The results of theoretical calculations of associative desorption of CH4 and H2 from the Ni(111) surface are presented. Both minimum-energy paths and classical dynamics trajectories were generated using density-functional theory to estimate the energy and atomic forces. In particular, the recombination of a subsurface H atom with adsorbed CH3 (methyl) or H at the surface was studied. The calculations do not show any evidence for enhanced CH4 formation as the H atom emerges from the subsurface site. In fact, there is no minimum-energy path for such a concerted process on the energy surface. Dynamical trajectories started at the transition state for the H-atom hop from subsurface to surface site also did not lead to direct formation of a methane molecule but rather led to the formation of a thermally excited H atom and CH3 group bound to the surface. The formation (as well as rupture) of the H–H and C–H bonds only occurs on the exposed side of a surface Ni atom. The transition states are quite similar for the two molecules, except that in the case of the C–H bond, the underlying Ni atom rises out of the surface plane by 0.25 Å. Classical dynamics trajectories started at the transition state for desorption of CH4 show that 15% of the barrier energy, 0.8 eV, is taken up by Ni atom vibrations, while about 60% goes into translation and 20% into vibration of a desorbing CH4 molecule. The most important vibrational modes, accounting for 90% of the vibrational energy, are the four high-frequency CH4 stretches. By time reversibility of the classical trajectories, this means that translational energy is most effective for dissociative adsorption at low-energy characteristic of thermal excitations but energy in stretching modes is also important. Quantum-mechanical tunneling in CH4 dissociative adsorption and associative desorption is estimated to be important below 200 K and is, therefore, not expected to play an important role under typical conditions. An unexpected mechanism for the rotation of the adsorbed methyl group was discovered and illustrated a strong three-center C–H–Ni contribution to the methyl-surface bonding.

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