Efficient photocatalytic reduction of CO₂ to formate by a molecular noble metal-free system

Reducing CO₂ to carbon-neutral solar fuels can not only alleviate the energy demand but also mitigate the greenhouse effect. At present, developing an efficient non-noble metal photo/electrocatalytic system for converting CO₂ to a specific product beyond CO is still a big challenge. Herein, we report a new cobalt complex bearing 2,2’-bi-1,10-phenanthroline ligand that can photocatalytically reduce CO₂ to formate (HCOO^-). Turnover number (TON) for HCOO^- up to 677 with 99% selectivity can be achieved using organic dyes purpurin as photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as sacrificial reductant in a CO₂-saturated NMP/TEA (v/v, 3:1) solution under visible light irradiation. Controlled potential electrolysis at -1.41 V (vs. Ag/AgCl) in the presence of 12 mM Et₃NHBF₄ also produced HCOO^- with Faradaic efficiency of 39%. Spectroscopic and electrochemical studies suggest that a [Co^I-H] species is the key intermediate for CO₂ reduction.