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 desorption, infrared reflection-absorption spectroscopy of adsorbed CO, scanning tunneling and atomic force microscopies (STM and AFM), and catalytic CO oxidation at both low pressure and elevated pressure conditions. The CO oxidation reactions on Pd(111), Pd(110) and Pd(100) have also been investigated. By evaporating palladium onto silica thin films (100 Angstrom), followed by an anneal to 900 K, the size of the palladium particles can be controlled in a range of 30-500 Angstrom. The surface of the palladium particles consists mainly of (111) and (100) facets, and exhibit catalytic activity similar to palladium single crystals for CO oxidation at both low pressure (10(-8)-10(-6) Torr) and high pressure (15 Torr) conditions. At low pressures, the rate of CO oxidation increases with temperature, reaches a maximum at 500-600 K, and then declines. At high pressures, the activation energy and turnover frequency 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 silica thin film provides an excellent model to bridge between single crystal and high-surface-area catalysts. The CO oxidation reactions on Pd(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, respectively) and turnover frequencies, indicating a subtle structure-sensitivity for CO oxidation on different crystal planes of palladium.