STRUCTURAL AND CATALYTIC PROPERTIES OF MODEL SILICA-SUPPORTED PALLADIUM CATALYSTS - A COMPARISON TO SINGLE-CRYSTAL SURFACES

The structural and catalytic properties of model silica-programmed des orption, infrared reflection-absorption spectroscopy of adsorbed CO, s canning tunneling and atomic force microscopies (STM and AFM), and cat alytic CO oxidation at both low pressure and elevated pressure conditi ons. The CO oxidation reactions on Pd(111), Pd(110) and Pd(100) have a lso been investigated. By evaporating palladium onto silica thin films (100 Angstrom), followed by an anneal to 900 K, the size of the palla dium particles can be controlled in a range of 30-500 Angstrom. The su rface of the palladium particles consists mainly of (111) and (100) fa cets, and exhibit catalytic activity similar to palladium single cryst als for CO oxidation at both low pressure (10(-8)-10(-6) Torr) and hig h pressure (15 Torr) conditions. At low pressures, the rate of CO oxid ation increases with temperature, reaches a maximum at 500-600 K, and then declines. At high pressures, the activation energy and turnover f requency for the CO oxidation reaction on the model catalysts compare favorable with analogous results from single crystal and high-surface- area catalysts. Thus, this system of metal particles supported on a si lica thin film provides an excellent model to bridge between single cr ystal and high-surface-area catalysts. The CO oxidation reactions on P d(111), Pd(110) and Pd( 100) have similar, but distinctive activation energies (28.1 +/- 0.4, 30.7 +/- 0.5 and 29.4 +/- 0.3 kcal/mol, respec tively) and turnover frequencies, indicating a subtle structure-sensit ivity for CO oxidation on different crystal planes of palladium.