Interaction between Pd and Ag on the surface of silica

Abstract

Palladium, silver and palladium–silver catalysts supported on silica were prepared by coimpregnation of support with solution of AgNO 3 and Pd(NO 3) 2. The catalysts were characterized by X-ray powder diffraction (XRD), temperature programmed reduction (TPR), time of flight ion mass spectrometry (ToF-SIMS), chemisorption of carbon monoxide and were tested in the reaction of selective oxidation of glucose to gluconic acid. XRD and TPR studies have shown that an interaction between Pd and Ag on the surface of silica after oxidation at 500 °C and reduction at 260 °C leads to the formation of solid solutions. ToF-SIMS images of the surface of 5% Ag/SiO 2 catalyst after oxidation at 500 °C and reduction at 260 °C show that Ag atoms supported on silica are not distributed homogenously but tend to form regions of enhanced Ag concentration. Positive ions images of the surface of 5% Pd/SiO 2 catalyst also display regions of enhanced concentration of Pd atoms, but they are more homogenously distributed on silica. ToF-SIMS peak intensity ratio 108Pd +/ 107Ag + for bimetallic 5% Pd–5% Ag/SiO 2 catalysts has a lower value than that obtained for physical mixture 5% Pd/SiO 2–5% Ag/SiO 2 which indicates that the surface of bimetallic catalyst is enriched with silver atoms.