JP2007181798A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for cleaning exhaust gas, which has such a new constitution that the cleaning efficiency can be increased even when the composition and amount of a catalyst component to be deposited on a carrier base material are the same as before. <P>SOLUTION: The catalyst for cleaning exhaust gas is composed of the carrier base material A having a plurality of cells 2 each of which is partitioned by partition walls 10 and penetrated to the predetermined direction and catalyst layers 3 deposited on wall surfaces 14 of partition walls 10. Each of catalyst layers 3 is deposited while leaving a gap 13 penetrated to the predetermined direction between the corresponding catalyst layer and the corresponding wall surface 14. Since the gap 13 penetrated to the predetermined direction is left between the corresponding wall surface 14 and the corresponding catalyst layer 3, the exhaust gas flowing through the catalyst for cleaning exhaust gas is made to flow into not only the conventional flow passage 31 surrounded by the catalyst layers 3 but also the gap 13. As a result, the surface area of each of catalyst layers 3 to be in contact with the exhaust gas can be increased and the cleaning efficiency of the catalyst for cleaning exhaust gas can also be increased. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine such as an automobile.

  An internal combustion engine is equipped with an exhaust gas purifying catalyst for purifying exhaust gas discharged. For example, a three-way catalyst is widely used as a catalyst for purifying NOx, CO, and HC in exhaust gas from automobiles and the like. The three-way catalyst is formed by supporting a noble metal on a porous oxide carrier base, and oxidizes and purifies HC and CO and reduces and purifies NOx. In order to increase the purification efficiency, various types of exhaust gas purification catalysts have been proposed, such as use in combination with the type, loading amount, loading position, and NOx occlusion material of noble metals.

Moreover, in patent document 1, the purification performance is improved by increasing the adhesion amount of the washcoat applied to the honeycomb carrier to form a thick washcoat layer. Specifically, the washcoat 2 is applied to the honeycomb carrier (1) having a plurality of convex portions (7) on the inner wall of the through hole using a slurry. A thick washcoat 2 is formed on the corner (3) formed by the convex portion (7) by the surface tension of the slurry (see FIG. 1 of Patent Document 1).
JP-A-11-333305

  In the exhaust gas purifying catalyst, the exhaust gas is largely purified at the surface layer portion of the washcoat layer containing the catalyst component. Therefore, there is a problem that even if the wash coat layer is simply formed thick, the exhaust gas is not diffused to the deep part of the coat layer, and the entire coat layer cannot be effectively used. Accordingly, the present inventor has come up with the idea of improving the purification performance by increasing the contact area between the exhaust gas and the catalyst component by utilizing the portion that did not contact the exhaust gas in the conventional configuration.

  In view of the above problems, the present invention provides an exhaust gas purifying catalyst having a novel configuration capable of improving purification efficiency even when the composition and amount of the catalyst component supported on the carrier base material are the same as conventional ones. The purpose is to do.

  The exhaust gas purifying catalyst of the present invention comprises a carrier base material having a plurality of cells partitioned by partition walls and penetrating in a predetermined direction, and a catalyst layer supported on the wall surface of the partition wall, It is supported with a gap penetrating in a predetermined direction between the wall surface and the wall surface.

  The wall surface has a groove surface in which a plurality of grooves that open to the inside of the cell and extend in a direction penetrating the cell are formed in the partition wall, and the void is formed between the catalyst layer and the groove surface. It is preferably located in between. For example, the groove may be formed so as to straddle the wall surface adjacent to the corner portion of the cell formed by intersecting the adjacent wall surfaces.

  In the exhaust gas purifying catalyst of the present invention, the exhaust gas flowing into the exhaust gas purifying catalyst of the present invention is surrounded by the catalyst layer because there is a gap penetrating in a predetermined direction between the wall surface of the carrier substrate and the catalyst layer. It flows not only into the conventional flow path but also into the air gap. As a result, the surface area of the catalyst layer in contact with the exhaust gas is increased, and the purification efficiency is improved.

  In addition, it is preferable that a groove surface is provided on the wall surface of the carrier substrate, and a gap is located between the catalyst layer and the groove surface. The position where the groove is formed is not particularly limited. For example, if the groove is formed at the corner of the cell, the surface of the catalyst layer is formed at the corner of the cell where the catalyst layer is formed thick due to the surface tension of the slurry. Since the back surface is in efficient contact with the exhaust gas, the exhaust gas is efficiently diffused into the catalyst layer, so that the purification efficiency is improved.

  In order to describe the exhaust gas purifying catalyst of the present invention in more detail, the best mode for carrying out the present invention will be described below.

  The exhaust gas purifying catalyst of the present invention mainly has a carrier substrate and a catalyst layer.

  The carrier base material is a so-called honeycomb structure having a plurality of cells partitioned by partition walls and penetrating in a predetermined direction. The “predetermined direction” corresponds to the axial direction of the carrier base material when the carrier base material is a columnar or prismatic columnar body, and is usually a flow path direction in which exhaust gas flows. FIG. 5 is a diagram schematically showing a general exhaust gas purifying catalyst, and a “predetermined direction” in the carrier substrate A can be indicated by an arrow X.

  There is no particular limitation on the size and number of each cell, and the specification can be appropriately selected according to the required performance as long as the specification is normally used for an exhaust gas purifying catalyst. For example, when used as an exhaust gas purification catalyst for automobiles, the cell density is preferably in the range of 200 to 900 cpsi, and the partition wall thickness is preferably 1 to 6 mil. If the thickness of the partition wall is in the above range, a carrier substrate having a good balance between strength and heat capacity is obtained.

  The material of the partition is preferably made of ceramics or metal. If the carrier substrate is a ceramic carrier substrate made of ceramics, various oxide or non-oxide ceramics can be used. Specifically, cordierite, mullite, alumina, spinel, silicon carbide, silicon nitride, aluminum nitride, zirconia, lithium aluminum silicate, aluminum titanate, and the like can be used. A carrier substrate using ceramics is lightweight and high in strength, and has excellent heat resistance. If the carrier substrate is a metal carrier substrate made of metal, various alloys such as stainless steel and Al—Cr—Fe alloy having excellent heat resistance and corrosion resistance can be used. Since the metal carrier base material has good heat conduction, the exhaust gas purifying catalyst using the metal carrier base material is excellent in low temperature characteristics.

  As for the shape of the cell, the shape of the cross section (a cross section obtained by cutting a cell penetrating in a predetermined direction in a direction perpendicular to the predetermined direction) is a polygon such as a triangle, a quadrangle, a hexagon, etc. It may be a carrier base material made of a laminate of corrugated plate and corrugated plate.

  The catalyst layer is supported on the wall surface of the partition wall. At this time, the catalyst layer may be supported in contact with at least a part of the wall surface. There are no particular limitations on the type and loading of the catalyst layer, and it may be appropriately selected according to the purpose. There is no particular limitation on the method for supporting the catalyst layer, and various conventional supporting methods in which the slurry containing the catalyst component is coated by wash coating, dip coating or the like and then fired at a predetermined temperature can be used.

  In the exhaust gas purifying catalyst of the present invention, the catalyst layer is supported with a space penetrating in the cell penetrating direction (predetermined direction) between the catalyst layer and the wall surface. Since the gap penetrates in a predetermined direction, it can be used as a flow path for the exhaust gas. Therefore, it is preferable that the gap is formed in parallel with the direction through which the cell passes. And since a space | gap becomes a flow path of exhaust gas and purification | cleaning of exhaust gas is performed in a space | gap, if even one space | gap exists, the improvement effect of purification efficiency can be expected, but it is preferable that a several space | gap exists. Furthermore, it is good to form a space | gap so that the area which the waste gas which flows through a space | gap and a catalyst layer may contact becomes large. However, if there are too many voids, or if the area where the exhaust gas flowing through the voids contacts the catalyst layer is too large, the catalyst layer may not be satisfactorily held on the carrier substrate. Therefore, in one cell, “the area of the wall surface that is in contact with the catalyst layer”: “the area of the wall surface that is not in contact with the catalyst layer (when a groove described later is formed in the partition wall of the carrier substrate, the opening area of the groove) ) "= 9: 1 to 5: 5.

  As a method of supporting the catalyst layer with a gap between the wall surface of the carrier base material, for example, a void can be easily formed by using a burned-out material that burns away at a predetermined temperature. Specifically, first, a burned-out material that is burned down at a firing temperature or lower in a subsequent firing step is disposed in a portion of the wall surface of the carrier base material where a void is to be formed. By coating the slurry on the carrier base material on which the burned-out material is disposed and firing, the burned-out material is burned off, so that an exhaust gas purifying catalyst having the same shape as the burned-out material can be produced. The burnout material is not particularly limited as long as it is made of a material that burns at a firing temperature or lower. Therefore, in addition to resin, paper or wood may be used, and the material thereof is not limited.

  Moreover, the internal space of a groove | channel can be made into a space | gap by forming in the partition the groove | channel opened inside the cell of a support base material. That is, in the carrier substrate, it is preferable that grooves are formed in the partition walls that partition the cells. Specifically, by forming a groove on the partition wall, a groove surface is formed as a cell wall surface. If a catalyst layer is supported with a space between the groove surface, the internal space of the groove is used as a space. it can. And by setting it as the groove | channel extended in the direction which the cell penetrates, a space | gap (internal space of a groove | channel) can be utilized as a flow path of waste gas.

  The groove formation position is not particularly limited. For example, in addition to forming a plurality of grooves on the flat surface of the cell (an example is shown in FIG. 1), if the cross-sectional shape of the cell is a polygon, the adjacent wall surface You may form in a corner | angular part of the cell formed by crossing so that an adjacent wall surface may be straddled (an example is shown in FIG. 2). Also, the cross-sectional shape of the groove is not particularly limited, and the cross-sectional shape is U-shaped (see FIG. 1), semicircular, and the cross-section of the groove bottom is V-shaped (L-shaped in FIG. 2) or U-shaped. It is good to do.

  However, by forming the groove on the wall surface, the strength of the carrier substrate may be lowered depending on the position and size of the groove. Therefore, in a cross section when cut in a direction perpendicular to a predetermined direction through which the cell passes, “cross-sectional area of partition walls of carrier substrate”: “cross-sectional area of grooves formed in carrier substrate” = 9: 1 If it is in the range of ˜5: 5, the strength of the carrier base material will not be greatly reduced.

  When forming a carrier substrate having grooves in the partition walls, if the carrier substrate is a ceramic carrier substrate, conventional methods such as extrusion using a mold corresponding to the cross-sectional shape of the desired carrier substrate Can be manufactured. That is, a desired groove can be formed in the partition only by changing the shape of the mold.

  When there is a groove in the partition wall of the carrier base material, the method of supporting the catalyst layer with a gap between the groove surface of the carrier base material can be easily formed in the groove by using the above-mentioned burned-out material. . The burned-out material may be buried in the groove, and after the burned-out material is burned out by firing, a void is formed between the catalyst layer and the groove surface (inner space of the groove).

The catalyst layer only needs to be able to purify each emission component such as NOx, CO, HC, H ₂ S contained in the exhaust gas. Therefore, a three-way catalyst in which a noble metal such as Pt (platinum), Rh (rhodium) or Pd (palladium) is supported on a porous oxide such as alumina, ceria, zirconia, or ceria-zirconia solid solution, or an alkali metal or alkaline earth A catalyst layer containing a catalyst component usually used as a catalyst for purifying exhaust gas, such as a NOx storage catalyst using a complex oxide containing a metal, may be used.

  The exhaust gas purifying catalyst of the present invention is not limited to the above embodiment. For example, as long as the effect of the exhaust gas purifying catalyst of the present invention is not impaired, another substance may be added as necessary to add another function.

  Examples of the exhaust gas purifying catalyst of the present invention will be specifically described below together with comparative examples with reference to FIGS.

[Example 1]
A molded body was obtained using an extrusion mold having a predetermined shape. The obtained molded body was dried and then cut into a desired length to prepare a carrier substrate having the same overall shape as the carrier substrate shown in FIG. This carrier base material is referred to as “carrier base material A-1”. The carrier substrate A-1 had a diameter of 103 mm, a length of 105 mm, a cell density of 600 cpsi, and a partition wall thickness of 3 mils (maximum thickness). FIG. 1 shows a part of a cross section obtained by cutting the carrier base material A-1 perpendicularly to the axial direction (the arrow X direction in FIG. 5). The carrier substrate A-1 includes a plurality of cells 2 having a substantially square cross section that is partitioned by the partition wall 10 and penetrates in the axial direction. The cell 2 is partitioned by partition walls 10 that intersect in a lattice pattern. The partition wall 10 has a plurality of grooves 13 having a U-shaped cross-section opening inside each cell 2. That is, the catalyst layer 3 is supported on the wall surface 14 constituting the inner surface of each cell 2, but the wall surface 14 includes the groove surface 11 of the groove 13 in addition to the support surface 12 that directly supports the catalyst layer 3. The grooves 13 are formed on the four inner surfaces (wall surfaces 14) of the partition wall 10 partitioning each cell 2, and four grooves 13 are formed on one surface. The four grooves 13 are parallel to each other and extend linearly from one end surface E of the carrier base material A-1 to the other end surface E ′ (see FIG. 5) and penetrate therethrough. Note that “cross-sectional area of the partition wall 10 of the carrier substrate”: “cross-sectional area of the groove 13 formed in the carrier substrate” = 8: 2.

  Next, a resin was embedded in the internal space of the groove 13 of the carrier base material A-1 so as to be flush with the support surface 12 of the wall surface 14. With the resin embedded in all the grooves 13, the catalyst layer 3 was formed on the carrier base material A- 1 by the following procedure, and the exhaust gas-purifying catalyst of Example 1 was produced.

CeO ₂ —ZrO ₂ powder and distilled water were mixed and stirred, and a predetermined amount of platinum nitrate solution and rhodium nitrate solution were added. The obtained mixed liquid was distilled and dried, and calcined at 500 ° C. for 2 hours to obtain CeO ₂ —ZrO ₂ powder supporting platinum and rhodium. Next, a predetermined amount of the obtained Pt—Rh / CeO ₂ —ZrO ₂ powder and alumina sol were mixed to prepare a catalyst slurry S.

  The obtained catalyst slurry S was poured into a carrier substrate A-1 embedded with a resin and sucked to form a coat layer. Then, it baked at 550 degreeC for 2 hours. By baking at 550 ° C., the resin previously embedded in the groove 13 was burned out, and the catalyst layer 3 was supported on the wall surface 14 with the gap 13 in the portion where the resin was burned out.

  In the exhaust gas purifying catalyst of Example 1, the amount of noble metal supported was noble metal equivalent weight per liter of catalyst, Pt was 1.5 g, and Rh was 0.4 g. Further, “the area of the catalyst layer carrying surface 12”: “the opening area of the groove 13” = 8: 2.

[Example 2]
Exhaust gas purification catalyst of Example 2 was produced in the same manner as in Example 1 except that the carrier substrate A-2 was used as the carrier substrate. The shape of the carrier substrate A-2 will be described below.

  The carrier substrate A-2 had a diameter of 103 mm, a length of 105 mm, a cell density of 600 cpsi, and a partition wall thickness of 3 mils (maximum thickness). FIG. 2 shows a part of a cross section obtained by cutting the carrier substrate A-2 perpendicularly to the axial direction (the arrow X direction in FIG. 5). The carrier base material A-2 is the same as the carrier base material A-1, except that the groove 13 'is positioned so as to straddle the adjacent wall surface 14 at the corner of the cell formed by the intersection of the adjacent wall surfaces 14. is there. Note that “cross-sectional area of the partition wall 10 of the carrier substrate”: “cross-sectional area of the groove 13 ′ formed in the carrier substrate” = 9: 1.

  When the catalyst layer 3 is formed on the carrier base A-2 using the catalyst slurry S in the same procedure as in Example 1, the wall surface 14 has a gap 13 'having an L-shaped cross section at the portion where the resin has been burned out. Catalyst layer 3 was supported. In addition, “the area of the support surface 12 of the catalyst layer”: “the opening area of the groove 13 ′” = 9: 1

[Comparative Example 1]
Exhaust gas purification catalyst of Comparative Example 1 was produced in the same manner as in Example 1 except that Carrier Base A-3 was used as the carrier base. The shape of the carrier substrate A-3 will be described below.

  The carrier substrate A-3 had a plurality of cells 2 having a square cross section, a diameter of 103 mm, a length of 105 mm, a cell density of 600 cpsi, and a partition wall thickness of 3 mils. FIG. 3 shows a part of a cross section obtained by cutting the carrier substrate A-3 perpendicularly to the axial direction (the arrow X direction in FIG. 5). The carrier substrate A-3 is the same as the carrier substrate A-1 except that it does not have a groove. That is, the catalyst layer 3 was supported on the entire wall surface 14 and no void was formed.

[Catalyst performance evaluation]
The exhaust gas purifying catalysts of Examples 1 and 2 and Comparative Example 1 are mounted as underfloor catalysts of a vehicle equipped with an in-line 4-cylinder 2.5L gasoline engine, combustion controlled at the theoretical air-fuel ratio, and the gas space velocity is adjusted. The purification rate of the HC component, CO component, and NOx component was measured while raising the temperature from 200 ° C. to 450 ° C. at a rate of 10 ° C./min. From the result, the temperature at which each component could be purified by 50% was calculated. The calculated temperature is shown in FIG. 4 as “50% purification temperature”.

  In the exhaust gas purifying catalysts of Example 1 and Example 2 in which the catalyst layer is supported with the gap 13 between the wall surface 12, not only the conventional flow path (31) surrounded by the catalyst layer 3 but also the gap Exhaust gas also flows into 13. Therefore, the surface area of the catalyst layer 3 in contact with the exhaust gas is increased, and the exhaust gas is purified even in the gap 13, so that the purification efficiency is improved as compared with the exhaust gas purification catalyst of Comparative Example 1.

  The exhaust gas purification catalyst of Example 1 exhibited particularly excellent purification performance because the contact area between the catalyst layer and the exhaust gas and the total cross-sectional area of the voids were larger than those of the exhaust gas purification catalyst of Example 2.

It is radial direction (perpendicular | vertical direction with respect to the direction which the cell penetrates) of the exhaust gas purification catalyst of Example 1, Comprising: It is the enlarged view which showed typically one of several cells. It is radial direction sectional drawing of the exhaust gas purification catalyst of Example 2, Comprising: It is the enlarged view which showed typically one of several cells. It is radial direction sectional drawing of the exhaust gas purification catalyst of the comparative example 1, Comprising: It is the enlarged view which showed typically one of several cells. 6 is a graph showing 50% purification temperatures of exhaust gas components for exhaust gas purification catalysts of Examples 1 and 2 and Comparative Example 1. It is a figure which shows typically an example of the general exhaust gas purification catalyst, Comprising: It is the whole figure of the exhaust gas purification catalyst.

Explanation of symbols

A: Support base material 2: Cell 3: Catalyst layer 10: Partition wall 11: Wall surface (groove surface)
12: Wall surface (holding surface)
13: Air gap (groove)
14: Wall surface

Claims (3)

A carrier substrate having a plurality of cells partitioned by partition walls and penetrating in a predetermined direction, and a catalyst layer supported on the wall surface of the partition walls,
The catalyst for exhaust gas purification, wherein the catalyst layer is supported with a gap penetrating in the predetermined direction between the wall surface and the wall surface. The wall surface has a groove surface in which a plurality of grooves that open in the cell and extend in a direction through the cell are formed in the partition wall,
The exhaust gas purifying catalyst according to claim 1, wherein the gap is located between the catalyst layer and the groove surface.   The exhaust gas-purifying catalyst according to claim 2, wherein the groove is formed so as to straddle the wall surface adjacent to the corner portion of the cell formed by intersecting the adjacent wall surfaces.

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