Supported Sub-Nanometer Gold Cluster Catalyzed Transfer Hydrogenation of Aldehydes to Alcohols

The ability of subnanometer sized Au-clusters to activate small molecule is well-known. Nevertheless, typical experimental situations involve loading of the bare Au-n (n < 10) on oxide surfaces. Recent plethora of literature indicate that supported gold clusters are extremely potent in catalyzing molecular transformation. In this work, we examine the role of the supporting substrate namely TiO2(110) in enhancing the activity of a small Au8 cluster for an industrially important reaction, namely, conversion of benzaldehyde into benzyl alcohol. The barrier for rate limiting C-H bond activation of the solvent/H-donor gets reduced by similar to 4 kcal/mol over a TiO2(110) surface with respect to its unsupported analogue. The activation energy (E-a) for the catalytic transfer hydrogenation (CTH) involving transfer of two hydrogens, one each from the solvent and the hydrogenated Au-cluster simultaneously to the aldehyde is also reduced significantly. Strong Au-8-TiO2(110) interactions result in charge transfer thereby making Au-cluster plectron deficient which assists by activating the rate limiting C-H bond cleavage. The present article provides a microscopic picture for catalysis of synthetically important complex reactions by supported Au-clusters.