Photocatalytic CO₂ reduction into value-added fuels has garnered considerable attention as a strategy to mitigate global warming and fossil fuel depletion. However, under practical aerobic conditions, photocatalytic activity often declines dramatically due to undesirable O₂-photoreduction. Here, we show that deep eutectic solvents (DESs) can provide a protective reaction field against O₂ while maintaining robust CO₂ reduction performance using a Ru(II)-complex/Ag/polymeric carbon nitride (PCN) ternary hybrid photocatalyst. The turnover number of formic acid reached 1300 with 96% selectivity, and the apparent quantum yield was 2.7% in ethaline, composed of choline chloride and ethylene glycol, under pure CO₂ conditions. Notably, ethaline retained 84% of its formic acid productivity under aerobic conditions with high selectivity, whereas the same catalyst showed only 63%, 42%, 28%, and 4% productivity in DMSO, DMA, MeOH, and MeCN, respectively. The protective nature of ethaline against O₂ was also found in another hybrid photocatalyst consisting of a binuclear Ru(II) complex and Ag/PCN. This superior protective reaction field against O₂ stems primarily from the low oxygen solubility and the low oxygen diffusion coefficient of ethaline. At the same time, its high CO₂ solubility, biodegradability, and nonvolatility make it a promising solvent for CO₂ reduction in O₂-containing environments─an important step toward practical photocatalytic applications.

Protective Reaction Fields Created by Deep Eutectic Solvents against Molecular Oxygen in CO₂ Reduction over Ru(II)-Complex/Ag/Polymeric Carbon Nitride Hybrid Photocatalysts

Jo Onodera, Xian Zhang, Toshiya Tanaka, Ryuichi Nakada, Megumi Okazaki, Naoki Tarutani, Kiyofumi Katagiri, Kazuhiko Maeda*

ACS Catal. 2026, in press.