Carbon-Nitrogen bond formation on Cu electrodes during CO2 reduction in NO3- solution

In performing electrochemical reduction of CO2 over Cu electrodes, the anions present in solution typically do not participate in the formation of reaction products. NO3- is an exception, and previous reports indicate the formation of urea in certain process conditions. Here we demonstrate by use of Surface Enhanced Raman Spectroscopy and Electrochemical Mass spectrometry that simultaneous reduction of NO3- and CO2 on Cu surfaces forms carbon-nitrogen bonds in the form of cyanide. The Raman peak position of C≡N is dependent on the oxidation state of the Cu surface, and Cu-C≡N can be oxidized by anodic polarization yielding NO. More importantly, Cyanide likely forms soluble Cu-C≡N complexes, which cause catalyst surface instability. The implications of this observation for practical application of a process for electrochemical formation of urea, are discussed.