Catalytic photo-redox of simulated air into ammonia over bimetallic MOFs nanosheets with oxygen vacancies

Solar-driven conversion of nitrogen (N₂) into ammonia (NH₃) is a promising alternative to the Haber-Bosch process, while still suffers from low conversion efficiency due to inactive N[tbnd]N bond. Herein, a novel pathway of photocatalytic air redox reaction (ARR) to ammonia via NO is proposed and tested over an effective catalyst of oxygen-vacancy-rich bimetallic Cu-Co organic framework ultrathin nanosheets (O_VR-CuCo-MOFs NS) under visible light. The catalyst with unique oxygen defective sites shows an excellent NH₃ synthesis rate from air (287.76 ± 7.02 μmol g^-1·h^-1), which is 5.4-fold higher than that from pure N₂. Moreover, experiments and theoretical calculations indicate that the transformation of air mainly follows a redox pathway, in which N₂ and O₂ can be trapped at the oxygen vacancies to generate nitric oxide (*NO) and further be reduced to ammonia by visible light. The ARR process shows a lower barrier of free energies in the onset activation step (*N₂ → *N-NO, −0.08 eV) and rate-limiting step (*NO → *NHO, 1.23 eV) compared with those of traditional nitrogen reduction (*N₂ → *N-NH, 1.48 eV and H₂N-NH₂ → *NH₂, 1.29 eV, respectively). This work provides a new and sustainable pathway for photo-driven ammonia synthesis.