Catalytic wall-flow filter having a membrane

Abstract

The present invention relates to a catalytic wall-flow monolith for use in an emission treatment system, the monolith comprising a porous substrate and having a first face and a second face defining a longitudinal direction therebetween and first and second pluralities of channels extending in the longitudinal direction, the first plurality of channels provides a first plurality of inner surfaces and is open at the first face and closed at the second face, and the second plurality of channels is open at the second face and closed at the first face, a first catalytic material is distributed within the porous substrate, a microporous membrane is provided in the first plurality of channels on a first portion, extending in the longitudinal direction, of the first plurality of inner surfaces, and the first portion extends from the first face for 75 to 95% of a length of the first plurality of channels.

Claims

1. A catalytic wall-flow monolith for use in an emission treatment system, the monolith comprising a porous substrate and having a first face and a second face defining a longitudinal direction therebetween and first and second pluralities of channels extending in the longitudinal direction, wherein the first plurality of channels provides a first plurality of inner surfaces and is open at the first face and closed at the second face, and wherein the second plurality of channels is open at the second face and closed at the first face, wherein a first catalytic material is distributed within the porous substrate, wherein a microporous membrane is provided in the first plurality of channels on a first portion, extending in the longitudinal direction, of the first plurality of inner surfaces, and wherein the first portion extends from the first face for 75 to 95% of a length of the first plurality of channels, wherein the length is the internal length of a cavity from the first face to the internal sealed end of the first plurality of channels.

2. The catalytic wall-flow monolith according to claim 1, wherein the first portion extends 80 to 90% of a length of the first plurality of channels.

3. The catalytic wall-flow monolith according to claim 1, wherein the microporous membrane has a thickness which decreases along the longitudinal direction, such that the thickness is greatest in a region adjacent the first face.

4. The catalytic wall-flow monolith according to claim 1, wherein the porous substrate has a first section extending in the longitudinal direction from the first face and a second section extending in the longitudinal direction from the second face and extending to the first section, and wherein the first catalytic material is distributed throughout the second section.

5. The catalytic wall-flow monolith according to claim 4, wherein a ratio of a length of the first section in the longitudinal direction to the length of the second section in the longitudinal direction is from 5:95 to 15:85.

6. An emission treatment system for treating a flow of a combustion exhaust gas, the system comprising the catalytic wall-flow monolith according to claim 1, wherein the second face is downstream of the first face.

7. A method for the manufacture of a catalytic wall-flow monolith according to claim 1, the method comprising: providing a porous substrate having a first face and an second face defining a longitudinal direction therebetween and first and second pluralities of channels extending in the longitudinal direction, wherein the first plurality of channels is open at the first face and closed at the second face, and wherein the second plurality of channels is open at the second face and closed at the first face, wherein the first plurality of channels provide a first plurality of inner surfaces and wherein the second plurality of channels provide a second plurality of inner surfaces, infiltrating the porous substrate with a washcoat containing a first catalytic material to provide the first catalytic material distributed within the porous substrate; and forming a microporous membrane on the first plurality of inner surfaces.

8. A method for treating a flow of a combustion exhaust gas comprising NO.sub.x and particulate matter, the method comprising passing the exhaust stream through the monolith of claim 1, wherein the second face is downstream of the first face.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in relation to the following non-limiting figures, in which:

(2) FIG. 1 is a perspective view that schematically shows a wall flow monolith filter 1 according to the present invention.

(3) FIG. 2 is an A-A line cross-sectional view of the wall flow monolith filter 1 shown in FIG. 1.

(4) FIG. 3A-C show different embodiments of the position and relative thicknesses of the microporous membrane described herein.

(5) FIG. 4 is a perspective view that schematically shows a wall flow monolith filter 1 according to the present invention.

(6) FIG. 5 shows an embodiment of the position and relative thicknesses of the microporous membrane described herein.

(7) FIG. 6 shows a schematic diagram of an exhaust gas treatment system for a diesel engine.

DETAILED DESCRIPTION OF THE INVENTION

(8) A wall flow monolith 1 according to the present invention is shown in FIG. 1 and FIG. 2. It includes a large number of channels arranged in parallel with each other in the longitudinal direction (shown by a double-sided arrow a in FIG. 1A) of the monolith 1. The large number of channels includes a first subset of channels 5 and a second subset of channels 10.

(9) The channels are depicted such that the second subset of channels 10 is narrower than the first subset of channels 5. This has been found to provide an increased ash storage capacity in the filter. However, the channels may alternatively be the same size.

(10) The first subset of channels 5 is open at an end portion on a first end face 15 of the wall flow monolith 1 and is sealed with a sealing material 20 at an end portion on a second end face 25.

(11) On the other hand, the second subset of channels 10 is open at an end portion on the second end face 25 of the wall flow monolith 1 and is sealed with a sealing material 20 at an end portion on the first end face 15.

(12) The porous material 40 of the wall-flow monolith 1 is provided with a catalytic material, such as a zeolite, within pores of the channels walls 35. This may be provided with a washcoat application method, as is known in the art and is discussed elsewhere in the specification. Preferably the catalytic material is distributed throughout the porous material 40, except in certain embodiments as described below.

(13) The channel walls 35 of the first subset of channels 5 are provided with a microporous membrane 36 on at least a portion thereof. The channel walls of the second subset of channels 10 are not coated. FIG. 2 shows how the wall-flow filter works but does not show the microporous membrane 36. Embodiments of the microporous membrane 36 are shown in FIGS. 3A-3C and FIG. 4.

(14) When the wall flow monolith is used for a urea SCR device, exhaust gases G (in FIG. 2, G indicates exhaust gases and the arrow indicates a flowing direction of exhaust gases) introduced to the first subset of channels 5 will pass through the channel wall 35 interposed between the channel 5a and the channels 10a and 10b, and then flow out from the monolith 1. Accordingly, particulate matter in exhaust gases is captured by the channel wall 35.

(15) The zeolite supported in the channel wall 35 of the monolith 1 functions as a catalyst for catalytic reduction which acts on NO.sub.x in combination with a reducing agent such as ammonia to reduce NO.sub.x to N.sub.2.

(16) Therefore, when the wall flow monolith 1 is used for a urea SCR device, NO.sub.x in exhaust gases is reduced to N.sub.2 by the action of the zeolite supported on the cell wall 35 and the action of ammonia derived from urea water sprayed from a urea spray nozzle of the urea SCR device while the exhaust gases pass through the cell wall 35.

(17) In FIG. 3A the microporous coating is provided on the channel walls 35 of the first subset of channels 5 such that about 90% of the channel length (measured from the first end face 15 to the sealed end of the channel) and starting from the first end face 15 has the microporous coating. The thickness of the coating is substantially uniform.

(18) The inventors have found that by providing the on-wall coating along the channel 5, but for less than the full length of the channel 5, they can affect the build-up of soot within the channel 5. In particular, in a conventional uncoated wall-flow monolith, the soot tends to build up particularly towards the end of the channel. When this is removed by a combustion regeneration step, the presence of the additional soot leads to a steep temperature gradient within the monolith. This leads to cracking and a significant shortening of the effective life of the filter.

(19) By providing the on-wall coating in the form of the microporous membrane 36, this stops soot going into the wall for a majority of the channels and hence the soot loaded backpressure is lower where there is on-wall coating. The soot in these regions forms a surface layer but does not have the same inhibiting effect on the gas flow. The region in the rear zone does not have a microporous coating so that soot can be deposited in the pores of the walls and this results in a backpressure which is higher in the rear. After initial loading, this elevated back pressure reduces the relative amount of soot which will load in the rear portion of the channels 5. This consequentially reduces the exotherms in that region during active soot regeneration, improving the durability of the filter against cracking/peak temps.

(20) In FIG. 3B the microporous coating is provided on the channel walls 35 of the first subset of channels 5 such that 90% of the channel length (measured from the first end face 15 to the sealed end of the channel) and starting from the first end face 15 has the microporous coating. The thickness of the coating is decreases along the channels 5, such that the coating is thickest close to the first end face 15.

(21) As in the embodiment discussed above, applying the microporous membrane 36 helps to prevent soot going into the wall in the first portion of the channels 5. However, by providing a decreasing thickness of the microporous membrane 36, a gradually increasing amount of soot will enter into the porous substrate. This provides an increasing backpressure in the channel 5 towards the back of the channel. This helps to further reduce the temperature gradients which occur on thermal regeneration.

(22) In FIG. 3C the microporous coating is provided on the channel walls 35 of the first subset of channels 5 such that 90% of the channel length (measured from the first end face 15 to the sealed end of the channel) and starting from the first end of the channel 5 has the microporous coating. The thickness of the coating is substantially uniform. However, the porous substrate 40 in this embodiment is divided into two sections. A first section 50 starting from the first face which does not include any catalytic material, and a second section 51 (the remainder) which has the catalytic material distributed throughout.

(23) As a result, since the first section 50 has a high porosity the back pressure is very low. Accordingly, soot build up on the surface of the microporous membrane 36 is increased and this, consequentially, reduces the soot loading in the rear portion and results in better durability.

(24) FIG. 4 is a perspective view that schematically shows a wall flow monolith filter 1 according to the present invention. The wall flow monolith shows the first section 50 and the second section 51.

(25) In FIG. 5 the microporous coating is provided on the channel walls 35 of the first subset of channels 5 such that 100% of the channel length (measured from the first end face 15 to the sealed end of the channel) has the microporous coating. The thickness of the coating on the channels 5 is such that the coating is thickest close to the closed second end face 25. The catalytic material is distributed throughout the porous material.

(26) The inventors have found that when the extra on-wall coating is increased in the rear portion beyond the level required to prevent soot deposition, the coating can have a direct effect on increasing the backpressure in the rear portion of the first channels. In particular, the coating should be sufficient such that the soot loaded back pressure is 20% higher in the rear than the front by virtue of the increased coating thickness in the rear section. This reduces soot deposition towards the rear of the monolith and helps to prevent undue heat build-up in this portion during regeneration.

(27) In the embodiment of the exhaust gas treatment system 100 shown in FIG. 6 an ammonia reductant 105 is injected into the flow of exhaust gas 110 upstream of the wall flow monolith 1. The exhaust gas 110 is passed from the engine 115 through ducting 120 to the exhaust system 125. The ammonia reductant 105 is dispensed from a reservoir 130 as required (as determined by controller 135) through an injection nozzle 140 and mixes with the exhaust gas prior to reaching the monolith 1 which acts as an SCR device.

(28) It should be noted that the wall flow monolith is described herein as a single component. Nonetheless, when forming an emission treatment system, the monolith used may be formed by adhering together a plurality of channels or by adhering together a plurality of smaller monoliths as described herein. Such techniques are well known in the art, as well as suitable casings and configurations of the emission treatment system.

(29) The membrane may be on the inlet or outlet side of the porous wall. The membrane may cover 10-90% of the filter length, measure from either in the inlet face or the outlet face. For example, the membrane may cover 10-25%, 25-50%, 50-75%, 35-75%, or 75-70% of the filter length.

(30) The catalytic and/or adsorbent material of the membrane may comprise an SCR catalyst as defined here, a NOx trap, a soot oxidation catalyst, a hydrolysis catalyst, an adsorbent for metals such as V, Pt, Pd, Rh, Ru, Na, Cu, Fe, Co, Nu, and Cr, or for other poisons such as ash and/or sulphur oxides. Examples of catalytic and/or adsorbent materials include metal loaded zeolites, such as Cu/CHA, Cu/AEI, Fe/CHA, Pd/CHA, and the like, H-form zeolites, supported platinum group metals, etc.

(31) Preferably, the membrane is applied as a catalyst washcoat that contains the catalyst and optionally one or more other constituents such as binders (e.g., metal oxide particles), fibers (e.g., glass or ceramic non-woven fibers), masking agents, rheology modifiers, and pore formers.

(32) All or any combination of these features in a membrane can improve soot-loaded backpressure (particularly in combination with filter efficiency), reduce exotherms during filter regeneration, improve filter thermal and mechanical durability (e.g. avoid cracking, peeling, etc.), protect temperature sensitive catalyst from high temperature spike, improve catalyst performance overall and on per weight basis, lower N2O formation, allows better NH3 utilization, and capture poisons such as Pt, ash, sulfur oxides, Na, Fe, and mitigate the potential loss of metals via volatilization.

(33) Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the invention or of the appended claims.