Multiscale Hydrogen Chemistry of Direct Iron Reduction in Decarbonizing Steelmaking

Steelmaking contributes to 8% of the total annual CO₂ emissions globally. One attractive approach to decarbonizing steelmaking is to shift from the conventional coal-based reduction of iron ores in blast furnaces to hydrogen-based direct reduction of iron, preventing CO₂ emissions from coal. Despite its opportunity, hydrogen-based steelmaking has been slow to scale up because iron ore reduction with hydrogen is slow and energy-intensive (i.e. endothermic). While extremely important, the reaction kinetics and mass transfer in hydrogen-based steelmaking are poorly understood. Herein, we measure the kinetics of magnetite (Fe₃O₄) reduction with H₂, comparing industrial Fe₃O₄ to the lab-synthesized nanoscale Fe₃O₄ samples. Using <i>in situ</i> synchrotron X-ray diffraction and Rietveld analysis, we obtain the phase fraction of Fe₃O₄, wustite (FeO), and metallic Fe as reduction propagates on the full temperature dependence from 300 ^oC to 800 ^oC. We further map out the reaction kinetics and reveal the associated structural/morphology changes. Using <i>in situ</i> small-angle X-ray scattering, we demonstrate how the kinetics evolve simultaneously with mesoscopic structural changes. Ptychographic X-ray computed tomography reveals how evolved nano-/microstructure influence reduction reaction kinetics in H₂-based steelmaking. Our work provides insights on microstructure design to accelerate reduction reaction kinetics, further paving the way for commercializing sustainable steelmaking.