Adsorption On Solid Surface
description
Transcript of Adsorption On Solid Surface
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Adsorption Adsorption is a process in which molecules from gas (or liquid) phase land
on, interact with and attach to solid surfaces. The reverse process of adsorption, i.e. the process in which adsorbed
molecules escape from solid surfaces, is called Desorption. Molecules can attach to surfaces in two different ways because of the
different forces involved. These are Physisorption (Physical adsorption) & Chemisorption (Chemical adsorption)
Physisorption Chemisorption
force van de Waal chemcal bondnumber of adsorbed layers multi only one layer
adsorption heat low (10-40 kJ/mol) high ( > 40 kJ/mol)selectivity low high
temperature to occur low high
Adsorption On Solid SurfaceCatalysis & Catalysts
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Catalyst composition Active phase
Where the reaction occurs (mostly metal/metal oxide)
Promoter Textual promoter (e.g. Al - Fe for NH3 production) Electric or Structural modifier Poison resistant promoters
Support / carrier Increase mechanical strength Increase surface area (98% surface area is supplied within the porous structure) may or may not be catalytically active
Solid CatalystsCatalysis & Catalysts
CatalystActiv
e ph
ase
Support
Promoter
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Some common solid support / carrier materials
Alumina Inexpensive Surface area: 1 ~ 700 m2/g Acidic
Silica Inexpensive Surface area: 100 ~ 800 m2/g Acidic
Zeolite mixture of alumina and silica, often exchanged metal ion present shape selective acidic
Solid CatalystsCatalysis & Catalysts
Other supports Active carbon (S.A. up to 1000 m2/g) Titania (S.A. 10 ~ 50 m2/g) Zirconia (S.A. 10 ~ 100 m2/g) Magnesia (S.A. 10 m2/g) Lanthana (S.A. 10 m2/g)
poreporous solid
Active site
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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 Solids
b
d
a
dwdd
d
wt
b awt
d
Cumulative curve Frequency curve
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Adsorption processAdsorbent and adsorbate Adsorbent (also called substrate) - The solid that provides surface for adsorption
high surface area with proper pore structure and size distribution is essential good mechanical strength and thermal stability are necessary
Adsorbate - The gas or liquid substances which are to be adsorbed on solid
Surface coverage, The solid surface may be completely or partially covered by adsorbed molecules
Adsorption heat Adsorption is usually exothermic (in special cases dissociated adsorption can be
endothermic) The heat of chemisorption is in the same order of magnitude of reaction heat;
the heat of physisorption is in the same order of magnitude of condensation heat.
Adsorption On Solid SurfaceCatalysis & Catalysts
define = = 0~1number of adsorption sites occupiednumber of adsorption sites available
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Applications of adsorption process Adsorption is a very important step in solid catalysed reaction processes
Adsorption in itself is a common process used in industry for various purposes Purification (removing impurities from a gas / liquid stream) De-pollution, de-colour, de-odour Solvent recovery, trace compound enrichment etc…
Usually adsorption is only applied for a process dealing with small capacity The operation is usually batch type and required regeneration of saturated adsorbent
Common adsorbents: molecular sieve, active carbon, silica gel, activated alumina.
Physisorption is 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 SurfaceCatalysis & Catalysts
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Adsorption On Solid Surface Characterisation of adsorption system
Adsorption isotherm - most commonly used, especially to catalytic reaction system, T=const.The amount of adsorption as a function of pressure at set temperature
Adsorption isobar - (usage related to industrial applications)The amount of adsorption as a function of temperature at set pressure
Adsorption Isostere - (usage related to industrial applications)Adsorption pressure as a function of temperature at set volume
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
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The Langmuir adsorption isotherm Basic assumptions
surface uniform (Hads does not vary with coverage) monolayer adsorption, and no interaction between adsorbed molecules and adsorbed molecules immobile
Case I - single molecule 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
CC
B PB P
s 0
01des
ads
kkB 0
PBk/kPk/k
desads
desads
0)(1)(
case I
A
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Adsorption On Solid Surface The Langmuir adsorption isotherm (cont’d)
Case II - single molecule adsorbed dissociatively on one site
A-B(g) + M(surface site) A-M-B
the rate of A-B adsorption rads=kads (1))PAB=kads (1)2PAB
the rate of A-B desorption rdes=kdes=kdes2
at equilibrium rads = rdes kads (1)2PAB= kdes2
rearrange it for
Let.
case II
A BBA
==
1/20
1/20
)(1)(
AB
ABs
PBPB
CC
des
ads
kkB 0
)(1
)(
ABdesads
ABdesads
Pk/kPk/k
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The Langmuir adsorption isotherm (cont’d) Case III - two molecules adsorbed on two sites
A(g) + B(g) + 2M(surface site) A-M + B-M
the rate of A 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
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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
1CCs 1
CCs
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
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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
1CCs
PBCCs
0
Strong adsorption kads>> kdes
Weak adsorption kads<< kdes
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Five types of physisorption isotherms are found over all solids
Type I is found for porous materials with small pores e.g. charcoal. It is clearly Langmuir monolayer type, but the other 4 are not
Type II for non-porous materials
Type III porous materials with cohesive force between adsorbate molecules greater than the adhesive force between adsorbate molecules and adsorbent
Type IV staged adsorption (first monolayer then build up of additional layers)
Type V porous materials with cohesive force between adsorbate molecules and adsorbent being greater than that between adsorbate molecules
Adsorption On Solid Surface
I
II
III
IV
V
relative pres. P/P0
1.0
amou
nt a
dsor
bed
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Other adsorption isothermsMany other isotherms are proposed in order to explain the observations
The Temkin (or Slygin-Frumkin) isotherm Assuming the adsorption enthalpy H decreases linearly with surface coverage
From ads-des equilibrium, ads. rate des. rate
rads=kads(1-)P rdes=kdes
where Qs is the heat of adsorption. When Qs is a linear function of i. Qs=Q0-iS (Q0 is a constant, i is the number and S represents the surface site),
the overall coverage
When b1P >>1 and b1Pexp(-i/RT) <<1, we have =c1ln(c2P), where c1 & c2 are constants
Valid for some adsorption systems.
Adsorption On Solid Surface
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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[
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The Freundlich isotherm assuming logarithmic change of adsorption enthalpy H with surface coverage
From ads-des equilibrium, ads. rate des. rate
rads=kads(1-)P rdes=kdes
where Qi is the heat of adsorption which is a function of i. If there are Ni types of surface sites, each can be expressed as Ni=aexp(-Q/Q0) (a and Q0 are constants), corresponding to a fractional coverage i,
the overall coverage
the solution for this integration expression at small is:
ln=(RT/Q0)lnP+constant, or
as is the Freundlich equation normally written, where c1=constant, 1/c2=RT/Q0
Freundlich isotherm fits, not all, but many adsorption systems.
Adsorption On Solid 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
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BET (Brunauer-Emmett-Teller) isotherm Many physical adsorption isotherms were found, such as the types II and III, that the
adsorption does not complete the first layer (monolayer) before it continues to stack on the subsequent layer (thus the S-shape of types II and III isotherms)
Basic assumptions the same assumptions as that of Langmuir but allow multi-layer adsorption the heat of ads. of additional layer equals to the latent heat of condensation based on the rate of adsorption=the rate of desorption for each layer of ads.
the following BET equation was derived
Where P - equilibrium pressureP0 - saturate vapour pressure of the adsorbed gas at the 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
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Comment on the BET isotherm BET equation fits reasonably well all known adsorption isotherms observed so far
(types I to V) for various types of solid, although there is fundamental defect in the theory because of the assumptions made (no interaction between adsorbed molecules, surface homogeneity and liquefaction heat for all subsequent layers being equal).
BET isotherm, as well as all other isotherms, gives accurate account of adsorption isotherm only within restricted pressure range. At very low (P/P0<0.05) and high relative pressure (P/P0>0.35) it becomes less applicable.
The most significant contribution of BET isotherm to the surface science is that the theory provided the first applicable means of accurate determination of the surface area of a solid (since in 1945).
Many new development in relation to the theory of adsorption isotherm, most of them are accurate for a specific system under specific conditions.
Adsorption On Solid Surface
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Use of BET isotherm to determine the surface area of a solid At low relative pressure P/P0 = 0.05~0.35 it is found that
Y = a + b 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 AVVs 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
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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
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The BET isotherm
00a
0mm0a
ppsI
ppvp
or
pp
Cv1C
Cv1
ppvp
Theoretical development based on several assumptions: multimolecular adsorption 1st layer with fixed heat of adsorption H1
following layers with heat of adsorption constant (= latent heat of condensation)
constant surface (i.e. no capillary condensation) gives
OT fig1.3
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The BET isotherm, cont.
00a ppsI
ppvp
Plot of left side vs. p/p0 should give straight line with slope s and intercept I
Reorganizing gives
Knowledge of S0 (specific area for a volume of gas then allows the calculation of the specific surface area Sg:
where mp is the mass of the sample
IsICand
Is1vm
OT fig1.5
p
0mg m
SvS
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BET cont’d
BET method useful, but has limitations microporous materials: mono - multilayer adsorption cannot occur, (although BET surface
areas are reported routinely) assumption about constant packing of N2 molecules not always correct? theoretical development dubious (recent molecular simulation studies, statistical
mechanics) - value of C is indication o f the shape of the isotherm, but not necessarily related to heat of adsorption
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Simplified method
1-point method simplefied BET assuming value of C 100 (usually the case), gives
usually choose p/p0 0,15 method underestimates the surface area by approx. 5%.
0
0a'm
0'm0mm0a
pppvv
pvp
pp
Cv1C
Cv1
ppvp
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Adsorbates An adsorbate molecule covers an area , calculated assuming dense packing of the
molecules in the multilayer. The corresponding area per volume gas is S0:
Gas Temp.[K]
σ[Å2/molecule]
S0[m2/cm3 gas (STP)]
N2 77,5 16,2 4,36Kr 77,5 19,5 5,24Ar 77,5 14,6 3,92
H2O 298 10,8 2,90C2H6 90 22,5 6,05CO2 195 19,5 5,24
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Porosity and pore size
The pore structure (porosity, pore diameter, pore shape) is important for the catalytic properties pore diffusion may influence rates pores may be too small for large molecules to diffuse into
Measurement techniques: Hg penetration interpretation of the adsorption - desorption isotherms electron microscopy techniques
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Hg penetration
Based on measuring the volume of a non-wetting liquid forced into the pores by pressure (typically mercury)
Surface tension will hinder the filling of the pores, at a given pressure an equilibrium between the force due to pressure and the surface tension is established:
where P = pressure of Hg, is surface tension and is the angle of wetting Common values used: = 480 dyn/cm and = 140° give average pore radius
valid in the range 50 - 50000Å
cosr2rP 2
Å]cm/kp[P
75000r 2
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Pore size distribution
If the Hg-volume is recorded as a function of pressure and this curve is differentiated we can find the pore size distribution function V(r)=dV/dr
OT fig 2.3.
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The Kelvin equation
0
_
ln
2
ppRT
Vrk
If adsorbent is mesoporous we get Type IV isotherm
Deviation upwards is due to filling of mesopores by capillary condensation - curved liquid meniscus in narrow pores with radius rk:
V is molar volume of the liquid, minus sign introduced since in the actual range of measurement 0 < p/p0 <1
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The Kelvin equation
Since capillary condensation is preceeded by multilayer adsorption on the wall the value is corrected with t, the thickness of this layer:Cylindrical pores: rp = rk + t
Parallell sided slits: dp = rk + 2t
Value of t determined from measurements without capillary condensation Practical experience, typical values give for circular pores:
Values for t have been found to be a function of rk, e.g. for rk > 20Å:
][ln
547,9
0
Å
pp
rk
Årt k61,2ln429,0
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Adsorption-desorption hysteresis
Hysteresis is classified by IUPAC (see fig.) Traditionally desorption branch used for
calculation H1: narrow distribution of mesopores H2: complex pore structure, network
effects, analysis of desorption loop misleading H2: typical for activated carbons
H3 & 4: no plateau, hence no well-defined mesopore structure, analysis difficult H3: typical for clays
Handbookfig 2 s 431