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.