Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
1999-02-09
2001-02-06
Jeffery, John A. (Department: 3742)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S299000
Reexamination Certificate
active
06182443
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of treating diesel engine exhaust gases with two catalyst components located in the exhaust gas passage. The first component material made of precious metal on a particular porous support absorbs nitrogen oxides at low temperatures and desorbs them as the temperature is raised during engine operation. The second component, located downstream of the first component, is a catalyst capable of reducing the desorbed nitrogen oxides, such as a lean-NOx catalyst or a selective catalytic reduction (SCR) catalyst.
BACKGROUND OF THE INVENTION
Catalysts are employed in the exhaust system of diesel vehicles to oxidize carbon monoxide (CO) and hydrocarbons (HC), including the volatile organic fraction of particulates, produced during engine operation into carbon dioxide (CO
2
). Current diesel after-treatment technologies are being developed to convert nitrogen oxides (NOx), in addition to the other emissions, into more desirable gases. These technologies involve lean NOx catalysts, selective catalytic reduction (SCR) catalysts, and lean NOx traps.
Lean NOx catalysts are catalysts that convert NOx in a lean, i.e., O
2
-rich, environment with the aid of low levels of hydrocarbons. In the case of diesel, hydrocarbon emissions are too low to achieve significant lean NOx conversion, so hydrocarbons need to be added by injection of diesel fuel into the pre-catalyst exhaust stream. Diesel lean NOx catalysts generally include such materials as e.g., precious metals or base metal zeolites. In particular, platinum is used because of its lean NOx activity at low temperatures, i.e., usually less than 230° C. This low temperature activity is needed, e.g., during European urban-type driving where diesel exhaust gas temperatures typically measure between 100-300° C. at the engine exhaust manifold. Overall diesel catalysts need to be able to operate over a wide temperature range, e.g., up to 500° C. Base metal zeolites and precious metal other than Pt have lean NOx activity at higher temperatures, usually greater than 230° C. Precious metal and base metal zeolite materials are deficient, however, because they have a limited temperature range of lean NOx activity and a limited level of NOx conversion over their active temperature range. Platinum has an additional shortcoming of reducing NOx predominantly to N
2
O rather than N
2
.
Selective reduction catalysts (SCR), in contrast to using hydrocarbons for NOx conversion, use urea or ammonia to provide NOx conversion in O
2
-rich exhaust. Base metal zeolite materials are used for SCR catalysts on diesel vehicle. SCR catalysts provide much higher NOx conversion than lean NOx catalysts. However, similar to lean NOx catalysts, SCR catalysts are deficient because they have a limited temperature range of operation. Their NOx conversion activity is usually confined to temperatures greater than 230° C.
NOx traps operate on lean-burn gasoline vehicles by absorbing NOx on a material like barium oxide during lean-burn operation, i.e., engine air/fuel (A/F) ratio is ca. 20/1 and exhaust is O
2
-rich. Then, the NOx trap is subjected to engine exhaust during stoichiometric or fuel-rich operation, i.e., A/F ratio is 14.7 or lower and exhaust becomes O
2
deficient. This is done to desorb the NOx and convert it over precious metal in the trap formulation. In diesel systems, the exhaust gases generated by the engine are always oxidizing, i.e., lean, the engine A/F ratio being generally from 20/1 to 60/1. Using a NOx trap like barium oxide on alumina described above is unlikely with a diesel engine. This is because it is impractical to run the engine fuel-rich or near stoichiometric to release and reduce NOx. Hence, using such NOx traps in gasoline lean-burn engine exhaust systems is more commercially appropriate since the air/fuel ratio can more conveniently be made stoichiometric or fuel-rich.
The present invention method overcomes deficiencies of prior diesel exhaust purification methods and provides a catalyst system capable of efficiently reducing nitrogen oxides as well as oxidizing hydrocarbons and carbon monoxide in the relatively cool oxidizing conditions of diesel exhaust gases.
DISCLOSURE OF THE INVENTION
The invention is an exhaust gas treatment system for diesel engine exhaust. It comprises a first catalyst component which contains a nitrogen oxide absorbent material, and is located in an exhaust gas passage of a diesel engine. The system also comprises a second catalyst component which contains a lean-NOx catalyst or a selective reduction catalyst, and is located downstream of the first component. The nitrogen oxide absorbent material comprises (a) porous support material selected from the group consisting of alumina, zeolite, zirconia, titania, lanthana, and mixtures of any of them and (b) at least 0.1 wt. % precious metal selected from the group consisting of platinum, palladium, and rhodium or a mixture of any of them based on the weight of a support for the precious metal. In the system, the diesel exhaust gas flowing into said first component material is always oxidizing. At low temperatures the first component material absorbs nitrogen oxides from the exhaust gas. These absorbed nitrogen oxides are released from the absorbent material at higher temperatures and converted to nitrogen (N
2
) or nitrous oxide (N
2
O) over the second catalyst component. The second component may be a lean-NOx catalyst or SCR catalyst.
Preferably the precious metal of the first component includes platinum and its support is alumina. The specific temperature ranges of NOx storage and of NOx release of the first component catalyst will depend on it particular formulation. In some instances, the NOx will be absorbed at temperatures of up to about 230° C. and be released above that temperature to travel to the second component for reduction.
In another aspect, this invention is a method for treating diesel exhaust gases which involves the system disclosed above where the oxidizing diesel exhaust gases pass through the first component with NOx being absorbed at low temperatures and later desorbed at elevated temperatures to be reduced over the second component.
For optimum operation, the amount of nitrogen oxide absorbed on the first component may be monitored so that the NOx absorbed on the first component material can be desorbed by elevating the temperature before the material has reached capacity for nitrogen oxide absorption. Advantageously, however, diesel exhaust temperatures during typical driving, exemplified in the MVEuro cycle, are within the range of NOx desorption often enough that NOx storage capacity in the first component is not likely to be reached.
REFERENCES:
patent: 4985210 (1991-01-01), Minami
patent: 5125231 (1992-06-01), Patil et al.
patent: 5269140 (1993-12-01), Take et al.
patent: 5388406 (1995-02-01), Takeshima et al.
patent: 5455012 (1995-10-01), Machida et al.
patent: 5473887 (1995-12-01), Takeshima et al.
patent: 5649421 (1997-07-01), Wakabayashi et al.
patent: 5943857 (1999-08-01), Ansell et al.
patent: 560991 (1993-09-01), None
patent: 54-52213 (1979-04-01), None
patent: 61-112715 (1986-05-01), None
patent: 62-106844 (1987-05-01), None
patent: 62-106826 (1987-05-01), None
patent: 62-136245 (1987-06-01), None
patent: 5-240034 (1993-09-01), None
patent: 6-336 (1994-01-01), None
patent: 7-166854 (1995-06-01), None
patent: 9-85054 (1997-03-01), None
patent: 10-156144 (1998-06-01), None
patent: 11-13462 (1999-01-01), None
patent: 9743031 (1997-11-01), None
Masakazu Iwamoto, Tetsu Zengyo, Angel M. Hernandez, Hirotaka Araki, “Intermediate addition of reductant between an oxidation and a reduction catalyst for highly selective reduction of NO in excess oxygen,” Applied Catalysis B: Environmental 17 (1998) 259-266.
Hideaki Hamada, “Cooperation of catalytic species for the selective reduction of nitrogen monoxide with hydrocarbons,” Catalysis Surveys from Japan 1 (1997) 53-60.
Karen M. Adams, John V. Cavataio, Robert H. Hammerle, Applied Catalysis B: Environmental 10 (1996) 157-181.
Adams Karen Marie
Jarvis Mottlene
Ford Global Technologies Inc.
Jeffery John A.
Killworth, Gottman Hagan & Schaeff LLP
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