ADSORPTION OF SIMPLE AND COMPLEX FLUIDS ON NON-GRAPHITIZED CARBON BLACK AND ACTIVATED CARBON - TRANSITION FROM SUB-CRITICAL TO SUPERCRITICAL ADSORPTION AND A NEW METHOD TO DETERMINE PORE SIZE DISTRIBUTION OF ACTIVATED CARBON

摘要

Description of adsorption on carbonaceous surfaces and in porous carbon requires first a thorough understanding of adsorption of simple and complex gases on a homogeneous graphite surface. The relative homogeneity of the surface of a highly graphitized carbon black makes it a perfect model to test the potential of any adsorption model for its capability in the prediction of adsorption in more complicated carbon structure. By starting with perfect homogeneous surface allows one to delineate the surface effects from the others such as the confinement effects, the defects on the surface and the effects of impurities (functional groups). The test of adsorption model for graphitized thermal carbon black is aided with the numerous excellent data in the literature for simple as well as complex gases. Among the many adsorption models that have been proposed in the literature, the ones that have foundation in molecular interactions are the best in terms of probing the molecular origins of adsorption. In this talk I will concentrate on the molecular models using Monte Carlo simulation as this is more suitable than others (such as density functional theory and molecular dynamics) in obtaining information on adsorption equilibria. The adsorption molecular models require mainly the detailed information of interaction between molecules, and if that information is accurately available the Monte Carlo simulation should provide a full description of adsorption. We will test this assertion with adsorption of noble gases first, and although the description of isotherm is reasonably adequate we still need to consider in a subtle manner the way how the surface would interact with fluid particles and the way how the surface would influence the interactions among the fluid particles. This surface influence will then be studied with some complex fluids, and I will illustrate this with some commonly used gases, such as nitrogen, carbon tetrachloride and benzene. Complex gases tend to show a reasonably complex behaviour in adsorption isotherm and isosteric heat. Finally I will consider the strongly associating fluids such as those exhibiting strong hydrogen bonding, for example water, methanol and ammonia. Their adsorption behaviour is drastically different from that of simple fluids, and with the use of molecular simulation we are able to have a deeper insight into the way how molecules interact with the surface. The strong association of fluid particles highlight the significance of functional groups (or impurities), no matter how low the concentration of the functional group is. These functional groups are undetectable with the use of simple fluids. The association between fluid particles is competing against the association between the functional group and the fluid particle, and in this respect, the adsorption isotherm and the isosteric heat behave differently from those observed with simple gases. Next, the various aspects of adsorption under supercritical conditions will be highlighted with the aid of molecular simulation. Once we understand how fluids behave on a homogeneous graphite surface, I will discuss how adsorption is affected when surface has defects. A model for defective surface is developed for non-graphitized carbon black, and it has been shown that this model can describe reasonably well the various features of carbon black with varying degrees of graphitization. Finally I talk about the pore size distribution and present a new method to derive the pore size distribution.