The two groups into which phenomenon of catalysis can be divided are:
(i) Homogeneous catalysis: When the reactants and the catalyst are in the same phase, the catalysis is said to be homogeneous catalysis. For example, SO2 is oxidised to SO3 in the presence of nitric oxide, NO as a catalyst.
2SO2(g) + O2(g) NO(g) → 2SO3(g)
(ii) Heterogeneous catalysis: When the reactants are in a different phase than the catalyst, the catalysis is said to be heterogeneous. For example, the combination of dinitrogen and dihydrogen to form ammonia in the presence of finely divided iron as a catalyst.
N2(g) + 3H2(g) Fe(s) → 2NH3(g).