The Sabatier Principle

When the concept of catalysis was first formulated, the idea that the catalytic reaction is actually a catalytic cycle was not at aU obvious. In 1836 Berzelius deflned catalytic force as the process responsible for catalysis in which the decomposition of bodies was caused by the action of another simple or compound body. Faraday later showed that a catalytically reactive surface was chemically altered by contact with reacting gases. It was not, however, until after chemical thermodynamics had been developed that a more scientific understanding of catalysis was formulated. In 1896 Van t Hoff demonstrated that the rate of a catalytic reaction depended up)on the amount of catalyst. Soon after Ostwald defined a catalyst to be a substance that changes the velocity of a reaction without itself being altered in the process. A catalyst, however, must operate within the thermodynamic limits of the reacting system PI. 

The reaction conditions for a specific system are defined by the overall thermodynamics of the reaction. The catalyst facilitates the adsorption of the reactants and their subsequent conversion into products. An important feature, however, is that the products must be rapidly removed from the surface in order to regenerate active surface sites. These ideas led to the concept that the catalytic reaction is comprised of a cycle which is made up of elementary physicochemical processes. At the most basic level, catalysis is comprised of at least five elementary steps  

The incorporation of these steps into a cycle and the overall concept of the catalytic cycle are illustrated for the catalytic decomposition of N2O over a catalytic substrate in Fig. 2.1. N2O is an environmentally detrimental molecule. It is produced as an undesirable product 

The order for a monomolecular reaction changes from positive in the reactant concentration to the left of the Sabatier maximmn to zero or negative order in the reactant 

Sabatier-type volcano plots have been constructed for a number of different commercially relevant systemsl l. A simple kinetic expression that simulates the Sabatier result is found when one realizes that the decomposition of molecules requires a vacant site for molecular fragments to adsorb on. For instance, in the N2O decomposition reaction, the dominant surface species (most abundant reaction intermediate) is atomic oxygen (O), which is in equilibrium with the gas phase. When the slow step in the reaction is dissociative adsorption of N2O, the mean-field kinetic rate expression for N2O decomposition, normalized per unit surface area of catalyst, becomes  

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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. 

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... 

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.

The curve is a graphical representation of the Sabatier principle according to which the best catalysts are those adsorbing relevant species neither too weakly nor too strongly. Volcano curves are known also for catalytic reactions (on the other hand the principles are precisely the same), the only difference being that they are called Balandin curves. 

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. 

Clearly, an optimum for the interaction of the catalytically active surface and the adsorbates exists, resulting in a maximum for the reaction rate (the Sabatier principle). To the left of the maximum the reaction has a positive order in the reactants, whereas to the right the order has become negative. 

These relationships, when incorporated into microkinetics models of catalytic reaction cycles, enable remarkable new predictive insights into the control of heterogeneously catalyzed reactions. Predictive models of catalytic activity as a function of catalyst composition as well as reaction conditiorvs have been constructed (22-24). The resultant volcano curves can be considered to be an application of the Sabatier principle (25,26). 

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. 

Figure 3.6 demonstrates on both the Pt-skin as well as Pt-skeleton surfaces the relationship between the specific activity and the d-band center position exhibits a volcano shape, with the maximum catalytic activity obtained for PtjCo. This behavior is apparently a consequence of the Sabatier principle discussed earlier, and published in many recent studies [77, 78]. For metal surfaces that bind oxygen too strongly, as in the case of Pt, the d-band center is too close to the Fermi level and the rate of the ORR is limited by the availability of spectator-free Pt sites. 

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]. 

Inorganic. - Aray et correlated the topology of p of pyrite-type transition metal sulfides with their catalytic activity in hydrodesulfurization. The most active catalysts are characterized by intermediate values at the M-S BCP. This result supports the consistency of transition-metal-sulfide-catalysed hydrodesulfurization with the Sabatier principle. 

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. 

In this chapter we introduced the basic physical chemistry that governs catalytic reactivity. The catalytic reaction is a cycle comprised of elementary steps including adsorption, surface reaction, desorption, and diffusion. For optimum catalytic performance, the activation of the reactant and the evolution of the product must be in direct balance. This is the heart of the Sabatier principle. Practical biological, as well as chemical, catalytic systems are often much more complex since one of the key intermediates can actually be a catalytic reagent which is generated within the reaction system. The overall catalytic system can then be thought of as nested catalytic reaction cycles. Bifunctional or multifunctional catalysts realize this by combining several catalytic reaction centers into one catalyst. Optimal catalytic performance then requires that the rates of reaction at different reaction centers be carefully tuned. 

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. 

Other metals, including iron or noble metals, are worthwhile for examination to compare the selectivity between hydrodesulfiirization and hydrogenation, although a series of metals are compared in the relationship between strength of metal-S and -H bonds and catalytic activity to confirm the Sabatier principle... 

These findings can also be related to Pt being the most active metal in electrochemical hydrogen evolution, owing to the ideal chemisorption strength of the adsorbed reaction intermediate corresponding to the Sabatier principle [80]. 

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