Aligned Carbon Nanotubes as Porous Materials for Selective Gas Adsorption and Desorption

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Summary Our efforts during funding period three will continue to be aimed at gaining a solid understanding of the adsorption of gases in highly ordered arrays of carbon nanotubes (CNTs). The technological incentive is the potential of such arrays as adsorbents for noxious components of flue gases, such as CO2, SO2, NOx and H2S, i.e. for gas purification. On a fundamental scientific level, however, CNT arrays are also perfect model systems for the investigation of gas adsorption in carbon materials. We can manufacture them reproducibly with well-controlled geometry parameters (tube diameter, intertube distance, length), which makes them ideal for comparison with theoretical simulation studies. Building on our results from funding periods 1 and 2, we will continue to use experimental techniques (targeted preparation, spectroscopic characterization and specific adsorption measurements under ambient and high pressure) in tandem with theoretical methods (grand-canonical Monte Carlo simulations of adsorption thermodynamics, molecular dynamics for transport processes, and analytical models) to obtain a unified picture of the adsorption mechanisms at the molecular level. The range of scientific questions and the methods to be used will be expanded. The specific foci of our research in funding period three will be on: Adsorption of mixtures. This includes the explanation of adsorption selectivities for certain components with a view to separation and purification processes, as well as an understanding of positive and negative mutual interference in the adsorption of multiple gases. As part of this subtopic, we will also investigate mixtures with one component being water, as humidity is omnipresent in technical applications. Charged nanotube arrays. Our theoretical studies have predicted that the adsorption of CO2 can be modulated by electrical charges placed on the CNT arrays. We will now try and verify these predictions in an experimental set-up to measure gas adsorptivity of electrically contacted CNT arrays whose charge can be externally controlled. Simultaneously, further simulations for other gases will be performed. We will also investigate the possibility of using electrically contacted blocks of CNT arrays as electrostatic swing adsorption/desorption devices. Chemically modified CNTs. We have shown that plasma treatment and high-temperature CO2 treatment are able to chemically modify the CNT surfaces, sometimes leading to drastically increased gas adsorption. These studies will be continued and accompanied by realistic simulations with the aim of obtaining a molecular understanding of the adsorption capability of individual surface groups and defects, their mutual interaction and their selectivity. The results of these combined efforts will be used to suggest target surface functionalisations with tailored adsorption and selectivity profiles.