Molecular H2 as the Reducing Agent in Low-Temperature Oxide Reduction Using Calcium Hydride

Low-temperature synthesis is crucial for advancing sustainable manufacturing and accessing novel metastable phases. Metal hydrides have shown great potential in facilitating the reduction of oxides at low temperatures, yet the underlying mechanism-whether driven by H-, H2, or atomic H-remains unclear. In this study, we employ in situ electrical transport measurements and first-principles calculations to investigate the CaH2-driven reduction kinetics in epitaxial alpha-Fe2O3 thin films. Intriguingly, samples in direct contact with or separated from CaH2 powders exhibit similar apparent activation energies for H2 reduction, although direct contact significantly increases the reduction rate. These findings indicate that molecular H2 is the dominant reducing species in the low-temperature reduction of oxides using CaH2, with a key aspect of the hydrides' superior reducing power attributed to their ability to eliminate residual moisture. This work underscores the critical role of moisture control in enabling effective low-temperature oxide reduction for advanced material synthesis.