Absorption of gases in porous media using Grand Canonical Monte Carlo simulations

The severe consequences of the global warming effect ,as observed through the detrimental alterations in the climate and natural environment of our planet undoubtedly give rise to alerting predictions all over the world. Certain well-known flue-gases emissions , such as nitrogen and carbon oxide, involves harmful effects for both human health and the environment.
 
Among the available storage technologies for gases, the most appropriate and efficient choice is possibly the most appropriate and efficient choice. A promising solid sorbing material has to exhibit a variety of unique physical and chemical properties: a) sufficient storage capacity for a given gas at room temperature, b) fast and quantitative recovery of the adsorbed gas without any need to increase the temperature, c) chemical stability and d) low cost of preparation and operation.
 
After the discovery of carbon nanotubes (CNTs) [1] much attention has been devoted to their potential applications, especially as storage mediums.
 
One of the most common simulation procedures of the adsorption phenomenon of gases in microporous solids may be summarized in the following two steps: Initially, the excess chemical potential of each pure gas is calculated at the thermodynamic state points of interest, using  effective force field models employed to describe the interactions among gas molecules. In order to accomplish the aforementioned task, we used NPT MC simulation and the Widom’s [2,3] test particle insertion method. In the second step, we use GCMC [2,3 ] simulation technique to determine adsorption of these gases in our carbon nanotube models assuming that no chemical process occurs. In the framework of each GCMC simulation we obtainrd the density profiles and a the weight percent (wt %) of the adsorbed gas inside and outside the tubes, which indicates the adsorption capacity of the system at the selected thermodynamic states.
 
We have chosen the temperature of 298 K and pressures in the range 0.01 – 2.0 Mpa. The adsorption isotherms are presented in figure 1. Throughout all simulations we have used the towhee simulation engine.
 
c8-f1a-n2-in-cnt

c8-f1b-co-in-cnt

c8-f1c-ch4-in-cnt

c8-f1d-co2-in-cnt
Figure 1: Simulated and experimental adsorption isotherms of 298 K [2]
Through python scripting we have calculated the density profiles which are presented in Figure 2.
 

c8-f2-density-profile-of-co2-in-cnts
Figure 2: Density profiles of methane adsorbed in CNTs and the GUI of the corresponding script.

The simulated amount of the adsorbed gases show a similar trend with experiment, namely follows the order CO2 >CO> N2, CH4.

The difference in the uptake values can be ascribed to a number of possible factors

(i) the employed model calculations

(ii) the possible remained carbonaceous impurities in the sample

a proportion of close ended tubes, even contained in the experimental sample after preparation

A snapshot of a simulation box configuration of CO2 adsorbed on (9, 9) SWCNTs model from the GCMC simulation of the system at 298K is presented in figure 3.
 

c8-f3-gcmc-simulation-of-co2-in-cnts
Figure 3: Snapshot of the GCMC simulation of CO2 . The red atoms are oxygens and the grey carbons.

As a summary, carbon nanotubes are considered a promising material for gas adsorption and separation. In order to optimize their performance various modifications on their surface may be accomplished in the future (alkali doped etc)

Furthermore using the same technique and tools we have studied the adsorption of  CO and CO on activated graphite. In order to activate the graphite model we used carboxyl and hydroxyl groups at surface densities of  0.89, 1.33, 2.22  nm-2. The temperature was 298 K and the pressure range between 0.01 – 2.0 Mpa. The simulated isotherms are presented in figure 4.
 
c8-f4a-co2-graphite

c8-f4b-co-graphite

c8-f4c-ch4-graphite
Figure 4:Adsorption isotherms of various gases in activated graphite

In figure 5 a snapshot of the simulation box is presented.
 

c8-f5a-co2-22b-30b-in-activated-graphite

c8-f5b-co-ch4-in-activated-graphite
Figure 5: Snapshots of the simulation boxes of the adsorbed gases in graphite pores with activation site density 2.22 at 298 K.

In figure 6 the density profiles of CO in activated graphite are presented. We may observe that density increases and this is an indication of capillary condensation.
 

c8-f6-density-profiles-co2-in-activated-graphite-at-22-and-30-bar
Figure 6: Density profiles of CO2 in activated graphite at 22 and 30 bar.

In summary, the simulation results for aspirin demonstrate that they are within acceptable range from the reported experimental values.
 
In summary we could conclude that the simulated amount of the adsorbed gases follows the order CO2 > CH4 > CO. The adsorption capacity of graphene increases with the density of the activation sites.
 
The density profiles obtained from the simulation show that the majority of the gas adsorbed molecules are distributed near the pore wall

 

References:

  1. Iijima, S., Nature, 1991, 354, 56
  2. George P. Lithoxoos, Anastasios Labropoulos, Loukas D. Peristeras, Nikolaos Kanellopoulos, Jannis Samios, Ioannis G. Economou J. of Supercritical Fluids 55 (2010) 510–523
  3. George P. Lithoxoos, Loukas D. Peristeras, Georgios C. Boulougouris, Ioannis G. Economou
    Molecular Physics Special Issue Thermodynamics 2012, 110, 11-12, 1153