Ortho-para hydrogen conversion over 3d transition-metal-based catalysts for hydrogen liquefaction and energy storage

Enhancing the para‑hydrogen concentration through ortho-para hydrogen conversion (OPHC) is critical for liquid hydrogen (LH₂) storage, and efficient OPHC catalysts can significantly improve hydrogen liquefaction efficiency while reducing equipment size. Herein, to elucidate the superiority of precipitation-derived α-Fe₂O₃ (FeO), a series of 3d transition metal-based catalysts was prepared to demonstrate the influence of paramagnetic active centers and synthetic methods. Low-temperature OPHC tests revealed that the reaction rate constants (k) of these samples exhibited a strong linear relationship with the square of the effective magnetic moments (μ_eff). The exceptional performance of FeO (k = 0.86 × 103 mol·L−1·s−1) is attributed to its remarkably high μ_eff (5.96 μ_B), high specific surface area (239.22 m2/g), and minimal average pore diameter (3.62 nm). Furthermore, the moderate synthesis temperature (298 K), substantial mechanical strength (0.6 GPa) and high granulation efficiency (69.8%) underscore the practical applicability of precipitation-derived α-Fe₂O₃ for hydrogen liquefaction across a broad temperature range. This work proposes key application criteria for catalysts critical to LH₂ storage, providing a reference for establishing a more comprehensive evaluation framework.