Unveiling the Key Magnetic Factors Governing Efficient <i>Ortho-Para</i> Hydrogen Conversion on Superparamagnetic Iron Oxide

Ortho-para hydrogen (o-p H2) conversion is a key step in enabling efficient liquid hydrogen storage and transportation. Nevertheless, the development of highly efficient o-p H2 conversion catalysts is still limited by an incomplete understanding of how structural and magnetic properties control the catalytic performance. Herein, this work reports the controlled synthesis of superparamagnetic iron oxides (FeOx) via a modified thermal decomposition method and elucidates the influence of their magnetic properties on the o-p H2 conversion efficiency. Notably, FeOx-2 (pyrolyzed at 473 K) exhibits a rate constant 1.6 times higher than that of the commercial catalyst. Results from structural characterization and vibrating sample magnetometry measurements reveal that the specific surface area and magnetization significantly influence the o-p H2 conversion efficiency. Furthermore, low-temperature (2.6−298 K) 57Fe Mössbauer spectroscopy reveals that superparamagnetic FeOx exhibiting a lower blocking temperature and shorter relaxation time promotes rapid magnetic moment reversal, thereby enhancing the o-p H2 conversion. Additionally, low-temperature H-D exchange experiments suggest the nondissociative magnetic reaction mechanism over superparamagnetic FeOx. This study clarifies the critical role of intrinsic magnetic properties in superparamagnetic iron oxides for efficient o-p H2 conversion, providing fundamental principles for the design of advanced catalysts for hydrogen energy applications.