JP2006326476A - Exhaust gas purification catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To position each component separately in the same cell according to the type of the catalyst component even when plural types of catalyst components (carrier, catalyst metal, etc.) coexist in the same cell of the carrier substrate It is an object of the present invention to provide an exhaust gas purifying catalyst having a novel structure supported on a catalyst.
SOLUTION: An exhaust gas purifying catalyst 1 of the present invention includes a carrier substrate 2 having a plurality of cells 2C that are partitioned by a partition wall 2 'and penetrates in a predetermined direction, and a wall surface of the partition wall 2' that partitions the same cell 2C. The first catalyst layer 2 _C1 supported on at least one of the wall surfaces and the second catalyst layer 2 _C2 supported on the other wall surface.
[Selection] Figure 1

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, and more particularly to an exhaust gas purifying catalyst carrying a plurality of types of catalyst components.

  An internal combustion engine is equipped with an exhaust gas purifying catalyst for purifying exhaust gas discharged. In particular, 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 such as Pt, Rh, and Pd on a porous oxide carrier such as alumina, ceria, zirconia, and ceria-zirconia solid solution, and oxidizes and purifies HC and CO. At the same time, NOx is reduced and purified.

  In the field of three-way catalysts, various arrangements of porous oxide supports and noble metals that improve the catalyst performance have been proposed so far. For example, Patent Document 1 describes that the catalyst performance is improved by loading a noble metal with an appropriate density distribution in the exhaust gas flow direction or the catalyst radial direction.

Since the three-way catalyst exhibits purification performance by arranging the base metals close to each other, most of the catalysts in Patent Document 1 and the like are arranged in the presence of a plurality of types of porous oxide carriers and noble metals. Yes. Therefore, depending on the combination of the porous oxide support and the noble metal, there is a problem that grain growth or alloying occurs and the catalyst performance is lowered.
JP 63-162045 A

  In the present invention, in view of the above-described problems, even when a plurality of types of catalyst components (such as a support and a catalyst metal) coexist in the same cell of the support substrate, each component is added according to the type of the catalyst component. An object of the present invention is to provide an exhaust gas purifying catalyst having a novel structure supported at different positions in the same cell.

  The exhaust gas purifying catalyst of the present invention is supported on at least one wall surface of a carrier base material having a plurality of cells that are partitioned by partition walls and penetrates in a predetermined direction, and a wall surface of the partition walls that partition the same cell. It has the 1st catalyst layer and the 2nd catalyst layer carry | supported by the other wall surface, It is characterized by the above-mentioned.

  At this time, it is preferable that the first catalyst layer includes a first support and / or the first catalyst metal, and the second catalyst layer includes a second support and / or the second catalyst metal.

  Here, the names “first” and “second” are merely names for convenience in order to distinguish the catalyst layer and the like. For example, the components of the first catalyst layer and the second catalyst layer may be different such that the first catalyst layer contains Pt and Rh but not the second catalyst layer.

  In the exhaust gas purifying catalyst of the present invention, the first catalyst layer is supported on at least one wall surface of the partition walls partitioning the same cell, and the second catalyst layer is supported on the other wall surface. With this configuration, for example, catalyst metals that are likely to be alloyed, catalyst metals and carriers that are likely to be solid-solved with each other, and carriers and catalyst metals on which the supported catalyst metals are likely to grow grains can be contained in the same cell. Can be carried on different parts. As a result, grain growth and alloying of the supported noble metal and the like can be suppressed, and the catalytic performance (specifically, durability and purification performance) of the exhaust gas purification catalyst is improved.

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

  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 a catalyst for exhaust gas purification for automobiles, the cell density is preferably in the range of 0.7 to 155 cpsi, and the partition wall thickness is 50 to 2000 μm, more preferably 200 to 1800 μm. 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.

  In the exhaust gas purifying catalyst of the present invention, the first catalyst layer is supported on at least one wall surface of the partition walls that partition the same cell. The second catalyst layer is supported on the other wall surface. At this time, it is preferable that the first catalyst layer mainly includes the first support and / or the first catalyst metal, and the second catalyst layer mainly includes the second support and / or the second catalyst metal.

The first carrier and the second carrier are not particularly limited as long as they are used, for example, as a porous oxide carrier supporting a noble metal in a conventional three-way catalyst. Specifically, CeO ₂ , ZrO ₂ , _{al 2 O 3, Nd 2 O} 3, La 2 O 3 and the like. Moreover, it is good to use noble metals, such as Pt, Pd, and Rh, as a 1st catalyst metal and a 2nd catalyst metal. In addition, at least one of an alkali metal element and an alkaline earth metal element as a NOx occlusion material, a metal oxide as an SOx adsorption component (hereinafter referred to as “addition component”) may be supported.

Here, FIG. 1 is an explanatory view schematically showing an example of the exhaust gas purifying catalyst of the present invention. The exhaust gas-purifying catalyst 1 shown in FIG. 1 has a cylindrical carrier substrate 2, and a plurality of cells having a square cross section partitioned by a plurality of partition walls 2 ′ penetrate in the axial direction of the carrier substrate 2. The catalyst layers are supported on the four wall surfaces that define the cell 2C, which is one of the plurality of cells. However, in the exhaust gas purifying catalyst of the present invention, at least one of the four wall surfaces (3 in FIG. 1). one of the walls) the first catalyst layer 2 _C1 is formed on the second catalyst layer 2 _C2 the components from the first catalyst layer 2 _C1 are different are formed on the remaining wall. At this time, the first catalyst layer 2 _C1 and the second catalyst layer 2 _C2 are present in the same cell 2C, but the components of both are not mixed. Therefore, for example, when an exhaust gas purification catalyst is used, the carrier and the catalyst metal, which are inconvenient when they react with each other to form an alloy or grow grains, are separated into the first catalyst layer 2 _C1 and the second catalyst layer 2 _C2 , It can be carried on a substrate.

That is, the first catalyst layer 2 _C1 and the second catalyst layer 2 _C2 are separately supported so that carriers and catalyst metals that are likely to react with each other and may reduce the catalyst performance due to the reaction do not coexist in the same catalyst layer. If used, the durability and purification performance of the exhaust gas purification catalyst can be improved. Specifically, when the first catalyst metal is Pt, CeO ₂ is used as the first support, and when the first catalyst metal is Pd, ZrO ₂ is used as the first support, so that Pt and Pd can be used. Grain growth can be suppressed. Moreover, when using Rh as a 1st catalyst metal, it is preferable to use Pd as a 2nd catalyst metal, and alloying with Rh and Pd can be suppressed. Further, when Rh is used as the first catalyst metal, it is possible to dissolve Rh in these second supports by using Al ₂ O ₃ , Nd ₂ O ₃ , La ₂ O ₃ as the second supports. Can be suppressed. In addition, since the additive component described above reduces the activity of the noble metal during use, the catalyst metal and the additive component are preferably separated and supported on each catalyst layer.

  The exhaust gas purifying catalyst of the present invention having the above-described configuration can be produced by the procedure described below. The exhaust gas purifying catalyst of the present invention is different from the conventional exhaust gas purifying catalyst in that a carrier base material is formed after coating each catalyst layer.

  When the carrier substrate is made of a plastically workable material, that is, when the partition wall is a metal carrier substrate made of metal or a ceramic carrier substrate made of ceramics that can be bent, a flat plate It is desirable to form a honeycomb body by alternately laminating the base material and the corrugated corrugated base material. When laminating a flat plate substrate and a corrugated plate substrate, the surface on which the first catalyst layer is supported faces the surface on which the second catalyst layer is supported. Laminate as follows. For example, the first catalyst layer is supported on the front and back surfaces of the flat plate base material, the second catalyst layer is supported on the front and back surfaces of the corrugated plate base material, and both are alternately laminated. A space (cell) defined by the flat plate substrate and the corrugated plate substrate is formed by a surface on which the first catalyst layer is supported and a surface on which the second catalyst layer is supported.

  Further, by joining and winding the belt-like flat plate base material and the corrugated plate base material, the flat plate base material and the corrugated plate base material are alternately laminated in the radial direction, and the cylindrical wound type carrier base It is good also as a material. The flat substrate has a first plane on which the first catalyst layer is supported, and a second plane on which the second catalyst layer is supported opposite to the first plane. The corrugated substrate has a first wavefront on which the first catalyst layer is supported and a second wavefront on the opposite side of the first wavefront and on which the second catalyst layer is supported. If the flat plate substrate and the corrugated plate substrate are joined and wound so that the first plane and the second wavefront face each other, the second plane and the first wavefront face each other. A space (cell) partitioned by the base material is formed by a surface on which the first catalyst layer is supported and a surface on which the second catalyst layer is supported. In addition, when using the ceramic which can be bent, it becomes a ceramic support base material with high intensity | strength by baking after lamination | stacking.

  Further, even when the carrier substrate is made of a material that cannot be plastically processed, that is, when it is a ceramic carrier substrate made of ordinary ceramics, the exhaust gas purifying catalyst of the present invention can be produced. It is. For example, the support substrate is cut in the direction penetrating the cell (or a part having the same shape as that of the cut is prepared in advance) to expose the wall surface of the cell, and the first catalyst layer and the inner wall surface of the cell are exposed. After supporting the second catalyst layer, the respective parts are joined. When joining, the joining may be performed so that the wall surface carrying the first catalyst layer and the wall surface carrying the second catalyst layer face each other.

  In each of the above production methods, the positions where the first catalyst layer and the second catalyst layer are supported may be appropriately selected according to the lamination method and the arrangement at the time of joining. As a method of supporting the first catalyst layer and the second catalyst layer, it is possible to support by a wash coat or a dip coat which is usually used when manufacturing an exhaust gas purifying catalyst. When the first carrier and the second carrier are supported, various vapor deposition methods may be used in addition to the above-described normal method. That is, the first carrier and the second carrier may be vapor deposited films formed on the wall surfaces. As a vapor deposition method, a film can be formed by a CVD method (chemical vapor deposition method) such as a plasma CVD method or a thermal CVD method, a PVD method (physical vapor deposition method) such as a vacuum vapor deposition method or sputtering. In particular, if the carrier substrate is a metal carrier substrate, it is desirable to use a thermal CVD method, and the adhesion between the metal substrate and the carrier is high. Cracking is reduced.

  In addition, the first catalyst metal and the second catalyst metal are usually adsorbed and supported using an aqueous solution used for producing an exhaust gas purifying catalyst, as well as a carrier substrate or a first carrier or second carrier by a coating method or a spray method. It can be carried by adhering to two carriers.

  The amount of the first catalyst layer and the second catalyst layer supported is not particularly limited and may be appropriately selected according to the purpose. When the first carrier and the second carrier are vapor-deposited films, the film thickness is 0. It is desirable to be 5 to 500 μm. If the thickness of the deposited film is in the above range, it is possible to achieve both the adhesion to the carrier substrate and the function as the carrier of the catalyst metal.

  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 with reference to FIGS.

[Example 1]
The exhaust gas purification catalyst of Example 1 is an exhaust gas purification catalyst using a stainless steel wound metal carrier base material. Below, the structure of the exhaust gas purifying catalyst of the present embodiment will be described together with the manufacturing method with reference to FIG. In addition, FIG. 2 is explanatory drawing which shows the manufacturing process of the catalyst for exhaust gas purification of a present Example in order. FIG. 2A and FIG. 2B are cross-sectional views in the thickness direction schematically showing a part of the strip-shaped flat substrate 3. FIG. 2C is a cross-sectional view showing a part of the strip-shaped flat substrate 3 and the strip-shaped corrugated plate substrate 3 ′ that are joined to each other. FIG. 2-D is a perspective view showing an exhaust gas purifying catalyst obtained by winding a joined body and a schematic view showing a part of a plurality of cells.

  The belt-like flat plate base material 3 is a flat and long belt-like stainless steel foil material (disclosed in JP-A-6-389). A ceria film 10 (first carrier 10) was formed on the surface (first flat surface 31) of the strip-shaped flat substrate 3 by a thermal CVD method. Moreover, the zirconia film | membrane 20 (2nd support | carrier 20) was formed into the back surface (2nd plane 32) of the strip | belt-shaped flat base material 3 facing away from the 1st plane 31 with the thermal CVD method. The ceria film 10 and the zirconia film 20 had a thickness of 12.5 μm. (See above, Fig. 2-A)

  Next, a dinitrodiammine platinum aqueous solution and a rhodium nitrate aqueous solution having a predetermined concentration were applied to the surface of the ceria film 10 (on the first plane 31 side). Similarly, a palladium nitrate aqueous solution having a predetermined concentration was applied to the surface of the zirconia film 20 (on the second plane 32 side). Then, the belt-shaped flat substrate 3 after coating is heated at 120 ° C. for 2 hours, dried and then fired at 500 ° C. for 1 hour, and Pt, Rh (first catalyst metal 11 is formed on the first flat surface 31 side of the belt-shaped flat substrate 3. , 12) was loaded with Pd (second catalytic metal 21) on the second plane 32 side. (See above, Fig. 2-B)

  Moreover, after carrying | supporting Pt, Rh, and Pd, except having continuously formed the corrugation located in the longitudinal direction of a stainless steel foil material at equal intervals, in the procedure similar to preparation of the strip | belt-shaped plate base material 3, Corrugated substrate 3 'was produced. The strip-shaped corrugated plate base 3 ′ includes a first wavefront 31 ′ supporting the first carrier 10 and the first catalyst metals 11 and 12, and a second wavefront 32 ′ supporting the second carrier 20 and the second catalyst metal 21. , Have. Since the peak portion on the first wavefront 31 'side and the peak portion on the second wavefront 32' side are parts that are brazed at the time of joining and winding, which will be described later, the supported carrier and the catalyst metal are each linear. Scraped off.

  Next, it arrange | positions so that the 1st plane 31 of the strip | belt-shaped flat base material 3 and 2nd wave surface 32 'of strip | belt-shaped corrugated board base material 3' may oppose, Were joined together to obtain a joined body obtained by joining the strip-like flat plate base material 3 and the strip-like corrugated plate base material 3 ′ (FIG. 2-C). This joined body was continuously wound in the direction of the arrow in FIG. 2C, and the strip-like flat plate base material 3 and the strip-like corrugated plate base material 3 'were alternately laminated in the radial direction. Thus, a cylindrical honeycomb body in which a plurality of cells were partitioned with the strip-shaped flat substrate 3 and the strip-shaped corrugated substrate 3 'as partition walls was obtained. The honeycomb body was accommodated in a cylindrical outer cylinder 30 to obtain an exhaust gas purifying catalyst using a wound metal carrier base material as a carrier base material. (See above, Fig. 2-D)

  As described above, the exhaust gas purifying catalyst of the present embodiment is a joined body in which the strip-like flat plate base material 3 and the strip-like corrugated plate base material 3 ′ are joined so that the first flat surface 31 and the second wave front 32 ′ face each other. Is wound, the strip-shaped flat plate substrate 3 and the strip-shaped corrugated plate substrate 3 ′ are alternately laminated in the radial direction. As a result, a layer is formed in which the first wave surface 31 ′ of the strip-shaped corrugated base 3 ′ and the second plane 32 of the strip-shaped flat base 3 are opposed to each other. Therefore, the exhaust gas purifying catalyst of the present embodiment is divided into a first plane 31 (shown by a solid line) and a second wavefront 32 ′ (shown by a dotted line) as schematically shown in FIG. And a plurality of cells 36 partitioned by a second plane 32 (shown by dotted lines) and a first wavefront 31 ′ (shown by solid lines).

  Each of the cells 33 and 36 includes a surface on which the first support 10 (ceria) and the first catalyst metals 11 and 12 (Pt, Rh) are supported, a second support 20 (zirconia) and a second catalyst metal 21 (Pd). And a surface on which is supported. That is, Pd and Rh, which are easily alloyed, are supported separately, and Pt is supported on a ceria carrier that is unlikely to sinter. Therefore, the exhaust gas purifying catalyst of the present embodiment is excellent in durability and exhaust gas purifying performance because grain growth and alloying of the catalyst metal are suppressed.

[Example 2]
The exhaust gas purifying catalyst of Example 2 is an exhaust gas purifying catalyst using a ceramic support substrate made of cordierite and having a square cell cross-sectional shape. Hereinafter, the configuration of the exhaust gas purifying catalyst of this embodiment will be described together with the manufacturing method with reference to FIG. In addition, FIG. 3 is explanatory drawing which shows the manufacturing process of the catalyst for exhaust gas purification of a present Example in order. 3A and 3B are cross-sectional views in the thickness direction schematically showing a part of the corrugated plate member 4. FIG. 3C is a cross-sectional view showing the arrangement of the plurality of corrugated plate members 4. FIG. 3-D is a perspective view showing the obtained exhaust gas-purifying catalyst.

  The corrugated plate member 4 is a plate-shaped member having a corrugated shape that is continuously bent so that two adjacent surfaces intersect at a right angle. The intersecting parts are located at equal intervals. A flat portion 45 is formed at a portion where two adjacent surfaces intersect. This flat part 45 is bonded to the flat part 45 of another corrugated plate member 4 in a later step, and a plurality of cells having a square cross-sectional shape are defined between the two corrugated plate members 4.

  A ceria film 10 (first carrier 10) was formed on the surface (first wall surface 41) of the corrugated plate member 4 by a thermal CVD method. A zirconia film 20 (second carrier 20) was formed on the back surface (second wall surface 42) of the corrugated member 4 facing away from the first wall surface 41 by a thermal CVD method. The ceria film 10 and the zirconia film 20 had a thickness of 12.5 μm. (See FIG. 3-A above)

  Next, a dinitrodiammine platinum aqueous solution and a rhodium nitrate aqueous solution having a predetermined concentration were applied to the surface of the ceria film 10 (on the first wall surface 41 side). Similarly, a palladium nitrate aqueous solution having a predetermined concentration was applied to the surface of the zirconia film 20 (on the second wall surface 42 side). Then, the corrugated plate member 4 after coating is heated at 120 ° C. for 2 hours, dried and then fired at 500 ° C. for 1 hour, and Pt, Rh (first catalyst metals 11, 12) are formed on the first wall surface 41 side of the corrugated plate member 4. ) Was supported on the second wall surface 42 side with Pd (second catalytic metal 21). In addition, since the support | carrier and catalyst metal which were carry | supported by each flat part 45 were adhere | attached in a next process, it shaved off (refer above FIG. 3-B).

  A plurality of corrugated plate members 4 were produced by the above procedure. And each corrugated plate member 4 is laminated | stacked and arrange | positioned so that the 1st wall surface 41 and the 2nd wall surface 42 of the corrugated plate member 4 may oppose, and the contact surface 450 which flat part 45 contact | abuts was adhere | attached ( FIG. 3-C). Thereafter, the shape of the outer peripheral surface was adjusted by processing to obtain an exhaust gas purifying catalyst using a ceramic carrier base material in which cells having a square cross section with the corrugated plate member 4 as a partition wall as a carrier base material. (See Fig. 3-D above)

  The exhaust gas purifying catalyst of the present embodiment has a plurality of cells 44 having a square cross section partitioned by a first wall surface 41 and a second wall surface 42. Each cell 44 has a surface on which the first support 10 (ceria) and the first catalyst metals 11 and 12 (Pt, Rh) are supported, and a second support 20 (zirconia) and the second catalyst metal 21 (Pd). And is made of a surface. For this reason, the exhaust gas purifying catalyst of the present embodiment is excellent in durability and exhaust gas purifying performance because grain growth and alloying of the catalyst metal are suppressed as in the first embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS It is explanatory drawing which shows an example of the exhaust gas purification catalyst of this invention, Comprising: It is the schematic diagram (right figure) of an exhaust gas purification catalyst, and the radial direction sectional drawing (left figure) of the principal part of an exhaust gas purification catalyst. . It is explanatory drawing which shows the manufacturing process of the catalyst for exhaust gas purification of Example 1 in order, Comprising: FIG. 2-A and FIG. 2-B are thickness direction sectional drawings which show typically a part of strip | belt-shaped flat base material, FIG. 2-C is a cross-sectional view of the strip-shaped flat plate base material and the strip-shaped corrugated plate base material joined together, and FIG. 2-D is a perspective view showing the exhaust gas-purifying catalyst obtained by winding the joined body and the cell. It is a schematic diagram which shows a shape. FIGS. 3A and 3B are explanatory views sequentially showing a manufacturing process of the exhaust gas purifying catalyst of Example 2, and FIGS. 3A and 3B are thickness direction cross-sectional views schematically showing a part of the corrugated plate member. 3-C is a sectional view showing the arrangement of a plurality of corrugated plate members, and FIG. 3-D is a perspective view showing the obtained exhaust gas-purifying catalyst.

Explanation of symbols

2 ′: partition wall 2C: cell 2 _C1 : first catalyst layer 2 _C2 : second catalyst layer 3: strip-shaped plate base material (partition wall) 3 ′: strip-shaped corrugated plate substrate (partition wall)
33, 36: Cell 4: Corrugated plate member (partition) 44: Cell 10: First carrier (ceria film) 11, 12: First catalyst metal (Pt, Rh)
20: Second support (zirconia membrane) 21: Second catalyst metal (Pd)

Claims (6)

A carrier substrate having a plurality of cells partitioned by partition walls and penetrating in a predetermined direction;
A first catalyst layer supported on at least one of the wall surfaces of the partition walls defining the same cell;
A second catalyst layer supported on another wall surface;
An exhaust gas purifying catalyst characterized by comprising:   2. The exhaust gas purifying catalyst according to claim 1, wherein the first catalyst layer includes a first support and / or the first catalyst metal, and the second catalyst layer includes a second support and / or the second catalyst metal. The first support is ceria, the first catalytic metal is Pt and / or Rh;
The exhaust gas purifying catalyst according to claim 2, wherein the second carrier is zirconia and the second catalytic metal is Pd.   The exhaust gas purifying catalyst according to claim 2, wherein the first carrier and / or the second carrier is a vapor deposition film formed on the wall surface.   2. The exhaust gas purifying catalyst according to claim 1, wherein the carrier substrate is a ceramic carrier substrate made of ceramics or a metal carrier substrate made of metal. The metal carrier substrate is
A flat plate substrate having a first plane on which the first catalyst layer is supported, and a second plane on which the second catalyst layer is supported opposite to the first plane;
A corrugated base material having a corrugated first wavefront on which the first catalyst layer is supported and a second wavefront facing the first wavefront and on which the second catalyst layer is supported; Mochi,
The exhaust gas according to claim 5, which is a wound metal carrier base material obtained by joining and winding the flat plate base material and the corrugated base material so that the first flat surface and the second wave front face each other. Purification catalyst.

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