Compensation behavior

Circulation flow system, measurement of reaction rate, 28 175-178 Clausius-Clapeyron equation, 38 171 Clay see also specific types color tests, 27 101 compensation behavior, 26 304-307 minerals, ship-in-bottle synthesis, metal clusters, 38 368-379 organic syntheses on, 38 264-279 active sites on montmorillonite for aldol reaction, 38 268-269 aldol condensation of enolsilanes with aldehydes and acetals, 38 265-273 Al-Mont acid strength, 38 270-271, 273 comparison of catalysis between Al-Mont and trifluorometfaanesulfonic acid, 38 269-270... 

Combustion, 27 189, 190 reaction, sites for, 33 161-166 reaction scheme, 27 190, 196 Commercial isomerization, 6 197 CoMo catalysts, 40 181 See also Cobalt (nickel)-molybdenum-sulfide catalysts Compact-diffuse layer model, 30 224 Compensation behavior, 26 247-315 active surface, 26 253, 254 Arrhenius parameters, see Arrhenius parameters... 

I/Ru ratio critical, 34 112 proposed mechanism, 34 112 ruthenium-carbonyl complexes, 34 113 species involved, 34 110-113 -catalyzed homologation, 34 115 proposed mechanism, 34 115 compensation behavior of, 26 285, 286 complex catalyst... 

Compensation temperatures have been found to be identical for the retention of a variety of eluites on three different types of reversed phase columns over a wide range of eluent composition (/ 77). The authors noted that if compensation behavior occurs in chromatography, the retention... 

Appendix I. Compensation Behavior Resulting from Temperature-... 

None of the mechanistic explanations of compensation behavior have enabled the values of Arrhenius parameters for untested systems to be predicted. Thus, every compensation plot consists of a number of individual points (log Ai,E1 log A2, E2 log A3, E3 ... log Ah f ...) each point is defined by a single reaction, and the line through these yields the characteristic values of B and e for that series of related reactions. In the absence of control over the magnitudes of A and of E, Eq. (2) is not a realizable continuous function. In principle, this might be achieved by appropriate variations in conditions if a meaningful mechanistic explanation of the surface behavior were available. 

If reactant adsorption can be described by the above equations it is to be expected (59) that groups of related reactions will exhibit compensation behavior. 

The following list mentions several factors that may control or influence the magnitude of one or both Arrhenius parameters and, in consequence, possibly result in the appearance of compensation behavior. Some of these parameters closely resemble, or represent alternative variations of, the reaction models described in Sections 1 -6. 

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. 

It is reasonable to suppose that between the members of a group of related reactions there will be modifications, but not drastic changes, in the positions of surface equilibria and in the temperature dependences of c1 c2, and As. Such variations, when subject to appropriate constraints, are capable of providing an explanation of compensation behavior (Appendix I and Section II, A, 5). From this it follows that the compensation effect appears as a general or at least a widely occurring characteristic feature of surface processes, rather than an exceptional phenomenon that requires an exceptional explanation. 

In Section II, D we propounded a case to suggest that values of /ls, ct, and c2 may vary with temperature we now discuss the probable form of this variation of the value of a in Eq. (7) with reference to compensation behavior (Appendix I). 

A more rigorous treatment of adsorption equilibria would include due allowance for the total number of surface sites available, which sets an upper limit on the surface concentrations to be used in the above equations, the variation in the heat of adsorption with coverage, and surface heterogeneity. The significant feature of Eq. (11), relevant to the present discussion of compensation behavior, is that this predicted temperature dependence of variations of c i and c2 results in no deviation from obedience to the Arrhenius equation. If a given set of kinetic results obey Eq. (1) the condition for a fit to Eq. (9) is... 

One aspect of compensation behavior that would appear to have received less attention than perhaps it deserves is the use of the constants B and e, or the isokinetic temperature / and the isokinetic reaction rate constant lip, as quantitative measurements of reactivities between series of related reactions. In the literature, comparisons of relative reaction rates are often based on the values of k at a particular temperature, arbitrarily selected, though often within the range of measurements, or the temperature at which a specified value of k is attained (137). It can be argued, however, that where compensation exists, a more complete description of kinetic behavior is given by B and e. The magnitudes of these parameters define the temperature range within which reaction rates become significant and that at which these become comparable there is also the possibility that such behavior may be associated with the operation of a common reaction mechanism or intermediate. 

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