Catalysis by Metal Clusters

The potential of metal clusters as homogeneous catalysts has been one important component in the driving force that has permitted the spectacular development of this chemistry observed in the last 15 years. 

Catalytic activity of cluster species has been detected and investigated in a number of processes. There are some detailed studies on mechanisms involved in such processes and many of them correspond indeed to reactions and mechanisms described in the previous Section. However, it is in general difficult to establish with precision which are the species actually responsible for the catalytic phenomenon. 

In order to illustrate the potential of cluster chemistry in catalysis, the catalytic activity of cluster species in a number of selected processes will be described. The processes to be considered-hydrogenation and isomerization of unsaturated hydrocarbons, hydroformylation of alkenes, hydrogen reductions of carbon monoxide, and water-gas shift reaction-are interesting not only from the point of view of fundamental knowledge but also because of their great industrial and economical relevance. 

One of us has reviewed catalysis by metal clusters through to mid-1979. Emphasis was placed on the potential applications of metal clusters in catalysis and on the problem area of characterization especially as it relates to our knowledge of systems under reaction conditions. The difficulty of estab-hshing beyond reasonable doubt that the catalytically active entity in a given reaction is indeed a cluster compound, and not a product of dissociation or aggregation, was also stressed. We now complement the earlier review by a consideration of subsequent developments and of attempts that have been 

Pierantozzi, K. J. McQuade, and B. C. Gates, Paper B18, 7th International Congress on Catalysis, Tokyo, 1980. 

Whyman, in Transition Metal Clusters , ed. B. F. G. Johnson, Wiley, London, 1980, Ch. 8. 

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There are only a few weU-documented examples of catalysis by metal clusters, and not many are to be expected as most metal clusters are fragile and fragment to give metal complexes or aggregate to give metal under reaction conditions (39). However, the metal carbonyl clusters are conceptually important because they form a bridge between catalysts commonly used in solution, ie, transition-metal complexes with single metal atoms, and catalysts commonly used on surfaces, ie, small metal particles or clusters. 

R.C. Baetzold, Molecular orbital description of catalysis by metal clusters, J. Catal. 29... 

Heterogeneous Catalysis by Metal Clusters. - Present activity in this area reflects the view that supported metal clusters could form the basis of a new generation of heterogeneous catalysts. Characterization of such materials, especially under reaction conditions, will be difficult. Such catalysts will be valuable if they exhibit activities and selectivities that differ from those afforded by conventionally prepared catalysts unfortunately, it is a deficiency of much of the work so far reported that such comparisons have not been made. However, evidence has recently become available that shows that cluster-derived heterogeneous catalysts may exhibit distinctive behaviour. 

These and other aspects of catalysis by metal clusters in zeolites have been reviewed recently by Gallezot, and by Uytterhoeven. ... 

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