Covalent grafting of molecular photosensitizer and catalyst on MOF-808: effect of pore confinement toward visible light-driven CO2 reduction in water

The photocatalytic reduction of CO2 in water using a single integrated system utilizing sunlight is the ultimate goal for artificial photosynthesis. Here, we report the design and multistep synthesis of Zr-MBA-Ru/Re-MOF for photocatalytic CO2 reduction via post-synthetic linker exchange (PSE) followed by metalation on MOF-808. The simultaneous covalent immobilization of the molecular [Ru(bpy)(3)](2+) photosensitizer and [Re(bpy)CO3Cl] catalyst in the confined space of the MOF resulted in highly efficient CO2-to-CO formation with a maximum production rate of 440 mu mol g(-1) h(-1) in aqueous medium without any sacrificial electron donor (with selectivity >99%, QE = 0.11). In parallel, under sunlight, this assembly also produces 210 mu mol g(-1) of CO in 6 h in aqueous medium. In addition, a maximum production rate of 180 mu mol g(-1) h(-1) is observed in MeCN/H2O (2 : 1) mixed solvent medium with BNAH and TEOA as the sacrificial electron donor (with CO selectivity 69%, QE = 0.22). The high surface area-based Zr-MOF (MOF-808) is robust and water-tolerant, and its post-synthetically modifiable pore surface allows us to covalently attach the molecular photosensitizer and catalyst in the confined nanospace. Covalent grafting of the [Ru(bpy)(3)](2+) photosensitizer significantly enhances the lifetime of the photoexcited electrons, and the proximity of the catalytic site shortens the transport distance of charge carriers during the reaction, resulting in an efficient catalytic activity. The reaction intermediates are characterized using in situ diffuse reflectance FT-IR (DRIFT), which is well-supported by DFT calculations, and the catalytic cycle involving the reaction mechanism is established.

Automated summary

The study reports the successful design and synthesis of Zr-MBA-Ru/Re-MOF for photocatalytic CO2 reduction, demonstrating high catalytic activity and selectivity in aqueous medium. Covalent immobilization of the photosensitizer and catalyst enhances the lifetime of photoexcited electrons and shortens charge carrier transport distance, leading to efficient CO2 reduction. In situ DRIFT characterization of reaction intermediates and DFT calculations support the established catalytic cycle involving the reaction mechanism.