Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-03-29
2001-10-02
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S213500
Reexamination Certificate
active
06296822
ABSTRACT:
FIELD OF THE INVENTION
This invention is related to a method of manufacturing a NOx trap material tolerant to sulfur for use in lean-burn exhaust gas treatment systems. More particularly, the material is made by providing barium and platinum on an alumina-ceria washcoat and overcoating with lithium.
BACKGROUND OF THE INVENTION
Significant research has been undertaken in the area of lean-burn engines, where the A/F ratio is higher than stoichiometry, in order to improve fuel economy. One approach to treating such exhaust gasses involves NOx absorbents, i.e., materials which are able to absorb nitrogen oxides from the exhaust gas during lean-burn operation and then later release them when the oxygen concentration in the exhaust gas is reduced. For example, when the A/F ratio is made rich or stoichiometric. Conventional NOx absorbents are alkaline earth metals like barium with a precious metal catalyst like platinum carried on alumina. The widely held mechanism for this absorption phenomena is that during lean-burn operation the platinum first oxidizes NO to NO
2
and the NO
2
subsequently forms a nitrate complex with the trapping material, e.g., the barium. In the regeneration mode, under a stoichiometric or rich environment, the nitrate decomposes and the NOx released is reduced catalytically over the platinum with reducing species like HC or CO in the exhaust gas.
Such conventional absorbent materials have a serious deficiency in that the barium reacts with sulfur oxides generally present in exhaust gas to form barium sulfate. This inactivates the barium for NOx absorption. It is suggested that to decompose the barium sulfate it should be subjected to elevated temperatures of at least 600° C. or more in reducing gas conditions. One negative aspect of this regeneration process is that it leads to detrimental changes in the NOx absorbent such as reduced surface area and crystallization of the aluminate phases thereby reducing the efficiency of the NOx absorbent. It is also difficult to carry out on-board a vehicle and the required rich operating conditions results in emission of CO and HC. Alkali metals like potassium have also been suggested as NOx absorbents, however, they are even more easily deactivated by sulfur than alkaline earth metals like barium.
It would be desirable if a NOx absorbent could be found which would be more resistant to sulfur poisoning while being a good NOx absorbent during leanburn engine operation. The present invention overcomes the deficiencies of prior art materials. The material and method of making are disclosed herein.
SUMMARY OF THE INVENTION
The invention is a method of manufacturing a NOx trap material which includes the steps of first providing a support material of alumina/ceria including at least 0.5 wt. % ceria. Then this material is provided with barium and precious metal including at least platinum, preferably from a solution of their soluble salts, to provide at least 1.0 wt. % barium and 0.2 wt. % platinum on the material. Subsequently, the material containing the barium and platinum is overcoated with a layer of 0.5-5 wt. % lithium based on the weight of the barium-precious metal containing support material, i.e., the washcoat. According to another aspect, the invention is the NOx trap material manufactured as disclosed above. In another embodiment, the invention is a method for treating automotive exhaust gas by subjecting the above NOx trap material placed in an exhaust gas passage to lean burn and then stoichiometric or rich exhaust gas.
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Denari John
Hepburn Jeffrey Scott
Watkins William Lewis Henderson
Coughlin William J.
Ford Global Technologies, Inc
Griffin Steven P.
Melotik Lorraine S.
Nguyen Cam N.
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