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)
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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
CH4003 Lecture Notes 11 (Erzeng Xue)
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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 CatalystsCatalysis & Catalysts
reactant
reaction process
uncatalytic
product
ener
gy
catalytic
CH4003 Lecture Notes 11 (Erzeng Xue)
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It is important to remember that the use of catalyst DOES NOT vary G & 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 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
CH4003 Lecture Notes 11 (Erzeng Xue)
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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
CH4003 Lecture Notes 11 (Erzeng Xue)
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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
CH4003 Lecture Notes 11 (Erzeng Xue)
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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
CH4003 Lecture Notes 11 (Erzeng Xue)
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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
CH4003 Lecture Notes 11 (Erzeng Xue)
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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 ProcessesCatalysis & Catalysts
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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
Catalytic Reaction ProcessesCatalysis & Catalysts
CH4003 Lecture Notes 12 (Erzeng Xue)
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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.
Catalytic Reaction ProcessesCatalysis & Catalysts
CH4003 Lecture Notes 12 (Erzeng Xue)
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Example Heterogeneous Catalytic Reaction Process The long journey for reactant molecules to
. travel within gas phase
. cross gas-liquid phase boundary
. travel within liquid phase/stagnant layer
. cross liquid-solid phase boundary
. reach outer surface of solid
. diffuse within pore
. arrive at reaction site
. be adsorbed on the site and activated. 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
gas phase
poreporous solid
liquid phase /stagnant layer
gas phasereactant molecule
Catalysis & Catalysts
CH4003 Lecture Notes 12 (Erzeng Xue)
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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
CatalystAct
ive
phas
e
Support
Prom
oter
CH4003 Lecture Notes 12 (Erzeng Xue)
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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 CatalystsCatalysis & 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)
poreporous solid
Active site
CH4003 Lecture Notes 12 (Erzeng Xue)
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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+ 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 CatalystsCatalysis & Catalysts
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 metal precursor
Amount
adsorb
ed
Concentration
Support
Drying & firing
CH4003 Lecture Notes 12 (Erzeng Xue)
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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.
Solid CatalystsCatalysis & Catalysts
0
25
50
75
100
500 600 700 800 900
Temperature °C
BE
T S
.A.
m2/g
0
40
0 10Time / hours
BE
T S
.A.
Act
ivit
y
Time
Normal use
Induction period
dead
CH4003 Lecture Notes 12 (Erzeng Xue)
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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 SurfaceCatalysis & Catalysts
CH4003 Lecture Notes 13 (Erzeng Xue)
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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,
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.
Adsorption On Solid SurfaceCatalysis & Catalysts
define = = 0~1number of adsorption sites occupied
number of adsorption sites available
CH4003 Lecture Notes 13 (Erzeng Xue)
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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)
Adsorption On Solid SurfaceCatalysis & Catalysts
CH4003 Lecture Notes 13 (Erzeng Xue)
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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
Catalysis & Catalysts
Pressure
Vol
. ad
sorb
ed T1
T2 >T1
T3 >T2
T4 >T3
T5 >T4
Vol
. ad
sorb
ed
Temperature
P1
P2>P1
P3>P2
P4>P3
Pre
ssur
e
Temperature
V2>V1
V1
V3>V2
V4>V3
Adsorption Isotherm Adsorption Isobar Adsorption Isostere
CH4003 Lecture Notes 13 (Erzeng Xue)
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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 adsorption
when adsorption is in a dynamic equilibrium
A(g) + M(surface site) AM
the rate of adsorption rads = kads (1-) P
the rate of desorption rdes = kdes
at equilibrium rads = rdes kads (1-) P = kdes
rearrange it for
let B0 is adsorption coefficient
Adsorption On Solid SurfaceCatalysis & Catalysts
C
C
B P
B Ps 0
01des
ads
k
kB 0
PBk/k
Pk/k
desads
desads
0)(1
)(
case I
A
CH4003 Lecture Notes 13 (Erzeng Xue)
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Adsorption On Solid Surface The Langmuir adsorption isotherm (cont’d)
Case II - single molecule adsorbed dissociatively on one site
A-B(g) + M(surface site) A-M-B
the rate of A-B adsorption rads=kads (1))PAB=kads (1)2PAB
the rate of A-B desorption rdes=kdes=kdes2
at equilibrium rads = rdes kads (1)2PAB= kdes2
rearrange it for
Let.
Catalysis & Catalysts
case II
A B
BA
==
1/20
1/20
)(1
)(
AB
ABs
PB
PB
C
C
des
ads
k
kB 0
)(1
)(
ABdesads
ABdesads
Pk/k
Pk/k
CH4003 Lecture Notes 13 (Erzeng Xue)
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The Langmuir adsorption isotherm (cont’d) Case III - two molecules adsorbed on two sites
A(g) + B(g) + 2M(surface site) A-M + B-M
the rate of A adsorption rads,A = kads,A (1) PA
the rate of B adsorption rads,B = kads,B (1) PB
the rate of A desorption rdes,A = kdes,A
the rate of B desorption rdes,B = kdes,B
at equilibrium rads ,A = rdes ,A and rads ,B = rdes ,B
kads,A(1)PA=kdes,A and kads,B(1)PB=kdes,B
rearrange it for
where are adsorption coefficients of A & B.
Adsorption On Solid SurfaceCatalysis & Catalysts
B,des
B,adsB,
A,des
A,adsA, k
kB
k
kB 00 and
BB,AA,
BB,B,sB
BB,AA,
AA,A,sA PBPB
PB
C
C
PBPB
PB
C
C
00
0
00
0
1
1
case III
A B
CH4003 Lecture Notes 13 (Erzeng Xue)
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The Langmuir adsorption isotherm (cont’d)
Adsorption On Solid SurfaceCatalysis & Catalysts
B,des
B,adsB,
A,des
A,adsA, k
kB
k
kB 00 and
BB,AA,
BB,B,sB
BB,AA,
AA,A,sA
PBPB
PB
C
CPBPB
PB
C
C
00
0
00
0
1
1
Adsorption
Strong kads>> kdes kads>> kdes
B0>>1 B0>>1
Weak kads<< kdes kads<< kdes
B0<<1 B0<<1
1/20
1/20
)(1
)(
AB
ABs
PB
PB
C
C
des
ads
k
kB 0
case II
A B
C
C
B P
B Ps 0
01
des
ads
k
kB 0
case I
A
1C
Cs 1C
Cs
PBC
Cs0
1/20 )( PB
C
Cs
Adsorption
A, B both strong
A strong, B weak
A weak, B weak
BB,AA,
BB,B,sB
BB,AA,
AA,A,sA
PBPB
PB
C
CPBPB
PB
C
C
00
0
00
0
BB,B,sB
AA,A,sA
PBC/CPBC/C
0
0
A
BA,B,B,sB
A,sA
P
PB/BC/C
C/C
)(
1
00
case III
A B
CH4003 Lecture Notes 13 (Erzeng Xue)
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Langmuir adsorption isotherm
case I
case II
Case III
Adsorption On Solid SurfaceCatalysis & Catalysts
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 Physisorption of gas molecules on a solid surface can be more than one layer
BB,AA,
BB,B,sB
BB,AA,
AA,A,sA
PBPB
PB
C
CPBPB
PB
C
C
00
0
00
0
1
1
1/20
1/20
)(1
)(
AB
ABs
PB
PB
C
C
C
C
B P
B Ps 0
01
large B0 (strong adsorp.)
small B0 (weak adsorp.)
moderate B0
Pressure
Am
ou
nt
ad
sorb
ed
mono-layer
1C
Cs
PBC
Cs0
Strong adsorption kads>> kdes
Weak adsorption kads<< kdes
CH4003 Lecture Notes 14 (Erzeng Xue)
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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
Adsorption On Solid SurfaceCatalysis & Catalysts
I
II
III
IV
V
relative pres. P/P0
1.0
am
ou
nt
ad
sorb
ed
CH4003 Lecture Notes 14 (Erzeng Xue)
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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-)P rdes=kdes
where Qs is the heat of adsorption. When Qs is a linear function of i. 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 =c1ln(c2P), where c1 & c2 are constants
Valid for some adsorption systems.
Adsorption On Solid SurfaceCatalysis & Catalysts
1
1 1
1
0
0
Peb
Peb
PB
PBRT/Q
RT/Q
s s
s
H
of
ad
s
Langmuir
Temkin
RTiRT/Q
RT/Q
s expP
P
i
RTdS
Peb
PebdS
s
s
1
11
01
11
0 b1
b1ln
(1
[
CH4003 Lecture Notes 14 (Erzeng Xue)
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The Freundlich isotherm assuming logarithmic change of adsorption enthalpy H with surface coverage
From ads-des equilibrium, ads. rate des. rate
rads=kads(1-)P rdes=kdes
where Qi is the heat of adsorption which is a function of i. 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 i,
the overall coverage
the solution for this integration expression at small is:
ln=(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.
Adsorption On Solid SurfaceCatalysis & Catalysts
0
0 11
0
0
e
e)](1[
dQa
dQaPeb/Peb
N
N
Q/Q
Q/QRT/QRT/Q
ii
iii
1
1 1
1
0
0
Peb
Peb
PB
PBRT/Q
RT/Q
i i
i
H
of
ad
s
Langmuir
Freundlich
211
C/pc
CH4003 Lecture Notes 14 (Erzeng Xue)
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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.
Adsorption On Solid SurfaceCatalysis & Catalysts
)(11
1 00
0 P/PcV
c
cV)P/P(V
P/P
mm
CH4003 Lecture Notes 14 (Erzeng Xue)
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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.
Adsorption On Solid SurfaceCatalysis & Catalysts
CH4003 Lecture Notes 14 (Erzeng Xue)
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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
Adsorption On Solid SurfaceCatalysis & Catalysts
)( )(11
1 000
0 P/PP/PcV
c
cV)P/P(V
P/P
mm
P P
V P P
/
( / )0
01
P/P0
P P
V P P
/
( / )0
01
A A N AV
Vs m m mm
T P
,
.6 022 1023Vm - 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
CH4003 Lecture Notes 14 (Erzeng Xue)
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Summary of adsorption isotherms
Name Isotherm equation Application Note
Langmuir
Temkin =c1ln(c2P)
Freundlich
BET
Adsorption On Solid SurfaceCatalysis & Catalysts
)(11
1 00
0 P/PcV
c
cV)P/P(V
P/P
mm
C
C
B P
B Ps 0
01
211
C/pc
Chemisorption andphysisorption
Chemisorption
Chemisorption andphysisorption
Multilayer physisorption
Useful in analysis of reaction mechanism
Chemisorption
Easy to fit adsorption data
Useful in surface area determination
CH4003 Lecture Notes 14 (Erzeng Xue)
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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) A(ads) + B(ads) P (the desorption of P is not r.d.s.)
The rate of reaction ri=k[A][B]=kAB
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 ReactionCatalysis & Catalysts
BB,AA,
BB,B
BB,AA,
AA,A
PBPB
PBPBPB
PB
00
0
00
0
1
1
BB,AA,
BAB,A,
BB,AA,
BB,
BB,AA,
AA,i PBPB
PPBkB
PBPB
PB
PBPB
PBkr
00
00
00
0
00
0
111
BABAB,A,i PP'kPPBkBr 00
BBB,AA,
BAB,A,i P''kPkB
PB
PPBkBr 0
0
00
A B+ P
CH4003 Lecture Notes 15 (Erzeng Xue)
34
Eley-Rideal mechanism This mechanism deals with the surface-catalysed reaction in which
one reactant, A, adsorbs on a surface without dissociation andother reactant, B, approaches from the gas phase to react with A
A(g) A(ads) P (the desorption of P is not r.d.s.)
The rate of reaction ri=k[A][B]=kAPB
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
Mechanism of Surface Catalysed ReactionCatalysis & Catalysts
AA,
AA,A PB
PB
0
0
1
AA,
BAA,B
AA,
AA,i PB
PPkBP
PB
PBkr
0
0
0
0
11
BABAA,i PP'kPPkBr 0
BAA,
BAA,i kP
PB
PPkBr
0
0
A P
B
+ B(g)
CH4003 Lecture Notes 15 (Erzeng Xue)
35
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) A(ads) + D(ads) P
(the des. of P is not r.d.s.)
The rate of reaction ri=k[A][B]=kADPB
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
Mechanism of Surface Catalysed ReactionCatalysis & Catalysts
21
0
210
1 /ADAD,
/ADAD,
ADPB
PB
21
0
210
210
210
11 /ADAD,
B/
ADAD,B/
ADAD,
/ADAD,
iPB
PPBkP
PB
PBkr
B/
ADB/
ADAD,i PP'kPPBkr 21210
B/
ADAD,
B/
ADAD,i kP
PB
PPBkr 21
0
210
+ B(g) P
B
A B
CH4003 Lecture Notes 15 (Erzeng Xue)
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
…
Mechanism of Surface Catalysed ReactionCatalysis & Catalysts
CH4003 Lecture Notes 15 (Erzeng Xue)
37
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 SurfaceCatalysis & Catalysts
CH4003 Lecture Notes 15 (Erzeng Xue)
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 …
Solids and Solid SurfaceCatalysis & Catalysts
CH4003 Lecture Notes 15 (Erzeng Xue)
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.
Solids and Solid SurfaceCatalysis & Catalysts
Terrace Step
CH4003 Lecture Notes 15 (Erzeng Xue)
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 awt
d
Cumulative curve Frequency curve
CH4003 Lecture Notes 15 (Erzeng Xue)
41
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 Rž (the dot here signifies the radical carrying unpaired electron)
2. Propagation step - attacking reactant molecules to generate new chain carriers
Rž + M R + Mž
3. Termination step - two chain carriers combining resulting in the end of chain growth
Rž + žM R-M
There are also other reactions occur during chain reaction:
Retardation step - chain carriers attacking product molecules breaking them to reactant Rž + R-M R + Mž(leading to net reducing of the product
formation rate)
Inhibition step - chain carriers being destroyed by reacting with wall or foreign matter Rž + W R-W(leading to net reducing of the number of
chain carriers)
Chain Reactions - ProcessComplex Reactions
E
CH4003 Lecture Notes 16 (Erzeng Xue)
42
Rate law of chain reaction
Example: overall reaction H2(g) + Br2(g) 2HBr(g) observed:
elem step rate law
a. Initiation: Br2 2Brž ra=ka[Br2]
b. Propagation: Brž + H2 HBr + Hžrb=kb[Br][H2]
Hž + Br2 HBr + Bržr’b=k’b[H][Br2]
c. Termination: Brž + žBr Br2rc=kc[Br][Br]=kc[Br]2
Hž + žH H2 (practically less important therefore neglected)
Hž + žBr HBr (practically less important therefore neglected)
d. Retardn (obsvd.) Hž + HBr H2 + Bržrd=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 LawComplex Reactions
[HBr]][Br
]][Br[H[HBr]
2
3/222
'k
k
dt
d
[HBr]][Br
]][Br[H2[HBr]
2
3/222
1/2
bd
cab
'k/k
k/kk
dt
d
CH4003 Lecture Notes 16 (Erzeng Xue)
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 xRž (usually r.d.s.) ri=ki[I]
Rž + M žM1 (fast)
Propagation: M + žM1 ž(MM1) žM2 (fast)
M + žM2 ž(MM2) žM3 (fast)
… … … … … … … … …M + žMn-1 ž(MMn-1) žMn rp=kp[M][žM] (ri is the r.d.s.)
Termination: žMn + žMm (MnMm) Mm+n rt=kt[žM]2
Apply s.s.a. to [žM] formed
The rate of propagation or the rate of M consumption or the rate of chain growth
Chain Reactions - PolymerisationComplex Reactions
[I] ][M
ikxdt
d
initiator chain-carrier
21
2
2
[I] ][M 0][M2[I] 2
][M/
t
itipi k
kxk-kxrrx
dt
d
[M][I]2
[M] i.e. ][M][M
[M] 1/2
21/
t
ippp k
kxk
dt
dkr
dt
d
is the yield of Ix to xR
CH4003 Lecture Notes 16 (Erzeng Xue)
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]
ktkk
dt
d
0
0
0
0
[A]1
[A]
[A]
[A][A]
kt
ktX B
CH4003 Lecture Notes 16 (Erzeng Xue)
45
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 žO2H + Hž
Propagation: H2 + žO2H žOH + H2O (non-branching)
H2 + žOH žH + H2O (non-branching)
O2 + žH žOž + žOH (branching)
žOž + H2 žOH + žH (branching)
Chain Reactions - ExplosionComplex Reactions
Lead to explosion
CH4003 Lecture Notes 16 (Erzeng Xue)
46
Type II Explosion: Thermal explosionA 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
H T r H T r H …
A combination of chain-branching reaction with heat accumulation can occur simultaneously
Explosion ReactionsComplex Reactions
CH4003 Lecture Notes 16 (Erzeng Xue)
47
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,
E.g. HI + hv H + I primary quantum yield =2 (one H and one I)
H + HI H2 + I
2I I2 overall quantum yield =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. = 104
The quantum yield of a photochemical reaction depends on the wavelength of light used
Photochemical ReactionsComplex Reactions
CH4003 Lecture Notes 16 (Erzeng Xue)
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* C6H537Cl + 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
CH4003 Lecture Notes 16 (Erzeng Xue)
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
CH4003 Lecture Notes 17 (Erzeng Xue)
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 ms-1 (in vacuum) wave-length, - the distance between adjacent crests of the wave
wave number, v’, - the number of waves per unit distance v’ =-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=6.626x10-34Js
Electromagnetic RadiationIntroductory to Spectroscopy
c'vc
v
c'hvhc
hvE
CH4003 Lecture Notes 17 (Erzeng Xue)
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
CH4003 Lecture Notes 17 (Erzeng Xue)
52
Interaction of electromagnetic radiant with matter The wave-length, , 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
Introductory to Spectroscopy
CH4003 Lecture Notes 17 (Erzeng Xue)
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
nsi
ty
Introductory to Spectroscopy
CH4003 Lecture Notes 17 (Erzeng Xue)
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
0.0350 400 450
wave length cm-1
inte
nsi
ty
Introductory to Spectroscopy
CH4003 Lecture Notes 17 (Erzeng Xue)
55
Spectral properties, applications, and interactions of electromagnetic radiation
absorptionemissionfluorescence
Magneticallyinduced spinstates
Electronparamagnetresonance
Infrared
Wave numberv’
cm-1
Wavelength
cm
Frequencyv
Hz
Energy
kcal/molElectronvole 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 ray
emission
Electronic(outer shell)
Molecularrotation
Molecularvibration
Nuclear magneticresonance
Microwaveabsorption
UV absorption
IR absorptionRaman
VacUVVis
Electronic(inner shell)
Introductory to Spectroscopy
CH4003 Lecture Notes 17 (Erzeng Xue)
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 = h6.63 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 = h = hc/(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 = h
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
.
Introductory to Spectroscopy
CH4003 Lecture Notes 17 (Erzeng Xue)
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
En
erg
y
n=1
n=2
n=3
n=4
1s2s2p
3s3p
4s3d4p
4d4f
CH4003 Lecture Notes 18 (Erzeng Xue)
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
En
erg
y
n=1
n=2
n=3
n=4
1s2s2p
3s3p
4s3d4p
4d4f
n = 1
n = 2
n = 3, etc.
energy
E
Introductory to Spectroscopy
CH4003 Lecture Notes 18 (Erzeng Xue)
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
En
erg
y
n=1
n=2
n=3
n=4
1s2s2p
3s3p
4s3d4p
4d4f
n = 1
n = 2
n = 3, etc.
energy
E
Introductory to Spectroscopy
CH4003 Lecture Notes 18 (Erzeng Xue)
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
T (Triplet) - two e-’s spin parallel
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
v2
v3
v4
v1
v2
v3
v4
v1
v2
v3
v4
v1
v2
v3
v4
CH4003 Lecture Notes 18 (Erzeng Xue)
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, 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 SpectraSpectroscopy
1022 v,v, SS EE'hcv
v1
v2
v3
v4
S0
T1
S2
S1
v1
v2
v3
v4
v1
v2
v3
v4
v1
v2
v3
v4
absorptionA
A
CH4003 Lecture Notes 18 (Erzeng Xue)
62
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 SpectraSpectroscopy
S0
T1
S2
S1
v1
v2
v3
v4
v1
v2
v3
v4
v1
v2
v3
v4
v1
v2
v3
v4
Inter- systemcrossing
Internaltransition
B
B
E1
E2
C
F
A
B
Fluorescence
D
Fluorescence
Jablonsky diagram
CH4003 Lecture Notes 18 (Erzeng Xue)
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. s)
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 SpectraSpectroscopy
S0
S2
v1
v2
v3
v4
v1
v2
v3
v4
B
Aphosphor-enscence
D
Fluore-scence
T1
v1
v2
v3
v4
F
CH4003 Lecture Notes 18 (Erzeng Xue)
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
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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 m
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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
UV & Visible SpectrophotometrySpectroscopy Application
Incident light I0
Emergent lightI
C
b
I'kdxNcs
dI
2I0
dx
bx
s
s
I
number of moleculesN-Avogadro number
light absorbed
fraction of light
acdxdxNcs'kI
dI 2
acbI
Idxac
I
dI bbI
I
b 0
0ln
0
AabcI
I 0log
Absorbance
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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 , 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
UV & Visible SpectrophotometrySpectroscopy Application
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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
UV & Visible SpectrophotometrySpectroscopy Application
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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 & 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*) < (n*) < ( *) < ( *)
Molecules which can be analysed by UV-visible absorption Chromophores
functional groups each of which absorbs a characteristic UV or visible radiation.
UV & Visible SpectrophotometrySpectroscopy Application
*
*
n
Antibonding Antibonding
non-bonding
Bonding
Energy
* * n * n *
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The functional groups & the wavelength of UV-visible absorption
Group Example max, nm Group Example max, 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 -carotene (11 C=C) 480
chloromethane 170
bromomethane 210 each additional C=C +30
iodomethane 260
UV & Visible SpectrophotometrySpectroscopy Application
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Instrumentation
UV visible
Light source Hydrogen discharge lamp Tungsten-halogen lamp
Cuvette QUARTZ glass
Detectors photomultiplier photomultiplier
UV & Visible SpectrophotometrySpectroscopy Application
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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.
Spectroscopy Application
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IR-Spectroscopy IR region
The part of electromagnetic radiation between the visible and microwave regions 0.8 m to 50 m (12,500 cm-1-200 cm-1).
Most interested region in Infrared Spectroscopy is between 2.5m-25 m
(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
Spectroscopy Application
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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 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
Spectroscopy Application
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IR Spectrum
Region freq. (cm-1) what is found there??
XH region 3800 - 2600 OH, NH, CH (sp, sp2, sp3) stretchestriple bond 2400 - 2000 CC, CN, 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
Spectroscopy Application
Principal Correlation ChartOH 3600 cm-1
NH 3500 cm-1
CH 3000 cm-1
CN 2250 cm-1
CC 2150 cm-1
C=O 1715 cm-1
C=C 1650 cm-1
CO 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
Spectroscopy Application
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
DetectorSignal 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
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Atomic Emission Spectrometer (AES)Spectroscopy Application
Source
Sample
P Wavelength Selector
DetectorSignal Processor
Readout
Type Method of Atomization Radiation 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
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Atomic Fluorescence Spectrometer (AFS)
Spectroscopy Application
Source
Sample
P P0
Chopper
90o
Wavelength Selector
DetectorSignal Processor
Readout
Type Method of Atomization Radiation Source
atomic (flame) sample solution aspirated
into a flame sample
atomic (nonflame) sample solution sample
evaporated & 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 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.
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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
E4
E3
E2
E1
E0
ground state
excitedstate
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)
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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 have the same frequency (or energy) and the same phase (coherent) as the stimulating photon (P1).
Laser - Stimulated EmissionSpectroscopy Application
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….
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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 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 & ConditionsSpectroscopy Application
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Laser - Functional Elements
Spectroscopy Application
Energy pumping mechanism
Energy input
Lasing medium
Highreflectance
mirror
Partially transmitting
mirror
Outputcoupler
Feedback mechanism
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Laser ActionSpectroscopy Application
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
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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.
Spectroscopy Application
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
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
Spectroscopy Application
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