This paper tries firstly to remember the origin of kinetic models in hetero
geneous catalysis. The change of catalysis from an art to science induced b
y the introduction of the concepts of Langmuir is presented in Section 1.
In Section 2, deficiencies contained in these concepts as applied, where on
e site is associated to one chemisorbed radical, are developed. Namely the
curious contradictions obtained in the analysis of the hydrogen inhibiting
term on the rate of hydrogenolysis reactions are analysed.
In Section 3, attention is drawn on information obtained by use of labelled
molecules. These studies indicated a similar H-2 inhibiting term on the on
ly adsorption step. The interpretation of that effect results in the develo
pment of a multisite adsorption model.
Section 4 introduces the interest and advantages of the use of the chemical
transient method. Simultaneously, its technical limitations are briefly pr
esented.
Sections 5 and 6 are devoted to the application of the transient method to
two very different but important catalytic reactions. The first of these ex
amples makes evident the interest in the combination of labelled molecules
with chemical transient effects. to determine the rate, direct and inverse,
of the successive elementary steps of the model reaction that is ethane hy
drogenolysis. The second example, the CO-Hp reaction, is much more complex
as the catalytically working surface is built during the transient phase, t
hus dramatically changing the surface properties. If it does not allow, as
in the previous example, to obtain quantitative determinations of the rate
of the elementary steps, it anyway provides unique information leading one
to propose a new reaction scheme.
One of the aims of this paper is to support the still large importance of k
inetic studies in heterogeneous catalysis, a topic dear to Michel Boudart w
ith whom one of us (A. Frennet) had the honour and pleasure to work. (C) 20
00 Published by Elsevier Science B.V.