AVS 50th International Symposium
    Surface Science Monday Sessions
       Session SS2-MoM

Paper SS2-MoM1
Atomic Level Explanation of the Compensation Effect in Heterogeneous Catalysis

Monday, November 3, 2003, 8:20 am, Room 327

Session: Catalysis I: Adsorption and Reactions of Small Molecules at Surfaces
Presenter: T. Bligaard, Center for Atomic-scale Materials Physics, Denmark
Authors: T. Bligaard, Center for Atomic-scale Materials Physics, Denmark
K. Honkala, Center for Atomic-scale Materials Physics, Denmark
A. Logadottir, Center for Atomic-scale Materials Physics, Denmark
J.K. Norskov, Technical University of Denmark
S. Dahl, Haldor Topsoe A/S, Denmark
C.J.H. Jacobsen, Haldor Topsoe A/S, Denmark
Correspondent: Click to Email
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. Secondly, 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, N@sub 2@ dissociation. We compare N@sub 2@ 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%. In order 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.