Using Photochemical Strategies to Reduce Carbon Dioxide to Formate

The increase in the concentration of carbon dioxide (CO₂) in the atmosphere is one of the main reasons for the greenhouse effect, which can bring catastrophic climate change issues. On the other hand, the indispensable fossil fuels in human production and life will also face depletion in the near future. To simultaneously address these two serious challenges, using sunlight to convert CO₂ into usable fuel is considered a win-win approach. The use of different strategies for CO₂ reduction reaction (CO₂RR) has been a research topic of great concern worldwide.

Recently, some researchers proposed a CO₂RR catalyst system without transition metals. The system uses a benzimidazole-based organic hydride catalyst for the CO₂RR process under visible light conditions, and uses carbazole-based compounds as photosensitizers to regenerate the benzimidazole. The system has a turnover number (TON) of over 8000 and does not produce other reduction products such as H₂, CO or oxalate.

Firstly, researchers noticed that BIH, as a strong reducing agent, has the potential to reduce CO₂. In order to enable the regeneration cycle of BIH in the photoreaction system, researchers studied the photochemical reduction process of BI⁺(I^-) (the oxidation product of BIH). BI⁺(I^-) requires a highly reducing photosensitizer to provide electrons to reduce it. Through a series of screening, the researchers selected carbazole compounds as photosensitizers (excited state oxidation potential -2.75 vs. SCE), vitamin C as electron donor, and photocatalytic reduction of BI⁺(I^-) to its reduced state BIH. Under visible light irradiation for 15 minutes, the yield of BIH was 66%. In addition, researchers also studied the stability of BI⁺(I^-) and BIH systems. After 20 hours of irradiation under the same conditions, the total molar amounts of BI⁺(I^-) and BIH remained almost unchanged before and after the reaction. This result indicates that BI⁺(I^-) and BIH have sufficient stability to withstand long-term catalytic cycles.

In order to further elucidate the reaction mechanism, the researchers conducted the following research on the mechanism of the CO₂RR process: (1) When using ¹³C-labeled CO₂ for experiments, ¹³C-labeled formate was generated. This experimental fact proves that formate is indeed converted from CO₂. (2) Using time-resolved spectroscopy, it is possible to explore the process of electron movement in the carbazole photosensitizer after absorbing light. The results showed that after absorbing light, the carbazole photosensitizer transferred electrons to the catalyst and CO₂, respectively, on a nanosecond to microsecond time scale. (3) After CO₂ accepts electrons from the carbazole photosensitizer, it is transformed into CO₂radical anion. The computational analysis of the subsequent conversion of the CO₂ radical anion to formate showed that the CO₂ radical anion and the catalyst underwent hydrogen atom exchange to form formate. (4) Dehydrovitamin C (an oxidation product of vitamin C) can also act as a reducing agent, indicating that each molecule of vitamin C can donate 4 electrons in this CO₂RR process.

Reference

1. Metal-free reduction of CO₂ to formate using a photochemical organohydride-catalyst recycling strategy
   Weibin Xie, Jiasheng Xu, Ubaidah Md Idros, Jouji Katsuhira, Masaaki Fuki, Masahiko Hayashi, Masahiro Yamanaka, Yasuhiro Kobori & Ryosuke Matsubara