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

ABSTRACT 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 H 2 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 ‐Fe 2 O 3 , γ ‐Fe 2 O 3 , and Fe 3 O 4 . The bulk and surface properties of these catalysts were characterized by XRD, BET, TG, IR, magnetic analysis, hydrogen adsorption, and 57 Fe‐Mössbauer spectrum. It was suggested that ortho–para H 2 conversion is linearly correlated with the specific surface area of α ‐Fe 2 O 3 which governs the residual magnetic properties as well as the adsorption capacity of molecular H 2 on the catalysts, and a nondissociation mechanism of ortho–para H 2 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 H 2 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 H 2 conversion was found to follow a trend similar to that of α ‐Fe 2 O 3 ≈ γ ‐Fe 2 O 3 > Fe 3 O 4 . The findings of this study provide valuable insights for the subsequent research on the mechanism of ortho–para H 2 conversion and the design of high‐performance hydrogen liquefaction catalysts.