Highly Efficient and Selective Visible-light Photocatalytic CO2 Reduction to CO Using a 2D Co(II)-Imidazole MOF as Cocatalyst and Ru(bpy)3Cl2 as Photosensitizer

The first application of an imidazole MOF, the 2D Co(II)- imidazole framework, {[Co(TIB)(2)(H2O)(4)]SO4} (TIB stands for 1,3,5-tris(1-imidazolyl) benzene) (CoTIB) in photocatalytic CO2 reduction was carried out, and compared with that of ZIF-67. The CO2/CoTIB (1.0 mg)/Ru(bpy)(3)Cl-2 (bpy=2,2'-bipyridine) (11.3 mg)/CH3CN (40 mL)/TEOA (10 mL)/H2O (400 & mu;L) system produced 76.9 & mu;mol of CO in 9 h, corresponding to the efficiency of 9.4 mmol g(-1) h(-1) (TOF: 7.3 h(-1)) with a >99% selectivity. Its catalytic activity is even higher than that of ZIF-67 based on TOF values. However, CoTIB is non-porous and has a very poor CO2 adsorption capacity and poor conductivity. Extensive photocatalytic experiments and energy-level diagrams suggest that the reduction did not depend on the CO2 adsorption by the cocatalyst, but can occur by the direct electron transfer from conduction-band maximum (CBM) of the cocatalyst to the zwitterionic alkylcarbonate adduct formed by the reaction of TEOA and CO2. In addition, the process utilizes the short-lived singlet state ((MLCT)-M-1), not the long-lived triplet state ((MLCT)-M-3) of Ru(bpy)(3)Cl-2 to transfer electrons to the CBM of CoTIB. We found that the high efficiency of a cocatalyst, a photosensitizer, or a photocatalytic system depends on the matching of all related energy levels of the photosensitizer, the cocatalyst, CO2, and the sacrificial agent in the reaction system.