Physisorption Kinetics in Carbon Nanotube Bundles

Abstract

The kinetics of gas uptake on different regions of carbon nanotube bundles is investigated by means of a kinetic Monte Carlo scheme. A lattice-gas description is used to model the adsorption of particles on a onedimensional chain of sites under two types of dynamics: (a) external kinetics, in which the chain is on the bundle's external surface directly exposed to the gas, and (b) pore-like kinetics, expected to occur inside the tubes and interstitial channels, where adsorption occurs via gas diffusion from the ends. From the time evolution of the coverage at a fixed temperature, equilibration times are obtained as a function of chemical potential (or amount adsorbed). The equilibration time of the external phase decreases linearly as the coverage increases toward monolayer completion; the rate at which this occurs strongly depends on the ratio between the binding energy and the temperature. Because of this dependence, unexpectedly long waiting times can be observed for very low coverages in systems with relatively high binding energies. The adsorption rate in pore-like phases is typically 2 orders of magnitude slower than that of external phases. We show how this large disparity between adsorption rates can hinder the observation of adsorption inside the tubes and in the interstitial channels during measurements of adsorption isotherms.

Non-Monotonic Kinetics of Alkane Adsorption on Single-Walled Carbon Nanotubes

“…The theoretical studies have also found that a completely different mechanism for reaching equilibrium exists for nanotubes that have open ends. 13 In previous publications, in which we studied gas adsorption kinetics on closed-and openended nanotubes, we determined experimentally that these predictions were correct. 4,5 Here we explore the dependence of the equilibration time on the length of the adsorbed molecule, and, for each adsorbate, we also explore the dependence of the equilibration time on fractional coverage.…”

Carbon nanotubes as adsorbents in environmental pollution management: A review

“…It is interesting to note that the adsorption reaches equilibrium much faster on external sites (grooves and outer surfaces) than on the internal sites (interstitial channels and inside the tube) under the same pressure and temperature conditions. The external sites are directly exposed to the adsorbing material; the adsorption process on internal sites has to be initiated on the ends of the pore, followed by diffusion to the sites at the interior [49,50]. Second, the fraction of opened and unblocked nanotubes can greatly influence the overall adsorption capacity.…”

Gas adsorption on single-wall carbon nanotube bundles and charcoal samples

“…It is interesting to note that recent theoretical calculations by Calbi et al, have predicted a two orders of magnitude longer adsorption time of gases to IC sites compared to OG sites. 50 Their prediction was confirmed by experimental adsorption studies of Ar on open-ended SWNT bundles by Migone et al, 51 To do a quantitative analysis of the data, we tried applying a random-walk diffusion model 52 to our data. According to this model, an initial packet of particles would diffuse out over time (t) through collisions with the environment similarly to a random-walk process, and this packet's mean-square displacement would be described as (in one dimension) where x 2 is the mean-square displacement and D is the diffusion coefficient.…”

“…14 Also, Calbi et al, have predicted much slower adsorption of gases to IC sites due to diffusive adsorption process. 50 So we interpret the y intercept value as the number of atoms adsorbed onto OG site. 55 Using some further assumptions [the mean-square displacement divided by the adsorbate distance is the number of adsorbates in the diffusion channel (IC sites)], we obtained the diffusion coefficient from the slope as well.…”