SORBATE PROPERTIES AND CAGE-TO-CAGE DIFFUSION OF ARGON IN NACAA - A MOLECULAR-DYNAMICS STUDY

Detailed molecular dynamics simulations of argon in zeolite NaCaA are reported. Thermodynamic, structural, and dynamical properties of the s orbate as a function of temperature have been obtained. The properties calculated include various site-site radial distribution functions, d ifferent energy distribution functions, self- diffusion coefficients, the power spectra, and properties relating to cage-to-cage diffusion. The results suggest that sorbate is delocalized above 300 K. Both mode s of cage-to-cage diffusion-the surface-mediated and centralized diffu sion-are associated with negative barrier heights. Surprisingly, rate of cage-to-cage diffusion is associated with negative and positive act ivation energies below and above 500 K. The observed differences in th e behavior of the rate of cage-to-cage diffusion between Xe-NaY and Ar -NaCaA systems and the nature of the potential energy surface are disc ussed. Presence of sorbate-zeolite interactions results in significant enhancement in the rate of cage-to-cage diffusion and rate of cage vi sits. It is shown that properties dependent on the long-time behavior such as the diffusion coefficient and the rate of cages visited exhibi t the expected Arrhenius dependence on temperature.