Activated diffusion in relaxed porous clusters

We study diffusion through disordered porous structures, where the size of the diffusing particle is of the order of the pore diameter, and focus on the effect of particle-wall interactions. Such 'configurational diffusion' is important in catalysis, separation, and membrane technology, yet it is still poorly understood. We consider three types of cluster cluster aggregation (CCA) structures using simulations carried out on a cubic lattice. Namely, (1) next-neighbor (classic) CCA, where each site may have up to six nearest neighbors; (2) CCA where each site may have up to 26 nearest neighbor in its first coordination shell; and (3) a relaxed CCA solid formed by a process of energy minimization. We find that at low interaction strengths, diffusivity through the various types of CCA structure of similar porosity is independent of the mode of preparation and can be well predicted. However, at high interaction strengths which inhibit the diffusivity considerably, clusters characterized by lower surface-to-volume ratio display lower diffusivities. We suggest a simple model that accounts for the dependence of the diffusivity on the interaction parameter.