Adsorption on nonporous and microporous carbons: Experiment, simulation, and theory
Abstract
Microporous carbons are used for many separation processes including air and water purification. The measurement of nitrogen adsorption isotherms is widely used as a means of characterizing porous carbons. Pressures as low as P/Psb0≈ 10sp{-9} may be needed to observe adsorption in the smallest possible carbon micropores, which have the highest selectivity for separations. Existing commercial sorptometers can typically measure down to about 10sp{-5}. Classical methods for interpreting such data rely on thermodynamic treatments that do not take proper account of either the fluid-wall interactions or the highly inhomogeneous nature of the adsorbate near the surface. As a result they fail for small pores, with diameters {<}80{-}100 A. Density functional theory provides an improvement over the classical methods, but it requires accurate knowledge of the fluid-wall interactions. The goal of this work is to predict the behavior of fluids adsorbed in carbon micropores with molecular simulation, to quantify the adsorbate-carbon interaction by measuring adsorption, and to improve the existing methods for obtaining the pore size distribution by measuring adsorption at the temperature most favorable to resolve the smallest micropores. Molecular simulation was used to calculate methane adsorption and diffusion in a 19 A wide carbon slit at temperatures between 60 and 135 K. Layering, capillary condensation, and freezing transitions were observed. Diffusion near the carbon surface is much slower than in the bulk. Adsorption was measured at pressures as low as 2× 10sp{-4} torr, which corresponds to P/Psb0=3× 10sp{-7} for nitrogen at 77 K. The LJ energy parameter varepsilonsb{sf}/ksb{B} for the interaction between nitrogen (methane) and the carbon wall was found to be 58.8 K (57.5 K) for a Lennard-Jones model. Experiments at 88.0 K on AX21, a microporous carbon, show its pores have a bimodal distribution with peaks at 13 and 27 A. Use of a cryostat to control the temperature at 88 K decreased equilibration times and reduced the minimum pore size which could be resolved to 8 A. These improvements can be readily applied to many other microporous materials.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- December 1997
- Bibcode:
- 1997PhDT.......123R