Single-File and Anomalous Diffusion in Porous Carbons

The aim of this project is to obtain a fundamental understanding of single-file and anomalous diffusion in porous carbons. For this purpose we report studies for four different systems: (1) the diffusion of argon in isolated carbon nanotubes; (2) the diffusion of argon/neon, argon/krypton, and argon/xenon mixtures in isolated carbon nanotubes; (3) the diffusion of argon in hexagonally arranged carbon nanotube bundles; and (4) the diffusion of argon in a disordered model of activated carbon and a carbon replica of zeolite. In (1) we find that argon exhibits a mean-squared displacement with a square root of time dependence when the nanotube is small enough in diameter that molecules cannot pass each other. In (2) we find that argon/neon and argon/xenon mixtures exhibit bimodal diffusion in some diameters of carbon nanotubes, where the larger component diffuses in single-file with a square root of time dependence of the mean-squared displacement and the smaller component by a much faster Fickian mechanism with a linear time dependence of the mean-squared displacement. In (1) and (2) the square root of time dependence is observed due to the influence of a stochastic thermostat, which mimics diffusive reflection between the adsorbate atoms and the pore wall. In (3) we observe a square root of time dependence for argon diffusing between carbon nanotubes in the interstitial sites using only microcanonical simulations. Natural corrugation results from the outside walls of the carbon nanotubes. We also show that one-dimensional diffusion within atomically detailed carbon nanotubes can result in artifacts in the simulation including size correlations and center of mass drift which can only be fully corrected for in the limit of infinite size. When this correction is made, influences including the pore flexibility become negligible for the diffusion of argon. Finally in (4), we observe anomalous, slower modes of diffusion within activated carbon due to argon atoms being trapped within small, highly attractive pores. This is most observable at low relative pressures and short times. In a carbon replica of zeolite, we find that anomalous regions appear at high relative pressures due to argon atoms competing to diffuse through windows and constrictions within the material.