On the Compensation Effect in Heterogeneous Catalysis

For a class of heterogeneously catalyzed reactions, we explain the compensation effect in terms of a switching of kinetic regimes leading to a concomitant change in the apparent activation energy and in the prefactor for the overall rate of the reaction. We first use the ammonia synthesis to illustrate the effect. Both experiments and a detailed kinetic model show a compensation effect. Second, we use density functional theory calculations to show that the compensation effect is not only due to changes in the activation barrier and prefactor of the rate-determining step, N2 dissociation. We compare N2 dissociation on Ru and Pd. The barrier for dissociation differs by more than 2 eV (200 kJ/mol), but calculations of the prefactor based on harmonic transition-state theory shows a difference of less than 10%. To analyze the origin of the compensation effect, we construct a general kinetic model for a surface-catalyzed reaction and show that the effect can be related to a shift in kinetic regime, from one dominated by the rate of activation of the reactants to a regime where the stability of the reaction products on the surface becomes increasingly important. Finally, we present arguments why this should be a general effect for a broad class of reactions. We will show that the compensation effect in the rate is intimately linked to the underlying linear relationships between activation energy and stability of intermediates, which have been found to hold for a number of surface reactions.