Page 16 - Adsorption by Powders and Porous Solids

CHAPTER 1

Introduction

1.1. Importance of adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2. Historical aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3. General definitions and terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4. Physisorption and chemisorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
1.5. Adsorption interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
1.6. Mobility of adsorbed molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
1.7. Energetics of physisorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
1.8. Types of adsorption isotherms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
1.8.1. Physisorption of gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
1.8.2. Chemisorption of gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
1.8.3. Adsorption from solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
1.9. Molecular modelling of adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
1.9.1. Intermolecular potential functions . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
1.9.2. Molecular simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
Monte Carlo (MC) simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
.
Molecular dynamics (MD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
1.9.3. Density functional theory (DFT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

1.1. Importance of Adsorption

Adsorption occurs whenever a solid surface is exposed to a gas or liquid: it is defined
as the enrichment of material or increase in the density of the fluid in the vicinity of
an interface. Under certain conditions, there is an appreciable enhancement in the
concentration of a particular component and the overall effect is then dependent on
the extent of the interfacial area. For this reason, all industrial adsorbents have large
specific surface areas (generally well in excess of 100mZg-I) and are therefore
highly porous or composed of very fine particles.
Adsorption is of great technological importance. Thus, some adsorbents are used
on a large scale as desiccants, catalysts or catalyst supports; others are used for the
separation of gases, the purification of liquids, pollution control or for respiratory
protection. In addition, adsorption phenomena play a vital role in many solid state
reactions and biological mechanisms.
Another reason for the widespread use of adsorption techniques is the importance
now attached to the characterization of the surface properties and texture of fine
powders such as pigments, fillers and cements. Similarly, adsorption measurements
are undertaken in many academic and industrial laboratories on porous materials