Tungsten Nitride/Tungsten Oxide Nanosheets for Enhanced Oxynitride Intermediate Adsorption and Hydrogenation in Nitrate Electroreduction to Ammonia

Abstract

Electrochemical NO₃^- reduction reaction (NO₃RR) is a promising technique for green NH₃ synthesis. Tungsten oxide (WO₃) has been regarded as an effective electrocatalyst for electrochemical NH₃ synthesis. However, the weak adsorption and the sluggish hydrogenation of oxynitride intermediates (NO_<i>x</i>, e.g., *NO₃ and *NO₂) over WO₃ materials hinder the efficiency of converting NO₃^- to NH₃. Herein, we design a heterostructure of tungsten nitride (WN) and WO₃ (WN/WO₃) nanosheets to optimize *NO₃ and *NO₂ adsorptions and facilitate *NO₂ hydrogenations to achieve a highly efficient electrochemical NO₃RR to produce NH₃. Theoretical calculations predict that locally introducing WN into WO₃ will shorten the distance between adjacent W atoms, resulting in *NO₃ and *NO₂ being strongly adsorbed on W active sites in the form of bidentate ligands instead of the relatively weak monodentate ligands. Furthermore, WN facilitates H₂O dissociation to supply the requisite protons, which is beneficial for *NO₂ hydrogenations. Inspired by theoretical prediction, WN/WO₃ nanosheets are successfully fabricated through a high-temperature nitridation process. The transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption near-edge spectroscopy investigations confirm that the amorphous WN has been locally introduced in situ into WO₃ nanosheets to form a composite heterostructure. The as-prepared WN/WO₃ nanosheets exhibit a high Faraday efficiency of 88.9 ± 7.2% and an appreciable yield rate of 8.4 mg h^-1 cm^-2 toward NH₃ production, which is much higher than that of individual WO₃ and WN. The enhanced adsorption and hydrogenation behaviors of *NO_<i>x</i> over WN/WO₃ are characterized by in situ Fourier-transform infrared spectroscopy, consistent with the theoretical predictions. This work develops facile and effective heterostructure nanomaterials to tune the adsorption and hydrogenation of NO_<i>x</i> for boosting the efficiency from NO₃^- to NH₃.