Rate constants of desorption

K,m rate constant of desorption in the maximum of the desorption rate... 

The rate constants in table 4 for Ru/AlaOs should be considered as initial rate constants since it was not possible to achieve a higher coverage of N— than 0.25. Furthennorc, it was not possible to detect TPA peaks for Ru/AlaOs within the experimental detection limit of about 20 ppm. Ru/MgO is a heterogeneous system with respect to the adsorption and desorption of Na due to the presence of promoted active sites which dominate under NH3 synthesis conditions. The rate constant of desorption given in table 4 for Ru/MgO refers to the unpromoted sites [19]. The Na TPD, Na TPA and lER results thus demonstrate the enhancing influence of the alkali promoter on the rate of N3 dissociation and recombination as expected based on the principle of microscopic reversibility. Adding alkali renders the Ru metal surfaces more uniform towards the interaction with Na. 

The transition state for the dissociative chemisorption and for desorption is strongly constrained. This assumption allows us to set the partition function in the transition state equal to 1, and the prefactors in the Arrhenius expressions for the rate constants of desorption of AB and re-desorption of A2 thus become... 

Figure 2. Influence of surface pressure on steady-state rate constant of desorption...

Consequently, they are distributed on the surface randomly even if there is no surface migration. Therefore, in contrast to the discussion of adsorption and desorption processes in Section X, the mechanism (188) needs no assumed rapid surface migration (it is evident, of course, that if the migration occurs, it does not affect the results). The analysis of adsorption and desorption rates given in Section X needs only minor alterations to its application to stage 1 namely, products kbPb> would be substituted for rate constants of desorption on separate surface sites, k, and the fugacity of adsorbed particles I, px for p. Therefore, in analogy to (148) and (157), we obtain... 

The rate constants of desorption (kdes) and reverse reactions (k7, kd) are calculated using the detailed balance relations. 

The obtained expression for cAads can now be used to obtain the reaction rate for B, as shown in Equation (13). If the rate constant of desorption (k i) is much higher than the rate of conversion to the product (k2), cAads can be expressed from the adsorption constant K by cAais - K x cA. ... 

V, is the volume of the solid phase in the catalyst bed, and k are the rate constants of desorption and of adsorption of the sample, 6 is the surface coverage, is the amount of the sample adsorbed per unit volume of the solid phase when 6 = 1, and F is the carrier gas flow rate. 

The simplest case to be considered is that of a homogeneous surface the rate constant of desorption k ) is not a function of 6 and its temperature dependence is given by the Arrhenius equation... 

Pa is the probability that a site occupied by A has environment i. The transition-state expression for the effective rate constant of desorption can be rewritten as... 

Coadsorbed species can interact in many different ways, when adsorbed at close distance from each other. Chapter 4 mentioned the decreased pre-exponents for rate constants of desorption of adatoms adsorbed at high coverage. The pre-exponent decreases with coverage, because of the decreased mobility of the atoms in the transition state due to site blocking by the other adatoms. When a surface is... 

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