HETEROGENEOUS CATALYSIS

AN INTRODUCTION

Paul Ratnasamy

National Chemical Laboratory

Pune-411008, India

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Why R& D in catalysis is important

-27 % of GNP and 90 % of chemical industry involve products made using catalysts (food, fuels, polymers, textiles, pharma/agrochemicals,etc)

-For discovery/use of alternate sources of energy/fuels/ raw material for chem industry.

-For Pollution control-Global warming.- For preparation of new materials (organic &

inorganic-eg: Carbon Nanotubes).

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Catalysis is multidisciplinary (physics,chemistry & chem engg)

• The catalyst is an inorganic solid;Catalysis is a surface phenomenon;solid state and surface structures play important roles.

• Adsorption,desorption and reaction are subject to thermodynamic, transport and kinetic controls(chem engg);

• adsorbate-substrate and adsorbate - adsorbate interactions are both electrostatic and chemical(physical chemistry).

• The chemical reaction is organic chemistry.

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Green Chemistry is Catalysis

• Pollution control(air and waste streams; stationary and mobile)

• Clean oxidation/halogenation processes using O2,H2O2(C2H4O, C3H6O, ECH)

• Avoiding toxic chemicals in industry

( HF,COCl2 etc.)

• Fuel cells( H2 generation)

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Catalysis in Nanotechnology

Methods of Catalyst preparation are most suited for the preparation of nanomaterials .

• Nano dimensions of catalysts.

• Common prep methods.

• Common Characterization tools.

• Catalysis in the preparation of carbon nanotubes.

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Hetrogeneous Catalysis-Milestones in Evolution-1

• 1814- Kirchhoff-starch to sugar by acid.• 1817-Davy-coal gas(Pt,Pd selective but not Cu,Ag,Au,Fe)• 1820s –Faraday H2 + O2 H2O(Pt);C2H4 and S • 1836- Berzelius coins”Catalysis”; • 1860-Deacon’s Process ;2HCl+0.5O2 H2O + Cl2; • 1875-Messel.SO2 SO3 (Pt);• 1880-Mond.CH4+H2O CO+3H2(Ni);• 1902-Ostwald-2NH3+2.5O2 2NO+3H2O(Pt);• 1902-Sabatier.C2H4+H2 C2H6(Ni).• 1905-Ipatieff.Clays for acid catalysed reactions;

isomerisation, alkylation, polymerisation.

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Milestones in Evolution-2• 1910-20: NH3 synthesis (Haber,Mittasch) ; Langmuir• 1920-30-Methanol syn(ZnO-Cr2O3); Taylor;BET • 1930-Lang-Hinsh &Eley -Rideal models ;FTsyn;EO;• 1930-50:Process Engg; FCC / alkylates;acid-base catalysis;Reforming

and Platforming.• 1950-70: Role of diffusion; Zeolites, Shape Selectivity; Bifunctional

cata;oxdn cat-HDS; Syngas and H2 generation.• 1970- Surface Science approach to catalysis(Ertl)• 1990 - Assisted catalyst design using : -surface chem of metals/oxides, coordination chemistry - kinetics,catalytic reaction engg - novel materials(micro/mesoporous materials)

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Catalysis in the Chemical Industry

• Hydrogen Industry(coal,NH3,methanol, FT, hydrogenations/HDT,fuel cell).

• Natural gas processing (SR,ATR,WGS,POX)• Petroleum refining (FCC, HDW,HDT,HCr,REF• Petrochemicals(monomers,bulk chemicals).• Fine Chem.(pharma, agrochem, fragrance,

textile,coating,surfactants,laundry etc)• Environmental Catalysis(autoexhaust, deNOx,

DOC)

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PHYSICAL ADSORPTION• Steps in a catalytic Reaction: - Diffusion of reactant (bulk, Film, surface) - Adsorption( physical chemical) -Surface reaction - Desorption and diffusion of products • Physical Adsorption: - Van der Waals forces;BET surface area • Pore Size distribution ( Wheeler, de Boer, BJH)• Influence of pore size on reaction order,

temperature coefficient, selectivity, Influence of poisons …

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CHEMISORPTION

• Langmuir isotherm; Langmuir –Hinshelwood and Eley- Rideal mechanisms of surface reactions;Kinetics of adsorption-Elovich equation.

Uses of chemisorption (1)probes (H2,CO,NH3, pyridine,CO2) for fraction of catalytically active surface (only 0.1% in cracking);(2)Do chemisorbed species actually participate in reactions(isotope exchange);(3) changes in surface structures on adsorption(S, H2, O2, H2O2…).

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The Sabatier Principle

“There is an optimum of the rate of a catalytic reaction as a function of the heat of adsorption”- Sabatier,1905: If the adsorption is too weak,the catalyst has little effect;If too strong, the adsorbates will be unable to desorb from the surface;Hence,the interaction between reactants or products with surface should be neither too strong nor too weak.

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Sabatier Principle -Optimal basicity results in high carbonate yields (MMM 90(2006)314)

340 360 380 400 420 440 460 480

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40

60

80

100

393 K - React. temp.

Chemisorbed - pri. amine

Chemisorbed - sec. amine

Chemisorbed - tert. amine

Physisorbed - surfaceC

hlo

rop

rop

ene

carb

onat

e yi

eld

(%

)

CO2 desorption temperature (K)

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How catalysts accelerate rates of chemical reactions

• H2+0.5O2 H2O; G 0298 = -58 Kcal/mol;

In the gas phase:• D(H-H) = 103 and D(O-O)=117 Kcal/mol;• E# ~ 10 Kcal/mol for H+O2 or H2+O HO2 or

H2O.Hence,kinetically gas-phase reaction improbable. Pt forms Pt-H and Pt-O bonds with E# ~ 0;Moreover,

Pt-H + Pt-O Pt-OH Pt -OH2 has E# ~ 0 .

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Turnover frequencies, Rates and numbers

CATALYSIS IS A KINETIC PHENOMENONSequence of elementary steps in steady state: diffusion

(bulk,film,surface) - adsorption-reaction-desorption-diffusion

TOF= number of product molecules formed per unit area per sec(molecules.cm-2.sec-1)

TOF= number of product molecules formed per active site per sec(molecules.sec-1) only if active site is known.

TOT= 1/TOF = turnover time, time necessary to form a product molecule(sec);

TOR = Turnover rate = TOF X Surface areaTON= TOF X total reaction time;TON=1( stoichiometry); TON must be >100 to be industrially useful.

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Conversions,Rates and Rate constants

• Conversion = % Reactant converted;

• Reaction rate = kp X f(Pi) or kc X f(Ci)

• k = Aexp(-E#/RT);A is temp independent.• TOFs between 0.0001 and 100 in industry; Temp

adjusted to get the desired rates. E# ~ 35-45 Kcal/mol for isom,cyclisation,

cracking,dehydo/hydrogenolysis;HighT needed. E# ~ 6-12 Kcal/mol for hydrogenation;

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The Compensation Effect

• k = A exp(-E#/RT); • For a given reaction, over different catalysts, A

increases linearly with E# so that k remains constant:

ln A = + (E# / R ); is a constant and is the isokinetic temp,when the rates on all catalysts are equal; A plot of ln A vs E# gives a linear plot with +ve slope.

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Compensation effect for the methanation reactionLogarithm of preexponential factor vs apparent

activation energy

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The Active SiteH.S.Taylor,Proc Roy Soc (London)A108(1925)105

• “There will be all extremes between the case in which all the atoms in the surface are active and that in which relatively few are so active “.

• “The amount of surface which is catalytically active is determined by the reaction catalyzed”.

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Active Sites-Metals:Structure sensitivity of Catalytic reactions over metals

• Structure Sensitive if rate changes markedly when crystallite/particle size is changed; “active site” comprises ensemble of many metal atoms;steps & edges. eg:hydrogenolysis,H2-D2 exch, steam reform,coking, aromatization etc

• Structure Insensitive if rate is independent of crystallite /particle size; each surface metal atom is a potential active site; example: hydrogenation, dehydrogenation

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Active Sites-Oxides /Sulfides.Catalysis by Ions at surfaces

• Bronsted & Lewis acids in solution

• Solid acid catalysts-Historical(acid-washed clays for cat cracking)

• L acidity of ions:Na+< Ca 2+<Y3+<Th 4+.

increases with charge/radius ratio.

• B acidity by ion substitution (Al for Si) in clays, zeolites, Al phosphates etc.

• Acidity measurement ( Total, L & B ).

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Heterolytic adsorption on Ionicoxide surfaces

Oxide Surface: M+ -O - - M+ - O - - M+

Lewis acid(e- acceptor) Bronsted base(H acceptor)

H+ H-

H2: M+- O - - M+ - O - - M+

H+ OH-

H2O: M+ -O --M+ - O - - M+ ( B acid and B base)

C2H5 H H OCH3

C2H6 & CH3OH: M+ -O - - M+ - O - - M+

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“Life Cycle” of a Catalyst• Catalyst Preparation• Activation• Surface reconstruction during catalytic

run - Beneficial-Sulfiding of Re in PtRe - Harmful (carbon formation)• Deactivation( poisons,coke, SA loss,

leaching)• Regeneration• Catalyst Unloading

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Activity, Selectivity, Stability and Accessibility

• High activity per unit volume .• High selectivity for desired product at

adequate conversion level (STY for product > 1mol / ml/sec)

• High Accessibility;Role of transport rates of mass and heat.

• Long life time; Regenerability.• Thermal/mechanical strength in reaction

conditions(sintering,crushing,attrition)• Reproducible/economic/safe manufacture.

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CATALYST CHARACTERIZATION

• Bulk Physical Properties

• Bulk Chemical properties

• Surface chemical properties

• Surface Physical Properties

• Catalytic Performance

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Bulk Chemical Properties• Elemental composition( of the final

catalyst ), EPMA

• XRD,electron microscopy (SEM,TEM).

• Thermal Analysis(DTA/TGA).

• NMR/IR/UV-Vis/ EPR/ Mossbauer

• TPR/TPO/TPD

• EXAFS

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Surface Properties• XPS,Auger, SIMS(bulk & surface

structure).• Texture :Surface area- porosity.• Counting “Active” Sites:

-Selective chemisorption (H2,CO,O2, NH3, Pyridine,CO2);Surface reaction (N2O).

• Spectra of adsorbed species (IR/EPR/ NMR / EXAFS etc)

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Physical properties of formulated catalysts

• Bulk density

• Crushing strength & attrition loss (comparative)

• Particle size distribution

• Porosimetry( micro(<2 nm),macro(>35 nm) and meso.

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Catalyst Activity Testing :Definitions- Activity

• Activity may be expressed as: -Rate constants or TON from kinetics -Rates/weight -Rates/volume -Conversions at constant P,T,and SV. - Temp required for a given conversion at constant

partial & total pressures - Space velocity required for a given conversion at

constant pressure and temp

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Catalyst Activity TestingDefinitions- Selectivity

• Selectivity = % concentration of product(s) among all the products excluding coke.

• Yield = conversion X selectivity.• Selectivities may depend on T,P,SV,diffusion,

catalyst particle size and shape , reactor geometry etc.

• Always compare selectivities at constant T,P and most important,conversion.

• Selectivity w.r.t. each of the reactants(H2O2).

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Catalyst Testing- 1

• What is the objective ?Testing a solid for its catalytic properties in many reactions?screening for a particular reaction? Exploring Kinetics?Industrial development?

• Activity;comparison at non-diffusion & non-thermodynamically limited, kinetically controlled conditions;

• 10-20 mesh;dreactor >10diacat(wall effects)• Bed length/ dreactor >5 to avoid channeling;• Comparison of Selectivity at similar activity;

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Catalyst Testing-2

Only at intermediate conversions and at low temp can the quality of the catalyst, expressed in an optimum of kinetically controlled conversion,be analyzed.At high temp or at high conversions,all catalysts are almost equal for either slow kinetic control or thermodynamically limited conversion.

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Start-Up Procedures Affect Catalyst Performance

Activated Rapidly

Activated as per manfacturers instruction

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Temperature dependence of catalytic activity

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Catalyst Preparation & Formulation -1

Catalyst Formulation - Size and shape is a compromise between the wish

to minimize pore diffusion effects( small size)and pressure drop( large size);

- Pelleting,extrusion,granulation,spray drying; Choice depends on properties of powder, size/shape/density/ required strength of catalyst particle;

-Loading of graded sized pellets.

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Catalyst Preparation & Formulation-2

• Unsupported Metals

- very high activity(small area adequate )

- High purity feedstock

eg: NH3 NO ( Pt-Rh gauze).

CH3OH HCHO (Ag granules)

- Raney Ni,Co,Cu for H2 ion (residual Al2O3 present!).

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Catalyst Preparation & Formulation- 3

• Fused catalysts.

eg: Triply promoted Fe ( + Ca,K,Al as oxides) catalyst for NH3 synthesis.

Fe3O4 + H2(N2 +H2) Fe(1600C)

Melt the mixture at 1600 C,cool,crush,size.

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Catalyst Preparation & Formulation- 4

• Wet methods of catalyst manufacture:

(A) Precipitation :pH of precipitating medium critical !!

(B)Precipitation-deposition: texture of support important.

Influence of Ageing,digestion; filterability;

washability of salts;

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The pH of precipitation affects chemical composition, particle size and other physical

properties of Cu/ZnO/Al2O3 WGS shift catalyst

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Catalyst Preparation & Formulation- 5

• Supported Metal(especially noble metals) Catalysts:• Used Extensively in industry: -autoexhaust, diesel oxidation, DeNOx, stationary

power sources - Hydrocracking,Naptha reforming,xylene isom,

isomerisations, Hydrogenations, etc - Fuel cell catalysts - Major issues: high cost and loss of activity due to

sintering .

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Why the need for high dispersion of PM

• PM are expensive: hence impregnation and not coprecipitation

• Activity depends on metal surface area (MSA)

• MSA increases with dispersion

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Metal Dispersion

• Metal Dispersion, D = No of Pt surface atoms / No of Total Pt atomsD is an operational definition (defined by technique used)N total= from chemical compositionN surface is obtained by physical or chemical methodsPhysical methods: Crystallite size from XRD, SEM/TEM

Chemical methods: Chemisorption of H2, CO, H2-O2 titration

PM distribution Profiles

a.Uniformb.Egg shellc.Egg whited.Egg yolk

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PM distribution profiles• Optimal dispersion depends on

– reaction kinetics and mode of catalyst poisoning

– Attrition strength of catalyst

– Egg shell favors • Reactions with positive order

• Fast reactions

- Egg Yolk favors• Reactions with negative order

- Pore mouth poisoning egg white or egg yolk

- Low attrition strength egg white or egg yolk

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Factors affecting dispersion of PM -1

1. Concentration of PMa. Low concentration – high dispersion

2. Presence of competing ions in impregnating solution increases D.a. Citric acid in H2PtCl6 impregnation on Al2O3 platforming)

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Factors influencing dispersion of PM -2

3. Functional groups on substrate surface for binding the PM precursor – Point of zero charge (PZC) influences dispersion of PM

Anions and neutral complexes disperse better on gamma Al2O3 at pH<8

PZC gamma alumina=8-9; SiO2~3

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Factors influencing dispersion of PM -3

4. Crystallite size of substrate

Al2O3, CeO2, CZO, TiO2 etc

Small crystallite sizes have large dispersion

5. Partially reducible oxide supports increase D eg Pt-CeO2

6. Ion exchange of PM increases D, eg: Pt in zeolites

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Sintering of PM

• Leads to lower dispersion, MSA and activity• Increases with PM loading

• Increases with T, TOS, H2O, O2, S, Cl

• Increases with crystallite size of support

• Increases with hydrophobicity of support (Pt-SiO2 sinters more than Pt-Al2O3)

• Suppressed by spacers (ZrO2 in CZO)

• Suppressed by “binding” groups on surface (OH, Cl-, SO3H- etc)

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Reverse Micro Emulsion (RME) method enables use of lower amount of Pt in DeNOx

• Nissan WO 2005/063391A1, PCT WO 2006/067912 A1 and othersPCT WO 2006/067912 A1 and others

• Catalyst was first used in a Nissan engine using gasoline fuel for 30 hrs at 700ºC.

• After engine durability test for 50 hrs at 70ºC, catalyst was tested in test rig at 350ºC for DeNOx activity.

• Catalyst=100g/l in honeycomb; Pt-Co(Ce)-Al2O3

Method Pt Co Ce Alsource NOx convRME 0.5 5 NA iso prop 49

Conventional impregnation-1 3 NA 5 Al2O3 49

Conventional impregnation-2 3 5 NA Al2O3 50

At 350ºC after endurance test at 700ºC for 30 hrs

0.5% Pt is as effective as 3%wt Pt

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Some Developments in Industrial catalysis-1

1900- 1920sIndustrial Process Catalyst

1900s:CO + 3H2 CH4 + H2O Ni

Vegetable Oil + H2 butter/margarine Ni1910s:Coal Liquefaction Ni

N2 +3 H2 2NH3 Fe/K

NH3 NO NO2 HNO3 Pt

1920s: CO +2 H2 CH3OH (HP) (ZnCr)oxide Fischer-Tropsch synthesis Co,Fe

SO2 SO3 H2SO4 V2O5

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Heterogeneous Catalysis.Some Challenges Ahead

• Selective oxdn of long chain paraffins to terminal alcohols/ald/acids;

• CH4 CH3OH.

• Activation of CO2 & its use as raw material;

CO2 + H2O/ CH3OH/C2H5OH C2 +

• Chiral catalysis with high ee.

• H2 generation from H2O without using HC .

• Photocatalysis with Sunlight.

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Industrial catalysis-21930s and 1940s

1930s:Cat Cracking(fixed,Houdry) Mont.Clay

C2H4 C2H4O Ag

C6H6 Maleic anhydride V2O5

1940s:Cat Cracking(fluid) amorph. SiAl

alkylation (gasoline) HF/acid- clay

Platforming(gasoline) Pt/Al2O3

C6H6 C6H12 Ni

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Industrial catalysis-3 1950s

C2H4 Polyethylene(Z-N) Ti

C2H4 Polyethylene(Phillips) Cr-SiO2 Polyprop &Polybutadiene(Z-N) Ti

Steam reforming Ni-K- Al2O3

HDS, HDT of naphtha (Co-Mo)/Al2O3

C10H8 Phthalic anhydride (V,Mo)oxide

C6H6 C6H12 (Ni)

C6H11OH C6H10O (Cu)

C7H8+ H2 C6H6 +CH4 (Ni-SiAl)

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Industrial catalysis-4 1960sButene Maleic anhydride (V,P) oxides

C3H6 acrolein (BiMo)oxides

C3H6 acrylonitrile(ammox) -do-

Bimetallic reforming PtRe/Al2O3

Metathesis(2C3 C2+C4) (W,Mo,Re)oxidesCatalytic cracking Zeolites

C2H4 vinyl acetate Pd/Cu

C2H4 vinyl chloride CuCl2

O-Xylene Phthalic anhydride V2O5/TiO2

Hydrocracking Ni-W/Al2O3

CO+H2O H2+CO2 (HTS) Fe2O3/Cr2O3/MgO

--do-- (LTS) CuO-ZnO- Al2O3

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Industrial catalysis-5 1970s

Xylene Isom( for p-xylene) H-ZSM-5

Methanol (low press) Cu-Zn/Al2O3

Toluene to benzene and xylenes H-ZSM-5

Catalytic dewaxing H-ZSM-5

Autoexhaust catalyst Pt-Pd-Rh on oxide

Hydroisomerisation Pt-zeolite

SCR of NO(NH3) V/ Ti

MTBE acidic ion exchange resin

C7H8+C9H12 C6H6 +C8H10 Pt-Mordenite

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Industrial catalysis-6

1980sEthyl benzene H-ZSM-5Methanol to gasoline H-ZSM-5Vinyl acetate PdOxdn of t-butanol to MMA Mo oxidesImproved Coal liq NiCo sulfidesSyngas to diesel CoHDW of kerosene/diesel.GO/VGO Pt/ZeoliteMTBE cat dist ion exchange resinCyclar Ga-ZSM-5Oxdn of methacrolein Mo-V-P heteropolyacid N-C6 to benzene Pt-L zeolite

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Industrial catalysis-7

1990+

DMC from acetone Cu chloride

NH3 synthesis Ru/CPhenol to HQ and catechol TS-1Isom of butene-1(MTBE) H-FerrieriteAmmoximation of cyclohexanone TS-1 Isom of oxime to caprolactam TS-1Ultra deep HDS Co-Mo-AlOlefin polym Supp. metallocene catsEthane to acetic acid Multi component oxide Fuel cell catalysts Rh, Pt, ceria-zirconiaCr-free HT WGS catalysts Fe,Cu- based

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Industrial catalysis-8 2000 +

• Solid catalysts for biodiesel

- solid acids, Hydroisom catalysts

• Catalysts for carbon nanotubes

- Fe (Ni)-Mo-SiO2

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ACKNOWLEDGEMENT

• Members of the catalysis division at NCL