Surface Moderation by Coadsorption of Organic Molecules

Heterogeneous supported transition-metal catalysts, on the other hand, lack individual molecular centers and instead are comprised of surfaces with different metal ensembles. Surfaces that contain step edges can begin to exploit the stereochemistry of the step to induce topologically controlled stereoselectivity. For instance, enantioselective catalysis has been realized electrochemically by the use of oriented transition-metal catalysts with 

An elegant polymerization experiment that illustrates the importance of these edges is shown in Fig. 2.27. 

The layered MoS2-based catalyst (discussed in Chapter 5) that are used in the hydrodesulfurization of crude oil can also be considered a solid-state chemical surface ana- 

We have now seen that specific steric factors can control catalytic selectivity by guiding specific reactants to specific products. Most heterogeneous catalysts, with the exception of zeolites, can be expected to be intrinsically less selective, since they typically do not contain a three-dimensional architecture that can help to guide the formation of specific products. Therefore, they are optimum for those applications where thermodynamics prescribes the formation of one particular product over the others. The careful choice of transition metals and promoters, however, can appreciably alter chemical reactivity and bias specific reactions, thus altering the relative rates of competing elementary reaction steps. 

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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. 

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