Aspects of carbon monoxide oxidation over supported gold catalysts and characteristics of the active site

Gold has historically been considered one of the most inert elements, and its catalytic properties were ignored until relatively recently, when it was discovered that nanosize gold particles supported on metal oxides are the most active catalysts for low temperature CO oxidation. Despite intense study of this reaction, the origin of the high activity of these supported gold nanoparticles remains unknown, providing a hindrance to the development of more active or selective catalysts. In this thesis research, Au/gamma-Al 2O3 and Au/TiO2 were studied in order to determine the necessary requirements for CO oxidation activity and to investigate the degree to which these requirements depend on the catalyst support.; Water was found to be profoundly important in generating and maintaining CO oxidation activity. It was required in the activation of as-prepared Au/gamma-Al 2O3 samples. Examination by XANES and TPR showed that these samples were initially composed entirely of ionic Au. Such samples were inactive for CO oxidation at room temperature, indicating that ionic Au alone is insufficient for activity. Metallic Au was required in order to effect CO oxidation, and this reduction could be accomplished by treatment in a reducing gas followed by treatment in water vapor at 100°C, as it was discovered that water enhanced the reduction of the Au in the catalyst. Moreover, the presence of water on the catalyst is required in order for the CO oxidation reaction to occur. An active Au/gamma-Al2O3 catalyst could be deactivated by thermal treatment in dry gas at temperatures as low as 100°C and regenerated by treatment in water vapor at room temperature. Consequently, an active site consisting of an ensemble of metallic Au atoms and Au +-OH-, along with a CO oxidation reaction mechanism involving the formation and decomposition of a bicarbonate intermediate, was proposed.; Investigation of Au/TiO2, especially the effect of H 2O on this catalyst, revealed that Au/TiO2 exhibits behavior very similar to that of Au/gamma-Al2O3. This suggests that the requirements for CO oxidation activity are independent of the catalyst support.