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