Symmetric Transition State Analysis: An Analysis of Dissociative Methane Adsorption on Rh{111} Using Quantum Chemical Calculations

Abstract

The chemical bonding aspects of the transition state (TST) of methane activation on a Rh{111} surface are analyzed. Three methods are compared: The barrier decomposition analysis of Hu et al. in which the bond between CH is assumed completely broken in the TST (Satterfield, Heterogeneous catalysis in industrial practice, 2nd ed., 1996; Chorkendorff and Niemantsverdriet, Concepts of modern catalysis and kinetics, 2003; Somorjai, Introduction to surface chemistry and catalysis, 1994); the activation strain model of Bickelhaupt in which the CH bond is assumed to be equal to the gasphase CH interaction energy (Christmann, Surface science reports, 1988; Nørskov and Christensen, Science, 2006; Forsberg, Chemical engineering progress, 2005); and a model in which the interaction energies between CH, and of the H atom and CH3 with the catalyst are all given equal attention, the symmetric transition state analysis. This symmetric transition state analysis would not yield a result different from the traditional methods if all bonds were additive and decoupled. But, as our results show, that is not in general the case. The position of the maximum in non-additivity can be considered a descriptor for the position of the TST on the reaction coordinate. At the TST, we find that the three interactions are of comparable strength.