Sabatier s principle

Hence, we intuitively feel that the successful combination of catalyst and reaction is that in which the interaction between catalyst and reacting species is not too weak, but also not too strong. This is a loosely formulated version of Sabatier s Principle, which we encounter in a more precise form in Chapter 2 and in detail in Section 6.5.3.5. 

Sabatier s Principle is illustrated in Fig. 6.40 where the ammonia rate is plotted for similar conditions versus the type of transition metals supported on graphite. The theory outlined so far readily explains the observed trends metals to the left of the periodic table are perfectly capable of dissociating N2 but the resulting N atoms will be bound very strongly and are therefore less reactive. The metals to the right are unable to dissociate the N2 molecule. This leads to an optimum for metals such as Fe, Ru, and Os. This type of plot is common in catalysis and is usually referred to as a volcano plot. 

Identification of such universal relations between activation energies and heats of adsorption for particular classes of reaction can be seen as a more precise and more quantitative formulation of Sabatier s Principle. It is promising tool in the search for new materials on the basis of optimized interaction strength between relevant intermediates and the surface. 

Figure 9.7. The hydrodesulfurization activity oftransition metal sulfides obeys Sabatier s principle (Section 6.5.3.5) the curve is a so-called volcano plot. [Adapted from T.A. Pecoraro and R.R. Chianelli.J, Catal. 67 (1981) 430 P.Raybaud,). Hafner, G. Kresse,...

Early in the last century, Paul Sabatier1 pointed out A most important property of an excellent catalyst is that it has an ability to bind many molecules but not too strongly . This Sabatier s principle is also the principle for how an excellent catalyst for electrochemical reactions works. In electrochemical terms, an active... 

Note that the catalytic active site is often not the energetically preferred site for adsorption (i.e., there are other sites on the surface where adsorption would be more exothermic). The most favored site for adsorption may be too good , in the sense that any species that are adsorbed on it would simply stay there. The active site follows Sabatier s principle - reactants should be able to be adsorbed there, but after the reaction they should also be able to leave. 

Sabatier s principle of the optimum site activity. Only optimum sites contribute to the reaction, resulting in an apparently uniform behaviour... 

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-... 

The energetics of the CO oxidation reaction is illustrated in Fig. 5.8. The activation energy of the homogeneous gas phase reaction between CO and Oz would be largely determined by the energy needed to break the 0-0 bond of O2, some 500 kj/mol. The catalyst easily dissociates the O2 molecule, and the ratedetermining step has been shifted to the reaction between COads and Oads/ which is only about 100 kJ/mol on palladium. The CO oxidation thus illustrates nicely that the essential action of the catalyst lies in the dissociation of a bond. Once this has been accomplished, the subsequent reactions follow, provided the intermediates are not held too strongly by the catalyst as expressed in Sabatier s Principle (see e.g. Ref. [50]). 

Fig. 9.3.1 Sabatier s principle. Comparison of various catalysts for the formic acid decom x)sition. [W. J. M., Rootsaert, and W. M. H. Sachtler, Z- Physik Chem., 26, 16 (I960))...

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. 

We now explore Sabatier s principle, named for the French chemist P Sabatier (1854-1941), w ho won the 1912 Nobel Prize in Chemistry for his work on catalysis. This principle explains why the best catalysts are surfaces that bind reactants neither too weakly nor too strongly. 

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