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.