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 NO_x^- 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.