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Bridging the Materials Gap in Catalysis: Reactivity Studies of Nanostructured Titania

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Bridging the Materials Gap in Catalysis: Reactivity Studies of Nanostructured Titania University of Pennsylvania ScholarlyCommons Publicly Accessible Penn Dissertations 2016 Bridging The Materials Gap In Catalysis: Reactivity Studies Of Nanostructured Titania David Alan Bennett University of Pennsylvania, [email protected] Follow this and additional works at: https://repository.upenn.edu/edissertations Part of the Chemical Engineering Commons, and the Chemistry Commons Recommended Citation Bennett, David Alan, "Bridging The Materials Gap In Catalysis: Reactivity Studies Of Nanostructured Titania" (2016). Publicly Accessible Penn Dissertations. 2187. https://repository.upenn.edu/edissertations/2187 This paper is posted at ScholarlyCommons. https://repository.upenn.edu/edissertations/2187 For more information, please contact [email protected]. Bridging The Materials Gap In Catalysis: Reactivity Studies Of Nanostructured Titania Abstract ABSTRACT BRIDGING THE MATERIALS GAP IN CATALYSIS: REACTIVITY STUDIES OF NANOSTRUCTURED TITANIA David A. Bennett John M. Vohs Surface science studies of defect-free, single-crystal model catalysts have provided vital knowledge in the form of structure-activity relationships and elementary reaction mechanisms. However, there is some difficultly in extending this understandingo t more complex systems, since these model catalysts typically lack the range of features that occur on the high surface area catalysts used in industry. This project seeks to bridge the gap between these two classes of materials by studying thin films of well-defined TiO2 nanocrystals with tunable size and morphology using traditional surface science techniques. This enables the controlled introduction of features lacking in single-crystal model catalysts, allowing for the formulation of more complex structure-activity relationships. The thermal- and photocatalytic reactions of methanol to produce methane, formaldehyde, dimethyl ether, and methyl formate on these TiO2 nanoparticles were investigated using temperature programmed desorption in ultra high vacuum. Results show clear effects of nanocrystal size and shape on the activity and selectivity of these reactions. This demonstrates the value of studying nanostructured metal-oxide catalysts to better understand the relationship between fundamental catalytic knowledge and the behavior of complex heterogeneous catalysts. Degree Type Dissertation Degree Name Doctor of Philosophy (PhD) Graduate Group Chemical and Biomolecular Engineering First Advisor John M. Vohs Keywords anatase, catalysis, surface science, titanium dioxide Subject Categories Chemical Engineering | Chemistry This dissertation is available at ScholarlyCommons: https://repository.upenn.edu/edissertations/2187 BRIDGING THE MATERIALS GAP IN CATALYSIS: REACTIVITY STUDIES OF NANOSTRUCTURED TITANIA David A. Bennett A DISSERTATION in Chemical and Biomolecular Engineering Presented to the Faculties of the University of Pennsylvania in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy 2016 Supervisor of Dissertation _____________________________________________________ Dr. John M. Vohs, Professor, Chemical and Biomolecular Engineering Graduate Group Chairperson _____________________________________________________ Dr. John C. Crocker, Professor, Chemical and Biomolecular Engineering Dissertation Committee Raymond J. Gorte, Professor, CBE Talid R. Sinno, Professor, CBE Christopher B. Murray, Professor, Chemistry ACKNOWLEDGEMENTS First and foremost, I want to thank my thesis advisor, John Vohs, for the guidance, advice, and patience he has shown me throughout the past five years. It goes without saying this project owes a large part of its success to his expertise and judgement. But beyond that, the balance he struck between mentorship and independence was exactly what I needed to develop as a scientist. I hope to learn from this example. I am also indebted to my collaborators, Matteo Cargnello, Thomas Gordon, and Benjamin Diroll, for providing the materials used in these studies. Without their proficiency in synthesis of nanostructured materials, this project would not have been possible. I also need to thank my co-workers in the Vohs and Gorte labs, in particular Eddie Martono and Jesse McManus for teaching me all about surface science. Lastly, I am extremely grateful for the support I have had from my family and from all the friends I have made in Philadelphia, in the chemical engineering department and elsewhere at Penn, at my church, and on our (seven-time champion) ultimate frisbee team. ii ABSTRACT BRIDGING THE MATERIALS GAP IN CATALYSIS: REACTIVITY STUDIES OF NANOSTRUCTURED TITANIA David A. Bennett John M. Vohs Surface science studies of defect-free, single-crystal model catalysts have provided vital knowledge in the form of structure-activity relationships and elementary reaction mechanisms. However, there is some difficultly in extending this understanding to more complex systems, since these model catalysts typically lack the range of features that occur on the high surface area catalysts used in industry. This project seeks to bridge the gap between these two classes of materials by studying thin films of well-defined TiO2 nanocrystals with tunable size and morphology using traditional surface science techniques. This enables the controlled introduction of features lacking in single-crystal model catalysts, allowing for the formulation of more complex structure-activity relationships. The thermal- and photocatalytic reactions of methanol to produce methane, formaldehyde, dimethyl ether, and methyl formate on these TiO2 nanoparticles were investigated using temperature programmed desorption in ultra high vacuum. Results show clear effects of nanocrystal size and shape on the activity and selectivity of these reactions. This demonstrates the value of studying nanostructured metal-oxide catalysts to better understand the relationship between fundamental catalytic knowledge and the behavior of complex heterogeneous catalysts. iii TABLE OF CONTENTS ACKNOWLEDGEMENTS ............................................................................................. ii ABSTRACT ...................................................................................................................... iii TABLE OF CONTENTS ................................................................................................ iv LIST OF TABLES ......................................................................................................... viii LIST OF FIGURES ......................................................................................................... ix Chapter 1. Introduction .................................................................................................. 1 1.1 Thesis Outline ........................................................................................... 5 Chapter 2. Background .................................................................................................. 6 2.1 Structure of TiO2 ....................................................................................... 6 2.1.1 Bulk structure .................................................................................... 6 2.1.2 Surface structure ............................................................................... 8 2.2 Thermally-driven reactions of oxygenates on TiO2 ................................ 14 2.2.1 Single crystal studies ....................................................................... 15 2.2.2 Studies of TiO2 powder .................................................................... 29 2.3 Photo-induced reactions of oxygenates on TiO2 ..................................... 31 2.3.1 Overview of photocatalysis ............................................................. 31 2.3.2 Single crystal studies ....................................................................... 37 2.3.3 Other photocatalytic studies of TiO2 ............................................... 40 iv Chapter 3. Experimental Methods .............................................................................. 44 3.1 Ultra-high vacuum environment ............................................................. 44 3.1.1 Gas behavior in high vacuum.......................................................... 45 3.1.2 Pressure measurement and management ........................................ 48 3.2 Sample preparation .................................................................................. 55 3.2.1 Synthesis .......................................................................................... 55 3.2.2 Temperature measurement and manipulation................................. 58 3.2.3 Cleaning Procedure ........................................................................ 60 3.3 Temperature programmed desorption ..................................................... 61 3.3.1 Theory.............................................................................................. 61 3.3.2 Analysis ........................................................................................... 65 3.3.3 Equipment........................................................................................ 69 3.4 X-ray photoelectron spectroscopy ........................................................... 74 3.4.1 Theory.............................................................................................. 74 3.4.2 Analysis ........................................................................................... 79 3.4.3 Equipment.......................................................................................
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