Doping-induced Structural Transformations in Maghemite for Enhanced Ortho-Para Hydrogen Conversion

Abstract

Ortho-para hydrogen (o-p H₂) conversion is a critical process for liquid hydrogen storage and transportation. Developing efficient catalysts for such process is the key at the heart of hydrogen economy while limited by the puzzling structure/reactivity correlation. In this contribution, a series of secondary transition metal doped Fe₂O₃ nanoparticles (M-Fe₂O₃, M = Cu, Ni, Zn) were controllably prepared via a simple thermal treatment approach and thoroughly characterized for clarifying the structure-function relationship toward o-p H₂ conversion. The reaction rate for o-p H₂ conversion reaction rate for o-p H₂ conversion over Fe₂O₃ was significantly enhanced by doping with Cu (0.41x10⁻³ kmol L⁻¹ s⁻¹) and Ni (0.32x10⁻³ kmol L⁻¹ s⁻¹), showing notably higher compared to that of Zn-Fe₂O₃ (0.13x10⁻³ kmol L⁻¹ s⁻¹). Results from Mössbauer measurements revealed that doping with Cu and Ni increases the proportion of Fe³⁺ in tetrahedral-site (A-site) of γ-Fe₂O₃, which is responsible for the enhanced o-p H₂ conversion. Furthermore, the higher specific surface area resulting from Cu and Ni doping were also demonstrated as non-innocent factor involved in o-p H₂ conversion due to enhanced active sites exposure and H₂ diffusion. Our work clarifies the crucial role of transition metal doping in iron oxides for enhancing o-p H₂ catalytic conversion, which may pave the way for the development of novel efficient o-p H₂ conversion catalysts for advancing the broader utilization of hydrogen energy.