Kinetics Sabatier principle

A volcano plot correlates a kinetic parameter, such as the activation energy, with a thermodynamic parameter, such as the adsorption energy. The maximum in the volcano plot corresponds to the Sabatier principle maximum, where the rate of activation of reactant molecules and the desorption of product molecules balance. 

A proper kinetic description of a catalytic reaction must not only follow the formation and conversion of individual intermediates, but should also include the fimdamental steps that control the regeneration of the catalyst after each catalytic turnover. Both the catalyst sites and the surface intermediates are part of the catalytic cycle which must turn over in order for the reaction to remain catalytic. The competition between the kinetics for surface reaction and desorption steps leads to the Sabatier principle which indicates that the overall catalytic reaction rate is maximized for an optimal interaction between the substrate molecule and the catalyst surface. At an atomic level, this implies that bonds within the substrate molecule are broken whereas bonds between the substrate and the catalyst are made during the course of reaction. Similarly, as the bonds between the substrate and the surface are broken, bonds within the substrate are formed. The catalyst system regenerates itself through the desorption of products, and the self repair and reorganization of the active site and its environment after each catalytic cycle. 

The dependence of the overall rate of the catalytic reaction on adsorption is extremely important in analyzing the kinetics for the overall rate of a zeolite-catalyzed reaction. We have already met this subject in Chapter 2 when analyzing the basis of the Sabatier principle. A proper understanding of adsorption effects is essential for establishing a theory of zeolite catalysis that predicts the dependence of kinetics on zeolite-micropore shape and connectivity. 

There are some multifunctional biometals like Mg and Mn(II) the complexes of which are rather labile in both kinetic and thermodynamic terms (Taube 1952 Riedel 2004 Jordan 1994), in spite of their high AC orders. This situation goes beyond the description by Sabatier s principle every (bio-)autocatalytic process promoted by such metal ions is going to have an exit order near 1 at least, summing up to an exit order 1 for snch metals. This also holds for photosynthe-... 

Thus, according to Sabatier the hydrogenations of olefins and ketones are the same type of reaction, while according to the multiplet classification these two reactions are of different types, and indeed, the two reaction types require two different catalysts. Naturally, in the application of a given classification the thermodynamic nature of the reactions should be taken into account as well as their structural aspects. This classification and thermodynamic requirements do not yet deal with the kinetics of processes. The latter is involved in the principles of structural and energetic correspondence of the multiplet theory. 

This chapter proceeds with a general discussion of the overall catalytic cycle and Sabatier s principle in order to illustrate the comparison of relative kinetic and thermodynamic steps in the overall cycle. This is followed by a fundamental discussion of the intrinsic surface chemistry and the application of transition state theory to the description of the surface reactivity. We discuss the important problem of the pressure and material gap in relating intrinsic rates with overall catalytic behavior and then describe the influence of the tatic reaction environment including promoters, cluster size, support, defects, ensemble, coadsorption and stereochemistry. Lastly, we discuss the transient changes to the surface structure as well as intermediates and their influence on catalytic performance. 

Sabatier s principle provides a kinetic rmderstanding of the catalytic cycle and its corresponding elementary reaction steps which include adsorption, surface reaction, desorption and catalyst self repair. The nature of the catalytic cycle implies that bonds at the surface of the catalyst that are disrupted during the reaction must be restored. A good catalyst has the unique property that it reacts with the reagent, but readily becomes liberated when the product is formed. This will be further discussed in Section 2.2, where we describe the kinetics of elementary surface reactions and their free energy relationships. 

Irrespective of the microscopic mechanism, a four-electron process must involve the breaking of an 0-0 bond and the formation of O-H bonds [17]. Surfaces that strongly bind an adsorbate tend to enhance the kinetics of bond-breaking steps. On the other hand, surfaces that bind species weakly tend to facilitate the kinetics of bond-making steps. Hence, according to principle of Sabatier [18], the catalyst which strikes the best balance between O2 adsorption and ORR intermediates removal will be the most active for ORR. 

Sabatier s principle has an interesting consequence for the kinetics of reactions catalyzed by systems with a wide reactivity distribution of active sites. Since the rate of reaction is at maximum for those sites having interaction energies close to the optimum of Figure 6.22, the overall rate of reaction is dominated by these sites. For this reason the kinetics of the reaction can often be modeled by equations corresponding to one type of catalytically reactive site only. However, depending... 

In this section, the emphasis is on the role of competitive adsorption phenomena in the kinetics of a few catalytic reactions. Trends in catalytic reactivity will again be discussed in terms of Sabatier s principle. 

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