Revealing CaH₂-Driven Metal Oxide Reduction Kinetics with In-Situ Transport Measurements

Metal hydrides, such as CaH₂, have recently emerged as highly promising reducing agents for facilitating the low-temperature reduction of metal oxides. One unique advantage of hydride reduction is its capability to synthesize metastable materials that are inaccessible through conventional high-temperature reactions. Notably, researchers have harnessed hydride reduction techniques to create unusual NiO₄ square-planar coordination in nickelates to host superconductivity [1]. Beyond the realm of novel materials discovery, metal hydrides also hold substantial potential in applied engineering. For instance, previous studies have shown that CaH₂ can lower the temperature required for gas-phase H₂ reduction of iron oxide, which can benefit clean hydrogen-based ironmaking [2]. Given these promising features, there is a compelling motivation to delve deeper into the mechanics of the hydride reduction process.<br/><br/>In this study, we investigate the CaH₂-induced reduction kinetics of metal oxides using epitaxial α-Fe₂O₃ thin films as a model system. To elucidate the intrinsic reducing capability of CaH₂, we seal the iron oxide thin-film samples along with CaH₂ in an evacuated quartz tube and analyze the reduction behavior of iron oxide within this closed system. In particular, we have developed an experimental platform that enables real-time monitoring of the CaH₂ reduction process through transport measurements. Using this setup, we have successfully quantified the phase transformation kinetics from iron oxide to metallic iron by continuously tracking the evolution of electrical resistivity in the thin-film sample. Our results demonstrate that CaH₂ alone can effectively reduce α-Fe₂O₃ into metallic iron within a one-hour reduction treatment at 400 °C, whereas 5% H₂/Ar (99.999% purity) failed to reduce the sample at identical conditions. These findings can advance our fundamental understanding of the hydride reduction process, opening new avenues to harness this phenomenon for the exploration of emergent materials properties and the development of environmentally friendly engineering applications.<br/><br/>[1] D. Li <i>et al.</i>, Nature <b>572</b>, 624 (2019).<br/>[2] T. Tsuchida <i>et al.</i>, Journal of Solid State Chemistry <b>302</b>, 122441 (2023).