Date of Award
Doctor of Philosophy
Five systems consisting of different sorbate-sorbent combinations were studied using experimental volumetric adsorption techniques. Multiple adsorption isotherms were measured at low temperatures and low pressures for all of the systems studied which included CO2 adsorption on single walled carbon nanotubes (CO2 – SWCNT), Ethane adsorption on closed carbon nanohorns (Ethane-cNH), Ar adsorption on open carbon nanohorns (Ar – oNH), CO2 adsorption on zeolitic imidazolate framework-8 (CO2 – ZIF-8), and O2 adsorption on ZIF-8 (O2 – ZIF-8). Each of these systems offers a unique study of the relationship between the physical properties of the adsorbate and substrate and the effects of these properties on the thermodynamics and kinetics of adsorption. In addition to being of fundamental interest, the thermodynamics and kinetics of adsorption are important to understand for practical considerations in research fields such as gas storage and gas separation via adsorption processes, among other applications. CO2 – SWCNT is a system with a small linear molecular adsorbate with a permanent quadrupole moment adsorbing on a substrate with quasi-1D grooves and convex outer adsorption sites. Ethane-cNH is a system with a linear alkane adsorbing on a substrate with conical pores and convex outer adsorption sites. Ar – oNH is a system with a spherical atom sorbing in a substrate with two different groups of conical adsorption sites and both convex and concave surface sites. CO2 – ZIF-8 and O2 – ZIF-8 are both systems with small linear molecules sorbing in a flexible microporous scaffold-like substrate. Adsorption isotherms were analyzed to identify features corresponding to adsorbate-adsorbate and adsorbate-substrate interactions. Namely, the presence of substeps in the semi-logarithmic data were identified and interpreted to correspond to groups of adsorption sites of similar binding energy which likely depend on the morphology and/or structural flexibility of the substrates. All of the systems, with the exception of CO2 - SWCNTs, yielded at least some isotherms with substeps at pressures below that corresponding to saturation. Effective specific surface areas for all adsorbent-substrate combinations were calculated using the BET and Point-B methods for the sake of comparison. These surface area measurements are very dependent on the porosity and morphology of the substrate as well as the size and shape of the adsorbate atoms/molecules and therefore the values vary greatly between the different systems. The isosteric heat of adsorption was calculated using isotherms over the full range of temperatures for each system. A variant of the Clausius-Clapeyron equation was used for this purpose and the results were analyzed based on adsorbate-adsorbate and adsorbate-substrate interactions. Plateaus in the isosteric heat data for Ethane – cNH and Ar – oNH were related to the morphology of the substrates and properties of the adsorbate species. For CO2 – SWCNTs, the isosteric heat at all but the lowest coverages is below the latent heat of deposition. This is due to the quadrupole moment of CO2. For both of the studies of adsorption on ZIF-8, the isosteric heat contains peaks in the data which likely are the result of the flexibility of the ZIF-8 structure. The kinetics of adsorption (or, the rates at which the adsorption systems approach equilibrium) were analyzed as functions of isotherm temperature and coverage, vapor pressure, and fractional uptake point by point along individual isotherms using the linear driving force model. Certain trends in the kinetics of adsorption are consistent for all the systems studied and others vary depending on the specific adsorbate-substrate combination. As with the thermodynamic results, trends in the kinetics of adsorption are discussed in terms of the effects of adsorbate-adsorbate and adsorbate-substrate interactions.
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