Pd-Au/SiO2: Characterization and Catalytic Activity

Abstract

A series of Pd-Au/SiO2 catalysts was prepared by incipient wetness coimpregnation of silica with respective chloride solutions, followed by calcination in air and reduction in hydrogen at 380°C for 15 h. A variety of techniques, chemisorption of hydrogen and carbon monoxide, temperature-programmed reduction, X-ray diffraction, X-ray photoelectron spectroscopy, and neopentane conversion as a catalytic probe, were employed to characterize metal dispersion and interaction between two metal components and the support. It has been discovered that the prepared materials are not uniform: a palladium-rich phase of very high metal dispersion (particle size ∼ 1.3 nm) coexists with much less dispersed gold-rich phase (particle size ∼ 15 nm). X-ray photoelectron spectroscopy revealed some presence of Si<IV species (as reduced silicon or ex-palladium silicide) in reduced silica-supported Pd-Au catalysts. Because the amount of Si<IV positively correlates with metal dispersion it is suggested that these species are in a close vicinity to the highly dispersed Pd (Pd-rich) material, forming a kind of "chemical glue" between metal and support. In contrast to other Pd-based systems (e.g., Pd-Cu and Pd-Zn), coimpregnation of silica with aqueous solutions of palladium and gold chlorides does not produce well-homogenized bimetallic material.