Chemical Reactions on Solid Surfaces - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 430

9.1.5. Pore Size and Surface Characterization
As we can conclude from the above discussions, the amount of adsorbate that can be
accommodated is directly proportional to the available surface sites or the surface area. To
achieve high specific surface area (surface area per mass of solid adsorbent material), solid
catalysts are commonly manufactured to be porous. Therefore, the structural characteristics
of solid (catalyst) materials are important. Pore volume, pore size, and specific pore surface
area are important parameters that dependent on the catalyst preparation procedures. These
parameters are commonly determined experimentally.
Complete coverage via physisorption is a preferred method of characterizing the surface
area of catalyst. In this case, BET isotherm can be applied to determine the surface area as n s 1
or monolayer coverage can be determined from BET isotherm, Eqn (9.77) , from experimental
measurements on the total amount of molecules adsorbed as a function of pressure. The total
surface area can be computed via

a T ¼ n s1 N Av a s (9.83)

10 23 mol 1 , and a s is the projected area of an
adsorbate molecule. Table 9.2 shows the parameters of commonly used adsorbate
molecules.
Pore volume and pore size are measured by Mercury penetration. Because Mercury is
a nonwetting liquid,

where N Av is the Avogadro's number, 6.023

it is not capable of penetrating pores smaller than 75,000 ˚

(or

10 7 m) at atmospheric conditions. At high pressure, Mercury can be forced into fine
pores as the surface tension is overcome by external pressure. The pore size and the pressure
is related by

7.5

10 7 m $ bar
p

7 : 5

r P ¼ (9.84)

By measuring the amount of Mercury penetration as the pressure is increased, we can deter-
mine the pore size distributions as well as the pore volume.

TABLE 9.2 Properties of Commonly Used Adsorbates for Catalyst Surface Characterization

Adsorption
temperature, K

Saturation vapor
pressure

Projected molecular
area a s ,10 L20 m 2 Adsorbate p ,MPa

N 2 77.4 0.10132 16.2

10 4 Kr 77.4 3.456 19.5

Ar 77.4 0.03333 14.6

10 3 C 6 H 6 293.2 9.879 40

CO 2 195.2 0.10132 19.5

CH 3 OH 293.2 0.01280 25