CATALYSIS LECTURE
Part-II
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)
CATALYTIC PROMOTERS
• The activity of a catalyst can often be increased by addition of a small
quantity of a second material. This second substance is either not a
catalyst itself for the reaction or it may be a feeble catalyst.
• Promoters are generally defined as substances added during preparation
of catalysts that improve the activity or selectivity or stabilize the catalytic
agents. The promoter is present in a small amount and by itself has little
or no activity.
• Example of Promoters:
– Molybdenum (Mo) or aluminium oxide (Al O ) promotes the activity of
iron catalyst in the Haber synthesis for the manufacture of ammonia.
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Catalytic Promoters
– In the synthesis of methanol (CH3OH) from carbon monoxide and
hydrogen, a mixture of zinc and chromium oxide is used as a catalyst.
• Promoters are termed as physical or chemical promoter depending on the
manner they improve the catalyst performance.
• The additives that maintain physical integrity of the support and/or
deposited catalytic agents are termed as physical promoters. For example,
addition of small quantities of alumina to an iron catalyst employed in
ammonia synthesis prevents sintering of the iron crystallites. Thus, for this
catalyst, alumina is a physical promoter.
• The addition of K2O to the same catalyst (Fe) increases the intrinsic activity
of the iron crystallites and therefore acts as a chemical promoter.
Catalytic promoters: Mechanism of Action
• The theory of promotion of a catalyst is not clearly understood. Probably:
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(1) Change of Lattice Spacing: The lattice spacing of the catalyst is changed thus
enhancing the spaces between the catalyst particles. The absorbed molecules of
the reactant (say H2) are further weakened and cleaved. This makes are reaction
go faster.
(2) Increase of Peaks and Cracks. The presence of the promoter increases the
peaks and cracks on the catalyst surface. This increases the concentration of the
reactant molecules and hence the rate of reaction.
Major promoters used Industrially
NEGATIVE CATALYSIS:NEGATIVE PROMOTERS OR INHIBITORS
• When a catalyst reduces the rate of a reaction, it is called a Negative
catalyst or Inhibitor. This phenomenon is called Negative catalysis or
Inhibition. Negative catalysis is useful to slow down or stop altogether an
unwanted reaction.
MECHANISM OF NEGATIVE CATALYSIS
CATALYTIC POISONING
• A substance which destroys the activity of the catalyst to accelerate a
reaction, is called a poison and the process is called Catalytic poisoning.
Explanation of Catalytic Poisoning
•
The poison is adsorbed on the catalyst surface in preference to the reactants. Even a
monomolecular layer renders the surface unavailable for further adsorption of the
reactants. The poisoning by As2O3 or CO appears to be of this kind.
•
The catalyst may combine chemically with the impurity. The poisoning
of iron catalyst by H2S falls in this class.
INHIBITORS/NEGATIVE PROMOTERS
• Inhibitors act opposite to promoters. When added in small amounts, these
can reduce catalyst activity, selectivity or stability.
• An inhibitor is a substance that reduces the rate of a catalytic reaction,
often as a result of bonding chemically to the catalyst.
• Examples are:
– Pt or Pd catalysts on CaCO3, poisoned by Pb (Lindlar’s catalyst). They enable
selective reduction of triple bonds to double bonds.
– Aprotic solutions (which act by forming H bonds with catalyst molecules in
competition with other reactant molecules).
• Inhibitor is particularly useful for reducing the activity of a catalyst
for undesirable side reactions.
• A strong inhibitor is a catalyst poison, e. g. sulfur for Ni hydrogenation
catalysts.
AUTOCATALYSIS
• When one of the products of reaction itself acts as a catalyst
for that reaction the phenomenon is called Autocatalysis.
• In autocatalysis the initial rate of the reaction rises as the catalytic product
is formed, instead of decreasing steadily (Fig. 21.4). The curve plotted
between reaction rate and time shows a maximum when the reaction is
complete.
Examples of autocatalysis
• (1) Hydrolysis of an Ester: The hydrolysis of ethyl acetate forms acetic
acid (CH3COOH) and ethanol. Of these products, acetic acid acts as a
catalyst for the reaction.
• (2) Oxidation of Oxalic acid: When oxalic acid is oxidised by acidified
potassium permanganate, manganese sulphate produced during the
reaction acts as a catalyst for the reaction.
• (3) Decomposition of Arsine: The free arsenic produced by the
decomposition of arsine (AsH3) autocatalyses the reaction.
Reaction Rate and Catalysis
• Catalyst is a substance that increases the rate of the reaction
at which a chemical system approaches equilibrium , without
being substantially consumed in the process.
• RATE OF REACTION: Function of reactants (or products)
concentrations (or pressure), temperature, presence of
catalyst (or inhibitor)
– α, β, ... are the orders of the reactants
– k is the rate constant
• Example: rate of chemical adsorption
R ads = k P x
Chemical Reaction and Catalysis
• Thermodynamics says NOTHING about the
rate of a reaction.
• Thermodynamics : Will a reaction occur ?
• Kinetics
: If so, how fast ?
Kinetic Vs. Thermodynamic
•A reaction may have a large, negative DGrxn, but the
rate may be so slow that there is no evidence of it
occurring.
Conversion of graphite to diamonds is a
thermodynamic favor process (DG -ve ).
C (graphite)  C (diamond)
Kinetics makes this reaction nearly impossible
(Requires a very high pressure and temperature over long time)
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ACTIVATION ENERGY AND CATALYSIS
•
According to the collision theory, a reaction occurs by the collisions between the
reactant molecules (or ions). At ordinary temperature, the molecules do not
possess enough energy and hence the collisions are not effective. However, when
the temperature of the system is raised, the kinetic energy of the molecules
increases. But the molecules do not react unless they attain a minimum amount of
energy. The minimum amount of energy required to cause a chemical reaction is
known as the Activation Energy. The activated molecules on collision first form
an Activated Complex. As a result of breaking and forming of new bonds, the
activated complex dissociates to yield product molecules.
Kinetic Vs. Thermodynamic
Reaction path for conversion of A + B into AB
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ACTIVATION ENERGY AND CATALYSIS
• A catalyst lowers the activation energy of the reaction by providing a
new pathway (mechanism). the activation energy of the catalytic reaction
is lowered compared to the uncatalyzed reaction as shown in Fig 2.
Activation Energy
Activation Energy : The energy required to overcome the reaction
barrier. Usually given a symbol Ea or ∆G≠
The Activation Energy (Ea) determines how fast a reaction occurs, the higher
Activation barrier, the slower the reaction rate. The lower the Activation
barrier, the faster the reaction
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Activation Energy
Catalyst lowers the activation energy for both forward and
reverse reactions.
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Activation Energy
This means , the catalyst changes the reaction path by
lowering its activation energy and consequently the
catalyst increases the rate of reaction.
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9/15/2020
DR. RABIUL HUSSAIN : 966882081
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