We derive a model for the dissociative chemisorption of methane on a Ni(100) surface, based on the reaction path Hamiltonian, that includes all 15 molecular degrees of freedom within the harmonic approximation. The total wavefunction is expanded in the adiabatic vibrational states of the molecule, and close-coupled equations are derived for wave packets propagating on vibrationally adiabatic potential energy surfaces, with non-adiabatic couplings linking these states to each other. Vibrational excitation of an incident molecule is shown to significantly enhance the reactivity, if the molecule can undergo transitions to states of lower vibrational energy, with the excess energy converted into motion along the reaction path. Sudden models are used to average over surface impact site and lattice vibrations. Computed dissociative sticking probabilities are in good agreement with experiment, with respect to both magnitude and variation with energy. The ν(1) vibration is shown to have the largest efficacy for promoting reaction, due to its strong non-adiabatic coupling to the ground state, and a significant softening of the vibration at the transition state. Most of the reactivity at 475 K is shown to result from thermally assisted over-the-barrier processes, and not tunneling.

We use density functional theory to examine 24 transition states for methane dissociation on five different metal surfaces. In our calculations, the nonlocal exchange-correlation effects are treated within the generalized gradient approximation using the Perdew-Burke-Ernzerhof functional. In all cases, the minimum energy path for dissociation is over a top site. The barriers are large, 0.66-1.12 eV, and relatively insensitive to the rotational orientation of the (nonreacting) methyl group and the azimuthal orientation of the reactive C-H bond. There is a strong preference on the Pt surfaces for the methyl fragment to bond on the top site, while on the Ni surfaces there is a preference for the hollow or bridge sites. Thus, during the dissociation on Pt, only the low mass H atom needs to significantly move or tunnel, while on Ni, both the dissociating H and the methyl fragment move away from the top site. For all 24 configurations there is a strong force at the transition state to pucker the metal atom over which the reaction occurs. The resulting magnitude of the variation in the barrier height with the motion of this atom varies a bit from surface-to-surface, but is of the order of 1 eV/A. We derive a model for the effective reaction barrier height that includes the effects of lattice motion and substrate temperature and compare with recent experiments and other theoretical studies.