Cerium dioxide-supported gold catalysts for carbon monoxide oxidation: Synthesis, reactivity, activation, and characterization for site-isolated mononuclear species and clusters

Site-isolated, mononuclear gold complexes supported on CeO 2 were synthesized by adsorption of Au(CH3)2(acac) (acac is acetylacetonate, C5H7O2) on partially dehydroxylated CeO2 powder (giving the "as-prepared" sample). Extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), and infrared (IR) spectroscopies gave evidence that the as-prepared sample contained gold complexes that were structural analogues of the organometallic precursor.;The as-prepared sample did not show any detectable activity for CO oxidation catalysis at room temperature. However, the as-prepared sample was active at 353 K. The activity increased and reached conversions of about 100% within 10 h of operation in a flow reactor. The catalyst was stable for more than 96 h. EXAFS data indicate the formation of gold clusters as the catalyst functioned. The cluster diameter increased with time on stream, and clusters of approximately 9, 15, 17, and 26 Au atoms each, on average, were found in samples that had been on stream for 24, 36, 48, and 96 h, respectively. XANES data show that the gold was reduced as the clusters formed.;Kinetics experiments carried out with the activated catalyst showed the roles of CO, O2, and CO2. The reaction order in CO is slightly negative, and CO2 inhibits the reaction. Apparent activation energies were determined for CO oxidation catalyzed by the mononuclear gold species and the gold clusters. The activation energy characterizing the latter is a third of that characterizing the mononuclear gold species, which are much less active than the clusters.;This work provides the first clear demonstration of the separate catalytic characteristics of supported gold complexes and supported gold clusters.