Electrocatalytic nitrate-to-ammonia conversion using Mo-S-Fe3O4 nanoplates on foam iron electrode: Synergistic effects of Lewis-acid and Lewis-base sites

The NO 3 − electroreduction process presents a promising alternative to the Haber-Bosch method by offering dual benefits: removal of nitrates from wastewater coupled with efficient NH 3 production. Herein, Mo and S doped Fe 3 O 4 nanoplates grown on foam Fe (Mo-S-Fe 3 O 4 @FF) with electron-rich Mo δ+ state was synthesized through a facile hydrothermal approach. Notably, Mo-S-Fe 3 O 4 @FF electrode achieved a nearly fourfold enhancement in ammonium yield compared to foam Fe electrode, while also enhancing the NH 3 Faradaic efficiency. At − 1.3 V vs . Ag/AgCl in a K 2 SO 4  + KNO 3 solution, the Mo-S-Fe 3 O 4 @FF electrode attained a NH 3 Faradaic efficiency of 91.3 %. Furthermore, performance was observed to improve with increasing solution pH levels as the reaction proceeded. From both kinetic and thermodynamic perspectives, our findings demonstrate that high NH 3 production activity is characteristic of Mo-S-Fe 3 O 4 @FF—indicating significant enhancements in conversion rates from NO 3 − →NO 2 − →NH 3 . Theoretical calculation, in conjunction with experimental analyses, reveal that the co-modification of Fe 3 O 4 with Mo and S enhances electron transfer capacity during NO 3 − RR. This modification forms Lewis-acid (Fe-Mo) pairs that increase nitrate adsorption energy alongside Lewis-base (S-O) sites that attract protons, thereby improving overall NO 3 − RR activity. This report elucidates the synergistic effects of Lewis-acid and Lewis-base sites on optimizing catalytic performance for NO 3 − RR using foam Fe-based electrodes.