Structure Effects on Rates and Equilibria in Catalysis

The structure of a reacting molecule can be used as the chemical probe for the reaction mechanism in several ways. Ample experience is available with these methods from the research of noncatalytic homogeneous reactions, and their possibilities and limitations are well known. However, the solid catalyst restricts the scope to some extent on the one hand, but opens new applications on the other. For this reason, the methods of physical organic and inorganic chemistry developed for noncatalytic reactions cannot simply be transferred into the field of heterogeneous catalysis. The following remarks should identify some of the problems. 

The analysis of the influence of substituents in organic molecules upon rates and equilibria has led to the recognition that they operate in two different ways, either by changing the electronic density, in comparison with a reference substituent, at the reaction center of the molecule or by blocking the access to the reaction center. The same is true for heterogeneous catalytic reactions. However, the interaction of a molecule with a surface can disturb the normal effect of a substituent. 

Consider a molecule X Y Z, where X is the substituent, Y the constant part of the molecule (the link between X and Z), and Z the reaction center. Now we wish to compare its reactivity with the molecule H Y Z bearing hydrogen as the reference substituent. The electronic influence of the substituent X may depend on whether it comes into contact with the surface or not. In terms of adsorption this means whether the molecule will be attached to the surface horizontally or perpendicularly  

In Case A, the proximity of the group to the surface or to a greater extent its bonding to the surface necessarily causes a shift in the original electron density distribution within the group X and, in consequence, also in the whole 

The steric effects may be more pronounced in heterogeneous catalysts than in homogeneous reactions in solution. The rigid, solid surface restricts the approach of the reactants to the active centers and interaction between the reactants. The steric requirements are quite stringent when a two-point adsorption is necessary and when, in consequence, the internal motion of the adsorbed molecules is limited. In this way, the stereoselectivity of some heterogeneous catalytic reactions, for example, the hydrogenation of alkenes on metals (5) or the dehydration of alcohols on alumina and thoria (9), have been explained. 

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