Operando UV/vis Absorption Spectroscopy for Studying the Nitrate to Ammonia Conversion on Cu₂O‑Based Electrodes

The electrochemical conversion of nitrate to ammonia was proposed as a feasible strategy for ammonia production. However, a deep understanding of the reaction mechanism and catalyst transformation is still needed to reach real applications. Herein, operando 2D-UV/vis absorption spectroelectrochemistry (2D-UV/vis SEC) was used to study in detail the reaction dynamics, as well as the transformation of the catalyst for the nitrate reduction to ammonia on Cu₂O-based electrodes. 2D-UV/vis SEC combines two simultaneous UV/vis absorption spectroelectrochemistry measurements to obtain information on a given system from two different points of view: normal and parallel configurations. The UV/vis signal in the parallel arrangement facilitates tracking of the reduction of nitrate to nitrite, providing kinetic information about the process. Conversely, the UV/vis signal in the normal configuration reveals the transformation of the catalyst, providing more detailed information about the changes occurring on the electrode surface during the reaction. Spectroelectrochemistry responses have shed more light on the kinetics of the process under the operating conditions. The results show that at the selected potential window for nitrate reduction, the Cu₂O surface is mostly reduced to Cu⁰, as evidenced by the changes observed in absorbance in the normal configuration. In addition, experiments performed at different initial nitrate concentrations reveal that nitrite reduction is hampered by the nitrate concentration, demonstrating the key role of the nitrate concentration in the reaction pathway. This work highlights the usefulness of 2D-UV/vis SEC in untangling the complex dynamics of the reaction intermediates and products as well as the transformation of the catalyst surface in electrocatalytic processes.