Surface Area and Porosity Outline

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Dec 4, 2012 - ... of pressure, volume, and temperature ... Surface area models neglect the effects of localized .... Probe Temperature, K Reference P ratio.
Surface Area and Porosity 1



Outline Background ✦

• ✦

Techniques

Surface area ✦

Total - physical adsorption



External

Porosity ✦

meso



micro 2

Tuesday, December 4, 12

Length 1 nm



10

100

1 µm

10

100

1 mm

macro meso micro

10-10m C-C bond

metal crystallite

10-9m

10-8m

Carbon nanotube

cell membrane

10-7m

10-6m

10-5m red blood cell

Transistor gate

10-4m human hair

10-3m red ant

3

Techniques Mercury intrusion

• Adsorption Physical Chemical Temperature Programmed Methods

4 Tuesday, December 4, 12

Physical Adsorption 5

Characterization via Adsorption Material Characterization

• •

Physical properties Differentiate

Gas Adsorption



Quantity adsorbed on a surface as a function of pressure, volume, and temperature



Modeled properties

• • •

Surface area Pore structure

Non-destructuve

6 Tuesday, December 4, 12

Static Adsorption P V

X

X

G

X

7

Adsorption Quantity adsorbed - always normalized for mass cm3/g or moles/g Relative pressure - equilibrium pressure divided by saturation pressure - p/po



Equilibrium pressure - vapor pressure above the sample - corrected for temperature (thermal transpiration)



Saturation pressure - vapor pressure above a liquid

Surface energy - solid/fluid interaction, strength, and heterogeneity 8 Tuesday, December 4, 12

Sample Preparation Clean the surface Remove volatiles

• • •

Water CO2 Solvents

Controlled environment!

• •

Inert purge or vacuum Temperature control

Avoid Phase Changes

9

Physical Adsorption Molecules from the gas phase strike the surface. At equilibrium the molecule adsorbs, lose the heat of adsorption, and subsequently desorb from surface. At equilibrium the rate of condensation = the rate of desorption Constant surface coverage at equilibrium.

Surface features change the adsorption potential. Surface area models neglect the effects of localized phenomenon. Curve surfaces or roughness provide enhanced adsorption potential.

10 Tuesday, December 4, 12

Physical Adsorption Not activated (no barrier) 60

Rapid

40

Potential Energy, kJ/mol

Weak (< 38 kJ/mol) Atomic/Molecular Reversible Non-specific May form multilayers

20 0 −20 −40 −60 −80

van der Waals/dipole interactions

−100

0

1

2

3

4

5

6

7

Distance from Surface, Å

Often measured near the condensation temperature

11

Chemical Adsorption May be activated

60

Covalent, metallic, ionic

Potential Energy, kJ/mol

40 20

Strong (> 35 kJ/mol)

0

May be dissociative

−20 −40

Often irreversible

−60

Specific - surface symmetry

−80 −100

0

1

2

3 4 Distance from Surface, Å

5

6

7

Limited to a monolayer Wide temperature range

12 Tuesday, December 4, 12

Isotherm Types

nads

I

II

III

IV

V

VI

P

• •

Constant temperature Quantity adsorbed as a function of pressure



• •

Six classifications Quantity is normalized for sample mass

Vacuum to atmospheric

13

Classical View of Adsorption As the system pressure is increased the formation of a monolayer may be observed.

A

qads

IV

A

p/po

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Adsorbed Layer Density

• •

The first layer begins to form below 1x10-4 p/po



The density continues to increase with pressure/adsorption

The monolayer is completed below 0.1 p/po

15

Classical View of Adsorption As the system pressure is increased (gas concentration also increases) multiple layers sorb to the surface.

A

B

qads

IV

B A

p/po

16 Tuesday, December 4, 12

Adsorbed Layer Density

• •

The monolayer is completed below 0.1 p/po



The second layer continues to form as pressure is increased

The third layer appears at < 0.5 p/ po

17

Classical View of Adsorption As pressure is further increased we may observe capillary condensation in mesopores.

A

B-C

qads

IV

C B A

p/po

18 Tuesday, December 4, 12

Adsorbed Layer Density



Layer formation continues as p/po increases



As p/po approaches 1, the density becomes constant or nearly liquid-like 19

Classical View of Adsorption As pressure approaches the saturation pressure, the pores are filled and we may estimate total pore volume.

D

D

qads

IV

A

B-C

C B A

p/po

20 Tuesday, December 4, 12

Adsorptives Nitrogen Argon Krypton

21

Nitrogen Broad usage

• • •

Limitations Strong interactions

t-plot

• •

Pore size distributions



Reduced precision for materials with < 1m2/g (10µmol/g monolayer)

Surface area



BJH - bulk fluid properties



NLDFT - excess density

Slow diffusion < 0.5 nm pores

22 Tuesday, December 4, 12

250

ZSM-5 Faujasite

Vads, cm3/g

200

150

100

50

0 1e-08

1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1

1

p/po

Confinement 23

Argon Pore size distributions

• •

H-K calculations NLDFT - excess density

Benefits



Reduced interaction compared to N2



Molecular size < N2 and faster diffusion due to size and T (87K)

Limitations



Ar molecular area not a generally accepted value



Statistical t-curves based upon N2



Not used for BJH bulk fluid methods

24 Tuesday, December 4, 12

Faujasite (H+) 250

Nitrogen Argon

Vads, cm3/g

200

150

100

50

0 1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1

1

p/po

Y zeolite, Ar Adsorption 25

ZSM-5 (LN2) 200 180

Nitrogen Argon

160

Vads, cm3/g

140 120 100 80 60 40 20 0 1e-08

1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1

1

p/po

ZSM-5, Ar Adsorption 26 Tuesday, December 4, 12

140

Adsorption Desorption

120

3

Vads, cm /g

100 80 60 40 20 0 1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1

1

p/po

ZSM-5 Low P Desorption 27

Krypton Surface area estimates BET



Low specific surface area (< 1m2/g)



Low absolute area limited sample quantity

Benefits



High precision, low pressure analysis

Limitations



Pressure range limited to < 1 torr at 77 K (