Highly Efficient and Selective Visible-light Photocatalytic CO₂ Reduction to CO Using a 2D Co(II)-Imidazole MOF as...

The first application of an imidazole MOF, the 2D Co(II)- imidazole framework, {[Co(TIB)₂ (H₂ O)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 Cl₂ (bpy=2,2'-bipyridine) (11.3 mg)/CH₃ CN (40 mL)/TEOA (10 mL)/H₂ O (400 μL) system produced 76.9 μmol 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 (¹ MLCT), not the long-lived triplet state (³ MLCT) of Ru(bpy)₃ Cl₂ 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.