PHYSISORPTION, CHEMISORPTION AND WORK FUNCTION CHANGE INDUCED

BY ADSORBATES

ANEETTA DAVISMSc NS & T, 2nd SEMMOUNT CARMEL COLLEGE

ADSORPTION

The accumulation of molecular species at the surface of a solid or liquid rather than in the bulk is called adsorption.

Free materials

Adsorbed material

Types of Adsorption

Physisorption Chemisorption

Adsorption

Physical Adsorption

or Physisorption

If accumulation of gas molecules on the surface of solids occurs due to weak van der Waals’ forces of attraction, the adsorption is called physisorption.

Characteristics of Physisorption

1. Non-specific nature: An adsorbent does not show any preference for a gas as the van der Waals’ forces are universal.

2. Easily liquefiable gases like CO2, SO2, NH3 etc, are readily adsorbed.

3. Reversible nature: Physisortion of a gas by a solid is reversible.

4. Increases by increase of pressure. 5. Surface area of adsorbant: When the surface area of the adsorbent

increases, more gas is adsorbed, ie extent of adsorption increases.

Since adsorption is exothermic, physisorption takes place readily at low temperature and desorption takes place at higher temperature.

Chemical Adsorption

or Chemisorption

When atoms or molecules of gases are held by solids on its surface by chemical bonds, the adsorption is called chemisorption.

Characteristics of Chemisorption

1. High specificity: It is highly specific and will occur only if chemical bond formation takes between adsorbate and adsorbent.

2. Irreversibility: Chemisorption is irreversible because the chemical bond formed is difficult to break.

3. Chemisorption increases with temperature. 4. Increases by increase of pressure. 5. Surface area of adsorbent: When the surface area of the

adsorbent increases, more gas is adsorbed, ie extent of adsorption increases.

Physisorption Chemisorption1 Occurs due to van der Waals’ force Chemical Bond

2 Reversible Irreversible

3 Not specific Specific

4 Enthalpy of adsorption is low Enthalpy of adsorption is high

5 More liquefiable gases are adsorbed readily

Gases which form compounds with adsorbent alone undergo chemisorption

6 Decreases with increase of temperature Increases with increase of temperature

7 Low temperature is favorable. High temperature is favorable

8 High pressure favors physisorption and decrease of pressure causes desorption

High pressure is favorable but decreases of pressure does not cause desorption

9 Results in multimolecular layers Only unimolecular layer are formed

10 No activation energy is needed High activation energy is needed

11 It is instantaneous It is a slow process

Isotherm models

Langmuir isotherm

In 1916, Irving Langmuir published an isotherm for gases adsorbed on

solids, which retained his name. It is an empirical isotherm derived

from a proposed kinetic mechanism. It is based on four hypotheses:

1. The surface of the adsorbent is uniform, that is, all the adsorption

sites are equal.

2. Adsorbed molecules do not interact.

3. All adsorption occurs through the same mechanism.

4. At the maximum adsorption, only a monolayer is formed: molecules

of adsorbate do not deposit on other, already adsorbed, molecules of

adsorbate, only on the free surface of the adsorbent.

Shape of Langmuir Isotherm

The Freundlich (or Classical) adsorption isotherm

The relation between the concentration of a solute on the surface of an adsorbent to the concentration of the solute in the liquid with which it is in contact. The variation of adsorption with pressure can often be represented (especially at moderately low pressures) by the equation

where k and n are constants, n usually being greater than unity.Taking logarithms,

Shape of Freundlich Isotherm

V

Work Function change induced by adsorption

Different material have different f. Moreover, chemisorptions change the charge distribution and cause f change. For semiconductor even band bending is changed.

The dipole of the adsorbate can change f.

Under different condition (T for example), different adsorption bonding for the same adsorbate can be formed on the same surface with different f.