A covalent organic framework integrating photocatalytic center and electron reservoir for photocatalytic CO2 reduction to HCOOH

Direct and efficient photocatalytic conversion of CO2 into fuels and high value-added chemicals under sunlight is of great significance in alleviating the energy crisis and environmental issues. Covalent organic frameworks (COFs) have attracted much attention in the photocatalytic field as materials that can be pre-designed on demand. Taking advantage of the pre-designable properties of COFs materials, the integration of organic building blocks with photocatalytic activity and photoelectron-resident properties into frameworks can improve the electron-hole separation efficiency and photocatalytic performance. Here, we report a newly designed COF (COF-TVBT-N) with triazine and alkenyl groups that can be used as a photocatalyst to accomplish the photoreduction of CO2 to HCOOH under simulated sunlight without the assistance of metal sites and photosensitizers. Furthermore, we unambiguously reveal that the photocatalytic active center in COF-TVBT-N is TVBT through the combination of experiments and theoretical calculations. DFT calculations suggest that the adsorption of H near the active center is beneficial for the adsorption and photoreduction of CO2 in this photocatalytic system. The photoexcitation process and photocatalytic CO2 reduction mechanism of COF-TVBT-N are also reasonably deduced based on DFT calculations. This work provides ideas for the design of COFs as photocatalysts for CO2 reduction and gives insights into the mechanism of CO2 conversion to HCOOH.