Abstracts Chemistry

Aligned Carbon Nanotubes as Porous Materials for Selective Gas Adsorption

by Mahshid Rahimi

Carbon dioxide and sulfur dioxide are environmentally noxious components of flue and exhaust gases. Hence, new solutions for carbon dioxide and sulfur dioxide sequestration and storage are highly important. We used grand-canonical Monte Carlo simulations to understand the adsorption of carbon dioxide and sulfur dioxide in bundles of regular parallel arrays of carbon nanotubes of different tube diameters and different intertube distances. Such carbon nanotube arrays have recently become available experimentally; they are not only promising as sorption materials but, because of their reproducibility and regularity, serve also as ideal model systems for the study of gas adsorption in carbon-based materials. The geometrical properties of the nanotube arrays were varied in order to optimize the sorption capacity of the material. We also investigated how the adsorption changes when the nanotube arrays are positively or negatively charged, as electrically contacted carbon nanotube arrays are a possible functional material for electric swing adsorption devices. The adsorption isotherms showed that the intertube distance plays a more important role than the nanotube diameter. The highest adsorption among the intertube distance of carbon nanotubes depends strongly on applied pressure. For lower pressures, the lower intertube distances show higher adsorption. As the pressure increases, the intertube distance that maximizes the adsorption shifts to a higher value. Moreover, optimizing the intertube distance can increase the adsorption up to ~40 %, depending on the system and pressure. This is in agreement with experiments and shows the importance of the geometry optimization. Charging the carbon nanotubes with +0.04 e per atom of carbon increases the adsorption of carbon dioxide by up to 35% at p=1.88 bar while adding the same amount of negative charge to the carbon nanotubes causes the adsorption to decrease. This increase/decrease is due to the change in the potential energy for the interaction between the individual carbon dioxide molecules and the nanotube. The separation behavior of binary mixtures in carbon nanotubes is investigated using grand-canonical Monte Carlo simulations. The results show that the quantity and quality of the selectivity for each system depend on the intertube distance of nanotubes and the type of the adsorbate molecules. The main reason for having different selectivities is the difference between the strength of interactions between the nanotubes and the individual molecules of one gas compared to the other one. This difference is the highest for the SO2-N2 mixture, followed by CO2-N2 and finally SO2-CO2. Selectivity also follows the same order with SO2-N2 being the best in terms of selectivity. Furthermore, for a binary mixture of SO2-CO2, the selectivity towards sulfur dioxide is characterized as a nonlinear behavior as a function of intertube distance. On the other hand for CO2-N2 and SO2-N2, the close-packed nanotubes show the highest selectivity over the entire pressure range studied. The… Advisors/Committee Members: Müller-Plathe, Florian (advisor), van Der Vegt, Nico (advisor).