ABC’s of Electrochemistry series
Materials Characterization
techniques:
Surface Area and Pore Size
Distribution
Ana María Valenzuela-Muñiz
February 9, 2012
Department of Chemical and Biomolecular
Engineering
Outline
• Introduction
• Principles
• Applications
• Summary
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Introduction
Basic concepts
Specific Surface Area:
 Is a material property of solids which measures the total surface
area per unit of mass, solid or bulk volume, or cross-sectional area
 It is a derived scientific value that can be used to determine the type and
properties of a material (e.g. soil). It is defined either by surface area
divided by mass (with units of m²/kg), or surface area divided by the volume
(units of m²/m³ or m-1)
 It has a particular importance for adsorption, heterogeneous
catalysis, and reactions on surfaces
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Introduction
Basic concepts
Adsorption:
Is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or
dissolved solids to a surface.
This process creates a film of the adsorbate (the molecules or atoms being
accumulated) on the surface of the adsorbent
- Desorption is the reverse process of adsorption Physisorption:
Is a process in which the electronic structure of the atom or molecule is
barely perturbed upon adsorption.
The weak bonding of physisorption is due to the induced dipole moment of a
nonpolar adsorbate interacting with its own image charge in the polarizable
solid
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Introduction
Basic concepts
Adsorption ≠ Absorption
Physisorption ≠ Chemisorption
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Introduction
Basic concepts
Adsorption Isotherm:
Describes the equilibrium of the adsorption of a material at a surface at
constant temperature. Is obtained by measuring the amount of gas
adsorbed across a wide range of relative pressures at a constant
temperature (typically liquid N2, 77K). Conversely desorption Isotherms
are achieved by measuring gas removed as pressure is reduced
Adsorption isotherms are often used as empirical models, which do not
make statements about the underlying mechanisms and measured
variables
Adsorption hysteresis:
Chemical potential of adsorbate during desorption is lower; hence true
equilibrium exists. Differences in contact angle during ads/des may also
lead to hysteresis. Presence of ink-bottle type pores-narrow neck & wide
body. Differences in the shape of the meniscus in the case of cylindrical
pores with both ends open.
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Introduction
Basic concepts
Gas adsorption provides a rapid and quantitative technique
for specific surface area and to determine other textural
properties of a solid as pore size, total pore volume, and
pore volume distribution
Pore Volume - Volume of pores accessible to condensed adsorbate
Pore size classification
Micropores - Less than 2 nm
Mesopores - Between 2 and 50 nm
Macropores - Greater than 50 nm
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How it works?
1.Adsorbate is introduced in to the manifold
2.The valve to the sample cell is opened allowing the adsorbate to
interact with the sample material
3.The pressure is repeatedly measured for the preset equilibration
time, if the pressure drops dosing recurs and measurement
proceeds until a stable reading is achieved
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How it works?
Multilayer formation
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What will you get?
Adsorption isotherm
30
RawCoal
RawCoal
ElecCoal
ElecCoal
28
Quantity Adsorbed (cm³/g STP)
26
24
ads
des
#42 ads
#42 des
22
20
18
16
14
12
10
8
6
4
2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Relative Pressure (p/p°)
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Isotherm types
II
III
V
VI
I
IV
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Isotherm types
IUPAC classification – 6 types of isotherms
Type I Microporous solids (Langmuir isotherm)
Type II Multilayer adsorption on non-porous / macroporous solids
Type III
Adsorption on non-porous /macroporous solids with weak
adsorption
Type IV Adsorption on meso porous solids with hysteresis loop
Type V Same as IV type with weak adsorbate-adsorbent interaction
Type VI
Stepped adsorption isotherm, on different faces of solid and/or
strong Interaction with surface
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Surface area calculation
Most common methods
• Langmuir (1918)
Monolayer adsorption
• BET (1938)
Multilayer adsorption
Va
bP

Vm 1  bP
Va

Vm
CP

P
Po  P 1  C  1 
Po 

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Langmuir Equation
p
1
p
 a
 a
a
n
nm  b nm
• Assumes adsorption limited to
one monolayer
• The Langmuir equation
describes Microporus material
p = pressure
exhibiting Type I Isotherms
na = amount of gas adsorbed, mol/g
nam = mono-layer capacity of sample, mol/g
b = Langmuir constant
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BET Equation

P
1
C  1 P


Va Po  P  VmC VmC Po
V= weight of gas adsorbed
• Multiple-layer adsorption
• Type II Isotherm
P/P0 =relative pressure
Vm = weight of adsorbate as monolayer
• Approaches essentially
infinite amount adsorbed
C = BET constant
Stephen Brunauer, Paul Emmett, Edward Teller; Fixed Nitrogen Laboratory (1938).
Second most cited chemistry reference over fifty-year period
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BET Equation
Linear plot

P
1
C  1 P


Va Po  P  VmC VmC Po
1
m
Vm 
, and C  1 
bm
b
Intercept
slope

1
C  1
b
, and m 
VmC
VmC
Ce
q
qo
RT
a

qa = heat of adsorption, J/mol
qo = heat of liquefaction, J/mol
R = ideal gas constant, 8.31 J/mol*K
T = absolute temperature, K
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BET Equation
Vm = weight of adsorbate as monolayer
1
Vm 
bm
Total Surface area (SA )can then be derived
VmN
SA 
Aacs
M
N = Avagadro’s number (6.023x1023)
M = Molecular weight of Adsorbate
Acs = Adsorbate cross sectional area (16.2Å2 for Nitrogen)
Specific Surface Area (S) is then determined by total Surface area by
sample weight
SA
SSA 
w
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BET Equation
Assumptions
 Adsorption energy of first layer is greater than that for higher layers
 Adsorption energies for second and higher layers are equal
 All adsorption sites on the adsorbent are equivalent
 Lateral adsorbate attractive forces are ignored
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Porosity
Pore Volume
Total pore volume is derived from the amount of vapour adsorbed at a
relative temperature close to unity (assuming pores are filled with liquid
adsorbate).
Vads = volume of gas adsorbed
PaVadsVm
Vliq 
RT
Vliq = volume of liquid N2 in pores
Vm = molar vol. of liquid adsorbate (N2=34.7cm3/mol)
Pa = ambient pressure
T = ambient temperature
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Porosity
Pore Radius
The average pore size can be estimated from the pore volume.
Assuming cylindrical pore geometry (type A hysteresis) average pore
radius (rp) can be expressed as:
2Vliq
rp 
S
Other pore geometry models may require further information
on the isotherm hysteresis before applying appropriate model.
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Porosity
Classification of pores
IUPAC
a - closed pores
b,f - pen only at one end
c,d,g - open
e - open at two ends (through)
 Different types and/or shapes of
pores will generate different hysteresis
types in the adsorption-desorption
isotherms
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Data reduction methods
to analyze textural properties
Langmuir: Provides a means of determining surface area based on a monolayer
coverage of the solid surface by the adsorptive
BET: The method of Brunauer, Emmet, and Teller is employed to determine surface
area on a model of adsorption which incorporates multilayer coverage
BJH: The method of Barrett, Joyner, and Halenda is a procedure for calculating
pore size distributions from experimental isotherms using the Kelvin model of pore
filling. It applies only to the mesopore and small macropore size range
deBoer t-Plot: Commonly used to determine the external surface area and
micropore volume of microporous materials. It is based on standard isotherms and
thickness curves which describe the statistical thickness of the film of adsorptive on
a nonporous reference surface
- DFT Plus - MP-Method - Dubinin Plots - Medek
- Horvath-Kawazoe technique - Deconvolution by Classical Model Fitting
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Available system
General Overview: TriStar II 3020
Located in CEER’s Analytical Lab (room 049) at OU
•
Surface Area/Porosity Analyzer
•
Three Sample Positions
•
Saturation Pressure Tube
•
30 Hour Dewar
•
Two Gas Inlets for Adsorptive Gases
•
Inlet for He for Free Space
•
Monolithic Manifold
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How to do the analysis
Steps
• Degasification
• Measure Free Space
• Measure P0
• Dose
• Equilibrate
Repeat throughout Isotherm
• Backfill
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Sample preparation
Sample preparation is an absolute prerequisite for the analysis
 Make sure the sample is dry and free of any solvent
 Degasification (degasification time and temperature depends on the
sample’s characteristics)
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Surface Area and Pore Size Distribution
Strengths
 Accurate
 Different gasses can be used
Limitations
 Size of the sample
 Minimum specific area of
-N2, He, CO2
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Surface Area and Pore Size Distribution
Applications
 Paints and coatings
 Pharmaceutics
 Aerospace
 Nanotubes
 Carbon Black
 Electronics
 Ceramics
 Catalysts
 Activated Carbons
 Fuel Cell Electrodes
 Adsorbents
http://www.micromeritics.com/Product-Showcase/TriStar-II-3020/TriStar-II-3020-Applications.aspx
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Examples of analysis
Specific surface area, pore size, pore volume
Surf Area Microp Ext Surf Area
[m²/g] Area [m²/g]
[m²/g]
RC 44
7.2941
0.8086
6.4854
Raw
9.5043
1.549
7.9554
RC 210
EC 44
12.6047
2.4809
10.1237
Electrolyzed
14.5666
1.3906
13.176
EC 210
4.3037
9.0005
EC 44Eth 13.3042
5.2594
11.9232
Extracted Eth EC 210Eth 17.1826
11.4953
3.9265
7.5688
EC 44Tol
2.0833
8.7963
Extracted Tol EC 210Tol 10.8796
Sample
Microp Vol
[cm³/g]
0.000444
0.000855
0.001366
0.000736
0.002386
0.002905
0.002181
0.001147
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Pore diam
AVE [nm]
13.0572
10.7825
14.81595
14.34755
16.2652
15.99825
17.67975
17.89785
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Examples of analysis
Specific surface area, pore size, pore volume
25
Total Area
Micropores
m2/g
20
15
10
5
0
RC 44
RC 210
EC 44
EC 210
EC 44Eth
EC
210Eth
EC 44Tol
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EC
210Tol
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Examples of analysis
BJH using a Reference Curve
BJH Adsorption Cumulative Pore Volume
10
0.8
8
0.6
6
0.4
4
0.2
2
0.0
10
50
100
Pore Volume (cm³/g)
Pore Volume (cm³/g)
1.0
MCM-41
BJH Adsorption dV/dlog(w) Pore Volume
0
500
Pore Width (Å)
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Examples of analysis
DFT – Beyond Porosity
• DFT can also be used to characterize the surface energy
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Summary
BET method
 The method is based on adsorption of gas on a surface
 The amount of gas adsorbed at a given pressure allows
to determine the surface area
 It is a cheap, fast and reliable method
 It is very well understood and applicable in many fields
 Not applicable to all types of isotherms
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Summary
Pore structure analysis
Adsorption Isotherm
Pore size distribution
BET plot
Pore radius/Pore volume
Surface area
Hysteresis yype
Isotherm type
t-Curve
Pore type, Shape, Geometry
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Related Literature
At CEER
Analytical Methods in Fine Particle Technology; Paul A Webb and
Clyde Orr (1997)
 Porosity and Specific Surface Area Measurements for Solid
Materials; Peter Klobes, Klaus Meyer and Ronald G. Munro (2006)
[electronic resource]
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Related Literature
WebPages
 http://www.micromeritics.com/Library/Application-Notes.aspx
 http://www.micromeritics.com/Library/Archived-Webinars/MicroActiveInteractive-Software/BET-microporous-sample.aspx
 http://www.micromeritics.com/Library/ArchivedWebinars/Physisorption/Physical-Adsorption.aspx
http://en.wikipedia.org/wiki/BET_theory
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Acknowledgments
 Analytical Lab in CEER at Ohio University
 Micromeritics Instrument Corporation for providing
information
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Questions!
For more information visit:
http://www.ohio.edu/ceer/
Contact:
[email protected]