MODELING THE TRANSIENT KINETICS OF HETEROGENEOUS CATALYSTS - CO-OXIDATION OVER SUPPORTED CR AND CU

This article illustrates the capabilities of the step response method in the determination of the mechanism and kinetics of a heterogeneousl y catalyzed reaction. The step response method was applied to investig ate the CO oxidation over alumina-supported chromium and copper. Detai led mechanisms of this reaction were determined by measuring the isoth ermal reduction and oxidation of the catalysts via switching the gas-p hase composition from inert (He) to either reducing (GO) or oxidizing (O-2) conditions. Modelling the obtained response curves has given inf ormation on the reaction rate constants of the elementary steps in the CO oxidation over the Cr and Cu catalyst. Here, only a minimum number of reaction steps and parameters have been used, but they are still p hysically acceptable. The reduction of the Cr catalyst was found to pr oceed via reaction of adsorbed CO. The activation energy of this proce ss is 95 kJ mol(-1). The dispersion of the Cr catalyst is 20%, whereas only half the metal atoms at the catalyst surface participate in the oxidation and reduction. The latter metal atoms are coordinated to abo ut four removable oxygen atoms. Reduction of the Cu catalyst takes pla ce in three steps: first oxygen on the surface is removed via reaction of CO from the gas phase. Next the oxygen in the surface is removed v ia reaction of adsorbed CO, and finally the oxygen from the bulk of th e catalyst that has diffused to the surface is removed. The activation energy of all three processes has been determined. The dispersion of the Cu catalyst was found to be about 22%.