Kinetic restriction of simple gases in porous carbons: Transition-state theory study

Nguyen, Thanh X. and Bhatia, Suresh K. (2008) Kinetic restriction of simple gases in porous carbons: Transition-state theory study. Langmuir, 24 1: 146-154. doi:10.1021/la702471d


Author Nguyen, Thanh X.
Bhatia, Suresh K.
Title Kinetic restriction of simple gases in porous carbons: Transition-state theory study
Journal name Langmuir   Check publisher's open access policy
ISSN 0743-7463
Publication date 2008-01-01
Year available 2007
Sub-type Article (original research)
DOI 10.1021/la702471d
Open Access Status
Volume 24
Issue 1
Start page 146
End page 154
Total pages 9
Editor D. G. Whitten
J. F. Holzwarth
R. G. Nuzzo
R. Crooks
Place of publication Washington D.C., USA
Publisher American Chemical Society
Language eng
Subject C1
090404 Membrane and Separation Technologies
Abstract The separation of simple gases such as N-2, Ar, CO2, and CH4 is an industrially important problem, particularly for the mitigation of greenhouse emissions. Furthermore, these gases are widely accepted as standard probing gases for the characterization of the microstructure of porous solids. However, a consistent set of microstructural parameters of a microporous solid deter-mined from the use of adsorption measurements of these different gases is not always achieved because of differences in their pore accessibility. This is a long-standing and poorly understood problem. Here, we present the calculated results of the crossing time of N-2, Ar, CO2, and CH4 between two neighboring cages through a constricted window in a realistic structural model of saccharose char, generated from hybrid reverse Monte Carlo (HRMC) simulation (Nguyen, T. X.; Bhatia, S. K.; Jain, S. K.; Gubbins, K. E. Mol. Simul. 2006,32,567-577) using transition state theory (TST), as described in our recent work (Nguyen, T. X.; Bhatia, S. K. J. Phys. Chem. 2007, 111, 2212-2222). The striking feature in these results is that whereas very fast diffusion of carbon dioxide within the temperature range of 273-343 K, with crossing time on the molecular dynamics scale (10(-4)-10(-6) S), leads to instantaneous equilibrium and no hysteresis on the experimental time scale, slower diffusion of Ar and N-2 at the low temperature of analysis indicates an accessibility problem. These results rationalize the experimental results of hysteresis for N-2 at 77 K and At at 87 K but not for CO2 at 273 K in Takeda 3 angstrom carbon molecular sieves. Furthermore, it is shown that CH4 diffusion through narrow pore mouths can be hindered even at ambient temperature. Finally, we show that the use of pore size and wall thickness distributions extracted from the adsorption of At at 87 K using the finite wall thickness (FWT) model (Nguyen, T. X.; Bhatia, S. K. Langmuir 2004,20,3532-3535 and Nguyen, T. X.; Bhatia, S. K. J. Phys. Chem. B 2004, 108, 14032-14042) provides the correct prediction of experimental CO2 adsorption in BPL and PCB carbons whereas that from N-2 at 77 K gives a significant underprediction for both CO2 and CH4 in the BPL carbon. These trends are in excellent agreement with those predicted using the calculated crossing times.
Keyword Density-functional Theory
High-pressure Adsorption
Pore-size
Activated Carbons
Molecular-sieve
Microporous Carbons
Co2 Adsorption
Supercritical Gases
heat-treatment
Sorption
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ
Additional Notes Publication Date (Web): November 29, 2007

Document type: Journal Article
Sub-type: Article (original research)
Collections: 2009 Higher Education Research Data Collection
School of Chemical Engineering Publications
 
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Created: Tue, 07 Apr 2009, 23:49:07 EST by Katherine Montagu on behalf of School of Chemical Engineering