Computer simulation is now considered to be a crucial stage in
the design of automotive catalysts due to the increasing complexity
of modern aftertreatment systems. The resulting models almost
invariably include surface reaction kinetics that are measured
under controlled conditions similar to those found on a vehicle.
Repeatability of the measurements used to infer surface reaction
rates is fundamental to the accuracy of the resulting catalyst
model. To achieve the required level of repeatability, it is
necessary to ensure that the catalyst sample in question is stable
and that its activity does not change during the test phase. It is
therefore essential that the catalyst has been lightly aged, or
"de-greened" before testing begins. It is also known that
the state of the catalyst's surface prior to testing has an
impact on its subsequent light-off performance and that test
history can play an important role in catalyst activity. Suitably
pre-treating the catalyst surface can ensure that a reference point
is reached prior to a light-off test. The work summarized in this
paper includes a study of both the de-greening phase and the state
of the catalyst's surface on its activity, with the aim of
developing a robust test protocol that provides repeatable kinetic
data under realistic operating conditions.
To establish a protocol for initial stabilization of new
catalyst samples, a series of tests were conducted on catalysts
that had been thermally aged at 600°C and 750°C respectively until
the sample exhibited stability. The activity of the sample was
assessed by repeating identical CO light-off tests following each
period of time in the oven. A period of 8 hours at 750°C was found
to sufficiently stabilize the sample.
In the pre-treatment study, an investigation into the effect of
several pre-treatment protocols on CO light-off was carried out on
a three-way catalyst of commercial formulation in order to
establish a robust protocol to ensure test repeatability. The
investigation focused on pre-treating the catalyst sample in
hydrogen, oxygen or nitrogen environments at specified conditions
of flow and temperature. Following pre-treatment, identical CO
light-off tests were performed in order to assess the respective
influences of each pre-treatment. A trend of lower light-off
temperatures was observed for all pre-treatment strategies on the
first of two consecutive light-off temperature ramps with a notable
increase in light-off temperature observed on the second ramp. As a
result of this study a pre-treatment protocol was established. The
protocol involves treating the catalyst sample in flowing nitrogen
up to 600°C using a temperature ramp of 15°C/min.