Adsorption rate determining

Fig. 3 a Model of vesicle rupture induced by adhesion involving two rate constants fcaj represents the adsorption rate determined by the interaction potential and fcrup, the constant of vesicle rupture, b Surface coverage after different time periods during the Monte Carlo simulation, c Impact of increasing fc up on the frequency shift of the QCM covered with biotinylated vesicles... 

The sequence of events in a surface-catalyzed reaction comprises (1) diffusion of reactants to the surface (usually considered to be fast) (2) adsorption of the reactants on the surface (slow if activated) (3) surface diffusion of reactants to active sites (if the adsorption is mobile) (4) reaction of the adsorbed species (often rate-determining) (5) desorption of the reaction products (often slow) and (6) diffusion of the products away from the surface. Processes 1 and 6 may be rate-determining where one is dealing with a porous catalyst [197]. The situation is illustrated in Fig. XVIII-22 (see also Ref. 198 notice in the figure the variety of processes that may be present). 

Some early observations on the catalytic oxidation of SO2 to SO3 on platinized asbestos catalysts led to the following observations (1) the rate was proportional to the SO2 pressure and was inversely proportional to the SO3 pressure (2) the apparent activation energy was 30 kcal/mol (3) the heats of adsorption for SO2, SO3, and O2 were 20, 25, and 30 kcal/mol, respectively. By using appropriate Langmuir equations, show that a possible explanation of the rate data is that there are two kinds of surfaces present, 5 and S2, and that the rate-determining step is... 

NO, the monomer C is CO, and the products are A2 = N2 and CB = CO2. The adsorption probability of C species (Fc) is the parameter of the model. The slow rate-determining step in this sequence is the dissociation of NO which requires a neighboring site to proceed. Since product formation liberates more vacant sites than those necessary for the dissociation of NO, an autocatalytic production of vacant sites takes place. 

It was found that [5-7] the rate of flocculation of particles produced by the bridging action of polymer is the slower process and, consequently, the rate-determining step. The primary adsorption of polymer is fairly rapid, but the slow attainment of the adsorption equilibrium under agitation arises at least in part from the breakdown of floes offering new surfaces for adsorption. Thus, the bridging step is slow because a polymer adsorbed on one particle must find another particle having a free surface available to complete the bridge. 

The simplest case to be analyzed is the process in which the rate of one of the adsorption or desorption steps is so slow that it becomes itself rate determining in overall transformation. The composition of the reaction mixture in the course of the reaction is then not determined by kinetic, but by thermodynamic factors, i.e. by equilibria of the fast steps, surface chemical reactions, and the other adsorption and desorption processes. Concentration dependencies of several types of consecutive and parallel (branched) catalytic reactions 52, 53) were calculated, corresponding to schemes (Ila) and (lib), assuming that they are controlled by the rate of adsorption of either of the reactants A and X, desorption of any of the products B, C, and Y, or by simultaneous desorption of compounds B and C. 

Fig. 4. Dependence of relative concentrationa nj/nt of reaction components A, B, and C on time variable r (arbitrary units) in the case of consecutive (— — ) reactions according to scheme (Ha) or parallel (C ) reactions according to scheme (lib). Ads X, Ads A, Des Y denotes that the rate determining step in the overall transformation is adsorption or desorption of the respective substance Des (B + C) denotes that the overall rate is determined by simultaneous desorption of the substance B and C. Ki/Ki = 0.5 for consecutive, and Ki /Ki — 0.5 for parallel reactions, b nxVn. 0 = 2.5 for consecutive reactions Kt = 0.5, and for parallel reactions Ki/Ki — 0.5. c nxVnA0 = 2.5 fcdesBKi Ky/fcdesoXj Kx = 10 [cf. (53)]. d Ki = 1.75 for consecutive, and Ki/Ki = 1.75 for parallel reactions.

They varied only the values of the adsorption and desorption rate constants of the reaction intermediate B, and by using the simplest Langmuir kinetics, they calculated time-concentration curves of compounds A, B, and C shown in Fig. 5. Also from this example, which does not consider any step as clearly rate determining, it is evident how very different concentration versus time plots can be obtained for the same sequence of surface reactions if adsorption and desorption of the intermediate B proceed by different rates, which are, however, comparable with the rate of surface reactions. In particular, the curves in the first and second columns of Fig. 5 simulate the parallel formation of substances B and C, at least... 

Using similar arguments as with the demethylation of xylenes (p. 31) we can assume from the form of the integral dependences shown in Fig. 7 that here also neither adsorption nor desorption is a rate-determining step. This is, after all, in agreement with the form of the best equations (19a) and (19b) found for the initial reaction rates of single reactions. 

If the rate-determining step were not a surface reaction but adsorption of competing reactants, we should obtain from the corresponding equations the expression which is formally identical with Eq. (15), in which, however, the relative reactivity is given by the expression S = fc dsA/ adsB. On the basis of data on competitive reactions only, these two cases cannot there-... 

From the first reaction, assuming that the anion adsorption process is rate determining, and neglecting the potential difference as shown in Eq. (75a), the average dissolution current density is written as a function of the surface concentration of the aggressive ions, (CJx, y, 0, /)), i.e.,... 

When the adsorption step determines the rate, component A no longer retards the reaction. Any A that is adsorbed will quickly react, and the concentration of [AS] sites will be low. Note that the desorption step is now treated as being reversible. Thus, any P in the gas phase will retard the reaction even if the surface reaction is irreversible, kr = 0. 

It is important to realize that the assumption of a rate-determining step limits the scope of our description. As with the steady state approximation, it is not possible to describe transients in the quasi-equilibrium model. In addition, the rate-determining step in the mechanism might shift to a different step if the reaction conditions change, e.g. if the partial pressure of a gas changes markedly. For a surface science study of the reaction A -i- B in an ultrahigh vacuum chamber with a single crystal as the catalyst, the partial pressures of A and B may be so small that the rates of adsorption become smaller than the rate of the surface reaction. 

The evolution of methylchlorosilanes between 450 and 600 K is consistent with the 550 - 600 K typical for the catalytic Rochow Process [3]. It is also reasonably consistent with the evolution of methylchlorosilanes at 500 - 750 K reported by Frank and Falconer for a temperature programmed reaction study of the monolayer remaining on a CuaSi surface after catalytic formation of methylchlorosilanes from CHaCl at higher pressures [5]. Both of these observations suggest that the monolayer formed by methyl and chlorine adsorption on pure CuaSi is similar to that present on active catalysts. For reference, methylchlorosilanes bond quite weakly to tiie surface and desorb at 180 - 220 K. It can thus be concluded that the rate-determining step in the evolution of methylchlorosilanes at 450 - 600 K is a surface reaction rather an product desorption. 

In our previous work [11], it has been shown that the reduction of NO with CH4 on Ga and ln/H-ZSM-5 catalysts proceeds through the reactions (1) and (2), and that CH4 was hardly activated by NO in the absence of oxygen on these catalysts. Therefore, NO2 plays an important role and the formation of NO2 is a necessary step for the reduction of NO with CH4. In the works of Li and Armor [17] and Cowan et al. [18], the rate-determining step in NO reduction with CH4 on Co-ferrierite and Co-ZSM-5 catalysts is involved in the dissociative adsorption of CH4, and the adsorbed NO2 facilitates the step to break the carbon-hydrogen bond in CH4. It is suggested that NO reduction by use of CH4 needs the formation of the adsorbed NO2, which can activate CH4. 

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