N2 dissociation on W(110): An ab initio molecular dynamics study on the effect of phonons.

Accurately modeling the chemisorption dynamics of N2 on metal surfaces is of both practical and fundamental interest. The factors that may have hampered this achievement so far are the lack of an accurate density functional and the use of approximate methods to deal with surface phonons and non-adiabatic effects. In the current work, the dissociation of molecular nitrogen on W(110) has been studied using ab initio molecular dynamics (AIMD) calculations, simulating both surface temperature effects, such as lattice distortion, and surface motion effects, like recoil. The forces were calculated using density functional theory, and two density functionals were tested, namely, the Perdew-Burke-Ernzerhof (PBE) and the revised PBE (RPBE) functionals. The computed dissociation probability considerably differs from earlier static surface results, with AIMD predicting a much larger contribution of the indirect reaction channel, in which molecules dissociate after being temporally trapped in the proximity of the surface. Calculations suggest that the surface motion effects play a role here, since the energy transfer to the lattice does not allow molecules that have been trapped into potential wells close to the surface to find their way back to the gas phase. In comparison to experimental data, AIMD results overestimate the dissociation probability at the lowest energies investigated, where trapping dominates, suggesting a failure of both tested exchange-correlation functionals in describing the potential energy surface in the area sampled by trapped molecules.