Characterization Of Nanoporous Materials Using Gas Adsorption Isotherms: Evaluating Their Potential For Gas Storage And Separation Applications

In order to find/design porous materials that could be used in practical applications involving adsorption, it is important to investigate the basic properties (i.e. isosteric heat, specific surface area, binding energy, pore size, pore volume, etc.) of each material. With this aim in mind we have looked at three different types of materials: single-walled carbon nanotubes (prepared by the HiPco and laser methods), single-walled nanohorns (dahlia-like and bud-like) and metal-organic frameworks (Cu-BTC and RPM-1). For these substrates we have measured volumetric adsorption isotherms using several gases such as neon, argon, tetrafluoromethane (CF4), xenon, and methane (not all gases for all substrates). Experimental adsorption isotherms were measured using methane, argon, xenon, and neon gases on unpurified single-walled carbon nanotubes prepared by the HiPco method. The main idea behind these experiments was to investigate, using different size gas molecules, the sites available for adsorption on this type of porous material. We found that surface area occupied by these adsorbates on the sample is the same, regardless of their size. This means that all the gases have access to the same group of adsorption sites. Since the biggest adsorbate in this experiment was Xe, and since it is unlikely that it could penetrate the interstitial channels in the nanotube bundles, we conclude that none of the gases, including the smallest one - Ne, are able to adsorb in the interstitial channels in bundles of single-walled carbon nanotubes. For the case of argon on laser produced single-walled carbon nanotubes we measured 21 adsorption isotherms using argon gas temperatures between 40 and 153 K that were used to determine the isosteric heat of adsorption for this system. Our experimental results were compared to the ones from computer simulations performed by J. K. Johnson (from the University of Pittsburgh) for the same gas on heterogeneous and homogenous bundles. It was observed that the isosteric heat data matches better with data computed for heterogeneous nanotube bundles. This indicates that at the lowest pressure and coverages argon might be adsorbing in the defect-induced interstitial channels. We studied Cu3(Benzene-1,3,5-tricarboxylate)2(H2O)3 (abbreviated as Cu-BTC) metal-organic framework with argon to determine the sites available for adsorption on this material. Volumetric adsorption isotherms were measured at temperatures between 66 and 143 K. We found two substeps in the isotherm data, indicating that there are two types of pores present in the material: tetrahedrally-shaped side pockets and the main channels. Our experimental results were compared with data from simulations conducted using the Grand Canonical Monte Carlo method. We determined that the theoretical results match reasonably well with ours if the coverage is scaled down by a factor of 1.6. We explored the potential of two different metal-organic framework materials (Cu-BTC and RPM-1) for gas separation application. We used argon and tetrafluoromethane (CF4) gases to check if this can be achieved through kinetic and steric mechanisms. We found that Cu-BTC has excellent potential in gas separation using a steric mechanism, since argon easily adsorbs into the small pores present in the sample, while CF4 is excluded from them. Adsorption properties of RPM-1 showed that it could be employed in gas separation using a kinetic mechanism - argon gas adsorbs and reaches equilibrium in the pores of the sample more than the order of magnitude faster than CF4. Closed-ended dahlia-like nanohorns were studied with neon and tetrafluoromethane gases. In the first layer of neon and tetrafluoromethane adsorbed on dahlia-like nanohorns we found two substeps. These results were compared with results of computer simulations performed by Prof. M. Calbi. We determined, after comparison with the simulation isotherms, that the lower pressure substeps correspond to adsorption of Ne and CF4 in the narrowest parts of interstitial channels of the aggregates. Surface area calculated from neon isotherms was found to be higher than the one obtained using CF4, meaning that the smaller Ne molecule has the access to the parts of the interstitial channels that are not accessible for the bigger CF4 molecule. Features that appeared in neon adsorption isotherms on bud-like nanohorn aggregates were quite different from the ones on dahlia-like aggregates. We measured neon adsorption isotherms on this type of sample at temperatures between 22 and 49 K. In the monolayer regime we observed one single substep whose origin we can not definitely identify, because the structure of the bud-like nanohorns is not well-known. The binding energy value that was calculated from the isotherm data was lower than the value for neon adsorbed in the grooves of nanotube bundles but higher than for neon on graphite