Emission treatment system and method using a scr filter

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DESCRIPTION

Emission treatment system and method using an SCR filter.

The present invention relates to an emission treatment system having an oxidation catalyst upstream of a soot filter coated with a material effective in the selective catalytic reduction (SCR) of NOx by a reductant, for example ammonia. In one embodiment, the system provides an efficient method of simultaneous decontamination of nitrogen oxides (NOx), particulate matter, and gaseous hydrocarbons present in diesel engine exhaust gas streams.

Diesel engine exhaust gases are a heterogeneous mixture that contains not only gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and nitrogen oxides (“NOx”), but also materials in condensed phase (liquids and solids) that constitute the so-called particles or particulate matter. Often times, the catalyst compositions and substrates on which the compositions are disposed are provided in diesel engine exhaust systems to convert some or all of these exhaust gas components to harmless components. For example, diesel exhaust systems may contain one or more than a diesel oxidation catalyst, a soot filter, and a NOx reduction catalyst.

Oxidation catalysts containing platinum group metals, base metals and combinations thereof are known to facilitate the treatment of diesel engine exhaust gas, promoting the conversion of both HC and CO gaseous pollutants and some proportion of particulate matter by oxidizing these pollutants to carbon dioxide and water. Such catalysts have generally been contained in units called Diesel Oxidation Catalysts (DOC), which are placed in the exhaust of diesel engines to treat the exhaust gases before they are discharged to the atmosphere. In addition to the conversions of gaseous HC, CO and particulate matter, oxidation catalysts containing platinum group metals (which are normally dispersed on a refractory oxide support) also promote the oxidation of nitric oxide (NO) to NO2.

Total emissions of particulate matter from diesel exhaust gases comprise the three main components. One component is the solid, dry solid carbonaceous fraction or soot fraction. This dry carbonaceous matter contributes to the visible soot emissions commonly associated with diesel exhaust gases. A second component of particulate matter is the soluble organic fraction ("SOF"). The soluble organic fraction is sometimes referred to as the volatile organic fraction ("VOF"), terminology that will be used herein. VOF can exist in diesel exhaust gases either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust gases. It is generally present as condensed liquids at the standard particulate collection temperature of 52 ° C in dilute exhaust gases, as indicated by a standard measurement test, such as the US Federal Transitional Vehicle Test Procedure. Heavy These fluids result from two sources: (1) lubricating oil swept from the walls of the engine cylinders each time the pistons rise and fall; and (2) unburned or partially burned diesel fuel.

The third component of particulate matter is the so-called sulfate fraction. The sulfate fraction is formed from small amounts of sulfur components present in diesel fuel. Small proportions of SO3 are formed during diesel combustion, which in turn rapidly combines with water in the exhaust gas to form sulfuric acid. Sulfuric acid is collected as a condensed phase with the particles as an aerosol, or adsorbed onto the other particulate components, and thus added to the TPM mass.

A key aftertreatment technology for the reduction of high particulate matter is the diesel particulate filter. There are many known filter structures that are effective in removing particulate matter from diesel exhaust gas, such as honeycomb wall flow filters, rolled or compacted fiber filters, open cell foams, sintered metal filters, etc. However, ceramic wall flow filters, described below, receive the most attention. These filters can remove more than 90% of the particulate material from diesel exhaust gas. The filter is a physical structure to remove particles from the exhaust gas, and the particles that accumulate will increase the filter back pressure on the engine. Thus, the particles that accumulate have to be continuously or periodically burned from the filter to maintain an acceptable back pressure. Unfortunately, carbon soot particles require temperatures above 500 ° C to burn under oxygen-rich (lean) exhaust gas conditions. This temperature is higher than what is normally present in diesel exhaust gas.

Supplies are generally introduced to reduce the combustion temperature of the soot in order to provide passive regeneration of the filter. The presence of a catalyst promotes the combustion of the soot, thus regenerating the filters to accessible temperatures within the diesel engine exhaust gases under realistic load cycles. In this way, a catalyzed soot filter (CSF) or catalyzed diesel particulate filter

ES 2 551 691 T3 (CDPF) is effective in providing> 80% reduction in particulate matter in conjunction with passive combustion of accumulated soot, thus promoting filter regeneration.

Future emission standards adopted around the world will also address NOx reductions from diesel exhaust gases. A proven NOx reduction technology applied to stationary sources with lean exhaust gas conditions is Selective Catalytic Reduction (SCR). In this process, NOx are reduced with ammonia (NH3) to nitrogen (N2) over a catalyst normally composed of basic metals. The technology is capable of reducing NOx in excess of 90%, and thus represents one of the best approaches to achieving aggressive NOx reduction targets. SCR is under development for mobile applications, with urea (normally present in an aqueous solution) as the source of ammonia. SCr provides efficient NOx conversions as long as the exhaust gas temperature is within the active catalyst temperature range.

Although separate substrates each containing catalysts can be provided to treat the discrete exhaust gas components in an exhaust system, the use of fewer substrates is desired to reduce the overall size of the system, to facilitate assembly of the system, and to reduce the overall cost of the system. One approach to achieve this goal is to coat the soot filter with a catalyst composition effective for the conversion of NOx to harmless components. With this approach, the catalyzed soot filter takes on two catalyst functions: the removal of the particulate component from the exhaust gas stream and the conversion of the NOx component from the exhaust gas stream to N2.

Coated soot filters that can achieve NOx reduction goals require a sufficient load of SCR catalyst composition on the soot filter. The gradual loss of catalytic effectiveness of compositions that occurs over time through exposure to certain deleterious components of the exhaust gas stream increases the need for higher catalyst loadings from the SCR catalyst composition. However, preparing wall flow soot filters coated with higher catalyst loads can lead to unacceptably high back pressure within the exhaust gas system. Therefore, coating techniques that allow higher catalyst loads on the wall flow filter, while still allowing the filter to maintain flow characteristics that achieve acceptable back pressures, are desirable.

An additional aspect to consider in coating the wall flow filter is the selection of the appropriate SCR catalyst composition. First, the catalyst composition must be durable so that it maintains its SCR catalytic activity even after prolonged exposure to higher temperatures that are characteristic of filter regeneration. For example, combustion of the soot fraction of particulate matter frequently leads to temperatures in excess of 700 ° C. Such temperatures render many commonly used SCR catalyst compositions, such as mixed vanadium and titanium oxides, less catalytically effective. Second, SCR catalyst compositions preferably have a wide enough operating temperature range that they can accommodate the varying temperature ranges that the vehicle operates at. Temperatures below 300 ° C are typically encountered, for example, under low load conditions, or at startup. SCR catalyst compositions are preferably capable of catalyzing the reduction of the exhaust gas NOx component to achieve NOx reduction objectives, even at lower exhaust gas temperatures.

The prior art contains descriptions of the use of SCR catalyst compositions, soot filters and combinations thereof for the reduction of both NOx components and diesel exhaust particulate. These references are described below.

The publication called "Kokai" of Japanese patent application 3-130522, for example, discloses the treatment of diesel exhaust gases characterized by the use of an ammonia injector and porous ceramic filter that has a denitration catalyst within the pores. . The filter is installed in the wake of diesel engine exhaust gases. The porous ceramic filter comprises an upstream fine-pored path layer and a downstream side path ceramic particle layer on which the denitration catalyst was supported. The thin layer can support a platinum or palladium or other hydrocarbon combustion catalyst. The diesel exhaust gas containing unburned carbon passes through the porous ceramic filter and the carbon particles are filtered on the surface. The gas containing nitric oxides and ammonia passes through the side containing the denitration catalyst of the filter and the nitric oxides are reduced to nitrogen and water. The oxidation catalyst on the upstream side causes the particulate component to catalytically burn.

US Patent No. 4,912,776 discloses an oxidation catalyst, an SCR catalyst downstream and adjacent to the SCR catalyst, and a source of reductant introduced to the exhaust gas stream between the oxidation catalyst and the SCR catalyst. Providing a higher feed containing a high ratio of NO2 to NO to the SCR reactor is said to allow the use of lower temperatures and higher space velocities than are possible with a NO feed.

WO 99/39809 discloses a system for treating combustion exhaust gas containing NOx and particulates having an oxidation catalyst effective to convert at least a portion of the NO to the NOx into

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NO2, a particulate trap, a reducing fluid source, and an SCR catalyst. The particle trap is downstream of the oxidation catalyst; the source of reducing fluid is downstream of the particle trap; and the SCR catalyst is downstream of the reducing fluid source. The reducing fluids disclosed include ammonia, urea, ammonium carbamate, and hydrocarbons (eg, diesel fuel).

A catalytic wall flow filter for an exhaust gas system of a combustion engine is described in WO 01/12320. The wall flow filter has channels that are in a honeycomb arrangement, in which some of the channels are blocked at the upstream end and some of the channels that are unblocked at the upstream end are blocked at the upstream end. downstream. An oxidation catalyst is disposed over a gas-impermeable zone at an upstream end of the channels that are blocked at the downstream end. The filter has a gas permeable filter zone that is downstream of the oxidation catalyst that is to trap soot. The oxidation catalyst is disclosed to be capable (when in an exhaust gas system) of generating NO2 from NO to burn trapped soot continuously at temperatures below 400 ° C. The oxidation catalyst preferably includes a platinum group metal. The NO-containing exhaust gas streams are initially passed over the oxidation catalyst to convert NO to NO2 before filtering to remove soot. The exhaust gas containing NO2 is then used to burn the soot trapped on the filter.

In some embodiments of the wall flow filter described in WO 01/12320, the channels downstream of the soot filter contain a catalyst for a NOx absorber and an SCR catalyst downstream of the NOx absorber. The SCR catalyst can be a material based on copper, platinum, a mixed oxide of vanadium and titania or a zeolite, or mixtures of two or more thereof.

Document WO03 / 054364 discloses an emission treatment system comprising an oxidation catalyst, an injector downstream of the oxidation catalyst and a particulate filter comprising an SCR catalyst.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to an emission treatment system for treating an exhaust gas stream containing NOx and particulate matter. The emission treatment system includes an oxidation catalyst, an injector that periodically doses ammonia or an ammonia precursor into the exhaust gas stream; and a 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 contains an SCR catalyst composition, is in fluid communication with the injector, and is located downstream of the injector.

The wall flow monolith has a plurality of longitudinally extending passages formed by longitudinally extending walls that limit and define said passages. The passages include inlet passages that have an open inlet end and a closed outlet end, and the outlet passages that have a closed inlet end and an open outlet end. The wall flow monolith contains an SCR catalyst composition that permeates the walls at a concentration of 79.3 g / l (1.3 g / in³) to 146 g / l (2.4 g / in³) (and preferably 97.6 to 146 g / l (1.6 to 2.4 g / in³)). The wall flow monolith has a wall porosity of at least 50% with an average pore size of at least 5 microns.

In the emission treatment system, the SCR catalyst composition contains a zeolite and base metal component selected from one or more than one copper and iron component. Preferably, the base metal component is a copper component. Preferred zeolites of the SCR catalyst composition have a silica to alumina ratio of at least 10. For example, a beta zeolite can be used in the SCR catalyst composition.

Among other things, the oxidation catalyst in the system is useful for burning substantial portions of the particulate matter, and in particular, the VOF, entrapped in the exhaust gas. In addition, a substantial portion of the NO in the NOx component is oxidized to NO2 on the oxidation catalyst. In preferred embodiments, the oxidation catalyst is disposed on a flow-through honeycomb monolith substrate or an open cell foam substrate. Preferably, the oxidation catalyst includes a platinum group metal component, and in particular, a platinum component. In some embodiments, the oxidation catalyst can also contain a zeolite component.

In another preferred embodiment of the emission treatment system, the system also has a diesel engine that is located upstream of, and in fluid communication with, the oxidation catalyst.

Another aspect of the invention relates to a method for treating emissions produced in an exhaust gas stream containing NOx and particulate matter. The method includes:

(a) passing the exhaust gas stream through an oxidation catalyst in which at least the

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20% of the NO in the NOx component is oxidized to NO2 to provide a NO2-enriched exhaust gas stream;

(b) dosing at periodic intervals ammonia or an ammonia precursor into the NO2-enriched exhaust gas stream; and, (c) subsequently passing the exhaust gas stream through a wall flow monolith in which particulate matter is filtered and a substantial portion of NOx is reduced to N2.

Here again, the wall flow monolith has a plurality of longitudinally extending passages formed by longitudinally extending walls that limit and define said passages. The passages include inlet passages that have an open inlet end and a closed outlet end, and outlet passages that have a closed inlet end and an open outlet end. The wall flow monolith contains an SCR catalyst composition that permeates the walls at a concentration of 79.3 g / l (1.3 g / in³) to 146 g / l (2.4 g / in³) (and preferably 97.6 to 146 g / l (1.6 to 2.4 g / in³)). The wall flow monolith has a wall porosity of at least 50% with an average pore size of at least 5 microns.

In another aspect, the present disclosure relates to a method of laying an SCR catalyst composition on a wall flow monolith. The method includes:

(a) immersing the wall flow monolith in an aqueous suspension comprising the SCR catalyst composition from a first direction to deposit the SCR catalyst composition over the inlet passages;

(b) removing excess suspension from the inlet passages by forcing a stream of compressed gas through the outlet passages and applying a vacuum to the inlet passages;

(c) immersing the wall flow monolith into the aqueous suspension from a second direction, opposite to the first direction, to deposit the SCR catalyst composition on the outlet passages;

(d) removing excess suspension from the outlet passages by forcing a stream of compressed gas through the inlet passages and applying a vacuum to the outlet passages; and (e) drying and calcining the coated wall flow monolith.

The wall flow monolith used in the method has a porosity of at least 50% (eg 50 to 75%) having an average pore size of at least 5 microns (eg 5 to 30 microns). ).

The SCR catalyst composition permeates the walls at a concentration of 79.3 g / l (1.3 g / in³) to 146 g / l (2.4 g / in³) (and preferably 97.6 to 146 g / l (1.6 to 2.4 g / in³)).

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1A and 1B are schematic representations of two embodiments of the emission treatment system of the invention;

Figure 2 shows a perspective view of a wall flow filter substrate;

Figure 3 shows a cross-sectional view of a section of a wall flow filter substrate;

Figure 4 shows an embodiment of the emission treatment system of the invention that includes a urea reservoir and injector;

Figure 5 is a plot of DTA signal in microvolts versus temperature for two SCR catalyst compositions mixed with a mass of model particles (carbon black and lubricating oil);

Figure 6 shows pressure drop as a function of air flow for various coated wall flow filter substrates and one uncoated wall flow filter substrate; and Figure 7 is a schematic representation of a laboratory bench system used to evaluate NOx and particulate reduction for an exemplary emissions treatment system of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an emission treatment system that efficiently provides simultaneous treatment of particulate matter, NOx and other gaseous components of diesel engine exhaust gases. The emission treatment system uses an integrated soot filter and SCR catalyst to significantly minimize the weight and volume required for the emissions system. Furthermore, due to the choice of catalyst compositions implemented in the system, effective pollutant reduction is provided for exhaust gas streams of varying temperatures. This feature is advantageous for operating diesel vehicles under varying loads and vehicle speeds that significantly affect the temperatures of the exhaust gases emitted from the engines of such vehicles.

Integration of the NOx reduction and particulate removal functions into a single catalyst article is accomplished using a flow-through substrate coated with an SCR catalyst composition. Applicants have found a method of applying an SCR catalyst composition to a wall flow substrate to form a substrate that can be used in an application where high efficiency of

ES 2 551 691 T3 filtration. For example, a substrate formed by this method is suitable for effectively removing particulate matter (eg, greater than 80%) in the emission treatment system of the invention. The coating method disclosed herein allows wall flow substrates to be loaded with practical levels of SCR catalyst without causing excessive back pressure through the coated article when implemented in emission treatment systems.

Achieving practical levels of SCR catalyst composition on the wall flow substrate is important to provide sufficient catalytic activity to achieve the required NOx reduction levels, and to reduce the combustion temperature of the trapped soot fraction on the filter. . Achieving adequate levels of SCR thin coating compositions on the soot filter is also important to ensure adequate durability for the catalyst. During prolonged use of the emission treatment system, catalysts are invariably exposed to various levels of catalyst poisons that can come from the breakdown of lubricating oils, or can result from impurities in diesel fuel. Examples of such catalyst poisons include phosphorous, zinc, alkali and alkaline earth elements. Thus, the highest levels of catalyst compositions are typically deposited on catalyst substrates to overcome the inevitable loss of catalytic activity.

An embodiment of the inventive emission treatment system is schematically depicted in Figure 1A. As can be seen in Figure 1A, exhaust gas containing gaseous pollutants (including unburned hydrocarbons, carbon monoxide, and NOx) and particulate matter is transported from engine 15 to oxidation catalyst 11. In oxidation catalyst 11, unburned non-volatile gaseous hydrocarbons (ie, VOFs) and carbon monoxide are largely burned to form carbon dioxide and water. The removal of substantial proportions of the VOF using the oxidation catalyst, in particular, helps prevent a large deposition of particulate matter on the soot filter 12 (ie, clogging), which is located downstream in the system. In addition, a substantial proportion of the NO in the NOx component is oxidized to NO2 in the oxidation catalyst.

Downstream of the oxidation catalyst is a reducing agent, in this case ammonia, which is injected as a spray through a nozzle (not shown) into the exhaust gas stream. Aqueous urea displayed in one line 18 can serve as a precursor to ammonia that can be mixed with air in another line 19 at a mixing station.

16. Valve 14 can be used to meter precise amounts of aqueous urea which is converted in the exhaust gas stream to ammonia. The exhaust gas stream with the added ammonia is conveyed to the soot filter 12 which is coated with an SCR catalyst composition. Passing through the soot filter, the NOx component is converted, by selective catalytic reduction of NOx with ammonia, to nitrogen. The high proportion of NO2 in NOx due to the catalytic action of the upstream oxidation catalyst facilitates the reduction of NOx compared to exhaust gas streams containing smaller proportions of NO2 in the NOx component.

Depending on the desired level of NOx removal, additional SCR catalyst may be provided downstream of the soot filter. For example, the additional SCR catalyst may be disposed on a monolithic honeycomb flow-through substrate or ceramic foam substrate downstream of the soot filter. Even in these embodiments, the use of the coated SCR soot filter still achieves a reduction in the total volume of catalyst required to meet the NOx reduction goals.

The particulate matter including the soot fraction and VOF is also largely removed (greater than 80%) by the soot filter. The particulate matter deposited on the soot filter is burned through regeneration of the filter, a process that is also aided by the presence of the SCR catalyst composition. The temperature at which the soot fraction of the particulate matter burns is lowered by the presence of the catalyst composition disposed on the soot filter.

An optional configuration is shown in Figure 1B, in which the emission treatment system is provided with a displacement oxidation catalyst 13 downstream of the coated soot filter 12. The synthesis oxidation catalyst can be coated, for example , with a composition containing base metals and less than 0.5% by weight of platinum. This arrangement can be used to oxidize any excess NH3 before it is discharged to the atmosphere.

SCR catalyst compositions suitable for use in the system can effectively catalyze the reduction of the NOx component at temperatures below 600 ° C, so that adequate NOx levels can be treated even under low load conditions that are normally associated with lower temperatures. of exhaust gases. Preferably, the catalyst article can convert at least 50% of the NOx component to N2, depending on the amount of reducing agent added to the system. Furthermore, SCR catalyst compositions for use in the system are also ideally capable of assisting in filter regeneration by reducing the temperature at which the soot fraction of the particulate matter burns. Another desirable attribute for the composition is that it possesses the ability to catalyze the reaction of O2 with any excess NH3 in N2 and H2O, so that NH3 is not released into the atmosphere.

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Useful SCR catalyst compositions used in the inventive system also have thermal resistance at temperatures above 650 ° C. Such high temperatures are frequently encountered during regeneration of soot filters. Additionally, SCR catalyst compositions must resist degradation upon exposure to sulfur components, which are frequently present in diesel exhaust gas compositions.

Suitable SCR catalyst compositions are described, for example, in US Patent Nos. 4,961,917 (the 917 patent) and 5,516,497. The compositions disclosed in the '917 patent include one or both of an iron and copper promoter present in a zeolite in an amount of 0.1 to 30 percent by weight, preferably 1 to 5 percent by weight, of the total weight. of the promoter plus zeolite. In addition to their ability to catalyze the reduction of NOx with NH3 to N2, the disclosed compositions can also promote the oxidation of excess NH3 with O2, especially for those compositions having higher concentrations of promoter.

The zeolites used in such compositions are resistant to sulfur poisoning, maintain a high level of activity for the SCR process, and are capable of oxidation of excess ammonia with oxygen. These zeolites have a sufficiently large pore size to allow adequate movement of the reactant molecules NO and NH3 into, and the product molecules N2 and H2O out of, the pore system in the presence of sulfur oxide molecules resulting from the short-term sulfur poisoning, and / or sulfate deposits resulting from long-term sulfur poisoning. The properly sized pore system is interconnected with the three crystallographic dimensions. As is well known to those skilled in the zeolite art, the crystal structure of zeolites has a complex pore structure having, for example, more or less regularly recurring connections or intersections. Pores having a particular characteristic, such as a given dimensional diameter or cross-sectional configuration, are said to be one-dimensional if those pores do not intersect with other similar pores. If pores cross only within a given plane with other similar pores, the pores of that characteristic are said to cross in two dimensions (crystallographic). If the pores intersect with other similar pores that are both in the same plane and in other planes, such similar pores are said to intersect in three dimensions, that is, they are "three-dimensional". Zeolites have been found to be highly resistant to sulfur poisoning and to provide good activity for both the SCR process and the oxidation of ammonia with oxygen, and to retain good activity even when subjected to high temperatures, hydrothermal conditions, and chemical poisons. sulfate, are zeolites that have pores that have a pore diameter of at least 7 Angstroms and intersect in all three dimensions. Without wishing to be bound by any specific theory, it is believed that the crossing of pores of at least 7 Angstroms in diameter in all three dimensions provides good mobility of sulfate molecules through the zeolite structure, thus allowing the sulfate molecules to are released from the catalyst to release a large number of the adsorbent sites available for the NOx and NH3 reactant molecules and the NH3 and O2 reactant molecules. Any zeolite that meets the above criteria is suitable for use in the practice of the present invention; Specific zeolites that meet these criteria are USY, Beta, and ZSM-20. Other zeolites can also satisfy the criteria mentioned above.

When deposited onto wall flow monolith substrates, such SCR catalyst compositions are deposited at a concentration of 79.3 g / l (1.3 g / in³) to 146 g / l (2.4 g / in³) to ensure that the desired levels of NOx reduction and particulate removal are achieved and to ensure adequate catalyst durability during extended use. In a preferred embodiment, there are 97.6 to 146 g / l (1.6 to 2.4 g / in³), arranged on the wall flow monolith.

Wall flow substrates useful for supporting SCR catalyst compositions have a plurality of fine, substantially parallel gas flow passages that extend along the longitudinal axis of the substrate. Typically, each passage is blocked at one end of the substrate body, with alternate passages blocked at opposite end faces. Such monolithic supports can contain up to 108.5 or more flow passages (or "cells") per square cm (700 or more flow passages (or "cells") per square inch) of cross section, although much fewer can be used. For example, the vehicle may have 1.08 to 93.02, more typically 15.5 to 62, cells per square cm (7 to 600, more typically 100 to 400, cells per square inch ("cpsi") ). Cells can have cross sections that are rectangular, square, circular, oval, triangular, hexagonal, or are of other polygonal shapes. Flow wall substrates typically have a wall thickness between 0.05 and 2.54 mm (0.002 and 0.1 inches). Preferred wall flow substrates have a wall thickness of between 0.05 and 0.38 mm (0.002 and 0.015 inches).

Figures 2 and 3 illustrate a wall flow filter substrate 30 having a plurality of passages 52. The passages are tubularly enclosed by internal walls 53 of the filter substrate. The substrate has an inlet end 54 and an outlet end 56. Alternate passages are plugged at the inlet end with inlet plugs 58 and at the outlet end with outlet plugs 60 to form opposing checkerboard patterns at the inlet 54 and exit 56. A gas stream 62 enters through the inlet of the uncapped channel 64, is stopped by the outlet plug 60, and diffuses through the walls of the channel 53 (which are porous) to the outlet side 66. The gas cannot pass back to the inlet side of the walls due to the inlet plugs 58.

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Preferred wall flow filter substrates are composed of ceramic type materials such as cordierite, α-alumina, silicon carbide, silicon nitride, zirconia, mullite, spodumene, alumina-silica-magnesia or zirconium silicate, or metal porous refractory. Wall flow substrates can also be formed from ceramic fiber composites. Preferred wall flow substrates are formed from cordierite and silicon carbide. Such materials can withstand the environment, particularly high temperatures, encountered in the treatment of exhaust gas streams.

Preferred wall flow substrates for use in the inventive system include thin porous walled honeycombs (monoliths) through which the fluid stream passes without causing a large increase in back pressure or pressure through the article. Typically, the presence of a clean flow wall article will create a back pressure of 249 to 68900Pa (1 inch of water column at 10 psig). The ceramic wall flow substrates used in the system are formed of a material having a porosity of at least 50% (for example, 50 to 75%) having an average pore size of at least 5 microns (for example, 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 these porosities and these average pore sizes are coated with the techniques described below, suitable levels of SCR catalyst compositions can be loaded onto the substrates to achieve excellent NOx conversion efficiency. These substrates are still capable of retaining adequate exhaust gas flow characteristics, ie, acceptable back pressures, despite sCr catalyst loading. US Patent No. 4,329,162 discloses suitable wall flow substrates.

Typical wall flow filters in commercial use are typically formed with lower wall porosities, eg, 35% to 50%, than the wall flow filters used in the invention. In general, the pore size distribution of commercial wall flow filters is typically very broad, with an average pore size smaller than 17 microns.

The porous wall flow filter used in the present invention is catalyzed in that the wall of said element has on it or is contained therein one or more catalytic materials. The catalytic materials may be present on the inlet side of the element wall alone, the outlet side alone, both the inlet and outlet sides, or the wall itself may consist of 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 on the inlet and / or outlet walls of the element.

To coat the wall flow substrates with the SCR catalyst composition, the substrates are dipped vertically into a portion of the catalyst suspension so that the top of the substrate is located just above the surface of the suspension. In this way, the suspension comes into contact with the entrance face of each wall of the honeycomb, but is prevented from coming into contact with the exit face of each wall. The sample is left in suspension for approximately 30 seconds. The substrate is removed from the suspension, and excess suspension is removed from the wall flow substrate by first allowing it to drain from the channels, then by blowing with compressed air (against the direction of penetration of the suspension), and then applying a vacuum from the suspension penetration direction. Using this technique, the catalyst suspension permeates the walls of the substrate, the pores are not yet blocked to the point that excessive back pressure builds up in the finished substrate. As used herein, the term "permeate", when used to describe the dispersion of the catalyst suspension over the substrate, means that the catalyst composition is dispersed through the wall of the substrate.

Coated substrates are typically dried at about 100 ° C and calcined at a higher temperature (eg, 300 to 450 ° C). After calcination, the catalyst loading can be determined by calculating the coated and uncoated weights of the substrate. As will be apparent to those skilled in the art, the catalyst loading can be modified by altering the solids content of the coating suspension. Alternatively, repeated dipping of the substrate into the coating suspension may be performed, followed by removal of the excess suspension as described above.

A reductant metering system is provided upstream of the soot filter and downstream of the oxidation catalyst to inject a NOx reducer into the exhaust gas stream. As disclosed in US Patent No. 4,963,332, NOx can be detected upstream and downstream of the catalytic converter, and a pulsed metering valve can be controlled by the upstream and downstream signals. In alternative configurations, the systems disclosed in US Patent No. 5,522,218, in which the reducer injector pulse width is controlled from maps of exhaust gas temperature and engine operating conditions such as engine rpm, transmission gear, and engine speed. Reference is also made to the discussion of redundant pulse dosing systems in US Patent No. 6,415,602.

In the embodiment of Figure 4, an aqueous urea reservoir 22 stores a urea / water solution on board the vehicle that is pumped through a pump 21 that includes a filter and pressure regulator to a urea injector 16. The urea injector 16 is a mixing chamber that receives regulated pressure air in line 19 which is

ES 2 551 691 T3 pulsed by a control valve to the urea injector 16. An atomized urea / water / air solution results that is injected by pulses through a nozzle 23 into the exhaust pipe 24 upstream of the soot filter Integrated SCR Catalyst Coated 12.

The present invention is not limited to the aqueous urea dosage arrangement shown in Figure 4. It is contemplated that a gaseous nitrogen-based reagent will be used. For example, a urea or cyanuric acid bead injector can dose solid urea pellets into a chamber heated by the exhaust gas to gasify the solid reductant (sublimation temperature range 300 to 400 ° C). Cyanuric acid will gasify to isocyanic acid (HNCO) and urea will gasify to ammonia and HNCO. With any reducer a hydrolysis catalyst can be provided in the chamber and a displacement stream of exhaust gas metered into the chamber (the exhaust contains sufficient water vapor) to hydrolyze (temperatures 150 to 350 ° C) HNCO to produce ammonia.

In addition to urea and cyanuric acid, other nitrogen-based or reducing reagents especially suitable for use in the control system of the present invention include Amelide, Amelin, Ammonium Cyanate, Biuret, Cyanuric Acid, Ammonium Carbamate, Melamine, Tricianourea , and mixtures of any number of these. However, the invention, in a broader sense, is not limited to nitrogen-based reducers, but can include any hydrocarbon-containing reducing agent such as distillate fuels including alcohols, ethers, organic nitro compounds, and the like (eg, methanol, ethanol , diethyl ether, etc.) and various amines and their salts (especially their carbonates), including guanidine, methylamine carbonate, hexamethylamine, etc.

Upstream of the reducer dosing system is an oxidation catalyst (or DOC). The oxidation catalyst can be formed from any composition that provides efficient combustion of non-volatile, gaseous hydrocarbons (ie, VOF) without burning and carbon monoxide. In addition, the oxidation catalyst must be effective in converting a substantial proportion of the NO in the NOx component to NO2. As used herein, the term "substantial conversion of NO from the NOx component to NO2" means at least 20%, and preferably between 30 and 60%. Catalyst compositions having these properties are known in the art, and include compositions based on platinum group metals and based on base metals. The catalyst compositions can be coated onto honeycomb flow-through monolith substrates formed of refractory metallic or ceramic materials (eg, cordierite). Alternatively, oxidation catalysts can be formed on metallic or ceramic foam substrates which are well known in the art. These oxidation catalysts, by virtue of the substrate on which they are coated (eg, open cell ceramic foam), and / or by virtue of their intrinsic oxidation catalytic activity, provide some level of particulate removal. Preferably, the oxidation catalyst removes some of the particulate matter from the exhaust gas stream upstream of the wall flow filter, since the reduction in particulate mass on the filter possibly prolongs the time before forced regenerations.

A preferred oxidation catalyst composition that can be used in the emission treatment system contains a platinum group component (eg platinum, palladium or rhodium components) dispersed over a high surface area, refractory oxide support (eg , γ-alumina) that is combined with a zeolite component (preferably a beta zeolite). A preferred platinum group metal component is platinum. When the composition is placed on a refractory oxide substrate, for example a flow-through honeycomb substrate, the platinum concentration is typically 0.35 to 4.24 g / l (10 to 120 g / ft³) platinum.

Platinum group metal-based compositions suitable for use in oxidation catalyst formation are also described in US Patent No. 5,100,632 (the 632 patent). The '632 patent describes compositions having 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 with an atomic ratio of the group metal platinum and the alkaline earth metal from 1: 250 to 1: 1, and preferably 1:60 to 1: 6.

Suitable catalyst compositions for the oxidation catalyst can also be formed using base metals as catalytic agents. For example, US Patent No. 5,491,120 discloses oxidation catalyst compositions that include a catalyst material having a BET surface area of ​​at least 10 µm.²/ g and consist essentially of a second bulk metal oxide which can be one or more of titania, zirconia, ceria-zirconia, silica, alumina-silica and α-alumina.

Also useful are the catalyst compositions disclosed in US Patent No. 5,462,907 (the '907 patent). The '907 patent teaches compositions that include a catalytic material containing ceria and alumina each having a surface area of ​​at least 10 µm.²/ g, for example, ceria and activated alumina in a weight ratio of 1.5: 1 to 1: 1.5. Optionally, platinum can be included in the compositions disclosed in the '907 patent in amounts effective to promote gas phase oxidation of CO and unburned hydrocarbons, but which are limited to exclude excessive oxidation of SO to SO2. Alternatively, palladium in any desired amount can be included in the catalyst material.

ES 2 551 691 T3

The following examples further illustrate the present invention, but of course, they should not be construed as limiting its scope in any way.

Example 1 - Coating ceramic wall flow filters

Cordierite ceramic wall flow filter substrates (product name C611, NGK Insulators, Ltd.) having dimensions of 14.4 x 15.2 cm (5.66 x 6 inches), a wall thickness of 0.305 mm (0.012 in), 25 micron average pore size and 60% wall porosity for preparing catalyst coated soot filters.

A catalyst slurry containing 27% by weight solids content was formed from zeolite beta exchanged with copper (containing 2% by weight of copper), additional CuSO4 (sufficient to provide 9.5% by weight of copper ) and 7% by weight of ZrO2 and deionized water (% by weight based on the weight of the beta zeolite). The copper exchanged zeolite beta was prepared as in US Patent No. 5,516,497.

An identical procedure was used to prepare two of the filter substrates according to a preferred embodiment of the invention. The wall flow substrate:

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

(2) the substrate was air-cut from the side opposite the coating direction (ie, the dry side);

(3) vacuum from the coated side;

(4) dried at 93 ° C for 1 h in circulating air, and calcined at 400 ° C for 1 h; and (5) then Steps (1) to (4) were repeated from the opposite side.

These filter substrates (designated catalysts A1 and A2) contained a catalyst loading of 128 g / l (2.1 g / in³). The amount of copper contained on these catalysts was approximately 12.2 g / l (0.2 g / in³).

Another filter substrate, designated Catalyst B1, was prepared by coating a single side of the substrate alone, following Steps (1) to (4). To achieve the same catalyst loading as catalyst A1, the suspended solids content increased to 38%. The composition of the catalyst remained the same. Catalyst B1 had a catalyst loading of 122 g / l (2.0 g / in³). The amount of copper contained on this catalyst was also approximately 12.2 g / l (0.2 g / in³).

A reference sample, catalyst D1, was prepared as a flow-through type catalyst. To prepare such a catalyst, a filter substrate of the type described above was cut through its diameter at one end, just below the depth of the plugs. Thus, the wall flow filter became a flow-through substrate with half of the front area effectively blocked. This substrate was coated to obtain a catalyst loading of 122 g / l (2.0 g / in³) of the copper-exchanged exchange zeolite beta catalyst composition.

Example 2 - Backpressure evaluation for coated soot filters

The pressure drop across the uncoated and coated filters was evaluated using commercially available automated equipment, Super Flow SF 1020, (Probench). This equipment is specifically designed to measure pressure drops as a function of air flow. The data from this equipment provides a diagram of pressure drop at ambient conditions as a function of air flow. Pressure drop is a measure of how easily air circulates through the filter. In diesel engine applications, lower pressure losses are desirable, as the engine must expend power to move air. Therefore, the greater the pressure drop, the greater the amount of engine power that is lost in pumping air. This lost power reduces the engine power that is available to the wheels.

Figure 6 summarizes the pressure drop across the coated filters, catalyst A1, A2 and B1, as well as an uncoated filter of identical dimensions. Filters coated according to Steps (1) to (5) of Example 1, that is, catalyst A1 and A2, showed a pressure drop that was approximately 25% higher than that of the uncoated filter. Unlike catalysts A1 and A2, the non-optimized filter, catalyst B1, showed pressure drops that were greater than 100% higher than the uncoated filter after coating. The pressure drop exhibited by catalyst B1 was so high that testing of the engine of this filter was impossible. Although it is possible to achieve lower pressure drops across the coated filter in the same way as catalyst B1 which reduces catalyst loading, lower SCR catalyst loading levels lead to unacceptable NOx reduction levels.

Example 3 - Demonstration of particle removal by SCR catalyst

When applied to the wall flow filter, the catalyst composition should ideally aid in filter regeneration. Therefore, the SCR catalyst composition disposed on the filter is preferably capable of

ES 2 551 691 T3 catalyze the oxidation of soot and VOF portions of the particle. To be effective in reducing NOx and particulate mass, the SCR catalyst should preferably not oxidize ammonia or SO2 to form SO3. One way to evaluate the ability of a catalyst to oxidize carbon and VOF is by using thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) combined. The TGA measures the weight loss of a sample while the DTA measures the change in the thermal capacity of the sample against a reference. In this experiment, a dried and calcined portion of the catalyst suspension was mixed with 6% by weight of lubricating oil, to simulate the VOF portion of the lubricant, and 14% by weight of carbon black, to simulate the soot fraction. of the particle. The mixture was loaded into an instrument that performs a combined TGA and DTA. Although different gas compositions can be passed through the sample, these tests were performed in air. The system was heated at a known rate to determine weight loss and heat release as a function of temperature. An advantage of the technique is its ability to separate the weight loss from various soot components, and to relate these weight losses to thermal changes. Catalysts effective in burning soot will reduce the soot combustion onset temperature.

Figure 5 represents the DTA signal in microvolts as a function of temperature for two catalyst compositions; (1) a reference composition, TiO2-10% by weight WO3-2% by weight V2O5 catalyst, and (2) the catalyst composition used to coat catalyst A1. The composition based on TiO2 is typical of the current state of the art in SCR catalysts and has wide application. Dried and calcined suspension powders of each catalyst were mixed with 6% lubricating oil and 14% by weight carbon black. These samples were heated at a rate of 20 ° C per minute, in air, from room temperature to 800 ° C. The resulting DTA signal shows two peaks, one at temperatures below 400 ° C corresponding to VOF combustion, and the second peak at higher temperatures corresponding to carbon black combustion. The results show that both catalyst compositions were effective in burning the lubricating oil portion of the simulated particle, but the preferred catalyst composition was much more effective in burning the carbon portion as demonstrated by reducing the combustion temperature of the simulated particle. soot. As will be seen in later examples, this advantage is maintained without compromising the NOx reduction activity.

Example 4 - Evaluation of NOx Conversion and Particulate Removal for Coated Soot Filters

Filtration efficiency and simultaneous NOx reduction were determined using a 4L turbocharged post-cooled diesel engine, prototype V6, which is representative of the current state of the art in diesel technology. The engine was mounted on a steady state operated test bed to provide reproducible and stable emissions. Engine speed and load were controlled to provide a filter inlet temperature of 370 ° C and a NOx concentration of approximately 950 ppm. Particle measurements were determined according to the procedures outlined in Code of Federal Regulations, Title 40, Part 86, paragraph 1312-88, but instead of a full dilution tunnel, a mini dilution tunnel was used. The dilution ratio was determined from the CO2 concentration. NOx removal in the diesel engine was achieved by injecting a urea solution after catalyst oxidation and before the SCR coated filter substrate. The experimental setup is illustrated in Figure 7. NOx and ammonia were measured using an FTIR instrument equipped with a heated sample line and analysis cell. NOx, CO and HC were also determined using a Horiba test bench, specially designed for raw diesel exhaust gas analysis.

Additional catalysts were prepared and aged for 1000 hours in a stationary diesel engine using an aging cycle that simulated passenger car driving. The aging cycle was an adaptation of the procedure described in "Durability Driving Program for Light Vehicles and Light Trucks" Code of Federal Regulations, Part 86 paragraph 836-01. The test cycle described in it specifies periodic speeds and stops for a vehicle driven around a test track. From previous work, the temperature profile of this cycle was measured, and then simulated on an engine bench. Aging and evaluation used ARCO ECD diesel fuel. This fuel has a sulfur content of 12 ppm, consistent with the fuel that is expected to be available during the expected application of the technology.

Using the experimental setup shown in Figure 7, NOx conversion and particulate removal were determined for three catalyst substrates. As seen in Figure 7, the experimental setup included a urea injector, upstream of the catalyzed soot filter, and an oxidation catalyst (DOC), upstream of the urea injector. To eliminate any of a variation due to DOC, all tests were performed with the same DOC. The oxidation catalyst composition was set on a 14.4 x 15.2 cm (5.66 x 6 in) flow-through cordierite substrate. The oxidation catalyst composition contained 3.18 g / l (90 g / ft³) dispersed on γ-alumina, and contained 27% by weight of zeolite beta exchanged with hydrogen ion. The DOC was aged 1000 hours.

In the tests performed in this experiment, the SCR catalyst composition was laid out on both a wall flow monolith substrate and a flow through monolith substrate. The SCR catalyst composition was identical to that used to coat the substrates in Example 1, ie, it contained a copper exchanged zeolite with a zirconia binder. In particular, the substrates used in the experiment were: a fresh catalyst substrate prepared identically to catalyst A1 in Example 1 (designated fresh catalyst A1); a

ES 2 551 691 T3 separate catalyst substrate also coated identically to catalyst A1, but aged 1000 hours (designated catalyst A1 aged); and finally the third catalyst substrate which was of the flow-through type, prepared identically to catalyst D1 (designated catalyst D1 fresh).

Table 1 below summarizes the particulate filtration efficiency and NOx reduction for the three catalyst substrates. Filtration efficiency was determined with and without urea injection.

Table 1

<td>Proof #</td><td>Catalyst</td><td>Substrate Type</td><td>NH / NOx</td><td>% NOx conv.</td><td>NH slip, ppm</td><td>Total particles removed,%</td>

<td> 1</td><td>D1fresh</td><td>Flow through</td><td> 0</td><td> &lt;5</td><td> 0</td><td> &lt;10</td>

<td> 2</td><td>D1fresh</td><td>Flow through</td><td> 0,3</td><td> 30</td><td> 0</td><td> &lt;10</td>

<td> 3</td><td>A1fresh</td><td>Wall flow</td><td> 0</td><td> &lt;5</td><td> 0</td><td> 82</td>

<td> 4</td><td>A1fresh</td><td>Wall flow</td><td> 0,5</td><td> 51</td><td> 0</td><td> 85</td>

<td> 5</td><td>A1 aged</td><td>Wall flow</td><td> 0</td><td> &lt;5</td><td> 0</td><td> 81</td>

<td> 6</td><td>A1 aged</td><td>Wall flow</td><td> 0,5</td><td> 55</td><td> 0</td><td> 85</td>

As can be seen from Table 1, the arrangement of the SCR catalyst composition on the wall flow monolith did not result in a loss of NOx removal efficiency. Also, the removal efficiency is not affected by urea injection. While the SCR coated flow monolith provided NOx removal function, it lacked the high filtration efficiency demonstrated by the coated wall flow monoliths. Thus, these coated SCR filter substrates of the invention demonstrate high integrated NOx and particulate removal efficiency.

Furthermore, the durability of the SCR catalyst composition is demonstrated by the data in Table 1. Aging of the coated substrate caused neither loss of filtration efficiency nor of NOx removal efficiency.

Although the present invention has been described with an emphasis on preferred embodiments, it will be obvious to those of ordinary skill in the art that variations in preferred devices and methods may be used and that it is intended that the invention may be practiced otherwise than specifically described herein. Accordingly, the present invention includes all modifications encompassed within the scope of the invention, as defined by the claims that follow.