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Characterization
of Materials
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Why Porous Materials?
Porous materials are also of scientific and technological
importance because of their vast ability to adsorb and interact
with atoms, ions and molecules on their large interior surfaces
and in the nanometer sized pore space.
They offer new opportunities in areas of inclusion chemistry,
guest-host synthesis and molecular manipulations and
reaction in the nanoscale for making nanoparticles, nanowires
and other quantum nanostructures.
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What are Porous Materials?
Porous materials are defined as solids containing pores.
Porous materials have porosity of 0.2-0.95.
Nature abhors a vacuum - always find ways to fill void
space.The interaction of the voids with guest species is the
subject of adsorption, catalysis, transport phenomena,
etc.
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Main Characteristics of Powders and
Porous Solids
Particle size
Surface area Porosity
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Why Do 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
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What is Particle Size?
SEM of real ibuprofen particles
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A Concept of Equivalent Sphere
3
6
1dV
Due to symmetry, size of sphere is
completely determined by only
one parameterits 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
2dS 3
6dm
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Different Equivalent Spheres
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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
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And What Do They Measure?
Direct observation (image analysis) usually some 2-Drepresentation of a particle. Which dimension isviable?;
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 (SSAback extrapolation to size);
Light scattering equivalent scatterers;
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Particle Size by Direct Observation
Google for
ImageJ
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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. Smallerparticles are kicked further by the solvent molecules and move morerapidly.
The velocity of Brownian motion is defined by a property known as thetranslational diffusion coefficient (D).
The size of a particle is calculated from the translational diffusioncoefficient by using the Stokes-Einstein equation:
d(H) hydrodynamic diameter, D translational diffusion coefficient, kBoltzmanns constant, T temperature, - viscosity
D
kTHd
3)(
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What Do We Measure in DLS?
The diameter that is measured in DLS isa value that refers to how a particlediffuses within a fluid so it is referred toas a hydrodynamic diameter
The diameter that is obtained by thistechnique is the diameter of a sphere
that has the same translational diffusioncoefficientas the particle
The translational diffusion coefficientwill depend not only on the size of theparticle core, but also on any surface
structure, as well as the concentrationand 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 andresulting in a larger, apparent
hydrodynamic diameter.
Thus, the measurements are
usually done in 10mM
NaCl (ISO13321 Part 8 1996)
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How DLS Works
The dark spaces in the speckle pattern produced by light scattering are where the phaseadditions of the scattered light are mutually destructive. The bright spots of light in thespeckle pattern are where the light scattered from the particles arrives with the samephase and interfere constructively.
The observed signal depends on the phase addition of the scattered light falling on thedetector. In example A, two beams interfere and cancel each other out resulting in adecreased intensity detected. In example B, two beams interfere and enhance eachother resulting in an increased intensity detected.
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How DLS Works
For a system of particles undergoing Brownian motion, a speckle pattern isobserved where the position of each speckle is seen to be in constant motion. Thisis because the phase addition from the moving particles is constantly evolving andforming new patterns.
The rate at which these intensity fluctuations occur will depend on the size of theparticles. 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 theintensity fluctuations arising from the Brownian motion of particles, but it isinefficient to do so. The best way is to use a device called a digital auto correlator.
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How an Auto Correlator Works
If the intensity of a signal is compared with itself at a particular point in time and a time muchlater, then for a randomly fluctuating signal it is obvious that the intensities are not going to berelated 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+t), 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+2t, t+3t, t+4t 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 longtime. If the particles are small and moving rapidly then correlation will reduce more quickly.
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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 wouldbe obtained (because the intensity of scattering is proportional to d6from Rayleighs approximation).
In DLS, the distribution obtained from a measurement is based on intensity.
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Schematics of Zetasizer Nano
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Measurement of Porosity andSpecific Surface Area by
Gas Adsorption
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F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and PorousSolids, 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 beused to perform specific function.Almost all solids are porous except for ceramics fired at extremely hightemperatures
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Measure of Porosity
Pore size andits 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
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Types of Pores
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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 PorousSolids, Academic Press, 1-25, 1999
Open pores are accessiblewhereas closed pores areinaccessible pores. Open porescan be inter-connected, passingor dead end.
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Size of Pores (IUPAC Standard)
2 nm 50 nm
Micropores Mesopores Macropores
Zeolite,Activated
carbon,Metal organicframework
Mesoporous silica,Activated carbon
Sintered metalsand ceramics
Porous material are classified according to the size of pores: material withpores less than 2 nm are called micropores, materials with pores between 2and 50 nm are called mesopores, and material with pores greater than 50 nmare macrospores
Sing, K. S. W. et al. Reporting Physisorption Data for Gas/Solid Systems. Pure &
Appl. Chem. 57, 603-619 (1985). 30
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Shapes of Pores
Conical
Interstices
SlitsCylindrical
Spherical orInk Bottle
PoreShapes
F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and PorousSolids, Academic Press, 1-25, 1999
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Experimental Techniques
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Techniques for Porosity Analysis
Mercuryporosimetry
TEM
SEM
Small angle
X-rayscattering
SmallAngle
Neutronscattering
Gasadsorption
Techniques
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Techniques for Porosity Analysis
Mercuryporosimetry
TEM
SEM
Small angleX-ray
scattering
SmallAngle
Neutronscattering
Gasadsorption
Techniques
Can measure only open pores Pore size : 0.4 nm 50 nm
Easy Established technique
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Techniques for Porosity Analysis
Mercuryporosimetry
TEM
SEM
Small angle
X-rayscattering
SmallAngle
Neutronscattering
Gasadsorption
Techniques
Similar to gasadsorption
Can measure onlyopen pores
Pore size >1.5 nm Easy Established technique
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Mercuryporosimetry
TEM
SEM
Small angle
X-rayscattering
SmallAngle
Neutronscattering
Gasadsorption
Techniques
Provide informationregarding poreconnectivity
Pore size can bemeasured if thematerials containsordered pores
Rarely used for poreanalysis
Techniques for Porosity Analysis
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Techniques for Porosity Analysis
Mercuryporosimetry
TEM
SEM
Small angle
X-rayscattering
SmallAngle
Neutronscattering
Gasadsorption
Techniques
Pore size > 5nm Rarely used for pore
analysis 37
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Techniques for Porosity Analysis
Mercuryporosimetry
TEM
SEM
Small angle
X-rayscattering
SmallAngle
Neutronscattering
Gasadsorption
Techniques
Any pore size Open + Closeporosity
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Techniques for Porosity Analysis
Mercuryporosimetry
TEM
SEM
Small angle
X-rayscattering
SmallAngle
Neutronscattering
Gasadsorption
Techniques
Any pore size Open & Close
porosity
Costly
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Theory of Adsorption
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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
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Ph i ti Ch i ti
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PHYSISORPTION CHEMISORPTION
WEAK, LONG RANGE BONDING
Van der Waals interactions
STRONG, SHORT RANGE BONDING
Chemical bonding involved.
NOT SURFACE SPECIFIC
Physisorption takes place between allmolecules on any surface providing the
temperature is low enough.
SURFACE SPECIFIC
E.g. Chemisorption of hydrogen takes place ontransition metals but not on gold or mercury.
Hads= 5 .. 50 kJ mol-1 Hads= 50 .. 500 kJ mol-1
Non activated with equilibrium achievedrelatively quickly. Increasing temperature
always reduces surface coverage.
Can be activated, in which case equilibrium canbe slow and increasing temperature can favour
adsorption.
No surface reactions. Surface reactions may take place:- Dissociation,reconstruction, catalysis.
MULTILAYER ADSORPTION
BET Isotherm used to model adsorptionequilibrium.
MONOLAYER ADSORPTION
Langmuir Isotherm is used to model adsorptionequilibrium.
Physisorption vs. Chemisorption
http://www.soton.ac.uk 42
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Adsorption Process
1. Diffusion to adsorbent surface2. Migration into pores of adsorbent
3. 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 SurfaceArea and Porosity, 3rd Ed. Chapman & Hall,New York, 1991
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Adsorption Process
adsorptiveofpressuresaturated
adsorbateofpressure
where
:aswrittenbecanequation
abovetheconstant,madeareIandT,W,If
adsorbent.andadsorbatebetweenninteractio
re;temperatu
adsorbate;theofpressure
adsorbent;ofweight
adsorbed;gasofvolume
where
),,,(
p
p
p
p
f
I
T
P
W
PITWf
o
o
V
V
V
a
a
a
Equation of adsorption
isotherm
Adsorbent
Adsorbate
S. Lowell & J. E. Shields, Powder Surface Area andPorosity, 3rd Ed. Chapman & Hall, New York, 1991
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h
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Gas Sorption: Isotherm
adsorptiveofpressuresaturated
adsorbateofpressure
where
p
p
p
pf
o
o
Va
Adsorption isotherm Isotherm is a measureof the volume of gasadsorbed at a constanttemperature as a
function of gaspressure.
Isotherms can begrouped into six
classes.
Va
Desorption isotherm
ppo
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G S ti I th
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Gas Sorption: Isotherm
S. Lowell & J. E. Shields, Powder Surface Area and Porosity,3rd Ed. Chapman & Hall, New York, 1991
Va
1P/Po
Type Ior
Langmuir
Concave to the P/PoaxisExhibited by microporous
solids ( < 2nm )
Exhibited by nonporous ormacroporous solids ( > 50nm )
Unrestricted monolayer-multilayeradsorption
Point B indicates the relativepressure at which monolayercoverage is complete
1P/P
o
Type II
B
V
a
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G S i I h
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Gas Sorption: Isotherm
Va
1P/Po
Type III Convex to the P/Po axisExhibited by nonporous solids
P/Po
Va
1
Type IVExhibited by mesoporous
solidsInitial 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 48
Gas Sorption: Isotherm
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Gas Sorption: Isotherm
Va
1P/Po
Type VHighly uncommonExhibited by mesoporous solids
S. Lowell & J. E. Shields, Powder Surface Area andPorosity, 3rd Ed. Chapman & Hall, New York, 1991
1P/Po
Type VI
Exhibited by nonporous solids
with an almost completelyuniform surfaceV
a
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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 type Ican have a hysteresis. Types IV and V exhibit hysteresis.
1P/Po
HysteresisV
a
S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed.Chapman & Hall, New York, 1991
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Gas Sorption: Hysteresis
Cylindrical Slits Conical Bottle neck
Va
1P/Po
Type A Type B
1P/Po 1P/Po
Type C Type D
1P/Po
Type
E
1P/Po
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Adsorption Theories: Langmuir
adsorbate.ofpressure
andconstant;empirical
monolayer;formtorequiredgasofvolume
;pressureatadsorbedgasofvolume
where
1
P
b
V
PV
V
P
bVV
P
m
a
mma
Assumptions:
homogeneous surface (all adsorption
sites energetically identical) monolayer adsorption (no multilayer
adsorption)
no interaction between adsorbedmolecules
Adsorbate
Adsorbent
I. Langmuir The Constitution and Fundamental
Properties of Solids and Liquids. Part I. Solids.J. Am. Chem. Soc., 1916, 38 (11), 2221-2295
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Adsorption Theories: BET
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Adsorption Theories: BET
adsorbate.ofpressurerelative
andlayer);1stofadsorptionofenergyto(relatedconstantBETC
monolayer;formtorequiredgasofvolume
;pressureatadsorbedgasofvolume
where
)1(1
)(
o
m
a
o
mm
o
a
P
P
V
PV
P
P
CV
C
CVPPV
P
Modification of Langmuirisotherm
Both monolayer and multilayeradsorption
Assumptions:
(a) gas molecules physicallyadsorb on a solid in layersinfinitely;
(b) there is no interaction betweeneach 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 309319
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S ifi S f A C l l ti
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Specific Surface Area Calculation
CVP
P
CV
C
PPV
P
m
o
m
o
a
1)1(
)(
imXY
imVm
1
adsorbateofWeightareasurfaceTotal csavm ANV
P/Po
1
V[(Po/P)-1]
0-1 0-2 0-3
At least three data points in therelative pressure range 0.05 to 0.30
sampleofWeight
areasurfaceTotalarea)surface(SpecificSSA
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Porosity Analyzer
Analysis station
Outgassing station
Liquid nitrogenbath
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Steps for Measurement
3.Interpretation
2.Adsorption Analysis
1.Sample Preparation
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Sample Preparation (Outgassing)
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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 IUPACstandards, materialsshould be outgassed forat least 16 hours.
Adsorbate
Helium
Vacuum
Po
Outgassingstation
Analysis station
SampleCell
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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
Outgassingstation
Analysis station
SampleCell
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I t t ti
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Interpretation
Points P/PoVolumeadsorbed
1
2
3
Pore shape
Specificsurface area
Pore volume
Pore size
&distribution
Results
Weight of sample
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C Ad b
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Common Adsorbates
Gas Temperature Cross sectionalarea (nm2)
N2 -195.8oC (liquid nitrogen)
-183 oC (liquid argon).
0.162
Ar -183 oC (liquid argon).
-195.8 oC (liquid nitrogen)
0.142
CO2 -78oC, -25 oC, 0 oC 0.195
CO -183 oC (liquid argon) 0.163
Kr -195.8 oC (liquid nitrogen) 0.205
O2 -183oC (liquid argon) 0.141
C4H10 0oC, 25 oC 0.469
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Ch i f Ad i
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Choice of Adsorptive
Oxy
gen
Ar
gon
Nitro
gen
Carbo
nmon
ooxide
Carbo
ndio
xide
Krypt
on
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
Cross-se
ctionalarea,nm
2
N2(g) in N2(l) is the most
commonly usedadsorbate.
Not completely inert. Dipole movement and
thus can havelocalized adsorption.
Cross-sectional area of0.162 nm2 isquestionable.
S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rdEd. Chapman & Hall, New York, 1991Quantachrome Autosorb-I Operational Manual
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Choice of Adsorptive
Ox
ygen
A
rgon
Nitr
ogen
Carbo
nmono
oxid
e
Carbo
ndioxid
e
Krypton
n-but
ane
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
Cross-s
ectionalarea,nm
2
S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rdEd. Chapman & Hall, New York, 1991Quantachrome Autosorb-I Operational Manual
Ar(g) in Ar(l)is preferable
but because ofunavailability of Ar(l) (87K),N2(l) (77 K) is used.
Ar can reach to somewhatsmaller pores than N2.
Accurate measurement ofmicropores is possibleusing Ar.
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Choice of Adsorptive
O
xygen
Arg
on
Nitrog
en
Carbo
nmon
ooxide
Carbo
nd
ioxide
Krypt
on
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
Cross-sectionalarea,nm
2
S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rdEd. Chapman & Hall, New York, 1991
Quantachrome Autosorb-I Operational Manual
In case of activatedcarbon, CO2 is oftenthe most preferred
adsorbate. Adsorption analysis of
CO2 takes less time. Limited to micropore
analysis.
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Shape of Microporous Materials
Type I isotherms dont have
hysteresis.
Pore shape cannot bedetermined by isotherm.
As various methods for poresize calculation are based onshape of pores, reliability ofpore size calculation isquestionable.
Va
1P/Po
Type Ior
Langmuir
F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous
Solids, Academic Press, 439-446, 1999 72
Choice of Method
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Methods Assumption
Pore Shape Based on ..
Brunauer MP method Cylindrical or Slit shaped de Boers t-method
Dubinin-Astakhov method - Polanyi potentialtheory
Independent ofKelvin equation
HK (Horvath-Kawazoe) method Slit Everett and Powl
methodIndependent ofKelvin equation
Saito-Foley method Cylindrical HK method
2 nm 50 nm
Micropores Mesopores Macropores
Choice of Method
P. A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics, 53 152, 1997
Quantachrome Autosorb-I Operational Manual 73
Choice of Method
-
7/29/2019 Sutarno KM 1
66/67
2 nm 50 nm
Micropores Mesopores Macropores
MethodsAssumption
Pore Shape Based on ..
BJH (Barrett, Joyner andHalenda) 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, 1997Quantachrome Autosorb-I Operational Manual
74
Choice of Method
-
7/29/2019 Sutarno KM 1
67/67
50 nm2 nm
Micropores Mesopores Macropores
Methods Assumption
Pore Shape Based on ..NLDFT (Non Local DensityFunctional Theory) and MonteCarlo simulation method
Cylindrical and slit Statisticalthermodynamics
Choice of Method
P. A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics, 53 152, 1997Quantachrome Autosorb-I Operational Manual