surface area, pore size and more :

Surface Area, Pore Size and More:

Theory and Application of Porous Materials Characterization Methods

• Gas Adsorption Measurements with particular focus on Microporous Materials• Liquid Intrusion Porosimetry with particular focus on Meso- and Macroporous Materials• Other Methods of Pore Size measurement - Capillary Flow Porometry and Electroacoustics• Catalyst Characterization using Chemisorption and Temperature Programmed Analyses• Dynamic Water Vapor Sorption - Adsorption, Absorption, Hydrophobicity, Hydrophilicity

Inert Gas Adsorption– What can be measured using this technique?– Who would be interested in such results?– A brief overview of measurement fundamentals.– Microporous materials

• Carbons• Zeolites• Metal organic frameworks

– Instrument selection for these materials– Specific features of benefit to analyzing microporous materials– Mesoporous/nonporous materials

• Carbon black• Ceramics• Pigments• Alumina• Silica• Metal powders• Pharmaceuticals

– Instrument selection for these materials– Specific features of benefit to analyzing meso-/nonporous materials

Inert Gas Adsorption– What can be measured using this technique?

• Specific Surface Area– How low?

» Depends on instrument sensitivity and amount of sample (more later!)

– How high?» No limit

• Pore Size Distribution– Min, max?

» As small as the smallest gas molecule that can be adsorbed

– Pore Volume» No limit

• Heats of Adsorption» More later

Inert Gas Adsorption

– Who would be interested in such results?– Everyone who needs to understand how pore

structure affects material performance.• Surface Area

– affects dissolution rates.– affects electron/ion current density at electrode interface

with electrolyte.– affects adsorption capacity.– represents surface free energy available for bonding in

tabletting and sintering.

Inert Gas Adsorption

– Who would be interested in such results?– Everyone who needs to understand how pore

structure affects material performance.• Pore Size Distribution

– affects diffusion rates.– affects molecular sieving properties.– affects surface area per unit volume.

Measurement Overview

• Two techniques available• Dynamic flow (uses different concentrations

of the adsorbing gas, i.e. gas mixtures)… this will only be covered in discussion session

• Vacuum-volumetric, better to say “Manometric” (uses different pressures of the adsorbing gas)… our main focus

What is a Gas Sorption Analyzer?

• Does it actually measure surface area and pore size?

• NO!! It simply records various pressures of gas in the sample cell due to adsorption and desorption. The instrument then calculates the amount (as STP

volume) of gas adsorbed/desorbed. Surface area, pore size are calculated by PC software (iQWin, NovaWin, Quadrawin).

• Pressure measurements are critical!

Adsorption/Desorption

• Adsorption is the sticking of gas molecules onto the surface of a solid… all available surfaces including that surface inside open pores.

• Increasing the pressure of gas over a solid causes increasing adsorption.

• Temperature dependent

Adsorption/Desorption

• Desorption is the removal of gas molecules from the surface of a solid… all available surfaces including that surface inside open pores.

• Decreasing the pressure of gas over a solid causes increasing desorption.

• Done at same temperature as adsorption.

Movie time!

So, How Does It Work?

• Basic Construction– Removable sample cell– Dosing manifold– Pressure transducers– Vacuum system– Analysis gas– Valves to move gas in and out of manifold and

sample cell– Sample thermostat (dewar, furnace, cryostat)


• Basic Construction– Removable sample cell

• A long stemmed piece of glassware that holds the sample during degassing (preparation) and analysis.

• Available in different stem diameters and bulb sizes.


• Basic Construction– Dosing manifold

• A chamber of known (i.e. calibrated) physical volume from which gas is added to and removed from the sample cell during adsorption and desorption respectively (think burette).


• Basic Construction– Pressure transducers

• Used to both quantitatively determine the amount of gas adsorbed/desorbed and the pressures at which the sorption is measured.


• Basic Construction– Vacuum system

• Vacuum pump(s) generate sub-atmospheric pressure conditions.

• Rotary oil pumps for low vacuum applications.• Turbo pump backed by oil-free diaphragm pump

for high vacuum applications.


• Basic Construction– Analysis gas

• Nitrogen is used most often.• Argon is recommended for micropore size

measurements.• Krypton is used for very low surface area and thin

film applications.• Multiple gases can be connected at one time,

though only one is actively used.


• Basic Construction– Valves to move gas in and out of manifold and

of sample cell• Automatically operated to fill the dosing manifold to

a pressure sufficient to yield a datum point at a specified target pressure (or at target sorbed amount)

• Magnetic latching valves… no heat generated during pressure equilibration


• Basic Construction– Sample thermostat (dewar, furnace, cryostat)

• Dewar holds cryogenic liquids (liquefied gases) like liquid nitrogen (LN2) and liquid argon (LAr)

• Furnace: used for chemisorption measurements at temperatures above ambient

• Cryostat: for advanced research applications, overcomes limitations of restricted choice of temperatures available with liquefied gases in a dewar.

Basic Construction

Sample cell

Analysis gas to vacuumManifold

Pressure transducer(s)

Basic Operation

Sample cell



Manifold, transducer and sample cell are evacuated.

Basic Operation

Sample cell



Manifold, transducer and sample cell are evacuated… and cell is cooled.

Basic Operation

Sample cell



Intermediate valve status.

Basic Operation

Sample cell



Analysis gas is admitted to build some pressure in the manifold.

Basic Operation

Sample cell



A steady pressure in the manifold is recorded, P1.

Basic Operation

Sample cell



Gas expands from manifold into sample cell; pressure drops in the manifold, rises in sample cell.

Basic Operation

Sample cell



Gas is adsorbed by the sample, pressure drops further in both volumes.

Basic Operation

Sample cell



Eventually the pressure equilbrates. Final pressure, P2, is recorded.

Basic Operation

Sample cell



Process is repeated at higher and higher pressures.

Basic Operation

Sample cell



Adsorption measurements are complete... Getting ready to desorb!

Basic Operation

Sample cell



In desorption, some gas is removed from the manifold while the sample cell remains isolated.

Basic Operation

Sample cell



Manifold is isolated and desorption P1 is measured.

Basic Operation

Sample cell



Gas is expanded from sample cell into manifold, pressure drops in the sample cell, rises in the manifold… P2 (desorption)

A More Realistic Representation

Sample Temperature Control

• As the coolant evaporates, the level sensor signals the dewar drive to compensate for the change in level, thereby maintaining a constant and small cold zone.

cabinet

level sensor

sample cell

90 hr dewar

drive shaft

dewar support arm

What’s Really Measured

• The pressure of gas not currently adsorbed by the sample, just filling the void volume.

• To know quantitatively what is adsorbed, the instrument calculates:– The dose amounts, i.e. amount of gas moved into

(adsorption) or out of (desorption) the cell by the end of an equilibration period.

– The amount of gas remaining unadsorbed (in the void volume) at that time.

– The difference is what is adsorbed.

What’s Measured

• To calculate the gas amounts dosed (in/out) the instrument must know:– P1– P2– Volume of the manifold– Temperature of the manifold

What’s Measured

• To know the volume of the manifold:– it is calibrated using a special cell and glass

cyclinder (rod).– All instrument manifolds are factory calibrated.

• To know the temperature of the manifold:– it is constantly monitored by a solid state

sensor.

What’s Measured

• To calculate the gas amounts not adsorbed the instrument must know:– Volume of the void volume (sample cell)– Temperature of the void volume (sample cell)

What’s Measured

• To know the volume of the sample cell the instrument can:– Measure it by expanding helium from the

manifold (as part of initializing the analysis)– Use a previously measured value– Use a stored value based on expanding

nitrogen into an empty cell, correcting for sample volume (the so-called NOVA method)

What’s Measured

• To know the temperature of the sample cell (in coolant) the instrument:– is told it as an analysis parameter.

• To ensure that the volume of cell in coolant remains constant:– a coolant level sensor and dewar elevator

mechanism combine to maintain level of coolant around the sample cell.

Small Cold Zone = Sensitivity

Coolant level controlled here creates a small cold zone.

Quantachrome’s instruments

Leve

l sen

sor

Working Equation

PV = nRT

nads = ndosed - nvoid

nads = (PV/RT)man. - (PV/RT)cell

Refinements

• Corrections for “non-ideality” of gas, especially at cryogenic temperatures.

• Compensation for the slight change in temperature of that part of the sample cell not in coolant (“TempComp”).

• Determination of “saturation vapor pressure” of the coolant, known as Po.

What Is The Result?A

mou

nt a

dsor

bed

Equilibrium pressure

It’s called an “isotherm”


mou

nt a

dsor

bed

Equilibrium pressure

The values on the y-axis are calculated from pressure measurements (and temperature values)

The values on the x-axis are pressure measurements.


mou

nt a

dsor

bed

Relative pressure

Desorption curve may overlay on, or appear to left of, the adsorption curve

The values on the x-axis are in fact expressed as relative pressure, P/Po

Very Low Pressure Behavior (micropore filling)

Relative Pressure (P/Po)

Am

ount

Ads

orbe

d

Low Pressure Behavior (monolayer)

The “knee”


Am

ount

Ads

orbe

d

Medium Pressure Behavior (multilayer)


Am

ount

Ads

orbe

d

High Pressure Behavior (capillary condensation)


Am

ount

Ads

orbe

d

Instrument Features• Multiple transducers

– 1000 torr• Used for usual BET (surface area) range and mesopore

analyses

– 10 torr• Used for krypton BET areas and shifted BET range (e.g.

zeolites)• Used to cover intermediate pressure range between 1 torr

and 1000 torr• Always associated with turbo pump

– 1 torr• Used for krypton BET areas and micropore measurements• Always associated with turbo pump

– 0.1 torr (in place of 1 torr, iQ-XR only)

• Extended range micropore

Instrument Features

• Degassing– Is done on the degassing ports– Is not for grossly wet samples– Is done without a filler rod*– Should include a “test”– Dirty filters can reduce effectiveness

– Should be done using LN2 in cold trap– *When using a Cell-Seal a filler rod is added first, so

degassing is done with the rod.

Applications I

– Microporous materials• Carbons• Zeolites• Metal organic frameworks

– Instrument selection for these materials– Specific features of benefit to analyzing

microporous materials

Applications I

– Microporous materials• Activated carbons

– The small size of their pores gives them great surface area… they can adsorb a large amount of gas directly on to their surface. Popular support for some catalyst metals (especially palladium and platinum). ρ~ 2g/cm3

• Zeolites– The narrow size distribution of their pores makes them very

useful for gas separation. Also used as catalysts because of acid sites in the pores. ρ~ 4g/cm3

• Metal organic frameworks– Their huge surface area and pore volume makes them

potentially useful for gas sequestration/storage. ρ< 0.5g/cm3

Activated Carbons

– Made from a variety of materials:• Rice husk• Coconut fiber• Nut shells• Waste biomass

– plant– animal

Activated Carbons

– Activation is done chemically and thermally.– It creates spaces between layers of carbon

(graphene) of non-uniform micropore size.– It usually produces a chemically

heterogeneous surface.• Presents a problem for accurate pore size

calculations.

N2 , Ar (at 77.35 K) vs. CO2 (273.15 K) Adsorption on Activated Carbon Fiber (ACF-10) and

NLDFT-PSD Histograms

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

4 6 8 10 12 14 16 18 20

Pore Size Å

Pore

Vol

ume,

cc/

g

CO

N2

2

N2/77.35 K

CO2/273.15 K

Analysis Time: CO2 = 3 h N2 = 40 h

0

100

200

300

400

1E-06 1E-05 0.0001 0.001 0.01 0.1 1

Relative Pressure

Amou

nt A

dsor

bed,

cc(

STP)

/g

N2 (77 K)Ar (77 K)

CO2 (273 K)

CO2,

N2

Ar

Quantachrome’s Powder Technote 35

Nitrogen (77.35 K and Water Sorption (298.4K) in Activated Carbon Fibers (ACF),

(M. Thommes, et al., FOA 8, 2004)

0 2.10-1 4.10-1 6.10-1 8.10-1 100

P/P0

0

100

200

300

400

500

600

700

Vol

ume

[cc/

g] S

TP

A5A5A10A10A15A15

Nitrogen, 77.35 K

Microporous Carbons: the Standard way

5 10-6 5 10-5 5 10-4 5 10-3 5 10-2 5 10-1 5 100

P/P0

0

120

240

360

480

600

Vol

ume

[cc/

g] S

TP

A5A5A10A10A15A15

Nitrogen (77.35 K and Water Sorption (298.4K) in Activated Carbon Fibers (ACF), (M. Thommes, et al., FOA 8, 2004

Nitrogen, 77.35 K

Featureless Isotherms

6 8 10 20 40 60 80 100Pore Diameter [Å]

0

0.16

0.32

0.48

0.64

0.8

NL

DF

T P

ore

Vol

ume

[cc/

g]

A5A5A10A10A15 A15

A 15

A 5

A 10

NLDFT


State of the Art Cryogenic Differentiation

0

100

200

300

400

500

600

700

800

0 0.2 0.4 0.6 0.8 1

A5 25CA10 25CA15 25C

A15

A10

A5

Water, 25 C


The Special Behavior of Water

Zeolites– Micropores are part of their crystal structure:

• Most are synthetic• Alumino-silicates• Silicalite = no aluminum

• Cation can be H+, Na+, Ca2+, NH4+, etc

• Pore shape needs to be incorporated into pore size calculation for accurate results

• Some adsorbates are better than others

Adsorption of Nitrogen (77.35 K) and Argon (87.27 K) on a Zeolite

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

350

Vol

ume

[cm

3 ]

N2/77KN2/77KAr/87 KAr/87 K

ZEOLITE | 10.5.2001

N2/77.35 K

Ar/87.27 K

Faujasite: Ar and N2 Adsorption

.

Different Sized Pores Fill at Different P/Po

Pore Shape is Important for Accurate Pore Size Analysis of Zeolites

(M.Thommes et al., presented at the International Zeolite Conference, Cape Town, 2004)

10-6 5 10-5 5 10-4 5 10-3 5 10-2 5 10-1 5 100

P/P0

0

60

120

180

240

300

Vol

ume

[cc/

g]

H-MordeniteH-Mordenite13X13X

NLDFT_Zeolite Fit_(spherical pore model)NLDFT_Zeolite Fit_(spherical pore model)NLDFT-Zeolite Fit (cylindrical pore model)NLDFT-Zeolite Fit (cylindrical pore model)

4 12 20 28 36 44Pore Diameter Å

0

0.14

0.28

0.42

0.56

0.7

dV[c

c/Å

/g]

MCM-41 (NLDFT_Silica_cylindrical pore model)MCM-41 (NLDFT_Silica_cylindrical pore model)Zeolite X_type (NLDFT_Zeolite spherical pore model)Zeolite X_type (NLDFT_Zeolite spherical pore model)Mordenite-type (NLDFT_Zeolite_cylindrical pore model)Mordenite-type (NLDFT_Zeolite_cylindrical pore model)

Mordenite structure(cylindrical pores)

X-Zeolite structure(spherical pores)

10-5 5 10-4 5 10-3 5 10-2 5 10-1 5 100

P/P0

0

60

120

180

240

300

Vol

ume

[cc/

g]

Zeolite X- typeZeolite X- typeDFT-Fitting : cylindrical pore modelDFT-Fitting : cylindrical pore modelDFT-Fitting : spherical pore modelDFT-Fitting : spherical pore model

Metal Organic FrameworksMOFs

– Synthetic materials– Also called coordination polymers– Similar materials without metals are called

COFs… covalent coordination polymers– Still a very active research area

Metal Organic FrameworksMOFs

ZnO4 tetrahedra (blue) are joined by organic linkers (O, red, C, black), giving an extended 3D cubic framework with inter-connected pores of 11.2 Â aperture width and 18.5Â pore (yellow sphere) diameter

Microporous Materials

– Instrument selection for these materials• A micropore size distribution requires an isotherm to be

measured at low enough pressures to see the micropore filling, and accurately enough to yield an accurate pore size analysis.

– Why?: • Best high vacuum performance = lowest starting

pressure.• Best (i.e. lowest) leak rate = data quality.• Lowest pressure measurement possible (0.1 torr xducer)

= greatest confidence at smallest pore filling pressure.• Largest dewar = Longest unattended analysis time =

even the slowest measurements are possible.• Optional second station = no sharing transducers =

significantly increased throughput (almost double!).

Microporous Materials

– Instrument selection for these materials• No high vacuum available? = no micropore size

distribution except when using CO2 at 0degC on carbons.

• Can still measure total BET surface area including contribution from micropores.

• Can determine micropore area and micropore volume using t-plot method.

The Autosorb-iQ• Basic specs

– Transducers: (optional 0.1 torr), 1 torr, 10 torr, 1000 torr– Vacuum system: turbo pump (dry pump is standard)– Multiple gas inputs– Large dewar (90 hour)– Two degas ports each with own mantle– Programmable degassing– Po port– Dosing algorithms

etc

The Autosorb-iQ

• Advanced specs– Metal seals and very low leak rate allow us to

measure very low pressure isotherms even when using helium void volume mode. No need to disconnect helium and all other gases from the unit when measuring micropore isotherm!

– Two stations data quality are the same as one station (see next slide). No transducer or dosing manifold sharing.

– Dedicated Po transducer. Sample station(s) NOT interrupted to re-measure.

This plot actually shows THREE isotherms. One generated using just one station, and a pair generated simultaneously using both stations of the iQ2.

Accurate pore size calculations

• Accurate pore size calculations– QSDFT for activated carbons…accounts for

surface heterogeneity.– Argon NLDFT models for different pore

shapes (zeolites and MOFs)

• Full and proper equilibration

correct

incorrect

Applications II

– Mesoporous/nonporous materials• Carbon black• Ceramics

Pigments• Alumina• Silica• Metal powders• Pharmaceuticals

– Instrument selection for these materials– Specific features of benefit to analyzing

meso-/nonporous materials

Applications II– Mesoporous/nonporous materials

• Carbon black– Essential for tires and other rubber applications. BET (NSA) and t-plot

(STSA) are important.• Ceramics

– Particle size affects surface area, surface area remains after particle size is history. Pore size affects wicking of liquids.

• Pigments– Surface area and porosity “immobilize” liquids and alter rheology.

• Alumina– Surface area and pore size are the dominant quality control parameter.

Often used as a catalyst support.• Silica

– Surface area and pore size are the dominant quality control parameter.• Metal powders

– Surface area supports particle size data especially fines.• Pharmaceuticals

– Surface area is lost during tabletting (however pore size affects wicking of liquids) but after ingestion (and dissolution of excipient) s.s.a. of active controls release rate.

Carbon Black

Aluminas

Mesoporous Templated Carbons

Mesoporous Oxides

Mesoporous Oxides (Calcination Temperature)

Applications II– Mesoporous/nonporous materials

• Materials Research– Templated silicas

» MCM41 is the most famous example. Pore size by gas adsorption is an essential part of characterization.

– Templated carbons– Thin films

» For low-k (dielectric) applications. Difficulty is associated with very small amount of porous material.

Mesopore Analysis Significant progress in the pore size analysis of

porous materials made in the last few years, mainly because of the following reasons:

• (i) The discovery of novel ordered mesoporous molecular sieves which were used as model adsorbents to test theories of gas adsorption

• (ii) The development of microscopic methods, such as Non-Local-Density Functional Theory (NLDFT) and Quenched Solid Density Functional Theory (QSDFT)

• (iii) Carefully performed adsorption experiments… something at which Quantachrome excels.

What Does a Model Adsorbent Look Like?

TEM of MCM-41 Silica

Sorption, Pore Condensation and Hysteresis Behavior of a Fluid in a Single Cylindrical Mesopore

From: M Thommes, “ Physical adsorption characterization of ordered and amorphous mesoporous materials”, Nanoporous Materials- Science and Engineering” (edited by Max Lu, X.S Zhao), Imperial College Press, Chapter 11, 317-364 (2004)

SEM- of Mesoporous TiO2

Pore Size Can Also be Controlled by Granulation

0 0.2 0.4 0.6 0.8 1P/P0

0

30

60

90

120

150

Vol

ume

STP

[cc

/g]

Sachtopore 60Sachtopore 60Sachtopore 100Sachtopore 100Sachtopore 300Sachtopore 300Sachtopore 1000Sachtopore 1000Sachtopore 2000Sachtopore 2000

6 nm

10 nm

30 nm

100 nm

H. Kueppers, B. Hirthe, M.Thommes, G.I.T, 3 (2001) 110


Nitrogen Sorption at 77 K into Mesoporous TiO2


0.0 0.2 0.4 0.6 0.8 1.0100

200

300

400

500

600

ads des

MCM-41A

MCM-41B

MCM-41C

VO

LU

ME

[10

-6m

3 /g]

RELATIVE PRESSURE p/p0

3.3nm

3.6 nm

4.2 nm Argon 77K/

MCM-41

In : S. Lowell, J. Shields, M. Thomas, M. Thommes, Characterization of porous solids and Powders: Surface Area, Pore Size and Density, Kluwer Academic Publ, 2004,

3.3nm 3.6 nm4.2 nm

Different Temperatures Cause Same Sized Pores to Fill at Different P/Po

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40

50

60

70

desads

87 K 77 K

Ar / 77 K and 87 K

volu

me

[10

-6m

3 /g]

relative pressure p/p0

Argon/ MCM-48 (d = 4.01nm)

87 K

77 K

M. Thommes,, R. Koehn and M. Froeba et al. J. Phys. Chem B 104, (2000), 7933

Some History of Pore Size Analysis of Mesoporous Materials (a) Methods based on (modified) Kelvin Equation • e.g., - Barrett-Joyner-Halenda (BJH) (1951) - Dollimore-Heal (DH) (1964) - Broeckhoff de Boer (BdB) (1967/68) - Kruk-Jaroniec-Sayari (KJS)) (1997) - Bhatia et al (mod. BdB) (1998/2004) - D.D.Do & Ustinov (mod. BdB) (2004/2005)

(b) Density Functional Theory (DFT / NLDFT): e.g.- Evans and Tarazona (1985/86) - Seaton (1989), - Lastoskie and Gubbins (1993) - Sombathley and Olivier (1994) - Neimark and Ravikovitch (1995 ……)

(c) Quenched Solid DFT (QSDFT): - Neimark and Ravikovitch (1995 ……)

(d) Monte Carlo (MC) and Molecular dynamics (MD), e.g. - Gubbins et. al. (1986…. ) - Walton and Quirke (1989…) - Gelb (1999- ….) - Neimark and Ravikovitch (1995….)

Theoretical Predictions of Pore Filling P/Po as Function of Pore Size

. Neimark AV, Ravikovitch P.I., Grün M., Schüth F., Unger K.K, (1998) J. Coll. Interface Sci. 207,159

N2 / 77K in cylindrical silica pores

XX

BJH and NLDFT Compared

0 0.2 0.4 0.6 0.8 1RELATIVE PRESSURE p/p0

140

210

280

350

420

490

560

Vol

ume

[10-6

m3 /g

]

N2 (77 K): adsN2 (77 K): adsN2 (77 K): desN2 (77 K): des

DFT-FittingDFT-Fitting

15 23 31 39 47 55Pore Diameter [Å]

0

0.05

0.1

0.15

0.2

0.25

0.3

Dv(

d) [

cc/Å

/g]

BJH-Pore size distribution BJH-Pore size distribution DFT-Pore size distributionDFT-Pore size distribution

BJH NLDFT

NLDFT method: N2/77K cylindrical-silica pore model

X

Combined Micro/Mesopore Analysis by NLDFT(can’t be done by BJH)

0

5

10

15

20

25

0.000001 0.00001 0.0001 0.001 0.01 0.1 1

P/Po

Ads

orpt

ion,

[mm

ol/g

]

MCM-41

ZSM-5

50-50

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 [c

m3/g

]

histogram

integral

ZSM-5

MCM-41

S. Lowell, J.E. Shields, M.A. Thomas and M. Thommes, Characterization of porous solids and powders: Surface Area, Pore Size and Density, Kluwer Academic Publisher, 2004

Argon adsorption at 87 K on a 50:50 mixture of ZSM-5 + MCM-41:

0 0.2 0.4 0.6 0.8 1RELATIVE PRESSURE P/P0

0

100

200

300

400

500

600

700

VO

LU

ME

(ST

P)

[cc/

g]

VycorVycorSBA-15 SBA-15 Controlled-Pore Glass (CPG)Controlled-Pore Glass (CPG)SE3030SE3030

Studying Pore Geometry, Connectivity and Disorder

Nitrogen Sorption at 77 K into various Mesoporous Silica Materials

IUPAC Classification of Hysteresis

CylindricalPores

Cylindrical & Spherical Pores

Disordered; lamellar pore structures, slit & wedge, shape pores

Micro/Mesoporous adsorbents

Due to intrinsic fluid property

Due to pore blocking / cavitation (wide bodies, narrow necks)

Why Does Type H1 Exist?

spinodal condensation

spinodal evaporationequilibrium transition

0.05

0.04

0.03

0.02

0.01

0.2 0.4 0.6 0.8 10

0

Experimental (des)

Experimental (ads)

NLDFT in 4.8nm pore

Ads

orpt

ion,

m

mol

/m2

Relative pressure, P/P0

It can be clearly seen that the experimental desorption branch is associated with the equilibrium gas-liquid phase transition, whereas the condensation step corresponds to the spinodal spontaneous transition (i.e. delayed until nucleation occurs).

(a)Neimark A.V., Ravikovitch P.I. and Vishnyakov A. (2000) Phys. Rev. E 62, R1493; (b)Neimark A.V. and Ravikovitch P.I. (2001) Microporous and Mesoporous Materials 44-56, 697.

NLD

FT

ad

sorp

tion

iso

the

rm o

f ar

gon

at 8

7K in

a

cylin

dric

al p

ore

of d

iam

eter

4.8

nm

in c

omp

aris

on w

ith

the

appr

opr

iate

exp

erim

enta

l so

rptio

n is

oth

erm

on

MC

M-

41.

25 45 65 85 105 125Pore Diameter [Å]

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.22

Dv(

d) [

cc/Å

/g]

Ads (NLDFT-spinodal condensation)Ads (NLDFT-spinodal condensation)Des (NLDFT- equilibrium transition)Des (NLDFT- equilibrium transition)

0 0.2 0.4 0.6 0.8 1Relative Pressure P/P0

0

100

200

300

400

500

600

700

Vol

ume

STP

[cc

/g]

M. Thommes, in Nanoporous Materials- Science and Engineering” (edited by Max Lu), Imperial College Press, Chapter 11 p. 317 - 364 (2004)

Pore Size from H1 Can be Calculated from Ads and/or Des using NLDFT (but not BJH)

Nitrogen adsorption/desorption at 77.35 K in SBA-15 and pore size distributions

Nitrogen sorption at 77 K in CPG (Controlled Pore Glass)


0

70

140

210

280

350

420

Vol

ume

STP

[cc

/g]

40 90 140 190 240Pore Diameter [Å]

0

0.013

0.026

Dv(

d) [

cc/Å

/g]

Ads (NLDFT-spinodal condensation)Ads (NLDFT-spinodal condensation)Des (NLDFT- equilibrium transition)Des (NLDFT- equilibrium transition)


Pore Size from H1 Can be Calculated from Ads and/or Des using NLDFT (but not BJH)

Why Does Type H2 Exist?

Two Problems for Pore Size Analysis:

Adsorption Branch: metastable pore fluid delayed pore condensation

Desorption Branch: pore blocking,percolation delayed evaporation

How to Solve:

Application of novel NLDFT approaches

Type H2 Hysteresis

Nitrogen sorption at 77 K in porous Vycor Glass and pore size distributions from adsorption- (NLDFT spinodal condensation kernel) and desorption (NLDFT equilibrium transition kernel)

25 50 75 100 125 150Pore Diameter [Å]

0

0.008

0.016

0.024

0.032

0.04

Dv(

d) [

cc/Å

/g]

Ads (NLDFT- spinodal condensation)Ads (NLDFT- spinodal condensation)Des (NLDFT- equilibrium transition)Des (NLDFT- equilibrium transition)

VYCOR(PSD) | 12.11.20020 0.2 0.4 0.6 0.8 1Relative Pressure p/p0

0

30

60

90

120

150

Vol

ume

STP

[cm

3 /g]


Body Pore Size from H2 Calculated from Ads and Neck Size from Des using NLDFT

(but not BJH)

H3 Hysteresis


30

60

90

120

150

180

210

240

Vol

ume

STP

[cc

/g]

AdsorptionAdsorptionDesorptionDesorption

10 50 100 500 1000Pore Diameter [Å]

0

0.2

0.4

0.6

0.8

1

Dv(

log

d) [

cc/g

]

AdsorptionAdsorptionDesorptionDesorption

BJH-PSD

Artifact

N2/77K sorption on disordered alumina catalyst

M. Thommes, In Nanoporous Materials Science and Engineering, (Max Lu and X Zhao, eds.), World Scientific, in press (2004)

0 0.2 0.4 0.6 0.8 1P/P0

0

100

200

300

400

500

Vol

ume

STP

[cc

/g]

Nitrogen (77 K)Nitrogen (77 K)

H4 Hysteresis

Nitrogen adsorption at 77.4 K in activated carbon

H2 versus “H2”/H3/H4

M. Thommes, B. Smarsly, P.I. Ravokovitch, A.V. Neimark et al.. Langmuir, 22, 765 (2006)

Neck size

Pore body size

NO size information

Pore body size

NO size information; Cavitation is a property of the liquid

Product Selection

• Mesopore analysis needs:– Regular vacuum– 1000 torr pressure range– 24 hour dewar– Po station (usually)

– A simple BET does not need a long life dewar and Po is less critical

Product Selector 1ONE SAMPLE Quantachrome

Model Nova1 iQ Nova2

Stations 1 1 1+1

Po ports 0 1 (1)

Full ads y y y

Full des y y y

Degas stn 2(vac /

flow)

2(vacuum)

2(vac /

flow)

Xducer (s) specification

0.11% f.s.

0.11% f.s.

0.11% f.s.

Product Selector 2-3TWO-THREE

SAMPLESQuantachrome Quantachrome

Model Nova2 Quad 2 iQ2 Nova 3 Quad 3 Nova 4

Stations 1+1 2 2 2+1 3 3+1

Po ports (1) 2 1 (1) 3 (1)

Full ads y y y y y y

Full des y y y y y y

Degas stn 2(vac /

flow)

- 2 (vacuum)

4(vac /

flow)

- 4(vac /

flow)

Xducer (s) specification

0.11% f.s.

0.11% f.s.

0.11% f.s. 0.11% f.s.

0.11% f.s.

0.11% f.s.

Product Selector 4+

FOUR or MORE SAMPLES

Quantachrome Q’chrome

Model N4 Quad AS6B

Stations 3+1 4 6

Po ports (1) 4 6

Full ads y y y

Full des y y y

Degas stn 4 (vac/flow)

- -

Xducer (s)specification

0.11% f.s. 0.11% f.s. 0.11% f..s.

Workshop topics

• Selecting sample cells

• Degassing conditions

• BET points

• Mesopore points

• Micropore points

surface area, pore size and more :

Documents

pressure of gas

adsorbing gas

gas sorption analyzer

removal of gas molecules

sticking of gas molecules

various pressures of

smallest gas molecule

pore structure