1 heterogeneous catalysis 6 lectures dr. adam lee surface chemistry & catalysis group
TRANSCRIPT
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Heterogeneous Catalysis
6 lectures
Dr. Adam LeeSurface Chemistry & Catalysis Group
Pd2+ Al3+ O2-
Air
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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 +
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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
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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!
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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
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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
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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
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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
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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
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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
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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
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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)
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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)
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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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