The self-diffusion of hydrogen in Ca2+-, Mg2+- and Ba2+-exchanged X zeolites (Mg46X, Ca46X, and Ba46X) has been studied by\nmolecular dynamics (MD) simulations for various temperatures and loadings. The results indicate that in the temperature range of\n77ââ?¬â??298 K and the loading range of 1ââ?¬â??80 molecules/cell, the self-diffusion coefficients are found to range from 1.2 Ã?â?? 10-9 m2Ã?·s-1 to\n2.3 Ã?â?? 10-7 m2Ã?·s-1 which are in good agreement with the experimental values from the quasielastic neutron scattering (QENS) and\npulse field gradients nuclear magnetic resonance (PFG NMR) measurements. The self-diffusion coefficients decrease with loading\ndue to packing of sorbate-sorbatemoleculeswhich causes frequent collusion among hydrogenmolecules in pores and increases with\nincreasing temperature because increasing the kinetic energy of the gas molecules enlarges themean free path of gas molecule. The\nmechanism of diffusion of hydrogenmolecules in these zeolites is transition diffusion. Knudsen diffusion occurs at low loading and\nthe molecular bulk diffusion occurs at higher loading. For given temperature and loading, the self-diffusion coefficients decrease\nin the order Ba46X < Mg46X < Ca46X, due to the different sizes and locations of the divalent cations. Moreover, the effect of\nconcentration of molecular hydrogen on self-diffusion coefficient also is analyzed using radial distribution function (RDF).
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