Electrosynthesis of nylon-6 precursor via heteroatom-doping- regulated oxygen vacancies engineering over ZnO

Abstract

Electrocatalysis is a green alternative to directly synthesize cyclohexanone oxime (CHOX, Nylon-6 precursor) via electrocatalytic reduction of nitrogen oxides to NH₂OH and spontaneous C–N coupling with cyclohexanone, but suffering from a low-yield or poor Faradaic efficiency (FE) because of the scaling relationship. Herein, a strategy of heteroatom-doping-regulated oxygen vacancies (Vo) engineering was proposed to design a robust Cu-ZnO_1−x catalyst for efficient electrosynthesis of cyclohexanone oxime (ESCO). The complete characterizations and theoretical studies revealed that the doped-Cu can reduce Vo sites formation energy and regulate their local electronic state. The synergistic effect between doped Cu and Vo led to the break of the scaling relationship, presenting the enhancement of NO₃⁻ adsorption/dissociation, the weakness of H* adsorption, and the balance of the NH₂OH adsorption for further C–N coupling reaction. Therefore, the hydrogen evolution reaction and NH₂OH reduction to NH₃ side reactions can be suppressed. The optimized Cu-ZnO_1−x delivered an outstanding activity with a 1238.8 μmol h⁻¹ mg_cat.⁻¹ yield and 68.2% FE. Lastly, the reaction mechanism was established following *NO₃ → *NO₃H → *NO₂ → *NO₂H → *NO → *HNO → *NHOH → *NH₂OH and spontaneous C–N coupling with cyclohexanone to form CHOX. The outstanding ESCO performance on Cu-ZnO_1−x catalyst demonstrates the effectiveness of this heteroatom-doping-regulated Vo engineering strategy.