Enhanced Electrosynthesis of Valuable C═N Compounds through Nitrite Reduction via Mesoporous Carbon Supporting Highly Accessible FeN _<i>x</i> Sites

Electrocatalytic C-N reductive coupling provides a promising alternative for wastewater remediation and the sustainable synthesis of cyclohexanone oxime (CHO), a key C═N compound for manufacturing nylon-6. However, it still suffers from low selectivity and Faraday efficiency (FE) due to the reactive intermediates (*NOH, *NHOH, *NH₂OH) involved in nitrogen source (e.g., NO_<i>x</i>^-, NO_<i>x</i>) reduction, which preferably convert to NH₃ rather than react with cyclohexanone. Based on the favorable properties of single-atom sites for *NH₂OH generation and the moderate adsorption of NH₂OH on the Fe surface, herein, we prepared an Fe-SAC-MIL-101 catalyst enriched with accessible FeN_<i>x</i> monatomic sites by the pyrolysis of NH₂-MIL-101(Al), adsorption of the Fe(II)-phenanthroline complex, and secondary annealing. Since a high utilization of FeN_<i>x</i> sites was achieved for NO₂^- reduction to selectively generate *NH₂OH, in CHO electrosynthesis from NO₂^- and cyclohexanone, almost 100% yield and 51.5% FE for CHO were realized at -0.9 V vs Ag/AgCl in H-type cell, with a yield rate of 8.53 g h^-1 g_cat^-1 and a TOF of 0.15 s^-1. Both yield (78.8%) and FE (40.6%) were also attained even when the NO₂^- concentration was decreased to 93 mM. The promotion of exposed FeN_<i>x</i> active sites in electrocatalysis performance was demonstrated by a series of electrochemical tests. In situ spectroscopic characterizations also evidenced the selective generation of *NH₂OH for cyclohexanone oximation. This work provides a feasible catalyst modification method for promoting *NH₂OH generation to facilitate CHO electrosynthesis from nitrogen-containing wastes.