 and γ-AlOOH. Prior to use, the γ-AlOOH powder was dried in air for 2 h at 120 °C and then calcined in stagnant air by heating from room temperature to 350 °C at 10 °C/min, with the final temperature held for 7 h. The -AlOOH was water dispersible and after adding the water and γ-AlOOH, the suspensions were stirred for 1 h prior to use. The pH of all the suspensions remained approximately constant st pH of 6-7.  2.3 Preparation of the Monolith Catalysts  The monolith was dried for 2 h at 120 °C prior to dip coating for 5 min in the previously prepared suspension. After removal from the suspension, pressurized air was used to remove any excess suspension trapped in the channels of the monolith. The washcoated monolith was subsequently dried for 2 h at 120 °C and calcined in stagnant air by heating from room temperature to 450 °C at 10 °C/min and then holding the final temperature for 7 h before cooling to room temperature. Following application of the washcoat, the drying step is critical and must be done slowly because fast drying leads to cracks in the washcoat layer [132]. The dip coating was repeated to obtain the desired washcoat loading of ~25 wt % on the monolith. The calcined and washcoated monolith was then wet impregnated with a Pd solution for 1 min, dried for 2 h at 45  120 °C and calcined in air flow of 100 cm3(STP)·min−1 while heating from 25 to 450 °C at 10 °C/min and holding the final temperature for 15 h. The monolith was subsequently cooled to room temperature. The final catalyst nominal compositions are reported in Table 2.2. The catalysts in Table 2.2 were used for the experiments of Chapter 3. Appendix C.2 and C.3 provides details on a series of preliminary experiments that were conducted to establish the most appropriate washcoat loading and calcination time.  Table 2.1 Nominal compositions of washcoat suspensions. Washcoat suspensions: A B C D Mass of colloidal -Al2O3 (25 wt% -Al2O3), g 80 80 80 80 Mass of water, g 0 15 30 46 Mass of -AlOOH, g 0 5 10 16 Total mass of suspension, g 80 100 120 142 Solid content of suspension, wt% 25.0 25.0 25.0 25.0 -Al2O3 content, wt % 25.0 20.0 16.7 14.1 -AlOOH content, wt % - 5.0 8.3 11.2   Table 2.2 Nominal composition of monolith catalysts with different loading of -AlOOH. Sample Catalyst composition, wt %   Pd -AlOOH -Al2O3 Cordierite Pd0B   0.5 0 24.9 74.6 Pd5B   0.5 5.3 21.2 73.0 Pd8B  0.5 8.5 18 73.0 Pd11B  0.5 11.5 15.0 73.0 46  To add CeO2 to the catalysts, the calcined and washcoated monolith was wet impregnated with the Ce solution for 1 min, dried for 2 h at 120 °C and calcined in air flow of 100 cm3(STP)·min−1 while heating from 25 to 450 °C at 10 °C/min and holding the final temperature for 15 h. The monolith was subsequently cooled to room temperature. Then, the washcoated monolith with CeO2 was wet impregnated with the Pd solution for 1 min, dried for 2 h at 120 °C and calcined in air flow of 100 cm3(STP)·min−1 while heating from 25 to 450 °C at 10 °C/min and holding the final temperature for 15 h. The monolith was subsequently cooled to room temperature. For all monolith catalysts with CeO2, the nominal Pd loading was held constant at 0.5 wt %. For the Pd0Ce catalyst, after the washcoat was deposited on the monolith, the Pd was added according to the method already described, but without adding the CeO2. The final catalyst nominal compositions are reported in Table 2.3. The catalysts in Table 2.3 were used for the experiments of Chapter 4.  Table 2.3 Nominal composition of monolith catalysts with different loading of CeO2. Sample Catalyst composition, wt %  Pd Ce -AlOOH -Al2O3 Cordierite Pd0Ce 0.5 0 5.3 21.2 73.0 Pd1Ce 0.5 1.0 5.1 20.4 73.0 Pd2Ce 0.5 2.0 4.9 19.6 73.0 Pd4Ce 0.5 4.0 4.5 18.0 73.0   For the monolith catalyst with Pd-Pt and Ce (PdPtCe-WC), the Pd solution was replaced with the Pd-Pt solution (Pd:Pt in the solution is 9:1 by mass) prior to impregnation of the active phase. A range of optimum Pd:Pt ratios for bimetallic CH4 oxidation catalysts have been reported in the 47  literature. In this study, the same ratio of Pd:Pt (9:1) as that reported by Wilburn and Epling [21] was used. These authors reported adding a low amount of Pt to Pd improve the activity and the stability of the catalysts for CH4 oxidation [21].   Following the preparation of the monolith catalysts according to the method already described, the washcoat overlayer was added by again dip coating the catalyst in the same γ-AlOOH/γ-Al2O3 suspension for 5 min. The monolith was subsequently dried for 2 h at 120 °C and calcined in stagnant air, heating from room temperature to 450 °C at 10 °C/min and then holding the final temperature (450°C) for 7 h before cooling to room temperature. The catalysts with the washcoat overlayer are designated as O-Pd-WC, O-PdCe-WC and O-PdPtCe-WC.  Note that the catalysts with the overlayer have more washcoat than the catalysts without the overlayer.  To assess the impact of the additional washcoat (and resulting reduction in the monolith channel cross-section) a PdCe catalyst was prepared on the same monolith that had been dip coated 3 times (rather than 2) and this sample is designated PdCe-2WC. Table 2.4 provides a summary of the nominal compositions of the catalysts prepared in this study and used in Chapter 5.        48  Table 2.4 Nominal composition of monolith catalysts with and without washcoat overlayer. Sample Washcoat Overlayer Pd Pt Ce -AlOOH -Al2O3 Cordierite  g g mass % Pd-WC 0.27 0 0.35 - - 5.45 21.2 73.0 PdCe-WC 0.27 0 0.35 - 1.5 5.0 20.15 73.0 PdPtCe-WC 0.27 0 0.31 0.04 1.5 5.0 20.15 73.0 O-Pd-WC 0.27 0.10 0.30 - - 6.3 25.4 68.0 O-PdCe-WC 0.27 0.10 0.30 - 1.3 6.2 25.2 67.0 O-PdPtCe-WC 0.27 0.10 0.27 0.03 1.3 6.2 25.2 67.0 PdCe-2WC 0.37 0 0.30 - 1.3 6.2 25.2 67.0   2.4 Characterization of the Monolith Catalysts  The fresh or used monolith catalysts were characterized by physical and chemical characterization methods to determine physical properties such as pore volume, pore distribution, and BET surface area of the catalysts, as well as chemical properties such as surface composition and metal dispersion.  2.4.1 N2 Adsorption-desorption  The monolith catalyst BET (Brunauer-Emmett-Teller) surface area, pore volume and average pore size were determined from N2 adsorption-desorption isotherms measured at 77 K using a 49  Micromeritics ASAP 2020 analyzer. The monolith catalysts were crushed to a powder and dried at 110 oC under vacuum for 3 h to remove moisture prior to the analysis. -Al2O3 powder was obtained from washcoat Suspension A (Table 2.1) by first drying the suspension in stagnant air followed by heating from room temperature to 450 °C at 10 °C/min and then holding the final temperature for 7 h. A γ-AlOOH powder sample was also obtained by first drying the γ-AlOOH powder in air and then calcining in stagnant air by heating from room temperature to 350 °C at 10 °C/min and then holding the final temperature for 7 h. A 20% -AlOOH-80% -Al2O3 powder sample was obtained from Suspension B (Table 2.1) by first drying the suspension in stagnant air followed by heating from room temperature to 450 °C at 10 °C/min and then holding the final temperature for 7 h. The detailed calculations related to the surface area measurements are giving in the Appendix A.1. In addition, the repeatability of the BET experiments, assessed in Appendix A.1, show the error associated with estimates of the BET surface area, pore volume and average pore size. For the BET surface area the error was ± 3 m2/g and for the average pore size the estimated error was 1 ± nm.  2.4.2 Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy   The monoliths were also analyzed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). A JEOL JSM-5510 LV SEM was used to assess the surface morphology of the powder and the washcoated monolith. In the latter case, the monolith was sectioned to yield an internal channel for analysis. The presence of Pd in the analyzed zone was verified by EDX.   50  2.4.3 X-Ray Diffraction (XRD)  X-ray diffraction (XRD) was used to identify crystalline phases present in the precursor powders and the catalysts and to estimate crystallite size using Scherrer's equation. XRD patterns of the catalysts were collected using a Bruker D8 Focus (LynxEye detector) with CoKα1 radiation (λ = 1.79 Å) a 35-kV source, angle scan ranges from 3° to 80° with a 0.04° step size and 0.8 s time steps. Appendix A.2 provides more details about the XRD analysis.  2.4.4 X-ray Photoelectron Spectroscopy (XPS)  The sectioned monolith was also analyzed by X-ray photoelectron spectroscopy (XPS) to determine the properties of the washcoat surface. A Leybold MAX200 X-ray photoelectron spectrometer with Al Kα X-ray source was used to determine the Pd oxidation states. The spectra were obtained at a survey pass energy of 192 eV and a narrow pass energy of 48 eV. XPSPEAK41 was used to analyze the spectra after background subtraction by the nonlinear Shirley method.  2.4.5 CO Chemisorption  The dispersion of Pd was estimated from CO pulse chemisorption measurements done using a Micromeritics AutoChemII 2920 analyzer. The powdered monolith catalyst was dried in an Ar flow at 200 °C for 2h prior to being reduced in a 50 cm3(STP)· min−1 flow of 9.5vol% H2/Ar (Praxair) at 100 °C for 1 h. After cooling to 50 °C in He the CO uptake was measured by passing 51  pulses of 9.93vol% CO/He (Praxair) at 50 °C over the sample and measuring the CO adsorbed using a thermal conductivity detector (TCD). The catalyst reduction transformed the PdO to Pd0, suitable for CO adsorption. Details of CO Chemisorption analysis are provided in Appendix A.3.  2.4.6 Adhesion and Thermal Stability  Ultrasonic vibration was used to evaluate the mechanical strength of the bond between the washcoat and the cordierite monolith [67]. A monolith with washcoat was placed in an ultrasonic water bath (frequency 40 kHz with power of 8 watts) for 1 h. The sample was subsequently dried at 120 oC for 2 h and weighed. The weight loss of the washcoat following this treatment gave a relative measure of the adhesion of the washcoat to the cordierite monolith.   To assess the thermal stability of the washcoat, the same monolith was placed in an oven in air while heating to 1000 oC at 10 oC/min and holding the final temperature for 7 hours. Subsequently, the monolith was subjected to the same ultrasonic vibration test as before, dried at 120 oC for 2 h and weighed to determine the mass loss.       52  2.5 Catalyst Testing  2.5.1 Experimental Setup  The experimental setup for catalyst testing (see Figure 2.1) consisted of a stainless steel tube in which the mini-monolith (length: 2.54 cm; inside diameter: 1 cm) was placed.  The reactor tube was placed inside a furnace with a PID temperature controller. Two thermocouples (K-type), inserted inside the reactor, measured the temperature at the inlet and outlet of the monolith. Electric mass flow controllers were used to set feed flows. A separate furnace was used to heat the gas flow line to 120 °C before it entered the reactor furnace. A pump was used to pump H2O to the system and mix with the feed gases prior to the pre-heater. A gas exhaust analyzer and a mass spectrometer were employed to analyze reactants and products of the reaction.   53  FMFC1PreheaterFMFC2FFMFC3FCH4COCO2N2/HeAirMonolith ReactorL=1"ID=0.5"Temperature ControllerMass SpectroscopyCO AnalyzerH2ODeionized H2OFSignal LineFilterMass Flow ControllerCheck ValveGate ValveCondenserVentFFNeedle ValveMFC4MFC5Thermocouple Heated line P-59 P-60 Figure 2.1 Schematic diagram of experimental setup. 54  2.5.2 Temperature Programmed Oxidation  After calcination, the mini-monolith was cooled to room temperature and the initial activity of the catalyst was measured by temperature-programmed CH4 oxidation (TPO). The total feed gas flow was 1025 cm3(STP)·min−1, corresponding to a GHSV of 36,000 h−1. The dry feed gas (0 vol % H2O) composition was 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 in N2 and He. TPO experiments done in wet feed used 2 vol% and 5 vol% H2O added to the dry feed gas. The reactor temperature was increased from 25 to 550 °C at 5 °C.min−1 and the outlet gas of the reactor was monitored continuously using an RGA-200 quadrupole mass spectrometer (Stanford Research Systems) to quantity the CH4 conversion. The m/z ratio monitored for CH4 was 15, to avoid interference with water or oxygen. The m/z ratio monitored for He was 4 and for CO2 was 44. A GEA-5002 exhaust gas analyzer (Emissions Systems Inc) was used to quantify the CO conversion. The detailed calculations related to the CH4 conversion measurements are provided in Appendix C.  In addition, the repeatability of the TPO experiments, assessed in Appendix C, show that the error associated with estimates of T10, T50 and T80 (i.e. the temperature at which CH4 conversion reached 10, 50 and 80%, respectively) was typically less than ± 5 °C.   2.5.3 Time-on-Stream Experiments  The stability of the catalysts was quantified using time-on-stream (TOS) tests in which the CH4 conversion was measured over a 7-200 h period at a constant temperature and feed H2O (and SO2) concentration. Total feed gas flow rate was 1026 cm3(STP)·min−1 corresponding to a GHSV of 36,000h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 10 vol% 55  H2O in N2 and He. In the TOS experiments, the temperature of the reactor was increased from 25 to 425 or 550 °C at 5 °C·min−1 in the presence of the reactants but excluding H2O. Once the system had stabilized, the H2O was injected into the gas feed flow. The temperature was then held constant as the reaction proceeded for a period of 7-200 h. The outlet gas of the reactor was continuously monitored as before to quantify both the CH4 and CO conversions. Then, the H2O flow was stopped and the CH4 conversion was monitored for a further 1-3h. Table 2.5 summarizes the reaction conditions for the TOS tests. Further details of the CH4 conversion calculation are given in Appendix C.  In addition, the analysis of the TOS experiment repeatability, reported in Appendix C, shows that the error in the measured CH4 conversion was less than ± 3.5 %.  Table 2.5 Reaction conditions used in the present study for TOS tests. Parameter Definition  TOS_1 TOS_2 TOS_3 T Reaction temperature (oC)  425 550 500 P Total pressure (kPa)  101.3 101.3 101.3 GHSV Gas hourly space velocity (h-1) 36000 36000 36000 Time Hour 7 7 24 𝑦𝐶𝐻4 CH4 concentration (ppmv)  700 700 700 𝑦𝐶𝑂2 CO2 concentration (vol%) 8 8 8 𝑦𝐶𝑂 CO concentration (ppmv) 600 600 600 𝑦𝑂2 O2 concentration (vol%) 8.5 8.5 8.5 𝑦𝐻2𝑂 H2O concentration (vol%) 10 10 10 𝑦𝑆𝑂2 SO2 concentration (ppmv) 0 0 5 56  Chapter 3: Beneficial Effect of Adding -AlOOH to the -Al2O3 Washcoat of a PdO Catalyst for Methane Oxidation  3.1 Introduction   As noted in the literature review, most studies of monolith catalysts for NGVs have focused on the preparation of the washcoat and the effect of the suspension properties, with fewer studies reporting on the stability and activity of these catalysts for low temperature CH4 oxidation in the presence of H2O, CO and CO2. The objective of this chapter was to investigate the impact of adding AlOOH to the γ-Al2O3 suspension on the deposition and adhesion of the washcoat as well as to evaluate the impact of the AlOOH on the performance of the catalyst for CH4 oxidation in the presence of H2O.  In this chapter, monolith catalysts for low temperature (200 – 550 °C) CH4 oxidation under lean burn conditions, washcoated with -AlOOH added to a γ-Al2O3 suspension, are reported. The beneficial effect of the -AlOOH on the stability and activity of the PdO/-Al2O3 catalysts during CH4 oxidation in the presence of water is demonstrated.   3.2 Results   3.2.1 Catalyst Characterization   Table 3.1 reports the textural properties of the washcoat precursors (-AlOOH and -Al2O3) analyzed as powders. The BET surface area of the as received -AlOOH decreased from 217 57  m2/g to 189 m2/g after treatment at 350 oC for 7h due to sintering of small particles [133]. The average pore diameter of the -AlOOH also increased as the micropores were removed by sintering [133, 134]. Adding -AlOOH (20 wt %) to the -Al2O3 suspension marginally increased the surface area of the washcoat powder from 179 m2/g (for the -Al2O3) to 184 m2/g. A comparison of the SEM images of the powdered precursors shows that the -Al2O3 has large macropores, approximately 0.5 m in diameter (Figure 3.1A). Figure 3.1B shows particles of -AlOOH aggregating because of hydrogen bonds between Al–OH groups [134]. Adding -AlOOH to the -Al2O3 enhanced particle aggregation and reduced the presence of the macropores, as shown in Figure 3.1C. Figure 3.2 reports the XRD patterns of the powdered precursors of -Al2O3, -AlOOH calcined at 350 oC and the 80 wt %-Al2O3-20 wt % -AlOOH washcoat calcined at 450 °C. The -AlOOH peaks (at 2  14.3, 28.3,  38.5, 49.3, and 55.5°) are broad, suggesting the crystallites of -AlOOH are small [67]. The average crystallite size of the  -AlOOH, estimated using Scherrer's equation was 6 nm. Dehydration of 𝛾-AlOOH to yield 𝛾-Al2O3 depends on the conditions of the heat treatment and the -AlOOH precursors [135, 136]. Priya et al. [137] reported the complete transition of  -AlOOH  to 𝛾-Al2O3 required calcination at 600 oC [137]. The 80 wt %-Al2O3-20 wt % -AlOOH washcoat shows features of both -AlOOH and -Al2O3, even after calcination at 450 oC.   The cordierite monolith has a very low surface area (~2 m2/g, Table 3.1) compared to the -Al2O3 and -AlOOH. Table 3.1 shows that as the -AlOOH content of the washcoat increased, the surface area of the monolith catalyst increased due to increased area of the -AlOOH compared to the -Al2O3 and a small increase in washcoat loading (Table 2.2). As shown in 58  Table 3.1 the BET surface area of Pd0B and Pd5B catalysts was ~50 m2/g, similar to the 54 m2/g reported for a commercial 0.41Pd/0.042Rh/0.6CaO/7.5CeO2/21.5Al2O3/70wt% monolith catalyst that decreased to 30 m2/g after use [138]. The BET results show that without adding 𝛾-AlOOH to the washcoat, the surface area decreased from 50 to 15 m2/g (Pd0B catalyst) after use. For Pd5B catalyst, with 5.4 wt % 𝛾-AlOOH present in the washcoat, the surface area decreased to 45 m2/g after use at the same reaction conditions (GHSV of 36000 h−1 with 0.07vol % CH4, 8.5vol % O2, 0.06vol % CO, 8 vol % CO2 and 10vol % H2O in N2 and He at 550 oC for 7 h).              59  Table 3.1 Textural properties of washcoat precursors and monolith catalysts. Sample BET area Pore volume Average Pore Diameter  m2/g cm3/g nm Precursor powders:      -Al2O3a  179 0.45 10   -AlOOH, as received 217 0.30 4   -AlOOH calcined at 350 oC for 7h 189 0.47 9   Washcoat powder - 80% Al2O3+20% AlOOH, calcined at 450 oC for 7h 184 0.55 12 Monolith catalysts: -AlOOH, wt %    Cordierite monolith - 2 0.01 30 Pd0B  0 50 0.19 17 Pd5B  5.3 55 0.14 9 Pd8B 8.5 62 0.27 17 Pd11B 11.5 71 0.26 14 Pd0B - Used catalyst 0 15 0.08 21 Pd5B – Used catalyst 5.3 45 0.12 10 aPowder recovered from the 25 wt%  -Al2O3 suspension A after drying at 450 °C for 7 h 60     Figure 3.1 SEM image of washcoat powders: (A) -Al2O3 calcined at 450 oC for 7 h; (B) -AlOOH calcined at 350 oC for 7 h and (C) 20%-AlOOH/80%-Al2O3 calcined at 450 oC for 7 h. 0 10 20 30 40 50 60 70 80 90(251)(151)(051)(031)(120)Intensity2theta ()(A)(B)(C)(020) Figure 3.2 XRD patterns for washcoat powders: (A) -Al2O3 calcined at 450 oC for 7 h; (B) -AlOOH calcined at 350 oC for 7 h and (C) 20%-AlOOH/80%-Al2O3 calcined at 450 oC for 7 h.  B A C 61  Figure 3.3 A reports the relationship between the washcoat loading and the amount of -AlOOH in the suspension used for dip coating of the monolith. The data show that with the addition of -AlOOH to the γ-Al2O3 suspension, the washcoat loading increased by about 2 wt%, even though the solid content of the suspension was held constant (25 wt %). More importantly, Figure 3.3 B shows the beneficial effect of the -AlOOH on the adhesion and thermal stability of the applied washcoat. Without -AlOOH there was a significant weight loss after the first ultrasonic vibration test (total loss of 3 wt % versus 1.8 wt % loss if the sample was placed in the water bath for 1 hour without vibration); whereas, the loss decreased to ~1 wt % with -AlOOH added to the suspension, indicative of improved washcoat adhesion in the presence of the -AlOOH. The thermal stability of the washcoat was also enhanced by the -AlOOH, as indicated by the decrease in weight loss after the second vibration test that followed the high temperature treatment (1000 °C for 7 h) of the washcoated monolith. Hence, we can conclude that the addition of the -AlOOH improved both the adhesion and thermal stability of the washcoat when applied to the cordierite monolith.   62   Figure 3.3 Effect of adding -AlOOH to the washcoat: (A) on the amount of washcoat deposited; (B) Relative washcoat adhesion and thermal stability as measured by relative weight loss after sonication.  SEM images of the monolith sections, with and without -AlOOH in the washcoat, are compared in Figure 3.4 and support the above conclusions. Figures 3.4 A-B show the washcoat deposition without adding -AlOOH to the washcoat suspension. The calcined monolith shows a significant number of surface cracks, reducing the washcoat homogeneity. In addition, the washcoat distribution on the cordierite is not uniform with significantly more accumulation in the corners of the square channels. In contrast, Figures 3.4 C-D show that adding -AlOOH to the washcoat suspension enhanced the uniformity of the washcoat surface with few surface cracks visible (Pd5B with 5.3 wt % -AlOOH). Figures 3.4 E-F show that the quality of the washcoat declined as the -AlOOH content increased to 12 wt %. Figure 3.5 shows the SEM-EDX analysis of the channel x-section, indicating a uniform dispersion of the Al and Pd in the channel of the Pd5B (-AlOOH content of 5.3 wt%) catalyst. The EDX analysis was done at different locations of the 63  monolith sample and Table 3.2 reports the average and standard deviation of the elemental analyses. The data are in good agreement with the Pd nominal loading, given that the EDX analysis does not include the cordierite monolith because of the sampling depth of the method.  Hence the nominal Pd loading in the washcoat (1.8 wt % Pd) is in good agreement with the EDX analysis results.   Table 3.2 EDX elemental analysis of the monolith catalysts.  O, wt % Al, wt % Pd, wt % Pd0B  51.1± 1.5 47.4 ± 2.8 1.5 ± 0.1 Pd5B  53.2± 1.0 45.1± 2.1 1.7± 0.2 Pd8B 50.7± 1.7 47.7± 2.7 1.6± 0.3 Pd11B 52.0± 1.9 46.3± 2.3 1.7± 0.2                    64        Figure 3.4 SEM images of the monolith catalysts:(A) Cross-section of Pd0B; (B) Channel wall of Pd0B; (C) Cross -section of Pd5B;(D) Channel wall of Pd5B; (E) Cross -section of Pd8B; (F) Cross -section of Pd11B. B C D E F A E F C D 65    Figure 3.5 SEM-EDX analysis showing distribution of Al and Pd in the monolith channel for Pd5B: (A) Al (B) Pd.  CO chemisorption analysis was done for both the fresh and used Pd0B and Pd5B catalysts (Table 3.3). The fresh Pd0B catalyst (no -AlOOH in the washcoat) had a higher CO uptake and dispersion than Pd5B catalyst (5.3 wt% -AlOOH in the washcoat). However, the CO uptake for the used Pd0B catalyst decreased significantly (from 26 mol/gcat to 8 mol/gcat); whereas, for Pd5B catalyst the loss in Pd dispersion was much less significant (from 19 mol/gcat to 10 mol/gcat). The loss in Pd surface sites was also confirmed by XPS analysis. Table 3.3 presents the XPS results for Pd0B and Pd5B catalysts before and after use, showing the binding energy (B.E.) of the Pd 3d5/2 and 3d3/2, the surface compositions of Pd, Al, and O, as well as the Pd/Al surface atom ratio. Consistent with the CO uptake data, the Pd/Al ratio for the fresh Pd0B catalyst was higher than the Pd5B catalyst, indicative of a higher Pd dispersion on Pd0B. However, after use, the Pd/Al ratio for the Pd0B catalyst decreased significantly (from 2.2 to 0.43); whereas, for the Pd5B catalyst the decrease was less significant (from 0.51 to 0.45; Table 3.3). Figure 3.6 shows the XPS Pd 3d spectral analysis for Pd0B and Pd5B catalysts fresh and B A 66  used samples. The binding energy for the Pd 3d5/2 of the fresh (336.6 eV) and used (336.8/336.9 eV) Pd0B and Pd5B catalysts are all similar. Also, from the XPS results it can be concluded that the PdO phase is stable and there is no evidence of Pd0 (335.6 eV) nor Pd(OH)2 (338.5 eV) in the used monolith catalysts, in agreement with the literature [12, 50, 139].    Table 3.3 CO uptake and XPS analysis of fresh and used Pd0B and Pd5B catalysts.  Monolith Catalyst CO uptake mol/gcat Pd dispersion % Binding Energy eV  Surface Composition    at% Surface %    Pd 3d5/2 Pd 3d3/2  Pd Al O Pd/Al Pd0B  26 55 336.6 341.9 342.1 342.2 342.2 0.84 37.3 61.86 2.2 Pd5B  19 41 336.6 0.2 39.1 60.7 0.51 Pd0B -used 8 16 336.9 0.16 37.6 62.24 0.43 Pd5B -used 10 21 336.9 0.17 37.5 62.3 0.45     67   Figure 3.6 XPS Pd 3d spectra measured for monolith catalysts: (A) Fresh Pd0B, (B) Pd0B-Used; (C) Fresh Pd5B (D) Pd5B-Used. 68  3.2.2 Monolith Catalyst Activity and Stability   The activities of the monolith catalysts, measured by TPO, are reported in Figure 3.7 and the T50 temperatures (the temperature required for 50% CH4 conversion) are summarized in Table 3.4. In dry feed gas (Figure 3.7A), all the catalysts reached approximately 100% CH4 conversion at 450 °C with T50 < 350 oC, for all catalysts (Table 3.4). In wet feed gas the light-off curves were shifted to higher temperatures for all catalysts (Figure 3.7B and 3.7C) which reflect a loss in catalyst activity in the presence of H2O. Addition of -AlOOH to the -Al2O3 washcoat decreased the catalyst activity when measured in the dry feed gas.  However, when H2O (2 and 5 vol%) was present in the feed, the -AlOOH improved the catalyst activity relative to the catalyst without -AlOOH (Pd0B catalyst), as shown in Figures 3.7 B-C. Note that a monolith with 27 wt% washcoat /73 wt% cordierite and no PdO had a CH4 conversion of < 1 % at the same reaction conditions, indicating that the reactor tube walls and monolith did not contribute to the conversion of CH4 as shown in Figure C.1 (Appendix C).  The results of the TOS experiments with 10 vol% H2O in the feed at 425 and 550 °C are shown in Figure 3.8A and B, respectively. Figure 3.8 A shows that for all the catalysts at the lower reaction temperature, the CH4 conversion decreased with the addition of H2O to the reactor feed gas. At the higher temperature (550 oC), with 10 vol% H2O, all the monoliths show very similar CH4 conversion and relative stability. Pd5B catalyst had the highest stability; whereas, Pd0B catalyst (without -AlOOH in the washcoat) was the least stable in the presence of H2O. At 425 oC, an exponential loss in activity occurred within the first 30 mins after the 10 vol% H2O was added to the feed for all the monoliths, followed by a linear loss up to TOS ~ 7 h. After the H2O 69  was removed, the CH4 conversion increased by about 20%. At high temperature (550 oC), CH4 conversion increased after H2O removal to approximately the same level as was observed for the dry gas.    Table 3.4 T50 from TPO of methane in dry and wet feed gas. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5vol% H2O in N2 and He. Monolith T50 Dry Gases T50 Wet Gases  2 vol% H2O T50 Wet Gases  5 vol% H2O Pd0B  309 406 450 Pd5B  317 378 401 Pd8B 333 395 411 Pd11B 342 404 419     70   Figure 3.7 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5 vol% H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:Pd5B.      71   Figure 3.8 TOS results after adding 10 vol % H2O to the dry feed gas.  Reacting conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 10 vol% H2O in N2 and He. The period of the experiment was 7h A: at 425 oC and B: at 550 oC.  After the TOS experiments the catalysts were purged using N2 at 120 °C for 12 h and then re-assessed by TPO in dry feed gas. The results illustrate that the Pd5B catalyst had the highest activity among all the used catalysts as shown in Figure 3.9. The T50 for Pd5B catalyst increased from 317 oC for the fresh catalyst to 362 oC for the used sample, an increase of 45 oC. In contrast, for Pd0B catalyst the T50 increased by 91 oC. Hence, one concludes from both the TPO and TOS results that the addition of -AlOOH to the -Al2O3 washcoat improved the stability and the activity of the monolith catalyst for methane oxidation with H2O present in the feed gas. Note that during TPO the catalyst activity for CO oxidation was also measured, as reported in Figure 3.10. The results confirm that all the catalysts were very active for CO oxidation with complete conversion occurring at < 180C, and T50 < 140 oC. Also, the conversion of CO was 100% for all the samples during the TOS at 425 oC and 550 oC. 72   Figure 3.9 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature after the TOS tests. The total feed gas flow is 1025 cm3(STP)·min−1, corresponding to a GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He.  Figure 3.10 Temperature-programmed oxidation profile for CO conversion as function of temperature for the catalysts. The total feed gas flow is 1025 cm3(STP)·min−1, corresponding to a GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He. 73  3.3 Discussion  The XRD results of Figure 3.2 show that both 𝛾-AlOOH and -Al2O3 were present in the washcoat after calcination at 450 oC, as expected because complete transition of -AlOOH to 𝛾-Al2O3 requires calcination above 600 oC [137]. The presence of -AlOOH in the used monolith catalysts could not be confirmed by XRD because of the dominance of cordierite reflections.  However, in the presence of 10 vol % H2O dehydration of the -AlOOH will not be favoured at the reaction conditions.  Jiang et al. [66] reported that washcoat loading increased linearly with increased solid content of the suspension, but that the washcoat formed cracks with weight loss following ultrasonic vibration and thermal shock of the washcoat. In this study similar observations are reported and depositing the washcoat with -AlOOH is shown to improve the bonding of the washcoat to the cordierite surface [66, 67, 77]. The presence of small amounts (~5 wt%) of -AlOOH improved the thermal and mechanical strength of the washcoat because the -AlOOH enhances the adhesion of the washcoat to the wall of the cordierite monolith, as presented in Figure 3.3. When the -AlOOH content of the suspension was increased above 5 wt%, the applied layer was less uniform with some cracks on the walls and corners (Figure 3.4 E-F), resulting in increased weight loss. A solid content in the suspension of 20 wt% -Al2O3 and 5 wt% -AlOOH, yielding ~27 wt% washcoat after two dip coatings, showed the best adhesion properties. Note that minor cracks appeared in all the washcoat layers because of differences in thermal expansion between cordierite and alumina [67].  74  The -AlOOH also impacted the textural properties of the monolith catalysts (Table 3.1). The decreased surface area of the used catalyst  is due to hydrothermal aging in the wet feed gas and has been reported in other studies [12, 13, 140]. Sintering and the collapse of pores [141] is accelerated in the presence of H2O [140] and this leads to reduced surface area and increased average pore size [142]. -AlOOH improved the stability of the washcoat textural properties by providing a partially hydrated surface that reduced the adsorption of H2O (Table 3.1, Pd0B versus Pd5B).   After the catalyst was calcined at 450 °C for 15 h in air, XPS confirmed the presence of PdO on Pd0B and Pd5B catalysts. The XPS and CO chemisorption results (Table 3.3) show a loss of Pd dispersion after CH4 oxidation on the PdO/-Al2O3 that was significantly greater than that of the PdO/-AlOOH/-Al2O3. Similarly, Narui et al.[93] reported that PdO dispersion decreased from 14% to 11% for a 0.5%Pd/Al2O3 catalyst after CH4 combustion at 350 oC for 6 h and  Xu et al.[31] reported a Pd dispersion decrease from 3.93% to 0.98% after aging a 1.1%Pd/Al2O3 catalyst under isothermal conditions at 900 oC in 10 mol% H2O/N2 for 92 h [31].  Stasinska et al.[139] studied the effect of particle size/dispersion of PdO on CH4 oxidation activity of a Pd/Al2O3 with the same loading of Pd. They reported a catalyst with an average Pd particle size of 4.6 nm had high dispersion and a Pd/Al ratio measured by XPS of 0.555. In contrast, a catalyst with a Pd average particle size of 13 nm had low dispersion and a Pd/Al ratio of 0.217 [139]. These data support the data of the present study that are indicative of more sintering of the PdO on the PdO/-Al2O3 than the PdO/-AlOOH/-Al2O3, suggesting that the PdO-support interaction is much stronger on the PdO/-AlOOH/-Al2O3 catalyst. 75  The most significant impact of the -AlOOH in the washcoat is on the catalyst CH4 oxidation activity and stability. Pd0B catalyst, with no -AlOOH in the washcoat, had the highest activity in dry feed gas and the highest Pd dispersion for the fresh catalyst compared to the other catalysts of the present study, in agreement with the results of Stasinska et al. [139]. In the presence of water in the feed gas, the activities of all the catalysts declined, but the catalysts with -AlOOH were more active and stable than Pd0B (without -AlOOH).  The reversible inhibition of CH4 oxidation that occurs in the presence of water, is thought to be due to the adsorption of H2O on the active sites of PdO [15, 36]. Alyani and Smith [12] described the rate of inhibition of CH4 oxidation by H2O using a kinetic model that assumed the deactivation of the catalyst at low temperature was due to the reversible adsorption of H2O on PdO sites [12]. Kikuchi et al.[26] reported the activity of 1.1wt%Pd/Al2O3 and 1.1wt%Pd/Al2O3-36NiO for dry and wet CH4 oxidation and assessed their data using a kinetic model that also assumed that H2O adsorption on the PdO surface was the main cause of reaction inhibition. They concluded that NiO decreased the H2O adsorption on the PdO surface and thereby enhanced the activity of the catalysts in the presence of H2O [26]. The TPO results obtained in the presence of 2 and 5 vol% H2O illustrated a similar benefit of AlOOH addition to the washcoat that suggests AlOOH decreases H2O adsorption on the PdO surface.   The H2O inhibition is also influenced by the nature of the support [14, 15, 17, 128, 143, 144]. Schwartz et al. [38] investigated the kinetics of CH4 oxidation over a 3wt%PdO/Al2O3 by assuming the RDS was the H2O desorption at temperature >500 oC and the surface reaction was the RDS at temperature <500 oC [38]. They proposed that hydroxyl accumulation on the oxide 76  support (Al2O3) is the main cause of CH4 oxidation inhibition by H2O, since hydroxyl accumulation impedes the O-exchange (needed for the PdO  Pd* + Os cycle) between the support oxygen (Os) and the active phase (PdO/Pd*) and decreases the rate of H2O desorption [38]. A support with high hydrophobicity plays a role in reducing the adsorption of H2O and hydroxyl accumulation on the surface of the catalysts [128, 143, 144]. Rinaldi et al. [134] reported that  Al2O3 adsorbs more water than the AlOOH because the AlOOH already has hydroxyl groups in its structure [134]. Hence, adding -AlOOH to -Al2O3 would increase the hydrophobicity of the washcoat, reducing hydroxyl adsorption and increasing the O-exchange rate, which leads to improved activity and stability of the catalyst in the presence of H2O.   Pd catalysts used for CH4 oxidation are known to deactivate in the presence of H2O and the effect of H2O decreases with increased temperature [17, 26, 104] which is in agreement with the TOS and TPO results of this study. The structural collapse of the catalyst support [15, 29], sintering [30-33] and PdO conversion to Pd [15, 34, 35] have all been stated as the reason for the deactivation of Pd catalysts [6]. Kucharczyk et al. [78] studied the effect of adding La2O3 and SiO2 or ZrSiO4 to the washcoat of a PdO/-Al2O3 catalyst and reported that these promoter oxides improved the activity and stability and enhanced the thermal resistance of the PdO/-Al2O3 catalysts after aging at 1060 oC for 24h. They concluded that decreased Pd dispersion led to a decrease in the activity of the catalyst, and ZrSiO4 and SiO2 improved the thermal resistance of PdO/-Al2O3 and decreased the rate of sintering of PdO [78]. Cargnello et al.[145] reported that encapsulating PdO in CeO2 limits the sintering of PdO particles. Moreover, Lu et al.[146] has reported that applying a layer of Al2O3 over PdO/Al2O3 catalysts, suppresses sintering of 77  PdO particles. In the present study, the PdO particles of the PdO/-Al2O3 catalyst (Pd0B) sintered significantly after use. Although this catalyst had a higher PdO dispersion initially compared to the PdO/-AlOOH/-Al2O3 (Pd5B), the former catalyst was sintered much more severely after use (Table 3.3). Hence, adding -AlOOH to the washcoat suppressed the sintering of PdO particles, consistent with a stronger metal-support interaction (MSI) on the -AlOOH/-Al2O3 than the -Al2O3 washcoat, as noted above. However, the Pd dispersion data of Table 3.3 indicate that the used Pd0B catalyst had very similar dispersion to Pd5B after use. Consequently, the different CH4 conversions reported during the TOS experiments (Figure 3.8) indicate that PdO dispersion is not the only factor determining the catalyst activity.  As noted above the interaction of H2O with the support and the PdO are also critical in determining the catalyst activity.    The results of the present study clearly show that adding -AlOOH to the -Al2O3 washcoat provides significant resistance to the suppression of the catalyst activity by H2O.  Several mechanisms appear to play a role: (i) the -AlOOH improves the resistance to structural collapse of the catalyst support, (ii) the -AlOOH decreases the adsorption of H2O on the support because the AlOOH has hydroxyl groups in its structure [134] and (iii) the -AlOOH decreases the rate of PdO sintering in the presence of H2O, possibly due to a stronger MSI in the presence of -AlOOH.     78  3.4 Conclusions  A series of catalysts supported on ceramic cordierite monoliths (400 CPI, 1 cm x 2.54 cm), washcoated with different supports and loaded with PdO, were evaluated for CH4 oxidation. The activity and stability of the PdO/-Al2O3 monolith catalysts was dependent on the presence of -AlOOH in the washcoat. The -AlOOH improved the thermal and mechanical strength of the washcoat by enhancing washcoat adhesion and improving washcoat distribution on the monolith. The CH4 oxidation activity of the catalyst in dry feed gas decreased when -AlOOH was added to the -Al2O3 washcoat. However, when H2O was present in the feed gas, the -AlOOH enhanced the activity and the stability of the catalyst. 79  Chapter 4: CeO2 Reduces the Impact of H2O and SO2 on CH4 Oxidation over PdO/-AlOOH/-Al2O3 Monolith Catalysts   4.1 Introduction   The literature provides significant evidence from studies of powdered catalysts that Ce improves the activity and the stability of Pd catalysts for CH4 oxidation in the presence of H2O. For monolith catalysts, the effect of Ce has been reported mostly at high temperatures in dry feed gas, conditions that are relevant to emissions from gasoline engines. This chapter is focused on the impact of Ce addition to Pd catalysts supported on 𝛾-Al2O3/AlOOH washcoated cordierite monoliths (400 cells per square inch (CPI); 1 cm diameter x 2.54 cm length). The monolith catalysts have been assessed at high space velocity and at temperatures  550 oC in the presence of CO, CO2, H2O and SO2 to closely mimic the exhaust gas from lean-burn NGVs. The objective of this chapter was to determine the impact of Ce on the adhesion and stability of the washcoat, and to determine the CH4 oxidation activity and stability of the Pd/CeO2/-AlOOH/-Al2O3 catalysts operated in the presence of H2O, CO, CO2 and SO2.   4.2 Results   4.2.1 Monolith Characterization Figure 4.1 shows the effect of the Ce loading on the adhesion properties and thermal stability of the applied washcoat. The catalysts have very similar adhesion as reflected in the total weight 80  loss of 1-2 wt% following the vibration test, even though the samples with CeO2 were calcined twice at 450 oC for 15 h. The thermal stability of the washcoat was not significantly influenced by adding CeO2 either, as indicated by the similar weight loss among all samples recorded after the second vibration test that followed the high temperature treatment (1000 °C for 7 h). Hence, one can conclude that the addition of the CeO2 does not have a significant impact on both the adhesion and thermal stability of the washcoat when applied to the cordierite monolith.    Figure 4.1 Effect of CeO2 loading on the adhesion properties and thermal stability of the washcoat.  Table 4.1 reports the textural properties of the monolith catalysts prepared in this study with varying amounts of CeO2. For the Pd0Ce catalyst without CeO2 in the washcoat, the surface area was 55 m2/g. As the CeO2 content of the catalyst increased, the surface area decreased to 46 m2/g for the catalyst with 4 wt % Ce (Pd4Ce). For the Pd0Ce-used catalyst, analysed after both the 81  TPO and TOS tests, the surface area decreased marginally to 45 m2/g while it decreased to 42 m2/g for the Pd2Ce-used catalyst.   CO chemisorption analysis was done after reduction of both the fresh and used Pd0Ce and Pd2Ce catalysts (Table 4.1). The fresh Pd0Ce (no CeO2 in the catalyst) had a higher CO uptake than Pd2Ce (2 wt% CeO2 in the catalyst) and generally the CO uptake decreased as the CeO2 loading increased. However, the CO uptake for the used Pd0Ce decreased significantly (from 19.4 mol/gcat to 10 mol/gcat); whereas, for Pd2Ce the loss in CO uptake was much less significant (from 13.5 mol/gcat to 8.8 mol/gcat).  Table 4.1 Textural properties and CO uptake of PdO/CeO2/-AlOOH/-Al2O3 monolith catalysts with varying Ce content. Sample Ce content BET area Pore volume Average Pore Diameter CO uptake Pd dispersion  wt % m2/g cm3/g nm mol/gcat % Pd0Ce 0 55 0.14 9 19.4 41 Pd1Ce 1 51 0.17 13 13.5 29 Pd2Ce 2 49 0.14 11 11.5 25 Pd4Ce 4 46 0.12 10 8.5 18 Pd0Ce – used  0 45 0.12 10 10 21 Pd2Ce – used  2 42 0.11 10 8.8 19   82  SEM images of monolith sections of the Pd0Ce (no CeO2) and Pd2Ce (2 wt % CeO2) catalysts are compared in Figure 4.2 A-D. Differences in thermal expansion between cordierite and alumina creates minor cracks on the washcoat surface of both catalysts [67]. Figure 4.3 shows the SEM-EDX analysis of the monolith x-section, indicating a uniform dispersion of the Al, Ce and Pd in the channels of the Pd1Ce catalyst. The EDX analysis was done at different locations of each catalyst and Table 4.2 reports the average and standard deviation of the analyses. Note that the EDX analysis results do not include the cordierite because of the sampling depth (a few microns) of the method. Accounting for the mass of cordierite (73 wt %), the Pd data of Table 4.2 are in very good agreement with the nominal compositions of Table 2.3. However, the Ce content is lower than the nominal composition, suggesting a greater penetration depth of Ce within the washcoat, in part because Ce was added in the first impregnation.         83       Figure 4.2 SEM images of the monolith catalysts Pd0Ce (A, B) and Pd2Ce, (C, D).  Table 4.2 EDX elemental analysis of the monolith catalysts.  O, wt % Al, wt % Ce, wt % Pd, wt % Pd0Ce 53.2± 1.0 45.1± 2.1 0 1.7± 0.2 Pd1Ce 50.1± 1.9 45.2± 2.4 3.2±0.8 1.5± 0.4 Pd2Ce 48.2± 1.6 44.3± 2.1 5.9±0.7 1.6± 0.3 Pd4Ce 45.9± 1.9 41.1± 1.5 11.3±0.5 1.7± 0.2    A B C D 84      Figure 4.3 SEM-EDX analysis of Pd1Ce catalyst: (A) Al, (B) Ce, (C) Pd, and (D)Pd-Ce-Al.  Table 4.3 presents the Pd 3d5/2 and 3d3/2 binding energies (B.E.s), the Pd, Al and O surface compositions and the Pd/Al ratio for all the catalysts, as measured by XPS. The data show that as the loading of Ce increased, the concentration of Pd on the surface increased, as did the Pd/Al ratio. Figure 4.4 shows the XPS Pd 3d spectral analysis for all the samples. For Pd2Ce the Pd 3d5/2 B.E was 337.1 eV, higher than for Pd0Ce (336.6 eV). Figure 4.5 shows the Pd 3d XPS data for the fresh and used Pd0Ce catalyst and the fresh and used Pd2Ce catalyst. The Pd 3d5/2 B.E increased by 0.3 eV for Pd0Ce and by 0.6 eV for Pd2Ce after use. Furthermore, the surface Pd/Al ratio decreased after use for both catalysts. From the fresh and used B.E. data it can be concluded that the Pd surface was in an oxidized state [147] and the decrease in the Pd/Al ratio after use is B A C D 85  due to catalyst aging, as has been reported previously for powdered catalysts operated under similar conditions [12, 50].  Table 4.3  XPS analysis of PdO/CeO2/-AlOOH/-Al2O3 monolith catalysts with varying Ce content. Monolith Catalyst Pd B.E.  Surface composition  3d5/2 3d3/2  Pd Ce Al O Pd/Al  eV eV  at % % Pd0Ce 336.6 342.1  0.2 0 39.1 60.7 0.51 Pd1Ce 337.1 342.3  0.28 0.29 35.13 64.3 0.80 Pd2Ce 337.1 342.3  0.30 0.45 31.25 68 0.96 Pd4Ce 336.9 342.1  0.69 0.51 35.4 63.4 1.95 Pd0Ce-Used 336.9 342.2  0.17 0 37.5 62.3 0.45 Pd2Ce-Used 337.7 342.9  0.26 0.64 35.7 63.4 0.73  86   Figure 4.4 XPS Pd 3d spectra measured for catalysts: A – Pd0Ce; B- Pd1Ce; C: Pd2Ce; D:Pd4Ce. 87    Figure 4.5 XPS Pd 3d spectra measured for catalysts: A - Pd0Ce; B - Pd0Ce-used; C - Pd2Ce; D - Pd2Ce-used.   88  4.2.2 Catalysts Activity and Stability   The activities of the monolith catalysts, measured by TPO, are reported in Figure 4.6 and the T50 temperatures (the temperature required for 50% CH4 conversion) are summarized in Table 4.4. In dry feed gas (i.e. no H2O in the feed gas), all the catalysts reached approximately 100% CH4 conversion at 450 °C. TPO analysis of the catalysts without water in the feed gas yield T50 of  362 oC for all the catalysts, as shown in Table 4.4. With H2O added to the feed gas, the light-off curves shifted to higher temperatures for all catalysts, reflecting the loss in catalyst activity in the presence of H2O. Addition of CeO2 to the washcoat decreased the catalyst activity when measured in the dry feed gas; whereas, when H2O (2 and 5 vol%) was present in the feed, the CeO2 improved the catalyst activity relative to the catalyst without CeO2 (Pd0Ce), as shown in Figures 4.6 B-C. From the results in Table 4.4 and Figure 4.6 it can be concluded that the Pd2Ce (2 wt % Ce) catalyst had the highest catalytic activity in the presence of H2O in the feed gas.  Table 4.4 Temperature-programmed oxidation T50 conversion for the catalysts in dry and wet feed gas. [Reaction conditions: GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol % O2, 0.06 vol % CO, 8 vol % CO2 and 0, 2 or 5 vol% H2O in N2 and He]. Monolith T50 Dry Gas  T50 Wet Gas 2 vol% H2O T50 Wet Gas 5 vol% H2O Pd0Ce 317 379 401 Pd1Ce 322 370 385 Pd2Ce 333 352 370 Pd4Ce 362 377 390  89   Figure 4.6 Temperature-programmed CH4 oxidation profile showing the initial activity of the catalysts as a function of temperature. Reaction conditions = GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5vol% H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:Pd2Ce.  The results of the time-on-stream (TOS) experiments with 10 vol% H2O in the feed at 425 °C and 550 °C are shown in Figure 4.7A and B, respectively. For all the catalysts, Figure 4.7A shows that at the lower reaction temperature, the CH4 conversion decreased with the addition of H2O to the reactor feed gas. The Pd0Ce (no CeO2) catalyst deactivated very rapidly with the conversion of CH4 decreasing to 60 % within 2 h and after 7 h the conversion decreased to 50 %. 90  After the H2O was removed from the feed gas, the conversion recovered to 58 %. The Pd2Ce was the most stable catalyst in the presence of 10 vol% H2O at 425 oC.  The conversion decreased to 70% in 7 h and recovered to 80% after the H2O was removed. At 550 oC all the catalysts showed good stability in the presence of 10 % H2O (Figure 4.7B). Figure 4.7C shows, however, that in the presence of both 10 % H2O and 5 ppmv SO2, the catalyst deactivation was severe, although the catalyst with the CeO2 (Pd2Ce) deactivated more slowly and reached a slightly higher conversion after 20 h than the catalyst without CeO2 (Pd0Ce).   91   Figure 4.7 TOS results after adding: (A) 10 vol % H2O added to the dry feed gas at 425 °C, (B) : (A) 10 vol % H2O added to the dry feed gas at 550 °C and (C) 10 vol % H2O and 5 ppmv SO2 added to the dry feed gas at 500 °C.  Reaction conditions: GHSV = 36000 h-1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 in N2 and He.  After the TOS experiments in H2O, the catalysts were purged using N2 at 120 °C for 12 h and then re-assessed by TPO in dry feed gas. The results illustrate that the Pd2Ce catalyst had the highest activity among all the used catalysts, as shown in Figure 4.8. The T50 for the Pd2Ce catalyst increased from 333 oC for the fresh catalyst to 350 oC for the used sample, an increase of 17 oC. In contrast, for the Pd0Ce catalyst T50 increased by 45 oC. Hence, one concludes from both 92  the TPO and TOS results that the addition of 2 wt % CeO2 to the catalyst/washcoat improved the stability and the activity of the monolith catalyst for CH4 oxidation in the presence of H2O in the feed gas. In the presence of H2O and SO2 the benefit of CeO2 remains, but is less pronounced.  The CO oxidation was measured to quantify the activity of the catalyst for CO oxidation with and without CeO2 in the washcoat. The results show that adding CeO2 did not have a significant impact on the activity of the catalysts for CO oxidation and all catalysts reached 100% CO conversion at temperature < 220 oC, as reported in Figure 4.9. Also, CO conversion, measured during the TOS test reached 100% for all the catalysts at 425 oC and 550 oC.   Figure 4.8 Temperature-programmed CH4 oxidation profile: the initial activity of the catalysts as a function of temperature after the catalysts were regenerated. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He.  93   Figure 4.9 Temperature-programmed oxidation profile for CO conversion as function of temperature for the catalysts. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, and 8 vol% CO2 in N2 and He.  4.3 Discussion  The BET surface area and the pore volume of the catalysts decreased with increased CeO2 loading, in agreement with results in the literature [12, 148, 149].  Alyani and Smith [12] reported that the BET surface area of a 6.5%Pd/Al2O3 catalyst decreased from 218 m2/g to 206 m2/g, when 2.6 wt% CeO2 was added to the catalyst. Moreover, Colussi et al. [90] reported that the BET surface area of Al2O3, 10%Pd/ Al2O3, 10%Pd/15%CeO2/Al2O3 was 148 m2/g, 124 m2/g and 110 m2/g, respectively. With increased CeO2 loading, the Al2O3 pores were filled by CeO2 leading to a decrease in the surface area and the pore volume [150-152]. For both Pd0Ce and 94  Pd2Ce, the BET surface area and the pore volume decreased after the TPO and TOS experiments, which likely resulted from some sintering of the washcoat [67].  The CO chemisorption results showed that CeO2 loading had an impact on the CO uptake. The CO uptake for Pd0Ce (0 wt % Ce) and Pd4Ce (4 wt % Ce) was 19.4 μmol/gcat and 8.5 μmol/gcat, respectively, although they had the same Pd loading. A similar loss in CO uptake in the presence of CeO2 has been noted previously [12, 149].  Alyani and Smith [12] reported that the CO uptake for a 6.5%Pd/Al2O3 catalyst decreased from 204 to 153 μmol/gcat after 2.9 wt% CeO2 was added to the catalyst. Moreover, Haneda et al.[149] measured the dispersion of Pd by CO chemisorption for a 10% Pd/Al2O3 catalyst and reported that the Pd dispersion decreased from 7 % for the 10 wt% Pd/Al2O3 catalyst to 4% after adding 20 wt% CeO2 to the 10 wt% Pd/Al2O3. The catalysts were reduced in H2 for 1 h at 100 oC before measuring the CO uptake. The decrease in the CO uptake with increased CeO2 loading is thought to be a consequence of a decrease in the reduction of PdO to Pd in H2 prior to the CO uptake measurement, caused by the presence of CeO2 [36, 148]. Yao et al. [153] and Oh et al.[89] reported that adding CeO2 to Pd/Al2O3 stabilized the PdO, making it difficult to reduce. Xiao et al. [46] reported the absence of Pd0 in the XPS spectra of a 2 wt%Pd/CeO2 catalyst after reduction in H2 for 1 h at 300 oC and concluded that Pd was present only as PdO because of a strong interaction between Pd and Ce that suppressed the reduction of the PdO. The decrease in CO uptake for Pd0Ce and Pd2Ce following their use, results from a loss in active surface sites after exposure to high amounts of H2O at 425 oC and 550 oC for 14 h [37].  95  After the catalysts were calcined at 450 °C for 15 h in air flow, the Pd was present as PdO [6, 154, 155] , confirmed by the Pd5/2 B.E. of 336.6 ± 0.3 eV (Table 4.3). The Pd5/2 B.E. increased by 0.5 eV and the surface Pd/Al ratio increased with 1 and 2 wt% CeO2, in agreement with Alyani and Smith [12]. Adding CeO2 leads to a marginal increase in the binding energy of Pd 3d5/2 due to charge transfer from CeO2 to PdO [12, 156].  The binding energy of the Pd 3d5/2 for catalysts Pd0Ce and Pd2Ce also increased after exposure to 10 vol% H2O at 425 oC for 7 h and 550 oC for 7 h, ascribed to the reconstruction of Pd species [46]. Shyu et al. [156] investigated the role of CeO2 on the oxidation state of Pd by comparing Pd 3d spectra of 0.6%Pd/CeO2 and 6.8%Pd/Al2O3 catalysts. After the catalysts were calcined at 800°C a peak at 337 eV, corresponding to PdO was observed for both catalysts. The catalysts were subsequently reduced at 920°C and exposed to ambient air, whereupon a peak at 337.0 eV was observed for the 0.6%Pd/CeO2 catalyst but not the 6.8%Pd/Al2O3, confirming that CeO2 increased the oxidation state of Pd.   Even though adding 1 – 4 wt % CeO2 to Pd/Al2O3 increased the surface Pd/Al ratio, the TPO data using a dry feed gas showed that adding CeO2 decreased the activity of the catalyst, consistent with several previous studies [89, 153, 156]. Oh et al. [89] studied the effect of adding CeO2 to Pd/Al2O3 and Pt/Al2O3 catalysts on their CH4 oxidation activity and conducted TPO tests in a dry feed gas containing 0.2 vol% CH4, 0.1 vol% CO, 1 vol% O2 balanced by N2. They reported that the activity of the catalysts decreased when the CeO2 was added and suggested this was due to a strong interaction between the CeO2 and the Pd that led to highly oxidized but less active PdO catalysts [89, 156]. Oh et al. [89]  proposed that adding CeO2 to Pd/Al2O3 increased the extent of Pd oxidation which leads to a tendency of the catalysts to completely oxidize the 96  CH4 to CO2 rather than partially oxidize it to CO, which also results in the catalyst with CeO2 being less active in dry feed. Moreover, Yao et al. [153] reported the same effect of adding CeO2 to Pd/Al2O3, showing that the catalyst activity for oxidation of lower alkanes (C1 to C4) decreased upon CeO2 addition to the Pd/Al2O3 catalyst. Shyu et al. [156], studied the effect of adding 10 wt% CeO2 to 10 wt%Pd/Al2O3 on the activity of C3H8 oxidation using a powder catalyst. The catalyst TPO activity test was done in a dry feed gas consisting of 0.1 vol% C3H8, 2 vol% O2 in N2, and it was found that the addition of CeO2 increased T50 from 288 to 343 oC, indicative of a decrease in the activity with CeO2 addition [156]. In this study, the results of CO uptake and TPO in dry feed gas suggest a decrease in the activity of the catalyst resulting from a strong interaction between CeO2 and Pd that retains PdO species on the catalyst surface.  Ciuparu et al. [91] reported that the oxygen exchange between the support of the catalyst and Pd-vacancies is interrupted by H2O adsorption on the support. Colussi et al. [157] showed that adding CeO2 to Pd/Al2O3 improved oxygen exchange capacity of the catalyst, which enhanced Pd re-oxidation. Thus, the deactivation of Pd/Al2O3 by H2O could be minimized by adding CeO2 to Al2O3 since CeO2 has a high oxygen exchange capacity. Yao et al. [83] also reported that the thermal stability of Al2O3 is improved by adding CeO2 to Al2O3. They reported that as the CeO2 loading increased the O2 uptake decreased, due to reduced CeO2 dispersion with increased CeO2 loading. The TPO data from the present study using a wet feed gas (2 vol% H2O) show that the Pd2Ce catalyst had the highest activity with T50 of 372 oC. The activity in the presence of 2 vol% H2O decreased in order: Pd2Ce (2 wt% Ce) > Pd1Ce (1 wt% Ce) > Pd4Ce (4 wt% Ce) > Pd0Ce (0 wt% Ce).  The same trend was obtained in the presence of 5 vol% H2O. For the Pd0Ce catalyst, T50 increased from 317 oC (dry feed gas) to 401oC in a wet feed gas with 5 vol% H2O, 97  an increase of 84 oC. Although the Pd4Ce catalyst was the least active among the catalysts tested in dry or wet feed gas, adding CeO2 reduced the T50 increase to 28 oC in the presence of 5 vol% H2O in the feed. A similar reduction in activity loss in the presence of water was observed for the Pd1Ce and Pd2Ce catalysts.  Hence, one concludes that adding CeO2 improved the activity of the catalysts in the presence of H2O in the feed gas.  In agreement with literature results, we assume that the higher oxygen exchange capacity of CeO2 (compared to the -AlOOH/-Al2O3 washcoat) enhances oxygen exchange between the support and Pd vacancies, thereby enhancing re-oxidation of the Pd which in turn enhances the CH4 oxidation rate observed in the presence of a wet feed gas.  Some studies report that Pd catalysts used for CH4 oxidation deactivate in the presence of H2O at low temperature (< 500°C) as a result of Pd(OH)2 formation, [104, 158, 159] even though Pd(OH)2 is formed at low temperatures (<  250°C) in the presence of H2O and decomposes above 250°C [104]. Moreover, PdO is favored thermodynamically over Pd(OH)2 and Pd0 at the chosen operating conditions. In the present study, there was no evidence of Pd0 (335.6 eV) nor Pd(OH)2 (338.5 eV) from the XPS analysis. Pd was present as PdO in both the fresh and used catalysts, in agreement with the literature [12, 50, 139].    The adsorption of H2O on the active sites of PdO also leads to an inhibition of the Pd catalysts for CH4 oxidation [15, 36]. Moreover, the nature of the support for the catalyst has an impact on the activity and stability of CH4 oxidation catalysts in the presence of H2O [15, 17, 128, 143, 144]. A support with high hydrophobicity plays a role in reducing the adsorption of H2O on the surface of the catalysts [128, 143, 144]. Carchini et al. [160] investigated H2O adsorption and 98  hydrophilicity of CeO2 and Al2O3, and reported that Al2O3 had a higher affinity for H2O than CeO2, the latter being more hydrophobic. Thus, adding CeO2 to the -AlOOH/-Al2O3 washcoat likely also increased the hydrophobicity of the washcoat and thereby reduced the adsorption of H2O. Alyani and Smith [12] studied the role of adding 2.6%Ce to 6.5%Pd/Al2O3 in the presence of 2 % H2O at 330oC and 350 oC. At 330°C both catalysts showed a high loss of activity compared to the results at 350°C due to an increase in H2O adsorption with decreased temperature [12]. They developed a kinetic model of CH4 oxidation in the presence of H2O to recognize the effect of adding CeO2 to a 6.5Pd/Al2O3 catalyst and reported that the equilibrium constant for H2O adsorption on the 6.5Pd/Al2O3 catalyst was higher than on the 2.9Ce/6.5Pd/Al2O3 catalyst and that the rate of H2O desorption was higher on the Ce-promoted catalyst [12]. Therefore, adding CeO2 to a 6.5Pd/Al2O3 catalyst led to reduced inhibition of the catalyst activity following the addition of H2O [12]. The TOS results of the present study at 550 oC in the presence of 10 vol% H2O in the feed (Figure 4.7B), showed that the temperature plays a major role in the deactivation of the catalysts since all the catalysts showed high stability at 550 oC. The inhibition effect of H2O decreased with increased temperature [17, 26, 104]. The TOS results from the present study show that at low temperature (425 oC), the deactivation in the presence of 10 vol% H2O is very significant. For the Pd0Ce catalyst the conversion of CH4 decreased to 50% and recovered to 60% after the H2O was removed from the feed gas. For the Pd2Ce catalyst with 2 wt% Ce the conversion at 425 oC dropped to 70% and recovered to 80% after the H2O was removed from the feed. The results of TOS are in agreement with the TPO results for the wet feed. From the TOS and TPO results it can be concluded that adding 2 wt% CeO2 to the washcoat enhanced the activity and the stability of the Pd/-AlOOH/-Al2O3 catalyst in the presence of H2O, suggesting adding CeO2 accelerates the Pd re-oxidation which enhances 99  the catalysts activity when using a wet feed gas. Furthermore, an increase in the hydrophobicity of the washcoat leads to a decrease in H2O adsorption on the surface of the washcoat, and hence higher activity.  Finally, the data of Figure 4.7C show that in the presence of H2O and SO2, severe deactivation occurs, but the presence of CeO2 mitigates this behaviour, albeit to a small degree. Ordonez et al. [131] reported similar effects on a powdered Pd/Al2O3 catalyst that was shown to be due to the formation of PdSO4. The formation of Al2(SO4)3 can also occur [11, 130] resulting in changed textural properties that contribute to the loss in catalyst activity. In the presence of CeO2, one expects that the more hydrophobic surface reduces the adsorption of sulphate species.  4.4 Conclusions  A series of catalysts prepared on a ceramic cordierite monolith (400 CPI, 1 cm diameter x 2.54 cm length; ~52 cells), washcoated with different supports that contained CeO2 and loaded with PdO, were evaluated for CH4 oxidation. The BET surface area and CO uptake decreased with increased CeO2 loading; whereas, the Pd/Al surface ratio increased. Moreover, adding CeO2 enhanced the dispersion of Pd. The CH4 oxidation activity of the catalyst in dry feed gas decreased when CeO2 was added to the washcoat. However, when H2O was present in the feed gas, the CeO2 enhanced the activity and the stability of the catalyst by enhancing oxygen transfer to the active phase and reducing water adsorption on the catalyst.   100  Chapter 5: Washcoat overlayer for improved activity and stability of natural gas vehicle (NGV) monolith catalysts operating in the presence of H2O and SO2  5.1 Introduction   The literature provides significant evidence that Pd is more active than Pt for CH4 oxidation and some studies report that adding Pt to Pd improves the activity and stability of Pd catalysts in the presence of H2O and SO2. The CH4 oxidation data using catalyst powders show that encapsulation of metal NPs improves their stability during CH4 oxidation and prevents Pd particle sintering. Moreover, adding a washcoat overlayer to monolith catalysts enhances the control of the wall temperature of the monolith. In the present study, the effect of adding a washcoat overlayer on top of the Pd, Pd/Ce, Pd-Pt or Pd-Pt/Ce active phase of CH4 oxidation monolith catalysts is reported at low temperatures (200 – 550 °C) in the presence of CO, CO2, H2O and SO2, relevant to NGVs.  Figure 5.1 illustrates the geometry of the washcoated monolith with the washcoat overlayer. The addition of the overlayer is thought to provide a diffusion barrier between the active catalyst layer and the convective gas flow through the monolith. On the one-hand the diffusion barrier will decrease the concentration of reactants (CH4 and O2) and catalyst poisons (SOx) in contact with the catalyst layer. The overlayer may also limit sintering of the active phase and provide additional sites for water (or –OH) and SOx adsorption, reducing the inhibition effects of these two components that arise by adsorption on the active metal.  101   Figure 5.1 Configuration of monolith catalyst with washcoat overlayer.  5.2 Results   5.2.1 Monolith characterization  Figure 5.2 shows the effect of the washcoat overlayer on the adhesion properties and thermal stability of the applied washcoat. All the catalysts have good adhesion properties and thermal stability as reflected in the total weight loss of < 3 wt% after the vibration tests. Comparing the standard catalysts (PdCe-WC and PdPtCe-WC) with those that have a washcoat overlayer shows a small increase in weight loss following each of the vibration tests. The weight loss is not significantly impacted by the catalyst composition, suggesting that the washcoat thickness is the 102  main determining factor for the loss of mass. Figure 5.2 also shows that following the TOS test for 200 h at 500 oC in the presence of 10 vol% H2O and 2 ppm SO2 in the feed, the weight loss increased to about 2.5 wt% total weight loss. Hence, we conclude that the adhesion and thermal stability of the monolith catalysts are satisfactory and that the catalyst washcoat and overlayer remain structurally sound during the performance assessment.    Figure 5.2 The effect of adding the washcoat overlayer on the adhesion properties and thermal stability of the fresh and used catalysts (PdPtCe-WC and O- PdPtCe-WC) operated for 200 h in the presence of 10 vol% H2O and 2 ppm SO2 at 500 oC.   103  The textural properties of the monolith catalysts are summarized in Table 5.1. For the Pd-WC catalyst without the washcoat overlayer, the surface area was 69 m2/g and this increased upon addition of the washcoat overlayer to 84 m2/g (O-Pd-WC). Similar increases were observed for the other catalyst compositions, indicating that the textural properties are primarily determined by the washcoat and that the additional drying and calcination steps following impregnation of the Pd, Pt or Ce did not significantly impact the washcoat textural properties. The surface area of the PdPtCe-WC catalyst following use (TPO reaction and TOS test for 24 h at 500 °C in the presence of 10 vol% H2O and 5 ppm SO2) decreased to 48 m2/g while it decreased to 50 m2/g for the O-PdPtCe-WC catalyst, reflecting some aging or sintering of the washcoat.                 104  Table 5.1 Textural properties of washcoat monolith catalysts. Sample Fresh  Useda  BET area Pore volume Average Pore Diameter  BET area Pore volume Average Pore Diameter  m2/g cm3/g nm  m2/g cm3/g nm Pd-WC 69 0.17 10  35 0.08 8 PdCe-WC 64 0.12 8  43 0.07 6 PdPtCe-WC 65 0.16 10  48 0.08 6 O-Pd-WC 84 0.17 8  49 0.14 11 O-PdCe-WC 80 0.15 7  51 0.15 11 O-PdPtCe-WC 82 0.17 8  50 0.13 10 PdCe-2WC 91 0.17 7  - - - aUsed catalysts are recovered after TPO test and TOS for 24 h in the presence of 10 vol% H2O and 5 ppm SO2 at 500 oC   SEM of the monolith sections of the fresh PdPtCe-WC and the O-PdPtCe-WC catalysts are reported in Figure 5.3 A and C, respectively.  The data show that at least qualitatively, washcoat surface homogeneity is not significantly influenced by the washcoat overlayer. Figure 5.3 B and D show the washcoat layer for the same catalysts after the 200 h TOS test at 500 oC in the presence of 2 ppm SO2 and 10 vol% H2O. The washcoat morphology appears very similar for both the fresh and used samples and the integrity of the washcoat layer is retained, in good agreement with the adhesion test results. Figures 5.4 and 5.5 show the SEM-EDX analysis of a 105  sectioned channel of the used PdPtCe-WC and the used O-PdPtCe-WC catalysts, showing a uniform distribution of Al, Ce, Pd and S in the channel. Table 5.2 reports the element average concentration and standard deviation as measured from several EDX spot analyses of the sectioned monolith. For the fresh PdPtCe-WC the EDX analysis results are in good agreement with the nominal Pd loading in the washcoat (1 wt % Pd, excluding the cordierite monolith). For the corresponding catalyst with the overlayer (O-PdPtCe-WC) the Ce and Pd content was significantly reduced, a consequence of the washcoat overlayer and the limited sampling depth of the EDX probe.      Figure 5.3 SEM images of selected monolith catalyst cross-sections: A: PdPtCe-WC; B: PdPtCe-WC used; C: O-PdPtCe-WC;  and D:O-PdPtCe-WC used. A B C D 106        Figure 5.4 SEM/EDX mapping of sectioned PdPtCe-WC monolith catalyst after use: A: SEM of  the washcoat; B - Al; C - O; D - Ce; E – Pd and F – S.    A B C D E F 107      Figure 5.5 SEM/EDX mapping of sectioned O-PdPtCe-WC monolith catalyst after use: A -SEM of the washcoat; B - Al, C - O; and D - S.  Table 5.2 EDX elemental analysis of the monolith catalysts.  O, wt % Al, wt % Ce, wt % Pd, wt % S, wt % PdPtCe-WC 48.6± 1.0 45.2± 1.0 5.3±0.4 0.9± 0.2 0 PdPtCe-WC-Useda 46.8± 0.6 44.3± 1.2 6.4±0.7 1.2± 0.3 1.3± 0.4 O-PdPtCe-WC 51.9± 1.0 47.85± 1.5 0.18±0.05 0.07± 0.03 0 O-PdPtCe-WC-Useda 49.8± 1.2 47.54± 1.0 0.65 ±0.12 0.13± 0.07 1.98± 0.5 aCatalysts after TPO and TOS reaction for 200 h in the presence of 10 vol% H2O and 2ppm SO2 at 500 oC  B A C D 108  Table 5.3 presents a summary of the XPS analysis of the monolith catalysts, reporting the Pd, Ce and S surface concentrations, as well as the surface Pd/Al and S/Al ratios of the fresh and used catalysts. (Note that the Pt signal could not be quantified because of its low concentration). The concentration of Pd or Ce at the surface of the catalyst decreased with the application of the washcoat overlayer, such that neither Pd nor S were detectable, indicating that the active phase was covered by the washcoat overlayer. In all cases the used catalysts showed high concentrations of S, indicative of the adsorption of SOx species. The used catalysts had similar S contents, with or without the washcoat overlayer, indicative of the fact that the SOx species were primarily adsorbed on the γ-AlOOH/γ-Al2O3 of the washcoat rather than selectively on the Pd.  Figure 5.6 shows the XPS Pd 3d spectral analysis of the fresh Pd-WC, O-Pd-WC and the PdPt-WC catalysts showing similar Pd B.E.s (Pd 3d3/2 at 342.2 ±0.2 eV and 3d5/2 at 337.0±0.3 eV), indicative of PdO surface species. Figure 5.7 reports the XPS Pd 3d spectra of the used catalysts that show that the Pd 3d B.E.s did not change significantly after use. Similarly, the S 2p XPS spectra of the used catalysts (Figure 5.8) had the same B.E. of 169.5 eV, corresponding to Al2(SO4)3 species.[161] From the B.E.s of all the samples fresh and used, it can be concluded that the Pd is present as PdO on the fresh and used catalysts and that the S species are present as sulphate, mostly associated with the Al oxides of the washcoat. [147] Note that Pd and Ce were observed by XPS analysis of the Pd-2WC catalyst, with the surface concentrations similar to the Pd-WC catalyst (Table 5.3), indicating that the extra washcoat thickness did not impact the Pd dispersion.     109  Table 5.3 Summary of XPS analysis of monolith catalysts. Catalyst Pd Ce S Pd/Al S/Al  at % % Pd-WC 0.16 - - 0.41 - Pd-WC - useda 0.16 - 1.6 0.45 4.6 O-Pd-WC 0 - - - - O-Pd-WC - used 0 - 2.2 0 6.5 PdCe-WC 0.20 0.90 - 0.62 - PdCe-WC - used 0.68 0.32 1.7 2.1 5.2 O-PdCe-WC 0 0 - - - O-PdCe-WC - used 0.10 0 1.5 0.30 4.4 PdPtCe-WC 0.17 0.21 - 0.43 - PdPtCe-WC - used 0.53 0.27 1.1 1.53 3.2 O-PdPtCe-WC 0 0 - - - O-PdPtCe-WC - used 0 0 1.6 0 4.3 PdCe-2WC 0.13 0.30 - 0.36 - aUsed catalyst recovered after TOS for 24 h at 500 oC in the presence of 10 vol% H2O and 5 ppm SO2  110   Figure 5.6 XPS Pd 3d spectra measured for fresh (A) Pd-WC, (B) O-Pd-WC, (C) PdPt-WC. 111   Figure 5.7 XPS Pd 3d spectra measured for used (A) Pd-WC, (B) O-Pd-WC, (C) PdPt-WC. 112   Figure 5.8 XPS S 2p spectra measured for fresh (A) PdWC, (B) O-PdWC, (C) PdCeWC and, (D) O-PdCeWC. 113  5.2.2 Catalysts Activity and Stability  The results of the catalyst initial activity measurements using wet and dry feed gas are presented in Figures 5.9 A-D for the Pd-WC, O-Pd-WC, PdPtCe-WC and O-PdPtCe-WC catalysts. All the catalysts reached approximately 90% CH4 conversion in the dry feed gas at temperatures below 450 °C, with the conversion significantly lower on the catalysts with the overlayer. Table 5.4 summarizes the TPO analysis of the catalysts without H2O in the feed gas.  Clearly, T50 increased with the addition of the overlayer and Figure 5.9 A shows that the decreased activity occurred over the entire TPO temperature range. In the case of wet feed gas (with 2, 5 and 10 vol% H2O) Figure 5.9 B-D clearly show that the catalysts with the overlayer were more active than those without the overlayer, at temperatures below approximately T80 (the temperature corresponding to 80% CH4 conversion). Above this temperature, the rate of increase in CH4 conversion with temperature was reduced, suggesting a diffusion limitation associated with the overlayer that resulted in lower activity compared to the corresponding catalyst without the washcoat overlayer.   The results of Table 5.4 and Figure 5.9 also demonstrate that Pt and Ce addition improved the activity of the Pd-WC catalyst when operated in both dry and wet feed gas. The PdPtCe-WC catalyst had the highest activity in dry feed among all the catalysts of Table 5.4 and the O-PdPtCe-WC had the highest catalytic activity in the presence of H2O in the feed gas. For both PdPtCe-WC and O-PdPtCe-WC the light-off curves shifted to higher temperatures with increasing H2O concentration in the feed. T50 for the PdPtCe-WC catalyst increased from 330 to 392 oC as the H2O concentration increased from 0 to 10 vol% H2O, an increase of 62 oC. For the O-PdPtCe-WC catalyst, T50 increased by 39 oC for the same increase in H2O concentration.  114  Similar effects of H2O were obtained for the PdCe-WC and O-PdCe-WC catalysts (Figure 5.10), showing higher activity of the catalyst with the washcoat overlayer in the presence of H2O. Note that the PdCe-2WC catalyst (Figure 5.10) had the lowest activity in the presence of H2O which indicates that the improved activity observed for the catalysts with the washcoat overlayer was not a consequence of the higher amount of washcoat applied in this case. Hence, we conclude that the catalyst activity, when measured in dry feed gas, was reduced by the washcoat overlayer.   However, in practice, the exhaust gas will contain significant amounts of H2O (up to 10%) and under these operating conditions the washcoat overlayer reduces the inhibition by H2O, such that the catalysts with the overlayer have higher activity than those without the overlayer.  Table 5.4 T50 from TPO of methane in dry and wet feed gas. Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5 vol% H2O in N2 and He. Monolith T50, °C  Feed gas H2O content, vol %  0 2 5 10 Pd-WC 340 397 423 461 O-Pd-WC 344 382 397 412 PdCe-WC 353 383 406 435 O-PdCe-WC 362 380 390 404 PdPtCe-WC 330 352 370 392 O-PdPtCe-WC 339 349 361 378 PdCe-2WC 366 387 409 442   115   Figure 5.9 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature. Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2, 5 or 10 vol% H2O in N2 and He.  (A) Dry feed 0 vol% H2O, (B) Wet feed 2 vol% H2O, (C) Wet feed 5 vol% H2O and (D) Wet feed 10 vol% H2O.  116   Figure 5.10 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature. Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2, 5 or 10 vol% H2O in N2 and He.  (A) Dry feed 0 vol% H2O, (B) Wet feed 2 vol% H2O, (C) Wet feed 5 vol% H2O and (D) Wet feed 10 vol% H2O.  Figure 5.11A reports the TOS results comparing the Pd-WC to the PdPtCe-WC catalysts, with and without the washcoat overlayer, operated at 425 °C with 10 vol% H2O in the feed gas. The CH4 conversion decreased with the addition of H2O to the reactor feed gas for all catalysts. The Pd-WC catalyst was the least stable in the presence of H2O; whereas, the O-PdPtCe-WC catalyst was the most stable (Figure 5.11A). The CH4 conversion decreased to 41% in the presence of 10 117  vol% H2O on the Pd-WC catalyst and upon removal of the H2O from the feed gas the conversion recovered to 56%. With the washcoat overlayer (O-Pd-WC) the conversion of CH4 decreased to 63% at the same reaction conditions and recovered to 75% after the H2O was removed. Similar benefits of the washcoat overlayer were demonstrated for all the catalysts of this study (Figure 5.11 and 5.12).  Comparing Figure 5.11B and Figure 5.12B also shows that both the addition of Pt and the washcoat overlayer improved the catalyst stability when operated in the presence of 5 ppm SO2 and 10 vol% H2O in the feed at 500 oC. The results show that after 5 ppm SO2 addition to the feed gas, the CH4 conversion decreased to < 10% after about 15 h TOS for the Pd-WC catalyst.  However, at the same operating conditions the CH4 conversion decreased to ~22% in the case of the monolith catalyst with the washcoat overlayer (O-Pd-WC). Furthermore, by comparing the PdCe (Figure 5.12) and the PdPtCe (Figure 5.11) catalysts, the results show that adding Pt improved the stability of the catalysts in the presence of SO2.   Figure 5.13 shows that with the washcoat overlayer, the catalyst (O-PdCe-WC) maintained higher conversion for an extended reaction period of 100 h in the presence of 10 vol% H2O at 500 oC, compared to the PdCe-WC catalyst without the washcoat overlayer. Also, Figure 5.14 shows that the O-PdPtCe-WC catalyst with the washcoat overlayer had higher stability than the PdPtCe-WC catalyst without the washcoat overlayer for the extended reaction period of 200 h in the presence of 10 vol% H2O and 2 ppm SO2 at 500 oC. Hence, one concludes from both the TPO and TOS results that the addition of Pt to Pd and washcoat layer to the catalysts improved 118  the stability and the activity of the monolith catalyst for CH4 oxidation in the presence of H2O and SO2 in the feed gas.   Figure 5.11 TOS results for adding 10 vol% H2O and 5 ppm SO2.  Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2, 10 vol% H2O and 0 or 5 ppm SOx in N2 and He. (A) 0 ppm SOx at 425 oC for 10h and (B) 5 ppm SO2 at 500 oC for 24h.  119   Figure 5.12 TOS results for adding 10 vol% H2O and 5 ppm SO2.  Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2, 10 vol% H2O and 0 or 5 ppm SOx in N2 and He. (A) 0 ppm SO2 at 425 oC for 10h and (B) 5 ppm SO2 at 500 oC for 24h.   Figure 5.13 TOS results for adding 10 vol% H2O to the feed at 500 oC.  Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 10 vol% H2O in N2 and He. 120   Figure 5.14 TOS results for adding 10 vol% H2O and 2 ppm SO2 to the feed at 500 oC.  Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2, 10 vol% H2O and 2 ppm SO2 in N2 and He.  5.3 Discussion  The BET results (Table 5.1) show that the catalyst surface area decreased after reaction (GHSV of 36000 h−1 with 0.07 vol % CH4, 8.5 vol % O2, 0.06 vol % CO, 8 vol % CO2 , 5 ppm SO2 and 10 vol % H2O in N2 and He at 500 oC for 24 h) for all catalysts, with or without the washcoat overlayer. The decreased surface area after use is due to hydrothermal aging and sulfating of the support in the wet feed gas with SO2, as has been reported in other studies [12, 13, 22, 130, 140].  The presence of H2O [140] and SOx [11, 21, 101] in the feed gas accelerates the sintering and the collapse of pores [141] and this leads to structural changes in the -Al2O3 and the -AlOOH 121  [142]. For the Pd-WC catalyst the pore volume and the average pore diameter decreased after use as has been observed previously [101, 130]. Honkanen et al.[130] investigated the effect of SO2 on a Pd/Al2O3 catalyst in the presence of H2O. They aged the catalysts at 400 oC for 5 h in 100 ppm SOx, 10 vol% H2O, and 2.1 vol% O2 balanced by N2 and reported relatively small decreases in surface area (from 180 to 175 m2/g), pore volume (from 0.38 to 0.35 cm3/g) and the average pore size (from 8.5 to 8 nm). The decreases were due to SOx bonding with Al2O3 to form bulk Al2(SO4)3. The higher losses in textural properties reported in the present study for the used catalysts are due to more severe reaction conditions and longer reaction times, but are consistent with the formation of Al2(SO4)3.   Figure 5.2 and 5.3 showed that all the catalysts prepared herein had good adhesion and thermal stability, even after the catalysts were used for extended periods (200 h) at high reaction temperatures (500 oC). The importance of -AlOOH in the washcoat to improve washcoat adhesion has been described in Chapter 3. Testing the catalysts for longer time periods at high temperatures emphasizes the role of -AlOOH in improving the quality and the properties of the washcoat.  The XPS results showed that the Pd/Al ratio of the PdPtCe-WC catalyst was marginally higher than that of the Pd-WC catalyst. Increased Pd dispersion in the presence of Pt has been reported in the literature [3, 93, 99]. Narui et al. [93] reported that adding Pt to PdO/α-Al2O3  increased the dispersion of Pd from 14% for the Pd monometallic catalyst to 27% for the  Pt-Pd bimetallic catalyst[93]. The XPS data of Table 5.3 also showed that neither Pd nor Ce were detected on the 122  fresh catalysts with the washcoat overlayer, meaning that both the Pd and Ce were covered by the overlayer [112, 146].    Following the 24 h TOS test at 500 oC and a GHSV of 36000 h−1 with 0.07 vol % CH4, 8.5 vol % O2, 0.06 vol % CO, 8 vol % CO2, 5 ppm SO2 and 10 vol % H2O in N2 and He feed gas, the XPS analysis showed the presence of S on all catalysts with the S/Al ratio varying from 3.2 to 6.5. Lampert et al. [11] studied the effect of SOx on the activity and the stability of Pt and Pd monolith supported catalysts for CH4 oxidation and reported that SOx was adsorbed by both the support (Al2O3) to form Al2(SO4)3 and the Pd to form PdSO4. Pt is more resistant to deactivation by SOx because of weaker adsorption on Pt than Pd and the low S content observed for the PdPtCe-WC catalyst compared to Pd-WC supports this observation. The XPS results are in good agreement with the EDX analysis of the fresh and used PdPtCe-WC catalyst, also showing the presence of S (Figure 5.4F) after the 200 h TOS test at 500 oC in the presence of 2 ppm SO2 and 10 vol% H2O. No evidence of Pd0, Pd(OH)2 nor PdSO4 in the used monolith catalysts was obtained from the XPS B.E. analysis, in agreement with several other literature studies [12, 21, 22, 50, 101, 139].   Lu et al.[146] reported that applying a layer of Al2O3 over PdO/Al2O3 catalysts, suppressed the sintering of PdO particles. They measured the accessibility of the Pd particles by IR analysis of CO chemisorption. For the Pd/Al2O3 without the Al2O3 overlayer, two strong peaks at 1929 and 1965 cm−1 and two weaker peaks at 2058 and 2081 cm−1 were observed. For the Pd/Al2O3 with the washcoat, no peaks of CO adsorption were observed, and the authors concluded that the Al2O3 overlayer covered the Pd particles. Furthermore, Habibi et al. [112] recently demonstrated 123  that encapsulation of Pd-Pt in SiO2 increased catalyst stability during CH4 oxidation in the presence of H2O. Three samples were prepared: PdPt@SiO2, PdPt/SiO2 and PdPt/Al2O3 with the loading of Pd and Pt fixed (4.2 wt% Pd, 6.98 wt% Pt). The surface concentration measured by XPS was 2.9 at.% Pd and 3.4 at.% Pt for the PdPt/Al2O3 catalyst and 3.1 at.% Pd and 2.6 at.% Pt for the PdPt/SiO2; whereas for the encapsulated catalyst PdPt@SiO2 the surface concentration was 0.3 at.% Pd and 0.5 at.% Pt. They concluded that the majority of the noble metal particles were coated within silica shells. In the present study, the catalysts operated for 24 h at 500 oC and a GHSV of 36000 h−1 with 0.07 vol % CH4, 8.5 vol % O2, 0.06 vol % CO, 8 vol % CO2 , 5 ppm SOx and 10 vol % H2O in N2 and He showed the presence of S on the catalyst surface, that might be bound as SOx with Al2O3 to form Al2(SO4)3, as reported in the literature [11, 101, 130]. The Pd/Al ratio increased after the TOS test, suggesting some restructuring of the surface in the presence of the SO2. In the case of the samples with the washcoat overlayer it was not possible to quantify the Pd signal because of the overlayer.   The TPO results from the dry feed gas showed that adding CeO2 decreased the activity of the catalyst, in agreement with Chapter 4. Figure 5.9A showed that adding Pt to the Pd catalyst improved the activity of the catalysts and this result is also in agreement with previous work [21, 93]. Wilburn and Epling [21] investigated the impact of the Pt-Pd ratio on the activity of Pt-Pd supported on 𝛾-Al2O3. They conducted TPO at a GHSV of 50000h−1 with 0.2 vol % CH4, 10 vol % O2 in N2 to measure the activity of the catalysts [21] and concluded that adding Pt to Pd in a low ratio enhanced the activity of the catalyst by providing some sinter resistance to the Pd catalyst [21]. In the present study, the activity of the catalysts increased with Pt addition (see Table 5.4) and one assumes that Pt dissociates O2 and the Pd adsorbed O atoms sustain the PdO 124  active sites, as previously suggested [92, 93, 96, 97].  The activity of the catalysts measured in the dry feed gas, decreased in the presence of the washcoat overlayer (Table 5.4). Similarly, Hayes et al. [102] prepared monolith catalysts with constant Pd loading (7.6%) but variable washcoat overlayer thickness from 0 to 10 µm. The CH4 conversion, measured at 452 oC in 1 vol% CH4 in air, decreased from 76% to 16 % with a washcoat overlayer thickness of 10 µm. The decreased catalyst activity in the presence of the washcoat overlayer is ascribed to the active phase being covered by the washcoat overlayer, as deduced from XPS and EDX analysis.  The TPO results for the catalysts operated in the presence of 2, 5 and 10 vol% H2O illustrated that adding CeO2 to the Pd-WC increased the activity of the catalyst. Moreover, adding Pt to the catalyst improved catalyst activity, in agreement with previous work [92, 96]. Lapisardi et al.[92] reported that for the same metal loading, 2.12 wt% Pd/Al2O3 and  Pd0.93-Pt0.07/ Al2O3 (molar ratio of 0.93:0.07) have the same initial activity in dry feed gas. However, in feed gas with 10 vol% H2O, the Pd0.93-Pt0.07/ Al2O3 showed higher activity compared to the Pd/Al2O3 catalyst. For the Pd-Pt bimetallic the T50 increased by 80 oC from 320 °C in dry feed gas to 400 °C in wet feed gas. For the monometallic Pd/Al2O3 the T50 increased by 105 oC since the monometallic Pd is more sensitive to H2O [92]. Furthermore, Pieck et al. [96] reported that adding 0.4% Pt to 0.8% Pd/Al2O3 improved the activity of the catalyst after aging in wet air (60 cm3 min−1 air flow with 0.356 cm3 h−1 H2O ) at 600 oC for 4h. The T50 for the 0.4% Pt-0.8% Pd/ Al2O3 was lower than 0.8% Pd/ Al2O3 by 50 oC after aging the catalysts. In this study the Pt-Pd bimetallic catalyst had the highest activity in the presence of H2O since the Pt is less sensitive to H2O inhibition, as reported in the literature [92, 96, 162].   125  The catalyst stability in feed gas with 10 vol% H2O at 425 oC improved when the washcoat overlayer was applied. Comparing the PdCe-2WC catalyst (35wt% of the washcoat) to the O-PdCe-WC catalyst (25wt% washcoat plus 10wt% overlayer) demonstrates that the catalysts with the overlayer have better stability in the presence of H2O due to the active phase being covered by the overlayer (as shown by the XPS data of Table 5.3 and Figure 5.12A). Inhibition of CH4 oxidation by adsorption of H2O on the active sites of PdO catalysts has been reported previously [15, 36]. The H2O adsorption is influenced by the nature of the support [14, 15, 17, 128, 143, 144]. Thus, H2O adsorption by the catalyst can be reduced by using a support with high hydrophobicity [128, 143, 144]. Rinaldi et al.[134] reported that AlOOH is more hydrophobic than γ-Al2O3. Thus, in the presence of H2O in the feed, applying a washcoat overlayer that covers the active phase (Pd or Pt-Pd) and reduces the H2O adsorption, improves the stability of the catalyst. Zou et al.[163] investigated the stability of NiO@PdO/Al2O3 (shell PdO and core NiO with a molar ratio Ni:Pd of 2:1), PdO/NiO/Al2O3 (impregnation method), and PdO/Al2O3 catalysts for CH4 oxidation in 1 vol% CH4 and 6 vol% H2O in air for 12h at 400 oC. The results showed that NiO@PdO/Al2O3 had the highest stability, suggesting that the large interface between Ni and Pd facilitate the O2 exchange between NiO and PdO which leads to increased OH desorption and increased stability of NiO@PdO/Al2O3 [163]. For the overlayer catalysts of this study, the Pd or Pt-Pd is covered by -AlOOH and -Al2O3 which may increase the O2 exchange between the support and the active phase and lead to increased OH desorption, thereby improving the catalyst performance in the presence of H2O. Applying a layer of Al2O3 over PdO/Al2O3 catalysts leads to decreased sintering of PdO particles as reported Lu et al.[146]. Moreover, Habibi et al. [112] reported that encapsulation of Pd-Pt in SiO2 suppresses the growth of Pd and Pt particles which leads to improved stability of the catalyst in the presence of H2O. 126  The XPS results of this study showed that Pd or Pd-Pt was covered by the washcoat overlayer. Since PdO sintering is a known cause of catalyst deactivation in the presence of H2O [30-33], adding the washcoat overlayer may also suppress sintering of the PdO particles.  The TOS results in the presence of H2O and SO2 showed that all the catalyst were deactivated by the SO2 very rapidly, even though the concentration of SO2 was very low at 2 or 5 ppm [11, 21, 101]. The PdPtCe-WC catalyst had the best stability compared to Pd-WC and PdCe-WC. Adsorbing SOx on the support and the active phase is the main cause for deactivation of the catalysts [11, 101]. Adding Pt to Pd decreases the adsorption of the SOx on the surface of the catalysts since SOx adsorption on Pt is less than Pd [11], resulting in the PdPtCe-WC catalyst  having better stability than the PdCe-WC and Pd-WC catalysts. In the present study, the stability of the catalysts in the presence of SOx and H2O was improved with the washcoat overlayer. Lampert et al.[11]  reported that the deactivation of catalysts by SOx with sulfating support such Al2O3 is slower than a non-sulfating support since Al2O3 adsorbs the SOx to form Al2(SO4)3.; whereas, for non-sulfating support the SOx was adsorbed by Pd. For the catalysts with the washcoat overlayer the SOx adsorbed on the overlayer, thereby protecting the active phase from SOx adsorption and enhancing the stability of the catalysts. XPS results showed that after the TOS tests in the presence of SOx and H2O, there was S on the catalyst surface but Pd and Ce were not detectable, and the EDX of the same used catalyst confirmed this observation. Hence, the SOx species must adsorb on the washcoat as indicated by the Pd 3d and S 2p B.E.s measured before and after reaction.  127  The results of this study clearly show that adding the washcoat overlayer to the catalysts reduces the suppression of the catalyst activity by H2O and SO2. The washcoat overlayer decreases the adsorption of H2O on the surface of the Pd catalyst due to coverage of the Pd particles by -AlOOH and -Al2O3.  In addition, the washcoat overlayer covers and surrounds the Pd particles which limits the adsorption of SOx species on the Pd active phase, with both XPS and EDX evidence suggesting that the SOx species bind to the overlayer, thereby reducing the exposure of the active phase to SOx. The coverage of the PdO by the overlayer may also lead to increased O2 exchange between the -AlOOH/-Al2O3 and PdO, which leads to increased catalyst activity.  Although the overlayer prevented an analysis of the PdO particle size after reaction, the possibility that the rate of PdO sintering in the presence of H2O is decreased by the washcoat overlayer also exists.  The ultimate test of the monolith catalysts is for them to be assessed using standard engine tests in which the engine exhaust is fed directly to the monolith catalyst.  To achieve such assessment, larger monoliths (at least 0.5 L) are required. Given the success of the O-PdPtCe-WC catalyst formulation, a 0.5 L monolith was prepared as detailed in Appendix E, for future engine testing. At the time of writing the sample had not yet been engine tested. However, as reported in  Appendix E, mini-monolith were cut from the 0.5 L sample to assess the impact of scale-up on the catalyst performance.  Although the scale-up resulted in some loss in performance, ways to mitigate these effects are described in Appendix E.    128  5.4 Conclusions  A PdO(Pt/CeO2)/-AlOOH/-Al2O3 washcoated cordierite (2MgO.2Al2O3.5SiO2) monolith catalyst is shown to enhance CH4 oxidation activity and stability at low temperature (< 500 C) in the presence of H2O and SO2 when a washcoat overlayer is applied to the catalyst. XPS and EDX analysis showed that adding the washcoat overlayer covered the Pd (and Pt/Ce when present). Adding Pt improved the activity and the stability of PdO/-Al2O3 in dry and wet feed gas. The CH4 oxidation activity of the catalyst in dry feed gas decreased when adding the washcoat overlayer; however, when H2O and SO2 were present in the feed gas, the washcoat overlayer enhanced the activity and the stability of the catalyst.  129  Chapter 6: Modeling of Monolith Catalytic Reactors  6.1 Introduction  The negative order in H2O of CH4 oxidation kinetics is well known and captures the inhibition effects of H2O on catalyst activity [6, 14, 15, 17, 45, 46]. In other studies, Langmuir-Hinshelwood kinetics have been used to describe the reaction and the impact of H2O. In these studies, the identification of a rate determining step (RDS) is necessary. In several studies[12, 14, 26, 113] the RDS has been assumed to be the cleavage of the first C-H bond on a PdO/Pd* site pair according to the reaction:  CH4 + PdO + Pd* → CH3-Pd + Pd-OH.                                                                                    6.1  In other studies, the desorption of OH species from the active site is assumed the RDS and more recently the migration of adsorbed OH species from the active phase to the support has been proposed as the RDS [15, 18, 91].  In addition, some authors have proposed that the transfer of O species from the support to the Pd* sites is required to ensure sufficient PdO/Pd* site pairs [18, 37, 38, 150, 157]. In the present study, three recent kinetic models reported in the literature, based on Langmuir-Hinshelwood kinetics have been applied to the data measured herein using the mini-monolith reactor.  Data obtained in dry feed gas and with 2 and 5% H2O are analysed. The kinetic analysis of the mini-monolith kinetic data is used to better understand the effect of adding CeO2 to the PdO catalyst and its operation in both wet and dry feed gas. The analysis has also been extended to describe the impact of the washcoat overlayer on the activity of the catalyst.  130  6.2 Mathematical Model  Modeling of the monolith reactor can be classified according to the dimensional and numerical complexity of the model as one-, two-, or three-dimensional models (1D, 2D or 3D, respectively) [123]. The 3D model is used to model the whole reactor instead of one channel when the washcoat deposited in the monolith is not homogeneous and temperature, concentrations and velocities change in both axial (flow direction, z) and radial (transverse to flow; r and ) directions. 2D models are used to model the reaction in one channel with changing temperature, concentrations and velocities in both axial (z) and radial (r) directions. 1D models are used to describe the reaction in one channel in one dimensional (flow direction, z), assuming that the profiles in the r and  directions are not significant.  The models can also be classified based on the scale as washcoat/catalyst scale models, channel models, or reactor scale models [121]. All of these classifications share the same principles and fundamentals of modeling [122]. Before deciding which monolith reactor model to use, there are several factors that must be considered, such as the nature of the reaction kinetics, the operating conditions of the monolith reactor, the numerical methods for solving the model equations, and the objective of the simulation [121]. To study the kinetics of the reaction a washcoat layer or a single channel model is adequate since this provides a good description of diffusion and reaction inside the catalyst washcoat and in the channels [119]. To evaluate experimental kinetic parameters and the effect of design variables on reactor performance, a 1D model is usually adequate because of the low CH4 concentration and a small temperature gradient in the washcoat layer,  as calculated in Appendix D [120]. 131  Mathematical and kinetic models of monolith reactors can be used to investigate how different experimental conditions, monolith catalysts and washcoat materials affect the observed rate of the chemical reaction [119]. In this study, a single channel of the monolith reactor will be modeled as a one-dimensional (1D) heterogeneous reactor with gaseous reactants and products and a solid catalyst. The assumptions for the modelling approach include the following [126]: 1) steady-state and isothermal conditions are assumed (the temperature gradient in the washcoat layer is small, as shown in Appendix D because of the low CH4 concentration and the thin layer of washcoat  [42, 103]), 2) the same hydrodynamic and thermal conditions within each monolith channel, 3) plug flow in a single channel (since the ratio of the length to hydraulic diameter of the channel is large enough to meet the plug flow criterion as shown in Appendix D.1 [126], 4) negligible pressure drop along the monolith channel [42, 126], 5) square geometry of the channel after washcoating (due to a thin layer of the washcoat) [126].  The calculations related to establishing plug flow, isothermal reactor conditions, no external mass transfer limitation and the extent of internal mass transfer are provided in Appendix D.  Figure 6.1 Scheme of square geometry of the single monolith channel. 132  The mole balance for CH4 flowing through the channel is: 𝜀𝑏𝜕(𝐶𝐶𝐻4)𝜕𝑡+ ∇𝐶𝐶𝐻4𝑢 + ∇. 𝐽 = −𝜂𝑟𝐶𝐻4                                                                                                6.2 𝜕(𝐶𝐶𝐻4)𝜕𝑡 represents the CH4 accumulation per unit volume, 𝜀𝑏 is the porosity of the washcoat, and ∇𝐶𝐶𝐻4𝑢 is the net rate of CH4 addition per unit volume caused by convection and ∇. 𝐽  is the net rate of CH4 addition per unit volume caused by diffusion. 𝑟𝐶𝐻4 is the intrinsic overall rate of CH4 oxidation in units of (mol.m-3.s-1),  𝜂 is the internal effectiveness factor, and 𝐶𝐶𝐻4is the concentration of CH4 in the bulk gas flow in (mol.m-3). 𝜀𝑏𝜕(𝐶𝐶𝐻4)𝜕𝑡  represents the CH4 accumulation per unit volume which is zero at steady state as per the assumption. The∇. 𝐽  term (diffusion transport) is negligible compared to the convective transport term, ∇𝐶𝐶𝐻4𝑢, as a result of the high superficial velocity (0.25 m.s-1) of the gas through the reactor channel. Based on these assumptions and from the mole balance across any channel in the monolith, equation (6.2) simplifies to the following:   𝑢𝜕(𝐶𝐶𝐻4)𝜕𝑍= −𝜂𝑟𝐶𝐻4                                                                                                                                    6.3  and 𝐴𝜌𝑤𝑎𝑠ℎ𝜕𝑍 = 𝜕𝑊𝑤𝑎𝑠ℎ                                                                                                                                   6.4 Since the CH4 oxidation reaction occurs in the washcoat layer, Equation 6.3 is transformed to Equation 6.5 where the mole balance is a function of mass of the washcoat:  𝐴𝑢𝜕(𝐶𝐶𝐻4)𝜕𝑊𝑤𝑎𝑠ℎ= −𝜂𝜌𝑤𝑎𝑠ℎ𝑟𝐶𝐻4                                                                                                                         6.5 133  where 𝜌𝑤𝑎𝑠ℎ  is the density of the washcoat in kg.m-3, 𝑊𝑤𝑎𝑠ℎ is the mass of the washcoat in kg, A is the cross-sectional area of the channel (m2), and 𝑢 is the superficial gas velocity in m.s-1 and 𝑣0 is total volumetric flowrate in (m3.s-1). 𝑣0𝜕(𝐶𝐶𝐻4)𝜕𝑊𝑤𝑎𝑠ℎ= −𝜂𝜌𝑤𝑎𝑠ℎ𝑟𝐶𝐻4                                                                                                                          6.6 𝜕(𝐶𝐶𝐻4)𝜕𝑊𝑤𝑎𝑠ℎ= −𝜂𝑣0𝑟𝐶𝐻4𝑚                                                                                                                                     6.7 𝑟𝐶𝐻4𝑚  is the CH4 oxidation reaction rate on a mass basis (mol.kgcat-1.s-1) 𝐶𝐶𝐻4 = 𝐶𝐶𝐻40 (1 − 𝑋𝐶𝐻4)                                                                                                                               6.8 𝐹𝐶𝐻4𝑜 = 𝑣0𝐶𝐶𝐻40                                                                                                                                                6.9 𝑋𝐶𝐻4 is the conversion of CH4, and  𝐹𝐶𝐻4𝑜  is the molar flow rate of CH4 in the inlet of the reactor in (mol.s-1) 𝜕𝑋𝜕𝑊𝑤𝑎𝑠ℎ=𝜂𝐹𝐶𝐻4𝑜 𝑟𝐶𝐻4𝑚                                                                                                                                    6.10  The effect of internal mass transfer across the washcoat is taken into account using the effectiveness factor calculated by [42]: 𝜂 =actual overall rate of reactionrate of reaction with no internal diffusion limitation                                                           6.11 𝜂 =tanh (𝜑)𝜑                                                                                                                                              6.12 and the Thiele modulus defined is given by 𝜑 = 𝐿𝑐√𝑘𝐷𝑒𝑓𝑓                                                                                                                                             6.13 where Lc is the thickness of the washcoat (m), 𝑘 is the rate constant of CH4 oxidation reaction in (s-1), and  𝐷𝑒𝑓𝑓 is effective diffusivity in (m2.s-1). 134  The Chapman-Enskog correlation is used to estimate the binary bulk diffusivity of CH4 in He [125]: 𝐷𝐶𝐻4,𝐻𝑒 = 1.8583. 10−3𝑇32 (1𝑀𝐶𝐻4+1𝑀𝐻𝑒)1/2𝑃𝜎𝐶𝐻4,𝐻𝑒2 Ω𝐶𝐻4,𝐻𝑒                                                                                   6.14   Where 𝐷𝐶𝐻4,𝐻𝑒 Binary bulk diffusivity (m2 s-1), 𝑀𝐶𝐻4Molecular weight of CH4 (kg.mol-1), 𝑀𝐻𝑒 Molecular weight of He (kg mol-1), 𝑇 temperature (K), 𝑃 pressure (kPa), 𝜎𝐶𝐻4,𝐻𝑒2 Constriction factor, and Ω𝐶𝐻4,𝐻𝑒 Collision integral. The bulk effective diffusivity is calculated by [125]: 𝐷𝐶𝐻4−𝐻𝑒𝑒𝑓𝑓 =𝜀𝑝𝜏𝐷𝐶𝐻4−𝐻𝑒                                                                                                                              6.15 Where 𝐷𝐶𝐻4−𝐻𝑒𝑒𝑓𝑓Effective bulk diffusivity (m2.s-1), 𝜀𝑝 Particle porosity and 𝜏 Tortuosity factor. Knudsen diffusivity is calculated as [125]: 𝐷𝐾 =8𝑑𝑝𝑜𝑟𝑒3√𝑅𝑇2𝜋𝑀𝑤𝑓𝑒𝑒𝑑                                                                                                                         6.16 Where 𝐷𝐾 Knudsen diffusivity (m2 s-1), 𝑑𝑝𝑜𝑟𝑒 Pore diameter (m), R Gas constant (kPa m3 K-1 mol-1). The effective Knudsen diffusivity is calculated from [125]: 𝐷𝐾𝑒𝑓𝑓 =𝜀𝑝𝜏𝐷𝐾                                                                                                                                                6.17                                                                                                                                Hence, the overall effective diffusivity is calculated by: 1𝐷𝑒𝑓𝑓=1𝐷𝐶𝐻4−𝐻𝑒𝑒𝑓𝑓 +1𝐷𝐾𝑒𝑓𝑓                                                                                                                            6.18    135  6.3 Determining the Reaction Kinetics of CH4 Oxidation:  A kinetic model of CH4 oxidation in the presence of H2O was developed by Alyani and Smith [12] to recognize the effect of adding Ce to a 6.5Pd/ Al2O3 catalyst. They assumed that adsorbed H2O was the most abundant surface intermediate (MASI) and that the RDS was the cleavage of the first C-H bond on a PdO/Pd* site pair. Their model explained the role of adding Ce to Pd and modelled their experimental data that showed that adding Ce to Pd reduced the negative effect of H2O by increasing the H2O desorption rate and reducing the amount of H2O adsorbed on the catalyst. In this study, their model will be applied to evaluate the performance of the monolith catalyst.  Habibi et al.[113] developed a kinetic model for Pd-Pt@SiO2 by assuming CH4 was activated by Pt and H2O adsorbed on the support which suppressed O-exchange between the active site and the support. The surface reaction between active oxygen (O*) and adsorbed CH4 was assumed to be the RDS. Their model explained why adsorbing CH4 on the metal is significant and how the support can contribute to increasing or decreasing inhibition by H2O. Thus, in this study the same concepts developed by Habibi et al.[113] will be applied but the CH4 will be adsorbed on Pd instead of Pt.  Ciuparu et al. [18] reported that the reaction rate of CH4 oxidation over Pd based catalysts in the presence of H2O in the feed depends on the temperature. They proposed at low temperature (<450 oC) the RDS is H2O desorption, and at high temperature (>450 oC) the RDS is CH4 activation. Also, they reported that H2O adsorbed on the support impedes the oxygen exchange 136  between the support and active site. Therefore, in this study the model proposed by Ciuparu et al. [18] will be assessed, assuming that H2O desorption is the RDS.    6.3.1 Model Proposed by Alyani and Smith  The model developed by Alyani and Smith[12] was adapted from that first proposed by  Kikuchi et al. [26]. The rate of reaction was assumed 1st-order in CH4 and dependent on the number of vacant active sites as follows:  −𝑟𝐶𝐻4𝑚 = 𝑘𝑟 𝑃𝐶𝐻4𝜃𝑣                                                                                                                                      6.19 where, 𝑘𝑟 is the rate constant (mol·gcat-1 s−1·Pa−1), 𝑃𝐶𝐻4  is the partial pressure of CH4 (Pa), 𝜃𝑣 is the fraction of vacant site pairs (both PdO and Pd-*).  The order of reaction with respect to CH4, O2 and CO2 was assumed 1, 0, and 0, respectively. Since the reaction occurred at low temperature (<500 °C) and the feed had high amounts of H2O (3.5 vol %), the authors assumed that hydroxyl formation and adsorption dominated the surface. Thus, the fraction of vacant sites, 𝜃𝑣, is given by: 𝜃𝑣 = 1 − 𝜃𝐻2𝑂                                                                                                                                             6.20 The rate-determining step (RDS) was assumed to be the C-H bond cleavage as shown in equation 6.21:  𝐶𝐻4 + 𝑃𝑑∗ + 𝑃𝑑𝑂 → 𝐶𝐻3. 𝑃𝑑 +  𝑂𝐻. 𝑃𝑑                                                                                            6.21 The hydroxyl/H2O adsorption on the the active site pair was assumed to be given by a Langmuir isotherm as in Equation 6.23: 𝐻2𝑂 + 𝑆 ↔ 𝐻2𝑂. 𝑆                                                                                                                                    6.22 137  𝜃𝑣 =11 + 𝐾𝐻2𝑂𝑃𝐻2𝑂                                                                                                                                   6.23 Thus, the reaction rate is given by: −𝑟𝐶𝐻4𝑚 =  𝑘𝑟 𝑃𝐶𝐻41 + 𝐾𝐻2𝑂𝑃𝐻2𝑂                                                                                                                           6.24 where, 𝑘𝑟 the rate constant (mol·gcat.-1.s−1 .Pa−1), 𝑃𝐶𝐻4  is the partial pressure of CH4 (Pa),  𝐾𝐻2𝑂 is equilibrium constant for H2O adsorption (Pa−1) and 𝑃𝐻2𝑂 is partial pressure of H2O (Pa).  6.3.2 Model Proposed by Habibi et al.   In a more recent study, a kinetic model of CH4 oxidation on a Pt-Pd bimetallic catalyst was reported by Habibi et al.[113] that accounted for the O-exchange between the active site and the support and the inhibition by H2O. The model was developed by assuming two active sites: (1) Pd sites for activating oxygen and (2) Pt sites for activating CH4 but herein CH4 is assumed to be activated on Pd. The RDS was assumed to be the surface reaction between active oxygen (O*) and adsorbed CH4. O2 from the gas phase was adsorbed by the Pd: 𝑂2 + 2𝑃𝑑 ↔ 2𝑃𝑑𝑂                                                                                                                                    6.25 The O2 exchange with the support (S) was assumed to occur as follows: 𝑃𝑑𝑂 + 𝑆 ↔ 𝑃𝑑 +  𝑆𝑂                                                                                                                               6.26 The O2 exchange was assumed to be impeded by H2O adsorption on the support, as shown by Equation 6.27 [28, 37, 38]:  𝑆𝑂 + 𝐻2𝑂 ↔ 𝑆𝑂. 𝐻2𝑂                                                                                                                               6.27 The support surface site balance can be written as: 138  𝜃𝑆𝑂 + 𝜃𝑆𝑂.𝐻2𝑂 + 𝜃𝑆 = 1                                                                                                                             6.28 The accumulation of hydroxyl/H2O is significant at temperatures below 450 °C; thus:  𝜃𝑆 ≈ 0 and by Equation 6.29, we can write: 𝜃𝑆𝑂.𝐻2𝑂 = 𝐾𝐻2𝑂𝜃𝑆𝑂𝑃𝐻2𝑂                                                                                                                            6.29 and hence, 𝜃𝑆𝑂 =1𝐾𝐻2𝑂𝑃𝐻2𝑂                                                                                                                                         6.30 CH4 is assumed to be activated on Pd sites: 𝐶𝐻4 + 𝑃𝑑 ↔ 𝑃𝑑. 𝐶𝐻4                                                                                                                               6.31 𝜃𝐶𝐻4 = 𝐾𝐶𝐻4  𝑃𝐶𝐻4𝜃𝑃𝑑                                                                                                                                  6.32 and the RDS of the reaction was assumed to be the reaction between 𝑃𝑑. 𝐶𝐻4  (adsorbed CH4) and activated oxygen 𝑆𝑂 as shown equation 6.33: 𝑃𝑑. 𝐶𝐻4 +  𝑆𝑂 − −> 𝐶𝑂2. 𝑃𝑑 + 𝑆𝑂. 𝐻2𝑂                                                                                          6.33 𝐶𝑂2. 𝑃𝑑 ↔ 𝐶𝑂2 + 𝑃𝑑                                                                                                                               6.34 The rate of reaction with 6.33 as the RDS is given by equation 6.35: −𝑟𝐶𝐻4𝑚 = 𝑘𝑟 𝜃𝐶𝐻4𝜃𝑆𝑂                                                                                                                                   6.35         CH4 is adsorbed on Pd, hence: 𝜃𝐶𝐻4 + 𝜃𝑃𝑑 + 𝜃𝐶𝑂2 = 1                                                                                                                             6.36 and assuming that 𝜃𝐶𝑂2 is negligible: 𝜃𝑃𝑑 =11 + 𝐾𝐶𝐻4𝑃𝐶𝐻4                                                                                                                                 6.37 Finally, the reaction rate is given by: −𝑟𝐶𝐻4𝑚 =𝑘𝑟 𝑃𝐶𝐻4(1 + 𝐾𝐶𝐻4𝑃𝐶𝐻4)(𝐾𝐻2𝑂𝑃𝐻2𝑂)                                                                                                   6.38 139  where 𝑘𝑟 the rate constant (mol·gcat.-1.s−1 .Pa−1),  𝑃𝐶𝐻4is the partial pressure of CH4 (Pa),  𝐾𝐶𝐻4 is equilibrium constant for CH4 adsorption (Pa−1), 𝐾𝐻2𝑂 is the equilibrium constant for H2O adsorption (Pa−1),  𝑃𝐻2𝑂 is partial pressure of H2O (Pa).  Note that Equation 6.38 implies an infinite rate as the H2O partial pressure approaches zero.  To avoid this incorrect feature, Equation 6.32 can be derived without assuming that 𝜃𝑆 ≈ 0 so that:   𝜃𝑆𝑂 =11 +  𝐾𝐻2𝑂𝑃𝐻2𝑂                                                                                                                                6.39 and  −𝑟𝐶𝐻4𝑚 =  𝑘𝑟  𝑃𝐶𝐻4(1 + 𝐾𝐶𝐻4𝑃𝐶𝐻4)(1 +  𝐾𝐻2𝑂𝑃𝐻2𝑂)                                                                                         6.40  6.3.3 Model Proposed by Ciuparu et al.   In some studies, the desorption of adsorbed OH species from the active metal site is assumed the RDS of CH4 oxidation [18]. Hence in this model the study of Habibi et al.[113] was modified to account for this RDS and reaction on a Pd catalyst as follows: S1 represents vacant active sites of the active phase (Pd*/PdO) and S2 is the vacant sites of the support. The O2 is adsorbed on the Pd and CH4 is activated by the Pd  𝑂2 + 𝑆1 → 𝑂2. 𝑆1                                                                                                                                         6.41 𝐶𝐻4 + 𝑆1  → 𝐶𝐻4. 𝑆1                                                                                                                                  6.42 and assuming Langmuir adsorption, this yields: 𝜃𝐶𝐻4𝑆1 = 𝐾1 𝑃𝐶𝐻4𝜃𝑆1                                                                                                                                   6.43 140  The CH4 oxidation reaction can be summarized as follows: 𝐶𝐻4. 𝑆1 + 2𝑂2. 𝑆1  ↔ 𝐶𝑂2. 𝑆1 +  2𝐻2𝑂. 𝑆1                                                                                            6.44 𝐶𝑂2. 𝑆1  ↔ 𝐶𝑂2 + 𝑆1                                                                                                                                  6.45 𝜃𝐶𝑂2𝑆1 = 𝐾2 𝑃𝐶𝑂2𝜃𝑆1                                                                                                                                    6.46 The RDS is assumed to be the hydroxyl/H2O exchange between the Pd and the support: 𝐻2𝑂. 𝑆1 + 𝑆2  → 𝐻2𝑂. 𝑆2 +  𝑆1                                                                                                                6.47 followed by H2O desorption:  𝐻2𝑂. 𝑆2  ↔ 𝐻2𝑂 + 𝑆2                                                                                                                                6.48 The rate of the reaction can then be written from equation 6.47 as: −𝑟𝐶𝐻4𝑚 = 𝑘𝑟 𝜃𝐻2𝑂.𝑆1𝜃𝑆2                                                                                                                                6.49 𝜃𝑆2 =11 + 𝐾𝐻2𝑂𝑃𝐻2𝑂                                                                                                                                  6.50 The equilibrium constant for equation 6.44 𝐾3 =𝜃𝐶𝑂2𝑆1  (𝜃𝐻2𝑂𝑆1)2𝜃𝐶𝐻4𝑆1  (𝜃𝑂2𝑆1)2                                                                                                                              6.51 𝐾3 =𝐾2 𝑃𝐶𝑂2𝜃𝑆1(𝜃𝐻2𝑂𝑆1)2𝐾1  𝑃𝐶𝐻4𝑃𝑂2𝜃𝑆12                                                                                                                     6.52 𝜃𝐻2𝑂𝑆1 =𝑘 𝑃𝐶𝐻4𝑃𝑂2𝜃𝑆1𝑃𝐶𝑂2                                                                                                                             6.53 𝜃𝑆1 =11 + 𝐾𝐶𝐻4𝑃𝐶𝐻4𝑃𝑂2𝑃𝐶𝑂2                                                                                                                           6.54 Then, the reaction rate is written as: 141  −𝑟𝐶𝐻4𝑚 =𝑘𝑟  𝑃𝐶𝐻4𝑃𝑂2𝑃𝐶𝑂2(1 + 𝐾𝐶𝐻4𝑃𝐶𝐻4𝑃𝑂2𝑃𝐶𝑂2) (1 + 𝐾𝐻2𝑂𝑃𝐻2𝑂)                                                                                   6.55  For all three kinetic models the rate equation arises from different assumptions and mechanisms.  The three equations have similar form, especially at low partial pressures of CH4.   The goal of the kinetic analysis reported herein was not to discriminate between the models, but rather determine if each of these proposed mechanisms, lead to kinetic equations that decsribe the mesured TPO data, as the amount of H2O added to the feed is varied.   Model proposed by Alyani and Smith (AS) The final equation that will be solved using MATLAB for the first model (AS), which accounts for the surface reaction as the rate limiting step and H2O as the MASI is given by: 𝜕𝑋𝜕𝑊𝑤𝑎𝑠ℎ=𝜂𝐹𝐶𝐻4𝑜 𝑟𝐶𝐻4𝑚 =𝜂𝐹𝐶𝐻4𝑜𝑘𝑟 𝑃𝐶𝐻41 + 𝐾𝐻2𝑂𝑃𝐻2𝑂                                                                                         6.56 Model proposed by Habibi et al. (HSH): For the second model (HSH) which accounts for CH4 adsorbed on the Pd*/PdO active site and H2O adsorbed on the support, the surface reaction is the RDS and the final model equation is: 𝜕𝑋𝜕𝑊𝑤𝑎𝑠ℎ=𝜂𝐹𝐶𝐻4𝑜𝑘𝑟 𝑃𝐶𝐻4(1 + 𝐾𝐶𝐻4𝑃𝐶𝐻4)(1 + 𝐾𝐻2𝑂𝑃𝐻2𝑂)                                                                              6.57  Model proposed by Ciuparu et al. (CP): The final equation for the third model (CP) which accounts for CH4 adsorbed on the active site and H2O adsorbed on the support, with H2O desorption as the RDS:  142  𝜕𝑋𝜕𝑊𝑤𝑎𝑠ℎ=𝜂𝐹𝐶𝐻4𝑜𝑘𝑟  𝑃𝐶𝐻4𝑃𝑂2𝑃𝐶𝑂2(1 + 𝐾𝐶𝐻4𝑃𝐶𝐻4𝑃𝑂2𝑃𝐶𝑂2) (1 + 𝐾𝐻2𝑂𝑃𝐻2𝑂)                                                                      6.58 The temperature dependence of the three unknown parameters of 6.56, 6.57 and 6.58 are given by 𝑘𝑟 = 𝑘𝑟0exp [−𝐸𝑎𝑅(1𝑇−1𝑇0)]                                                                                                                    6.59 𝐾𝐻2𝑂 = 𝐾𝐻2𝑂0 exp [−Δ𝐻𝐻2𝑂𝑅(1𝑇−1𝑇0)]                                                                                                   6.60 𝐾𝐶𝐻4 = 𝐾𝐶𝐻40 exp [−Δ𝐻𝐶𝐻4𝑅(1𝑇−1𝑇0)]                                                                                                    6.61 Where, 𝐸𝑎 Activation energy (kJ.mol-1), Δ𝐻𝐻2𝑂 Enthalpy of H2O adsorption (kJ.mol-1) and Δ𝐻𝐶𝐻4 Enthalpy of CH4 adsorption (kJ.mol-1)  6.3.4 CP Model with the Effect of the Washcoat Overlayer (Diffusion Barrier)  Figure 6.2 shows the expected CH4 concentration profile through the diffusion barrier. We assume the washcoat overlayer provides more S2 sites for H2O desorption. Hence, with the same RDS: 𝐻2𝑂. 𝑆1 + 𝑆2  → 𝐻2𝑂. 𝑆2 +  𝑆1                                                                                                                6.62 Also, the concentration profile for O2, H2O and CO2 are assumed to be fixed through the washcoat overlayer and CO is completely converted at <200 oC. Thus, only the CH4 concentration is significantly impacted by the overlayer. This assumption is based on the 143  relatively high concentrations of the other components that will not change significantly through the overlayer after reaction with such low CH4 concentrations in the feed gas.  Figure 6.2 Profile of the CH4 concentration through the washcoat overlayer (diffusion barrier - DB).  The CH4 concentration profile through the diffusion barrier can be calculated from a CH4 flux balance as follows: 𝑁𝐶𝐻4 = 𝑘𝑐(𝐶𝑚 − 𝐶𝑚𝑖)                                                                                                                               6.63 Where 𝑁𝐶𝐻4 flux of CH4 (mol. m-2.s-1), 𝑘𝑐 external mass transfer coefficient, m.s-1 and 𝐶𝑚 CH4 concentration (mol. m-3). Assume CH4 is an ideal gas where 𝑃𝑚 the partial pressure of CH4 (Pa) : 𝐶𝑚 =𝑃𝑚𝑅𝑇                                                                                                                                                      6.64 𝑁𝐶𝐻4 =𝑘𝑐(𝑃𝑚 − 𝑃𝑚𝑖)𝑅𝑇= −𝜂𝑟𝐶𝐻4𝑚𝑎𝑖                                                                                                          6.65 144  𝑘𝑐 =𝐷𝑒𝑓𝑓𝐿𝑑𝑏                                                                                                                                                    6.66 Where, 𝑎𝑖 surface area, (m2.g-1) and 𝐿𝑑𝑏 thickness of diffusion barrier (m). 𝑘𝑐𝑖 =𝑘𝑐𝑅𝑇                                                                                                                                                      6.67 𝑘′ =𝑘𝑟  𝑃𝑂2𝑃𝐶𝑂2(1 + 𝐾𝐶𝐻4 𝑃𝐶𝐻4𝑃𝑂2𝑃𝐶𝑂2) (1 + 𝐾𝐻2𝑂𝑃𝐻2𝑂)                                                                                         6.68 −𝑟𝐶𝐻4𝑚 =𝑘𝑟  𝑃𝐶𝐻4𝑃𝑂2𝑃𝐶𝑂2(1 + 𝐾𝐶𝐻4 𝑃𝐶𝐻4𝑃𝑂2𝑃𝐶𝑂2) (1 + 𝐾𝐻2𝑂𝑃𝐻2𝑂)=  𝜂𝑘′𝑃𝑚𝑖                                                               6.69 𝑘𝑐𝑖(𝑃𝑚 − 𝑃𝑚𝑖) =𝜂𝑘′𝑃𝑚𝑖𝑎𝑖                                                                                                                         6.70 𝑃𝑚𝑖 = (𝑎𝑖𝑘𝑐𝑖𝑎𝑖𝑘𝑐𝑖 + 𝜂𝑘′)𝑃𝑚                                                                                                                             6.71 −𝑟𝐶𝐻4𝑚 =  𝜂𝑘′(𝑎𝑖𝑘𝑐𝑖𝑎𝑖𝑘𝑐𝑖 + 𝜂𝑘′)𝑃𝑚                                                                                                                 6.72 A non-linear least-squares method was used to estimate the model parameters.  Minimizing the residual sum of squares (RSS) of measured and calculated CH4 conversions was used as the objective function as follows: 𝑅𝑆𝑆 = ∑ ( 𝑋𝑜𝑏𝑠𝑖 − 𝑋𝑐𝑎𝑙𝑖)2𝑁𝑜𝑏𝑠𝑖=1                                                                                                                    6.73 The CH4 conversion was measured during TPO experiments, which are generally not at steady-state because the temperature is continuously ramped with time. However, the TPO ramp rate was typically 5 °C/min whereas the gas residence time was < 0.1 sec, so that a steady-state reactor model could be applied at each temperature to calculate the conversion as follows: 145  𝑋𝑐𝑎𝑙 = ∫𝑟𝑚𝐹𝑚0𝑑𝑤𝑊0= 𝑓(𝑇, 𝑤, 𝐹𝑚0, 𝑃𝐶𝐻4 , 𝑃𝐻2𝑂 , 𝐾𝐶𝐻4 , 𝐾𝐻2𝑂 , 𝑘𝑟)                                                       6.74 By measuring the CH4 conversion as a function of operating conditions, the parameters of the kinetic models can be determined by regression analysis [12]. 𝐾𝐻2𝑂 , 𝐾𝐶𝐻4, 𝑘𝑟, Δ𝐻𝐻2𝑂, Δ𝐻𝐶𝐻4and Ea were estimated using the Levenburgh−Marquardt nonlinear regression algorithm and numerical integration of Eq. 6.74  was done using a 4th-order Runge−Kutta algorithm [164]. MATLAB software was used to complete the numerical calculations. Furthermore, the effectiveness factor (η) was calculated by Eq. 6.11 to quantify the diffusion in the washcoat at the experimental conditions. Table 6.1 shows the catalysts used in this study and the monolith properties are presented in Table 6.2  Table 6.1 Nominal composition of monolith catalysts used in kinetic model analysis. Sample  Composition, wt %  Pt Pd Ce -AlOOH -Al2O3 Cordierite Pd0Ce 0 0.5 0 5.3 21.2 73.0 Pd1Ce 0 0.5 1.0 5.1 20.4 73.0 PdPtCe-WC 0.04 0.31 1.5 5 20.15 73.0 O-PdPtCe-WC 0.03 0.27 1.3 6.2 25.2 67.0       146  Table 6.2 Monolith properties. ID (cm) 0.92 Lch (cm) 2.54 Volume (cm3) 1.69 Number of channels 52 Cell ID (cm) 0.1 Volume of Cell (cm3) 2.53 Total feed gas flow is 1025 cm3(STP)·min−1 1025 GHSV 36000 h−1  6.4 Results   The TPO experimental data for Pd0Ce and Pd1Ce in dry (0 vol% H2O) and wet (2 and 5vol% H2O) feed gas were used to estimate the kinetics parameters of all three models. For Pd0Ce and Pd1Ce, Figure 6.3 and 6.4 show the plots of experimental and calculated CH4 conversions obtained by AS, HSH and CP models, along with the coefficient of determination (r2 =0.99). The results of Pd0Ce and Pd1Ce for dry and wet feed showed that the three models are a good fit to the experimental data. Table 6.3 presents the estimated parameters for Pd0Ce and Pd1Ce using the AS Model. According to the AS model, the activation energy (Ea) for Pd1Ce is marginally higher than Pd0Ce, and the reaction rate constant (𝑘𝑟0) is higher for Pd1Ce. The values of the equilibrium constant for H2O adsorption (𝐾𝐻2𝑂0 ) and the H2O adsorption enthalpy (Δ𝐻𝐻2𝑂)  of Pd0Ce are higher than the values for Pd1Ce, suggesting that the activity of Pd1Ce was less affected by H2O adsorption compared to Pd0Ce.   147  Table 6.3 Estimated values obtained from the AS Model for CH4 oxidation over Pd0Ce and Pd1Ce.  𝐸𝑎  kJ mol-1 𝑘𝑟0 mol·gcat-1 s−1·Pa−1 Δ𝐻𝐻2𝑂 kJ mol-1 𝐾𝐻2𝑂0  Pa-1 r2 Pd0Ce 102±0.5a 0.035±0.0003 -56±0.4 0.22±0.0008 0.99 Pd1Ce 110±0.6 0.032±0.0004 -41±0.5 0.12±0.0005 0.99 a- the std. error associated with the parameter estimate   Figure 6.3 Comparisons of modeled and observed CH4 conversions for Pd0Ce. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5vol % H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:Pd0Ce. 148   Figure 6.4 Comparisons of modeled and observed CH4 conversions for Pd1Ce. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5vol % H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:Pd1Ce initial activity.  The estimated parameters obtained for the HSH Model for the Pd0Ce and Pd1Ce catalysts are presented in Table 6.4. The HSH Model results are in the line with the results of the AS Model, with Ea higher for Pd0Ce and 𝑘𝑟0 higher for Pd0Ce. Also, the Pd1Ce has lower values of  𝐾𝐻2𝑂0 and Δ𝐻𝐻2𝑂 compared to Pd0Ce. The HSH Model showed that Ea (16 and 25 kJ mol-1 ) for both catalysts was lower  than the values of Ea for the AS Model (102 and 110 kJ mol-1) due to 149  the fact that the equilibrium constant for CH4 adsorption (𝐾𝐶𝐻40 ) and the CH4 adsorption enthalpy (Δ𝐻𝐶𝐻4) are not neglected in HSH Model. 𝐾𝐶𝐻40  and Δ𝐻𝐶𝐻4for Pd1Ce is higher than Pd0Ce. Table 6.5 showed how these parameters change with increasing temperature for both catalysts. 𝐾𝐻2𝑂 and 𝐾𝐶𝐻4 for both catalysts decreased with increasing temperature. At 500 oC the values of 𝐾𝐶𝐻4of Pd1Ce is 0.43 Pa -1 while it is 6.17 Pa-1  for Pd0Ce which illustrates the rate of decrease in 𝐾𝐶𝐻4 is higher for Pd1Ce since Pd1Ce has higher value of Δ𝐻𝐶𝐻4.   Table 6.4 Estimated values obtained from HSH Model for CH4 oxidation over Pd0Ce and Pd1Ce.  𝐸𝑎  kJ mol-1 𝑘𝑟0  mol·gcat-1 s−1·Pa−1 Δ𝐻𝐻2𝑂 kJ mol-1 𝐾𝐻2𝑂0  Pa-1 Δ𝐻𝐶𝐻4 kJ mol- 𝐾𝐶𝐻40  Pa-1 r2 Pd0Ce 16±0.5a 0.52±0.003 -87±1.2 0.42±0.03 -60±0.3 36±1.9 0.99 Pd1Ce 26±0.4 0.38±0.0.002 -80±1.1 0.21±0.01 -94±1.4 7±0.4 0.99 a- the std. error associated with the parameter estimate       150  Table 6.5 Estimated values of rate constant, equilibrium constant for H2O adsorption, and equilibrium constant for CH4 adsorption for Pd0Ce and Pd1Ce using HSH Model. Temperature (K) Pd0Ce  Pd1Ce 𝑘𝑟 mol·gcat-1 s−1·Pa−1 𝐾𝐻2𝑂 Pa-1 𝐾𝐶𝐻4 Pa-1  𝑘𝑟  mol·gcat-1 s−1·Pa−1 𝐾𝐻2𝑂 Pa-1 𝐾𝐶𝐻4 Pa-1 410 0.599 0.204 22.00  0.473 0.105 3.11 440 0.676 0.105 13.89  0.575 0.057 1.52 470 0.755 0.057 9.11  0.689 0.033 0.78 500 0.837 0.032 6.17  0.813 0.019 0.43  The estimated parameters values obtained from the CP Model for Pd0Ce and Pd1Ce are reported in Table 6.6. The values are similar to the values that were obtained using the HSH Model with some minor differences. Table 6.7 reports the parameter values with increasing temperature. As the temperature increased 𝑘𝑟 increased while  𝐾𝐻2𝑂 and 𝐾𝐶𝐻4  decreased with increasing temperature. Moreover, both models have the same value for the effectiveness factor (η) (for dry feed) as shown in Figure 6.5; whereas, the AS Model has higher values of effectiveness factor η. For HSH and CP models the effectiveness factor η is small which indicates the importance of internal diffusion in these cases.       151  Table 6.6 Estimated values obtained from the CP Model for CH4 oxidation over Pd0Ce and Pd1Ce.  𝐸𝑎  kJ mol-1 𝑘𝑟0  mol·gcat-1 s−1·Pa−1 Δ𝐻𝐻2𝑂 kJ mol-1 𝐾𝐻2𝑂0  Pa-1 Δ𝐻𝐶𝐻4 kJ mol- 𝐾𝐶𝐻40  Pa-1 r2 Pd0Ce 18±0.6a 0.49±0.003 -85±1.2 0.48±0.02 -58±0.3 25±1.2 0.99 Pd1Ce 25±0.4 0.36±0.002 -80±1.1 0.21±0.01 -93±1.3 6±0.4 0.99 a- the std. error associated with the parameter estimate  Table 6.7 Estimated values of rate constant, equilibrium constant for H2O adsorption, and equilibrium constant for CH4 adsorption for Pd0Ce and Pd1Ce using CP Model. Temperature (K) Pd0Ce  Pd1Ce 𝑘𝑟  mol·gcat-1 s−1·Pa−1 𝐾𝐻2𝑂 Pa-1 𝐾𝐶𝐻4 Pa-1  𝑘𝑟  mol·gcat-1 s−1·Pa−1 𝐾𝐻2𝑂 Pa-1 𝐾𝐶𝐻4 Pa-1 410 0.570 0.240 15.53  0.447 0.107 2.81 440 0.655 0.125 9.97  0.542 0.058 1.37 470 0.743 0.069 6.64  0.647 0.033 0.71 500 0.835 0.040 4.56  0.762 0.020 0.39   152   Figure 6.5 Comparisons of effectiveness factor η value of Pd1Ce for the three models. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He.  For the PdPtCe-WC catalyst, the AS and CP models were used to estimate the kinetic parameters and as can be seen in Figure 6.6, both models showed a good fit with the experimental data (TPO data in dry and wet (2 and 5vol% H2O) feed gas with a coefficient of determination (r2) of 0.99 for both models. Table 6.8 reports the estimated kinetics parameters for the AS Model. Comparing the results of AS and CP models for PdPtCe-WC, the value of 𝐸𝑎estimated by AS Model (124 kJ mol-1) is higher than the value that estimated by CP Model (26 kJ mol-1). Δ𝐻𝐻2𝑂 (-42±1 kJ mol-1) and 𝐾𝐻2𝑂0 ( 0.04±0.01 Pa-1) for PdPtCe-WC estimated by using AS Model are less than the values for the same parameters that were estimated using the CP Model( Δ𝐻𝐻2𝑂 -153  55±1 kJ mol-1) and 𝐾𝐻2𝑂0 ( 0.05±0.03 Pa-1)). The difference between these results is due to an assumption of different RDS.  Table 6.8 Estimated values obtained from the AS Model for CH4 oxidation over PdPtCe-WC.  𝐸𝑎  kJ mol-1 𝑘𝑟0  mol·gcat-1 s−1·Pa−1 Δ𝐻𝐻2𝑂 kJ mol-1 𝐾𝐻2𝑂0  Pa-1 r2 PdPtCe-WC 124±1a 0.038±0.0006 -42±1 0.04±0.0005 0.99 a- the std. error associated with the parameter estimate  Table 6.9 Estimated values obtained from the CP Model for CH4 oxidation over PdPtCe-WC and O-PdPtCe-WC.  𝐸𝑎  kJ mol-1 𝑘𝑟0  mol·gcat-1 s−1·Pa−1 Δ𝐻𝐻2𝑂 kJ mol-1 𝐾𝐻2𝑂0  Pa-1 Δ𝐻𝐶𝐻4 kJ mol- 𝐾𝐶𝐻40  Pa-1 r2  PdPtCe-WC 26±2a 0.45±0.01 -55±1 0.05±0.001 -71±1 36±3 0.99  O-PdPtCe-WC 88±3 0.91±0.003 -55±1 0.05±0.001 -71±1 36±3 0.98  a- the std. error associated with the parameter estimate  154   Figure 6.6 Comparisons of modeled and observed CH4 conversions for PdPtCe-WC using AS and CP models. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5vol% H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:PdPtCe-WC initial activity.  The parameters (𝐾𝐻2𝑂0 , Δ𝐻𝐻2𝑂, 𝐾𝐶𝐻40 , Δ𝐻𝐶𝐻4) that were estimated for PdPtCe-WC using CP Model were also used to fit the experimental data of the O-PdPtCe-WC (i.e. the same catalyst with the washcoat overlayer). PdPtCe-WC and O-PdPtCe-WC have the same loading of the active phase and CeO2; but O-PdPtCe-WC has the washcoat overlayer applied on the top of the active phase, as Figure 6.2 presented. The CP Model and the parameters of the PdPtCe-WC catalyst were used to estimate the 𝑘𝑟0 the reaction rate constant, the activation energy Ea and the 155  CH4 concentration profile through the washcoat overlayer. Figure 6.7 reports the good fit between the experimental data (TPO data in dry and wet (2 and 5vol% H2O) feed gas) and the model results with a coefficient of determination (r2) of 0.98. The estimated parameters for PdPtCe-WC and O-PdPtCe-WC are presented in Table 6.9, and the parameter estimated for O-PdPtCe-WC from the model showed that O-PdPtCe-WC had higher activation energy (86±1.1 kJ mol-1) and reaction rate constant compared to the PdPtCe-WC.     Figure 6.7 Comparisons of modeled and observed CH4 conversions for O-PdPtCe-WC using CP models. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5vol% H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:O-PdPtCe-WC initial activity. 156  The estimated value of the effectiveness factor η for the dry feed are presented in Figure 6.8. As shown, the O-PdPtCe-WC had an effectiveness factor η  1 and it decreased with increasing temperature due to the fact that the rate of reaction increased faster than the diffusion rate as temperature increased (300-500 oC).  Also, the effectiveness factor η increased with increased concentration of H2O in the feed. At high temperature the diffusion barrier (washcoat overlayer) increased the reaction rate which leads to lower values of effectiveness factor η for O-PdPtCe-WC compared to PdPtCe-WC.  Adding the washcoat overlayer limits the diffusion of CH4 which reduce the reaction rate.    157   Figure 6.8 Comparisons of effectiveness factor η value of PdPtCe-WC and O-PdPtCe-WC. Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5 vol% H2O in N2 and He. A:Dry feed 0 vol% H2O; B:Wet feed 2 vol% H2O; C:Wet feed 5 vol% H2O; D:O-PdPtCe-WC η.       158  6.5 Discussion   The results of the AS Model for the Pd0Ce and Pd1Ce showed that adding CeO2 to the Pd catalyst increased the activation energy marginally from 102 to 110 kJ mol-1 and reduced the strength of the H2O adsorption (Δ𝐻𝐻2𝑂 from -56 to -41kJ mol-1) and adsorption equilibrium constant for H2O (𝐾𝐻2𝑂0 from 0.44 to 0.21 Pa-1). The high negative value of the enthalpy of H2O adsorption for Pd0Ce(without CeO2) indicates a strong adsorption of H2O on Pd0Ce [12]. The results of adsorption equilibrium constant for H2O are in agreement with Alyani and Smith [12]. Their kinetic analysis of CH4 oxidation over 6.5%Pd/Al2O3 and 2.6%CeO2/ 6.5%Pd/Al2O3 catalysts using the same model (AS Model), found that adding the CeO2 to 6.5%Pd/ Al2O3 decreased the adsorption equilibrium constant for H2O from 0.005 to 0.001 Pa-1, suggesting that adding CeO2 increased the desorption of hydroxyl/H2O. Moreover, Kikuchi et al.[26] used the same model (AS Model) for studying the kinetics of 1.1%Pd/Al2O3. They reported an activation energy of 81 kJ mol-1 and an enthalpy of H2O adsorption of -49 kJ mol-1 which are in agreement with the results of Pd0Ce (-56 kJ mol-1) [26]. From the results of the AS Model it can be concluded that adding CeO2 decreased the enthalpy of H2O adsorption Δ𝐻𝐻2𝑂 and adsorption equilibrium constant for H2O 𝐾𝐻2𝑂0 which leads to improved stability and activity of the catalyst in the presence of H2O.   In the AS Model, adsorbed H2O is assumed to be the most abundant surface intermediate (MASI) and oxidation of CH4 is the RDS; thus, the Ea is higher than HSH and CP models. In the HSH Model, adsorption of CH4 on the Pd and adsorption of H2O on the support are accounted for. The results for the HSH and CP models showed that the adsorption of CH4 has a major 159  impact on the reaction rate for both Pd0Ce and Pd1Ce catalysts. The results showed that adding CeO2 increased strength of the CH4 adsorption (Δ𝐻𝐶𝐻4from -60 to -90 kJ.mol-1). In the HSH model the results showed that CH4 adsorbed more on Pd1Ce catalysts, suggesting that adding CeO2 increased the desorption of H2O that leads to more active sites available to adsorb CH4 and these results are in the line with Palfi et al. [165]. They reported a calorimetric study to investigate the heat of adsorption of CH4 and other hydrocarbons on Pt and the heat of adsorption of CH4 was in the range -60 to -150 kJ.mol-1 depending on the methane coverage. The heat of adsorption decreased as the number of vacant sites decreased [165]. Also, Broclawik et al. [166] investigated the heat of adsorption of CH4 on PdO by means of density functional theory and reported that the heat of adsorption of CH4 on PdO is 102 kJ.mol-1 [166]. The values of (Δ𝐻𝐶𝐻4)  and the activation energy Ea are in agreement with Habibi et.al [113] although the catalysts are different. Habibi et al. used the same kinetic model (HSH) but CH4 was assumed to be adsorbed by Pt. They reported that the enthalpy of CH4 adsorption (Δ𝐻𝐶𝐻4) is -83.59 kJ.mol-1 and the activation energy is 36 kJ.mol-1 [113]. The results of the HSH Model showed that adding CeO2 to the catalysts decreased the enthalpy of H2O adsorption Δ𝐻𝐻2𝑂 and adsorption equilibrium constant for H2O 𝐾𝐻2𝑂0 , in agreement with the AS Model. From the results of the HSH Model it can concluded that CH4 adsorption has a significant impact on the reaction rate and adding the CeO2 enhanced the activity of the catalysts in the presence of H2O by increasing the rate of H2O desorption which leads to more active sites available for CH4.  The CP Model showed almost the same results as the HSH Model even though the RDS is different. The CP Model assumed H2O desorption as the RDS. The adsorption strength of CH4 is 160  higher than H2O for both models since we assumed that CH4 adsorbed on the active phase and H2O adsorbed on the support and the hydroxyl/H2O produced from the reaction migrated from the active site to the support. Also, the effectiveness factor η for both models are the same. The effective diffusivity Deff in this study was estimated using tortuosity factor and constriction factor valuesof τ = 8 and σ = 0.8, respectively and the value of Deff for all the models for all the catalysts were obtained in the order of 10-7 m2/s, in agreement with the data of Hayes et al. [116]. They estimated the effect of the tortuosity factor on the diffusivity of CH4 in the washcoat (Al2O3) of the monolith. For τ = 2.44 they obtained a value of 5.6E10-7 m2/s for Deff. For τ = 8.1 the value of Deff was 1.7E10-7 m2/s. The higher value of the tortuosity factor leads to a decreased rate of diffusion which leads to a lower value of the effectiveness factor η.  At 480 oC the η is about 0.05 which is in good agreement with Hayes et al. [116] who reported an activation energy of 127 kJ mol-1 and an η at 480 oC of 0.04 for a Pd/Al2O3 monolith catalyst [116]. The AS Model has a higher effectiveness factor compared to the HSH and CP models since the reaction rate constant (kr) for HSH and CP models is higher than the AS Model [12]. The Weisz-Parter criterion (CWP) was calculated at 250 oC and a CH4 conversion below 10 %.  The value of CWP was 0.1 ( < 1)  which indicates no internal mass transfer limitation at low temperature, even though the value is not very small. At high temperature the reaction rate is signifcantly higher which leads to a high value of CWP, indicating that at high temperature the reaction is controlled by internal mass transfer. From the result of CWP, the value of the effectiveness factor η for model AS is more realistic than that reported for the HSH and CP mdels (Figure 6.5). For model HSH and CP the value of the effectiveness factor η is very small since the reaction rate constant is higher for these models and the rate constants are highly correlated with η.  Alyani and Smith reported that the 6.5Pd/Al2O3 had a higher reaction rate constant compared to co-161  2.9Ce/6.5Pd/Al2O3 which leads to lower effectiveness factor with respect to co-2.9Ce/6.5Pd/Al2O3 catalyst [12]. It can be concluded from the results from all three models applied to the CH4 conversion data measured over the  Pd0Ce and Pd1Ce catalysts, that adding CeO2 decreased the strength of H2O adsorption Δ𝐻𝐻2𝑂 and the adsorption equilibrium constant for H2O 𝐾𝐻2𝑂0  which explain why catalyst Pd1Ce has higher activity than Pd0Ce in wet feed (2 or 5% vol H2O) gas.   Comparing the results of the Pd0Ce, Pd1Ce and PdPtCe-WC catalysts using the CP Model, shows the estimated activation energies in the range of 16 -26 kJ mol-1, and the reaction rate constant between 0.36 and 0.47 mol·gcat.-1.s−1 .Pa−1. However, PdPtCe-WC showed better resistance to the H2O since the enthalpy of H2O adsorption Δ𝐻𝐻2𝑂 (-55 kJ mol-1) and adsorption equilibrium constant for H2O 𝐾𝐻2𝑂0  (0.05 Pa-1 ) are lower than the values for Pd0Ce and Pd1Ce, suggesting that adding Pt improves the activity and the stability by decreasing the H2O adsorption on the catalyst.  For the case of the washcoat overlayer, the RDS is assumed to be the desorption of H2O from the active phase to the support. From the results of PdPtCe-WC and O-PdPtCe-WC catalysts, O-PdPtCe-WC had higher reaction rate and lower effectiveness factor which explains the benefits of adding the washcoat overlayer by increasing the rate of reaction due to increasing the rate of H2O desorption. The washcoat overlayer covers and surrounds the Pd particles which leads to an accelerated transfer of H2O from the active phase (S1) to the support (S2) which makes more active sites available for the reaction. Then the H2O desorbs from the support.  162  𝐻2𝑂. 𝑆1 + 𝑆2  → 𝐻2𝑂. 𝑆2 +  𝑆1                                                                                                                6.47 followed by H2O desorption:  𝐻2𝑂. 𝑆2  ↔ 𝐻2𝑂 + 𝑆2                                                                                                                                6.48 Schwartz et al. [38] investigated the kinetics of CH4 oxidation over a 3wt%PdO/Al2O3 by assuming the RDS was the H2O desorption at temperature >500 oC and the surface reaction was the RDS at temperature <500 oC. They proposed that hydroxyl accumulation on the oxide support (Al2O3) was the main cause for CH4 oxidation catalyst inhibition by the H2O since hydroxyl accumulation impedes the O2 exchange between the support and the active phase and decreases the rate of H2O desorption [38]. Their assumption was applied for the CP model. Hence, by adding the washcoat overlayer, the oxide support covers the active phase and suppresses the effect of hydroxyl accumulation by providing more oxide support around the active phase which leads to reduced inhibition by H2O on the active phase of the catalyst.  Cullis et al. [158] proposed that competition of H2O with CH4 for surface active sites is the cause of the inhibition of Pd catalysts. Also van Giezen et al.[114] suggested adsorption of H2O on the active site is the main cause of Pd catalyst inhibition in the presence of H2O.  In Chapter 5 it was proposed that the stability of the catalyst was enhanced by adding the washcoat overlayer due to the reduced adsorption of H2O on the active site. The fit of the experimental data to the model that accounts for additional H2O adsorption by the overlayer provides one possible mechanism that explains the role of the washcoat overlayer.     Hayes et. al.[42] studied the kinetics for CH4 oxidation on Pd/Al2O3 monolith catalysts with feed 2 vol% CH4 in air.  They reported an activation energy for their model of 58 kJ mol-1 and the 163  enthalpy of H2O adsorption (Δ𝐻𝐻2𝑂) of -43 kJ mol-1. The concentration of the H2O varied from 1 to 7 vol%. The found that the effectiveness factor (η) increased with increasing H2O concertation in the feed, in agreement with the results of PdPtCe-WC and O-PdPtCe-WC in Figure 6.9. It can be concluded from the results of the CP model that adding the washcoat overlayer increases the rate of the reaction when H2O desorption is the RDS since it accelerates the hydroxyl/H2O desorption by surrounding the active phase by the support.  6.6 Conclusion   The experimental results for different catalysts were used to model the reaction of CH4 oxidation on monolith catalysts.  All three selected models provided a good fit with the experimental data obtained in dry and wet feed gas.  All three models showed that adding CeO2 enhanced the activity and the stability of the catalysts by decreasing H2O adsorption. The AS model showed that the activation energy for CH4 oxidation is ~100 kJ mol -1 if it is assumed that adsorbed H2O is the most abundant surface intermediate (MASI). The HSH and CP models showed that CH4 adsorption has a major impact on the reaction rate and that adding CeO2 increased the heat of CH4 adsorption since the rate of H2O desorption increased. Also, the results of PdPtCe-WC illustrated the benefit of adding Pt to Pd catalysts by suppressing H2O inhibition. The CP Model showed that adding a washcoat overlayer increased the reaction rate in the presence of H2O since the desorption of H2O accelerated.    164  Chapter 7: Conclusions and Recommendations  7.1 Conclusions  A series of catalysts supported on ceramic cordierite monoliths (400 CPI, 1 cm x 2.54 cm), washcoated with different supports and loaded with PdO, were evaluated for CH4 oxidation. Catalysts with improved washcoat formulations that enhanced the activity and stability of the monolith catalysts, when operated in the presence of H2O, CO, CO2 and SO2, have been identified. Results of this study showed that H2O adsorption on the catalyst surface plays a major role in inhibiting the catalyst activity and SO2 introduced in the feed gas deactivates the catalyst. Decreasing the H2O and SO2 absorbed on the surface of the catalyst, either by changing the washcoat formulation or by adding a washcoat overlayer, leads to improved activity and stability of the catalyst.  The activity and stability of the PdO/-Al2O3 monolith catalysts was dependent on the presence of -AlOOH in the washcoat. An optimum solid content of 25 wt% in the washcoat suspension was used to obtain a ~25 wt% washcoat on the monolith. The presence of -AlOOH enhanced the thermal and mechanical stability of the washcoat, provided that the -AlOOH content was < 8 wt%. Temperature-programmed methane oxidation (TPO) showed that addition of -AlOOH to the -Al2O3 washcoat decreased the catalyst activity. However, when H2O (2 and 5 vol%) was present in the feed gas, the -AlOOH improved the catalyst activity and stability. A -AlOOH 165  content of ~5 wt % in the washcoat was determined to provide the highest catalyst activity and stability for CH4 oxidation in the presence of H2O.  The inhibition of CH4 oxidation by H2O at low temperature (< 500 C) is suppressed by adding CeO2 to a PdO/-AlOOH/-Al2O3 washcoated monolith catalyst. The CeO2 loading in the washcoat was varied from 1 to 4 wt%. Increased CeO2 loading increased the Pd/Al surface ratio; whereas, the BET surface area and CO uptake decreased. The highest catalyst activity and stability for CH4 oxidation in the presence of H2O was obtained by adding 2 wt% CeO2 to the washcoat. Adding CeO2 enhances oxygen exchange with the active phase and reduces water adsorption on the catalyst which leads to improved activity and stability of the catalysts in the presence of H2O.  A washcoat overlayer applied to PdO(Pt/CeO2)/-AlOOH/-Al2O3 washcoated cordierite monolith catalysts, is shown to enhance methane oxidation activity at low temperature (< 500 C) in the presence of water. A washcoat overlayer was applied by dip coating the monolith catalyst using a -AlOOH/-Al2O3 suspension. XPS and EDX analysis confirmed that the overlayer covered the Pd (and Ce and Pt when present). Temperature-programmed methane oxidation (TPO) showed that adding the washcoat overlayer decreased the catalyst activity in a dry feed gas; whereas, when H2O (2, 5 and 10 vol%) was present in the feed gas, the catalyst activity and stability increased compared to the same catalyst prepared without the washcoat overlayer. Time-on-stream experiments operated at 500oC and the same feed gas conditions but with 10 vol% H2O and 5 ppmv SO2 demonstrated that the washcoat overlayer also improved the 166  stability of the catalyst. The stability and activity of the catalysts in dry and wet feed gas also increased with Pt addition. The washcoat overlayer provided additional sites for both SOx and H2O adsorption, thereby reducing their adsorption on the active phase of the catalyst. O-PdPtCe-WC was the most active and stable catalyst in the presence of H2O and SO2. Hence, this catalyst has been scaled up and a 0.5 L monolith prepared for engine testing of this catalyst formulation.  TPO data obtained in dry feed gas and with 2 and 5 vol% H2O for different catalysts were used to model the reaction of CH4 oxidation on monolith catalysts based on Langmuir-Hinshelwood kinetics. Three recent kinetic models reported in the literature were applied to evaluate the performance of the monolith catalysts with different assumptions and RDS. All three models provide a good fit of the experimental data. The results of these models showed that adding CeO2 to Pd-based catalysts decreased H2O adsorption which led to enhanced activity of the catalyst in the presence of H2O in the feed. Also, the results of models showed that the desorption of H2O is accelerated by adding the washcoat overlayer which increased the reaction rate in the presence of H2O.  7.2 Recommendations  7.2.1 Kinetic Modeling for a Wider Range of CH4 Concentrations In this study the reactor model was developed by describing the reaction on the catalyst surface, assuming a one dimensional (flow direction, z) reactor model. The model was applied to the TPO experimental data for CH4 oxidation with low and fixed CH4 concertation (0.07 vol%) and varying H2O concentration (0 to 5 vol%). Also, the GHSV was fixed at 36000 h-1. It is 167  recommended that the kinetics of CH4 oxidation be investigated with varying CH4 concentrations (0.1 to 2 vol%) in the presence of H2O (10-15 vol%) and SO2 (1-5 ppm) as the GHSV is varied from 10000 to 55000 h-1. Also, the model should consider the mass and heat transfer through the washcoat layer.    7.2.2 Study the Effect of the Support on Monolith Catalyst  In this study, the effect of CeO2 and -AlOOH was investigated and the results showed that the support plays a major role in the activity and the stability of the CH4 catalyst. Furthermore, in this study it was proposed that a support with higher hydrophobicity reduced the adsorption of H2O on the surface of the catalysts. Thus, a study of the effect of the support hydrophobicity on H2O adsorption is recommended. Also, it was proposed that CeO2 increased the O2 exchange between the support and the active phase which leads to increased activity of the catalyst. Thus, supports with high oxygen mobility such as ZrO2 should be added to the washcoat and compared to the performance of to PdO/CeO2/AlOOH/Al2O3 to understand the role of promoter oxides.  7.2.3 Scale-up and Engine Test   In this study a 0.5 L monolith catalyst of the O-PdPtCeWC formulation that had the best activity and stability among all the catalysts of this study was prepared. This 0.5 L sample will be subjected to engine testing. In the preparation of the 0.5 L monoliths we found the catalyst showed low activity and less stability due to the calcination in the furnace, which is not adequate for large samples, resulting in a monolith interior temperature of < 450 oC. To overcome this 168  obstacle the temperature and the time of the calcination in the furnace were increased, which might effect the catalyst activity. 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The isotherm data (P/P° < 0.3) is used to fit the linear form of The BET (Brunauer-Emmett-Teller) equation 𝑃𝑉(𝑃0 − 𝑃)=1𝑉𝑚𝐶+𝐶 − 1𝑉𝑚𝐶(𝑃𝑃𝑜)                                                                                                            𝐴. 1 𝑥 =𝑃𝑃𝑜                                                                                                                                                          𝐴. 2 𝑦 =𝑃𝑉(𝑃𝑜−𝑃) = 1𝑉(1𝑥−1)                                                                                                                              𝐴. 3  Thus,  𝑦 = 𝑃𝑉𝑚𝐶+𝐶−1𝑉𝑚𝐶𝑥                                                                                                                                      𝐴. 4  P is the partial pressure of N2, Po is the saturation pressure at the adsorption temperature, V is the volume of adsorbed gas at P, Vm is volume adsorbed at monolayer coverage, and C is a constant.  By plotting x versus y for the P/Po < 0.3 Vm would be obtained. Thus, BET surface area is calculated from this equation and the error was typically less than ± 3 m2/g.: 𝑆𝐵𝐸𝑇 =𝑉𝑚𝜎𝑁𝐴𝑉0                                                                                                                                            𝐴. 5 184   σ is the occupied area by adsorption of a single molecule of N2 (16.2 Å2) and NA is the Avogadro constant. 0.0 0.2 0.4 0.6 0.8 1.0050100150200250Vads(cc STP/gcat)Relative Pressure (P/Po) Adsorption Isotherm Desorption Isotherm Figure A.1 Isotherm data for calcined AlOOH.  185   Figure A.2 Pore size distribution of the Al2O3, AlOOH and 20% AlOOH/80% Al2O3.           186  Table A.1 BET analysis repeatability. Monolith catalysts:  BET area Pore volume Average Pore Diameter   m2/g cm3/g nm Cordierite monolith Run1 2 0.01 30 Run2 1 0.009 23 Average  1.5 0.01 26.5 SD  0.7 0.001 4.9 Pd0B  Run1 50 0.19 17 Run2 46 0.2 16 Average  48 0.195 16.5 SD  2.8 0.01 0.7 Pd0B - Used catalyst Run1 15 0.08 21 Run2 13 0.07 24 Average  14 0.075 22.5 SD  1.4 0.01 2.1       A.2 XRD   The Scherrer equation is used to calculate the crystallite size (d) as follows:  d𝑐𝑟𝑦𝑠𝑡𝑎𝑙 =Kλ(βcosθ)                                                                                                                                     𝐴. 6 187  dcrystal is the crystallite size (nm), K is an instrument constant (usually 1), λ is the X-ray wavelength (1.7902 Å), β is the width of the peak (full width at half maximum-FWHM), and θ is the angle of reflection.   A.3 CO Chemisorption  CO chemisorption is commonly used to calculate the dispersion of metals. Metal dispersion is determined by this equation:   Dispersion =Ns(surface metal atoms)Nt(total number of metal atoms)                                                                               𝐴. 7  The CO chemisorption analysis for Pd5B showed that the CO uptake was 11.5 µmolgcat and the Pd molecular weight 106.42X10−6gPdµmol   the metal dispersion calculated as:   Metal dispersion (%) =CO uptake X metal molecular weightmetal wt%X100                                         𝐴. 8  Pd dispersion (%) =11.5µmolgcat  X 106.42X10−6 gPdµmol0.005gPdgcatX100 = 24.5%     188  Appendix B  MFC and MS Calibration  B.1 MFC Calibration  Mass flow controllers (MFC) were used to control the flow rate of CH4, CO, CO2, Air, N2 and He gases to the monolith reactor. A bubble flow meter is used to calibrate each MFC in a wide range of flow rate. Tables B.1 and Figure B.1 present the 1 vol% CH4/N2 MFC calibration results. Tables B.2 and Figure B.2 present the 5 vol% CO/N2 MFC calibration results. Tables B.3 and Figure B.3 present the 50 vol% CO2/N2 MFC calibration results. Tables B.4 and Figure B.4 present the Air MFC calibration results. Tables B.5 and Figure B.5 present the N2 MFC calibration results. Tables B.5 and Figure B5 present the He MFC calibration results.  Table B.1 MFC calibration using a bubble flow meter for 1vol% CH4/N2. Set Point  % Volume  cc Time s Flow rate cc(STP).min-1 40 6 7.10 46.4 50 6 6.0 54.3 60 6 5.2 62.9 70 10 7.7 71.7 80 10 6.8 80.7 90 10 6.2 88.2 189   Figure B.1 Calibration equation obtained for 1vol%CH4/N2.  Table B.2 MFC calibration using a bubble flow meter for 5 vol% CO /N2. Set Point  % Volume  cc Time s Flow rate cc(STP).min-1 10 1 6.9 8.0 20 2 6.8 16.2 30 3 6.4 25.9 40 5 7.9 34.8 50 5 6.3 43.3     y = 0.8483x + 12.231R² = 0.99953050709011030 50 70 90 110Measured Flow Rate (sccm)Flow SP (%)190   Figure B.2 MFC Calibration equation obtained for 5 vol% CO/N2.  Table B.3 MFC calibration using a bubble flow meter for 50 vol% CO2/N2. Set Point  % Volume  cc Time s Flow rate cc(STP).min-1 40 10 7.4 74.1 50 10 5.7 96.3 60 10 6.8 121.3 70 10 5.5 150.5 80 15 6.3 174.8 90 15 5.4 202.1    y = 0.8929x - 1.1608R² = 0.9994010203040500 20 40 60Measured Flow Rate (sccm)Flow SP191   Figure B.3 MFC Calibration equation obtained for 50vol%CO2/N2.  Table B.4 MFC calibration using a bubble flow meter for Air. Set Point  % Volume  cc Time s Flow rate cc(STP).min-1 100 10 7.6 72.7 400 15 3.0 278.4 500 20 3.1 351.0 600 20 2.6 421.5 700 20 2.2 492.7 800 25 2.5 551.5   y = 2.584x - 31.463R² = 0.99876010014018022020 40 60 80 100Measured Flow Rate (sccm)Flow SP192   Figure B.4 MFC Calibration equation obtained for Air.  Table B.5 MFC calibration using a bubble flow meter for N2. Set Point  % Volume  cc Time s Flow rate cc(STP).min-1 200 8 3.6 246.8 250 8 2.8 313.5 300 15 4.2 395.2 350 15 3.5 466.6 450 25 3.8 604.3 500 30 2.4 674.7  y = 0.6911x + 4.239R² = 0.999502004006000 200 400 600 800 1000Measured Flow Rate (sccm)Flow SP193   Figure B.5 MFC Calibration equation obtained for N2.  Table B.6 MFC calibration using a bubble flow meter for He. Set Point  % Volume  cc Time s Flow rate cc(STP).min-1 5 2 9.9 11.0 10 2 4.4 24.8 15 5 7.1 38.9 20 5 5.3 51.6 30 10 7.3 75.8 40 10 5.5 99.5  y = 1.4313x - 38.851R² = 0.99940200400600800150 250 350 450 550Measured Flow Rate (sccm)Flow SP194   Figure B.6 MFC Calibration equation obtained for He.  B.2 MS Calibration   The change in the concentration of CH4 during the reaction must be quantified to calculate CH4 conversion. Thus, the MS needs to be calibrated from time to time. For TPO or TOS test the signal intensities of CH4 and of He are recorded, and all mass signals of gases are normalized based on mass signal of He. Table B.7 showed the change in 𝐼𝐶𝐻4𝐼𝐻𝑒  single ratio intensities and  𝑌𝐶𝐻4𝑌𝐻𝑒 ratio flow rate.     y = 2.5172x - 0.0693R² = 0.9987040801200 10 20 30 40 50Measured Flow Rate (sccm)Flow SP195  Table B.7 MS calibration for CH4 using He. 𝐼𝐻𝑒 Torr 𝐼𝐶𝐻4 Torr 𝐼𝐶𝐻4𝐼𝐻𝑒   𝑌𝐶𝐻4𝑌𝐻𝑒  2.79E-06 3.48E-07 1.25E-01 2.06E-02 2.75E-06 2.92E-07 1.06E-01 1.64E-02 2.74E-06 2.33E-07 8.51E-02 1.23E-02 2.71E-06 1.72E-07 6.36E-02 8.22E-03 2.72E-06 1.17E-07 4.31E-02 4.11E-03 2.78E-06 7.88E-8 2.83E-02 1.06E-03   Figure B.7 MS Calibration equation for CH4.  y = 0.2002x - 0.0046R² = 0.99960.00000.00600.01200.01800.02400.0000 0.0400 0.0800 0.1200 0.1600Flow RatioIntensity Ratio 196  Appendix C  Reaction System C.1 Washcoat monolith without Pd (blank run)  A blank experiment with a monolith that was loaded with washcoat but no active phase was carried out to make sure the washcoat and cordierite did not influence the catalyst activity data. A monolith with 27 wt% washcoat was used to measure the initial activity of the washcoat by temperature-programmed CH4 oxidation (TPO). The total feed gas flow was 1025 cm3(STP)·min−1, corresponding to a GHSV of 36,000 h−1. The dry feed gas composition was 0.07 vol% CH4, 8.5vol% O2, 0.06 vol% CO, 8 vol% CO2 in N2 and He. Figure C.1 presents the results of the blank test.    0 100 200 300 400 500 600020406080100CH4 Conversion (mol %)Temperature (OC) 27% Washcoat No Pd Figure C.1 Temperature-programmed oxidation profile: the initial activity of the washcoat 6%AlOOH/21%Al2O3/73% cordierite (no Pd) as a function of temperature. The total feed gas flow is 1025 cm3(STP)·min−1, corresponding to a GHSV of 36000 h−1 with 0. 07vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He.  197  C.2 Washcoat Loading   A sries of preliminary tests were conducted to determine the best loading of the washcoat for the present study.  Three samples were prepared as presented in Table C.1, with varying washcoat mass loaded onto the cordierite.  The activities of the monolith catalysts were measured by TPO at a total feed gas flow of 1025 cm3(STP)·min−1, GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 5vol% H2O in N2 and He. Figure C.2 reports the results of the TPO and from the results it can be observed that the Pd2WC was the most active sample using wet feed gas. The loading of Pd2WC sample was 27 wt% loading of the washcoat relative to the weight of the monolith and washcoat. Thus, in this study all the samples were loaded with 27 wt% of washcoat.  Table C.1 Nominal composition of monolith catalysts with different loading of washcoat. Sample Washcoat Pd Ce -AlOOH -Al2O3 Cordierite   g mass % Pd1WC 0.15 0.37 1.6 2.9 11.8 83 Pd2WC 0.27 0.4 1.5 5.0 20.2 73 Pd2WC 0.37 0.3 1.3 6.2 25.2 67  198   Figure C.2 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 5 vol% H2O in N2 and He.  C.3 Effect of Calcination Duration   The calcination time for the washcoat and the active phase may effect the catalyst activity. In this section, three samples were prepared using different calcination times. Table C.2 provides detailsfor the samples. The catalyst activity was determined by TPO in the presence of 5 vol% H2O and Figure C.3 reports the results of the tests. The most active catalyst was Pd4Clin for which the washcoat was calcined for 7 h at 450 oC and the active phase (Pd) was calcined for 15 199  h at 450 oC. Thus, in this study all the samples were calcined at 450 oC for 7 h to prepare the washcoat and at 450 oC for 15 h to prepare the active phase.   Table C.2 Calcination time.  Sample Pd Calcination time, h Washcoat Calcination time, h Pd1Clin 7 4 Pd2Clin 15 4 Pd3Clin 7 7 Pd4Clin 15 7   200   Figure C.3 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature. Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 5 vol% H2O in N2 and He.  C.4 CH4 Conversion by Total Carbon Flow Rate   To determine the activity of the catalysts the CH4 conversion must be determined. The conversion of CH4 was calculated based on the flow rate of CH4, CO2 and He. The mass signal intensities by the MS for CH4 , CO2 and He are recorded. A constant He flow rate (50 cc(STP).min-1) was fed to the reactor to measure the change of signal intensities of CH4 and CO2 during the experiments based on the He signal. Before starting the experiments CH4, Air, N2 and He pass through the monolith reactor and the MS to measure the relative signal intensities and 201  record the signals for 15 min at ambient temperature to ensure the average flow of these gases from the MS matched the bubble flow meter flow to minimize the error for calculating the CH4 conversion. After the experiment was completed, the reactor was switched off and the flow of the gases was measured by the MS and the bubble flow meter to ensure no fluctuation in the flow during the experiments. The relation between the CH4, CO2 and He signal intensities is given by:  𝐼𝑟𝑒𝐶𝐻4 =  𝐼𝐶𝐻4𝐼𝐻𝑒                                                                                                                                                𝐶. 1   𝐼𝑟𝑒𝐶𝑂2 =  𝐼𝐶𝑂2𝐼𝐻𝑒                                                                                                                                                𝐶. 2  𝑌𝑟𝑒𝐶𝐻4 =𝑌𝐶𝐻4𝑌𝐻𝑒                                                                                                                                                𝐶. 3  𝑌𝑟𝑒𝐶𝑂2 =𝑌𝐶𝑂2𝑌𝐻𝑒                                                                                                                                                𝐶. 4    𝐹𝐶𝑂2 = 𝑌𝑟𝑒𝐶𝑂2 ∗ 𝐹𝐻𝑒                                                                                                                                     𝐶. 5  𝐹𝐶𝐻4 = 𝑌𝑟𝑒𝐶𝐻4 ∗ 𝐹𝐻𝑒                                                                                                                                     𝐶. 6  However, the flow of CO2 needs to be corrected since a small amount of CO2 is observed by MS before the experiment is started. 𝐹𝐶𝑂2)𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 = 𝐹𝐶𝑂2 − 𝐹𝐶𝑂2𝑜                                                                                                                    𝐶. 7  The total carbon balance is calculated to determine the CH4 conversion  𝐹𝐶)𝑡𝑜𝑡𝑎𝑙 = 𝐹𝐶𝑂2)𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 + 𝐹𝐶𝐻4                                                                                                             𝐶. 8  𝑋𝐶𝐻4 =𝐹𝐶)𝑡𝑜𝑡𝑎𝑙−𝐹𝐶𝐻4𝐹𝐶)𝑡𝑜𝑡𝑎𝑙X100                                                                                                                         𝐶. 9   Table C.1 presented the CH4 conversion calculations for TPO experiment for Pd5B catalysts with 5°C.min-1 from 20 oC to 500°C. The feed Total feed gas flow = 1025 cm3(STP)·min−1, 202  GHSV of 36000 h−1, Feed gas composition: 0.07 vol% CH4, 8.5 vol% O2 in N2 and He. Table C.2 presented the CH4 conversion calculations for TPO experiments for the same catalysts and feed compaction but different GHSV of 18000 h−1.  C.5 CH4 Conversion by CH4 Flow Rate  To calculate the conversion using the CH4 flow rate, the same method and steps for the calculating the CH4 conversion by using total carbon was used. However, the flow rate and the gas compositions are different. For real condition in NGVs there is CO2, CO and H2O with CH4 which the same conditions that we used for the experiments of this thesis. High amount of CO2 (8 vol%) and CO (600 ppm) are present in the feed whereas the CH4 (700 ppm) is very low compared to CO2. Thus, calculating the conversion for CH4 by carbon mole balance is not accurate and the conversion of CH4 should be calculated based on the CH4 flow rate. The CH4 flow rate should be monitored for 15 min before the experiment starts and the flow rate of the MC and bubble flow meter have to be in agreement because the calculation depends on the CH4 initial flow rate. Also, the flow rates will be checked after the experiment is completed and the reactor was cooled to 25 oC to make sure the flow rate of CH4 and other gases are not fluctuating.  The relation between the CH4 and He signal intensities is as follow: CH4, CO2 and He signal intensities as: 𝐼𝑟𝑒𝐶𝐻4 =  𝐼𝐶𝐻4𝐼𝐻𝑒                                                                                                                                                𝐶. 1   𝑌𝑟𝑒𝐶𝐻4 =𝑌𝐶𝐻4𝑌𝐻𝑒                                                                                                                                                𝐶. 3  𝐹𝐶𝐻4 = 𝑌𝑟𝑒𝐶𝐻4 ∗ 𝐹𝐻𝑒                                                                                                                                     𝐶. 6  203  Calculating the Conversion of CH4 as: 𝑋𝐶𝐻4 =𝐹𝐶𝐻4𝑜 − 𝐹𝐶𝐻4𝐹𝐶𝐻4𝑜 X100                                                                                                                       𝐶. 10 Table C.1 and C.2 showed the different between these two methods with different GHSV. These two methods showed almost the same results as presented as well in Figure C.2 and C.3.  204  Table C.3 CH4 conversion calculation for Pd5B catalysts during TPO experiment. [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4 and 8.5 vol% O2 in N2 and He. Temp oC IHe  Torr ICH4 Torr ICO2 Torr ICH4/IHe ICO2/IHe YCH4/YHe YCO2/YHe He Flow cc(STP) .min-1 CH4 Flow  cc(STP). min-1 CO2 Flow cc(STP). min-1 CO2 Flow cc(STP).min-1 Corrected Σ C Flow cc(STP).min-1 X1C  mol.% X2CH4  mol.% 25 2.51E-06 2.33E-07 5.18E-08 9.28E-02 2.06E-02 1.40E-02 2.93E-03 5.00E+01 6.99E-01 1.47E-01 8.16E-03 7.07E-01 1.2 0.0 29 2.52E-06 2.34E-07 5.10E-08 9.29E-02 2.02E-02 1.40E-02 2.87E-03 5.00E+01 7.00E-01 1.44E-01 5.32E-03 7.05E-01 0.8 0.0 32 2.52E-06 2.35E-07 5.09E-08 9.33E-02 2.02E-02 1.41E-02 2.87E-03 5.00E+01 7.03E-01 1.43E-01 5.04E-03 7.09E-01 0.7 -0.6 36 2.51E-06 2.36E-07 5.27E-08 9.40E-02 2.10E-02 1.42E-02 2.98E-03 5.00E+01 7.11E-01 1.49E-01 1.07E-02 7.22E-01 1.5 -1.7 39 2.50E-06 2.34E-07 5.24E-08 9.36E-02 2.10E-02 1.41E-02 2.98E-03 5.00E+01 7.07E-01 1.49E-01 1.04E-02 7.17E-01 1.5 -1.1 43 2.52E-06 2.36E-07 5.19E-08 9.37E-02 2.06E-02 1.41E-02 2.92E-03 5.00E+01 7.07E-01 1.46E-01 7.86E-03 7.15E-01 1.1 -1.1 46 2.50E-06 2.35E-07 5.18E-08 9.40E-02 2.07E-02 1.42E-02 2.94E-03 5.00E+01 7.11E-01 1.47E-01 8.75E-03 7.20E-01 1.2 -1.6 50 2.52E-06 2.35E-07 5.34E-08 9.33E-02 2.12E-02 1.41E-02 3.01E-03 5.00E+01 7.03E-01 1.50E-01 1.21E-02 7.16E-01 1.7 -0.6 53 2.53E-06 2.32E-07 5.22E-08 9.17E-02 2.06E-02 1.38E-02 2.93E-03 5.00E+01 6.88E-01 1.46E-01 8.12E-03 6.96E-01 1.2 1.7 57 2.54E-06 2.32E-07 5.22E-08 9.13E-02 2.06E-02 1.37E-02 2.92E-03 5.00E+01 6.84E-01 1.46E-01 7.55E-03 6.92E-01 1.1 2.2 60 2.52E-06 2.33E-07 5.19E-08 9.25E-02 2.06E-02 1.39E-02 2.92E-03 5.00E+01 6.96E-01 1.46E-01 7.86E-03 7.03E-01 1.1 0.6 64 2.54E-06 2.33E-07 5.18E-08 9.17E-02 2.04E-02 1.38E-02 2.90E-03 5.00E+01 6.88E-01 1.45E-01 6.43E-03 6.95E-01 0.9 1.6 67 2.53E-06 2.33E-07 5.13E-08 9.21E-02 2.03E-02 1.38E-02 2.88E-03 5.00E+01 6.92E-01 1.44E-01 5.60E-03 6.97E-01 0.8 1.1 71 2.52E-06 2.32E-07 5.03E-08 9.21E-02 2.00E-02 1.38E-02 2.83E-03 5.00E+01 6.92E-01 1.42E-01 3.35E-03 6.95E-01 0.5 1.1 74 2.52E-06 2.36E-07 4.95E-08 9.37E-02 1.96E-02 1.41E-02 2.79E-03 5.00E+01 7.07E-01 1.39E-01 1.10E-03 7.09E-01 0.2 -1.1 78 2.53E-06 2.35E-07 4.95E-08 9.29E-02 1.96E-02 1.40E-02 2.78E-03 5.00E+01 7.00E-01 1.39E-01 5.47E-04 7.00E-01 0.1 0.0 81 2.54E-06 2.35E-07 5.01E-08 9.25E-02 1.97E-02 1.39E-02 2.80E-03 5.00E+01 6.96E-01 1.40E-01 1.68E-03 6.98E-01 0.2 0.5 85 2.53E-06 2.32E-07 5.13E-08 9.17E-02 2.03E-02 1.38E-02 2.88E-03 5.00E+01 6.88E-01 1.44E-01 5.60E-03 6.94E-01 0.8 1.7 88 2.54E-06 2.32E-07 5.64E-08 9.13E-02 2.22E-02 1.37E-02 3.15E-03 5.00E+01 6.84E-01 1.58E-01 1.93E-02 7.04E-01 2.7 2.2 92 2.53E-06 2.34E-07 5.22E-08 9.25E-02 2.06E-02 1.39E-02 2.93E-03 5.00E+01 6.96E-01 1.46E-01 8.12E-03 7.04E-01 1.2 0.5 95 2.54E-06 2.33E-07 5.22E-08 9.17E-02 2.06E-02 1.38E-02 2.92E-03 5.00E+01 6.88E-01 1.46E-01 7.55E-03 6.96E-01 1.1 1.6 99 2.54E-06 2.34E-07 5.19E-08 9.21E-02 2.04E-02 1.38E-02 2.90E-03 5.00E+01 6.92E-01 1.45E-01 6.71E-03 6.99E-01 1.0 1.0 102 2.53E-06 2.33E-07 5.18E-08 9.21E-02 2.05E-02 1.38E-02 2.91E-03 5.00E+01 6.92E-01 1.45E-01 7.00E-03 6.99E-01 1.0 1.1 106 2.54E-06 2.32E-07 5.34E-08 9.13E-02 2.10E-02 1.37E-02 2.99E-03 5.00E+01 6.84E-01 1.49E-01 1.09E-02 6.95E-01 1.6 2.2 109 2.54E-06 2.32E-07 5.22E-08 9.13E-02 2.06E-02 1.37E-02 2.92E-03 5.00E+01 6.84E-01 1.46E-01 7.55E-03 6.92E-01 1.1 2.2 113 2.52E-06 2.34E-07 5.22E-08 9.29E-02 2.07E-02 1.40E-02 2.94E-03 5.00E+01 7.00E-01 1.47E-01 8.71E-03 7.08E-01 1.2 0.0 116 2.54E-06 2.32E-07 5.19E-08 9.13E-02 2.04E-02 1.37E-02 2.90E-03 5.00E+01 6.84E-01 1.45E-01 6.71E-03 6.91E-01 1.0 2.2 205  Temp oC IHe  Torr ICH4 Torr ICO2 Torr ICH4/IHe ICO2/IHe YCH4/YHe YCO2/YHe He Flow cc(STP) .min-1 CH4 Flow  cc(STP). min-1 CO2 Flow cc(STP). min-1 CO2 Flow cc(STP).min-1 Corrected Σ C Flow cc(STP).min-1 X1C  mol.% X2CH4  mol.% 120 2.53E-06 2.33E-07 5.18E-08 9.21E-02 2.05E-02 1.38E-02 2.91E-03 5.00E+01 6.92E-01 1.45E-01 7.00E-03 6.99E-01 1.0 1.1 123 2.54E-06 2.34E-07 5.13E-08 9.21E-02 2.02E-02 1.38E-02 2.87E-03 5.00E+01 6.92E-01 1.43E-01 5.03E-03 6.97E-01 0.7 1.0 127 2.53E-06 2.33E-07 5.13E-08 9.21E-02 2.03E-02 1.38E-02 2.88E-03 5.00E+01 6.92E-01 1.44E-01 5.60E-03 6.97E-01 0.8 1.1 130 2.52E-06 2.32E-07 5.64E-08 9.21E-02 2.24E-02 1.38E-02 3.18E-03 5.00E+01 6.92E-01 1.59E-01 2.05E-02 7.12E-01 2.9 1.1 134 2.52E-06 2.32E-07 5.44E-08 9.21E-02 2.16E-02 1.38E-02 3.07E-03 5.00E+01 6.92E-01 1.53E-01 1.49E-02 7.06E-01 2.1 1.1 137 2.52E-06 2.34E-07 5.44E-08 9.29E-02 2.16E-02 1.40E-02 3.07E-03 5.00E+01 7.00E-01 1.53E-01 1.49E-02 7.14E-01 2.1 0.0 141 2.53E-06 2.34E-07 5.47E-08 9.25E-02 2.16E-02 1.39E-02 3.07E-03 5.00E+01 6.96E-01 1.54E-01 1.51E-02 7.11E-01 2.1 0.5 144 2.52E-06 2.33E-07 5.13E-08 9.25E-02 2.04E-02 1.39E-02 2.89E-03 5.00E+01 6.96E-01 1.45E-01 6.17E-03 7.02E-01 0.9 0.6 148 2.53E-06 2.33E-07 5.03E-08 9.21E-02 1.99E-02 1.38E-02 2.82E-03 5.00E+01 6.92E-01 1.41E-01 2.79E-03 6.95E-01 0.4 1.1 151 2.54E-06 2.33E-07 4.95E-08 9.17E-02 1.95E-02 1.38E-02 2.77E-03 5.00E+01 6.88E-01 1.38E-01 0.00E+00 6.88E-01 0.0 1.6 155 2.53E-06 2.32E-07 4.95E-08 9.17E-02 1.96E-02 1.38E-02 2.78E-03 5.00E+01 6.88E-01 1.39E-01 5.47E-04 6.88E-01 0.1 1.7 158 2.53E-06 2.34E-07 5.01E-08 9.25E-02 1.98E-02 1.39E-02 2.81E-03 5.00E+01 6.96E-01 1.41E-01 2.23E-03 6.98E-01 0.3 0.5 162 2.52E-06 2.34E-07 5.13E-08 9.29E-02 2.04E-02 1.40E-02 2.89E-03 5.00E+01 7.00E-01 1.45E-01 6.17E-03 7.06E-01 0.9 0.0 165 2.52E-06 2.33E-07 5.64E-08 9.25E-02 2.24E-02 1.39E-02 3.18E-03 5.00E+01 6.96E-01 1.59E-01 2.05E-02 7.16E-01 2.9 0.6 169 2.53E-06 2.33E-07 5.44E-08 9.21E-02 2.15E-02 1.38E-02 3.05E-03 5.00E+01 6.92E-01 1.53E-01 1.43E-02 7.06E-01 2.0 1.1 172 2.53E-06 2.33E-07 5.44E-08 9.21E-02 2.15E-02 1.38E-02 3.05E-03 5.00E+01 6.92E-01 1.53E-01 1.43E-02 7.06E-01 2.0 1.1 176 2.52E-06 2.32E-07 5.47E-08 9.21E-02 2.17E-02 1.38E-02 3.08E-03 5.00E+01 6.92E-01 1.54E-01 1.57E-02 7.07E-01 2.2 1.1 179 2.53E-06 2.36E-07 5.39E-08 9.33E-02 2.13E-02 1.41E-02 3.03E-03 5.00E+01 7.04E-01 1.51E-01 1.29E-02 7.17E-01 1.8 -0.6 183 2.50E-06 2.35E-07 5.26E-08 9.40E-02 2.10E-02 1.42E-02 2.99E-03 5.00E+01 7.11E-01 1.49E-01 1.10E-02 7.22E-01 1.5 -1.6 186 2.50E-06 2.35E-07 5.34E-08 9.40E-02 2.14E-02 1.42E-02 3.03E-03 5.00E+01 7.11E-01 1.52E-01 1.33E-02 7.24E-01 1.8 -1.6 190 2.51E-06 2.32E-07 5.22E-08 9.24E-02 2.08E-02 1.39E-02 2.95E-03 5.00E+01 6.95E-01 1.48E-01 9.29E-03 7.05E-01 1.3 0.6 193 2.51E-06 2.32E-07 5.22E-08 9.24E-02 2.08E-02 1.39E-02 2.95E-03 5.00E+01 6.95E-01 1.48E-01 9.29E-03 7.05E-01 1.3 0.6 197 2.52E-06 2.33E-07 5.19E-08 9.25E-02 2.06E-02 1.39E-02 2.92E-03 5.00E+01 6.96E-01 1.46E-01 7.86E-03 7.03E-01 1.1 0.6 200 2.52E-06 2.33E-07 5.18E-08 9.25E-02 2.06E-02 1.39E-02 2.92E-03 5.00E+01 6.96E-01 1.46E-01 7.58E-03 7.03E-01 1.1 0.6 204 2.51E-06 2.33E-07 5.44E-08 9.28E-02 2.17E-02 1.40E-02 3.08E-03 5.00E+01 6.99E-01 1.54E-01 1.55E-02 7.15E-01 2.2 0.0 207 2.50E-06 2.32E-07 5.47E-08 9.28E-02 2.19E-02 1.40E-02 3.11E-03 5.00E+01 6.99E-01 1.55E-01 1.70E-02 7.16E-01 2.4 0.1 211 2.52E-06 2.36E-07 5.39E-08 9.37E-02 2.14E-02 1.41E-02 3.04E-03 5.00E+01 7.07E-01 1.52E-01 1.35E-02 7.21E-01 1.9 -1.1  206  Temp oC IHe  Torr ICH4 Torr ICO2 Torr ICH4/IHe ICO2/IHe YCH4/YHe YCO2/YHe He Flow cc(STP) .min-1 CH4 Flow  cc(STP). min-1 CO2 Flow cc(STP). min-1 CO2 Flow cc(STP).min-1 Corrected Σ C Flow cc(STP).min-1 X1C  mol.% X2CH4  mol.% 214 2.50E-06 2.35E-07 5.26E-08 9.40E-02 2.10E-02 1.42E-02 2.99E-03 5.00E+01 7.11E-01 1.49E-01 1.10E-02 7.22E-01 1.5 -1.6 218 2.52E-06 2.35E-07 5.34E-08 9.33E-02 2.12E-02 1.41E-02 3.01E-03 5.00E+01 7.03E-01 1.50E-01 1.21E-02 7.16E-01 1.7 -0.6 221 2.53E-06 2.32E-07 5.22E-08 9.17E-02 2.06E-02 1.38E-02 2.93E-03 5.00E+01 6.88E-01 1.46E-01 8.12E-03 6.96E-01 1.2 1.7 225 2.54E-06 2.32E-07 5.22E-08 9.13E-02 2.06E-02 1.37E-02 2.92E-03 5.00E+01 6.84E-01 1.46E-01 7.55E-03 6.92E-01 1.1 2.2 228 2.52E-06 2.33E-07 5.19E-08 9.25E-02 2.06E-02 1.39E-02 2.92E-03 5.00E+01 6.96E-01 1.46E-01 7.86E-03 7.03E-01 1.1 0.6 232 2.54E-06 2.34E-07 5.18E-08 9.21E-02 2.04E-02 1.38E-02 2.90E-03 5.00E+01 6.92E-01 1.45E-01 6.43E-03 6.99E-01 0.9 1.0 235 2.53E-06 2.33E-07 5.13E-08 9.21E-02 2.03E-02 1.38E-02 2.88E-03 5.00E+01 6.92E-01 1.44E-01 5.60E-03 6.97E-01 0.8 1.1 239 2.52E-06 2.32E-07 5.03E-08 9.21E-02 2.00E-02 1.38E-02 2.83E-03 5.00E+01 6.92E-01 1.42E-01 3.35E-03 6.95E-01 0.5 1.1 242 2.52E-06 2.32E-07 4.95E-08 9.21E-02 1.96E-02 1.38E-02 2.79E-03 5.00E+01 6.92E-01 1.39E-01 1.10E-03 6.93E-01 0.2 1.1 246 2.53E-06 2.34E-07 4.95E-08 9.25E-02 1.96E-02 1.39E-02 2.78E-03 5.00E+01 6.96E-01 1.39E-01 5.47E-04 6.96E-01 0.1 0.5 249 2.54E-06 2.34E-07 5.01E-08 9.21E-02 1.97E-02 1.38E-02 2.80E-03 5.00E+01 6.92E-01 1.40E-01 1.68E-03 6.94E-01 0.2 1.0 253 2.53E-06 2.29E-07 5.13E-08 9.05E-02 2.03E-02 1.35E-02 2.88E-03 5.00E+01 6.76E-01 1.44E-01 5.60E-03 6.82E-01 0.8 3.4 256 2.54E-06 2.23E-07 5.64E-08 8.78E-02 2.22E-02 1.30E-02 3.15E-03 5.00E+01 6.49E-01 1.58E-01 1.93E-02 6.68E-01 2.9 7.2 260 2.53E-06 2.15E-07 6.68E-08 8.50E-02 2.64E-02 1.24E-02 3.75E-03 5.00E+01 6.21E-01 1.87E-01 4.91E-02 6.70E-01 7.3 11.3 263 2.53E-06 2.03E-07 7.94E-08 8.02E-02 3.14E-02 1.15E-02 4.46E-03 5.00E+01 5.73E-01 2.23E-01 8.45E-02 6.58E-01 12.8 18.1 267 2.52E-06 1.93E-07 9.42E-08 7.66E-02 3.74E-02 1.07E-02 5.31E-03 5.00E+01 5.37E-01 2.65E-01 1.27E-01 6.64E-01 19.1 23.3 270 2.52E-06 1.87E-07 1.04E-07 7.42E-02 4.13E-02 1.03E-02 5.86E-03 5.00E+01 5.13E-01 2.93E-01 1.55E-01 6.67E-01 23.2 26.7 274 2.52E-06 1.84E-07 1.15E-07 7.30E-02 4.56E-02 1.00E-02 6.48E-03 5.00E+01 5.01E-01 3.24E-01 1.86E-01 6.87E-01 27.0 28.4 277 2.53E-06 1.75E-07 1.22E-07 6.92E-02 4.82E-02 9.25E-03 6.85E-03 5.00E+01 4.62E-01 3.42E-01 2.04E-01 6.66E-01 30.6 33.9 281 2.52E-06 1.70E-07 1.30E-07 6.75E-02 5.16E-02 8.91E-03 7.33E-03 5.00E+01 4.45E-01 3.66E-01 2.28E-01 6.73E-01 33.9 36.3 284 2.53E-06 1.66E-07 1.35E-07 6.56E-02 5.34E-02 8.54E-03 7.58E-03 5.00E+01 4.27E-01 3.79E-01 2.40E-01 6.67E-01 36.0 39.0 288 2.54E-06 1.62E-07 1.42E-07 6.38E-02 5.59E-02 8.17E-03 7.94E-03 5.00E+01 4.08E-01 3.97E-01 2.59E-01 6.67E-01 38.8 41.6 291 2.53E-06 1.59E-07 1.50E-07 6.28E-02 5.93E-02 7.98E-03 8.42E-03 5.00E+01 3.99E-01 4.21E-01 2.83E-01 6.82E-01 41.5 42.9 295 2.53E-06 1.53E-07 1.55E-07 6.05E-02 6.13E-02 7.51E-03 8.70E-03 5.00E+01 3.75E-01 4.35E-01 2.97E-01 6.72E-01 44.1 46.3 298 2.52E-06 1.49E-07 1.62E-07 5.91E-02 6.43E-02 7.24E-03 9.13E-03 5.00E+01 3.62E-01 4.56E-01 3.18E-01 6.80E-01 46.8 48.3 302 2.52E-06 1.42E-07 1.72E-07 5.63E-02 6.83E-02 6.68E-03 9.69E-03 5.00E+01 3.34E-01 4.85E-01 3.46E-01 6.80E-01 50.9 52.2 305 2.53E-06 1.39E-07 1.76E-07 5.49E-02 6.96E-02 6.40E-03 9.88E-03 5.00E+01 3.20E-01 4.94E-01 3.56E-01 6.76E-01 52.6 54.3   207  Temp oC IHe  Torr ICH4 Torr ICO2 Torr ICH4/IHe ICO2/IHe YCH4/YHe YCO2/YHe He Flow cc(STP) .min-1 CH4 Flow  cc(STP). min-1 CO2 Flow cc(STP). min-1 CO2 Flow cc(STP).min-1 Corrected Σ C Flow cc(STP).min-1 X1C  mol.% X2CH4  mol.% 309 2.53E-06 1.37E-07 1.83E-07 5.42E-02 7.23E-02 6.24E-03 1.03E-02 5.00E+01 3.12E-01 5.14E-01 3.75E-01 6.87E-01 54.6 55.4 312 2.52E-06 1.30E-07 1.93E-07 5.16E-02 7.66E-02 5.73E-03 1.09E-02 5.00E+01 2.86E-01 5.44E-01 4.05E-01 6.92E-01 58.6 59.1 316 2.53E-06 1.27E-07 1.97E-07 5.02E-02 7.79E-02 5.45E-03 1.11E-02 5.00E+01 2.72E-01 5.53E-01 4.14E-01 6.87E-01 60.3 61.0 319 2.54E-06 1.23E-07 2.03E-07 4.84E-02 7.99E-02 5.09E-03 1.13E-02 5.00E+01 2.55E-01 5.67E-01 4.29E-01 6.84E-01 62.7 63.6 323 2.53E-06 1.18E-07 2.14E-07 4.66E-02 8.46E-02 4.74E-03 1.20E-02 5.00E+01 2.37E-01 6.01E-01 4.62E-01 6.99E-01 66.1 66.1 326 2.53E-06 1.16E-07 2.17E-07 4.58E-02 8.58E-02 4.58E-03 1.22E-02 5.00E+01 2.29E-01 6.09E-01 4.71E-01 7.00E-01 67.3 67.3 330 2.51E-06 1.13E-07 2.21E-07 4.50E-02 8.80E-02 4.41E-03 1.25E-02 5.00E+01 2.21E-01 6.25E-01 4.87E-01 7.07E-01 68.8 68.5 333 2.53E-06 1.08E-07 2.24E-07 4.27E-02 8.85E-02 3.95E-03 1.26E-02 5.00E+01 1.97E-01 6.29E-01 4.90E-01 6.88E-01 71.3 71.8 337 2.52E-06 1.02E-07 2.30E-07 4.05E-02 9.13E-02 3.50E-03 1.30E-02 5.00E+01 1.75E-01 6.48E-01 5.10E-01 6.85E-01 74.4 75.0 340 2.53E-06 9.88E-08 2.40E-07 3.91E-02 9.49E-02 3.22E-03 1.35E-02 5.00E+01 1.61E-01 6.74E-01 5.35E-01 6.96E-01 76.9 77.0 344 2.53E-06 9.54E-08 2.44E-07 3.77E-02 9.64E-02 2.95E-03 1.37E-02 5.00E+01 1.47E-01 6.85E-01 5.46E-01 6.94E-01 78.7 78.9 347 2.52E-06 9.22E-08 2.49E-07 3.66E-02 9.88E-02 2.72E-03 1.40E-02 5.00E+01 1.36E-01 7.02E-01 5.63E-01 6.99E-01 80.5 80.5 351 2.52E-06 8.93E-08 2.50E-07 3.54E-02 9.92E-02 2.49E-03 1.41E-02 5.00E+01 1.25E-01 7.04E-01 5.66E-01 6.91E-01 81.9 82.2 354 2.53E-06 8.67E-08 2.55E-07 3.43E-02 1.01E-01 2.26E-03 1.43E-02 5.00E+01 1.13E-01 7.16E-01 5.77E-01 6.90E-01 83.6 83.8 358 2.52E-06 8.48E-08 2.61E-07 3.37E-02 1.04E-01 2.14E-03 1.47E-02 5.00E+01 1.07E-01 7.35E-01 5.97E-01 7.04E-01 84.8 84.7 361 2.53E-06 8.09E-08 2.64E-07 3.20E-02 1.04E-01 1.80E-03 1.48E-02 5.00E+01 9.01E-02 7.41E-01 6.03E-01 6.93E-01 87.0 87.1 365 2.52E-06 7.90E-08 2.69E-07 3.13E-02 1.07E-01 1.68E-03 1.52E-02 5.00E+01 8.38E-02 7.58E-01 6.20E-01 7.03E-01 88.1 88.0 368 2.52E-06 7.74E-08 2.71E-07 3.07E-02 1.08E-01 1.55E-03 1.53E-02 5.00E+01 7.75E-02 7.64E-01 6.25E-01 7.03E-01 89.0 88.9 372 2.53E-06 7.58E-08 2.68E-07 3.00E-02 1.06E-01 1.40E-03 1.50E-02 5.00E+01 6.99E-02 7.52E-01 6.14E-01 6.84E-01 89.8 90.0 375 2.54E-06 7.50E-08 2.73E-07 2.95E-02 1.07E-01 1.31E-03 1.53E-02 5.00E+01 6.56E-02 7.63E-01 6.25E-01 6.90E-01 90.5 90.6 379 2.52E-06 7.37E-08 2.76E-07 2.92E-02 1.10E-01 1.26E-03 1.56E-02 5.00E+01 6.28E-02 7.78E-01 6.39E-01 7.02E-01 91.1 91.0 382 2.53E-06 7.15E-08 2.78E-07 2.83E-02 1.10E-01 1.06E-03 1.56E-02 5.00E+01 5.29E-02 7.80E-01 6.42E-01 6.95E-01 92.4 92.4 386 2.52E-06 7.06E-08 2.76E-07 2.80E-02 1.10E-01 1.01E-03 1.56E-02 5.00E+01 5.04E-02 7.78E-01 6.39E-01 6.90E-01 92.7 92.8 389 2.53E-06 7.08E-08 2.76E-07 2.80E-02 1.09E-01 1.00E-03 1.55E-02 5.00E+01 5.01E-02 7.75E-01 6.36E-01 6.86E-01 92.7 92.8 393 2.55E-06 6.99E-08 2.77E-07 2.74E-02 1.09E-01 8.88E-04 1.54E-02 5.00E+01 4.44E-02 7.71E-01 6.33E-01 6.77E-01 93.4 93.7 396 2.55E-06 7.01E-08 2.84E-07 2.75E-02 1.11E-01 9.04E-04 1.58E-02 5.00E+01 4.52E-02 7.91E-01 6.52E-01 6.98E-01 93.5 93.5 400 2.54E-06 6.85E-08 2.81E-07 2.70E-02 1.11E-01 7.99E-04 1.57E-02 5.00E+01 4.00E-02 7.85E-01 6.47E-01 6.87E-01 94.2 94.3   208  Temp oC IHe  Torr ICH4 Torr ICO2 Torr ICH4/IHe ICO2/IHe YCH4/YHe YCO2/YHe He Flow cc(STP) .min-1 CH4 Flow  cc(STP). min-1 CO2 Flow cc(STP). min-1 CO2 Flow cc(STP).min-1 Corrected Σ C Flow cc(STP).min-1 X1C  mol.% X2CH4  mol.% 403 2.53E-06 6.77E-08 2.81E-07 2.68E-02 1.11E-01 7.57E-04 1.58E-02 5.00E+01 3.79E-02 7.89E-01 6.50E-01 6.88E-01 94.5 94.6 407 2.52E-06 6.65E-08 2.85E-07 2.64E-02 1.13E-01 6.83E-04 1.61E-02 5.00E+01 3.42E-02 8.03E-01 6.65E-01 6.99E-01 95.1 95.1 410 2.53E-06 6.61E-08 2.87E-07 2.61E-02 1.13E-01 6.31E-04 1.61E-02 5.00E+01 3.15E-02 8.05E-01 6.67E-01 6.99E-01 95.5 95.5 414 2.54E-06 6.49E-08 2.86E-07 2.56E-02 1.13E-01 5.15E-04 1.60E-02 5.00E+01 2.58E-02 7.99E-01 6.61E-01 6.87E-01 96.2 96.3 417 2.51E-06 6.39E-08 2.90E-07 2.55E-02 1.16E-01 4.97E-04 1.64E-02 5.00E+01 2.48E-02 8.20E-01 6.82E-01 7.07E-01 96.5 96.4 421 2.53E-06 6.35E-08 2.88E-07 2.51E-02 1.14E-01 4.25E-04 1.62E-02 5.00E+01 2.12E-02 8.08E-01 6.70E-01 6.91E-01 96.9 97.0 424 2.54E-06 6.21E-08 2.90E-07 2.44E-02 1.14E-01 2.95E-04 1.62E-02 5.00E+01 1.47E-02 8.11E-01 6.72E-01 6.87E-01 97.9 97.9 428 2.51E-06 6.10E-08 2.90E-07 2.43E-02 1.16E-01 2.65E-04 1.64E-02 5.00E+01 1.33E-02 8.20E-01 6.82E-01 6.95E-01 98.1 98.1 431 2.51E-06 6.12E-08 2.92E-07 2.44E-02 1.16E-01 2.81E-04 1.65E-02 5.00E+01 1.41E-02 8.26E-01 6.88E-01 7.02E-01 98.0 98.0 435 2.52E-06 6.09E-08 2.87E-07 2.42E-02 1.14E-01 2.38E-04 1.62E-02 5.00E+01 1.19E-02 8.09E-01 6.70E-01 6.82E-01 98.3 98.3 438 2.54E-06 6.00E-08 2.93E-07 2.36E-02 1.15E-01 1.29E-04 1.64E-02 5.00E+01 6.46E-03 8.19E-01 6.81E-01 6.87E-01 99.1 99.1 442 2.53E-06 5.92E-08 2.96E-07 2.34E-02 1.17E-01 8.45E-05 1.66E-02 5.00E+01 4.23E-03 8.31E-01 6.92E-01 6.97E-01 99.4 99.4 445 2.53E-06 5.97E-08 2.98E-07 2.36E-02 1.18E-01 1.24E-04 1.67E-02 5.00E+01 6.20E-03 8.36E-01 6.98E-01 7.04E-01 99.1 99.1 449 2.52E-06 5.92E-08 2.99E-07 2.35E-02 1.19E-01 1.03E-04 1.68E-02 5.00E+01 5.16E-03 8.42E-01 7.04E-01 7.09E-01 99.3 99.3 452 2.53E-06 5.83E-08 2.97E-07 2.30E-02 1.17E-01 1.33E-05 1.67E-02 5.00E+01 6.65E-04 8.33E-01 6.95E-01 6.96E-01 99.9 99.9 456 2.53E-06 5.72E-08 2.95E-07 2.26E-02 1.17E-01 -7.37E-05 1.66E-02 5.00E+01 -3.69E-03 8.28E-01 6.89E-01 6.86E-01 100.5 100.5 459 2.51E-06 5.74E-08 3.00E-07 2.29E-02 1.20E-01 -2.17E-05 1.70E-02 5.00E+01 -1.09E-03 8.49E-01 7.10E-01 7.09E-01 100.2 100.2 463 2.54E-06 5.65E-08 3.06E-07 2.22E-02 1.20E-01 -1.47E-04 1.71E-02 5.00E+01 -7.34E-03 8.55E-01 7.17E-01 7.10E-01 101.0 101.0 466 2.52E-06 5.67E-08 2.96E-07 2.25E-02 1.17E-01 -9.55E-05 1.67E-02 5.00E+01 -4.78E-03 8.34E-01 6.96E-01 6.91E-01 100.7 100.7 470 2.52E-06 5.49E-08 2.96E-07 2.18E-02 1.17E-01 -2.39E-04 1.67E-02 5.00E+01 -1.19E-02 8.34E-01 6.96E-01 6.84E-01 101.7 101.7 473 2.51E-06 5.43E-08 2.96E-07 2.16E-02 1.18E-01 -2.69E-04 1.67E-02 5.00E+01 -1.34E-02 8.37E-01 6.99E-01 6.85E-01 102.0 101.9 477 2.51E-06 5.40E-08 2.94E-07 2.15E-02 1.17E-01 -2.93E-04 1.66E-02 5.00E+01 -1.46E-02 8.32E-01 6.93E-01 6.79E-01 102.2 102.1 480 2.51E-06 5.36E-08 3.02E-07 2.14E-02 1.20E-01 -3.25E-04 1.71E-02 5.00E+01 -1.62E-02 8.54E-01 7.16E-01 7.00E-01 102.3 102.3 484 2.51E-06 5.32E-08 2.99E-07 2.12E-02 1.19E-01 -3.57E-04 1.69E-02 5.00E+01 -1.78E-02 8.46E-01 7.07E-01 6.90E-01 102.6 102.5 487 2.54E-06 5.18E-08 3.04E-07 2.04E-02 1.20E-01 -5.17E-04 1.70E-02 5.00E+01 -2.59E-02 8.50E-01 7.11E-01 6.86E-01 103.8 103.7 491 2.53E-06 5.24E-08 3.06E-07 2.07E-02 1.21E-01 -4.54E-04 1.72E-02 5.00E+01 -2.27E-02 8.59E-01 7.20E-01 6.98E-01 103.3 103.2 494 2.52E-06 5.28E-08 3.00E-07 2.10E-02 1.19E-01 -4.05E-04 1.69E-02 5.00E+01 -2.03E-02 8.45E-01 7.07E-01 6.87E-01 103.0 102.9 498 2.52E-06 5.29E-08 3.00E-07 2.10E-02 1.19E-01 -3.97E-04 1.69E-02 5.00E+01 -1.99E-02 8.45E-01 7.07E-01 6.87E-01 102.9 102.8 501 2.53E-06 5.23E-08 3.07E-07 2.07E-02 1.21E-01 -4.61E-04 1.72E-02 5.00E+01 -2.31E-02 8.62E-01 7.23E-01 7.00E-01 103.3 103.3 209  Table C.4 CH4 conversion calculation for Pd5B catalysts during TPO experiment. [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 18000 h−1, Feed composition: 0.07 vol% CH4 and 8.5 vol% O2 in N2 and He. Temp oC IHe  Torr ICH4 Torr ICO2 Torr ICH4/IHe ICO2/IHe YCH4/YHe YCO2/YHe He Flow cc(STP) .min-1 CH4 Flow  cc(STP). min-1 CO2 Flow cc(STP). min-1 CO2 Flow cc(STP).min-1 Corrected Σ C Flow cc(STP).min-1 X1C  mol.% X2CH4  mol.% 25 3.02E-06 2.77E-07 6.20E-09 9.17E-02 2.05E-03 1.35E-02 2.71E-04 2.50E+01 3.37E-01 6.77E-03 3.84E-03 3.40E-01 1.1 1.1 29 3.01E-06 2.79E-07 6.50E-09 9.25E-02 2.16E-03 1.36E-02 2.85E-04 2.50E+01 3.40E-01 7.12E-03 4.20E-03 3.45E-01 1.2 0.0 32 3.00E-06 2.76E-07 5.70E-09 9.21E-02 1.90E-03 1.35E-02 2.51E-04 2.50E+01 3.38E-01 6.27E-03 3.35E-03 3.42E-01 1.0 0.6 36 3.02E-06 2.76E-07 6.50E-09 9.13E-02 2.15E-03 1.34E-02 2.84E-04 2.50E+01 3.34E-01 7.09E-03 4.17E-03 3.38E-01 1.2 1.8 39 3.00E-06 2.80E-07 5.70E-09 9.32E-02 1.90E-03 1.38E-02 2.51E-04 2.50E+01 3.44E-01 6.27E-03 3.35E-03 3.47E-01 1.0 -1.0 43 3.02E-06 2.78E-07 4.80E-09 9.19E-02 1.59E-03 1.35E-02 2.10E-04 2.50E+01 3.37E-01 5.24E-03 2.31E-03 3.40E-01 0.7 0.9 46 3.04E-06 2.78E-07 4.20E-09 9.17E-02 1.38E-03 1.35E-02 1.83E-04 2.50E+01 3.36E-01 4.57E-03 1.64E-03 3.38E-01 0.5 1.2 50 3.05E-06 2.79E-07 3.60E-09 9.15E-02 1.18E-03 1.34E-02 1.56E-04 2.50E+01 3.35E-01 3.90E-03 9.74E-04 3.36E-01 0.3 1.5 53 3.02E-06 2.77E-07 3.00E-09 9.17E-02 9.92E-04 1.35E-02 1.31E-04 2.50E+01 3.36E-01 3.27E-03 3.51E-04 3.37E-01 0.1 1.2 57 3.05E-06 2.78E-07 2.70E-09 9.13E-02 8.86E-04 1.34E-02 1.17E-04 2.50E+01 3.35E-01 2.92E-03 0.00E+00 3.35E-01 0.0 1.7 60 3.04E-06 2.77E-07 6.50E-09 9.13E-02 2.14E-03 1.34E-02 2.83E-04 2.50E+01 3.34E-01 7.07E-03 4.14E-03 3.39E-01 1.2 1.8 64 3.02E-06 2.76E-07 5.70E-09 9.14E-02 1.88E-03 1.34E-02 2.49E-04 2.50E+01 3.35E-01 6.22E-03 3.30E-03 3.38E-01 1.0 1.6 67 3.02E-06 2.78E-07 5.70E-09 9.21E-02 1.88E-03 1.35E-02 2.49E-04 2.50E+01 3.38E-01 6.22E-03 3.30E-03 3.42E-01 1.0 0.6 71 3.04E-06 2.80E-07 4.70E-09 9.21E-02 1.55E-03 1.35E-02 2.04E-04 2.50E+01 3.38E-01 5.11E-03 2.19E-03 3.41E-01 0.6 0.6 74 3.05E-06 2.78E-07 5.70E-09 9.12E-02 1.87E-03 1.34E-02 2.47E-04 2.50E+01 3.34E-01 6.17E-03 3.25E-03 3.37E-01 1.0 1.9 78 3.04E-06 2.78E-07 6.50E-09 9.16E-02 2.14E-03 1.34E-02 2.83E-04 2.50E+01 3.36E-01 7.07E-03 4.14E-03 3.40E-01 1.2 1.4 81 3.05E-06 2.80E-07 7.30E-09 9.17E-02 2.40E-03 1.35E-02 3.16E-04 2.50E+01 3.37E-01 7.90E-03 4.98E-03 3.42E-01 1.5 1.1 85 3.04E-06 2.77E-07 7.60E-09 9.13E-02 2.50E-03 1.34E-02 3.30E-04 2.50E+01 3.34E-01 8.26E-03 5.34E-03 3.40E-01 1.6 1.8 88 3.05E-06 2.79E-07 7.40E-09 9.17E-02 2.43E-03 1.35E-02 3.20E-04 2.50E+01 3.36E-01 8.01E-03 5.09E-03 3.41E-01 1.5 1.2 92 3.05E-06 2.78E-07 6.80E-09 9.13E-02 2.23E-03 1.34E-02 2.94E-04 2.50E+01 3.35E-01 7.36E-03 4.44E-03 3.39E-01 1.3 1.7 95 3.04E-06 2.77E-07 6.00E-09 9.13E-02 1.98E-03 1.34E-02 2.61E-04 2.50E+01 3.34E-01 6.52E-03 3.60E-03 3.38E-01 1.1 1.8 99 3.05E-06 2.79E-07 5.10E-09 9.15E-02 1.67E-03 1.34E-02 2.21E-04 2.50E+01 3.35E-01 5.52E-03 2.60E-03 3.38E-01 0.8 1.5 102 3.05E-06 2.77E-07 4.50E-09 9.09E-02 1.48E-03 1.33E-02 1.95E-04 2.50E+01 3.33E-01 4.87E-03 1.95E-03 3.35E-01 0.6 2.3 106 3.02E-06 2.78E-07 5.70E-09 9.21E-02 1.88E-03 1.35E-02 2.49E-04 2.50E+01 3.38E-01 6.22E-03 3.30E-03 3.42E-01 1.0 0.6 109 3.05E-06 2.77E-07 4.80E-09 9.09E-02 1.57E-03 1.33E-02 2.08E-04 2.50E+01 3.33E-01 5.20E-03 2.27E-03 3.35E-01 0.7 2.3 113 3.04E-06 2.77E-07 4.20E-09 9.13E-02 1.38E-03 1.34E-02 1.83E-04 2.50E+01 3.34E-01 4.57E-03 1.64E-03 3.36E-01 0.5 1.8 116 3.05E-06 2.80E-07 3.60E-09 9.18E-02 1.18E-03 1.35E-02 1.56E-04 2.50E+01 3.37E-01 3.90E-03 9.74E-04 3.38E-01 0.3 1.0 210  Temp oC IHe  Torr ICH4 Torr ICO2 Torr ICH4/IHe ICO2/IHe YCH4/YHe YCO2/YHe He Flow cc(STP) .min-1 CH4 Flow  cc(STP). min-1 CO2 Flow cc(STP). min-1 CO2 Flow cc(STP).min-1 Corrected Σ C Flow cc(STP).min-1 X1C  mol.% X2CH4  mol.% 120 3.04E-06 2.78E-07 3.00E-09 9.17E-02 9.88E-04 1.35E-02 1.30E-04 2.50E+01 3.36E-01 3.26E-03 3.38E-04 3.37E-01 0.1 1.2 123 3.02E-06 2.79E-07 2.70E-09 9.22E-02 8.93E-04 1.36E-02 1.18E-04 2.50E+01 3.39E-01 2.95E-03 2.32E-05 3.39E-01 0.0 0.4 127 3.02E-06 2.77E-07 6.50E-09 9.17E-02 2.15E-03 1.35E-02 2.84E-04 2.50E+01 3.36E-01 7.09E-03 4.17E-03 3.40E-01 1.2 1.2 130 3.02E-06 2.77E-07 5.70E-09 9.17E-02 1.88E-03 1.35E-02 2.49E-04 2.50E+01 3.36E-01 6.22E-03 3.30E-03 3.40E-01 1.0 1.2 134 3.04E-06 2.80E-07 5.70E-09 9.21E-02 1.88E-03 1.35E-02 2.48E-04 2.50E+01 3.38E-01 6.20E-03 3.27E-03 3.42E-01 1.0 0.6 137 3.02E-06 2.80E-07 4.70E-09 9.25E-02 1.55E-03 1.36E-02 2.05E-04 2.50E+01 3.40E-01 5.13E-03 2.21E-03 3.42E-01 0.6 0.1 141 3.04E-06 2.79E-07 5.70E-09 9.18E-02 1.88E-03 1.35E-02 2.48E-04 2.50E+01 3.37E-01 6.20E-03 3.27E-03 3.40E-01 1.0 1.0 144 3.05E-06 2.79E-07 6.50E-09 9.15E-02 2.13E-03 1.34E-02 2.81E-04 2.50E+01 3.36E-01 7.04E-03 4.11E-03 3.40E-01 1.2 1.4 148 3.04E-06 2.80E-07 7.30E-09 9.22E-02 2.40E-03 1.36E-02 3.17E-04 2.50E+01 3.39E-01 7.93E-03 5.01E-03 3.44E-01 1.5 0.5 151 3.04E-06 2.80E-07 7.60E-09 9.21E-02 2.50E-03 1.35E-02 3.30E-04 2.50E+01 3.38E-01 8.26E-03 5.34E-03 3.44E-01 1.6 0.6 155 3.02E-06 2.79E-07 7.40E-09 9.24E-02 2.45E-03 1.36E-02 3.23E-04 2.50E+01 3.40E-01 8.08E-03 5.15E-03 3.45E-01 1.5 0.2 158 3.02E-06 2.80E-07 6.80E-09 9.25E-02 2.25E-03 1.36E-02 2.97E-04 2.50E+01 3.40E-01 7.42E-03 4.50E-03 3.45E-01 1.3 0.1 162 3.04E-06 2.80E-07 6.00E-09 9.21E-02 1.98E-03 1.35E-02 2.61E-04 2.50E+01 3.38E-01 6.52E-03 3.60E-03 3.42E-01 1.1 0.6 165 3.04E-06 2.80E-07 5.10E-09 9.22E-02 1.68E-03 1.36E-02 2.22E-04 2.50E+01 3.39E-01 5.54E-03 2.62E-03 3.41E-01 0.8 0.5 169 3.02E-06 2.78E-07 4.50E-09 9.21E-02 1.49E-03 1.35E-02 1.96E-04 2.50E+01 3.38E-01 4.91E-03 1.99E-03 3.40E-01 0.6 0.6 172 3.04E-06 2.76E-07 3.90E-09 9.09E-02 1.28E-03 1.33E-02 1.70E-04 2.50E+01 3.33E-01 4.24E-03 1.32E-03 3.34E-01 0.4 2.3 176 3.00E-06 2.78E-07 3.30E-09 9.28E-02 1.10E-03 1.37E-02 1.45E-04 2.50E+01 3.42E-01 3.63E-03 7.07E-04 3.43E-01 0.2 -0.4 179 3.00E-06 2.76E-07 3.00E-09 9.20E-02 1.00E-03 1.35E-02 1.32E-04 2.50E+01 3.38E-01 3.30E-03 3.77E-04 3.38E-01 0.1 0.7 183 3.01E-06 2.76E-07 3.20E-09 9.16E-02 1.06E-03 1.34E-02 1.40E-04 2.50E+01 3.36E-01 3.51E-03 5.83E-04 3.37E-01 0.2 1.3 186 3.01E-06 2.78E-07 6.20E-09 9.23E-02 2.06E-03 1.36E-02 2.72E-04 2.50E+01 3.39E-01 6.79E-03 3.87E-03 3.43E-01 1.1 0.3 190 3.02E-06 2.79E-07 7.00E-09 9.23E-02 2.31E-03 1.36E-02 3.06E-04 2.50E+01 3.39E-01 7.64E-03 4.72E-03 3.44E-01 1.4 0.3 193 3.02E-06 2.77E-07 7.30E-09 9.17E-02 2.41E-03 1.35E-02 3.19E-04 2.50E+01 3.36E-01 7.97E-03 5.04E-03 3.41E-01 1.5 1.2 197 3.01E-06 2.80E-07 7.10E-09 9.28E-02 2.36E-03 1.37E-02 3.11E-04 2.50E+01 3.42E-01 7.78E-03 4.86E-03 3.47E-01 1.4 -0.5 200 3.00E-06 2.77E-07 6.50E-09 9.23E-02 2.17E-03 1.36E-02 2.86E-04 2.50E+01 3.39E-01 7.15E-03 4.23E-03 3.43E-01 1.2 0.3 204 3.02E-06 2.79E-07 5.70E-09 9.23E-02 1.88E-03 1.36E-02 2.49E-04 2.50E+01 3.39E-01 6.22E-03 3.30E-03 3.43E-01 1.0 0.3 207 3.00E-06 2.75E-07 4.80E-09 9.16E-02 1.60E-03 1.34E-02 2.11E-04 2.50E+01 3.36E-01 5.28E-03 2.36E-03 3.38E-01 0.7 1.3 211 3.02E-06 2.76E-07 4.20E-09 9.14E-02 1.39E-03 1.34E-02 1.83E-04 2.50E+01 3.35E-01 4.58E-03 1.66E-03 3.37E-01 0.5 1.6  211  Temp oC IHe  Torr ICH4 Torr ICO2 Torr ICH4/IHe ICO2/IHe YCH4/YHe YCO2/YHe He Flow cc(STP) .min-1 CH4 Flow  cc(STP). min-1 CO2 Flow cc(STP). min-1 CO2 Flow cc(STP).min-1 Corrected Σ C Flow cc(STP).min-1 X1C  mol.% X2CH4  mol.% 214 3.04E-06 2.78E-07 3.60E-09 9.15E-02 1.18E-03 1.34E-02 1.56E-04 2.50E+01 3.35E-01 3.91E-03 9.87E-04 3.36E-01 0.3 1.5 218 3.07E-06 2.79E-07 3.00E-09 9.09E-02 9.78E-04 1.33E-02 1.29E-04 2.50E+01 3.33E-01 3.23E-03 3.04E-04 3.33E-01 0.1 2.3 221 3.02E-06 2.80E-07 2.70E-09 9.26E-02 8.93E-04 1.36E-02 1.18E-04 2.50E+01 3.41E-01 2.95E-03 2.32E-05 3.41E-01 0.0 -0.1 225 3.05E-06 2.81E-07 7.30E-09 9.21E-02 2.40E-03 1.35E-02 3.16E-04 2.50E+01 3.39E-01 7.90E-03 4.98E-03 3.44E-01 1.4 0.5 228 3.04E-06 2.77E-07 6.00E-09 9.13E-02 1.98E-03 1.34E-02 2.61E-04 2.50E+01 3.34E-01 6.52E-03 3.60E-03 3.38E-01 1.1 1.8 232 3.02E-06 2.80E-07 6.70E-09 9.25E-02 2.22E-03 1.36E-02 2.92E-04 2.50E+01 3.40E-01 7.31E-03 4.39E-03 3.45E-01 1.3 0.1 235 3.04E-06 2.81E-07 5.70E-09 9.25E-02 1.88E-03 1.36E-02 2.48E-04 2.50E+01 3.40E-01 6.20E-03 3.27E-03 3.44E-01 1.0 0.0 239 3.05E-06 2.79E-07 6.70E-09 9.16E-02 2.20E-03 1.34E-02 2.90E-04 2.50E+01 3.36E-01 7.25E-03 4.33E-03 3.40E-01 1.3 1.3 242 3.05E-06 2.75E-07 9.50E-09 9.02E-02 3.12E-03 1.31E-02 4.11E-04 2.50E+01 3.29E-01 1.03E-02 7.36E-03 3.36E-01 2.2 3.4 246 3.04E-06 2.62E-07 8.40E-09 8.64E-02 2.77E-03 1.24E-02 3.65E-04 2.50E+01 3.10E-01 9.13E-03 6.21E-03 3.16E-01 2.0 8.9 249 3.05E-06 2.59E-07 1.28E-08 8.50E-02 4.20E-03 1.21E-02 5.54E-04 2.50E+01 3.03E-01 1.39E-02 1.09E-02 3.14E-01 3.5 11.0 253 3.05E-06 2.56E-07 2.24E-08 8.39E-02 7.35E-03 1.19E-02 9.70E-04 2.50E+01 2.97E-01 2.43E-02 2.13E-02 3.19E-01 6.7 12.7 256 3.02E-06 2.42E-07 3.12E-08 8.02E-02 1.03E-02 1.11E-02 1.36E-03 2.50E+01 2.79E-01 3.40E-02 3.11E-02 3.10E-01 10.0 18.1 260 3.05E-06 2.29E-07 4.78E-08 7.52E-02 1.57E-02 1.02E-02 2.07E-03 2.50E+01 2.54E-01 5.18E-02 4.88E-02 3.03E-01 16.1 25.4 263 3.04E-06 2.21E-07 7.12E-08 7.27E-02 2.35E-02 9.66E-03 3.10E-03 2.50E+01 2.42E-01 7.74E-02 7.45E-02 3.16E-01 23.6 29.1 267 3.05E-06 2.14E-07 8.00E-08 7.02E-02 2.62E-02 9.16E-03 3.46E-03 2.50E+01 2.29E-01 8.66E-02 8.37E-02 3.13E-01 26.8 32.8 270 3.04E-06 2.00E-07 9.20E-08 6.60E-02 3.03E-02 8.31E-03 4.00E-03 2.50E+01 2.08E-01 1.00E-01 9.71E-02 3.05E-01 31.8 38.9 274 3.02E-06 1.97E-07 9.51E-08 6.51E-02 3.14E-02 8.13E-03 4.15E-03 2.50E+01 2.03E-01 1.04E-01 1.01E-01 3.04E-01 33.2 40.3 277 3.02E-06 1.88E-07 1.10E-07 6.23E-02 3.64E-02 7.57E-03 4.80E-03 2.50E+01 1.89E-01 1.20E-01 1.17E-01 3.06E-01 38.2 44.4 281 3.02E-06 1.82E-07 1.22E-07 6.03E-02 4.03E-02 7.18E-03 5.33E-03 2.50E+01 1.79E-01 1.33E-01 1.30E-01 3.10E-01 42.1 47.3 284 3.04E-06 1.78E-07 1.34E-07 5.85E-02 4.41E-02 6.81E-03 5.83E-03 2.50E+01 1.70E-01 1.46E-01 1.43E-01 3.13E-01 45.6 50.0 288 3.02E-06 1.66E-07 1.45E-07 5.48E-02 4.79E-02 6.06E-03 6.33E-03 2.50E+01 1.52E-01 1.58E-01 1.55E-01 3.07E-01 50.6 55.5 291 3.04E-06 1.61E-07 1.62E-07 5.30E-02 5.34E-02 5.70E-03 7.04E-03 2.50E+01 1.43E-01 1.76E-01 1.73E-01 3.16E-01 54.8 58.1 295 3.05E-06 1.51E-07 1.69E-07 4.96E-02 5.54E-02 5.03E-03 7.32E-03 2.50E+01 1.26E-01 1.83E-01 1.80E-01 3.06E-01 58.9 63.1 298 3.04E-06 1.39E-07 1.78E-07 4.58E-02 5.86E-02 4.28E-03 7.74E-03 2.50E+01 1.07E-01 1.93E-01 1.91E-01 2.98E-01 64.0 68.6 302 3.04E-06 1.34E-07 1.85E-07 4.43E-02 6.09E-02 3.96E-03 8.04E-03 2.50E+01 9.91E-02 2.01E-01 1.98E-01 2.97E-01 66.7 70.9 305 3.02E-06 1.28E-07 1.95E-07 4.25E-02 6.45E-02 3.60E-03 8.51E-03 2.50E+01 9.00E-02 2.13E-01 2.10E-01 3.00E-01 70.0 73.6   212  Temp oC IHe  Torr ICH4 Torr ICO2 Torr ICH4/IHe ICO2/IHe YCH4/YHe YCO2/YHe He Flow cc(STP) .min-1 CH4 Flow  cc(STP). min-1 CO2 Flow cc(STP). min-1 CO2 Flow cc(STP).min-1 Corrected Σ C Flow cc(STP).min-1 X1C  mol.% X2CH4  mol.% 309 3.02E-06 1.25E-07 2.07E-07 4.13E-02 6.85E-02 3.36E-03 9.04E-03 2.50E+01 8.41E-02 2.26E-01 2.23E-01 3.07E-01 72.6 75.3 312 3.04E-06 1.17E-07 2.13E-07 3.84E-02 7.02E-02 2.78E-03 9.26E-03 2.50E+01 6.96E-02 2.32E-01 2.29E-01 2.98E-01 76.7 79.6 316 3.04E-06 1.12E-07 2.21E-07 3.69E-02 7.28E-02 2.49E-03 9.61E-03 2.50E+01 6.23E-02 2.40E-01 2.37E-01 3.00E-01 79.2 81.7 319 3.02E-06 1.06E-07 2.28E-07 3.51E-02 7.54E-02 2.12E-03 9.95E-03 2.50E+01 5.31E-02 2.49E-01 2.46E-01 2.99E-01 82.2 84.4 323 3.04E-06 9.73E-08 2.36E-07 3.21E-02 7.77E-02 1.52E-03 1.03E-02 2.50E+01 3.79E-02 2.57E-01 2.54E-01 2.92E-01 87.0 88.9 326 3.05E-06 9.28E-08 2.43E-07 3.04E-02 7.97E-02 1.19E-03 1.05E-02 2.50E+01 2.98E-02 2.63E-01 2.60E-01 2.90E-01 89.7 91.2 330 3.04E-06 8.63E-08 2.51E-07 2.84E-02 8.27E-02 7.89E-04 1.09E-02 2.50E+01 1.97E-02 2.73E-01 2.70E-01 2.90E-01 93.2 94.2 333 3.04E-06 8.08E-08 2.58E-07 2.66E-02 8.50E-02 4.25E-04 1.12E-02 2.50E+01 1.06E-02 2.80E-01 2.78E-01 2.88E-01 96.3 96.9 337 3.01E-06 7.74E-08 2.66E-07 2.57E-02 8.83E-02 2.45E-04 1.17E-02 2.50E+01 6.11E-03 2.91E-01 2.89E-01 2.95E-01 97.9 98.2 340 3.04E-06 7.46E-08 2.67E-07 2.46E-02 8.79E-02 2.19E-05 1.16E-02 2.50E+01 5.48E-04 2.90E-01 2.87E-01 2.88E-01 99.8 99.8 344 3.02E-06 7.07E-08 2.71E-07 2.34E-02 8.96E-02 -2.21E-04 1.18E-02 2.50E+01 -5.52E-03 2.96E-01 2.93E-01 2.87E-01 101.9 101.6 347 3.05E-06 6.84E-08 2.81E-07 2.24E-02 9.22E-02 -4.07E-04 1.22E-02 2.50E+01 -1.02E-02 3.04E-01 3.01E-01 2.91E-01 103.5 103.0 351 3.05E-06 6.58E-08 2.85E-07 2.16E-02 9.35E-02 -5.81E-04 1.23E-02 2.50E+01 -1.45E-02 3.09E-01 3.06E-01 2.91E-01 105.0 104.3 354 3.04E-06 6.37E-08 2.87E-07 2.10E-02 9.45E-02 -6.98E-04 1.25E-02 2.50E+01 -1.75E-02 3.12E-01 3.09E-01 2.92E-01 106.0 105.1 358 3.05E-06 6.20E-08 2.92E-07 2.04E-02 9.58E-02 -8.25E-04 1.26E-02 2.50E+01 -2.06E-02 3.16E-01 3.13E-01 2.93E-01 107.0 106.1 361 3.05E-06 5.98E-08 2.93E-07 1.96E-02 9.61E-02 -9.75E-04 1.27E-02 2.50E+01 -2.44E-02 3.17E-01 3.14E-01 2.90E-01 108.4 107.2 365 3.02E-06 5.84E-08 2.96E-07 1.93E-02 9.79E-02 -1.03E-03 1.29E-02 2.50E+01 -2.58E-02 3.23E-01 3.20E-01 2.94E-01 108.8 107.6 368 3.05E-06 5.76E-08 2.99E-07 1.89E-02 9.81E-02 -1.12E-03 1.29E-02 2.50E+01 -2.79E-02 3.24E-01 3.21E-01 2.93E-01 109.5 108.2 372 3.04E-06 5.69E-08 3.08E-07 1.87E-02 1.01E-01 -1.15E-03 1.34E-02 2.50E+01 -2.87E-02 3.35E-01 3.32E-01 3.03E-01 109.5 108.4 375 3.05E-06 5.53E-08 3.03E-07 1.81E-02 9.94E-02 -1.27E-03 1.31E-02 2.50E+01 -3.17E-02 3.28E-01 3.25E-01 2.93E-01 110.8 109.3 213   Figure C.4 CH4 conversion calculation for Pd5B catalyst with GHSV =36000 h-1.   Figure C.5 CH4 conversion calculation for Pd5B catalyst with GHSV =18000 h-1.   214  Table C.5 CH4 conversion calculation for Pd5B catalyst during TPO experiment. [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He. Temp oC IHe  Torr ICH4 Torr ICH4/IHe YCH4/YHe He Flow cc(STP) .min-1 CH4 Flow  cc(STP). min-1 X2CH4  mol.% 25 2.64E-06 2.45E-07 9.28E-02 1.40E-02 5.00E+01 7.01E-01 0.0 29 2.65E-06 2.44E-07 9.20E-02 1.39E-02 5.00E+01 6.94E-01 1.0 32 2.63E-06 2.44E-07 9.27E-02 1.40E-02 5.00E+01 7.01E-01 0.0 36 2.63E-06 2.43E-07 9.23E-02 1.39E-02 5.00E+01 6.97E-01 0.6 39 2.63E-06 2.43E-07 9.24E-02 1.39E-02 5.00E+01 6.97E-01 0.5 43 2.64E-06 2.43E-07 9.20E-02 1.39E-02 5.00E+01 6.93E-01 1.1 46 2.63E-06 2.41E-07 9.16E-02 1.38E-02 5.00E+01 6.89E-01 1.7 50 2.64E-06 2.43E-07 9.20E-02 1.39E-02 5.00E+01 6.93E-01 1.1 53 2.63E-06 2.42E-07 9.20E-02 1.39E-02 5.00E+01 6.93E-01 1.1 57 2.65E-06 2.43E-07 9.16E-02 1.38E-02 5.00E+01 6.90E-01 1.6 60 2.64E-06 2.41E-07 9.12E-02 1.37E-02 5.00E+01 6.86E-01 2.2 64 2.63E-06 2.40E-07 9.12E-02 1.37E-02 5.00E+01 6.85E-01 2.2 67 2.62E-06 2.43E-07 9.27E-02 1.40E-02 5.00E+01 7.01E-01 0.0 71 2.63E-06 2.42E-07 9.20E-02 1.39E-02 5.00E+01 6.93E-01 1.1 74 2.63E-06 2.42E-07 9.20E-02 1.39E-02 5.00E+01 6.93E-01 1.1 78 2.64E-06 2.42E-07 9.16E-02 1.38E-02 5.00E+01 6.89E-01 1.6 81 2.63E-06 2.42E-07 9.20E-02 1.39E-02 5.00E+01 6.93E-01 1.1 85 2.62E-06 2.41E-07 9.19E-02 1.39E-02 5.00E+01 6.93E-01 1.2 88 2.62E-06 2.43E-07 9.27E-02 1.40E-02 5.00E+01 7.01E-01 0.0 92 2.62E-06 2.43E-07 9.27E-02 1.40E-02 5.00E+01 7.01E-01 0.0 95 2.63E-06 2.43E-07 9.23E-02 1.39E-02 5.00E+01 6.97E-01 0.6 99 2.64E-06 2.41E-07 9.12E-02 1.37E-02 5.00E+01 6.86E-01 2.2 102 2.63E-06 2.43E-07 9.23E-02 1.39E-02 5.00E+01 6.97E-01 0.6 106 2.61E-06 2.42E-07 9.27E-02 1.40E-02 5.00E+01 7.00E-01 0.1 109 2.63E-06 2.43E-07 9.23E-02 1.39E-02 5.00E+01 6.97E-01 0.6 113 2.63E-06 2.41E-07 9.16E-02 1.38E-02 5.00E+01 6.89E-01 1.7 116 2.62E-06 2.40E-07 9.17E-02 1.38E-02 5.00E+01 6.90E-01 1.6 120 2.62E-06 2.43E-07 9.28E-02 1.40E-02 5.00E+01 7.01E-01 -0.1 123 2.61E-06 2.42E-07 9.27E-02 1.40E-02 5.00E+01 7.00E-01 0.1 127 2.61E-06 2.42E-07 9.27E-02 1.40E-02 5.00E+01 7.01E-01 0.0 130 2.63E-06 2.42E-07 9.20E-02 1.39E-02 5.00E+01 6.93E-01 1.1 134 2.65E-06 2.42E-07 9.13E-02 1.37E-02 5.00E+01 6.86E-01 2.1 137 2.64E-06 2.41E-07 9.12E-02 1.37E-02 5.00E+01 6.86E-01 2.2 141 2.63E-06 2.42E-07 9.20E-02 1.39E-02 5.00E+01 6.93E-01 1.1 144 2.62E-06 2.41E-07 9.20E-02 1.39E-02 5.00E+01 6.93E-01 1.1 148 2.63E-06 2.40E-07 9.14E-02 1.37E-02 5.00E+01 6.87E-01 2.0 151 2.63E-06 2.40E-07 9.13E-02 1.37E-02 5.00E+01 6.86E-01 2.1 155 2.64E-06 2.39E-07 9.05E-02 1.36E-02 5.00E+01 6.78E-01 3.2 158 2.63E-06 2.38E-07 9.06E-02 1.36E-02 5.00E+01 6.79E-01 3.1 215  Temp oC IHe  Torr ICH4 Torr ICH4/IHe YCH4/YHe He Flow cc(STP) .min-1 CH4 Flow  cc(STP). min-1 X2CH4  mol.% 162 2.62E-06 2.40E-07 9.16E-02 1.38E-02 5.00E+01 6.89E-01 1.7 165 2.62E-06 2.37E-07 9.04E-02 1.35E-02 5.00E+01 6.77E-01 3.4 169 2.62E-06 2.35E-07 8.96E-02 1.34E-02 5.00E+01 6.69E-01 4.5 172 2.63E-06 2.35E-07 8.95E-02 1.34E-02 5.00E+01 6.68E-01 4.7 176 2.64E-06 2.35E-07 8.90E-02 1.33E-02 5.00E+01 6.63E-01 5.4 179 2.63E-06 2.35E-07 8.93E-02 1.33E-02 5.00E+01 6.66E-01 5.0 183 2.61E-06 2.34E-07 8.98E-02 1.34E-02 5.00E+01 6.71E-01 4.2 186 2.63E-06 2.34E-07 8.91E-02 1.33E-02 5.00E+01 6.63E-01 5.3 190 2.62E-06 2.34E-07 8.97E-02 1.34E-02 5.00E+01 6.69E-01 4.5 193 2.62E-06 2.34E-07 8.94E-02 1.33E-02 5.00E+01 6.66E-01 4.9 197 2.62E-06 2.34E-07 8.93E-02 1.33E-02 5.00E+01 6.66E-01 5.0 200 2.62E-06 2.34E-07 8.92E-02 1.33E-02 5.00E+01 6.64E-01 5.2 204 2.62E-06 2.33E-07 8.91E-02 1.33E-02 5.00E+01 6.64E-01 5.3 207 2.63E-06 2.33E-07 8.89E-02 1.32E-02 5.00E+01 6.62E-01 5.5 211 2.63E-06 2.33E-07 8.87E-02 1.32E-02 5.00E+01 6.60E-01 5.9 214 2.63E-06 2.33E-07 8.86E-02 1.32E-02 5.00E+01 6.59E-01 6.0 218 2.62E-06 2.32E-07 8.85E-02 1.32E-02 5.00E+01 6.58E-01 6.1 221 2.63E-06 2.32E-07 8.83E-02 1.31E-02 5.00E+01 6.55E-01 6.5 225 2.63E-06 2.32E-07 8.80E-02 1.31E-02 5.00E+01 6.53E-01 6.8 228 2.63E-06 2.31E-07 8.79E-02 1.30E-02 5.00E+01 6.51E-01 7.1 232 2.63E-06 2.30E-07 8.75E-02 1.29E-02 5.00E+01 6.47E-01 7.7 235 2.63E-06 2.29E-07 8.73E-02 1.29E-02 5.00E+01 6.46E-01 7.9 239 2.63E-06 2.29E-07 8.70E-02 1.29E-02 5.00E+01 6.43E-01 8.3 242 2.63E-06 2.29E-07 8.70E-02 1.28E-02 5.00E+01 6.42E-01 8.4 246 2.63E-06 2.27E-07 8.65E-02 1.28E-02 5.00E+01 6.38E-01 9.0 249 2.63E-06 2.27E-07 8.62E-02 1.27E-02 5.00E+01 6.34E-01 9.5 253 2.63E-06 2.26E-07 8.57E-02 1.26E-02 5.00E+01 6.29E-01 10.2 256 2.64E-06 2.24E-07 8.50E-02 1.24E-02 5.00E+01 6.22E-01 11.2 260 2.64E-06 2.23E-07 8.45E-02 1.23E-02 5.00E+01 6.17E-01 12.0 263 2.64E-06 2.21E-07 8.36E-02 1.22E-02 5.00E+01 6.08E-01 13.2 267 2.63E-06 2.19E-07 8.34E-02 1.21E-02 5.00E+01 6.05E-01 13.6 270 2.63E-06 2.17E-07 8.25E-02 1.19E-02 5.00E+01 5.97E-01 14.9 274 2.63E-06 2.14E-07 8.16E-02 1.17E-02 5.00E+01 5.87E-01 16.2 277 2.62E-06 2.12E-07 8.07E-02 1.16E-02 5.00E+01 5.78E-01 17.6 281 2.63E-06 2.09E-07 7.95E-02 1.13E-02 5.00E+01 5.66E-01 19.3 284 2.63E-06 2.05E-07 7.80E-02 1.10E-02 5.00E+01 5.50E-01 21.5 288 2.63E-06 2.01E-07 7.66E-02 1.07E-02 5.00E+01 5.36E-01 23.4 291 2.63E-06 1.98E-07 7.52E-02 1.04E-02 5.00E+01 5.22E-01 25.5 295 2.63E-06 1.94E-07 7.39E-02 1.02E-02 5.00E+01 5.09E-01 27.4 298 2.63E-06 1.89E-07 7.21E-02 9.80E-03 5.00E+01 4.90E-01 30.1 302 2.63E-06 1.85E-07 7.04E-02 9.46E-03 5.00E+01 4.73E-01 32.5 305 2.63E-06 1.81E-07 6.87E-02 9.11E-03 5.00E+01 4.56E-01 35.0 309 2.63E-06 1.76E-07 6.69E-02 8.74E-03 5.00E+01 4.37E-01 37.6 312 2.63E-06 1.71E-07 6.49E-02 8.33E-03 5.00E+01 4.17E-01 40.6 216  Temp oC IHe  Torr ICH4 Torr ICH4/IHe YCH4/YHe He Flow cc(STP) .min-1 CH4 Flow  cc(STP). min-1 X2CH4  mol.% 316 2.64E-06 1.65E-07 6.28E-02 7.90E-03 5.00E+01 3.95E-01 43.6 319 2.64E-06 1.60E-07 6.06E-02 7.45E-03 5.00E+01 3.73E-01 46.8 323 2.64E-06 1.54E-07 5.84E-02 7.01E-03 5.00E+01 3.50E-01 50.0 326 2.63E-06 1.48E-07 5.61E-02 6.54E-03 5.00E+01 3.27E-01 53.4 330 2.64E-06 1.42E-07 5.39E-02 6.10E-03 5.00E+01 3.05E-01 56.5 333 2.64E-06 1.36E-07 5.16E-02 5.62E-03 5.00E+01 2.81E-01 59.9 337 2.64E-06 1.30E-07 4.94E-02 5.18E-03 5.00E+01 2.59E-01 63.1 340 2.64E-06 1.25E-07 4.73E-02 4.76E-03 5.00E+01 2.38E-01 66.1 344 2.64E-06 1.19E-07 4.50E-02 4.29E-03 5.00E+01 2.14E-01 69.4 347 2.64E-06 1.14E-07 4.32E-02 3.92E-03 5.00E+01 1.96E-01 72.1 351 2.64E-06 1.09E-07 4.13E-02 3.52E-03 5.00E+01 1.76E-01 74.9 354 2.64E-06 1.04E-07 3.96E-02 3.18E-03 5.00E+01 1.59E-01 77.3 358 2.64E-06 1.00E-07 3.80E-02 2.85E-03 5.00E+01 1.43E-01 79.6 361 2.65E-06 9.69E-08 3.66E-02 2.57E-03 5.00E+01 1.29E-01 81.6 365 2.64E-06 9.35E-08 3.54E-02 2.33E-03 5.00E+01 1.17E-01 83.4 368 2.65E-06 9.03E-08 3.41E-02 2.05E-03 5.00E+01 1.03E-01 85.3 372 2.64E-06 8.73E-08 3.31E-02 1.85E-03 5.00E+01 9.23E-02 86.8 375 2.65E-06 8.48E-08 3.20E-02 1.62E-03 5.00E+01 8.11E-02 88.4 379 2.65E-06 8.26E-08 3.11E-02 1.45E-03 5.00E+01 7.26E-02 89.6 382 2.66E-06 8.06E-08 3.03E-02 1.29E-03 5.00E+01 6.45E-02 90.8 386 2.65E-06 7.87E-08 2.97E-02 1.16E-03 5.00E+01 5.79E-02 91.7 389 2.66E-06 7.67E-08 2.89E-02 9.95E-04 5.00E+01 4.98E-02 92.9 393 2.65E-06 7.50E-08 2.83E-02 8.67E-04 5.00E+01 4.34E-02 93.8 396 2.65E-06 7.36E-08 2.77E-02 7.56E-04 5.00E+01 3.78E-02 94.6 400 2.65E-06 7.24E-08 2.73E-02 6.71E-04 5.00E+01 3.36E-02 95.2 403 2.65E-06 7.17E-08 2.70E-02 6.08E-04 5.00E+01 3.04E-02 95.7 407 2.65E-06 7.03E-08 2.65E-02 5.13E-04 5.00E+01 2.57E-02 96.3 410 2.65E-06 6.95E-08 2.62E-02 4.47E-04 5.00E+01 2.24E-02 96.8 414 2.65E-06 6.89E-08 2.60E-02 4.03E-04 5.00E+01 2.02E-02 97.1 417 2.65E-06 6.81E-08 2.57E-02 3.35E-04 5.00E+01 1.68E-02 97.6 421 2.66E-06 6.75E-08 2.54E-02 2.88E-04 5.00E+01 1.44E-02 97.9 424 2.66E-06 6.70E-08 2.52E-02 2.38E-04 5.00E+01 1.19E-02 98.3 428 2.67E-06 6.65E-08 2.50E-02 1.92E-04 5.00E+01 9.58E-03 98.6 431 2.66E-06 6.62E-08 2.48E-02 1.67E-04 5.00E+01 8.37E-03 98.8 435 2.66E-06 6.58E-08 2.47E-02 1.45E-04 5.00E+01 7.24E-03 99.0 438 2.67E-06 6.55E-08 2.45E-02 1.03E-04 5.00E+01 5.13E-03 99.3 442 2.67E-06 6.54E-08 2.45E-02 9.66E-05 5.00E+01 4.83E-03 99.3 445 2.67E-06 6.51E-08 2.44E-02 6.85E-05 5.00E+01 3.42E-03 99.5 449 2.67E-06 6.46E-08 2.42E-02 2.94E-05 5.00E+01 1.47E-03 99.8 452 2.67E-06 6.44E-08 2.42E-02 2.73E-05 5.00E+01 1.36E-03 99.8 456 2.66E-06 6.43E-08 2.42E-02 3.05E-05 5.00E+01 1.52E-03 99.8 459 2.66E-06 6.40E-08 2.41E-02 7.80E-06 5.00E+01 3.90E-04 99.9 463 2.67E-06 6.40E-08 2.40E-02 -2.68E-06 5.00E+01 -1.34E-04 100.0 466 2.67E-06 6.42E-08 2.40E-02 -2.55E-06 5.00E+01 -1.27E-04 100.0 217   Figure C.6 Comparison of TPO raw data and TPO average data using Pd5B catalyst ; [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO and 8 vol% CO2 in N2 and He.   Figure C.7 Comparison of TPO results with and without CO and CO2 for PdCe-WC [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol% O2, 0 or 0.06 vol% CO and 0 or 8 vol% CO2 in N2 and He.   218   Figure C.8. Comparison of TPO results with and without CO and CO2 for O-PdCe-WC [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol% O2, 0 or 0.06 vol% CO and 0 or 8 vol% CO2 in N2 and He.          219  C.6 TPO and TOS Reaction Repeatability  Table C.6 TPO Reaction Repeatability. [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 0, 2 or 5 vol% H2O in N2 and He]. Monolith Catalyst  Light-off Temperature (°C)    T10 T50 T80    PdCe-WC dry run1  289.0 353.0 389 PdCe-WC dry run2  285.0 348.0 382 Average   287.0 350.5 385.5 SD  2.8 3.5 4.9 PdCe-WC 5 vol% H2O run1  351.0 409.0 441 PdCe-WC 5 vol% H2O run2  340.0 403.0 436 Average   345.5 406.0 438.5 SD  7.8 4.2 3.5 O-PdCe-WC dry run1  306.0 361.0 402 O-PdCe-WC dry run2  300.0 357.0 400 Average   303.0 359.0 401.0 SD  4.2 2.8 1.4 O-PdCe-WC 5 vol% H2O run1  358.0 394.0 423 O-PdCe-WC 5 vol% H2O run2  351.0 392.0 420 Average   354.5 393.0 421.5 SD  4.9 1.4 2.1  220  Table C.7 TOS Reaction Repeatability at 425 oC. [Reaction conditions: Total feed gas flow = 1025 cm3(STP)·min−1, GHSV = 36000 h−1, Feed composition: 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 10 vol% H2O in N2 and He]. Monolith Catalyst  Time-on-Stream (h)      0 2 4 8 10     mol (%) PdCe-WC 5 vol% H2O run1  100.0 62.0 54 53.0 51.0 PdCe-WC 5 vol% H2O run2  100.0 63.0 57 54.0 53.0 Average   100.0 62.5 55.5 53.5 52.0 SD  0.0 0.7 2.1 0.7 1.4 O-PdCe-WC 5 vol% H2O run1  95.0 75.0 72.0 71 69.0 O-PdCe-WC 5 vol% H2O run2  96.0 80.0 76 73.0 70.0 Average   95.5 77.5 74.0 72.0 69.5 SD  0.7 3.5 2.8 1.4 0.7            221  Appendix D  Supporting calculations   This section presents the calculations regarding the kinetic modeling of CH4 oxidation.  Table D.1 Physical properties of catalyst Pd0Ce. Parameter Definition  Value  T Reaction temperature (K)  523 P Total pressure (Pa)  101325 R Gas constant (Pa.m3.mol-1.K-1)  8.314 𝑦𝐶𝐻4 CH4 volume fraction  0.0007 𝑦𝑂2 O2 volume fraction  0.085 𝑦𝐻𝑒 He volume fraction  0.05 𝑀𝑤𝑓𝑒𝑒𝑑 Feed molecular weight (g.mol-1)  34 𝑃𝐶𝐻4𝑜  Feed CH4 partial pressure (Pa)  71 v0 Total volumetric flow rate (m3.s-1)  1.7e-5 𝐹𝐶𝐻4𝑜  Feed CH4 molar flow (mol.s-1 (STP))  5.35e-7 rt Radius of reactor (m)  4.6e-3 Ac Cross sectional area of the reactor (m2)  6.4e-5 u Superficial gas velocity (m.s-1)  0.26 G Superficial mass velocity (kg.m-2.s-1)  0.4 T* Tstar for collision integral 4.11 ΩD Collision integral  0.81 τ Tortuosity factor  8 σ Constriction factor  0.8 𝐷𝐶𝐻4,𝐻𝑒 Binary bulk diffusivity, (m2 s-1) 1.04e-04 𝐷𝐶𝐻4−𝐻𝑒𝑒𝑓𝑓 Effective bulk diffusivity, (m2 s-1) 6.96e-06 DK Knudsen diffusivity, (m2 s-1) 2.5e-06 DK_eff Effective Knudsen diffusivity, (m2 s-1) 2.1e-07 Deff Effective diffusivity, (m2 s-1) 1.67E-07  222  Table D.2 Washcoat properties. Parameter Definition  Value  Wwash Mass of washcoat (kg) 2.7e-4 Lch Length of channel (m) 2.54e-2 Lc Washcoat thickness (m) 3.5e-5 Vr Volume of monolith (m3) 1.69e-6 SBET BET surface area (m2.g-1)  54 dpore Pore size (m)  1.8e-8 ρwash Washcoat density (kg.m-3)  1340 εb Washcoat porosity  0.67 τ Tortuosity factor  8  D.1 Plug Flow Criterion:  It is critical to ensure that the gas flow pattern inside the channel of the monolith is ideal. The channel length (Lch) should at least 10x’s the channel hydraulic diameter (𝑑ℎ) to ensure the wall of the channel doesn’t have a major impact on the flow pattern [167]. Since the channel Reynolds number is small, a higher Lch /𝑑ℎ ratio is required to ensure plug flow in the channel. Moreover, the Peclet number and Lch/𝑑ℎ must be higher than the minimum Peclet number and the minimum Lch/𝑑ℎ ratio to satisfy the plug flow criteria [167]. 𝑂𝑝𝑒𝑛 𝐹𝑟𝑜𝑛𝑡𝑎𝑙 𝐴𝑟𝑒𝑎 (𝑂𝐹𝐴) =(𝑙 − 𝑡𝑤)2𝑙2                                                                                             𝐷. 1 𝐺𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐 𝑆𝑢𝑟𝑓𝑎𝑐𝑒  𝐴𝑟𝑒𝑎 (𝐺𝑆𝐴) =4(𝑙 − 𝑡𝑤)𝑙2                                                                                𝐷. 2 where 𝑙 = width of channel in inches and 𝑡𝑤 = thickness of the wall in inches 𝐻𝑦𝑑𝑟𝑎𝑢𝑙𝑖𝑐 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 (𝑑ℎ) =4 𝑂𝐹𝐴𝐺𝑆𝐴                                                                                                   𝐷. 3 Channel Reynolds number is calculated as follows: 223  𝑅𝑒 =𝑑ℎ𝑢𝑠𝜌𝑔𝜇                                                                                                                                               𝐷. 4 Where 𝐿𝑐 is the washcoat thickness, μ is the dynamic gas viscosity, and 𝜖𝑑 is the washcoat porosity, 𝜌𝑔 the gas density and 𝑢 the superficial gas velocity, 𝑀𝑤𝑓𝑒𝑒𝑑 feed molecular weight (kg.kmol-1)  are defined as: 𝜌𝑔 =𝑃𝑀𝑤𝑓𝑒𝑒𝑑𝑅𝑇                                                                                                                                             𝐷. 5 𝑢𝑠 =𝑣0𝐴                                                                                                                                                          𝐷. 6 For gas phase Peclet number (Pe) is giving as follow [167]: 𝑃𝑒 = 0.087𝑅𝑒0.23Lch𝑑ℎ                                                                                                                                𝐷. 7 Minimum Peclet number (Pe) is giving as follow [167]: 𝑃𝑒𝑚𝑖𝑛 = 8𝑛𝐿𝑛 (11 −𝑋100)                                                                                                                     𝐷. 8  For gas phase minimum (Lch /𝑑ℎ)min is calculated as: (Lch/𝑑ℎ)min = 92𝑅𝑒−0.23𝑛𝐿𝑛 (11 −𝑋100)                                                                                         𝐷. 9           224  Table D.3 Calculations for verification of the plug flow assumption and absence of axial dispersion. Parameter Definition  Value  T Reaction temperature (K)  523 P Total pressure (Pa)  101325 R Gas constant (Pa.m3.mol-1.K-1)  8.314 𝑀𝑤𝑓𝑒𝑒𝑑 Feed molecular weight (kg.kmol-1)  34 𝑃𝐶𝐻4𝑜  Feed CH4 partial pressure (Pa)  71 𝐹𝐶𝐻4𝑜  Feed CH4 molar flow (mol.s-1 (STP))  5.35e-7 OFA Open frontal area (m2/ m2) 0.75 GSA Geometric surface area (m/ m2) 3.04 dh Hydraulic diameter (m) 9.8e-4 rt Radius of reactor (m)  4.6e-3 Lch Length of channel (m) 2.54e-2 Lc Washcoat thickness (m) 3.5e-5 Ac Cross sectional area of the reactor (m2)  6.4e-5 us Superficial gas velocity (m.s-1)  0.26 ρg Gas density (kg.m3)  0.78  µ Gas dynamic viscosity (kg.m-1.s-1)  3.87e-5 Re Reynolds Number  5.13 n Order of CH4 oxidation reaction  1 Pe Peclet number 3.27 Pemin minimum Peclet number 0.41 (Lch/𝑑ℎ) Ratio of Lchannel to 𝑑ℎ 25.8 (Lch/𝑑ℎ)min Minimum ratio of (Lchannel /𝑑ℎ)min 3.2  From the results of Pe is higher than Pemin and (Lchannel /𝑑ℎ) is higher than (Lchannel /𝑑ℎ)min; thus, the flow of the gas inside the channel meets the plug flow criteria.      225  D.2 External and Internal Mass Transfer Calculation  Schmidt number is given by:  𝑆𝑐 =𝜇𝜌𝑔𝐷𝐶𝐻4,𝐻𝑒                                                                                                                                         𝐷. 10 In a gas phase system with Re < 2000 and 0.4 < 𝜖𝑑 < 0.79, 𝑗𝐷factor is calculated 𝑗𝐷𝜖𝑑 = 0.35𝑅𝑒−0.359                                                                                                                                𝐷. 11 The Sherwood number is calculated as follows 𝑆ℎ = 𝑗𝐷𝑅𝑒𝑆𝑐13                                                                                                                                            𝐷. 12 The external mass transfer coefficient (𝑘𝑐 ) is calculated as follows: 𝑘𝑐 =𝐷𝐶𝐻4,𝐻𝑒𝑆ℎ𝐿𝑐                                                                                                                                         𝐷. 13 Mears criterion is calculated as follows [125, 168] 𝐶𝑀 =𝑟𝐶𝐻4𝑚 𝜌𝑤𝑎𝑠ℎ 𝐿𝑐𝑘𝑐𝐶𝐶𝐻4                                                                                                                                   𝐷. 14 Weisz-Prater Number is calculated as follows [125, 168] 𝐶𝑊𝑃 =𝑟𝐶𝐻4𝑚 𝜌𝑤𝑎𝑠ℎ 𝐿𝑐2𝐷𝑒𝑓𝑓𝐶𝐶𝐻4                                                                                                                               𝐷. 15 The mass transfer from the bulk gas phase to the surface of the washcoat is negligible if CM is < 0.15. CM is obtained as 0.0002 in this study which indicts the absence of external mass transfer limitations since CM is < 0.15. Also, internal mass transfer through the washcoat is negligible if CWP is < 0.3. CWP is obtained as 0.104 in this study which indicts the absence of internal mass transfer limitations since CWP is < 0.3.  226  Table D.4 Details of calculations for Mears criterion factor and Weisz-Prater number for Pd0Ce at 523 K. Parameter Definition   Value  T Reaction temperature (K)   523 P Total pressure (Pa)   101325 R Gas constant (Pa.m3.mol-1.K-1)   8.314 𝑀𝑤𝑓𝑒𝑒𝑑 Feed molecular weight (kg.kmol-1)   34 𝑃𝐶𝐻4𝑜  Feed CH4 partial pressure (Pa)   71 𝐹𝐶𝐻4𝑜  Feed CH4 molar flow (mol.s-1 (STP))   5.35e-7 OFA Open frontal area (m2/ m2)  0.75 GSA Geometric surface area (m/ m2)  3.04 dh Hydraulic diameter (m)  9.8e-4 rt Radius of reactor (m)   4.6e-3 Ac Cross sectional area of the reactor (m2)   6.4e-5 us Superficial gas velocity (m.s-1)   0.26 εb Washcoat porosity   0.67 𝐷𝐶𝐻4,𝐻𝑒 Binary bulk diffusivity, (m2 s-1)   1.04e-04 Deff Effective diffusivity, (m2 s-1)  1.67E-07 ρg Gas density (kg.m3)   0.78  ΩD Collision integral   0.81 µ Gas dynamic viscosity (kg.m-1.s-1)   3.87e-5 Re Reynolds Number   5.13 Sc Schmidt Number   0.5 jD jD factor   0.29 Sh Sherwood number  1.15 kc External mass transfer coefficient, (m s-1)  3.43 𝐶𝐶𝐻4 CH4 concentration, (mol m-3)  0.029 −𝑟𝐶𝐻4𝑚  CH4 oxidation reaction rate (mol.kgcat-1.s-1)  3.06E-04 ρwash Washcoat density (kg.m3)   1340 n Order of CH4 oxidation reaction   1 CM Mears criterion factor   0.0002 CWP Weisz-Prater Numbe  0.104 227  D.3 Isothermal Conditions In modeling the CH4 oxidation it was assumed isothermal condition. Thus, the maximum temperature inside the washcoat is calculated by using equation D.3 to make sure the assumption of the isothermal condition is valid [125]. 𝑇𝑚𝑎𝑥 = 𝑇𝑠 −Δ𝐻𝑟𝐷𝑒𝑓𝑓𝐶𝐶𝐻4𝑘𝑝                                                                                                                    𝐷. 15 Table D.5 Calculations the maximum temperature inside the washcoat. Parameter Definition  Value  Ts Surface temperature (K)  523 Δ𝐻𝑟 Heat of reaction at 298 K, (kJ mol-1) -891 Deff Effective diffusivity, (m2 s-1) 1.67E-07 𝐶𝐶𝐻4 CH4 concentration, (mol m-3) 0.029 𝑘𝑝 Thermal conductivity of washcoat, (kJ m-1 s-1 K-1) 0.025 Tmax Reaction temperature (K)  523+1.7e-4    From the results Tmax is equal to TS; thus, the temperature gradient in the washcoat layer will be small due to the low CH4 concentration and the thin layer of washcoat. From this result the assumption of isothermal condition is valid.           228  Appendix E  Scale up  Details about the preparation of 0.5L monolith catalyst and determined the activity of this catalyst was described in this section.  E.1 Preparation the 0.5 L Monolith Catalyst  For scaling up and engine test, a 0.5 L monolith catalyst was prepared by using the same method that was described in Section 2.3. O-PdPtCeWC catalyst was prepared for the 0.5L monolith since showed the best performance. However, during preparing the big monolith we found that a few channels were plugged due to the viscosity of the washcoat suspension. Thus, to overcome this the solid content in the suspension was decreased. Also, the catalyst shows low activity and less stability due to the calcination in the furnace (monolith interior temperature < 450 oC). To solve this problem, the temperature of calcination increased from 450 to 500 oC ( to confirm monolith interior temperature reached 450 oC) and decreased ramping rate of the temperature from 10 oC /min to 5 oC /min. The final catalyst nominal compositions are reported in Table E.1  Table E.1 Nominal composition of 0.5L monolith catalysts. Sample Washcoat Overlayer Pd Pt Ce -AlOOH -Al2O3 Cordierite g 52 16 0.49 0.05 3.2 17 51 155 Mass wt% 23 7 0.21 0.02 1.4 8 23 68   229    Figure E.1 0.5L monolith (A) after deposited the washcoat and (B) after deposited the active phase.  The monoliths were also analyzed using scanning electron microscopy (SEM). SEM was used to assess the surface morphology the washcoated monolith. In the latter case, the monolith was sectioned to yield an internal channel for analysis. SEM images of the monolith sections are presented in Figure E.2. Also, the figure shows some cracks in the layer of the washcoat which resulting from differences in thermal expansion between cordierite and alumina [67].       A B 230    Figure E.2 SEM images of the 0.5L monolith catalysts:(A) X-section of the catalyst; (B) Channel wall of the catalyst.  E.2 Catalyst Testing  The activity of the monolith catalysts for CH4 oxidation were assessed by temperature-programmed CH4 oxidation (TPO) at a GHSV of 36000 h−1 in a feed gas with composition 0.07 vol % CH4, 8.5 vol % O2, 0.06 vol % CO and 8 vol % CO2 in N2 and He. The stability of the catalysts at 500 oC was determined by using a time-on-stream (TOS) test at the same GHSV with a feed gas composition of 0.07 vol % CH4, 8.5 vol % O2, 0.06 vol % CO, 8 vol % CO2 and 10 vol % H2O in N2 and He. The monolith was cut in the middle and selected three different locations (Top, Middle and Bottom) from the monolith to be cut as the same size for the mini monolith (52 cells and 1 cm x 2.54 cm). Then the actvivty was measured for each one of these mini monolith.  A B 231  The activity of the monolith catalysts, measured by TPO, are reported in Figure E.3 and compared to O-PdPtCeWC (results for the mini monolith in Chapter 5). Figure E.3A showed that when the catalyst tested direct after being cut from the 0.5L monolith, the activity is less than O-PdPtCeWC, suggesting due to insufficient calcination time and less temperature in the furnace. Figure E.3B showed that after the sample recalcined in the reactor in air flow of 100 cm3(STP)·min−1 while heating from 25 to 450 °C at 10 °C/min and holding the final temperature for 15 h, the activity improved for the catalyst. From this result it is clear that calcined the catalyst in in the furnace which is not adequate for large sample since the monolith interior temperature was < 450 oC is the main cause for decreasing the activity.  The results of the TOS experiments with 10 vol% H2O in the feed at 500 °C are shown in Figure E.4. Figure E.4 shows that the mini monolith (Mid) that was cut from the 0.5L monolith has lower stability than O-PdPtCeWC. From the results of TPO and TOS it is recommended to calcine the monolith with enough time and heat to have active and stable catalysts for engine test.     232   Figure E.3 Temperature-programmed oxidation profile: the initial activity of the catalysts as a function of temperature. Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, and 8 vol% CO2 in N2 and He. (A) calcined in the furnace and (B) recalcined in the reactor.  Figure E.4 TOS results for adding 10 vol% H2O at 500 oC for 24h.  Reaction conditions: GHSV of 36000 h−1 with 0.07 vol% CH4, 8.5 vol% O2, 0.06 vol% CO, 8 vol% CO2 and 10 vol% H2O in N2 and He.    233  Appendix F  MATLAB M-files Code  Main m-file contains reading the data from Excel file, beginning calculation using the Levenberg-Marquardt nonlinear regression by calling objective function m-file and printing the results as follows: clc clear all global nvar nx %x0 y0  global verbose  verbose(1:2) = 1; more off format short e   % Obtain kinetic parameters for methane oxidation in the presence of H2O by % TPMO data % multiresponse data: % x is the indep varaibale vector e.g. time measurements % y is matrix of responses % columns of y are responses y1, y2 (e.g. mol frac of component 1 and 2) % rows of y are y values at the value of the indep variable (time) in x % first row of y is initial value of response % the program uses the Levenberg-Marquardt method to estimate parameters % and calc statistics - done in leasqr and dfdp % these two matlab m-files are designed for single repsonse % the input data is re-aarnaged to yoied a single respone vector y % the L-M requires the model to be calculated -this is done in modelmulti.m % and assume sthe model is a series of ODEs, with the number of odes equalt  % to the number of responses.  The ODEs are calcualte din ODEfunm.  Note that % this function must use teh correct model for each y % % input number of responses % nvar=1; % 234  % NO BD layer % x1_7c_0h = xlsread('input_data_temp_convBD_H.xlsx','0_H2O','a5:a50');    % Temperature, K....first indep vari x2_7c_0h = xlsread('input_data_temp_convBD_H.xlsx','0_H2O','e5:e50');    % H2O partial pressure, kpa,...second indep vari x3_7c_0h = xlsread('input_data_temp_convBD_H.xlsx','0_H2O','f5:f50');    % CH4 volume fraction x1_7c_2h = xlsread('input_data_temp_convBD_H.xlsx','2_H2O','a5:a50');    % Temperature, K....first indep vari x2_7c_2h = xlsread('input_data_temp_convBD_H.xlsx','2_H2O','e5:e50');    % H2O partial pressure, kpa,...second indep vari x3_7c_2h = xlsread('input_data_temp_convBD_H.xlsx','2_H2O','f5:f50');    % CH4 volume fraction% x1_5c_2h = xlsread('input_data_temp_conv.xlsx','2_H2O','a5:a60');    % Temperature, K....first indep vari x1_7c_5h = xlsread('input_data_temp_convBD_H.xlsx','5_H2O','a5:a50');    % Temperature, K....first indep vari x2_7c_5h = xlsread('input_data_temp_convBD_H.xlsx','5_H2O','e5:e50');    % H2O partial pressure, kpa,...second indep vari x3_7c_5h = xlsread('input_data_temp_convBD_H.xlsx','5_H2O','f5:f50');    % CH4 volume fraction% x1_5c_2h = xlsread('input_data_temp_conv.xlsx','2_H2O','a5:a60');    % Temperature, K....first indep vari  x1=[x1_7c_0h;x1_7c_2h;x1_7c_5h]; x2=[x2_7c_0h;x2_7c_2h;x2_7c_5h]; x3=[x3_7c_0h;x3_7c_2h;x3_7c_5h];   x = [x1 x2 x3]; oldx = x; nx = length(x); % %NO BD LAYER % y_7c_0h = xlsread('input_data_temp_convBD_H.xlsx','0_H2O','j5:j50'); 235  y_7c_2h = xlsread('input_data_temp_convBD_H.xlsx','2_H2O','j5:j50'); y_7c_5h = xlsread('input_data_temp_convBD_H.xlsx','5_H2O','j5:j50');   y=[y_7c_0h;y_7c_2h;y_7c_5h]; newy = y(:)./100; oldy = reshape(newy,nx,nvar); newx = x; %  %INPUT DATA NOW IN CORRECT COLUMN FORMAT %   x = newx y = newy %        %  provide initial parameter guesses % % pin(1) = frequency factor, mol/min/gcat/kpa^1+a % pin(2) = activation energy, kJ/mol   theta = [0.20 -50.0 0.1 -15.0  -0.5  -10.0];                                             np = length(theta) pin = theta   % % Begin calculation by calling L-M leat squares routine % [f,p,kvg,iter,corp,covp,covr,stdresid,Z,r2]=leasqr(x,y,pin,'modelmultiHKSDe',0.1); disp('RESPONSE:') if kvg ==1     disp ('PROBELM CONVERGED')     elseif kvg == 0     disp('PROBLEM DID NOT CONVERGE') end   oldf=reshape(f,nx,nvar); 236  oldr=reshape(y-f, nx, nvar);       disp ('X-values:')     disp (oldx)      disp ('Y-values')     disp(oldy)      disp('f-values - i.e. model calculated responses')     disp(oldf)     disp('Residuals:')     disp (oldr)     disp ('Final SSQ')     disp (stdresid)     disp ('Estimated parameter values are;')     disp (p)     disp ('Covariance of estimated parameters')     disp (covp)     disp('R2 values is:')     disp (r2)    subplot(1,3,1),    plot (oldx(1:46,1),oldy(1:46),'d'), hold, plot (oldx(1:46,1),oldf(1:46))     title('0.07% CH4, 0% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('CH4 conversion,a.u.')     subplot(1,3,2), plot (oldx(47:92,1),oldy(47:92),'d'), hold, plot (oldx(47:92,1),oldf(47:92))     title('0.07% CH4, 2% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('CH4 conversion,a.u.')     subplot(1,3,3),     plot (oldx(93:138,1),oldy(93:138),'d'), hold, plot (oldx(93:138,1),oldf(93:138))     title('0.07% CH4, 5% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('CH4 conversion,a.u.')       237    nx1=length (x1); nx3=length(x3); %eta0=zeros(1,50);   %  Using Hayes etal Appl. Catal. B: 25(2000) 93-104 to calcuate eta  % % Assuming CH4 in N2 % R = 0.008314; % Gas constant, kJ/mol.K epsilon = 0.67;  %Bed voidage dpore=1.8e-9;   % pore diameter - assuming 11 nm; Hamads BET data ai=12000; % area per mass of washcoat, m2/kg db = 168.0; % Bed density, kgcat/m3 dwash = 1.34e3; % washcoat density of catalyst, kg/m3  Rt = 4.7e-3; % radius of reactor, m Lc = 3.5e-5; % Thickness of washcoat of reactor, m %ac = 6*(1-epsilon)/3.5e-5; % external surface area/volume of solids, m2/m3 vis = 3.87e-5; % dynamic viscosity...assumed const for all temperatures, kg/m/s tf = 8; % tortuosity factor MCH4=16.0; MN2=28.0; epsi= 3.778; % equation (12) Mwtterm= sqrt((MCH4+MN2)/(MCH4*MN2)); phi = 2.17e-7-0.5e-7*(Mwtterm); % % constants for omega - equn 14 % At=0.06036; Bt = 0.15610; Ct = 0.19300; Dt = 0.47635; Et = 1.03587; Ft = 1.52996; Gt = 1.76474; 238  Ht = 3.89411;   v0 = 1000; %Total volumetric feed rate, cm3/min Po20 = 0.0867*101.325; %Feed O2, assuming CO is consumed, kPa Pco20=0.0706*101.325; %Feed CO2, assuming CO is consumed a0=exp(pin(1)); b0=exp(pin(3)); c0=exp(pin(5)); Pterm=zeros(1,100000);Deff=zeros(1,100000); k1=zeros(1, 100000); lograte=zeros(1,100000);invt=zeros(1,100000);thi=zeros(1,100000); eta0=zeros(1,100000);pmi=zeros(1,100000);  for i=1:nx1   Fm0 = x3(i)*v0/22414; % Feed CH4 flow, mol/min Pm0 = x3(i)*101.325; % Feed CH4 partial pressure, kpa Mwfeed = 34.21; % molecular weight of feed, g/mol   a = a0*exp(-1*pin(2)/R*(1/x1(i)-1/650));  % rate constant, mol/min/gcat/kPa b = b0*exp(-1*pin(4)/R*(1/x1(i)-1/650)); c = b0*exp(-1*pin(6)/R*(1/x1(i)-1/650)); kr=1/a; Kch4=b*kr; KH2O=c*kr;   Pterm = Pm0*(1-y(i))*(Po20/Pm0-2*y(i))/(Pco20/Pm0+2*y(i));   k1(i) = kr/60*1000*0.008314*x1(i)/(1+Kch4*Pterm)/(1+KH2O*(x2(i)/Pm0+2*y(i))); % rate constant, m3/kgcat/s   % % 1st -oder apparent constant % k4e = k1*(Po20/Pm0-2*y(i))/(Pco20/Pm0+2*y(i)); lograte(i)=log(k4e(i)); 239  invt(i)=1/x1(i)-1/650;   Tstar= 9.708e-3*x1(i); D1 = (Tstar)^Bt; D2 = exp(Dt*Tstar); D3 = exp(Ft*Tstar); D4 = exp(Ht*Tstar); omega = At/D1+Ct/D2+Et/D3+Gt/D4; Dbulk = phi*(x1(i)^1.5)*Mwtterm/1/(epsi^2)/omega;   Dk = 48.5*dpore*(x1(i)/Mwtterm)^0.5; % Knudsen diffusion, m2/s  Equn 16 DMi=1/Dbulk+1/Dk ; DM = 1/DMi; Deff(i) = epsilon*DM/tf; % effective  diffusion, m2/s   thi(i) = Lc*sqrt(k4e(i)*dwash/Deff(i)); % Thiele modulus eta0(i) = tanh(thi(i))/thi(i); % internal effectiveness factor   end   figure  subplot(1,3,1), plot (x1(1:46,1),eta0(1:46),'d')     title('0.07% CH4, 0% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('Overall Effectiveness Factor')     subplot(1,3,2), plot (x1(47:92,1),eta0(47:92),'s')     title('0.07% CH4, 2% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('Overall Effectiveness Factor')     subplot(1,3,3), plot (x1(93:138,1),eta0(93:138),'d')     title('0.07% CH4, 5% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('Overall Effectiveness Factor')        240           figure    plot (lograte,invt,'d')          figure  subplot(1,3,1), plot (x1(1:46,1),k4e(1:46),'d')     title('0.07% CH4, 0% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('k4e')     subplot(1,3,2), plot (x1(47:92,1),k4e(47:92),'s')     title('0.07% CH4, 2% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('k4e')     subplot(1,3,3), plot (x1(93:138,1),k4e(93:138),'d')     title('0.07% CH4, 5% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('k4e')        figure  subplot(1,3,1), plot (x1(1:46,1),k1(1:46),'d')     title('0.07% CH4, 0% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('k1')     subplot(1,3,2), plot (x1(47:92,1),k1(47:92),'s')     title('0.07% CH4, 2% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('k1')     subplot(1,3,3), plot (x1(93:138,1),k1(93:138),'d')     title('0.07% CH4, 5% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('k1') figure  subplot(1,3,1), plot (x1(1:46,1),Deff(1:46),'d')     title('0.07% CH4, 0% H2O, 1012sccm')     xlabel('Temperature, K') 241      ylabel('Deff')     subplot(1,3,2), plot (x1(47:92,1),Deff(47:92),'s')     title('0.07% CH4, 2% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('Deff')     subplot(1,3,3), plot (x1(93:138,1),Deff(93:138),'d')     title('0.07% CH4, 5% H2O, 1012sccm')     xlabel('Temperature, K')     ylabel('Deff')    The objective function solves the mole balance equation by calling ODEfunm.m through ODE45 function as follows: function f = modelmultiHKSDe(x,pin) % find the solution at sepcified x values - corresponding to measured data % Temp = x(:,1)...Operatig temperature, K % Pw0 = x(:,2)...H2O partial pressure, kpa % param = [Temp ym0 Pw0 pin(1) pin(2) pin(3) pin(4)...] global effect param = [1 0 0 pin(1) pin(2) pin(3) pin(4) pin(5)  pin(6) 0]; % param = [1 0 0 pin(1) pin(2)];   yzero = 0.0000000; nxx=length(x); for i = 1:nxx         param(1) = x(i,1); % Temperature, K     param(2) = x(i,3); % CH4 volume fraction     param(3) = x(i,2); % Pw0, kpa     [xmodel,ymodel] = ode45 (@ODEfunm_0BD_HKSDe,[0,0.1],yzero,[],param);     yfinal(i,:)=ymodel(end,:);    end  f = yfinal(:);   242  Mole balance incorporated with mass transfer calculations are defined in the following function   function dXdw=ODEfunm_0BD_HKSDe(w,X,param) % param = [Temp ym0 Pw0 pin(1) pin(2) pin(3) pin(4)...] % system properties % % %  Using Hayes etal Appl. Catal. B: 25(2000) 93-104 to calcuate eta  % % Assumin CH4 in N2 % R = 0.008314; % Gas constant, kJ/mol.K epsilon = 0.67;  %Bed voidage dpore=1.8e-9;   % pore diameter - assuming 18nm - Hamads BET data ai=12000; % area per mass of washcoat, m2/kg db = 168.0; % Bed density, kgcat/m3 dwash = 1.34e3; % washcoat denisty, kg/m3  Rt = 4.7e-3; % radius of reactor, m Lc = 3.5e-5; % Thickness of washcoat of reactor, m %ac = 6*(1-epsilon)/3.5e-5; % external surface area/volume of solids, m2/m3 vis = 3.87e-5; % dynamic viscosity...assumed const for all temperatures, kg/m/s tf = 8; % tortuosity factor MCH4=16.0; MN2=28.0; epsi= 3.778; % equation (12) Mwtterm= sqrt((MCH4+MN2)/(MCH4*MN2)); phi = 2.17e-7-0.5e-7*(Mwtterm); % % constants for omega - equn 14 % At=0.06036; Bt = 0.15610; Ct = 0.19300; Dt = 0.47635; Et = 1.03587; Ft = 1.52996; Gt = 1.76474; 243  Ht = 3.89411;   v0 = 1000; %Total volumetric feed rate, cm3/min Fm0 = param(2)*v0/22414; % Feed CH4 flow, mol/min Pm0 = param(2)*101.325; % Feed CH4 partial pressure, kpa Po20 = 0.0867*101.325; %Feed O2, assuming CO is consumed, kPa Pco20=0.0706*101.325; %Feed CO2, assuming CO is consumed   a0=exp(param(4)); b0=exp(param(6)); c0=exp(param(8));   a = a0*exp(-1*param(5)/R*(1/param(1)-1/650));  % rate constant, mol/min/gcat/kPa b = b0*exp(-1*param(7)/R*(1/param(1)-1/650)); c = c0*exp(-1*param(9)/R*(1/param(1)-1/650)); kr=1/a; Kch4=b*kr; KH2O=c*kr;   % conc terms for this model % xterm = Pm0*(1-X)*(Po20/Pm0-2*X)/(Pco20/Pm0+X); k1 = kr/60*1000*0.008314*param(1)/(1+Kch4*xterm)/(1+KH2O*(param(3)/Pm0+2*X)); % reaction rate, m3/kgcat/s % % 1st -oder apparent constant % k4eta = k1*(Po20/Pm0-2*X)/(Pco20/Pm0+X); %k1 = kr/60*1000*0.008314*param(1)/(1+Kch4*Pm0*(1-X));   Tstar= 9.708e-3*param(1); D1 = (Tstar)^Bt; D2 = exp(Dt*Tstar); D3 = exp(Ft*Tstar); D4 = exp(Ht*Tstar); 244  omega = At/D1+Ct/D2+Et/D3+Gt/D4; Dbulk = phi*(param(1)^1.5)*Mwtterm/1/(epsi^2)/omega;   Dk = 48.5*dpore*(param(1)/Mwtterm)^0.5; % Knudsen diffusion, m2/s  Equn 16 DMi=1/Dbulk+1/Dk ; DM = 1/DMi; Deff = epsilon*DM/tf; % effective  diffusion, m2/s   thi = Lc*sqrt(k4eta*dwash/Deff); % Thiele modulus eta = tanh(thi)/thi; % internal effectiveness factor   eta0=eta; % Set eta yo unity for now   dXdw = eta0/Fm0*xterm/(a+b*xterm)/(a+c*(param(3)/Pm0+2*X));       Levenberg-Marquardt nonlinear regression m-files are as follows: function [f,p,kvg,iter,corp,covp,covr,stdresid,Z,r2]= ...       leasqr(x,y,pin,F,stol,niter,wt,dp,dFdp,options) %function[f,p,kvg,iter,corp,covp,covr,stdresid,Z,r2]= %                   leasqr(x,y,pin,F,{stol,niter,wt,dp,dFdp,options}) % % Version 3.beta %  {}= optional parameters % Levenberg-Marquardt nonlinear regression of f(x,p) to y(x), where: % x=vec or mat of indep variables, 1 row/observation: x=[x0 x1....xm] % y=vec of obs values, same no. of rows as x. % wt=vec(dim=length(x)) of statistical weights.  These should be set %   to be proportional to (sqrt of var(y))^-1; (That is, the covariance %   matrix of the data is assumed to be proportional to diagonal with diagonal %   equal to (wt.^2)^-1.  The constant of proportionality will be estimated.), %   default=ones(length(y),1). % pin=vector of initial parameters to be adjusted by leasqr. 245  % dp=fractional incr of p for numerical partials,default= .001*ones(size(pin)) %   dp(j)>0 means central differences. %   dp(j)<0 means one-sided differences. % Note: dp(j)=0 holds p(j) fixed i.e. leasqr wont change initial guess: pin(j) % F=name of function in quotes,of the form y=f(x,p) % dFdp=name of partials M-file in quotes default is prt=dfdp(x,f,p,dp,F) % stol=scalar tolerances on fractional improvement in ss,default stol=.0001 % niter=scalar max no. of iterations, default = 20 % options=matrix of n rows (same number of rows as pin) containing %   column 1: desired fractional precision in parameter estimates. %     Iterations are terminated if change in parameter vector (chg) on two %     consecutive iterations is less than their corresponding elements %     in options(:,1).  [ie. all(abs(chg*current parm est) < options(:,1)) %      on two consecutive iterations.], default = zeros(). %   column 2: maximum fractional step change in parameter vector. %     Fractional change in elements of parameter vector is constrained to be %     at most options(:,2) between sucessive iterations. %     [ie. abs(chg(i))=abs(min([chg(i) options(i,2)*current param estimate])).], %     default = Inf*ones(). % %          OUTPUT VARIABLES % f=vec function values computed in function func. % p=vec trial or final parameters. i.e, the solution. % kvg=scalar: =1 if convergence, =0 otherwise. % iter=scalar no. of interations used. % corp= correlation matrix for parameters % covp= covariance matrix of the parameters % covr = diag(covariance matrix of the residuals) % stdresid= standardized residuals % Z= matrix that defines confidence region % r2= coefficient of multiple determination   % All Zero guesses not acceptable % Richard I. Shrager (301)-496-1122 246  % Modified by A.Jutan (519)-679-2111 % Modified by Ray Muzic 14-Jul-1992 %       1) add maxstep feature for limiting changes in parameter estimates %          at each step. %       2) remove forced columnization of x (x=x(:)) at beginning. x could be %          a matrix with the ith row of containing values of the %          independent variables at the ith observation. %       3) add verbose option %       4) add optional return arguments covp, stdresid, chi2 %       5) revise estimates of corp, stdev % Modified by Ray Muzic 11-Oct-1992 %   1) revise estimate of Vy.  remove chi2, add Z as return values % Modified by Ray Muzic 7-Jan-1994 %       1) Replace ones(x) with a construct that is compatible with versions %          newer and older than v 4.1. %       2) Added global declaration of verbose (needed for newer than v4.x) %       3) Replace return value var, the variance of the residuals with covr, %          the covariance matrix of the residuals. %       4) Introduce options as 10th input argument.  Include %          convergence criteria and maxstep in it. %       5) Correct calculation of xtx which affects coveraince estimate. %       6) Eliminate stdev (estimate of standard deviation of parameter %          estimates) from the return values.  The covp is a much more %          meaningful expression of precision because it specifies a confidence %          region in contrast to a confidence interval..  If needed, however, %          stdev may be calculated as stdev=sqrt(diag(covp)). %       7) Change the order of the return values to a more logical order. %       8) Change to more efficent algorithm of Bard for selecting epsL. %       9) Tighten up memory usage by making use of sparse matrices (if %          MATLAB version >= 4.0) in computation of covp, corp, stdresid. % Modified by Sean Brennan 17-May-1994 %          verbose is now a vector: %          verbose(1) controls output of results %          verbose(2) controls plotting intermediate results % % References: 247  % Bard, Nonlinear Parameter Estimation, Academic Press, 1974. % Draper and Smith, Applied Regression Analysis, John Wiley and Sons, 1981. % %set default args   % argument processing %   plotcmd='plot(x(:,1),y,''+'',x(:,1),f); shg'; %if (sscanf(version,'%f') >= 4), vernum= sscanf(version,'%f'); if vernum(1) >= 4,   global verbose   plotcmd='plot(x(:,1),y,''+'',x(:,1),f); figure(gcf)'; end; if (exist('OCTAVE_VERSION'))   global verbose end;   if(exist('verbose')~=1), %If verbose undefined, print nothing     verbose(1)=0    %This will not tell them the results     verbose(2)=0    %This will not replot each loop end; if (nargin <= 8), dFdp='dfdp'; end; if (nargin <= 7), dp=.001*(pin*0+1); end; %DT if (nargin <= 6), wt=ones(length(y),1); end;    % SMB modification if (nargin <= 5), niter=20; end; if (nargin == 4), stol=.0001; end; %   y=y(:); wt=wt(:); pin=pin(:); dp=dp(:); %change all vectors to columns % check data vectors- same length? m=length(y); n=length(pin); p=pin;[m1,m2]=size(x); if m1~=m ,error('input(x)/output(y) data must have same number of rows ') ,end;   248  if (nargin <= 9),   options=[zeros(n,1) Inf*ones(n,1)];   nor = n; noc = 2; else   [nor noc]=size(options);   if (nor ~= n),     error('options and parameter matrices must have same number of rows'),   end;   if (noc ~= 2),     options=[options(noc,1) Inf*ones(noc,1)];   end; end; pprec=options(:,1); maxstep=options(:,2); %   % set up for iterations % f=feval(F,x,p); fbest=f; pbest=p; r=wt.*(y-f); sbest=r'*r; nrm=zeros(n,1); chgprev=Inf*ones(n,1); kvg=0; epsLlast=1; epstab=[.1 1 1e2 1e4 1e6];   % do iterations % for iter=1:niter,   pprev=pbest;   prt=feval(dFdp,x,fbest,pprev,dp,F);   r=wt.*(y-fbest);   sprev=sbest;   sgoal=(1-stol)*sprev;   for j=1:n, 249      if dp(j)==0,       nrm(j)=0;     else       prt(:,j)=wt.*prt(:,j);       nrm(j)=prt(:,j)'*prt(:,j);       if nrm(j)>0,         nrm(j)=1/sqrt(nrm(j));       end;     end     prt(:,j)=nrm(j)*prt(:,j);   end; % above loop could ? be replaced by: % prt=prt.*wt(:,ones(1,n)); % nrm=dp./sqrt(diag(prt'*prt)); % prt=prt.*nrm(:,ones(1,m))';   [prt,s,v]=svd(prt,0);   s=diag(s);   g=prt'*r;   for jjj=1:length(epstab),     epsL = max(epsLlast*epstab(jjj),1e-7);     se=sqrt((s.*s)+epsL);     gse=g./se;     chg=((v*gse).*nrm); %   check the change constraints and apply as necessary     ochg=chg;     for iii=1:n,       if (maxstep(iii)==Inf), break; end;       chg(iii)=max(chg(iii),-abs(maxstep(iii)*pprev(iii)));       chg(iii)=min(chg(iii),abs(maxstep(iii)*pprev(iii)));     end;      if (verbose(1) & any(ochg ~= chg)),        disp(['Change in parameter(s): ' ...           sprintf('%d ',find(ochg ~= chg)) 'were constrained']);      end;     aprec=abs(pprec.*pbest);       %--- % ss=scalar sum of squares=sum((wt.*(y-f))^2).     if (any(abs(chg) > 0.1*aprec)),%---  % only worth evaluating function if 250        p=chg+pprev;                       % there is some non-miniscule change       f=feval(F,x,p);       r=wt.*(y-f);       ss=r'*r;       if ss<sbest,         pbest=p;         fbest=f;         sbest=ss;       end;       if ss<=sgoal,         break;       end;     end;                          %---   end;   epsLlast = epsL; %   if (verbose(2)), %     eval(plotcmd); %   end;   if ss<eps,     break;   end   aprec=abs(pprec.*pbest); %  [aprec chg chgprev]   if (all(abs(chg) < aprec) & all(abs(chgprev) < aprec)),     kvg=1;     if (verbose(1)),       fprintf('Parameter changes converged to specified precision\n');     end;     break;   else     chgprev=chg;   end;   if ss>sgoal,     break;   end; end;   251  % set return values % p=pbest; f=fbest; ss=sbest; kvg=((sbest>sgoal)|(sbest<=eps)|kvg); if kvg ~= 1 , disp(' CONVERGENCE NOT ACHIEVED! '), end;   % CALC VARIANCE COV MATRIX AND CORRELATION MATRIX OF PARAMETERS % re-evaluate the Jacobian at optimal values jac=feval(dFdp,x,f,p,dp,F); msk = dp ~= 0; n = sum(msk);           % reduce n to equal number of estimated parameters jac = jac(:, msk);  % use only fitted parameters   %% following section is Ray Muzic's estimate for covariance and correlation %% assuming covariance of data is a diagonal matrix proportional to %% diag(1/wt.^2). %% cov matrix of data est. from Bard Eq. 7-5-13, and Row 1 Table 5.1   if vernum(1) >= 4,   Q=sparse(1:m,1:m,(0*wt+1)./(wt.^2));  % save memory   Qinv=inv(Q); else   Qinv=diag(wt.*wt);   Q=diag((0*wt+1)./(wt.^2)); end; resid=y-f;                                    %un-weighted residuals covr=resid'*Qinv*resid*Q/(m-n);                 %covariance of residuals Vy=1/(1-n/m)*covr;  % Eq. 7-13-22, Bard         %covariance of the data   jtgjinv=inv(jac'*Qinv*jac);         %argument of inv may be singular covp=jtgjinv*jac'*Qinv*Vy*Qinv*jac*jtgjinv; % Eq. 7-5-13, Bard %cov of parm est d=sqrt(abs(diag(covp))); corp=covp./(d*d'); 252    covr=diag(covr);                 % convert returned values to compact storage stdresid=resid./sqrt(diag(Vy));  % compute then convert for compact storage Z=((m-n)*jac'*Qinv*jac)/(n*resid'*Qinv*resid);   %%% alt. est. of cov. mat. of parm.:(Delforge, Circulation, 82:1494-1504, 1990 %%disp('Alternate estimate of cov. of param. est.') %%acovp=resid'*Qinv*resid/(m-n)*jtgjinv   %Calculate R^2 (Ref Draper & Smith p.46) % r=corrcoef(y,f); if (exist('OCTAVE_VERSION'))   r2=r^2; else   r2=r(1,2).^2; end   % if someone has asked for it, let them have it %  if (verbose(2)), eval(plotcmd); end,  if (verbose(1)),    disp(' Least Squares Estimates of Parameters')    disp(p')    disp(' Correlation matrix of parameters estimated')    disp(corp)    disp(' Covariance matrix of Residuals' )    disp(covr)    disp(' Correlation Coefficient R^2')    disp(r2)    sprintf(' 95%% conf region: F(0.05)(%.0f,%.0f)>= delta_pvec''*Z*delta_pvec',n,m-n)    Z %   runs test according to Bard. p 201.   n1 = sum((f-y) < 0); 253    n2 = sum((f-y) > 0);   nrun=sum(abs(diff((f-y)<0)))+1;   if ((n1>10)&(n2>10)), % sufficent data for test?     zed=(nrun-(2*n1*n2/(n1+n2)+1)+0.5)/(2*n1*n2*(2*n1*n2-n1-n2)...       /((n1+n2)^2*(n1+n2-1)));     if (zed < 0),       prob = erfc(-zed/sqrt(2))/2*100;       disp([num2str(prob) '% chance of fewer than ' num2str(nrun) ' runs.']);     else,       prob = erfc(zed/sqrt(2))/2*100;       disp([num2str(prob) '% chance of greater than ' num2str(nrun) ' runs.']);     end;   end; end   % A modified version of Levenberg-Marquardt % Non-Linear Regression program previously submitted by R.Schrager. % This version corrects an error in that version and also provides % an easier to use version with automatic numerical calculation of % the Jacobian Matrix. In addition, this version calculates statistics % such as correlation, etc.... % % Version 3 Notes % Errors in the original version submitted by Shrager (now called version 1) % and the improved version of Jutan (now called version 2) have been corrected. % Additional features, statistical tests, and documentation have also been % included along with an example of usage.  BEWARE: Some the the input and % output arguments were changed from the previous version. % %     Ray Muzic     <rfm2@ds2.uh.cwru.edu> %     Arthur Jutan  <jutan@charon.engga.uwo.ca>     254  function prt=dfdp(x,f,p,dp,func) % numerical partial derivatives (Jacobian) df/dp for use with leasqr % --------INPUT VARIABLES--------- % x=vec or matrix of indep var(used as arg to func) x=[x0 x1 ....] % f=func(x,p) vector initialsed by user before each call to dfdp % p= vec of current parameter values % dp= fractional increment of p for numerical derivatives %      dp(j)>0 central differences calculated %      dp(j)<0 one sided differences calculated %      dp(j)=0 sets corresponding partials to zero; i.e. holds p(j) fixed % func=string naming the function (.m) file %      e.g. to calc Jacobian for function expsum prt=dfdp(x,f,p,dp,'expsum') %----------OUTPUT VARIABLES------- % prt= Jacobian Matrix prt(i,j)=df(i)/dp(j) %================================ m=length(x);n=length(p);      %dimensions ps=p; prt=zeros(m,n);del=zeros(n,1);       % initialise Jacobian to Zero for j=1:n       del(j)=dp(j) .*p(j);    %cal delx=fract(dp)*param value(p)            if p(j)==0            del(j)=dp(j);     %if param=0 delx=fraction            end       p(j)=ps(j) + del(j);       if del(j)~=0, f1=feval(func,x,p);            if dp(j) < 0, prt(:,j)=(f1-f)./del(j);            else            p(j)=ps(j)- del(j);            prt(:,j)=(f1-feval(func,x,p))./(2 .*del(j));            end       end       p(j)=ps(j);     %restore p(j) end return  

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