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Preexponential factor desorption

With the aid of (B1.25.4), it is possible to detennine the activation energy of desorption (usually equal to the adsorption energy) and the preexponential factor of desorption [21, 24]. Attractive or repulsive interactions between the adsorbate molecules make the desorption parameters and v dependent on coverage [22]- hr the case of TPRS one obtains infonnation on surface reactions if the latter is rate detennming for the desorption. [Pg.1863]

Energy of Desorption, of the Order of Desorption and of the Preexponential Factor. 365... [Pg.343]

When the temperature of the analyzed sample is increased continuously and in a known way, the experimental data on desorption can serve to estimate the apparent values of parameters characteristic for the desorption process. To this end, the most simple Arrhenius model for activated processes is usually used, with obvious modifications due to the planar nature of the desorption process. Sometimes, more refined models accounting for the surface mobility of adsorbed species or other specific points are applied. The Arrhenius model is to a large extent merely formal and involves three effective (apparent) parameters the activation energy of desorption, the preexponential factor, and the order of the rate-determining step in desorption. As will be dealt with in Section II. B, the experimental arrangement is usually such that the primary records reproduce essentially either the desorbed amount or the actual rate of desorption. After due correction, the output readings are converted into a desorption curve which may represent either the dependence of the desorbed amount on the temperature or, preferably, the dependence of the desorption rate on the temperature. In principle, there are two approaches to the treatment of the desorption curves. [Pg.346]

Suggest a transition state for the desorption of a molecule when the preexponential factor is 10 s . Do the same for a desorption when the prefac-... [Pg.405]

A pre-exponential factor and activation energy for each rate constant must be established. All forward rate constants involving alkyne adsorption (ki, k2, and ks) are assumed to have equal pre-exponential factors specified by the collision limit (assuming a sticking coefficient of one). All adsorption steps are assumed to be non-activated. Both desorption constants (k.i and k ) are assumed to have preexponential factors equal to 10 3 sec, as expected from transition-state theory [28]. Both desorption activation energies (26.1 kcal/mol for methyl acetylene and 25.3 kcal/mol for trimethylbenzene) were derived from TPD results [1]. [Pg.304]

We use this knowledge to derive preexponential factors from (2-20) for a few desorption pathways (see Fig. 2.15). The simplest case arises if the partition functions Q and Q in (2-20) are about equal. This corresponds to a transition state that resembles the ground state of the adsorbed molecule. In order to compare (2-20) with the Arrhenius expression (2-15) we need to apply the definition of the activation energy ... [Pg.46]

In conclusion, TDS of adsorbates on single crystal surfaces measured in ultrahigh vacuum systems with sufficiently high pumping speeds provides information on adsorbate coverage, the adsorption energy, the existence of lateral interactions between the adsorbates, and the preexponential factor of desorption, which in turn depends on the desorption mechanism. Analysis of spectra should be done with care, as simplified analysis procedures may easily give erroneous results. [Pg.48]

The simultaneous desorption peaks observed at 560-580 K in TPR are of reaction-limited desorption. The peak temperatures of these peaks do not depend on the coverage of methoxy species, indicating that the desorption rate (reaction rate) on both surfaces has a first-order relation to the coverage of methoxy species. Activation energy (Ea) and the preexponential factor (v) for a first-order process are given by the following Redhead equation [12] ... [Pg.239]

From this expression, one can show that, for reasonable parameter values, there can be no multiple steady states for very large Ts. This will also be the case, independent of Ts, if r is larger than both dA and dB. However, if Er is less than either desorption activation energy, then it is possible to reduce T below the critical value required for multiple steady states by decreasing the surface temperature. The actual values of the reactant partial pressure and surface temperature for which multiple steady states occur can vary by orders of magnitude due to the sensitivity of (46) to the parameters (and the fact that preexponential factors can vary by orders of magnitude between different systems). [Pg.305]

Interpretation of the mechanisms of the hydrocarbon desorption reactions mentioned above was considered (31,291) with due regard for the possible role of clay dehydration. While this water evolution process is not regarded as a heterogeneous catalytic reaction, it is at least possible that water loss occurs at an interface (293) so that estimations of preexponential factors per unit area can be made. On this assumption, Arrhenius parameters (in the units used throughout the present review) were calculated from the available observations in the literature and it was found (Fig. 9, Table V, S) that compensation trends were present in the kinetic data for the dehydration reactions of illite (+) (294), kaolinite ( ) (293,295 298), montmorillonite (x) (294) and muscovite (O) (299). If these surface reactions are at least partially reversible,... [Pg.305]

A similar interpretation holds for the preexponential factor of the rate constants for the dissociative adsorption, desorption, reaction between the adspecies and their migration. The CM is distinguished by the fact that the preexponential factor is dependent on the properties of the starting reagents only and is independent of the transition state whereas the rate constant depends on the activation barrier height, which is governed by the transition state energy. [Pg.394]

The rate coefficients of reactions (15)-(27) were taken from the results of ab initio calculations. Reactions (28) and (29) describe the process of surface dehydroxylation/hydroxylation. We used a value of 1013 sec-1 as an estimation of the preexponential factor (this value corresponds to the characteristic frequency of internal vibrations of the reaction center) for the desorption reaction. To describe the experimental dependence of the growth rate vs. temperature adequately, we considered that the water adsorption energy is a linear function of the hydroxylation degree / ... [Pg.496]


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