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

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slide1

Heterogeneous Catalysis

6 lectures

Dr. Adam Lee

Surface Chemistry & Catalysis Group

slide2

Synopsis

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.

Topics:

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

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)
slide3

Lecture 1 Overview

  • 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
slide4

How can we accelerate a chemical reaction?

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?

Use reagents

- stoichiometric

- separation problems

- TOXIC waste

- Industrial fine chemicals

processes developed

- Carry on using reagents

slide5

Permanganate, Manganese dioxide,

Chromium (VI)

(<0.10 ppm)

Metal Hydrides, (NaBH

, LiAlH

)

4

4

Reducing metals (

Na, Fe, Mg,

Zn)

Potassium

butoxide,

diisopropylamine

Tetramethyl guanidine

AlCl

, BF

, ZnCl

, H

SO

3

3

2

2

4

Typical Reagents

Oxidation

Reduction

Basic reagents

Acidic reagents

C-C Coupling

Homogeneous Pd based complexes

slide6

Nobel Prize in Chemistry 2007 – Gerhard Ertl

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
slide7

HDPE

LDPE

Polymerisation (1957/1991)

nC2H2

Zeigler-Natta

/Metallocene

Catalytic Cracking (1964)

CxH2x+2 Cx-2H2x-2

CxH2x+2 Cx-2H2x-4

Faujasitic

zeolites

Historical Evolution

slide8

HC + CO + NOX CO2 + H2O + N2

Pt/Rh/Al2O3

Chiral pocket

Automotive Emission Control (1976)

Chiral Catalysis (1988)

slide9

Advantages of Catalytic Technology

‘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.

slide10

Classes of Catalyst

  • 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
slide11

kforward

Reactants Products

kback

Catalyst Definitions

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

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
slide12

Uncatalysed

Catalysed

Catalyst Definitions

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

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

Catalysts do not affect K!

slide13

Catalyst Definitions

Goal of catalytic research is improved activity & selectivity

Alter rate constants: k

For simple reax. A  B + C

  • Activity =
  • Selectivity=

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

mol . s-1 rate of reaction

%relative formation

of specific product

slide14

Triglyceride transesterification

Catalyst Efficiency: 1

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)

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

dt 20

Conversion = 20 %

Biodiesel

slide15

Triglyceride transesterification

[FAME]

[Diglyceride]+[Monoglyceride]+[FAME]

45

20+10+45

x 100

=

x 100

Tri-glyceride

Methyl-butanoate

(FAME)

Di-glyceride

Selectivity to FAME?

= 60 %

slide16

Catalyst Efficiency: 2

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 COCO2

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

e.g. 1 mmol Pd converts 1000 mmols of COCO2 in 10 s

To be valid TOF must be measured in absence of:

- mass transport limitations

- deactivation effects

TON = 1000

TOF = 100 s-1

slide17

Catalyst Constituents

Active Phase

- transition-metal

- highly dispersed

- reduced/oxidic/sulphided

state

‘Inert’ Support

- high surface area oxide

- high porosity

- high thermal/mechanical

stability

Sn - Naptha reforming

Cl - Ethylene epoxidation

K2O - NH3 synthesis

C - Catalytic cracking

S, Pb - Car exhaust catalysts

active component
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 Electron

Micrograph

slide19

Support

Main function is to maintain high surface area for active phase

Other features include:

  • porosity
  • mechanical properties
  • stability
  • dual functional activity
  • modification of active component

Types:

  • high melting point oxides (silica, alumina)
  • clays
  • carbons
slide20

Advantages and Limitations of Heterogeneous Catalysts

  • 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
slide21

Why the Implementation Delay??

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

slide22

Dr. Paul Anastas

Director of Green Chemical Inst.

Washington D.C.

ex. White House Asst. Director

for Environment

slide23

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

Chemical

Process

No waste

slide24

Only required product

C + D + E + F ...

A + B

C (only product)

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

Selectivity

slide25

Filtration

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

High Activity

Heating

Cooling

Stirring

Distillation

Compression

Pumping

Separation

Energy requirement

(electricity)

Global

warming

Burn fossil

fuel

CO2 to

atmosphere

slide26

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

Selectivity

slide27

CONCLUSION:

“Selective catalysts are superior to stoichiometric reagents”

Stoichiometric

4-Chlorobenzophenone

Catalytic

slide28

Catalysis in Action: C2H2 on Pd(111)

Scanning Tunnelling Microscope movie

- real-time molecular rotation

Further Info

Even More Info!

slide29

Lecture 3/4 Overview

  • Reaction kinetics and diffusion limitations
  • Langmuir adsorption isotherm
  • Unimolecular reaction
  • Bimolecular reactions
  • Surfaces
slide30

Kinetics of Catalysed Reactions

  • 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.

slide31

Comparison

Batch

Reactor

Batch/Flow

Reactor

slide32

Key Considerations

  • Diffusional effects -

(Mass Transfer)

  • Adsorption strength -
  • Mechanism -
  • Heat transfer -

Solvent polarity

Ratio of reactant

Competitive adsorption

Adsorption of product/by products (e.g. H2O)

Site blocking

Solvent adsorption

Study rate as function of concentration

and compare theoretical profile

Hot spots?

In exothermic reactions rapid removal

of heat from active site is essential

slide33

Reactant film

O2

Diffusion Parameters

k1 k7

k2 k6

k3 k4 k5

Porous catalyst structure

A B

k1 = Film mass transfer to ext. surface

k2 = Diffusion into Catalyst Pore (Bulk or Knudsen Diffusion)

k3 = Adsorption on surface

k4 = Reaction

k5 = Desorption of Product

k6 = Diffusion of Product

k7 = Film mass transfer away ext. surface

Reax. Mix

Gas diffusion kinetics important in liquid oxidation/hydrogenation

- high pressure needed to increase solubility

slide34

Henry’s Law

Dissolution is

EXOTHERMIC

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

Raise PRESSURE

Not temperature

slide35

Arrhenius const

Activation Energy

Diffusion control

ln kapp

Reaction control

1/T

Activation Energy - Diffusion Limitation?

  • At low T reaction processes dominate
  • At high T diffusional effects become rate limiting
  • Typical Arrhenius plot

kapp = Aexp (-Eapp/RT)

lnkapp = LnA - Eapp/RT

slide36

Test for Diffusion Limitation

  • 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 Step

Small T dep (T1/2 or T3/2) High T dep

Ea ~ 20-200kJmol-1

slide37

Surface Terminology

  • Substrate (adsorbent)

- the solid surface where adsorption occurs

 Adsorbate

- the atomic/molecular species adsorbed on the substrate

slide38

Adsorbed NH3 reacting over Fe

 = 1

Langmuir

Adsorption

Isotherm

  • Adsorption

- the process in whichspecies ‘bind’ to surface of another phase

  • Coverage

- the extent of adsorption of

a species onto a surface ()

slide39

GAS/LIQUID

reactants, products

solvents

CATALYST

absorbate

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

slide40

S* + M

S----M

[S----M]  

adsorbate coverage

[S*]  vacancies

 (1- )

Reactants

[M]  gas pressure

 P

Products

Equilibrium between the gas molecules M, empty surface sites S and adsorbates

e.g. for non-dissociative adsorption

Assumption 1:

Fixed number of identical, localised surface sites

slide41

b

Langmuir Adsorption Isotherm

Equilibrium constant, b is

Rearrange in terms of ,

- b called sticking-probability and depends on Hads

Assumption 2:

Hads and thus b is temperature & pressure independent

slide42

Unimolecular Decomposition

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

Assumption 3:

Hads is coverage independent

Assumption 4:

Only 1 adsorbate per site

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

slide43

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

Rate = k q

(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 q 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 )

and Rate ~ k.b.P

then ( 1 + b.P ) ~ 1

 steady state surface q of reactantv. small

slide44

Limit 2 : b.P >> 1 ; then ( 1 + b.P ) ~ b.P and Rate ~ ki.e. zero order reaction (with respect to A)

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

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

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

slide45

Bimolecular Reactions:1

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 qA qB

Since q = 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

slide46

Competitive Adsorption

Rate

Pure A

Pure B

[A]/[B]

Substituting these into the rate expression gives :

Look at several extreme limits:

  • Limit 1 : bAPA<< 1 & bBPB<< 1

In this limit qA & qB are both very low , and

Rate®k . bAPA . bBPB= k' . PA. PB1st order in both reactants

  • Limit 2 :bAPA<< 1 <<bBPB

In this limit qA® 0 , qB® 1 , and

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

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

1st order in A

negative 1st order in B

slide48

Rate = k qA [B]

Rate

[A ]/ [B]

Bimolecular Reactions:2

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

A varied

where [B] is pressure/conc

in gas or liquid phase

slide49

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 qA
  • vary ratio of or over wide range

slow

fast

fast

Langmuir-Hinshelwood

not Eley-Rideal.

need free sites

slide50

O

CO

Calculated energy diagram

Example 1

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

CO(g)+½O2(g)

CO(g)+O(a)

Reaction pathway

slide51

O atoms

Ru catalyst

Calculated energy diagram

GAS

SURFACE

Transition state

Example 2

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

CO(g)+O(a)

slide52

Oscillating reactions of carbon monoxide oxidation on platinum.

‘Inert’ towards O2

Can adsorb CO

Good for

oxididation

slide53

Kinetics Summary

  • 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....
slide54

Lecture 4 Overview

  • Surfaces
  • Structure
  • Geometric factors - dispersion, particle size effects
  • Electronic factors - alloys
slide55

Face Centred Cubic unit cell

Surfaces

Most technologically important catalysts contain active metal surfaces

  • Most possess simple fcc structures e.g. Pt, Rh, Pd
  • Low index faces are most commonly studied surfaces with unique:

- Surface symmetry

- Surface atom coordination

- Surface reactivity

slide56

(111)

(100)

(110)

Surface Symmetry

  • Surface are regions of high energy

- cohesive energy is lost in their creation

Principle Low Index Surfaces

  • “Close-packed” surfaces have higher coord. nos

- more stable lowsurface energy

  • Open (rough) surfaces low coord. nos

- unstable  highsurface energy

slide57

E

T.S.

P

Reax. Co-ordinate

R

Geometric Factors

For any reaction the pathway depends on:

- reactant geometry

- reactant energy

relative to transition complex

Monitor adsorption geometry via vibrational spectroscopy

(RAIRS, HREELS, ARUPS)

e.g. C2H4 dehydrogenation

slide58

CH2 CH2

Ni Ni

x 5

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)

slide59

Geometric Factors: C2H4 dehydrogenation

  • Spectroscopy shows

- same adsorption mode (HREELS)

- strength (TPD)

Volcano Plot

(110)

(111)

Trend reflects C2H4geometrysurface structure important

slide60

Pt(111)

Temperature-programmed desorption

C2H3

Stepwise

decomposition

Quadrupole Mass

Spectrometer

CH3

CH2

H2

slide61

Structure Sensitivity

  • 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

slide62

(111)

(100)

hex

square

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

slide63

Spherical particles

Consider total fraction of available surface sites:

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

slide64

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
slide65

-H2

-CHx

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

slide66

Bimetal

2s-band

Energy

1s-band

Electronic Factors: Alloys

 Electronic properties of crystalline solids described by Band Theory

Alkali-metals

→ 1 valence e-/atom

 Bimetal may transfer e- to/from active metal

changes adsorbate binding strength

slide67

Bimetallic Alloys

  • Requirements:

- Intimate contact between components

- Direct chemical coordination (bonding) between metal neigbours

‘True’ alloy versus surface decoration?

Minimise excess bimetal deposits on support

slide68

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

slide69

 Methodology

- construct relevant

model catalyst

- add gold (Au)

promoter

- perform chemistry

over Pd/Au alloys

Au

Zoom

C2H2

C2H2

C6H6

C6H6

Pd(111)

Pd(111)

Pd(111)

Au

  • Incorporation of Au

improved activity, selectivity & lifetime

slide70

Chemistry

- products include C6H6, C6H14, C6H14

- add heteroatoms O, S..C5 heterocycles

BUT ~50 % of C2H2 decomposes over Pd

slide72

Summary

Au/Pd alloys  reactant/product decomposition vs. Pd

 Au  selectivity to benzene

  • Au  long-term activity

Bothensemble & ligand effects are important

Au breaks up active site

Au ‘softens’ Pd chemistry

slide73

Lecture 6

Preparation of Heterogeneous Catalysts

  • Sol-gel synthesisFormation 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 modificationActive site is ‘grafted’ onto pre-formed support via

reaction with surface groups (often OH)

slide74

ImpregnationPore filling with catalyst precursor followed by

evaporation of solvent

Traditional method for supported metals

  • Ion ExchangeEquilibrium amount of cation or anion is adsorbed at

active sites containing H+ or OH-

SOH + C+ = SOC + H+

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

  • PrecipitationCatalyst precursor is precipitated in form of hydroxide or carbonate.
slide76

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
slide78

Surfactant

Templated Sol-Gel

Surfactant + Solvent Þ Micelle

Lauric Acid

(coconut oil)

Al precursor

Template extraction

Surfactant

micelle

Ordered (hexagonal)

array

Alumino-surfactant

mesostructure

Mesostructured

Al2O3

slide79

Characterisation

Porosimetry

  • N2 physisorption used to surface area, pore structure, pore shape
  • Typical adsorption isotherms
  • BET model surface area during monolayer adsorption
slide80

Use hysteresis on desorption to deduce pore shape

A

B

E

According to IUPAC

Type A = cylindrical pores

Type B = slit shaped pores

Type E = Bottle neck pores

slide81

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)

Powder X-Ray Diffraction

  • Well developed laboratory technique
  • Gives satisfactory results (<5 h per sample)
  • 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

slide83

d(100)

XRD of modified MCM supports

  • Typical XRD lattice parameter for

MCM = 35Å

  • Estimate pore wall thickness
slide84

Infrared Spectroscopy

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

slide85

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