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10.5 Catalytic reaction
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2.1 catalysts and catalysis
Examples for catalytic reaction 1)
decomposing of KClO3 to produce oxygen with MnO2
as catalyst 2) oxidation of NH3 to NO with
Pt-Rh as catalyst 3) combination of H2 and O2
in sealed lead battery 4) synthesis of ammonia
from N2 and H2 over iron catalyst.
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Catalytic oxidation of ammonia to nitrogen
monooxide over Pt/Rh alloy
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catalyst
substance that changes the rate of a chemical
reaction without themselves undergoing any
chemical change.
substance that appears in the rate equation to
a power that is higher than that to which it
appears in the stoichiometric equation.
C12H22O11 H2O ?? C6H12O6 C6H12O6
catalysis
The phenomenon of acceleration or retardation
of the speed of a chemical reaction by addition
of small amount of foreign substances to the
reactants.
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2.2 types of catalysis
Homogeneous catalysis Heterogeneous catalysis
Biological catalysis / enzyme catalysis
Homogeneous catalysis
the catalyst is present in the same phase as the
reactant.
Examples
1) Oxidation of SO2 to SO3 with the aid of NO.
2) Hydrolysis of sucrose with inorganic acid (in
physical chemistry Lab).
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Heterogeneous catalysis
the catalyst constitutes a separate phase from
the reaction.
Examples
Habers process for ammonia synthesis contact
oxidation of sulphur dioxide Hydrogenation of
alkene, aldehyde, etc.
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 2.3 General characteristics of catalyzed
reactions
1) Catalyst takes part in the reaction, alters
the reaction path and cause significant change in
apparent activation energy and reaction rate.
(CH3)3COH ? (CH3)2CCH2 H2O
with HBr as catalyst
2) t-Bu-Br ? (CH3)2CCH2 HBr
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Mechanism
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without catalyst k
4.8 ? 1014 exp(-32700/T) s-1 with HBr as
catalyst kc 9.2 ?
1012 exp(-15200/T) dm3?mol-1?s-1
By altering reaction path, catalyst lower
activation energy of the overall reaction
significantly and change the reaction rate
dramatically.
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2) No impact on the thermodynamic features of the
reaction
(1) Catalyst cannot start or initiate a
thermodynamically non-spontaneous reaction
(2) Catalyst can change the rate constant of
forward reaction and backward reaction with the
same amplitude and does not alter the final
equilibrium position.
As a state function, ?rGm? only related to
the initial and the final state. Therefore,
catalyst has no impact on equilibrium constant
of the reaction.
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Catalyst can shorten the time for reaching
equilibrium.
(3) Catalyst is effective both for forward
reaction and backward reaction.
Study on the catalyst for ammonia synthesis can
be done with easy by making use of the
decomposition of ammonia.
decomposition of methanol over ZnO catalyst?
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3) Selectivity of catalysts
1) The action of catalyst is specific. Different
reaction calls for different catalyst.
Hydrogenation? Isomerization?
2) The same reactants can produce different
products over different catalysts.
CCl4 HF ? CCl3F CCl2F2 CClF3
b. p. 23.7 -29.8 -81.1
SbCl5 9 90 0.5
FeCl3 20 75
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4) Other characteristics
1) The chemical composition of catalyst remains
unchanged at the end of the reaction 2) Only
a small amount of catalyst is required 3)
Catalyst has optimum temperature 4) Catalyst
can be poisoned by the presence of small amount
of poisons anti-poisoning. 5) The activity of
a catalyst can be enhanced by promoter 6)
catalyst usually loaded on support with high
specific area , such as activated carbon, silica.

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2.4 kinetics of homogeneous catalysis
For homogeneous reaction, the reactant is usually
named as substrate.
When C is some acid, rate constant is
proportional to dissociation constant (Ka) as
pointed out by Brønsted et al. in the 1920s
Where Ga and ? is experimental constants.
? ranges between 0 1.
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In aqueous solution, the acid may be H or
H3O but in general it may be any species HA
capable of being a proton donor (Brønsted acid)
or a electron acceptor (Lewis acid).
Dehydration of acetaldehyde catalyzed by
different acids.
For base-catalyzed reaction there also exists
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CH2 CH2 Br2 ? CH2Br?CH2Br
This reaction proceeds readily in a glass
vessel at 470 K. it was found that this reaction
proceeds much more rapidly in smaller reaction
vessels. When the vessel is packed with glass
beads, the rate is enhance.When the inside of the
vessel is coated with paraffin, the rate is
reduced.
If formic acid is passed through a heated
glass tube, the reaction is about one-half
dehydration (1) and one-half dehydrogenation (2).
However, if the tube is packed with Al2O3, only
reaction (1) occurs but if packed with ZnO, only
reaction (2) occurs. The properties of the
solid surface has great effect on reaction.
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10.5.1 basic principal of heterogeneous catalysis
Interaction between molecule and catalyst on
catalytic activity
The potential curve of adsorption
When the interaction between molecules
and catalyst is weak, the activation is
insufficient. When the interaction between
molecules and catalyst is very strong, it is
difficult for the succeeding reaction to occur.
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10.5.2 Mechanism of heterogeneous catalysis
A surface reaction can usually be divided
into five elementary steps
1) diffusion of reactants to surface 2)
adsorption of reactants at surface 3) reaction
on the surface 4) desorption of products from
surface 5) diffusion of products away from the
surface.
Which is r.d.s.?
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Many surface reactions can be treated
successfully on the basis of the following
assumptions 1) the r.d.s. is a reaction of
adsorbed molecules 2) the reaction rate per
unit surface area is proportional to ?, the
fraction of surface covered by reactant.
For unimolecular reaction over catalyst
Catalyzed isomerization and decomposition
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For bimolecular reaction over catalyst
Langmuir-Hinshelwood mechanism (L-H mechanism)
Langmuir-Rideal mechanism (L-R mechanism)
Synthesis of ammonia from dinitrogen and
dihydrogen
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Hydrogenation of ethylene
Synthesis of ammonia
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10.5.3 kinetics for heterogeneous catalysis
For unimolecular reaction
According to Langmuir isotherm
First-order reaction
Under low pressure, when bAPA ltlt 1
Increase in pressure will accelerate reaction
rate.
zeroth-order reaction
At high pressure, when bAPA gtgt 1
Equilibrium adsorption has been reached. Change
in pressure has no effect on the reaction rate.
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when competing adsorption exists
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When bApA ltlt 1 bBpB
The adsorption of competing species inhibits
the reaction.
For example
Decomposition of N2O over Ag, CuO, or CdO.
When bBpB gtgt 1
For example
Decomposition of ammonia over Pt
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The situation of the L-R mechanism is the
same as that of unimolecular reaction over
catalyst.
For L-H mechanism, small modification should be
made.
Rate partial pressure relation of L-H mechanism
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10.5.4 Active sites
Ununiformity of solid surface and catalysis
10-9 PH3, which is insufficient for formation
of monolayer, can destroy completely the activity
of Pt catalyst toward oxidation of ammonia.
1926, Talyor proposed the active site model for
explanation
1) Only the molecules adsorbed on the active
sites can lead to reaction.
2) The fraction of active sites on the catalyst
surface is very low.
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Fe(110)
Active sites in iron catalyst for ammonia
synthesis
Fe(210)
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Where are the active sites?
The active site is in fact atom cluster
comprising of several metal atoms.
Atom cluster
Increase of the degree of subdivision will
increase the ununiformity of catalyst surface and
increase the number of active sites.
Adsorption of species on the edges of a calcites
crystal
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10.5.5 Poison of catalyst
If bB is very large, even at low pB, ?A will
be very small. The reaction of A will be greatly
retarded. The impurities with high b is catalyst
poison.
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2.5 Enzyme catalysis
Enzymes are biologically developed catalysts,
each usually having some one specific function in
a living organism.
Enzymes are proteins, ranging in molecular
weight from about 6000 to several million. Some
150 kinds have been isolated in crystalline form.
The diameter of enzyme usually ranges between
10 100 nm. Therefore, the enzyme catalysis
borders the homogeneous catalysis and the
heterogeneous catalysis.
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Kinds of enzymes 1) hydrolytic enzymes
2) oxidation-reduction
enzymes
Important hydrolytic enzymes
pepsin Hydrolysis of proteins
diastase Hydrolysis of starch
urease hydrolysis of urea
invertase hydrolysis of sucrose
zymase hydrolysis of glucose
maltase Hydrolysis of maltose
oxidation-reduction enzymes
SOD(Superoxide Dismutase) Decomposition of superoxide (O2-)
Nitrogenase Dinitrogen fixation
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2.5.1) Kinetics of enzyme catalysis
A rather widely applicable kinetic framework
for enzymatic action is that known as the
Michaelis-Menten Mechanism (1913).
?
Enzyme-substrate complex
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Using stationary-state approximation
Michaelis constant
Discussion
1) When S gtgt kM
is zeroth order with respect of S.
2) When S ltlt kM
is first order with respect of S.
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When S kM
At r ½ rm, S kM
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Slope S kM/rm
Lineweaver-Burk plot
intercept I 1/rm
Both rm and kM can be obtained by solving the
equations.
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Many enzyme systems are more complicated
kinetically than the foregoing treatment
suggests.
There may be more than one kind of
enzyme-substrate binding site sites within the
same enzyme may interact cooperatively. Often, a
cofactor is involved.
Luciferase is a generic name for enzymes commonly
used in nature for bioluminescence.
http//en.wikipedia.org/wiki/ImageLuciferase-1BA3
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Outstanding characteristics of enzyme catalysis
1) High selectivity
Lock and key
Even 10-7 mol dm-3 urease can catalyze the
hydrolysis of urea (NH2CONH2) effectively.
However, it has no effect on CH3CONH2.
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Chirality of enzyme catalysis
John Warcup Cornforth
1975 Noble Prize Great Britain 1917/09/07 for his
work on the stereochemistry of enzyme-catalyzed
reactions
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2) High efficiency
Activation energy of hydrolysis of sucrose is
107 kJ mol-1 in presence of H, while that is 36
kJ mol-1 in presence of a little amount of
saccharase, corresponding to a rate change of
1022.
A superoxide Dismutase can catalytically
decompose 105 molecules of hydrogen peroxide in
at ambient temperature in 1 s, while Al2(SiO3)3,
an industrial catalyst for cracking of petroleum,
can only crack one alkane molecules at 773K in 4
s.
3) Moderate conditions
Nitrogenase in root-node can fix dinitrogen
from dinitrogen and water at ambient pressure and
atmospheric pressure with 100 conversion.
While in industry, the conversion of dinitrogen
and dihydrogen to ammonia over promoted iron
catalyst at 500 atm and 450 480 oC for single
cycle is only 1015.
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4 autocatalysis and B-Z oscillation
The phenomenon that the intermediate or
product of a reaction acts as catalyst for the
reaction is called autocatalysis.
For example, Mn2, one of the products in the
titration of (COOH)2 with KMnO4, has catalytic
effect on the reaction. Acetic acid also has
catalytic effect on the hydrolysis of acetyl
acetate.
Owing to the autocatalysis, the reaction
accelerates after a induction period.
Induction period
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B-Z oscillation
For consecutive reaction
A B B
C
The equilibrium will finally reach. DEAD?
When the backward reaction is inhibited, then
A B B
C
For open system, stationary state can be
maintained.
In closed system, A was depleted, C was
produced and B can attain a maximum
concentration, and no stationary state can be
reached.
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It was interesting that, for some open system
far apart from equilibrium, the intermediate
concentration oscillates with time. These
reactions is called chemical oscillating reaction.
The first oscillating reaction was observed by
Belousov in 1958. Latterly, Zhabotinshii reported
other systems that can generate chemical
oscillation. We now call chemical oscillation
Belousov-Zhabotinshii oscillation (B-Z
oscillation).
The first B-Z oscillation system is the
cerium-ion-catalyzed oxidation of malonic acid by
bromate.
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The oscillation system 0.25 mol dm-3 malonic
acid, 0.06 mol dm-3 KBrO4 in 1.5 mol dm-3 H2SO4
with 0.002 mol dm-3 Ce(NH4)2(NO3)5 as catalyst
and a trace of the redox indicator Ferroin is
present to make the changes more evident.
3H 3BrO3- 5CH2(COOH)2 ?
3BrCH(COOH)2 2HCOOH 4CO2
5H2O
A periodic color changes from blue to violet can
back again can be observed.
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Field, Koros, and Noyes proposed a mechanism
for explanation of the B-Z oscillation, which is
named as FKN mechansim.
A series
Ce(IV)/ Ce(III)
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Periodic change of Br- and Ce(IV)/Ce(III)
In 1910, Lotka showed that the system
G S ? 2S k1 S W ? 2W k2 W
? inert k3
k2S k3lnS k2W k1GlnW constant
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If A is constant, the system gives undamped
oscillations in (X) and (Y)
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Conditions for oscillation
1) open system
2) Bistable state
3) far apart from equilibrium
4) feedback mechanics
In such a system, order may generate from
chaos.
Flow reactor in chemical engineering process is
open system. When the system with bistability far
apart from equilibrium, chemical oscillation may
occur. Chemical oscillation is a common phenomena
is chemical industry.
The above systems are all with negative
feedback mechanism. Thermal explosion is a
oscillation system with positive feedback
mechanics.
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Oscillation around equilibrium is not allowed.
Ilya Prigogine
1977 Noble Prize Russia 1917/1/25 for his
contributions to non-equilibrium thermodynamics,
particularly the theory of dissipative
structures.
Oscillation around a quasi-steady state that is
approaching equilibrium
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Cloud patterns