Evidence for Distinct Active Sites on Oxide-Derived Cu for Electrochemical Nitrate Reduction

Cu is a promising catalyst for electrochemical nitrate (NO₃^-) reduction. However, desorption of the nitrite (NO₂^-) intermediate can occur, leading to lowered ammonia productivity and Faradaic efficiency. Here, we discovered that this does not occur with oxide-derived Cu due to the presence of at least two distinct types of cooperative active sites: one for NO₃^- → NO₂^- and another for NO₂^- → NH₃. As a result, oxide-derived Cu exhibits enhanced ammonia productivity with a mixed NO₃^-/NO₂^- feed relative to pure NO₃^- or NO₂^-. In contrast, this was not observed with a standard Cu sample, implying the presence of only a single type of active site. Our dual-site hypothesis was supported by attenuated total reflection surface enhanced infrared absorption spectroscopy and isotopic labeling experiments involving co-reduction of ¹⁵NO₃^-/¹⁴NO₂^-. We also successfully simulated our experimental results using a mathematical model involving two different adsorption sites. These findings motivate the need for further study and rational design of such active sites.