§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

612 22 11 2[C H O ][H O] [H ]r k

Substance that appears in the rate equation to a power that is higher than that to which it appears in the stoichiometric equation.

the catalyst constitutes a separate phase from the reaction.

2) Heterogeneous catalysis:

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) dm3mol-1s-

1

23

14

12

101.432700

exp108.4

15200exp102.9

T

Tk

kc

with HBr as catalyst:

2) t-Bu-Br (CH3)2C=CH2 + HBr

1) t- Bu-OH + HBr t-Bu-Br + H2O

1

1

2

A C A C

A C + B A B + C

k

k

k

1 2

1

[A][B][A][B]

k kr k

k

1,2,1,, aaaappa EEEE

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.

Catalyst can shorten the time for reaching equilibrium.

e

e

ln ( )( )

xk k t kt

x x e

e

ln ( )( )

xk k t kt

x x

(3) 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.

.

2 2 3N 3H 2NHCat

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.

CH2Ag

200~300 oCCH2 CH2

O

+21

O2CH2

CH2200~300 oC

+21

O2CH2

PdCl2 CuCl2CH3 C

O

H

(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.

4) Other characteristics:

5.4 kinetics of homogeneous catalysis

1 2

1 2

[S][C][S][C] '[S]

k kr k k

k k

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:

aaa KGk

Where Ga and is experimental constants.

aaa KGk lglglg

ranges between 0 ~ 1.

S C M P C1 2

1

k k

k

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).

0 2 4 6 8 10

-2

-1

0

1

2

3

log

ka

- lgKa

Dehydration of acet-aldehyde catalyzed by different acids.

For base-catalyzed reaction there also exists:bbb KGk

5.5 Some phenomena of heterogeneous catalysis

The potential curve of adsorption

(1) basic principal of heterogeneous catalysis

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

adsorption

reaction

desorption

diffusion

1) diffusion of reactants to surface;

2) adsorption of reactants at surface;

3) reaction on the surface;

4) desorption of product from surface;

5) diffusion of product from surface.

Which is r.d.s.?

Many surface reactions can be treated successfully on the basis of

the following assumptions:

For unimolecular reaction over catalyst

Catalyzed isomerization or decomposition

A (g) +

A

B +

B

1) the r.d.s. is a reaction of adsorbed molecules;

2) the reaction rate per unit surface area is proportional to .

1) the r.d.s. is a reaction of adsorbed molecules;

2) the reaction rate per unit surface area is proportional to .

For bimolecular reaction over catalyst

Langmuir-Hinshelwood mechanism (L-H mechanism)

Langmuir-Rideal mechanism (L-R mechanism)

A (g)B (g)

A B

Transition state

A-B

+

A (g) +

A-B +

A

+ B (g)

A B

Synthesis of ammonia

Langmuir-Hinshelwood mechanism (L-H mechanism)

Hydrogenation of ethylene

Langmuir-Rideal mechanism (L-R mechanism)

For unimolecular reactionAr Akr

According to Langmuir isotherm

1

bp

bp

1A A

A A

kb pr

b p

（ 3 ） kinetics for heterogeneous catalysis

Under low pressure, when bAPA << 1

At high pressure, when bAPA >> 1

kr

1A A

A A

kb pr

b p

A Ar kb p

pA

r

rmax

When bApA << 1 + bBpB 1

A A

B B

kb pr

b p

The adsorption of competing species inhibits the reaction.

1A A

A A B B

kb pr

b p b p

For example:

Decomposition of N2O over Ag, CuO or CdO.

2

2

[N O]

1 [O ]

kr

b

When bBpB >> 1 1

A A

B B

kb pr

b p

' A

B

pr k

p

For example

Decomposition of ammonia over Pt

3

2

[NH ]

[H ]r k

1A A

AA A B B

b p

b p b p

when competing adsorption exists:

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.

A Br k A A B B2

A A B B(1 )

kb p b pr

b p b p

Rate~ partial pressure relation of L-H mechanism

pA

pB = constant

r

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

（ 4 ） Active sites

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(100)Fe(111) Fe(211)

Fe(110)Fe(210)

Active sites in iron catalyst for ammonia synthesis

C7: active sites

Where are the active sites?

Atom cluster

Adsorption of species on the edges of a calcites crystal

The active site is in fact atom cluster comprising of several metal atoms.

Increase of the degree of

subdivision will increase the

ununiformity of catalyst

surface and increase the

number of active sites.

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) Poison of catalyst

1A A

A A B B

kb pr

b p b p

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:

pepsin Hydrolysis of proteins

diastase Hydrolysis of starch

urease hydrolysis of urea

invertase hydrolysis of sucrose

zymase hydrolysis of glucose

maltase Hydrolysis of maltose

Important hydrolytic enzymes

oxidation-reduction enzymes

SOD(Superoxide Dismutase) Decomposition of superoxide (O2-)

Nitrogenase Dinitrogen fixation

1) hydrolytic enzymes

2) oxidation-reduction enzymes

(2) 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

3

[P][ES]

dk

dt 1 2 3

[ES][E][S] [ES] [ES]

dk k k

dt

0[E] [E] [ES]

1 0 1 2 3

[ES][E] [S] [ES][S] [ES] [ES]

dk k k k

dt

1 3

2

S E SE P Ek k

k

?

Using stationary-state approximation

1 0

1 2 3

[E] [S][ES]

[S]

k

k k k

1 3 0

1 2 3

[E] [S][P]

[S]

k kd

dt k k k

3 0 3 0

2 3

1

[E] [S] [E] [S]

[S][S] M

k kr

k k kk

Michaelis constant

Discussion: 1) When [S] >> kM:

3 0[E]mr k

2) When [S] << kM:

30[E] [S]

M

kr

k

When [S] = kM:

3 0 3 0[E] [S] [E] 1

2[S] 2 2 m

k kr r

3 0[E] [S]

[S] M

kr

k

3 0[E]mr k

[S]

[S]m M

r

r k

1 1 1

[S]M

m m

k

r r r

Lineweaver-Burk plotSlope: S = kM/rm

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.

http://en.wikipedia.org/wiki/Image:Luciferase-1BA3.png

Luciferase ( 荧光素酶 ) is a generic name for enzymes commonly used in nature for bioluminescence.

(2) Outstanding characteristics of enzyme catalysis

1) High selectivity:

substrate

enzyme

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.

NH

N

O

O

H OO

OHN

Multiple optically active centers produced by imidase catalysis

OHHO

R2HH R1

O O

O O

NH

OH

O

R1

R2 R1 R2 R1 R2

R2R1R2

R1

HHH

H HOOH

Imidase

Chirality of enzyme catalysis

1975 Noble Prize

Great Britain 1917/09/07

for his work on the stereochemistry of enzyme-catalyzed reactions

John Warcup Cornforth

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%.