Tracking Copper Oxidation State during Nitrate Electrochemical Reduction Reaction

Abstract

The synthesis of ammonia via Haber-Bosch process, obtained from nitrogen catalytic hydrogenation, was a great revolution for humankind that allowed mass production of food with the availability of fertilizers.1 However, it caused a disturbance in the global nitrogen cycle due to the production of ammonia from a non-reactive source (N2), fixing 108 tons of nitrogen into reactive species per year.2 Among nitrogenous contaminants, nitrate is the main pollutant of wastewater and the most oxidized species, being accumulated in the ecosystems.2 Besides that, because of the need of H2 as reactant, the current ammonia production emits 500 million tons of carbon dioxide per year, making ammonia the largest CO2 emitting chemical process.1 Finding sustainable alternatives to restore the nitrogen cycle without compromising ammonia production and consequently food supply is urgently needed.1,3 In this context, nitrate electrochemical reduction reaction (NO3RR) could be a sustainable way to produce ammonia, removing a pollutant from wastewater and helping ammonia decarbonization.2,3 Finding a catalyst that presents high electrochemical performance, high availability, and good stability is still challenging. Among transition metals, copper has the fastest rate-determining step of nitrate reduction to nitrite, offering the highest electrocatalytic kinetics and exchange current densities for NO3RR.4 In addition to metallic copper, its oxides are also attracting attention for catalytic NO3RR.5–13 Different nanomaterials that combine copper metallic and oxide phases have been reported in the literature for ammonia synthesis, with Faradaic efficiencies up to 98%.4 This enhancement is attributed to the fact that Cu2O requires lower activation energy to *NH2O hydrogenation, enabling the faster formation of *NH2OH at catalyst surface.13 Our preliminary results show that there are also synergistic enhancements of Cu and Cu2O, i.e., a prepared mixture of Cu and Cu2O is more active than either compound alone.Herein, we investigate structural and compositional changes at a Cu/Cu2O nanocomposite after and during NO3RR.14 Our findings suggest that under the potentials in which ammonia production from NO3RR takes place, Cu2O is also being reduced to Cu. Figure 1a presents linear sweep voltammetries (LSV) measurements with and without nitrate. LSV obtained in the absence of nitrate (black line) shows a peak related to Cu2O reduction (centered in -0.2 V vs. RHE). In the presence of nitrate (red line), the onset potential is around -0.1 V vs. RHE, from which a cathodic current takes place until -0.8 V vs. RHE. It indicates that Cu2O starts to reduce close to the onset potential for NO3RR.We evaluated the catalyst performance from -0.2 to -0.6 V vs. RHE and the highest faradaic efficiency (66 ± 7%) for NH3 was obtained at -0.4 V vs. RHE (Figure 1b). Then, the composite was ex-situ characterized with spectroscopic and microscopic techniques before and after an 1h-electrolysis at -0.4 V vs. RHE. Though X-ray photoelectron spectroscopy, we show that copper oxidation state changes after potentiostatic electrolysis experiments at -0.4 V vs. RHE (Figures 1c and 1d), where the satellite peak related to Cu2O disappears. Significant structural changes were detected with atomic force microscopy (Figures 1e and 1f), evidenced by the change of the average roughness from 222.5 to 173.1 nm. In-situ Raman spectroscopy was employed to track copper oxidation state during NO3RR. Cu2O has some characteristic peaks at Raman spectra: 150 cm-1 related to infrared active mode F1u (Τ15), 520 cm-1 related to Raman allowed mode 3T’25(F 2g) and 630 cm-1 related to infrared active mode F1u (Τ15).15 The spectra obtained (Figure 1g) showed that these peaks disappear at a less negative potential than the window that ammonia production from NO3RR takes place (-0.2 V vs. RHE), which is in accordance with LSV results, indicating that Cu2O reduces at a more positive potential than nitrate. These results suggest that higher activity for nitrate reduction could be associated with the formation of defects at composite structure, which will be evaluated later with kinetic studies.This work shows strong experimental indicatives that copper oxide is not stable at the potential window employed to ammonia production from NO3RR, although its reduction can induce some defects at copper structure that enhances the catalytic performance. These findings provide important clues towards designing new copper-based materials for electrochemical reduction of nitrate to ammonia. Figure 1