Variables associated with the catalyst

Metal hydrogenation catalysts may be employed in any one of a variety of forms (a) macroscopic forms as wires, foils or granules (b) microscopic forms as powders obtained by chemical reduction, colloidal suspensions, blacks or evaporated metal films (c) supported catalysts where varying concentrations of metal are dispersed to a varying degree on a carrier such as alumina, silica or carbon. 

Clearly, there is an infinite number of variations of the physical form of the catalyst which may be employed. One of the major problems is the production of a reproducible metal surface. Irreproducibility may be due to any one of a number of factors, the following being some of the more important ones (a) variations in the degree of cleanliness and state of reduction of the surface (b) variation in the degree of exposure of certain crystallographic planes (c) variation in the concentration of surface defects and (d) variation in the distribution of particle sizes. Most of these factors are not readily controllable and may not be without effect upon the rate and mechanism of the reaction being catalysed it is important, therefore, that the effects of each of the variables is assessed independently. 

Metal single crystals and evaporated metal films have been used in an attempt to overcome some of the difficulties of irreproducibility. How- 

Supported metal catalysts are much easier to employ and have obvious attractions for industrial use from their ease of handling and economic considerations of obtaining maximum utilisation of the catalytically active metal, by using very small particles with a high surface-to-volume ratio, which are stable on the support and not susceptible to sintering. In spite of the inherent difficulties of variable activity, kinetics and activation energies [11] associated with their use, supported metals have been extensively used as hydrogenation catalysts. 

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