1 Heterogeneous Catalysis 6 lectures Dr. Adam Lee Surface Chemistry & Catalysis Group

1

Heterogeneous Catalysis

6 lectures

Dr. Adam LeeSurface Chemistry & Catalysis Group

Pd2+ Al3+ O2-

Air

2

Synopsis

Topics:• Heterogeneous catalysts: definitions, types, advantages • Catalyst surfaces: adsorption processes, kinetics• Structure-sensitivity: dispersion, active site• Bimetallic catalysts: selectivity control• Catalyst preparation• Catalyst characterisation

Heterogeneous Catalysis is crucial to diverse industries ranging from fuels to food and pharmaceuticals. This course will introduce a wide range of heterogeneous catalysts and associated industrial processes.

Methods for the preparation, characterisation and testing of solid catalysts will be discussed.

Fundamentals of surface reactions and catalyst promotion are addressed, and finally some applied aspects of catalyst reactor engineering will be considered.

Recommended Texts:• Basis and Applications of Heterogeneous Catalysis: Mike Bowker,Oxford Primer, (1998) • Catalytic Chemistry: B.C.Gates, Wiley (1992)• Heterogeneous Catalysis: G.C.Bond OUP 2nd Ed (1987)

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• What are catalysts and why are they beneficial

‘Why haven’t they been used more widely when so many examples in petrochemical industry?’

• Types of catalysts • Properties of catalysts

• Calculation of TON & measurement of kinetic parameters

• Overview of typical classes of reactions and catalysts used

• Environmental considerations

Lecture 1 Overview

4

Organic Chemistry (1805) Physical Chemistry

Discovery of Catalysis (1835)

- Petrochemical & oil refining industry recognise promise

- Catalytic technology generates >10 trillion $/yr

- Clean technology (1990?) - applications in plastics, fabrics, food, fuel

Why don’t we use a catalyst?

How can we accelerate a chemical reaction?

Use reagents - stoichiometric - separation problems - TOXIC waste

- Industrial fine chemicals processes developed

- Carry on using reagents

5

Typical Reagents

•Oxidation Permanganate, Manganese dioxide,Chromium (VI)(<0.10 ppm)

•Reduction Metal Hydrides, (NaBH , LiAlH )4 4

Reducing metals (Na, Fe, Mg, Zn)

•Basic reagents Potassium butoxide, diisopropylamineTetramethyl guanidine

•Acidic reagents SO, HAlCl3, BF3, ZnCl2 2 4

•C-C Coupling Homogeneous Pd based complexes

T H-Br +

6

Importance of Heterogeneous Catalysis

Chemicals Industry:>90% of global chemical output relies upon heterogeneous catalysed processes

Economics:• ~20% of world GNP dependent on processes or derived products• Equates to $10,000 billion/year!!

Environment:• Ozone depletion catalysed over aerosol surfaces in Polar Stratospheric Clouds• Pollution control (catalytic converters, VOC destruction) • Clean synthesis (waste minimisation, benign solvents, low temperature) • Power generation

Nobel Prize in Chemistry 2007 – Gerhard Ertl

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Faujasiticzeolites

Polymerisation (1957/1991)

Zeigler-Natta/Metallocene

nC2H2

Catalytic Cracking (1964)

CxH2x+2 Cx-2H2x-2

CxH2x+2 Cx-2H2x-4

HDPE LDPE

Historical Evolution

8

Automotive Emission Control (1976)

Pt/Rh/Al2O3

HC + CO + NOX CO2 + H2O + N2

Chiral Catalysis (1988)

Chiral pocket

9

‘A catalyst is a material that enhances the rate and selectivity of a chemical reaction without itself being consumed in the reaction.’

Swedish Chemist - Jöns Jakob Berzelius (1779-1848)

Minimize FEEDSTOCK and reduce ENERGY costs

More efficient use of raw materials.

Advantages of Catalytic Technology

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•Heterogeneous - active site immobilised on solid support - tuneable selectivity- easily separated

•Homogeneous - organometallic complexes widely used - more active than heterogeneous, - high selectivity - difficult to separate

•Bio-catalysts - enzymes, bacteria, fungi - highly selective

•Phase transfer - Reagent soluble in separate phase to substrate - use PTC to transfer reagent into organic

Classes of Catalyst

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Catalyst: a material that enhances the rate and selectivity of a chemical reaction without itself being consumed in the reaction.

Catalyst Definitions

Rates (kinetics):

Rate = rate constant x [reactant]n

Rate constant (k or k’) = A exp (-EAct/RT)

Consider,

All catalysts work by providing alternative pathways:

- different, lower EAct

- accelerates both forward AND reverse reactions (increase kf and kb)

- catalysts do not influence how MUCH product forms

Reactants Productskforward

kback

12

http://www.chemguide.co.uk/physical/basicrates/catalyst.html#top

Catalyst Definitions

Uncatalysed Catalysed

Energetics:

Reactants do not all have same energy: Boltzmann distribution

So what determines theoretical product yield??- thermodynamic driving force, G = -nRT ln(K)

Large –ve G large +ve ln(K) huge K ~100 % Yield

Catalysts do not affect K!

13

Goal of catalytic research is improved activity & selectivity

Alter rate constants: k

For simple reax. A B + C

• Activity =

• Selectivity =

= Yield of Desired Product x 100 % Total Yield of all Product

dt

]A[d

100x]C[]B[

]B[

Catalyst Definitions

mol . s-1 rate of reaction

% relative formation of specific product

14

0

20

40

60

80

100

120

0 50 100 150 200

Time / s

[Rea

ctan

t] /

mm

ols

Conversion• The % of reactant that has reacted

Conversion = (Amt of Reactant at t0) - (Amt of Reactant at t1) x 100(Amt of Reactant at t0)

Catalyst Efficiency: 1

Triglyceride transesterification

Biodiesel

Activity = -d[Tributyrin] = 20 = 1 mmol.s-1

dt 20

Conversion = 20 %

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Triglyceride transesterification

Tri-glyceride

Di-glycerideMethyl-butanoate

(FAME)

Selectivity to FAME?

[FAME][Diglyceride]+[Monoglyceride]+[FAME]

x 10045

20+10+45x 100= = 60 %

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Reagents are often stoichiometric - single use

• By definition catalysts must be regenerated once product formed. • Need a parameter to compare efficiency of catalysts.

Turn over number (TON) - Number of reactions a single site can achieve

e.g. 1 mmol Pd converts 1000 mmols of COCO2

Turn over frequency (TOF) - Number of reactions per site per unit time.

e.g. 1 mmol Pd converts 1000 mmols of COCO2 in 10 s

To be valid TOF must be measured in absence of: - mass transport limitations - deactivation effects

Catalyst Efficiency: 2

TON = 1000

TOF = 100 s-1

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C - Catalytic cracking

S, Pb - Car exhaust catalysts

Active Phase - transition-metal - highly dispersed - reduced/oxidic/sulphided state

‘Inert’ Support - high surface area oxide - high porosity - high thermal/mechanical stability

Sn - Naptha reformingCl - Ethylene epoxidationK2O - NH3 synthesis

Catalyst Constituents

Solid Phase(powder, wire, gauze or pellet)

Promoters Poisons

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Active Component

Responsible for the principal chemical reaction

Features:• activity, selectivity, purity

• surface area, distribution on support, particle size

Types:• Metals

• Semiconductor oxides and sulphides

• Insulator oxides and sulphides

Platinum particles on a porous carbon support

Transmission ElectronMicrograph

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Other features include:• porosity

• mechanical properties

• stability

• dual functional activity

• modification of active component

Types:• high melting point oxides (silica, alumina)

• clays

• carbons

Main function is to maintain high surface area for active phase

Support

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• Ease of removal from reaction and possible to recycle

• Diffusional effects - reaction rates may be limited by diffusion into/out of pores.

• May need to re-optimise plants (often batch reactors) for solid-liquid processes - separation technology

• Opportunity to operate continuous processes

Advantages and Limitations of Heterogeneous Catalysts

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Apathy - Fine chemicals synthesis often on small scale, magnitude of waste not appreciated.

Cost - Conventional reagents are cheap, catalysts require development………(i.e. Investment!)

Time - Fine chemicals have a short life cycle compared to bulk chemicals:‘Time to market’ is critical.

‘…classical methods are broadly applicable and can be implemented relatively quickly. ..…the development of catalytic

technology is time consuming and expensive.’

R.A.Sheldon & H.Van Bekkum - Eds. Fine chemicals through heterogeneous catalysis

Why the Implementation Delay??

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The 12 Principles of Green Chemistry

1) It is better to prevent waste than to treat or clean up waste after it is formed.

2) Synthetic methods should be designed to maximise the incorporation of all materials used into the final product.

3) Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment.

4) Chemical products should be designed to preserve efficacy of function while reducing toxicity.

5) The use of auxiliary substances (e.g. solvents, separation agents, etc) should be made unnecessary wherever

possible and, innocuous when used.

6) Energy requirements should be recognised for their environmental and economic impacts & should be minimised.

Synthetic methods should be conducted at ambient temperature and pressure.

7) A raw material of feedstock should be renewable rather than depleting wherever technically and economically possible.

8) Unnecessary derivatisation (blocking group, protection/deprotection, temporary modification of physical/chemical

processes) should be avoided whenever possible.

9) Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.

10) Chemical products should be designed to preserve efficacy of function while reducing toxicity.

11) Analytical methodologies need to be developed to allow for real-time, in-process monitoring and control prior

to the formation of hazardous substances.

12) Substances and the form of a substance used in a chemical process should be chosen as to minimise

the potential for chemical accidents, including releases, explosions and fires.

Dr. Paul AnastasDirector of Green Chemical Inst.

Washington D.C.

ex. White House Asst. Director for Environment

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“It is better to prevent waste than to treat or clean up waste after it is formed”

ChemicalProcess

No waste

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“Synthetic methods should be designed to maximise the incorporation of all materials used into the final product”

A + B C + D + E + F ...

Only required product

C (only product)

SelectivitySelectivity

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“Energy requirements should be recognised for their environmental impacts and minimised. Synthetic methods should be conducted at ambient pressure and temperature”

HeatingCoolingStirringDistillationCompressionPumpingSeparation

Energy requirement(electricity)

Burn fossilfuel

CO2 toatmosphere

Globalwarming

High ActivityHigh Activity FiltrationFiltration

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“Unnecessary derivatisation (blocking group, protection/deprotection..) should be avoided wherever possible”

HO

R

O

H

R

O

protecting group H

R

OH

protecting group

Protect Reduction

Deprotection

HO

R

OHSpecific reduction agent

SelectivitySelectivity

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CONCLUSION:

“Selective catalysts are superior to stoichiometric reagents”

+AlCl3

ClO

Cl

O

Cl

AlCl3

H2O

exothermic

O

Cl+ Al (OH)3 + HCl

+

ClO

Cl

O2N

ENVIROCATEPZG

135 oC/6h

O Cl

NO2

Stoichiometric

Catalytic

4-Chlorobenzophenone

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Catalysis in Action: C2H2 on Pd(111)

Scanning Tunnelling Microscope movie- real-time molecular rotation

Further Info

Even More Info!

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• Reaction kinetics and diffusion limitations

• Langmuir adsorption isotherm • Unimolecular reaction

• Bimolecular reactions

• Surfaces

Lecture 3/4 Overview

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• Kinetics of heterogeneously catalysed liquid phase reactions are largely governed by diffusion limitation within the porous solid

•Require a new range of heterogeneous catalysts tailored for liquid phase organic reactions offering...

- pore structure

- ease of separation

- high activity

- high selectivity to desired products.

Kinetics of Catalysed Reactions

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Homogeneous vs Heterogeneous

Quench& Neutralise

Separate

Product Waste

AddHeterogeneousCatalyst

Filter

Product Catalyst

Homogeneous Reaction

BatchReactor

Batch/FlowReactor

Comparison

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• Diffusional effects - (Mass Transfer)

• Adsorption strength -

• Mechanism -

• Heat transfer -

Key Considerations

Solvent polarityRatio of reactant

Competitive adsorptionAdsorption of product/by products (e.g. H2O)Site blockingSolvent adsorption

Study rate as function of concentrationand compare theoretical profile

Hot spots? In exothermic reactions rapid removal of heat from active site is essential

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Porous catalyst structure

k1 k7

k2 k6

k3 k4 k5

k1 = Film mass transfer to ext. surfacek2 = Diffusion into Catalyst Pore (Bulk or Knudsen Diffusion)k3 = Adsorption on surfacek4 = Reactionk5 = Desorption of Productk6 = Diffusion of Product k7 = Film mass transfer away ext. surface

A B

Diffusion ParametersReactant film

Gas diffusion kinetics important in liquid oxidation/hydrogenation- high pressure needed to increase solubility

Reax. Mix

O2

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For dissolution of oxygen in water, O2(g) <--> O2(aq), enthalpy change under standard conditions is -11.7 kJ/mole.

Dissolution isEXOTHERMIC

Henry’s Law

Raise PRESSURENot temperature

35

At low T reaction processes dominate

At high T diffusional effects become rate limiting

Typical Arrhenius plot

Reaction control

Diffusion control

ln kapp

1/T

Activation Energy - Diffusion Limitation?

kapp = Aexp (-Eapp/RT)

lnkapp = LnA - Eapp/RT

Activation EnergyArrhenius const

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Rate [Cat]n n=1 if no diffusion limitation

Rate with agitation, or gas flow

Eapp is low 10-15 kJmol-1

Diffusional Step Chemical StepSmall T dep (T1/2 or T3/2) High T dep

Ea ~ 20-200kJmol-1

Test for Diffusion Limitation

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Surface Terminology

• Substrate (adsorbent) - the solid surface where adsorption occurs

Adsorbate - the atomic/molecular species adsorbed on the substrate

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• Adsorption - the process in which species ‘bind’ to surface of another phase

•Coverage - the extent of adsorption of a species onto a surface ()

Adsorbed NH3 reacting over Fe

LangmuirAdsorption Isotherm

= 1

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Langmuir Adsorption Isotherm:refresher

• Predicts adsorbate coverage () calculate reaction rates

optimise reaction conditions (T, pressure)

• Chemical equilibria exist during all reactions

- stabilities of adsorbate vs. gas/liquid

- temperature (surface and reaction media)

- pressure (liquid conc.) above catalyst

GAS/LIQUIDreactants, products

solvents

CATALYSTabsorbate

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Equilibrium between the gas molecules M, empty surface sites S

and adsorbates

e.g. for non-dissociative adsorption

S* + M S----M

Assumption 1: Fixed number of identical, localised surface sites

[S----M] adsorbate coverage

[S*] vacancies (1- ) [M] gas pressure

PReactants Products

41

Equilibrium constant, b is

P)1(]tstanac[Re]oducts[Pr

b

Rearrange in terms of ,

)bP1(bP

Langmuir Adsorption Isotherm

- b called sticking-probability and depends on Hads

Assumption 2: Hads and thus b is temperature & pressure independent

b

42

Consider the surface decomposition of a molecule A , i.e.

A (g) A (ads) Products

Let us assume that :

• decomposition occurs uniformly across surface sites

(not restricted to a few special sites)

• products are weakly bound to surface and, once formed, rapidly desorb

• the rate determining step (rds) is the surface decomposition step

Under these circumstances, the molecules of A on the surface are in equilibrium with those in the gas phase

predict surface conc. of A from Langmuir isotherm

Unimolecular Decomposition

= b.P / ( 1 + b.P )

Assumption 3: Hads is coverage independent

Assumption 4: Only 1 adsorbate per site

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Rate of surface decomposition (reaction) is given by an equation:

Rate = k

(assuming that the decomposition of Aads occurs in unimolecular elementary reaction

step and that kinetics are 1st order in surface concentration of intermediate Aads)

Substituting for the gives us equation for the rate in terms of gas pressure above surface

Two extreme cases:

• Limit 1 : b.P << 1 ;

i.e. a 1st order reaction (with respect to A) with an 1st order rate constant , k' = k.b .

This is low pressure (weak binding) limit :

Rate = k b P / ( 1 + b P )

then ( 1 + b.P ) ~ 1 and Rate ~ k.b.P

steady state surface of reactant v. small

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Limit 2 : b.P >> 1 ; then ( 1 + b.P ) ~ b.P and Rate ~ k

i.e. zero order reaction (with respect to A)

This is the high pressure (strong binding) limit : steady state surface of reactant ~100%

Rate shows the same pressure variation as (not surprising since rate !)

Rate = k b P / ( 1 + b P )

45

Langmuir-Hinshelwood type reaction :

Assume that surface reaction between two adsorbed species is the rds.

If both molecules are mobile on the surface and intermix then reaction rate given by following equation for bimolecular surface combination step:

Rate = k

Since b.P / ( 1 + b.P ), when A& B are competing for same adsorption sites the relevant equations are:

A (g) A (ads)

B (g) B (ads)

A (ads) + B (ads) AB (ads) AB (g)rds fast

Bimolecular Reactions:1

46

Look at several extreme limits:

Limit 1 : bA PA << 1 & bB PB << 1

In this limit A & B are both very low , and

Rate k . bAPA . bBPB = k' . PA. PB 1st order in both reactants

Limit 2 : bA PA << 1 << bB PB

In this limit A 0 , B 1 , and

Rate k . bA PA / (bB PB ) = k' . PA / PB

Substituting these into the rate expression gives :

1st order in A

negative 1st order in B

= b.P / ( 1 + b.P )

Rat

e

Pure A Pure B[A]/[B]

Competitive Adsorption

47

48

Eley-Rideal type reaction :

Consider same chemistry

A (g) A (ads)

A (ads) + B (gas) AB (ads) AB (gas)

last step is direct reax between adsorbed A* and gas-phase B.

A + B AB

rds fast

Rate = k

where [B] is pressure/conc

in gas or liquid phase

[A ]/ [B]

Rat

e A varied

Bimolecular Reactions:2

49

However

Without extra evidence cannot conclude above reaction is Eley-Rideal mechanism…

last step may be composite and consist of the following stages

B (g) B (ads)

A (ads) + B (ads) AB (ads) AB (g)

with extremely small steady-state coverage of adsorbed B

Test by monitoring rate

• vary

• vary ratio of or over wide range

fast fast

slow

Langmuir-Hinshelwood

not Eley-Rideal.

B

A

pp

]B[]A[ need free sites

50

Calculated energy diagram

Langmuir-Hinshelwood: CO oxidation over Pt

Highest rate of CO2 production under slightly oxidising conditions:

- a high concentration (~0.75 monolayer) of surface O

- significant no. of Oa vacancies (empty sites)

- CO adsorbs in vacancy with only small energy barrier

Reaction pathway

CO

O

Example 1

CO(g)+O(a)

CO(g)+½O2(g)

51

Ru catalyst

O atoms

Eley-Rideal: CO oxidation over Ru

Highest rate of CO2 production under oxidizing conditions:

- a high concentration (1 monolayer) of surface O

- no surface CO detectable

Example 2

Calculated energy diagram

Transition state

GAS

SURFACE

CO(g)+O(a)

52

Oscillating reactions of carbon monoxide oxidation on platinum.

Good for oxididation

‘Inert’ towards O2

Can adsorb CO

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• Important to verify whether reaction kinetics (esp. liquid phase) are determined by mass transport limitations.

• Homogeneous reaction conditions may not be directly transferable

• Reactions involving Solid-Liquid-Gas particularly challenging!

• Relative ‘sticking probability’ of reactants plays a major role in determining surface coverage and optimum reaction conditions.

• Use of promoters can help with coverage effects....

Kinetics Summary

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• Surfaces

• Structure

• Geometric factors - dispersion, particle size effects

• Electronic factors - alloys

Lecture 4 Overview

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Surfaces

Most technologically important catalysts contain active metal surfaces

• Most possess simple fcc structures e.g. Pt, Rh, Pd

Face Centred Cubic unit cell

• Low index faces are most commonly studied surfaces with unique:

- Surface symmetry

- Surface atom coordination

- Surface reactivity

56

Surface Symmetry

(111) (100) (110)

• Surface are regions of high energy - cohesive energy is lost in their creation

• “Close-packed” surfaces have higher coord. nos - more stable low surface energy

• Open (rough) surfaces low coord. nos - unstable high surface energy

Principle Low Index Surfaces

57

For any reaction the pathway depends on:- reactant geometry - reactant energy

relative to transition complex

Monitor adsorption geometry via vibrational spectroscopy(RAIRS, HREELS, ARUPS)

Geometric Factors

Reax. Co-ordinate

T.S.

E

R

P

e.g. C2H4 dehydrogenation

58

Calculate Ni-C-C bond angle,

for different Ni surfaces,

Ni-Ni = 0.25 = 103 , bond twists to stabilise ethene “ = 0.35 = 123 , destabilisation of C-H bond

Observe R(110) > R(100) > R(111)

(110) (100) (111)

0.35 nm

0.25 0.25

Ni Ni

CH2 CH2

x 5

59

log Rate

Atom Spacing0.40 0.45

WTa

Ni

RhPd

Pt

Fe

W

Ta

Large Strain

LowStrain

log Rate

Atom Spacing0.40 0.45

WTa

Ni

RhPd

Pt

Fe

W

Ta

log Rate

Atom Spacing0.40 0.45

WTa

Ni

RhPd

Pt

Fe

W

Ta

Large Strain

LowStrain

• Spectroscopy shows - same adsorption mode (HREELS) - strength (TPD)

Geometric Factors: C2H4 dehydrogenation

Volcano Plot

• Trend reflects C2H4 geometry surface structure important

(111) (110)

60

Quadrupole MassSpectrometer

H2

Temperature-programmed desorption

Pt(111)

Temperature / K 100 200 300 400 500 650

H2

Des

orpt

ion 3 L C2H4

Stepwisedecomposition

C2H3

CH3

CH2

61

Supported metal particle can expose different crystal faces.

In addition there are steps & defects within each particle. - these are low coordination sites

- region of high potential energy

facilitate bond dissociation

Structure Sensitivity

Pd{557} surface with - {111} terraces - {100} steps

Pd{111} 9-coordinate

Pd{100} 8-coordinate

Defect sites

Terrace sites

62

Structure Sensitivity occurs when reaction requires specific active sites:(any mix of step, terrace, kink atoms)

The density of steps and dominant crystal face reflects the metal particle size

changing particle size modifies rate

Stepped surfaces Stepped + kinked surface

(100)

square

(111)

hex

63

100xNN

(%)DispersionT

S

Consider total fraction of available surface sites:

Spherical particles

if Ns = total no. of surface atoms NT = total atoms in particle

For small particles (< 20Å) Dispersion 1

if Activity SA, then particle size will rate (per mass of catalyst)

provided exposed surface atom arrangement unchanged

64

Structure sensitive test:

Consider CO + 3H2 CH4 + H2O

Compare specific TON (per surface site)

Ni (100)

9% Ni/Al2O3

5% Ni/Al2O3

If reaction requires specific (4-coord) active site expect

• constant Eact observed

• higher rate over surfaces with most (100) sites larger particles

65

Structure sensitive vs insensitive reaction:

Cyclohexane hydrogenolysis

• High step/kink densities high rates• Reaction requires defect sites

contrast with (de)hydrogenation which proceeds over diverse surface arrangements

Reaction kinetics tell us about the active site

-H2

-CHx

66

Electronic Factors: Alloys

Electronic properties of crystalline solids described by Band Theory

Bimetal may transfer e- to/from active metal changes adsorbate binding strength

Ene

rgy

1s-orbital

Anti-Bo. MO

Bo. MO

Band of tightly-spaced MO’s

1s-band

2s-band

Energy

Bimetal

Alkali-metals→ 1 valence e-/atom

67

Bimetallic Alloys

• ‘True’ alloy versus surface decoration?

• Requirements: - Intimate contact between components

- Direct chemical coordination (bonding) between metal neigbours

Al2O3

Pt/Rh

Al2O3

Pt/Rh

vs. Rh

• Minimise excess bimetal deposits on support

68

Acetylene Coupling over Pd/Au

Reaction mechanism well understood

Unique chemistry - low temperature (25°C) & high selectivity - operates from 10-13 - 10 atmospheres

Reaction requires 7-atom ensemble

69

Pd(111)

C2H2 C6H6

Pd(111)

Methodology- construct relevant model catalyst

- add gold (Au) promoter

- perform chemistry over Pd/Au alloys Pd(111)

Au

C2H2 C6H6

• Incorporation of Au improved activity, selectivity & lifetime

Zoom

Au

70

0

20

40

60

80

100

% B

enze

ne P

rodu

ctio

n

0 20 40 80 60 100

% Gold

Trace surface Au enhances

benzene synthesis over Pd catalysts

Pd6Au

Chemistry - products include C6H6, C6H14, C6H14

- add heteroatoms O, S..C5 heterocycles

BUT ~50 % of C2H2 decomposes over Pd

71

Au/Pd alloys promote cyclisation

Auger shows surface C build-up

- Au prevents sterically-demanding

hydrogenolysis reax. (C-C breaking)

C6H6 desorption temperature

- Au destabilises product binding

- benzene tilts (IR)

AES/XPS Au Pd charge-transfer

vs.

72

Summary

Au/Pd alloys reactant/product decomposition vs. Pd

Au selectivity to benzene Au long-term activity

Both ensemble & ligand effects are important

Au breaks up active site

Au ‘softens’ Pd chemistry

73

•Sol-gel synthesis Formation of inorganic oxide via acid or base

initiated hydrolysis of liquid precursor (e.g. Si(OEt)4).

Can incorporate active sites directly in ‘one-pot’ route.

•Post modification Active site is ‘grafted’ onto pre-formed support via

reaction with surface groups (often OH)

Lecture 6Preparation of Heterogeneous Catalysts

74

•Impregnation Pore filling with catalyst precursor followed by

evaporation of solvent

Traditional method for supported metals

•Ion Exchange Equilibrium amount of cation or anion is adsorbed at

active sites containing H+ or OH-

SOH + C+ = SOC + H+

S(OH)- + A- = SA- + (OH)-

•Precipitation Catalyst precursor is precipitated in form of hydroxide or carbonate.

75

Incipient-Wetness (wet-impregnation)

76

• Increased rate of drying temperature gradient across pore forces precursor to be deposited at the pore mouth.

• Concentration of solution for impregnation will alter loading and particle size

77

Precipitation

78

Surfactantmicelle

Alumino-surfactant mesostructure

Ordered (hexagonal)array

MesostructuredAl2O3

Surfactant + Solvent Micelle

Lauric Acid(coconut oil)

Template extractionAl precursor

Templated Sol-Gel

Surfactant

79

Porosimetry• N2 physisorption used to surface area, pore structure, pore shape

• Typical adsorption isotherms

• BET model surface area during monolayer adsorption

Characterisation

80

A B

E

• Use hysteresis on desorption to deduce pore shape

According to IUPAC

Type A = cylindrical poresType B = slit shaped poresType E = Bottle neck pores

81

• Well developed laboratory technique

• Gives satisfactory results (<5 h per sample)

Powder X-Ray Diffraction

• Complications - Minimum amount of material is required (usually 1-5wt%)

- Diffraction lines broaden as crystallite size decreases

hard to measure crystallites < 2nm diameter

peakwidth yields particle size

- Lines from different components often overlap or interfere with each other

dCos

893.0B

B = line width at ½ height (in degrees)d = crystallite size (in nm) = X-Ray wave length (0.154nm for Cu K)

= Diffraction angle (in degrees)

Measure intensity of diffraction peaks as a function of sample and analyser angle (2)

82

XRD of Cu/CeO2 Catalyst

83

• Typical XRD lattice parameter for MCM = 35Å

• Estimate pore wall thickness

d(100)

XRD of modified MCM supports

84

Can make vibrational measurements of adsorbates on catalyst surface!

• Transmission Mode – using KBr Self Supporting Wafer

- e.g. CO adsorption on metal crystallites

• Diffuse Reflectance Mode (DRIFTS) – acquire data directly from a catalyst powder

Infrared Spectroscopy

85

COURSE SUMMARY

Learning Objectives

• Catalysis Definitions - activity, selectivity, conversion, TON and TOF

• Reaction Kinetics - diffusion limitations, Langmuir adsorption, unimolecular and bimolecular reactions

• Surface structure - terminology, symmetry, geometric vs. electronic factors

• Structure-Sensitivity - definition, particle size effects, dispersion

• Catalyst Preparation - simple methodologies

• Catalyst Characterisation - simple methodologies, surface vs. bulk insight