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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
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Catalysts- Functionalities Basic Activity Selectivity Stability Applied Manufacturing Aging Deactivation Regenerability Evalua -tion Character -izattion Prepa -ration Catalyst Development Cycle Why do we Characterize? Provides answers to WHY & HOW Integral part of Catalyst development cycle
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Catalysts-Characteristics Chemical composition Active elements, promoters, stabilizers Structural features Crystalline/Amorphous, Crystal structure Phase composition, Phase transformations- TiO 2— 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
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Catalysts- Shape factor
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Catalysts- Shape effect
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Characterization of Catalysts PreparationCharacterizati on EvaluationAgeingSpent Concn. of active elements Phase composition In-situ Spectroscopy Solid state transformations Inactive phases Species in Solution phase Electronic stateTransient surface species Structural transformations Poisons Solid state transformations Structural featuresReactants & Products Surface composition Analysis of coke Preparation techniques Dispersion & Distribution Kinetics & mechanism Surface composition Evolve active phase Ensure desired characteristics Surface reactions Catalyst lifeDeactivation & Regeneration Catalysts Characterization- From Cradle to Coffin
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Textural properties CatalystsAdsorbents Metals Metal oxides Metal sulfides Metal chlorides Zeolites Heteropoly acids Alumina Silica Carbon Mol.sieves Clays Surface area Pore structure Pore size-Area-Volume-Distribution-Geometry Textural properties Porous solids External Internal Geometric shape/size Porosity / Pores
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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
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Origin of pores Crystal structure- Intrinsic voids A tomic/molecular Preparation- Voids due to leaving groups Hydroxides, carbonates, Oxalates- Ni(OH) 2, MgCO 3, ZnC 2 O 4 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
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Origin of pores Pores Inherent in any solid structure Intrinsic intra particle pores Voids 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 Al 2 O 3 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
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Origin & types of pores K.Kaneko,J.Membrane Science, 96,59,1994
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Pore size% pore volume % surface area Micro30 - 60>95 Meso< 10< 5 Macro25 - 30negligible
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Intrinsic pores in zeolites ME Davis, Nature,412,813, (2002)
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Classification of pores
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Experimental techniques
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2015/5/4Aerosol & 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); reversible Chemisorption: chemical bonding by reaction; exothermic (10 Kcal/mole); irreversible Physical vs Chemical
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2015/5/4Aerosol & 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 Carbon Bulk Density22-34 lb/ft 3 Heat Capacity0.27-0.36 BTU/lb o F Pore Volume0.56-1.20 cm 3 /g Surface Area600-1600 m 2 /g Average Pore Diameter 15-25 Å Regeneration Temperature (Steaming) 100-140 o C Maximum Allowable Temperature 150 o C http://www.activatedcarbonindia.com/activate d_carbon.htm
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2015/5/4Aerosol & Particulate Research Lab 21 Adsorption Mechanism
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Measurement of Textural properties Adsorption isotherms- v = f (p/p o ) T Adsorbates – N 2 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)
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Measurement of adsorption
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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 /macro- porous 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
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Types of Isotherms - Brunauer
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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/p 0 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
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Types of hysteresis loops- de Boer
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Hysteresis Loops IUPAC
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Surface area by BET method p/v( p 0 -p) = 1/v m C + (C-1)p/ Cv m p 0 - Plot of p/v(p 0 -p) Vs p/p 0 P 0 - Sat. pressure; p- actual equilibrium Pressure; V m -mono layer volume V- adsorbed vol. at equilibrium pressure p C- constant signifying adsorbate-adsorbent extent of interaction Applicable in the range p/p 0 - 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/v m Surface area = v m N A m / 22414 x 10 -20 m 2 N- Avogadro’s number; A m -cross sectional area of adsorbate molecule Mono layer volume by Point B method in Type II isotherms
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Pore geometries- models
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t- method of Lippens & deBoer Standard isotherms- Plot of V a /V m Vs p/p 0 gives a straight line t = 0.354( V a /V m ) = f 1 (p/p 0 ) – for multilayer adsorption of nitrogen t is independent of the nature of adsorbent if it is non-porous Plot of t Vs V a then passes through origin and the slope of the line can be used to calculate SA s t = 1.547 x 10 6 dV a /dt with t expressed in nm s t Surface area by t-method As long as multilayer adsorption takes place, V a –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
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α s - method of Sing Comparison of experimental isotherm with that of standard one Thickness t replaced by a specific V a /V m ratio for non-porous solid Ratio of volume adsorbed at specific p/p 0 to volume adsorbed at p/p 0 = 0.4 is designated as α s α s = V a /V m = f(p/p 0 ) ; α s = 1 at p/p 0 =0.4 Basis - mono layer coverage completed and multilayer adsn. starts at p/p 0 = 0.4
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t - Plots for various pore size ranges
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Pore size distribution- BJH method Based on Kelvin equation for capillary condensation for spherical meniscus lnp/p 0 = -2vλ Cosθ/ r k RT –θ- contact angle –λ- surface tension –r k - Kelvin radius –V-molar volume With θ =0, γ = 8.85.dynes/cm2 V= 34.6 cc/mole r k = 4.14/ln(p/p 0 ) t = 3.5[5/ln(p/p 0 )] 1/3 Pore radius r p = r k + t rprprprp rkrkrkrk t Model calculations For cylindrical pores - Gregg & Sing – p.164 For parallel plates - RB Anderson - p.66
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Calculation of t, r k & r p
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dV = dv f +dv k dV k = dV-dV f dV f = 0.064xΔtx ∑dS p dS p = 31.2 dV p /r* p dV p = dV k (r* p /r* k )
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Micro porous solids Follow Type I isotherm- Langmuir isotherm Large uptake of adsorbate at very low pressures, up to p/p 0 =0.15 BET model applicable up to pores 1 nm For <1nm Dubinin model applicable Dubinin- Radushhkevich equation for micro porous solids log 10 V a = log 10 V 0 - D( log 10 X) 2 V a - Vol adsorbed per unit mass of adsorbent V 0 – largest volume of adsorbate, total pore volume X- p/p 0 ; D- factor varying with temp & asorbent/adsorbate Langmuir equation 1/n = 1/n m + 1/(n m K) X 1/p/p 0 n- moles adsorbed per gram of adsorbent; n m - monolayer volume Plot of 1/n.Vs. 1/p/p 0 gives a straight line with intercept 1/n m Surface area can be calculated from n m Total pore volume from the uptake at horizontal plateau
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Mercury porosimetry Intrusion of mercury into the pores by applying pressure r p = (2 γ/ P) cosθ - γ- Surface tension 480 dynes/ cm θ - Contact angle, 141 r p = 7260/p with p-atmos. r p -nm r p = 7x 10 -4 cm = 70000Å ; 100Å – 700 atm.; 20Å- 3500 atm. Pressure range – 0.1 to 400 Kpa Pore radius – 75000 to 18Å
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Pore structure Analysis - Summary Adsorption Isotherm BET Plot Isotherm Type Pore size distribution Hysteresis Type t-Curve Surface area area Pore radius/ Pore volume Pore type, Shape, Geometry
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