Bilayer electrified-membrane with pair-atom tin catalysts for near-complete conversion of low concentration nitrate to dinitrogen

Discharge of wastewater containing nitrate (NO₃^-) disrupts aquatic ecosystems even at low concentrations. However, selective and rapid reduction of NO₃^- at low concentration to dinitrogen (N₂) is technically challenging. Here, we present an electrified membrane (EM) loaded with Sn pair-atom catalysts for highly efficient NO₃^- reduction to N₂ in a single-pass electrofiltration. The pair-atom design facilitates coupling of adsorbed N intermediates on adjacent Sn atoms to enhance N₂ selectivity, which is challenging with conventional fully-isolated single-atom catalyst design. The EM ensures sufficient exposure of the catalysts and intensifies the catalyst interaction with NO₃^- through mass transfer enhancement to provide more N intermediates for N₂ coupling. We further develop a reduced titanium dioxide EM as the anode to generate free chlorines for fully oxidizing the residual ammonia (<1 mg-N L^-1) to N₂. The sequential cathode-to-anode electrofiltration realizes near-complete removal of 10 mg-N L^-1 NO₃^- and ~100% N₂ selectivity with a water resident time on the order of seconds. Our findings advance the single-atom catalyst design for NO₃^- reduction and provide a practical solution for NO₃^- contamination at low concentrations.