Characteristic heats of adsorption for slit pore and defected pore models.

The characteristics of the heat of adsorption from a slit pore model of carbon are presented. This is shown to have a few key features that are always present, regardless of the pore size distribution used, as long as there is a reasonable range of pore sizes considered. The adsorption in a slit pore model is compared against the adsorption for a defected pore model. The isotherms of the defected pore model are qualitatively different from those of the slit pore and similar to those of amorphous carbon models presented in the literature. The heat of adsorption of the defected pore model is qualitatively different from the slit pore model, and its behavior falls between those of the slit pore model and the amorphous carbon models in the literature.

Comparative simulation study of nitrogen and ammonia adsorption on graphitized and nongraphitized carbon blacks.

Grand canonical Monte Carlo simulation is used to study the adsorption of nitrogen at 77 K and ammonia at 240 K to represent weakly polar and polar molecules, respectively, on infinite and finite graphite surfaces. These graphite surfaces were modeled with different percentages of carbons removed (defects) from the top graphite layer. Increasing the number of defects increases the adsorption and the isosteric heat of nitrogen at low pressure. At moderate pressures the amount adsorbed is less due to the disruption in the packing of the nitrogen in the first layer. In contrast, the adsorption of ammonia at all pressures is reduced as the percentage of defects is increased. This is due to the disruption in ammonia bonding caused by the defects. The condensation-like step change in the ammonia isotherm on the perfect graphite surface is not observed for any of these surfaces with defects even for the case of only 10% defects. At high percentage of defects the adsorption isotherm is close to Henry law behavior for much of the pressure range. The adsorption on finite surfaces shows that the amount adsorbed for both molecules decreases compared with that of the infinite surfaces, resulting from interaction potentials with the surface and other fluid molecules at the edge. The decrease is much greater for the ammonia adsorption because the bonding between ammonia molecules is disrupted, meaning that the adsorption cannot follow the mechanism of condensation seen for the infinite surface.

On the heat capacity of adsorbed phases using molecular simulation.

The heat capacities of argon, ammonia, and methanol on carbon black at 87.3, 240, and 300 K, respectively, have been investigated. The carbon black surface has been modeled with and without carbonyl groups. Part of this investigation is a decomposition of the heat capacity into its contributions from the different interaction potentials of an adsorption system. All systems show a spectrum of heat capacity versus loading, and this behavior depends on the carbonyl configuration present on the surface. For methanol and ammonia the variation of the heat capacity between the two for the same carbonyl configurations is greater than the variation in the heat of adsorption. Heat capacities of methanol and ammonia are generally dominated by fluid-fluid interactions due to the strong association of fluid particles through hydrogen bonding. The difference in the heat capacity behavior of the two fluids is an indicator of their different clustering behaviors on the carbon black surface. The presence of carbonyl groups reduces the fluid-fluid contributions to the heat capacity. This is due to the compensation of fluid-fluid interactions with fluid-functional group interactions. At 87.3 K a first layer transition to a solidlike state is present for argon and results in a large peak in the heat capacity on a bare surface. The presence of functional groups greatly reduces this peak in the heat capacity by disrupting the packing of argon on the surface and preventing a transition to a solidlike state.

Correct procedures for the calculation of heats of adsorption for heterogeneous adsorbents from molecular simulation.

Several procedures for calculating the heat of adsorption from Monte Carlo simulations for a heterogeneous adsorbent are presented. Simulations have been performed to generate isotherms for nitrogen at 77 K and methane at 273.15 K in graphitic slit pores of various widths. The procedures were then applied to calculate the heat of adsorption of an activated carbon with an arbitrary pore size distribution. The consistency of the different procedures shows them to be correct in calculating interaction energy contributions to the heat of adsorption. The currently favored procedure for this type of calculation, from the literature, is shown to be incorrect and in serious error when calculating the heat of adsorption of activated carbon.