RU2776705C2 - Catalytic product with active scr substrate, layer of ammonia slip neutralization catalyst, and scr layer for use in exhaust treatment system - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to an improved catalytic product for use in an exhaust treatment system. A catalytic product for treatment of a flow of combustion exhaust gases contains: catalytically active substrate containing one or more channels passing along its axial length, through which the flow of combustion exhaust gases flows during use, wherein one or more channels have the first surface for contact with the flow of combustion exhaust gases. In this case, substrate is formed by extruded material of a vanadium-containing SCR catalyst, while the first layer is located on at least part of the first surface, while the first layer contains a catalytic composition for ammonia slip neutralization, containing one or more platinum group metals deposited on titanium dioxide, mixed oxide of silicon dioxide-titanium dioxide, containing 5-25% by wt. of silicon dioxide, mixed Ce-Zr oxide having a ratio of cerium oxide to zirconium dioxide from 60:40 to 30:70 per total oxide, or a mixture thereof, and the second layer is located on at least part of the first layer and contains a catalytic SCR composition.

EFFECT: provision of improved resistance to poisoning, especially relatively to vanadium, for ammonia slip neutralization catalysts (ASC), while the degree of vanadium poisoning is much lower.

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Description

The present invention relates to an improved catalyst article for use in an exhaust treatment system. In particular, it relates to an ammonia slip neutralization catalyst composition supported on an extruded vanadium-containing SCR catalyst with improved resistance to vanadium poisoning.

The combustion of hydrocarbons in diesel engines, stationary gas turbines and other systems produces exhaust gases that must be treated to remove oxides of nitrogen (NO _x ), which include NO (nitrogen oxide) and NO ₂ (nitrogen dioxide), where NO is a large part of the formed NO _x . NO _x is known to cause a number of health problems in humans, as well as a number of adverse environmental effects, including the formation of smog and acid rain. In order to reduce human and environmental exposure to NO _x in exhaust gases, it is desirable to eliminate these undesirable components, preferably in a manner that does not form other harmful or toxic substances.

Exhaust gases from lean-burn and diesel engines are generally oxidative. NO _x must be selectively reduced with a catalyst and a reducing agent in a process known as selective catalytic reduction (SCR), which converts NO _x to elemental nitrogen (N ₂ ) and water. In the SCR process, a gaseous reducing agent, typically anhydrous ammonia, aqueous ammonia, or urea, is added to the exhaust gas stream prior to contact with the catalyst. The reducing agent is absorbed onto the catalyst and NO _x is reduced as the gases pass through or over the catalytic substrate.

In order to maximize NO _x conversion efficiency, it is often necessary to add more than stoichiometric amount of ammonia to the gas stream. However, the release of excess ammonia into the atmosphere can harm human health and the environment. In addition, ammonia is corrosive, especially in its aqueous form. Condensation of ammonia and water in the exhaust pipe sections downstream of the exhaust catalysts can form a corrosive mixture that can damage the exhaust system. Therefore, the release of ammonia into the exhaust gases must be avoided. In many conventional exhaust systems, an ammonia oxidation catalyst (also known as an ammonia slip catalyst or "ASC") is installed after the SCR catalyst to remove ammonia from the exhaust by converting it to nitrogen. The use of ammonia slip catalysts can provide NO _x conversions in excess of 90% over a typical diesel engine driving cycle.

Known catalysts for the oxidation of excess ammonia in exhaust gases. WO2016205506 describes an example of an ammonia slip neutralization catalyst. This invention includes examples of selective ASC coatings containing Pt on alumina deposited on extruded vanadium-containing catalysts. The coatings are said to have good resistance to vanadium poisoning.

WO2011092521 describes a flow wall filter containing an extruded solid mass for use in the treatment of nitrogen oxides in exhaust emissions from internal combustion engines from stationary sources and mobile applications. This filter substrate contains vanadium. WO2009093071 and WO2013017873 describe similar substrates.

WO2015011452 describes an exhaust gas treatment catalytic article.

Accordingly, it is desirable to provide an improved ammonia slip catalytic product and/or solve at least some of the problems associated with the prior art, or at least offer a commercially viable alternative thereof.

According to a first aspect, a catalytic article for treating a combustion exhaust stream is provided, the article comprising:

a catalytically active substrate comprising one or more channels extending along its axial length through which a combustion exhaust stream flows during use, the one or more channels having a first surface for contact with the combustion exhaust stream;

in this case, the substrate is formed by an extruded material of a vanadium-containing SCR catalyst,

wherein the first layer is located on at least a part of said first surface, wherein the first layer contains an ammonia slip neutralization catalytic composition containing one or more platinum group metals supported on titanium dioxide, mixed oxide silicon dioxide–titania, mixed oxide Ce– Zr, or a mixture thereof, and

the second layer is located on at least a portion of the first layer and contains the SCR catalyst composition.

The present invention will now be described in more detail. In the following paragraphs, various aspects/embodiments of the invention are defined in more detail. Each aspect/embodiment so defined may be combined with any other aspect/embodiment or aspects/embodiments, unless expressly stated otherwise. In particular, any feature or features identified as preferred or advantageous may be combined with any other feature or features identified as preferred or advantageous. It is contemplated that features disclosed in connection with a product may be combined with features disclosed in connection with a method, and vice versa.

In use, the extruded material of the vanadium-containing SCR catalyst in the presence of a nitrogen reducing agent serves to reduce _NOx in the exhaust gases. Any excess nitrogen reducing agent contacts the ASC in the first layer and is converted to nitrogen. The presence of a second layer containing additional SCR material solves the problem of NO _x recovery that can occur when ASC is in contact with a nitrogen reducing agent. As a result of this configuration, NO _x conversion levels are high.

However, the inventors have found that vanadium can migrate from the substrate into the PGM ASC-coating layer. This leads to deactivation, especially after 580°C/100 h aging in the engine, corresponding to end-of-life simulation in a vehicle, and is observed where the PGM carrier is based on alumina or silica-alumina. Surprisingly, it has been found that by using a different carrier material for the PGM in the ASC coating, as described herein, the functionality of the ASC becomes much more stable. In particular, the inventors have unexpectedly found that the use of a PGM support material based on titanium dioxide, silica-titania or mixed Ce-Zr oxide provides improved resistance to poisoning, especially against vanadium, for ammonia slip catalysts (ASCs) supported onto extruded substrates of the vanadium-containing catalyst. This has been demonstrated, in particular, in comparison with conventional silica-alumina support materials.

The inventors unexpectedly found that the migration of vanadium still occurs, even in the presence of local areas containing up to 1% of the mass. vanadium in fresh catalyst. However, the degree of vanadium poisoning appears to be much lower. Without being bound by any theory, it is believed that vanadium and PGM bind to different sites in the carrier material, whereby vanadium does not affect the catalytic properties of PGM.

The present invention relates to a catalytic article. Under the catalytic product refers to the structure described here, with catalytic properties. The catalytic properties are due to the materials included in the structure or deposited on it in the form of a coating. The product defined herein includes both the coated catalytic substrate described herein and a processed and wrapped SCR and/or ASC module suitable for installation on a vehicle. The catalytic article is a heterogeneous oxidation catalyst that is effective in reducing ammonia slip when used for or after the SCR process.

The catalytic article is intended for treating/purifying a combustion exhaust gas stream. Thus, the catalytic article can be used to treat/purify exhaust gases produced in the combustion process, for example, from an internal combustion engine (mobile or stationary), a gas turbine for stationary, marine or moving equipment, and for coal-fired or power plants. oil. The product can also be used to treat/purify gas from industrial processes such as oil refining, from refinery heaters and boilers, furnaces, chemical process industries, coke ovens, municipal waste treatment plants and incinerators, etc. In a particular embodiment, the method is used to treat/purify exhaust gases obtained from a gas turbine or lean-burn engine.

The catalytic article contains a catalytically active substrate containing one or more channels extending along its axial length through which the flow of combustion exhaust gases flows during use. In use, one or more channels have a first surface for contact with the combustion exhaust stream. The catalytically active substrate provides SCR functionality, whereby the _NOx in the combustion gases will be converted to nitrogen and water.

The substrate is formed by an extruded material of a vanadium-containing SCR catalyst. Examples of such extruded vanadium-containing substrates are given in WO2011092521, WO2009093071 and WO2013017873, the contents of which are incorporated herein by reference. The use of such a vanadium-containing substrate provides effective SCR properties, but creates the risk of vanadium poisoning. Preferably the substrate contains 1-3% wt., more preferably 1.5-2.5% wt. vanadium oxide. Such levels are suitable for good SCR properties.

The extruded SCR catalyst material preferably also contains a transition metal promoted molecular sieve. For example, a preferred substrate is formed from a mixture of vanadium/tungsten/titanium dioxide and iron promoted ZSM-5 zeolite as described in WO 2014/027207 A1, the entire content of which is incorporated herein by reference. Other suitable transition metals and molecular sieves are well known in the art discussed here.

Preferably the substrate is a flow-through monolithic substrate. Honeycomb configurations are more compact than plate configurations, but have higher pressure drops and clog (become blocked) much more easily. For most mobile applications, preferred substrates include flow-through monoliths having a so-called honeycomb geometry that contains a plurality of adjacent parallel channels that are open at both ends and typically extend from the inlet end face to the outlet end face of the substrate and result in high ratio of surface area to volume. For some applications, the honeycomb flow monolith preferably has a high cell density, such as about 600 to 800 cells per square inch (93 to 124 cells per cm ² ), and/or an average inner wall thickness of about 0.18 to 0.35 mm , preferably about 0.20-0.25 mm. For some other applications, the honeycomb flow monolith preferably has a low cell density, about 150-600 cells per square inch (16-93 cells per cm ² ), more preferably about 200-400 cells per square inch (31-62 cells per cm ² ). Preferably, the honeycomb monoliths are porous. Alternatively, the substrate can be the channels of a so-called flow wall filter.

The first layer is located on at least part of the inner walls of the channels of the substrate, i. "first surface". According to one embodiment, the first layer covers up to 50% of the axial length of the substrate, preferably 10-40% of the axial length, and preferably extends from the end of the product. In this embodiment, the remaining uncoated substrate serves as an SCR catalyst for NO _x treatment in the presence of a nitrogen reducing agent. The coated portion created by the first layer then serves as the ASC. Preferably, the coated portion extends from the lower end of the substrate during use, whereby the ASC attacks any residual ammonia (or the like) before it leaves the substrate.

According to an alternative embodiment, the first layer covers up to 50% of the axial length of the substrate, preferably 100% of the axial length. In this embodiment, the device can function entirely as an ASC, with the ASC properties provided by the first layer, and it can be located downstream of a separate SCR catalyst in the aftertreatment/treatment system.

The first layer contains the catalytic composition for neutralizing the ammonia slip. The ammonia slip neutralization catalyst composition contains one or more platinum group metals (PGMs) supported on titanium dioxide, silica-titania mixed oxide, Ce-Zr mixed oxide, or a mixture thereof. PGMs are selected from ruthenium, rhodium, palladium, osmium, iridium, platinum, and mixtures of two or more of these metals. Preferably the PGM is platinum, palladium, or a combination thereof, and most preferably is platinum. Preferably the first layer contains 0.05-0.5% wt. platinum group metal, more preferably 0.1-0.2% of the mass. Such PGM contents provide desirable ASC properties. Below the lower limit, the durability of the ASC coating is reduced due to the sintering of the PGM when heated. Above the upper limit, the selectivity of the catalyst decreases.

Titanium dioxide, if present in the first layer, is preferably anatase because it has a higher surface area. Mixed silica-titanium dioxide, if present, can be characterized by a balance of silica to titanium dioxide. Preferably, the mixed silica-titanium dioxide contains less than 50 wt. silicon dioxide, preferably 5-25% wt., and more preferably 7-15% wt. silicon dioxide. The Ce–Zr mixed oxide, if present, can be characterized by a balance of cerium oxide to zirconia. Preferably, the Ce-Zr mixed oxide has a ratio of cerium oxide to zirconia from 60:40 to 30:70 based on total oxide, i.e. CeO ₂ :ZrO ₂ .

In addition to the PGMs on the media listed, the first layer may contain additional components. For example, components such as fillers, binders, stabilizers, rheological modifiers and other additives. In some embodiments, the porous oxide coating contains blowing agents such as graphite, cellulose, starch, polyacrylate, and polyethylene, and the like. These additional components do not necessarily catalyze the desired reaction, but instead improve the performance of the catalytic material, for example, by increasing its operating temperature range, increasing the contact surface area of the catalyst, increasing the adhesion of the catalyst to the substrate, and the like. Typically, the only additional component may be a binder. Typically, the metal oxide particles used as binders differ from the metal oxide particles used as carriers in terms of particle size, with the binder particles being significantly larger compared to the carrier particles. Preferably, the additional components are less than 25% wt., preferably less than 15% wt., and most preferably less than 10% wt. the first layer, with the rest being PGM and titanium dioxide, mixed silica-titania, mixed Ce-Zr oxide, or a mixture of these.

Preferably the first layer contains less than about 5% of the mass. and more preferably less than about 1% of the mass. vanadium in a fresh state, after the initial calcination to fix the layers.

The second layer is located on top of the first layer, and the second layer contains the SCR catalyst composition. The second layer contains a catalyst for the accumulation of NH₃ and selective reduction of NO_x with NH₃ in the presence of oxygen, also referred to here as the SCR catalyst. Preferably the second layer covers the entire first layer inside the channels. Indeed, most preferably the second layer surrounds the first layer in such a way that no part of the first layer can directly contact the exhaust gas stream. This configuration provides improved N selectivity.₂ and is characterized by minimal formation of N₂O and NO and/or NO₂. Partial overlap or complete exclusion of the upper SCR layer can increase NH conversion₃ due to selectivity (more formation of N₂O and NO and/or NO₂ via ASC). Top layer with some usable NH₃ or without it can lead to lower selectivity with increased N production₂O and NO and/or NO₂.

Preferably, the second layer SCR catalyst composition comprises a copper-promoted zeolite, an iron-promoted zeolite, or a combination thereof. The first SCR catalyst is preferably Cu-SCR catalyst, Fe-SCR catalyst or mixed oxide, more preferably Cu-SCR catalyst or Fe-SCR catalyst, most preferably Cu-SCR catalyst. Cu-SCR-catalyst contains copper and zeolite. Fe–SCR–catalyst contains iron and zeolite.

Zeolite is a microporous aluminosilicate having any of the framework structures listed in the "Database of Zeolite Structures" published by the International Zeolite Association (IZA). Framework structures include, without limitation, CHA, FAU, BEA, MFi, MOR types. Non-limiting examples of zeolites having these structures include chabasite, faujasite, zeolite Y, ultrastable zeolite Y, beta zeolite, mordenite, silicalite, zeolite X, and ZSM-5. The aluminosilicate zeolites may have a silica to alumina molar ratio (SAR, defined as SiO ₂ /Al ₂ O ₃ ) of at least about 5, preferably at least about 20, with suitable ranges from about 10 to 200.

Preferably the second layer has at least some ammonia storage up to 0.1 g per gram of the second layer measured at 200° C., preferably 0.01-0.05 g/g and most preferably about 0.025 g/g.

As used herein, the terms "first layer" and "second layer" are used to describe the relative positions of the catalyst layers in a catalyst article with respect to the normal direction of flow of exhaust gases through, after, and/or over the catalyst article. Under normal exhaust gas flow conditions, the exhaust gases contact the second layer prior to contacting the first layer and will then contact the second layer again before exiting the catalyst article. It should be noted that the first and second layers are porous due to their structure and allow exhaust gases to pass within and through the material of the layer. The first layer is applied to the flow honeycomb substrate as the bottom layer and the second layer is the top layer that is applied over the first layer.

Methods for applying the first and second layers are well known in the art and include applying a porous oxide coating to the surfaces to be coated. After applying the layers to the product, they are usually calcined to fix the layers. The calcination is well known in the art and can be carried out in air at temperatures of about 500°C.

Preferably, the catalyst article described herein is encapsulated and ready for use in an automotive aftertreatment system.

In accordance with a further aspect, an exhaust treatment system for treating a combustion exhaust gas stream is provided, the system comprising a combustion exhaust gas source in fluid communication with a catalyst article described herein and a nitrogen reducing agent source located upstream of said article. Preferably, the source of combustion exhaust gases is a diesel engine.

Thus, the catalyst article of the present invention may be part of an exhaust gas treatment system in which the catalyst article is located downstream of a nitrogen reductant source. In accordance with one embodiment, the article is an SCR catalyst containing a downstream portion of the ASC catalyst. For example, an ammonia slip neutralization catalyst is placed at the downstream end of the flow substrate, and an SCR catalyst is placed at the upstream end of the flow substrate. According to another embodiment, the article is an ASC catalyst to be located downstream of a separate Selective Catalytic Reduction (SCR) catalyst. Thus, the ammonia slip catalyst and the SCR catalyst are placed on separate modules inside the exhaust system. These individual modules may be adjacent and in contact with each other, or may be separated by a certain distance, provided they are in fluid communication with each other, and provided that the SCR catalyst module is located upstream. flow from the catalyst module to neutralize the slip of ammonia. In both of these embodiments, the ammonia slip neutralization catalyst oxidizes at least a portion of any nitrogen reductant that is not consumed in the selective catalytic reduction process.

The nitrogen reducing agent may be ammonia itself, hydrazine, or an ammonia precursor selected from the group consisting of urea ((NH ₂ ) ₂ CO), ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate, and ammonium formate. Ammonia is the most preferred.

Preferably, the first layer covers up to 50% of the axial length of the substrate and extends from the downstream end of the article during use.

In accordance with a further aspect, a method is provided for treating a combustion exhaust stream, the method comprising contacting the combustion exhaust stream with a catalyst article described herein in the presence of a nitrogen reducing agent.

Although the present invention describes the first and second layers, it is also contemplated that the first and second layers can be made as a single mixed layer, including all those components that are described here for the first and second layers.

The present invention will now be described with reference to the following non-limiting drawings, in which:

figure 1 shows a cross section of a flowing monolithic substrate;

Figure 2 is a graph showing the activity of fresh and aged catalysts described herein and in the prior art.

Figure 1 shows one channel 5 within the flow substrate 2 in accordance with the catalytic product 1 of the present invention. Substrate 1 has walls 10 formed from extruded vanadium-containing SCR material. The walls 10 define a channel 5 through which the exhaust gas 15 passes. Each wall 10 has a surface 20 for contact with the exhaust gas 15.

At the downstream end 25 of the surfaces 20 is a first layer 30. The first layer 30 contains an ammonia slip neutralization catalyst material containing Pt on mixed silica-titania.

The first layer 30 is completely covered by the second layer 35 containing the SCR catalyst composition.

During use, the exhaust gas 15 contacts the SCR material in the substrate 1 in the presence of ammonia. This causes the NO _x in the exhaust gas 15 to be converted into nitrogen and water.

The excess ammonia in the exhaust gas 15 then contacts the ASC in the first layer 30 and is converted to nitrogen. This conversion may also generate additional NO _x which then comes into contact with the SCR in the second layer 35 and is converted back to nitrogen.

In a preferred embodiment, flow substrate 2 is an extruded blend of vanadium/tungsten/titanium dioxide and iron promoted ZSM-5 zeolite. It has a first layer 30 deposited as a porous oxide coating containing about 0.15 wt. Pt, promoted by iron zeolite and less than 10% of the mass. silica sol as a binder. The binder promotes the adhesion of the layer to the substrate, but is preferably present in a minimal amount to avoid an increase in back pressure.

Then a second layer 35 of the SCR composition containing the iron-promoted zeolite is applied. It is applied with alumina as a binder. The first and second layers 30, 35 can be dried after application and then calcined in air at a temperature of about 500°C to fix them.

The finished product 1 is then enclosed in a shell for installation in the exhaust system.

The present invention will now be described with reference to the following non-limiting examples.

EXAMPLES

Example 1 Preparation of Extruded Honeycomb Substrate

The extruded honeycomb substrate catalyst according to WO 2014/027207 A1 was first prepared by mixing an MFI aluminosilicate zeolite that had been ion exchanged with >1% wt. iron with 2% wt. V ₂ O ₅ - WO ₃ /TiO ₂ other components, with inorganic auxiliary substances to improve the rheological properties of extrusion and increase the mechanical strength of the extrudate. Suitable organic auxiliaries such as extrusion lubricants and plasticizers may be added to facilitate mixing to form a uniform extrudable mass. Organic excipients may include cellulose, water soluble resins such as polyethylene glycol, and burn out from the final substrate during calcination. Appropriate proportions of zeolite, V ₂ O ₅ - WO ₃ /TiO ₂ , inorganic excipients were chosen so that after the removal of organic excipients, the substrate contained 16% of the mass. Fe/zeolite component, 72% wt. V ₂ O ₅ - WO ₃ /TiO ₂ components, 12% wt. inorganic excipients. The extrudable mass was extruded into honeycomb bodies 10.5 x 7.0 inches in diameter (26.7 x 17.8 cm) long and 400 cells per square inch (62 cells per cm ² ) in a flow configuration (i.e. cells open both ends) having a honeycomb cell wall thickness of 0.011 inches (0.028 cm). The extruded honeycomb substrate was then dried and calcined to obtain the final product.

Example 2: Obtaining a porous oxide coating of a copper-substituted zeolite SCR catalyst (second layer)

The commercially available synthetic aluminosilicate zeolite CHA was ion exchanged with NH4 ⁺ in NH ₄ NO ₃ solution, which was then filtered. The resulting materials were added to an aqueous solution of Cu(NO ₃ ) ₂ with stirring. The suspension was filtered, then washed and dried. The procedure could be repeated to achieve a metal concentration of 3% wt. The final product was calcined.

Comparative Example 3: Preparation of Catalytic Coating of Porous Pt Oxide/Silica Doped Alumina Catalyst for Ammonia Surge Neutralization Catalyst (First Layer)

The platinum nitrate solution was impregnated onto commercially available silica-doped alumina to form a slurry in which the target Pt content in the silica-doped alumina was 0.2 wt% and the nominal Pt loading was 3 g/ft ³ (0.007 kg / m ³ ).

Example 4: Preparation of a porous oxide catalytic coating containing Pt/silica-titania Ammonia slip neutralization catalyst (first layer)

A solution of platinum nitrate was impregnated onto a commercially available mixed silica-titania having a silica content of about 10 wt.% to form a slurry in which the Pt content in the mixed silica-titania was 0.2 wt.% and the nominal loading Pt was 3 g/ft ³ (0.007 kg/m ³ ).

Example 5 Catalytic Coating of Pt Oxide/Ce-Zr Mixed Oxide Ammonia Surge Neutralization Catalyst (First Layer)

A solution of platinum nitrate was impregnated onto a commercially available mixed cerium oxide-zirconia having a cerium oxide content of about 50 wt.% to form a slurry in which the Pt content in the mixed cerium oxide-zirconia was 0.2 wt.% and the nominal loading Pt was 3 g/ft ³ (0.007 kg/m ³ ).

Example 6 Coating of Substrates with First Layer Catalyst Compositions

Each of the three honeycomb substrates prepared according to Example 1 was coated at one end of the honeycomb substrate with a porous oxide coating of the first layer catalyst compositions described in Examples 3 (comparative), 4 and 5 using the method described in WO 99/47260 A1, to a depth of 2 inches (5.1 cm), i.e. a method comprising the steps of: (a) placing a retention agent on top of a substrate, (b) dispensing a predetermined amount of a liquid component into said retention agent, either in the order of (a) followed by (b), or (b) followed by (a), and ( c) applying a vacuum to draw all of said amount of the liquid component into at least a portion of the substrate and retain substantially all of said amount within the carrier, without recycling. The coated substrates were then dried and calcined.

Example 7 Coating Substrates with Second Layer Catalyst Compositions

Each of the three honeycomb substrates prepared in Example 6 was further coated with the Cu/zeolite catalyst composition of Example 2 using the same technique as described in Example 6, ie. in WO 99/47260 A1. Coatings of the second layer were applied to the same end of the honeycomb substrate in such a way that the second layer overlapped the first layer to such an extent that no part of the first layer was left exposed, i. a second layer was applied as a slight overlap over the first layer at the end of the first layer remote from the end of the coated substrate to eliminate an exposed first layer oxidizing the ammonia in the incoming gas. The coated substrates were then dried and calcined.

Example 8 Aging Conditions

The extruded catalytic honeycomb substrates prepared in Example 7 were thermally aged (without water) in an accelerated aging step, either by heating them in an oven at over 600° C. for 2 hours (referred to herein as "fresh") or at 650 °C for 100 h (referred to here as "aged") to simulate the expected impact of honeycomb substrates on vehicle exhaust gases over the life of a vehicle in accordance with the European emission standard.

Example 9: Test Conditions

Cores 1 inch (2.54 cm) in diameter were cut from the fresh and aged substrates from Example 7 and each was loaded into a laboratory Synthetic Catalyst Catalyst Tester (SCAT) to test each sample's ability to oxidize NH ₃ in model exhaust gas, containing 500 ppm NH ₃ , 4.5% of the mass. CO, 5 wt. H ₂ O, 200 ppm CO ₂ , 12% wt. O ₂ and the rest - N ₂ . The test was carried out at an exhaust gas space velocity of 150,000 h ^–1 . The results as % conversion of NH ₃ versus temperature are shown in FIG. 2.

As shown in FIG. 2, the silica-alumina-supported catalyst exhibits good activity in the fresh state, but decreases with aging. Supported catalysts of silica–titania and cerium oxide–zirconium dioxide have comparable activities in the fresh state, which decreases significantly less with aging than for the silica–aluminum oxide sample.

The foregoing detailed description has been presented for purposes of explanation and illustration and is not intended to limit the scope of the appended claims. Many changes to the currently preferred embodiments illustrated herein will be apparent to those skilled in the art and remain within the scope of the appended claims and their equivalents.

For the avoidance of doubt, the entire contents of all documents cited herein are incorporated herein by reference.

Claims (17)

1. A catalytic article for treating a combustion exhaust stream, comprising: a catalytically active substrate comprising one or more channels extending along its axial length through which a combustion exhaust stream flows during use, the one or more channels having a first surface for contact with the combustion exhaust stream; wherein the substrate is formed by an extruded material of a vanadium-containing SCR catalyst, wherein the first layer is located on at least part of the first surface, wherein the first layer contains an ammonia slip neutralization catalytic composition containing one or more platinum group metals deposited on titanium dioxide, a mixed oxide of silicon dioxide–titania, containing 5-25% mass. silicon dioxide, a mixed Ce-Zr oxide having a ratio of cerium oxide to zirconia from 60:40 to 30:70 based on total oxide, or a mixture thereof, and the second layer is located on at least a portion of the first layer and contains the SCR catalyst composition. 2. The article of claim 1 wherein the substrate is a honeycomb flow monolithic substrate. 3. An article according to any of the preceding claims, wherein the substrate contains 1-3% by weight, more preferably 1.5-2.5% by weight, vanadium oxide. 4. The product according to any of the preceding paragraphs, in which the first layer contains 0.05-0.5% of the mass. platinum group metal. 5. An article according to any one of the preceding claims, wherein the platinum group metal is Pt. 6. An article according to any one of the preceding claims, wherein the first layer covers up to 50% of the axial length of the substrate, preferably 10-40% of the axial length, and preferably extends from the end of the article. 7. An article according to any one of claims 1 to 5, wherein the first layer covers up to 50% of the axial length of the substrate, preferably 100% of the axial length. 8. An article according to any one of the preceding claims, wherein the second layer SCR catalyst composition comprises a copper-promoted zeolite, an iron-promoted zeolite, or a combination thereof. 9. An article according to any one of the preceding claims, wherein the second layer covers 100% of the area of the first layer. 10. An exhaust treatment system for treating a combustion exhaust gas stream, wherein the system comprises a combustion exhaust gas source in fluid communication with the catalyst article of any one of the preceding claims and a source of nitrogen reductant located upstream of said article. 11. The system of claim 10 wherein the first layer covers up to 50% of the axial length of the substrate and extends from the downstream end of the article. 12. The system of claim 10 or 11, wherein the combustion exhaust source is a diesel engine. 13. A method for treating a combustion exhaust gas stream, comprising: contacting a combustion exhaust gas stream with a catalytic article according to any one of claims 1 to 9 in the presence of a nitrogen reducing agent.

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