An Infrared and Kinetic Study of CO Oxidation on Model Silica - Supported Palladium Catalysts from to 15 Torr

The catalytic CO Oxidation on model silica-supported palladium catalysts has been investigateed with in situ infrared reflection-absorption spectroscopy (IRAS), temperature-programmed desorption/reaction, and kinetic measurements in the pressure range of lC9-15 Torr and the temperature range of 350-1000 K. The model catalysts were prepared by evaporating palladium onto a silica thin film supported on a Mo( 1 10) substrate, an arrangement that facilitates the use of electron spectroscopies and IRAS. The CO oxidation reaction was studied in three pressure ranges: (1) coadsorption and reaction of CO and 0 under UHV conditions, (2) CO oxidation between 10-8 and 1O-a Torr (350-1000 K), and (3) at 15 Torr (500-650 K). A generic rate law is derived that adequately describes the observed kinetic behavior for both lowand high-pressure conditions. At low temperatures, the oxidation rate is only proportional to the ratio of 02 and CO pressure and exhibits an activation energy of 27 kcal/mol. The rate increases with temperature to a maximum (500-600 K for a pressure of 1O-a Torr) and then decreases. The temperature of the rate maximum increases with CO pressure, decreases with oxygen pressure, and increases with the total pressure for a constant Po2 f PCO ratio. The reaction order with respect to the CO pressure also changes from negative to positive with an increase in temperature. This change in the kinetic behavior is attributed to a change in the rate-determining step with temperature. At temperatures above the rate maximum, the steady-state CO coverage is near zero while the oxygen coverage increases with temperature. Both the activation enthalpy and the entropy of the surface CO + 0 C02 reaction are found to be dependent on the oxygen coverage. The transition state of the reaction of CO and 0 lies nearer the reactants along the reaction coordinate for low steady-state CO and 0 coverage than for high 0 coverages.