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+[double bond, length as m-dash]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.

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

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Article information

Article type
Paper
Submitted
11 Feb 2025
Accepted
29 May 2025
First published
02 Jun 2025
This article is Open Access
Creative Commons BY license

Phys. Chem. Chem. Phys., 2025, Advance Article

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

K. Balogun, Q. Adesope, S. Amagbor, A. Tochi, A. Vass, G. Mul, C. Baeumer, G. Katsoukis and J. A. Kelber, Phys. Chem. Chem. Phys., 2025, Advance Article , DOI: 10.1039/D5CP00554J

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