.3. Catalyst Activity Assessment
3.3.1. Light-off tests with mini scale catalysts
The activity of the catalyst as a function of its
temperature is a critical feature of its performance
and is affected by a number of exothermic reactions.
Arrhenius plots, derived from dedicated measure-
ments, yield information regarding the reaction
kinetics of single reactions. Figure 4 presents, as an
example, a typical Arrhenius plot obtained for the
CO-O2 reaction on a Pt-Rh catalyst. 4° Four
temperature regions characterized by their own
activation energy may be recognized in this plot. In
the first the overall reaction is controlled by the
reaction kinetics, whereas in the second the conver-
sion is limited by the pore diffusion in the alumina
washcoat. The third region corresponds to mass-
transfer controlled conditions and in the fourth, high
temperature region the homogeneous reaction
becomes the controlling phenomenon.
An easier way to assess the catalytic activity in the
laboratory is to measure the efficiency with which
hydrocarbons, carbon monoxide and nitrogen oxides,
contained in synthetic exhaust gas, are converted as
functions of catalyst temperature. Such tests are per-
formed on a monolithic catalyst core or representative