CONICET | Buscador de Institutos y Recursos Humanos

Dissociation of methane on Iridium surfaces shows, under certain conditions, a particular behavior: the reactive sticking probability strongly decreases when the impact energy decreases but below ~0.15 eV, the sticking curves present an upturn putting in evidence two different mechanisms taking place at low and at high energies. 1,2 This points to aprecursor mediated mechanism as the responsible for the increase in the sticking probability at low energies not easily reconciled with the relatively high activation energy barriers reported in the literature (~0.7 eV). In order to shed some light on the prevailing methane dissociation mechanisms, we have developed a reactive force field (RFF) based on Density Functional Theory (DFT) calculations which we use in quasi-classical molecular dynamics (MD) simulations. This has allowed us to reproduce very well the experimental sticking probabilities an its dependence with temperature, incident kinetic energy and the initial vibrational state of the methane molecules. Moreover, our MD results compared with those of a sudden-lattice approximation show that for high surface temperatures (e.g. 1000K) the strong temperature effect is mainly governed by the different dissociation barriers that result from surface deformation due to thermal fluctuations but with a negligible dynamic coupling between the surface and molecular degrees of freedom. We also show that this dynamic coupling becomes relevant only at surface temperatures below room temperature for which methane is rather inert. In addition, we have found that the upturn of the reactive sticking probability observed experimentally at very low impact energies is due to the combination of trapping in a shallow physisorption combined with very-low-activation-energy pathways that the molecule can found at high surface temperatures due to lattice distortionsinduced by thermal fluctuations.