Integration of Ru(II)-Bipyridyl and Zinc(II)-Porphyrin Moieties in a Metal-Organic Framework for Efficient Overall CO2 Photoreduction

Abstract

Efficiently converting CO₂ and H₂O into value-added chemicals using solar energy is a viable approach to address global warming and the energy crisis. However, achieving artificial photocatalytic CO₂ reduction using H₂O as the reductant poses challenges is due to the difficulty in efficient cooperation among multiple functional moieties. Metal-organic frameworks (MOFs) are promising candidates for overall CO₂ photoreduction due to their large surface area, diverse active sites, and excellent tailorability. In this study, we designed a metal-organic framework photocatalyst, named PCN-224(Zn)-Bpy(Ru), by integrating photoactive Zn(II)-porphyrin and Ru(II)-bipyridyl moieties. In comparison, two isostructural MOFs just with either Zn(II)-porphyrin or Ru(II)-bipyridyl moiety, namely PCN-224-Bpy(Ru) and PCN-224(Zn)-Bpy were also synthesized. As a result, PCN-224(Zn)-Bpy(Ru) exhibited the highest photocatalytic conversion rate of CO₂ to CO, with a production rate of 7.6 μmol·g⁻¹·h⁻¹ in a mixed solvent of CH₃CN and H₂O, without the need for co-catalysts, photosensitizers, or sacrificial agents. Mass spectrometer analysis detected the signals of ¹³CO (m/z = 29), ¹³C¹⁸O (m/z = 31), ¹⁶O¹⁸O (m/z = 34), and ¹⁸O₂ (m/z = 36), confirming that CO₂ and H₂O acted as the carbon and oxygen sources for CO and O₂, respectively, thereby confirming the coupling of photocatalytic CO₂ reduction with H₂O oxidation. In contrast, using PCN-224-Bpy(Ru) or PCN-224(Zn)-Bpy as catalysts under the same conditions resulted in significantly lower CO production rates of only 1.5 and 0 μmol·g⁻¹·h⁻¹, respectively. Mechanistic studies revealed that the lowest unoccupied molecular orbital (LUMO) potential of PCN-224(Zn)-Bpy(Ru) is more negative than the redox potentials of CO₂/CO, and the highest occupied molecular orbital (HOMO) potential is more positive than that of H₂O/O₂, satisfying the thermodynamic requirements for overall photocatalytic CO₂ reduction. In comparison, the HOMO potential of PCN-224(Zn)-Bpy without Ru(II)-bipyridyl moieties is less positive than that of H₂O/O₂, indicating that the Ru(II)-bipyridyl moiety is thermodynamically necessary for CO₂ reduction coupled with H₂O oxidation. Additionally, photoluminescence spectroscopy revealed that the fluorescence of PCN-224(Zn)-Bpy(Ru) was almost completely quenched, and a longer average photoluminescence lifetime compared to PCN-224(Zn)-Bpy and PCN-224-Bpy(Ru) was observed. These suggest a low recombination rate of photogenerated carriers in PCN-224(Zn)-Bpy(Ru), which also supported by the higher photocurrent observed in PCN-224(Zn)-Bpy(Ru) compared to PCN-224(Zn)-Bpy and PCN-224-Bpy(Ru). In summary, the integrated Zn(II)-porphyrin and Ru(II)-bipyridyl moieties in PCN-224(Zn)-Bpy(Ru) play important roles of a photosensitizer and CO₂ reduction as well as H₂O oxidation sites, and their efficient cooperation optimizes the band structure, thereby facilitating the coupling of CO₂ reduction with H₂O oxidation and resulting in high-performance artificial photocatalytic CO₂ reduction.

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