CH 4003 Lecture Notes 11 Erzeng Xue Catalysis

  • Slides: 88
Download presentation
CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts q Facts and Figures

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts q Facts and Figures about Catalysts Life cycle on the earth Catalysts (enzyme) participates most part of life cycle e. g. forming, growing, decaying m Catalysis contributes great part in the processes of converting sun energy to various other forms of energies e. g. photosynthesis by plant CO 2 + H 2 O=HC + O 2 m Catalysis plays a key role in maintaining our environment m Chemical Industry ca. $2 bn annual sale of catalysts m ca. $200 bn annual sale of the chemicals that are related products m 90% of chemical industry has catalysis-related processes m Catalysts contributes ca. 2% of total investment in a chemical process m 1

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts What is Catalysis q

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts What is Catalysis q Catalysis m 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 Ø taking part in the reaction • Ø altering the rates of reactions • Ø it will change itself during the process by interacting with other reactant/product molecules 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 2

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Action of Catalysts q

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Action of Catalysts q 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 Ø The reaction activation energy is altered Ø The intermediates formed are different from those formed in non-catalytic reaction Ø The rates of reactions are altered (both desired and undesired ones) m uncatalytic energy m reactant product reaction process Reactions proceed under less demanding conditions Ø Allow reactions occur under a milder conditions, e. g. at lower temperatures for those heat sensitive materials 3

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Action of Catalysts q

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Action of Catalysts q 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 m 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. m 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 CH 4(g) + CO 2(g) = 2 CO(g) + 2 H 2(g) DG° 373=151 k. J/mol (100 °C) DG° 973 =-16 k. J/mol (700 °C) 100°C, DG° 373=151 k. J/mol > 0. There is no thermodynamic driving force, the reaction won’t proceed with or without a catalyst Ø At 700°C, DG° 373= -16 k. J/mol < 0. The thermodynamic driving force is there. However, simply putting CH 4 and CO 2 together in a reactor does not mean they will react. Without a proper catalyst heating the mixture in reactor results no conversion of CH 4 and CO 2 at all. When Pt/Zr. O 2 or Ni/Al 2 O 3 is present in the reactor at the same temperature, equilibrium conversion can be achieved (<100%). Ø At 4

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Types of Catalysts &

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Types of Catalysts & Catalytic Reactions q The types of catalysts m Classification based on the its physical state, a catalyst can be Ø gas Ø liquid Ø solid m Classification based on the substances from which a catalyst is made Ø Inorganic Ø Organic m (gases, metal oxides, inorganic acids, bases etc. ) (organic acids, enzymes etc. ) Classification based on the ways catalysts work Ø Homogeneous - both catalyst and all reactants/products are in the same phase (gas or liq) Ø Heterogeneous m - reaction system involves multi-phase (catalysts + reactants/products) Classification based on the catalysts’ action Ø Acid-base catalysts Ø Enzymatic Ø Photocatalysis Ø Electrocatalysis, etc. 5

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Applications of Catalysis q

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Applications of Catalysis q Industrial applications Almost all chemical industries have one or more steps employing catalysts m Petroleum, energy sector, fertiliser, pharmaceutical, fine chemicals … Advantages of catalytic processes m Achieving better process economics and productivity Ø Increase reaction rates - fast Ø Simplify the reaction steps - low investment cost Ø Carry out reaction under mild conditions (e. g. low T, P) - low energy consumption m Reducing wastes Ø Improving selectivity toward desired products - less raw materials required, less unwanted wastes Ø Replacing harmful/toxic materials with readily available ones m Producing certain products that may not be possible without catalysts m Having better control of process (safety, flexible etc. ) m Encouraging application and advancement of new technologies and materials m And many more … 6

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Applications of Catalysis q

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Applications of Catalysis q Environmental applications m Pollution controls in combination with industrial processes Ø Pre-treatment - reduce the amount waste/change the composition of emissions Ø Post-treatments Ø Using - once formed, reduce and convert emissions alternative materials … m Pollution reduction Ø gas - converting harmful gases to non-harmful ones Ø liquid Ø solid - de-pollution, de-odder, de-colour etc - landfill, factory wastes … m q And many more … Other applications m Catalysis and catalysts play one of the key roles in new technology development. 7

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Research in Catalysis q

CH 4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Research in Catalysis q Research in catalysis involve a multi-discipline approach m Reaction kinetics and mechanism Ø Reaction paths, intermediate formation & action, interpretation of results obtained under various conditions, generalising reaction types & schemes, predict catalyst performance… m Catalyst development Ø Material m synthesis, structure properties, catalyst stability, compatibility… Analysis techniques Ø Detection limits in terms of dimension of time & size and under extreme conditions (T, P) and accuracy of measurements, microscopic techniques, sample preparation techniques… m Reaction modelling Ø Elementary m reactions and rates, quantum mechanics/chemistry, physical chemistry … Reactor modelling Ø Mathematical interpretation and representation, the numerical method, micro-kinetics, structure and efficiency of heat and mass transfer in relation to reactor design … m Catalytic process Ø Heat and mass transfers, energy balance and efficiency of process … 8

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Catalytic Reaction Processes q

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Catalytic Reaction Processes q Understanding catalytic reaction processes m A catalytic reaction can be operated in a batch manner Ø 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 Ø Type of reactor is usually simple, basic requirements Withstand required temperature & pressure è Some stirring to encourage mass and heat transfers è Provide sufficient heating or cooling è m Catalytic reactions are commonly operated in a continuous manner Ø Reactants, which are usually in gas or liquid phase, are fed to reactor in steady rate (e. g. mol/h, kg/h, m 3/h) Ø Usually a target conversion is set for the reaction, based on this target required quantities of catalyst is added è required heating or cooling is provided è required reactor dimension and characteristics are designed accordingly. è 9

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Catalytic Reaction Processes m

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Catalytic Reaction Processes m Catalytic reactions in a continuous operation (cont’d) Ø Reactants in continuous operation are mostly in gas phase or liquid phase è easy transportation è The heat & mass transfer rates in gas phase is much faster than those in liquid Ø 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 è 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. m è 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), è 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 10

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Catalytic Reaction Processes q

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Catalytic Reaction Processes q General requirements for a good catalyst m Activity - being able to promote the rate of desired reactions m 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 NO 2 in the presence of SO 2) m m Stability - a good catalyst should resist to deactivation, caused by è the presence of impurities in feed (e. g. lead in petrol poison TWC. è thermal deterioration, volatility and hydrolysis of active components è 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. 11

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Example Heterogeneous Catalytic Reaction

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Example Heterogeneous Catalytic Reaction Process q 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 q Product molecules must follow the same track in the reverse direction to return to gas phase q Heat transfer follows similar track gas phase reactant molecule j k l gas phase liquid phase / stagnant layer mn o porous solid pore pq r 12

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Solid Catalysts Catalyst composition

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Solid Catalysts Catalyst composition Active phase Promoter Support / carrier r m Catalyst ote Textual promoter (e. g. Al - Fe for NH 3 production) Ø Electric or Structural modifier Ø Poison resistant promoters Ø m Pro m Where the reaction occurs (mostly metal/metal oxide) se Ø ep ha m Ac tiv q Support Increase mechanical strength Ø Increase surface area (98% surface area is supplied within the porous structure) Ø may or may not be catalytically active Ø 13

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Solid Catalysts q Some

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Solid Catalysts q Some m common solid support / carrier materials Alumina m Inexpensive Ø Surface area: 1 ~ 700 m 2/g Ø Acidic Ø m Silica Inexpensive Ø Surface area: 100 ~ 800 m 2/g Ø Acidic Ø m Other supports Ø Ø Ø Active carbon (S. A. up to 1000 m 2/g) Titania (S. A. 10 ~ 50 m 2/g) Zirconia (S. A. 10 ~ 100 m 2/g) Magnesia (S. A. 10 m 2/g) Lanthana (S. A. 10 m 2/g) Active site Zeolite mixture of alumina and silica, Ø often exchanged metal ion present Ø shape selective Ø acidic Ø porous solid pore 14

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Solid Catalysts q Preparation

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Solid Catalysts q Preparation of catalysts Precipitation To form non-soluble precipitate by desired reactions at certain p. H and temperature m Adsorption & ion-exchange Cationic: S-OH+ + C+ ® SOC+ + H+ Drying & firing precursor add acid/base solution with p. H control Support Anionic: S-OH- + A- ® SA- + OHI-exch. m S-Na+ + Ni 2+ D S-Ni 2+ + Na+ Impregnation precipitate or deposit precipitation Dry mixing Drying & firing Concentration Support Fill the pores of support with a metal salt solution of sufficient concentration to give the correct loading. m filter & wash the resulting precipitate Amount adsorbed m Support Drying & firing Soln. of metal precursor Pore saturated pellets Physically mixed, grind, and fired 15

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Solid Catalysts q Preparation

CH 4003 Lecture Notes 12 (Erzeng Xue) Catalysis & Catalysts Solid Catalysts q Preparation of 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. 40 Commonly used Pre-treatments m Reduction Ø if elemental metal is the active phase m Sulphidation Ø if a metal sulphide is the active phase m 100 BET S. A. q BET S. A. m 2/g m Catalysts 75 50 25 0 500 600 700 800 900 0 0 Time / hours 10 Temperature °C Activation Ø q 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 Induction period Typical catalyst life span m Can be many years or a few mins. Activity Ø Some Normal use Time dead 16

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface q Adsorption m Adsorption is a process in which molecules from gas (or liquid) phase land on, interact with and attach to solid surfaces. m The reverse process of adsorption, i. e. the process n which adsorbed molecules escape from solid surfaces, is called Desorption. m 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 k. J/mol) high ( > 40 k. J/mol) selectivity low high temperature to occur low high 17

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface q Adsorption process Adsorbent and adsorbate m Adsorbent (also called substrate) - The solid that provides surface for adsorption Ø high surface area with proper pore structure and size distribution is essential Ø good mechanical strength and thermal stability are necessary m 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 define number of adsorption sites occupied q= number of adsorption sites available q = 0~1 Adsorption heat m Adsorption is usually exothermic (in special cases dissociated adsorption can be endothermic) m 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. 18

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface q Applications of adsorption process m Adsorption is a very important step in solid catalysed reaction processes m Adsorption in itself is a common process used in industry for various purposes Ø Purification (removing impurities from a gas / liquid stream) Ø De-pollution, Ø Solvent de-colour, de-odour recovery, trace compound enrichment Ø etc… Ø Usually Ø The adsorption is only applied for a process dealing with small capacity operation is usually batch type and required regeneration of saturated adsorbent Common adsorbents: molecular sieve, active carbon, silica gel, activated alumina. m 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) 19

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts Characterisation of adsorption system

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts Characterisation of adsorption system m Adsorption isotherm - most commonly used, especially to catalytic reaction system, T=const. The amount of adsorption as a function of pressure at set temperature m Adsorption isobar - (usage related to industrial applications) The amount of adsorption as a function of temperature at set pressure m Adsorption Isostere - (usage related to industrial applications) Adsorption pressure as a function of temperature at set volume V 3>V 2 V 2>V 1 T 2 >T 1 T 3 >T 2 T 4 >T 3 P 3>P 2 P 2>P 1 V 4>V 3 V 1 Pressure T 1 Vol. adsorbed P 4>P 3 Vol. adsorbed q Adsorption On Solid Surface T 5 >T 4 Pressure Adsorption Isotherm Temperature Adsorption Isobar Temperature Adsorption Isostere 20

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts q The m Adsorption

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts q The m Adsorption On Solid Surface Langmuir adsorption isotherm Basic assumptions Ø surface uniform (DHads does not vary with coverage) monolayer adsorption, and Ø no interaction between adsorbed molecules and adsorbed molecules immobile Ø m Case I - single molecule adsorption A 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 = kdes q at equilibrium rads = rdes Þ kads (1 -q) P = kdes q case I rearrange it for q let Þ B 0 is adsorption coefficient 21

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts q The m Adsorption

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts q The m Adsorption On Solid Surface 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 -q. A )(1 -q. B)PAB=kads (1 -q )2 PAB q=q. A =q. B the rate of A-B desorption rdes=kdesq. Aq. B =kdesq 2 at equilibrium rads = rdes Þ kads (1 -q )2 PAB= kdesq 2 A A B B case II rearrange it for q Let. Þ 22

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts q The m Adsorption

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts q The m Adsorption On Solid Surface Langmuir adsorption isotherm (cont’d) Case III - two molecules adsorbed on two sites A(g) + B(g) + 2 M(surface site) D A-M + B-M the rate of A adsorption rads, A = kads, A (1 - q. A- q. B) PA the rate of B adsorption rads, B = kads, B (1 - q. A- q. B) PB the rate of A desorption rdes, A = kdes, A q. A the rate of B desorption rdes, B = kdes, B q. B at equilibrium Þ rads , A = rdes , A A B case III and Þ rads , B = rdes , B kads, A(1 -q. A-q. B)PA=kdes, Aq. A and kads, B(1 -q. A-q. B)PB=kdes, Bq. B rearrange it for q where adsorption coefficients of A & B. 23

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts q The Adsorption On

CH 4003 Lecture Notes 13 (Erzeng Xue) Catalysis & Catalysts q The Adsorption On Solid Surface Langmuir adsorption isotherm (cont’d) A A A B B case III case II Adsorption Strong Weak kads>> kdes B 0>>1 kads<< kdes B 0<<1 A, B both strong A strong, B weak A weak, B weak 24

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface q Langmuir adsorption isotherm case I kads>> kdes Weak adsorption kads<< kdes Amount adsorbed case II Strong adsorption Case III mono-layer large B 0 (strong adsorp. ) moderate B 0 small B 0 (weak adsorp. ) Pressure Ø Langmuir Ø It adsorption isotherm established a logic picture of adsorption process fits many adsorption systems but not at all Ø The assumptions made by Langmuir do not hold in all situation, that causing error Solid surface is heterogeneous 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 § 25

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface q Five types of physisorption isotherms are found over all solids I amount adsorbed II IV V 1. 0 relative pres. P/P 0 m 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 m Type II for non-porous materials m Type III porous materials with cohesive force between adsorbate molecules greater than the adhesive force between adsorbate molecules and adsorbent m Type IV staged adsorption (first monolayer then build up of additional layers) m Type V porous materials with cohesive force between adsorbate molecules and adsorbent being greater than that between adsorbate molecules 26

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface q Other adsorption isotherms Many other isotherms are proposed in order to explain the observations The Temkin (or Slygin-Frumkin) isotherm m Assuming the adsorption enthalpy DH decreases linearly with surface coverage From ads-des equilibrium, ads. rate des. rate rads=kads(1 -q)P rdes=kdesq DH of ads q Langmuir Temkin where Qs is the heat of adsorption. When Qs is a linear function of qi. Qs=Q 0 -i. S (Q 0 is a constant, i is the number and S represents the surface site), q the overall coverage When b 1 P >>1 and b 1 Pexp(-i/RT) <<1, we have q =c 1 ln(c 2 P), where c 1 & c 2 are constants Ø Valid for some adsorption systems. 27

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface The Freundlich isotherm m assuming logarithmic change of adsorption enthalpy DH with surface coverage From ads-des equilibrium, ads. rate des. rate rads=kads(1 -q)P rdes=kdesq DH of ads q Langmuir Freundlich q 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/Q 0) (a and Q 0 are constants), corresponding to a fractional coverage qi, the overall coverage the solution for this integration expression at small q is: lnq=(RT/Q 0)ln. P+constant, or as is the Freundlich equation normally written, Ø where c 1=constant, 1/c 2=RT/Q 0 Freundlich isotherm fits, not all, but many adsorption systems. 28

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface q BET (Brunauer-Emmett-Teller) isotherm m 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) m Basic assumptions Ø the same assumptions as that of Langmuir but allow multi-layer adsorption Ø the heat of ads. of additional layer equals to the latent heat of condensation Ø 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 P 0 - saturate vapour pressure of the adsorbed gas at the temperature P/P 0 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[(H 1 -HL)/RT], where H 1 is adsorption heat of 1 st layer & HL is liquefaction heat, so that the adsorption heat can be determined from constant c. 29

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface q Comment on the BET isotherm m 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 theory because of the assumptions made (no interaction between adsorbed molecules, surface homogeneity and liquefaction heat for all subsequent layers being equal). m BET isotherm, as well as all other isotherms, gives accurate account of adsorption isotherm only within restricted pressure range. At very low (P/P 0<0. 05) and high relative pressure (P/P 0>0. 35) it becomes less applicable. m The most significant contribution of BET isotherm to the surface science is that theory provided the first applicable means of accurate determination of the surface area of a solid (since in 1945). m Many new development in relation to theory of adsorption isotherm, most of them are accurate for a specific system under specific conditions. 30

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface q Use of BET isotherm to determine the surface area of a solid m At low relative pressure P/P 0 = 0. 05~0. 35 it is found that Y Ø The = a +b X P/P 0 principle of surface area determination by BET method: A plot of against P/P 0 will yield a straight line with slope of equal to (c-1)/(c. Vm) and intersect 1/(c. Vm). 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, 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, m 2 * In practice, measurement of BET surface area of a solid is carried out by N 2 physisorption at liquid N 2 temperature; for N 2, Am = 16. 2 x 10 -20 m 2 31

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface

CH 4003 Lecture Notes 14 (Erzeng Xue) Catalysis & Catalysts Adsorption On Solid Surface q Summary of adsorption isotherms Name Isotherm equation Application Note Chemisorption and physisorption Useful in analysis of reaction mechanism Chemisorption Freundlich Chemisorption and physisorption Easy to fit adsorption data BET Multilayer physisorption Useful in surface area determination Langmuir Temkin q =c 1 ln(c 2 P) 32

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Mechanism of Surface Catalysed

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Mechanism of Surface Catalysed Reaction q Langmuir-Hinshelwood m 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 m The rate of reaction A + B "P (the desorption of P is not r. d. s. ) ri=k[A][B]=kq. Aq. B From Langmuir adsorption isotherm (the case III) we know m We then have Ø When both A and B are weakly adsorbed (B 0, APA<<1, B 0, BPB<<1), 2 nd order reaction Ø When A is strongly adsorbed (B 0, APA>>1) and B weakly adsorbed (B 0, BPB<<1 <<B 0, APA) 1 st order w. r. t. B 33

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Mechanism of Surface Catalysed

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Mechanism of Surface Catalysed Reaction q Eley-Rideal B mechanism m 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 + B(g) A(g) D A(ads) P (the desorption of P is not r. d. s. ) m The rate of reaction A "P ri=k[A][B]=kq. APB From Langmuir adsorption isotherm (the case I) we know m We then have Ø When both A is weakly adsorbed or the partial pressure of A is very low (B 0, APA<<1), 2 nd order reaction Ø When A is strongly adsorbed or the partial pressure of A is very high (B 0, APA>>1) 1 st order w. r. t. B 34

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Mechanism of Surface Catalysed

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Mechanism of Surface Catalysed Reaction q Mechanism of surface-catalysed reaction with dissociative adsorption m The mechanism of the surface-catalysed reaction in which one reactant, AD, dissociatively adsorbs on one surface site + B(g) AD(g) D A(ads) + D(ads) P B A B "P (the des. of P is not r. d. s. ) m The rate of reaction ri=k[A][B]=kq. ADPB From Langmuir adsorption isotherm (the case I) we know m We then have Ø When both AD is weakly adsorbed or the partial pressure of AD is very low (B 0, ADPAD<<1), The reaction orders, 0. 5 w. r. t. AD and 1 w. r. t. B Ø When A is strongly adsorbed or the partial pressure of A is very high (B 0, APA>>1) 1 st order w. r. t. B 35

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Mechanism of Surface Catalysed

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Mechanism of Surface Catalysed Reaction q Mechanisms of surface-catalysed rxns involving dissociative adsorption m q In a similar way one can derive mechanisms of other surface-catalysed reactions, in which Ø dissociatively adsorbed one reactant, AD, (on one surface site) reacts with another associatively adsorbed reactant B on a separate surface site Ø dissociatively adsorbed one reactant, AD, (on one surface site) reacts with another dissociatively adsorbed reactant BC on a separate site Ø … The use of these mechanism equations m Determining which mechanism applies by fitting experimental data to each. m Helping in analysing complex reaction network m Providing a guideline for catalyst development (formulation, structure, …). m Designing / running experiments under extreme conditions for a better control m … 36

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Solids and Solid Surface

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Solids and Solid Surface q Bulk and surface m The composition & structure of a solid in bulk and on surface can differ due to Ø Surface è Bombardment by foreign molecules when exposed to an environment Ø Surface è contamination enrichment Some elements or compounds tend to be enriched (driving by thermodynamic properties of the bulk and surface component) on surface than in bulk Ø Deliberately è Coating (conductivity, hardness, corrosion-resistant etc) è Doping the surface of solid with specific active components in order perform certain function such as catalysis Ø… m made different in order for solid to have specific properties 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 37

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Solids and Solid Surface

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Solids and Solid Surface q Morphology of a solid and its surface m A solid, so as its surface, can be well-structured crystalline (e. g. diamond C, carbon nano-tubes, Na. Cl, sugar etc) or amorphous (non-crystallised, e. g. glass) m Mixture of different crystalline of the same substance can co-exist on surface (e. g. monoclinic, tetragonal, cubic Zr. O 2) m Well-structured crystalline and amorphous can co-exist on surface m Both well-structured crystalline and amorphous are capable of being used adsorbent and/or catalyst m … 38

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Solids and Solid Surface

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Solids and Solid Surface q Defects m A ‘perfect crystal’ can be made in a controlled way m Surface defects m Ø terrace Ø step Ø kink Ø adatom / vacancy Terrace Step Dislocation Ø q and dislocation on surface crystalline structure 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. 39

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Pores of Porous Solids

CH 4003 Lecture Notes 15 (Erzeng Xue) Catalysis & Catalysts Pores of Porous Solids q Pore sizes m micro pores dp <20 -50 nm m meso-pores 20 nm <dp<200 nm m macro pores dp >200 nm m Pores can be uniform (e. g. polymers) or non-uniform (most metal oxides) q Pore m m size distribution Typical curves to characterise pore size: Ø Cumulative curve Ø Frequency curve Uniform size distribution (a) & non-uniform size distribution (b) dw dd wt Dwt a b Dd a b d Cumulative curve d Frequency curve 40

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Process q

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Process q Many reactions proceed via chain reaction polymerisation m explosion m … m q 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 E 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 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) 41

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Rate Law

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Rate Law q Rate law of chain reaction Example: overall reaction H 2(g) + Br 2(g) ® 2 HBr(g) observed: elem step rate law a. Initiation: Br 2 ® 2 Brž ra=ka[Br 2] b. Propagation: Brž + H 2 ® HBr + Hž rb=kb[Br][H 2] Hž + Br 2 ® HBr + Brž r’b=k’b[H][Br 2] c. Termination: Brž + žBr ® Br 2 rc=kc[Br]=kc[Br]2 Hž + žH ® H 2 (practically less important therefore neglected) Hž + žBr ® HBr (practically less important therefore neglected) d. Retardn (obsvd. ) Hž + HBr ® H 2 + Brž rd=kd[H][HBr] HBr net rate: r. HBr= rb+ r’b- rd or d[HBr]/dt=kb[Br][H 2]+k’b[H][Br 2]-kd[H][HBr] Apply s. s. a. or d[H]/dt=kb[Br][H 2]- k’b[H][Br 2]-kd[H][HBr]=0 r. H= rb- r’b- rd r. Br= 2 ra-rb+r’b-2 rc +rd or d[Br]/dt=2 ka[Br 2]-kb[Br][H 2]+k’b[H][Br 2]-2 kc[Br]2 +kd[H][HBr]=0 solve the above eqn’s we have 42

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Polymerisation q

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Polymerisation q Monomer - the individual molecule unit in a polymer q 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. f is the yield of Ix to x. R initiator chain-carrier rate law Initiation: Ix ® x. Rž (usually r. d. s. ) ri=ki[I] m Propagation: Termination: Rž + M ® žM 1 (fast) M + žM 1 ® ž(MM 1) ® žM 2 M + žM 2 ® ž(MM 2) ® žM 3 …………… M + žMn-1 ® ž(MMn-1) ® žMn + žMm ® (Mn. Mm) ® Mm+n (fast) rp=kp[M][žM] rt=kt[žM]2 (ri is the r. d. s. ) Apply s. s. a. to [žM] formed The rate of propagation or the rate of M consumption or the rate of chain growth 43

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Polymerisation q

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Polymerisation q 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} H 2 N(CH 2)6 NH 2 + HOOC(CH 2)4 COOH ® H 2 N(CH 2)6 NHOC(CH 2)4 COOH + H 2 O ® H-HN(CH 2)6 NHOC(CH 2)4 CO-OH ® H-[HN(CH 2)6 NHOC(CH 2)4 CO]n-OH (1) … (n) Note: If a small molecule is dropped as a result of reaction, like a H 2 O dropped in rxn (1), this type of reaction is called condensation reaction. Protein molecules are formed in this way. m 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 2 nd order the conversion of B (-OH containing substance) at time t is 44

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Explosion q

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Chain Reactions - Explosion q 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: 2 H 2(g) + O 2(g) ® 2 H 2 O(g) Initiation: H 2 + O 2 ® žO 2 H + Hž Propagation: H 2 + žO 2 H ® žOH + H 2 O (non-branching) H 2 + žOH ® žH + H 2 O (non-branching) O 2 + žH ® žOž + žOH (branching) žOž + H 2 ® žOH + žH (branching) Lead to explosion 45

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Explosion Reactions q Type II

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Explosion Reactions q 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 m Must be exothermic reaction m Must be in a confined space and within short time DH ® T ®r ® DH ® T ® r ® DH ®… m A combination of chain-branching reaction with heat accumulation can occur simultaneously 46

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Photochemical Reactions q Photochemical reaction

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Photochemical Reactions q Photochemical reaction The reaction that is initiated by the absorption of light (photons) q 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) m 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 m 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 ® H 2 + I 2 I ® I 2 overall 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 47

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Photochemical Reactions q Wave-length selectivity

CH 4003 Lecture Notes 16 (Erzeng Xue) Complex Reactions Photochemical Reactions q Wave-length selectivity of photochemical reaction m A light with a specific wave length may only excite a specific type of molecule Ø Quantum m yield of a photochemical rxn may vary with light (wave-length) used Isotope separation (photochemical reaction Application) Ø Different isotope species - different mass - different frequencies required to match their vibration-rotational energys e. g. I 36 Cl + I 37 Cl 508 nm light I 36 Cl + I 37 Cl* (only 37 Cl molecules are excited) C 6 H 5 Br + I 37 Cl* ® C 6 H 537 Cl + IBr m Photosensitisation (photochemical reaction Application) Ø 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 + H 2 254 nm light Hg* + H 2 (Hg is, but H 2 is not excited by 254 nm light) Hg* + H 2 ® Hg + 2 H* & Hg* + H 2 ® Hg. H + H* H* CO H 2 2 HCO ® HCHO + H* HCHO + CO 48

CH 4003 Lecture Notes 17 (Erzeng Xue) Spectroscopy Introduction to Spectroscopy q What is

CH 4003 Lecture Notes 17 (Erzeng Xue) Spectroscopy Introduction to Spectroscopy q 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 q What to be discussed m Theoretical background of spectroscopy m Types of spectroscopy and their working principles in brief m Major components of common spectroscopic instruments m Applications in Chemistry related areas and some examples 49

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Electromagnetic Radiation q Electromagnetic

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Electromagnetic Radiation q Electromagnetic radiation (e. m. r. ) m Electromagnetic radiation is a form of energy m Wave-particle duality of electromagnetic radiation Ø Wave nature - expressed in term of frequency, wave-length and velocity Ø 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 q Characteristics of wave m Frequency, v - number of oscillations per unit time, unit: hertz (Hz) - cycle per second m velocity, c - the speed of propagation, for e. m. r c=2. 9979 x 108 m×s-1 (in vacuum) m wave-length, l - the distance between adjacent crests of the wave number, v’, - the number of waves per unit distance v’ =l-1 q 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. 626 x 10 -34 J×s 50

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Electromagnetic Radiation q Electromagnetic

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Electromagnetic Radiation q Electromagnetic radiation X-ray, light, infra-red, microwave and radio waves are all e. m. r. ’s, difference being their frequency thus the amount of energy they possess q Spectral region of e. m. r. 51

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Interaction of e. m.

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Interaction of e. m. r. with Matter q Interaction of electromagnetic radiant with matter m The wave-length, l, and the wave number, v’, of e. m. r. changes with the medium it travels through, because of the refractive index of the medium; the frequency, v, however, remains unchanged m Types of interactions Ø Absorption absorption Ø Reflection transmission Ø Transmission Ø Scattering Ø Refraction reflection scattering refraction m Each interaction can disclose certain properties of the matter m When applying e. m. r. of different frequency (thus the energy e. m. r. carried) different type information can be obtained 52

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Spectrum q Spectrum is

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Spectrum q Spectrum is the display of the energy level of e. m. r. as a function of wave number of electromagnetic radiation energy The energy level of e. m. r. is usually expressed in one of these terms m absorbance (e. m. r. being absorbed) m transmission (e. m. r. passed through) m Intensity The term ‘intensity’ has the meaning of the radiant power that carried by an e. m. r. intensity 1. 0 . 0. 5 0. 0 350 400 wave length 450 cm-1 53

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Spectrum q What an

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Spectrum q What an spectrum tells m A peak (it can also be a valley depending on how the spectrum is constructed) represents the absorption or emission of e. m. r. at that specific wavenumber Ø The wavenumber at the tip of peak is the most important, especially when a peak is broad ØA broad peak may sometimes consist of several peaks partially overlapped each other mathematic software (usually supplied) must be used to separate them case of a broad peak (or a valley) observed Ø The height of a peak corresponds the amount absorption/emission thus can be used as a quantitative information (e. g. concentration), a careful calibration is usually required Ø The ratio in intensity of different peaks does not necessarily means the ratio of the quantity (e. g. concentration, population of a state etc. ) intensity 1. 0 0. 5 . 0. 0 350 400 wave length cm-1 450 54

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Spectral properties, applications, and

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Spectral properties, applications, and interactions of electromagnetic radiation Wave number v’ Electron kcal/mol vole e. V cm-1 Energy Wavelength l cm Frequency v Hz 9. 4 x 107 4. 1 x 106 3. 3 x 1010 3. 0 x 10 -11 1021 9. 4 x 105 4. 1 x 104 3. 3 x 108 3. 0 x 10 -9 1019 9. 4 x 103 4. 1 x 102 3. 3 x 106 3. 0 x 10 -7 1017 9. 4 x 101 4. 1 x 100 3. 3 x 104 3. 0 x 10 -5 1015 Type of radiation Gamma ray Visible Infrared 9. 4 x 10 -1 4. 1 x 10 -2 3. 3 x 102 3. 0 x 10 -3 1013 9. 4 x 10 -3 4. 1 x 10 -4 3. 3 x 100 3. 0 x 10 -1 1011 9. 4 x 10 -5 4. 1 x 10 -6 3. 3 x 10 -2 3. 0 x 101 109 9. 4 x 10 -7 4. 1 x 10 -8 3. 3 x 10 -4 3. 0 x 103 107 Gamma ray emission Electronic (inner shell) Vac UV absorption UV Vis absorption emission fluorescence IR absorption Raman Molecular vibration Microwave absorption Electron paramagnet resonance Radio Nuclear X-ray absorption emission X-ray Ultra Violet Type of quantum transition Type of spectroscopy Nuclear magnetic resonance Electronic (outer shell) Molecular rotation Magnetically induced spin states 55

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Examples 1. A laser

CH 4003 Lecture Notes 17 (Erzeng Xue) Introductory to Spectroscopy Examples 1. A laser emits light with a frequency of 4. 69 x 1014 s-1. (h = 6. 63 x 10 -34 Js) A) What is the energy of one photon of the radiation from this laser? B) If the laser emits 1. 3 x 10 -2 J during a pulse, how many photons are emitted during the pulse? Ans: A) Ephoton = hn = 6. 63 x 10 -34 Js x 4. 69 x 1014 s-1 = 3. 11 x 10 -19 J B) No. of photons = (1. 3 x 10 -2 J )/(3. 11 x 10 -19 J) = 4. 2 x 1016 2. The brilliant red colours seen in fireworks are due to the emission of red light at a wave length of 650 nm. What is the energy of one photon of this light? (h = 6. 63 x 10 -34 Js) Ans: Ephoton = hc/l =(6. 63 x 10 -34 Js x 3 x 108 ms-1)/650 x 10 -9 m = 3. 06 x 10 -19 J 3: Compare the energies of photons emitted by two radio stations, operating at 92 MHz (FM) and 1500 k. Hz (MW)? Ans: . Ephoton = hn 92 MHz = 92 x 106 Hz (s-1) => E = (6. 63 x 10 -34 Js) x (92 x 106 s-1) = 6. 1 x 10 -26 J 1500 k. Hz = 1500 x 103 Hz (s-1) E = (6. 63 x 10 -34 Js) x (1500 x 103 s-1) = 9. 9 x 10 -28 J 56

CH 4003 Lecture Notes 18 (Erzeng Xue) Introductory to Spectroscopy Atomic Spectra q Shell

CH 4003 Lecture Notes 18 (Erzeng Xue) Introductory to Spectroscopy Atomic Spectra q Shell structure & energy level of atoms m In an atom there a number of shells and of subshells where e-’s can be found m The energy level of each shell & subshell are different and quantised Excited state ground state n=1 n=2 n = 3, etc. Ø The e-’s in the shell closest to the nuclei has energy the lowest energy. The higher shell number DE is, the higher energy it is Ø The exact energy level of each shell and subshell varies with substance Ground state and excited state of e-’s m m Under normal situation an stays at the lowest possible shell - the e- is said to be at its ground state n=4 e- Upon absorbing energy (excited), an e- can change its orbital to a higher one - we say the e- is at is excited state. Energy q n=3 n=2 n=1 4 f 4 d 4 p 3 d 4 s 3 p 3 s 2 p 2 s 1 s 57

CH 4003 Lecture Notes 18 (Erzeng Xue) Introductory to Spectroscopy Atomic Spectra Electron excitation

CH 4003 Lecture Notes 18 (Erzeng Xue) Introductory to Spectroscopy Atomic Spectra Electron excitation m m m The excitation can occur at different degrees Ø low E tends to excite the outmost e-’s first Ø when excited with a high E (photon of high v) an e- can jump more than one levels Ø even higher E can tear inner e-’s away from nuclei energy n=1 DE n=2 n = 3, etc. An e- at its excited state is not stable and tends to return its ground state If an e- jumped more than one energy levels because of absorption of a high E, the process of the e- returning to its ground state may take several steps, - i. e. to the nearest low energy level first then down to next … n=4 Energy q n=3 n=2 n=1 4 f 4 d 4 p 3 d 4 s 3 p 3 s 2 p 2 s 1 s 58

CH 4003 Lecture Notes 18 (Erzeng Xue) Introductory to Spectroscopy Atomic Spectra Atomic spectra

CH 4003 Lecture Notes 18 (Erzeng Xue) Introductory to Spectroscopy Atomic Spectra Atomic spectra m The level and quantities of energy supplied to excite e-’s can be measured & studied in terms of the frequency and the intensity of an e. m. r. - the absorption spectroscopy energy n=1 DE n=2 n = 3, etc. m The level and quantities of energy emitted by excited e-’s, as they return to their ground state, can be measured & studied by means of the emission spectroscopy m The level & quantities of energy absorbed or emitted (v & intensity of e. m. r. ) are specific for a substance m Atomic spectra are mostly in UV (sometime in visible) regions n=4 Energy q n=3 n=2 n=1 4 f 4 d 4 p 3 d 4 s 3 p 3 s 2 p 2 s 1 s 59

CH 4003 Lecture Notes 18 (Erzeng Xue) Spectroscopy Molecular Spectra q Motion & energy

CH 4003 Lecture Notes 18 (Erzeng Xue) Spectroscopy Molecular Spectra q Motion & energy of molecules m Molecules are vibrating and rotating all the time, two main vibration modes being stretching - change in bond length (higher v) Ø bending - change in bond angle (lower v) (other possible complex types of stretching & bending are: scissoring / rocking / twisting Ø m Molecules are normally at their ground state (S 0) S (Singlet) - two e-’s spin in pair v 4 v 3 v 2 v 1 S 2 E S 1 T (Triplet) - two e-’s spin parallel J m m Upon exciting molecules can change to high E states (S 1, S 2, T 1 etc. ), which are associated with specific levels of energy The change from high E states to low ones can be stimulated by absorbing a photon; the change from low to high E states may result in photon emission v 4 v 3 v 2 v 1 T 1 v 4 v 3 v 2 v 1 S 0 60

CH 4003 Lecture Notes 18 (Erzeng Xue) Spectroscopy Molecular Spectra q Excitation of a

CH 4003 Lecture Notes 18 (Erzeng Xue) Spectroscopy Molecular Spectra q Excitation of a molecule m The energy levels of a molecule at each state / sub-state are quantised m To excite a molecule from its ground state (S 0) to a higher E state (S 1, S 2, T 1 etc. ), the exact amount of energy equal to the difference between the two states has to be absorbed. (Process A) v 4 v 3 v 2 v 1 S 2 S 1 i. e. to excite a molecule from S 0, v 1 to S 2, v 2, e. m. r with wavenumber v’ must be used m m The values of energy levels vary with the (molecule of) substance. Molecular absorption spectra are the measure of the amount of e. m. r. , at a specific wavenumber, absorbed by a substance. v 4 v 3 v 2 v 1 A v 4 v 3 v 2 v 1 T 1 absorption A v 4 v 3 v 2 v 1 S 0 61

CH 4003 Lecture Notes 18 (Erzeng Xue) Spectroscopy Molecular Spectra q Energy change of

CH 4003 Lecture Notes 18 (Erzeng Xue) Spectroscopy Molecular Spectra q Energy change of excited molecules An excited molecules can lose its excess energy via several processes m m Process B - Releasing E as heat when changing from a sub-state to the parental state occurs within the same state B B A C m Process E 2 - Undergoing intersystem crossing to a triplet sublevel of the excited state Process F - Radiating E from triplet to ground state (triplet quenching) - Phosphorescence D Fluorescence T 1 Fluorescence v 4 v 3 v 2 v 1 E 2 S 1 Process D - Emitting photons when falling back to the ground state - Fluorescence Process E 1 - Undergoing internal transition within the same mode of the excited state v 4 v 3 v 2 v 1 E 1 S 2 The remaining energy can be release by one of following Processes (C, D & E) m m v 4 v 3 v 2 v 1 Process C - Transfer its remaining E to other chemical species by collision Inter- system crossing Internal transition F B S 0 Jablonsky diagram 62

CH 4003 Lecture Notes 18 (Erzeng Xue) Spectroscopy Molecular Spectra q Two types of

CH 4003 Lecture Notes 18 (Erzeng Xue) Spectroscopy Molecular Spectra q Two types of molecular emission spectra m Fluorescence Ø In the case fluorescence the energy emitted can be the same or smaller (if heat is released before radiation) than the corresponding molecular absorption spectra. e. g. adsorption in UV region - emission in UV or visible region (the wavelength of visible region is longer than that of UV thus less energy) Ø Ø m v 4 v 3 v 2 v 1 B S 2 T 1 Fluorescence can also occur in atomic adsorption spectra A Fluorescence emission is generally short-lived (e. g. s) Phosphorescence Ø Phosphorescence generally takes much longer to complete (called metastable) than fluorescence because of the transition from triplet state to ground state involves altering the e-’s spin. If the emission is in visible light region, the light of excited material fades away gradually v 4 v 3 v 2 v 1 Fluorescence D phosphorenscence F v 4 v 3 v 2 v 1 S 0 63

CH 4003 Lecture Notes 18 (Erzeng Xue) Introductory to Spectroscopy Atomic Spectra & Molecular

CH 4003 Lecture Notes 18 (Erzeng Xue) Introductory to Spectroscopy Atomic Spectra & Molecular Spectra q Comparison of atomic and molecular spectra Atomic spectra Molecular spectra Adsorption spectra Yes Emission spectra Yes Energy required for excitation high low change of e-’s orbital change of vibration states UV mainly visible simple complex Change of energy level related to Spectral region Relative complexity of spectra q Quantum mechanics is the basis of atomic & molecular spectra m The transitional, rotational and vibrational modes of motion of objects of atomic / molecular level are well-explained. 64

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q Observations Incident light, I 0 (UV or visible) ultraviolet visible infra-red 200 - 400 - 800 - 15 nm nm nm m Emergent light, I C b When a light of intensity I 0 goes through a liquid of concentration C & layer thickness b m m Ø The emergent light, I, has less intensity than the incident light I 0 Ø scattering, reflection Ø absorption by liquid There are different levels of reduction in light intensity at different wavelength Ø detect by eye - colour change Ø detect by instrument The method used to measure UV & visible light absorption is called spectrophotometry (colourimetry refers to the measurement of absorption of light in visible region only) 65

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q Theory of light absorption Quantitative observation m m Emergent light I Incident light I 0 The thicker the cuvette - more diminishing of light in intensity Higher concentration the liquid - the less the emergent light intensity C b These observations are summarised by Beer’s Law: Successive increments in the number of identical absorbing molecules in the path of a beam of monochromatic radiation absorb equal fraction of the radiation power travel through them Thus light absorbed b fraction of light x s I 0 s dx I number of molecules N-Avogadro number Absorbance 66

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q Terms, units and symbols for use with Beer’s Law Name alternative name symbol definition Path length - b (or l) - cm Liquid concentration - c - mol / L Transmittance T I / I 0 - T% 100 x I / I 0 % A log(I / I 0) - a (or e, k) A/(bc) [bc]-1 A/(bc) AM/(bc’) ] M-molar weight Transmission Percent transmittance Absorbance Optical density, extinction Absorptivity Extinction coeff. , absorbance index Molar absorptivity Molar extinction coeff. , molar absorbancy index a [or a. M unit c’ -gram/L 67

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q Use of Beer’s Law ü Beer’s law can be applied to the absorption of UV, visible, infra-red & microwave 7 The limitations of the Beer’s Law m Effect of solvent - Solvents may absorb light to a various extent, e. g. the following solvents absorb more than 50% of the UV light going through them m 180 -195 nm sulphuric acid (96%), water, acetonitrile 200 -210 nm cyclopentane, n-hexane, glycerol, methanol, ethanol 210 -220 nm n-butyl alcohol, isopropyl alcohol, cyclohexane, ethyl ether 245 -260 nm chloroform, ethyl acetate, methyl formate 265 -275 nm carbon tetrachloride, dimethyl sulphoxide/formamide, acetic acid 280 -290 nm benzene, toluene, m-xylene 300 -400 nm pyridine, acetone, carbon disulphide Effect of temperature Ø Varying temperature may cause change of concentration of a solute because of è thermal expansion of solution è changing of equilibrium composition if solution is in equilibrium 68

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry What

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry What occur to a molecule when absorbing UV-visible photon? A UV-visible photon (ca. 200 -700 nm) promotes a bonding or non-bonding electron into antibonding orbital - the so called electronic transition e-’s appear in s & p molecular orbitals; non-bonding in n Ø Bonding transition can occur between various states; in general, the energy of e-’s transition increases in the following order: (n®p*) < (n®s*) < (p ®p*) < (s ®s*) q n n * Ø Antibonding Ø e-’s Antibonding * Energy orbitals correspond to the bonding ones * * m * q Antibonding non-bonding Bonding Molecules which can be analysed by UV-visible absorption m Chromophores functional groups each of which absorbs a characteristic UV or visible radiation. 69

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q The functional groups & the wavelength of UV-visible absorption Group C=C C=O C-X Example 1 -octane lmax, nm 180 lmax, nm Group Example arene benzene 260 naphthalene 280 methanol 290 phenenthrene 350 propanone 280 anthracene 375 ethanoic acid 210 pentacene 575 ethyl ethanoate 210 ethanamide 220 1, 3 -butadiene 220 1, 3, 5 -hexatriene 250 conjugated methanol 180 2 -propenal 320 trimethylamine 200 b-carotene (11 C=C) 480 chloromethane 170 bromomethane 210 each additional C=C +30 iodomethane 260 70

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q Instrumentation UV visible Light source Hydrogen discharge lamp Tungsten-halogen lamp Cuvette QUARTZ glass Detectors photomultiplier 71

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q

CH 4003 Lecture Notes 19 (Erzeng Xue) Spectroscopy Application UV & Visible Spectrophotometry q Applications m Analysis of unknowns using Beer’s Law calibration curve m Absorbance vs. time graphs for kinetics m Single-point calibration for an equilibrium constant determination m Spectrophotometric titrations – a way to follow a reaction if at least one substance is colored – sudden or sharp change in absorbance at equivalence point 72

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application IR-Spectroscopy q Atoms in a

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application IR-Spectroscopy q Atoms in a molecule are constantly in motion m. There are two main vibrational modes: Stretching - (symmetrical/asymmetrical) change in bond length - high frequency Ø Bending - (scissoring/stretch/rocking/twisting) change in bond angle - low freq. Ø m. The rotation and vibration of bonds occur in specific frequencies Ø Every type of bond has a natural frequency of vibration, depending on è the mass of bonded atoms (lighter atoms vibrate at higher frequencies) è the stiffness of bond (stiffer bonds vibrate at higher frequencies) è the force constant of bond (electronegativity) è the geometry of atoms in molecule Ø The same bond in different compounds has a slightly different vibration frequ. Ø Functional groups have characteristic stretching frequencies. 73

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application IR-Spectroscopy q IR region m

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application IR-Spectroscopy q IR region m The part of electromagnetic radiation between the visible and microwave regions 0. 8 m to 50 m (12, 500 cm-1 -200 cm-1). m Most interested region in Infrared Spectroscopy is between 2. 5 m-25 m (4, 000 cm-1 -400 cm-1), which corresponds to vibrational frequency of molecules q Interaction of IR with molecules m Only molecules containing covalent bonds with dipole moments are infrared sensitive m Only the infrared radiation with the frequencies matching the natural vibrational frequencies of a bond (the energy states of a molecule are quantitised) is absorbed m Absorption of infrared radiation by a molecule rises the energy state of the molecule Ø m increasing the amplitude of the molecular rotation & vibration of the covalent bonds è Rotation - Less than 100 cm-1 (not included in normal Infrared Spectroscopy) è Vibration - 10, 000 cm-1 to 100 cm-1 The energy changes thr. infrared radiation absorption is in the range of 8 -40 KJ/mol 74

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application IR-Spectroscopy q Use of Infra-Red

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application IR-Spectroscopy q Use of Infra-Red spectroscopy m IR spectroscopy can be used to distinguish one compound from another. Ø No two molecules of different structure will have exactly the same natural frequency of vibration, each will have a unique infrared absorption spectrum. Ø A fingerprinting type of IR spectral library can be established to distinguish a compounds or to detect the presence of certain functional groups in a molecule. m Obtaining structural information about a molecule Ø Absorption of IR energy by organic compounds will occur in a manner characteristic of the types of bonds and atoms in the functional groups present in the compound Ø Practically, examining each region (wave number) of the IR spectrum allows one identifying the functional groups that are present and assignment of structure when combined with molecular formula information. m The known structure information is summarized in the Correlation Chart 75

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application IR Spectrum Principal Correlation Chart

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application IR Spectrum Principal Correlation Chart O H N H C N C C C=O C=C C O 3600 cm-1 3500 cm-1 3000 cm-1 2250 cm-1 2150 cm-1 1715 cm-1 1650 cm-1 1100 cm-1 Dispersive (Double Beam) IR Spectrophotometer IR Source Lenz Prism or Diffraction Grating Region freq. (cm-1) what is found there? ? XH region triple bond double bond fingerprint 3800 - 2600 2400 - 2000 1900 - 1500 - 400 1400 - 900 1500 - 1300 1000 - 650 Split Beam Slit Air Sample OH, NH, CH (sp, sp 2, sp 3) stretches C C, C N, C=C=C stretches C=O, C=N, C=C stretches many types of absorptions C-O, C-N stretches CH in-plane bends, NH bends CH out-of-plane (oop) bends Photometer Recorder 76

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application Atomic Absorption/Emission Spectroscopy q Atomic

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application Atomic Absorption/Emission Spectroscopy q Atomic absorption/emission spectroscopes involve e-’s changing energy states q Most useful in quantitative analysis of elements, especially metals q These spectroscopes are usually carried out in optical means, involving m conversion of compounds/elements to gaseous atoms by atomisation. Atomization is the most critical step in flame spectroscopy. Often limits the precision of these methods. m excitation of electrons of atoms through heating or X-ray bombardment m UV/vis absorption, emission or fluorescence of atomic species in vapor is measured q Instrument easy to tune and operate q Sample preparation is simple (often involving only dissolution in an acid) Source: R. Thomas, “Choosing the Right Trace Element Technique, ” Today’s Chemist at Work, Oct. 1999, 42. 77

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application Atomic Absorption Spectrometer (AA) Source

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application Atomic Absorption Spectrometer (AA) Source P 0 P Wavelength Selector Detector Signal Processor Readout Chopper Sample Type Method of Atomization Radiation Source atomic (flame) sample solution aspirated into a flame Hollow cathode lamp (HCL) atomic (nonflame) sample solution evaporated & ignited HCL x-ray absorption none required x-ray tube 78

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application Atomic Emission Spectrometer (AES) P

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application Atomic Emission Spectrometer (AES) P Wavelength Selector Source Type Sample Detector Signal Processor Readout Method of Atomization Radiation Source arc sample heated in an electric arc sample spark sample excited in a high voltage spark sample argon plasma sample heated in an argon plasma sample flame sample solution aspirated into a flame sample none required; sample bombarded w/ e- sample x-ray emission 79

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application Atomic Fluorescence Spectrometer (AFS) P

CH 4003 Lecture Notes 20 (Erzeng Xue) Spectroscopy Application Atomic Fluorescence Spectrometer (AFS) P 0 e pp P Wavelength Selector r Detector Signal Processor Readout o h C 90 o Type Method of Atomization atomic (flame) sample solution aspirated into a flame Source atomic (nonflame) Sample solution evaporated & ignited x-ray fluorescence none required Radiation Source sample 80

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser - Characteristics q Laser

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser - Characteristics q Laser - is a special type of light sources or light generators. The word LASER represents Light Amplification by Stimulated Emission of Radiation q Characteristics of light produced by Lasers Monochromatic (single wavelength) m Coherent (in phase) m Directional (narrow cone of divergence) m The first microwave laser was made in the microwave region in 1954 by Townes & Shawlow using ammonia as the lasing medium. The first optical laser was constructed by Maiman in 1960, using ruby (Al 2 O 3 doped with a dilute concentration of Cr+3) as the lasing medium and a fast discharge flash-lamp to provide the pump energy. Incandescent lamp • Chromatic • Incoherent • Non-directional Monochromatic light source • Coherent • Non-directional 81

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser - Stimulated Emission q

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser - Stimulated Emission q When excited atoms/molecules/ions undergo de-excitation (from excited state to ground state), light is emitted q Types of light emission m Spontaneous Ø Excited E 4 E 3 emission - chromatic & incoherent e-’s when returning to ground states emit light spontaneously (called spontaneous emission). Ø Photons emitted when e-’s return from different excited states to ground states have different frequencies (chromatic) Ø Spontaneous emission happens randomly and requires no event to trigger the transition (various phase or incoherent) E 2 E 1 Ep 4 excited state Ep 2 ground state E 0 Ep 1=(E 1 – E 0) = hv 1 Ep 2=(E 2 – E 0) = hv 2 Ep 4=(E 4 – E 0) = hv 4 82

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser - Stimulated Emission q

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser - Stimulated Emission q Types of light emission (cont’d) m Stimulated emission - monochromatic & coherent Ø While an atom is still in its excited state, one can bring it down to its ground state by stimulating it with a photon (P 1) having an energy equal to the energy difference of the excited state and the ground state. In such a process, the incident photon (P 1) is not absorbed and is emitted together with the photon (P 2), The latter will have the same frequency (or energy) and the same phase (coherent) as the stimulating photon (P 1). q E 4 E 3 E 2 E 1 Ep 1=(E 2–E 0)=hv 2 Ep 2=(E 2–E 0)=hv 2 E 0 Laser uses the stimulated emission process to amplify the light intensity As in the stimulated emission process, one incident photon (P 1) will bring about the emission of an additional photon (P 2), which in turn can yield 4 photons, then 8 photons, and so on…. 83

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser - Formation & Conditions

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser - Formation & Conditions q The conditions must be satisfied in order to sustain such a chain reaction: m Population Inversion (PI), a situation that there are more atoms in a certain excited state than in the ground state PI can be achieved by a variety means (electrical, optical, chemical or mechanical), e. g. , one may obtain PI by irradiating the system of atoms by an enormously intense light beam or, if the system of atoms is a gas, by passing an electric current through the gas. m Presence of Metastable state, which is the excited state that the excited e-’s can have a relatively long lifetime (>10 -8 second), in order to avoid the spontaneous emission occurring before the stimulated emission In most lasers, the atoms/molecules/ions in the lasing medium are not “pumped” directly to a metastable state. They are excited to an energy level higher than a metastable state, then drop down to the metastable state by spontaneous non-radiative de-excitation. m Photon Confinement (PC), the emitted photons must be confined in the system long enough to stimulate further light emission from other excited atoms This is achieved by using reflecting mirrors at the ends of the system. One end is made totally reflecting & the other is slight transparent to allow part of the laser beam to escape. 84

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser - Functional Elements Feedback

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser - Functional Elements Feedback mechanism Output coupler Lasing medium High reflectance mirror Energy input Energy pumping mechanism Partially transmitting mirror 85

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser Action Lasing medium at

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser Action Lasing medium at ground state Pump energy Population inversion Pump energy Start of stimulated emission Pump energy Stimulated emission building up Pump energy Laser in full operation 86

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Types of Lasers q There

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Types of Lasers q There are many different types of lasers The lasing medium can be gas, liquid or solid (insulator or semiconductor) m Some lasers produce continuous light beam and some give pulsed light beam m Most lasers produce light wave with a fixed wave-length, but some can be tuned to produce light beam of wave-length within a certain range. m Laser type Physical form of lasing medium Wave length (nm) Helium neon laser Gas 633 Carbon dioxide laser Gas 10600 (far-infrared) Argon laser Gas 488, 513, 361 (UV), 364 (UV) Nitrogen laser Gas 337 (UV) Dye laser Liquid Tunable: 570 -650 Ruby laser Solid 694 Nd: Yag laser Solid 1064 (infrared) Diode laser Semiconductor 630 -680 87

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser - Applications q Laser

CH 4003 Lecture Notes 21 (Erzeng Xue) Spectroscopy Application Laser - Applications q Laser m can be applied in many areas Commerce Compact disk, laser printer, copiers, optical disk drives, bar code scanner, optical communications, laser shows, holograms, laser pointers m Industry Measurements (range, distance), alignment, material processing (cutting, drilling, welding, annealing, photolithography, etc. ), non-destructive testing, sealing m Medicine Surgery (eyes, dentistry, dermatology, general), diagnostics, ophthalmology, oncology m Research Spectroscopy, nuclear fusion, atom cooling, interferometry, photochemistry, study of fast processes m Military Ranging, navigation, simulation, weapons, guidance, blinding 88