Zoned catalyzed soot filter

#The present invention relates to a zoned catalyzed soot filter. This soot filter comprises a wall flow substrate which comprises an inlet end, an outlet end, a substrate axial length extending between the inlet end and the outlet end, and a plurality of passages defined by internal walls of the wall flow substrate. The plurality of said passages comprise inlet passages having an open inlet end and a closed outlet end, and outlet passages having a closed inlet end and an open outlet end. The internal walls of the inlet passages comprise a first inlet coating that extends from the inlet end to a first inlet coating end, thereby defining a first inlet coating length, wherein the first inlet coating length is x % of the substrate axial length with 0<x<100. The internal walls of the outlet passages comprise a first outlet coating that extends from the outlet end to a first outlet coating end, thereby defining a first outlet coating length, wherein the first outlet coating length is 100-x % of the substrate axial length. The first inlet coating length thereby defines an upstream zone, and the first outlet coating length defines a down-stream zone. According to a preferred embodiment, both the first inlet coating and the first outlet coating comprise an oxidation catalyst. According to the present invention, the first inlet coating and the first outlet coating are present on the wall flow substrate at a specific coating loading ratio defined as the inlet coating loading relative to the outlet coating loading. In particular, said coating loading ratio is less than 0.5.

#Operation of lean burn engines such as diesel engines provide the user with excellent fuel economy and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines also offer significant advantages over gasoline engines in terms of their fuel economy, durability, and their ability to generate high torque at low speed. However, there are certain materials contained in diesel engine exhaust gas which are known to cause pollution and therefore may have severe influence on the environment. Apart from gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbon (“HC”) and nitrogen oxides (“NOx”), diesel engine exhaust also contains condensed phase materials, i.e. liquids and solids, which constitute the so-called particulate matter (“PM”). The total particulate matter emissions comprised in diesel exhaust comprises, apart from the soluble organic fraction (“SOF”) and the so-called the sulfate fraction, the solid and dry carbonaceous fraction which is also known as “soot” fraction. This soot contributes to the visible soot emissions commonly associated with diesel exhaust. The soluble organic fraction can exist in diesel exhaust either as vapour or as an aerosol, i.e. fine droplets of liquid condensate, depending on the temperature of the diesel exhaust. Generally, it is present as condensed liquid at the standard particulate collection temperature of 52° C. in diluted exhaust, as prescribed by a standard measurement test such as the U.S. Heavy Duty Transient Federal Test Procedure. These liquids are believed to arise from two sources: one the one hand, lubricated oil swept from cylinder walls of the engine each time the pistons go up and down, and on the hand, unburned or only partially burned diesel fuel. The sulfate fraction is believed to be formed from small quantities of sulfur components present in the diesel fuel.

#Catalyzed filters are typically provided in diesel engine exhaust systems to achieve high particulate matter reduction, in particular soot reduction, and to convert certain or all of the exhaust components to innocuous components. Known filter structures that remove particulate matter from diesel exhaust include honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal filters, etc. However, ceramic wall flow filters, described below, receive the most attention. Typical ceramic wall flow filter substrates are composed of refractory materials such as cordierite or silicon-carbide. Wall flow substrates are particularly useful to filter particulate matter from diesel engine exhaust gases. A common construction is a multi-passage honeycomb structure having the ends of alternate passages on the inlet and outlet sides of the honeycomb structure plugged. This construction results in a checkerboard-type pattern on either end. Passages plugged on the inlet axial end are open on the outlet axial end. This permits the exhaust gas with the entrained particulate matter to enter the open inlet passages, flow through the porous internal walls and exit through the channels having open outlet axial ends. The particulate matter is thereby filtered on the internal walls of the substrate. The gas pressure forces the exhaust gas through the porous structural walls into the channels closed at the upstream axial end and open at the downstream axial end. The accumulating particles will increase the back pressure from the filter on the engine. Thus, the accumulating particles have to be continuously or periodically burned out of the filter to maintain an acceptable back pressure. Catalyst compositions deposited along the internal walls of the wall flow substrate assist in the regeneration of the filter substrates by promoting the combustion of the accumulated particulate matter. The combustion of the accumulated particulate matter restores acceptable back pressures within the exhaust system. These processes may be either passive or active regeneration processes. Both processes utilize an oxidant such as O2 or NO2 to combust the particulate matter. Passive regeneration processes combust the particulate matter at temperatures within the normal operating range of the diesel exhaust system. Preferably, the oxidant used in the regeneration process is NO2 since the soot fraction combusts at much lower temperatures than those needed when O2 serves as the oxidant. While O2 is readily available from the atmosphere, NO2 can be actively generated though the use of upstream oxidation catalysts which oxidize NO in the exhaust stream.

#FIG. 1 a sketch of the zoned catalysed soot filter (CSF) with a higher washcoat loading in the outlet zone of the CSF with a 50% over length inlet coating and a 50% over length outlet coating. In FIG. 1, 1 shows the inlet coating. 2 shows the outlet coating having higher washcoat loading than the inlet coating. 3 shows the plug of the filter substrate to close the inlet channel. 4 shows the plug of the filter substrate to close the outlet channel. The arrow indicates the direction of flow through the CSF.

#FIG. 2 shows a sketch of the zoned catalysed soot filter with a higher washcoat loading in the outlet zone of the CSF with a washcoat gradient over the axial substrate length. In FIG. 2, 1 shows the inlet coating. 2 shows the outlet coating having higher washcoat loading than the inlet coating. 3 shows the second outlet coating on top of the outlet coating 2. 4 shows the plug of the filter substrate to close the inlet channel. 5 shows the plug of the filter substrate to close the outlet channel. The arrow indicates the direction of flow through the CSF.

#FIG. 3 shows the placement of thermocouples 1 to 4 in samples A and B for the drop to idle regeneration according to the Example. Thermocouple 1 is located 1 inch from the inlet end of the CSF, thermocouple 2 is located 3 inches from the inlet end of the CSF, thermocouple 3 is located 4 inches from the inlet end of the CSF, and thermocouple 4 is located 4 inches from the inlet end of the CSF. The arrow indicates the direction of flow through the CSF.

#The present invention relates to a catalyzed soot filter, comprising a wall flow substrate comprising an inlet end, an outlet end, a substrate axial length extending between the inlet end and the outlet end, and a plurality of passages defined by internal walls of the wall flow substrate; wherein the plurality of passages comprise inlet passages having an open inlet end and a closed outlet end, and outlet passages having a closed inlet end and an open outlet end; wherein the internal walls of the inlet passages comprise a first inlet coating that extends from the inlet end to a first inlet coating end, thereby defining a first inlet coating length, wherein the first inlet coating length is x % of the substrate axial length with 0<x<100; wherein the internal walls of the outlet passages comprise a first outlet coating that extends from the outlet end to a first outlet coating end, thereby defining a first outlet coating length, wherein the first outlet coating length is 100-x % of the substrate axial length; wherein the first inlet coating length defines an upstream zone and the first outlet coating length defines a downstream zone; wherein the first inlet coating and the first outlet coating are present on the wall flow substrate at a coating loading ratio of less than 0.5, calculated as ratio of the loading of the first inlet coating (g/inch3 (g/(2.54 cm)3)): loading of the first outlet coating (g/inch3 (g/(2.54 cm)3)).

#According to a preferred embodiment of the present invention, said first inlet coating contains an oxidation catalyst. According to a further preferred embodiment of the present invention, said first outlet coating contains an oxidation catalyst. More preferably, both the first inlet coating and the first outlet coating comprise an oxidation catalyst.

#According to the present invention, the washcoat loading ratio calculated as ratio of the loading of the first inlet coating:loading of the first outlet coating is less than 0.5. Surprisingly, it was found that by applying said first inlet coating and said first outlet coating such that the washcoat loading ratio is less than 0.5, the SML could be increased and the maximum temperature necessary for drop to idle regeneration could be decreased. Further it was found that such washcoat loading ratios of less than 0.5 allowed for a lower frequency of active regeneration of the catalyzed soot filter during driving.