10.1 Positive and negative catalysis

2.1 Characteristics of Adsorption
2.2 Types of Adsorption

4.1 Freundlich adsorption isotherm
4.2 The Langmuir adsorption isotherm

7.1 Different Colloidal Systems
7.2 Classification of Colloidal System
7.3 Preparation of Colloidal System
7.4 Purification of Colloidal System
7.5 Properties of colloidal system

8.1 Protective colloids

9.1 Foams
9.2 Applications of colloids and emulsions

10.1 Positive and negative catalysis
10.2 Types of catalysis

12.1 Characteristics of Enzymes
12.2 Mechanism of enzyme catalysis
12.3 Autocatalysis
12.4 Induced catalysis

If a catalyst increases (accelerates) the speed of a reaction, it is called a positive catalyst and the phenomenon is called positive catalysis.

On the other hand, if a catalyst decreases (retards) the speed of a reaction, it is called a negative catalyst and the phenomenon is called negative catalysis.

It may be noted that a positive catalyst lowers the potential energy barrier, i.e., lowers the activation energy of the reaction, a negative catalyst raises the potential energy barrier, i.e., increases the activation energy.

Certain substances like arsenic, $CO$, etc. if present, lowers the activity of the catalyst. These substances are called catalytic poisons. This seems to be due to the fact that these poisons are preferentially adsorbed at the active sites of the catalyst as compared to the reactants.

A substance, if present along with the catalyst, enhances the activity of the catalyst, it is called a catalytic promoter. The best example for this is, molybdenum acts as a promoter for iron catalyst in the manufacture of ammonia by Haber's process. This seems to be due to the fact that the promoter increases the roughness of the catalyst surface. As a result, free valencies increase. Hence, adsorption increases and so does the rate.