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

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 (ZnO1-x NPs) to enhance catalytic activity. The ZnO1-x NPs achieved a high NH3-N selectivity of 92.4% and NH3-N production rate of 1007.9 mgNH3-N h- 1 m- 2 at -0.65 V vs. RHE in 22.5 mg L- 1 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 NO-3 (nitrate) and key intermediate of NO-2 (nitrite). The strong adsorption of NO-3 on Ov decreased the energy barrier of potential determining step (NO-3 ->*NO3) from 0.49 to 0.1 eV, boosting the reaction rate. Furthermore, the strong adsorption of NO-2 on Ov prevented its escape from the active sites, thereby minimizing 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 mol- 1 flow rate below 112 mL min- 1 for the treatment of nitrate-polluted lake water. These results demonstrate that ZnO1-x NPs are a reliable catalytic material for NO3RR, enabling the development of a sustainable technology for nitrogen recovery from nitrate-polluted surface water.