Sustainable removal of nitrite waste to value-added ammonia on Cu@Cu2O core-shell nanostructures by pulsed laser technique

Abstract

The biochemical reduction of nitrite (NO₂^-) ions to ammonia (NH₃) requires six electrons and is catalyzed by the cytochrome c NO₂^- reductase enzyme. This biological reaction inspired scientists to explore the reduction of nitrogen oxyanions, such as nitrate (NO₃^-) and NO₂^- in wastewater, to produce the more valuable NH₃ product. It is widely known that copper (Cu)-based nanoparticles (NPs) are selective for the NO₃^- reduction reaction (NO₃^-RR), but the NO₂^-RR has not been well explored. Therefore, we attempted to address the electrocatalytic conversion of NO₂^- to NH₃ using Cu@Cu₂O core-shell NPs to simultaneously treat wastewater by removing NO₂^- and producing valuable NH₃. The Cu@Cu₂O core-shell NPs were constructed using the pulsed laser ablation of Cu sheet metal in water. The core-shell nanostructure of these particles was confirmed by various characterization techniques. Subsequently, the removal of NO₂^- and the ammonium (NH₄⁺)-N yield rate were estimated using the Griess and indophenol blue methods, respectively. Impressively, the Cu@Cu₂O core-shell NPs exhibited outstanding NO₂^-RR activity, demonstrating a maximum NO₂^- removal efficiency of approximately 94% and a high NH₄⁺-N yield rate of approximately 0.03 mmol h^-1.cm^-2 at -1.6 V vs. a silver/silver chloride reference electrode under optimal conditions. The proposed NO₂^-RR mechanism revealed that the (111) facet of Cu favors the selective conversion of NO₂^- to NH₃ via a six-electron transfer. This investigation may offer a new insight for the rational design and detailed mechanistic understanding of electrocatalyst architecture for the effective conversion of NO₂^- to NH₄⁺.

Keywords: Cu@Cu(2)O core–Shell; Electrocatalyst; NH(3) production; NO(2)(−) removal; Nitrite reduction reaction; Pulsed laser ablation.