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.