Electrocatalytic Nitrate and Nitrite Reduction toward Ammonia Using Cu 2 O Nanocubes: Active Species and Reaction Mechanisms.

The electrochemical reduction of nitrate (NO ₃ ^- ) and nitrite (NO ₂ ^- ) enables sustainable, carbon-neutral, and decentralized routes to produce ammonia (NH ₃ ). Copper-based materials are promising electrocatalysts for NO _x ^- conversion to NH ₃ . 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 NO ₃ ^- /NO ₂ ^- to NH ₃ . On the contrary, Cu(I) species are predominant in a potential region where the two-electron reduction of NO ₃ ^- to NO ₂ ^- is the major reaction. Electrokinetic analysis and in situ Raman spectroscopy was also used to propose possible steps and intermediates leading to NO ₂ ^- and NH ₃ , 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 NO ₃ ^- /NO ₂ ^- electrocatalytic reduction.