Energy of activation for adsorption

TABLE 3.4 Effect of Kinetic Method on Energies of Activation for Adsorption (Ej in Systems Studied ... 

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 ... 

To further substautiate these insights for the ORR mechanism, Wang and co-workers analyzed the four-electron ORR in acidic media using an intrinsic kinetic equation with the free energies of activation and adsorption as the kinetic parameters [Wang JX et al., 2007]. The kinetic model consisted of four essential elementary reactions ... 

At room temperature the energy of activation for the decomposition of O2 molecules is more readily available than at 90.5°K. Therefore, an additional increase of Aft is observed even at 10 mm. Hg (Table IV). High pressures do not cause any further increase. The specific increase of resistance h.R./d is independent of the temperature, as shown in Fig. 16. Considering the high values of 0 obtained at room temperature, it is probable that O atoms adsorbed above 0=1 also enter the surface and interact electronically. Scheuble s (57a) adsorption measurement.s support this view. 

Energy of activation for activated adsorption on uncovered surface E°ds... 

Fig. 26. Comparison of heat of adsorption of hydrogen and energy of activation for H-D exchange.

The experimental rate of the evolution of molecular hydrogen from a layer of hydrogen atoms on glass, which is only partially occupied, may be understood by accepting the concept of localized adsorption of the hydrogen atoms and an energy of activation for the reaction of 25.1 kcal./mole. 

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. 

A palladium-hydrogen-mordenite catalyst with a 10.8/1 silica/alumina mole ratio was evaluated for the hydroisomerization of cyclohexane. The rate of reaction followed a first-order, reversible reaction between cyclohexane and methylcyclopentane. The energy of activation for this reaction between 400° and 500°F was 35.5 it 2.4 kcal/mole. Cyclohexane isomerization rates decreased with increasing hydrogen and cyclohexane-plus-methylcyclopentane partial pressure. These effects are compatible with a dual-site adsorption model. The change of the model constants with temperature was qualitatively in agreement with the expected physical behavior for the constants. 

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

In a study of the effect of gas pressure on the chemisorption of hydrogen on chromium oxide gel, Taylor and Burwell (6f) found it necessary to resort to an arbitrary process of subtraction from experimentally observed amounts of adsorbed gas in order to display a relatively uniform area of gel for hydrogen chemisorption. Within close limits, at temperatures of 457 and 491° K. and at pressures of 1, 0.5 and 0.25 atmospheres, the energy of activation for the adsorption process was nearly independent of the amounts of adsorption (0-35 cc.) and the activation energy was about 21 kcal. In the temperature range 383 to 457° K. no such uniformity was observed. For example, if the amounts adsorbed with time at 405 and 427° K. be used to calculate activation energies the values received increase from 0 to 18.5 kcal. as the amount of gas adsorbed on the surface increases from 1.6 to 8.5 cc. Similar results can be drawn from the earlier measurements of Kohlschutter (17). 

Since there is no evidence for absorption of Cl atoms into the Au surface [46,47], only adsorption on the surface is considered. As the diffusing species in the ease of ehlotine adsorption a Au adatom with a chlorine atom on top was proposed to be the diffusing species in the case of chlorine adsorption [11]. Table 3.7 compares the energies of activation for the most important processes in the presence and absence of an adsorbed Cl atom. With few exceptions, the Cl atom significantly reduces the barrier. 

TABLE 3.8 Calculated energies of activation for hydrogen adsorption (Volmer step) on various... 

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]). 

The standard free energies of activation of the cathodic partial currents of reactions 17 and 19 may be written in the form of Eq. 5. For simplicity the factor (q + (l—q)C) is put equal to (1—a) for reaction 17 and equal to (1 —P) for reaction 19. Similar expressions that contain a and P as factors hold for the standard free energies of activation for the anodic direction. The parameters a and P are called transfer coefficients. It is assumed that the adsorption of H atoms follows a Langmuir isotherm while adsorption of H2 molecules is negligible. The effect of the strength of adsorption is reflected entirely by the equilibrium coverage Gq at Ph2= 1 Introducing rj, the rates of reactions 17 and 19 are ... 

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])...

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