Facts and Figures about Catalysts - PowerPoint PPT Presentation

Loading...

PPT – Facts and Figures about Catalysts PowerPoint presentation | free to download - id: 3c165b-MWExO



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Facts and Figures about Catalysts

Description:

CH4003 Lecture Notes 11 (Erzeng Xue) Catalysis & Catalysts Catalysis & Catalysts Facts and Figures about Catalysts Life cycle on the earth Catalysts (enzyme ... – PowerPoint PPT presentation

Number of Views:301
Avg rating:3.0/5.0
Slides: 89
Provided by: www3UlIe
Learn more at: http://www3.ul.ie
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Facts and Figures about Catalysts


1
Catalysis Catalysts
CH4003 Lecture Notes 11 (Erzeng Xue)
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 H2OHC
    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
What is Catalysis
CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis Catalysts
  • 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
Action of Catalysts
CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis 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
Action of Catalysts
CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis 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)
    DG373151 kJ/mol (100 C)
  • DG973 -16 kJ/mol (700 C)
  • At 100C, DG373151 kJ/mol gt 0. There is no
    thermodynamic driving force, the reaction wont
    proceed with or without a catalyst
  • At 700C, DG373 -16 kJ/mol lt 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 (lt100).

5
Types of Catalysts Catalytic Reactions
CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis Catalysts
  • 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
Applications of Catalysis
CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis Catalysts
  • 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
Applications of Catalysis
CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis Catalysts
  • 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
Research in Catalysis
CH4003 Lecture Notes 11 (Erzeng Xue)
Catalysis Catalysts
  • 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
Catalytic Reaction Processes
CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis Catalysts
  • 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
Catalytic Reaction Processes
CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis Catalysts
  • Catalytic reactions in a continuous operation
    (contd)
  • 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
Catalytic Reaction Processes
CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis Catalysts
  • 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
Example Heterogeneous Catalytic Reaction Process
CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis Catalysts
  • 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
Solid Catalysts
CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis 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
Solid Catalysts
CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis 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
Solid Catalysts
CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis 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
Solid Catalysts
CH4003 Lecture Notes 12 (Erzeng Xue)
Catalysis 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
Adsorption On Solid Surface
CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis Catalysts
  • 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 ( gt 40
    kJ/mol)
  • selectivity low high
  • temperature to occur low high

18
Adsorption On Solid Surface
CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis Catalysts
  • 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
Adsorption On Solid Surface
CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis Catalysts
  • 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 an useful technique for
    determining the surface area, the pore shape,
    pore sizes and size distribution of porous solid
    materials (BET surface area)

20
Adsorption On Solid Surface
CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis Catalysts
  • Characterisation of adsorption system
  • Adsorption isotherm - most commonly used,
    especially to catalytic reaction system, Tconst.
  • 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
Adsorption On Solid Surface
CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis Catalysts
  • The Langmuir adsorption isotherm
  • Basic assumptions
  • surface uniform (DHads does 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 kdes q
  • at equilibrium rads rdes Þ kads (1-q) P
    kdes q
  • rearrange it for q
  • let Þ B0 is adsorption coefficient

22
Adsorption On Solid Surface
CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis Catalysts
  • The Langmuir adsorption isotherm (contd)
  • 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 radskads (1-qA
    )(1-qB)PABkads (1-q )2PAB
  • the rate of A-B desorption rdeskdesqAqB
    kdesq2
  • at equilibrium rads rdes Þ kads (1-q
    )2PAB kdesq2
  • rearrange it for q
  • Let. Þ

qqAqB
23
Adsorption On Solid Surface
CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis Catalysts
  • The Langmuir adsorption isotherm (contd)
  • 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,A qA
  • the rate of B desorption rdes,B kdes,B qB
  • at equilibrium rads ,A rdes ,A and Þ
    rads ,B rdes ,B
  • Þ kads,A(1-qA-qB)PAkdes,AqA and
    kads,B(1-qA-qB)PBkdes,BqB
  • rearrange it for q
  • where are adsorption
    coefficients of A B.

24
Adsorption On Solid Surface
CH4003 Lecture Notes 13 (Erzeng Xue)
Catalysis Catalysts
  • The Langmuir adsorption isotherm (contd)

Adsorption A, B both strong A strong, B
weak A weak, B weak
Adsorption Strong kadsgtgt kdes kadsgtgt
kdes B0gtgt1 B0gtgt1 Weak kadsltlt kdes kadsltlt
kdes B0ltlt1 B0ltlt1
25
Adsorption On Solid Surface
CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis Catalysts
  • 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
Adsorption On Solid Surface
CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis Catalysts
  • 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
Adsorption On Solid Surface
CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis Catalysts
  • 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 º des. rate
  • radskads(1-q)P º rdeskdesq
  • where Qs is the heat of adsorption. When Qs is a
    linear function of qi. QsQ0-iS (Q0 is a
    constant, i is the number and S represents the
    surface site),
  • the overall coverage
  • When b1P gtgt1 and b1Pexp(-i/RT) ltlt1, we have q
    c1ln(c2P), where c1 c2 are constants
  • Valid for some adsorption systems.

28
Adsorption On Solid Surface
CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis Catalysts
  • The Freundlich isotherm
  • assuming logarithmic change of adsorption
    enthalpy DH with surface coverage
  • From ads-des equilibrium, ads. rate º des. rate
  • radskads(1-q)P º rdeskdesq
  • 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 Niaexp(-Q/Q0) (a
    and Q0 are constants), corresponding to a
    fractional coverage qi,
  • the overall coverage
  • the solution for this integration expression at
    small q is
  • lnq(RT/Q0)lnPconstant, or
  • as is the Freundlich equation normally written,
    where c1constant, 1/c2RT/Q0
  • Freundlich isotherm fits, not all, but many
    adsorption systems.

29
Adsorption On Solid Surface
CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis Catalysts
  • 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 adsorptionthe 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
    cexp(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
Adsorption On Solid Surface
CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis Catalysts
  • 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/P0lt0.05) and high relative pressure
    (P/P0gt0.35) 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
Adsorption On Solid Surface
CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis Catalysts
  • Use of BET isotherm to determine the surface
    area of a solid
  • At low relative pressure P/P0 0.050.35 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
Adsorption On Solid Surface
CH4003 Lecture Notes 14 (Erzeng Xue)
Catalysis Catalysts
  • 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
Mechanism of Surface Catalysed Reaction
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis Catalysts
  • Langmuir-Hinshelwood mechanism
  • This mechanism deals with the surface-catalysed
    reaction in which
  • that 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 rikABkqAqB
  • From Langmuir adsorption isotherm (the case III)
    we know
  • We then have
  • When both A B are weakly adsorbed (B0,APAltlt1,
    B0,BPBltlt1),
  • 2nd order reaction
  • When A is strongly adsorbed (B0,APAgtgt1) B
    weakly adsorbed (B0,BPBltlt1 ltltB0,APA)
  • 1st order w.r.t. B

34
Mechanism of Surface Catalysed Reaction
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis Catalysts
  • Eley-Rideal mechanism
  • This mechanism deals with the surface-catalysed
    reaction in which
  • that one reactant, A, adsorb on surface without
    dissociation and
  • other reactant, B, approaching from gas to react
    with A
  • A(g) D A(ads) P (the
    desorption of P is not r.d.s.)
  • The rate of reaction rikABkqAPB
  • 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,APAltlt1),
  • 2nd order reaction
  • When A is strongly adsorbed or the partial
    pressure of A is very high (B0,APAgtgt1)
  • 1st order w.r.t. B

"
35
Mechanism of Surface Catalysed Reaction
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis Catalysts
  • Mechanism of surface-catalysed reaction with
    dissociative adsorption
  • The mechanism of the surface-catalysed reaction
    in which one
  • reactant, AD, dissociatively adsorbed 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 rikABkqADPB
  • 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,ADPADltlt1),
  • The reaction 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,APAgtgt1)
  • 1st order w.r.t. B

36
Mechanism of Surface Catalysed Reaction
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis Catalysts
  • 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
Solids and Solid Surface
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis Catalysts
  • 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
Solids and Solid Surface
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis Catalysts
  • 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
Solids and Solid Surface
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis Catalysts
  • 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
Pores of Porous Solids
CH4003 Lecture Notes 15 (Erzeng Xue)
Catalysis Catalysts
  • Pore sizes
  • micro pores dp lt20-50 nm
  • meso-pores 20nm ltdplt200nm
  • macro pores dp gt200 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
Chain Reactions - Process
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex Reactions
  • 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
Chain Reactions - Rate Law
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex Reactions
  • Rate law of chain reaction
  • Example overall reaction H2(g) Br2(g)
    2HBr(g) observed
  • elem step rate law
  • a. Initiation Br2 2Brž rakaBr2
  • b. Propagation Brž H2 HBr
    Hž rbkbBrH2
  • Hž Br2 HBr Brž rbkbHBr2
  • c. Termination Brž žBr Br2 rckcBrBrk
    cBr2
  • Hž žH H2 (practically less important
    therefore neglected)
  • Hž žBr HBr (practically less important
    therefore neglected)
  • d. Retardn (obsvd.) Hž HBr H2
    Brž rdkdHHBr
  • HBr net rate rHBr rb rb- rd
    or dHBr/dtkbBrH2kbHBr2-kdHHBr
  • Apply s.s.a. rH rb- rb- rd
    or dH/dtkbBrH2- kbHBr2-kdHHBr
    0
  • rBr 2ra-rbrb-2rc rd
    or dBr/dt2kaBr2-kbBrH2kbHBr2-2
    kcBr2 kdHHBr0
  • solve the above eqns we have

43
Chain Reactions - Polymerisation
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex Reactions
  • 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.) rikiI
  • Rž M žM1 (fast)
  • Propagation M žM1 ž(MM1) žM2 (fast)
  • M žM2 ž(MM2) žM3 (fast)
  • M žMn-1 ž(MMn-1) žMn rpkpMžM (ri
    is the r.d.s.)
  • Termination žMn žMm (MnMm) Mmn
    rtktžM2
  • 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
Chain Reactions - Polymerisation
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex Reactions
  • 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-(ab)-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)4COn-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
Chain Reactions - Explosion
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex Reactions
  • 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
Explosion Reactions
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex 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
Photochemical Reactions
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex 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 I2 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

48
Photochemical Reactions
CH4003 Lecture Notes 16 (Erzeng Xue)
Complex 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
Introduction to Spectroscopy
CH4003 Lecture Notes 17 (Erzeng Xue)
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
Electromagnetic Radiation
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
  • 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
    c2.9979 x 108 ms-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 Plancks constant h6.626x10-34Js

51
Electromagnetic Radiation
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
  • 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
  • Spectral region of e.m.r.

52
Interaction of e.m.r. with Matter
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
  • Interaction of electromagnetic radiant with
    matter
  • 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
  • Types of interactions
  • Absorption
  • Reflection
  • Transmission
  • Scattering
  • Refraction
  • Each interaction can disclose certain properties
    of the matter
  • When applying e.m.r. of different frequency (thus
    the energy e.m.r. carried) different type
    information can be obtained

53
Spectrum
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
  • 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
  • absorbance (e.m.r. being absorbed)
  • transmission (e.m.r. passed through)
  • Intensity
  • The term intensity has the meaning of the
    radiant power that carried by an e.m. r.

.
54
Spectrum
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
  • What an spectrum tells
  • 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
  • The wavenumber at the tip of peak is the most
    important, especially when a peak is broad
  • A 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
  • 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
  • The ratio in intensity of different peaks does
    not necessarily means the ratio of the quantity
    (e.g. concentration, population of a state etc.)

.
55
Spectral properties, applications, and
interactions of electromagnetic radiation
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
56
Examples
CH4003 Lecture Notes 17 (Erzeng Xue)
Introductory to Spectroscopy
  • 1. A laser emits light with a frequency of
    4.69x1014 s-1. (h 6.63 x 10-34Js)
  • A) What is the energy of one photon of the
    radiation from this laser?
  • B) If the laser emits 1.3x10-2J during a pulse,
    how many photons are emitted during the pulse?
  • Ans A) Ephoton hn 6.63 x 10-34Js x 4.69x1014
    s-1 3.11 x 10-19 J
  • B) No. of photons (1.3x10-2J )/(3.11 x 10-19J)
    4.2x1016
  • 2. The brilliant red colours seen in fireworks
    are due to the emission of red light at a wave
    length of 650nm. What is the energy of one
    photon of this light? (h 6.63 x 10-34Js)
  • Ans Ephoton hn hc/l (6.63 x 10-34Js x 3 x
    108ms-1)/650x10-9m 3.06x10-19J
  • 3 Compare the energies of photons emitted by
    two radio stations, operating at 92 MHz (FM) and
    1500 kHz (MW)?
  • Ans Ephoton hn
  • 92 MHz 92 x 106 Hz (s-1) gt
  • E (6.63 x 10-34 Js) x (92 x 106 s-1) 6.1 x
    10-26J
  • 1500 kHz 1500 x 103 Hz (s-1)
  • E (6.63 x 10-34 Js) x (1500 x 103 s-1) 9.9
    x 10-28J

.
57
Atomic Spectra
CH4003 Lecture Notes 18 (Erzeng Xue)
Introductory to Spectroscopy
  • Shell structure energy level of atoms
  • In an atom there are a number of shells and of
    subshells where e-s can be found
  • The energy level of each shell subshell are
    different and quantised
  • The e-s in the shell closest to the nuclei has
    the lowest energy. The higher shell number is,
    the higher energy it is
  • The exact energy level of each shell and subshell
    varies with substance
  • Ground state and excited state of e-s
  • Under normal situation an e- stays at the lowest
    possible shell - the e- is said to be at its
    ground state
  • Upon absorbing energy (excited), an e- can change
    its orbital to a higher one - we say the e- is at
    is excited state.

58
Atomic Spectra
CH4003 Lecture Notes 18 (Erzeng Xue)
Introductory to Spectroscopy
  • Electron excitation
  • The excitation can occur at different degrees
  • low E tends to excite the outmost e-s first
  • when excited with a high E (photon of high v) an
    e- can jump more than one levels
  • even higher E can tear inner e-s away from
    nuclei
  • 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

59
Atomic Spectra
CH4003 Lecture Notes 18 (Erzeng Xue)
Introductory to Spectroscopy
  • Atomic spectra
  • 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
  • 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
  • The level quantities of energy absorbed or
    emitted (v intensity of e.m.r.) are specific
    for a substance
  • Atomic spectra are mostly in UV (sometime in
    visible) regions

energy DE
n 1 n 2 n 3, etc.
4f
4d
n4
4p
3d
4s
Energy
n3
3p
3s
n2
2p
2s
n1
1s
60
Molecular Spectra
CH4003 Lecture Notes 18 (Erzeng Xue)
Spectroscopy
  • Motion energy of molecules
  • Molecules are vibrating and rotating all the
    time, two main vibration modes being
  • stretching - change in bond length (higher v)
  • bending - change in bond angle (lower v)
  • (other possible complex types of stretching
    bending are scissoring / rocking / twisting
  • Molecules are normally at their ground state (S0)
  • S (Singlet) - two e-s spin in pair
    E
About PowerShow.com