Activation energy, of adsorption

Fig. XVIII-13. Activation energies of adsorption and desorption and heat of chemisorption for nitrogen on a single promoted, intensively reduced iron catalyst Q is calculated from Q = Edes - ads- (From Ref. 130.)...

The comparison of experimental data on adsorption of various particles on different adsorbents indicate that absorbate reaction capacity plays a substantial role in effects of influence of adsorption on electric conductivity of oxide semiconductors. For instance, the activation energy of adsorption of molecular oxygen on ZnO is about 8 kcal/mole [83] and molecular hydrogen - 30 kcal/mole [185]. Due to such high activation energy of adsorption of molecular hydrogen at temperatures of adsorbent lower than 100 C (in contrast to O2) practically does not influence the electric conductivity of oxides. The molecular nitrogen and... 

Figure 7. Change in activation energy of adsorption/desorptlon due to EDL. The solid and dashed lines in the free energy diagram represent the free energy path in the presence and absence of the EDL activation potential.

The desorption of anions from iron oxides as a result of changing the anion concentration in solution is often very slow. It can be accelerated by increasing the pH. The partial irreversibility of anion adsorption has been attributed by some authors to a high activation energy of adsorption resulting from the formation of multiden-tate surface complexes, whereas others attribute it to a slow diffusion out of micropores. 

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. 

The explanation for the apparent correlation between catalytic activity and electron affinity of metals cannot be as simple as that which has been advanced for the homogeneous catalysts. This is because chemisorption on metals (unlike the splitting of hydrogen by metal ions in solution ) is an exothermic process and, hence, as shown earlier, catalytic activity depends not only on a low activation energy of adsorption but also on a low heat of adsorption. The interpretation applied earlier to homogeneous catalysts can account for an inverse dependence of Ea on the work function, but does not suggest any obvious reason why Q should show a similar dependence. 

Taylor and Strother found that the activation energy of Type A adsorption from 0 to 56°C varied, with increasing surface coverage, from 3 to 6 Kcal./mole. The activation energy of adsorption from 184° to 218°C, with the same surface coverage, varied from 10 to 11 Kcal./mole. The latter may be qualitatively compared to the activation energy for Type B adsorption of 0.8 e.v. or 18 Kcal./mole. Two sources of error may be mentioned here (1) the observed Type B adsorption may have... 

It was later demonstrated that if the reaction mechanism corresponds to scheme (295) and the linear relation between standard Gibbs energy of adsorption and Gibbs activation energy of adsorption is obeyed [see (91)], then the kinetic (305) corresponds in general to the exponential nonuniformity of the surface with even nonuniformity included as a particular case (44). In the general case the exponent m is not equal to transfer coefficient a, but is connected with it according to (143).5... 

Energy models in these models a constant number of reactive sites is supposed, while the activation energy of adsorption changes with the amount adsorbed. Two models, mathematically equivalent, have been used site heterogeneity and induced heterogeneity. 

Adsorption of sulfur on monocrystalline, polycrystalline, and supported metallic catalysts has been extensively studied. For all adsorption reactions, thermodynamics states that A//llds = Ea - Ed (where A//ads is the enthalpy of adsorption, Ea is the activation energy of adsorption, and Ed... 

It now appears clear, however, that the rate of adsorption upon metal surfaces, and charcoal also, and the activation energy of adsorption (if this is the rate-determining factor) depends very much upon the cleanliness of the surfaces, previously adsorbed films slowing down the rate considerably. The very detailed work of Taylor and his school on oxide catalysts, which are porous, at present forms the main support for the theory that the rate of activation either of the adsorbed molecules, or of the surface molecules of the solid with which they combine, is the ratedetermining step in the adsorption. 

Figure 5.9 Typical reaction coordinate for adsorption interactions, indicating the activation energy of adsorption ) and desrnption (Ej).

On the viewpoint here presented poisons and promoters become impurity centres in the normal lattice of the catalyst, the former tending to raise the activation energy of adsorption, the latter to lower it. 

Thus spillover is reversible and complex and depends upon the existence of surface states which are incompletely understood. The multidimensional problem cannot be properly represented in two, as attempted in Figure 1, but some further obvious features can be seen from the juxtaposed potential boxes of Figure 2 wherein the activation energies of adsorption of Figure 1 are omitted. The hatched area of the box representing the good adsorber (a) shows the... 

Similarly, the activation energy of adsorption depends on surface coverage... 

As Temkin (32) has shown, the adsorption laws always agree with the assumption analogous to the Brensted rule saying that the change of activation energy of adsorption is a part of the change of heat of adsorption. Then, the activation energy of adsorption is... 

The exchange in molecular oxygen, as it has already been shown, proceeds, presumably, on the sections whose degree of coverage approaches a, where a is the ratio of the variation of activation energy of adsorption to the one of the heat of adsorption ... 

No2f is the concentration of the physically adsorbed gaseous molecules of O2 on the surface of the interactive system of the gas-oxide SE. m is the mass of the gas molecule, co is the probability of adsorption (co = 1 at calculations [32]). S is the effective square of the adsorbed O2 molecules. N is the number of adsorption centers. P is the partial pressure on the surface of the SE. k = kg s, the Boltzmann constant. V is the probability of desorption of adsorbed O2 molecules, n = 1/to and to are the minimum time for the gaseous component to be at the adsorption state. Q is the activation energy of adsorbed atoms, which is equal to the activation energy of adsorption of gaseous molecules plus the dissociation energy of the molecule Q =... 

The activation energy of desorption Ej is related to the heat (- AH),i and the activation energy of adsorption by the equation. 

The equation describing the kinetics of adsorption on a nonuniform surface with a linear increase of activation energy of adsorption with coverage was first deduced by Brunauer et al. (82). The surface is divided into a series of equal areas ds each area constitutes a uniform element of surface having an activation energy for adsorption given by = Eq (x s, where s is the reference number of the uniform patch and a is a constant. The rate of adsorption is then given by... 

In general, the rate of adsorption of component 2 is affected by the presence of component 1 and so differs from that on a clean surface. Different types of kinetics are obtained depending on (i) whether the activation energy of adsorption of component 2 is increased or decreased by the presence of component 1, i.e., whether g-y of Eq. (68) is positive or negative (ii) whether component 1 is present on all patches 0 to m or only on the high-energy patches k to m-, and (iii) whether a surface complex is formed. 

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