Low-temperature ortho-para hydrogen conversion on rare earth (4f) hydroxides with high effective magnetic moments

Enhancing para-hydrogen (p-H 2 ) content through ortho-para hydrogen conversion (OPHC) constitutes a key approach to extend liquid hydrogen (LH 2 ) storage duration, with high-efficiency catalysts serving as the pivotal component in this process. The unique electronic structure of 4f orbitals endows rare earth elements with exceptional magnetic properties, making them intrinsically suitable as paramagnetic active centers for OPHC. Herein, to evaluate the potential of 4f metals as active centers for OPHC catalysts, a series of rare earth (4f) hydroxides (Y–O samples) were synthesized using a simple precipitation method and applied for OPHC at 77 K. The crystal structures, micromorphology, elemental oxidation states, mechanical strengths, and magnetic properties of these catalysts were characterized by XRD, SEM, XPS, BET, EPR, and PPMS, etc. Although the rare earth atoms are subject to the crystal field effect, resulting in an effective magnetic moment (μ eff ) smaller than that of the free-ion intrinsic state, they generally exceed those μ eff of 3d metals (3d 5 configurations). Low-temperature OPHC tests revealed that the reaction rate constant (k) of the catalysts at different volume space velocities (VSV) is proportional to μ eff 2 . The Dy–O sample exhibited the highest μ eff (6.90 μ B ), with a p-H 2 content of 48.8 % in the converted H 2 at a VSV of 30000 h −1 , and the normalized rate constant is more than 7 times greater than that of the commercial catalyst, providing a new perspective on the selection of active centers for OPHC catalysts.