Adsorption on Gold: Effects on Surface Morphology and Reactivity
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ADSORPTION ON GOLD: EFFECTS ON SURFACE MORPHOLOGY AND REACTIVITY A thesis presented by THOMAS ARTHUR BAKER to The Department of Chemistry and Chemical Biology in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the subject of Chemical Physics HARVARD UNIVERSITY Cambridge, Massachusetts May 2009 © 2009 – Thomas Arthur Baker All rights reserved. Dissertation Advisors: Professors Efthimios Kaxiras and Cynthia M. Friend Thomas Arthur Baker ADSORPTION ON GOLD: EFFECTS ON SURFACE MORPHOLOGY AND REACTIVITY ABSTRACT Computational tools are used in conjunction with experiments to understand adsorption and reactions on gold surfaces. We focus on systems important for oxidation, a class of reactions gold can perform quite readily. Experimental results have illustrated that the structure of gold is sensitive to adsorbate type and coverage, temperature, and many other factors. We find the adsorption of atomic oxygen and chlorine can significantly affect the morphology of the surface, either by favorably interacting with defects on the surface or by incorporating gold into the adsorbate layer, a process that is energetically favorable at higher coverages. The adsorbate-gold interaction becomes more covalent upon gold incorporation, which lowers the partial negative charge on the adsorbate, allowing for closer packing on the surface. The temperature dependence on the oxygen-gold interaction motivated the use of ab-initio molecular dynamics to model the dynamic surface modification during adsorption of atomic oxygen. We successfully match calculated vibrational spectra with experimental high-resolution electron energy loss spectroscopy results and suggest chemisorbed oxygen (oxygen bound in a locally flat three-fold coordination site) as the reactive species for oxidation on the surface. Since reactivity is closely related to surface morphology we studied the reaction of propene iii with atomic oxygen on different defect covered gold surfaces. Reaction barriers for allylic hydrogen abstraction and oxametallacycle formation can differ by at least a factor of two and different pathways can be selected by modifying the surface structure. Furthermore, we find the correct pathway for phenol formation on cyclohexene and use ab-initio molecular dynamics to model the oxygen-covered gold surface for the oxidation of CO. We qualitatively reproduce experimental trends in reactivity and gain insight into the mechanism of CO oxidation on the oxygen-covered gold surface, confirming our previous result that chemisorbed oxygen is the reactive species on the surface. iv TABLE OF CONTENTS Acknowledgments…………………………………………………………………….. vii Glossary of Acronyms…………………………………………………………………. ix Chapter 1 – Introduction………………………………………………………………. 1 Chapter 2 – Theoretical Approach…………………………………………………….. 4 PART I – FUNDAMNTAL UNDERSTANDING OF ADSORPTION Chapter 3 – Nature of Cl Bonding on the Au(111) Surface………………………….16 Chapter 4 – Chlorine Interaction with Defects on the Au(111) Surface…………….27 Chapter 5 – Chlorine Adsorption on Au(111): Chlorine Overlayer or Surface Chloride?......................................................................................................45 Chapter 6 – Atomic Oxygen Adsorption on Au(111) Surfaces with Defects……….. 66 Chapter 7 – Atomic Oxygen on Au(111): An High-resolution Electron Energy Loss Spectroscopy and Ab-initio Molecular Dynamics Study.........................93 Chapter 8 – Effects of Chlorine and Oxygen Coverage on the Structure of the Au(111) Surface………………………………………………………….106 PART II – APPLICATIONS TO REACTIVITY AND CATALYSIS Chapter 9 – Selectivity Switch Induced by Defects: Adsorption and Reaction of Propene on O-Covered Au(111)………………………………………...131 Chapter 10 – Ab-initio Molecular Dynamics Investigation of Carbon Monoxide Oxidation on Atomic Oxygen Covered Au(111)……………………... 160 v PART III – APPENDIX Appendix A – A Pathway for NH Addition to Styrene Promoted by Gold……….. 185 Appendix B – Transient Hydroxyl Formation from Water on Oxygen-Covered Au(111)…………………………………………………………………198 Appendix C – Phenol Formation on Cyclohexene: A DFT Mechanistic Study……226 vi ACKNOWLEDGEMENTS ‘To give anything less than your best, is to sacrifice the gift’ -Steve Prefontaine It is quite a gift. But I do not mean a natural physical talent or intellect, but rather the gift of great teachers, friends, co-workers, family, and mentors who were extremely instrumental in my growth as a scientist. I’ve been truly blessed to have some incredible mentors. Brett Criswell (Central Columbia High School) inspired me to study science, cultivated my interest, gave me room to explore, and was always willing to discuss new ideas with me as I grew during those important times as an adolescence. Ronald See (Indiana University of Pennsylvania) introduced me to a rigorous treatment of science and chemistry. He taught me most of the fundamental concepts of chemistry and how to apply those to solving a scientific problem. He taught me to believe in myself and he never seemed to think there was a ceiling to my potential. Along with being a wonderful advisor, he was a great friend, teaching me some of the finer points of life. Cynthia Friend and Tim Kaxiras (Harvard University) have been incredible advisors during my Ph.D work. They have taught me how to do science and how to make an impact. I have learned a great deal from them. I have been extremely lucky that they are dedicated advisors who care deeply about the well-being of their students. Any extremely busy professor who offers to buy you groceries when you have been deathly ill for a week is ok in my book. I also appreciate the help of Alán Aspuru-Guzik (Harvard University) who has been almost a third advisor, going incredibly out of his way to offer advice, write letters, and look out for me. I can’t thank him enough for all his help. vii I have had the pleasure of working with people a lot smarter than me from whom I have learned a great deal: Lauren Benz, Anne Co, Xingyi Deng, Maria Fyta, Weiwei Gao, Jan Haubrich, Stephen Jensen, Byoung-Koun Min, Andreas Klust, Xiaoying Liu, Sheng Meng, Yina Mo, Dilini Pinnaduwage, Su Ying Quek, Ryan Quiller, Mike Stopa, Weili Wang, Bingjun Xu, Ling Zhou, and Wenguang Zhu. I thank everyone for all the help over the years. I thank the following organizations for their financial or computational support; this is your tax dollars at work: National Science Foundation (Graduate School Fellowship and MRSEC), Department of Energy, National Nanotechnology Infrastructure Network, Teragrid, Harvard SEAS HPC, and Harvard FAS HPTC. Finally, I thank some of my closer friends (Brian and Chun) for their support and humor. I thank Dilini for being a passionate and great friend who has always been there for me. I thank my family for their support and belief in me and finally I thank again all those who have helped and inspired me (and apologize for those I failed to mention) to complete this thesis. viii GLOSSARY OF ACRONYMS AIMD Ab-initio Molecular Dynamics BOMD Born-Oppenheimer Molecular Dynamics CN Coordination Number DFT Density Function Theory HREELS High-Resolution Electron Energy Loss Spectroscopy GGA Generalized Gradient Approximation KS Kohn-Sham LDA Local Density Approximation LEED Low Energy Electron Diffraction ML Monolayer PDOS Partial Density of States PBE Perdew-Burke-Ernzerhof exchange-correlation functional PW91 Perdew-Wang 91 exchange-correlation functional RPBE Revised-Perdew-Burke-Ernzerhof exchange-correlation functional STM Scanning Tunneling Microscopy VASP Vienna Ab-initio Simulation Package XPS X-ray Photoelectron Spectroscopy ix CHAPTER 1 INTRODUCTION It can be argued that for any heterogeneous system, all the important science occurs at the interface or the surface between two phases. Corrosion of a metal occurs at the metal-air (or liquid) interface, ion-channels located in the protein lipid bilayer at the surface of a cell regulate salt content, and even the interface between land and the ocean is typically prime real estate. The interesting processes occurring at interfaces, however, present challenges for experimentalists and theorists to explore, model, and describe. For example, a statistical mechanical treatment reserved for the bulk of a material can fail at the important interface. Often a thermodynamic equilibrium is not established at an interface because of the dynamic chemical and physical phenomena occurring at the surface. Nevertheless, both experimental and theoretical tools have been developed and are routinely used to understand and study important surfaces. We use such tools in this thesis to understand the surface of gold. Gold is typically thought as a ‘noble’ metal: unreactive and stable. However in the 1980s, Haruta and co-workers discovered that Au could be reactive and used as a heterogeneous catalyst, specifically for the oxidation of CO1 and propene2. However the exact nature of gold’s catalytic activity and the mechanism by which gold can oxide reagents is not completely known. To understand the reactivity of gold, this work is split into two parts. In the first part, Fundamental Understanding of Adsorption, we study the adsorption of chlorine and oxygen on the Au(111) surface. We gain a fundamental understanding in Chapter 3 of the type of bond created between chlorine and the gold 1 surface, finding that the interaction is covalent in nature, in contrast to the binding of most other halogens on metal surfaces. In chapter 4, we study the adsorption of chlorine on gold defects. The motivation for