Adsorption activated chemisorption

When gaseous or liquid molecules adhere to thesurface of the adsorbent by means of a chemical reaction and the formation of chemical bonds, the phenomenon is called chemical adsorption or chemisorption. Heat releases of 10 to 100 kcal/g-mol are typical for chemisorption, which are much higher than the heat release for physisorption. With chemical adsorption, regeneration is often either difficult or impossible. Chemisorption usually occurs only at temperatures greater than 200 C when the activation energy is available to make or break chemical bonds. 

Effect of temperature on amount adsorbed for simultaneous physical adsorption and activated chemisorption. (Adapted from Chemical Engineering Kinetics by J. M. Smith. Copyright 1970. Used with permission of McGraw-Hill Book Company.)... 

Adsorption. Adsorption refers to the adsorption of gas on the gas-solid interface only. It may be van der Waal s adsorption or chemisorption, the latter including activated adsorption. 

Activated Adsorption. Activated adsorption—that is, adsorption with a measurable rate of adsorption and a measurable temperature coefficient of rate of adsorption—is a type of chemisorption which is, for instance, found in the adsorption of nitrogen on certain metals at elevated temperatures. The difficulties of deciding whether or not true van der Waal s adsorption exists in cases where the heats of adsorption exceed considerably the heats of condensation will become apparent later in the text. 

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 transition from physical adsorption to chemisorption occurs at point A. The potential energy at A is in excess of that for the adsorbate and the adsorbent when separated and represents the activation energy required for chemisorption, A fl. If curve I resided more to the right or curve II more to the left, then the transition from physical to chemical adsorption would occur with no activation energy since the crossover point would reside beneath zero potential energy. 

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. 

Although it is evident that the reaction is almost entirely homogeneous, Stoddart403,404 presented evidence that the rate of reaction is influenced by the history of the reaction vessel it diminishes from experiment to experiment when starting with a new vessel it also diminishes with time during the course of a single experiment. However, the termolecular rate constant does not depend on the surface-to-volume ratio. He concluded that these effects are due to activated adsorption or chemisorption of nitrosyl chloride on the adsorbed moisture of the... 

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. 

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. 

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.

IN THIS SURVEY of current concepts in adsorption and chemisorption, it is pointed out that entropy relations, both thermodynamic and kinetic, have made a relatively late appearance on the scene of adsorption research. Exaggerated preoccupation with heats of adsorption and energies of activation has led to a frozen formalism which appears to have outlived much of its usefulness. This situation is now being corrected by more attention to molecular structure of adsorbed layers and its relation to entropies of adsorption. 

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. 

ABC physical adsorption DFGH activated chemisorption DFH rton activated chemisorption... 

The essential parts of a volumetric apparatus are a closed system of known volume, a source of adsorbate and a pressure measuring device. This type of apparatus is more commonly used for the determination of adsorption isotherms, but kinetics can be measured provided that the adsorbent surface is large and the response of the pressure measuring device is much faster than the rate of adsorption. These criteria are usually only satisfied by powders or films which show activated adsorptions. Bond has discussed the phenomenon of activated adsorption. Although the results may often be ascribed to contamination or incorporation into the bulk of the material, in other instances a genuine activated chemisorption is found. 

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. 

The last point deserves a special discussion. In fact, due to the short-range nature of the forces involved in chemisorption, E Q) is expected to depend on 0 only for 0 1/c, where c is the coordination number i.e., the number of nearest neighbours) of each adsorption site. Since the Elovich behaviour is associated with the final stages of the process, this is not a serious difficulty in activated chemisorption, where the appearance of the logarithmic kinetics is associated with high coverage. 

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