Catalytic reactions on metal-supported semiconductors. Oxidation of CO over ZnO films on silver

Abstract

The catalytic activity of silver-supported zinc oxide films in CO oxidation has been studied. Attention has been directed to the influence of the semiconductor–metal contact on the catalytic behaviour of the semiconductor. Auger electron spectroscopy and electron microscopy have shown the presence of zinc oxide films uniform both in thickness and composition, which were free of contaminants. The kinetics of the catalytic reaction have been studied in a gas-flow system. The results fit two rate equations leading to two possible mechanisms: one with a rate-determining step which is a reaction between adsorbed CO and adsorbed oxygen, without mutual displacement; the other which corresponds to the Eley–Rideal mechanism. Both mechanisms assume a dissociative, asymmetric, acceptor-type adsorption of the oxygen molecule, and a donor-type adsorption of the CO molecule. It has been found that the elementary constants in the rate equations for CO oxidation depend on the semiconductor film thickness. Good agreement between the experimental and the theoretical results has been found. The Schottky model, together with the rigid band model, have been used in the theoretical considerations. The Schottky barrier height for the ZnO–Ag system has been calculated using the kinetic results, and is in very good agreement with the measured values reported in the literature.