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Investigating Metal Oxide Nanoparticles as Photocatalysts for Carbon Dioxide Reduction Reactions by « Thomas Wood » A thesis submitted in conformity with the requirements for the degree of Doctorate of Philosophy Chemical Engineering and Applied Chemistry University of Toronto © Copyright by Thomas Wood 2017 ii Investigating Metal Oxide Nanoparticles as Photocatalysts for Carbon Dioxide Reduction Reactions Thomas Wood Doctorate of Philosophy Chemical Engineering and Applied Chemistry University of Toronto 2017 Abstract Metal oxide nanoparticles were investigated for their application as photocatalysts for the gas- phase photocatalytic conversion of carbon dioxide to value-added chemicals and fuels. A multi- photoreactor catalyst screening system was developed to screen a broad set of nanoparticles for 13 photocatalytic activity. Carbon-13 labelled carbon dioxide ( CO2) isotope tracing was carried out to discriminate whether the products arose from CO2 conversion or from reactions involving residual carbon contamination. A set of metal oxide photocatalysts were investigated at o o temperatures ranging from 80 C to 190 C in atmospheres containing both H2 and CO2. A nanoparticle heterostructure composed of Cu sputtered into a mesoporous TiO2 nanoparticle film was synthesized. These films showed evidence of increased hydrocarbon production under illumination, but no 13C labelled product was measured. iii Hydroxylated indium sesquioxide (In2O3-xOHy) nanoparticles were synthesized and investigated for the photocatalytic conversion of CO2. It was found that In2O3-xOHy nanoparticles are capable of photocatalytically and thermally driving the reverse water gas shift (RWGS) reaction at o 13 13 temperatures above 130 C. Both CO and trace CH4 were measured. A strong relationship between the CO2 adsorption capacity and the relative surface concentrations of surface defects, such as hydroxides and oxygen vacancies, and photocatalytic activity was established. It was found that there is strong dependence on illumination intensity and wavelength, suggesting that the improved activity under illumination is from light. A combined computational and experimental study investigating the (111) surface and kinetics of the RWGS reaction over In2O3-xOHy was carried out. It was found that a surface site containing a surface hydroxide adjacent to a surface oxygen vacancy was capable of the heterolytic splitting of H2 and the subsequent reduction of CO2 into CO. This proposed mechanism is analogous to a surface frustrated Lewis pair (FLP) and this discovery represents the first time such a system is shown to be present on the surface of a photocatalyst. Kinetic st measurements revealed a 1 order dependence on CO2 in the dark and light, whereas a zero-order dependence was observed for H2. These observations are consistent with the proposed mechanism from theory, which is similar to the Eley-Rideal mechanism. iv Acknowledgments I am truly grateful for the opportunity, support, and guidance offered to me by Professor Charles A. Mims and Professor Geoffrey A. Ozin. Over the years, their unwavering supported made this work possible. I would also like to also thank my colleagues and collaborators who have helped, guided me, and put a lot of hard work into this project: Dr. Peter Brodersen, Professor Cathy Chin, Yuchan Dong, Dr. Mo Hamdah, Johnny Haui, Dr. Le He, Dr. Laura Hoch, Abdinoor Jelle, Dr. Kulbir. Kulman, Dr. Kristine Liao, Joel Loh, Dr. Benoit Mahler, Sue Mamiche, Azer Mehmoodi, Neha Mithia, Dr. Jonathon Moir, Dr. Paul Obrien, Jia Jia Qie, Dr. Engelbert Redel, Laura Reyes, Amit Sandhel, Professor Chandra Veer Signh, Dr. Rana Sodhi, Dr. Navid Soheilnia, Annabelle Wong, Andy Zhou. From the University of Western Ontario: Dr. Jose E. Herrera and Dr. Dong Min. v Table of Contents Acknowledgments.......................................................................................................................... iv Table of Contents .............................................................................................................................v List of Tables ................................................................................................................................. ix List of Figures ..................................................................................................................................x List of Appendices ...................................................................................................................... xvii Chapter 1 Introduction .....................................................................................................................1 1.1 Setting the stage for Solar Fuels ..........................................................................................1 1.2 Solar Fuels at the University of Toronto..............................................................................4 1.3 Thesis Objectives .................................................................................................................4 1.4 Research Context for Gas-phase CO2 Photocatalysis ..........................................................5 1.5 Research Context for Screening Photocatalysts ..................................................................6 1.6 Research Context for Hydroxylated Indium Oxide Nanoparticles ......................................8 1.7 Thesis Outline ......................................................................................................................9 Chapter 2 Literature Review ..........................................................................................................11 2.1 Introduction to Photocatalysis............................................................................................11 2.2 Fundamentals of Photocatalysis .........................................................................................12 2.3 Semiconductor Photocatalysis ...........................................................................................14 2.4 Energetics of Photocatalytic Reactions ..............................................................................17 2.4.1 Thermal Catalysis Kinetics and Equilibrium .........................................................17 2.4.2 Thermodynamics of Photocatalysis .......................................................................20 2.5 Material Properties of Semiconductor Photocatalysts .......................................................22 vi 2.5.1 Bulk properties .......................................................................................................22 2.6 Surface Chemistry ..............................................................................................................26 2.6.1 Surface Oxygen Defects ........................................................................................28 2.6.2 Acidic and Basic Sites ...........................................................................................28 2.7 Multi-component Systems .................................................................................................29 2.8 Background on In2O3 .........................................................................................................32 Chapter 3 Experimental Methods ..................................................................................................34 3.1 Photocatalytic Testing ........................................................................................................34 3.2 Batch photoreactor testing systems ....................................................................................35 3.2.1 Multi-photoreactor Material Screening System .....................................................38 3.2.2 Dual Arc Lamp Photoreactor System ....................................................................42 3.2.3 Fixed Bed Photocatalytic Reactor ..........................................................................44 3.3 Isotope tracing experiments ...............................................................................................46 3.4 Material Characterization...................................................................................................48 3.4.1 X-ray Photoelectron Spectroscopy ........................................................................48 3.4.2 Fourier Transform Infrared Spectroscopy .............................................................49 Chapter 4 Fact or Fiction: Results from Preliminary Screening of Semiconductor Photocatalysts ............................................................................................................................50 4.1 Introduction ........................................................................................................................50 4.2 Results and Discussion ......................................................................................................52 4.2.1 Case Study: Cu/TiO2 Photocatalysts ......................................................................53 4.2.2 Case Study 2: Ru/TiO2 ...........................................................................................61 4.2.3 Case Study 3: CH4 Production from Indium Oxide based Semiconductors ..........63 4.3 Conclusions ........................................................................................................................66 vii Chapter 5 The Rational Design of a Single-Component Photocatalyst for Gas-Phase CO2 Reduction Using Both UV and Visible Light ...........................................................................68