Thin-shell Fe@SiO2 catalysts for efficient and stable ortho-para hydrogen conversion at 77 K

Liquid hydrogen is attractive for large-scale hydrogen storage and long-distance transport because of its high volumetric energy density. However, exothermic ortho-para hydrogen conversion at cryogenic temperature causes boil-off loss and requires efficient catalysts. Iron-based catalysts are widely used, but their practical application is limited by poor stability under water, oxygen, and impurity attack. Herein, Fe@SiO 2 core-shell catalysts with tunable shell thicknesses were prepared by precipitation combined with hydrolytic coating and evaluated at 77 K. An amorphous SiO 2 shell was uniformly formed on the iron-based particles without destroying the active phase. The optimized catalyst achieved an outlet para-H 2 content of 47.5% and remained stable over 120 h. The improved catalytic performance is attributed to the SiO 2 coating, which enhances the structural stability of the iron-based particles while retaining accessible mesoporous transport pathways. These pathways facilitate H 2 transport to the iron-based active phase while reducing its exposure to the external environment.