Abstract
Increasing levels of CO2 in the atmosphere is a problem that must be urgently resolved if the rise in current global temperatures is to be slowed. Chemically reducing CO2 into compounds that are useful as energy sources and carbon-based materials could be helpful in this regard. However, for the CO2 reduction reaction (CO2RR) to be operational on a global scale, the catalyst system must: use only renewable energy, be built from abundantly available elements and not require high-energy reactants. Although light is an attractive renewable energy source, most existing CO2RR methods use electricity and many of the catalysts used are based on rare heavy metals. Here we present a transition-metal-free catalyst system that uses an organohydride catalyst based on benzimidazoline for the CO2RR that can be regenerated using a carbazole photosensitizer and visible light. The system is capable of producing formate with a turnover number exceeding 8,000 and generates no other reduced products (such as H2 and CO).

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Data availability
The additional discussions and the data supporting the plots within this Article and other findings of this study, such as 1H NMR and 13C NMR spectra, cyclic voltammograms, optical spectra, experimental procedures, and quantum chemical calculations, are available in the Supplementary Information. Source data are provided with this Article.
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Acknowledgements
R.M. is thankful for financial support from ENEOS TonenGeneral Research/Development Encouragement & Scholarship Foundation, Takahashi Industrial and Economic Research Foundation, the Takano Science Foundation, Kanamori Foundation, and Fukuoka Naohiko Memorial Foundation. This work was partially supported by JSPS KAKENHI Grant-in-Aid for Transformative Research Areas, “Dynamic Exciton” (JP20H05832 to Y.K.) and grant numbers JP20K21174, JP20KK0120 and JP22K19008 to Y.K. We thank T. Amimoto from the Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University for mass analysis measurements. We also thank T. Tachikawa and Z. Zhang at Kobe University for their assistance on GC analysis and T. Harada at Kobe University for determining the mechanism elucidation hint.
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R.M. conceived and directed the project. R.M., W.X. and Y.K. wrote the paper. W.X., J.X. and U.M.I. performed most experiments under the supervision of R.M. and M.H. J.K., M.F. and Y.K. conducted transient absorption spectroscopy and fluorescence lifetime measurements. M.Y. conducted the computational calculation for the transition states. All authors discussed the results and commented on the manuscript.
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Extended data
Extended Data Fig. 1 13C-labelling experiment.
a, A solution of 1 (1.5 mM), BI+(I−) (2.5 mM), H2A (50 mM), and NaOH (110 mM) was irradiated (λmax = 400 nm) for 20 h under 1 atm of 13CO2 atmosphere. According to 1H NMR analysis, 13C-formate was obtained in a yield of 45 mM, along with a small amount of 12C-formate. This confirmed that the carbon source of the product formate is gaseous CO2. b, 1H NMR spectrum of the reaction mixture. The formation of 13C-formate was characterized by a set of doublet peaks (9.06 and 8.58 ppm). The singlet peak at 6.57 ppm is 1,3,5-trimethoxybenzene (internal standard). c, Proton-decoupled 13C (13C{1H}) NMR spectrum of the reaction mixture. An intense peak attributed to 13C-formate was observed at 169.4 ppm.
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Supplementary Figs. 1–47, Sections 1–32 and Tables 1–15.
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Source Data Fig. 4a
Numerical source data for spectroscopy and calculation.
Source Data Fig. 4b
Numerical source data for spectroscopy.
Source Data Fig. 4c
Numerical source data for spectroscopy.
Source Data Fig. 4d
Numerical source data for spectroscopy.
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Xie, W., Xu, J., Md Idros, U. et al. Metal-free reduction of CO2 to formate using a photochemical organohydride-catalyst recycling strategy. Nat. Chem. (2023). https://doi.org/10.1038/s41557-023-01157-6
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DOI: https://doi.org/10.1038/s41557-023-01157-6