Heterogeneity of a silver surface used in the oxidation of carbon monoxide

Abstract

The mechanism of the oxidation of CO by nitrous oxide and oxygen over a silver catalyst has been studied by an analysis of the transient behaviour at 20 °C. Half of the total surface sites are active for the adsorption of oxygen and one-third of the active sites are active for the decomposition of N2O, indicating the heterogeneity of the silver surface. At a steady state of CO oxidation in the N2O + CO and the O2+ CO systems, the surface for the N2O + CO reaction is more reduced than that for the O2+ CO reaction. The activation energy is 14 kcal mol–1 for the O2+ CO reaction and 10 kcal mol–1 for the N2O + CO reaction, suggesting different rate-controlling steps.Adsorbed oxygen (presumed to be diatomic) formed from gaseous oxygen and not from N2O is responsible for the oxidation of CO. An Eley–Rideal-type mechanism is proposed and the rate of oxidation of CO by diatomic oxygen is estimated to be ca. 14 times faster than the rate of decomposition of N2O. Graphical analysis of the transient response curves obtained for the two reactions leads to the conclusion that the rate of formation of the diatomic oxygen species is the rate-determining step in the N2O + CO reaction.Computer simulation of the transient response curves proved the validity of the proposed reaction mechanism and gave estimates of the values of the kinetic parameters (kj) for all the elementary steps. Comparing the kj obtained for the two reactions, the dissociation of diatomic oxygen in the O2+ CO reaction was determined to be one hundred times faster than in the N2O + CO reaction, again indicating the heterogeneity of the working silver surface.