Molecular Simulation of CO2 Adsorption in the Presence of Water in Single-Walled Carbon Nanotubes

The adsorption of carbon dioxide in the presence of water in single-walled carbon nanotubes is studied using Monte Carlo simulation, at 300, 325, and 350 K. We also investigate the influence of the diameter and chirality of the nanotubes on the adsorption isotherms of CO 2 . It is observed that increasing the nanotube diameter from 1.36 nm (10, 10) to 2.03 nm (15, 15) leads to enhanced CO 2 capacity, while change in chirality has little effect on the adsorption capacity of carbon nanotubes. Our results show that the influence of preadsorbed water on CO 2 adsorption is dependent on both the effects of excluded volume and H 2 O–CO 2 interactions. The maximum adsorbed amount of CO 2 decreases linearly with the loading of water, and drops more rapidly in narrower nanotubes. The structure of water in hydrophobic nanopores is in the form of hydrogen-bonded clusters, and its adsorption does not affect the arrangement and orientation of CO 2 molecules (i.e., it does not affect the mechanism of CO 2 adsorption). The average size of water clusters coexisting with CO 2 depends strongly on the adsorbed amount of CO 2; however, it is shown that splitting large water clusters into smaller ones can lead to significant enhancement of CO 2 adsorption, due to the resulting stronger water–CO 2 interaction. The maximum percentage increase in the excess adsorption of CO 2 is as high as 53.4% when a single cluster is split into multiple smaller clusters. This finding demonstrates that the efficiency of CO 2 capture from flue gas can be significantly improved by controlling the structure of coexisting water in carbon nanotubes.