Heterogeneous catalysis

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Heterogeneous catalysis

• Lec 9 week 12

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Comparison of homogeneous and heterogeneous
catalysts
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Comparison of homogeneous and heterogeneous
catalysts
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Heterogeneous Catalysis Fundamentals

• Individual Steps in Heterogeneous Catalysis
• Heterogeneously catalyzed reactions are composed
  of purely chemical and purely physical reaction
  steps.
• For the catalytic process to take place, the
  starting materials must be transported to the
  catalyst. Thus, apart from the actual chemical
  reaction, diffusion, adsorption, and desorption
  processes are of importance for the progress of
  the overall reaction.

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Steps in catalytic reactions
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• We will now consider the simplest case of a
  catalytic gas reaction on a porous catalyst.

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• In heterogeneous catalysis chemisorption of the
  reactants and products on the catalyst surface is
  of central importance, so that the actual
  chemical reaction step can not be considered
  independently from adsorption and desorption
  steps.
• The measured reaction rate, known as the
  effective reaction rate, is determined by the
  most strongly inhibited and therefore slowest
  step of the reaction sequence. This
  rate-determining step also determines the
  reaction order.

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The Importance of Adsorption in Heterogeneous
Catalysis

• For the moment, let us focus our attention on
  gas-phase reactions catalyzed by solid surfaces.
• For a catalytic reaction to occur, at least one
  and frequently all of the reactants must become
  attached to the surface. This attachment is known
  as adsorprion and takes place by two different
  processes
• physical adsorption (physisorption) and chemical
  adsorption chemisorption.
• Physisorption is the result of van der Waals
  forces, and the accompanying heat of adsorption
  is comparable in magnitude to the heat of
  evaporation of the adsorbate.
• chemisorption, chemical bonds are formed between
  the catalyst and the starting material. The
  resulting surface molecules are much more
  reactive than free adsorbate molecules, and the
  heats of adsorption are comparable in magnitude
  to heats of chemical reaction.
• both types of adsorption are exothermic.

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Comparison between physisorption and
chemisorption.

• Physisorption is fast, and equilibrium is rapidly
  reached, even at low temperature. Chemisorption
  generally requires high activation energies. The
  rate of adsorption is low at low temperatures,
  but the process can be rapid at higher
  temperatures.
• The rate of both types of adsorption is strongly
  dependent on pressure. Chemisorption leads only
  to a monolayer, whereas in physisorption
  multilayers can form.
• The type of adsorption that affects the rate of a
  chemical reaction is chernisorption

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Factors affect the extent of adsorption on the
catalyst surfaces

• (1) Nature of the adsorbate (gas) and adsorbent
  (solid)
• (2) Surface area of the solid adsorbent
• (3) Effect of pressure on the adsorbate gas
• (4) Effect of temperature

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Factors which affect the extent of adsorption on
the catalyst surfaces

•  The following are the factors which affect the
  adsorption,
• (1) Nature of the adsorbate (gas) and adsorbent
  (solid)
• (i) In general, easily liquefiable gases e.g.,
  CO2, NH3, Cl2 and SO2 etc. are adsorbed to a
  greater extent than the elemental gases e.g. H2,
  O2, N2, He etc.
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• (ii) Porous and finely powdered solid e.g.
  charcoal, fullers earth, adsorb more as compared
  to the hard non-porous materials. Due to this
  property powdered charcoal is used in gas masks.

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Factors which affect the extent of adsorption on
the catalyst surfaces

• (2) Surface area of the solid adsorbent
• (i) The extent of adsorption depends directly
  upon the surface area of the adsorbent, i.e.
  larger the surface area of the adsorbent, greater
  is the extent of adsorption.
• (ii) Surface area of a powdered solid adsorbent
  depends upon its particle size. Smaller the
  particle size, greater is its surface area.
•  

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Factors which affect the extent of adsorption on
the catalyst surfaces

• (3) Effect of pressure on the adsorbate gas
• (i) An increase in the pressure of the adsorbate
  gas increases the extent of adsorption.
• (ii) At low temperature, the extent of adsorption
  increases rapidly with pressure.
• (iii) Small range of pressure, the extent of
  adsorption is found to be directly proportional
  to the pressure.
• (iv) At high pressure (closer to the saturation
  vapour pressure of the gas), the adsorption tends
  to achieve a limiting value.

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Factors which affect the extent of adsorption on
the catalyst surfaces

• (4) Effect of temperature
• (i) As adsorption is accompanied by evolution of
  heat, so according to the Le-Chateliers
  principle, the magnitude of adsorption should
  decrease with rise in temperature.

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The important factors influencing the reaction
kinetics

• 1) Adsorption is a necessary step preceding the
  actual chemical reaction on solid catalyst
  surfaces.
• 2) Heterogeneous catalysis involves
  Chemisorption, which has the characteristics of a
  chemical reaction in that the molecules of the
  starting material react with the surface atoms of
  the catalyst.
• 3) Catalyst surfaces have heterogeneous
  structures, and chemisorption takes place
  preferentially at active sites on the surface.

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Fundamental laws of adsorption

• 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)

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• 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.

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Adsorption Mechanism
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Adsorption Isotherms
Data relating adsorbed concentration (g/g of bed
weight) to equilibrium gas phase concentration
(g/ml of stream) is given in terms of adsorption
isotherms.
Wads f (P,T)

• Three common types of isotherms
• Langmuir
• Freundlich
• BET

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• 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.

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Langmuir Isotherm
The earliest model of gas adsorption suggested by
Langmuir (1916). The classical Langmuir model is
limited to monolayer adsorption. It is assumed
that gas molecules striking the surface have a
given probability of adsorption. Molecules
already adsorbed similarly have a given
probability of desorption. At equilibrium, equal
numbers of molecules desorb and adsorb at any
time. The probabilities are related to the
strength of the interaction between the adsorbent
surface and the adsorbate gas.
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Langmuir Isotherm (contd)
Rate of adsorption,
Rate of desorption,
At equilibrium,
where, Wads the mass of gas adsorbed at
pressure P Wmax the mass of gas which covers
the entire adsorbing surface with a monolayer P
the partial pressure of interest in the gas
phase ? coverage C a constant for the
gas/solid combination ka/kd ka the
adsorption rate coefficient kd the desorption
rate coefficient.
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Langmuir Isotherm (contd)
Some physisorption and most chemisoption
processes follow this isotherm. It is the one
with the best theoretical basis, which assumes
that adsorption is limited to one monolayer on
the surface.
One can obtain the two constants by linearization
of the isotherm take the reciprocal and rearrange
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Langmuir Isotherm (contd)
It is particularly suited to represent binary and
ternary systems.
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Assignment

• Define the Langmuir Isothermin case of liquid
  phase.

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Freundlich Isotherm
The Fruendlich isotherm model is valid for
heterogeneous surfaces, monolayer coverage.
Common for most adsorption work since it fits
almost all data. It is empirical in nature,
although some theoretical foundations do exit.
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Freundlich Isotherm
The expression Wads KF P 1/n
(KF and n are experimentally determined
parameters)

• When n 1, the reaction is linear and called
  partitioning.
• When n gt 1, the reaction is said to be
  favorable as the incremental change in amount
  sorbed decreases with increasing concentrations.
• While n lt 1 is called unfavorable because the
  reverse is true.
• Most natural adsorbents exhibit either linear or
  favorable adsorption.
• The Langmuir and Fruendlich models for n lt 1 are
  concave downwards, so both models can be
  calibrated to similar data..

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Freundlich Isotherm (contd)
lnWads lnKF 1/n lnP
Wads KF P 1/n
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Freundlich Isotherm Parameters
Available for a wide variety of organic vapors on
various activated carbon types
Wads KF P 1/n
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Brunauer-Emmett-Teller (BET) Isotherm

• Brunauer, Emmett and Teller (BET) developed
  several models for gas adsorption on solids which
  have become the effective standard for surface
  area measurements.
• BET isotherm is valid for multiple layers on
  homogeneous surfaces.

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Brunauer-Emmett-Teller (BET) Isotherm

• The assumptions underlying the simplest BET
  isotherm are
• Gas adsorbs on a flat, uniform surface of the
  solid with a uniform heat of adsorption due to
  van der Waals forces between the gas and the
  solid.
• There is no lateral interaction between the
  adsorbed molecules.
• After the surface has become partially covered by
  adsorbed gas molecules, additional gas can adsorb
  either on the remaining free surface or on top of
  the already adsorbed layer. The adsorption of
  the second and subsequent layers occurs with a
  heat of adsorption equal to the heat of
  liquefaction of the gas.

multi-layers adsorption
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BET Isotherm (contd)
Work for almost any type of data on the
adsorption of gases on solids. It describes
every type of isotherm including the linear, and
Langmuir isotherms. The theoretical basis is
sound.
For single component the equation is,
for n ? ?
for finite n
Note that n is the number of adsorbed monolayers,
and x P/P0. Where, P is the actual partial
pressure of gas in the stream and P0 is the vapor
pressure of the pure gas.
Note The BET simplifies to the Langmuir when
relative pressure x lt 0.01 and C gt100 (Valsaraj
et al., 1992).
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BET Isotherm (contd)
To obtain the parameters in the BET equation, one
needs to linearize the equation
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The most common isotherm models Dastgheib and
Rockstraw, 2002