Identification of Structural Factors in Iron Oxide Triggering Ortho–Para Hydrogen Conversion

The ortho–para hydrogen (o–p H₂) conversion is an essential intensification process for the liquefaction, storage, and transportation of H₂, which is highly desired for harnessing liquid hydrogen. Iron-based catalysts have been demonstrated as the most promising candidates for o–p H₂ conversion because of their inexpensive and stable catalytic properties; however, the reactive active sites and catalytic mechanism remain hitherto unclear. In this contribution, a series of γ-Fe₂O₃ nanoparticles were prepared via a simple thermal treatment process and thoroughly characterized for exploring their structure–function relationship as o–p H₂ conversion catalysts. A tremendous increased reaction rate constant for o–p H₂ conversion was achieved using Fe₂O₃-2 as a catalyst, which is 2 orders of magnitude higher than that of the pristine γ-Fe₂O₃. Results from Mössbauer measurements demonstrated a positive correlation between the Fe³⁺ in the tetrahedral-site (A-site) and the o–p H₂ conversion performance. Additionally, the higher saturation magnetization, pore volume, and specific surface area were also demonstrated to be critical for o–p H₂ conversion. Our work provides an in-depth insight into the key structural factors in iron oxide triggering o–p H₂ conversion, which shall pave the way toward the design of novel efficient catalysts for practical application.