Theoretical investigation of CH4 decomposition on Ni: electronic structure calculations and dynamics

Abstract

The dissociative chemisorption of CH₄ on an Ni(111) surface has been studied using different cluster models. Density functional theory is used to determine the transition state and the dissociated state of CH₄ on the substrate. The transition state is explicitly determined on a one-layer Ni₇ cluster. We find a transition-state barrier of 210.3 kJ mol^–1, which is much higher than our one-atom result of 40.7 kJ mol^–1. The higher barrier can be understood in terms of the intrinsic lower reactivity of the central nickel atom in the cluster and a more extended CH bond. If we use as a substrate an Ni₁₃ cluster, the barrier drops to 99.7 kJ mol^–1. Vibrational frequencies are abstracted from the potential-energy surface at the transition state and the dissociated state. We have used transition-state theory to compute rate constants in terms of rotational, vibrational and translational partition functions. We have also determined sticking coefficients and activation energies for CH₄ decomposition as well as CH₃–H recombination of the surface. Sticking coefficients are small, which is consistent with experimental values. At 500 K a kinetic isotope effect of 6.2 is found for CH₄ adsorption, while only a factor of 2.0 is found for CH₃–H association at this temperature. The isotope effect on the activation energy for adsorption is 6.3 kJ mol^–1, while essentially no effect for the activation energy for association is found.