Surface phenomena desorption

Adsorption is a surface phenomenon. When a multi-component fluid mixture is contacted with a solid adsorbent, certain components of the mixture (adsorbates) are preferentially concentrated (selectively adsorbed) near the solid surface creating an adsorbed phase. This is because of the differences in the fluid-solid molecular forces of attraction between the components of the mixture. The difference in the compositions of the adsorbed and the bulk fluid phases forms the basis of separation by adsorption. It is a thermodynamically spontaneous process, which is exothermic in nature. The reverse process by which the adsorbed molecules are removed from the solid surface to the bulk fluid phase is called desorption. Energy must be supplied to carry out the endothermic desorption process. Both adsorption and desorption form two vital and integral steps of a practical adsorptive separation process where the adsorbent is repeatedly used. This concept of regenerative use of the adsorbent is key to the commercial and economic viability of this technology. 

Figure 3.9 The phenomena of adsorption and partition. Contrary to absorption, adsorption is a surface phenomenon. Separation is due to a series of adsorption/desorption steps. Absorption is due to solute partitioning between the two phases.

The adsorption in a catalytic reaction is an exothermic phenomenon. However, either the adsorption of molecules at the surface or desorption from the surface occurs under different strengths, decreasing the degree of freedom that facilitates the reaction. The energy of activation of a catalytic reaction is, therefore, lower than the energy... 

Substances differ in their adsorption-desorption behaviour between a moving solvent (a liquid or a gas) and a stationary solid phase. This behaviour of a substance can be exploited to achieve its separation. Adsorption is a surface phenomenon which signifies a higher concentration at an interface as compared to that present in the surrounding medium (see chapter 7). Adsorption should not be confused with absorption, which signifies the penetration of one substance into the body of another. 

In this maimer, it can also be seen that molecules will desorb as the surface temperature is raised. This is the phenomenon employed for TPD spectroscopy (see section Al.7.5.4 and section BT25). Note tliat some adsorbates may adsorb and desorb reversibly, i.e. the heats of adsorption and desorption are equal. Other adsorbates, however, will adsorb and desorb via different pathways. 

Desorption of the organic molecules at potentials where is large is due to a phenomenon known in electrostatics In any charged electrostatic capacitor, forces are operative that tend (when this is possible) to replace a medium with a low e value with a medium with a higher s value. Therefore, regardless of any chemical interaction of the organic molecules with the surface, they are expelled electrostatically from the EDL at a certain value of Qs , and replaced by water molecules. 

Surface diffusion is yet another mechanism that is often invoked to explain mass transport in porous catalysts. An adsorbed species may be transported either by desorption into the gas phase or by migration to an adjacent site on the surface. It is this latter phenomenon that is referred to as surface diffusion. This phenomenon is poorly understood and the rate of mass... 

Asymmetry of the response curve to the point of the exposition end reflects the different nature of the exposition and relaxation output signals. A transition from an exposition into relaxation phase corresponds to a return of gas-sensitive matter contact with the initial atmosphere. A variety of processes take place simultaneously in that phase. They may include oxidation of adsorbed molecules by the air oxygen, desorption of the previously adsorbed molecules, competitive adsorption of the ambient atmosphere components. These circumstances cause a complicated shape of the relaxation curve. In general, its course reflects the dynamics of the surface concentration of conductivity clusters. Almost all relaxation curves are characterized by presence of a maximum. It is often more prominent that the corresponding exposition maximum. The origin of this phenomenon is determined by higher conductivity of clusters formed by the oxidized molecules of compounds adsorbed during the exposition phase. 

When the applied electric field reaches a few volts per angstrom range, atoms on a surface, irrespective of whether they are lattice atoms or adsorbed atoms and of whether the surface temperature is high or low, may start to emit out of the surface in the form of ions. This high electric field produced evaporation phenomenon is usually called field evaporation if the surface atoms are lattice atoms, and is called field desorption if they are adsorbed atoms. From a theoretical point of view there are no fundamental differences. We will use the term field desorption for general purposes, especially for theoretical discussions, since desorption is the term used in many other adsorption-desorption phenomena. When we specifically mean removal of lattice atoms by electric field the term field evaporation will be used. Sometimes field evaporation is used where it may mean both field evaporation and field desorption. 

Although other possibilities cannot as yet be absolutely ruled out, the evidence strongly indicates that in this study the desorption of product molecules from the surface (pore mouths) of the zeolite crystallites is a rate-limiting step. Further, product desorption limitations are probably also responsible for the maxima in rates previously reported (7, 8, 9) and may be a more general phenomenon for zeolite systems. Such limitations... 

Sevastianov et al.73,74) have developed a model which considers the effect of surface heterogeniety on the adsorption process. They define centers of irreversible adsorption , labeled P, and centers of irreversible desorption , labeled D. They argue, in agreement with Soderquist and Walton, that desorbed material is conformationally altered and thus cannot readsorb — hence desorption is irreversible. The results of this model are given as Fig. 14, taken from Ref. 7J). The model also includes the case where adsorption may be transport limited. The model fits commonly observed adsorption data, including the overshoot phenomenon (Fig. 14, top) (discussed in Ref. 72)) to be discussed later. 

Now the kinetic concept of surface fluctuations and relaxation enters into the theory of adsorption. A nontrivial dependence is predicted between the adsorbed quantity and the rate of pressure or concentration change of the adsorbent. Also predicted is the phenomenon of hysteresis during adsorption and desorption over a time comparable to the relaxation time of the surface. 

Surfactant molar masses range from a few hundreds up to several thousands. As there will be a balance between adsorption and desorption (due to thermal motions) the interfacial condition requires some time to establish. Because of this, surface activity should be considered a dynamic phenomenon. This can be seen by measuring surface tension versus time for a freshly formed surface. 

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