Dinesh Rawat

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.

Dependence of single-walled carbon nanotube adsorption kinetics on temperature and binding energy.

We present results for the isothermal adsorption kinetics of methane, hydrogen, and tetrafluoromethane on closed-ended single-walled carbon nanotubes. In these experiments, we monitor the pressure decrease as a function of time as equilibrium is approached, after a dose of gas is added to the cell containing the nanotubes. The measurements were performed at different fractional coverages limited to the first layer. The results indicate that, for a given coverage and temperature, the equilibration time is an increasing function of E/(k(B)T), where E is the binding energy of the adsorbate and k(B)T is the thermal energy. These findings are consistent with recent theoretical predictions and computer simulations results that we use to interpret the experimental measurements.

Possible existence of a higher coverage quasi-one-dimensional phase of argon adsorbed on bundles of single-walled carbon nanotubes.

We present results of Ar adsorption isotherms at very low coverages in the first layer and, beyond monolayer completion, on bundles of close-ended single-walled carbon nanotubes. The low coverage results were used to determine the isosteric heat of adsorption and the binding energy of Ar in the groove sites in the first layer. The higher coverage results show evidence of the possible formation of a second-layer groove phase, beyond monolayer completion. Our results for higher coverages are compared with recent computer simulations for this system.

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.

Surface area measurements with linear adsorbates: an experimental comparison of different theoretical approaches.

The specific area of a substrate was determined from the results of adsorption isotherms performed with a sequence of four alkanes, from methane to butane, using three different approaches. The data were first analyzed using the BET equation and the point B methods; these results were compared with those obtained using a new equation designed for examining the case of multisite occupancy. The new model specifically accounts for sites that are left uncovered in the case of adsorption by linear adsorbates. Of these three, only the last method gives essentially the same value for the specific surface area of the substrate when different adsorbates are used to measure it. The other two, more traditional, approaches give values of the specific surface area that decrease as the length of the adsorbate used increases.

Ethylene Films Adsorbed onto Purified HiPco Single Walled Carbon Nanotubes: A Comparison with Ethane and Longer Alkanes

The results of adsorption isotherm measurements for ethylene on purified HiPco SWNTs conducted at 11 different temperatures (between 110 K and 220 K) are reported, together with a comparison with the results we have obtained for ethane on the same substrate. Two groups of distinct binding sites were observed for ethylene on bundles of carbon nanotubes; this is in agreement with previously reported observations for ethane on this same sorbent. In addition, a consistent value of the specific surface area of the substrate was obtained from the ethylene and ethane data measured at approximately the same temperature. We have also determined the coverage dependence of the isosteric heat of adsorption for ethylene on the HiPco nanotube bundles. The reported experimental isosteric heat results are compared with simulation results for ethane and ethylene.