Catalyst is a substance that changes the rate of the reaction
without being consumed in the process. Catalyst affects only the
rate of the reaction,i.e. Kinetics. It changes neither the
thermodynamics of the reaction nor the equilibrium
composition.
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HOW?
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Do you remember this man?
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A catalyst provides an alternative route for the reaction with
a lower activation energy. It does not lower the activation energy
of the reaction.
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Since a catalyst makes it possible to obtain an end product by
a different mechanism, it can affect both the yield and
selectivity. A catalyst can either accelerate or slow the formation
of a particular product species.
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CATEGORIES OF CATALYSIS
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Concerns processes in which a catalyst is in a solution with at
least one of the reactants. Homogeneous Catalysis Involves more
than one phase; usually the catalyst is a solid and the reactants
and products are in liquid or gaseous form. Heterogeneous
Catalysis
Most examples of heterogeneous catalysis go through the same
stages: One or more of the reactants are adsorbed on to the surface
of the catalyst at active sites. Adsorption is where something
sticks to a surface. It isn't the same as absorption where one
substance is taken up within the structure of another. Be careful!
An active site is a part of the surface which is particularly good
at adsorbing things and helping them to react. There is some sort
of interaction between the surface of the catalyst and the reactant
molecules which makes them more reactive. This might involve an
actual reaction with the surface, or some weakening of the bonds in
the attached molecules.
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The reaction happens. At this stage, both of the reactant
molecules might be attached to the surface, or one might be
attached and hit by the other one moving freely in the gas or
liquid. The product molecules are desorbed. Desorption simply means
that the product molecules break away. This leaves the active site
available for a new set of molecules to attach to and react.
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A good catalyst needs to adsorb the reactant molecules strongly
enough for them to react, but not so strongly that the product
molecules stick more or less permanently to the surface. Silver,
for example, isn't a good catalyst because it doesn't form strong
enough attachments with reactant molecules. Tungsten, on the other
hand, isn't a good catalyst because it adsorbs too strongly. Metals
like platinum and nickel make good catalysts because they adsorb
strongly enough to hold and activate the reactants, but not so
strongly that the products can't break away.
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EXAMPLE
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The hydrogenation of a carbon-carbon double bond
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Hydrogen molecules are also adsorbed on to the surface of the
nickel. When this happens, the hydrogen molecules are broken into
atoms. These can move around on the surface of the nickel.
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If a hydrogen atom diffuses close to one of the bonded carbons,
the bond between the carbon and the nickel is replaced by one
between the carbon and hydrogen.
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That end of the original ethene now breaks free of the surface,
and eventually the same thing will happen at the other end.
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As before, one of the hydrogen atoms forms a bond with the
carbon, and that end also breaks free. There is now space on the
surface of the nickel for new reactant molecules to go through the
whole process again.
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Presence of Inhibitor
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The animation to the right shows CO reacting with O 2 with the
help of a metal catalyst. Oxygen (white) dissociates on the
catalyst surface. Carbon (red), which is already attracted to the
catalytic surface, picks up an unbonded oxygen atom to form CO
2.
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REACTIONS
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Adsorption Concentration of vacant active sites A Occupied Site
Vacant Site
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Surface Reaction Langmuir- Hinshelwood Kinetics
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Desorption
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EXERCISE
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SHEET 8
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1.Derive a Hoogen & Watson type model for the overall
reaction 2A --------- P Assuming that the surface reaction is the
rate determining step, if: A + S = = = = AS (1) 2AS -------- PS + S
(2) PS = = = = P + S (3)
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3.The catalytic hydrogenation of butyraldehyde (B) to butanol
(C): H 2 + C 3 H 7 CHO = = = = C 3 H 7 CH 2 OH Has a reported rate
equation: R = [k o e E/RT (p A p B p c /K eq )] / [1+K 1 p A +K 2 p
B +K 3 p C ] 2 Develop a surface reaction model for the observed
kinetics