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Design and synthesis of new ceria-based materials for low-temperature methane oxidation Vickers, Susan Michaela 2015

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Design and Synthesis of New Ceria-Based Materials for Low-Temperature Methane Oxidation  by SUSAN MICHAELA VICKERS M. Sci., University of Glasgow, 2009  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (CHEMISTRY)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) September 2015  ©Susan Michaela Vickers, 2015   ii Abstract   The synthesis and characterization of novel ceria-based materials is reported. Modification of synthetic routes reported in the literature produced several new doped and undoped ceria samples, which were tested for catalytic activity for low-temperature methane oxidation. These tests showed that the presence of a second, redox active metal oxide results in materials with higher catalytic activity than those with only ceria. In addition, several new routes to ceria-based materials with various morphologies, including nanorods and hollow, mesoporous nanospheres, were developed. The nanospheres were successfully doped with lanthanum, giving rise to the first non-hydrothermal route to hollow, mesoporous nanospheres of both doped and undoped ceria.  Further examination of materials containing two redox active metal oxides generated mesoporous cobalt oxide with doped and undoped ceria in the pores. The cobalt oxide was templated with KIT-6 silica to produce a material with highly ordered mesopores. The ceria/cobalt oxide materials have remarkably high activity for low-temperature methane oxidation given that they contain no noble metals. However, surprisingly, the mesoporous cobalt oxide on its own exhibited the highest catalytic activity with 50% complete methane conversion to carbon dioxide and water below 400 ˚C.  Cerium-based precursors were also used as a starting material to synthesize Pd/ceria materials via a new method termed surface-assisted reduction. Surface-assisted reduction produces ceria   iii with PdO highly dispersed on the surface. These materials showed exceptionally high activity for methane oxidation, with the best materials exhibiting 50% methane conversion below 300 ˚C. Exploration of the scope of surface-assisted reduction successfully produced ceria materials with gold or silver deposited on their surface.    iv Preface   All the work in this thesis was carried out under the guidance of Prof. Mark J. MacLachlan. Chapters 2-5 involved collaboration with Dr. Kevin J. Smith and Rahman Gholami of the Department of Chemical and Biological Engineering, University of British Columbia, Canada, who performed all the methane oxidation testing described throughout this thesis. X-ray photoelectron spectroscopy analysis was performed by Dr. Ken Wong of the Interfacial Analysis and Reactivity Laboratory, University of British Columbia. Elemental analysis was carried out by microanalytical services, University of British Columbia.  Chapter 2: Stanley Shi synthesized Comp3-X%M where M= Gd, Er, Zn and Cr.  I performed all the experiments reported in Chapter 2.  Chapter 3: Portions of this chapter will be submitted for publication: Susan M. Vickers, Loryn Arnett, Gomathi Anandhanatarajan and Mark J. MacLachlan, “A simple route to ceria and La-doped ceria hollow spheres” 2015. Loryn Arnett and Dr. Gomathi Anandhanatarajan conducted experiments to refine the synthetic conditions used for comp9. Dr. Nick White collected the NMR spectrum for comp9. I performed all of the other experiments reported in chapter 3.  Portions of Chapter 4 have previously been published as: Susan M. Vickers, Rahman Gholami, Kevin J. Smith, and Mark J. MacLachlan, “Mesoporous Mn- and La-Doped Cerium   v Oxide/Cobalt Oxide Mixed Metal Catalysts for Methane Oxidation.” ACS Appl. Mater. Interfaces 2015, 7, 11460–11466. Dr. Georg Meseck collected the SEM images for comp12. I performed all of the other experiments reported in chapter 4.  Portions of Chapter 5 will be published: Gomathi, Anandhanatarajan, Susan M. Vickers, Rahman Gholami, Mina Alyani, Renee W. Y. Man, Mark J. MacLachlan, Kevin J. Smith and Michael O. Wolf, “Nanostructured Materials Prepared by Surface-Assisted Reduction: New Catalysts for Methane Oxidation”, and Susan M. Vickers, Gomathi Anandhanatarajan, Rahman Gholami, Kevin J. Smith and Mark J. MacLachlan, “The Scope of Surface-Assisted Reduction”. Surface-assisted reduction was discovered by Gomathi Anandhanatarajan, who also first synthesized and characterized Comp13. I performed all other experiments reported in Chapter 5.      vi Table of Contents Abstract ................................................................................................................................ ii!Preface ................................................................................................................................. iv!Table of Contents ................................................................................................................ vi!List of Tables ..................................................................................................................... xii!List of Figures ................................................................................................................... xiii!List of Schemes ................................................................................................................. xxi!List of Symbols and Abbreviations .................................................................................. xxii!Acknowledgements ......................................................................................................... xxiv!Dedication ........................................................................................................................ xxv!Chapter 1: Introduction ........................................................................................................ 1!1.1! Nanomaterials ......................................................................................................... 1!1.1.1! Overview .......................................................................................................... 1!1.1.2! Synthesis of nanomaterials .............................................................................. 2!1.2! Mesoporous materials ............................................................................................. 4!1.2.1! Overview .......................................................................................................... 4!1.2.2! Mesoporous silica ............................................................................................ 5!1.2.3! Mesoporous inorganic materials ...................................................................... 9!1.2.4! Nanomaterials in catalysis ............................................................................. 13!1.3! Low-temperature methane oxidation .................................................................... 15!1.3.1! Natural gas ..................................................................................................... 15!1.3.2! Catalysts for low-temperature methane oxidation ......................................... 17!1.4! Cerium oxide ......................................................................................................... 20!  vii 1.4.1! Overview ........................................................................................................ 20!1.4.2! Doped ceria .................................................................................................... 24!1.4.3! Nanostructured ceria ...................................................................................... 26!1.4.4! Ceria in catalysis ............................................................................................ 30!1.4.5! Ceria as a support ........................................................................................... 31!1.5! Analytical techniques ............................................................................................ 32!1.5.1! X-ray diffraction ............................................................................................ 32!1.5.2! Transmission electron microscopy ................................................................ 35!1.5.3! Scanning electron microscopy ....................................................................... 37!1.5.4! Energy dispersive X-ray spectroscopy ........................................................... 38!1.5.5! Nitrogen adsorption ....................................................................................... 39!1.5.6! X-ray photoelectron spectroscopy ................................................................. 43!1.6! Goals and scope of this thesis ............................................................................... 44!Chapter 2: Synthesis Methods and Doping of CeO2 .......................................................... 46!2.1! Introduction ........................................................................................................... 46!2.2! Experimental ......................................................................................................... 48!2.2.1! General ........................................................................................................... 48!2.2.2! Microscopy .................................................................................................... 48!2.2.3! Preparation of plate-like CeO2 materials ....................................................... 49!2.2.4! Preparation of doped plate-like CeO2 materials ............................................. 49!2.2.5! Preparation of pseudo-spherical CeO2 particles ............................................ 49!2.2.6! Preparation of doped pseudo-spherical CeO2 particles .................................. 50!2.2.7! Preparation of high surface area CeO2 ........................................................... 50!  viii 2.2.8! Preparation of doped high surface area CeO2 ................................................ 51!2.2.9! Pd impregnation onto doped high surface area CeO2 .................................... 51!2.2.10! Preparation of CeO2 from a chloride precursor ........................................... 51!2.2.11! Methane oxidation testing ............................................................................ 52!2.3! Results and discussion .......................................................................................... 54!2.3.1! Plate-like CeO2 materials ............................................................................... 54!2.3.2! Doped plate-like CeO2 materials ................................................................... 58!2.3.3! Pseudo-spherical CeO2 particles .................................................................... 66!2.3.4! High surface area cerium oxide ..................................................................... 69!2.3.5! High surface area CeO2  as a support for Pd .................................................. 77!2.3.6! Effects of a chloride precursor ....................................................................... 80!2.4! Conclusions ........................................................................................................... 81!Chapter 3: Controlling the Morphology of Nanostructured CeO2 ..................................... 83!3.1! Introduction ........................................................................................................... 83!3.2! Experimental ......................................................................................................... 85!3.2.1! General ........................................................................................................... 85!3.2.2! Microscopy .................................................................................................... 85!3.2.3! Preparation of CeO2 nanoflowers .................................................................. 86!3.2.4! Preparation of doped CeO2 nanoflowers ........................................................ 86!3.2.5! Preparation of ceria-coated hydroxyapatite ................................................... 87!3.2.6! Removal of hydroxyapatite from ceria-coated hydroxyapatite ...................... 88!3.2.7! Preparation of CTAC templated ceria nanorods ............................................ 88!3.2.8! Preparation of hollow ceria spheres ............................................................... 89!  ix 3.2.9! Preparation of hollow, doped ceria spheres ................................................... 89!3.3! Results and discussion .......................................................................................... 90!3.3.1! CeO2 nanoflowers .......................................................................................... 90!3.3.2! Ceria-coated hydroxyapatite .......................................................................... 95!3.3.3! Hydroxyapatite template removal .................................................................. 99!3.3.4! CTAC templated ceria nanorods .................................................................. 101!3.3.5! Hollow ceria spheres .................................................................................... 103!3.3.6! Hollow, doped ceria spheres ........................................................................ 109!3.4! Conclusions ......................................................................................................... 113!Chapter 4: Mesoporous Doped Cerium Oxide/Cobalt Oxide Mixed Metal Catalysts ..... 114!4.1! Introduction ......................................................................................................... 114!4.2! Experimental ....................................................................................................... 116!4.2.1! General ......................................................................................................... 116!4.2.2! Microscopy .................................................................................................. 117!4.2.3! Preparation of KIT-6 .................................................................................... 118!4.2.4! Preparation of mesoporous Co3O4 ............................................................... 118!4.2.5! Preparation of Co3O4/CeO2 composite ........................................................ 119!4.2.6! Preparation of Co3O4/doped-CeO2 composite. ............................................ 119!4.2.7! Preparation of chiral nematic Co3O4 ............................................................ 119!4.2.8! Methane oxidation testing ............................................................................ 120!4.3! Results and discussion ........................................................................................ 120!4.4! Chiral nematic mesoporous cobalt oxide ............................................................ 140!4.4.1! Conclusions .................................................................................................. 143!  x Chapter 5: Surface-assisted Reduction: A New Method for the Preparation of Noble Metal/Ceria Catalysts ....................................................................................................... 145!5.1! Introduction ......................................................................................................... 145!5.2! Experimental ....................................................................................................... 147!5.2.1! General ......................................................................................................... 147!5.2.2! Microscopy .................................................................................................. 148!5.2.3! Preparation cerium formate hollow spheres via a solvothermal method ..... 148!5.2.4! Preparation of cerium hydroxycarbonate rods ............................................. 149!5.2.5! Preparation of cerium hydroxycarbonate stacked sheets ............................. 149!5.2.6! Surface-assisted reduction of Pd(NO3)2 with ceria precursors .................... 149!5.2.7! Non-hydrothermal route to cerium formate ................................................. 150!5.2.8! Surface-assisted reduction of HAuCl4 with ceria precursors ....................... 150!5.2.9! Simultaneous surface-assisted reduction of Pd(NO3)2 and HAuCl4 with ceria precursors ................................................................................................................. 151!5.2.10! Sequential surface-assisted reduction of Pd(NO3)2 and HAuCl4 with ceria precursors ................................................................................................................. 152!5.2.11! Surface-assisted reduction of AgNO3 with ceria precursors ...................... 152!5.2.12! Methane oxidation testing .......................................................................... 152!5.3! Results and discussion ........................................................................................ 153!5.3.1! Novel cerium-containing nanomaterials ...................................................... 153!5.3.2! Surface-assisted reduction ........................................................................... 155!5.3.3! Scale up of surface-assisted reduction ......................................................... 160!5.3.4! Non-hydrothermal route to cerium formate for surface-assisted reduction . 162!  xi 5.3.5! Surface-assisted reduction of Pd using CF hollow spheres ......................... 165!5.3.6! Surface-assisted reduction of HAuCl4 with ceria precursors ....................... 167!5.3.7! Surface-assisted reduction of Pd(NO3)2 and HAuCl4 with ceria precursors 172!5.3.8! Scope of surface-assisted reduction ............................................................. 176!5.3.9! Conclusions .................................................................................................. 179!Chapter 6: Conclusions and Future Directions ................................................................ 181!6.1! Conclusions ......................................................................................................... 181!6.2! Future directions ................................................................................................. 185!References ........................................................................................................................ 189!Appendix A ...................................................................................................................... 203!Appendix B ...................................................................................................................... 206!Appendix C ...................................................................................................................... 208!     xii List of Tables  Chapter 2 Table 2-1: BET surface areas of Comp1-X%Mn, Comp1-X%La and Comp1-X%Cu. ......... 63!Table 2-2: Catalytic activity of Comp1 and Comp1-X%M for complete methane oxidation expressed in terms of T50%. ..................................................................................................... 64!Table 2-3: BET surface area of Comp3-15%M and Comp3-50%M ...................................... 75!Table 2-4: Catalytic activity of Comp3 and Comp3-X%M for complete methane oxidation expressed in terms of T50% ...................................................................................................... 77!Chapter 4 Table 4-1: Crystallite sizes estimated using the Scherrer equation for Comp10, Comp11, Comp11-10%La and Comp11-10%Mn. ............................................................................... 123!Table 4-2: High resolution Ce 3d XPS binding energies and peak assignments for Comp11, Comp11-10%La and Comp11-10%Mn. ............................................................................... 132!Table 4-3: High resolution Co 2p XPS binding energies for Comp10, Comp11, Comp11-10%La and Comp11-10%Mn. .............................................................................................. 133!Table 4-4: Catalytic performance of the materials described in this chapter and materials from the literature. ................................................................................................................ 139!Chapter 5 Table 5-1: Quantities of reagents used in the Pd SAR experiments. .................................... 150!Table 5-2: Quantities of reagents used in the Au SAR experiments. ................................... 151!Table 5-3: High resolution Au 4f XPS binding energies for Comp16-CF/Au(0.5mM), Comp16-CF/Au(1mM), Comp16-CHC/Au(0.5mM), and Comp16-CHC/Au(1mM). ......... 171!Table 5-4: Standard reduction potentials for selected half reactions in aqueous solution at 25 ˚C. .......................................................................................................................................... 177!    xiii List of Figures  Chapter 1 Figure 1-1: Schematic of a Teflon® lined stainless steel autoclave. ......................................... 4!Figure 1-2: Definition of micro-, meso- and macroporous materials with representative examples. .................................................................................................................................. 5!Figure 1-3: a) Chemical formula and properties of P123 and b) structure of a P123 micelle. . 7!Figure 1-4: TEM images of SBA-15 mesoporous silica with different average pore sizes: A) 60 Å, B) 89 Å, C) 200 Å and D) 260 Å. Reproduced from reference 40. Reprinted with permission from AAAS. ........................................................................................................... 8!Figure 1-5: Schematic drawing of expected morphologies of porous metal oxides templated by (a) SBA-15, (b) KIT-6, (c) FDU-12 and (d) SBA-16. The two interpenetrating pores in KIT-6 (b) are highlighted with black and white. Reprinted from reference number 62 with permission from Elsevier. ....................................................................................................... 11!Figure 1-6: A schematic of hard templating synthesis using KIT-6 and SBA-15. Reproduced from reference 68 with permission of The Royal Society of Chemistry. ................................ 12!Figure 1-7: Kilograms of air pollutants (CO, NOx, SO2, particulates and CO2) produced per billion kilojoules of energy extracted from natural gas, coal and oil.89 .................................. 16!Figure 1-8: a) The fluorite structure of CeO2 with the unit cell shown, b) the (100) plane of CeO2 and c) the (110) plane of CeO2. Cerium atoms are yellow and oxygen atoms are red. 21!Figure 1-9: Ceria centered at the cubic oxygen sublattice containing two 3+ dopant cations (M) and an oxygen vacancy. ................................................................................................... 25!Figure 1-10: TEM images of CeO2 nanopolyhedra, CeO2 nanorods and CeO2 nanocubes. Adapted with permission from reference 28. Copyright 2005 American Chemical Society. .. 27!Figure 1-11: TEM image of pore channels in CeO2 formed from cerium acetate polymeric chains aligned with P123. Reprinted from reference 125 with permission from Elsevier. ....... 28!Figure 1-12: (a) TEM images of ceria templated with SBA-15 (b) HR-TEM micrograph of the crystalline framework (c). Ceria templated with SBA-15 along the (100) direction (d) and (111) direction. Reprinted from reference number 126 with permission from Elsevier. .......... 29!Figure 1-13: Bragg diffraction. Incident X-ray strikes a set of planes, with an interplanar spacing of d, at an angle of θ. ................................................................................................. 33!  xiv Figure 1-14: Illustration of an X-ray beam diffracting off a powder sample, resulting in Debye-Scherrer cones. ............................................................................................................ 34!Figure 1- 15: A schematic representation of a transmission electron microscope. ................. 37!Figure 1-16: Six types of gas adsorption isotherms. Image adapted from reference 146. ........ 41!Figure 1-17: IUPAC classification of hysteresis loops in nitrogen adsorption isotherms. ..... 42!Chapter 2 Figure 2-1: Schematic diagram of TPO experimental setup; MFCs: mass flow controllers; SVs: switch valves. ................................................................................................................. 53!Figure 2-2: SEM images of Comp1 showing plate-like particles (scale bar = 1 µm). ............ 55!Figure 2-3: TEM image of Comp1 showing porous plates (scale bar = 100 nm). ................. 55!Figure 2-4: a) PXRD pattern of Comp1 (!= CeO2, JCPDS-34-0394). b) N2 adsorption-desorption isotherms for Comp1. Solid line represents adsorption and dashed line represents desorption, inset)  BJH pore size distribution curve for Comp1. ............................................ 56!Figure 2-5: a) SEM (scale bar = 1 µm) and b) TEM (scale bar = 100 nm) images of Comp1 synthesized without P123. ...................................................................................................... 57!Figure 2-6: TPO curve of Comp1. .......................................................................................... 58!Figure 2-7: PXRD patterns of Comp1-10%Mn, (yellow), Comp1-20%Mn (blue), Comp1-50%Mn (green) and Comp1-80%Mn (red). != CeO2 (JCPDS-34-0394), != Mn3O4 (JCPDF-24-0734). ................................................................................................................................. 59!Figure 2-8: SEM images of a) Comp1-10%Mn, b) Comp1-20%Mn, c) Comp1-50%Mn and d) Comp1-80%Mn showing an increasing amount of a compound with an irregular morphology, thought to be Mn3O4 (scale bar = 1 µm). ........................................................... 60!Figure 2-9: EDX mapping images for a) Comp1-20%Mn and b) Comp1-80%Mn. Purple = Ce, green = Mn and red = O. Scale bars = 500 µm. ................................................................ 61!Figure 2-10: TEM images of a) Comp1-10%Mn, b) Comp1-20%Mn, c) Comp1-50%Mn and d) Comp1-80%Mn (scale bar = 100 nm). ............................................................................... 62!Figure 2-11: N2 adsorption-desorption isotherms for a) Comp1-10%Mn (yellow), b) Comp1-20%Mn (blue), c) Comp1-50%Mn (green) and d) Comp1-80%Mn (red). Solid lines represent adsorption and dashed lines represent desorption. .................................................. 63!Figure 2-12: TPO curve for for a) Comp1-10%Mn (yellow), Comp1-20%Mn (blue), Comp1-50%Mn (green), Comp1-80%Mn (red) and Comp1-100%Mn (purple), b) Comp1-10%La   xv (yellow), Comp1-50%La (green), Comp1-80%La (red), Comp1-100%La (purple) and c) Comp1-20%Cu (blue). ............................................................................................................ 65!Figure 2-13: PXRD patters of a) Comp2 (black), b) Comp2-10%Mn (blue) and c) Comp2-20%Mn (red). ! = CeO2 (JCPDS-34-0394). .......................................................................... 67!Figure 2-14: Nitrogen adsorption isotherms for Comp2 (black), Comp2-10%Mn (blue) and Comp2-20%Mn (red). Solid lines represent adsorption and dashed lines represent desorption. ................................................................................................................................................. 68!Figure 2-15: TEM images of a) Comp2, b) Comp2-10%Mn and c) Comp2-20%Mn (scale bars = 100 nm) and d) SEM image of Comp2-10%Mn (scale bar = 1 µm). .......................... 69!Figure 2-16: a) PXRD pattern (!= CeO2, JCPDS-34-0394) and b) TEM image of Comp3 (scale bar = 100 nm). .............................................................................................................. 70!Figure 2-17: N2 adsorption-desorption isotherm of Comp3. Solid line represents adsorption and dashed line represents desorption. .................................................................................... 71!Figure 2-18: EDX mapping images for a) Comp3-15%Fe, b) Comp3-15%Mn, c) Comp3-15%Gd, d) Comp3-15%Er, and e) Comp3-15%Zn and f) Comp3-15%Cr. Scale bars = 20 µm. .......................................................................................................................................... 72!Figure 2-19: PXRD patterns of a) Comp3-15%Cr (pink), Comp3-15%Zn (purple), Comp3-15%Er (blue), Comp3-15%Gd (orange), Comp3-15%Mn (green), and Comp3-15%Fe (red) and b) Comp3-50%Cr (pink), Comp3-50%Zn (purple), Comp3-50%Er (blue), Comp3-50%Gd (orange), Comp3-50%Mn (green), and Comp3-50%Fe (red). Dashed lines represent CeO2 (JCPDS-34-0394). ......................................................................................................... 73!Figure 2-20: TEM images of a) Comp3-15%Fe, b) Comp3-50%Fe, c) Comp3-15%Mn and d) Comp3-50%Mn (scale bars = 100 nm). .................................................................................. 74!Figure 2-21: N2 adsorption-desorption isotherms for a) Comp3-15%Cr (pink), Comp3-15%Zn (purple), Comp3-15%Er (blue), Comp3-15%Gd (orange), Comp3-15%Mn (green), and Comp3-15%Fe (red) and b) Comp3-50%Cr (pink), Comp3-50%Zn (purple), Comp3-50%Er (blue), Comp3-50%Gd (orange), Comp3-50%Mn (green), and Comp3-50%Fe (red). Solid lines represent adsorption and dashed lines represent desorption. ................................ 75!Figure 2-22: TPO curve of for a) Comp3 (black), Comp3-15%Zn (purple), Comp3-15%Er (blue), Comp3-15%Gd (orange), Comp3-15%Mn (green), and Comp3-15%Fe (red) and b) Comp3 (black), Comp3-50%Cr (pink), Comp3-50%Zn (purple), Comp3-50%Er (blue), Comp3-50%Gd (orange), Comp3-50%Mn (green), and Comp3-50%Fe (red). ..................... 76!Figure 2-23: High resolution Pd 3d XPS spectra of Pd/Comp3-50%Fe. ................................ 78!Figure 2-24: TEM images of 10%PdComp3-50%Fe (scale bars = 100 nm). ......................... 79!  xvi Figure 2-25: TPO curve for 10%Pd/Comp3-50%Fe. ............................................................. 80!Figure 2-26: TPO curve for Comp4. ....................................................................................... 81!Chapter 3 Figure 3-1: TEM images of Comp5, Comp5-10%Mn, Comp5-20%Mn and Comp5-50%Mn (scale bars = 500 nm). ............................................................................................................. 91!Figure 3-2: PXRD pattern of Comp5 (purple), Comp5-10%Mn (green), Comp5-20%Mn (blue) and Comp5-50%Mn (orange). != CeO2 (JCPDS-34-0394). ....................................... 92!Figure 3-3: N2 adsorption-desorption isotherms for Comp5 (purple), Comp5-10%Mn (green), Comp5-20%Mn (blue) and Comp5-50%Mn (orange). Solid lines represent adsorption and dashed lines represent desorption. .................................................................. 93!Figure 3-4: BJH pore size distribution curves for a) Comp5, b) Comp5-10%Mn, c) Comp5-20%Mn and d) Comp5-50%Mn. ............................................................................................. 94!Figure 3-5: TEM image of commercial hydroxyapatite. Scale bar = 100 nm. ....................... 96!Figure 3-6: Molecular structure of CTAC .............................................................................. 96!Figure 3-7: TEM images of a) Comp6-0.5Ce, b) Comp6-1Ce and c) SEM image of Comp6-1Ce. TEM scale bars = 100 nm and SEM scale bar = 2 µm. .................................................. 97!Figure 3-8: PXRD patterns of commercial HAp (yellow), Comp6-0.5Ce (blue) and Comp6-1Ce (green). != CeO2 (JCPDS-34-0394). .............................................................................. 97!Figure 3-9: a) N2 adsorption-desorption isotherms, and b) and c) BJH pore size distribution curves for Comp6-0.5Ce (blue) and Comp6-1%Ce (green). Solid lines represent adsorption and dashed lines represent desorption. .................................................................................... 98!Figure 3-10: PXRD patterns of Comp7-0.5Ce (blue) and Comp7-1Ce (green). != CeO2 (JCPDS-34-0394). ................................................................................................................... 99!Figure 3-11: TEM images of a and b) Comp7-0.5Ce and c) Comp7-1Ce. Scale bars = 100 nm. ........................................................................................................................................ 100!Figure 3-12: a) N2 adsorption-desorption isotherms, and b) and c) BJH pore size distribution curves for Comp7-0.5Ce (blue) and Comp7-1%Ce (green). Solid lines represent adsorption and dashed lines represent desorption. .................................................................................. 101!Figure 3-13: PXRD pattern and TEM images of Comp8. != CeO2 (JCPDS-34-0394). Scale bars = 100 nm. ....................................................................................................................... 102!Figure 3-14: a) FT-IR spectrum and b) PXRD pattern of Comp9. ....................................... 104!  xvii Figure 3-15: TEM images of a and b) Comp9 before calcination, c and d) Comp9 after calcination (scale bars = 100 nm), and e and f) SEM images of Comp9 after calcination (scale bars = 2 µm). ............................................................................................................... 107!Figure 3-16: a) PXRD pattern of Comp9 after calcination. != CeO2 (JCPDS-34-0394). b) N2 adsorption-desorption isotherm for Comp9. Solid line represents adsorption and dashed line represents desorption. ........................................................................................................... 108!Figure 3-17: 1H NMR and 13C NMR spectra of liquid remaining after synthesis of Comp9 (25 ˚C). .................................................................................................................................. 108!Figure 3-18: FT-IR spectra of a) Comp9-10%La, b) Comp9-30%La, c) Comp9-10%Fe, d) Comp9-30%Fe, Comp9-10%Mn, and d) Comp9-30%Mn. .................................................. 109!Figure 3-19: PXRD pattern of a) as-synthesized Comp9-10%Mn (green), Comp9-10%Fe (red) and Comp9-10%La (purple), b) as-synthesized Comp9-30%Mn (green), Comp9-30%Fe (red) and Comp9-30%La (purple), c) calcined Comp9-10%Mn (green), Comp9-10%Fe (red) and Comp9-10%La (purple) and d) calcined Comp9-30%Mn (green), Comp9-30%Fe (red) and Comp9-30%La (purple). != CeO2 (JCPDS-34-0394). ............................. 110!Figure 3-20: TEM images of as-synthesized a) Comp9-10%Mn, b) Comp9-10%Fe c) Comp9-10%La d) Comp9-30%Mn, e) Comp9-30%Fe and f) Comp9-30%La. Scale bar = 100 nm .................................................................................................................................. 111!Figure 3-21: TEM images of calcined a) Comp9-10%Mn, b) Comp9-10%La c) Comp9-30%Mn and d) Comp9-30%La. Scale bar = 100 nm. ........................................................... 112!Figure 3-22: TEM images of Comp9-30%La prepared at a) 150 ˚C, b) 170 ˚C and c) 180 ˚C. Scale bar = 100 nm, .............................................................................................................. 113!Chapter 4 Figure 4-1: a) N2 adsorption-desorption isotherm, b) BJH pore size distribution curves and c) TEM image of KIT-6. ........................................................................................................... 122!Figure 4-2: PXRD patterns of a) Comp10, b) Comp11, c) Comp11-10%La and d) Comp11-10%Mn. PXRD pattern of Co3O4 matches JCPDS-78-1969 for Co3O4 (!) and the PXRD pattern of CeO2 matches JCPDS-34-0394 (!). .................................................................... 123!Figure 4-3: N2 adsorption-desorption isotherms for a) Comp10, b) Comp11, c) Comp11-10%La and d) Comp11-10%Mn. Solid lines represent adsorption and dashed lines represent desorption. ............................................................................................................................. 125!Figure 4-4: BJH pore size distribution curves for a) Comp10, b) Comp11, c) Comp11-10%La and d) Comp11-10%Mn. .......................................................................................... 126!  xviii Figure 4-5: Low angle PXRD spectra for a) Comp10, b) Comp11, c) Comp11-10%La and d) Comp11-10%Mn. .................................................................................................................. 127!Figure 4-6: TEM images of a) KIT-6, b) Comp10, c) Comp10/Ce(NO3)3⋅6H2O d) Comp11  e) Comp11-10%La and f) Comp11-10%Mn. ....................................................................... 128!Figure 4-7: EDX mapping images of Comp11-10%La. ....................................................... 129!Figure 4-8: EDX mapping images of Comp11-10%Mn. ...................................................... 130!Figure 4-9: High resolution Ce 3d XPS spectra of a) Comp11, b) Comp11-10%La and c) Comp11-10%Mn. .................................................................................................................. 131!Figure 4-10: High resolution Co 2d XPS spectra of a) Comp10, b) Comp11, c) Comp11-10%La and d) Comp11-10%Mn. .......................................................................................... 133!Figure 4-11: High resolution La 3d XPS spectrum of Comp11-10%La. ............................. 134!Figure 4-12: High resolution Mn 2p XPS spectrum of Comp11-10%Mn. ........................... 134!Figure 4-13: TPO curve of fresh Comp10 (blue) and used (purple). .................................... 135!Figure 4-14: Variable-temperature PXRD patterns of Comp10. .......................................... 136!Figure 4-15: TPO curve of Comp10. .................................................................................... 137!Figure 4-16: TPO curves for Comp11 (yellow), Comp11-10%La (blue) and Comp11-10%Mn (pink). ...................................................................................................................... 138!Figure 4-17: SEM images of chiral nematic silica films. a) Side view of a cracked film shows the stacked layers that result from the helical pitch of the chiral nematic phase (scale bar = 3"µm). b) Higher magnification reveals the helical pitch distance to be of the order of several hundred nanometres (scale bar = 2"µm). Reprinted by permission from Macmillan Publishers Ltd: Nature, reference 206, copyright 2010. ........................................................................... 140!Figure 4-18: PXRD pattern of Comp12. Pattern matches JCPDS-78-1969 for Co3O4 (!). 141!Figure 4-19: SEM images of a and b) Comp12 before silica removal and c and d) after silica removal. ................................................................................................................................ 142!Figure 4-20: N2 adsorption-desorption isotherms for Comp12. Solid lines represent adsorption and dashed lines represent desorption. ................................................................ 142!Chapter 5 Figure 5-1: Synthesis of new nanostructured cerium-containing materials. Scale bars = 100 nm. ........................................................................................................................................ 154!  xix Figure 5-2: PXRD patterns of Comp13-CF (purple), Comp13-CHC-f (green) and Comp13-CHC (orange). Comp13-CF corresponds to CF (JCPDS-49-1245), and Comp13-CHC-f and Comp13-CHC correspond to CHC (JCPDS-52-0352). ........................................................ 155!Figure 5-3: PXRD patterns of Comp14-CF/Pd (purple) and Comp14-CHC-f/Pd (green). Both patterns indicate a layered structure. ..................................................................................... 158!Figure 5-4: L-T MOX for Comp14-CF/Pd(0.3mM) (red), Comp14-CF/Pd(0.4mM) (blue), Comp14-CF/Pd(1mM) (purple), and Comp14-CHC-f/Pd(1mM) (green). ........................... 159!Figure 5-5: High resolution a) Ce 3d and b) Pd 3d XPS spectra of scaled up Comp14-CF/Pd(1mM). ........................................................................................................................ 161!Figure 5-6: TPO of Comp14-CF/Pd(1mM). ......................................................................... 162!Figure 5-7: a) PXRD pattern and b) FT-IR spectrum of Comp15-CF. ! = cerium formate (JCPDS 49-1245). ................................................................................................................. 163!Figure 5-8: TEM image of Comp15-CF. Scale bar = 100 nm. ............................................. 164!Figure 5-9:  a) PXRD pattern indicating a layered structure and b) TEM image of Comp15-CF/Pd scale bar = 100 nm. .................................................................................................... 165!Figure 5-10: a) PXRD pattern indicating a layered structure and b) TEM image of Comp15. Scale bar = 500 nm. .............................................................................................................. 166!Figure 5-11: PXRD patterns of as-synthesized a) Comp16-CF/Au(0.5mM) (red) and Comp16-CF/Au(1mM) (green), and b) Comp16-CHC/Au(0.5mM) (purple) and Comp16-CHC/Au(1mM) (blue). All patterns indicate a layered structure. ........................................ 168!Figure 5-12: TEM images of as-synthesized a) Comp16-CF/Au(0.5mM) , b) Comp16-CF/Au(1mM), c) Comp16-CHC/Au(0.5mM), and d) Comp16-CHC/Au(1mM). Scale bars = 100 nm. ................................................................................................................................. 169!Figure 5-13: PXRD patterns of calcined Comp16-CF/Au(0.5mM) (red), Comp16-CF/Au(1mM) (green), Comp16-CHC/Au(0.5mM) (purple) and Comp16-CHC/Au(1mM) (blue). != CeO2 (JCPDS-34-0394). ...................................................................................... 170!Figure 5-14: High resolution Au 4f XPS spectra for calcined a) Comp16-CF/Au(0.5mM), b) Comp16-CF/Au(1mM), c) Comp16-CHC/Au(0.5mM) and d) Comp16-CHC/Au(1mM). .. 171!Figure 5-15: PXRD pattern of a) as-synthesized Comp17-PdAu-sim (green) and Comp17-PdAu-seq (purple), indicating a layered structure and b) calcined Comp17-PdAu-sim (green) and Comp17-PdAu-seq (purple). != CeO2 (JCPDS-34-0394). ............................................ 174!Figure 5-16: EDX mapping images of a) Pd and b) Au for Comp17-PdAu-sim, and c) Pd and d) Au for Comp17-PdAu-seq. Scale bars = 100 µm. ............................................................ 175!  xx Figure 5-17: TPO curve for calcined Comp17-PdAu-sim (green) and Comp17-PdAu-seq (purple). ................................................................................................................................. 176!Figure 5-18: PXRD pattern for Comp18-Ag as-synthesized (pink) and calcined (grey). != CeO2 (JCPDS-34-0394). ....................................................................................................... 178!Figure 5-19: High resolution a) Ce 3d and b) Ag 3d  XPS spectra for calcined Comp18-Ag. ............................................................................................................................................... 179       xxi List of Schemes Scheme 1-1: Synthesis of MCM-41. Reproduced in part from reference 39 with permission of The Royal Society of Chemistry. ......................................................................................... 6!Scheme 1-2: Kinetic model for the reduction of ceria. 1) Dissociation of chemisorbed hydrogen to form hydroxyl groups at the surface, 2) formation of anionic vacancies and reduction of neighboring Ce4+ ions, 3) desorption of water by recombination of hydrogen and hydroxyl groups and 4) diffusion of surface anionic vacancies into the bulk material. ......... 23!Chapter 3 Scheme 3-1: Oxidation of ethylene glycol to formic acid. ................................................... 103!Chapter 4 Scheme 4-1: Synthesis of mesoporous Co3O4-based materials using KIT-6 as a template. a) (i) 2.075 g of KIT-6 ground with 4.016 g of Co(NO3)2⋅6H2O; (ii) calcination at 500 ˚C; (iii) etching in 2M NaOH(aq). b) (i) 0.220 g Ce(NO3)3⋅6H2O in 5 mL EtOH added to 0.400 g Co3O4 under vacuum; (ii) calcination at 500 ˚C. c) (i) 0.198 g of Ce(NO3)3⋅6H2O and 0.022 g of La(NO3)3⋅6H2O in 5 mL of EtOH added to 0.400 g Co3O4 under vacuum; (ii) calcination at 500 ˚C. d) (i) ) 0.198 g of Ce(NO3)3⋅6H2O and 0.013 g of Mn(NO3)2⋅4H2O in 5 mL of EtOH added to 0.400 g Co3O4 under vacuum; (ii) calcination at 500 ˚C. ............................ 121!Chapter 5 Scheme 5-1: The reaction of Comp13-CHC-f or Comp13-CF with or CHC-f with Pd2+ gives black precipitates of as-synthesized Comp14-CHC-f/Pd or Comp14-CF/Pd. ...................... 156!    xxii List of Symbols and Abbreviations  θ Bragg angle λ wavelength BET Brunauer-Emmett-Teller BJH Barret-Joyner-Halenda CF cerium formate CHC cerium hydroxycarbonate CTAB cetyl trimethylammonium bromide DI de-ionized EDX energy dispersive X-ray EG ethylene glycol FT-IR Fourier transform infrared GHG greenhouse gas GWP global warming potential IUPAC International Union for Pure and Applied Chemistry L-T MOX low-temperature methane oxidation NGV natural gas vehicle NMR nuclear magnetic resonance OSC oxygen storage capacity PEO polyethylene oxide PPO polypropylene oxide   xxiii PXRD powder X-ray diffraction SAR surface-assisted reduction SEM scanning electron microscopy T50% light-off temperature TEM transmission electron microscopy TGA thermal gravimetric analysis TPO temperature programmed oxidation TWC three-way catalyst UHP ultra high purity UV ultraviolet VOC volatile organic compound WHSV weight hourly space velocity XPS X-ray photoelectron spectroscopy     xxiv Acknowledgements  I would like to gratefully and sincerely thank Dr. Mark MacLachlan for his guidance, understanding and energy towards my research at UBC. His enthusiasm and passion combined with his impressive knowledge of chemistry have been an inspiration. Mark has encouraged me to have a well-rounded experience during my PhD and allowed me to explore career options outside a traditional research setting. I feel incredibly lucky to have had a supervisor who encourages individuality and independence and who supported both my research and long-term career goals.  The MacLachlan group is the most amazing, kind, fun, knowledgeable and weird group of people I have ever met, and I am so proud to be part of it. We have had so many memorable lunchtimes, nights out and adventures together. I am leaving the group with so many great friends and I know that we will have many more adventures together in the future.   A very special thank you goes out to Dr. Gomathi. This thesis would not exist if it weren’t for her. She is one of the best chemists I have met and I am so grateful for her patience, kindness, generosity and friendship during the years she spent at UBC.  A huge thank you also goes to Dr. Georg Meseck and Dr. Vitor Zamarion for proof reading my thesis and providing helpful feedback.  I was privileged to supervise two incredibly talented undergraduate students during my PhD, Stanley Shi and Loryn Arnett. I am grateful for all their hard work and it brings me a lot of joy to see them forging their own successful careers.   There are many people at UBC who helped me to complete the work in this thesis. Dr. Kevin Smith and Dr. Rahman Gholami carried out all the methane oxidation testing. Anita Lam trained me to run and interpret PXRD and her support and advice was greatly appreciated. I am also thankful to Derrick Horne and Bradford Ross who were a great help with SEM, TEM and EDX analysis and Dr. Ken Wong who ran XPS analysis.  I would also like to say a heartfelt thank you to my family for always believing in me.  My mum has been a huge encouragement to me and it is because of her that I was able to pursue my dreams. She proof read my entire thesis and flew to Vancouver from Scotland to cheer me on during my defence. I could not ask for a more supportive mum. My sister, Camille, has shown so much strength over the past few years and is my inspiration for facing life head on. She kept me smiling as I wrote my thesis with videos and photos of my beautiful niece, Holly.  I love you all.   There are so many people who have supported me and believed in me. My family and friends mean so much to me and I would like to thank every one of them for the love, support, and happiness they give me.   xxv Dedication          I dedicate this thesis to the memory of my dad, Iain Vickers, who taught me to work hard and laugh harder; and to my mum, Katie Vickers, who taught me to be curious and pursue my dreams  1  Chapter 1: Introduction  1.1 Nanomaterials 1.1.1 Overview  Nanoscience is the field of study of materials with at least one dimension in the nanometer (10-9 m) range.1,2 Physicist Richard Feynman, often considered to be the father of the field, first introduced his ideas about nanoscience in a 1959 lecture, “There’s Plenty of Room at the Bottom”.3 In his talk Feynman envisaged scientists being able to manipulate individual atoms and molecules. However, the term “nanotechnology” was not coined until 1974, in a paper entitled “On the Basic Concept of Nanotechnology" by Norio Taniguchi.4 Following this, Eric Drexler popularized the concept of nanotechnology, publishing the first book on molecular nanotechnology, “Engines of Creation: The Coming Era of Nanotechnology”, in 1986.5 These three scientists paved the way for modern nanoscience and nanotechnology, which has grown to become a vast and active research area.  Nanomaterials have remarkable properties arising from the size, shape, and surface of the particle at the nanometer scale.6 Nanoparticles show characteristics that are distinctive from those displayed by the corresponding bulk solids or isolated molecules, including optical,7 magnetic8,9 and electronic properties.10 These properties have been shown to be advantageous for several important applications including catalytic systems,11,12 gas storage,13,14 electronics,15 and drug delivery.16,17 It is anticipated that advances in the preparation, 2  characterization and exploitation of nanomaterials will lead to applications in industries such as energy, chemical, electronics, health, and space.18   1.1.2 Synthesis of nanomaterials  The construction of nanomaterials presents a major challenge as they are difficult to assemble using traditional methods. New synthetic routes have been investigated in order to overcome this challenge and the surge in nanomaterial research over the last two decades means that several innovative methods are now available to researchers. Generally speaking, nanomaterials are manufactured using strategies that can be classified as either bottom-up or top-down. The top-down approach uses macroscopic initial structures that are reduced to the nanoscale using techniques such as lithography, a method that can achieve atomic-scale precision, and ball milling, a method that is simpler but gives no control over particle morphology. In the bottom-up approach nanomaterials are built using atomic or molecular precursors as basic building blocks. The concept of molecular self-assembly is widely used in this approach, where molecules or nano-sized building blocks are designed so that a specific conformation is favoured due to weak, non-covalent bonds.   Chemical methods for synthesizing metal nanoparticles are based on the reduction of metal ions or decomposition of precursors to form atoms, which then aggregate to form particles. Metal oxide nanoparticles are often synthesized using methods such as coprecipitation, sol-gel, and hydrothermal/solvothermal synthesis. The sol-gel process involves conversion of a precursor, usually inorganic metal salts or metal organic compounds, into a colloidal solution 3  (sol) that acts as the precursor for an integrated network (gel) of discrete particles or network polymers. After the solution condenses into a gel the solvent is removed. The size of the nanoparticles formed can be tuned by controlling the solution composition, pH and temperature.19 Several metal oxide nanoparticles, such as TiO2, SnO2, Fe2O3/Fe3O4 and CeO2, have been successfully prepared using the sol-gel method.20–26   Hydrothermal synthesis involves reactions of precursors in aqueous solutions at high temperatures and high pressures (>100 ˚C, >1 atm).27 Steel autoclaves with a Teflon® liner designed to withstand these temperatures and pressures, as shown in Figure 1-1, are usually used as the reaction vessel. When solvents other than water are used, the process is called solvothermal synthesis. The nature of the nanomaterials formed depends on a variety of factors including the temperature, pressure, solvent used and reaction time. Hydrothermal methods generally produce materials with narrow particle size distribution, controlled particle morphology and product purity, and have been used to synthesize metal oxide nanomaterials with a variety of shapes and sizes.27–31 However, while efforts are being made to develop rational design approaches to hydrothermal synthesis, most discoveries have been made using time-consuming trial and error methods.32 4   Figure 1-1: Schematic of a Teflon® lined stainless steel autoclave.  1.2 Mesoporous materials 1.2.1 Overview  One important area of nanomaterials research involves materials with pores on the nanometer scale. Mesoporous - from the Greek prefix meso meaning “in between”- materials contain pores between 2 and 50 nm in diameter. A summary of pore size definitions according to IUPAC is presented in Figure 1-2.33 However, “pore size” can only be used as a precise definition when the shape of the pores is well defined and known, otherwise averages or estimations are used and may not realistically represent the pores present in a material. The term “nanoporous” is also used, meaning pores on the nanometer scale, but it is not clearly defined.  5   Figure 1-2: Definition of micro-, meso- and macroporous materials with representative examples.   The first report of an ordered mesoporous material was in a patent in 1969,34 however incomplete characterization meant that the true properties of this silica material were not documented until 28 years later.35 The first silica material recognized to have ordered mesopores was MCM-41 (Mobil Composition of Matter No. 41), a mesoporous silica discovered by Mobil Oil Corporation (Scheme 1-1).36  1.2.2 Mesoporous silica  MCM-41 has a hexagonal arrangement of uniform mesopores with diameters larger than 1.5 nm. MCM-41 is part of a larger family of silicate/aluminosilicate mesoporous materials termed S41S.37,38 The S41S family can be prepared from a variety of silica sources via hydrothermal synthesis with the help of a structure-directing quaternary ammonium surfactant. These surfactants (CnH2n+1(CH3)3N+) form micelles with a liquid crystalline phase that are used to template the pores in these materials. The silica precursor undergoes hydrolysis and condensation reactions to form a solid silica network around the liquid crystal template (referred to as soft templating). The pore size of these materials can be controlled by 6  using surfactants with different alkyl chain lengths, while altering the ratio of surfactant to silica gives rise to materials with different geometries.   Scheme 1-1: Synthesis of MCM-41. Reproduced in part from reference 39 with permission of The Royal Society of Chemistry.  The discovery of the S41S family of mesoporous materials paved the way for the creation of numerous surfactant-templated mesoporous materials, such as SBA-15,40,41 SBA-16,41,42 KIT-643 and FDU-12,44,45 with various pore geometries. These mesoporous silica materials are templated using amphiphilic triblock copolymers. Non-ionic triblock copolymers were first patented in 1973 and are known under the trademark Pluronics.46 They consist of hydrophobic polypropylene oxide (PPO) segments and hydrophilic polyethylene oxide (PEO) segments. The family of Pluronics contains a number of different polymers with various PPO/PEO ratios and hence varying molecular weights. The nomenclature used for Pluronics® describes the appearance, molecular weight and PPO/PEO ratio of each polymer. The name consists of a letter defining the appearance of the polymer: F for flake, P for paste and L for liquid. The first one or two numbers multiplied by 300 approximates the molecular weight of the PPO block and the last number multiplied by 10 approximates the PEO weight fraction. For example Pluronic® P123 represents a paste with ~3600 g/mol PPO and 30 wt% 7  PEO. In aqueous solutions P123 self-assembles into micelles with a hydrophobic core of PPO chains surrounded by a hydrophilic shell of PEO chains, as shown in Figure 1-3.  Figure 1-3: a) Chemical formula and properties of P123 and b) structure of a P123 micelle.  In general, mesoporous silicas templated with amphiphilic triblock copolymers have larger pores and thicker walls when compared to silicas synthesized with ionic surfactants. SBA-15, first synthesized by Zhao et al. in 1998, has a hexagonal pore arrangement (shown in Figure 1-4) similar to that of MCM-41.40 However, in contrast to MCM-41, the cylindrical pores of SBA-15 are interconnected with small channels making the whole pore system three-dimensional. SBA-15 has large, tunable pore sizes up to 30 nm and thicker silica walls than MCM-41 resulting in greater stability. A range of reaction mixture compositions and conditions can be used to synthesize SBA-15 under acidic conditions. By heating the reaction solution at different temperatures or for different lengths of time and by using various triblock copolymers along with the addition of a swelling agent the pore size and silica wall thickness can be tuned.    8   Figure 1-4: TEM images of SBA-15 mesoporous silica with different average pore sizes: A) 60 Å, B) 89 Å, C) 200 Å and D) 260 Å. Reproduced from reference 40. Reprinted with permission from AAAS.  KIT-6 mesoporous silica was first synthesized by Kleitz et al. in 2003. Using a triblock copolymer (P123)-butanol mixture, cubic Ia3d silica was formed with a pair of interpenetrating bicontinuous networks of channels.43 The diameter of these pores can be tuned between 4 and 12 nm by altering the reaction temperature. It is believed that the butanol causes the hydrophobic section of the micelles to swell leading to decreased curvature in the micellar aggregates producing a lamellar phase, which later evolves into the cubic mesophase as the reaction proceeds. 9  1.2.3 Mesoporous inorganic materials  Since the discovery of mesoporous silica, many other mesoporous materials have been reported including non-siliceous metal oxides,47 non-oxide materials48 and metals.49 Many of these materials have unique magnetic, catalytic or electronic properties when compared to the analogous bulk and nanoparticulate material.50,51 The synthesis of non-siliceous mesoporous metal oxides presents more of a challenge when compared to silica-based mesoporous materials. This is because hydrolysis and condensation of silica precursors can be easily controlled leading to large extended networks of silica that are stable during calcination. However, inorganic species have a strong tendency to crystallize into bulk phases from aqueous solutions. Furthermore, the hydrolysis and condensation of non-silica precursors is more difficult to control, meaning there is not sufficient time for a well-ordered mesostructure to develop. In addition, mesoporous metal oxides with redox activity tend to collapse due to the occurrence of redox conditions during calcination.52   In spite of these difficulties, the use of ligands and judicious choice of precursors, templates and solvents has given rise to a variety of methods for soft templating metal oxide materials.47,53,54 After many attempts failed to remove soft templates from transition metal oxides with retention of the mesostructure,55,56 Antonelli et al. successfully prepared mesoporous TiO2 (a non-redox active transition metal oxide) in 1995 via a ligand-assisted method.57 Titanium acetylacetonate tri-isopropoxide was used as a precursor with tetradecylphosphate as a template. The acetylacetonate slows down hydrolysis allowing for a 10  more controlled reaction. Variations of the ligand-assisted method have since been used to prepare numerous other mesostructured metal oxides.58–60  Yang et al. successfully prepared mesoporous metal oxides (including TiO2, ZrO2, Al2O3, Nb2O5, Ta2O5, WO3, HfO2, SnO2, and mixed oxides SiAlO3.5, SiTiO4, ZrTiO4, Al2TiO5 and ZrW2O8) using P123 as a soft template in non-aqueous media.61 The use of metal halides as precursors in non-aqueous solutions slows down the rate of hydrolysis and condensation allowing more thermally stable products to form. These materials have hexagonally arranged pores with thick walls containing nanocrystalline domains.   Another approach for the synthesis of mesostructured metal oxides is hard templating. Crystals of the metal oxides grow inside the pores of a mesoporous silica. Upon removal of this template, a negative replication of the silica pore network is obtained (Figure 1-5 and Figure 1-6). The first example of a non-siliceous metal oxide template using mesoporous silica was the preparation of mesoporous chromium oxide using SBA-15 by Zhu et al. in 2002.  11   Figure 1-5: Schematic drawing of expected morphologies of porous metal oxides templated by (a) SBA-15, (b) KIT-6, (c) FDU-12 and (d) SBA-16. The two interpenetrating pores in KIT-6 (b) are highlighted with black and white. Reprinted from reference number 62 with permission from Elsevier.  Hard templated mesoporous metal oxides can be prepared using various impregnation methods such as the surface modification,63 dual-solvent,64 evaporation65,66 and solid-liquid methods.67 The simplest is perhaps the evaporation method.65,66 In this procedure, mesoporous silica is mixed with the metal oxide precursor in a solvent, which is then allowed to evaporate. As the solvent evaporates, the precursor is drawn into the pores through capillary action and the precursor is then calcined to give the oxide. The solid-liquid method is another simple method, but is only viable for metal oxides with precursors that have melting points lower than their decomposition temperatures. Examples of precursors include the nitrates of Co, Ni, Ce and Cr.67 The precursor and silica template are ground together and 12  heated slowly. Then, as the precursor melts, it is drawn into the pores via capillary action before it decomposes to give the metal oxide.   Figure 1-6: A schematic of hard templating synthesis using KIT-6 and SBA-15. Reproduced from reference 68 with permission of The Royal Society of Chemistry.  Developing new methods for synthesizing nanomaterials produces novel materials with unique properties.2,18 The ability to control the morphology and pore structure of catalytic material can result in a higher surface area, resulting in higher catalytic activity.69 Therefore, there is extensive interest in developing new methods for the synthesis of nanostructured materials for catalysis.   13  1.2.4 Nanomaterials in catalysis  The increased surface area of nanomaterials when compared to their bulk analogues contributes to better performance in numerous applications including gas storage and catalysis. At the macro scale, the properties of a material will be determined by chemical structure and composition, with surface atoms contributing to a negligible amount of the total number of atoms. However, when the size of an object is reduced to the nanometer scale the proportion of surface atoms increases. About 20% of the atoms in a spherical particle with a diameter of 5 nm are surface atoms, and this number rises to 50% in a particle with a diameter of 2 nm.70 A high surface to volume ratio is beneficial for catalysis as the number of active surface sites is maximized. The high surface energy of nanoparticles can also lead to higher catalytic activity than seen in bulk materials. However, this also makes nanoparticles prone to aggregation, often necessitating the use of an inert support material.71   Nanomaterials have also been investigated as photocatalysts in applications such as anti-fogging mirrors and anti-microbial coatings.72 Titania (TiO2) is the most widely investigated photocatalytic nanomaterial due to its low cost and toxicity combined with its ability to generate excited ions by promoting electrons across their band gaps.73 The resulting electrons and holes use surrounding water molecules to form hydroxyl radicals and protons, which are powerful oxidants in degrading organics. While UV light is required to achieve this effect in pure titania, doping may be used to narrow the band gap and thus allow absorption of visible light.74 The small particle size of nanostructured titania is beneficial due to the increased surface area and greater charge carrier separation. However, some studies have found that 14  mesoporous titania has a lower quantum yield for photocatalytic reactions than a standard titania catalyst due to its poor crystallinity, with defect sites acting as electron-hole traps.75  Mesoporous metal oxide catalysts is an active area of research. Many mesoporous materials have been reported to be active catalysts for a large variety of reactions including SnO2/WO3 for oleic acid esterification,76 Fe2O3 for cyclohexane oxidation,77 and Mn3O4 for the total oxidation of VOCs (volatile organic compounds).78 The large amount of interest in using mesoporous materials as catalysts is primarily due to their high surface areas combined with pore sizes that are large enough to allow reactant molecules to diffuse into the pores.69 Ceria nanomaterials have been used in automotive emission-control catalysts for many years and will be discussed in more detail in section 1.4 of this introduction. A Mg-substituted Co3O4 mesoporous catalyst was templated using KIT-6 mesoporous silica by Rosen et al. to produce a catalyst with surface areas up to 250 m2/g and high activity for oxygen evolution from water.79 Mesoporous Co3O4 templated with SBA-15 or KIT-6 has also been used as a highly active catalyst for the oxidation of compounds such as methanol and toluene due to its high surface area and high oxygen adspecies concentration.80    15  1.3 Low-temperature methane oxidation 1.3.1 Natural gas  The personal and societal benefits of contemporary transportation are innumerable, with the economic development of entire regions depending on easy access to people and goods. It has been estimated that there are currently 1.2 billion vehicles on the world’s roads, with this number projected to rise to 2 billion by 2035.81 This rapidly growing number of vehicles is having a hugely detrimental effect on the environment, with around 24% of Canada’s greenhouse gas (GHG) emissions being generated by the transportation sector.82 By signing onto the Copenhagen Accord in 2009, Canada committed to reducing its GHG emissions from all sectors to 17% below 2005 levels by 2020.83,84 In order to reduce the amount of GHG emissions from the transportation sector, alternative non-hydrocarbon fuels with costs and performances comparable to gasoline and diesel need to be developed. Vehicles are continually being modified to be safer and faster and the increase in environmental damage associated with this progress is often overlooked by both developers and consumers in favour of more high performance vehicles that are easily maintained and reliable, leading to a large, unmet need for clean burning, cheap and efficient vehicle fuel.  Natural gas is seen as a viable alternative to gasoline and diesel as a vehicle fuel. Natural gas is composed of 70-90% methane and, depending on the source, ethane, propane, butane and/or CO2. O2, N2 and H2S may also be present in small amounts.85 It has been estimated that shale formations worldwide hold over 220 x 1012 m3 of recoverable natural gas, enough to power the world for 65 years at current consumption rates.86 In addition to shale gas, 16  synthetic natural gas is an attractive option for replacing declining natural gas reserves.87 Natural gas is the cleanest burning hydrocarbon-based fuel available (Figure 1-7) and for this reason it is seen as the best temporary solution for the replacement of gasoline and diesel in vehicles until renewable sources are feasible options. Currently, there are over 17 million natural gas vehicles (NGVs) in operation worldwide,88 and this number is expected to increase as public awareness of the environmental concerns associated with gasoline and diesel increases.   Figure 1-7: Kilograms of air pollutants (CO, NOx, SO2, particulates and CO2) produced per billion kilojoules of energy extracted from natural gas, coal and oil.89  While there are many advantages to using natural gas as a vehicle fuel, its full potential cannot yet be realized due to unburned methane expelled in the exhaust; 500-1000 ppm of methane can be released into the atmosphere in NGV exhaust emissions. Methane is the second largest contributor to global warming after carbon dioxide, in part due to how efficient it is at trapping radiation. The global warming potential (GWP) compares the 17  amount of heat trapped by a certain mass of a GHG compared to the amount of heat trapped by the same mass of carbon dioxide. The GWP of methane is estimated to be 23, meaning it is 23 times more potent a GHG than carbon dioxide. Therefore, it is of utmost importance to minimize the amount of unburned methane emitted from NGVs by oxidizing it to carbon dioxide before it is enters the atmosphere. The overall reaction for the total oxidation of methane is CH4 + 2O2 → CO2 + 2H2O However, other reactions may also be involved including steam reforming accompanied by the water gas shift reaction: CH4 + 3/2O2 → CO + 2H2O CH4 + H2O → CO + 3H2 2H2 + O2 → 2H2O CO + H2O → CO2 + H2   1.3.2 Catalysts for low-temperature methane oxidation  Methane is more difficult to oxidize than other hydrocarbons due to its high C-H bond strength of ~435 kJ mol-1, thus current noble metal-based catalytic converters typically found in gasoline and diesel engines cannot efficiently oxidize unburned methane. The main obstacle to developing a post-engine catalyst for the complete oxidation of methane in NGVs is the low temperature at which the engines operate. Methane exhibits a light-off temperature (the temperature at which the conversion rate is 50%, T50%) in the range of 400-450 ˚C, while NGV exhaust gas has relatively low temperatures (450-550 ˚C). Moreover, the low 18  concentrations of methane present in NGV engine exhaust (500-1000 ppm), large amounts of water vapour (10-15%) and CO2 (15%), and the presence of SOx and NOx can present problems.90,91 Although NGVs usually operate at low temperatures, any catalytic material must be thermally stable due to the possibility of temperature increases occurring during fast acceleration or engine misfiring.   Due to their high activity for the complete oxidation of hydrocarbons, the first materials investigated for low-temperature methane oxidation (L-T MOX) were noble metal catalysts. Several reports have shown that supported Pt and Pd have high activities.90,92 Most early reports focused on alumina as a support for both Pt and Pd, with studies showing that many variables influence the catalytic activity of the material, including precursor, Pt or Pd loading, surface area and morphology.90,92,93 Pd has shown to be more active than Pt under lean-burn combustion conditions,90 and so this discussion will focus on the use of Pd. The full nature of the active sites on Pd-based catalysts is unknown and further study is needed in order to better understand the reaction. However, many reports have been published that attempt to understand methane oxidation over Pd, with sometimes conflicting results. In most cases the oxidation of methane over Pd is first-order with respect to methane and zero-order with respect to O2.93 In the presence of oxygen, Pd oxidizes to form PdO between 300 and 400 ˚C. Studies have shown that at operating temperatures of natural gas engines, formation of the oxide is essential for high catalytic activity, and PdO is the main active phase. It has been reported that fully oxidized PdO is more active than Pd coated with a thin surface layer of PdO or Pd with O chemisorbed to the surface.94 However, methane does not adsorb onto completely oxidized Pd, with oxygen blocking the actives sites. It has also been reported that 19  some metallic Pd is required to allow CH4 to dissociatively adsorb onto the surface. This can be demonstrated by an initial period during which the rate of reduction of PdO by CH4 is first-order with respect to the amount of Pd formed as small particles of metallic Pd form on the surface. After this initial phase, rapid reduction occurs and the reaction becomes zero-order with respect to the amount of Pd.95 At higher temperature, methane also dissociates over metallic Pd; however, due to the temperatures in NVG engines, this is less relevant to this thesis. A dual site mechanism for the oxidation of methane over Pd has been proposed, where methane dissociatively adsorbs onto Pd metal but the adsorbed methyl or methylene species react with oxygen at a PdO site.96   Another mechanism, proposed by Garbowski et al., involves a PdO/Pd redox reaction during which superficial palladium oxide is formed through the adsorption of O2 onto Pd0 followed by adsorption of methane which is then oxidized, returning the Pd2+ to the reduced Pd0 state.97 In contrast to this mechanism, Schmal et al. conducted XPS studies on Pd/alumina catalysts, which show no metallic Pd after pre-treatment of the sample in a chamber with CH4 and O2 at 400 ˚C.98 The authors suggest that the oxidation of methane does not involve the reduction of PdO to metallic Pd, but rather the formation of Pd oxides of different oxidation states. Their results are in agreement with the following mechanism proposed by Li et al.:99 O2+PdO→PdOx PdOx+CH4→HCOO- +PdOy+H2O                  HCOO- +O2→m-CO32- + H2O m-CO32- + PdOy→ CO2 + PdO 20  The contradictory nature of findings relating to methane oxidation over Pd catalysts demonstrates the complex nature of the reaction and the necessity of further investigation.   In order to further the understanding of L-T MOX under operating conditions, Nilsson et al. employed oxygen rich-poor cycling conditions for methane oxidation over Pd supported on alumina.100 It was observed that the methane conversion changed according to oxidation and reduction of the Pd. When oxygen is introduced into the system there is a brief decrease in activity, which is suggested to be due to chemisorbed oxygen on the Pd surface. There is again a decrease in activity when oxygen is removed from the system.100 This observation illustrates the need for an oxygen buffer within the catalyst.   1.4 Cerium oxide 1.4.1 Overview  Cerium oxide (ceria) is a widely used heterogeneous catalyst and is a key component in three-way catalysts. It has been identified as a promising candidate for use as a catalyst with high activity in L-T MOX. Cerium has a 4f25d06s2 electronic configuration and can exist in both the 3+ and 4+ oxidation states allowing ceria to exist as CeO2, Ce2O3, and intermediate oxides. Pure CeO2 has a fluorite-type structure, with a face-centered cubic (fcc) unit cell where each Ce3+ cation is coordinated to eight O2- anions at the corner of each cube. Each O2- anion is tetrahedrally coordinated to four Ce3+ cations, as illustrated in Figure 1-8. The space group of CeO2 is Fm3m with lattice constant a = 0.541 nm at room temperature. Many of the properties that make ceria of interest as a heterogeneous catalyst arise from its unique redox 21  properties. Ceria can switch between Ce3+ and Ce4+, allowing it to undergo substantial changes in oxygen stoichiometry, while retaining its fluorite crystal structure, therefore allowing it to act as an oxygen (oxide) reservoir.  Figure 1-8: a) The fluorite structure of CeO2 with the unit cell shown, b) the (100) plane of CeO2 and c) the (110) plane of CeO2. Cerium atoms are yellow and oxygen atoms are red.  The predominant types of intrinsic defects observed in non-stoichiometric ceria are anion Frenkel pairs and anion vacancies. In the Frenkel type defect an O2- anion is displaced from its position in the ceria lattice to an interstitial site. Under ambient conditions these defects are present in low concentrations and do not affect the overall charge or stoichiometry of the lattice. This defect type can be illustrated using the Kröger and Vink defect notation, where 22  O0 represents oxygen ion in its lattice site, O’’i an oxygen ion in an interstitial position and Vö an oxygen ion vacancy, a dot indicates each positive charge and a prime each negative charge as: O0 ↔ O’’i + Vö  Anion vacancies mainly occur at high temperatures and low oxygen pressures. O2- is removed from the lattice as a neutral species, 0.5O2, creating a vacant site and leaving behind two electrons. The accessible Ce3+/Ce4+ redox cycle allows charge balance to be maintained through the reduction of two Ce4+ to Ce3+ for every one O2- removed from the lattice, as shown in the following equation, where ☐ represents a vacancy: 4Ce4+ + O2- → 4Ce4+ + 2e-/☐ + 0.5O2 → 2Ce4+ + 2Ce3+ ☐ + 0.5O2 This can also be illustrated using Kröger and Vink defect notation, where O0 and Cece represent oxygen and cerium in their lattice sites and Vö an oxygen vacancy: O0 + 2CeCe ↔ Vö + 2Ce’Ce + 0.5O2(g)  While CeO2 retains its crystal structure during substantial changes in oxygen stoichiometry, it undergoes lattice expansion upon reduction due the larger size of Ce3+ ions compared to Ce4+ ions (lattice spacing of 1.14 Å and 0.97 Å, respectively).  Due to the importance of this redox behavior to ceria applications, particularly as a catalyst, it has been extensively studied. A four step kinetic model for the reduction of ceria has been proposed by El Fallah et al. and is illustrated in Scheme 1-2.101 23   Scheme 1-2: Kinetic model for the reduction of ceria. 1) Dissociation of chemisorbed hydrogen to form hydroxyl groups at the surface, 2) formation of anionic vacancies and reduction of neighboring Ce4+ ions, 3) desorption of water by recombination of hydrogen and hydroxyl groups and 4) diffusion of surface anionic vacancies into the bulk material.  Ceria’s redox properties as well as the high mobility of oxygen ions in the ceria lattice allow it to release and absorb oxygen and hence act as a buffer during alternating redox conditions, a property measured by the oxygen storage capacity (OSC).  This is particularly useful in automobile engines, which operate under cycling oxygen rich and oxygen deficient conditions. During the oxygen deficient portion of the cycle, ceria can release oxygen for the oxidation of CO and hydrocarbons then replace this oxygen during the oxygen rich portion of the cycle. Recently, a large amount of research has focused on increasing the OSC of ceria and ceria-based materials in the hope of creating improved materials for catalysts in automobile engines.  24  1.4.2 Doped ceria  One way to increase the OSC of ceria is to introduce dopant cations. Dopant cations can be isovalent (dopant cation has the same oxidation state as the cation it is replacing) or aliovalent (dopant cation has a different oxidation state to the cation it is replacing). The addition of isovalent cations to the ceria lattice, such as Zr4+, Hf4+, Ti4+, can decrease the Ce4+/Ce3+ reduction energy and preserve oxygen defects.102,103 It has also been suggested that doping with Zr4+ ions, which are smaller than Ce4+ ions, can increase the number of oxygen vacancies by reducing the lattice strain introduced with formation of the larger Ce3+ ions.102 Ceria-based materials doped with aliovalent cations are also widely reported as a significant number of oxygen vacancies can be introduced. The most commonly used aliovalent dopant cations are in the +3 oxidation state, for example Mn3+, La3+, and Fe3+. It has been shown that doping with M3+ ions in ceria occurs via the vacancy compensation mechanism,104 illustrated in Figure 1-9 and represented using Kröger and Vink defect notation (where MCe represents a dopant cation in a cerium lattice site) as: xMO1.5 + (1-x)CeO2 ↔ xM’Ce + 0.5xVö + (1-x)CeCe + (2-0.5x)O0   25   Figure 1-9: Ceria centered at the cubic oxygen sublattice containing two 3+ dopant cations (M) and an oxygen vacancy.  The precise nature of ceria doping and its effect on OSC is not fully understood and varying results have been reported in the literature. For example, Balducci et al. reported that the lower Ce4+/Ce3+ reduction energy could be related to a larger ionic radius of the dopant cation when using divalent and trivalent dopants (Ca2+, Mn2+, Ni2+, Zn2+, Gd3+, La3+, Mn3+, Sc3+, and Y3+),105 while Reddy et al. reported that the reduction energy did not depend on the ionic radius when doping with 4+ ions (Si4+, Ti4+ and Zr4+).106 In actuality, the nature of oxygen vacancies probably depends on a variety of factors, as shown by Chen et al. who determined that the oxygen vacancy formation energy is composed of two main components: the interaction energy of the metal and oxygen and the relaxation energy.107 The interaction energy is the energy needed to remove the oxygen bonded to the dopant cation and the relaxation energy is energy gained from structural relaxation. = Ce4+ = M3+ = O2- = anion     vacancy 26  1.4.3 Nanostructured ceria  Efforts to increase the catalytic activity of ceria have also focused on increasing the surface area of both undoped and doped ceria-based materials to create more adsorption and reaction sites through the synthesis of nanostructured ceria. It is worth noting that care needs to be taken when comparing the surface area of non-siliceous mesoporous materials with silica-based materials. While mesoporous silica generally has surface areas close to 1000 m2/g, ceria has rarely been reported with a surface area over 200 m2/g. This is because the density of the oxide wall is considered in the calculation for surface area reported in m2/g. Quartz has a density of 2.6 g/cm3 while ceria has a density of 7.7 g/cm3.104,108 Assuming that mesoporous materials have identical densities to the crystalline bulk material, 340 m2/g of ceria corresponds to about 1000 m2/g of silica if density is taken into consideration.  Many different synthetic routes to ceria nanoparticles have been used, leading to a large variety of morphologies. Precipitation or co-precipitation methods allow fine particles to be precipitated out of solution. Precursors are generally inorganic salts such as Ce(NO3)3, (NH4)2Ce(NO3)6 or CeCl3 that are precipitated using alkali solutions, usually NaOH or NH4OH as follows: Ce3+(aq) + 3OH-(aq) → Ce(OH)3(s)" The resulting gels are calcined to give CeO2.109 Doped CeO2 can be synthesized by adding the corresponding salt of the dopant cation to the solution before coprecipitation.110,111 Although high surface areas can be achieved due to the small particle size obtained, typical precipitation processes produce particles without a well-defined size or morphology. 27  Hydrothermal methods are probably the most commonly used routes to obtaining ceria-based nanomaterials and have been successfully used to synthesize a number of morphologies including cubes,28,112 flowers,113,114 hollow spheres,115 rods28,116 and polyhedra.28 The size and morphology of the particles produced depends on the precursors and reaction temperature, as demonstrated by Mai et al., who heated cerium nitrate (Ce(NO3)3) with different concentrations of NaOH in a Teflon® lined autoclave to temperatures between 100 and 180 ˚C.28 Nanopolyhedra, nanorods or nanocubes were obtained depending on the concentration of NaOH and reaction temperature used. For example, 6 M NaOH heated to 100 ˚C produced rods while 6 M NaOH heated to 180 ˚C produced cubes and 0.01 M NaOH heated to 100 or 180 ˚C formed polyhedra. These different morphologies are shown in Figure 1-10. Differences in dissolution/recrystallization rates under various concentrations of NaOH and temperatures are believed to play a key part in the size and shape selectivity of this reaction.  Figure 1-10: TEM images of CeO2 nanopolyhedra, CeO2 nanorods and CeO2 nanocubes. Adapted with permission from reference 28. Copyright 2005 American Chemical Society.  Sol-gel methods have been used to obtain ceria nanostructures with various architectures.117–122 Recently, mesoporous ceria was successfully synthesized via a sol-gel method with 28  cerium nitrate, cerium acetate (Ce(CH3CO2)3) or cerium acetylacetonate (Ce(C5H7O2)3) as the starting material.123 Dimethyloctylamine, monoethanolamine, or tetraethylammonium hydroxide surfactants were added to the reaction mixture, with the identity of the precursor and surfactant affecting the final porosity. BET surface areas up to 129 m2/g were achieved using cerium acetate and tetraethylammonium hydroxide. It is suggested that the surfactant adsorbs onto the surface of ceria particles, preventing coagulation and forming a high surface area material.  Other surfactant-assisted methods have also been successfully used to synthesize mesoporous ceria. For example, Lyons et al. used cerium acetate to form a cerium acetate coordination polymer chain around a hexadecylamine surfactant.124 Hydrogen bonding between the precursor and surfactant assisted in the formation of an ordered hexagonal mesoporous phase. The calcined ceria product had an unusually high surface area of 245 m2/g. The cerium acetate coordination polymer has also been used by Ni et al. in combination with P123 to form ceria particles with highly aligned pores (Figure 1-11).125   Figure 1-11: TEM image of pore channels in CeO2 formed from cerium acetate polymeric chains aligned with P123. Reprinted from reference 125 with permission from Elsevier. 29  Mesoporous ceria has been synthesized through hard templating with KIT-6 and SBA-15 mesoporous silica. Rossinyol et al. used the evaporation method to produce mesoporous ceria.126 Cerium nitrate was added to the pores of KIT-6 and SBA-15 by stirring the precursor and template in ethanol with phosphotungstic acid then drying at room temperature. The template was then removed using HF to produce ceria with the inverse structure of its template, as illustrated in Figure 1-12. A similar method was used by Shen et al. to make KIT-6 templated mesoporous ceria and CuO-loaded mesoporous ceria.127   Figure 1-12: (a) TEM images of ceria templated with SBA-15 (b) HR-TEM micrograph of the crystalline framework (c). Ceria templated with SBA-15 along the (100) direction (d) and (111) direction. Reprinted from reference number 126 with permission from Elsevier.  30  1.4.4 Ceria in catalysis  Ceria-based materials are widely used in three-way catalysts (TWCs) for the treatment of emissions from gasoline engines due to their OSC. The name “three-way catalyst” is derived from the catalyst’s ability to remove CO, NOx and hydrocarbons simultaneously. Most TWCs consist of a ceramic monolithic honeycomb support loaded with noble metals, cerium-based oxides and alumina. The ceria-based materials are used as oxygen buffers to increase the operating window of the catalyst during the cycling oxygen rich and oxygen deficient conditions. Ceria-based materials have also been investigated for use as L-T MOX catalysts for use in NGV engines. Ceria species can act as active centres for methane oxidation and ceria maintains its nonstoichiometric structure even after L-T MOX has taken place.92 Kinetics studies over Ce0.75Zr0.25O2 have suggested that the oxidation of methane occurs via the Langmuir-Hinshelwood mechanism in which the surface reaction of dissociated oxygen with methane chemisorbed to the catalyst surface is the rate-determining step.128 It has been shown that ceria doped with ions such as Ca2+, Mn3+, Nd3+, La3+ and Pr3+ generally has higher activity for L-T MOX than pure ceria due to an increase number of oxygen vacancies and increased oxygen mobility.111,114,129,130 The preparation method is known to cause the L-T MOX activity of ceria-based catalysts to vary to a large degree,111 however there have not been enough studies to be able to consistently and rationally design synthetic routes to materials with high activity. To date no ceria-based materials have had activity for L-T MOX that is high enough to make them viable options for use in commercial NGVs and so ceria is often combined with precious metals to produce effective catalysts.  31  1.4.5 Ceria as a support  As mentioned in section 1.3.2, there is a need for an oxygen buffer within noble metal-based catalysts for L-T MOX. Many researchers have looked at the possibility of using ceria as an active support for noble metals to serve this purpose. Tompos et al. used combinatorial methods to identify good target compositions for L-T MOX and found that the highest activities were achieved for Pt/Pd mixtures deposited on CeO2.131 L-T MOX is expected to be carried out more effectively due to synergistic effects between Pd and ceria when compared with ceria alone. It has been shown that Pd supported on non-stoichiometric ceria has higher activity than Pd supported on alumina or silica.132 The active sites for L-T MOX on Pd/ceria-based catalysts are surface Pd atoms, with the oxygen buffering effect of the ceria support allowing Pd to maintain its active oxidized form through oxygen spillover. Hence the interaction between the Pd and ceria support is important, with high activity being linked to cationic PdOδ+ species in close contact with the support133 and Pd-O-Ce linkages.134 This interaction can be affected by both the morphology and oxidation state of the Pd nanoparticles, which vary with different methods of synthesis and pretreatment.100  Recently, Pd@CeO2 core-shell nanostructures on hydrophobic alumina were reported.135 The synthesis of the core-shell nanoparticles is based on self-assembly between functionalized Pd nanoparticles with diameters of roughly 2 nm protected by 11-mercaptoundecanoic acid and cerium alkoxide.136 The high activity of the catalyst was attributed to strong interaction between palladium and ceria due to their core-shell nature. The porous ceria shell allows access to the Pd and helps to maintain an oxidized Pd core, increasing the L-T MOX activity 32  of the catalyst. The hydrophobic alumina adsorbed the Pd@CeO2 nanostructures to a much higher degree than pristine alumina. This catalyst demonstrated complete conversion of CH4 to H2O and CO2 at temperatures below 400 ˚C and is one of the best cerium-based L-T MOX catalysts published to date.  1.5 Analytical techniques  The characterization of inorganic nanomaterials can present unique challenges compared to their bulk analogues due to their small size and distinctive properties. Many methods are good at elucidating one or a few aspect of nanomaterials, however in order to fully characterize the materials a combination of multiple techniques is usually used.  The following section briefly describes some of the analytical techniques employed in this thesis.  1.5.1 X-ray diffraction  In 1912, Max von Laue predicted that atoms in a crystal lattice have a periodic structure with equal interplanar distance on the same order of magnitude as the wavelength of X-ray light, and so crystalline materials could be used to diffract X-rays.137 His prediction was correct and led to the field of X-ray crystallography. Powder X-ray diffraction (PXRD) is a particularly useful tool for identifying materials that will not form single crystals large enough for single crystal X-ray diffraction as each crystalline solid has a unique “fingerprint” PXRD pattern. Once the identity of the material has been determined the pattern can be used to determine structural information about the material.  33  The X-rays used in PXRD are generated by a cathode ray tube that produces X-rays with a number of different wavelengths. Radiation with a single wavelength is preferred and so the beam is passed through a monochromator and then collimated to concentrate the beam. The beam is directed at the sample where coherent scattering occurs. Constructive interference, and hence a peak, occurs when the angle of diffraction satisfies Bragg’s law, !" = 2! !"# ! where n is a positive integer, λ is the wavelength of the incident X-rays, d is the interplanar spacing of the crystal and θ is the angle of incidence (Figure 1-13). At angles other than the Bragg angle, θ, diffracted beams are out of phase and destructive interference occurs.138   Figure 1-13: Bragg diffraction. Incident X-ray strikes a set of planes, with an interplanar spacing of d, at an angle of θ.  Powder samples usually contain tens of thousands of randomly oriented crystallites, producing a “Debye-Scherrer ring” of diffraction around the beam axis, as illustrated in Figure 1-14, as opposed to the discrete Laue spots observed in single crystal diffraction. A 34  linear diffraction pattern is formed as the detector scans through an arc that intersects each Debye cone at a single point, giving the produced spectra the appearance of discrete diffraction peaks. Only a small fraction of the crystallites in the sample are in the correct orientation to produce diffraction and so the sample is sometimes spun to increase the number of crystallites contributing to the diffraction pattern collected.  Once a diffraction pattern has been collected it can be compared to reference patterns from a database or from a calculation for identification. For historical reasons, PXRD patterns are a plot of 2θ on the x-axis versus intensity on the y-axis.   Figure 1-14: Illustration of an X-ray beam diffracting off a powder sample, resulting in Debye-Scherrer cones.  Crystals are usually thought of as perfect, infinite arrangements of atoms, molecules or ions. However, when characterizing nanomaterials by PXRD it is important to consider the fact that the number of planes is no longer infinite.139 Infinitely large, ideal crystals will produce diffraction maxima only at the Bragg angle. However, nanomaterials do not have enough planes contributing to the diffraction, so as the particles become smaller, the diffraction peaks become wider. Consider, for example, a nanoparticle 10 nm in diameter with d spacing 35  of 0.25 nm. This particle will only contain about 40 atomic layers, which is not enough to form complete destructive interference at angles close to θ so the peak is broadened. The crystallite size can be calculated using the width of the peaks using Scherrer equation,140–142 L= KλBcosθ where λ is the X-ray wavelength in nanometers (nm), B is the peak width of the diffraction peak profile at half maximum height resulting from small crystallite size (radians), and K is the Scherrer constant related to crystallite shape. However, while the Scherrer equation can be useful, the presence of polydisperse particles imparts uncertainty to the results. Also, as the Scherrer equation measures crystallite size and not particle size, care must be taken when analysing polycrystalline particles. Other factors that could affect peak width, and so affect the use of the Scherrer equation, involve the instrument peak profile and non-uniform lattice distortions such as strain.   1.5.2 Transmission electron microscopy  Transmission electron microscopy (TEM) is a technique used to image specimens in two dimensions on the nanoscale, with resolutions below 1 nm. This high resolution is a result of the small de Broglie wavelength of high energy electrons, which enables electron microscopes to image at significantly higher resolutions than light microscopes. During TEM imaging, electrons are emitted from a tungsten filament and accelerated through a high voltage of 20-300 kV, giving them enough energy to be transmitted through an ultra-thin sample (<200 nm) and a smaller de Broglie wavelength according to the equation: 36  ! = ℎ!" where λ = wavelength, h = Plank’s constant, m = mass and v = velocity.   Several electromagnetic lenses are used in the microscope. In a simplified picture, the condenser lens illuminates the sample by gathering the electrons and focusing them on the specimen, the objective lens focuses and magnifies the resulting image, which is further magnified by the projector lens. The condenser aperture reduces spherical aberration and the objective aperture enhances specimen contrast. A high-resolution image is formed from the interaction of the electrons transmitted through the specimen onto a phosphor screen that emits photons when irradiated by the electron beam. Most TEMs are equipped with a digital camera to capture these images. The image contrast is a result of absorption of electrons in the material due to the thickness and composition of the material.138,143   37   Figure 1- 15: A schematic representation of a transmission electron microscope.   1.5.3 Scanning electron microscopy  Scanning electron microscopy (SEM) is a technique used to image the surface of solid samples. In SEM, electrons are emitted from a filament, most commonly made of tungsten, and accelerated through a high voltage, usually between 2 and 30 kV. The optical column of the microscope is kept under high vacuum to prevent the filament from oxidizing and so that the beam is not hindered by the presence of other molecules. The electron beam is focused by one or two condenser lenses and scanned back and forth over the sample. As the electron beam hits the sample, various interactions occur and produce primary backscattered electrons (with an energy more than 50 eV), secondary electrons (with an energy less than 50 eV), and 38  X-rays. Backscattered electrons are electrons that have collided inelastically with atoms in the specimen, secondary electrons are emitted due to ionization of the material, and X-rays are produced when outer shell electrons move into inner shell “holes” left by secondary electrons. Most SEM analysis uses secondary electrons to create an image of the surface. Due to the energy of the backscattered electrons being so low, a detector with a positive bias is used to attract the electrons. More electrons are able to leave the sample at edges leading to increased brightness while fewer electrons can leave a depression resulting in images with a well-defined, 3D appearance.   Normally, SEM imaging requires a conductive connection between the sample and the ground to prevent charging, an effect caused by the build-up of electrons in the sample and their uncontrolled discharge. Charging can cause unwanted artifacts in secondary electron images and is particularly problematic for insulating samples and nanomaterials as a large number of the particles may not be connected to the sample mount. Sputter coating samples with a thin conductive layer of metal, such as gold, can help to inhibit charging, however layers of coating may hide nanosized surface features.  1.5.4 Energy dispersive X-ray spectroscopy  Energy dispersive X-ray spectroscopy (EDX or EDS) can be used in conjunction with SEM or TEM to provide elemental analysis of a sample. When a secondary electron is emitted from an atom, due to ionization by the electron beam, it leaves a “hole” which is filled by an electron from a higher energy shell. During this process X-rays with energies equal to the 39  energy difference between the higher-energy shell and lower-energy shell are emitted. The energy of the X-ray is characteristic of the atom from which it is derived. By scanning the electron beam across the sample, an image of each element in the material can be acquired. A beryllium window is used to protect the detector from higher energy X-rays and hence absorption by the window prevents the detection of elements below an atomic number of 4.  1.5.5 Nitrogen adsorption  Nitrogen adsorption can be used to determine the surface area of a solid sample and provide information about the pore structure and size. The surface area can be calculated using Brunauer-Emmett-Teller (BET) theory. BET theory expands upon Langmuir theory by considering multilayered gas molecule adsorption, where one layer does not need to be complete before a second layer starts. However BET theory makes several assumptions including:144 • The surface is homogeneous • At equilibrium the rate of adsorption is equal to the rate of desorption • Adsorptions only occur on well-defined sites of the sample surface with one molecule per active site • There is no lateral interaction between the molecules  By plotting the amount of adsorbate on the adsorbent as a function of the relative pressure at a constant temperature, adsorption isotherms are obtained. The shape of these isotherms 40  provides information about the sample surface (including the monolayer capacity) and pore structure. IUPAC has classified 6 major types of isotherms (see Figure 1-16): 145 Type I: Commonly observed in microporous materials with relatively small external surfaces. The steep increase at low relative pressures indicates that the available microporous volume is being occupied. Type II: Obtained with non-porous or macroporous materials. This isotherm arises from unrestricted monolayer-multilayer adsorption. Point B indicates the point at which a monolayer of adsorbate is complete and multilayer adsorption begins. Type III: This isotherm is very uncommon and corresponds to systems in which the interactions between the adsorbent and the adsorbate are weak in comparison with the adsorbate-adsorbate interactions. Type IV: The hysteresis loop of type IV isotherms is associated with capillary condensation taking place inside mesopores. The shape of the hysteresis loop gives information relating to the structure of the mesopores. Type V: This isotherm is related to the Type III isotherm and is also very uncommon. It is obtained with a few porous adsorbents. Type VI: Multilayer adsorption on a uniform, non-porous surface produces the Type VI isotherm. The steps represent the completion of each monolayer.  41   Figure 1-16: Six types of gas adsorption isotherms. Image adapted from reference 146.   Determining the monolayer capacity of a pre-weighed sample using the slope of the isotherm, and then multiplying this number by the cross sectional area of the adsorbate molecule (0.162 nm2 for N2), gives the BET surface area of the sample. In addition to BET surface area, nitrogen desorption data can also be used to calculate the Barrett-Joyner-Halenda (BJH) pore size distribution. This method assumes capillary condensation of nitrogen within the pores and relates the amount of nitrogen removed from the pores of the sample to the size of the pores as the relative pressure is decreased from high to low.  42  Hysteresis loops seen in nitrogen adsorption isotherms are associated with capillary condensation in mesopores. These hysteresis loops, shown in Figure 1-17, can be classified into four different types according to IUPAC: 145 H1: Type H1 hysteresis loops are given by materials with a narrow distribution of uniform pores. H2: Type H2 hysteresis loops are a consequence of interconnected pores with irregular size and shape. H3: Type H3 hysteresis loops are usually given by aggregates of plate-like particles containing slit-shaped pores. H4: Type H4 hysteresis loops are also given by slit-shaped pores, however the pore size distribution is in the micropore range.    Figure 1-17: IUPAC classification of hysteresis loops in nitrogen adsorption isotherms.     43  1.5.6 X-ray photoelectron spectroscopy  X-ray photoelectron spectroscopy (XPS) (also known as electron spectroscopy for chemical analysis) is a technique based on the photoelectric effect, which was first explained by Einstein in 1905.147 Kai Siegbahn received the Nobel Prize for Physics in 1981 for developing XPS. XPS is a surface-sensitive spectroscopic technique that measures the elemental composition of the outer 1-10 nm of a sample. When a material is irradiated with a beam of X-rays, photon-electron interactions with complete energy transfer occur. If the beam energy exceeds the electron binding energy, a photoelectron with a discrete energy is emitted from the sample. The kinetic energy that remains on the emitted electrons can be measured and used to calculate the binding energy using the equation:148 !!"#$"#% = !!!!"!# − !!"#$%"& + !  Where Ebinding is the binding energy of the electron, Ephoton is the energy of the X-ray photons, Ekinetic is the kinetic energy of the electron measured by the instrument and φ is the work function of the instrument. The number of electrons that escape from the sample can also be measured as a function of binging energy, giving a plot of the binding energy on the x-axis versus the number of electrons detected on the y-axis. The binding energy is characteristic of each element and is also sensitive to the oxidation state. Atoms of a higher positive oxidation state tend to have higher binding energies due to reduced shielding of the nucleus. Doublets are observed for p, d, and f peaks due to spin orbital splitting. XPS data is usually collected by scanning over the full range of accessible energies then taking high-resolution data from specific areas of interest.   44  1.6 Goals and scope of this thesis  The main goal of this thesis is the development of new ceria-based, mesoporous materials for catalysis of low-temperature methane oxidation (L-T MOX) in natural gas powered vehicles (NGVs). In order to function as L-T MOX catalysts these materials must efficiently oxidize methane below 500 ˚C. Natural gas is a more environmentally friendly fuel than gasoline or diesel and is seen as a stepping-stone to renewable energy sources. There are several reasons why this research focuses on ceria-based materials. First, ceria is cheaper than the noble metals traditionally used in catalytic converters, so using ceria to reduce or eliminate these expensive metals is desirable. Secondly, ceria acts as an oxygen buffer by storing and releasing oxygen during the cycling oxygen rich and oxygen deficient atmospheres in a vehicle engine. This allows ceria to be an active support for precious metals during L-T MOX. Lastly, ceria is currently used in three-way catalysts and can be synthesized at industrial quantities. Its advantageous properties for catalysis have given rise to a large body of literature relating to the synthesis, characterization and properties of both bulk and nanostructured ceria. However, to date no ceria-based materials have been successfully employed in NGV engines as L-T MOX catalysts and so further research is required to meet this important goal.  In this thesis new synthetic routes to ceria-based materials are investigated, giving rise to numerous novel materials. In Chapter 2 I investigate the ways in which the synthesis and doping of ceria-based materials can affect their catalytic activity for L-T MOX. In Chapter 3 I describe efforts to prepare ceria-based nanostructured materials with various morphologies 45  including the first mesoporous, hollow spheres of doped ceria synthesized via a non-hydrothermal method. Chapter 4 highlights the synthesis and characterization of mesoporous ceria/cobalt oxide mixed metal materials and their high activity for L-T MOX catalysis. In chapter 5 I describe the development of a new method, termed surface-assisted reduction (SAR), for preparing Pd/ceria catalysts. The scope of SAR is investigated, resulting in new Au/ceria and Ag/ceria materials. Finally, in Chapter 6 I draw conclusions based on my research and outline future directions for this work.    46  Chapter 2: Synthesis Methods and Doping of CeO2  2.1 Introduction  Several synthetic methods have been used for the preparation of ceria particles with nano-scale features. These include hydrothermal,112,114 sonochemical,149 combustion,150 soft templating,125 hard templating126 and homogeneous precipitation151 methods. The large range of synthetic routes available give rise to a wide range of ceria-based materials with different morphologies, such as mesoporous materials,125,127 nanofibers,152 nanoflowers,114 hollow nanospheres,153 nanorods28,116 and nanocubes.28,154 It is important to continue to study the synthesis and properties of ceria-materials in order to develop new, efficient catalysts for low-temperature methane oxidation (L-T MOX). While the mechanism for L-T MOX is not fully understood, the preparation method,111 surface area,155 morphology,28,127 oxygen vacancy defects,156 dopants157,158 and other properties of ceria-based catalysts are known to affect the activity.   Particular attention has been paid to the synthesis of ceria-based catalysts with high surface areas, which increases the catalytic activity by creating more accessible surface sites for the reaction to take place. Mesoporous ceria is of interest as the pores are small enough to give high surface areas but large enough to allow access to molecules with a range of different sizes. Soft templating methods are attractive as they eliminate the need to design, synthesize and remove a hard template, however the high condensation rate of metal-containing 47  precursors as well as the structural transformation and hence collapse of metal oxide mesostructures during calcination makes it a challenging route.   Efforts to increase the oxygen storage capacity of ceria by the introduction of cation dopants have been successful. When an aliovalent cation (with a valence lower than 4+) is added to the ceria lattice, intrinsic defects occur through anion vacancies throughout the lattice in order to balance the charges. Ceria-based materials have been prepared with a variety of dopant aliovalent cations, including Mn3+, La3+, Pr3+, Gd3+ and Fe3+.111,114,130,159,160 Most of these materials showed enhancement of both oxygen vacancy concentration and oxygen storage capacity as well as of redox activities when compared to undoped ceria. Doping can also affect the crystallite size of ceria-based materials, with Mn doping decreasing the crystallite size of doped ceria samples with increasing dopant concentrations.161  It is also possible to increase the rate of diffusion of O2- through bulk ceria by doping with isovalent elements. This creates extrinsic structural defects by variation of the cell parameter, which increases both total and kinetic oxygen storage.162,163 Optimizing synthesis routes as well as the type and amount of doping are key factors for developing new ceria-based materials with properties beneficial for catalysts. This chapter describes the development of new materials based on literature procedures, which are then tested for L-T MOX. Specifically, multiple high surface area ceria-based materials doped with different types and amounts of dopant cations were synthesized. The properties of these catalysts were investigated and this information was used to determine if any correlation between the properties of the catalyst and its activity for L-T MOX could be found. 48  2.2 Experimental 2.2.1 General  All solvents and reagents were purchased from commercial sources and used without further purification. Thermogravimetric analysis was performed on a PerkinElmer Pyris 6 thermogravimetric analyzer. All samples were run under air. Gas adsorption studies were performed using a Micromeritics Accelerated Surface Area & Porosity (ASAP) 2000 system. All samples were degassed for 2 hours under vacuum at 120 °C immediately prior to analysis. BJH pore size distributions were calculated from the adsorption branch of the isotherm. Powder X-ray diffraction patterns were collected using a D8 advance X-ray diffractometer.  2.2.2 Microscopy  TEM images were collected on a Hitachi H7600 electron microscope. Samples were prepared by suspending the powders in ethanol and then dropcasting them onto a carbon-coated copper TEM grid and air-drying. SEM images were obtained with a Hitachi S-4700 field emission scanning electron microscope. Samples were prepared using double-sided carbon tape or dropcasting the dispersion directly onto aluminum stubs. The samples were then sputter-coated with 5 nm of gold. EDX analyses were acquired using a Hitachi S-2600N variable pressure scanning electron microscope. Samples were analyzed without sputter coating.  49  2.2.3 Preparation of plate-like CeO2 materials  After dissolving 1.00 g of Pluronic P123 in 20 mL ethanol, Ce(OAc)3"1.5H2O (3.443 g, 10.00 mmol) was added. After 2 h of stirring the reaction mixture was placed in an oven at 60 ˚C and maintained at this temperature for 3 d. The resulting gel was calcined at 400 ˚C for 5 h to give a pale yellow solid. Yield: 1.668 g. This sample is referred to as Comp1.  2.2.4 Preparation of doped plate-like CeO2 materials  After dissolving 1.00 g of Pluronic P123 in 20 mL ethanol, a total of 10.00 mmol of Ce(OAc)3"1.5H2O and Mn(OAc)2"4H2O (or Cu(OAc)2"H2O or La(OAc)3"1.5H2O) were added in proportion with Mn2+ (or Cu2+ or La3+) in a molar fraction ranging from 0.1 to 0.8. After 2 h of stirring, the reaction mixture was placed in an oven at 60 ˚C and maintained at this temperature for 3 d. The resulting gel was calcined at 400 ˚C for 5 h to give a solid. These samples are referred to as Comp1-X%M where M is the dopant metal.  2.2.5 Preparation of pseudo-spherical CeO2 particles  0.250 g of Pluronic P123 was first dissolved in 7 mL of propanol then Ce(NO3)2"6H2O (2.605 g, 6.000 mmol) was added. The reaction mixture was placed in an oven at 45 ˚C for 7 d then heated to 120 ˚C for 5 h. Once cool, the solid was stirred in ethanol overnight to extract the P123 and isolated by filtration. The solids were then washed with 2 x 20 mL 50  ethanol, dried under suction and calcined at 400 ˚C (heating rate 1˚C min-1) under air to afford a pale yellow solid. Yield: 0.252 g. This sample is hereafter referred to as Comp2.  2.2.6 Preparation of doped pseudo-spherical CeO2 particles  0.250 g of Pluronic P123 was first dissolved in 7 mL of propanol then a total of 6.000 mmol of Ce(NO3)2"6H2O and Mn(NO3)2"xH2O were added in proportion with Mn2+ molar fractions of 0.1 and 0.2. The reaction mixture was placed in an oven at 45 ˚C for 7 d then heated to 120 ˚C for 5 h. Once cool, the solid was stirred in ethanol overnight to extract the P123 and isolated by filtration. The solids were then washed with 2 x 20 mL ethanol, dried under suction and calcined at 400 ˚C (heating rate 1˚C min-1) under air. These samples are hereafter referred to as Comp2-X%Mn.  2.2.7 Preparation of high surface area CeO2  (NH4)2Ce(NO3)6 (5.000 mmol, 2.741 g) was dissolved in 30 mL of water with stirring. The reaction mixture was heated to 50 ˚C and 2M NaOH was added dropwise until a yellow precipitate formed and litmus paper showed a pH of ~10. The mixture was aged at 50 ˚C for 2 h then filtered. The yellow solids were washed with 2 x 20 mL water, dried under suction and calcined at 500 ˚C (heating rate 1˚C min-1) under air. Yield: 0.841 g. This sample is hereafter referred to as Comp3.  51  2.2.8 Preparation of doped high surface area CeO2  A total of 5.000 mmol of Ce(OAc)3"1.5H2O and M(OAc)x"YH2O (where M= Fe, Mn, Gd, Er, Zn or Cr)) in proportion with a molar fraction for M of 0.15 and 0.5 was dissolved in 30 mL of water with stirring. The reaction mixture was heated to 50 ˚C and 2M NaOH was added dropwise until a yellow precipitate formed and litmus paper showed a pH of ~10. The mixture was aged at 50 ˚C for 2 h then filtered. The solids were washed with 2 x 20 mL water, dried under suction and calcined at 500 ˚C (heating rate 1˚C min-1) under air. These samples are hereafter referred to as Comp3-X%M where M is the dopant metal.  2.2.9 Pd impregnation onto doped high surface area CeO2  Pd(NO3)2"xH2O (0.065 g, 0.282 mmoles) was dissolved in 30 mL of distilled water and Comp3-50%Fe (0.270 mg) was added. The reaction mixture was covered and stirred at room temperature overnight then placed in an oven uncovered at 80 ˚C until all the liquid evaporated. The resulting solids were calcined at 500 ˚C under air for 4 h to give a grey solid, hereafter referred to as 10%Pd/Comp3-50%Fe.  2.2.10 Preparation of CeO2 from a chloride precursor  CeCl3"7H2O (5.600 g, 15.00 mmol) was stirred with cetyltrimethylammonium bromide (CTAB) (4.700 g, 13.00 moles) in 275 mL of water. NH4OH (125 mL, 25 wt%) was added slowly and a yellow precipitate formed. The reaction mixture was stirred for 24 h at 90 ˚C, 52  collected by filtration and washed with 500 mL of water and 250 mL of ethanol. The solid was dried overnight at 60 ˚C then calcined at 500 ˚C under air for 3 h to give a yellow solid. Yield: 1.473 g. This sample is hereafter referred to as Comp4.  2.2.11 Methane oxidation testing  Temperature-programmed CH4 oxidation (TPO) was used to evaluate the activities of the prepared catalysts. The TPO setup (Figure 2-1) consisted of a stainless steel, fixed-bed, micro-reactor (length = 4.5 cm; i.d. = 0.7 cm) placed inside an electric tube furnace with PID temperature control. Two thermocouples (K-type), placed co-axially in the reactor, measured the temperature at the top and bottom of the catalyst bed. Flowrates of CH4 (0.76 (v./v.)% CH4/Ar, Praxair, certified purity), O2 (Praxair, UHP), Ar (Praxair, UHP) and He (Praxair, UHP) were set using electric mass flow controllers (Brooks 5850 TR) and mixed to yield a feed gas of 1000 ppmv CH4, 20 (v./v.)% O2, balance He/Ar at a total feed gas flowrate of 300 mL (STP) min-1 (WHSV 180,000 cm3 (STP) g-1 h-1).  The feed gas was pre-heated to 373 K using a separate furnace before entering the reactor.  The catalyst (0.1000 g ; 90 – 354 µm) was diluted four times (volume/volume) with inert SiC pellets (90-354 µm) to ensure isothermal reactor operation. The diluted catalyst was flushed in 100 cm3 (STP) min-1 flow of Ar at 393 K for 1 h prior to introducing the reactant gas to the catalyst bed. The reactor temperature was simultaneously increased linearly at 5 K·min-1 from 393 K to 873 K while monitoring the reactor exit gas composition using a VG ProLab quadrupole mass spectrometer (ThermoFisher Scientific). Mass numbers corresponding to CH4, CO2, and He 53  were monitored and their intensity calibrated using standard gas mixtures (Praxair, certified purity), from which the CH4 conversion and overall C balance were calculated.   Figure 2-1: Schematic diagram of TPO experimental setup; MFCs: mass flow controllers; SVs: switch valves.   54  2.3 Results and discussion 2.3.1 Plate-like CeO2 materials  The synthesis of Comp1 was adapted from work by Ni et al., who synthesized irregularly shaped CeO2 particles with aligned pores using P123 as a soft template in ethanol.125 The aligned pores originate from a polymeric chain, formed by the acetate source, hydrogen bonding with the surfactant head group. Initially, difficulty was encountered when trying to dissolve cerium acetate in ethanol. Subsequent contact with the paper’s authors revealed the necessity for 60% humidity for the complete dissolution of the Ce(OAc)3"1.5H2O. However, interesting results were obtained when the experimental conditions allowed the starting materials to remain undissolved and so this reaction was further investigated.   The morphology of Comp1 was examined by SEM, as shown in Figure 2-2. It can be seen that the calcined sample forms plate-like particles. TEM imaging indicates that these plates are porous (Figure 2-3). It is proposed that Ce(OAc)3"1.5H2O is very poorly soluble in pure ethanol and so forms a solubility equilibrium, allowing polymeric chains of cerium acetate to align into plates slowly over the three day aging period.  55   Figure 2-2: SEM images of Comp1 showing plate-like particles (scale bar = 1 µm).   Figure 2-3: TEM image of Comp1 showing porous plates (scale bar = 100 nm).  PXRD analysis of Comp1 (Figure 2-4a) clearly shows peaks corresponding to the crystalline cubic fluorite ceria phase (JCPDS-34-0394). This material was thermally stable up to 900 ˚C by TGA analysis under air. The nitrogen adsorption/desorption isotherm measured for a representative sample of Comp1 (Figure 2-4b) can be classified as a Type IV isotherm with a type H3 hysteresis loop, according to IUPAC classification.145 This isotherm is typical of a compound composed of aggregates of plate-like particles with non-rigid slit-shaped pores, 56  which is in agreement with the SEM studies. Comp1 has a BET surface area of 129 m2/g. The BJH pore size distribution curve, shown in the inset of Figure 2-4b, shows pores with an average diameter of about 3.5 nm, corresponding to the pores seen in the plates by TEM. There is also a wide range of large pore sizes, which is to be expected as the slit shaped pores are not uniform.  Figure 2-4: a) PXRD pattern of Comp1 (!= CeO2, JCPDS-34-0394). b) N2 adsorption-desorption isotherms for Comp1. Solid line represents adsorption and dashed line represents desorption, inset)  BJH pore size distribution curve for Comp1.   Ni et al. state that cerium acetate polymeric chains are aligned in solution with P123 micelles, forming the pores seen in their irregularly shaped particles. When P123 was omitted from the reaction, a decreased CeO2 surface area of 109 m2/g was obtained. SEM imaging shows some indication of the formation of plate-like particles, however they are less defined than samples synthesized with P123, and TEM imaging shows these particles are porous (Figure 2-5a and b, respectively). This work suggests that cerium acetate polymeric chains align to form pores even in the absence of P123, but the surfactant helps to stabilize the formation of the CeO2 sheets through interactions between the hydrophilic segments of 57  P123 and the acetate group. The poor solubility of the precursor in ethanol helps to control the reaction and gives a higher degree of organization. To confirm that the acetate group is required for the formation of plate-like porous particles, Ce(NO3)3·6H2O was used in place of Ce(OAc)3"1.5H2O. The CeO2 product had a surface area of only 37 m2/g and no plate-like morphology.   Figure 2-5: a) SEM (scale bar = 1 µm) and b) TEM (scale bar = 100 nm) images of Comp1 synthesized without P123.  Comp1 was tested for catalytic activity for complete methane oxidation. The catalytic activity is reported in terms of the light-off temperature at 50% conversion (T50%). Catalyst light-off is a term used to describe the results obtained from catalytic testing when the temperature of the inlet gas is gradually increased. As shown in Figure 2-6, Comp1 shows a T50% of 538 ˚C and does not reach 100% conversion when tested up to 600 ˚C. While it is promising that some activity is present in this sample, it is not at a temperature low enough to be considered a candidate for a L-T MOX catalyst.  58   Figure 2-6: TPO curve of Comp1.  2.3.2 Doped plate-like CeO2 materials  In an attempt to produce oxygen vacancies and improve oxygen mobility for higher catalytic activity, Comp1 was doped with Mn, La and Cu at various doping concentrations: 10, 20, 50 and 80 mol%. At Mn doping levels of 10 and 20% (Comp1-10%Mn and Comp1-20%Mn) only peaks corresponding to CeO2 are visible in the PXRD pattern, as shown in Figure 2-7. Above this level, peaks corresponding to both CeO2 and manganese oxide (Mn3O4) are present. 59   Figure 2-7: PXRD patterns of Comp1-10%Mn, (yellow), Comp1-20%Mn (blue), Comp1-50%Mn (green) and Comp1-80%Mn (red). != CeO2 (JCPDS-34-0394), != Mn3O4 (JCPDF-24-0734).  The fact that the peaks from Comp1-10%Mn and Comp1-20%Mn correspond only to CeO2, suggests that Mn ions are incorporated into the ceria lattice until a saturation level is reached. Above this saturation level metal oxides of the dopant form in addition to doped CeO2. However, due to the broad nature of the peaks no significant peak shifting is observed, and thus the possibility that the manganese is present in the form of an amorphous oxide cannot be ruled out. As a control experiment, ceria was preformed by calcining Ce(OAc)3"1.5H2O at 500 ˚C under air for 5 hours. The ceria product was combined with both 10 and 20 mol% of Mn(OAc)2"4H2O in ethanol, dried at 60 ˚C and then calcined at 400 ˚C, so Mn doping of the ceria could not occur. Peaks corresponding to crystalline Mn3O4 are present in the PXRD pattern of the control sample (see Appendix Figure A-1), indicating that any manganese oxides present in Comp1-10%Mn and Comp1-20%Mn would be 60  crystalline and should also appear in the PXRD pattern. Because only peaks corresponding to CeO2 are observed it is likely that the Mn ions have entered into the ceria lattice.   The presence of an additional compound at doping levels of 50% and above is confirmed by SEM, (Figure 2-8) which shows plate-like particles and an increasing quantity of a compound with irregular morphology, presumed to be Mn3O4. EDX mapping, shown in Figure 2-9, shows an increased amount of Mn in Comp1-80%Mn compared to Comp1-20%Mn, but the resolution is not high enough to determine how these elements are distributed in the samples.    Figure 2-8: SEM images of a) Comp1-10%Mn, b) Comp1-20%Mn, c) Comp1-50%Mn and d) Comp1-80%Mn showing an increasing amount of a compound with an irregular morphology, thought to be Mn3O4 (scale bar = 1 µm). 61    Figure 2-9: EDX mapping images for a) Comp1-20%Mn and b) Comp1-80%Mn. Purple = Ce, green = Mn and red = O. Scale bars = 500 µm.  TEM images of Comp1-X%Mn show particles with pores similar to those in undoped Comp1 (Figure 2-10). Aligned channels can be seen in Figure 2-10a, presumably from a particle lying perpendicular to those where the pore openings are visible. Similar PXRD patterns, SEM images and TEM images were obtained for Comp1-X%La and Comp1-X%Cu (See Appendix A).  62   Figure 2-10: TEM images of a) Comp1-10%Mn, b) Comp1-20%Mn, c) Comp1-50%Mn and d) Comp1-80%Mn (scale bar = 100 nm).  As shown in Table 2-1, BET surface areas of Comp1-X%M samples decrease as the ratio of Ce:dopant decreases, with the exception of Comp1-50%Cu. Small channels with diameters of a few nanometers within the plate-like particles are present in all samples. This indicates that the majority of the surface area of ceria arises from the plate-like particles seen by SEM, which decrease proportionately as doping increases. This is supported by the N2 adsorption-desorption isotherms (Figure 2-11) of the Comp1-X%M samples, which lose Type IV,H3 character as the doping level increases. The same pattern of decreasing surface area 63  corresponding to the increasing presence of dopant cation metal oxides is also seen in Comp1-X%La and Comp1-X%Cu.  Table 2-1: BET surface areas of Comp1-X%Mn, Comp1-X%La and Comp1-X%Cu. Mn doping level BET surface area (m2/g) La doping level BET surface area (m2/g) Cu doping level BET surface area (m2/g) 10% 123 10% 107 10% 117 20% 119 20% 110 20% 91 50% 98 50% 76 50% 105 80% 46 80% 62 - -    Figure 2-11: N2 adsorption-desorption isotherms for a) Comp1-10%Mn (yellow), b) Comp1-20%Mn (blue), c) Comp1-50%Mn (green) and d) Comp1-80%Mn (red). Solid lines represent adsorption and dashed lines represent desorption.  Due to constraints related to the availability of catalytic activity testing, only selected samples samples of Comp1-X%M were tested for catalytic activity for L-T MOX (Table 2-2 64  and Figure 2-12). It can be seen that there is no clear correlation between doping level, morphology or surface area and catalytic activity. While all of these compounds do show activity for L-T MOX, the achieved values were not high enough for them to be considered candidates for further testing.  However, this study illustrated that CeO2 can be easily doped with a variety of dopant cations up to 20 mol% and resulted in a new method for producing porous, plate-like particles of doped and undoped CeO2 with high surface area.  Table 2-2: Catalytic activity of Comp1 and Comp1-X%M for complete methane oxidation expressed in terms of T50%.    Compound T50% (˚C) Comp1 538 Comp1-10%Mn 548  Comp1-20%Mn 524 Comp1-50%Mn 520 Comp1-80%Mn 538 Comp1-100%Mn >600 Comp1-10%La 545 Comp1-50%La 552 Comp1-80%La 559 Comp1-100%La 559 Comp1-20%Cu >600 65   Figure 2-12: TPO curve for for a) Comp1-10%Mn (yellow), Comp1-20%Mn (blue), Comp1-50%Mn (green), Comp1-80%Mn (red) and Comp1-100%Mn (purple), b) Comp1-10%La (yellow), Comp1-50%La (green), Comp1-80%La (red), Comp1-100%La (purple) and c) Comp1-20%Cu (blue).  66  2.3.3 Pseudo-spherical CeO2 particles  In order to determine if it is necessary to form a polymeric chain derived from Ce(OAc)3"1.5H2O  to obtain high surface area CeO2, the synthesis of high surface area CeO2 using Ce(NO3)3·6H2O as a precursor was attempted. Suzuki et al. have previously synthesized mesoporous ceria with a high surface area via a sol gel route using Ce(NO3)3·6H2O in n-propanol employing Pluronic F127 as a soft template.164 While the authors give no explanation for the high surface area of their materials, it is likely that the surfactant adsorbs onto the surface of the ceria nanoparticles, forming a protective layer and preventing aggregation. In this way P123 was used as a surfactant resulting in similar materials to those reported in the literature. Mn(NO)3"xH2O was added at doping concentrations of 10 and 20 mol%. The PXRD patterns of Comp2, Comp2-10%Mn and Comp2-20%Mn, shown in Figure 2.13, correspond to ceria with fluorite-like cubic structure. The peaks are too broad to determine if there are any changes in the lattice parameters, however no peaks corresponding to oxides of Mn were seen in the PXRD patterns of Comp2-10%Mn or Comp2-20%Mn, again suggesting that the Mn has been incorporated into the CeO2 lattice.   67   Figure 2-13: PXRD patters of a) Comp2 (black), b) Comp2-10%Mn (blue) and c) Comp2-20%Mn (red). !  = CeO2 (JCPDS-34-0394).  The N2 adsorption/desorption isotherms of Comp2, Comp2-10%Mn and Comp2-20%Mn are shown in Figure 2-14. All examples exhibit the type IV isotherm with H1-type hysteresis loops.  Highly ordered mesoporous materials typically have isotherms of Type IV. A hysteresis loop of Type H1 is characteristic for porous materials with cylindrical pores and a high degree of pore size uniformity. The irreversibility is associated with capillary condensation taking place in mesopores.145 Interestingly, the BET surface areas do not decrease gradually with an increase in doping contents as was seen in previous experiments but shows an irregular pattern. Comp2-10%Mn (178 m2/g) has a higher BET surface area than Comp2 (169 m2/g) and Comp2-20%Mn (156 m2/g) has the lowest. The high surface area of Comp2 arises from the small particle size and porous nature of the pseudo-spherical particles, as shown by SEM and TEM (Figure 2-15). 20 30 40 50 602Θ / degreesIntensity / a.u.68    Figure 2-14: Nitrogen adsorption isotherms for Comp2 (black), Comp2-10%Mn (blue) and Comp2-20%Mn (red). Solid lines represent adsorption and dashed lines represent desorption.   69   Figure 2-15: TEM images of a) Comp2, b) Comp2-10%Mn and c) Comp2-20%Mn (scale bars = 100 nm) and d) SEM image of Comp2-10%Mn (scale bar = 1 µm).  2.3.4 High surface area cerium oxide  Materials with a high surface area are of particular interest due to the well-known relationship between particle size and catalytic activity.165 Shi et al. synthesized MnOx–CeO2 with a 1:1 ratio of Mn:Ce via a coprecipitation route, yielding samples with a surface area of 75 m2/g. This synthetic route was employed to synthesize undoped ceria (Comp3) as well as several samples of doped ceria (Comp3-X%M). The PXRD pattern of Comp3 is shown in Figure 2-16a. The diffraction peaks, which correspond to ceria, are very broad, indicating a 70  small crystallite size. TEM imaging also suggests that the sample is composed of small crystallites that have aggregated into larger particles (Figure 2-16b). Comp3 is stable up to 900 ˚C in air by TGA.  Figure 2-16: a) PXRD pattern (!= CeO2, JCPDS-34-0394) and b) TEM image of Comp3 (scale bar = 100 nm).  Comp3 has a high BET surface area of 210 m2/g and a BJH average pore diameter of 6.7 nm.  As shown in Figure 2-17, the nitrogen adsorption isotherm is type IV with a hysteresis loop, indicating a mesoporous structure.  71   Figure 2-17: N2 adsorption-desorption isotherm of Comp3. Solid line represents adsorption and dashed line represents desorption.  Due to the facile synthesis and high surface area of Comp3, doping was attempted with a number of different ions using the corresponding nitrate salt. Fe(NO3)3, Mn(NO3)2, Gd(NO-3)3, Er(NO3)3, Cr(NO3)3 and Zn(NO3)2 were added to the starting materials for Comp3 in amounts that resulted in doped ceria with 15 mol%  and 50 mol% of dopant.   The PXRD patterns of Comp3-15%M are shown in Figure 2-19a. Only peaks associated with CeO2 are present, with no diffraction peaks relating to any other metal oxides. As with other samples, no shifts in the ceria diffraction peaks were observed. However, due to the broad peaks it is difficult to be certain that no small shift has taken place. While the possibility of the metal oxides forming amorphous coatings on the CeO2 cannot be excluded, this seems unlikely due to the high calcination temperatures and previous control 72  experiments, described in section 2.3.2. In addition, EDX mapping shows a homogeneous distribution of the Ce and dopant cations (Figure 2-18). Figure 2-19b shows the PXRD patterns of Comp4-50%Fe. While peaks corresponding to CeO2 are still the dominant feature of the spectra, peaks relating to the oxide of the dopant metal can be seen in Comp3-50%Cr, Comp3-50%Zn, and, to a lesser extent, Comp3-50%Fe. There is a slight shift apparent in the CeO2 peaks of Comp3-50%Fe, Comp3-50%Mn, Comp3-50%Gd, and Comp3-50%Er. As anticipated, Fe and Mn doping produce the largest peak shifts to higher values of 2Θ. Fe3+ and Mn3+ both have smaller ionic radii than the Ce4+ ion resulting in a decrease in d-spacing when substitutional doping occurs. Gd3+ and Er3+ are both only slightly smaller than Ce4+. Er3+ doping produces a small increase in 2θ as expected, however, Gd3+ produces a small decrease in 2θ, which may be due to interstitial doping.   Figure 2-18: EDX mapping images for a) Comp3-15%Fe, b) Comp3-15%Mn, c) Comp3-15%Gd, d) Comp3-15%Er, and e) Comp3-15%Zn and f) Comp3-15%Cr. Scale bars = 20 µm. 73    Figure 2-19: PXRD patterns of a) Comp3-15%Cr (pink), Comp3-15%Zn (purple), Comp3-15%Er (blue), Comp3-15%Gd (orange), Comp3-15%Mn (green), and Comp3-15%Fe (red) and b) Comp3-50%Cr (pink), Comp3-50%Zn (purple), Comp3-50%Er (blue), Comp3-50%Gd (orange), Comp3-50%Mn (green), and Comp3-50%Fe (red). Dashed lines represent CeO2 (JCPDS-34-0394).  TEM images of Comp3-15%M and Comp3-50%M (examples shown in Figure 2-20) are similar to those of Comp3, meaning doping does not significantly change the morphology of the materials. While TEM does not show a significant change in morphology, nitrogen adsorption suggests that doping causes changes within the pore structure, probably due to defects in the crystal structure creating less pore uniformity. The hysteresis of most of the doped samples is altered to a type H2 (Figure 2-21), which is indicative of a complex pore structure composed of interconnected networks of pores of different size. The shape and surface area of the doped samples decreases when compared to the undoped sample (Table 2-3). 74   Figure 2-20: TEM images of a) Comp3-15%Fe, b) Comp3-50%Fe, c) Comp3-15%Mn and d) Comp3-50%Mn (scale bars = 100 nm).   75   Figure 2-21: N2 adsorption-desorption isotherms for a) Comp3-15%Cr (pink), Comp3-15%Zn (purple), Comp3-15%Er (blue), Comp3-15%Gd (orange), Comp3-15%Mn (green), and Comp3-15%Fe (red) and b) Comp3-50%Cr (pink), Comp3-50%Zn (purple), Comp3-50%Er (blue), Comp3-50%Gd (orange), Comp3-50%Mn (green), and Comp3-50%Fe (red). Solid lines represent adsorption and dashed lines represent desorption.  Table 2-3: BET surface area of Comp3-15%M and Comp3-50%M Doping metal BET Surface Area (m2/g) Comp3-15%M Comp3-50%M Fe 96 94 Mn 133 113 Gd 56 67 Er 81 77 Zn 42 45 Cr 69 106  Comp3 and Comp3-X%M were tested for catalytic activity for L-T MOX. As shown in Figure 2-22a and Table 2-4, Comp3 and Comp3-15%M have fairly low activity for L-T MOX, with only Comp3-15%Fe and Comp3-15%Mn having above 50% conversion at 600 ˚C. However, Comp3-50%Fe and Comp3-50%Mn have reasonable activity for L-T MOX, with T50%s of 598 ˚C and 589 ˚C, respectively (Figure 2-22b). It is interesting to note that 76  Comp3-50%Fe and Comp3-50%Mn exhibited the largest decrease in d-spacing by PXRD. In addition, both Mn3O4 and Fe2O3 are mixed valence compounds that have themselves been studied as potential candidates for methane oxidation catalysts.166 Any Fe or Mn that is not incorporated into the ceria lattice forms the metal oxide, which may interact synergistically with the ceria and contribute to L-T MOX.167 In contrast, Comp3-50%Cr and Comp3-50%Zn, which have relatively intense PXRD peaks relating to the dopant metal oxide, have the lowest activity for L-T MOX, with activities even lower than the undoped Comp3. It is probable that these dopant metal oxides add mass to the catalyst without participating in the catalysis and so they decrease activity.  Figure 2-22: TPO curve of for a) Comp3 (black), Comp3-15%Zn (purple), Comp3-15%Er (blue), Comp3-15%Gd (orange), Comp3-15%Mn (green), and Comp3-15%Fe (red) and b) Comp3 (black), Comp3-50%Cr (pink), Comp3-50%Zn (purple), Comp3-50%Er (blue), Comp3-50%Gd (orange), Comp3-50%Mn (green), and Comp3-50%Fe (red).   77  Table 2-4: Catalytic activity of Comp3 and Comp3-X%M for complete methane oxidation expressed in terms of T50% Doping metal  T50% (˚C) Comp3 Comp3-15%M Comp3-50%M None >600 - - Fe - 547 498 Mn - 563 489 Gd - >600 575 Er - >600 586 Zn - >600 >600 Cr - - >600  2.3.5 High surface area CeO2  as a support for Pd  In order to determine the feasibility of using Comp3-X%M as an active support in a L-T MOX catalyst, 10 wt% palladium was deposited onto Comp3-50%Fe using a facile wet impregnation route. Comp3-50%Fe was chosen because it was one of the most promising materials from the Comp3-X%M series. Using this method reduced the chances of the support being altered in any way, however there is no certainty that all the Pd was deposited onto the support with none depositing onto the walls of the beaker used.   The presence of Pd was confirmed by EDX and XPS and the chemical state of the Pd was examined by XPS. The high resolution Pd XPS spectrum, shown in Figure 2-23, shows binding energy of Pd3d5/2 to be 337.2 eV which is characteristic of PdO, the most active phase of Pd for L-T MOX.168  78   Figure 2-23: High resolution Pd 3d XPS spectra of Pd/Comp3-50%Fe.  Wet impregnation of Pd does not alter the surface area (95 m2/g for Pd/Comp3-50%Fe and 94 m2/g for Comp3-50%Fe) or morphology of the support, as seen in Figure 2-24b. However, Figure 2-24a shows rod and sphere shaped particles that were not present in Comp3-50%Fe. This shows at least some of the PdO precipitated as nanoparticles rather than depositing onto the support surface. It is also possible that some PdO is present in the form of small crystallites of similar size to the Comp3-50%Fe particles or finely dispersed as a layer on the surface. 79   Figure 2-24: TEM images of 10%PdComp3-50%Fe (scale bars = 100 nm).  10%PdComp3-50%Fe was tested for complete oxidation of methane at low temperatures and shows promising activity with a T50% of 307 ˚C and 100% conversion at 510 ˚C (Figure 2-25). However, it should also be noted that 10wt% Pd would render a catalyst too expensive for commercial use and so, while the results of this experiment were taken into consideration when designing further catalysts, these materials are not suitable candidates for commercial L-T MOX catalysts.  80   Figure 2-25: TPO curve for 10%Pd/Comp3-50%Fe.  2.3.6 Effects of a chloride precursor  It is known that chloride can inhibit L-T MOX over Pd-based catalysts and that chloride can poison ceria-based catalysts. Residual chloride from precursor salts has been shown to reduce the catalytic activity of Pd catalysts; however the reasons for this are not clear.90 Very little research has been undertaken to determine the effects of chloride on L-T MOX catalyzed by ceria-based catalysts. It is advantageous to have as many precursors as possible available for the synthesis of ceria, so I synthesized ceria from a known procedure using CeCl3"7H2O (Comp4).169 Comp4 has a surface area of 125 m2/g with a nitrogen adsorption isotherm typical of mesoporous solids and is CeO2 by PXRD. As shown in Figure 2-26, the catalyst had lower activity for L-T MOX than other pure ceria catalysts, potentially due to residual chloride and so CeCl3"7H2O was not used as a precursor towards L-T MOX catalysts. 100 200 300 400 500 600Temperature (˚C)020406080100Methane Conversion (%)81    Figure 2-26: TPO curve for Comp4.  2.4 Conclusions  A number of different ceria-based materials were synthesized and characterized and several of these materials were tested for activity in L-T MOX. This study has shown that, while surface area is beneficial for catalytic activity, it is not the most important property. Indeed, plate-like ceria materials with a surface area of 129 m2/g show higher activity for L-T MOX than ceria prepared through a precipitation method with a surface area of 210 m2/g. Doping plate-like CeO2 did not significantly change its activity for L-T MOX, however 50% doping of high surface area ceria with 50% Mn3+ and Fe3+significantly increased the catalytic activity. Impregnation of high surface area ceria with 50 mol % Fe3+ with 10 weight % Pd gave rise to a catalyst with high activity, but it is not suitable for industrial applications unless the amount of Pd can be significantly reduced while maintaining activity. 82  A number of variables can affect the catalytic activity of mesoporous ceria, including synthetic technique, solvent, cerium source and doping level. This chapter has demonstrated the varied and unpredictable nature of ceria-based catalysts and highlighted the complexity of creating new catalytic materials. The unexpected finding that the presence of a second, redox active metal oxide in ceria-based catalysts is more advantageous than a high surface area is important for the rational design of new catalysts for L-T MOX.    83  Chapter 3: Controlling the Morphology of Nanostructured CeO2  3.1 Introduction  The synthesis of ceria with well-defined morphologies is an important area of research for advancing the use of ceria as an oxidation catalyst as it gives rise to novel, reproducible materials with high surface areas and good catalytic activity. Numerous morphologies of ceria have been reported including mesoporous structures,124,127 nanocubes,28 nanorods,28,116,170 nanotubes,171,172 nanopolyhedra,28,173 and nanoflowers.113,114 There have been several studies published discussing morphology-dependent properties of ceria, confirming that nanoparticles with a defined structure can be used for different applications when compared to bulk ceria. For example, mesoporous materials (materials containing pores with a diameter of 2-50 nm) have a high surface area, increasing the number of available active sites for catalysis while the pore size is favourable for transport of the reactant molecules to the active sites.127 Ceria nanotubes with large cavities and thin walls show enhanced oxygen storage capacity (OSC) and reducibility.170 In addition, there have been several studies published describing the relationship between catalytic activity and crystal surface of ceria.28,170,174  Controlling the synthesis of ceria in one, two or three dimensions presents synthetic challenges and requires the fine-tuning of reaction conditions, often through trial and error. After nucleation takes place, the aggregation of crystallites must occur in a controlled manner 84  in order to be able to reproducibly direct their assembly into monodisperse nanoparticles with a particular morphology. Surfactants,124 hard templates,126,127 intermediates175 and hydrothermal conditions173 can be used to direct the growth of particles.126,127 Hollow spheres of metal oxides with mesoporous shells have received considerable attention because of their potential to be used in applications including catalysts, molecular sieves and host-guest systems.31,176,177 There have been few reports of porous, hollow nanospheres of ceria in the literature. The first synthesis of ceria hollow nanospheres was reported by Gao et al. in 2008153 and mesoporous hollow nanospheres using a hydrothermal method were first reported by Yang et al. in 2010.115 Synthetic routes to hollow nanospheres frequently employ energy intensive hydrothermal routes that are challenging to use on a large scale or templating methods that have low atom economy. Furthermore, the synthesis of doped ceria hollow spheres has rarely been achieved.178 Therefore, the development of new preparation methods to control its morphology is vital for tapping the full potential of ceria.  In this chapter, modifications of a published synthesis for ceria nanoflowers to produce Mn-doped ceria nanoflowers are described. In addition, new methods for preparing high surface area nanocrystalline ceria via a hydroxyapatite templated route and ceria nanorods via a surfactant templated route are reported. Finally, the first report of doped ceria mesoporous hollow nanospheres synthesized without the use of a hard template or hydrothermal methods is described.   85  3.2 Experimental 3.2.1 General  All solvents and reagents were purchased from commercial sources and used without further purification. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 Advance X-ray diffractometer in the Bragg-Brentano configuration, using Cu Kα radiation at 40 kV, 40 mA. Gas adsorption studies were performed using a Micromeritics Accelerated Surface Area & Porosity (ASAP) 2020 system. All samples were degassed for two hours under vacuum at 120 °C immediately prior to analysis. Barrett-Joyner-Halenda (BJH) pore size distributions were all calculated from the adsorption branch of the isotherm. FT-IR (Fourier transform infrared) spectra were recorded on powdered solids on a Thermo Scientific Nicolet 4700 spectrometer.  3.2.2 Microscopy  Transmission electron microscopy (TEM) images were collected on a Hitachi H7600 electron microscope operating at an accelerating voltage of 100 kV. Samples were prepared by suspending the powders in ethanol and then dropcasting them onto a carbon-coated copper TEM grid and air-drying. SEM images were obtained with a Hitachi S-4700 field emission scanning electron microscope. Samples were prepared using double-sided carbon tape or dropcasting the dispersion directly onto aluminum stubs. The samples were then sputter coated with 5 nm of gold. Energy dispersive X-ray analysis (EDX) was collected on a Hitachi S-2600N variable pressure scanning electron microscope (SEM) equipped with an X-86  ray detector coupled to Quartz Imaging Systems Xone software. Samples were analyzed without sputter coating. 1H and 13C NMR spectra were recorded on a BRUKER AV300 spectrometer, operating at 300 MHz and 76 MHz, respectively.  3.2.3 Preparation of CeO2 nanoflowers  Glucose (1.800 g, 10.00 mmol) and glacial acetic acid (0.900 g, 15.00 mmol) were first dissolved in water (80 mL) then Ce(OAc)3#xH2O (1.585 g, 5.000 mmol) was added. NH4OH(aq) (28 wt%) was added dropwise to this solution until a gel formed at pH ~10. After 5 h the solution was split between 8 x 20 mL Teflon lined autoclaves (10 mL in each), heated to 180 ˚C and maintained at this temperature for 3 d, after which the autoclaves were left to cool to room temperature. The reaction mixture was filtered and the solid was washed with water (6 x 20 mL) and ethanol (6 x 20 mL) consecutively and then dried overnight at 80 ˚C. The resulting solid was calcined at 600 ˚C (heating rate 20 ˚C min-1) for 6 h under nitrogen followed by calcination at 500 ˚C under air for 3 h to afford a pale yellow solid. This sample is referred to as Comp5.  3.2.4 Preparation of doped CeO2 nanoflowers  Glucose (10.00 mmol, 1.800 g) and glacial acetic acid (15.00 mmol, 0.900 g) were first dissolved in water (80 mL) then a total of 5.000 mmol of Ce(OAc)3#xH2O and Mn(OAc)2#4H2O were added in proportion with Mn2+ molar fraction ranging from 0.1 to 0.5. Ammonia solution (28 wt%) was added dropwise to this solution until a gel formed at pH 87  ~10. After 5 h, the solution was split between 8 x 20 mL Teflon lined autoclaves (10 mL in each), heated to 180 ˚C and maintained at this temperature for 3 d after which the autoclaves were left to cool to room temperature. The reaction mixture was filtered and the solid was washed with water (6 x 20 mL) and ethanol (6 x 20 mL) consecutively and then dried overnight at 80 ˚C. The resulting solid was calcined at 600 ˚C (heating rate 20 ˚C min-1) for 6 h under nitrogen followed by calcination at 500 ˚C under air for 3 h. These samples are referred to as Comp5-X%Mn.  3.2.5 Preparation of ceria-coated hydroxyapatite  Ca10(PO4)6(OH)2 nanopowder (<200 nm particle size, 0.400 g, 0.800 mmols) was added to water (15 mL) and sonicated for 10 minutes. CTAC (25 wt% in water, 2.0 mL) was added and the solution stirred for 10 min. NH4OH (28 wt% in water, 2.0 mL) and Ce(NO3)3#6H2O (0.174 mg, 0.400 mols or 0.348 mg, 0.800 mmols) in 15 mL of water was added to the solution and the reaction mixture turned cloudy and peach coloured. The reaction mixture was stirred at room temperature for 24 h then the solids were collected and washed with water (20 mL) then ethanol (20 mL) via centrifuge (15 min, 4500 rpm). The solids were dried overnight at 80 ˚C then calcined at 500 ˚C for 5 h under air to give a pale green powder. These samples are referred to as Comp6-xCe where x is mols of Ce/mols of HAp.   88  3.2.6 Removal of hydroxyapatite from ceria-coated hydroxyapatite  Comp6-xCe (0.250 mg) was stirred in 50 mL of 1 M HCl for 24 h. The solid was then collected via centrifugation (15 min, 4500 rpm) and washed with 2 x 20 mL of water and 2 x 20 mL of ethanol consecutively to give a yellow solid. These samples are labeled Comp7-xCe where x corresponds to the starting material nomenclature.   3.2.7 Preparation of CTAC templated ceria nanorods  CTAC (25 wt% in H2O, 1.0 mL) was added to 15 mL of water and stirred for 10 min. NH4OH (28 wt% in water, 2.0 mL) and Ce(NO3)3#6H2O (0.174 mg, 0.400 mmols) were added and the reaction turned cloudy and peach coloured. The reaction mixture was stirred at room temperature for 24 h then the solids were collected and washed with water (20 mL) then ethanol (20 mL) via centrifugation (15 min, 4500 rpm). The solids were dried overnight at 45 ˚C then calcined at 500 ˚C for five h under air to give a yellow powder. This sample is referred to as Comp8.    89  3.2.8 Preparation of hollow ceria spheres  Ethylene glycol (120 mL) and HNO3 (6.0 mL, 21.4 M) were heated at 155 ˚C in a round bottom flask attached to a condenser, without stirring. During this time a brown gas evolved and the solution turned pale yellow. After 1 h, Ce(NO3)3#6H2O (2.605 g, 6.000 mmols) was added and the reaction mixture was heated for a further 30 min, during which time an off-white precipitate formed. The mixture was cooled to room temperature and the precipitate collected via centrifugation (45 min, 3500 rpm). The solid was washed with 2 x 20 mL ethanol then dried at 60 ˚C overnight to give an off-white solid. Yield: 0.162 g. This sample is referred to as Comp9. The product was then calcined at 500 ˚C for 4 h under air to give a yellow solid.  3.2.9 Preparation of hollow, doped ceria spheres  Ethylene glycol (120 mL) and HNO3 (6.0 mL, 21.4 M) were heated at 155 ˚C in a round bottom flask attached to a condenser, without stirring. During this time a brown gas evolved and the solution turned pale yellow. After 1 h a total of 6.000 mmols of Ce(NO3)3#6H2O and M(OAc)x"YH2O (where M= Fe, Mn or La) in proportion with a molar fraction for M of 0.1 and 0.33 was added and the reaction mixture was heated for a further 30 min, during which time an off-white precipitate formed. The mixture was cooled to room temperature and the precipitate collected via centrifugation (45 min, 3500 rpm). The solid was washed with 20 mL ethanol then dried at 60 ˚C overnight to give an off-white solid. These samples are 90  referred to as Comp9-X%M where M is the dopant metal. The product was then calcined at 500 ˚C for 4 h under air to give a yellow solid.  3.3 Results and discussion 3.3.1 CeO2 nanoflowers  Flower-like mesoporous ceria microspheres with a BET surface area of 181 m2/g have been prepared by Li et al. via a hydrothermal route with the aid of glucose and acrylic acid.114 The microspheres are composed of interconnected, mesoporous nanosheets. It is also possible to dope these microspheres with La and Pr. In line with previous observations in this thesis, the surface area of the microspheres decreases as the amount of doping increases. In this study, the synthetic route employed by Li et al. was successfully reproduced using cerium acetate in place of cerium nitrate and acetic acid in place of acrylic acid to give high BET surface area (177 m2/g) microspheres which can be seen by TEM (Figure 3-1). It is clear from these results that in this case, the cerium counterion has little or no effect on the resulting ceria.  The flower-like microspheres were doped with Mn using mixtures of cerium acetate and manganese acetate. Figure 3-1 shows the TEM images of Comp5-10%Mn, Comp5-20%Mn and Comp5-50%Mn microspheres with uneven edges, consistent with the concept of interweaved nanosheets. Comp5 appears to be composed of finer sheets than the doped samples, suggesting that doping may cause some structural collapse or aggregation.   91   Figure 3-1: TEM images of Comp5, Comp5-10%Mn, Comp5-20%Mn and Comp5-50%Mn (scale bars = 500 nm).  The PXRD patterns of ceria microspheres with various Mn doping contents are shown in Figure 3-2. The results show diffraction peaks corresponding to the ceria cubic structure with no diffraction peaks of manganese oxide. However, unlike Comp3-50%Mn, there is no observable peak shift suggesting that the lattice parameter has not changed due to doping with Mn3+ ions. Mn was present in all doped samples by EDX spectroscopy. It is unlikely that manganese oxide is present in an amorphous form after calcination at 500 ˚C under air.179 92   Figure 3-2: PXRD pattern of Comp5 (purple), Comp5-10%Mn (green), Comp5-20%Mn (blue) and Comp5-50%Mn (orange). != CeO2 (JCPDS-34-0394).  The BET surface areas of Comp5, Comp5-10%Mn, Comp5-20%Mn and Comp5-50%Mn are 177 m2/g, 157 m2/g, 123 m2/g and 126 m2/g, respectively, showing that the surface area of the microspheres decreases with increasing Mn doping levels until 20% doping, with Comp5-20%Mn and Comp5-50%Mn having similar surface areas. The isotherms produced by Comp5 and Comp5-10%Mn are type IV with a H3 hysteresis loop, indicative of plate-like particles with slit-like pores (Figure 3-3). However, the Comp5-20%Mn and Comp5-50%Mn isotherms have more type II character, suggesting the pores are large with a broad size distribution. A peak coresponding to a pore size of ~ 4 nm can be seen in the BJH pore size distribution curves for Comp5 and Comp5-10%Mn, but is absent for Comp5-20%Mn 93  and Comp5-50%Mn (Figure 3-4). Again, this suggests that doping with Mn has disrupted the morphology of the spheres and caused pore collapse.    Figure 3-3: N2 adsorption-desorption isotherms for Comp5 (purple), Comp5-10%Mn (green), Comp5-20%Mn (blue) and Comp5-50%Mn (orange). Solid lines represent adsorption and dashed lines represent desorption.    94   Figure 3-4: BJH pore size distribution curves for a) Comp5, b) Comp5-10%Mn, c) Comp5-20%Mn and d) Comp5-50%Mn.   95  3.3.2 Ceria-coated hydroxyapatite  Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) has several favourable properties for use as a hard template. First, it is inexpensive and can be synthesized on the nanometer scale with high levels of monodispersity. Second, it is soluble in dilute acids and thus, it can be easily removed from templated materials. Hydroxyapatite-based materials have also shown activity for oxidation reactions,180,181 including the partial oxidation of methane,182,183 and so investigating materials that combine hydroxyapatite and ceria could lead to new effective methane oxidation catalysts.  Commercial hydroxyapatite with a spherical morphology and irregular size of less than 200 nm, as shown in Figure 3-5, was used to template ceria. The method used is similar to one reported in the literature for the synthesis of porous hollow silica nanostructures.184 Williamson et al. used hydroxyapatite nanoparticles of various morphologies as hard templates and cetyltrimethylammonium chloride (CTAC, Figure 3-6) as a directing agent for the formation of mesopores in the silica. The authors note that, while the mechanism for the formation of dual templated mesoporous silica remains unclear, two routes have been suggested. Tan and Rankin proposed that aggregates of silica and CTAC adsorb onto the template surface,185 while Wang et al. suggest that CTAC, which has a positive charge, adsorbs onto the negatively charged surface of the template before the subsequent deposition of the silicate species.186  96   Figure 3-5: TEM image of commercial hydroxyapatite. Scale bar = 100 nm.   Figure 3-6: Molecular structure of CTAC  A similar method was used to determine whether ceria could be deposited onto hydroxyapatite, creating hollow spheres of mesoporous ceria with high surface area. Various molar ratios of Ce:HAp were used, with 1:2 (Comp6-0.5Ce) and 1:1 (Comp6-1Ce) ratios giving the most promising results by TEM and SEM imaging. As shown in Figure 3-7, after calcination the texture of the HAp surface has been altered, suggesting that the ceria has deposited onto the surface. However, from these images it cannot be determined whether the ceria has completely coated the surface or has formed islands of aggregates. The PXRD patterns of Comp6-0.5Ce and Comp6-1Ce both show peaks corresponding to commercial HAp and CeO2.  97   Figure 3-7: TEM images of a) Comp6-0.5Ce, b) Comp6-1Ce and c) SEM image of Comp6-1Ce. TEM scale bars = 100 nm and SEM scale bar = 2 µm.    Figure 3-8: PXRD patterns of commercial HAp (yellow), Comp6-0.5Ce (blue) and Comp6-1Ce (green). != CeO2 (JCPDS-34-0394).  98  Nitrogen adsorption measurements for Comp6-0.5Ce and Comp6-1Ce give BET surface areas of 66 m2/g and 58 m2/g, respectively. The BET isotherms show a hysteresis loop suggesting porosity while the BJH pore size distribution curves show a small pore volume related to pores with diameters of less than 5 nm. It is likely that these pores originate from CTAC templating, however, the pore volume and surface area are low due to the majority of pores collapsing during calcination.    Figure 3-9: a) N2 adsorption-desorption isotherms, and b) and c) BJH pore size distribution curves for Comp6-0.5Ce (blue) and Comp6-1%Ce (green). Solid lines represent adsorption and dashed lines represent desorption.   99  3.3.3 Hydroxyapatite template removal  In an attempt to synthesize hollow spheres of ceria, Comp6-0.5Ce and Comp6-1Ce were stirred in 1 M HCl to try to selectively etch the HAp while leaving the ceria intact (samples called ceria Comp7-0.5Ce and Comp7-1Ce respectively). As can be seen in Figure 3-10, the PXRD pattern of the resulting compound contains only peaks corresponding to CeO2, demonstrating that the crystalline HAp was successfully removed by dissolution in HCl.    Figure 3-10: PXRD patterns of Comp7-0.5Ce (blue) and Comp7-1Ce (green). != CeO2 (JCPDS-34-0394).  Unfortunately, removal of the HAp template resulted in collapse of the CeO2 and did not yield any hollow spheres. This indicates that the ceria had formed small discrete particles on the surface of the HAp rather than a continuous shell. TEM images of Comp7-0.5Ce and 100  Comp7-1Ce show aggregates of small nanoparticles, mainly with no well-defined morphology (Figure 3-11). However, a few aggregates appear to be porous distorted spheres, seen in Figure 3-11b, and probably arise from the ceria nanoparticles on the surface of one HAp sphere aggregating together upon template removal. These materials show high BET surface areas of 129 m2/g for Comp7-0.5Ce and 109 m2/g for Comp7-1Ce. There is a significant increase in surface area after HAp template removal, especially considering that the density of HAp is lower than that of ceria. The BET isotherm is type IV with a H2 hysteresis loop, as show in Figure 3-12a. This is indicative of a highly interconnected pore system, which in this case is likely to be the result of voids forming as the structure collapses and the irregularly shaped nanoparticles aggregate together. The BJH pore size distribution curves (figures Figure 3-12b and c) show that the pores have a diameter of around 5 nm.   Figure 3-11: TEM images of a and b) Comp7-0.5Ce and c) Comp7-1Ce. Scale bars = 100 nm.  101   Figure 3-12: a) N2 adsorption-desorption isotherms, and b) and c) BJH pore size distribution curves for Comp7-0.5Ce (blue) and Comp7-1%Ce (green). Solid lines represent adsorption and dashed lines represent desorption.  3.3.4 CTAC templated ceria nanorods  In order to determine whether the HAp templating was having any effect on the overall morphology of Comp7-0.5Ce and Comp7-1Ce, the reaction was repeated without the addition of HAp to give Comp8. As can be seen in Figure 3-13, the final product was ceria comprised mainly of nanorods with lengths of several hundred nanometers. Smaller nanoparticles are also seen attached to the nanorods. It is likely that, in the presence of the HAp template, small nanoparticles adsorb onto the surface of the HAp, preventing further aggregation. In the case of Comp8, the nanoparticles aggregate together to make rods, 102  formed due to CTAC acting as a capping agent. This is the first time ceria nanorods have been synthesized using CTAC as a structure-directing agent.   It is proposed that, while HAp templating did not successfully lead to the formation of hollow spheres, HAp-CTAC-ceria structures prevent the ceria from extensive aggregation. The nanoparticulate structure of the ceria after removal of the template gives it a high surface area.    Figure 3-13: PXRD pattern and TEM images of Comp8. != CeO2 (JCPDS-34-0394). Scale bars = 100 nm.  103  3.3.5 Hollow ceria spheres  Ethylene glycol (EG) has many beneficial properties as a solvent for metal nanoparticle synthesis. Its boiling point of 195 ˚C allows reactions to be carried out at relatively high temperatures and its high dielectric constant of 37 enhances the solubility of inorganic salts. EG is also a strong reducing agent and so can reduce metal ion precursors. EG has been used previously to synthesis ceria nanoparticles by Ho et al. who heated cerium ammonium nitrate, poly(vinylpyrrolidone) (PVP) and EG under reflux for various lengths of time.175 Nanospheres were obtained after four hours of heating and larger rods were obtained after 24 hours of heating. The authors suggest that the spheres obtained at the start of the reaction are composed of ceria, produced by the decomposition of the cerium ammonium nitrate. The Ce4+ cations of the ceria are reduced to Ce3+ by EG then further react with products from EG oxidation (Scheme 3-1) to form cerium formate (CF).   Scheme 3-1: Oxidation of ethylene glycol to formic acid.  The spherical and rod morphology is derived from capping by the PVP surfactant. However, no hollow spheres were observed in this reaction. Herein, the first non-hydrothermal or solvothermal, template-free route to hollow ceria spheres is reported.  104  EG and nitric acid (HNO3) were heated at 155 ˚C for one hour then cerium nitrate was added and the temperature was maintained at 155 ˚C for a further 30 minutes. During the reaction brown gases, assumed to be NOx, evolved and an off-white solid, Comp9, precipitated out. FT-IR spectroscopy of the solid is shown in Figure 3-14a. The broad band near 3200 cm-1 is assigned to OH groups from water. The peak around 1100 cm-1 is due to surface adsorbed EG.187 The peaks at 1570 cm-1 and 1300 cm-1 correspond to the stretching mode of C=O in an oxalate group, while the peak at 790 cm-1 corresponds to its bending mode. These peaks are typical for metal fomate salts. The PXRD of Comp9, shown in Figure 3-14b, indicates an amorphous material. These results suggest that the sample is amorphous cerium formate (CF, Ce(CHOO)3).  Figure 3-14: a) FT-IR spectrum and b) PXRD pattern of Comp9.  TEM images of Comp9 show spheres with diameters of around 250 and 350 nm (Figure 3-15). The darker rim compared with the brighter centers of these spheres suggests that the spheres are hollow. They appear to be composed of smaller nanoparticles, suggesting the hollow spheres may have been produced by Ostwald ripening.188 Ostwald ripening has been 105  observed previously for the formation of ceria hollow spheres via a hydrothermal route using CeCl3"7H2O as a precursor.115 Core-shell particles could be isolated from this hydrothermal route, however no core-shell particles were observed in the EG-mediated non-hydrothermal reaction, suggesting the hollow spheres are produced through a core hollowing Ostwald ripening process. While no non-hollow spheres were seen in TEM images of Comp9, they were seen in doped samples described in section 3.3.6. It is plausible that the formation of hollow spheres occurs very rapidly due to the presence of HNO3 and the one hour pretreatment of HNO3 and EG. The highly oxidizing HNO3 will convert Ce3+ to Ce4+, which in turn reacts with EG oxidation products formed during the pre-treatment step, meaning the hollow spheres form quickly and non-hollow particles are not isolated.  TEM and SEM images show Comp9 retains its morphology during calcination, to give hollow spheres with porous shells (Figure 3-15c,d,e and f). The PXRD pattern of these spheres can be indexed to CeO2, as shown in Figure 3-16a. Surprisingly, Comp9 has a low BET surface area of only 22 m2/g. This can be explained by the lack of extended pore network, with the main surface area being the outside and inside surfaces of the shells. The isotherm, shown in Figure 3-16b, is an intermediate between types II and IV, related to the presence of large mesopores and macropores with a broad size distribution. Pores are seen in TEM images between the crystallites that comprise the shell and the hollow interiors of the spheres may also contribute to this hysteresis loop.  In an attempt to elucidate the mechanism for the formation of hollow CF spheres, the residual liquid was investigated by 1H and 13C NMR spectroscopy (Figure 3-17). The 1H spectrum 106  showed peaks at 4.45 and 3.39 ppm, while the 13C spectrum showed a single peak at 62.9 ppm. Both of these are consistent with wet EG. For comparison, 1H and 13C NMR spectra were collected for a solution of EG containing 5% water and the resulting spectra were very similar to those of the supernatant Comp9 (see Appendix Figure B-1).  107   Figure 3-15: TEM images of a and b) Comp9 before calcination, c and d) Comp9 after calcination (scale bars = 100 nm), and e and f) SEM images of Comp9 after calcination (scale bars = 2 µm). 108   Figure 3-16: a) PXRD pattern of Comp9 after calcination. != CeO2 (JCPDS-34-0394). b) N2 adsorption-desorption isotherm for Comp9. Solid line represents adsorption and dashed line represents desorption.   Figure 3-17: 1H NMR and 13C NMR spectra of liquid remaining after synthesis of Comp9 (25 ˚C).    109  3.3.6 Hollow, doped ceria spheres  In order to test whether the hollow spheres produced via an EG-mediated synthesis would retain their morphology when dopant cations are added, Fe3+, Mn3+ and La3+ nitrate salts were added to the reaction mixture to give dopant:Ce mol% values of 10% and 30% in the final product. The IR spectra of all the doped as-synthesized materials, shown in Figure 3-18, are similar to that of the undoped sample. The PXRD patterns (Figure 3-19) also show that the samples are amorphous. These results suggest that metal formate salts have been formed.    Figure 3-18: FT-IR spectra of a) Comp9-10%La, b) Comp9-30%La, c) Comp9-10%Fe, d) Comp9-30%Fe, Comp9-10%Mn, and d) Comp9-30%Mn. 110   Figure 3-19: PXRD pattern of a) as-synthesized Comp9-10%Mn (green), Comp9-10%Fe (red) and Comp9-10%La (purple), b) as-synthesized Comp9-30%Mn (green), Comp9-30%Fe (red) and Comp9-30%La (purple), c) calcined Comp9-10%Mn (green), Comp9-10%Fe (red) and Comp9-10%La (purple) and d) calcined Comp9-30%Mn (green), Comp9-30%Fe (red) and Comp9-30%La (purple). != CeO2 (JCPDS-34-0394).  TEM images of Comp9-10%Mn and Comp9-30%Mn show almost spherical particles, which appear not to be hollow (Figure 3-20). The Fe-doped product does not have a homogeneous morphology, while the La-doped product forms hollow spheres, which are indistinguishable from the undoped sample. The ionic radius of La3+ is similar to that of Ce3+, 111  meaning the CF structure will experience minimal disruption with La doping, whereas Mn or Fe doping appears to have either caused the structure collapse or prevented it from forming.    Figure 3-20: TEM images of as-synthesized a) Comp9-10%Mn, b) Comp9-10%Fe c) Comp9-10%La d) Comp9-30%Mn, e) Comp9-30%Fe and f) Comp9-30%La. Scale bar = 100 nm  TEM images of Comp9-10%Mn and Comp9-30%Mn after calcination (Figure 3-21a and b) show mainly non-hollow spheres with a few hollow spheres, giving further evidence of the Ostwald ripening process. Doping may slow the Ostwald ripening process by adding disruption to the CF crystallites forming. TEM images of Comp9-10%La and Comp9-30%La after calcination (Figure 3-21c and d) show that the morphology has been maintained and hollow spheres of 10 and 30% doped ceria are formed.   112   Figure 3-21: TEM images of calcined a) Comp9-10%Mn, b) Comp9-10%La c) Comp9-30%Mn and d) Comp9-30%La. Scale bar = 100 nm.  The synthesis of Comp9-30%La was attempted at 150, 170 and 180 ˚C to test how sensitive to temperature this reaction is. The TEM images in Figure 3-22 show that hollow spheres are produced when the reaction is heated to 150 and 170 ˚C; however, at 180 ˚C small crystallites with no defined morphology are produced. The PXRD and IR data are the same for all samples regardless of the temperature they were synthesized at, meaning the temperature affects the morphology but not the chemical composition.   113    Figure 3-22: TEM images of Comp9-30%La prepared at a) 150 ˚C, b) 170 ˚C and c) 180 ˚C. Scale bar = 100 nm,  3.4 Conclusions  Several new, straightforward ways for synthesizing ceria-based nanostructures have been developed. It was demonstrated that a literature procedure for the synthesis of ceria nanoflowers can be succesfully altered to utilize different precursors and the ceria can be doped with up to 30% manganese while still retaining the nanoflower morphology. Ceria-coated hydroxyapatite nanospheres were synthesized for the first time, and removal of the hydroxyapatite through dissolution in HCl yields high surface area ceria nanoparticles. The same reaction carried out without the addition of hydroxyapatite yields the first report of CTAC templated ceria nanorods. Finally, the first non-hydrothermal route to mesoporous, hollow spheres of cerium formate is reported. These spheres can be doped with up to 30% La and calcined to give ceria or La-doped ceria while still retaining their morphology. Future work will aim to try to fully understand the mechanism behind the formation of these materials and further develop them for use as oxidation catalysts.  114  Chapter 4: Mesoporous Doped Cerium Oxide/Cobalt Oxide Mixed Metal Catalysts*  4.1 Introduction  In chapter 2 it was demonstrated that ceria combined with a second redox active metal oxide produces catalysts that have higher activity for low-temperature methane oxidation (L-T MOX) than ceria alone. Recently there has been substantial interest in Co-Ce composite oxides as catalysts for the oxidation of CO, N2O, volatile organic compounds (VOCs), and propene.189–192 It has been shown that the combination of cerium and cobalt oxides leads to higher activity when compared to the individual component oxides in these reactions due to synergistic effects between the two metal oxides.167 There have been relatively few studies using Co-Ce composite oxides as catalysts for L-T MOX. Materials prepared by co-precipitation methods have shown high activity but have low surface areas.193,194 Li et al. successfully prepared a Co3O4/CeO2 composite oxide using a modified citrate sol-gel method to achieve high surface area materials reaching a light-off temperature (T50%) as low as 401 ˚C.193 However, there is a significant need to explore new Ce-Co-O composite materials with tunable structures and properties beneficial for L-T MOX catalysis.                                                 * Portions of this chapter have been published as: S. M. Vickers, R Gholami, K. J. Smith, M. J. MacLachlan, “Mesoporous Mn- and La-Doped Cerium Oxide/Cobalt Oxide Mixed Metal Catalysts for Methane Oxidation.” ACS Appl. Mater. Interfaces 2015, 7, 11460–11466. 115  Materials containing mesopores are promising candidates for use as L-T MOX catalysts because of their large surface area, interconnected pores and controllable pore wall compositions. KIT-6 is a mesoporous silica material with a three-dimensional cubic structure (space group Ia3d) that is synthesized using a non-ionic triblock copolymer template.43 Non-ionic triblock copolymers were first patented in 1973 and are known under the trademark Pluronics®46. They consist of hydrophobic polypropylene oxide (PPO) chains and hydrophilic polyethylene oxide (PEO) chains. The family of Pluronics® contains a number of different polymers with various PPO/PEO ratios and hence varying molecular weights. The nomenclature used for Pluronics® describes the appearance, molecular weight and PPO/PEO ratio of each polymer. The name consists of a letter defining the appearance of the polymer: F for flake, P for paste and L for liquid. The first one or two numbers multiplied by 300 approximates the molecular weight of the PPO block and the last number multiplied by 10 approximates the PEO weight fraction. In the case of KIT-6 the Pluronic® used is P123, a paste with ~3600 g/mol PPO and 30 wt% PEO.  In aqueous solutions P123 self-assembles into micelles with a hydrophobic core of PPO chains surrounded by a hydrophilic shell of PEO chains. KIT-6 is synthesized in water with n-butanol as a cosolvent. KIT-6 has a bi-continuous pore network with tunable pore sizes in a range of 4-12 nm, a large surface area and thick walls. These properties have led to KIT-6 attracting attention for applications in catalysis,195,196 adsorption and separation197 and as a template for other mesoporous materials.127,198 Various mesoporous metal oxides have been successfully templated using KIT-6.51,67,126,199 Co3O4 templated with KIT-6 shows high activity for CO oxidation, resulting from the high surface area and open pore system of the 116  material.200,201 Mesoporous cobalt oxide materials templated by KIT-6 have also shown high activities for various hydrocarbon oxidations.202–204 However, an extensive literature search did not uncover any reports of these materials being evaluated for L-T MOX.   In this chapter a straightforward and scalable method to make catalysts based on templating with KIT-6 is developed. Cobalt oxide was prepared in the channels of KIT-6 and released after the silica was etched. The mesoporous Co3O4 was then used to template CeO2, La-doped CeO2, and Mn-doped CeO2 within its pores. These hierarchically organized catalysts show high activity for L-T MOX.  4.2 Experimental 4.2.1 General  All solvents and reagents were purchased from commercial sources and used without further purification. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 Advance X-ray diffractometer in the Bragg-Brentano configuration, using Cu Kα radiation at 40 kV, 40 mA. Crystallite size was estimated from the broadening of the (440) peak for Co3O4 and the (111) peak for CeO2 using the Scherrer equation. X-ray photoelectron spectroscopy (XPS) was carried out on a Leybold Max200 spectrometer using an aluminum Kα X-ray source for samples containing Mn and a magnesium Kα X-ray source for all other samples, operating at a base pressure of 1 × 10−9 Torr. Initial survey scans were acquired with a pass energy of 192 eV, while higher resolution scans were acquired with a pass energy of 48 eV. XPS spectra were deconvoluted using the XPSPEAK program by curve fitting with a mixed 117  Gaussian-Lorentzian function after the Shirley-type background subtraction. Gas adsorption studies were performed using a Micromeritics Accelerated Surface Area & Porosity (ASAP) 2020 system. All samples were degassed for two hours under vacuum at 120 °C immediately prior to analysis. BJH pore size distributions were all calculated from the adsorption branch of the isotherm.  4.2.2 Microscopy  Transmission electron microscopy (TEM) images were collected on a Hitachi H7600 electron microscope operating at an accelerating voltage of 100 kV. Samples were prepared by suspending the powders in ethanol and then dropcasting them onto a carbon-coated copper TEM grid and air-drying. SEM images were obtained with a Hitachi S-4700 field emission scanning electron microscope. Samples were prepared using double-sided carbon tape or dropcasting the dispersion directly onto aluminum stubs. The samples were then sputter coated with 5 nm of gold. Energy dispersive X-ray analysis (EDX) was collected on a Hitachi S-2600N variable pressure scanning electron microscope (SEM) equipped with an X-ray detector coupled to Quartz Imaging Systems Xone software. Samples were analyzed without sputter coating.   118  4.2.3 Preparation of KIT-6  Mesoporous silica, KIT-6, was synthesized according to the established method.43 P123 (2.00 g) was dissolved in a mixture of H2O (72 mL) and concentrated HCl (4.3 mL). 2.00 g of n-BuOH was added and the mixture was stirred at 35 ˚C. After 1 h of stirring TEOS (4.300 g) was added and the mixture continued to be stirred at 35 ˚C. After 24 h the mixture was heated to 100 ˚C inside a sealed Teflon bottle for 24 h. The solids were collected and dried via suction then stirred in a mixture of H2O (150 mL) and concentrated HCl (10 mL) to remove the P123. The solids were collected, washed with 3 x 50 mL H2O and 3 x 50 mL EtOH then dried via suction. The solid was calcined at 550 ˚C under air for 4 hours to give a white solid. Yield: 0.872 g  4.2.4 Preparation of mesoporous Co3O4  Mesoporous Co3O4 was synthesized using a procedure similar to that used by Yue et al.205 KIT-6 (2.075 g) was ground with Co(NO3)2⋅6H2O (4.016 g, 13.79 mmol) using a pestle and mortar then heated to 500 ˚C at a ramp rate of 1 ˚C/min in a muffle furnace. The furnace was maintained at 500 ˚C for 3 h before cooling under ambient conditions. During the slow heating, Co(NO3)2⋅6H2O melts at 55 ˚C and enters the KIT-6 pores via capillary action before it decomposes to Co3O4 at 74 ˚C. After cooling, the silica was then removed by stirring the product in 2 M NaOH for 2 h. The resulting black solid was collected and washed with H2O (2 × 20 mL) and EtOH (2 × 20 mL) via centrifugation then dried overnight at 70 ˚C to give a black powder.  Yield: 1.233 g.  This sample is referred to as Comp10 . 119  4.2.5 Preparation of Co3O4/CeO2 composite  As-synthesized Comp10 (0.400 g, 1.66 mmol) was placed in a Schlenk flask and evacuated for 1 h. Ce(NO3)3⋅6H2O (0.220 g, 0.506 mmol) was dissolved in 5 mL of EtOH then added dropwise via syringe. The sample was then left to dry under vacuum overnight and calcined at 500 ˚C under air for 5 h to give a black powder. Yield: 0.420 g. This sample is herein referred to as Comp11.  4.2.6 Preparation of Co3O4/doped-CeO2 composite.   As-synthesized Comp10 (0.400 g, 1.66 mmol) was placed in a Schlenk flask and evacuated for 1 h. Ce(NO3)3⋅6H2O (0.198 g, 0.469 mmol) and La(NO3)3⋅6H2O (0.022 g, 0.051 mmol) or Mn(NO3)2⋅4H2O (0.013 g, 0.051 mmol) were dissolved in 5 mL of EtOH then added dropwise via syringe. The sample was then left to dry under vacuum overnight and calcined at 500 ˚C under air for 5 h to give a black powder. These sample is referred to as Comp11-10%M where M = La or Mn.  4.2.7 Preparation of chiral nematic Co3O4  Chiral nematic silica templated with nanocrystalline cellulose206 (2.075 g) was ground with Co(NO3)2⋅6H2O (4.016 g, 13.79 mmol) using a pestle and mortar then heated to 500 ˚C at a ramp rate of 1 ˚C/min in a muffle furnace. The furnace was maintained at 500 ˚C for 3 h 120  before cooling under ambient conditions. After cooling, the silica was then removed by stirring the product in 2 M NaOH for 2 h. The resulting black solid was collected and washed with H2O (2 × 20 mL) and EtOH (2 × 20 mL) via centrifugation then dried overnight at 70 ˚C to give a black powder.  Yield: 1.053 g.  This sample is herein referred to as Comp12.  4.2.8 Methane oxidation testing  Temperature-programmed CH4 oxidation (TPO) done as described in section 2.2.11 of this thesis.  4.3 Results and discussion  KIT-6, a well-known mesoporous silica, was used as a template to construct novel hybrid catalytic materials. KIT-6 has large, uniform, easily accessible pores, which make it ideal for use as a template. Nitrogen adsorption measurements of the KIT-6 used as a template for mesoporous cobalt oxide show an H1 hysteresis loop and an average pore size of 7 nm (Figure 4-1a,b). The ordered pores, as seen by TEM in Figure 4-1c, afford a large surface area of 802 m2 g-1. The Co3O4 based catalysts templated with KIT-6 were prepared using the route shown in Scheme 4-1. First, KIT-6 was ground together with cobalt nitrate, then the composite was calcined to give a SiO2/Co3O4 material. Etching of the silica with NaOH(aq) afforded Comp10. In a second step, the pores of the Comp10 were infiltrated with CeO2 by dissolving Ce(NO3)3⋅6H2O in EtOH and injecting it into a Schlenk flask containing Comp10 121  under vacuum. Samples were also prepared with CeO2 enriched with La or Mn using a similar procedure.   Scheme 4-1: Synthesis of mesoporous Co3O4-based materials using KIT-6 as a template. a) (i) 2.075 g of KIT-6 ground with 4.016 g of Co(NO3)2⋅6H2O; (ii) calcination at 500 ˚C; (iii) etching in 2M NaOH(aq). b) (i) 0.220 g Ce(NO3)3⋅6H2O in 5 mL EtOH added to 0.400 g Co3O4 under vacuum; (ii) calcination at 500 ˚C. c) (i) 0.198 g of Ce(NO3)3⋅6H2O and 0.022 g of La(NO3)3⋅6H2O in 5 mL of EtOH added to 0.400 g Co3O4 under vacuum; (ii) calcination at 500 ˚C. d) (i) ) 0.198 g of Ce(NO3)3⋅6H2O and 0.013 g of Mn(NO3)2⋅4H2O in 5 mL of EtOH added to 0.400 g Co3O4 under vacuum; (ii) calcination at 500 ˚C.  122   Figure 4-1: a) N2 adsorption-desorption isotherm, b) BJH pore size distribution curves and c) TEM image of KIT-6.  PXRD patterns of the calcined catalysts are shown in Figure 4-2. Diffraction peaks associated with cubic Co3O4, space group Fd3m, were observed in all samples. Comp11, Comp11-10%La and Comp11-10%Mn also display peaks corresponding to cubic CeO2 with fluorite-like cubic structure. There are no diffraction peaks of manganese or lanthanum oxide, suggesting that Mn or La has entered into the CeO2 lattices rather than phase separating. No shift in the ceria diffraction peaks that one would typically expect for a solid solution were observed, but the peaks did become broader. This is consistent with previous reports of doped ceria.207 Crystallite sizes were estimated using the Scherrer equation, as shown in Table 4-1. 123   Figure 4-2: PXRD patterns of a) Comp10, b) Comp11, c) Comp11-10%La and d) Comp11-10%Mn. PXRD pattern of Co3O4 matches JCPDS-78-1969 for Co3O4 (!) and the PXRD pattern of CeO2 matches JCPDS-34-0394 (!).   Table 4-1: Crystallite sizes estimated using the Scherrer equation for Comp10, Comp11, Comp11-10%La and Comp11-10%Mn.   Crystallite size (Å) Catalyst Co3O4 CeO2 Comp10 134 - Comp11  123 82 Comp11-10%La 133 72 Comp11-10%Mn 125 73  124  The as-synthesized materials were examined by nitrogen adsorption/desorption measurements to determine the Brunauer-Emmett-Teller (BET) surface areas and the pore size distributions (Figure 4-3 and Figure 4-4). The BET surface area of the Comp10 decreases from 97 m2 g-1 to 68 m2 g-1 upon addition of Ce(NO3)3⋅6H2O followed by calcination. This relatively small decrease in surface area is due to partial filling of the pores with CeO2, which allows the surface area to remain relatively high. This is supported by TEM images, which show both filled and empty pores (Figure 4-6c). The confinement of most of the CeO2 within the Comp10 pores is thought to prevent the CeO2 from any significant sintering upon calcination at 500 ˚C and allows the Co3O4 and CeO2 to maintain intimate interactions in the solid state, interactions that are expected to be beneficial for catalysis. Comp11-10%La and Comp11-10%Mn have similar surface areas of 67 m2 g-1 and 55 m2 g-1, respectively.  Figure 4-3 shows N2 adsorption curves for all of the materials, indicating that they all show typical type IV isotherms, as defined by IUPAC.145 This shows that the materials do not lose their mesoporous structure when some of the pores are filled with doped or undoped CeO2.   125   Figure 4-3: N2 adsorption-desorption isotherms for a) Comp10, b) Comp11, c) Comp11-10%La and d) Comp11-10%Mn. Solid lines represent adsorption and dashed lines represent desorption.   126   Figure 4-4: BJH pore size distribution curves for a) Comp10, b) Comp11, c) Comp11-10%La and d) Comp11-10%Mn.  PXRD measurements (Figure 4-5) show peaks at low angles for all synthesized materials due to the ordered nature of the mesopores. KIT-6 itself has cubic Ia3d symmetry and this ordered structure is retained in the templated products. The pore size distribution curves of Comp10, Comp11, Comp11-10%La and Comp11-10%Mn are presented in . Comp10 has an average pore size of 4.4 nm, while Comp11, Comp11-10%La and Comp11-10%Mn have pore diameters of 4.8 nm, 7.0 nm and 7.3 nm, respectively. The doped CeO2 samples have larger pore sizes and greater pore size distributions, possibly due to the dopant disrupting the CeO2 lattice structure. Any CeO2 that does not enter the Comp10 forms small particles with pore-like voids between them. 127     Figure 4-5: Low angle PXRD spectra for a) Comp10, b) Comp11, c) Comp11-10%La and d) Comp11-10%Mn.  TEM of the samples show that for Comp10 the inverse KIT-6 morphology is maintained upon removal of the silica template, Figure 4-6. After addition of Ce(NO3)3⋅6H2O the material is calcined under air for 5 h and again the mesoporous morphology of the Comp10 remains intact in Comp11, with some of the pores filled with CeO2.     128   Figure 4-6: TEM images of a) KIT-6, b) Comp10, c) Comp10/Ce(NO3)3⋅6H2O d) Comp11  e) Comp11-10%La and f) Comp11-10%Mn.  The chemical composition of the new materials was examined by energy dispersive X-ray (EDX) spectroscopy. The EDX spectra from Comp10 show the elements cobalt and oxygen, with a trace amount of silicon remaining from the KIT-6 template. After the CeO2 was added, cobalt, oxygen and cerium are all observed by EDX of Comp11. Similarly, EDX measurements indicate that cobalt, oxygen, cerium and lanthanum or manganese are present 129  in Comp11-10%La and Comp11-10%Mn respectively. EDX mapping (Figure 4-7 and Figure 4-8) shows homogeneous dispersion of all elements present, which is expected as the resolution is not high enough to image the individual mesopores.   Figure 4-7: EDX mapping images of Comp11-10%La. 130   Figure 4-8: EDX mapping images of Comp11-10%Mn.  X-ray photoelectron spectroscopy (XPS) was also used to determine the chemical composition of the materials. Figure 4-9 shows the high resolution Ce 3d XPS spectra collected from Comp11, Comp11-10%La and Comp11-10%Mn. Following convention, two sets of multiplets u and v (3d3/2 and 3d5/2 spin–orbit components) have been labeled, with splitting of 18.4 eV, in agreement with the literature.193,208 Three pairs of peaks denoted as V/U, V’’/U’’, V’’’/U’’’ are assigned to Ce4+ species and arise from different Ce 4f electron configurations in the final states (Table 4-2). The couple V’/U’ is attributed to the electron configuration of the final state of the Ce3+ species.208–210 131   Figure 4-9: High resolution Ce 3d XPS spectra of a) Comp11, b) Comp11-10%La and c) Comp11-10%Mn.    132  Table 4-2: High resolution Ce 3d XPS binding energies and peak assignments for Comp11, Comp11-10%La and Comp11-10%Mn.       Binding energy (eV)       Catalyst v v' v'' v''' u u' u'' u''' Comp11  882.9 885.3 889.1 898.7 901.3 903.7 907.5 917.1 Comp11-10%La 883.1 885.9 889.8 898.8 901.5 904.3 907.5 917.2 Comp11-10%Mn 883.1 885.6 889.1 898.8 901.5 904.0 907.2 917.2 *The small difference in the binding energies could be due to different metal ions’ environments near the cerium sites as various dopants are added as well as errors in the fitting process.   The position of the Co 2p3/2 and 2p1/2 peaks in the XPS spectra for all samples confirms the presence of Co3O4, as illustrated in Figure 4-10 and Table 4-3.211,212 An extremely weak satellite structure symptomatic of shake-up from the minor Co2+ component can be seen in Co3O4, but it is completely absent from Comp11, Comp11-10%La and Comp11-10%Mn.213,214 These peaks are not strong enough to be assigned as the shake-up peaks characteristic of the Co2+ in CoO and it is likely that they arise from the very small amount of Co2+ present in mixed-valence Co3O4. XPS data for Comp11-10%La and Comp11-10%Mn also confirms the presence of La and Mn, respectively (Figure 4-11 and Figure 4-12).  133   Figure 4-10: High resolution Co 2d XPS spectra of a) Comp10, b) Comp11, c) Comp11-10%La and d) Comp11-10%Mn.  Table 4-3: High resolution Co 2p XPS binding energies for Comp10, Comp11, Comp11-10%La and Comp11-10%Mn. Catalyst Binding energy (eV) Comp10 780.9 796.2 Comp11  780.6 795.8 Comp11-10%La  780.5 795.7 Comp11-10%Mn 780.6 795.9  134   Figure 4-11: High resolution La 3d XPS spectrum of Comp11-10%La.    Figure 4-12: High resolution Mn 2p XPS spectrum of Comp11-10%Mn.  135  All of the materials prepared were investigated for LT-MOX catalytic activity. Interestingly, when tested for catalytic activity for L-T MOX, one batch of Comp10 reproducibly showed substantial activity at ~175 ˚C during the first catalytic test (45% methane conversion, Figure 4-13). This activity returned to 10% before rising again to 100% complete methane conversion at 475 ˚C. When the measurement was repeated with the same sample, this initial, low temperature peak was no longer present.   Figure 4-13: TPO curve of fresh Comp10 (blue) and used (purple).  Comp10 does not undergo any change in structure due to heating up to 200 ˚C (confirmed by variable-temperature PXRD, Figure 4-14) that could be causing this surprising catalytic activity. Subsequent analysis of these results showed that there was more carbon in the reactor exit gas than in the feed gas at 175 ˚C. Examination of the reactor did not reveal any sources for this carbon and, due to the fact that the peak was reproducible, it is assumed it is not due to any leaks in the reactor. However, the catalyst only lost mass due to water during TGA analysis up to 900 ˚C in air and no carbon was detected by elemental analysis. The 136  source of this carbon has not yet been identified, but it is an issue that should be noted in case it is encountered in future experiments.    Figure 4-14: Variable-temperature PXRD patterns of Comp10.  A second batch of Comp10 was synthesized and, when tested for catalytic activity for L-T MOX, showed high activity at low temperatures with 50% complete methane conversion (T50%) at 390 ˚C and the activity seen at low temperatures in the previous sample was not present (Figure 4-15; compared to T50% of 250 °C for a conventional precious metal-containing catalyst (7.7 wt% Pd/SiO2) tested under the same TPO reaction conditions215). While the complete sequence of elementary steps governing hydrocarbon oxidation on metal oxide surfaces is not completely understood, it is thought to occur through C---H activation with a simultaneous reduction of metal oxide surface sites.190,216 As seen by the very weak 137  nature of the Co2+ satellite peaks in the XPS spectrum, there is almost no Co2+ on the surface of the catalysts. Therefore, I propose that Co3+ sites on the surface of the Comp10 are reduced to Co2+ by activation of CH4, and may generate surface hydroxide ions. This is similar to the mechanism proposed for hydrocarbon oxidation over other metal oxides. 216,217   Figure 4-15: TPO curve of Comp10.  Comp11, Comp11-10%La and Comp11-10%Mn show high catalytic activity for materials that do not contain noble metals as seen in Figure 4-16 and Table 4-4, with the T50% at 400 ˚C for Comp11-10%La and 445 ˚C for Comp11-10%Mn. The weight hourly space velocity (WHSV) of 180,000 mL/g/h is significantly higher than most literature examples of similar catalysts, indicating that the materials described here are much more active for L-T MOX. At 100% conversion, the only products were CO2 and H2O. It is probable that the reduction in surface area associated with the addition of CeO2 or doped CeO2 causes the slight decrease in catalytic activity. It is also possible that Co3O4 catalysts containing CeO2 have metal-metal 138  interactions between the Co and Ce that alter the redox properties of the materials, with electron transfer between the two metal oxides preventing the reduction of the Comp10 surface at low temperatures. While the introduction of CeO2 and doped CeO2 did not have any significant effect on catalytic activity, it is important to note that doping was successful and the high catalytic activity maintained. This will allow the catalysts to be more easily modified to increase chances of success in various applications for which L-T MOX is required. These catalytic materials could also serve as active supports for noble metals.  Figure 4-16: TPO curves for Comp11 (yellow), Comp11-10%La (blue) and Comp11-10%Mn (pink).  139       Table 4-4: Catalytic performance of the materials described in this chapter and materials from the literature.  Composition Preparation method T50% Catalyst mass (mg)  WHSV (mL/g/h) CH4 in feed gas (Vol.%) Reference Comp10 As described 390 ˚C 100 180,000  0.1 This work Comp11  As described 407 ˚C 100 180,000  0.1 This work Comp11-10%La As described 400 ˚C 100 180,000  0.1 This work Comp11-10%Mn As described 445 ˚C 100 180,000  0.1 This work 30%Co3O4–CeO2 Co-precipitation 400 ˚C 50 12,000  0.3 218 25%Co-Ce-O composite Modified Citrate Sol–Gel Method 401 ˚C 100 30,000  1.0 193 40%La/CeO2 Hydrothermal <500 ˚C 100 30,000  1.0 114 MnCo2O4 Fast heating Mn/Co alkoxyacetate precursors  405 ˚C 200 7,020 1.0 219      140 4.4 Chiral nematic mesoporous cobalt oxide  Given that the templation of cobalt oxide with a porous silica template was successful, the feasibility of templating chiral cobalt oxide with chiral nematic mesoporous silica previously developed in the MacLachlan group was tested (Figure 4-17).206 This silica was produced through templating with chiral nematic nanocrystalline cellulose and the resulting films have a helical pitch distance of several hundred nanomaters. Chirality has been introduced into other materials by templation with this chiral nematic silica, including carbon and titanium dioxide, however, a solid-liquid route such as used for the synthesis of Comp10 has not been used before. The template was ground with Co(NO3)2⋅6H2O and heating slowly in a muffle furnace to 500 ˚C in a similar manner to Comp10. The silica template was then removed by stirring in 2 M NaOH to give Comp12.    Figure 4-17: SEM images of chiral nematic silica films. a) Side view of a cracked film shows the stacked layers that result from the helical pitch of the chiral nematic phase (scale bar = 3!µm). b) Higher magnification reveals the helical pitch distance to be of the order of several hundred nanometres (scale bar = 2!µm). Reprinted by permission from Macmillan Publishers Ltd: Nature, reference 206, copyright 2010.     141 Comp12 is Co3O4 by PXRD analysis, as shown in Figure 4-18. SEM images before removal of the silica template (Figure 4-19a and b) shows no distinct morphology, and the fact that chiral nematic ordering cannot be seen suggests that the cobalt oxide has infiltrated the pores. Upon removal of the silica, sections of Comp12 appear to have a layered structure with possible chiral nematic ordering, however it is not as highly ordered as the silica, as shown in Figure 4-19c and d. Comp12 has a BET surface area of 86 m2/g and a type IV isotherm with a type H2 hysteresis loop, reflecting that of the mesoporous silica template used.    Figure 4-18: PXRD pattern of Comp12. Pattern matches JCPDS-78-1969 for Co3O4 (!).   142  Figure 4-19: SEM images of a and b) Comp12 before silica removal and c and d) after silica removal.   Figure 4-20: N2 adsorption-desorption isotherms for Comp12. Solid lines represent adsorption and dashed lines represent desorption.    143  The black colour of cobalt oxide prevents iridescence and so it is difficult to deduce whether Comp12 has chiral nematic ordering. However, SEM images suggest that the template has imparted a layered structure on the cobalt oxide. Further investigation is needed to determine the true structure of Comp12, but initial results are encouraging and suggest that this technique will successfully lead to the first example of chiral nematic metal oxides obtained through a solid-liquid technique and the first example of chiral nematic cobalt oxide.   4.4.1 Conclusions  Mesoporous Co3O4 and Co3O4/CeO2 based catalysts that have excellent activity for L-T MOX was discovered. These materials are formed using a simple KIT-6 templating method followed by impregnation. Comp10 on its own shows unusually high L-T MOX activity with a light-off temperature of 390 ˚C. Comp11, Comp11-10%La and Comp11-10%Mn show high activity for total oxidation of methane, with Comp11-10%La having a light-off temperature of 400 ˚C. The ability to modify these materials without significantly affecting their catalytic activity means they have the potential to be used for a number of applications in which L-T MOX would be an advantage, as well as used as active supports for noble metals. The wide availability of silica combined with the simplicity of this approach makes it plausible that large quantities of these materials could be produced for commercial uses. Future work will aim to further improve the catalytic activity of these materials by altering the ratio of Ce to Co as well as the type and ratio of dopant cation. It is also possible that the metal oxides could be substituted for other materials that show promise as L-T MOX   144 catalysts and that the materials described in this chapter could be used to support precious metals such as Pt or Pd. Finally, the stability of these materials during repetitive LT-MOX testing requires investigation.     145 Chapter 5: Surface-assisted Reduction: A New Method for the Preparation of Noble Metal/Ceria Catalysts1  5.1 Introduction  To date, no transition metal catalysts have high enough activity for low-temperature methane oxidation (L-T MOX) to completely eliminate the use of expensive noble metals in natural gas vehicles (NGVs). However, reducing the amount of noble metals in catalyts through judicious choice of support as well as catalyst composition, morphology, synthetic route and precursors used has the potential to lead to inexpensive, effective catalysts for use in NGVs.  Pd/CeO2 catalysts are amongst the most promising materials for L-T MOX in vehicle engines.134,220,221 As shown previously in this thesis, ceria is an active support for methane oxidation catalysts due to its accessible Ce3+/Ce4+ redox cycle, which results in high oxygen storage capacity (OSC) and high oxygen mobility. Electron and oxygen transfer studies on model Pt/CeO2 catalysts show favorable interactions on nanostructured ceria that enhance activity222 and the hydrophobic nature of ceria may help stabilize it under the harsh conditions found in exhaust streams.223 Many methods are available for the synthesis of                                                 1 Portions of this chapter have been submitted for publication as: Gomathi, Anandhanatarajan, Susan M. Vickers, Rahman Gholami, Mina Alyani, Renee W. Y. Man, Mark J. MacLachlan, Kevin J. Smith and Michael O. Wolf, “Nanostructured Materials Prepared by Surface-Assisted Reduction: New Catalysts for Methane Oxidation”.    146 Pd/CeO2 materials including wet impregnation, co-precipitation, deposition-precipitation, specific adsorption, and combustion synthesis.134,224–227 However, the structure of the material is difficult to fine tune using these methods and they often lead to catalysts with relatively high light-off temperatures (T50%) for L-T MOX, as demonstrated for wet impregnation in section 2.3.5 of this thesis. Pd@CeO2 core shell particles on alumina have resulted in catalysts with complete oxidation of methane at temperatures below 400 ˚C.135 The high activity of these materials was mainly attributed to strong interactions between Pd and ceria, as well as high dispersion of the Pd@CeO2 nanostructures. However, reaching T50% of less than 300 °C for L-T MOX remains a formidable challenge and new methods for synthesizing catalytic materials are highly desirable.   Given the wide range of reactions for Pd/CeO2 based catalysts, the discovery of new preparation methods could lead to more efficient catalysts for a large number of processes. Additionally, ceria is commonly used as a support for other noble metals, furthering the importance of new methods for synthesizing noble metal/ceria composite catalysts. For example, Au/ceria catalysts have been reported to have activity for CO oxidation,228,229 hydrocarbon oxidation,228 and the water-gas shift reaction.230,231 Ag/ceria catalysts have been used for CO oxidation,232,233 formaldehyde oxidation,234,235 and hydrocarbon oxidation.233  In this chapter the synthesis of novel cerium formate (CF) and cerium hydroxy carbonate (CHC) nanostructures is described. These new materials are then used as precursors in a new, straightforward and scaleable method for synthesizing Pd/ceria catalysts, termed surface-assisted reduction (SAR). The new catalytic materials have exceptional activity for L-T   147 MOX, with T50%’s below 300 ˚C. The scope of SAR is investigated and Au/ceria, PdAu/ceria and Ag/ceria materials are successfully formed.   5.2 Experimental 5.2.1 General  All solvents and reagents were purchased from commercial sources and used without further purification. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 Advance X-ray diffractometer in the Bragg-Brentano configuration, using Cu Kα radiation at 40 kV, 40 mA. X-ray photoelectron spectroscopy (XPS) was carried out on a Leybold Max200 spectrometer using a magnesium Kα X-ray source for all other samples, operating at a base pressure of 1 × 10−9 Torr. Initial survey scans were acquired with a pass energy of 192 eV, while higher resolution scans were acquired with a pass energy of 48 eV. XPS spectra were deconvoluted using the XPSPEAK program by curve fitting with a mixed Gaussian-Lorentzian function after the Shirley-type background subtraction. Gas adsorption studies were performed using a Micromeritics Accelerated Surface Area & Porosity (ASAP) 2020 system. All samples were degassed for two hours under vacuum at 120 °C immediately prior to analysis. Barrett-Joyner-Halenda (BJH) pore size distributions were all calculated from the adsorption branch of the isotherm. FT-IR (Fourier transform infrared) spectra were recorded on powdered solids on a Thermo Scientific Nicolet 4700 spectrometer.     148 5.2.2 Microscopy  Transmission electron microscopy (TEM) images were collected on a Hitachi H7600 electron microscope operating at an accelerating voltage of 100 kV. Samples were prepared by suspending the powders in ethanol and then dropcasting them onto a carbon-coated copper TEM grid and air-drying. SEM images were obtained with a Hitachi S-4700 field emission scanning electron microscope. Samples were prepared using double-sided carbon tape or dropcasting the dispersion directly onto aluminum stubs. The samples were then sputter coated with 5 nm of gold. Energy dispersive X-ray analysis (EDX) was collected on a Hitachi S-2600N variable pressure scanning electron microscope (SEM) equipped with an X-ray detector coupled to Quartz Imaging Systems Xone software. Samples were analyzed without sputter coating.  5.2.3 Preparation cerium formate hollow spheres via a solvothermal method  Ce(NO3)3!6H2O (1.300 g, 3.000 mmol) was added to ethylene glycol (EG) (15 mL) in a 23 mL Teflon lined stainless steel autoclave. The reaction vessel was heated at 145 ˚C for 15 h then cooled to room temperature. The resulting violet product was collected by centrifugation (10 min, 4500 rpm), washed with ethanol (3 × 20 mL) and dried overnight at 50 ˚C to give a pale violet coloured powder. These samples are referred to as Comp13-CF.     149 5.2.4  Preparation of cerium hydroxycarbonate rods  Ce(NO3)3!6H2O (0.870 g, 2.000 mmol) was added to EG (15 mL) in a 23 mL Teflon lined stainless steel autoclave. The reaction mixture was maintained at 180 ˚C for 48 h and was then cooled to room temperature. The product, a pale yellow gel, was collected by centrifugation  (10 min, 4500 rpm), washed with ethanol (3 × 20 mL), and dried at 50 ˚C to give a tan coloured powder. These samples are referred to as Comp13-CHC-f.  5.2.5 Preparation of cerium hydroxycarbonate stacked sheets  Ce(NO3)3!6H2O (0.870 g, 2 mmol) was added to EG (15 mL) in a 23 mL Teflon lined stainless steel autoclave. The reaction mixture was maintained at 200 ˚C for 24 h and was then cooled to room temperature. The product, a white gel, was collected by centrifugation  (10 min, 4500 rpm), washed with ethanol (3 × 20 mL), and dried at 50 ˚C to give a white powder. Yield = 0.429 g. These samples are referred to as Comp13-CHC.  5.2.6 Surface-assisted reduction of Pd(NO3)2 with ceria precursors  In a typical reaction, calculated quantities of ceria precursor were added to the appropriate amount of Pd(NO3)2 dissolved in distilled water. The solid was collected by centrifugation (10 min, 3000 rpm) washed with 20 mL of distilled water and dried at 60 ˚C overnight. The grey powder was calcined at 400 ˚C to give the dark grey, 1wt%Pd-ceria samples. Table 5-1 gives the quantities of reagents used for each sample.   150 Table 5-1: Quantities of reagents used in the Pd SAR experiments. Sample ID Mass of Pd(NO3)2 used (mg) Volume of H2O (mL) Quantity of CHC-f used (mg) Quantity of CF used (mg) Comp14-CHC-f/Pd(1mM) 4.3 20 250 - Comp14-CF/Pd(1mM) 4.3 20 - 320 Comp14-CF/Pd (0.4mM) 4.3 50 - 320 Comp14-CF/Pd /Pd(0.3mM) 4.3 70 - 320 Comp15-CF/Pd 4.3 20 - 320  5.2.7 Non-hydrothermal route to cerium formate  Ce(NO3)3!6H2O (1.310 g, 3.000 mmol) was added to EG (15 mL) and heated to 155 ˚C in a round bottomed flask with a condenser.  After 40 h the purple reaction mixture was left to cool to room temperature before being collected by centrifugation (10 min, 4500 rpm), washed with ethanol (3 × 20 mL), and dried at 50 ˚C to give a pale violet powder. Yield = 0.620 g. These samples are referred to as Comp15-CF.  5.2.8 Surface-assisted reduction of HAuCl4 with ceria precursors  In a typical reaction, quantities of ceria precursor were added to the appropriate amount of HAuCl4!xH2O in distilled water. The reaction mixture was stirred at room temperature for 4 h. The solid was collected by centrifugation (10 min, 3000 rpm) washed with 20 mL of distilled water and dried at 60 ˚C overnight. The powder was calcined at 400 ˚C to give the   151 red or violet 1wt%Au/ceria samples. Table 5-2 gives the quantities of reagents used for each sample.  Table 5-2: Quantities of reagents used in the Au SAR experiments. Sample ID Mass of HAuCl4 used (mg) Volume of H2O (mL) Quantity of CHC used (mg) Quantity of CF used (mg) Comp16-CF/Au(0.5mM) 6.9 20 - 633 Comp16-CF/Au(1mM) 6.9 40 - 633 Comp16-CHC/Au(0.5mM) 6.9 20 500 - Comp16-CHC/Au(1mM) 6.9 40 500 -  5.2.9 Simultaneous surface-assisted reduction of Pd(NO3)2 and HAuCl4 with ceria precursors  Pd(NO3)2 (4.3 mg, 0.02 mmol) and HAuCl4 (2.0 mg, 0.006 mmol) were each dissolved in 10 mL of distilled water then both solutions were added to Comp13-CHC-f (0.252 g). The reaction mixture was stirred at room temperature for 4 h. The solid was collected by centrifugation (10 min, 3000 rpm), washed with 20 mL of distilled water and dried at 60 ˚C overnight. Grey powder was calcined at 400 ˚C to give the dark grey solid 1%wtPd-0.5wt%Au/ceria samples, referred to as Comp17-PdAu-sim.     152 5.2.10 Sequential surface-assisted reduction of Pd(NO3)2 and HAuCl4 with ceria precursors  Pd(NO3)2 (4.3 mg, 0.02 mmol) was dissolved in 18.0 mL of distilled water then added to Comp13-CHC-f (0.252 g). The reaction mixture was stirred at room temperature for 30 min then HAuCl4 (2.0 mg, 0.006 mmol) dissolved in 2.0 mL of distilled water was added and the mixture stirred for another 3.5 h. The solid was collected by centrifugation (10 min, 3000 rpm), washed with 20 mL of distilled water and dried at 60 ˚C overnight. Grey powder was calcined at 400 ˚C to give the dark grey solid 1%wtPd-0.5wt%Au /ceria samples, referred to as Comp17-PdAu-seq.  5.2.11 Surface-assisted reduction of AgNO3 with ceria precursors  Ag(NO3) (3.1 mg, 0.02 mmol) was dissolved in 20 mL of distilled water then added to Comp13-CF (0.320 g). The reaction mixture was stirred at room temperature for 4 h. The solid was collected by centrifugation (10 min, 3000 rpm) washed with 20 mL of distilled water and dried at 60 ˚C overnight. Grey powder was calcined at 400 ˚C to give the pale grey solid 1%wtAg/ceria samples, referred to as Comp18-Ag.  5.2.12 Methane oxidation testing  Temperature-programmed CH4 oxidation (TPO) done as described in section 2.2.11 of this thesis.   153 5.3 Results and discussion 5.3.1 Novel cerium-containing nanomaterials  A novel synthetic route to cerium hydroxycarbonate (CHC) and cerium formate (CF) nanomaterials was discovered through collaboration with other members of the MacLachlan lab. This route utilized ethylene glycol (EG), which is known to be a reducing agent that is itself oxidized to aldehydes then acids and finally CO2, in a solvothermal reaction. By altering the reaction temperature, different morphologies were obtained, as illustrated in Figure 5-1. At 120 ˚C cerium nitrate and EG reacted to produce CeO2. At 145 ˚C the Comp13-CF nanospheres formed were composed of CF by PXRD and FT-IR analysis (Figure 5-2). The FT-IR spectrum shows an intense band at 1570 cm-1 that is characteristic of the asymmetric COO stretching mode and a band at 776 cm-1 that arises from δ(OCO) of the formate group, along with bands due to residual EG (~1040-1080 cm-1). PXRD and FT-IR analysis of the Comp13-CHC-f nanorods and Comp13-CHC stacked sheets, formed at 180 ˚C and 200 ˚C, respectively, show that increasing the temperature of the reaction reduces the amount of formate groups in the sample and increases the amount of carbonate groups until, at 200 ˚C, the product is entirely CHC. The PXRD patterns of Comp13-CHC-f and Comp13-CHC are both indexed to CHC. However, the FT-IR spectra of Comp13-CHC-f contains peaks corresponding to formate in addition to those corresponding to carbonate and residual EG while Comp13-CHC has comparably less residual formate and EG. Given that formic acid and carbon dioxide are both known decomposition products of EG, it is proposed that Comp13-CHC-f is composed of CHC with formate groups coating the surface.     154  Figure 5-1: Synthesis of new nanostructured cerium-containing materials. Scale bars = 100 nm.     155  Figure 5-2: PXRD patterns of Comp13-CF (purple), Comp13-CHC-f (green) and Comp13-CHC (orange). Comp13-CF corresponds to CF (JCPDS-49-1245), and Comp13-CHC-f and Comp13-CHC correspond to CHC (JCPDS-52-0352).  5.3.2 Surface-assisted reduction  These nanostructured cerium-containing materials were employed in the development of a new method for creating Pd/CeO2 catalysts, termed surface-assisted reduction (SAR). The addition of palladium nitrate to either formate-coated Comp13-CHC-f nanorods or   156 Comp13-CF hollow spheres in water yielded a black precipitate, as shown in Scheme 5-1. This colour change is suggestive of the reduction of Pd2+ to Pd0.    Scheme 5-1: The reaction of Comp13-CHC-f or Comp13-CF with or CHC-f with Pd2+ gives black precipitates of as-synthesized Comp14-CHC-f/Pd or Comp14-CF/Pd.  It is believed that the formate groups present in both Comp13-CF and Comp13-CHC-f are responsible for the reduction of Pd2+. It is known that sodium formate can act as a reducing agent in the synthesis of Pd nanoparticles, however, the use of formate groups on the surface of a support material has not been reported. The amount of formate groups on the surface of the Ce3+ precursor correlates with the rate of Pd2+ reduction. The reaction mixture turns black in less than 5 minutes when Comp13-CF, the precursor with the largest amount of formate, is used as the reducing agent. When Comp13-CHC-f (CHC with formate groups on the surface) and Comp13-CHC (the precursor with the least amount of formate) are used the reaction takes 1 hour and 12 hours, respectively, to turn black. The formate peaks present in the Comp13-CF FT-IR spectra are replaced with peaks characteristic of carbonate (1385 and   157 847 cm-1) in Comp14-CF/Pd, meaning the reduction of Pd2+ by formate yields carbonate as a by product.  The PXRD patterns of Comp14-CF/Pd and Comp14-CHC-f/Pd do not show peaks relating to crystalline palladium or palladium oxide (Figure 5-3). We have not been able to index the peaks into a unit cell, but the patterns are similar to those obtained by layered metal oxide structures.236,237 Interestingly, Comp13-CHC-f stirred in water for 12 hours without added palladium nitrate also transformed into the same crystalline structure. When the Comp13-CF or Comp13-CHC was treated with water there was no indication of the layered structure forming.     158 Figure 5-3: PXRD patterns of Comp14-CF/Pd (purple) and Comp14-CHC-f/Pd (green). Both patterns indicate a layered structure.  Based on these results it is proposed that the formation of the layered structures in water is facilitated by carbonate and residual EG from the precursor synthesis. Comp13-CF has residual EG but no carbonate while Comp13-CHC has carbonate but no residual EG and so neither forms the layered structure in water. However, during reduction of Pd2+, carbonate is formed as a by-product and Comp14-CF/Pd also has the layered structure.  The reduction of Pd2+ was conducted at various concentrations of Pd(NO3)2. Comp13-CHC-f successfully reduced Pd2+ in a 1 mM solution, while Comp13-CF was successful at reducing Pd2+ at concentrations as low as 0.3 mM. At lower concentrations the reaction mixture took a longer time to turn black, indicating that the reaction time is inversely dependent on Pd2+ concentration.   These samples were calcined under air to give catalysts with 1 wt% Pd supported on ceria. The PXRD patterns of the calcined samples show CeO2 with no peaks relating to Pd or PdO, probably due to the particles being too small to be detected by PXRD. EDX and XPS analysis was collected by A. Gomathi and confirmed the presence of Pd in all samples.  Comp14-CF/Pd(0.3mM) and Comp14-CHC-f/Pd(1mM) show remarkably high activity of L-T MOX, with T50%’s of well below 300 ˚C and complete methane oxidation by 400 ˚C. As far as we are aware, this activity is better than all other Pd/CeO2 samples reported in the literature. Comp14-CF/Pd(1mM) and Comp14-CF/Pd(0.4mM) both show lower activity   159 for L-T MOX than Comp14-CF/Pd(0.3mM). Slower reaction times for Pd2+ reduction give rise to slower nucleation of Pd0. This leads to less aggregation of Pd0 and so better dispersion and more active sites. For this reason, Comp14-CF/Pd(0.3mM) and Comp14-CHC-f/Pd(1mM) exhibit the highest catalytic activity. It is thought that the excellent activity of these materials is a result of the formate groups on the surface of the precursors reducing the Pd2+, leading to high dispersion of the Pd nanoparticles and enhanced metal-support interactions.   Figure 5-4: L-T MOX for Comp14-CF/Pd(0.3mM) (red), Comp14-CF/Pd(0.4mM) (blue), Comp14-CF/Pd(1mM) (purple), and Comp14-CHC-f/Pd(1mM) (green).     160 5.3.3 Scale up of surface-assisted reduction  In order for PdO/CeO2 synthesized by SAR to be a viable option for use in commercial vehicles powered by natural gas, the reaction needs to be easily scalable. Several test reactions were performed in order to determine how scalable SAR is. SAR is very sensitive to the concentration of Pd(NO3)3, and so the amount of water had to be scaled proportionately with the amount of precursor. However, with larger volumes of water, stirring becomes more uneven throughout the reaction mixture and the solid cerium precursors cannot be as evenly distributed as with smaller volumes. To scale up Comp14-CHC/Pd by 8 times, 2 g of CHC was used to reduce 34.4 mg of Pd(NO3)2 in 80 mL of water. No reduction took place after 12 hours, so the mixture was then gently heated to 40 ˚C. However, still no reaction occurred after 24 hours at 40 ˚C indicating that the synthesis of Comp14-CHC/Pd is not scalable in its current form and further work will have to be done if the material is to be used commercially. Advanced stirring apparatus or differently shaped glassware may help to distribute the reactions more evenly and allow the reaction to proceed. The synthesis of Comp14-CF/Pd(1mM) was successfully scaled by 8 times in 160 mL of water to give a dark grey product similar in appearance to the small scale product. CF reduces Pd2+ more readily on a smaller scale than CHC and so this result is not unexpected. XPS analysis for the scaled up product is similar to the small scale product (Figure 5-5a). Two sets of multiplets, u and v, (3d3/2 and 3d5/2 spin–orbit components) have been labeled, with splitting of 18.4 eV.193,208 The pairs of peaks denoted as V/U, V’’/U’’, V’’’/U’’’ are assigned to Ce4+ species and the couple V’/U’ is attributed to the Ce3+ species.208–210 The high resolution Pd 3d XPS spectra shows the Pd to be in the form of PdO, with binding energies of 336.9 eV and 342.2 eV (Figure 5-5b).   161   The T50% measured for Comp14-CF/Pd(1mM) produced when the reaction is scaled up  is  329 ˚C, which is comparable to the small-scale reaction and is encouraging for possible commercial uses of  Comp14-CF/Pd.   Figure 5-5: High resolution a) Ce 3d and b) Pd 3d XPS spectra of scaled up Comp14-CF/Pd(1mM).   162  Figure 5-6: TPO of Comp14-CF/Pd(1mM).  5.3.4 Non-hydrothermal route to cerium formate for surface-assisted reduction  In order to be able to scale up the production of Pd/CeO2 for commercial uses it is also important to create more straightforward routes to the precursors. To develop a non-hydrothermal route to CF, Ce(NO3)3!6H2O was stirred in EG for 40 h at 155 ˚C to give Comp15-CF. The resulting material was CF by PXRD, as seen in Figure 5-7a. The FT-IR spectrum is similar to that of Comp13-CF, with peaks corresponding to formate groups and residual EG. The CF has no distinct morphology, in contrast to the CF formed by hydrothermal synthesis (Figure 5-8).    163  Figure 5-7: a) PXRD pattern and b) FT-IR spectrum of Comp15-CF. "  = cerium formate (JCPDS 49-1245).    164  Figure 5-8: TEM image of Comp15-CF. Scale bar = 100 nm.  The reaction between Comp15-CF and Pd(NO3)2 in water to give Comp15-CF/Pd turned black and so, based on previous results, was assumed to have successfully reduced Pd2+ to Pd0. The PXRD pattern, shown in Figure 5-9a is similar to that of Comp14-CF/Pd, indicating that the lamellar structure formed from carbonate and residual EG has formed. TEM images are also indicative of this layered structure (Figure 5-9b). Unfortunately, this material has not yet been tested for L-T MOX due to limited availability of the TPO setup.    165  Figure 5-9:  a) PXRD pattern indicating a layered structure and b) TEM image of Comp15-CF/Pd scale bar = 100 nm.  5.3.5 Surface-assisted reduction of Pd using CF hollow spheres  Section 3.3.5 of this thesis describes the synthesis of cerium formate hollow spheres via a non-hydrothermal EG-mediated route. Given that these hollow spheres are composed of CF and synthesized in EG, it was proposed that they would also be able to reduce Pd2+ to form Pd/ceria catalysts. The PXRD pattern, shown in Figure 5-10a, is similar to the pattern collected for Comp14-Cf-Pd with an additional large, very broad peak usually associated with amorphous material. TEM images show some of the material has a layered structure, however, surprisingly, a large amount of the sample is still hollow spheres (Figure 5-10b), which explains the amorphous region in the PXRD. This was unexpected as the morphology of all other SAR precursor materials fully transformed to a layered structure. It is possible that only some of the spheres participated in SAR, however, a deeper understanding of the nature of SAR is needed before a reason for this can be proposed.     166   Figure 5-10: a) PXRD pattern indicating a layered structure and b) TEM image of Comp15. Scale bar = 500 nm.     167 5.3.6 Surface-assisted reduction of HAuCl4 with ceria precursors  Gold nanoparticles supported on redox active oxides have been shown to have high catalytic activity for many oxidation reactions. To develop new Au/CeO2 catalytic materials via SAR, The synthesis of 1wt%Au/CeO2 was attempted by reacting chloroauric acid with a hydrothermally synthesized cerium containing precursor. Comp13-CF and Comp13-CHC were both used to reduce 0.5 and 1 mM choroauric acid solutions to give Comp16. After 4 hours the reaction mixtures had all turned purple. Purple is generally associated with colloidal suspensions of larger Au nanoparticles, while spherical colloidal suspensions of Au nanoparticles less than 100 nm are usually red. However, metal-support interactions may change the plasmon resonances of nanoparticles.  PXRD patterns of as-synthesized Comp16, shown in Figure 5-11, are similar to those obtained when Pd2+ is reduced by CF and CHC-f. This indicates that the layered structure formed by carbonate and residual EG in water has been formed. No peaks corresponding to Au are seen, as in the Pd samples. TEM images, shown in Figure 5-12, also show a material with a layered structure similar to that obtained previously. FT-IR analysis confirmed that the characteristic peaks of formate have disappeared and peaks characteristic of carbonate (1385 and 857 cm-1) appear or are enhanced.       168  Figure 5-11: PXRD patterns of as-synthesized a) Comp16-CF/Au(0.5mM) (red) and Comp16-CF/Au(1mM) (green), and b) Comp16-CHC/Au(0.5mM) (purple) and Comp16-CHC/Au(1mM) (blue). All patterns indicate a layered structure.    169  Figure 5-12: TEM images of as-synthesized a) Comp16-CF/Au(0.5mM) , b) Comp16-CF/Au(1mM), c) Comp16-CHC/Au(0.5mM), and d) Comp16-CHC/Au(1mM). Scale bars = 100 nm.  All Comp16 samples were calcined under air, resulting in CeO2 by PXRD. No peaks assigned to Au can be seen, suggesting that the Au is highly dispersed over the ceria support. XPS analysis confirms the presence of Au with binding energies around 84.0 eV and 87.7 eV (Figure 5-14 and Table 5-3). Interestingly, both Comp16-CF/Au samples are pale purple while both Comp16-CHC/Au samples have a more red appearance (See Appendix Figure C-1). While the dielectric constant of the environment can change the surface plasmon   170 resonance of gold nanoparticles, all samples are composed of ceria and so it is expected that the nanoparticles experience similar environments. Aggregation of nanoparticles can cause colour transitions from red to purple due to plasmonic coupling between particles,238 suggesting the Au in Comp16-CF/Au has aggregated more than in Comp16-CHC/Au. This is consistent with the observation during Pd2+ reduction that slower reduction by precursors with less formate groups leads to higher dispersion of the precious metal.  All Comp16 samples have been sent to collaborators to be tested for catalytic activity in a number of oxidation reactions, however to date no results have been obtained.   Figure 5-13: PXRD patterns of calcined Comp16-CF/Au(0.5mM) (red), Comp16-CF/Au(1mM) (green), Comp16-CHC/Au(0.5mM) (purple) and Comp16-CHC/Au(1mM) (blue). "= CeO2 (JCPDS-34-0394).   171   Figure 5-14: High resolution Au 4f XPS spectra for calcined a) Comp16-CF/Au(0.5mM), b) Comp16-CF/Au(1mM), c) Comp16-CHC/Au(0.5mM) and d) Comp16-CHC/Au(1mM).  Table 5-3: High resolution Au 4f XPS binding energies for Comp16-CF/Au(0.5mM), Comp16-CF/Au(1mM), Comp16-CHC/Au(0.5mM), and Comp16-CHC/Au(1mM).         Catalyst Binding energy (eV) Comp16-CF/Au(0.5mM) 83.6 87.3 Comp16-CF/Au(1mM) 83.8 87.4 Comp16-CHC/Au(0.5mM) 83.9 87.6 Comp16-CHC/Au(1mM) 84.0 87.6   172 5.3.7 Surface-assisted reduction of Pd(NO3)2 and HAuCl4 with ceria precursors  Gold is known to increase the catalytic activity of supported Pd for a number of reactions. For example Kapoor et al. demonstrated increased activity for AuPd/ceria compared to Pd/ceria for methanol decomposition.239 The addition of small amounts of gold led to a dramatic increase in catalytic activity, which leveled out with the addition of larger amounts of gold. Therefore, it is proposed that AuPd clusters are highly active for methanol decomposition, with a rise in activity stopping once the Pd particles are saturated with Au. The authors suggest that Au may contribute to the reaction by adsorbing molecular hydrogen, however, further investigation is needed to determine a mechanism for the increased activity. In addition, Au has an affinity with sulfur compounds and so the presence of Au in a automotive catalyst can protect Pd from being poisoned.240  1wt%Pd-0.5wt%Au/ceria catalysts were prepared using Comp13-CHC-f via both a simultaneous and a sequential route. In the simultaneous route Pd(NO3)3 and HAuCl4, each dissolved in 10.0 mL of water, were added to Comp13-CHC-f at the same time to give Comp17-PdAu-sim. In the sequential route Pd(NO3)3 dissolved in 18.0 mL of water was added to Comp13-CHC-f then, once the reaction mixture had turned black after 30 minutes, HAuCl4 dissolved in 2.0 mL of water was added to give Comp17-PdAu-seq. The simultaneous reaction is more likely to form AuPd mixed metal nanoparticles while the sequential reaction is more likely to form separate Au and Pd nanoparticles.     173 As expected, both reactions led to a layered structure, confirmed by PXRD (Figure 5-15a). After calcination in air, only peaks corresponding to CeO2 were seen in the PXRD pattern for both samples, again suggesting that Pd and Au are highly dispersed on the surface of the support. EDX mapping of Comp17-PdAu-sim and Comp17-PdAu-seq reveals both Pd and Au distributed evenly over the surface of the ceria support (Figure 5-16). This means that after the reduction of Pd2+ onto the surface of the CHC-f there were still formate groups available for the reduction of Au3+ in the sequential reaction. If the gold had not been reduced during the reaction it is expected that it would remain in solution and so not appear in EDX mapping.     174  Figure 5-15: PXRD pattern of a) as-synthesized Comp17-PdAu-sim (green) and Comp17-PdAu-seq (purple), indicating a layered structure and b) calcined Comp17-PdAu-sim (green) and Comp17-PdAu-seq (purple). "= CeO2 (JCPDS-34-0394).    175  Figure 5-16: EDX mapping images of a) Pd and b) Au for Comp17-PdAu-sim, and c) Pd and d) Au for Comp17-PdAu-seq. Scale bars = 100 µm.  Comp17-PdAu-sim and Comp17-PdAu-seq were tested for L-T MOX, as shown in Figure 5-13. The T50% for the product of simultaneous reduction is 301 ˚C while the T50% for the product of sequential reduction is 290 ˚C. These results are very similar to those obtained by the corresponding Pd/ceria catalyst by SAR and suggest that Au does not have an effect on the catalytic oxidation of methane by Pd/ceria catalysts produced via SAR. However, the potential benefits of Au reducing sulfur poisoning of Pd in an automotive catalyst have not been tested, and so further testing is required in order to determine if PdAu/ceria catalysts would be superior to Pd/ceria catalysts in NGV engines.   176   Figure 5-17: TPO curve for calcined Comp17-PdAu-sim (green) and Comp17-PdAu-seq (purple).  5.3.8 Scope of surface-assisted reduction  Catalysts synthesized by SAR show high catalytic activity for L-T MOX, likely due to high dispersion of the precious metal on the support. This property is desirable for most supported heterogeneous catalysts and so SAR has the potential to lead to a new class of catalytic materials.  Therefore, it is important to investigate the scope of SAR to pave the way to new, highly active catalytic materials. Test reactions were run on a small scale using 2.0 mL of 2 mM solutions of Co(NO3)2, Cu(NO3)2 or AgNO3 and a spatula tip of Comp13-CF. After 24 hours only the reaction mixture containing AgNO3 had undergone any changes. As seen in Table 5-4, Au3+, Pd2+ and Ag+ have higher reduction potentials than Cu2+ and Co2+ and so   177 have a higher tendency to be reduced. The reduction potential for CO2 to formate is -0.42 V. Therefore, the sum of the half-reactions for Au3+, Pd2+ and Ag+ reduction by formate is positive and the reactions are spontaneous, while the sum of the half reactions for Cu2+ and Co2+ reduction by formate is negative.   Table 5-4: Standard reduction potentials for selected half reactions in aqueous solution at 25 ˚C. Oxidized form Reduced form E0 volts Au3+ + 3e- Au 1.50 Pd2++ 2 e- Pd 0.92 Ag+ + e- Ag 0.80 Cu2+ + 2 e- Cu 0.34 Co2+ + 2e- Co -0.28  The reduction of Ag+ with Comp13-CF was successful using a 1mM solution of Ag(NO3) to produce 1%Ag/CeO2, Comp18-Ag. The PXRD pattern of the as-synthesized product was analogous to other SAR products, showing a layered structure while the calcined product showed only CeO2 with no peaks attributed to Ag (Figure 5-18). XPS analysis (Figure 5-19) again reveals 4 pairs of peaks, indicating Ce3+ and Ce4+ oxidation states are both present. The presence of metallic Ag is also confirmed with two peaks, one at 367.7 eV and one at 373.7 eV.    178  Figure 5-18: PXRD pattern for Comp18-Ag as-synthesized (pink) and calcined (grey). "= CeO2 (JCPDS-34-0394).    179  Figure 5-19: High resolution a) Ce 3d and b) Ag 3d  XPS spectra for calcined Comp18-Ag.  5.3.9 Conclusions  This chapter describes the synthesis and characterization of novel nanostructured ceria containing materials. These materials were used as precursors for the synthesis of Pd/ceria catalysts by a new method termed surface-assisted reduction. SAR employs formate groups   180 on the surface of the precursor to reduce the noble metal, leading to high dispersion of the metal and expected beneficial metal-support interactions. The Pd/ceria and PdAu/ceria catalysts have exceptional activity for L-T MOX, with T50%’s below 300 ˚C. This activity is higher than all other Pd/CeO2 catalysts reported in the literature. SAR is straightforward and can be carried out with a variety of cerium-based precursors, provided formate groups and residual EG are present. Furthermore, the reaction was shown to be scalable and these materials have the potential to be used in catalytic converters of commercial NGVs.   SAR was also successfully utilized to produce Au/ceria, PdAu/ceria and Ag/ceria materials. The wide scope of SAR has given rise to a new family of materials with the potential to have outstanding activity for the large number of oxidation reactions that can be catalyzed by ceria-supported noble metals.     181 Chapter 6: Conclusions and Future Directions  6.1 Conclusions  Natural gas vehicles (NGVs) are a viable, more environmentally friendly alternative to gasoline or diesel powered vehicles. However, a major obstacle to the widespread implementation of NGVs is the unburned methane emitted in their exhaust. Methane is an extremely potent greenhouse gas and the conventional three-way catalytic (TWC) converters currently employed in gasoline and diesel powered vehicles do not break it down to carbon dioxide and water due to the low temperatures (450-550 ˚C) present in NGV engines.   Ceria-based materials have been suggested as a possible solution to this problem as they function as oxygen buffers during the cycling of oxygen rich and oxygen lean conditions in an engine, making them active supports for noble metal catalysts. The aim of this thesis has been to explore the synthesis, characterization and catalytic activity of ceria-based materials.   In Chapter 2, I adapted published synthetic routes to investigate how the synthesis, morphology, surface area and doping of ceria affected the catalytic activity. It was discovered that mixed metal oxides, where the second metal oxide is also redox active, have higher catalytic activity for methane oxidation than ceria alone. Mixed metal oxides tended to have a lower surface area than the analogous ceria-only material, but still had higher activity when the second metal oxide was redox active. These findings were employed in Chapter 4   182 to create catalysts with high activity for methane oxidation. In addition, 10 wt% palladium was deposited onto one of the most active catalysts via wet impregnation. For this catalyst to be commercially viable, however, the amount of palladium would have to be significantly reduced, highlighting the need for new, more effective methods for palladium deposition.   In Chapter 3, I described the synthesis and characterization of several novel ceria-based materials with various morphologies. Ceria was successfully deposited onto the surface of hydroxyapatite spheres in the presence of CTAC surfactant. Unfortunately when the hydroxyapatite template was removed the ceria spheres collapsed. Nevertheless, the ceria product has a high surface area, likely due to the hydroxyapatite preventing significant aggregation of the ceria particles during calcination. When this reaction was repeated without the presence of hydroxyapatite nanorods of ceria were obtained, resulting in the first reported route to ceria nanorods using CTAC as a soft template. Additionally, an ethylene glycol-mediated route was used to synthesize cerium formate hollow, mesoporous nanospheres. These spheres retained their morphology during calcination to give hollow, mesoporous nanospheres of ceria. Doping with 10 mol% Fe3+ prevented the spheres forming, however doping with 10 mol% Mn3+ produced non-hollow spheres, and mesoporous, hollow nanospheres of 10 mol% La3+ doped ceria were successfully prepared. It is proposed that these spheres are formed through a hollowing Ostwald ripening process. This synthetic route is simple and less energy intensive than other published routes to hollow ceria nanospheres and is the first reported non-hydrothermal synthesis of hollow, mesoporous ceria-based nanospheres.     183 In Chapter 4, I used the discovery from Chapter 2 that a combination of two redox active metal oxides is a more efficient catalyst than one on its own to create mesoporous ceria/cobalt oxide materials. Cobalt oxide was templated with KIT-6, a highly ordered mesoporous silica using a solid-liquid route then cerium nitrate was introduced into the pores by wet impregnation. The ceria was also doped with 10 mol% La3+ and Mn3+. After calcination, these materials all showed high activity for methane oxidation, with 50% methane conversion at around 400-450 ˚C without the use of any noble metals. Surprisingly, mesoporous cobalt oxide alone exhibited the highest activity with 50% methane conversion at 390 ˚C. This is very high for a catalyst with no noble metal. The slight decrease in activity when ceria was added to the pores may be due to the associated decrease in surface area, but further investigation is needed to fully understand the high activity of the cobalt oxide. In addition, chiral nematic silica was used to template cobalt oxide via the same solid-liquid route used for KIT-6 templated silica. While further investigation is required, initial results indicate that the cobalt oxide has a chiral nematic structure, even after removal of the silica template, producing the first report of a chiral nematic metal oxide prepared through a templated, solid-liquid route.  Chapter 5 explores a new method for the deposition of noble metals onto ceria, termed surface-assisted reduction. Palladium nitrate is reduced by formate groups on the ceria precursor to give highly dispersed palladium on the surface. These materials, which contain only 1 wt% Pd, have outstanding activity for methane oxidation, with 50% methane conversion below 300 ˚C. These reactions have been successfully scaled up by eight times in an effort to produce the catalyst for prototype catalytic converters if further testing is to take   184 place. The scope of surface-assisted reduction was investigated, resulting in the successful synthesis of silver or gold deposited on ceria. Cobalt and copper could not be reduced by the ceria precursor and this could be explained by examination of the standard reduction potentials of the metal ions. This means that one drawback to surface-assisted reduction is that it is restricted to only a few metals with standard reduction potentials high enough to be reduced by formate. Palladium and gold were reduced both sequentially and simultaneously onto the ceria support. Catalytic testing showed that the resulting materials did not have a higher activity for methane oxidation than the analogous materials without gold. However, gold and palladium on ceria have shown activity for numerous oxidation reactions and this bimetallic catalyst may have applications in reactions other than methane oxidation.   Taken as a whole, the work presented in this thesis has met the initial aim of developing new catalysts for low-temperature methane oxidation. New materials both with and without noble metals have been obtained that have some of the highest catalytic activities published to date. These materials all employ simple synthetic routes and it is feasible that, with further testing and modification, they could be used commercially in NGVs.  This work will hopefully inspire further investigation into catalysts for low-temperature methane oxidation, which will eventually promote the widespread implementation of NGVs worldwide.     185 6.2 Future directions  This thesis has described the synthesis of many new ceria-based materials, and, while extensive characterization was carried out on all these materials, many unanswered questions still remain. A fully understanding of the structure and composition of these materials will help further understanding of the catalytic mechanism and aid in the rational design of new catalysts.   Studying the location of dopant cations in ceria materials is challenging, especially in nanomaterials when the PXRD peaks are too broad to observe peak shifting due to a change in the lattice parameter. While it has been deemed unlikely that the dopant cation forms an amorphous layer on the surface of the ceria, I was unable to fully eliminate this possibility. TEM coupled with EDX could be used to search for any areas containing only the dopant cation and eliminate the possibility of an amorphous layer, giving further indication that the dopant cation had inserted into the ceria lattice. Additionally, nanoscale spatially resolved electron energy loss spectroscopy could be employed to investigate if there are redox differences between doped and undoped ceria, as would be expected with doping with aliovalent ions.   The mechanism for the formation of the mesoporous, hollow nanospheres described in Chapter 3 is still unclear. The synthesis of hollow metal oxide spheres remains a challenge, and is usually undertaken using templating or hydrothermal methods. It would be interesting to investigate the mechanism of formation for these nanoparticles and determine if it is   186 possible to use this synthetic route for other lanthanide or non-lanthanide metal oxides. Carrying out the reaction at a range of temperatures and monitoring it by TEM may help to confirm or reject the Ostwald ripening hypothesis. Coupling TEM imaging with in situ IR spectroscopy could help to identify how and when the cerium formate product is formed, as the reaction is believed to proceed via ceria.   The materials described in Chapter 4 are a good starting point for the development of a family of mixed metal oxide materials for methane oxidation. While only ceria and cobalt oxide were tested, there are numerous combinations of metal oxides that could be used. Tin oxide is also known to be active for low-temperature methane oxidation241,242 and, while it was not used in this thesis, it would be interesting to replace ceria with tin oxide in these mixed metal oxides and have them tested for catalytic activity. It is currently unclear why cobalt oxide has higher activity for methane oxidation than mixed metal oxides, as this is contrary to the results obtained in Chapter 2 with other metal oxides. The synthesis, characterization and catalytic activity testing of other cobalt oxide materials would be valuable to determine if the high activity is due to the pore structure, morphology or some other property of the cobalt oxide. Temperature programmed reduction could also be used to estimate the reducibility of the catalysts to determine if cobalt oxide alone is more easily reducible compared to ceria/cobalt oxide mixed metal catalysts. Finally, if these materials are to be used commercially, it is essential that methods for depositing palladium onto the surface are developed to obtain high catalytic activity with small amounts of palladium. It would be interesting to attempt to convert the cerium nitrate to cerium formate once it is inside the pores of the cobalt by heating in ethylene glycol then deposit palladium onto the   187 catalysts using surface-assisted reduction. However, it is possible that the cerium nitrate would be washed out of the pores during this process. Wet impregnation and deposition-precipitation could also be tried.   Further development of the chiral nematic cobalt oxide synthesized in Chapter 4 is currently underway. Different ratios of silica to cobalt are being tested and the removal of the silica template with HF instead of NaOH will be tested to limit disruption of the cobalt oxide. It would be advantageous if a film of chiral nematic cobalt oxide could be produced, however this is very challenging using the solid-liquid route.  In Chapter 5 I described the development and scope of surface-assisted reduction. I believe that this work has very positive prospects and should be further investigated in order to fully elucidate the mechanism for surface-assisted reduction. The ability to quantify the formate groups on the surface and develop a relationship between the dilution and rate of reduction is important and could help to further explain the high activity of these catalysts. The gold/ceria catalysts are currently being tested for a variety of oxidation reactions, including CO oxidation, and it is anticipated that they will have high activity due to good dispersion of the gold on the surface. Further analysis of these materials for a variety of oxidation reactions could give rise to new, highly active catalysts for a variety of industrial processes.   Further work involving precursors to metal oxides other than ceria is already being explored. One example of this is the synthesis of cobalt formate, which will hopefully give rise to Pd/cobalt oxide via surface-assisted reduction. This will combine results from Chapters 4 and   188 5 and extend the scope of surface-assisted reduction past ceria, opening up the possibility of new catalytic materials for an even wider range of reactions.   The novel synthetic routes developed in this thesis may be applicable for other metal oxide materials, especially lanthanide oxides, and it would be interesting to determine to what extent they can be applied. While this thesis has focused on using the materials developed as catalysts for low-temperature methane oxidation, ceria-based materials have been used for a wide range of applications including CO oxidation,174 fuel cells122 and water treatment.115 It would be exciting to explore applications beyond methane oxidation for the materials developed in this thesis.       189 References  (1)  Rao, C. N. R.; Muller, A.; Cheetham, A. K. The Chemistry of Nanomaterials: Synthesis, Properties and Applications - Volume 1; Wiley-VCH, 2004. (2)  Ozin, G. A.; Arsenault, A. C.; Cademartiri, L. Nanochemistry: A Chemical Approach to Nanomaterials; Royal Society of Chemistry, 2008. (3)  Feynman, R. P. Eng. Sci. 1960, 23, 22–36. (4)  Taniguchi, N. In Proceedings of the International Conference on Production Engineering; Tokyo: Japan Society of Precision Engineering: Tokyo, 1974. (5)  Drexler, K. E. Engines of Creation: The Coming Era of Nanotechnology; Anchor Press/Doubleday, 1986. (6)  Roduner, E. Chem. Soc. Rev. 2006, 35, 583–592. (7)  Huang, W.-C.; Lue, J.-T. J. Phys. Chem. Solids 1997, 58, 1529–1538. (8)  Seo, W. S.; Jo, H. H.; Lee, K.; Kim, B.; Oh, S. J.; Park, J. T. Angew. Chem. Int. Ed. Engl. 2004, 43, 1115–1117. (9)  Song, Q.; Zhang, Z. J. J. Am. Chem. Soc. 2004, 126, 6164–6168. (10)  Seifert, G.; Terrones, H.; Terrones, M.; Jungnickel, G.; Frauenheim, T. Phys. Rev. Lett. 2000, 85, 146–149. (11)  Campelo, J. M.; Luna, D.; Luque, R.; Marinas, J. M.; Romero, A. A. ChemSusChem 2009, 2, 18–45. (12)  Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181–190. (13)  Varin, R. A.; Czujko, T.; Wronski, Z. S. Nanomaterials for Solid State Hydrogen Storage; Springer US, 2009. (14)  Bekyarova, E.; Murata, K.; Yudasaka, M.; Kasuya, D.; Iijima, S.; Tanaka, H.; Kahoh, H.; Kaneko, K. J. Phys. Chem. B 2003, 107, 4681–4684. (15)  Ahn, J.-H.; Kim, H.-S.; Lee, K. J.; Jeon, S.; Kang, S. J.; Sun, Y.; Nuzzo, R. G.; Rogers, J. A. Science 2006, 314, 1754–1757.   190 (16)  Slowing, I. I.; Trewyn, B. G.; Giri, S.; Lin, V. S.-Y. Adv. Funct. Mater. 2007, 17, 1225–1236. (17)  Bianco, A.; Kostarelos, K.; Prato, M. Curr. Opin. Chem. Biol. 2005, 9, 674–679. (18)  Rao, C. N. R.; Cheetham, A. K. J. Mater. Chem. 2001, 11, 2887–2894. (19)  Haas-Santo, K.; Fichtner, M.; Schubert, K. Appl. Catal., A 2001, 220, 79–92. (20)  Ansari, A. A.; Kaushik, A.; Solanki, P. R.; Malhotra, B. D. Electrochem. commun. 2008, 10, 1246–1249. (21)  Fan, J.; Boettcher, S. W.; Stucky, G. D. Chem. Mater. 2006, 18, 6391–6396. (22)  Jagadale, T. C.; Takale, S. P.; Sonawane, R. S.; Joshi, H. M.; Patil, S. I.; Kale, B. B.; Ogale, S. B. J. Phys. Chem. C 2008, 112, 14595–14602. (23)  Niederberger, M. Acc. Chem. Res. 2007, 40, 793–800. (24)  Gu, F.; Wang, S. F.; Lü, M. K.; Zhou, G. J.; Xu, D.; Yuan, D. R. J. Phys. Chem. B 2004, 108, 8119–8123. (25)  Trewyn, B. G.; Slowing, I. I.; Giri, S.; Chen, H.; Lin, V. S.-Y. Acc. Chem. Res. 2007, 40, 846–853. (26)  Lu, Y.; Yin, Y.; Mayers, B. T.; Xia, Y. Nano Lett. 2002, 2, 183–186. (27)  Hayashi, H.; Hakuta, Y. Materials. 2010, 3, 3794–3817. (28)  Mai, H.-X.; Sun, L.-D.; Zhang, Y.-W.; Si, R.; Feng, W.; Zhang, H.-P.; Liu, H.-C.; Yan, C.-H. J. Phys. Chem. B 2005, 109, 24380–24385. (29)  Wang, X.; Li, Y. J. Am. Chem. Soc. 2002, 124, 2880–2881. (30)  Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 4430–4431. (31)  Titirici, M.-M.; Antonietti, M.; Thomas, A. Chem. Mater. 2006, 18, 3808–3812. (32)  Song, H.; Rioux, R. M.; Hoefelmeyer, J. D.; Komor, R.; Niesz, K.; Grass, M.; Yang, P.; Somorjai, G. A. J. Am. Chem. Soc. 2006, 128, 3027–3037. (33)  Rouquerolt, J.; Avnir, D.; Fairbridge, C. W.; Everett, D. H.; Haynes, J. H.; Pernicone, N.; Ramsay, J. D. F.; Sing, K. S. W.; Unger, K. K. Pure Appl. Chem. 1994, 66, 1739–1758.   191 (34)  Chiola, V.; Ritsko, J. E.; Vanderpool, C. D. Process for producing low-bulk density silica. US3556725, January 19, 1971. (35)  Di Renzo, F.; Cambon, H.; Dutartre, R. Microporous Mater. 1997, 10, 283–286. (36)  Beck, J. S.; Borghard, W. S.; Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C. Synthetic porous crystalline material its synthesis and use. WO1991011390, August 1991. (37)  Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710–712. (38)  Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W. J. Am. Chem. Soc. 1992, 114, 10834–10843. (39)  Gibson, L. T. Chem. Soc. Rev. 2014, 43, 5163–5172. (40)  Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548–552. (41)  Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024–6036. (42)  Ravikovitch, P. I.; Neimark, A. V. Langmuir 2002, 18, 9830–9837. (43)  Kleitz, F.; Hei Choi, S.; Ryoo, R. Chem. Commun. 2003, 2136–2137. (44)  Fan, J.; Yu, C.; Gao, F.; Lei, J.; Tian, B.; Wang, L.; Luo, Q.; Tu, B.; Zhou, W.; Zhao, D. Angew. Chem. Int. Ed. Engl. 2003, 42, 3146–3150. (45)  Fan, J.; Yu, C.; Lei, J.; Zhang, Q.; Li, T.; Tu, B.; Zhou, W.; Zhao, D. J. Am. Chem. Soc. 2005, 127, 10794–10795. (46)  Schmolka, I. R. Polyoxyethylene-Polyoxypropylene Aqueous Gels. US3740421, June 19, 1973. (47)  Ren, Y.; Ma, Z.; Bruce, P. G. Chem. Soc. Rev. 2012, 41, 4909–4927. (48)  Shi, Y.; Wan, Y.; Zhao, D. Chem. Soc. Rev. 2011, 40, 3854–3878. (49)  Yamauchi, Y.; Kuroda, K. Chem. Asian J. 2008, 3, 664–676. (50)  Jiao, F.; Harrison, A.; Jumas, J.-C.; Chadwick, A. V; Kockelmann, W.; Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 5468–5474.   192 (51)  Jiao, F.; Shaju, K. M.; Bruce, P. G. Angew. Chem. Int. Ed. Engl. 2005, 44, 6550–6553. (52)  Park, S.-E. P.; Ryoo, R.; Ahn, W.-S.; Lee, C. W.; Chang, J.-S. Studies in Surface Science and Catalysis; Elsevier, 2003; Vol. 146. (53)  Schüth, F. Chem. Mater. 2001, 13, 3184–3195. (54)  Brinker, C. J.; Lu, Y.; Sellinger, A.; Fan, H. Adv. Mater. 1999, 11, 579–585. (55)  Ciesla, U.; Demuth, D.; Leon, R.; Petroff, P.; Stucky, G.; Unger, K.; Schüth, F. J. Chem. Soc. Chem. Commun. 1994, 1387–1388. (56)  Zhao, D.; Goldfarb, D. Chem. Mater. 1996, 8, 2571–2578. (57)  Antonelli, D. M.; Ying, J. Y. Angew. Chem. Int. Ed. Engl. 1995, 34, 2014–2017. (58)  Antonelli, D. M.; Ying, J. Y. Angew. Chem. Int. Ed. Engl. 1996, 35, 426–430. (59)  Zhang, J.; Deng, Y.; Gu, D.; Wang, S.; She, L.; Che, R.; Wang, Z.-S.; Tu, B.; Xie, S.; Zhao, D. Adv. Energy Mater. 2011, 1, 241–248. (60)  Takahara, Y.; Kondo, J. N.; Takata, T.; Lu, D.; Domen, K. Chem. Mater. 2001, 13, 1194–1199. (61)  Yang, P. D.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152–155. (62)  Yue, W.; Zhou, W. Prog. Nat. Sci. 2008, 18, 1329–1338. (63)  Zhu, K.; Yue, B.; Zhou, W.; He, H. Preparation of three-dimensional chromium oxide porous single crystals templated by SBA-15; The Royal Society of Chemistry, 2003; pp. 98–99. (64)  Jiao, K.; Zhang, B.; Yue, B.; Ren, Y.; Liu, S.; Yan, S.; Dickinson, C.; Zhou, W.; He, H. Chem. Commun. 2005, 5618–5620. (65)  Tian, B.; Liu, X.; Yang, H.; Xie, S.; Yu, C.; Tu, B.; Zhao, D. Adv. Mater. 2003, 15, 1370–1374. (66)  Dickinson, C.; Zhou, W.; Hodgkins, R. P.; Shi, Y.; Zhao, D.; He, H. Chem. Mater. 2006, 18, 3088–3095. (67)  Yue, W.; Zhou, W. Chem. Mater. 2007, 19, 2359–2363. (68)  Qiu, M.; Zhan, S.; Yu, H.; Zhu, D.; Wang, S. Nanoscale 2015, 7, 2568–2577.   193 (69)  Taguchi, A.; Schüth, F. Microporous Mesoporous Mater. 2005, 77, 1–45. (70)  Bréchignac, C.; Houdy, P.; Lahmani, M. Nanomaterials and nanochemistry; Springer, 2008. (71)  Andrievski, R. A. J. Mater. Sci. 2014, 49, 1449–1460. (72)  Nanostructured Materials for Engineering Applications; Bergmann, C. P.; de Andrade, M. J., Eds.; Springer, 2011. (73)  Pelaez, M.; Nolan, N. T.; Pillai, S. C.; Seery, M. K.; Falaras, P.; Kontos, A. G.; Dunlop, P. S. M.; Hamilton, J. W. J.; Byrne, J. A.; O’Shea, K.; Entezari, M. H.; Dionysiou, D. D. Appl. Catal., B 2012, 125, 331–349. (74)  Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271. (75)  Stone, V. F.; Davis, R. J. Chem. Mater. 1998, 10, 1468–1474. (76)  Sarkar, A.; Ghosh, S. K.; Pramanik, P. J. Mol. Catal. A Chem. 2010, 327, 73–79. (77)  Srivastava, D. N.; Perkas, N.; Gedanken, A.; Felner, I. J. Phys. Chem. B 2002, 106, 1878–1883. (78)  Piumetti, M.; Fino, D.; Russo, N. Appl. Catal., B 2015, 163, 277–287. (79)  Rosen, J.; Hutchings, G. S.; Jiao, F. J. Am. Chem. Soc. 2013, 135, 4516–4521. (80)  Xia, Y.; Dai, H.; Jiang, H.; Zhang, L. Catal. Commun. 2010, 11, 1171–1175. (81)  Transportation Forecast: Light Duty Vehicles Light Duty Stop-Start, Hybrid Electric, Plug-In Hybrid Electric, Battery Electric, Natural Gas, Fuel Cell, and Conventional Vehicles: Global Market Forecasts, 2015-2035, Navigant Research, 2015. http://www.navigantresearch.com/research/transportation-forecast-light-duty-vehicles (accessed Jun 15, 2015). (82)  Greenhouse gas sources and sinks in Canada: National Inventory Report 1990-2013, Government of Canada, 2013. http://www.ec.gc.ca/ges-ghg/default.asp?lang=En&n=5B59470C-1 (accessed Apr 4, 2015). (83)  Copenhagen Accord Decision -/CP.15. United Nations Climate Change Conference 2009., 2009. (84)  Canada’s Emissions Trends, Government of Canada, Environment Canada, 2013. https://ec.gc.ca/Publications/default.asp?lang=En&xml=1723EA20-77AB-4954-9333-69D1C4EBD0B2 (accessed Jun 15, 2015).   194 (85)  Overview of Natural Gas http://naturalgas.org/overview/background/ (accessed May 29, 2015). (86)  Technically Recoverable Shale Oil and Shale Gas Resources, US Energy Information Administration, 2013. http://www.eia.gov/analysis/studies/worldshalegas/pdf/fullreport.pdf (accessed May 29, 2015). (87)  Kopyscinski, J.; Schildhauer, T. J.; Biollaz, S. M. A. Fuel 2010, 89, 1763–1783. (88)  Worldwide NGVs and Refuelling Stations, NGVA Europe and the GVR, 2013. http://www.ngvaeurope.eu/worldwide-ngv-statistics (accessed May 29, 2015). (89)  Natural Gas 1998 Issues and Trends, U.S. Energy Information Administration, 1999. http://www.eia.gov/oil_gas/natural_gas/analysis_publications/natural_gas_1998_issues_and_trends/it98.html (accessed May 29, 2015). (90)  Gélin, P.; Primet, M. Appl. Catal., B 2002, 39, 1–37. (91)  Gholami, R.; Alyani, M.; Smith, K. J. Catalysts 2015, 5, 561–594. (92)  Li, Z.; Hoflund, G. B. J. Nat. Gas Chem. 2003, 12, 153–160. (93)  Ahlström-Silversand, A. F.; Odenbrand, C. U. I. Appl. Catal., A 1997, 153, 157–175. (94)  Burch, R. Catal. Today 1997, 35, 27–36. (95)  Su, S. C.; Carstens, J. N.; Bell, A. T. J. Catal. 1998, 176, 125–135. (96)  Lyubovsky, M.; Pfefferle, L. Appl. Catal., A 1998, 173, 107–119. (97)  Garbowski, E.; Feumi-Jantou, C.; Mouaddib, N.; Primet, M. Appl. Catal., A 1994, 109, 277–291. (98)  Schmal, M.; Souza, M. M. V. M.; Alegre, V. V.; da Silva, M. A. P.; César, D. V.; Perez, C. A. C. Catal. Today 2006, 118, 392–401. (99)  Li, Z.; Xu, G.; Hoflund, G. B. Fuel Process. Technol. 2003, 84, 1–11. (100)  Nilsson, J.; Carlsson, P.-A.; Fouladvand, S.; Martin, N. M.; Gustafson, J.; Newton, M. A.; Lundgren, E.; Grönbeck, H.; Skoglundh, M. ACS Catal. 2015, 5, 2481–2489. (101)  El Fallah, J.; Boujana, S.; Dexpert, H.; Kiennemann, A.; Majerus, J.; Touret, O.; Villain, F.; Le Normand, F. J. Phys. Chem. 1994, 98, 5522–5533.   195 (102)  Mamontov, E.; Egami, T.; Brezny, R.; Koranne, M.; Tyagi, S. J. Phys. Chem. B 2000, 104, 11110–11116. (103)  Balducci, G.; Kašpar, J.; Fornasiero, P.; Graziani, M.; Islam, M. S. J. Phys. Chem. B 1998, 102, 557–561. (104)  Trovarelli, A. Catalysis by ceria and related materials; Imperial College Press, 2005; Vol. 2. (105)  Balducci, G.; Islam, M. S.; Kašpar, J.; Fornasiero, P.; Graziani, M. Chem. Mater. 2003, 15, 3781–3785. (106)  Reddy, B. M.; Khan, A.; Lakshmanan, P.; Aouine, M.; Loridant, S.; Volta, J.-C. J. Phys. Chem. B 2005, 109, 3355–3363. (107)  Chen, H.-L.; Chang, J.-G.; Chen, H.-T. Chem. Phys. Lett. 2011, 502, 169–172. (108)  Encyclpedia of Polymeric Nanomaterials; Kobayashi, S.; Müllen, K., Eds.; Springer, 2015. (109)  Ketzial, J. J.; Nesaraj, A. S. J. Ceram. Process. Res. 2011, 12, 74–79. (110)  Godinho, M. J.; Gonçalves, R. F.; Santos, L. P. S.; Varela, J. A.; Longo, E.; Leite, E. R. Mater. Lett. 2007, 61, 1904–1907. (111)  Shi, L.; Chu, W.; Qu, F.; Luo, S. Catal. Lett. 2007, 113, 59–64. (112)  Yu, H.; Bai, Y.; Zong, X.; Tang, F.; Lu, G. Q. M.; Wang, L. Chem. Commun. 2012, 48, 7386–7388. (113)  Li, H.; Lu, G.; Dai, Q.; Wang, Y.; Guo, Y.; Guo, Y. ACS Appl. Mater. Interfaces 2010, 2, 838–846. (114)  Li, H.; Lu, G.; Wang, Y.; Guo, Y.; Guo, Y. Catal. Commun. 2010, 11, 946–950. (115)  Yang, Z.; Wei, J.; Yang, H.; Liu, L.; Liang, H.; Yang, Y. Eur. J. Inorg. Chem. 2010, 2010, 3354–3359. (116)  Zhou, K.; Wang, X.; Sun, X.; Peng, Q.; Li, Y. J. Catal. 2005, 229, 206–212. (117)  Gnanam, S.; Rajendran, V. J. Sol-Gel Sci. Technol. 2011, 58, 62–69. (118)  Periyat, P.; Laffir, F.; Tofail, S. A. M.; Magner, E. RSC Adv. 2011, 1, 1794–1798. (119)  Niederberger, M.; Garnweitner, G. Chemistry 2006, 12, 7282–7302.   196 (120)  Xiao, H.; Ai, Z.; Zhang, L. J. Phys. Chem. C 2009, 113, 16625–16630. (121)  Thundathil, M. A.; Lai, W.; Noailles, L.; Dunn, B. S.; Haile, S. M. J. Am. Ceram. Soc. 2004, 87, 1442–1445. (122)  Laberty-Robert, C.; Long, J. W.; Lucas, E. M.; Pettigrew, K. A.; Stroud, R. M.; Doescher, M. S.; Rolison, D. R. Chem. Mater. 2006, 18, 50–58. (123)  Zagaynov, I. V.; Kutsev, S. V. Appl. Nanosci. 2013, 4, 339–345. (124)  Lyons, D. M.; Ryan, K. M.; Morris, M. A. J. Mater. Chem. 2002, 12, 1207–1212. (125)  Ni, C.; Li, X.; Chen, Z.; Li, H.-Y. H.; Jia, X.; Shah, I.; Xiao, J. Q. Microporous Mesoporous Mater. 2008, 115, 247–252. (126)  Rossinyol, E.; Arbiol, J.; Peiró, F.; Cornet, A.; Morante, J. R. R.; Tian, B.; Bo, T.; Zhao, D. Sens. Actuators, B 2005, 109, 57–63. (127)  Shen, W.; Dong, X.; Zhu, Y.; Chen, H.; Shi, J. Microporous Mesoporous Mater. 2005, 85, 157–162. (128)  Pengpanich, S.; Meeyoo, V.; Rirksomboon, T.; Bunyakiat, K. Appl. Catal., A 2002, 234, 221–233. (129)  Kundakovic, L.; Flytzani-Stephanopoulos, M. J. Catal. 1998, 179, 203–221. (130)  Palmqvist, A. E. C.; Johansson, E. M.; Järås, S. G.; Muhammed, M. Catal. Lett. 1998, 56, 69–75. (131)  Tompos, A.; Margitfalvi, J. L.; Tfirst, E.; Végvári, L.; Jaloull, M. A.; Khalfalla, H. A.; Elgarni, M. M. Appl. Catal., A 2005, 285, 65–78. (132)  Haneda, M.; Mizushima, T.; Kakuta, N. J. Phys. Chem. B 1998, 102, 6579–6587. (133)  Xiao, L.-H.; Sun, K.-P.; Xu, X.-L.; Li, X.-N. Catal. Commun. 2005, 6, 796–801. (134)  Colussi, S.; Gayen, A.; Camellone, M. F.; Boaro, M.; Llorca, J.; Fabris, S.; Trovarelli, A. Angew. Chem. Int. Ed. Engl. 2009, 48, 8481–8484. (135)  Cargnello, M.; Jaen, J. J. D.; Garrido, J. C. H.; Bakhmutsky, K.; Montini, T.; Gamez, J. J. C.; Gorte, R. J.; Fornasiero, P. Science 2012, 337, 713–717. (136)  Cargnello, M.; Wieder, N. L.; Montini, T.; Gorte, R. J.; Fornasiero, P. J. Am. Chem. Soc. 2010, 132, 1402–1409.   197 (137)  Eckert, M. Ann. Phys. 2012, 524, A83–A85. (138)  Fultz, B.; Howe, J. M. Transmission electron microscopy and diffractometry of materials; Springer, 2008. (139)  Scardi, P.; Leoni, M.; Beyerlein, K. R. Zeitschrift für Krist. 2011, 226, 924–933. (140)  Scherrer, P. Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Math. Klasse 1918, 1918, 98–100. (141)  Langford, J. I.; Wilson, A. J. C. J. Appl. Crystallogr. 1978, 11, 102–113. (142)  Patterson, A. L. Phys. Rev. 1939, 56, 978–982. (143)  Williams, D. B.; Carter, C. B. Transmission Electron Microscopy: A Textbook for Materials Science; Springer, 2009; Vol. 3. (144)  McMillan, W. G.; Teller, E. J. Phys. Chem. 1951, 55, 17–20. (145)  Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquérol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603–619. (146)  Kajiro, H.; Kondo, A.; Kaneko, K.; Kanoh, H. Int. J. Mol. Sci. 2010, 11, 3803–3845. (147)  Einstein, A. Am. J. Phys. 1965, 33, 367–374. (148)  Van der Heide, P. X-ray Photoelectron Spectroscopy: An introduction to Principles and Practices; John Wiley & Sons, 2011; Vol. 1. (149)  Yin, L.; Wang, Y.; Pang, G.; Koltypin, Y.; Gedanken, A. J. Colloid Interface Sci. 2002, 246, 78–84. (150)  Hwang, C.-C.; Huang, T.-H.; Tsai, J.-S.; Lin, C.-S.; Peng, C.-H. Mater. Sci. Eng. B 2006, 132, 229–238. (151)  Ntainjua N., E.; Garcia, T.; Solsona, B.; Taylor, S. H. Catal. Today 2008, 137, 373–378. (152)  Gu, F.; Wang, Z.; Han, D.; Shi, C.; Guo, G. Mater. Sci. Eng. B 2007, 139, 62–68. (153)  Gao, Y.; Ding, X.; Zheng, Z.; Peng, Y. e-Polymers 2013, 8, 1308–1315. (154)  Yang, S.; Gao, L. J. Am. Chem. Soc. 2006, 128, 9330–9331.   198 (155)  Carrettin, S.; Concepción, P.; Corma, A.; López Nieto, J. M.; Puntes, V. F. Angew. Chem. Int. Ed. Engl. 2004, 43, 2538–2540. (156)  Lawrence, N. J.; Brewer, J. R.; Wang, L.; Wu, T.-S.; Wells-Kingsbury, J.; Ihrig, M. M.; Wang, G.; Soo, Y.-L.; Mei, W.-N.; Cheung, C. L. Nano Lett. 2011, 11, 2666–2671. (157)  Palmqvist, A. E. C.; Wirde, M.; Gelius, U.; Muhammed, M. Nanostruct. Mater. 1999, 11, 995–1007. (158)  Zhang, Y.; Andersson, S.; Muhammed, M. Appl. Catal., B 1995, 6, 325–337. (159)  Yang, T.; Xia, D. Mater. Chem. Phys. 2010, 123, 816–820. (160)  Bali, S.; Huggins, F. E.; Ernst, R. D.; Pugmire, R. J.; Huffman, G. P.; Eyring, E. M. Ind. Eng. Chem. Res. 2010, 49, 1652–1657. (161)  Ouzaouit, K.; Benlhachemi, A.; Benyaich, H.; Aneflous, L.; Marrouche, A.; Gavarri, J. R.; Musso, J. J. Phys. IV 2005, 123, 125–130. (162)  De Leitenburg, C.; Trovarelli, A.; Llorca, J.; Cavani, F.; Bini, G. Appl. Catal., A 1996, 139, 161–173. (163)  Ouyang, J.; Yang, H. J. Phys. Chem. C 2009, 113, 6921–6928. (164)  Suzuki, K.; Sinha, A. K. J. Mater. Chem. 2007, 17, 2547. (165)  Evans, G.; Kozhevnikov, I. V.; Kozhevnikova, E. F.; Claridge, J. B.; Vaidhyanathan, R.; Dickinson, C.; Wood, C. D.; Cooper, A. I.; Rosseinsky, M. J. J. Mater. Chem. 2008, 18, 5518–5523. (166)  Brown, A. S. C.; Hargreaves, J. S. J.; Rijniersce, B. Catal. Today 1998, 45, 47–54. (167)  Kang, M.; Song, M. W.; Lee, C. H. Appl. Catal., A 2003, 251, 143–156. (168)  Brun, M.; Berthet, A.; Bertolini, J. C. J. Electron Spectros. Relat. Phenom. 1999, 104, 55–60. (169)  Terribile, D.; Trovarelli, A.; Llorca, J.; de Leitenburg, C.; Dolcetti, G. J. Catal. 1998, 178, 299–308. (170)  Zhou, K.; Yang, Z.; Yang, S. Chem. Mater. 2007, 19, 1215–1217. (171)  Hua, G.; Zhang, L.; Fei, G.; Fang, M. J. Mater. Chem. 2012, 22, 6851–6855.   199 (172)  Fang, J.; Cao, Z.; Zhang, D.; Shen, X.; Ding, W.; Shi, L. J. Rare Earths 2008, 26, 153–157. (173)  Yan, L.; Yu, R.; Chen, J.; Xing, X. Cryst. Growth Des. 2008, 8, 1474–1477. (174)  Pan, C.; Zhang, D.; Shi, L. J. Solid State Chem. 2008, 181, 1298–1306. (175)  Ho, C.; Yu, J. C.; Kwong, T.; Mak, A. C.; Lai, S. Chem. Mater. 2005, 17, 4514–4522. (176)  Xia, Y.; Mokaya, R. J. Mater. Chem. 2005, 15, 3126–3131. (177)  Davis, M. E. Nature 2002, 417, 813–821. (178)  Strandwitz, N. C.; Shaner, S.; Stucky, G. D. J. Mater. Chem. 2011, 21, 10672–10675. (179)  Chang, J.-K.; Chen, Y.-L.; Tsai, W.-T. J. Power Sources 2004, 135, 344–353. (180)  Mori, K.; Hara, T.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc. 2004, 126, 10657–10666. (181)  Yamaguchi, K.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc. 2000, 122, 7144–7145. (182)  Kim, K. H.; Lee, S. Y.; Yoon, K. J. Korean J. Chem. Eng. 2006, 23, 356–361. (183)  Jun, J. H.; Jeong, K. S.; Lee, T.-J.; Kong, S. J.; Lim, T. H.; Nam, S.-W.; Hong, S.-A.; Yoon, K. J. Korean J. Chem. Eng. 2004, 21, 140–146. (184)  Williamson, P. A.; Blower, P. J.; Green, M. A. Chem. Commun. 2011, 47, 1568–1570. (185)  Tan, B.; Rankin, S. E. Langmuir 2005, 21, 8180–8187. (186)  Wang, J.-X.; Wen, L.-X.; Liu, R.-J.; Chen, J.-F. J. Solid State Chem. 2005, 178, 2383–2389. (187)  Li, Y.; Xie, X.; Liu, J.; Cai, M.; Rogers, J.; Shen, W. Chem. Eng. J. 2008, 136, 398–408. (188)  Voorhees, P. W. J. Stat. Phys. 1985, 38, 231–252. (189)  Luo, J. Y.; Meng, M.; Li, X.; Li, X. G.; Zha, Y. Q.; Hu, T. D.; Xie, Y. N.; Zhang, J. J. Catal. 2008, 254, 310–324. (190)  Liotta, L. F.; Wu, H.; Pantaleo, G.; Venezia, A. M. Catal. Sci. Technol. 2013, 3, 3085–3102.   200 (191)  Xue, L.; Zhang, C.; He, H.; Teraoka, Y. Appl. Catal., B 2007, 75, 167–174. (192)  Liotta, L. F.; Ousmane, M.; Di Carlo, G.; Pantaleo, G.; Deganello, G.; Marcì, G.; Retailleau, L.; Giroir-Fendler, A. Appl. Catal., A 2008, 347, 81–88. (193)  Li, H.; Lu, G.; Qiao, D.; Wang, Y.; Guo, Y.; Guo, Y. Catal. Lett. 2011, 141, 452–458. (194)  Liotta, L. F.; Di Carlo, G.; Pantaleo, G.; Venezia, A. M.; Deganello, G. Appl. Catal., B 2006, 66, 217–227. (195)  Hu, B.; Liu, H.; Tao, K.; Xiong, C.; Zhou, S. J. Phys. Chem. C 2013, 117, 26385–26395. (196)  Pirez, C.; Caderon, J.-M.; Dacquin, J.-P.; Lee, A. F.; Wilson, K. ACS Catal. 2012, 2, 1607–1614. (197)  Wang, J.; Li, Y.; Zhang, Z.; Hao, Z. J. Mater. Chem. A 2015, 3, 8650–8658. (198)  Dai, W.; Zheng, M.; Zhao, Y.; Liao, S.; Ji, G.; Cao, J. Nanoscale Res. Lett. 2010, 5, 103–107. (199)  Yue, W.; Zhou, W. J. Mater. Chem. 2007, 17, 4947–4952. (200)  Ren, Y.; Ma, Z.; Qian, L.; Dai, S.; He, H.; Bruce, P. G. Catal. Lett. 2009, 131, 146–154. (201)  Tüysüz, H.; Comotti, M.; Schüth, F. Chem. Commun. 2008, 4022–4024. (202)  Garcia, T.; Agouram, S.; Sánchez-Royo, J. F.; Murillo, R.; Mastral, A. M.; Aranda, A.; Vázquez, I.; Dejoz, A.; Solsona, B. Appl. Catal., A 2010, 386, 16–27. (203)  Du, Y.; Meng, Q.; Wang, J.; Yan, J.; Fan, H.; Liu, Y.; Dai, H. Microporous Mesoporous Mater. 2012, 162, 199–206. (204)  Ma, C. Y.; Mu, Z.; Li, J. J.; Jin, Y. G.; Cheng, J.; Lu, G. Q.; Hao, Z. P.; Qiao, S. Z. J. Am. Chem. Soc. 2010, 132, 2608–2613. (205)  Yue, W.; Hill, A. H.; Harrison, A.; Zhou, W. Chem. Commun. 2007, 2518–2520. (206)  Shopsowitz, K. E.; Qi, H.; Hamad, W. Y.; Maclachlan, M. J. Nature 2010, 468, 422–425. (207)  Pereira, G. J.; Castro, R. H. R.; de Florio, D. Z.; Muccillo, E. N. S.; Gouvêa, D. Mater. Lett. 2005, 59, 1195–1199.   201 (208)  Alexandrou, M.; Nix, R. M. Surf. Sci. 1994, 321, 47–57. (209)  Burroughs, P.; Hamnett, A.; Orchard, A. F.; Thornton, G. J. Chem. Soc. Dalt. Trans. 1976, 1686–1698. (210)  Bera, P.; Anandan, C. RSC Adv. 2014, 4, 62935–62939. (211)  McIntyre, N. S.; Cook, M. G. Anal. Chem. 1975, 47, 2208–2213. (212)  Garbowski, E.; Guenin, M.; Marion, M.-C.; Primet, M. Appl. Catal. 1990, 64, 209–224. (213)  Xu, R.; Wang, J.; Li, Q.; Sun, G.; Wang, E.; Li, S.; Gu, J.; Ju, M. J. Solid State Chem. 2009, 182, 3177–3182. (214)  Chuang, T. J.; Brundle, C. R.; Rice, D. W. Surf. Sci. 1976, 59, 413–429. (215)  Gholami, R.; Smith, K. J. Appl. Catal., B 2015, 168-169, 156–163. (216)  Baldi, M.; Escribano, V. S.; Amores, J. M. G.; Milella, F.; Busca, G. Appl. Catal., B 1998, 17, L175–L182. (217)  Wang, Y.; Yang, X.; Hu, L.; Li, Y.; Li, J. Chinese J. Catal. 2014, 35, 462–467. (218)  Liotta, L.; Di Carlo, G.; Pantaleo, G.; Deganello, G. Catal. Commun. 2005, 6, 329–336. (219)  Liu, L.; Zhang, X.; Liu, J. Mater. Lett. 2014, 136, 209–213. (220)  Bozo, C.; Guilhaume, N.; Herrmann, J.-M. J. Catal. 2001, 203, 393–406. (221)  Mayernick, A. D.; Janik, M. J. J. Catal. 2011, 278, 16–25. (222)  Vayssilov, G. N.; Lykhach, Y.; Migani, A.; Staudt, T.; Petrova, G. P.; Tsud, N.; Skála, T.; Bruix, A.; Illas, F.; Prince, K. C.; Matolín, V.; Neyman, K. M.; Libuda, J. Nat. Mater. 2011, 10, 310–315. (223)  Azimi, G.; Dhiman, R.; Kwon, H.-M.; Paxson, A. T.; Varanasi, K. K. Nat. Mater. 2013, 12, 315–320. (224)  Shen, W. J.; Ichihashi, Y.; Matsumura, Y. Catal. Lett. 2002, 79, 125–127. (225)  Thevenin, P. O.; Alcalde, A.; Pettersson, L. J.; Järås, S. G.; Fierro, J. L. G. J. Catal. 2003, 215, 78–86.   202 (226)  Le Normand, F.; Hilaire, L.; Kili, K.; Krill, G.; Maire, G. J. Phys. Chem. 1988, 92, 2561–2568. (227)  Luo, M.-F.; Hou, Z.-Y.; Yuan, X.-X.; Zheng, X.-M. Catal. Lett. 1998, 50, 205–209. (228)  Bera, P.; Hegde, M. S. Catal. Lett. 2002, 79, 75–81. (229)  Camellone, M. F.; Fabris, S. J. Am. Chem. Soc. 2009, 131, 10473–10483. (230)  Fu, Q.; Weber, A.; Flytzani-Stephanopoulos, M. Catal. Lett. 2001, 77, 87–95. (231)  Si, R.; Flytzani-Stephanopoulos, M. Angew. Chem. Int. Ed. Engl. 2008, 47, 2884–2887. (232)  Chang, S.; Li, M.; Hua, Q.; Zhang, L.; Ma, Y.; Ye, B.; Huang, W. J. Catal. 2012, 293, 195–204. (233)  Bera, P.; Patil, K. C.; Hegde, M. S. Phys. Chem. Chem. Phys. 2000, 2, 3715–3719. (234)  Imamura, S.; Uchihori, D.; Utani, K.; Ito, T. Catal. Lett. 1994, 24, 377–384. (235)  Ding, H.-X.; Zhu, A.-M.; Lu, F.-G.; Xu, Y.; Zhang, J.; Yang, X.-F. J. Phys. D. Appl. Phys. 2006, 39, 3603–3608. (236)  Zhong, L.-S.; Hu, J.-S.; Cao, A.-M.; Liu, Q.; Song, W.-G.; Wan, L.-J. Chem. Mater. 2007, 19, 1648–1655. (237)  Larcher, D.; Sudant, G.; Patrice, R.; Tarascon, J.-M. Chem. Mater. 2003, 15, 3543–3551. (238)  Ghosh, S. K.; Pal, T. Chem. Rev. 2007, 107, 4797–4862. (239)  Kapoor, M. P.; Ichihashi, Y.; Nakamori, T.; Matsumura, Y. J. Mol. Catal. A Chem. 2004, 213, 251–255. (240)  Venezia, A. M.; La Parola, V.; Nicolı̀, V.; Deganello, G. J. Catal. 2002, 212, 56–62. (241)  Sekizawa, K.; Widjaja, H.; Maeda, S.; Ozawa, Y.; Eguchi, K. Catal. Today 2000, 59, 69–74. (242)  Vishnyakov, A. V; Gridasova, T. P.; Chashchin, V. A.; Rodina, K. V. Kinet. Catal. 2011, 52, 733–738. (243)  Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997, 62, 7512–7515.    203 Appendix A Additional characterization for Chapter 1.   Figure A-1: PXRD patterns for Comp1 control experiments. a) 10 mol% and b) 20 mol% of Mn(OAc)2!4H2O was added to preformed CeO2 in ethanol, dried at 60 ˚C then calcined at 400 ˚C. "= MnO.   204  Figure A-2: PXRD patterns of Comp1-10%La, (yellow), Comp1-20%La (blue), Comp1-50%La (green), Comp1-80%La (red) and Comp1-100%La.   Figure A-3: PXRD patterns of Comp1-10%Cu, (yellow), Comp1-20%Cu (blue), Comp1-50%Cu (green)   205   Figure A-4: a) SEM and b) TEM images of Comp1-50%La. SEM scale bar = 4 µm and TEM scale bar = 100 nm.    206 Appendix B Details of NMR spectroscopy from Chapter 3.  The residual liquid from the synthesis of Comp9 shows two major resonances in the d6-DMSO 1H NMR spectrum, both are singlets, at 3.39 and 4.45 ppm. Gottlieb et al. have reported that the CH proton of ethylene glycol resonates at 3.34 ppm in d6-DMSO (no value was given for the OH resonance).243 This difference may be explained by the samples being recorded at different concentrations or due to the presence of cerium salts, but to further investigate this, we prepared a sample of “wet” ethylene glycol in d6-DMSO consisting of 19:1 v/v ethylene glycol:water. The 1H NMR spectrum of this showed a CH resonance at 3.39 ppm, the same as the residual liquid.  The relative areas of the two peaks integrate to 2.4:1 for the 3.39 ppm and 4.45 ppm peaks, respectively. We would expect 2:1 for these peaks (4 CH protons and 2 OH protons); the slight difference is probably due to water overlapping with the CH resonance, thus increasing its intensity (water typically resonates around 3.3 ppm in d6-DMSO).  Several other compounds are clearly visible in the 1H NMR spectrum of the residual liquid (Figure B-1), although their relative intensity is very low (all integrate to < 1% of the ethylene glycol resonance at 3.39 ppm), and none could be immediately identified.  The 13C NMR spectrum of the residual liquid has one singlet resonance at 62.9 ppm.  Gottlieb et al. report that ethylene glycol resonates at 62.8 ppm in d6-DMSO.243 The sample   207 of “wet” ethylene glycol shows one peak at 63.2 ppm, showing that concentration/water content can cause small shifts in the position of this peak. Thus both the 1H and 13C NMR data are entirely consistent with the vast majority of the residual liquid consisting of ethylene glycol, as well as some water and inorganic salts, and trace amounts of other organic molecules.   Figure B-1: 1H NMR spectrum of residual liquid showing trace organic molecules in baseline of the spectrum. The spectrum has been “zoomed in” to show these molecules truncating the two major peaks: the resonance at 3.39 ppm extends approximately ten times the height of the image, while the resonance at 4.45 ppm extends approximately four times the height of the image. Asterisk indicates the residual DMSO solvent signal (300 MHz, 25 ˚C).    208 Appendix C Additional characterization for Chapter 5.   Figure C-1: Photographs of calcined a) Comp16-CF/Au(0.5mM), b) Comp16-CF/Au(1mM), c) Comp16-CHC/Au(0.5mM, and d) Comp16-CHC/Au(1mM).    

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