Performance of composite oxygen carriers for plasma-assisted nitrogen fixation and ammonia synthesis

Abstract

NH₃, a vital chemical feedstock, has also been recognized in recent years as a hydrogen energy carrier with significant potential. Achieving low-energy, high-efficiency nitrogen fixation has attracted considerable attention from researchers. The conventional Haber–Bosch process requires stringent production conditions of high temperature and pressure, resulting in high energy consumption and substantial carbon emissions. This paper presents a mild nitrogen-fixation process in which dielectric-barrier discharge plasma is coupled with chemical-looping technology, enabling a substantial reduction in nitridation temperature and achieving an ammonia yield of 2.23 μmol g⁻¹. Nitrogen is fixed onto the oxygen carrier under the voltage applied, and the resulting substances are hydrolyzed under mild conditions to produce NH₃. The mutual enhancement between Mg-based composite nitrogen carriers and plasma is emphasized. The experimental system investigated the optimization of oxygen carriers via ball milling and additive modifications (TiO₂ and ZrO₂), elucidating the O_v-mediated nitrogen-conversion mechanism for ammonia synthesis. Real-time optical emission spectroscopy monitoring further confirmed synergistic effects between the composite nitrogen carrier and plasma-generated excited N₂ and N₂⁺ species. Isotopic evidence indicates that the ammonia released during hydrolysis originates from plasma-fixed N₂, although a minor background contribution cannot be completely excluded. The sustained formation of reactive nitrogen species, coupled with dynamic changes in oxygen vacancy concentration, directly drives enhanced nitrogen-fixation performance and nitrogen-carrier generation. This work has significant implications for developing novel, highly efficient energy-storage processes for nitrogen fixation and ammonia synthesis.