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 Prepa -ration Cycle Character -izattion

Why do we Characterize?

Provides answers to WHY & HOW Integral part of Catalyst development cycle

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

Catalysts- Shape factor

Catalysts- Shape effect

Characterization of Catalysts

Preparation Characterizati on

Concn. of active elements Phase composition

Evaluation

In-situ Spectroscopy Species in Solution phase Solid state transformations Preparation techniques

Evolve active phase

Electronic state Structural features Dispersion & Distribution Surface composition

Ensure desired characteristics

Transient surface species Reactants & Products Kinetics & mechanism

Surface reactions

Ageing

Solid state transformations Structural transformations Surface composition

Catalyst life

Spent

Inactive phases Poisons Analysis of coke

Deactivation & Regeneration Catalysts Characterization

-

From Cradle to Coffin

Geometric shape/size

Textural properties Porous solids

External

Surface area

Internal Catalysts Metals Metal oxides Metal sulfides Metal chlorides Zeolites Heteropoly acids Pore structure Adsorbents Alumina Silica Carbon Mol.sieves

Clays Porosity / Pores

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 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

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

Origin & types of pores

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

Pore size Micro Meso Macro % pore volume 30 - 60 % surface area >95 < 10 < 5 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

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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/ft 3 Heat Capacity Pore Volume Surface Area 0.27-0.36 BTU/lb o F 0.56-1.20 cm 3 /g 600-1600 m 2 /g Average Pore Diameter 15-25 Å Regeneration Temperature 100-140 o C (Steaming) Maximum Allowable Temperature 150 o C

http://www.activatedcarbonindia.com/activate d_carbon.htm

Air Pollution Engineering Manual., 1992

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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)

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 /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

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/p 0 that on adsorption for in desorption branch is lower than • 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( 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 N m 2 Avogadro’s number; A m -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 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 line passing through origin –t plot is a straight • 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

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 of volume adsorbed at specific p/p 0 p/p 0 = 0.4 is designated as α s ratio for non-porous solid to volume adsorbed at • 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

t - Plots for various pore size ranges

Pore size distribution- BJH method

• Based on Kelvin equation for capillary condensation for spherical meniscus • lnp/p 0 = -2v λ 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

r k

V= 34.6 cc/mole = 4.14/ln(p/p 0 )

t r k r p

• •

t

= 3.5[5/ln(p/p 0 )] 1/3

Pore radius r p = r k + t

Model calculations For cylindrical pores - Gregg & Sing – p .164

For parallel plates - RB Anderson - p.66

Calculation of t, r

k

& r

p

dV = dv f +dv k dV k = dV-dV f dV f = 0.064x

Δtx ∑dS p dS p = 31.2 dV p /r* p dV p = dV k (r* p /r* k )

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

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 r p p = 7260/p with p-atmos. r p = 7x 10 -4 -nm cm = 70000 Å ; 100Å – 700 atm.; 20Å- 3500 atm.

Pressure range – 0.1 to 400 Kpa Pore radius – 75000 to 18Å

Pore structure Analysis - Summary

Adsorption Isotherm BET Plot

Surface area

Hysteresis Type Isotherm Type

Pore type, Shape, Geometry

Pore size distribution

Pore radius/ Pore volume

t-Curve