(i) Positive catalysis: The catalyst increases the rate of a reaction.
(ii) Negative catalysis (Inhibitor or retarder): Chemical reactions are sometimes retarded by the presence of a foreign substance the substance is known as a negative catalyst.
Alcohol, Acetanilide: Prevents oxidation of Na2SO3 by air
H3PO4: Prevents decomposition of H2O2
(iii) Auto catalysis: In this type of catalysis, one of the products of the reaction catalyses the reaction. In the oxidation of oxalic acid by KMnO4, Mn2+ ion formed is known to accelerate the reaction. So, when KMnO4 solution is run into warm solution of oxalic acid (+ dil. H2SO4), initially there is a time lag before decolourisation occurs; as more KMnO4 is added, the decolourisation becomes almost instantaneous.