Preparation and Evaluation of Bimetallic Au Nano-Catalyst with Aerobic Oxidation of 1-Phenylethanol

Contributions of electronic (or ligand) and geometric (or ensemble) effects on the AuM bimetallic nano-catalyst were elucidated by using a simple aerobic oxidation of 1-phenylethanol to acetophenone on the basis of difference in the ionization energy values (Ei) between Au and M elements. The poly(N-vinylpyrrolidone) (PVP)-protected Au_M_ bimetallic NPs (M = Ag, Cu, Pd, Pt and Ir) were prepared with a polyol reduction method, and stabilized onto the solid base hydrotalcite support affording the Au_M_-PVP/HT catalysts. The yields for acetophenone were observed as the following order; Au_Pd_-PVP/HT (>99%) >> Au_Ag_-PVP/HT (17.4%) > Au_Cu_-PVP/HT (13.8%) > Au_Pt_-PVP/HT (7.1%) > Au_Ir_-PVP/HT (5.5%), at 343 K for 6 h. Differences in the Ei between Au and M (Ei_-Ei_M) indicted that the yields over the Ag, Cu, Pt, and Ir incorporated Au catalysts were well-understood on the ligand effects theory, though geometric factors such as differences in nanostructure around Au atom in Au_M_ NPs on HT should be further considered as other contributed factors. The significant activity on Au_Pd_-PVP/HT was studied in terms of the electron density of Pd atoms. It was observed that the Pd 4d density was varied by the amount of Au loading. According to these observations combined with our previous studies, we suggest that the advantages in AuPd bimetallic catalyst are not only in the ligand effect serving negatively-charged Au but also the ensemble effect of neighbor Pd, and they synergistically contribute to the novel activity for aerobic alcoholoxidation over AuPd catalyst

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

The PVP-protected bimetallic gold/palladium nanoclusters (Au_xPd_y-PVP NCs) were prepared onto 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 contents. Aerobic oxidations of 1-phenylethanol over the Au_xPd_y-PVP/HT catalysts exhibited that their catalytic activities were significantly affected by the Pd contents. Correlations between charge transfer between Au and Pd and catalytic activity of the Au_xPd_y-PVP/HT catalyst 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 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 Au 5d orbital in the Au_xPd_y-PVP/HT catalysts estimated by the Au L_3-XANES spectra correlated well with their catalytic activities. Moreover, the kinetic studies also proposed that the electron rich Au 5d states, resulted from the intermetallic electron transfer from Pd atoms, strongly contributed to the rate-determination step in the alcohol oxidation. It was concluded that the electronic negativity in the Au 5d states controlled by the Pd contents accelerated the rate-determination step in alcohol oxidation through highly active radical-like intermediates