Unit 9: Revisiting, Editing, and Revising (Dec 8 - Dec 14) Unit 9 Seminar.
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Transcript of Seminar core slides malaysia dec 2013
Characterization of Powders & Porous Solids
A sharing session . . .
Mr Mohd Zulkiffli A Bakar
Itinerary (11.12.13)
TIME TOPIC REMARKS
0900 ~ 1300 Gas Sorption
1300 ~ 1400 Lunch
1400~1700 Mercury porosimetryChemisorption
1700~1730 Q & A
Itinerary (12.12.13)
TIME TOPIC REMARKS
0900 ~ 1300 Gas Sorption
1300 ~ 1400 Lunch
1400~1700 Microporosity
1700~1730 Q & A
Itinerary (13.12.13)
TIME TOPIC REMARKS
0900 ~ 1300 Gas Sorption , Microporosimetry
1300 ~ 1400 Lunch
1400~1700 MPOB
1700~1730 Q & A
History of sorption science
History sorption science
Pioneers of sorption science
Main Characteristics of Powders and Porous Solids
� Particle size� Surface area� Porosity
Why We Care About Particle Size and Surface Area� These characteristics control many properties of
materials:� Flowability;� “Filter-ability”� Viscosity-Reology;� Agglomeration;� Dusting tendency;� Settling rate;� Activity/Reactivity rate (e.g. of catalyst);� Dissolution rate (of pharmaceutical);� Gas absorption;� Hydration rate (of cement);� Moisture absorption;� Entry into lungs (shape dependency too);� Combustion rate (of fuel)� Etc…
What is Particle Size?
SEM of real ibuprofen particles
A Concept of Equivalent Sphere� Due to symmetry, size of sphere is
completely determined by only one parameter – it’s diameter (radius)
� Other properties of sphere are easily computed from its size:
� Sphere is just a convenient model! This is why it is found throughout the particle size analysis
3
6
1dV π= 2
dS π= 3
6dm π
ρ=
Different Equivalent Spheres
Particle Size Measurement Techniques
� Direct observation (image analysis)� Sieving;� Sedimentation – settling rate;� Coulter counter – electrozone sensing;� Gas adsorption – BET (SSA back extrapolation
to size);� Permeability (gas or liquid) e.g. Blaine, FSSS� Light scattering – laser diffraction and Photon
Correlation Spectroscopy / Dynamic Light Scattering
And What Do They Measure� Direct observation (image analysis) – usually
some 2-D representation of a particle. Which dimension is viable?;
� Sieving – combination of particle size and shape;
� Sedimentation – settling rate. Stokes Law (spheres, straight line settling);
� Coulter counter – electrozone sensing;� Gas absorption / Permeability – surface area.
Extrapolate to average particle size only. – BET (SSA back extrapolation to size);
� Light scattering – equivalent scatterers;
Particle Size by Direct Observation
Google for ImageJ
Dynamic Light Scattering (DLS)� DLS measures Brownian motion and relates this to the size of the
particles.
� The larger the particle the slower the Brownian motion will be. Smaller particles are “kicked” further by the solvent molecules and move more rapidly.
� The velocity of Brownian motion is defined by a property known as the translational diffusion coefficient (D).
� The size of a particle is calculated from the translational diffusion coefficient by using the Stokes-Einstein equation:
d(H) – hydrodynamic diameter, D – translational diffusion coefficient, k – Boltzmann’s constant, T – temperature, η - viscosity
D
kTHd
πη3)( =
What We Measure in DLS?� The diameter that is measured in
DLS is a value that refers to how a particle diffuses within a fluid so it is referred to as a hydrodynamic diameter
� The diameter that is obtained by this technique is the diameter of a sphere that has the same translational diffusion coefficient as the particle
� The translational diffusion coefficient will depend not only on the size of the particle “core”, but also on any surface structure, as well as the concentration and type of ions in the medium
Particle core
Shell formed by solvent particles, ions etc. Low conductivity medium will produce an extended double layer of ions around the particle, reducing the diffusion speed and
resulting in a larger, apparenthydrodynamic diameter.
Thus, the measurements are usually done in 10mM
NaCl (ISO13321 Part 8 1996)
How DLS Works
� The dark spaces in the speckle pattern produced by light scattering are where the phase additions of the scattered light are mutually destructive. The bright spots of light in the speckle pattern are where the light scattered from the particles arrives with the same phase and interfere constructively.
� The observed signal depends on the phase addition of the scattered light falling on the detector. In example A, two beams interfere and “cancel each other out” resulting in a decreased intensity detected. In example B, two beams interfere and “enhance each other” resulting in an increased intensity detected.
How DLS Works
� For a system of particles undergoing Brownian motion, a speckle pattern is observed where the position of each speckle is seen to be in constant motion. This is because the phase addition from the moving particles is constantly evolving and forming new patterns.
� The rate at which these intensity fluctuations occur will depend on the size of the particles. Figure above schematically illustrates typical intensity fluctuations arising from a dispersion of large particles and a dispersion of small particles.
� The small particles cause the intensity to fluctuate more rapidly than the large ones.
� It is possible to directly measure the spectrum of frequencies contained in the intensity fluctuations arising from the Brownian motion of particles, but it is inefficient to do so. The best way is to use a device called a digital auto correlator.
How an Auto Correlator Works
� If the intensity of a signal is compared with itself at a particular point in time and a time much later, then for a randomly fluctuating signal it is obvious that the intensities are not going to be related in any way, i.e. there will be no correlation between the two signals.
� However, if the intensity of signal at time t is compared to the intensity a very small time later (t+δt), there will be a strong relationship or correlation between the intensities of two signals.
� Perfect correlation is indicated by unity (1.00) and no correlation is indicated by zero (0.00).
� If the signals at t+2δt, t+3δt, t+4δt etc. are compared with the signal at t, the correlation of a signal arriving from a random source will decrease with time until at some time, effectively t = ∞, there will be no correlation.
� If the particles are large the signal will be changing slowly and the correlation will persist for a long time. If the particles are small and moving rapidly then correlation will reduce more quickly.
Different Forms of Particle Size Distribution
� Consider 2 populations of spherical particles of diameter 5nm and 50nm present in equal numbers.
� If a number distribution of these 2 particle populations is plotted, a plot consisting of 2 peaks (positioned at 5 and 50nm) of a 1 to 1 ratio would be obtained.
� If this number distribution was converted into volume, then the 2 peaks would change to a 1:1000 ratio (because the volume of a sphere is proportional to d3).
� If this was further converted into an intensity distribution, a 1:1000000 ratio between the 2 peaks would be obtained (because the intensity of scattering is proportional to d6 from Rayleigh’s approximation).
� In DLS, the distribution obtained from a measurement is based on intensity.
Schematics of Zetasizer Nano
Measurement of Porosity and Specific Surface Area by
Gas Adsorption
Name 2 methods to measure particle size
- Laser scattering
- Optical ( microscopy)A
?
? ?
??
?
?
Quiz
F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 1-25, 1999
What are Porous Materials?
Non-porous solid� Low specific surface area� Low specific pore volume
Porous solid� High specific surface area� High specific pore volume
Porous materials have highly developed internal surface area that can be used to perform specific function.Almost all solids are porous except for ceramics fired at extremely high temperatures
Looking at the diagram, how to tell if a particle is porous?
Porous if and only if value of pore depth is larger than value of pore width
A
?
? ?
??
?
?
Quiz
Measure of Porosity
Pore size and its distribution
Specific Surface Area, m2/g =
Porosity
There are three parameters used as a measure of porosity; specific surface area, specific pore volume or porosity, and pore size and its distribution.
Mass of the solid, g
Total surface area, m2
Specific Pore volume, cm3/g
Mass of the solid, g
Total pore volume, cm3
=
Porosity, % =
Volume of solid (including pores)
Volume of poresX 100
Concept of Porosity: Open vs. Closed Pores
Dead end (open)
ClosedInter-connected (open)
Passing (open)
F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 1-25, 1999
Open pores are accessible whereas closed pores are inaccessible pores. Open pores can be inter-connected, passing or dead end.
Size of Pores (IUPAC Standard)
2 nm 50 nm
Micropores Mesopores Macropores
Zeolite,Activated carbon,Metal organicframework
Mesoporous silica, Activated carbon
Sintered metals and ceramics
Porous material are classified according to the size of pores: material with pores less than 2 nm are called micropores, materials with pores between 2 and 50 nm are called mesopores, and material with pores greater than 50 nm are macrospores
Sing, K. S. W. et al. Reporting Physisorption Data for Gas/Solid Systems. Pure & Appl. Chem. 57,603-619 (1985).
Shapes of Pores
Conical
Interstices
SlitsCylindrical
Spherical orInk Bottle
Pore Shapes
F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 1-25, 1999
Will pore size be the same as particle size ?
Particle size measures external cross-sectional diameter, while pore size measures measuresmean internal pore diameter
A
?
? ?
??
?
?
Quiz
Experimental Techniques
Techniques for Porosity Analysis
Mercuryporosimetry
TEM
SEM
Small angleX-ray
scattering
SmallAngle
Neutron scattering
Gas adsorption
Techniques
Mercuryporosimetry
TEM
SEM
Small angleX-ray
scattering
SmallAngle
Neutron scattering
Gas adsorption
Techniques
� Can measure only open pores� Pore size : 0.4 nm – 50 nm� Easy� Established technique
Techniques for Porosity Analysis
Mercuryporosimetry
TEM
SEM
Small angleX-ray
scattering
SmallAngle
Neutron scattering
Gas adsorption
Techniques
� Similar to gas adsorption
� Can measure only open pores
� Pore size >1.5 nm� Easy� Established technique
Techniques for Porosity Analysis
Mercuryporosimetry
TEM
SEM
Small angleX-ray
scattering
SmallAngle
Neutron scattering
Gas adsorption
Techniques
� Provide information regarding pore connectivity
� Pore size can be measured if the materials contains ordered pores
� Rarely used for pore analysis
Techniques for Porosity Analysis
Mercuryporosimetry
TEM
SEM
Small angleX-ray
scattering
SmallAngle
Neutron scattering
Gas adsorption
Techniques
� Pore size > 5nm� Rarely used for pore
analysis
Techniques for Porosity Analysis
Mercuryporosimetry
TEM
SEM
Small angleX-ray
scattering
SmallAngle
Neutron scattering
Gas adsorption
Techniques
� Any pore size� Open + Close
porosity
Techniques for Porosity Analysis
Mercuryporosimetry
TEM
SEM
Small angleX-ray
scattering
SmallAngle
Neutron scattering
Gas adsorption
Techniques
� Any pore size� Open & Close
porosity� Costly
Techniques for Porosity Analysis
Theory of Adsorption
Adsorption Process
Adsorption is brought by the forces acting between the solid and themolecules of the gas. These forces are of two kinds: physical(physiosorption) and chemical (chemisorption)
Adsorbent - the solid where adsorption takes place
Adsorbate - the gas adsorbed on the surface of solids
Adsorptive - adsorbate before being adsorbed on the surface
PHYSISORPTION CHEMISORPTIONWEAK, LONG RANGE BONDING
Van der Waals interactionsSTRONG, SHORT RANGE BONDING
Chemical bonding involved.
NOT SURFACE SPECIFICPhysisorption takes place between all
molecules on any surface providing the temperature is low enough.
SURFACE SPECIFICE.g. Chemisorption of hydrogen takes place on
transition metals but not on gold or mercury.
∆Hads = 5 ….. 50 kJ mol-1 ∆Hads = 50 ….. 500 kJ mol-1
Non activated with equilibrium achieved relatively quickly. Increasing temperature
always reduces surface coverage.
Can be activated, in which case equilibrium can be slow and increasing temperature can favour
adsorption.
No surface reactions. Surface reactions may take place:- Dissociation, reconstruction, catalysis.
MULTILAYER ADSORPTIONBET Isotherm used to model adsorption
equilibrium.
MONOLAYER ADSORPTIONLangmuir Isotherm is used to model adsorption
equilibrium.
Physisorption vs Chemisorption
http://www.soton.ac.uk
Adsorption Process
1. Diffusion to adsorbent surface2. Migration into pores of adsorbent3. Monolayer builds up of adsorbate
1 2 3
�Gas molecules admittedunder increasing pressure toa clean, cold surface.
�Data treatment techniquesfind the quantity of gas thatforms the first layer.1 2 3
S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991
Adsorption Process
Adsorbent
Adsorbate
adsorptive of pressure saturated
adsorbate of pressure
where
:as written becan equation
above theconstant, made are I and T, W,If
adsorbent. and adsorbatebetween n interactio
re; temperatu
adsorbate; theof pressure
adsorbent; of weight
adsorbed; gas of volume
where
),,,(
=
=
=
=
=
=
=
=
p
p
p
pf
I
T
P
W
PITWf
o
oV
V
V
a
a
a
Equation of adsorption isotherm
S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991
Gas Sorption: Isotherm
Adsorption isotherm�Isotherm is a measure of the volume of gas adsorbed at a constant temperature as a function of gas pressure.�Isotherms can be grouped into six classes.
adsorptive of pressure saturated
adsorbate of pressure
where
=
=
p
p
p
pf
o
oV a
Va
Desorption isotherm
ppo
Gas Sorption: IsothermV
a
1P/Po
Type Ior
Langmuir
S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991
�Concave to the P/Po axis�Exhibited by microporous solids ( < 2nm )
1P/Po
Type II
�Exhibited by nonporous ormacroporous solids ( > 50nm )�Unrestricted monolayer-multilayeradsorption�Point B indicates the relativepressure at which monolayercoverage is complete
B
Va
Gas Sorption: IsothermV
a
1P/Po
Type III �Convex to the P/Po axis�Exhibited by nonporous solids
Va
1P/Po
Type IV�Exhibited by mesoporous solids�Initial part of the type IV follows the same path as the type II
S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991
Gas Sorption: IsothermV
a
1P/Po
Type V
1P/Po
Type VI
�Highly uncommon�Exhibited by mesoporous solids
�Exhibited by nonporous solids with an almost completely uniform surface
S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991
Va
Gas Sorption: Hysteresis
�Hysteresis indicates the presence of mesopores.
�Hysteresis gives information regarding pore shapes .
�Types I, II and III isotherms are generally reversible but typeI can have a hysteresis. Types IV and V exhibit hysteresis.
1P/Po
Hysteresis
Va
S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991
Gas Sorption: HysteresisV
a
1P/Po
Type A
Cylindrical Slits
Type B
1P/Po 1P/Po
Type C Type D
1P/Po
Type E
1P/Po
Conical Bottle neck
Adsorption Theories: Langmuir
Adsorbate
Adsorbent
Assumptions:
� homogeneous surface (all adsorption sites energetically identical)
� monolayer adsorption (no multilayer adsorption)
� no interaction between adsorbed molecules
adsorbate. of pressure
and constant; empirical
monolayer; form torequired gas of volume
; pressureat adsorbed gas of volume
where
1
=
=
=
=
+=
P
b
V
PV
V
P
bVV
P
m
a
mma
I. Langmuir The Constitution and Fundamental Properties of Solids and Liquids. Part I. Solids. J. Am. Chem. Soc., 1916, 38 (11), 2221-2295
� 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 adsorption rads = kads (1-θ) Pthe rate of desorption rdes = kdes θ
at equilibrium rads = rdes ⇒ kads (1-θ) P = kdes θ
rearrange it for θ
let ⇒ B0 is adsorption coefficient
56
θ = =+∞
C
C
B P
B P
s 0
01des
ads
k
kB =0
PBk/k
Pk/k
desads
desads
0)(1
)(
+=θ
case I
A
)
� The Langmuir adsorption isotherm (cont’d)�Case II - single molecule adsorbed
dissociatively on one siteA-B(g) + M(surface site) ���� A-M-B
the rate of A-B adsorption rads=kads (1−θΑ )(1−θΒ)PAB=kads (1−θ )2PAB
the rate of A-B desorption rdes=kdesθΑθΒ =kdesθ2
at equilibrium rads = rdes ⇒ kads (1−θ )2PAB= kdesθ2
rearrange it for θ
Let. ⇒
57
case II
A B
BAθ=θΑ=θ
Β
1/20
1/20
)(1
)(
AB
ABs
PB
PB
C
C
+==
∞
θdes
ads
k
kB =0
)(1
)(
ABdesads
ABdesads
Pk/k
Pk/k
+=θ
)
� The Langmuir adsorption isotherm (cont’d)�Case III - two molecules adsorbed on two sites
A(g) + B(g) + 2M(surface site) ���� A-M + B-M
the rate of A adsorption rads,A = kads,A (1− θΑ− θΒ) PA
the rate of B adsorption rads,B = kads,B (1− θΑ− θΒ) PB
the rate of A desorption rdes,A = kdes,A θΑ
the rate of B desorption rdes,B = kdes,B θΒ
at equilibrium rads ,A = rdes ,A and ⇒ rads ,B = rdes ,B
⇒ kads,A(1−θΑ−θΒ)PA=kdes,AθΑ and kads,B(1−θΑ−θΒ)PB=kdes,BθΒ
rearrange it for θ
where are adsorption coefficients of A & B.
58
B,des
B,ads
B,
A,des
A,ads
A,k
kB
k
kB == 00 and
BB,AA,
BB,B,s
B
BB,AA,
AA,A,s
APBPB
PB
C
C
PBPB
PB
C
C
00
0
00
0
1
1 ++==
++==
∞∞
θθ
case III
A B
� The Langmuir adsorption isotherm (cont’d)
59
B,des
B,ads
B,
A,des
A,ads
A,k
kB
k
kB == 00 and
BB,AA,
BB,B,s
B
BB,AA,
AA,A,s
A
PBPB
PB
C
C
PBPB
PB
C
C
00
0
00
0
1
1
++==
++==
∞
∞
θ
θ
Adsorption
Strong kads>> kdes kads>> kdes
B0>>1 B0>>1
Weak kads<< kdes kads<< kdes
B0<<1 B0<<1
1/20
1/20
)(1
)(
AB
ABs
PB
PB
C
C
+==
∞
θ
des
ads
k
kB =0
case II
A B
θ = =+∞
C
C
B P
B P
s 0
01
des
ads
k
kB =0
case I
A
1→=∞C
Csθ 1→=∞C
Csθ
PBC
Cs0==
∞
θ 1/20 )( PB
C
Cs ==∞
θ
Adsorption
A, B both strong
A strong, B weak
A weak, B
weak
BB,AA,
BB,B,s
B
BB,AA,
AA,A,s
A
PBPB
PB
C
C
PBPB
PB
C
C
00
0
00
0
+==
+==
∞
∞
θ
θ
BB,B,sB
AA,A,sA
PBC/C
PBC/C
0
0
==
==
∞
∞
θ
θA
BA,B,B,sB
A,sA
P
PB/BC/C
C/C
)(
1
00==
→=
∞
∞
θ
θ
case III
A B
�Langmuir adsorption isotherm
case I
case II
Case III
60
� 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 (error?) � Solid surface is heterogeneous , heat of adsorption is not a constant at different θ� Physisorption of gas molecules on a solid surface can be more than one layer
BB,AA,
BB,B,s
B
BB,AA,
AA,A,s
A
PBPB
PB
C
C
PBPB
PB
C
C
00
0
00
0
1
1
++==
++==
∞
∞
θ
θ
1/20
1/20
)(1
)(
AB
ABs
PB
PB
C
C
+==
∞
θ
θ = =+∞
C
C
B P
B P
s 0
01
large B0 (strong adsorp.)
small B0 (weak adsorp.)
moderate B0
Pressure
Am
ou
nt
adso
rbed
mono-layer
1→=∞C
Csθ
PBC
Cs0==
∞
θ
Strong adsorption kads>> kdes
Weak adsorption kads<< kdes
Adsorption Theories: BET
adsorbate. of pressure relative
and layer);1st of adsorption ofenergy to(relatedconstant BET C
monolayer; form torequired gas of volume
; pressureat adsorbed gas of volume
where
)1(1
)(
=
=
=
=
−+=
−
o
m
a
o
mm
o
a
P
P
V
PV
P
P
CV
C
CVPPV
P
� Modification of Langmuir isotherm
� Both monolayer and multilayer adsorption
� Assumptions:(a) gas molecules physically
adsorb on a solid in layers infinitely;
(b) there is no interaction between each adsorption layer;
(c) the Langmuir theory can be applied to each layer.
Adsorbate
Adsorbent
S.Brunauer, P.Emmett, E.Teller Adsorption of Gases in Multimolecular Layers, J. Am. Chem. Soc., 1938, 60 (2), pp 309–319
Specific Surface Area Calculation
CVP
P
CV
C
PPV
P
m
o
m
o
a
1)1(
)(+
−=
−
imXY +=
imVm
+=
1
P/Po
1
V[(Po/P)-1]
0-1 0-2 0-3
At least three data points in the relative pressure range 0.05 to 0.30
adsorbate ofWeight area surface Total csavm ANV
=
sample ofWeight
area surface Totalarea) surface (SpecificSSA =
Single Point BET�Single-point method offers the advantage of simplicity andspeed, often with little loss in accuracy.
( )o
am PPVV −= 1 i.e. Vm = 1/slope�A relative pressure of 0.3 gives good general agreement with the multi-point method.
� Correction of single point “error” at P/P0 = 0.3 by multiplying the single point BET value by C/C-2 decreases the difference.
Sample No.
Multi-point BET
(m2/g)
Uncorrected single-point
(m2/g)
Uncorrecteddifference
(%)
Corrected single –
point
(m2/g)
Correcteddifference
(%)
1 4.923 4.241 -13.9 4.948 0.51
2 4.286 3.664 -14.5 4.275 -0.26
3 8.056 6.867 -14.8 8.011 -0.56
4 5.957 5.194 -12.8 6.060 +1.73
Pore Size DistributionV
a
Pore diameter, d
Narrow pore size distribution
Broad pore size distribution
Unimodal pore size distribution
Pore diameter, d
Multimodal pore size distribution
The distribution of pore volume with respect to pore size is called a pore size distribution.
Va
∑=d
aV volumePore
Pore Size Distribution
∆Gads = RT(lnPads - lnP0)
∆Gdes = RT(lnPdes - lnP0)∆Gdes < ∆Gads
1
P/Po
(P/Po)des (P/Po)ads
�Adsorption or desorption isotherm.
�The desorption isotherm is preferred over adsorption isotherm.
Va
Pore Size: Kelvin Equation
�Multilayer formation occurs in parallel to capillary condensation. �Capillary condensation is described by the Kelvin equation.
phase. condensed and solid ebetween th anglecontact
re; temperatu
constant; gas real
meniscus; liquid theof curvature of radiusmean
adsorbate; condensed of memolar volu
tension;surface liquid
;adsorbate of pressure saturated
adsorbate of pressure
where
cos2
ln
=
=
=
=
=
=
=
=
θ
γ
θγ
T
R
V
RT
V
r
p
p
rp
p
k
o
k
o
θ
kr
Pore Size: Kelvin Equation
trr kp +=
Actual radius of the pore
Kelvin radius of the pore
Thickness of the adsorbed layer
Prior to condensation, some adsorption has taken placeon the walls of the pore, rk does not represent the actualpore radius.
θ
tkr
Adsorbed layer
Methods for Calculation of Pore Size Distribution�BJH (Barrett, Joyner andHalenda) method
�DH (Dollimore Heal) method
�Dubinin-Astakhov method
�HK (Horvath-Kawazoe) method
�Saito-Foley method
Mesoporous solids
Microporous solids
�NLDFT (Non Local Density Functional Theory) and Monte Carlo simulation method
Microporous and Mesoporous solids
Questions . . . anyone ?
Porosity Analyzer
Outgassing station
Analysis station
Liquid nitrogen bath
Steps for Measurement
3. Interpretation
2. Adsorption Analysis
1. Sample Preparation
Sample Preparation (Outgassing)� Surface contamination is
removed by applicationof:� Temperature� Flowing gas (helium or
nitrogen) or vacuum
� Backfill can be doneusing helium or adsorbategas.
� According to IUPAC standards, materials should be outgassed for at least 16 hours.
Adsorbate
Helium
Vacuum
Po
Outgassing station
Analysis station
Sample Cell
Adsorption Analysis
� Adsorbate (nitrogen,argon, carbon dioxide,krypton)
� Analysis temperature(liquid nitrogen, liquidargon, 0 oC)
� Quantity of sample (1mg sample is sufficient)
� Number of points(single point, fivepoints, seven points,eleven points, fullanalysis)
Adsorbate
Helium
Vacuum
Po
Outgassing station
Analysis station
Sample Cell
Interpretation
Points P/Po Volume adsorbed
123
Weight of sample
Pore shape
Specific surface area
Pore volume
Pore size&
distribution
Results
Common Adsorbates
Gas Temperature Cross sectional area (nm2)
N2 � -195.8 oC (liquid nitrogen)� -183 oC (liquid argon).
0.162
Ar � -183 oC (liquid argon).� -195.8 oC (liquid nitrogen)
0.142
CO2 � -78 oC, -25 oC, 0 oC 0.195
CO � -183 oC (liquid argon) 0.163
Kr � -195.8 oC (liquid nitrogen) 0.205
O2 � -183 oC (liquid argon) 0.141
C4H10 � 0 oC, 25 oC 0.469
Choice of Adsorptive
� N2(g) in N2(l) is the most commonly used adsorbate.
� Not completely inert. � Dipole movement and
thus can have localized adsorption.
� Cross-sectional area of 0.162 nm2 is questionable.
�S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991�Quantachrome Autosorb-I Operational Manual
Oxy
gen
Arg
on
Nitr
ogen
Car
bon
mon
ooxi
deC
arbo
n di
oxid
e
Kry
pton
n-bu
tane
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
Cro
ss-s
ecti
on
al a
rea,
nm
2
Oxy
gen
Arg
on
Nitr
ogen
Car
bon
mon
ooxi
deC
arbo
n di
oxid
e
Kry
pton
n-bu
tane
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
Cro
ss-s
ecti
on
al a
rea,
nm
2
Choice of Adsorptive
�S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991�Quantachrome Autosorb-I Operational Manual
� Ar(g) in Ar(l) is preferable but because of unavailability of Ar(l) (87K), N2(l) (77 K) is used.
� Ar can reach to somewhat smaller pores than N2.
� Accurate measurement of micropores is possible using Ar.
Oxy
gen
Arg
on
Nitr
ogen
Car
bon
mon
ooxi
deC
arbo
n di
oxid
e
Kry
pton
n-bu
tane
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
Cro
ss-s
ecti
on
al a
rea,
nm
2
Choice of Adsorptive
�S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991�Quantachrome Autosorb-I Operational Manual
� In case of activated carbon, CO2 is often the most preferred adsorbate.
� Adsorption analysis of CO2 takes less time.
� Limited to microporeanalysis.
Validity of BET - Method
� The BET method depends on the cross-sectional area of adsorbate.
� Monolayer structure is same on all the surface.
� Localized monolayer coverage.
K. S. W. Sing, The Use of Nitrogen Adsorption for the Characterisation of Porous Materials, Colloids and Surfaces, 187 – 188, 2001, 3 - 9
−+=
− o
mm
oP
P
CV
C
CVPPV
P )1(1
)(
M
ALVSSA
av=
Adsorbate
Adsorbent
Validity of Kelvin Equation
θγ
cos2
lnRT
V
rp
p
k
o=
� Is relation between the
meniscus curvature and the pore size and shape valid?
� Is it applicable for micropores and narrow mesopores?
� Does surface tension varies with pore width?
θ
F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 203, 1999
kr
Shape of Microporous Materials
Va
1P/Po
Type Ior
Langmuir
�Type I isotherms don’t have hysteresis.
�Pore shape cannot be determined by isotherm.
�As various methods for pore size calculation are based on shape of pores, reliability of pore size calculation is questionable.
F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 439-446, 1999
2 nm 50 nm
Micropores Mesopores Macropores
Methods Assumption
Pore Shape Based on ..
Brunauer MP method Cylindrical or Slit shaped de Boer’s t-method
Dubinin-Astakhov method - �Polanyi potential theory
�Independent of Kelvin equation
HK (Horvath-Kawazoe) method Slit �Everett and Powl method
�Independent of Kelvin equation
Saito-Foley method Cylindrical HK method
Choice of Method
�P. A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics, 53 –152, 1997�Quantachrome Autosorb-I Operational Manual
2 nm 50 nm
Micropores Mesopores Macropores
Methods Assumption
Pore Shape Based on ..
BJH (Barrett, Joyner and Halenda) method
Cylindrical, Slit-shaped Kelvin equation
DH (Dollimore Heal) methodCylindrical t-method
Choice of Method
�P. A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics, 53 –152, 1997�Quantachrome Autosorb-I Operational Manual
2 nm 50 nm
Micropores Mesopores Macropores
Methods Assumption
Pore Shape Based on ..
NLDFT (Non Local Density Functional Theory) and Monte Carlo simulation method
Cylindrical and slit Statistical thermodynamics
Choice of Method
�P. A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics, 53 –152, 1997�Quantachrome Autosorb-I Operational Manual
Physisorption
Methods and Techniques
QuantachromeI N S T R U M E N T S
Micro and Mesopore Size Determination by Gas Sorption
First: Quantitative estimation of micropore volume and area…
T-plot and DR methods.
Multilayer adsorption
Type II, IV
Relative Pressure (P/Po)
Vol
ume
adso
rbed
After the knee, micropores cease to contribute to the adsorption process.
Low slope region in middle of isotherm indicates first few multilayers, on external surface including meso and macropores… before the onset of capillary condensation
Estimation of Micropores...the t-plot method
This method uses a mathematical representation of multi-layer adsorption. The thickness, t, of an adsorbate layer increases with increasing pressure. The t-curve so produced is very similar in appearance to a type II isotherm. For every value of P/Po, the volume adsorbed is plotted against the corresponding value of “t”.
If the model describes the experimental data a straight line is produced on the t-plot...
The t-plotResembles a type II
Relative Pressure (P/Po)
Sta
tistic
al th
ickn
ess
A statistical monolayer
A statistical multilayer
t-plot Method (mesoporous only)
1 2 3 4 5 6 7
t (�)
Slope = V/t = A
t-plot Methodshowing a “knee”
Slope A - slope B = area contribution by micropores size C
1 2 3 4 5 6 7
t (�)
XX
X
XX
XXC
A
B
A
C
B
What is an αs plot?
αs (for Ken Sing) is a comparison plot like the t-plot but its slope does not give area directly.
A
?
? ?
??
?
?
Quiz
Estimation of MicroporesDubinin-Radushkevich (DR) Theory
−−−−====
P
Plog
TBexpWW 02
2
0β
W = volume of the liquid adsorbateW0 = total volume of the microporesB = adsorbent constantβ = adsorbate constant
A linear relationship should be found between log(W) and log2(Po/P)...
Log2(Po/P)
Log
(W
)
Extrapolation yields Wo
Estimation of MicroporesDubinin-Radushkevich (DR) Plot
0
Pore Size Determination
Requires a recognition and understanding of different basic
isotherm types.
t-plot Method(in the presence of micropores)
1 2 3 4 5 6 7
t (�)
Intercept = micropore volume
Types of Isotherms
Type I
Type II
Type III
Type IV
Relative Pressure (P/Po)
Vol
ume
adso
rbed
Type V
Types of Isotherms
Type I or
pseudo-“Langmuir”
Relative Pressure (P/Po)
Vol
ume
adso
rbed
Steep initial region due to very strong adsorption, for example in micropores.
Limiting value (plateau) due to filled pores and essentially zero external area.
Why pseudo Langmuir?
Langmuir applies to monolayerlimit, not volume filling limit.
A
?
? ?
??
?
?
Quiz
Types of Isotherms
Type II
Relative Pressure (P/Po)
Vol
ume
adso
rbed
Rounded knee indicates approximate location of monolayer formation.
Absence of hysteresis indicates adsorption on and desorption from a non-porous surface..
Low slope region in middle of isotherm indicates first few multilayers
Types of Isotherms
Type III
Relative Pressure (P/Po)
Vol
ume
adso
rbed
Lack of knee represents extremely weak adsorbate-adsorbent interaction
BET is not applicable
Example: krypton on polymethylmethacrylate
Types of IsothermsType IV
Relative Pressure (P/Po)
Vol
ume
adso
rbed
Rounded knee indicates approximate location of monolayer formation.
Low slope region in middle of isotherm indicates first few multilayers
Hysteresis indicates capillary condensation in meso and macropores.Closure at P/Po~0.4 indicates
presence of small mesopores (hysteresis would stay open longer but for the tensile-strength-failure of the nitrogen meniscus.
Types of Isotherms
Type V
Relative Pressure (P/Po)
Vol
ume
adso
rbed
Lack of knee represents extremely weak adsorbate-adsorbent interaction
BET is not applicable
Example: water on carbon black
Types of Hysteresis
Large pores/voids
Gel
Mesopores
MCM
Vol
ume
adso
rbed
Relative Pressure (P/Po)
MesoPore Size
by Gas
Sorption(BJH)
Analyzer measures volume of pores: Yes or No?
NO! It measures what leavessupernatent gas phase
A
?
? ?
??
?
?
Quiz
Pore Size Distribution
Hysteresis is indicative of the presence of mesopores and the pore size distribution can be calculated from the sorption isotherm.
Whilst it is possible to do so from the adsorption branch, it is more normal to do so from the desorption branch...
Mesopore (Greek meso = middle): 2nm - 50 nm diameter
Macropore (Greek macro = large): >50 nm diameter
Micropore (Greek micro = small): 0 nm - 2 nm diameter
Adsorption / Desorption
Adsorption =
multilayer formation
Desorption =
meniscus development
Kelvin* Equation
)P/Plog(
.)A(rk
0
154====
* Lord Kelvin a.k.a. W.T. Thomson
θγ
= cos2
ln0 rRT
V
P
P
Pore Size
trr kp ++++====
rp = actual radius of the pore
rk = Kelvin radius of the pore
t = thickness of the adsorbed film
Statistical Thickness, t
• Halsey equation
• Generalized Halsey
• deBoer equation
• Carbon Black STSA
BJH Method(Barrett-Joyner-Halenda)
trr Kelvinpore ++++====
Pore volume requires assumption of liquid density!
Pore Size Distribution
40 Pore Diameter (angstrom)
dV/d
logD
Artifact
Relative Pressure (P/Po)
Am
ou
nt
adso
rbed
~ 0.42
Pore Size Data• Volume and size of pores can be expressed from
either adsorption and/or desorption data.
• The total pore volume, V, is taken from the maximum amount of gas adsorbed at the “top” of the isotherm and conversion of gas volume into liquid volume.
• The mean pore diameter is calculated from simple cylindrical geometry:
A
Vd
4= where A is the BET
surface area.
Pore size analysis of MCM 41 (Templated silica) by N2 sorption
at 77 K
0 0.2 0.4 0 .6 0 .8 1P/P 0
100
200
300
400
500
600
Vol
ume
[cc/
g]
Exp. N itrogen sorption at 77 K in M C M 41Exp. N itrogen sorption at 77 K in M C M 41D FT- IsothermD FT- Isotherm
Pore size analysis of MCM 41: Calculations compared
15 23 31 39 47 55Pore Diameter [Å]
0
0.05
0.1
0.15
0.2
0.25
0.3
Dv(
d) [c
c/Å
/g]
BJH-Pore size distribution BJH-Pore size distribution DFT-Pore size distributionDFT-Pore size distribution
Calculation
Models
Comparisons• Gas Sorption Calculation Methods
P/Po range Mechanism Calculation model1x10-7 to 0.02 micropore filling DFT, GCMC, HK, SF, DA, DR0.01 to 0.1 sub-monolayer formation DR0.05 to 0.3 monolayer complete BET, Langmuir> 0.1 multilayer formation t-plot (de-Boer,FHH),> 0.35 capillary condensation BJH, DH
0.1 to 0.5 capillary filling DFT, BJHin M41S-type materials
Different Theories of Physisorption
Surface area Pore volume Pore sizeBET Total pore vol DR ave
Langmuir t-plot (µpore vol) BJHDR DR (µpore vol) DH
MP and t-plot DA DFT
αs plot BJH HK(BJH) (DFT) SF(DH) (DH)(DFT)
HK & SFHorvath-Kawazoe & Saito-Foley
• HK• Direct mathematical relationship between relative
pressure (P/Po) and pore size. Relationship calculatedfrom modified Young-Laplace equation, and takes intoaccount parameters such as magnetic susceptibility.Based on slit-shape pore geometry (e.g. activatedcarbons). Calculation restricted to micropore region (≤2nm width).
• SF• Similar mathematics to HK method, but based on
cylindrical pore geometry (e.g. zeolites). Calculationrestricted to micropore region (≤ 2 nm diameter).
DA & DRDubinin-Astakov and Dubinin-Radushkevic
• DA• Closely related to DR calculation based on pore filling mechanism.
Equation fits calculated data to experimental isotherm by varying twoparameters, E and n. E is average adsorption energy that is directlyrelated to average pore diameter, and n is an exponent that controlsthe width of the resulting pore size distribution. The calculated poresize distribution always has a skewed, monomodal appearance(Weibull distribution).
• DR• Simple log(V) vs log2(Po/P) relationship which linearizes the isotherm
based on micropore filling principles. “Best fit” is extrapolated tolog2(Po/P) (i.e. where P/Po = 1) to find micropore volume.
BET• The most famous gas sorption model. Extends Langmuir
model of gas sorption to multi-layer. BET equationlinearizes that part of the isotherm that contains the“knee” , i.e. that which brackets the monolayer value.Normally solved by graphical means, by plotting1/(V[(Po/P)]-1) versus P/Po. Monolayer volume (Vm) isequal to 1/(s+i) where s is the slope and i is the y-intercept.Usually BET theory is also applied to obtain the specificsurface area of microporous materials, although from ascientific point of view the assumptions made in the BETtheory do not take into account micropore filling. Pleasenote, that for such samples the linear “BET” range isfound usually at relative pressures< 0.1, in contrast to theclassical BET range, which extends over relativepressures between 0.05 – 0.3.
Langmuir
• Adsorption model limited to the formation of amonolayer that does not describe most realcases. Sometimes can be successfully appliedto type I isotherms (pure micropore material) butthe reason for limiting value (plateau) is notmonolayer limit, but due to micropore filling.Therefore type I physisorption isotherm wouldbe better called “pseudo-Langmuir” isotherm.
t-plotStatistical Thickness
• Multi-layer formation is modeled mathematically to calculate a layer “thickness, t” as a function of increasing relative pressure (P/Po). The resulting t-curve is compared with the experimental isotherm in the form of a t-plot. That is, experimental volume adsorbed is plotted versus statistical thickness for each experimental P/Po value. The linear range lies between monolayer and capillary condensation. The slope of the t-plot (V/t) is equal to the “external area”, i.e. the area of those pores which are NOT micropores. Mesopores, macropores and the outside surface is able to form a multiplayer, whereas micropores which have already been filled cannot contribute further to the adsorption process.
• It is recommended to initially select P/Po range 0.2 – 0.5, and subsequently adjust it to find the best linear plot.
BJH & DHBarrett, Joyner, Halenda and Dollimore-Heal
• BJH• Modified Kelvin equation. Kelvin equation predicts
pressure at which adsorptive will spontaneously condense (and evaporate) in a cylindrical pore of a given size. Condensation occurs in pores that already have some multilayers on the walls. Therefore, the pore size is calculated from the Kelvin equation and the selected statistical thickness (t-curve) equation.
• DH• Extremely similar calculation to BJH, which gives very
similar results. Essentially differs only in minor mathematical details.
Other Methods
• FRACTAL DIMENSION• The geometric topography of the surface
structure of many solids can be characterized by the fractal dimension D, which is a kind of roughness exponent. A “flat” surface is considered D is 2, however for an irregular (real) surface D may vary between 2 and 3 and expresses so the degree of roughness of the surface and/or porous structure. The determination of the surface roughness can be investigated by means of the modified Frenkel-Halsey Hill method, which is applied in the range of multilayer adsorption.
Example Data : Microporous Carbon
BET : Not strictly applicable
Example Data : Microporous Carbon
• Tag all adsorption points
• Analyze behavior• Note knee – transition
from micropore filling to limitedmultilayering (plateau).
Example Data : Microporous Carbon
• Use Langmuir (Monolayer model) / DR for Surface Area, Micropore Volume
• Usue Langmuir in range of 0.05 -> 0.2 (monolayer)
Example Data : Microporous Carbon
• Langmuir Surface Area
Example Data : Microporous Carbon
• DR Method for surface area, micropore volume
• Choose low relative pressure points (up to P/P0 = 0.2)
Example Data : Microporous Carbon
• Reports micropore surface area, and micropore volume.
• Note Langmuir, DR surface areas very close (1430 m2/g vs. 1424 m2/g)
Example Data : Macroporous Sample
Little or no “knee”, isotherm closes at
0.95
Example Data : Macroporous Sample
• BET Plot = OK• Surface area ca. 8m2/g (low)• Note hysteresis above P/P0 = 0.95 ∴Pores > 35 nm
Example Data : Macroporous Sample
Intercept = (-), no micropore
volume.
Example Data : Macroporous Sample
BJH Shows pores > 20nm, to over
200 nm
Example Data : Mesoporous Silica
Hysteresis => mesoporesAlso micropores ?? Test using t-
method
Example Data : Mesoporous Silica
BET Surface area = 112m2/gClassic mesoporous silica !
Example Data : Mesoporous Silica
Statistical Thickness => Use de Boer for oxidic surfaces = silicas
Intercept ~ 0Look at tabular data
MP SA = 8m2/g (total SA = 112)
Example Data : Mesoporous Silica
Use BJH – shows narrow pore size distribution in 14-17nm range (mesopores)
Questions from audience?
MicroPore Size
by Gas
Sorption
Available
Calculation
Models
Pore filling pressures for nitrogen in cylindrical pores at 77 K,
(Gubbins et al. 1997)
Pore filling pressures for nitrogen in cylindrical silica pores at 77 K
(Neimark et al., 1998)
Pore size analysis of MCM 41 by silica by N2 sorption at 77 K
0 0.2 0.4 0.6 0.8 1P/P0
100
200
300
400
500
600
Vol
ume
[cc/
g]
Exp. Nitrogen sorption at 77 K in MCM 41Exp. Nitrogen sorption at 77 K in MCM 41DFT- IsothermDFT- Isotherm
15 23 31 39 47 55Pore Diameter [Å]
0
0.05
0.1
0.15
0.2
0.25
0.3
Dv(
d) [c
c/Å
/g]
BJH-Pore size distribution BJH-Pore size distribution DFT-Pore size distributionDFT-Pore size distribution
Gas- and liquid density profiles in a slit pore by GCMC
(Walton and Quirke,1989)
NLDFT / GCMC (Monte Carlo) Kernel File
Applicable Pore Diameter Range
Examples
NLDFT– N2 - carbon kernel at 77 K based on a slit-pore model
0.35nm-30 nm Carbons with slit-like pores, such as activated carbons and others.
NLDFT– N2 – silica equilibrium transition kernel at 77 K, based on a cylindrical pore model
0.35nm- 100nm Siliceous materials such as some silica gels, porous glasses, MCM-41, SBA-15, MCM-48 and other adsorbents which show type H1 sorption hysteresis.
NLDFT– N2 - silica adsorption branch kernel at 77 K, based on a cylindrical pore model
0.35nm-100nm Siliceous materials such as some controlled pore glasses, MCM-41, SBA-15, MCM-48, and others. Allows to obtain an accurate pore size distribution even in case of type H2 sorption hysteresis
NLDFT– Ar zeolite/silica equilibrium transition kernel at 87 K based on a cylindrical pore model
0.35nm -100nm Zeolites with cylindrical pore channels such as ZSM5, Mordenite, and mesoporous siliceous materials (e.g., MCM-41, SBA-15, MCM-48, some porous glasses and silica gels which show type H1 sorption hysteresis).
NLDFT / GCMC (Monte Carlo) Kernel File
Applicable Pore Diameter Range
Examples
NLDFT – Ar-zeolite/silica adsorption branch kernel at 87 K based on a cylindrical pore model
0.35nm-100nm Zeolites with cylindrical pore channels such as ZSM5, Mordenite etc., and mesoporous siliceous materials such as MCM-41, SBA-15, MCM-48, porous glasses some silica gels etc). Allows to obtain an accurate pore size distribution even in case of H2 sorption hysteresis.
NLDFT – Ar-zeolite / silicaequilibrium transition kernel based on a spherical pore model (pore diameter < 2 nm) and cylindrical pore model (pore diameter > 2 nm)
0.35nm-100nm Zeolites with cage-like structures such as Faujasite, 13X etc. , and mesoporous silica materials (e.g., MCM-41, SBA-15, porous glasses, some silica gels which show H1 sorption hysteresis).
NLDFT – Ar-zeolite / silica adsorption branch kernel at 87 K based on a spherical pore model (pore diameter < 2 nm) and cylindrical pore model (pore diameter > 2 nm)
0.35nm-100nm Zeolites with cage-like structures such as Faujasite, 13X, and mesoporous silica materials (e.g., MCM-41, SBA-15, controlled-pore glasses and others). Allows to obtain an accurate pore size distribution even in case of H2 sorption hysteresis.
NLDFT / GCMC (Monte Carlo) Kernel File
Applicable Pore Diameter Range
Examples
NLDFT – Ar - carbon kernel at 77 K based on a slit-pore model
0.35 nm - 7 nm Carbons with slit-like pores, such as activated carbons etc.
NLDFT - CO2 - carbon kernel at 273 K based on a slit-pore model
0.35nm-1.5 nm Carbons with slit-like pores, such as activated carbons etc.
GCMC – CO2 - carbon kernel at 273 K based on a slit-pore model
0.35nm-1.5 nm Carbons with slit-like pores, such as activated carbons etc.
RECENT ADVANCES IN THE PORE SIZE ANALYSIS OF
MICRO- AND MESOPOROUS MOLECULAR SIEVES BY ARGON
GAS ADSORPTION
Micropore Size Characterization
• Physical adsorption in micropores, e.g. zeolites occurs at relative pressures substantially lower than in case of adsorption in mesopores.
• Adsorption measurements using nitrogen at 77.4 K is difficult, because the filling of 0.5 - 1 nm pores occurs at P/Po of 10-7 to 10-5
, where the rate of diffusion and adsorption equilibration is very slow.
Advantages of Using Argon
• Advantage to analyze such narrow micropores by using argon at liquid argon
temperature (87.3 K).
• Argon fills these micropores (0.5 – 1nm) at much higher relative pressures (i.e., at relative pressures 10-5 to 10-3) compared to nitrogen.
Advantages of Higher Temperature & Pressure
• Accelerated diffusion.
• Accelerated equilibration processes.
• Reduction in analysis time.
Argon Adsorption at 87.3 K versus Nitrogen Adsorption at 77.4 K
10-6 5 10-5 5 10-4 5 10-3 5 10-2 5 10-1 5 100
P/P0
0
70
140
210
280
350V
olum
e [c
m3 ]
N2/77KN2/77KAr/87 KAr/87 K
ZEOLITE | 10.5.2001
The different pore filling ranges for argon adsorption at 87.3K and nitrogen adsorption at 77.4K in faujasite-type zeolite are illustrated above.
Micropore Size Calculation
• Difficulties are associated with regard to the analysis of micropore adsorption data.
• Classical, macroscopic, theories [1] like DR and semiempirical treatments such those of HK and SF do not give a realistic description of micropore filling
• This leads to an underestimation of pore sizes for micropores and even smaller mesopores [2].
[1] F. Rouquerol, J. Rouquerol & K. Sing, Adsorption by Powders & Porous Solids, Academic Press, 1999
[ 2 ] P. I Ravikovitch, G.L. Haller, A.V. Neimark, Advcances in Colloid and Interface Science 76-77 , 203 (1998)
New Calculation
• To overcome the above mentioned problems weintroduce a new method for micropore analysisbased on a Non-local Density Functional Theory(NLDFT) model by Neimark and Co-workers [3-5].
• The new DFT-method is designed for micro-mesopore size characterization of zeoliticmaterials ranging in size from 0.44 to 20 nm usinghigh-resolution low-pressure argon adsorptionisotherms at 87.3 K.
[3] P.I. Ravikovitch, G.L. Haller, A.V. Neimark, Advances in Colloid and Interface Science, 76 – 77 (1998), 203 -207
[4] A.V. Neimark, P.I Ravikovitch, M. Gruen, F. Schueth, and K.K. Unger, J. Coll. Interface Sci., 207, (1998) 159
[5] A.V. Neimark, P.I. Ravikovitch, Microporous and Mesoporous Materials (2001) 44-45, 697
Systematic, Experimental Study
• To evaluate the application of argon sorption formicro- and mesopore size analysis of zeolites andmesoporous silica materials including novelmesoporous molecular sieves of type MCM-41and MCM-48.
• The sorption isotherms were determined using a static volumetric technique
• Samples were outgassed for 12 h under vacuum (turbomolecular pump) at elevated temperatures (573 K for the zeolites and 393 K for MCM-41/MCM-48).
Results
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1 P/Po
Ads
orpt
ion
, [m
mo
l/g]
MCM-41
ZSM-5
50-50
Argon adsorption isotherms at 87 K on MCM-41, ZSM-5 and their 50-50 mixture.
Results
0
5
10
15
20
25
0.000001 0.00001 0.0001 0.001 0.01 0.1 1
P/Po
Ad
sorp
tio
n, [
mm
ol/g
]
MCM-41
ZSM-5
50-50
0
0.02
0.04
0.06
0.08
0.1
0.12
1 10 100 1000D, [Å]
dV
/dD
[cm
3 /g
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Vcu
m, [
cm3 /
g]
histogram
integral
ZSM
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
1 10 100 1000
D, [Å]
dV
/dD
[cm
3 /g
]/g
]
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Vcu
m, [
cm3 /
g]
histogram
integral
MCM
Evaluation of DFT Algorithm
0
2
4
6
8
10
12
14
16
18
20
0.000001 0.00001 0.0001 0.001 0.01 0.1 1
P/Po
Ads
orpt
ion,
[mm
ol/g
]
experimental
NLDFT fit
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
1 10 100 1000
D, [Å]
dV
/dD
[cm
3 /g
]
0
0.1
0.2
0.3
0.4
0.5
0.6
Vcu
m [
cm3 /
g]
histogram
integral
Pore Size Distribution
Discussion• Argon sorption at 77 K is limited to pore
diameters smaller than 12 nm.i.e. no pore filling/pore condensation can be observed at this
temperature for silica materials containing larger pores.
• This lack of argon condensation for pores larger than ca. 12 nm is associated with the fact, that 77 K is ca. 6.8 K below the bulk triple point [4,5] .
[4] M. Thommes, R. Koehn and M. Froeba, J. Phys. Chem. B (2000), 104, 7932
[5] M. Thommes, R. Koehn and M. Froeba, Stud. Surf. Sci. Catal., (2001), 135 17
Discussion
• These limitation do not exist for argon sorption at its’ boiling temperature, i.e. ca. 87 K.
• Pore filling and pore condensation can be observed over the complete micro- and mesopore size range .
Discussion
• Results of classical, and semi-empirical methods (e.g., BJH, SF etc) indicate that these methods underestimate the pore size considerably.
• Deviations from the DFT-results are often in a range of ca. 20 % for pore diameters < 10 nm.
Summary
• Our results indicate that argon sorption data at 87 K combined with the new NLDFT-methods provides a convenient way to achieve an accurate and comprehensive pore size analysis over the complete micro-and mesopore size range for zeolites, catalysts, and mesoporous silica materials.
Acknowledgements
• Special thanks go to Alex Neimark and Peter Ravikovitch at TRI Princeton, New Jersey, USA.
Referencesto research work of nitrogen, argon and krypton
in MCM-48/MCM-41 materials
(1) M. Thommes, R. Koehn and M. Froeba, “ Systematic Sorption studies on surface and pore size characteristics of different MCM-48 silica materials”, Studies in Surface Science and Catalysis 128, 259 (2000)
(2) M. Thommes, R. Koehn and M. Froeba, “Sorption and pore condensation behavior of nitrogen, argon and krypton in mesoporous MCM-48 silica materials” J. Phys. Chem. B 104, 7932 (2000)
(3)M. Thommes, R. Koehn and M. Froeba, “Sorption and pore condensation behavior of pure fluidsin mesoporous MCM-48 silica, MCM-41 silica and controlled pore glass, Studies in SurfaceScience and Catalysis, 135, 17 (2001)
(4)M. Thommes, R. Koehn and M. Froeba, “Characterization of porous solids: Sorption and porecondensation behavior of nitrogen, argon and krypton in ordered and disordered mesoporoussilica materials (MCM-41, MCM-48, SBA-15, controlled pore glass, silica gel) at temperaturesabove and below the bulk triple point”, Proceedings of the first topical conference on
nanometer scale science and engineering” (G.U. Lee, Ed) AIChE Annual Meeting, Reno,
Nevada, November 4-9, 2001
(5)M. Thommes, R. Koehn and M. Froeba, “Sorption and pore condensation behavior of pure fluidsin mesoporous MCM-48 silica, MCM-41 silica and controlled pore glass at temperaturesabove and below the bulk triple point”, submitted to Applied Surface Science, (2001)
Rapid Micropore Size Analysis by CO2
Adsorption
�
CO2 Adsorption at 0oCon Carbon
RAPID MICROPORE ANALYSIS
• The advantages of micropore analysis with Quantachrome’s Density Functional Theory (DFT) and CO2 include:
• Speed of analysis; with the higher diffusion rate at 273.15K, analysis times are reduced as much as 90%.
• Carbon dioxide at 273.15K permits probing pores from about 2 angstroms (0.2 nm).
DFT ADVANTAGE
DFT has recently been applied to describe the behavior of fluids that are confined in small pores. The current popular gas sorption models, e.g. BJH, HK, SF, DA, etc., assume that the density of the adsorbed phase remains constant, regardless of the size of the pores that are being filled. Packing considerations suggest that these models are less than satisfactory for analyses of pores less than 2 nm.
DFT “Fitting”
• For a given adsorbate-adsorbent system, DFT calculates the most likely summation of "ideal isotherms“ calculated from "ideal pores" of fixed sizes needed to match the experimental results.
CO2 for Speed!
• Typically, micropore analyses with nitrogen as adsorbate will require 24 hours or more to run.
• Using carbon dioxide as adsorbate provides several advantages. – Carbon dioxide molecules are slightly thinner than
nitrogen molecules (2.8 angstroms radius vs. 3.0 angstroms) and will fill smaller pores than nitrogen.
– The use of carbon dioxide allows the measurements to be made at 273.15K, typically with an ice/water bath.
– There is no longer any need to provide and maintain or replenish a level of liquid nitrogen during the analysis.
CO2 Benefits
• At this temperature, the diffusion rate of molecules moving through small and tortuous micropores is much higher than at 77.35K. This so-called "activated adsorption" effect led to the popularization of the use of carbon dioxide to characterize carbonaceous material since the early 1960s.
CO2 Benefits
• This higher diffusion rate is responsible for reducing the analysis time to a few hours for a complete adsorption experiment. The faster rate also provides for the possibility of using larger samples than with nitrogen adsorption, thus reducing sample weighing errors.
• Pore size distributions thus obtained are comparable to those from a 24-hour nitrogen/77.35K analysis.
N2 Adsorption @ 77K: 40 hours
CO2 adsorption at 273K: 2.75 hours
CO2 Adsorption at 0oC
Density Functional Theory Micropore Distribution
CO2 Adsorption at 0oC
Monte Carlo Simulation Micropore Distribution
How to do it?
• Hardware requirements for this new method are minimal: – a wide- mouth dewar and – a water-level sensor.
• The proprietary Quantachrome Autosorb® software provides the DFT data reduction capabilities to do the rest. Pore size distributions from about 2 angstroms can be determined from the data taken at 273.15K.
• Currently, calculation parameters are optimized for studies on carbon surfaces.
BIBLIOGRAPHY for Rapid Micropore Size Analysis by CO2 Adsorption
1. J. Garrido, A. Linares-Solano, J.M. Martin-Martinez, M. Molina-Sabio, F. Rodriguez-Reinoso, R. Torregosa Langmuir, 3, 76, (1987)
2. F. Carrasco-Martin, M.V. López-Ramón, C. Moreno-Castilla. Langmuir, 9, 2758 (1993)
3. P. Tarazona. Phys.Rev.A 31, 2672 (1985)
4. N.A. Seaton, J.P.R.B. Walton, N. Quirke. Carbon, 27, 853 (1989)
5. C. Lastoskie, K.E. Gubbins, N. Quirke. J.Phys.Chem., 97, 4786 (1993)
6. J.J. Olivier. Porous Materials 2, 9 (1995)
7. P.I. Ravikovitch, S.C. Ó Domhnaill, A.V. Neimark, F. Schüth, K.K. Unger. Langmuir, 11, 4765 (1995)
8. A.V. Neimark, P.I. Ravikovitch, M. Grün, F. Schüth, K.K. Unger. COPS-IV, 1997 (in press)
9. P.I. Ravikovitch P.I., D. Wei, W.T. Chuen, G.L. Haller,A.V. Neimark. J.Phys.Chem., May 1997
10. E.J. Bottani, V. Bakaev, W.A. Steele. Chem.Eng.Sci. 49, 293 (1994)
11. M.M. Dubinin. Carbon 27, 457 (1989)
Questions from the floor ?
CHEMISORPTION &
CATALYSIS
Catalysis & Catalysts
� Facts and Figures about CatalystsLife cycle on the earth� Catalysts (enzyme) participates most part of life cycle
e.g. forming, growing, decaying� Catalysis contributes great part in the processes of converting sun energy to
various other forms of energies e.g. photosynthesis by plant CO2 + H2O=HC + O2
� Catalysis plays a key role in maintaining our environment
Chemical Industry� ca. $2 bn annual sale of catalysts� ca. $200 bn annual sale of the chemicals that are related products� 90% of chemical industry has catalysis-related processes� Catalysts contributes ca. 2% of total investment in a chemical process
189
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
190
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)
� Reactions proceed under less demanding conditions
� Allow reactions occur under a milder conditions, e.g. at lower temperatures for those heat sensitive materials 191
reactant
reaction process
uncatalytic
product
ener
gy
catalytic
Action of Catalysts� It is important to remember that the use of catalyst DOES NOT vary ∆G &
Keq values of the reaction concerned, it merely change the PACE of the process
� Whether a reaction can proceed or not and to what extent a reaction can proceed is solely determined by the reaction thermodynamics, which is governed by the values of ∆G & Keq, NOT by the presence of catalysts.
� In another word, the reaction thermodynamics provide the driving force for a rxn; the presence of catalysts changes the way how driving force acts on that process.
e.g CH4(g) + CO2(g) = 2CO(g) + 2H2(g) ∆G°373=151 kJ/mol (100 °C)
∆G°973 =-16 kJ/mol (700 °C)
� At 100°C, ∆G°373=151 kJ/mol > 0. There is no thermodynamic driving force, the reaction won’t proceed with or without a catalyst
� At 700°C, ∆G°373= -16 kJ/mol < 0. The thermodynamic driving force is there. However, simply putting CH4 and CO2 together in a reactor does not mean they will react. Without a proper catalyst heating the mixture in reactor results no conversion of CH4and CO2 at all. When Pt/ZrO2 or Ni/Al2O3 is present in the reactor at the same temperature, equilibrium conversion can be achieved (<100%).
192
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.
193
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 …
194
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.
195
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 …
196
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 continuousmanner
� Reactants, which are usually in gas or liquid phase, are fed to reactor in steady rate (e.g. mol/h, kg/h, m3/h)
� Usually a target conversion is set for the reaction, based on this target� required quantities of catalyst is added� required heating or cooling is provided� required reactor dimension and characteristics are designed accordingly.
197
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
198
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 NO2 in the presence of SO2) 199
Catalytic reaction processes
�Stability - a good catalyst should resist to deactivation, caused by
� the presence of impurities in feed (e.g. lead in petrol poison TWC.
� thermal deterioration, volatility and hydrolysis of active components
� attrition due to mechanical movement or pressure shock
�A solid catalyst should have reasonably large surface area needed for reaction (active sites). This is usually achieved by making the solid into a porous structure.
Example Heterogeneous Catalytic Reaction Process
� The long journey for reactant molecules to
1. travel within gas phase
2. cross gas-liquid phase boundary3. travel within liquid phase/stagnant layer4. cross liquid-solid phase boundary5. reach outer surface of solid6. diffuse within pore7. arrive at reaction site8. be adsorbed on the site and activated9. 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 track201
1
9
gas phase
poreporous solid
liquid phase /stagnant
layer
2
345
6
78
gas phasereactant molecule
Solid Catalysts
� 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 202
Catalyst
Support
Solid Catalysts� 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
203
� 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
Solid Catalysts� Preparation of catalysts
� PrecipitationTo 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
204
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 metalprecursor
Am
ou
nt
adso
rbed
Concentration
Support
Drying & firing
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.
� Commonly used Pre-treatments
� Reduction � if elemental metal is the active phase
� Sulphidation � if a metal sulphide is the active phase
� Activation� Some catalysts require certain activation steps in order to receive the best
performance. � Even when the oxide itself is the active phase it may be necessary to pre-treat
the catalyst prior to the reaction
� Typical catalyst life span
�Can be many years or a few mins.205
0
25
50
75
100
500 600 700 800 900Temperature °C
BE
T S
.A. m
2 /g
0
40
0 10Time / hours
BE
T S
.A.
Act
ivit
y
Time
Normal use
Induction period
dead
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)
Physisorption Chemisorption
force van de Waal chemcal bond
number of adsorbed layers multi only one layer
adsorption heat low (10-40 kJ/mol) high ( > 40 kJ/mol)
selectivity low high
temperature to occur low high
206
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, θθθθ
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.
207
define θ θ θ θ = θθθθ = 0~1number of adsorption sites occupiednumber of adsorption sites available
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 an useful technique for determining the surface area, the pore shape, pore sizes and size distribution of porous solid materials (BET surface area)
208
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
209
Pressure
Vol
. ads
orbe
d T1
T2 >T1
T3 >T2
T4 >T3
T5 >T4
Vol
. ads
orbe
d
Temperature
P1
P2>P1
P3>P2
P4>P3
Pre
ssur
e
Temperature
V2>V1
V1
V3>V2
V4>V3
Adsorption Isotherm Adsorption Isobar Adsorption Isostere
Adsorption On Solid Surface� 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 adsorbatemolecules greater than the adhesive force between adsorbatemolecules and adsorbent
� Type IV staged adsorption (first monolayer then build up of additional layers)
� Type V porous materials with cohesive force between adsorbatemolecules and adsorbent being greater than that between
adsorbate molecules
210
I
II
III
IV
V
relative pres. P/P0
1.0
amou
nt a
dsor
bed
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 ∆∆∆∆H decreases linearly with surface coverage
From ads-des equilibrium, ads. rate ≡ des. rate
rads=kads(1-θ)P ≡ rdes=kdesθ
where Qs is the heat of adsorption. When Qs is a linear function of θi. 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 θθθθ =c1ln(c2P), where c1 & c2 are constants
� Valid for some adsorption systems.
211
1
1 1
1
0
0
Peb
Peb
PB
PBRT/Q
RT/Q
ss
s
+=⇒
+= θθ ∆∆ ∆∆H
of
ads
θθθθ
Langmuir
Temkin
( )
−+
+=
+== ∫∫
RTiRT/Q
RT/Q
sexpP
P
i
RTdS
Peb
PebdS
s
s
1
11
01
11
0 b1
b1ln
(1
[θθ
Adsorption On Solid Surface� The Freundlich isotherm
� assuming logarithmic change of adsorption enthalpy ∆∆∆∆H with surface coverageFrom ads-des equilibrium, ads. rate ≡ des. rate
rads=kads(1-θ)P ≡ rdes=kdesθ
where Qi is the heat of adsorption which is a function of θi. 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 θi, the overall coverage
the solution for this integration expression at small θ is:
lnθ=(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.
212
∫
∫∑
∑∞
∞
⋅+==
0
0 11
0
0
e
e)](1[
dQa
dQaPeb/Peb
N
N
Q/Q
Q/QRT/QRT/Q
i
i
i
iiθ
θ
1
1 1
1
0
0
Peb
Peb
PB
PBRT/Q
RT/Q
ii
i
+=⇒
+= θθ ∆∆ ∆∆
H o
f ad
s
θθθθ
Langmuir
Freundlich
211
C/pc=θ
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 was derived
Where P - equilibrium pressureP0 - saturate vapour pressure of the adsorbed gas at the temperature
P/P0 is called relative pressureV - volume of adsorbed gas per kg adsorbentVm -volume of monolayer adsorbed gas per kg adsorbentc - constant associated with adsorption heat and condensation heatNote: for many adsorption systems c=exp[(H1-HL)/RT], where H1 is adsorption heat of 1st layer & HL is liquefaction heat, so that the adsorption heat can be determined from constant c.
213
)(11
1 00
0 P/PcV
c
cV)P/P(V
P/P
mm
−+=
−
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/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.
214
Adsorption On Solid Surface� Use of BET isotherm to determine the surface area of a solid
� At low relative pressure P/P0 = 0.05~0.35 it is found that
Y = a + b X
�The principle of surface area determination by BET method:
A plot of against P/P0 will yield a straight line with slope of equal to (c-
1)/(cVm) and intersect 1/(cVm).
For a given adsorption system, c and Vm are constant values, the surface area of a solid material can be determined by measuring the amount of a particular gas adsorbed on the surface with known molecular cross-section area Am,
* In practice, measurement of BET surface area of a solid is carried out by N2
physisorption at liquid N2 temperature; for N2, Am = 16.2 x 10-20 m2215
)( )(11
1 000
0 P/PP/PcV
c
cV)P/P(V
P/P
mm
∝−
+=−
P P
V P P
/
( / )0
01−
P/P0
P P
V P P
/
( / )0
01−
A A N AV
Vs m m m
m
T P
= = × ×,
.6022 1023 Vm - volume of monolayer adsorbed gas molecules calculated from the plot, L
VT,P - molar volume of the adsorbed gas, L/mol
Am - cross-section area of a single gas molecule, m2
Adsorption On Solid Surface� Summary of adsorption isotherms
Name Isotherm equation ApplicationNote
Langmuir
Temkin θθθθ =c1ln(c2P)
Freundlich
BET
216
)(11
1 00
0 P/PcV
c
cV)P/P(V
P/P
mm
−+=
−
θ= =+∞
C
C
B P
B P
s 0
01
211
C/pc=θ
Chemisorption andphysisorption
Chemisorption
Chemisorption andphysisorption
Multilayer physisorption
Useful in analysis of reaction mechanism
Chemisorption
Easy to fit adsorption data
Useful in surface area
determination
Mechanism of Surface Catalysed Reaction
�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]=kθAθB
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
217
++=
++=
BB,AA,
BB,
B
BB,AA,
AA,
A
PBPB
PB
PBPB
PB
00
0
00
0
1
1
θ
θ
BB,AA,
BAB,A,
BB,AA,
BB,
BB,AA,
AA,
iPBPB
PPBkB
PBPB
PB
PBPB
PBkr
00
00
00
0
00
0
111 ++=
++
++=
BABAB,A,i PP'kPPBkBr == 00
BBB,
AA,
BAB,A,
i P''kPkBPB
PPBkBr === 0
0
00
A B+ � P
Mechanism of Surface Catalysed Reaction
�Eley-Rideal mechanism� This mechanism deals with the surface-catalysed reaction in which
that one reactant, A, adsorb on surface without dissociation &other 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]=kθAPB
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
218
AA,
AA,
APB
PB
0
0
1+=θ
AA,
BAA,
B
AA,
AA,
iPB
PPkBP
PB
PBkr
0
0
0
0
11 +=
+=
BABAA,i PP'kPPkBr == 0
B
AA,
BAA,
i kPPB
PPkBr ==
0
0
A� P
B
+ B(g)
Mechanism of Surface Catalysed Reaction� Mechanism of surface-catalysed reaction with dissociative adsorption
� The mechanism of the surface-catalysed reaction in which onereactant, 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]=kθADPB
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. B219
( )( ) 21
0
210
1 /
ADAD,
/
ADAD,
ADPB
PB
+=θ
( )( )
( )( ) 21
0
210
210
210
11 /
ADAD,
B
/
ADAD,
B/
ADAD,
/
ADAD,
iPB
PPBkP
PB
PBkr
+=
+=
( ) B
/
ADB
/
ADAD,i PP'kPPBkr2121
0 ==
( )( ) B/
ADAD,
B
/
ADAD,
i kPPB
PPBkr == 21
0
210
+ B(g)� P
B
A B
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
� …220
Need to ask ?
© 2004 Quantachrome Instruments
Chemisorption
QuantachromeI N S T R U M E N T S
3
© 2004 Quantachrome Instruments
3. Chemisorption Techniques
3.1 Introduction: Physisorption/Chemisorption
3.2 Classical Models
3.3 Active Metal Area Measurement
3.4 Adsorption Thermodynamics
3.5 Pulse vs. Static
3.6 Temperature Programmed Analyses
© 2004 Quantachrome Instruments
The Nature of Gas Sorption at a Surface
• When the interaction between a surface and an adsorbate is relatively weak only physisorption takes place.
• However, surface atoms often possess electrons or electron pairs which are available for chemical bond formation.
• This irreversible adsorption, or chemisorption, is characterized by large interaction potentials which lead to high heats of adsorption.
© 2004 Quantachrome Instruments
Physisorption vs Chemisorption
Property Physisorption Chemisorption
Forces van der Waals Chemical bonding
∆Hads
(kJ mol-1) < 40 50-200
Ea
(kJ mol-1) Rare 60–100
Isothermal Reversibility Complete Slow or none
Extent Multilayers Monolayer
© 2004 Quantachrome Instruments
On The Nature of Chemisorption
• Chemisorption is often found to occur at temperatures far above the critical temperature of the adsorbate.
• As is true for most chemical reactions, chemisorption is usually associated with an activation energy, which means that adsorbate molecules attracted to a surface must go through an energy barrier before they become strongly bonded to the surface.
© 2004 Quantachrome Instruments
Adsorption PotentialsP
oten
tial E
nerg
yP
C
∆Hc
∆Hp
A
Potential energy curves for molecular (non-dissociative) adsorption
© 2004 Quantachrome Instruments
Pot
entia
l Ene
rgy
X + X
∆Hact.
X2
P
C
∆Hdissoc.
A
Adsorption Potentials
Potential energy curves for activated adsorption
© 2004 Quantachrome Instruments
Adsorption Potentials
Potential energy curves for non-activated adsorption
Pot
entia
l Ene
rgy
C
P
X + X
X2
∆Hdissoc.
A
© 2004 Quantachrome Instruments
Isobars
Isobaric variation in quantity adsorbed with temperature. Physisorption isobar (a) represents lower heat of adsorption than chemisorption isobar (b).
Temperature
Quantity adsorbed
(a)
(b)
(c)
© 2004 Quantachrome Instruments
On The Nature of Chemisorption
• Because chemisorption involves a chemicalbond between adsorbate and adsorbent,unlike physisorption, only a single layer ofchemisorbed species can be realized onlocalized active sites such as those found inheterogeneous catalysts.
• However, further physical adsorption on topof the chemisorbed layer and diffusion of thechemisorbed species into the bulk solid canobscure the fact that chemisorbed materialcan be only one layer in depth
© 2004 Quantachrome Instruments
Classical Models
QuantachromeI N S T R U M E N T S
3.2
© 2004 Quantachrome Instruments
3.2 Classical Models
3.2.1 Langmuir
3.2.2 Freundlich
3.2.3 Temkin
© 2004 Quantachrome Instruments
Adsorption Process
Active Sites (Adsorbent)
Adsorbate Adsorptive
© 2004 Quantachrome Instruments
Graduated as a metallurgical engineer from the School of Mines at Columbia University in 1903
1903-1906 M.A. and Ph.D. in 1906 from Göttingen.
1906-1909 Instructor in Chemistry at Stevens Institute of Technology, Hoboken, New Jersey.
1909 –1950 General Electric Company at Schenectady where he eventually became Associate Director
1913 -Invented the gas filled, coiled tungsten filament incandescent lamp.
1919 to 1921, his interest turned to an examination of atomic theory, and he published his "concentric theory of atomic structure" . In it he proposed that all atoms try to complete an outer electron shell of eight electrons
Irving Langmuir (1881-1957)
© 2004 Quantachrome Instruments
1927 Coined the use of the term "plasma" for an ionized gas.
1932 The Nobel Prize in Chemistry "for his discoveries and investigations in
surface chemistry"
1935-1937 With Katherine Blodgett studied thin films.
1948-1953 With Vincent Schaefer discovered that the introduction of dry ice and iodide into a sufficiently moist cloud of low temperature could induce precipitation.
Irving Langmuir (1881-1957)
© 2004 Quantachrome Instruments
3.2.1 Langmuir’s “Kinetic” Approach
rate of adsorption = ka P(1-θ)
where θ is the fraction of the surface already covered with adsorbate, i.e.,θ = V/Vm
rate of desorption = kd θ
Suggests a dynamic equilibrium. Is it?
© 2004 Quantachrome Instruments
Langmuir (continued…)At equilibrium (any pressure)
ka P(1-θ) = kd θ
from which
θ = V/Vm = KP/(1+KP)
where K = ka / kd.
In its linear form, the above equation can be expressed as:
1/V = 1/Vm + 1/(VmKP)
© 2004 Quantachrome Instruments
Confining adsorption to a monolayer, the Langmuirequation can be written
where V is the volume of gas adsorbed at pressure P,Vm is the monolayer capacity (i.e. θ=1) expressed asthe volume of gas at STP and K is a constant for anygiven gas-solid pair. Rearranging in the form of astraight line (y=ab+x) gives
KP
KP
V
V
m +=1
mm V
P
KVV
P+=
1
Or, if you prefer…
© 2004 Quantachrome Instruments
Langmuir Plot
1/P
1/V
Slope = 1/(VmK)
Intercept = 1/Vm
1/V = 1/Vm + 1/(VmKcP1/s)
© 2004 Quantachrome Instruments
Temperature Dependent Models
generally
K = Ko exp(q/RT)where Ko is a constant, R is the universal gas constant, T is the
adsorption temperature and q is the heat of adsorption
• Langmuir:K is constant;q is constant at all θ• Temkin: assumed that q decreases linearly with
increasing coverage• Freundlich: assumed that q decreases
exponentially with increasing coverage
© 2004 Quantachrome Instruments
TemkinTemkin assumed that q decreases linearly with increasing coverage,that is,
Q=qo(1- λ θ)
Where qo is a constant equal to the heat of adsorption at zero coverage (θ = 0) and λ is a proportionality constant.
© 2004 Quantachrome Instruments
Temkinθ = A ln P + B
or, since θ = V/Vm
V = Vm A lnP + VmB
Where A = RT/qo λ θ andB = A ln Ko + 1/ λ θ
© 2004 Quantachrome Instruments
Temkin Plot
Ln(P)
V
Slope = VmA
Intercept = VmB
V = Vm A lnP + VmB
© 2004 Quantachrome Instruments
Multiple Temkin Plots to find
Ln(P)
V
Temp H Temp M Temp L
*mV
* denotes “temperature invariant” or “thermally irreversible” quantity
experimental extrapolated
© 2004 Quantachrome Instruments
FreundlichTemkin assumed that q decreases
exponentially with increasing coverage, that is,
Q = -qm lnθ
Where qm is a constant equal to the heat of adsorption at θ = 0.3679
© 2004 Quantachrome Instruments
Freundlich
lnθ = C lnP + D or, since θ = V/Vm
ln(V/Vm) = C lnP + D
Where C=RT/ qm and D = C lnKo
© 2004 Quantachrome Instruments
Freundlich (continued…)
Ln(P)
Ln(V
)
Slope = C
Intercept = D + ln(Vm)
Ln(V/Vm) = C lnP + D
© 2004 Quantachrome Instruments
Multiple Temkin Plots to find
Ln(P)
Ln(V
)
Temp H Temp M Temp L
*mV
* denotes “temperature invariant” or “thermally irreversible” quantity
experimental extrapolated
© 2004 Quantachrome Instruments
Active Metal Area
QuantachromeI N S T R U M E N T S
3.3
© 2004 Quantachrome Instruments
3.3 Active Metal Area
3.3.1 Principles of Calculation
3.3.2 Choice of Adsorbate
3.3.3 Active Site Size Calculation
3.3.4 Metal Dispersion
3.3.5 Accessible vs non-accessible sites
© 2004 Quantachrome Instruments
Active Site Quantification
• Because the formation of a chemicalbond takes place between an adsorbatemolecule and a localized, or specific,site on the surface of the adsorbent, thenumber of active sites on catalysts canbe determined simply by measuring thequantity of chemisorbed gas
© 2004 Quantachrome Instruments
Active Site on a Catalyst
• Metal on support.• Island-like crystallites• Not all metal atoms exposed.• Adsorption technique perfectly suited.(cf Chemical analysis of entire metal
content )
© 2004 Quantachrome Instruments
3.3.1 Principles of Calculation
Monolayer Volume, Vm= volume of gas chemisorbed in a monomolecular layer
© 2004 Quantachrome Instruments
Methods to Determine Vm
•Extrapolation
• Bracketing
• Langmuir
• Temkin
• Freundlich
= volume of gas chemisorbed in a monomolecular layer
© 2004 Quantachrome Instruments
Vm
Vol
ume
Ads
orbe
d
Pressure (mm Hg)
Extrapolation method
First (only?)isotherm
© 2004 Quantachrome Instruments
Vol
ume
Ads
orbe
d
Pressure (mm Hg)
The second isotherm
combined
Weak only
© 2004 Quantachrome Instruments
Vol
ume
Ads
orbe
d
Pressure (mm Hg)
The difference isotherm
combined
Weak only
Strong
© 2004 Quantachrome Instruments
Vm from Pulse Titration
… will be covered in 3.5.2
© 2004 Quantachrome Instruments
Metal Area Calculation
To Calculate Metal Surface Area:A = (Vm) x (MXSA) x (S) x 6.03 x 10-3 (units m2/g)
where MXSA = metal cross sectional area (Å2)and S = stoichiometry = metal atoms per gas molecule
To calculate metal area per gram of metal, Am:Am = A x l00/L
where L = metal loading (%) = known value from chemical analysis
© 2004 Quantachrome Instruments
Stoichiometry
The gas-sorption stoichiometry is defined as the number of metal atoms with which each gas molecule reacts.
Since, in the gas adsorption experiment to determine the quantity of active sites in a catalyst sample, it is the quantity of adsorbed gas which is actually measured, the knowledge of (or at least a reasonably sound assumption of) the stoichiometry involved is essential in meaningful active site determinations (area, size, dispersion).
© 2004 Quantachrome Instruments
3.3.2 Choice of Adsorbate
Chemisorption
• CO or H2 on Pt, Pd
at 40 oC
• CO or H2 on Ni
For metal-only area
(& dispersion etc)
Physisorption
• N2 at 77K
• Ar at 87K
• Kr at 77K
• CO2 at 273KFor total surface area
and pore size
© 2004 Quantachrome Instruments
3.3.3 Active Site Size Calculation
To calculate average crystallite size:
d = (L x 100 x f )/AD (units Å)
where f = shape factor = 6ρ = density of metal (g/ml)
© 2004 Quantachrome Instruments
Shape Factor & Crystallite Size
The default shape factor of 6 is for assumed cubic geometry.Consider a cube of six sides (faces) each of length l. then the total surface area, ΣA = 6l2.
The volume of the cube is given by l3 or, in terms of total area, substitute ΣA /6 for l2 to give
V= lΣA/6
For a cube whose mass is unit mass, its volume is given by 1/ ρ(where ρ is the density of the material).
V=1/ρ
© 2004 Quantachrome Instruments
Shape Factor & Crystallite Size
For the same cube of unit mass, the area is then the area per unit mass A and l is rewritten d (crystallite size), the length required to give a cube whose mass is unity. Equating both terms for volume:
dA/6=1/ ρ
ord=6/A ρ
For a supported metal, the loading, L, must be taken into consideration.
d=L6/A ρ
Other geometries can be treated in a similar fashion. For example, a rectangular particle whose length is three times its width has a shape factor of 14/3.
© 2004 Quantachrome Instruments
Supported metalsIt is most likely that the catalyst exists as a
collection of metal atoms distributed over an inert, often refractory, support material such as alumina.
At the atomic level it is normal that these atoms are assembled into island-like crystallites on the surface of the support.
3.3 Metal Dispersion
© 2004 Quantachrome Instruments
3.3 Metal Dispersion• In the case of supported metal catalysts, it is
important to know what fraction of the active metal atoms is exposed and available to catalyze a surface reaction.
• Those atoms that are located inside metal particles do not participate in surface reactions, and are therefore wasted.
© 2004 Quantachrome Instruments
Exposed metal atomsSince these islands vary in size due to both the intrinsic
nature of the metal and the support beneath, plus themethod of manufacture more or less of the metalatoms in the whole sample are actually exposed atthe surface. It is evident therefore that the method ofgas adsorption is perfectly suited to the determinationof exposed active sites.
support
Exposed active sitesAdsorbed gas
© 2004 Quantachrome Instruments
Metal Dispersion
• Dispersion is defined as the percentage of all metal atoms in the sample that are exposed.
• The total amount of metal in the sample is termed the loading, χ , as a percentage of the total sample mass, and is known from chemical analysis of the sample.
© 2004 Quantachrome Instruments
Metal Dispersion
• The dispersion, δ, is calculated from:
• Where M is the molecular weight of the metal, Na is the number of exposed metal atoms found by adsorption and WS is the mass of the sample.
%WL
NM
SAv
a 100100
×χ
×=δ
© 2004 Quantachrome Instruments
3.3.5Accessible vs. Non-accessible Sites1. Adventitious moisture2. Reducing gas accessibility3. Diffusion4. Purge5. Physisorption blocks6. Bulk hydride7. Spillover8. Stoichiometry9. Characterization gas vs. Process gas
© 2004 Quantachrome Instruments
Spatial Ordering
There may exist a number of different adsorption sites that involve different numbers of metal atoms per adsorbate molecule.
© 2004 Quantachrome Instruments
Adsorption Thermodynamics
QuantachromeI N S T R U M E N T S
3.4
© 2004 Quantachrome Instruments
3.4 Adsorption Thermodynamics
3.4.1 Isosteric Heats from Isotherms
See also activation energy under 3.6.1
© 2004 Quantachrome Instruments
3.4.1 Heats of Adsorption
• Whenever a gas molecule adsorbs on a surface, heat is (generally) released, i.e. the process of adsorption is exothermic.
• This heat comes mostly from the loss of molecular motion associated with the change from a 3-dimensional gas phase to a 2-dimensional adsorbed phase.
• Heats of adsorption provide information about the chemical affinity and the heterogeneity of a surface, with larger amounts of heat denoting stronger adsorbate-adsorbent bonds.
• There are at least two ways to quantify the amount of heat released upon adsorption: in terms of (i) differential heats, q, and (ii) integral heat, Q.
mRq −= ∫θ
θ
θ=max
min
qV
Qm d
22414
© 2004 Quantachrome Instruments
Differential Heats of Adsorption• q, is defined as the heat released upon adding
a small increment of adsorbate to the surface. • Its value depends on (i) the strength of the
bonds formed and (ii) the degree to which surface is already covered.
• i.e a plot of q vs. θ provides a curve illustrating the energetic heterogeneity of the surface.
• Use it to fingerprint surface energetics and to test of the validity of any Vm evaluation method used (see earlier) since each method assumes a different relationship between q and θ.
© 2004 Quantachrome Instruments
Differential Heats of Adsorption• Since q can, and most often does, vary with θ,
it is convenient to express it as an isosteric heat of adsorption, that is, at equal surface coverage for different temperatures.
• Thus, obtain two or more isotherms at different temperatures.
• Determine pressures corresponding to equal coverage at different temperatures.
• Construct an Arrhenius plot of (lnP) versus (1/T). Values for q at any given coverage, θ, can be calculated from the Arrhenius slopes, m.
© 2004 Quantachrome Instruments
Slopes of (lnP) vs. (1/T).
mRq −=
where
m = d lnP/d(1/T)and R is the universal gas constant.
© 2004 Quantachrome Instruments
Integral Heat of Adsorption• This is simply defined as the total
amount of heat released, Q, when one gram of adsorbent takes up X grams of adsorbate. It is equivalent to the sum, or integral, of q over the adsorption range considered, that is:
where Vm is expressed in mL at STP, and θ ideally ranges from
θmin = 0 to θmax = maximum coverage attained experimentally.
∫θ
θ
θ=max
min
qV
Qm d
22414
© 2004 Quantachrome Instruments
Experimental Approaches
QuantachromeI N S T R U M E N T S
3.5
© 2004 Quantachrome Instruments
3.5 Experimental Approaches
3.5.1 Pulse
3.5.2 Static
© 2004 Quantachrome Instruments
Preparation Techniques
• Sample is heated under inert flow to
remove adsorbed moisture. Whilst
reduction step creates moisture, we don’t
ant the reducing gas to compete for diffusion
to surface.
• Reduce with H2: can be pure hydrogen or
diluted with nitrogen or argon. Higher
concentrations give higher space velocities
for the same volumetric flow rate.
© 2004 Quantachrome Instruments
Preparation Techniques (continued…)
• Purging with inert gas (normally helium) strips
excess reducing gas quickly. Can shorten prep
time and/or give more reproducible data since
hydrogen is difficult to pump.
• Cooling is done under vacuum/flow to ensure
continued removal of residual reducing gas…
though it is the hot removal step (above) which is
critical. That is, don’t cool before removing as
much reducing gas as possible.
© 2004 Quantachrome Instruments
Chemisorption Techniques
• Vacuum method
• Flow methods
© 2004 Quantachrome Instruments
Vacuum Technique
• Sample is heated under inert flow
• Reduced with H2
• Purged with inert, cooled under vacuum/flow
• Adsorbate dosed to obtain isotherm
• Calculate the amount adsorbed
© 2004 Quantachrome Instruments
Static (volumetric) Setup
furnace
manifold
adsorptives
vent
diaphragm pump
Turbo-molecular
(drag) pump
Flow “U” cell
© 2004 Quantachrome Instruments
Setup
Filler rod goes here
Quartz wool
sample capillary
© 2004 Quantachrome Instruments
3.5.2Flow (Pulse) Chemisorption
© 2004 Quantachrome Instruments
Flow Types of Analysis
� TPR
� TPO
� TPD
� Monolayer by Titration
� BET
support
active sites
A flow system permits multi-functional catalyst characterization :
© 2004 Quantachrome Instruments
OverviewAnalysis is done by detecting changes in gas
composition downstream of sample.
• Detector senses – abstraction of reactive species during
adsorption – evolution of previously adsorbed species during
desorption– decomposition products
• Signal detection– Standard: thermal conductivity detector– Optional: mass spectrometer
© 2004 Quantachrome Instruments
ChemBET™ 3000 TPR
© 2004 Quantachrome Instruments
Flow Diagram
AB
1
2
3
4
A
IN
OUT
CLICK FOR
BYPASS &
LONGPATH
CLICK FOR
BYPASS &
LONGPATH
CLICK FOR
BYPASS &
LONGPATH
© 2004 Quantachrome Instruments
Flow/Static (FloStat™) Flow Diagram
12
3
4
5
tomass spec
(optional)
to vent
B
A
oil-free high vacuum
vapor source
(optional)
heater
Schematic representation only. Some vacuum volumetric components omitted for clarity.
heated zone (vapor option)
© 2004 Quantachrome Instruments
TPRWin™ Software
Data Acquisition
© 2004 Quantachrome Instruments
Overview• Quartz flow-through
cell allows – high-temperature (up to
1100 degC) – in-cell temperature
monitoring– Two t/c’s if necessary,
one to DAQ, one to MassSpec.
– mass spectrometer sampling port.
T/C #1T/C #2
Modified cell holder
Capillary to mass spec.
Gas flow
© 2004 Quantachrome Instruments
Pulse Titration• Metal area, dispersion and crystallite size are
calculated from the amount of analysis (reactive) gas adsorbed.
• Variable volumes of analysis gas are injected into the inert carrier gas stream, which continuously flows over the sample.
• Detector measures the volume of gas that remains unadsorbed by the sample. Subtraction from the total amount injected gives the total amount adsorbed to within 1uL accuracy.
© 2004 Quantachrome Instruments
Titration� Pulse Titration of Active Sites
− H2 or CO titration
− N2 and He carrier respectively
− Constant temperature (room temp?)
− Multiple injections until saturation
M M MM
HH
H H H
H2 CON2
He
© 2004 Quantachrome Instruments
Titration
Data Acquisition
© 2004 Quantachrome Instruments
Titration
injections
LOAD INJECT
© 2004 Quantachrome Instruments
Titration Calculations
1. Calculate total nominal volume of reactive gas adsorbed by comparison with calibration injection or average of last n (three) peaks
(note: peak area represents gas not adsorbed!)
Total vol adsorbed = (Peak Avg - Peak1) + (Peak Avg - Peak2) +
(Peak Avg - Peak3) etcx nominal injection volume = Vnom (units µl)
© 2004 Quantachrome Instruments
Titration Calculations
2. Convert to STP:(Vnom) x (273/rt) x (Pamb/760) = Vstp (units µl)
3. Convert to specific volume adsorbed:Vstp /sample wt = Vsv (units µl/g)
4. Convert to micromoles per gram (weight as supplied ):
Vsv / 22.4 = Vm (units µmole/g)
© 2004 Quantachrome Instruments
Requirements for Different Analysis Types
Long cell
Short cell
Std. cell
5% H2
100%H2
5% O2
100%N2
100%He
30% N2
Inj. Furnace
Mantle Dewar Long path
TPR ()
TPO
TPD
Metal Area* () () * *
BET ()
* Using H2 active gas. If using CO, substitute 100% CO for 100% H2 & 100% He for 100% N2.
L
© 2004 Quantachrome Instruments
Temperature Programmed (TP)
Experiments
QuantachromeI N S T R U M E N T S
3.5
© 2004 Quantachrome Instruments
3.6 Temperature Programmed (TP) Experiments
3.6.1 TP-Reduction
3.6.2 TP-Oxidation
3.6.3 TP-Desorption
3.6.4 TP-Reaction
© 2004 Quantachrome Instruments
3.6.1 TP-Reduction
• Metal oxides are readily characterized by their ease of reduction.
CeO2 � CeO2-x + x/2O2
• TPR profiles represent that ease of reduction as reduction rate as a function of increasing temperature.
2CeO2 + H2 → Ce2O3 + H2O
© 2004 Quantachrome Instruments
Temperature Programmed Reduction
• A low concentration of pre-mixed hydrogen (e.g.5%) in nitrogen or argon (or other reducing gas for custom research applications) flows over the sample as it is heated during a linear increase (ramp) in temperature.
• Peak reduction temperature is also a function of heating rate and may be used to calculate activation energy for the reduction process.
© 2004 Quantachrome Instruments
TPR� Temperature Programmed Reduction
− Metal oxide to metal
− 5% hydrogen reactive gas
− Balance N2 or Ar (not He ! ...unless MS)
− Ramp rate
− Activation Energy
H2
MO MO MOMO
H2O
M M MM
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TPR
temperature
tmax
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TPRLinearly ramped
furnace is essential for standard TP
profiles
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time
tmax
TPR Profiles for Different Heating Rates
1
2
3
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TPR Profiles for Different Heating Rates
800 1000
0 20 40 60 80
100 120 140 160 180 3
1
2
Sig
nal
Temperature / K
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TPR Profile
Heating Rate ββββ (K-1)
Peak Temperature (Tmax)
1 10 874
2 15 902
3 20 928
Heating Rate & Peak Temperature
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Kissinger (Redhead) Equation
1.08 1.10 1.12 1.14
-11.2
-11.1
-11.0
-10.9
-10.8
-10.7
s lope = -8.6
Ea = 72 kJ mol-1
ln(β
Tm
ax-2
)
1000 /Tmax
(K -1)
max
a2max T
1
R
EK
Tln
+=
β
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3.6.2 TP-Oxidation
• Temperature programmed oxidation (using 2%-5% O2 in He for example) is performed in a manner analogous to TPR.
• TPO can be particularly useful for looking at carbons:– Carbon supports (graphite vs. amorphous)– Carbon deposits from coking– Carbides
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TPO� Temperature Programmed Oxidation
− Metals and carbon to oxides
− 2-5% oxygen reactive gas
− balance He (not N2 !)
− Ramp rate
− Activation Energy
O2
C C CC
CO + CO2
M M MM
carbon
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TPO: Signal vs. Temperature
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TPO: Signal & Temp. vs. Time
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Temperature Programmed Oxidation
Zhang and Verykios reported that three types of carbonaceous species designated as Cα, Cβ, and Cγwere found over Ni/Al2O3 and Ni/CaO±Al2O3 catalysts in the TPO experiments.
Zhang ZL and Verykios XE,. Catal. Today 21 589-595 (1994).
Goula et al identified two kinds of carbon species on Ni/CaO Al2O3 catalysts from TPO experiments. The high-temperature peak was assigned to amorphous and/or graphite forms of carbon. The lower temperature peak suggested a filamentous form.
Goula MA, Lemonidou AA and Efstathiou AM, J Catal 161 626-640 (1996).
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3.6.3 Temperature Programmed Desorption
• The monitoring of desorption processes is equally easy.
• A pure unreactive carrier gas carries evolved species from the sample to the detector as the user-programmable furnace heats the sample.
• This technique is commonly employed to determine the relative-strength distribution of acidic sites by means of ammonia desorption.
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TPD� Temperature Programmed Desorption
− Remove previously adsorbed species
− Helium/Nitrogen purge
− Ramp rate
− Activation Energy
NH3MO MO MOMO
NH3
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Ammonia TPD
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Pyridine TPD
Physisorbed pyridine is clearly evident in the first sample (low temp.), but absent in the second.
Multiple acid sites revealed by peak deconvolution
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TPD
temperature
tmaxIncreasing mass
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Overview• Quartz flow-through
cell allows – high-temperature (up
to 1100 degC) – in-cell temperature
monitoring– Two t/c’s if necessary,
one to DAQ, one to MassSpec.
– mass spectrometer sampling port.
T/C #1T/C #2
Modified cell holder
Capillary to mass spec.
Gas flow
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With Mass Spectrometer
Capillary or capillary connector to mass spectrometer
Tube endsjust below port connection
In-situ thermocouple
¼” swagelok®
compression fitting
T/C #1T/C #2
Modified cell holder
Capillary to mass spec.
Gas flow
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3.6.4 TP-Reaction
• Essentially everything that is not standard TPR or TPO!!
• Can be a single reactive gas, or a mixture of reactants… akin to microreactor work.
• Need not be done over a bare metal surface… might have one reactive species preadsorbed on the surface
e.g. ( ) OHCHNiCONiH
n 2422 ++→
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Questions so far ?
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Mercury Porosimetry
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Pore Size Analysis Using Liquid Methods
• What can be measured using these techniques?• Who would be interested in such results?• A brief overview of measurement fundamentals.• Meso-/macroporous solids
– Ceramics– Batteries and Fuel Cells– Geological samples– Cement, concrete, stone and bricks– Pharmaceuticals– Filters– Membranes
• Instrument selection for these materials• Specific features of benefit to such materials
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Pore Size Analysis Using Liquid Methods
• What can be measured using these techniques?– Pore size distributions (meso/macro, not micro)– Pores too large for gas sorption– Through-pores (porometry)
• Who would be interested in such results?– Anyone who forms powders into solids– Anyone who makes non-woven fabrics– Membrane manufacturers
© 2004 Quantachrome Instruments
Meso-/macroporous solids– Ceramics
• Strength, absorbence, filtration
– Batteries and Fuel Cells• Electrolyte contact, separator efficiency
– Geological samples• Oil and gas, strength, liquid permeation
– Cement, concrete, stone and bricks• Curing, strength, freeze/thaw resistance
– Pharmaceuticals• Tablet structure, strength, dissolution
– Filters & Membranes• Efficiency
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Ceramics
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Bioceramics
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Battery Pores
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Electrode Pores
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Separator Pores
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Geological
sandstone Diatomaceous earth
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Cement, Concrete, Mortar etc
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Pharmaceuticals
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Filters & MembranesNitrocellulose membrane
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Filters & Membranes
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Washburn methods
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Wetting / Contact Angles
Wetting θ < 90°
Non-wetting θ > 90°
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Washburn Equation
θγ−= cos2Pr
m/N480=γ
This image cannot currently be displayed.
and
r
736.0P =
Where P is in MPa and r in µm
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Sample Cell
The sample cell or penetrometer (sometimes
called a dilatometer) is used both to contain the sample and to facilitate
the measurement of intrusion and extrusion
volumes.
Max measurable intrusion volume
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Low Pressure Intrusion
Volume (capacitance) sensing circuit
Mercury reservoir
Vacuum
Cold trap
Sample
Metal cap
Concentric sheath
Mercury level sensorPressure transducer
Dry gas (e.g. 400 kPa)
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High Pressure Intrusion
Pressure transducer
Cylinder
Polished shaft
Motorand gearbox
Worm gear
Check valve
Rupture disk
Oil return line
Oil filter
Oil reservoir
Oil pump
Contact electrode
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PressurePressure
Vol
ume
Increasing Pressure Causes Intrusion
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Mercury Porosimetry - Overview
Apparent pore size (log scale)
volume
Powder compaction
Intrusion into powder voids
Intrusion into internal pores
Compression of solid (rare)
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Results Overview
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Hysteresis
• Intrusion curves are not retraceable.
(Extrusion curves lie above the intrusion curve)
• Can be explained by changes in θ betweenintrusion and extrusion.
•Some mercury remains in the pores…
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Entrapment
• Mercury left behind in the pores:
entrapment.
• Entrapment ceases after the first
few cycles.
• Complex network of pores responsible
for such entrapment.
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THE state-of-the-art porometer
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sample holders support the sample
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Real-time data presentation
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Repeatability
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Application/Technique Selector
Mercury Porosimeter
Capillary Porometer
3D structures � -
2D structures - �
© 2004 Quantachrome Instruments
What Defines a Mercury Intrusion Porosimeter?
• Pressure Range– Lowest pressure defines largest pore.– Highest pressure defines smallest pore.
• NOTE: Effect of Contact Angle– A lower contact angle shifts pore size
range to smaller values. Merely mathematical.
– A higher contact angle shifts pore size to larger values. Merely mathematical.
© 2004 Quantachrome Instruments
The 3G Series 3G micro 3G Macro 3G z 3G zhPore size minimum 0.09 µm
or 0.06 µm0.09 µm <0.04 µm <0.02 µm
Pore size maximum 100 µm >500 µm 500 µm 500 µm
Pressure controllers 1 2 2 2
Controller #1 0-100 psi or 0-150 psi
0-5 psi 0-30 psi 0-30 psi
Controller #2 n/a 0-100 psi 0-300 psi 0-500 psi
Pressure sensors 2 2 3 3
Sensor #1 0-5 psi 0-5 psi 0-5 psi 0-5 psi
Sensor #2 0-100 psi or 0-150 psi
0-100 psi 0-100 psi 0-100 psi
Sensor #3 n/a n/a 0-250 psi 0-500 psi
Flow sensors 1 1 1 or 2 2
Sensor #1 0-100 L/min or 0-200 L/minor 0-20 L/min
0-200 L/min 0-100 L/min 0-10 L/min
Sensor #2n/a n/a
Optional 5, 50, 200 L/min
0-200 L/min
Flow sensor switching
n/a n/a manualauto
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APPLICATIONS OF HG POROSIMETRY
IN PWHGM
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371
Path to a PWHGM
HGM
Heat Treatment
580°C
600°C
Acid Treatment
PWHGM
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372
Impact of Heat Treatment
Non heat treated 8 hours 600°C
Pore size is extremely small in sample with no heat treatment– At 200,000X pores are barely detectable (Pore diameter: ~100 Ǻ)
Heat treatment enhances the formation of the interconnected microstructure– Pores are clearly visible at only 50,000X (Pore diameter: ~1000 Ǻ)
Baseline composition
600 nm150 nm
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373
Impact of Heat Treatment
• Considerable increase in pore volume with heat treatment
• Pore diameter increases from ~100 Ǻ to ~1000 Ǻ
0.0
0.5
1.0
1.5
2.0
2.5
3.0
10 100 1000 10000
Pore Diameter (Angstroms)
Lo
g D
iffer
entia
l In
tru
sio
n V
olu
me
(mL
/g)
no heat treatment
600°C 8 hrs.Shift to larger pore diameters
Baseline composition – Mercury Porosimetry Data
© 2004 Quantachrome Instruments
374
Impact of Heat Treatment Temperature
• Microstructure is strongly influenced by temperature – Only a 20°C difference in temperature
• Mercury porosimetry results are inconclusive for 8 hours at 580°C – Sample treated at 600°C for 8 hours has a pore diameter of ~1000 Ǻ
8 hours at 580°C 8 hours 600°C
Baseline composition – Same Magnification
600 nm 600 nm
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375
Impact of Heat Treatment Time
8 hours at 580°C 24 hours 580°C
Variation in microstructure is minimal for heat treatment times of 8 – 24 hours
Heat treatment time is not as effective as heat treatment temperature
Baseline composition
Apparent “cracking” is due to sample preparation
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376
Impact of Heat Treatment Time
0.0
0.5
1.0
1.5
2.0
2.5
3.0
10 100 1000 10000
Pore Diameter (Angstroms)
Lo
g D
iffer
entia
l In
tru
sio
n V
olu
me
(mL
/g)
600°C 8 hrs.
600°C 24 hrs.
Baseline composition – Mercury Porosimetry Data
Very little (if any) increase in pore volume No noticeable shift in pore diameter
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377
Impact of Composition
+3 SiO2
B/R +0.5
B/R -0.5
Similar microstructures….
Base
+6 SiO2-6 SiO2
-3 SiO2
Heat treatment for 8 hours at 600°C
Images taken at same magnification
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378
Impact of Composition• All compositions yield interconnected
morphology
• Possible influence of composition on microstructure– Varying degrees of porosity
– Mercury porosimetry data is inconclusive
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379
Impact of Composition
+3 SiO2
B/R +0.5
B/R -0.5
Similar microstructures….
Base
+6 SiO2-6 SiO2
-3 SiO2
Heat treatment for 8 hours at 600°C
Images taken at same magnification
© 2004 Quantachrome Instruments
380
Impact of Composition• All compositions yield interconnected
morphology
• Possible influence of composition on microstructure– Varying degrees of porosity
– Mercury porosimetry data was inconclusive
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381
Conclusions• Task Objectives
– Determine the impact of heat treatment time and temperatureand composition on porosity
• TEMPERATURE – PRIMARY EFFECTNo HT 580°C 8 hrs. 600°C 8 hrs.
Increase in the degree of phase separation/porosity with increasing heat treatment temperature
~100 Ǻ ~1000 Ǻ
© 2004 Quantachrome Instruments
382
Conclusions• COMPOSITION – SECONDARY EFFECT*
– Micrographs indicate variations in the degree of porosity– *Assuming no confounding effects of HGM diameter/wall
thickness
• HEAT TREATMENT TIME – NO EFFECT (8 – 24 hours) 580°C 8 hrs. 580°C 24 hrs.
No change with heat treatment time
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ANY QUESTIONS ?
© 2004 Quantachrome Instruments