Evidence for the oxidation of CO by molecular oxygen adsorbed on Ag(110)

The oxidation of CO with molecularly adsorbed oxygen has been investigated on the Ag(110) surface by means of the surface titration technique. By directly measuring the CO, production rate with a mass spectrometer, substantial reaction has been observed upon the admission of CO via a molecular beam onto an O-2 layer well below the dissociation temperature of molecular oxygen, which is strong evidence for O-2 participating in the primary step. The data at 100 K clearly exhibit an induction period as a characteristic of the reaction, indicating a Langmuir-Hinshelwood mechanism. The effect of thermal dissociation of adsorbed molecular oxygen is apparent in the titration curves. The influence of external parameters such as substrate temperature, pre-coverage and magnitude of the CO flux, has been studied by systematic variation and the results have been used to set up a model system of reaction equations which include the temporary formation of an intermediate complex of CO and O-2. The numerical solution of the corresponding system of rate equations has been fitted to the experimental data with the rate constants acting as fit parameters. Both the activation energy and the frequency factor for the reaction steps could be calculated from the resulting values.