Levitation effect and its relationship with the underlying potential energy landscape

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Abstract
Positions of potential energy minima for spherical monatomic sorbates in zeolite NaY have been identified for different sizes of the sorbate. It is found that for small sorbates (σ⩽4.96 Å) there are only six adsorption sites per α-cage, which are located close to the inner surface of the α-cage. For larger sorbates, additional sites of comparable energies appear close to the 12-ring window which forms the bottleneck for intercage diffusion. Minimum energy paths between these sites have been computed. These suggest that the barriers for both intracage and intercage site-to-site migrations are comparable and decrease with increase in sorbate size. Earlier simulation studies on the diffusion of monatomic sorbates in zeolites indicated that there is a dramatic change in the nature of dependence of D on sorbate size around 4.96 Å, for zeolite NaY. Therefore, the present results suggest that the dependence of D on sorbate size and the changes in the potential energy landscape are correlated. The sorbate–zeolite system is characterized by a flatter potential energy landscape when the sorbate size is large.

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