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.