Carbon monoxide oxidation catalyzed by highly dispersed gold on supports: Catalyst activation, deactivation, and active gold species

Site-isolated, mononuclear Au(III) complexes were synthesized on various metal oxides and zeolite by bringing an organometallic precursor, Au(CH 3)2(acac) (acac is acetylacetonate, C5H7 O2) in contact with partially dehydroxylated support. Extended X-ray absorption fine structure (EXAFS) and infrared (IR) characterizations confirmed that the sample contained gold complexes that were structural analogues to the molecular precursor. Gold clusters can be formed when the initially prepared samples were treated in flowing helium at increasing temperatures.;The initially prepared mononuclear Au(III) species on MgO powder was not active for CO oxidation at room temperature. As the reaction temperature was increased to >373 K with the sample in flowing CO + O2 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 of the sample after treatment in flowing CO + O2 at 408 K indicated that the gold was present in clusters consisting of approximately 4--6 Au atoms each, on average. These extremely small gold clusters were suggested to be the catalytically active species.;The as-prepared sample was also activated by treatment in flowing helium at 473 K. X-ray absorption spectra showed that the activated catalyst contained zerovalent gold clusters with an average diameter of approximate 10 A. During operation in a flow reactor, the catalyst underwent deactivation, accumulating species such as carbonates on its surface, as indicated by IR spectra. Accumulation of such species on MgO took place during the initial period of operation of the catalyst, whereas the accumulation of such species on the gold continued throughout the operation, consistent with the inference that these species blocked catalytically active sites on the gold. During CO oxidation catalysis in a batch reactor, at least some of the gold was oxidized, as evidenced by the appearance of an Audelta+--CO band at 2151 cm -1 in the IR spectrum, suggesting the possible involvement of cationic gold in the catalytic reaction. Similar results were obtained for a La 2O3-supported gold sample prepared in essentially the same way, demonstrating the oxidation of gold by CO2 on various supports.