Catalytic converter

Abstract

Catalytic converter comprising: a substrate having a cell structure through which exhaust gas flows; and a catalyst layer formed on a cell wall surface of the substrate, wherein the catalyst layer includes a lower catalyst layer and an upper catalyst layer, wherein the lower catalyst layer is formed on a surface of the substrate and the upper catalyst layer is formed on a surface of the lower catalyst layer, the upper catalyst layer comprises at least one zirconium oxide support with rhodium supported thereon and two types of cerium oxide-zirconia-based composite oxides with different specific surface areas, wherein the cerium oxide-zirconia-based composite oxides each have no rhodium supported thereon, the lower catalyst layer comprises an alumina support with platinum supported thereon and a ceria-zirconia-based composite oxide, and of the two types of ceria-zirconia-based composite oxides with different specific surface areas the cerium oxide-zirconia-based composite oxide with a larger specific surface area has a specific surface area greater than or equal to 40 m 2 / g and is contained in the upper catalyst layer in an amount greater than or equal to 12 g / l, and the cerium oxide-zirconia-based composite oxide with a smaller specific surface area has a specific surface area less than or equal to 4 m 2 / g and is contained in the upper catalyst layer in an amount greater than or equal to 8 g / l.

Description

PRIORITY CLAIM

[0001] This application claims priority from Japanese patent application JP 2015-059384 filed on March 23, 2015, the contents of which are hereby incorporated by reference into this application. BACKGROUNDTechnical field

[0002] The present invention relates to a catalytic converter which is fixedly accommodated in a pipe of an exhaust system for exhaust gas. Current state of the art

[0003] A wide range of industries worldwide have made diverse efforts to reduce environmental impacts and pollution. In particular, in the automotive industry, efforts have been made to promote the popularization of not only fuel-efficient gasoline vehicles, but also so-called environmentally friendly vehicles, such as hybrid vehicles and electric vehicles, and to further improve the performance of such vehicles. In addition to the development of such environmentally friendly vehicles, active research has also been conducted on an exhaust gas purification catalyst for purifying exhaust gas emitted from an engine. An exhaust gas purification catalyst includes an oxidation catalyst, a three-way catalyst, a NOx storage / reduction catalyst, and the like.A noble metal catalyst, such as platinum (Pt), palladium (Pd), or rhodium (Rh), exhibits catalytic activity in the exhaust gas purification catalyst. The noble metal catalyst is typically used while supported on a carrier made of a porous oxide, such as alumina (Al2O3).

[0004] A catalytic converter for purifying exhaust gases is typically located in an exhaust system connecting a vehicle engine and a muffler. The engine can sometimes emit environmentally harmful substances such as CO, NO xand unburned HC and VOC. To convert such harmful substances into environmentally safe substances, exhaust gas is passed through a catalytic converter in which a catalyst layer with a supported noble metal catalyst, such as Rh, Pd or Pt, is arranged on the cell wall surface of a substrate, so that CO is converted into CO2 and NO x is converted into N2 and O2, while HC and VOC are burned to produce CO2 and H2O.

[0005] As a support on which a noble metal catalyst is supported, a cerium oxide-zirconia-based composite oxide (also referred to as a CeO2-ZrO2 solid solution, a CZ material, and the like) can be used. This is also referred to as a promoter and is an essential component of the aforementioned three-way catalyst for simultaneously removing CO and NO. xand HC, which are harmful components in the exhaust gas. Examples of the essential component of the promoter include CeO2. CeO2 has a property in that its oxidation number changes depending on the partial pressure of oxygen in the exhaust gas to which CeO2 is exposed, for example, to Ce 3+ or Ce 4+ changes, and has the function of absorbing and releasing oxygen, as well as the function of storing oxygen (OSC: oxygen storage capacity) to compensate for the deficiency and excess of electrical charges. Furthermore, CeO2 can cushion and mitigate fluctuations in the exhaust gas atmosphere and keep the air / fuel ratio at approximately the theoretical level to maintain a purification window of the three-way catalyst.

[0006] Incidentally, the question of how to reduce the amount of precious metal catalyst used in the aforementioned three-way catalyst is an important factor to consider from a cost competitiveness perspective. However, if the amount of precious metal catalyst in a three-way catalyst is significantly reduced, the catalytic activity also decreases significantly. This leads to significantly reduced OSC performance and low-temperature activity, as well as significantly reduced NO x -Cleaning performance in a high-temperature environment, etc. This is because a significantly reduced amount of a noble metal catalyst results in a significantly reduced number of active sites, and a significantly reduced number of catalytic reaction sites results in significantly reduced cleaning performance.

[0007] Of the noble metal catalysts such as Pt, Pd and Rh, which are particularly used for a three-way catalyst, Rh has the most outstanding NO x -purification performance, but is traded at the highest market price per unit weight. It is known that higher OSC performance is achieved when Rh is supported on a carrier containing cerium oxide (ceria). However, it is also known that increasing the amount of cerium oxide in the carrier increases NO x -purification performance, which is a characteristic property of Rh. Thus, when Rh is used as a noble metal catalyst for a three-way catalyst, the production of a catalyst with high OSC performance as well as NO x -Cleaning performance of optimal three-way catalyst is an urgent problem to be solved in the technical field.

[0008] In this regard, Patent Document 1 discloses an exhaust gas purification catalyst comprising a first oxygen storage material on which no noble metal is supported and which has a pyrochlore phase-type regular array structure; and a second oxygen storage material having a higher oxygen storage rate and a lower oxygen storage capacity than the first oxygen storage material, wherein a platinum group noble metal is supported on the second oxygen storage material. According to such an exhaust gas purification catalyst, it is possible to provide an exhaust gas purification catalyst having a high NO x -Cleaning performance after continuous use.

[0009] Meanwhile, Patent Document 2 discloses an exhaust gas purification catalyst having a first catalyst layer and a second catalyst layer sequentially formed on a support substrate, the first catalyst layer having rhodium supported thereon, and the second catalyst layer having platinum and palladium supported thereon, wherein the ratio of the supported amount of palladium (y) to the supported amount of platinum (x) (y / x; molar ratio) satisfies 0 < y / x ≤ 1.0. According to such an exhaust gas purification catalyst, it is possible to provide an exhaust gas purification catalyst of a NO x -storage / reduction type that provides more outstanding NO x -Cleaning performance.

[0010] If rhodium is supported on cerium oxide, metallization of the rhodium is prevented and thus the NO x-Cleaning performance is reduced, as described above. However, if the amount of cerium oxide is increased to increase the oxygen storage capacity, pressure loss is increased.

[0011] That is, even when using the exhaust gas purification catalyst disclosed in Patent Document 1 or 2, it is unclear whether a catalyst with an OSC performance that is superior to that with NO x -Cleaning performance of excellent catalytic converters can be provided or not.

[0012] Furthermore, Patent Document 3 discloses a generic exhaust gas purification catalyst. RELATED PRIOR ART DOCUMENTS Patent documents Patent Document 1: JP 2012-024701 A Patent Document 2: JP 2010-201284 A Patent document 3: EP 1 174 174 A1 SUMMARY

[0013] The present invention has been made in view of the above problems, and an object of the present invention is to provide a catalytic converter having excellent OSC performance and NO x -Cleaning performance.

[0014] To achieve the above object, a catalytic converter according to the present invention includes a substrate having a cell structure through which exhaust gas flows, and a catalyst layer formed on a cell wall surface of the substrate. The catalyst layer includes a lower catalyst layer and an upper catalyst layer, wherein the lower catalyst layer is formed on a surface of the substrate and the upper catalyst layer is formed on a surface of the lower catalyst layer. The upper catalyst layer includes at least one zirconia support with rhodium supported thereon and two types of ceria-zirconia-based composite oxides having different specific surface areas, each of the ceria-zirconia-based composite oxides having no rhodium supported thereon.The lower catalyst layer contains an alumina support with platinum supported thereon and a ceria-zirconia-based composite oxide.

[0015] The catalytic converter of the present invention is characterized by comprising a catalyst layer having a two-layer structure of a lower catalyst layer formed on the surface of a substrate and an upper catalyst layer formed thereon, wherein the upper catalyst layer includes at least one zirconia support with rhodium supported thereon and two types of ceria-zirconia-based composite oxides having different specific surface areas, each of the ceria-zirconia-based composite oxides having no rhodium supported thereon. The inventors have verified that the activity of rhodium is increased and thus the NO x-Purification rate is increased because rhodium is supported on a carrier that does not contain cerium oxide. In addition, since the upper catalyst layer contains two types of cerium oxide-zirconia-based composite oxides with different specific surface areas, more specifically, since the upper catalyst layer contains a cerium oxide-zirconia-based composite oxide with a large specific surface area, the NO x -purification rate is increased, while the fact that the upper catalyst layer also contains a ceria-zirconia-based composite oxide with a small specific surface area can suppress an increase in pressure loss.

[0016] In addition to cordierite, which is made of a composite oxide of magnesium oxide, aluminum oxide, and silicon dioxide, ceramic materials such as silicon carbide, or materials other than ceramic materials, such as metal materials, can be used as the substrate with a cellular structure. Furthermore, the substrate can have a so-called honeycomb structure with a series of cells whose lattice contour is a square, hexagon, octagon, or the like.

[0017] The lower catalyst layer formed on the cell wall surface of the substrate contains alumina (Al2O3) which is a carrier and platinum (Pt) supported thereon and further contains a ceria-zirconia-based composite oxide (CeO2-ZrO2 composite oxide).

[0018] Meanwhile, as described above, the upper catalyst layer may contain zirconia (ZrO2) which is a carrier and rhodium (Rh) supported thereon, and also contains two kinds of ceria-zirconia-based composite oxides (CeO2-ZrO2 composite oxides) with different specific surface areas (SSA), and may further contain alumina (Al2O3).

[0019] In the catalytic converter according to the present invention, of the two types of ceria-zirconia-based composite oxides having different specific surface areas, the ceria-zirconia-based composite oxide having a larger specific surface area has a specific surface area greater than or equal to 40 m 2 / g and is contained in the upper catalyst layer in an amount greater than or equal to 12 g / l, while the cerium oxide-zirconia-based composite oxide with a smaller specific surface area has a specific surface area less than or equal to 4 m 2 / g and is also present in the upper catalyst layer in an amount greater than or equal to 8 g / l.

[0020] In the present invention, of the two kinds of ceria-zirconia-based composite oxides having different specific surface areas, the “ceria-zirconia-based composite oxide having a larger specific surface area” has a specific surface area greater than or equal to 40 m 2 / g, while the “ceria-zirconia-based composite oxide with a smaller specific surface area” has a specific surface area less than or equal to 4 m 2 / g.

[0021] The inventors have found that it is possible to suppress pressure loss while maintaining excellent OSC performance when the ceria-zirconia-based composite oxide having a larger specific surface area is contained in an amount greater than or equal to 12 g / L in the upper layer and the ceria-zirconia-based composite oxide having a smaller specific surface area is also contained in the upper catalyst layer in an amount greater than or equal to 8 g / L.

[0022] In addition, the zirconium oxide support with rhodium supported thereon is preferably contained in an amount of 40 g / l in the upper catalyst layer.

[0023] Increasing the amount of rhodium-supported zirconia support improves the low-temperature activity, in particular. However, if the amount of rhodium support is increased too much, pressure loss increases. Therefore, the preferable amount of rhodium-supported zirconia support is defined as 40 g / L from the perspective of achieving both excellent low-temperature activity and suppressed pressure loss.

[0024] Furthermore, in another embodiment of the catalytic converter according to the present invention, the upper catalyst layer is formed within 80% of the total length of the substrate from an end of the substrate on the downstream side of the exhaust gas flow direction, while the lower catalyst layer is formed within 80% of the total length of the substrate from an end of a substrate on the upstream side of the exhaust gas flow direction. Note that each of the upper catalyst layer and the lower catalyst layer is preferably formed within 65 to 95% of the total length of the substrate.

[0025] The catalytic converter of the present invention preferably comprises a cordierite honeycomb support with excellent heat shock resistance. Alternatively, the catalytic converter may be an electrically heated converter (EHC). The electrically heated catalytic converter of this type comprises a honeycomb catalyst and a pair of electrodes attached thereto. When current is supplied to the pair of electrodes to heat the honeycomb catalyst, the activity of the honeycomb catalyst is increased, so that exhaust gas passing through the honeycomb catalyst is detoxified. When such a converter is applied to an exhaust system for exhaust gas that connects a vehicle engine and a muffler, it is possible to purify exhaust gas at room temperature and also purify exhaust gas at a cold temperature by activating the catalyst through electrical heating.

[0026] As can be understood from the above description, the catalytic converter of the present invention comprises a catalyst layer having a two-layer structure of a lower catalyst layer formed on the surface of a substrate and an upper catalyst layer formed thereon. The upper catalyst layer includes at least one zirconia support with rhodium supported thereon and two types of ceria-zirconia-based composite oxides having different specific surface areas, each of which does not have rhodium supported thereon. Thus, it is possible to obtain a catalytic converter with excellent OSC performance and NO x -Provide cleaning performance while suppressing pressure loss. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view of a catalytic converter of the present invention. Fig. Figure 2 is a partially enlarged view of a cell. Fig. 3 is a longitudinal sectional view illustrating one embodiment of a catalyst layer. Fig. Figure 4 is a graph showing the experimental results for determining the relationship between the Ce concentration in the Rh support and the NO x -Cleaning rate shows. Fig. Figure 5 is a graph showing the experimental results for determining the relationship between the added amounts of a high SSA composite oxide and a low SSA composite oxide in the upper catalyst layer and the NO x -Cleaning rate shows. Fig. Figure 6 is a graph showing the experimental results for determining the relationship between the amount of Rh carrier and a pressure loss and the relationship between the amount of Rh carrier and the purification temperature of low-temperature active NO x shows. DETAILED DESCRIPTION OF THE EMBODIMENT(S)

[0027] Embodiments of a catalytic converter of the present invention will be described below with reference to the drawings. The catalytic converter shown in the drawings has an upper catalyst layer formed within 80% of the total length of a substrate from an end of the substrate on the downstream side of the exhaust gas flow direction, and also has a lower catalyst layer formed within 80% of the total length of the substrate from an end of the substrate on the upstream side of the exhaust gas flow direction. Note that the length over which each of the upper catalyst layer and the lower catalyst layer is formed is preferably within 65 to 95% of the total length of the substrate. (Exhaust system for exhaust gas)

[0028] First, an exhaust system for exhaust gas in which the catalytic converter of the present invention is incorporated will be briefly described. An exhaust system for exhaust gas to which the catalytic converter of the present invention is applied has a configuration in which an engine, a catalytic converter, a three-way catalytic converter, a sub-muffler, and a main muffler are arranged and connected to each other via system piping, so that exhaust gas generated in the engine flows through each component via the system piping and is then exhausted. Next, an embodiment of the catalytic converter will be described. (Embodiment of the catalytic converter)

[0029] Fig. Figure 1 is a schematic view of the catalytic converter of the present invention. Fig. Figure 2 is a partially enlarged view of a cell. Fig.3 is a longitudinal sectional view illustrating one embodiment of a catalyst layer.

[0030] One in Fig. 1 generally includes a cylindrical substrate 1 having a series of cells and a catalyst layer 3 formed on the surface of a cell wall 2 of each cell, as shown in Fig. 2 shown.

[0031] Here, examples of the substrate 1 include cordierite made of a composite oxide of magnesium oxide, aluminum oxide, and silicon dioxide, ceramic materials such as silicon carbide, and materials other than ceramic materials such as metal materials.

[0032] The substrate 1 has a honeycomb structure with a series of cells whose lattice contours are quadrilaterals, hexagons, octagons, or the like. Exhaust gas that has entered a cell at one end of the substrate 1 on the upstream side (Fr side) of the exhaust flow direction flows through the substrate 1 and is thereby purified. The purified exhaust gas then flows out of one end of the substrate 1 on the downstream side (Rr side) of the exhaust flow direction (x direction).

[0033] Next, an embodiment of the catalyst layer will be described with reference to Fig. 2 and Fig. 3 described.

[0034] The Fig. 2 and Fig. 3 includes a lower catalyst layer 4 formed on the surface of a cell wall 2 and an upper catalyst layer 5 formed on the surface of the lower catalyst layer 4.

[0035] The lower catalyst layer 4 is formed in the range of 80% of the total length of the substrate 1 from the end of the substrate 1 on the upstream side Fr of the exhaust gas flow direction, while the upper catalyst layer 5 is formed in the range of 80% of the total length of the substrate 1 from the end of the substrate 1 on the downstream side Rr of the exhaust gas flow direction.

[0036] The lower catalyst layer 4 contains an aluminum oxide support (Al2O3) with platinum (Pt) supported thereon and a cerium oxide-zirconia-based composite oxide (CeO2-ZrO2 composite oxide).

[0037] Meanwhile, the upper catalyst layer 5 contains a zirconia support (ZrO2) with rhodium (Rh) supported thereon and two kinds of ceria-zirconia-based composite oxides (CeO2-ZrO2 composite oxides) with different specific surface areas and further contains alumina (Al2O3).

[0038] Regarding the two types of ceria-zirconia-based composite oxides with different specific surface areas, the specific surface area of ​​the ceria-zirconia-based composite oxide with a larger specific surface area is greater than or equal to 40 m 2 / g, while the specific surface area of ​​the ceria-zirconia-based composite oxide with a smaller specific surface area less than or equal to 4 m 2 / g is.

[0039] In the upper catalyst layer 5, rhodium (Rh) is supported only on zirconium oxide (ZrO2), which does not contain cerium oxide. Such a structure can reduce the NO x -Improve the cleaning rate. This was verified by the experiments described below.

[0040] Since the upper catalyst layer 5 contains a ceria-zirconia-based composite oxide with a large specific surface area and a ceria-zirconia-based composite oxide with a small specific surface area, specifically, since the upper catalyst layer 5 contains a composite oxide with a large specific surface area in an amount greater than or equal to 12 g / L and also contains a composite oxide with a small specific surface area in an amount greater than or equal to 8 g / L, it is possible to suppress pressure loss while maintaining excellent OSC performance. This was also verified by the experiments described below.

[0041] In addition, the zirconium oxide support with rhodium supported thereon is contained in the upper catalyst layer 5 in an amount of 40 g / l.

[0042] Increasing the amount of zirconia support with rhodium on it significantly improves low-temperature activity. However, if the amount of zirconia support is increased too much, pressure loss increases. Therefore, the amount of zirconia support with rhodium on it is defined as 40 g / L from the perspective of achieving both excellent low-temperature activity and suppressed pressure loss. This was also verified by the experiments described below.

[0043] (Experiments to determine the relationship between the Ce concentration in the Rh carrier and the NO x -Purification rate, experiments to determine the relationship between the added amounts of a high SSA composite oxide and a low SSA composite oxide in the upper catalyst layer and the NO x-Purification rate and experiments to determine the relationship between the amount of Rh carrier and pressure loss as well as the relationship between the amount of Rh carrier and the purification temperature of low-temperature active NO x )

[0044] The inventors conducted experiments to determine the relationship between the Ce concentration in the Rh layer and the NO x -purification rate, experiments to determine the relationship between the added amounts of a high SSA composite oxide and a low SSA composite oxide in the upper catalyst layer and the NO x -purification rate, and experiments to determine the relationship between the amount of Rh carrier and pressure loss as well as the relationship between the amount of Rh carrier and the purification temperature of low-temperature active NO xto determine. Reference Examples 1-12, Example 1, and Comparative Examples 1-4 were prepared using the procedures described below. <Referenzbeispiel 1>

[0045] In Reference Example 1, the lower catalyst layer contains Pt as a catalyst (Pt(0.2) / Al2O3(25) + CZ(30)), and the upper catalyst layer contains Rh as a catalyst (Rh(0.12) / Ce-Zr composite oxide A(40) + Al2O3(20)). The unit of the numerical values ​​in the parentheses is g / L. Using nitric acid-Pt, Pt / Al2O3 (i.e., Material 1) in which Pt is supported on Al2O3 was first prepared. Impregnation was used as a method to cause Pt to be supported on Al2O3. Next, a slurry 1 was prepared by pouring Material 1, a CZ material, and an Al2O3-based binder into distilled water with stirring. Further, the prepared slurry 1 was poured into a substrate, and unnecessary portions were removed with a blower, so that the wall surface of the substrate was coated with the slurry 1.This time, the coating material for the Pt layer was prepared such that the Pt content, the content of Material 1, and the content of CZ material were 0.2 g / L, 25 g / L, and 30 g / L, respectively, based on the volume of the substrate. Finally, moisture was dried with a dryer maintained at 120°C for 2 hours, and firing was performed with an electric furnace at 500°C for 2 hours. Similarly, using nitric acid-Rh, a Rh / Ce-Zr composite oxide A (i.e., Material 2) in which Rh is supported on a Ce-Zr composite oxide A was prepared. Here, the content of Ce in the Ce-Zr composite oxide A was 30 mass%. Next, a slurry 2 was prepared by pouring the material 2, Al2O3 and an Al2O3-based binder into distilled water with stirring such that the materials were suspended in the distilled water.The prepared Slurry 2 was poured into the coated substrate, and the excess was removed with a blower, so that the wall surface of the substrate was coated with Slurry 2. This time, the coating material for the Rh layer was prepared such that the Rh content, the content of Material 2, and the Al2O3 content were 0.12 g / L, 40 g / L, and 20 g / L, respectively, based on the volume of the substrate. Finally, moisture was removed by drying with a dryer maintained at 120°C for two hours, and firing was performed with an electric furnace at 500°C for two hours. <Referenzbeispiel 2, 3 und 4>

[0046] In each of Reference Examples 2, 3, and 4, the lower catalyst layer contains Pt as a catalyst (Pt(0.2) / Al2O3(25) + CZ(30)), and the upper catalyst layer contains Rh as a catalyst (Rh(0.12) / Ce-Zr composite oxide B,C,D(40) + Al2O3(20)). A slurry was prepared by changing the specifications of the Rh support (Material 2) used for Slurry 2 in Reference Example 1, and then coating, drying, and firing were performed. Regarding the catalyst, the process was not changed except for changing the composition of Material 2 shown in Reference Example 1. In Reference Example 2, a Ce-Zr composite oxide containing a Ce concentration of 0 wt% (B) was used; In Reference Example 3, a Ce-Zr composite oxide containing a Ce concentration of 20 Ma%(C) was used; and in Reference Example 4, a Ce-Zr composite oxide containing a Ce concentration of 60 Ma%(D) was used. <Referenzbeispiel 5, 6, 7 und 8>

[0047] In each of Reference Examples 5, 6, 7, and 8, the lower catalyst layer contains Pt as a catalyst (Pt(0.2) / Al2O3(25) + CZ(30)), and the upper catalyst layer contains Rh as a catalyst (Rh layer Rh(0.12) / Ce-Zr composite oxide B(40) + Al2O3(20) + Ce-Zr low SSA composite oxide(4,8,12,16)). A slurry was prepared by further pouring each amount of a low SSA Ce-Zr composite oxide into Slurry 2 in Reference Example 1, and then coating, drying, and firing were performed. Regarding the catalyst, the process was not changed except that a low SSA Ce-Zr composite oxide was added in the step of preparing Slurry 2 in Reference Example 1.In Reference Example 5, 4 mass parts of a low SSA Ce-Zr composite oxide were added; in Reference Example 6, 8 mass parts of a low SSA Ce-Zr composite oxide were added; in Reference Example 7, 12 mass parts of a low SSA Ce-Zr composite oxide were added; and in Reference Example 8, 16 mass parts of a low SSA Ce-Zr composite oxide were added. Note that the "low SSA Ce-Zr composite oxide" herein means a material initially having an SSA of less than or equal to 4 μm. 2 / g has. <Referenzbeispiel 9, 10, 11 und 12>

[0048] In each of Reference Examples 9, 10, 11, and 12, the lower catalyst layer contains Pt as a catalyst (Pt(0.2) / Al2O3(25) + CZ(30)), and the upper catalyst layer contains Rh as a catalyst (Rh(0.12) / Ce-Zr composite oxide B(40) + Al2O3(20) + Ce-Zr high SSA composite oxide (4, 8, 12, 16)). A slurry 2 was prepared by further pouring each amount of a high SSA Ce-Zr composite oxide into the slurry 2 in Reference Example 1, and then coating, drying, and firing were performed. Regarding the catalyst, the process was not changed except that a high SSA Ce-Zr composite oxide was added in the step of preparing the slurry 2 in Reference Example 1.In Reference Example 9, 4 parts by mass of a high SSA Ce-Zr composite oxide were added; in Reference Example 10, 8 parts by mass of a high SSA Ce-Zr composite oxide were added; in Reference Example 11, 12 parts by mass of a high SSA Ce-Zr composite oxide were added; and in Reference Example 12, 16 parts by mass of a high SSA Ce-Zr composite oxide were added. Note that the "high SSA Ce-Zr composite oxide" herein means a material initially having an SSA of greater than or equal to 40 μm. 2 / g has. <Beispiel 1 und Vergleichsbeispiel 1, 2, 3 und 4>

[0049] In each of Example 1 and Comparative Examples 1, 2, 3 and 4, the lower catalyst layer contains Pt as a catalyst (Pt(0.2) / Al2O3(25) + CZ(30)), and the upper catalyst layer contains Rh as a catalyst (Rh(0.12) / Ce-Zr composite oxide B(x) + Al2O3(20) + Ce-Zr composite oxide low SSA(8) + Ce-Zr composite oxide high SSA(12)). A slurry was prepared by changing the Rh support of Slurry 2 in Reference Example 1 to a Ce-Zr composite oxide B containing 0 mass% Ce, and pouring 8 mass parts of a low SSA Ce-Zr composite oxide and 12 mass parts of a high SSA Ce-Zr composite oxide into Slurry 2. Coating, drying, and firing were then performed. Regarding the catalyst, the process was not changed except for changing the material used in the step of preparing Slurry 2 in Reference Example 1.In Comparative Example 1, 24 mass parts of a Ce-Zr composite oxide B were added; in Comparative Example 2, 32 mass parts of a Ce-Zr composite oxide B were added; in Example 1, 40 mass parts of a Ce-Zr composite oxide B were added; in Comparative Example 3, 48 mass parts of a Ce-Zr composite oxide B were added; and in Comparative Example 4, 56 mass parts of a Ce-Zr composite oxide B were added. <beurteilungsmethode>

[0050] A 4.3-liter 8-cylinder V-shaped gasoline engine was used, and the bed temperature of a catalyst on the downstream side was set to 950°C, so that a cycle including recirculation, fuel cut, rich and lean per minute as one condition was carried out for 50 hours.

[0051] To the NO x To measure the purification rate under fluctuating A / F ratio conditions, a catalyst layer containing an aged catalytic converter was mounted on the upstream side, and the purification rate was measured as the atmosphere of the incoming gas was periodically switched between the rich and lean sides of the A / F ratio. In addition, to measure the NO x To determine the purification rate under the constant rich condition, an aged catalytic converter was mounted, and the purification rate was measured when the atmosphere of the incoming gas was continuously kept on the rich side of the A / F ratio. Furthermore, to measure a pressure loss, a pressure loss was measured when air was added to a catalyst at a constant flow rate (6 m 3 / minute). Furthermore, with regard to the NO x -Light-off or starting performance NO x -T50 measured the temperature that had risen from a low temperature in the rich environment and at which the NOx purification rate reached 50%.

[0052] Table 1 below shows the materials used herein. [Table 1] Area Name of the material Manufacturer composition Upper catalyst layer (Rh layer) Ce-Zr composite oxide TOYOTA MOTOR CORPORATION CeO2 (x mass%),ZrO2 (100-x mass%) Ce-Zr composite oxide high, low SSA TOYOTA MOTOR CORPORATION Al2O3 (30 Ma%), CeO2 (20 Ma%), ZrO2 (40 Ma%), Y2O3 (4 Ma%), Nd2O3 (2 Ma%),La2O3 (4 Ma%) aluminum oxide Sasol Al2O3 (99 mass%), La2O3 (1 mass%) Lower catalyst layer (Pt layer) CZ material Rhodia CeO2 (30Ma%), ZrO2 (60Ma%), La2O3 (5Ma%), Y2O3 (5Ma%) aluminum oxide Sasol Al2O3 (99 wt%), La2O3 (1 Ma%) <Ergebnisse des Experiments>

[0053] The Fig. 4 to 6 show the experimental results. Fig. 4 a graph showing the experimental results for determining the Ce concentration in the Rh carrier and the NO x cleaning rate. In addition, Fig. 5 a graph showing the experimental results for determining the relationship between the added amounts of a high SSA composite oxide and a low SSA composite oxide in the upper catalyst layer and the NO x -cleaning rate. Furthermore, Fig. 6 a graph showing the experimental results for determining the relationship between the amount of Rh carrier and a pressure loss as well as the relationship between the amount of Rh carrier and the NO x -starting performance.

[0054] Fig. Figure 4 confirms that, with respect to the Ce concentration in the Rh support, the catalytic activity is highest when the concentration is 0%. Accordingly, a Ce concentration of 0% is chosen in Reference Example 2. It is assumed that the activity of Rh is improved with a reduction in the amount of bases in the Rh support.

[0055] In addition, Fig. 5 From the difference compared to the required OSC performance, it can be confirmed that the OSC performance of a low-SSA OSC material is saturated when added in an amount greater than or equal to 8 g / L, and the OSC performance of a high-SSA OSC material is also saturated when added in an amount greater than or equal to 12 g / L, and thus that the low-SSA OSC material is preferably added in an amount greater than or equal to 8 g / L, and the high-SSA OSC material is preferably added in an amount greater than or equal to 12 g / L. It should be noted that the low-SSA OSC material is useful for reducing pressure drop because of its small volume relative to its weight. However, with the low-SSA OSC material, the oxygen release capacity of the OSC performance is limited.Thus, the use of a high SSA OSC material can compensate for the insufficient OSC performance of the low SSA OSC and achieve both excellent OSC performance and pressure loss reduction.

[0056] In addition, Fig. 6 confirm that increasing the amount of Rh carrier improves starting performance but increases pressure loss.

[0057] Increasing the amount of Rh support is beneficial for increasing Rh dispersibility, and the starting performance improves with an increase in the amount of Rh support. However, since pressure loss tends to increase, a Rh support amount of 40 g / L can be defined as the optimal amount of Rh support, as demonstrated by Example 1.

[0058] It should be noted that the Fig. 6 do not suggest that Comparative Examples 1-4 should be excluded from the scope of the present invention, but rather that Example 1 has an optimal amount of the Rh support that can achieve both excellent OSC performance and a reduction in pressure loss.

[0059] Although the embodiments of the present invention have been described in detail with reference to the drawings, specific structures are not limited thereto, and any design changes that may arise within the spirit and scope of the present invention are all included in the present invention. DESCRIPTION OF SYMBOLS 1 substrate 2 cell wall 3 Catalyst layer 4 Lower catalyst layer 5 Upper catalyst layer 10 Catalytic converter Fr Upstream side of the exhaust gas flow direction Rr Downstream side of the exhaust gas flow direction< / beurteilungsmethode>

Claims

[1] Catalytic converter comprising: a substrate having a cell structure through which exhaust gas flows; and a catalyst layer formed on a cell wall surface of the substrate, wherein the catalyst layer includes a lower catalyst layer and an upper catalyst layer, wherein the lower catalyst layer is formed on a surface of the substrate and the upper catalyst layer is formed on a surface of the lower catalyst layer, the upper catalyst layer comprises at least one zirconium oxide support with rhodium supported thereon and two types of cerium oxide-zirconia-based composite oxides with different specific surface areas, wherein the cerium oxide-zirconia-based composite oxides each have no rhodium supported thereon, the lower catalyst layer comprises an alumina support with platinum supported thereon and a ceria-zirconia-based composite oxide, and of the two types of ceria-zirconia-based composite oxides with different specific surface areas the cerium oxide-zirconia-based composite oxide with a larger specific surface area has a specific surface area greater than or equal to 40 m 2 / g and is contained in the upper catalyst layer in an amount greater than or equal to 12 g / l, and the cerium oxide-zirconia-based composite oxide with a smaller specific surface area has a specific surface area less than or equal to 4 m 2 / g and is contained in the upper catalyst layer in an amount greater than or equal to 8 g / l. [2] The catalytic converter according to claim 1, wherein the zirconia carrier having rhodium supported thereon is contained in the upper catalyst layer in an amount of 40 g / l.

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