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Abstract Understanding the hydrogenation behavior of *NO during electrochemical nitrate reduction reaction (NO 3 RR) is essential for designing catalysts with high selectivity toward ammonia (NH 3 ) and valuable intermediates like hydroxylamine. Spinel cobalt oxides (Co 3 O 4 ) are promising NO 3 RR 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. Co 3 O 4 nanoparticles enriched with (111) facets (o‐Co 3 O 4 ) and (100) facets (c‐Co 3 O 4 ) were synthesized to expose 4‐ and 6‐coordinated Co sites, respectively. Theoretical results show that o‐Co 3 O 4 favors nitrogen‐site hydrogenation (), while c‐Co 3 O 4 promotes oxygen‐site hydrogenation (), leading to distinct rate‐determining steps. This selectivity is experimentally supported by in situ spectroscopy, which detects the *NH 2 OH intermediate exclusively on o‐Co 3 O 4 . Consequently, o‐Co 3 O 4 achieves superior NO 3 RR performance, with a Faradaic efficiency of 99.4% and an NH 3 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 NO 3 RR.
Niu et al. (Wed,) studied this question.