Surface Chemistry on Oxide-Supported Au Nanoparticle Catalysts and High Surface Area Oxide Supports

Green, Isabel Xiaoye, Department of Chemistry, University of Virginia
Pate, Brooks, Department of Chemistry, University of Virginia
Yates, John, Department of Chemistry, University of Virginia
Neurock, Matthew, Department of Chemistry, University of Virginia

Catalytic reactions are of central importance to today's industrial development and productivity, especially when a metal that is normally unreactive becomes active. Gold nanoparticles supported on TiO2 powder have been found to be an outstanding catalyst for various reactions, while neither the TiO2 powder nor the Au nanoparticles by themselves have shown such property. The catalytic activity of a Au/TiO2 catalyst in oxidation reactions was studied using transmission infrared (IR) spectroscopy in this project. Combining high vacuum-cryogenic temperature experimental techniques and density functional theory (DFT) calculations, a special kind of Au-Ti 4 + dual perimeter site located at the interface of Au and TiO2 was identified as the key active site for oxygen activation. We demonstrated the working mechanism of this special site via three different reactions with O2 gas: CO oxidation, H2 oxidation, and acetic acid partial oxidation. Spectroscopic evidence unambiguously identified the active reaction zone at the Au-Ti 4 + dual perimeter sites. In addition, the surface properties of high area metal oxide supports were investigated. The adsorption, diffusion, and desorption of pyridine molecules on powdered nano-crystalline TiO2 and MgO was studied, demonstrating the utility of transmission IR spectroscopy in monitoring such processes. Nano-particle surface crystalline structure, surface acidity, and surface defect sites were found to play important roles in controlling adsorbate diffusivity. The influence of humidity (H2O) on the surface diffusion/desorption behavior of pyridine was also studied, where a dramatic increase in pyridine surface diffusivity over MgO was observed when modified by H2O adsorption to produce Brensted acid sites from Lewis acid sites. A weakly bound IRactive molecular O2 species was detected on the TiO2 surface at low temperatures, and its ii heat of adsorption was measured to be between -16 and -21 kJ mol -1 . It is suspected to be the precursor to "active" O2 species in various oxidation reactions on the catalyst surface.

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PHD (Doctor of Philosophy)
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