Anomalous and Single-File Diffusion for Fluids Confined in Microporous Carbons: Activated Carbon, Carbon Replicas, and Carbon Nanotube Systems

Microporous carbons have found widespread use in industry due to their low production cost and high adsorption capacity.  Disordered carbons, such as activated carbon, have been used extensively in environmental applications as adsorbents for the removal of toxic and odorous industrial chemicals.  A second class of materials which has had recent interest are carbon templates of silica materials such as MCM-41, SBA-15 or various zeolites¹.  This type of material is inherently highly ordered.  These materials have shown potential application in the areas of hydrogen storage and in the development of hydrogen fuel cells².  Realistic, atomically detailed models of these systems have previously been constrictued^3,4.

Using Grand Canonical Monte Carlo (GCMC) and equilibrium molecular dynamics (EMD) simulations, we have compared the adsorption and diffusion properties of fluids confined in realistic models of activated carbon with those of carbon replicas of zeolites.  Both materials exhibit a maximum in the diffusion coefficient in the pore filling region of the isotherm as well as an anomalous (e.g., t^1/2 dependence of the mean squared displacement) for short times followed by a normal diffusive (Fickian) region. In order to explain the seemingly dual mode diffusion mechanism in these materials, we compare to diffusion of fluids confined in carbon nanotube systems (isolated carbon nanotubes and carbon nanotube bundles) which have distinct, bi-modal diffusion mechanisms.