Following the monoxide

Abstract

The electrochemical reduction of CO2, especially when powered by renewable electricity, would enable the sustainable production of value-added chemicals and fuels. Among the various metal catalysts for this reaction, copper is the only one that promotes the formation of multi-carbon products such as ethylene or ethanol. Despite being the focus of a large number of works, the precise reaction mechanism, however, remains unclear. Now, Ward van der Stam, Bert Weckhuysen and colleagues at Utrecht University perform time-resolved surface-enhanced Raman spectroscopy (SERS) measurements to track the formation and evolution of a CO intermediate during CO2 electroreduction. The researchers use a mechanically polished polycrystalline copper electrode, which is oxidized and subsequently reduced in situ to increase the roughness and form copper nanoparticles. These nanostructures, in turn, enhance the Raman signal and the selectivity for C–C products — acting as both SERS hotspots and active sites, and allowing sub-second time resolution. The researchers are able to monitor the formation of these surface nanostructures upon switching from anodic to cathodic potentials below –0.4 VRHE. At potentials of –0.7 to –0.9 VRHE, adsorbed CO on the in-situ-formed copper nanoparticles is immediately observed through the CO stretching peaks — namely, the bridged and lowand high-frequency linear peaks. By studying their time evolution after switching from the anodic potential to –0.7, –0.8 or –0.9 VRHE, the researchers identify a unique behaviour of adsorbed CO. At –0.7 VRHE, CO adsorbs onto undercoordinated defect sites and is rather static as it desorbs to gaseous CO product. On the other hand, at –0.9 VRHE, the Faradaic efficiencies decrease for CO and H2 but increase for ethylene. This correlates with the time-resolved SERS showing preferential adsorption of CO on step-edge sites with highly dynamic behaviour, which promotes its dimerization and subsequent formation of ethylene.