Microenvironment modulation of Fe-porphyrinic metal-organic - organic frameworks for CO2 photoreduction

Photocatalytic CO2 reduction to fuels and chemicals is a promising pathway towards carbon resource recovery. Herein, three isomorphic Fe-porphyrinic MOFs, Zr6O4(OH)(4)(Fe-TCPP)(3) (MOF-525, Fe-TCPP = iron 5,10,15,20-tetra(4-carboxyphenyl)-porphyrin), Zr6O4(OH)(4)(Fe-TCPP-NO2)(3) (MOF-525-NO2, Fe-TCPP-NO2 = iron 5,10,15,20-tetra(2-nitro-4-carboxyphenyl)-porphyrin), and Zr6O4(OH)(4)(Fe-TCBPP-NO2)(3) (MOF-526-NO2, Fe-TCBPP-NO2 = iron 5,10,15,20-tetra[4-(4 '-carboxyphenyl)-2-nitrophenyl]-porphyrin) were synthesized and employed as photocatalysts for CO2 reduction. Among them, MOF-525-NO2 exhibited the highest catalytic activity with CO and H-2 yields of 10.36 and 0.46 mmol<middle dot>g(-1) without any photosensitizer under visible light. Mechanism investigations suggested that the micro-environments of these MOFs were adjusted by introducing porphyrinic fragments with different lengths and functional groups, resulting in stronger CO2 affinity, faster photocurrent response, and efficient photogenerated electron-hole separation and transfer, which finally promoted the efficiency for photocatalytic CO2 reduction.