Presentation is loading. Please wait.

Presentation is loading. Please wait.

Heterogeneous Catalysis Surface Chemistry & Catalysis Group

Similar presentations


Presentation on theme: "Heterogeneous Catalysis Surface Chemistry & Catalysis Group"— Presentation transcript:

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

2 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)

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 catalysts Properties of catalysts Calculation of TON & measurement of kinetic parameters Overview of typical classes of reactions and catalysts used Environmental considerations

4 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? 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 Reduction Basic reagents Acidic reagents
Permanganate, Manganese dioxide, Chromium (VI) (<0.10 ppm) Reduction Metal Hydrides, (NaBH , LiAlH ) 4 Reducing metals ( Na, Fe, Mg, Zn) Basic reagents Potassium butoxide, diisopropylamine Tetramethyl guanidine Acidic reagents SO , H AlCl 3 , BF , ZnCl 2 4 C-C Coupling Homogeneous Pd based complexes

6 Importance of Heterogeneous Catalysis
Chemicals Industry: >90% of global chemical output relies upon heterogeneous catalysed processes Economics: ~20% of world GNP dependent on processes or derived products Equates to $10,000 billion/year!! Environment: Ozone depletion catalysed over aerosol surfaces in Polar Stratospheric Clouds Pollution control (catalytic converters, VOC destruction) Clean synthesis (waste minimisation, benign solvents, low temperature) Power generation Nobel Prize in Chemistry 2007 – Gerhard Ertl

7 Historical Evolution Polymerisation (1957/1991)
HDPE LDPE Polymerisation (1957/1991) Zeigler-Natta /Metallocene nC2H2 Catalytic Cracking (1964) CxH2x+2 Cx-2H2x-2 CxH2x+2 Cx-2H2x-4 Faujasitic zeolites

8 Automotive Emission Control (1976)
Pt/Rh/Al2O3 HC + CO + NOX CO2 + H2O + N2 Chiral 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 costs More efficient use of raw materials.

10 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

11 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 Reactants Products kforward kback

12 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 Catalysed Uncatalysed Catalysts do not affect K!

13 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 Product x % Total Yield of all Product mol . s rate of reaction % relative formation of 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 transesterification Activity = -d[Tributyrin] = = 1 mmol.s-1 dt Conversion = 20 % Biodiesel

15 [Diglyceride]+[Monoglyceride]+[FAME]
Triglyceride transesterification Tri-glyceride Methyl-butanoate (FAME) Di-glyceride Selectivity to FAME? [FAME] [Diglyceride]+[Monoglyceride]+[FAME] x 100 45 x 100 = = 60 %

16 Reagents are often stoichiometric - single use
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

17 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

18 Transmission Electron
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

19 Support Other features include: Types:
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

20 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

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 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

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 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 Anastas Director of Green Chemical Inst. Washington D.C. ex. White House Asst. Director for Environment

23 “It is better to prevent waste than to treat or clean up waste after it is formed”
Chemical Process No waste

24 “Synthetic methods should be designed to maximise the incorporation of all materials used into the final product” Selectivity Only required product C (only product) A + B C + 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 Activity Filtration Heating Cooling Stirring Distillation Compression Pumping Separation Energy requirement (electricity) Global warming Burn fossil fuel CO2 to atmosphere

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” Stoichiometric 4-Chlorobenzophenone Catalytic

28 Catalysis in Action: C2H2 on Pd(111)
Scanning Tunnelling Microscope movie - real-time molecular rotation Further Info Even More Info!

29 Lecture 3/4 Overview Reaction kinetics and diffusion limitations
Langmuir adsorption isotherm Unimolecular reaction Bimolecular reactions Surfaces

30 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.

31 Comparison Batch Reactor Batch/Flow Reactor

32 Key Considerations Diffusional effects - Adsorption strength -
(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

33 Porous catalyst structure
Diffusion Parameters Reactant film k k7 k k6 k k 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 O2 Reax. Mix Gas 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 PRESSURE Not temperature

35 Activation Energy - Diffusion Limitation?
At low T reaction processes dominate At high T diffusional effects become rate limiting Typical Arrhenius plot Activation Energy Arrhenius const kapp = Aexp (-Eapp/RT) Reaction control Diffusion control ln kapp 1/T lnkapp = LnA - Eapp/RT

36 Test for Diffusion Limitation
Rate  [Cat]n n=1 if no diffusion limitation Rate  with agitation, or gas flow Eapp is low kJmol-1 Diffusional Step Chemical Step Small T dep (T1/2 or T3/2) High T dep Ea ~ 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 phase Adsorbed NH3 reacting over Fe Langmuir Adsorption Isotherm  = 1 Coverage - the extent of adsorption of a 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 catalyst GAS/LIQUID reactants, products solvents CATALYST absorbate

40 Equilibrium between the gas molecules M, empty surface sites S and adsorbates
e.g. for non-dissociative adsorption S* M S----M [S*]  vacancies  (1- ) [M]  gas pressure  P [S----M]   adsorbate coverage Reactants Products Assumption 1: Fixed number of identical, localised surface sites

41 Langmuir Adsorption Isotherm
b Equilibrium constant, b is Rearrange in terms of , Langmuir Adsorption Isotherm - b called sticking-probability and depends on Hads Assumption 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) ® 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 )

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 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 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 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

46 Rate ® k . bAPA . bBPB = k' . PA. PB 1st order in both reactants
Competitive Adsorption Substituting these into the rate expression gives : Rate Pure A Pure B [A]/[B] Look at several extreme limits: Limit 1 : bA PA << 1 & bB PB << 1 In this limit qA & qB 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 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

47

48 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 ]/ [B] Rate Rate = k qA [B] A varied where [B] is pressure/conc in gas or liquid phase

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

50 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 O Calculated energy diagram CO(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 detectable Ru catalyst O atoms Calculated energy diagram Transition state GAS SURFACE CO(g)+O(a)

52 Oscillating reactions of carbon monoxide oxidation on platinum.
Good for oxididation ‘Inert’ towards O2 Can adsorb CO Oscillating reactions of carbon monoxide oxidation on platinum.

53 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....

54 Lecture 4 Overview Surfaces Structure
Geometric factors - dispersion, particle size effects Electronic factors - alloys

55 Surfaces Most technologically important catalysts contain active metal surfaces Most possess simple fcc structures e.g. Pt, Rh, Pd Face Centred Cubic unit cell Low index faces are most commonly studied surfaces with unique: - Surface symmetry - Surface atom coordination - Surface reactivity

56 Principle Low Index Surfaces
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

57 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 Reax. Co-ordinate T.S. E R P

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 bond Observe R(110) > R(100) > R(111) Ni Ni CH2 CH2 x 5

59 Geometric Factors: C2H4 dehydrogenation
Spectroscopy shows - same adsorption mode (HREELS) - strength (TPD) Volcano Plot (111) (110) Trend reflects C2H4 geometry  surface structure important

60 Temperature-programmed desorption
C2H3 Stepwise decomposition Quadrupole Mass Spectrometer CH3 CH2 H2 Pt(111)

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 size changing particle size modifies rate (111) hex (100) square Stepped surfaces Stepped + kinked surface

63 Consider total fraction of available surface sites:
Spherical particles if Ns = total no. of surface atoms NT = total atoms in particle For small particles (< 20Å) Dispersion  1 if Activity  SA, then  particle size will  rate (per mass of catalyst) provided exposed surface atom arrangement unchanged

64 Structure sensitive test:
Consider CO + 3H2  CH4 + H2O Compare specific TON (per surface site) Ni (100) 9% Ni/Al2O3 5% Ni/Al2O3 If reaction requires specific (4-coord) active site expect constant Eact observed higher rate over surfaces with most (100) sites larger particles

65 Structure sensitive vs insensitive reaction:
Cyclohexane hydrogenolysis High step/kink densities  high rates Reaction requires defect sites contrast with (de)hydrogenation which proceeds over diverse surface arrangements Reaction kinetics tell us about the active site -H2 -CHx

66 Electronic Factors: Alloys
 Electronic properties of crystalline solids described by Band Theory Alkali-metals → 1 valence e-/atom 1s-band 2s-band Energy Bimetal  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

69 Incorporation of Au  Methodology Zoom
 improved activity, selectivity & lifetime Pd(111) C2H2 C6H6  Methodology - construct relevant model catalyst - add gold (Au) promoter - perform chemistry over Pd/Au alloys Au Zoom Au

70  Chemistry - products include C6H6, C6H14, C6H14 - add heteroatoms O, S..C5 heterocycles BUT ~50 % of C2H2 decomposes over Pd

71

72 Au/Pd alloys  reactant/product decomposition vs. Pd
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 Preparation of Heterogeneous Catalysts
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)

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 Templated Sol-Gel Surfactant + Solvent Þ Micelle Surfactant
Lauric Acid (coconut oil) Al precursor Template extraction Surfactant micelle Ordered (hexagonal) array Alumino-surfactant mesostructure Mesostructured Al2O3

79 Characterisation Porosimetry
N2 physisorption used to surface area, pore structure, pore shape Typical adsorption isotherms BET model  surface area during monolayer adsorption

80 E B A Use hysteresis on desorption to deduce pore shape
According to IUPAC Type A = cylindrical pores Type B = slit shaped pores Type E = Bottle neck pores

81 Powder X-Ray Diffraction
Well developed laboratory technique Gives 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 other 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)

82 XRD of Cu/CeO2 Catalyst

83 XRD of modified MCM supports
Typical XRD lattice parameter for MCM = 35Å Estimate pore wall thickness

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

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


Download ppt "Heterogeneous Catalysis Surface Chemistry & Catalysis Group"

Similar presentations


Ads by Google