Adsorption of Lennard-Jones Fluids in Carbon Slit Pores of a Finite Length. A Computer Simulation Study

Abstract

ABSTRACT:The adsorption of simple Lennard-Jones fluids in a carbon slit pore of finite length was studied with Canonical Ensemble (NVT) and Gibbs Ensemble Monte Carlo Simulations (GEMC). The Canonical Ensemble was a collection of cubic simulation boxes in which a finite pore resides, while the Gibbs Ensemble was that of the pore space of the finite pore. Argon was used as a model for Lennard-Jones fluids, while the adsorbent was modelled as a finite carbon slit pore whose two walls were composed of three graphene layers with carbon atoms arranged in a hexagonal pattern. The Lennard-Jones (LJ) 12-6 potential model was used to compute the interaction energy between two fluid particles, and also between a fluid particle and a carbon atom.Argon adsorption isotherms were obtained at 87.3 K for pore widths of 1.0, 1.5 and 2.0 nm using both Canonical and Gibbs Ensembles. These results were compared with isotherms obtained with corresponding infinite pores using Grand Canonical Ensembles. The effects of the number of cycles necessary to reach equilibrium, the initial allocation of particles, the displacement step and the simulation box size were particularly investigated in the Monte Carlo simulation with Canonical Ensembles. Of these parameters, the displacement step had the most significant effect on the performance of the Monte Carlo simulation. The simulation box size was also important, especially at low pressures at which the size must be sufficiently large to have a statistically acceptable number of particles in the bulk phase. Finally, it was found that the Canonical Ensemble and the Gibbs Ensemble both yielded the same isotherm (within statistical error); however, the computation time for GEMC was shorter than that for canonical ensemble simulation. However, the latter method described the proper interface between the reservoir and the adsorbed phase (and hence the meniscus).

Pore Size Distribution of Carbon with Different Probe Molecules

“…The adjacent carbon-carbon distance is 1.42 Å [6]. Figure 1 shows the schematic diagram of a carbon pore, which is finite in length in the x and y directions [7] In this study all graphene layers are assumed to be square and equal in size, and they have a linear dimension of 60 Å, because it is reported in the literature [5] that the size of the graphene layer is between 20 and 70 Å. We choose values for pore width from 6.3 to 30 Å, to represent the micropore and mesopore in activated carbons.…”

“…In the larger pores, we can observe the presence of meniscus; the shape of meniscus is cylindrical and becomes flat at pressures close to the vapor pressure. It is noted that a study of meniscus is not possible with the simulation of infinite pores 7 . In the case of CO 2 the snapshots also support that the greater adsorption occurs at lower temperature at the same pressure and width.…”

“…However, the infinite pore model is too ideal to reflect the adsorption behavior of activated carbon whose length is finite [5] and it contains functional groups on the basal graphene layers [6]. Therefore a real carbon pore of finite length and carbon surfaces as graphene layers comprising of carbon atoms arranged in a hexagonal pattern [7] is used in this study to model adsorption behavior of nitrogen and carbon dioxide.…”

The effects of energy sites on adsorption of Lennard–Jones fluids and phase transition in carbon slit pore of finite length a computer simulation study

“…While in the case of finite pores, molecules are confined within the simulation box, and if a particle is displaced to a new position which is outside the box that move is rejected [17]. This simulation box is the same as that used in our previous study with the Gibbs ensemble [18]. The ideal gas equation of state is used to determine the pressure of the bulk gaseous phase corresponding to a given chemical potential.…”

On the influence of heterogeneity of graphene sheets in the determination of the pore size distribution of activated carbons

“…Unfortunately, real surfaces are far from that ideal situation, and assuming a perfect surface to study adsorption in pores could lead to serious errors in the determination of adsorption isotherms (Do and Do 2006). Several authors have been trying to introduce defects in an explicit way in their simulations, e.g., edge sites (Seaton et al 1997), pore wall heterogeneity with variable number of layers (Nguyen and Bhatia 2004) randomly oriented crystallites (Segarra and Glandt 1994), rumpled graphite surfaces (Bakaev 1995), pores of finite length (Wongkoblap et al 2005), rough amorphous carbon surfaces (Neimark et al 2009) and reverse Monte Carlo carbon reconstruction (Thomson and Gubbins 2000). More recent works have also included heterogeneity in the geometric shape of the pores .…”