AN INFRARED SPECTROSCOPIC STUDY OF CO AND NO ADSORPTION ON BIMETALLICPD-RH SIO2 CATALYSTS/

Pure Pd/SiO2, pure Rh/SiO2, and bimetallic PdRh/SiO2 catalysts, with P d and Rh in the atomic ratios of 75/25 at% and 50/50 at%, were studied by means of infrared (IR) spectroscopy. Spectra of adsorbed CO and NO were recorded in the temperature range from 298 to 573K. The IR spect ra obtained for the bimetallic catalysts exhibit characteristic featur es of both Pd and Rh. The results point to the presence of alloy cryst allites with properties between those of the constituent metals. Most likely the PdRh(1:1)/SiO2 catalyst consists of small Rh clusters or at oms, surrounded by Pd, whereas for the PdRh(3:1)/SiO2 catalyst the sur face consists primarily of Pd. However, in the presence of NO, Rh migr ates to the surface, due to the dissociation of NO and the concomitant oxidation of the metal particles. No indication of mixed sites, consi sting of both Pd and Rh atoms, was found. The results are compared wit h those previously reported for Pt-Rh catalysts.