Adsorption of N 2, CH 4, CO and CO 2 gases in single walled carbon nanotubes: A combined experimental and Monte Carlo molecular simulation study

Abstract

In this study, the adsorption capacity of single-wall carbon nanotubes (SWCNTs) bundles with regard to the pure CH 4, N 2, CO and CO 2 gases at 298 K and pressure range from 0.01 up to 2.0 MPa has been investigated experimentally and computationally. Experimental work refers to gravimetric surface excess adsorption measurements of each gas studied in this nanomaterial. Commercial samples of pristine SWCNTs, systematically prepared and characterized at first, were used for the evaluation of their adsorption capacity. A Langmuir type equation was adopted to estimate the total adsorption isotherm based on the experimental surface excess adsorption data for each system studied. Computational work refers to Monte Carlo (MC) simulation of each adsorbed gas on a SWCNTs model of the type (9, 9) in the grand canonical (GC) ensemble at the same conditions with experiment using Scienomics’ MAPS platform software simulation packages such as Towhee. The GCMC simulation technique was employed to obtain the uptake wt% of each adsorbed gas by considering a SWCNTs model of arrays with parallel tubes exhibiting open-ended cylindrical structures as in experiment. Both experimental and simulation adsorption data concerning these gases within the examined carbon material are presented and discussed in terms of the adsorbate fluid molecular characteristics and corresponding interactions among adsorbate species and adsorbent material. The adsorption isotherms obtained exhibited type I (Langmuir) behavior, providing enhanced gas–substrate interactions. We found that both the experimental as well as the simulated adsorption uptake of the examined SWCNTs at these conditions with regard to the aforementioned fluids and in comparison with adsorbate H 2 on the same material increase similarly and in the following order: H 2 ≪ N 2 ≈ CH 4 < CO ≪ CO 2. Furthermore, for each adsorbate fluid the calculations exhibit somewhat greater gas uptake with pressure compared to the corresponding experiment. The difference in the absolute uptake values between experiment and simulation has been discussed and ascribed to the following implicit factors: (i) to the employed model calculations, (ii) to the remained carbonaceous impurities in the sample, and (iii) to a proportion of close ended tubes, contained in the experimental sample even after preparation.