Atomic Pathways of Ammonia-Driven Fe ₃ O ₄ 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.