Desorption-readsorption equilibrium

Taking i m = 0, we obtain an equation for the desorption process alone, i.e. for the case where the rate of readsorption is negligible. On the other hand, if only the last term in the brackets is considered, one has the equation for the equilibrium desorption. 

Step (18) in the above is the analog of step (8), which is required for H2—D2 equilibration it is a necessary step if we view the jr-allyl as an immobile species on the surface. The products of step (19) can be viewed as propylene in the form of a loosely held w-complex which on desorption yields isomerized propylene. Readsorption of the isomerized propylene or further reaction of the x-complex would yield surface OD groups. When equilibrium is achieved, the concentration of surface OD groups should equal 40% of the initial concentration of OH groups. Figure 21 shows a plot versus time of the intensity (multiplied by a scale factor to yield concentration) of the surface OH and OD. The expected equilibrium points are indicated by arrows. Corresponding data for CD3—CH=CH2 are also shown. Except for the OH species from CD3—CH=CH2, which is a relatively weak band on the side of a surface hydroxyl, the curves approach the expected value. 

At the beginning stage of dehydrogenation, the substrate organic hydride is adsorbed onto the catalyst surface from the liquid phase directly and easily. Catalytic reaction processes will succeed it, until the surface sites are filled with the adsorbed reactant and products. Once product desorption starts to form and grow a bubble, product readsorption becomes unfavorable due to the increment of translational entropy of the product molecule in the bubble, if compared with that in the solution, shifting the adsorption equilibrium for the product and suppressing its effect of rate retardation. 

In a hysteresis experiment, the movable barrier would be reversed at a time, designated as t, so that the monolayer comes under an expansion process at the same speed, v. The increase of surface area causes a reduction in the surface pressure. For a reversibly adsorbed monolayer, the desorption of segments may continue during the first period of expansion until the surface pressure is reduced to its equilibrium value. On further expansion, readsorption occurs because the surface pressure is below its equilibrium value. 

The activation energy for desorption comprises the heat of adsorption and the activation energy of adsorption, (see Fig. 1), but, as the adsorption of alkali metals and most gases on clean metal surfaces is non-activated, the activation energy of desorption is, in fact, equal to that of adsorption. Two classes of measurements have been made (1) those in which desorption occurred without subsequent readsorption, and (2) those where equilibrium conditions were approached during the desorption process. A true desorption velocity is observed in the first case only. 

It was found that chemisorption equilibrium is rapidly attained in most reacting systems through rapid desorption and readsorption. With a few exceptions, chemisorbed molecules can be regarded as immobile since statistical-mechanical calculations of the chemisorption equilibrium agree well with the experiment if two-dimensional translations and rotations of the chemisorbed molecules are assumed to be nonexistent. The chemisorbed state of di- or triatomic molecules can be molecular or atomic, depending on the nature of the adsorbent. For example, the carbon dioxide molecule is chemisorbed with complete dissociation into its three atoms on metallic surfaces, while on oxidic catalysts it is chemisorbed with only partial dissociation. 

The adsorption kinetics are found to be completely different for the two molecules, AHS and DHS. The rate of dissolution of molecules into solution from fully formed monolayers at room temperature is negligible, so equilibrium carmot be established by desorption and readsorption of monolayer components in the complete mono-layer. Equilibration could proceed through the physisorbed thiol. Rapid equilibration between the physisorbed molecule and the molecules in solution would be followed by relatively slow conversion of the physisorbed thiols to chemisorbed thiolates. If the rate constant for conversion of thiol to surface thiolate is independent of the structure of the thiol, which is likely, a chemisorbed layer would be kinetically trapped. The adsorption rate constant would then be determined by the equilibration between the physisorbed thiol and the thiols in solution, which might explain the observed large differences between AHS and DHS. A similar argument has been used by Bain et al. to explain the observed composition of mixed monolayers. This would explain the major role of the solvent in the adsorption kinetics and possibly also in the resulting film structure. A further consideration is the expected hy-... 

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