Energy Level Engineering: Ru Single Atom Anchored on Mo-MOF with a [Mo8O26(im)2]4– Structure Acts as a Biomimetic Photocatalyst

Energy level engineering by controlling the defect level in the band gap and constructing a single-site heterojunction via MOF-based biomimetic catalysts brings out opportunities for photocatalytic nitrogen fixation. Herein, isolated Ru single atoms were successfully implanted onto well-organized Mo-MOFs with a [Mo8O26(im)2]4– structure to form Ruδ+-O3-Mo3 single-site heterojunctions. Both Ru doping level and Mo5+ defect level have been incorporated into the band structure of Mo-MOF simultaneously with enhanced visible-light absorption up to 700 nm. Ru@Mo-MOF-tri showed significantly higher photocatalytic nitrogen fixation activity than Ru@Mo-MOF-mono because of the higher amount of Ruδ+-O3-Mo3 active sites and bending down at the interface of Ru@Mo-MOF-tri heterojunction. DFT calculations based on a Ru single atom at the Mo-MOF-tri structure prove that the formation of the Ruδ+-O3-Mo3 single site with Mo5+ results in moving up of the Fermi level with a high electron energy; thus, the energy barrier is dramatically reduced for the formation of NNH* as the rate-limiting step of nitrogen fixation. The clear structure–activity relationship obtained in this work provides clues for designing single-site heterojunctions as biomimetic photocatalysts and regulating the energy band structures rationally, integrating the advantages of single atom metal and photosensitive MOF.