ch4003_lecture notes 11-21

1

Facts and Figures about CatalystsLife cycle on the earth Catalysts (enzyme) participates most part of life cycle

e.g. forming, growing, decaying Catalysis contributes great part in the processes of converting sun energy to various

other forms of energies e.g. photosynthesis by plant CO2 + H2O=HC + O2

Catalysis plays a key role in maintaining our environment

Chemical Industry ca. $2 bn annual sale of catalysts ca. $200 bn annual sale of the chemicals that are related products 90% of chemical industry has catalysis-related processes Catalysts contributes ca. 2% of total investment in a chemical process

Catalysis & CatalystsCatalysis & Catalysts

CH4003 Lecture Notes 11 (Erzeng Xue)

2

Catalysis Catalysis is an action by catalyst which takes part in a chemical reaction process

and can alter the rate of reactions, and yet itself will return to its original form without being consumed or destroyed at the end of the reactions (This is one of many definitions)

Three key aspects of catalyst action taking part in the reaction

• it will change itself during the process by interacting with other reactant/product molecules

altering the rates of reactions • in most cases the rates of reactions are increased by the action of catalysts; however, in

some situations the rates of undesired reactions are selectively suppressed

Returning to its original form• After reaction cycles a catalyst with exactly the same nature is ‘reborn’• In practice a catalyst has its lifespan - it deactivates gradually during use

What is CatalysisCatalysis & Catalysts


3

Catalysis action - Reaction kinetics and mechanism Catalyst action leads to the rate of a reaction to change. This is realised by changing the course of reaction (compared to non-catalytic reaction) Forming complex with reactants/products, controlling the rate of elementary steps

in the process. This is evidenced by the facts that

The reaction activation energy is altered

The intermediates formed are different from those formed in non-catalytic reaction

The rates of reactions are altered (bothdesired and undesired ones)

Reactions proceed under less demanding conditions

Allow reactions occur under a milder conditions, e.g. at lower temperatures for those heat sensitive materials

Action of CatalystsCatalysis & Catalysts

reactant

reaction process

uncatalytic

product

ener

gy

catalytic


4

It is important to remember that the use of catalyst DOES NOT vary ∆G & Keqvalues of the reaction concerned, it merely change the PACE of the process

Whether a reaction can proceed or not and to what extent a reaction can proceed is solely determined by the reaction thermodynamics, which is governed by the values of ∆G & Keq, NOT by the presence of catalysts.

In another word, the reaction thermodynamics provide the driving force for a rxn; the presence of catalysts changes the way how driving force acts on that process.

e.g CH4(g) + CO2(g) = 2CO(g) + 2H2(g) ∆G°373=151 kJ/mol (100 °C)

∆G°973 =-16 kJ/mol (700 °C)

At 100°C, ∆G°373=151 kJ/mol > 0. There is no thermodynamic driving force, the reaction won’t proceed with or without a catalyst

At 700°C, ∆G°373= -16 kJ/mol < 0. The thermodynamic driving force is there. However, simply putting CH4 and CO2 together in a reactor does not mean they will react. Without a proper catalyst heating the mixture in reactor results no conversion of CH4 and CO2 at all. When Pt/ZrO2 or Ni/Al2O3 is present in the reactor at the same temperature, equilibrium conversion can be achieved (<100%).

Action of CatalystsCatalysis & Catalysts


5

The types of catalysts Classification based on the its physical state, a catalyst can be

gas liquid solid

Classification based on the substances from which a catalyst is made Inorganic (gases, metals, metal oxides, inorganic acids, bases etc.) Organic (organic acids, enzymes etc.)

Classification based on the ways catalysts work Homogeneous - both catalyst and all reactants/products are in the same phase (gas or liq) Heterogeneous - reaction system involves multi-phase (catalysts + reactants/products)

Classification based on the catalysts’ action Acid-base catalysts Enzymatic Photocatalysis Electrocatalysis, etc.

Types of Catalysts & Catalytic ReactionsCatalysis & Catalysts


6

Industrial applicationsAlmost all chemical industries have one or more steps employing catalysts Petroleum, energy sector, fertiliser, pharmaceutical, fine chemicals …

Advantages of catalytic processes Achieving better process economics and productivity

Increase reaction rates - fast Simplify the reaction steps - low investment cost Carry out reaction under mild conditions (e.g. low T, P) - low energy consumption

Reducing wastes Improving selectivity toward desired products - less raw materials required, less unwanted wastes Replacing harmful/toxic materials with readily available ones

Producing certain products that may not be possible without catalysts Having better control of process (safety, flexible etc.) Encouraging application and advancement of new technologies and materials And many more …

Applications of CatalysisCatalysis & Catalysts


7

Environmental applications Pollution controls in combination with industrial processes

Pre-treatment - reduce the amount waste/change the composition of emissions Post-treatments - once formed, reduce and convert emissions Using alternative materials

…

Pollution reduction gas - converting harmful gases to non-harmful ones liquid - de-pollution, de-odder, de-colour etc solid - landfill, factory wastes

…

And many more …

Other applications Catalysis and catalysts play one of the key roles in new technology development.

Applications of CatalysisCatalysis & Catalysts


8

Research in catalysis involve a multi-discipline approach Reaction kinetics and mechanism

Reaction paths, intermediate formation & action, interpretation of results obtained under various conditions, generalising reaction types & schemes, predict catalyst performance…

Catalyst development Material synthesis, structure properties, catalyst stability, compatibility…

Analysis techniques Detection limits in terms of dimension of time & size and under extreme conditions (T, P)

and accuracy of measurements, microscopic techniques, sample preparation techniques…

Reaction modelling Elementary reactions and rates, quantum mechanics/chemistry, physical chemistry …

Reactor modelling Mathematical interpretation and representation, the numerical method, micro-kinetics,

structure and efficiency of heat and mass transfer in relation to reactor design …

Catalytic process Heat and mass transfers, energy balance and efficiency of process …

Research in CatalysisCatalysis & Catalysts


9

Understanding catalytic reaction processes A catalytic reaction can be operated in a batch manner

Reactants and catalysts are loaded together in reactor and catalytic reactions (homo- or heterogeneous) take place in pre-determined temperature and pressure for a desired time / desired conversion

Type of reactor is usually simple, basic requirements Withstand required temperature & pressure Some stirring to encourage mass and heat transfers Provide sufficient heating or cooling

Catalytic reactions are commonly operated in a continuous mannerReactants, which are usually in gas or liquid phase, are fed to reactor in

steady rate (e.g. mol/h, kg/h, m3/h)Usually a target conversion is set for the reaction, based on this target

required quantities of catalyst is added required heating or cooling is provided required reactor dimension and characteristics are designed accordingly.

Catalytic Reaction ProcessesCatalysis & Catalysts


10

Catalytic reactions in a continuous operation (cont’d)Reactants in continuous operation are mostly in gas phase or liquid phase

easy transportation The heat & mass transfer rates in gas phase is much faster than those in liquid

Catalysts are pre-loaded, when using a solid catalyst, or fed together with reactants when catalyst & reactants are in the same phase and pre-mixed It is common to use solid catalyst because of its easiness to separate catalyst

from unreacted reactants and products Note: In a chemical process separation usually accounts for ~80% of cost. That

is why engineers always try to put a liquid catalyst on to a solid carrier. With pre-loaded solid catalyst, there is no need to transport catalyst which is

then more economic and less attrition of solid catalyst (Catalysts do not change before and after a reaction and can be used for number cycles, months or years),

In some cases catalysts has to be transported because of need of regeneration

In most cases, catalytic reactions are carried out with catalyst in a fixed-bed reactor (fluidised-bed in case of regeneration being needed), with the reactant being gases or liquids



11

General requirements for a good catalyst Activity - being able to promote the rate of desired reactions Selective - being to promote only the rate of desired reaction and also

retard the undesired reactions Note: The selectivity is sometime considered to be more important

than the activity and sometime it is more difficult to achieve(e.g. selective oxidation of NO to NO2 in the presence of SO2)

Stability - a good catalyst should resist to deactivation, caused by the presence of impurities in feed (e.g. lead in petrol poison TWC. thermal deterioration, volatility and hydrolysis of active components attrition due to mechanical movement or pressure shock

A solid catalyst should have reasonably large surface area needed for reaction (active sites). This is usually achieved by making the solid into a porous structure.



12

Example Heterogeneous Catalytic Reaction Process The long journey for reactant molecules to

j. travel within gas phase k. cross gas-liquid phase boundaryl. travel within liquid phase/stagnant layerm. cross liquid-solid phase boundaryn. reach outer surface of solido. diffuse within porep. arrive at reaction siteq. be adsorbed on the site and activatedr. react with other reactant molecules, either

being adsorbed on the same/neighbour sites or approaching from surface above

Product molecules must follow the same track in the reverse direction to return to gas phase

Heat transfer follows similar track

j

r

gas phase

poreporous solid

liquid phase /stagnant layer

kl

mn

o

pq

gas phasereactant molecule

Catalysis & Catalysts


13

Catalyst composition Active phase

Where the reaction occurs (mostly metal/metal oxide)

Promoter Textual promoter (e.g. Al - Fe for NH3 production) Electric or Structural modifier Poison resistant promoters

Support / carrier Increase mechanical strength Increase surface area (98% surface area is supplied within the porous structure) may or may not be catalytically active

Solid CatalystsCatalysis & Catalysts

Catalyst

Support


14

Some common solid support / carrier materials

Alumina Inexpensive Surface area: 1 ~ 700 m2/g Acidic

Silica Inexpensive Surface area: 100 ~ 800 m2/g Acidic

Zeolite mixture of alumina and silica, often exchanged metal ion present shape selective acidic


Other supports Active carbon (S.A. up to 1000 m2/g) Titania (S.A. 10 ~ 50 m2/g) Zirconia (S.A. 10 ~ 100 m2/g) Magnesia (S.A. 10 m2/g) Lanthana (S.A. 10 m2/g)

poreporous solid

Active site


15

Preparation of catalysts Precipitation

To form non-soluble precipitate by desired reactions at certain pH and temperature

Adsorption & ion-exchangeCationic: S-OH+ + C+ → SOC+ + H+

Anionic: S-OH- + A- → SA- + OH-

I-exch. S-Na+ + Ni 2+ D S-Ni 2+ + Na+

ImpregnationFill the pores of support with a metal salt solution of sufficient concentration to give the correct loading.

Dry mixing Physically mixed, grind, and fired


precipitate or deposit

precipitation

filter & wash the resultingprecipitate

Drying& firing

precursorsolution

Support

add acid/basewith pH control

Support

Drying & firing

Pore saturated pellets

Soln. of metalprecursor

Amou

ntad

sorb

ed

Concentration

Support

Drying & firing


16

Preparation of catalystsCatalysts need to be calcined (fired) in order to decompose the precursor and to

received desired thermal stability. The effects of calcination temperature and time are shown in the figures on the right.

Commonly used Pre-treatments Reduction

if elemental metal is the active phase

Sulphidation if a metal sulphide is the active phase

Activation Some catalysts require certain activation steps in order to receive the best performance. Even when the oxide itself is the active phase it may be necessary to pre-treat the

catalyst prior to the reaction

Typical catalyst life span

Can be many years or a few mins.


0255075

100

500 600 700 800 900Temperature °C

BET

S.A

. m2 /g

0

40

0 10Time / hours

BET

S.A

.

Activ

ity

Time

Normal use

Induction period

dead


17

Adsorption Adsorption is a process in which molecules from gas (or liquid) phase land

on, interact with and attach to solid surfaces. The reverse process of adsorption, i.e. the process n which adsorbed

molecules escape from solid surfaces, is called Desorption. Molecules can attach to surfaces in two different ways because of the

different forces involved. These are Physisorption (Physical adsorption) & Chemisorption (Chemical adsorption)

Physisorption Chemisorption

force van de Waal chemcal bondnumber of adsorbed layers multi only one layer

adsorption heat low (10-40 kJ/mol) high ( > 40 kJ/mol)selectivity low high

temperature to occur low high

Adsorption On Solid SurfaceCatalysis & Catalysts


18

Adsorption processAdsorbent and adsorbate Adsorbent (also called substrate) - The solid that provides surface for adsorption

high surface area with proper pore structure and size distribution is essential good mechanical strength and thermal stability are necessary

Adsorbate - The gas or liquid substances which are to be adsorbed on solid

Surface coverage, θThe solid surface may be completely or partially covered by adsorbed molecules

Adsorption heat Adsorption is usually exothermic (in special cases dissociated adsorption can be

endothermic) The heat of chemisorption is in the same order of magnitude of reaction heat;

the heat of physisorption is in the same order of magnitude of condensation heat.


define θ = θ = 0~1number of adsorption sites occupiednumber of adsorption sites available


19

Applications of adsorption process Adsorption is a very important step in solid catalysed reaction processes

Adsorption in itself is a common process used in industry for various purposes Purification (removing impurities from a gas / liquid stream) De-pollution, de-colour, de-odour Solvent recovery, trace compound enrichment etc…

Usually adsorption is only applied for a process dealing with small capacity The operation is usually batch type and required regeneration of saturated adsorbent

Common adsorbents: molecular sieve, active carbon, silica gel, activated alumina.

Physisorption is an useful technique for determining the surface area, the pore shape, pore sizes and size distribution of porous solid materials (BET surface area)



20

Adsorption On Solid Surface Characterisation of adsorption system

Adsorption isotherm - most commonly used, especially to catalytic reaction system, T=const.The amount of adsorption as a function of pressure at set temperature

Adsorption isobar - (usage related to industrial applications)The amount of adsorption as a function of temperature at set pressure

Adsorption Isostere - (usage related to industrial applications)Adsorption pressure as a function of temperature at set volume


Pressure

Vol.

adso

rbed

T1T2 >T1

T3 >T2

T4 >T3

T5 >T4

Vol.

adso

rbed

Temperature

P1

P2>P1

P3>P2P4>P3

Pre

ssur

e

Temperature

V2>V1

V1

V3>V2

V4>V3

Adsorption Isotherm Adsorption Isobar Adsorption Isostere


21

The Langmuir adsorption isotherm Basic assumptions

surface uniform (∆Hads does not vary with coverage) monolayer adsorption, and no interaction between adsorbed molecules and adsorbed molecules immobile

Case I - single molecule adsorptionwhen adsorption is in a dynamic equilibrium

A(g) + M(surface site) D AMthe rate of adsorption rads = kads (1-θ) Pthe rate of desorption rdes = kdes θ

at equilibrium rads = rdes ⇒ kads (1-θ) P = kdes θ

rearrange it for θ

let ⇒ B0 is adsorption coefficient


θ = =+∞

CC

B PB P

s 0

01des

ads

kkB =0

PBk/kPk/k

desads

desads

0)(1)(

+=θ

case I

A


22

Adsorption On Solid Surface The Langmuir adsorption isotherm (cont’d)

Case II - single molecule adsorbed dissociatively on one siteA-B(g) + M(surface site) D A-M-B

the rate of A-B adsorption rads=kads (1−θΑ )(1−θΒ)PAB=kads (1−θ )2PAB

the rate of A-B desorption rdes=kdesθΑθΒ =kdesθ2

at equilibrium rads = rdes ⇒ kads (1−θ )2PAB= kdesθ2

rearrange it for θ

Let. ⇒


case II

A BBA

θ=θΑ=θΒ

1/20

1/20

)(1)(

AB

ABs

PBPB

CC

+==

∞

θdes

ads

kkB =0

)(1

)(

ABdesads

ABdesads

Pk/kPk/k

+=θ


23

The Langmuir adsorption isotherm (cont’d) Case III - two molecules adsorbed on two sites

A(g) + B(g) + 2M(surface site) D A-M + B-M

the rate of A adsorption rads,A = kads,A (1− θΑ− θΒ) PA

the rate of B adsorption rads,B = kads,B (1− θΑ− θΒ) PB

the rate of A desorption rdes,A = kdes,A θΑ

the rate of B desorption rdes,B = kdes,B θΒ

at equilibrium rads ,A = rdes ,A and ⇒ rads ,B = rdes ,B

⇒ kads,A(1−θΑ−θΒ)PA=kdes,AθΑ and kads,B(1−θΑ−θΒ)PB=kdes,BθΒ

rearrange it for θ

where are adsorption coefficients of A & B.


B,des

B,adsB,

A,des

A,adsA, k

kB

kk

B == 00 and

BB,AA,

BB,B,sB

BB,AA,

AA,A,sA PBPB

PBCC

PBPBPB

CC

00

0

00

0

1

1 ++==

++==

∞∞

θθ

case III

A B


24

The Langmuir adsorption isotherm (cont’d)


B,des

B,adsB,

A,des

A,adsA, k

kB

kk

B == 00 and

BB,AA,

BB,B,sB

BB,AA,

AA,A,sA

PBPBPB

CC

PBPBPB

CC

00

0

00

0

1

1

++==

++==

∞

∞

θ

θ

AdsorptionStrong kads>> kdes kads>> kdes

B0>>1 B0>>1

Weak kads<< kdes kads<< kdes

B0<<1 B0<<1

1/20

1/20

)(1)(

AB

ABs

PBPB

CC

+==

∞

θ

des

ads

kkB =0

case II

A B

θ = =+∞

CC

B PB P

s 0

01

des

ads

kkB =0

case I

A

1→=∞C

Csθ 1→=∞C

Csθ

PBCCs

0==∞

θ 1/20 )( PB

CCs ==

∞

θ

AdsorptionA, B both strong

A strong, B weak

A weak, B weak

BB,AA,

BB,B,sB

BB,AA,

AA,A,sA

PBPBPB

CC

PBPBPB

CC

00

0

00

0

+==

+==

∞

∞

θ

θ

BB,B,sB

AA,A,sA

PBC/CPBC/C

0

0

====

∞

∞

θθ

A

BA,B,B,sB

A,sA

PPB/BC/C

C/C

)(

1

00==

→=

∞

∞

θ

θ

case III

A B


25

Langmuir adsorption isothermcase I

case II

Case III


Langmuir adsorption isotherm established a logic picture of adsorption process

It fits many adsorption systems but not at all

The assumptions made by Langmuir do not hold in all situation, that causing error Solid surface is heterogeneous thus the heat of adsorption is not a constant at different θ Physisorption of gas molecules on a solid surface can be more than one layer

BB,AA,

BB,B,sB

BB,AA,

AA,A,sA

PBPBPB

CC

PBPBPB

CC

00

0

00

0

1

1

++==

++==

∞

∞

θ

θ

1/20

1/20

)(1)(

AB

ABs

PBPB

CC

+==

∞

θ

θ = =+∞

CC

B PB P

s 0

01

large B0 (strong adsorp.)

small B0 (weak adsorp.)moderate B0

Pressure

Amou

nt a

dsor

bed

mono-layer

1→=∞C

Csθ

PBCCs

0==∞

θ

Strong adsorption kads>> kdes

Weak adsorption kads<< kdes


26

Five types of physisorption isotherms are found over all solids

Type I is found for porous materials with small pores e.g. charcoal. It is clearly Langmuir monolayer type, but the other 4 are not

Type II for non-porous materials

Type III porous materials with cohesive force between adsorbate molecules greater than the adhesive force between adsorbate molecules and adsorbent

Type IV staged adsorption (first monolayer then build up of additional layers)

Type V porous materials with cohesive force between adsorbate molecules and adsorbent being greater than that between adsorbate molecules


I

II

III

IV

V

relative pres. P/P0

1.0

amou

nt a

dsor

bed


27

Other adsorption isothermsMany other isotherms are proposed in order to explain the observations

The Temkin (or Slygin-Frumkin) isotherm Assuming the adsorption enthalpy ∆H decreases linearly with surface coverage

From ads-des equilibrium, ads. rate ≡ des. rate

rads=kads(1-θ)P ≡ rdes=kdesθ

where Qs is the heat of adsorption. When Qs is a linear function of θi. Qs=Q0-iS (Q0 is a constant, i is the number and S represents the surface site),

the overall coverage

When b1P >>1 and b1Pexp(-i/RT) <<1, we have θ =c1ln(c2P), where c1 & c2 are constants

Valid for some adsorption systems.


1

1 1

1

0

0

PebPeb

PBPB

RT/Q

RT/Q

s s

s

+=⇒

+= θθ ∆H

of a

ds

θ

LangmuirTemkin

( )

−+

+=

+== ∫∫

RTiRT/Q

RT/Q

s expPP

iRTdS

PebPebdS

s

s

1

11

01

11

0 b1b1ln

(1[θθ


28

The Freundlich isotherm assuming logarithmic change of adsorption enthalpy ∆H with surface coverage

From ads-des equilibrium, ads. rate ≡ des. rate

rads=kads(1-θ)P ≡ rdes=kdesθ

where Qi is the heat of adsorption which is a function of θi. If there are Ni types of surface sites, each can be expressed as Ni=aexp(-Q/Q0) (a and Q0 are constants), corresponding to a fractional coverage θi, the overall coverage

the solution for this integration expression at small θ is:

lnθ=(RT/Q0)lnP+constant, or

as is the Freundlich equation normally written, where c1=constant, 1/c2=RT/Q0

Freundlich isotherm fits, not all, but many adsorption systems.


∫∫

∑∑

∞

∞⋅+

==

0

0 11

0

0

e

e)](1[

dQa

dQaPeb/Peb

N

N

Q/Q

Q/QRT/QRT/Q

ii

iiiθ

θ

1

1 1

1

0

0

PebPeb

PBPB

RT/Q

RT/Q

i i

i

+=⇒

+= θθ ∆H

of a

ds

θ

LangmuirFreundlich

211

C/pc=θ


29

BET (Brunauer-Emmett-Teller) isotherm Many physical adsorption isotherms were found, such as the types II and III, that the

adsorption does not complete the first layer (monolayer) before it continues to stack on the subsequent layer (thus the S-shape of types II and III isotherms)

Basic assumptions the same assumptions as that of Langmuir but allow multi-layer adsorption the heat of ads. of additional layer equals to the latent heat of condensation based on the rate of adsorption=the rate of desorption for each layer of ads.

the following BET equation was derived

Where P - equilibrium pressureP0 - saturate vapour pressure of the adsorbed gas at the temperature

P/P0 is called relative pressureV - volume of adsorbed gas per kg adsorbentVm -volume of monolayer adsorbed gas per kg adsorbentc - constant associated with adsorption heat and condensation heatNote: for many adsorption systems c=exp[(H1-HL)/RT], where H1 is adsorption heat of 1st layer &

HL is liquefaction heat, so that the adsorption heat can be determined from constant c.


)(111 0

0

0 P/PcVc

cV)P/P(VP/P

mm

−+=

−


30

Comment on the BET isotherm BET equation fits reasonably well all known adsorption isotherms observed so far

(types I to V) for various types of solid, although there is fundamental defect in the theory because of the assumptions made (no interaction between adsorbed molecules, surface homogeneity and liquefaction heat for all subsequent layers being equal).

BET isotherm, as well as all other isotherms, gives accurate account of adsorption isotherm only within restricted pressure range. At very low (P/P0<0.05) and high relative pressure (P/P0>0.35) it becomes less applicable.

The most significant contribution of BET isotherm to the surface science is that the theory provided the first applicable means of accurate determination of the surface area of a solid (since in 1945).

Many new development in relation to the theory of adsorption isotherm, most of them are accurate for a specific system under specific conditions.



31

Use of BET isotherm to determine the surface area of a solid At low relative pressure P/P0 = 0.05~0.35 it is found that

Y = a + b XThe principle of surface area determination by BET method:

A plot of against P/P0 will yield a straight line with slope of equal to (c-1)/(cVm) and intersect 1/(cVm). For a given adsorption system, c and Vm are constant values, the surface area of a solid material can be determined by measuring the amount of a particular gas adsorbed on the surface with known molecular cross-section area Am,

* In practice, measurement of BET surface area of a solid is carried out by N2 physisorption at liquid N2 temperature; for N2, Am = 16.2 x 10-20 m2


)( )(111 00

0

0 P/PP/PcVc

cV)P/P(VP/P

mm

∝−

+=−

P PV P P

/( / )

0

01−

P/P0

P PV P P

/( / )

0

01−

A A N AV

Vs m m mm

T P= = × ×

,.6 022 1023

Vm - volume of monolayer adsorbed gas molecules calculated from the plot, L

VT,P - molar volume of the adsorbed gas, L/molAm - cross-section area of a single gas molecule, m2


32

Summary of adsorption isotherms

Name Isotherm equation Application Note

Langmuir

Temkin θ =c1ln(c2P)

Freundlich

BET


)(111 0

0

0 P/PcVc

cV)P/P(VP/P

mm

−+=

−

θ = =+∞

CC

B PB P

s 0

01

211

C/pc=θ

Chemisorption andphysisorption

Chemisorption

Chemisorption andphysisorption

Multilayer physisorption

Useful in analysis of reaction mechanism

Chemisorption

Easy to fit adsorption data

Useful in surface area determination


33

Langmuir-Hinshelwood mechanism This mechanism deals with the surface-catalysed reaction in which

that 2 or more reactants adsorb on surface without dissociation

A(g) + B(g) D A(ads) + B(ads) " P (the desorption of P is not r.d.s.)

The rate of reaction ri=k[A][B]=kθAθB

From Langmuir adsorption isotherm (the case III) we know

We then have

When both A & B are weakly adsorbed (B0,APA<<1, B0,BPB<<1),

2nd order reaction When A is strongly adsorbed (B0,APA>>1) & B weakly adsorbed (B0,BPB<<1 <<B0,APA)

1st order w.r.t. B

Mechanism of Surface Catalysed ReactionCatalysis & Catalysts

++=

++=

BB,AA,

BB,B

BB,AA,

AA,A

PBPBPB

PBPBPB

00

0

00

0

1

1

θ

θ

BB,AA,

BAB,A,

BB,AA,

BB,

BB,AA,

AA,i PBPB

PPBkBPBPB

PBPBPB

PBkr

00

00

00

0

00

0

111 ++=

++

++=

BABAB,A,i PP'kPPBkBr == 00

BBB,AA,

BAB,A,i P''kPkB

PBPPBkB

r === 00

00

A B+ " P


34

Eley-Rideal mechanism This mechanism deals with the surface-catalysed reaction in which

that one reactant, A, adsorb on surface without dissociation andother reactant, B, approaching from gas to react with A

A(g) D A(ads) P (the desorption of P is not r.d.s.)

The rate of reaction ri=k[A][B]=kθAPB

From Langmuir adsorption isotherm (the case I) we know

We then have

When both A is weakly adsorbed or the partial pressure of A is very low (B0,APA<<1),

2nd order reaction When A is strongly adsorbed or the partial pressure of A is very high (B0,APA>>1)

1st order w.r.t. B


AA,

AA,A PB

PB

0

0

1+=θ

AA,

BAA,B

AA,

AA,i PB

PPkBP

PBPB

kr0

0

0

0

11 +=

+=

BABAA,i PP'kPPkBr == 0

BAA,

BAA,i kP

PBPPkB

r ==0

0

A" P

B

+ B(g)


35

Mechanism of surface-catalysed reaction with dissociative adsorption The mechanism of the surface-catalysed reaction in which one

reactant, AD, dissociatively adsorbed on one surface site

AD(g) D A(ads) + D(ads) P

(the des. of P is not r.d.s.)

The rate of reaction ri=k[A][B]=kθADPB

From Langmuir adsorption isotherm (the case I) we know

We then have

When both AD is weakly adsorbed or the partial pressure of AD is very low (B0,ADPAD<<1),

The reaction orders, 0.5 w.r.t. AD and 1 w.r.t. B When A is strongly adsorbed or the partial pressure of A is very high (B0,APA>>1)

1st order w.r.t. B


( )( ) 21

0

210

1 /ADAD,

/ADAD,

AD PBPB

+=θ

( )( )

( )( ) 21

0

210

210

210

11 /ADAD,

B/

ADAD,B/

ADAD,

/ADAD,

i PBPPBk

PPB

PBkr

+=

+=

( ) B/

ADB/

ADAD,i PP'kPPBkr 21210 ==

( )( ) B/

ADAD,

B/

ADAD,i kP

PBPPBk

r == 210

210

+ B(g)" P

B

A B


36

Mechanisms of surface-catalysed rxns involving dissociative adsorption In a similar way one can derive mechanisms of other surface-catalysed reactions,

in which dissociatively adsorbed one reactant, AD, (on one surface site) reacts with

another associatively adsorbed reactant B on a separate surface site dissociatively adsorbed one reactant, AD, (on one surface site) reacts with

another dissociatively adsorbed reactant BC on a separate site …

The use of these mechanism equations

Determining which mechanism applies by fitting experimental data to each.

Helping in analysing complex reaction network

Providing a guideline for catalyst development (formulation, structure,…).

Designing / running experiments under extreme conditions for a better control

…



37

Bulk and surface The composition & structure of a solid in bulk and on surface

can differ due toSurface contamination

Bombardment by foreign molecules when exposed to an environment

Surface enrichment Some elements or compounds tend to be enriched (driving by thermodynamic

properties of the bulk and surface component) on surface than in bulk

Deliberately made different in order for solid to have specific properties Coating (conductivity, hardness, corrosion-resistant etc) Doping the surface of solid with specific active components in order perform certain

function such as catalysis…

To processes that occur on surfaces, such as corrosion, solid sensors and catalysts, the composition and structure of (usually number of layers of) surface are of critical importance

Solids and Solid SurfaceCatalysis & Catalysts


38

Morphology of a solid and its surface A solid, so as its surface, can be well-structured crystalline (e.g. diamond C,

carbon nano-tubes, NaCl, sugar etc) or amorphous (non-crystallised, e.g. glass)

Mixture of different crystalline of the same substance can co-exist on surface (e.g. monoclinic, tetragonal, cubic ZrO2)

Well-structured crystalline and amorphous can co-exist on surface Both well-structured crystalline and amorphous are capable of being used

adsorbent and/or catalyst …



39

Defects and dislocation on surface crystalline structure A ‘perfect crystal’ can be made in a controlled way Surface defects

terrace step kink adatom / vacancy

Dislocation screw dislocation

Defects and dislocation can be desirable for certain catalytic reactions as these may provide the required surface geometry for molecules to be adsorbed, beside the fact that these sites are generally highly energised.


Terrace Step


40

Pore sizes micro pores dp <20-50 nm meso-pores 20nm <dp<200nm macro pores dp >200 nm Pores can be uniform (e.g. polymers) or non-uniform (most metal oxides)

Pore size distribution Typical curves to characterise pore size:

Cumulative curve Frequency curve

Uniform size distribution (a) & non-uniform size distribution (b)

Pores of Porous SolidsCatalysis & Catalysts

b

d

a

dwdd

∆d

wt

b a∆wt

d

Cumulative curve Frequency curve


41

Many reactions proceed via chain reaction polymerisation explosion …

Elementary reaction steps in chain reactions1. Initiation step - creation of chain carriers (radicals, ions, neutrons etc, which are capable of

propagating a chain) by vigorous collisions, photon absorptionR R (the dot here signifies the radical carrying unpaired electron)

2. Propagation step - attacking reactant molecules to generate new chain carriersR + M → R + M

3. Termination step - two chain carriers combining resulting in the end of chain growthR + M → R-M

There are also other reactions occur during chain reaction: Retardation step - chain carriers attacking product molecules breaking them to reactant

R + R-M → R + M(leading to net reducing of the product formation rate)

Inhibition step - chain carriers being destroyed by reacting with wall or foreign matterR + W → R-W (leading to net reducing of the number of chain carriers)

Chain Reactions - ProcessComplex Reactions

E


42

Rate law of chain reactionExample: overall reaction H2(g) + Br2(g) → 2HBr(g) observed:

elem step rate law

a. Initiation: Br2 → 2Br ra=ka[Br2]b. Propagation: Br + H2 → HBr + H rb=kb[Br][H2]

H + Br2 → HBr + Br r’b=k’b[H][Br2]c. Termination: Br + Br → Br2 rc=kc[Br][Br]=kc[Br]2

H + H → H2 (practically less important therefore neglected)H + Br → HBr (practically less important therefore neglected)

d. Retardn (obsvd.) H + HBr → H2 + Br rd=kd[H][HBr]

HBr net rate: rHBr= rb+ r’b- rd or d[HBr]/dt=kb[Br][H2]+k’b[H][Br2]-kd[H][HBr]

Apply s.s.a. rH= rb- r’b- rd or d[H]/dt=kb[Br][H2]- k’b[H][Br2]-kd[H][HBr]=0rBr= 2ra-rb+r’b-2rc +rd or d[Br]/dt=2ka[Br2]-kb[Br][H2]+k’b[H][Br2]-2 kc[Br]2 +kd[H][HBr]=0

solve the above eqn’s we have

Chain Reactions - Rate LawComplex Reactions

[HBr]][Br]][Br[H[HBr]

2

3/222

'kk

dtd

+=

( )( )[HBr]][Br

]][Br[H2[HBr]

2

3/222

1/2

bd

cab

'k/kk/kk

dtd

+=


43

Monomer - the individual molecule unit in a polymer Type I polymerisation - Chain polymerisation

An activated monomer attacks another monomer, links to it, then likes another monomer, so on…, leading the chain growth eventually to polymer.

rate lawInitiation: Ix → xR (usually r.d.s.) ri=ki[I]

R + M → M1 (fast)

Propagation: M + M1 → (MM1) → M2 (fast)M + M2 → (MM2) → M3 (fast)… … … … … … … … …M + Mn-1 → (MMn-1) → Mn rp=kp[M][M] (ri is the r.d.s.)

Termination: Mn + Mm → (MnMm) → Mm+n rt=kt[M]2

Apply s.s.a. to [M] formed

The rate of propagation or the rate of M consumption or the rate of chain growth

Chain Reactions - PolymerisationComplex Reactions

[I] ][Mikx

dtd φ=

•

initiator chain-carrier

212

2[I] ][M 0][M2[I] 2 ][M

/

t

itipi k

kxk-kxrrxdt

d

=•⇒=•=−=

• φφφ

[M][I]2

[M] i.e. ][M][M[M] 1/221/

t

ippp k

kxkdt

dkrdt

d

−=•−=−=

φ

φ is the yield of Ix to xR


44

Type II polymerisation - Stepwise polymerisationA specific section of molecule A reacts with a specific section of molecule B forming chain

(a-A-a’) + (b’-B-b) → {a -A-(a’b’)-B-b}

H2N(CH2)6NH2 + HOOC(CH2)4COOH → H2N(CH2)6NHOC(CH2)4COOH + H2O (1)→ H-HN(CH2)6NHOC(CH2)4CO-OH … → H-[HN(CH2)6NHOC(CH2)4CO]n-OH (n)

Note: If a small molecule is dropped as a result of reaction, like a H2O dropped in rxn (1), this type of reaction is called condensation reaction. Protein molecules are formed in this way.

The rate law for the overall reaction of this type is the same as its elementary step involving one H- containing unit & one -OH containing unit, which is the 2nd order

the conversion of B (-OH containing substance) at time t is

Chain Reactions - PolymerisationComplex Reactions

0

02

[A]1[A][A]or [A][A][-OH][A]kt

kkdt

d+

=−=−=

0

0

0

0

[A]1[A]

[A][A][A]

ktktX B +

=−

=


45

Type I Explosion: Chain-branching explosionChain-branching - During propagation step of a chain reaction one attack by a

chain carrier can produce more than one new chain carriersChain-branching explosion

When chain-branching occurs the number carriers increases exponentially the rate of reaction may cascade into explosionExample: 2H2(g) + O2(g) → 2H2O(g)

Initiation: H2 + O2 → O2H + H

Propagation: H2 + O2H → OH + H2O (non-branching)H2 + OH → H + H2O (non-branching)

O2 + H → O + OH (branching)O + H2 → OH + H (branching)

Chain Reactions - ExplosionComplex Reactions

Lead to explosion


46

Type II Explosion: Thermal explosionA rapid increase of the rate of exothermic reaction with temperatureStrictly speaking thermal explosion is not caused by multiple production of chain carriers

Must be exothermic reaction

Must be in a confined space and within short time

∆H → T ↑ → r ↑ → ∆H ↑↑ → T ↑↑ → r ↑↑ → ∆H ↑↑ ↑ → … A combination of chain-branching reaction with heat accumulation can occur

simultaneously

Explosion ReactionsComplex Reactions


47

Photochemical reactionThe reaction that is initiated by the absorption of light (photons)

Characterisation of photon absorption - quantum yieldA reactant molecule after absorbing a photon becomes excited. The excitation may lead to product formation or may be lost (e.g. in form of heat emission)

The number of specific primary products (e.g. a radical, photon-excited molecule, or an ion) formed by absorption of each photon, is called primary quantum yield, φ

The number of reactant molecules that react as a result of each photon absorbed is call overall quantum yield, Φ

E.g. HI + hv → H + I primary quantum yield φ =2 (one H and one I)H + HI → H2 + I

2I → I2 overall quantum yield Φ =2 (two HI molecules reacted)

Note: Many chain reactions are initiated by photochemical reaction. Because of chain reaction overall quantum yield can be very large, e.g. Φ = 104

The quantum yield of a photochemical reaction depends on the wavelength of light used

Photochemical ReactionsComplex Reactions


48

Wave-length selectivity of photochemical reaction A light with a specific wave length may only excite a specific type of molecule

Quantum yield of a photochemical rxn may vary with light (wave-length) used Isotope separation (photochemical reaction Application)

Different isotope species - different mass - different frequencies required to match their vibration-rotational energys

e.g. I36Cl + I37Cl I36Cl + I37Cl* (only 37Cl molecules are excited)C6H5Br + I37Cl* → C6H5

37Cl + IBr Photosensitisation (photochemical reaction Application)

Reactant molecule A may not be activated in a photochemical reaction because it does not absorb light, but A may be activated by the presence of another molecule B which can be excited by absorbing light, then transfer some of its energy to A.

e.g. Hg + H2 Hg* + H2 (Hg is, but H2 is not excited by 254nm light)Hg* + H2 → Hg + 2H* & Hg* + H2 → HgH + H*

H* HCO HCHO + H*2HCO → HCHO + CO

Photochemical ReactionsComplex Reactions

508 nm light

254 nm light

CO H2


49

What is Spectroscopy The study of structure and properties of atoms and molecule by means of the spectral information obtained from the interaction of electromagnetic radiant energy with matter

It is the base on which a main class of instrumental analysis and methods is developed & widely used in many areas of modern science

What to be discussed

Theoretical background of spectroscopy Types of spectroscopy and their working principles in brief Major components of common spectroscopic instruments Applications in Chemistry related areas and some examples

Introduction to SpectroscopySpectroscopy


50

Electromagnetic radiation (e.m.r.) Electromagnetic radiation is a form of energy Wave-particle duality of electromagnetic radiation

Wave nature - expressed in term of frequency, wave-length and velocity Particle nature - expressed in terms of individual photon, discrete packet of energy

when expressing energy carried by a photon, we need to know the its frequency

Characteristics of wave Frequency, v - number of oscillations per unit time, unit: hertz (Hz) - cycle per second velocity, c - the speed of propagation, for e.m.r c=2.9979 x 108 m⋅s-1 (in vacuum) wave-length, λ - the distance between adjacent crests of the wave

wave number, v’, - the number of waves per unit distance v’ =λ-1

The energy carried by an e.m.r. or a photon is directly proportional to the

frequency, i.e. where h is Planck’s constant h=6.626x10-34J⋅s

Electromagnetic RadiationIntroductory to Spectroscopy

c'vcv ==λ

c'hvhchvE ===λ


51

Electromagnetic radiation X-ray, light, infra-red, microwave and radio waves are all e.m.r.’s, difference being their frequency thus the amount of energy they possess

Spectral region of e.m.r.

Electromagnetic RadiationIntroductory to Spectroscopy


52

Interaction of electromagnetic radiant with matter The wave-length, λ, and the wave number, v’, of e.m.r. changes with the medium it

travels through, because of the refractive index of the medium; the frequency, v, however, remains unchanged

Types of interactions

AbsorptionReflectionTransmissionScatteringRefraction

Each interaction can disclose certain properties of the matter

When applying e.m.r. of different frequency (thus the energy e.m.r. carried) different type information can be obtained

Interaction of e.m.r. with Matter

refraction

transmission

absorption

reflection scattering

Introductory to Spectroscopy


53

Spectrum is the display of the energy level of e.m.r. as a function of wave number of electromagnetic radiation energy

The energy level of e.m.r. is usually expressed in one of these terms absorbance (e.m.r. being absorbed)

transmission (e.m.r. passed through)

Intensity

The term ‘intensity’ has the meaning of the radiant power that carried by an e.m. r.

Spectrum

.

1.0

0.5

0.0350 400 450

wave length cm-1

inte

nsity



54

What an spectrum tells A peak (it can also be a valley depending on how the spectrum is constructed)

represents the absorption or emission of e.m.r. at that specific wavenumber The wavenumber at the tip of peak is the most important, especially when a peak is broad

A broad peak may sometimes consist of several peaks partially overlapped each other -mathematic software (usually supplied) must be used to separate them case of a broad peak (or a valley) observed

The height of a peak corresponds the amount absorption/emission thus can be used as a quantitative information (e.g. concentration), a careful calibration is usually required

The ratio in intensity of different peaks does not necessarily means the ratio of the quantity (e.g. concentration, population of a state etc.)

Spectrum

.

1.0

0.5

0.0 350 400 450wave length cm-1

inte

nsity



55

Spectral properties, applications, and interactions of electromagnetic radiation

absorptionemissionfluorescence

Magneticallyinduced spinstates

Electronparamagnetresonance

Infrared

Wave numberv’

cm-1

Wavelengthλ

cm

Frequencyv

Hz

Energy

kcal/mol Electronvole eV

Type of radiation

Type of spectroscopy

Type of quantum transition

9.4x107 4.1x106 3.3x1010 3.0x10-11 1021

9.4x105 4.1x104 3.3x108 3.0x10-9 1019

9.4x103 4.1x102 3.3x106 3.0x10-7 1017

9.4x101 4.1x100 3.3x104 3.0x10-5 1015

9.4x10-1 4.1x10-2 3.3x102 3.0x10-3 1013

9.4x10-3 4.1x10-4 3.3x100 3.0x10-1 1011

9.4x10-5 4.1x10-6 3.3x10-2 3.0x101 109

9.4x10-7 4.1x10-8 3.3x10-4 3.0x103 107

Gamma ray

X-ray

Ultra Violet

Visible

Microwave

Radio

X-rayabsorption emission

NuclearGamma rayemission

Electronic(outer shell)

Molecularrotation

Molecularvibration

Nuclear magneticresonance

Microwaveabsorption

UV absorption

IR absorptionRaman

VacUVVis

Electronic(inner shell)



56

1. A laser emits light with a frequency of 4.69x1014 s-1. (h = 6.63 x 10-34Js)A) What is the energy of one photon of the radiation from this laser? B) If the laser emits 1.3x10-2J during a pulse, how many photons are emitted during the pulse?

Ans: A) Ephoton = hν = 6.63 x 10-34Js x 4.69x1014 s-1 = 3.11 x 10-19 JB) No. of photons = (1.3x10-2J )/(3.11 x 10-19J) = 4.2x1016

2. The brilliant red colours seen in fireworks are due to the emission of red light at a wave length of 650nm. What is the energy of one photon of this light? (h = 6.63 x 10-34Js)

Ans: Ephoton = hν = hc/λ =(6.63 x 10-34Js x 3 x 108ms-1)/650x10-9m = 3.06x10-19J

3: Compare the energies of photons emitted by two radio stations, operating at 92 MHz (FM) and 1500 kHz (MW)?

Ans: Ephoton = hν92 MHz = 92 x 106 Hz (s-1) => E = (6.63 x 10-34 Js) x (92 x 106 s-1) = 6.1 x 10-26J

1500 kHz = 1500 x 103 Hz (s-1) E = (6.63 x 10-34 Js) x (1500 x 103 s-1) = 9.9 x 10-28J

Examples

.



57

Shell structure & energy level of atoms In an atom there are a number of shells and

of subshells where e-’s can be found The energy level of each shell & subshell

are different and quantised The e-’s in the shell closest to the nuclei has

the lowest energy. The higher shell number is, the higher energy it is

The exact energy level of each shell and subshell varies with substance

Ground state and excited state of e-’s Under normal situation an e- stays at the

lowest possible shell - the e- is said to be at its ground state

Upon absorbing energy (excited), an e- can change its orbital to a higher one - we say the e- is at is excited state.

Atomic SpectraIntroductory to Spectroscopy

n = 1

n = 2

n = 3, etc.

energy∆E

groundstate

Excitedstate

Ener

gyn=1

n=2

n=3

n=4

1s2s2p3s3p

4s3d4p4d4f


58

Electron excitation The excitation can occur at different degrees

low E tends to excite the outmost e-’s first when excited with a high E (photon of high v)

an e- can jump more than one levels even higher E can tear inner e-’s away from

nuclei

An e- at its excited state is not stable and tends to return its ground state

If an e- jumped more than one energy levels because of absorption of a high E, the process of the e- returning to its ground state may take several steps, - i.e. to the nearest low energy level first then down to next …

Atomic Spectra

Ener

gyn=1

n=2

n=3

n=4

1s2s2p3s3p

4s3d4p4d4f

n = 1

n = 2

n = 3, etc.

energy∆E



59

Atomic spectraThe level and quantities of energy supplied

to excite e-’s can be measured & studied in terms of the frequency and the intensity of an e.m.r. - the absorption spectroscopy

The level and quantities of energy emitted by excited e-’s, as they return to their ground state, can be measured & studied by means of the emission spectroscopy

The level & quantities of energy absorbed or emitted (v & intensity of e.m.r.) are specific for a substance

Atomic spectra are mostly in UV (sometime in visible) regions

Atomic Spectra

Ener

gyn=1

n=2

n=3

n=4

1s2s2p3s3p

4s3d4p4d4f

n = 1

n = 2

n = 3, etc.

energy∆E



60

Motion & energy of molecules Molecules are vibrating and rotating all the time,

two main vibration modes being stretching - change in bond length (higher v) bending - change in bond angle (lower v)

(other possible complex types of stretching & bending are: scissoring / rocking / twisting

Molecules are normally at their ground state (S0)S (Singlet) - two e-’s spin in pair ET (Triplet) - two e-’s spin parallel J

Upon exciting molecules can change to high E states (S1, S2, T1 etc.), which are associated with specific levels of energy

The change from high E states to low ones can be stimulated by absorbing a photon; the change from low to high E states may result in photon emission

Molecular SpectraSpectroscopy

S0

T1

S2

S1

v1

v2v3v4

v1

v2v3v4

v1

v2v3v4

v1

v2v3v4


61

Excitation of a molecule The energy levels of a molecule at

each state / sub-state are quantised To excite a molecule from its ground

state (S0) to a higher E state (S1, S2, T1etc.), the exact amount of energy equal to the difference between the two states has to be absorbed. (Process A)i.e. to excite a molecule from S0,v1 to S2,v2, e.m.r with wavenumber v’ must be used

The values of energy levels vary with the (molecule of) substance.

Molecular absorption spectra are the measure of the amount of e.m.r., at a specific wavenumber, absorbed by a substance.


1022 v,v, SS EE'hcv −=

v1

v2v3v4

S0

T1

S2

S1

v1

v2v3v4

v1

v2v3v4

v1

v2v3v4

absorptionA

A


62

Energy change of excited moleculesAn excited molecules can lose its excessenergy via several processes Process B - Releasing E as heat when changing

from a sub-state to the parental state occurs within the same state

The remaining energy can be release by one of following Processes (C, D & E)

Process C - Transfer its remaining E to other chemical species by collision

Process D - Emitting photons when falling back to the ground state - Fluorescence

Process E1 - Undergoing internal transitionwithin the same mode of the excited state

Process E2 - Undergoing intersystem crossing to a triplet sublevel of the excited state

Process F - Radiating E from triplet to ground state (triplet quenching) - Phosphorescence


S0

T1

S2

S1

v1

v2v3v4

v1

v2v3v4

v1

v2v3v4

v1

v2v3v4

Inter- systemcrossing

Internaltransition

B

B

E1

E2

C

F

A

B

FluorescenceD

Fluorescence

Jablonsky diagram


63

Two types of molecular emission spectra Fluorescence

In the case fluorescence the energy emitted can be the same or smaller (if heat is released before radiation) than the corresponding molecular absorption spectra.

e.g. adsorption in UV region - emission in UV or visible region (the wavelength of visible region is longer than that of UV thus less energy)

Fluorescence can also occur in atomic adsorption spectra

Fluorescence emission is generally short-lived (e.g. µs)

Phosphorescence

Phosphorescence generally takes much longer to complete (called metastable) than fluorescence because of the transition from triplet state to ground state involves altering the e-’s spin. If the emission is in visible light region, the light of excited material fades away gradually


S0

S2

v1

v2v3v4

v1

v2v3v4

B

Aphosphor-enscence

D

Fluore-scence

T1

v1

v2v3v4

F


64

Comparison of atomic and molecular spectra

Quantum mechanics is the basis of atomic & molecular spectra The transitional, rotational and vibrational modes of motion of objects of atomic /

molecular level are well-explained.

Atomic Spectra & Molecular SpectraIntroductory to Spectroscopy

Atomic spectra Molecular spectra

Adsorption spectra Yes Yes

Emission spectra Yes Yes

Energy required for excitation high low

Change of energy level related to change of e-’s orbital change of vibration states

Spectral region UV mainly visible

Relative complexity of spectra simple complex


65

Observations

When a light of intensity I0 goes through a liquid of concentration C & layer thickness b The emergent light, I, has less intensity than the incident light I0

scattering, reflection absorption by liquid

There are different levels of reduction in light intensity at different wavelength detect by eye - colour change detect by instrument

The method used to measure UV & visible light absorption is called spectrophotometry(colourimetry refers to the measurement of absorption of light in visible region only)

UV & Visible SpectrophotometrySpectroscopy Application

Incident light, I0(UV or visible)

Emergent light, I

C

b

ultraviolet visible infra-red

200 - 400 400 - 800 800 - 15nm nm nm nm nm µm


66

Theory of light absorptionQuantitative observation The thicker the cuvette

- more diminishing of light in intensity

Higher concentration the liquid- the less the emergent light intensity

These observations are summarised by Beer’s Law:Successive increments in the number of identical absorbing molecules in the path of a beam of monochromatic radiation absorb equal fraction of the radiation power travel through themThus


Incident lightI0

Emergent lightI

C

b

I'kdxNcs

dI−=2I0

dx

bx

s

sI

number of moleculesN-Avogadro number

light absorbed

fraction of light

acdxdxNcs'kI

dI−=−=⇒ 2

acbIIdxac

IdI bbI

I

b −=⇒−=⇒ ∫∫0

0ln

0

AabcII

≡=⇒ 0log Absorbance


67

Terms, units and symbols for use with Beer’s Law

Name alternative name symbol definition unit

Path length - b (or l) - cm

Liquid concentration - c - mol / L

Transmittance Transmission T I / I0 -Percent transmittance - T% 100x I / I0 %

Absorbance Optical density, A log(I / I0) -extinction

Absorptivity Extinction coeff., a (or ε, k) A/(bc) [bc]-1absorbance index

Molar absorptivity Molar extinction coeff., a A/(bc)molar absorbancy index [or aM AM/(bc’) ] M-molar weight

c’ -gram/L



68

Use of Beer’s Law

Beer’s law can be applied to the absorption of UV, visible, infra-red & microwave

The limitations of the Beer’s Law Effect of solvent - Solvents may absorb light to a various extent,

e.g. the following solvents absorb more than 50% of the UV light going through them180-195nm sulphuric acid (96%), water, acetonitrile200-210nm cyclopentane, n-hexane, glycerol, methanol, ethanol210-220nm n-butyl alcohol, isopropyl alcohol, cyclohexane, ethyl ether245-260nm chloroform, ethyl acetate, methyl formate265-275nm carbon tetrachloride, dimethyl sulphoxide/formamide, acetic acid280-290nm benzene, toluene, m-xylene300-400nm pyridine, acetone, carbon disulphide

Effect of temperature Varying temperature may cause change of concentration of a solute because of

thermal expansion of solution changing of equilibrium composition if solution is in equilibrium



69

What occur to a molecule when absorbing UV-visible photon? A UV-visible photon (ca. 200-700nm) promotes a bonding or non-bonding

electron into antibonding orbital - the so called electronic transition Bonding e-’s appear in σ & π molecular

orbitals; non-bonding in n

Antibonding orbitals correspond to the bonding ones

e-’s transition can occur between variousstates; in general, the energy of e-’stransition increases in the following order:

(n→π*) < (n→σ*) < (π →π*) < (σ →σ*)

Molecules which can be analysed by UV-visible absorption Chromophores

functional groups each of which absorbs a characteristic UV or visible radiation.


σ *

π*

n

π

Antibonding

Antibonding

non-bonding

Bonding

Ene

rgy σ

→σ*

π→π*

n →

σ*

n →

π*

σ


70

The functional groups & the wavelength of UV-visible absorption

Group Example λmax, nm Group Example λmax, nm

C=C 1-octane 180 arene benzene 260naphthalene 280

C=O methanol 290 phenenthrene 350propanone 280 anthracene 375ethanoic acid 210 pentacene 575ethyl ethanoate 210ethanamide 220 conjugated 1,3-butadiene 220

1,3,5-hexatriene 250C-X methanol 180 2-propenal 320

trimethylamine 200 β-carotene (11 C=C) 480chloromethane 170bromomethane 210 each additional C=C +30iodomethane 260



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Instrumentation

UV visibleLight source Hydrogen discharge lamp Tungsten-halogen lamp

Cuvette QUARTZ glass

Detectors photomultiplier photomultiplier



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UV & Visible Spectrophotometry Applications

Analysis of unknowns using Beer’s Law calibration curve

Absorbance vs. time graphs for kinetics

Single-point calibration for an equilibrium constant determination

Spectrophotometric titrations – a way to follow a reaction if at least one substance is colored – sudden or sharp change in absorbance at equivalence point

Spectroscopy Application


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IR-Spectroscopy Atoms in a molecule are constantly in motion

There are two main vibrational modes: Stretching - (symmetrical/asymmetrical) change in bond length - high frequency Bending - (scissoring/stretch/rocking/twisting) change in bond angle - low freq.

The rotation and vibration of bonds occur in specific frequencies Every type of bond has a natural frequency of vibration, depending on

the mass of bonded atoms (lighter atoms vibrate at higher frequencies) the stiffness of bond (stiffer bonds vibrate at higher frequencies) the force constant of bond (electronegativity) the geometry of atoms in molecule

The same bond in different compounds has a slightly different vibration frequ.

Functional groups have characteristic stretching frequencies.



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IR-Spectroscopy IR region

The part of electromagnetic radiation between the visible and microwave regions 0.8 µm to 50 µm (12,500 cm-1-200 cm-1).

Most interested region in Infrared Spectroscopy is between 2.5µm-25 µm(4,000cm-1-400cm-1), which corresponds to vibrational frequency of molecules

Interaction of IR with molecules Only molecules containing covalent bonds with dipole moments are infrared sensitive

Only the infrared radiation with the frequencies matching the natural vibrational frequencies of a bond (the energy states of a molecule are quantitised) is absorbed

Absorption of infrared radiation by a molecule rises the energy state of the molecule increasing the amplitude of the molecular rotation & vibration of the covalent bonds

Rotation - Less than 100 cm-1 (not included in normal Infrared Spectroscopy) Vibration - 10,000 cm-1 to 100 cm-1

The energy changes thr. infrared radiation absorption is in the range of 8-40 KJ/mol



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IR-Spectroscopy Use of Infra-Red spectroscopy

IR spectroscopy can be used to distinguish one compound from another. No two molecules of different structure will have exactly the same natural

frequency of vibration, each will have a unique infrared absorption spectrum. A fingerprinting type of IR spectral library can be established to distinguish a

compounds or to detect the presence of certain functional groups in a molecule.

Obtaining structural information about a moleculeAbsorption of IR energy by organic compounds will occur in a manner

characteristic of the types of bonds and atoms in the functional groups present in the compound

Practically, examining each region (wave number) of the IR spectrum allows one identifying the functional groups that are present and assignment of structure when combined with molecular formula information.

The known structure information is summarized in the Correlation Chart



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IR SpectrumRegion freq. (cm-1) what is found there??

XH region 3800 - 2600 OH, NH, CH (sp, sp2, sp3) stretchestriple bond 2400 - 2000 C≡C, C≡N, C=C=C stretchesdouble bond 1900 - 1500 C=O, C=N, C=C stretchesfingerprint 1500 - 400 many types of absorptions

1400 - 900 C-O, C-N stretches1500 - 1300 CH in-plane bends, NH bends1000 - 650 CH out-of-plane (oop) bends


Principal Correlation ChartO−H 3600 cm-1

N−H 3500 cm-1

C−H 3000 cm-1

C≡N 2250 cm-1

C≡C 2150 cm-1

C=O 1715 cm-1

C=C 1650 cm-1

C−O 1100 cm-1

Dispersive (Double Beam) IR Spectrophotometer

Prismor

DiffractionGrating Slit

Photometer

IR Source Recorder

SplitBeam Air

Lenz Sample


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Source: R. Thomas, “Choosing the Right Trace Element Technique,” Today’s Chemist at Work, Oct. 1999, 42.

Atomic Absorption/Emission Spectroscopy Atomic absorption/emission spectroscopes involve e-’s changing energy states

Most useful in quantitative analysis of elements, especially metals


These spectroscopes are usually carried out in optical means, involving conversion of compounds/elements to gaseous

atoms by atomisation. Atomization is the most critical step in flame spectroscopy. Often limits the precision of these methods.

excitation of electrons of atoms through heating or X-ray bombardment

UV/vis absorption, emission or fluorescence of atomic species in vapor is measured

Instrument easy to tune and operate

Sample preparation is simple (often involving only dissolution in an acid)


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Atomic Absorption Spectrometer (AA)Spectroscopy Application

Source

Sample

P P0

Chopper

Wavelength Selector Detector Signal Processor

Readout

Type Method of Atomization RadiationSource

atomic (flame) sample solution aspirated Hollow cathode into a flame lamp (HCL)

atomic (nonflame) sample solution HCLevaporated & ignited

x-ray absorption none required x-ray tube


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Atomic Emission Spectrometer (AES)Spectroscopy Application

Source

Sample

P Wavelength Selector Detector Signal Processor

Readout


arc sample heated in an electric arc sample

spark sample excited in a high voltage spark sample

argon plasma sample heated in an argon plasma sample

flame sample solution aspirated into a flame sample

x-ray emission none required; sample bombarded w/ e- sample


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Atomic Fluorescence Spectrometer (AFS)Spectroscopy Application

Source

Sample

P P0

90o

Wavelength Selector Detector Signal Processor

Readout


atomic (flame) sample solution aspirated into a flame sample

atomic (nonflame) sample solution sampleevaporated & ignited

x-ray fluorescence none required sample


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Laser - is a special type of light sources or light generators. The word LASER represents Light Amplification by Stimulated Emission of Radiation

Characteristics of light produced by Lasers Monochromatic (single wavelength) Coherent (in phase) Directional (narrow cone of divergence)

Laser - CharacteristicsSpectroscopy Application

Incandescent lamp• Chromatic• Incoherent• Non-directional

Monochromatic light source

• Coherent• Non-directional

The first microwave laser was made in the microwave region in 1954 by Townes & Shawlow using ammonia as the lasing medium.

The first optical laser was constructedby Maiman in 1960, using ruby (Al2O3doped with a dilute concentration of Cr+3)as the lasing medium and a fastdischarge flash-lamp to provide the pumpenergy.


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When excited atoms/molecules/ions undergo de-excitation (from excited state to ground state), light is emitted

Types of light emission

Laser - Stimulated EmissionSpectroscopy Application

E4E3

E2

E1

E0

ground state

excitedstate

Ep1=(E1 – E0) = hv1Ep2=(E2 – E0) = hv2Ep4=(E4 – E0) = hv4

Ep1

Ep4

Ep2

Spontaneous emission - chromatic & incoherent

Excited e-’s when returning to ground states emit light spontaneously (called spontaneous emission).

Photons emitted when e-’s return from different excited states to ground states have different frequencies (chromatic)

Spontaneous emission happens randomly and requires no event to trigger the transition (various phase or incoherent)


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Types of light emission (cont’d)

Stimulated emission - monochromatic & coherent While an atom is still in its excited state, one can

bring it down to its ground state by stimulating it with a photon (P1) having an energy equal to the energy difference of the excited state and the ground state. In such a process, the incident photon (P1) is not absorbed and is emitted together with the photon (P2), The latter will havethe same frequency (or energy) and the same phase (coherent) as the stimulating photon (P1).

Laser - Stimulated EmissionSpectroscopy Application

E4E3

E2

E1

E0

Ep1=(E2–E0)=hv2Ep2=(E2–E0)=hv2

Ep1=(E2–E0)=hv2

Laser uses the stimulated emission process to amplify the light intensity

As in the stimulated emission process, one incident photon (P1) will bring about the emission of an additional photon (P2), which in turn can yield 4 photons, then 8 photons, and so on….


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The conditions must be satisfied in order to sustain such a chain reaction: Population Inversion (PI), a situation that there are more atoms in a certain excited

state than in the ground statePI can be achieved by a variety means (electrical, optical, chemical or mechanical), e.g., one may obtain PI by irradiating the system of atoms by an enormously intense light beam or, if the system of atoms is a gas, by passing an electric current through the gas.

Presence of Metastable state, which is the excited state that the excited e-’s can have a relatively long lifetime (>10-8 second), in order to avoid the spontaneous emission occurring before the stimulated emissionIn most lasers, the atoms/molecules/ions in the lasing medium are not “pumped” directly to a metastable state. They are excited to an energy level higher than a metastable state, then drop down to the metastable state by spontaneous non-radiative de-excitation.

Photon Confinement (PC), the emitted photons must be confined in the system long enough to stimulate further light emission from other excited atomsThis is achieved by using reflecting mirrors at the ends of the system. One end is made totally reflecting & the other is slight transparent to allow part of the laser beam to escape.

Laser - Formation & ConditionsSpectroscopy Application


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Laser - Functional ElementsSpectroscopy Application

Energy pumping mechanism

Energy input

Lasing medium

Highreflectance

mirror

Partially transmitting

mirror

OutputcouplerFeedback mechanism


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Laser ActionSpectroscopy Application

Lasing medium at ground state

Population inversion

Start of stimulated emission

Stimulated emissionbuilding up

Laser infull operation

Pump energy

Pump energy

Pump energy

Pump energy


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Types of Lasers There are many different types of lasers

The lasing medium can be gas, liquid or solid (insulator or semiconductor) Some lasers produce continuous light beam and some give pulsed light beam Most lasers produce light wave with a fixed wave-length, but some can be tuned

to produce light beam of wave-length within a certain range.


Laser type Physical form of lasing medium Wave length (nm)Helium neon laser Gas 633

Carbon dioxide laser Gas 10600 (far-infrared)

Argon laser Gas 488, 513, 361 (UV), 364 (UV)

Nitrogen laser Gas 337 (UV)

Dye laser Liquid Tunable: 570-650

Ruby laser Solid 694

Nd:Yag laser Solid 1064 (infrared)

Diode laser Semiconductor 630-680


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Laser - Applications Laser can be applied in many areas

CommerceCompact disk, laser printer, copiers, optical disk drives, bar code scanner, optical communications, laser shows, holograms, laser pointers

IndustryMeasurements (range, distance), alignment, material processing (cutting, drilling, welding, annealing, photolithography, etc.), non-destructive testing, sealing

MedicineSurgery (eyes, dentistry, dermatology, general), diagnostics, ophthalmology, oncology

ResearchSpectroscopy, nuclear fusion, atom cooling, interferometry, photochemistry, study of fast processes

MilitaryRanging, navigation, simulation, weapons, guidance, blinding



ch4003_lecture notes 11-21

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