| Exhaust system for a diesel engine comprising a nox- trap -> Monitor Keywords |
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Exhaust system for a diesel engine comprising a nox- trapRelated Patent Categories: Power Plants, Internal Combustion Engine With Treatment Or Handling Of Exhaust Gas, Methods, Anti-pollutionExhaust system for a diesel engine comprising a nox- trap description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060117736, Exhaust system for a diesel engine comprising a nox- trap. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The present invention relates to an exhaust system for a diesel engine, which system comprising a first nitrogen oxide (NO.sub.x)-trap comprising at least one first NO.sub.x absorbent and platinum. [0002] Exhaust gases from compression ignition engines, such as diesel engines, comprise a mixture of pollutants including carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NO.sub.x) and particulate matter (PM) including volatile and soluble organic fractions (VOF and SOF respectively). The NO.sub.x component can comprise nitrogen monoxide NO) and nitrogen dioxide (NO.sub.2). The level of these pollutants in exhaust gases from internal combustion engines it is permissible to exhaust to atmosphere is regulated by legislation. Such legislation can be met by engine design, engine management and/or exhaust gas after treatment, and typically a combination of all three measures. [0003] In conventional diesel engines, present legislated limits for HC and CO can be met using a platinum (Pt)-based diesel oxidation catalyst (DOC). Diesel nitrogen oxides (NO.sub.x) emissions are presently controlled by engine management, such as exhaust gas recirculation (EGR). As a consequence, however, PM emissions including VOF and SOF are increased. The DOC is used to treat VOF and SOF in order to meet presently legislated limits for PM. [0004] Another prior art exhaust system primarily for treating diesel exhaust comprises an oxidation catalyst for oxidising NO in the exhaust gas to NO.sub.2 and a downstream filter for trapping PM. A process for treating diesel PM that uses this arrangement is described in EP 0341382 or U.S. Pat. No. 4,902,487, both of which are incorporated herein by reference. The process comprises passing diesel exhaust gas including PM and NO unfiltered over an oxidation catalyst to convert the NO to NO.sub.2, collecting soot on the filter and combusting the collected soot by reaction with the NO.sub.2. This technology is commercially available as Johnson Matthey's Continuously Regenerating Trap or CRT.RTM.. Combustion of the PM in NO.sub.2 results in CO and NO, with a potential side-reaction leading to the complete reduction of the NO.sub.2 to N.sub.2 as described in SAE 890404. [0005] A problem with the process described in EP 0341382 and in the use of exhaust gas after treatment catalysts such as DOC is that, as exhaust emission legislation is tightened, legislative bodies have begun to discuss limiting the amount of NO.sub.2 it is permissible to exhaust to atmosphere. For example, the California Air Resources Board (CARB) has proposed that a maximum of 20% of tailpipe NO.sub.x of the relevant drive cycle is emitted as NO.sub.2 (see CARB's Diesel Risk Reduction Program and Final Regulation Order Chapter 14 "Verification procedure, warranty and in-use compliance requirements for in-use strategies to control emissions from diesel engines", section 2706). NO.sub.2 is toxic and can cause headaches, dizziness and nausea in low doses. It also has an objectionable smell. In the process of EP 0341832, if there is insufficient PM on the filter to react with NO.sub.2 generated over the oxidation catalyst or the temperature of the exhaust gas is below a preferred range for combustion of PM in NO.sub.2, NO.sub.2 can slip past the filter and be undesirably exhausted to atmosphere. [0006] At low exhaust gas temperatures, e.g. about 200-250.degree. C., and part-load, a modern diesel engine produces a relatively high amount of NO.sub.x in the form of NO.sub.2. If, as is typical in an exhaust system for a lean-burn internal combustion engine including a NO.sub.x-trap, the exhaust gas is first passed over a platinum oxidation catalyst, the NO.sub.2 can be removed by facile low temperature oxidation of hydrocarbons present in the exhaust gas. This process is known as lean NO.sub.x catalysis. In any event, the NO.sub.x absorbent material can poison the lean NO.sub.x activity of the platinum, thereby reducing the conversion efficiency of the process. At higher temperatures, e.g. above about 300-350.degree. C., the oxidation of NO to NO.sub.2 becomes sufficiently rapid for significant quantities of NO.sub.2 to be formed and a conventional platinum containing NO.sub.x-trap then starts to become effective. [0007] A NO.sub.x-trap is purposely designed for absorbing and storing NO.sub.x in lean exhaust gases (lambda >1 conditions), and releasing and catalytically reducing the stored NO.sub.x in rich exhaust gases (1 >lambda conditions). To remove the stored NO.sub.x typically the engine is controlled so that periodically it is run rich during normal lean-running operation. A NO.sub.x-trap typically includes active materials for three functions: an oxidation catalyst, such as platinum; a NO.sub.x absorbent, for example a compound of an alkali metal e.g. a potassium and/or a caesium, an alkaline earth compound typified by barium oxide or a compound of a rare earth metal, such as lanthamum; and a reduction catalyst, such as rhodium. In the exhaust gas the or each alkali metal and alkaline earth metal compound is present as an oxide, although it may also be present in use as a hydroxide or a carbonate. [0008] A convention used herein is to refer to a "NO.sub.x-trap" as a composition comprising a NO.sub.x absorbent and at least one catalytic material, such as a platinum group metal. NO.sub.x absorbent refers to a material, e.g. alkaline earth metal compound, alkali metal compound or rare earth metal compound, capable of absorbing NO.sub.2 in lean exhaust gas. [0009] An oxidation catalyst is purposely designed to provide for the reaction of gaseous components with oxygen, typically in as wide a temperature range as possible, especially at lower temperatures. The catalyst oxidises whenever oxygen is available for reaction in the gas stream Active components of an oxidation catalyst can include platinum, palladium or a base metal active for oxidation such as manganese, copper, molybdenum, cobalt or any other transition element that is active for oxidation. [0010] An oxygen storage component (OSC) is purposely designed to absorb oxygen from lean exhaust gases and to release oxygen in rich exhaust gas conditions. Examples of suitable OSC include ceria doped with transition metals, e.g. zirconium, or other rare earth metals and manganese-based materials. [0011] According to the present specification, "adsorb" and "absorb" in relation to the storage of NO.sub.x on suitable basic materials have the same meaning. [0012] Our WO 02/18753 (incorporated herein by reference) describes an exhaust system for a lean-burn internal combustion engine, including a diesel engine, comprising an oxidation catalyst upstream of a NO.sub.x-trap for oxidising relatively large amounts of unburned hydrocarbons present in exhaust gas in normal lean-running conditions and for oxidising NO in the exhaust gas to NO.sub.2 in lean-running conditions. In one embodiment, a clean-up catalyst comprising an oxygen storage component such as ceria, an oxidation component, such as platinum, a NO.sub.x reducing component, for example rhodium, and a component for suppressing H.sub.2S, for example NiO, Fe.sub.2O.sub.3, MnO.sub.2, CoO and CrO.sub.2, is disposed downstream of the NO.sub.x-trap. [0013] In WO 01/94760, we disclose an exhaust system for a diesel engine comprising a solid NO.sub.x absorbent for absorbing NO.sub.x from relatively cool exhaust gas and desorbing adsorbed NO.sub.x and passing it to atmosphere by intermittently increasing the temperature of the exhaust gas. [0014] We have now found that by using a NO.sub.x-absorbent which is free of platinum to absorb relatively low temperature engine-derived NO.sub.2 upstream of a NO.sub.x-trap comprising platinum when the NO.sub.x-trap is too cool to catalytically reduce the NO.sub.2 to N.sub.2 and to thermally release stored NO.sub.x when the NO.sub.x reducing function of the NO.sub.x-trap is up to temperature, the exhaust system as a whole is able to treat NO.sub.x more efficiently. More particularly, we have found that the system slips less NO.sub.2 to atmosphere than a system comprising a conventional NO.sub.x-trap comprising a NO.sub.x absorbent and platinum as the sole NO.sub.x absorbent-containing component. [0015] According to one aspect, the invention provides an exhaust system for a diesel engine, which system comprising a first NO.sub.x-trap comprising at least one first NO.sub.x absorbent and platinum, characterised in that at least one second NO.sub.x absorbent is disposed upstream of the first NO.sub.x-trap, which at least one second NO.sub.x absorbent is not associated with platinum. [0016] A further advantage of this system over the above-mentioned prior art system is that the system of the invention generates less N.sub.2O by lean NO.sub.x catalysis over the platinum component of the first NO.sub.x-trap. [0017] An aspect of the present invention is the concept of absorbing NO.sub.2 on the second NO.sub.x absorbent at relatively low temperature when the downstream first NO.sub.x trap is too cool to catalyse the reduction of NO.sub.x to N.sub.2, and thermally releasing the NO.sub.x when the first NO.sub.x-trap is up to temperature, e.g. >200.degree. C., such as >225.degree. C., >250.degree. C., >275.degree. C. or >300.degree. C. [0018] As is typical for engines including exhaust systems comprising NO.sub.x-traps, the engine can include an engine control unit (ECU) programmed, in use, intermittently to adjust to the exhaust gas composition to the rich side for regenerating the at least one first NO.sub.x absorbent. [0019] Whilst the at least one second NO.sub.x-absorbent can be associated with a base metal catalyst, e.g. a manganese compound, a cobalt compound or a copper compound, for oxidising nitrogen monoxide (NO) to nitrogen dioxide (NO.sub.2) in lean exhaust gas or a non-platinum platinum group metal, such as rhodium or iridium for reducing NO.sub.x to N.sub.2 in rich exhaust gas, or other non-platinum PGMs such as palladium, osmium and ruthenium, preferably the at least one second NO.sub.x-absorbent is free from components other than the NO.sub.x-absorbent and optional support. [0020] Each first and second NO.sub.x absorbent can be selected from alkaline earth metal compounds, alkali metal compounds or rare-earth metal compounds. Suitable alkaline earth metals include calcium, magnesium, strontium and barium Alkali metals can be potassium and/or caesium and rare earth metals can be cerium, yttrium or praseodymium. [0021] The first and second NO.sub.x absorbent can be supported on a suitable support, such as particulate alumina, silica, zirconia, titania, ceria or a mixture or composite oxide according to any two or more thereof, such as ceria-zirconia or alumina-silica "Composite oxide" as defined herein means a largely amorphous oxide material comprising oxides of at least two elements which are not true mixed oxides consisting of the at least two elements. [0022] Alternatively, the NO.sub.x-absorbent can comprise the support e.g. ceria per se or alumina per se. [0023] In use, compounds according to the invention can be oxides, although they may be present as nitrates, hydroxides or carbonates in exhaust gas containing NO.sub.x, CO and H.sub.2O. [0024] The first NO.sub.x-trap can be coated on a flow-through monolith, but in one embodiment, it is coated on a particulate filter. In a further embodiment according to the latter aspect, the particulate filter can include a DOC, optionally comprising supported platinum and/or palladium Alternatively, the first NO.sub.x trap can be disposed downstream of the filter, which arrangement adopting the process of EP 0341832 and disclosed in EP 0758713. Continue reading about Exhaust system for a diesel engine comprising a nox- trap... 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