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CH4003 Lecture Notes 5 (Erzeng Xue)
ADSORPTION
BY:M.R.DEHGHANI
2
Drop on a Solid Surface
•Young’s equation relates interfacial tensions and contact angle
LV
SV
Solid, “S”
Liquid, “L”
Vapor, “V”
SL
cosLVSLSV
Contact angle, (reflects the degree of wetting)
3
Wetting Phenomena
> 90o = 90o < 90o
= 0o =180o
Absolute wettingNo wetting
4
Wettability of Powders
Solid
Liquid
Vapor
LV
SV
SL
cosLVSLSV
5
Surface Activity of Solutes
Solute, c
Solutes that decrease surface tension are called “surfactants”
0dcd
Solute, c
6
c
NaOH
butanol
soap
Control of wettinguse of surfactants
7
Adsorption
More solute we have less surface tension (free energy) is - favorableAs a result solute concentrates onto surface - adsorption
More solute we have more surface tension (free energy) is - unfavorableSolute is repelled bysurface
0dcd
0dcd
Surface active materials Surface inactive
materials
8
Specific Adsorption
- thickness of adsorption layer
Surface area,
. = Volume of adsorption layer,c= Solute concentration in Bulk,c= Surface concentration,= Specific adsorption.
nc
ccnn
The units for the specific adsorption are concentration units multiplied by the distance units.
9
Gibbs Law
ccnn
dcd
RTc
add
RT
ln
1 o
0dcd
Solute, c
Gibbs law relates specific adsorption with the surface activity of the solute. It provides quantitative expression for the conclusion that the
adsorption of surfactant is positive.
10
Justification: Gibbs Law
cRTc
cRTcRTn
nnpVTSGnGG
oTpTp
ln)ln(;
:equationMaxwell
dddddd':Introduce
...,...,
Gibbs law:
Surface area,
dcd
RTc
To derive the Gibbs law we introduce a new function of state G’ by subtracting the product of the chemical potential and number of moles from the Gibbs energy
11
Adsorption Isotherms
cc
max
Langmuir isotherm
pp
'max
(for a solution)
(for a gas)
c or p
max
12
More Adsorption Isotherms
c or P
c or P
c or P
c or P
(a) Langmuir (b) Multilayer formation
(c) “Positive cooperativity” (d) Hysteresis
first layer
13
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
14
Catalyst composition Active phase
Where the reaction occurs (mostly metal/metal oxide)
Promoter Textual promoter (e.g. Al - Fe for NH3 production) Electric or Structural modifier Poison resistant promoters
Support / carrier Increase mechanical strength Increase surface area (98% surface area is supplied within the porous structure) may or may not be catalytically active
Solid Catalysts
CatalystActiv
e ph
ase
Support
Promoter
15
Some common solid support / carrier materials
Alumina Inexpensive Surface area: 1 ~ 700 m2/g Acidic
Silica Inexpensive Surface area: 100 ~ 800 m2/g Acidic
Zeolite mixture of alumina and silica, often exchanged metal ion present shape selective acidic
Solid Catalysts
Other supports Active carbon (S.A. up to 1000 m2/g) Titania (S.A. 10 ~ 50 m2/g) Zirconia (S.A. 10 ~ 100 m2/g) Magnesia (S.A. 10 m2/g) Lanthana (S.A. 10 m2/g)
poreporous solid
Active site
16
Preparation of catalysts Precipitation
To form non-soluble precipitate by desired reactions at certain pH and temperature
Adsorption & ion-exchangeCationic: S-OH+ + C+ SOC+ + H+
Anionic: S-OH- + A- SA- + OH-
I-exch. S-Na+ + Ni 2+ S-Ni 2+ + Na+
ImpregnationFill the pores of support with a metal salt solution of sufficient concentration to give the correct loading.
Dry mixing Physically mixed, grind, and fired
Solid 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
Amou
ntad
sorb
ed
Concentration
Support
Drying & firing
17
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 spanCan be many years or a few mins.
Solid Catalysts
0255075
100
500 600 700 800 900Temperature °C
BET
S.A
. m2 /g
0
40
0 10Time / hours
BET
S.A
.
Act
ivity
Time
Normal use
Induction period
dead
18
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 chemical bondnumber of adsorbed layers multi only one layer
adsorption heat low (10-40 kJ/mol) high ( > 40 kJ/mol)selectivity low high
temperature to occur low high
Adsorption On Solid Surface
19
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 Surface
define = = 0~1number of adsorption sites occupiednumber of adsorption sites available
20
Applications of adsorption process Adsorption is a very important step in solid catalysed reaction processes Adsorption in itself is a common process used in industry for various purposes
Purification (removing impurities from a gas / liquid stream) De-pollution, de-colour, de-odour Solvent recovery, trace compound enrichment etc…
Usually adsorption is only applied for a process dealing with small capacity The operation is usually batch type and required regeneration of saturated adsorbent
Common adsorbents: molecular sieve, active carbon, silica gel, activated alumina.
Physisorption is an useful technique for determining the surface area, the pore shape, pore sizes and size distribution of porous solid materials (BET surface area)
Adsorption On Solid Surface
21
Adsorption On Solid Surface Characterisation of adsorption system
Adsorption isothermAdsorption isotherm - most commonly used, especially to catalytic reaction system, T=const.The amount of adsorption as a function of pressure at set temperature
Adsorption isobarAdsorption isobar - (usage related to industrial applications)The amount of adsorption as a function of temperature at set pressure
Adsorption IsostereAdsorption Isostere - (usage related to industrial applications)Adsorption pressure as a function of temperature at set volume
Pressure
Vol
. ads
orbe
d T1T2 >T1
T3 >T2
T4 >T3
T5 >T4
Vol
. ads
orbe
d
Temperature
P1
P2>P1
P3>P2P4>P3
Pre
ssur
e
Temperature
V2>V1
V1
V3>V2
V4>V3
Adsorption Isotherm Adsorption Isobar Adsorption Isostere
22
The Langmuir adsorption isotherm Basic assumptions
Surface uniform (Hads does not vary with coverage) Monolayer adsorption No interaction between adsorbed molecules and adsorbed molecules immobile
Case I - single molecule adsorptionwhen adsorption is in a dynamic equilibrium A(g) + M(surface site) AMthe rate of adsorptionrads = kads (1-) P
the rate of desorptionrdes = kdes
at equilibrium rads = rdes kads (1-) P = kdes
rearrange it for
let B0 is adsorption coefficient
Adsorption On Solid Surface
PBPB
CCs
0
0
1
des
ads
kkB 0
PkkPkk
desads
desads
)/(1)/(
case I
A
23
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.
case II
A BBA
==
1/20
1/20
)(1)(
AB
ABs
PBPB
CC
des
ads
kkB 0
)(1
)(
ABdesads
ABdesads
Pk/kPk/k
24
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 adsorptionrads,A = kads,A (1) PA
the rate of B adsorptionrads,B = kads,B (1) PB
the rate of A desorptionrdes,A = kdes,A
the rate of B desorptionrdes,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 Surface
B,des
B,adsB,
A,des
A,adsA, k
kB
kk
B 00 and
BB,AA,
BB,B,sB
BB,AA,
AA,A,sA PBPB
PBCC
PBPBPB
CC
00
0
00
0
1
1
case III
A B
25
The Langmuir adsorption isotherm (cont’d)
Adsorption On Solid Surface
B,des
B,adsB,
A,des
A,adsA, k
kB
kk
B 00 and
BB,AA,
BB,B,sB
BB,AA,
AA,A,sA
PBPBPB
CC
PBPBPB
CC
00
0
00
0
1
1
AdsorptionStrong kads>> kdes kads>> kdes
B0>>1 B0>>1
Weak kads<< kdes kads<< kdes
B0<<1 B0<<1
1/20
1/20
)(1)(
AB
ABs
PBPB
CC
des
ads
kkB 0
case II
A B
CC
B PB P
s 0
01
des
ads
kkB 0
case I
A
1C
Cs 1C
Cs
PBCCs
0
1/20 )( PB
CCs
AdsorptionA, B both strong
A strong, B weak
A weak, B weak
BB,AA,
BB,B,sB
BB,AA,
AA,A,sA
PBPBPB
CC
PBPBPB
CC
00
0
00
0
BB,B,sB
AA,A,sA
PBC/CPBC/C
0
0
A
BA,B,B,sB
A,sA
PPB/BC/C
C/C
)(
1
00
case III
A B
26
Langmuir adsorption isothermcase I
case II
Case III
Adsorption On Solid Surface
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
PBPBPB
CC
PBPBPB
CC
00
0
00
0
1
1
1/20
1/20
)(1)(
AB
ABs
PBPB
CC
CC
B PB P
s 0
01
large B0 (strong adsorp.)
small B0 (weak adsorp.)moderate B0
Pressure
Am
ount
ads
orbe
d
mono-layer
1C
Cs
PBCCs
0
Strong adsorption kads>> kdes
Weak adsorption kads<< kdes
27
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 Surface
I
II
III
IV
V
relative pres. P/P0
1.0
amou
nt a
dsor
bed
28
CH4003 Lecture Notes 5 (Erzeng Xue)
1) Type I (Langmuir isotherm) - for microporous 1) Type I (Langmuir isotherm) - for microporous materialsmaterials
Adsorbate – nitrogenAdsorbate – nitrogen
Adsorbent - ironAdsorbent - iron
There are 5 main types of adsorptions isothermThere are 5 main types of adsorptions isotherm
29
CH4003 Lecture Notes 5 (Erzeng Xue)
2 ) Type II - for nonporous materials2 ) Type II - for nonporous materials
Adsorbate - oxygenAdsorbate - oxygen
BB
Adsorbent - carbonAdsorbent - carbon
30
CH4003 Lecture Notes 5 (Erzeng Xue)
2 ) Type III – rare– Adsorption heat equal to or less 2 ) Type III – rare– Adsorption heat equal to or less than the heat of liquefactionthan the heat of liquefaction
Adsorbate - bromineAdsorbate - bromine
Adsorbent – silica gelAdsorbent – silica gel
31
CH4003 Lecture Notes 5 (Erzeng Xue)
2 ) Type IV – for mesopore materials 2 ) Type IV – for mesopore materials
Adsorbate – benzeneAdsorbate – benzene
Adsorbent – ferric oxideAdsorbent – ferric oxide
32
CH4003 Lecture Notes 5 (Erzeng Xue)
2 ) Type V – Hysteresis loop (the amount adsrobed 2 ) Type V – Hysteresis loop (the amount adsrobed for given pressure is always greater on for given pressure is always greater on the desorption branch)the desorption branch)
Adsorbate – waterAdsorbate – water
Adsorbent – carbonAdsorbent – carbon
33
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 Surface
1
1 1
1
0
0
PebPeb
PBPB
RT/Q
RT/Q
s s
s
H
of a
ds
LangmuirTemkin
RTiRT/Q
RT/Q
s expPP
iRTdS
PebPebdS
s
s
1
11
01
11
0 b1b1ln
(1[
34
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 Surface
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
PebPeb
PBPB
RT/Q
RT/Q
i i
i
H
of a
ds
Langmuir
Freundlich
211
C/pc
35
CH4003 Lecture Notes 5 (Erzeng Xue)
BET TheoryBET TheoryBET - BET - Brunauer-Emmett-Teller - 1938Brunauer-Emmett-Teller - 1938
AssumptionsAssumptions1. Homogenous surface equal energy surface1. Homogenous surface equal energy surface
2. 2. Only the uppermost molecules of a multilayered adsorbate are in Only the uppermost molecules of a multilayered adsorbate are in dynamic equilibrium with the vapordynamic equilibrium with the vapor
3. Heats of adsorption of the second and higher layers equal to the heat 3. Heats of adsorption of the second and higher layers equal to the heat of condensationof condensation
4. 4. A molecule covered by another molecule cannot evaporateA molecule covered by another molecule cannot evaporate
5. At saturation the number of layers becomes infinite5. At saturation the number of layers becomes infinite
6. 6. No lateral interaction between adsorbed moleculesNo lateral interaction between adsorbed molecules
36
CH4003 Lecture Notes 5 (Erzeng Xue)
Methods for determination of BET surface areaMethods for determination of BET surface areaVolumetric methodsVolumetric methods
Set P/PSet P/P00
PP00 (From table ) (From table )
calculate and set Pcalculate and set P
measure Vmeasure V
open valve of sampleopen valve of sample
measure Pmeasure P
calculate n (pv = nrt)calculate n (pv = nrt)
Not equal
Not equal
EqualEqual
V=V=nRT/P with/with out samplenRT/P with/with out sample
VVadsads=V=Vtotaltotal-V-Vvacantvacant
37
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 temperatureP/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 Surface
)(111 0
0
0 P/PcVc
cV)P/P(VP/P
mm
38
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 Surface
39
Use of BET isotherm to determine the surface area of a solid At low relative pressure P/P0 = 0.05~0.35 it is found that
Y = a + b XThe principle of surface area determination by BET method:
A plot of against P/P0 will yield a straight line with slope of equal to (c-1)/(cVm) and
intersect 1/(cVm).
For a given adsorption system, c and Vm are constant values, the surface area of a solid material can be determined by measuring the amount of a particular gas adsorbed on the surface with known molecular cross-section area Am,
* In practice, measurement of BET surface area of a solid is carried out by N2 physisorption at liquid N2 temperature; for N2, Am = 16.2 x 10-20 m2
Adsorption On Solid Surface
)( )(111 00
0
0 P/PP/PcVc
cV)P/P(VP/P
mm
P PV P P
/( / )
0
01
P/P0
P PV P P
/( / )
0
01
A A N AV
Vs m m mm
T P
,
.6 022 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
40
Summary of adsorption isotherms
Name Isotherm equation Application Note
Langmuir
Temkin =c1ln(c2P)
Freundlich
BET
Adsorption On Solid Surface
)(111 0
0
0 P/PcVc
cV)P/P(VP/P
mm
CC
B PB P
s 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
41
Langmuir-Hinshelwood mechanism This mechanism deals with the surface-catalysed reaction in which
that 2 or more reactants adsorb on surface without dissociation
A(g) + B(g) 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 & B are weakly adsorbed (B0,APA<<1, B0,BPB<<1),
2nd order reaction When A is strongly adsorbed (B0,APA>>1) & B weakly adsorbed (B0,BPB<<1 <<B0,APA)
1st order w.r.t. B
Mechanism of Surface Catalysed Reaction
BB,AA,
BB,B
BB,AA,
AA,A
PBPBPB
PBPBPB
00
0
00
0
1
1
BB,AA,
BAB,A,
BB,AA,
BB,
BB,AA,
AA,i PBPB
PPBkBPBPB
PBPBPB
PBkr
00
00
00
0
00
0
111
BABAB,A,i PP'kPPBkBr 00
BBB,AA,
BAB,A,i P''kPkB
PBPPBkB
r 00
00
A B+ P
42
Eley-Rideal mechanism This mechanism deals with the surface-catalysed reaction in which
that one reactant, A, adsorb on surface without dissociation andother reactant, B, approaching from gas to react with A
A(g) 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
PBPB
kr0
0
0
0
11
BABAA,i PP'kPPkBr 0
BAA,
BAA,i kP
PBPPkB
r 0
0
A P
B
+ B(g)
CH4003 Lecture Notes 15 (Erzeng Xue)
43
Mechanism of surface-catalysed reaction with dissociative adsorption The mechanism of the surface-catalysed reaction in which one
reactant, AD, dissociatively adsorbed on one surface site
AD(g) 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,
AD PBPB
21
0
210
210
210
11 /ADAD,
B/
ADAD,B/
ADAD,
/ADAD,
i PBPPBk
PPB
PBkr
B/
ADB/
ADAD,i PP'kPPBkr 21210
B/
ADAD,
B/
ADAD,i kP
PBPPBk
r 210
210
+ B(g) P
B
A B
CH4003 Lecture Notes 15 (Erzeng Xue)
44
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)
45
Bulk and surface The composition & structure of a solid in bulk and on surface
can differ due toSurface contamination
Bombardment by foreign molecules when exposed to an environmentSurface 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)
46
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
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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
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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
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Many reactions proceed via chain reaction polymerisation explosion …
Elementary reaction steps in chain reactions1. Initiation step - creation of chain carriers (radicals, ions, neutrons etc, which are capable of propagating a chain) by vigorous collisions, photon absorption
R Rž (the dot here signifies the radical carrying unpaired electron)
2. Propagation step - attacking reactant molecules to generate new chain carriersRž + M R + Mž
3. Termination step - two chain carriers combining resulting in the end of chain growthRž + žM R-MThere 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
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Rate law of chain reactionExample: 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
'kk
dtd
[HBr]][Br
]][Br[H2[HBr]
2
3/222
1/2
bd
cab
'k/kk/kk
dtd
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Monomer - the individual molecule unit in a polymer Type I polymerisation - Chain polymerisation
An activated monomer attacks another monomer, links to it, then likes another monomer, so on…, leading the chain growth eventually to polymer.rate law
Initiation: Ix 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] ][Mikx
dtd
initiator chain-carrier
212
2[I] ][M 0][M2[I] 2 ][M
/
t
itipi k
kxk-kxrrxdt
d
[M][I]2
[M] i.e. ][M][M[M] 1/221 /
t
ippp k
kxkdt
dkrdt
d
is the yield of Ix to xR
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Type II polymerisation - Stepwise polymerisationA specific section of molecule A reacts with a specific section of molecule B forming chain
(a-A-a’) + (b’-B-b) {a -A-(a’b’)-B-b}
H2N(CH2)6NH2 + HOOC(CH2)4COOH H2N(CH2)6NHOC(CH2)4COOH + H2O (1)
H-HN(CH2)6NHOC(CH2)4CO-OH …
H-[HN(CH2)6NHOC(CH2)4CO]n-OH (n)Note: If a small molecule is dropped as a result of reaction, like a H2O dropped in rxn (1), this type of reaction is called condensation reaction. Protein molecules are formed in this way.
The rate law for the overall reaction of this type is the same as its elementary step involving one H- containing unit & one -OH containing unit, which is the 2nd order
the conversion of B (-OH containing substance) at time t is
Chain Reactions - PolymerisationComplex Reactions
0
02
[A]1[A][A]or [A][A][-OH][A]kt
kkdt
d
0
0
0
0
[A]1[A]
[A][A][A]
ktktX B
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Type I Explosion: Chain-branching explosionChain-branching - During propagation step of a chain reaction one attack by a
chain carrier can produce more than one new chain carriersChain-branching explosion
When chain-branching occurs the number carriers increases exponentially the rate of reaction may cascade into explosionExample: 2H2(g) + O2(g) 2H2O(g)
Initiation: H2 + O2 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
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Type II Explosion: Thermal explosionA rapid increase of the rate of exothermic reaction with temperatureStrictly speaking thermal explosion is not caused by multiple production of chain carriers
Must be exothermic reaction Must be in a confined space and within short time
H T r H T r H … A combination of chain-branching reaction with heat accumulation can occur
simultaneously
Explosion ReactionsComplex Reactions
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Photochemical reactionThe reaction that is initiated by the absorption of light (photons)
Characterisation of photon absorption - quantum yieldA reactant molecule after absorbing a photon becomes excited. The excitation may lead to product formation or may be lost (e.g. in form of heat emission) The number of specific primary products (e.g. a radical, photon-excited molecule, or an ion)
formed by absorption of each photon, is called primary quantum yield, 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
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Wave-length selectivity of photochemical reaction A light with a specific wave length may only excite a specific type of molecule
Quantum yield of a photochemical rxn may vary with light (wave-length) used Isotope separation (photochemical reaction Application)
Different isotope species - different mass - different frequencies required to match their vibration-rotational energys
e.g. I36Cl + I37Cl I36Cl + I37Cl* (only 37Cl molecules are excited)C6H5Br + I37Cl* C6H5
37Cl + IBr Photosensitisation (photochemical reaction Application)
Reactant molecule A may not be activated in a photochemical reaction because it does not absorb light, but A may be activated by the presence of another molecule B which can be excited by absorbing light, then transfer some of its energy to A.
e.g. Hg + H2 Hg* + H2 (Hg is, but H2 is not excited by 254nm light)Hg* + H2 Hg + 2H* & Hg* + H2 HgH + H*
H* HCO HCHO + H* 2HCO HCHO + CO
Photochemical ReactionsComplex Reactions
508 nm light
254 nm light
CO H2
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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
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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'vcv
c'hvhchvE
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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
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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
AbsorptionReflectionTransmissionScatteringRefraction
Each interaction can disclose certain properties of the matter
When applying e.m.r. of different frequency (thus the energy e.m.r. carried) different type information can be obtained
Interaction of e.m.r. with Matter
refraction
transmission
absorption
reflection scattering
Introductory to Spectroscopy
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Spectrum is the display of the energy level of e.m.r. as a function of wave number of electromagnetic radiation energy
The energy level of e.m.r. is usually expressed in one of these terms absorbance (e.m.r. being absorbed) transmission (e.m.r. passed through) Intensity
The term ‘intensity’ has the meaning of the radiant power that carried by an e.m. r.
Spectrum
.
1.0
0.5
0.0350 400 450
wave length cm-1
inte
nsity
Introductory to Spectroscopy
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What an spectrum tells A peak (it can also be a valley depending on how the spectrum is constructed)
represents the absorption or emission of e.m.r. at that specific wavenumber The wavenumber at the tip of peak is the most important, especially when a peak is broad
A broad peak may sometimes consist of several peaks partially overlapped each other - mathematic software (usually supplied) must be used to separate them case of a broad peak (or a valley) observed
The height of a peak corresponds the amount absorption/emission thus can be used as a quantitative information (e.g. concentration), a careful calibration is usually required
The ratio in intensity of different peaks does not necessarily means the ratio of the quantity (e.g. concentration, population of a state etc.)
Spectrum
.
1.0
0.5
0.0 350 400 450wave length cm-1
inte
nsity
Introductory to Spectroscopy
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Spectral properties, applications, and interactions of electromagnetic radiation
absorptionemissionfluorescence
Magneticallyinduced spinstates
Electronparamagnetresonance
Infrared
Wave numberv’
cm-1
Wavelength
cm
Frequencyv
Hz
Energy
kcal/mol Electronvole eV
Type of radiation
Type of spectroscopy
Type of quantum transition
9.4x107 4.1x106 3.3x1010 3.0x10-11 1021
9.4x105 4.1x104 3.3x108 3.0x10-9 1019
9.4x103 4.1x102 3.3x106 3.0x10-7 1017
9.4x101 4.1x100 3.3x104 3.0x10-5 1015
9.4x10-1 4.1x10-2 3.3x102 3.0x10-3 1013
9.4x10-3 4.1x10-4 3.3x100 3.0x10-1 1011
9.4x10-5 4.1x10-6 3.3x10-2 3.0x101 109
9.4x10-7 4.1x10-8 3.3x10-4 3.0x103 107
Gamma ray
X-ray
Ultra Violet
Visible
Microwave
Radio
X-rayabsorption emission
NuclearGamma ray
emission
Electronic(outer shell)
Molecularrotation
Molecularvibration
Nuclear magneticresonance
Microwaveabsorption
UV absorption
IR absorptionRaman
VacUVVis
Electronic(inner shell)
Introductory to Spectroscopy
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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
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Shell structure & energy level of atoms In an atom there are a number of shells and
of subshells where e-’s can be found The energy level of each shell & subshell
are different and quantised The e-’s in the shell closest to the nuclei has
the lowest energy. The higher shell number is, the higher energy it is
The exact energy level of each shell and subshell varies with substance
Ground state and excited state of e-’s Under normal situation an e- stays at the
lowest possible shell - the e- is said to be at its ground state
Upon absorbing energy (excited), an e- can change its orbital to a higher one - we say the e- is at is excited state.
Atomic SpectraIntroductory to Spectroscopy
n = 1
n = 2
n = 3, etc.
energy E
groundstate
Excitedstate
Ener
gyn=1
n=2
n=3
n=4
1s2s2p3s3p
4s3d4p4d4f
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Electron excitation The excitation can occur at different degrees
low E tends to excite the outmost e-’s first when excited with a high E (photon of high v)
an e- can jump more than one levels even higher E can tear inner e-’s away from
nuclei
An e- at its excited state is not stable and tends to return its ground state
If an e- jumped more than one energy levels because of absorption of a high E, the process of the e- returning to its ground state may take several steps, - i.e. to the nearest low energy level first then down to next …
Atomic Spectra
Ener
gyn=1
n=2
n=3
n=4
1s2s2p3s3p
4s3d4p4d4f
n = 1
n = 2
n = 3, etc.
energy E
Introductory to Spectroscopy
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Atomic spectraThe level and quantities of energy supplied
to excite e-’s can be measured & studied in terms of the frequency and the intensity of an e.m.r. - the absorption spectroscopy
The level and quantities of energy emitted by excited e-’s, as they return to their ground state, can be measured & studied by means of the emission spectroscopy
The level & quantities of energy absorbed or emitted (v & intensity of e.m.r.) are specific for a substance
Atomic spectra are mostly in UV (sometime in visible) regions
Atomic Spectra
Ener
gyn=1
n=2
n=3
n=4
1s2s2p3s3p
4s3d4p4d4f
n = 1
n = 2
n = 3, etc.
energy E
Introductory to Spectroscopy
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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
v2v3
v4
v1
v2v3
v4
v1
v2v3
v4
v1
v2v3
v4
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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
v2v3
v4
S0
T1
S2
S1
v1
v2v3
v4
v1
v2v3
v4
v1
v2v3
v4
absorptionA
A
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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
v2v3
v4
v1
v2v3
v4
v1
v2v3
v4
v1
v2v3
v4
Inter- systemcrossing
Internaltransition
B
B
E1
E2
C
F
A
B
FluorescenceD
Fluorescence
Jablonsky diagram
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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
v2v3
v4
v1
v2v3
v4
B
Aphosphor-enscence
D
Fluore-scence
T1
v1
v2v3
v4
F
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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 themThus
UV & Visible SpectrophotometrySpectroscopy Application
Incident light I0
Emergent lightI
C
b
I'kdxNcs
dI2I0
dx
bx
s
sI
number of moleculesN-Avogadro number
light absorbed
fraction of light
acdxdxNcs'kI
dI 2
acbIIdxac
IdI b
bI
I
b 0
0ln
0
AabcII
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, ethanol210-220nm n-butyl alcohol, isopropyl alcohol, cyclohexane, ethyl ether245-260nm chloroform, ethyl acetate, methyl formate265-275nm carbon tetrachloride, dimethyl sulphoxide/formamide, acetic acid280-290nm benzene, toluene, m-xylene300-400nm pyridine, acetone, carbon disulphide
Effect of temperature Varying temperature may cause change of concentration of a solute because of
thermal expansion of solution changing of equilibrium composition if solution is in equilibrium
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 various
states; 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 375ethanoic acid 210 pentacene 575ethyl ethanoate 210ethanamide 220 conjugated 1,3-butadiene 220
1,3,5-hexatriene 250 C-X methanol 180 2-propenal 320
trimethylamine 200 -carotene (11 C=C) 480chloromethane 170bromomethane 210 each additional C=C +30iodomethane 260
UV & Visible SpectrophotometrySpectroscopy Application
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Instrumentation
UV visibleLight 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 SpectrumRegion 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 Detector Signal Processor
Readout
Type Method of Atomization Radiation Source
atomic (flame) sample solution aspirated Hollow cathode into a flame lamp (HCL)
atomic (nonflame) sample solution HCL evaporated & ignited
x-ray absorption none required x-ray tube
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Atomic Emission Spectrometer (AES)Spectroscopy Application
Source
Sample
P Wavelength Selector Detector Signal 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 Detector Signal 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 statePI can be achieved by a variety means (electrical, optical, chemical or mechanical), e.g., one may obtain PI by irradiating the system of atoms by an enormously intense light beam or, if the system of atoms is a gas, by passing an electric current through the gas.
Presence of Metastable state, which is the excited state that the excited e-’s can have a relatively long lifetime (>10-8 second), in order to avoid the spontaneous emission occurring before the stimulated emissionIn most lasers, the atoms/molecules/ions in the lasing medium are not “pumped” directly to a metastable state. They are excited to an energy level higher than a metastable state, then drop down to the metastable state by spontaneous non-radiative de-excitation.
Photon Confinement (PC), the emitted photons must be confined in the system long enough to stimulate further light emission from other excited atomsThis is achieved by using reflecting mirrors at the ends of the system. One end is made totally reflecting & the other is slight transparent to allow part of the laser beam to escape.
Laser - Formation & ConditionsSpectroscopy Application
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Laser - Functional ElementsSpectroscopy Application
Energy pumping mechanism
Energy input
Lasing medium
Highreflectance
mirror
Partially transmitting
mirror
OutputcouplerFeedback mechanism
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Laser ActionSpectroscopy Application
Lasing medium at ground state
Population
inversion
Start of stimulated emission
Stimulated 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
CommerceCompact disk, laser printer, copiers, optical disk drives, bar code scanner, optical communications, laser shows, holograms, laser pointers
IndustryMeasurements (range, distance), alignment, material processing (cutting, drilling, welding, annealing, photolithography, etc.), non-destructive testing, sealing
MedicineSurgery (eyes, dentistry, dermatology, general), diagnostics, ophthalmology, oncology
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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