Structure and Chemistry of Flat and Stepped Rh Surfaces during NO Dissociation near 1 mbar

The dissociation of NO is a critical step in its catalytic reduction to N₂, which is key to automotive exhaust treatment. Here, we examine the role of Rh atomic steps in the NO dissociation reaction under 0.05 mbar NO. We use a Rh crystal sample curved around the (111) direction and ambient-pressure X-ray photoelectron spectroscopy to probe different Rh surfaces subject to the very same reaction conditions. At the dissociation onset, this approach allows us to quantitatively determine the NO species involved in the reaction, and to rationally assess the process in terms of diffusion and dissociation probability at terraces and steps. At a higher temperature we trigger surface oxidation, which begins preferentially on flat Rh(111) and B-type stepped surfaces, as compared to A-type stepped surfaces. Surface X-ray diffraction performed on single crystal samples reveals similar oxide structures at the atomic scale, but while B-type Rh(553) and Rh(111) surfaces do not reconstruct, A-type Rh(223) facet exhibits faceting. These findings underscore the structural sensitivity of NO dissociation and its potential impact on Rh-catalyzed NO reduction.