Ultrafine fragments of a 2D Co(II)-imidazole MOF as cocatalysts for highly efficient and selective visible-light-driven reduction of CO₂ to CO

We discovered that triethanolamine (TEOA) can break Co-N bonds in a 2D Co(II)-imidazole MOF, {[Co(TIB)(2)(H2O)(4)]SO4} (TIB stands for 1,3,5-tris(1-imidazolyl) benzene) (CoTIB) to form the ultrafine fragments of CoTIB with sizes <5.6 nm under stirring, which are highly efficient and selective (>99%) heterogenous cocatalysts for the visible-light-driven photocatalytic CO2 reduction reaction to CO. The efficiency of the ultrafine fragments of CoTIB (2.0-4.2 nm) reaches 42.7 mmol g(-1) h(-1) or a TOF of 33.7 h(-1) based on Co atoms, which are among the best of heterogenous cocatalyst for photocatalytic CO2RR to CO systems. They are also highly stable under stirring with a TON greater than 909 in 54 h. The ultrafine fragments of CoTIB in mixed solvents were obtained by filtration, using a 0.22-mu m filter head, of the 9-h stirred mixture of CoTIB/CH3CN/TEOA/trace-H2O or the 9-h reaction mixture using 1.0 mg of CoTIB. The fragments of CoTIB in these filtrates were characterized by high-resolution mass spectrometry (HRMS), size distribution and Zeta-potential measurements, ICP-MS, as well as Linear-Sweep-Voltammetry (LSV) measurements. The formation process of these nanoparticles and the mechanism of the photocatalytic CO(2)RRs were proposed. In addition, this work provides an easy method for making ultrafine fragments (<10 nm) of a Co(II)-imidazole MOF or other MOFs, which could also be viewed as MOF nanoparticles or quantum dots stabilized by coordination of TEOA molecules.