Ethane/ethylene adsorption on carbon nanotubes: temperature and size effects on separation capacity.

We present the results of Monte Carlo simulations of the adsorption of single-component ethane and ethylene and of equimolar mixtures of these two gases on bundles of closed, single-walled carbon nanotubes. Two types of nanotube bundles were used in the simulations: homogeneous (i.e., those in which all the nanotubes have identical diameters) and heterogeneous (those in which nanotubes of different diameters are allowed). We found that at the same pressure and temperature more ethane than ethylene adsorbs on the bundles over the entire range of pressures and temperatures explored. The simulation results for the equimolar mixtures show that the pressure at which maximum separation is attained is a very sensitive function of the diameter of the nanotubes present in the bundles. Simulations using heterogeneous bundles yield better agreement with single-component experimental data for isotherms and isosteric heats than those obtained from simulations using homogeneous bundles. Possible applications of nanotubes in gas separation are discussed. We explored the effect of the diameter of the nanotubes on the separation ability of these sorbents, both for the internal and for the external sites. We found that substrate selectivity is a decreasing function of temperature.

Study of a butane monolayer adsorbed on single-walled carbon nanotubes.

We present the results of a study of first-layer butane films adsorbed on single-walled carbon nanotubes. We measured 12 isotherms between 180 and 311 K. Butane molecules bind more strongly than shorter alkanes to the nanotubes. We measured a value of 391 meV for the low-coverage isosteric heat of butane. This value is 1.21 times larger than that for butane adsorbed on planar graphite and 1.27 times larger than the value for ethane on nanotubes at comparable coverages. We also compared the characteristics of the adsorption isotherms for butane with those we determined for ethane at the same relative temperatures. This comparison allowed us to infer that there is a change in the adsorption behavior of linear alkanes which occurs as a function of increasing carbon chain length. While ethane isotherms display two substeps in the first layer (corresponding to adsorption on different groups of adsorption sites), one of these steps is significantly smeared for butane isotherms, becoming essentially impossible to resolve above 220 K.

Adsorption of xenon on purified HiPco single walled carbon nanotubes.

We have measured adsorption of xenon on purified HiPco single-walled carbon nanotubes (SWNTs) for coverages in the first layer. We compare the results on this substrate to those our group obtained in earlier measurements on lower purity arc-discharge produced nanotubes. To obtain an estimate for the binding energy of Xe, we measured five low-coverage isotherms for temperatures between 220 and 260 K. We determined a value of 256 meV for the binding energy; this value is 9% lower than the value we found for arc discharge nanotubes and is 1.59 times the value found for this quantity on planar graphite. We have measured five full monolayer isotherms between 150 and 175 K. We have used these data to obtain the coverage dependence of the isosteric heat. The experimental values obtained are compared with previously published computer simulation results for this quantity.