Activation energy and heat of adsorption

The activation energy of desorption is related to the heat of adsorption and activation energy for adsorption E by the... 

The catalytic properties of the PGM in the heterogeneous catalysis are based on the moderate values of the heats of adsorption which correspond to the dissociation energies of the reactant molecules. Figure 3.1-251 [1.276] and Table 3.1-188 [1.218] give some values of the heat of adsorption and binding energies between adsorbates and surface atoms on various noble metal single crystals. The heat of adsorption increases for different orientations of the crystal surface planes of the fee crystals in the order [111] < [100] < [110] (Table 3.1-189 [1.218]). The catalytic activities are element-specific for different re-... 

For surface diffusion, (e v) and (sbv ) may be assumed to be the heats of adsorption (Chen and Yang, 1992). For diffusion in zeolite, the derivation is also valid, provided that the activation energies for diffusion are used for sav ) and bbv). In contrast to surface diffusion, in which the activation energy is always lower than the heat of adsorption, the activation energy for diffusion in zeolite can be higher than heat of adsorption because the repulsive forces between the diffusing molecule and the aperture or pore surfaces can influence the diffusion process. 

The objectives of this research are therefore 1) to see whether rate expressions such as Equations 11 and 12 provide adequate descriptions of reaction rates and, if not, what rate expressions are appropriate, 2) to determine reaction activation energies, heats of adsorption, and pre-exponential factors, and 3) to compare these quantities with those measured under UHV conditions to determine whether the same processes and surface species might be involved. 

The final rate expressions, which were used in the present work, were given by Hou and Hughes (2001). In these rate expressions all reaction rate and equilibrium constants were defined to be temperature-dependent through the Arrhenius and van t Hoff equations. The particular values for the activation energies, heats of adsorption, and pre-exponential constants are available in the original reference and were used in our work without alteration. 

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. 

Step 19. Fit C7 heats of adsorption and k activation energy by using data at 111 and 794 K. 

Heats of Adsorption and Desorption and Activation Energies Connected with Chemisorption on Metals... 

In adsorption with redistribution in which there is no functional relationship between E and Q, it is assumed that the velocity of redistribution is larger than the velocity of adsorption. Consequently the various sections of the surface will be occupied in the order of decreasing heats of adsorption and at all times all sections will be involved in the adsorption irrespective of the magnitude of their activation energy. However, due to the exponential dependence of the velocity of adsorption upon the activation energy the character of the adsorption will be determined principally by the processes occurring upon sections with minimal values of E. These sections form on the graph p(E), as shown in Fig. 8, a relatively narrow vertical band the position of which determines the... 

IN THIS SURVEY of current concepts in adsorption and chemisorption, it is pointed out that entropy relations, both thermodynamic and kinetic, have made a relatively late appearance on the scene of adsorption research. Exaggerated preoccupation with heats of adsorption and energies of activation has led to a frozen formalism which appears to have outlived much of its usefulness. This situation is now being corrected by more attention to molecular structure of adsorbed layers and its relation to entropies of adsorption. 

A rise in temperature can have both a positive and a negative effect on the permeability. Transport through the membrane is an activated process, so it will benefit from a rise in temperature. However, as the temperature increases, the concentration in the membrane diminishes. These effects counteract each other, and one of them will dominate, depending on the difference in activation energy for diffusion and the heat of adsorption and the temperature. This will be discussed further in Section III. 

Thus, on a nonuniform surface on which the activation energies for the rate of adsorption varies with coverage, it is expected that there will also be some corresponding variation in the heat of adsorption. However, one energy is associated with an activation barrier E, and the other with an equilibrium heat of adsorption —A.H there is therefore no a priori reason for a specific functional relationship to exist between... 

Activation Energies for the Heats of Adsorption and Kinetic Equation for Oxygen Chemisorption on Semiconductor Catalysts... 

The adsorption of oxygen on certain semiconductors, such as NiO and Cu20 was studied in fair detail. The activation energies, heats of adsorption and kinetic laws for oxygen sorption on simple semiconducting catalysts are summarized in Table VI. 

This model was used to fit the adsorption of 02 and N2 on various metal films [435, 436] and good agreement is claimed between the experim-mental and theoretical work. The authors also note that the length of the plateau is a measure of the ratio of heat of solution to activation energy for diffusion. Thus, for a given metal, the length of plateaus for O and N adsorption/adsorption should be rationally related and the ratio of the two plateau lengths calculable. 

Figure 2 A plot of the heat of adsorption ( ) and surface diffusion activation energy ( ) for SO2 against the corresponding parameters for CF2CI2 measured at the same value of 6.

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