Page 29 - Principles of Catalyst Development
P. 29
CATALYTIC FUNCTIONS IS
giving
( 1.20)
Rates of pore diffusion-controlled reactions can be increased by decreasing
the particle radius or increasing the diffusivity. The latter can only be done
by increasing the pore radius (without decreasing () or Sg).
Another important conclusion results from equation (1.20). For the
simple-order rate equation
(1.21)
substitution from equation (1.20) gives
(1.22 )
Extreme pore diffusion limitation leads to one half the normal activation
energy and moves the order closer to unity. These facts will be useful when
we discuss diagnostics.
1.4.3. Adsorption
Having made its way to the interior surface of the porous particle,
molecule A is now ready for the first chemical step, adsorption on the
surface. In catalysis, adsorption is almost always chemisorption. Chemisorp-
tion results from chemical bonds between the molecule (adsorbate) and the
solid surface (adsorbent). It is therefore very specific,(23) and receptive sites
for chemisorption must exist. Physical adsorption comes from general van
der Waals forces, which are physical in origin, weaker than chemisorption,
and not specific. Chemisorption stops when a monolayer of adsorbed
molecules is formed. It is activated with energies around 10 kcal mole-I, is
exothermic with enthalpy changes of -15 to -40 kcal mole -\ is slowly
reversible or even irreversible, and is the key step in activation of reaction
intermediates.
The rate of chemisorption is governed by the frequency of collisions
with the surface and the probability of "sticking" with chemical bond
formation. The former is a physical phenomenon, dependent on temperature
and pressure. For example, at one atmosphere pressure and 25°C, 3 x 10 23
molecules strike each square centimeter of surface each second. If all "stick,"
the surface is covered in 3 x 10- sec.(26) The probability of chemical bonding
9
is exponentially proportional to the enthalpy change, flHa , and activation