European Master

Adsorption

Modeling of physisorption in porous materials Part 1

Bogdan Kuchta

Laboratoire MADIREL

Université 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 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.

[c]

What is adsorption ?

Concentration of the adsorbed phase Concentration of the gas phase

z

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 Gaz phase Adsorbate Adsorbed phase Adsorbent 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

Chemical (chemisorption) versus physical (physisorption) adsorption.

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

Energie d’activation

n / m s Adsorption physique Adsorption chimique

Adsorption physique Adsorption chimique

P = const.

The energy of chemisorption is stronger.

T adsorption

c

How do we represent the adsorbed quantity?

(a) interface layer Surface : A Solide : v s Adsorbed quantity: v a Gas : v g

Adsorbed quantity n

a

n a



A

0  

c

.

dz

0 

z

c

How do we represent the adsorbed quantity ?

(b) GIBBS representation Surface of Gibbs Solide : v s

dn dv

c g c g .v

a Excess adsorbed quantity: n  Volume (total) : v g,0

z

Excess quantity on surface n



n

 

n i g



n g f

n a = n  + c g .v

a T  : n a  n 

c = dn/dV

Representations of adsorption equilibrium

Interface layer

solide couche adsorbée gaz

Gibbs representation

solide gaz

c

V S

Surface

V a V g V S,0

GDS

V g,0

(imaginary surface)

c = dn/dV

I II III gas in volume V g (unknown) I II gas in volumeV g,0 (measured with He)

c g F C S = 0 0

Adsorbed quantity n a : 

n a



A

0  

z

c

.

dz

c g F C S = 0 z 0

n

 

n i g



n f g



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

)  .

a



n



m s



f

(

p

).

g

(

T

)

n



m s

5 4 3 2 1 0 0

Isotherm of argon at 77.4 K

0.2

0.4

0.6

 .

a



n



m s



f

(

p

)

p / p o

0.8

1

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".

n



m s

5 4 3 2 1 0 0 0.2

0.4

0.6

0.8

1

p / p o

Adsorption

Special case – materials with large surface

:

porous materials

: specific surface between 0.1 and 2600 m 2 g -1  mesopores (nano-pores): 2 – 50 nm in diameter Micropores <2 nm  “

nanomaterials ”

of technological interests with 1, 2 or 3 dimensions  100 nm Macropores >50 nm f d b a c a : corrugation b c : bottle neck : opening d : interconnection e : close-ended f : closed (isolated)

Adsorption mechanism: – localized adsorption – filling of micropores – monolayer adsorption – multilayer adsorption – capillary condensation

Adsorption

n  /m s Classification of isotherms of physical adsorption :

Fundamental questions

: 1. Mechanism of adsorption – influence of the geometry and pore structure 2. Influence of confined geometry (interaction with walls) on structural transformations 3. Capillary condensation 4. Hysteresis and metastability n  /m s p/p 0 p/p °

Different stages of adsorption on heterogeneous adsorbent

Ultramicropore Specific sites External surface Supermicropore Internal surface Mesopores (nanopores)

Adsorption on purely microporous samples: d  0.4 à 2 nm

d d

( (

pore

)  1

à

5

gaz

)

n



/ m s

  Filling of micropores   ultramicropores supermicropores  cooperative mechanism

p / p 0

Adsorption on purely non-porous sample

 Building statistical monolayer

n



/ m s

B  Multilayer  Adsorption

p / p 0

Adsorption on purely mesoporous sample : d  2 à 50 nm  Building statistical monolayer  Multilayer  Adsorption  Capillary  condensation

n



/ m s

 B Hysteresis

d d

( (

pore

)  5

à

125

gaz

)

p / p 0

n  /m s Adsorption on heterogeneous sample p/p °

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..)

n a / m s

C

0

A

0.2

B

0.4

p / p 0

0.6

0.8

D

1

Interpretation of isotherms of physical adsorption

n a / m s

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

D C B A

0 0.2

0.4

p / p 0

0.6

0.8

1

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.

Interpretation of isotherms of physical adsorption

n a / m s

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

multilayer adsorption

D C

0

A

0.2

B

0.4

p / p 0

0.6

0.8

1 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)

Different domains of physical adsorption n  /m s Capillary condensation Building of monolayer Multilayer adsorption Localised dsorption Filling of micropores p/p °

n  Classification de l'IUPAC des isothermes d'adsorption physique I II III /m s IV B V VI B p/p 0 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.

I n  /m s IV B II B V p/p 0 III VI

Classification (II)

The isotherms of adsorption of

type II

are characterized par increase adsorbed quantity, of continuous way, in as the a a function of the equilibrium pressure.

This type of isotherms is observed in non-porous adsorbents or in macropores. It indicates the multilayer adsorption.

.

I n  /m s IV B II B V p/p 0 III 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 I which is observed at different pressures. This type of isotherms n  /m s is observed when adsorption IV happens in nanoporous (mesoporous) materials where the capillary observed.

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

B II V p/p 0 III 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. I n  /m s IV B II B V p/p 0 III VI

Classification (V)

Step-wise adsorption isotherms (type VI) of are observed in adsorption on homogeneous surfaces where the layers are formed one after another.

I n  /m s IV The proposed classification shows the typical adsorbents.

The real isotherms are very often composed of different types discussed above.

B II B V p/p 0 III VI