Compensation behavior theoretical explanations

The theoretical and mechanistic explanations of compensation behavior mentioned above contain common features. The factors to which references are made most frequently in this context are surface heterogeneity, in one form or another, and the occurrence of two or more concurrent reactions. The theoretical implications of these interpretations and the application of such models to particular reaction systems has been discussed fairly fully in the literature. The kinetic consequence of the alternative general model, that there are variations in the temperature dependence of reactant availability (reactant surface concentrations, mobilities, and active areas Section 5) has, however, been much less thoroughly explored. 

No single theoretical explanation of compensation behavior has been recognized as having general application. It is appropriate, therefore, to consider in this context the conditions obtaining on a catalyst surface during reaction, with particular reference to the factors that control the rate of product evolution and to the interpretation of kinetic measurements. This discussion of surface behavior precedes a critical assessment of the significance of measured values of A and E. 

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. 

From the critical comparison of all observations included in the determination of the compensation trends in Table V, A-C, and also remembering the scatter of data and the interrelationship between errors in B and e, we conclude that the kinetic results for all three rate processes discussed can probably be regarded as a single compensation trend. Allowing for standard error, there is overlap of the values of e at 0.049. This common pattern of kinetic behavior is consistent with the conclusion reached by Winter (263, 264) that the three rate processes involve the same rate-limiting step, oxygen desorption. No mechanistic interpretations of these examples of compensation beha vior have been provided and it is not known which of the theoretical models described in Section II, A gives the most satisfactory explanation of the systematic variation of log A with E. 

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