Theoretical calculations of CH4 and H2 associative desorption from Ni(111):: Could subsurface hydrogen play an important role? -: art. no. 044706

The results of theoretical calculations of associative desorption of CH4 and H-2 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 A. 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. (c) 2006 American Institute of Physics.