Synergism and kinetics in CO oxidation over palladium-rhodium bimetallic catalysts

Abstract

The activity and kinetics of CO oxidation over alumina-supported Pd-Rh bimetallic catalysts were investigated. One bimetallic catalyst, Pd-Rh(2), was prepared by two-step impregnation and another, Pd-Rh(l), by simultaneous impregnation. Monometallic catalysts as well as a physical mixture of them were also prepared. The catalysts were characterized by selective chemisorption of both H2 and CO, and an attempt was made to determine the surface compositions of the bimetallic catalysts. The bimetallic catalysts showed different kinetic behavior, such as higher turnover frequencies (TOFs), lower apparent activation energies and/or negative reaction orders for CO which were smaller in the absolute value, from that of the monometallic catalysts as well as a physical mixture of them. It is suggested that this Pd-Rh synergism is due to an interaction on the catalyst surface, such that adsorbed CO or oxygen on one metal migrates to the other metal site so that the reaction rate is facilitated and also that the particles of Pd and Rh are located close enough to each other for the interaction to occur. On the surface of Pd-Rh (2) most of the Pd and Rh particles existed as separate entities, while a great portion of the particles on Pd-Rh(l) exhibited the surface enrichment of Pd. This explains the higher TOF and the negative reaction orders for CO over Pd-Rh(2) which were smaller in the absolute value than those over Pd-Rh(l).