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Catalysis & Catalysts

Catalysis & Catalysts

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Catalysis & Catalysts

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  1. CH4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Catalysis & Catalysts • Facts and Figures about Catalysts Life cycle on the earth • Catalysts (enzyme) participates most part of life cycle e.g. forming, growing, decaying • Catalysis contributes great part in the processes of converting sun energy to various other forms of energies e.g. photosynthesis by plant CO2 + H2O=HC + O2 • Catalysis plays a key role in maintaining our environment Chemical Industry • ca. $2 bn annual sale of catalysts • ca. $200 bn annual sale of the chemicals that are related products • 90% of chemical industry has catalysis-related processes • Catalysts contributes ca. 2% of total investment in a chemical process

  2. CH4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts What is Catalysis • Catalysis • Catalysis is an action by catalyst which takes part in a chemical reaction process and can alter the rate of reactions, and yet itself will return to its original form without being consumed or destroyed at the end of the reactions (This is one of many definitions) Three key aspects of catalyst action • taking part in the reaction • it will change itself during the process by interacting with other reactant/product molecules • altering the rates of reactions • in most cases the rates of reactions are increased by the action of catalysts; however, in some situations the rates of undesired reactions are selectively suppressed • Returning to its original form • After reaction cycles a catalyst with exactly the same nature is ‘reborn’ • In practice a catalyst has its lifespan - it deactivates gradually during use

  3. CH4003 Lecture Notes 11 (Erzeng Xue) uncatalytic catalytic reactant energy product reaction process Catalysis & Catalysts Action of Catalysts • Catalysis action - Reaction kinetics and mechanism Catalyst action leads to the rate of a reaction to change. This is realised by changing the course of reaction (compared to non-catalytic reaction) • Forming complex with reactants/products, controlling the rate of elementary steps in the process. This is evidenced by the facts that • The reaction activation energy is altered • The intermediates formed are different from those formed in non-catalytic reaction • The rates of reactions are altered (both desired and undesired ones) • Reactions proceed under less demanding conditions • Allow reactions occur under a milder conditions, e.g. at lower temperatures for those heat sensitive materials

  4. CH4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Action of Catalysts • It is important to remember that the use of catalyst DOES NOT vary DG & Keq values of the reaction concerned, it merely change the PACE of the process • Whether a reaction can proceed or not and to what extent a reaction can proceed is solely determined by the reaction thermodynamics, which is governed by the values of DG & Keq, NOT by the presence of catalysts. • In another word, the reaction thermodynamics provide the driving force for a rxn; the presence of catalysts changes the way how driving force acts on that process. e.g CH4(g) + CO2(g) = 2CO(g) + 2H2(g) DG°373=151 kJ/mol (100 °C) DG°973 =-16 kJ/mol (700 °C) • At 100°C, DG°373=151 kJ/mol > 0. There is no thermodynamic driving force, the reaction won’t proceed with or without a catalyst • At 700°C, DG°373= -16 kJ/mol < 0. The thermodynamic driving force is there. However, simply putting CH4 and CO2 together in a reactor does not mean they will react. Without a proper catalyst heating the mixture in reactor results no conversion of CH4 and CO2 at all. When Pt/ZrO2 or Ni/Al2O3 is present in the reactor at the same temperature, equilibrium conversion can be achieved (<100%).

  5. CH4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Types of Catalysts & Catalytic Reactions • The types of catalysts • Classification based on the its physical state, a catalyst can be • gas • liquid • solid • Classification based on the substances from which a catalyst is made • Inorganic (gases, metals, metal oxides, inorganic acids, bases etc.) • Organic (organic acids, enzymes etc.) • Classification based on the ways catalysts work • Homogeneous - both catalyst and all reactants/products are in the same phase (gas or liq) • Heterogeneous - reaction system involves multi-phase (catalysts + reactants/products) • Classification based on the catalysts’ action • Acid-base catalysts • Enzymatic • Photocatalysis • Electrocatalysis, etc.

  6. CH4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Applications of Catalysis • Industrial applications Almost all chemical industries have one or more steps employing catalysts • Petroleum, energy sector, fertiliser, pharmaceutical, fine chemicals … Advantages of catalytic processes • Achieving better process economics and productivity • Increase reaction rates - fast • Simplify the reaction steps - low investment cost • Carry out reaction under mild conditions (e.g. low T, P) - low energy consumption • Reducing wastes • Improving selectivity toward desired products - less raw materials required, less unwanted wastes • Replacing harmful/toxic materials with readily available ones • Producing certain products that may not be possible without catalysts • Having better control of process (safety, flexible etc.) • Encouraging application and advancement of new technologies and materials • And many more …

  7. CH4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Applications of Catalysis • Environmental applications • Pollution controls in combination with industrial processes • Pre-treatment - reduce the amount waste/change the composition of emissions • Post-treatments - once formed, reduce and convert emissions • Using alternative materials … • Pollution reduction • gas - converting harmful gases to non-harmful ones • liquid - de-pollution, de-odder, de-colour etc • solid - landfill, factory wastes … • And many more … • Other applications • Catalysis and catalysts play one of the key roles in new technology development.

  8. CH4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Research in Catalysis • Research in catalysis involve a multi-discipline approach • Reaction kinetics and mechanism • Reaction paths, intermediate formation & action, interpretation of results obtained under various conditions, generalising reaction types & schemes, predict catalyst performance… • Catalyst development • Material synthesis, structure properties, catalyst stability, compatibility… • Analysis techniques • Detection limits in terms of dimension of time & size and under extreme conditions (T, P) and accuracy of measurements, microscopic techniques, sample preparation techniques… • Reaction modelling • Elementary reactions and rates, quantum mechanics/chemistry, physical chemistry … • Reactor modelling • Mathematical interpretation and representation, the numerical method, micro-kinetics, structure and efficiency of heat and mass transfer in relation to reactor design … • Catalytic process • Heat and mass transfers, energy balance and efficiency of process …

  9. CH4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Catalytic Reaction Processes • Understanding catalytic reaction processes • A catalytic reaction can be operated in a batch manner • Reactants and catalysts are loaded together in reactor and catalytic reactions (homo- or heterogeneous) take place in pre-determined temperature and pressure for a desired time / desired conversion • Type of reactor is usually simple, basic requirements • Withstand required temperature & pressure • Some stirring to encourage mass and heat transfers • Provide sufficient heating or cooling • Catalytic reactions are commonly operated in a continuous manner • Reactants, which are usually in gas or liquid phase, are fed to reactor in steady rate (e.g. mol/h, kg/h, m3/h) • Usually a target conversion is set for the reaction, based on this target • required quantities of catalyst is added • required heating or cooling is provided • required reactor dimension and characteristics are designed accordingly.

  10. CH4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Catalytic Reaction Processes • Catalytic reactions in a continuous operation (cont’d) • Reactants in continuous operation are mostly in gas phase or liquid phase • easy transportation • The heat & mass transfer rates in gas phase is much faster than those in liquid • Catalysts are pre-loaded, when using a solid catalyst, or fed together with reactants when catalyst & reactants are in the same phase and pre-mixed • It is common to use solid catalyst because of its easiness to separate catalyst from unreacted reactants and products Note: In a chemical process separation usually accounts for ~80% of cost. That is why engineers always try to put a liquid catalyst on to a solid carrier. • With pre-loaded solid catalyst, there is no need to transport catalyst which is then more economic and less attrition of solid catalyst (Catalysts do not change before and after a reaction and can be used for number cycles, months or years), • In some cases catalysts has to be transported because of need of regeneration • In most cases, catalytic reactions are carried out with catalyst in a fixed-bed reactor (fluidised-bed in case of regeneration being needed), with the reactant being gases or liquids

  11. CH4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Catalytic Reaction Processes • General requirements for a good catalyst • Activity - being able to promote the rate of desired reactions • Selective - being to promote only the rate of desired reaction and also retard the undesired reactions Note: The selectivity is sometime considered to be more important than the activity and sometime it is more difficult to achieve (e.g. selective oxidation of NO to NO2 in the presence of SO2) • Stability - a good catalyst should resist to deactivation, caused by • the presence of impurities in feed (e.g. lead in petrol poison TWC. • thermal deterioration, volatility and hydrolysis of active components • attrition due to mechanical movement or pressure shock • A solid catalyst should have reasonably large surface area needed for reaction (active sites). This is usually achieved by making the solid into a porous structure.

  12. CH4003 Lecture Notes 12 (Erzeng Xue) gas phase reactant molecule j gas phase k liquid phase / stagnant layer l mn o pore porous solid r q p Catalysis & Catalysts Example Heterogeneous Catalytic Reaction Process • The long journey for reactant molecules to j. travel within gas phase k. cross gas-liquid phase boundary l. travel within liquid phase/stagnant layer m. cross liquid-solid phase boundary n. reach outer surface of solid o. diffuse within pore p. arrive at reaction site q. be adsorbed on the site and activated r. react with other reactant molecules, either being adsorbed on the same/neighbour sites or approaching from surface above • Product molecules must follow the same track in the reverse direction to return to gas phase • Heat transfer follows similar track

  13. CH4003 Lecture Notes 12 (Erzeng Xue) Active phase Promoter Catalyst Support Catalysis & Catalysts Solid Catalysts • Catalyst composition • Active phase • Where the reaction occurs (mostly metal/metal oxide) • Promoter • Textual promoter (e.g. Al - Fe for NH3 production) • Electric or Structural modifier • Poison resistant promoters • Support / carrier • Increase mechanical strength • Increase surface area (98% surface area is supplied within the porous structure) • may or may not be catalytically active

  14. CH4003 Lecture Notes 12 (Erzeng Xue) Active site pore porous solid Catalysis & Catalysts Solid Catalysts • Some common solid support / carrier materials • Alumina • Inexpensive • Surface area: 1 ~ 700 m2/g • Acidic • Silica • Inexpensive • Surface area: 100 ~ 800 m2/g • Acidic • Zeolite • mixture of alumina and silica, • often exchanged metal ion present • shape selective • acidic • Other supports • Active carbon (S.A. up to 1000 m2/g) • Titania (S.A. 10 ~ 50 m2/g) • Zirconia (S.A. 10 ~ 100 m2/g) • Magnesia (S.A. 10 m2/g) • Lanthana (S.A. 10 m2/g)

  15. CH4003 Lecture Notes 12 (Erzeng Xue) Support Drying & firing add acid/base with pH control precursor solution Support Amount adsorbed Drying & firing Concentration Support Drying & firing Pore saturated pellets Soln. of metal precursor Catalysis & Catalysts Solid Catalysts • Preparation of catalysts • Precipitation To form non-soluble precipitate by desired reactions at certain pH and temperature • Adsorption & ion-exchange Cationic: S-OH+ + C+® SOC+ + H+ Anionic: S-OH- + A-® SA- + OH- I-exch. S-Na+ + Ni 2+ D S-Ni 2+ + Na+ • Impregnation Fill the pores of support with a metal salt solution of sufficient concentration to give the correct loading. • Dry mixing Physically mixed, grind, and fired filter & wash the resulting precipitate precipitate or deposit precipitation

  16. CH4003 Lecture Notes 12 (Erzeng Xue) 40 100 75 BET S.A. BET S.A. m2/g 50 25 0 10 0 0 Time / hours 500 600 700 800 900 Temperature °C Induction period Normal use dead Activity Time Catalysis & Catalysts Solid Catalysts • Preparation of catalysts • Catalysts need to be calcined (fired) in order to decompose the precursor and to received desired thermal stability. The effects of calcination temperature and time are shown in the figures on the right. • Commonly used Pre-treatments • Reduction • if elemental metal is the active phase • Sulphidation • if a metal sulphide is the active phase • Activation • Some catalysts require certain activation steps in order to receive the best performance. • Even when the oxide itself is the active phase it may be necessary to pre-treat the catalyst prior to the reaction • Typical catalyst life span • Can be many years or a few mins.

  17. CH4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface • Adsorption • Adsorption is a process in which molecules from gas (or liquid) phase land on, interact with and attach to solid surfaces. • The reverse process of adsorption, i.e. the process n which adsorbed molecules escape from solid surfaces, is called Desorption. • Molecules can attach to surfaces in two different ways because of the different forces involved. These are Physisorption (Physical adsorption) & Chemisorption(Chemical adsorption) Physisorption Chemisorption force van de Waal chemcal bond number of adsorbed layers multi only one layer adsorption heat low (10-40 kJ/mol) high ( > 40 kJ/mol) selectivity low high temperature to occur low high

  18. CH4003 Lecture Notes 13 (Erzeng Xue) number of adsorption sites occupied number of adsorption sites available define q = q= 0~1 Catalysis & Catalysts Adsorption On Solid Surface • Adsorption process Adsorbent and adsorbate • Adsorbent(also called substrate)- The solid that provides surface for adsorption • high surface area with proper pore structure and size distribution is essential • good mechanical strength and thermal stability are necessary • Adsorbate- The gas or liquid substances which are to be adsorbed on solid Surface coverage, q The solid surface may be completely or partially covered by adsorbed molecules Adsorption heat • Adsorption is usually exothermic (in special cases dissociated adsorption can be endothermic) • The heat of chemisorption is in the same order of magnitude of reaction heat; the heat of physisorption is in the same order of magnitude of condensation heat.

  19. CH4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface • Applications of adsorption process • Adsorption is a very important step in solid catalysed reaction processes • Adsorption in itself is a common process used in industry for various purposes • Purification (removing impurities from a gas / liquid stream) • De-pollution, de-colour, de-odour • Solvent recovery, trace compound enrichment • etc… • Usually adsorption is only applied for a process dealing with small capacity • The operation is usually batch type and required regeneration of saturated adsorbent Common adsorbents: molecular sieve, active carbon, silica gel, activated alumina. • Physisorption is a useful technique for determining the surface area, the pore shape, pore sizes and size distribution of porous solid materials (BET surface area)

  20. CH4003 Lecture Notes 13 (Erzeng Xue) V3>V2 V2>V1 V4>V3 V1 Pressure P4>P3 P3>P2 T1 T2 >T1 P2>P1 Temperature T3 >T2 Vol. adsorbed Vol. adsorbed P1 T4 >T3 T5 >T4 Temperature Pressure Adsorption Isostere Adsorption Isobar Adsorption Isotherm Catalysis & Catalysts Adsorption On Solid Surface • Characterisation of adsorption system • Adsorption isotherm - most commonly used, especially to catalytic reaction system, T=const. The amount of adsorption as a function of pressure at set temperature • Adsorption isobar - (usage related to industrial applications) The amount of adsorption as a function of temperature at set pressure • Adsorption Isostere - (usage related to industrial applications) Adsorption pressure as a function of temperature at set volume

  21. CH4003 Lecture Notes 13 (Erzeng Xue) A case I Catalysis & Catalysts Adsorption On Solid Surface • The Langmuir adsorption isotherm • Basic assumptions • surface uniform (DHadsdoes not vary with coverage) • monolayer adsorption, and • no interaction between adsorbed molecules and adsorbed molecules immobile • Case I - single molecule adsorption when adsorption is in a dynamic equilibrium A(g) +M(surface site)D AM the rate of adsorption rads = kads(1-q) P the rate of desorption rdes = kdesq at equilibrium rads = rdesÞkads(1-q) P = kdesq rearrange it for q let ÞB0 is adsorption coefficient

  22. CH4003 Lecture Notes 13 (Erzeng Xue) A B B A case II Catalysis & Catalysts Adsorption On Solid Surface • The Langmuir adsorption isotherm (cont’d) • Case II - single molecule adsorbed dissociatively on one site A-B(g) +M(surface site)D A-M-B the rate of A-B adsorption rads=kads (1-qA )(1-qB)PAB=kads (1-q)2PAB the rate of A-B desorption rdes=kdesqAqB =kdesq2 at equilibrium rads = rdesÞ kads (1-q)2PAB= kdesq2 rearrange it for q Let. Þ q=qA=qB

  23. CH4003 Lecture Notes 13 (Erzeng Xue) A B case III Catalysis & Catalysts Adsorption On Solid Surface • The Langmuir adsorption isotherm (cont’d) • Case III - two molecules adsorbed on two sites A(g) +B(g) +2M(surface site)D A-M + B-M the rate of A adsorption rads,A = kads,A (1- qA- qB) PA the rate of B adsorption rads,B = kads,B (1- qA- qB) PB the rate of A desorption rdes,A = kdes,AqA the rate of B desorption rdes,B = kdes,BqB at equilibrium rads ,A = rdes ,A and Þrads ,B = rdes ,B Þ kads,A(1-qA-qB)PA=kdes,AqAand kads,B(1-qA-qB)PB=kdes,BqB rearrange it for q whereare adsorption coefficients of A & B.

  24. CH4003 Lecture Notes 13 (Erzeng Xue) A B A A B case III case I case II Catalysis & Catalysts Adsorption On Solid Surface • The Langmuir adsorption isotherm (cont’d) Adsorption A, B both strong A strong, B weak A weak, B weak Adsorption Strongkads>> kdes kads>> kdes B0>>1 B0>>1 Weakkads<< kdes kads<< kdes B0<<1 B0<<1

  25. CH4003 Lecture Notes 14 (Erzeng Xue) Strong adsorptionkads>> kdes Weak adsorptionkads<< kdes Catalysis & Catalysts Adsorption On Solid Surface • Langmuir adsorption isotherm case I case II Case III mono-layer Amount adsorbed large B0 (strong adsorp.) moderate B0 small B0 (weak adsorp.) Pressure • Langmuir adsorption isotherm established a logic picture of adsorption process • It fits many adsorption systems but not at all • The assumptions made by Langmuir do not hold in all situation, that causing error • Solid surface is heterogeneous thus the heat of adsorption is not a constant at different q • Physisorption of gas molecules on a solid surface can be more than one layer

  26. CH4003 Lecture Notes 14 (Erzeng Xue) I II III amount adsorbed IV V 1.0 relative pres. P/P0 Catalysis & Catalysts Adsorption On Solid Surface • Five types of physisorption isotherms are found over all solids • Type I is found for porous materials with small pores e.g. charcoal. It is clearly Langmuir monolayer type, but the other 4 are not • Type II for non-porous materials • Type III porous materials with cohesive force between adsorbate molecules greater than the adhesive force between adsorbate molecules and adsorbent • Type IV staged adsorption (first monolayer then build up of additional layers) • Type V porous materials with cohesive force between adsorbate molecules and adsorbent being greater than that between adsorbate molecules

  27. CH4003 Lecture Notes 14 (Erzeng Xue) Langmuir DH of ads Temkin q Catalysis & Catalysts Adsorption On Solid Surface • Other adsorption isotherms Many other isotherms are proposed in order to explain the observations • The Temkin (or Slygin-Frumkin) isotherm • Assuming the adsorption enthalpy DH decreases linearly with surface coverage From ads-des equilibrium, ads. rate ºdes. rate rads=kads(1-q)Pº rdes=kdesq where Qs is the heat of adsorption. When Qs is a linear function of qi. Qs=Q0-iS (Q0 is a constant, i is the number and S represents the surface site), the overall coverage When b1P >>1 and b1Pexp(-i/RT) <<1, we haveq=c1ln(c2P), where c1 & c2 are constants • Valid for some adsorption systems.

  28. CH4003 Lecture Notes 14 (Erzeng Xue) Langmuir DH of ads Freundlich q Catalysis & Catalysts Adsorption On Solid Surface • The Freundlich isotherm • assuming logarithmic change of adsorption enthalpy DH with surface coverage From ads-des equilibrium, ads. rate ºdes. rate rads=kads(1-q)Pº rdes=kdesq where Qi is the heat of adsorption which is a function of qi. If there are Ni types of surface sites, each can be expressed as Ni=aexp(-Q/Q0) (a and Q0 are constants), corresponding to a fractional coverage qi, the overall coverage the solution for this integration expression at small qis: lnq=(RT/Q0)lnP+constant, or as is the Freundlich equation normally written, where c1=constant, 1/c2=RT/Q0 • Freundlich isotherm fits, not all, but many adsorption systems.

  29. CH4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface • BET (Brunauer-Emmett-Teller) isotherm • Many physical adsorption isotherms were found, such as the types II and III, that the adsorption does not complete the first layer (monolayer) before it continues to stack on the subsequent layer (thus the S-shape of types II and III isotherms) • Basic assumptions • the same assumptions as that of Langmuir but allow multi-layer adsorption • the heat of ads. of additional layer equals to the latent heat of condensation • based on the rate of adsorption=the rate of desorption for each layer of ads. the following BET equation was derived Where P - equilibrium pressure P0 - saturate vapour pressure of the adsorbed gas at the temperature P/P0 is called relative pressure V - volume of adsorbed gas per kg adsorbent Vm - volume of monolayer adsorbed gas per kg adsorbent c - constant associated with adsorption heat and condensation heat Note: for many adsorption systems c=exp[(H1-HL)/RT], where H1 is adsorption heat of 1st layer & HL is liquefaction heat, so that the adsorption heat can be determined from constant c.

  30. CH4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface • Comment on the BET isotherm • BET equation fits reasonably well all known adsorption isotherms observed so far (types I to V) for various types of solid, although there is fundamental defect in the theory because of the assumptions made (no interaction between adsorbed molecules, surface homogeneity and liquefaction heat for all subsequent layers being equal). • BET isotherm, as well as all other isotherms, gives accurate account of adsorption isotherm only within restricted pressure range. At very low (P/P0<0.05) and high relative pressure (P/P0>0.35) it becomes less applicable. • The most significant contribution of BET isotherm to the surface science is that the theory provided the first applicable means of accurate determination of the surface area of a solid (since in 1945). • Many new development in relation to the theory of adsorption isotherm, most of them are accurate for a specific system under specific conditions.

  31. CH4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface • Use of BET isotherm to determine the surface area of a solid • At low relative pressureP/P0 = 0.05~0.35 it is found that Y = a + b X • The principle of surface area determination by BET method: A plot of against P/P0 will yield a straight line with slope of equal to (c-1)/(cVm) and intersect 1/(cVm). For a given adsorption system, c and Vm are constant values, the surface area of a solid material can be determined by measuring the amount of a particular gas adsorbed on the surface with known molecular cross-section area Am, * In practice, measurement of BET surface area of a solid is carried out by N2 physisorption at liquid N2 temperature; for N2, Am = 16.2 x 10-20 m2 P/P0 Vm - volume of monolayer adsorbed gas molecules calculated from the plot, L VT,P - molar volume of the adsorbed gas, L/mol Am - cross-section area of a single gas molecule, m2

  32. CH4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface • Summary of adsorption isotherms Name Isotherm equation Application Note Langmuir Temkin q=c1ln(c2P) Freundlich BET Useful in analysis of reaction mechanism Chemisorption Easy to fit adsorption data Useful in surface area determination Chemisorption and physisorption Chemisorption Chemisorption and physisorption Multilayer physisorption

  33. CH4003 Lecture Notes 15 (Erzeng Xue) A B P + " Catalysis & Catalysts Mechanism of Surface Catalysed Reaction • Langmuir-Hinshelwood mechanism • This mechanism deals with the surface-catalysed reaction in which 2 or more reactants adsorb on surface without dissociation A(g) + B(g)D A(ads) + B(ads)" P (the desorption of P is not r.d.s.) • The rate of reaction ri=k[A][B]=kqAqB From Langmuir adsorption isotherm (the case III) we know • We then have • When both A and B are weakly adsorbed (B0,APA<<1, B0,BPB<<1), 2nd order reaction • When A is strongly adsorbed (B0,APA>>1) and B weakly adsorbed (B0,BPB<<1 <<B0,APA) 1st order w.r.t. B

  34. CH4003 Lecture Notes 15 (Erzeng Xue) A P B + B(g) Catalysis & Catalysts Mechanism of Surface Catalysed Reaction • Eley-Rideal mechanism • This mechanism deals with the surface-catalysed reaction in which one reactant, A, adsorbs on a surface without dissociation and other reactant, B, approaches from the gas phase to react with A A(g)D A(ads) P (the desorption of P is not r.d.s.) • The rate of reaction ri=k[A][B]=kqAPB From Langmuir adsorption isotherm (the case I) we know • We then have • When both A is weakly adsorbed or the partial pressure of A is very low (B0,APA<<1), 2nd order reaction • When A is strongly adsorbed or the partial pressure of A is very high (B0,APA>>1) 1st order w.r.t. B "

  35. CH4003 Lecture Notes 15 (Erzeng Xue) P B A B " + B(g) Catalysis & Catalysts Mechanism of Surface Catalysed Reaction • Mechanism of surface-catalysed reaction with dissociative adsorption • The mechanism of the surface-catalysed reaction in which one reactant, AD, dissociatively adsorbs on one surface site AD(g)D A(ads) + D(ads) P (the des. of P is not r.d.s.) • The rate of reaction ri=k[A][B]=kqADPB From Langmuir adsorption isotherm (the case I) we know • We then have • When both AD is weakly adsorbed or the partial pressure of AD is very low (B0,ADPAD<<1), Thereaction orders, 0.5 w.r.t. AD and 1 w.r.t. B • When A is strongly adsorbed or the partial pressure of A is very high (B0,APA>>1) 1st order w.r.t. B

  36. CH4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Mechanism of Surface Catalysed Reaction • Mechanisms of surface-catalysed rxns involving dissociative adsorption • In a similar way one can derive mechanisms of other surface-catalysed reactions, in which • dissociatively adsorbed one reactant, AD, (on one surface site) reacts with another associatively adsorbed reactant B on a separate surface site • dissociatively adsorbed one reactant, AD, (on one surface site) reacts with another dissociatively adsorbed reactant BC on a separate site • … • The use of these mechanism equations • Determining which mechanism applies by fitting experimental data to each. • Helping in analysing complex reaction network • Providing a guideline for catalyst development (formulation, structure,…). • Designing / running experiments under extreme conditions for a better control • …

  37. CH4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Solids and Solid Surface • Bulk and surface • The composition & structure of a solid in bulk and on surface can differ due to • Surface contamination • Bombardment by foreign molecules when exposed to an environment • Surface enrichment • Some elements or compounds tend to be enriched (driving by thermodynamic properties of the bulk and surface component) on surface than in bulk • Deliberately made different in order for solid to have specific properties • Coating (conductivity, hardness, corrosion-resistant etc) • Doping the surface of solid with specific active components in order perform certain function such as catalysis • … • To processes that occur on surfaces, such as corrosion, solid sensors and catalysts, the composition and structure of (usually number of layers of) surface are of critical importance

  38. CH4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Solids and Solid Surface • Morphology of a solid and its surface • A solid, so as its surface, can be well-structured crystalline (e.g. diamond C, carbon nano-tubes, NaCl, sugar etc) or amorphous (non-crystallised, e.g. glass) • Mixture of different crystalline of the same substance can co-exist on surface (e.g. monoclinic, tetragonal, cubic ZrO2) • Well-structured crystalline and amorphous can co-exist on surface • Both well-structured crystalline and amorphous are capable of being used adsorbent and/or catalyst • …

  39. CH4003 Lecture Notes 15 (Erzeng Xue) Terrace Step Catalysis & Catalysts Solids and Solid Surface • Defects and dislocation on surface crystalline structure • A ‘perfect crystal’ can be made in a controlled way • Surface defects • terrace • step • kink • adatom / vacancy • Dislocation • screw dislocation • Defects and dislocation can be desirable for certain catalytic reactions as these may provide the required surface geometry for molecules to be adsorbed, beside the fact that these sites are generally highly energised.

  40. CH4003 Lecture Notes 15 (Erzeng Xue) wt Dwt a a b b d Dd d Cumulative curve Frequency curve dw dd Catalysis & Catalysts Pores of Porous Solids • Pore sizes • micro pores dp <20-50 nm • meso-pores 20nm <dp<200nm • macro pores dp >200 nm • Pores can be uniform (e.g. polymers) or non-uniform (most metal oxides) • Pore size distribution • Typical curves to characterise pore size: • Cumulative curve • Frequency curve • Uniform size distribution (a) & non-uniform size distribution (b)

  41. CH4003 Lecture Notes 16 (Erzeng Xue) E Complex Reactions Chain Reactions - Process • Many reactions proceed via chain reaction • polymerisation • explosion • … • Elementary reaction steps in chain reactions 1.Initiation step - creation of chain carriers (radicals, ions, neutrons etc, which are capable of propagating a chain) by vigorous collisions, photon absorption R Rž(the dot here signifies the radical carrying unpaired electron) 2.Propagation step - attacking reactant molecules to generate new chain carriers Rž + M ® R + Mž 3.Termination step - two chain carriers combining resulting in the end of chain growth Rž + žM ® R-M There are also other reactions occur during chain reaction: Retardation step - chain carriers attacking product molecules breaking them to reactant Rž + R-M ® R + Mž (leading to net reducing of the product formation rate) Inhibition step - chain carriers being destroyed by reacting with wall or foreign matter Rž + W ® R-W(leading to net reducing of the number of chain carriers)

  42. CH4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Rate Law • Rate law of chain reaction Example:overall reaction H2(g) + Br2(g)®2HBr(g) observed: elem steprate law a.Initiation:Br2® 2Bržra=ka[Br2] b.Propagation:Brž + H2® HBr + Hž rb=kb[Br][H2] Hž + Br2® HBr + Brž r’b=k’b[H][Br2] c.Termination: Brž + žBr ® Br2rc=kc[Br][Br]=kc[Br]2 Hž + žH ® H2 (practically less important therefore neglected) Hž + žBr ® HBr (practically less important therefore neglected) d. Retardn (obsvd.) Hž + HBr ® H2 + Brž rd=kd[H][HBr] HBr net rate: rHBr=rb+ r’b- rdor d[HBr]/dt=kb[Br][H2]+k’b[H][Br2]-kd[H][HBr] Apply s.s.a. rH=rb- r’b- rdor d[H]/dt=kb[Br][H2]- k’b[H][Br2]-kd[H][HBr]=0 rBr= 2ra-rb+r’b-2rc +rdor d[Br]/dt=2ka[Br2]-kb[Br][H2]+k’b[H][Br2]-2 kc[Br]2 +kd[H][HBr]=0 solve the above eqn’s we have

  43. CH4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Polymerisation • Monomer - the individual molecule unit in a polymer • Type I polymerisation - Chain polymerisation • An activated monomer attacks another monomer, links to it, then likes another monomer, so on…, leading the chain growth eventually to polymer. rate law Initiation: Ix® xRž(usually r.d.s.)ri=ki[I] Rž + M ®žM1(fast) Propagation: M + žM1®ž(MM1) ®žM2(fast) M + žM2®ž(MM2) ®žM3(fast) … … … … … … … … … M + žMn-1®ž(MMn-1) ®žMnrp=kp[M][žM] (ri is the r.d.s.) Termination: žMn + žMm® (MnMm) ® Mm+nrt=kt[žM]2 Apply s.s.a. to [žM] formed The rate of propagation or the rate of M consumption or the rate of chain growth f is the yield of Ix to xR initiator chain-carrier

  44. CH4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Polymerisation • Type II polymerisation - Stepwise polymerisation A specific section of molecule A reacts with a specific section of molecule B forming chain (a-A-a’) + (b’-B-b) ® {a -A-(a’b’)-B-b} H2N(CH2)6NH2 + HOOC(CH2)4COOH ® H2N(CH2)6NHOC(CH2)4COOH + H2O (1) ® H-HN(CH2)6NHOC(CH2)4CO-OH … ® H-[HN(CH2)6NHOC(CH2)4CO]n-OH (n) Note: If a small molecule is dropped as a result of reaction, like a H2O dropped in rxn (1), this type of reaction is called condensation reaction. Protein molecules are formed in this way. • The rate law for the overall reaction of this type is the same as its elementary step involving one H- containing unit & one -OH containing unit, which is the 2nd order the conversion of B (-OH containing substance) at time t is

  45. CH4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Explosion • Type I Explosion: Chain-branching explosion Chain-branching - During propagation step of a chain reaction one attack by a chain carrier can produce more than one new chain carriers Chain-branching explosion When chain-branching occurs the number carriers increases exponentially the rate of reaction may cascade into explosion Example: 2H2(g) + O2(g)® 2H2O(g) Initiation:H2 + O2®žO2H + Hž Propagation:H2 + žO2H ®žOH + H2O (non-branching) H2 + žOH ®žH + H2O (non-branching) O2 + žH ®žOž + žOH (branching) žOž + H2®žOH + žH (branching) Lead to explosion

  46. CH4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Explosion Reactions • Type II Explosion: Thermal explosion A rapid increase of the rate of exothermic reaction with temperature Strictly speaking thermal explosion is not caused by multiple production of chain carriers • Must be exothermic reaction • Must be in a confined space and within short time DH ®T ­ ® r ­®DH ­­ ®T ­­ ® r ­­® DH ­­­ ® … • A combination of chain-branching reaction with heat accumulation can occur simultaneously

  47. CH4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Photochemical Reactions • Photochemical reaction The reaction that is initiated by the absorption of light (photons) • Characterisation of photon absorption - quantum yield A reactant molecule after absorbing a photon becomes excited. The excitation may lead to product formation or may be lost (e.g. in form of heat emission) • The number of specific primary products (e.g. a radical, photon-excited molecule, or an ion) formed by absorption of each photon, is called primary quantum yield, f • The number of reactant molecules that react as a result of each photon absorbed is call overall quantum yield, F E.g. HI + hv ® H + I primary quantum yield f =2 (one H and one I) H + HI® H2 + I 2I® I2overall quantum yield F =2 (two HI molecules reacted) Note: Many chain reactions are initiated by photochemical reaction. Because of chain reaction overall quantum yield can be very large, e.g. F = 104 The quantum yield of a photochemical reaction depends on the wavelength of light used

  48. CH4003 Lecture Notes 16 (Erzeng Xue) 508 nm light 254 nm light CO H2 Complex Reactions Photochemical Reactions • Wave-length selectivity of photochemical reaction • A light with a specific wave length may only excite a specific type of molecule • Quantum yield of a photochemical rxn may vary with light (wave-length) used • Isotope separation (photochemical reaction Application) • Different isotope species - different mass - different frequencies required to match their vibration-rotational energys e.g. I36Cl + I37Cl I36Cl + I37Cl* (only 37Cl molecules are excited) C6H5Br + I37Cl* ® C6H537Cl + IBr • Photosensitisation (photochemical reaction Application) • Reactant molecule A may not be activated in a photochemical reaction because it does not absorb light, but A may be activated by the presence of another molecule B which can be excited by absorbing light, then transfer some of its energy to A. e.g. Hg + H2 Hg* + H2(Hg is, but H2 is not excited by 254nm light) Hg* + H2® Hg + 2H* & Hg* + H2® HgH + H* H* HCO HCHO + H* 2HCO ®HCHO + CO

  49. CH4003 Lecture Notes 17 (Erzeng Xue) Spectroscopy Introduction to Spectroscopy • What is Spectroscopy The study of structure and properties of atoms and molecule by means of the spectral information obtained from the interaction of electromagnetic radiant energy with matter It is the base on which a main class of instrumental analysis and methods is developed & widely used in many areas of modern science • What to be discussed • Theoretical background of spectroscopy • Types of spectroscopy and their working principles in brief • Major components of common spectroscopic instruments • Applications in Chemistry related areas and some examples

  50. CH4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Electromagnetic Radiation • Electromagnetic radiation (e.m.r.) • Electromagnetic radiation is a form of energy • Wave-particle duality of electromagnetic radiation • Wave nature - expressed in term of frequency, wave-length and velocity • Particle nature - expressed in terms of individual photon, discrete packet of energy when expressing energy carried by a photon, we need to know the its frequency • Characteristics of wave • Frequency, v - number of oscillations per unit time, unit: hertz (Hz) - cycle per second • velocity, c - the speed of propagation, for e.m.r c=2.9979 x 108 m×s-1 (in vacuum) • wave-length, l - the distance between adjacent crests of the wave wave number, v’, - the number of waves per unit distance v’ =l-1 • The energy carried by an e.m.r. or a photon is directly proportional to the frequency, i.e. where h is Planck’s constant h=6.626x10-34J×s