The presentation is based on the Undergraduate Physical Chemistry Syllabus of Mumbai University

1. Catalysis
Dr. Pravin U. Singare
Department of Chemistry,
N.M. Institute of Science, Bhavan’s College,
Andheri (West), Mumbai 400 058

2. Introduction
• A substance which accelerates the rate of Chemical Reactions by remaining chemically unchanged
at the end of reaction is called Catalysts.
• The important characteristics features of catalysts are as follows:
 In case of reversible reactions, the catalysts equally increase the rate of both forward and
reverse reaction. As a result, the equilibrium constant (K) which is the ratio of
reaction rate k (forward reaction) / reaction rate k (reverse reaction) will remain constant.
Hence the magnitude of equilibrium constant remain unchanged.
 Presence of catalysts increases (accelerates) the rate of chemical reaction by lowering the
energy of activation. The rate of reaction (k) is related to energy of activation (Ea) by Arrhenius
equation
k = A.𝑒− Ea RT
log (k) = log (A) -
Ea
2.303RT
log (k) = log (A) +
2.303RT
Ea
Based on the above equation, reaction rate (k) is inversely related to the energy of activation
(Ea). Hence, catalysed reactions which proceeds with lowering in Ea will have high reaction rate.

3. Important characteristics features of catalysts (continued ----)
 The presence of catalysts only increases the rate of chemical reaction but
does not start the chemical reaction.
 At the end of chemical reaction, the catalysts may undergo physical change
i.e. the catalysts in solid form may get converted to powder form. At the end
of reaction, sometimes the surface properties and particle size of catalyst
participating in the reaction may change. However, the mass and chemical
composition of catalysts will remain unchanged at the end of chemical
reaction.
 Only the small quantity (trace quantity) of catalysts in finely divided form will
be sufficient to increase (accelerate) the rate of the chemical reaction. For
example in order to bring about decomposition of 108 dm3 of hydrogen
peroxide (H2O2) only 1 gm of colloidal platinum (Pt) catalyst is sufficient.

4. Important characteristics features of catalysts (continued ----)
 Sometimes the catalysts used are specific in their action. A catalyst used for
performing one particular type of reaction can not be used to catalysed other
reaction. For example decomposition of KClO3 to give KCl and O2 is brought
about in presence of MnO2 as a catalyst. But the same MnO2 as a catalyst can
not be used for reaction of SO2 and O2 during the manufacturing of H2SO4.
 The presence of catalyst does not have any effect on the nature of products
formed during the chemical reaction. For example decomposition of KClO3 will
give the same product KCl and O2 irrespective of the fact that if reaction is
performed in presence of MnO2 as a catalyst or if the reaction is performed
without the catalyst.

5. Promoters , inhibitors & Catalytic Poisons
 The substance which increases the catalytic activity is called Promoters.
 The Promoters themselves has no catalytic properties.
 For example during the manufacturing of ammonia by Hyber’s process using iron as a
catalyst, molybdenum (Mo) is used as a promoter which increases the catalytic activity of
iron.
 Sometimes during the chemical reactions, certain substances instead of accelerating
(increasing) the rate of chemical reaction will retard (slow down) the rate of chemical
reaction.
 Such substances are called negative catalysts or inhibitors.
 For example, decomposition of hydrogen peroxide (H2O2) using colloidal platinum (Pt) as a
catalyst is retarded due to the presence of small amount of glycerine which act as an
inhibitor.

6. Promoters , inhibitors & Catalytic Poisons
 Presence of small amount of impurities may poison the catalyst by deactivating the catalyst.
 Such impurities are called catalytic poison.
 Presence of mercury (Hg), hydrogen sulphide (H2S), carbon disulphide (CS2), carbon
monoxide (CO), free halogens, compounds of phosphorous, lead and arsenic compounds
act as a catalytic poisons for platinum (Pt) and Nickel (Ni) catalysts.
 The catalytic poison will suppress the activity of catalyst in reversible or irreversible
manner.
 In case of reversible poisoning, the surface activity of catalyst is suppressed in temporary
manner and the catalytic activity can be restored again. Such type of poisoning is called
temporary poisoning.
 For example, CO or CS2 will cause temporary poisoning of Pt catalyst by blocking the active
surface of catalyst.
 However, the Pt catalyst surface can be made active again by introducing the Pt catalyst in a
pure reactant gases (free of CO or CS2).

7. Promoters , inhibitors & Catalytic Poisons
 In case of irreversible poisoning, the surface activity of catalyst is suppressed permanently
and the catalytic activity can not be restored again. Such type of poisoning is called
permanent poisoning.
 For example, Pt catalyst is totally deactivated in an irreversible or permanent manner in
presence of H2S gas.

8. Different types of catalysed reactions
Catalysed reactions are classified into two types
 Heterogeneous type of catalysed reaction
 Homogeneous type of catalysed reaction
• Heterogeneous type of catalysed reaction
 The reactions in which the reactants and catalysts are in different physical form are
called Heterogeneous type of catalysed reaction
 Generally the catalyst used is in solid form on which the reactant molecules get
adsorbed.
 The reaction take place on the surface of the catalyst leading to the formation of
products.
 The product molecules thus formed gets desorbed from the surface of catalyst thereby
making the catalyst surface available for the fresh reactant molecules get adsorbed.
 Increase in surface area of catalyst will result in an increase in catalytic activity.
 The surface area of catalyst can be increased by using the catalyst in finely divided form.
 For the same reason, in industrial manufacturing process, the powdered catalysts are
used which increase the rate of chemical reaction.

9. Heterogeneous type of catalysed reaction (continued----)
 Some examples of heterogeneous type of catalysed reactions are
1. Platinum (Pt) catalyst used to convert SO2 to SO3 during the manufacturing of H2SO4
2SO2 (g) + O2 (g) 2SO3 (g)
SO3 (g) + H2O (l) H2SO4 (aq)
2. During the manufacture of ammonia by Haber’s process, Iron (Fe) is used as a catalyst
to which traces of Molybdenum (Mo) is added as a promoter to improve the catalytic
activity of iron.
N2(g) + 3H2(g) 2NH3(g)
Pt (s)
Fe(s)
Mo

10. Homogeneous type of catalysed reaction
• Homogeneous type of Catalysed reactions
 The reactions in which the catalyst and reactants both are in the same physical form are
known as homogeneous type of catalysed reactions.
 Such reactions proceed in a single phase generally in gaseous phase or in liquid phase.
 Examples of homogeneous type of catalysed reactions are as follows:
1. Acid catalysed hydrolysis of methyl acetate (CH3COOCH3)is a homogeneous type of
catalysed reaction which proceed in liquid state
CH3COOCH3 + H2O CH3COOH + CH3OH
In the above reaction, the reactants namely methyl acetate (CH3COOCH3) and H2O as
well as the catalyst HCl all are in the same liquid phase. Hence HCl is acting as a
Homogeneous catalyst.
HCl

11. Homogeneous type of catalysed reaction (Continued----)
2. Enolisation of acetone is catalysed by hydroxyl OH- ions. The mechanism is as follows:
(i) + OH- + H2O
(ii)
(iii) + H-OH + OH-
Thus OH- used as a catalyst in the step (i) is regenerated in last step (iii). Thus the above reaction is
said to be base catalysed.
The net reaction (obtained by adding the above three steps) is
OH- Catalyst
O
CH3-C-CH2
-
O
CH3-C-CH3
acetone (Ketone)
O
CH3-C-CH2
- CH3-C=CH2
O
-
CH3-C=CH2
O
-
CH3-C=CH2
OH
(enol form)
O
CH3-C-CH3
acetone (Keto form)
CH3-C=CH2
OH
(enol form)

12. Enzyme catalysed reactions
• The biochemical reactions which are catalysed by the presence of enzymes are known as
enzyme catalysed reactions.
• Enzymes are complex proteins occurring in living cells and can catalysed variety of different
chemical reactions.
• This reactions are also classified as a homogeneous type of catalysed reactions.
• The enzymes are highly specific catalysts which are used to catalysed the particular
biochemical reaction.
• For example, the hydrolysis of urea is brought about using enzyme Urease
H2N-CO-NH2 + H2O CO2 + 2NH3
The catalytic activity of urease is effective at room temperature, while at a temperature of
50oC and above the catalytic activity get desensitized.
The catalytic activity is also dependent on pH of the reaction medium.

13. Enzyme catalysed reactions (continued------)
• Similarly, the conversion of starch into maltose sugar is brought about by the enzyme
diastase.
2(C6H10O5)n + nH2O nC12H22O11
• Conversion of maltose sugar into glucose is carried in presence of enzyme maltase.
C12H22O11 + H2O 2C6H12O6
• Conversion of glucose into ethyl alcohol take place with the help of catalyst zymase.
C6H12O6 2C2H5OH + 2CO2
• All the above enzyme catalysed reactions are highly temperature sensitive having maximum
catalytic activity at the temperature ranging from 15oC to 25oC.
Starch Maltose sugar
Diastase
Maltose sugar Glucose
maltase
Glucose Ethanol
zymase

14. Acid & Base Catalysed Reactions
• A reaction which is catalysed by an acid or any substance which has a tendency to loose a proton
(H+) is called acid catalysed reaction.
• Acid catalysed reactions proceeds by transfer of proton (H+) from an acid to a substrate molecule.
• For example hydrolysis reaction of ester (ethyl acetate) is accelerated in presence of hydrogen ions.
CH3COOC2H5 (ester ethyl acetate) + H2O CH3COOH + C2H5OH
• When the rate of acid catalysed reaction is directly proportional to the concentration of acid catalyst
and the concentration of substrate, the condition is called general acid catalysis.
• When the rate of acid catalysed reaction is directly proportional to only the concentration of
hydrogen (H+) ions, the condition is called specific hydrogen ion catalysis.
• A reaction which is catalysed by a base or any substance having tendency to gain proton is called
base catalysed reaction.
• Base catalysed reactions proceeds by transfer of proton (H+) from a substrate molecule to the base.
• The reaction rate of base to catalyse a reaction is directly proportional to OH- ions concentration.
acid

15. Nanoparticles as Catalysts
• By tuning the morphology of catalyst at the nanoscale one can quantitatively increase the active
sites at the surface of catalyst.
• By decreasing the size of catalyst particles, the surface density of active sites at the surface of
catalyst increases proportionately.
• Greater the density of active sites at the catalytic surface, greater will be the catalytic activity.
• By reducing the size of catalytic nano particles it is possible to maximise the catalytic performance of
active sites.
• Nanoparticles are a useful material for catalysis due to their high relative surface area – if a
nanoparticle system has the same volume as a bulk material, its relative active surface area is
greater than the bulk material.
• Therefore, nanoparticles are added in as heterogeneous catalysts to act as a binding/adsorption
site, or as catalytic support, their high active surface area utilized for various types of reactions.
• Heterogeneous catalysis represents one of the oldest commercial practices of
nanoscience; nanoparticles of metals, semiconductors, oxides, and other compounds have been
widely used for important chemical reactions.
• Metal nanoparticles have high surface area, which can increase catalytic activity.
• Nanoparticle catalysts can be easily separated and recycled.
• They are typically used under mild conditions to prevent decomposition of the nanoparticles