Waste gas treatment system

Abstract

A method and system for an exhaust gas treatment system comprising, in order: (i) a first device for injecting a nitrogen-containing reducing agent; (ii) a first selective catalytic reduction (SCR) catalyst; (iii) an ammonia slip catalyst (ASC); and (iv) a second selective catalytic reduction (SCR) catalyst, wherein the ASC comprises an SCR catalyst and a supported palladium (Pd) component.

Description

[0001] The present invention relates to an improved exhaust gas treatment system having improved NOx treatment and reduced ammonia slip. In particular, the present invention relates to the use of a palladium-containing ammonia slip catalyst (ASC) configured to allow more aggressive ammonia dosing for NOx removal without the expected increase in ammonia slip.

[0002] The combustion of hydrocarbon fuels produces engine exhaust or smoke, which contains relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2) in large portions. However, the exhaust also contains relatively small portions of harmful and / or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from unburned fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (primarily soot). In order to mitigate the environmental impact of exhaust emissions to the atmosphere, it is desirable to eliminate or reduce the amount of undesirable components, preferably by a process that does not in turn produce other harmful or toxic substances.

[0003] NOx, which includes nitrogen monoxide (NO), nitrogen dioxide (NO2) and nitrous oxide (N2O), is a burden component to be removed from the exhaust gas produced by lean-burn engines. Reducing NOx to N2 is particularly problematic in lean-burn exhaust gases because the exhaust gas contains enough oxygen to promote oxidation reactions rather than reduction. Nevertheless, NOx can be reduced by a method generally referred to as selective catalytic reduction (SCR). The SCR method involves converting NOx into elemental nitrogen (N2) and water in the presence of a catalyst and with the aid of a nitrogen-containing reducing agent such as ammonia. In the SCR method, a gaseous reducing agent such as ammonia is added to the exhaust gas flow before the exhaust gas contacts the SCR catalyst. The reducing agent is adsorbed onto the catalyst, and when the gas passes through or over the catalytic substrate, the NO reduction reaction occurs. The chemical equation for the stoichiometric SCR reaction using ammonia is:

[0004] 4NO+4NH3+O2→4N2+6H2O

[0005] 2NO2+4NH3+O2→3N2+6H2O

[0006] NO+NO2+2NH3→2N2+3H2O

[0007] Some SCR methods utilize a stoichiometric excess of ammonia to maximize the conversion of NOx. Ammonia that passes through the SCR process without reacting (also known as "ammonia slip") is undesirable because, if released, the leaked ammonia can react with other combustion substances and / or have a negative impact on the atmosphere. To reduce ammonia slip, the SCR system can include an ammonia oxidation catalyst (AMOX) (also known as an ammonia slip catalyst (ASC)) downstream of the SCR catalyst.

[0008] Catalysts for oxidizing excess ammonia in exhaust gases are known. For example, U.S. Pat. No. 7,393,511 describes an ammonia oxidation catalyst containing a noble metal such as platinum, palladium, rhodium or gold on a support such as titanium dioxide, aluminum oxide, silicon dioxide, zirconium oxide, etc. These catalysts oxidize NH3 to produce N2 and / or secondary NOx+H2O as shown below:

[0009] 4NH3+7O2→4NO2+6H2O

[0010] 4NH3+5O2→4NO+6H2O

[0011] 2NH3+2O2→N2O+3H2O

[0012] 4NH3+3O2→2N2+6H2O

[0013] To remedy this secondary NOx formation, a typical ASC contains a top catalyst layer (which contains a conventional SCR catalyst) and a bottom catalyst layer (which contains an ammonia oxidation catalyst). Exhaust containing slipped NH3 and little or no NOx is passed through the top SCR layer of the ASC, where the SCR catalyst stores a portion of the NH3. Another portion of the NH3 continues to permeate through the catalyst until it reaches the bottom layer, where it is oxidized to secondary NOx and H2O. Secondary NOx permeates back through the top layer, where it reacts with the stored NH3 to produce N2 and H2O.

[0014] The top and bottom layers of the ASC are separated to prevent NH3 from being immediately oxidized (which would result in the formation of untreated secondary NOx in the exhaust gas stream). Therefore, the top layer in the ASC is typically free of precious metals such as platinum group metals (PGMs). In addition, the bottom layer containing the PGM-based oxidation catalyst is completely covered by the top layer to prevent untreated secondary NOx from entering the exhaust gas stream.

[0015] The construction and composition of ammonia slip catalysts are well known in the art. As mentioned above, ASCs are usually placed at the end of the exhaust system as a gatekeeper to prevent ammonia from slipping from the system.

[0016] Pt is commonly used in ammonia slip catalysts (ASCs). Pt has a lower light-off temperature (~250°C) than Pd (~400°C). This means that it is preferred to use Pt rather than Pd in ​​an ASC at the end of the exhaust system, as Pt is able to achieve lower NH3 slip levels than Pd at low temperatures. Pt is most useful for cleaning exhaust emissions at cold starts in an ASC, before the SCR reaches temperature and uses ammonia to convert NOx.

[0017] Although Pt ASC is a commercial product, ASC containing palladium is also contemplated. For example, US20150037233 discloses providing ASC components containing palladium.

[0018] It is an object of the present invention to provide an improved exhaust gas treatment system that addresses problems associated with the prior art and / or at least provides a commercially viable alternative option.

[0019] Therefore, the present invention provides an exhaust gas treatment system, which comprises in order:

[0020] (i) a first means for injecting a nitrogen-containing reducing agent;

[0021] (ii) a first selective catalytic reduction (SCR) catalyst;

[0022] (iii) an ammonia slip catalyst (ASC); and

[0023] (iv) a second selective catalytic reduction (SCR) catalyst,

[0024] The ASC comprises an SCR catalyst and a supported palladium (Pd) component.

[0025] The present invention will now be further described. In the following paragraphs, different aspects / embodiments of the present invention are defined in more detail. Each aspect / embodiment defined in this way can be combined with any other one or more aspects / embodiments, unless there is a clear contrary indication. Specifically, any feature indicated as being preferred or advantageous can be combined with any other one or more features indicated as being preferred or advantageous.

[0026] The inventors have now discovered that the disclosed exhaust system can now be operated in a different manner than that which relies on a conventional ASC. Rather than a cleanup catalyst used occasionally at the end of the system (cold start and high temperature ammonia slip events), the ASC is used continuously as a buffer which controls the ammonia feed to the second SCR. That is, the system does not have an additional injector upstream of the second SCR, but the ASC acts as a buffer which controls the ammonia that the second SCR is exposed to. This system is able to operate with lower NOx emissions and lower NH3 slip, but with a more aggressive ANR target upstream of the first SCR.

[0027] Specifically, a single injector upstream of the SCR / ASC / SCR configuration is run aggressively with an ammonia to NOx ratio (ANR) ratio > 1 (greater than stoichiometric ammonia). This means that NOx reduction on the first SCR component can be very high. However, this results in increased NH3 leakage from the first SCR component, which needs to be mitigated. It is mitigated in this case by an ASC containing Pd.

[0028] In order to operate the ASC in this manner, a Pd ASC (i.e. not a Pt ASC) needs to be used. The Pd ASC is highly selective for N2 relative to NOx. This means that in the operating window above 400°C, above the NH3 oxidation light-off temperature observed on the Pd ASC, the Pd ASC is able to cope with intentional ammonia slip without excessive NOx regeneration. If there is high NOx regeneration, this will make higher SCR NOx conversion rates meaningless. Since the Pt ASC has a lower selectivity for N2 formation (i.e. there is a higher amount of NOx regeneration), when exposed to intentional NH3 slip with an aggressive dosing strategy, there will be a large amount of NOx regeneration at high temperatures, which ineffectively reduces the high NOx reduction achieved relative to the SCR, which results in increased NOx emissions.

[0029] In new continuous use, scheme Pd is ideal because it has better selectivity for N2 formation. In addition, since this component is not exactly at the end of the system (see conventional ASC), it encounters conditions closer to the optimal operating conditions of Pd.

[0030] The present invention relates to an exhaust gas treatment system. An exhaust gas treatment system is a conventional arrangement of catalyst components provided in an airtight flow path. The system extends from an exhaust manifold of an exhaust gas source to an outlet to release the treated exhaust gas into the atmosphere. Typically, the exhaust manifold comes from an engine such as a diesel automobile engine. The exhaust gas from a diesel engine contains hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx). These gaseous components need to be treated to convert them before being released into the atmosphere.

[0031] The exhaust treatment system disclosed herein relies on known exhaust system components provided in a new configuration. That is, while the Pd-containing ASC is not common, it is not disclosed in the SCR / ASC / SCR configuration disclosed herein. Additional components, including, for example, DOC, SCR, CSF components may all take a range of different forms and configurations and are not overly limited. These components are all within the knowledge of those skilled in the art and may be selected and configured as needed for the final configured system.

[0032] The system includes a number of components "in sequence." That is, the components are provided in fluid connection with each other and arranged so that exhaust passing through the exhaust treatment system contacts each component in turn in the sequence described. For example, the exhaust to be treated must pass through the first SCR before passing through the ASC.

[0033] The system comprises a first means for injecting a nitrogenous reductant. Such means is typically a spray injector. The nitrogenous reductant is typically urea or ammonia, although other precursor chemicals may be injected which decompose to provide ammonia in situ. For simplicity, the present invention generally refers to ammonia, but in those embodiments where the nitrogenous reductant is not ammonia itself, this is shorthand for an ammonia equivalent. Preferably, the nitrogenous reductant is ammonia.

[0034] Preferably, the first means for injecting the nitrogenous reductant is configured to dispense the nitrogenous reductant at an ANR greater than 1, preferably 1.1-1.5. The ANR or ammonia to NOx ratio is well known in the art. In conventional systems, it is often the case that the ANR is configured to target an ANR of 1. That is, the stoichiometric ratio of ammonia (or ammonia equivalent) to NOx is 1. This configuration may be based on measurements of a first sensor for determining NOx levels in the exhaust gas.

[0035] Preferably, the system further comprises a first sensor for determining the NOx level in the exhaust gas. The first sensor may be a NOx measurement sensor, which is arranged downstream of the first device for injecting a nitrogenous reductant. Preferably, when the first sensor is a NOx sensor, it is downstream of the first SCR catalyst. In this way, feedback can be provided based on the NOx passing through the first SCR to determine the nitrogenous dosage required to achieve the desired ANR. Optionally, the first sensor is one or more of a temperature sensor, an engine speed sensor, and an engine load sensor. Based on such measurements, a lookup table can be used to determine the estimated NOx level in the exhaust gas.

[0036] The system comprises a first selective catalytic reduction (SCR) catalyst. SCR catalysts are well known in the art. There are no particular limitations on the SCR catalyst of the present invention, provided that it can selectively reduce NOx in an oxidizing environment in the presence of a reducing agent. Preferably, the first SCR catalyst comprises a Cu or Fe doped zeolite component.

[0037] The SCR catalyst preferably comprises at least one promoter metal on a high surface area support, the support comprising a refractory metal oxide and a molecular sieve such as an aluminosilicate (zeolite), a silicoaluminophosphate (SAPO) or an aluminophosphate (AlPO). In certain embodiments, the SCR catalyst has little to no NH3 oxidation capability, but can store and release NH3 as a function of temperature, as a function of lean or rich environments, or both. As used herein, the term "lean environment" refers to an exhaust gas produced by burning a fuel with a superstoichiometric air-fuel mixture (e.g., excess air) or an exhaust gas containing an amount of oxygen at least equivalent to that of a lean burn exhaust gas. As used herein, the term "rich environment" refers to an exhaust gas produced by burning a rich mixture of air-fuel.

[0038] Preferred promoter metals are selected from V, Cr, Co, Cu, Fe, Hf, La, Ce, In, V, Mn, Ni, Zn, Ga, alone or in combination. The promoter metal may be a free metal or a metal ion and may be incorporated onto or into the support by various techniques including ion exchange, incipient wetness, direct coating or in situ during the synthesis of the support material. Preferred promoter metals include Cu and Fe, particularly when the promoter metal is supported on and / or into a molecular sieve, preferably a small pore zeolite.

[0039] The promoter metal oxide can be vanadium such as free vanadium, vanadium ions or vanadium oxides or derivatives thereof. Preferably, the form of vanadium is vanadium oxide (V2O5). In addition to vanadium, the promoter metal oxide can include other catalytically active metal oxides, such as tungsten oxides and / or molybdenum oxides. As used herein, a "catalytically active" metal oxide is such that it directly participates in the catalytic reduction of NOx and / or the oxidation of NH3 or other nitrogen-based SCR reductants as a molecular component. In certain embodiments, the SCR catalyst is V2O5 / WO3 / TiO2, optionally comprising MoO3.

[0040] Preferred molecular sieve supports include zeolites and SAPOs having a framework selected from the group consisting of AEI, AFX, CHA, KFI, LEV, ERI, DDR, UEI, RHO, EAB, PAU, MER, GOO, YUG, GIS, UFI, VIN, AEI / CHA intergrowth, BEA, MFI, MOR and FER. In certain embodiments, the framework is selected from the group consisting of AEI, CHA and intergrowth thereof. Preferred aluminosilicate molecular sieves have a silica to alumina ratio of about 10 to about 50, preferably about 15 to about 25.

[0041] Particularly preferred SCR catalysts include V2O5 / WO3 / TiO2, which optionally includes MoO3; Cu supported on an aluminosilicate molecular sieve having a framework selected from AEI, CHA or a combination or inter-organism thereof; and Fe supported on an aluminosilicate molecular sieve having a framework selected from BEA and FER.

[0042] The system comprises an ammonia slip catalyst (ASC). The ASC comprises an SCR catalyst and a supported palladium (Pd) component. The SCR catalyst component can be any of the above-mentioned SCR catalysts.

[0043] The palladium component of the load means that the palladium is loaded on a carrier material. Such materials are well known in the art. There is no particular restriction on the type of carrier used for palladium, provided that it is a particle with a large surface area, is inert, and is suitable for post-processing systems. Examples of carrier materials include refractory metal oxides, such as aluminum oxide, silicon dioxide, zirconium oxide, titanium dioxide, ceria and physical mixtures or composites thereof, and aluminum oxide and ceria are particularly preferred. In certain embodiments, the carrier has wide pores (e.g., 100-350Å) or both wide pores and narrow pores. In certain embodiments, the BET surface area of ​​the carrier is at least 50 m 2 / g, preferably about 50-500m 2 / g, more preferably about 50-300m 2 / g, or about 150-250m 2 The refractory metal oxide support preferably has a pore volume of about 0.1 to 0.5 g / cc, such as about 0.2 to 0.4 g / cc, preferably as measured by mercury intrusion porosimetry.

[0044] Other palladium supports for the second oxidation catalyst include molecular sieves such as aluminosilicates, silicoaluminophosphates and aluminophosphates having a zeolite-type framework, such as AEI, AFX, CHA, KFI, LEV, ERI, DDR, UEI, RHO, EAB, PAU, MER, GOO, YUG, GIS, UFI, VIN, AEI / CHA intergrowth, BEA, MFI, MOR and FER. Preferably, the palladium component is supported on a particulate metal oxide or zeolite, preferably alumina.

[0045] Preferably, the ASC is substantially free of Pt. The presence of less selective Pt in the exhaust system will lead to undesirable NOx production. Therefore, its content should be minimized. Preferably, the ASC contains less than 10 wt% Pt, preferably less than 2 wt%, preferably less than 1 wt%, based on the total weight of PGM in the ASC, and preferably it is free of Pt.

[0046] Although it is known to provide an ASC component comprising a mixture of Pt and SCR catalyst, it is preferred that the ASC used in the system of the present invention has a layered structure. That is, it is preferred that the Pd is provided in a layer separate from the SCR catalyst. Preferably, in such a layered structure, the upper layer comprises the SCR catalyst, and the lower layer comprises a supported palladium (Pd) component. The SCR catalyst introduced into the ASC is used to treat secondary NOx and is physically separate and different from the SCR catalysts (first, second and third) discussed herein for treating NOx generated by more general fuel combustion.

[0047] The system comprises a second selective catalytic reduction (SCR) catalyst. The SCR catalyst component may be any of the above-mentioned SCR catalysts. Preferably, the second SCR catalyst comprises a Cu or Fe doped zeolite component.

[0048] Preferably, there is no additional means for injecting nitrogenous reductant after the first SCR.That is, the Pd-ASC provides all necessary ammonia control for the second SCR catalyst required in the system.

[0049] In one embodiment, a first selective catalytic reduction (SCR) catalyst and an ammonia slip catalyst (ASC) share a common substrate. In another embodiment, an ammonia slip catalyst (ASC) and a second selective catalytic reduction (SCR) catalyst share a common substrate. Alternatively, all three may be disposed on separate substrates. Providing one or more components on a common substrate provides space and volume efficiencies for the exhaust system.

[0050] When they share a common substrate, preferably the ASC is provided over less than 50%, more preferably 10-40% of the axial length of the common substrate, while the SCR catalyst (first or second) is provided over the remaining length of the common substrate.

[0051] The exhaust gas treatment preferably further comprises, upstream of the first means for injecting nitrogenous reductant, in order: a second means for injecting nitrogenous reductant; a third selective catalytic reduction (SCR) catalyst; and a catalyzed soot filter (CSF). The third SCR catalyst may be any material discussed above in relation to the SCR catalyst.

[0052] A catalyzed soot filter is a component known in the art. Specifically, it is a catalyst coated diesel particulate filter (DPF). It catalyzes the oxidation of gaseous emissions, converting carbon monoxide and hydrocarbons into carbon dioxide and water by the presence of precious metals (such as Pt and Pd) coated on a wall flow monolith. It also oxidizes NO to NO2 to provide sufficient NO2 for passive regeneration of soot and to provide a sufficient NO2 / NOx ratio for SCR downstream. The wall flow format means that it also filters out particulate matter, which decomposes under hot conditions when the filter is regenerated.

[0053] When the exhaust system contains a CSF, it is undesirable to have ammonia leak onto it, as the PGM will oxidize it there to produce more NOx. Therefore, it is desirable to run the system containing the CSF with underdosed ammonia on the upstream SCR (here, the third SCR catalyst, which is the upstream or "front" SCR in this system). This means that there is much less potential for leaks, while still allowing a satisfactory amount of NOx reduction to be performed on the SCR.

[0054] In use, the system has a front SCR (third SCR) that performs well under cold start conditions. However, given the temperature dependence of the SCR reaction, its performance decreases under higher temperature conditions. The first and second SCRs are in a later, cooler position, so under high load conditions in the engine (when it is fully hot), these can replace the required SCR performance. Therefore, the configuration of the catalyst disclosed herein allows a new ammonia management scheme, thereby reducing ammonia more at the upstream injector and increasing ammonia at the downstream injector to make the most efficient use of ammonia. Therefore, one aspect of the present invention here is based on the temperature conditions of the entire exhaust system (at the front when colder, then towards the rear when warmer), with a controlled and variable dosing between the two injectors.

[0055] Preferably, the exhaust system further comprises a diesel oxidation catalyst, which is upstream of the second device for injecting a nitrogenous reductant or between the third SCR catalyst and the CSF. Diesel oxidation catalysts are well known in the art. Diesel oxidation catalysts (DOC) are catalytic converters designed specifically for diesel engines and diesel equipment to reduce emissions of carbon monoxide (CO), hydrocarbons (HC) and particulate matter (PM). They also oxidize NO to NO2 to provide a sufficient NO2 / NOx ratio downstream of the SCR. Modern catalytic converters typically include a monolithic honeycomb substrate (flow-through substrate) coated with a platinum group metal catalyst. The honeycomb structure with many small parallel channels presents a high catalytic contact area with the exhaust gas. When the hot gases contact the catalyst, several exhaust pollutants are converted into harmless substances: carbon dioxide and water.

[0056] Preferably, the exhaust gas treatment system further comprises a second sensor for determining the NOx level in the exhaust gas, and wherein based on the measurement of the second sensor, the second device for injecting the nitrogenous reductant is configured to dose the nitrogenous reductant at an ANR of less than 1, preferably 0.5-0.9. Like the first sensor, the second sensor can be a NOx measurement sensor, which is arranged downstream of the second device for injecting the nitrogenous reductant, preferably downstream of the third SCR catalyst. Optionally, the second sensor is one or more of a temperature sensor, an engine speed sensor and an engine load sensor. In this second option, the use of these other sensors can be used alone, or in combination, and a lookup table is used to estimate the NOx level and thus achieve the desired ANR dosing.

[0057] In a preferred embodiment, the system further comprises a CSF, or a DOC and a CSF in sequence downstream of the second SCR.

[0058] In a preferred embodiment, the system does not contain any catalytic components downstream of the second SCR.

[0059] Several preferred embodiments of the exhaust system according to the present invention will now be described. In the following embodiments only, the numerical markings of the components will be modified to provide a more logical, increasing numbering from the front to the rear of the exhaust system. Therefore, hereinafter, in the case where there are two devices for injecting nitrogenous reductant, the markings of "first" and "second" are reversed. Similarly, hereinafter, in the case where there are three SCR catalysts, the first, second and third SCR catalysts correspond to the third, first and second SCR catalysts discussed above, respectively. In all of these embodiments, the ASC is as described herein (i.e., based on Pd).

[0060] according to Figure 3 In a preferred embodiment shown, the exhaust system comprises in order:

[0061] (i) a first means for injecting a nitrogen-containing reducing agent;

[0062] (ii) a first selective catalytic reduction (SCR) catalyst;

[0063] (iii) Catalyzed soot filter (CSF);

[0064] (iv) a second means for injecting a nitrogen-containing reducing agent;

[0065] (v) a second selective catalytic reduction (SCR) catalyst;

[0066] (vi) an ammonia slip catalyst (ASC); and

[0067] (vii) A third selective catalytic reduction (SCR) catalyst.

[0068] according to Figure 4 In a preferred embodiment shown, the exhaust system comprises in order:

[0069] (i) diesel oxidation catalyst;

[0070] (ii) Catalyzed Soot Filter (CSF).

[0071] (iii) a first means for injecting a nitrogen-containing reducing agent;

[0072] (ii) a first selective catalytic reduction (SCR) catalyst;

[0073] (vi) an ammonia slip catalyst (ASC); and

[0074] (vii) A second selective catalytic reduction (SCR) catalyst.

[0075] according to Figure 5 In a preferred embodiment shown, the exhaust system comprises in order:

[0076] (i) diesel oxidation catalyst;

[0077] (ii) a first means for injecting a nitrogen-containing reducing agent;

[0078] (iii) a first selective catalytic reduction (SCR) catalyst;

[0079] (iv) catalyzed soot filter (CSF);

[0080] (v) a second means for injecting a nitrogen-containing reducing agent;

[0081] (vi) a second selective catalytic reduction (SCR) catalyst;

[0082] (vii) an ammonia slip catalyst (ASC); and

[0083] (viii) A third selective catalytic reduction (SCR) catalyst.

[0084] according to Figure 6 In a preferred embodiment shown, the exhaust system comprises in order:

[0085] (i) a first means for injecting a nitrogen-containing reducing agent;

[0086] (ii) a first selective catalytic reduction (SCR) catalyst;

[0087] (iii) diesel oxidation catalyst;

[0088] (iv) catalyzed soot filter (CSF);

[0089] (v) a second means for injecting a nitrogen-containing reducing agent;

[0090] (vi) a second selective catalytic reduction (SCR) catalyst;

[0091] (vii) an ammonia slip catalyst (ASC); and

[0092] (viii) A third selective catalytic reduction (SCR) catalyst.

[0093] according to Figure 7 In a preferred embodiment shown, the exhaust system comprises in order:

[0094] (i) a first means for injecting a nitrogen-containing reducing agent;

[0095] (ii) a first selective catalytic reduction (SCR) catalyst that is close coupled (i.e. within 100 cm, preferably within 50 cm, of the engine manifold);

[0096] (iii) diesel oxidation catalyst;

[0097] (iv) catalyzed soot filter (CSF);

[0098] (v) a second means for injecting a nitrogen-containing reducing agent;

[0099] (vi) a second selective catalytic reduction (SCR) catalyst;

[0100] (vii) an ammonia slip catalyst (ASC); and

[0101] (viii) A third selective catalytic reduction (SCR) catalyst.

[0102] according to Figure 8In a preferred embodiment shown, the exhaust system comprises in order:

[0103] (i) diesel oxidation catalyst;

[0104] (ii) a first means for injecting a nitrogen-containing reducing agent;

[0105] (iii) a first selective catalytic reduction (SCR) catalyst;

[0106] (iv) an ammonia slip catalyst (ASC); and

[0107] (v) A second selective catalytic reduction (SCR) catalyst.

[0108] (vi) Catalyzed Soot Filter (CSF).

[0109] according to Fig. 9 In a preferred embodiment shown, the exhaust system comprises in order:

[0110] (i) a first diesel oxidation catalyst;

[0111] (ii) a first means for injecting a nitrogen-containing reducing agent;

[0112] (iii) a first selective catalytic reduction (SCR) catalyst;

[0113] (iv) an ammonia slip catalyst (ASC); and

[0114] (v) A second selective catalytic reduction (SCR) catalyst.

[0115] (vi) a second diesel oxidation catalyst;

[0116] (vii) Catalyzed Soot Filter (CSF).

[0117] Each of the aforementioned embodiments may further comprise a (further) catalyzed soot filter (CSF), or a further DOC and a (further) CSF downstream of the last selective catalytic reduction (SCR) catalyst.

[0118] According to another aspect, a diesel combustion and exhaust treatment system is provided, the system comprising a diesel engine and an exhaust treatment system as described herein, arranged to treat exhaust generated by the diesel engine.

[0119] According to another aspect, a method of treating diesel engine exhaust is provided, the method comprising passing the exhaust through the exhaust treatment system described herein.

[0120] definition

[0121] As used herein, the singular form "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0122] The use of the term "comprising / including" is intended to be interpreted as including such features but not excluding other features, and is also intended to include options that are necessarily limited to those features. In other words, the term also includes limitations such as "essentially consisting of..." (intended to indicate that certain other components may be present, provided that they do not materially affect the basic characteristics of the features) and "consisting of..." (intended to indicate that other features may not be included, so that if the components are expressed in their proportional percentages, they add up to 100%, while taking into account any unavoidable impurities), unless the context clearly indicates otherwise.

[0123] As used herein, the term "on" is intended to mean "directly on," such that there are no intervening layers between one material claimed to be "on" another material. Spatially relative terms such as "below," "beneath," "lower," "above," "upper," and the like may be used herein to facilitate the specification in describing the relationship of one element or feature to another. It should be understood that the spatially relative terms are intended to encompass different orientations of the catalyst in use or operation in addition to the orientations shown in the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0124] The invention will now be further described with reference to the following non-limiting drawings, in which:

[0125] Figure 1 The basic components of the exhaust system described herein are shown.

[0126] Figure 2 A preferred embodiment of the exhaust system described herein is shown.

[0127] Figure 3 A preferred embodiment of the exhaust system described herein is shown.

[0128] Figure 4 A preferred embodiment of the exhaust system described herein is shown.

[0129] Figure 5 A preferred embodiment of the exhaust system described herein is shown.

[0130] Figure 6 A preferred embodiment of the exhaust system described herein is shown.

[0131] Figure 7 A preferred embodiment of the exhaust system described herein is shown.

[0132] Figure 8 A preferred embodiment of the exhaust system described herein is shown.

[0133] Fig. 9 A preferred embodiment of the exhaust system described herein is shown.

[0134] Fig. 10A and 10B Comparative NOx conversion and N2O generation of Pd and Pt ASC components in a conventional SCR / ASC configuration are shown.

[0135] Fig.11A and 11B Comparative NH3 conversion and slip for components containing Pt and Pd in ​​a conventional SCR / ASC configuration are shown.

[0136] 12 and 13 compare the performance of a conventional SCR / SCR / ASC arrangement with the SCR / ASC / SCR configuration described herein. Fig. 12A and 12B NH3 conversion and slip are presented separately. Fig.13A and 13B NOx conversion and N2O generation are presented separately.

[0137] Fig.14A and 14B Data obtained when the exhaust configuration was tested on an engine is shown.

[0138] like Figure 1 As shown, an exhaust gas treatment system 1 is provided, which includes, in order from an engine 5 to an exhaust outlet 10: a first device 15 for injecting a nitrogenous reductant, a first selective catalytic reduction (SCR) catalyst 20, an ammonia slip catalyst (ASC) 25; and a second selective catalytic reduction (SCR) catalyst 30. The system further includes a NOx sensor 31.

[0139] The ASC 25 comprises an SCR catalyst layer 35 overlying a layer 40 containing supported palladium (Pd).

[0140] In use, the first device 15 for injecting nitrogenous reducing agent is actively operated with an ANR ratio higher than 1. This allows a thorough SCR reaction on the first selective catalytic reduction (SCR) catalyst 20. The first device 15 for injecting nitrogenous reducing agent controls the dosing of the nitrogenous reducing agent based on measurements made by the NOx sensor 31.

[0141] The Ammonia Slip Catalyst (ASC) 25 is used to regulate the ammonia sent to the second Selective Catalytic Reduction (SCR) catalyst 30. This ensures no ammonia slip and minimal NOx release. The location of the Ammonia Slip Catalyst (ASC) 25 in the exhaust gas treatment system 1 means that it operates under optimal conditions for the palladium component so that it has a high selectivity for NOx.

[0142] like Figure 2 As shown, Figure 1 The exhaust gas treatment system 1 is now provided with a second upstream device 45 for injecting a nitrogenous reducing agent, a third selective catalytic reduction (SCR) catalyst 50, and a catalyzed soot filter (CSF) 55. The system further comprises a further NOx sensor 51.

[0143] In use, the second means for injecting nitrogenous reductant 45 is operated with an ANR of less than 1. This ensures that there is no (or minimal) ammonia slip onto the CSF 55. Any ammonia passing through the system is processed by the downstream SCR catalyst 20. The ANR of the second means for injecting nitrogenous reductant 45 may be controlled based on measurements made by another NOx sensor 51.

[0144] At cold start, the third selective catalytic reduction (SCR) catalyst 50 handles most of the required SCR reactions. However, as the exhaust gas temperature increases, the performance of the overheated upstream third selective catalytic reduction (SCR) catalyst 50 will decrease. Therefore, ammonia can be saved by shifting the balance of ammonia distribution from the upstream and hot second device 45 for injecting nitrogenous reductant to the cooler and downstream first device 15 for injecting nitrogenous reductant. This means that the performance of the system can be enhanced by reducing the use of nitrogenous reductant.

[0145] Figure 3-9 Shows the same Figure 1 and Figure 2 Like parts have like reference numerals. Additional parts are represented by their usual acronyms as discussed herein. Example

[0146] The invention will now be further described with reference to the following non-limiting examples.

[0147] The exhaust system was prepared and tested as described below.

[0148] Reactor testing

[0149] The test reactor was run with the following settings:

[0150] High temperature (HT) aging: 650℃ / 264h / 10%H2O / air

[0151] SV整个ASC : 210,000h -1

[0152] Raw gas: 350ppm NO, 500ppm NH3, 10% O2, 6.5% H2O, 7% CO2, balance N2

[0153] Tested ASC: 2g / ft 3 Pt or Pd, or 5g / ft 3 Pd, based on PGM.Al2O3 bottom layer (0.35 g / in 3 WCL, excluding PGM), Cu.CHA based top layer (3.33 wt% Cu, 2.4 g / in 3 WCL)

[0154] Tested SCR: Based on Cu.CHA (3.33wt% Cu, 2.4g / in 3 WCL)

[0155] Temperature: 500°C / 30min, pre-conditioned in base gas, then ramped to 200°C, started NO / NH3 flow, and measured emissions during ramp at 5°C / min to 600°C

[0156] Fig. 10A and 10B Results for NOx conversion and N2O generation are shown for comparison of Pt and Pd ASC components. The parts have been high temperature aged at 650°C in 10% H2O / air for 264 hours. They were tested in a feed gas containing 350 ppm NO, 500 ppm NH3, 10% O2, 6.5% H2O, 7% CO2, balance N2, using the reactor setup described above.

[0157] Fig. 10A The improved NOx conversion at high temperatures for two Pd-containing ASCs is shown. The graph labels are the three lines corresponding in order (from top to bottom) at 550°C. Fig. 10B The ASC containing Pd shows the reduced N2O generation. At 550℃, the bottom line is Pd (2g / ft 3 ) ASC. The uppermost line at 250°C is the Pt ASC.

[0158] Fig.11A and 11B Results for NH3 conversion and NH3 slip are shown for comparison of Pt and Pd ASC compositions. It can be seen that the Pd ASC exhibits a slower NH3 light-off, but approaches similar conversions as the Pt ASC at high temperatures. Fig.11AIn the figure, the order of the lines at 550°C is the opposite of the order in which they are listed in the figure. Fig. 11B The order of the lines at 550°C is the same as their order at Figure 8 The order listed in A is matched.

[0159] Fig. 12A The SCR / ASC / SCR configuration disclosed herein shows improved NH3 conversion at higher temperatures. Fig. 12B The SCR / ASC / SCR configuration disclosed herein is shown to reduce NH3 slip at higher temperatures.

[0160] Fig.13A Improved NOx conversion at higher temperatures is shown for the SCR / ASC / SCR configuration disclosed herein. Fig. 13B The SCR / ASC / SCR configuration disclosed herein shows reduced N2O generation at temperatures below about 500°C (and comparable performance described above).

[0161] As shown in Figures 10-13, the Pd ASC exhibits higher NOx conversion than the Pt ASC at high temperature, and NH3 conversion is close to that of the Pt ASC. At low temperature, significantly reduced N2O generation is observed on the Pd ASC compared to the Pt ASC. The disclosed ASC configuration with the Pd ASC exhibits higher NOx conversion and slightly higher N2O generation at high temperature than the conventional configuration with the Pt ASC.

[0162] Fig.14A and 14B Engine performance data for the exhaust system configuration described herein is shown. As shown, NOx conversion increases with lower NOx output, although the upstream SCR allows for greater NOx output due to a slight change in ammonia dosing. Fig.14A The NH3 slip is shown to be reduced (ie, essentially zero).

[0163] Engine Test Example

[0164] DOC + injection (ANR 0.9) + SCR1 + CSF + injection (ANR 1.2) + SCR2 (with and without Pd ASC) + SCR3

[0165] DOC (Ba promoter, based on PGM.SiO2 / Al2O3, 2.6 g / in 3 WCL)

[0166] SCR (4.25wt% Cu, based on Cu.CHA, 2.4g / in 3 WCL, no ASC)

[0167] CSF (based on PGM.Al2O3, 0.2g / in 3 WCL)

[0168] SCR2 (3.33wt% Cu, based on Cu.CHA, 2.4g / in 3 WCL), with or without 5L Pd ASC (0:1 / 2)

[0169] SCR3 is the same as SCR2 (without ASC)

[0170] The above configuration was tested and demonstrated reduced NOx output (mg / mi), and increased NOx conversion during the US06 cycle running on a 6.6L light-duty diesel engine.

[0171] The foregoing detailed description has been provided by way of illustration and description, and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments shown herein will be apparent to those of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

Claims

1. An exhaust gas treatment system comprising, in order: (i) a first means for injecting a nitrogen-containing reducing agent; (ii) a first selective catalytic reduction (SCR) catalyst; (iii) an ammonia slip catalyst (ASC); and (iv) a second selective catalytic reduction (SCR) catalyst, The ASC comprises an SCR catalyst and a supported palladium (Pd) component. 2 . The exhaust gas treatment system according to claim 1 , wherein the ASC has a layered structure in which an upper layer comprises the SCR catalyst, and a lower layer comprises the supported palladium (Pd) component.

3. An exhaust gas treatment system according to claim 1 or 2, wherein the system further comprises a first sensor for determining the NOx level in the exhaust gas, Therein, based on the measurement of the first sensor, the first device for injecting the nitrogenous reductant is configured to dose the nitrogenous reductant at an ANR greater than 1, preferably 1.1-1.

5.

4. An exhaust gas treatment system according to claim 3, wherein the first sensor is a NOx measurement sensor, which is arranged downstream of the first device for injecting nitrogenous reducing agent, preferably downstream of the first SCR catalyst, or wherein the first sensor is one or more of a temperature sensor, an engine speed sensor and an engine load sensor.

5. An exhaust gas treatment system according to any one of the preceding claims, further comprising, upstream of the first means for injecting a nitrogenous reducing agent, in sequence: a second means for injecting a nitrogenous reducing agent; a third selective catalytic reduction (SCR) catalyst; Catalyzed Soot Filter (CSF).

6. The exhaust gas treatment system of claim 5, further comprising a diesel oxidation catalyst upstream of the second means for injecting a nitrogenous reductant, or between the third SCR catalyst and the CSF.

7. An exhaust gas treatment system according to claim 5 or 6, wherein the exhaust gas treatment system further comprises a second sensor for determining the NOx level in the exhaust gas, and wherein based on the measurement of the second sensor, the second device for injecting the nitrogenous reductant is configured to dose the nitrogenous reductant at an ANR of less than 1, preferably 0.5-0.

9.

8. An exhaust treatment system according to any one of the preceding claims, wherein a first selective catalytic reduction (SCR) catalyst and the ammonia slip catalyst (ASC) share a common substrate, or wherein the ammonia slip catalyst (ASC) and a second selective catalytic reduction (SCR) catalyst share a common substrate.

9. An exhaust gas treatment system according to any one of the preceding claims, wherein the first and / or second SCR catalyst comprises a Cu or Fe doped zeolite component.

10. The exhaust treatment system of any one of the preceding claims, wherein the ASC is substantially free of Pt.

11. An exhaust gas treatment system according to any one of the preceding claims, wherein the palladium component is supported on a particulate metal oxide or zeolite, preferably alumina.

12. An exhaust gas treatment system according to any one of the preceding claims, wherein the system further comprises a CSF, or a DOC and a CSF in sequence downstream of the second SCR, or wherein the system does not contain any catalytic component downstream of the second SCR.

13. A diesel combustion and exhaust treatment system comprising a diesel engine and an exhaust treatment system according to any preceding claim, configured to treat exhaust generated by the diesel engine.

14. A method of treating exhaust gas from a diesel engine, the method comprising passing the exhaust gas through an exhaust gas treatment system according to any one of claims 1 to 12.

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