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Metal cations drive carbon dioxide’s chemical reduction

27 August 2021

Experimental evidence confirms one long-standing hypothesis: Positively charged ions stabilize the reaction’s intermediate molecules.

The industrial production of ammonia, quicklime, and other chemical compounds generates carbon dioxide as a by-product. A more sustainable system would redirect that CO2 waste stream as an input for the synthesis of methanol, biofuels, and other industrially useful chemicals.

One approach dissolves the CO2 in an aqueous electrolyte solution and strips one of its electrons using a metal catalyst and an applied current between two electrodes. If the catalyst is gold or silver, then the intermediate anion reacts with a proton from a water molecule to form carbon monoxide. Further processing then yields methanol, for example. Copper and other catalysts drive reactions that produce ethylene, ethanol, and other chemicals.

To better understand the electrochemical reduction of CO2, PhD candidate Mariana Monteiro, her adviser Marc Koper (both at Leiden University in the Netherlands), and their colleagues investigated how metal cations such as cesium in the electrolyte solution affect the reaction. Previous research hinted that they generate a local electric field near the electrode that boosts the reaction’s effectiveness. The researchers at Leiden University have now found that the metal cations not only accelerate the reaction but are indispensable to it. Without them, the reaction doesn’t yield the expected CO product.

The researchers chose to use a polycrystalline gold electrocatalytic system in their experiments because of its stability and simplicity. In that system, CO and molecular hydrogen are the only two products of CO2. Monteiro and her colleagues tested for the cation effect by running the reduction reaction with and without varying amounts of Cs ions or other alkali metals in the electrolyte.

The experimental results clearly showed that CO was generated only with Cs+ in solution. Molecular dynamics simulations clarified that the Cs+ thermodynamically favors the adsorption of CO2 on the electrode surface and coordinates with the adsorbed intermediate molecule CO2, as illustrated in the schematic. The interaction stabilizes CO2 and allows for the next reaction steps to take place, which yields CO.

Molecular representation of the chemical reaction
Credit: M. C. O. Monteiro et al., Nat. Catal. 4, 654 (2021)

Monteiro and her colleagues observed that cations enable the CO2 reduction reaction on copper and silver electrodes in addition to gold ones. To achieve any future commercial applications, engineers and designers of electrocatalytic systems should therefore consider not only the catalyst but also the electrolyte and the metal cations in it. In a follow-up paper, Monteiro, Koper, and two other colleagues demonstrated a proof of concept for the first step in CO2 electrochemical reduction, at conditions similar to those of an industrial setting. (M. C. O. Monteiro et al., Nat. Catal. 4, 654, 2021.)

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