The restricted diffusion of nickel etioporphyrin is experimentally measured in six granular catalyst support materials. These materials include five γ-aluminas and one small-pore silica-alumina. Pore size distribution is narrow for three of the supports and roughly bimodal for the remaining three. Diffusivities are determined by a transient uptake technique utilizing a Freundlich equilibria adsorption isotherm.
Experimentally determined diffusivities agree well with restricted diffusion theory based on a circular pore model when the appropriate characteristic length scales are used to describe the molecule and pore size. For the restricted diffusion of dilute solute molecules in neutral pores, the appropriate measure of granular pore size is the hydraulic pore radius. Molecule size, in this case, is best described by a mean projected molecule radius for partitioning and a Stokes-Einstein radius for hydrodynamic aspects of restricted diffusion.