Activation energy of adsorption and desorption

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 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. 

From a physical viewpoint, since the Elovich equation has been found to be obeyed for adsorption rates on finely divided powders and on metal films, one might consider the total surface to comprise a small number of different crystal faces for which the activation energy for adsorption (and desorption) and their area would differ. Owing to the imperfect nature of real crystal planes, these will not have a set of completely uniform sites but such imperfections will not affect the following considerations about the minimum number of different sets of sites required so that the Elovich equation describes the adsorption rate... 

Figure 4. Relationship of the activation energies to adsorption ( ) and desorption ( ) and the reaction energy, .

Contrary to what is found in the analysis of TPD spectra based on ART, the preexponential coefficients and were considered to be independent of surface coverage and of adsorption energy e. Also, the assumption was made that the activation energies for adsorption and desorption, and e, are linear functions of the adsorption energy e [41]. 

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

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. 

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

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 ... 

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 chemical mechanism of the conversion. This includes the determination of reaction intermediates, the rate-determining step in the mechanism, the nature of the transition state (i.e., the high energy transient state that dictates the activation energy). For catalytic systems, one needs to examine the role and nature of adsorption and desorption of feed and product on the catalyst surface, and the occurrence of physical changes or solid state reactions in the catalyst under process conditions (oxidation/reduction, sintering, carbon deposition, etc.). 

The values given in Table II are based on the use of Eq. (11), valid for the simple case when no readsorption occurs. If readsorption does occur, as is likely to be the case with such adsorbents as alumina and silica-alumina, use of Eq. (19) is more appropriate. As already pointed out, the E values in Table II represent then the heats of desorption ( H). Since the activation energies of adsorption are probably neg-... 

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