Solvent-free selective oxidation of alcohols by molecular oxygen over gold nanoparticles supported on β-MnO₂ nanorods

The performances of a series of MnO₂-supported gold nanocatalysts with varying gold loading were studied in the solvent-free selective oxidation of alcohols by molecular oxygen. The gold was deposited onto the β-MnO₂ support, featuring one dimensional rod-like morphology, by means of homogeneous deposition-precipitation. The catalytic activity of the catalyst for the solvent-free aerobic oxidation of benzyl alcohol was found to increase with increasing gold loading, with the highest specific activity in terms of turnover frequencies (TOF) being achieved over the catalyst with 5 wt.% gold loading. Both XRD and TEM results showed that the majority of Au particles of various catalysts have diameters in the range of 2-6 nm. Temperature-programmed reduction (TPR) results indicated that the reducibility of MnO₂ was significantly enhanced with the surface-deposited gold nanoparticles. X-ray photoelectron spectroscopy (XPS) revealed the coexistence of both oxidized and metallic gold species on the surface of MnO₂, the composition of which depends critically on the gold loading. The superior specific activity of the catalyst with 5% gold loading was attributed to the presence of higher amounts of positive gold species and more surface oxygen vacancy sites resulting from stronger metal-support interactions as compared to other catalysts.