Catalysts
Learning Objectives
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Discuss types of catalyst
Differentiate between homogeneous and heterogeneous catalyst
Describe properties of catalyst
Study theories of catalysis
Catalyst
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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
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
bond
Nickel
Types of catalyst
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.
Fe2+
S2O82- + 2I2 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
Ni
CH3-CH3
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
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MnO2
(i) 2KClO3
2KCl + 3O2
Pt
(ii) H2O2
H2O + [O]
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Negative Catalysis
The catalyst which decreases the rate of reaction is called
negative catalyst and phenomenon is called negative catalysis
Examples are
Acetanilide
(i) H2O2
H2 O + O
(ii) Knocking of petrol by tetraethyl lead
Properties of catalyst
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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,
Fe ,Mo/ Al2O3
N2 + 3H2
2NH3
[Fe] is the catalyst and presence of Mo or Al2O3 increase the activity
of iron and, therefore, acts as promoter.
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
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Pt ,CO
2H2 + O2
2H2O
Activity of catalyst Pt is poisoned by presence of CO
Fe ,H2S
N2 + 3H2
2NH3
Presence of H2S reduces the activity of Fe
Theories of catalysis
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:
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[NO]
2 SO2 + O2
Proceeds as: 2NO + O2
NO2 + SO2
2 SO3
2NO2
SO3 + NO
II) Adsorption Theory
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The heterogeneous catalysis e.g. gaseous reaction on a
solid surface, is explained by this theory as follows:
Catalyst
A(g) + B(g)
C(g) +D(g)
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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.