On the mechanism and kinetics of the CO-oxidation reaction on polycrystalline palladium: II. The kinetics

Abstract

The kinetics of the oxidation of carbon monoxide have been studied extensively over polycrystalline palladium foils which were prepared by exposure to relatively large amounts of oxygen at high temperatures. Reaction orders with respect to CO 2, CO and O 2 were determined over various intervals of partial pressure, total pressure and temperature; 0.1 ⩽ P CO p O2 ⩽ 20, 10 −7 < p < 10 −3 Pa, and 450 < T < 873 K . The order with respect to CO 2 was zero under all the conditions tested. The orders with respect to O 2 and CO depended on the partial pressure ratio and showed very little dependence on total pressure. For p O2 < p CO the rate was first order in oxygen and negative 0.5 to 0.8 order in CO below 473 K. When p O 2 > p CO in this temperature range, the orders with respect to oxygen and carbon monoxide were zero and unity respectively. The inhibitory effect of CO vanished at T = 573 K. In this case the orders were the same as above except when p O 2 < p CO the order with respect to CO was zero. The amount of adsorbed oxygen was measured under working conditions at 473 K using a CO-titration method. For p O 2 >] p CO the amount of adsorbed oxygen is almost independent of p O 2 . The amount decreases sharply around P O 2 = P CO and is very small for p O 2 < p CO. The kinetic behaviour can be explained in terms of a change in the rate determining step, depending on the amount of adsorbed oxygen.