Theoretical calculations of CH4 and H2 associative desorption from Ni(111): Could subsurface hydrogen play an important role?

Abstract

The results of theoretical calculations of associative desorption of CH(4) 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 CH(3) (methyl) or H at the surface was studied. The calculations do not show any evidence for enhanced CH(4) 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 CH(3) 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 CH(4) 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 CH(4) molecule. The most important vibrational modes, accounting for 90% of the vibrational energy, are the four high-frequency CH(4) 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 CH(4) 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.

Theoretical Study of the Dissociative Adsorption of Methane on Ir(111): The Role of Steps and Surface Distortions at High Temperatures

“…0.66 eV for the tbe TS geometry, allowing surface relaxation) is in reasonably good agreement with the k-point converged value mentioned in ref. 19 (0.7 eV), and 0.27 eV smaller than the one reported by Qi et al 8 : 0.93 eV. The latter significant discrepancy might be partially due to the use of a 2×2 supercell in ref.…”

On the origin of mode- and bond-selectivity in vibrationally mediated reactions on surfaces

“…For example, in the transition state for methane's dissociative chemisorption on Ni(111), [35][36][37][38][39][40] only one C-H bond is significantly extended, but the infrared-and Raman-active vibrations available for gas phase excitation are collective excitations of all four C-H oscillators. Isotopic substitution can change the identity of the molecule's normal mode vibrations and vibrational eigenstates, and is therefore a useful tool for expanding the range of vibrational motions available for probing transition state access.…”

Structure‐sensitivity of CH3 dissociation on Ni(100)from first‐principles calculations

“…The first Brillouin zone of the p(3 × 3) supercell is sampled with a 2 × 2 × 1 k -point mesh which is evidenced to be sufficient for such a large cell. [19] The energies of localized vibrations are substantial for light atoms such as H atoms. It is necessary to make zero-point energy (ZPE) correction to the adsorption energies and activation energy barriers.…”

“…They found that the threefold hollow sites were also energetically favorable for the CH 2 adsorption, and the activation energy for the CH 3 dehydrogenation was calculated to be more than 1 eV. Ciobica and van santen [5] carried out ab initio calculations to study the CH x adsorption and methane dissociation on Ru (11)(12)(13)(14)(15)(16)(17)(18)(19)(20). In their study, CH 2 appeared to be the most stable species on the surface and the energy barrier for CH 3 dissociation was 11 kJ/mol.…”