Acta Phys. -Chim. Sin. ›› 2024, Vol. 40 ›› Issue (4): 2306046.doi: 10.3866/PKU.WHXB202306046

Special Issue: Carbon Dioxide Valorization

• COMMUNICATION • Previous Articles     Next Articles

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

Hui-Ying Chen, Hao-Lin Zhu*(), Pei-Qin Liao*(), Xiao-Ming Chen*()   

  1. School of Chemistry, MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Sun Yat-Sen University, Guangzhou 510275, China
  • Received:2023-06-27 Revised:2023-07-14 Accepted:2023-07-14 Published:2023-07-20
  • Contact: Email: zhuhlin3@mail2.sysu.edu.cn (Hao-Lin Zhu)liaopq3@mail.sysu.edu.cn (Pei-Qin Liao)cxm@mail.sysu.edu.cn (Xiao-Ming Chen)
  • Supported by:
    the National Key Research and Development Program of China(2021YFA1500401); the National Natural Science Foundation of China(21890380); the National Natural Science Foundation of China(21821003); the Local Innovative and Research Teams Project of the Guangdong Pearl River Talents Program, China(2017BT01C161); the Technology Innovation Strategy Special City and County Science and Technology Innovation Support Project, China(STKJ2023078)

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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.

Key words: Metal-organic framework, Zn(Ⅱ) porphyrin, Ru(Ⅱ) bipyridyl complex, Photogenerated charge, CO2 photoreduction