Adsorption phenomena are increasingly studied by computer molecular simulation because it can be used to minimize the experimentation effort or to predict adsorption isotherms at temperatures other than those used in experiments. Activated carbon represents an important group of adsorbents because of its desirable properties for a broad range of applications, including purification of gases and liquids, membrane technology, catalysis, energy storage and environmental technology. Traditionally molecular simulations of activated carbons rely on the modelling of pore as a parallel pair of infinite graphite layers. In reality pores are neither infinite nor uniform and they contain functional groups, chemical impurities and defects on the basal graphene layers. Therefore to describe correctly the physical adsorption on activated carbon, a real carbon pore of finite length and carbon surfaces as graphene layers with structural and chemical heterogeneities should be taken into account. The aim of this thesis is to extend the understanding of the effects of pore geometries and surface heterogeneities on the adsorption of polar and non-polar fluids in porous carbons.
This thesis investigates the adsorption behaviour of finite length pores using various Monte Carlo simulation techniques (a Canonical (NVT), a Gibbs (GEMC) and a Grand Canonical (GCMC), ensembles). First, the behaviour of non-polar molecules such as argon and methane is investigated. The sub-critical adsorption of these species in a finite pore is significantly different from that in an infinite slit pore, in terms of capacity and hysteresis loop. The slant hysteresis loop observed for finite pores is very similar to hysteresis loops typically observed for activated carbons. The placement of high energy sites and pore constrictions also alters the adsorption behaviour. Basically it shifts the onset of adsorption to lower pressure and the adsorption isotherms for these heterogeneous pores are generally greater than that for corresponding homogeneous pores. In the case of pore constriction, the adsorbed phase is generally started by forming the initial two contact layers at the constriction then at the larger cavity and subsequently filling the inner cores. To model non-graphitized surface, a defective surface model is
used and it is found to be an excellent model to describe argon and nitrogen adsorption on Cabot Non-Graphitized Carbon Blacks.
In addition to non-polar molecules, the adsorption of water in finite pores has also been studied. The adsorption of water on activated carbon is very complex, due to the strong hydrogen bonding, and it depends on the concentration and position of the functional group. The onset of adsorption shifts to lower pressure when the concentration of functional group increases or when the functional group is positioned at the centre of the graphene surface. In all cases investigated, the hysteresis loop always exists, and the loop size depends on the concentration of functional group and its position. Similar to the chemical heterogeneity due to functional group, the structural defects also have significant effects on the adsorption isotherm in shifting the pore filling to a lower pressure when it is located at a position away from the pore entrance. We found that the molecular simulation results agree well with the experimental data of a commercial activated carbon when the model porous solid is composed of pores having widths in the range between 8 and 30 Å and the functional groups positioned at the centre of the graphitic wall, and the simulated isotherm of water on a heterogeneous surface can describe the behaviour of water on Graphitized Thermal Carbon Black (GTCB) satisfactorily.
For both cases of non-polar and polar fluids, we particularly investigated the effects of curvature on the behaviour of adsorption isotherms. For the homogeneous cylinder, the pore filling occurs at a pressure lower than the saturation pressure while it is greater in the case of homogeneous slit pore of a comparable size. The size of hysteresis loop is more sensitive to the length of cylinder than that of slit, and it increases with a decrease in pore length. In the case of argon adsorption in homogeneous cylinder, the curvature effects lead to an early onset of adsorption isotherm and a lower amount at saturation (P → P0).