Porphyrin Coordination Polymer with Dual Photocatalytic Sites for Efficient Carbon Dioxide Reduction

The resurgence of visible light photocatalysis for carbon dioxide reduction reaction (CO₂RR) has resulted in the generation of various homogeneous and heterogeneous paradigms. Herein, a new system has been established by incorporating dual catalytic sites into porous coordination polymer toward the photocatalysis of CO₂RR. A functional ligand, 5,10,15,20-tetrakis[4'-(terpyridinyl)phenyl]porphyrin (TTPP), has been used to assemble discrete divalent nickel ions into the coordination polymer (TTPP-Ni) through metal bis(terpyridine) nodes. Both the porphyrin and terpyridine moieties prefer to bind with nickel ions, giving rise to TTPP-Ni with dual active catalytic sites. By controlling different molar ratios of ligand and metal and the reaction temperature, four samples including TTPP-Ni-n (n = 1, 2, 3, and 4) with different molar ratios of nickel porphyrin and nickel bis(terpyridine) subunits have been fabricated. The predesigned two-dimensional chemical structures of TTPP-Ni samples have been fully characterized using powder X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and IR and UV-vis spectroscopies. The photocatalytic activities of these coordination polymers have been screened using [Ru(bpy)₃]Cl₂·6H₂O as a photosensitizer together with triisopropanolamine as the sacrificial electron donor in CH₃CN and H₂O. Among these photocatalysts, TTPP-Ni-3 and TTPP-Ni-4 with almost saturated metal sites are able to display extraordinary photocatalytic performance including a CO generation rate of ca. 3900 μmol g^-1 h^-1 and 98% selectivity. The mechanism associated with dual active sites has been rationalized on the basis of theoretical simulations.