Correlating reductive vanadium oxide transformations with electrochemical N2 activation and ammonia formation

Abstract

The electrochemical reduction of nitrogen to ammonia (E-NRR) could become an environmentally friendly approach, yet its molecular-scale reaction mechanisms remain difficult to elucidate. Here, we use in situ electrochemical infrared reflection–absorption spectroscopy (EC-IRRAS) to examine vanadium oxide electrodes in neutral aqueous electrolyte (pH 7). Ex situ XPS reveals that the vanadium oxide electrode initially consists predominantly of V⁵⁺ species in the form of V₂O₅. However, in neutral aqueous electrolyte (pH 7), the surface evolves into anionic ortho-, meta-, and polyvanadate species at potentials above +0.6 V vs. RHE. Upon cathodic sweeping these anionic vanadates undergo progressive reduction toward V₂O₄. In an N₂ saturated electrolyte, subsequent reduction and redeposition of these anionic vanadates remove a distinct vanadyl (V⁴⁺O) feature – likely associated with an undercoordinated site, i.e. oxygen vacancies or grain boundaries – while the appearance of a broad, red-shifted band suggests the formation of vanadyl intermediates that interact with N₂. Crucially, we find that ammonia (or ammonium) formation initiates at −0.28 V versus RHE, coinciding with a phase transition from V₂O₄ to V₂O₃ and continues until this transition completes. This onset is accompanied by the appearance of adsorbed N₂ at −0.28 to −0.38 V versus RHE, indicating an associative mechanism. Overall, these findings emphasize the pivotal role of transient redox transitions (V⁵⁺ → V⁴⁺ → V³⁺) in enabling N₂ activation – beyond the static presence of V₂O₃ alone – and highlight the promise of vanadium oxides as dynamic platforms for E-NRR.