An XPS and UPS study of the kinetics of carbon monoxide oxidation over Ag(111)

Abstract

The oxidation of carbon monoxide over a Ag(111) catalyst has been studied by XPS and UPS. The kinetics have been determined over the temperature range of 180 to 400 K and found to be of the Langmuir-Hinshelwood type, although the Eley-Rideal mechanism is mimicked. A negative activation energy, −1.7 kcal/mole, and a preexponential, 6 × 10 −18 cm 2, are found. The former corresponds to the difference in the activation energies for carbon monoxide desorption and for carbon monoxide oxidation (leading to CO 2 desorption). At 90 K, upon carbon monoxide exposure to the active oxygen precovered surface, the O ls and C ls spectral regions show the formation of CO 2-like and carbonate species; the latter is stable to at least room temperature. That is, at 90 K, the residence time and mobility of CO 2 formed at the surface permits a new surface reaction — the formation of stable surface carbonate. The identifications are based on C and O coverages and on line positions from the literature for Cu/CO 2 and several bulk carbonates. With UPS, the 1π g, the unresolved doublet 1π u and 3σ g, and the 4σ g molecular orbitals of adsorbed CO 2-like species are identified, as well as the unresolved triplet 1α′ 2, 1e″ and 4e′ and the unresolved triplet 3e′, 1α″ 2 and 4a′ molecular orbitals of the carbonate species. Surface CO 2-like species formed by surface oxidation of CO seem to be more strongly bound than reversibly adsorbed CO 2.