Improved Photoreduction of CO2 with Water by Tuning the Valence Band of Covalent Organic Frameworks

Porous covalent organic frameworks (COFs), as an emerging material, have the characteristics of high stability, large series of components, easy synthesis, modification, and adjustable amplitude. They have the potential to become good catalysts. Bromine, as a halogen, has attracted intensive interest for the modification of photocatalysts for photocatalytic reactions. It is feasible to enhance the activity and selectivity of the material by facile functionalization of the reticular parent structure's electron-withdrawing groups. In addition, the conjugation effect of bromine, further delocalizing the electrons of the COF, is beneficial to the progress of many photocatalytic reactions. Reports on the modification of COFs by bromine functional groups to improve the catalytic performance have not been found so far. Here, TAPP [5,10,15,20-tetrakis(4-aminophenyl)porphyrin] and 2,5-dibromo-1,4-benzenedialdehyde instead of terephthalaldehyde were chosen to synthesize a porphyrin-based COF (TAPBB-COF) by the solvothermal method. As expected, the valence band (VB) of TAPBB-COF is thus adjusted to a more suitable position. Additionally, the CO production when using TAPBB-COF under full-wavelength light for 12 h was 295.2 μmol g-1 , which was three times that of COF-366, and the new material has good recycling stability and selectivity (95.6 %). Theoretical calculations indicate that the nitrogen of the porphyrin ring and the Schiff base, and the bromine in TAPBB-COF contribute greatly to the activation of H2 O and the conversion of CO2 in the photoreaction.