Re: why metallic catalysts,such as iron and platinum are often deposited as cry

Rodney,
      The reason why metallic catalysts are usually used in the form you 
mention is related to the chemical mechanisms by which they act.  The 
catalysis occurs on the surface of the metal; the rest of the metal does 
nothing.  So, the greater the surface area, the more efficient the catalyst 
will be on a pound-for-pound basis.  Some metal catalysts such as palladium 
are expensive, which provides a financial incentive to maximize the amount 
of metal that is catalytically active.  Thus catalysts are often used as a 
film over some other, cheaper support material.

In most cases the metal surface serves to bind the two reactants and to 
lower the activation energy for the chemical reaction between them.  One 
example is a catalytic converter in an automobile.  The metal catalyst 
binds oxygen molecules from the air.  Exhaust gases, containing 
incompletely oxidized hydrocarbons, pass over the catalyst.  Some of these 
organic molecules bind to the surface where they are further oxidized by 
the metal-bound oxygen before being released.