Physical forms and their preparation

Solid catalytic materials can be divided into two main groups  

Bulk metals can be used in traditional engineering forms, more particularly as fine wire woven into gauzes. Such forms are generally used only in high-temperature processes, such as the partially oxidative dehydrogenation of methanol (over Ag) or ammonia oxidation (over Pt-Rh) at about 500-600°C and 850-900°C respectively. Mechanical stability is of greater importance than high surface area. 

There are two major reasons for the use of supports, to provide a stable extended surface over which an active component can be dispersed, and to confer mechanical strength. In either case, the desired property may well have to be maintained to a high temperture. 

Synthetic silicas, aluminas and carbon blacks—there are many variations 

Preparation most frequently involves absorption of a solution of a suitable metal compound into the support, followed by drying, thermal decomposition and reduction. This treatment, and the reductant employed (such as hydrazine in solution, or hydrogen gas), often has a significant effect on the physical form and dispersion of the resulting metal. There are few rules, and this area is far too complex for discussion here. 

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