Physical adsorption and chemisorption

The nature of the adsorption bond depends on the nature of the solid and the gas-phase molecules. It is customary practice to distinguish physical and chemical adsorption. In the former case, no collectivization of the electrons of the adsorbent and adsorbate occurs, while in the latter case a conventional chemical bond forms with redistribution of the electrons. 

If the values of the energies Ex and E2 are close and have the order of magnitude of the thermal energy of the adsorbed particles, one has to do with non-localized adsorption. It occurs most frequently in physical adsorption. Here too lowering of the temperature may be attended by a 

A type of the adsorption is connected with a type of the phase transition in which adspecies could be involved. The adspecies condensation (the first type of the phase transition) as usually occurs at the physical adsorption, while the adspecies ordering (the second type of the phase transition) occurs at the chemical adsorption. However, more complex phase transitions (ordering with condensation) are quite often realized at the chemical adsorption. 

In the condensed phase the AC permanently interacts with its neighbors, therefore a change in the local phase composition (as were demonstrated on Figs. 8.1 and 8.2) affects the activation barrier level (Fig. 8.6). Historically the first model used for surface processes is the analogy of the collision model (CM) [23,48,57]. This model uses the molecular-kinetic gas theory [54]. It will be necessary to count the number of the active collisions between the reagents on the assumption that the molecules represent solid spheres with no interaction potential between them. Then the rate constant can be written down as follows (instead of Eq. (6))  

In the Eq. (59) P0 is the steric factor EAB is the rate activation energy Kab is the frequency cofactor or the preexponential rate constant factor d — Ra + Rb, here Rt and are the radius of the sphere and mass of species 

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. 

It is in many cases difficult to decide whether a certain adsorption phenomenon belongs to the physical adsorption type or whether it is a case of chemisorption. If we define physical adsorption as the phenomenon which occurs when the molecules are bound to the surface of the adsorbent by van der Waals cohesion forces in the widest sense of the word, hence 

Chemisorption is often identified with the so-called activated adsorption, where the rate of adsorption is governed by an activation energy. We shall have to conclude in our following discussions that this criterion is not valid. 

One might suppose that in chemisorption phenomena the adsorbed atoms or molecules occupy fixed places on the surface and that physically adsorbed molecules show some freedom of movement over the surface. We shall see, however, that mobile adsorption and adsorption on definite adsorption sites may occur in both cases. 

The absorption of light by certain molecules is sometimes drastically changed when they are adsorbed. This phenomenon undoubtedly has some relation to the nature of the binding forces and one might be inclined to consider it as chemisorption. We shall be compelled in our following discussions to shatter this conception. 

The presence of unbalanced attractions at the surface of a solid—say, a metal such as nickel—means that small molecules will tend to become rather loosely attached to the surface in one or (more likely) several molecular layers with an exothermic adsorption energy ranging to about —20 kJ mol for nonpolar molecules. (The term adsorption is used to denote surface sorption without penetration of the bulk solid, which would be called absorption.) No chemical bonds are formed or broken. This state is usually called physical adsorption or physisorption. If, however, the adsorbate forms chemical bonds with the surface atoms, the adsorption process is called chemisorption. Chemisorption can be quite strongly exothermic (—40 to —800 kJ mol ) but involves only the first monomolecular layer of adsorbate. 

As Fig. 6.1 shows, with reference to the adsorption of hydrogen on nickel. 

In fact, the most catalytically active transient oxygen species on surfaces is often the singly charged atom 0.  

With the advent of sophisticated experimental techniques for studying surfaces, it is becoming apparent that the structure of chemisorbed species may be very different from our intuitive expectations. For example, ethylene (ethene, H2C=CH2) chemisorbs on platinum, palladium, or rhodium as the ethylidyne radical, CH3 — (Fig. 6.2). The carbon with no hydrogens is bound symmetrically to a triangle of three metal atoms of a close-packed layer [known as the (111) plane of the metal crystal] the three carbon-metal bonds form angles close to the tetrahedral value that is typical of aliphatic hydrocarbons. The missing H atom is chemisorbed separately. Further H atoms can be provided by chemisorption of H2, and facile reaction of the metal-bound C atom with three chemisorbed H atoms dif- 

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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. 

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. 

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.

These methods suggested in the present form by Caunt83) rely on inhibition (retardation) effects of strong catalyst poisons on polymerization. Typical poisons potentially usable for this purpose are carbon oxides, carbonyl sulfide, carbon disulfide, acetylenes and dienes. All these substances exhibit a strong unsaturation they have either two double bonds or one triple bond. Most of the works devoted to application of the poisons to determination of active centers 10,63 83 102 1O7) confirm a complicated nature of their interaction with the catalytic systems. To determine the active centers correctly, it is necessary to recognize and — as much as practicable — suppress side processes, such as physical adsorption and chemisorption on non-propagative species, interaction with a cocatalyst, oligomerization and homopolymerization of the poison and its copolymerization with the main chain monomer. 

For the moment, let us focus our attention on gas-phase reactions catalyzed by solid smfaces. For a catalytic reaction to occur, at least one and frequently all of the reactants must become attached to the smface. Tliis attachment,is known as adsorption and takes place by two different processes physical adsorption and chemisorption. Physical adsorption is similar to condensation. The process is exothermic, and the heat of adsorption is relatively small, being on the order of 1 to 15 kcal/g mol. The forces of attraction between the gas molecules and the solid smface are weak. These van der Waals forces consist of interaction between permanent dipoles, between a permanent... 

The complexities of solid surfaces and onr inability to characterize exactly their interactions with adsorbed molecules hmits our understanding of the adsorption process. It does not, however, prevent development of an exact thennodynamic description of adsorption equilibrium, applicable alike to physical adsorption and chemisorption and equally to monolayer and multilayer adsorption. The thermodynamic frame work is independent of any particular theoretical or empirical description of material behavior. However, in application such a description is essential, and meaningful results require appropriate models of behavior. 

Some experiments were made with the adsorption of propylene at 100 torr but the data did not permit a clear-cut separation of physical adsorption and chemisorption. However, no substantial drift in weight occurred at 100 torr, most of the adsorbed propylene was liberated by helium flushing at 100° and all, by the original temperature of activation. 

Three main subjects are discussed in this chapter the effect of adsorption on catalysis, the distinction and the differences between physical adsorption and chemisorption with regard to rates of adsorption, and heats of adsorption. 

The adsorbing molecule loses entropy since its motion on the surface is more restricted than in the gas phase. The free energy of the system also decreases as the surface valencies become saturated so it can be concluded that the adsorption process is always exothermic. There is no single criterion which distinguishes between physical adsorption and chemisorption in all systems, but there are a few which are generally valid ... 

Such behavior is consistent with what one would expect from the chemical nature of the adsorbate and the adsorbent surfaces. The platinum surface is unreactive and the behavior of the stearic acid film on it is typical of what is generally regarded as physical adsorption, while the NiO surface is reactive and the stearic acid is considered to be chemisorbed. In Section 10.3.2 below we shall examine the difference between physical adsorption and chemisorption in detail. 

At a first approach we can take the feasibility of desorption as the distinguishing difference between physically adsorbed and chemisorbed films. Even though this criterion may break down both experimentally and semantically in certain cases, it is workable as an initial guideline and it keeps us from becoming enmeshed in exceptions and modifications before we are ready for them. Chemisorbed films can be put on the adsorbing surfaces by the same techniques as physically adsorbed films retraction from the melt or from the liquid, retraction from solution, vapor deposition, etc. Chemisorbed films iHu respond to probes for the nature of the film—e.g. drop contact angle or surface potential — in the same way as physically adsorbed films. It is not until we attempt to desorb the film that we become aware of the difference between physical adsorption and chemisorption, as exemplified by the observations of Timmons and Zisman cited above [10]. 

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