In situ spectroscopic examination of a low overpotential pathway for carbon dioxide conversion to carbon monoxide

Lowering the overpotential for the electrochemical conversion of CO ₂ to useful products is one of the grand challenges in the Department Of Energy report, "Catalysis for Energy". In a previous paper, we showed that CO ₂ conversion occurs at low overpotential on a 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF ₄)-coated silver catalyst in an aqueous solution of EMIM-BF4. One of the surprises in the previous paper was that the selectivity to CO was better than 96% on silver, compared with ∼80% in the absence of ionic liquid. In this article, we use sum frequency generation (SFG) to explore the mechanism of the enhancement of selectivity. The study used platinum rather than silver because previous workers had found that platinum is almost inactive for CO production from CO ₂. The results show that EMIM-BF ₄ has two effects: it suppresses hydrogen formation and enhances CO ₂ conversion. SFG shows that there is a layer of EMIM on the platinum surface that inhibits hydrogen formation. CO ₂, however, can react with the EMIM layer to form a complex such as CO ₂-EMIM at potentials more negative than -0.1 V with respect to a standard hydrogen electrode (SHE). That complex is converted to adsorbed CO at cathodic potentials of -0.25 V with respect to SHE. These results demonstrate that adsorbed monolayers can substantially lower the barrier for CO ₂ conversion on platinum and inhibit hydrogen formation, opening the possibility of a new series of metal/organic catalysts for this reaction.