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The Catalysts Themselves

 

What is desired in a catalyst is

 

 

 

 

 

Different kinds of substances are effective in different kinds of reactions. Examples are:

Catalyst Function Examples
metals hydrogenation
dehydrogenation

Fe, Ni, Pt, Ag, Ru
semiconducting oxidation NiO, ZnO,
insulating dehydration Al2O3, SiO2, MgO
Acids polymerisation
isomerisation
cracking
alkylation

H3PO4, H2SO4, SiO2/ Al2O3, zeolites

 

The activity of heterogeneous catalysts depends on adsorption. The surface needs to be covered by adsorbed reactants, but if they are held too strongly the molecules become immobilised on the catalyst surface, and other reactant molecules cannot react.

Catalyst activity for metals is highest towards the middle of the d-block.

 

Catalyst activity

Turnover factor is the number of molecules converted by each catalytic site in a unit of time. The most active enzymes have a turnover factor of 1010/hr. This activity is matched by the acid sites in some model zeolite catalysts. Typical industrial catalysts have turnover factors of between 103 and 105/hr.

 

 

 

 

 

 

 

 

 

 

 

 

Selectivity

A selectivity of 99.99% means that the catalyst makes one mistake in 10,000 conversions. This is the performance achieved by enzymes in living systems, though most enzymes do better than this.
Few synthetic industrial catalysts achieve this degree of control over the chemistry they catalyse.

 

Stability

The number of reactions performed by each active site before it decays or becomes inactive, or the catalyst has to be disposed of is of great importance. Fine chemical manufacture demands in excess of 1,000,000 reactions per active site.