Catalytic Mechanism Studies of Ortho–para H2 Conversion Over Iron Oxide Catalysts

Hydrogen serves as an ideal clean energy with zero carbon emissions, whereas its large-scale application relies on its liquidation, by which the catalytic conversion of ortho–para H₂ at cryogenic temperature is inevitable with iron oxides as a promising catalyst. In this research, iron oxides with varied surface area and diverse phases were synthesized from the precursor of hydrous ferric oxide, including α-Fe₂O₃, γ-Fe₂O₃, and Fe₃O₄. The bulk and surface properties of these catalysts were characterized by XRD, BET, TG, IR, magnetic analysis, hydrogen adsorption, and ⁵⁷Fe-Mössbauer spectrum. It was suggested that ortho–para H₂ conversion is linearly correlated with the specific surface area of α-Fe₂O₃ which governs the residual magnetic properties as well as the adsorption capacity of molecular H₂ on the catalysts, and a nondissociation mechanism of ortho–para H₂ conversion was revealed at cryogenic temperature. The hydrate that contributed to the surface area of iron oxides shows a negative effect on the ortho–para H₂ conversion. Moreover, by estimating the reaction rate based on the per surface area of iron oxides, the Fe(III) exposed on surfaces exhibited a superior activity irrespective of the bulk magnetism of iron oxides, and the intrinsic activity of iron oxides for ortho–para H₂ conversion was found to follow a trend similar to that of α-Fe₂O₃ ≈ γ-Fe₂O₃ > Fe₃O₄. The findings of this study provide valuable insights for the subsequent research on the mechanism of ortho–para H₂ conversion and the design of high-performance hydrogen liquefaction catalysts.