Page 158 - Principles of Catalyst Development

146 CHAPTER 7
7.3.4. Surface Area

The term ··texture" refers to the general pore structure of particles and
includes surface area, pore size distribution, and shape. Total surface area,
S!', m g I, is possibly the most important particle parameter specified
2
without regard to the type of surface. No attempt is made to distinguish
one component from another.
Measurement of surface area involves the principles of physical adsorp-
tion, which ditTer from those of chemical adsorption, as shown in Table
7.3. 1211
Physical adsorption is equilibrium coverage similar to surface liquefac-
tion. Produced by van der Waals forces originating in surface atoms,
interactions with the surface are similar to those between molecules and
are approximately the same for all materials. Coverage proceeds first with
adsorption on surface atoms but is quickly followed by the generation of
additional layers even before a complete monolayer forms. Since the process
is exothermic and at equilibrium, the amount decreases as temperature
increases. Easily measurable quantities are found close to the normal boiling
point of the adsorbate. Equilibrium isotherms follow the shape shown in
Fig. 7.9, where the volume adsorbed is plotted against pi Po (p is the pressure,
Po the saturation pressure at measurement temperature). Three regions are
included. At low pressures monolayer formation follows the Langmuir
equation

v Kp/po
(7.4 )

TABLE 7.3. Differences between Physical and Chemisorption

Property Physical adsorption Chemisorption

I. Forces responsible Physical van der Waals Chemical bonds, ionic,
electrostatic forces covalent
2. Heat of adsorption Low «lOkcalmole'l) Moderate to high
( exothermic) similar to liquefaction (l0-50 kcal mole' 1) similar
to reactions
3. Activation energy None Low « 15 kcal mole-I) to
moderate (25 kcal mole 1 )
4, Specificity None Specific adsorbate-
adsorbent interactions
5. Reversibility Complete, rapid Slow
6, Extent Multilayers Monolayer saturation