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Kinetic and deactivation studies of methane oxidation over palladium catalysts in the presence of water Gholami Shahrestani, Rahman 2015

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  Kinetic and Deactivation Studies of Methane Oxidation over Palladium Catalysts in the Presence of Water  by  Rahman Gholami Shahrestani M.Sc., Isfahan University of Technology, 2007 B.Sc., Ferdowsi University of Mashhad, 2005  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)  October 2015  © Rahman Gholami Shahrestani, 2015 ii  Abstract  Natural gas vehicles (NGVs) generate considerably fewer emissions of CO, NOx and CO2 in comparison to conventional diesel and gasoline vehicles, although these benefits are mitigated by the presence of significant amounts of CH4 in the exhaust. The relatively low temperature (423–823 K) and high concentrations of CO2 and H2O in the NGV exhaust gas make current catalytic converters inefficient for the removal of unburned CH4. Although Pd is the most active metal for CH4 oxidation, Pd catalysts deactivate after long time exposure to the NGV exhaust conditions. This thesis develops an understanding of the deactivation mechanisms of Pd supported catalysts following thermal treatments and examines the kinetics of CH4 oxidation, accounting for the effect of H2O.  Hydrothermal aging (HTA) at high temperatures (673–973 K) is shown to significantly deactivate PdO/SiO2 catalysts used for CH4 oxidation. PdO occlusion by the SiO2 support is responsible for catalyst deactivation at low HTA temperatures (673 K), whereas a combination of PdO sintering and PdO occlusion contributes to significant deactivation at high HTA temperatures (973 K). The stability of PdO catalysts during HTA is dependent upon the support. PdO/α-Al2O3 is found to have the highest catalyst stability during HTA at 973 K for up to 65 h and its high stability is attributed to a strong Pd-support interaction. Although PdO crystallites sinter and are occluded by the support during HTA, PdO occlusion only affects PdO/SiO2 performance significantly. The deactivation of PdO/γ-Al2O3 and PdO/SnO2 during HTA at 973 K is attributed to PdO sintering and a PdO  Pd0 transformation.   iii  The kinetics of CH4 oxidation over a PdO/γ-Al2O3 catalyst is also reported. A power law model can accurately predict the observed temperature-programmed CH4 oxidation data profiles measured for PdO/γ-Al2O3 at conversions < 40% when 0.1 v.% CH4 with no H2O or 0.5 v.% CH4 with ≤ 2 v.% H2O are present in the feed. At high H2O concentration, model fit is improved with 0.1 v.% CH4 in the feed but shows significant deviation with 0.5 v.% CH4 in the feed.                    iv  Preface  This thesis includes eight chapters. Versions of Chapter 2 and 4 were published in peer reviewed journals and a version of Chapter 5 was submitted for publication in a peer reviewed journal. Chapter 6 and 7 are in preparation in manuscript format to be submitted in peer review journals.   Literature review, catalyst preparation, catalyst characterization, catalyst aging and durability experiments, catalyst testing experiments and kinetic experiments and modeling were all conducted by Rahman Gholami Shahrestani under supervision of Professor Kevin J. Smith in Department of Chemical and Biological Engineering at the University of British Columbia. Thesis writing and papers for publication in peer reviewed journals were all done by Rahman Gholami Shahrestani under supervision and final approval of Professor Kevin J. Smith in Department of Chemical and Biological Engineering at the University of British Columbia.  The list of publications and author contributions are shown as follows:  Rahman Gholami, Kevin J. Smith, Activity of PdO/SiO2 Catalysts for CH4 Oxidation Following Thermal Treatments. Applied Catalysis B: Environmental 168 (2015) 156-163.  The manuscript was prepared and written by Rahman Gholami Shahrestani under supervision and final approval of Professor Kevin J. Smith.  v  Rahman Gholami, Mina Alyani, Kevin J. Smith, Deactivation of Pd Catalysts by Water during Low Temperature Methane Oxidation Relevant to Natural Gas Vehicle Converters. Catalysts 5 (2015) 561-594.  This article was published as part of a special issue in Catalysts journal. Sections 2, 2.1, 2.2, 2.4 and 4 of that paper were prepared and written by Rahman Gholami Shahrestani under supervision and final approval of Professor Kevin J. Smith and versions of these sections were included in Chapter 2. Sections 2.3 and 3 of that paper were prepared and written by Mina Alyani under supervision and final approval of Professor Kevin J. Smith. All authors contributed equally to sections 1, 5 and abstract of that paper.                 vi  Table of Contents  Abstract ........................................................................................................................................... ii Preface............................................................................................................................................ iv Table of Contents ........................................................................................................................... vi List of Tables ................................................................................................................................ xii List of Figures .............................................................................................................................. xvi Nomenclature .............................................................................................................................. xxv List of Abbreviations ................................................................................................................. xxxi Acknowledgements .................................................................................................................. xxxiv Chapter 1 Introduction .................................................................................................................... 1 1.1 Objectives ....................................................................................................................... 8 1.2 Approach ......................................................................................................................... 9 Chapter 2 Literature Review ......................................................................................................... 12 2.1 Effect of H2O on CH4 Oxidation over Pd Catalysts at Low to Moderate Temperatures ..   ....................................................................................................................................... 13 2.1.1 H2O Concentration and Reaction Temperature Effects on CH4 Oxidation Activity of Pd Catalysts ...................................................................................................................... 13 2.1.2 H2O Inhibition and Hydroxyl Formation ................................................................ 21 2.2 Catalyst Sintering through Hydrothermal Aging of Pd Catalysts ................................. 30 vii  2.3 Support Effects.............................................................................................................. 35 2.4 Kinetic Consequences of H2O on CH4 Oxidation over Pd Catalysts............................ 40 2.5 Summary ....................................................................................................................... 45 Chapter 3 Experimental ................................................................................................................ 48 3.1 Catalyst Preparation ...................................................................................................... 48 3.2 Catalyst Characterization .............................................................................................. 49 3.2.1 N2 Adsorption-desorption ....................................................................................... 49 3.2.2 Atomic Absorption Spectroscopy ........................................................................... 50 3.2.3 CO Chemisorption .................................................................................................. 50 3.2.4 Temperature-programmed Oxidation ..................................................................... 51 3.2.5 X-ray Diffraction .................................................................................................... 51 3.2.6 X-ray Photoelectron Spectroscopy ......................................................................... 52 3.2.7 Transmission Electron Microscopy ........................................................................ 52 3.2.8 Raman Spectroscopy ............................................................................................... 53 3.2.9 Thermal Gravimetric Analysis ................................................................................ 53 3.3 Catalyst Testing ............................................................................................................ 53 3.3.1 Experimental Setup ................................................................................................. 53 3.3.2 Temperature-programmed CH4 Oxidation ............................................................. 55 3.3.3 Hydrothermal or Thermal Aging ............................................................................ 56 3.3.4 Kinetic Experiments................................................................................................ 57 viii  Chapter 4 Activity of PdO/SiO2 Catalysts for CH4 Oxidation Following Thermal Treatments .. 58 4.1 Introduction ................................................................................................................... 58 4.2 Results ........................................................................................................................... 59 4.2.1 Catalyst Aging in Air and Air/H2O ......................................................................... 59 4.2.2 Effect of Pd Loading ............................................................................................... 70 4.3 Discussion ..................................................................................................................... 74 4.4 Conclusions ................................................................................................................... 80 Chapter 5 Effect of Hydrothermal Aging Conditions on the Activity of a PdO/SiO2 Catalyst for CH4 Oxidation ............................................................................................................................... 82 5.1 Introduction ................................................................................................................... 82 5.2 Results ........................................................................................................................... 82 5.3 Discussion ..................................................................................................................... 93 5.4 Conclusions ................................................................................................................... 97 Chapter 6 Effect of Oxide Supports on the Activity of PdO Catalysts during Hydrothermal Aging in CH4 Oxidation........................................................................................................................... 98 6.1 Introduction ................................................................................................................... 98 6.2 Results ........................................................................................................................... 98 6.2.1 HTA Effect on PdO/γ-Al2O3 versus PdO/SiO2 for CH4 Oxidation ...................... 100 6.2.2 Effect of HTA on Low Surface Area PdO Catalysts for CH4 Oxidation.............. 107 6.3 Discussion ................................................................................................................... 115 ix  6.4 Conclusions ................................................................................................................. 121 Chapter 7 Kinetics of CH4 Oxidation over PdO Catalysts in the Presence of Water ................. 122 7.1 Introduction ................................................................................................................. 122 7.2 Kinetic Models ............................................................................................................ 123 7.3 Reactor Modeling........................................................................................................ 127 7.3.1 Differential Mode.................................................................................................. 127 7.3.2 Integral Mode ........................................................................................................ 127 7.4 Optimization Method .................................................................................................. 130 7.5 Kinetic Model Discrimination .................................................................................... 132 7.6 Results ......................................................................................................................... 132 7.6.1 Effect of Reactants and H2O Concentration on CH4 Conversion ......................... 134 7.6.2 Differential Experiments and Intrinsic Kinetic Rate ............................................ 137 7.6.3 Prediction of TPMO Data Using Differential Kinetic Parameters ....................... 141 7.6.4 TPMO Data and Re-estimating the Kinetic Parameters ....................................... 145 7.7 Discussion ................................................................................................................... 154 7.8 Conclusions ................................................................................................................. 158 Chapter 8 Conclusions and Recommendations ........................................................................... 159 8.1 Conclusions ................................................................................................................. 159 8.2 Recommendations ....................................................................................................... 161 8.2.1 Kinetic Model Development to Predict the TPMO Data Profiles ........................ 161 x  8.2.2 Catalyst Aging Durability Studies in a Prototype Reactor ................................... 162 8.2.3 Kinetic Model Development in a Prototype Reactor ............................................ 162 Bibliography ............................................................................................................................... 163 Appendices .................................................................................................................................. 180 Appendix A Catalyst Characterization ....................................................................................... 181 A.1 BET ............................................................................................................................. 181 A.2 XRD ............................................................................................................................ 182 Appendix B Reaction System ..................................................................................................... 183 B.1 Blank Run ................................................................................................................... 183 B.2 CH4 Conversion Calculation and Catalyst Testing Experiment Analysis .................. 184 B.3 Repeatability of TPMO Results .................................................................................. 195 B.4 Repeatability and Error Analysis of Differential Results ........................................... 198 Appendix C Mass Spectrometer Calibration .............................................................................. 202 Appendix D Plug Flow Operation .............................................................................................. 205 Appendix E Isothermal Operation .............................................................................................. 208 Appendix F Mass Transfer Effects ............................................................................................. 212 F.1 Theoretical Calculations ............................................................................................. 212 F.2 Diagnostic Tests .......................................................................................................... 217 Appendix G Supplementary Figures for Chapter 6 .................................................................... 219 Appendix H Kinetics................................................................................................................... 221 xi  H.1 Supplementary Information ........................................................................................ 221 H.2 MATLAB M-files for Estimating Kinetic Parameters Using Differential Data ......... 232 H.3 MATLAB Codes for Predicting the TPMO Data Profiles Using Differential Kinetic Parameters ............................................................................................................................... 245 H.4 MATLAB M-files for Estimating the Kinetic Parameters Using TPMO Data Profiles ...   ..................................................................................................................................... 249                  xii  List of Tables  Table ‎1.1. Alternative fuel prices in the USA (adapted from [6]) .................................................. 1 Table ‎1.2. Light-duty vehicle exhaust emissions standards in the USA (adapted from [12, 14]). . 4 Table ‎2.1. Effect of H2O addition on T30 (temperature at 30% CH4 conversion) during temperature-programmed CH4 oxidation over Pd catalysts. ...................................... 15 Table ‎2.2. H2O inhibition over Pd catalysts during CH4 oxidation............................................... 16 Table ‎2.3. Effect of acid strength of support materials on CH4 oxidation activity of Pd catalysts (adapted from [56]) .................................................................................................... 37 Table ‎2.4. Kinetic parameters for CH4 oxidation over Pd catalysts. ............................................ 42 Table ‎2.5. Estimated kinetic parameters for CH4 oxidation using the rate equation                                        [22]............................................................................................... 44 Table ‎4.1. Thermal aging effects on the properties of 7.7 wt.% Pd/SiO2. .................................... 63 Table ‎4.2. HTA effect on surface properties; Pd/SiO2, pre-oxidized at 723 K in air 15 h (Fresh), aged at 973 K in 6.5 v.% H2O in air for 16 h (Aged). ............................................... 71 Table ‎5.1. HTA temperature and time effects on activity and properties of 6.8 wt.% PdO/SiO2 catalysts. ..................................................................................................................... 86 Table ‎6.1. Properties of catalysts used in this study. .................................................................... 99 Table ‎6.2. A list of B.E.s and Pd0/PdO ratios of fitted peaks presented in Figure ‎6.6‎for‎PdO/γ-Al2O3 as a function of HTA time. ............................................................................ 107 Table ‎6.3.‎The‎fresh‎and‎HTA‎PdO/γ-Al2O3,‎PdO/α-Al2O3 and PdO/SnO2 catalyst properties and then activities for CH4 oxidation. ............................................................................. 110 xiii  Table ‎6.4. A list of B.E.s of fitted peaks presented in Figure  ‎G.1‎and‎G.2‎for‎PdO/γ-Al2O3 and PdO/SnO2 before and after HTA. ............................................................................ 115 Table ‎7.1. Proposed rate expressions .......................................................................................... 126 Table ‎7.2.‎Catalyst‎properties‎of‎PdO/γ-Al2O3 ........................................................................... 134 Table ‎7.3. Calculated kinetic parameters for various kinetic models by fitting to differential data .................................................................................................................................. 140 Table ‎7.4. Calculated kinetic parameters for power law models by fitting to TPMO data. ....... 148 Table ‎7.5. Calculated kinetic parameters for various kinetic models by fitting to TPMO data. 149 Table ‎7.6. Calculated kinetic parameters for MVK kinetic models by fitting to TPMO data .... 150  Table ‎B.1.‎ Experimental‎ analysis‎ for‎ TPMO‎ of‎ 0.8‎ wt.%‎ Pd/γ-Al2O3 catalyst; Reaction condition: 0.1 v.% CH4, 20% O2 and balance He and Ar, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1 ...................................................................................................... 186 Table ‎B.2. Error calcualtions for TPMO results at T10, T50 and T90. .......................................... 197 Table ‎B.3.‎ Repeatability‎ and‎ error‎ analysis‎ of‎ differential‎ runs‎ using‎ 0.8‎ wt.%‎ Pd/γ-Al2O3 catalyst. Reaction condition follows Figure  ‎B.5a. .................................................. 199 Table‎ B.4.‎ Repeatability‎ and‎ error‎ analysis‎ of‎ differential‎ runs‎ using‎ 0.8‎ wt.%‎ Pd/γ-Al2O3 catalyst. Reaction condition follows Figure  ‎B.5b. .................................................. 199 Table ‎B.5. Pure error analysis for differential experiments. ....................................................... 200 Table ‎B.6. Student t-test to compare the mean TOFs at different total flow rates and temperatures. ............................................................................................................ 201 Table ‎C.1. Gas and mass signal profile in QMS. ........................................................................ 202 Table ‎C.2. Experimental plan for calibrating CH4 flow. ............................................................ 203 xiv  Table ‎D.1. Rules of thumb to ensure plug flow operation. ......................................................... 205 Table ‎D.2. Details of calculations for ensuring plug flow operation for laboratory scale fixed-bed reactors. .................................................................................................................... 207 Table ‎E.1. Criteria for ensuring isothermal operation of reactor ................................................ 208 Table ‎E.2. Details of calculations for parameters involved in ensuring isothermal operation criteria. ..................................................................................................................... 210 Table ‎E.3. Values for different isothermal operation criteria ..................................................... 211 Table ‎F.1. Catalyst properties and operating conditions used in mass transfer calculations. ..... 213 Table ‎F.2. Details of calculations for effective diffusivity ......................................................... 215 Table ‎F.3. Details of calculations for internal mass transfer criterion. ....................................... 216 Table ‎F.4. Details of calculations of external mass transfer criterion. ....................................... 217 Table ‎H.1. Details of calculations for mass transfer criteria and maximum temperature in observed TPMO data when no H2O added to the feed. Reaction condition follows Figure ‎7.7 ................................................................................................................. 222 Table ‎H.2. Details of calculations for mass transfer criteria and maximum temperature in observed TPMO data when 2 v.% H2O added to the feed. Reaction condition follows Figure ‎7.7 ................................................................................................................. 223 Table ‎H.3. Details of calculations for mass transfer criteria and maximum temperature in observed TPMO data when 5 v.% H2O added to the feed. Reaction condition follows Figure ‎7.7 ................................................................................................................. 224 Table ‎H.4. Calculated kinetic parameters for power law models by fitting to differential data. 225 Table ‎H.5. Calculated kinetic parameters for various kinetic models by fitting to differential data .................................................................................................................................. 226 xv  Table ‎H.6. Calculated kinetic parameters for MVK kinetic models by fitting to differential data .................................................................................................................................. 227                     xvi  List of Figures  Figure ‎2.1. Effect of H2O vapor on the activity for CH4 combustion over Pd/Al2O3 (■);‎2:1PdPt/Al2O3 (); and 1:1PdPt/Al2O3 (o) at 773 K; 5 v.% of H2O was added to the 1.5 v.%CH4/air feed gas, GHSV = 100,000 h-1, for 5 h. [42] Copyright © 2007 Elsevier. ................................................................................................................... 14 Figure ‎2.2. Delay in the H2O peak with respect to other products obtained by passing pulses of CH4/O2/He (closed square) and 1v.%O2/3.45%H2O/He (open square) over Pd/ZrO2 at different temperatures. Reproduced with permission from [50]. Copyright © 2001 Elsevier. .......................................................................................................... 19 Figure ‎2.3. Temperature-programmed reactions during pulsed or continuous flow of reactants over Pd/ZrO2 with or without H2O in the feed. Reproduced with permission from [50]. Copyright © 2001 Elsevier. ............................................................................ 20 Figure ‎2.4. FTIR spectra of 5 wt.% PdO/Al2O3 at 473 K (a) during the CH4─O2 reaction, (b) desorption when CH4 was removed. Reproduced with permission from [42]. Copyright  2007 Elsevier. ..................................................................................... 22 Figure ‎2.5. FTIR spectra at highest surface coverage and 623 K on (1) PdO/Al2O3 during CH4-O2 reaction, (2) PdO/Al2O3 and (3) Al2O3 when injecting H2O pulses. Reproduced with permission from [62]. Copyright © 2004 Elsevier. ................................................. 23 Figure ‎2.6. The normalized peak areas of different surface OH species generated during lean-CH4-O2 reaction at 448 K. Reproduced with permission from [62]. Copyright © 2004 Elsevier. .......................................................................................................... 25 xvii  Figure ‎2.7. Oxygen exchange of (a) 3wt.% Pd18O/Al216O3 (top) and (b) 3wt.% Pd18O/Mg16O (bottom) with catalyst supports in a flow of 18O2/He at 673 K. H216O was injected at some time to probe its effect on oxygen exchange. Reproduced with permission from [45]. Copyright 2012 American Chemical Society. .................................... 29 Figure ‎2.8. Schematic of oxygen exchange during CH4 oxidation using labeled pulsed experiments. Reproduced with permission from [77]. Copyright © 2002 Elsevier. 29 Figure ‎2.9. CH4 conversion over fresh 1 wt.% Pd/ZrO2-Ce catalyst compared to1 wt.% Pd/ZrO2-Ce thermally aged in air at 823 K (Pd/ZrO2-Ce-550) and hydrothermally aged at different temperatures in 2 v.% H2O/air or air (identified as Pd/ZrO2-Ce-TTTh where TTT is the aging temperature in C). Reproduced with permission from [16]. Copyright © 2008 Elsevier. ..................................................................................... 33 Figure ‎2.10. Hydrothermal stability of Pd/Al2O3 modified by metal oxides (Mn, Fe, La, Mg, Ni) in reaction of 0.4 v.% CH4, 4 v.% H2O balance of air at 873 K and  GHSV 80,000 h-1. Reproduced with permission from [67]. Copyright © 2013 Elsevier. .............. 36 Figure ‎2.11. Effect of support hydrophobicity on CH4 oxidation over Pd catalysts at 598 K; (A) Pd/Aerosil 130; (B) Pd/R972; reaction condition: 1.5 v.% CH4, 6 v.% O2, and 3 v.% H2O added in wet feed, GHSV 27,000 cm3 g-1 h-1. Reproduced with permission from [20]. Copyright © 2005 Elsevier. ................................................................... 39 Figure ‎3.1. Schematic diagram of experimental setup. ................................................................. 55 Figure ‎4.1. Schematic diagram of Pd/SiO2 catalyst thermal treatements. .................................... 60 Figure ‎4.2. Thermal aging effect on 7.7 wt.%  Pd/SiO2 catalysts;‎(■)‎Catalyst‎B,‎(●)‎Catalyst‎D,‎(▲)‎Catalyst‎F;‎Catalyst‎identity‎follows‎Table ‎4.1. TPMO condition: 0.1 v.% CH4 and 20% O2 in Ar and He, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1. ............... 61 xviii  Figure ‎4.3. TEM images and cluster size distributions of 7.7 wt.% Pd/SiO2 catalysts. Catalyst identity follows Table ‎4.1. ....................................................................................... 64 Figure ‎4.4. XRD patterns of 7.7 wt.% Pd/SiO2 catalyst following thermal treatments at the conditions described in Table ‎4.1. Catalyst identity also follows Table ‎4.1. .......... 66 Figure ‎4.5. Raman spectrum of SiO2 support and 7.7 wt.% Pd/SiO2 following thermal treatments as identified in Table ‎4.1. ........................................................................................ 67 Figure ‎4.6. XPS narrow scan spectra of 7.7 wt.% Pd/SiO2 following thermal treatments as identified in Table ‎4.1. ............................................................................................. 69 Figure ‎4.7. The T50 as a function of recycle number for the 7.7 wt.% Pd/SiO2 catalyst with HTA in 6.5 v.% H2O/air at 973 K for 4 h after each TPMO; TPMO condition: 0.1 v.% CH4, 20% O2 in Ar & He, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1. ............... 70 Figure ‎4.8. Effect of HTA on (a) 0.6, (b) 1.7, (c) 7.7 wt.% Pd/SiO2 catalysts; (■) Fresh sample, pre-oxidized in air at 723 K for 15 h; (▲) Sample hydrothermally aged at 973 K in 6.5 v.% H2O/air for 16 h; TPMO condition: 0.1 v.% CH4, 20% O2 in Ar & He, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1. ................................................................. 72 Figure ‎4.9. The T50 as a function of recycle number for the 1.7 wt.% Pd/SiO2 catalyst:‎ (■)‎thermally aged in air at 973 K for 4‎h‎after‎each‎TPMO;‎(▲)‎hydrothermally‎aged‎in 6.5 v.% H2O/air at 973 K for 4 h after each TPMO. TPMO condition: 0.1 v.% CH4, 20% O2 in Ar & He, 5 K min-1, GHSV 180,000 cm-3 (STP) g-1 h-1. .............. 73 Figure ‎4.10. Schematic of catalyst sintering through (a) Ostwald ripening (adapted from [12]), (b) particle migration and coalescence (adapted from [12]) and (c) PdO occlusion by the support .......................................................................................................... 77 xix  Figure ‎4.11. HRTEM images of 7.7 wt.% Pd/SiO2 showing amorphous layers on top of PdO: (a) Catalyst D; (b) Catalyst  F. Catalyst identity follows Table ‎4.1. ............................. 80 Figure ‎5.1. Effect of HTA temperature on CH4 oxidation activity of the 6.78 wt.% PdO/SiO2 catalysts. Catalysts hydrothermally aged for 16 h in 6.5 v.% H2O/air prior to TPMO evaluation. ................................................................................................................ 83 Figure ‎5.2. Effect of HTA time on the CH4 oxidation activity results of the 6.78 wt.% PdO/SiO2 catalysts. Catalyst identity follows Table ‎5.1. ......................................................... 84 Figure ‎5.3. XRD patterns of the pre-treated 6.8 wt.% PdO/SiO2 catalysts; PdO‎ (▼).‎  Catalyst identity follows Table ‎5.1. ....................................................................................... 85 Figure ‎5.4. PdO size of 6.8 wt.% PdO/SiO2 catalysts after HTA for 16 h at various temperatures measured by XRD and TEM. .................................................................................. 87 Figure ‎5.5. TEM images of the pre-treated 6.8 wt.% PdO/SiO2, evidence of PdO cluster growth after HTA; (A) Catalyst A, (B) Catalyst E. Catalyst identity follows Table ‎5.1 ..... 88 Figure ‎5.6. TEM cluster size distribution of the pre-treated 6.8 wt.% PdO/SiO2 catalysts. Catalyst identity follows Table ‎5.1. ....................................................................................... 89 Figure ‎5.7. XPS narrow scan spectra of Pd atom in the pre-treated 6.8 wt.% PdO/SiO2 catalysts. Catalyst identity follows Table ‎5.1. ......................................................................... 90 Figure ‎5.8. Pore size distribution of the pre-treated 6.8 wt.% PdO/SiO2 catalysts. Catalyst identity follows Table ‎5.1 ........................................................................................ 92 Figure ‎5.9. TGA results of the pre-treated 6.8 wt.% PdO/SiO2 catalysts, Catalyst A (top) and Catalyst D (mid); commercial 20 wt.% Pd(OH)2/activated carbon (bottom) dried at 393 K. Catalyst identity follows Table ‎5.1 .............................................................. 93 xx  Figure ‎5.10. HRTEM images of the pre-treated 6.8 wt.% PdO/SiO2 catalysts, evidence of PdO cluster occlusion by SiO2 during HTA; (A) Catalyst A, (B) Catalyst D. Catalyst identity follows Table ‎5.1 ........................................................................................ 96 Figure ‎6.1. Catalyst activity in CH4 oxidation for various calcined PdO supported catalysts. TPMO condition: 0.1 v.% CH4, 10% O2 and balance He and Ar, 5 K min-1, 180,000 cm3 (STP) g-1 h-1 .................................................................................................... 100 Figure ‎6.2.‎ Effect‎ of‎ HTA‎ on‎ catalyst‎ activity‎ of‎ 7.4‎ wt.%‎ PdO/γ-Al2O3 in CH4 oxidation. Catalysts were hydrothermally aged at 973 K in 6.5 v.% H2O/air prior to TPMO evaluation. .............................................................................................................. 101 Figure ‎6.3. Effect of HTA on catalyst properties of PdO/SiO2 and‎ PdO/γ-Al2O3 as a function HTA time. M: Si or Al. .......................................................................................... 103 Figure ‎6.4. Pore size distribution (a) PdO/SiO2 and‎(b)‎PdO/γ-Al2O3 as a function HTA time (0 h: fresh catalyst). Catalysts were hydrothermally aged at 973 K in 6.5 v.% H2O/air. ............................................................................................................................... 104 Figure ‎6.5.‎XRD‎patterns‎of‎(a)‎PdO/γ-Al2O3 and (b) PdO/SiO2 before and after HTA at 973 K for‎a‎specified‎time‎(0‎h:‎fresh‎catalyst).‎PdO‎(▼),‎Pd0 (),‎γ-Al2O3 (). ........... 105 Figure ‎6.6. XPS spectra‎of‎PdO/γ-Al2O3 catalysts before and after HTA at 973 K for a specified time (0 h: fresh catalyst). ....................................................................................... 106 Figure ‎6.7.‎ Effect‎ of‎ HTA‎ on‎ catalyst‎ activity‎ of‎ (a)‎ PdO/α-Al2O3, (b) PdO/SnO2 in CH4 oxidation. Catalysts were hydrothermally aged at 973 K in 6.5 v.% H2O/air prior to TPMO evaluation. ................................................................................................. 108 Figure ‎6.8.‎Pore‎size‎distribution‎of‎PdO/α-Al2O3 catalyst before and after HTA. Catalysts were hydrothermally aged at 973 K in 6.5 v.% H2O/air (0 h: fresh catalyst). ............... 109 xxi  Figure ‎6.9. Al2O3 phase changes in XRD patterns of fresh (0 h) and hydrothermally-aged (at 973 K‎for‎65‎h)‎PdO/α-Al2O3.‎PdO‎(▼),‎α-Al2O3 (),‎θ-Al2O3(). .......................... 112 Figure ‎6.10. (a) XRD diffractogram of fresh (0 h) and HTA (at 973 K for 65 h) PdO/SnO2. (b) A zoomed-in plot of XRD diffractogram in the range of 46-58°.‎PdO‎(▼),‎Pd0 (), SnO2 (). ............................................................................................................... 113 Figure ‎6.11. TEM image and cluster size distribution of fresh PdO/SnO2 catalyst. .................. 114 Figure ‎6.12.‎ PdO‎ crystallite‎ size‎ as‎ a‎ function‎ of‎ pore‎ size‎ for‎ PdO/γ-Al2O3 as HTA time increases from 0 to 65 h. ........................................................................................ 117 Figure ‎6.13.‎XRD‎patterns‎of‎PdO/γ-Al2O3; (A) fresh catalyst; (B) catalyst (A), thermally aged in 19.6 v.% O2/Ar at 973 K for 16 h; (C) catalyst (A), hydrothermally aged in 6.5 v.% H2O/air‎at‎973‎K‎for‎16‎h.‎PdO‎(▼),‎Pd0 (),‎γ-Al2O3 (). ................................. 119 Figure ‎7.1. Steps for the kinetic model development. ................................................................ 133 Figure ‎7.2. Effect of H2O and CH4 concentration‎on‎activity‎of‎PdO/γ-Al2O3 catalyst during CH4 oxidation; (a) 0.1 v.% CH4 (b) 0.5% CH4, 20% O2 in balance of He and Ar, GHSV 180,000 cm3 (STP) g-1 h-1. ..................................................................................... 135 Figure ‎7.3. T50 extracted from TPMO profiles in Figure ‎7.2 as a function of H2O concentration; TPMO conditions follow Figure ‎7.2a. ................................................................... 136 Figure ‎7.4. Effect of O2 concentration‎on‎activity‎of‎PdO/γ-Al2O3 catalyst during CH4 oxidation; TPMO condition: 0.1 v.% CH4, O2 (figure legend) in He and Ar, GHSV 180,000 cm3 (STP) g-1 h-1. ................................................................................................... 137 Figure ‎7.5. TOF as a function of inverse temperature at different feed concentrations of H2O. Reaction condition: 0.5 v.% CH4, 20% O2, 0 or 2% H2O and balance He and Ar, GHSV 270,000 cm3 (STP) g-1 h-1, each temperature for 0.5 h. ............................. 138 xxii  Figure ‎7.6. Comparisons of calculated and observed CH4 conversion as a function of H2O concentration (0 and 2 v.%). Reaction condition follows Figure ‎7.5. ................... 139 Figure ‎7.7.  Weisz-Prater criterion calculated as a function of CH4 conversion for observed TPMO data. Reaction condition: (a) 0.1 v.% or (b) 0.5 v.% CH4, 20 v.% O2, H2O, 5 K min-1, GHSV 180,000 cm3 g-1 h-1. ...................................................................... 142 Figure ‎7.8. Kinetic model comparisons to predict TPMO data as a function of CH4 and H2O concentrations. Reaction condition: 0.1v.% (a, c and e) or 0.5% (b, d and f) CH4, 20% O2 and 0% (a and b), 2% (c and d) or 5% (e and f)  H2O and balance He and Ar, 5 K min-1, GHSV 180,000 cm3 g-1 h-1. ............................................................ 144 Figure ‎7.9. Kinetic model comparisons to predict TPMO data as a function of H2O concentration. Reaction condition: 0.1v.% CH4, 20% O2 and 3% (a) or 10% (b) H2O and balance of He and Ar, 5 K min-1, GHSV 180,000 cm3 g-1 h-1. ............... 145 Figure ‎7.10. Comparisons of calculated and observed CH4 conversions as a function of CH4 and H2O concentration for models A (a and b) and B2 (c and d) as described in Table ‎7.4. Reaction condition follows Figure ‎7.2. 0.1v.% (a and c) or 0.5% (b and d) CH4 in the feed. ................................................................................................. 151 Figure ‎7.11. Comparisons of modeled and observed CH4 conversions as a function of CH4 and H2O concentration for models C (a and b) and D (c and d) as described in Table ‎7.5. Reaction condition follows Figure ‎7.2. 0.1v.% (a and c) or 0.5% (b and d) CH4 in the feed. ................................................................................................. 152 Figure ‎7.12. Comparisons of modeled and observed CH4 conversions as a function of CH4 and H2O concentration for models E (a and b) and I (c and d) as described in Table ‎7.5 xxiii  and 7.6. Reaction condition follows Figure ‎7.2. 0.1 v.% (a and c) or 0.5% (b and d) CH4 in the feed. ..................................................................................................... 153  Figure A.1. Isotherm data for calcined 0.7 wt.% Pd/Al2O3 ........................................................ 181 Figure B.1. TPMO runs with no catalyst loaded in the reactor; reaction condition: 0.1 v.% CH4, 20% O2, 0 or 3% H2O, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1. .................. 184 Figure ‎B.2.‎Comparison‎of‎TPMO‎raw‎data‎and‎TPMO‎average‎data‎using‎0.8‎wt.%‎Pd/γ-Al2O3 catalyst; reaction condition: 0.1 v.% CH4, 20% O2, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1. .......................................................................................................... 195 Figure B.3. Repeatability of TPMO‎ runs‎ using‎ commercial‎ 1‎wt.%‎Pd/γ-Al2O3 catalyst in the absence of H2O; reaction condition: 0.1 v.% CH4, 20% O2, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1. ..................................................................................... 196 Figure ‎B.4.‎Repeatability‎of‎TPMO‎runs‎using‎0.8‎wt.%‎Pd/γ-Al2O3 catalyst in the presence of H2O; reaction condition: 0.1 v.% CH4, 20% O2, 5% H2O, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1. ..................................................................................... 197 Figure ‎B.5.‎ Repeatability‎ of‎ differential‎ runs‎ using‎ 0.8‎ wt.%‎ Pd/γ-Al2O3 catalyst; reaction condition: 0.5 v.% CH4, 20% O2 and balance He and Ar, GHSV (a) 180,000; (b) 270,000 cm3 (STP) g-1 h-1. ..................................................................................... 198 Figure ‎C.1. CH4 volume fraction as a function of CH4 mass signal normalized to He (calibration curve). .................................................................................................................... 204 Figure ‎F.1. Diagnostic test to ensure negligible external mass transfer resistance. Reaction condition: 0.5 v.% CH4, 20% O2 in balance of He and Ar, 538 K ........................ 218 xxiv  Figure ‎G.1.‎ XPS‎ spectra‎ of‎ PdO/α-Al2O3 catalysts. (a) fresh – calcined catalyst, (b) aged – catalyst hydrothermally aged in 6.5 v.% H2O/air at 973 K for 65 h catalyst. ....... 219 Figure ‎G.2. XPS spectra of PdO/SnO2 catalysts. (a) fresh – calcined catalyst, (b) aged – catalyst hydrothermally aged in 6.5 v.% H2O/air at 973 K for 65 h catalyst. .................... 220 Figure ‎H.1. Mass transfer effects on calculated TPMO results with 0.1v.% CH4 and no H2O in the feed. Reaction condition is similar to Figure ‎7.8. ............................................ 228 Figure ‎H.2. Mass transfer effects on calculated TPMO results with 0.1v.% CH4 and 2% H2O in the feed. Reaction condition is similar to Figure ‎7.8. ............................................ 229 Figure ‎H.3. Mass transfer effects on calculated TPMO results with 0.5v.% CH4 and no H2O in the feed. Reaction condition is similar to Figure ‎7.8. ............................................ 230 Figure ‎H.4. Mass transfer effects on calculated TPMO results with 0.5v.% CH4 and 2% H2O in the feed. Reaction condition is similar to Figure ‎7.8. ............................................ 231           xxv  Nomenclature  *  An O vacancy on the PdO surface ac  External catalyst surface area per catalyst bed volume, cm2 cm-3 C  Constant number in BET equation Cm  CH4 concentration, mol cm-3 CM  Mears criterion Cm0  CH4 concentration at reactor inlet, mol cm-3 (STP) cp  Heat capacity, kJ g-1 K-1 Cs  CH4 concentration on external surface of catalyst, mol cm-3 (STP) CWP  Weisz-Prater criterion db  Catalyst bed diameter, cm DCH4-Ar  Binary bulk diffusivity, cm2 min-1 DCH4-Ar_eff Effective bulk diffusivity, cm2 min-1 dcrystal  Crystallite size, nm Deff  Effective diffusivity, cm2 min-1 df  Degree of freedom DK  Knudson diffusivity, cm2 min-1 DK_eff  Effective Knudson diffusivity, cm2 min-1 dp  Particle diameter, cm dpore  Pore diameter, cm DPd  Pd dispersion Eac  Activation energy, kJ mol-1 xxvi  Fm0  CH4 molar flow at reactor inlet, mol min-1 h  Heat transfer coefficient, kJ cm-2 min-1 K-1 Ii  Mass signal of component i = CH4, CO2 and He, torr Ire  Relative mass signal (based on He) (         ) jD   jD factor k  Boltzmann's constant, kJ mol-1 K  Instrument constant (often 1)     First order rate constant, cm3 min-1 gcat-1     Adsorption equilibrium constant for component i, kPa-1      Adsorption equilibrium frequency factor for component i, kPa-1 kb  Effective thermal conductivity of catalyst bed, kJ cm-1 min-1 K-1 kc  External mass transfer coefficient, cm min-1 kg  Thermal conductivity of gas, kJ cm-1 min-1 K-1 ki  Rate constant i = 1, 2 and 3      Rate constant, s-1 kPa-1  kp  Thermal conductivity of catalyst particle, kJ cm-1 min-1 K-1 kr  Rate constant, mol min-1 gcat-1 Lb  Catalyst bed length, cm LPd  Pd loading, wt.% m  Number of different observations for TOF Mw  Molecular weight, g mol-1 Mwfeed  Average gas molecular weight at reactor inlet, g mol-1 MwCH4-Ar Reduced mass of CH4 and Ar xxvii  n  Reaction order ni  Number of repeat observations of TOF  NNu  Nusselt number Nobs  Number of observed data Npar  Number of parameters NPe  Peclet number         Minimum Peclet number NPr  Prandtl number NRe  Particle Reynolds number NSc  Schmidt number P  Pressure, kPa P°  Saturation vapor pressure, kPa Pc  Critical pressure, kPa Pd-*  Pd vacancy site Pi  Partial pressure of component i = CH4, H2O, CO2 and O2, kPa        CH4 partial pressure, kPa        CH4 partial pressure at reactor inlet, kPa R  Gas constant, 8.3144621E3 kPa cm3 K-1 mol-1       Reaction rate based on catalyst volume, mol min-1 cm-3 r2  Coefficient of determination rki  Effective pore radius, Å rm  Reaction rate based on catalyst mass, mol min-1 gcat-1 rpi  Pore radius, Å rt  Catalyst bed radius, cm xxviii  SBET  BET surface area, cm2 g-1 sd  Standard deviation between two means S.D.  Standard deviation sp2  Pooled variance ti  Layer thickness, Å t  Time, min T  Temperature, K Tb  Temperature of bulk fluid, K Tc  Critical temperature, K Ti  Temperature at which i % of CH4 is converted in TPMO profile, K Tm  Average temperature of experiments, K Tmax  Maximum temperature, K TOF  Turnover frequency, s-1 TOFm  Mean TOF, s-1 Ts  Temperature of catalyst surface, K TSTP  Standard temperature, 273.15 K Tw  Temperature of reactor wall, K V  Superficial gas velocity, cm min-1 V0  Average pore volume, cm3 g-1  Vm  Monolayer volume, cm3 (STP) g-1  Vpi  Pore volume in each desorption step (i), cm3 (STP) w  Catalyst mass, g X  CH4 conversion, % xxix        Calculated CH4 conversion, %       Observed CH4 conversion, %      Ar volume fraction       CH4 volume fraction      He volume fraction      O2 volume fraction Yre  Relative volume fraction (based on He) (         )  Greek Letters  β  FWHM, Radians ΔH  Heat of adsorption, kJ mol-1 ΔHads  Heat of adsorption, kJ mol-1 ΔHr  Heat of reaction at 298 K, kJ mol-1 Δti  Change in layer thickness in each desorption step (i), Å ΔVi  Gas volume adsorbed in each desorption step (i), cm3 (STP) g-1      Lennard-Jones energy of component i = CH4 and Ar, kJ mol-1 εb  Bed porosity εp   Particle porosity η  Internal effectiveness factor ηo  Overall effectiveness factor θ  Angle of reflection, ° θ1  Characteristic parameter, min xxx  θi  Fractional coverage of component i or i-* θv  Fraction of vacant sites or * λ  X-ray wave length, nm µ  Gas dynamic viscosity, g cm-1 min-1 ζ  Constriction factor ζi  Lennard-Jones characteristic length of component i = CH4 and Ar, Å η  Tortuosity factor ν  Total volumetric flow rate, cm3 (STP) min-1 ν0  Total volumetric flow rate at reactor inlet, cm3 (STP) min-1 νi  Volumetric flow rate of component i, cm3 (STP) min-1 ρb  Bed density, g cm-3        SiC bed density, g cm-3 ρg   Gas density (ideal gas law), g cm-3 ρp  Particle density, g cm-3 ρs   Solid density, g cm-3 ρSiC   SiC solid density, g cm-3 θ1  Thiele modulus ΣC  Carbon balance, cm3 (STP) min-1  ΣC0  Carbon balance at reactor inlet, cm3 (STP) min-1  ΩD  Collision integral   xxxi  List of Abbreviations  AAS  Atomic absorption spectroscopy 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, cm3 (STP) g-1 h-1 or h-1 h  Hour(s) H/C  Hydrogen/Carbon HC  Hydrocarbon HRTEM  High resolution transmission electron microscopy HTA  Hydrothermal aging LH  Langmuir-Hinshelwood LMA  Levenberg-Marquardt algorithm xxxii  LNG  Liquefied Natural Gas MFC  Mass flow controller mi  Mile MRSS  Mean residual sum of squares MVK  Mars van Krevelen NG  Natural gas NGV  Natural gas vehicle NMOG Non-methane organic gases NOx  Nitrogen oxides nv  Not valid Obs  Observed ODE  Ordinary differential equation PDE  Partial differential equation PEMS  Pure error mean square 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) SIIC  Spark-ignited internal combustion SOx  Sulfur oxides xxxiii  SRSS  Sum of RSS TC  Thermocouple TCD  Thermal conductivity detector TEM   Transmission electron microscopy Temp  Temperature TGA  Thermal gravimetric analysis TOF  Turnover frequency TOS  Time on stream TPMO  Temperature-programmed CH4 oxidation TPO  Temperature-programmed oxidation TPR  Temperature-programmed reduction TWC  Three way catalytic v  Volume basis wt  Weight basis XPS  X-ray photoelectron spectroscopy XRD  X-ray diffraction yr  Year       xxxiv  Acknowledgements  I would like to thank my doctoral supervisor, Professor Kevin J. Smith for giving me the opportunity to do my PhD program here at University of British Columbia. I had the pleasure to have a supervisor who was patient, supportive, helpful, smart and insightful with a solid background and experience in his field. I should confess that I learned a lot from him both academically and in general during my time here at University of British Columbia.    I would like to thank my PhD committee, Professor Fariborz Taghipour and Professor Mark MacLachlan for their useful discussions and suggestions throughout my PhD program.  I would like to express my gratitude to Helsa Leong, Richard Ryoo and all other Chemical and Biological Engineering Department staffs for their always smiling face and all of administrative services they provided me during these years.  I would like to acknowledge the financial support from Natural Sciences and Engineering Research Council of Canada (NSERC), Westport Innovations Inc. and The University of British Columbia that make my PhD program possible.  I would like to thank my colleages in CHBE catalysis group specially Hooman Rezaei, Farnaz Sotoodeh, Mina Alyani, Shahin Goodarznia, Ross Kukard and Pooneh Ghasvareh for lab assistance and equipment trainings.  xxxv  I would also like to thank my parents for their always best wishes and their emotional and optimistic support they provided me throughout my academic life.  Finally, I would like to thank my loving, beautiful and adoring wife, Samira, for being supportive and patient at all times while I was doing my PhD program.                        xxxvi     To my loving ones…  My wife, Samira and daughter, Ayla My parents   1  Chapter 1 Introduction  Natural gas (NG) is a non-renewable energy resource with the highest H/C ratio of all fossil fuels which is mainly used for heating, electricity production and more recently, transportation. NG contains primarily CH4 (87‒97 mol%) with varying amounts of other hydrocarbons (HCs) and trace amounts of N, S (3 ppm to 30 ppm [1]), and O [2, 3]. NG is known to have a lower amount of N and S than gasoline and diesel. NG is cheaper than gasoline on an energy equivalent basis (as shown in Table ‎1.1) and is available in large quantities [1]. The total NG reserves in North America are estimated at > 11 trillion cubic meters of NG [4]. Nevertheless, NG occupies more volume than liquid fuels per unit mass and needs to be compressed (Compressed Natural Gas (CNG)) or liquefied (Liquefied Natural Gas (LNG)) for transportation and for use as a fuel in motor vehicles [5]. Compared to gasoline, NG has a very high octane rating, allowing operation in spark-ignited internal combustion (SIIC) engines at higher compression ratios and thus, higher fuel efficiency [1].  Table ‎1.1. Alternative fuel prices in the USA (adapted from [6]) Fuel Nationwide average fuel price ($/GGE)  April 2015 January 2015 CNG 2.09 2.11 Gasoline 2.42 2.30 Diesel 2.56 2.72  The use of natural gas vehicles (NGVs) is growing at about 20% per year worldwide, with > 16.7 million NGVs currently in use [5]. NGVs generate considerably fewer emissions in comparison 2  to conventional diesel and gasoline vehicles. Light-duty NGVs reduce emissions of CO by 90-97%, NOx by 35-60%, CO2 by 25%, and also emit little or no particulate matter [7]. Natural gas engines have lower CO2 emissions because of the high H/C ratio of CH4, compared to gasoline or diesel engines. These engines can also operate under lean-burn conditions which increases fuel efficiency and decreases emissions of nitrogen oxides (NOx) [8]. The benefits of natural gas engines are mitigated by the presence of significant quantities of unburned CH4 in the exhaust. CH4 is a strong greenhouse gas (GHG) that contributes to global warming with a GHG impact 23x’s‎greater than that of CO2 [8, 9].  The transportation sector is one of the main sources of GHG emissions in the world. In the USA, the transportation sector is responsible for 28% of GHG emissions, mostly arising from CO2 emissions. Over half of these GHG emissions come from passenger cars, light-duty trucks and minivans [10]. By growing the number of NGVs, CH4 emissions become another important source of GHG emissions. Increasing CH4 emissions, increasing societal demands and recognition of health impacts have made environmental agencies set lower CH4 emission standards for natural gas engines in recent years. In Europe, starting from 2014, CH4 emissions from heavy duty vehicles must be less than 0.5 g/kWh (Euro VI) [11]. This brings a new challenge for automobile companies to find a solution to reduce the CH4 emissions from NGVs.  A typical automotive SIIC engine releases energy by combusting an injected air/fuel mixture inside a chamber. The energy released in the chamber is transformed to mechanical energy that is transferred to a shaft that drives the vehicle. The products of combustion (H2O and CO2) and undesired products such as CO, unburned HCs, and NOx are emitted in the engine exhaust gas. 3  The concentration of emissions is strictly related to the A/F ratio. For example, when the A/F ratio is lowest (A/F < 14.6), the concentration of CO and HCs in the exhaust is highest. As A/F ratio approaches the stoichiometric point (A/F = 14.6), the NOx and O2 concentration increases while limited HCs and CO leave the exhaust. At fuel-lean conditions (A/F > 14.6), the HC concentration tends to increase because of irregularity in the combustion process [12]. The exhaust emissions must be removed before reaching the atmosphere. Governments of developed countries have set emission standards for vehicles since the early 1970s [12]. To reduce the emissions from vehicles, the automotive industry agreed to place catalytic converters, which consist of active catalysts designed to oxidize CO and HCs and reduce NOx emissions, in the engine exhaust [13], thereby reducing exhaust emissions to meet government standards (Table ‎1.2). 4    Table ‎1.2. Light-duty vehicle exhaust emissions standards in the USA (adapted from [12, 14]). Region Standard Limit (g/mia)  Durability   NMOGb+NOx NMOGc CO NOx GHG  USA Tier 1/1994  0.25 3.4 0.4  50000 mi/ 5 yrs  Tier 2/2004  0.125 3.4 0.07  50000 mi/ 5 yrs  Tier 3/bin 160/2017 0.160  4.2   150000 mi California LEV/Tier 1  0.25 3.4 0.4  50000 mi/ 5 yrs  LEV II/LEV  0.075 3.4 0.05  50000 mi/ 5 yrs  LEV III/LEV160 0.160  4.2   150000 mi  GHG Emission/Near Term     233   GHG Emission/Medium Term      205  a For GHG g CO2/mi (CO2-Equivalent = CO2 + 296 * N2O + 23 * CH4 – AC Allowances); b NMOG: non-methane organic gases.; c For Tier 1/1994 and LEV/Tier 1.  5  Catalytic converters are made of ceramic or metal honeycomb monoliths with a thin layer of washcoat (15‒60 µm) covering the walls of the monolith. The washcoat is a high surface area γ-Al2O3 on which precious metals such as Pd, Pt and Rh (0.1‒0.15 wt.% loading of metal with a Pt:Pd mass ratio of 2.5:1 at oxidation converters, constructed in 1970s) are dispersed. Other metal oxides such as La2O3, BaO, ZrO2 and CeO2 are typically added to the washcoats in order to increase their stability and oxygen storage capacity [12, 15]. Catalysts used in these converters have been developed according to new emissions standards and operating conditions over time. The A/F ratio of the engine, having a considerable effect on the performance of catalytic converters, is controlled through an engine management system (an oxygen sensor controls the oxygen content by comparing the exhaust gas with ambient air). Three way catalytic (TWC) converters, the second generation catalytic converters, achieve the concurrent oxidation of CO and HC emissions and reduction of NOx emissions from the exhaust gas on a single catalyst bed, provided that the oxygen sensor keeps the A/F ratio at 14.6 (stoichiometric conditions) within a narrow tolerance. On the other hand, dual-bed converters first reduce the NOx emissions and then oxidize the CO and HC emissions with added air in a separate catalyst bed. Lean-burn engines working at high A/F ratios (> 14.6) improve the fuel efficiency and decrease substantially the CO2 and NOx emissions although significant unburned HCs are still present in the exhaust due to inconsistency of combustion. The design of catalytic converters for these engines is complex as exhaust temperatures are low and NOx emissions cannot be reduced in a high oxygen content environment. Typically, a mix of NOx trap and TWC converters is proposed in the exhaust of lean-burn gasoline engines and is shown to be successful [12, 15]. One novel lean-burn engine/NOx storage-reduction catalyst system operates the engine regularly under lean-burn conditions and periodically fuel-rich or stoichiometric conditions. Under lean-burn, the NOx is 6  stored in an alkaline metal oxide such as BaO attached to the TWC converters and during transitions to fuel-rich or stoichiometric conditions, the TWC converter reduces the stored NOx [12].  The exhaust of NGVs mainly contain varying concentrations of CH4 (400 - 1500 ppmv), CO (200 - 700 ppmv), CO2 (5 - 10 v.%), NOx, SOx  (~ 1 ppm) and H2O (10 - 15 v.%) (e.g. [8, 16]). The NGVs usually operate in stoichiometric or lean-burn combustion modes. The stoichiometric combustion systems are mainly used for light-duty vehicles such as passenger cars. The lean-burn combustion systems, on the other hand, are primarily used in heavy-duty vehicles such as buses [17]. The lower temperature (423 - 823 K) of the exhaust of lean-burn NGVs and the presence of high concentration of CO2 and H2O makes conventional catalytic converters inefficient to meet current CH4 emission standards. CH4 has the lowest reactivity compared to other hydrocarbons because of the high strength of the C-H bond, so that CH4 is difficult to oxidize [18]. In order to achieve complete CH4 conversion under catalytic combustion conditions with current catalytic converters, temperatures of 673 - 723 K are required [15]. Current catalysts in the market might be able to fully oxidize CH4 at low temperatures but the issue is their instability in the presence of high concentrations of oxidation products such as H2O.  Combustion of CH4 has been extensively studied with a focus on improving the catalytic efficiency of platinum group metals (PGMs; such as Pd, Pt, Rh, Au) and transition metal oxides (such as Co, Cu, Mn, Cr oxides) [9]. Mostly, Pd catalysts supported on metal oxides (Al2O3, SiO2, ZrO2, CeO2, SnO2, zeolites, and perovskites) promoted by noble metals like Pt and Rh have been considered for CH4 oxidation [8, 9, 19-31]. Recently, PdO sites on Pd-O-Ce surface 7  have shown high CH4 oxidation activity at low temperatures [32, 33]. Despite a high initial activity, Pd catalysts deactivate after long periods of exposure to exhaust gas conditions [9, 16]. The structural collapse of the catalyst support [34, 35], sintering [23, 36-38] and PdO  Pd conversion [34, 39, 40], have all been reported as contributors to catalyst deactivation during CH4 oxidation.   Water, present in the exhaust gas of NGVs with high concentrations, has been shown to deactivate or inhibit Pd catalysts in CH4 oxidation [8, 16, 41-43]. The inhibitory effect of H2O was shown to be reversible and decrease with increased temperature [8]. Competitive adsorption between CH4 and H2O on the active sites of PdO has been suggested as the cause of catalyst inhibition [34, 44]. Other reports suggest that in the presence of H2O, PdO is converted to inactive Pd(OH)2 [8] whereas Persson et al. claimed that H2O produced during reaction does not cause deactivation through physical restructuring of PdO [42]. Previous work also suggests that the accumulation of hydroxyls on the Pd catalyst surface impedes the O exchange between the gas phase, PdO and the support and this limits catalyst activity [43, 45]. These studies mostly report the effect of H2O on CH4 oxidation over Pd catalysts at temperatures below 723 K.   At higher temperatures (typically > 773 K), the deactivation effect of H2O on Pd catalysts is shown to be irreversible [1, 16, 36, 46]. In practice, catalysts are typically aged in the presence of H2O for extended periods of time at high temperatures prior to catalyst activity assessment. HTA of Pd catalysts at temperatures above about 1078 K (with O2 concentrations < 21 v.%), where PdO decomposes to Pd0 [47], results in sintering of Pd0 [1, 36], a process that is influenced by the support [34, 35]. The effect of HTA at lower temperatures has also been reported. Escandon et 8  al. [16] hydrothermally aged a Pd/ZrO2-Ce catalyst at 573–823 K prior to CH4 combustion and reported an irreversible decrease in CH4 conversion for the hydrothermally aged catalyst compared to the Pd/ZrO2-Ce that was not hydrothermally aged [16]. More recently, Park et al. [48] compared a Pd/ZrO2 and a Pd/-Al2O3 catalyst, both hydrothermally aged at 873 K for 100 h. The Pd/ZrO2 was more active and had higher stability than the Pd/-Al2O3, illustrating the importance of support effects on the stability of PdO catalysts. Although many studies address the issue of H2O in NGVs' exhaust and its effect on Pd catalysts, studies dealing with detailed deactivation mechanisms of CH4 oxidation over Pd catalysts in the presence of H2O at temperatures where PdO is thermodynamically stable are required.  H2O also affects the kinetics of CH4 oxidation over Pd catalysts. The H2O reaction order (from -1 to 0) and the apparent activation energy change significantly at temperatures below 600 K to around 880 K during CH4 oxidation over PdO catalysts [49, 50], reflecting H2O inhibition effects on PdO catalysts. Literature studies of CH4 oxidation kinetics consider competitive adsorption between CH4 and H2O on Pd active sites, which forms a reversible inactive Pd(OH)2, as the H2O inhibition mechanism [51]. No studies have incorporated recent proposals for the mechanism of H2O inhibition in which H2O interrupts the O exchange between the support and Pd active sites [45], in the kinetic models for CH4 oxidation.  1.1 Objectives  The aim of the present study is to determine the effect of hydrothermal (in air/H2O) and thermal (in air) aging at various conditions (473–973 K and 0–65 h) on PdO supported catalysts used for 9  CH4 oxidation. A second objective is to develop a kinetic model for CH4 oxidation over PdO supported catalysts that incorporates the effect of H2O and that predicts CH4 conversion during temperature-programmed CH4 oxidation (TPMO) at conditions applicable to lean-burn NGV converters.  1.2 Approach  1. In Chapter 1, a brief introduction of the importance of CH4 oxidation as an effective route for removal of CH4 emissions in the exhaust of NGVs and also the need for an active and stable catalyst that can tolerate the low to moderate temperature (423 - 823 K) of NGV exhausts with high concentration of H2O, CO2 and O2 is reported. Chapter 2 reviews the studies relevant to the effect of H2O on CH4 oxidation over Pd catalysts, which is the focus of this thesis.     2. Chapter 3 contains the details of the experimental procedures used to develop the activity, stability, and kinetic data and also the deactivation mechanisms associated with the hydrothermally and thermally aged catalysts. Catalyst preparation including the primary catalyst treatments, reactor design, experimental equipment, reaction test plans and methods (steady-state or TPMO), various catalyst characterization techniques and their specification, are all included in this chapter.   3. In Chapter 4, the effect of various thermal treatments on PdO/SiO2 catalysts used for CH4 oxidation is reported. Catalyst aging in the presence of dry air versus wet air, at temperatures 10  relevant to NGV applications (< 973 K) and below the PdO thermodynamic decomposition temperature, are compared. The loss in catalyst activity is discussed in terms of characterization data obtained before and after thermal aging.  4. In Chapter 5, the HTA of a PdO/SiO2 catalyst at temperatures between 673–973 K (in moderate temperatures) in high H2O concentrations for various time periods is reported. Detailed deactivation mechanisms occurring during HTA using several characterization techniques are discussed. The effect of aging temperature and time on Pd catalyst activity is reported.  5. In Chapter 6, the HTA of PdO catalysts supported on various metal oxides (high surface area γ-Al2O3 and SiO2 as well as low surface area (α-Al2O3 and SnO2)), is reported. The role of the support on deactivation or stability of the PdO catalysts during HTA is discussed.     6. Chapter 7 reports the effect of H2O, O2 and CH4 concentration on CH4 oxidation over Pd/γ-Al2O3 and these data are used to develop a kinetic model at temperatures relevant to NGV exhaust that accounts for reaction inhibition by H2O. The kinetic model is further used to predict the TPMO profiles measured for the same catalyst.    7. Chapter 8 contains the conclusions associated with this project, recommendations and future work. The Appendices provide details on the mass spectrometer calibration, ensuring isothermal reactor operation and plug flow condition, and the assessment of internal and external mass transfer during reaction. Details of the experimental workplan, kinetic model 11  development, kinetic model MATLAB codes, and detailed results are also provided in the Appendices.                    12  Chapter 2 Literature Review1  Pd supported on various substrates is reportedly the most active metal for the complete oxidation of CH4 (CH4 + 2O2   CO2 + 2H2O; ΔHr = -891 kJ mol-1) [16, 34, 40, 52-56]. C-H bond activation and interaction with lattice oxygen is reported to be the rate determining step in CH4 oxidation [57]. Precise steady-state rate measurements on Pd single crystals at 600 K have confirmed bulk oxidation of Pd during reaction [58]. This indicates that PdO is the active phase, especially at the lower exhaust gas temperatures of NGVs where PdO is thermodynamically stable. However, morphological changes to the PdO and/or the support affect the Pd-O binding energy and contribute to the hysteresis effects observed during temperature-programmed CH4 oxidation [52, 59]. Despite a high initial activity, Pd catalysts deactivate after long periods of exposure to exhaust gas conditions [9, 16]. The structural collapse of the catalyst support [34, 35], sintering [36-38] and PdO  Pd conversion [34, 39, 40], have all been reported as contributors to catalyst deactivation during CH4 oxidation.  Water, a major component of exhaust gas and a product of CH4 combustion, affects the catalyst performance in catalytic converters through various mechanisms. In TWC converters, H2O performs as the oxidant for CO conversion by the water-gas shift reaction and hydrocarbon conversion by steam reforming [60]. H2O may also affect the thermal stability of the catalysts by sintering of noble metals [38, 60, 61], supports [60] or changing the metal oxidation state [46]. In lean-burn NGV converters, where Pd is the metal of choice due to its high activity in CH4 oxidation, the effect of H2O on the performance of Pd catalysts is shown to be temperature                                                  1 A‎version‎of‎this‎chapter‎was‎already‎published‎at‎“Rahman‎Gholami,‎Mina‎Alyani,‎Kevin‎J.‎Smith,‎Deactivation of Pd Catalysts by Water during Low Temperature Methane Oxidation Relevant to Natural Gas Vehicle Converters. Catalysts 5 (2015) 561-594.” 13  dependent. At low temperatures (< 723 K), H2O addition to the feed during CH4 oxidation significantly reduces the Pd catalyst activity even though this decrease is partially reversible when H2O is removed from the feed (H2O inhibition) [62, 63]. On the other hand, at higher temperatures (> 723 K), the effect of H2O on Pd catalysts during CH4 oxidation is not reversible [16, 39]. In this chapter, literature studies reporting on the effect of H2O on Pd catalysts during CH4 oxidation are reviewed.  2.1 Effect of H2O on CH4 Oxidation over Pd Catalysts at Low to Moderate Temperatures   2.1.1 H2O Concentration and Reaction Temperature Effects on CH4 Oxidation Activity of Pd Catalysts  With the growing interest in NGVs, recent studies have focused on the effects of H2O on Pd catalysts during CH4 combustion [22, 30, 42, 49, 62-68]. The deactivation or inhibition effects of H2O are dependent upon several factors including catalyst formulation, reaction temperature, catalyst time-on-stream history and H2O concentration. Table ‎2.1 summarizes selected data that show effects of H2O added to the feed gas during CH4 light-off experiments over Pd catalysts. The light-off temperature (here reported as the temperature corresponding to 30% CH4 conversion during temperature-programmed reaction; T30) increases as the H2O concentration increases, showing a clear inhibition or deactivation effect that increases in magnitude with increasing H2O concentration.  14  In several cases the effects of H2O have been examined by measuring the CH4 conversion at steady-state, with and without H2O added to the feed gas. A typical set of data, reported by Persson et al. [42], is shown in Figure ‎2.1 using several Pd/Al2O3 catalysts reacted at 773 K. These data also show that added H2O significantly suppresses CH4 conversion, but the effect is at least partially reversible. Similar effects of H2O addition have been reported in the literature, as summarized in Table ‎2.2. These reports confirm that H2O acts as an inhibitor of CH4 oxidation over Pd catalysts and that upon removal of the H2O from the CH4/O2 reactant, the inhibition is partially reversible [63, 65].   Figure ‎2.1. Effect of H2O vapor on the activity for CH4 combustion over Pd/Al2O3 (■); 2:1PdPt/Al2O3 (); and 1:1PdPt/Al2O3 (o) at 773 K; 5 v.% of H2O was added to the 1.5 v.%CH4/air feed gas, GHSV = 100,000 h-1, for 5 h. [42] Copyright © 2007 Elsevier.  15    Table ‎2.1. Effect of H2O addition on T30 (temperature at 30% CH4 conversion) during temperature-programmed CH4 oxidation over Pd catalysts. Reference [22]  [69]  1.1%Pd/Al2O3 1.1%Pd/SnO2 1.1%Pd/Al2O3-36NiO  0.9%Pd/ZrO2a 0.9%Pd/ZrO2b GHSV, h-1 48,000  50,000 Dry feed gas composition, v.% 1%CH4/20%O2 in N2  0.4%CH4/0.05%CO/5%CO2/10%O2 in N2  T30, K  T30, K Added H2O, v.%        0 618 563 645  633 573 1 673 588 645  - - 5 703 608 693  - - 10 733 633 698  683 623 20 783 638 718  - -  a calcined at 873 K for 6 h; b calcined at 1273 K for 6h   16     Table ‎2.2. H2O inhibition over Pd catalysts during CH4 oxidation.   H2O addition  Reaction conditions  CH4 conversion, %   Catalyst Temp GHSV Feed gas Conc. Perioda Before H2O addition During H2O addition After H2O addition Comments Refs.  K h-1 v.% v.% min      0.1wt.% Pd/H-beta 673 120,000 0.2% CH4/10%O2 in N2 10 100 75 15 58 Conversion after 400 min TOS with periodic water addition [68] 1.3wt.% Pd/HTNU-10c 673 120,000 1%CH4/4%O2 in N2 5 900 43 8 40 Conversion  after 35 h TOS with periodic water addition [65] 5wt.% Pd/Al2O3 773 100,000 1.5%CH4 in air 5 300 95 13 30 Initial activity [42] 2wt.% Pd/Al2O3 823 160,000 0.4%CH4 in air 10 60 95 79 92 Conversion after 400 min TOS with periodic water addition [64] 1wt.% Pd/Al2O3 873 160,000 0.4% in air 8 60 95 90 93 Conversion after 300 min TOS with periodic water addition [67] a Period: Time length of H2O addition period; b TOS: Time-on-stream; c HTNU-10 is the H-form of a medium pore zeolite with Si/Al = 7.1    17  Reaction temperature is another key variable determining the overall effect of H2O addition. Although the data of Table ‎2.2 cannot be compared directly because of the different operating conditions, they do show that at 873 K, the decrease in CH4 conversion with H2O addition is much less significant than at lower temperatures (673 K). Several authors have proposed that the deactivation is related to the reaction of H2O with active PdO sites [22, 30, 63, 70, 71], PdO + H2O  Pd(OH)2, resulting in the formation of inactive Pd(OH)2, as first proposed by Cullis et al. [70]. Burch et al. [63] also reported a strong inhibitory effect of H2O on Pd catalysts up to 723 K. However, at higher temperatures the negative influence of H2O on the activity is very small, suggesting that above 723 K the reverse reaction (Pd(OH)2  PdO + H2O) occurs. Eriksson et al. [71] observed a significant decrease in CH4 conversion over a much wider range of temperatures (473 - 1073 K) after adding 18 v.% H2O to a CH4/O2 feed over a Pd/ZrO2 catalyst, which was likely due to the relatively high H2O concentration used in this study. Different results were reported by Kikuchi et al. [22] when adding 1 v.% H2O during CH4 oxidation over a Pd/Al2O3 catalyst i.e. a decrease in activity was observed up to about 723 K and no H2O inhibition was observed at higher temperatures. However, during addition of 20 v.% H2O, the inhibiting effect could be observed up to 873 K, in qualitative agreement with Eriksson et al. [71].  Further insight into the H2O adsorption/desorption phenomena on Pd/ZrO2 catalysts has been obtained using pulsed-flow experiments [50, 72]. Accordingly, pulses of CH4/O2/He were passed over a Pd/ZrO2 catalyst at various temperatures and the products monitored by mass spectrometer. The time at which the peak maximum for H2O appeared in each spectrum, compared to other products, was reported as the delay in the H2O peak. The data (Figure ‎2.2) 18  show that the H2O generated during CH4 oxidation lags other products, suggesting a slow H2O adsorption/desorption equilibrium, which might include spillover to the support. As the temperature increases above 723 K, the desorption rate of H2O increases and the delay in the H2O peak compared to the other products is insignificant. This behavior is in agreement with observations from other studies [45, 62, 63] that the desorption rate of H2O produced during CH4 oxidation is slow and on the order of seconds below 723 K, even though CO2, the other product of reaction, desorbs very quickly. Increasing temperature above 723 K removes the desorption time gap between CO2 and H2O, and thus, no inhibition by H2O occurs. Ciuparu et al. [50] also pulsed gas containing 3.45 v.% H2O/O2/He but no CH4 (and hence no reaction) through the same catalyst bed (Figure ‎2.2), showing that the H2O generated from CH4 oxidation lags the H2O added to the feed (Figure ‎2.2). These data indicate that the adsorption/desorption of H2O from the Pd catalyst surface is temperature dependent and reaches equilibrium at temperatures above ~723 K, even for H2O added in the gas phase.    19   Figure ‎2.2. Delay in the H2O peak with respect to other products obtained by passing pulses of CH4/O2/He (closed square) and 1v.%O2/3.45%H2O/He (open square) over Pd/ZrO2 at different temperatures. Reproduced with permission from [50]. Copyright © 2001 Elsevier.  Figure ‎2.3 compares temperature-programmed reaction (TPR) profiles for CH4 oxidation obtained over a Pd/ZrO2 catalyst, from both pulsed and continuous flow experiments with or without H2O added [50, 72]. The pulsed flow TPR profile is obtained by injecting pulses of the reaction mixture into a He stream every 3 min while ramping the temperature at 0.5 K min-1. Between consecutive pulses the catalyst is purged in flowing He. The pulsed flow data of Figure ‎2.3 show that at temperatures above 723 K, there is no H2O‎inhibition‎since‎the‎“dry”‎and‎“wet”‎reaction‎mixtures‎have‎the‎same‎conversion.‎At‎<‎723 K, inhibition is observed due to a low H2O desorption rate. When H2O is added to the gas phase, the H2O adsorption rate is enhanced and the rate of desorption is further decreased. With continuous flow of reactants and a 20  higher H2O concentration, H2O inhibition occurs at high temperatures due to re-adsorption. The addition of H2O to the feed directs the equilibrium towards more H2O adsorption on the surface and hence a greater decrease in catalyst activity during CH4 oxidation.   Figure ‎2.3. Temperature-programmed reactions during pulsed or continuous flow of reactants over Pd/ZrO2 with or without H2O in the feed. Reproduced with permission from [50]. Copyright © 2001 Elsevier.  The above observations are consistent with the following hypotheses: (1) product inhibition by H2O of CH4 oxidation on PdO catalysts occurs at temperatures below 723 K; (2) product inhibition by H2O is enhanced by its slow rate of desorption from the PdO catalyst relative to a higher rate of CH4 oxidation; (3) PdO and H2O may interact via the reversible reaction: PdO + H2O‎↔‎Pd(OH)2 to an inactive Pd(OH)2 phase, thus reversibly deactivating PdO catalysts as first proposed by Cullis et al. [70]; and (4) the extent of the CH4 oxidation reaction increases with 21  increasing temperature and H2O concentration in the gas phase, while the extent of inhibition by H2O during reaction, decreases with increasing temperature and increases with increasing H2O concentration. The possible role of hydroxyl groups in reversible deactivation is discussed in Section ‎2.1.2.  2.1.2 H2O Inhibition and Hydroxyl Formation  Although Pd(OH)2 has been assumed as a cause for deactivation of PdO catalysts in the presence of H2O [30, 63, 64, 70], and while this mechanism is consistent with many of the observations discussed above, recent evidence obtained from FTIR and isotopic labeling experiments that monitor the formation and conversion of hydroxyls on the catalyst surface during reaction suggest an alternative mechanism of deactivation.  Using DRIFTS, Persson et al. [42] reported an increase in signal intensity from surface hydroxyls (OHs) weakly H-bonded to the support (3200‒3800 cm-1) [73] after introducing 1.5 v.% CH4 in air to a PdO/Al2O3 catalyst at low temperature (473 K; Figure ‎2.4). The peak at 3016 cm-1 in Figure ‎2.4a, assigned to gas phase CH4, increases with time-on-stream because of catalyst deactivation. The hydroxyls have characteristic adsorptions at 3733, 3697, 3556 and 3500 cm-1, with the hydroxyls at 3697 and 3733 cm-1 assigned to bridged and terminal isolated hydroxyl species, respectively. Upon CH4 removal from the feed (Figure ‎2.4b), the peaks associated with OH species remain highly consistent with a slow desorption of OH species produced during CH4 oxidation. Hence, Persson et al. [42] suggested that catalyst deactivation on PdO/Al2O3 might be due to the formation and accumulation of hydroxyls on the catalyst surface, 22  bound either to the PdO, Al2O3 or the interface between the two [62]. Gao et al. [64] reported similar hydroxyl bands at 3733, 3697, 3556 and 3500 cm-1 during lean-burn CH4 oxidation (0.4 v.% CH4 in air) at 523 K. The FTIR spectra from reaction with 2 v.% H2O added to the CH4-O2 feed also yield a broad band at 3445 cm-1 that is associated with OH species on Al2O3 [64]   Figure ‎2.4. FTIR spectra of 5 wt.% PdO/Al2O3 at 473 K (a) during the CH4─O2 reaction, (b) desorption when CH4 was removed. Reproduced with permission from [42]. Copyright  2007 Elsevier. 23    Figure ‎2.5. FTIR spectra at highest surface coverage and 623 K on (1) PdO/Al2O3 during CH4-O2 reaction, (2) PdO/Al2O3 and (3) Al2O3 when injecting H2O pulses. Reproduced with permission from [62]. Copyright © 2004 Elsevier.  Ciuparu et al. [62] also identified three well-defined peaks at 3732 (OHI), 3699 (OHII), and 3549 (OHIII) cm-1 associated with surface hydroxyls generated during CH4 oxidation on a PdO/Al2O3 catalyst at 623 K using a feed gas of 0.128 v.% CH4/17.283% O2 in He/N2 (Figure ‎2.5). The spectrum was compared to that measured at the same temperature when injecting pulses of ~3% H2O into an air flow over the PdO/Al2O3 catalyst and the Al2O3 support (see Figure ‎2.5). Since Al2O3 has been shown to have a significantly lower hydroxyl coverage compared to PdO/Al2O3 when injecting H2O pulses at 623 K (the spectrum of Al2O3 is magnified by a factor of 15 in Figure ‎2.5), they conclude that the three peaks are associated with the presence of OH adsorbed 24  on the PdO catalyst surface. The higher hydroxyl coverage during CH4 oxidation compared to pulse injection of H2O onto PdO/Al2O3 catalyst, indicates that (1) adsorbed H2O is dissociated on the surface of PdO/Al2O3, and (2) hydroxyls formed from H2O pulses are less strongly bound to the surface than hydroxyls produced by the CH4 oxidation reaction.     Since the frequencies of the OHI and OHII species are shifted to higher wavenumbers for OH species more weakly bound to Pd, Ciuparu et al. [62] suggested that the high frequency peaks (OHI, OHII) can be assigned to terminal and bridged hydroxyl species, respectively, and the low frequency peak at ~3549 cm-1 with broad maximum values can be associated with OH species bound to different sites (multi-bound OHs; OHIII) (Figure ‎2.5). Transient temperature experiments show that the hydroxyl binding energy increases in the order OHI < OHII < OHIII [62].  25   Figure ‎2.6. The normalized peak areas of different surface OH species generated during lean-CH4-O2 reaction at 448 K. Reproduced with permission from [62]. Copyright © 2004 Elsevier.  The peak areas of the terminal, bridged, and multi-bound hydroxyls were monitored with time-on-stream at different temperatures during reaction, as illustrated by Figure ‎2.6 for reaction at 448 K [62]. Upon removal of CH4 from the feed, the peak areas for the bridged and multi-bound OH species continue to increase, whereas the area of the terminal OH species decreases (Figure ‎2.6). This decrease is attributed to the conversion of terminal OH species to bridged or multi-bound OH species. Based on the intensities of the various hydroxyl species at different temperatures, the authors propose the inter-conversion among the OH species as:                       where only terminal OH species recombine and desorb as H2O and the transformation of bridged OH species to terminal OH species is the rate determining step (RDS) for hydroxyl desorption and hence low temperature CH4 oxidation [62]. Importantly the authors 26  show that the surface coverage by the hydroxyls (Figure ‎2.6) correlates with the activity loss at low temperature, meaning that the activity loss and surface coverage have similar timescales, from which they conclude that the former is likely an effect of the latter [62].  FTIR spectra measured during CH4 oxidation at 598 K and 0.1v.%CH4/4%O2 in He over a series of 3 wt.% PdO catalysts supported on Al2O3, MgO, TiO2, and MCM-41 [45] show that the hydroxyl coverage is dependent on the support. On Al2O3, well defined peaks similar to those identified by Ciuparu et al. [62] were observed, but no common peak among all catalysts, that would provide evidence for Pd-OH bond formation, was present. A large contribution of OH bonding on the support makes it impossible to directly identify the presence of Pd(OH)2 on these supports [45, 64]. However, using 18O isotopic labelling and FTIR, these authors found that peaks associated with the accumulation of hydroxyls on PdO were not present. Hence, the more recent evidence suggests that deactivation by Pd(OH)2 formation is unlikely, in agreement with the experimental observation that Pd(OH)2/C decomposes in N2 at about 523 K [74]. In addition, evidence from temperature-programmed desorption studies of H2O adsorbed on PdO (101) thin films, suggests the formation of an OH-H2O complex at low temperature (< 400 K) and low coverage (< ½ monolayer), whereas H2O preferentially chemisorbs in molecular form at higher coverages [75].  Schwartz et al. [45] show, however, that catalyst deactivation during CH4 oxidation correlates with hydroxyl accumulation on the oxide support. The redox mechanism for CH4 combustion on Pd/PdO generally assumes dissociation of a CH4 molecule to yield a methyl fragment and a hydroxyl group (CH4 + Pd-O + Pd-*  Pd-OH + Pd-CH3, where Pd-* represents a vacancy) [44, 27  76]. H atoms are abstracted sequentially from the methyl group by neighboring Pd-O to form surface hydroxyl groups (Pd-OH) [44, 76]. Recombination of surface hydroxyl groups yields H2O and a surface vacancy (2Pd-OH  H2O + Pd-O +Pd-*), that is re-generated by O (2Pd-* + O2  2Pd-O) [44, 76]. Based on their experimental studies, Schwartz et al. [43, 45], proposed that during lean-burn CH4 oxidation, O2 molecules dissociate on Pd-* sites and exchange with O on the support so that Pd active sites are re-oxidized with oxygen from the support during the catalytic reaction as follows: Pd-O + S-*  Pd-* + S-O          ‎2.1 Pd-* + S-Os  Pd-Os + S-*          ‎2.2 and overall:  Pd-O + S-Os  Pd-Os + S-O          ‎2.3 where S represents the support, S-* is an O vacancy on the support and Os represents an O atom associated with the solid oxide. This mechanism suggests the possibility that a primary cause for catalyst deactivation is hydroxyl accumulation on the support, which hinders the oxygen migration and exchange process.  Isotopic labeling experiments are summarized in Figure ‎2.7, during which isotopically labeled catalysts Pd18O/Al216O3 and Pd18O/Mg16O [43] were exposed to 18O2/He flow at 673 K. An increase in 16O18O signal intensity with time is proposed to arise from oxygen exchange with the catalyst support [45]. The 16O18O signal (see lower separate dashed line in Figure ‎2.7) is reduced when H216O is injected to the feed and is recovered when H216O is removed. Apparently, hydroxyl groups tend to migrate to the oxide support rather than desorb. By increasing the concentration of hydroxyl groups, through addition and dissociation of H2O, the O exchange of 28  Pd-* active sites with the oxide support (S-Os) is interrupted. Thus, the number of PdO sites participating in the CH4 oxidation reaction decreases with time, as H2O dissociates and OH coverage of the support increases, with a consequent decrease in CH4 conversion [45]. This proposed mechanism of catalyst deactivation is believed to occur at temperatures below 723 K. Finally, the authors note that the rate of deactivation on the Pd/Al2O3 catalysts, with higher concentration of hydroxyl during reaction, is higher than on catalysts containing a support with higher oxygen mobility (Pd/MgO) [43, 45].  Ciuparu et al. [77] also reported on pulsed experiments with 18O2 over pure Pd and Pd/ZrO2 catalysts, oxidized before reaction, to clarify the effect of hydroxyls on the surface oxygen exchange. They determined that due to the slow recombination of hydroxyls and hence H2O desorption from the Pd catalyst surface during CH4 oxidation (2Pd-OH  H2O + Pd-O +Pd-*), the isotopic exchange of oxygen with the Pd sites (see Figure ‎2.8) occurs before H2O desorption from the surface. The oxygen vacancies on the PdO surface resulting from H2O desorption are thus rapidly filled by oxygen from the PdO bulk or oxide support (Pd-* + S-Os  Pd-Os + S-*). In fact, in this unsteady-state experiment, the labeled oxygen pulsed through the catalyst bed, is purged from the reactor before H2O is desorbed [77]. These observations are in agreement with the studies of Schwartz et al. [43, 45] already discussed and confirm that the accumulation of hydroxyls on the Pd catalyst surface impedes the oxygen exchange and limits Pd catalyst activity.   29   Figure ‎2.7. Oxygen exchange of (a) 3wt.% Pd18O/Al216O3 (top) and (b) 3wt.% Pd18O/Mg16O (bottom) with catalyst supports in a flow of 18O2/He at 673 K. H216O was injected at some time to probe its effect on oxygen exchange. Reproduced with permission from [45]. Copyright 2012 American Chemical Society.   Figure ‎2.8. Schematic of oxygen exchange during CH4 oxidation using labeled pulsed experiments. Reproduced with permission from [77]. Copyright © 2002 Elsevier.  30  2.2 Catalyst Sintering through Hydrothermal Aging of Pd Catalysts  Thermal aging in the presence of H2O (HTA) has been used to evaluate the stability of the catalysts for application in catalytic converters [1, 16, 23, 35-37, 39, 48, 78]. At high temperatures (> 1073 K) where PdO  Pd0 transformation is complete [38, 47], high temperature aging results in Pd0 sintering [1, 36]. Continuous Pd0 sintering during HTA in 10 v.% H2O/N2 over Pd/Al2O3 at 1173 K for up to 192 h, is suggested to be responsible for the loss in catalyst performance reported in this study [36]. Pd loading is shown to have a limited effect on Pd0 sintering. The Pd dispersion of various Pd loaded catalysts (from 0.1 wt.% to 7 wt.% Pd) ranges from 0.9% to 1.9% after HTA at 1173 K for 96 h [36]. McCarty et al. [39] also observed that thermal aging of a 15 wt.% Pd supported on SiO2-stabilized Al2O3 in O2/H2O/CO2/SO2/N2 at 1173 K and 1000 kPa for a period of 4000 h led to catalyst deactivation as a consequence of Pd0 sintering. The Pd0 sintering mechanism is attributed to either Ostwald ripening (atomic migration) or particle migration and coalescence, which might be influenced by support phase transformations [39].  The alumina support undergoes significant sintering and surface area loss during HTA as a consequence of phase transformations [35, 36, 39]. It is likely that Al2O3 phase transformations move Pd particles from mesopores or small macropores into macropores and hence lead to particle migration [39]. However, Xu et al. [35] reported that support sintering made only a small contribution to Pd0 sintering. The hydrothermal stability of a ball-milled Pd/Al2O3 catalyst, a process used to prepare washcoats that accelerates Al2O3 phase transformations, is compared to that of a fresh Pd/Al2O3 catalyst. The ball-milled catalyst had a considerable surface area loss 31  (e.g. 89.7 m2/g (fresh) to 23.8 m2/g (aged)) after HTA for 192 h in 10 v.% H2O/N2 compared to the non-milled catalyst (e.g. 92.7 m2/g (fresh) to 68.8 m2/g (aged)) even though the Pd dispersion remained almost identical for both catalysts (e.g. ball-milled catalyst: 3.60% (fresh) to 0.57% (aged); non-milled catalyst: 2.33% (fresh) to 0.60% (aged)) [35].  Even though Pd0 sintering through particle migration and coalescence is likely [39] during HTA of Pd catalysts at 1173 K, Ostwald ripening is suggested as the governing Pd0 sintering mechanism [36, 39]. According to Hansen et al. [37], the sintering rate of metal nanoparticles depends on their size. For nanoparticles < 3 nm in diameter, Ostwald ripening is the most likely sintering mechanism. For larger particles (3 - 10 nm), both Ostwald ripening and particle migration and coalescence may occur, but the sintering rate is much slower than for the smaller particles [37]. The particle sintering rate has also been shown to correlate with the vapor pressure of the surface species [60]. Pd is unique among the PGMs in that the oxide (PdO) has a much lower vapor pressure than the metal (Pd), and consequently, one would expect a very low sintering rate of PdO by Ostwald ripening [60]. The rate of sintering is also dependent on the support. Lamber et al. [79] suggested that on SiO2 in the presence of H2O, the formation of silanol (Si-OH) groups favors the migration and coalescence of Pd, whereas in the absence of H2O, Ostwald ripening is favored. Sintering suppression has been demonstrated for Pt catalysts, using supports that enhance the metal-support interaction [61]. Nagai et al. [80] demonstrated a correlation between the O electron density of the support, the strength of the Pt-O interaction and the resulting particle size. Thus, supports with a stronger metal-support interaction have a higher O electron density and yield smaller stable Pt particles in the order SiO2 < Al2O3 < ZrO2 < TiO2 < CeO2 [61, 80]. 32   At low to moderate temperatures (< 1073 K), where PdO is thermodynamically stable [47], limited studies report on thermal aging of PdO catalysts with or without added H2O. Escandon et al. [16] examined the effect of HTA on a 1 wt.% Pd/ZrO2-Ce catalyst. The catalyst was hydrothermally aged at 573, 698, and 823 K in 2 v.% H2O/air for 30 h, before being evaluated for CH4 oxidation under lean-burn conditions (0.5 v.% CH4 in dry air). The results, shown in Figure ‎2.9, are compared with the same catalyst, thermally aged at 823 K in dry air for 30 h (identified as Pd/ZrO2-Ce-550 in Figure ‎2.9) [16]. A significant irreversible decrease in CH4 conversion occurs and the extent of catalyst deactivation increases with aging temperature (Figure ‎2.9). The T50 increases from 648 K (375 C) for the fresh oxidized catalyst (identified as Pd/ZrO2-Ce in Figure ‎2.9), to 723 K (450 C) for the air-aged catalyst and > 823 K (550 C) for the hydrothermally aged catalyst. The Pd dispersion and BET surface area of the aged catalysts remain unchanged [16]. Comparing the activity results of the catalyst thermally aged in air (Pd/ZrO2-Ce-550) with that aged in 2 v.% H2O/air at 823 K (Pd/ZrO2-Ce-550h), confirms that catalyst deactivation increases in the presence of H2O. The stability of the aged catalysts was also evaluated, using both isothermal deactivation experiments at 773 K and light-off measurements made after 50 h reaction with 0.5 v.% CH4 in air. The catalyst aged in the presence of H2O at 573 K shows strong deactivation whereas the catalyst aged in the presence of H2O at 698 K has much stronger resistance to deactivation, and after 25 h time-on-stream is the most active of all the catalysts examined. XRD analysis of the catalysts show that the more stable catalysts are associated with the most stable supports [16].  33   Figure ‎2.9. CH4 conversion over fresh 1 wt.% Pd/ZrO2-Ce catalyst compared to1 wt.% Pd/ZrO2-Ce thermally aged in air at 823 K (Pd/ZrO2-Ce-550) and hydrothermally aged at different temperatures in 2 v.% H2O/air or air (identified as Pd/ZrO2-Ce-TTTh where TTT is the aging temperature in C). Reproduced with permission from [16]. Copyright © 2008 Elsevier.  Other studies in this temperature region report the effect of HTA on PdO catalysts by monitoring the H2O–CH4–O2 reaction with time on stream [48, 67, 81]. The effect of long-term aging of 0.6 wt.% Pd/NiO-Al2O3 catalyst with different loading of Ni in lean-CH4 oxidation (0.4 v.% CH4, 4% H2O, balance of air; GHSV 80,000 h-1) at 873 K up to 3200 h was reported [81]. It is claimed that the formation of NiAl2O4 crystals increases Pd dispersion on the surface of the support and enhances the hydrothermal stability of the catalyst in lean-CH4 oxidation [81], whereas no information about Pd dispersion after HTA has been reported. Zhang et al. [82] reported that CH4 conversion during long-term aging (for up to 100 h) of a Pd/ZSM-5 catalyst, prepared by 34  either impregnation or ion-exchange methods and reacted at 703 or 753 K in H2O–CH4–O2, was stabilized after a small initial decrease in activity, whereas in CH4–O2, the CH4 conversion continuously decreased. The authors realized that the change in CH4 conversion correlated with the change in Pd dispersion and H2O addition not only slowed down the rate of change in Pd dispersion but also stabilized the CH4 conversion [82]. On the other hand, Liu et al. [67] reported that HTA of PdO supported catalysts in a CH4-H2O-O2 reaction at 873 K for up to 150 h resulted in PdO sintering (section ‎2.3; Figure ‎2.10) [67]. It is clear that there is no agreement on the effect of H2O on PdO catalysts during HTA in this temperature region.   Several studies have demonstrated that catalyst sintering can be reduced by encapsulating Pd/PdO nanoparticles in support materials. Sinter-resistant Pd catalysts have been prepared by atomic layer deposition of Al2O3 overlayers on Pd [83], as well by the synthesis of Pd/SiO2 core-shell structures [84, 85]. Cargnello et al. [32] reported a Pd/CeO2 core-shell catalyst supported on Al2O3 for CH4 oxidation‎that‎was‎about‎200’xs‎more‎active‎than‎an‎equivalent‎Pd-CeO2/Al2O3 catalyst prepared by wet impregnation. The authors demonstrate that the Pd cores remain isolated even after heating the catalyst to 1123 K and that the CH4 light-off curves (measured at GHSV of 200,000 h-1 in a feed gas of 0.5 v.% CH4, 2% O2 in Ar) are the same for the fresh catalyst and one that has been aged at 1123 K for 12 h. The Pd nanoparticles encapsulated by CeO2 enhance the metal-support interaction that leads to exceptionally high CH4 oxidation activity and good thermal stability [32].   35  2.3 Support Effects  The effect of H2O on CH4 oxidation over Pd catalysts has been shown to depend on the support. The metal-support interaction can affect the chemical or thermal stability and hence the dispersion of Pd catalysts [22]. Kikuchi et al. [22]  examined the CH4 oxidation activity of three oxide supported catalysts (Al2O3, SnO2, Al2O3-36NiO) with identical Pd loadings in the presence of 0‒20 v.% H2O (T30 and other reaction conditions for these catalysts were presented in Table ‎2.1) and showed that the activity of Pd/SnO2 and Pd/Al2O3-36NiO decreased and then stabilized with an increase in H2O concentration while the Pd/Al2O3 activity decreased monotonically. The deactivation behavior of Pd/Al2O3 in the presence of high concentrations of H2O is related to higher H2O heat of adsorption and hence, higher coverage of H2O on the catalyst surface during CH4 oxidation, compared to Pd/SnO2 and Pd/Al2O3-36NiO [22]. Eriksson et al. [71] also reported the support effect by measuring the CH4 oxidation activity of various Pd supported catalysts and showed that the Pd activity was affected by the support materials and increased in the following order: 5 wt.% Pd/La-CeO2 < 5 wt.% Pd/Zr-CeO2 < 5 wt.% Pd/ZrO2. Adding 18 v.% H2O to the feed significantly decreased Pd activity for all three catalysts and did not change the above order. Liu et al. [67] examined the catalyst activity and hydrothermal stability of 1 wt.% Pd/Al2O3 modified by various metal oxides (Mn, Fe, La, Mg, and Ni) in CH4 oxidation with added H2O at 873 K (Figure ‎2.10). Addition of MgO and NiO to Al2O3 support forms spinel phases of MgAl2O4 and NiAl2O4 in the supports, respectively, not observed in other modified Pd/Al2O3 catalysts. Formation of spinel phases correlates with improving catalyst activity and stability of Pd/Al2O3 catalysts [67].     36   Figure ‎2.10. Hydrothermal stability of Pd/Al2O3 modified by metal oxides (Mn, Fe, La, Mg, Ni) in reaction of 0.4 v.% CH4, 4 v.% H2O balance of air at 873 K and GHSV 80,000 h-1. Reproduced with permission from [67]. Copyright © 2013 Elsevier.  The acid strength of the oxide supports was also reported to influence the catalyst activity and stability of Pd catalysts in CH4 oxidation. Yoshida et al. [56] investigated the activity of Pd catalysts in CH4 oxidation as a function of acid strength of various oxide supports (Table ‎2.3). A decrease in the acid strength of oxide support shows an increase in the Pd oxidation state and also an increase in the Pd activity in CH4 oxidation at 623 K. Nevertheless, basic supported catalysts (e.g. Pd/MgO), in spite of highest Pd oxidation state, indicates lower CH4 oxidation activity. The authors believe that the excess stabilization of PdO phase on the basic supports in the form of Pd oxo-anions (e.g. PdOxδ-), which prevents them from participating in catalysis, is the cause of lower CH4 oxidation activity [56]. Liu et al. [67] also reported that the higher activity and hydrothermal stability of Pd/Al2O3 catalysts modified by MgO and NiO could be attributed to a decrease in the acid strength of the support by formation of spinel phases (MgAl2O4 and NiAl2O4) [67]. The authors clarify that the weak acidity of the support with its 37  non-electrophilic behavior decreases the Pd oxidation state and hence prevents from formation of inactive Pd(OH)2, contrary to Yoshida et al.'s [56] observations. Furthermore, the interaction of spinel phases with Pd clusters plays a key role in maintaining the higher Pd dispersion of these catalysts after CH4-H2O-O2 reaction [67], resulting in greater catalyst stability in the presence of H2O, as also mentioned in other study by Liu et al. [81].  Table ‎2.3. Effect of acid strength of support materials on CH4 oxidation activity of Pd catalysts (adapted from [56]) Support Acid strengtha (H0) CH4 conversionb MgO 22.3 12 ZrO2 9.3 4 Al2O3 3.3 59 SiO2 -5.6 57 SiO2-ZrO2 -8.2 21 SiO2-Al2O3 -11.99 9 SO42--ZrO2 -13.16 10 a measured by Hammett indicator; b reacted in 0.25 kpa CH4 and 3 kpa O2 at 623 K  A more recent study prepared Pd/ZrO2 catalysts with the ZrO2 supports previously calcined at various temperatures (973–1273 K). The catalyst activity and stability of Pd/ZrO2 in CH4 oxidation in the presence or absence of 3 v.% H2O at 873 K shows a strong dependency on support calcination temperature and is highest when the ZrO2 support is calcined at 1173 K. The higher Pd/ZrO2 catalyst activity and stability in CH4 oxidation is attributed to a higher Pd oxidation state [48]. 38   The effect of support hydrophobicity on CH4 oxidation in the presence or absence of H2O was also reported [20]. Commercial hydrophilic SiO2, Aerosil 130, and hydrophobic SiO2, R972, with similar surface areas were loaded with 1 wt.% Pd. The hydrophobic character of the Pd/R972 catalyst is a result of the OH groups being replaced with methyl groups, as confirmed by IR spectroscopy [20]. As shown in Figure ‎2.11A and B, both catalysts have similar behavior in CH4 oxidation at 598 K when no H2O is added to the feed (dry feed). By passing the dry feed through the catalysts, CH4 conversion first increases for 1 - 2 h to reach a maximum and then gradually decreases as a function of time on stream. Introducing 3 v.% H2O to the feed (wet feed) after 2 h of reaction considerably decreases the CH4 conversion, reflecting the inhibition effect of H2O discussed in detail in section ‎2.1. When H2O addition to the feed is stopped, the CH4 conversion partially returns to the level observed before the addition of H2O. However, the catalysts show some deactivation when exposed to H2O compared to the case where only dry feed gas is used. Calculating the adsorption constant (KH2O) from equation (2.1) when taking into account 3 v.% H2O in the feed (wet feed) leads to a value of KH2O = 8500 L mol-1 for Pd/Aerosil-130 and 23700 L mol-1 for Pd/R972, suggesting higher H2O inhibition on the hydrophobic catalyst (Pd/R972). The authors clarify this behavior by the fact that hydrophilic supports with higher tendency for adsorbing H2O (e.g. Aerosil-130) reduce the local H2O concentration around the PdO active sites, resulting in considerable decrease in H2O adsorption compared to the hydrophobic supports (e.g. R972). Therefore, the high hydrophobicity of the support cannot decrease the inhibition or deactivation effect of H2O in CH4 oxidation over Pd catalysts [20].   39   Figure ‎2.11. Effect of support hydrophobicity on CH4 oxidation over Pd catalysts at 598 K; (A) Pd/Aerosil 130; (B) Pd/R972; reaction condition: 1.5 v.% CH4, 6 v.% O2, and 3 v.% H2O added in wet feed, GHSV 27,000 cm3 g-1 h-1. Reproduced with permission from [20]. Copyright © 2005 Elsevier.  Supports with higher oxygen surface mobility are shown to decrease the inhibitory effect of H2O on PdO catalysts [62]. FTIR studies of the CH4-O2 reaction at 623 K on the surface of PdO supported on various metal oxides (Al2O3, ZrO2 and Ce0.1Zr0.9O2) reveal that higher oxygen mobility in the supports, increasing in the order of Al2O3 < ZrO2 < Ce0.1Zr0.9O2, leads to lower surface hydroxyl coverage and also higher rate of hydroxyl desorption from the catalyst surface during CH4 oxidation [62]. Schwartz et al. [45] also reported that the rate of deactivation of 40  PdO/MgO was lower compared to PdO/Al2O3 in CH4 oxidation, correlated to higher oxygen mobility of MgO support previously discussed in Section ‎2.1.2, although PdO/MCM-41, which had lower oxygen mobility, did not show any deactivation in CH4 oxidation.          2.4 Kinetic Consequences of H2O on CH4 Oxidation over Pd Catalysts  The rate of CH4 oxidation over Pd catalysts is influenced by temperature, reactant partial pressures, the state of the Pd at reaction conditions (Pd0, PdO or a sub-oxide), possibly Pd crystallite size (i.e. may be structure-sensitive) and inhibition by products, H2O and CO2. Consequently, kinetic parameters reported in the literature vary over a wide range and this is especially true of the apparent activation energy for CH4 oxidation [24]. As noted by Carstens et al. [86], rate data must account for the inhibition effect of H2O when determining the activation barrier, but Ciuparu et al. [72] has shown that the correction is complicated by the fact that the effect of H2O inhibition is temperature dependent.  For example, the apparent activation energy for CH4 oxidation over a Pd/ZrO2 catalyst was estimated to be 180 kJ/mol from data measured at temperatures below 465 K, whereas a value of 87 kJ/mol was obtained at temperatures above 465 K [50]. The higher value of the apparent activation energy at lower temperatures was attributed to the strong inhibiting effect of H2O on the Pd catalyst.   Zhu et al. [52] reported kinetic parameters for CH4 oxidation over a series of model Pd and PdO surfaces and foils, and compared the values of literature data on supported Pd catalysts (Table ‎2.4). From Table ‎2.4 the reaction orders for CH4 and O2 are probably not sensitive to the structure of the Pd catalyst, although on the supported catalysts the reaction orders for H2O vary 41  from -0.25 to -1.3. Taking account of the error in the Eac estimates ( 20 kJ/mol), Zhu et al. [52] concluded that on the large single-crystal model catalysts, the activation energies are similar and the combustion of CH4 over Pd is not sensitive to the structure of the catalyst. Larger Eac values are reported for the Pd/oxide-supports (150–185 kJ/mol) corrected for the effect of H2O (assuming an order of -1) [86], whereas the much smaller Eac for the Pd/zeolite catalysts (72–77 kJ/mol) are possibly associated with the high acidity and high OH surface concentration of zeolites, in obvious contrast to the observed inhibition by OH groups for PdO supported on conventional supports. The negative orders of reaction for H2O are indicative of varying degrees of inhibition of CH4 oxidation by H2O on Pd and PdO surface and catalysts.               42    Table ‎2.4. Kinetic parameters for CH4 oxidation over Pd catalysts.  Eac Reaction order Temp. range Refs Catalyst kJ/mol CH4 O2 H2O K  Model Catalysts:       Pd foil 125 0.7 -0.1 0.05 568-633 [87] Pd (111) 140 0.7 -0.1 0.05 568-633 [52] Pd (100) 130 0.9 0.01 0.07 568-633 [52] PdO foil 125 0.7 0.2 -0.9 568-633 [52] PdO(111) 140 0.8 -0.1 -0.9 568-633 [52] PdO(100) 125 0.8 0.1 -1.0 568-633 [52] Supported Catalysts:       Pd black 135 0.7 0.1 -0.8 568-633 [87] 8.5%Pd/Al2O3 150 1 0 -1 505-633 [52, 59] 0.5%Pd/Al2O3a 60 0.90 0.08 -1.3 to -0.9 513-673 [51] 10%Pd/ZrO2 185 1 0 -1 505-633 [52, 59] 5%Pd/ZrO2 185 1.1 0.1 -1.0 523-553 [44] 1%Pd/ZrO2 172 1 0 -1.0 500-714 [20] 1%Pd/SiO2 - 1 0 -0.25 500-714 [9, 20] 2.8%Pd/H-mordenite 77 0.7 -0.1 -0.4 615-690 [65] 2.5%Pd-H-beta 72 0.5 0.2 -0.5 615-690 [65] a Eac, determined under dry reaction conditions, correction for H2O inhibition   43  The role of H2O in the inhibition of PdO catalysts during CH4 oxidation has been related to the adsorption and slow desorption of H2O on active sites during reaction. Kikuchi et al. [22] proposed a kinetic model assuming competitive adsorption between H2O and CH4 on PdO sites, where dissociative CH4 adsorption was assumed to be the rate determining step (RDS) and the coverage by C-species was assumed to be negligible. The main elementary steps of the reaction are postulated as follows:               -            ‎2.4              -   -           ‎2.5 Which yield the kinetic equation [22]:                               ‎2.6      is exponentially dependent upon the H2O adsorption enthalpy (     ). To increase the activity and durability of the Pd catalysts in the presence of H2O,      should be small according to the above reaction model. Based on measured      values for supported Pd catalysts, Pd/Al2O3 had the highest adsorption enthalpy (       -49 kJ mol-1) compared to Pd/SnO2 (-31 kJ mol-1) and Pd/Al2O3-36NiO (-30 kJ mol-1) (Table ‎2.5) despite the lower activation energy calculated for Pd/Al2O3 (see Table ‎2.5) [22]. A higher         implies stronger H2O adsorption on the surface and is evidence for a higher coverage of active sites by H2O molecules on Pd/Al2O3 catalysts and consequent lower catalyst activity. However, it also predicts for Pd/Al2O3 a more rapid decrease in      with increasing temperature.    44  Table ‎2.5. Estimated kinetic parameters for CH4 oxidation using the rate equation                       [22]. Catalyst Pd loading  wt.%     kJ/mol       for H2O kJ/mol Pd/Al2O3 1.1 81 -49 Pd/SnO2 1.1 111 -31 Pd/Al2O3-36NiO 1.1 90 -30           The larger negative value in the order of H2O for the 1% Pd/ZrO2 catalyst, compared to the Pd/SiO2 catalyst, as reported by Araya et al. [20] (Table ‎2.4), reflects a stronger H2O adsorption on ZrO2 than on the SiO2 [20].  Hurtado et al. [51] observed a change in the power-law reaction order of H2O from -1.3 to -0.9 as the temperature increased from 573 K to 623 K using a H2O-CH4-O2 reactant mixture and a commercial 0.5 wt.% Pd/γ-Al2O3 catalyst. Considering the equation proposed by Kikuchi et al. [22], with           , the H2O reaction order will reduce to -1 and if          is small, the H2O reaction order reduces to a value approaching zero.   Hurtado et al. [50] also attributed the inhibition effect of H2O during reaction to the adsorption of H2O on Pd catalysts. Based on this assumption the authors examined several Eley-Rideal, Langmuir-Hinshelwood and Mars-van-Krevelen kinetic models finding that by considering competitive adsorption between H2O and CH4 on Pd oxide sites and slow desorption of products, the following kinetic model:                      (          )                ⁄                       ‎2.7 45  where the ki are rate constants. This model provides the best fit of their measured rate data. The       estimated from Equation 2.3 was -54.5 kJ/mol, in agreement with the data of Table ‎2.5.  The inhibiting effects of H2O are assumed to be a consequence of a competitive adsorption between CH4 and H2O on PdO sites. Deactivation by H2O was previously thought to be due to formation of inactive Pd(OH)2 that does not participate in the CH4 oxidation reaction. Hurtado et al. [50] also note that the formation of Pd(OH)2 is thermodynamically favored from PdO sites rather than from Pd0. However, the more recent mechanism involving H2O inhibition of the O exchange between Pd-* sites and oxide supports, proposed by Schwartz et al. [42, 44] (see earlier discussion) appears to be supported by more definitive data.  2.5 Summary  The performance of Pd catalysts in CH4 oxidation is significantly affected by the presence of H2O. The effects of H2O on Pd catalysts are summarized as follows:   (a) H2O acts an inhibitor for CH4 oxidation over Pd catalysts at low temperatures (< 723 K). The H2O inhibition is related to slow desorption of H2O from the catalyst surface compared to the higher CH4 oxidation rate. The extent of H2O inhibition during the CH4-O2 reaction decreases with increasing reaction temperature and decreasing H2O concentration in the feed.  (b) Formation of inactive Pd(OH)2 as a result of the PdO + H2O ↔ Pd(OH)2 reaction is postulated to play a significant role in reversible deactivation of Pd catalysts in CH4 oxidation at low temperatures. 46   (c) Alternatively, hydroxyl accumulation on the catalyst surface during CH4 oxidation is proposed to deactivate Pd catalysts at low temperatures. Oxygen migration and exchange between Pd-* vacancy sites and the oxide support, shown to be an important step in CH4-O2 reaction, is interrupted by the formation of hydroxyls on the oxide supports. A high concentration of hydroxyls on the support surface and similar hydroxyl peaks during FTIR studies on Pd supported catalysts provide no evidence for formation Pd-OH bonds in CH4 oxidation.  (d) HTA of Pd catalysts at higher temperatures (> 773 K) is shown to deactivate catalysts. At temperatures where Pd0 is thermodynamically favored, catalyst deactivation occurs through Pd0 sintering, which might be influenced by support sintering. At temperatures where PdO is thermodynamically favored, a detailed study dealing with the deactivation mechanisms of PdO catalysts in the presence of H2O on CH4 oxidation is lacking. There is no agreement about governing deactivation mechanisms during HTA of PdO catalysts at these temperature conditions.  (e) Oxide support properties (e.g. acid strength and oxygen surface mobility) seem to affect Pd-support interactions through a changing Pd oxidation state, Pd dispersion, and tendency for hydroxyl accumulation. Oxide supports are shown to affect the stability of Pd catalysts in CH4 oxidation in the presence of H2O.  47  (f) Reaction rates of CH4 oxidation are shown to have a negative order with respect to H2O as a consequence of the H2O inhibition effect. The kinetic models in the literature are based on the assumption that there is competitive adsorption between CH4 and H2O on PdO active sites, which forms inactive Pd(OH)2, as the consequence of reversible reaction of PdO + H2O ↔ Pd(OH)2. None of the kinetic studies consider the recent mechanism of interruption of oxygen exchange between PdO and support by hydroxyl accumulation on the support. Furthermore, kinetic studies in the literature mostly consider a limited range of temperatures (Table ‎2.4), so that a kinetic model able to cover the wide range of temperatures (423–823 K) and incorporate the effect of H2O relevant to exhaust condition of NGV converters is lacking.              48  Chapter 3 Experimental  3.1 Catalyst Preparation  All Pd catalysts investigated in this thesis were prepared using incipient wetness impregnation. Generally, oxide supports were ground and sieved to a particle size of 90–354 µm, dried at 378 K for 8 h, before being placed inside a desiccator for 2 days. Then, 5 g of the dried oxide support was impregnated using a Pd precursor solution with known concentration. The impregnated catalysts were equilibrated for 48 h before being dried for 8 h at 393 K in ambient air. The dried catalysts were sieved again to 90–354 µm before further treatment.  Pd/SiO2 catalysts reported in Chapter 4 were prepared by impregnating SiO2 (Sigma-Aldrich, 1.15 cm3 g-1 pore volume, 60–200 mesh) with a PdCl2 (Sigma-Aldrich, 99% purity) solution, prepared by adding a few drops of 0.5 N HCl and heating to enhance dissolution, a procedure described by Sotoodeh and Smith [88]. The impregnated sample was equilibrated and dried as noted above before being calcined in He (Praxair, UHP) for 6 h at 773 K following heat-up at a rate of 5 K min-1, to ensure removal of residual Cl [88]. The calcined sample was then reduced in H2 (Praxair, UHP) for 1 h at 673 K and finally cooled in He to room temperature (Catalyst A of Table ‎4.1). The reduced catalyst was further oxidized in situ in a flow of dry air (Praxair, Extra dry air) at 723 K for 15 h (Catalyst B of Table ‎4.1) to ensure stabiliziation of PdO, following heat-up at a rate of 10 K min-1, before activity assessment and characterization. Other thermal treatments of Catalyst A and B investigated in Chapter 4 are presented in Table ‎4.1.  49  Pd catalysts in Chapters 5, 6, and 7 were prepared by impregnating SiO2, γ-Al2O3 (Sasol, 0.75 cm3 g-1 pore volume, 2.5 mm diameter, 97.6% purity), SnO2 (Sigma-Aldrich, 99.99% purity, 6.95 g/mL density), and α-Al2O3 (prepared by heating the γ-Al2O3 powders at 1273 K for 4 h in ambient air) with Pd(NO3)2 (Sigma-Aldrich, 99% purity) dissolved in 10 wt.% HNO3 (prepared from concentrated HNO3 (BDH, 69-70% purity)) solution. The impregnated sample was equilibrated and dried as already described, calcined in situ in dry air for 15 h at 723 K following a heat-up at 10 K min-1, and then cooled to room temperature before further treatment.  3.2 Catalyst Characterization  3.2.1 N2 Adsorption-desorption  BET (Brunauer-Emmett-Teller) surface area, pore volume, average pore size, and pore size distribution of the catalysts was determined from the N2 adsorption-desorption isotherms measured at 77 K using a Micromeritics ASAP 2020 analyzer. About 0.15 g of sample was placed inside a glass tube with a seal frit and degassed in vacuum at 473 K for 4 h prior to analysis. The surface area was calculated using the BET isotherm. The pore volume was calculated based on the total amount of N2 adsorbed on the surface at 0.995 relative pressure (P/P°). The desorption isotherm for each sample was analyzed using the Barrett-Joyner-Halenda (BJH) method to determine the average pore size and pore size distribution. More details are provided in Section ‎A.1.  50  3.2.2 Atomic Absorption Spectroscopy  The Pd loading of the prepared catalysts was determined using flame atomic absorption spectroscopy (AAS; GBC 904 AA). A known amount of sample (10‒40 mg depending upon the nominal Pd loading of the sample) was digested in a concentrated acid mixture (adding stepwise 1 cm3 HNO3 (20% purity), 2 cm3 HCl (20% purity) and 1 cm3 H2SO4 (96% purity)) for a week. The digested samples were diluted in a 100 cm3 flask with H2O (distilled and deionized) prior to AAS analysis. Standard solutions with various Pd concentrations (0‒15 µg cm-3) were also prepared to calibrate the AAS. All Pd loadings reported in the thesis are after the AAS analysis.        3.2.3 CO Chemisorption  The CO uptake of the catalysts was measured by pulsed CO chemisorption carried out using a Micromeritics AutoChem 2920 analyzer. A sample (~0.2 g) of the catalyst was degassed at 473 K for 2 h in a 50 cm3 (STP) min-1 flow of Ar (Praxair, UHP), cooled to 373 K, held for 1 h, reduced in a 50 cm3 (STP) min-1 flow of 10 v.% H2/Ar (Praxair, certified purity) for 1 h, flushed in He and finally cooled to chemisorption temperature, 298 K. Pulses of CO (0.0405 cm3 (STP)) were injected into the reduced catalyst bed until no further CO uptake was observed, using a thermal conductivity detector (TCD) to monitor the exit gas CO concentration.    51  3.2.4 Temperature-programmed Oxidation  The AutoChem 2920 analyzer was also used to estimate the degree of Pd oxidation of Pd/SiO2 catalysts in Chapter 4 and to verify the temperature at which Pd oxidation occurs. The dried SiO2 impregnated with PdCl2 was calcined in Ar (Praxair, UHP) for 6 h at 773 K following heat-up at 5 K min-1 from room temperature. After cooling to room temperature, the calcined catalyst was reduced in a 30 cm3 (STP) min-1 flow of 10 v.% H2/Ar for 1 h at 673 K following heat-up from room temperature at a rate of 10 K min-1. After cooling to room temperature in He, the reduced sample was then oxidized in a 30 cm3 (STP) min-1 flow of 10 v.% O2/He (Praxair, certified purity) while heating at a rate of 10 K min-1 and monitoring the O2 uptake in the exit flow with a TCD. The oxidized sample was again cooled in He and re-oxidized at the same conditions to confirm that no further oxygen consumption occurred. The amount of O2 consumed during the experiment was used to estimate PdO content after oxidation by assuming the reaction stoichiometry 2Pd + O2  2PdO. Hence the degree of oxidation based on the total amount of Pd on the catalyst before temperature-programmed oxidation (TPO), was calculated.  3.2.5 X-ray Diffraction  X-ray diffraction (XRD) patterns of the catalysts were collected using a Bruker D8 Focus (LynxEye‎detector)‎with‎CoKα1‎radiation (λ = 1.79 Å) a 35-kV source, an angle scan range from 3° to 80° with a 0.04° step size and 0.8 s time steps. The crystallite size was estimated using the Scherrer equation (see Section ‎A.2).   52  3.2.6 X-ray Photoelectron Spectroscopy  X-ray photoelectron spectroscopy (XPS) was carried out using a Omicron & Leybold Max200 X-ray‎photoelectron‎spectrometer.‎Al‎Kα‎was‎used‎as‎the‎photon‎source‎generated‎at‎15‎kV‎and‎20 mA. The pass energy was set at 192 eV for the survey scan and 48 eV for the narrow scan. The XPS spectra were corrected to the C1s peak at 285.0 eV. Besides the survey scans indicating the catalyst surface components, narrow scans (of e.g. Pd, Si, Al, Sn, and O) were used to determine their oxidation states, and Pd/metal atomic ratio representing the Pd dispersion.   3.2.7 Transmission Electron Microscopy  Transmission electron microscopy (TEM) images were obtained using a FEI Tecnai G2 instrument, 200kV LaB6 filament, with a 1.4Å point-to-point resolution. The samples were ground in a pestle and mortar for a set period, dispersed in ethanol and sonicated for 5 min. A single drop of the dispersion was placed on a 300 mesh copper grid coated with Formvar-Carbon film before analysis. More than 150 clusters were counted and measured from the TEM images of each pre-treated sample and then fitted by a lognormal distribution to determine the average particle size. The high resolution transmission electron microscopy (HRTEM) images were also obtained by the same unit to identify the d-spacing of Pd/PdO crystals and the possible Pd particle occlusion by the support.    53  3.2.8 Raman Spectroscopy  Raman spectra were collected using a ReniShaw Invia Raman Microscope and a 785 nm laser, with 10% laser power and 200 s acquisition time. Sample aggregates (~ 160 µm size) were analysed at room temperature to identify PdO in pre-treated Pd/SiO2 catalysts in Chapter 4. The laser power and acquisition time were optimized to obtain the highest signal intensity for the PdO peak at ~650 nm. Raman spectra of the pure SiO2 support were compared to the PdO/SiO2 catalyst to identify PdO.  3.2.9 Thermal Gravimetric Analysis  Thermal gravimetric analysis (TGA) was performed using a Shimadzu TGA-50 thermogravimetric analyzer. The sample was located‎in‎a‎α-Al2O3 ceramic crucible, flushed with a 50 cm3 (STP) min-1 N2 (Praxair, UHP), dried at 393 K (10 K min-1) for 2 h, and then heated to a maximum temperature of 973 K (5 K min-1) while monitoring the weight changes of the sample. Note the commercial Pd(OH)2/C (Sigma-Aldrich) was heated to 773 K (5 K min-1).  3.3 Catalyst Testing  3.3.1 Experimental Setup  The experimental setup for catalyst testing (see Figure ‎3.1) consisted of a stainless steel, fixed-bed microreactor operated in plug flow (length: 4.5 cm; inside diameter: 0.7 cm) placed inside an 54  electric tube furnace with a PID temperature controller. Two thermocouples (K-type), inserted inside the reactor, measured the temperature at the top and bottom of the catalyst bed. The catalyst (90–354‎ μm) was diluted four times (volume/volume) with inert SiC pellets (90–354 μm) to ensure isothermal operation. Feed flows were set using electric mass flow controllers (Brooks 5850 TR) and a syringe pump (Harvard Apparatus Pump 11 Elite), able to provide the desired flowrates of the feed gas components CH4 (0.76 v.% CH4/Ar, Praxair, certified purity or 9.93 v.% CH4/He, Praxair, certified purity), O2 (Praxair, UHP), H2O (distilled and deionized), Ar (Praxair, UHP), He (Praxair, UHP) and air (Praxair, Extra dry air). Note that instead of using N2, the most abundant component of the exhaust gas, Ar and He were used. At the exhaust gas temperatures of NGVs, N2 is expected to act as an inert, as do Ar and He. A separate furnace (pre-heater) in the gas flow line before the reactor furnace heated the reactants to 373 K. This was especially important in the case of adding H2O to the gas stream, where H2O was pumped, mixed with other gas reactants, and then transformed to vapor in the pre-heater before reaching the reactor. The gas flow lines connecting the pre-heater, the reactor and the quadropole mass spectrometer (QMS) were held at the same temperature as the pre-heater (373 K) using heating tapes. Theoretical calculations using the appropriate criteria from the literature, summarized in ‎Appendix D and E, ensured the isothermal operation and plug flow pattern of the fixed-bed reactor.   Reactants and products were analyzed by a VG ProLab quadropole mass spectrometer (ThermoFisher Scientific) that continuously monitored the reactor exit gas line, through detecting and recording the signal intensity of mass peaks corresponding to CH4, O2, H2O, CO2, He, and Ar. The QMS was calibrated by passing a mix of CH4, CO2 (0.1859 v.% CO2/Ar, 55  Praxair, certified purity), and inert gases to obtain the gas concentration at the exit of reactor and quantify CH4 converted during reaction (see ‎Appendix C).    Figure ‎3.1. Schematic diagram of experimental setup.  3.3.2 Temperature-programmed CH4 Oxidation  Temperature-programmed CH4 oxidation (TPMO) was used to evaluate the catalyst activities. Accordingly, 0.1 g of the dried catalyst diluted by 2.5 g SiC pellets was loaded inside the reactor before calcining the catalyst in situ by flowing 100 cm3 (STP) min-1 of dry air and heating to 723 K (10 K min-1) for 15 h and cooling to 393 K. Then, a gas mixture of 0.1 or 0.5 v.% CH4, 20% O2, 0, 2, 3, 5 or 10% H2O and balance of He and Ar (desired feed gas) flowed over the calcined catalyst (GHSVs 90,000‒270,000 cm3 (STP) g-1 h-1) before the reactor temperature was ramped 56  from 393 K to 873 K at 5 K min-1 while the exit stream composition was continuously monitored using a QMS. Subsequently the CH4 conversion as a function of temperature was determined to provide the initial activity of the catalyst for CH4 oxidation. Blank TPMO runs with no catalyst loaded in the reactor with or without added H2O to the feed revealed that CH4 conversion started to increase at temperatures above 700 K and reached to ~25% at 873 K (see Section ‎B.1), reflecting the catalytic effect of walls as a function of temperature.   3.3.3 Hydrothermal or Thermal Aging  The stability of the catalysts was assessed by thermal aging of the catalysts in the reactor at various temperatures and times before evaluation by TPMO. Similar to the TPMO experiment, 0.1 g of the dried catalyst diluted with 2.5 g SiC particles was loaded inside the reactor, calcined in situ by a flow of 100 cm3 (STP) min-1 of dry air at 723 K (10 K min-1) for 15 h, and cooled to ambient temperature. Then, the calcined catalyst was flowed with 100 cm3 (STP) min-1 of air or 6.5 v.% H2O/air followed by heating up at 10 K min-1 to the desired temperature, held at that temperature for the desired period of time, cooled to 393 K in Ar and finally purged in Ar for 1h. The thermal aging temperature and time conditions were chosen according to the NGV exhaust conditions (temperature and water content) and an aging period of sufficient time to ensure that deactivation of the PdO catalysts and their related property changes could be observed.       57  3.3.4 Kinetic Experiments  Kinetic experiments were performed on the 0.8 wt.% Pd/Al2O3 catalyst at steady-state with CH4 conversions below 10% (differential method) to obtain intrinsic kinetic data. A diagnostic test and theoretical calculations, summarized in ‎Appendix F, ensured that kinetic data were not mass-transfer-controlled. Similar to the TPMO experiment, 0.1 g of the dried catalyst diluted with 2.5 g SiC particles was loaded inside the reactor, calcined in situ by a flow of 100 cm3 (STP) min-1 of dry air at 723 K (10 K min-1) for 15 h, and cooled to the desired reaction temperature. A gas mixture of 0.5 v.% CH4, 20% O2, 0 or 2% H2O and balance of He and Ar flowed over the calcined catalyst (GHSV 270,000 cm3 (STP) g-1 h-1) and held for 0.5 h at the desired temperature while monitoring the exit flow composition by QMS. Afterwards, the experiment was continued by increasing the temperature to another setpoint. To ensure the repeatability of the experiment and also to see any changes in the catalyst activity during the course of experiment, the temperature was decreased to its first setpoint and the experiment was repeated.          58  Chapter 4 Activity of PdO/SiO2 Catalysts for CH4 Oxidation Following Thermal Treatments2  4.1 Introduction  HTA has been used to evaluate the stability of catalysts in catalytic converters [16, 36, 39]. The effect of HTA on Pd supported catalysts is shown to depend upon the temperature. At temperatures where PdO  Pd0 transformation is complete, catalyst deactivation occurs through Pd0 sintering that is influenced by the support [36, 39]. At temperatures where PdO is the stable phase, a detailed study focusing on HTA of PdO catalysts and deactivation mechanisms in CH4 oxidation relevant to NGV catalytic converters is lacking. In this chapter, a conventional Pd/SiO2 catalyst was prepared as described in Section ‎3.1. The effect of thermal treatment of PdO/SiO2 catalysts in air versus air/H2O at temperatures < 973 K and below the PdO thermodynamic decomposition temperature is reported. The loss in catalyst activity is discussed in terms of characterization data obtained before and after thermal aging.                                                       2 A version‎ of‎ this‎ chapter‎ was‎ already‎ published‎ at‎ “Rahman Gholami, Kevin J. Smith, Activity of PdO/SiO2 Catalysts for CH4 Oxidation Following Thermal Treatments. Applied Catalysis B: Environmental 168 (2015) 156-163.” 59  4.2 Results  4.2.1 Catalyst Aging in Air and Air/H2O  The effect of different thermal environments (shown in Figure ‎4.1) on the catalyst properties is investigated first. The reduced 7.7 wt.% Pd/SiO2 catalyst (A) was calcined at 723 K (Catalyst B) or 873 K (Catalyst C) for 15 h in a 100 cm3 (STP) min-1 air flow and then cooled to room temperature in air. Subsequently, Catalyst B was thermally aged for a further 16 h in 100 cm3 (STP) min-1 of dry air at 973 K (Catalyst D), or hydrothermally aged in a 100 cm3 min-1 STP flow of 6.5 v.% H2O in air at the same temperature (973 K; Catalyst F). The activity of catalysts B, D and F was evaluated by TPMO (Figure ‎4.2). The data show that HTA significantly deactivate the PdO/SiO2 catalyst (F) when compared to thermal aging in dry air (D), as reflected in the shift in the light-off curves of the HTA sample towards higher temperature (Figure ‎4.2). The T50 (the temperature at which 50% of the CH4 had been converted) increases from 521 K for the calcined catalyst (B) to 544 K for the air-aged sample (D), compared to 623 K for the HTA sample (F).   60     Figure ‎4.1. Schematic diagram of Pd/SiO2 catalyst thermal treatements.   Reduced in H2 at 673 K for 1 h Catalyst A Catalyst B Catalyst C Catalyst D Catalyst E Catalyst F He-calcined Pd/SiO2 Heated in air at 723 K for 15 h Heated in air at 873 K for 15 h Thermally aged in air at 973 K for 16 h Reduced in H2 at 673 K for 1 h Hydrothermally aged in air/H2O at 973 K for 16 h 61   Figure ‎4.2. Thermal aging effect on 7.7 wt.%  Pd/SiO2 catalysts; (■) Catalyst B, (●) Catalyst D, (▲) Catalyst F; Catalyst identity follows Table  4.1. TPMO condition: 0.1 v.% CH4 and 20% O2 in Ar and He, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1.  Table ‎4.1 summarizes the properties of the 7.7 wt.% Pd/SiO2 catalysts after the thermal treatments, showing significant differences in the properties after the thermal treatment in air versus air/H2O. The TEM analysis of the catalysts, summarized in Figure ‎4.3, shows that the PdO TEM cluster size increases from 2.2 nm (Catalyst B) when the reduced Pd/SiO2 (Catalyst A) is calcined at 723 K, to 3.2 nm when calcined in air at 873 K (Catalyst C). Aging in dry air at 973 K for a further 16 h (Catalyst D), yields an average PdO cluster size of 13.5 nm, compared to 3.3 nm after aging in air/H2O at 973 K for a further 16 h (Catalyst F).  These data show that calcination of the reduced Pd/SiO2 (Pd cluster size ~16 nm), yields smaller PdO particles, that grow upon further thermal treatment in air or H2O, but the PdO growth rate is slower in H2O 62  than in air.  Furthermore, the data for Catalyst E show that reduction of the small PdO particles (Catalyst B) yields Pd particles about half the size of those present in the unused Pd/SiO2 catalyst (Catalyst A) i.e. the thermal oxidation/reduction of the catalyst and the associated change in PdO/Pd size, is partly reversible. The data of Table ‎4.1 also show that the PdO/Pd cluster size data trends based on the TEM analysis are consistent with the XRD and CO chemisorption data. However, the PdO size measured by XRD is larger than that measured by TEM (e.g. for Catalyst F), and this difference is attributed to the limitation of XRD in detecting small crystallites (< 5 nm). As shown in Figure ‎4.3, a portion of the PdO particles (e.g. for Catalyst F) is very small (< 5 nm) and cannot be detected by XRD. Another contributor to the difference is the fact that TEM cluster size is a number-based average but XRD PdO size is a volume-based average calculated by the Scherrer equation.  63     Table  4.1. Thermal aging effects on the properties of 7.7 wt.% Pd/SiO2.      Cluster size  Catalyst Treatment BET area Pd/Sia Phase XRD TEM CO uptakeb   m2/g at% - nm nm µmol/g A Reduced in H2 at 673 K for 1 h 314 0.92 Pd 15 (111) 16±11 62 B Catalyst A heated in air at 723 K for 15 h 291 1.29 PdO 5 (101) 2±1 174 C Catalyst A heated in air at 873 K for 15 h 291 1.55 PdO 6 (101) 3±1 160 D Catalyst B air-aged at 973 Kc for 16 h 268 1.31 PdO 23 (101) 14±6 36 E Catalyst B reduced in H2 at 673 K for 1 h 304 0.82 Pd 7 (111) 8±5 254 F Catalyst B H2O-aged at 973 Kd for 16 h 274 0.98 PdO 11 (101) 3±1 137 a Determined by XPS; b CO chemisorption after reduction in H2 at 373 K; c Thermally aged at 973 K in 100 cm-3 (STP) min-1 of air for 16 h; d Hydrothermally aged at 973 K in 100 cm-3 (STP) min-1 of 6.5 v.% H2O/air for 16 h    64    Figure ‎4.3. TEM images and cluster size distributions of 7.7 wt.% Pd/SiO2 catalysts. Catalyst identity follows Table  4.1.     65  The XRD patterns of Figure ‎4.4 show that as the severity of the oxidation in air increases from 723 K for 15 h (Catalyst B) to an additional 16 h at 973 K (Catalyst D), there is less broadening of the PdO reflections (increased cluster size) and the intensity of the x-ray reflections increases. The transformation of Pd to PdO in the presence of air is known to be a relatively slow process, that is dependent upon temperature [34, 89]. Consequently the changes in the XRD patterns likely reflect both increased conversion of Pd to PdO and growth of the PdO crystallites as the oxidation severity increases. The Raman spectra measured for selected catalysts (Figure ‎4.5) are consistent with increased Pd to PdO conversion with increased oxidation severity. The peak at approximately 650 cm-1 in Figure ‎4.5, present in all four samples (B, C, D and F), corresponds to PdO [48, 90, 91]. By comparing the Raman spectra of SiO2 support and other samples (B, C, D and F) in Figure ‎4.5, one may conclude that the peak at approximately 625 cm-1 is associated with the SiO2 support. Interpretation of the spectra is complicated by the fact that the PdO signal intensity is only proportional to the amount of PdO present for PdO crystallites < 10 nm in size [90, 92]. Nevertheless, the spectra clearly show that increased severity of the oxidation increases the amount of PdO on the catalyst, since the most intense signal is obtained for Catalyst D, which has the largest PdO clusters. Hence we conclude that increased oxidation severity results in both increased PdO cluster size and more complete conversion of Pd to crystalline PdO.  The Raman spectra of the hydrothermally aged catalyst (F), when compared to the air aged catalyst (D), also show that in the presence of H2O there is both less growth of the PdO crystallites and less conversion of Pd to PdO. The XRD data of Figure ‎4.4 do not show the presence of Pd in the oxidized catalysts D or F, likely because of the low concentration of Pd in  66  the sample and the disordered Pd/PdOx structure that results after the oxidation at relatively low temperatures [92].   Figure ‎4.4. XRD patterns of 7.7 wt.% Pd/SiO2 catalyst following thermal treatments at the conditions described in Table  4.1. Catalyst identity also follows Table  4.1.  The oxidation of the Pd present in the reduced Catalyst A, to yield PdO, is further demonstrated by the XPS analysis of the 7.7 wt.% Pd/SiO2 catalyst (Figure ‎4.6). The XPS data show that after oxidation of Catalyst A at 723 and 873 K, PdO is present on the surface (Catalyst B and C), and re-reduction of this calcined sample, yields Pd0 (Catalyst E). The Pd/Si atomic ratio, which can be taken as a measure of Pd dispersion [93], increases after oxidation of Pd, corresponding to a reduced cluster size as observed from the TEM, XRD and CO chemisorption data. The reduction  67  of Catalyst B results in a decrease of Pd/Si atomic ratio to 0.82. It is apparent that the oxidized samples have higher dispersion in comparison to the reduced samples.   Figure ‎4.5. Raman spectrum of SiO2 support and 7.7 wt.% Pd/SiO2 following thermal treatments as identified in Table  4.1.  Experiments in which the stability of the 7.7 wt.% Pd/SiO2 catalyst was examined through repeated reaction and aging cycles were also completed. Catalyst B was examined by TPMO. Subsequently the catalyst was hydrothermally aged in a flow of 100 cm3 (STP) min-1 of 6.5 v.% H2O/air at 973 K for 4 h. The hydrothermally-aged sample was then cooled to 393 K in Ar and purged in Ar for 1 h. The TPMO and HTA steps were subsequently repeated several times. The T50 as a function of cycle number, reported in Figure ‎4.7, increases from 521 K to 549 K after 10 cycles. After the 2nd cycle the extent of catalyst deactivation, reflected in the increase in the T50  68  temperature at each cycle, is the same since the T50 increases linearly with cycle number. Note that the T50 decreases after the 1st cycle, a consequence of previously reported PdO restructuring and hysteresis effects that are a result of the exposure of the catalyst to temperatures above the calcination temperature (723 K) during the first TPMO test [34, 89]. Also note that the T50 after four cycles, corresponding to a 16 h cumulative HTA time, is 535 K, significantly lower than the T50 of the hydrothermally-aged Catalyst F reported in Figure ‎4.2 (623 K). Clearly, continuous aging for 16 h has more impact on the catalyst compared to the case where the catalyst is exposed to CH4/O2 at high temperature during TPMO between each of the 4-h HTA cycles. The sequential aging results show that the deactivation that occurs following exposure to air/H2O at high temperature is attenuated by the lower average partial pressure of H2O that exists during sequential aging, compared to continuous aging.              69     Figure ‎4.6. XPS narrow scan spectra of 7.7 wt.% Pd/SiO2 following thermal treatments as identified in Table  4.1.   70   Figure ‎4.7. The T50 as a function of recycle number for the 7.7 wt.% Pd/SiO2 catalyst with HTA in 6.5 v.% H2O/air at 973 K for 4 h after each TPMO; TPMO condition: 0.1 v.% CH4, 20% O2 in Ar & He, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1.  4.2.2 Effect of Pd Loading  The effect of HTA on Pd/SiO2 catalysts with different Pd loadings was also investigated. Catalysts with 0.6, 1.7 and 7.7 wt.% Pd supported on SiO2 were hydrothermally aged at 973 K in 6.5 v.% H2O/air for 16 h and then evaluated by TPMO. Figure ‎4.8 shows the activity results as a function of Pd loading. Note that an increase in Pd loading expectedly shifts the light-off curves towards lower temperatures because as the catalyst Pd loading increases, the mass of Pd per unit volume of reactor also increases since the mass of catalyst is held constant for each experiment. HTA significantly deactivates the Pd-loaded catalysts, reflected in the considerable shift in light-0 2 4 6 8 10515520525530535540545550  Temperature at 50% conversion (K)Recycle 71  off curves of the hydrothermally-aged catalysts toward higher temperatures (Figure ‎4.8). The surface area of the calcined and hydrothermally-aged catalysts with different Pd loadings are summarized in Table ‎4.2. Although there is a decrease in surface area after HTA, the pore size remains unchanged.   Table ‎4.2. HTA effect on surface properties; Pd/SiO2, pre-oxidized at 723 K in air 15 h (Fresh), aged at 973 K in 6.5 v.% H2O in air for 16 h (Aged).   Surface Area Pore size  Pore volume Pd loadinga  CO uptake Fresh Aged Fresh Aged Fresh Aged wt % µmol/gPd m2 g-1 nm cm3 g-1 0b - 310 266 11 11 1.1 1.0 0.6 500 344 282 10 10 1.1 0.9 1.7 2471 322 239 10 10 1.0 0.8 7.7 2260 291 274 11 10 1.0 0.9 a Pd/SiO2; fresh, calcined He 773 K, Reduced H2 673 K, then air, 723 K; b SiO2 support   72   Figure ‎4.8. Effect of HTA on (a) 0.6, (b) 1.7, (c) 7.7 wt.% Pd/SiO2 catalysts; (■) Fresh sample, pre-oxidized in air at 723 K for 15 h; (▲) Sample hydrothermally aged at 973 K in 6.5 v.% H2O/air for 16 h; TPMO condition: 0.1 v.% CH4, 20% O2 in Ar & He, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1.  The effect of Pd loading on the deactivation observed during repeated HTA and TPMO cycles is also investigated. For the 1.7 wt.% Pd/SiO2, the T50 after the 1st cycle (4 h HTA) decreases from 643 K to 577 K, showing an increase in catalyst activity (Figure ‎4.9). Further cycles show the expected catalyst deactivation, as observed for the 7.7 wt.% Pd/SiO2 catalyst (Figure ‎4.7). The T50 increases from 577 K to 765 K after four cycles. To clarify whether an increase in the  73  catalyst activity after the first 4 h HTA cycle is a result of the added H2O in the gas stream or not, the cycle of aging and activity measurements was carried out on the calcined 1.7 wt.% Pd/SiO2 catalyst, but the catalyst was aged in dry air rather than air/H2O. Figure ‎4.9 shows a sharp decrease in the T50 from 643 K to 584 K as observed in the previous experiment with H2O (Figure ‎4.7, after the 1st cycle) and then, the T50 decreases further to 564 K (Figure ‎4.9). If structural changes that occur during aging allow the PdO to re-structure and crystallize, yielding a more active catalyst, as noted in the literature [34, 89], then the data show again that restructuring of the catalyst in the presence of H2O is inhibited compared to that which occurs in a dry atmosphere.    Figure ‎4.9. The T50 as a function of recycle number for the 1.7 wt.% Pd/SiO2 catalyst: (■) thermally aged in air at 973 K for 4 h after each TPMO; (▲) hydrothermally aged in 6.5 v.% H2O/air at 973 K for 4 h after each TPMO. TPMO condition: 0.1 v.% CH4, 20% O2 in Ar & He, 5 K min-1, GHSV 180,000 cm-3 (STP) g-1 h-1. 1 2 3 4 5550600650700750800  Temperature at 50% conversion (K)Recycle 74  4.3 Discussion  Previous studies have shown that the oxidation of Pd0 occurs first through rapid uptake of a monolayer of O2, followed by the formation of a sub-oxide (PdOx) as Pd-O islands. Due to strong metal-support interactions, the islands tend to separate from each other and PdO re-dispersion occurs. The sub-oxide is slowly transformed to an active crystalline PdO phase [8, 34, 39, 63, 89, 91, 92, 94]. Crystalline PdO is believed to form at approximately 775 K [34]. Complete oxidation of the Pd is therefore difficult and by TPO of Catalyst A (not shown), we estimated about 35% of the Pd was converted to PdO following the initial calcination in air at 723 K for 15 h. Similar data have been reported previously [89, 91]. The data of Table ‎4.1 are consistent with the PdO nucleation mechanism described above. The PdO cluster size after calcination in air at 723 K is significantly smaller than the Pd cluster size, reflecting the re-dispersion of PdO. A further increase in temperature to 973 K sinters the PdO clusters and increases the Pd  PdO conversion (as shown by the Raman spectra). However, the data of Table ‎4.1 also show that HTA inhibited the sintering of the PdO. Furthermore, the catalyst activity after both the continuous aging and cyclic aging experiments shows significant deactivation after HTA.   Several studies have demonstrated the inhibition effect of H2O on PdO catalysts during CH4 combustion [34, 50, 62, 63]. The inhibition is strongly dependent on temperature, with inhibition less important at temperatures > 723 K because of desorption [50]. Surface hydroxyls that interrupt O-exchange are thought to be the cause of the inhibition [43, 45]. Studies in which H2O was added to the reactant gas have shown the inhibition to be reversible such that if the added  75  H2O is removed, the catalyst activity partially returns to that observed prior to H2O addition [16, 20]. On the other hand, when Pd catalysts are hydrothermally aged at elevated temperatures, they undergo significant, irreversible deactivation, as observed in the present study. Xu et al. [36] reported that the main deactivation mechanism of Pd/Al2O3 catalysts following exposure to 10 v.% H2O/N2 at 1173 K for up to 200 h was due to Pd sintering. A substantial decrease in Pd dispersion from 3.7% to 0.9% over 7 wt.% Pd/Al2O3 and similar decrease over other Pd loadings after 96 h HTA was observed. The BET surface area of the 7 wt.% Pd/Al2O3 catalyst was also reduced from 82.3 to 31.9 m2 g-1 with aging time, attributed to Al2O3 phase transformations. As noted by Xu et al. [36], aging the catalyst at 1173K ensured that PdO decomposition was complete and consequently the sintering observed was relevant to the behaviour of Pd0 rather than PdO. Furthermore, they showed that phase transformation of the Al2O3 during HTA did not considerably contribute to Pd sintering [35].   Escandon et al. [16] have examined the effect of HTA at lower temperatures, where PdO is thermodynamically stable. A significant irreversible decrease in CH4 conversion over 1 wt.% Pd/ZrO2-Ce after thermal aging in 2 v.% H2O/air at 573, 698, and 823 K for 30 h is reported, and the extent of catalyst deactivation increases with aging temperature. The T50 increases from 648 K for the fresh oxidized catalyst, to 723 K for the air-aged catalyst and > 823 K for the HTA catalyst. The Pd dispersion and BET surface area of the aged catalysts remain unchanged [16].  The data of Figure ‎4.2 show similar trends, where the largest deactivation of the PdO/SiO2 catalyst occurrs after HTA at 973 K (T50 = 623 K) compared to aging in dry air (T50 = 544 K ). However, the data of Table ‎4.1 show that sintering of PdO is inhibited in the presence of the  76  added H2O, so that the observed activity loss could not be a consequence of PdO cluster size increase alone.    The growth of catalytic particles on SiO2 supports is complex. According to Hansen et al. [37], the sintering rate of metal nanoparticles depends on their size.  For nanoparticles < 3 nm in diameter, Ostwald ripening (Figure ‎4.10a) is the most likely sintering mechanism. For larger particles (3–10 nm), both Ostwald ripening and particle migration and coalescence (Figure ‎4.10b) may occur, but the sintering rate is much slower than for the smaller particles [37]. The particle sintering rate has also been shown to correlate with the vapor pressure of the surface species [60].  Pd is unique among the PGMs in that the oxide (PdO) has a much lower vapor pressure than the metal (Pd) [60], and consequently, one would expect a very low sintering rate of PdO by Ostwald ripening [60]. On supported catalysts Ostwald ripening may lead to particles greater in size than the size of the support pores [37]. The data of Table ‎4.1 and Table ‎4.2 show that the PdO size is greater than the SiO2 support pore size (10-11 nm) only after extended aging in air at 973 K. These data are consistent with a very slow Ostwald ripening of the PdO particles.   77   Figure  4.10. Schematic of catalyst sintering through (a) Ostwald ripening (adapted from [12]), (b) particle migration and coalescence (adapted from [12]) and (c) PdO occlusion by the support  The rate of sintering is also very dependent on the support. For example, Lamber et al. [79] suggested that on SiO2 in the presence of H2O, the formation of silanol (Si-OH) groups favors the migration and coalescence of Pd, whereas in the absence of H2O, Ostwald ripening is favored. Furthermore, thermal aging of Pd/Al2O3 catalysts under lean conditions (16% O2, 10% CO2 balance of N2) at 973 K leads to no change in PdO cluster size [95], reflecting the stronger metal-support interaction of Al2O3 compared to SiO2 which limits PdO sintering. Nevertheless, the data reported herein suggest that the SiO2 also plays a key role in the catalyst deactivation observed in the present study. The sintering behavior of Pt catalysts on different supports is shown to be dependent upon metal-support interactions [61]. The sintering rate of Pt clusters correlates with electron density of oxygen in the oxide supports during thermal aging of Pt          (a) (b) Support Support     (c) Support Support  78  catalysts in air at 1073 K. As the electron density of oxygen in the support increases (CZY > CeO2 > TiO2 > ZrO2 > Al2O3 > SiO2), the Pt-oxide-support bond strengthens, which contributes to a decrease in sintering rate [80].  SiO2 desorbs physisorbed H2O at ~370 K whereas chemisorbed H2O (silanol groups – Si-OH) desorb at ~670 K [79].  The formation of hydroxyls according to the reaction:                                           ‎4.1                            is feasible at temperatures above 973 K [96, 97], the HTA temperature of the present study. The hydroxyls are expected to change the metal-support interaction [37, 79] and interrupts the oxygen exchange between the support and Pd active sites which is the key to high activity [45]. Furthermore, at high temperature, the hydroxyls on the support surface are mobile. Zhu et al. [98] reported the encapsulation of PdO active sites by SiO2 during CH4 oxidation at 598 K and during reduction in H2 at 923 K over a Pd/SiO2 catalyst. The authors suggest that high temperature, the H2O formed during reaction and the formation of Pd silicide during reduction followed by oxidation in O2 are all important factors promoting the encapsulation of PdO by the SiO2 (Figure ‎4.10c). The generation of SiO2 overlayers on Pd clusters by low temperature H2 reduction (573 K) has also been reported [99] and the migration of SiO2 over metal catalysts in systems containing H2O or  H2 are reported in the literature [79, 100, 101]. Given the H2 reduction conditions (673 K) of the present study, the occlusion of Pd clusters even before aging or reaction is therefore likely. HRTEM images and CO uptakes of thermally-aged catalysts from the present study (Figure ‎4.11) show the presence of amorphous layers on top of PdO after thermal aging in air (Catalyst D) or air/H2O (Catalyst F) and hence, these data do not clarify if the PdO occlusion is exacerbated in the presence of H2O. The XPS data of Table ‎4.1, on the  79  other hand, show that the Pd/Si ratio is significantly lower for the HTA PdO/SiO2 catalyst (Catalyst F) compared to the thermally aged catalyst (Catalyst D), despite smaller PdO particles on the former, and consistent with SiO2 overlayers reducing PdO growth, but also reducing activity by limiting access to PdO by the gas reactants. These data support the notion that migration of SiO2 and occlusion of PdO is responsible for the larger deactivation and inhibition by H2O observed for the HTA catalyst compared to the thermally aged catalyst. As shown above, PdO sintering is inhibited significantly by H2O during thermal aging in air/H2O versus air. It is expected that the migration of SiO2 and PdO occlusion establishes a strong metal-support interaction [79, 100], which hinders PdO sintering (Table ‎4.1). A similar mechanism of decreased sintering by oxide support overlayers has been demonstrated for Pd and Au catalysts. Several studies have reported enhanced sintering resistance of Pd catalysts after encapsulation by SiO2 [32, 84, 85, 102] and Au nanoparticles have been stabilized on oxide supports using overlayers of Al2O3 [83] and SiO2 [103] and by TiO2 modification of a silica support surface [104]. In addition, an increase in the surface area of Pd/SiO2 core-shell catalysts, with porous SiO2 shells and hence lower mass transfer hindrance for access of reactants to active sites, is believed to enhance catalyst activity, compared to conventional Pd/SiO2 catalysts [102]. In the present study, the encapsulation appears to also reduce PdO sintering, however, the support loses surface area and porosity and the thickness of the SiO2 overlayer is not controlled so that the catalyst activity decreases significantly after occlusion. Finally, we note that the CO uptake data of Table ‎4.2 suggest that occlusion of smaller PdO particles is more likely than for larger particles, since the CO uptake was lowest for the catalyst with the lowest Pd loading, which is expected to have the smallest PdO particles.    80   Figure ‎4.11. HRTEM images of 7.7 wt.% Pd/SiO2 showing amorphous layers on top of PdO: (a) Catalyst D; (b) Catalyst  F. Catalyst identity follows Table  4.1.  The possibility that catalyst deactivation was caused by other deactivation mechanisms was also considered.  Deactivation by carbon deposition was eliminated since the used catalysts were free of carbon. Although poisoning by other elements may also be possible in practise [12], the use of high purity gases in the experimental work presented here, makes this unlikely.  Furthermore, these deactivation mechanisms could not provide a plausible explanation for the observed changes in catalyst properties and performance.  4.4 Conclusions  PdO/SiO2 catalysts used for CH4 oxidation deactivate when aged in air or air/H2O at elevated temperatures (723–973 K). HTA of PdO catalysts results in a significant irreversible decrease in CH4 oxidation activity of the catalysts, compared to aging in air at the same temperature. The PdO catalysts sinter during aging in air and air/H2O, although the growth and restructuring of the  81  PdO clusters is suppressed by the presence of added H2O. The higher degree of catalyst deactivation in the presence of air/H2O compared to air, is mostly due to occlusion of PdO by SiO2. Amorphous overlayers of SiO2 on the PdO are confirmed by HRTEM and XPS analysis. Although HTA and H2-reduction likely both contribute to the PdO occlusion by SiO2, the XPS data show a significant increase in PdO occlusion after HTA that increases catalyst deactivation and decrease PdO sintering.                 82  Chapter 5 Effect of Hydrothermal Aging Conditions on the Activity of a PdO/SiO2 Catalyst for CH4 Oxidation  5.1 Introduction  In Chapter 4, HTA was found to significantly deactivate PdO/SiO2 catalysts used for CH4 oxidation, compared to thermal aging in dry air and several mechanisms of catalyst deactivation were determined. This chapter examines the effect of HTA conditions on a PdO/SiO2 catalyst for CH4 oxidation at HTA temperatures of 673‒973 K in high concentrations of H2O. Deactivation mechanisms during HTA are also identified based on several characterization techniques conducted before and after catalyst HTA, combined with measurements of the effect of catalyst aging temperature and time on PdO catalyst activity.  5.2 Results  The calcined catalysts were hydrothermally aged in 6.5 v.% H2O/air at 673, 773 and 973 K for 16 h prior to being examined by TPMO. The CH4 conversion as a function of HTA temperature demonstrates a considerable activity loss after HTA (see Figure ‎5.1). An increase in HTA temperature shifts the light-off curves and the temperature corresponding to 50% CH4 conversion (T50) to higher temperatures, reflecting a significant deactivation of PdO following HTA. The data of Figure ‎5.1 show that a significant HTA effect occurs at T > 673 K.    83   Figure ‎5.1. Effect of HTA temperature on CH4 oxidation activity of the 6.78 wt.% PdO/SiO2 catalysts. Catalysts hydrothermally aged for 16 h in 6.5 v.% H2O/air prior to TPMO evaluation.  The effect of aging time during HTA of PdO/SiO2 catalysts is reported in Figure ‎5.2, where TPMO data for the PdO/SiO2 catalysts hydrothermally aged for 16 and 32 h at low (673 K) and high (973 K) temperatures are shown. At low HTA temperature (673 K), an increase in aging time does not affect the catalyst activity significantly, whereas significant catalyst deactivation is observed after increasing the aging time at high HTA temperature (973 K), as reflected in the shift in T10 of HTA for 16 h (Catalyst D) compared to 32 h (Catalyst E) at 973 K (see Table ‎5.1).   84   Figure  5.2. Effect of HTA time on the CH4 oxidation activity results of the 6.78 wt.% PdO/SiO2 catalysts. Catalyst identity follows Table ‎5.1.  The fresh and HTA catalysts were characterized to identify changes in catalyst properties following HTA (see Table ‎5.1). The T10, T30, and T50 of the catalysts, representing a relative measure of the catalyst activity at the test conditions, increase consistently as HTA temperature increases and as aging time increases for the high HTA temperatures. The XRD patterns of the fresh and HTA PdO/SiO2 catalysts are presented in Figure ‎5.3. Similar diffraction patterns indicative of PdO are present before and after HTA, independent of the HTA temperature or time. The PdO crystallite size, estimated using the PdO (101) peak at 39.56°, is 7 nm for the fresh catalyst (A) and remains constant after HTA at 673 K (Catalysts B and C) (see Table ‎5.1). The data suggest that the PdO size increases as HTA temperature increases from 673 K to 973 K, although large standard deviation associated with the PdO size estimate from TEM (Figure ‎5.4)  85  suggest this effect is not large. Data in Table ‎5.1 also show that the PdO crystallite size remains unchanged as HTA time increases from 16 to 32 h at both low and high HTA temperatures (673 and 973 K, respectively).    Figure ‎5.3. XRD patterns of the pre-treated 6.8 wt.% PdO/SiO2 catalysts; PdO (▼).  Catalyst identity follows Table ‎5.1.      86      Table ‎5.1. HTA temperature and time effects on activity and properties of 6.8 wt.% PdO/SiO2 catalysts.  HTA conditions     PdO size (nm)     Catalyst Temperature Time BET area Pore size Pore volume Pd/Si XRD TEM CO uptake T10b T30b T50b  K h m2/g nm cm3/g at% nm nm µmol/g K K K A ‒a ‒ 300 11 1 1.39 7 7±3 72 490 521 542 B 673 16 277 12 0.9 1.05 7 8±2 74 500 533 553 C 673 32 270 11 0.9 0.92 7 7±2 77 495 530 555 D 973 16 277 11 0.9 1.07 10 9±4 65 641 780 872 E 973 32 250 11 0.8 1.15 10 10±3 46 817 > 873 > 873 a Fresh catalyst‒calcined at 723 K for 15 h; b Temperature at which 10, 30, and 50% CH4 in the feed is converted with 0.1 v.% CH4, 20% O2 and balance He and Ar, 5 K/min, GHSV 180,000 cm3 (STP) g-1 h-1   87   Figure ‎5.4. PdO size of 6.8 wt.% PdO/SiO2 catalysts after HTA for 16 h at various temperatures measured by XRD and TEM.  TEM images of the PdO/SiO2 catalysts show the morphology of PdO clusters on the surface of the SiO2 support (see Figure ‎5.5). Although in some regions small individual PdO clusters of 7 nm size can be observed, other regions show agglomeration of PdO. The size distributions of PdO are reported in Figure ‎5.6. Compatible with the XRD crystallite size estimates, the TEM data show that the PdO size remains constant after HTA at low temperature (673 K) but increases after HTA at high temperature (973 K). Despite of a broad cluster size distribution before and after HTA, the cluster size distributions (Figure ‎5.6) display a gradual shift towards larger sizes as the HTA temperature and time increase.    88         Figure ‎5.5. TEM images of the pre-treated 6.8 wt.% PdO/SiO2, evidence of PdO cluster growth after HTA; (A) Catalyst A, (B) Catalyst E. Catalyst identity follows Table ‎5.1          89   Figure ‎5.6. TEM cluster size distribution of the pre-treated 6.8 wt.% PdO/SiO2 catalysts. Catalyst identity follows Table ‎5.1.  XPS narrow scan data of Pd for the fresh and HTA PdO/SiO2 catalysts are reported in Figure ‎5.7. XPS peak analysis indicates that PdO (BE = 337.4 eV) is present both before and after HTA, but there is no evidence of Pd0 (335.6 eV) or Pd-OH (338.5 eV) formation during HTA. The Pd/Si atom ratio, representing the dispersion of PdO on the surface of the catalysts, is reported in Table ‎5.1. The Pd/Si ratio decreases significantly after HTA, e.g. from a value of 1.39 for the fresh catalyst (Catalyst A) to 1.05 (32% decrease) for the catalyst HTA at 673 K (Catalyst B).  90  The Pd/Si ratio remains almost unchanged as the HTA temperature and time increase (see Table ‎5.1; Pd/Si ratio of the PdO/SiO2 catalyst hydrothermally aged at 773 K for 16 h is 1.08 close to other HTA catalysts in Table ‎5.1).   Figure ‎5.7. XPS narrow scan spectra of Pd atom in the pre-treated 6.8 wt.% PdO/SiO2 catalysts. Catalyst identity follows Table ‎5.1.  CO uptake of the PdO/SiO2 catalysts increases marginally from 72 µmol/gcat for Catalyst A to 77 µmol/gcat (7% increase) after HTA at 673 K (Catalyst B and C), whereas it decreases to 65  91  µmol/gcat (Catalyst D) and 46 µmol/gcat (Catalyst E) after HTA at 973 K (56% decrease), suggesting considerable agglomeration of PdO clusters after high temperature HTA, compatible with the change in XPS Pd/Si ratio and increased PdO crystal size, as observed by TEM and XRD.  The BET surface areas of the PdO/SiO2 catalysts show an 8% decrease (from 300 to 277 m2/g) after 16 h HTA (not dependent on the HTA temperature; Catalyst B and D). An increase in aging time from 16 to 32 h causes a 3% decrease in the BET surface area (277 to 270 m2/g) at 673 K (Catalyst B and C), whereas at 973 K (Catalyst D and E) the BET surface area decreases by 11% (from 277 to 250 m2/g). The pore size and pore volume of the catalysts before and after HTA remain unchanged (11 nm and ~0.9 cm3/g, respectively). The pore size distribution (see Figure ‎5.8) also remains unchanged after HTA, even though these data suggest a partial collapse of a portion of the pores (homogenously from all pore sizes) specifically after 32 h HTA. BET surface area data reflect that the collapse of SiO2 support structure is gradual after HTA.     92   Figure ‎5.8. Pore size distribution of the pre-treated 6.8 wt.% PdO/SiO2 catalysts. Catalyst identity follows Table ‎5.1   TGA was used in an attempt to identify the formation of Pd(OH)2 during HTA. The catalysts were dried at 393 K in N2 for 1 h prior to ramping the temperature at a rate of 5 K/min in N2 up to 973 K and then monitoring the weight change. In order to determine the decomposition temperature of Pd(OH)2 (Pd(OH)2  PdO + H2O), a commercial 20 wt.% Pd(OH)2/C (Sigma-Aldrich) was also examined. The TGA results of Figure ‎5.9 indicate a major loss in the weight of the commercial sample (91% decrease in the hydroxide content of the dried commercial catalyst) at ~650 K. Previous work has reported 523 K as the Pd(OH)2 decomposition temperature by TGA in N2 when heated at a rate of 10 K/min over a 6% Pd(OH)2/C [74]. Performing the same TGA on the catalysts A and D reveals no changes in the catalyst weight (Figure ‎5.9). Together,  93  results of TGA, XRD, and XPS reported herein provide evidence that Pd(OH)2 is not formed during HTA and that PdO is the stable bulk phase of PdO/SiO2 catalyst during HTA.   Figure ‎5.9. TGA results of the pre-treated 6.8 wt.% PdO/SiO2 catalysts, Catalyst A (top) and Catalyst D (mid); commercial 20 wt.% Pd(OH)2/activated carbon (bottom) dried at 393 K. Catalyst identity follows Table ‎5.1  5.3 Discussion  Previous studies have reported significant deactivation of Pd catalysts during HTA [16, 39]. Escandón et al. [16] reported the effect of HTA at various temperatures (573–823 K) for 50 h on Ce-modified Pd/ZrO2 catalysts for CH4 combustion and observed a significant decrease in catalyst activity as HTA temperature increased, although no significant change in the Ce-  94  Pd/ZrO2 catalyst properties after HTA were observed. In the present study, however, significant changes in the properties of the PdO/SiO2 following HTA were observed (see Table ‎5.1).  PdO size increases at high HTA temperature and time (see Table ‎5.1 and Figure ‎5.4), corresponding to a decrease in catalyst activity (Figure ‎5.1). A decrease in PdO area with a loss in catalyst activity has also been reported during fuel-lean CH4 oxidation [26, 105]. HTA of Pd0 catalysts at elevated temperatures (> 1073 K, where Pd0 is stable rather than PdO) showed continuous growth of Pd clusters with time (up to 4000 h) [37, 39]. Pd0 sintering is attributed to both Ostwald ripening (atomic migration) and particle coalescence and migration. A more recent study identified three phases of Pd0 sintering during thermal deactivation at elevated temperatures i.e. phase one (clusters < 3 nm) characterized by a rapid decrease in dispersion and loss of activity, phase two (clusters 3–10 nm) where sintering slows as the majority of small clusters are consumed, and phase three (clusters > 10 nm) where sintering rate approaches zero [37]. The authors concluded that except for phase two, in which both Ostwald ripening and particle coalescence/migration mechanisms, play a role, Ostwald ripening is the governing sintering mechanism, as deduced from in-situ TEM studies [37]. According to the above, both sintering mechanisms might contribute to the growth of PdO clusters during HTA in the present work. However, Pd has a very high vapor pressure compared to its oxide phase (PdO) [60], suggesting that atomic migration is much less likely for PdO than for Pd. Thermal aging of Pd catalysts in oxidizing environments (at various oxygen concentrations) versus reducing environments at 873 and 1173 K also confirmed that PdO has more sintering resistance than Pd0 [38]. The fact that the PdO sizes of thepresent study (< 10 nm) are less than the catalyst pore size  95  (~11 nm) is evidence that most of the PdO sintering is also a result of particle migration and coalescence, rather than Ostwald ripening.  Although the PdO cluster size determined by XRD and TEM is similar for fresh and HTA catalysts (Table ‎5.1), XPS results shows a somewhat different trend. In previous work (‎Chapter 4 and [106]), a major decrease in the surface Pd/Si ratio (as measured by XPS) on a PdO/SiO2 catalyst during HTA at 973 K, compared to aging in air, was attributed to PdO occlusion by the SiO2 support and a change in the Pd-support interaction as a consequence of mobile hydroxyls (Si-OH) formed by adsorption of H2O on SiO2. The same phenomenon is suggested by the results of Table ‎5.1. The significant decrease in Pd/Si ratio after HTA at 673 K shown in Table ‎5.1, without a corresponding change is PdO size, is indicative of PdO clusters occluded by the SiO2. HRTEM images (Figure ‎5.10) of the fresh and HTA PdO/SiO2 catalysts (A and D, respectively) show evidence of crystalline PdO occluded by amorphous SiO2 (XRD data of Figure ‎5.3), indicating that the amorphous layers on top of the PdO clusters are the migrated SiO2 [98].   The data in Table ‎5.1 suggest that PdO occlusion by SiO2 occurs even at low HTA temperatures (673 K) i.e. the significant decrease in Pd/Si ratio (as measured by XPS) can be compared to the PdO cluster size after HTA at 673 K (Catalysts B and C in Table ‎5.1), which shows no change. However, the CO uptake results (Table ‎5.1), which provide a measure of the number of catalyst active sites, do not show a significant decrease at low HTA temperatures. Hence we conclude that at low HTA temperatures (673 K), the SiO2 support occludes the edges of PdO clusters so that most of PdO clusters remain accessible to participate in CH4 oxidation, whereas a further  96  increase in HTA temperature to 973 K, results in both PdO sintering and PdO occlusion by the SiO2 support that lead to a significant decrease in CO uptake and a loss in catalyst activity (Figure ‎5.1).   Figure ‎5.10. HRTEM images of the pre-treated 6.8 wt.% PdO/SiO2 catalysts, evidence of PdO cluster occlusion by SiO2 during HTA; (A) Catalyst A, (B) Catalyst D. Catalyst identity follows Table ‎5.1  An alternative deactivation mechanism for Pd catalysts during CH4 oxidation in the presence of added H2O is related to the formation of inactive Pd(OH)2 [20, 64, 68, 70]. Gao et al. [64] reported a small weight loss during TGA of a Pd/Al2O3 catalyst aged in the presence of H2O (0.4 v.% CH4, 10% H2O and air; 823 K; 4 h).  The weight loss occurred at ~493 K and was attributed to Pd(OH)2 decomposition. The HTA temperatures of this study (> 673 K) are above the decomposition temperature of Pd(OH)2 (< 650 K), as reflected in the above TGA studies. Furthermore, the data of Figure ‎5.9 show that PdO is the stable phase after exposing PdO/SiO2 catalysts to H2O at > 673K and there is no evidence of Pd(OH)2 formation at these conditions.  97  5.4 Conclusions  The effect of HTA time and temperature on the activity of PdO/SiO2 catalysts for CH4 oxidation is reported. HTA is shown to play a key role in deactivating the PdO/SiO2 catalysts at temperatures  673 K. An increase in HTA time significantly decreases the PdO activity at high HTA temperatures (973 K). PdO occlusion by the SiO2 support is responsible for deactivation of PdO/SiO2 catalysts at low HTA temperatures (673 K), whereas a combination of PdO sintering and PdO occlusion by the SiO2 support causes more significant catalyst deactivation at high HTA temperatures (973 K). The hydroxyl mobility on the catalyst surface as a result of interaction of SiO2 and H2O is believed to occlude the PdO clusters after HTA, and this occurs at low temperature (673 K). Bulk Pd(OH)2 formation at the HTA conditions of this study was not observed.            98  Chapter 6 Effect of Oxide Supports on the Activity of PdO Catalysts during Hydrothermal Aging in CH4 Oxidation  6.1 Introduction  Chapters 4 and 5 conclude that the PdO/SiO2 catalyst activity for CH4 oxidation decreases following HTA at high temperatures ( 673 K) for extended periods of time. A combination of PdO sintering and PdO occlusion by the SiO2 support are shown to contribute to the deactivation. Clearly, support stability during HTA plays a significant role in the performance of the PdO catalyst during CH4 oxidation. In this chapter data are reported to better understand the effect of HTA on PdO dispersed on various oxide supports. The effects of HTA on PdO catalysts with high surface area supports (γ-Al2O3 and SiO2) as well as low surface area supports (α-Al2O3 and SnO2) are investigated. The catalyst performance before and after HTA are compared and possible deactivation mechanisms are discussed.  6.2 Results  The PdO catalysts used in this study were prepared on different supports as described in Section ‎3.1 with similar Pd loadings, as measured by AAS (see Section ‎3.2.2). The catalyst properties are summarized in Table ‎6.1. The Pd loadings and PdO crystallite size of the catalysts are almost identical although the catalyst surface areas are significantly different. PdO/SiO2 and PdO/γ-Al2O3 are well-known catalysts for CH4 oxidation and have high surface area compared to  99  PdO/α-Al2O3 and PdO/SnO2. Despite a very low surface area (3 m2 g-1), PdO/SnO2 has been reported to have high catalyst activity for CH4 oxidation at low temperatures [22].    Table ‎6.1. Properties of catalysts used in this study. Catalyst Pd loading BET area PdO crystallite size  wt.% m2 g-1 nm PdO/SiO2 6.78 300 7 (101) PdO/γ-Al2O3 7.43 196 6 (101) PdO/α-Al2O3 7.54 64 8 (101) PdO/SnO2 5.94 3 6 (110)   The calcined PdO catalysts were examined using TPMO to compare their initial activity for CH4 oxidation (see Figure ‎6.1). The TPMO experiment was done using a mixed flow of 0.1 v.% CH4, 20% O2 and balance He and Ar (GHSV 180,000 cm3 (STP) g-1 h-1) as described earlier in Section ‎3.3.2. The CH4 conversion for all PdO catalysts in this study reached 100% before the temperature reached 673 K, indicating high catalyst activity for CH4 oxidation. PdO dispersed on high surface area supports (SiO2 and γ-Al2O3) have higher activity for CH4 oxidation than when dispersed on low surface area supports (α-Al2O3 and SnO2). In addition, the light-off curves for PdO/α-Al2O3 and PdO/SnO2 catalysts overlap, reflecting identical catalyst performance in CH4 oxidation.    100   Figure ‎6.1. Catalyst activity in CH4 oxidation for various calcined PdO supported catalysts. TPMO condition: 0.1 v.% CH4, 10% O2 and balance He and Ar, 5 K min-1, 180,000 cm3 (STP) g-1 h-1  6.2.1 HTA Effect on PdO/γ-Al2O3 versus PdO/SiO2 for CH4 Oxidation  PdO/γ-Al2O3 catalysts were hydrothermally aged in 6.5 v.% H2O/air at 973 K for different periods of time (0–65 h) and then evaluated by TPMO (see Figure ‎6.2). The same TPMO conditions as reported in Figure ‎6.1 were used to assess the catalyst activity after HTA. The CH4 conversion as a function of temperature shifts towards higher temperatures with increasing HTA time, indicating catalyst activity loss during HTA. The extent of catalyst deactivation increases as HTA time increases to 32 h whereas a further increase in HTA time to 65 h does not affect the catalyst activity. Comparing the catalyst activity results of PdO/SiO2 in Figure ‎5.2 after HTA at  101  973 K for 16 and 32 h to those on PdO/γ-Al2O3 in Figure ‎6.2, suggests a higher stability of the PdO/γ-Al2O3 catalyst during HTA.     Figure ‎6.2. Effect of HTA on catalyst activity of 7.4 wt.% PdO/γ-Al2O3 in CH4 oxidation. Catalysts were hydrothermally aged at 973 K in 6.5 v.% H2O/air prior to TPMO evaluation.   The change in catalyst properties of PdO/γ-Al2O3 and PdO/SiO2 as function of HTA time are reported in Figure ‎6.3. On SiO2, the PdO crystallite size increases from 7 nm to 10 nm after 16 h of HTA and then remains unchanged after 32 h of HTA, whereas on γ-Al2O3, PdO crystallite size increases from ~6 nm to ~7.5 nm after 16 h HTA followed by a gradual increase to ~8 nm after 65 h HTA (see Figure ‎6.3a). CO uptake, representing the Pd dispersion, continuously decreases as HTA time increases for both supports (see Figure ‎6.3b). CO uptake on PdO/SiO2 decreases by 56% after 32 h of HTA whereas CO uptake on PdO/γ-Al2O3 decreases by 21% after 65 h of HTA. The Pd/M (M: Si or Al) surface atomic ratio measured by XPS (Figure ‎6.3c) represents the  102  Pd dispersion and decreases during the course of HTA, compatible with the CO uptake results. The Pd/M ratio decreases from 2.1 to 1.5 (51% decrease) after 65 h of HTA of the PdO/γ-Al2O3 while it decreases from 1.4 to ~1.1 (27% decrease) after 32 h of HTA of the PdO/SiO2.   The BET surface area and pore size distribution of the fresh and HTA catalysts are reported in Figure ‎6.3d and Figure ‎6.4, respectively. The surface area decreases linearly as HTA time increases. The pore size distribution results (Figure ‎6.4) suggest a different sintering mechanism for each catalyst. The pore size distribution of PdO/γ-Al2O3 shifts towards larger pore sizes as HTA time increases although this phenomenon occurs rapidly in the first 16 h of HTA and then slows down as HTA increases. The pore size distribution of PdO/SiO2, on the other hand, indicates no shift towards larger or smaller pore sizes but suggests a homogenous collapse in all pore sizes as HTA time increases.   Comparing the results of PdO/γ-Al2O3 and PdO/SiO2 catalysts, one may conclude that the deactivation mechanisms for both catalysts during HTA at 973 K follow the same trends, even though their activity results suggest better catalyst stability of the PdO/γ-Al2O3 in comparison to the PdO/SiO2 during HTA.     103   Figure ‎6.3. Effect of HTA on catalyst properties of PdO/SiO2 and PdO/γ-Al2O3 as a function HTA time. M: Si or Al.    104   Figure ‎6.4. Pore size distribution (a) PdO/SiO2 and (b) PdO/γ-Al2O3 as a function HTA time (0 h: fresh catalyst). Catalysts were hydrothermally aged at 973 K in 6.5 v.% H2O/air.    XRD patterns of the PdO/γ-Al2O3 and PdO/SiO2 catalysts as a function of HTA time are shown in Figure ‎6.5. The presence of PdO is confirmed by XRD for both fresh and HTA catalysts. However, the XRD pattern of the PdO/γ-Al2O3 after HTA at 973 K also reveals the formation of Pd0, not seen after HTA of PdO/SiO2 at 973 K (see Figure ‎6.5b). XPS analysis of PdO/γ-Al2O3 confirms the existence of two peaks with B.E.s consistant with Pd0 and PdO species on the catalyst surface after HTA at 973 K (see Figure ‎6.6 and Table ‎6.2). The difference between B.E.s of Pd 3d5/2 associated with PdO (336.9 eV) and Pd0 (335.4 eV) should be about 1.5 eV [107], compatible with the data of Table ‎6.2 with difference of 1.2−1.6 eV. The higher Pd0/PdO of the fresh PdO/γ-Al2O3 catalyst compared to the HTA catalysts evidences that there is no Pd0 on the surface of the fresh catalyst. The Pd0/PdO of HTA catalysts remains constant as HTA time  105  increases (see Table ‎6.2). XPS spectra of the PdO/SiO2 catalysts after HTA at 973 K, on the other hand, do not show any evidence of Pd0 on the catalyst surface (see Figure ‎5.7).    Figure ‎6.5. XRD patterns of (a) PdO/γ-Al2O3 and (b) PdO/SiO2 before and after HTA at 973 K for a specified time (0 h: fresh catalyst). PdO (▼), Pd0 (), γ-Al2O3 ().         106     Figure ‎6.6. XPS spectra of PdO/γ-Al2O3 catalysts before and after HTA at 973 K for a specified time (0 h: fresh catalyst).      107  Table ‎6.2. A list of B.E.s and Pd0/PdO ratios of fitted peaks presented in Figure ‎6.6 for PdO/γ-Al2O3 as a function of HTA time.  Pd 3d5/2 B.E.  HTA time Pd0 PdO Pd0/PdO h eV eV  0 335.9 336.8 0.04 16 336.0 337.3 0.39 32 335.9 337.1 0.36 65 335.4 337.0 0.35  6.2.2 Effect of HTA on Low Surface Area PdO Catalysts for CH4 Oxidation   The activity of the fresh and HTA low surface area PdO catalysts for CH4 oxidation are reported in Figure ‎6.7. The same TPMO conditions as reported in Figure ‎6.1 were used to evaluate the catalyst activity of PdO/α-Al2O3 and PdO/SnO2 after HTA. The light-off curve for the PdO/α-Al2O3 catalyst remains unchanged after HTA in 6.5 v.% H2O/air at 973 K for 65 h, reflecting high catalyst stability during HTA treatment. The activity of PdO/SnO2 catalyst after 16 h HTA at 973 K also remains unchanged, although a further increase in HTA time deactivates the catalyst (see Figure ‎6.7b). T10, T50 and T90 in Table ‎6.3, representing the catalyst activity in CH4 oxidation, demonstrates that the PdO/α-Al2O3 has the highest catalyst stability during HTA at 973 K for up to 65 h among the catalysts tested in this study.  108   Figure ‎6.7. Effect of HTA on catalyst activity of (a) PdO/α-Al2O3, (b) PdO/SnO2 in CH4 oxidation. Catalysts were hydrothermally aged at 973 K in 6.5 v.% H2O/air prior to TPMO evaluation.  Catalyst properties of the fresh and HTA PdO/α-Al2O3 and PdO/SnO2 are summarized in Table ‎6.3 and compared with PdO/γ-Al2O3. The BET surface area of all three catalysts decreases as the catalysts undergo HTA treatment for 65 h, even though the reduction in surface area is not considered significant for the low surface area catalysts. The average pore size of the PdO/α-Al2O3 decreases from 13 nm to 12 nm after HTA, whereas as shown in Figure ‎6.4, HTA shifts the pore size distribution and the average pore size of PdO/γ-Al2O3 catalysts towards larger pore size (from 7 to 9 nm). The pore size distribution of PdO/α-Al2O3, in agreement with its average pore size shifts towards smaller pores during HTA (see Figure ‎6.8). It is clear that HTA results preferentially in the collapse of larger pores compared to smaller pores of the PdO/α-Al2O3, but in the case of PdO/γ-Al2O3 smaller pores collapse first. The PdO/SnO2 catalyst, due to its very  109  low surface area (2–3 m2 g-1), is essentially non-porous, which means that PdO (> 6 nm; Table ‎6.3) mostly forms on the external surface of the SnO2 support.     Figure ‎6.8. Pore size distribution of PdO/α-Al2O3 catalyst before and after HTA. Catalysts were hydrothermally aged at 973 K in 6.5 v.% H2O/air (0 h: fresh catalyst).    110      Table ‎6.3. The fresh and HTA PdO/γ-Al2O3, PdO/α-Al2O3 and PdO/SnO2 catalyst properties and then activities for CH4 oxidation. Catalyst BET area Pore size XRD PdO size CO uptake Pd/Mc T10d T50d T90d  m2 g-1 nm nm µmol g-1 at.% K K K  Fresha Agedb Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged Fresh Aged PdO/γ-Al2O3 196 148 7 9 6 (101) 8 (101) 166 137 2.1 1.4 477 502 521 555 555 589 PdO/α-A2O3 64 45 13 12 8 (101) 9 (101) 57 79 7.1 4.4 499 518 565 559 612 590 PdO/SnO2 3 2 – – 6 (110) 15 (110) 5 11 255.7 83.1 503 535 564 604 626 848 a Calcined catalyst; b Catalysts were hydrothermally aged in 6.5 v.% H2O/air at 973 K for 65 h; c Determined by XPS (M: Al or Sn); d Temperature at which 10, 50 or 90% of CH4 is converted during TPMO reaction.     111  The XRD pattern of PdO/α-Al2O3 after HTA reveals Al2O3 phase transformations (see Figure ‎6.9) i.e. the θ-Al2O3 in the PdO/α-Al2O3 catalyst is transformed to α-Al2O3 as HTA occurs, and this phase change correlates with a decrease in surface area, as seen in Table ‎6.3. The XRD patterns of PdO/γ-Al2O3 and PdO/SnO2 do not show significant change after HTA (see Figure ‎6.5a and Figure ‎6.10), although the crystallinity of PdO/γ-Al2O3 increases as HTA time increases. Similar to PdO/γ-Al2O3, the XRD patterns of PdO/SnO2 show the coexistence of PdO/Pd0 after HTA, whereas the XPS and XRD analysis of PdO/α-Al2O3 after HTA show no formation of Pd0 (see Figure ‎G.1 and Figure ‎6.9, respectively). XPS analysis of Pd/SnO2 catalysts does not show co-existance of PdO/Pd0 after HTA but indicates a 0.5 eV change in B.E. of the fitted peak towards lower oxidation state after HTA (see Table ‎6.4). There is a small contribution of a fitted peak at 339.4 eV before and after HTA of Pd/SnO2 that should belong to higher oxidation of Pd (PdOx, x > 1) as shown in Figure ‎G.2.      112   Figure ‎6.9. Al2O3 phase changes in XRD patterns of fresh (0 h) and hydrothermally-aged (at 973 K for 65 h) PdO/α-Al2O3. PdO (▼), α-Al2O3 (), θ-Al2O3().  The data of Table ‎6.3 also shows that the PdO crystals sinter during HTA. The increase in PdO crystallite size of PdO/α-Al2O3 after HTA is small compared to the other catalysts. For PdO/SnO2, the PdO crystallite size increases from 6 to 15 nm during HTA. Note that the PdO crystallite size of PdO/SnO2 catalyst was calculated by the peak associated with the (110) crystal plane of PdO, due to overlapping the (101) crystal plane of PdO with SnO2, and this had a relatively low signal intensity in the XRD (see Figure ‎6.10). For comparison, the PdO cluster size of the fresh PdO/SnO2 (3 ± 1 nm) was measured from TEM images (shown in Figure ‎6.11) and found to be close to the PdO crystallite size reported in Table ‎6.3.    113       Figure ‎6.10. (a) XRD diffractogram of fresh (0 h) and HTA (at 973 K for 65 h) PdO/SnO2. (b) A zoomed-in plot of XRD diffractogram in the range of 46-58°. PdO (▼), Pd0 (), SnO2 ().    114   Figure ‎6.11. TEM image and cluster size distribution of fresh PdO/SnO2 catalyst.  The CO uptake and Pd/M atomic ratio (M: Al or Sn) determined by XPS, of the PdO catalysts before and after HTA are shown in Table ‎6.3. The Pd/M ratio for low surface area catalysts (e.g. PdO/α-Al2O3) is significantly higher than for the high surface area catalysts (e.g. PdO/γ-Al2O3) despite similar Pd loadings. In addition, the Pd/M ratio significantly decreases after HTA for all  115  three catalysts representing a loss in Pd active sites during HTA. The CO uptake of PdO/α-Al2O3 and PdO/SnO2 (low surface area catalysts) increases after HTA, not in agreement with their Pd/M ratios. However, the CO uptake of PdO/γ-Al2O3 is shown to decrease significantly after HTA, in line with the change in the Pd/M ratio.  Table ‎6.4. A list of B.E.s of fitted peaks presented in Figure ‎G.1 and G.2 for PdO/γ-Al2O3 and PdO/SnO2 before and after HTA. Catalyst Pd 3d5/2 B.E. (eV)  Fresha Agedb PdO/α-A2O3 337.2 337.5 PdO/SnO2 337.0 336.5 a Calcined catalyst; b Catalysts were hydrothermally aged in 6.5 v.% H2O/air at 973 K for 65 h;  6.3 Discussion  In Chapter 4 and 5, the deactivation of PdO/SiO2 catalysts during HTA was attributed to a combination of PdO sintering and PdO occlusion by the support. Characterization results of PdO/SiO2 and PdO/γ-Al2O3 (see Figure ‎6.3) show that the catalyst properties for both catalysts follow similar trends as HTA time increases, suggesting similar deactivation mechanisms occurring for both catalysts. A significant decrease in Pd/M ratio of PdO/γ-Al2O3, determined by XPS after HTA, which continues as HTA time increases, in comparison to a much less significant PdO sintering, suggests PdO occlusion by the Al2O3 support. CO uptake of PdO/γ-Al2O3 also continuously decreases as HTA time increases, further supporting the PdO occlusion  116  mechanism, even though the rate of decrease does not follow the Pd/M ratio decrease rate. This can explain why the PdO/γ-Al2O3 is more stable during HTA at 973 K than PdO/SiO2. It is likely that although the γ-Al2O3 support undergoes re-structuring during HTA, as reflected in surface area decrease (see Figure ‎6.3), it only occludes the edges of PdO crystallites, and hence PdO active sites remain accessible to participate in catalysis, allowing a higher activity in CH4 oxidation after HTA, compared to PdO/SiO2 catalysts.  PdO sintering is also observed during HTA of both PdO/SiO2 and PdO/γ-Al2O3 catalysts. Unlike PdO/SiO2, a plot of PdO crystallite size versus average pore size for PdO/γ-Al2O3 as HTA time increases (0–65 h) suggests a linear relationship between PdO sintering and pore structure collapse (Figure ‎6.12). The PdO crystallite size is always smaller than pore size and increases as the pore size increases. This reflects that the PdO sintering is dependent upon pore size and PdO crystals sinter until their size becomes equal to the pore size. Similarly for PdO/SiO2 and PdO/α-Al2O3 catalysts, the PdO crystallite size is smaller than the catalyst pore size and after HTA, PdO sinters but remains smaller than the pore size. This phenomenon and low vapor pressure of PdO compared to other metal oxides [60], leads to the conclusion that PdO sintering occurs mostly through particle migration and coalescence. As discussed in Section ‎5.3, the Pd0 sintering mechanism during HTA at elevated temperatures is shown to depend upon the particle size and for Pd0 particle sizes in the range of 3−10 nm, a combination of Ostwald ripening and particle migration and coalescence is involved in Pd0 sintering, not compatible with the results of this study.    117   Figure ‎6.12. PdO crystallite size as a function of pore size for PdO/γ-Al2O3 as HTA time increases from 0 to 65 h.   XRD and XPS analysis of PdO/γ-Al2O3 and PdO/SnO2 suggests that PdO  Pd0 transformation occurs during HTA at 973 K, whereas this transformation does not occur on PdO/SiO2 and PdO/α-Al2O3. The PdO phase is thermodynamically stable (                 ) at the HTA treatment conditions (973 K, 101.3 kPa and 19.6 v.% O2) of this study [34, 47]. Thermal aging of PdO/γ-Al2O3 in 19.6 v.% O2 and 80.4% Ar at 973 K for 16 h (similar to treatment condition of HTA but without H2O) does not produce any Pd0 (see Figure ‎6.13). Hence, it is likely that the PdO  Pd0 transformation is a consequence of the presence of H2O. Schwartz et al. [45] proposed that the H2O/hydroxyls, produced during CH4 oxidation at temperatures below 723 K, accumulate on the support surface and interrupt the O exchange between Pd-* sites and the oxide support (  -    -      -     - ), resulting in deactivation of the Pd catalysts over time. O2  118  molecules in the gas phase dissociate on the Pd-* sites (          -        ) and migrate to the support whereas O atoms from the oxide support are transported to the Pd-* sites. This O exchange between the oxide support and Pd-* sites is an important requirement for PdO catalysts to have a high activity in CH4 oxidation [43, 45]. The same phenomenon possibly occurs during HTA. H2O molecules during HTA adsorb on the catalyst surface and interrupt the O exchange with the oxide support by formation OH groups on the surface ( -    -        -     -  ), leading to a decrease in the metal-support interaction and hence formation of Pd0 crystals (PdO  Pd-*  Pd0). The effect of the PdO  Pd0 transformation on the CH4 oxidation activity over Pd catalysts at low temperatures is well-known [34, 47, 49]. Pd0 is less active than PdO for CH4 oxidation. The kinetic rate of CH4 oxidation on a Pd foil at 907 K (0.04 v.% CH4, 0.2% O2) is reported to decrease from 3 to 0.3 s-1 as PdO decomposes to Pd0 [49]. Farrauto et al. [47] studied the CH4 oxidation over Pd/γ-Al2O3 by heating and cooling the catalyst to transition temperatures, where PdO decomposition occurs. They reported that the decomposition of PdO to Pd0 occurs during heating of the PdO/γ-Al2O3 above 1023 K in 1 v.% CH4 (balance air) resulting in a hysteresis effect during CH4 conversion. CH4 conversion is shown to significantly decrease during cooling profile compared to the heating profile. In the former case the Pd0 phase is stable (from 1173 to 923 K), whereas after re-oxidization of Pd0 below 923 K, the CH4 conversion measured during the cooling or heating profile overlap. Hence the PdO  Pd0 transformation occurring during HTA of PdO/γ-Al2O3 and PdO/SnO2 contributes to a decrease in catalyst activity during CH4 oxidation. One explanation for the absence of the Pd0 phase on the PdO/SiO2 and PdO/α-Al2O3 catalysts after HTA is because of a strong Pd-support interaction on these catalysts. A strong metal-support interaction also explains the limited PdO sintering observed after HTA of PdO/α-Al2O3 and no further PdO  119  sintering on PdO/SiO2 as the HTA time increases from 16 to 32 h. As discussed in Section ‎4.3, the migration of SiO2 overlayers on PdO clusters and hence, PdO occlusion are expected to establish a strong Pd-support interaction, which limits further PdO sintering (see Figure ‎6.3) and also PdO  Pd0 transformation (see Figure ‎6.5).   Figure ‎6.13. XRD patterns of PdO/γ-Al2O3; (A) fresh catalyst; (B) catalyst (A), thermally aged in 19.6 v.% O2/Ar at 973 K for 16 h; (C) catalyst (A), hydrothermally aged in 6.5 v.% H2O/air at 973 K for 16 h. PdO (▼), Pd0 (), γ-Al2O3 ().  PdO/α-Al2O3 retains its catalyst activity for CH4 oxidation after HTA whereas the other catalysts of this study are shown to deactivate. Although PdO/SnO2 retains its activity during 16 h of HTA, a further increase in HTA time up to 65 h, deactivates the catalyst. Besides the PdO  Pd0 transformation discussed above, the PdO crystallite size results shown in Table ‎6.3 indicate that  120  PdO sintering also plays a key role in the deactivation of the PdO/SnO2 during CH4 oxidation following HTA. A comparison of the XRD and XPS results also shows that although Pd/M ratio for both PdO/SnO2 and PdO/α-Al2O3 decrease significantly after HTA, the PdO does not sinter to the same degree, reflecting PdO occlusion by the support during HTA, as discussed in Chapters 4 and 5. PdO occlusion by the support is not suggested to significantly affect the catalyst activity in CH4 oxidation for the low surface area catalysts (at least for PdO/α-Al2O3). The reason can be attributed to the higher Pd/M ratio of the low surface area PdO catalysts, allowing the high catalyst activity to be maintained despite losing a portion of the active sites. As also discussed for PdO/γ-Al2O3, occlusion in low surface area catalysts might occur on the edges of PdO crystallites, that is why PdO sites remain active after HTA.  PdO/α-Al2O3 as the most stable catalyst in this study undergoes phase transformations (from θ-Al2O3 to α-Al2O3) during HTA, which has no effect on PdO activity in CH4 oxidation despite contributing to a surface area decrease. Previous work reported that support sintering through Al2O3 phase transformations did not significantly contribute to Pd sintering during HTA in 10 v.% H2O/N2 at 1173 K for up to 200 h, in line with the observations of this study as PdO crystallites of PdO/α-Al2O3 do not change considerably during HTA [35]. On the other hand, the surface area of the PdO/γ-Al2O3 catalyst decreases during HTA and pores are enlarged but no phase transformation occurs, in line with the observations of Johnson [108]. The surface area decrease of γ-Al2O3 at < 1073 K is attributed to the collapse of the pores [109] and is expected to be accelerated in the presence of H2O [108]. Alumina phase transformations (γ‎ δ‎ θ‎ α) mostly occur at > 1073 K [110, 111].     121  6.4 Conclusions  PdO catalysts with various oxide supports were hydrothermally aged and tested in CH4 oxidation. PdO/α-Al2O3 was found to have the highest catalyst stability during HTA at 973 K for up to 65 h. A comparison of catalyst properties reveals that the PdO catalysts sinter during HTA and PdO crystallites are occluded by the support. PdO occlusion by the support, as discussed in Chapters 4 and 5, plays a key role in the deactivation of PdO/SiO2 catalysts during HTA but does not significantly affect the performance of the other catalysts investigated in the present study during HTA. The deactivation of PdO/γ-Al2O3 and PdO/SnO2 catalysts during HTA in CH4 oxidation are mostly related to PdO sintering and the PdO  Pd0 transformation. XRD and XPS results confirm the coexistence of PdO and Pd0 after HTA in these catalysts, associated with the presence of H2O. Small PdO sintering and absence of Pd0 phase in PdO/α-Al2O3 suggests stronger Pd-support interactions, leading to higher catalyst stability during HTA.            122  Chapter 7 Kinetics of CH4 Oxidation over PdO Catalysts in the Presence of Water  7.1 Introduction  H2O is known to affect the kinetics of CH4 oxidation over PdO catalysts. Previous studies report that the reaction order with respect to H2O partial pressure changes from -1 to 0 as temperature increases from below 600 K to around 880 K [49, 50], reflecting inhibition effects of H2O on PdO catalysts. The decreasing inhibition effect with increasing temperature [8, 49] is attributed to enhanced OH desorption [49] and improved CH4-O2 reaction rates. Over PdO, the C-H bond activation of CH4, generally assumed to be the rate-determining step (RDS) of the reaction, is interrupted by an adsorbed layer of H2O on the catalyst surface [34, 66] that blocks CH4 access to the PdO active sites [42, 62] during CH4 oxidation. The formation of inactive Pd(OH)2 has also been proposed as the cause of H2O inhibition. However, more recent studies suggest that the interruption of oxygen exchange between the support and Pd sites by OH accumulation on the support are the cause of inhibition. This chapter reports the effect of H2O and CH4 concentration on CH4 combustion over a PdO/γ-Al2O3 catalyst. The data are used to develop a kinetic model that accounts for the inhibition of the reaction rate by H2O at temperatures below 723 K. The kinetic model is further used to predict TPMO profiles measured for the same catalyst.     123  7.2 Kinetic Models  Power law models are tested first (Model A, B1 and B2 in Table ‎7.1). A reaction order of 1, 0, -1 and 0 for CH4, O2, H2O and CO2, respectively, is used in Model A following the literature (see Table ‎2.4). To improve the data-fitting, general forms of the power law model that consider reaction orders with respect to H2O and CO2 as variables, are also used (Model B1 and B2). Mechansitic kinetic models to incorporate the H2O effect on the rate of CH4 combustion are also derived and tested (see Table ‎7.1) according to the following elementary steps:             -             -         ‎7.1    -     -    -           ‎7.2              -           ‎7.3 where PdO sites represented by O-* and * is an O vacancy on the PdO surface. The O2 dissociative adsorption (           - ) on the catalyst surface is assumed to be kinetically insignificant, in line with zero order dependency of O2 reported in the literature (see Table ‎2.4) and confirmed in Figure ‎7.4. CH4 combustion over PdO is proposed to occur on a site pair of O-*–O-*, O-*–* or *–*, dependent on the experimental conditions (e.g. temperature and O2 concentration) [76]. In the present study, initial C-H bond cleavage of CH4 is assumed to occur on a site pair (O-*–*) irreversibly and is the RDS of the CH4 combustion reaction (Equation 7.1). H abstraction from CH3 is also irreversible and occurs sequentially to form adsorbed OH (OH-*) and CO2, not kinetically significant [44]. Based on Equation 7.1 and zero order O2 kinetics, the reaction rate reduces to:                         ‎7.4  124  The value of n is 2 when a site pair is kinetically essential for dissociation of CH4 (Model D, E and G), as reflected in Equation 7.1. Equation 7.4 with n = 1 arises if CH4 is assumed to be adsorbed on the surface and reacts with O2 from the gas phase or when O coverage (O-*) is not kinetically relevant (Model C and F).  Recombination of OH groups on the surface generates H2O and a surface vacancy (Equations 7.2 and 7.3) [44]. H2O adsorption on the catalyst surface is fast but its slow desorption leads to H2O accumulation in the form of adsorbed H2O and/or OH groups, accounting for the H2O inhibition effect on the CH4 combustion rate [44, 45, 62, 72]. In Models C and D, adsorbed H2O is postulated to be the most abundant surface intermediate so that the surface site balance is:                        ‎7.5 Assuming pseudo steady state equilibrium for Equation 7.3, H2O coverage (     or H2O-*) is calculated:                             ‎7.6 Replacing H2O coverage in Equation 7.5 with Equation 7.6 and re-arranging Equation 7.5 gives the vacant sites. Finally, by replacing the vacant sites in Equation 7.4, Model C and D are obtained. To seek a mechanistic kinetic model that can improve data fitting, the inhibition effect of CO2 and CH4 on the rate of combustion is also tested. In Model E, adsorbed CH4 (              - ) and adsorbed H2O are assumed as the most abundant surface intermediates so that the site balance is:                               ‎7.7 Similar to Model C and D, by calculating the CH4 and H2O coverage and replacing in Equation 7.4, Model E can be derived [19]. Similarly, by considering adsorbed CO2 (              - )  125  and adorbed H2O as the most abundant surface intermediates (Equation 7.8), Model F and G can be derived.                               ‎7.8  Mars van Krevlen (MVK) kinetic models have been used successfully to describe the CH4 oxidation over Pd catalysts [51, 112]. The reaction is assumed to occur through alternative oxidation and reduction of the catalyst surface. O2 dissociatively chemisorbed on the catalyst surface from gas phase provides surface O (              -         - ). CH4 is irreversibly chemisorbed on the oxidized sites, reacts with O on the catalyst surface in sequential steps and produces H2O and CO2 (          -              ) [51, 112]. Model H and I are derived by assuming a negligible amount of chemisorbed O, fast desorption of CO2 and slow desorption of H2O (depending upon reduced (*) or oxidized (O-*) sites is affected by adsorption/desorption of H2O, Model H or I  is drived, respectively) to incorporate the inhibition effect of H2O (e.g. [51]).    Compared to the power law models, the reaction order with respect to H2O in all mechanistic models, shown in Table ‎7.1, varies. While Model A has a negative first order with respect to H2O, all mechanistic models with an adsorption equilibrium constant in the denominator (dependent on temperature) are almost zero order at high temperatures and negative first order at low temperatures with respect to H2O.     126    Table ‎7.1. Proposed rate expressions  Model Rate expression A    =   PCH4PH2O-1  B1    =   PCH4PH2O  B2    =   PCH4PH2O P  2  C   =‎  PCH41+KH2OPH2O D    =‎  PCH4(1+KH2OPH2O)2 E           (                   )  F           (                   ) G           (                   )  H                                (          ) I                       (          )               127  7.3 Reactor Modeling  7.3.1 Differential Mode  In differential experiments, CH4 conversions are kept < 10% and hence the change in CH4, O2 and H2O concentration through the reactor do not significantly affect the kinetic rate. The rate of CH4 oxidation can then be calculated using the design equation for a differential fixed-bed reactor as follows:                      ‎7.9 The turnover frequency (TOF) is calculated as follows:                                     ‎7.10  7.3.2 Integral Mode  In TPMO experiments, the reactor operates in integral mode and hence CH4 concentration in the gas phase changes as the reactant gases are transported through the catalyst bed and also as temperature changes. Hence: Cm = f (w, T)           ‎7.11 Temperature has a linear relationship with time as follows: T = 5t + 393           ‎7.12 Writing a mole balance on the catalyst bed to calculate the CH4 concentration profile results in:  - Cmν w-ηorm=‎ Cm t.εbρb          ‎7.13  128  Equation 7.13 is a partial differential equation (PDE) that can be solved by transformation to an ordinary differential equation (ODE) via the method of characteristics:                                  ‎7.14 By solving the 1st and 2nd terms in equation 7.14, the following constant as the characteristic parameter is derived:     -                   ‎7.15 and hence the following ODE is obtained that can be solved numerically using Runge-Kutta 4th order (RK4) integration: dCmdw= -r mηo 0           ‎7.16 Cm 0,t =‎Cm0           ‎7.17 The above reactor model is used when TPMO data is predicted using the rate expressions in Table ‎7.1. In this case, the kinetic parameters are already estimated by fitting the differential data to the rate expressions.  To estimate the kinetic parameters by fitting TPMO data to the rate expressions in Table ‎7.1, it is assumed that each recorded CH4 conversion as a function of temperature in TPMO data profiles is measured at pseudo steady-state under isothermal conditions, so that the Equation 7.13 is simplified as: dCmdw = -r mηo 0           ‎7.18 Cm 0  =‎Cm0           ‎7.19 Equations 7.18 and 7.19 are also solved numerically using RK4 integration.   129  The overall effectiveness factor (ηo) to incorporate mass transfer effects is calculated by assuming that the CH4-O2 reaction is 1st order [113]. This assumption has been successfully used for estimating the kinetic parameters for CH4 oxidation over commercial Pt and Pt-Pd catalysts using light-off data [19]. ηo=‎η1+ηk ρb kcac⁄           ‎7.20 The rate constant (kr; see Table ‎7.1) needs to be re-parameterized to calculate the 1st order rate constant (  ). For example, the 1st order rate constant in Model A is calculated as follows:           -                ‎7.21 External catalyst surface area per catalyst bed volume for fixed-bed reactors is calculated as follows:      ( -  )              ‎7.22 The internal effectiveness factor is calculated based on a 1st order reaction rate assumption and is given by [112]:       (        - )         ‎7.23  where the Thiele modulus is defined as:       √                      ‎7.24 The binary bulk diffusivity of CH4 in Ar can be calculated from the Chapman-Enskog correlation [114, 115]: DCH4-Ar=0.001858‎T32⁄ ‎MwCH4-Ar-1 2⁄PζCH4-Ar2 ΩD,‎T‎<‎1000‎K,‎P‎<‎70‎atm‎     ‎7.25 and the bulk effective diffusivity is given by [113]:  130  DCH4-Areff=DCH4-Ar‎εp‎ζη          ‎7.26 Knudsen diffusivity (when molecular mean free path >> pore diameter) is calculated as [113]: DK=48.5‎dpore (TMwfeed⁄ )12⁄          ‎7.27 and the effective Knudsen diffusivity is calculated from [113]: DKeff=εp‎DKη           ‎7.28  Hence, the overall effective diffusivity is calculated based on contributions from both bulk and Knudsen diffusivities as follows [113]: Deff=(1DCH4-Areff+1DKeff)-1         ‎7.29  The mass transfer coefficient (kc)  is calculated using the jD-factor correlation as follows [114]:              ⁄              ‎7.30                                   ‎7.31 Other parameters in this section are defined in Appendices E and F.   7.4 Optimization Method  The measured reaction rates calculated from Equation 7.9 for the differential fixed-bed reactor, are fitted to the reaction rate models consisting of independent variables measured during the experiment and the unknown parameters (rate constants (   ) and adsorption constants (  )) that need to be determined as follows:        (                           )        ‎7.32  131  The parameters are estimated using a non-linear least-squares method. The objective function is to minimize the residual sum of squares (RSS) of reaction rate as follows:       ∑ (  obsi-  cali)2Nobsi=1          ‎7.33   For the integral fixed-bed reactors as described in Section ‎7.3.2, the conversion changes throughout the catalyst bed so that the conversion at steady-state generally is calculated as follows: X al= ∫rmFm0dww0=f (T,w, Fm0,PCH40,‎PH2O0,‎kri,‎Ki‎)      ‎7.34 The parameters are also estimated using non-linear least-squares method. The objective function here is to minimize the RSS of conversion as follows:       ∑ (     -     )                 ‎7.35  The Levenberg-Marquardt algorithm (LMA) is used as the optimization method to solve these non-linear least square regression problems [116]. The MATLAB m-file for LMA is presented in section H.2.  To avoid correlation between parameters and improve the data fitting convergence, the Arrhenius equation and adsorption equilibrium constant were re-parameterized as follows [51, 117, 118]:             (     (  -   ))          ‎7.36          (    (  -   ))          ‎7.37  132  7.5 Kinetic Model Discrimination  To discriminate between kinetic models, mean residual sum of squares (MRSS) is calculated as follows [51]: MRSS=‎∑ (Xobsi-Xcali)2Nobsi=1Nobs-Npar         ‎7.38 MRSS needs to be close to 0 for a kinetic model to have a satisfactory fitting. The best fitted kinetic models are determined by comparing their MRSS values, appearance of parity plots i.e. whether the data are uniformly distributed around y = x line, and standard deviations of the estimated kinetic parameters (i.e. should be <10%).  7.6 Results  A flowchart of the steps followed for the kinetic model development is summarized in Figure ‎7.1.  Both differential and integral reactor data have been used in the analysis as described below.          133     Figure ‎7.1. Steps for the kinetic model development.     Conduct steady-state differential experiments on a PdO/Al2O3 catalyst to measure intrinsic kinetic rate. Fit differential data to kinetic models in Table 7.1 using LMA method and estimate intrinsic kinetic parameters. Conduct TPMO experiments on a PdO/Al2O3 catalyst at various CH4, O2 and H2O concentrations. Predict the TPMO data using differential data-fitted kinetic models. Fit TPMO data to kinetic models in Table 7.1 using LMA method and estimate kinetic parameters.  134  7.6.1 Effect of Reactants and H2O Concentration on CH4 Conversion  Kinetic experiments were carried out on a PdO/γ-Al2O3 catalyst, prepared using incipient wetness impregnation as described in Section ‎3.1. The characterization results for this catalyst are summarized in Table ‎7.2.   Table ‎7.2. Catalyst properties of PdO/γ-Al2O3 Catalyst Pd loading BET area Pore size XRD PdO size CO uptake Pd dispersion  wt.% m2 g-1 nm nm µmol g-1  PdO/γ-Al2O3 0.8 237 7 7 18 0.24  The effect of H2O (0–10 v.%) and CH4 (0.1–0.5 v.%) concentration on the activity of the PdO/γ-Al2O3 catalyst during CH4 oxidation was evaluated by TPMO (see Figure ‎7.2). Increasing the CH4 and H2O concentration shifts the light-off curve towards higher temperatures, a consequence of the inhibition effect of H2O, generated during CH4 oxidation or added to the feed. Previous work showed that an increase in H2O concentration from 0−20 v.% in the feed significantly shifted the light-off curve towards higher temperature during CH4 combustion over different Pd supported catalysts [22], in agreement with the results presented here. A decrease in CH4 conversion as CH4 concentration increases is also reported in [51]. Note that an increase in CH4 concentration from 0.1 to 0.5 v.% increases T10 from 541 to 563 K (22 K increase) and T90 from 664 to 701 K (37 K increase) when no extra H2O is added to the feed (see Figure ‎7.2). The higher increase in T90 is attributed to the higher H2O concentration produced when 0.5 v.% CH4 is in the feed compared to the case with 0.1 v.% CH4, because of the reaction stoichiometry (CH4 + 2O2  CO2 + 2H2O).  135   Figure ‎7.2. Effect of H2O and CH4 concentration on activity of PdO/γ-Al2O3 catalyst during CH4 oxidation; (a) 0.1 v.% CH4 (b) 0.5% CH4, 20% O2 in balance of He and Ar, GHSV 180,000 cm3 (STP) g-1 h-1.    As the H2O concentration increases from 0 to 5 v.% in the feed, the T50 for PdO/γ-Al2O3 catalyst increases from 597 to 675 K (Figure ‎7.3) but a further increase in H2O concentration results in a smaller increase in T50, reflecting that H2O adsorption/desorption on the catalyst surface reaches an equilibrium when H2O concentration increases to 5 v.% or more at elevated temperatures.     136   Figure ‎7.3. T50 extracted from TPMO profiles in Figure ‎7.2 as a function of H2O concentration; TPMO conditions follow Figure ‎7.2a.  The effect of O2 concentration (3–20 v.%) on the activity of the PdO/γ-Al2O3 during CH4 oxidation is reported in Figure ‎7.4. The PdO/γ-Al2O3 catalyst was evaluated using TPMO with 0.1 v.% CH4, 20% O2 in a balance of He and Ar (GHSV 180,000 cm3 (STP) g-1 h-1). Subsequently, the reactor was cooled to ambient temperature in Ar, and re-evaluated using TPMO but with 10 v.% O2. The process was repeated for 6 and 3 v.% O2 concentrations. To ensure that the catalyst did not deactivate during subsequent TPMOs, TPMO with 20 v.% O2 was repeated at the end of sequence. The data show that the change in O2 concentration does not have a significant effect on PdO activity during CH4 oxidation, in line with literature data that suggest that CH4 oxidation is zero order with respect to O2 (see Table ‎2.4).     137   Figure ‎7.4. Effect of O2 concentration on activity of PdO/γ-Al2O3 catalyst during CH4 oxidation; TPMO condition: 0.1 v.% CH4, O2 (figure legend) in He and Ar, GHSV 180,000 cm3 (STP) g-1 h-1.  7.6.2 Differential Experiments and Intrinsic Kinetic Rate   To measure the intrinsic kinetic rate of CH4 oxidation over the PdO/γ-Al2O3 catalyst and compare with the kinetic rate calculated from TPMO data, a set of experiments was carried out at steady-state with CH4 conversions below 10% (i.e. differential method), so that the kinetic parameters were not affected by mass transfer resistances. Theoretical calculations and diagnostic tests, summarized in Appendix F, ensured that under differential conditions the measured rate was not influenced by mass transfer. The experimental conditions are described in detail in Section ‎3.3.4. The measured TOF as a function of inverse temperature is shown in Figure ‎7.5. An increase in temperature exponentially increases the TOF, as expected whereas an  138  increase in H2O concentration from 0 to 2 v.% significantly decreases the TOF, in agreement with the shift of TPMO profiles to higher temperature as a consequence of adding H2O to the feed (Figure ‎7.2).   Figure ‎7.5. TOF as a function of inverse temperature at different feed concentrations of H2O. Reaction condition: 0.5 v.% CH4, 20% O2, 0 or 2% H2O and balance He and Ar, GHSV 270,000 cm3 (STP) g-1 h-1, each temperature for 0.5 h.  A MATLAB m-file (see Section H.2) was developed to estimate the optimum kinetic parameters using the LMA to fit the differential data to various kinetic models, as summarized in Table ‎7.3. The kinetic models that resulted in poor fitting or invalid parameters were rejected and are summarized in Table ‎H.4, H.5 and H.6. Note that Model A and B1 considered the effect of CH4 and H2O concentration on the reaction rate show an identical fit. The difference between Model  139  A and B1 is the H2O reaction order that in Model A is set to be -1 but in Model B1 is allowed to be optimized to improve data-fitting (see Table ‎7.3 and H.4).  The coefficient of determination (r2) and the parity plot in Figure ‎7.6 for the kinetic models in Table ‎7.3, suggests that all three kinetic models are good fits to the experimental data, even though the standard deviation of the pre-exponential factor for the H2O adsorption constant (     ) for Model C and D is large and suggests that this parameter might be highly correlated with other parameters (kr0 or  HH2O . It is concluded that Model A is the best fitting model of the differential data.    Figure ‎7.6. Comparisons of calculated and observed CH4 conversion as a function of H2O concentration (0 and 2 v.%). Reaction condition follows Figure ‎7.5.   140     Table ‎7.3. Calculated kinetic parameters for various kinetic models by fitting to differential data Model    a Eac             r2 MRSS  s-1 kPa-1 kJ mol-1 kPa-1 kJ mol-1   A 1.88E13 ± 3.36E3 165 ± 2 – – 0.959 1.93E-2 C 1.01E9 ± 7.73E3 98 ± 35 1.63E-4 ± 4.11E-1 -61 ± 38 0.957 1.87E-2 D 4.17E6 ± 7.38E1 78 ± 9 5.61E-7 ± 3.04E-3 -74 ± 11 0.964 1.58E-2 a Model A: s-1    141  7.6.3 Prediction of TPMO Data Using Differential Kinetic Parameters   The intrinsic kinetic parameters reported in Table ‎7.3 are valid for CH4 conversions < 10% (differential conditions) and over a relatively limited‎ temperature‎ range‎(538−643‎K). In NGV catalytic converters, CH4 conversions of 0−100% and temperatures of‎ 423−823‎ K are encountered and these conditions are also encountered during the TPMO experiments. Hence in this section, the intrinsic kinetic parameters of Table ‎7.3 are used to predict the TPMO data profiles. It is assumed that the reactor operates in plug flow (see Appendix D). To understand at what conditions the observed TPMO data are mass-transfer-controlled, theoretical calculations based on reaction conditions at T10, T50 and T90 of the observed TPMO data are done to estimate the influence of mass transfer as shown in Figure ‎7.7. The details of the calculations are summarized in Section H.1. A value of the Weisz-Prater criterion of < 1 ensures negligible internal mass transfer resistances in a fixed-bed reactor [113]. Figure ‎7.7 shows that an increase in H2O concentration significantly decreases the Weisz-Prater criterion at all CH4 conversion levels, although only at CH4 conversions < 50% is the value < 1 and hence, internal mass transfer resistances can only be considered negligible when the CH4 conversion is < 50%. The Mears criterion was also calculated to ensure the negligibility of external mass transfer resistances [113] and it was determined that at all CH4 conversion levels for the observed TPMO data, the external mass transfer resistance was negligible (see Section H.1).   142   Figure ‎7.7.  Weisz-Prater criterion calculated as a function of CH4 conversion for observed TPMO data. Reaction condition: (a) 0.1 v.% or (b) 0.5 v.% CH4, 20 v.% O2, H2O, 5 K min-1, GHSV 180,000 cm3 g-1 h-1.  The kinetic models and calculated parameters from Table ‎7.3 and the mole balance equations incorporated with the mass transfer calculations‎ (ηo) described in Section ‎7.3, were used to calculate TPMO profiles measured for the PdO/γ-Al2O3 catalyst (see MATLAB m-files in Section H.3). The TPMO calculated results as a function of CH4 and H2O concentrations are shown in Figure ‎7.8 and 7.9 (mass transfer effects on calculated TPMO results are also shown in Appendix H). MRSS values for Models A, C and D, indicating the goodness-of-fit, are shown in Table ‎7.3, suggesting relatively similar fit for all three models when predicting TPMO data profiles. As also discussed in Section ‎7.6.2, Model C and D, which consider the effect of temperature on H2O desorption from the catalyst surface, do not significantly improve data fitting compared to the simpler Model A. The proposed kinetic models show a good fit for  143  conversions up to 40% when 0.1 v.%  CH4 with no H2O or 0.5 v.% CH4 with up to 2 v.% H2O present in the feed. An increase in H2O concentration from 0 to 10 v.% with 0.1 v.% CH4 present in the feed increases the goodness of fit and the calculated and observed data overlap at high concentrations of H2O (≥ 3 v.%), whereas the data fitting gets worse as H2O concentration increases when 0.5 v.% CH4 is present in the feed.      144   Figure ‎7.8. Kinetic model comparisons to predict TPMO data as a function of CH4 and H2O concentrations. Reaction condition: 0.1v.% (a, c and e) or 0.5% (b, d and f) CH4, 20% O2 and 0% (a and b), 2% (c and d) or 5% (e and f)  H2O and balance He and Ar, 5 K min-1, GHSV 180,000 cm3 g-1 h-1.    145   Figure ‎7.9. Kinetic model comparisons to predict TPMO data as a function of H2O concentration. Reaction condition: 0.1v.% CH4, 20% O2 and 3% (a) or 10% (b) H2O and balance of He and Ar, 5 K min-1, GHSV 180,000 cm3 g-1 h-1.  7.6.4 TPMO Data and Re-estimating the Kinetic Parameters   To address the lack of fit between the calculated and observed TPMO data in Figure ‎7.8, intrinsic kinetic parameters were re-estimated using the TPMO data directly (instead of using the differential data). As described in Section ‎7.6.3, the TPMO data were mass-transfer controlled so that the overall effectiveness factor (ηo) was calculated and incorporated in the mole balance calculations (see MATLAB m-files in Section H.4). All kinetic models, shown in Table ‎7.1, were investigated and kinetic parameters were determined as summarized in Table ‎7.4, 7.5 and 7.6.   146   Model A and B1 considered the effect of CH4 and H2O concentration on the reaction rate and show a relatively similar goodness-of-fit. The rate constant calculated for Model B1 is significantly different from Model A although the H2O reaction order for Model B1 is -0.6 close to -1 for Model A. The goodness-of-fit for power law models is improved when the CO2 partial pressure term is added (Model B2). The CO2 reaction order was calculated to be -0.7, suggesting that CO2 might inhibit rate of CH4 oxidation. The value of other kinetic parameters in Model B2 is close to Model A.   To understand the effect of H2O, CH4 and CO2 on the rate of CH4 combustion, mechanistic models were derived as discussed in Section ‎7.2. Model E that considers slow consumption of CH4 and slow desorption of H2O, and Model F and G that consider the slow desorption of H2O and CO2 indicate a significant improvement in data fit compared to Model C and D that only consider slow desorption of H2O from catalyst surface (see Table ‎7.5). Even though the MRSS value for Model E, F and G is significantly lower than the Model C and D in Table ‎7.5, Model E, F and G have parameters with no physical interpretation. The standard deviation associated with the pre-exponential factor of the H2O adsorption constant for Model C and D in Table ‎7.5 is improved, compared to the differential data-fitting in Table ‎7.3, but it remains large.   MVK models (Model H and I) shown in Table ‎7.6 reveal similar goodness-of-fit although only the kinetic parameters calculated for Model I that considers the H2O effect on oxide sites have physical interpretations. Note that the H2O adsorption enthalpy calculated for Model I is small, compared to literature values (e.g. -49 kJ mol-1 in [22]).   147   The parity plots for TPMO-data-fitted kinetic models (Model A, B2, C, D, E and I) are shown in Figure ‎7.10, 7.11 and 7.12. Generally speaking, all kinetic models act in the same way and can accurately predict the observed data at conversions < 40% when 0.1 v.% CH4 with no H2O or 0.5 v.% CH4 with ≤ 2 v.% H2O present in the feed. An increase in H2O concentration when 0.1 v.% CH4 present in the feed improves the fit, even though the fit gets worse by an increase in H2O concentration when 0.5 v.% CH4 is present in the feed. Model A (a power law model) that has parameters with physical meaning can be reported as the best fitting model in this study.        148      Table ‎7.4. Calculated kinetic parameters for power law models by fitting to TPMO data. Model     Eac a b r2 MRSS  s-1 kPa-1-a-b kJ mol-1     A 8.2E12 ± 1.7E5 161 ± 7 – – 0.856 1.57E-2 B1 9.0E8 ± 1.6E3 113 ± 8  -0.6 ± 0.1 – 0.870 1.17E-2 B2 8.6 ± 1.8E5 186 ± 4 -1.0 ± 0.0 -0.7 ± 0.0 0.931 6.32E-3       149    Table ‎7.5. Calculated kinetic parameters for various kinetic models by fitting to TPMO data. Model     Eac K0H2O  HH2o K0ia  Hi r2 MRSS  s-1 kPa-1 kJ mol-1 kPa-1 kJ mol-1 kPa-1 kJ mol-1   C 1.8E7 ± 2.4E2 85 ± 4 3.6E-3 ± 6.5E-3 -33 ± 5 – – 0.885 1.05E-2 D 3.3E6 ± 4.9E1 77 ± 2 2.2E-4 ± 3.6E-4 -40 ± 3 – – 0.887 1.05E-2 E 1.6E9 ± 8.9E2 106 ± 1 4.2E-3 ± 6.9E-4 -26 ± 1 1.0E5 ± 8.8E0 57 ± 1 0.972 2.66E-3 F 6.1E14 ± 4.9E7 163 ± 15 1.3E5 ± 7.7E2 45 ± 15 1.4E6 ± 7.9E3 46 ± 13 0.923 7.48E-3 G 2.6E11 ± 7.7E4 128 ± 3 1.1E-1 ± 1.4E-2 -12 ± 2 1.6E1 ± 9.1E-1 1 ± 2 0.944 5.22E-3 a i = CH4 or CO2   150      Table ‎7.6. Calculated kinetic parameters for MVK kinetic models by fitting to TPMO data Model     Eac kO20       K0H2O  HH2o r2 MRSS  s-1 kPa-1 kJ mol-1 s-1 kPa-1 kJ mol-1 kPa-1 kJ mol-1   H 2.5E5 ± 9.7E0 62 ± 1 2.7E14 ± 4.5E5 207 ± 20 2.2E9 ± 3.7E5 86 ± 20 0.946 5.04E-3 I 1.8E10 ± 9.7E3 117 ± 2 3.3E-2 ± 3.0E-5 30 ± 2 1.4E0 ± 1.0E-1 -5 ± 3 0.938 5.94E-3   151   Figure ‎7.10. Comparisons of calculated and observed CH4 conversions as a function of CH4 and H2O concentration for models A (a and b) and B2 (c and d) as described in Table ‎7.4. Reaction condition follows Figure ‎7.2. 0.1v.% (a and c) or 0.5% (b and d) CH4 in the feed.  152   Figure ‎7.11. Comparisons of modeled and observed CH4 conversions as a function of CH4 and H2O concentration for models C (a and b) and D (c and d) as described in Table ‎7.5. Reaction condition follows Figure ‎7.2. 0.1v.% (a and c) or 0.5% (b and d) CH4 in the feed.   153   Figure ‎7.12. Comparisons of modeled and observed CH4 conversions as a function of CH4 and H2O concentration for models E (a and b) and I (c and d) as described in Table ‎7.5 and 7.6. Reaction condition follows Figure ‎7.2. 0.1 v.% (a and c) or 0.5% (b and d) CH4 in the feed.   154  7.7 Discussion  Finding a kinetic model that can predict the TPMO data successfully at different reaction conditions applicable to NGV catalytic converters is challenging. Abbasi et al. [19] performed a kinetic study on commercial Pt and Pt-Pd catalysts during CH4 oxidation to evaluate kinetic parameters that could predict the light-off curves (TPMO data) in a micro-reactor. The kinetic parameters associated with power law and LH models were optimized using the light-off curve data with mass transfer effects and their best fit parameters were able to predict the light-off curve data even though the fit was biased [19]. Hayes et al. [119] designed a single channel monolith reactor, assumed to operate adiabatically, and evaluated the kinetic parameters for CO oxidation using light-off curve data collected in that reactor, which needed to solve the mole and energy balance simultaneously using a finite-element-based method called a “generalized pattern search algorithm”. Pandya et al. [120] developed an empirical kinetic model for a diesel oxidation catalyst that incorporated oxidation of CO, HC and H2 and reduction of NO using the “generalized pattern search algorithm”.‎ The‎model‎ is successfully able to predict the light-off curve data collected in a single channel monolith reactor whereas the kinetic parameters have no mechanistic interpretations [120].   The kinetic models in this study are among those already applied on CH4 combustion over Pd catalysts [20, 22, 44, 51]. As discussed in the results section, the kinetic models fitted to the differential data are able to accurately predict the observed TPMO data under certain reaction conditions. To improve the prediction of TPMO profiles, kinetic parameters were directly optimized using TPMO data with similar kinetic models used in the differential data-fitting. The  155  fitting results do not show any improvement and parameter values estimated from differential data and TPMO data are very close to each other (e.g. see the results for Model A in Table ‎7.3 and 7.4). The activation energies and H2O adsorption enthalpy obtained for Model A, C and D using both TPMO data-fitting and differential data-fitting are within the range reported in the literature (e.g. see activation energies for supported catalysts in Table ‎2.4 and activation energies and H2O adsorption enthalpy in Table ‎2.5). Except for Model A, C and D, other proposed kinetic models in this study have parameters with no physical meaning. Model E as discussed in Section ‎7.6.4 was the best fitting model that can predict observed TPMO-data in this study although its parameter value representing the CH4 adsorption constant has no physical meaning and the kinetic model must be considered empirical. The parameter values in Model E reflect that the reaction order with respect to CH4 should be smaller than 1 to obtain a good fit. Model C with CH4 reaction order less than 1 (e.g. 0.5) was tested and led to invalid kinetic parameters, although this model also had a fit similar to Model E.    The lack of fit between calculated and observed TPMO data in the present study might be associated with the limited data set available. The proposed kinetic models in Table ‎7.3, fitted to the differential data at‎a‎limited‎temperature‎(538─643‎K)‎and‎concentration‎(0.5‎v.%‎CH4 and 0─2%‎ H2O) range, were used to extrapolate the TPMO data at a wider range of conditions (temperature‎ (393─873‎K),‎CH4 (0.1─0.5‎v.%)‎ and‎H2O (0─10‎v.%)). This extrapolation was partially successful. The proposed kinetic models (the best one Model A) are able to predict TPMO data at temperature conditions similar to the conditions that differential data were measured plus when H2O concentration increases above 3 v.% with 0.1 v.% CH4 in the feed.    156  The limited data set can also affect the parameter optimization in the mechanistic models of Table ‎7.1. For example, a large standard deviation associated to       for Model C and D is evident that more kinetic data over a wider range of conditions are required. Using TPMO data with a wider range of conditions for parameter optimization slightly improves the standard deviation associated to       for Model C and D, compared to using limited differential data set in this study. The large standard deviation of parameters might also reflect a high correlation between parameters. In the case of Model C and D, KH2O0  cannot be correlated to  HH2O due to use of Equation 7.37 to perform data-fitting that limits the correlation between  HH2O and KH2O0. Another possibility is that the value of KH2OPH2O is much higher than 1 in the denominator at different reaction conditions so that kr0 and KH2O0are highly correlated. The calculation of KH2OPH2O with parameter values in Table ‎7.3 and 7.5 at different reaction conditions (for both Model C and D) shows that KH2OPH2O is generally much higher than 1 by an increase in H2O concentration in the feed or a decrease in temperature.  In TPMO, where the reactor operates in integral mode, product concentrations change significantly throughout the catalyst bed and over time (e.g. no H2O at zero conversion can be compared to 0.2 v.% or 1 v.% H2O at 100% conversion with 0.1 v.% or 0.5 v.% CH4 in the feed, respectively.). The proposed kinetic models, optimized by both differential and TPMO data, work well in fitting the TPMO data at conversions < 40% when limited H2O and CO2 are generated, compared to higher conversions, where the lack of fit occurs (see Figure ‎7.10, 7.11 and 7.12). The reason can be attributed to the effect of oxidation products on the CH4 oxidation rate. H2O is shown to inhibit the CH4 oxidation rate over Pd catalysts (see Figure ‎7.5). H2O inhibition is dependent on reaction conditions and decreases as temperature increases or as H2O  157  concentration decreases [8, 49]. The effect of CO2 as the other oxidation product on the rate of CH4 combustion over Pd catalysts remains unclear. CO2 is suggested to inhibit CH4 oxidation rate at high concentrations (> 3%) [44] and hence a reaction order from -2 to 0 with respect to CO2 is suggested in the literature [44, 53, 63], although the effect of H2O is found to be dominant when both reaction products are present during CH4 oxidation [63].    The lack of fit between calculated and observed TPMO data might also be associated with some of the assumptions and empirical correlations used in the reactor model: (a) The reaction rate used in mass transfer calculations was simplified to a 1st-order CH4 reaction rate. This assumption might not be a good approximation. (b) The catalyst bed is diluted with SiC to increase heat dissipation and hence, it is assumed to operate under isothermal conditions during steady-state experiments, which might not be correct. (c) Effective diffusivity, which significantly affects the overall effectiveness factor and hence calculated TPMO profile, is calculated from empirical correlations and not measured at the process conditions of this study. Other parameters such as tortuosity factor, particle porosity and constriction factor are also approximations from the literature. (d) CH4 combustion is a highly exothermic reaction (ΔHr = -891 kJ mol-1) and occurs on a high surface area Al2O3 (237 m2 g-1) loaded with Pd. This might cause a siginicant difference between the temperature at the external surface of the pellet and the temperature inside the pellet so that the CH4 conversion measured at a specified temperature is higher than expected, accounting for a shift in TPMO data towards lower temperatures, although theoretical calulations suggest no temperature difference at T10, T50 and T90 reaction conditions (shown in Table ‎H.1, H.2 and H.3).   158  7.8 Conclusions  The kinetics of CH4 combustion over a PdO/γ-Al2O3 catalyst over a wide range of conditions (CH4 (0.1−0.5‎ v.%),‎ O2 (3−20‎ v.%)‎ and‎ H2O‎ (0−10‎ v.%)‎ at‎ 393−873‎ K) relevant to NGV exhaust gas conditions through differential and TPMO methods is reported. Changing the O2 concentration does not have a significant effect on reaction kinetics so that a zero reaction order for O2 in all kinetic models except MVK models is assumed, whereas CH4 and H2O significantly affect the rate of CH4 combustion. The TPMO data profiles are best fitted by Model A (a power law model) with parameter values that have mechanistic interpretations. The proposed kinetic model can accurately predict the observed TPMO data profiles measured for PdO/γ-Al2O3 catalyst at conversions < 40% when 0.1 v.% CH4 with no H2O or 0.5 v.% CH4 with ≤ 2 v.% H2O are present in the feed under both differential and TPMO conditions. An increase in H2O concentration when 0.1 v.% CH4 present in the feed improves the fit and the calculated and observed data overlap at high concentrations of H2O‎(≥‎3‎v.%), even though the fit gets worse by an increase in H2O concentration when 0.5 v.% CH4 is present in the feed. Re-estimating the kinetic parameters with TPMO data to improve the data fitting leads to almost identical kinetic parameter values. Other kinetic models such as Model E considering slow desorption of H2O and slow consumption of CH4 show a good data-fit but their parameter values have no physical meaning. The lack of fit between calculated and observed TPMO data is attributed to a limited data set available for kinetic analysis and assumptions and empirical correlations used in the reactor models.    159  Chapter 8 Conclusions and Recommendations  8.1 Conclusions  This thesis reports the effects of H2O on CH4 combustion over PdO supported catalysts. Catalyst aging, durability and kinetic studies at various reaction conditions are reported. HTA and thermal aging in air at high temperatures (723–973 K) is shown to deactivate PdO/SiO2 catalysts used for CH4 oxidation, whereas significant catalyst activity loss is observed during HTA. PdO clusters sinter during both HTA and thermal aging in air although the PdO sintering is suppressed in the presence of H2O. Besides PdO sintering, the deactivation of PdO/SiO2 catalysts during HTA is attributed to PdO occlusion by the SiO2 support. Amorphous SiO2 overlayers on PdO clusters were identified by XPS and HRTEM studies. Both H2-reduction and HTA contribute to PdO occlusion although a significant decrease in XPS Pd/Si atom ratio represents a higher PdO occlusion during HTA compared to H2-reduction. The hydroxyl mobility on the catalyst surface as a consequence of SiO2 and H2O interactions is believed to occlude PdO clusters during HTA and H2-reduction.  The influence of HTA temperature (673–973 K) and time (0–32 h) on PdO/SiO2 catalysts used for CH4 oxidation is also reported. An increase in HTA temperature significantly decreases the PdO activity in CH4 combustion, whereas an increase in HTA time only affects the PdO activity at high HTA temperatures (973 K). PdO occlusion by the SiO2 support is responsible for deactivation of PdO/SiO2 catalysts at low HTA temperatures (673 K), whereas a combination of PdO sintering and PdO occlusion by the SiO2 support is found to contribute to the significant  160  catalyst deactivation at high HTA temperatures (973 K). Bulk Pd(OH)2 formation as the altenative deactivation mechanism was not observed at the HTA conditions of this study.  The stability of PdO catalysts during HTA for CH4 oxidation is shown to depend upon the type of oxide support. PdO/α-Al2O3 is found to be the most stable catalyst for CH4 oxidation during HTA at 973 K for up to 65 h. A comparison of catalyst properties before and after HTA at 973 K reflects that PdO supported catalysts sinter and PdO crystallites are occluded by the support. PdO occlusion by the support plays a key role in deactivation of PdO/SiO2 catalysts but is believed not to significantly affect the performance of other PdO supported catalysts tested. The deactivation of PdO/γ-Al2O3 and PdO/SnO2 catalysts during HTA at 973 K is attributed to PdO sintering and PdO  Pd0 transformation. XPS and XRD studies demonstrate the co-existance of PdO/Pd0 after HTA of these catalysts, associated with the presence of H2O. The high stability of PdO/α-Al2O3 catalyst during HTA is attributed to strong Pd-support interactions, accounting for small PdO sintering and no formation of Pd0 phase during HTA.  The kinetics of CH4 oxidation over a PdO/γ-Al2O3 catalyst using differential and TPMO experiments over a range of conditions (CH4 (0.1−0.5‎v.%),‎O2 (3−20‎v.%)‎and‎H2O‎(0−10‎v.%) concentrations) at 393−873‎K‎ is reported. A change in O2 concentration does not significantly affect the PdO catalyst activity so that the O2 dependency in all kinetic models of this study except MVK models is assumed negligible, whereas H2O and CH4 concentration have a significant effect on rate of CH4 combustion. The TPMO data profiles are best fitted by a power law model with parameters values that have mechanistic interpretations. Generally, the proposed kinetic models provide a good fit for‎ TPMO‎ data‎ measured‎ for‎ the‎ PdO/γ-Al2O3 at CH4  161  conversions < 40% when 0.1 v.% CH4 with no H2O or 0.5 v.% CH4 with ≤ 2 v.% H2O present in the feed under both differential and TPMO data-fitting. An increase in H2O concentration increases data-fitting when 0.1 v.% CH4 present in the feed whereas the fit gets worse as H2O concentration increases when 0.5 v.% CH4 present in the feed. The lack of fit between calculated and modeled conversions occurs at high conversions and is attributed to limited data set available for kinetic analysis and assumptions and empirical correlations used in reactor models..        8.2 Recommendations  8.2.1 Kinetic Model Development to Predict the TPMO Data Profiles  A kinetic model that can work under design condition of NGV catalytic converters is needed. Most of the kinetic models in the literature are developed under differential conditions with limited range of component concentrations and temperatures and hence, are not applicable under catalytic converter design conditions. In Chapter 7, several kinetic models were tested to predict TPMO data measured under exhaust gas condition of NGV catalytic converters but none of proposed models works under entire range of conditions with meaningful parameters. It is recommended that kinetic model development should be continued. A new complete set of kinetic experiments considering effect of CH4, H2O and CO2 concentrations at steady-state and under differential mode (at conversions < 10%) to limit the effect of oxidation products at various temperatures (423─823 K) applicable to NGV converters is required. Reactor model also needs to be improved (see Section ‎7.7).   162  8.2.2 Catalyst Aging Durability Studies in a Prototype Reactor  In this thesis, a laboratory scale fixed-bed reactor, designed to operate in plug flow, was used to perform the experiments with the focus on the effect of H2O over Pd catalysts. It is recommended that catalyst aging and durability studies be investigated in a prototype reactor, which has design condition similar to catalytic converters. The catalysts should be washcoated on a honeycomb monolith structure. In a real engine exhaust, catalytic converters are exposed to various operation conditions such as cold start, hot start, acceleration, and steady state so that catalyst aging experiments should simulate the same experience for the prototype catalyst. Furthermore, catalyst aging should be performed in a mixture of gases present in the exhaust instead of only H2O and air for various temperatures.     8.2.3  Kinetic Model Development in a Prototype Reactor   The kinetic models developed in Section ‎8.2.1  should be tested and optimized in a prototype reactor. To have reaction conditions similar to catalytic converters, the reactor should operate in integral mode, where the conversion significantly changes through the catalyst bed. Prototype reactor should be modeled by considering heat transfer, mass transfer, axial dispersion and perhaps pressure drop effects on the performance of catalysts and hence a set of partial differential equations should be solved numerically. Kinetic experiments should also be repeated for the new reactor at various reaction conditions.    163  Bibliography   [1] R.G. Silver, J.C. Summers, in: A. Frennet and J.M. 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D  ez,‎Kinetics‎of‎ the‎Deep‎Oxidation‎of‎Benzene, Toluene, N-Hexane and their Binary Mixtures Over a Platinum on γ-Alumina Catalyst. Appl. Catal. B 38 (2002) 139.                       180            Appendices            181  Appendix A Catalyst Characterization  A.1 BET  The isotherm data (P/P° < 0.3; e.g. see Figure ‎A.1) is fitted to the linear form of BET equation:   (  - )       -    (   )           ‎A.1 and then constants e.g. monolayer volume (Vm) are estimated. The BET surface area is calculated from the following equation:                        ‎A.2   Figure ‎A.1. Isotherm data for calcined 0.7 wt.% Pd/Al2O3   182  The BJH equation used to obtain the pore size distribution and average pore size is as follows: Vpi=rpi2 rki+ ti 2 Vi-rpi2 rki+ ti 2 ti ∑rpj-tjrpjApji-1j=1         ‎A.3 Api=2Vpirpi             ‎A.4 This equation is applied for each desorption step (i; desorption isotherm in e.g. Figure ‎A.1). Layer thickness (ti) in each desorption step is determined from t-plot equation as follows:    (   )       -                    ‎A.5 Kelvin equation is used to obtain the effective pore radius (rki) in each desorption step as follows:    (   )  -                    ‎A.6 Pore radius (rpi) in desorption step is obtained as follows:                       ‎A.7 So that pore volume (Vpi) and pore size (2rpi) from desorption isotherm data are obtained and plotted as cumulative pore size distribution. The first derivative of this plot, which gives dPore volume/dPore size as a function of pore size, is defined as the pore size distribution in this thesis. The maximum pore size in pore size distribution plot is reported as average pore size.  A.2 XRD  The Scherrer equation, which attributes the crystallite size (dcrystal) to FWHM (full width at half maximum; β) of the peak associated to specified crystal, is as follows:                            ‎A.8 K‎is‎an‎instrument‎constant‎(often‎1),‎θ‎is‎angle‎of‎reflection,‎and‎λ‎is‎the‎X-ray wave length.   183  Appendix B Reaction System  B.1 Blank Run  To make sure that reactor walls and the diluent (SiC) do not affect the catalyst activity data, blank runs with no catalyst were carried out. The stainless steel reactor was loaded with 2.5 g of SiC pellets with no catalyst, flowed by 100 cm3 (STP) min-1 STP of dry air, heated to 723 K (10 K min-1) for 15 h and cooled to 393 K (calcination treatment). Then, a gas mixture of 0.1 v.% CH4, 20% O2, 0, or 3% H2O and balance He and Ar flowed over the calcined bed (GHSVs 180,000 cm3 (STP) g-1 h-1) before the reactor temperature was ramped from 393 K to 873 K at 5 K min-1 while the exit stream composition was continuously monitored using a QMS. The CH4 conversion as a function of temperature is. The activity data, shown in Figure ‎B.1, demonstrate no CH4 conversion up to ~700 K, whereas further increase in temperature up to 873 K increases the CH4 conversion to ~25% with or without H2O added to gas stream.   184   Figure ‎B.1. TPMO runs with no catalyst loaded in the reactor; reaction condition: 0.1 v.% CH4, 20% O2, 0 or 3% H2O, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1.  B.2 CH4 Conversion Calculation and Catalyst Testing Experiment Analysis  In each catalyst testing experiment (TPMO or steady state), temperature, reactant flow and mass signal intensities of CH4, CO2 and He (using QMS) as a function of time are recorded. At beginning of the experiment, when the temperature is at ambient and no reaction occurs, a reactant gas mixture flows through the catalyst bed and mass signal intensities are collected for a specified time (e.g. 10 min). Averages of mass signal intensities for that period of time are calculated and used as reactant values at reactor inlet (subscript 0). The mass signal intensities of CH4 and CO2 are normilized based on mass signal of He.                        ‎B.1  185  i is either CH4 or CO2. By having calibration equations for CH4 and CO2 (as described in Appendix C), the relative volume fraction of CH4 and CO2 is calculated.                          ‎B.2 He flow is constant at the inlet and outlet of the reactor. By measuring the He flow at the inlet of reactor, CH4 and CO2 flows are calculated.                                   ‎B.3  CO2 flow should be subtracted from CO2 of environment, which is always present in QMS. In all experiments of this thesis, CO2 flow at reactor inlet should be zero.                               -                    ‎B.4 To reduce the errors associated with calculating CH4 conversion in this thesis, CH4 conversion is calculated based on carbon balance (total moles of carbon (C) leaving the reactor are always constant and here equal to moles of CH4 in the feed.) Note that according to stoichiometery of the CH4 oxidation reaction (                  ), the reaction can be assumed to be constant-volume so that conversion can be calculated based on volumetric flow rates: ∑                                        ‎B.5                                       ‎B.6 Percentage error of carbon balance is calculated as follows: ∑         ∑  - ∑       ∑            ‎B.7 A sample experimental analysis for TPMO of an in situ-calcined 0.8 wt.% Pd/γ-Al2O3 is shown in Table ‎B.1.    186   Table ‎B.1. Experimental analysis for TPMO of 0.8 wt.% Pd/γ-Al2O3 catalyst; Reaction condition: 0.1 v.% CH4, 20% O2 and balance He and Ar, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1 Temp. IHe                                       CH4 flow CO2 flow Corrected CO2 flow ∑C‎flow ∑C‎Error X K Torr Torr Torr     cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 % % 298a 8.88E-09 6.43E-10 1.12E-10 0.0724 0.0126 0.0156 0.0015 0.3125 0.0296 0.0000 0.3125 – – 389 8.68E-09 6.24E-10 1.01E-10 0.0719 0.0117 0.0155 0.0014 0.3103 0.0273 -0.0022 0.3080 1.44 -0.73 395 8.63E-09 6.23E-10 9.98E-11 0.0722 0.0116 0.0156 0.0014 0.3118 0.0271 -0.0025 0.3093 1.02 -0.79 400 8.60E-09 6.23E-10 9.88E-11 0.0724 0.0115 0.0156 0.0013 0.3124 0.0269 -0.0026 0.3098 0.87 -0.85 405 8.68E-09 6.18E-10 9.87E-11 0.0712 0.0114 0.0154 0.0013 0.3072 0.0266 -0.0029 0.3043 2.62 -0.96 408 8.59E-09 6.17E-10 9.82E-11 0.0718 0.0114 0.0155 0.0013 0.3100 0.0268 -0.0028 0.3072 1.69 -0.90 411 8.59E-09 6.18E-10 9.84E-11 0.0720 0.0115 0.0155 0.0013 0.3106 0.0268 -0.0027 0.3079 1.47 -0.88 413 8.60E-09 6.18E-10 9.96E-11 0.0718 0.0116 0.0155 0.0014 0.3101 0.0272 -0.0024 0.3077 1.55 -0.78 415 8.59E-09 6.15E-10 9.86E-11 0.0716 0.0115 0.0155 0.0013 0.3091 0.0269 -0.0026 0.3065 1.93 -0.86 416 8.54E-09 6.16E-10 9.92E-11 0.0721 0.0116 0.0156 0.0014 0.3112 0.0272 -0.0023 0.3088 1.18 -0.76 417 8.55E-09 6.16E-10 9.88E-11 0.0720 0.0116 0.0155 0.0014 0.3109 0.0271 -0.0025 0.3084 1.32 -0.80 418 8.53E-09 6.12E-10 9.87E-11 0.0717 0.0116 0.0155 0.0014 0.3096 0.0271 -0.0024 0.3072 1.71 -0.80 419 8.51E-09 6.06E-10 9.83E-11 0.0712 0.0115 0.0154 0.0014 0.3071 0.0271 -0.0025 0.3046 2.53 -0.82 419 8.46E-09 6.07E-10 9.89E-11 0.0717 0.0117 0.0155 0.0014 0.3094 0.0274 -0.0022 0.3072 1.71 -0.71 420 8.44E-09 6.03E-10 9.90E-11 0.0714 0.0117 0.0154 0.0014 0.3084 0.0275 -0.0021 0.3063 2.00 -0.68 421 8.41E-09 5.99E-10 1.00E-10 0.0712 0.0119 0.0154 0.0014 0.3072 0.0279 -0.0017 0.3055 2.25 -0.56  187  Temp. IHe                                       CH4 flow CO2 flow Corrected CO2 flow ∑C‎flow ∑C‎Error X K Torr Torr Torr     cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 % % 421 8.43E-09 5.99E-10 9.97E-11 0.0711 0.0118 0.0153 0.0014 0.3069 0.0277 -0.0018 0.3050 2.40 -0.60 423 8.43E-09 6.02E-10 1.01E-10 0.0713 0.0120 0.0154 0.0014 0.3079 0.0280 -0.0015 0.3064 1.96 -0.51 424 8.34E-09 5.97E-10 1.02E-10 0.0716 0.0123 0.0155 0.0014 0.3091 0.0288 -0.0007 0.3083 1.34 -0.24 426 8.29E-09 5.94E-10 1.04E-10 0.0716 0.0125 0.0155 0.0015 0.3092 0.0293 -0.0002 0.3090 1.13 -0.07 429 8.24E-09 5.92E-10 1.04E-10 0.0719 0.0126 0.0155 0.0015 0.3102 0.0296 0.0000 0.3102 0.73 0.00 431 8.30E-09 5.92E-10 1.05E-10 0.0713 0.0127 0.0154 0.0015 0.3076 0.0297 0.0001 0.3077 1.55 0.03 435 8.30E-09 5.91E-10 1.07E-10 0.0712 0.0128 0.0154 0.0015 0.3074 0.0301 0.0005 0.3079 1.48 0.18 438 8.31E-09 5.90E-10 1.05E-10 0.0711 0.0127 0.0153 0.0015 0.3068 0.0298 0.0002 0.3070 1.78 0.06 442 8.22E-09 5.86E-10 1.05E-10 0.0713 0.0128 0.0154 0.0015 0.3076 0.0300 0.0004 0.3080 1.44 0.14 446 8.23E-09 5.83E-10 1.06E-10 0.0708 0.0128 0.0153 0.0015 0.3055 0.0301 0.0005 0.3061 2.07 0.17 450 8.20E-09 5.81E-10 1.05E-10 0.0708 0.0128 0.0153 0.0015 0.3057 0.0299 0.0004 0.3061 2.05 0.12 454 8.18E-09 5.79E-10 1.04E-10 0.0707 0.0127 0.0153 0.0015 0.3053 0.0298 0.0003 0.3056 2.21 0.08 458 8.16E-09 5.78E-10 1.04E-10 0.0708 0.0127 0.0153 0.0015 0.3056 0.0299 0.0003 0.3059 2.13 0.10 463 8.16E-09 5.75E-10 1.03E-10 0.0705 0.0127 0.0152 0.0015 0.3043 0.0297 0.0001 0.3045 2.58 0.04 467 8.13E-09 5.77E-10 1.04E-10 0.0710 0.0128 0.0153 0.0015 0.3063 0.0299 0.0003 0.3066 1.88 0.11 470 8.13E-09 5.73E-10 1.04E-10 0.0704 0.0128 0.0152 0.0015 0.3039 0.0300 0.0004 0.3044 2.60 0.15 474 8.10E-09 5.70E-10 1.03E-10 0.0704 0.0127 0.0152 0.0015 0.3039 0.0297 0.0001 0.3040 2.72 0.04 478 8.12E-09 5.69E-10 1.03E-10 0.0701 0.0127 0.0151 0.0015 0.3024 0.0297 0.0001 0.3025 3.20 0.04 481 8.10E-09 5.63E-10 1.05E-10 0.0696 0.0129 0.0150 0.0015 0.3002 0.0303 0.0007 0.3009 3.71 0.23 484 8.07E-09 5.64E-10 1.06E-10 0.0699 0.0132 0.0151 0.0015 0.3015 0.0309 0.0013 0.3028 3.12 0.42  188  Temp. IHe                                       CH4 flow CO2 flow Corrected CO2 flow ∑C‎flow ∑C‎Error X K Torr Torr Torr     cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 % % 487 8.06E-09 5.62E-10 1.08E-10 0.0697 0.0135 0.0150 0.0016 0.3009 0.0315 0.0020 0.3029 3.09 0.65 490 8.02E-09 5.59E-10 1.08E-10 0.0697 0.0135 0.0150 0.0016 0.3006 0.0317 0.0021 0.3028 3.12 0.70 493 7.97E-09 5.53E-10 1.10E-10 0.0693 0.0138 0.0150 0.0016 0.2992 0.0324 0.0028 0.3020 3.37 0.92 496 8.02E-09 5.54E-10 1.13E-10 0.0690 0.0141 0.0149 0.0017 0.2978 0.0331 0.0036 0.3014 3.57 1.18 498 8.02E-09 5.51E-10 1.14E-10 0.0688 0.0142 0.0148 0.0017 0.2968 0.0332 0.0037 0.3004 3.87 1.22 501 7.92E-09 5.45E-10 1.15E-10 0.0687 0.0146 0.0148 0.0017 0.2966 0.0341 0.0046 0.3012 3.63 1.51 503 7.92E-09 5.41E-10 1.17E-10 0.0683 0.0148 0.0147 0.0017 0.2947 0.0348 0.0052 0.2999 4.05 1.73 506 7.99E-09 5.39E-10 1.21E-10 0.0675 0.0151 0.0146 0.0018 0.2912 0.0355 0.0059 0.2971 4.92 1.98 509 7.98E-09 5.37E-10 1.24E-10 0.0673 0.0156 0.0145 0.0018 0.2905 0.0365 0.0069 0.2975 4.82 2.33 511 7.93E-09 5.34E-10 1.28E-10 0.0674 0.0161 0.0145 0.0019 0.2909 0.0377 0.0082 0.2990 4.32 2.74 514 7.87E-09 5.28E-10 1.32E-10 0.0671 0.0167 0.0145 0.0020 0.2896 0.0393 0.0097 0.2993 4.23 3.24 517 7.91E-09 5.28E-10 1.36E-10 0.0668 0.0172 0.0144 0.0020 0.2881 0.0402 0.0107 0.2988 4.40 3.57 520 7.87E-09 5.20E-10 1.41E-10 0.0660 0.0179 0.0143 0.0021 0.2851 0.0419 0.0124 0.2974 4.83 4.16 523 7.85E-09 5.15E-10 1.46E-10 0.0655 0.0186 0.0141 0.0022 0.2829 0.0436 0.0141 0.2969 4.99 4.74 526 7.85E-09 5.10E-10 1.53E-10 0.0649 0.0195 0.0140 0.0023 0.2803 0.0457 0.0161 0.2964 5.15 5.44 529 7.87E-09 5.07E-10 1.60E-10 0.0644 0.0204 0.0139 0.0024 0.2779 0.0478 0.0182 0.2961 5.26 6.15 532 7.83E-09 4.99E-10 1.67E-10 0.0637 0.0213 0.0137 0.0025 0.2749 0.0500 0.0205 0.2954 5.48 6.93 535 7.77E-09 4.93E-10 1.77E-10 0.0634 0.0227 0.0137 0.0027 0.2737 0.0533 0.0238 0.2974 4.82 7.99 538 7.82E-09 4.89E-10 1.85E-10 0.0625 0.0236 0.0135 0.0028 0.2697 0.0554 0.0258 0.2955 5.43 8.74 541 7.84E-09 4.82E-10 1.96E-10 0.0615 0.0250 0.0133 0.0029 0.2656 0.0587 0.0291 0.2947 5.69 9.89  189  Temp. IHe                                       CH4 flow CO2 flow Corrected CO2 flow ∑C‎flow ∑C‎Error X K Torr Torr Torr     cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 % % 544 7.84E-09 4.77E-10 2.08E-10 0.0608 0.0265 0.0131 0.0031 0.2625 0.0621 0.0325 0.2951 5.58 11.03 547 7.82E-09 4.68E-10 2.19E-10 0.0599 0.0280 0.0129 0.0033 0.2586 0.0657 0.0362 0.2947 5.69 12.27 550 7.80E-09 4.58E-10 2.30E-10 0.0587 0.0295 0.0127 0.0035 0.2533 0.0691 0.0395 0.2928 6.31 13.50 553 7.84E-09 4.51E-10 2.43E-10 0.0575 0.0310 0.0124 0.0036 0.2481 0.0727 0.0431 0.2913 6.80 14.81 557 7.92E-09 4.40E-10 2.56E-10 0.0556 0.0324 0.0120 0.0038 0.2401 0.0758 0.0463 0.2864 8.37 16.16 560 7.83E-09 4.32E-10 2.70E-10 0.0552 0.0345 0.0119 0.0040 0.2381 0.0809 0.0514 0.2894 7.39 17.75 563 7.82E-09 4.25E-10 2.87E-10 0.0543 0.0367 0.0117 0.0043 0.2344 0.0859 0.0564 0.2908 6.96 19.39 566 7.80E-09 4.15E-10 3.04E-10 0.0532 0.0390 0.0115 0.0046 0.2296 0.0914 0.0618 0.2914 6.76 21.20 569 7.83E-09 4.03E-10 3.19E-10 0.0514 0.0408 0.0111 0.0048 0.2220 0.0956 0.0660 0.2879 7.86 22.92 572 7.80E-09 3.92E-10 3.37E-10 0.0502 0.0432 0.0108 0.0051 0.2168 0.1014 0.0718 0.2886 7.65 24.88 575 7.76E-09 3.83E-10 3.56E-10 0.0493 0.0458 0.0106 0.0054 0.2128 0.1074 0.0778 0.2906 7.02 26.78 578 7.80E-09 3.72E-10 3.72E-10 0.0477 0.0477 0.0103 0.0056 0.2057 0.1118 0.0822 0.2879 7.88 28.56 581 7.75E-09 3.55E-10 3.89E-10 0.0458 0.0502 0.0099 0.0059 0.1976 0.1177 0.0881 0.2857 8.58 30.85 584 7.76E-09 3.43E-10 4.09E-10 0.0442 0.0528 0.0095 0.0062 0.1910 0.1237 0.0941 0.2851 8.78 33.02 587 7.85E-09 3.34E-10 4.29E-10 0.0425 0.0547 0.0092 0.0064 0.1835 0.1282 0.0986 0.2821 9.72 34.95 590 7.78E-09 3.23E-10 4.49E-10 0.0415 0.0577 0.0090 0.0068 0.1792 0.1354 0.1058 0.2850 8.82 37.12 593 7.71E-09 3.08E-10 4.69E-10 0.0400 0.0608 0.0086 0.0071 0.1726 0.1425 0.1130 0.2856 8.61 39.56 596 7.76E-09 2.99E-10 4.88E-10 0.0385 0.0630 0.0083 0.0074 0.1661 0.1476 0.1181 0.2842 9.07 41.55 599 7.72E-09 2.84E-10 5.08E-10 0.0368 0.0658 0.0079 0.0077 0.1586 0.1543 0.1248 0.2834 9.32 44.02 602 7.74E-09 2.73E-10 5.26E-10 0.0353 0.0680 0.0076 0.0080 0.1523 0.1593 0.1298 0.2821 9.74 46.00  190  Temp. IHe                                       CH4 flow CO2 flow Corrected CO2 flow ∑C‎flow ∑C‎Error X K Torr Torr Torr     cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 % % 605 7.67E-09 2.59E-10 5.47E-10 0.0338 0.0713 0.0073 0.0084 0.1458 0.1672 0.1376 0.2834 9.32 48.55 608 7.77E-09 2.47E-10 5.70E-10 0.0318 0.0733 0.0069 0.0086 0.1374 0.1718 0.1423 0.2797 10.51 50.88 611 7.69E-09 2.33E-10 5.91E-10 0.0303 0.0768 0.0065 0.0090 0.1309 0.1799 0.1504 0.2813 9.99 53.45 614 7.72E-09 2.23E-10 6.11E-10 0.0289 0.0791 0.0062 0.0093 0.1246 0.1854 0.1559 0.2805 10.26 55.57 617 7.71E-09 2.10E-10 6.29E-10 0.0272 0.0817 0.0059 0.0096 0.1174 0.1915 0.1619 0.2793 10.62 57.97 620 7.72E-09 1.98E-10 6.53E-10 0.0257 0.0845 0.0055 0.0099 0.1108 0.1981 0.1685 0.2794 10.61 60.32 623 7.77E-09 1.85E-10 6.75E-10 0.0239 0.0869 0.0052 0.0102 0.1030 0.2038 0.1742 0.2773 11.28 62.84 626 7.78E-09 1.78E-10 6.95E-10 0.0229 0.0894 0.0049 0.0105 0.0987 0.2097 0.1801 0.2788 10.80 64.60 629 7.76E-09 1.61E-10 7.14E-10 0.0207 0.0920 0.0045 0.0108 0.0894 0.2158 0.1862 0.2756 11.81 67.55 632 7.72E-09 1.52E-10 7.32E-10 0.0197 0.0948 0.0043 0.0111 0.0851 0.2222 0.1927 0.2777 11.13 69.37 635 7.70E-09 1.39E-10 7.49E-10 0.0180 0.0972 0.0039 0.0114 0.0779 0.2279 0.1983 0.2762 11.62 71.80 638 7.73E-09 1.31E-10 7.67E-10 0.0169 0.0992 0.0036 0.0116 0.0730 0.2326 0.2031 0.2761 11.66 73.56 641 7.71E-09 1.20E-10 7.80E-10 0.0156 0.1012 0.0034 0.0119 0.0672 0.2371 0.2076 0.2748 12.08 75.54 644 7.71E-09 1.11E-10 8.00E-10 0.0143 0.1038 0.0031 0.0122 0.0619 0.2432 0.2137 0.2755 11.83 77.54 647 7.70E-09 1.01E-10 8.13E-10 0.0132 0.1057 0.0028 0.0124 0.0569 0.2477 0.2182 0.2751 11.98 79.31 650 7.70E-09 8.85E-11 8.31E-10 0.0115 0.1079 0.0025 0.0126 0.0496 0.2528 0.2232 0.2728 12.70 81.82 653 7.69E-09 7.91E-11 8.43E-10 0.0103 0.1097 0.0022 0.0129 0.0444 0.2571 0.2276 0.2720 12.96 83.66 656 7.71E-09 7.42E-11 8.56E-10 0.0096 0.1109 0.0021 0.0130 0.0415 0.2600 0.2305 0.2720 12.98 84.74 659 7.71E-09 6.60E-11 8.69E-10 0.0086 0.1126 0.0018 0.0132 0.0370 0.2640 0.2344 0.2714 13.16 86.38 662 7.68E-09 5.41E-11 8.77E-10 0.0070 0.1142 0.0015 0.0134 0.0304 0.2677 0.2381 0.2685 14.08 88.68  191  Temp. IHe                                       CH4 flow CO2 flow Corrected CO2 flow ∑C‎flow ∑C‎Error X K Torr Torr Torr     cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 % % 665 7.68E-09 4.51E-11 8.88E-10 0.0059 0.1156 0.0013 0.0135 0.0254 0.2709 0.2413 0.2666 14.68 90.49 668 7.65E-09 3.71E-11 8.98E-10 0.0048 0.1174 0.0010 0.0138 0.0209 0.2753 0.2457 0.2667 14.67 92.15 671 7.70E-09 3.38E-11 9.11E-10 0.0044 0.1184 0.0009 0.0139 0.0189 0.2774 0.2479 0.2668 14.63 92.90 674 7.68E-09 2.69E-11 9.20E-10 0.0035 0.1197 0.0008 0.0140 0.0151 0.2806 0.2510 0.2661 14.84 94.32 677 7.72E-09 2.56E-11 9.29E-10 0.0033 0.1203 0.0007 0.0141 0.0143 0.2820 0.2524 0.2667 14.67 94.64 680 7.69E-09 1.92E-11 9.32E-10 0.0025 0.1213 0.0005 0.0142 0.0108 0.2843 0.2547 0.2655 15.05 95.95 683 7.70E-09 1.33E-11 9.42E-10 0.0017 0.1224 0.0004 0.0143 0.0074 0.2870 0.2574 0.2649 15.25 97.19 686 7.69E-09 9.26E-12 9.47E-10 0.0012 0.1232 0.0003 0.0144 0.0052 0.2887 0.2591 0.2643 15.43 98.03 689 7.73E-09 6.85E-12 9.52E-10 0.0009 0.1233 0.0002 0.0144 0.0038 0.2889 0.2593 0.2632 15.79 98.55 692 7.73E-09 2.59E-12 9.53E-10 0.0003 0.1234 0.0001 0.0145 0.0014 0.2893 0.2597 0.2611 16.44 99.45 695 7.65E-09 -8.56E-13 9.56E-10 -0.0001 0.1249 0.0000 0.0146 -0.0005 0.2927 0.2632 0.2627 15.94 100.18 698 7.62E-09 -1.46E-12 9.57E-10 -0.0002 0.1256 0.0000 0.0147 -0.0008 0.2944 0.2649 0.2640 15.51 100.31 701 7.65E-09 -4.30E-12 9.64E-10 -0.0006 0.1259 -0.0001 0.0148 -0.0024 0.2951 0.2656 0.2631 15.80 100.92 704 7.64E-09 -7.34E-12 9.65E-10 -0.0010 0.1264 -0.0002 0.0148 -0.0041 0.2962 0.2666 0.2625 16.01 101.58 707 7.60E-09 -1.02E-11 9.69E-10 -0.0013 0.1275 -0.0003 0.0149 -0.0058 0.2988 0.2692 0.2634 15.70 102.20 710 7.58E-09 -9.23E-12 9.69E-10 -0.0012 0.1279 -0.0003 0.0150 -0.0053 0.2999 0.2703 0.2651 15.18 101.98 713 7.61E-09 -1.00E-11 9.71E-10 -0.0013 0.1276 -0.0003 0.0150 -0.0057 0.2990 0.2695 0.2638 15.60 102.15 716 7.58E-09 -1.18E-11 9.72E-10 -0.0016 0.1282 -0.0003 0.0150 -0.0067 0.3005 0.2709 0.2642 15.47 102.54 719 7.57E-09 -1.35E-11 9.73E-10 -0.0018 0.1286 -0.0004 0.0151 -0.0077 0.3014 0.2719 0.2642 15.47 102.92 722 7.57E-09 -1.46E-11 9.73E-10 -0.0019 0.1286 -0.0004 0.0151 -0.0083 0.3013 0.2718 0.2635 15.70 103.15  192  Temp. IHe                                       CH4 flow CO2 flow Corrected CO2 flow ∑C‎flow ∑C‎Error X K Torr Torr Torr     cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 % % 725 7.57E-09 -1.66E-11 9.77E-10 -0.0022 0.1292 -0.0005 0.0151 -0.0095 0.3028 0.2732 0.2637 15.61 103.59 728 7.53E-09 -1.62E-11 9.78E-10 -0.0022 0.1299 -0.0005 0.0152 -0.0093 0.3046 0.2750 0.2657 14.97 103.50 731 7.54E-09 -1.73E-11 9.77E-10 -0.0023 0.1296 -0.0005 0.0152 -0.0099 0.3038 0.2743 0.2644 15.40 103.74 734 7.53E-09 -1.78E-11 9.74E-10 -0.0024 0.1294 -0.0005 0.0152 -0.0102 0.3033 0.2738 0.2636 15.65 103.86 737 7.49E-09 -1.51E-11 9.79E-10 -0.0020 0.1307 -0.0004 0.0153 -0.0087 0.3064 0.2769 0.2681 14.20 103.25 740 7.48E-09 -1.63E-11 9.83E-10 -0.0022 0.1313 -0.0005 0.0154 -0.0094 0.3078 0.2782 0.2688 13.98 103.49 743 7.52E-09 -1.83E-11 9.80E-10 -0.0024 0.1303 -0.0005 0.0153 -0.0105 0.3053 0.2758 0.2652 15.13 103.96 746 7.52E-09 -1.85E-11 9.79E-10 -0.0025 0.1302 -0.0005 0.0153 -0.0106 0.3052 0.2756 0.2650 15.21 104.00 749 7.50E-09 -1.86E-11 9.82E-10 -0.0025 0.1310 -0.0005 0.0154 -0.0107 0.3070 0.2774 0.2668 14.65 104.01 752 7.52E-09 -1.89E-11 9.84E-10 -0.0025 0.1310 -0.0005 0.0153 -0.0108 0.3070 0.2774 0.2666 14.70 104.07 755 7.52E-09 -1.94E-11 9.81E-10 -0.0026 0.1304 -0.0006 0.0153 -0.0111 0.3057 0.2762 0.2650 15.20 104.21 758 7.50E-09 -1.92E-11 9.80E-10 -0.0026 0.1307 -0.0006 0.0153 -0.0110 0.3064 0.2768 0.2658 14.95 104.15 761 7.48E-09 -2.05E-11 9.81E-10 -0.0027 0.1313 -0.0006 0.0154 -0.0118 0.3077 0.2781 0.2663 14.80 104.45 764 7.51E-09 -2.13E-11 9.85E-10 -0.0028 0.1312 -0.0006 0.0154 -0.0122 0.3074 0.2779 0.2656 15.00 104.61 767 7.55E-09 -2.07E-11 9.87E-10 -0.0027 0.1306 -0.0006 0.0153 -0.0118 0.3062 0.2767 0.2648 15.27 104.47 770 7.52E-09 -2.26E-11 9.86E-10 -0.0030 0.1311 -0.0006 0.0154 -0.0130 0.3074 0.2778 0.2649 15.25 104.89 773 7.52E-09 -2.13E-11 9.85E-10 -0.0028 0.1310 -0.0006 0.0154 -0.0122 0.3070 0.2774 0.2652 15.13 104.61 776 7.52E-09 -2.14E-11 9.87E-10 -0.0028 0.1312 -0.0006 0.0154 -0.0123 0.3076 0.2781 0.2658 14.96 104.62 779 7.48E-09 -2.08E-11 9.86E-10 -0.0028 0.1318 -0.0006 0.0155 -0.0120 0.3090 0.2794 0.2674 14.43 104.49 782 7.49E-09 -2.21E-11 9.88E-10 -0.0030 0.1319 -0.0006 0.0155 -0.0127 0.3092 0.2796 0.2669 14.60 104.78  193  Temp. IHe                                       CH4 flow CO2 flow Corrected CO2 flow ∑C‎flow ∑C‎Error X K Torr Torr Torr     cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 % % 785 7.50E-09 -2.13E-11 9.87E-10 -0.0028 0.1315 -0.0006 0.0154 -0.0122 0.3083 0.2787 0.2665 14.73 104.60 788 7.49E-09 -2.17E-11 9.89E-10 -0.0029 0.1320 -0.0006 0.0155 -0.0125 0.3095 0.2799 0.2674 14.43 104.68 791 7.47E-09 -2.21E-11 9.89E-10 -0.0030 0.1324 -0.0006 0.0155 -0.0128 0.3103 0.2807 0.2679 14.27 104.76 794 7.53E-09 -2.31E-11 9.88E-10 -0.0031 0.1312 -0.0007 0.0154 -0.0132 0.3075 0.2780 0.2647 15.29 105.00 797 7.42E-09 -2.27E-11 9.88E-10 -0.0031 0.1332 -0.0007 0.0156 -0.0132 0.3122 0.2826 0.2694 13.80 104.91 800 7.42E-09 -2.12E-11 9.94E-10 -0.0029 0.1340 -0.0006 0.0157 -0.0123 0.3140 0.2845 0.2721 12.93 104.54 803 7.42E-09 -2.06E-11 9.95E-10 -0.0028 0.1340 -0.0006 0.0157 -0.0120 0.3140 0.2845 0.2725 12.81 104.40 806 7.40E-09 -2.02E-11 9.95E-10 -0.0027 0.1344 -0.0006 0.0158 -0.0118 0.3151 0.2855 0.2737 12.41 104.29 810 7.44E-09 -2.20E-11 9.98E-10 -0.0029 0.1340 -0.0006 0.0157 -0.0127 0.3141 0.2846 0.2719 13.01 104.68 813 7.49E-09 -2.09E-11 9.99E-10 -0.0028 0.1334 -0.0006 0.0156 -0.0121 0.3128 0.2832 0.2712 13.24 104.45 816 7.44E-09 -2.12E-11 9.99E-10 -0.0028 0.1342 -0.0006 0.0157 -0.0123 0.3147 0.2851 0.2728 12.70 104.50 819 7.47E-09 -2.27E-11 1.01E-09 -0.0030 0.1350 -0.0007 0.0158 -0.0131 0.3164 0.2869 0.2738 12.41 104.80 822 7.50E-09 -2.17E-11 1.01E-09 -0.0029 0.1346 -0.0006 0.0158 -0.0125 0.3154 0.2858 0.2733 12.54 104.57 825 7.46E-09 -2.10E-11 1.01E-09 -0.0028 0.1353 -0.0006 0.0159 -0.0122 0.3172 0.2877 0.2755 11.85 104.42 828 7.42E-09 -2.17E-11 1.02E-09 -0.0029 0.1369 -0.0006 0.0160 -0.0126 0.3209 0.2913 0.2786 10.84 104.54 831 7.46E-09 -2.07E-11 1.02E-09 -0.0028 0.1360 -0.0006 0.0159 -0.0120 0.3188 0.2892 0.2772 11.29 104.32 834 7.47E-09 -2.09E-11 1.02E-09 -0.0028 0.1366 -0.0006 0.0160 -0.0121 0.3202 0.2906 0.2785 10.88 104.34 837 7.49E-09 -1.89E-11 1.02E-09 -0.0025 0.1366 -0.0005 0.0160 -0.0109 0.3202 0.2906 0.2797 10.51 103.90 840 7.48E-09 -2.18E-11 1.03E-09 -0.0029 0.1374 -0.0006 0.0161 -0.0126 0.3220 0.2924 0.2798 10.46 104.49 843 7.44E-09 -2.14E-11 1.03E-09 -0.0029 0.1381 -0.0006 0.0162 -0.0124 0.3236 0.2940 0.2816 9.89 104.41  194  Temp. IHe                                       CH4 flow CO2 flow Corrected CO2 flow ∑C‎flow ∑C‎Error X K Torr Torr Torr     cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 cm3 (STP) min-1 % % 846 7.43E-09 -2.02E-11 1.03E-09 -0.0027 0.1386 -0.0006 0.0162 -0.0117 0.3248 0.2953 0.2835 9.27 104.13 849 7.40E-09 -2.18E-11 1.03E-09 -0.0029 0.1397 -0.0006 0.0164 -0.0127 0.3275 0.2979 0.2853 8.72 104.45 852 7.43E-09 -1.91E-11 1.04E-09 -0.0026 0.1393 -0.0006 0.0163 -0.0111 0.3266 0.2970 0.2859 8.51 103.87 855 7.40E-09 -2.11E-11 1.04E-09 -0.0028 0.1405 -0.0006 0.0165 -0.0123 0.3294 0.2999 0.2876 7.98 104.27 858 7.37E-09 -2.10E-11 1.04E-09 -0.0029 0.1412 -0.0006 0.0165 -0.0123 0.3310 0.3014 0.2891 7.50 104.26 861 7.38E-09 -2.27E-11 1.05E-09 -0.0031 0.1416 -0.0007 0.0166 -0.0133 0.3318 0.3023 0.2890 7.53 104.60 864 7.38E-09 -2.17E-11 1.05E-09 -0.0029 0.1427 -0.0006 0.0167 -0.0127 0.3345 0.3049 0.2922 6.49 104.34 867 7.40E-09 -2.37E-11 1.06E-09 -0.0032 0.1430 -0.0007 0.0168 -0.0138 0.3351 0.3055 0.2917 6.65 104.73 869 7.38E-09 -2.06E-11 1.06E-09 -0.0028 0.1434 -0.0006 0.0168 -0.0120 0.3362 0.3066 0.2946 5.75 104.08 871 7.40E-09 -1.88E-11 1.06E-09 -0.0025 0.1437 -0.0005 0.0168 -0.0110 0.3368 0.3072 0.2962 5.21 103.70 873 7.41E-09 -2.13E-11 1.06E-09 -0.0029 0.1437 -0.0006 0.0168 -0.0124 0.3369 0.3074 0.2949 5.63 104.21 a Values at reactor inlet    195  Temperature and mass signal intensities were recorded every ~40 s during the course of experiment. For presentation purposes, every three data points in TPMO results were averaged and the average data points were reported. A sample of raw data versus average data for TPMO of an in situ-calcined 0.8 wt.% Pd/γ-Al2O3 is shown in Figure ‎B.2.   Figure ‎B.2. Comparison of TPMO raw data and TPMO average data using 0.8 wt.% Pd/γ-Al2O3 catalyst; reaction condition: 0.1 v.% CH4, 20% O2, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1.  B.3 Repeatability of TPMO Results  To determine the repeatability of TPMO results, a fresh commercial 1 wt.% Pd/γ-Al2O3 (Sigma Aldrich) catalyst, calcined in situ in dry air at 723 K for 15 h, was evaluated by TPMO in the absence of H2O in separate trials (shown in Figure ‎B.3).   196    Figure ‎B.3. Repeatability of TPMO runs using commercial 1 wt.% Pd/γ-Al2O3 catalyst in the absence of H2O; reaction condition: 0.1 v.% CH4, 20% O2, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1.  Repeatability of TPMO experiments in the presence of H2O was also determined. An in situ- calcined 0.8 wt.% Pd/γ-Al2O3 catalyst, prepared using incipient wetness impregnation, was evaluated by TPMO in the presence of 5 v.% H2O in separate trials (shown in Figure ‎B.4). TPMO results with/without extra H2O are shown to be repeatable with error < 2% (Table  ‎B.2).   197   Figure ‎B.4. Repeatability of TPMO runs using 0.8 wt.% Pd/γ-Al2O3 catalyst in the presence of H2O; reaction condition: 0.1 v.% CH4, 20% O2, 5% H2O, 5 K min-1, GHSV 180,000 cm3 (STP) g-1 h-1.  Table  ‎B.2. Error calcualtions for TPMO results at T10, T50 and T90.  Commercial‎1wt.%‎Pd/γ-Al2O3, no H2O 0.8‎wt.%‎Pd/γ-Al2O3, 5 v.% H2O X (%) 10 50 90 10 50 90 Repeat 1 489 546 591 630 676 717 Repeat 2 495 545 581 640 685 730 Repeat 3 495 548 590 643 686 725        Average 493 546 587 638 682 724 S.D. 3 2 6 7 6 7 % Error 0.7 0.3 0.9 1.1 0.8 0.9   198  B.4 Repeatability and Error Analysis of Differential Results  To determine the repeatability of differential results, an in situ- calcined 0.8 wt.% Pd/γ-Al2O3 catalyst, prepared using incipient wetness impregnation, was evaluated by differential steady-state experiments at different temperatures and GHSVs in separate trials (shown in Figure ‎B.5). The results show a good repeatability for differential experiments.   Figure ‎B.5. Repeatability of differential runs using 0.8 wt.% Pd/γ-Al2O3 catalyst; reaction condition: 0.5 v.% CH4, 20% O2 and balance He and Ar, GHSV (a) 180,000; (b) 270,000 cm3 (STP) g-1 h-1.  To evaluate the error in differential experiments, pure error was calculated (see Table ‎B.3, B.4 and B.5). RSS for each temperature is calculated as follows:  199       ∑(    -    )           ‎B.8 Sum of RSS (SRSS) is calculated as follows:       ∑               ‎B.9 Pure error mean square (PEMS) is calculated as follows:           ∑  -             ‎B.10  Table ‎B.3. Repeatability and error analysis of differential runs using 0.8 wt.% Pd/γ-Al2O3 catalyst. Reaction condition follows Figure ‎B.5a. T TOF Mean S.D. ni RSS K s-1 s-1 s-1  s-2 538 2.54E-02 1.86E-02 2.24E-02 1.95E-02 2.26E-02 2.17E-02 2.71E-03 5 2.93E-05 548 2.61E-02 3.16E-02 – – – 2.88E-02 3.90E-03 2 1.52E-05 558 4.82E-02 3.65E-02 4.29E-02 – – 4.25E-02 5.88E-03 3 6.91E-05 568 6.34E-02 4.95E-02 5.65E-02 6.12E-02 – 5.76E-02 6.13E-03 4 1.13E-04  Table ‎B.4. Repeatability and error analysis of differential runs using 0.8 wt.% Pd/γ-Al2O3 catalyst. Reaction condition follows Figure ‎B.5b. T TOF Mean S.D. ni RSS K s-1 s-1 s-1  s-2 538 3.00E-02 3.11E-02 3.05E-02 3.06E-02 5.39E-04 3 5.82E-07 548 4.20E-02 4.14E-02 – 4.17E-02 4.27E-04 2 1.82E-07 558 5.79E-02 5.52E-02 – 5.66E-02 1.85E-03 2 3.42E-06    200  Table ‎B.5. Pure error analysis for differential experiments. Σni SRSS PRMS S.D.  s-2 s-2 s-1 21 2.30E-04 1.65E-05 4.06E-03  To determine if the difference between TOFs observed by changing the total flow rates from 300 to 450 cm3 (STP) min-1 and hence superficial gas velocity in the reactor is significant, student t-tests were carried out as shown in Table ‎B.6. Pooled variance is calculated as follows:    =‎PRMS‎(ni1-1)+‎PRMS‎(ni2-1)‎‎(ni1-1)+‎(ni2-1)          ‎B.11 Standard deviation of the two means is calculated as follows:       √                         ‎B.12 t-value is calculated as follows:  -       |     -      |            ‎B.13 Given the degree of freedom (                ) and confidence level, t-values from student’s‎ t‎ tables‎ (t-table) can be determined. If calculated t-value is less than t-table, the difference between the two means (here TOFm) will be insignificant. As shown in Table ‎B.6, the difference between TOFs as a consequence of an increase in total flow rates from 300 to 450 cm3 (STP) min-1 at different temperatures is insignificant.     201       Table ‎B.6. Student t-test to compare the mean TOFs at different total flow rates and temperatures.  450 cm3 (STP) min-1 300 cm3 (STP) min-1         T ni TOFm ni TOFm PRMS sp2 sd t-value df Confidence level t-table Check K  s-1  s-1         538 3 3.06E-02 5 2.17E-02 1.65E-05 9.87E-05 7.26E-03 1.220 6 95% 2.450 Insignificant 548 2 4.17E-02 2 2.88E-02 1.65E-05 3.29E-05 5.74E-03 2.252 2 95% 4.303 Insignificant 558 2 5.66E-02 3 4.25E-02 1.65E-05 4.94E-05 6.41E-03 2.184 3 95% 3.180 Insignificant     202  Appendix C  Mass Spectrometer Calibration  Table ‎C.1. Gas and mass signal profile in QMS. Gas Mass signal monitored He 4 CH4 15 CO2 44 O2 32 H2O 18 Ar 40  All mass signals of gases are normalized based on mass signal of He. To calibrate the CH4 as an example, He and CH4 flow through QMS while recording the He and CH4 mass signals for a specified period of time (e.g. 10 min). Subsequently, CH4 flow is changed to another level and the mass signals are recorded accordingly (see Table ‎C.2). The calibration equation is obtained by fitting a line over the calibration data (see Figure ‎C.1) as follows:                      -                    ‎C.1        203         Table ‎C.2. Experimental plan for calibrating CH4 flow. Gas CH4 νHe (cm3 min-1 STP) 20 20 20 20 20 νCH4 (cm3 min-1 STP) 0.30 0.24 0.18 0.11 0.06 IHe (torr) 7.72E-8 8.75E-8 1.03E-7 1.30E-7 1.65E-7 S.D.a (torr) 1.19E-9 5.46E-10 2.85E-10 2.10E-10 5.09E-10 ICH4 (torr) 5.83E-9 5.11E-9 4.40E-9 3.46E-9 2.29E-9 S.D. (torr) 1.06E-10 6.81E-11 2.88E-11 1.03E-11 1.17E-11 Ireb 7.55E-2 5.84E-2 4.27E-2 2.66E-2 1.39E-2 Yrec 1.52E-2 1.22E-2 9.1E-3 5.7E-3 3.0E-3 a Standard deviation. b Relative mass signal (based on He) c Relative volume fraction (based on He)       204        Figure ‎C.1. CH4 volume fraction as a function of CH4 mass signal normalized to He (calibration curve).         205  Appendix D Plug Flow Operation  It is essential to make sure that the gas flow pattern inside the reactor is ideal otherwise data interpretation is challenging. A catalyst bed‎diameter‎at‎least‎10x’s‎of‎particle‎diameter‎ensures‎the negligible wall effect on flow pattern. Axial dispersions are also negligible when the catalyst bed length/particle diameter ratio is much higher than 50 [121]. According to Table ‎D.1, the reactor design in this thesis passes the rules of thumb to ensure plug flow operation.  Table ‎D.1. Rules of thumb to ensure plug flow operation. db (cm) 0.70   Lb (cm) 4.5   dp (cm) 0.0222       ⁄  32 > 10     ⁄  203 > 50  It is suggested that as particle Reynolds number is small in laboratory-scale fixed-bed reactors, much higher Lb/dp ratio is needed to ensure plug flow pattern [121]. For laboratory-scale reactors, Peclet number and Lb/dp must be higher than minimum Peclet number and minimum Lb/dp ratio, respectively, to ensure plug flow operation, which is the case in this thesis (see Table ‎D.2).  Average gas molecular weight at reactor inlet was calculated as follows: Mwfeed= yCH4‎MwCH4+yO2 ‎MwO2+yHe‎MwHe+yAr‎MwAr     ‎D.1  Gas density was calculated as follows (ideal gas law):                         ‎D.2  206  Particle Reynolds number was calculated as follows: NRe=ρgVdpμ           ‎D.3 Peclet number was calculated as follows (for gas phase operation) [121]:                       ⁄          ‎D.4 Minimum Peclet number was calculated as follows [121]:             (  -    ⁄)         ‎D.5  Minimum Lb/dp was calculated as follows (for gas phase operation) [121]:     ⁄            -         (11-    ⁄)        ‎D.6                207     Table ‎D.2. Details of calculations for ensuring plug flow operation for laboratory scale fixed-bed reactors. Parameter Units Value T K 558 P kPa 101.325 R kpa cm3 mol-1 K-1 8.31E+03       0.0050      0.2000 yHe  0.0898 yAr  0.7052 Mwfeed g mol-1 35.01 ρg g cm-3 7.65E-04 µ g cm-1 min-1 2.32E-02 rt cm 3.52E-01 ν0 cm3 (STP) min-1 450 V cm min-1 2367 X % 7.16 n  1 dp cm 2.22E-02 NRe  1.73 NPe  20.01        0.59     ⁄   203     ⁄      6      208  Appendix E Isothermal Operation  Prior to running any experiment, ensuring isothermal operation of the reactor is required. A small temperature variation in the catalyst bed can considerably influence the reaction rate. The temperature gradients arising from non-ideality of the catalyst bed can be divided in three levels as follows: (a) interparticle; (b) interphase (c) intraparticle [121, 122]. Table ‎E.1 contains criteria to check the existence of temperature gradients in any of the above levels.  Table ‎E.1. Criteria for ensuring isothermal operation of reactor [121, 122] Interparticle |   ||    |                 [121, 122] Interphase |   ||  |                   [121, 122] Intraparticle |   ||  |                      0.1 g of in situ-calcined 0.8 wt.% Pd/Al2O3 diluted by 2.5 g SiC was examined in steady-state CH4-O2 reaction at various temperatures (each for 0.5 h) and CH4 conversions below 10%, as described in section ‎3.3.4.   Particle density was calculated from the following equation:        ⁄               ‎E.1 Reaction rates were calculated by the design equation of fixed-bed reactor assuming the differential method (CH4 conversion below 10%) as follows:  209    =Fm0.Xw            ‎E.2 r   =ρp.             ‎E.3 The kinetic parameters e.g. activation energy was calculated by assuming the following reaction rate incorporated with the Arrhenius equation:    = k1PCH4PH2O-1           ‎E.4 k1 = k10exp (-EacRT)          ‎E.5 The heat transfer coefficient was calculated by the following empirical equation [113]: NNu=hdpkg           ‎E.6 NPr=cpμkg           ‎E.7 NNu= 2 + 1.8NRe12⁄ NPr13⁄           ‎E.8 Prandtl number of Ar at 500 K was used to calculate Nusselt number. Thermal conductivity of SiC was used as kb, thermal conductivity of Al2O3 was used as the kp and thermal conductivity of Ar at the desired temperature was used as kg. The values of different criteria in Table ‎E.3 shows that the reactor in this thesis operates isothermally.         210  Table ‎E.2. Details of calculations for parameters involved in ensuring isothermal operation criteria. Parameter Units Value T K 558 P kPa 101.325 R kpa cm3 mol-1 K-1 8.31E+03 w G 0.1000 V0 cm3 g-1 5.00E-01 ρs g cm-3 4.03 ρp g cm-3 1.34E+00 dp cm 2.22E-02       0.0050      0.2000 yHe  0.0898 yAr  0.7052 Mwfeed g mol-1 35.01 ρg g cm-3 7.65E-04 Fm0 mol min-1 1.00E-04 µ g cm-1 min-1 2.32E-02 rt cm 3.52E-01 ν0 cm3 (STP) min-1 450 V cm min-1 2367 X % 7.1648 rm mol min-1 gcat-1 7.19E-05 r''' mol min-1 cm-3 9.61E-05 NRe  1.73 ΔHr kJ mole-1 -891 kb kJ cm-1 min-1 K-1 0.294 kp kJ cm-1 min-1 K-1 0.0108 kg kJ cm-1 min-1 K-1 0.00001695 NPr  0.6630 NNu  4.065 h kJ cm-2 min-1 K-1 0.0031 Eac kJ mole-1 153.00  211  Table ‎E.3. Values for different isothermal operation criteria Criterion Our system Limit Intrareactor 2.13E-03 < 0.2 Interphase 1.81E-02 < 0.15 Intraparticle 5.78E-05 < 0.75                     212  Appendix F Mass Transfer Effects  F.1 Theoretical Calculations  Weisz-Prater and Mears criteria were calculated to ensure that the kinetic data in this thesis were not internal and external mass-transfer-controlled, respectively.  Particle porosity was calculated as follows: εp=V01ρs⁄ +V0           ‎F.1 Average pore size was calculated as follows: dpore=4V0SBET           ‎F.2 Bed density was calculated based on catalyst mass and diluted bed volume as follows: ρb=w rt2             ‎F.3 Bed porosity was calculated based on the void volume between SiC (catalyst diluent) pellets: ρbSiC=SiC‎ ass+w rt2            ‎F.4 εb=1-ρbSiCρSiC           ‎F.5       213  Table ‎F.1. Catalyst properties and operating conditions used in mass transfer calculations. Parameter Unit Value T K 558 P kPa 101.325 R kPa cm3 mol-1 K-1 8.314E+03 yCH4  5.00E-03 yO2  2.00E-01 yHe  8.98E-02 yAr  7.05E-01 Mwfeed g mol-1 35.01 ρg g cm-3 7.65E-04       kPa 0.507 Fm0 mol min-1 1.00E-04 µ g cm-1 min-1 2.32E-02 rt cm 3.52E-01 ν0 cm3 (STP) min-1 450 V cm min-1 2367 w g 0.1000 SBET cm2 g-1 2.37E+06 V0 cm3 g-1 0.5 ρs g cm-3 4.03 ρb g cm-3 0.0572 ρp g cm-3 1.34 εp  0.67 dp cm 2.22E-02 η  4 ζ  0.8 dpore cm 1.10E-06 ρbSiC g cm-3 1.49 ρSiC g cm-3 3.21 εb  0.54   214  Binary bulk diffusivity of CH4 in Ar was calculated based on Chapman-Enskog correlation [114, 115]: ζi=2.44 (TciPci⁄ )13⁄          ‎F.6 ζCH4-Ar=(ζCH4+ζAr)2⁄          ‎F.7  ik⁄ =0.75Tci           ‎F.8     -   ⁄ =(     ⁄      ⁄ )12⁄        ‎F.9 T*= kT  CH4-Ar⁄           ‎F.10 ΩD= (44.54‎T*-4.909+1.911‎T*-1.575)0.1        ‎F.11 MwCH4-Ar=MwCH4 .MwArMwCH4+MwAr         ‎F.12 DCH4-Ar=0.001858‎T32⁄ ‎MwCH4-Ar-1 2⁄PζCH4-Ar2 ΩD,‎T‎<‎1000‎K,‎P‎<‎70‎atm‎      F.13 Bulk effective diffusivity was calculated as follows [113]: DCH4-Areff=DCH4-Ar‎εp‎ζη          ‎F.14 Knudson diffusion (when molecular mean free path >> pore diameter) was calculated as follows [113]: DK=48.5‎dpore (TMwfeed⁄ )12⁄         ‎F.15 Effective Knudsen diffusion was calculated as follows [113]: DKeff=εp‎DKη           ‎F.16  215  Effective diffusivity was calculated based on contributions from both bulk and Knudsen diffusions as follows [113]: Deff=(1DCH4-Areff+1DKeff)-1         ‎F.17  Table ‎F.2. Details of calculations for effective diffusivity Parameter Unit Value      g mol-1 39.95      kPa 4863      K 150.69       g mol-1 16.04       kPa 4640       K 190.70     ⁄   113.02 ζAr Å 3.57      ⁄   143.03 ζCH4 Å 3.93  CH4-Ar k⁄   127.14 ζCH4-Ar Å 3.75 T* K 4.39 ΩD  0.86 MwCH4-Ar  11.44 DCH4-Ar cm2 min-1 36.00 DCH4-Ar_eff cm2 min-1 24.06 DK cm2 min-1 1.28 DK_eff cm2 min-1 0.85 Deff cm2 min-1 0.82  Thiele modulus was calculated as follows:  216        √                      ‎F.18 Internal effectiveness factor was calculated based on first order reaction rate as follows [113]:       (        - )         ‎F.19 Weisz-Prater criterion [113] to ensure the negligibility of internal mass transfer resistance was assessed as follows:  C P‎=‎η.θ12‎<‎1          ‎F.20  Table ‎F.3. Details of calculations for internal mass transfer criterion. Parameter Unit Value X % 7.1648      kPa 0.470 Cm mol cm-3 (STP) 2.072E-07 Cs mol cm-3 (STP) 2.072E-07 rm mol min-1 gcat-1 7.19E-05      mol min-1 cm-3 9.61E-05 θ1  0.26 η  1.00 Cwp  0.07  The mass transfer coefficient was calculated using jD-factor correlation as follows [114]:            -              ‎F.21                  ‎F.22              ⁄             ‎F.23                                 ‎F.24  217  Mears criterion [113] to ensure the negligibility of external mass transfer resistance is assessed as follows:                               ‎F.25  Table ‎F.4. Details of calculations of external mass transfer criterion. Parameter Unit Value NRe  1.73 NSc  0.84 jD  0.55 kc cm min-1 1447.41 CM  1.52E-04  F.2 Diagnostic Tests  To ensure negligible external mass transfer, a diagnostic test with changing total flow rates under differential condition has been done [121] (see Figure ‎F.1). A change in superficial gas velocity does not indicate significant change in TOF at > 800 cm min-1 by considering the pure error associated to reactor operation under differential condition in this thesis. Based on the results of Figure ‎F.1 and student t-test in Section ‎B.4, differential experiments were carried out at total flow rates of 450 cm3 min-1 (or superficial gas velocity of 1170 cm min-1) to ensure negligibility of external mass transfer resistance.   218   Figure ‎F.1. Diagnostic test to ensure negligible external mass transfer resistance. Reaction condition: 0.5 v.% CH4, 20% O2 in balance of He and Ar, 538 K   Previous study used the same reactor design and configuration with WHSV of 500 h-1 and showed that catalyst particles below 500 μm ensured negligible internal mass transfer resistances [51, 123]. WHSV for performing differential experiments was 421 h-1 and catalyst particles in this thesis are approximately 222 μm.        219  Appendix G Supplementary Figures for Chapter 6    Figure ‎G.1. XPS spectra of PdO/α-Al2O3 catalysts. (a) fresh – calcined catalyst, (b) aged – catalyst hydrothermally aged in 6.5 v.% H2O/air at 973 K for 65 h catalyst.    220     Figure ‎G.2. XPS spectra of PdO/SnO2 catalysts. (a) fresh – calcined catalyst, (b) aged – catalyst hydrothermally aged in 6.5 v.% H2O/air at 973 K for 65 h catalyst.     221  Appendix H Kinetics  H.1 Supplementary Information  Details of calculations of mass transfer criteria for observed TPMO data at various CH4 and H2O concentration are summarized in Table ‎H.1, H.2 and H.3. The same equations and correlations as in Appendices D, E and F are used to calculate the mass transfer criteria. Reaction rates are calculated using Model A parameters in Table ‎7.3. The maximum temperature, representing the temperature inside the pellet, is calculated as follows [113]:        -                           ‎H.1 In Equation H.1, the surface CH4 concentration (Cs) and external surface temperature (Ts) are assumed to be equal to the fluid bulk CH4 concentration and temperature, respectively.   222  Table ‎H.1. Details of calculations for mass transfer criteria and maximum temperature in observed TPMO data when no H2O added to the feed. Reaction condition follows Figure ‎7.7  Parameter Unit Value CH4 v.% 0.1 0.1 0.1 0.5 0.5 0.5 X % 10 50 90 10 50 90 T K 541 607 664 563 636 701 Cm mol cm-3 (STP) 4.02E-08 2.23E-08 4.46E-09 2.01E-07 1.12E-07 2.23E-08 rm mol min-1 gcat-1 1.85E-05 8.30E-05 1.25E-04 6.99E-05 3.31E-04 5.38E-04 r    mol min-1 cm-3 2.47E-05 1.11E-04 1.66E-04 9.34E-05 4.42E-04 7.19E-04 Deff cm2 min-1 2.01E-01 2.14E-01 2.25E-01 2.10E-01 2.24E-01 2.36E-01 kc cm min-1 1072.05 1269.43 1447.20 1122.87 1342.38 1546.89 θ1  0.61 1.69 4.52 0.52 1.48 4.10 η  0.98 0.85 0.52 0.98 0.88 0.55 Cwp  0.37 2.43 10.57 0.27 1.92 9.32 CM  2.72E-04 1.86E-03 1.22E-02 1.97E-04 1.40E-03 9.88E-03 ΔHr kJ mole-1 -891 -891 -891 -891 -891 -891 kp kJ cm-1 min-1 K-1 0.0108 0.0108 0.0108 0.0108 0.0108 0.0108 Tmax K 541 607 664 563 636 701  223  Table ‎H.2. Details of calculations for mass transfer criteria and maximum temperature in observed TPMO data when 2 v.% H2O added to the feed. Reaction condition follows Figure ‎7.7  Parameter Unit Value CH4 v.% 0.1 0.1 0.1 0.5 0.5 0.5 X % 10 50 90 10 50 90 T K 594 640 688 617 677 737 Cm mol cm-3 (STP) 4.02E-08 2.23E-08 4.46E-09 2.01E-07 1.12E-07 2.23E-08 rm mol min-1 gcat-1 3.81E-06 1.89E-05 2.70E-05 5.82E-05 3.82E-04 6.02E-04 r    mol min-1 cm-3 5.09E-06 2.52E-05 3.61E-05 7.77E-05 5.10E-04 8.04E-04 Deff cm2 min-1 2.13E-01 2.21E-01 2.30E-01 2.22E-01 2.33E-01 2.44E-01 kc cm min-1 1225.13 1366.32 1518.21 1279.10 1464.58 1657.08 θ1  0.27 0.79 2.08 0.46 1.56 4.27 η  1.00 0.96 0.79 0.99 0.87 0.54 Cwp  0.07 0.60 3.44 0.21 2.10 9.81 CM  4.91E-05 3.93E-04 2.53E-03 1.44E-04 1.48E-03 1.03E-02 ΔHr kJ mole-1 -891 -891 -891 -891 -891 -891 kp kJ cm-1 min-1 K-1 0.0108 0.0108 0.0108 0.0108 0.0108 0.0108 Tmax K 594 640 688 617 677 737  224  Table ‎H.3. Details of calculations for mass transfer criteria and maximum temperature in observed TPMO data when 5 v.% H2O added to the feed. Reaction condition follows Figure ‎7.7  Parameter Unit Value CH4 v.% 0.1 0.1 0.1 0.5 0.5 0.5 X % 10 50 90 10 50 90 T K 630 675 716 657 718 763 Cm mol cm-3 (STP) 4.02E-08 2.23E-08 4.46E-09 2.01E-07 1.12E-07 2.23E-08 rm mol min-1 gcat-1 9.00E-06 3.45E-05 3.24E-05 1.47E-04 8.19E-04 6.92E-04 r    mol min-1 cm-3 1.20E-05 4.61E-05 4.33E-05 1.97E-04 1.09E-03 9.26E-04 Deff cm2 min-1 2.22E-01 2.30E-01 2.37E-01 2.31E-01 2.42E-01 2.50E-01 kc cm min-1 1327.52 1468.06 1599.58 1393.49 1585.74 1732.08 θ1  0.41 1.05 2.24 0.72 2.23 4.52 η  0.99 0.93 0.77 0.97 0.77 0.52 Cwp  0.16 1.03 3.88 0.50 3.85 10.55 CM  1.07E-04 6.68E-04 2.88E-03 3.34E-04 2.94E-03 1.14E-02 ΔHr kJ mole-1 -891 -891 -891 -891 -891 -891 kp kJ cm-1 min-1 K-1 0.0108 0.0108 0.0108 0.0108 0.0108 0.0108 Tmax K 630 675 716 657 718 763  225  The fit of Model B2 was strongly dependent on the initial guess and a change in the initial guess results in new set of parameters. Two sets of parameters estimated are reported in Table ‎H.4. The parameter values are found significantly different when comparing two separate trials with different initial guess and hence, Model B2 was rejected.    Table ‎H.4. Calculated kinetic parameters for power law models by fitting to differential data. Model     Eac a b r2  s-1 kPa-1-a-b kJ mol-1    B1 2.0E13 ± 2.8E3 165 ± 3  -1.0 ± 0.0 – 0.959 B2 4.9E4 ± 1.4E1 55 ± 4 -0.3 ± 0.0 0.7 ± 0.0 0.997 3.5E12 ± 8.1E3 155 ± 11 -0.9 ± 0.1 0.1 ± 0.1 0.967     226     Table ‎H.5. Calculated kinetic parameters for various kinetic models by fitting to differential data Model     Eac K0H2O  HH2o K0ia  Hi r2  s-1 kPa-1 kJ mol-1 kPa-1 kJ mol-1 kPa-1 kJ mol-1  E 5.8E8 ± nvb 102 ± nv 1.3E-4 ± nv -47 ± nv 0.0E0 ± nv -4.1E11 ± nv 0.960 F 2.8E11 ± 3.3E4 152 ± 1124 2 ± 353 3E1 ± 6E4 -8.4E-24 ± 1.3E-3 -188E0 ± 4E10 0.632 G 3.2E11 ± 2.9E4 152 ± 1235 27 ± 401 37E0 ± 2E4 -7.0E-14 ± 0.8E0 -101E0 ± 2E8 0.624 a i = CH4 or CO2; b nv = Not valid      227       Table ‎H.6. Calculated kinetic parameters for MVK kinetic models by fitting to differential data Model     Eac kO20       K0H2O  HH2o r2  s-1 kPa-1 kJ mol-1 s-1 kPa-1 kJ mol-1 kPa-1 kJ mol-1  H nv ± nv -1E-41 ± 1E17 2.5E5 ± 2.0E3 70E0 ± 1E3 -1.5E-41± 2.8E-9 -3E2 ± 1E14 0.436 I nv ± nv 5517 ± nv 8.9E6 ± nv 82 ± nv 0.0E0 ± nv -1E4 ± nv 0.474     228     Figure ‎H.1. Mass transfer effects on calculated TPMO results with 0.1v.% CH4 and no H2O in the feed. Reaction condition is similar to Figure ‎7.8.     229     Figure ‎H.2. Mass transfer effects on calculated TPMO results with 0.1v.% CH4 and 2% H2O in the feed. Reaction condition is similar to Figure ‎7.8.    230     Figure ‎H.3. Mass transfer effects on calculated TPMO results with 0.5v.% CH4 and no H2O in the feed. Reaction condition is similar to Figure ‎7.8.    231     Figure ‎H.4. Mass transfer effects on calculated TPMO results with 0.5v.% CH4 and 2% H2O in the feed. Reaction condition is similar to Figure ‎7.8.    232  H.2 MATLAB M-files for Estimating Kinetic Parameters Using Differential Data   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  global verbose verbose(1:2) = 1; % Obtain kinetic parameters for CH4 oxidation in the presence of H2O % using the differential 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  % % Reading the data from excel file % nvar=1;   233  x1_0 = xlsread('Data.xlsx','sheet1','g5:g12');    % Temperature, C...first indep vari x2_0 = xlsread('Data.xlsx','sheet1','i5:i12');    % CH4 conversion,...second indep vari x3_0 = xlsread('Data.xlsx','sheet1','j5:j12');    % H2O partial pressure, kPa....third indep vari x1_2 = xlsread('Data.xlsx','sheet1','b5:b16');    % Temperature, C....first indep vari x2_2 = xlsread('Data.xlsx','sheet1','d5:d16');    % CH4 conversion,...second indep vari x3_2 = xlsread('Data.xlsx','sheet1','e5:e16');    % H2O partial pressure, kPa....third indep vari  x1 = [x1_0;x1_2]; x2 = [x2_0;x2_2]; x3 = [x3_0;x3_2]; x = [x1 x2 x3];  oldx = x; nx = length(x);  y_0 = xlsread('Data.xlsx','sheet1','h5:h12');    % rate, umol/s/gpd....observed values dep vari y_2 = xlsread('Data.xlsx','sheet1','c5:c16');    % rate, umol/s/gpd....observed values dep vari   y = [y_0;y_2];  newy = y(:); oldy = reshape(newy,nx,nvar); newx = x;   %  %INPUT DATA NOW IN CORRECT COLUMN FORMAT %  x = newx  y = newy  %        %  provide initial parameter guesses % theta = [27 52];                                             np = length(theta) pin = theta   % % Begin calculation by calling L-M leat squares routine  234  % [f,p,kvg,iter,corp,covp,covr,stdresid,Z,r2]=leasqr(x,y,pin,'modelmulti',0.0001);  disp('RESPONSE:') if kvg ==1     disp ('PROBELM CONVERGED')     elseif kvg == 0     disp('PROBLEM DID NOT CONVERGE') end oldf=reshape(f,nx,nvar); 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(1,2,1),plot(oldy(1:8),oldy(1:8)), hold on     plot(oldy(1:8),oldf(1:8),'d')     xlabel('Observed Rate, umol/s/gPd')     ylabel('Calculated Rate, umol/s/gPd')      subplot(1,2,2),plot(oldy(9:20),oldy(9:20)), hold on     plot(oldy(9:20),oldf(9:20),'d')  Objective function m-file, which calculates modeled reaction rate, is defined as follows:  function f = modelmulti (x,pin) % find the solution at sepcified x values - corresponding to measured data % Temp = x(:,1)...Operatig temperature, K % Pw0 = x(:,3)...H2O partial pressure, kPa  235   nxx=length(x); Pm0 = 0.005*101.325; %Feed CH4 partial pressure, kPa  for i = 1:nxx     k = pin(1)*exp(-1*pin(2)/8.314e-3*(1/(x(i,1)+273)-1/490));         r(i,:) = k*Pm0*(1-x(i,2))/(x(i,3)+2*Pm0*x(i,2)); end  f = r(:);  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. % 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)  236  % 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 % 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  237  %       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: % Bard, Nonlinear Parameter Estimation, Academic Press, 1974. % Draper and Smith, Applied Regression Analysis, John Wiley and Sons, 1981. % %set default args    238  % 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;   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);  239  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,     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  240      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       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;  241    end;   if ss>sgoal,     break;   end; end;   % 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  242  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');   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);   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)...  243        /((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>  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  244  % 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          245  H.3 MATLAB Codes for Predicting the TPMO Data Profiles Using Differential Kinetic Parameters  The ODE and its boundary condition shown in equations 7.9 and 7.10 are solved using ODE45 function of MATLAB. Main m-file is as follows:  % Fixed-bed reactor with time and temperature variation % Isothermal, constant volumetric flowrate % This solution assumes reaction rate = kPch4/Ph2O % clear all                   % clears prev. vars from memory   % Variables global Tstep knt eta0 thi eta k1 Deff D1m;   % %set-up integration for each time interval   % define time span t0 = 0; tfinal = 96; nstep = 24; Tstep = (tfinal-t0)/nstep; eta0 = 0; eta = 0; thi = 0;   for knt = 1:nstep; % Time steps W0 = 0; WF = 0.1;  %Catalyst mass, g initial = 0.00000001; [W,X] = ode45('P2b', [W0,WF], initial);     %X is the solution matrix Xfinal(knt,:) = X(end,:);  tfinal(knt,:) = Tstep*knt; thif(knt,:) = thi; etaf(knt,:) = eta; eta0f(knt,:) = eta0; k1f(knt,:) = k1; Defff(knt,:) = Deff; D1mf(knt,:) = D1m;  246  end; tfinal Xfinal thif %etaf %eta0f %k1f subplot(1,5,1) plot (tfinal,Xfinal) xlabel ('Time, min') ylabel ('CH4 Conversion') subplot(1,5,2) plot(tfinal,etaf) xlabel ('Time, min') ylabel ('eta') subplot(1,5,3) plot(tfinal,thif) xlabel ('Time, min') ylabel ('Thi') subplot(1,5,4) plot(tfinal,Defff) xlabel ('Time, min') ylabel ('Deff, m2/s ') subplot(1,5,5) plot(tfinal,D1mf) xlabel ('Time, min') ylabel ('DCH4-Ar, m2/s ')  Objective function solving the mole balance equation incorporated with mass transfer calculations is as follows:  %USS FBR % Program contains calculations for odes which are to be solved by ODE45        %   function xprime = P2b(W,X)   % Variables % global Tstep knt eta0 thi eta k1 Deff D1m;   % system properties % epsilon = 0.54;  %Bed voidage v0 = 300; %Total volumetric feed rate, cm3/min  247  Fm0 = 0.005*v0/22414; % Feed CH4 flow, mol/min Pm0 = 0.005*101.325; % Feed CH4 partial pressure, kpa Pw0 = 0.0*101.325; % Feed H2O partial pressure, kpa db = 0.0572; % Bed density, gcat/cm3 k0 = 3.36626e16*0.01/10^6*60; % frequency factor, mol/min/gcat Ea = 164.7737; % activation energy, kJ/mol R = 0.008314; % Gas constant, kJ/mol.K Mwfeed = 34.21; % molecular weight of feed, g/mol denp = 1.34e3; % particle density of catalyst, kg/m3  Rt = 3.52e-3; % radius of reactor, m ac = 6*(1-epsilon)/2.22e-4; % external surface area/volume of solids, m2/m3 vis = 3.87e-5; % dynamic viscosity...assumed const for all temperatures, kg/m/s tf = 4; % tortuosity factor   time = Tstep*knt; theta = time-epsilon*W/db/v0;   % effectiveness factor calculations k1 = k0*exp(-1*Ea/R/(5*(theta+epsilon*W/v0/db)+393))...     /60*1000*0.008314*273/(Pw0+2*Pm0*X); % rate constant, m3/kgcat/s Dk = 48.5*1.1e-8*((5*(theta+epsilon*W/v0/db)+393)/Mwfeed)^0.5; % Knudson diffusion, m2/s Dkeff = Dk*0.67/tf; % effective knudson diffusion, m2/s T0 = 127.14^-1*(5*(theta+epsilon*W/v0/db)+393); % Tstar for collision integral ci = (44.54*T0^-4.909+1.911*T0^-1.575)^0.1; % Collision integral D1m = 0.001858*(5*(theta+epsilon*W/v0/db)+393)^1.5*11.44^(-0.5)/1/3.75^2/ci*10^-4; % diffusivity of CH4 in Ar, m2/s D1meff = D1m*0.67*1/tf; % Mixture effective diffusivity, m2/s Deff = (1/Dkeff+1/D1meff)^-1; % effective diffusivity, m2/s thi = 2.22e-4/3*sqrt(k1*denp/Deff); % Thiele modulus eta = 3/thi^2*(thi*coth(thi)-1); % internal effectiveness factor deng = 101.325/0.008314/(5*(theta+epsilon*W/v0/db)+393)*Mwfeed/1000; % gas density, kg/m3 U = v0/Rt^2/3.14*(5*(theta+epsilon*W/v0/db)+393)/273/10^6/60; % superficial velocity at T, m/s G = deng*U; % mass velocity of fluid, kg/m2/s NRe = 2.22e-4*G/vis; % Reynolds number NSc = vis/deng/D1m; % Schmidth number jD = 0.357/NRe^0.359/epsilon; % jD factor kc = jD*G/deng/NSc^(2/3); % mass transfer coefficient, m/s eta0 = eta/(1+eta*k1*db*1000/kc/ac);     248    xprime = eta0*k0/Fm0*exp(-1*Ea/R/(5*(theta+epsilon*W/v0/db)+393))...     *Pm0*(1-X)/(Pw0+2*Pm0*X);                        249  H.4 MATLAB M-files for Estimating the Kinetic Parameters Using TPMO Data Profiles  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 global verbose verbose(1:2) = 1; % 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;   % Reading data from excel file    250  x1_5c_0h = xlsread('input_data_temp_conv.xlsx','sheet2','a5:a26');    % Temperature, K....first indep vari x2_5c_0h = xlsread('input_data_temp_conv.xlsx','sheet2','c5:c26');    % H2O partial pressure, kpa,...second indep vari x3_5c_0h = xlsread('input_data_temp_conv.xlsx','sheet2','d5:d26');    % CH4 volume fraction x1_5c_2h = xlsread('input_data_temp_conv.xlsx','sheet3','a5:a26');    % Temperature, K....first indep vari x2_5c_2h = xlsread('input_data_temp_conv.xlsx','sheet3','c5:c26');    % H2O partial pressure, kpa,...second indep vari x3_5c_2h = xlsread('input_data_temp_conv.xlsx','sheet3','d5:d26');    % CH4 volume fraction x1_5c_5h = xlsread('input_data_temp_conv.xlsx','sheet1','a5:a24');    % Temperature, K....first indep vari x2_5c_5h = xlsread('input_data_temp_conv.xlsx','sheet1','c5:c24');    % H2O partial pressure, kpa,...second indep vari x3_5c_5h = xlsread('input_data_temp_conv.xlsx','sheet1','d5:d24');    % CH4 volume fraction x1_1c_0h = xlsread('input_data_temp_conv.xlsx','sheet4','a5:a23');    % Temperature, K....first indep vari x2_1c_0h = xlsread('input_data_temp_conv.xlsx','sheet4','c5:c23');    % H2O partial pressure, kpa,...second indep vari x3_1c_0h = xlsread('input_data_temp_conv.xlsx','sheet4','d5:d23');    % CH4 volume fraction x1_1c_2h = xlsread('input_data_temp_conv.xlsx','sheet5','a5:a18');    % Temperature, K....first indep vari x2_1c_2h = xlsread('input_data_temp_conv.xlsx','sheet5','c5:c18');    % H2O partial pressure, kpa,...second indep vari x3_1c_2h = xlsread('input_data_temp_conv.xlsx','sheet5','d5:d18');    % CH4 volume fraction  251  x1_1c_5h = xlsread('input_data_temp_conv.xlsx','sheet7','a5:a16');    % Temperature, K....first indep vari x2_1c_5h = xlsread('input_data_temp_conv.xlsx','sheet7','c5:c16');    % H2O partial pressure, kpa,...second indep vari x3_1c_5h = xlsread('input_data_temp_conv.xlsx','sheet7','d5:d16');    % CH4 volume fraction x1_1c_3h = xlsread('input_data_temp_conv.xlsx','sheet6','a5:a17');    % Temperature, K....first indep vari x2_1c_3h = xlsread('input_data_temp_conv.xlsx','sheet6','c5:c17');    % H2O partial pressure, kpa,...second indep vari x3_1c_3h = xlsread('input_data_temp_conv.xlsx','sheet6','d5:d17');    % CH4 volume fraction x1_1c_10h = xlsread('input_data_temp_conv.xlsx','sheet8','a5:a13');    % Temperature, K....first indep vari x2_1c_10h = xlsread('input_data_temp_conv.xlsx','sheet8','c5:c13');    % H2O partial pressure, kpa,...second indep vari x3_1c_10h = xlsread('input_data_temp_conv.xlsx','sheet8','d5:d13');    % CH4 volume fraction     x1 = [x1_5c_0h;x1_5c_2h;x1_5c_5h;x1_1c_0h;x1_1c_2h;x1_1c_5h;x1_1c_3h;x1_1c_10h]; x2 = [x2_5c_0h;x2_5c_2h;x2_5c_5h;x2_1c_0h;x2_1c_2h;x2_1c_5h;x2_1c_3h;x2_1c_10h];  x3 = [x3_5c_0h;x3_5c_2h;x3_5c_5h;x3_1c_0h;x3_1c_2h;x3_1c_5h;x3_1c_3h;x3_1c_10h];   x = [x1 x2 x3]; oldx = x; nx = length(x);   y_5c_0h = xlsread('input_data_temp_conv.xlsx','sheet2','b5:b26');    % Conversion X, %....observed values dep vari  252  y_5c_2h = xlsread('input_data_temp_conv.xlsx','sheet3','b5:b26');    % Conversion X, %....observed values dep vari y_5c_5h = xlsread('input_data_temp_conv.xlsx','sheet1','b5:b24');    % Conversion X, %....observed values dep vari y_1c_0h = xlsread('input_data_temp_conv.xlsx','sheet4','b5:b23');    % Conversion X, %....observed values dep vari y_1c_2h = xlsread('input_data_temp_conv.xlsx','sheet5','b5:b18');    % Conversion X, %....observed values dep vari y_1c_5h = xlsread('input_data_temp_conv.xlsx','sheet7','b5:b16');    % Conversion X, %....observed values dep vari y_1c_3h = xlsread('input_data_temp_conv.xlsx','sheet6','b5:b17');    % Conversion X, %....observed values dep vari y_1c_10h = xlsread('input_data_temp_conv.xlsx','sheet8','b5:b13');    % Conversion X, %....observed values dep vari     y = [y_5c_0h;y_5c_2h;y_5c_5h;y_1c_0h;y_1c_2h;y_1c_5h;y_1c_3h;y_1c_10h];  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 % pin(2) = activation energy, kJ/mol   theta = [0.001 100];                                             np = length(theta) pin = theta    253  %help  % Begin calculation by calling L-M leat squares routine % [f,p,kvg,iter,corp,covp,covr,stdresid,Z,r2]=leasqr(x,y,pin,'modelmulti',0.0001); disp('RESPONSE:') if kvg ==1     disp ('PROBELM CONVERGED')     elseif kvg == 0     disp('PROBLEM DID NOT CONVERGE') end oldf=reshape(f,nx,nvar); 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(1:22,1),oldy(1:22),'d'), hold, plot (oldx(1:22,1),oldf(1:22))     title('0.5% CH4, 0% H2O, 300sccm')     xlabel('Temperature, K')     ylabel('CH4 conversion,a.u.')     subplot(1,3,2), plot (oldx(23:44,1),oldy(23:44),'d'), hold, plot (oldx(23:44,1),oldf(23:44))     title('0.5% CH4, 2% H2O, 300sccm')     xlabel('Temperature, K')     ylabel('CH4 conversion,a.u.')     subplot(1,3,3),     plot (oldx(45:64,1),oldy(45:64),'d'), hold, plot (oldx(45:64,1),oldf(45:64))     title('0.5% CH4, 5% H2O, 300sccm')     xlabel('Temperature, K')     ylabel('CH4 conversion,a.u.')     figure  254      subplot(1,3,1), plot (oldx(65:83,1),oldy(65:83),'d'), hold, plot (oldx(65:83,1),oldf(65:83))     title('0.1% CH4, 0% H2O, 300sccm')     xlabel('Temperature, K')     ylabel('CH4 conversion,a.u.')     subplot(1,3,2),plot (oldx(84:97,1),oldy(84:97),'d'), hold, plot (oldx(84:97,1),oldf(84:97))     title('0.1% CH4, 2% H2O, 300sccm')     xlabel('Temperature, K')     ylabel('CH4 conversion,a.u.')     subplot(1,3,3)     plot (oldx(98:109,1),oldy(98:109),'d'), hold, plot (oldx(98:109,1),oldf(98:109))     title('0.1% CH4, 5% H2O, 300sccm')     xlabel('Temperature, K')     ylabel('CH4 conversion,a.u.')     figure      subplot(1,2,1)     plot (oldx(110:122,1),oldy(110:122),'d'), hold, plot (oldx(110:122,1),oldf(110:122))     title('0.1% CH4, 3% H2O, 300sccm')     xlabel('Temperature, K')     ylabel('CH4 conversion,a.u.')      subplot(1,2,2),     plot (oldx(123:131,1),oldy(123:131),'d'), hold, plot (oldx(123:131,1),oldf(123:131))     title('0.1% CH4, 10% H2O, 300sccm')     xlabel('Temperature, K')     ylabel('CH4 conversion,a.u.')  The objective function solves the mole balance equation by calling ODEfunm.m through ODE45 function as follows:   function f = modelmulti (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)...] param = [1 0 0 pin(1) pin(2)]; yzero = 0.00000001; nxx=length(x); for i = 1:nxx  255      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,[0,0.1],yzero,[],param);     yfinal(i,:)=ymodel(end,:); end  f = yfinal(:);  Mole balance incorporated with mass transfer calculations are defined in the following function. Note this function considers Model A as the rate expression.    function dXdw=ODEfunm(w,X,param) % param = [Temp ym0 Pw0 pin(1) pin(2) pin(3) pin(4)...] % system properties % epsilon = 0.54;  %Bed voidage v0 = 300; %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 db = 0.0572; % Bed density, gcat/cm3 R = 0.008314; % Gas constant, kJ/mol.K Mwfeed = 34.21; % molecular weight of feed, g/mol denp = 1.34e3; % particle density of catalyst, kg/m3  Rt = 3.52e-3; % radius of reactor, m ac = 6*(1-epsilon)/2.22e-4; % external surface area/volume of solids, m2/m3 vis = 3.87e-5; % dynamic viscosity...assumed const for all temperatures, kg/m/s tf = 4; % tortuosity factor   kr = param(4)*exp(-1*param(5)/R*(1/param(1)-1/650));  % rate constant, mol/min/gcat   % effectiveness factor calculations k1 = kr/60*1000*0.008314*273/(param(3)+2*Pm0*X); % rate constant, m3/kgcat/s Dk = 48.5*1.1e-8*(param(1)/Mwfeed)^0.5; % Knudson diffusion, m2/s Dkeff = Dk*0.67/tf; % effective knudson diffusion, m2/s T0 = 127.14^-1*param(1); % Tstar for collision integral ci = (44.54*T0^-4.909+1.911*T0^-1.575)^0.1; % Collision integral D1m = 0.001858*param(1)^1.5*11.44^(-0.5)/1/3.75^2/ci*10^-4; % diffusivity of CH4 in Ar, m2/s D1meff = D1m*0.67*0.8/tf; % Mixture effective diffusivity, m2/s  256  Deff = (1/Dkeff+1/D1meff)^-1; % effective diffusivity, m2/s thi = 2.22e-4/2*sqrt(k1*denp/Deff); % Thiele modulus eta = 3/thi^2*(thi*coth(thi)-1); % internal effectiveness factor deng = 101.325/0.008314/param(1)*Mwfeed/1000; % gas density, kg/m3 U = v0/Rt^2/3.14*param(1)/273/10^6/60; % superficial velocity at T, m/s G = deng*U; % mass velocity of fluid, kg/m2/s NRe = 2.22e-4*G/vis; % Reynolds number NSc = vis/deng/D1m; % Schmidth number jD = 0.357/NRe^0.359/epsilon; % jD factor kc = jD*G/deng/NSc^(2/3); % mass transfer coefficient, m/s eta0 = eta/(1+eta*k1*db*1000/kc/ac);        dXdw = eta0*kr/Fm0*Pm0*(1-X)/(param(3)+2*Pm0*X); 

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