Henry constant and isosteric heat at zero-loading for gas adsorption in carbon nanotubes

Abstract

The Henry constant and the isosteric heat of adsorption at zero loading in a carbon nanotube bundle are studied with Monte Carlo integration for the adsorption of gases over a range of temperatures. The spacing between nanotubes in a bundle is determined from the minimization of potential energy of interaction between these tubes. We study different tube configurations with bundles of 2, 3, 4 and 7 tubes. Depending on the configuration it is found that the spacing is of between 0.31 to 0.333 nm, and this falls within the range reported in the literature. The Henry constant has been carefully defined so that it will not become negative at high temperatures. This is done with the aid of accessible volume, rather than the usual absolute void volume. We show that linearity of the van't Hoff plot for the Henry constant is not strictly followed. Furthermore the slope of this plot is not equal to the isosteric heat of adsorption at zero loading, which is found to be a strong function of temperature. From the results we find that the Henry constant and the heat of adsorption depend on the tube configuration. In general the adsorption in the cusp interstices is strongest followed by that inside the tube and finally on the outer surface. However for very small tubes adsorption occurs inside the tube first. For molecules with orientation, the behaviour is even more interesting and the shape of the isosteric heat versus temperature depends on the degree of orientation, tube configuration and the domain of adsorption (interstices, inside the tube and on the outer surface).

The role of accessibility in the characterization of porous solids and their adsorption properties

“…The question that one has to raise is: how would one benefit from this accessible volume, should we decide to use it? Do et al (2008bDo et al ( , 2008c have shown that by using the accessible volume the Henry constant is always positive (which should be the case if one would expect it to have physical significance). The surface excess of adsorption of argon on a graphitic surface is calculated from:…”

“…Details of this analysis are given in Do et al (2008cDo et al ( , 2008dDo et al ( , 2008e, 2009. For a given volume, , containing solid atoms and void space in which adsorption is taking place, the interaction energy between a fluid particle placed at a point r and all solid atoms be φ(r).…”

“…A derivation for the isosteric heat at zero loading in the grand canonical ensemble has been provided by Do et al (2008b). Figure 5 shows the plot of the isosteric heat at zero loading versus temperature for adsorption of argon inside a single carbon nanotube (Do et al 2008c). It is seen that the heat calculated using the helium void volume (inset) at high temperatures is too high to be physically realistic and it even becomes negative for higher temperatures.…”

A novel and consistent method (TriPOD) to characterize an arbitrary porous solid for its accessible volume, accessible geometrical surface area and accessible pore size

“…It is known that helium adsorbs, especially in pores of molecular dimensions, and if its adsorption is neglected the gas phase volume is over estimated. This overestimation could compromise the determination of the Henry constant and isosteric heat (Do et al 2008b(Do et al , 2008c.…”

Fundamental study of adsorption and desorption process in porous materials with functional groups

“…In simulation, the intrinsic Henry constant can be defined as the ratio of the excess amount to the bulk gas density, which is given by [117][118][119] :…”