Ferric Porphyrin-Based Porous Organic Polymers for CO₂ Photocatalytic Reduction to Syngas with Selectivity Control

Visible light-induced photocatalytic CO2 reduction reaction (CO2RR) is a feasible and promising option to tackle the greenhouse effect and energy crisis. Herein, two ferric porphyrin-based porous organic polymer semiconductors, hereafter referred to as POPn-Fe (porous organic polymers, n = 1 or 2, corresponding to a benzene/biphenyl unit as a linker between porphyrin units), are synthesized for the visible light-driven CO2RR to produce syngas. The CO/H2 evolution rates for POP2-Fe under irradiation >420 nm are found to successfully reach up to 3043 and 3753 μmol g–1 h–1, respectively. Interestingly, the experiment results imply that the ferric porphyrin site could be responsible for CO evolution and the uncoordinated porphyrin unit in POPn or POPn-Fe semiconductors may be obligate for H2 formation. Furthermore, as evidenced by Mott–Schottky plots, the extended π-conjugation with the biphenyl linker makes POP2-Fe a lower conduction band potential, which helps the ferric porphyrin sites capture electrons from the photosensitizer, thus producing more CO to realize selectivity control. Also, the efficient catalytic activity of POP2-Fe is presumably attributed to the accelerated charge transfer as well as facilitates photogenerated electron and hole separation. This work offers an elegant strategy to design and optimize earth-abundant metal visible light photocatalysis for CO2 reduction to syngas with CO/H2 ratio control.