Broensted-Polanyi relation

In addition, a variety of correlations often are used in the catalysis sci ence, which relate to the reaction rates and energies of the reaction inter mediates formation upon interaction of the catalyst with the initial reactants and transformation products. Many of these relations are known as the Broensted Polanyi relations. 

While identifying the rate determining parameters of the catalytic trans formation schemes under consideration, we understand better the reason for possible energy correlations in catalysis, the Broensted Polanyi relations for the transition state energies of surface transformations being typically apphed. Such an approach was intensively used in 1960s—1970s to... 

Let us estimate variations in the rate of a heterogeneous homogeneous reac tion on changes of the active phase (e.g., metal) particle size by considering the changes in the standard thermodynamic potential of the substance of the phase in terms of the correlation Broensted Polanyi relations. Consider the formation of adsorbate Kj in the course, for example, of an elementary gas solid phase reaction (4.85). Rate v of the elementary reaction is written as... 

One may suppose that changes in the particle size should result in changes of of the surface transition state, the changes being described correctly enough by the Broensted Polanyi relation with the increment Ak of substance K at its dispersing ... 

Applying the Broensted Polanyi correlations is sometimes useful for describing the dependence of the reaction rate on the size of the catalyti caUy active component. A huge amount of experimental data have been compiled to date regarding the effect of the particle size of the catalyst active components on the specific catalytic activity, SCA, as well as on the turnover frequency, TOP, of the active center. Both parameters do not relate to the total surface area of the catalyticaUy active phase or to the total number of active centers and, therefore, characterize directly the properties of the active center. There are also some experimental data on the size dependence of the adsorption properties of small metal parti cles, as well as on the selectivity of a few catalytic processes. 

Figure 4.6 A graphical interpretation in terms of the Broensted-Polanyi correlations of variations in standard energy characteristics (the Gibbs potential) of participants of (A) reaction (4.85) and (B) catalytic process (4.3)- 4.4) with disperse active phase K and one thermalized molecular intermediate Ki. Sign / indicates the transition state TS in elementary reaction (4.85) /I and 2 relate to transition states TSq and TS2 in elementary reactions 1 and 2 of scheme (4.4).

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