Aldo D. Migone

Adsorbate binding energy and adsorption capacity of xenon on carbon nanohorns

Xenon adsorption studies were performed on aggregates of single-walled carbon nanohorns (SWNHs). The SWNHs were prepared in two different batches; the adsorption results from the two batches were essentially identical. Isotherms were performed in two groups of experiments: low-coverage data were used to measure the binding energy of Xe on the SWNHs; and full isotherms were measured to determine specific surface areas. The binding energy value for Xe on the SWNHs was intermediate between those for Xe on single-walled carbon nanotubes, and that for Xe on graphite. The specific surface area of the SWNHs was of the order of 250 m2 g-1.

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.

Neon and CO2 adsorption on open carbon nanohorns.

We present the results of a thermodynamics and kinetics study of the adsorption of neon and carbon dioxide on aggregates of chemically opened carbon nanohorns. Both the equilibrium adsorption characteristics, as well as the dependence of the kinetic behavior on sorbent loading, are different for these two adsorbates. For neon the adsorption isotherms display two steps before reaching the saturated vapor pressure, corresponding to adsorption on strong and on weak binding sites; the isosteric heat of adsorption is a decreasing function of sorbent loading (this quantity varies by about a factor of 2 on the range of loadings studied), and the speed of the adsorption kinetics increases with increasing loading. By contrast, for carbon dioxide there are no substeps in the adsorption isotherms; the isosteric heat is a nonmonotonic function of loading, the value of the isosteric heat never differs from the bulk heat of sublimation by more than 15%, and the kinetic behavior is opposite to that of neon, with equilibration times increasing for higher sorbent loadings. We explain the difference in the equilibrium properties observed for neon and carbon dioxide in terms of differences in the relative strengths of adsorbate-adsorbate to adsorbate-sorbent interaction for these species.

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.

Methane Adsorption on Planar WS2 and on WS2-Fullerene and -Nanotube Containing Samples

Adsorption-desorption cycles were measured for methane on non-irradiated WS2, and on irradiated WS2 (which contained, in part, WS2 fullerenes and nanotubes). Both types of samples were further subdivided into three sets: one set received no further treatment, another set was heated under vacuum, and the last set was acid-treated and heated. The specific surface area was determined for each set; so was the presence or absence of a hysteresis loop in the adsorption-desorption cycles. The results of these two groups of measurements were correlated with the space available for adsorption. The implications of the results for the experimental determination of the dimensionality of gas adsorbed at the interior of nanotubes are discussed.

Sorption Kinetics on Open Carbon Nanohorn Aggregates: The Effect of Molecular Diameter

We present the results of a study of the kinetics of adsorption on aggregates of open carbon nanohorns using argon and CF4 sorbates. We measured the equilibration times for each value of the sorbent loading along eight adsorption isotherms (four isotherms for each sorbate species). We found that: the equilibration times decrease as the sorbent loading (and the equilibrium pressure of the coexisting gas) increases, for a given temperature; and, that, for a given value of the sorbent loading, the equilibration times decrease with increasing temperature. When considering the effect of scaling of the temperatures by the respective critical temperatures we found that, at the same scaled temperature and at comparable loadings, the equilibration times for CF4 were longer than those for argon. We discuss a possible explanation for this result.

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.