Some examples of negative catalyst are alcohol, acetanilide which prevents the oxidation of Na2SO3 by air. Another type of catalysis is auto catalysis in which products of the reaction catalyses the reaction.For example, in the oxidation of oxalic acid by potassium permanganate results the formation of Mn(II) ions which accelerates the reaction and acts as auto-catalyst. In the homogeneous catalysis reactants and catalyst remains in the same phase such as in the lead chamber process, nitric oxide (Catalyst) and reactant molecules (sulphur dioxide and oxygen) are in same phase.