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