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
1Å
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
14 Tuesday, December 4, 12
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 (
