Simultaneous Presence of Cationic and Reduced Gold in Functioning MgO-Supported CO Oxidation Catalysts: Evidence from X-ray Absorption Spectroscopy

Abstract

Gold clusters on the surface of MgO powder (calcined at 673 K) were prepared from adsorbed [Au(CH3)2(acac), where acac is C5H7O2] and characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near edge spectroscopy (XANES). One sample initially contained gold predominantly in the form of clusters approximated as Au6 on the basis of the EXAFS data showing first- and second-shell Au−Au coordination numbers of 4.0 ± 0.4 and 1.0 ± 0.1, respectively. The other sample initially contained larger clusters, with an average diameter of about 30 A (containing about 100 atoms each, on average), as shown by the EXAFS first- and second-shell Au−Au coordination numbers of 9.4 ± 0.9 and 3.5 ± 0.4, respectively. The samples, in each of the three gases CO, O2, and He and in the presence of CO + O2 during CO oxidation catalysis, were investigated by EXAFS spectroscopy and XANES in a cell that was also a flow reactor. Data obtained during steady-state CO oxidation indicate the presen...