Highly Efficient and Selective Visible‐light Photocatalytic CO<sub>2</sub> Reduction to CO Using a 2D Co(II)‐Imidazole MOF as Cocatalyst and Ru(bpy)<sub>3</sub>Cl<sub>2</sub> as Photosensitizer
Lin Wang, Hongyan Zhang, Zhoujie Zhang, Jiajia Zhang, Yuhan He, Qi Li, Jianchun Bao, Min Fang, Yong Wu
Abstract
Abstract The first application of an imidazole MOF, the 2D Co(II)‐ imidazole framework, {[Co(TIB) 2 (H 2 O) 4 ]SO 4 } (TIB stands for 1,3,5‐tris(1‐imidazolyl) benzene) (CoTIB) in photocatalytic CO 2 reduction was carried out, and compared with that of ZIF‐67. The CO 2 /CoTIB (1.0 mg)/Ru(bpy) 3 Cl 2 (bpy=2,2’‐bipyridine) (11.3 mg)/CH 3 CN (40 mL)/TEOA (10 mL)/H 2 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 CO 2 adsorption capacity and poor conductivity. Extensive photocatalytic experiments and energy‐level diagrams suggest that the reduction did not depend on the CO 2 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 CO 2 . In addition, the process utilizes the short‐lived singlet state ( 1 MLCT), not the long‐lived triplet state ( 3 MLCT) 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, CO 2 , and the sacrificial agent in the reaction system.