Electrochemical Synthesis of Ammonia from Nitrate on Preferentially Oriented Cu2 O

Abstract

Electrochemistry-based processes to produce ammonia have recently gained widespread interest by the scientific community because they provide a way to reduce the high carbon emissions currently associated with ammonia production. Global NH3 demand is expected to rise since it is included in various energy scenarios as a possible green transportation fuel and storage medium of renewable electricity involving fuel cells.1 Several electrochemical and hybrid electrochemical processes with plasma integration are currently being investigated to produce NH3 via different nitrogen sources, such as N2, NO3 -/NO2 - or NO. The utilization of NO3 - from wastewater sources can be valuable for the environmental and health issues associated with NO3 - contamination. Additionally, NO3 - electroreduction in aqueous media is an attractive approach to investigate the catalysis due to its high solubility and the low dissociation energy needed to break the N=O bond. Cu and Cu-based materials have extensively been studied for NH3 production through NO3 - electroreduction. Earlier studies on Cu single crystals2 in alkaline media showed that Cu (100) performs better than Cu (111) in reducing the NO2 - intermediate to hydroxylamine, which is believed to be the precursor to NH3. On the contrary, recent density functional theory calculations3 showed that a Cu−Cu couple with 2.57 Å distance serves as a catalytic center, leading to the conclusion that Cu (111) is the most active surface termination. In this work, preferentially oriented Cu2O crystals with (111), (100), and (110) terminations were obtained via electrodeposition and evaluated for their performance in NO3 - electroreduction to NH3 (Figure 1). To understand which oxidation state of Cu represents the active phase, the samples were investigated by in-situ Raman spectroscopy. According to a recent study, the active phase was a combination of Cu and Cu+.4 Our results point to the immediate reduction of remaining Cu+ species during application of -0.3V vs. Ag/AgCl and formation of Cu(OH)2 at more negative potentials. The presence of Cu(OH)2 was also apparent from ex-situ XPS and UV-Vis analysis. Moreover, post-reaction XRD analysis showed a retainment of the crystal orientation on the reduced Cu surfaces. All the nanostructured catalysts were active for NO3 - reduction and produced NH3 with faradaic efficiencies above 40%. Cu2O (111) was the most efficient for NH3 synthesis reaching a faradaic efficiency of up to 99%.1 D. Anastasiadou, Electrochem. Sci. Adv.,submitted. 2 E. Pérez-Gallent, Electrochim. Acta, 2017, 227, 77–84.3 T. Hu, ACS Catal., 2021, 11, 14417–14427.4 Y. Wang, Angew. Chemie Int. Ed., 2020, 59, 5350–5354. Figure 1