First-Principles Analysis of Coverage, Ensemble, and Solvation Effects on Selectivity Trends in NO Electroreduction on Pt₃Sn Alloys

Contamination of groundwater by runoff of NO_x species from agricultural land is a growing problem with increasingly negative consequences for public health. Among other technologies, denitrification of runoff streams via electrocatalysis is a promising solution, but a lack of fundamental insights into the relevant reaction mechanisms and a limited understanding of the molecular-level factors that control selectivity to desired products have hindered progress. Previous work suggests that, while Pt electrocatalysts produce primarily undesirable ammonium (NH₄⁺) products from electrochemical NO_x reduction at room temperature, addition of promoters such as Sn may tune the solution-phase product selectivity to yield a mixture of hydroxylamine (NH₃OH⁺) and ammonium. To elucidate the atomic-scale features responsible for this significant selectivity shift, we investigate the reaction mechanism for NO electroreduction on Pt₃Sn­(111) surfaces using periodic density functional theory calculations combined with detailed treatments of surface ensemble, coverage, and solvation effects. High coverages of NO* on the catalyst surface (up to 5/9 ML) are explicitly considered to mimic experimental reaction conditions and to ascertain the effect of adsorbate–adsorbate interactions on the reaction mechanism. Activation barriers for electrochemical reduction and N–O cleavage in adsorbed NO* and subsequent reaction intermediates, as well as chemical N–O bond breaking in similar species, are determined using a combination of transition state search algorithms and explicit electrochemical double-layer models. The resulting reaction free energy landscape highlights the importance of both interactions between the adsorbed reaction intermediates and NO* and solvation effects in promoting hydroxylamine production over ammonium on Pt-Sn alloys. Such effects lead to enhanced stability of the HNO* reaction intermediate, in comparison to pure Pt, and to its subsequent conversion to hydroxylamine. The results provide a compact interpretation of how Sn alters the product selectivity during NO electroreduction on Pt and point to design principles by which selectivity may be controlled in denitrification and related electrocatalytic processes.