Metallo-Porous Organic Polymer as a CO2 Reduction Catalyst toward Selective Solar Fuel Production

In photocatalytic CO2 reduction for solar fuel production, selectivity and efficiency are crucial. Here, we report the design and synthesis of a donor–acceptor imine-based porous organic polymer (POP) Tpa-Phenda and a metallo-porous organic polymer (M-POP) Tpa-Phenda-Ru, by reacting tris(4-formylphenyl)amine (Tpa) and Phenda/[Ru(Phenda)(bpy)2]2+ (Phenda = 4,4′-(1,10-phenanthroline-3,8-diyl)dianiline; bpy = 2,2′-bipyridine) using acid-catalyzed Schiff base condensation reaction under solvothermal conditions. Here, the donor–acceptor dyads in both polymers harvested the visible light and transferred the photoexcited electrons to the active catalytic center, which is elucidated through in situ UV–vis spectroscopy. Both Tpa-Phenda and Tpa-Phenda-Ru produced CO in the acetonitrile–water medium using 1-benzyl-1,4-dihydronicotinamide (BNAH) and triethylamine (TEA) as sacrificial electron donors. Tpa-Phenda and Tpa-Phenda-Ru produced 0.92 and 9.77 mmol g–1 of CO, respectively. Tpa-Phenda-Ru exhibited a higher rate of CO formation and selectivity compared to bare Tpa-Phenda. This can be attributed to the presence of the coordinated RuII center in Tpa-Phenda-Ru, which acts as a catalytic site. Interestingly, Tpa-Phenda showed a low exciton binding energy (78 meV), which enhances the charge transfer efficiency and minimizes the energy loss. From an in situ diffuse reflectance FTIR spectroscopy (DRIFTS) study together with DFT calculation, a possible catalytic cycle for CO formation was constructed.