Arrhenius parameters desorption kinetics

Arrhenius parameters for these reactions are shown on Fig. 8, which also includes results for the reactions of n-dodecanol in the absence of volatile products (x) (292) and of cyclohexanol in the presence (O) and the absence ( ) of volatile material (291). This pattern of kinetic observations emphasizes the sensitivity of the values of log A and E determined for these reactions to the availability of volatile reactants, including water vapor. The values of A extend beyond the range attributable to a simple rate-limiting desorption process such magnitudes cannot be directly identified with the frequency of occurrence of the reaction situation and do not provide a measure of the concentration of surface-active sites. 

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,... 

Common features in the various theoretical explanations of compensation behavior referred to in Section II, A, 1-7 are the occurrence of parallel reactions that are characterized by different values of the kinetic parameters (A, E) and/or a systematic change in the effective concentrations of reactants across the temperature interval used in the measurements of the Arrhenius parameters. Both influences are based on reaction models for which the kinetic behavior cannot be represented as a single desorption step and, indeed, the overall surface interactions could be much more complicated. 

D Evelyn et al. [45] modeled desorption experiments by assuming that the desorption rate was proportional to the density of doubly occupied dimers, taken from the lattice gas model. For large values of E f, most H is paired on doubly occupied sites and desorption is close to first order. As E pair approaches zero, the predicted kinetics become second order. D Evelyn et al. [45] reanalyzed the isothermal measurements of Wise et al. using this model and determined slightly different Arrhenius parameters than obtained from a strictly first-order analysis (E = 54.9 kcal/mol, prefactor = 5.6 X 10 s" ). 

The kinetic parameters for desorption from the monohydride phase on Si(100)-2 X 1 have been controversial, but consensus has now been reached on the reaction order and Arrhenius parameters. The rotational and vibrational energy distributions are also well-characterized. Almost everything else about this reaction requires clarification. In this section, some of the remaining issues for theory and experiment are outlined. The motivations for these issues have been addressed in more detail in the main body of this chapter. 

This matches the functional form of the BET isotherm when the parameter k is given by the ratio of adsorbate partial pressure pa to its saturation vapor pressure at the experimental temperature T, PA,saturation(P)- Let s consider the parameter f), which was defined above as the ratio of kq to k. If the adsorption and desorption kinetic rate constants for chemisorption follow Arrhenius temperature dependence, then kq for chemisorption on the bare surface is expressed as... 

Generally, adsorption steps were taken as temperature independent, whereas the rate parameters of surface reactions and desorption steps were described by Arrhenius equations. The kinetic rate parameters for CO oxidation (steps 1-10) and the catalyst properties were taken from [24] with minor adaptation as mentioned. The rate parameters, e.g. activation energies and pre-exponential factors, for steps 11-28 were determined by non-linear regression. It was found [25] that the rates for NO reactions on ceria are independent of the oxidation state of ceria, so the rate parameters for the corresponding steps were taken as the same (i.e. steps 11 and 12 for oxygen, steps 25-28 for NO). 

In the discussion so far, orders of one or two have been considered. Surface processes with other orders (and fractional orders) have been found and changes in Arrhenius plot slopes can be caused by changes in desorption mechanism. Equation (94) shows the general form of the dependence of the kinetic parameters on the desorption order in terms of initial coverages. Another important relationship can be derived from the work of Falconer [280] who showed that... 

The adsorption of oxygen is assumed as irreversible and dissociative with a rate proportional to the fraction of vacant sites [1]. In contrast to Sant et al. [2] adsorption of both ethene and ethyne is also assumed to be first order in the vacant sites. The rate of the surface reactions between adsorbed ethene and adsorbed oxygen, and between adsorbed ethyne and adsorbed oxygen, is considered as proportional to the product of the involved surface coverages. The adsorption rate coefficients are obtained fi om the kinetic gas theory, while Arrhenius-type expressions are used for the rate coefficients of desorption,dissociation and the surface reactions. The kinetic parameter values used in this study are shown in Table 2. 

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