Adsorbance adsorption

Below the critical temperature of the adsorbate, adsorption is generally multilayer in type, and the presence of pores may have the effect not only of limiting the possible number of layers of adsorbate (see Eq. XVII-65) but also of introducing capillary condensation phenomena. A wide range of porous adsorbents is now involved and usually having a broad distribution of pore sizes and shapes, unlike the zeolites. The most general characteristic of such adsorption systems is that of hysteresis as illustrated in Fig. XVII-27 and, more gener-... 

The channels in zeoHtes are only a few molecular diameters in size, and overlapping potential fields from opposite walls result in a flat adsorption isotherm, which is characterized by a long horizontal section as the relative pressure approaches unity (Fig. 6). The adsorption isotherms do not exhibit hysteresis as do those in many other microporous adsorbents. Adsorption and desorption are reversible, and the contour of the desorption isotherm foUows that of adsorption. 

If an adsorbed chemical group (anchor) is more strongly bound to the surface than a solvent molecule would be at that site, an equiHbrium expression may be written for the displacement of solvent by adsorbate. Adsorption is particularly strong if the chemical nature of the adsorbed group is similar to that of the particle surface for example, in aqueous systems perfluoroalkane groups adsorb weU on polytetrafluoroethene particles and aromatic polyethene oxides adsorb weU on polystyrene. 

This reaction is catalyzed by iron, and extensive research, including surface science experiments, has led to an understanding of many of the details (72). The adsorption of H2 on iron is fast, and the adsorption of N2 is slow and characterized by a substantial activation energy. N2 and H2 are both dis so datively adsorbed. Adsorption of N2 leads to reconstmction of the iron surface and formation of stmctures called iron nitrides that have depths of several atomic layers with compositions of approximately Fe N. There is a bulk compound Fe N, but it is thermodynamically unstable when the surface stmcture is stable. Adsorbed species such as the intermediates NH and NH2 have been identified spectroscopically. 

Adsorption for gas purification comes under the category of dynamic adsorption. Where a high separation efficiency is required, the adsorption would be stopped when the breakthrough point is reached. The relationship between adsorbate concentration in the gas stream and the solid may be determined experimentally and plotted in the form of isotherms. These are usually determined under static equilibrium conditions but dynamic adsorption conditions operating in gas purification bear little relationship to these results. Isotherms indicate the affinity of the adsorbent for the adsorbate but do not relate the contact time or the amount of adsorbent required to reduce the adsorbate from one concentration to another. Factors which influence the service time of an adsorbent bed include the grain size of the adsorbent depth of adsorbent bed gas velocity temperature of gas and adsorbent pressure of the gas stream concentration of the adsorbates concentration of other gas constituents which may be adsorbed at the same time moisture content of the gas and adsorbent concentration of substances which may polymerize or react with the adsorbent adsorptive capacity of the adsorbent for the adsorbate over the concentration range applicable over the filter or carbon bed efficiency of adsorbate removal required. 

Usually, physisorption is carried out using nitrogen as an adsorbate at 77.3 K, the boiling point of liquid nitrogen. The solid material is called the adsorbent. Adsorption can be measured in many different ways. So-called volumetric adsorption, in which volumes of gas are introduced. sequentially while simultaneously measuring the pressure, is a commonly used technique. Fig. 3.41 shows a schematic of the equipment used (referred to as barometric equipment ). 

Adsorption is a process in which molecules of adsorbate become attached to the surface of a solid adsorbent. Adsorption processes can be divided into two broad classes ... 

The Langmuir Equation for the Case Where Two or More Species May Adsorb. Adsorption isotherms for cases where more than one species may adsorb are of considerable significance when one is dealing with heterogeneous catalytic reactions. Reactants, products, and inert species may all adsorb on the catalyst surface. Consequently, it is useful to develop generalized Langmuir adsorption isotherms for multicomponent adsorption. If 0t represents the fraction of the sites occupied by species i, the fraction of the sites that is vacant is just 1 — 0 where the summation is taken over all species that can be adsorbed. The pseudo rate constants for adsorption and desorption may be expected to differ for each species, so they will be denoted by kt and k h respectively. 

Rouquerol, F., Rouquerol, J., Sing, K., 1999, Chapter 12. Properties of some novel adsorbents, Adsorption by Powders and Porous Solids Principles, Methodology and Applications, Academic Press, London, 415... 

It is necessary to remind ourselves, that the adsorption of humic acids or fulvic acids correspond to the adsorption of a mixture of adsorbates. Adsorption equations derived for the adsorption of a single adsorbate (Langmuir, Frumkin or Gibbs Equation) cannot be used for mechanistic interpretation of the data even if these data can be fitted to such equations (Tomaic and Zutic, 1988). 

The variety of the different framework structures result in different adsorbent characteristics acid strength, size of molecule adsorbed, adsorption/desorption rate of different molecules, capacity and stability. As a result, these differences characterize the adsorbent s selectivity to a specific molecule and adsorbent-adsorbate interactions. Take for example, the difference in selectivity of BaY and Ba-Mordenite [24] to p-xylene (PX), m-xylene (MX) and o-xylene (OX) ... 

Adsorption is the net accumulation of matter on the sohd phase at the interface with an aqueous solution or gaseous phase. In this process, the solid surface is the adsorbent and the matter that accumulates is the adsorbate. Adsorption also may be defined as the excess concentration of a chemical at the subsurface solid interface compared to that in the bulk solution, or the gaseous phase, regardless of the nature of the interface region or the interaction between the adsorbate and the sohd surface that causes the excess. Surface adsorption is due to interactions between electrical charges, or nonionized functional groups, on mineral and organic constituents. 

Adsorbents. See Adsorption and Adsorbents Adsorption and Adsorbents. Adsorption may be defined as the ability of a substance (adsorbent) to hold on its surface, including inner pores or cracks, thin layers of gases, liquids or dissolved substances (adsorbates). Adsorption is a surface phenomenon and should not be confused with absorption (qv). Adsorption may be divided into physical and chemical (also called chemisorption). In physical adsorption the forces are those betw the adsorbing surface and the molecules of the adsorbate, and are similar to Van der Waals forces. In chemisorption, which in eludes ion exchange, the forces are much stronger than those of physical adsorption and depend on chemical bond formation. 

An important question is how much of a material is adsorbed to an interface. This is described by the adsorption function T = /(/, T), which is determined experimentally. It indicates the number of adsorbed moles per unit area. In general, it depends on the temperature. A graph of T versus P at constant temperature is called an adsorption isotherm. For a better understanding of adsorption and to predict the amount adsorbed, adsorption isotherm equations are derived. They depend on the specific theoretical model used. For some complicated models the equation might not even be an analytical expression. 

In adsorption, we call the gas or solution solute the adsorbate and the solid the adsorbent. Monolayer adsorption involves up to one layer of adsorbate on the adsorbent, whereas multilayer adsorption involves more than one layer of adsorbate. Adsorption may be either physical adsorption (physisorption), where the adsorbate is bound to the surface by relatively weak physical forces (AT/desorp < 40 kJ/mol) or chemical adsorption (chemisorption), where the binding forces are stronger (A//desorb > 40 kJ/mol).6 Because chemical adsorption involves chemical-type bonds between adsorbate and adsorbent, it is limited to the first monolayer on the surface. Physical adsorption can involve multiple layers and physical adsorption can occur on top of chemisorbed layer. 

The forces (repulsive or attractive) between the adsorbed molecules are negligible, when compared to the adsorbent-adsorptive interactions. 

Surface complexation — is complexation of metal ions by ligands immobilized on the electrode surface (-> electrode surface area). The ligands may be incorporated in the structure of a -> carbon paste electrode, covalently bound to the surface of a chemically modified electrode (-> surface-modified electrodes), or adsorbed (-> adsorption) on the electrode surface etc. Surface complexation is not confined to electrodes. It can occur on many surfaces, e.g., minerals, when in contact with metal ion solutions or solutions containing complexing ions (in the first case, the surface provides the ligand and the solution the metal ion, whereas in the second case, the surface provides the metal ion and the solution the ligand). Surface complexation can be an important step in the dissolution of solid phases [ii]. 

Consider a solvent A with a low concentration of a solute B (Figure C3-1). If the solute does not like the solvent it may concentrate strongly on an interface. This can be any interface with a gas, with another liquid or with a solid. This accumulation on an interface is known as adsorption. The phase on the other side of the interface is the adsorbent. Adsorption can be characterized by the fraction of the surface that is occupied by the solute - this can usually only be determined indirectly. This surface fraction is often much larger than the fraction of the solute in the bulk of the liquid. Here we consider only a solute that dissolves in a phase to one side of the interface dissolution in both phases is also possible. 

While previous work has often been conducted under conditions where only trace quantities of lead or other heavy metals have been placed in contact with an adsorbent, very few of these approaches have dealt with the problems faced as the adsorbent sites begin to be filled. The usefulness of the VSC-VSP model in taking this into account is illustrated here by demonstration of the effect of charged adsorbed species on the electrostatic potential which acts on the adsorbing ions. When a given number of equivalents of adsorbent are placed in contact with a comparatively large number of moles of cations, some of which will attach to the adsorbent, adsorption will be further opposed in two ways. First, of course, the process of adsorption will reduce the number of sites available for further adsorption. Second, the Gouy potential is said by Bowden ad. (7) to decrease from the... 

The phenomenon of adsorption was introduced in sec. 1.1.2. There one can find definitions of elementary notions, including those of adsorbent, adsorbate, adsorptive, desorption, specific surface area, adsorption isotherm (equation) and two-dimensional equation of state. Adsorbed amounts can eonveniently be expressed as moles adsorbed (n ), moles adsorbed per unit area or surface concentration (F = n /A) or, if the adsorption in a monolayer r(max) is known, as... 

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