Electrocatalytic Nitrate and Nitrite Reduction toward Ammonia Using Cu2O Nanocubes: Active Species and Reaction Mechanisms

The electrochemical reduction of nitrate (NO3–) and nitrite (NO2–) enables sustainable, carbon-neutral, and decentralized routes to produce ammonia (NH3). Copper-based materials are promising electrocatalysts for NOx– conversion to NH3. However, the underlying reaction mechanisms and the role of different Cu species during the catalytic process are still poorly understood. Herein, by combining quasi in situ X-ray photoelectron spectroscopy (XPS) and operando X-ray absorption spectroscopy (XAS), we unveiled that Cu is mostly in metallic form during the highly selective reduction of NO3–/NO2– to NH3. On the contrary, Cu(I) species are predominant in a potential region where the two-electron reduction of NO3– to NO2– is the major reaction. Electrokinetic analysis and in situ Raman spectroscopy was also used to propose possible steps and intermediates leading to NO2– and NH3, respectively. This work establishes a correlation between the catalytic performance and the dynamic changes of the chemical state of Cu, and provides crucial mechanistic insights into the pathways for NO3–/NO2– electrocatalytic reduction.