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