Covalent triazine framework featuring single electron Co2+ centered in intact porphyrin units for efficient CO2 photoreduction

Covalent triazine frameworks (CTFs) are porous materials constructed by the trimerization of aromatic nitriles. They have frequently been utilized in heterogeneous catalysis due to rich nitrogen sites, excellent stability and porosity. In the past decade, many methods have been developed to synthesize CTFs, however, the roles of structural differences of CTFs synthesized by different methods in catalytic performance remain unclear. Herein, we synthesized two Co porphyrin-based CTFs using same porphyrin monomer by different approaches. CTF-TfOH-Co formed by trifluoromethanesulfonic acid vapor catalysis retains original porphyrin units, and Co (active catalytic sites) feature single electrons. In contrast, CTF-ZnCl2-Co synthesized by the ionothermal method no longer preserves the original porphyrin units, and the Co sites have no single electrons. CTF-TfOH-Co exhibits efficient photocatalytic activity for CO2 reduction with a CO yield rate of 2562.8 μmol g-1h−1, which greatly exceeds that of CTF-ZnCl2-Co. Those results highlight that the integrity of the porphyrin units and the electronic state of Co significantly are vital to their photocatalytic performance. Steady-state photoluminescence tests confirm that CTF-TfOH-Co has a better ability to separate photogenerated electron-hole pairs than CTF-ZnCl2-Co, which accounts for its superior photoreduction performance.