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

Area of Science:

• Materials Science
• Surface Chemistry
• Solid-State Chemistry

Background:

• Low-temperature synthesis is vital for sustainable manufacturing and discovering new metastable materials.
• Metal hydrides show promise for low-temperature oxide reduction, but the exact reduction mechanism (H⁻, H₂, or atomic H) is debated.

Purpose of the Study:

• To elucidate the mechanism of calcium hydride (CaH₂) driven reduction of α-iron(III) oxide (α-Fe₂O₃) thin films at low temperatures.
• To investigate the role of molecular hydrogen (H₂) as the reducing species and the impact of moisture.

Main Methods:

• Utilized in situ electrical transport measurements to monitor reduction kinetics.
• Employed first-principles calculations to support experimental findings.
• Investigated epitaxial α-Fe₂O₃ thin films in contact with and separated from CaH₂.

Main Results:

• Similar apparent activation energies for H₂ reduction were observed for samples in direct contact with or separated from CaH₂.
• Direct contact with CaH₂ significantly accelerated the reduction rate.
• Molecular hydrogen (H₂) was identified as the dominant reducing species.

Conclusions:

• The superior reducing power of CaH₂ at low temperatures is primarily due to molecular H₂ and its ability to scavenge residual moisture.
• Effective moisture control is critical for efficient low-temperature oxide reduction in advanced material synthesis.