AN INFRARED AND KINETIC-STUDY OF CO OXIDATION ON MODEL SILICA-SUPPORTED PALLADIUM CATALYSTS FROM 10-9 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 10(-9)-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(110) 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 O under UHV conditions, (2) CO oxidation between 10(-8) and 10(-6) 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 low-and high-pressure conditions. At low temperatures, the oxidation rate is only proportional to the ratio of O2 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 10(-6) 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 P(O2)/P(CO) 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 + O --> CO2 reaction are found to be dependent on the oxygen coverage. The transition state of the reaction of CO and O lies nearer the reactants along the reaction coordinate for low steady-state CO and O coverage than for high O coverages.