Adsorption Modeling of physisorption in porous materials Part 1 European Master 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.

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 Adsorption Chemical (chemisorption) versus physical (physisorption) adsorption. n / m s T adsorption Adsorption physique Adsorption chimique The energy of chemisorption is stronger. P = const.

How do we represent the adsorbed quantity? (a) interface layer z c Gas : v g Adsorbed quantity: v a Solide : v s Surface : A Adsorbed quantity n a 0 

Volume (total) : v g,0 How do we represent the adsorbed quantity ? (b) GIBBS representation Excess adsorbed quantity: n  Surface of Gibbs cgcg z c Solide : v s T  : n a  n  n a = n  + c g.v a c g.v a Excess quantity on surface n 

Representations of adsorption equilibrium gas in volumeV g,0 (measured with He) c solide gaz c g F z III C S = 0 c = dn/dV V S,0 V g,0 GDS (imaginary surface) 0 Gibbs representation solide gaz couche adsorbée V S V a V g c = dn/dV C S = 0 0 c g F z IIIIII  Surface gas in volume V g (unknown) Interface layer Excess quantity on surface n  Adsorbed quantity n a :

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 )

Isotherm of argon at 77.4 K p / p o

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 / p o

Special case – materials with large surface : c a f e d b a : corrugation b : bottle neck c : opening d : interconnection e : close-ended f : closed (isolated) porous materials: specific surface between 0.1 and 2600 m 2 g -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  /m s 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 structure 2. Influence of confined geometry (interaction with walls) on structural transformations 3. Capillary condensation 4. Hysteresis and metastability

Different stages of adsorption on heterogeneous adsorbent Specific sitesExternal surface Internal surfaceSupermicropore Ultramicropore Mesopores (nanopores)

Adsorption on purely microporous samples: d  0.4 à 2 nm n  / m s p / p 0  Filling of micropores  ultramicropores   supermicropores  cooperative mechanism

Adsorption on purely non-porous sample n  / m s p / p 0 B  Building statistical monolayer  Multilayer  Adsorption

Adsorption on purely mesoporous sample : d  2 à 50 nm n  / m s p / p 0  Multilayer  Adsorption B  Building statistical monolayer  Capillary  condensation  Hysteresis

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

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..)

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. 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) Interpretation of isotherms of physical adsorption

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

Classification de l'IUPAC des isothermes d'adsorption physique n  /m s p/p 0 B B I IIIII 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  /m s p/p 0 B B I IIIII 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  /m s p/p 0 B B I IIIII 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  /m s p/p 0 B B I IIIII 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  /m s p/p 0 B B I IIIII 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  /m s p/p 0 B B I IIIII IV V VI Classification (V)