Reactions Catalyzed by Metal Surfaces

FIGURE 23.19 Reduction of carbon-carbon multiple bonds using Adams catalyst. 

Heterogeneous catalysts predominate in industrial processes, because of their ability to be recovered and recycled relatively easily. Both activity and selectivity may depend significantly on the method of catalyst preparation and the precise conditions of the reaction. 

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The skeletal rearrangements of hydrocarbons represent an important class of reactions catalyzed by metal surfaces, which have few counterparts in homogeneous catalysis. Although two reviews dealing with the subject (7, 2) have recently been published, this review seemed worthwhile on account... 

Intimately bound up with the possibility of discovering catalytically active surface phases is the basic query whether or not the efficiency need be as low as is commonly experienced in commercial applications. In a typical industrial reaction the catalytic efficiency per impinging reactant molecule is often about 10" °, or even lower. This can be explained simply in three different ways (i) Catalytically active sites are extremely sparse, yet very efficient, (ii) Catalytically active sites are many in number but extremely inefficient, (iii) The surface is almost completely poisoned. If the view (i) should be true, the LEED approach alone cannot possibly be used to identify such a dilute concentration of sites. Yet views (ii) and (iii) seem at least as likely, and in either case we can expect LEED analysis to help pinpoint the nature of the sites. This will be the case particularly for those reactions with high catalytic efficiency, such as a number of simple reactions catalyzed by metal surfaces. 

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