Activation of dimethyl gold complexes on MgO for CO oxidation: Removal of methyl ligands and formation of catalytically active gold clusters

A supported CO oxidation catalyst was synthesized by the reaction of Au(CH 3) 2(acac) (acac is acetylacetonate) with partially dehydroxylated MgO powder. The as-prepared sample was found by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies to incorporate dimethyl gold complexes that were bonded to the support; it lacked measurable catalytic activity for CO oxidation at room temperature. As the temperature was increased to >373 K with the sample in flowing CO + O 2 at atmospheric pressure, removal of methyl ligands from the gold was observed by IR and EXAFS spectroscopies. Simultaneously, the sample became active for CO oxidation catalysis. EXAFS characterization of the sample right after the activation indicated that the gold had aggregated into clusters consisting of approximately 4–6 Au atoms each, on average. These are among the smallest supported gold clusters yet reported, and they are inferred to be the catalytically active species. The XANES data suggest that the gold in the activated catalyst had not been reduced to the metallic state. EXAFS: Cluster formation. XANES: Gold positively charged.