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

Electrochemical NO 3 - reduction reaction (NO 3 RR) is a promising technique for green NH 3 synthesis. Tungsten oxide (WO 3 ) has been regarded as an effective electrocatalyst for electrochemical NH 3 synthesis. However, the weak adsorption and the sluggish hydrogenation of oxynitride intermediates (NO x , e.g., *NO 3 and *NO 2 ) over WO 3 materials hinder the efficiency of converting NO 3 - to NH 3 . Herein, we design a heterostructure of tungsten nitride (WN) and WO 3 (WN/WO 3 ) nanosheets to optimize *NO 3 and *NO 2 adsorptions and facilitate *NO 2 hydrogenations to achieve a highly efficient electrochemical NO 3 RR to produce NH 3 . Theoretical calculations predict that locally introducing WN into WO 3 will shorten the distance between adjacent W atoms, resulting in *NO 3 and *NO 2 being strongly adsorbed on W active sites in the form of bidentate ligands instead of the relatively weak monodentate ligands. Furthermore, WN facilitates H 2 O dissociation to supply the requisite protons, which is beneficial for *NO 2 hydrogenations. Inspired by theoretical prediction, WN/WO 3 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 WO 3 nanosheets to form a composite heterostructure. The as-prepared WN/WO 3 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 NH 3 production, which is much higher than that of individual WO 3 and WN. The enhanced adsorption and hydrogenation behaviors of *NO x over WN/WO 3 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 x for boosting the efficiency from NO 3 - to NH 3 .