5.2. Catalytically Coated Filters
Catalytic coatings on wall-flow filters have not
proven very effective in lowering regeneration
temperatures. It is well known that even under the
most favorable conditions, such as a low exhaust
flow rate, high oxygen content and low filter loading,
less than 100°C decrease in ignition temperatures is
attained.
The experience with catalytically coated filters may
be summarized as follows
1. They attain a small reduction in soot ignition
temperature (30-100°C). Thus, a thermal regeneration back up device is still needed.
2. The use of precious metal coatings necessitates a
very low sulfur content in fuel (ca 0.05% as in
the case of catalytic converters).
3. The catalytic coating is more effective with filters
with high porosity and mean pore size.
4. Catalytic activity is more pronounced at low
filter loadings.
5. Reliability and high life expectancy of the
catalytic coating are crucial for the design of a
reliable diesel filter system.
In a catalytically coated filter the micro-scale
contact, among the carbonaceous particle matrix
and the catalyst active sites, is relatively poor. This
explains the limited efficiency of these systems in
lowering the regeneration temperature as compared
to those employing fuel additives