Selective Photochemical Conversion of Carbon Dioxide to Formic Acid at Gas–Water Interface of Microbubbles

We report a selective photochemical conversion of carbon dioxide (CO₂) to formic acid (HCOOH, FA) at the gas-water interface (GWI) of microbubbles. The microbubbles with an average diameter of 42 μm are produced by passing CO₂ gas through a porous thermoplastic bubbler immersed in an aqueous solution of the copper(II)-phenanthroline complex [Cu(Phen)₂]²⁺. The average FA production rate at room temperature is found to be 47.5 μM h^-1 for 5 mM of [Cu(Phen)₂]²⁺. When 5 mM iodide (I^-) is added to the system, the FA production rate increases to a maximum value of 63.8 μM h^-1. We also demonstrated that both acidic and alkaline conditions stimulate FA formation. Mechanistic investigations indicate that H^• at the GWI plays a crucial role in the reduction of CO₂ via the formation of the ^•COOH intermediate, which was captured using (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as a spin trap as well as by the molecular copper catalyst. As microbubbles are continuously formed in water, the reactions at the GWI of microbubbles can be sustained over extended periods, making it easier to scale up production, which often is an issue with droplet-generated products. These findings demonstrate the promising potential of gas microbubbles in water to drive unexpected chemistry, thereby removing a greenhouse gas such as CO₂ by converting it into valuable products. The present study is a first step toward a practical demonstration, but does not constitute an industrial process at present.