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

Abstract

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.