Synergetic effect of hollowrization and sulfonation on improving the photocatalytic performance of covalent porphyrin polymers in the reduction of CO₂

As a new kind of photocatalytic system, covalent organic polymers (COPs) exhibit wide spectral absorption and high surface area, but poorer photocatalytic performance relative to traditional inorganic semiconductor materials due mainly to the lower separation efficiency of photogenerated carriers. Herein, a tetrabiphenylporphyrin-based COP (COP-P) was investigated in photocatalyzing the reduction of CO2, and nearly exclusive production of CO was observed with a rate of 1.55 µmol g-1 within one hour, which is a little higher than commercial TiO2 (P25) with a rate of 0.92 µmol g-1. Meanwhile, hollowrization and post-sulfonation were utilized to modulate the photocatalytic performance of COP-P, resulting in just moderately improved and slightly deteriorated performance, respectively. Surprisingly, the synergistic action of these two modulations gives 4.8 times enhanced production of CO (7.54 µmol g-1 within 1 hour). Its revealed that post-sulfonation would improve the separation and transfer ability of photogenerated carriers based on transient amperometric I-t curve analyses, but reduces significantly the surface area of COP-P polymers and thereby decreases CO2 uptake through gas adsorption analyses, which fortunately could be remedied by hollowrization, leading to much improved photocatalytic production of CO. This work reveals the complicated influences of post-modification and paves a facile way to optimize the photocatalytic efficiency of organic polymers through synergetic effects.