Page 15 - Principles of Catalyst Development

CATALYTIC FUNCTIONS

What the Catalyst Does

1.1. A DEFINITION

This chapter deals with what a catalyst does and what it can be expected
to do or not do. We start the discussion with a definition:
A catalyst is a substance that increases tile rate at which a chemical reaction
approaches equilibrium without itself becoming permallently involved in the
reaction.
Key words in the definition are italicized. A catalyst is itself a chemical
substance and as such becomes involved in the reaction, although not
permanently. The chemical state of the catalyst is subject to all the rules of
chemistry in its interaction with reactants but remains unchanged at the
end of the reaction. Primarily, the catalyst accelerates the kinetics of the
reaction toward thermodynamic completion by introducing a less difficult
path for molecules to follow. Figure 1.1 illustrates this feature with an
industrially important reaction, the synthesis of ammonia 'l ):

(1.1)

Nitrogen and hydrogen molecules combine homogeneously (without
a catalyst) at an extremely slow rate. Breaking their bonds to form reactive
fragments requires large amounts of energy, leading to an activation energy
of 57 kcal mole -1. The probability of finding these fragments together is
small. Spontaneous ammonia formation at moderate conditions is
infinitesimal. The catalyst, however, assists dissociation through chemisorp-
tion and recombination with a series of surface interactions: