Highly selective and efficient photocatalytic CO2 reduction to HCOO- by in situ hybridization of cobalt quaterpyridine complex with carbon nitride.

The design of non-noble metal photocatalysts that are highly active, selective, robust, and easily separable remains a key challenge in the photocatalytic conversion of CO2 into storable chemical fuels like HCOOH. This process is one of the promising strategies to simultaneously address global energy and environmental crisis. Here, a hybrid system was in situ formed through the Schiff base condensation reaction between a Co quaterpyridine complex bearing a benzaldehyde group (Coqpy-BA, qpy-BA = 4-(2,2':6',2':6',2'''-quaterpyridin]-4-yl)benzaldehyde) and carbon nitride (C3N4). Highly efficient formate (HCOO-) production (13.11 μmol) was obtained with a turnover number (TON) of up to 2185 (three times higher than that of non-covalently linked Coqpy/C3N4) and 99.5 % selectivity in the 1-methyl-2-pyrrolidinone (NMP) solution containing 1,3-dimethyl-2-phenylbenzimidazoline (BIH) and triethylamine (TEA) as sacrificial reductants under visible light irradiation. In addition, characterization experiments show that the excellent performance is attributed to the establishment of efficient electron transport channels via covalent bonding between cobalt catalyst and the C3N4 semiconductor, which enhances charge separation, facilitates electron transfer, and suppresses electron-hole (e--h⁺) recombination within the semiconductor. A plausible photocatalytic CO2 reduction mechanism with Coqpy-BA@C3N4 hybrid system is proposed based on the ultraviolet-visible diffuse reflection spectroscopy (UV-Vis DRS), photoluminescence spectroscopy (PL), and electrochemical characterization results. This approach offers a novel strategy for the in situ construction of low-cost hybrid composite systems aimed at photocatalytic CO2 reduction to HCOO-.