experimental methods in catalysis (emc) m.tech-catalysis technology ii semester ct-503...
TRANSCRIPT
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
19
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
20
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
21
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