Methane on a stepped surface: Dynamical insights on the dissociation of CHD 3 on Pt(111) and Pt(211).

The simulation of the dissociation of molecules on metal surfaces is a cornerstone for the understanding of heterogeneously catalyzed processes. However, due to high computational demand, the accurate dynamical simulation of the dissociative chemisorption of polyatomic molecules has been limited mostly to flat low-index metal surfaces. The study of surfaces that feature "defected" sites, such as steps, is crucial to improve the understanding of the overall catalytic process due to the high reactivity of under-coordinated sites for this kind of reaction. In this work, we have extensively analyzed more than 10 000 ab initio molecular dynamics trajectories where a CHD ₃ molecule is impinging either on the flat Pt(111) surface or on the stepped Pt(211) surface for different initial rovibrational states and collision energies. The results have been compared in order to get insight into the effect of the step in the dissociation of methane. We have found that, despite a large difference in the activation barrier and consequently in reactivity, the geometry of the lowest transition states is very similar on the two surfaces and this results in a similar dissociation dynamics. Furthermore, the trapping observed on the Pt(211) surface can be explained with energy transfer to parallel translational motion induced by the geometry of the slab and by a larger energy transfer to phonons for the stepped Pt(211) surface.