1. Catalyst

2. Learning Objectives Discuss types of catalyst
Differentiate between homogeneous and heterogeneous catalyst
Describe properties of catalyst
Study theories of catalysis

3. Catalyst
A catalyst is a substance which speeds up a reaction, but is chemically unchanged at the end of the reaction.
When the reaction has finished, you would have exactly the same mass of catalyst as you had at the beginning.
Catalysis is the change in rate of a chemical reaction
 due to the participation of a catalyst
Some examples Fe

4. Decomposition of hydrogen peroxide Manganese(IV) oxide, MnO2
Reaction
Catalyst
Decomposition of hydrogen peroxide
Manganese(IV) oxide, MnO2
Nitration of benzene
Concentrated sulphuric acid
Manufacture of ammonia by the Haber Process
Iron
Conversion of SO2 into SO3 during the Contact Process to make sulphuric acid
Vanadium(V) oxide, V2O5
Hydrogenation of a C=C double bSSSond
Nickel

5. Types of catalyst Fe2+ Ni
Catalysts can be divided into two main types – heterogeneous and homogeneous i) Homogeneous catalyst - the catalyst is in the same phase as the reactants. Example- The reaction between persulphate ions and iodide ions. S2O I- 2 SO42- + I2 ii) Heterogeneous catalyst - the catalyst is in a different phase from the reactants. Example- The hydrogenation of a carbon-carbon double bond. CH2=CH2 + H2 CH3-CH3
Fe2+ Ni

6. Properties of catalyst
Positive Catalysis
The catalyst which increases the rate of a chemical reaction is called positive catalyst and the phenomenon is known as
positive catalysis
Examples are
(i) 2KClO KCl + 3O2
(ii) H2O H2O + [O]
Negative Catalysis
The catalyst which decreases the rate of reaction is called
negative catalyst and phenomenon is called negative catalysis
(i) H2O H2O + O
(ii) Knocking of petrol by tetraethyl lead []
MnO2 Pt
Acetanilide

7. Properties of catalyst
Promotors
Sometimes the activity of a catalyst may be increased by addition of small amount of a second substance. The second substance, which though itself is not a catalyst, promotes the activity of a catalyst, is called the promoter 
For example,
N2 + 3H NH3
[Fe] is the catalyst and presence of Mo or Al2O3 increase the activity
of iron and, therefore, acts as promoter.
Fe ,Mo/ Al2O3

8. Properties of catalyst
Poison
Sometimes the rate of a catalysed reaction is reduced by the presence of a small amount of same substance (may be as impurities in the reactants).
Such a substance, which destroys the activity of a catalyst, is called poison and the process is called catalytic poisoning
For example, in reaction
2H2 + O H2O
Activity of catalyst Pt is poisoned by presence of CO
N2 + 3H NH3
Presence of H2S reduces the activity of Fe
Pt ,CO
Fe ,H2S

9. Theories of catalysis I) Intermediate Compound Formation Theory
The two main theories of catalysis are: (i) intermediate compound
formation theory and (ii) adsorption theory
I) Intermediate Compound Formation Theory
According to this theory, the catalyst reacts with one of the reactants to give an intermediate, which reacts with another reactant to yield products and the catalyst as follows:
A + [ Catalyst] [Intermediate]
[Intermediate] + B Product + [Catalyst]
Examples:
2 SO2 + O SO3
Proceeds as: 2NO + O NO2
NO2 + SO SO3 + NO
[NO]

10. II) Adsorption Theory
The heterogeneous catalysis e.g. gaseous reaction on a solid surface, is explained by this theory as follows:
A(g) + B(g) C(g) +D(g)
Following four steps are involved in the heterogeneous catalysis:
(i)Diffusion of reactants at the surface of the catalyst.
(ii)Adsorption of reactants at the surface.
(iii)Reaction of reactants at the surface.
(iv)Desorption of products from the surface.
Catalyst