Theories and Mechanism of Catalysis

1. CATALYSIS: Theories of Catalysis
Dr.Rabiul Hussain
School of Material Science & Engineering
Jimma Institute of Technolgy, Jimma University
E-mail: rabiul786@gmail.com
Ph.No. +251-0966882081 (Ethiopia)
+91-9508832510 (India)

2. Theories of Catalysis
• There are two main theories of catalysis :
– Intermediate Compound Formation theory
– The Adsorption theory.
• Intermediate compound Formation Theory:
• i) This theory was proposed by Clement and Desormes in
1806.
• According to this theory, the desired reaction is brought about
by a path involving the formation of an unstable intermediate
compound, followed by its decomposition into the desired
end products with the regeneration of the catalyst.
• (ii) The intermediate compound may be formed in either of
two ways:
– (a) When the intermediate compound is reactive and reacts with the
other reactants.

3. Theory of Catalysis
– b) When the intermediate is unstable and decomposes to give the final
product.
• Where, A, B and C are the reactant molecules and X is the molecule of the
catalyst.

4. CATALYSIS BY INTERMEDIATE COMPOUND
FORMATION

5. CATALYSIS BY INTERMEDIATE COMPOUND
FORMATION

6. The Adsorption Theory
• This theory explains the mechanism of a reaction between two gases
catalyzed by a solid(Heterogeneous or Contact Catalysis). Here the catalyst
functions by adsorption of the reacting molecules on its surface.
• Generally speaking, four steps can be put forward for heterogeneous catalysis. For
example, if the reaction is :
• Step1:Adsorption of Reactant molecules
The reactant molecules A and B strike the catalyst surface. They are held
up at the surface by weak van der Waals forces (Physical adsorption) or by partial
chemical bonds (Chemisorption).
• STEP 2: Formation of Activated complex
The particles of the reactants adjacent to one another join to form an
intermediate complex(A – B). The activated complex is unstable.

7. The Adsorption Theory
• Step 3. Decomposition of Activated complex: The activated complex
breaks to form the products C and D. The separated particles of the
products hold to the catalyst surface by partial chemical bonds.
• Step 4. Desorption of Products: The particles of the products are
desorbed or released from the surface. They are stable and can lead an
independent existence.

8. Representative example of heterogeneous catalysis
• HYDROGENATION OF ETHENE (ETHYLENE) IN PRESENCE OF NICKEL:
Ethene adds hydrogen in the presence of nickel as a catalyst to yield ethane.
The catalyst Ni operates by the following steps:
• Step 1. Adsorption of Hydrogen molecules: Hydrogen molecules are
adsorbed on the nickel surface due to the residual valence bonds of the
nickel atoms.
• Step. 2 H–H Bonds are broken: The H–H bond is smaller (0.74Å) than Ni–
Ni bond. Therefore, the H–H bond of the adsorbed hydrogen molecule is
stretched and weakened. The weakened bond breaks, separating the
hydrogen atoms. The separated hydrogen atoms are held to the nickel
surface by chemical bonds.

9. • Step 3. Formation of the Activated complex The chemisorbed hydrogen
atoms then attach to ethene molecule by partial chemical bonds.The
unstable activated complex is thus formed.
• Step 4. Decomposition of the Activated complex and desorption of
ethane molecules: The unstable activated complex decomposes to release
ethane molecules. The freed catalyst surface is again available for further
action.
Representative example of heterogeneous catalysis

11. Acid-base Catalysis
• A number of homogeneous catalytic reactions are known which are
catalyzed by acids or bases, or both acids and bases. These are often
referred to as Acid-Base catalysts.
• Arrhenius pointed out that acid catalysis was, in fact, brought about by H+
ions supplied by strong acids, while base catalysis was caused by OH– ions
supplied by strong bases.
• Examples of Acid-Base catalysis:

12. Mechanism of Acid-Base catalysis
• (a) In acid catalysis, the H+ (or a proton donated by Bronsted acid) forms
an intermediate complex with the reactant, which then reacts to give back
the proton. For example, the mechanism of keto-enol tautomerism of
acetone is :

13. Mechanism of Acid-Base Catalysis
• (b) In base catalysis, the OH– ion (or any Bronsted base) accepts a proton
from the reactant to form an intermediate complex which then reacts or
decomposes to regenerate the OH– (or Bronsted base). For example, the
decomposition of nitramide by OH– ions and CH3COO– ions may be
explained as follows :

14. ENZYME CATALYSIS
• Numerous organic reactions are taking place in the body of animals and
plants to maintain the life process. These reactions being slow remarkably
catalysed by the organic compounds known as Enzymes.
• Enzymes are protein molecules which act as catalysts to speed up organic
reactions in living cells. The catalysis brought about by enzymes is known
as Enzyme Catalysis.
• Importance: Enzyme catalytic processes are extensively used in food
industries for centuries such as in production of bread, beverages,
yoghurt, cheese, vinegar etc. Enzymes have properties similar to
homogeneous catalysts. New enzymes are continuously being discovered.
At present, more than 3000 enzymes are reported.

15. What is an Enzyme?
• Enzymes are large macromolecular polypeptide (polymers of amino acid
monomers) proteins. Molecular weight is in the range of 104-106 .
• Each enzyme has a unique three-dimensional structure with a binding site
or active site that is chemically and geometrically compatible with a
particular reactant molecule and thereby can give upto 100 % selectivity.
• Enzymes are formed in living systems by condensation and/or dehydration
of amino acids that have the composition of H2N-CHR-COOH, to form
peptide C-N bonds. Large structure contains hundreds of amino acids and
there is enormous number of possible structures. Only few are
characterized and well known.
• Enzymes are synthesized by living organisms and can be extracted from
their biological source, purified and used in laboratory and industrial
processes. Enzymes can also be synthesized in vitro that is in an artificial
environment outside the living organism. Enzymes are only active within a
limited range of pH and temperature.

16. EXAMPLE OF AN ENZYME
Lysozyme enzyme:
– Catalyze splitting of polysaccharide chains
– 129 amino acids residues joined by peptide linkage
– Glutamic acid and aspartic acid are important functional groups at
the active sites

17. SOME IMPORTANT BIOLOGICAL REACTIONS INVOLVING ENZYMES
• Enzyme catalyze a variety of biological reactions such as :
– Breakdown of proteins and sugars
– Photosynthesis
– Oxidation–reduction that converts food to CO2, water and energy
– Production of hormones

19. 9/15/2020 19
DR. RABIUL HUSSAIN : 966882081