NUS scientists have discovered a new mechanism for selective electrochemical reduction of carbon dioxide to ethanol using copper-silver (Cu-Ag) composite catalysts. A paper on their work is published in the journal ACS Catalysis.

Credit: ACS Catalysis

The electrochemical reduction of CO₂ to fuels and chemicals, when powered by renewable electricity, is a step forward in alleviating carbon emissions. Copper (Cu) materials are choice catalysts for this process because they have the highest electrochemical activities towards multi-carbon products.

However, their selectivity towards ethanol (C₂H₅OH), a valuable fuel and chemical feedstock, is always lower than towards ethylene (C₂H₄). The preference towards production of ethylene compared to ethanol arises from the CO dimerization mechanism for producing C₂ molecules from CO₂, where the formation of ethylene, which has a lower energy barrier, is favored over ethanol.

A research team led by Prof YEO Boon Siang, Jason from the Department of Chemistry at NUS, in collaboration with a team led by Dr Federico CALLE-VALLEJO from the University of Barcelona, has shown that an influx of CO molecules, provided by silver (Ag) co-catalysts, activates an otherwise-locked mechanistic pathway on Cu which converts CO₂ gas to ethanol.

Graphs showing how the enhancement of ethanol (green) and ethylene (blue) production on copper-silver (Cu-Ag) composites varies with different (a) Ag/Cu ratios and (b) Ag particle sizes. A five-fold enhancement was observed for ethanol, while ethylene was not significantly impacted. (c) Energy level diagram showing CO+CO (red) and CO+CH (orange) coupling steps on Cu (111). A lower barrier is required for CO+CH coupling, making it more favorable than the CO+CO step. (d) Scheme showing the mechanism for CO₂ reduction to ethanol on Cu-Ag composites via CO+CH coupling. Credit: ACS Catalysis.

The researchers tested a series of Cu-Ag composite catalysts, fabricated from a mixture of oxide-derived Cu nanowires and Ag powders, for their electrochemical CO₂ reduction activities. During CO₂ reduction, Ag converts CO₂ to CO and these CO molecules migrate to the Cu active sites for further reduction to hydrocarbons (ethylene) and alcohols (ethanol).

The researchers varied the Ag/Cu ratio and the Ag particle sizes in the composites to increase the CO influx from Ag to the active sites on the Cu material. The experimental results showed that the increased CO influx enhanced the production of ethanol by up to five times, with little impact on ethylene production.

Theoretical simulations on the reaction mechanism show that instead of the CO+CO step which results in ethylene formation, the CO+CH_x step was the dominant C-C bond formation step at the Cu-Ag interface. Ethanol was the sole product when the reaction proceeds through the CO+CH_x step, which was found to occur on active sites different from those that facilitated ethylene formation via the CO+CO step.

Further plans arising from this finding by the research team include maximizing the active sites through catalyst design and upscaled production using a high throughput flow cell configuration.

The concept that a previously closed pathway can be opened by an influx of intermediates, as demonstrated in this work, opens new possibilities to uncover new synthetic mechanisms which might previously be inaccessible.

—Prof YEO

Resources

• Ting LRL; Pique O; Lim SY; Tanhaei M; Calle-Vallejo F*; Yeo BS. (2020) “Enhancing CO2 Electroreduction to Ethanol on Copper–Silver Composites by Opening an Alternative Catalytic Pathway” ACS Catalysis Volume: 10 Issue: 7 Pages: 4059-4069 doi: 10.1021/acscatal.9b05319