Catalysis III C presentation for measurement

*
 Facts and Figures about Catalysts
Life 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
Catalysis & Catalysts
CH4003 Lecture Notes 11 (Erzeng Xue)

*
 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 Catalysis

*
 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 (both
desired 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 Catalysts
reactant
reaction process
uncatalytic
product
energy
catalytic

*
 It is important to remember that the use of catalyst DOES NOT vary DG & Keq
values 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 DG & 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.
Action of Catalysts

*
 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 Reactions

*
 Industrial applications
Almost 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 Catalysis

*
 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 Catalysis

*
 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 Catalysis

*
 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 manner
 Reactants, 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 Processes

*
 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

*
 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.

*
Example Heterogeneous Catalytic Reaction Process
 The long journey for reactant molecules to
j. travel within gas phase
k. cross gas-liquid phase boundary
l. travel within liquid phase/stagnant layer
m. cross liquid-solid phase boundary
n. reach outer surface of solid
o. diffuse within pore
p. arrive at reaction site
q. be adsorbed on the site and activated
r. 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
pore
porous
solid
liquid phase /
stagnant layer
k
l
mn
o
p q
gas phase
reactant molecule

*
 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 Catalysts
Catalyst
Support

*
 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
Solid Catalysts
 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)
pore
porous
solid
Active site

*
Preparation of catalysts
 Precipitation
To form non-soluble precipitate by desired
reactions at certain pH and temperature
 Adsorption & ion-exchange
Cationic: S-OH+ + C+  SOC+ + H+
Anionic: S-OH- + A-  SA- + OH-
I-exch. S-Na+ + Ni 2+ D S-Ni 2+ + Na+
 Impregnation
Fill the pores of support with a metal salt
solution of sufficient concentration to give
the correct loading.
 Dry mixing
Physically mixed, grind, and fired
Solid Catalysts
precipitate
or deposit
precipitation
filter & wash
the resulting
precipitate
Drying
& firing
precursor
solution
Support
add acid/base
with pH control
Support
Drying
& firing
Pore saturated
pellets
Soln. of metal
precursor
Amount
adsorbed
Concentration
Support
Drying
& firing

*
 Preparation of catalysts
Catalysts 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.
Solid Catalysts
0
25
50
75
100
500 600 700 800 900
Temperature °C
BET
S.A.
m
2
/g
0
40
0 10
Time / hours
BET
S.A.
Activity
Time
Normal use
Induction period
dead

*
 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 bond
number 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 Surface

*
 Adsorption process
Adsorbent 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, q
The 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 q = q = 0~1
number of adsorption sites occupied
number of adsorption sites available

*
 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 a useful technique for determining the surface area, the pore
shape, pore sizes and size distribution of porous solid materials (BET surface area)

*
 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.
adsorbed
T1
T2 >T1
T3 >T2
T4 >T3
T5 >T4
Vol.
adsorbed
Temperature
P1
P2>P1
P3>P2
P4>P3
Pressure
Temperature
V2>V1
V1
V3>V2
V4>V3
Adsorption Isotherm Adsorption Isobar Adsorption Isostere

*
 The Langmuir adsorption isotherm
 Basic assumptions
 surface uniform (DHads does not vary with coverage)
 monolayer adsorption, and
 no interaction between adsorbed molecules and adsorbed molecules immobile
 Case I - single molecule adsorption
when adsorption is in a dynamic equilibrium
A(g) + M(surface site) D AM
the rate of adsorption rads = kads (1-q) P
the rate of desorption rdes = kdes q
at equilibrium rads = rdes  kads (1-q) P = kdes q
rearrange it for q
let  B0 is adsorption coefficient
q  


C
C
B P
B P
s 0
0
1
des
ads
k
k
B 
0
P
B
k
/
k
P
k
/
k
des
ads
des
ads
0
)
(
1
)
(


q
case I
A

*
 The Langmuir adsorption isotherm (cont’d)
 Case II - single molecule adsorbed dissociatively on one site
A-B(g) + M(surface site) D A-M-B
the rate of A-B adsorption rads=kads (1-qA )(1-qB)PAB=kads (1-q )2PAB
the rate of A-B desorption rdes=kdesqAqB =kdesq2
at equilibrium rads = rdes  kads (1-q )2PAB= kdesq2
rearrange it for q
Let. 
case II
A B
B
A
q=qA=qB
1/2
0
1/2
0
)
(
1
)
(
AB
AB
s
P
B
P
B
C
C




q
des
ads
k
k
B 
0
)
(
1
)
(
AB
des
ads
AB
des
ads
P
k
/
k
P
k
/
k


q

*
 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- qA- qB) PA
the rate of B adsorption rads,B = kads,B (1- qA- qB) PB
the rate of A desorption rdes,A = kdes,A qA
the rate of B desorption rdes,B = kdes,B qB
at equilibrium rads ,A = rdes ,A and  rads ,B = rdes ,B
 kads,A(1-qA-qB)PA=kdes,AqA and kads,B(1-qA-qB)PB=kdes,BqB
rearrange it for q
where are adsorption coefficients of A & B.
B
,
des
B
,
ads
B
,
A
,
des
A
,
ads
A
,
k
k
B
k
k
B 
 0
0 and
B
B
,
A
A
,
B
B
,
B
,
s
B
B
B
,
A
A
,
A
A
,
A
,
s
A
P
B
P
B
P
B
C
C
P
B
P
B
P
B
C
C
0
0
0
0
0
0
1
1 









q
q
case III
A B

*
B
,
des
B
,
ads
B
,
A
,
des
A
,
ads
A
,
k
k
B
k
k
B 
 0
0 and
B
B
,
A
A
,
B
B
,
B
,
s
B
B
B
,
A
A
,
A
A
,
A
,
s
A
P
B
P
B
P
B
C
C
P
B
P
B
P
B
C
C
0
0
0
0
0
0
1
1










q
q
Adsorption
Strong kads>> kdes kads>> kdes
B0>>1 B0>>1
Weak kads<< kdes kads<< kdes
B0<<1 B0<<1
1/2
0
1/2
0
)
(
1
)
(
AB
AB
s
P
B
P
B
C
C




q
des
ads
k
k
B 
0
case II
A B
q  


C
C
B P
B P
s 0
0
1
des
ads
k
k
B 
0
case I
A
1



C
Cs
q 1



C
Cs
q
P
B
C
Cs
0



q 1/2
0 )
( P
B
C
Cs



q
Adsorption
A, B both strong
A strong, B weak
A weak, B weak
B
B
,
A
A
,
B
B
,
B
,
s
B
B
B
,
A
A
,
A
A
,
A
,
s
A
P
B
P
B
P
B
C
C
P
B
P
B
P
B
C
C
0
0
0
0
0
0








q
q
B
B
,
B
,
s
B
A
A
,
A
,
s
A
P
B
C
/
C
P
B
C
/
C
0
0






q
q
A
B
A
,
B
,
B
,
s
B
A
,
s
A
P
P
B
/
B
C
/
C
C
/
C
)
(
1
0
0






q
q
case III
A B

*
 Langmuir adsorption isotherm
case 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 q
 Physisorption of gas molecules on a solid surface can be more than one layer
B
B
,
A
A
,
B
B
,
B
,
s
B
B
B
,
A
A
,
A
A
,
A
,
s
A
P
B
P
B
P
B
C
C
P
B
P
B
P
B
C
C
0
0
0
0
0
0
1
1










q
q
1/2
0
1/2
0
)
(
1
)
(
AB
AB
s
P
B
P
B
C
C




q
q  


C
C
B P
B P
s 0
0
1
large B0 (strong adsorp.)
small B0 (weak adsorp.)
moderate B0
Pressure
Amount
adsorbed
mono-layer
1



C
Cs
q
P
B
C
Cs
0



q
Strong adsorption kads>> kdes
Weak adsorption kads<< kdes

*
 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
amount
adsorbed

*
 Other adsorption isotherms
Many other isotherms are proposed in order to explain the observations
 The Temkin (or Slygin-Frumkin) isotherm
 Assuming the adsorption enthalpy DH decreases linearly with surface coverage
From ads-des equilibrium, ads. rate  des. rate
rads=kads(1-q)P  rdes=kdesq
where Qs is the heat of adsorption. When Qs is a linear function of qi. 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 q =c1ln(c2P), where c1 & c2 are constants
 Valid for some adsorption systems.
1
1 1
1
0
0
P
e
b
P
e
b
P
B
P
B
RT
/
Q
RT
/
Q
s s
s




 q
q
D
H
of
ads
q
Langmuir
Temkin
( 







-





 
 RT
i
RT
/
Q
RT
/
Q
s
exp
P
P
i
RT
dS
P
e
b
P
e
b
dS s
s
1
1
1
0
1
1
1
0 b
1
b
1
ln
(1
[
q
q

*
 The Freundlich isotherm
 assuming logarithmic change of adsorption enthalpy DH with surface coverage
From ads-des equilibrium, ads. rate  des. rate
rads=kads(1-q)P  rdes=kdesq
where Qi is the heat of adsorption which is a function of qi. 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 qi,
the overall coverage
the solution for this integration expression at small q is:
lnq=(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
1
1
0
0
e
e
)]
(1
[
dQ
a
dQ
a
P
e
b
/
P
e
b
N
N
Q/Q
Q/Q
RT
/
Q
RT
/
Q
i
i
i
i
i
q
q
1
1 1
1
0
0
P
e
b
P
e
b
P
B
P
B
RT
/
Q
RT
/
Q
i i
i




 q
q
D
H
of
ads
q
Langmuir
Freundlich
2
1
1
C
/
p
c

q

*
 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 pressure
P0 - saturate vapour pressure of the adsorbed gas at the temperature
P/P0 is called relative pressure
V - volume of adsorbed gas per kg adsorbent
Vm -volume of monolayer adsorbed gas per kg adsorbent
c - constant associated with adsorption heat and condensation heat
Note: 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.
)
(
1
1
1
0
0
0
P
/
P
cV
c
cV
)
P
/
P
(
V
P
/
P
m
m
-


-

*
 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.

*
 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 X
 The 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
)
(
)
(
1
1
1
0
0
0
0
P
/
P
P
/
P
cV
c
cV
)
P
/
P
(
V
P
/
P
m
m

-


-
P P
V P P
/
( / )
0
0
1-
P/P0
P P
V P P
/
( / )
0
0
1-
A A N A
V
V
s m m m
m
T P
   
,
.
6022 1023
Vm - volume of monolayer adsorbed gas molecules calculated from the plot, L
VT,P - molar volume of the adsorbed gas, L/mol
Am - cross-section area of a single gas molecule, m2

*
 Summary of adsorption isotherms
Name Isotherm equation Application Note
Langmuir
Temkin q =c1ln(c2P)
Freundlich
BET
)
(
1
1
1
0
0
0
P
/
P
cV
c
cV
)
P
/
P
(
V
P
/
P
m
m
-


-
q  


C
C
B P
B P
s 0
0
1
2
1
1
C
/
p
c

q
Chemisorption and
physisorption
Chemisorption
Chemisorption and
physisorption
Multilayer physisorption
Useful in analysis of
reaction mechanism
Chemisorption
Easy to fit adsorption
data
Useful in surface area
determination

*
 Langmuir-Hinshelwood mechanism
 This mechanism deals with the surface-catalysed reaction in which
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]=kqAqB
From Langmuir adsorption isotherm (the case III) we know
 We then have
 When both A and B are weakly adsorbed (B0,APA<<1, B0,BPB<<1),
2nd order reaction
 When A is strongly adsorbed (B0,APA>>1) and B weakly adsorbed (B0,BPB<<1 <<B0,APA)
1st order w.r.t. B
Mechanism of Surface Catalysed Reaction













B
B
,
A
A
,
B
B
,
B
B
B
,
A
A
,
A
A
,
A
P
B
P
B
P
B
P
B
P
B
P
B
0
0
0
0
0
0
1
1
q
q
B
B
,
A
A
,
B
A
B
,
A
,
B
B
,
A
A
,
B
B
,
B
B
,
A
A
,
A
A
,
i
P
B
P
B
P
P
B
kB
P
B
P
B
P
B
P
B
P
B
P
B
k
r
0
0
0
0
0
0
0
0
0
0
1
1
1 























B
A
B
A
B
,
A
,
i P
P
'
k
P
P
B
kB
r 
 0
0
B
B
B
,
A
A
,
B
A
B
,
A
,
i P
'
'
k
P
kB
P
B
P
P
B
kB
r 

 0
0
0
0
A B
+ " P

*
 Eley-Rideal mechanism
 This mechanism deals with the surface-catalysed reaction in which
one reactant, A, adsorbs on a surface without dissociation and
other reactant, B, approaches from the gas phase 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]=kqAPB
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
A
A
,
A
A
,
A
P
B
P
B
0
0
1

q
A
A
,
B
A
A
,
B
A
A
,
A
A
,
i
P
B
P
P
kB
P
P
B
P
B
k
r
0
0
0
0
1
1 











B
A
B
A
A
,
i P
P
'
k
P
P
kB
r 
 0
B
A
A
,
B
A
A
,
i kP
P
B
P
P
kB
r 

0
0
A
" P
B
+ B(g)

*
 Mechanism of surface-catalysed reaction with dissociative adsorption
 The mechanism of the surface-catalysed reaction in which one
reactant, AD, dissociatively adsorbs 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]=kqADPB
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
( 
(  2
1
0
2
1
0
1
/
AD
AD
,
/
AD
AD
,
AD
P
B
P
B


q
( 
( 
( 
(  2
1
0
2
1
0
2
1
0
2
1
0
1
1
/
AD
AD
,
B
/
AD
AD
,
B
/
AD
AD
,
/
AD
AD
,
i
P
B
P
P
B
k
P
P
B
P
B
k
r




(  B
/
AD
B
/
AD
AD
,
i P
P
'
k
P
P
B
k
r 2
1
2
1
0 

( 
(  B
/
AD
AD
,
B
/
AD
AD
,
i kP
P
B
P
P
B
k
r 
 2
1
0
2
1
0
+ B(g)
" P
B
A B

*
 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
 …

*
 Bulk and surface
 The composition & structure of a solid in bulk and on surface
can differ due to
 Surface 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 Surface

*
 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
 …

*
 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

*
 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 Solids
b
d
a
dw
dd
Dd
wt
b a
Dwt
d
Cumulative curve Frequency curve

*
 Many reactions proceed via chain reaction
 polymerisation
 explosion
 …
 Elementary reaction steps in chain reactions
1. Initiation step - creation of chain carriers (radicals, ions, neutrons etc, which are capable of
propagating a chain) by vigorous collisions, photon absorption
R R (the dot here signifies the radical carrying unpaired electron)
2. Propagation step - attacking reactant molecules to generate new chain carriers
R + M  R + M
3. Termination step - two chain carriers combining resulting in the end of chain growth
R + 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 matter
R + W  R-W (leading to net reducing of the number of chain carriers)
Chain Reactions - Process
Complex Reactions
E

*
 Rate law of chain reaction
Example: 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]=0
rBr= 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 Law
Complex Reactions
[HBr]
]
[Br
]
][Br
[H
[HBr]
2
3/2
2
2
'
k
k
dt
d


( 
( [HBr]
]
[Br
]
][Br
[H
2
[HBr]
2
3/2
2
2
1/2
b
d
c
a
b
'
k
/
k
k
/
k
k
dt
d



*
 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 law
Initiation: 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 - Polymerisation
Complex Reactions
[I]
]
[M
i
k
x
dt
d



initiator chain-carrier
2
1
2
2
[I]
]
[M
0
]
[M
2
[I]
2
]
[M
/
t
i
t
i
p
i
k
k
x
k
-
k
x
r
r
x
dt
d














-

 


[M]
[I]
2
[M]
i.e.
]
[M][M
[M] 1/2
2
1/
t
i
p
p
p
k
k
x
k
dt
d
k
r
dt
d








-


-

-


 is the yield of Ix to xR

*
 Type II polymerisation - Stepwise polymerisation
A 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 - Polymerisation
Complex Reactions
0
0
2
[A]
1
[A]
[A]
or
[A]
[A][-OH]
[A]
kt
k
k
dt
d


-

-

0
0
0
0
[A]
1
[A]
[A]
[A]
[A]
kt
kt
XB


-


*
 Type I Explosion: Chain-branching explosion
Chain-branching - During propagation step of a chain reaction one attack by a
chain carrier can produce more than one new chain carriers
Chain-branching explosion
When chain-branching occurs the number carriers increases exponentially
the rate of reaction may cascade into explosion
Example: 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 - Explosion
Complex Reactions
Lead to explosion

*
 Type II Explosion: Thermal explosion
A rapid increase of the rate of exothermic reaction with temperature
Strictly 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
DH  T  r  DH  T  r  DH  …
 A combination of chain-branching reaction with heat accumulation can occur
simultaneously
Explosion Reactions
Complex Reactions

*
 Photochemical reaction
The reaction that is initiated by the absorption of light (photons)
 Characterisation of photon absorption - quantum yield
A 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, 
 The number of reactant molecules that react as a result of each photon absorbed is call
overall quantum yield, F
E.g. HI + hv  H + I primary quantum yield  =2 (one H and one I)
H + HI  H2 + I
2I  I2 overall quantum yield F =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. F = 104
The quantum yield of a photochemical reaction depends on the wavelength of light used
Photochemical Reactions
Complex Reactions

*
 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 Reactions
Complex Reactions
508 nm light
254 nm light
CO H2

*
 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 Spectroscopy
Spectroscopy

*
 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, l - the distance between adjacent crests of the wave
wave number, v’, - the number of waves per unit distance v’ =l-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 Radiation
Introductory to Spectroscopy
c
'
v
c
v 

l
c
'
hv
hc
hv
E 


l

*
 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 Radiation

*
 Interaction of electromagnetic radiant with matter
 The wave-length, l, 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
 Absorption
 Reflection
 Transmission
 Scattering
 Refraction
 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

*
 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
0.0
350 400 450
wave length cm-1
intensity

*
 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
350 400 450
wave length cm-1
intensity

*
Spectral properties, applications, and
interactions of electromagnetic radiation
absorption
emission
fluorescence
Magnetically
induced spin
states
Electron
paramagnet
resonance
Infrared
Wave number
v’
cm-1
Wavelength
l
cm
Frequency
v
Hz
Energy
kcal/mol
Electron
vole 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-ray
absorption
emission
Nuclear
Gamma ray
emission
Electronic
(outer shell)
Molecular
rotation
Molecular
vibration
Nuclear magnetic
resonance
Microwave
absorption
UV absorption
IR absorption
Raman
Vac
UV
Vis
Electronic
(inner shell)

*
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 = hn  6.63 x 10-34Js x 4.69x1014 s-1 = 3.11 x 10-19 J
B) 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 = hn = hc/l (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 = hn
92 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
.

*
 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 Spectra
n = 1
n = 2
n = 3,
etc.
energy
DE
ground
state
Excited
state
Energy
n=1
n=2
n=3
n=4
1s
2s
2p
3s
3p
4s
3d
4p
4d
4f

*
 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
Energy
n=1
n=2
n=3
n=4
1s
2s
2p
3s
3p
4s
3d
4p
4d
4f
n = 1
n = 2
n = 3,
etc.
energy
DE

*
 Atomic spectra
The 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
Energy
n=1
n=2
n=3
n=4
1s
2s
2p
3s
3p
4s
3d
4p
4d
4f
n = 1
n = 2
n = 3,
etc.
energy
DE

*
 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 E
T (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 Spectra
Spectroscopy
S0
T1
S2
S1
v1
v2
v3
v4
v1
v2
v3
v4
v1
v2
v3
v4
v1
v2
v3
v4

*
 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, T1
etc.), 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.
Molecular Spectra
Spectroscopy
1
0
2
2 v
,
v
, S
S E
E
'
hcv -

v1
v2
v3
v4
S0
T1
S2
S1
v1
v2
v3
v4
v1
v2
v3
v4
v1
v2
v3
v4
absorption
A
A

*
 Energy change of excited molecules
An excited molecules can lose its excess
energy 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 transition
within 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
Molecular Spectra
Spectroscopy
S0
T1
S2
S1
v1
v2
v3
v4
v1
v2
v3
v4
v1
v2
v3
v4
v1
v2
v3
v4
Inter- system
crossing
Internal
transition
B
B
E1
E2
C
F
A
B
Fluorescence
D
Fluorescence
Jablonsky diagram

*
 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. ms)
 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
Molecular Spectra
Spectroscopy
S0
S2
v1
v2
v3
v4
v1
v2
v3
v4
B
A
phosphor-
enscence
D
Fluore-
scence
T1
v1
v2
v3
v4
F

*
 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 Spectra
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

*
 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 Spectrophotometry
Spectroscopy Application
Incident light, I0
(UV or visible)
Emergent light, I
C
b
ultraviolet visible infra-red
200 - 400 400 - 800 800 - 15
nm nm nm nm nm mm

*
 Theory of light absorption
Quantitative 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 them
Thus
Incident light
I0
Emergent light
I
C
b
I
'
k
dx
Ncs
dI
-

2
I0
dx
b
x
s
s
I
number of
molecules
N-Avogadro number
light
absorbed
fraction
of light
acdx
dx
Ncs
'
k
I
dI
-

-

 2
acb
I
I
dx
ac
I
dI b
b
I
I
b
-


-

 
 0
0
ln
0
A
abc
I
I


 0
log
Absorbance

*
 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 e, k) A/(bc) [bc]-1
absorbance index
Molar absorptivity Molar extinction coeff., a A/(bc)
molar absorbancy index [or aM AM/(bc’) ] M-molar weight
c’ -gram/L

*
 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 them
180-195nm sulphuric acid (96%), water, acetonitrile
200-210nm cyclopentane, n-hexane, glycerol, methanol, ethanol
210-220nm n-butyl alcohol, isopropyl alcohol, cyclohexane, ethyl ether
245-260nm chloroform, ethyl acetate, methyl formate
265-275nm carbon tetrachloride, dimethyl sulphoxide/formamide, acetic acid
280-290nm benzene, toluene, m-xylene
300-400nm 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

*
 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 s & p molecular
orbitals; non-bonding in n
 Antibonding orbitals correspond to the
bonding ones
 e-’s transition can occur between various
states; in general, the energy of e-’s
transition increases in the following order:
(np*) < (ns*) < (p p*) < (s s*)
 Molecules which can be analysed by UV-visible absorption
 Chromophores
functional groups each of which absorbs a characteristic UV or visible radiation.
s *
p*
n
p
Antibonding
Antibonding
non-bonding
Bonding
Energy
s
s*
pp*
n
s*
n
p*
s

*
 The functional groups & the wavelength of UV-visible absorption
Group Example lmax, nm Group Example lmax, nm
C=C 1-octane 180 arene benzene 260
naphthalene 280
C=O methanol 290 phenenthrene 350
propanone 280 anthracene 375
ethanoic acid 210 pentacene 575
ethyl ethanoate 210
ethanamide 220 conjugated 1,3-butadiene 220
1,3,5-hexatriene 250
C-X methanol 180 2-propenal 320
trimethylamine 200 b-carotene (11 C=C) 480
chloromethane 170
bromomethane 210 each additional C=C +30
iodomethane 260

*
 Instrumentation
UV visible
Light source Hydrogen discharge lamp Tungsten-halogen lamp
Cuvette QUARTZ glass
Detectors photomultiplier photomultiplier

*
 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

*
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.

*
IR-Spectroscopy
 IR region
 The part of electromagnetic radiation between the visible and microwave regions 0.8
mm to 50 mm (12,500 cm-1-200 cm-1).
 Most interested region in Infrared Spectroscopy is between 2.5mm-25 mm
(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

*
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 molecule
 Absorption 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

*
IR Spectrum
Region freq. (cm-1) what is found there??
XH region 3800 - 2600 OH, NH, CH (sp, sp2, sp3) stretches
triple bond 2400 - 2000 CC, CN, C=C=C stretches
double bond 1900 - 1500 C=O, C=N, C=C stretches
fingerprint 1500 - 400 many types of absorptions
1400 - 900 C-O, C-N stretches
1500 - 1300 CH in-plane bends, NH bends
1000 - 650 CH out-of-plane (oop) bends
Principal Correlation Chart
O-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
Prism
or
Diffraction
Grating
Slit
Photometer
IR Source Recorder
Split
Beam Air
Lenz Sample

*
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)

*
Atomic Absorption Spectrometer (AA)
Source
Sample
P
P0
Chopper
Wavelength
Selector
Detector
Signal Processor
Readout
Type Method of Atomization Radiation
Source
atomic (flame) sample solution aspirated Hollow cathode
into a flame lamp (HCL)
atomic (nonflame) sample solution HCL
evaporated & ignited
x-ray absorption none required x-ray tube

*
Atomic Emission Spectrometer (AES)
Source
Sample
P
Wavelength
Selector
Detector
Signal Processor
Readout
Source
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

*
Atomic Fluorescence Spectrometer (AFS)
Source
Sample
P
P0
90o
Wavelength
Selector
Detector
Signal Processor
Readout
Source
atomic (flame) sample solution aspirated
into a flame sample
atomic (nonflame) sample solution sample
evaporated & ignited
x-ray fluorescence none required sample

*
 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 - Characteristics
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 constructed
by Maiman in 1960, using ruby (Al2O3
doped with a dilute concentration of Cr+3)
as the lasing medium and a fast
discharge flash-lamp to provide the pump
energy.

*
 When excited atoms/molecules/ions undergo de-excitation (from excited state
to ground state), light is emitted
 Types of light emission
Laser - Stimulated Emission
E4
E3
E2
E1
E0
ground
state
excited
state
Ep1=(E1 – E0) = hv1
Ep2=(E2 – E0) = hv2
Ep4=(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)

*
 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 have
the same frequency (or energy) and the same
phase (coherent) as the stimulating photon (P1).
Laser - Stimulated Emission
E4
E3
E2
E1
E0
Ep1=(E2–E0)=hv2
Ep2=(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….

*
 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 state
PI 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 emission
In 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 atoms
This 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 & Conditions

*
Laser - Functional Elements
Energy pumping
mechanism
Energy
input
Lasing medium
High
reflectance
mirror
Partially
transmitting
mirror
Output
coupler
Feedback mechanism

*
Laser Action
Lasing medium
at ground state
Population
inversion
Start of stimulated
emission
Stimulated emission
building up
Laser in
full operation
Pump energy
Pump energy
Pump energy
Pump energy

*
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

*
Laser - Applications
 Laser can be applied in many areas
 Commerce
Compact disk, laser printer, copiers, optical disk drives, bar code scanner, optical
communications, laser shows, holograms, laser pointers
 Industry
Measurements (range, distance), alignment, material processing (cutting, drilling,
welding, annealing, photolithography, etc.), non-destructive testing, sealing
 Medicine
Surgery (eyes, dentistry, dermatology, general), diagnostics, ophthalmology,
oncology
 Research
Spectroscopy, nuclear fusion, atom cooling, interferometry, photochemistry, study
of fast processes
 Military
Ranging, navigation, simulation, weapons, guidance, blinding

Catalysis III C presentation for measurement

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Catalysis III C presentation for measurement