§10.5 Catalytic reaction

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.

Catalytic oxidation of ammonia to nitrogen monooxide over Pt/Rh alloy

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.

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.

C12H22O11 + H2O C6H12O6 + C6H12O6

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

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.

1) Oxidation of SO2 to SO3 with the aid of NO.

2) Hydrolysis of sucrose with inorganic acid (in physical chemistry Lab).

Examples:

the catalyst constitutes a separate phase from the reaction.

Heterogeneous catalysis:

Examples:

Haber’s process for ammonia synthesis;

contact oxidation of sulphur dioxide;

Hydrogenation of alkene, aldehyde, etc.

 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)2C=CH2 + H2O

with HBr as catalyst:

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

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

Mechanism:

1 2

1

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

k kr k

k

1,2,1,, aaaappa EEEE

1

1

2

A C A C

A C + B A B + C

k

k

k

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

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) 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.

r m lnG RT K y y

As a state function, rGm only related to the initial and t

he final state. Therefore, catalyst has no impact on equilibrium constant of the reaction.

[A] [B]a br k

[G] [H]g hr k

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.

.

2 2 3N 3H 2NHcat

e

e

ln ( )( )

xk k t kt

x x e

e

ln ( )( )

xk k t kt

x x

decomposition of methanol over ZnO catalyst?

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.

totalreact

demandprod

n

nselectivty

,

,

CCl4 + HF CCl3F + CCl2F2 + CClF3

b. p. 23.7 -29.8 -81.1

SbCl5 9% 90% 0.5%

FeCl3 20% 75%

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:

2.4 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.

1 2

1

S C M P Ck 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 acetaldehyde catalyzed by different acids.

For base-catalyzed reaction there also exists:

bbb KGk

CH2= CH2 + Br2 CH2BrCH2Br

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.

The potential curve of adsorption

10.5.1 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.

10.5.2 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 products from surface;

5) diffusion of products away from the surface.

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 , the fraction of surface covered by reactant.

For unimolecular reaction over catalyst

Catalyzed isomerization and decomposition

A (g) +A

B +

B

For bimolecular reaction over catalyst

Langmuir-Hinshelwood mechanism (L-H mechanism)

Langmuir-Rideal mechanism (L-R mechanism)

Synthesis of ammonia from dinitrogen and dihydrogen

A (g) +

A-B +

A

+ B (g)

A B

A (g)B (g)

A B

Transition state

A-B

+

Hydrogenation of ethylene

Synthesis of ammonia

For unimolecular reactionAr Akr

According to Langmuir isotherm

1

bp

bp

1A A

A A

kb pr

b p

10.5.3 kinetics for heterogeneous catalysis

Under low pressure, when bAPA << 1 A Ar kb p First-order reaction

Increase in pressure will accelerate reaction rate.

At high pressure, when bAPA >> 1 kr zeroth-order reaction

Equilibrium adsorption has been reached. Change in pressure has no effect on the reaction rate.

when competing adsorption exists:

1A A

AA A B B

b p

b p b p

1A A

A A B B

kb pr

b p b p

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

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 oxi

dation of ammonia.

1926, Talyor proposed the active site model for explanation

10.5.4 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 su

bdivision will increase the un

uniformity of catalyst surface

and increase the number of a

ctive sites.

If bB is very large, even at low pB, A will be very small. T

he reaction of A will be greatly retarded. The impurities w

ith high b is catalyst poison.

10.5.5 Poison of catalyst

1A A

A A B B

kb pr

b p b p

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.

Kinds of enzymes: 1) hydrolytic enzymes

2) oxidation-reduction 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 enzymesSOD(Superoxide Dismutase) Decomposition of superoxide (O2

-)

Nitrogenase Dinitrogen fixation

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

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

is zeroth order with respect of [S].

2) When [S] << kM: 30[E] [S]

M

krk

is first order with respect of [S].

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

2[S] 2 2 m

k kr r

When [S] = kM:

At r = ½ rm, [S] = kM

3 0[E] [S]

[S] M

kr

k

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 is a generic name for enzymes commonly used in nature for bioluminescence.

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 (NH2C

ONH2) 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%.

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

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 CFor 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.

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 ca

n generate chemical oscillation. We now call chemical oscill

ation Belousov-Zhabotinshii oscillation (B-Z oscillation).

The first B-Z oscillation system is the cerium-ion-catalyzed

oxidation of malonic acid by bromate.

The oscillation system:

0.25 mol dm-3 malonic acid, 0.06 mol dm-3 KBrO4 in 1.5 mol d

m-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.

Field, Koros, and Noyes proposed a mechanism for explanation of the B-Z oscillation, which is named as FKN mechansim.

Ce(IV)/ Ce(III)

A series

BrO3- + CH2(COOH)2

BrCH(COOH)2

HCOOH + CO2 + Br-

Br-

Inhibited by Br-B series

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

k2[S] – k3ln[S] + k2[W] + k1[G]ln[W] = constant

If A is constant, the system gives undamped oscillations in (X) and (Y)

Conditions for oscillation

1) open system 1) open system

2) Bistable state2) Bistable state

3) far apart from equilibrium3) far apart from equilibrium

4) feedback mechanics4) 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.

Oscillation around equilibrium is not allowed.

Oscillation around a quasi-steady state that is approaching equilibrium

1977 Noble Prize

Russia 1917/1/25

for his contributions to non-equilibrium thermodynamics, particularly the theory of dissipative structures.

Ilya Prigogine

Cloud patterns