Gas Adsorption in Carbon Nanohorns: Equilibrium and Kinetics

A study of gas adsorption has been carried out with the focus of better understanding the relationships between the individual properties of the adsorbent/adsorbate (e.g. material structure, interactions, gas size and shape, etc.) and the overall adsorptive properties of the combined system (e.g. capacity, binding strength, equilibration time, etc.) as a function of thermodynamical variables. This is useful from the perspective of a comprehensive and fundamental understanding as well as for practical applications. The equilibrium regime of adsorption on carbon nanostructure materials (nanohorns, nanotubes, and graphite) is investigated using molecular statics (MS) and grand canonical monte carlo (GCMC) methods for a variety of gas species (carbon dioxide, ethane, argon, etc.). Through the controlled variation and comparison of these simulations, interaction and structural models are developed to help interpret and understand experimental observations. For the case of the adsorption of ethane on closed carbon nanohorns, the lack of distinct features in the adsorption data was found to be a result of binding on exterior sites of the aggregate as well as the increased degrees of freedom of the molecular species. The isosteric heat of adsorption of carbon dioxide on both carbon nanotubes and nanohorns has been experimentally shown to trend through a minimum before approaching the bulk value, which contradicts what is observed for all other adsorbate species. Here it is shown that carbon dioxide\u27s unique behavior is due to the increased gas-gas interactions which are present due to its quadrupole moment. In order to study the effect of complex geometries and inhomogeneous interaction profiles in the kinetic regime, such as those present in carbon nanohorns, a general 3D on-lattice KMC modelling scheme was developed. A lattice model for carbon nanohorns was developed within this scheme and preliminary calculations show the variation of binding energy along the length of the pore serves to reduce the time to reach equilibrium as well as causes higher site occupancy near the bottom of the pore

Thermodynamics and Kinetics of Carbon Dioxide Adsorption on HiPco Nanotubes

We present the results of a combined experimental and computational study of CO2 adsorption on purified HiPco nanotubes. Isotherms were measured at six temperatures between 147 and 207 K, below the bulk triple point for CO2. Unlike the case of other adsorbates on HiPco nanotube bundles, adsorption isotherms at corresponding temperatures do not reveal the presence of any resolvable substeps. The isosteric heat values derived from the measured isotherms are lower than the latent heat of sublimation for most of the loadings, with the exception of a narrow range at very low coverage. Results from grand canonical Monte Carlo simulations show that this is due to the much larger contribution of the CO2–CO2 interactions (owing mostly to the presence of the electrostatic component) that greatly exceeds the size of the gas–surface interaction as the coverage increases beyond the monolayer. Measurements of the kinetics of adsorption show that the equilibration time increases with sorbent loading, which is typical of systems with relatively larger adsorbate–adsorbate interactions

Ethane Adsorption on Aggregates of Dahlia-like Nanohorns: Experiments and Computer Simulations

This is a report on a study of the adsorption characteristics of ethane on aggregates of unopened dahlia-like carbon nanohorns. This sorbent presents two main groups of adsorption sites: the outside surface of individual nanohorns and deep, interstitial spaces between neighbouring nanohorns towards the interior of the aggregates. We have explored the equilibrium properties of the adsorbed ethane films by determining the adsorption isotherms and isosteric heat of adsorption. Computer simulations performed on different model structures indicate that the majority of ethane adsorption occurs on the outer region of the aggregates, near the ends of the nanohorns. We have also measured the kinetics of adsorption of ethane on this sorbent. The measurements and simulations were conducted along several isotherms spanning the range between 120 K and 220 K