Revealing the mechanisms of non-thermal plasma-enabled iron oxide reduction through nanoscale operando TEM

H₂ plasma-enabled reduction of iron ore is a promising green alternative for reducing CO₂ emissions in the iron and steelmaking industry. In this work, we develop an operando plasma transmission electron microscopy (TEM) technique enabling the direct and real-time observation of magnetite (Fe₃O₄) nanoparticle reduction by non-thermal H₂ plasma with a spatial resolution of ~1 nm. Our operando results show a decrease in particle size accompanied by crack formation on timescales of ~10 s. We reveal that these observations are due to the oxide reduction, which induces a change in crystal structure from magnetite to iron, driven by the hydrogen radical, H^•. The operando reduction in particle volume by the plasma is well described by a shrinking-core reaction model. Our findings provide critical insights into mechanisms and rate-controlling processes of non-thermal iron ore reduction at the nanoscale. The developed operando plasma TEM technique is expected to find widespread application with the advent of non-thermal plasma technologies and the growing demands for diagnostic techniques to enhance mechanistic understandings in the field of plasma-nanoengineering.