periods of time at temperatures greater than 700°C.
This deactivation is known to be related to zeolite
dealumination and subsequent loss in the number of
active exchanged Cu cations.13° The low hydrocarbon
selectivity of this catalyst can be compensated by
injecting additional hydrocarbon reductants in the
exhaust upstream of the catalyst. Due to the very
low HC emissions of diesel engines, secondary fuel
addition strategy is expected to play a large role in
practical application.
Also, it is very important to adapt the catalyst to
exhaust gas temperature because whole or only a part
of the catalyst contributes to NOx conversion,
depending on the temperature. It has been found
that the precious metal catalysts, which have
relatively low sulfate suppression ability, can reduce
NOx at low temperatures, but particulate greatly
increases by the sulfate formation because of the
exhaust gas temperature increase, caused by the
addition of secondary fuel. On the other hand,
the surplus secondary fuel must be readily oxidized by
the catalyst, in order to avoid significant HC and CO
emission increases (as is the case with high light-off
catalysts). Simultaneous 12% NOx and 25% particu-
late reduction have been reported with base metal
catalyst by minor optimization of the amount and
pattern of secondary fuel addition, with 3% fuel
penalty. 129 Higher amounts of secondary fuel are not
favorable because they result in an increase of
particulate emissions and fuel economy penalty.
Furthermore, it is important to note that diesel fuel,
which is readily available on diesel powered vehicles,
is not the best means of producing the necessary HC
for reduction of NOx. Current research is aimed at the
optimization of secondary fuel addition strategies
(e.g. by exploitation of the post injection capabilities
of modern common rail diesel injection systems),
combined with the development of improved catalyst
formulations) 31