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Exhaust after-treatment system for a lean burn internal combustion engineRelated Patent Categories: Power Plants, Internal Combustion Engine With Treatment Or Handling Of Exhaust Gas, Methods, Anti-pollutionExhaust after-treatment system for a lean burn internal combustion engine description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20050241296, Exhaust after-treatment system for a lean burn internal combustion engine. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] This invention relates to exhaust after-treatment systems and more particularly to exhaust after-treatment systems for lean burn internal combustion engines. BACKGROUND AND SUMMARY [0002] As is known in the art, precious metal three-way catalysts are generally used as a means for removing pollutants from the exhaust gas of an internal combustion engine. These three-way catalysts remove CO, HC, and NO.sub.x simultaneously from engine exhaust gases under stoichiometric conditions. However, under lean fuel conditions, which are desired for optimal fuel efficiency, the three-way catalyst is ineffective for the removal of NO.sub.x. Accordingly, to achieve NO.sub.x control under fuel lean conditions, exhaust after-treatment systems have included a lean NO.sub.x trap (LNT). [0003] An LNT has 3 essential components: [0004] 1) a NO.sub.x storage medium (also called compound or component). Prototypically, this is barium. Barium never exists by itself; it will always be present in the form of a compound in the trap, e.g., barium carbonate. Other storage components are those of the alkali metal group (especially potassium and cesium) and other alkaline earth elements besides Ba (e.g., strontium and magnesium). [0005] 2) a NO oxidation component. NO.sub.x is present in engine exhaust gases as a mixture of NO and NO.sub.2. It is stored as a nitrate species (NO.sub.3). To convert to the nitrate form, both the NO and NO.sub.2 must be oxidized (i.e. reacted with oxygen from the exhaust gas). Platinum is the prototypical metal for doing that, but other metals have oxidation capability. [0006] 3) a reducing component. Regeneration of the trap involves driving the exhaust gas to rich conditions (i.e. excess of reductant species such as carbon monoxide, hydrogen, and hydrocarbons) and reacting the adsorbed nitrate back to nitrogen. This is similar to the way NO.sub.x is treated in a three-way catalyst. Rhodium is the prototypical element for NO.sub.x reduction and it is used in most LNTs for the purpose of regenerating the trap. [0007] Those are the three main components. Additionally, a high surface support phase is used such as alumina over which all the components are dispersed to create finally divided, small particles of all the active components. Various stabilizers and so-called oxygen storage materials are often added as well. [0008] An additional function of the Pt in the LNT is to combust reductants such as CO, H.sub.2, and HC to release heat needed to raise the operating temperature of the LNT to the high temperature levels required for removal of stored sulfur. [0009] Thus, the LNT includes material to oxidize the CO and HC and material to store NO.sub.x. Presently, however, the performance of NO.sub.x trap technology is limited in several respects. NO.sub.x trap performance is affected by the relatively narrow operating temperature window of current trap formulations. At temperatures outside this window, the system may not operate efficiently and NO.sub.x emissions can increase. [0010] Both three-way catalysts and lean NO.sub.x traps (LNT) are generally inefficient at ambient temperatures and must reach high temperatures before they are activated. Typically, contact with high-temperature exhaust gases from the engine elevates the temperature of the catalyst or LNT. The temperature at which a catalytic converter can convert 50% of CO, HC, or NO.sub.x is referred to as the "light-off" temperature of the converter. [0011] During start up of the engine, the amount of CO and HC in the exhaust gas is typically higher than during normal engine operation. While a large portion of the total emissions generated by the engine is generated within the first few minutes after start up, the catalysts are relatively ineffective because they will not have reached the "light-off" temperature. In other words, the catalysts are the least effective during the time they are needed the most. [0012] As noted above, in order to achieve NO.sub.x control in lean burn engines, exhaust after-treatment systems have included an additional NO.sub.x storage device often referred to as a lean NO.sub.x trap (LNT). Presently, however, the performance of NO.sub.x trap technology is limited in several respects. NO.sub.x trap performance is affected by the operating temperature and requires a relatively narrow temperature-operating window of the exhaust gases. At temperatures outside this window, the system will not operate efficiently and NO.sub.x emissions will increase. Exposure to high temperature will also result in permanent degradation of the NO.sub.x trap capacity. [0013] The LNT is purged periodically to release and convert the oxides of nitrogen (NO.sub.x) stored in the trap during the preceding lean operation. To accomplish the purge, the engine has to be operated at an air-to-fuel ratio that is rich of stoichiometry. As a result of the rich operation, substantial amounts of feedgas carbon monoxide (CO) and hydrocarbons (HC) are generated to convert the stored NO.sub.x. Typically, the purge mode is activated on the basis of estimated trap loading. That is, when the estimated mass of NO.sub.x stored in the trap exceeds a predetermined threshold, a transition to the purge mode is initiated. The rich operation continues for several seconds until the trap is emptied of the stored NO.sub.x, whereupon the purge mode is terminated and the normal lean operation is resumed. The end of the purge is usually initiated by a transition in the reading of the HEGO sensor located downstream of the trap, or based on the model prediction of the LNT states. Since the engine is operated rich of stoichiometry during the purge operation, the fuel economy advantage of the lean operation is lost. [0014] In addition to normal trap regeneration, the LNT may also be subjected to a much higher temperature regeneration process for the removal of stored sulfur (typically temperatures in excess of 600 degrees Celsius). Furthermore, if the LNT is contained in an exhaust system that also contains a diesel particulate filter (DPF), the LNT may also be subjected to temperatures in excess of 500 degrees Celsius during regeneration of the DPF (i.e. removal of accumulated carbonaceous (i.e. soot) material via combustion with oxygen in the exhaust gas). Both of these processes can result in permanent, gradual deterioration in NO.sub.x trap performance--more so even than normal trap regeneration to remove stored NO.sub.x. [0015] More particularly, as noted above, a LNT has both functions of oxidation of HC and CO, etc. and storage/reduction of NO.sub.x. In a conventional LNT, as shown in FIG. 1, an oxidation material (namely platinum, Pt) used to oxidize the HC and CO is included along with additional components such as rhodium (Rh), used for NO.sub.x reduction, and barium (Ba) used to store the NO.sub.x. The inventors have discovered that the exposure of the lean NO.sub.x trap (LNT) to temperatures in the range of 600 to 700 degrees Celsius, especially under the oxidizing conditions required for DPF regeneration, can cause the deterioration of the LNT especially its "light off" function, and largely reduces its low temperature NO.sub.x reduction efficiency. The inventors speculate that it is one or more of the major components of the LNT (i.e., such as rhodium (Rh) and barium (Ba)) that interacts with the Pt in a deleterious way following the high temperature operation of the LNT required for de-sulfurization and/or DPF regeneration (if such DPF is serially connected in the system). For example, it is known that Rh and Pt can form alloys, and it may turn out that the high temperature conditions required for LNT desulfurization and/or DPF regeneration causes the Pt and Rh to alloy in the LNT in such a way that the oxidation activity of the Pt is adversely affected. [0016] In accordance with the present invention, an exhaust gas after-treatment system is provided having a NO.sub.x storage material in a NO.sub.x storage section and an HC and CO oxidation catalyst in a separate HC and CO oxidation section, such oxidation section being substantially free of the NO.sub.x storage material. [0017] In one embodiment the oxidation section is substantially free of Rh. [0018] With such an arrangement, the HC and CO oxidation catalyst is physically separated from the NO.sub.x storage material. Thus, the oxidation catalyst used in the oxidation section will not become adversely affected by any alloying or other types of interactions with components contained in the NO.sub.x storing section. [0019] In one embodiment, the oxidation catalyst is Pt for generating heat required to "light off". Thus, while it is known that Pt is an effective NO.sub.x oxidation catalyst, the negative effects described above of using the Pt completely in conjunction with the NO.sub.x storage material such as Ba and reducing components such as Rh are avoided by separating part of the Pt out into a separate oxidation (combustion) catalyst preceding the NO.sub.x storage section. [0020] In one embodiment, an exhaust gas after-treatment system is provided. The system includes in one section thereof, a NO.sub.x oxidation component, a NO.sub.x storage component, and a NO.sub.x reduction component, and, in a separate section thereof, a catalytic HC and CO combustion section substantially free of the NO.sub.x storage component and the NO.sub.x reduction component. [0021] In accordance with another feature of the invention, a method is provided for treating exhaust gas produced by an internal combustion engine. The method includes oxidizing hydrocarbons and carbon monoxide present in the exhaust gas and storing NO.sub.x in the exhaust gas; wherein the oxidizing and NO.sub.x storing are performed as separate, sequential processes on the exhaust gas. [0022] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 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