Electrocatalytic Reduction of Nitrate Using Magnéli Phase TiO₂ Reactive Electrochemical Membranes Doped with Pd-Based Catalysts

Abstract

This research focused on synthesis, characterization, and application of point-of-use catalytic reactive electrochemical membranes (REMs) for electrocatalytic NO₃^- reduction. Deposition of Pd-Cu and Pd-In catalysts to the REMs produced catalytic REMs (i.e., Pd-Cu/REM and Pd-In/REM) that were active for NO₃^- reduction. Optimal performance was achieved with a Pd-Cu/REM and upstream counter electrode, which reduced NO₃^- from 1.0 mM to below the EPAs regulatory MCL (700 μM) in a single pass through the REM (residence time ∼2 s), obtaining product selectivity of <2% toward NO₂^-/NH₃. Nitrate reduction was not affected by dissolved oxygen and carbonate species and only slightly decreased in a surface water sample due to Ca²⁺ and Mg²⁺ scaling. Energy consumption to treat surface water was 1.1 to 1.3 kWh mol^-1 for 1 mM NO₃^- concentrations, and decreased to 0.19 and 0.12 kWh mol^-1 for 10 and 100 mM NaNO₃ solutions, respectively. Electrocatalytic reduction kinetics were shown to be an order of magnitude higher than catalytic NO₃^- reduction kinetics. Conversion of up to 67% of NO₃^-, with low NO₂^- (0.7-11 μM) and NH₃ formation (<10 μM), and low energy consumption obtained in this study suggest that Pd-Cu/REMs are a promising technology for distributed water treatment.