Nanoscopic reduction mechanism of hematite by hydrogen revealed via X-ray spectromicroscopy

ABSTRACT The reduction of fine hematite (Fe 2 O 3 ) grains (< a few tens μm) by hydrogen, rather than coke (carbon), is a key technology for carbon-neutral ironmaking. Because bulk oxide grain reduction occurs heterogeneously, understanding its mechanism is essential for process optimization. However, observing chemical state heterogeneity within a single fine grain is challenging. This study employed X-ray spectromicroscopy with a spatial resolution of less than 50 nm to nondestructively visualize changes in chemical state heterogeneity in 3D. The results reveal that two competing reduction mechanisms emerge at the early stage of reduction owing to slight but significant differences in the phase stability of Fe 2 O 3 , Fe 3 O 4 , and Fe 1-x O. At 973 K, a topochemical mode dominates, where the surface layer of the grain rapidly reduces to Fe and covers Fe 1-x O or Fe 3 O 4 domains, saturating the reduction degree to approximately 70 %. At 1173 K, a topotaxial mode dominates, with reduction progressing along crystallographic directions, achieving a rate of over 95%. These findings demonstrate that reduction conditions cannot be optimized solely by enhancing hydrogen gas diffusion through thermal control. Changes in coexisting phases and microstructures, particularly those influencing void and pore formation, are crucial. These findings highlight the significance of 3D X-ray spectromicroscopy in understanding heterogeneous reduction processes and optimizing gas reactions for efficient ironmaking and other redox reactions.