ABC’s of Electrochemistry series Materials ... · 3 Specific Surface Area: Is a material property...

Ana María Valenzuela-Muñiz February 9, 2012

Department of Chemical and Biomolecular

Engineering

ABC’s of Electrochemistry series

Materials Characterization

techniques:

Surface Area and Pore Size

Distribution

2 Ohio University - Avionics Engineering Center

• Introduction

• Principles

• Applications

• Summary

Outline

3

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

Introduction

Center for Electrochemical Engineering Research, Ohio University

Basic concepts

4

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

5

Introduction


Basic concepts

Physisorption ≠ Chemisorption

Adsorption ≠ Absorption

6

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.


Introduction

Basic concepts

7

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

Introduction

Basic concepts

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


8

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

9

How it works?


Multilayer formation

10

What will you get?

Adsorption isotherm


0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

Qua

ntity

Ads

orbe

d (c

m³/g

STP

)

Relative Pressure (p/p°)

RawCoal ads

RawCoal des

ElecCoal #42 ads

ElecCoal #42 des

11

I

II III

IV V VI

Isotherm types


12 Center for Electrochemical Engineering Research, Ohio University

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

Isotherm types

IUPAC classification – 6 types of isotherms

13

• Langmuir (1918)

Monolayer adsorption

• BET (1938)

Multilayer adsorption

bP

bP

V

V

m

a

1

o

o

m

a

P

PCPP

CP

V

V

11

Most common methods

Surface area calculation


14

p = pressure

na = amount of gas adsorbed, mol/g

nam = mono-layer capacity of sample, mol/g

b = Langmuir constant

a

m

a

m

a n

p

bnn

p

1

Langmuir Equation

• Assumes adsorption limited to

one monolayer

• The Langmuir equation

describes Microporus material

exhibiting Type I Isotherms


15

ommoa P

P

CV

C

CVPPV

P 11

V= weight of gas adsorbed

P/P0 =relative pressure

Vm = weight of adsorbate as monolayer

C = BET constant

BET Equation

Stephen Brunauer, Paul Emmett, Edward Teller; Fixed Nitrogen Laboratory (1938).

Second most cited chemistry reference over fifty-year period

• Multiple-layer adsorption

• Type II Isotherm

• Approaches essentially

infinite amount adsorbed


16

ommoa P

P

CV

C

CVPPV

P 11

Intercept slope

CV

Cmand

CVb

mm

1,

1

b

mCand

mbVm

1,

1 TR

qq oa

eC

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

BET Equation

Linear plot


17

mbVm

1Vm = weight of adsorbate as monolayer

Total Surface area (SA )can then be derived

N = Avagadro’s number (6.023x1023)

M = Molecular weight of Adsorbate

Acs = Adsorbate cross sectional area (16.2Å2 for Nitrogen)

acs

mA

M

NVSA

Specific Surface Area (S) is then determined by total Surface area by

sample weight

w

SASSA

BET Equation


18

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


19

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

Vliq = volume of liquid N2 in pores

Vm = molar vol. of liquid adsorbate (N2=34.7cm3/mol)

Pa = ambient pressure

T = ambient temperature

Porosity

Pore Volume

RT

VVPV

madsaliq


20

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:

Other pore geometry models may require further information

on the isotherm hysteresis before applying appropriate model.

Porosity

Pore Radius

S

Vr

liqp

2


21

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


22

to analyze textural properties


Data reduction methods

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


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

24

How to do the analysis

• Degasification

• Measure Free Space

• Measure P0

• Dose

• Equilibrate

• Backfill

Repeat throughout Isotherm

Steps



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)

26

Surface Area and Pore Size Distribution


Strengths Limitations

Accurate

Different gasses can be used

-N2, He, CO2

Size of the sample

Minimum specific area of


Pharmaceutics

Aerospace

Applications

Ceramics

Adsorbents Activated Carbons

Carbon Black

Catalysts

Paints and coatings

Electronics

Fuel Cell Electrodes

http://www.micromeritics.com/Product-Showcase/TriStar-II-3020/TriStar-II-3020-Applications.aspx

Nanotubes

Surface Area and Pore Size Distribution


Examples of analysis

Sample Surf Area

[m²/g]

Microp

Area [m²/g]

Ext Surf Area

[m²/g]

Microp Vol

[cm³/g]

Pore diam

AVE [nm]

Raw RC 44 7.2941 0.8086 6.4854 0.000444 13.0572

RC 210 9.5043 1.549 7.9554 0.000855 10.7825

Electrolyzed EC 44 12.6047 2.4809 10.1237 0.001366 14.81595

EC 210 14.5666 1.3906 13.176 0.000736 14.34755

Extracted Eth

EC 44Eth 13.3042 4.3037 9.0005 0.002386 16.2652

EC 210Eth 17.1826 5.2594 11.9232 0.002905 15.99825

Extracted Tol

EC 44Tol 11.4953 3.9265 7.5688 0.002181 17.67975

EC 210Tol 10.8796 2.0833 8.7963 0.001147 17.89785

Specific surface area, pore size, pore volume



Specific surface area, pore size, pore volume

0

5

10

15

20

25

RC 44 RC 210 EC 44 EC 210 EC 44Eth EC 210Eth

EC 44Tol EC 210Tol

Total Area

Micropores

m2/g

30

Pore Width (Å)10 50 100 500

Po

re V

olu

me (

cm

³/g

)

0.00.0

0.2

0.4

0.6

0.8

1.0

Po

re V

olu

me (c

m³/g

)

00

2

4

6

8

10

MCM-41BJH Adsorption dV/dlog(w) Pore Volume

BJH Adsorption Cumulative Pore Volume


BJH using a Reference Curve


31

• DFT can also be used to characterize the surface energy


DFT – Beyond Porosity



Summary

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

BET method


Pore structure analysis

Summary

Adsorption Isotherm

BET plot

Isotherm type

Pore size distribution

Hysteresis yype t-Curve

Surface area Pore radius/Pore volume

Pore type, Shape, Geometry


Related Literature

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]

At CEER


http://www.micromeritics.com/Library/Application-Notes.aspx

http://www.micromeritics.com/Library/Archived-Webinars/MicroActive-

Interactive-Software/BET-microporous-sample.aspx

http://www.micromeritics.com/Library/Archived-

Webinars/Physisorption/Physical-Adsorption.aspx

http://en.wikipedia.org/wiki/BET_theory

Related Literature

WebPages

36

Acknowledgments

Analytical Lab in CEER at Ohio University

Micromeritics Instrument Corporation for providing

information


Questions!

For more information visit:

http://www.ohio.edu/ceer/

Contact:

[email protected]

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