Atomically Resolved Transition Pathways of Iron Redox

The redox transition between iron and its oxides is of the utmost importance in heterogeneous catalysis, biological metabolism, and geological evolution. The structural characteristics of this reaction may vary based on surrounding environmental conditions, giving rise to diverse physical scenarios. In this study, we explore the atomic-scale transformation of nanosized Fe₃O₄ under ambient-pressure H₂ gas using in-situ environmental transmission electron microscopy. Our results reveal that the internal solid-state reactions dominated by iron diffusion are coupled with the surface reactions involving gaseous O or H species. During reduction, we observe two competitive reduction pathways, namely Fe₃O₄ → FeO → Fe and Fe₃O₄ → Fe. An intermediate phase with vacancy ordering is observed during the disproportionation reaction of Fe²⁺ → Fe⁰ + Fe³⁺, which potentially alleviates stress and facilitates ion migration. As the temperature decreases, an oxidation process occurs in the presence of environmental H₂O and trace amounts of O₂. A direct oxidation of Fe to Fe₃O₄ occurs in the absence of the FeO phase, likely corresponding to a change in the water vapor content in the atmosphere. This work elucidates a full dynamical scenario of iron redox under realistic conditions, which is critical for unraveling the intricate mechanisms governing the solid-solid and solid-gas reactions.