The process in which the catalyst and the reactants exist in a different phase is called a heterogeneous catalysis.

The reactants come in contact with the surface of catalyst which behaves like surface molecules of a liquid not surrounded by atoms or molecules of their type thereby having unbalanced attractive forces on the surface holding the adsorbate in order to perform the reaction. Here the molecule of reactant (gas) gets and produces an adsorption complex. Increase in concentration of reactants on the surface of the catalyst also increases the rate of reaction.