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

Abstract

We report kinetics measurements for the adsorption of methane, ethane, propane, butane, and pentane on close-ended single-walled carbon nanotube bundles. The measurements were conducted by monitoring the adsorption equilibration times. For methane and ethane, equilibration times were found to decrease as the fractional coverage on the surface of the nanotube bundles increased, approaching monolayer completion. However, for propane, butane, and pentane, the opposite trend was observed: for these longer alkanes, there was an increase in the equilibration time with increasing fractional coverage. We propose a plausible explanation for our experimental findings, and we suggest that different mechanisms for attaining equilibrium may be in place for short and long alkane chains.

Adsorption kinetics of diatomic molecules

“…In this work, we focused on the effect of attractive interactions mainly because we are considering submonolayer coverages; also, this is the kind of interaction that could lead to longer equilibration times due to effectively increasing the binding, 5 which is the effect observed for longer molecules. 18 Fig. 4 shows the isotherms obtained from the simulation results (symbols) together with predictions from solving the corresponding lattice gas model of adsorption 25 for the number of adsorbed molecules (lines) as follows.…”

“…During the last decade, the adsorption behavior of linear hydrocarbons on nanotube bundles has been investigated via experiments and simulations. [9][10][11][12][13][14][15][16][17][18] Most of these studies are focused on equilibrium properties; only very recently has the kinetics of adsorption of linear alkanes been explored experimentally. 18 In that study, the equilibration time of a series of alkane molecules of increasing length (from ethane to pentane), adsorbing on the exterior of a nanotube bundle, was measured.…”

“…[9][10][11][12][13][14][15][16][17][18] Most of these studies are focused on equilibrium properties; only very recently has the kinetics of adsorption of linear alkanes been explored experimentally. 18 In that study, the equilibration time of a series of alkane molecules of increasing length (from ethane to pentane), adsorbing on the exterior of a nanotube bundle, was measured. This was done as a function of the final coverage on the surface, up to monolayer completion, and at the same relative temperature with respect to the critical temperature of the species.…”

“…In those cases, the equilibration time would initially increase, with coverage reaching some maximum value before eventually decreasing to very low values as the coverage approaches a single monolayer. 18 We have previously explained the occurrence of the decreasing trend in the monomer case (methane) with the aid of a simple model and simulations. 5,7,8 This decreasing trend basically occurs as a consequence of the increase in the external gas pressure necessary to achieve higher coverages, and that is the main cause of the accelerated adsorption seen as monolayer completion is approached.…”

“…This allows us to directly compare with the experimental results mentioned above for ethane adsorption on closed-end nanotube bundles. 15,18 Although we will be somewhat focusing on this system as a particular example (due to the availability of experimental results for the whole alkane series), the model and results presented here are in fact relevant for the kinetic behavior of other homo-nuclear diatomic adsorbates such as H 2 , N 2 , and O 2 . Diatomic molecules are the shortest non-spherical adsorbates possible, and this work is a sensible first step to later analyze the increasingly larger effects of the non-spherical nature of longer molecules.…”