WO2009089156A1

WO2009089156A1 - Mitigation of particulates and nox in engine exhaust

Info

Publication number: WO2009089156A1
Application number: PCT/US2009/030112
Authority: WO
Inventors: R. Samuel Boorse; Marius Vaarkamp
Original assignee: Basf Catalysts Llc
Priority date: 2008-01-07
Filing date: 2009-01-05
Publication date: 2009-07-16
Also published as: US20090173063A1

Abstract

An emission treatment system and associated method for treating an exhaust stream containing nitrogen oxides and particulate matter are disclosed. One embodiment of the system comprises a flow-through oxidation catalyst, a reductant injector downstream from the oxidation catalyst, a particulate filter downstream from the reductant injector, an SCR catalyst downstream from the particulate filter and an ammonia oxidation catalyst downstream from the SCR catalyst. An embodiment of the method comprises passing the exhaust gas stream through the oxidation catalyst, injecting a reductant into the exhaust gas stream, passing the exhaust gas stream through the particulate filter, passing the exhaust gas stream through an SCR catalyst, and passing the exhaust gas stream through an ammonia oxidation catalyst.

Description

MITIGATION OF PARTICULATES AND NOx IN ENGINE EXHAUST

FIELD OF THE INVENTION

[0001] Embodiments of the invention relate generally to exhaust treatment systems and methods. More particularly, embodiments of the present invention pertain to exhaust treatment systems and methods that efficiently mitigate both nitrogen oxides and particulate matter in emissions.

BACKGROUND

[0002] Both diesel engines and gasoline engines that run lean produce significant amounts of particulates in addition to NOx. Compression ignition diesel engines have great utility and advantage as vehicle power trains because of their inherent fuel economy and high torque at low speed. Diesel engines run at a high air to fuel ("A/F") ratio under very lean fuel conditions. Because of this, they have very low emissions of gas phase hydrocarbons and carbon monoxide. However, diesel exhaust is characterized by relatively high concentrations of nitrogen oxides ("NOx") and particulates. Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide ("CO"), unburned hydrocarbons ("HC") and nitrogen oxides, but also condensed-phase materials (liquids and solids) which constitute the so-called particulates or particulate matter. Emissions treatment systems for diesel engines must treat all of the components of the exhaust to meet emissions standards set by various regulatory agencies throughout the world.

[0003] The total particulate matter emissions of diesel exhaust contain three main components. One component is the solid, dry, solid carbonaceous fraction or soot fraction. This dry carbonaceous fraction contributes to the visible soot emissions commonly associated with diesel exhaust. A second component of the particulate matter is the soluble organic fraction ("SOF"). The SOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust. It is generally present as condensed liquids at the Standard particulate collection temperature of 52° C in diluted exhaust, as prescribed by a standard measurement test such as the U.S. Heavy Duty Transient Federal Test Procedure. These liquids arise from two sources: (1) lubricating oil swept from the cylinder walls of the engine each time the pistons go up and down; and (2) υnburned or partially burned diesel fuel. The third component of the particulate matter is the so-called sulfate fraction, which is formed from small quantities of sulfur components present in the diesel fuel. [0004] Catalyst compositions and substrates on which the compositions are disposed are typically provided in diesel engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For instance, oxidation catalysts, which may be referred to as diesel oxidation catalysts ("DOCs"), containing platinum group metals, base metals and combinations thereof, facilitate the treatment of diesel engine exhaust by promoting the conversion of both unburned HC and CO gaseous pollutants, and some proportion of the particulate matter through oxidation of these pollutants to carbon dioxide and water. Such catalysts have generally been disposed on various substrates (e.g., honeycomb flow through monolith substrates), which are placed in the exhaust of diesel engines to treat the exhaust before it vents to the atmosphere. Certain oxidation catalysts also promote the oxidation of NO to NO₂.

[0005] In addition to the use of oxidation catalysts, particulate filters are used to achieve high particulate matter reduction in diesel emissions treatment systems. Known filter structures that remove particulate matter from diesel exhaust include honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal filters, etc. However, ceramic wall flow filters, described below, receive the most attention. These filters are capable of removing over 90% of the particulate material from diesel exhaust. [0006] Typical ceramic wall flow filter substrates are composed of refractory materials such as cordierite or silicon-carbide. Wall flow substrates are particularly useful to filter particulate matter from diesel engine exhaust gases. A common construction is a multi- passage honeycomb structure having the ends of alternate passages on the inlet and outlet sides plugged. This construction results in a checkerboard-type pattern on either end of the honeycomb structure. Passages plugged on the inlet axial end are open on the outlet axial end. This permits the exhaust gas with the entrained particulate matter to enter the open inlet passages, flow through the porous internal wails and exit through the channels having open outlet axial ends. The particulate matter is thereby filtered on to the internal walls of the substrate. The gas pressure forces the exhaust gas through the porous structural walls into the channels closed at the upstream axial end and open at the downstream axial end. The accumulating particles will increase the back pressure from the filter on the engine. Thus, the accumulating particles have to be continuously or periodically burned out of the filter to maintain an acceptable back pressure. [0007] Catalyst compositions deposited along the internal walls of the wall flow substrate assist in the regeneration of the filter substrates by promoting the combustion of the accumulated particulate matter. The combustion of the accumulated particulate matter restores acceptable back pressures within the exhaust system. Soot combustion can be passive (e.g., with catalyst on the wall flow filter and adequately high exhaust temperatures), though for many applications active soot combustion is also required (e.g., production of a high temperature exotherm in the exhaust up-stream of the filter). Both processes utilize an oxidant such as O₂ or NO₂ to combust the particulate matter. During active regeneration, CO is sometime generated by the regeneration process. [0008] Passive regeneration processes combust the particulate matter at temperatures within the normal operating range of the diesel exhaust system. Preferably, the oxidant used in the regeneration process is NO₂ since the soot fraction combusts at much lower temperatures than those needed when O₂ serves as the oxidant. While O₂ is readily available from the atmosphere, NO₂ can be generated through the use of upstream oxidation catalysts to oxidize NO in the exhaust stream.

[0009] In spite of the presence of the catalyst compositions and provisions for using NO₂ as the oxidant, active regeneration processes are generally needed to clear out the accumulated particulate matter, and restore acceptable back pressures within the filter. The soot fraction of the particulate matter generally requires temperatures in excess of 500° C to burn under oxygen-rich (lean) conditions, which are higher temperatures than those typically present in diesel exhaust. Active regeneration processes are normally initiated by altering the engine management to raise temperatures in front of the filter up to 570-630° C. One common way that has been developed to accomplish active regeneration is the introduction of a combustible material (e.g., diesel fuel) into the exhaust and burning it across a flow-thru DOC mounted upstream of the filter. The exotherm from this auxiliary combustion provides the sensible heat (e.g. about 550-700° C) needed to burn soot from the filter in a short period of time (e.g. about 2-20 min.). Depending on driving mode, high exotherms can occur inside the filter when the cooling during regeneration is not sufficient (low speed/low load or idle driving mode). Such exotherms may exceed 800° C or more within the filter.

[0010] Current particulate filter systems that include the capability for active filter regeneration (soot combustion) under high temperature conditions (e.g., about 550-650° C) typically consist of a light-off DOC upstream of a downstream particulate filter, with a selective catalytic reduction ("SCR") catalyst disposed between the DOC and the particulate filter. However, an emission system that reduces both particulates and NOx can have several other configurations. For example, there are systems in which the NOx removal catalyst (e.g., SCR, LNT or LNC) is upstream as a separate device from the particulate filter. In other systems, the NOx removal catalyst is placed downstream as a separate device. Another configuration involves integration of the NOx catalyst and particulate removal, i.e. SCR, LNT, or LNC on a particulate filter. [OOllJ Certain conventional coating designs for wall flow substrates have a homogeneous distribution of catalyst along the entire axial length of the internal walls. In such designs the platinum group metal concentration is typically adjusted to meet the emissions requirements under the most stringent conditions. Most often, such conditions refer to the catalyst's performance after the catalyst has aged. The cost associated with the required platinum group metal concentration is often higher than is desired. [0012] Other conventional coating designs for wall flow substrates employ concentration gradients of catalyst along the axial length of the substrate. In these designs, certain catalyst zones (e.g., upstream zones) have a higher concentration of platinum group metals than do adjacent axial zones (e.g., downstream zones). Typically, the internal walls of the axial zone where the higher amount of catalyst is disposed will have a lower permeability than an adjacent zone having a lower washcoat loading. A gas stream passing along the length of the inlet passage will preferentially travel through the internal wall in the segments that have the highest permeability. Thus, the gas stream will tend to flow through the internal wall segments that have lower amounts of washcoat. This differential flow pattern can result in inadequate pollutant conversion. For instance, certain gaseous pollutants, e.g., unburned saturated hydrocarbons, require contact with higher concentrations of platinum group metal components than do unsaturated hydrocarbons to achieve sufficient levels of combustion. This requirement is exacerbated during operating conditions where the exhaust temperatures are cooler, e.g., at startup. [0013] A typical emission treatment system is shown in FIG. 1. The exhaust gas stream emitted from a diesel engine 10 travels through exhaust gas conduit 11. A hydrocarbon ("HC") injector 24 is provided downstream from the engine. The HC injector 24 can inject numerous compounds containing carbon and hydrogen upstream from oxidation catalyst 12 that mix with the exhaust gas stream from the engine 10. The oxidation catalyst 12, which in a diesel exhaust system is a diesel oxidation catalyst ("DOC") on a flow-through substrate, is located downstream from the HC injector 24. The exhaust gas stream passes through the oxidation catalyst 12, which promotes the oxidation of its constituent compounds and down to a soot or particulate filter 16. A reductant injector 14 is positioned downstream from the particulate filter 16 upstream from SCR catalyst 18. A mixer 17 is positioned upstream from the SCR catalyst to mix injected reductant. The system may further include an ammonia oxidation catalyst 20. [0014] As can be appreciated from the above, current systems pose a number of issues concerning size, in particular, the length of the system to accommodate the need for adequate mixing. Longer systems result in more heat loss, which is not good for catalysts efficiency. In addition, in SCR systems that inject urea or other reductants into the system, the mixer 17 is required to adequately mix the reductant. Mixers create turbulence, and turbulence leads to pressure drop. High backpressure adversely affects engine operation. Furthermore, in existing systems, not all of the urea decomposes to ammonia especially at lower temperatures, for example, below 300° C. Accordingly, it would be desirable to provide engine exhaust treatment systems and methods that alleviate one or more of these issues. SUMMARY

[0015] According to an embodiment of the invention, an emission treatment system for treating an exhaust stream containing NO_x and particulate matter is provided, comprising a flow-through oxidation catalyst, a reductant injector for introducing a reductant into the exhaust stream located downstream from the oxidation catalyst, a particulate filter that does not contain an SCR catalyst for removing the particulate matter from the exhaust stream located downstream from the reductant injector, an SCR catalyst for promoting chemical reactions between the nitrogen oxides and the reductant located downstream from the particulate filter, the particulate filter being effective to disperse the reductant prior to the reductant contacting the SCR catalyst. Tn one embodiment, an optional ammonia oxidation catalyst is located downstream from the SCR catalyst. Various reductants, including urea, can be utilized. In one embodiment, the particulate filter is uncatalyzed. An alternative embodiment provides a particulate filter that is catalyzed. In certain embodiments, the ammonia oxidation catalyst and a CO oxidation catalyst can be combined onto the same substrate and located downstream of the SCR catalyst. The function of the CO oxidation catalyst is to provide oxidation of CO generated during active regeneration.

[0016] In one or more embodiments, the particulate filter and the SCR catalyst are closely coupled. According to one embodiment, the particulate filter and the SCR catalyst are positioned inside a single canister. According to one or more embodiments, the reductant is urea and the particulate filter is configured so that substantially all of the urea that exits the particulate filter is converted to ammonia and isocyanic acid. [0017] Additional embodiments of the emission treatment system include a hydrocarbon injector for introducing hydrocarbon compounds into the exhaust stream located upstream of the oxidation catalyst and a metering system in fluid communication with the reductant injector. In certain embodiments of the invention, the SCR catalyst and the ammonia oxidation catalyst are located on the same substrate.

[0018] The particulate filter in at least one embodiment has a first catalyst zone for creating NO₂ from the oxidation of NO. According to one embodiment, the catalyst zone extends for at least a portion of the axial length of the filter. In other embodiments, the particulate filter also includes a second catalyst zone for the oxidation of CO. Furthermore, in certain embodiments of the invention, the particulate filter also comprises a urea hydrolysis catalyst and/or a base metal oxide for burning soluble organic fraction of soot captured in the filter. [0019] According to some embodiments of the invention, a method of treating an exhaust gas stream containing nitrogen oxides and particulate matter is provided. The method comprises passing the exhaust gas stream containing hydrocarbon compounds through a oxidation catalyst, injecting a reductant into the exhaust gas stream downstream from the oxidation catalyst and upstream of a particulate filter, passing the exhaust gas stream through the particulate filter to cause the reductant to mix with the exhaust gas stream, and passing the exhaust gas stream exiting the particulate filter through an SCR catalyst. In one embodiment, the method may optionally include passing the exhaust gas stream exiting the SCR catalyst through an ammonia oxidation catalyst. In another embodiment, the method optionally includes injecting hydrocarbon compounds into the exhaust gas stream upstream of the oxidation catalyst. In certain embodiments, injecting hydrocarbon compounds into the exhaust gas stream is regulated by a first control system. Additional embodiments employ a second control system to regulate the injecting of a reductant into the exhaust gas stream, The hydrocarbons from the first injector will be oxidized and generate and exotherm, and the heat generated could be used to heat the system quickly during start-up. Another other use of this heat is for active regeneration. In one embodiment, hydrocarbon is injected in the second injector and a hydrocarbon SCR catalyst is located downstream of the filter. [0020] In some embodiments of the method, passing the exhaust gas stream through the particulate filter comprises contacting the exhaust gas stream with a base metal oxide catalyst for burning soluble organic fractions or with a urea hydrolysis catalyst on the filter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Before describing several exemplary embodiments of the invention, it is to be understood that the invention is not limited to the details of construction or process steps set forth in the following description. The invention is capable of other embodiments and of being practiced or being carried out in various ways.

[0022] Figure 1 is a schematic illustration of an emission treatment system in accordance with the prior art; and [0023] Figure 2 is a schematic illustration of a of an emission treatment system in accordance with one embodiment of the invention.

DETAILED DESCRIPTION

[0024] Before describing several exemplary embodiments of the invention, it is to be understood that the invention is not limited to the details of construction or process steps set forth in the following description. The invention is capable of other embodiments and of being practiced or being carried out in various ways.

[0025] An embodiment of the inventive emission treatment system is shown in FIG. 2, The exhaust gas stream emitted from a diesel engine 110 travels through exhaust gas conduit 111. In one embodiment, an optional hydrocarbon ("HC") injector 124, which can assume various configurations, including that of a nozzle, is provided downstream from the engine. The HC injector 124 can inject numerous compounds containing carbon and hydrogen upstream from oxidation catalyst 112 that mix with the exhaust gas stream from the engine 110. A suitable control system selected from those known in the art can be employed to regulate the HC injection process For example, a closed feedback loop can be configured to operated based on the desired temperature behind the oxidation catalyst, A higher desired temperature can be achieved by increasing the amount of hydrocarbon injected. A temperature sensor can be utilized to determine whether the minimum temperature for injection is met. The oxidation catalyst 112, which in a diesel exhaust system is a diesel oxidation catalyst ("DOC") on a flow-through substrate is located downstream from the HC injector 124.

[0026] The exhaust gas stream passes through the oxidation catalyst 112, which promotes the oxidation of its constituent compounds and down to a soot or particulate filter 116. A reductant injector 114 is positioned upstream of the particulate filter 116. The filter 116 does not contain an SCR catalyst The system may further include an optional ammonia oxidation catalyst 120. The reductant injector 114 introduces aqueous urea into the exhaust gas stream. Water quickly evaporates from the urea to yield gaseous urea:

(NH₂)₂CO (aq) → (NH₂)₂CO (g) + H₂O (g)

The urea then decomposes to ammonia in two steps, the first of which is thermolysis of urea and the second of which involves hydrolysis of isocyanic acid: (NHs)₂CO → NH₃(g) + HNCO(g) HNCO (g) + H₂O(g) → NH₃ (g) + CO₂(g)

[0027] The gas stream exiting the oxidation catalyst is passed through particulate filter 116, Nearly all of the ammonia formed by the decomposition of urea passes through the filter 116. Unreacted urea accumulates on the large surface area of the filter 116, According to one or more embodiments of the present invention, the filter 116 prevents unreacted urea from traveling any further downstream. Unreacted urea may have a tendency to deactivate the SCR catalyst. Another function served by the filter 116 is to facilitate the mixing of ammonia and urea with the exhaust gas without increasing backpressure in the system. An introduction port or valve can be used to meter precise amounts of the reductant A suitable control system can be utilized to regulate the reductant injection process. Examples of control systems include a NOx sensor upstream of the injector (open loop control); a NOx sensor downstream of the SCR catalyst (closed loop control); predictive control based on engine conditions; and combinations of these systems.

[0028] The filter 116, which can be either catalyzed with a NOx oxidation catalyst or uncatalyzed, facilitates the mixing of the reductant and the exhaust gas stream. If the filter is catalyzed, a catalyst that does not cause the oxidation of ammonia should be utilized. SCR catalyst 118 is present downstream from the filter 116. In certain embodiments there is a gap 130 between the filter 116 and the SCR catalyst 1 18. In addition, certain embodiments of the filter 116 comprise a base metal oxide for burning soluble organic fraction of soot captured therein. Other embodiments comprise a urea hydrolysis catalyst. Optionally, the filter 116 and the SCR catalyst 118 can be closely coupled. For example, as depicted in FIG. 2, they can be positioned in adjacent "bricks" inside a single canister. A "brick" of material, such as cordϊerite or the like, is a portion of a honeycomb-type carrier member having a plurality of fine gas-flow passages extending from the front portion to the rear portion of the carrier member. Downstream from the SCR catalyst 118, the exhaust gas stream passes through an optional ammonia oxidation catalyst 120. The SCR catalyst 118 can be located on the same substrate as the ammonia oxidation catalyst 120. WALL FLOW SUBSTRATES

[0029] Wall flow substrates useful for filtration of particulate matter from exhaust streams have a plurality of fine, substantially parallel gas flow passages extending along the longitudinal axis of the substrate. Typically, each passage is blocked at one end of the substrate body, with alternate passages blocked at opposite end-faces. Such monolithic carriers may contain up to about 400 or more flow passages (or "cells") per square inch of cross section, although far fewer may be used. For example, the carrier may have from about 100 to 350, cells per square inch ("cpsi"). Wall flow substrates typically have a wall thickness between about 0.012 to 0.020 inches. Examples of suitable wall flow substrates have a wall thickness of between 0.012 and 0.015 inches. An exemplary aspect ratio for a filter is 0.75 to about 1,5,

[0030] Suitable wall flow filter substrates are composed of ceramic-like materials such as cordierite, ά-alumina, silicon carbide, silicon nitride, zirconia, mullite, spodumene, alumina-silica-magnesia, alumina-titanate, or zirconium silicate, or of porous, refractory metal. Wall flow substrates may also be formed of ceramic fiber composite materials. Examples of suitable wall flow substrates are formed from cordierite, alumina-titanate, and silicon carbide. Such materials are able to withstand the environment, particularly high temperatures, encountered in treating the exhaust streams.

[0031] Suitable wall flow substrates for use in the inventive system include thin porous walled honeycombs (monoliths) through which the fluid stream passes without causing too great an increase in back pressure or pressure across the article. Normally, the presence of a clean wall flow article will create a back pressure of I inch water column to 10 psig. According to embodiments of the invention, ceramic wall flow substrates used in the system are formed of a material having a porosity of at least 40% (e.g., from 50 to 75%) having a mean pore size of at least 5 microns (e.g., from 5 to 30 microns). In certain embodiments, the substrates have a porosity of at least 55% and have a mean pore size of at least 10 microns. The porous wall flow filter used according to embodiments of the invention can be catalyzed in that the wall of said element has thereon or contained therein one or more catalytic materials. Catalytic materials may be present on the inlet side of the element wall alone, the outlet side alone, both the inlet and outlet sides, or the wall itself may consist all, or in part, of the catalytic material. After coating with catalyst, the substrates are dried typically at about 100° C and calcined at a higher temperature (e.g., 300 to 450° C). After calcining, the catalyst loading can determined be through calculation of the coated and uncoated weights of the substrate. As will be apparent to those of skill in the art, the catalyst, loading can be modified by altering the solids content of the coating slurry. Alternatively, repeated immersions of the substrate in the coating slurry can be conducted, followed by removal of the excess slurry as described above.

DOC CATALYST COMPOSITIONS

[0032] The oxidation catalyst formed on the particulate filter can be formed from any composition that provides effective combustion of unburned gaseous and non-volatile hydrocarbons (i.e., the VOF) and carbon monoxide. In addition, the oxidation catalyst should be effective to convert a substantial proportion of the NO of the NOx component to NO₂. As used herein, the term "substantial conversion of NO of the NOx component to NO₂" means at least 20%, and preferably between 30 and 60%, Catalyst compositions having these properties are known in the art, and include platinum group metal- and base metal-based compositions. An example of oxidation catalyst composition that may be used in the emission treatment system contains a platinum group component (e.g., platinum, palladium or rhodium components) dispersed on a high surface area, refractory oxide support (e.g., γ-alumina) which is combined with a zeolite component (for example, a beta zeolite). A suitable platinum group metal component is platinum. 10033] Platinum group metal ("PGM") based compositions suitable for use in forming the oxidation catalyst may have a mixture of platinum, palladium, rhodium, and ruthenium and an alkaline earth metal oxide such as magnesium oxide, calcium oxide, strontium oxide, or barium oxide with an atomic ratio between the platinum group metal and the alkaline earth metal of about 1 :250 to about 1 :1 , and particularly about 1 :60 to about 1 :6. Catalyst compositions suitable for the oxidation catalyst may also be formed using base metals as catalytic agents.

[0034] The catalyst loading in the DOC can be varied to between about 40 g/ft³ and 100 g/ft³. In specific embodiments, the catalyst is can be chosen from Pt and/or Pd, both of which are good oxidation catalysts for hydrocarbons. The current price of platinum is much higher than for palladium, thus the latter offers the advantage of reduced cost; however, this may change in the future depending on catalyst demand. Platinum is very active for hydrocarbon oxidation reactions and is rather resistant to poisoning. Palladium can be less active and is susceptible to poisoning, e.g. by sulfur. However, under lean exhaust conditions and temperatures that might exceed 800° C, platinum can experience thermal sintering and thereby reduction in oxidation activity. Addition of palladium and its interaction with the platinum results in a substantial reduction in the high temperature sintering of the platinum and thereby maintenance of its oxidation activity. If the temperatures of exposure are kept low, Pt-only may be a good option to obtain the highest possible oxidation activity. However, in configurations in which high temperatures (e.g. 800° C) are anticipated, especially internal to the filter, inclusion of some Pd is desired. Pt:Pd ratios to obtain acceptable Pt stability with the highest oxidation activity are between about 10:1 and 4:1; however, ratios as low as 2:1 and 1 :1 are also within the scope of the invention. Higher Pd contents (e.g., 1 :2) need to be are also within the scope of the present invention. In certain embodiments, Pd with no platinum may be used.

[0035] The catalyst is dispersed on a suitable support material such as a refractory oxide with high surface area and good thermal stability such as a high surface area aluminum oxide. Suitable aluminas include aluminas stabilized with lanthana, for example 4 wt. % lanthana. Mixture of such aluminas in a 50:50 wt blend can be utilized as a suitable support material]. Other aluminas that are doped or treated with oxides such as SiO₂, ZrO₂, TiO₂, etc.) to provide stabilization or improved surface chemistries can also be utilized. Other suitable support materials, include, but are not limited to, ZrO₂ and TiO₂ can be used. In addition to the catalyst support oxides discussed above, it might prove useful to include other catalytically functional oxides to incorporate into the catalytic zone. Examples of these include CeO₂, Pr₆On, V₂O₅, and MnO₂ and combinations thereof and solid solution oxide mixtures, etc. These oxides can contribute to burning of hydrocarbons, especially heavy fuel derived hydrocarbons, and deposited coke/soot derived from disproportination of the injected fuel and in this way give additional combustion activity to the catalytic zone, plus prevent deactivation of the catalyst by the deposition hydrocarbon derived coke. FILTER CATALYST COATING

[0036] The loading of the oxidation catalyst in the zone on the filter substrate is typically is limited to control the contribution of the physical volume of the catalyst coating filling the pore volume of the filter substrate and thereby adversely affecting the flow resistance through the filter wall and thus the back-pressure. On the other hand, with high loadings of catalyst on the support oxide we have to provide sufficient surface area for good catalyst dispersion. As an example, a catalyst loading on the inlet zone of about 60 g/ft³, a dry gain (DG) of 0.5 g/in³ in the zone is acceptable. The DG can be adjusted taking into consideration the optimum catalyst loading, alumina to other (denser) oxide weight ratio, and other factors.

[0037] The ratio of the zone length/volume to total filter length/volume can vary between about 0.20 to 1.0, for example, this value can be 0,25, 0.5 or 0.75. Thus, for example, an 11.25" diameter x 14.0" long filter substrate a zone length/depth of ca. 3.0" could be used or a ratio of 0.21 of total length/volume of the filter. However, determination of the most effective zone length/volume ratio will be part of catalyzed filter optimization for a particular exhaust system design.

NOx REDUCTION CATALYSTS

[0038] For most US heavy duty diesel applications, starting in 2007, the after treatment system utilizes engine design and calibration. However, in the United States, particularly starting in 2010, stricter NOx emissions standards are not expected to be met by engine design and calibration measures alone and a NOx reduction after treatment catalyst will be required. The NOx reducing catalyst according to one or more embodiments of the invention can comprise an SCR catalyst, a lean NOx catalyst, a lean NOx trap (LNT), or a combination of these.

[0039] It should be noted that the engine-out NOx is mainly in the form of NO with low levels of NO₂ and that the catalyst loadings and ratios employed in the zone and body of the zoned particulate filter can be tailored to control the level of filter-out NO₂ versus NO. The oxidation reaction, represented by NO + 14 O₂ --> NO₂, can be controlled by the catalyst function. The effectiveness of the down-stream SCR or LNT can be enhanced by control of the N0₂/N0 ratio. [0040] For an SCR reaction, there three reaction conditions can be considered depending on the N0₂/N0 ratio:

[0041] (1) Standard : 4 NH₃ + 4 NO + O₂ --> 4 N₂ + 6 H₂O [0042] (2) "Fast": 4 NH₃ + 2 NO + 2 NO₂ -> 4 N₂ + 6 H₂O [0043] (3) "Slow": 4 NH₃ + 3 NO₂ ~> 3.5 N₂ + 6 H₂O.

[0044] From the above three conditions, it can be seen that the desired "fast" or more efficient SCR reaction occurs if the NO₂ to NO ratio is 1 :1 and relative to engine-out it is expected to require an oxidation function to increase the relative amount of NO₂. According to embodiments of the invention, the catalyst on the zoned particulate filter can contribute to this function and tailoring the catalyst loading and ratio can be used to achieve this. It is believed that the 1 :1 ratio will give the best down-stream SCR reaction. Higher levels of NO₂ are detrimental in that it gives a slower SCR reaction. For the LNT operation, it is necessary to oxidize engine-out NO as fully as possible to NO₂ as LNT's absorb NOx principally as nitrates. Tailoring the zoned-CSF's catalyst loading and ratio would achieve this.

[0045] Suitable SCR catalyst compositions for use in the system are able to effectively catalyze the reduction of the NOx component at temperatures below 600° C, so that adequate NOx levels can be treated even under conditions of low load which typically are associated with lower exhaust temperatures. Preferably, the catalyst article is capable of converting at least 50% of the NOx component to N₂, depending on the amount of reductant added to the system. Useful SCR catalyst compositions used in the inventive system also have should resist degradation upon exposure to sulfur components, which are often present in diesel exhaust gas compositions.

[0046] The NOx reducing catalyst may comprise a lean NOx catalyst. Lean NOx catalysts are typically classified as either a low temperature NOx catalyst or a high temperature NOx catalyst. The low-temperature lean NOx catalyst is platinum based (Pt- based) and does not have to have a zeolite present to be active, but Pt/zeolite catalysts appear to have better selectivity against formation of N₂O as a by-product than other catalysts, such as Pt/alumina catalysts. Generally, a low temperature lean NOx catalyst has catalytically active temperature ranges of about 180 to 350° C with highest efficiencies at a temperature of about 250° C. High temperature lean NOx catalysts have base metal/zeolite compositions, for example Cu/ZSM-5. High temperature NOx catalysts have a lower temperature range of about 300-350° C with highest efficiency occurring around 400° C. Different embodiments of the present invention use either high or low temperature lean NOx catalysts with an HC reductant.

[0047] The NOx reducing catalyst may comprise a lean NOx trap. In general, a lean NOx trap containing a combination of a NOx sorbent and an oxidation catalyst, which sorbs NOx onto the trap member during selected periods of time, e.g., when the temperature of the gaseous stream is not suited for catalytic lean NOx abatement. During other periods of time, e.g., when the temperature of the gaseous stream being treated is suitable for catalytic lean NOx abatement, the combustible component on the trap is oxidized to thermally desorb the NOx from the trap member. A lean NOx trap typically comprises a catalytic metal component such as one or more platinum group metals and/or a base metai catalytic metal component such as oxides of one or more of copper, cobalt, vanadium, iron, manganese, etc.

[0048] Reference throughout this specification to "one embodiment," "certain embodiments," "one or more embodiments" or "an embodiment" means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as "in one or more embodiments," "in certain embodiments," "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments.

[0049] Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the method and apparatus of the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include modifications and variations that are within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. An emission treatment system for treating an exhaust stream containing nitrogen oxides and particulate matter comprising: a flow-through oxidation catalyst; a reductant injector for introducing a reductant into the exhaust stream located downstream from the oxidation catalyst; a particulate filter that does not contain an SCR catalyst for removing the particulate matter from the exhaust stream located downstream from the reductant injector; and an SCR catalyst for promoting chemical reactions between the nitrogen oxides and the reductant located downstream from the particulate filter, the particulate filter being effective to disperse the reductant prior to the reductant contacting the SCR catalyst.

2. The emission treatment system of claim 1, wherein the particulate filter does not contain any catalyst.

3. The emission treatment system of claim 1, wherein the particulate filter and the SCR catalyst are closely coupled in the same canister.

4. The system of claim i, further comprising an ammonia oxidation catalyst located downstream from the SCR catalyst.

5. The emission treatment system of claim 1, wherein the particulate filter is catalyzed.

6. The emission treatment system of claim 1, wherein the reductant is urea and the particulate filter is configured so that substantially all of the urea that enters the particulate filter is converted to ammonia and isocyanic acid when it exits.

7. The emission treatment system of claim 1, further comprising a hydrocarbon injector for introducing hydrocarbon compounds into the exhaust stream located upstream of the oxidation catalyst.

8. The emission treatment system of claim 7, wherein the system further includes a metering system in fluid communication with the reductant injector.

9. The emission treatment system of claim 6, wherein the reductant comprises urea and water.

10. The emission treatment system of claim 1 , wherein the SCR catalyst and the ammonia oxidation catalyst are located on the same substrate.