Kinetics of adsorption and desorption

Third possibility to obtain the adsorption heats is to extract them from the data acquired from temperature-programmed desorption experiments. This possibility will be exposed in detail later, in the Sect.4.5.2. However, for that purpose, it is obligatory to know some basic postulates about the kinetics of adsorption and desorption what is given in the following section. 

In the case of gas-phase adsorbate, the siuface coverage 9 is dependent on the gas pressure. Adsorption isotherms relay the surface coverage and the gas pressure (at constant temperature) the most known equation of this type is Langmuir adsorption isotherm. It is based on the following assumptions [6]  

The rates of adsorption (rads) and desorption (fdes) are proportional to the numbers of empty or occupied active sites, 0 or (1 - 0), respectively  

If adsorption is a reversible process (i.e. backward process—desorption, passes through exactly the same states), the rates of both processes can be described using the same equation  

However, in contrast to adsorption which may or may not be activated process, desorption is always activated, with a minimum activation energy denoted as activation energy for desorption (AE ). The rate constant for desorption can be expressed by Arrhenius equation  

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The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

Detailed derivations of the isothemi can be found in many textbooks and exploit either statistical themio-dynaniic methods [1] or independently consider the kinetics of adsorption and desorption in each layer and set these equal to define the equilibrium coverage as a function of pressure [14]. The most conmion fomi of BET isothemi is written as a linear equation and given by ... 

Kinetics of Adsorption and Desorption and the Elovich Equation C. Aharoni and F. C. Tompkins... 

Eeulner P, Menzel D. 1985. The adsorption of hydrogen on Ru(OOOl) Adsorption states, dipole moments and kinetics of adsorption and desorption. Surf Sci 154 465. 

This equation has been used, especially in soil science, to describe the kinetics of adsorption and desorption on soils and soil minerals. 

This set of observations, then, leads to the conclusion that the kinetics of adsorption and desorption may severely effect the dispersion of nuclides in geomedia. The following paper in this symposium will describe in detail further experiments aimed at quantifying these effects. 

D. Kinetics of Adsorption and Desorption The rate of adsorption onto a solid surface is given by... 

The kinetics of adsorption and desorption and the Elovich equation have been the matter of a comprehensive review by Aharoni and Tompkins in 1970 [14]. At that time, however, concepts now pervasive in physical chemistry of surfaces like fractality were not known, the mathematical theory of adsorption equilibrium on heterogeneous surfaces was at its beginning, and the notion of equilibrium surfaces had not demonstrated yet its usefulness in the understanding of adsorption phenomena on real surfaces. In view of these facts there is a space for another work, which however does not intend to be as comprehensive as that of Aharoni and Tompkins, but rather aims to study the Elovich behaviour met in new situations, to elucidate the theoretical origin of Eq. (3), and to relate the macroscopic empiric parameters te, and t and to microscopic quantities. 

Bruemmer, G. W., Gerth, J., and Tiller, K. G. (1988). Reaction kinetics of adsorption and desorption of nickel, zinc and cadmium by goethite, I Adsorption and diffusion of metals. J. Soil Sci. 39, 37-52. 

In this chapter, we shall review the literature on the detailed kinetics of adsorption and desorption of oxygen on Ag with a view to establishing the nature of the selective oxygen and the surface structure of the oxygen overlayer under reaction conditions. We shall review the literature on the bonding of ethylene to O atoms on Ag and will include new data on the detailed kinetics of the bonding of ethylene to unoxidised and oxidised versions of a commercial Ag/a-AhOa catalyst and to unoxidised and oxidised versions of these catalysts promoted by Cs and Cl separately, and in combination. We shall review the various theories of the mechanism and of the role of the promoters in relation to these postulated mechanisms. 

Campbell and Paffett reported that adsorbed Cl atoms had no effect on the kinetics of adsorption and desorption of oxygen on Ag(llO) [40]. To an extent, this result was not surprising since Campbell and Paffett dosed the Cl atoms on to the Ag(llO) from CI2 gas at 300 K, producing ordered overlayers of Cl, discemable by LEED, and areas that were Cl free [40]. 

The initial work at Bartlesville has concentrated on measurements of enthalpy changes from dilution and adsorption for surfactant systems. From the observed dilution enthalpy changes, critical micelle concentrations have been determined, and standard state enthalpies of micel lization have been calculated. In the studies on adsorption, several properties are of interest the enthalpy of adsorption, the amount of surfactant adsorbed, the surface area of the solid and determining whether the adsorption is reversible. The kinetics of adsorption and desorption are also of interest. 

In gas-solid chromatography, the kinetics of adsorption and desorption determine the C terms in Figure 13.1. 

Chemical adsorption on metals is usually considered as nonacti-vated. For H2 on Ni/Al203, however, the isobars pass through a maximum at about 100°C 144), meaning that at 25°C kinetic factors (adsorption time) have an important influence on the amount of H2 adsorbed. For Rh/Si02 144) the amount of H2 adsorbed is found to depend very little on time the isobar descends smoothly as the temperature is increased. Thus each metal has its own behavior, which must be taken into account. The thermodynamics and kinetics of adsorption and desorption are discussed in the literature 145, 146). 

Adsorption isotherms can be applied to any surface. In the following we focus our attention on surfaces covered with adsorption layers under dynamic conditions, the kinetics of adsorption and desorption of surfactants to and from soluble adsorption layers for example. Another phenomenon is the spread of surfactant molecules tangential to the surface that effect takes place if the adsorption layer is inhomogeneous (cf. Fig. 1.1). 

Figure 25. Illustration of the kinetics of adsorption and desorption, calculated assuming first-order kinetics and rate constants obtained from coreflood experiments. (Reproduced with permission from reference 128. Copyright 1991 Royal Society of Chemistry.)...

It has been noted above that the influence of micelles on the adsorption process is determined by their ability to be a source or sink of monomers. The capacity of this source is determined by the parameter ncm- Flowever, only a small part of these monomers can participate in the fast process. Therefore, at low micellar concentration, the influence of the slow process (the change of the total number of micelles in the system) on the kinetics of adsorption and desorption of surfactants will be more significant. 

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