Adsorption, structure and dynamics of fluids in ordered and disordered models of porous carbons

Abstract

Grand Canonical Monte Carlo and Molecular Dynamics simulations are used to investigate the adsorption and dynamics of argon in ordered and disordered models of porous carbons. The ordered porous carbon (model A) is a regular slit pore made up of graphene sheets. The disordered porous carbon (model B) is a structural model that reproduces the morphological (pore shape) and topological (pore connectivity) disorders of saccharose-based porous carbons. Three pore widths, H = 7, 11, and 15 Å, are selected for model A; they correspond to the smaller, mean, and larger pore sizes of model B. The filling pressures for the graphite slit pores are lower than those for the disordered porous carbon. It is also found that model A is not able to capture the behaviour of the isosteric heat of adsorption of model B. For all pressures, the confined phase in model A is composed of well-defined layers, which crystallize into hexagonal 2D crystals after complete filling of the pores. In contrast, the structure of argon in the disordered porous carbon remains liquid-like overall. It is also found that the slit pore model cannot reproduce the dynamics of argon in the disordered porous carbon. While the self-diffusivity of argon in model A decreases with increasing loading, it exhibits a maximum for model B. Such a non-monotonic behaviour of the self-diffusivity for the disordered porous carbon can be explained by the surface (energetic) heterogeneities of the material.

Acknowledgements

We thank the National Science Foundation (grant no. CTS-0211792) and the U.S. Department of Energy (grant DE-FG02-98ER14847) for support of this research. The Laboratoire de Physicochimie de la Matiere Condensee is UMR 5617 of Centre National de la Recherche Scientifique (CNRS, France). We also thank Roland Pellenq (CRMCN, Marseille, France) for very helpful discussions. Pierre Sans (LPMC, Montpellier, France) and Dominique Caron (LCVN, Montpellier, France) are gratefully acknowledged for their help with computer resources.