ELECTRONIC-STRUCTURE CALCULATIONS AND DYNAMICS OF THE CHEMISORPTION OF METHANE ON A NI(111) SURFACE

The dissociative chemisorption of CH4 on a 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 kJ/mol, which is considerably higher than our single atom result of 41 kJ/mol. The overall reaction is endothermic by 136 kJ/mol. The higher barrier can be attributed to the intrinsic lower reactivity of the central nickel atom embedded in the cluster and a more extended CH bond. If we use a Ni13 Cluster as a substrate, the barrier reduces to 100 kJ/mol. Vibrational frequencies are obtained 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 on the surface. Sticking coefficients are small, which is consistent with experiment. 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 kJ/mol, while there is essentially no effect on the activation energy for association.