The document discusses catalysts and how they work. It defines a catalyst as a substance that speeds up a chemical reaction but remains unchanged after the reaction. It states that catalysts lower the activation energy of reactions, allowing more effective collisions between reactant molecules. The document distinguishes between homogeneous and heterogeneous catalysts, providing examples of each type. It explains that catalysts can work by initiating reactions through faster intermediate steps or by stabilizing reactive intermediates.

1. 2020
Mujeeb UR Rahman
17CH106
10/21/2020
CATALYST
Submitted to: Prof.Dr. Imran Nazir
Associate Professor
Department of Chemical Engineering
Mehran UET Jamshoro,Pakistan

2. Catalyst:
• A catalyst is a substance that affects the rate of a
reaction but emerges from the process unchanged.
• A catalyst usually changes a reaction rate by promoting a
different molecular path ("mechanism") for the reaction.
• Catalyst affect yield and selectivity
• Changes only the rate of reaction; it does not affect the
equilibrium.
• Selectively enhances the rate of a reaction
• Changes the reaction pathway
• Neither consumed nor formed
• Does not affect equilibrium

3. Types of Catalysts
Homogeneous Catalyst
• Catalysts can operate by increasing the number of
effective collisions.
• That is, from the Arrhenius equation: catalysts increase k
be increasing A or decreasing E
a
.
• A catalyst may add intermediates to the reaction.
• Example: In the presence of Br
-
, Br
2
(aq) is generated as
an intermediate in the decomposition of H
2
O
2
.
• When a catalyst adds an intermediate, the activation
energies for both steps must be lower than the activation
energy for the uncatalyzed reaction. The catalyst is in a
different phase than the reactants and products.
• Acids or bases
• Metal salt
• Enzymes
• Radical initiators
• Solvents
Heterogeneous Catalyst
• Typical example: solid catalyst, gaseous reactants and
products (catalytic converters in cars).
Most industrial catalysts are heterogeneous
• First step is adsorption (the binding of reactant molecules
to the catalyst surface).
• Adsorbed species (atoms or ions) are very reactive.
• Molecules are adsorbed onto active sites on the catalyst
surface.

4. How Catalysts Work:
Catalyst can initiate reactions. The mechanisms are similar to
the mechanism without a catalyst, but the initiation process is
much faster with the catalyst.

5. Intermediate Stabilization
104
k
52
252
k
62
42
k
52
524
k
623
3
k
62
HCHC2
:ationminTer
HHCHCH
HHCHC
HCCHHCCH
:opagationPr
CH2HC
:Initiation
CATALISTNO
5
4
3
2
1





104
k
52
252
k
62
42
k
52
252
k
622
HCHC2
:ationminTer
HHCHCH
HHCHC
:opagationPr
HNOHCHCNO
:Initiation
CATALISTWITH
5
4
3
1




Reactant + Catalyst  Stable Complex  Products + Catalyst