Local microenvironment modulation of zirconium-porphyrinic frameworks for CO2 reduction

Solar-powered photocatalysis offers a sustainable strategy to convert carbon dioxide into valuable fuels, which mitigates the energy crisis and the greenhouse effect. Nevertheless, exploring efficient, selective, stable, and environmentally friendly photocatalysts for CO2 reduction continues to meet a significant challenge. Herein, two iron-porphyrinic zirconium-based metal-organic frameworks, Zr6O4(OH)4(FeTCBPP)3 (MOF-526-H) and Zr6O4(OH)4(FeTCBPP-NH2)3 (MOF-526-NH2) were achieved by the integration of the light-harvesting sites and catalytic centers into one phase (FeTCBPP = iron 5,10,15,20-tetra[4-(4 '-carboxyphenyl)phenyl]-porphyrin, FeTCBPP-NH2 = iron 5,10,15,20-tetra[4-(4 '-carboxyphenyl)-2-aminophenyl]-porphyrin), which were employed as a couple of models for CO2 photoreduction. MOF-526-NH2 exhibited excellent CO2 reduction with a CO yield of 21.2 mmol center dot g(-1) in the absence of any photosensitizer under visible light, which was 4-fold higher than that of MOF-526-H. Additionally, MOF-526-NH2 showed no significant reduction in CO production during the four rounds, indicating that MOF-526-NH2 possessed good stability. The presence of amino groups in MOF-526-NH2 played an important role in adjusting the micro-environment and electronic structure, which was beneficial to CO2 enrichment and light absorption, and thus improved CO2 photoreduction performance. Based on the density functional theory calculations, a possible mechanism of photocatalytic CO2 reduction over porphyrinic MOFs was proposed.