Identifying the active sites and intermediates on copper surfaces for electrochemical nitrate reduction to ammonia.

Copper (Cu) is a widely used catalyst for the nitrate reduction reaction (NORR), but its susceptibility to surface oxidation and complex electrochemical conditions hinders the identification of active sites. Here, we employed electropolished metallic Cu with a predominant (100) surface and compared it to native oxide-covered Cu. The electropolished Cu surface rapidly oxidized after exposure to either air or electrolyte solutions. However, this oxide was reduced below 0.1 V RHE, thus returning to the metallic Cu before NORR. It was distinguished from the native oxide on Cu, which remained during NORR. Fast NO and NO reduction on the metallic Cu delivered 91.5 ± 3.7% faradaic efficiency for NH at -0.4 V RHE. In contrast, the native oxide on Cu formed undesired products and low NH yield. Operando shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) analysis revealed the adsorbed NO, NO, and NO species on the electropolished Cu as the intermediates of NH. Low overpotential NO and NO adsorptions and favorable NO reduction are key to increased NH productivity over Cu samples, which was consistent with the DFT calculation on Cu(100).