On the kinetics and mechanism of the CO oxidation over polycrystalline iridium

Abstract

The kinetics of the CO oxidation was studied over polycrystalline iridium under ultrahigh vacuum conditions. The reaction was first-order in CO and zero-order in O 2 for low CO pressures, while it was inhibited by CO above a certain critical CO pressure. Above that pressure the reaction was first-order in O 2 and negative-order in CO. The amount of adsorbed CO and oxygen during the catalyzed reaction was determined with flash-desorption, AES, and UPS. Above the critical CO pressure the amount of adsorbed CO was equal to that in the CO/Ir equilibrium (nonworking) system. It decreased sharply around the CO pressure and was very small at lower pressures. The amount of adsorbed oxygen decreased smoothly as the CO pressure increased and became very small around the critical CO pressure. This kinetic behavior can be explained in terms of a Langmuir-Hinshelwood mechanism and a change in the rate-limiting process.