Defect Engineering in Wüstite: Unlocking Control Over Iron Morphologies in Gas-Solid Reduction.

Hydrogen-based direct reduction (HyDR) technology has emerged as a promising pathway for sustainable steelmaking. However, the efficiency and stability of HyDR are critically influenced by the microstructure evolution of iron during gas-solid reduction reactions. Despite significant research on the reduction mechanisms of hydrogen (H) and carbon monoxide (CO) with iron oxides, key aspects of the interplay between internal defects, pore dynamics, and reduction chemistry remain unresolved. In this study, the morphological evolution of iron during reduction with H and CO across a full concentration range at 900 °C is explored, establishing a direct link between lattice distortions in wüstite (FeO) and the resultant iron microstructure. Complementary analyses reveal that the concentration of defects in FeO governs these distortions. Specifically, low CO concentrations (< 80%) induce limited large-scale defects, leading to single-point nucleation and the growth of filament-shaped iron whiskers. Conversely, H₂ and high CO concentrations (> 80%) create a high density of large-scale defects, promoting multi-point nucleation and the aggregation of tumor-shaped iron structures. This work provides a multiscale perspective on how defect engineering in FeO modulates the morphologies of iron during reduction, offering valuable insights into optimizing reaction pathways to enhance efficiency and sustainability in materials processing.