Surface reaction bimolecular

Continuing the formal development of the influence of the adsorption isotherm on the apparent reaction kinetics, we next consider the case of a reac- 

Only two of the many possible special cases need be considered. Thus if the products and reactants are weakly adsorbed, 

If A is weakly adsorbed as well as the products but B is strongly adsorbed, one finds 

C(ads) + H2 CH2(ads) followed by fast steps. The corresponding rate expression proposed is 

Rate laws have also been observed that correspond to there being two kinds of surface, one adsorbing reactant A and the other reactant B and with the rate proportional to 5a x 5b- For traditional discussions of Langmuir-Hinshelwood rate laws, see Refs. 240-242. Many catalytic systems involve a series of intermediates, and the simplifying assumption of steady-state equilibrium is usually made. See Boudart and co-workers [243-245] for a contemporary discussion of such complexities. 

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Studies of the influence of total pressure on the initial reaction rate for pure reactants present in stoichiometric proportions provide a means of discriminating between various classes of Hoqgen-Watson models. Isolation of a class of probable models by means of plots of initial reaction rate versus total pressure, feed composition, and temperature constitutes the first step n developing a Hougen-Watson rate model. Hougen (14) has considered the influence of total pressure for unimolecular and bimolecular surface reactions the analysis that follows is adopted from his monograph. 

In initial rate studies no products need be present in the feed, and the terms in the rate expression involving the partial pressures of these species may be omitted under appropriate experimental conditions. The use of stoichiometric ratios of reactants may also cause a simplification of the rate expression. If one considers a reversible bimolecular surface reaction between species A and ,... 

Similarly, the inhibition equation can also be derived for a bimolecular surface reaction. Let us consider a reaction between two reactants A and B and inhibition by a inhibitor (I). If Pu P2 and P are the respective partial pressures of A, B and I, then the fractions of the surface covered by A and B are given as... 

Bimolecular surface reactions reactants adsorption, 29 111-112 with single reactant, 29 108-109 1,1 -Binaphthyl, dehydrocyclization, 28 318 Binary oxides, 32 119 Binding energy, 32 160-162 chemisorbed sulfur, 37 281 hydrogen, sulfur effect, 37 295-296 shift, Pd, 37 62-64 ZnO/SiOj, 37 21-22 Binor-S, see Norbomadiene Biological systems, hydrogen in, activation of, 11 301... 

Bimolecular surface reaction mobile reactants rotating activated complex (42)... 

Bimolecular surface reaction one reactant appreciably adsorbed (F,F2)/(Cg,C,2) ... 

Bimolecular surface reaction both reactants appreciably absorbed 2Flc (50)... 

Bimolecular Surface Reaction with a Single Reactant... 

When the C expression for Step 7 of Table I is used in Eq. (10), the standard TST equation for a bimolecular surface reaction is produced. If the two molecules are the same, we have after rearrangement... 

We studied the metathesis of propylene over W03-S102, an example of a single-reactant, bimolecular surface reaction. We calculated L for various conceivable rate-determining steps, including the step for which Eq. (34) applies (12). 

Bimolecular Surface Reaction Both Reactants Appreciably Adsorbed... 

Steps 10 and 11 of Table I give the appropriate expressions for C to be used in Eq. (10) when the adsorption of only one reactant is significant. If in a bimolecular surface reaction a significant amount of each reactant is adsorbed, then the number of unoccupied sites is consequently small. Therefore,... 

The general equation for a bimolecular surface reaction is, neglecting the small factor that takes into account the number of adjacent active sites (9),... 

This is a simple example of a bimolecular surface reaction, which we will use to illustrate bimolecular reactions. 

Recently, Praharso et al also developed a Langmuir-Hinshelwood type of kinetic model for the SR kinetics of i-Cg over a Ni-based catalyst. In their model, it was assumed that both the hydrocarbon and steam dissociatively chemisorb on two different dual sites on the catalyst surface. The bimolecular surface reaction between dissociated adsorbed species was proposed as the ratedetermining step. The following generalized rate expression was proposed ... 

McKarnin, M. A., Schmidt, L. D., and Aris, R. (1988). Forced oscillations of a self-oscillating bimolecular surface reaction model. Proc. R. Soc., A417, 363-88. 

Fig. 1. Variation of rate with reactant pressure for a bimolecular surface reaction proceeding by a Rideal—-Eley or a Langmuir—Hinshelwood mechanism.

APPLICATION OF ABSOLUTE RATE THEORY TO BIMOLECULAR SURFACE REACTIONS... 

A comparative study was done by Kevrekidis and published as I. G. Kevrekidis, L. D. Schmidt, and R. Aris. Some common features of periodically forced reacting systems. Chem. Eng. Sci. 41,1263-1276 (1986). See also two papers by the same authors Resonance in periodically forced processes Chem. Eng. Sci. 41, 905-911 (1986) The stirred tank forced. Chem. Eng. Sci. 41,1549-1560 (1986). A full study of the Schmidt-Takoudis vacant site mechanism is to be found in M. A. McKamin, L. D. Schmidt, and R. Aris. Autonomous bifurcations of a simple bimolecular surface-reaction model. Proc. R. Soc. Lond. A 415,363-387 (1988) Forced oscillations of a self-oscillating bimolecular surface reaction model. Proc. R. Soc. Lond. A 415,363-388 (1988). 

K. Autonomous Bifurcations of a Simple Bimolecular Surface-Reaction Model... 

The second model results from a bimolecular surface reaction, A + B — products, with competitive Langmuir-Hinshelwood kinetics, which occurs in a heterogeneous differential reactor with perfectly mixed gas phase. The reaction is first order in both adsorbed A and B, and two vacant sites are required in the reaction mechanism. If the reaction products desorb immediately, the... 

AUTONOMOUS BIFURCATIONS OF A SIMPLE BIMOLECULAR SURFACE-REACTION MODEL... 

In both these cases there is an autocatalytic element, i.e. one which is both the product of the reaction and which tends to increase its rate. This is the substance B in the first case and heat in the second. It is this element of feedback that is the source of the interesting behaviour. The first two terms in both of these equations represent the access to the site of reaction, in this case the stirred tank, in accordance with the criterion of actuality. The feasibility of these simple reaction schemes can be established by showing that they can be embedded in a fully reversible mechanism and the simple system recovered by limiting processes that do not violate the laws of thermodynamics or kinetics (for example, the Wegscheider condition). Yablonskii and his coworkers (Bykov et al. 1978,1979a, b, c Yablonskii Bykov 1979 Gol dshtein et al. 1986) have considered a number of simple models from which it is clear that the autocatalytic feature is essential. In the bimolecular surface reaction the autocatalytic role is played by the vacant sites which are indeed the product of the main reaction which releases those previously held by adsorbate and, at the same time, are a positive influence on the rate of reaction. 

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