Scientists at the National University of America discovered a new mechanism using copper-silver (Cu-Ag) composite catalysts to selectively electrochemically reduce carbon dioxide (CO 2) to ethanol.
When generating electricity from renewable sources, electrochemical reduction of CO 2 to fuels and chemicals is a step toward reducing carbon emissions. Copper (Cu) materials are the preferred catalysts for this process due to their highest electrochemical activity toward multi-carbon products. However, their selectivity toward ethanol (C 2 H 5 OH)—a valuable fuel and chemical feedstock—has consistently been lower than that toward ethylene (C 2 H 4). The preference for ethylene production over ethanol stems from the CO dimerization mechanism forming CO 2 into CO 2, where the pathway with a lower energy barrier leading to ethylene is more favorable.
A research team led by Professor Jason Yeo Boon Siang from the Department of Chemistry at the University of New South Wales, in collaboration with Dr. Federico Calle-Vallejo's group at the University of Barcelona, demonstrated that a surge of CO molecules provided by silver (Ag) co-catalysts activates previously locked reaction pathways on copper (Cu), converting CO 2 gas into ethanol.
We evaluated the electrochemical CO 2 reduction activity of Cu-Ag composite catalysts composed of a mixture of oxide-derived Cu nanowires and Ag powder. During CO 2 reduction, Ag converts CO 2 into CO; these CO molecules then migrate to Cu active sites where they are further reduced to hydrocarbons (ethylene) and alcohols (ethanol). Researchers varied the Ag/Cu ratio and Ag particle size in the composites to enhance CO flux from Ag to the Cu active sites. Experimental results showed that increasing CO flux boosted ethanol production by up to fivefold while having minimal impact on ethylene formation. Theoretical simulations of the reaction mechanism revealed that, contrary to the CO + CO pathway for ethylene formation, the CO + CHₓ step is the dominant C–C coupling step at the Cu-Ag interface. When the reaction proceeds via the CO + CHₓ step, ethanol is the sole product, and its active sites differ from those facilitating ethylene formation through the CO + CO pathway.
The research team's next steps include scaling up production via catalyst design and high-throughput flow cell configurations to maximize active sites.
Professor Yeo stated: "As described in this paper, the concept of opening previously inaccessible pathways through the introduction of numerous intermediates offers new possibilities for discovering novel synthetic mechanisms that were previously unattainable."