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- FunctionalitiesCatalysts- Functionalities

BasicActivitySelectivityStability

AppliedManufacturingAgingDeactivationRegenerability

Evalua-tion

Character-izattion

Prepa-ration

CatalystDevelopment

Cycle

Why do we Characterize?Why do we Characterize? Provides answers to WHY & HOW Integral part of Catalyst development cycle

Catalysts-CharacteristicsCatalysts-CharacteristicsChemical composition

Active elements, promoters, stabilizersStructural features

Crystalline/Amorphous, Crystal structurePhase composition, Phase transformations- TiO2—Anatase/Rutile

Surface PropertiesComposition, -Bulk Vs Surface, in-situ techniquesCo-ordination, Geometry/ Structure- Spectroscopic methods

Dispersion & distribution of active phasesConcentration profile, Crystallite size

Electronic propertiesRedox character, Chemisorption

Textural propertiesSurface area, Pore volume, Pore-size & distribution

Physical propertiesSize, Shape, Strength

Chemical properties Surface reactivity/Acidity/Basicity

Enabling Structure-Activity correlationsEnabling Structure-Activity correlations

Catalysts- Shape factorCatalysts- Shape factor

Catalysts- Shape effectCatalysts- Shape effect

Characterization of CatalystsCharacterization of Catalysts

Preparation Characterization

Evaluation Ageing Spent

Concn. of active elements

Phase composition

In-situ Spectroscopy

Solid state transformations

Inactive

phases

Species in Solution phase

Electronic state Transient surface species

Structural transformations

Poisons

Solid state transformations

Structural features Reactants & Products

Surface composition

Analysis

of coke

Preparation techniques

Dispersion & Distribution

Kinetics & mechanism

Surface composition

Evolve active phase

Ensure desired characteristics

Surface reactions

Catalyst life Deactivation & Regeneration

Catalysts Characterization- From Cradle to Coffin

Textural propertiesTextural properties

Catalysts Adsorbents

MetalsMetal oxidesMetal sulfidesMetal chloridesZeolitesHeteropoly acids

AluminaSilicaCarbonMol.sievesClays

Surface area

Pore structure

Pore size-Area-Volume-Distribution-Geometry

Textural propertiesTextural properties

Porous solids

External InternalGeometric shape/size

Porosity /Pores

Textural properties- SignificanceTextural 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 timeSelectivity

Extent of coking

Thermal & mechanical stability

Textural properties-Integral part of catalyst architectureTextural properties-Integral part of catalyst architecture

Origin of poresOrigin of poresCrystal structure- Intrinsic voids

Atomic/molecular

Preparation- Voids due to leaving groupsHydroxides, carbonates, Oxalates- Ni(OH)2, MgCO3, ZnC2O4

Structural modifications-Intercalation/PillaringGraphite/ Clay

Aggregation/Coalescence- PreparationFormation of secondary particles from primary particlesFlexible pores- dispersion of particles

Agglomeration/Sintering- Pre-treatmentsRigid pores

CompactingShaping

Origin of poresOrigin of pores

Pores Inherent in any solid structure

Intrinsic intra particle poresVoids created by specific arrangement of atoms / molecules- Zeolites- 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 poresOrigin & types of pores

K.Kaneko,J.Membrane Science, 96,59,1994

Pore size % pore volume

% surface area

Micro 30 - 60 >95

Meso < 10 < 5

Macro 25 - 30 negligible

Intrinsic pores in zeolitesIntrinsic pores in zeolites

ME Davis, Nature,412,813, (2002)

Classification of poresClassification of pores

Classification of poresClassification of pores

Classification of poresClassification of pores

Experimental techniquesExperimental techniques

112/04/18 Aerosol & Particulate Research Lab

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Definition

The concentration of gases, liquids or dissolved substances (adsorbate) on the surface of solids (adsorbent)

Physical Adsorption (van der Waals adsorption): weak bonding of gas molecules to the solid; exothermic (~ 0.1 Kcal/mole); reversibleChemisorption: chemical bonding by reaction; exothermic (10 Kcal/mole); irreversible

Physical vs Chemical

112/04/18 Aerosol & Particulate Research Lab

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Sorbent Materials• Activated Carbon• Activated Alumina

Air Pollution Engineering Manual., 1992

• Silica Gel

• Molecular Sieves (zeolite)

Polar and Non-polar adsorbents

Properties of Activated CarbonBulk Density 22-34 lb/ft3

Heat Capacity 0.27-0.36 BTU/lboFPore Volume 0.56-1.20 cm3/gSurface Area 600-1600 m2/gAverage Pore Diameter

15-25 Å

Regeneration Temperature (Steaming)

100-140 oC

Maximum Allowable Temperature

150 oC

http://www.activatedcarbonindia.com/activated_carbon.htm

112/04/18 Aerosol & Particulate Research Lab

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Adsorption Mechanism

Measurement of Textural propertiesMeasurement 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 adsorptionMeasurement of adsorption

Types of adsorption isotherms -IUPACTypes of adsorption isotherms -IUPACReveal the type of pores & degree of adsorbate-adsorbent interactions

IUPAC classification – 6 types of isotherms

Type-I - Microporous solids Langmuir isothermType-II - Multilayer adsorption on non-porous / macroporous solidsType-III - Adsorption on non-porous /macro- porous solids with weak adsorptionType-IV - Adsorption on meso porous solids with hysteresis loopType-V - Same as IV type with weak adsorbate-adsorbent interactionType-VI - Stepped adsorption isotherm, on different faces of solid

Original classification by Brunauer covers upto Type-5

Types of Isotherms - BrunauerTypes of Isotherms - Brunauer

Origin of HysteresisOrigin 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 BoerTypes of hysteresis loops- de Boer

Hysteresis Loops IUPACHysteresis Loops IUPAC

Surface area by BET methodSurface 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 volumeV- adsorbed vol. at equilibrium pressure pC- constant signifying adsorbate-adsorbent extent of interaction

Applicable in the range p/p0- 0.05-0.35 & Only from Type II &IV isothermsSurface 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 moleculeMono layer volume by Point B method in Type II isotherms

Pore geometries- modelsPore geometries- models

t- method of Lippens & deBoert- 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, ink-bottle type, spheroidal cavities

• Downward deviation- micro pores, with slit shaped geometry

• Higher the pressure at which deviation occurs, the larger the pore size

ααss- method of Sing- 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 rangest - Plots for various pore size ranges

Pore size distribution- BJH methodPore 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 volumeWith 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

rrpprrkk

tt

Model calculationsFor cylindrical pores - Gregg & Sing – p .164For parallel plates - RB Anderson - p.66

Calculation of t, rCalculation of t, rkk & r & rpp

dV = dvf +dvk

dVk= dV-dVf

dVf= 0.064xΔtx ∑dSp

dSp= 31.2 dVp/r*p

dVp= dVk(r*p/r*k)

Micro porous solidsMicro porous solidsFollow 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 porosimetryMercury porosimetryIntrusion 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 KpaPore radius – 75000 to 18Å

Pore structure Analysis - SummaryPore structure Analysis - Summary

Adsorption Isotherm

BET Plot

Isotherm Type

Pore size distribution

Hysteresis Type t-Curve

SurfaceSurface areaarea

Pore radius/Pore radius/Pore volumePore volume

Pore type, Shape, GeometryPore type, Shape, Geometry