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1

NIOK CAIA November 2009 1

University of TwenteHeterogeneous

CatalysisIntroduction

By Leon LeffertsAcknowledgement Hans Niemantsverdriet

Faculty of Science and Technology, IMPACT

Catalytic Processes & Materials


University of Twente


University of Twente Introduction• Catalysis: definition

– influence on reaction rates– thermodynamics not changed

– catalyst not consumed

A Bk1

k-1

K=k1/k-1

K constantk1 and k-1 vary

2



Catalysis relevant for society ?


University of Twente Branches of Catalysis

• Bio-catalysis– living cells– enzymes

• Homogeneous catalysis

• Heterogeneous catalysis


University of TwenteAssignment I:

• define your branch• provide one practical example in two

other branches• Stick to flap-over

– Bio-catalysis• living cells• enzymes

– Homogeneous catalysis– Heterogeneous catalysis

3


University of Twente Examplesl Biocatalysis

» living cells» enzymes

l Homogeneous catalysis

l Heterogeneous catalysis

•beer

•wine

•cheese

•bread

•Detergents

•Water clearance



bonding

reaction

separation

enzyme

substrate 1

substrate 2

product

Enzymes: Nature’s Catalysts


University of Twente Examples• Biocatalysis

– living cells– enzymes

• Homogeneous catalysis• Heterogeneous catalysis

Practical examples?

CH3OH+CO acetic acid Rh, Ir

Propene + CO/H2 butanal Co, Rh

Oxidation xylene and toluene Co, Cu

Polymerization of olefins

OMO Power Mn2complexNN

NiBr Br

4


University of Twente Examples

• Biocatalysis– living cells– enzymes

• Homogeneous catalysis

• Heterogeneous catalysis

Practical examples?

•Auto-exhaust catalyst

•Cracking heavy oil fractions

•NH3 production

•NOx removal with NH3

•Hydrogenation edible oils

•Ziegler-Natta polymerisation

supported catalyst


University of Twente Summary branches

Bio

Homo

Hetero

Cells

Enzymes

Synzymes

Complexes

Immobilized complexes

Solids



ContinuousBatchType of process

EasyMay be problematicProduct separation

Mainly physical chemistry and chemical engineering,

ChemistryScientific discipline

Bulk chemicals, fuels, environmental clean up.

Fine chemicals, specialtiesApplication Large processesMany small processesScale of applicationLessHighMechanistic understandingLow (via promotors)High (via ligands)TunabilityNot necessaryExpensive Catalyst recovery

HighNoneDiffusion problemsHighLowSensitivity to poisonsLong (most cases)VariableService life of catalystHarshMildReaction conditionsVariableHighSelectivityVariableHighActivity

Gas or liquidGenerally liquidMediumSolid Molecule, complex;Catalyst

HeterogeneousCatalysis

Homogeneous Catalysis

Property

Comparison homogeneous and heterogeneous catalysis

5


University of TwenteLanguage differences…

adsorptionassociationDissociative adsorptionOxidative additionMetal… ion,oxide,compoundmetalmetalMetal blackLangmuir HinshelwoodHougon-WatsonDesorptionDecompositionReactantSubstrate

HeterogeneousCatalysis

Homogeneous Catalysis


Catalytic Cycle and Elementary StepsCatalytic Cycle and Elementary StepsLanguage differencesLanguage differences

++ HHYY- L

+ L

Ln-2MHH

YY

RLn-2M

YY

RHH

-- LL

Ln-1MHH

YY

MMLLnn MMLLnn--11

R YYHH

dissociationdissociation

ν = 18, 16ν = 18, 16 ν = 16, 14ν = 16, 14

ν = 18, 16ν = 18, 16

ν = 16, 14ν = 16, 14

ν = 16, 14ν = 16, 14

Dissociationdesorption

AssociationAssociative adsorptionν = 18, 16ν = 18, 16

oxidative additionoxidative additionDissociative adsorptionDissociative adsorption

reductive eliminationreductive eliminationdesorptiondesorption

insertioninsertionassociationassociation

ν = 18, 16ν = 18, 16

Catgen_sw 1


University of TwenteWhy do we use catalysts?

6


University of TwenteWhy do we use catalysts?

• Process intensification– Smaller volumes – Lower T and P

• Selectivity– Catalyze the right reaction– Limit waste– Limit feedstock consumption– Limit separation cost (variable, investment,

energy)



A catalyst enhances the rate of reaction

enables reactions under practical conditions

Major applications:

• Oil processing, fuels

• Production of bulk and fine chemicals

• Environmental pollution control



The Importance of Catalysis

• 90% all chemicals at least one catalytic process

• Catalyst business 20.000 M€ (1999)

Chemicals Refining

Environment

7


University of Twente Importance of Chemical Industry for NL (2008)

• € 50 billion

• 66.000 employees

• € 1.1 billion in R&D

• Trade balance + €17 billion

• 10% of employment

• 15% of production

• 16% of GDP

• 17% of the export

• 25% of R&D spending

http://www.vnci.nl


University of Twente The European Chemical Industry

http://www.cefic.org

• Dfl 470 Billions

• 2 million employees

• 40.000 companies


University of Twente EU


http://www.vnci.nl



8













9


University of Twente Wrap up (1)

• Thermodynamics versus kinetics• Bio-hetero-homo• Economic driver

Next: heterogeneous catalysts


University of TwenteWhat does a catalyst

look like ?

Surface phenomenon:• small particles• on an inert, porous support

(silica, alumina, carbon)

gas in gas outreactor

supported catalyst


University of Twente Supported catalyst

Why is support necessary?

10


University of TwenteHeterogeneous catalysts

• Adsorption• Reaction• Mass and

heat transfer

Externaldiffusion

Inte

rnal

di

ffus

ion

Adsorption

Surface reaction

Aa d d

surface.

a

A + +B

B C

C D

D


University of Twente Truly heterogeneous



Catalysis+reactors+processes

microscopic

catalyticsurface

catalytically active particles on a support

shaped catalyst particles

catalyst bed in a reactor

1 nm

10 mm

1 µm

1 m

mesoscopic macroscopic

I. Chorkendorff & J.W. Niemantsverdriet, Concepts of Modern Catalysis and Kinetics, Wiley-VCH, 2003.

11



pellets

extrudates

fused catalyst

Courtesy of Haldor Topsoe A/S

Shaped catalysts


University of Twente Net

e.g. NH3combustion to NOx:•high T•short contact time


University of Twente Typical applications

• In fuels, chemicals and environmental• Historical perspective

12


University of Twente Oil ProcessingCatalytic cracking • largest scale application of catalysis• zeolites

Hydrotreating• removal of S, N, metals• molybdenum sulfide-based catalysts

Reforming • increasing octane number of gasoline• isomerisation• Pt, Pt-Re, or Pt-Ir catalyst



atmosphericdistillation

crudeoil

gas

naphta

kerosene

gas oil

vacuum gas oil

atmosphericresidue

vacuumresidue

vacuumdistillation

LPG

gasoline

kerosene

diesel

low sulfur fuel oil

reforming

hydrotreating

hydrocracking

hydrotreating

residue conversion

fcc

Oil Refinery

Adapted from J.W. Gosselink, CaTTech 4 (1999)


University of TwenteFraction Boiling point °C Volume % Density (kg/l) Sulfur (wt %)

Gas, LPG 1.5 0.5 – 0.6 0

Light naphtha (≤ C5)

< 80°C 6 0.66 0

Heavy naphtha(C5 – C10)

80-170°C 15 0.74 0.02

Kerosene 170-220 °C 9 0.79 0.1

Gas oil 220-360 °C 25 0.83-0.87 0.8

Vacuum gas oil (C20-C40)

360-530 °C 23 0.92 1.4

Residue (≥ C40) > 530 °C 20 1.02 2.2

Source: AKZO-Nobel, private communication to Niemantsverdriet

Average Characteristics Crude Oil

13



Catalyst20% Zeolite Y80% Matrix

Circulating fluid-

bed reactor

FCC – Fluidized Catalytic Cracking


University of TwenteZeolites

Zeolite A Zeolite Y


University of TwenteWhy Are

Zeolites Catalytically

Active?

14



O O O OSi Al Si

O O O O O O

H

Bridging OH: Bronsted Acid



Carbonium ion formation: H+ + CnH2n+2 → [CnH2n+3]+ Decomposition to carbenium ion: [CnH2n+3]+ → [CnH2n+1]+ + H2 [CnH2n+3]+ → [Cn-xH2n-2x-1]+ + CxH2x Propagation: CmH2m+2 + [CnH2n+1]ads

+ → CnH2n+2 + [CmH2m+1]ads

+

Mechanism of cracking on acid sites

exist only in the transition states


University of TwenteAverage FCC Unitwho took this risk?

Catalyst inventory 200 tonFresh catalyst rate 2-3 ton / dayFeed rate 3300 ton / dayCat / oil ratio 5-6Linear velocity in riser reactor 10 m / sCatalyst recycle 200 kg/sec

(from H W Kouwenhoven and B. de Kroes, 1991)

15



thiophene di-methyl di-benzothiophene

pyrrole

indole

carbazole

pyridine

quinoline

acridine

HDS&

HDN



Thiophene DesulfurizationHigh H2 pressures

thiophenetetra hydrothiophene

butadiene1-butene

2-butenes

- H2S

H2

H2

R. Prins, in “Handbook of Heterogeneous Catalysis”

?


University of Twente HDS/HDN

• What was/is the driver here?

16


University of Twente Wrap up (2)

• Applications of catalysis in oil-refining

• Next: chemicals– Ammonia and hydrogen– Monomers


University of TwenteAmmonia Synthesis

N2 + 3 H2 2 NH3 + 50 kJ/mole

equilibrium reaction !

Assignment: give arguments for operation at low-high pressure and temperature.Consider:• size and cost of reactor• heat transfer• recycle cost unconverted feed


University of Twente The Ammonia Synthesis

CH4 + H2O CO + 3 H2steam reforming

CO + H2O CO2 + H2water gas shift

Sources of hydrogen: natural gas and steam

Ni700 - 800 C

FeOxCu / ZnO

17

52


Comparison steam vsautothermal reforming

• Steam reforming

• CH4 + H2O → CO + 3 H2 ΔH = 226 kJ/ mol-1

• Deep oxidation

• CH4 + 2O2→ CO2 + 2 H2O ΔH = -803 kJ/ mol-1

53


• Deep oxidation

• CH4 + 2O2→ CO2 + 2 H2O ΔH = -803 kJ/ mol-1

•Steam reforming

• CH4 + H2O → CO + 3 H2 ΔH = 226 kJ/ mol-1

Comparison steam vs autothermalreforming


University of Twente Ammonia

• We know how to make hydrogen• What about nitrogen?

18


University of Twente Ammonia• We know how to make hydrogen• What about nitrogen?


University of TwenteAmmonia Synthesis

SteamReforming

Water GasShift

Methanation

AmmoniaSynthesis

CO2Absorption

NH3

CH4 air steam


University of Twente Ammonia Synthesis(1908 - 1913)

Haber Bosch Mittasch• screening of 2500catalysts• in 6000 tests• 1913: plant opened at Oppau near Ludwigshafen

•Why did this happen in early 1900?

19



Cr Mn Fe Co Ni Cu

Mo Tc Ru Rh Pd Ag

W Re Os Ir Pt Au

N2 dissociation

no N2 dissociation = no reaction

Metal Catalysts in NH3 Synthesis

Nitride formation

Interaction

Rate(log)



N2 + 3H2 = 2NH3

From: Per Stoltze,Technical University of Denmark

AmmoniaSynthesis



The Oppau explosion occurred on September 21, 1921 when a stock of 4500 tonnes of a mixture

of ammonium sulfate and ammonium nitrate fertilizer exploded at a BASF plant in Oppau – now

part of Ludwigshafen, Germany – killing 500–600 people and injuring about 2000 more.

20


University of TwenteBulk chemicals

• Boosted in second half of 20th century– Left-over of the fuel production from oil, e.g.

• Light alkanes and olefins• BTX: benzene-toluene-xylenes

– cyclohexane

– Catalysts developed to make useful materials– “BRANDHOUT”

€?


University of Twente PE from ethylene


University of Twente Poly-esther

21



H2

H2

H2

NH3

NH3

NH3

H2SO4

H2SO4

Caprolactam; integrated process

• Caprolactam

• But also:– H2

– NH3

– HNO3

– H2SO4

O COH

HNO 3 NH3OH(HSO 4)+

NOH

NO

N

O

O

N


University of TwenteIntegrated processes


University of Twente Wrap up (3)• Bulk chemicals

– Ammonia– Monomers, connection to energy– Caprolactam; integrated processes

• Fine chemicals– Mainly hydrogenation– Selective oxidation needed– Tension between time to market and catalyst

development

• Next: Environment

22


University of TwenteAutomotive ExhaustMostly N2 , CO2 , H2O

and (approx quantities)

• hydrocarbons (HC), 750 ppm

• NOx 1000 ppm

• CO 0.70 vol%

• H2 0.25 vol%

• O2 0.50 vol%

• Pb, P, etc traces



Automotive emissionsin gram / km, without catalyst

0

2

4

6

8

10

urban /cold

urban /warm

highway USA1991

USA2004

CO–HC-NO

gram

/ k

m

How to remove CO, Hydrocarbons and NO simultaneously?

CO

HC

NO HCNO

CO

CO

CO

CO

NO

NONO

HC

HCHC

University of TwenteThree-way Catalyst

• CO + O2 = CO2

• CxHy + O2 = CO2 + H2O

• CO + NO = CO2 + N2

Pt, Pd

Pt, Pd

Rh, Pd

23





University of TwenteThree way catalyst

• Initiated in California, 1970• Legislation after technical demonstration by

Engelhard• Spread around the world

From end-of-pipe to prevention (HDS)

24



Environmental Pollution Control

• DeNOXNO + NH3à N2 + H2

• Oxidation organic waste

• Soot oxidation (diesel)



Drivers for NOx/SOx abatement in 70s/80s

• Smog– California

• Acid rain– German woods– Acidification lakes

• From end-of-pipe to prevention– HDS/HDN



What determines applicability of a

catalyst?

25


University of Twente Catalyst applicable?• Costs of, related to activity and selectivity

– Depends on added value (product-feedstock) and scale of product

– process equipment and energy cost:• reactor• separation (products and feedstock)• heat exchange (RT not practical)

– catalyst manufacture and recycling• Catalyst stability

– replacement– downtime– loss


University of TwenteCatalyst applicable?• Important factors

– regeneration– pressure drop– reactor filling, mechanical strength– separation from products– robustness– constant quality – No single producer– fit existing hardware (conditions, heat transfer,

downstream processing, steam generation



Is catalysis mature and “ready”?

26





Benzene + Propylene Cumene

G. Belussi and C Perego, CaTTech 4 (2000) 4



Cumulative R&D Effort

Matu

rity

of an

Inve

ntion

fundamental researchemerging technology

industrial applicationmature technology

research development

The S-Curve

27







Scientific point of view:No: more tomorrow




Practical point of view:

28


University of TwenteChallenges

• Trends– Increase population– regional strong

development

• Decrease footprint– Less CO2

– Less waste


University of Twente Challenges• Regional strong development

– Strongly growing demand, esp. China and India

0

200

400

600

800

1.000

1.200

2000 2010 2020 2030 2040 2050

[EJ]

Asia Eastern Europe and Former Soviet Union Latin America and Africa North America Europe, Japan, Oceania

Asia

”RUS”LA + AfrNA

EU, Japan, Oceania



• World energy use depends on fossil fuels:– Supply security– Climate change

Nuclear6%

Renewables6%

Fossil Fuels 88%Source: BP review of World Energy 2004

World energy use in 2003

29


University of TwenteProblems facing the world energy system

• Supply security– Oil and Gas reserves in unstable regions: renewed

interest in coal and nuclear.– Strong driver in US

Oil reserves in Middle East Gas reserves in Middle East and Russia



• World energy use depends on fossil fuels:– Supply security– Climate change

Nuclear6%

Renewables6%

Fossil Fuels 88%Source: BP review of World Energy 2004

World energy use in 2003



Problems facing the world energy system

• Climate change– Combustion of fossil fuels gives rise to an

increased CO2 concentration in the atmosphere.

Source: IPCC

1000 1200 1400 1600 1800 2000

30


University of Twente Challenges

Source: IPCC


University of Twente Challenges

• Climate change– Combustion of fossil fuels gives rise to

increased CO2 concentration in the atmosphere.

– The greenhouse effect probably leads to temperature rise.

Arctic sea ice, 1979Arctic sea ice, 2003

Source: IPCC


University of TwenteDo you believe Al Gore?• Politics does!!• Demand increases• Supply will decrease/cost increase

– Less accessible oil fields– More contaminated

• What will happen with price?– Continue to be high, may even increase

further– Price is determined by demand-supply,

not by “running out”• Other schemes will take share of the

market: business opportunity

• When?????

31


University of Twente Energy sources: past to present

Source: IIASA, 2001


University of Twente Energy sources• Alternative energy

sources are needed…


University of Twente Role of catalysis?

• Energy saving• Produce energy carriers from new sources• Produce chemicals from byproducts of

production energy carriers

32


University of TwenteWaste of methane

Red spots = flaring (simple burning) of natural gas, which is a by-product of oil production.

Source: ICAT

“Stranded gas” refers to remote locations but also to economic barrier to “sweetening” or removing contaminants such as CO2 (e.g. when CO2 > 15%)



What is the Fischer-Tropsch Process?Catalytic conversion of

synthesis gas into hydrocarbons

CO + 2 H2 CnH2n(+2) + H2OFe, Co catalyst

10-50 bar; 180-350°Cfluidized bed; slurry phase

C7-C11

≥ C20

≤ C2

C12-C19

C3-C4

German inventionwhy?



• Energy saving– Stranded gas to hydrocarbons via syngas

(H2/CO) and Fischer-Tropsch– Decrease energy consumption of chemical

industry (e.g. steam-cracking)• Produce energy carriers from new sources• Produce chemicals from byproducts of

production energy carriers

33



Energy carriers from new sources(Gasification)

Bio-oilSyngas

(H2,CO)(Pyrolysis) (Reforming)

Inert atmosphere 500 C, τ = < 1s

Drivers:-Small scale production-Transport oil instead of biomass-Ashes back to field


University of TwenteFlash pyrolysisFlash pyrolysis

Drivers

• Simple process

• Mild conditions

• Energy densification

• Decoupling of productionand application

Barriers

• Oil quality- acidity (corrosive)- solids- viscous- unstable, polymerization- low heating-value

• Oxygen is key problem– Carboxilic acid– Aldehydes



• Energy saving• Produce energy carriers from new

sources– Catalytic upgrading of bio-oil

34



CHx

H2, COx

Pt

ZrO2

H2, CO2, CH4, CO C C

OH

OH

H

H

1

H2O

HO

HO

2

Proposed mechanism for steam reforming on Pt-based catalysts

• K. Takanabe, K. Aika, K. Inazu, T. Baba, K. Seshan, L. Lefferts. J. Catal 243, 263 (2006)


University of TwenteRole of catalysis?• Energy saving

– Mineral energy carriers remain very important• Produce energy carriers from new

sources– New processes and catalysts to fuels– New processes and catalysts to chemicals– Use everything: bio-feed refinery– Preferred technology site depending




Catalysis is a crucial tool to meet future challenges

Wrap up (3)

35


University of Twente end

1



CatalysisFundamentalsBy Leon Lefferts

Acknowledgement Hans Niemantsverdriet

Faculty of Science and Technology, IMPACT

Catalytic Processes & Materials



Fundamentals of catalysis•Metals

•The potential energy picture

•The kinetic picture

•The chemical bonding picture

•Acid-base

•Redox



catalysts• Adsorption• Reaction• Mass and

heat transfer

Externaldiffusion

Inte

rnal

di

ffusi

on

Adsorption Desorption

Surface reaction

Aa d d

surface.

a

A + +B

B C

C D

D

2







•Acid-base

•Redox


University of TwentePotential energy surfaces, activated complex

A-B + C → A···B···C → A + B-C

Bond length A-B 0

Bond

leng

th B

-C

0



A-B + C → A···B···C → A + B-C

Bond length A-B 0

Bon

d le

ngth

B-C

0

3



A-B + C → A···B···C → A + B-C

Bond length A-B 0

Bon

d le

ngth

B-C

0



A-B + C → A···B···C → A + B-C

Bond length A-B 0

Bond

leng

th B

-C

0



A-B + C → A···B···C → A + B-C

Bond length A-B 0

Bond

leng

th B

-C

0

4



A-B + C → A···B···C → A + B-C

Bond length A-B 0

Bond

leng

th B

-C

0



A-B + C → A···B···C → A + B-C

Bond length A-B 0

Bond

leng

th B

-C

0E

Reaction coordinate

A-B C

A B-C

Eact

A…B…C

DH


University of TwenteFundamentals of catalysis•Metals


•The kinetic picture•general chemical•catalytic


•Acid-base

•Redox

5



SvanteArrhenius1859 - 1927

Nobel Prize 1903

k = v e

The Arrhenius Equation

-Eact /RT

Eact

reaction parameter

E

+

+ k

A B AB

r = = k [A] [B]d[AB]

dt

Empirical !


University of TwenteTransition State Theory

Henry Eyring1901 - 1981

k KTST = #

R R# PK# k#

k#


University of Twente Imagine the transition state

• Rate determined by– Frequency of encounters– Right orientation– Sufficient energy to stretch N-O bond

N-O H à N…O…H à N O-H

6



Transition State Theory: The Assumptions

Eb

reaction coordinate

E

R

P

R# • passage over barrier only inforward direction

• equilibrium between reactantsand products for all degrees offreedom, except for reactioncoordinate

• One specific vibration responsible for reaction

R R# PK#

#k



# # ## exp expG S HK

RT R RT −∆ ∆ ∆

= = −

R R# PK#

#k

_# # . b

TSTk Tk k K Kh

κνν

= =

Transmission factorFrequency vibration

(disappearing)

Equil. ConstantExcept vibration

Partition function of vibration


University of Twente Agrees with Arrhenius

# #_

0

12 10

. . exp

exp

~ 10 sec

b bTST

act

k T k T S Hk Kh h R RT

EART

A

κ κ

−

∆ ∆= = − =

−∆ =

≈

7



Eb

reaction coordinate

E

R

P

R#

Eb

reaction coordinate

E

R

P

R#

Loose TST:q# >> q

Tight TST:q# << q

1013 < A0 < 1017 s-1109 < A0 < 1013 s-1

∆S# < 0 ∆S# > 0


University of Twente Summary

• Kinetic description– Transition state theory for elementary steps– Pre-exp. determined by entropy– In agreement with Arrhenius equation


University of TwenteFundamentals of catalysis•Metals


•The kinetic picture•general chemical

•catalytic


•Acid-base

•Redox

8


University of TwenteRates in heterogeneous

catalysis• Adsorption• Reaction• Mass and

heat transfer

Externaldiffusion

Inte

rnal

di

ffusi

on

Adsorption Desorption

Surface reaction

Aa d d

surface.

a

A + +B

B C

C D

D



• worked at General Electrics• oxygen adsorption on tungsten

filaments of light bulbs• 1932: Nobel Prize in Chemistry• Langmuir Adsorption Isotherm:

θA = KA [A]

1 + KA [A]

Irving Langmuir(1881 - 1957)


University of Twente Types of adsorption

• Physisorption– van der Waals interaction

– Physisorbed molecule retains its identity (may be distorted). Enthalpy change is insufficient for bond breaking

• Chemisorption– Chemical bond formation– A chemisorbed molecule

may be broken apart(dissociation)

molecule ∆H0ads (kJ.mol-1)

CH4 -21H2O -59N2 -21

molecule ∆H0ads (kJ.mol-1)

Cr Fe NiC2H4 -487 -285 -209CO -192H2 -188 -134NH3 -188 -155

9


University of TwenteMonolayer surface coverage θ

= empty site = occupied site

σ = Σ

+σmax = Σ

θ = +ΣΣ

surface coverage: θ = σ/σmax0 ≤ θ ≤ 1

maximum surface concentration: σmax

surface concentration: σ



• Describes equilibrium

– All adsorbing sites equivalent and independent of each other– Only monolayer coverage occurs– Adsorbed molecules stay on the surface for a finite amount

of time (τ ≠ 0)– Heat of adsorption is independent of coverage

Langmuir adsorption isotherm

adsorption desorption

solid phase

gas phase

rads rdes

ads

des

kgas adskA A→←


University of Twente Assignment• Deduce Langmuir adsorption isotherm

– Adsorption and desorption are elementary reactions

– Assume equilibrium– Derive equation expressing θA as function of PAA

ads

des

kgas adsk

A A→←

10



Langmuir - Hinshelwood Kinetics

Irving Langmuir1881 - 1957

Nobel Prize 1932

Cyril NormanHinshelwood

1897 - 1967Nobel Prize 1956

1915 Langmuir: Adsorption Isotherm

1927 Hinshelwood:Kinetics of Catalytic Reactions

• Consistent with Sabatier’s Principle

• Coverage dependence: Volcano plot

• Temperature dependence: Volcano plot

Next:


University of TwenteReaction Mechanism:

A + * ⇔ Aads equilibrium; KA

B + * ⇔ Bads equilibrium; KB

Aads + Bads → ABads + * r.d.s; k

ABads → AB + * fast

Coverages:

θA = KA pAθ*

θB = KB pB θ*

θ*

= 1

1+KApA+KBpB

Reaction rate: r =N* k KAKB pApB

(1 + KApA + KBpB)2



A

B

Eads (A) = Eads (B) = 125 kJ/mol

Eact = 50 kJ/mol

T = 600 K; p is fixed

Rate of a Catalytic Reaction:Pressure Dependence

reaction orderpositive in pAnegative in pB

reaction ordernegative in pApositive in pB

10

θ,no r

mali z

ed

r ate

p /pB

1.0

0.8

0.6

0.4

0.2

0.0θ*

θAθB

rate

0.1 1.0

N*

k KAKB pA pB

(1 + KA pA + KB pB )2

11



Eads (A) = 135 kJ/molEads (B) = 125 kJ/molPA = 0.1 PBEact = 50 kJ/mol

0,0

0,2

0,4

0,6

0,8

1,0

100 300 500 700 900

θ ,norm

aliz

edra

te

T (K)

θA

θB

θ*rate

Rate of a Catalytic Reaction:Temperature Dependence

reaction ordernegative in pApositive in pB

reaction orderpositive in pA and pB


University of Twente Assigment

• Expression for apparent activation energy?

0,0

0,2

0,4

0,6

0,8

1,0

100 300 500 700 900

θ,nor

mal

ized

rate

T (K)

θA

θB

θ*rate

θ ,no

rmal

ized

rate

p /pB

1.0

0.8

0.6

0.4

0.2

0.0θ*

θAθB

rate

0.1 1.0

N* k KAKB pApB

(1+ KApA + KBpB)2


University of TwenteThe Sabatier Effect

metal - adsorbate bond strength

cata

lytic

activ

ity optimum interaction catalyst - adsorbate:• not too strong• not too weak

optimum coverage

12



Cr Mn Fe Co Ni Cu

Mo Tc Ru Rh Pd Ag

W Re Os Ir Pt Au

N2 dissociation

no N2 dissociation = no reaction

Metal Catalysts in NH3 Synthesis

Nitride formation

Interaction

Rate(log)



NO/H2; exampleO COH

HNO3 NH3OH(HSO 4)+

NOH

NO

N

O

O

N

NO + H2



NOa + 3Haà NH2OH+4*

Na + OaHaNH4

+

NOa

N2O

NO + *à NOa

H2 + 2*à 2 Ha

*

Reaction scheme

•Pt/graphite•Sulphoric acid•Slurry process

13


University of Twente Coverage and selectivity

• Dissociation requires empty sites

• Low coverage, dissociation, ammonia

• High coverage, N2O• Why is this?

PNO

Yiel

dNH2OH

NH4+ N2O

2NO + 3H2à 2 NH2OH



NOa + 3Haà NH2OH+4*

Na + OaHaNH4

+

NOa

N2O

NO + *à NOa

H2 + 2*à 2 Ha

*

PNO

Yiel

d

NH2OH

NH4+ N2O


University of Twente Selectivity?

• Via surface coverage: thermodynamic selectivity

• Via Eact : kinetic selectivity

14


University of TwenteKinetics of Surface Reactions

Langmuir-Hinshelwood (mean field) description:

rate = k θ θ

Deviations:

• inequivalent sitessteps, vacancies, defects

• interactions between adsorbatesordering, segregation, modified bonding

CO oxidation


University of Twente Observation of islands• O islands on Ru(0001)

– STM• CO oxidation on Pt(110)

– PEEM; work-function varies with adsorbate• Pt-O > Pt-CO

dark grey

Real time

R. Imbihl, G. Ertl, Chem. Rev., 95 (1995) 697G. Ertl, in Handbook of Heterog. Catal., 1033-1051


University of Twente So what?

• Islands of adsorbates• Reaction fronts

• General kinetics equations assumes mixing of species

15



Why does ∆H vary with coverage?

•Coordination number vary•Number of dangling bonds influences chemisorption•How many different sites?


University of Twente Why does ∆H vary with coverage?

•Coordination number vary•Number of dangling bonds influences chemisorption•How many different sites?



Why does ∆H vary with coverage?

• Polarisation effects– Molecules polarised on adsorption – Polar molecules – repulsion effects

+-

+-

+-

16


University of Twente Wrap up coverage effects• Kinetics of elementary steps

– Coverage of reactants should be significant and notcomplete

• Influence on strength of interaction with surface– Surface inhomogeneous– Lateral interactions, e.g. steric repulsion– Surface reconstruction

• But: what determines strength of interaction with catalyst? – chemical bonding picture



What is catalysis•The potential energy picture


•The chemical bonding picture•Chemical bonds with catalyst

•Breaking bonds







•Acid-base

•Redox

17



Catalysis by Metals: Trends in Reactivity

Cr Mn Fe Co Ni Cu

Mo Tc Ru Rh Pd Ag

W Re Os Ir Pt Au

stable againstoxide, carbide, nitride formation

stable oxides, carbides, nitridesstrong, dissociative adsorption

Weak, molecular adsorption

WHY?



The minimum you need to know about .

. . . . . Molecular Orbitals

atom molecule atom

atomic molecular atomicorbital orbitals orbital

antibonding

bonding

strong weak no bond

much / little overlap



18



and about . . . . .

Bonding in Metals

sp - band

d-band

energ

y

density of statesatom metal

4p

4s

3d



EF

Filling of bands determines strength of between metal atoms

EF

Coh

esiv

e E

nerg

y (e

V)

0

2

4

6

8

10

5d-series

4d-series

3d-series

Ca Sc Ti V Cr Mn Fe Co Ni Cu ZnSr Y Zr Nb Mo Tc Ru Rh Pd Ag CdBa La Hf Ta W Re Os Ir Pt Au Hg


University of Twente Atom on d-metal:

d-metal adsorbed freeatom atom

Evac

EF

a) b)


antibonding

bonding

antibonding

bonding

d-band

s-band

Evac

EF

19




Evac

EF


antibonding

bonding

antibonding

bonding

d-band

s-band

Evac

EF

Cr Mn Fe Co Ni Cu

Mo Tc Ru Rh Pd Ag

W Re Os Ir Pt Au

Strong atomic adsorption

Weaker adsorption

Tre

nds

in c

hem

isorp

tion

Volc

ano p

lot

reas

onin

g

d-band < half filledstrong bond

d-band > half filledweaker bond

585 564

543

531 531 N/Metal, kJ/mol



d-metal free molecule

σ-orbitals σ*-orbitals

Molecular Adsorption on ad-metal

σ*

σ

1s 1s

Evac

EF

antibonding

bonding

antibonding

bonding

adsorbed molecule


University of TwenteCO orbitals

or 5σ

20



5σ

2π*

Metal orbitals

CO orbitals

Repulsive Attactive

C O



2π*

5σ

Evac

EF

d-metal free molecule

antibonding

bonding

antibonding

bonding

d-5σ d-2π*

-CO adsorption on a d metal

adsorbed molecule

“back donation”binds molecule to surfaceweakens internal CO bond! Th

is p

ictu

re is

the

key

to u

nder

stan

ding

ca

talysis

in t

erms

of o

rbital t

heor

y



2143 cm-1

IR IR

1800-1880 1880-2000 2000-2130 cm-1

CO on a metal surface

more overlap = more back donation = lower frequency

21



0.0000

0.0020

0.0040

0.0060

0.0080

0.0100

0.0120

165017501850195020502150

Wavenumber (cm-1)

ATR

-IR a

bsor

banc

e (a

.u.)

Aqueous phase

Gas phase

1870

1910

1934

2034

20911978

0

0.002

0.004

0.006

0.008

0.01

0.012

165017501850195020502150Wavenumber (cm-1)

ATR

-IR a

bsor

banc

e (a

.u.)

2071 cm-1

Time

2065 cm-1

Adsorbing CO

Water versusgas-phase

-huge red-shift

-intensity *4!

Pt gas-phase

Pd gasphase


University of TwenteCO on Pd

0.0000

0.0020

0.0040

0.0060

0.0080

0.0100

0.0120

165017501850195020502150

Wavenumber (cm-1)

ATR

-IR abs

orba

nce (a.u.)

Aqueous phase

Gas phase

1870

1910

1934

2034

20911978

Lin

Bridged


University of Twente CO on Pd, effect of pH

202 0

202 2

202 4

202 6

202 8

203 0

203 2

203 4

203 6

4 5 6 7 8 9 10pH

(B)

Infr

ared

freq

uenc

y of

lin

ear C

O (

cm )-1

(B)

Infr

ared

freq

uenc

y of

lin

ear C

O (

cm )-1

22


University of Twente Assignment

• Explain difference of water on adsorbed CO on Pd

• Explain the effect of pH on CO wave-numbers– Consider H20 + Oa + 2e- ßà 2OH-

(or H+ ßà Ha + e-)



Sune D. Ebbesen, Barbara L. Mojet and Leon Lefferts, Phys. Chem. Chem. Phys., 2009; DOI: 10.1039/b814605e; Journal of Catalysis 246 (2007) 66–73


University of Twente Wrap up adsorption

• Theory describes– Adsorption strength of atoms and

molecules – Weakening of bonds in molecules– Non activated dissociation possible

• Not generally the case

23



∆Hads (AB)–150 kJ/mol Eact

75 kJ/mol

∆Hads (A+B)–600 kJ/mol

Energetics of Dissociationon a transition metal such as Fe, Ru

DrivingForce




Evac

EF


antibonding

bonding

antibonding

bonding

d-band

s-band

Evac

EF

Cr Mn Fe Co Ni Cu

Mo Tc Ru Rh Pd Ag

W Re Os Ir Pt Au


Weaker adsorption

Tre

nds

in c

hem

isorp

tion

d-band < half filledstrong bond

d-band > half filledweaker bond

585 564

543

531 531 N/Metal, kJ/mol



∆Hads (AB) δEact

δ ∆Hads (A+B)

Dissociation on Different Metals e.g. Rh and Fe

δEact ≈ ½ δ ∆Hads (A+B)Bronstedt-Polanyi Relation

Rh

Fe

24



Cr Mn Fe Co Ni Cu

Mo Tc Ru Rh Pd Ag

W Re Os Ir Pt Au

easy dissociation

no dissociation

Tre

nds

in c

hem

isorp

tion

∆Hads (AB) δEact

δ ∆Hads (A+B)

Dissociation on Different Metalse.g. Rh and Fe

δEact ≈ ½δ ∆Hads (A+B)Bronstedt-Polanyi Relation

Rh

Fe


University of TwenteCr Mn Fe Co Ni Cu

Mo Tc Ru Rh Pd Ag

W Re Os Ir Pt Au

easy dissociation

no dissociation

Tre

nds

in c

hem

isorp

tion

∆Hads (AB) δEact

δ ∆Hads (A+B)

Dissociation NO e.g. Pt and Rh

δEact ≈ ½δ ∆Hads (A+B)Bronstedt-Polanyi Relation

Pt

Rh



Logatottir, Rod, Nørskov, Hammer, Dahl, Jacobsen, J. Catal. 197, 229 (2001)

The Brønsted-Evans-Polanyi relation

25



Coordinative unsaturation: higher reactivity

energ

y

density of states

Fermi level

energ

y

density of states

fcc (111)9 neighbours per atom

less dense surfaceor defects

<9 neighbours per atom

sp - band

d-band



Coordinative unsaturation: higher reactivity

energ

y

density of states

Fermi level

energ

y

density of states

fcc (111)9 neighbours per atom

less dense surfaceor defects

<9 neighbours per atom

sp - band

d-band

Evac

EF

antibonding

bonding

antibonding

bonding

stronger

bond!


University of Twente Wrap up

• Atoms bonded strongly induce facile dissociation– Bronsted-Polyani

• Low coordination sites more active– Did not discuss effect of site structure!

26



Cr Mn Fe Co Ni Cu

Mo Tc Ru Rh Pd Ag

W Re Os Ir Pt Au


Weaker adsorption

• Rh dissociates NO easier– Great for de-NOx; not for NH2OH synthesis

• Surface coverage always influences strongly– “Empty site” is reactant in dissociation;

Sabatier again


University of Twente Summary

• Chemical bonding approach– Simple molecular orbital theory

explains on d-metals• Adsorption strength• Dissociation

– Also effects of• Structure• coverage



Dispersion

N Atoms in the particle

Ns Surface Atoms

Dispersion D = Ns/N

expressed as • fraction 0 < D ≤ 1• percentage

Catalyst Particles: Dispersion and Specific Area

Specific Area

Surface: A = 6 a2

Volume: V = a3

Mass: M = ρa3

Specific Area: A/M = 6 a2/ρa3 = 6/ρa

Platinuma = 1 µm SA = 0.28 m2/ga = 1 nm SA = 280 m2/g

SiO2, Al2O3:SA = 50 - 500 m2/g

a

27


University of TwenteEffects of particles sizes

• Very small particles: incomplete d-band, electronic structure effect (< 2nm)

• Small particles: (2-4 nm)– Coordination number effects– Ensemble effects– Support interaction effect

• Larger particles:– Step site effects


University of TwenteParticle Size Effect on Catalytic Activity

S.H. Oh and C.C. Eickel, J. Catal. 128 (1991) 526



Statistics of Surface Sites on Metal CrystalsR van Hardeveld and F HartogSurf. Sci. 15 (1969) 189

28



if particle:

and ensemble:

ensembles on particles of different size



The CO molecule dissociates in the transition state:

optimal overlap between d- and 2π*-orbitals

De Koster and Van Santen

Dissociation: on “ensemble”



G.L. Bezemer, J.H. Bitter, H.P.C.E. Kuipers, H. Oosterbeek, J.E. Holewijn,X. Xu, F. Kapteijn, A.J. van Dillen and K.P. de Jong,

J. Am. Chem. Soc. 128 (2006) 3956

Particle Size Dependence FTS Cobalt on Carbon Nano Fibres - 35 bar, 210 °C

C5+ selectivityTOF

Optimum: Co particles of 6-8 nm

29



Dissociation easier on steps

K. Honkala et al., Science 307, 555 -558 (2005)

Diameter approximately 6 nm:Smallest size that supports steps



Steps may dominate the kinetics!

Ib Chorkendorff and coworkers, TU Denmark


University of TwenteEffects of particles sizes

• Very small particles: incomplete d-band, electronic structure effect (< 2nm)

• Small particles: (2-4 nm)– Coordination number effects– Ensemble effects– Support interaction effect

• Larger particles:– Step site effects

30


University of Twente So far

• Concentrated on L-H reactions on (metal-) surfaces







•Acid-base

•Redox


University of TwenteZeolites

Zeolite A Zeolite Y

31



Zeolite A



MORDENITE


University of TwenteWhy Are

Zeolites Catalytically

Active?

32



O O O OSi Al Si

O O O O O O

H

Bridging OH: Brønsted Acid

4+ 3+ 4+

+



catalytic cycle involving•proton donation from an acidic catalyst HA to a reactant R•rearrangement of the protonated intermediate HR+ to HR’+, •proton return to the catalyst by the reaction intermediate •desorption of the isomerized product R’

Catalysis by Solid Acids

Bruce Gates, Encyclopedia of Catalysis, Wiley, 2002



Zeolite A X Y MOR MFI Silicalite-1Si/Al > 1 1 2.5 5 10 OO

acid strength proton

thermal stability

hydrophilic - hydrophobic

affinity for affinity for polar molecules apolar molecules

number of cations

Zeolite Properties: Trends

33



Shape-Selective Catalysis

S. Csicsery, I. Kiricsi, Encyclopedia of Catalysis, Wiley, 2002

reactant selectivity





product selectivity





Restricted transition state selectivity

34







•Acid-base

•Redox


University of Twente Selective oxidation• LH: reaction between adsorbed species• Mars-van Krevelen mechanism, e.g. toluene



oxygenates by heterogeneous catalysis

J.C. Vedrine in “Catalytic Oxidation”, NIOK, World Scientific 1995

35



Oxygenates

Acetic acid

Phtalic anhydride

Maleic anhydride

Acrylic acid

Terephthalic acidMethyl ethyl ketoneMethyl metacrylate Vinyl acetate

Acrylonitrile

Acetonitrile


University of TwenteSurface Chemistry on Oxides:

• adsorption and dissociation(often heterolytic)

• reaction:Mars-van Krevelen Mechanism

Elementary steps less known



Oxide Surface

Mδ+ - O δ- - M δ+ - O δ- - M δ+Lewis BronstedAcid Baseelectron protonacceptor acceptor

36


University of TwenteChemisorption on Oxide:Heterolytic Dissociation

H-

H+ O

Mδ+ - Oδ- - Mδ+ - Oδ- - Mδ+Bronsted Bronsted

acid base



Adsorption of n-butane on a VPO catalyst

F Trifiro and F Cavani, Chem Tech (April 1994), p 18



Mechanism Butane to Maleic Anhydride

Oxidation of hydrocarbon molecules is a multistep process

consecutive abstraction of H-atoms and addition of O-atoms:

From: Jerzy Haber

Selective Oxidation – HeterogeneousEncyclopedia of Catalysis

John Wiley & Sons, Inc. 2002

F Trifiro, F Cavani, Chem Tech(April 1994), p 18

37



Selective Oxidation is Structure Sensitive

Selectivities in oxidation of o-xyleneon V2O5 catalysts as a function of the ‘morphological factor’

M. Gasior, T. Machej,

J. Catal. 83 (1983) 472

From: Jerzy Haber

Selective Oxidation – HeterogeneousEncyclopedia of Catalysis

John Wiley & Sons, Inc. 2002



• polar surfaces – acid / basic sites

• charged intermediates

• heterolytic dissociation routes

• correlations of catalytic activity with ‘electron donor power’ and ionization potential reactants and products

• Mars-van Krevelen mechanisms

• Elementary steps less well known than inmetal catalysis

Catalysis by Oxides:


University of TwenteNeed for new technology

Cavani&Trifirò: Cat. Today 24 (1995)

C3H 8

COx + H2O

k1

k 2k3

C3H6

38


University of Twente Need for new technology

Hodnett, Heterogeneous Catalytic Oxidation, Wiley 2000.


University of TwenteSeparated reaction zones• Transient operation

or moving bed

Contractor, CHEM. ENG. SCI., 54(22): 5627-5632 NOV 1999

O2-: nucleophilicoxygen insertion

O2-, O-: electrophilicdeep oxidation

J. Haber (1997), 5th world congressoxidation catalysis

Safety



Separated reaction zones; alternative approach

oxygenatesalkane

Reaction

latticeO2-

MEMBRANE

2O gas

Reoxidation

O2-

Catalytic Membrane ReactorCatalytic Membrane Reactor(keeping the reactants separated, avoiding undesired products)(using lattice oxygen species for reaction)

39



Li acts as an activator

0

0.2

0.4

0 10[Li+O-] mol/m2

Pr

opan

e co

nver

sion

(10-6

mol

e/m

2 .s)

C3H8 + ½ O2 → C3H6 + H2O→ COx + H2O

Selective oxidation over non-redox oxides; Li-MgO


University of Twente oxy-cracking C4

Mg2+ O2- Mg2+ O2- Mg2+ O2-

O2- Mg2+ O2- Mg2+ O2- Mg2+

Li+ O-O-

L. Leveless, Thesis 2002, U. Twente


University of Twente Propose mechanism

• Li is activator• Main products: C3H6, C2H4, CH4

• Oxygen cannot be removed• Conversion increases with free

volume in catalyst bed: explain?

40


University of Twente Proposed mechanism

Li+ O-

C HH3CH

CH3

?


University of Twente ·CH3C3H8

C3H8

CH4

H2

C2H4

C3H6

iC3H7·

nC3H7·

H·

HO2·

O2H2O22 HO·

½

½1

1Propagation

O2 COxCH2O


University of Twente Scientific challenges

• Control over conditions• Control over active site

41





University of TwenteHigh-temperature microreactor

v T-max: 725 Cv Continuous operatedv Fast heat transfer; even safe with explosive mixtures

promotieonderzoek Roald Tiggelaar/Gardeniers UT, i.s.m. groep Schouten, TU Eindhoven

gas ingas uit



High P reactors

≥ 700 bar !!

42


University of Twente Concentrations• Fast diffusion in

super-porous layers

• Microstructuredreactors

• Monoliths

Densely grown jungle of entangled CNFs !






What do we need in the field of nanoparticles ?

q Methods to establish the morphology of a particle and to quantitate the number of each type of site

q Methods for the preparation of mono-disperse particles with selected sizes

q Insight in particle support interaction • Morphology • Electronic properties• Specific sites on supported particles

q Insight in the reactivity of • Particles as a function of size / morphology• Sites as a function of size • Particles / sites on different supports

q and much more …….

43



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