CO Oxidation on a Au/TiO2 Nanoparticle Catalyst via the Au-Assisted Mars-van Krevelen Mechanism

Recently, there has been increasing evidence that CO oxidation on TiO2 supported Au catalysts proceeds predominantly via a Au-assisted Mars-van Krevelen mechanism for reaction temperatures of 80 degrees C and above. We here present results of a combined experimental and theoretical study, aiming at the identification of activated steps in this reaction. O-2 multipulse experiments, performed in a temporal analysis of products (TAP) reactor at different temperatures between -80 and +240 degrees C, revealed that the replenishment of surface lattice oxygen vacancies at perimeter sites, at the perimeter of the interface between TiO2 support and Au nanoparticles, proceeds with essentially constant efficiency, independent of the reaction temperature. Hence, this reaction step is barrier-free. Previous studies (Widmann and Behm Angew. Chem. Int. Ed. 2011, 50, 10241) had shown that the preceding step, the formation of a surface lattice oxygen vacancy at these sites, is activated, requiring temperatures above room temperature. Density functional theory based calculations, performed on a Au nanorod supported on a TiO2 anatase (101) substrate confirmed that the presence of the Au nanorod leads to a significant reduction of the vacancy formation energy at these sites, resulting in a barrier of only similar to 0.9 eV for vacancy formation by reaction with adsorbed CO. The reverse process, replenishing the vacancies by reaction with O-2, was found to be activated in the case of individual vacancies but essentially barrier-free for the case of pairs of neighbored vacancies. Consequences of these findings for the mechanism of the CO oxidation reaction on these catalysts, which can be considered as a model system for Au catalysts supported on reducible oxides, are discussed.