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 h^-1 m^-2 at -0.65 V vs. RHE in 22.5 mg L^-1, 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 (nitrate) and key intermediate of (nitrite). The strong adsorption of on Ov decreased the energy barrier of potential determining step ( →*NO₃) from 0.49 to 0.1 eV, boosting the reaction rate. Furthermore, the strong adsorption of on Ov prevented its escape from the active sites, thereby minimizing 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 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.