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Adsorption

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Catalysis & Catalysts Adsorption On Solid Surface Adsorption Adsorption is a process in which molecules from gas (or liquid) phase land on, interact with and attach ... – PowerPoint PPT presentation

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Title: Adsorption


1
Adsorption On Solid Surface
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 in 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

2
Solid Catalysts
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

3
Solid Catalysts
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)

4
Pores of Porous Solids
  • 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)

5
Adsorption On Solid Surface
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.

6
Adsorption On Solid Surface
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)

7
Adsorption On Solid Surface
  • 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

8
Adsorption On Solid Surface
  • 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

9
Adsorption On Solid Surface
  • 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
10
Adsorption On Solid Surface
  • 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.

11
Adsorption On Solid Surface
  • 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
12
Adsorption On Solid Surface
  • Langmuir adsorption isotherm
  • case I
  • case II
  • Case III

mono-layer
Amount adsorbed
large B0 (strong adsorp.)
moderate B0
small B0 (weak adsorp.)
Pressure
  • Langmuir adsorption isotherm established a logic
    picture of adsorption process
  • It fits many adsorption systems but not at all
  • The assumptions made by Langmuir do not hold in
    all situation, that causing error
  • Solid surface is heterogeneous thus the heat of
    adsorption is not a constant at different q
  • Physisorption of gas molecules on a solid surface
    can be more than one layer

13
Adsorption On Solid Surface
  • Five types of physisorption isotherms are found
    over all solids
  • Type I is found for porous materials with small
    pores e.g. charcoal.
  • It is clearly Langmuir monolayer type, but the
    other 4 are not
  • Type II for non-porous materials
  • Type III porous materials with cohesive force
    between adsorbate molecules greater than the
    adhesive force between adsorbate molecules and
    adsorbent
  • Type IV staged adsorption (first monolayer then
    build up of additional layers)
  • Type V porous materials with cohesive force
    between adsorbate molecules and adsorbent being
    greater than that between adsorbate molecules

14
Adsorption On Solid Surface
  • Other adsorption isotherms
  • Many other isotherms are proposed in order to
    explain the observations
  • The Temkin (or Slygin-Frumkin) isotherm
  • Assuming the adsorption enthalpy DH decreases
    linearly with surface coverage
  • From ads-des equilibrium, ads. rate 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.

15
Adsorption On Solid Surface
  • 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.

16
Adsorption On Solid Surface
  • BET (Brunauer-Emmett-Teller) isotherm
  • Many physical adsorption isotherms were found,
    such as the types II and III, that the adsorption
    does not complete the first layer (monolayer)
    before it continues to stack on the subsequent
    layer (thus the S-shape of types II and III
    isotherms)
  • Basic assumptions
  • the same assumptions as that of Langmuir but
    allow multi-layer adsorption
  • the heat of ads. of additional layer equals to
    the latent heat of condensation
  • based on the rate of 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.

17
Adsorption On Solid Surface
  • Comment on the BET isotherm
  • BET equation fits reasonably well all known
    adsorption isotherms observed so far (types I to
    V) for various types of solid, although there is
    fundamental defect in the theory because of the
    assumptions made (no interaction between adsorbed
    molecules, surface homogeneity and liquefaction
    heat for all subsequent layers being equal).
  • BET isotherm, as well as all other isotherms,
    gives accurate account of adsorption isotherm
    only within restricted pressure range. At very
    low (P/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.

18
Adsorption On Solid Surface
  • 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
19
Adsorption On Solid Surface
  • Summary of adsorption isotherms
  • Name Isotherm equation Application Note
  • Langmuir
  • Temkin q c1ln(c2P)
  • Freundlich
  • BET

Useful in analysis of reaction mechanism
Chemisorption Easy to fit adsorption data
Useful in surface area determination
Chemisorption and physisorption Chemisorption
Chemisorption and physisorption Multilayer
physisorption
20
The BET isotherm
  • Theoretical development based on several
    assumptions
  • multimolecular adsorption
  • 1st layer with fixed heat of adsorption H1
  • following layers with heat of adsorption constant
    ( latent heat of condensation)
  • constant surface (i.e. no capillary condensation)
    gives

OT fig1.3
21
The BET isotherm, cont.
  • Plot of left side vs. p/p0 should give straight
    line with slope s and intercept I
  • Reorganizing gives
  • Knowledge of S0 (specific area for a volume of
    gas then allows the calculation of the specific
    surface area Sg
  • where mp is the mass of the sample

OT fig1.5
22
BET contd
  • BET method useful, but has limitations
  • microporous materials mono - multilayer
    adsorption cannot occur, (although BET surface
    areas are reported routinely)
  • assumption about constant packing of N2 molecules
    not always correct?
  • theoretical development dubious (recent molecular
    simulation studies, statistical mechanics) -
    value of C is indication o f the shape of the
    isotherm, but not necessarily related to heat of
    adsorption

23
Simplified method
  • 1-point method
  • simplefied BET assuming value of C ? 100 (usually
    the case), gives
  • usually choose p/p0 ? 0,15
  • method underestimates the surface area by approx.
    5.

24
Adsorbates
  • An adsorbate molecule covers an area ?,
    calculated assuming dense packing of the
    molecules in the multilayer. The corresponding
    area per volume gas is S0

25
Porosity and pore size
  • The pore structure (porosity, pore diameter, pore
    shape) is important for the catalytic properties
  • pore diffusion may influence rates
  • pores may be too small for large molecules to
    diffuse into
  • Measurement techniques
  • Hg penetration
  • interpretation of the adsorption - desorption
    isotherms
  • electron microscopy techniques

26
Hg penetration
  • Based on measuring the volume of a non-wetting
    liquid forced into the pores by pressure
    (typically mercury)
  • Surface tension will hinder the filling of the
    pores, at a given pressure an equilibrium between
    the force due to pressure and the surface tension
    is established
  • where P pressure of Hg, ? is surface tension
    and ? is the angle of wetting
  • Common values used ? 480 dyn/cm and ?
    140 give average pore radius
  • valid in the range 50 - 50000Å

27
Pore size distribution
  • If the Hg-volume is recorded as a function of
    pressure and this curve is differentiated we can
    find the pore size distribution function
    V(r)dV/dr

OT fig 2.3.
28
The Kelvin equation
  • If adsorbent is mesoporous we get Type IV
    isotherm
  • Deviation upwards is due to filling of mesopores
    by capillary condensation - curved liquid
    meniscus in narrow pores with radius rk
  • V is molar volume of the liquid, minus sign
    introduced since in the actual range of
    measurement 0 lt p/p0 lt1

29
The Kelvin equation
  • Since capillary condensation is preceeded by
    multilayer adsorption on the wall the value is
    corrected with t, the thickness of this layer
  • Cylindrical pores rp rk t
  • Parallell sided slits dp rk 2t
  • Value of t determined from measurements
    without capillary condensation
  • Practical experience, typical values give for
    circular pores
  • Values for t have been found to be a function of
    rk, e.g. for rk gt 20Å

30
Adsorption-desorption hysteresis
  • Hysteresis is classified by IUPAC (see fig.)
  • Traditionally desorption branch used for
    calculation
  • H1 narrow distribution of mesopores
  • H2 complex pore structure, network effects,
    analysis of desorption loop misleading
  • H2 typical for activated carbons
  • H3 4 no plateau, hence no well-defined
    mesopore structure, analysis difficult
  • H3 typical for clays

Handbook fig 2 s 431
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