Atichat Wongkoblap

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

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).

Adsorption of polar and non-polar fluids in carbon nanotube bundles: Computer simulation and experimental studies

The effects of adsorbate on the adsorption in a bundle of carbon nanotubes are investigated to explore the preferential adsorption over various adsorption sites: inside the tube, in the cusp interstices and in the square interstices outside the tubes. This is carried out with the Grand Canonical Monte Carlo simulation and the simulation results are tested against the experimental results of bundles of single wall carbon nanotubes (SWCN). With regard to adsorbate, we choose argon and nitrogen to represent simple fluids and water to represent strong associating fluids with strong orientation interaction. The preferential adsorption of argon and nitrogen depends on the tube size. For tube size smaller than 10.8 A, adsorption inside the tube is preferred because the solid-fluid potential is greatest in the tube interiors. While for larger tubes adsorption occurs initially in the small cusp interstices between the tubes, and as adsorption is progressed adsorption occurs inside the tube as well as the larger square interstices. At higher pressures capillary condensation occurs in the square interstices. For water, however, the adsorption mechanism is different. Its adsorption occurs dominantly inside the tube, irrespective of the tube size. This is due to the requirement of appropriate geometry to allow hydrogen bonding among water molecules to occur. The small cusp interstices do not provide proper space for clusters of hydrogen bonded waters, while the larger square interstices are too large and hence the solid-fluid potential is not strong enough to induce adsorption unless the partial pressure is sufficiently high. Finally the model of these fluids and carbon nanotube is tested with the experimental data of a commercial SWCN, and the simulation results are in agreement with the data.

The effects of curvature and surface heterogeneity on the adsorption of water in finite length carbon nanopores: a computer simulation study

The effects of pore curvature and surface heterogeneity on the adsorption of water on a graphitic surface at 298 K were investigated using a Grand Canonical Monte Carlo (GCMC) simulation. Slit and cylindrical pores are used to study the curvature effects. To investigate the surface heterogeneity the functional group and the structural defect on the surface were specifically considered. The hydroxyl group (OH) is used as a model for the functional group and the water potential model proposed by Müller et al. is used to calculate the water interaction. For the homogeneous cylinder, the pore filling occurs at a pressure lower than the saturation pressure of the water model, while it is greater in the case of homogeneous slit pore. The size of hysteresis loop is more sensitive to the length of cylinder than that of the slit, and it increases with decreasing pore length. The isotherms of water in cylindrical pores are found to depend on the position and the concentration of the functional group. The pore filling pressure is lower with an increased number and/or with the position of the functional group. The structural defect shows significant effects on the adsorption isotherm in shifting to a lower pore filling pressure when it is located at a position away from the pore entrance. The adsorption of water on the heterogeneous surface was studied and it was found that the simulated isotherm can describe the behaviour of water on Graphitized Thermal Carbon Black (GTCB) satisfactorily. The water cluster grows mostly along the surface for the case of finite extent surface, while for the slit the pore grows in all directions but the preference is a direction perpendicular to the pore wall. Reasons for the direction of growth will be discussed.

ADSORPTION OF POLAR AND NONPOLAR FLUIDS IN FINITE-LENGTH CARBON SLIT PORE: A MONTE CARLO SIMULATION STUDY

A grand canonical Monte Carlo simulation (GCMC) method is used to study adsorption in a carbon slit pore of finite length. Methane is used as a model for the nonpolar molecule, while the polar molecule is modeled as water. The behavior of a nonpolar molecule in the finite-length pore is investigated and compared with that obtained for a commonly used infinite slit pore. The adsorption of methane in the finite pore is significantly different from that in the infinite pore, both the capacity and the hysteresis loop. In addition to nonpolar molecules, the adsorption of water in finite pores has also been studied. The adsorption of water on activated carbon is very complex, and it strongly depends on the concentration as well as the position of the functional groups in the pore. For both cases of nonpolar and polar fluids, effects of pore size on the behavior of adsorption isotherms are also investigated.

Effect of pore constriction on adsorption behaviour in nanoporous carbon slit pore: a computer simulation

A Grand Canonical Monte Carlo simulation (GCMC) method is used to study the effects of pore constriction on the adsorption of argon at 87.3 K in carbon slit pores of infinite and finite lengths. It is shown that the pore constriction affects the pattern of adsorption isotherm. First, the isotherm of the composite pore is greater than that of the uniform pore having the same width as the larger cavity of the composite pore. Secondly, the hysteresis loop of the composite pore is smaller than and falls between those of uniform pores. Two types of hysteresis loops have been observed, irrespective of the absence or presence of constriction and their presence depend on pore width. One hysteresis loop is associated with the compression of adsorbed particles and this phenomenon occurs after pore has been filled with particles. The second hysteresis loop is the classical condensation–evaporation loop. The hysteresis loop of a composite pore depends on the sizes of the larger cavity and the constriction. Generally, it is found that the pore blocking effect is not manifested in composite slit pores, and this result does not support the traditional inkbottle pore hypothesis.

Effects of Graphene Layer Size on the Adsorption of Fluids on Graphitized Thermal Carbon Black. A Computer Simulation Study

The adsorption of Lennard-Jones fluids (argon and nitrogen) onto a graphitized thermal carbon black surface was studied with a Grand Canonical Monte Carlo Simulation (GCMC). The surface was assumed to be finite in length and composed of three graphene layers. When the GCMC simulation was used to describe adsorption on a graphite surface, an over-prediction of the isotherm was consistently observed in the pressure regions where the first and second layers are formed. To remove this over-prediction, surface mediation was accounted for to reduce the fluid–fluid interaction. Do and co-workers have introduced the so-called surface-mediation damping factor to correct the over-prediction for the case of a graphite surface of infinite extent, and this approach has yielded a good description of the adsorption isotherm. In this paper, the effects of the finite size of the graphene layer on the adsorption isotherm and how these would affect the extent of the surface mediation were studied. It was found that this finite-surface model provides a better description of the experimental data for graphitized thermal carbon black of high surface area (i.e. small crystallite size) while the infinite-surface model describes data for carbon black of very low surface area (i.e. large crystallite size).