Homogeneous and Heterogeneous Catalysis

Catalysis splits into two broad classes according to the phases of reactants and catalyst.

(a) Homogeneous catalysis

The reactants, products and catalyst are all in the same phase (all gas or all liquid).

• Oxidation of sulphur dioxide with nitric oxide catalyst (lead chamber process):

• Acid-catalysed hydrolysis of methyl acetate ($H^+$ from HCl):

• Acid-catalysed hydrolysis of sugar ($H^+$ from $H_2SO_4$):

(glucose + fructose). In each case reactants and catalyst share one phase.

(b) Heterogeneous catalysis

The reactants and catalyst are in different phases — most often gaseous reactants over a solid catalyst.

• Oxidation of $SO_2$ over platinum:

• Ammonia synthesis over finely divided iron (Haber's process): $N_2(g) + 3H_2(g) \xrightarrow{\;Fe(s)\;} 2NH_3(g)$ …