All-in-one fabrication of bimetallic PdIn-decorated porous PES membranes for the catalytic flow-through reduction of NO3− to NH3 with formic acid in water

We utilized a novel all-in-one method to fabricate porous polyethersulfone (PES) membranes containing bimetallic PdIn as catalyst and a cationic ionomer to introduce charged surface groups. Our fabrication is based on in situ solidification of dissolved palladium and indium ions in a PES-ionomer casting-reaction solution, directly followed by preparation of porous catalytic membranes through film casting and phase separation in water. We employed as-prepared membranes in the catalytic reduction of NO₃⁻ (50 mg/L, 0.81 mM) to NH₃ in water using formic acid (FA) as electron source. The in situ solidification of PdCl₄²⁻ and In³⁺ generated PdIn species with the highest catalytic activity, compared to Pd²⁺ and In³⁺ or a sequential solidification of Pd followed by In. Moreover, positively charged ionomer in a Pd₃In₁-PES membrane boosted NO₃⁻ conversion rate in flow-through at pH 7 from 2.5 to 17 mmol/m²h and NH₃ selectivity from 4 % to 34 %, likely by promoting interaction between nitrate and formate anions with catalyst sites. Reducing the flow rate from 100 to 50 L/m²h further enhanced NH₃ selectivity to 55 % (NH₃ production rate of 189 µg/h mg), illustrating that a longer residence time in the membrane promotes NH₃ formation. Additionally, we achieved 90 % electron efficiency for NO₃⁻ reduction with FA in flow-through compared to 60 % in batch, highlighting that a short contact time between catalyst and FA limits excess consumption through dehydrogenation. Finally, we demonstrated continuous NH₃ production from NO₃⁻ for 11 h of flow-through, and found indications that PdIn catalyzes NO₃⁻ reduction through an electron/oxygen transfer cycle.