On the nature of active oxygen on supported Au catalysts - formation, stability and CO oxidation activity

Abstract

The objective of the present work was to resolve mechanistic details, on a molecular scale, as well as the physical origin of the high activity for the CO oxidation reaction over oxide supported Au catalysts. Special attention was given to the most controversially discussed problem in oxidation reactions over oxide supported Au catalysts, the activation of molecular oxygen and the nature of the resulting active oxygen species present under working conditions. For this reason, quantitative Temporal Analysis of Products (TAP) reactor measurements were performed, which allow a precise determination of the amount of active oxygen species present on the catalyst surface and, moreover, the determination of the overall oxygen storage capacity (OSC). Applying a combination of i) quantitative TAP measurements and ii) conventional measurements of the catalytic activity for the CO oxidation under atmospheric pressure, it is shown that the active oxygen species for CO oxidation over Au catalysts supported on reducible metal oxides is surface lattice oxygen located at the perimeter of the interface between Au nanoparticles (NPs) and support. This leads directly to the conclusion, that the CO oxidation reaction proceeds via a Au-assisted Mars-van Krevelen reaction mechanism. Different activities of differently supported Au catalysts (support effects) are originating from differences in energy levels and barriers during the reaction of CO adsorbed on the Au NP with surface lattice oxygen from the support. Moreover, for Au/titania it is also demonstrated that the oxygen content in the reaction atmosphere, the oxidation state of the catalysts surface under reaction conditions and the activity for CO oxidation are strictly correlated, and that the catalyst surface quickly adapts to the gas phase composition. These results further corroborate the Au-assisted Mars-van Krevelen mechanism proposed above, including stable, atomic lattice oxygen at perimeter sites as active oxygen species.