Catalyst is a substance which can change the speed of a chemical reaction without being used up in that reaction and the phenomenon is known as catalysis. Catalyst is divided into two types:
Positive catalyst: If a catalyst increases the speed of a reaction, it is called a positive catalyst and the phenomenon is called positive analysis. For example: oxidation of SO2 to SO3 in presence of NO or in presence of V2O5 are examples of positive catalysis.
Negative catalyst: If a catalyst decreases the speed of a reaction, it is called a negative catalyst and the phenomenon is called negative catalyst. For example: decomposition of H2O2 in presence of phosphoric acid and oxidation of chloroform in presence of alcohol.