Adsorption and Desorption Processes

An especially readable discussion of this topic is provided by Hayward and Trapnell [1]. This section will focus only on chemisorption (chemical adsorption) because physisorption (physical adsorption) is addressed in Chapter 3 as it relates to catalyst characterization. 

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The simplest case to be analyzed is the process in which the rate of one of the adsorption or desorption steps is so slow that it becomes itself rate determining in overall transformation. The composition of the reaction mixture in the course of the reaction is then not determined by kinetic, but by thermodynamic factors, i.e. by equilibria of the fast steps, surface chemical reactions, and the other adsorption and desorption processes. Concentration dependencies of several types of consecutive and parallel (branched) catalytic reactions 52, 53) were calculated, corresponding to schemes (Ila) and (lib), assuming that they are controlled by the rate of adsorption of either of the reactants A and X, desorption of any of the products B, C, and Y, or by simultaneous desorption of compounds B and C. 

Measurements of the double-layer capacitance provide valuable insights into adsorption and desorption processes, as well as into the structure of film-modified electrodes (6). 

Principal differences between catalysis by dissolved electrolytes and by resins are that with resins as catalysts catalysis overlaps with diffusion, adsorption, and desorption processes, while this is not the case with electrolytes (Naumann, 1959). Also, the matrix of the resin with fixed ionic group may have some influence on the course of reaction. 

Hougen- Watson Models for Cases where Adsorption and Desorption Processes are the Rate Limiting Steps. When surface reaction processes are very rapid, the overall conversion rate may be limited by the rate at which adsorption of reactants or desorption of products takes place. Usually only one of the many species in a reaction mixture will not be in adsorptive equilibrium. This generalization will be taken as a basis for developing the expressions for overall conversion rates that apply when adsorption or desorption processes are rate limiting. In this treatment we will assume that chemical reaction equilibrium exists between various adsorbed species on the catalyst surface, even though reaction equilibrium will not prevail in the fluid phase. 

The sulphate adsorption and desorption processes run cyclically by supplying brine and water to the resin column under automatic control. A number of optional operation parameters are available. 

Fig. 12.7 Change of outflow concentrations in the adsorption and desorption processes. For adsorption NaCI = 302.3 g I-1 I = 1.0549 ppm pH = 2 room temperature. For desorption pH = 12 room temperature.

Fig-l- Schematic description of the different zones in an SMB system and the adsorption and desorption processes in these zones... 

Note that the redox reaction of electron transfer via adsorption intermediates requires the adsorption and desorption processes to occur as the preceding and... 

In the chromatographic liquid adsorptive separation process, the adsorption and desorption processes must occur simultaneously. After the desorption step, both the rejected product (product with lower selectivity, resulting in less adsorption by adsorbent) and the extracted product (product with higher selectivity, resulting in strong adsorption by adsorbent) contain desorbent In general, the desorbent is recovered by fractionation or evaporation and recycled back into the system. 

In heterogeneous catalysis, the catalyst often exists in clusters spread over a porous carrier. Experimentally, it is well established that reactivity and selectivity of heterogeneous reactions change enormously with cluster size. Thus, theoretical studies on clusters are particularly important to establish a basis for the determination of their optimal size and geometry. Cluster models are also important for studying the chemistry and reactivity of perfect crystal faces and the associated adsorption and desorption processes in heterogeneous catalysis (Bauschlicher et al, 1987). 

Porosity refers to the volume of pores in a solid. It contributes to the internal surface area of the sample and can influence the kinetics of adsorption. Diffusion into and out of pores is often considered responsible for slow adsorption and desorption processes. Pores vary in size and shape. They have been classified according to their average widths as micropores which are of the order of molecular dimensions (<2 nm), meso- or transitional pores which are between 2-50 nm and macropores which are larger than 50 nm (Sing et al., 1985). The sum of all the pores is called the pore volume (porosity). 

Even a very brief description of numerous papers published in the recent years was often beyond the scope of this work. Instead, a concise presentation of general properties of adsorption and desorption processes and electrochemical properties of such modified electrodes will be given. 

Introduction of a suppression device between the column and the detector can be expected to cause some degree of peak broadening due to diffusional effects. The shape of the analyte band will also be influenced by hydrophobic adsorption effects, especially when the adsorption and desorption processes are slow. Examination of peak shapes and analyte losses can therefore provide important insight into the use of suppressors with organic analytes which are weakly acidic or weakly basic. It can be expected that peak area recovery rates after suppression are governed by a combination of hydrophobic interactions with the suppressor and permeation through the membranes with the balance between these mechanisms being determined by eluent composition, suppression conditions and analyte properties. 

In ion-exchange chromatography, adsorption and desorption processes are determined by the properties of the three interacting entities ... 

A C.P.D. method was adopted by Bosworth and Rideal (95, 119) to investigate the evaporation of Na from a W filament. Desorption was accompanied by a negative drift in the S.P. when the coated filament was held at a temperature in the range 610° to 795° K., and the resulting S.P.-time curves were converted into coverage-time curves by the use of calibration data previously obtained. The results represent the mutual effect of adsorption and desorption processes on the W filament. Hence, the heat of evaporation E wav iiaay be calculated from the temperature coefficient of... 

There are several types of mechanisms that could account for this behavior. (A) There could be abnormal flow paths, such as fissures which allow a small amount of activity to migrate more quickly than the main body. (b) This activity could be carried on colloidal, non-absorbing particles. Or (C) the flow rate could be too fast for equilibrium to occur for the adsorption and desorption processes. These possibilities can be examined to determine which appears the most important. 

Non-linearities arising from non-reactive interactions between adsorbed species will not be our main concern. In this section we return to variations of the Langmuir-Hinshelwood model, so the adsorption and desorption processes are not dependent on the surface coverage. We are now interested in establishing which properties of the chemical reaction step (12.2) may lead to multiplicity of stationary states. In particular we will investigate situations where the reaction step requires the involvement of additional vacant sites. Thus the reaction step can be represented in the general form... 

The simple model just discussed shows multistability even when the system is clean but requires the involvement of a poison for oscillations. One reason for this is that the latter is needed to provide a second independent surface concentration, so we theij. have a two-variable system. It was mentioned in 12.3.1 that implicit in the rate law used above may be the adsorption of a second reactant which participates in the reaction step. The latter did not provide a second concentration variable there since its adsorption and desorption processes were assumed to be on a very much faster (instantaneous) timescale. 

Comparison to Solvent Extraction. The methodology for using solid adsorbents is described in detail in a later section. Briefly, it involves adsorption and desorption processes. In the adsorption process, the liquid matrix is water containing very small amounts of organic... 

In Sect. 1.2, we chose to ignore the effects of the double layer as well as complexities due to adsorption and desorption processes. Similarly, we will also choose to ignore the presence of non-faradaic currents, though in practice they may be important. Hence, throughout this chapter only faradaic currents will be considered. 

All adsorption and desorption processes depend on transport of solute to and from the interface. There are basically four major transport mechanisms (Fig. 3) ... 

Since mass action law for elementary reactions in ideal adsorbed layers (including also adsorption and desorption processes) coincides in its form with mass action law for elementary reactions in volume ideal systems, general results of the theory of steady-state reactions are equally applicable to volume and to surface reactions. They are very useful when the reaction mechanism is complicated. 

Adsorption and desorption processes are particular cases of stage 1 namely, when substance Bt is absent, and I coincides with Aj, rA is the rate of adsorption and rB is the rate of desorption. Since the equilibrium of stage 2 is maintained as a result of mutually reverse elementary reaction, the particles I very frequently leave some surface sites and appear on others. 

Consequently, they are distributed on the surface randomly even if there is no surface migration. Therefore, in contrast to the discussion of adsorption and desorption processes in Section X, the mechanism (188) needs no assumed rapid surface migration (it is evident, of course, that if the migration occurs, it does not affect the results). The analysis of adsorption and desorption rates given in Section X needs only minor alterations to its application to stage 1 namely, products kbPb> would be substituted for rate constants of desorption on separate surface sites, k, and the fugacity of adsorbed particles I, px for p. Therefore, in analogy to (148) and (157), we obtain... 

Duinker, 3.C., 1980. Suspended matter in estuaries adsorption and desorption processes. In E. Olausson and I. Cato (eds), Chemistry and Biogeochemistry of Estuaries. Wiley, New York, pp. 121-151. 

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