Abstract:
Disclosed herein is a method of distributing a washcoat along channels of a particulate filter substrate, the method comprising: disposing a washcoat slurry comprising a catalytically active material within a channel of a particulate filter element such that a concentration of the catalytically active material varies anisotropically along the length of the channels. In one embodiment, a solids content of the washcoat slurry is selected to provide an anisotropic variation in the concentration of the catalytically active material. The washcoat slurry may be pushed a predetermined distance into the channels, the predetermined distance being less than or equal to the full length of the channels, and the pushing may be performed once. The solids content of the washcoat composition may be increased to provide an increase in a concentration of the catalytically active material proximate an end of the particulate filter substrate. Alternatively, the solids content of the washcoat composition may be decreased to increase the uniformity of in the distribution of the catalytically active material throughout a length of the particulate filter substrate.

Description:
BACKGROUND  
         [0001]    Federal and state requirements have mandated substantial reductions in carbon particulate matter (so called soot), hydrocarbons and carbon monoxide emission for internal combustion engines. Attempts at reducing such emissions, in particular attempts to reduce particulate matter emissions from diesel engines, include catalytic diesel particulate filters (DPF) in which a catalytically active material is disposed within a substrate in the particulate filter. The catalyst promotes removal (e.g., oxidation) of particulate matter and other byproducts of diesel fuel combustion in the exhaust gas stream.  
           [0002]    Applying a catalytic material (also referred to herein as a washcoat) to the particulate filter substrate is one of the most promising technologies for effective removal (combustion) of trapped particulate matter. In general, better catalytic performance is achieved by increasing washcoat loading so as to increase the concentration of catalytically active material present. However, an increase in washcoat loading can result in an increase in flow restriction when exhaust gas flows through particulate filter. The increase in exhaust gas flow restriction results in an increase in exhaust line backpressure, which is detrimental to engine performance and fuel economy. In general, the lower the backpressure of the exhaust line, the better the engine performance and fuel economy will be. Accordingly, a need exists for a particulate filter that provides for reduction of particulate matter emissions in the exhaust gas while minimizing backpressure caused by the particulate filter.  
         SUMMARY OF THE INVENTION  
         [0003]    Disclosed herein is a method of distributing a washcoat along channels of a DPF substrate, the method comprising: dispersing a washcoat slurry comprising a catalytically active material within a channel of a particulate filter such that a concentration of the catalytically active material varies anisotropically along the length of the channels. In one embodiment, a solids content of the washcoat slurry is selected to provide an anisotropic variation in the concentration of the catalytically active material. The washcoat slurry may be forced to travel a predetermined distance into the channels, the predetermined distance being less than or equal to the full length of the channels, and the pushing may be performed once. The solids content of the washcoat composition may be increased to provide an increase in a concentration of the catalytically active material proximate an end of the particulate filter substrate. Alternatively, the solids content of the washcoat composition may be decreased to increase the uniformity of distribution of the catalytically active material throughout a length of the particulate filter substrate. 
       
    
    
     FIGURES  
       [0004]    [0004]FIG. 1 shows a cross-sectional view of a particulate filter including a catalytic particulate filter substrate, which is shown in partial cut-away view.  
         [0005]    [0005]FIG. 2 shows a perspective view of an embodiment of the catalytic particulate filter substrate;  
         [0006]    [0006]FIG. 3 shows an isoscan plot of a particulate filter substrate having washcoat evenly distributed, and a particulate filter substrate having washcoat biased toward one end; and  
         [0007]    [0007]FIG. 4 shows the relative backpressure increase after washcoating of particulate filter with different washcoat distribution patterns. 
     
    
     DETAILED DESCRIPTION  
       [0008]    Referring to FIG. 1, a particulate filter  10  may be part of an exhaust gas emission control system in which an inlet  12  on the particulate filter  10  is in fluid communication with an exhaust manifold of an internal combustion engine (e.g., a diesel engine), and an outlet  14  on the filter  10  is in fluid communication with an exhaust gas destination, such as atmosphere. In addition, the system may comprise various other emission control devices including catalytic converters, evaporative emission devices, scrubbing devices, adsorbers/absorbers, non-thermal plasma reactors, mufflers, and the like, as well as combinations comprising at least one of the foregoing devices.  
         [0009]    The particulate filter  10  comprises a particulate filter substrate  16  enclosed within a housing or canister  18 . The canister  18  may have an input collar  20  connectable to the exhaust manifold or other components in the system such as a turbocharger, and an output collar  22  connectable to the tailpipe or other components in the system. Located between the particulate filter substrate  16  and the interior of the canister  18  is a retention or support material  24  that supports and protects the particulate filter substrate  16 , and insulates the canister  18  from both the high exhaust gas temperatures and the exothermic catalytic reaction occurring within the particulate filter substrate  16 .  
         [0010]    The particulate filter substrate  16 , which is shown in a partial cut-away view, may comprise a gas permeable ceramic material having a honeycomb structure consisting of a plurality of channels, preferably parallel channels. The channels may be divided into alternating inlet channels  26  and outlet channels  28 . The inlet channels  26  are open at an inlet end  30  of the particulate filter substrate  16  and preferably plugged at an outlet end  32  of the substrate  16 . Conversely, outlet channels  28  are preferably plugged at the inlet end  30  and open at the outlet end  32 . The inlet and outlet channels  26 ,  28  are formed and separated by thin porous longitudinal sidewalls  34 , which permit exhaust gases  36  to pass from the inlet channels  26  to the outlet channels  28  along their length. As shown in FIG. 2, the particulate filter substrate  16  may be a generally cylindrical structure with a plurality of inlet and outlet channels  26 ,  28  disposed therein. The inlet and outlet channels  26 ,  28  may have a substantially rectangular cross-sectional shape. However, the channels  26 ,  28  may have any multi-sided or rounded shape, with substantially square, triangular, hexagonal, or octagonal or similar geometries. The dimensions of the inlet and outlet channels  26 ,  28  depend on various design considerations, including space limitations, projected washcoat loading, and end use requirements.  
         [0011]    The particulate filter substrate  16  may comprise any material designed for use in the environment and which may remove particulate matter from a gaseous stream. Some possible materials include ceramics (e.g., extruded ceramics, such as Cordierite, aluminum oxide, aluminum phosphate and the like), metals (e.g., extruded, sintered metals. metal mesh materials), silicon nitrate and silicon carbide, and the like, and mixtures comprising at least one of the foregoing materials.  
         [0012]    Dispersed on the sidewalls  34  forming the inlet and outlet channels  26 ,  28  are one or more washcoat compositions comprising one or more catalytically active materials. While the washcoat compositions are said to be dispersed “on” the sidewalls  34 , it is also possible that at least a portion of the washcoat compositions will be located within the sidewalls  34 . Thus, as used herein, a washcoat dispersed on the sidewalls includes washcoat dispersed on and/or in the sidewalls.  
         [0013]    The washcoat compositions need not be present along the entire length of the channels  26 ,  28 , and may be dispersed in one or both of the inlet and outlet channels  26 ,  28 . For example, a washcoat may be coated throughout the length “L” of the inlet channel  26 , or only through part of the length “L” near the inlet end  30 . For certain application the washcoat may also be coated through the entire length “L”, or a portion of the length “L”, of the outlet channels  28 . For certain application, washcoat may also be coated on a portion of the length “L”, of the inlet channels  26  and a portion of the length “L”, of the outlet channels  28 . The choice of the locations, amount, and type of washcoat depend on the application of the particulate filter  10 .  
         [0014]    The catalytic material used in the washcoat composition may be any components capable of reducing the concentration of at least one component in the exhaust gas. Thus, the catalyst may comprise one or more catalytic materials. Possible catalyst materials include metals, such as barium, cesium, vanadium, molybdenum, niobium, tungsten platinum, palladium, rhodium, iridium, ruthenium, zirconium, yttrium, cerium, lanthanum, and the like, as well as oxides, alloys, and combinations comprising at least one of the foregoing catalyst materials, and other catalysts.  
         [0015]    In operation, exhaust gas  36  generated by the internal combustion engine passes through the exhaust gas manifold into the inlet channels  26  of the particulate filter substrate  16 . The exhaust gas passes through the sidewalls  34  into the outlet channels  28 , and the porous sidewalls  34  permit the exhaust gas  36  to pass from the inlet channels  26  to the outlet channels  28  such that the inlet channels  26  collect particulate matter contained in the exhaust gas  36 . The catalyst material dispersed on sidewalls  34  promotes removal (e.g., oxidation) of the particulate matter collected at inlet channel  26 . From the outlet channels  28 , the exhaust gas  36  flows toward the exhaust gas destination.  
         [0016]    It has been discovered that backpressure caused by the particulate filter  10  can be adjusted using an anisotropic washcoat application throughout the length “L” of the inlet and/or outlet channels  26 ,  28 . In other words, the loading (concentration) of catalytic material varies along the length “L” of the inlet and/or outlet channels  26 ,  28 . The anisotropy need not be linear or uniform, and preferably only varies from one location (e.g., the inlet end  30 ) of the particulate filter substrate  16  to another location (e.g., the outlet end  32 ) of the particulate filter substrate  16 .  
         [0017]    The additional backpressure caused by washcoating the particulate filter substrate  16  is mainly determined by the thickness of the washcoat dispersed on substrate wall surface or in the substrate porous wall where exhaust gas is passing by. Thus, the backpressure of caused by the coated particulate filter substrate  16  may be controlled by selectively locating the washcoat in the particulate filter substrate  16 . For example, to minimize exhaust gas backpressure, washcoat may be heavily applied in a portion of the substrate channel. This portion of the channel will have relatively high flow restriction due to the washcoat loading. When exhaust gas flows through the channel, it will tend to pass through another portion of the channel that does not contain washcoat or that contains relatively less washcoat, which results in less flow restriction. The anisotropy in the catalytic material concentration provides for an increase in catalytic loading without causing a substantial increase in backpressure, as compared to a uniform loading of the catalytic material dispersed throughout.  
         [0018]    In general, the more catalytically active material that can be applied to the substrate, the better activity the converter can achieve. In addition, selectively locating the washcoat in one portion of the substrate may also help to improve the catalytic activity of particulate matter reduction. For example, temperature is critical for ignition of particulate matter combustion, regardless of whether it is performed catalytically or non-catalytically. In the catalytic converter design of FIG. 1, inlet exhaust gas is always hotter than outlet exhaust gas due to the heat loss to the converter. Locating the catalyst at inlet end  30  will allow the catalyst to heat up faster and will allow for the faster catalytic ignition of soot. Thus, controlling the washcoat location is beneficial to both lowering backpressure of converter and improving particulate matter reduction performance.  
         [0019]    It has been determined that the location of the washcoat within the channels can be controlled by adjusting the solid content of the washcoat slurry and applying the washcoat slurry in a single-pass. Applying a washcoat slurry with a higher solid content yields a washcoat deposited (loaded) in the channels proximate the end of the particulate filter substrate  16  through which the washcoat slurry was forced into, while applying a washcoat with a lower solid content yields a more uniform washcoat loading throughout the distance that the washcoat slurry was pushed into the channels. For example, if higher washcoat loading at the inlet end of the inlet channels is desired, a washcoat slurry having a higher solid content (e.g., a solid content in the range of between about 5-50%) would be forced to travel a predetermined distance (e.g., 30% of L, 50% of L, 70% of L, 100% of L) into the inlet channels via the inlet end of the filter substrate. After the washcoat slurry is forced into the channels, excess washcoat slurry is removed from the channels by using a vacuum, air blowing, or the like. Because a washcoat slurry having a higher solid content is used, the washcoat will be more heavily loaded toward the inlet end, with the washcoat loading extending no further than the predetermined distance. Higher washcoat loading at the outlet end of the outlet channels can be similarly obtained by pushing a washcoat slurry having the higher solid content into the outlet channels via the outlet end of the filter substrate.  
         [0020]    If a more uniform washcoat loading in the inlet channels is desired, a washcoat slurry having a lower solid content (e.g., a solid content in the range of between about 1-10%) would be pushed a predetermined distance (e.g., 30% of L, 50% of L, 70% of L, 100% of L) into the inlet channels via the inlet end of the filter substrate. After the washcoat slurry is forced into the channels, excess washcoat slurry is removed from the channels by using a vacuum, air blowing, or the like. Because a washcoat slurry having a lower solid content is used, the washcoat will be more uniformly attached to the substrate wall and uniformly loaded throughout the predetermined distance, with the washcoat loading extending no further than the predetermined distance. Uniform washcoat loading in the outlet channels can be similarly obtained by pushing a washcoat slurry having the lower solid content into the outlet channels via the outlet end of the filter substrate.  
         [0021]    By controlling the solid content of the washcoat slurry and the distance that the slurry traveled in the channels, the position of the washcoat along the length “L” of the inlet and/or outlet channels  26 ,  28  in the particulate filter substrate  16  can be controlled. In this manner, the washcoat loading may be biased towards either the inlet end  30  or the outlet end  32  of either the inlet channels  26 , the outlet channels  28 , or both the inlet and outlet channels of the particulate filter substrate  16 .  
         [0022]    After the washcoat slurry is applied to the substrate  16  and the excess slurry is removed, the coated part is calcined at high temperature (e.g. &gt;450° C.) in oven to decompose any organic component from raw material, evaporate water, and fix the washcoat to the substrate  16 .  
       EXAMPLES  
     Comparative Example 1 (Coating  1  of FIGS.  3  and  4 )  
       [0023]    In a comparative example, one particulate filter substrate was washcoated using five passes (applications) of a single washcoat slurry. In each pass, the washcoat slurry was fully pushed into the channels of the filter substrate via one end. The resulting washcoat loading after each of the five passes were:  
         [0024]    Pass I—20.2 grams/part  
         [0025]    Pass II—43.0 grams/part  
         [0026]    Pass III—63.5 grams/part  
         [0027]    Pass IV—80.3 grams/part  
         [0028]    Pass V—85.2 grams/part  
         [0029]    Referring to FIG. 3, an isoscan plot of the particulate filter of the comparative example is shown as Coating  1 . As can be seen by the isoscan plot of Coating  1 , by applying multiple coating passes, washcoat loading on the substrate increases with each pass while distribution of the washcoat loading is substantially even along the length of the filter substrate.  
       Example 1 (Coating  2  of FIGS.  3  and  4 )  
       [0030]    Seven particulate filter substrates were each washcoated using an embodiment of the method of the present invention. For each filter substrate, a washcoat slurry having a predetermined solid content was prepared and the washcoat slurry was applied in a single-pass application. The washcoat slurry used in Example 1 was the same as the slurry used in Comparative Example 1, only with higher solids content. The solids content in the washcoat slurry was increased for each filter substrate from A to G. The single-pass application included pushing the washcoat slurry the entire length of the channels (100% of L) from one side of the filter substrate and clearing the excess washcoat slurry from the channels. The resulting washcoat loading for each washcoat pass was as follows:  
         [0031]    Particulate Filter Substrate (part) A—15 grams/part  
         [0032]    Particulate Filter Substrate (part) B—25.6 grams/part  
         [0033]    Particulate Filter Substrate (part) C—37.9 grams/part  
         [0034]    Particulate Filter Substrate (part) D—45.9 grams/part  
         [0035]    Particulate Filter Substrate (part) E—72.5 grams/part  
         [0036]    Particulate Filter Substrate (part) F—104.3 grams/part  
         [0037]    Particulate Filter Substrate (part) G—174.5 grams/part  
         [0038]    As can be seen, coating with a higher solid content slurry yields a higher washcoat loading of the particulate filter. Referring to FIG. 3, isoscan plots of the particulate filters of this example are shown as Coating  2 . As can be seen by the isoscan plots of Coating  2 , as the solid content of the washcoat slurry is increased, the washcoat loading becomes more anisotropic along the length of the particulate filter, with more of the washcoat loading being disposed towards one end of the particulate filter. With a washcoat having a lower solid content, the washcoat distribution is more homogeneous throughout the length of the particulate filter.  
         [0039]    Referring to FIG. 4, the backpressure increase for Coating  1  and Coating  2 , as a percent increase over an uncoated (raw) substrate, are shown as functions of washcoat loading on the DPF substrate. As shown for Coating  1 , the comparative example, backpressure increases exponentially at washcoat loading higher than 40 grams/part. In comparison, the single-pass method of the present invention, as embodied in Coating  2 , provides only a mild backpressure increase for increased amounts of washcoat loading, even for washcoat loading as high as 175 grams/part.  
         [0040]    Described herein is a method for controlling the washcoat distribution along the channels of a coated particulate filter substrate. The method allows for anisotropically coating a particulate filter substrate, which provides for an uneven distribution of catalytically active material within the channels of a particulate filter substrate. By controlling the location of the washcoat in the particulate filter substrate, the backpressure caused by the coated particulate filter substrate and the flow of the exhaust gasses through various portions of the filter substrate may be controlled.  
         [0041]    While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for substrates thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.