Exploring Ortho–Para Hydrogen Conversion Catalysts Based on Surface Electric Field Gradient

According to our hypothesis that ortho (O) to para (P) hydrogen conversion is promoted by an inhomogeneous electric field on the surface of the insulating oxide with high ionicity, we searched for OP conversion catalysts using lattice energy as an indicator of high ionicity. As a result, we found new oxide catalysts, including SiO₂, γ-Al₂O₃, and CeO₂ combined with 3d late transition metal cocatalysts. The fraction of para-H₂ on 5%Fe-loaded SiO₂ powder reached 50% (equilibrium value at 77 K) within 20 min. The activities of these catalysts are significantly superior to those of benchmark catalysts, such as Mn₃O₄. The cocatalyst nanoparticles of 1-12 nm size dispersed on the oxide catalysts adsorb hydrogen well without dissociating it. The nuclear spin state of ortho-H₂ adsorbed at the asymmetric site (nonzero electric field gradient) of the oxide surface is thermally excited by nuclear quadrupole interaction, and the OP conversion rate is increased by nuclear spin relaxation.