Structure and chemistry of flat and stepped Rh surfaces under ambient NO dissociation

We explore the effect of Rh steps on the NO dissociation process at 0.05 mbar NO, which is a crucial step of the NO reduction reaction. By using a curved Rh(111) crystal and ambient-pressure X-ray photoemission spectroscopy, data demonstrate a lower temperature onset for NO dissociation at stepped surfaces compared to Rh(111), and suggest that dissociation occurs predominantly at the lower side of the step. We also observe that surface oxidation arises at a lower temperature for Rh(111) and its B-type vicinals, with respect to A-type stepped surfaces. Surface X-ray diffraction was employed to characterize the Rh surface oxides. At the atomic scale, these were found structurally similar in flat Rh(111), A-type Rh(223) and B-type Rh(553), but they exhibited different growth behavior depending on the substrate orientation. These results showcase the structural sensitivity of NO dissociation, which is likely to affect the reactivity of these Rh surfaces under operando NO reduction conditions.