Electrocatalytic conversion of nitrate to ammonia on the oxygen vacancy engineering of zinc oxide for nitrogen recovery from nitrate-polluted surface water

Abstract

Nitrate pollution in surface water poses a significant threat to drinking water safety. The integration of electrocatalytic reduction reaction of nitrate (NO₃RR) to ammonia with ammonia collection processes offers a sustainable approach to nitrogen recovery from nitrate-polluted surface water. However, the low catalytic activity of existing catalysts has resulted in excessive energy consumption for NO₃RR. Herein, we developed a facile approach of electrochemical reduction to generate oxygen vacancy (Ov) on zinc oxide nanoparticles (ZnO_1-x NPs) to enhance catalytic activity. The ZnO_1-x NPs achieved a high NH₃-N selectivity of 92.4% and NH₃-N production rate of 1007.9 ${\text{mg}}_{{\text{NH}}_3‐N}$ h⁻¹ m⁻² at −0.65 V vs. RHE in 22.5 mg L⁻¹ ${\text{NO}}_3^{‐}‐N$, surpassing both pristine ZnO and the majority of catalysts reported in the literature. DFT calculations with in-situ Raman spectroscopy and ESR analysis revealed that the presence of Ov significantly increased the affinity for the ${\text{NO}}_3^{‐}$ (nitrate) and key intermediate of ${\text{NO}}_2^{‐}$ (nitrite). The strong adsorption of ${\text{NO}}_3^{‐}$ on Ov decreased the energy barrier of potential determining step (${\text{NO}}_3^{‐}$ →∗NO₃) from 0.49 to 0.1 eV, boosting the reaction rate. Furthermore, the strong adsorption of ${\text{NO}}_2^{‐}$ on Ov prevented its escape from the active sites, thereby minimizing ${\text{NO}}_2^{‐}$ by-product formation and enhancing ammonia selectivity. Moreover, the NO₃RR, when coupled with a membrane separation process, achieved a 100% nitrogen recycling efficiency with low energy consumption of 0.55 kWh ${\mathrm{mol}}_N^{-1}$ at a flow rate below 112 mL min⁻¹ for the treatment of nitrate-polluted lake water. These results demonstrate that ZnO_1-x NPs are a reliable catalytic material for NO₃RR, enabling the development of a sustainable technology for nitrogen recovery from nitrate-polluted surface water.