Turning CO2 into Formate Using a Dual Single-Atom Catalyst

The increasing concentration of CO₂ in the atmosphere strongly contributes to climate change. In addition to the reduction of CO₂emissions, the removal of CO₂ from the atmosphere could help to tackle this global issue. The sustainability of CO₂ removal improves significantly if the CO₂ captured from air is converted to value-added products instead of just being stored. This process is called direct air capture and conversion (DACC). Most CO₂ reduction catalysts require pure CO₂ as a feed gas and cannot work with the relatively small amount of CO₂ in the air. The presence of oxygen poses an additional challenge for the activity of the catalytic centers.

Guoqing Ren, Weiqiao Deng, Shandong University, Qingdao, China, and colleagues have developed a heteronuclear dual single-atom catalyst for the conversion of CO₂ to formate under ambient conditions. The catalyst contains palladium and cobalt atoms supported on a covalent triazine framework (CTF). CO₂ is captured from the air by an aqueous solution of triethylamine, in which bicarbonate is formed. The catalyst converts this to formate, which can serve as a feedstock for further conversions or as a liquid hydrogen storage material.

The team synthesized the CTF from 2,6-pyridinedicarbonitrile, which was heated with zinc chloride under vacuum. After removal of ZnCl₂, the desired palladium and cobalt-functionalized material was formed by the reaction of metal precursors (palladium(II) acetylacetonate and cobalt(III) acetylacetonate) with the CTF in ethanol at 50 °C over 12 h. The catalyst was characterized by Fourier-transform infrared (FT-IR) spectroscopy, X-ray absorption spectroscopy, and different microscopy techniques.

The capture and hydrogenation of CO₂ were carried out at 30 °C under 1 bar of hydrogen. The optimized catalyst exhibits a high TON of 59.6 under ambient conditions. It can be used to directly hydrogenate the CO₂ captured from air into formate with a conversion of up to 84.6 %. The catalyst was successfully recycled four times. The addition of cobalt improved the turnover number by a factor of four compared to using only palladium. The cobalt binds and activates the CO_2, while the palladium activates the hydrogen.