KINETICS OF COADSORPTION OF DIOXYGEN AND AMMONIA AT A ZN(0001) SURFACE - A THEORETICAL-MODEL

The experimentally observed kinetics of the coadsorption of dioxygen and ammonia at a Zn(0001) surface have been modelled theoretically. The rate-limiting step involves a precursor dioxygen-ammonia surface complex, formed via ammonia and dioxygen surface diffusion. The system of differential equations describing the model provides a solution that is in good agreement with the experimental results when account is taken of the dependence of the enthalpy of ammonia adsorption on the coverage of chemisorbed oxygen O2-(a). Thermodynamic arguments are shown to account for the observed large increase (ca. 10(3)) in the probability of dioxygen dissociation during coadsorption, compared with that observed with dioxygen alone. Electrostatic interactions within the precursor {NH3-O2-} zinc complex are suggested to provide a low-energy pathway to the observed chemisorbed products.