MEL471 Porosity

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Outline

• Use of mercury porosimetry and nitrogen adsorption for characterization of the porosity

• Estimation of particle size from the porosity measurements

• Application of porous materials

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Porous materials

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Types of pore

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Measurement techniques

6http://www.micromeritics.com/

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http://www.micromeritics.com/

8http://www.micromeritics.com/

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http://www.micromeritics.com/

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Mercury porosimetry

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Mercury porosimetry

Pore diameter (nm)

Vo

lum

e (m

l/g)

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Mercury porosimetry• Total volume (Vtot): total intruded volume of mercury at Pmax

• Total pore surface area (S):

• Mean pore diameter:

∫=totV

pdVS0|cos|

1

θγ

S

Vd

totmean ×= 4

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Limitations of mercury porosimetry

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Adsorption vs Absorption

H H H H H H H H H

H H H H H H H H H

H2 adsorption onpalladium

H

H

HH

H

HH H

H

H

HH

HH

H

H

H H

H2 absorption →palladium hydride

adsorbent

adsorbate

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Quantachrome Instruments

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Adsorption and Desorption Isotherms

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www.quantachrome.com

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Adsorption and Desorption Isotherms�As P/Po changes to 1� coverage of surface passes from monolayer to multilayer

� pressure ratio at which condensation takes place on a concave surface of radius “r” is smaller than that for a flat surface (P/Po=1)

� as condensation progresses in a pore → added layer of condensate reduce the pore radius → pressure ratio for this pore is further reduced

� any multilayer which has developed on the walls of a wider capillary tube will evaporate and condense in the narrower tubes

RTr

V

P

P

K

0 2ln

γ=

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Video on Gas adsorption processhttp://www.youtube.com/watch?v=ZF-sk5hYSwA

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Information from Adsorption and Desorption Isotherms

• Surface area (BET)

• Pore size distribution– Mesopore: Kelvin equation, BJH method– Micropore: Surface area of microporous by Langmuir,

t-plot, HK, SF

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Brunauer-Emmett-Teller (BET)� surface of a monolayer adsorbate of N2-molecules at boiling

temperature of N2 at Po=1 atm, 77 K

� determination of surface area of solid materials

−+=

− ommo P

P

CW

C

CWPPW

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)1)/((

1W: weight of gas adsorbed at P/Po

Wm: weight of monolayer adsorbate

C: related to energy of adsorption in the first layer

0.05 < P/Po < 0.3MNAW

Sis

WCW

iCW

Cs

sXiY

csmtm

mm=

+=−=

+=

= ,1

,1

,1

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Pore size distribution (Mesopore)

� desorption isotherm is closer to true thermodynamic stability

� using Barrett, Joyner and Haldena (BJH) method

2/1

034.0)/log(

99.13

)/ln(

2

+=

−=

+=

PPt

trr

PPRT

Vr

o

Kp

o

mK

γ

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Pore size distribution (Micropore)

� Langmuir equation – adsorption of single molecular layer of adsorbate, Surface area calculation

� t-method: determination of micropore volume in presence of mesopore

�HK method, SF method: pore size calculation independetly from Kelvin equation

)/(1)/(

o

o

m PPCPPC

WW

+=

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IUPAC Isotherm classification

III

n ad

p/p0

VI

n ad

p/p0

V

n ad

p/p0

In a

d

p/p0 p/p

II

n ad

0

B

IV

n ad

p/p0

B

III

n ad

p/p0

VI

n ad

p/p0

V

n ad

p/p0

In a

d

p/p0

In a

d

p/p0 p/p

II

n ad

0

Bp/p

II

n ad

0

B

IV

n ad

p/p0

B

IV

n ad

p/p0

B

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Isotherms

Assumptions:

• homogeneous surface

(all adsorption sites energetically identical)

• monolayer adsorption (so no multilayer adsorption)

pKpK

nnn mmad +⋅=θ⋅=1

n ad

p/p0

Type I Langmuir Adsorption Isotherm

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Isotherms

Multilayer adsorption (starting at B)

Common for pore-free materials

p/p

n ad

0

B

Type II

Type IV

Similar to II at low p

Pore condensation at high p

n ad

p/p0

B

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Type III

Type V

n ad

p/p0

Weak adsorbate-adsorbent interactions, Strong cohesion force between adsorbed molecules,

n ad

p/p0

Similar to III at low p

Pore condensation at high p

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Type VI

n ad

p/p0

� Highly uniform surface� Layer by layer adsorption� Stepped isotherm

Example:� Adsorption of simple non-porous

molecules on uniform surfaces

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Isotherm: γ-Al2O3 (Mesoporous)

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BET specific surface area: γ-Al2O3

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Pore size distribution: γ-Al2O3

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Isotherm: Activated carbon (Microporous)

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Pore size distribution: Activated carbon

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Limitations of N2 adsorption method�Micropores are not accessible to N2 at 77 K

�Micropores can not accommodate 2 layers of adsorptive molecules

underestimates surface area

�Ar, Kr based measurements

� samples with low surface area can be characterized by Kr gas adsorption

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Properties of adsorbates

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Hysteresis?

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Application of porous materials

• Gas separation membranes• Sensors• Hydrogen storage • Catalysis