Synthesis and characterization of Au-Pd/SiO2 bimetallic catalysts prepared by electroless deposition

A series of Au-Pd/SiO2 catalysts have been prepared by the electroless deposition of Au onto a Pd/SiO2 catalyst. A kinetically stable, electroless bath consisting of Au(CN)(2)(-); and N2H4 (i.e., Au source and reducing agent) was developed and optimized, allowing for incremental coverages of Au on Pd to be obtained, while avoiding deposition onto the SiO2 support. The structural and electronic properties of the catalysts were characterized using hydrogen titration of oxygen-precovered Pd, scanning transmission electron microscopy with energy dispersed X-ray spectroscopy, Fourier transform infrared spectroscopy, and Xray photoelectron spectroscopy. The results suggest that Au was deposited on all types of Pd surface sites (e.g., planes, steps, kinks, edges) in a non-discriminatory fashion, with a net electron transfer from Pd to Au. The catalysts were evaluated for propylene hydrogenation, revealing significantly enhanced turnover frequencies at elevated fractional coverage of Au on Pd (theta(Au) >= 0.60). The enhanced catalytic activity can be explained by the disruption of continuous Pd ensembles by Au deposition, which prevents formation of the multiply bonded and less reactive propylidyne, while permitting formation of highly reactive and weakly pi-bonded propylene. (C) 2010 Elsevier Inc. All rights reserved.