Quick Notes Heterogeneous Catalysis

  • In heterogeneous catalysis, the catalyst is in a different phase to the reactants.
  • With solid-based catalysts and gaseous or aqueous reactants, the catalysis process occurs in stages:
    • reactants diffuse onto catalyst surface
    • reactants are adsorbed onto the catalyst
    • the reaction starts and an intermediate is formed
    • the reaction completes and products are made
    • products desorb (leave) the surface of the catalyst

Full Notes Heterogeneous Catalysis

There are two types of catalysis: heterogeneous and homogeneous.

Heterogeneous catalysis is where the catalyst is in a different phase to the reactants. For example, in a car exhaust the catalysts in a catalytic converter are solid metals, but the reactants are in gaseous states.

simple reaction showing catalyst

In the above reaction, in the presence of a catalyst two reactants (A and B) react to produce C.

heterogeneous catalysis diffusion transition metal a-level chemistry catalyst

For solid-based catalysts and gaseous reactants: the reactants diffuse (through other gases) to the catalyst and ‘land’ on the surface.

heterogeneous catalysis absorbed absorbing transition metal a-level chemistry catalyst

The reactants then get adsorbed onto the surface of the catalyst.

heterogeneous catalysis intermediate transition metal

After adsorption has taken place, bonds within the reactants are weakened to the point where the reaction can start to take place, and an intermediate is formed.

heterogeneous catalysis products catalyst

After the reaction has completed, the product remains adsorbed on the surface of the catalyst.

heterogeneous catalysis products desorb desorbing from catalyst

The product will desorb from the surface of the catalyst and will diffuse away.

The Contact Process

The contact process is a frequently used example of heterogeneous catalysis, where vanadium (V) oxide is used as a catalyst in the production of sulfur trioxide from sulfur dioxide. (Sulfur trioxide is a reactant used to produce sulfuric acid.)

contact process heterogeneous catalysis sulfur dioxide and oxygen sulfur trioxide

The vanadium (V) catalyst is used to produce the sulfur trioxide, then it’s reformed when reacted with oxygen.

contact process heterogeneous catalysis vanadium oxide catalyst V2O5 a-level chemistry

The vanadium (V) oxide oxidises the sulfur dioxide, going from vanadium (V) oxide to vanadium (IV) oxide.

contact process heterogeneous catalysis oxidation of sulfur reduction of vanadium

The vanadium (IV) oxide then gets oxidised itself by the oxygen from the sulfur trioxide. The final product is vanadium (V) oxide – the same as at the start of the reaction.

contact process heterogeneous catalysis reduction of oxygen oxidation of vanadium

The key to the whole process is the fact that the vanadium in the catalyst can be reduced (to form the sulfur trioxide), then oxidised (to reform the catalyst). The variable oxidation state of vanadium allows this to happen.