Experimental Methods in
Catalysis (EMC)
M.Tech-Catalysis Technology
II Semester
CT-503
Dr.K.R.Krishnamurthy
National Centre for Catalysis Research
Indian Institute of Technology
Chennai-600036
Catalysts- Functionalities
Basic
Activity
Selectivity
Stability
Applied
Manufacturing
Aging
Deactivation
Regenerability
Evalua
-tion
Catalyst
Development
Cycle
Character
-izattion
Why do we Characterize?
Provides answers to WHY & HOW
Integral part of Catalyst development cycle
Prepa
-ration
Catalysts-Characteristics
Chemical composition
Active elements, promoters, stabilizers
Structural features
Crystalline/Amorphous, Crystal structure
Phase composition, Phase transformations- TiO2—Anatase/Rutile
Surface Properties
Composition, -Bulk Vs Surface, in-situ techniques
Co-ordination, Geometry/ Structure- Spectroscopic methods
Dispersion & distribution of active phases
Concentration profile, Crystallite size
Electronic properties
Redox character, Chemisorption
Textural properties
Surface area, Pore volume, Pore-size & distribution
Physical properties
Size, Shape, Strength
Chemical properties
Surface reactivity/Acidity/Basicity
Enabling Structure-Activity correlations
Catalysts- Shape factor
Catalysts- Shape effect
Characterization of Catalysts
Preparation Characterizati Evaluation
on
Ageing
Spent
Concn. of
Phase
active elements composition
In-situ
Spectroscopy
Solid state
transformations
Inactive
phases
Species in
Solution phase
Transient
surface species
Structural
transformations
Poisons
Solid state
Structural features
transformations
Reactants &
Products
Surface
composition
Analysis
of coke
Preparation
techniques
Kinetics &
mechanism
Catalyst life
Deactivation
&
Regeneration
Electronic state
Dispersion &
Distribution
Surface
composition
Evolve active
phase
Ensure desired
characteristics
Surface
reactions
Catalysts Characterization- From Cradle to Coffin
Textural properties
Porous solids
Geometric
shape/size
External
Surface area
Catalysts
Internal
Porosity /
Pores
Adsorbents
Alumina
Silica
Carbon
Mol.sieves
Clays
Metals
Metal oxides
Metal sulfides
Metal chlorides
Zeolites
Heteropoly acids
Pore structure
Pore size-Area-Volume-Distribution-Geometry
Textural properties
Textural properties- Significance
Surface area/Pore volume - Dispersion of active phase
Pore size & distribution
Molecular traffic-Diffusion of reactants & products
Heat & mass transfer
Diffusion rates- residence time
Selectivity
Extent of coking
Thermal & mechanical stability
Textural properties-Integral part of catalyst architecture
Origin of pores
Crystal structure- Intrinsic voids
Atomic/molecular
Preparation- Voids due to leaving groups
Hydroxides, carbonates, Oxalates- Ni(OH)2, MgCO3, ZnC2O4
Structural modifications-Intercalation/Pillaring
Graphite/ Clay
Aggregation/Coalescence- Preparation
Formation of secondary particles from primary particles
Flexible pores- dispersion of particles
Agglomeration/Sintering- Pre-treatments
Rigid pores
Compacting
Shaping
Origin of pores
Pores
Inherent in any solid structure
Intrinsic intra particle pores
Voids created by specific arrangement of atoms / moleculesZeolites- Cages & channels –Structurally intrinsic pores
Voids formed due to missing/removed molecules, atoms,
particles- Dehydration of AlOOH to Al2O3
Removal of Na from Na silicate glass
Interstitial space between graphitic plates in CF
Extrinsic intra particle pores
Voids created by removal of combustible additives- Addition of
surfactants-fillers in alumina precursor to increase pore
volume/size
Origin & types of pores
K.Kaneko,J.Membrane Science, 96,59,1994
Pore size
Micro
% pore
volume
30 - 60
% surface
area
>95
Meso
< 10
<5
Macro
25 - 30
negligible
Intrinsic pores in zeolites
ME Davis, Nature,412,813, (2002)
Classification of pores
Classification of pores
Classification of pores
Experimental techniques
Definition
The concentration of gases, liquids or dissolved
substances (adsorbate) on the surface of solids
(adsorbent)
Physical vs Chemical
Physical Adsorption (van der Waals adsorption):
weak bonding of gas molecules to the solid;
exothermic (~ 0.1 Kcal/mole);
reversible
Chemisorption:
chemical bonding by reaction;
exothermic (10 Kcal/mole);
irreversible
2015/4/7
Aerosol & Particulate Research
Lab
19
Sorbent Materials
• Activated Carbon
• Activated Alumina
• Silica Gel
• Molecular Sieves (zeolite)
Polar and Non-polar adsorbents
Properties of Activated Carbon
Bulk Density
22-34 lb/ft3
Heat Capacity
0.27-0.36 BTU/lboF
Pore Volume
0.56-1.20 cm3/g
Surface Area
600-1600 m2/g
Average Pore
15-25 Å
Diameter
Regeneration
100-140 oC
Temperature
(Steaming)
Maximum Allowable 150 oC
Temperature
http://www.activatedcarbonindia.com/activate
d_carbon.htm
Air Pollution Engineering Manual., 1992
2015/4/7
Aerosol & Particulate Research
Lab
20
Adsorption Mechanism
2015/4/7
Aerosol & Particulate Research
Lab
21
Measurement of Textural properties
• Adsorption isotherms- v = f (p/po)T
• Adsorbates – N2 Ar, Kr
• Methods – Volumetric – static/dynamic- Manual/automated
Gravimetric
• Samples to be pre-treated to remove adsorbed impurities/moisture
• Different molecules depending upon the size can be used as probes
to elucidate pore structure - Molecular resolution porosimetry
• Isotherms/Isobars/Isosters – ( P,V,T)
Measurement of adsorption
Types of adsorption isotherms -IUPAC
Reveal the type of pores & degree of
adsorbate-adsorbent interactions
IUPAC classification – 6 types of isotherms
Type-I - Microporous solids
Langmuir isotherm
Type-II - Multilayer adsorption on
non-porous / macroporous solids
Type-III - Adsorption on non-porous /macroporous solids with weak adsorption
Type-IV - Adsorption on meso porous solids
with hysteresis loop
Type-V - Same as IV type with weak
adsorbate-adsorbent interaction
Type-VI - Stepped adsorption isotherm, on
different faces of solid
Original classification by Brunauer covers upto Type-5
Types of Isotherms - Brunauer
Origin of Hysteresis
• Normally observed in Type IV & V and sometimes in II &III
• Absence of hysteresis- Type-I Micro porous structure
• At any given value for Va, p/p0 for in desorption branch is lower than
that on adsorption
• Chemical potential of adsorbate during desorption is lower; hence
true equilibrium exists
• Differences in contact angle during ads/des may lead to hysteresis
• Presence of ink-bottle type pores-narrow neck & wide body. This
could mean that adsorption branch represents equilibrium
• Differences in the shape of the meniscus in the case of cylindrical
pores with both ends open
Types of hysteresis loops- de Boer
Hysteresis Loops IUPAC
Surface area by BET method
p/v( p0-p) = 1/vmC + (C-1)p/ Cvmp0 - Plot of p/v(p0-p) Vs p/p0
P0- Sat. pressure; p- actual equilibrium Pressure; Vm-mono layer volume
V- adsorbed vol. at equilibrium pressure p
C- constant signifying adsorbate-adsorbent extent of interaction
Applicable in the range p/p0- 0.05-0.35 & Only from Type II &IV
isotherms
Surface heterogeneity and interactions between adsorbates in adsorbed
state are not accounted for
Slope + Intercept – 1/vm
Surface area = vmN Am/ 22414 x 10-20 m2
N- Avogadro’s number; Am-cross sectional area of adsorbate molecule
Mono layer volume by Point B method in Type II isotherms
Pore geometries- models
t- method of Lippens & deBoer
• Standard isotherms- Plot of Va/Vm Vs p/p0 gives a straight line
• t = 0.354( Va/Vm) = f1(p/p0) – for multilayer adsorption of nitrogen
t is independent of the nature of adsorbent if it is non-porous
• Plot of t Vs Va then passes through origin and the slope of the line
can be used to calculate SA
• st = 1.547 x 106 dVa/dt with t expressed in nm
st Surface area by t-method
• As long as multilayer adsorption takes place, Va –t plot is a straight
line passing through origin
• At higher t values deviations occur;
• Upward deviation – capillary condensation, cylindrical pores, inkbottle type, spheroidal cavities
• Downward deviation- micro pores, with slit shaped geometry
• Higher the pressure at which deviation occurs, the larger the pore
size
αs- method of Sing
• Comparison of experimental isotherm with that of standard one
• Thickness t replaced by a specific Va/Vm ratio for non-porous solid
• Ratio of volume adsorbed at specific p/p0 to volume adsorbed at
p/p0 = 0.4 is designated as αs
• αs= Va/Vm = f(p/p0) ; αs= 1 at p/p0=0.4
• Basis - mono layer coverage completed and multilayer adsn. starts
at p/p0 = 0.4
t - Plots for various pore size ranges
Pore size distribution- BJH method
• Based on Kelvin equation for capillary condensation for spherical
meniscus
• lnp/p0 = -2vλ Cosθ/ rkRT
–
–
–
–
θ- contact angle
λ- surface tension
rk- Kelvin radius
V-molar volume
t
rk
rp
With θ =0,
γ = 8.85.dynes/cm2
V= 34.6 cc/mole
rk = 4.14/ln(p/p0)
• t = 3.5[5/ln(p/p0)]1/3
• Pore radius rp = rk+ t
Model calculations
For cylindrical pores - Gregg & Sing – p .164
For parallel plates - RB Anderson - p.66
Calculation of t, rk & rp
dV = dvf +dvk
dVk= dV-dVf
dVf= 0.064xΔtx ∑dSp
dSp= 31.2 dVp/r*p
dVp= dVk(r*p/r*k)
Micro porous solids
Follow Type I isotherm- Langmuir isotherm
Large uptake of adsorbate at very low pressures, up to p/p0=0.15
BET model applicable up to pores 1 nm
For <1nm Dubinin model applicable
Dubinin- Radushhkevich equation for micro porous solids
log10Va = log10V0 - D( log10X)2
Va- Vol adsorbed per unit mass of adsorbent
V0 – largest volume of adsorbate, total pore volume
X- p/p0 ; D- factor varying with temp & asorbent/adsorbate
Langmuir equation
1/n = 1/nm+ 1/(nmK) X 1/p/p0 n- moles adsorbed per gram of
adsorbent; nm- monolayer volume
Plot of 1/n .Vs. 1/p/p0 gives a straight line with intercept 1/nm
Surface area can be calculated from nm
Total pore volume from the uptake at horizontal plateau
Mercury porosimetry
Intrusion of mercury into the pores by applying pressure
rp= (2 γ/ P) cosθ - γ- Surface tension 480 dynes/ cm
θ - Contact angle, 141
rp = 7260/p with p-atmos. rp -nm
rp= 7x 10-4 cm = 70000Å ; 100Å – 700 atm.; 20Å- 3500 atm.
Pressure range – 0.1 to 400 Kpa
Pore radius – 75000 to 18Å
Pore structure Analysis - Summary
Adsorption Isotherm
Pore size distribution
BET Plot
Pore radius/
Pore volume
Surface
area
Hysteresis Type
Isotherm Type
Pore type, Shape, Geometry
t-Curve