Atomic Pathways of Ammonia-Driven Fe₃O₄ Reduction Revealed by First-Principles Calculations

The direct reduction of iron ore using hydrogen faces challenges associated with hydrogen storage, transport, and on-site handling. Ammonia (NH₃), with its high hydrogen content, established distribution infrastructure, and economic viability, has emerged as a promising alternative reductant. Here, we employ density functional theory calculations to elucidate the atomic-scale mechanisms governing NH₃ adsorption, dehydrogenation, and nitrogen incorporation on the Fe₃O₄(001) surface. Our results show that NH₃ preferentially adsorbs upright at the surface Fe sites, initiating a sequence of dehydrogenation steps. Among the three dehydrogenation reaction pathways examined, H migration is identified as the rate-determining step for H₂O formation and desorption, a process that generates surface oxygen vacancies. The resulting NH and N species strongly bind to the surface through multiple Fe-N and Fe-NH coordination bonds. Notably, the most favorable configurationNH binds adjacent to an oxygen vacancyfacilitates further NH dissociation into N and H. The generated vacancies migrate favorably into the subsurface, enabling N incorporation into the lattice and promoting the formation of Fe nitride. Concurrently, N atoms that do not incorporate recombine to form N₂, thereby preventing excessive N accumulation on the surface. These results provide atomistic insights into NH₃-driven Fe₃O₄ reduction and reveal the coupled vacancy dynamics, H mobility, and N incorporation pathways that underpin NH₃-based ironmaking, highlighting the mechanistic opportunities for optimizing sustainable iron ore reduction and advancing NH₃-enabled catalytic processes.