Identifying the Active Site of Cu/Cu₂O for Electrocatalytic Nitrate Reduction Reaction to Ammonia

Abstract

Copper oxide-derived electrocatalysts are reported to have high activity and selectivity for nitrate electrochemical reduction reaction (NO₃RR) to ammonia. However, the cause underlying their enhanced NH₃ formation, i.e., the active catalytic site, remains unclear. We measure the NO₃RR activity of Cu/Cu₂O catalysts that have been pre-reduced to different degrees and compare them to metallic Cu to identify the active site for NO₃RR to ammonia in sodium sulfate electrolyte. We find that in the absence of a buffer, catalyst kinetic performance is similar to alkaline electrolytes (pH 12) due to pH increases during nitrate reduction. Higher pH values improve catalyst performance for NO₃RR to NH₃ compared to electrolyte buffered at pH 5.8. Using in-situ Raman and X-ray absorption near edge structure spectroscopies, we detect that Cu₂O reduces at −0.6 V vs. SHE. We use in-situ Fourier-transform infrared spectroscopy and detect the presence of hydroxylamine, a NO₃RR intermediate, from −0.7 V vs. SHE to −1.0 V vs. SHE. Based on kinetic analysis, ex-situ and in-situ spectroscopies, we propose that oxygen vacancies formed by Cu₂O reduction promote NO₃RR to ammonia from −0.6 to −0.77 V vs. SHE, while copper itself is the active site at −1.1 V vs. SHE.

Bio

Raphael Nagao, was born in São Paulo, Brazil, in 1984. He received his B.Sc, M.Sc and Ph.D. in the São Carlos Institute of Chemistry, University of São Paulo (USP), Brazil. He joined the Electrochemistry Group at USP in 2003 under supervision of Prof. Dr. Hamilton Varela and remained in this group until the defense of his doctoral thesis in 2013. Meanwhile, he participated in a scientific program of fourteen-month period in the Department of Chemistry and Volen Center for Complex Systems, Brandeis University, USA, under supervision of Prof. Dr. Irving R. Epstein (2011-2012). He was also a postdoctoral fellow in the Department of Chemistry, Saint Louis University, USA, under supervision of Prof. Dr. István Z. Kiss (2014-2016). His work was mainly devoted to comprehending the control and evolution of self-organized patterns in oscillating electrochemical reactions. Currently, Dr. Nagao is an Associate Professor in the Institute of Chemistry, University of Campinas (UNICAMP), and head of the Laboratory of Electrocatalysis for Sustainable Energy Conversion (ElectroEnergy), Brazil. His interests are now focused on the development of nanostructured materials applied for energy conversion reactions such as the sustainable electrosynthesis of green hydrogen and ammonia besides derivatives from the reduction of carbon dioxide. He is an associate researcher at the Center for Innovation on New Energies (CINE), the chair of the Brazilian Section at The Electrochemical Society (ECS) and the director of the Physical Chemistry division of the Brazilian Chemical Society (SBQ).