Adsorption and phase transitions on nanoporous carbonaceous materials: insights from molecular simulations

Abstract

Three different examples of adsorption and phase transitions on nanoporous carbonaceous materials are investigated using molecular simulations. In the first example, N2 adsorption on the surface of and within a C60 crystal is studied using the grand canonical Monte Carlo simulation. On the crystal surface, the predicted isotherm is of type II in good agreement with experiment, and N2 molecules are found to initially occupy the octahedral voids, then the tetrahedral voids, and finally on the top of the C60 molecules with increasing coverage. Within the crystal, the predicted isotherm is of type I, and N2 molecules intercalate the octahedral voids. In the second example, N2 adsorption on two types of carbon nanotube bundles is studied using the Gibbs ensemble Monte Carlo simulation to explore the role of the external surface of the nanotube bundle in the character of adsorption isotherm. The isotherm for an infinite bundle without an external surface is found to be of type I independent of temperature, while for a finite bundle with an external surface the isotherm changes from type II to I as temperature changes from subcritical to supercritical. In the third example, the capillary phase transitions of n-alkanes (from C1 to C8) in a carbon nanotube are studied using the gauge-cell Monte Carlo simulation implemented with the configurational-bias technique. At subcritical temperatures the isotherms exhibit sigmoid van der Waals loops, including stable, metastable, and unstable regions, and the coexisting vapor−liquid phases are determined from a Maxwell construction along the isotherms. The confinement of an n-alkane in a carbon nanotube decreases its critical temperature, increases its critical density, and narrows its binodal curve.