Tailored covalent organic frameworks with metal sites enabling high-efficiency ammonia generation through nitrate...

Electrocatalytic reduction of nitrate to ammonia (NRA) has emerged as an attractive route for renewable NH₃ electrosynthesis. Most currently investigated catalytic systems suffer from low metal-atom utilization efficiency, while metal-organic frameworks (MOFs) with theoretically high atomic dispersion face structural instability. Metal covalent organic frameworks (MCOFs) represent an advanced catalytic platform that simultaneously achieves atomic dispersion of metal active sites and exceptional structural stability. Here, we synthesize a hydroxyl-functionalized COF (COF-OH) through polycondensation of 2,5-dihydroxyterephthalaldehyde and melamine, followed by post-synthetic metalation to introduce Cu sites. The hydroxyl groups in the framework serve dual functions: (i) Stabilizing atomic Cu centers through strong CuO coordination. (ii) Enhancing catalytic activity by modulating the electronic structure of Cu sites. The experimental results demonstrated that the introduction of hydroxyl groups led to a significant increase in both ammonia yield (19.90 mg h^-1 mg_cat^-1) and faraday efficiency (91.91%). Critically, the COF-OH model with ordered single-metal sites can serve as an exemplary platform for investigating the catalytic mechanisms of NRA. Theoretical and experimental results indicate that the single metal sites can facilitate the subsequent reduction of nitrate, and the activities of metal species and organic N species: OCuN > CuN > OFeN > OCoN > ONiN.