Paper SS-TuP5
Two-faced Steps: How Molecular Alignment does and does not Impacts O₂ Sticking Dynamics on Pt.

Tuesday, October 23, 2018, 6:30 pm, Room Hall B

Adsorption and dissociation of O₂ on Pt are intensely studied reactions. In heterogeneous catalysis, it is claimed to be of high importance to automotive air pollution control. In electrochemistry, the oxygen reduction reaction limits the efficiency of hydrogen fuel cells. From previous dynamical and spectroscopic studies, it is well-established that O₂ does not dissociate as an elementary reaction on Pt(111). It requires trapping in a chemisorbed molecular state prior to dissociating. Surface corrugation enhances the sticking probability for O₂ at low incident energy, whereas at higher incident energy the influence depends on the step type causing corrugation. In this contribution, we study the origins of these findings combining two recently developed techniques. First, we apply a curved single crystal approach in combination with supersonic molecular beam techniques with high spatial resolution. At low incident energy, the enhanced reactivity is linear with step density and nearly identical for two different step types over a step density range covering approximately two orders of magnitude. At high incident energy, the enhancement mostly disappears. These result suggest energy-dependent dominance of parallel mechanisms causing sticking and dissociation. Second, we use spin-rotation state-selected and aligned O₂ to determined how rotation parallel and perpendicular to the (111) plane and the step direction affect dissociation on ‘flat’ Pt(533), Pt(553) and Pt(111) single crystals. At lower incident energy, activated molecular chemisorption on the (111) plane strongly favors helicoptering O₂ molecules. This effect diminishes with increasing kinetic energy. The A- and B-stepped surfaces show, on the contrary, at low incident energy no dependence on the orientation of the molecular axis upon impact. This is ascribed to the dominance of initial scattering into a physisorbed state preceding molecular chemisorption and dissociation. At higher incident energy, this mechanism loses its dominance and steps become stereodynamically selective. We observe a clear preference for O₂ molecules impacting with the molecular axis parallel to the step facet.