We recently summarized the strategies to improve the selectivity and faradaic efficiency of the photochemical and electrochemical reduction of nitrogen to ammonia (NRR).^[1] One of the possibilities is to follow a Mars-van-Krevelen mechanism (MvK) on transition metal nitrides (TMN).^[2] Moreover, TMNs possess a good chemical stability, high conductivity, flexible electronic structure and avoid the use of precious metals.^[3] Skúlason and co-workers have published theoretical, density functional theory (DFT) based catalyst screening of TMNs as novel catalysts for NRR; vanadium nitride (VN) was found among the most promising active, selective, and vacancy regenerating mononitrides; for VN films especially the (100) facet is stable.^[4] Also experimentally, VN has been reported to be an active electrochemical NRR catalyst.^[5] However, commercial VN suffers from severe instability, both chemical and electrochemical experiments lead to significant vanadium and nitrogen leaching.^[6] Thus, Pan and coworkers established an alternative NRR pathway over vanadium oxynitride; they proposed that mixed anion arrangement improve the adsorption of N₂, the stability of active surface states, and their activity and selectivity over the unwanted hydrogen evolution.^[7]

We have synthesized VN powders, VN nanotubes and VN sheets starting from various vanadium precursors (VOCl₃, V₂O₅, VO_x nanotubes, NH₄VO₃) and comparing different routes (microwave assisted hydrothermal, urea glass, carbothermal). Pure VN phases were obtained with the urea glass route operating at 800°C and the carbothermal route heated up to 1200°C. Best morphology stability of nanotubes and nanosheets was found with the urea glass route. Linear sweep analysis in N₂ atmosphere indicated NRR activity somewhat preferred over hydrogen evolution, most pronounced for VN nanotubes prepared by the urea glass route.

Ongoing work includes ammonolysis to obtain stable VN morphologies and to combine VN with titanium nitride (TiN) as a stabilizing substrate. Based on DFT calculations enhanced catalytic performance due to optimized proton adsorption was proposed for VN/TiN alloys.^[8] The synthesis of such alloys with TiN or other TMNs should be possible with slight modifications of our VN synthesis routes. Actual synthesis results and electrochemical NRR data employing indophenol test and ¹⁵N experiments will be presented in the talk.