AdsorptionModeling of physisorption in porous materials

Part 1

European Master

Bogdan KuchtaLaboratoire MADIRELUniversité Aix-Marseille

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)

Adsorption

• Phenomenon of adsorptionWhen a fluid is in the vicinity of solid surface, its concentration

increases close to the interface.

• DefinitionAdsorption 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.

Concentration of the adsorbed phase

z

[c]

Concentration of the gas phase

What is 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

Adsorbent

Gaz phase Adsorbate

Adsorbed phase

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.

Adsorption

In chemisorption there is an exchange of electrons between the surface and the adsorbed molecules .

Adsorption physique

Adsorption chimique

Energie d’activation

Energie

AdsorptionChemical (chemisorption) versus physical (physisorption) adsorption.

n / ms

Tadsorption

Adsorptionphysique

Adsorptionchimique

The energy of chemisorption is stronger.

P = const.

How do we represent the adsorbed quantity?(a) interface layer

z

c

Gas : vg

Adsorbed quantity: va

Solide : vs

Surface : AAdsorbed quantity na

0

dzcAna .

0

Volume (total) : vg,0

How do we represent the adsorbed quantity ?(b) GIBBS representation

dv

dnExcess adsorbed quantity: n

Surface of Gibbs

cg

z

cSolide : vs

T : na n

na = n + cg.vacg.va

Excess quantity on surface n

gf

gi nnn

Representations of adsorption equilibrium

gas in volumeVg,0

(measured with He)

csolide gaz

cgF

z

I II

CS = 0

c = dn/dV

VS,0 Vg,0

GDS

(imaginary surface)

0

Gibbs representationsolide gaz

coucheadsorbée

VS Va Vg

c = dn/dV

CS = 0

0

cgF

z

I II III

Surface

gas in volume Vg

(unknown)

Interface layer

Excess quantity on surface n

gf

gi nnn Adsorbed quantity na:

0

dzcAna .

Equilibre d'adsorption

An equilibrium of adsorption , at temperature T, is characterized par quantity that depends on pressure of gas , p :

)().( . Tgpfm

na

s

• = 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)

Isotherm of argon at 77.4 K

)( . pfm

na

s

p / po0

1

2

3

4

5

0 0.2 0.4 0.6 0.8 1

sm

n

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".

p / po0

1

2

3

4

5

0 0.2 0.4 0.6 0.8 1

sm

n

Special case – materials with large surface:

c

a

f

ed

ba : corrugationb : bottle neckc : openingd : interconnectione : close-endedf : closed (isolated)

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

Adsorption

Micropores <2 nm

Macropores >50 nm

p/p°

n/ms

Adsorption Adsorption mechanism:

– localized adsorption– filling of micropores– monolayer adsorption– multilayer adsorption– capillary condensation

Classification of isotherms of physical adsorption :

Fundamental questions :1. Mechanism of adsorption – influence of the

geometry and pore structure2. Influence of confined geometry (interaction

with walls) on structural transformations3. Capillary condensation4. Hysteresis and metastability

n/ms

p/p0

B

B

I I I I I I

I V V VI

n/ms

p/p0

B

B

I I I I I I

I V V VI

Different stages of adsorption on heterogeneous adsorbent

Specific sites External surface

Internal surfaceSupermicropore

Ultramicropore

Mesopores(nanopores)

Adsorption on purely microporous samples: d 0.4 à 2 nm

n / ms

p / p0

Filling of micropores ultramicropores

supermicropores cooperative mechanism

5 1 )(

)(à

gazd

pored

Adsorption on purely non-porous sample

n / ms

p / p0

B

Building statistical monolayer

Multilayer Adsorption

Adsorption on purely mesoporous sample : d 2 à 50 nm

n / ms

p / p0

Multilayer Adsorption

B

Building statistical monolayer

Capillary condensation

Hysteresis

125 5 )(

)(à

gazd

pored

Adsorption on heterogeneous sample

p/p°

n/ms

Interpretation of isotherms of physical adsorption

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

0 0.2 0.4 0.6 0.8 1

p / p0

na / m

s

A

B

C

D

• 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..)

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.

0 0.2 0.4 0.6 0.8 1

p / p0

na / m

s

A

B

C

D

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)

0 0.2 0.4 0.6 0.8 1

p / p0

na / m

s

A

B

C

D

Interpretation of isotherms of physical adsorption

Capillary condensation

Building of monolayerMultilayer adsorption

Localised dsorptionFilling of micropores

Different domains of physical adsorption

p/p°

n/ms

Classification de l'IUPAC des isothermes d'adsorption physique

n/ms

p/p0

B

B

I II III

IV V VI

T < Tc

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.

n/ms

p/p0

B

B

I II III

IV V VI

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..

n/ms

p/p0

B

B

I II III

IV V VI

Classification (III)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.

Desorption is very often nonreversible; one observes a hysteresis of desorption with respect to adsorption.

n/ms

p/p0

B

B

I II III

IV V VI

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.

n/ms

p/p0

B

B

I II III

IV V VI

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.

n/ms

p/p0

B

B

I II III

IV V VI

Classification (V)