Balogun Kabirat, Adesope Qasim, Amagbor Stella, Tochi Agbara, Vass Adam, Mul Guido, Baeumer Christoph, Katsoukis Georgios, Kelber Jeffry A
Dept. of Chemistry, University of North Texas, Denton, TX, 76203, USA.
MESA+ Institute for Nanotechnology, Faculty of Science and Technology, University of Twente, Drienerlolaan 5, 7522 NB, Enschede, The Netherlands.
Phys Chem Chem Phys. 2025 Jun 10. doi: 10.1039/d5cp00554j.
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 electrochemical infrared reflection-absorption spectroscopy (EC-IRRAS) to examine vanadium oxide electrodes in neutral aqueous electrolyte (pH 7). XPS reveals that the vanadium oxide electrode initially consists predominantly of V species in the form of VO. However, in neutral aqueous electrolyte (pH 7), the surface evolves into anionic -, -, and polyvanadate species at potentials above +0.6 V RHE. Upon cathodic sweeping these anionic vanadates undergo progressive reduction toward VO. In an N saturated electrolyte, subsequent reduction and redeposition of these anionic vanadates remove a distinct vanadyl (VO) feature - likely associated with an undercoordinated site, oxygen vacancies or grain boundaries - while the appearance of a broad, red-shifted band suggests the formation of vanadyl intermediates that interact with N. Crucially, we find that ammonia (or ammonium) formation initiates at -0.28 V RHE, coinciding with a phase transition from VO to VO and continues until this transition completes. This onset is accompanied by the appearance of adsorbed N at -0.28 to -0.38 V RHE, indicating an associative mechanism. Overall, these findings emphasize the pivotal role of transient redox transitions (V → V → V) in enabling N activation - beyond the static presence of VO alone - and highlight the promise of vanadium oxides as dynamic platforms for E-NRR.
将氮气电化学还原为氨（E-NRR）可能成为一种环境友好的方法，但其分子尺度的反应机制仍难以阐明。在此，我们使用电化学红外反射吸收光谱（EC-IRRAS）来研究中性水电解质（pH 7）中的氧化钒电极。X射线光电子能谱（XPS）表明，氧化钒电极最初主要由VO形式的V物种组成。然而，在中性水电解质（pH 7）中，在高于可逆氢电极（RHE）+0.6 V的电位下，表面演变成阴离子、和多钒酸盐物种。在阴极扫描时，这些阴离子钒酸盐会逐渐还原为VO。在氮气饱和的电解质中，这些阴离子钒酸盐随后的还原和再沉积消除了一个明显的钒氧基（VO）特征——可能与配位不足的位点、氧空位或晶界有关——而一个宽的、红移的带的出现表明形成了与N相互作用的钒氧基中间体。至关重要的是，我们发现氨（或铵）的形成在-0.28 V RHE开始，与从VO到VO的相变同时发生，并持续到该转变完成。这种起始伴随着在-0.28至-0.38 V RHE处吸附N的出现，表明是一种缔合机制。总体而言，这些发现强调了瞬态氧化还原转变（V→V→V）在实现N活化方面的关键作用——不仅仅是VO的静态存在——并突出了氧化钒作为E-NRR动态平台的前景。
