Characteristics Of Adsorption Of Simple Gases On Graphitized Carbon Surfaces

The study of adsorption of simple gases on a graphitized carbon surface plays an important role in the adsorption area. Many theories in adsorption area are based on the assumption of the adsorption on a homogeneous surface. Experimentally, many works have been done for the investigation of the adsorption behavior of various gases on the graphitized surface. Also, in nowadays, with the availability of the powerful computation resources the molecular simulation has been used widely as a useful tool for the studying of the adsorption behavior. Through simulation, knowing the microscopic information mechanism of various adsorption phenomena becomes possible. However, before any attempt at modeling adsorption in a heterogeneous system, a successful description of the adsorption behavior of a fluid on a graphitized surface should be achieved as the stepping stone.A vital parameter for the successful modeling of the adsorption behavior is the model for the calculation of potential energy in the system, which includes the potential energy between fluid-fluid and fluid-solid interactions. The effects of adsorption potentials of argon and methane on their adsorption behavior on a graphitized carbon surface were studied with the aid of the Grand Canonical Monte Carlo simulation. Two commonly used intermolecular potential equations are been considered: the Lennard-Jones 12-6 and Buckingham Exp-6. The interaction energy between a fluid particle and a flat carbon surface for the LJ 12-6 and Exp-6 equations is described by the Steele 10-4-3 equation and the Crowell-Chang equation, respectively. The main difference between the Lennard-Jones 12-6 and Buckingham Exp-6 model is the repulsive component for the potential energy. In Exp-6 equation, it is known to have a theoretical basis while for LJ 12-6 model the repulsion term is set mainly due to the computation simplicity rather than theoretical deduction. Since the Buckingham Exp-6 has been claimed to be a better equation to describe bulk properties, such as the vapor-liquid equilibria, it is essential to investigate the role of the Crowell-Change equation in its description of adsorption and its comparison with the Steele equation.For the simplest adsorption system, i.e. the adsorption of argon on graphitized thermal carbon black, when comparing the simulation results with the experimental data, it has been found the simulation failed to capture exactly the features that been observed experimentally. The reason for that is the commonly used models do not account for the nonadditivity between the fluid-fluid interaction and the solid-fluid interaction. The concept of surface mediation is well known with its effects on the intermolecular interaction of adsorbed molecules close to the surface. To evaluate the effects of the surface mediation on the adsorption behavior, the adsorption isotherm and isosteric heat of argon adsorption on graphitized thermal carbon black over a range of temperatures are been tested with a combined experimental measurement and Grand Canonical Monte Carlo simulation. By matching the simulation results against the experimental data, we have found that the surface mediation is extended up to the fourth layer, rather than only the first as suggested by Kim and Steele, and the extent of this mediation is reduced with distance from the surface. This reinforces the important role of surface on the intermolecular interaction. With regard to the heat of adsorption, we found that the isosteric heats obtained directly from the simulation agree fairly well with the heats calculated from the application of Clausius-Clapeyron equation on experimental isotherms of 77 and 87.3 K. The temperature dependence of the isosteric heat was investigated with the GCMC simulation results. One interesting observation is the existence of a heat spike at 77 K and its absence at higher temperatures, a phenomenon which is common to both simulation results and experimental data. This lends good support to the molecular model with surface mediation as a proper one to describe adsorption of simple gases on the graphitized thermal carbon black.Being confident with the models for the simulation of simple gases on the graphitized thermal carbon black, an extensive computer simulation of argon adsorption on a graphite surface over a wide range of temperatures, from below the triple point to well above the critical point are presented. Adsorption over such a wide temperature range has not been reported previously, in the form of adsorption isotherms and enthalpy change in adsorption. The adsorption isotherms can be classified broadly into four categories: Below the triple point, the isotherms show stepwise character (strict layering mechanism) with 2D condensation; the Type II (according to the IUPAC classification) is followed by isotherms at temperatures above the triple point and below the critical point and a sharp spike is seen for isotherms in the neighborhood of the critical point; and finally the typical behavior of a maximum is observed for isotherms above the critical point. For the isosteric heat, the heat curve (plotted against loading) remains finite for subcritical conditions, but is infinite (singularity) at the maximum in excess loading for supercritical adsorption. For the latter case, a better representation of the energy change is the use of the integral molecular enthalpy, as this does not exhibit a singularity as in the case of isosteric heat. The differential and integral molecular enthalpies for the subcritical and supercritical adsorptions are compared.