Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2002-01-29
2004-11-09
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
By means producing a chemical reaction of a component of the...
C060S274000, C060S311000, C060S297000
Reexamination Certificate
active
06813884
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to a method of treating compression ignition engine exhaust gases such as diesel engine exhaust gases. More particularly, the present invention relates to a method which utilizes a catalyst to convert NO in diesel exhaust gas streams to NO
2
at low temperatures. The NO
2
(nitrogen dioxide) generated by the catalyst can then be used to promote regeneration of a downstream diesel particulate filter at low temperatures.
2. Background of the Invention
In recent years, environmental regulations in the United States and Europe for diesel particulate emissions have required the effective removal of particulate (generally consisting of carbonaceous particulates in the form of soot) from the diesel engine. A well-known method for removing soot from engine exhaust has been to collect the soot using a particulate filter or porous ceramic trap made from cordierite or SiC followed by oxidation of accumulated particulate at the high temperatures encountered in the exhaust.
However, lean-burn engines such as diesel engines operate at much cooler temperatures than gasoline engines. As a result, the lower temperature of the exhaust gases hinders the oxidation of particulate captured in the particulate filter. The use of catalytic traps capable of promoting particulate filtration and regeneration have been proposed as one solution to this problem. It has also been found that the presence of NO
2
in diesel exhaust results in oxidation of particulate at lower temperatures up to 300° C. NO
2
reaction with soot occurs even at room temperature, but the reaction time takes up to 2 orders of magnitude longer. Accordingly, it would be desirable to be able to use a catalyst to convert NO in diesel exhaust gas to NO
2
at the lowest possible temperature in order to increase the rate of particulate combustion.
Diesel catalysts have been proposed containing platinum, which is active at temperatures below about 300° C. Among catalysts studied, the Pt/silica catalyst exhibited the best activity; however, it exhibited low activity below 250° C. The same catalyst was the most active in particulate combustion due to the effective NO oxidation to NO
2
. There are thermodynamic limitations of NO conversion to NO
2
at temperatures exceeding 250° C. While it is impossible to achieve high conversion of NO to NO
2
at higher temperatures due to the thermodynamic limitations, the development of catalysts which are active at temperatures between about 175 to 250° C. could lead to practically complete conversion. It would be desirable to improve the activity of catalysts at low temperatures and improve the thermal stability of catalysts for NO oxidation to NO
2
.
Accordingly, there is still a need in the art for a method of treating compression ignition exhaust gases such as diesel engine exhaust gases which can successfully convert NO to NO
2
at temperatures of less than 250° C.
SUMMARY OF INVENTION
The present invention meets that need by providing an improved method for converting NO in diesel engine exhaust gases to NO
2
at low temperatures utilizing an oxidation catalyst comprising platinum and a support material comprising zirconia-silica. The resulting NO
2
promotes regeneration of the diesel particulate filter (i.e., combustion of trapped carbonaceous particulate) at a temperature less than about 250° C., or accumulates on NOx trap material for particulate combustion at higher temperatures.
According to one aspect of the present invention, a diesel exhaust gas treatment system is provided comprising an oxidation catalyst positioned in an exhaust gas passage of a diesel engine for converting at least a portion of NO contained in the exhaust gas to NO
2
. The oxidation catalyst comprises platinum and a support material comprising zirconia-silica. The system also includes a particulate filter for receiving the exhaust gas containing NO
2
, where the NO
2
enables oxidation of particulate trapped on the filter, thereby promoting regeneration of the filter.
The conversion of NO to NO
2
preferably occurs at a temperature range of between about 175 to 350° C., and more preferably at a temperature of between about 200 and 250° C. The oxidation of particulate preferably occurs at a temperature less than about 250° C.
The oxidation catalyst is preferably positioned between the exhaust passage and the particulate filter. The oxidation catalyst preferably comprises from about 1 to 5 wt. % platinum on the support. The support contains from about 3 to 20 wt. % zirconia, and the balance silica. To increase the activity of the catalyst, or decrease Pt loading at the same level of performance, the catalyst also preferably includes one or more oxides selected from the group consisting of TiO
2
, P
2
O
5
, WO
3
, B
2
O
3
, and Al
2
O
3
, or a heteropolyacid selected from H
3
PW
12
O
40
and H
4
SiW
12
O
40
.
The system may optionally include a second catalyst positioned downstream from the particulate filter for NOx reduction. The second catalyst preferably comprises a selective reduction catalyst, which is capable of reducing nitrogen oxides such as NO
2
and NO into nitrogen (N
2
) or nitrous oxide (N
2
O). In addition, reductants such as ammonia, urea, or diesel fuel may be added to the second catalyst to aid in reduction.
In another embodiment of the invention, the system may include a NOx trap positioned downstream from the oxidation catalyst. This NOx trap may be combined with a particulate filter according to the method developed by Toyota Motor Corp. (Automotive Engineering International/October 2000, p.119) for simultaneous removal of NOx and particulate. According to another aspect of the present invention, a method for treating diesel exhaust gases is provided in which an oxidation catalyst is positioned in the exhaust gas passage of a diesel engine, where the catalyst comprises platinum and a support material comprising zirconia-silica. The oxidation catalyst is exposed to diesel exhaust gas such that at least a portion of NO contained in the exhaust gas is converted to NO
2
. The NO
2
is then passed through a particulate filter provided downstream of the oxidation catalyst in an amount sufficient to oxidize particulate trapped on the filter.
The method preferably includes pretreating the oxidation catalyst in a gas mixture containing NO, O
2
and N
2
prior to positioning the catalyst in the exhaust stream. The gas mixture preferably comprises about 500 ppm of NO, about 3% by volume O
2
, and the balance N
2
. The pretreatment is preferably carried out a temperature of between about 500 to 650° C.
The method also preferably includes adding one or more oxides (such as, for example, TiO
2
, P
2
O
5
, WO
3
, B
2
O
3
, and Al
2
O
3
) to the catalyst to further improve the performance prior to positioning the catalyst in the exhaust stream, or adding a heteropolyacid such as, for example, H
3
PW
12
O
40
and H
4
SiW
12
O
40
.
The method of the present invention results in a high conversion of NO contained in the exhaust gas to NO
2
of from about 60% to about 96% at a temperature of about 175 to 250° C.
The method may further include providing a second catalyst downstream of the particulate filter to aid in reducing NOx. The second catalyst is preferably a selective reduction catalyst as discussed above.
The method may also include positioning an NOx trap downstream from the oxidation catalyst (between the oxidation catalyst and the particulate filter) or combining the trap with the particulate filter.
Accordingly, it is a feature of the present invention to provide a diesel exhaust treatment system and method for converting NO to NO
2
at low temperatures. This, and other features and advantages of the present invention, will become apparent from the following detailed description, the accompanying drawings, and the appended claims.
REFERENCES:
patent: 3193491 (1965-07-01), Cramer et al.
patent: 4717708 (1988-01-01), Cheng et al.
patent: 4902487 (1990-02-01), Cooper et al.
patent: 4934142 (1990-06-01), Hayashi et al.
patent: 50396
Carberry Brendan Patrick
McCabe Robert Walter
Shigapov Albert Nazipovich
Denion Thomas
Ford Global Technologies LLC
Porcari Damian
Tran Diem
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