Removal of particulates from the exhaust gas of internal combustion engines operated with a predominantly stoichiometric air/fuel mixture
The exhaust gas of internal combustion engines operated with a predominantly stoichiometric air/fuel mixture contains, as well as the gaseous hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NO x ) pollutants, also ultrafine particulates. There is disclosed a catalytically active particulate filter, an exhaust gas cleaning system and a process for cleaning the exhaust gases of predominantly stoichiometrically operated internal combustion engines, as well as the gaseous CO, HC and NO x pollutants, also for removing particulates from the exhaust gas. The particulate filter comprises a filter body and a catalytically active coating consisting of two layers. Both layers contain alumina. The first layer contains palladium. The second layer contains rhodium. The latter is disposed above the first layer.
1. An exhaust gas cleaning system for cleaning the exhaust gases of internal combustion engines operated with a predominantly stoichiometric air/fuel mixture, comprising:
a particulate filter comprising a filter body,
a first catalytically active layer, which comprises alumina and palladium and is free of oxygen-storing cerium/zirconium mixed oxides, layered on and in direct contact with the filter body, and
a second catalytically active layer, which comprises alumina, rhodium and an oxygen-storing cerium/zirconium mixed oxide, completely covering the first catalytically active layer on its exhaust gas side,
wherein the palladium and the rhodium are spatially separated, and
wherein the catalytically active particulate filter is disposed in a position close-coupled to the engine.
2. The exhaust gas cleaning system as claimed in claim 1 , wherein
a three-way catalytic converter comprising a ceramic or metallic flow substrate and a catalytically active coating is present upstream or downstream of the particulate filter.
3. The exhaust gas cleaning system as claimed in claim 2 , wherein
the three-way catalytic converter is disposed upstream of the particulate filter and contains less than 100 g/l of oxygen-storing cerium/zirconium mixed oxide, based on the volume of the three-way catalytic converter.
4. The exhaust gas cleaning system as claimed in claim 2 , wherein
the three-way catalytic converter is disposed downstream of the particulate filter and contains more than 100 g/l of oxygen-storing cerium/zirconium mixed oxide, based on the volume of the three-way catalytic converter.
5. The exhaust gas cleaning system as claimed in claim 3 , wherein
the three-way catalytic converter additionally comprises a nitrogen oxide storage material selected from the group consisting of the alkali metal oxides, alkaline earth metal oxides, and rare earth metal oxides, and combinations thereof.
6. The exhaust gas cleaning system as claimed in claim 4 , wherein
the three-way catalytic converter additionally comprises a nitrogen oxide storage material selected from the group consisting of the alkali metal oxides, alkaline earth metal oxides, rare earth metal oxides, and combinations thereof.
7. A process for cleaning an exhaust gas from an internal combustion engine operated with a predominantly stoichiometric air/fuel mixture which comprises:
passing the exhaust gas, which contains carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NO x ) and particulates, through a particulate filter comprising a filter body and a catalytically active coating which consists of a first layer and a second layer arranged one on top of the other, the first layer is on and in direct contact with the filter body, the second layer completely covers the first layer and is capable of being in direct contact with an incoming exhaust gas, the first layer and the second layer both comprise alumina, wherein the alumina in the first layer is catalytically activated with palladium, while the alumina in the second layer is catalytically activated with rhodium,
and only the second layer contains an oxygen-storing cerium/zirconium mixed oxide.
8. The process as claimed in claim 7 , wherein the particulate filter is regenerated by passing an oxygen-rich exhaust gas through at exhaust gas temperatures between 400 and 700° C.
9. The process as claimed in claim 8 , wherein the oxygen-rich exhaust gas is generated by means of a power cutoff.
10. The process as claimed in claim 9 , wherein the oxygen-rich exhaust gas is generated by blowing secondary air into the exhaust gas line.