Phenomenological Kinetics of Real Gas-Adsorption-Systems: Isothermal Adsorption

Abstract

Since Langmuir published the derivation of his famous adsorption isotherm equation in 1918, hundreds of papers on the kinetics of gas adsorption on solid surfaces have appeared in the world literature. Until the early ®fties of the 20th century, these were mainly papers reporting on experimental studies of the kinetics of adsorption in various adsorption systems. Surprisingly, although the Langmuir isotherm described the adsorption equilibria fairly well, the related kinetic expressions generally failed to describe the adsorption kinetics. So, beginning in the early ®fties, vigorous attempts were made to improve that kinetic approach based on ideas of the Absolute Rate Theory (ART). Since then, one has been able to see more progress on the theoretical side than in the related experimental studies. However, the vast majority of these papers treated hypothetical adsorption kinetics in virtual adsorption systems with well-de®ned surfaces. This was especially true in the case of the theoretical studies of isothermal adsorption kinetics. Certain progress was made in theoretical studies of the kinetics of thermodesorption, where the energetic heterogeneity of the real solid surfaces was demonstrated in an impressive way. Here, almost all the attempts to take energetic surface heterogeneity into account were based on a further generalization of the ART approach.In the early eighties a new family of fundamental approaches appeared, linking the rate of adsorption/desorption processes to the chemical potentials of the bulk and the adsorbed molecules. Among them the so-called Statistical Rate Theory (SRT) has received the most advanced theoretical attention. The new SRT approach has been generalized during recent years for the case of energetically heterogeneous surfaces. This has been done for both the isothermal kinetics and the kinetics of thermodesorption. So we have only two fundamental approaches which have been Brought to you by | Purdue University Libraries Authenticated Download Date | 6/4/15 11:42 PM generalized for consideration of the kinetics of gas adsorption in the real systems, i.e. the ART and the SRT approaches.The purpose of the present review is to discuss the features of the adsorption kinetics in real adsorption systems, where the solid surface energetic heterogeneity is the main factor determining these features. For that purpose we will thoroughly analyze existing and possible future generalizations of both the ART and SRT approaches, taking this crucial physical factor into account. Next, we will apply the obtained theoretical expressions to quantitatively simulate the observed adsorption kinetics in real adsorption systems.Our study does not refer to the special case of the kinetics of sorption by porous media where the exchange of mass between the gas and the adsorbed phase may not be assumed as the only possible rate-controlling step. 150 W. Rudzinski, T. Panczyk

Effects of adsorbed phase on diffusion of subcritical hydrocarbons in activated carbon at low pressures

“…(9) are the surface di¤usion permeabilities in meso/macropores and in micropores, respectively. It is generally accepted that adsorption in activated carbon is a transition process from gas phase molecules to liquid-like molecules in the pores [13]. The di¤usion of adsorbed molecules is contributed by weakly adsorbed molecules in meso/macropores and by strongly adsorbed molecules in micropores, and it will be discussed in the next section.…”

Theoretical description of the kinetics of solute adsorption at heterogeneous solid/solution interfaces

“…and the experimentally monitored total surface coverage u ðeÞ t is then given by the following average [1,2]:…”

“…By considering the surface heterogeneity one might expect a large variety of adsorption behaviour to be observed. Surprisingly, it appears that, to the first approximation, the majority of all the reported experimental adsorption isotherms can be correlated fairly well using a number of simple empirical isotherms [2,3]. These have been, for instance, the Freundlich isotherm, applicable at low surface coverages, next its generalized form -the Langmuir-Freundlich isotherm, applicable also at higher surface coverages, and the Dubinin-Radushkevich isotherm used to describe adsorption on microporous surfaces.…”

Sticking coefficient and pressure dependence of desorption rate in the statistical rate theory approach to the kinetics of gas adsorption. Carbon monoxide adsorption/desorption rates on the polycrystalline rhodium surface

“…2,3 It was, however, soon generalized for the case of open adsorption systems, 4 including temperature programmed desorption, 5 and for adsorption systems where the adsorbing surface is not energetically uniform. [6][7][8][9][10] However, despite many successful applications of the SRT kinetic equation in the analysis of experimental data, [3][4][5][6][7][8][9][10] one can notice some peculiar features of this equation. These are the infinitely high adsorption rate for the zero coverage limit, infinitely high desorption rate at full saturation of the surface and dependence of the desorption rate on the adsorbate pressure.…”