The reduction of iron oxide is a critical process in catalysis and iron making. In particular, there is a growing interest in exploring how non-equilibrium plasma-induced reactions can contribute to catalytic processes and iron making. Nonetheless, the underpinning processes remain not well understood.

We have developed a novel in-situ plasma transmission electron microscopy (TEM) technique (Fig. 1). This powerful technique can uniquely visualize plasma-material interactions in real-time at ~ 1 nm spatial resolution under atmospheric pressure conditions. It can provide mechanistic insights that are inaccessible through conventional post-mortem characterization methods which are still valuable but limited in that only snapshots of transient states can be observed, and the specimen can be exposed to external environments.

We investigated the reduction of magnetite nanoparticles (Fe₃O₄) by He-H₂ plasma through in-situ TEM, which directly captured particle shrinkage and subsequent fracturing on a timescale of seconds. Compositional analyses confirmed a direct, non-thermal reduction to metallic iron (Fe) driven by plasma-produced hydrogen radicals (H), which bypasses the typical high-temperature wüstite (FeO) intermediate. The unique advantage of in-situ plasma TEM technique enabled the time-resolved kinetic analysis by tracking the volume of individual nanoparticles in real-time, revealing that rates are controlled by the H-enabled reduction at the Fe-Fe₃O₄ phase boundary.

This work elucidates mechanisms of plasma-enabled iron oxide reduction and demonstrates in-situ plasma TEM technique as a transformative analytical tool. The ability to directly monitor morphological changes of materials induced by plasma-chemistry opens new opportunities for fundamental studies such as plasma-catalysis and nanomaterial synthesis.

Geometry of developed in-situ plasma holder

Peter J. Bruggeman

University of Minnesota Twin Cities

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