A Metal-Free Donor-Acceptor Covalent Organic Framework Photocatalyst for Visible-Light-Driven Reduction of CO2 with H2O

Visible-light-driven CO2 reduction to valuable chemicals without sacrificial agents and cocatalysts remains challenging, especially for metal-free photocatalytic systems. Herein, a novel donor-acceptor (D-A) covalent organic framework (CT-COF) was constructed by the Schiff-base reaction of carbazole-triazine based D-A monomers and possessed a suitable energy band structure, strong visible-light-harvesting, and abundant nitrogen sites. CT-COF as a metal-free photocatalyst could reduce CO2 with gaseous H2O to CO as the main carbonaceous product with approximately stoichiometric O-2 evolution under visible-light irradiation and without cocatalyst. The CO evolution rate (102.7 mu mol g(-1) h(-1)) was 68.5 times that of g-C3N4 under the same conditions. In situ Fourier-transform (FT)IR analysis indicated that CT-COF could adsorb and activate the CO2 and H2O molecules and that COOH* species may be a key intermediate. DFT calculations suggested that nitrogen atoms in the triazine rings may be photocatalytically active sites.