Transcript Year 2 – Heterogeneous Catalysis Lectures 1-6

Heterogeneous Catalysis
6 lectures
Dr. Adam Lee
Surface Chemistry & Catalysis Group
1
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)
2
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
3
How can we accelerate a chemical reaction?
Organic Chemistry (1805)
Physical Chemistry
Use reagents
- stoichiometric
- separation problems
- TOXIC waste
Discovery of Catalysis (1835)
- Industrial fine chemicals
processes developed
- Petrochemical & oil refining
industry recognise promise
- Carry on using reagents
- Catalytic technology
generates >10 trillion $/yr
- Clean technology (1990?)
Why don’t we use a
catalyst?
- applications in plastics,
fabrics, food, fuel
4
Typical Reagents
•Oxidation
Permanganate, Manganese dioxide,
Chromium (VI)
(<0.10 ppm)
•Reduction
Metal Hydrides, (NaBH4, LiAlH4)
Reducing metals (Na, Fe, Mg, Zn)
•Basic reagents
Potassium butoxide, diisopropylamine
Tetramethyl guanidine
•Acidic reagents
AlCl3, BF3, ZnCl2, H2SO4
•C-C Coupling
Homogeneous Pd based complexes
+
T
H-Br
5
Importance of Heterogeneous Catalysis
Chemicals Industry:
>90% of global chemical output relies upon heterogeneous catalysed processes
Economics:
Nobel Prize in Chemistry 2007 – Gerhard Ertl
• ~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
6
Historical Evolution
Polymerisation (1957/1991)
nC2H2
HDPE
LDPE
Zeigler-Natta
/Metallocene
Catalytic Cracking (1964)
CxH2x+2
Cx-2H2x-2
CxH2x+2
Cx-2H2x-4
Faujasitic
zeolites
7
Automotive Emission Control (1976)
HC + CO + NOX
Pt/Rh/Al2O3
CO2 + H2O + N2
Chiral Catalysis (1988)
Chiral pocket
8
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.
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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 10
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)
kforward
Consider,
Reactants
Products
kback
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
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Catalyst Definitions
Energetics:
Reactants do not all have same energy: Boltzmann distribution
Uncatalysed
Catalysed
http://www.chemguide.co.uk/physical/basicrates/catalyst.html#top
So what determines theoretical product yield??
- thermodynamic driving force, G = -nRT ln(K)
Catalysts do not affect K!
Large –ve G  large +ve ln(K)  huge K  ~100 % Yield
12
Catalyst Definitions
Goal of catalytic research is improved activity & selectivity
Alter rate constants: k
For simple reax.
• Activity =
• Selectivity
AB+C
 d[A]
dt
mol . s-1
[B]
x 100
= [B]  [C]
%
rate of reaction
relative formation
of specific product
= Yield of Desired Product x 100
Total Yield of all Product
%
13
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)
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[Reactant] / mmols
Triglyceride transesterification
Activity = -d[Tributyrin] = 20
dt
20
100
= 1 mmol.s-1
80
Conversion = 20 %
60
40
20
0
Biodiesel
0
50
100
150
200
Time / s
14
Triglyceride transesterification
Tri-glyceride
Methyl-butanoate
Di-glyceride
(FAME)
Selectivity to FAME?
45
[FAME]
x 100 = 60 %
x 100 =
20+10+45
[Diglyceride]+[Monoglyceride]+[FAME]
15
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
TON = 1000
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
TOF = 100 s-1
To be valid TOF must be measured in absence of:
- mass transport limitations
- deactivation effects
16
Catalyst Constituents
‘Inert’ Support
Active Phase
- transition-metal
- highly dispersed
- reduced/oxidic/sulphided
state
- high surface area oxide
- high porosity
- high thermal/mechanical
stability
Solid Phase
(powder, wire, gauze or pellet)
Promoters
Sn - Naptha reforming
Cl - Ethylene epoxidation
K2O - NH3 synthesis
Poisons
C - Catalytic cracking
S, Pb - Car exhaust catalysts
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Active Component
Responsible for the principal chemical reaction
Features:
• activity, selectivity, purity
• surface area, distribution on support, particle size
Types:
Platinum particles on a porous
carbon support
• Metals
• Semiconductor oxides and sulphides
• Insulator oxides and sulphides
Transmission Electron
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Micrograph
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
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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
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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
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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.
Dr. Paul Anastas
Director of Green Chemical Inst.
Washington D.C.
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.
ex. White House Asst. Director
for Environment
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.
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“It is better to prevent waste than to treat or clean
up waste after it is formed”
Chemical
Process
No waste
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“Synthetic methods should be designed to
maximise the incorporation of all materials
used into the final product”
Selectivity
Only required product
A+B
C + D + E + F ...
C (only product)
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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”
Heating
Cooling
Stirring
Distillation
Compression
Pumping
Separation
High Activity
Energy requirement
(electricity)
Burn fossil
fuel
Filtration
Global
warming
CO2 to
atmosphere
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“Unnecessary derivatisation (blocking group,
protection/deprotection..) should be avoided
wherever possible”
O
protecting
group
H
Protect
O
protecting
group
H
Reduction
O
R
H
OH
R
R
Selectivity
Deprotection
O
Specific reduction agent
H
OH
R 26
CONCLUSION:
“Selective catalysts are superior to stoichiometric reagents”
Cl
O
AlCl3
AlCl3
O
+
Cl
Cl
H2O
Stoichiometric
exothermic
O
4-Chlorobenzophenone
+
Al (OH)3
+
Cl
Catalytic
O
+
Cl
Cl
ENVIROCAT
EPZG
O
Cl
135 oC/6h
O2N
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NO2
HCl
Catalysis in Action: C2H2 on Pd(111)
Scanning Tunnelling Microscope movie
- real-time molecular rotation
Further Info
Even More Info!
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Lecture 3/4 Overview
• Reaction kinetics and diffusion limitations
• Langmuir adsorption isotherm
• Unimolecular reaction
• Bimolecular reactions
• Surfaces
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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.
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Comparison
Homogeneous vs Heterogeneous
Batch
Reactor
Add
Heterogeneous
Catalyst
Quench&
Neutralise
Filter
Separate
Product
Batch/Flow
Reactor
Homogeneous
Reaction
Waste
Product
Catalyst
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Key Considerations
• Diffusional effects (Mass Transfer)
Solvent polarity
Ratio of reactant
• Adsorption strength -
Competitive adsorption
Adsorption of product/by products (e.g. H2O)
Site blocking
Solvent adsorption
• Mechanism
-
Study rate as function of concentration
and compare theoretical profile
• Heat transfer
-
Hot spots?
In exothermic reactions rapid removal
of heat from active site is essential
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Diffusion Parameters
Reactant film
k1
k7
k2
k6
k3
Porous catalyst structure
k4
A
k5
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
O2
Reax. Mix
Gas diffusion kinetics important in liquid oxidation/hydrogenation
- high pressure needed to increase solubility
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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
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Activation Energy - Diffusion Limitation?
 At low T reaction processes dominate
 At high T diffusional effects become rate limiting
 Typical Arrhenius plot
Arrhenius const
Activation Energy
kapp = Aexp (-Eapp/RT)
lnkapp = LnA - Eapp/RT
ln kapp
Diffusion
control
Reaction
control
1/T
35
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
Small T dep (T1/2 or T3/2)
Chemical Step
High T dep
Ea ~ 20-200kJmol-1
36
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
Adsorbed NH3 reacting over Fe
• Coverage
- the extent of adsorption of
a species onto a surface ()
=1

Langmuir
Adsorption
Isotherm
38
Langmuir Adsorption Isotherm:refresher
• Predicts adsorbate coverage ()
 calculate reaction rates
 optimise reaction conditions (T, pressure)
• Chemical equilibria exist during all reactions
GAS/LIQUID
reactants, products
solvents
CATALYST
absorbate
- stabilities of adsorbate vs. gas/liquid
- temperature (surface and reaction media)
- pressure (liquid conc.) above catalyst
39
Equilibrium between the gas molecules M, empty surface sites S
and adsorbates
e.g. for non-dissociative adsorption
S* + M
S----M
[S*]  vacancies
Reactants
Products
[M]

gas
pressure
 (1- )
P
[S----M]  
adsorbate coverage
Assumption 1:
Fixed number of identical, localised surface sites
40
Equilibrium constant, b is
b
[Pr oducts]

b 

[Re ac tan ts] (1  )P
Rearrange in terms of ,
bP
 
(1  bP)
Langmuir Adsorption Isotherm
- b called sticking-probability and depends on Hads
Assumption 2:
Hads and thus b is temperature & pressure independent
41
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)
Assumption 3:
Hads is coverage independent
Assumption 4:
Only 1 adsorbate per site
• 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
 = b.P / ( 1 + b.P )
42
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
Rate = k b P / ( 1 + b P )
Two extreme cases:
• Limit 1 : b.P << 1 ; then
( 1 + b.P ) ~ 1
and
Rate ~ k.b.P
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 :  steady state surface  of reactant v. small
43
 Limit 2 : b.P >> 1 ; then
( 1 + b.P ) ~ b.P
i.e. zero order reaction (with
Raterespect
= k btoPA)
/(
and
Rate ~ k
1+bP)
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  !)
44
Bimolecular Reactions:1
Langmuir-Hinshelwood type reaction :
A (g)  A (ads)
B (g)  B (ads)
A (ads) + B (ads)
rds
AB (ads)
fast
AB (g)
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 A B
Since   b.P / ( 1 + b.P ), when A& B are competing for same adsorption sites the
relevant equations are:
45
Competitive Adsorption
Substituting these into the rate expression gives :
Pure A
Look at several extreme limits:
Limit 1 :
bA PA << 1
&
=
In this limit A & B are both very low , and
Pure B
bB PB << 1
b.P / ( 1 + b.P )
Rate  k . bAPA . bBPB = k' . PA. PB
Limit 2 :
[A]/[B]
1st order in both reactants
bA PA << 1 << bB PB
In this limit A  0 , B  1 , and
Rate  k . bA PA / (bB PB ) = k' . PA / PB
1st order in A
negative 1st order in B
46
47
Bimolecular Reactions:2
Eley-Rideal type reaction :
A + B  AB
Consider same chemistry
A (g)  A (ads)
A (ads) + B (gas)
rds
AB (ads)
fast
AB (gas)
last step is direct reax between adsorbed A* and gas-phase B.
Rate = k A [B]
A varied
where [B] is pressure/conc
in gas or liquid phase
[A ]/ [B]
48
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)
fast
slow
AB (ads)
with extremely small steady-state coverage of adsorbed B 
fast
AB (g)
Langmuir-Hinshelwood
not Eley-Rideal.
Test by monitoring rate
• vary A
pA
[A]
• vary ratio of
or
over wide range
pB
[ B]
need free sites
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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
Calculated energy diagram
O
CO
CO(g)+½O2(g
)
CO(g)+O(a)
Reaction pathway
50
Example 2
Eley-Rideal: CO oxidation over Ru
Highest rate of CO2 production under oxidizing conditions:
- a high concentration (1 monolayer) of surface O
O atoms
- no surface CO detectable
Ru catalyst
Calculated energy diagram
GAS
CO(g)+O(a)
SURFACE
Transition state
51
‘Inert’ towards O2
Good for
oxididation
Can adsorb CO
Oscillating reactions of carbon
monoxide oxidation on platinum.
52
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....
53
Lecture 4 Overview
• Surfaces
• Structure
• Geometric factors - dispersion, particle size effects
• Electronic factors - alloys
54
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
55
Surface Symmetry
• Surface are regions of high energy
- cohesive energy is lost in their creation
(111)
(100)
(110)
Principle Low Index Surfaces
• “Close-packed” surfaces have higher coord. nos
- more stable  low surface energy
• Open (rough) surfaces low coord. nos
- unstable  high surface energy
56
Geometric Factors
For any reaction the pathway depends on:
- reactant geometry
- reactant energy
relative to transition complex
e.g. C2H4 dehydrogenation
E
T.S.
P
R
Reax. Co-ordinate
Monitor adsorption geometry via vibrational spectroscopy
(RAIRS, HREELS, ARUPS)
57
Calculate Ni-C-C bond angle,
CH2  CH2
Ni
Ni
for different Ni surfaces,
(110)
(100)
0.25
(111)
0.25
0.35 nm
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)
x5
58
Geometric Factors: C2H4 dehydrogenation
• Spectroscopy shows
- same adsorption mode (HREELS)
- strength (TPD)
Rh
log Rate
Low
Strain
Pd
Pt
Fe
Large
Strain
Volcano Plot
W
Ni
W
Ta
(111)
Ta
(110)
0.45
0.40
Atom Spacing
• Trend reflects C2H4 geometry  surface structure important
59
Temperature-programmed desorption
C2H3
H2 Desorption
Stepwise
decomposition
3 L C2H4
Quadrupole Mass
Spectrometer
CH3
CH2
H2
100
200
300
400
500
650
Temperature / K
60
Structure Sensitivity
 Supported metal particle can expose different crystal faces.
 Defect sites
Pd{111}
9-coordinate
 Terrace sites
Pd{100}
8-coordinate
Pd{557} surface with
- {111} terraces
- {100} steps
 In addition there are steps & defects within each particle.
- these are low coordination sites
- region of high potential energy
 facilitate bond dissociation
61
Structure Sensitivity occurs when reaction requires specific active sites:
(any mix of step, terrace, kink atoms)
(111)
hex
(100)
square
Stepped surfaces
Stepped + kinked surface
The density of steps and dominant crystal face reflects the metal particle
size
changing particle size modifies rate
62
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)
NS
Dispersion (%) 
x 100
NT
Dispersion  1
if Activity  SA, then  particle size will  rate (per mass of catalyst)
provided exposed surface atom arrangement unchanged
63
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
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Structure sensitive vs insensitive reaction:
Cyclohexane hydrogenolysis
• High step/kink densities  high rates
• Reaction requires defect sites
-H2
-CHx
contrast with (de)hydrogenation which proceeds over diverse surface arrangements
Reaction kinetics tell us about the active site
65
Electronic Factors: Alloys
Energy
 Electronic properties of crystalline solids described by Band Theory
Alkali-metals
→ 1 valence e-/atom
1s-orbital
Anti-Bo. MO
Bimetal
Bo. MO
Energy

Band of  tightly-spaced
MO’s
2s-band
1s-band
 Bimetal may transfer e- to/from active metal
 changes adsorbate binding strength
66
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
Pt/Rh
Pt/Rh
vs.
Al2O3
Al2O3
Rh
67
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
68
• Incorporation of Au
 improved activity, selectivity & lifetime
C2H2
C6H6
 Methodology
- construct relevant
model catalyst
Pd(111)
Au
Au
- add gold (Au)
promoter
Pd(111)
C2H2
Pd(111)
C6H6
- perform chemistry
over Pd/Au alloys
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 Chemistry
- products include C6H6, C6H14, C6H14
- add heteroatoms O, S..C5 heterocycles
~50 % of C2H2 decomposes over Pd
% Benzene Production
BUT
Pd6Au
100
80
% Gold
60
40
20
0
0
20
40
60
80
100
Trace surface Au enhances
benzene synthesis over Pd catalysts
70

Au/Pd alloys promote cyclisation

Auger shows  surface C build-up
-
Au prevents sterically-demanding
hydrogenolysis reax. (C-C breaking)
vs.

C6H6 desorption temperature 
-
Au destabilises product binding
-
benzene tilts (IR)
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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
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Lecture 6
Preparation of Heterogeneous Catalysts
•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)
73
•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 OHSOH + C+
= SOC + H+
S(OH)- + A- = SA- + (OH)-
•Precipitation
Catalyst precursor is precipitated in form of hydroxide
or carbonate.
74
Incipient-Wetness (wet-impregnation)
75
• 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
76
Precipitation
77
Templated Sol-Gel
Surfactant + Solvent  Micelle
Surfactant
Lauric Acid
(coconut oil)
Al precursor
Template extraction
Surfactant
micelle
Ordered (hexagonal) Alumino-surfactant
array
mesostructure
Mesostructured
Al2O3
78
Characterisation
Porosimetry
• N2 physisorption used to surface area, pore structure, pore shape
• Typical adsorption isotherms
• BET model  surface area during monolayer adsorption
79
• Use hysteresis on desorption to deduce pore shape
E
A
B
According to IUPAC
Type A = cylindrical pores
Type B = slit shaped pores
Type E = Bottle neck pores
80
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
0.893
B 
 hard to measure crystallites < 2nm diameter
dCos
 peakwidth yields particle size
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)
- Lines
fromintensity
different
often
with
each other
Measure
of components
diffraction peaks
as aoverlap
functionorofinterfere
sample and
analyser
angle (2)
81
XRD of Cu/CeO2 Catalyst
82
XRD of modified MCM supports
d(100)
• Typical XRD lattice parameter for
MCM = 35Å
• Estimate pore wall thickness
83
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
84
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
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