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Adsorption: Journal of the International Adsorption Society (v.3, #3)
Theoretical basis for the Dubinin-Radushkevitch (D-R) adsorption isotherm equation by Nick D. Hutson; Ralph T. Yang (pp. 189-195).
The Dubinin-Radushkevitch (D-R) equation, which was originally proposed as an empirical adaptation The Polanyi adsorption potential theory, has been the fundamental equation to quantitatively describe the adsorption gases and vapors by microporous sorbents. The equation, based on the postulate that the mechanism for adsorption in micropores is that of pore-filling rather than layer-by-layer surface coverage, generally applies well to adsorption systems involving only van der Waals forces and is especially useful to describe adsorption on activated ???. The ability of the D-R equation to describe gas adsorption on porous materials has inspired many to undertake studies, both experimental and theoretical, to explain the source of the success of the D-R equation in ??? of molecular properties at the gas-solid interface. In many cases, these studies have led to extensions or modifications of the original D-R equation. Many of these attempts and the resulting extensions are reviewed and discussed here. Recently, an isotherm equation was derived for adsorption of gases and vapors on microporous ??? from statistical mechanical principles. It was shown that the D-R equation is an approximated form of this potential theory isotherm. This development is also reviewed and discussed.
Keywords: Dubinin-Radushkevitch equation; Dubinin-Astakhov equation; adsorption; micropore adsorption
An analytical method of micropore filling of a supercritical gas by K. Kaneko; K. Murata (pp. 197-208).
The supercritical gas adsorbed in the micropore having a strong molecular field was presumed to transform into the quasi-vapor to be filled in the micropore (quasi-vaporization adsorption mechanism). The Dubinin-Radushkevitch (DR) equation for micropore filling of vapor was extended to the quasi-vaporized supercritical gas using the quasi-saturated vapor pressureP 0 q and the inherent micropore volumeW L . The reason why the concepts ofP 0q andW L were introduced was explained with the molecule-pore interaction potential theory which is based on the Lennard-Jones interaction. The extended DR equation was named the supercritical DR equation. TheW L was evaluated by the Langmuir plot of the adsorption isotherm for a supercritical gas and both ofP 0q andW L provided the single reduced adsorption isotherms of supercritical NO, N2, and CH4 on activated carbon fibers and high surface area carbons were analyzed by the supercritical DR plots. The wide applicability of the reduced adsorption isotherm to these adsorption data was explicity shown. The two phase model of the organized and confined fluids was proposed in order to improve the quasi-vaporization adsorption mechanism.
Keywords: micropore filling; supercritical gas; carbon micropore; nitrogen monooxide; methane; molecular assembly
Critical discussion of simple adsorption methods used to evaluate the micropore size distribution by Michal Kruk; Mietek Jaroniec; Jerzy Choma (pp. 209-219).
The well-known simple adsorption methods used to evaluate the micropore size distribution from low pressure adsorption isotherms were examined by employing model isotherms for slit-like graphite micropores obtained from nonlocal density functional theory. It was shown that in the range of pore sizes from about 0.4 to 0.9 nm, the Horvath Kawazoe (HK) method satisfactorily reproduces the shape of the micropore size distribution, but the pore sizes are underestimated. In the case of micropores wider than 0.9 nm, the method fails as the formation of the monolayer on the pore walls produces a peak corresponding to 0.6 nm micropores on the HK pore size distribution. Therefore, the HK method indicates the presence of microporosity even for nonporous samples. The Dubinin-Astakhov adsorption isotherms were also examined and it was shown that their application to represent local adsorption isotherms for homogeneous pores is questionable. However, the adsorption potential distributions seem to be promising for micropore analysis.
Keywords: carbons; porous structure; Horvath-Kawazoe method; Dubinin-Astakhov equation; gas adsorption
On the porous structure of coals: Evidence for an interconnected but constricted micropore system and implications for coalbed methane recovery by L. R. Radovic; V. C. Menon; C. A. Leon Y Leon; T. Kyotani; R. P. Danner; S. Anderson; P. G. Hatcher (pp. 221-232).
An experimental and theoretical study of adsorption and diffusion of carbon dioxide and methane in coals of widely varying rank was carried out. Low pressures adsorption isotherms of CO2 were obtained and analyzed using Dubinin's theory of volume filling of micropores. High-pressure adsorption isotherms of CH4 were obtained and analyzed using tracer pulse chromatography in conjunction with an appropriate adsorption/diffusion model. A preliminary129Xe NMR analysis of chemical shifts experienced by xenon atoms in particles of different sizes is also reported.The heretofore undocumented and/or underestimated effects of activated diffusion of CO2 at 273–298 K complicate the elucidation of the true microporous structure of coals, especially its dependence on coal rank. Activated diffusion of both CO2 and methane at room temperature does not allow reliable estimates of coalbed gas content to be made. A model of an interconnected network of pores which includes randomly distributed, numerous and ultramicroporous constrictions (at any size scale) is consistent with all these experimental and theoretical findings.
Keywords: coalbed methane; coal (porous structure, micropores); adsorption (of xenon, carbon dioxide, methane); Xe-129 NMR spectroscopy
Steam-carbon gasification catalyzed by calcium: Assessment of the porous structure of active carbons from plum stones and synthetic active carbons by A. Dąbrowski; Z. Fekner; R. Leboda; J. Goworek (pp. 233-242).
A need for an elaboration of the methods for synthesis and characterization of activated carbons with a requisite porous structure has existed for a long time. One of the methods giving possibility for creating controlled mesopore and micropore structures deals with the steam gasification of various carbon materials. In this work the effects of calcium catalyst on the catalytic steam gasification of active carbons from plum stones and porous polymers are presented. Determination of micropores capacity and specific adsorption in mesopores have been performed by means of theα s method, but adsorption on the heterogeneous solids was described by the integral equation with various local isotherms. This equation has been solved by the regularization method. Based on this method the changes in structural parameters of active carbons depending on the amount of calcium catalyst were estimated.
Keywords: natural active carbons; synthetic active carbons; porous structure; pore size distribution; modification of active carbons