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Ammonia is the molecular foundation of the global food system, supplying the nitrogen fertilizer that sustains roughly half the world's population — yet its synthesis consumes an estimated 1–2% of global energy. Iron nitrides are earth-abundant materials central to greening this nitrogen economy: candidate catalysts for nitrogen reduction and ammonia synthesis, ammonia decomposition for hydrogen storage, and rare-earth-free magnets. Each function demands phase-selective control over iron nitride formation, requiring a mechanistic understanding of how iron oxides are simultaneously reduced and nitrided.
Departing from multi-step reduction using carbonaceous resources followed by nitriding, we develop a single-step pathway using ammonia as both reducing and nitriding agent — cheaper to transport than hydrogen and uniquely enabling simultaneous reduction and nitriding to tailor metallic versus nitride products in one step. Using Fe₂O₃ as a case study, we apply operando USAXS/SAXS/WAXS to resolve these poorly understood coupled kinetics from sub-nanometer to micrometer scales, tracking phase evolution alongside morphology relevant to scale-up.
Reducing Fe₂O₃ in ammonia at 500–800 °C reveals a temperature-independent pathway but a temperature-dependent product. Reduction proceeds Fe₂O₃ → Fe₃O₄ → FeO; FeO is then reduced and nitrided to Fe₄N, which decomposes to Fe. Temperature tunes the reduction–nitriding balance: incomplete reduction with mixed Fe/Fe₄N/Fe₃N at 500 °C, single-phase Fe₄N at 600 °C, a Fe/Fe₄N mixture at 700 °C, and pure Fe at 800 °C. USAXS shows agglomeration only at 600–800 °C, with the smoothest surfaces for single-phase products.
This work shows the iron product can be tailored by composition and morphology, establishing operando cross-scale scattering for scale-up of ammonia-based reduction–nitriding. Both materials and method feed back into sustainable-nitrogen technologies: the iron nitrides here are catalyst candidates for greening ammonia — and therefore fertilizer — production, while the scattering approach transfers to nitrogen speciation and metal cycling in soils and plants.