Structure- and Coverage-Sensitive Mechanism of NO Reduction on Platinum Electrodes

The interaction of nitric oxide with metal surfaces has been a traditional model system for (electrochemical) surface science. Moreover, NO is an important intermediate within the currently imbalanced nitrogen cycle. Here, we study the electrochemical reduction of adsorbed NO on Pt(111) and Pt(100) electrodes by means of experimental and computational tools. Using linear sweep voltammetry, we find that the onset potentials on Pt(111) for the reduction of *NO on top and on fcc hollow sites (approximately +0.40 and +0.25 VRHE, respectively) are independent of the surface coverage. On the other hand, *NO adsorbed at a low coverage on Pt(100) is more reactive than a compact, saturated *NO adlayer is, and the reaction kinetics switches from first- to second-order from high to low coverage. Density functional theory calculations offer an explanation for the experimental observations by suggesting that the stability of the first hydrogenation product (*NHO or *NOH) and thus the reaction mechanism strongly depends on the *NO coverage and the surface facet. Therefore, *NO reduction on platinum exemplifies a reaction in which not only the rate but also the mechanism is sensitive to structure and coverage. These observations hint at the need for a wider scope in materials design methodologies, as facet- and coverage-independent reaction pathways are typically used for materials screening.