Sabatier principle

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. 

Here max Rt is the maximal rate of reaction step i, which is calculated by assuming optimal coverages for that reaction step. This (usually multi-dimensional) volcano-curve we shall refer to as the Sabatier volcano-curve, as it is intimately linked to the original Sabatier principle [132,133]. This principle states that desorption from a reactive metal catalyst is slow and will increase on less reactive metals. On very noble metals the large energy barrier for dissociation will, however decrease the dissociation rate. The best catalyst must be a compromise between the two extremes. As has been shown above, this does not necessarily mean that the optimal compromise is obtained exactly where the maximal desorption and dissociation rates are competing. That is only the case far from equilibrium. Close to equilibrium the maximum will often be attained while dissociation is the rate-determining step, and the maximum of the volcano-curve will then be reached due to a lack of free sites to dissociate into. 

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. 

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