1 Heterogeneous Catalysis Surface Chemistry & Catalysis Group 6 lecturesDr. Adam LeeSurface Chemistry & Catalysis Group
2 SynopsisHeterogeneous 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, advantagesCatalyst surfaces: adsorption processes, kineticsStructure-sensitivity: dispersion, active siteBimetallic catalysts: selectivity controlCatalyst preparationCatalyst characterisationRecommended 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)
3 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 catalystsProperties of catalystsCalculation of TON & measurement of kinetic parametersOverview of typical classes of reactions and catalysts usedEnvironmental considerations
4 How can we accelerate a chemical reaction? Organic Chemistry (1805) Physical ChemistryDiscovery of Catalysis (1835)- Petrochemical & oil refining industry recognise promise- Catalytic technologygenerates >10 trillion $/yr- Clean technology (1990?) - applications in plastics, fabrics, food, fuelWhy don’t we use acatalyst?How can we accelerate a chemical reaction?Use reagents- stoichiometric- separation problems- TOXIC waste- Industrial fine chemicalsprocesses developed- Carry on using reagents
6 Importance of Heterogeneous Catalysis Chemicals Industry:>90% of global chemical output relies upon heterogeneous catalysed processesEconomics:~20% of world GNP dependent on processes or derived productsEquates to $10,000 billion/year!!Environment:Ozone depletion catalysed over aerosol surfaces in Polar Stratospheric CloudsPollution control (catalytic converters, VOC destruction)Clean synthesis (waste minimisation, benign solvents, low temperature)Power generationNobel Prize in Chemistry 2007 – Gerhard Ertl
8 Automotive Emission Control (1976) Pt/Rh/Al2O3HC + CO + NOX CO2 + H2O + N2Chiral Catalysis (1988)Chiral pocket
9 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 ( )Minimize FEEDSTOCK and reduce ENERGY costsMore efficient use of raw materials.
10 Classes of CatalystHeterogeneous - active site immobilised on solid support tuneable selectivity- easily separatedHomogeneous - organometallic complexes widely used- more active than heterogeneous,- high selectivity- difficult to separateBio-catalysts - enzymes, bacteria, fungi- highly selectivePhase transfer - Reagent soluble in separate phase to substrate use PTC to transfer reagent into organic
11 Catalyst DefinitionsCatalyst: 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]nRate constant (k or k’) = A exp (-EAct/RT)Consider,All catalysts work by providing alternative pathways:different, lower EActaccelerates both forward AND reverse reactions(increase kf and kb)catalysts do not influence how MUCH product formsReactants Productskforwardkback
12 Catalyst Definitions Energetics: Reactants do not all have same energy: Boltzmann distributionSo what determines theoretical product yield??- thermodynamic driving force, G = -nRT ln(K)Large –ve G large +ve ln(K) huge K ~100 % YieldCatalysedUncatalysedCatalysts do not affect K!
13 Catalyst DefinitionsGoal of catalytic research is improved activity & selectivityAlter rate constants: kFor simple reax. A B + CActivity =Selectivity == Yield of Desired Product x % Total Yield of all Productmol . s rate of reaction% relative formationof specific product
14 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)Triglyceride transesterificationActivity = -d[Tributyrin] = = 1 mmol.s-1dtConversion = 20 %Biodiesel
15 [Diglyceride]+[Monoglyceride]+[FAME] Triglyceride transesterificationTri-glycerideMethyl-butanoate(FAME)Di-glycerideSelectivity to FAME?[FAME][Diglyceride]+[Monoglyceride]+[FAME]x 10045x 100== 60 %
16 Reagents are often stoichiometric - single use Catalyst Efficiency: 2Reagents are often stoichiometric - single useBy 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 achievee.g. 1 mmol Pd converts 1000 mmols of COCO2Turn over frequency (TOF) - Number of reactions per site per unit time.e.g. 1 mmol Pd converts 1000 mmols of COCO2 in 10 sTo be valid TOF must be measured in absence of:- mass transport limitations- deactivation effectsTON = 1000TOF = 100 s-1
17 Catalyst Constituents Active Phase- transition-metal- highly dispersed- reduced/oxidic/sulphidedstate‘Inert’ Support- high surface area oxide- high porosity- high thermal/mechanicalstabilitySn - Naptha reformingCl - Ethylene epoxidationK2O - NH3 synthesisC - Catalytic crackingS, Pb - Car exhaust catalysts
18 Transmission Electron Active ComponentResponsible for the principal chemical reactionFeatures:activity, selectivity, puritysurface area, distribution on support, particle sizeTypes:MetalsSemiconductor oxides and sulphidesInsulator oxides and sulphidesPlatinum particles on a porous carbon supportTransmission ElectronMicrograph
19 Support Other features include: Types: Main function is to maintain high surface area for active phaseOther features include:porositymechanical propertiesstabilitydual functional activitymodification of active componentTypes:high melting point oxides (silica, alumina)clayscarbons
20 Advantages and Limitations of Heterogeneous Catalysts Ease of removal from reaction and possible to recycleDiffusional effects- reaction rates may be limited by diffusion into/out of pores.May need to re-optimise plants (often batch reactors) forsolid-liquid processes- separation technologyOpportunity to operate continuous processes
21 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 tobulk 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
22 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 possesslittle 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 whereverpossible 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 economicallypossible.8) Unnecessary derivatisation (blocking group, protection/deprotection, temporary modification of physical/chemicalprocesses) 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 priorto the formation of hazardous substances.12) Substances and the form of a substance used in a chemical process should be chosen as to minimisethe potential for chemical accidents, including releases, explosions and fires.Dr. Paul AnastasDirector of Green Chemical Inst.Washington D.C.ex. White House Asst. Directorfor Environment
23 “It is better to prevent waste than to treat or clean up waste after it is formed” ChemicalProcessNo waste
24 “Synthetic methods should be designed to maximise the incorporation of all materials used into the final product”SelectivityOnly required productC (only product)A + BC + D + E + F ...
25 High Activity Filtration “Energy requirements should be recognised for their environmental impacts and minimised. Synthetic methods should be conducted at ambient pressure and temperature”High ActivityFiltrationHeatingCoolingStirringDistillationCompressionPumpingSeparationEnergy requirement(electricity)GlobalwarmingBurn fossilfuelCO2 toatmosphere
26 “Unnecessary derivatisation (blocking group, protection/deprotection “Unnecessary derivatisation (blocking group, protection/deprotection..) should be avoided wherever possible”Selectivity
27 “Selective catalysts are superior to stoichiometric reagents” CONCLUSION:“Selective catalysts are superior to stoichiometric reagents”Stoichiometric4-ChlorobenzophenoneCatalytic
28 Catalysis in Action: C2H2 on Pd(111) Scanning Tunnelling Microscope movie- real-time molecular rotationFurther InfoEven More Info!
30 Kinetics of Catalysed Reactions Kinetics of heterogeneously catalysed liquid phase reactions arelargely governed by diffusion limitation within the porous solidRequire a new range of heterogeneous catalysts tailored for liquidphase organic reactions offering...- pore structure- ease of separation- high activity- high selectivity to desired products.
32 Key Considerations Diffusional effects - Adsorption strength - (Mass Transfer)Adsorption strength -Mechanism -Heat transfer -Solvent polarityRatio of reactantCompetitive adsorptionAdsorption of product/by products (e.g. H2O)Site blockingSolvent adsorptionStudy rate as function of concentrationand compare theoretical profileHot spots?In exothermic reactions rapid removalof heat from active site is essential
33 Porous catalyst structure Diffusion ParametersReactant filmk k7k k6k k k5Porous catalyst structureA Bk1 = Film mass transfer to ext. surfacek2 = Diffusion into Catalyst Pore (Bulk or Knudsen Diffusion)k3 = Adsorption on surfacek4 = Reactionk5 = Desorption of Productk6 = Diffusion of Productk7 = Film mass transfer away ext. surfaceO2Reax. MixGas diffusion kinetics important in liquid oxidation/hydrogenation- high pressure needed to increase solubility
34 Henry’s Law Dissolution is EXOTHERMIC Raise PRESSURE Not temperature For dissolution of oxygen in water, O2(g) <--> O2(aq), enthalpy change under standard conditions is kJ/mole.Raise PRESSURENot temperature
35 Activation Energy - Diffusion Limitation? At low T reaction processes dominateAt high T diffusional effects become rate limitingTypical Arrhenius plotActivation EnergyArrhenius constkapp = Aexp (-Eapp/RT)Reaction controlDiffusion controlln kapp1/Tlnkapp = LnA - Eapp/RT
36 Test for Diffusion Limitation Rate [Cat]n n=1 if no diffusion limitationRate with agitation, or gas flowEapp is low kJmol-1Diffusional Step Chemical StepSmall T dep (T1/2 or T3/2) High T depEa ~ kJmol-1
37 Surface Terminology Substrate (adsorbent) Adsorbate - the solid surface where adsorption occurs Adsorbate- the atomic/molecular species adsorbed on the substrate
38 Adsorption- the process in which species ‘bind’ to surface of another phaseAdsorbed NH3 reacting over FeLangmuirAdsorptionIsotherm = 1Coverage- the extent of adsorption ofa species onto a surface ()
39 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 catalystGAS/LIQUIDreactants, productssolventsCATALYSTabsorbate
40 Equilibrium between the gas molecules M, empty surface sites S and adsorbates e.g. for non-dissociative adsorptionS* MS----M[S*] vacancies (1- )[M] gas pressure P[S----M] adsorbate coverageReactantsProductsAssumption 1:Fixed number of identical, localised surface sites
41 Langmuir Adsorption Isotherm bEquilibrium constant, b isRearrange in terms of ,Langmuir Adsorption Isotherm- b called sticking-probability and depends on HadsAssumption 2:Hads and thus b is temperature & pressure independent
42 Unimolecular Decomposition Consider the surface decomposition of a molecule A , i.e.A (g) « A (ads) ® ProductsLet 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 desorbthe rate determining step (rds) is the surface decomposition stepUnder 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 isothermAssumption 3:Hads is coverage independentAssumption 4:Only 1 adsorbate per siteq = b.P / ( 1 + b.P )
43 Rate = k q Rate = k b P / ( 1 + b P ) 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 surfaceTwo 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.Pthen ( 1 + b.P ) ~ 1 steady state surface q of reactant v. small
44 Limit 2 : b. P >> 1 ; then. ( 1 + b. P ) ~ b. P. and. Rate ~ k i 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 q of reactant ~100%Rate shows the same pressure variation as q (not surprising since rate q!)Rate = k b P / ( 1 + b P )
45 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 qBSince 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)rdsfast
46 Rate ® k . bAPA . bBPB = k' . PA. PB 1st order in both reactants Competitive AdsorptionSubstituting these into the rate expression gives :RatePure APure B[A]/[B]Look at several extreme limits:Limit 1 : bA PA << 1 & bB PB << 1In this limit qA & qB are both very low , andRate ® k . bAPA . bBPB = k' . PA. PB 1st order in both reactantsLimit 2 : bA PA << 1 << bB PBIn this limit qA ® 0 , qB ® 1 , andRate ® k . bA PA / (bB PB ) = k' . PA / PBq = b.P / ( 1 + b.P )1st order in Anegative 1st order in B
48 Bimolecular Reactions:2 Eley-Rideal type reaction :Consider same chemistryA (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 ABrdsfast[A ]/ [B]RateRate = k qA [B]A variedwhere [B] is pressure/concin gas or liquid phase
49 A (ads) + B (ads) AB (ads) AB (g) HoweverWithout extra evidence cannot conclude above reaction is Eley-Rideal mechanism…last step may be composite and consist of the following stagesB (g) B (ads)A (ads) + B (ads) AB (ads) AB (g)with extremely small steady-state coverage of adsorbed B Test by monitoring ratevary qAvary ratio of or over wide rangeslowfastfastLangmuir-Hinshelwoodnot Eley-Rideal.need free sites
50 Calculated energy diagram Example 1Langmuir-Hinshelwood: CO oxidation over PtHighest 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 barrierCOOCalculated energy diagramCO(g)+½O2(g)CO(g)+O(a)Reaction pathway
51 Example 2 Eley-Rideal: CO oxidation over Ru GAS SURFACE Highest rate of CO2 production under oxidizing conditions:- a high concentration (1 monolayer) of surface O- no surface CO detectableRu catalystO atomsCalculated energy diagramTransition stateGASSURFACECO(g)+O(a)
52 Oscillating reactions of carbon monoxide oxidation on platinum. Good foroxididation‘Inert’ towards O2Can adsorb COOscillating reactions of carbon monoxide oxidation on platinum.
53 Kinetics SummaryImportant to verify whether reaction kinetics (esp. liquid phase)are determined by mass transport limitations.Homogeneous reaction conditions may not be directly transferableReactions involving Solid-Liquid-Gas particularly challenging!Relative ‘sticking probability’ of reactants plays a major role indetermining surface coverage and optimum reaction conditions.Use of promoters can help with coverage effects....
55 SurfacesMost technologically important catalysts contain active metal surfacesMost possess simple fcc structures e.g. Pt, Rh, PdFace Centred Cubic unit cellLow index faces are most commonly studied surfaces with unique:- Surface symmetry- Surface atom coordination- Surface reactivity
56 Principle Low Index Surfaces Surface SymmetrySurface 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 energyOpen (rough) surfaces low coord. nos- unstable high surface energy
57 Geometric Factors For any reaction the pathway depends on: - reactant geometry- reactant energyrelative to transition complexMonitor adsorption geometry via vibrational spectroscopy(RAIRS, HREELS, ARUPS)e.g. C2H4 dehydrogenationReax. Co-ordinateT.S.ERP
58 CH2 CH2 Ni Ni x 5 Observe R(110) > R(100) > R(111) Calculate Ni-C-C bond angle,for different Ni surfaces,Ni-Ni = = 103 , bond twists to stabilise ethene“ = = 123 , destabilisation of C-H bondObserve R(110) > R(100) > R(111)Ni NiCH2 CH2x 5
61 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
62 (any mix of step, terrace, kink atoms) 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 sizechanging particle size modifies rate(111)hex(100)squareStepped surfaces Stepped + kinked surface
63 Consider total fraction of available surface sites: Spherical particlesif Ns = total no. of surface atomsNT = total atoms in particleFor small particles (< 20Å) Dispersion 1if 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 + H2OCompare specific TON (per surface site)Ni (100)9% Ni/Al2O35% Ni/Al2O3If reaction requires specific (4-coord) active site expectconstant Eact observedhigher rate over surfaces with most (100) sites larger particles
65 Structure sensitive vs insensitive reaction: Cyclohexane hydrogenolysisHigh step/kink densities high ratesReaction requires defect sitescontrast with (de)hydrogenation which proceeds over diverse surface arrangementsReaction kinetics tell us about the active site-H2-CHx
66 Electronic Factors: Alloys Electronic properties of crystalline solids described by Band TheoryAlkali-metals→ 1 valence e-/atom1s-band2s-bandEnergyBimetal Bimetal may transfer e- to/from active metal changes adsorbate binding strength
67 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
68 Acetylene Coupling over Pd/Au Reaction mechanism well understood Unique chemistry- low temperature (25°C) & high selectivity- operates from atmospheres Reaction requires 7-atom ensemble
72 Au/Pd alloys reactant/product decomposition vs. Pd SummaryAu/Pd alloys reactant/product decomposition vs. Pd Au selectivity to benzeneAu long-term activityBoth ensemble & ligand effects are important Au breaks up active site Au ‘softens’ Pd chemistry
73 Preparation of Heterogeneous Catalysts Lecture 6Preparation of Heterogeneous CatalystsSol-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 viareaction with surface groups (often OH)
74 Impregnation Pore filling with catalyst precursor followed by evaporation of solventTraditional method for supported metalsIon Exchange Equilibrium amount of cation or anion is adsorbed atactive 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.
79 Characterisation Porosimetry N2 physisorption used to surface area, pore structure, pore shapeTypical adsorption isothermsBET model surface area during monolayer adsorption
80 E B A Use hysteresis on desorption to deduce pore shape According to IUPACType A = cylindrical poresType B = slit shaped poresType E = Bottle neck pores
81 Powder X-Ray Diffraction Well developed laboratory techniqueGives satisfactory results (<5 h per sample)Measure intensity of diffraction peaks as a function of sample and analyser angle (2)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 otherB = 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)
84 Infrared Spectroscopy Can make vibrational measurements of adsorbates on catalyst surface!Transmission Mode – using KBr Self Supporting Wafer- e.g. CO adsorption on metal crystallitesDiffuse Reflectance Mode (DRIFTS)– acquire data directly from a catalyst powder
85 COURSE SUMMARY Learning Objectives Catalysis Definitions - activity, selectivity, conversion, TON and TOFReaction Kinetics - diffusion limitations, Langmuir adsorption,unimolecular and bimolecular reactionsSurface structure - terminology, symmetry, geometric vs. electronic factorsStructure-Sensitivity - definition, particle size effects, dispersionCatalyst Preparation - simple methodologiesCatalyst Characterisation - simple methodologies, surface vs. bulk insight
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