Adsorption

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European Master
Adsorption Modeling of physisorption in porous
materials Part 1
Bogdan Kuchta Laboratoire MADIREL Université
Aix-Marseille
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Notion of Interface

• Interface Separation between two (volume)
  phases.
• Although words interface and surface mean
  practically the same, the word "interface" is
  often used when 2 condensed phases are in contact
  or when two phase are explicitly considered.
• ex interface solid/liquid interface
  solid/gaz.
• The word "surface" is used to describe interface
  of solid (when it is (or not) in a contact with
  different phase)

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Adsorption

• Phenomenon of adsorption
• When a fluid is in the vicinity of solid surface,
  its concentration increases close to the
  interface.
• Definition
• Adsorption is a process that occurs when a gas or
  liquid solute accumulates on the surface of a
  solid or, more rarely, a liquid (adsorbent),
  forming a molecular or atomic film (adsorbate).
• Word "adsorption" is also used to describe the
  transition of a fluid when it transforms into an
  adsorbed phase.

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What is adsorption ?
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Adsorption
Adsorption a phenomenon general which involves
the preferential partitioning of substances from
the gaseous or liquid phase onto the surface of a
solid substrate
Physical adsorption is caused mainly by van der
Waals forces and electrostatic forces between
adsorbate molecules and the atoms which compose
the adsorbent surface. Thus adsorbents are
characterized first by surface properties such as
surface area, structure (roughness) and polarity.
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Adsorption
Chemical (chemisorption) versus physical
(physisorption) adsorption.
In chemisorption there is an exchange of
electrons between the surface and the adsorbed
molecules .
n / ms
The energy of chemisorption is stronger.
P const.
Tadsorption
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How do we represent the adsorbed quantity?(a)
interface layer
Surface A
Solide vs
c
z
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How do we represent the adsorbed quantity ?(b)
GIBBS representation
c
Surface of Gibbs
Solide vs
Excess adsorbed quantity n?
Excess quantity on surface n?
Volume (total) vg,0
cg
na ns cg.va
cg.va
z
T? na ? n?
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Representations of adsorption equilibrium
Excess quantity on surface n?
Adsorbed quantity na
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Equilibre d'adsorption
An equilibrium of adsorption , at temperature T,
is characterized par quantity ? that depends on
pressure of gas , p

• ? n ?/A
• n ? est la quantité d'excès en surface
• A est l'étendue de l'interface
• a est l'aire spécifique ( A /m s)

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Isotherm of argon at 77.4 K
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Isotherme d'adsorption (définition)
Isotherm of adsorption is an ensemble of
equilibrium states, at temperature T, for all
pressures p between 0 et p (pressure of
saturated vapor of adsorbate at temperature T
). p/p is called  relative equilibrium
pressure".
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Adsorption
Special case materials with large surface
porous materials specific surface between 0.1
and 2600 m2g-1 ? mesopores
(nano-pores) 2 50 nm
in diameter ? nanomaterials of
technological interests with 1, 2 or 3
dimensions ? 100 nm
Micropores lt2 nm
Macropores gt50 nm
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Adsorption

• Adsorption mechanism
• localized adsorption
• filling of micropores
• monolayer adsorption
• multilayer adsorption
• capillary condensation
• Classification of isotherms of physical
  adsorption

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Different stages of adsorption on heterogeneous
adsorbent
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Adsorption on purely microporous samples d ? 0.4
à 2 nm
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Adsorption on purely non-porous sample
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Adsorption on purely mesoporous sample d ? 2 à
50 nm
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Adsorption on heterogeneous sample
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Interpretation of isotherms of physical adsorption

• Generally, adsorption starts at lower pressure
  when the interaction between the surface and the
  adsorbed particles is stronger

• When an adsorbate is in contact with an
  adsorbent, adsorption is first observed (that is,
  at lowest relative pressure, domain A) on the
  centers of adsorption that are the most strongly
  attractive (defaults, imperfections, etc..)

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Interpretation of isotherms of physical adsorption

• The filling of the micropores happens also
  at pressures relatively low (domain B) 

Domain C corresponds to pressures where the
adsorption monomolecular  is observed. At the
end of this domain , statistically, the whole
surface of solid is totally covered by a
monolayer adsorbed on the surface.
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Interpretation of isotherms of physical adsorption

• When the relative pressure increases,
  (domain D), the surface is covered by multilayer
  of increasing thickness multilayer adsorption

Starting at some pressure in the domain D,
one can observe a rapid acceleration of
adsorption, due to the phenomenon of capillary
condensation (in nanopores)
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Different domains of physical adsorption
ns/ms
p/p
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Classification de l'IUPAC des isothermes
d'adsorption physique
T lt Tc
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Classification (I)
The isotherms of adsorption of type I are
characterized par horizontal line which indicates
saturation of the adsorbent. This isotherm is
observed in adsorbents having only micropores
that are filled at low pressures. Lower pressure
of filling, smaller the size of the pores.
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Classification (II)
The isotherms of adsorption of type II are
characterized par increase of the adsorbed
quantity, in a continuous way, as a function of
the equilibrium pressure. This type of isotherms
is observed in non-porous adsorbents or in
macropores. It indicates the multilayer
adsorption..
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Classification (III)
Desorption is very often nonreversible one
observes a hysteresis of desorption with respect
to adsorption.
Isotherms of the type IV have the same shape as
the type II at lower pressures (approximately,
below 0.4 of the reduced pressure). At higher
pressures it is characterized par saturation
which is observed at different pressures. This
type of isotherms is observed when adsorption
happens in nanoporous (mesoporous) materials
where the capillary condensation is observed.
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Classification (IV)
The isotherms of adsorption of type III and V are
less frequently observed they are similar to
isotherm of type II and IV but they differ at low
pressures. This difference is attributed to weak
interaction between adsorbent and the adsorbed
molecules. For example, it is observed for
adsorption of water on hydrophobic surfaces.
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Classification (V)
Step-wise isotherms of adsorption (type VI) are
observed in adsorption on homogeneous surfaces
where the layers are formed one after another.
The proposed classification shows the typical
adsorbents. The real isotherms are very often
composed of different types discussed above.