Cation Diffusion-Mediated Displaced Reaction-Transformation in Green Steelmaking

Hydrogen-based direct reduction of iron oxides shows great potential for decarbonizing the iron and steelmaking industry. However, deciphering such complex, multistage reactions remains challenging due to the concurrent occurrence of multiple nonlinearly coupled processes. Here, the in-operando reduction dynamics of iron oxide ranging from 200 °C-1000 °C is revealed using time-resolved hydrogen-environmental scanning transmission electron microscopy (with a hydrogen gas pressure of 3 Pa) combined with bulk analysis and atomistic modeling. Proposing a general cation-mediated redox mechanism in Fe₃O₄, cation diffusion triggers a spatially displaced reaction-transformation and drives the ensuing dynamic phase transformations and reduction steps that extend significantly beyond the reaction front. At 727 °C the spatial displacement is 2-4 orders of magnitude above the previously assumed reaction-front thickness. The results underscore the cross-scale universality and pivotal role of cation diffusion in enabling rapid hydrogen-based reduction of metal oxides, offering not only fundamental atomic-scale insights to help render the steelmaking industry more sustainable, but also a better understanding of related processes in batteries and corrosion.