Reaction mechanism for direct propylene epoxidation by alumina-supported silver aggregates: The role of the particle/support interface

Subnanometer Ag aggregates on alumina supports have been found to be active toward direct propylene epoxidation to propylene oxide by molecular oxygen at low temperatures, with a negligible amount of carbon dioxide formation (Science 2010, 328, 224,). In this work, we computationally and experimentally investigate the origin of the high reactivity of the subnanometer Ag aggregates. Computationally, we study O₂ dissociation and propylene epoxidation on unsupported Ag₁₉ and Ag₂₀ clusters, as well as alumina-supported Ag₁₉. The O₂ dissociation and propylene epoxidation apparent barriers at the interface between the Ag aggregate and the alumina support are calculated to be 0.2 and 0.2-0.4 eV, respectively. These barriers are somewhat lower than those on sites away from the interface. The mechanism at the interface is similar to what was previously found for the silver trimer on alumina and can account for the high activity observed for the direct oxidation of propylene on the Ag aggregates. The barriers for oxygen dissociation on these model systems both at the interface and on the surfaces are small compared to crystalline surfaces, indicating that availability of oxygen will not be a rate limiting step for the aggregates, as in the case of the crystalline surfaces. Experimentally, we investigate Ultrananocrystalline Diamond (UNCD)-supported silver aggregates under reactive conditions of propylene partial oxidation. The UNCD-supported Ag clusters are found to be not measurably active toward propylene oxidation, in contrast to the alumina supported Ag clusters. This suggests that the lack of metal-oxide interfacial sites of the Ag-UNCD catalyst limits the epoxidation catalytic activity. This combined computational and experimental study shows the importance of the metal-oxide interface as well as the noncrystalline nature of the alumina-supported subnanometer Ag aggregate catalysts for propylene epoxidation.