Cobalt-Oxygen Coordination Steering *NO Hydrogenation in Nitrate Electroreduction

Abstract

Understanding the hydrogenation behavior of *NO during electrochemical nitrate reduction reaction (NO₃RR) is essential for designing catalysts with high selectivity toward ammonia (NH₃) and valuable intermediates like hydroxylamine. Spinel cobalt oxides (Co₃O₄) are promising NO₃RR electrocatalysts, yet the exact *NO hydrogenation mechanism remains unclear. Here, we integrate theoretical calculations and systematic experiments to reveal that the hydrogenation pathway is dictated by the coordination environment, which can be tuned via crystal facet engineering. Co₃O₄ nanoparticles enriched with (111) facets (o-Co₃O₄) and (100) facets (c-Co₃O₄) were synthesized to expose 4- and 6-coordinated Co sites, respectively. Theoretical results show that o-Co₃O₄ favors nitrogen-site hydrogenation ( ${}^{\ast }\text{NO}{\to }^{\ast }\text{NHO}$ ), while c-Co₃O₄ promotes oxygen-site hydrogenation ( ${}^{\ast }\text{NO}{\to }^{\ast }\text{NHO}$ ), leading to distinct rate-determining steps. This selectivity is experimentally supported by in situ spectroscopy, which detects the *NH₂OH intermediate exclusively on o-Co₃O₄. Consequently, o-Co₃O₄ achieves superior NO₃RR performance, with a Faradaic efficiency of 99.4% and an NH₃ yield rate of 12.8 mg h^-1 cm^-2 at -0.4 V. This work uncovers the coordination-governed mechanism of *NO hydrogenation and establishes facet engineering as an effective strategy for directing reaction pathways in NO₃RR.

Keywords: *NO hydrogenation mechanism; Bridged adsorption; Coordination number; Electrochemical nitrate reduction reaction; NH3 production.