Effects of surface step on molecular propane adsorption

Abstract

We have studied the effects of surface step on molecular propane adsorption using molecular-dynamics simulations and a model stepped surface, Pt(6 5 5). Incidences along the step edge (smooth azimuth) and perpendicular to the step edge with upstairs momentum (upstairs azimuth) and downstairs momentum (downstairs azimuth) are considered. In general, the surface step enhances the initial trapping probability of propane except for the downstairs incidences. The most efficient zone in facilitating adsorption is near the bottom of the surface step on the lower terrace where incident molecules experience stronger attraction and an “additional-layer” effect when crossing the step. The least efficient zone is the top of the surface step on the upper terrace due to an opposite “missing-layer” effect. Surface step also creates steric effects such that more incident molecules along the upstairs azimuth but significantly less molecules along the downstairs azimuth impact the step-bottom zone. The latter steric effect, a shadowing effect, undermines the high trapping efficiency of the step-bottom zone to cause the downstairs incidences to have the lowest trapping probabilities. While the shadowing effect can be enhanced by larger incident angles and lower incident energies, the other steric effect on the upstairs incidences is relatively insensitive to the incident energy. Overall, the influence of surface step on molecular adsorption diminishes at low incident energies and large incident angles because longer contact times and less normal momenta result in high trapping probability across the entire stepped surface.