Revealing the Role of Interfacial Water in pH-Dependent Hydroxylamine Electrosynthesis over Bi-Based Catalysts

Hydroxylamine (NH₂OH) is an important nitrogenous feedstock in the production of numerous valuable chemicals. The selective electroreduction of nitrate (NO₃^–) into NH₂OH paves a promising avenue for the synthesis of NH₂OH. Bi-based catalysts are regarded as the most promising candidates owing to the weak adsorption for NH₂OH, preventing the further reduction of NH₂OH. It is worth noting that Bi-based catalysts exhibited a dramatic activity gap in the acid and neutral/alkaline electrolytes toward the synthesis of NH₂OH. Unfortunately, the mechanistic origin of pH-dependent NH₂OH production has rarely been investigated, lacking an insightful interpretation on the atomic and molecular scale. Herein, taking Bi nanosheets (NSs) as a model catalyst, we investigated the underlying origin of the pH-dependent selectivity in NO₃^– electroreduction toward NH₂OH production. The Faradaic efficiency (FE) for NH₂OH of Bi NSs dramatically decreased from 86.6 to 12.0%, with the pH value ranging from 0 to 3. Combining the in situ Fourier transform infrared spectroscopy and theoretical calculations, we revealed that the dominant 4-coordinated hydrogen-bonded water in acid media favored the involvement of *H in the hydrogenation of *NO to *NHO, thereby facilitating the synthesis of NH₂OH. Moreover, 10 kinds of amino acids were successfully synthesized by Bi NSs, demonstrating the wide universality of Bi nanosheets for the synthesis of amino acids.