A COUPLED SCR SYSTEM

Abstract

System for the treatment of NO x containing exhaust gases from an engine, the system comprising the following: a flow monolith with a first catalytic composition for selective catalytic reduction of NO x and with a first volume; a closely coupled particulate filter with a second catalytic composition for the reduction of particulate matter and the selective catalytic reduction of NO x and with a second volume; wherein the first volume is 10% to 75% of the second volume; wherein the flow monolith is in fluid communication with the fine dust filter, and is installed upstream of it, and wherein the first and the second catalytic composition comprise a base metal-supported aluminosilicate molecular sieve or a silicoaluminophosphate molecular sieve, and wherein the first and the second catalytic composition are different; and furthermore an oxidation catalyst comprising at least one platinum group metal selected from platinum, palladium or a combination of platinum and palladium, wherein the platinum group metal is applied to a high surface area primer component selected from aluminum oxide, zeolite, silicon dioxide, non-zeolite silicon dioxide-aluminum oxide, cerium oxide, zirconia, titanium dioxide or a mixed or compound oxide containing both cerium oxide and zirconia, wherein the oxidation catalyst is arranged upstream of the flow monolith and is adapted to release a gas stream having a ratio of NO to NO2 of 4:1 to 1:3 per volume.

Description

AREA OF INVENTION

[0001] This invention relates to the exhaust gas purification of internal combustion engines. BACKGROUND OF THE INVENTION

[0002] One of the most harmful components of vehicle exhaust is NOx, which includes nitrogen oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). NOx emissions are particularly problematic for lean-burn engines, such as diesel engines. To reduce the environmental impact of NOx in exhaust gases, it is desirable to eliminate these undesirable components, preferably through a process that does not produce any other harmful or toxic substances.

[0003] In addition to emitting NOx gases, lean-burn engines have the disadvantage, due to their combustion characteristics, of producing particulate matter or soot, to which a variety of organic substances can be absorbed, including unburned hydrocarbons and sulfuric acid, which is produced by the oxidation of sulfur dioxide derived from sulfur compounds present in the fuel or lubricating oils. Diesel engine exhaust tends to contain more soot compared to gasoline engines.

[0004] Since exhaust gases from lean combustion engines contribute to air pollution, treatment systems are essential to minimize the harmful environmental impacts resulting from operating a lean combustion engine.

[0005] Two methods are commonly used to reduce pollutants in the exhaust gases of lean-burn engines. The first method converts NOx in diesel exhaust into less harmful substances, a process also known as selective catalytic reduction (SCR). An SCR process involves the conversion of NOx, in the presence of a catalyst and a reducing agent—typically anhydrous ammonia, aqueous ammonia solution, or urea—into elemental nitrogen (N2) and water. The second method reduces soot emissions by passing the soot-laden exhaust gas through a particulate filter. However, the accumulation of soot particles on the filter can cause an undesirable increase in the exhaust system's backflow while it is in operation, thus reducing efficiency.To renew the filter, the accumulated carbon-based soot must be removed from the filter, for example by regularly burning off the soot through passive or active oxidation at high temperatures.

[0006] WO 99 / 39 809 A1 discloses the combination of several separate components in an exhaust system, including an SCR catalyst, to treat, among other things, particulate matter and nitrogen oxides. For example, an exhaust stream from an engine, combined with a reducing agent, can first flow through a flow-through monolith containing the SCR catalyst to reduce NOx, and then the gases are further treated downstream to remove particulate matter by passing them through a particulate filter. The disadvantage of such configurations is that an increase in the number of exhaust aftertreatment components increases the overall cost of the exhaust system as well as its overall volume and weight, which is particularly detrimental for motor vehicles. The heavier a motor vehicle's overall exhaust system is, the more fuel the vehicle will need to carry it.

[0007] To counteract the aforementioned disadvantages, exhaust systems have been designed with a single component capable of reducing NOx and particulate matter. US Patent Publication US 2010 / 0 180 580 A1 discloses a system for combining an SCR catalyst with a wall-flow filter. Wall-flow filters contain multiple adjacent parallel channels, each sealed at one end, with the sealing occurring at the opposite ends of adjacent channels in an alternating pattern. Sealing the alternating channel ends prevents the gas entering the inlet side of the filter from flowing straight through and exiting the channel. Instead, the gas enters the front of the substrate and travels to approximately the middle of the channels, where it is forced over the channel walls before exiting through the outlet side of the substrate.The catalyst is typically applied to the walls of the wall flow filter in the form of an aqueous mixture or primer, and then calcined to adhere to the surface of the walls.

[0008] The disadvantage of certain SCR wall-flow filters is that a limited amount of catalyst can be applied to the surface. A thick primer will constrict the channels and, in some cases, the pores, inhibiting gas flow and contributing to backflow, thus negatively impacting the system's effectiveness. A known method for reducing backflow involves limiting the amount of primer applied. Less primer results in a reduced catalyst and the filter's ability to convert NOx. Finally, coating the surface of a wall-flow filter with an SCR catalyst contributes to the filter's overall weight. Increased mass will require more time and energy to heat the wall-flow filter to the temperatures necessary for catalyst activation, a significant disadvantage during start-up when the engine has not yet reached its normal steady-state operating temperature.To increase the heating rate of an SCR wall-flow filter, the filter can be positioned close to the engine.

[0009] A well-known method to counteract the disadvantages associated with an SCR wall-flow filter is to arrange an SCR flow monolith upstream of the wall-flow filter. Flow monoliths with a so-called honeycomb geometry comprise multiple adjacent parallel channels that are open at both ends and generally extend from the inlet side to the outlet side of the substrate. Each channel typically has a square, round, hexagonal, or triangular cross-section. Catalytic material is applied to the substrate, typically as a primer or other slurry, which may be contained on and / or within the walls of the substrate.

[0010] DE 10 2010 023 820 A1 relates to an exhaust gas treatment system for a diesel engine, which reduces lean nitrogen oxide (NOx) emissions. x )-catcher and a two-way catalyst, wherein the two-way catalyst comprises a urea-selective catalytic reduction (U-SCR) catalyst and a diesel particulate filter (DPF). SUMMARY OF THE INVENTION

[0011] The applicants have presented a system for the exhaust gas treatment of a lean-burn engine that produces NO x and reduced soot, it was discovered. The system comprises a compact SCR flow monolith located upstream of a tightly coupled SCR wall-flow filter.

[0012] As used herein, the term “closely coupled” refers to a component position in an engine’s exhaust aftertreatment system that is less than approximately 1 meter downstream of the engine’s exhaust manifold or exhaust turbocharger, preferably approximately 0.05 to approximately 0.5 meters. During startup or when an engine is operating under heavy load, components in a closely coupled position are typically exposed to higher exhaust gas temperatures compared to components located further downstream. It has been found that the combination of a compact SCR flow monolith and a separate, downstream closely coupled SCR wall-flow filter exhibits a synergistic effect that is not observed with combinations of larger SCR flow monoliths and downstream SCR wall-flow filters, or SCR wall-flow filters that are not closely coupled to the engine.Furthermore, this effect does not occur in a single SCR wall-flow filter with a volume and catalyst loading equivalent to the combined SCR flow-through monolith and SCR wall-flow filter of the present invention. In particular, the synergistic combination of SCR components results in a soot combustion efficiency that is higher than that of conventional SCR flow-through monolith and SCR wall-flow filter combinations. Moreover, the synergistic combination of components generates a higher NO. x -conversion, compared to a single, tightly coupled SCR wall-flow filter with a volume and catalyst loading comparable to the synergistic component combination. That is, the synergistic component combination of the present invention improves the overall NO conversion. x-Conversion efficiency of an SCR / soot filtration system without the need for an additional catalyst, thereby reducing the cost of the system, while also providing high temperature stability of the wall flow filter to enable continuous filter renewal.

[0013] The compact SCR flow monolith is characterized by its lower heat capacity compared to the downstream SCR wall-flow filter. This lower heat capacity can result from the use of an SCR flow material with a lower heat capacity and / or a smaller volume or mass relative to the SCR wall-flow filter.

[0014] Accordingly, one aspect of the invention provides a system for treating exhaust gases containing NO. x from an engine, wherein the system (a) includes a flow monolith with a first catalytic composition for selective catalytic reduction of NO xand a first volume; (b) a closely coupled particulate matter filter with a second catalytic composition for the reduction of particulate matter and the selective catalytic reduction of NO x and a second volume; and (c) comprising a volume ratio of the first volume to the second volume of less than approximately 1:2; wherein the flow monolith is in fluid communication with, and integrated upstream of, the particulate filter.

[0015] According to another aspect of the invention, a method is provided for treating an engine exhaust stream containing NO. x and contains soot, comprising: bringing the exhaust gas stream into contact, in the presence of a reducing agent, with a flow-through monolith having a first SCR catalyst composition loading and a first volume to generate an intermediate gas stream, wherein a first part of the NO xhas been converted into N2 and O2; bringing the intermediate gas stream into contact with a closely coupled catalytic particulate filter with a second SCR catalyst composition loading and a second volume, wherein the second volume is at least approximately twice the first volume, in order to capture some of the soot and produce a clean gas stream, wherein a second part of the NO xhas been converted into N2 and O2; the oxidation of part of the soot at a soot oxidation temperature to regenerate the catalytic particulate filter; the heating of the catalytic closely coupled flow monolith to an SCR start-up temperature before heating the catalytic particulate filter to an SCR start-up temperature; and the maintenance, under low-load conditions, of the soot oxidation temperature of the catalytic particulate filter for a longer period of time compared to a catalytic particulate filter with a volume equal to the first and second combined volume. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic representation of an embodiment of an engine exhaust gas treatment system according to the present invention; and Fig. Figure 2 graphically represents the total mass of NOx that is converted during engine start-up by a system comprising only an SCR wall-flow filter and a system according to the present invention. DETAILED DESCRIPTION

[0016] This invention presents a novel system for the treatment of exhaust gases from a lean-burn combustion engine, comprising a temperature-stabilized series of SCR catalysts to achieve improved NO reduction. x -conversion and higher soot combustion efficiency. In particular, the invention provides a system for soot reduction and NO treatment. xin an exhaust gas comprising a compact SCR flow monolith arranged upstream of a tightly coupled SCR wall-flow filter. The invention is intended for particular application in exhaust gases from heavy diesel engines, especially motor vehicle engines, for example, truck or bus engines, but it is not intended to be limited to these. Other applications could include LDD (light diesel engines), GDI (gasoline direct injection), CNG (compressed natural gas) engines, ships, or stationary sources. For the sake of simplicity, however, the majority of this description relates to such motor vehicle engines.

[0017] Since the SCR wall-flow filter is tightly coupled, the overall length of the compact SCR flow monolith used in the present invention should be less than the space provided between the SCR wall-flow filter and the engine outlet to accommodate other exhaust system components, such as the reducing agent injection point or an oxidation catalyst. The compact SCR flow monolith should also be lightweight to heat up quickly to the catalyst's activation temperature during engine start-up. Prior to the present invention, the structural integrity of such a lightweight, compact flow monolith was questionable.

[0018] The compact monolith contributes to the NO x-Reduction, especially during engine start-up. The heavier SCR wall-flow filter, after heating to the activation temperature of its SCR catalyst (i.e., the start-up temperature), will then filter a larger proportion of NO at the engine's steady-state operating temperatures. x in the exhaust gases following commissioning. Since the SCR wall-flow filter is heavier and closely coupled to the engine, cooling and therefore maintenance of the NOₓ concentration is prevented. x -Conversion takes longer while the engine is operating under low-load conditions or idling, thereby improving its soot oxidation efficiency.

[0019] Since the compact SCR flow-through monolith of the present invention is small and therefore has a limited surface area onto which a catalyst can be applied, it is preferred that the monolith be heavily loaded with a catalyst per unit volume of the monolith. Furthermore, it is preferred that the compact SCR flow-through monolith has a catalyst loading per unit volume that is greater than the catalyst loading per unit volume of the substrate on the SCR wall-flow filter used in the present inventive system. The compact monolith has a preferred catalyst loading range of approximately 3 to approximately 15 g / in³. 3 , preferably approximately 4 to approximately 10 g / in 3 In comparison, the SCR wall-flow filter preferably has a catalyst loading range of approximately 1 to approximately 2.8 g / in³. 3 , for example 1.5 to 2.5 g / in 3Therefore, in some embodiments, the ratio of SCR catalyst loading per unit volume on the compact monolith, compared to the wall-flow particulate filter, is approximately 15:1 to approximately 1.5:1, for example, approximately 4:1 to approximately 2:1. The SCR catalyst on the flow-through monolith and the SCR catalyst on the wall-flow filter can be the same catalyst or different catalysts. In certain embodiments, the SCR catalyst on the upstream flow-through monolith is an iron-accelerated zeolite, and the SCR catalyst on the downstream wall-flow filter is a copper-accelerated zeolite.

[0020] The dimensions of the compact monolith are partly selected based on the desired heat capacity per unit volume. The "volume," as used here, is determined by the external dimensions, such as the length and diameter of the monolith or filter. As mentioned above, the compact size and weight of the SCR flow-through monolith allow it to heat up quickly to the catalyst's activation temperature when the engine is started, enabling an exhaust aftertreatment system with improved NOₓ reduction. x-conversion. The desired dimensions of the compact monolith can be expressed in terms of the size of the downstream SCR wall-flow filter. The volume of the compact monolith of the present invention is preferably approximately 10% to approximately 75% of the volume of the SCR wall-flow filter, more preferably approximately 15% to approximately 50%, more preferably approximately 15% to approximately 40%, and most preferably approximately 20% to approximately 25%. Preferably, the ratio of the volume of the flow-through monolith to the volume of the wall-flow filter is less than approximately 1:2, for example approximately 1:10 to approximately 1:2, or approximately 1:6 to approximately 1:4.

[0021] In addition to selecting the size of the compact monolith, the type of materials used to form the compact monolith is chosen based on the desired heat capacity per unit volume of the compact monolith, since the heat capacity depends on the material properties of the object to be heated. Similar to volume, the desired heat capacity per unit volume (for example, a specific heat capacity) of the compact monolith can be expressed relative to the specific heat capacity of the wall-flow filter. The specific heat capacity of the compact flow-through monolith is preferably approximately 20% to 80% of the specific heat capacity of the wall-flow filter, more preferably 25% to 75%, and most preferably 35% to 65%.

[0022] In a first preferred embodiment, the compact monolith used in the present invention is extruded. The extruded monolith is produced by first combining the starting materials, comprising a catalyst, a binder, and optionally inorganic fibers, to form a suspension. The suspension is further processed by additional mixing and / or kneading in an acidic or alkaline aqueous mixture. An organic reagent is added to the aqueous mixture to produce a composition suitable for extrusion. After the composition has been extruded into the form of a monolith, it is dried and calcined. The result is a monolith with sufficient mechanical stability and effective activity for long-term use.

[0023] By extruding a catalyst composition into the form of a monolith, there is no longer a need for a substrate coated with a catalyst; this is replaced by a catalyst body that contains the catalyst material throughout. Therefore, extruded monoliths typically contain more catalyst per unit volume than inert substrates onto which a primer containing a catalytic component is applied. An extruded catalytic monolith can be compact and lightweight, allowing it to be positioned upstream of an SCR wall-flow filter in the present inventive system.

[0024] In a second embodiment, the compact monolith used in the present invention is produced by first preparing an aqueous mixture comprising a catalytic component and optionally a binder, and then applying this aqueous mixture to a non-reactive substrate in the form of a monolith, which is subsequently dried and calcined. Since a lightweight monolith is desired, a material should be selected that will provide thin walls to allow the application of an effective amount of catalyst to its surface without compromising the mechanical stability of the monolith for long-term use in the present invention.

[0025] The flow-through monolith is preferably a honeycomb with a plurality of channels open at both ends and extending through the monolith in an approximately parallel direction. The cross-sectional shape of the channels is not particularly defined and can be, for example, square, round, oval, rectangular, triangular, hexagonal, or the like. Preferably, the flow-through monolith (either the extruded catalyst or the inert substrate) contains approximately 150 to approximately 800 channels per square inch (cpsi), and, more preferably, approximately 300 to approximately 400 cpsi or approximately 600 to approximately 800 cpsi. In certain embodiments, the flow-through monolith (either the extruded catalyst or the inert substrate) can have walls with an average wall thickness of less than approximately 0.30 mm, less than approximately 0.25 mm, less than approximately 0.22 mm, or less than approximately 0.20 mm.In certain embodiments, the cell walls will have an average thickness of approximately 0.30 mm to approximately 0.25 mm, approximately 0.25 mm to approximately 0.22 mm, or approximately 0.22 mm to approximately 0.20 mm.

[0026] The flow monolith is preferably constructed from one or more materials comprising, as a predominant phase, ceramics, cermet, metals, oxides, and combinations thereof. Combinations are understood to be physical or chemical combinations, such as mixtures, compounds, or compositions. Some materials particularly suitable for the application of the present invention are those made from cordierite, mullite, clay, talc, zirconium, zirconia, spinel, aluminum oxide, silicon oxide, boride, lithium aluminosilicate, aluminum oxide-silicate material, feldspar, titanium oxide, quartz glass, nitride, boride, carbide, for example, silicon carbide, silicon nitride, or mixtures thereof. Silicon carbide is a particularly preferred material.

[0027] According to the present invention, the system comprises an SCR wall-flow filter located downstream of the compact SCR flow-through monolith, but closely coupled to the engine. Such SCR wall-flow filters are known in the prior art and may include the same or different catalysts as the compact flow-through filter used in the present inventive system. The catalyst is integrated into the wall-flow filter by applying a catalyst primer to a substrate prior to calcination. A vacuum is typically used to draw the primer through the filter walls. Conventional wall-flow filter substrates for diesel engines, which typically have several parallel channels, contain approximately 250–800 cpsi, for example, approximately 250–350 cpsi, and are provided in either a ceramic honeycomb or a metal honeycomb form.The channels are defined by porous walls, and each channel has a cap on either the inlet or outlet side of the substrate. Wall-flow filter substrates for use in automotive exhaust systems, such as this one, are commercially available from a variety of sources and can have any shape suitable for use in an exhaust system.

[0028] The walls of the wall-flow filter have a porosity and pore size that make them gas-permeable, yet allow the capture of a large portion of the fine dust, such as soot, from the exhaust gas as it passes through the wall. The substrate can be made of a porous material with a porosity of at least approximately 35%, preferably approximately 45–55%. The mean pore size of the porous substrate is also important for filtration. The mean pore size can be determined by any acceptable method, including mercury porosimetry. The mean pore size of the porous substrate should be high enough to promote a low backflow while providing adequate efficiency, either through the substrate itself, by the support of a soot filter layer on the substrate surface, or by a combination of both.Preferred porous substrates have a mean pore size of approximately 10 to approximately 40 µm, for example approximately 20 to approximately 30 µm, approximately 10 to approximately 25 µm, approximately 10 to approximately 20 µm, approximately 20 to approximately 25 µm, approximately 10 to approximately 15 µm, and approximately 15 to approximately 20 µm.

[0029] Preferred wall-flow substrates are high-efficiency filters. The efficiency is determined by the weight percentage of fine dust of a specific size that is removed from the untreated exhaust gas when passing through a wall-flow substrate. Therefore, the efficiency refers to soot and other particles of similar size and to particle concentrations typically found in conventional diesel exhaust. The size of particles in diesel exhaust can range from 0.05 microns to 2.5 microns. Thus, the efficiency is based on this range. Wall-flow filters for the application of the present invention preferably have an efficiency of at least 70%, at least approximately 75%, at least approximately 80%, or at least approximately 90%.In certain embodiments, the effectiveness will preferably be between approximately 75 and approximately 99%, between approximately 75 and approximately 90%, between approximately 80 and approximately 90%, or between approximately 85 and approximately 95%.

[0030] Catalysts for use according to the present invention comprise any suitable catalyst capable of reducing nitrogen oxides in the presence of a reducing agent. Catalysts include metal-loaded support materials. Suitable catalysts include vanadium, titanium dioxide, tungsten, or combinations thereof, and also metal-supported, particularly base metal, molecular sieves, comprising, but not limited to, copper- and / or iron-supported aluminosilicates and silicoaluminophosphates. A particularly preferred metal is copper. In one embodiment, the transition metal loading is approximately 0.1 to approximately 10 wt.% of the molecular sieve, for example, approximately 0.5 wt.% to approximately 5 wt.%, approximately 0.5 wt.% to approximately 1 wt.%, and approximately 2 wt.% to approximately 5 wt.%. The type and concentration of the transition metal can vary according to the host molecular sieve and the application.Aluminosilicates preferably have a silicon dioxide to aluminum oxide ratio (SAR) of approximately 15 to approximately 50, for example of approximately 20 to approximately 40 or of approximately 25 to approximately 30.

[0031] The molecular sieves have a framework suitable for SCR processes, including but not limited to Beta, CHA, AEI, LEV, MFI, ERI, and mixtures or intergrowths thereof.

[0032] The arrangement of the SCR catalyst, which is either in the flow-through monolith or the flow-through filter, in a manner that minimizes any restriction of the exhaust gas flow through the component is highly preferred. More than one catalyst can be stacked on top of the other. The catalyst material can also be arranged in such a way that one or more concentration gradients are formed along the channel walls or across the channels between the upstream and downstream sides of the wall. Different catalysts can be loaded along the channel walls or on the upstream and corresponding downstream sides of the walls in a wall flow filter.

[0033] The system of the present invention may also include a reducing agent source. The reducing agent (also known as a reducing agent) for SCR processes generally means any compound that promotes the reduction of NOx in an exhaust gas. Examples of reducing agents useful in the present invention include ammonia, hydrazine, or any suitable ammonia precursor, for example, urea ((NH₂)₂CO), ammonium carbonate, ammonium carbamate, ammonium bicarbonate, or ammonium formate, and hydrocarbons, for example, diesel fuel and the like. Nitrogen based on ammonia is a particularly preferred reducing agent. Other reducing agents include hydrocarbons, for example, propene and diesel fuel.

[0034] The reducing agent liquid source can utilize existing technology to inject liquid into the gas stream. For example, a mass control unit can control the supply of compressed NH3, which can be injected through an annular injection ring located in an exhaust pipe. The injection ring can have a variety of injection ports arranged around its circumference. Conventional diesel fuel injection systems include pumps and injectors to inject urea. A compressed air stream can also be injected around the nozzle to provide thorough mixing and cooling. In a preferred embodiment of the invention, the injection point of a reducing agent is located upstream of the compact SCR flow-through monolith. A second injection point can optionally be located between the compact monolith and the SCR wall-flow filter.

[0035] One difficulty in treating NOx from mobile source applications is that the amount of NOx present in the exhaust gas is volatile, meaning it varies under driving conditions such as acceleration, deceleration, and driving at different speeds. The volatile nature of the NOx component in mobile application exhaust presents a number of engineering challenges, including the correct dosing of the nitrogen-based reducing agent to sufficiently reduce NOx without wasting or releasing nitrogen-based reducing agents into the atmosphere.

[0036] In practice, SCR catalysts can preferentially absorb (or store) a nitrogen-containing reducing agent, thereby providing a buffer for the adequate supply of available reducing agent. Technologists use this phenomenon to calibrate appropriate nitrogen-containing reducing agent injection into exhaust gases. Low storage capacity will necessitate more frequent injections of reducing agent into the operating system. A desirable SCR catalyst has sufficient NH3 storage capacity at a given temperature (to ensure that excess NH3 does not "slip" across the catalyst and to allow the conversion to continue when NH3 is not present in the feed) and exhibits high activity independent of the NH3 level (the level is defined relative to a saturated NH3 storage capacity).The NH3 level can be expressed as the amount of NH3 (e.g., in grams) present on the complete catalyst (e.g., in liters) relative to a maximum level under a given set of conditions. It may be desirable to integrate an ammonia slip catalyst downstream of the SCR wall-flow filter to remove any NH3 or its derivatives that might otherwise pass through unreacted or as byproducts. Ammonia slip catalysts can comprise a two-layer catalyst, consisting of one layer of a reductive component, such as a metal-accelerated zeolite, and one layer of an oxidative component, such as platinum or palladium.

[0037] The common inventive exhaust gas treatment system can optionally include an oxidation catalyst upstream of the compact SCR flow-through monolith. In one embodiment, the oxidation catalyst is adapted to release a gas stream entering the SCR zeolite catalyst, which has a NO to NO₂ ratio of approximately 4:1 to approximately 1:3 per volume, for example, at an exhaust gas temperature at an oxidation catalyst inlet of 250 °C to 450 °C. The oxidation catalyst can contain at least one platinum group metal (or combinations thereof), for example, platinum, palladium, or rhodium, deposited on a flow-through monolith substrate. In one embodiment, the at least one platinum group metal is platinum, palladium, or a combination of platinum and palladium.The platinum group metal may be deposited on a high surface primer component, for example aluminum oxide, zeolite, for example aluminosilicate zeolite, silicon dioxide, non-zeolite silicon dioxide-aluminum oxide, cerium, zirconia, titanium dioxide or a mixed or compound oxide containing both cerium and zirconia.

[0038] In another aspect, an engine is provided which is combined with an exhaust system according to the present invention. The engine can be a diesel engine, a lean-burn gasoline engine, or an engine powered by liquefied petroleum gas (LPG) or natural gas. As in Fig. Figure 1 shows a lean-burn engine (10) having an exhaust manifold (12) and an optional exhaust gas turbocharger (14), from which an exhaust gas stream from the lean-burn engine (10) first moves towards (30) an optional oxidation catalyst (28), then to a compact SCR flow monolith (20), and then to a closely coupled SCR wall-flow filter (22). An injection point for a reducing agent (24), for example ammonia or urea, is located close to and upstream of the compact monolith (20). A second, optional injection point (26) for a reducing agent is located between the compact monolith (20) and the wall-flow filter (22). The gas stream exiting the SCR wall-flow filter (22) is treated to reduce the concentration of NO. x-gases and particulate matter have been reduced compared to the exhaust gas exiting the engine. The location of the SCR wall-flow filter (22) relative to the manifold (12) or optionally the exhaust gas turbocharger (14) is such that the exhaust gas flows less than 0.5 meters or even less than 0.3 meters between the outlet of the manifold or exhaust gas turbocharger and the outlet of the wall-flow filter. The exhaust gas flow distance between the inlet of the compact monolith (20) and the outlet of the manifold (12) or exhaust gas turbocharger (14) should not be particularly limited and is preferably minimized to allow for the optional oxidation catalyst (28) and the corresponding reducing agent injection and mixing. The exhaust gas flow distance between the inlet of the wall flow filter (22) and the outlet of the compact monolith (20) is not particularly limited, provided that at least a certain distance separates the two components.Examples of suitable distances include 0.05 meters, 0.1 meters, and 0.2 meters. Components 20, 22, 24, 26, and 28 are all in fluid communication with each other via an exhaust system line or other means that guide the engine's exhaust gas through a treatment system.

[0039] According to a further aspect of the invention, a method for reducing NOx in an exhaust gas stream from an internal combustion engine is provided, comprising introducing a reducing agent into the exhaust gas stream, passing the exhaust gas stream through a compact SCR flow monolith, and finally passing the gas exiting the compact flow monolith through a tightly coupled SCR wall-flow filter. Particulate matter within the exhaust gas is simultaneously captured within the SCR wall-flow filter. In one embodiment, the temperature of the exhaust gas stream at the inlet of the tightly coupled SCR wall-flow filter is at least 600 °C under heavy load.

[0040] The process can also include the step of renewing the SCR wall-flow filter. During normal operation of the exhaust system, soot and other particles accumulate on the inlet sides of the walls, leading to an increase in backflow. To reduce this increase in backflow, the filter substrates are continuously or periodically renewed by active or passive methods, including the combustion of the accumulated soot by known methods, for example, in the presence of nitrogen dioxide generated by an upstream oxidation catalyst. As mentioned above, the present inventive exhaust treatment system maintains the SCR wall-flow filter in a closely coupled position, which therefore offers the advantage of being in close proximity to the heat source, thus facilitating simpler heat management and filter renewal.

[0041] The above process can be carried out for gas originating from a combustion process, for example, from an internal combustion engine (whether mobile or stationary), a gas turbine, and coal-fired or oil-fired power plants. The process can also be used to treat gas from industrial processes, such as refining processes, refinery heaters and boilers, blast furnaces, the chemical processing industry, coke ovens, municipal waste treatment plants, and waste incineration plants, etc. In a particular embodiment, the process is used for treating exhaust gas from a lean-burn engine in a motor vehicle, for example, a diesel engine, a lean-burn gasoline engine, or an engine powered by liquefied petroleum gas (LPG) or natural gas.

[0042] According to another aspect, when in use, the system may only include means for controlling the dosage of nitrogen-containing reducing agents into the exhaust gas flow if it is determined that the catalyst is capable of catalyzing NOx reduction at or above a desired efficiency, for example, above 100 °C, above 150 °C, or above 175 °C. The determination by the control devices may be supported by one or more appropriate sensor inputs indicating an engine condition selected from the group consisting of exhaust gas temperature, catalyst bed temperature, accelerator pedal position, exhaust gas mass flow in the system, manifold vacuum, ignition timing, engine speed, exhaust gas lambda value, the amount of fuel injected into the engine, the position of the exhaust gas recirculation (EGR) valve, and thus the extent of EGR and boost pressure.

[0043] In a particular embodiment, the metering is controlled in response to the amount of nitrogen oxides in the exhaust gas, either directly (using a suitable NOx sensor) or indirectly, for example, using pre-correlated lookup tables or maps—stored in the control devices—that correlate one or more of the aforementioned inputs as an indicator of an engine condition with the predicted NOx content of the exhaust gas. The metering of the nitrogen-containing reducing agent can be configured so that 60% to 200% of the theoretical ammonia is present in the exhaust gas entering the SCR catalyst calculated at 1:1 NH3 / NO and 4:3 NH3 / NO2 ratios. The control devices may include a pre-programmed processor, for example, an electronic control unit (ECU). EXAMPLE

[0044] The total NO x-Conversion during commissioning of the engine for a system according to the present invention was performed with the total NO x -Conversion of a system comprising only an SCR wall-flow filter was compared. The SCR wall-flow filter comprised a low-reactivity silicon carbide substrate with a porosity of 52% and a mean pore size of 20 microns. The volume of the wall-flow filter was 2.5 L. The volume of the compact monolith was 0.625 L. Prior to the review, the compact monolith and the SCR wall-flow filter were aged at 800 °C for 16 hours. The two systems were tested with a 1.9 L engine in a motor vehicle subjected to test conditions associated with the MVEG driving cycle with injected urea as the reducing agent at 180 °C. The resulting NOx emissions for the two systems and a control over time were plotted.

[0045] The NOx conversion potential provided by the present invention is in Fig. 2 shown, which represents a decrease in the total output of NO x The graph shows the NOx accumulation in the exhaust gases after passing through a system of the present invention compared to a system using only an SCR wall-flow filter. Line 1 represents the NOx accumulation in the exhaust gas produced by the engine; Line 2 represents the NOx accumulation using only an SCR wall-flow filter; and Line 3 represents the NOx accumulation using a system according to the present invention. While both systems significantly reduced the overall NOx emissions at engine start-up, the system according to the present invention showed an increased NOx conversion rate of approximately 15% to 20%.

[0046] Although preferred embodiments of the invention have been presented and described herein, it is obvious that such embodiments are intended only as examples. Those skilled in the art will encounter numerous variations, modifications, and substitute examples without deviating from the essence of the invention. Accordingly, it is intended that the appended claims cover all such variations and also fall within the essence and scope of protection of the invention.

[0047] Preferred embodiments of the invention are described below: 1. System for the treatment of NO x containing exhaust gases from an engine, the system comprising the following: a flow monolith with a first catalytic composition for selective catalytic reduction of NO x and with a first volume; a closely coupled particulate filter with a second catalytic composition for the reduction of particulate matter and the selective catalytic reduction of NO x and with a second volume; and a volume ratio of the first volume to the second volume of less than approximately 1:2, in which the flow monolith is in fluid communication with the fine dust filter, and is installed upstream of it. 2. System according to embodiment 1, wherein the volume ratio is approximately 1:10 to approximately 1:2. 3. System according to embodiment 1, wherein the volume ratio is approximately 1:6 to approximately 1:4. 4. System according to embodiment 1, wherein the flow monolith is an extruded catalyst brick. 5. System according to embodiment 4, wherein the fine dust filter is a non-reactive substrate coated and / or impregnated with the second catalytic composition. 6. System according to embodiment 5, wherein the substrate is primarily made of either cordierite or metal. 7. System according to embodiment 1, wherein the flow monolith has a lower heat capacity with respect to the fine dust filter. 8. System according to embodiment 1, wherein the flow monolith has a lower specific heat capacity with respect to the fine dust filter. 9. System according to embodiment 8, wherein the flow monolith has a specific heat of approximately 20 to approximately 80% of the specific heat capacity of the fine dust filter. 10. System according to embodiment 9, wherein the flow monolith has a specific heat of approximately 35 to approximately 65% ​​of the specific heat capacity of the fine dust filter. 11. System according to embodiment 1, wherein the first and the second catalytic composition comprise a base metal-supported aluminosilicate molecular sieve or a silicoaluminophosphate molecular sieve. 12. System according to embodiment 11, wherein the flow-through monolith has an SCR catalyst loading that is greater than an SCR catalyst loading on the particulate filter. 13. System according to embodiment 12, wherein the flow monolith has an SCR catalyst loading of approximately 3 to 15 g / in 3 exhibits. 14. System according to embodiment 1, wherein the first and the second catalytic composition are different, provided that at least one of the first and the second catalytic composition comprises a base metal-accelerated aluminosilicate molecular sieve or a silicoaluminophosphate molecular sieve. 15. System according to embodiment 1, wherein the second catalytic composition is for the selective catalytic reduction of NO x coated and / or impregnated on a downstream side of the fine dust filter. 16. System according to embodiment 1, wherein the second catalytic composition is for the selective catalytic reduction of NO x coated and / or impregnated on an upstream side of the fine dust filter. 17. System according to embodiment 1, wherein the fine dust filter is arranged approximately 0.01 to approximately 0.25 meters downstream of the flow monolith. 18. System according to embodiment 17, further comprising a source of a reducing agent injection, in fluid communication with and arranged between the flow monolith and the fine dust filter. 19. Procedures for treating NO x and soot-containing engine exhaust stream, comprising: the contacting of the exhaust gas stream in the presence of a reducing agent with a flow-through monolith having a first SCR catalyst composition loading and a first volume to generate an intermediate gas stream, wherein a first part of the NO x has been converted into N2 and O2; The contacting of the intermediate gas stream with a closely coupled catalytic particulate filter with a second SCR catalyst composition loading and a second volume, wherein the second volume is at least approximately twice the first volume, in order to capture a portion of the soot and produce a clean gas stream, wherein a second portion of the NO x has been converted into N2 and O2; the oxidation of the soot component at a soot oxidation temperature to renew the catalytic particulate filter; Heating the catalytic tightly coupled flow monolith to an SCR start-up temperature before the catalytic particulate filter is heated to an SCR start-up temperature; and Maintaining, under low load conditions, the soot oxidation temperature of the catalytic particulate filter for a longer period of time, compared to a catalytic particulate filter with a volume equal to the combined first and second volumes. 20. Method according to embodiment 19, wherein the steps of bringing the exhaust gas stream into contact and bringing the intermediate gas stream into contact, compared with a catalytic particulate filter having a volume equal to the combined first and second volumes and with an SCR catalyst loading equal to the first and second loadings, result in a higher conversion of NO. x exhibit.

Claims

[1] System for the treatment of NO x containing exhaust gases from an engine, the system comprising the following: a flow monolith with a first catalytic composition for selective catalytic reduction of NO x and with a first volume; a closely coupled particulate filter with a second catalytic composition for the reduction of particulate matter and the selective catalytic reduction of NO x and with a second volume; wherein the first volume is 10% to 75% of the second volume; wherein the flow monolith is in fluid communication with the fine dust filter, and is installed upstream of it, and wherein the first and the second catalytic composition comprise a base metal-supported aluminosilicate molecular sieve or a silicoaluminophosphate molecular sieve, and wherein the first and the second catalytic composition are different; and furthermore an oxidation catalyst comprising at least one platinum group metal selected from platinum, palladium or a combination of platinum and palladium, wherein the platinum group metal is applied to a high surface area primer component selected from aluminum oxide, zeolite, silicon dioxide, non-zeolite silicon dioxide-aluminum oxide, cerium oxide, zirconia, titanium dioxide or a mixed or compound oxide containing both cerium oxide and zirconia, wherein the oxidation catalyst is arranged upstream of the flow monolith and is adapted to release a gas stream having a ratio of NO to NO2 of 4:1 to 1:3 per volume. [2] System according to claim 1, wherein the volume ratio is 1:10 to 1:

2. [3] System according to claim 1, wherein the volume ratio is 1:6 to 1:

4. [4] System according to claim 1, wherein the flow monolith is an extruded catalyst brick. [5] System according to claim 4, wherein the fine dust filter is a non-reactive substrate coated and / or impregnated with the second catalytic composition. [6] System according to claim 5, wherein the substrate is primarily made of either cordierite or metal. [7] System according to claim 1, wherein the flow monolith has a lower heat capacity with respect to the fine dust filter. [8] System according to claim 1, wherein the flow monolith has a lower specific heat capacity with respect to the fine dust filter. [9] System according to claim 8, wherein the flow monolith has a specific heat capacity which is 20 to 80% of the specific heat capacity of the fine dust filter. [10] System according to claim 9, wherein the flow monolith has a specific heat capacity which is 35 to 65% of the specific heat capacity of the fine dust filter. [11] System according to claim 1, wherein the flow-through monolith has an SCR catalyst loading that is greater than an SCR catalyst loading on the particulate filter. [12] System according to claim 11, wherein the flow-through monolith has an SCR catalyst loading of 3 to 15 g / in 3 exhibits. [13] System according to claim 1, wherein the second catalytic composition for the selective catalytic reduction of NO x coated and / or impregnated on a downstream side of the fine dust filter. [14] System according to claim 1, wherein the second catalytic composition for the selective catalytic reduction of NO x coated and / or impregnated on an upstream side of the fine dust filter. [15] System according to claim 1, wherein the fine dust filter is arranged 0.01 to 0.25 meters downstream of the flow monolith. [16] System according to claim 15, further comprising a source of a reducing agent injection, in fluid communication with and arranged between the flow monolith and the fine dust filter. [17] Methods for treating NO x and soot-containing engine exhaust stream, comprising: the contacting of the engine exhaust stream with an oxidation catalyst comprising at least one platinum group metal selected from platinum, palladium or a combination of platinum and palladium, wherein the platinum group metal is applied to a high surface area primer component selected from aluminum oxide, zeolite, silicon dioxide, non-zeolite silicon dioxide-aluminum oxide, cerium oxide, zirconia, titanium dioxide or a mixed or compound oxide containing both cerium oxide and zirconia, wherein the oxidation catalyst is adapted to release a gas stream having a NO to NO2 ratio of 4:1 to 1:3 per volume; the contacting of the gas stream in the presence of a reducing agent with a flow monolith with a first SCR catalyst composition loading and a first volume to generate an intermediate gas stream; the contacting of the intermediate gas stream with a closely coupled catalytic particulate filter with a second SCR catalyst composition loading and a second volume to capture some of the soot and produce a clean gas stream; the oxidation of the soot component at a soot oxidation temperature to renew the catalytic particulate filter; Heating the catalytic tightly coupled flow monolith to an SCR start-up temperature before the catalytic particulate filter is heated to an SCR start-up temperature; and Maintaining, under low load conditions, the soot oxidation temperature of the catalytic particulate filter for a longer period of time, compared to a catalytic particulate filter with a volume equal to the combined first and second volumes; wherein the first volume is 10% to 75% of the second volume. [18] Method according to claim 17, wherein a system according to claim 1 is used.

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