The role of negatively charged Au states in aerobic oxidation of alcohols over hydrotalcite supported AuPd nanoclusters

Abstract

The PVP-protected bimetallic gold–palladium nanoclusters (Au_xPd_y-PVP NCs) were prepared on the solid base hydrotalcite (HT) with various Au : Pd (x : y) molar ratios. Transmission electron microscopy showed narrow particle size distributions of Au_xPd_y-PVP NCs with a mean diameter in the range of 2.6–3.0 nm regardless of Pd content. Aerobic oxidations of 1-phenylethanol over the Au_xPd_y-PVP/HT catalysts showed that their catalytic activities were significantly affected by the Pd content. Correlations between charge transfer between Au and Pd and catalytic activity of the Au_xPd_y-PVP/HT catalysts were investigated with X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), Michaelis–Menten kinetic studies for alcohol oxidation, and other analytical techniques. The peaks of Au 4f in the XPS spectra were shifted to the lower energy side with increase of Pd content, indicating the electron transfer from Pd to Au atoms according to Pauling's electronegativity protocol. The electron densities in the Au 5d orbital in the Au_xPd_y-PVP/HT catalysts estimated by the Au L₃-XANES spectra correlated well with their catalytic activities. Moreover, the kinetic studies also proposed that the electron rich Au 5d states, resulting from the intermetallic electron transfer from Pd atoms, strongly contributed to the rate-determining step in the alcohol oxidation. It was concluded that the electronic negativity of the Au 5d states controlled by the Pd content accelerated the rate-determining step in alcohol oxidation through highly active radical-like intermediates.