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

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 CH, 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 CH, 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 foun