Model catalyst studies of the CO oxidation reaction on Titania supported gold nanoclusters
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The chemical nature of gold has been determined to be much richer than previously thought. Recent discoveries that properly prepared gold catalysts (i.e. gold particle diameters in 2 – 5 nm size range) can catalyze the CO oxidation reaction at low temperature have spurned a renewed interest in the chemistry of gold. Despite the extensive research that has been performed regarding CO oxidation on Au based catalysts, many issues still remain unresolved. The origin of the particle size dependence of the reaction is not well understood. Also, details concerning the reaction mechanism, specifically identification of the active oxygen species, remain unresolved. In the following studies, ultra high vacuum, molecular beam, surface science techniques are used to study the CO oxidation reaction on titania supported gold nanoclusters (Au/TiO2). Using a radio frequency generated plasma-jet, it is possible to simultaneously populate the Au/TiO2 samples with atomically adsorbed and molecularly chemisorbed oxygen species, allowing for the opportunity to investigate the reactivity of each respective species. The reaction of CO with atomically adsorbed oxygen has been studied over a range of temperatures from 65 – 250 K as a function of gold coverage and oxygen coverage. The reaction is observed to be a strong function of both of the sample temperature and the oxygen coverage. The reaction is also relatively independent of the gold coverage on the sample, in contrast to findings for the reaction employing gas-phase reactants under moderate pressures. The formation and reactivity of molecularly chemisorbed oxygen on the samples following exposure to the plasma-jet was also investigated. Evidence is presented showing that some molecularly chemisorbed oxygen is formed as a result of recombination of impinging atoms on the model catalyst surface. Evidence is also presented showing that adsorption of an oxygen atom on the sample influences the chemisorption of molecular species from the gas phase. Finally, evidence is presented showing that the molecularly chemisorbed oxygen species can participate in the CO oxidation reaction at 77 K. This finding reveals a reaction channel for CO oxidation on Au/TiO2 model catalysts that does not require the dissociation of oxygen.