Adsorption and Chemisorption

Apart from the usual adsorptive dryer station, additional adsorption steps can serve to remove trace components (e.g. sulphur compounds, alcohols, aldehydes, ketones, ester, other odorants, aromatics, etc.). All molsieves and all types of activated carbon are used as adsorbents, but also zinc and ferric oxides which, in contrast to the dryers, are usually not regenerated in situ but have to be replaced by fresh adsorbents after saturation. According to the quantity of the component(s) to be removed, adsorption occurs in single adsorbers or in series-connected twin-adsorbers. This series-connection (also called Lead/Lag configuration) is designed in such a way that the container with the unloaded adsorbent follows the active one during normal operation of the plant. When the adsorption capacity of the active adsorber is exhausted, it is taken out of operation and the vessel with the fresh adsorbent is turned into the active one. The loaded adsorbent can be replaced by a fresh one while the plant is kept in operation. 

---

As also noted in the preceding chapter, it is customary to divide adsorption into two broad classes, namely, physical adsorption and chemisorption. Physical adsorption equilibrium is very rapid in attainment (except when limited by mass transport rates in the gas phase or within a porous adsorbent) and is reversible, the adsorbate being removable without change by lowering the pressure (there may be hysteresis in the case of a porous solid). It is supposed that this type of adsorption occurs as a result of the same type of relatively nonspecific intermolecular forces that are responsible for the condensation of a vapor to a liquid, and in physical adsorption the heat of adsorption should be in the range of heats of condensation. Physical adsorption is usually important only for gases below their critical temperature, that is, for vapors. 

In such an experiment the material actually adsorbed by the solid (the adsorbent) is termed the adsorbate, in contradistinction to the adsorptive which is the the general term for the material in the gas phase which is capable of being adsorbed. The adsorption is brought about by the forces acting between the solid and the molecules of the gas. These forces are of two main kinds—physical and chemical—and they give rise to physical (or van der Waals ) adsorption, and chemisorption respectively. The nature of the physical forces will be dealt with in the next section meanwhile it is convenient to note that they are the same in nature as the van der Waals forces which bring about the condensation of a vapour to the liquid state. 

Adsorption of dispersants at the soHd—Hquid interface from solution is normally measured by changes in the concentration of the dispersant after adsorption has occurred, and plotted as an adsorption isotherm. A classification system of adsorption isotherms has been developed to identify the mechanisms that may be operating, such as monolayer vs multilayer adsorption, and chemisorption vs physical adsorption (8). For moderate to high mol wt polymeric dispersants, the low energy (equiUbrium) configurations of the adsorbed layer are typically about 3—30 nm thick. Normally, the adsorption is monolayer, since the thickness of the first layer significantly reduces attraction for a second layer, unless the polymer is very low mol wt or adsorbs by being nearly immiscible with the solvent. 

Gas adsorber A device for the removal of gaseous impurities from a gas or liquid phase, two methods are in use, physical adsorption and chemisorption. 

What was evident in 1950 was that very few surface-sensitive experimental methods had been brought to bear on the question of chemisorption and catalysis at metal surfaces. However, at this meeting, Mignolet reported data for changes in work function, also referred to as surface potential, during gas adsorption with a distinction made between Van der Waals (physical) adsorption and chemisorption. In the former the work function decreased (a positive surface potential) whereas in the latter it increased (a negative surface potential), thus providing direct evidence for the electric double layer associated with the adsorbate. 

Chemiluminescent immunoassay systems, commercial, 14 151 Chemineer CD6 agitator, 1 739 Chemineer CD6 impeller, 16 673, 701, 703 Chemisorbed water, 23 71 Chemisorption, 1 583-584 for indoor air cleaning, 1 834 parameters of physical adsorption and chemisorption contrasted, l 583t Chemisorption chromatography, 6 405 Chemistry. See also Combinatorial... 

Gas adsorption is the most commonly used method for characterizing the surface area of catalysts. Both physical adsorption and chemisorption may be used. Furthermore, EM can provide supplementary information. A large surface area is desirable since activity is defined as the rate per unit active surface area ((per metre) ), and this necessitates porous catalysts. Eor an idealized porous system. 

The heat curves, themselves, are informative. The kaolin-based pellet catalyst has a few more active sites then attapulgite, but its site activity decreases rapidly and to values only about 3 kcal./mole above the heat of liquefaction of the liquid at maximum coverage. Obviously, a distinction cannot be made between physical adsorption and chemisorption for some of the amine adsorbed at full coverage on the cracking catalyst. On the other hand, attapulgite has a much narrower distribution of adsorption energies, and the lowest heats are about double the heat of liquefaction of butyl amine. Therefore, it appears safe to conclude that the amount remaining after evacuation at 25° is chemisorbed. 

One of the valuable features of this method is its ability to distinguish clearly between physical adsorption and chemisorption, since only the latter causes an appreciable change in magnetization. Thus, Selwood 97) has demonstrated conclusively that some hydrogen is rapidly chemisorbed by nickel even at —196°, but the amount is small compared with that at higher temperatures. 

Only monomolecular chemisorbed layers are possible. Chemisorption is a specific process which may require an activation energy and may, therefore, be relatively slow and not readily reversible. The nature of physical adsorption and chemisorption is illustrated by the schematic potential energy curves shown in Figure 5.2 for the adsorption of a diatomic gas X2 on a metal M. 

Figure 5.2 Potential energy curves for physical adsorption and chemisorption...

Solid surfaces are usually heterogeneous therefore, since adsorption at the more active sites is favoured, heats of both monolayer physical adsorption and chemisorption might, in this respect, be expected to become significantly less exothermic as the surface coverage increases, as, for example, shown at low pressures in Figures 5.12a and 5.12b. This, in turn would cause the initial slope of an adsorption isotherm to be steeper than that predicted according to the Langmuir equation or the BET equation. 

One might be inclined to think that only chemisorption would lead to an enhanced reactivity in this type of catalysis. Adsorption by physical forces only tends to lower the reactivity of the adsorbed molecules. It is, however, difficult to give such definitions of physical adsorption and chemisorption that the fields are clearly separated. We shall, therefore, discuss some of the differences and similarities between these two kinds of adsorption. 

The investigation of adsorption and chemisorption of various substances on M/SC nanoparticles in PVD-prepared films... 

According to the kind of interaction forces and binding relationship between the solid and the adsorbed particle, we distinguish the physical adsorption and chemisorption. The real state of most systems is between these two extremes. For a detailed study of both adsorption mechanisms see e.g. Fripat et al.80. ... 

In general, two types of adsorption are distinguished, physical adsorption and chemisorption, which depend on the type of interaction established between the adsorbent and the adsorptive. In a chemisorption process, specific chemical interactions between the adsorbent and the adsorptive occur, and the process is not reversible. On the other hand, physical adsorption includes attractive dispersion forces and, at very short distances, repulsive forces, as well as contribution from polarization and electrostatic forces between permanent electrical moments and the electrical field of the solid, if the adsorptive or the adsorbent has a polar nature. In this case, the process is fully reversible (or almost reversible). Thus, the overall interaction energy ( >(z) of a molecule of adsorptive at a distance z from the surface of the adsorbent is given by the general expression... 

Figure 1.2 Potential energy curves for the approach of a hydrogen molecule and of two hydrogen atoms to a metal surface E is the activation energy — AH is the heat of adsorption subscripts p and c are, respectively, physical adsorption and chemisorption.

---