Principle of Sabatier

This plot is, for obvious reasons, called a volcano curve and the principle that the points will fall on a smooth curve is called the principle of Sabatier... 

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. 

The above conclusion, based on a simplified model, must be considered in the light of the experimental results, according to the above conclusion that an iron catalyst for ammonia synthesis is about half-covered with nitrogen during the steady state reaction. Another way to look at these results is to consider them as expressions of the principle of Sabatier, who considered that the optimum catalyst was capable of adsorbing an intermediate compound sufficiently, but not to stably. 

The key step in the synthesis of NH3 form N2 + 3H2 is to dissociate the N-N triple bond in the N2 molecule. The direct gas phase reaction would involve extremely endothermic and exothermic reactions. The resulting activation energies would be prohibitively high according to the principle of Sabatier. 

A catalytic reaction is composed of several reaction steps. Molecules have to adsorb to the catalyst and become activated, and product molecules have to desorb. The catalytic reaction is a reaction cycle of elementary reaction steps. The catalytic center is regenerated after reaction. This is the basis of the key molecular principle of catalysis the Sabatier principle. According to this principle, the rate of a catalytic reaction has a maximum when the rate of activation and the rate of product desorption balance. 

Figure 1.2 Volcano plot illustrating the Sabatier principle. Catalytic rate is maximum at optimum adsorption strength. On the left of the Sabatier maximum, rate has a positive order in reactant concentration, and on the right of Sabatier maximum the rate has a negative order.

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. 

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. 

It is intriguing that analysis of the volcano curve predicts that the apex of the curve occurs at AH(H2)ads = 0 (formally, AG = 0) [26]. This value corresponds to the condition D(M-H) = 1/2D(H-H), that is, forming an M-H bond has the same energetic probability as forming an H2 molecule. This condition is that expressed qualitatively by the Sabatier principle of catalysis and corresponds to the situation of maximum electrocatalytic activity. Interestingly, the experimental picture shows that the group of precious transition metals lies dose to the apex of the curve, with Pt in a dominant position. It is a fact that Pt is the best catalyst for electrochemical H2 evolution however, its use is made impractical by its cost. On the other hand, Pt is the best electrocatalyst on the basis of electronic factors only, other conditions being the same. 

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

We have summarized these developments in two recent papers the first addresses the topic of structure sensitivity in combination with BEP relationships (14) and the second addresses the Sabatier principle (27). Sabatier-type volcano relationships can be deduced for activity as a function of adsorption energy, and they can also be used to predict trends regarding deactivation of Fischer-Tropsch catalysts by C—C recombination reactions (28). We refer to these texts as background information to the material presented here. 

The Sabatier principle of catalysis also finds extensive application in the area of electrocatalysis reactants should be moderately adsorbed on the catalyst/electro-catalyst surface. Very weak or very strong adsorption leads to low electrocatalytic activity. This has been demonstrated repeatedly in the literature by the use of volcano plots (Figs 23-25). In these plots, the electrocatalytic activity is plotted as a function of the adsorption energy of the key reactant or some other parameter related to it in a linear or near-linear fashion, such as the work function of the metal [5], or the metal—H bond strength when discussing the H2 evolution reaction (Fig. 24) [54] or the enthalpy of the lower-to-higher oxide transition when examining the O2 evolution reaction (Fig. 25) [55]. 

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. 

In other words, the optimum catalyst with Nmax is one with half the sites filled and half empty. This is a quantitative verification of Sabatier s Principle, which states that the best catalyst is one that forms an unstable intermediate compound at the surface which is neither too weakly nor too strongly adsorbed [2,3]. 

Since the ORR is a surface reaction that involves adsorbed intermediates, it is subdued to the Sabatier-volcano principle of heterogeneous catalysis (Balandin, 1969 Sabatier, 1920). This principle states that in order for a catalyzed surface reaction to proceed, some bonding of adsorbed intermediate(s) is necessary, whereas a bonding that is too strong will block the surface and slow down the reaction. Several examples of reactions in heterogeneous catalysis and electrochemistry that obey these principles are discussed by Parsons (2011). 

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. 

The rate of the ammonia synthesis shows an optimum for metals in the first column of the Group VIII metals and provides a classical example of Sabatier s principle. 

The palladium clusters and islands, which we examined in Section 1.5, show another important property of a good catalyst They have various adsorption sites for hydrogen with different adsorption energies, so that the reaction can pass through those sites whose energy is optimal in the sense of Sabatier s principle. They are therefore more active than a monolayer of Pd on Ah(lll), which offers only two sites, neither of which has the optimum energy. 

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. 

The Sabatier principle deals with the relation between catalytic reaction rate and adsorption energies of surface reaction intermediates. A very useful relation often... 

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. 

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