Surface Phase Stability of Fe ₂ O ₃ (001) in Hydrogen Reducing Environments: A DFT and XPS Analysis

This study combines density functional theory (DFT) and <i>ab initio</i> thermodynamics calculations with X-ray photoelectron spectroscopy (XPS) investigations to identify the reduction properties of the Fe₂O₃ (001) surface with implications for corrosion resistance, hydrogen transport, and energy safety. <i>Ab initio</i> thermodynamics modeling predicts fully hydroxylated surface stability across a broad range of pressures (1 × 10^-23 to 1 × 10⁵ mbar) and temperatures below 700 K, consistent with previous experimental studies. Above 800 K, exposures to 1 × 10^-4 mbar H₂, 1 × 10^-4 mbar O₂, or 1 × 10^-4 mbar H₂ + 1 × 10^-4 mbar O₂ each yield unique XPS signals indicating a loss of -OH coverage, aligning with DFT predictions. Insight into the mechanism of reduction as a function of H₂ exposure is provided, as well as conditions that promote further reduction toward Fe₃O₄. Theoretical and experimental investigations indicate the ability to maintain the Fe₂O₃ protective layer of iron oxides that have been exposed to H₂ environments by including trace amounts of aqueous O₂.