Page 18 - Principles of Catalyst Development
P. 18
4 CHAPTER I
Second, since the equilibrium constant, Kp , is unchanged, it follows
from the equality
(1.3 )
that the catalyst must accelerate both the forward rate constant, k, and the
reverse, k. Although not a factor in irreversible reactions, this feature is
important in appreciating the role of the catalyst in normally reversible
situations. For example, materials that are known to function as hydrogena-
tion catalysts will also be good for dehydrogenation, if compatible with the
necessarily different process conditions.
Another more subtle point emerges. Sometimes studying a forward
reaction is difficult, while the reverse is easy. An example is ammonia
synthesis. This reaction is reversible over the range of temperatures normally
encountered in ind ustrial operations, 200-1200°C. Figure 1.2 shows that the
exothermic synthesis reaction decreases in equilibrium conversion as the
temperature increases. Higher yields are obtained by decreasing the tem-
perature. But kinetic rates are lower, so precision suffers. Also, the
stoichiometry of reaction (1.1) indicates that increasing pressure will raise
the equilibrium conversion. In the case of NH3 synthesis, this amounts to
hundreds of atmospheres. High-pressure equipment is extremely incon-
venient for most laboratory studies.
Ammonia decomposition, on the other hand, may be carried out under
more favorable conditions. Stoichiometry favors low pressure, so normal
atmospheric-pressure equipment is sufficient. Equilibrium yields increase
with temperature and kinetic rates are measured with precision. This is why
ammonia decomposition, which is less interesting, has historically received
so much attention in the search for improved synthesis catalysts.(2i
The third implication from the definition is that more than one reaction
may be involved, leading to different thermodynamically feasible products.
A catalyst, in principle, promotes only one of these, leading to improvements
in selectivity as well as activity. Since the catalyst is a chemical, reacting
with reactants and products through chemisorption or complexing, its
reactivity depends upon its own chemical structure. We see this demon-
strated in the simple decomposition of formic acid:(JI
Dehydration: HCOOH --+ H 20 + CO
AI,Ol
(1.4 )
Dehydrogenation: HCOOH --+ H 2 +C0 2
Metals
More important industrial examples exist. For instance, by changing
the catalyst (and process conditions) we may convert H2 and CO mixtures