seminar core slides malaysia dec 2013

Characterization of Powders & Porous Solids

A sharing session . . .

Mr Mohd Zulkiffli A Bakar

Itinerary (11.12.13)

TIME TOPIC REMARKS

0900 ~ 1300 Gas Sorption

1300 ~ 1400 Lunch

1400~1700 Mercury porosimetryChemisorption

1700~1730 Q & A

TIME TOPIC REMARKS

0900 ~ 1300 Gas Sorption

1300 ~ 1400 Lunch

1400~1700 Microporosity

1700~1730 Q & A

TIME TOPIC REMARKS

0900 ~ 1300 Gas Sorption , Microporosimetry

1300 ~ 1400 Lunch

1400~1700 MPOB

1700~1730 Q & A

History of sorption science

History sorption science

Pioneers of sorption science

Main Characteristics of Powders and Porous Solids

� Particle size� Surface area� Porosity

Why We Care About Particle Size and Surface Area� These characteristics control many properties of

materials:� Flowability;� “Filter-ability”� Viscosity-Reology;� Agglomeration;� Dusting tendency;� Settling rate;� Activity/Reactivity rate (e.g. of catalyst);� Dissolution rate (of pharmaceutical);� Gas absorption;� Hydration rate (of cement);� Moisture absorption;� Entry into lungs (shape dependency too);� Combustion rate (of fuel)� Etc…

What is Particle Size?

SEM of real ibuprofen particles

A Concept of Equivalent Sphere� Due to symmetry, size of sphere is

completely determined by only one parameter – it’s diameter (radius)

� Other properties of sphere are easily computed from its size:

� Sphere is just a convenient model! This is why it is found throughout the particle size analysis

1dV π= 2

dS π= 3

6dm π

Different Equivalent Spheres

Particle Size Measurement Techniques

� Direct observation (image analysis)� Sieving;� Sedimentation – settling rate;� Coulter counter – electrozone sensing;� Gas adsorption – BET (SSA back extrapolation

to size);� Permeability (gas or liquid) e.g. Blaine, FSSS� Light scattering – laser diffraction and Photon

Correlation Spectroscopy / Dynamic Light Scattering

And What Do They Measure� Direct observation (image analysis) – usually

some 2-D representation of a particle. Which dimension is viable?;

� Sieving – combination of particle size and shape;

� Sedimentation – settling rate. Stokes Law (spheres, straight line settling);

� Coulter counter – electrozone sensing;� Gas absorption / Permeability – surface area.

Extrapolate to average particle size only. – BET (SSA back extrapolation to size);

� Light scattering – equivalent scatterers;

Particle Size by Direct Observation

Google for ImageJ

Dynamic Light Scattering (DLS)� DLS measures Brownian motion and relates this to the size of the

particles.

� The larger the particle the slower the Brownian motion will be. Smaller particles are “kicked” further by the solvent molecules and move more rapidly.

� The velocity of Brownian motion is defined by a property known as the translational diffusion coefficient (D).

� The size of a particle is calculated from the translational diffusion coefficient by using the Stokes-Einstein equation:

d(H) – hydrodynamic diameter, D – translational diffusion coefficient, k – Boltzmann’s constant, T – temperature, η - viscosity

πη3)( =

What We Measure in DLS?� The diameter that is measured in

DLS is a value that refers to how a particle diffuses within a fluid so it is referred to as a hydrodynamic diameter

� The diameter that is obtained by this technique is the diameter of a sphere that has the same translational diffusion coefficient as the particle

� The translational diffusion coefficient will depend not only on the size of the particle “core”, but also on any surface structure, as well as the concentration and type of ions in the medium

Particle core

Shell formed by solvent particles, ions etc. Low conductivity medium will produce an extended double layer of ions around the particle, reducing the diffusion speed and

resulting in a larger, apparenthydrodynamic diameter.

Thus, the measurements are usually done in 10mM

NaCl (ISO13321 Part 8 1996)

How DLS Works

� The dark spaces in the speckle pattern produced by light scattering are where the phase additions of the scattered light are mutually destructive. The bright spots of light in the speckle pattern are where the light scattered from the particles arrives with the same phase and interfere constructively.

� The observed signal depends on the phase addition of the scattered light falling on the detector. In example A, two beams interfere and “cancel each other out” resulting in a decreased intensity detected. In example B, two beams interfere and “enhance each other” resulting in an increased intensity detected.

How DLS Works

� For a system of particles undergoing Brownian motion, a speckle pattern is observed where the position of each speckle is seen to be in constant motion. This is because the phase addition from the moving particles is constantly evolving and forming new patterns.

� The rate at which these intensity fluctuations occur will depend on the size of the particles. Figure above schematically illustrates typical intensity fluctuations arising from a dispersion of large particles and a dispersion of small particles.

� The small particles cause the intensity to fluctuate more rapidly than the large ones.

� It is possible to directly measure the spectrum of frequencies contained in the intensity fluctuations arising from the Brownian motion of particles, but it is inefficient to do so. The best way is to use a device called a digital auto correlator.

How an Auto Correlator Works

� If the intensity of a signal is compared with itself at a particular point in time and a time much later, then for a randomly fluctuating signal it is obvious that the intensities are not going to be related in any way, i.e. there will be no correlation between the two signals.

� However, if the intensity of signal at time t is compared to the intensity a very small time later (t+δt), there will be a strong relationship or correlation between the intensities of two signals.

� Perfect correlation is indicated by unity (1.00) and no correlation is indicated by zero (0.00).

� If the signals at t+2δt, t+3δt, t+4δt etc. are compared with the signal at t, the correlation of a signal arriving from a random source will decrease with time until at some time, effectively t = ∞, there will be no correlation.

� If the particles are large the signal will be changing slowly and the correlation will persist for a long time. If the particles are small and moving rapidly then correlation will reduce more quickly.

Different Forms of Particle Size Distribution

� Consider 2 populations of spherical particles of diameter 5nm and 50nm present in equal numbers.

� If a number distribution of these 2 particle populations is plotted, a plot consisting of 2 peaks (positioned at 5 and 50nm) of a 1 to 1 ratio would be obtained.

� If this number distribution was converted into volume, then the 2 peaks would change to a 1:1000 ratio (because the volume of a sphere is proportional to d3).

� If this was further converted into an intensity distribution, a 1:1000000 ratio between the 2 peaks would be obtained (because the intensity of scattering is proportional to d6 from Rayleigh’s approximation).

� In DLS, the distribution obtained from a measurement is based on intensity.

Schematics of Zetasizer Nano

Measurement of Porosity and Specific Surface Area by

Gas Adsorption

Name 2 methods to measure particle size

- Laser scattering

- Optical ( microscopy)A

F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 1-25, 1999

What are Porous Materials?

Non-porous solid� Low specific surface area� Low specific pore volume

Porous solid� High specific surface area� High specific pore volume

Porous materials have highly developed internal surface area that can be used to perform specific function.Almost all solids are porous except for ceramics fired at extremely high temperatures

Looking at the diagram, how to tell if a particle is porous?

Porous if and only if value of pore depth is larger than value of pore width

Measure of Porosity

Pore size and its distribution

Specific Surface Area, m2/g =

Porosity

There are three parameters used as a measure of porosity; specific surface area, specific pore volume or porosity, and pore size and its distribution.

Mass of the solid, g

Total surface area, m2

Specific Pore volume, cm3/g

Mass of the solid, g

Total pore volume, cm3

Porosity, % =

Volume of solid (including pores)

Volume of poresX 100

Concept of Porosity: Open vs. Closed Pores

Dead end (open)

ClosedInter-connected (open)

Passing (open)

Open pores are accessible whereas closed pores are inaccessible pores. Open pores can be inter-connected, passing or dead end.

Size of Pores (IUPAC Standard)

2 nm 50 nm

Micropores Mesopores Macropores

Zeolite,Activated carbon,Metal organicframework

Mesoporous silica, Activated carbon

Sintered metals and ceramics

Porous material are classified according to the size of pores: material with pores less than 2 nm are called micropores, materials with pores between 2 and 50 nm are called mesopores, and material with pores greater than 50 nm are macrospores

Sing, K. S. W. et al. Reporting Physisorption Data for Gas/Solid Systems. Pure & Appl. Chem. 57,603-619 (1985).

Shapes of Pores

Conical

Interstices

SlitsCylindrical

Spherical orInk Bottle

Pore Shapes

Will pore size be the same as particle size ?

Particle size measures external cross-sectional diameter, while pore size measures measuresmean internal pore diameter

Experimental Techniques

Techniques for Porosity Analysis

Mercuryporosimetry

Small angleX-ray

scattering

SmallAngle

Neutron scattering

Gas adsorption

Techniques

Mercuryporosimetry

Small angleX-ray

scattering

SmallAngle

Neutron scattering

Gas adsorption

Techniques

� Can measure only open pores� Pore size : 0.4 nm – 50 nm� Easy� Established technique

Mercuryporosimetry

Small angleX-ray

scattering

SmallAngle

Neutron scattering

Gas adsorption

Techniques

� Similar to gas adsorption

� Can measure only open pores

� Pore size >1.5 nm� Easy� Established technique

Mercuryporosimetry

Small angleX-ray

scattering

SmallAngle

Neutron scattering

Gas adsorption

Techniques

� Provide information regarding pore connectivity

� Pore size can be measured if the materials contains ordered pores

� Rarely used for pore analysis

Mercuryporosimetry

Small angleX-ray

scattering

SmallAngle

Neutron scattering

Gas adsorption

Techniques

� Pore size > 5nm� Rarely used for pore

analysis

Mercuryporosimetry

Small angleX-ray

scattering

SmallAngle

Neutron scattering

Gas adsorption

Techniques

� Any pore size� Open + Close

porosity

Mercuryporosimetry

Small angleX-ray

scattering

SmallAngle

Neutron scattering

Gas adsorption

Techniques

� Any pore size� Open & Close

porosity� Costly

Theory of Adsorption

Adsorption Process

Adsorption is brought by the forces acting between the solid and themolecules of the gas. These forces are of two kinds: physical(physiosorption) and chemical (chemisorption)

Adsorbent - the solid where adsorption takes place

Adsorbate - the gas adsorbed on the surface of solids

Adsorptive - adsorbate before being adsorbed on the surface

PHYSISORPTION CHEMISORPTIONWEAK, LONG RANGE BONDING

Van der Waals interactionsSTRONG, SHORT RANGE BONDING

Chemical bonding involved.

NOT SURFACE SPECIFICPhysisorption takes place between all

molecules on any surface providing the temperature is low enough.

SURFACE SPECIFICE.g. Chemisorption of hydrogen takes place on

transition metals but not on gold or mercury.

∆Hads = 5 ….. 50 kJ mol-1 ∆Hads = 50 ….. 500 kJ mol-1

Non activated with equilibrium achieved relatively quickly. Increasing temperature

always reduces surface coverage.

Can be activated, in which case equilibrium can be slow and increasing temperature can favour

adsorption.

No surface reactions. Surface reactions may take place:- Dissociation, reconstruction, catalysis.

MULTILAYER ADSORPTIONBET Isotherm used to model adsorption

equilibrium.

MONOLAYER ADSORPTIONLangmuir Isotherm is used to model adsorption

equilibrium.

Physisorption vs Chemisorption

http://www.soton.ac.uk

Adsorption Process

1. Diffusion to adsorbent surface2. Migration into pores of adsorbent3. Monolayer builds up of adsorbate

�Gas molecules admittedunder increasing pressure toa clean, cold surface.

�Data treatment techniquesfind the quantity of gas thatforms the first layer.1 2 3

S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991

Adsorption Process

Adsorbent

Adsorbate

adsorptive of pressure saturated

adsorbate of pressure

:as written becan equation

above theconstant, made are I and T, W,If

adsorbent. and adsorbatebetween n interactio

re; temperatu

adsorbate; theof pressure

adsorbent; of weight

adsorbed; gas of volume

Equation of adsorption isotherm

Gas Sorption: Isotherm

Adsorption isotherm�Isotherm is a measure of the volume of gas adsorbed at a constant temperature as a function of gas pressure.�Isotherms can be grouped into six classes.

adsorptive of pressure saturated

Desorption isotherm

Gas Sorption: IsothermV

Type Ior

Langmuir

�Concave to the P/Po axis�Exhibited by microporous solids ( < 2nm )

Type II

�Exhibited by nonporous ormacroporous solids ( > 50nm )�Unrestricted monolayer-multilayeradsorption�Point B indicates the relativepressure at which monolayercoverage is complete

Type III �Convex to the P/Po axis�Exhibited by nonporous solids

Type IV�Exhibited by mesoporous solids�Initial part of the type IV follows the same path as the type II

Type V

Type VI

�Highly uncommon�Exhibited by mesoporous solids

�Exhibited by nonporous solids with an almost completely uniform surface

Gas Sorption: Hysteresis

�Hysteresis indicates the presence of mesopores.

�Hysteresis gives information regarding pore shapes .

�Types I, II and III isotherms are generally reversible but typeI can have a hysteresis. Types IV and V exhibit hysteresis.

Hysteresis

Gas Sorption: HysteresisV

Type A

Cylindrical Slits

Type B

1P/Po 1P/Po

Type C Type D

Type E

Conical Bottle neck

Adsorption Theories: Langmuir

Adsorbate

Adsorbent

Assumptions:

� homogeneous surface (all adsorption sites energetically identical)

� monolayer adsorption (no multilayer adsorption)

� no interaction between adsorbed molecules

adsorbate. of pressure

and constant; empirical

monolayer; form torequired gas of volume

; pressureat adsorbed gas of volume

I. Langmuir The Constitution and Fundamental Properties of Solids and Liquids. Part I. Solids. J. Am. Chem. Soc., 1916, 38 (11), 2221-2295

� The Langmuir adsorption isotherm� Basic assumptions

� surface uniform (∆Hads does not vary with coverage)� monolayer adsorption, and � no interaction between adsorbed molecules and adsorbed

molecules immobile

� Case I - single molecule adsorptionwhen adsorption is in a dynamic equilibrium

A(g) + M(surface site) �� AMthe rate of adsorption rads = kads (1-θ) Pthe rate of desorption rdes = kdes θ

at equilibrium rads = rdes ⇒ kads (1-θ) P = kdes θ

rearrange it for θ

let ⇒ B0 is adsorption coefficient

θ = =+∞

desads

case I

� The Langmuir adsorption isotherm (cont’d)�Case II - single molecule adsorbed

dissociatively on one siteA-B(g) + M(surface site) �� A-M-B

the rate of A-B adsorption rads=kads (1−θΑ )(1−θΒ)PAB=kads (1−θ )2PAB

the rate of A-B desorption rdes=kdesθΑθΒ =kdesθ2

at equilibrium rads = rdes ⇒ kads (1−θ )2PAB= kdesθ2

rearrange it for θ

Let. ⇒

case II

BAθ=θΑ=θ

ABdesads

� The Langmuir adsorption isotherm (cont’d)�Case III - two molecules adsorbed on two sites

A(g) + B(g) + 2M(surface site) �� A-M + B-M

the rate of A adsorption rads,A = kads,A (1− θΑ− θΒ) PA

the rate of B adsorption rads,B = kads,B (1− θΑ− θΒ) PB

the rate of A desorption rdes,A = kdes,A θΑ

the rate of B desorption rdes,B = kdes,B θΒ

at equilibrium rads ,A = rdes ,A and ⇒ rads ,B = rdes ,B

⇒ kads,A(1−θΑ−θΒ)PA=kdes,AθΑ and kads,B(1−θΑ−θΒ)PB=kdes,BθΒ

rearrange it for θ

where are adsorption coefficients of A & B.

kB == 00 and

BB,AA,

BB,B,s

BB,AA,

AA,A,s

1 ++==

∞∞

case III

� The Langmuir adsorption isotherm (cont’d)

kB == 00 and

BB,AA,

BB,B,s

BB,AA,

AA,A,s

Adsorption

Strong kads>> kdes kads>> kdes

B0>>1 B0>>1

Weak kads<< kdes kads<< kdes

B0<<1 B0<<1

case II

θ = =+∞

case I

1→=∞C

Csθ 1→=∞C

θ 1/20 )( PB

Cs ==∞

Adsorption

A, B both strong

A strong, B weak

A weak, B

BB,AA,

BB,B,s

BB,AA,

AA,A,s

BB,B,sB

AA,A,sA

BA,B,B,sB

PB/BC/C

case III

�Langmuir adsorption isotherm

case I

case II

Case III

� Langmuir adsorption isotherm established a logic picture of adsorption process

� It fits many adsorption systems but not at all

� The assumptions made by Langmuir do not hold in all situation (error?) � Solid surface is heterogeneous , heat of adsorption is not a constant at different θ� Physisorption of gas molecules on a solid surface can be more than one layer

BB,AA,

BB,B,s

BB,AA,

AA,A,s

θ = =+∞

large B0 (strong adsorp.)

small B0 (weak adsorp.)

moderate B0

Pressure

mono-layer

1→=∞C

Strong adsorption kads>> kdes

Weak adsorption kads<< kdes

Adsorption Theories: BET

adsorbate. of pressure relative

and layer);1st of adsorption ofenergy to(relatedconstant BET C

monolayer; form torequired gas of volume

; pressureat adsorbed gas of volume

� Modification of Langmuir isotherm

� Both monolayer and multilayer adsorption

� Assumptions:(a) gas molecules physically

adsorb on a solid in layers infinitely;

(b) there is no interaction between each adsorption layer;

(c) the Langmuir theory can be applied to each layer.

Adsorbate

Adsorbent

S.Brunauer, P.Emmett, E.Teller Adsorption of Gases in Multimolecular Layers, J. Am. Chem. Soc., 1938, 60 (2), pp 309–319

Specific Surface Area Calculation

imXY +=

V[(Po/P)-1]

0-1 0-2 0-3

At least three data points in the relative pressure range 0.05 to 0.30

adsorbate ofWeight area surface Total csavm ANV

sample ofWeight

area surface Totalarea) surface (SpecificSSA =

Single Point BET�Single-point method offers the advantage of simplicity andspeed, often with little loss in accuracy.

am PPVV −= 1 i.e. Vm = 1/slope�A relative pressure of 0.3 gives good general agreement with the multi-point method.

� Correction of single point “error” at P/P0 = 0.3 by multiplying the single point BET value by C/C-2 decreases the difference.

Sample No.

Multi-point BET

(m2/g)

Uncorrected single-point

(m2/g)

Uncorrecteddifference

Corrected single –

(m2/g)

Correcteddifference

1 4.923 4.241 -13.9 4.948 0.51

2 4.286 3.664 -14.5 4.275 -0.26

3 8.056 6.867 -14.8 8.011 -0.56

4 5.957 5.194 -12.8 6.060 +1.73

Pore Size DistributionV

Pore diameter, d

Narrow pore size distribution

Broad pore size distribution

Unimodal pore size distribution

Pore diameter, d

Multimodal pore size distribution

The distribution of pore volume with respect to pore size is called a pore size distribution.

aV volumePore

Pore Size Distribution

∆Gads = RT(lnPads - lnP0)

∆Gdes = RT(lnPdes - lnP0)∆Gdes < ∆Gads

(P/Po)des (P/Po)ads

�Adsorption or desorption isotherm.

�The desorption isotherm is preferred over adsorption isotherm.

Pore Size: Kelvin Equation

�Multilayer formation occurs in parallel to capillary condensation. �Capillary condensation is described by the Kelvin equation.

phase. condensed and solid ebetween th anglecontact

re; temperatu

constant; gas real

meniscus; liquid theof curvature of radiusmean

adsorbate; condensed of memolar volu

tension;surface liquid

;adsorbate of pressure saturated

Pore Size: Kelvin Equation

trr kp +=

Actual radius of the pore

Kelvin radius of the pore

Thickness of the adsorbed layer

Prior to condensation, some adsorption has taken placeon the walls of the pore, rk does not represent the actualpore radius.

Adsorbed layer

Methods for Calculation of Pore Size Distribution�BJH (Barrett, Joyner andHalenda) method

�DH (Dollimore Heal) method

�Dubinin-Astakhov method

�HK (Horvath-Kawazoe) method

�Saito-Foley method

Mesoporous solids

Microporous solids

�NLDFT (Non Local Density Functional Theory) and Monte Carlo simulation method

Microporous and Mesoporous solids

Questions . . . anyone ?

Porosity Analyzer

Outgassing station

Analysis station

Liquid nitrogen bath

Steps for Measurement

3. Interpretation

2. Adsorption Analysis

1. Sample Preparation

Sample Preparation (Outgassing)� Surface contamination is

removed by applicationof:� Temperature� Flowing gas (helium or

nitrogen) or vacuum

� Backfill can be doneusing helium or adsorbategas.

� According to IUPAC standards, materials should be outgassed for at least 16 hours.

Adsorbate

Helium

Vacuum

Outgassing station

Analysis station

Sample Cell

Adsorption Analysis

� Adsorbate (nitrogen,argon, carbon dioxide,krypton)

� Analysis temperature(liquid nitrogen, liquidargon, 0 oC)

� Quantity of sample (1mg sample is sufficient)

� Number of points(single point, fivepoints, seven points,eleven points, fullanalysis)

Adsorbate

Helium

Vacuum

Outgassing station

Analysis station

Sample Cell

Interpretation

Points P/Po Volume adsorbed

Weight of sample

Pore shape

Specific surface area

Pore volume

Pore size&

distribution

Results

Common Adsorbates

Gas Temperature Cross sectional area (nm2)

N2 � -195.8 oC (liquid nitrogen)� -183 oC (liquid argon).

Ar � -183 oC (liquid argon).� -195.8 oC (liquid nitrogen)

CO2 � -78 oC, -25 oC, 0 oC 0.195

CO � -183 oC (liquid argon) 0.163

Kr � -195.8 oC (liquid nitrogen) 0.205

O2 � -183 oC (liquid argon) 0.141

C4H10 � 0 oC, 25 oC 0.469

Choice of Adsorptive

� N2(g) in N2(l) is the most commonly used adsorbate.

� Not completely inert. � Dipole movement and

thus can have localized adsorption.

� Cross-sectional area of 0.162 nm2 is questionable.

�S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991�Quantachrome Autosorb-I Operational Manual

� Ar(g) in Ar(l) is preferable but because of unavailability of Ar(l) (87K), N2(l) (77 K) is used.

� Ar can reach to somewhat smaller pores than N2.

� Accurate measurement of micropores is possible using Ar.

� In case of activated carbon, CO2 is often the most preferred adsorbate.

� Adsorption analysis of CO2 takes less time.

� Limited to microporeanalysis.

Validity of BET - Method

� The BET method depends on the cross-sectional area of adsorbate.

� Monolayer structure is same on all the surface.

� Localized monolayer coverage.

K. S. W. Sing, The Use of Nitrogen Adsorption for the Characterisation of Porous Materials, Colloids and Surfaces, 187 – 188, 2001, 3 - 9

P )1(1

ALVSSA

Adsorbate

Adsorbent

Validity of Kelvin Equation

� Is relation between the

meniscus curvature and the pore size and shape valid?

� Is it applicable for micropores and narrow mesopores?

� Does surface tension varies with pore width?

F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 203, 1999

Shape of Microporous Materials

Type Ior

Langmuir

�Type I isotherms don’t have hysteresis.

�Pore shape cannot be determined by isotherm.

�As various methods for pore size calculation are based on shape of pores, reliability of pore size calculation is questionable.

2 nm 50 nm

Methods Assumption

Pore Shape Based on ..

Brunauer MP method Cylindrical or Slit shaped de Boer’s t-method

Dubinin-Astakhov method - �Polanyi potential theory

�Independent of Kelvin equation

HK (Horvath-Kawazoe) method Slit �Everett and Powl method

�Independent of Kelvin equation

Saito-Foley method Cylindrical HK method

Choice of Method

�P. A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics, 53 –152, 1997�Quantachrome Autosorb-I Operational Manual

2 nm 50 nm

Methods Assumption

BJH (Barrett, Joyner and Halenda) method

Cylindrical, Slit-shaped Kelvin equation

DH (Dollimore Heal) methodCylindrical t-method

Choice of Method

2 nm 50 nm

Methods Assumption

NLDFT (Non Local Density Functional Theory) and Monte Carlo simulation method

Cylindrical and slit Statistical thermodynamics

Choice of Method

Physisorption

Methods and Techniques

QuantachromeI N S T R U M E N T S

Micro and Mesopore Size Determination by Gas Sorption

First: Quantitative estimation of micropore volume and area…

T-plot and DR methods.

Multilayer adsorption

Type II, IV

Relative Pressure (P/Po)

After the knee, micropores cease to contribute to the adsorption process.

Low slope region in middle of isotherm indicates first few multilayers, on external surface including meso and macropores… before the onset of capillary condensation

Estimation of Micropores...the t-plot method

This method uses a mathematical representation of multi-layer adsorption. The thickness, t, of an adsorbate layer increases with increasing pressure. The t-curve so produced is very similar in appearance to a type II isotherm. For every value of P/Po, the volume adsorbed is plotted against the corresponding value of “t”.

If the model describes the experimental data a straight line is produced on the t-plot...

The t-plotResembles a type II

tistic

A statistical monolayer

A statistical multilayer

t-plot Method (mesoporous only)

1 2 3 4 5 6 7

t (�)

Slope = V/t = A

t-plot Methodshowing a “knee”

Slope A - slope B = area contribution by micropores size C

1 2 3 4 5 6 7

t (�)

What is an αs plot?

αs (for Ken Sing) is a comparison plot like the t-plot but its slope does not give area directly.

Estimation of MicroporesDubinin-Radushkevich (DR) Theory

−−−−====

TBexpWW 02

W = volume of the liquid adsorbateW0 = total volume of the microporesB = adsorbent constantβ = adsorbate constant

A linear relationship should be found between log(W) and log2(Po/P)...

Log2(Po/P)

Extrapolation yields Wo

Estimation of MicroporesDubinin-Radushkevich (DR) Plot

Pore Size Determination

Requires a recognition and understanding of different basic

isotherm types.

t-plot Method(in the presence of micropores)

1 2 3 4 5 6 7

t (�)

Intercept = micropore volume

Types of Isotherms

Type I

Type II

Type III

Type IV

Type V

Types of Isotherms

Type I or

pseudo-“Langmuir”

Steep initial region due to very strong adsorption, for example in micropores.

Limiting value (plateau) due to filled pores and essentially zero external area.

Why pseudo Langmuir?

Langmuir applies to monolayerlimit, not volume filling limit.

Types of Isotherms

Type II

Rounded knee indicates approximate location of monolayer formation.

Absence of hysteresis indicates adsorption on and desorption from a non-porous surface..

Low slope region in middle of isotherm indicates first few multilayers

Types of Isotherms

Type III

Lack of knee represents extremely weak adsorbate-adsorbent interaction

BET is not applicable

Example: krypton on polymethylmethacrylate

Types of IsothermsType IV

Rounded knee indicates approximate location of monolayer formation.

Low slope region in middle of isotherm indicates first few multilayers

Hysteresis indicates capillary condensation in meso and macropores.Closure at P/Po~0.4 indicates

presence of small mesopores (hysteresis would stay open longer but for the tensile-strength-failure of the nitrogen meniscus.

Types of Isotherms

Type V

Lack of knee represents extremely weak adsorbate-adsorbent interaction

BET is not applicable

Example: water on carbon black

Types of Hysteresis

Large pores/voids

Mesopores

MesoPore Size

by Gas

Sorption(BJH)

Analyzer measures volume of pores: Yes or No?

NO! It measures what leavessupernatent gas phase

Hysteresis is indicative of the presence of mesopores and the pore size distribution can be calculated from the sorption isotherm.

Whilst it is possible to do so from the adsorption branch, it is more normal to do so from the desorption branch...

Mesopore (Greek meso = middle): 2nm - 50 nm diameter

Macropore (Greek macro = large): >50 nm diameter

Micropore (Greek micro = small): 0 nm - 2 nm diameter

Adsorption / Desorption

Adsorption =

multilayer formation

Desorption =

meniscus development

Kelvin* Equation

)P/Plog(

.)A(rk

154====

* Lord Kelvin a.k.a. W.T. Thomson

= cos2

ln0 rRT

Pore Size

trr kp ++++====

rp = actual radius of the pore

rk = Kelvin radius of the pore

t = thickness of the adsorbed film

Statistical Thickness, t

• Halsey equation

• Generalized Halsey

• deBoer equation

• Carbon Black STSA

BJH Method(Barrett-Joyner-Halenda)

trr Kelvinpore ++++====

Pore volume requires assumption of liquid density!

40 Pore Diameter (angstrom)

Artifact

~ 0.42

Pore Size Data• Volume and size of pores can be expressed from

either adsorption and/or desorption data.

• The total pore volume, V, is taken from the maximum amount of gas adsorbed at the “top” of the isotherm and conversion of gas volume into liquid volume.

• The mean pore diameter is calculated from simple cylindrical geometry:

4= where A is the BET

surface area.

Pore size analysis of MCM 41 (Templated silica) by N2 sorption

at 77 K

0 0.2 0.4 0 .6 0 .8 1P/P 0

Exp. N itrogen sorption at 77 K in M C M 41Exp. N itrogen sorption at 77 K in M C M 41D FT- IsothermD FT- Isotherm

Pore size analysis of MCM 41: Calculations compared

15 23 31 39 47 55Pore Diameter [Å]

BJH-Pore size distribution BJH-Pore size distribution DFT-Pore size distributionDFT-Pore size distribution

Calculation

Models

Comparisons• Gas Sorption Calculation Methods

P/Po range Mechanism Calculation model1x10-7 to 0.02 micropore filling DFT, GCMC, HK, SF, DA, DR0.01 to 0.1 sub-monolayer formation DR0.05 to 0.3 monolayer complete BET, Langmuir> 0.1 multilayer formation t-plot (de-Boer,FHH),> 0.35 capillary condensation BJH, DH

0.1 to 0.5 capillary filling DFT, BJHin M41S-type materials

Different Theories of Physisorption

Surface area Pore volume Pore sizeBET Total pore vol DR ave

Langmuir t-plot (µpore vol) BJHDR DR (µpore vol) DH

MP and t-plot DA DFT

αs plot BJH HK(BJH) (DFT) SF(DH) (DH)(DFT)

HK & SFHorvath-Kawazoe & Saito-Foley

• HK• Direct mathematical relationship between relative

pressure (P/Po) and pore size. Relationship calculatedfrom modified Young-Laplace equation, and takes intoaccount parameters such as magnetic susceptibility.Based on slit-shape pore geometry (e.g. activatedcarbons). Calculation restricted to micropore region (≤2nm width).

• SF• Similar mathematics to HK method, but based on

cylindrical pore geometry (e.g. zeolites). Calculationrestricted to micropore region (≤ 2 nm diameter).

DA & DRDubinin-Astakov and Dubinin-Radushkevic

• DA• Closely related to DR calculation based on pore filling mechanism.

Equation fits calculated data to experimental isotherm by varying twoparameters, E and n. E is average adsorption energy that is directlyrelated to average pore diameter, and n is an exponent that controlsthe width of the resulting pore size distribution. The calculated poresize distribution always has a skewed, monomodal appearance(Weibull distribution).

• DR• Simple log(V) vs log2(Po/P) relationship which linearizes the isotherm

based on micropore filling principles. “Best fit” is extrapolated tolog2(Po/P) (i.e. where P/Po = 1) to find micropore volume.

BET• The most famous gas sorption model. Extends Langmuir

model of gas sorption to multi-layer. BET equationlinearizes that part of the isotherm that contains the“knee” , i.e. that which brackets the monolayer value.Normally solved by graphical means, by plotting1/(V[(Po/P)]-1) versus P/Po. Monolayer volume (Vm) isequal to 1/(s+i) where s is the slope and i is the y-intercept.Usually BET theory is also applied to obtain the specificsurface area of microporous materials, although from ascientific point of view the assumptions made in the BETtheory do not take into account micropore filling. Pleasenote, that for such samples the linear “BET” range isfound usually at relative pressures< 0.1, in contrast to theclassical BET range, which extends over relativepressures between 0.05 – 0.3.

Langmuir

• Adsorption model limited to the formation of amonolayer that does not describe most realcases. Sometimes can be successfully appliedto type I isotherms (pure micropore material) butthe reason for limiting value (plateau) is notmonolayer limit, but due to micropore filling.Therefore type I physisorption isotherm wouldbe better called “pseudo-Langmuir” isotherm.

t-plotStatistical Thickness

• Multi-layer formation is modeled mathematically to calculate a layer “thickness, t” as a function of increasing relative pressure (P/Po). The resulting t-curve is compared with the experimental isotherm in the form of a t-plot. That is, experimental volume adsorbed is plotted versus statistical thickness for each experimental P/Po value. The linear range lies between monolayer and capillary condensation. The slope of the t-plot (V/t) is equal to the “external area”, i.e. the area of those pores which are NOT micropores. Mesopores, macropores and the outside surface is able to form a multiplayer, whereas micropores which have already been filled cannot contribute further to the adsorption process.

• It is recommended to initially select P/Po range 0.2 – 0.5, and subsequently adjust it to find the best linear plot.

BJH & DHBarrett, Joyner, Halenda and Dollimore-Heal

• BJH• Modified Kelvin equation. Kelvin equation predicts

pressure at which adsorptive will spontaneously condense (and evaporate) in a cylindrical pore of a given size. Condensation occurs in pores that already have some multilayers on the walls. Therefore, the pore size is calculated from the Kelvin equation and the selected statistical thickness (t-curve) equation.

• DH• Extremely similar calculation to BJH, which gives very

similar results. Essentially differs only in minor mathematical details.

Other Methods

• FRACTAL DIMENSION• The geometric topography of the surface

structure of many solids can be characterized by the fractal dimension D, which is a kind of roughness exponent. A “flat” surface is considered D is 2, however for an irregular (real) surface D may vary between 2 and 3 and expresses so the degree of roughness of the surface and/or porous structure. The determination of the surface roughness can be investigated by means of the modified Frenkel-Halsey Hill method, which is applied in the range of multilayer adsorption.

Example Data : Microporous Carbon

BET : Not strictly applicable

• Tag all adsorption points

• Analyze behavior• Note knee – transition

from micropore filling to limitedmultilayering (plateau).

• Use Langmuir (Monolayer model) / DR for Surface Area, Micropore Volume

• Usue Langmuir in range of 0.05 -> 0.2 (monolayer)

• Langmuir Surface Area

• DR Method for surface area, micropore volume

• Choose low relative pressure points (up to P/P0 = 0.2)

• Reports micropore surface area, and micropore volume.

• Note Langmuir, DR surface areas very close (1430 m2/g vs. 1424 m2/g)

Example Data : Macroporous Sample

Little or no “knee”, isotherm closes at

• BET Plot = OK• Surface area ca. 8m2/g (low)• Note hysteresis above P/P0 = 0.95 ∴Pores > 35 nm

Intercept = (-), no micropore

volume.

BJH Shows pores > 20nm, to over

200 nm

Example Data : Mesoporous Silica

Hysteresis => mesoporesAlso micropores ?? Test using t-

method

BET Surface area = 112m2/gClassic mesoporous silica !

Statistical Thickness => Use de Boer for oxidic surfaces = silicas

Intercept ~ 0Look at tabular data

MP SA = 8m2/g (total SA = 112)

Use BJH – shows narrow pore size distribution in 14-17nm range (mesopores)

Questions from audience?

MicroPore Size

by Gas

Sorption

Available

Calculation

Models

Pore filling pressures for nitrogen in cylindrical pores at 77 K,

(Gubbins et al. 1997)

Pore filling pressures for nitrogen in cylindrical silica pores at 77 K

(Neimark et al., 1998)

Pore size analysis of MCM 41 by silica by N2 sorption at 77 K

0 0.2 0.4 0.6 0.8 1P/P0

Exp. Nitrogen sorption at 77 K in MCM 41Exp. Nitrogen sorption at 77 K in MCM 41DFT- IsothermDFT- Isotherm

15 23 31 39 47 55Pore Diameter [Å]

BJH-Pore size distribution BJH-Pore size distribution DFT-Pore size distributionDFT-Pore size distribution

Gas- and liquid density profiles in a slit pore by GCMC

(Walton and Quirke,1989)

NLDFT / GCMC (Monte Carlo) Kernel File

Applicable Pore Diameter Range

Examples

NLDFT– N2 - carbon kernel at 77 K based on a slit-pore model

0.35nm-30 nm Carbons with slit-like pores, such as activated carbons and others.

NLDFT– N2 – silica equilibrium transition kernel at 77 K, based on a cylindrical pore model

0.35nm- 100nm Siliceous materials such as some silica gels, porous glasses, MCM-41, SBA-15, MCM-48 and other adsorbents which show type H1 sorption hysteresis.

NLDFT– N2 - silica adsorption branch kernel at 77 K, based on a cylindrical pore model

0.35nm-100nm Siliceous materials such as some controlled pore glasses, MCM-41, SBA-15, MCM-48, and others. Allows to obtain an accurate pore size distribution even in case of type H2 sorption hysteresis

NLDFT– Ar zeolite/silica equilibrium transition kernel at 87 K based on a cylindrical pore model

0.35nm -100nm Zeolites with cylindrical pore channels such as ZSM5, Mordenite, and mesoporous siliceous materials (e.g., MCM-41, SBA-15, MCM-48, some porous glasses and silica gels which show type H1 sorption hysteresis).

Examples

NLDFT – Ar-zeolite/silica adsorption branch kernel at 87 K based on a cylindrical pore model

0.35nm-100nm Zeolites with cylindrical pore channels such as ZSM5, Mordenite etc., and mesoporous siliceous materials such as MCM-41, SBA-15, MCM-48, porous glasses some silica gels etc). Allows to obtain an accurate pore size distribution even in case of H2 sorption hysteresis.

NLDFT – Ar-zeolite / silicaequilibrium transition kernel based on a spherical pore model (pore diameter < 2 nm) and cylindrical pore model (pore diameter > 2 nm)

0.35nm-100nm Zeolites with cage-like structures such as Faujasite, 13X etc. , and mesoporous silica materials (e.g., MCM-41, SBA-15, porous glasses, some silica gels which show H1 sorption hysteresis).

NLDFT – Ar-zeolite / silica adsorption branch kernel at 87 K based on a spherical pore model (pore diameter < 2 nm) and cylindrical pore model (pore diameter > 2 nm)

0.35nm-100nm Zeolites with cage-like structures such as Faujasite, 13X, and mesoporous silica materials (e.g., MCM-41, SBA-15, controlled-pore glasses and others). Allows to obtain an accurate pore size distribution even in case of H2 sorption hysteresis.

Examples

NLDFT – Ar - carbon kernel at 77 K based on a slit-pore model

0.35 nm - 7 nm Carbons with slit-like pores, such as activated carbons etc.

NLDFT - CO2 - carbon kernel at 273 K based on a slit-pore model

0.35nm-1.5 nm Carbons with slit-like pores, such as activated carbons etc.

GCMC – CO2 - carbon kernel at 273 K based on a slit-pore model

0.35nm-1.5 nm Carbons with slit-like pores, such as activated carbons etc.

RECENT ADVANCES IN THE PORE SIZE ANALYSIS OF

MICRO- AND MESOPOROUS MOLECULAR SIEVES BY ARGON

GAS ADSORPTION

Micropore Size Characterization

• Physical adsorption in micropores, e.g. zeolites occurs at relative pressures substantially lower than in case of adsorption in mesopores.

• Adsorption measurements using nitrogen at 77.4 K is difficult, because the filling of 0.5 - 1 nm pores occurs at P/Po of 10-7 to 10-5

, where the rate of diffusion and adsorption equilibration is very slow.

Advantages of Using Argon

• Advantage to analyze such narrow micropores by using argon at liquid argon

temperature (87.3 K).

• Argon fills these micropores (0.5 – 1nm) at much higher relative pressures (i.e., at relative pressures 10-5 to 10-3) compared to nitrogen.

Advantages of Higher Temperature & Pressure

• Accelerated diffusion.

• Accelerated equilibration processes.

• Reduction in analysis time.

Argon Adsorption at 87.3 K versus Nitrogen Adsorption at 77.4 K

10-6 5 10-5 5 10-4 5 10-3 5 10-2 5 10-1 5 100

N2/77KN2/77KAr/87 KAr/87 K

ZEOLITE | 10.5.2001

The different pore filling ranges for argon adsorption at 87.3K and nitrogen adsorption at 77.4K in faujasite-type zeolite are illustrated above.

Micropore Size Calculation

• Difficulties are associated with regard to the analysis of micropore adsorption data.

• Classical, macroscopic, theories [1] like DR and semiempirical treatments such those of HK and SF do not give a realistic description of micropore filling

• This leads to an underestimation of pore sizes for micropores and even smaller mesopores [2].

[1] F. Rouquerol, J. Rouquerol & K. Sing, Adsorption by Powders & Porous Solids, Academic Press, 1999

[ 2 ] P. I Ravikovitch, G.L. Haller, A.V. Neimark, Advcances in Colloid and Interface Science 76-77 , 203 (1998)

New Calculation

• To overcome the above mentioned problems weintroduce a new method for micropore analysisbased on a Non-local Density Functional Theory(NLDFT) model by Neimark and Co-workers [3-5].

• The new DFT-method is designed for micro-mesopore size characterization of zeoliticmaterials ranging in size from 0.44 to 20 nm usinghigh-resolution low-pressure argon adsorptionisotherms at 87.3 K.

[3] P.I. Ravikovitch, G.L. Haller, A.V. Neimark, Advances in Colloid and Interface Science, 76 – 77 (1998), 203 -207

[4] A.V. Neimark, P.I Ravikovitch, M. Gruen, F. Schueth, and K.K. Unger, J. Coll. Interface Sci., 207, (1998) 159

[5] A.V. Neimark, P.I. Ravikovitch, Microporous and Mesoporous Materials (2001) 44-45, 697

Systematic, Experimental Study

• To evaluate the application of argon sorption formicro- and mesopore size analysis of zeolites andmesoporous silica materials including novelmesoporous molecular sieves of type MCM-41and MCM-48.

• The sorption isotherms were determined using a static volumetric technique

• Samples were outgassed for 12 h under vacuum (turbomolecular pump) at elevated temperatures (573 K for the zeolites and 393 K for MCM-41/MCM-48).

Results

0 0.2 0.4 0.6 0.8 1 P/Po

MCM-41

Argon adsorption isotherms at 87 K on MCM-41, ZSM-5 and their 50-50 mixture.

Results

0.000001 0.00001 0.0001 0.001 0.01 0.1 1

MCM-41

1 10 100 1000D, [Å]

histogram

integral

1 10 100 1000

D, [Å]

histogram

integral

Evaluation of DFT Algorithm

0.000001 0.00001 0.0001 0.001 0.01 0.1 1

experimental

NLDFT fit

1 10 100 1000

D, [Å]

histogram

integral

Discussion• Argon sorption at 77 K is limited to pore

diameters smaller than 12 nm.i.e. no pore filling/pore condensation can be observed at this

temperature for silica materials containing larger pores.

• This lack of argon condensation for pores larger than ca. 12 nm is associated with the fact, that 77 K is ca. 6.8 K below the bulk triple point [4,5] .

[4] M. Thommes, R. Koehn and M. Froeba, J. Phys. Chem. B (2000), 104, 7932

[5] M. Thommes, R. Koehn and M. Froeba, Stud. Surf. Sci. Catal., (2001), 135 17

Discussion

• These limitation do not exist for argon sorption at its’ boiling temperature, i.e. ca. 87 K.

• Pore filling and pore condensation can be observed over the complete micro- and mesopore size range .

Discussion

• Results of classical, and semi-empirical methods (e.g., BJH, SF etc) indicate that these methods underestimate the pore size considerably.

• Deviations from the DFT-results are often in a range of ca. 20 % for pore diameters < 10 nm.

Summary

• Our results indicate that argon sorption data at 87 K combined with the new NLDFT-methods provides a convenient way to achieve an accurate and comprehensive pore size analysis over the complete micro-and mesopore size range for zeolites, catalysts, and mesoporous silica materials.

Acknowledgements

• Special thanks go to Alex Neimark and Peter Ravikovitch at TRI Princeton, New Jersey, USA.

Referencesto research work of nitrogen, argon and krypton

in MCM-48/MCM-41 materials

(1) M. Thommes, R. Koehn and M. Froeba, “ Systematic Sorption studies on surface and pore size characteristics of different MCM-48 silica materials”, Studies in Surface Science and Catalysis 128, 259 (2000)

(2) M. Thommes, R. Koehn and M. Froeba, “Sorption and pore condensation behavior of nitrogen, argon and krypton in mesoporous MCM-48 silica materials” J. Phys. Chem. B 104, 7932 (2000)

(3)M. Thommes, R. Koehn and M. Froeba, “Sorption and pore condensation behavior of pure fluidsin mesoporous MCM-48 silica, MCM-41 silica and controlled pore glass, Studies in SurfaceScience and Catalysis, 135, 17 (2001)

(4)M. Thommes, R. Koehn and M. Froeba, “Characterization of porous solids: Sorption and porecondensation behavior of nitrogen, argon and krypton in ordered and disordered mesoporoussilica materials (MCM-41, MCM-48, SBA-15, controlled pore glass, silica gel) at temperaturesabove and below the bulk triple point”, Proceedings of the first topical conference on

nanometer scale science and engineering” (G.U. Lee, Ed) AIChE Annual Meeting, Reno,

Nevada, November 4-9, 2001

(5)M. Thommes, R. Koehn and M. Froeba, “Sorption and pore condensation behavior of pure fluidsin mesoporous MCM-48 silica, MCM-41 silica and controlled pore glass at temperaturesabove and below the bulk triple point”, submitted to Applied Surface Science, (2001)

Rapid Micropore Size Analysis by CO2

Adsorption

CO2 Adsorption at 0oCon Carbon

RAPID MICROPORE ANALYSIS

• The advantages of micropore analysis with Quantachrome’s Density Functional Theory (DFT) and CO2 include:

• Speed of analysis; with the higher diffusion rate at 273.15K, analysis times are reduced as much as 90%.

• Carbon dioxide at 273.15K permits probing pores from about 2 angstroms (0.2 nm).

DFT ADVANTAGE

DFT has recently been applied to describe the behavior of fluids that are confined in small pores. The current popular gas sorption models, e.g. BJH, HK, SF, DA, etc., assume that the density of the adsorbed phase remains constant, regardless of the size of the pores that are being filled. Packing considerations suggest that these models are less than satisfactory for analyses of pores less than 2 nm.

DFT “Fitting”

• For a given adsorbate-adsorbent system, DFT calculates the most likely summation of "ideal isotherms“ calculated from "ideal pores" of fixed sizes needed to match the experimental results.

CO2 for Speed!

• Typically, micropore analyses with nitrogen as adsorbate will require 24 hours or more to run.

• Using carbon dioxide as adsorbate provides several advantages. – Carbon dioxide molecules are slightly thinner than

nitrogen molecules (2.8 angstroms radius vs. 3.0 angstroms) and will fill smaller pores than nitrogen.

– The use of carbon dioxide allows the measurements to be made at 273.15K, typically with an ice/water bath.

– There is no longer any need to provide and maintain or replenish a level of liquid nitrogen during the analysis.

CO2 Benefits

• At this temperature, the diffusion rate of molecules moving through small and tortuous micropores is much higher than at 77.35K. This so-called "activated adsorption" effect led to the popularization of the use of carbon dioxide to characterize carbonaceous material since the early 1960s.

CO2 Benefits

• This higher diffusion rate is responsible for reducing the analysis time to a few hours for a complete adsorption experiment. The faster rate also provides for the possibility of using larger samples than with nitrogen adsorption, thus reducing sample weighing errors.

• Pore size distributions thus obtained are comparable to those from a 24-hour nitrogen/77.35K analysis.

N2 Adsorption @ 77K: 40 hours

CO2 adsorption at 273K: 2.75 hours

CO2 Adsorption at 0oC

Density Functional Theory Micropore Distribution

CO2 Adsorption at 0oC

Monte Carlo Simulation Micropore Distribution

How to do it?

• Hardware requirements for this new method are minimal: – a wide- mouth dewar and – a water-level sensor.

• The proprietary Quantachrome Autosorb® software provides the DFT data reduction capabilities to do the rest. Pore size distributions from about 2 angstroms can be determined from the data taken at 273.15K.

• Currently, calculation parameters are optimized for studies on carbon surfaces.

BIBLIOGRAPHY for Rapid Micropore Size Analysis by CO2 Adsorption

1. J. Garrido, A. Linares-Solano, J.M. Martin-Martinez, M. Molina-Sabio, F. Rodriguez-Reinoso, R. Torregosa Langmuir, 3, 76, (1987)

2. F. Carrasco-Martin, M.V. López-Ramón, C. Moreno-Castilla. Langmuir, 9, 2758 (1993)

3. P. Tarazona. Phys.Rev.A 31, 2672 (1985)

4. N.A. Seaton, J.P.R.B. Walton, N. Quirke. Carbon, 27, 853 (1989)

5. C. Lastoskie, K.E. Gubbins, N. Quirke. J.Phys.Chem., 97, 4786 (1993)

6. J.J. Olivier. Porous Materials 2, 9 (1995)

7. P.I. Ravikovitch, S.C. Ó Domhnaill, A.V. Neimark, F. Schüth, K.K. Unger. Langmuir, 11, 4765 (1995)

8. A.V. Neimark, P.I. Ravikovitch, M. Grün, F. Schüth, K.K. Unger. COPS-IV, 1997 (in press)

9. P.I. Ravikovitch P.I., D. Wei, W.T. Chuen, G.L. Haller,A.V. Neimark. J.Phys.Chem., May 1997

10. E.J. Bottani, V. Bakaev, W.A. Steele. Chem.Eng.Sci. 49, 293 (1994)

11. M.M. Dubinin. Carbon 27, 457 (1989)

Questions from the floor ?

CHEMISORPTION &

CATALYSIS

Catalysis & Catalysts

� Facts and Figures about CatalystsLife cycle on the earth� Catalysts (enzyme) participates most part of life cycle

e.g. forming, growing, decaying� Catalysis contributes great part in the processes of converting sun energy to

various other forms of energies e.g. photosynthesis by plant CO2 + H2O=HC + O2

� Catalysis plays a key role in maintaining our environment

Chemical Industry� ca. $2 bn annual sale of catalysts� ca. $200 bn annual sale of the chemicals that are related products� 90% of chemical industry has catalysis-related processes� Catalysts contributes ca. 2% of total investment in a chemical process

What is Catalysis

� Catalysis� Catalysis is an action by catalyst which takes part in a chemical reaction

process and can alter the rate of reactions, and yet itself will return to its original form without being consumed or destroyed at the end of the reactions (This is one of many definitions)

Three key aspects of catalyst action� taking part in the reaction

• it will change itself during the process by interacting with other reactant/product molecules

� altering the rates of reactions • in most cases the rates of reactions are increased by the action of catalysts;

however, in some situations the rates of undesired reactions are selectively suppressed

� Returning to its original form• After reaction cycles a catalyst with exactly the same nature is ‘reborn’• In practice a catalyst has its lifespan - it deactivates gradually during use

Action of Catalysts�Catalysis action - Reaction kinetics and

mechanism Catalyst action leads to the rate of a reaction to change.

This is realised by changing the course of reaction (compared to non-catalytic reaction)

� Forming complex with reactants/products, controlling the rate of elementary steps in the process. This is evidenced by the facts that

� The reaction activation energy is altered

� The intermediates formed are different from

those formed in non-catalytic reaction

� The rates of reactions are altered (both

desired and undesired ones)

� Reactions proceed under less demanding conditions

� Allow reactions occur under a milder conditions, e.g. at lower temperatures for those heat sensitive materials 191

reactant

reaction process

uncatalytic

product

catalytic

Action of Catalysts� It is important to remember that the use of catalyst DOES NOT vary ∆G &

Keq values of the reaction concerned, it merely change the PACE of the process

� Whether a reaction can proceed or not and to what extent a reaction can proceed is solely determined by the reaction thermodynamics, which is governed by the values of ∆G & Keq, NOT by the presence of catalysts.

� In another word, the reaction thermodynamics provide the driving force for a rxn; the presence of catalysts changes the way how driving force acts on that process.

e.g CH4(g) + CO2(g) = 2CO(g) + 2H2(g) ∆G°373=151 kJ/mol (100 °C)

∆G°973 =-16 kJ/mol (700 °C)

� At 100°C, ∆G°373=151 kJ/mol > 0. There is no thermodynamic driving force, the reaction won’t proceed with or without a catalyst

� At 700°C, ∆G°373= -16 kJ/mol < 0. The thermodynamic driving force is there. However, simply putting CH4 and CO2 together in a reactor does not mean they will react. Without a proper catalyst heating the mixture in reactor results no conversion of CH4and CO2 at all. When Pt/ZrO2 or Ni/Al2O3 is present in the reactor at the same temperature, equilibrium conversion can be achieved (<100%).

Types of Catalysts & Catalytic Reactions� The types of catalysts

� Classification based on the its physical state, a catalyst can be � gas � liquid� solid

� Classification based on the substances from which a catalyst is made� Inorganic (gases, metals, metal oxides, inorganic acids, bases etc.)� Organic (organic acids, enzymes etc.)

� Classification based on the ways catalysts work� Homogeneous - both catalyst and all reactants/products are in the same phase

(gas or liq)� Heterogeneous - reaction system involves multi-phase (catalysts +

reactants/products)

� Classification based on the catalysts’ action� Acid-base catalysts� Enzymatic� Photocatalysis� Electrocatalysis, etc.

Applications of Catalysis� Industrial applications

Almost all chemical industries have one or more steps employing catalysts

� Petroleum, energy sector, fertiliser, pharmaceutical, fine chemicals …

Advantages of catalytic processes� Achieving better process economics and productivity

� Increase reaction rates - fast� Simplify the reaction steps - low investment cost� Carry out reaction under mild conditions (e.g. low T, P) - low energy consumption

� Reducing wastes� Improving selectivity toward desired products - less raw materials required, less unwanted

wastes� Replacing harmful/toxic materials with readily available ones

� Producing certain products that may not be possible without catalysts� Having better control of process (safety, flexible etc.)� Encouraging application and advancement of new technologies and materials� And many more …

Applications of Catalysis� Environmental applications

� Pollution controls in combination with industrial processes� Pre-treatment - reduce the amount waste/change the composition of emissions� Post-treatments - once formed, reduce and convert emissions � Using alternative materials

� Pollution reduction� gas - converting harmful gases to non-harmful ones� liquid - de-pollution, de-odder, de-colour etc� solid - landfill, factory wastes

� And many more …

� Other applications� Catalysis and catalysts play one of the key roles in new technology

development.

Research in Catalysis� Research in catalysis involve a multi-discipline approach

� Reaction kinetics and mechanism� Reaction paths, intermediate formation & action, interpretation of results obtained under

various conditions, generalising reaction types & schemes, predict catalyst performance…

� Catalyst development� Material synthesis, structure properties, catalyst stability, compatibility…

� Analysis techniques� Detection limits in terms of dimension of time & size and under extreme conditions (T,

P) and accuracy of measurements, microscopic techniques, sample preparation techniques…

� Reaction modelling� Elementary reactions and rates, quantum mechanics/chemistry, physical chemistry …

� Reactor modelling� Mathematical interpretation and representation, the numerical method, micro-kinetics,

structure and efficiency of heat and mass transfer in relation to reactor design …

� Catalytic process� Heat and mass transfers, energy balance and efficiency of process …

Catalytic Reaction Processes� Understanding catalytic reaction processes

� A catalytic reaction can be operated in a batch manner� Reactants and catalysts are loaded together in reactor and catalytic

reactions (homo- or heterogeneous) take place in pre-determined temperature and pressure for a desired time / desired conversion

� Type of reactor is usually simple, basic requirements� Withstand required temperature & pressure � Some stirring to encourage mass and heat transfers� Provide sufficient heating or cooling

� Catalytic reactions are commonly operated in a continuousmanner

� Reactants, which are usually in gas or liquid phase, are fed to reactor in steady rate (e.g. mol/h, kg/h, m3/h)

� Usually a target conversion is set for the reaction, based on this target� required quantities of catalyst is added� required heating or cooling is provided� required reactor dimension and characteristics are designed accordingly.

Catalytic Reaction Processes� Catalytic reactions in a continuous operation (cont’d)

� Reactants in continuous operation are mostly in gas phase or liquid phase

� easy transportation� The heat & mass transfer rates in gas phase is much faster than those in liquid

� Catalysts are pre-loaded, when using a solid catalyst, or fed together with reactants when catalyst & reactants are in the same phase and pre-mixed

� It is common to use solid catalyst because of its easiness to separate catalyst from unreacted reactants and products Note: In a chemical process separation usually accounts for ~80% of cost. That is why engineers always try to put a liquid catalyst on to a solid carrier.

� With pre-loaded solid catalyst, there is no need to transport catalyst which is then more economic and less attrition of solid catalyst (Catalysts do not change before and after a reaction and can be used for number cycles, months or years),

� In some cases catalysts has to be transported because of need of regeneration

� In most cases, catalytic reactions are carried out with catalyst in a fixed-bed reactor (fluidised-bed in case of regeneration being needed), with the reactant being gases or liquids

Catalytic Reaction Processes

� General requirements for a good catalyst�Activity - being able to promote the rate of

desired reactions

�Selective - being to promote only the rate of desired reaction and also retard the undesired reactions

Note: The selectivity is sometime considered to be more important than the activity and sometime it is more difficult to achieve

(e.g. selective oxidation of NO to NO2 in the presence of SO2) 199

Catalytic reaction processes

�Stability - a good catalyst should resist to deactivation, caused by

� the presence of impurities in feed (e.g. lead in petrol poison TWC.

� thermal deterioration, volatility and hydrolysis of active components

� attrition due to mechanical movement or pressure shock

�A solid catalyst should have reasonably large surface area needed for reaction (active sites). This is usually achieved by making the solid into a porous structure.

Example Heterogeneous Catalytic Reaction Process

� The long journey for reactant molecules to

1. travel within gas phase

2. cross gas-liquid phase boundary3. travel within liquid phase/stagnant layer4. cross liquid-solid phase boundary5. reach outer surface of solid6. diffuse within pore7. arrive at reaction site8. be adsorbed on the site and activated9. react with other reactant molecules, either

being adsorbed on the same/neighbour sites or approaching from surface above

� Product molecules must follow the same track in the reverse direction to return to gas phase

� Heat transfer follows similar track201

gas phase

poreporous solid

liquid phase /stagnant

gas phasereactant molecule

Solid Catalysts

� Catalyst composition

�Active phase� Where the reaction occurs (mostly metal/metal oxide)

�Promoter � Textual promoter (e.g. Al - Fe for NH3 production)� Electric or Structural modifier� Poison resistant promoters

�Support / carrier� Increase mechanical strength� Increase surface area (98% surface area is supplied within the

porous structure) � may or may not be catalytically active 202

Catalyst

Support

Solid Catalysts� Some common solid support / carrier

materials

� Alumina� Inexpensive� Surface area: 1 ~ 700 m2/g� Acidic

� Silica� Inexpensive� Surface area: 100 ~ 800 m2/g� Acidic

� Zeolite� mixture of alumina and silica, � often exchanged metal ion present� shape selective� acidic

� Other supports

� Active carbon (S.A. up to 1000 m2/g)

� Titania (S.A. 10 ~ 50 m2/g)� Zirconia (S.A. 10 ~ 100 m2/g)

� Magnesia (S.A. 10 m2/g)� Lanthana (S.A. 10 m2/g)

poreporous solid

Active site

Solid Catalysts� Preparation of catalysts

� PrecipitationTo form non-soluble precipitate by desired reactions at certain pH and temperature

� Adsorption & ion-exchangeCationic: S-OH+ + C+ →→→→ SOC+ + H+

Anionic: S-OH- + A- →→→→ SA- + OH-

I-exch. S-Na+ + Ni 2+ �� S-Ni 2+ + Na+

� ImpregnationFill the pores of support with a metal salt solution of sufficient concentration to give the correct loading.

� Dry mixing Physically mixed, grind, and fired

precipitate or deposit

precipitation

filter & wash the resultingprecipitate

Drying& firing

precursorsolution

Support

add acid/basewith pH control

Support

Drying & firing

Pore saturated pellets

Soln. of metalprecursor

Concentration

Support

Drying & firing

Solid Catalysts� Preparation of catalysts

�Catalysts need to be calcined (fired) in order to decompose the precursor and to received desired thermal stability. The effects of calcination temperature and time are shown in the figures on the right.

� Commonly used Pre-treatments

� Reduction � if elemental metal is the active phase

� Sulphidation � if a metal sulphide is the active phase

� Activation� Some catalysts require certain activation steps in order to receive the best

performance. � Even when the oxide itself is the active phase it may be necessary to pre-treat

the catalyst prior to the reaction

� Typical catalyst life span

�Can be many years or a few mins.205

500 600 700 800 900Temperature °C

0 10Time / hours

Normal use

Induction period

Adsorption On Solid Surface�Adsorption

� Adsorption is a process in which molecules from gas (or liquid) phase land on, interact with and attach to solid surfaces.

� The reverse process of adsorption, i.e. the process n which adsorbed molecules escape from solid surfaces, is called Desorption.

� Molecules can attach to surfaces in two different ways because of the different forces involved. These are Physisorption (Physical adsorption) & Chemisorption (Chemical adsorption)

Physisorption Chemisorption

force van de Waal chemcal bond

number of adsorbed layers multi only one layer

adsorption heat low (10-40 kJ/mol) high ( > 40 kJ/mol)

selectivity low high

temperature to occur low high

Adsorption On Solid Surface� Adsorption process

Adsorbent and adsorbate

� Adsorbent (also called substrate) - The solid that provides surface for adsorption

� high surface area with proper pore structure and size distribution is essential� good mechanical strength and thermal stability are necessary

� Adsorbate - The gas or liquid substances which are to be adsorbed on solid

Surface coverage, θθθθ

The solid surface may be completely or partially covered by adsorbed molecules

Adsorption heat

� Adsorption is usually exothermic (in special cases dissociated adsorption can be endothermic)

� The heat of chemisorption is in the same order of magnitude of reaction heat; the heat of physisorption is in the same order of magnitude of condensation heat.

define θ θ θ θ = θθθθ = 0~1number of adsorption sites occupiednumber of adsorption sites available

Adsorption On Solid Surface

�Applications of adsorption process� Adsorption is a very important step in solid catalysed reaction processes

� Adsorption in itself is a common process used in industry for various purposes

� Purification (removing impurities from a gas / liquid stream)� De-pollution, de-colour, de-odour� Solvent recovery, trace compound enrichment� etc…

� Usually adsorption is only applied for a process dealing with small capacity� The operation is usually batch type and required regeneration of saturated

adsorbent

Common adsorbents: molecular sieve, active carbon, silica gel, activated alumina.

� Physisorption is an useful technique for determining the surface area, the pore shape, pore sizes and size distribution of porous solid materials (BET surface area)

Adsorption On Solid Surface� Characterisation of adsorption system

� Adsorption isotherm - most commonly used, especially to catalytic reaction system, T=const.

The amount of adsorption as a function of pressure at set temperature

� Adsorption isobar - (usage related to industrial applications)

The amount of adsorption as a function of temperature at set pressure

� Adsorption Isostere - (usage related to industrial applications)

Adsorption pressure as a function of temperature at set volume

Pressure

T2 >T1

T3 >T2

T4 >T3

T5 >T4

Temperature

Adsorption Isotherm Adsorption Isobar Adsorption Isostere

Adsorption On Solid Surface� Five types of physisorption isotherms are found over all solids

� Type I is found for porous materials with small pores e.g. charcoal. It is clearly Langmuir monolayer type, but the other 4 are not

� Type II for non-porous materials

� Type III porous materials with cohesive force between adsorbatemolecules greater than the adhesive force between adsorbatemolecules and adsorbent

� Type IV staged adsorption (first monolayer then build up of additional layers)

� Type V porous materials with cohesive force between adsorbatemolecules and adsorbent being greater than that between

adsorbate molecules

relative pres. P/P0

Adsorption On Solid Surface� Other adsorption isotherms

Many other isotherms are proposed in order to explain the observations

� The Temkin (or Slygin-Frumkin) isotherm� Assuming the adsorption enthalpy ∆∆∆∆H decreases linearly with surface coverage

From ads-des equilibrium, ads. rate ≡ des. rate

rads=kads(1-θ)P ≡ rdes=kdesθ

where Qs is the heat of adsorption. When Qs is a linear function of θi. Qs=Q0-iS (Q0 is a constant, i is the number and S represents the surface site),

the overall coverage

When b1P >>1 and b1Pexp(-i/RT) <<1, we have θθθθ =c1ln(c2P), where c1 & c2 are constants

� Valid for some adsorption systems.

PBRT/Q

+= θθ ∆∆ ∆∆H

θθθθ

Langmuir

Temkin

+== ∫∫

RTiRT/Q

Adsorption On Solid Surface� The Freundlich isotherm

� assuming logarithmic change of adsorption enthalpy ∆∆∆∆H with surface coverageFrom ads-des equilibrium, ads. rate ≡ des. rate

rads=kads(1-θ)P ≡ rdes=kdesθ

where Qi is the heat of adsorption which is a function of θi. If there are Ni types of surface sites, each can be expressed as Ni=aexp(-Q/Q0) (a and Q0 are constants), corresponding to a fractional coverage θi, the overall coverage

the solution for this integration expression at small θ is:

lnθ=(RT/Q0)lnP+constant, or

as is the Freundlich equation normally written, where c1=constant, 1/c2=RT/Q0

� Freundlich isotherm fits, not all, but many adsorption systems.

∫∑

∑∞

⋅+==

e)](1[

dQaPeb/Peb

Q/QRT/QRT/Q

PBRT/Q

+= θθ ∆∆ ∆∆

θθθθ

Langmuir

Freundlich

C/pc=θ

Adsorption On Solid Surface� BET (Brunauer-Emmett-Teller) isotherm

� Many physical adsorption isotherms were found, such as the types II and III, that the adsorption does not complete the first layer (monolayer) before it continues to stack on the subsequent layer (thus the S-shape of types II and III isotherms)

� Basic assumptions� the same assumptions as that of Langmuir but allow multi-layer adsorption� the heat of ads. of additional layer equals to the latent heat of condensation� based on the rate of adsorption=the rate of desorption for each layer of ads.

the following BET equation was derived

Where P - equilibrium pressureP0 - saturate vapour pressure of the adsorbed gas at the temperature

P/P0 is called relative pressureV - volume of adsorbed gas per kg adsorbentVm -volume of monolayer adsorbed gas per kg adsorbentc - constant associated with adsorption heat and condensation heatNote: for many adsorption systems c=exp[(H1-HL)/RT], where H1 is adsorption heat of 1st layer & HL is liquefaction heat, so that the adsorption heat can be determined from constant c.

0 P/PcV

cV)P/P(V

Adsorption On Solid Surface� Comment on the BET isotherm

� BET equation fits reasonably well all known adsorption isotherms observed so far (types I to V) for various types of solid, although there is fundamental defect in the theory because of the assumptions made (no interaction between adsorbed molecules, surface homogeneity and liquefaction heat for all subsequent layers being equal).

� BET isotherm, as well as all other isotherms, gives accurate account of adsorption isotherm only within restricted pressure range. At very low (P/P0<0.05) and high relative pressure (P/P0>0.35) it becomes less applicable.

� The most significant contribution of BET isotherm to the surface science is that the theory provided the first applicable means of accurate determination of the surface area of a solid (since in 1945).

� Many new development in relation to the theory of adsorption isotherm, most of them are accurate for a specific system under specific conditions.

Adsorption On Solid Surface� Use of BET isotherm to determine the surface area of a solid

� At low relative pressure P/P0 = 0.05~0.35 it is found that

Y = a + b X

�The principle of surface area determination by BET method:

A plot of against P/P0 will yield a straight line with slope of equal to (c-

1)/(cVm) and intersect 1/(cVm).

For a given adsorption system, c and Vm are constant values, the surface area of a solid material can be determined by measuring the amount of a particular gas adsorbed on the surface with known molecular cross-section area Am,

* In practice, measurement of BET surface area of a solid is carried out by N2

physisorption at liquid N2 temperature; for N2, Am = 16.2 x 10-20 m2215

)( )(11

0 P/PP/PcV

cV)P/P(V

∝−

( / )0

A A N AV

Vs m m m

= = × ×,

.6022 1023 Vm - volume of monolayer adsorbed gas molecules calculated from the plot, L

VT,P - molar volume of the adsorbed gas, L/mol

Am - cross-section area of a single gas molecule, m2

Adsorption On Solid Surface� Summary of adsorption isotherms

Name Isotherm equation ApplicationNote

Langmuir

Temkin θθθθ =c1ln(c2P)

Freundlich

0 P/PcV

cV)P/P(V

θ= =+∞

C/pc=θ

Chemisorption andphysisorption

Chemisorption

Chemisorption andphysisorption

Multilayer physisorption

Useful in analysis of reaction mechanism

Chemisorption

Easy to fit adsorption data

Useful in surface area

determination

Mechanism of Surface Catalysed Reaction

�Langmuir-Hinshelwood mechanism� This mechanism deals with the surface-catalysed reaction in which

that 2 or more reactants adsorb on surface without dissociation

A(g) + B(g) � A(ads) + B(ads) � P (the desorption of P is not r.d.s.)

� The rate of reaction ri=k[A][B]=kθAθB

From Langmuir adsorption isotherm (the case III) we know

� We then have

� When both A & B are weakly adsorbed (B0,APA<<1, B0,BPB<<1),

2nd order reaction

� When A is strongly adsorbed (B0,APA>>1) & B weakly adsorbed (B0,BPB<<1 <<B0,APA)

1st order w.r.t. B

BB,AA,

BAB,A,

BB,AA,

111 ++=

BABAB,A,i PP'kPPBkBr == 00

BAB,A,

i P''kPkBPB

PPBkBr === 0

A B+ � P

Mechanism of Surface Catalysed Reaction

�Eley-Rideal mechanism� This mechanism deals with the surface-catalysed reaction in which

that one reactant, A, adsorb on surface without dissociation &other reactant, B, approaching from gas to react with A

A(g) � A(ads) P (the desorption of P is not r.d.s.)

� The rate of reaction ri=k[A][B]=kθAPB

From Langmuir adsorption isotherm (the case I) we know

� We then have

� When both A is weakly adsorbed or the partial pressure of A is very low (B0,APA<<1),

2nd order reaction

� When A is strongly adsorbed or the partial pressure of A is very high (B0,APA>>1)

1st order w.r.t. B

BABAA,i PP'kPPkBr == 0

i kPPB

PPkBr ==

A� P

+ B(g)

Mechanism of Surface Catalysed Reaction� Mechanism of surface-catalysed reaction with dissociative adsorption

� The mechanism of the surface-catalysed reaction in which onereactant, AD, dissociatively adsorbed on one surface site

AD(g) � A(ads) + D(ads) P

(the des. of P is not r.d.s.)

� The rate of reaction ri=k[A][B]=kθADPB

From Langmuir adsorption isotherm (the case I) we know

� We then have

� When both AD is weakly adsorbed or the partial pressure of AD is very low (B0,ADPAD<<1),

The reaction orders, 0.5 w.r.t. AD and 1 w.r.t. B

� When A is strongly adsorbed or the partial pressure of A is very high (B0,APA>>1)

1st order w.r.t. B219

( )( ) 21

( )( )

( )( ) 21

ADAD,i PP'kPPBkr2121

( )( ) B/

i kPPB

PPBkr == 21

+ B(g)� P

Mechanism of Surface Catalysed Reaction� Mechanisms of surface-catalysed rxns involving dissociative

adsorption� In a similar way one can derive mechanisms of other surface-catalysed

reactions, in which

� dissociatively adsorbed one reactant, AD, (on one surface site) reacts with another associatively adsorbed reactant B on a separate surface site

� dissociatively adsorbed one reactant, AD, (on one surface site) reacts with another dissociatively adsorbed reactant BC on a separate site

� …

� The use of these mechanism equations

� Determining which mechanism applies by fitting experimental data to each.

� Helping in analysing complex reaction network

� Providing a guideline for catalyst development (formulation, structure,…).

� Designing / running experiments under extreme conditions for a better control

� …220

Need to ask ?

Chemisorption

3. Chemisorption Techniques

3.1 Introduction: Physisorption/Chemisorption

3.2 Classical Models

3.3 Active Metal Area Measurement

3.4 Adsorption Thermodynamics

3.5 Pulse vs. Static

3.6 Temperature Programmed Analyses

The Nature of Gas Sorption at a Surface

• When the interaction between a surface and an adsorbate is relatively weak only physisorption takes place.

• However, surface atoms often possess electrons or electron pairs which are available for chemical bond formation.

• This irreversible adsorption, or chemisorption, is characterized by large interaction potentials which lead to high heats of adsorption.

Physisorption vs Chemisorption

Property Physisorption Chemisorption

Forces van der Waals Chemical bonding

∆Hads

(kJ mol-1) < 40 50-200

(kJ mol-1) Rare 60–100

Isothermal Reversibility Complete Slow or none

Extent Multilayers Monolayer

On The Nature of Chemisorption

• Chemisorption is often found to occur at temperatures far above the critical temperature of the adsorbate.

• As is true for most chemical reactions, chemisorption is usually associated with an activation energy, which means that adsorbate molecules attracted to a surface must go through an energy barrier before they become strongly bonded to the surface.

Adsorption PotentialsP

tial E

Potential energy curves for molecular (non-dissociative) adsorption

∆Hact.

∆Hdissoc.

Adsorption Potentials

Potential energy curves for activated adsorption

Adsorption Potentials

Potential energy curves for non-activated adsorption

∆Hdissoc.

Isobars

Isobaric variation in quantity adsorbed with temperature. Physisorption isobar (a) represents lower heat of adsorption than chemisorption isobar (b).

Temperature

Quantity adsorbed

On The Nature of Chemisorption

• Because chemisorption involves a chemicalbond between adsorbate and adsorbent,unlike physisorption, only a single layer ofchemisorbed species can be realized onlocalized active sites such as those found inheterogeneous catalysts.

• However, further physical adsorption on topof the chemisorbed layer and diffusion of thechemisorbed species into the bulk solid canobscure the fact that chemisorbed materialcan be only one layer in depth

Classical Models

3.2 Classical Models

3.2.1 Langmuir

3.2.2 Freundlich

3.2.3 Temkin

Adsorption Process

Active Sites (Adsorbent)

Adsorbate Adsorptive

Graduated as a metallurgical engineer from the School of Mines at Columbia University in 1903

1903-1906 M.A. and Ph.D. in 1906 from Göttingen.

1906-1909 Instructor in Chemistry at Stevens Institute of Technology, Hoboken, New Jersey.

1909 –1950 General Electric Company at Schenectady where he eventually became Associate Director

1913 -Invented the gas filled, coiled tungsten filament incandescent lamp.

1919 to 1921, his interest turned to an examination of atomic theory, and he published his "concentric theory of atomic structure" . In it he proposed that all atoms try to complete an outer electron shell of eight electrons

Irving Langmuir (1881-1957)

1927 Coined the use of the term "plasma" for an ionized gas.

1932 The Nobel Prize in Chemistry "for his discoveries and investigations in

surface chemistry"

1935-1937 With Katherine Blodgett studied thin films.

1948-1953 With Vincent Schaefer discovered that the introduction of dry ice and iodide into a sufficiently moist cloud of low temperature could induce precipitation.

Irving Langmuir (1881-1957)

3.2.1 Langmuir’s “Kinetic” Approach

rate of adsorption = ka P(1-θ)

where θ is the fraction of the surface already covered with adsorbate, i.e.,θ = V/Vm

rate of desorption = kd θ

Suggests a dynamic equilibrium. Is it?

Langmuir (continued…)At equilibrium (any pressure)

ka P(1-θ) = kd θ

from which

θ = V/Vm = KP/(1+KP)

where K = ka / kd.

In its linear form, the above equation can be expressed as:

1/V = 1/Vm + 1/(VmKP)

Confining adsorption to a monolayer, the Langmuirequation can be written

where V is the volume of gas adsorbed at pressure P,Vm is the monolayer capacity (i.e. θ=1) expressed asthe volume of gas at STP and K is a constant for anygiven gas-solid pair. Rearranging in the form of astraight line (y=ab+x) gives

Or, if you prefer…

Langmuir Plot

Slope = 1/(VmK)

Intercept = 1/Vm

1/V = 1/Vm + 1/(VmKcP1/s)

Temperature Dependent Models

generally

K = Ko exp(q/RT)where Ko is a constant, R is the universal gas constant, T is the

adsorption temperature and q is the heat of adsorption

• Langmuir:K is constant;q is constant at all θ• Temkin: assumed that q decreases linearly with

increasing coverage• Freundlich: assumed that q decreases

exponentially with increasing coverage

TemkinTemkin assumed that q decreases linearly with increasing coverage,that is,

Q=qo(1- λ θ)

Where qo is a constant equal to the heat of adsorption at zero coverage (θ = 0) and λ is a proportionality constant.

Temkinθ = A ln P + B

or, since θ = V/Vm

V = Vm A lnP + VmB

Where A = RT/qo λ θ andB = A ln Ko + 1/ λ θ

Temkin Plot

Slope = VmA

Intercept = VmB

V = Vm A lnP + VmB

Multiple Temkin Plots to find

Temp H Temp M Temp L

* denotes “temperature invariant” or “thermally irreversible” quantity

experimental extrapolated

FreundlichTemkin assumed that q decreases

exponentially with increasing coverage, that is,

Q = -qm lnθ

Where qm is a constant equal to the heat of adsorption at θ = 0.3679

Freundlich

lnθ = C lnP + D or, since θ = V/Vm

ln(V/Vm) = C lnP + D

Where C=RT/ qm and D = C lnKo

Freundlich (continued…)

Slope = C

Intercept = D + ln(Vm)

Ln(V/Vm) = C lnP + D

Multiple Temkin Plots to find

Temp H Temp M Temp L

* denotes “temperature invariant” or “thermally irreversible” quantity

experimental extrapolated

Active Metal Area

3.3 Active Metal Area

3.3.1 Principles of Calculation

3.3.2 Choice of Adsorbate

3.3.3 Active Site Size Calculation

3.3.4 Metal Dispersion

3.3.5 Accessible vs non-accessible sites

Active Site Quantification

• Because the formation of a chemicalbond takes place between an adsorbatemolecule and a localized, or specific,site on the surface of the adsorbent, thenumber of active sites on catalysts canbe determined simply by measuring thequantity of chemisorbed gas

Active Site on a Catalyst

• Metal on support.• Island-like crystallites• Not all metal atoms exposed.• Adsorption technique perfectly suited.(cf Chemical analysis of entire metal

content )

3.3.1 Principles of Calculation

Monolayer Volume, Vm= volume of gas chemisorbed in a monomolecular layer

Methods to Determine Vm

•Extrapolation

• Bracketing

• Langmuir

• Temkin

• Freundlich

= volume of gas chemisorbed in a monomolecular layer

Pressure (mm Hg)

Extrapolation method

First (only?)isotherm

Pressure (mm Hg)

The second isotherm

combined

Weak only

Pressure (mm Hg)

The difference isotherm

combined

Weak only

Strong

Vm from Pulse Titration

… will be covered in 3.5.2

Metal Area Calculation

To Calculate Metal Surface Area:A = (Vm) x (MXSA) x (S) x 6.03 x 10-3 (units m2/g)

where MXSA = metal cross sectional area (Å2)and S = stoichiometry = metal atoms per gas molecule

To calculate metal area per gram of metal, Am:Am = A x l00/L

where L = metal loading (%) = known value from chemical analysis

Stoichiometry

The gas-sorption stoichiometry is defined as the number of metal atoms with which each gas molecule reacts.

Since, in the gas adsorption experiment to determine the quantity of active sites in a catalyst sample, it is the quantity of adsorbed gas which is actually measured, the knowledge of (or at least a reasonably sound assumption of) the stoichiometry involved is essential in meaningful active site determinations (area, size, dispersion).

3.3.2 Choice of Adsorbate

Chemisorption

• CO or H2 on Pt, Pd

at 40 oC

• CO or H2 on Ni

For metal-only area

(& dispersion etc)

Physisorption

• N2 at 77K

• Ar at 87K

• Kr at 77K

• CO2 at 273KFor total surface area

and pore size

3.3.3 Active Site Size Calculation

To calculate average crystallite size:

d = (L x 100 x f )/AD (units Å)

where f = shape factor = 6ρ = density of metal (g/ml)

Shape Factor & Crystallite Size

The default shape factor of 6 is for assumed cubic geometry.Consider a cube of six sides (faces) each of length l. then the total surface area, ΣA = 6l2.

The volume of the cube is given by l3 or, in terms of total area, substitute ΣA /6 for l2 to give

V= lΣA/6

For a cube whose mass is unit mass, its volume is given by 1/ ρ(where ρ is the density of the material).

V=1/ρ

Shape Factor & Crystallite Size

For the same cube of unit mass, the area is then the area per unit mass A and l is rewritten d (crystallite size), the length required to give a cube whose mass is unity. Equating both terms for volume:

dA/6=1/ ρ

ord=6/A ρ

For a supported metal, the loading, L, must be taken into consideration.

d=L6/A ρ

Other geometries can be treated in a similar fashion. For example, a rectangular particle whose length is three times its width has a shape factor of 14/3.

Supported metalsIt is most likely that the catalyst exists as a

collection of metal atoms distributed over an inert, often refractory, support material such as alumina.

At the atomic level it is normal that these atoms are assembled into island-like crystallites on the surface of the support.

3.3 Metal Dispersion

3.3 Metal Dispersion• In the case of supported metal catalysts, it is

important to know what fraction of the active metal atoms is exposed and available to catalyze a surface reaction.

• Those atoms that are located inside metal particles do not participate in surface reactions, and are therefore wasted.

Exposed metal atomsSince these islands vary in size due to both the intrinsic

nature of the metal and the support beneath, plus themethod of manufacture more or less of the metalatoms in the whole sample are actually exposed atthe surface. It is evident therefore that the method ofgas adsorption is perfectly suited to the determinationof exposed active sites.

support

Exposed active sitesAdsorbed gas

Metal Dispersion

• Dispersion is defined as the percentage of all metal atoms in the sample that are exposed.

• The total amount of metal in the sample is termed the loading, χ , as a percentage of the total sample mass, and is known from chemical analysis of the sample.

Metal Dispersion

• The dispersion, δ, is calculated from:

• Where M is the molecular weight of the metal, Na is the number of exposed metal atoms found by adsorption and WS is the mass of the sample.

a 100100

3.3.5Accessible vs. Non-accessible Sites1. Adventitious moisture2. Reducing gas accessibility3. Diffusion4. Purge5. Physisorption blocks6. Bulk hydride7. Spillover8. Stoichiometry9. Characterization gas vs. Process gas

Spatial Ordering

There may exist a number of different adsorption sites that involve different numbers of metal atoms per adsorbate molecule.

Adsorption Thermodynamics

3.4 Adsorption Thermodynamics

3.4.1 Isosteric Heats from Isotherms

seminar core slides malaysia dec 2013

size light

particle size analysis

average particle size

size of sphere

size permeability gas

brownian motion of particles

light scattering

solvent particles

Education

human networking syracuse university seminar dec 2016

science note frm free education malaysia [2014 seminar]

201512 dec jan seminar highlights

malaysia (mys) specimen questions dec 2015

[dec., 2016] zulkifli abdul majid · [zulkifli abdul majid]...

grl seminar dec 11, 2013

drawing! - hcii phd lunch seminar - dec 1, 2011

malaysia fit presentation dec 2010

overzicht studiedagen 11 & 12 dec 2013 ec2 seminar

seminar bumn malaysia

seminar-2 dec 2011 experimental investigation

telekom malaysia berhad converging -...

summary seminar,idea 18 dec 2012.pdf

icao regional language proficiency seminar - baku dec.2005

fast company 2 intro seminar 8 dec 2010

safety net project seminar in malaysia

eye tracking - seminar 6 dec 2013

dec seminar session 2 ic vs. employee

malaysia – egypt palm oil trade fair and seminar

motarme nui galway technology marketing seminar dec 2012