Transcript Document
CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis & Catalysts
Catalysis & Catalysts
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 CO 2 + H 2 O=HC + O 2 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 1
CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis & Catalysts
What is Catalysis
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 2
CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis & Catalysts
Action of Catalysts
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)
uncatalytic catalytic
reactant reaction process product Reactions proceed under less demanding conditions Allow reactions occur under a milder conditions, e.g. at lower temperatures for those heat sensitive materials 3
CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis & Catalysts
Action of Catalysts
It is important to remember that the use of catalyst DOES NOT vary D
G
&
K
eq
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 D
G
&
K eq
, 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 CH 4 (g) + CO 2 (g) = 2CO(g) + 2H 2 (g) D
G
° 373 =151 kJ/mol (100 °C) D
G
° 973 =-16 kJ/mol (700 °C) At 100 °C, D
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, D
G
° 373 = -16 kJ/mol < 0. The thermodynamic driving force is there. However, simply putting CH 4 and CO 2 together in a reactor does not mean they will react. Without a proper catalyst heating the mixture in reactor results no conversion of CH 4 and CO 2 at all. When Pt/ZrO 2 or Ni/Al 2 O 3 is present in the reactor at the same temperature, equilibrium conversion can be achieved (<100%).
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CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis & Catalysts
Types of Catalysts & Catalytic Reactions
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.
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CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis & Catalysts
Applications of Catalysis
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 … 6
CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis & Catalysts
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.
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CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis & Catalysts
Research in 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 … 8
CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis & Catalysts
Catalytic Reaction Processes
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, m 3 /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.
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CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis & Catalysts
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
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CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis & Catalysts
Catalytic Reaction Processes
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 NO 2 in the presence of SO 2 )
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.
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CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis & Catalysts
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 k l j porous solid mn o gas phase reactant molecule gas phase liquid phase / stagnant layer pore p q r 12
CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis & Catalysts
Solid Catalysts
Catalyst composition
Active phase Where the reaction occurs (mostly metal/metal oxide) Promoter Textual promoter (e.g. Al - Fe for NH 3 production) Electric or Structural modifier Poison resistant promoters
Catalyst
Support Support / carrier Increase mechanical strength Increase surface area (98% surface area is supplied within the porous structure) may or may not be catalytically active 13
CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis & Catalysts
Solid Catalysts
Some common solid support / carrier materials
Alumina Inexpensive Surface area: 1 ~ 700 m 2 /g Acidic Silica Inexpensive Surface area: 100 ~ 800 m 2 /g Acidic Zeolite mixture of alumina and silica, often exchanged metal ion present shape selective acidic Other supports Active carbon (S.A. up to 1000 m 2 /g) Titania (S.A. 10 ~ 50 m 2 /g) Zirconia (S.A. 10 ~ 100 m 2 /g) Magnesia (S.A. 10 m 2 /g) Lanthana (S.A. 10 m 2 /g) porous solid pore
Active site
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CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis & Catalysts
Solid Catalysts
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 -
Support precursor solution Support add acid/base with pH control precipitate or deposit precipitation filter & wash the resulting precipitate Drying & firing Drying & firing
I-exch. S-Na + + Ni 2+
D
S-Ni 2+ + Na + Concentration
Support Impregnation Fill the pores of support with a metal salt solution of sufficient concentration to give the correct loading.
Drying & firing Dry mixing Physically mixed, grind, and fired Sol n . of metal precursor Pore saturated pellets 15
CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis & Catalysts
Solid Catalysts
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.
40 100 Commonly used Pre-treatments 75 50
Reduction
25
if elemental metal is the active phase Sulphidation
0 500 600 700 800 Temperature °C 900 0 0 Time / hours
if a metal sulphide is the active phase
10
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
Induction period Normal use
Typical catalyst life span
dead
Can be many years or a few mins.
Time
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CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis & Catalysts
Adsorption On Solid Surface
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)
force number of adsorbed layers adsorption heat selectivity temperature to occur
Physisorption
van de Waal multi low (10-40 kJ/mol) low low
Chemisorption
chemcal bond only one layer high ( > 40 kJ/mol) high high 17
CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis & Catalysts
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 define q
=
number of adsorption sites occupied number of adsorption sites available
Adsorption heat
q = 0~1 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.
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CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis & Catalysts
Adsorption On Solid Surface
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) 19
CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis & Catalysts
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 T1 T2 >T1 T3 >T2 P4>P3 P3>P2 P2>P1 P1 V4>V3 V3>V2 V2>V1 V1 T4 >T3 T5 >T4 Pressure
Adsorption Isotherm
Temperature
Adsorption Isobar
Temperature
Adsorption Isostere
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CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis & Catalysts
Adsorption On Solid Surface
The Langmuir adsorption isotherm
Basic assumptions
surface uniform ( D
H ads
does not vary with coverage) monolayer adsorption, and no interaction between adsorbed molecules and adsorbed molecules immobile
Case I - single molecule adsorption A when adsorption is in a dynamic equilibrium A
(g) +
M
(surface site) D
AM
the rate of adsorption the rate of desorption at equilibrium
r ads = k ads r des = k des r ads = r des
(1
-
q ) q
P k ads
(1
-
q )
P = k des
q rearrange it for q let q 1 (
k ads
(
k ads / / k des
)
P k des
)
B
0
P B
0
k ads k des
q
C C
s
1
case I
B 0
is
adsorption coefficient
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CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis & Catalysts
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) D
A-M-B
the rate of
A-B
adsorption the rate of
A-B
desorption at equilibrium
r ads =k ads
(1 -q A ) (1 -q B )
P AB =k ads
(1 -q ) 2
P AB
q
=
q A
=
q B
r des =k des
q A q B
=k des
q
2 r ads = r des
k ads
(1 -q ) 2
P AB = k des
q
2
A B case II A
rearrange it for q Let.
q 1 (
k ads
(
k ads / / k des
)
P AB k des
)
P AB B
0
k ads k des
q
C C
s
1 (
B
0 (
B P
0
AB P
)
AB
1/2 ) 1/2
B
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CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis & Catalysts
Adsorption On Solid Surface
The Langmuir adsorption isotherm
(
cont’d
)
Case III - two molecules adsorbed on two sites A
(g) +
B
(g) +
2 M
(surface site) D
A-M + B-M
the rate of
A
adsorption the rate of
B
adsorption the rate of
A
desorption the rate of
B
desorption at equilibrium
r ads,A = k ads,A
(1 - q A - q B )
P A r ads,B = k ads,B
(1 - q A - q B )
P B r des,A = k des,A
q A
r des,B = k des,B
q B
A B case III
r ads ,A = r des ,A
and
r ads ,B = r des ,B k ads,A
(1 -q A -q B )
P A =k des,A
q A and
k ads,B
(1 -q A -q B )
P B =k des,B
q B rearrange it for q where q
A
C s , A C
B
0
, A
1
B
0
, A B
0
, A P A P A
B
0
, B P B
q
B
C s , B C
k ads , A k des , A
and
B
0
, B
k ads , B k des , B
1
B
0
B
0
, A P A , B P B
B
0
, B P B
are adsorption coefficients of
A
&
B
.
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CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis & Catalysts
Adsorption On Solid Surface
The Langmuir adsorption isotherm
(
cont’d
)
A A B A B case I
q
C C
s
1
B
0
k ads k des
case II
q
C C
s
1 (
B
0 (
B P AB
0
P
)
AB
1/2 ) 1/2
B
0
k ads k des
Adsorption
Strong Weak
k ads >> k des
B 0 >>1
q
C C
s
1
k ads << k des
B 0 <<1
q
C C
s
B
0
P k ads >> k des
B 0 >>1
q
C C
s
1
k ads << k des
B 0 <<1
q
C C
s
(
B
0
P
) 1/2 q q
A B
C s , A
C
C s , B C
B
0
, A
k
case III
1 1
B
0
, A B
0
, A P A P A
B
0
, B P B B
0
, A B
0
, B P A P B
B
0
, B P B ads , A k des , A
and
B
0
, B
k ads , B k des , B
Adsorption
A, B both strong A strong, B weak A weak, B weak
q
A
q
B
C s , A C
C s , B C
B
0
, A B
0
, A P A
P A B
0
, B P B B
0
, A B
0
, B P A
P B B
0
, B P B
q q q q
B A A B
C s , A / C
1
C s , B
C s , A C s , B / / / C
C
C
(
B
0
, B / B
0
, A
)
P B P A B
0
, A P A B
0
, B P B
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CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis & Catalysts
Adsorption On Solid Surface
Langmuir adsorption isotherm
Strong adsorption case I case II Case III
q
C s C
1 q q
A
q
B
C C
s
C s , A C C s C
1 (
B
0 (
B P AB
0
P
)
AB
1/2 ) 1/2
, B
1
B
0
B
0
, A , A P A P A
B
0
, B P B
1
B
0
B , A
0
P , B A P B
B
0
, B P B
Weak adsorption
k ads >> k des k ads << k des
q q
C
C
C s C
s
1
B
0
P
mono-layer large B 0 (strong adsorp.) moderate B 0 small B 0 (weak adsorp.)
Pressure
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 25
CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis & Catalysts
Adsorption On Solid Surface
Five types of physisorption isotherms are found over all solids I
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
II
Type II for non-porous materials
III IV V
1.0
relative pres. P/P 0 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 26
CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis & Catalysts
Adsorption On Solid Surface
Other adsorption isotherms
Many other isotherms are proposed in order to explain the observations
The Temkin (or Slygin-Frumkin) isotherm
Assuming the adsorption enthalpy D
H
decreases linearly with surface coverage From ads-des equilibrium,
ads. rate
des. rate
r
ads
=k
ads
(1
-
q )
P
r
des
=k
des
q Langmuir q 1
B
0
P B
0
P
q
s
1
b
1
e b
1
Q s e / Q s RT / P RT P
where
Q s
constant,
i
is the heat of adsorption. When
Q s
is a linear function of is the number and S represents the surface site), q
i
.
Q s
=Q 0 -iS
(
Q 0
Temkin is a q the overall coverage q 0 1 q
s dS
0 1 (1 [
b
1
e Q s / b
1
e Q s RT / P RT P dS
RT i
ln 1 b 1 1
P
b 1
exp P
(
RT
When
b 1 P
>>1 and
b 1 P
exp(-
i
/
RT
) <<1, we have q
=c
1
ln(c
2
P)
, where c 1 & c 2 are constants Valid for some adsorption systems
.
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CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis & Catalysts
Adsorption On Solid Surface
The Freundlich isotherm
assuming logarithmic change of adsorption enthalpy D
H
with surface coverage From ads-des equilibrium,
ads. rate
des. rate r ads =k ads
(1
-
q )
P
r des =k des
q Langmuir q 1
B
0
P B
0
P
q
i
1
b
1
e b
1
Q i e / Q i RT / P RT P
Freundlich q where
Q i
is the heat of adsorption which is a function of q
i
. If there are
N i
types of surface sites, each can be expressed as
N i
=aexp(-Q/Q
0
)
fractional coverage the overall coverage q
i
, q
i
i
q
i N N i i
0 [
b
1
e Q / RT P /
(1 0
a
e (
a b
1
e Q/Q
0 the solution for this integration expression at small q
Q /
and
Q 0 RT dQ P
)] is: are constants), corresponding to a
a
e
Q/Q
0
dQ
ln q
=
(
RT/Q 0
)ln
P+
constant, or as is the
Freundlich equation
normally written, q
c
1
p
1
/ C
2 where
c 1
=constant,
1/c 2
=
RT/Q 0
Freundlich
isotherm fits, not all, but many adsorption systems.
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CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis & Catalysts
Adsorption On Solid Surface
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
1
c
1 was derived
/ V (
1
P
-
/ P P
0
/ P
0
)
cV m
cV m
(
P P
0 )
Where P
- equilibrium pressure
P 0 V
- saturate vapour pressure of the adsorbed gas at the temperature
P/P 0
is called
relative pressure
- volume of adsorbed gas per kg adsorbent
V m
-volume of monolayer adsorbed gas per kg adsorbent
c
- constant associated with adsorption heat and condensation heat Note: for many adsorption systems
c
=exp[(
H 1
-
H L
)/
RT
], where
H 1
is adsorption heat of 1st layer &
H L
is liquefaction heat, so that the adsorption heat can be determined from constant
c
.
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CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis & Catalysts
Adsorption On Solid Surface
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/P 0 <0.05) and high relative pressure (P/P 0 >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. 30
CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis & Catalysts
Adsorption On Solid Surface
Use of BET isotherm to determine the surface area of a solid
At low
relative pressure P/P
0
P / V (
1 -
P P
0
/ P
0
)
1
cV m
c
1 (
P cV m /
= 0.05~0.35
P
0 ) (
P / P
0 ) it is found that Y = a + b X
V P P
0 ( 1 0 )
P/P 0
The principle of surface area determination by BET method: A plot of
V P P
0 ( 1 ) against 0 and intersect 1/(
cV m
).
P/P 0
will yield a straight line with slope of equal to (
c
-1)/(
cV m
) For a given adsorption system,
c
and
V m
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
A m
,
A s
m
A m V V m
.
23
V m
- volume of monolayer adsorbed gas molecules calculated from the plot, L
V T,P
- molar volume of the adsorbed gas, L/mol
A m
- cross-section area of a single gas molecule, m 2 * In practice, measurement of BET surface area of a solid is carried out by N 2 at liquid N 2 temperature; for N 2 ,
A m
= 16.2 x 10 -20 m 2 physisorption 31
CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis & Catalysts
Adsorption On Solid Surface
Summary of adsorption isotherms Name Langmuir Temkin Isotherm equation
q
C C
s
1 q
=c
1
ln(c
2
P) Application
Chemisorption and physisorption Chemisorption
Note
Useful in analysis of reaction mechanism Chemisorption
Freundlich BET
q
c
1
p
1
/ C
2
P / P
0
V (
1 -
P / P
0
)
1
cV m
c
-
cV m
1 (
P / P
0 ) Chemisorption and physisorption Easy to fit adsorption data Multilayer physisorption Useful in surface area determination 32
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis & Catalysts
Mechanism of Surface Catalysed Reaction
Langmuir-Hinshelwood mechanism
This mechanism deals with the surface-catalysed reaction in which 2 or more reactants adsorb on surface without dissociation
A + B
"
P
A (
g
) + B (
g
) D A (
ads
) + B (
ads
) " P (the desorption of
P
is not r.d.s.) The rate of reaction
r i
=
k
[A][B]=
k
We then have
r i
k
1
B
0
B
0
, A , A P A P A
B
0
, B P B
q
A
q
B
From Langmuir adsorption isotherm (the case III) we know q
A
1
B
0
B
0
, B , A P A P B
B
0
, B P B
1
kB
0
, A B
0
B
0
, A P A , B
P A P B B
0
, B P B
q
B
1 1
B
0
, A B
0
, A P A P A
B
0
, B P B B
0
, A B
0
, B P B P A
B
0
, B P B
When both A and B are weakly adsorbed (B 0,A P A <<1, B 0,B P B <<1),
r i
kB
0
, A B
0
, B P A P B
k ' P A P B
2nd order reaction When A is strongly adsorbed (B 0,A P A >>1) and B weakly adsorbed (B 0,B P B <<1 <
r i
kB
0
, A B
0
, B P A P B B
0
, A P A
kB
0
, B P B
k ' ' P B
1st order w.r.t. B 33
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis & Catalysts
Mechanism of Surface Catalysed Reaction
Eley-Rideal mechanism
B
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 + B (
g
) A (
g
) D A (
ads
) P (the desorption of
P
is not r.d.s.)
A
"
P
The rate of reaction
r i
=
k
[A][B]=
k
q
A P B
From Langmuir adsorption isotherm (the case I) we know We then have
r i
k
B
0
, A P A
1
B
0
, A P A
P B
kB
0
, A P A P B
1
B
0
, A P A
q
A
B
0
, A
1
B
0
P A , A P A
When both A is weakly adsorbed or the partial pressure of A is very low (B 0,A P A <<1),
r i
kB
0
, A P A P B
k ' P A P B
2nd order reaction When A is strongly adsorbed or the partial pressure of A is very high (B 0,A P A >>1)
r i
kB
0
, A P A P B B
0
, A P A
kP B
1st order w.r.t. B 34
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis & Catalysts
Mechanism of Surface Catalysed Reaction
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 + B (
g
) AD (
g
) D A (
ads
) + D (
ads
) P
B A B
"
P
(the des. of
P
is not r.d.s.) The rate of reaction
r i
=
k
[A][B]=
k
q
AD P B
From Langmuir adsorption isotherm (the case I) we know We then have
r i
k
1 (
B
0
,
(
B
0
AD , AD P AD P AD
1
/
1
/
2 2
P B
k
1 (
B
0
, AD
(
B
0
, P AD
1
AD P AD /
1 2
/ P B
2 q
AD
1 (
B
0 (
B
0
, AD , AD P AD P AD
1
/
1
/
2 2 When both AD is weakly adsorbed or the partial pressure of AD is very low (B 0,AD P AD <<1),
r i
k
(
B
0
, AD P AD
1
/
2
P B
k ' P
1
/ AD
2
P B
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 (B 0,A P A >>1)
r i
k
(
B
0 (
B
0
, AD , AD P AD P AD
1 1
/ /
2 2
P B
kP B
1st order w.r.t. B 35
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis & Catalysts
Mechanism of Surface Catalysed Reaction
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 … 36
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis & Catalysts
Solids and Solid Surface
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 37
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis & Catalysts
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 ZrO 2 ) 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 … 38
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis & Catalysts
Solids and Solid Surface
Defects and dislocation on surface crystalline structure
A ‘perfect crystal’ can be made in a controlled way
Surface defects
terrace
step
kink
adatom / vacancy Terrace Step
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.
39
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis & Catalysts
Pores of Porous Solids
Pore sizes
micro pores
d
p
<20-50 nm meso-pores 20nm <
d
p
<200nm macro pores
d
p
>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) & wt D wt non-uniform size distribution (b) b a
d
w
d
d b a D d d Cumulative curve d Frequency curve 40
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex Reactions
Chain Reactions - Process
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) 41
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex Reactions
Chain Reactions - Rate Law
Rate law of chain reaction
Example:
overall reaction H 2 (
g
) + Br 2 (
g
) 2HBr (
g
)
elem step rate law a. Initiation: b. Propagation: c. Termination: d. Retard
n
(obsvd.) observed:
d
[HBr]
dt
k
[H [Br 2 ] 2 ][Br 2
k '
] 3/2 [HBr]
Br 2
2Br
r a
=k
a
[Br 2 ] Br
+ H 2
HBr + H
r b
=k
b
[Br][H 2 ] H
+ Br 2
HBr + Br Br
H H
+
+ +
Br
H
Br
Br H 2 HBr 2
r’ r c b
= =k
k’ c b
[H][Br 2 ] [Br][Br]=k
c
[Br] 2
(practically less important therefore neglected) (practically less important therefore neglected)
H
+ HBr
H 2 + Br
r d
=k
d
[H][HBr] HBr net rate: r HBr = r
b
+
r’ b
- r
d
or
Apply s.s.a. r H = r
b
-
r’ b
- r
d r
Br = 2r
a
-r
b
+
r’ b
-2r
c
+r
d
or or
d[HBr]/dt=k
b
[Br][H 2 ]+
k’ b
[H][Br 2 ]-k
d
[H][HBr] d[H]/dt=k
b
[Br][H 2 ]-
k’ b
[H][Br 2 ]-k
d
[H][HBr]=0
d[Br]/dt=2
k a
[Br 2 ]-
k b
[Br][H 2 ]+
k’ b
[H][Br 2 ]-2
k c
[Br] 2 +
k d
[H][HBr]=0
solve the above eqn’s we have
d
[HBr]
dt
2
k b
(
k a
[Br 2 ]
/
k c
(
k d
1/2
/
[H
k ' b
2 ][Br 2 [HBr] ] 3/2 42
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex Reactions
Chain Reactions - Polymerisation
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.
initiator chain-carrier rate law is the yield of I x to xR
Initiation
:
I x R
xR
+ M
M 1
(usually r.d.s.)
r i =k i
[I]
(fast)
d
[M ]
dt
x
k i
[I]
Propagation Termination
: :
M + M +
M 1 M 2
(MM 1 )
(MM 2 )
M 2 M 3 … … … … … … … … … M + M n
M +
n-1 M m
(MM n-1 )
(M n M m )
M M n m+n
(fast) (fast)
r p
=k
p
[M][
r t
=k
t
[
M] 2 M]
(
r i
is the r.d.s.) Apply s.s.a. to [ M] formed The rate of propagation or the rate of M consumption or the rate of chain growth
d
[M ]
dt d
[M]
dt
x
r i
-
r p
2
r p
x
k i
[I]
-
2
k t
[M ] 2 -
k p
[M][M ] i.e.
d
[M]
dt
0 [M ] -
k p
x
k i
2
k t
1
/ x
k i
2 2
k t
[I] [I] 1/2 1 [M]
/
2 43
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex Reactions
Chain Reactions - Polymerisation
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}
H 2 N(CH 2 ) 6 NH 2 + HOOC(CH 2 ) 4 COOH
H 2 N(CH 2 ) 6 NHOC(CH 2 ) 4 COOH + H 2 O
H-HN(CH 2 ) 6 NHOC(CH 2 ) 4 CO-OH
H-[HN(CH 2 ) 6 NHOC(CH 2 ) 4 CO] n -OH (1) … (n)
Note: If a small molecule is dropped as a result of reaction, like a H 2 O 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 2 nd order
d
[A]
dt
-
k
[A][-OH] -
k
[A] 2 or [A] 1 [A] 0
kt
[A] 0 the conversion of
B
X
[A]
B
[A] 0 [A] 0 (-OH containing substance) at time
t
is 1
kt
[A] 0
kt
[A] 0 44
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex Reactions
Chain Reactions - Explosion
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: 2H 2
(
g
)
+ O 2
(
g
)
2H 2 O
(
g
)
Initiation: Propagation: H 2 + O 2
O 2 H + H
H 2 H 2 +
+
O 2 H
OH
OH + H
H + H 2 O 2 O
O 2 +
H
O
+ H 2
O
+
OH + OH
H
(non-branching) (non-branching)
(branching) (branching)
Lead to explosion
45
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex Reactions
Explosion Reactions
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
D
H
T
r
D
H
T
r
D
H
…
A combination of chain-branching reaction with heat accumulation can occur simultaneously
46
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex Reactions
Photochemical 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
overall quantum yield,
F photon absorbed is call
E.g.
HI + hv
H + I H + HI 2I
H 2
I 2 + I
primary quantum yield =2 (one H and one I) 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 = 10 4 The quantum yield of a photochemical reaction depends on the wavelength of light used 47
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex Reactions
Photochemical 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
508 nm light
e.g. I 36 Cl + I 37 Cl I 36 Cl + I 37 Cl*
(only 37 Cl molecules are excited)
C 6 H 5 Br + I 37 Cl*
C 6 H 5 37 Cl + 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 + H 2
254 nm light
Hg* + H 2 Hg* + H 2
Hg + 2H* & Hg* + H 2 CO H* HCO H 2 2HCO
HCHO + H* HCHO + CO
(Hg is, but H 2
HgH + H*
is not excited by 254nm light) 48
CH4003 Lecture Notes 17 (Erzeng Xue)
Spectroscopy
Introduction to Spectroscopy
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
49
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
Electromagnetic Radiation
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 10 8 m wave-length, l - the distance between adjacent crests of the wave wave number,
v’
, - the number of waves per unit distance
v’
= l -1 s -1 (in vacuum)
v
c
l
v ' c
The energy carried by an e.m.r. or a photon is directly proportional to the frequency, i.e.
E
hv
hc
l
hv ' c
where
h
is Planck’s constant
h
=6.626x10
-34 J s 50
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
Electromagnetic Radiation
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.
51
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
Interaction of e.m.r. with Matter
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 reflection absorption scattering transmission 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 52
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
Spectrum
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.
1.0
0.5
.
0.0
350 400 wave length cm -1 450 53
.
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
Spectrum
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.) 1.0
0.5
0.0
350 400 wave length cm -1 450 54
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
Spectral properties, applications, and interactions of electromagnetic radiation
Energy kcal/mol Wave number Electron vole eV v’ cm -1 Wavelength l cm Frequency v Hz Type of radiation Type of spectroscopy Type of quantum transition 9.4x10
7 4.1x10
6 3.3x10
10 3.0x10
-11 10 21 Gamma ray Gamma ray emission Nuclear 9.4x10
5 4.1x10
4 3.3x10
8 3.0x10
-9 10 19 X-ray X-ray absorption emission Electronic (inner shell) 9.4x10
3 4.1x10
2 3.3x10
6 3.0x10
-7 10 17 9.4x10
1 4.1x10
0 3.3x10
4 9.4x10
-1 4.1x10
-2 3.3x10
2 9.4x10
-3 4.1x10
-4 3.3x10
0 9.4x10
-5 4.1x10
-6 3.3x10
-2 9.4x10
-7 4.1x10
-8 3.3x10
-4 3.0x10
-5 3.0x10
-3 3.0x10
-1 3.0x10
1 3.0x10
3 10 15 10 13 10 11 10 9 10 7 Ultra Violet Visible Infrared Microwave Radio Vac UV Vis UV absorption absorption emission fluorescence IR absorption Raman Molecular vibration Microwave absorption Electron paramagnet resonance Nuclear magnetic resonance Electronic (outer shell) Molecular rotation Magnetically induced spin states 55
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
Examples
1. A laser emits light with a frequency of 4.69x10
14 s -1 . (
h
= 6.63 x 10 -34 Js) A) What is the energy of one photon of the radiation from this laser? B) If the laser emits 1.3x10
-2 J during a pulse, how many photons are emitted during the pulse?
Ans:
A) E photon = h n 6.63 x 10 -34 Js x 4.69x10
14 s -1 = 3.11 x 10 B) No. of photons = (1.3x10
-2 J )/(3.11 x 10 -19 J) = 4.2x10
16 -19 J 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 -34 Js)
Ans
: E photon = h n = hc/ l (6.63 x 10 -34 Js x 3 x 10 8 ms -1 )/650x10 -9 m = 3.06x10
-19 J .
3: Compare the energies of photons emitted by two radio stations, operating at 92 MHz (FM) and 1500 kHz (MW)?
Ans
: E photon = h n 92 MHz = 92 x 10 6 E = (6.63 x 10 -34 Hz (s -1 ) => Js) x (92 x 10 6 s -1 ) = 6.1 x 10 -26 J 1500 kHz = 1500 x 10 3 E = (6.63 x 10 -34 Hz (s -1 ) Js) x (1500 x 10 3 s -1 ) = 9.9 x 10 -28 J 56
CH4003 Lecture Notes 18 (Erzeng Xue)
Introductory to Spectroscopy
Atomic Spectra
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
energy
D
E
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 lowest possible shell - the e stays at the 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.
n=4 n=3 n=2 n=1
Excited state ground state
4f 4d 4p 3d 4s 3p 3s 2p 2s 1s n = 1 n = 2 n = 3, etc.
57
CH4003 Lecture Notes 18 (Erzeng Xue)
Introductory to Spectroscopy
Atomic Spectra
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
energy
D
E
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 … n=4 n=3 n=2 n=1 n = 1 n = 2 n = 3, etc.
4f 4d 4p 3d 4s 3p 3s 2p 2s 1s 58
CH4003 Lecture Notes 18 (Erzeng Xue)
Introductory to Spectroscopy
Atomic spectra
Atomic Spectra
energy
D
E
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 n=4 n=3 Atomic spectra are mostly in UV (sometime in visible) regions n=2 n=1 n = 1 n = 2 n = 3, etc.
4f 4d 4p 3d 4s 3p 3s 2p 2s 1s 59
CH4003 Lecture Notes 18 (Erzeng Xue)
Spectroscopy
Molecular Spectra
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 (S 0 ) S (Singlet) - two e ’s spin in pair E T (Triplet) - two e ’s spin parallel J
v 4 v v 3 2 v 1 S 2
Upon exciting molecules can change to high E states (S 1 , S 2, T 1 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
v 4 v v 3 2 v 1 S 0 S 1 T 1 v 4 v v 3 2 v 1 v 4 v v 3 2 v 1
60
CH4003 Lecture Notes 18 (Erzeng Xue)
Spectroscopy
Molecular Spectra
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 (S 0 ) to a higher E state (S 1 , S 2, T 1 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 S 0,v1 e.m.r with wavenumber
v’
to S 2,v2 must be used ,
hcv '
E S
2
, v
2 -
E S
0
, v
1
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. v 4 v v 3 2 v 1 S 2 v 4 v v 3 2 v 1 S 0 A S 1 absorption A v 4 v v 3 2 v 1 T 1 v 4 v v 3 2 v 1
61
CH4003 Lecture Notes 18 (Erzeng Xue)
Spectroscopy
Molecular Spectra
Energy change of excited molecules An excited molecules can lose its excess energy via several processes v 4 v v 3 2 v 1
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 E 1 -
Undergoing internal transition within the same mode of the excited state
Process E 2 -
Undergoing intersystem crossing to a triplet sublevel of the excited state
Process F -
Radiating E from triplet to ground state (triplet quenching) - Phosphorescence S 2 v 4 v v 3 2 v 1 S 0 A B B C Internal transition Inter- system crossing E 1 B v 4 v v 3 2 v 1 E 2 S 1 D
Fluorescence
T 1
Fluorescence
Jablonsky diagram F v 4 v v 3 2 v 1
62
CH4003 Lecture Notes 18 (Erzeng Xue)
Spectroscopy
Molecular Spectra
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.
v 4 v v 3 2 v 1
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)
S 2
Fluorescence can also occur in atomic adsorption spectra Fluorescence emission is generally short-lived (e.g. m s)
A B
Fluore scence
T 1
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
v 4 v v 3 2 v 1 S 0 D F v 4 v v 3 2 v 1
phosphor enscence 63
CH4003 Lecture Notes 18 (Erzeng Xue)
Introductory to Spectroscopy
Atomic Spectra & Molecular Spectra
Comparison of atomic and molecular spectra
Adsorption spectra Emission spectra Energy required for excitation Change of energy level related to Spectral region Relative complexity of spectra Atomic spectra Yes Yes high change of e ’s orbital UV simple Molecular spectra Yes Yes low change of vibration states mainly visible complex
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. 64
CH4003 Lecture Notes 19 (Erzeng Xue)
Spectroscopy Application
UV & Visible Spectrophotometry
Observations
Incident light,
I 0
(UV or visible) Emergent light,
I
ultraviolet visible infra-red
C
200 - 400 400 - 800 800 - 15 nm nm nm nm nm m m
b
When a light of intensity
I 0
goes through a liquid of concentration
C
& layer thickness
b
The emergent light,
I
, has less intensity than the incident light
I 0
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) 65
CH4003 Lecture Notes 19 (Erzeng Xue)
Spectroscopy Application
UV & Visible Spectrophotometry
Theory of light absorption
Quantitative observation The thicker the cuvette - more diminishing of light in intensity Incident light
I 0 C
Emergent light
I b
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
b
light absorbed fraction of light
I 0
x s s
I
dI Ncs
2
dx
-
k ' I
number of molecules
N
-Avogadro number
dI I
I
I
0
b
-
dI
I k ' Ncs
2
dx
-
ac
0
b
-
acdx dx
ln
I b I
0 Absorbance
dx
log
I
0 -
acb
abc
A I
66
CH4003 Lecture Notes 19 (Erzeng Xue)
Spectroscopy Application
UV & Visible Spectrophotometry
Terms, units and symbols for use with Beer’s Law
Name alternative name symbol Path length Liquid concentration Transmittance Percent transmittance Transmission Absorbance Optical density, extinction Absorptivity Molar absorptivity Extinction coeff., absorbance index Molar extinction coeff., molar absorbancy index
b
(
or l
)
c T T% A a
(
or
e
, k
)
a
[or
a M
definition -
I / I 0
100x
I / I 0
log(
I / I 0
)
A
/(
bc
)
A
/(
bc
)
AM
/(
bc’
) ] unit cm mol / L -
%
[
bc
] -1
M-molar weight c’ -gram/L
67
CH4003 Lecture Notes 19 (Erzeng Xue)
Spectroscopy Application
UV & Visible Spectrophotometry
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 200-210nm 210-220nm 245-260nm 265-275nm 280-290nm 300-400nm sulphuric acid (96%), water, acetonitrile cyclopentane, n-hexane, glycerol, methanol, ethanol n-butyl alcohol, isopropyl alcohol, cyclohexane, ethyl ether chloroform, ethyl acetate, methyl formate carbon tetrachloride, dimethyl sulphoxide/formamide, acetic acid benzene, toluene, m-xylene 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
68
CH4003 Lecture Notes 19 (Erzeng Xue)
Spectroscopy Application
UV & Visible Spectrophotometry
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
s *
orbitals; non-bonding in n
p *
Antibonding orbitals correspond to the
Antibonding Antibonding
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
*)
n s p non-bonding Bonding
Molecules which can be analysed by UV-visible absorption
Chromophores
functional groups each of which absorbs a characteristic UV or visible radiation.
69
CH4003 Lecture Notes 19 (Erzeng Xue)
Spectroscopy Application
UV & Visible Spectrophotometry
The functional groups & the wavelength of UV-visible absorption Group Example
l
max ,
nm
Group Example
l
max ,
nm C=C C=O C-X 1-octane methanol propanone ethanoic acid ethyl ethanoate ethanamide methanol trimethylamine chloromethane bromomethane iodomethane 180 290 280 210 210 220 180 200 170 210 260 arene conjugated benzene naphthalene phenenthrene anthracene pentacene 1,3-butadiene 1,3,5-hexatriene 2-propenal b -carotene (11 C=C) each additional C=C 260 280 350 375 575 220 250 320 480 +30 70
CH4003 Lecture Notes 19 (Erzeng Xue)
Spectroscopy Application
UV & Visible Spectrophotometry
Instrumentation Light source Cuvette Detectors UV
Hydrogen discharge lamp QUARTZ photomultiplier
visible
Tungsten-halogen lamp glass photomultiplier 71
CH4003 Lecture Notes 19 (Erzeng Xue)
Spectroscopy Application
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 72
CH4003 Lecture Notes 20 (Erzeng Xue)
Spectroscopy Application
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.
73
CH4003 Lecture Notes 20 (Erzeng Xue)
Spectroscopy Application
IR-Spectroscopy
IR region
The part of electromagnetic radiation between the visible and microwave regions 0.8 m m to 50 m m (12,500 cm -1 -200 cm -1 ).
Most interested region in Infrared Spectroscopy is between 2.5
m m-25 m 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 74
CH4003 Lecture Notes 20 (Erzeng Xue)
Spectroscopy Application
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
75
CH4003 Lecture Notes 20 (Erzeng Xue)
Spectroscopy Application
IR Spectrum
Principal Correlation Chart
O H N H C H C N C C C=O C=C C O 3600 cm -1 3500 cm -1 3000 cm -1 2250 cm -1 2150 cm -1 1715 cm -1 1650 cm -1 1100 cm -1
Region freq. (cm -1 ) what is found there??
XH region triple bond 3800 - 2600 2400 - 2000 double bond 1900 - 1500 fingerprint 1500 - 400 1400 - 900 1500 - 1300 1000 - 650 OH, NH, CH (sp, sp 2 , sp 3 ) stretches C C, C N, C=C=C stretches C=O, C=N, C=C stretches many types of absorptions C-O, C-N stretches CH in-plane bends, NH bends CH out-of-plane (oop) bends
Dispersive (Double Beam) IR Spectrophotometer
Split Beam Air Photometer IR Source Lenz Prism or Diffraction Grating Slit Sample Recorder 76
CH4003 Lecture Notes 20 (Erzeng Xue)
Spectroscopy Application
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)
Source
: R. Thomas, “Choosing the Right Trace Element Technique,”
Today’s Chemist at Work
, Oct. 1999, 42.
77
CH4003 Lecture Notes 20 (Erzeng Xue)
Spectroscopy Application
Atomic Absorption Spectrometer (AA)
Source
P 0 P
Wavelength Selector Detector Signal Processor Readout Chopper Sample
Type atomic (flame) Method of Atomization Radiation Source sample solution aspirated into a flame Hollow cathode lamp (HCL) HCL atomic (nonflame) sample solution evaporated & ignited x-ray absorption none required x-ray tube
78
CH4003 Lecture Notes 20 (Erzeng Xue)
Spectroscopy Application
Atomic Emission Spectrometer (AES)
P
Source Wavelength Selector Detector Signal Processor Readout Sample
Type Method of Atomization Radiation Source sample arc sample heated in an electric arc spark argon plasma flame x-ray emission sample excited in a high voltage spark sample heated in an argon plasma sample sample solution aspirated into a flame sample none required; sample bombarded w/ e sample sample
79
CH4003 Lecture Notes 20 (Erzeng Xue)
Spectroscopy Application
Atomic Fluorescence Spectrometer (AFS)
Source
P 0 P
Wavelength Selector Detector Signal Processor Readout 90 o
Type atomic (flame) Method of Atomization Radiation Source sample solution aspirated into a flame sample sample
Sample
atomic (nonflame) sample solution evaporated & ignited x-ray fluorescence none required sample
80
CH4003 Lecture Notes 21 (Erzeng Xue)
Spectroscopy Application
Laser - Characteristics
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) Incandescent lamp
• Chromatic • Incoherent • 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 (Al 2 O 3 doped with a dilute concentration of Cr +3 ) as the lasing medium and a fast discharge flash-lamp to provide the pump energy.
Monochromatic light source
• Coherent • Non-directional 81
CH4003 Lecture Notes 21 (Erzeng Xue)
Spectroscopy Application
Laser - Stimulated Emission
When excited atoms/molecules/ions undergo de-excitation (from excited state to ground state), light is emitted Types of light emission
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)
E 4 E 3 E 2
excited state
E 1 E p1 E p2 E p4
ground state
E 0 E p1 =(E 1 – E 0 ) = hv
1
E p2 =(E 2 – E 0 ) = hv
2
E p4 =(E 4 – E 0 ) = hv
4
82
CH4003 Lecture Notes 21 (Erzeng Xue)
Spectroscopy Application
Laser - Stimulated Emission
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 (P 1 ) having an energy equal to the energy difference of the excited state and the ground state. In such a process, the incident photon (P 1 ) is not absorbed and is emitted together with the photon (P 2 ), The latter will have
the same frequency
(or energy) and
the same phase
(coherent) as the stimulating photon (P 1 ).
E 4 E 3 E 2 E 1 E p1 =(E 2 – E 0 )=hv 2 E 0 Ep 1 =(E 2 –E 0 )=hv 2 Ep 2 =(E 2 –E 0 )=hv 2
Laser uses the stimulated emission process to amplify the light intensity
As in the stimulated emission process, one incident photon (P 1 ) will bring about the emission of an additional photon (P 2 ), which in turn can yield 4 photons, then 8 photons, and so on….
83
CH4003 Lecture Notes 21 (Erzeng Xue)
Spectroscopy Application
Laser - Formation & Conditions
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.
84
CH4003 Lecture Notes 21 (Erzeng Xue)
Spectroscopy Application
Laser - Functional Elements
Output coupler High reflectance mirror Feedback mechanism Lasing medium Energy input Energy pumping mechanism Partially transmitting mirror
85
CH4003 Lecture Notes 21 (Erzeng Xue)
Spectroscopy Application
Lasing medium at ground state Population inversion Start of stimulated emission Stimulated emission building up Laser in full operation
Laser Action
Pump energy Pump energy Pump energy Pump energy 86
CH4003 Lecture Notes 21 (Erzeng Xue)
Spectroscopy Application
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
Helium neon laser Carbon dioxide laser Argon laser Nitrogen laser Dye laser Ruby laser Nd:Yag laser Diode laser
Physical form of lasing medium
Gas Gas Gas Gas Liquid Solid Solid Semiconductor
Wave length (nm)
633 10600 (far-infrared) 488, 513, 361 (UV), 364 (UV) 337 (UV) Tunable: 570-650 694 1064 (infrared) 630-680 87
CH4003 Lecture Notes 21 (Erzeng Xue)
Spectroscopy Application
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 88