Page 30 - Principles of Catalyst Development
P. 30
16 CHAPTER I
energy Ea , so the stronger the bond formed the faster the rate and the longer
the adsorbed species remains on the surface. Current models for chemisorp-
tion include geometric (or ensemble) and electronic (ligand) effects in which
a given molecule finds the right symmetry or orbitals at the surface.(23)
Understanding these features is important in design of new or improved
catalysts and will be treated in Chapter 4.
Also part of the adsorption step is surface migration or diffusion.
Although we visualize the adsorptive process as chemical bonding with a
surface site, the adsorbed molecule does not stay at that site but "hops"
around from one site to another. Activation energy for this process is
approximately 0.3-0.5 of ilHa. For example, with ilHa = -30 kcal mole- 1
(a typical value) the molecule makes 5 x 10 14 "hops" per second!(25) The
surface is truly a dynamic system with molecules arriving, leaving, and
migrating constantly. It is within this framework that we consider the next
step of the catalytic mechanism.
1.4.4. Surface Reaction
In this sea of constant movement, two adsorbed molecules destined to
react must come together. For instance, the Ns species of mechanism (1.2)
finds itself in surface contact with Hs. If surface geometry and energetics
are appropriate then a surface reaction occurs. Thus
H
I
N H N
I I I I
S+S -+ S+S (1.23 )
If the Ns species is held too strongly it will either be too immobile to
find Hs or disinclined to break its bonds with S and react. If held too
weakly, it will desorb before reacting and the surface concentration of Ns
is low. Thus we find the situation shown in Fig. 1.7, in which the rate of
N H ~ synthesis over different metals passes through a maximum when plotted
against the enthalpy of nitrogen adsorption.(26)
Curves of this shape are often found when correlating catalytic rates
against some measure of adsorption strength and are called "volcano"
curves.
Formulating rate equations from rate-determining steps such as (1.23)
is indeed a formidable task. The reader is referred to standard texts on this
topic.(27.2S) For our purpose in appreciating the role of catalytic steps, it is
sufficient to note that most success is achieved with very simple models. By