Although highly dispersed Au catalysts with Au nanoparticles (NPs) of a few nanometers in diameter are well-known for their high catalytic activity for several oxidation and reduction reactions already at rather low temperatures for almost 30 years, central aspects of the reaction mechanism are still unresolved. While most studies focused on the active site, the active Au species, and the effect of the support material, the most crucial step during oxidation reactions, the activation of molecular oxygen and the nature of the resulting active oxygen species (Oact), received more attention just recently. This is topic of this Account, which focuses on the formation, location, and nature of the Oact species present on metal oxide supported Au catalysts under typical reaction conditions, at room temperature and above. It is mainly based on quantitative temporal analysis of products (TAP) reactor measurements, which different from most spectroscopic techniques are able to detect and quantify these species even at the extremely low concentrations present under realistic reaction conditions. Different types of pulse experiments were performed, during which the highly dispersed, realistic powder catalysts are exposed to very low amounts of reactants, CO and/or O2, in order to form and reactively remove Oact species and gain information on their formation, nature, and the active site for Oact formation. Our investigations have shown that the active oxygen species for CO oxidation on Au/TiO2 for reaction at 80 °C and higher is a highly stable atomic species, which at 80 °C is formed only at the perimeter of the Au-oxide interface and whose reactive removal by CO is activated, but not its formation. From these findings, it is concluded that surface lattice oxygen represents the Oact species for the CO oxidation. Accordingly, the CO oxidation proceeds via a Au-assisted Mars-van Krevelen mechanism, during which surface lattice oxygen close to the Au NPs is removed by reaction with CO, resulting in a partially reduced TiO2 surface, which is subsequently reoxidized by O2. We demonstrate that this is the dominant reaction pathway for Au catalysts based on reducible metal oxides in general, at typical reaction temperatures, while for less active Au catalysts based on nonreducible metal oxides, this reaction pathway is not possible and the remaining activity must arise from another pathway, most probably a Au-only mechanism. At lower reaction temperature, reactive removal of Oact becomes increasingly inhibited, leading to a change in the dominant reaction pathway.

No abstract

Electronic metal−support interactions (EMSIs) are demonstrated to severely affect the CO oxidation activity and the CO adsorption properties of Au/TiO 2 catalysts. Bulk oxygen vacancies, generated by a strongly reductive pretreatment of Au/TiO 2 at elevated temperature in 10% CO/N 2 , significantly lower the catalytic activity for CO oxidation at 80 °C. With time on stream, the activity slowly increases until reaching the same steady-state value as that obtained for a previously calcined and, hence, defect-poor Au/TiO 2 catalyst (activation period), where the time required for the activation period decreases with reaction temperature but is independent of the oxygen partial pressure. Considering the similar Au particle sizes and Au loadings, we conclude that the different activities originate from the presence of bulk oxygen vacancies generated during pretreatment, which are slowly replenished during reaction. In situ IR spectroscopy measurements reveal that the lower activity in the presence of bulk defects is coupled with and likely results from a strong modification of the CO adsorption strength on the reduced Au/TiO 2 catalysts due to EMSIs. A possible mechanism explaining how these EMSIs may be induced by the presence of bulk defects is discussed.

No abstract

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 °C and above. We here present results of a combined experimental and theoretical study, aiming at the identification of activated steps in this reaction. O2 multipulse experiments, performed in a temporal analysis of products (TAP) reactor at different temperatures between −80 and +240 °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 ∼0.9 eV for vacancy formation by reaction with adsorbed CO. The reverse process, replenishing the vacancies by reaction with O2, 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.