Short Notes #7

Catalysts

These are extremely important substances or materials used in numerous chemical processes. 

Catalysts are substances that increase the reaction rate without it itself being consumed in the process. Thus, catalysts do not appear in the overall stoichiometric reaction it is used it but appears in the elementary reactions in the mechanism for the catalyzed reaction. 

Examples of catalysts:

• Enzymes - These are proteins that act as catalysts in biochemical reactions.
• Homogeneous catalysts
• Heterogeneous (or surface) catalysts

Promoters: These are not catalysts by themselves but increase the effectiveness of a catalyst. For example, alumina Al2O3, is added to finely divided iron to increase the ability of the iron to catalyze the formation of ammonia from a mixture of nitrogen and hydrogen.

Poison: A substance that reduces the efficiency of a catalyst. For example lead compounds poison platinum catalysts. 

Homogenous Catalysis:

This is a process where a catalyst that is in the same phase from the reactants. Typically, everything will be present as a gas or contained in a single liquid phase.

Heterogeneous Catalysis:

This is process where a catalyst that is in a separate phase from the reactants. Typical examples involve a solid catalyst with the reactants as either liquids or gases.

Most examples of heterogeneous catalysis go through the same stages:

• One or more of the reactants are adsorbed (stick) on the surface of the catalysts on special locations called active sites (part of the surface where adsorbed materials can react with one another).
• Interaction between the catalyst surface and the reactant molecule making them for reactive through weakening of bonds in the attached molecules.
• The reaction occurs. Both the reacting molecules may attach to the surface or one might attach and the other moving freely in the liquid or gas medium. 
• The product molecules desorbs from the catalyst surface i.e. breaks free from the catalyst surface to allow other reacting molecules to attach themselves and the reaction cycle starts again. 

A good catalyst needs to adsorb the reactant molecules strongly enough for them to react, but not so strongly that the product molecule sticks permanently to the surface.

Effects of Catalysts

Catalyst lower the activation energy for a reaction. Activation energy is defined as the minimum amount of energy that is needed to a chemical reaction to occur. This is illustrated in the graph below.