Ab initio studies of propene oxide formation at gold nanocatalysts supported on anatase-TiO2

Small gold particles supported on TiO2 are promising catalysts for the direct epoxidation of propene. Some experimental studies suggest a critical role of the oxide/gold interfacial region, where a bidentate propoxy species is formed during the reaction. However, it is not yet clear that such complex is a true reaction intermediate, and not a mere spectator or even a deactivating species. In order to clarify this issue, we have performed extensive ab initio simulations of the propene oxide formation on a model catalyst formed by small gold clusters (Au-4, Au-5 and Au-8) supported on the anatase-TiO2 (101) surface. In a variety of situations (with or without the supported gold cluster, with a varying amount of coadsorbed water or hydroxyl groups, etc.) we have investigated the stability of the bidentate propoxy species adsorbed at TiO2, as well as the barrier for its desorption into propene oxide. In all cases, the bidentate propoxy bonded at two surface Ti sites is highly stable, with a binding energy of the order of 1.5 eV. Such stability leads to very high energy barriers for propene oxide formation, of almost 2 eV. This suggests that such species is likely to be an spectator. Alternative reaction pathways, with the propoxy species formed in direct contact with the gold catalyst, are a much more probable candidate for a true reaction intermediate towards propene oxide formation.