What are the different types of catalysis?

Catalysis reactions can broadly be categorized into two types – homogeneous catalysis and heterogeneous catalysis. 

Homogeneous Catalysis

Homogeneous catalysis occurs when the catalyst is in the same physical state or phase as the reactants. This type of catalysis is most common between catalysts and reactants in the gaseous state. 

Example of homogeneous catalysis:

Nitrogen oxide (NO) acts as a catalyst in oxidizing sulfur dioxide (SO2) to sulfur trioxide (SO3) with dioxygen (O2). The catalyst NO, as well as the two reactants are in the same phase. 

2SO2 (gas) + O2 (gas) → 2SO3 (gas) in the presence of gaseous NO

Heterogeneous Catalysis

Heterogeneous catalysis occurs when the catalyst is in a different physical state or phase as that of the reactant/s. In this type of catalysis, the reaction between the catalyst and reactant takes place at the interface between them, often referred to as the gas-solid border. 

Example of heterogeneous catalysis: 

Platinum (Pt), which is in a solid state, acts as a catalyst in oxidizing sulfur dioxide (SO2) to sulfur trioxide (SO3). Both reactants are in a gaseous state.  

2SO2 (gas) → 2SO3 (gas) in presence of solid Pt.