Elucidating Determining Factors in Heterogeneous Catalysis Using Surface Science Models

Abstract

This thesis investigates fundamental interactions between catalyst surfaces and adsorbates that can affect the performance of heterogeneous catalysts by using controlled, model surface science studies. These interactions include: adsorbate binding strength, van der Waals (vdW) interactions, adsorbate migration, surface electronic structure and dynamic interface rearrangement. First, the utilization of near-ambient pressure X-ray photoelectron spectroscopy for investigating model catalyst surfaces (Chapter 1) and development of an improved method for photoelectron spectra analysis by accounting for multi-electron interactions (Chapter 2) is described. Second, interactions on a gold surface is explored for the selective oxidation of organic acids facilitated by gold, where adsorbed oxygen initiates the reaction. The relative stability of surface adsorbates (carboxylates) is determined by an interplay between surface anchoring group strength and vdW interactions (Chapter 3). Adsorption of water on oxygen-covered gold redistributes adsorbed oxygen atoms (active sites) via transient hydroxyl formation and diffusion (Chapter 4). Highly stepped surface sites, which can be present on high curvature nanoparticles, readily reconstruct with thermal treatment and high concentration oxygen adsorption induces cluster formation which decreases reactivity (Chapter 5). Third, interactions on a palladium-silver surface is explored for a range of reactions, where generally palladium activates reactants and silver imparts selectivity. Dynamic surface restructuring is dependent on reaction conditions, where the absence of adsorbates favors silver termination while carbon monoxide adsorption or palladium oxidation favors palladium termination (Chapter 6). Oxophilicity facilitates the oxidation of palladium in contact with silver oxide, while a strong palladium-silver interaction promotes materials separation by formation of three-dimensional palladium particles on silver (Chapter 7). Metallic palladium dissociates dihydrogen where hydrogen atoms diffuse to silver, where adsorption is endothermic, through metastable adsorption sites at the palladium-silver interface (Chapter 8). Palladium oxide promotes silver oxide reduction by dihydrogen via interfacial intermediate migration involving transport of catalyst material (Chapter 9). An atomic layer film of silver on palladium promotes formic acid decomposition by modifying the electronic structure of silver (Chapter 10). Lastly, appendices explore carboxylate decomposition and stability on Au(110) (Chapter 11), methane reaction on oxidized Pd(111) (Chapter 12) and molecular hydrogen reaction on oxidized Ag(111) (Chapter 13).