Energy of activation for desorption

The energy of activation for desorption (Ej ) cannot be directly related to a heat of adsorption, because... 

Fig. 4.1 Potential energy curves for (7) physical and (2) chemical adsorption (a) non-activated (b) activated. Epot - potential energy, Qc - heats of chemisorption, Qp - heats of physisorption, Ead -energy of activation for desorption, Ediss - dissociation energy for the diatomic molecule. The sum AEdes = Ead + Qc is the the heat of hemisorption, in the activated processes [8]...

Table 17. Acidic properties of H-ZSM-5 and Ca,H-ZSM-5 samples obtained by solid-state ion exchange. A(OH), maximum absorbance of the band of acidic OH groups at 3610 cm i X(EB), conversion of ethylbenzene peak temperature obtained by TPD of NH3 from Bronsted acid sites (cf. [29]) E, most frequent energy of activation for desorption of NH3 from Bron-sted acid sites (cf. [29,222,223]) AHad, differential heat of adsorption of NH3 (cf. [222,223])...

S6). It depended on the variation of the number of latex particles formed iV with temperature. Unfortunately, they have overlooked the fact that the particle growth rate fi which appears to the power —f in the Smith-Ewart expression for the number of latex particles formed coitains the propa gation rate constant which is temperature dependent. It has also recently been realized that another factor on which JV depends, the area occupied by a surfactant molecule at the polymer-water interface Og, is also temperature dependent- Dunn et al. (1981) observed that the temperature dependence of N in the thermal polymerization of styrene differed from different emulsifiers. It seems unlikely that the differences ran be wholly explained by differing enthalpies of adsorption of the emulsifiers and, if not, this observation implies that the energy of activation for thermal initiation of styrene in emulsion depends on the emulsifier used. Participation of emulsifiers in thermal initiation (and probsbly also in initiation by oil-soluble initiators) is most probably attributable to transfer to emulsifier and desorption of the emulsifier radical frcan the micelle x>r latex particle into the aqueous phase the rates of these processes are likely to differ with the emulsifier. 

R is the ideal gas constant, T is the absolute temperature, and Ea and Ed denote energies of activation for adsorption and desorption, respectively. At equilibrium, the net flux is equal to zero, and we can solve (2-150) for the interface concentration ... 

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.)...

When studying the kinetics of diffusion of hydrogen through palladium, Farkas (28) noticed the difference in catalytic activity of both sides of the palladium disks or tubes for the parahydrogen conversion the energy of activation was greater on the inlet side than on the outlet side, where due to extensive desorption of the hydrogen its concentration could be lower. 

Equation 17.59 has been confirmed experimentally, suggesting that molecules move over a surface by hopping to adjacent adsorption sites. It may be assumed that this process involves a lower energy of activation than that required for complete desorption. The molecule will continue to hop until it finds a vacant adsorption site, thus explaining the increase of surface diffusion coefficient with coverage. 

Data on the energies of activation and frequency factors for the adsorp-tion/desorption in the exchange of propane over bulk metals 44) and over films 18) are shown in Tables XI and XII, respectively. 

From the kinetic theory of gases, the rate of adsorption is dependent on the pressure and the fraction of bare sites (1-0). The rate of desorption is dependent on 9 and on the energy of activation, E (i.e. equivalent to an energy of adsorption expressed as a positive quantity). Equilibrium is obtained for the values of 9 andp for which the rates of adsorption and desorption are equal. Thus, the net rate of adsorption is zero ... 

Calculated Free Energies of Activation (AGie,) and Rate Constants for Desorption of Monolayers at Different Surface Pressures... 

The enthalpy in turn depends on the difference between the activation energy of adsorption and desorption. Since the activation energy for adsorption is normally zero while that for desorption is significant, the heat of adsorption is normally exothermic with the result that the exponent of the equilibrium constant is positive and the numerators in isotherm equations 12.4 and 12.5 grow smaller with increased temperature. 

Alternatively, can be used to calculate the activation energy of adsorption for the chemisorbed base, which is proportional to the heat of adsorption. However, determination of heats of adsorption from ammonia temperature-programmed desorption (TPD), although popular for its simplicity, may lead to inconsistent data because the experimental protocols may greatly influence the results [21]. 

The evaporation (desorption) of an adsorbed molecule from the surface is essential an activated process in which the energy of activation, E, for desorption may be identified with -AH, the differential (isosteric) heat of desorption. The rate of evaporation can be written as... 

More recently, Wang et al. [28] derived an intrinsic kinetic equation for the four-electron (4e ) oxygen reduction reaction (ORR) in acidic media, by using free energies of activation and adsorption as the kinetic parameters, which were obtained through fitting experimental ORR data from a Pt(lll) rotating disk electrode (RDE). Their kinetic model consists of four essential elementary reactions (1) a dissociative adsorption (DA) (2) a reductive adsorption (RA), which yields two reaction intermediates, O and OH (3) a reductive transition (RT) from O to OH and (4) a reductive desorption (RD) of OH, as shown below [28] (Reproduced with permission from [28]). 

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