High selectivity of CO2 capture with single- and double-walled carbon nanotubes†

An excessive concentration of greenhouse gases, most significantly carbon dioxide (CO₂), in the atmosphere has led to the serious environmental issue of global warming. Carbon capture is a suitable strategy to reduce the increase of CO₂ in the atmosphere due to fossil fuel combustion. Innovative technologies for CO₂ capture are urgently required and this is an area of intensive study in order to improve efficiency and reduce operational costs. In this work, we applied molecular dynamics simulations to demonstrate the ability of single-walled carbon nanotubes (SWCNT) and double-walled carbon nanotubes (DWCNT) to capture CO₂ from flue gases. Both SWCNTs and DWCNTs prefer to adsorb CO₂ rather than N₂ and O₂, resulting in a separation effect. CO₂ molecules form a solid ice structure in the carbon nanotubes (CNT) while N₂ and O₂ remain gaseous. As a result, the potential energy of the CO₂ structure inside the CNTs is lower than that of the N₂ or O₂ structures. This implies that CO₂ is more stable in the CNTs. Therefore, the formation of these solid CO₂ structures plays an important role in the process of capturing CO₂via CNTs. Moreover, the van der Waals interactions between CO₂ molecules and the CNT walls make a significant contribution to the separation of CO₂ as well. The potential energy of the CO₂–CNT wall interactions is significantly lower than those of N₂–CNT wall or O₂–CNT wall interactions. In addition, the presence of a second wall in DWCNTs causes even stronger attractive CO₂–CNT wall van der Waals interactions than those found in SWCNTs. As a result, the CO₂ capturing effect of DWCNT is greater than that of SWCNT.