Ensemble Effects and Defect Sites

In zeolites as well as in enzymes, the cavity shape and form determine the rate of conversion and selectivity of the catalytic reaction (Chapters 4 and 7). The selectivity of a reaction is defined as the percentage of a particular product molecule formed with respect to the total amount of product molecules. 

The planar surfaces of transition-metal catalysts, however, have few steric possibilities to influence selectivity, since the steric constraints are absent. As a consequence, such heterogeneous catalysts very often only have a high selectivity when limited product op -tions exist. On such surfaces, the differences in rates of competitive reaction pathways are controlled by differences in activation entropies and energies of the elementary rate constants r-j. These are controlled by the intrinsic chemistry of the reactions. One geometric parameter that can critically affect the reaction rate is the size of the atomic surface ensemble necessary to activate bonds in molecules. 

In Chapter 7 we discuss the unique seven-atom surface-ensemble cluster on the Fe(lll) surface (shown in Fig. 2. IOC) that is optimum for N2 activation. Early suggestions that surface ensembles with a particular number of atoms are necessary for a particular reaction to occur are deduced from alloying studies of reactive transition-metal surfaces, with catalytically inert metals such as Au, Ag, Cu or Sn . For example, the infrared spectrum of CO adsorbed on Pd shows the characteristic signature of CO adsorbed one-fold, twofold or three-fold to surface Pd atoms . Alloying Pd with Ag, to which CO only weakly coordinates, dilutes the surface ensembles. One observes a decrease of the three-fold and the two-fold coordinated CO and the one-fold coordinated CO becomes the dominant species. The effect of alloying a reactive metal with a more inert metal is especially dramatic when one compares hydrocarbon hydrogenation reactions with hydrocarbon hydrogenolysis reactionst . 

One notes the small dependence of the dehydrogenation rate of cyclohexane on Cu concentration. Actually the dehydrogenation reaction rate shows a slight initial increase. In contrast, there is a strong decrease in the rate of the hydrogenolysis of hexane with 

The ensemble effect is typically inferred when one realizes that a molecule that dissociates requires at least two different surface sites to accommodate the molecular fragments. For instance, the rate constant for CO dissociation can in an elementary fashion be written 

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We conclude this introductory with a short outline of the sections that follow. In Section 2.3.2, we offer a brief introduction to pressrue and material gap problems that arise when model catalytic systems and conditions are used to emulate working catalytic systems. In Section 2.3.3, we describe the local aspects of the catalytically reactive sites in the section titled Ensemble effects and defect sites . This is followed by four sections on environmental influences on the reactivily entitled Cluster size and metal supports , Cooperativity , Surface moderation by coadsorption of organic molecules and Stereochemistry of homogeneous catalysts . The chapter is concluded with a section on surface kinetics, dealing with surface reconstruction and transient reaction intermediates. 

Principles of Molecular Heterogeneous Catalysis 41 2.3.3 Ensemble Effects and Defect Sites... 

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