The Principles of Catalysis

How does a heterogeneous catalyst work?

A reaction catalysed by a heterogeneous catalyst can be represented by a flow chart and animated diagram.

 

Step 1

reactant + catalyst

 

Step 2

reactant/catalyst complex

 

Step 3

product/catalyst complex

 

Step 4

product + catalyst

Making and breaking bonds

Reactant molecules are adsorbed at active sites onto the surface of the catalyst. This involves the formation of weak bonds between reactant molecules and the catalyst which causes other bonds in the reactant molecule to be stretched and weakened. The weakened structure is converted to another complex that is essentially the product attached to the catalyst. Finally, this complex breaks down to release the product molecule which moves away to leave the catalyst surface ready to interact with another reactant molecule.

Choosing a catalyst

The choice of suitable catalyst for a particular reaction often depends on the stability of the complexes formed between reactant and catalyst and product and catalyst. They must be stable enough to form and provide an alternative pathway to the uncatalysed reaction but they must not be too stable as this would lead to an increase in the activation enthalpy and would slow down the rate of reaction.

Not silver or tungsten

For example, the adsorption of molecules onto the surface of tungsten tends to be too strong while the adsorption onto silver tends to be too weak making them both less useful as catalysts in some reactions, compared to nickel or platinum.