Sutarno KM 1

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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)


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


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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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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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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


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Mercuryporosimetry

TEM

SEM

Small angle

X-rayscattering

SmallAngle

Neutronscattering

Gasadsorption

Techniques

Pore size > 5nm Rarely used for pore

analysis 37


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Mercuryporosimetry

TEM

SEM

Small angle

X-rayscattering

SmallAngle

Neutronscattering

Gasadsorption

Techniques

Any pore size Open + Closeporosity

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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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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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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



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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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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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Ch i f d i


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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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Ch i f Ad ti


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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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Sh f Mi M t i l


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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


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


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2 nm 50 nm


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

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Choice of Method


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50 nm2 nm


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

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