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.