Ammonia Slip Catalyst Filter

Abstract

A catalyzed wall-flow filter for treating exhaust gases is disclosed, comprising at least first, second, and third catalyst layers, the first catalyst layer extending from an inlet end of a substrate and comprising a first SCR composition, the second catalyst layer disposed in or on the walls of inlet channels extending from the inlet end of the substrate and comprising a PGM-containing composition, and the third catalyst layer disposed in or on the walls of outlet channels extending from the outlet end of the substrate and comprising a second SCR composition.

Description

[Technical Field]

[0001] The present invention relates to a catalyst article for use as an ammonia slip catalyst. In particular, the ammonia slip catalyst is provided on a wall-flow filter. The present invention further relates to an exhaust gas treatment system comprising the catalyst article and its use. The catalyst article is designed to address particulate emissions resulting from the upstream addition of a reducing agent, such as ammonia or urea, while maintaining good exhaust treatment performance. [Background technology]

[0002] Exhaust gases produced by lean-burn and diesel engines are generally oxidizing. NO x NOx must be selectively reduced using a catalyst and a reducing agent in a process known as selective catalytic reduction (SCR), which converts NOx to elemental nitrogen (N2) and water. In the SCR process, a gaseous reducing agent, usually anhydrous ammonia, aqueous ammonia, or urea, is added to the exhaust gas stream before the exhaust gas is contacted with a catalyst. The reducing agent is absorbed on the catalyst, and NO x NO is reduced as the gas passes through or over the catalyzed substrate. x To maximize the conversion of ammonia, it is often necessary to add more than the stoichiometric amount of ammonia to the gas stream. However, releasing excess ammonia into the atmosphere is harmful to human health and the environment. Furthermore, ammonia is caustic, especially in its aqueous form. Condensation of ammonia with water in the region of the exhaust line downstream of the exhaust catalyst can create a corrosive mixture that can damage the exhaust system. Therefore, it is necessary to eliminate the release of ammonia in the exhaust gas.

[0003] Even the best SCR catalysts produce the highest NO emissions in systems with uneven NH3 distribution. x The reduction will not be achieved. x The significant variation in concentration is due to the 1:1 ammonia / NO ratio required by the reaction stoichiometry. xThis makes it difficult to deliver NH3 to the catalyst at a low ammonia-NOx ratio (ANR). Uneven NH3 distribution leads to incomplete NOx production when the localized ANR is low. x conversion and may result in NH3 slip if the ANR is high.

[0004] In many conventional exhaust systems, an ammonia oxidation catalyst (also known as an ammonia slip catalyst or "ASC") is provided downstream of the SCR catalyst to remove ammonia from the exhaust gas by converting it to nitrogen. Ammonia slip catalysts are well known in the art. The purpose of the ammonia slip catalyst is to treat any ammonia that passes through the upstream SCR catalyst.

[0005] To overcome the difficulty of achieving ideal stoichiometry, ASC technology combines oxidation and SCR catalytic functions on a flow-through (FT) substrate to achieve NO while maintaining low NH3 slip. x ASC allows continuous operation of the upstream SCR at higher ANR, compensating for uneven NH3 distribution and improving NO reduction while maintaining low NH3 slip. x Increase conversion rate (on upstream SCR).

[0006] In ASC, the oxidation catalyst and the SCR catalyst can be coated on a substrate according to different design strategies, one of the most common being the so-called "two-layer ASC" design. The two-layer ASC design is based on the sequential coating of two layers (a PGM layer and an SCR layer) on the walls of the flow-through channel. A functional ASC design will convert only a small portion of the ammonia available in the exhaust in the oxidation layer. The remaining ammonia fraction will then be used as a reducing agent in the SCR layer, ideally with just water, elemental nitrogen, and the smallest possible amounts of NO and NO. x This allows for the overall conversion of ammonia to

[0007] A typical ASC is described in International Publication No. WO 2016205509. This publication discloses an ASC that includes a combination of platinum on a support with low ammonia storage capacity and a first SCR catalyst. A preferred combination is a dual-layer configuration with an upper layer containing the first SCR catalyst and a lower layer containing platinum on a support with low ammonia storage capacity. During use, some excess ammonia can be stored in the upper SCR catalyst layer. A portion of the excess ammonia can pass through the Pt, where it is converted into NO. x can then be oxidized to NO x The ammonia is passed back through the SCR bed and treated with the stored ammonia to provide N2. In this way, excess slipped ammonia is stored and used rather than released to the atmosphere.

[0008] Another configuration of an ASC is described in U.S. Patent Application Publication No. 20150037233.

[0009] Particulate emissions are a well-known problem for diesel engine exhaust gas treatment systems. It is known to use filters to address these emissions. Known types of filters include diesel particulate filters (DPFs) and catalyzed soot filters (CSFs). These trap soot from the exhaust and can be regenerated by increasing the exhaust temperature to burn the accumulated soot. The substrate for a conventional DPF or CSF is a so-called wall-flow filter.

[0010] EP 2483537 discloses a four-way catalyst for diesel exhaust. As the name implies, it removes all four main emissions in the exhaust, namely CO, HC, and NO. x , and soot are removed in a single component.

[0011] EP 2567081 discloses a catalytic article for treating an exhaust gas stream containing particulate matter, hydrocarbons, CO, and ammonia. In a first embodiment of the disclosure, the article comprises a substrate having an inlet end and an outlet end defining an axial length, a first catalytic coating comprising a platinum group metal, the first catalytic coating extending from the outlet end toward the inlet end over less than the entire axial length of the substrate, and a second catalytic coating comprising a catalyst for selective catalytic reduction (SCR) of nitrogen oxides, the second catalytic coating extending from the inlet end toward the outlet end over less than the entire axial length of the substrate and overlapping a portion of the first catalytic coating.

[0012] EP 3277411 and EP 3277403 disclose multi-zone catalyst articles, methods of making multi-zone catalyst articles, and methods for controlling emissions in diesel engine exhaust streams using multi-zone catalyst articles, and various embodiments of an emissions treatment system effectively treat diesel engine exhaust using a single multi-zone catalyst article.

[0013] EP 3357558 discloses a catalyst for purifying the exhaust gases of diesel engines, said catalyst comprising several material zones.

[0014] As emission standards become more stringent, the emission output of exhaust systems is increasingly being affected by particulate emissions as well as NO x It is becoming increasingly important to ensure that concerns regarding species such as CO₂ and NH₃ are addressed. Summary of the Invention

[0015] It is therefore an object of the present invention to provide an ammonia slip catalyst suitable for meeting expected future regulations on particulate emissions, or at least to address the problems associated therewith in the prior art, or to provide a commercially viable alternative thereto.

[0016] According to a first aspect, there is provided a catalytic article for the treatment of exhaust gases, the catalytic article comprising: a wall-flow filter substrate having an inlet channel, the inlet channel being open at an inlet end of the substrate and closed at an outlet end of the substrate, the inlet channel being adjacent to an outlet channel that is closed at the inlet end of the substrate and open at the outlet end of the substrate; the wall-flow filter comprising at least first, second, and third catalyst layers; (i) a first catalyst layer extending from the inlet end of the substrate and comprising a first SCR composition; (ii) a second catalyst layer is disposed in or on the walls of the inlet channels extending from the inlet end of the substrate and comprises a PGM-containing composition; When the second catalyst layer is disposed on the wall surface of the inlet channel, the first catalyst layer is disposed on the second catalyst layer; the first catalyst layer is disposed on the wall of the inlet channel when the second catalyst layer is disposed within the wall of the inlet channel; (iii) a third catalyst layer disposed within or on the walls of the outlet channel extending from the outlet end of the substrate, the third catalyst layer comprising a second SCR composition; g / ft 3 PGM in the second catalyst layer: g / in 3 The ratio of the total amounts of the first and second SCR compositions in the first and third catalyst layers of each unit is 1.5:8 to 5:8, preferably 2:8 to 4:8, and more preferably 2:8 to 3:8.

[0017] The present disclosure will now be further described. In the following sections, different aspects / embodiments of the present disclosure are defined in more detail. Each aspect / embodiment so defined may be combined with any other aspect / embodiment or aspects / embodiments, unless expressly indicated otherwise. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. It is intended that features disclosed with respect to a product may be combined with features disclosed with respect to a method, and vice versa.

[0018] Hereinafter, the term "ammonia slip catalytic filter" or "ASCF" will be used as shorthand for the catalytic article structure described herein.

[0019] It has recently been observed that the use of reducing agents (especially urea / ammonia) injection into the exhaust gas stream can lead to the formation of certain agglomerated compounds and ammonium salts. These are species formed by reaction and polymerization of the reducing agent before it can participate in the SCR reaction. The polymerized material then takes the form of additional particulate matter that can be released into the atmosphere, contrary to stringent emission requirements.

[0020] A discussion of the emissions of these polymerized particulates can be found in SAE 2017-01-0915, which explains that depending on the urea injector, decomposition reaction tube (DRT) design, and operating conditions, incomplete decomposition of injected urea can lead to the formation of solid urea deposits in diesel aftertreatment systems. The deposits formed can result in increased engine backpressure and NO emissions. x This can lead to a decrease in treatment performance. The formed urea deposits can be further converted to chemically more stable substances when exposed to high-temperature exhaust gases; therefore, understanding this conversion process is important. The results of our experiments show that: 1) below the urea melting point (130°C), the formed urea deposits remain primarily urea; 2) in the temperature range of 130–190°C, urea converts to a combination of urea, biuret, and CYA; and 3) above the biuret melting point (190°C), CYA is primarily formed, with some characteristics of ammelide. At exposures above 200°C, urea undergoes rapid chemical transformation via urea decomposition, biuret formation, and subsequent biuret decomposition, converting to CYA within a short period of time.

[0021] Thus, polymerization of urea and ammonia components in high-temperature exhaust gas conditions can lead to the formation of polymeric particulate matter, which may have some degree of thermal stability. This particulate matter is typically very fine, and it is precisely this type of fine matter that is now subject to stricter regulations. Furthermore, because particulate matter formation can only occur after a reductant is introduced into the exhaust gas treatment system, particulate matter generally forms both after any conventional particulate filter (DPF or CSF) in the system and in low-temperature environments (i.e., underfloor configurations) where conventional regeneration of such filters is difficult.

[0022] The inventors have now discovered that it is possible to address these concerns by providing the catalytic articles described herein. In particular, the articles can provide ASC performance comparable to standard flow-through monoliths while reducing or avoiding particulate emissions. Furthermore, the inventors have found that the NO and NO emissions of standard flow-through ASCs can be reduced. x It has been found that it is possible, and indeed desirable, to use reduced amounts of PGMs to tailor performance, thereby reducing costs and optimizing performance.

[0023] It should be noted that the ASC is often provided as the rear portion of an SCR catalyst article. That is, a flow-through monolith may include a PGM layer on the outlet surface portion, with an SCR layer along the entire length of the monolith. In other embodiments, the SCR and dual-layer ASC may be provided on separate monoliths, but may be arranged immediately consecutively. This performance comparison is made with respect to the ASC portion only (i.e., does not include the upstream SCR-only portion of such a combined monolith). In use, the ASCF disclosed herein will typically be preceded by a separate SCR monolith, as discussed below.

[0024] The present invention relates to a catalytic article. By catalytic article is meant a structure as described herein that has catalytic properties. The catalytic properties come from materials contained in or coated on the structure. Articles as defined herein include both coated catalytic substrates as described herein and treated and sealed ASCF units suitable for installation in automobiles. The catalytic article provides a catalyst that is effective in reducing ammonia slip and particulate emissions when used downstream of an SCR process.

[0025] The present invention relates to a catalytic article for the treatment of exhaust gases. That is, the catalytic article can be used to treat exhaust gases from combustion processes, such as internal combustion engines (either mobile or stationary), gas turbines for stationary, marine, or locomotive applications, and coal- or oil-fired power plants. The article can also be used to treat gases from industrial processes, such as refineries, refinery heaters and boilers, furnaces, chemical processing industries, coke ovens, municipal waste plants, and incinerators. In a specific embodiment, the method is used to treat exhaust gases from gas turbines or lean-burn engines. The treatment involves the reduction of NOx, NOx, and NOx. x This is done to remove undesirable components of the exhaust gas, such as CO and particulate matter. Other species, such as CO and unburned hydrocarbons (HC), may also be treated by components of the exhaust treatment system.

[0026] The catalyst article comprises a wall-flow filter substrate, which are well known in the art. Wall-flow filters have inlet channels that are open at the inlet end of the substrate and closed at the outlet end of the substrate, adjacent to outlet channels that are closed at the inlet end of the substrate and open at the outlet end of the substrate. The alternating open and closed channels mean that, in use, exhaust gases entering the inlet channels are forced through the walls separating the channels and exit through the open outlet channels. The porosity of the walls and the provision of specific coating areas and layers ensure that the gases are treated as they pass through the walls.

[0027] Typical ceramic wall-flow filter substrates are constructed from refractory materials such as cordierite or silicon carbide. A common construction is a multi-passage honeycomb structure in which alternating passages on the inlet and outlet sides of the honeycomb structure are plugged at both ends, resulting in a checkerboard-type pattern on either end.

[0028] The monolith substrate used in wall-flow filters has a cross-sectional area of ​​1 square inch (2.54 cm) 2 A carrier may contain up to about 400 channels (or "cells") per square inch, although far fewer channels may be used. For example, a carrier may have about 7 to 400, specifically 100 to 400, cells per square inch ("cpsi"). The cells may have a cross section that is rectangular, square, circular, oval, triangular, hexagonal, or other polygonal shape.

[0029] Wall-flow substrates are generally constructed from ceramic-like materials such as cordierite, alpha alumina, silicon carbide, silicon nitride, zirconia, mullite, spodumene, alumina-silica magnesia, or zirconium silicate, or refractory metals such as stainless steel. Such materials can withstand the high temperatures encountered in the environment, particularly when treating exhaust streams. Ceramic wall-flow substrates are typically formed from materials with a porosity of approximately 40-70. The term "porosity" as used in this context is understood to be determined according to mercury porosimetry according to DIN 66133. According to embodiments of the present invention, the wall-flow substrate is specifically one having a porosity in the range of 38-75.

[0030] The wall-flow filter of the present invention comprises at least first, second, and third catalyst layers. Preferably, these are the only layers present in the catalyst article. Nevertheless, it should be understood that other conventional layers may be present, or that the first, second, and third layers may themselves be comprised of one or more sublayers having the same or different formulations. That is, the first catalyst layer may be comprised of two layers containing SCR catalyst to create a single SCR catalyst formulation, or to provide two layers of different SCR catalyst formulations. In the most preferred embodiment, three catalyst layers are the only layers present, each provided as a separately applied layer.

[0031] The first catalyst layer extends from the inlet end of the substrate and comprises a first SCR composition. Preferably, the first catalyst layer extends up to 100% of the longitudinal length of the substrate, extending from the inlet end to the outlet end, and preferably 70-90% of the longitudinal length. Coating methods for applying such compositions, particularly for thin in-wall coatings, typically involve immersing the substrate in the washcoat, resulting in substantially 100% of the coating extending all the way along the inside of the inlet channel to the plug that blocks the outlet end of the channel. For wall coatings, these can be applied by drawing the washcoat into the substrate through the application of a strong, short vacuum.

[0032] A preferred coating of up to 90% of the length will leave a portion of the wall at the end of the channel adjacent the plug without any coating thereon. Thus, a most preferred coating of 70-90% of the length will provide an inlet region of the inlet channel where the composition is located and an area distal to the inlet end where the composition is absent. In use, the first catalyst layer is the first catalytic element of the article that contacts the exhaust gas being treated.

[0033] In some preferred embodiments, the second and third catalyst layers do not overlap and together extend 100% of the longitudinal length of the substrate extending from the outlet end to the inlet end. These embodiments have been found to achieve good conversion while minimizing backpressure. In these embodiments, preferably, the second catalyst layer is disposed on the walls of the inlet channels and the third catalyst layer is disposed on the walls of the outlet channels.

[0034] Preferably, the catalytically active SCR component of the first and / or second SCR compositions comprises, and preferably consists of, one or more metal-exchanged zeolites. This does not preclude the presence of additional non-catalytic binders or processing aids, such as those conventionally used in washcoat formulations, which are discussed further below.

[0035] Preferably, the first SCR composition comprises a copper-promoted zeolite, an iron-promoted zeolite, a manganese-promoted zeolite, or a combination thereof. The total amount of Cu, Mn, and Fe is preferably present in an amount of 0.1 to 5 wt. % of the promoted zeolite, most preferably 1 to 3 wt. %. The second SCR composition may independently be selected from the same materials described herein for the first SCR composition.

[0036] Zeolites are microporous aluminosilicates having any one of the framework structures listed in the database of zeolite structures published by the International Zeolite Association (LZA). Framework structures include, but are not limited to, CHA, FAU, BEA, MFI, and MOR types. Non-limiting examples of zeolites having these structures include chabazite, faujasite, Y-type zeolite, ultrastable Y-type zeolite, beta-type zeolite, mordenite, silicalite, X-type zeolite, and ZSM-5. Aluminosilicate zeolites can have a silica / alumina molar ratio (SAR), defined as SiO2 / Al2O3, of at least about 5, preferably at least about 20, with a useful range of about 10-200.

[0037] Preferably, the metal-exchanged zeolite is a small pore zeolite, preferably having a CHA framework structure. Preferably, the zeolite is exchanged with Cu and / or Mn. Preferably, the zeolite is copper-promoted and has a CHA framework. Preferably, the copper is present in an amount of 0.1 to 5 wt. % of the copper-promoted zeolite, most preferably 2 to 4 wt. %.

[0038] A second catalyst layer is disposed within or on the walls of the inlet channels extending from the inlet end of the substrate and comprises a PGM-containing composition, i.e., the second catalyst layer extends from the inlet end in the same manner as the first catalyst layer.

[0039] In use, the second catalyst layer is the second catalytic element of the article that contacts the exhaust gas to be treated, either by the second catalyst layer being disposed on the wall of the inlet channel and the first catalyst layer being disposed on top of the second catalyst layer, or by the second catalyst layer being disposed within the wall of the inlet channel and the first catalyst layer being disposed on the wall of the inlet channel.

[0040] Providing either an "in-wall" or "wall" coating is well known in the art. It is achieved by carefully adjusting the washcoat formulation to match the porosity of the filter wall. Thus, a "thinner" composition with finer materials (especially supported PGMs) may penetrate the wall more easily than a "thicker" composition containing coarser materials. Exemplary in-wall and wall coatings are provided in the examples. Thus, by tailoring the washcoat material, it is possible to ensure that the coating is provided as desired on the porous walls of the article.

[0041] The second catalyst layer comprises a PGM-containing composition. Suitable PGMs include Pd, Pt, and Rh. Preferably, the PGM-containing composition contains Pt, preferably as the only PGM. The PGM is preferably provided on a support material. Suitable support materials are known in the art. Preferably, the PGM-containing composition comprises alumina as a support for the PGM.

[0042] Preferably, the second catalyst layer extends up to 100% of the longitudinal length of the substrate extending from the inlet end to the outlet end, preferably 25-90%, more preferably 35-80%, more preferably 40-70%, and most preferably 45-65% of the longitudinal length. As with the first catalyst layer, the preferred coating of up to 90% of the length leaves a portion of the wall at the end of the channel adjacent to the plug without any coating thereon. Thus, the preferred coating of 40-70% of the length provides an inlet region of the inlet channel where the composition is provided and a region distal to the inlet end where the composition is absent.

[0043] In use, the second catalyst layer is the second catalytic element of the article that comes into contact with the exhaust gas to be treated. The first catalyst layer is preferably provided having a length at least as long as the second catalyst layer, and preferably at least 5% to 10% longer (relative to the overall length of the article) than the second catalyst layer. This ensures that the exhaust gas comes into contact with the first catalyst layer before the second catalyst layer.

[0044] A third catalyst layer is disposed within or on the walls of the outlet channel extending from the outlet end of the substrate and includes a second SCR composition. As noted above, the second SCR composition can be selected from the same materials as the first SCR composition. Preferably, the second SCR composition is a copper-promoted CHA zeolite.

[0045] The third catalyst layer extends from the outlet end of the substrate. Preferably, the third catalyst layer extends up to 100% of the longitudinal length of the substrate extending from the outlet end to the inlet end, preferably 20-90%, more preferably 30-80%, preferably 30-70%, and most preferably 40-60% of the longitudinal length. In embodiments without overlap with the second catalyst layer, the lengths are preferably complementary (i.e., 40-70% for the second layer and 60-30% for the third layer). In embodiments with overlap, the third layer may be longer, such as 60-90% of the length. Coating methods for applying such compositions typically involve immersing the substrate in the washcoat, resulting in substantially 100% coating extending all the way along the inside of the outlet channel to the plug that blocks the inlet end of the channel. A preferred coating of up to 90% of the length will leave a portion of the wall at the end of the channel adjacent the plug without any coating thereon. Thus, a most preferred coating of 70-90% of the length provides an outlet region of the outlet channel where the composition is located, and an area distal to the outlet end where the composition is absent. In use, the third catalyst layer is the final catalytic element of the article that comes into contact with the exhaust gas being treated.

[0046] When applying an in-wall catalyst composition, the washcoat may coat all of the wall pores. Therefore, when both second and third catalyst compositions are provided in the wall, there may be substantial overlap in the location of the catalyst compositions. Nevertheless, it is still possible to determine which side the compositions were applied from. This is particularly true because the inlet channel will contain the second composition up to the inlet lip, whereas the third composition cannot be coated there due to a plug blocking the inlet end of the outlet channel. Conversely, the outlet channel will contain the third composition up to the outlet lip, whereas the second composition cannot be coated there due to a plug blocking the outlet end of the inlet channel. Thus, even when both the second and third compositions are provided in the wall, there will still be an in-wall region of SCR only immediately adjacent the outlet of the outlet channel.

[0047] In a preferred embodiment, the third catalyst composition is provided on the wall. Preferably, the second catalyst layer is provided on the wall of the inlet channel, and the third catalyst layer is provided on the wall of the outlet channel. According to another preferred embodiment, the third catalyst composition is provided in the wall, and the second catalyst layer is provided on the wall of the inlet channel. In this case, if there is overlap, there is intimate contact between the particles of the second and third catalyst layers. In some embodiments, it is preferred that the second and third catalyst layers are adjacent and do not overlap.

[0048] Preferably, the compositions forming the first and third layers do not include any PGM components. Preferably, the composition forming the second layer does not include any SCR components. Nevertheless, it should be understood that when coating porous wall-flow filters, there may be a small amount of bleed-through between the layers.

[0049] When describing washcoat loading in a catalyst article, g / ft 3Typically, SCR materials provide PGM loadings in the order of g / in. This is a measure of the metals themselves in the applied layer. In contrast, washcoat loadings for SCR materials are typically much higher, in the order of g / in. 3 The SCR composition is given in units of 0.1 wt. sq. m / s. Unlike PGMs, such values ​​for SCR materials typically include all elements of the washcoat, including, for example, the binder. Thus, the SCR composition herein includes both the zeolite component and any binder. Thus, preferably, the SCR composition is synonymous with the first and third catalyst layers.

[0050] In a conventional ASC (i.e., the PGM-containing end portion of a combined SCR / ASC), g / ft 3 Total PGM loading in g / in 3 The ratio of total loadings of SCR compositions of units of 1000 to 10000 is typically greater than 1:1, more commonly greater than 5:4. The inventors have found that when ASCFs with the same loading of catalyst composition are formulated, excessive NO x and NO formation. Without wishing to be bound by theory, it was believed that the exhaust gas in the new configuration was exposed to PGM materials to a much greater extent. That is, in a flow-through ASC, the exhaust gas encounters PGMs only as it permeates through the layer of catalytic material, whereas providing an ASCF configuration passes all of the exhaust gas directly through the PGM-containing composition.

[0051] Therefore, the present inventors sought to address these concerns and arrived at an optimized relative loading of catalytic material within the layer. g / ft 3 PGM in the second catalyst layer: g / in 3 The ratio of the total amounts of the first and second SCR compositions in the first and third catalyst layers of each unit is 1.5:8 to 5:8, preferably 2:8 to 4:8, preferably 4.5:16 to 7:16, preferably 4.5:16 to 6:16. Preferably, g / ft 3 PGM in the second catalyst layer in units of g / in 3The ratio of the total amounts of the first and second SCR compositions in the first and third catalyst layers of the unit is about 5:16.

[0052] Given the underlying understanding of ASCs, it is surprising that the structure can even operate in a three-layer configuration as disclosed herein. In particular, the SCR layer of a conventional ASC functions only by the amount of ammonia that is trapped within the SCR layer during use. Sufficient ammonia can permeate through the first and second layers to the third catalyst layer and then trap any NO formed upon contact of the exhaust gas with the PGM in the second catalyst layer. x It is surprising that SCR can be performed at this level. Nevertheless, the performance demonstrates that this does occur, and the comparative example shows that in the absence of the outlet SCR composition, the exhaust produces undesirably high levels of NO. x and N2O, likely a result of contact with the PGM as the last layer before exiting the article.

[0053] Preferably, the weight ratio of the catalytically active components in the first and third catalyst layers is 2:3 to 3:2, preferably about 1:1. That is, the first and third catalyst layers are preferably provided in similar amounts. Indeed, from a manufacturing standpoint, it is particularly preferred that the first and third catalyst layers also have substantially the same composition.

[0054] Preferably, at least two of the first, second, and third catalyst layers, and preferably all three, extend 70-90% of the longitudinal length. It will be understood that if all three extend to such lengths, there will be overlap between all three layers. Thus, in embodiments where all three layers extend to this length, there will be portions of the inlet channel wall that are free of the first or second catalyst layer (and, of course, the third catalyst layer) and portions of the outlet channel wall that are free of the third catalyst layer (and, of course, neither the first nor second catalyst layer). This configuration facilitates layer formation and avoids buildup at plugged edges that can occur when attempting complete channel coating, but also helps ensure that the gas being processed first contacts the first catalyst layer and last contacts the third catalyst layer.

[0055] In embodiments in which the second and third catalyst layers do not overlap, preferably the first catalyst layer extends 70-90% of the longitudinal length, the second catalyst layer extends 30-60% of the longitudinal length, and the third catalyst layer extends 70-40% of the longitudinal length.

[0056] The SCR and PGM-containing composition may contain additional components, such as fillers, binders, stabilizers, rheology modifiers, and other additives. In certain embodiments, the washcoat contains pore formers, such as graphite, cellulose, starch, polyacrylate, and polyethylene. These additional components do not necessarily catalyze the desired reaction; instead, they improve the effectiveness of the catalytic material by, for example, increasing its operating temperature range, increasing the contact surface area of ​​the catalyst, or increasing the adhesion of the catalyst to the substrate. Typically, the only additional component is the binder. Preferably, the additional component forms less than 25 wt. %, preferably less than 15 wt. %, and most preferably less than 10 wt. % of the layer, with the remainder being SCR or supported PGM, respectively. The particle size of the binder particles will vary depending on whether the associated coating is intra- or extra-wall. A typical alumina binder for intra-wall applications will have a D90 of less than 7 μm, as measured by laser diffraction.

[0057] Techniques for applying the first and second layers are well known in the art and include applying a washcoat to the surface to be coated. After the layers are coated on the article, they are typically baked to set the layers. Baking is well known in the art and can be carried out in air at a temperature of about 500°C.

[0058] The inventors have envisioned two approaches for forming catalyst articles. According to one embodiment, there is provided a method for the manufacture of a catalyst article in the following sequence of steps: applying a washcoat to the outlet channels to form a third coating, followed by calcination; applying a washcoat to the inlet channels to form a second coating, followed by calcination; applying a washcoat to the inlet channels to form a first coating, followed by calcination. Each calcination step will generally be preceded by a drying step.

[0059] According to a more preferred embodiment, a method for producing a catalyst article is provided that involves fewer calcinations and involves the following sequence of steps: applying a washcoat to the inlet channels to form a second coating, followed by calcination; applying a washcoat to the outlet channels to form a third coating, followed by drying; applying a washcoat to the inlet channels to form a first coating, followed by calcination. Each calcination step will generally be preceded by a drying step. Avoiding calcination steps reduces energy and process costs.

[0060] Preferably, the catalyst article includes a means for electrically heating the catalyst article. Such means are well known in the art and generally rely on resistive heating to increase the operating temperature of the catalyst article during use. This approach has associated energy costs to achieve the required temperatures, but may be used for in-situ regeneration of the catalyst article to remove accumulated particulate deposits. Other approaches for regeneration are known, such as increasing the temperature of the exhaust gases by engine management or hydrocarbon dosing.

[0061] According to a further aspect, there is provided an exhaust gas treatment system comprising a catalytic article as described herein. Thus, the system implicitly comprises an initial manifold for receiving exhaust gas, such as from an engine, and an outlet for discharge of treated exhaust gas to the environment. In use, the catalytic article is adapted to treat excess NO x Any ammonia that slips from an upstream component can be processed to produce N2 without forming and releasing N2O or producing excessive N2O.

[0062] Preferably, the exhaust gas treatment system comprises, in order, a catalyzed soot filter (CSF) or diesel particulate filter (DPF), a means for injecting a nitrogen-based reductant, an SCR catalyst article, and a catalyst article as described herein. Preferably, the system further comprises a diesel oxidation catalyst (DOC) upstream of the CSF. All of these components and their formulations and compositions are well known in the art. The upstream means for injecting the nitrogen-based reductant and the SCR catalyst article are potentially responsible for both slipped ammonia (and thus ASCF) and the formation of reductant-derived particulate matter. Because this reductant-derived particulate matter is generated downstream of the CSF (or DPF), there is no other filter body in which it can be retained.

[0063] Providing a CSF upstream of the catalyst article (ASCF) prevents soot buildup on the ASCF. This is desirable because it is difficult to handle large buildups of material in end-of-system components due to their relatively lower temperatures. It would be more energy intensive to raise the temperature of the ASCF, as would be required if soot were to build up. In contrast, urea / ammonia-derived particulate buildup is relatively small, so regeneration can be performed infrequently, as needed.

[0064] The nitrogen-based reductant provided by the means for injecting a nitrogen-based reductant can be ammonia itself, hydrazine, or an ammonia precursor selected from the group consisting of urea ((NH)CO), ammonium carbonate, ammonium carbamate, ammonium bicarbonate, and ammonium formate. Ammonia and urea are the most preferred alternatives. Preferably, the means for injecting a nitrogen-based reductant further comprises a reservoir containing urea.

[0065] Preferably, the catalyst article, in use, is the last catalyst article encountered by the exhaust gases before being released into the atmosphere, meaning that the article can suitably function as an ammonia slip catalyst and simultaneously prevent the release of urea / ammonia derived particulate matter into the atmosphere.

[0066] According to a further aspect, there is provided a fuel combustion and exhaust gas treatment system comprising an engine and an exhaust gas treatment system as described herein. Preferably, the engine is a diesel or lean-burn engine.

[0067] According to a further aspect, there is provided a vehicle comprising a fuel combustion and exhaust treatment system as described herein, preferably at least the catalyst article as described herein, located in an underfloor location and / or experiencing exhaust gases at temperatures of 200-450°C, preferably 270-350°C, during normal use. The provision of the catalyst article in an underfloor location arises because the article is preferably at the end of the exhaust system. This creates a challenge where temperatures promote the formation of reductant-derived particulate matter but not its destruction, and therefore ASCF is a solution to meeting emission standards.

[0068] According to a further aspect of the present invention, there is provided a method for treating an exhaust gas, the method comprising passing the exhaust gas through a catalytic article as described herein or an exhaust gas treatment system as described herein. [Brief explanation of the drawings]

[0069] The invention will now be further explained in the following figures. [Figure 1] 1 shows a schematic cross-sectional view of a portion of a catalyst article described herein. [Figure 2] 1 shows a schematic cross-sectional view of a portion of a catalyst article described herein. [Figure 3] 1 shows the arrangement of exhaust gas treatment system components downstream of the engine. [Figure 4] 1 shows charts of NH3 conversion, N2O and NOx production for a comparative flow-through ASC versus an ASCF with different ratios of PGM to SCR loading (NH3 only - 500 ppm SCAT SV=90k). [Figure 5]Charts of NH3 conversion rate, N2O and NOx formation for the comparison flow-through ASC for ASCFs having different ratios of PGM to SCR loading are shown (NOx bias: 500 ppm NH3 and 500 ppm NO, SCAT SV = 90k). [Figure 6-1] The engine test data to be considered below is shown. [Figure 6-2] The engine test data to be considered below is shown. [Figure 6-3] The engine test data to be considered below is shown. [Figure 7A] Three preferred embodiments of the present invention are shown. [Figure 7B] Three preferred embodiments of the present invention are shown. [Figure 7C] Three preferred embodiments of the present invention are shown. [Figure 8] Test data when the embodiments of FIGS. 7A - 7C are compared with the standard is shown. [Figure 9] Test data when the embodiments of FIGS. 7A - 7C are compared with the standard is shown. [Figure 10] Test data when the embodiments of FIGS. 7A - 7C are compared with the standard is shown.

Mode for Carrying Out the Invention

[0070] FIG. 1 shows a schematic portion of the catalyst article 1 described herein. In particular, the portion shown focuses on a single inlet channel 5 and the corresponding single outlet channel 10. The inlet channel 5 is separated from the outlet channel 10 by a porous wall 15. The inlet channel 5 opens at the inlet face 20 of the catalyst article 1 and is plugged at the outlet face 30 of the catalyst article 1 by a plug 25. The outlet channel 10 opens at the outlet face 30 of the catalyst article 1 and is plugged at the inlet face 20 of the catalyst article 1 by a plug 25.

[0071] A first wall surface 40 on the inlet channel 5 is provided with a first catalyst layer 35. The first catalyst layer 35 is provided as a washcoat and includes an SCR composition, such as a Cu-CHA washcoat composition, along with conventional binders and processing aids. The first catalyst layer 35 extends from the inlet face 20 at least 70% of the total length 45 of the catalyst article 1.

[0072] Disposed within porous wall 15 is second catalyst layer 50. Second catalyst layer 50 is provided as a washcoat and includes a PGM-containing composition, such as Pt, supported on alumina along with conventional binders and processing aids. Second catalyst layer 50 extends at least 70% of the total length 45 of catalyst article 1 from inlet face 20. Second catalyst layer 50 is preferably slightly shorter than first catalyst layer 35.

[0073] A third catalyst layer 55 is provided on a second wall surface 60 above the outlet channel 10. The third catalyst layer 55 is provided as a washcoat and includes an SCR composition, such as a Cu-CHA washcoat composition, along with conventional binders and processing aids. The third catalyst layer 55 extends from the outlet face 30 for at least 70% of the total length 45 of the catalyst article 1.

[0074] In use, exhaust gases entering the catalyst article 1 through the inlet face 20 enter the inlet channels 5, pass through the porous wall 15, enter the outlet channels 10 and exit the article 1. This path is indicated by the large arrows. The gases pass sequentially through the first, second and third catalyst layers (35, 50, 55). Ammonia is converted to NO 2 produced on the second catalyst layer 50. x The ammonia / urea particulate matter is stored on the first and third catalyst layers (35, 55) until it can be used to perform an SCR reaction to destroy the ammonia. Particulate matter from the ammonia / urea accumulates in the inlet channel 5 rather than being released to the atmosphere. If necessary, the particulate matter can be burned to reduce excessive accumulation, which can be assisted by localized resistive heating using an electrical element (not shown) if necessary.

[0075] Figure 2 shows an alternative configuration of the first, second, and third catalyst layers (35, 50, 55) in the catalyst article 1. All reference numbers refer to the same components, and all composition details remain the same.

[0076] A second catalyst layer 50 is provided on the wall 40 of the inlet channel 5. The first catalyst layer 35 is provided on the second catalyst layer 50 and, since it is slightly longer, overlaps a small portion of the wall 40 of the inlet channel 5. A third catalyst layer 55 is provided within the porous wall 10, but could alternatively be provided on the surface 60 of the outlet channel 10.

[0077] 3 shows an exhaust gas treatment system including, downstream of an engine 65, a diesel oxidation catalyst (DOC) 70, then a catalyzed soot filter (CSF) 75, then a means for injection of a nitrogen-based reductant 80, then an SCR 85, and finally the catalyst article 1 described herein, i.e., an ASCF. The CSF 75 may be a diesel particulate filter (DPF). The presence of a CSF 75 or DPF upstream of the catalyst article 1 is desirable because such components are difficult to regenerate and soot buildup on the catalyst article 1 (which is prevented by the CSF 75 or DPF) would lead to unacceptable backpressure. The difficulty in regenerating a soot-laden catalyst article 1 is exacerbated by end-of-system locations (such as underfloor) where lower temperatures are not sufficient. [Example]

[0078] The invention will now be further described with reference to the following non-limiting examples.

[0079] Exemplary catalyst articles were fabricated and tested as described below.

[0080] Engine Testing The inventive examples used a wall-flow filter substrate measuring 10.5 inches by 6.0 inches. A reference filter measuring 10.5 inches by 4.0 inches was used, but coated with the same total mass (g) of washcoat as on the coated filter (varied on the filter as discussed below).

[0081] SCR "In-wall" Coating - "Coating 1": A spray-dried Cu (3.3 wt%) chabazite (SAR20) slurry was milled to a target particle size distribution characterized by a D90 of 3.8-3.9 μm and stirred overnight. The milled slurry was adjusted to a target pH of 9.9-10.2 by adding aqueous tetraethylammonium hydroxide and stirred for 5 minutes. Finally, a small-particle mixed oxide containing Al2O3 (91.2 wt%), La2O3 (4.8 wt%), and Nd2O3 (4.1 wt%), with a particle size distribution characterized by a D90 of 3.0-5.0 μm, was added under high-speed stirring at a target of 11 wt% relative to the calcined Cu chabazite weight. The washcoat with the above composition was stirred overnight and then readjusted to a pH of 9.9-10.2 by adding aqueous tetraethylammonium hydroxide. The pH adjusted washcoat was stirred for 30 minutes and applied at 0.8 g / in based on the volume of the ASCF brick through the rear end of the suitable DPF. 3 The washcoat was coated to a target washcoat loading of 1000 ppm.

[0082] The blocks were dried at 110°C for 30 minutes and calcined at 500°C for 2 hours.

[0083] PGM In-wall Coating - "Coating 2": Succinic acid target 40g / ft 3 The solution was dissolved in demineralized water to a concentration of 0.5 g / ft and the solution was stirred for 5 minutes. Platinum(IV) nitrate was added to this solution to a platinum target of 0.5 g / ft. 3 The mixture was stirred for 5 minutes. Finally, small particle size alumina PO (pre-milled to a target particle size distribution characterized by a D90 of 4.8-5.0 μm) was added as a slurry to a target concentration of 0.06 g / in. 3The washcoat having the above composition was stirred for 3 hours and coated onto the front end of an intermediate DPF containing the calcined SCR "Coating 1" (described above), then dried at 115°C for 30 minutes and calcined at 500°C for 2 hours.

[0084] All targets above refer to final loadings based on the volume of the ASCF brick.

[0085] SCR Porous Wall Coating - "Coating 3": A spray-dried slurry of Cu (3.3 wt%) chabazite (SAR20) was milled to a target particle size distribution characterized by a D90 of 3.8–3.9 μm and stirred overnight. The slurry was transferred to a cooling tank, and colloidal aluminum hydroxide (boehmite) was added under high-speed stirring to a target of 18 wt% based on the calcined Cu chabazite weight. The mixture was stirred for 30 minutes, and then a cellulose pore former (Arbocel UFC100) was added under high-speed stirring to a target of 54 wt% based on the calcined Cu chabazite weight. The mixture was stirred for 30 minutes, and then a cellulose thickener was added under high-speed stirring to a target of 0.2 wt% based on the total weight of the wet washcoat. The mixture was stirred at high speed for 30 minutes, and then the washcoat was stored in a sealed container for 2 days. The washcoat was then stirred at high speed for 3 minutes and applied at a rate of 0.8 g / in based on the volume of the finished catalyst to the front end of the pre-fabricated DPF containing calcined SCR "Coating 1" and calcined PGM "Coating 2" (described above). 3 The bricks were dried at 110°C for 45 minutes and then fired at 500°C for 2 hours.

[0086] SCAT exam The test was performed in a SCAT test. This type of test does not include any other catalysts upstream or downstream, but operates under specific operating conditions, i.e., 350 ppm CO / 500 ppm NH3 / 0 ppm NOx for "NH3 rich conditions," or "NO XFor the "bias" condition, the core is artificially provided with a gas composition representing 350 ppm CO / 500 ppm NH3 / 500 ppm NO.

[0087] All examples used a 1 in. x 5.53 in. wall-flow filter substrate. The space velocity was the same for all tests.

[0088] The wall SCR washcoat was applied to the inlet channel (0.8 g / in 3 ), with a PGM-containing in-wall composition (Pt on alumina with various loadings) applied from the inlet side and an in-wall SCR washcoat (0.8 g / in ) in the outlet channel. 3 The ASCF of the example was manufactured having an SCR layer (2.4 g / in ) on the upper layer. 3 ), the lower layer, 3g / ft 3 The PGM coating was applied to the rear 50% of the brick.

[0089] PGM level is 1g / ft 3 , 0.75g / ft 3 , 0.5g / ft 3 , and 0.25 g / ft 3 This was the case for the SCR materials discussed herein (1.6 g / in ) of 10:16, 7.5:16, 5:16, and 2.5:16. 3 ) ratio, where the SCR material is the copper-exchanged zeolite and binder in the washcoat layer. These are compared in the examples with standard ASC, which has a ratio of 30:24, which represents the conventional configuration.

[0090] Further SCAT tests showed that NH3 conversion was 1g Pt / ft 3 , 0.75g / ft 3 , 0.5g / ft 3 was comparable to 0.25g / ft 3 of N2O production is Pt0.5g / ft 3The results show that the Pt loadings were essentially the same as the standard flow-through, but were much worse for those examples with higher Pt loadings.

[0091] The ASCF catalyst showed improved filtration efficiency in preliminary measurements when compared to the std-ASC on FT. In particular, PN10 emissions were 70–90% lower when the ASCF catalyst was used instead of the std-ASC on FT.

[0092] Therefore, Pt0.5g / ft 3 Near values ​​(1.6g / in 3 The 5:16 ratio (paired with SCR) provides a sweet spot where oxidation performance matches SCR performance, NH conversion is sufficient without excessive NO production, and filtration performance addresses reductant-derived particulate matter. This is ideally observed at and around a 5:16 ratio.

[0093] In Figure 4, the lines at 300°C for NH3 conversion are, from top to bottom, Std, 1, 0.75, 0.5, and 0.25 g / ft 3 For NO production, the lines peak from top to bottom: 1, STD, 0.75, 0.5, and 0.25 g / ft 3 For NOx production, the lines at 400°C are, from top to bottom, 1g, 0.75g, 0.5g, 0.25g, and STD.

[0094] As shown in these examples, a three-layer ASCF was developed with the aim of converting the ASC concept from a flow-through to a filter substrate. The three-layer ASCF approach expands the variety of possible coating designs compared to standard two-layer ASCs, while also significantly reducing particulate matter emissions. In particular, the use of a three-layer ASCF can utilize different combinations of intra-wall and wall coatings, thus enabling finer control of ASC activity, backpressure, and filtration efficiency.

[0095] Subsequent testing was conducted using engine test data to show that the same benefits could be achieved under real-world conditions. In this case, either the ASCF or SCR / ASC bricks are downstream of another SCR flow-through brick (in this case, a VSCR). Further information is shown in Figure 6, where a standard ASCF is coated with the same mass (in grams) of SCR and PGM catalyst as the standard flow-through SCR / ASC, while a "low PGM" variant has exactly half the PGM amount but the same SCR washcoat amount. Steady-state (SS) engine test data was conducted at three different temperatures (270, 320, and 370°C), and the low PGM variant was much better in terms of secondary emissions while still comparable to the SCR / ASC reference in terms of NH3 conversion, providing a better tradeoff between NH3 conversion and secondary emissions for the low PGM variant.

[0096] In each cluster of results, the left column is a conventional SCR / ASC, the middle column is an ASCF with SCR and PGM levels comparable to the conventional SCR / ASC, and the right column has half the level of PGMs as the middle column (0.5 g / ft total). 3 of PGM and 1.6g / in 3 SCR).

[0097] Figure 6A shows that the ASCF of the present invention achieves sufficiently good NH3 slip control. Figure 6B shows the improvement in reduced NO production levels. Figure 6C shows the improvement in reduced NO x The SS numbers in each chart reflect the test temperature under steady state conditions, demonstrating improved production levels.

[0098] 7A-7C use the same reference numbers as FIGS. 1 and 2 for the same components.

[0099] 7A is similar to FIG. 2, but in an embodiment where the second catalyst layer 50 has a shorter length, extending approximately 50% of the total length 45 of the catalyst article 1. The first catalyst layer extends approximately 80% of the total length 45 of the catalyst article 1. Coating the PGM layer as a porous coating on the wall surface of FIG. 7A was an attempt to prevent / minimize intimate contact with the SCR catalyst particles (which are dispersed within the wall volume and then radically separated from the PGM on the wall surface).

[0100] 7B and 7C are embodiments in which the second catalyst layer 50 does not overlap the third catalyst layer 55. Both of these layers (50, 55) are provided as in-wall washcoats and together coat the entire length 45 of the catalyst article 1. In FIG. 7B, the second catalyst layer 50 is about 25% of the entire length 45, and the third catalyst layer 55 is about 75% of the entire length 45. In FIG. 7C, the second catalyst layer 50 is about 50% of the entire length 45, and the third catalyst layer 55 is about 50% of the entire length 45.

[0101] As supported by embodiments 7B and 7C, avoiding overlap between these layers avoids intimate contact between the platinum group metal and the SCR catalyst.

[0102] Figure 8 shows a comparison of Embodiments 7A-7C to a comparative example in which the second catalyst layer 50 is approximately 70% (in-wall) of the total length 45 and the third catalyst layer 55 is approximately 80% (in-wall) of the total length 45. The provision of these overlapping in-wall coatings means that there is an overlap within the wall of the platinum group metal and SCR catalyst. The columns, from left to right, are the comparative example, Embodiment 7C, Embodiment 7B, and Embodiment 7A, respectively. Figure 8 shows that the lowest backpressures are obtained for the embodiments of Figures 7C (62) and 7B (63).

[0103] Figure 9 shows NH conversion at different temperatures. Performance is consistently best for the embodiment of Figure 7C. The embodiment of Figure 7B has the worst performance. The columns, from left to right, are the comparative example, embodiment 7C, embodiment 7B, and embodiment 7A, respectively.

[0104] 10 shows NH, NO, and NO outputs at different temperatures and ANR ratios. The columns, from left to right, for each set of conditions are Comparative Example, Embodiment 7C, Embodiment 7B, and Embodiment 7A, respectively.

[0105] As used herein, the term "comprising" is interchangeable with the definitions "consisting essentially of" or "consisting of." The term "comprising" is intended to mean that the specified elements are essential, although other elements may be added and still form a structure within the scope of the claim. The term "consisting essentially of" limits the scope of the claim to the specified materials or steps and those that do not materially affect the basic and novel characteristics of the claimed invention. The term "consisting of" closes the claim to the inclusion of materials other than those recited, except for impurities ordinarily associated therewith.

[0106] The foregoing detailed description has been provided for purposes of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to those skilled in the art and will still fall within the scope of the appended claims and their equivalents.

[0107] For the avoidance of doubt, the entire contents of all documents identified herein are hereby incorporated by reference.

Claims

1. A catalytic article for the treatment of exhaust gases, said catalytic article comprising: a wall-flow filter substrate having an inlet channel that is open at an inlet end of the substrate and closed at an outlet end of the substrate, the inlet channel adjacent to an outlet channel that is closed at the inlet end of the substrate and open at the outlet end of the substrate; the wall-flow filter comprising at least first, second, and third catalyst layers; (i) the first catalyst layer extends from the inlet end of the substrate and comprises a first SCR composition; (ii) the second catalyst layer is disposed in or on the wall of the inlet channel extending from the inlet end of the substrate and comprises a PGM-containing composition; when the second catalyst layer is disposed on the wall surface of the inlet channel, the first catalyst layer is disposed on the second catalyst layer; when the second catalyst layer is disposed within the wall of the inlet channel, the first catalyst layer is disposed on the wall of the inlet channel; (iii) the third catalyst layer is disposed in or on the walls of the outlet channels extending from the outlet end of the substrate and comprises a second SCR composition; g / ft ３ PGM in the second catalyst layer in units of g / in ３ a ratio of the total amounts of said first and second SCR compositions in said first and third catalyst layers of units of 1.5:8 to 5:

8.

2. 10. The catalytic article of claim 1, wherein the second catalyst layer is disposed within the walls of the inlet channels and / or the third catalyst layer is disposed on the walls of the outlet channels.

3. (i) the first catalyst layer extends 70 to 90% of the longitudinal length of the substrate extending from the inlet end to the outlet end; and / or (ii) the second catalyst layer extends 25 to 90% of the longitudinal length of the substrate extending from the inlet end to the outlet end; and / or (iii) The catalyst article of claim 1, wherein the third catalyst layer extends 20 to 90% of the longitudinal length of the substrate extending from the outlet end to the inlet end.

4. 4. The catalyst article of claim 3, wherein at least two of the first, second, and third catalyst layers extend 70-90% of the longitudinal length.

5. 2. The catalyst article of claim 1, wherein the second and third catalyst layers are non-overlapping and together extend 100% of the longitudinal length of the substrate extending from the outlet end to the inlet end, the second catalyst layer disposed within the walls of the inlet channels, and the third catalyst layer disposed within the walls of the outlet channels.

6. g / ft ３ The PGM in the second catalyst layer in units of g / in ３ 2. The catalytic article of claim 1, wherein the ratio of the total amounts of said first and second SCR compositions in said first and third catalyst layers is about 5:

16.

7. The catalytic article of claim 1 , wherein the PGM-containing composition contains Pt.

8. The catalyst article of claim 1 , wherein the PGM-containing composition comprises alumina as a support for the PGM.

9. 10. The catalytic article of claim 1, wherein the catalytically active SCR component of the first and / or second SCR compositions comprises one or more metal-exchanged zeolites.

10. 10. The catalyst article of claim 9, wherein the metal-exchanged zeolite has a CHA framework structure, and the zeolite is exchanged with Cu and / or Mn.

11. 10. The catalyst article of claim 1, wherein the weight ratio of the first and second SCR compositions in the first and third catalyst layers is from 2:3 to 3:

2.

12. The catalytic article of claim 1 further comprising means for electrically heating the catalytic article.

13. An exhaust gas treatment system comprising the catalytic article of claim 1.

14. 14. The exhaust gas treatment system of claim 13, comprising, in sequence: a catalyzed soot filter (CSF); means for injecting a nitrogen-based reductant; an SCR catalyst article; and the catalyst article of claim 1.

15. A fuel combustion and exhaust gas treatment system comprising an engine and an exhaust gas treatment system according to claim 13 or 14.

16. 16. A vehicle comprising the fuel combustion and exhaust treatment system of claim 15, wherein at least the catalytic article of claim 1 is located in an underfloor location and / or the vehicle experiences exhaust gases at temperatures of 270-350°C in normal use.

17. A method for the treatment of exhaust gases, comprising passing the exhaust gases through the catalytic article of claim 1.

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