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§10.5 Catalytic reaction
5.1 Catalysts and catalysis
catalyst
Substance that changes the rate of a chemical reaction without
themselves undergoing any chemical change.
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.
5.2 type of catalysis
Homogeneous catalysis
Heterogeneous catalysis
Biological catalysis / enzyme catalysis
1) Homogeneous catalysis
the catalyst is present in the same phase as the reactant.
Example: Hydrolysis of sucrose with inorganic acid.
C12H22O11 + H2O  C6H12O6 + C6H12O6
r  k[C12 H 22O11 ][H 2O]6 [H  ]
Substance that appears in the rate equation to a power that is higher
than that to which it appears in the stoichiometric equation.
2) Heterogeneous catalysis:
the catalyst constitutes a separate phase from the reaction.
Examples:
Haber’s process for ammonia synthesis;
contact oxidation of sulphur dioxide;
Hydrogenation of alkene, aldehyde, etc.
5.3 General characteristics of catalyzed reactions
1) Catalyst takes part in the reaction.
(CH3)3COH  (CH3)2C=CH2 + H2O
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
 15200 
9.2 1012 exp  

kc
T

  4.110 23

k
 32700 
4.8 1014 exp  

T


with HBr as catalyst:
1) t- Bu-OH + HBr
t-Bu-Br + H2O
2) t-Bu-Br  (CH3)2C=CH2 + HBr
AC
k1
k1
AC
k2
A  C + B 
A  B + C
k1k2 [A][B]
r
 k[A][B]
k1
Ea ,app  Ea ,1  Ea , 2  Ea , 1
By altering reaction path, catalyst lower activation energy of the overall
reaction significantly and change the reaction rate dramatically.
2) No impact on the thermodynamic features of the reaction
(1) Cannot start or initiate a thermodynamically non-spontaneous
reaction;
(2) Can change the rate constant of forward reaction and backward
reaction with the same amplitude and does not alter the final
equilibrium position.
xe
ln
 (k  k )t  kt
( xe  x)
Catalyst can shorten the time
for reaching equilibrium.
(3) Is effective both for forward reaction and backward reaction.
N2  3H2
Cat .
2NH3
Study on the catalyst for ammonia synthesis can be done with easy by
making use of the decomposition of ammonia.
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.
CH2
CH2 +
Ag
1
O2
2
200~300 oC
CH2
CH2
O
CH2
CH2 +
PdCl2 CuCl2
1
O2
2
200~300 oC
O
CH3
C
H
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.
5.4 kinetics of homogeneous catalysis
For homogeneous reaction, the reactant is usually named as substrate.
S C
k1
k1
k1k2 [S][C]
M 
P  C r 
 k[S][C]  k '[S]
k1  k2
k2
When C is some acid, rate constant is proportional to dissociation
constant (Ka) as pointed out by Brønsted et al. in the 1920s:
k a  Ga K a
lg ka  lg Ga   lg K a
Where Ga and  is experimental constants.
 ranges between 0 ~ 1.
log k
a
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).
3
2
Dehydration of acetaldehyde catalyzed by
different acids.
1
0
-1
-2
0
2
4
6
- lgK
8
10
a
For base-catalyzed reaction there also exists:

kb  Gb K b
5.5 Some phenomena of heterogeneous catalysis
(1) basic principal of heterogeneous catalysis
The potential curve of adsorption
Interaction between molecule and
catalyst on catalytic activity
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.
（2） Mechanism of heterogeneous catalysis
A surface reaction can usually be divided into five elementary steps:
diffusion
1) diffusion of reactants to surface;
2) adsorption of reactants at surface;
adsorption
3) reaction on the surface;
4) desorption of product from surface;
reaction
5) diffusion of product from surface.
diffusion
desorption
Which is r.d.s.?
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 .
For unimolecular reaction over catalyst
A
B
A (g) +
B+
Catalyzed isomerization or decomposition
For bimolecular reaction over catalyst
Langmuir-Hinshelwood mechanism (L-H mechanism)
A
A (g)
B (g)
A-B
B
+
Transition state
Langmuir-Rideal mechanism (L-R mechanism)
A
A (g) +
A
+ B (g)
A-B +
B
Langmuir-Hinshelwood mechanism (L-H mechanism)
Synthesis of ammonia
Langmuir-Rideal mechanism (L-R mechanism)
Hydrogenation of ethylene
（3） kinetics for heterogeneous catalysis
For unimolecular reaction
r   A r  k A
According to Langmuir isotherm
bp

1  bp
kbA p A
r
1  bA p A
Under low pressure,
when bAPA << 1
At high pressure, when
bAPA >> 1
r
r k
rmax
r
kbA p A
1  bA p A
r  kbA pA
pA
when competing
adsorption exists:
When bApA << 1 + bBpB
r
kbA p A
1  bB pB
The adsorption of competing species inhibits
the reaction.
bA p A
A 
1  bA p A  bB pB
For example:
k[N 2 O]
Decomposition of N2O over Ag, r 
1  b[O2 ]
CuO or CdO.
kbA p A
r
1  bA pA  bB pB
When bBpB >> 1
r
kbA p A
1  bB pB
pA
r  k'
pB
For example
Decomposition of ammonia
over Pt
[NH3 ]
rk
[H 2 ]
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.
r  kAB
kbA pA bB pB
r
(1  bA pA  bB pB ) 2
r
pB = constant
Rate~ partial pressure relation
of L-H mechanism
pA
（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.
In 1926, Talyor proposed the active site model
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.
Fe(111)
Fe(211)
Fe(100)
C7: active sites
Fe(110)
Fe(210)
Active sites in iron catalyst
for ammonia synthesis
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
surface
increase
and
number of active sites.
catalyst
the
Adsorption of species on the
edges of a calcites crystal
(5) Poison of catalyst
kbA p A
r
1  bA pA  bB pB
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.
5.6 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.
（1）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
(2) Kinetics of enzyme catalysis
A rather widely applicable kinetic framework for enzymatic action
is that known as the Michaelis-Menten Mechanism (1913).
S E
k1
k2
SE  P  E
k3
Enzyme-substrate complex
d [P]
 k3[ES]
dt
d [ES]
 k1[E][S]  k2 [ES]  k3 [ES]
dt
[E]0  [E]  [ES]
d [ES]
 k1[E]0 [S]  k1[ES][S]  k2 [ES]  k3[ES]
dt
?
Using stationary-state approximation
k1[E]0 [S]
[ES] 
k1[S]  k2  k3
k1k3 [E]0 [S]
d [P]

dt
k1[S]  k2  k3
k3 [E]0 [S]
k3 [E]0 [S]
r

k 2  k3
[S]  k M
[S] 
k1
Michaelis constant
rm  k3[E]0
Discussion:
1) When [S] >> kM:
2) When [S] << kM:
When [S] = kM:
r
r
k3[E]0 [S] k3[E]0 1

 rm
2[S]
2
2
k3
[E]0 [S]
kM
k3 [E]0 [S]
r
[S]  k M
rm  k3[E]0
r
[S]

rm k M  [S]
1 kM 1
1



r
rm [S] rm
Slope: S = kM/rm
Lineweaver-Burk plot
intercept: I = 1/rm
Both rm and kM can be obtained by solving the equations.
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 ( 荧 光 素 酶 ) is a
generic name for enzymes
commonly used in nature for
bioluminescence.
http://en.wikipedia.org/wiki/Image:Luciferase-1BA3.png
(2) Outstanding characteristics of enzyme catalysis
1) High selectivity:
Lock and key
substrate
enzyme
Even 10-7 mol dm-3 urease can catalyze the hydrolysis of urea
(NH2CONH2) effectively. However, it has no effect on CH3CONH2.
Chirality of enzyme catalysis
Multiple optically active centers
produced by imidase catalysis
O
O
R1
R2
N
N
H
H
H
N
O
O
O
R2
R1
O
O
R1
R2
Imidase
John Warcup Cornforth
O
1975 Noble Prize
Great Britain 1917/09/07
for his work on the stereochemistry
of enzyme-catalyzed reactions
H
R1
H
R2
OH
N
H
O
OH HO
O
H R1 H R2
HO
OH
H R H R2
1
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 10~15%.