A series of Pd-Au/SiO2 catalysts was prepared by incipient wetness coi
mpregnation of silica with respective chloride solutions, followed by
calcination in air and reduction in hydrogen at 380 degrees C for 15 h
. A variety of techniques, chemisorption of hydrogen and carbon monoxi
de, temperature-programmed reduction, X-ray diffraction, X-ray photoel
ectron spectroscopy, and neopentane conversion as a catalytic probe, w
ere 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 hig
h metal dispersion (particle size similar to 1.3 nm) coexists with muc
h less dispersed gold-rich phase (particle size similar to 15 nm). X-r
ay photoelectron spectroscopy revealed some presence of Si-<IV species
(as reduced silicon or ex-palladium silicide) in reduced silica-suppo
rted Pd-Au catalysts. Because the amount of Si-<IV positively correlat
es with metal dispersion it is suggested that these species are in a c
lose vicinity to the highly dispersed Pd (Pd-rich) material, forming a
kind of ''chemical glue'' between metal and support. In contrast to o
ther Pd-based systems (e.g., Pd-Cu and Pd-Zn), coimpregnation of silic
a with aqueous solutions of palladium and gold chlorides does not prod
uce well-homogenized bimetallic material. (C) 1995 Academic Press, Inc
.