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Cationic Gold Supported on Nanocrystalline La2o3: Catalysts for Co Oxidation

Yalin Hao and Bruce C. Gates. Chemical engineering and materials science, University of California at Davis, One Shields Ave, Davis, CA 95616

Gold that is highly dispersed on metal oxide supports is catalytically active for CO oxidation at room temperature. The activity of supported gold is strongly affected by the choice of the support. Supports that facilitate the formation of reactive oxygen species have been suggested to improve the catalytic activity. Thus, La2O3 is a good candidate support, because O2 adsorbs dissociatively on La2O3, generating potentially reactive oxygen species. We used nano-structured La2O3 as a support to synthesize a supported gold catalyst from an organometallic precursor, Au(CH3)2(acac) [acac is the bidentate ligand CH3COCHCOCH3]. Extended X-ray absorption fine structure (EXAFS) spectra show that the as-prepared catalyst incorporated gold as site-isolated mononuclear species bonded to the support by AuO bonds, with a distance of approximately 2.15 . X-ray absorption near edge structure (XANES) spectra indicate that the Au oxidation state was +3. The activity of this catalyst was determined for CO oxidation in a once-through plug-flow reactor at room temperature. Products were analyzed by gas chromatography and mass spectrometry; the rate data show that the activity of the catalyst was comparable to those of the most active supported gold catalysts that have been reported. The catalyst was also characterized by XANES, EXAFS, and IR spectroscopies under reaction conditions. XANES spectra indicate the presence of cationic gold on the surface of the working catalyst. EXAFS spectra demonstrate the bonding of the cationic gold to the oxide support surface. A role of the La2O3 support is evidently to stabilize gold in a highly dispersed cationic state.