Gas Adsorption on Solids

Recently the new SRT approach has been generalized further to take into account the energetic heterogeneity of the actual adsorption systems and the possible role of the interactions between the adsorbed molecules. [10-20] Most recently, the authors have shown, that the SRT approach can be successfully applied to describe the multi-site-occupancy adsorption of molecules which do not dissociate after being adsorbed. [14] Compact simple analytical expressions were developed, and next used successfully to correlate experimental data for adsorption/desorption kinetics in various gas/solid systems. The purpose of this presentation is to show, that the new SRT approach can be also applied to represent the kinetics of dissociative gas adsorption on solids. That kinetics is of a crucial importance in a variety of catalytic reactions occurring on solid surfaces. 

The thickness of adsorbed layers depends on the extent of molecular interactions, but usually films of one molecule thickness, which are called monolayers, form at the liquid-vapor and liquid-liquid interfaces upon adsorption. In gas adsorption on solids, several molecular layers, which are called multilayers, form at high pressures, and mono-layers can only be formed if the gas pressure is sufficiently low. If van der Waals forces are operative during the adsorption process, it is called physical adsorption or physiosorption (see Section 8.3.1), whereas if chemical bonds are formed during the adsorption process, then it is called chemical adsorption or more preferably chemisorption (see Section 8.3.2). 

As we saw in Section 3.3, the concentration difference of one constituent of a gas or liquid solution at the surface of another phase is called adsorption. In other words, adsorption is the partitioning of a chemical species between a bulk phase and an interface. Desorption is the reverse of the adsorption process, showing that the molecules are leaving the interface towards the other phase. Adsorption is different from absorption where a species penetrates and is dissolved throughout the bulk phase of a liquid or a solid. For gas or liquid adsorption on solids, the solid material on which adsorption takes place is defined as the adsorbent the material in the adsorbed state (while bound to the solid surface) is called the adsorbate, and the gas, vapor or liquid molecule prior to being adsorbed is called the adsorptive. In this section, we will investigate only the principles and applications of gas adsorption on solids (see Section 9.4 for liquid adsorption on solids). 

Thermodynamics of gas adsorption on solids relation with the Gibbs adsorption equation... 

That equation and its integral form soon gained enormous popularity among scientists investigating the kinetics of gas adsorption on solid surfaces. Correlations of experimental data usually started with the application of that equation, and the frequently noted deviations from "Elovich behavior" were the form of analysis in dozens of published papers. Even reviews followed that general trend, discussing the Elovich equations and "deviations" from it. 

Rudzinski, W., et al.. On the theoretical origin and applicability of the potential theory approach to predict mixed-gas adsorption on solid surfaces from single-gas adsorption isotherms, Chem. Eng. Sci., 50(16), 2641-... 

As a final remark we would like to draw reader s attention to some neighboring fields of gas adsorption on solid surfaces which for obvious reasons could not be encountered here, namely... 

Gas Adsorption on Solid Measurement Methods 1.6.6.1 Volumetric Change Methods... 

The gas adsorption on solid surface data has been analyzed by different models. Gas recovery from shale deposits is a very important example of such surface phenomena. Other isotherm equations begin as an alternative approach to the developed equation of state for a two-dimensional ideal gas. As mentioned earlier, the ideal equation of state is found to be as... 

Regular solution theory employs simple statistical arguments to capture the nature of phase equilibria of solutions. Although simple, it has been used widely to predict the existence of phases and phase transitions. It has been used to explain different first-order transition phenomena, such as ferromagnetism and gas adsorption on solid surfaces. 

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