Emission treatment system and method using a scr filter

[0001] The present invention relates to an emission purification system that has an oxidation catalyst located above the soot filter which is coated with an effective material for selective catalytic reduction (SCR) NO_x with a reducing agent such as ammonia. In one embodiment, the system provides an effective method of simultaneous removal of nitrogen oxides (NO_x), particulates and gaseous hydrocarbons present in the exhaust gas streams of diesel engines.

[0002] Diesel exhaust is a heterogeneous mixture that contains not only gaseous pollutants such as carbon monoxide ("CO"), unburned hydrocarbons ("HC") and nitrogen oxides ("NO_x"), but also contains condensed phase material (liquids and solids), which are so-called dust or solid particles. Often, catalyst compositions and substrates on which these compositions are applied occur in diesel exhaust systems to transform some or all of the harmful ingredients to a harmless form. For example, diesel exhaust systems may contain one or more oxidation catalysts, a soot filter, and a NO reduction catalyst_x. [0003] Oxidation catalysts that contain platinum group metals, base metals and combinations thereof are used to improve the purification of diesel engine exhaust gas by enhancing the conversion of both unburned "HC" hydrocarbons and gas

CO and some particles due to the oxidation of these pollutants to carbon dioxide and water. Such catalysts are generally found in units called oxidation catalysts (DOCs), which are placed in the exhaust systems of diesel engines to purify pollutants from exhaust gases that escape into the atmosphere. In addition to gas conversions, "HC", CO and dust, an oxidation catalyst containing metals from the platinum group (which are usually applied to a refractory oxide substrate) also promotes the oxidation of nitric oxide (NO) to

NO2.

[0004] The total emission of particulate matter from diesel engines consists of three main elements. One of the components is a solid, dry carbon fraction or carbon black fraction. The dry carbon fraction contributes to the observable soot emissions that can often be seen in diesel engines. The second component of particulate emissions is the soluble organic fraction ("SOF"). The soluble organic fraction is sometimes referred to, according to the terminology used herein, as the volatile organic fraction ("VOF"). VOF can occur in the exhaust of diesel engines, either in the form of steam or as an aerosol (fine droplets of condensation) depending on the temperature of the exhaust from diesel engines. As determined by a standard measurement test, such as US Heavy Duty Transient Federal Test Procedure, generally it occurs in the form of condensed liquids at a standard solids collection temperature of 52 ° C in the diluted exhaust gas. These liquids come from two sources: (1) from lubricating oil entrained from the engine cylinder walls each time the pistons move up and down; and (2) from unburned or partly burned gas oil.

[0005] The third particulate component is the so-called sulfate fraction. The sulfate fraction is formed from small amounts of sulfur compounds present in diesel fuel. Small amounts of SO₃ they are formed during the combustion of diesel fuel, which in turn quickly combines with the water contained in the exhaust gas to form sulfuric acid. Sulfuric acid accumulates in the form of a condensed phase with the particles, giving an aerosol or is adsorbed on the particles of other components, and thus contributes to the growth of TPM.

[0006] One of the most important technologies used to reduce the particulate content is the diesel particulate filter used in diesel engines. There are many known structures of this filter that are effective in removing particulate matter from diesel exhaust gases, such as, for example, honeycomb flow filters, wound or packed fiber filters, open cell foams, sintered metal filters, etc. However, the ceramic flow baffle filters described below focus the most attention. These filters are capable of removing more than 90% of particulates from diesel exhaust gases. The filter is a physical structure that is used to remove particles from the exhaust gas, with the accumulated particles increasing the back pressure from the filter on the engine. Thus, the accumulating particles must be continuously or periodically burned to maintain acceptable back pressure. Unfortunately, carbon black particles for combustion require temperatures exceeding 500 ° C in conditions rich (poor) in oxygen. This temperature is higher than usually prevailing in exhaust gas from diesel engine. [0007] Regulations usually impose the use of lower soot combustion temperatures to provide passive regeneration of the filter. The presence of a catalyst promotes combustion of soot, and thus the regeneration of filters at temperatures usually occurring in exhaust from a diesel engine in real work cycles. In this way, the catalytic soot filter (CSF) or catalytic coating particle filter (CDPF) effectively provides> 80% reduction of particulate matter with passive burning of accumulating soot and thus promotes regeneration of the filter.

[0008] Future exhaust emission standards adopted around the world will also be directed towards NO reduction_x from diesel exhaust. Proven NO reduction technology_x, used for stationary sources in poor conditions, is selective catalytic reduction (SCR). In this process, NO_x is reduced with ammonia (NH₃) on a catalyst, usually of base metals, with the formation of nitrogen (N₂). This technology is capable of reducing over 90% NO_x, so it's one of the best ways to achieve instant NO reduction_x. Currently, SCR technology is under development in order to use it for mobile applications, using urea (usually found in aqueous solution) as a source of ammonia. SCR provides efficient conversion of nitrogen oxides NO_x, as long as the temperature of the exhaust gas is within the range of temperatures corresponding to the operation of the catalyst.

[0009] While each of the separate substrates containing catalysts, selectively directed to individual compounds in the exhaust gas, may be used in the exhaust system, it is still desirable to use a smaller number of substrates so as to reduce the overall size of the system to facilitate the assembly of such a system and also reduce overall layout costs. One way to achieve this is to coat the soot filter with a catalyst composition effective for NO conversion_x into the form of harmless ingredients. With this approach, the catalytic soot filter performs two catalytic functions: like removing solid particles from the exhaust stream and converting NO_x in the exhaust stream to

N2.

[0010] Coated soot filters that achieve NO reduction_x require sufficient loading of the SCR catalytic composition on the soot filter. The gradual loss of catalytic efficiency of the composition that occurs over time as a result of exposure to certain harmful components in the exhaust stream increases the need for a greater load on the SCR catalyst composition. However, the production of coated, flow baffle soot filters with higher catalyst loading can lead to an unacceptably high back pressure in the exhaust system. Application techniques that allow a higher load on the wall filter with a catalyst while maintaining a flow characteristic that allows an acceptable level of back pressure to be achieved are therefore desirable.

[0011] An additional aspect to consider when coating a wall flow filter is the selection of an appropriate SCR catalyst composition. First, the catalyst composition must be stable so as to maintain the activity of the SCR catalyst, even after prolonged exposure to the high temperatures that are characteristic of filter regeneration. For example, the combustion of a carbon black fraction from a particulate fraction often leads to temperatures above 700 ° C. Such temperatures contribute to the fact that many commonly used SCR catalyst compositions, such as mixed vanadium and titanium oxides, have lower catalytic efficiency. Secondly, the SCR catalyst compositions preferably have a relatively wide range of operating temperatures so that they can adapt to changing temperatures at which the vehicle operates. Temperatures below 300 ° C occur, for example, under low load conditions or at start-up. Preferably, the SCR catalyst compositions are capable of catalyzing NO reduction_x in exhaust gas to achieve the intended NO reduction_x, even at lower outlet temperatures.

[0012] The prior art describes the use of the SCR catalytic composition, soot filters and their combinations to reduce both NO emissions_x as well as particulates from diesel exhaust. These references are described below.

[0013] For example, Japanese Kokai document 3-130522 discloses a method of treating exhaust gas from a diesel engine, characterized in that it uses ammonia injection and a porous ceramic filter having a denitration catalyst that is embedded in the pores. The filter is installed in the exhaust gas stream from a diesel engine. The porous ceramic filter comprises a finely porous layer situated above and on the downstream side a layer of ceramic particles on which a denitration catalyst is applied. The finely porous layer can be a substrate for platinum or palladium or other catalyst used to burn hydrocarbons. Exhaust gas from a diesel engine containing unburned carbon flows through a porous ceramic filter, where carbon particles are filtered out onto its surface. The gas containing nitrogen oxides and ammonia passes through the denitrification catalyst and nitrogen oxides are reduced to nitrogen and water. The oxidation catalyst on the upstream side allows catalytic burning of solid components.

[0014] US 4,912,776 discloses an oxidation catalyst, an SCR catalyst located below, and adjacent to an SCR catalyst, and a reducer source introduced into the exhaust stream between the oxidation catalyst and the SCR catalyst. Thus providing a higher dose containing a high NO ratio₂ to NO introduced into the SCR reactor, which allows the use of lower temperatures and higher volumetric speeds than is possible with NO.

[0015] WO 99/39809 discloses a NO-containing flue gas purification system_x and particulates that has an oxidation catalyst effective to convert at least a portion of NO to NO_x to NO₂, particulate filter, source of fluid reducer and SCR catalyst. The particle filter is located below the oxidation catalyst; the source of the fluid reducer is located below the particulate filter; and the SCR catalyst is located below the fluid source of the reducer. The fluid reducer disclosed in the publication includes ammonia, urea, ammonium carbamate and hydrocarbons (e.g., diesel). [0016] A wall-mounted catalytic filter for an internal combustion engine exhaust system is described in WO 01/12320. The wall-flow filter has channels that are distributed in a honeycomb pattern, with some of the channels being blocked at the inlet end and some of the channels being unblocked at the inlet end and being blocked at the outlet end. The oxidation catalyst is applied to a gas impermeable zone at the inlet end of the channel which is blocked at the outlet end. The filter has a gas-permeable zone, which is located behind the oxidation catalyst, which is used to trap soot. The oxidation catalyst described can continuously (if it is in the exhaust system) produce NO₂ with NO by burning captured soot at temperatures below 400 ° C. The oxidation catalyst preferably contains a metal from the platinum group. Exhaust streams containing NO are initially passed through an oxidation catalyst to convert NO to NO₂ before the filtering stage so as to remove soot. Then flue gas containing NO₂ is used to burn soot captured on the filter.

[0017] In some embodiments of the wall flow filter described in WO 01/12320, the soot filter channels located below include a NO absorption catalyst_x and an SCR catalyst located below the NO absorber_x. The SCR catalyst may be made of a material based on copper, platinum, a mixture of vanadium oxides and titanium dioxide or zeolite, or a mixture of two or more of them.

[0018] WO03 / 054364 discloses an emission purification system comprising an oxidation catalyst, an injector downstream of the oxidation catalyst and a particulate filter containing an SCR catalyst.

SUMMARY OF THE INVENTION [0019] In one aspect, the invention relates to an emission purification system for purifying an exhaust stream that includes NO_x and solid particles. The emission purification system includes an oxidation catalyst, an injector that periodically doses ammonia or an ammonia precursor into the flue gas stream; and wall flow monolith. The injector is in fluid communication with the oxidation catalyst and is located downstream of the oxidation catalyst. The wall flow monolith comprises an SCR catalyst composition, is in fluid communication with the injector, and is located downstream of the injector.

[0020] The wall-flow monolith has a series of longitudinally extending channels formed by the limiting longitudinally extending walls that define said channels. The channels include inlet portions that have an open inlet end and a closed outlet end, and outlet channels that have a closed inlet end and an open outlet end. The wall flow monolith contains an SCR catalyst composition that is deposited on and within the walls at a concentration of 79.3 g / l (1.3 g / in³ ) up to 146 g / l (2.4 g / in³ ), (and preferably from 97.6 to 146 g / l (1.6 to

2.4 g / in³ )). The wall flow monolith has at least a 50% porosity of the walls, with an average pore size of at least 5 microns.

emissions, zeolite composition and metal [0021] The SCR catalyst purification system comprises a base component selected from one or more components such as copper and iron. Preferably, the base metal is copper. Preferred zeolites in the SCR catalyst composition have a silica to alumina ratio of at least 10. For example, beta zeolite can be used in the SCR catalyst composition. [0022] The oxidation catalyst in the system is useful, among others, for the combustion of a substantial part of the solid particles, and in particular VOF, which are entrained with the exhaust gas. In addition, a significant proportion of NO in NO_x oxidizes to NO₂under the influence of an oxidation catalyst. In preferred embodiments, the oxidation catalyst is supported on a flow monolithic honeycomb substrate, or on an open-celled foam substrate. Preferably, the oxidation catalyst comprises a metal component from the platinum group, and in particular platinum. In some embodiments, the oxidation catalyst may also contain zeolite.

[0023] In another preferred embodiment of the emission treatment system, the system is also equipped with a diesel engine which is located upstream and which is in fluid communication with the oxidation catalyst.

[0024] Another aspect of the invention relates to a method of reducing emissions generated in an exhaust stream that contain NO_xand solid particles. This method includes:

(a) passing the exhaust stream through an oxidation catalyst in which at least 20% NO of NO_x oxidizes to NO₂, obtaining enriched with NO₂ exhaust stream;

(b) measurement at regular intervals of ammonia or ammonia precursors in an NO-enriched exhaust stream₂; and (c) passing, exhaust gas stream through a monolith with a wall flow, on which solid particles are captured and a significant part of NO_x is reduced to N₂ .

[0025] As before, the wall-flow monolith has a series of longitudinally extending channels formed by the limiting longitudinally extending walls that define said channels. The channels include inlet portions that have an open inlet end and a closed outlet end, and outlet channels that have a closed inlet end and an open outlet end. The wall flow monolith contains an SCR catalyst composition that is deposited on and within the walls at a concentration of 79.3 g / l (1.3 g / in³) up to 146 g / l (2.4 g / in³), (and preferably from 97.6 to 146 g / l (1.6 to 2.4 g / in³)). The wall flow monolith has at least a 50% porosity of the walls with an average pore size of at least 5 microns. [0026] In another aspect, the present invention relates to a method of depositing an SCR catalyst composition on a wall flow monolith. This method includes:

(a) immersing one end of the wall flow monolith in an aqueous slurry containing the SCR catalyst composition to deposit the SCR catalyst composition on the inlet channels;

(b) removing excess slurry from the inlet channels by passing a stream of compressed gas through the outlet channels and applying reduced pressure from the inlet channels;

(c) immersing the second end of the monolith with the wall flow opposite the first end in an aqueous slurry to deposit the SCR catalyst composition on the exhaust channels;

(d) removing excess slurry from the outlet channels by passing a stream of compressed gas through the inlet channels and applying reduced pressure from the outlet channels; and (e) drying and roasting the wall flow monolith thus coated.

[0027] The wall flow monolith used in the invention has a porosity of at least 50% (e.g., 50 to 75%) with an average pore size of at least 5 micrometers (e.g., 5 to 30 microns).

[0028] The SCR catalyst composition is deposited on and in the walls at a concentration of 79.3 g / L (1.3 g / in³) up to 146 g / l (2.4 g / in³), (and preferably from 97.6 to 146 g / l (from 1.6 to 2.4 g / in³)).

BRIEF DESCRIPTION OF THE DRAWINGS [0029]

Figures 1A and 1B are schematic illustrations showing two embodiments of the emission treatment system according to the invention;

Figure 2 is a perspective view of the filter substrate with wall flow;

Figure 3 is a cross-sectional view of a section of the filter substrate with wall flow;

Figure 4 shows an embodiment of an emission treatment system according to the invention which comprises a urea tank and an injector;

Figure 5 is a graph of DTA signal in microvolts as a function of temperature for two SCR catalyst compositions mixed with a reference mass containing solid particles (soot and lubricating oil),

Figure 6 shows the pressure drop as a function of airflow for several coated wall flow substrates and for uncoated wall flow substrate; and

Figure 7 is a schematic representation of the laboratory system used to assess NO_x and particle reduction for an exemplary emission treatment system of the invention.

DETAILED DESCRIPTION OF THE INVENTION [0030] The invention relates to an emission purification system that provides efficient and simultaneous removal of solid particles, NO_x and other gaseous components derived from diesel exhaust gases. The emission treatment system uses an integrated soot filter and SCR catalyst to significantly reduce the necessary weight and size of the emission system. In addition, due to the choice of catalytic compositions used in the system, an effective reduction of pollutants is provided for a stream of exhaust gases with different temperatures. This feature is beneficial when operating vehicles with diesel engines under varying load conditions and at varying vehicle speeds that significantly affect the exhaust gas temperatures emitted from the engines of such vehicles. [0031] Integration of NO reduction_x together with the removal of particles in one catalyst is carried out using a wall flow substrate coated with an SCR catalyst composition. Applicant has developed a method of applying the SCR catalyst composition to a baffle-flow substrate to form a substrate that can be used in applications where high filtration efficiency is required. For example, the substrate formed by this method is suitable for effective removal of solid particles (e.g., 80% more) from the emission treatment system according to the invention. The coating method disclosed herein allows loading of the substrate with the wall flow of the SCR catalyst to practical levels, without causing excessive back pressure for the coated article, when applied to an emission treatment system.

[0032] Achieving practical levels of SCR catalyst composition on a wall with substrate is important to provide sufficient catalytic activity to achieve the required levels of NO reduction_xand to reduce the combustion temperature of the soot fraction retained on the filter. Achieving the appropriate levels of the SCR intermediate layer composition on the soot filter is also important to provide adequate catalyst strength. During the long-term use of the emission treatment system, catalysts are always exposed to varying levels of poisonous compounds, which may come from decomposed lubricating oils or may come from contaminants in diesel fuel. Examples of such catalyst poisons include phosphorus, zinc, alkalis and alkaline earth elements. Thus, in order to overcome the inevitable loss of catalytic activity, larger amounts of catalytic compositions are usually deposited on catalytic substrates.

[0033] One embodiment of the invented emission treatment system is schematically shown in Figure 1A. As can be seen in Figure 1A, the exhaust gas containing gaseous pollutants (including unburned hydrocarbons, carbon monoxide and NO_x) and solid particles are transported from the engine 15 to the oxidation catalyst 11. In the oxidation catalyst 11, unburned and non-volatile gaseous hydrocarbons (i.e. VOF) and carbon monoxide are largely burned to carbon dioxide and water. Removal of significant amounts of VOF using an oxidation catalyst prevents, in particular, excessive deposition of particulate matter on the soot filter 12 (i.e. clogging), which is further placed in the system. In addition, a significant proportion of NO with NO in the oxidation catalyst_x oxidizes to NO₂.

[0034] Below the oxidation catalyst is a reducer, in this case ammonia, which is injected into the exhaust stream as a spray through a nozzle (not shown). The aqueous urea solution represented by line 18 can serve as an ammonia precursor and can be mixed with the air from the second line 19 in the mixing station 16. The valve 14 can be used to precisely dispense the appropriate amounts of the aqueous urea solution that is converted into the ammonia flue gas stream. The flue gas stream with the addition of ammonia is fed to a soot filter 12 which is covered with a SCR catalyst composition. Passing through the soot filter, NO_x it is transformed due to the selective catalytic reduction of NO_x with ammonia, to nitrogen. More NO₂ at NO_x as a result of the catalytic process previously taking place on the catalyst, it allows reduction of NO emissions_x compared to the exhaust stream containing smaller NO proportions₂ at NO_x.

[0035] Depending on the desired NO removal rate_x, an additional SCR catalyst can be placed downstream of the soot filter. For example, an additional SCR catalyst may be placed on a monolithic flow-through honeycomb substrate or on a foam ceramic substrate located downstream of the soot filter. Even in these embodiments, the use of a coated SCR soot filter still achieves a reduction in the total amount of catalyst that is necessary for the purpose of NO reduction_x.

[0036] Particulates, including the soot fraction, and VOF are also largely removed (above 80%) through the soot filter. The solid particles accumulated on the soot filter are burned during the regeneration of the filter, which process is also aided by the presence of the SCR catalyst composition.

The temperature at which the soot fraction from the particulate fraction burns is lower due to the presence of the catalytic composition deposited on the soot filter.

[0037] The optional configuration of the emission purification system with the secondary oxidation catalyst 13 located below the coated soot filter 2 is shown in Figure 1B. The secondary oxidation catalyst may be coated, for example, with a composition containing base metals and less than 0.5 wt. platinum. This variant can be used to oxidize excess NH₃ before it gets into the atmosphere.

[0038] Suitable SCR catalyst compositions for use in the system are able to effectively catalyze NO reduction_xat temperatures below 600 ° C so that adequate amounts of NO_xthey can be removed even under low load conditions, which are usually associated with lower exhaust gas temperatures. Preferably, the catalyst is able to convert at least 50% NO depending on the amount of reducing agent added to the system_x to N₂. In addition, the SCR catalyst compositions used in the system are ideally suited for filter regeneration by reducing the temperature at which the carbon black fraction from the particulate fraction burns. Another desirable attribute of the composition is that it has the ability to catalyze the O reaction₂ with excess NH₃to N₂ and H₂About that way, NH₃ it is not emitted into the atmosphere.

[0039] Useful SCR catalyst compositions used in the invented system of the invention also have thermal resistance at temperatures greater than 650 ° C.

Such high temperatures often occur when regenerating soot filters. In addition, SCR catalytic compositions should be resistant to degradation occurring under the influence of sulfur compounds, which are often found in exhaust gases of diesel engines.

[0040] Suitable SCR catalyst compositions are described, for example, in US Patent 4,961,917 ('917 patent) and in Patent No. 5,516,497. The compositions disclosed in the '917 patent include one iron and copper promoter or both promoters present in zeolite in an amount of 0.1 to 30 weight percent, preferably 1 to 5 weight percent, based on the total weight of the promoter and zeolite. In addition to their ability to catalyze NO reduction_x using NH₃ to N₂, the disclosed compositions may also enhance the oxidation of excess NH₃ using O₂, especially for those compositions that contain higher promoter concentrations.

[0041] The zeolites used in such compositions are resistant to sulfur poisoning, maintain a high level of activity in the SCR process and are able to oxidize excess ammonia with oxygen. These zeolites have pores large enough to allow proper movement of NO and NH substrate molecules₃ to and particles of the product N₂and H₂O z, the pore system in the presence of sulfur oxide molecules from short-term sulfur poisoning and / or sulfate deposition due to long-term sulfur poisoning. An appropriate pore system is co-connected in all three crystallographic dimensions. It is well known to those skilled in the art of zeolites, for example, that the crystal structure of zeolites has a complex pore structure that has more or less regularly repeating joints or common parts. Pores with specific properties, such as diameter and distribution in cross-section, are referred to as one-dimensional if these pores do not intersect with other pores. If the pores intersect with other pores only within a given plane, such pores are said to be connected together in two dimensions (crystallographic). If the pores intersect with other pores that lie in the same plane and in other planes, such pores are considered to be interconnected in three dimensions, and therefore are "three-dimensional" pores. It has been found that zeolites that are very resistant to sulfate poisoning and provide good activity both in the SCR process and in the process of oxidation of ammonia with oxygen and which retain good activity even when exposed to high temperatures, hydrothermal conditions and sulfate poisoning are zeolites which have pores with a pore diameter of at least 7 angstroms and are interconnected in three dimensions. Without wishing to be bound by any particular theory, it is believed that the combination of pores with a diameter of at least 7 Angstroms in three dimensions provides good ability to move sulfate particles through the zeolite structure, and thus allows sulfate particles to leave the catalyst, thus freeing a large number of places available for adsorption for NO reagent molecules_x and NH molecules ₃ and NH reagent₃ and particles₂. Any zeolites meeting the above criteria are suitable for use in practice in accordance with the present invention; specific zeolites that meet these criteria are USY, Beta and ZSM-20. Other zeolites may also meet the above-mentioned criteria.

[0042] When depositing a wall flow monolith onto a substrate, such SCR catalyst compositions are applied at a concentration of 79.3 g / L (1.3 g / in³) up to 146 g / l (2.4 g / in³) to ensure the expected NO reduction_x and an adequate level of solids removal and ensure proper catalyst durability during prolonged use. In a preferred embodiment, it is applied to the monolith with a wall flow of 97.6 to 146 g / l (1.6 to 2.4 g / in³).

[0043] Wall flow substrates useful as carriers for SCR catalyst compositions have a plurality of small, substantially parallel gas flow channels that extend along the longitudinal axis of the substrate. Typically, each channel is blocked at one end of the substrate body along with alternating channels blocked at opposite ends. Such monolithic carriers may contain up to 108.5 or more flow channels (or "cells") per square centimeter (700 or more flow channels (or "cells") per square inch) in cross-section, however, a smaller amount may also be used. For example, the carrier may have from 1.08 to 93.02, usually from 15.5 to 62 cells per square centimeter (from 7 to 600, usually from 100 to 400, cells per square inch ("cpsi")). Cells can have sections that are rectangular, square, round, oval, triangular, hexagonal or other polygonal shapes. Wall flow substrates typically have a wall thickness of 0.05 to

2.54 mm (0.002 to 0.1 inches). Preferably, wall flow substrates have a wall thickness of 0.05 to 0.38 mm (0.002 and 0.015 inches).

[0044] Figures 2 and 3 show a wall flow filter substrate 30 that has a plurality of channels 52. The channels are cylindrical surrounded by the inner walls 53 of the filter substrate. The substrate has an inlet end 54 and an outlet end 56. The channels are obstructed alternately at the inlet by the closure 58 and at the outlet by the closure 60, forming checkerboard forms at the inlet 54 and the outlet 56. The gas stream 62 passes through the unblocked inlet channel 64 and is stopped by the outlet closure 60 and diffuses through the walls of the channel 53 (which are porous) to the outlet side 66. The gas cannot go back to the inlet side due to the inlet closure 58.

[0045] Preferably, the wall flow filter substrates consist of ceramic materials such as cordierite, α-alumina, silicon carbide, silicon nitride, zirconia, mullite, kodene, mixtures of alumina, silicon and magnesium oxides and zirconium silicate or of porous refractory metal. Wall flow substrates can also be formed from ceramic fibers of composite materials. Preferably, the wall flow substrates are formed of cordierite and silicon carbide. These materials are able to withstand high temperatures, in particular those occurring in flue gas streams.

[0046] Preferably, wall flow substrates for use in the invented system include thin-walled porous honeycomb (monolith) structures through which the liquid stream flows without causing too much increase in back pressure or pressure passing through the element. Typically, the presence of a clean wall-mounted element creates a pressure of 249 to 68900 Pa (1 inch water column to 10 psig). Wall-bound ceramic substrates used in the system are made of a material with a porosity of at least 50% (e.g., 50 to 75%), with an average pore size of at least 5 microns (e.g., 5 to 30 microns). More preferably, the substrates have a porosity of at least 55% and have an average pore size of at least 10 microns. When substrates with such porosity and with such an average pore size are coated with the techniques described below, then the SCR catalytic composition of the appropriate concentration can be applied to the substrate for excellent NO conversion efficiency_x. These substrates are able to additionally maintain adequate exhaust gas flow characteristics, i.e. the allowable back pressure, despite the SCR catalyst being applied. U.S. Patent No. 4,329, 162 discloses suitable wall flow substrates.

[0047] Typical wall flow filters used commercially are generally made of materials with a lower wall porosity, for example 35% to 50%, than the wall flow filters used in the invention. Generally, the pore size distribution in commercial wall-mounted filters is usually very wide and the average pore size is less than 17 microns.

[0048] The porous wall flow filter used in the present invention becomes a catalyst when one or more catalytic materials are present on the walls or walls of this element. Catalytic materials may only be present on the inlet side of the element walls, only on the outlet side, both on the inlet and outlet sides, or the walls may include all or part of the catalytic material. The present invention includes the use of one or more layers of catalytic materials and combinations of one or more layers of catalytic materials at the inlet and / or outlet walls of an element.

[0049] In order to cover wall substrates with SCR catalytic composition, the substrates are immersed vertically in a portion of the catalyst slurry so that the upper part of the substrate is just above the slurry surface. In this way, the slurry covers the fronts of each of the honeycomb walls, but does not contact the outlet side of each wall. The element is left in suspension for about 30 seconds. The substrate is removed from the suspension, and the excess suspension is removed from the wall with the wall flow by draining from the channels and then by blowing with compressed air (opposite to the direction of penetration of the suspension), after which a vacuum is applied from the side from which the suspension penetrated the element. Using this technique, the catalyst slurry penetrates the walls of the substrate, but the pores are not clogged, which would result in an increase in undesirable back pressure that would arise in the final substrate. The term "permeation," as used herein, when used to describe the dispersion of a catalyst slurry on a substrate, means that the catalytic composition is dispersed throughout the entire wall of the substrate.

[0050] The coated substrates are generally dried at about 100 ° C and calcined at a higher temperature (for example from 300 to 450 ° C). After calcination, the catalyst load can be determined by calculating the mass for the coated and uncoated substrate. It will be apparent to one skilled in the art that the amount of catalyst may be modified by changing the solids content of the coating suspension. Alternatively, multiple immersion of the substrate in the coating suspension may be performed followed by removal of excess suspension as described above.

[0051] The reducer dosing system is located above the soot filter and below the oxidation catalyst so as to inject the NO reducer_x to the exhaust stream. As disclosed in US Patent No. 4,963,332, NO_x they can be detected above and below the catalyst, and the pulse dosing valve can be controlled by signals coming from before and after the catalyst. In alternative configurations, the systems disclosed in US Patent No. 5,522,218 are such that the pulse length of the injected reducer is controlled by data on exhaust gas temperature and engine operating conditions such as RPM, gear ratio and engine speed. In US Patent 6,415, 602, systems for pulsed dosing of the reducer are also considered.

[0052] In the embodiment shown in Fig. 4, the aqueous urea solution tank 22 provides the vehicle with a urea / water solution supply. This solution is pumped through a pump 21 containing a filter and pressure regulator to the urea injector 16. The urea injector 16 is a mixing chamber into which from the line 19 through the control valve air is supplied at the appropriate pressure. The sprayed urea / water / air solution is pulsed through a nozzle 23 into exhaust pipe 24, located above the integrated soot filter 12 coated with an SCR catalyst. [0053] This invention is not limited to the dispensing system of the aqueous urea solution shown in Fig. 4. It is believed that nitrogen gas can also be used. For example, an urea or cyanuric acid granule injector can dispense solid urea granules into a chamber heated by the exhaust gas to gasify the solid reducer (sublimation in the temperature range from 300 to 400 ° C). Cyanuric acid will be gasified to isocyanic acid (HNCO), and urea will be gasified to ammonia and HNCO. To the chamber with both reducers, a hydrolysis catalyst and a secondary exhaust stream are dosed into the chamber (the exhaust gas contains enough water vapor) to carry out the hydrolysis process (temperatures from 150 to 350 ° C) HNCO to produce ammonia.

[0054] In addition to urea and cyanuric acid, other nitrogenous compounds that are reducing agents or reducing agents particularly suitable for use in the control system of the invention include ammelide, ammeline, ammonium cyanate, biuret, cyanuric acid, tricyanic urea carbamate and any of their invention in a broad sense is not limited to nitrogen reducing agents, but may contain any ammonium reducing agent, mixtures.

melamine,

However, containing hydrocarbons such as distilled fuels, including alcohols, ethers, organic nitrogen compounds and the like (e.g. methanol, ethanol, diethyl ether, etc.) and various amines and their salts (in particular their carbonates), including guanidine carbonate, methylamine carbonate, hexamethylamine, etc. [0055] Above the reducer dosing system is the oxidation catalyst (or DOC). The oxidation catalyst may be formed from any composition that provides effective combustion of unburned gaseous and non-volatile hydrocarbons (i.e. VOF) and carbon monoxide. In addition, the oxidation catalyst should be effective in converting a significant proportion of NO to NO_x to NO₂. The term "substantial conversion of NO from NO" as used herein_x to NO₂"means at least 20%, preferably between 30 and 60%. Catalytic compositions having these properties are known in the art and include platinum group metals and compositions based on base metals. Catalytic compositions may be applied to monolithic honeycomb flow substrates. formed from metallic or ceramic refractory materials (e.g. cordierite). Alternatively, oxidation catalysts may be formed on metal or ceramic foam substrates that are well known in the art. Oxidation catalysts, due to the substrate on which they are applied (e.g. open cellular ceramic foams) and / or because of their actual catalytic oxidation activity, provide a certain level of removal of solid particles. Preferably, the oxidation catalyst removes some of the solids from the stream above the wall-flow filter, as reducing the amount of solids on the filter potentially extends the time for forced regeneration.

[0056] One preferred oxidation catalyst composition that can be used in an emission purification system comprises a platinum group component (e.g., platinum, palladium or rhodium) dispersed over a large surface, a refractory oxide substrate (e.g., γ-alumina) which is combined with zeolite (preferably beta zeolite). The preferred metal from the platinum group is platinum. When the composition is placed on a refractory oxide substrate, e.g., a flow-through honeycomb substrate, then the concentration of platinum is usually from 0.35 to 4.24 g / l (from 10 to 120 g / ft³) platinum.

[0057] Metal based compositions from the platinum group, suitable for use in the preparation of an oxidation catalyst, are also described in US Patent No. 5,100,632 ('632 patent). The '632 patent describes compositions that are a mixture of platinum, palladium, rhodium and ruthenium and an alkaline earth metal oxide, such as magnesium oxide, calcium oxide, strontium oxide or barium oxide, in which the atomic ratio of the metal from the platinum group to the alkaline earth metal is from 1: 250 to 1: 1, preferably from 1:60 to 1: 6.

[0058] Catalyst compositions suitable for the oxidation catalyst may also be prepared by using base metals as catalysts. For example, US Patent No. 5,491,120 discloses oxidation catalyst compositions that contain catalytic material with a BET specific surface area of ​​at least 10 m²/ g and consisting essentially of a second metal oxide, which may be one or more of titanium oxide, zirconia, mixed cerium zirconia, silicon oxide, mixed aluminosilica and α-alumina.

[0059] Also useful are the catalyst compositions disclosed in US Patent No. 5,462,907 ('907 patent). The '907 patent discloses compositions that contain a catalytic substance containing cerium and alumina, each having a surface area of ​​at least 10 m²/ g, for example cerium dioxide and activated alumina in a weight ratio of 1.5: 1 to 1: 1.5. Optionally, platinum may be included in the compositions described in the '907 patent in an amount effective to promote the oxidation of CO in the gas phase and to oxidize unburned hydrocarbons, but which are limited to an amount that limits excessive oxidation of SO to SO₂. Alternatively, the catalyst material may contain palladium in any required amount.

[0060] The following examples further illustrate the present invention, but of course, are not to be construed as limiting its scope in any way.

Example 1 - Coating of ceramic flow filters [0061] Ceramic wall flow filter substrates made of cordierite (product name C611, NGK Insulators, Ltd.) with dimensions 14.4 x 15.2 cm (5.66 x 6 inches), thick 0.305 mm (0.012 inch) walls, 25 micron average pore size, and 60% porosity walls were used to produce catalyst-coated soot filters.

[0062] The catalyst slurry containing 27% by weight of solids was formed from a copper exchangeable beta zeolite (containing 2% by weight of copper), an additional amount of CuSO₄ (sufficient to deliver 9.5% by weight of copper) and 7% by weight ZrO₂ and deionized water (wt% based on the weight of beta zeolite). Beta copper-exchangeable zeolite was prepared as described in US Patent No. 5,516,497.

[0063] An identical procedure, according to a preferred embodiment of the invention, was used to prepare two filter media. The wall flow substrate was:

(1) immersed in suspension to a depth sufficient to coat the substrate channels along the entire axial length of the substrate in one direction;

(2) blown with air from the opposite side to the coating direction (e.g. from the dry side);

(3) subjected to a vacuum on the coated side;

(4) dried at 93 ° C for 1 h, in an air stream and calcined at 400 ° C for 1 h; and (5) then steps (1) to (4) on the opposite side were repeated.

[0064] These filter media (designated as catalysts A1 and A2) contained a catalyst at a concentration of 128 g / l (2.1 g / in³ ). The amount of copper contained in these catalysts was about

12.2 g / l (0.2 g / in³ ).

[0065] Another filter medium, designated as catalyst B1, was prepared by coating only one side of the medium, using steps (1) to (4). To achieve the same catalyst concentration as for catalyst A1, the solids content of the suspension increased to 38%. The composition of the catalyst remained the same. Catalyst B1 had a catalyst concentration of 122 g / L (2.0 g / in³). The amount of copper contained in this catalyst was also about

12.2 g / l (0.2 g / in³).

[0066] The reference sample, catalyst D1, was prepared as a flow type catalyst. To prepare this type of catalyst, a filter medium of the type described above was cut at one end along the diameter, just behind the partitions. In this way, the wall-flow filter was transformed into a flow medium with an effectively effective half of the blocked face. This medium was coated to give a catalyst concentration of 122 g / L (2.0 g / in³), which was a copper-exchangeable beta zeolite catalytic composition.

Example 2 - Back pressure estimation for coated soot filters [0067] The pressure drop across uncoated and coated filters was evaluated using a commercially available automated flow Super Flow SF 1020 (Probench). This device has been designed to measure the pressure drop as a function of air flow. Data from this device are presented as a graph of pressure drop in ambient conditions as a function of air flow. The pressure drop is a measure of how easily air flows through the filter. In diesel engines, smaller pressure drops are desirable because the engine must consume power for the airflow. Therefore, the greater the pressure drop, the greater the loss of engine power that is lost when pumping air. This loss of power is associated with a reduction in power that is transmitted to the wheels.

[0068] Fig. 6 shows pressure drops on coated filters, catalyst A1, A2 and B1, and on an uncoated filter of identical dimensions. Filters coated according to steps (1) to (5) according to example 1, i.e. catalyst A1 and A2, showed a pressure drop that was about 25% more than on the uncoated filter. Unlike catalysts A1 and A2, the non-optimized filter, catalyst B1, showed pressure drops that were greater than 100% compared to the uncoated filter after coating. The pressure drop exhibited by the B1 catalyst was so high that engine test testing for this filter proved impossible. Although it is possible to achieve a pressure drop in the coated filter, in the same way as in the B1 catalyst, by reducing the catalyst concentration, lower SCR catalyst concentration levels lead to an unacceptable level of NO reduction_x.

Example 3 - Demonstration of Particle Removal by an SCR Catalyst [0069] When a wall catalyst catalyst composition is used, it should ideally support filter regeneration. In this regard, the SCR catalyst composition deposited on the filter is preferably capable of catalytically oxidizing some carbon black and VOF particles. SCR catalyst to be effective in reducing NO_x and solids should preferably not cause oxidation of ammonia or SO₂ to form SO₃. The only way to assess the ability of a catalyst to oxidize carbon and VOF is to use combined thermogravimetric analysis (TGA) and differential thermal analysis (DTA). With the help of TGA, the weight loss of a sample is determined, while DTA means a change in the thermal capacity of the sample relative to the reference sample. In this experiment, the dried and calcined part of the catalyst suspension was mixed with 6 wt. oil so as to simulate the presence of grease-derived VOFs and 14% by weight of carbon black to simulate the carbon black fraction that is present in the particulate fraction. The mixture was introduced into the device so as to perform a combined TGA and DTA analysis. Although various gaseous compositions can be passed through the sample, these tests were carried out using air. The system was heated at a known rate to determine mass loss and heat release as a function of temperature. The advantage of this technique is its ability to separately analyze the weight loss depending on the various components of soot and take these mass losses into account as the temperature changes. Catalysts effective in soot combustion have a lower soot combustion start temperature.

[0070] Fig. 5 shows the DTA signal in microvolts as a function of temperature for two catalyst compositions; (1) TiO reference composition₂ - 10% by weight, WO₃ - 2 wt. catalyst V₂ABOUT₅and (2) a catalyst composition used to coat the A1 catalyst. TiO based composition in SCR catalysts₂ is typical in the light of the current state of the art and has wide application. The dried and calcined powder suspensions of each catalyst were mixed with 6% lubricating oil and 14% by weight. carbon black. at a rate of 20 ° C per minute, 800 ° C. DTA signals obtained

Samples were heated in air from room temperature to show two peaks, one below 400 ° C, corresponding to VOF combustion, and the other peak at higher temperature corresponding to soot burning. The results show that both catalyst systems are effective in burning part of the lubricating oil in a simulated particulate system, but preferred catalyst compositions are much more effective in burning the carbon part as evidenced by the reduction of soot combustion temperature. This will be seen in the following examples, because this advantage persists without compromising the NO reduction activity_x.

Example 4 - Assessment of NO conversion_x and particulate removal for coated soot filters [0071] Filtration efficiency and simultaneous reduction of NO_xwas determined using a V6 4L prototype turbocharged, cooled diesel engine that is representative of the current state of the art in diesel technology. The engine was mounted on a fixed test stand, achieving repeatable and stable exhaust emission conditions. The engine speed and load were controlled to ensure a filter inlet temperature of 370 ° C and NO concentration_x about 950 ppm. Particle measurements were determined according to the procedures described in the "Code of Federal Regulations", Title 40, Part 86, para. 1312-88, however, a mini-dilution tunnel was used instead of the full dilution tunnel. The dilution factor was determined from the CO concentration₂. NO removal_x from a diesel engine was obtained by injecting a urea solution downstream of the oxidation catalyst and upstream of the coated SCR filter substrate. The experimental setup is shown in Fig. 7. WELL_xand ammonia were measured using a FTIR analysis device equipped with a heated sample line and measuring chamber. WELL_x, CO and HC were also determined using a measuring apparatus from Horiba, specifically designed for analyzing raw exhaust gas from diesel engines.

[0072] Additional catalysts were prepared and aged for 1000 hours on a stationary diesel engine, using its cyclic operation to simulate driving a passenger car. The aging cycle was an adaptation of the procedure described in the "Durability Driving Schedule for Light Duty Vehicles and Light Duty Trucks" Code of Federal Regulations, Part 86 para. 836-01. The test cycle described there determines the speeds and periodic stops for a vehicle moving around the test track. Previously, the temperature profile was measured for such a cycle and simulation was carried out on a bench with the engine. ARCO ECD diesel fuel was used in the aging process and at the evaluation stage. This fuel has a sulfur content of 12 ppm, which corresponds to the fuel that is intended for use during the use of the technology.

[0073] Using the experimental setup shown in Fig. 7, NO conversion_x and particulate removal were determined for three catalytic substrates. As seen in Fig. 7, the experimental configuration included a urea injector located above the catalyzed soot filter and an oxidation catalyst (DOC) located in front of the placed (DOC) urea injector. To eliminate any differences due to DOC, all trials were performed using the same DOC. The oxidation catalyst composition was applied to a flow cordierite substrate with dimensions

14.6 x 15.2 cm (5.66 x 6 inches). The oxidation catalyst composition contained 3.18 g / L (90 g / ft³) dispersed in γ-alumina and contained 27 wt. hydrogen ions in exchangeable beta zeolite. DOC was aged for 1000 hours.

[0074] In the tests carried out in this experiment, the SCR catalyst composition was applied to either a monolithic wall flow substrate or a monolithic flow substrate. The SCR catalyst composition was identical to that used for coating the substrate in Example 1, i.e. it contained a zeolite capable of exchanging copper together with a zirconium binder. Basically, the substrates used in the experiment were: fresh catalytic substrate identically to the catalyst A1 of example 1 (designated as catalyst A1_{fresh greens}s); separate catalyst bed, coated identically to catalyst A1 but aged for 1000 hours (designated as catalyst A1_aged); and finally a third catalytic substrate that was of the flow type and was prepared in the same way as the D1 catalyst (designated as D1 catalyst_fresh).

[0075] Table 1 below summarizes the efficiency of particle filtration and NO reduction_x for three catalytic substrates. The filtration efficiency was determined with and without urea ejection.

Table 1

<td>Attempt</td><td>Catalyst</td><td>Substrate type</td><td>NH3 / NOx</td><td>about % conver sion NOx.</td><td>NH3 secondary, ppm</td><td>Removal particles permanent total, about %</td>

<td> 1</td><td>D¹ fresh</td><td>flow</td><td> 0</td><td> &lt;5</td><td> 0</td><td> &lt;10</td>

<td> 2</td><td>D¹ fresh</td><td>flow</td><td> 0,3</td><td> 30</td><td> 0</td><td> &lt;10</td>

<td> 3</td><td>^A1 fresh</td><td>with wall flow</td><td> 0</td><td> &lt;5</td><td> 0</td><td> 82</td>

<td> 4</td><td>^A1 fresh</td><td>with wall flow</td><td> 0,5</td><td> 51</td><td> 0</td><td> 85</td>

<td> 5</td><td>^A1 aged</td><td>with wall flow</td><td> 0</td><td> &lt;5</td><td> 0</td><td> 81</td>

<td> 6</td><td>^A1 aged</td><td>with wall flow</td><td> 0,5</td><td> 55</td><td> 0</td><td> 85</td>

[0076] As can be seen in Table 1, removal of the SCR catalyst composition on a wall flow monolith does not reduce the NO removal efficiency_x. Furthermore, urea injection also has no effect on filtration efficiency. Although the SCR catalyst-coated flow monolith is intended for NO removal_x, it does not have high filtration efficiency, which in turn is characterized by coated wall monolith. Thus, coated SCR filter media according to the invention show integrated, high efficiency in NO removal_x and solid particles.

[0077] Furthermore, the stability of the SCR catalytic composition is shown by the data in Table 1. The aging of the coated substrate did not result in either loss of filtration efficiency or NO removal efficiency_x.

[0078] Although the present invention has been described with an emphasis on preferred embodiments, it will be apparent to those skilled in the art that changes may be made to preferred devices and methods, and it should be understood that the invention may be implemented differently from that described herein. document. Accordingly, this invention includes all modifications that fall within the scope of the invention as defined in the following claims.

NZ: 25496 / PE / 15

EP 2 042 227