Dynamic surface composition in a Mars-van Krevelen type reaction: CO oxidation on Au/TiO2

Abstract

Aiming at a better mechanistic understanding of Mars-van Krevelen type catalytic reactions, we have systematically investigated possible correlations between the composition of the reaction gas atmosphere, the concentration of surface oxygen vacancies (surface oxidation state), and the catalytic activity of a Au/TiO2 catalyst in the CO oxidation reaction. In the meantime, there is considerable experimental and theoretical evidence that this reaction follows a Au-assisted Mars-van Krevelen mechanism at reaction temperatures ≥80 °C. Employing quantitative pulse experiments in a temporal analysis of products (TAP) reactor we found that both the surface oxidation state and the activity of the catalyst under steady-state conditions depend sensitively on the composition of the reaction gas atmosphere, specifically on the CO:O2 ratio, and that there is a strict correlation between these quantities. A simple kinetic model is introduced, which allows to quantitatively determine the ratio of the effective reaction rate constants for reduction and oxidation of the TiO2 support surface. These results and ideas are considered to be of general relevance for the understanding of Mars-van Krevelen type reactions and specifically of oxidation reactions on supported Au catalysts, and they lend further support to previous proposals of a Au-assisted Mars-van Krevelen mechanism for this reaction.