X-ray absorption study on the activation and deactivation of supported gold catalysts.

摘要

Gold has historically been regarded as chemically inert until the discovery that gold supported on metal oxides is the most active catalyst for low temperature CO oxidation. Despite intense study of this reaction, the origin of the activity of these supported gold catalysts remains unknown and the nature of the active gold species is still controversial. In this thesis research, Au/TiO 2 and Au/Al2O3 were studied with various in-situ characterization techniques to determine the active Au species responsible for CO oxidation activity and to investigate the properties of these gold species in effecting CO oxidation.; To understand the Au species that are responsible for CO oxidation, in-situ characterization methods, such as X-ray absorption spectroscopy (XAS) and infrared spectroscopy (FTIR), were used to follow the activation of Au/TiO 2 from its initial inactive state to an active form. The changes in the gold oxidation state and particle size were determined by XAS, which showed that metallic Au in the form of 1-2 nm particles is necessary for CO oxidation. In-situ FTIR results confirmed this finding and also identified that CO adsorbed on metallic gold is the active CO species for sub-ambient CO oxidation.; Deactivation of Au/TiO2 catalysts by poisoning with halide ions was also investigated. The halide ions' mobility and affinity to gold were found to be essential to their poisoning capabilities. Using XAS, bromide ions were shown to prevent the complete reduction of Au cations, and also had a direct poisoning effect on the activity of metallic Au particles. This was confirmed by FTIR results showing that the poisoning mechanism of Br is to prevent CO adsorption on the reduced Au/TiO2 catalyst for CO reaction.; H2 reduction of Au/Al2O3 catalyst to its metallic state is also found to be an effective method in activating Au/Al 2O3. In-situ XAS results showed that the resulting Au nanoparticles are highly reactive towards O2. Furthermore, CO adsorption on Au is accompanied by charge transfer from the gold site to the adsorbed CO. The structure sensitive activity of gold catalysts can be attributed to these unique properties of Au nanoparticles.