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 (NO 3 RR), 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 vs. RHE, thus returning to the metallic Cu before NO 3 RR. It was distinguished from the native oxide on Cu, which remained during NO 3 RR. Fast NO 3 - and NO reduction on the metallic Cu delivered 91.5 ± 3.7% faradaic efficiency for NH 3 at -0.4 V vs. RHE. In contrast, the native oxide on Cu formed undesired products and low NH 3 yield. Operando shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) analysis revealed the adsorbed NO 3 - , NO 2 , and NO species on the electropolished Cu as the intermediates of NH 3 . Low overpotential NO 3 - and NO adsorptions and favorable NO reduction are key to increased NH 3 productivity over Cu samples, which was consistent with the DFT calculation on Cu(100).