No abstract

Oxide-supported Au nanostructures are promising low-temperature oxidation catalysts. It is generally observed that Au supported on reducible oxides is more active than Au supported on irreducible oxides. Recent studies also suggest that cationic Au(delta+) is responsible for the unique Au/oxide catalytic activity, contrary to the conventional perception that oxide supports donate electronic charge to Au. We have utilized density functional calculations and ab initio thermodynamic studies to investigate the oxidation state of Au nanostructures deposited on reducible and irreducible supports. We find that there are fundamental differences in the electronic structure of Au deposited on the different oxides. We propose a simple model, grounded in the first principles calculations, which can explain the oxide-specific catalytic activity of Au nanostructures and which can account for the presence and the role of cationic Au(delta+).

The nature of the active sites involved in the gold catalyzed Sonogashira cross-coupling reaction between iodobenzene and phenylacetylene, and in the competitive homocoupling reactions, has been investigated by means of DFT calculations, kinetic measurements, and synthesis of catalysts with different gold surface species. Several catalyst models have been theoretically investigated to simulate gold nanoparticles of different size either isolated, supported on inert materials, or supported on CeO 2 . The mechanistic studies show that IB dissociation occurs on low coordinated Au 0 atoms present in small gold nanoparticles, either isolated or supported, while PA is preferentially adsorbed and activated on Au δ+ species existing at the metal−support interface. When this occurs, the activation energy of the rate-determining step of the Sonogashira reaction, which has been found experimentally to be the bimolecular coupling, is minimized. The product distribution obtained with Au/CeO 2 catalysts containing different ratios of Au 0 /Au δ+ sites confirms the positive role played by cationic gold in the Sonogashira cross-coupling reaction. Importantly, only metallic Au 0 atoms present in gold nanoparticles are required to perform the homocoupling of iodobenzene.