Physisorption and Ortho-Para Conversion of H2 on Maghemite γ-Fe2O3 (001) Surface: A Computational Study

GU Kuda-Singappulige and SO Akande and CJ Jameson and S Murad, JOURNAL OF PHYSICAL CHEMISTRY C, 129, 12679-12696 (2025).

DOI: 10.1021/acs.jpcc.5c01684

We study ortho-to-para transition events on gamma-Fe2O3 (maghemite), a component of commercial catalysts for this conversion accompanying the hydrogen liquefaction process. We adopt a semiclassical approach in which the physisorption is described by classical molecular dynamics, from which the instantaneous positions of the H nuclei are used to calculate dipole-dipole and Fermi contact contributions to transition integrals. Dipole-dipole interactions are calculated using magnetic moments from a quantum description of the (001) surface of a solid maghemite slab with ordered vacancies; Fermi contact interactions are calculated using the electron spin density on the same surface to obtain the hyperfine constant at each H position. This approximation is tested by fully quantum calculations including electrons on the H2 molecule. Naturally disordered maghemite with disordered vacancies is represented by four disordered unit cells extracted from a 100 x 100 x 1 supercell obtained by Monte Carlo migration simulations, using migration energies that have been calculated through Nudged Elastic Band theory. Quantum calculations on surface slabs of disordered maghemite prepared from these representative disordered cells provide magnetic moments in the solid and electron spin densities at the surface for transition integrals in the semiclassical model for o-p transition at a naturally disordered maghemite surface. Effects of disordered vacancies on magnetic moments providing the dipolar mechanism and on electron spin densities at/near the surface contributing to the Fermi contact mechanism are found to affect the proportions of dipolar to Fermi contributions, although dipolar contributions dominate the transition integral for maghemite with disordered vacancies, just as in the ordered case. Disordered vacancies increase the probability of ortho-para conversion events per encounter. The semiclassical approach describes reasonably well the o-p transition events at 77 K because the Fermi contact contribution, which it does not describe completely, is a much smaller contribution to the transition integral than the dipole-dipole term.

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