DE102013207922B4 - Method for treating an exhaust gas flow with a silver-backed, closely coupled NOx absorber - Google Patents

Abstract

A method of treating the exhaust gas stream flowing from a vehicle internal combustion engine during a period of time after the engine has been cold started, the exhaust gas stream comprising a mixture of nitrogen monoxide, carbon monoxide, hydrocarbons, hydrogen, water, carbon dioxide, oxygen and nitrogen, the exhaust gas having an initial temperature of below 75 ° C and heats up progressively during further engine operation, the method comprising: continuously directing the hydrogen-containing and nitrogen monoxide-containing exhaust gas stream into contact with a silver / aluminum oxide material, the particles of silver or a silver oxide, supported on particles of alumina, wherein the silver content is in the weight range of 0.5% to 10% of the total weight of the particles of silver or silver oxide and alumina, the silver / alumina material being deposited on a substrate that carries the flow of the exhaust gas stream in contact with the silver / aluminum oxide Material to oxidize at least a portion of the nitrogen monoxide to nitrogen dioxide, and temporarily absorb some of the nitrogen oxide and nitrogen dioxide product on the silver / alumina material, the exhaust gas stream next flowing in contact with a different absorber material selected to to serve as a NOx absorber during the period after the cold start of the engine, wherein the silver / aluminum oxide material is arranged such that the exhaust gas flow leaves an exhaust manifold of the engine in a defined exhaust gas flow path and flows in contact with the silver / aluminum oxide material, after flowing a distance of no more than 50 centimeters along the flow path after exiting the exhaust manifold; thereafter continuously and thereafter passing the exhaust gas stream through at least one downstream reactor for further oxidation or reduction of constituents of the exhaust gas as the temperature of the exhaust gas increases and each downstream reactor is heated to an oxidation or reduction operating temperature, andcontinuing the passing of the exhaust gas in contact with the silver / alumina material, the different absorber material and through each reactor during the duration of the engine operation, the silver / alumina material releasing its absorbed nitrogen oxides and ceasing to oxidize NO in the exhaust gas stream when it passes through the exhaust gas stream its absorption temperature range for the nitrogen oxides is warmed; wherein the silver / aluminum oxide material resumes its NO oxidation function and its nitrogen dioxide absorption after an engine cool-down period and a subsequent engine cold start.

Description

TECHNICAL AREA

This disclosure relates to an improvement in the temporary storage of nitrogen oxides (NOx) that are absorbed from the exhaust gas of a diesel engine during the first few minutes after a cold start of the engine. This disclosure relates more specifically to the use of particulate silver / aluminum oxide materials (Ag or silver oxide particles supported on Al ₂ O ₃ particles) to remove NOx from diesel engine exhaust at a relatively low temperature, which also contains relatively small amounts of hydrogen and incomplete contains burnt hydrocarbons. The silver material is used in the method according to the invention in combination with other material that oxidizes NO and / or absorbs NOx, especially when the exhaust gas flow has heated the silver / aluminum oxide material above its effective operating temperature.

BACKGROUND OF THE INVENTION

Over the past several decades, automobile manufacturers have met continuously decreasing limits for the amounts of carbon monoxide, unburned hydrocarbons and nitrogen oxides (collectively NOx) that are released into the atmosphere in exhaust gas from vehicle engines. These requirements with regard to reduced exhaust gas emissions are combined with the requirements for greater fuel economy. These combined requirements have required increasingly sophisticated engines, computer control of engines, and exhaust treatment systems, including catalytic reactors, in the exhaust stream.

Current exhaust treatment systems are quite effective in treating the exhaust gas from a warmed-up engine because the catalyst materials have been heated to temperatures (e.g. 250 ° C and above) at which they serve to effectively oxidize carbon monoxide and incompletely burned fuel components and nitrogen oxides to reduce. These treatment systems are quite effective for both gasoline-powered engines operating at a stoichiometric air-to-fuel ratio and diesel engines (and other lean-burn engines that operate with a significant excess of air (sometimes referred to as "lean burn"). It has been difficult to Treat exhaust emissions immediately after an engine cold start, before the exhaust has heated the catalytic reactor or other processing vessel to the effective temperatures of the catalyst materials. It is realized that such untreated emissions make up a significant portion of the total emissions in contract engine exhaust systems. The problem is with the treatment especially difficult of mixed nitrogen oxides in the exhaust gas of diesel engines. These nitrogen oxides include nitric oxide (NO) and nitrogen dioxide (NO ₂₎ and generally lower amounts of other nitrogen oxides, said mixture typi is often referred to as NOx. There is therefore a need for better systems for treating the exhaust gas from an engine after a cold start. The need is particularly acute in lean burn engines such as diesel engines, which tend to produce cooler exhaust gas streams due to the excess of air used in the combustion mixtures that are fed to its cylinders .

From the EP 1 541 219 A1 a method is known in which NOx in exhaust gases from a lean-burn engine is reduced without the addition of gaseous reducing agents using a silver / transition metal catalyst. The low temperature reduction is preceded by the oxidation of NO to NO ₂ at 220 to 350 ° C using the same catalyst.

In the EP 2 223 735 A1 an exhaust gas aftertreatment system is described which comprises a reforming agent, an adsorbent and a cleaning agent. The reforming agent is arranged upstream of the adsorbent and the adsorbent comprises silver oxide as a catalyst and one or more oxides from the group consisting of aluminum oxide, silicon oxide, zirconium oxide, titanium oxide, cerium oxide and zeolite.

SUMMARY OF THE INVENTION

This disclosure provides and illustrates techniques for absorbing NOx from exhaust gas pumped from the exhaust manifold of a diesel engine in the short period of time after a cold engine start. The techniques of the invention are also applicable in the cold start period of lean burn gasoline engines and other hydrocarbon fuel burning internal combustion engines (whether they are compression ignited or spark ignited) which are operated to produce a combination of nitric oxide and hydrogen as part of their exhaust gas stream, especially after a Starting the engine when it is at ambient temperature (sometimes called a cold start) or below its warmed-up operating temperature and its exhaust gas has not yet warmed the vehicle's exhaust treatment systems to their operating temperatures.

It has been found that silver-based materials, which comprise particles of silver (or a silver oxide, for example Ag ₂ O) in nanometer size deposited on aluminum oxide particles with a high specific surface area (Ag / Al ₂ O ₃ ), oxidize nitrogen monoxide to nitrogen dioxide and nitrogen dioxide from a flowing stream of diesel exhaust at relatively low temperatures, for example from about 75 ° C to about 250 ° C. In the process of the invention, the silver / aluminum oxide material contains one-half percent by weight to ten percent by weight based on the total weight of the silver / Alumina material. This silver-based absorbent material oxidizes NO and absorbs NOx from exhaust gas, which contains a few hundred parts per million of hydrogen in addition to other nitrogen oxides, carbon monoxide, carbon dioxide, unburned hydrocarbons, oxygen, water and nitrogen. At such relatively low exhaust gas temperatures and in the presence of about 100 ppm to about 1,000 ppm or more hydrogen, the silver / alumina material serves to oxidize NO _{and absorb NO 2} , even in the presence of some lower molecular weight hydrocarbons. In fact, the silver / alumina material performs better than platinum group metals for this purpose and under these cold start conditions.

In preferred embodiments of the invention, the particulate silver / alumina material is used in combination with NOx adsorbers that are closely coupled to the exhaust manifold of the engine, including those currently for use in systems for trapping and reducing nitrogen oxides produced during a typical lean-burn diesel engine operation. It is preferred that the silver / alumina material be within about five to about fifty centimeters of the engine's exhaust manifold or of a turbocharger positioned on the exhaust manifold to return some of the exhaust flow to the engine. The silver / aluminum oxide material supports and complements the storage capacity of such closely coupled NOx adsorbers by _{supplying them with NO 2} for storage (instead of NO), since the NOx adsorbers _{can store NO 2} at low temperatures, but they can do not produce from NO in hydrocarbon-containing exhaust gas streams at temperatures below about 200 ° C.

In one system, the silver / alumina material is used in combination with a tightly coupled passive NOx adsorber material that is composed to adsorb and release NOx at relatively low temperatures. An example of the composition of such a passive NOx adsorber material (PNA) is a platinum group metal or a mixture of these (PGM). For example, mixtures of platinum and palladium particles supported on ceria or ceria / alumina mixtures can be used. Oxides of metals other than non-PGM-PNA materials can also be used; for example ceria-based oxides, for example magnesium-ceria mixed oxides and the like. Such a material is considered a “passive” adsorber as it does not require any active regeneration activity, for example the production of a temporarily “rich” exhaust gas stream. In this system, the silver / alumina material supports and complements the passive adsorbent material by oxidizing NO and storing NO ₂ in the hydrogen-containing exhaust gas at a relatively low temperature. _{The NO 2} produced by the silver / alumina is also stored by the PNA until the exhaust stream is hot enough to remove _{stored NO 2.} After the exhaust gas stream has passed through the silver / alumina material and the passive NOx adsorber, it is typically subjected to a reduction process to reduce the NOx to nitrogen. One widely used NOx reduction process involves injecting an aqueous urea solution into the exhaust gas stream (forming ammonia as a reducing agent) at an underfloor location in the exhaust gas flow path and then passing the ammonia-containing exhaust gas stream over a catalyst material selected to the ammonia-NOx reduction system to support or favor. This technique is known as ammonia selective catalytic reduction (NH ₃ -SCR). Accordingly, one technique for using the silver / alumina material in question is in combination with a PNA material followed by an appropriate SCR process. In this system, the silver / aluminum oxide material serves oxidation and storage functions during the first few minutes after a cold engine start. After that, warmed up exhaust gas goes through the material, which remains available for the next cold start of the engine. With each cold start cycle, the combined effect of the silver / alumina material and the PNA results in greater NO ₂ storage than any material acting alone.

In another exhaust gas treatment technique, the silver / alumina material is used in combination with a second type of NOx treatment system known as a trapping system. These NOx trap systems are often referred to as Lean NOx Traps or LNT. LNT systems typically include three component materials; an oxidation catalyst for oxidizing NO to NO ₂ , an adsorbent material for adsorbing NOx, and a reduction catalyst for reducing NO and NO ₂ to nitrogen. Platinum group element materials (PGM) supported on particles of alumina are often called uses the oxidation catalyst portion of an LNT system to oxidize _{NO to NO 2;} however, the conversion is low until the catalyst temperature is above 200 ° C. The adsorbent material of an LNT system is typically an oxide of one or more metals, for example barium, calcium, strontium, manganese, cerium, magnesium, potassium, sodium, lithium, cesium, lanthanum or yttrium. Palladium or rhodium is often used as the reduction catalyst. Typically, the operation of the LNT system requires periodic short periods of fuel-rich engine operation during which stored NO ₂ (or NOx) is released from its temporary storage and reduced to nitrogen. In accordance with the techniques of this invention, however, an appropriate amount of a silver / alumina material is used to aid and complement the function of the LNT materials, especially during periods of a few minutes after a cold start of a diesel engine. The silver / aluminum oxide particles are used _{to oxidize NO to NO 2} at exhaust gas temperatures of up to about 200 ℃ and _{to absorb the NO 2} , possibly as silver nitrate or aluminum nitrate compounds. The LNT material also stores NO ₂ produced on the silver / alumina material, increasing the total NO ₂ storage of the combination of silver / alumina and LNT. The silver / alumina material then releases the NOx materials as the warming engine exhaust continues to heat the material beyond its oxidizing and absorbing temperature range. Thus, one advantage of the combination of silver / alumina and LNT catalysts is to provide a wider temperature window to effectively store and release NOx during typical LNT operation.

In one embodiment of the invention, silver / alumina / absorbent is deposited as a washcoat on the walls of an extruded cordierite monolith body that has many parallel flow channels extending from an inlet side to an outlet side. The monolithic body is typically round or elliptical in cross section and has, for example, 400 channels per square inch of inlet side surface, each with a square or hexagonal cross section and corresponding walls extending the length of the body. The monolithic body can be contained in a suitable high temperature and oxidation resistant container having an exhaust gas flow inlet and a downstream outlet and can be closely coupled to the exhaust manifold of the engine. The amount of such silver-containing material is determined to provide the oxidation of NO and the temporary storage of NO ₂ (NOx) based on the exhaust gas flow and the warm-up time for the piston impact displacement of the diesel engine from which the exhaust gas is flowing. A representative space velocity for flow through washcoated channels of the monolith may be about 50,000 h ^-1 . In other embodiments of the invention, the silver / aluminum oxide particles can be supported on suitable metallic substrates and other wall-flow substrates.

The silver / aluminum oxide particles can be arranged in different ways in a flow-through monolith or other structure in order to complement other adsorbent material, whether PNA material or LNT material. The silver / aluminum oxide particles can be arranged upstream of the other adsorbent material in the exhaust gas flow, or the silver / aluminum oxide particles can be applied in combination with PNA or LNT material. In general, however, the silver material should be arranged in such a way that the exhaust gas flow with its hydrogen content hits the silver material before it hits the PGM material. For example, other PNA material (which may include or exclude platinum group material) can be applied as a first washcoat to duct walls of a cordierite extrusion, and the silver / alumina washcoat can be applied as a second layer that coincides with the conventional PNA layer is present in the same area and covers it. Accordingly, the silver / alumina absorbent material can be used in different places and in different arrangements to other PNA material or LNT material in the temporary storage of NOx for a subsequent further oxidation of NO and / or a subsequent reduction of the NOx when downstream exhaust gas treatment reactors are heated to their appropriate operating temperatures.

Other objects and advantages of the invention will become apparent from descriptions of illustrative embodiments that follow this specification.

Figure list

• 1 Figure 3 is a first schematic flow diagram using numbered blocks representing an arrangement of diesel exhaust treatment elements. Exhaust gas flowing out of the engine's exhaust manifold (block 10 ), first enters an extruded flow-through cordierite body (block 12th ), which is closely coupled to the exhaust manifold and _{coated with separate washcoats of Ag / Al 2} O ₃ + passive NOx adsorber + a diesel oxidation catalyst (DOC). A reducing agent material can be added to the exhaust gas (block 14th ), and the exhaust gas then flows through an SCR Reactor (block 16 ), which is located downstream in the exhaust gas flow and is arranged under the vehicle body. The exhaust gas flow then flows through an underbody diesel particulate trap (block 18th ) before exiting the vehicle's tailpipe.
• 2 Figure 13 is an oblique side view of a cylindrical stainless steel flow-through container including an extruded cylindrical cordierite body having many parallel channels, each channel having a square cross-section and extending from a flat exhaust gas flow inlet side of the body to a flat exhaust gas flow outlet side of the body. The four walls of each channel are coated with a thin washcoat of silver / alumina catalyst and other materials for use in accordance with this disclosure. In this illustration, the container and the cordierite body are each shaped as a round cylinder, and a portion of the circular container wall is broken away to show the cordierite body. And the outer wall of the cordierite body has broken away and the solid lines represent the outer channels.
• 3 FIG. 13 is a partial side elevational view of the top and bottom walls of a single channel of the cordierite body shown in FIG 2 is shown. The top and bottom wall fragments have been coated with a single washcoat of a functional exhaust treatment material that can be modified as described in this specification.
• 4th FIG. 14 is a partial side view of the top and bottom walls of a single channel of the FIG 2 shown cordierite body. The top and bottom wall fragments were coated with two superimposed and identical washcoat layers made of functional exhaust gas treatment materials, which can be modified according to this description.
• 5 Figure 13 is a second schematic flow diagram using numbered blocks to illustrate a second arrangement of diesel exhaust treatment elements. Exhaust gas flowing out of the engine's exhaust manifold (block 10 ), first enters a tightly coupled, extruded flow-through cordierite body _{coated with Ag / Al 2} O ₃ plus the component materials of an LNT (block 20th ). The treated exhaust gas then flows through a DOC reactor (block 22nd ) and a diesel particulate trap (block 24 ) before it emerges from the tailpipe.
• 6th is a bar graph showing the cumulative amounts of NO in milligrams (vertical axis 30th ) from a synthetic diesel exhaust stream, each at 150 ° C, from (from left to right on the horizontal axis, 32 ) a commercial PGM-based LNT material (data bar 34 ), a combination of Ag / Al ₂ O ₃ and LNT material (data bar 36 ) and a combination of Ag / Al ₂ O ₃ and MnCeOx (data bar 38 ) were saved. The following three bar data show the cumulative amounts of NO in milligrams that were each stored by the same three materials at 200 ° C (data bar 40 , 42 or. 44 ). The synthetic diesel exhaust stream consisted of 500 ppm H ₂ , 150 ppm NO, 1,000 ppm C1 hydrocarbons, 10% CO2, 8% O2, 8% H ₂ O and nitrogen and recorded the storage materials at a space velocity of 50,000 H ^-1 .
• 7th is a diagram of NO ₂ (in ppm) (vertical axis, 70 ), absorbed by a commercial DOC material in a synthetic stream that also contains 1,500 ppm hydrogen (curve 74 ), and absorbed by four samples of 5% Ag / Al ₂ O ₃ with 1,500 ppm H ₂ (curve 76 ), 750 ppm H ₂ (curve 78 ), 300 ppm H ₂ (curve 80 ) and 0 H ₂ (curve 82 ) at gas stream inlet temperatures from 100 ° C to 300 ° C (horizontal axis, 72).

DESCRIPTION OF PREFERRED EMBODIMENTS

Exhaust emissions from a vehicle engine running on a dynamometer are often evaluated by operating the engine according to a specified test procedure in which the engine is cold started and then accelerated and decelerated as prescribed. One such procedure is the US Federal Test Procedure 75 Cycle. If a representative light diesel engine according to the FTP 75 Cycle, more than 50% of the tailpipe emissions of NOx will be emitted during the first two test cycles after a cold start. It is an object of this invention to provide a method and silver-based catalyst system for use in reducing tailpipe NOx emissions during such periods of engine operation.

During a warm-up operation, such diesel engines typically produce hot exhaust gas with relatively high levels of oxygen, water and nitrogen oxides (NOx). In the case of diesel engines, the temperature of the exhaust gas is typically in the range of 50 to 150 ° C from a cold engine and 200 to 400 ° C from a warmed up engine (depending, for example, on the engine load) and it has a representative composition, by volume, of about 10% oxygen, 6% carbon dioxide, 5% water, 0.1% carbon monoxide, 180 ppm hydrocarbons, 235 ppm NOx (mostly NO) and the remainder essentially nitrogen. The Exhaust gas often contains some very small carbon-rich particles. And to the extent that the hydrocarbon fuel contains sulfur, the exhaust from the combustion source may also contain sulfur dioxide. It is desirable to treat such exhaust gas compositions to minimize the discharge of any substance other than nitrogen, carbon dioxide and water to the atmosphere. A representative value of the flow rate of such a vehicle exhaust stream in terms of the effective volume of exhaust treatment devices is, for example, 50,000 h ^-1 .

The NOx gases, which typically include varying mixtures of nitrogen oxide (NO) and nitrogen dioxide (NO2), are difficult to reduce _{to nitrogen (N 2 ) due to the high oxygen (O 2} ) content in the hot exhaust gas stream. It is stated that when a large portion of the NO is _{oxidized to NO 2} , there are selective catalytic reduction compositions and catalytic flow reactor concepts for reducing a large portion of the NO and NO ₂ in the hot exhaust gas to nitrogen before the exhaust gas is discharged from the vehicle exhaust system . Thus, in many exhaust gas treatment systems for lean burn engines, a suitable flow through oxidation catalyst body is suitably located close to the engine exhaust manifold to promote the efficient and timely oxidation of NO and CO and HC in the exhaust gas. A second catalyst material is located downstream of the oxidation catalyst reactor in the flowing exhaust gas stream for the reduction of a large part of the NO and NO ₂ to nitrogen and water. Sometimes a reducing material is added to the exhaust gas to allow the selective reduction reaction and the engine can be run repeatedly, but very briefly, in a fuel-rich mode to add small amounts of unburned fuel as a reducing agent for the nitrogen oxides.

Other techniques for treating diesel exhaust use an LNT system, as described above, to adsorb NOx during lean engine operation and to release and reduce NOx during short fuel-rich periods of engine operation.

When starting or starting the cold engine, these oxidation and reduction catalyst materials and LNT system materials often have to be heated from ambient temperature to their respective working temperatures by the exhaust gas flow. It is necessary to convert most of the carbon monoxide and unburned hydrocarbons to carbon dioxide and water and to convert most of the NOx to nitrogen and water during all stages of engine operation, including the period when the exhaust system is being heated.

Techniques of this invention take advantage of the inventors' discovery that hydrogen can be present in amounts of about 100 ppm to 1,000 ppm or more in the exhaust gas of the diesel engine after a cold engine start. The engine is typically operated under a computerized engine governance system to manage the timing and amount of fuel injection and air flow, and during periods after a cold start, hydrogen can be caused to be present in the diesel engine exhaust during this stage of vehicle operation. The silver / alumina material used in the techniques of this invention takes advantage of the presence of appropriate (but relatively small) amounts of hydrogen in combination with the silver catalyst and storage material located near the engine's exhaust manifold To favor oxidation of NO to NO ₂ at exhaust temperatures in the range of 75 ° C to about 250 ° C and to temporarily _{complement PNA material or LNT material in the storage of some of the mixture of NO and NO 2} until the exhaust gas Silver material is heated above its best working temperature and other downstream waste gas treatment reactors take over the waste gas treatment functions. The silver / aluminum oxide material supports the conversion of NO to NO ₂ , which can be stored on PGM and non-PGM NOx adsorbers at temperatures below 200 ° C. Without silver / alumina, a NOx adsorbent material cannot store NO (unless it is very high in PGM), and it does not efficiently convert _{NO to NO 2 below 200 ° C in a hydrocarbon-containing feed.}

1 Figure 13 is a schematic flow diagram of a diesel exhaust system that is also useful for removing particulate matter from the exhaust. The relatively cold exhaust gas pumped out of the exhaust manifold of an engine such as a diesel engine by piston action is in 1 in block 10 displayed. Closely coupled to a short section of an exhaust pipe (for example, no more than about 5 centimeters to about 50 centimeters in length) to the exhaust manifold of a vehicle with a diesel engine is a flow-through reactor (block 12th ), for example an extruded cordierite reactor as described above in this specification. In techniques of this invention, the walls of the channels of this reactor vessel (or vessels) are washcoated with 5 wt% silver on activated alumina material, a conventional passive NOx adsorber material, and a conventional DOC material. When the NOx adsorber material is a non-platinum group metal material, for example MnCeOx, it becomes a DOC for further oxidation of exhaust gas stream components to be required. If the NOx adsorber is a platinum group material, it can also serve as a DOC. The function of this first stage reactor is _{to promote the oxidation of NO to NO 2} and the temporary storage of NO ₂ and other NOx components. The warmed up DOC material also serves to oxidize carbon monoxide to carbon dioxide and to oxidize unburned hydrocarbons to carbon dioxide and water.

In the exhaust gas flow treatment system this 1 comprise additional downstream reactors (i) means for injecting a reducing agent for NOx, for example an aqueous urea solution, into the exhaust gas flow (block 14th ) for an ammonia-selective catalytic reduction of NOx (SCR), (ii) an ammonia-SCR reactor (block 16 ) for the chemical reduction of NOx to N ₂ and (iii) an underfloor device for trapping and removing diesel particulate matter (PM, block 18th ) from the exhaust before the exhaust is discharged into the atmosphere through the tailpipe. An underbody DOC (in 1 not shown) is typically located just upstream of the PM unit to periodically burn injected raw fuel injected into the exhaust gas at the DOC so as to heat the exhaust gas to remove particulate carbon from the PM. These downstream exhaust gas treatment reactors are conventional in construction. However, the close-coupled reactor utilizes the silver / alumina material of this invention and requires further discussion.

In an in 1 The exhaust gas treatment sequence shown can be used in the close-coupled oxidation and storage flow-through reactor (through block 12th be coated with active materials in various ways. The oxidation and storage reactor will comprise a suitable arrangement of a silver / alumina material according to the invention, a conventional passive NOx adsorber material (PNA) and a diesel oxidation catalyst material (DOC). The DOC material is necessary for the oxidation of HC and CO in particular when the PNA material is a non-PGM composition. When the PNA is a PGM composition, it can also serve as a DOC.

An illustration of a suitable catalytic reactor 50 for the inclusion of a silver / aluminum oxide catalyst in the exhaust gas flow of a diesel engine is in 2 shown. The reactor 50 can be a round tubular stainless steel body 52 for tightly enclosing an extruded, round, cylindrical, honeycomb-shaped cordierite catalyst carrier body 54 that in two broken out windows in the side of the body 52 can be seen include. Catalyst carrier 54 may be formed from any other known and suitable high temperature resistant metal or ceramic material. In this embodiment, the cordierite catalyst support body 54 formed with many exhaust gas flow passages extending from an upstream exhaust gas inlet side 56 of the carrier body 54 through the length of the body to a downstream exhaust gas outlet side (in 4th not visible) of the body 54 extend. For example, 400 flow channels per square inch inlet side are typically formed during the extrusion of the ceramic body. The walls of these small flow channels are shown as crossing lines in the representation of the exhaust gas flow inlet side 56 shown. A silver-on-alumina particle catalyst, a PNA material and a DOC catalyst are each in the form of a particulate washcoat on the walls of each of the channels of the extruded ceramic support body 54 applied using techniques disclosed in the specification below. The diameter of the steel body 52 and the enclosed silver-based oxidation catalyst carrier body 54 is enlarged with respect to the upstream and downstream exhaust ducts so as to reduce the draft on the exhaust stream as it enters the inlet side 56 of the silver catalyst carrier body enters and flows through the channels coated with washcoat. Carrier body 54 is inside the steel body 52 sealed so that the exhaust gas flow is in contact with the silver-aluminum oxide washcoat on the duct wall surfaces of the carrier body 54 is steered. The catalyst carrier body is dimensioned with sufficient channel wall surface in such a way that it carries sufficient washcoat material to ensure sufficient catalyst contact with a flowing exhaust gas during its residence time in the reactor 50 provide.

As in 2 seen is the upstream end of the steel casing body 52 (as indicated by arrow indicating the exhaust gas flow 58 shown) from a flared stainless steel exhaust inlet section 60 locked in. Exhaust gas inlet 62 of the exhaust gas inlet section 60 is dimensioned and adapted in such a way that the exhaust gas flow from an exhaust pipe (in 4th not shown), which is closely coupled to the exhaust manifold of a diesel engine or other lean-burn engine.

Similarly, the downstream end is (exhaust gas flow arrow 64 ) of the steel housing body 52 through a converging exhaust outlet section 66 with an exhaust outlet 68 locked in. Outlet 68 is adapted to be welded or otherwise connected to an exhaust line to convey the exhaust to a further downstream treatment reactor, for example a DOC reactor or an SCR reactor.

3 represents a section of part of the length of a single channel 57 which is one of the many open parallel channels for exhaust gas flow extending from the inlet end 56 of the cordierite body 54 to the outlet (in 2 not visible) of the body 54 extends. However, the exhaust gas flow is received through the full length of each of the many channels. In the cross section of 3 are part of the length of the top wall 59 and part of the length of the bottom wall 61 from canal 57 to see. These thin walls 59 and 61 are molded from extruded and calcined cordierite composition. And each wall typically has an opposite side that acts as the wall side for another duct in the body 54 serves. On the top wall 51 and the bottom wall 61 from canal 57 there is a layer of washcoat material to perform a treatment function on the exhaust gas flowing through the duct. Although in 3 not shown are the exterior surfaces of the walls 59 and 61 when channel 57 an inner channel is coated with layers of washcoat material. Each type of washcoat material described in this specification is formed from an aqueous slurry that adheres to the walls of the channels of the body 54 deposited and then dried and calcined in place to form a thin adhesive coating on each of the four walls of the channels.

In a first embodiment, the Ag / Al ₂ O ₃ material, a non-PGM-PNA material, and a DOC material can be applied as separate washcoats in progressive downstream order on the channel walls of a single extruded cordierite body 54 (sometimes referred to as a "building block"), as shown in 2 is shown.

In other words, the silver / aluminum oxide material is as a sheet-like layer 63 on the canal walls leading to the inlet 56 closest to the body, the PNA material is applied as a sheet-like layer 63 Applied on channel walls in the central portion of the body and the DOC material is as a sheet-like layer 63 applied near the outlet of the cordierite brick. In other arrangements, the three coatings can be distributed as individual layers over two or three building blocks.

Other arrangements of the materials applied as washcoats can be used, where they are distributed as washcoats on the channel walls of a single building block or cordierite body. If the PNA material does not include platinum group materials, the three materials can be applied as two layers or as a single layer on the walls of a single body. 4th Figure 3 is a schematic representation of part of a channel 57 ' with top wall 59 ' and bottom wall 61 ' each with a first applied washcoat 65 bonded to the wall surfaces and a later applied washcoat 67 working on the first washcoat 65 adheres, wear. In a two-layer arrangement, the DOC material can be applied extensively to the walls of the building block as a first washcoat material, and a mixture of the Ag / Al ₂ O ₃ material and the PNA material can be applied as a second washcoat layer over the DOC layer must be applied. In another arrangement, a mixture of the three materials is applied as a single layer to the channel walls of the cordierite body. In each of these arrangements, the Ag / Al ₂ O ₃ material completes the function of the PNA material during the period a few minutes after an engine cold start by preventing the oxidation of NO to NO ₂ and the temporary storage of the NO ₂ in a hydrogen-containing one Performs diesel exhaust at low temperature. Again, it is emphasized that in most of the channels of a cordierite body, washcoat layers are formed on both sides of each channel wall.

5 Figure 13 is a block flow diagram illustrating the use of Ag / Al ₂ O ₃ material in a different arrangement of diesel exhaust treatment components. Exhaust gas that emerges from the exhaust manifold of an engine after a cold start (block 10 ), enters a reactor (block 20th ) comprising an Ag / aluminum oxide material in combination with LNT materials. The Ag / Al ₂ O ₃ material is used in combination with a Lean NOx Trap Reactor (LNT). In this example the Al / Al ₂ O ₃ material and LNT (block 20th ) upstream in the exhaust gas flow from an underbody diesel particulate filter (DPF, block 24 ) is used. A DOC (block 22nd ) can also be immediately upstream of the DPF (block 24 ) can be used to generate elevated temperatures for the removal of particulate matter from the DPF. For example, raw fuel can be found on the DOC (block 22nd ) so that it burns the fuel with oxygen in the lean exhaust gas to heat the exhaust gas to regenerate the DPF. Such heating is done well downstream of the silver / alumina material and the LNT.

However, in this embodiment of the invention, the Ag / Al ₂ O ₃ material is used in combination with the three materials of an LNT device - the DOC material, the NO ₂ adsorber material and the NOx reduction material. The Ag / Al ₂ O ₃ material is used when hydrogen is present in the exhaust gas after starting a cold diesel engine. Again, a washcoat of particles of a suitable Ag / Al ₂ O ₃ material, about 0.5 to contains about 10% silver, based on the total weight of the silver and the alumina, is used. _{The NO 2} produced on the silver / alumina and NOx stored thereon are eventually released from the silver / alumina. The NOx is stored on the LNT and converted to nitrogen during fuel-rich pulse engine cycles.

Accordingly, LNT typically serves as a NOx adsorber, converter of NOx to N ₂ and DOC. Much of the time during operation of the warmed-up engine, the LNT is exposed to the hot lean exhaust gas from the engine and serves its adsorbing and oxidizing functions in exhaust gas containing oxygen. However, engine operation is periodically adjusted by an engine management computer to burn fuel-rich to produce reductant in the exhaust so that the LNT can convert stored NOx to nitrogen. In certain exhaust systems, some of the NOx in the exhaust is converted to ammonia instead of N ₂ by LNT. Optionally, an underfloor urea SCR catalytic converter can be installed downstream of the LNT (in 5 not shown) to convert the ammonia and residual NOx into N ₂ .

When two cordierite building blocks are used, or when an upstream portion and a downstream portion of a single building block are used, the Ag / Al ₂ O ₃ material is used upstream of the separate LNT materials. In this manner, the Ag / Al ₂ O ₃ material is first exposed to the relatively low temperature exhaust gas and serves to oxidize _{NO to NO 2 and release NO 2} for later release to the LNT and other downstream exhaust gas flow treatment bodies. However, how further in relation to 5 is disclosed, the Ag / Al ₂ O ₃ material can be used with LNT materials as a washcoat on a single cordierite building block as follows.

The LNT materials can be deposited as a first washcoat layer on the channel walls of the building block and then a suitable Ag / Al ₂ O ₃ material can be deposited as a second layer (as in 4th shown) are deposited in the same area as the underlying layer of LNT materials. Alternatively, the Ag / Al ₂ O ₃ material and the LNT materials can be mixed and deposited as a single layer on the walls of a single cordierite brick, as shown in FIG 3 is shown.

The utility of using the Ag / Al ₂ O ₃ material to treat NOx in a relatively cold diesel exhaust stream is illustrated in the data presented in FIG 6th are presented, explained. 6th is a bar graph showing the cumulative amounts of NO in milligrams (vertical axis 30th ) shows that have been stored and that have been converted to NO ₂ by (from left to right on the X-axis, 32 ) a commercial PGN-based LNT material ( 34 ), a combination of Ag / Al ₂ O ₃ and the LNT material ( 36 ) and a combination of Ag / Al ₂ O ₃ and MnCeOx ( 38 ). The exhaust gas feed was as follows: 1,000 ppm C1-HCs (2: 1 unburned to partially oxidized HCs, unburned is 2: 1 dodecane to n-xylene, and partly burned is 2: 1 propylene to propane), 150 ppm NO, 500 ppm H ₂ , 8% O ₂ , 10% CO ₂ , 8% water, 1,500 ppm CO and nitrogen. In these three tests, the temperature of the synthetic exhaust gas stream inlet was 150 ° C.

This data shows that when a conventional LNT (barium-based at 145 g / ft ³ PGM) material is used to oxidize NO and adsorb NOx in a synthetic exhaust stream at 150 ° C, only about 30 milligrams of NO were saved. When the Ag / Al ₂ O ₃ material was combined with the LNT material, the storage of NO increased to nearly 500 milligrams of NO. And when the Ag / Al ₂ O ₃ material is combined with a non-PGM based NO adsorber, MnCeOx, the combination is almost as effective as the combination of the Ag / Al ₂ O ₃ material with the commercial PGM based LNT material.

The fourth ( 40 ), fifth ( 42 ) and sixth ( 44 Bar data represent the cumulative amounts of NO stored or converted to N ₂ by the same three materials at 200 ° C, in milligrams. This temperature approaches the normal operating temperature range of a conventional LNT material. The same pattern of improved storage and release is seen by the Ag / Al ₂ O ₃ material.

7th is a graph of NO ₂ (in ppm) (vertical axis, 70 ) produced by a commercial DOC in a synthetic stream (similar to that with the data from 6th , at 50,000 h ^-1 SV), also containing 1,500 ppm hydrogen (curve 74 ), and by four samples of 5% Ag / Al ₂ O ₃ with 1,500 ppm H ₂ (curve 76 ), 750 ppm H ₂ (curve 78 ), 300 ppm H ₂ (curve 80 ) or 0 H ₂ (curve 80 ) at gas stream inlet temperatures from 100 ° C to 300 ° C (horizontal axis, 72). It can be seen that when hydrogen (at 300 to about 1,500 ppm) is present in low temperature diesel exhaust, the Ag / Al ₂ O ₃ material of this invention is very effective _{in oxidizing NO to NO 2.} The Ag / Al ₂ O ₃ material of this invention is equally _{effective in temporarily storing the NO 2} until downstream selective reduction catalysts are warmed to their operating temperatures.

Accordingly, a silver / alumina catalyst is _{very effective in oxidizing NO to NO 2} and temporarily storing NOx in low temperature diesel exhaust streams where the exhaust gas contains relatively small amounts of hydrogen. _{In addition, the NO 2} generated by the silver / aluminum oxide can be stored by both a PNA material and an LNT. This is important because neither conventional PNA material nor LNT will oxidize NO below about 200 ° C unless they contain very high levels of platinum, which levels are prohibitively expensive. The silver content of the silver / aluminum oxide catalyst in the process according to the invention is in the range from 0.5% to 10%. This silver-based catalyst and NOx absorber provides a very effective complement for PNA materials in general and the PNA compositions in LNT material combinations. Engine operation can easily be regulated during such cold start periods so that hydrogen from the combustion of diesel fuel is provided for such temporary oxidation and storage functions of the silver catalyst material.

Claims (7)

A method of treating the exhaust gas stream flowing from a vehicle internal combustion engine during a period of time after the engine has been cold started, the exhaust gas stream comprising a mixture of nitrogen monoxide, carbon monoxide, hydrocarbons, hydrogen, water, carbon dioxide, oxygen and nitrogen, the exhaust gas having an initial temperature is below 75 ° C and heats up progressively during continued engine operation, the method comprising: continuously passing the hydrogen-containing and nitrogen monoxide-containing exhaust gas stream into contact with a silver / aluminum oxide material which comprises particles of silver or a silver oxide supported on particles of aluminum oxide, the silver content in the weight range from 0.5% to 10% of the total weight of the particles of silver or silver oxide and aluminum oxide, the silver / aluminum oxide material being deposited on a substrate which receives the flow of the exhaust gas stream in contact with the silver / aluminum oxide material to convert at least a portion of the nitric oxide to nitrogen dioxide oxidize, and temporarily absorb some of the nitrogen oxide and nitrogen dioxide product on the silver / alumina material, the exhaust gas stream next flowing in contact with a different absorber material selected to act as a NOx absorber during the period after the engine cold start serve, the silver / aluminum oxide mat erial is arranged so that the exhaust gas flow leaves an exhaust manifold of the engine in a defined exhaust gas flow path and flows into contact with the silver / aluminum oxide material after it has flowed a distance of not more than 50 centimeters along the flow path after having passed the exhaust manifold has left; then continuously and then Passing the exhaust gas stream through at least one downstream reactor to further oxidize or reduce constituents of the exhaust gas as the temperature of the exhaust gas increases and heats each downstream reactor to an oxidizing or reducing operating temperature, and Continuing to pass the exhaust gas into contact with the silver / alumina material, the different absorber material, and through each reactor for the duration of the engine operation, the silver / alumina material releasing its absorbed nitrogen oxides and ceasing to oxidize NO in the exhaust stream when it is heated by the exhaust gas stream above its absorption temperature range for the nitrogen oxides; wherein the silver / aluminum oxide material resumes its NO oxidation function and its nitrogen dioxide absorption after an engine cool-down period and a subsequent engine cold start. Method for treating the exhaust gas from a vehicle internal combustion engine according to Claim 1 wherein the exhaust gas containing hydrogen and nitrogen monoxide is in contact with the silver / alumina material and thereafter in contact with a platinum group element material which is composed to absorb nitrogen dioxide during the cold start period and to start oxidation of NO when it passes through the flowing exhaust gas is heated to a temperature of 190 ° C. or higher, the platinum group element material being a mixture of platinum particles and palladium particles deposited on carrier particles of ceria or particles of mixtures of ceria and alumina. Method for treating the exhaust gas from a vehicle internal combustion engine according to Claim 1 wherein the hydrogen content of the exhaust gas stream is in the range of 100 ppm to 1,000 ppm for a period after the engine has been cold started and the exhaust gas containing hydrogen and nitrogen monoxide is in contact with the silver / alumina material and thereafter in contact with a passive absorbent -Material which is composed so that it absorbs nitrogen dioxide during the cold start period and releases the nitrogen dioxide when it has been heated by the flowing exhaust gas stream to a temperature of 190 ° C or higher, the passive absorbent material being passed at least one of a cerium oxide and a manganese oxide. Method for treating the exhaust gas from a vehicle internal combustion engine according to Claim 1 , with the engine of the vehicle using Internal combustion engine is a diesel engine that is regulated to operate on an air to fuel ratio of 17: 1 for the majority of its operation. Method for treating the exhaust gas from a vehicle internal combustion engine according to Claim 1 wherein the exhaust gas stream flows in contact with the silver / alumina material, a passive NOx absorber material and with a selective reducing agent for the NOx. Method for treating the exhaust gas from a vehicle internal combustion engine according to Claim 1 wherein the exhaust gas flows in contact with the silver / alumina material and then with the constituents of a lean NOx trap, the constituents of the lean NOx trap being an oxidation catalyst for the oxidation of NO to NO ₂ , an adsorbent material for adsorbing NOx and a reduction catalyst for reducing NO and NO ₂ to nitrogen. Method for treating the exhaust gas from a vehicle internal combustion engine according to Claim 6 , wherein the oxidation catalyst comprises platinum particles supported on aluminum oxide particles, the adsorbent material comprises an oxide of one metal or more metals selected from the group consisting of barium, calcium, cerium, cesium, lanthanum, lithium, magnesium, manganese, potassium, Sodium, strontium or yttrium, and the reduction catalyst comprises palladium or rhodium.

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