Unveiling Catalyst Restructuring and Composition Evolution During Nitrate Electrocatalytic Reduction Through Correlated Operando Microscopy and Spectroscopy

The electrocatalytic 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.<br/>Here I will present our findings on structure/composition/reactivity correlations obtained for size-controlled Cu₂O nanocube pre-catalysts by combining quasi in situ X-ray photoelectron spectroscopy (XPS), operando X-ray absorption spectroscopy (XAS), transmission X-ray microscopy (TXM), Raman spectroscopy and electrochemical liquid cell transmission electron microscopy (EC-TEM). In particular, 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. Moreover, we revealed how redox kinetics determine the working catalyst morphology and found drastic differences in catalyst restructuring during operation and a strong dependency of its composition on the applied potential and the chemical environment, including the choice of electrolyte.<br/>Electrokinetic analysis 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 structure and chemical state of Cu, and provides crucial mechanistic insights into the pathways for NO₃–/NO₂– electrocatalytic reduction.