Adsorption is a very important phenomenon because of the significant role it plays from industry applications to everyday life. For example, the isosteric heat of adsorption is a critical design variable in many industrial processes, because it governs the changes in the local adsorption equilibrium and kinetics, and thus, the over-all separation efficiency of the process. Therefore, understanding the adsorption mechanism on porous solids will provide valuable information for improving the industrial processes. Owing to the rapid development of computer technology, molecular simulation has become a pivotal tool in the study of adsorption mechanisms. This work mainly focuses on the characterization of porous solids as well as the fundamental aspects of the 2D and 3D transitions of confined fluid in different shapes of pores by using the Grand Canonical Monte Carlo (GCMC) molecular simulation. This thesis focuses mainly on the following topics.
(1) The isosteric heat of argon adsorption in MCM-41 and carbon nanotubes has been investigated using GCMC simulation. We find that the isosteric heat during the capillary condensation is practically constant irrespective of temperature, pore-curvature and surface strength. We also study the microscopic behaviour during capillary condensation. Finally we propose a simple model for the heterogeneous surface of MCM-41 which is able to describe the experimental isosteric heat of argon adsorption at 87.3K in MCM-41.
(2) In 2011, Morishige synthesized a porous carbon material that has regular and non-connected hexagonal pores with graphitic walls. Classical theories (BJH and DFT) are unsuitable for characterizing this material because the kernel of local isotherms is based on model cylindrical pores. So we have developed a new characterization based on a GCMC simulation of hexagonal pores that can account for the pore size, the surface morphology and the heterogeneity arising from either structural or energetic defects. The molecular model describes the adsorption isotherms and the isosteric heat at 77.4 and 87.3K over a wide range of pressure. This new material and the successful molecular model provides us with an ideal system to study adsorption mechanisms in a confined space, including the effects of confinement on the 2D-transition for temperatures below the triple point and the 3D-capillary condensation for temperatures below the critical point of the bulk fluid.
(3) Adsorption isotherms and isosteric heats have been studied experimentally and by computer simulation for the krypton-graphitic hexagonal pore and krypton-graphite planar surface systems in the 60-109K temperature range. The existence of a 2D transition in the sub-monolayer film on the basal plane of graphite that is observed experimentally is confirmed by the computer simulation results, but this transition is not observed in graphitic hexagonal pores because the onset of adsorption occurs at the junctions of adjacent pore walls, and the mechanism of surface adsorption is the spreading of adsorbate from the junction towards the basal planes until the first layer is completed. This is followed by molecular layering of higher layers, and then by capillary condensation when the empty core is small enough.
(4) Adsorption isotherms and heats of adsorption of argon in pores having cylindrical, hexagonal and triangular cross-sectional shape have been studied. The shape of the cross section has a strong effect in the sub-monolayer region because of potential overlap from adjacent surfaces lowers the adsorption potential minimum in the triangular pores and to a lesser extent in the hexagonal pores, giving rise to higher adsorption and adsorption heat. This is reflected in the higher pore densities and adsorption heats at low pressure for the triangular pore than for the cylindrical pore. However this does not affect the condensation/evaporation (CE) pressure or the form of the isosteric heat curve in the CE region. This is because condensation only occurs when molecular build-up has reached distances away from the surface where the solid-fluid interaction makes negligible contribution to the total interaction. The surface strength does however affect the pressures at which CE occurs. A weaker surface requires a higher pressure (chemical potential) to build a layer thick enough for condensation to occur. In addition, very weak surfaces can result in partial wetting of the surface and consequently, pores are not filled when the pressure approaches the saturation vapour pressure.
In summary, the various topics focus on the microscopic adsorption behaviours of confined fluids, and some worthwhile results have been achieved both in fundamental studies and the applications.