JP5573710B2 - Exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to an exhaust gas purifying catalyst that removes harmful components contained in exhaust gas from engines such as automobiles and motorcycles, and more particularly to an exhaust gas catalyst that supports a different type of noble metal on the upstream side and downstream side of an exhaust gas passage. .

  In an exhaust gas purifying catalyst that purifies exhaust gas discharged from an engine such as an automobile, a catalyst layer is formed on the surface of the substrate. The catalyst layer includes a catalyst noble metal such as Pt, Pd, and Rh, and a carrier that supports the catalyst noble metal.

  As the exhaust gas purification catalyst, a three-way catalyst that simultaneously purifies HC, CO, and NOx is mainly used. However, in order to reduce material risk, reduction of the amount of the above-mentioned catalyst noble metal is required. At the same time, in recent years, exhaust gas regulations for automobiles have been strengthened globally, and there is a demand for improvement in catalyst performance in order to respond to the tightening regulations such as LEVIII and EURO6. HC contained in the exhaust gas is most frequently immediately after engine cold start, and in order to satisfy the above requirements, it is necessary to improve catalyst activity and catalyst warm-up at low temperatures. Development of exhaust gas purification technology that can realize these is required.

  Since the performance of catalyst precious metals and supports varies depending on the material, considering that there is a gradient in the temperature and exhaust gas components at each part of the catalyst so that the properties of the material can be effectively exhibited, Zone coat catalysts in which different materials are arranged on the upstream side and the downstream side, and catalysts in which different materials are arranged on the inner layer and the outer layer of the base material have been developed.

  For example, Patent Document 1 discloses that the upstream inner layer carrier is a ceria-rich ceria / zirconia composite oxide, and the upstream outer layer and downstream carrier are zirconia-rich zirconia / ceria composite oxides. Yes. The ceria-rich composite oxide on the upstream inner layer side adsorbs the exhaust gas component, and the adsorbed exhaust gas component is decomposed by the noble metal contained in the catalyst layer. Thereby, the purification performance of exhaust gas is enhanced, and in particular, the ignitability is enhanced.

  Patent Document 2 discloses a catalyst in which an inner layer includes ceria and a first palladium catalyst component, and an outer layer includes alumina and a second palladium catalyst component. In the inner layer, the ceria and the first palladium catalyst component are in intimate contact with each other, so that the redox characteristics exhibited by the ceria at a high temperature (eg, 500 ° C. or higher) are promoted. The second palladium catalyst component in the outer layer exhibits sufficient catalytic activity during initial heating and at an operating temperature of 500 ° C. or lower. In this way, by changing the type of support between the inner layer and the outer layer, the operating temperature window of the catalyst is widened to improve the performance of palladium.

  Further, Patent Document 3 includes Rh on the upstream side, Rh and a catalytic noble metal other than Rh on the downstream side, and more catalytic noble metal other than Rh is supported on the downstream side. As a result, Rh can suppress alloying with other noble metals on the upstream side where high temperatures are likely to occur. Further, on the downstream side, a purification action by a catalyst noble metal other than Rh is exhibited.

JP 2007-38072 A (paragraph number “0010”) JP-T 09-500570 (Claim 85) JP 2006-326428 A (paragraph numbers “0009” to “0011”)

  However, none of the exhaust gas purifying catalysts of Patent Documents 1 to 3 is obtained by disposing a catalyst layer at each site on the substrate in consideration of the purification characteristics of the catalyst noble metal. The catalytic precious metals Pt and Pd mainly contribute to the oxidation purification of CO and HC, and Rh mainly contributes to the reduction purification of NOx. There is a demand for a configuration that sufficiently exhibits the purification characteristics of these catalytic noble metals.

  In recent years, there have been an increasing number of vehicles such as hybrid vehicles that frequently stop and operate the engine. For this reason, improvement of the low-temperature purification performance of a catalyst is desired. For this purpose, it is conceivable to increase the amount of catalyst noble metal supported, but this leads to an increase in cost.

  Further, the warm-up performance of the catalyst is influenced by the activity of the catalyst noble metal in the upstream portion and the ease of warming of the catalyst, that is, the heat capacity. In the zone coat catalyst in which the catalyst layers of different components are arranged on the upstream side and the downstream side, the activity of the upstream catalyst noble metal is considered to depend on the length of the upstream upstream catalyst layer and the coating amount. It is considered that the ease of warming of the catalyst greatly depends on the coating amount and material of the previous catalyst layer. Therefore, it is considered that there is an optimum material for the optimum length and coating amount of the catalyst layer and the carrier for enhancing the warm-up property.

  Moreover, the downstream downstream catalyst layer is more susceptible to sulfur poisoning and atmospheric deterioration as it is located further downstream. As the latter catalyst layer is arranged on the upstream side, the utilization efficiency of the catalyst noble metal is improved. For this reason, it is considered that the post-catalyst layer has a length having an optimal balance. Therefore, it is important to set the optimum upstream and downstream catalyst layer lengths and coat amounts in order to maximize the performance of the zone coat catalyst.

  In addition, since the frequency of use of a three-way catalyst in a high load region is higher than in the past, it is also necessary to improve the catalyst performance in a high Ga (high intake air amount) region. For this purpose, it is considered effective to increase the amount of catalyst noble metal supported, but this leads to an increase in cost.

  The present invention has been made in view of such circumstances, and provides an exhaust gas purification catalyst that can sufficiently exhibit the exhaust gas purification characteristics of a catalyst noble metal, has a high low temperature purification performance of the catalyst, and is excellent in catalyst warm-up performance. With the goal.

The exhaust gas purifying catalyst of the present invention comprises a base material forming a gas flow path through which exhaust gas flows and a catalyst layer formed on the base material, and the catalyst layer is formed on the surface of the base material. A lower catalyst layer, a front upper catalyst layer covering the upstream side of the gas flow direction on the surface of the lower catalyst layer, and a gas flow higher than the front upper catalyst layer on the surface of the lower catalyst layer. A catalyst for exhaust gas purification comprising a downstream upper catalyst layer covering the downstream side in the direction, wherein the lower catalyst layer supports at least one of Pd and Pt, and the rear upper catalyst layer supports Rh The upstream catalyst layer supports Pd, and the carrier supporting Pd of the upstream catalyst layer is a composite oxide containing ZrO ₂ as a main component (hereinafter referred to as ZrO ₂ composite oxide). And

  The exhaust gas purifying catalyst of the present invention has a lower catalyst layer supporting at least one of Pd and Pt, a front upper catalyst layer containing Pd, and a rear upper catalyst layer containing Rh. In the upstream upper catalyst layer, oxidative purification of HC in the exhaust gas is mainly performed by Pd.

  HC (hydrocarbon) has a relatively large molecular structure and is difficult to diffuse inside the upstream catalyst layer. In addition, since HC has a higher molecular weight than CO and NOx and is difficult to be decomposed, the HC purification reaction requires higher activation energy than the CO and NOx purification reaction. The temperature should be high. Therefore, in the present invention, Pd capable of purifying HC is supported on the upstream upstream catalyst layer on the upper side (surface side) on which HC tends to diffuse and also on the upstream side where it tends to become high temperature. Therefore, Pd increases the chance of contact with HC and can purify HC with high efficiency at high temperatures.

Furthermore, in the present invention, the carrier supporting Pd in the upstream catalyst layer is a ZrO ₂ composite oxide. Since ZrO ₂ has a small specific heat and is easily warmed, the use of ZrO ₂ as a carrier improves the warm-up performance of the catalyst.

In addition, since the front part becomes high temperature, heat resistance is required. However, the addition of a predetermined amount of Y ₂ O ₃ improves the heat resistance of ZrO ₂ and prevents the warm-up performance from being lowered. Can be maximized. As described above, by using the ZrO ₂ composite oxide containing Y ₂ O ₃ as the support for the upper catalyst layer in the previous stage, while improving the temperature rise of the catalyst, the heat resistance is secured and the durable catalyst warm-up is ensured. You can get functionality.

As described above, in the present invention, the catalytic noble metal is disposed at an optimum site in accordance with the characteristics of the catalytic noble metal, and the purification characteristics of the catalytic noble metal are exhibited with high efficiency. Furthermore, by using a ZrO ₂ composite oxide containing Y ₂ O ₃ as the carrier for supporting Pd in the upstream catalyst layer, it is possible to improve the temperature rise of the catalyst while ensuring the heat resistance, thereby ensuring durability. A certain catalyst warm-up property can be obtained. Therefore, high exhaust gas purification performance can be exhibited even during cold start such as immediately after engine start or after intermittent engine stop of a hybrid vehicle.

Preferably, the ZrO ₂ composite oxide, based on the total mass of the ZrO ₂ composite oxide, including 20-30 wt% Y ₂ O _3. When the content of Y ₂ O ₃ is less than 20% by mass, the effect of increasing the heat resistance is reduced, and the warm-up performance may be reduced. Even when the content of Y ₂ O ₃ exceeds 30% by mass, the amount of ZrO ₂ decreases and the warm-up performance may be deteriorated.

Furthermore, high warm-up performance can be obtained by controlling the bulk density of the ZrO ₂ composite oxide in the upstream catalyst layer.

Preferably, the bulk density of the ZrO ₂ composite oxide is 0.98 to 1.52 g / ml. If the bulk density is lower than 0.98 g / ml, the temperature rise of the catalyst may be lowered. If the bulk density is higher than 1.52 g / ml, Pd sintering is likely to occur and the catalyst performance may be deteriorated. There is.

  Furthermore, in the present invention, it is preferable that the loading density of Pd supported on the upstream catalyst layer is 4.5 to 12% by mass. When the loading density of Pd is less than 4.5% by mass, the amount of HC purified by Pd may be reduced. Even when the Pd loading density exceeds 12% by mass, the HC purification amount may decrease. This is presumably because the sintering of Pd progresses and the surface area of Pd decreases.

  As described above, by increasing the loading density of Pd supported on the upstream catalyst layer of 4.5 to 12% by mass as compared with the prior art, it becomes possible to further increase the low-temperature activity of the catalyst and increase the cost. Can be suppressed.

  In the downstream upper catalyst layer on the downstream side, NOx in the exhaust gas is mainly reduced and purified by Rh. Rh generates hydrogen by a hydrogen reforming reaction from residual HC that has not been oxidized and purified in the upstream catalyst layer in the exhaust gas. Due to the reducing power of hydrogen, NOx in the exhaust gas is reduced and purified.

  CO in the exhaust gas component has a relatively small molecular structure and a relatively high diffusion rate. For this reason, CO can diffuse quickly into the lower catalyst layer disposed inside the upstream catalyst layer and the upstream catalyst layer. Therefore, Pt or Pd supported on the lower catalyst layer can mainly oxidize and purify CO and residual HC.

  The ratio of the length of the upper catalyst layer to the length of the lower catalyst layer is preferably 20 to 40%. In this case, the catalytic activity of the upstream catalyst layer can be sufficiently exhibited, and a space for providing the downstream catalyst layer on the downstream side is secured. Therefore, it is possible to enhance the HC purification performance of the upstream catalyst layer while fully exhibiting the NOx purification performance of Rh of the rear catalyst layer.

  The ratio of the length of the rear upper catalyst layer to the length of the lower catalyst layer is preferably 71 to 81%. In this case, the NOx purification performance by Rh supported on the rear upper catalyst layer is effectively exhibited.

  The lower catalyst layer preferably contains an oxygen storage / release material, more preferably the oxygen storage / release material content is 80 to 120 g / L, and the lower catalyst layer per liter of the base material. The coating amount of the layer is preferably 105 to 155 g.

The oxygen storage / release material is a material having oxygen storage / release (OSC) performance that stores oxygen in an oxidizing atmosphere and releases oxygen in a reducing atmosphere. Preferably, the oxygen storage / release material is a composite oxide containing CeO ₂ . By including the oxygen storage / release material in the lower catalyst layer where the oxygen concentration tends to be low, the purification reaction environment of the lower catalyst layer can be stably maintained. When the oxygen storage / release material has the above content, the lower catalyst layer can be maintained at an appropriate oxygen concentration, and the exhaust gas purification performance by Pd or Pt can be effectively exhibited.

  Preferably, when the coating amount of the lower catalyst layer per liter of the substrate is 105 to 155 g, a sufficient opening area of the gas flow path is secured and pressure loss is suppressed while maintaining a more sufficient OSC performance. be able to.

It is preferable that Total-GSA (total geometric surface area) of the catalyst layer after coating the catalyst layer is 25 to 36 cm ² / piece. In this case, high OSC performance can be obtained in a high Ga (high intake air amount) region, and catalyst purification performance in a high Ga (high intake air amount) region can be improved without increasing the amount of noble metal. it can.

According to the exhaust gas purifying catalyst of the present invention, various catalytic noble metals are arranged at appropriate sites on the substrate in accordance with the type of catalytic noble metal, that is, the lower catalyst layer is at least one of Pd and Pt. , The rear upper catalyst layer carries Rh, and the front upper catalyst layer carries Pd, so that the catalytic precious metal purification characteristics can be sufficiently exhibited, and the low temperature purification performance of the catalyst can be enhanced. In addition, by using a ZrO ₂ composite material added with Y ₂ O ₃ that has low specific heat and excellent heat resistance as a support material for the upper catalyst layer in the previous stage, heat resistance can be ensured while improving catalyst temperature rise. The catalyst warm-up property having durability can be obtained.

It is sectional drawing of the catalyst for exhaust gas purification of this invention. It is a perspective view of the base material used for the catalyst for exhaust gas purification of this invention. 2 is a cross-sectional view of an exhaust gas purifying catalyst of Comparative Example 1. FIG. It is a diagram which shows the relationship between the Pd carrying density of a front stage upper catalyst layer, and HC purification amount. It is a diagram which shows the relationship between the Pd carrying density of a pre-stage upper catalyst layer, and a pressure loss. It is a graph which shows the relationship between the material of a front | former stage upper catalyst layer, and HC50% purification | cleaning time. After heat treatment at 1100 ° C., and Y ₂ O ₃ content in the ZrO ₂ composite oxide, a graph showing the relationship between the specific surface area of ZrO ₂ composite oxide (SSA). It is sectional drawing of the catalyst for exhaust gas purification of this invention. It is a diagram which shows the relationship between the coating amount of a pre-stage upper catalyst layer, and HC50% purification time in Study 1. It is a diagram which shows the relationship between the length of the front | former stage upper catalyst layer, and HC50% purification | cleaning time in examination 2. FIG. It is a diagram which shows the relationship between the length of a back | latter stage upper catalyst layer, and NOx discharge | emission amount in the examination 2. FIG. It is a diagram which shows the relationship between the coating amount of a back | latter stage upper catalyst layer, and NOx discharge | emission amount in the examination 3. FIG. It is a diagram which shows the relationship between the coating amount of a lower catalyst layer, and OSC performance in examination 4. It is a diagram which shows the relationship between the coating amount of a lower catalyst layer, and a pressure loss in examination 4. FIG. 10 is a diagram showing the relationship between the coating amount of the lower catalyst layer and the HC 50% purification time in Study 5. It is a diagram which shows the relationship between the coating amount of a lower catalyst layer, and OSC performance in examination 5. It is a diagram which shows the relationship between Total-GSA by each Ga, and OSC performance in examination 6. FIG. Is a graph showing the relationship between the bulk density and the Atsushi Nobori of the ZrO ₂ composite oxide. Is a graph showing the relationship between bulk density of ZrO ₂ composite oxide and NOx50% purification time. It is a graph showing the relationship between the bulk density and the Pd crystallite size of ZrO ₂ composite oxide.

  The exhaust gas purifying catalyst of the present invention includes a base material, a lower catalyst layer formed on the surface of the base material, a front upper catalyst layer formed on the upstream side of the surface of the lower catalyst layer, and a lower catalyst layer And a rear upper catalyst layer formed on the downstream side of the surface.

  The substrate has a structure having a gas flow path, and for example, a substrate having a shape such as a honeycomb shape, a foam shape, or a plate shape is used. The material of the substrate is not particularly limited, and known materials such as cordierite, ceramics such as SiC, or metals can be used.

  The lower catalyst layer includes a catalyst noble metal and a carrier supporting the catalyst noble metal. The catalyst noble metal of the lower catalyst layer contains at least one of Pt and Pd. In addition to Pt and Pd, the catalyst noble metal may contain other catalyst noble metals such as Rh so as not to hinder the performance of Pt and Pd. In this specification, “per liter of substrate” means “per liter of the entire bulk volume including the volume of voids formed in the interior of the substrate in the pure volume of the substrate”. When the inside of the substrate is partitioned into cells, it means the total volume of 1 liter including the volume of the cell in the pure volume of the substrate. The mass of each component per liter of substrate may be expressed as g / L.

The lower catalyst layer support preferably contains CeO ₂ (ceria). In this case, the oxygen concentration in the lower catalyst layer where the oxygen concentration tends to be low can be maintained at a certain level, and the exhaust gas purification performance by the catalyst noble metal is improved.

The CeO ₂ content in the lower catalyst layer is preferably 21 to 30 g per liter of the base material. If it is less than 21 g, the oxygen concentration in the lower catalyst layer may not be stably maintained. If it exceeds 30 g, the NOx purification performance of the lower catalyst layer may be reduced.

The support of the lower catalyst layer preferably contains a CeO ₂ —ZrO ₂ composite oxide in which CeO ₂ and ZrO ₂ are dissolved. Grain growth of CeO ₂ is suppressed, and deterioration of OSC performance after endurance can be suppressed.

  The coating amount of the lower catalyst layer is preferably 105 to 155 g per liter of the base material, and more preferably 150 to 155 g per liter of the base material. “Coating amount” means “a mass obtained by converting the mass of the lower catalyst layer coated on the surface of the base material into the mass of the lower catalyst layer coated on 1 liter of the base material”. The 1 liter of the base material means “per 1 liter of the entire bulk volume including the volume of voids formed inside the base material in the pure volume of the base material”. When the coating amount of the lower catalyst layer is less than 105 g, the lower catalyst layer is too thin and the catalytic activity of Pd or Pt may be reduced. If it exceeds 155 g, the pressure loss may increase.

  The thickness of the lower catalyst layer is preferably 10 to 30 μm. In this case, the catalytic activity of the lower catalyst layer is high, and pressure loss can be suppressed.

The upstream catalyst layer includes Pd as a catalyst noble metal and a ZrO ₂ composite oxide as a support. Pd mainly purifies HC in the exhaust gas.

  The Pd loading density is preferably 4.5 to 12% by mass. “Pd loading density” refers to the ratio of the loading mass of Pd to the coating mass of the upper catalyst layer in the previous stage. A front upper catalyst layer and a rear upper catalyst layer coexist on the surface of the lower catalyst layer, and the length of the front upper catalyst layer containing Pd is shorter than the length of the lower catalyst layer. Therefore, the volume of the portion where Pd can react is narrower than before. In order to purify HC with high efficiency in a narrow reaction space, in the present invention, the loading density of Pd is made higher than in the prior art.

  Furthermore, the loading density of Pd is preferably 6 to 10% by mass. In this case, the HC purification performance by Pd is further improved, and the cost increase can be suppressed.

The carrier for supporting Pd in the upstream catalyst layer is a ZrO ₂ composite oxide. Conventionally, Al ₂ O ₃ having a large specific surface area is mainly used as a support. However, by using a ZrO ₂ composite oxide having a specific heat smaller than that of Al ₂ O ₃ as a support, the catalyst can be easily warmed. Can be improved. ZrO ₂ has lower heat resistance than Al ₂ O ₃ , but the heat resistance of ZrO ₂ can be improved by adding Y ₂ O ₃ . Y ₂ O ₃ has a phase stabilization effect that suppresses the change of ZrO ₂ to a monoclinic phase at high temperatures. As described above, by using the ZrO ₂ composite oxide containing Y ₂ O ₃ as a support for supporting Pd of the upstream catalyst layer, it is possible to improve the catalyst temperature rise and heat resistance of the upstream catalyst layer. As a result, the warm-up performance of the catalyst can be improved.

The content of Y ₂ O _3, relative to the total mass of the ZrO ₂ composite oxide containing Y ₂ O _3, preferably 20 to 30 mass%, further, desirably 23 to 27 mass%. In this case, the heat resistance improvement effect can be further enhanced.

The ZrO ₂ composite oxide may further contain other materials, for example, La and / or Nd. La and Nd have a large ionic radius and have a function of suppressing sintering between particles, and can improve the heat resistance of the ZrO ₂ composite oxide. The ZrO ₂ composite oxide preferably contains 10% by mass or less of La and / or Nd.

The bulk density of the ZrO ₂ composite oxide is preferably 0.98 to 1.52 g / ml, and more preferably 1.06 to 1.25 g / ml. In this case, the NOx 50% purification time can be further shortened.

  The ratio of the length of the previous upper catalyst layer to the length of the lower catalyst layer is preferably 20 to 40%, and more preferably 30 to 35%. In this case, a space for providing the rear upper catalyst layer downstream from the front upper catalyst layer is secured. Therefore, it is possible to enhance the HC purification performance of the upstream catalyst layer while fully exhibiting the NOx purification performance of Rh of the rear catalyst layer. Here, “length” refers to the length of the base material in the gas flow direction. When the substrate has a honeycomb structure, it means the length in the longitudinal direction of the substrate. When the ratio of the length of the upper catalyst layer in the previous stage to the length of the lower catalyst layer is less than 20%, the absolute amount of Pd decreases, and the HC purification performance may be deteriorated. If it exceeds 40%, the space for forming the rear upper catalyst layer is too narrow, and the amount of Rh supported is lowered, and the exhaust gas purification performance by Rh may be lowered. When the ratio of the length of the front upper catalyst layer to the length of the lower catalyst layer is 30 to 35%, the NOx purification performance by Rh of the rear upper catalyst layer is more fully exhibited, HC purification performance can be further enhanced.

  The coat amount of the upstream catalyst layer is preferably 40 to 50 g per liter of the base material. When the amount is less than 40 g, the workability of the coating of the upstream catalyst layer may be deteriorated. On the other hand, when the coating amount of the upstream catalyst layer exceeds 50 g per liter of the base material, the upstream catalyst Since the heat capacity of the layer becomes too large, it is difficult to quickly raise the temperature, and HC may not be easily purified when the engine is cold started.

  The thickness of the upstream catalyst layer is preferably 10 to 30 μm. In this case, an absolute amount of Pd sufficient for HC purification can be secured, and the heat capacity of the upstream catalyst layer becomes low, and the temperature of the upstream catalyst layer quickly rises when the engine is cold.

  The rear upper catalyst layer may contain other catalytic noble metal such as Pd to the extent that the performance of Rh is not impaired. The post-stage upper catalyst layer preferably does not contain Pt. This is to suppress a decrease in catalytic activity due to alloying of Rh and Pt.

  The thickness of the rear upper catalyst layer is preferably 10 to 30 μm. In this case, the NOx purification performance by Rh in the latter upper catalyst layer is high and the gas diffusibility to the lower catalyst layer is good, so the purification performance by Pt and Pd in the lower catalyst layer can also be enhanced.

  The ratio of the length of the rear upper catalyst layer to the length of the lower catalyst layer is preferably 70 to 85%, more preferably 71 to 81%. In this case, it is possible to carry a sufficient amount of Rh for NOx purification in the rear stage upper catalyst layer while securing a sufficient space to form the front stage upper catalyst layer upstream of the rear stage upper catalyst layer.

  The Rh loading density in the latter upper catalyst layer is preferably 0.1 to 0.4% by mass. If it is less than 0.1% by mass, the NOx purification performance by Rh may be reduced. If it exceeds 0.4 mass%, the purification performance corresponding to that cannot be expected, resulting in an increase in cost.

  The coating amount of the rear upper catalyst layer is preferably 50 to 200 g per liter of the base material. In this case, the NOx purification performance by Rh is enhanced while suppressing pressure loss.

It is preferable that Total-GSA after catalyst coating is 25-36 cm < ² > / piece. If it is less than 25 cm ² / piece, the OSC performance may deteriorate in a high load (high Ga) region. When it exceeds 36 cm ² / piece, the coating thickness becomes thin and there is a possibility that the coating cannot be performed.

  In the present invention, GSA (geometric surface area) is the surface area of the inner wall of the catalyst layer per unit length (axial direction) of one cell of the honeycomb substrate after the honeycomb substrate is washed with the catalyst. “Total-GSA” is a value derived by multiplying GSA by the number of cells included in the substrate and the length of the substrate.

  The downstream end portion of the rear upper catalyst layer is disposed directly above the downstream end portion of the lower catalyst layer. Further, the upstream end portion of the upstream catalyst layer is disposed directly above the upstream end portion of the lower catalyst layer. The upstream portion of the rear upper catalyst layer may be separated from the downstream portion of the front upper catalyst layer. Further, the upstream portion of the rear upper catalyst layer may overlap the downstream portion of the front upper catalyst layer. In this case, either the front upper catalyst layer or the rear upper catalyst layer may be on the upper side. If you want to preferentially function the upstream catalyst layer, cover the downstream part of the upstream catalyst layer with the upstream part of the upstream catalyst layer, and if you want the upstream catalyst layer to function preferentially, The upstream portion of the catalyst layer may be covered with the downstream portion of the upstream catalyst layer.

  In forming the lower catalyst layer on the surface of the base material, the slurry containing the carrier powder may be wash-coated on the surface of the base material, and the catalyst noble metal containing Pd or / and Pt may be supported on the surface of the base material. Alternatively, a catalyst powder previously loaded with a catalyst noble metal may be made into a slurry, and this may be wash coated on the substrate surface.

In forming the former upper catalyst layer on the surface of the lower catalyst layer, a slurry containing a carrier powder that is a ZrO ₂ composite oxide is washed on the upstream portion of the surface of the lower catalyst layer, and Pd is supported on it. Alternatively, a catalyst powder in which Pd is previously supported on a carrier powder that is a ZrO ₂ composite oxide may be used as a slurry, and this may be wash coated on the upstream portion of the surface of the lower catalyst layer.

  In forming the latter upper catalyst layer on the surface of the lower catalyst layer, the slurry containing the carrier powder may be wash-coated on the downstream portion of the surface of the lower catalyst layer, and Rh may be supported on the slurry, The catalyst powder supporting Rh may be made into a slurry, and this may be wash coated on the downstream portion of the surface of the lower catalyst layer.

  The present invention will be specifically described using reference examples, examples, and comparative examples.

(Reference Example 1)
As shown in FIG. 1, the exhaust gas purifying catalyst of this example includes a base material 1 and a catalyst layer 10 formed on the base material 1. The catalyst layer 10 includes a lower catalyst layer 2 formed on the surface of the substrate 1, a pre-stage upper catalyst layer 3 formed on the upstream portion of the surface of the lower catalyst layer 2, and a downstream side of the surface of the lower catalyst layer 2. It is comprised from the back | latter stage upper catalyst layer 4 formed in the part.

  As shown in FIG. 2, the base material 1 is a monolith base material having a cordierite honeycomb structure. The substrate 1 has a circular cross section with a diameter of 103 mm, a length of 105 mm, and an overall capacity of 875 cc. The interior of the substrate 1 is partitioned into a large number of cells by partition walls 11. A gas passage 6 is formed in each cell. As shown in FIG. 1, the lower catalyst layer 2 is formed on the surface of the partition wall 11 partitioning the cell, and the upstream catalyst layer 3 and the upstream catalyst layer on the upstream side and downstream side of the surface of the lower catalyst layer 2. 4 is formed.

The lower catalyst layer 2 includes Pd, Al ₂ O ₃ and an oxygen storage / release material. The amount of Pd supported in the lower catalyst layer 2 is 1.0 g / L, the coating amount of Al ₂ O ₃ is 25 g / L, and the oxygen storage / release material is a composite oxide of ZrO ₂ —CeO ₂ . The coating amount is 85 g / L.

The upstream catalyst layer 3 includes Pd, Al ₂ O ₃ and an oxygen storage / release material. The supported amount of Pd in the upstream catalyst layer 3 is 1.73 g / L, and the supported density of Pd in the upstream catalyst layer 3 is 5.8% by mass. The coating amount of the upstream catalyst layer 3 is 30 g, which is 34.3 g / L in terms of the coating amount per liter of the base material. The coating amount of Al ₂ O _{3 in} the upstream catalyst layer 3 is 18.6 g / L per liter of the base material, and the oxygen storage / release material is a composite oxide of ZrO ₂ —CeO ₂ , which absorbs and releases oxygen. The coating amount of the material is 5.0 g / L. The ratio of the coating amount of Al ₂ O _{3 to} the coating amount of the oxygen storage / release material is 3.7.

The rear upper catalyst layer 4 includes Rh, Al ₂ O _3, and an oxygen storage / release material. The amount of Rh supported in the rear upper catalyst layer 4 is 0.2 g / L, the coating amount of Al ₂ O ₃ is 75.2 g / L, and the oxygen storage / release material is a composite oxidation of ZrO ₂ —CeO ₂ . The coating amount of the oxygen storage / release material is 30.1 g / L. The ratio of the coating amount of Al ₂ O _{3 to} the coating amount of the oxygen storage / release material is 2.5.

  The length of the lower catalyst layer 2 is 105 mm, which is the same as the length of the substrate 1. The pre-stage upper catalyst layer 3 is formed from the upstream end portion of the lower catalyst layer 2 to the position of 31.5 mm toward the downstream side, and is 30% of the length of the lower catalyst layer 2. The rear stage upper catalyst layer 4 is formed from the downstream end of the lower catalyst layer 2 to a position of 84 mm toward the upstream side, and has a length of 80% with respect to the length of the lower catalyst layer 2.

  The thickness of the lower catalyst layer 2 is 20 μm, and the thicknesses of the upper catalyst layer 3 and the upper catalyst layer 4 are 20 μm.

In producing the exhaust gas purifying catalyst of this example, a carrier powder composed of Al ₂ O ₃ powder and a ZrO ₂ —CeO ₂ composite oxide powder is immersed in an aqueous palladium nitrate solution, calcined, and palladium on the carrier. Was supported. The support on which palladium was supported was mixed with an Al ₂ O ₃ binder to prepare a slurry, and the surface of the substrate 1 was wash coated. The coated slurry was dried and fired to form the lower catalyst layer 2.

Next, a carrier powder composed of an Al ₂ O ₃ powder and a ZrO ₂ —CeO ₂ composite oxide powder was immersed in an aqueous palladium nitrate solution and baked to support palladium on the carrier. An Al ₂ O ₃ binder was mixed with the carrier on which palladium was supported to prepare a slurry, and was washed on the upstream portion of the surface of the lower catalyst layer 2. The coated slurry was dried and calcined to form the upstream catalyst layer 3.

Next, a carrier powder composed of an Al ₂ O ₃ powder and a ZrO ₂ —CeO ₂ composite oxide powder was immersed in an aqueous rhodium nitrate solution and baked to support the rhodium on the carrier. A slurry on which rhodium was supported was mixed with an Al ₂ O ₃ binder to prepare a slurry, which was then coated on the downstream portion of the lower catalyst layer 2 surface. The coated slurry was dried and calcined to form the rear upper catalyst layer 4.

(Reference Example 2)
In the exhaust gas purifying catalyst of this example, the loading density of Pd in the upstream catalyst layer is 8.8% by mass, the coating amount of the upstream catalyst layer is 20 g, and the length of the upstream catalyst layer is 21 mm. It is. That is, in this example, the amount of Pd supported in the upstream catalyst layer is the same as that in Reference Example 1, and the coating amount of the upstream catalyst layer is reduced from that in Reference Example 1, thereby reducing the upstream catalyst layer. The Pd loading density in the inside is increased, and the length of the upstream catalyst layer is shortened. Others are the same as in Reference Example 1.

Example 1 Preparation of Catalyst Using ZrO ₂ Composite Oxide as Support for Previous Stage Catalyst Layer The catalyst was produced by the following method. First, slurry (A) for the lower catalyst layer was prepared. As starting materials, cerium nitrate, zirconium oxynitrate, lanthanum nitrate, and yttrium nitrate were dissolved in pure water to obtain a 0.3M precursor solution. Next, the precursor solution was dropped into ammonia water diluted with pure water, stirred with a homogenizer, water was removed with a centrifuge, and only the precipitate was collected. These precipitates were dried and pre-baked, and then crystallized by baking at 700 ° C. Then, it grind | pulverized with the blender and it was set as the powder. As a result, a powdery ZrO ₂ —CeO ₂ —La ₂ O ₃ —Y ₂ O ₃ composite oxide was obtained.

0.5 g / L Pd, 86 g / L ZrO ₂ —CeO ₂ —La ₂ O ₃ —Y ₂ O ₃ composite oxide, 40 g / L La-added Al ₂ O ₃ , 10 g / L BaSO ₄ and 3 g / L of Al ₂ O ₃ binder were mixed to prepare slurry (A) for the lower catalyst layer.

Next, a slurry (B) for the rear upper catalyst layer was prepared. As starting materials, aluminum nitrate, cerium nitrate, zirconium oxynitrate and lanthanum nitrate were dissolved in pure water to obtain a 0.3M precursor solution. Next, the precursor solution was dropped into ammonia water diluted with pure water, stirred with a homogenizer, water was removed with a centrifuge, and only the precipitate was collected. These precipitates were dried and pre-baked, and then crystallized by baking at 700 ° C. Then, it grind | pulverized with the blender and it was set as the powder. This powder is a composite oxide in which first particles made of La-added CeO ₂ —ZrO ₂ and _second particles made of La-added Al ₂ O ₃ are mixed at the primary particle level.

Neodymium nitrate was solubilized, and this powder was mixed and stirred. After stirring, it was dried and fired at 900 ° C. As a result, Nd ₂ O ₃ was segregated on the surface layer of the first particles and the second particles, and a powdery carrier was obtained.

This carrier was immersed in an aqueous rhodium nitrate solution and baked to carry 0.15 g / L of rhodium on the carrier. A slurry (B) for the catalyst catalyst layer on the rear stage was prepared by mixing 17 g / L of La-added Al ₂ O ₃ and 3 g / L of Al ₂ O ₃ binder with 46 g / L of the carrier supporting rhodium. .

Next, slurry (C) for the upstream catalyst layer was prepared. Similar to the composite oxide mixed in the slurry for the lower catalyst layer, except that zirconium oxynitrate, yttrium nitrate, and lanthanum nitrate were used as starting materials, ZrO ₂ —Y ₂ O ₃ —La ₂ O ₃ A composite oxide was prepared. 0.35 g / L Pd, 9.8 g / L La-added Al ₂ O ₃ , 29.4 g / L ZrO ₂ composite oxide, and 3 g / L Al ₂ O ₃ binder were mixed. A slurry (C) for the upstream catalyst layer was prepared. The content of Y ₂ O ₃ with respect to the total mass of the ZrO ₂ composite oxide was 25% by mass.

  The above three types of slurries (A), (B), and (C) were coated on a substrate. Similar to the base material of Reference Example 1, the base material has a cordierite honeycomb structure and is a cylindrical body having a diameter of 103 mm and a length of 105 mm. As shown in FIG. 8, the lower catalyst layer slurry (A) was coated on the entire surface of the substrate 1, dried and fired to form the lower catalyst layer 2. The front stage catalyst layer 3 was formed by coating the front stage part of the surface of the lower catalyst layer 2 with the slurry (C) for the front stage upper catalyst layer, drying and firing. The latter part of the surface of the lower catalyst layer 2 was coated with the slurry (B) for the latter upper catalyst layer, dried and fired. The length of the rear upper catalyst layer 4 is 85 mm, and is 81% of the length of the lower catalyst layer 2. The length of the front stage upper catalyst layer 3 is 37 mm, which is 35% of the length of the lower catalyst layer 2. The upstream portion 4 a of the rear upper catalyst layer 4 covers the downstream portion 3 a on the surface of the front upper catalyst layer 3.

The Pd loading density in the upstream catalyst layer 3 is 2.8% by mass. The coating amount of the upstream catalyst layer 3 is 30 g, which is 34.3 g / L in terms of the coating amount per liter of the base material. The coating amount of the ZrO ₂ composite oxide in the upstream catalyst layer 3 is 29.4 g / L per liter of the substrate.

(Example 2): Preparation of a catalyst using ZrO ₂ composite oxides having different bulk densities as supports for the upper catalyst layer in the front stage Slurry (A) for the lower catalyst layer and slurry (B) for the upper catalyst layer were carried out Prepared as in Example 1. For the slurry (C) for the upstream catalyst layer, the ZrO ₂ composite oxide has a composition of ZrO ₂ / La ₂ O ₃ / Y ₂ O ₃ = 68/7/25 (mass%). Process conditions are adjusted so that the bulk density of the catalyst layer is 0.98, 1.09, 1.52 (g / ml), and the loading density of Pd in the upstream catalyst layer is 0.5% by mass. This was prepared in the same manner as in Example 1 except that it was prepared as described above.

  The above three types of slurries (A), (B), and (C) were coated on a substrate in the same manner as in Example 1, dried at 120 ° C. for 2 hours, and calcined at 500 ° C. for 2 hours, respectively. Formed.

(Comparative Example 1)
In the exhaust gas purifying catalyst of this comparative example, the loading density of Pd in the upstream catalyst layer is 1.8% by mass, the coating amount of the upstream catalyst layer is 100 g, and the length of the upstream catalyst layer is 105 mm. That is, in this example, the amount of Pd supported in the upstream catalyst layer is the same as that in Reference Example 1, and the coating amount of the upstream catalyst layer is increased from that in Reference Example 1, so that the upstream catalyst is increased. The Pd carrying density in the layer is reduced as compared with the case of Reference Example 1, and the length of the upstream catalyst layer is the same as that of the lower catalyst layer. In this example, as shown in FIG. 3, the front upper catalyst layer 3 covers the entire surface of the lower catalyst layer 2, and the rear upper catalyst layer is not formed. Others are the same as in Reference Example 1.

(Comparative Examples 2, 3, 4)
In the exhaust gas purifying catalyst of Comparative Example 2, the loading density of Pd in the upstream catalyst layer is 3.5% by mass, the coating amount of the upstream catalyst layer is 50 g, and the length of the upstream catalyst layer is 52.5 mm.

  In the exhaust gas purifying catalyst of Comparative Example 3, the loading density of Pd in the upstream catalyst layer is 4.4% by mass, the coating amount of the upstream catalyst layer is 40 g, and the length of the upstream catalyst layer is 42 mm.

  That is, in Comparative Examples 2 and 3, the amount of Pd supported in the upstream catalyst layer is the same as that in Reference Example 1, and the coating amount of the upstream catalyst layer is increased compared to that in Reference Example 1, The Pd carrying density in the upstream catalyst layer is reduced, and the length of the upstream catalyst layer is made longer than that in Reference Example 1.

  In the exhaust gas purifying catalyst of Comparative Example 4, the loading density of Pd in the upstream catalyst layer is 17.5% by mass, the coating amount of the upstream catalyst layer is 10 g, and the length of the upstream catalyst layer is 10.5 mm.

  That is, in Comparative Example 4, the amount of Pd supported in the upstream catalyst layer is the same as that in Reference Example 1, and the coating amount of the upstream catalyst layer is reduced as compared with that in Reference Example 1, so that The Pd carrying density in the catalyst layer is increased, and the length of the upstream catalyst layer is made shorter than that in the case of Reference Example 1.

  Other configurations of Comparative Examples 2, 3, and 4 are the same as those of Reference Example 1.

(Comparative Example 5)
In the exhaust gas purifying catalyst of Comparative Example 5, the slurry of the upstream catalyst layer 3 was mixed with 0.35 g / L Pd, 39.2 g / L La-added Al ₂ O ₃ , and 3 g / L Al ₂ O. ₃ A slurry of the upstream catalyst layer 3 was prepared by mixing with a binder. That is, the carrier supporting Pd in the upstream catalyst layer was La-added Al ₂ O ₃ . Other configurations of Comparative Example 5 are the same as those of Example 1.

(Comparative Example 6)
The exhaust gas purifying catalyst of Comparative Example 6 was constructed in the same manner as Example 2 except that the bulk density of the upstream catalyst layer was 0.98 and 1.76 g / ml.

<Measurement of purification amount>
The amount of exhaust gas purification was measured for the catalysts of Reference Examples 1-2 and Comparative Examples 1-4.

  First, each catalyst was exposed to an exhaust gas flow exhausted by a V-type 8-cylinder engine (3UZ-FE), and an endurance test at a bed temperature of 1000 ° C. × 50 hours was performed. Each catalyst subjected to the durability test was mounted under the floor of a vehicle equipped with an in-line 4-cylinder 2.4L engine, combustion controlled at the theoretical air-fuel ratio, and exhaust gas circulated at an inlet gas temperature of 450 ° C. The HC concentration in the outgas was detected. By comparing the detected HC concentration in the output gas with the HC concentration in the input gas, the amount of HC purified per second was determined. The results are shown in Table 1 and FIG. The X axis in FIG. 4 indicates the Pd loading density, and the Y axis indicates the HC purification amount. In FIG. 4, the reference example is represented by “R”, and the comparative example is represented by “C”. The same applies to FIG.

<Measurement of catalyst pressure loss>
The gas pressure loss was measured about the catalyst of the said reference examples 1-2 and the comparative examples 1-4. A gas having a gas flow rate of 5 m ³ / min was allowed to flow into the catalyst from the upstream side of the catalyst. The gas pressure of the outgas was measured immediately after the downstream end of the catalyst. The difference between the pressure of the incoming gas and the pressure of the outgoing gas was determined, and this was shown as a pressure loss in Table 1 and FIG. The X axis in FIG. 5 indicates the loading density of Pd, and the Y axis indicates the pressure loss.

＜評価＞
上記測定結果より、参考例１〜２の触媒の浄化量は、比較例１〜４の触媒に比べて浄化量が高かった。比較例１では、Ｐｄの担持密度が低く、ＨＣ浄化性能が低かった。７質量％程度までは、前段上触媒層のＰｄ担持密度が高くなるほど、ＨＣ浄化性能が高くなった。

<Evaluation>
From the above measurement results, the purification amount of the catalysts of Reference Examples 1 and 2 was higher than that of Comparative Examples 1 to 4. In Comparative Example 1, the Pd loading density was low and the HC purification performance was low. Up to about 7% by mass, the higher the Pd carrying density of the upstream catalyst layer, the higher the HC purification performance.

  When the Pd carrying density of the upstream catalyst layer was 4.5 to 12% by mass, the HC purification amount was good. The amount of HC purification became maximum when the Pd carrying density of the upstream catalyst layer was 5.8 to 8.8% by mass, and the amount of HC purification gradually decreased when the Pd carrying density was higher than that. In addition, when the Pd carrying density of the upstream catalyst layer exceeds 12% by mass (Comparative Example 4), the amount of HC purification is lower than when the Pd carrying density is low (Comparative Example 1). This is presumably because the sintering of Pd progresses and the surface area of Pd decreases.

  In addition, the length of the upstream catalyst layer becomes shorter as the Pd carrying density of the upstream catalyst layer increases. When the Pd loading density of the upstream catalyst layer is 4.5 to 12% by mass, and the ratio of the length of the upstream catalyst layer to the total length of the catalyst is 20 to 40%, further 30 to 35% As described above, by reducing the length of the upstream catalyst layer in the previous stage and supporting Pd at a high density of 4.5 to 12% by mass on the upstream catalyst layer, the catalytic activity in the upstream catalyst layer can be increased. It turns out that it improves.

  As for the pressure loss of the catalyst, almost the same results were obtained in both Reference Examples 1-2 and Comparative Examples 1-4. This is because, in the catalysts of Reference Examples 1 and 2 and Comparative Examples 1 to 4, the thickness of the catalyst layer covering the substrate surface was made substantially the same. The pressure loss is almost the same.

<Measurement of catalyst warm-up improvement effect by carrier for upstream catalyst layer>
For the catalysts of Example 1 and Comparative Example 5, the amount of exhaust gas purification from the cold start was measured.

  First, each catalyst was exposed to an exhaust gas flow exhausted by a V-type 8-cylinder engine (3UZ-FE), and an endurance test at a bed temperature of 1000 ° C. × 50 hours was performed.

Each catalyst whose temperature was lowered to 50 ° C after the endurance test was installed under the floor of a vehicle equipped with an in-line 4-cylinder 2.4L engine, and the exhaust gas was circulated at an inlet gas temperature of 450 ° C. The amount of HC and the amount of HC in the output gas were measured, and the HC purification rate was continuously measured. The time when the amount of HC in the output gas became half the amount of HC in the input gas was calculated as the time when the HC purification rate reached 50% (THC 50% purification time) and is shown in FIG. Whether the air-fuel ratio (A / F) is rich, stoichiometric, or lean, using the ZrO ₂ composite oxide of the present invention reduces the HC50 purification time than using Al ₂ O ₃ as a carrier. It turns out that the effect to do is acquired.

<Measurement of heat resistance of ZrO ₂ composite oxide based on Y ₂ O ₃ content>
FIG. 7 shows the values of specific surface areas of the ZrO ₂ composite oxide powder samples having different Y ₂ O ₃ contents after heat treatment in an electric furnace at 1100 ° C. for 5 hours. The specific surface area was measured by the BET single point method. The specific surface area of each sample before heat treatment was 60 m ² / g. The heat treatment causes sintering of ZrO ₂ to reduce the specific surface area, but the specific surface area after heat treatment is kept large when the Y ₂ O ₃ content with respect to the total mass of the ZrO ₂ composite oxide is 20 to 30% by mass. In this case, it can be seen that the heat resistance is excellent.

In Example 1, the Pd support density in the upstream catalyst layer is 2.8% by mass. However, in the case of 4.5 to 12% by mass, the carrier supporting Pd in the upstream catalyst layer is often the ZrO ₂ composite oxide, ZrO ₂ composite oxide, with respect to the total mass of the ZrO ₂ composite oxide, it can be said that preferably contains 20-30 wt% Y ₂ O _3. This is because, regardless of the Pd carrying density of the upstream catalyst layer, the temperature rise of the catalyst improves as the specific heat of the carrier material carrying Pd of the upstream catalyst layer decreases, and the heat resistance of the carrier improves. .

<Measurement of warm-up improvement effect by bulk density of upper catalyst support in front stage>
Regarding the catalysts of Example 2 and Comparative Example 6, the exhaust gas purification amount from the cold start was measured. For each catalyst whose temperature was lowered to 50 ° C. after the endurance test, the gas composition: 3000 ppm C ₃ H ₆ +3300 ppm NO + 1500 ppm CO + 14.1% CO ₂ + 0-1% O ₂ + N ₂ balance, A gas with a flow rate of 10 L / min is circulated at an inlet gas temperature of 500 ° C., while measuring the catalyst bed temperature, the NOx amount in the inlet gas and the NOx amount in the outlet gas are measured, and the NOx purification rate is continuously measured. did.

The results of the catalyst bed temperature are shown in FIG. Further, the time when the NOx amount in the output gas becomes half of the NOx amount in the input gas was calculated as the time when the NOx purification rate reached 50% (TNOx 50% purification time) and is shown in FIG. As shown in FIG. 18, the higher the bulk density, the higher the temperature rise property. As shown in FIG. 19, when the bulk density of the ZrO ₂ composite oxide of the upstream catalyst layer support in the previous stage was 0.98 to 1.52 g / ml, there was a tendency for the 50% NOx purification time to be shortened.

<Measurement of sintering suppression effect of Pd by bulk density of ZrO ₂ composite oxide>
The same five types of ZrO ₂ composite oxide slurries supporting Pd for the upstream catalyst as used in Example 2 and Comparative Example 6 were dried at 120 ° C. for 2 hours and then at 500 ° C. for 2 hours. Firing was performed to prepare catalyst powder, and a molded body of about 1 mm square was produced with a cold isostatic pressing machine (CIP molding machine: 100 MPa, 120 seconds, 25 ° C.).

Each molded body was exposed to a gas flow having a rich composition of 2% CO / N ₂ balance and a lean composition of 5% O ₂ / N ₂ balance, and an endurance test at 1100 ° C. × 5 hours was performed. The gas flow rate was 20 L / min, and the rich and lean gases were switched every 5 minutes.

About the sample after the durability test of each molded object, the Pd crystallite diameter was computed by CO adsorption method. The result is shown in FIG. There was a tendency that sintering of Pd was suppressed as the bulk density of the ZrO ₂ composite oxide of the upstream catalyst layer support was lower.

From FIG. 18 to FIG. 20, the ZrO ₂ composite oxide of the upper stage catalyst support preferably has a bulk density range of 0.98 to 1.52 g / ml, which suppresses sintering of Pd and has high temperature rise property. Further, it can be seen that the catalyst warm-up performance can be further improved. In Example 2, the Pd loading density in the upstream catalyst layer is 0.5% by mass. However, in the case of 4.5 to 12% by mass, whether the ZrO ₂ composite oxide of the upstream catalyst layer carrier is used. The density is preferably 0.98 to 1.52 g / ml. This is because, when a ZrO ₂ composite oxide having a bulk density in the above range is used as the upstream catalyst layer support, regardless of the Pd loading density of the upstream catalyst layer, Pd sintering is suppressed and the temperature rise performance is increased. This is because the catalyst warm-up performance is improved.

<Examination 1: coating amount of upper catalyst layer and HC50% purification time>
The HC 50% purification time of the catalyst when the coating amount of the upper stage catalyst layer was changed was measured.

The catalyst was produced by the following method. First, slurry (A) for the lower catalyst layer was prepared. As starting materials, cerium nitrate, zirconium oxynitrate, lanthanum nitrate, and yttrium nitrate were dissolved in pure water to obtain a 0.3M precursor solution. Next, the precursor solution was dropped into ammonia water diluted with pure water, stirred with a homogenizer, water was removed with a centrifuge, and only the precipitate was collected. These precipitates were dried and pre-baked, and then crystallized by baking at 700 ° C. Then, it grind | pulverized with the blender and it was set as the powder. As a result, a powdery ZrO ₂ —CeO ₂ —La ₂ O ₃ —Y ₂ O ₃ composite oxide was obtained.

0.5 g / L Pd, 86 g / L ZrO ₂ —CeO ₂ —La ₂ O ₃ —Y ₂ O ₃ composite oxide, 40 g / L La-added Al ₂ O ₃ , 10 g / L BaSO ₄ and 3 g / L of Al ₂ O ₃ binder were mixed to prepare slurry (A) for the lower catalyst layer.

Next, a slurry (B) for the rear upper catalyst layer was prepared. As starting materials, aluminum nitrate, cerium nitrate, zirconium oxynitrate and lanthanum nitrate were dissolved in pure water to obtain a 0.3M precursor solution. Next, the precursor solution was dropped into ammonia water diluted with pure water, stirred with a homogenizer, water was removed with a centrifuge, and only the precipitate was collected. These precipitates were dried and pre-baked, and then crystallized by baking at 700 ° C. Then, it grind | pulverized with the blender and it was set as the powder. This powder is a composite oxide in which first particles made of La-added CeO ₂ —ZrO ₂ and _second particles made of La-added Al ₂ O ₃ are mixed at the primary particle level.

Neodymium nitrate was solubilized, and this powder was mixed and stirred. After stirring, it was dried and fired at 900 ° C. As a result, Nd ₂ O ₃ was segregated on the surface layer of the first particles and the second particles, and a powdery carrier was obtained.

This carrier was immersed in an aqueous rhodium nitrate solution and baked to carry 0.15 g / L of rhodium on the carrier. A slurry (B) for the catalyst catalyst layer on the rear stage was prepared by mixing 17 g / L of La-added Al ₂ O ₃ and 3 g / L of Al ₂ O ₃ binder with 46 g / L of the carrier supporting rhodium. .

Next, slurry (C) for the upstream catalyst layer was prepared. Similar to the composite oxide mixed in the slurry for the lower catalyst layer, except that cerium nitrate, zirconium oxynitrate, lanthanum nitrate, and praseodymium nitrate were used as starting materials, CeO ₂ —ZrO ₂ —La ₂ O ₃ the -pr ₆ O ₁₁ composite oxide was prepared. 1.0 g / L Pd, 14 to 50 g / L La-added Al ₂ O _3, and 5 to 15 g / L CeO ₂ —ZrO ₂ —La ₂ O ₃ —Pr ₆ O ₁₁ composite oxide (oxygen absorption) Release material), 7 g / L BaSO ₄ , and 3 g / L Al ₂ O ₃ binder were mixed to prepare slurry (C) for the upstream catalyst layer.

  The above three types of slurries (A), (B), and (C) were coated on a substrate. Similar to the base material of Reference Example 1, the base material has a cordierite honeycomb structure and is a cylindrical body having a diameter of 103 mm and a length of 105 mm. As shown in FIG. 8, the lower catalyst layer slurry (A) was coated on the entire surface of the substrate 1, dried and fired to form the lower catalyst layer 2. Next, the slurry for the upper catalyst layer (B) was coated on the latter part of the surface of the lower catalyst layer 2, dried and fired. The length of the rear upper catalyst layer 4 is 85 mm, and is 81% of the length of the lower catalyst layer 2. Next, the front stage catalyst layer 3 was formed by coating the front stage part of the surface of the lower catalyst layer 2 with the slurry (C) for the front stage upper catalyst layer, drying and firing. The length of the front upper catalyst layer 3 is 40 mm, which is 38% of the length of the lower catalyst layer 2. The downstream portion 3 a of the upstream catalyst layer 3 covers the upstream portion 4 a of the surface of the upstream catalyst layer 4.

  The catalyst produced as described above was exposed to an exhaust gas flow discharged from a V-type 8-cylinder engine (3UZ-FE), and an endurance test at a bed temperature of 1000 ° C. for 50 hours was performed.

  Exhaust gas in a stoichiometric atmosphere with an inlet gas temperature of 400 ° C. was passed through each catalyst whose temperature was lowered to room temperature after the endurance test was performed, and the HC purification rate was continuously measured. The time required for the HC purification rate to reach 50% (THC 50% purification time) was calculated and shown in FIG. As can be seen from FIG. 9, the smaller the coating amount of the upstream catalyst layer, the shorter the HC 50% purification time. This is thought to be because the heat capacity of the upstream catalyst layer is small because the thickness of the upstream catalyst layer is small, and it is quickly warmed by waste heat from the exhaust gas.

  In addition, when the coating amount of the upstream catalyst layer was 40 to 50 g / L, the HC 50% purification time hardly changed. When the coating amount of the upper stage catalyst layer exceeded 50 g / L, the HC50% purification time was further increased.

  On the other hand, when the coating amount of the upstream catalyst layer was less than 40 g / L, it was difficult to form the upstream catalyst layer in the manufacturing process. Therefore, in order to satisfy both catalyst performance and production conditions, it was found that the coating amount of the upstream catalyst layer is preferably 35 to 44 g / piece. This coating amount corresponds to 40 to 50 g per liter of the substrate.

  In Study 1, the Pd support density in the upstream catalyst layer is 1.4 to 3.6% by mass, but the coating amount of the upstream catalyst layer is 40 to 4.5% by mass. It can be said that ˜50 g / L is sufficient. This is because, regardless of the Pd carrying density of the upstream catalyst layer, the smaller the coating amount of the upstream catalyst layer, the shorter the HC50% purification time and the harder the coating.

<Examination 2: Length and catalyst performance of the upper catalyst layer>
The length of the rear upper catalyst layer was changed, and the HC 50% purification time and NOx emission amount of the catalyst were measured.

The slurry (A) for the lower catalyst layer is prepared by adding 0.5 g / L of Pd, 86 g / L of ZrO ₂ —CeO ₂ —La ₂ O ₃ —Y ₂ O ₃ composite oxide, and 30 g / L of La. A slurry (A) for the lower catalyst layer was prepared by mixing Al ₂ O ₃ , 10 g / L BaSO ₄ , and 3 g / L Al ₂ O ₃ binder.

The slurry (B) for the latter upper catalyst layer is a water-soluble composite oxide in which first particles composed of La-added CeO ₂ —ZrO ₂ and second particles composed of La-added Al ₂ O ₃ are mixed at the primary particle level. The mixed neodymium nitrate was mixed and stirred. After stirring, it was dried and fired at 900 ° C. As a result, Nd ₂ O ₃ was segregated on the surface layer of the first particles and the second particles, and a powdery carrier was obtained. This carrier was immersed in an aqueous rhodium nitrate solution and calcined, and 0.15 g / L of rhodium was supported on a 57 g / L carrier. 17 g / L of La-added Al ₂ O ₃ and 3 g / L of Al ₂ O ₃ binder were mixed on the carrier on which rhodium was supported to prepare slurry (B) for the upper catalyst layer.

The slurry (C) for the upper stage catalyst layer is 1.0 g / L Pd, 25 g / L La-added Al ₂ O ₃ , and 11 g / L CeO ₂ —ZrO ₂ —La ₂ O ₃ —Pr _6. O ₁₁ composite oxide (oxygen storage / release material), 7 g / L BaSO ₄ , and 3 g / L Al ₂ O ₃ binder were mixed to prepare slurry (C) for the upper catalyst layer.

  The above three types of slurries (A), (B), and (C) were coated on the substrate in the same manner as in Study 1. The coating length of the slurry (A) for the lower catalyst layer was 105 mm, and the entire surface of the substrate was coated. The coating length of the slurry (B) for the latter upper catalyst layer is 65 to 95 mm, and is 62 to 91% of the coating length of the lower catalyst layer. The coating length of the slurry (C) for the upstream catalyst layer is 35 mm, which is 33% of the length of the lower catalyst layer.

  The durability test was performed on the catalyst manufactured as described above in the same manner as in Study 1. About each catalyst after an endurance test, HC50% purification time was measured like examination 1, and the result was shown in FIG. Further, the NOx emission amount of each catalyst after the durability test was measured.

  From FIG. 10, it can be seen that the HC 50% purification time becomes shorter as the coat length of the rear upper catalyst layer becomes shorter.

  In measuring the NOx emission amount, each catalyst after the endurance test was attached to an exhaust system from which the engine discharged. The exhaust gas was circulated through each catalyst in a steady reducing atmosphere with an inlet gas temperature of 550 ° C., and the NOx emission amount of the outlet gas was detected. The NOx emission amount of each catalyst is shown in FIG.

  From FIG. 11, it can be seen that the NOx emission amount decreases as the coat length of the rear upper catalyst layer increases.

  From the above results, it can be seen that the HC 50% purification time is short and the NOx emission amount is small when the coating length of the upper catalyst layer at the rear stage is just around the center, that is, about 71 to 81%.

<Examination 3: Coating amount of the upper catalyst layer>
The NOx emission amount when the coating amount of the upper catalyst layer at the rear stage was changed was measured.

The slurry (A) for the lower catalyst layer was prepared by adding 0.5 g / L of Pd, 86 g / L of ZrO ₂ —CeO ₂ —La ₂ O ₃ —Y ₂ O ₃ composite oxide, and 40 g / L of La addition. A slurry (A) for the lower catalyst layer was prepared by mixing Al ₂ O ₃ , 10 g / L BaSO ₄ , and 3 g / L Al ₂ O ₃ binder.

The slurry (B) for the latter upper catalyst layer is a water-soluble composite oxide in which first particles composed of La-added CeO ₂ —ZrO ₂ and second particles composed of La-added Al ₂ O ₃ are mixed at the primary particle level. The mixed neodymium nitrate was mixed and stirred. After stirring, it was dried and fired at 900 ° C. As a result, Nd ₂ O ₃ was segregated on the surface layer of the first particles and the second particles, and a powdery carrier was obtained. This carrier was immersed in an aqueous rhodium nitrate solution and calcined, so that 0.15 g / L of rhodium was supported on a 46 to 80 g / L carrier. 6 to 29 g / L of La-added Al ₂ O ₃ and 3 g / L of Al ₂ O ₃ binder were mixed with the carrier on which rhodium was supported to prepare a slurry (B) for the upper catalyst layer.

The slurry (C) for the upper stage catalyst layer is 1.0 g / L Pd, 25 g / L La-added Al ₂ O ₃ , and 11 g / L CeO ₂ —ZrO ₂ —La ₂ O ₃ —Pr _6. O ₁₁ composite oxide (oxygen storage / release material), 7 g / L BaSO ₄ , and 3 g / L Al ₂ O ₃ binder were mixed to prepare slurry (C) for the upper catalyst layer.

  The above three types of slurries (A), (B), and (C) were coated on the substrate in the same manner as in Study 1. The coating length of the slurry (A) for the lower catalyst layer was 105 mm, and the entire surface of the substrate was coated. The coating length of the slurry (B) for the rear upper catalyst layer is 85 mm, and is 81% of the coating length of the lower catalyst layer. The coating length of the slurry (C) for the upstream catalyst layer is 40 mm and is 38% of the length of the lower catalyst layer. The coating amount of the catalyst layer on the latter stage of the catalyst produced as described above is changed between 60 and 115 g / L by increasing or decreasing the coating amount of the carrier.

  The durability test was performed on the catalyst manufactured as described above in the same manner as in Study 1. For each catalyst after the durability test, the NOx emission amount was measured in the same manner as in Study 2. The results are shown in FIG.

  As can be seen from FIG. 12, the NOx emission amount decreased as the coating amount of the rear upper catalyst layer increased. On the other hand, when the coating amount of the rear upper catalyst layer was less than 60 g / L, it was difficult to form the rear upper catalyst layer in the production process. Therefore, in order to satisfy both the catalyst performance and the production conditions, it can be seen that the coating amount of the upper stage catalyst layer is preferably 57.8 to 78.8 g / piece, which corresponds to 66 to 90 g per liter of the base material. To do.

<Examination 4: Coating amount of lower catalyst layer>
The OSC performance and the pressure loss were measured for each catalyst in which the coating amount of the lower catalyst layer was changed.

The slurry (A) of the lower catalyst layer is composed of 0.5 g / L of Pd, 69 to 120 g / L of ZrO ₂ —CeO ₂ —La ₂ O ₃ —Y ₂ O ₃ composite oxide (OSC material), 29 A slurry (A) for the lower catalyst layer was prepared by mixing ˜40 g / L La-added Al ₂ O ₃ , 10 g / L BaSO ₄ , and 3 g / L Al ₂ O ₃ binder.

The slurry (B) for the latter upper catalyst layer is a water-soluble composite oxide in which first particles composed of La-added CeO ₂ —ZrO ₂ and second particles composed of La-added Al ₂ O ₃ are mixed at the primary particle level. The mixed neodymium nitrate was mixed and stirred. After stirring, it was dried and fired at 900 ° C. As a result, Nd ₂ O ₃ was segregated on the surface layer of the first particles and the second particles, and a powdery carrier was obtained. This carrier was immersed in an aqueous rhodium nitrate solution and calcined, and 0.15 g / L of rhodium was supported on a 57 g / L carrier. 29 g / L of La-added Al ₂ O ₃ and 3 g / L of Al ₂ O ₃ binder were mixed on the carrier on which rhodium was supported to prepare slurry (B) for the upper catalyst layer.

The slurry (C) for the upper catalyst layer in the previous stage was prepared by adding 1.0 g / L of Pd, 25 g / L of La-added Al ₂ O ₃ , and 11 g / L of CeO ₂ —ZrO ₂ —La ₂ O ₃ —Pr _6. O ₁₁ composite oxide (oxygen storage / release material), 7 g / L BaSO ₄ , and 3 g / L Al ₂ O ₃ binder were mixed to prepare slurry (C) for the upper catalyst layer.

  The above three types of slurries (A), (B), and (C) were coated on the substrate in the same manner as in Study 1. The coating length of the slurry (A) for the lower catalyst layer was 105 mm, and the entire surface of the substrate was coated. The coating length of the slurry (B) for the rear upper catalyst layer is 85 mm, and is 81% of the coating length of the lower catalyst layer. The coating length of the slurry (C) for the upstream catalyst layer is 40 mm and is 38% of the length of the lower catalyst layer. The coating amount of the lower catalyst layer of the catalyst produced as described above is changed between 97 and 171 g / L by increasing or decreasing the coating amount of the carrier.

  About each manufactured catalyst, the durability test similar to examination 1 was implemented, and OSC performance and pressure loss were measured. Regarding the OSC performance, each catalyst after the endurance test was placed in the exhaust system of a 2.4 L engine, and the catalyst in the exhaust gas at a catalyst bed temperature of 450 ° C. and an air-fuel ratio of 14.1 to 15.1 The oxygen concentration was measured continuously. The amount of oxygen released between when the air-fuel ratio is high and when it is low is shown in FIG. 13 as the oxygen storage amount. The pressure loss was measured in the same manner as in Study 2, and the result is shown in FIG.

As can be seen from FIG. 13, the OSC performance improved as the coating amount of the lower catalyst layer increased. This is because the coating amount of the lower catalyst layer was changed while keeping the length of the lower catalyst layer constant, so that the thickness of the lower catalyst layer was changed, and as the thickness of the lower catalyst layer increased, ZrO ₂ —CeO ₂ —La ₂ The absolute amount of the O ₃ —Y ₂ O ₃ composite oxide is increased, so that the amount of oxygen that can be absorbed and released, that is, the OSC performance is increased.

  As known from FIG. 14, the pressure loss increased as the coating amount of the lower catalyst layer increased. This is because as the coating amount of the lower catalyst layer increases, the thickness of the lower catalyst layer increases, and as a result, the opening cross-sectional area of the gas flow path decreases.

  From the results of the OSC performance and the pressure loss, when the coating amount of the lower catalyst layer is around the middle of the applied amount, that is, around 105 to 155 g / L, further 150 to 155 g / L, It can be said that the OSC performance is good and the pressure loss can be reduced.

<Examination 5: Coating amount of lower catalyst layer>
The HC 50% purification temperature and pressure loss were measured for each catalyst in which the coating amount of the lower catalyst layer was changed.

The slurry (A) of the lower catalyst layer is composed of 1.5 g / L of Pd, 60 to 100 g / L of ZrO ₂ —CeO ₂ —La ₂ O ₃ —Y ₂ O ₃ composite oxide (OSC material), 24 A slurry (A) for the lower catalyst layer is prepared by mixing ˜40 g / L La-added Al ₂ O ₃ , 6-10 g / L BaSO ₄ , and 3-5 g / L Al ₂ O ₃ binder. did.

In the slurry (B) for the upper stage catalyst layer, the first particles composed of La ₂ O ₃ added Al ₂ O ₃ and the second particles composed of ZrO ₂ —CeO ₂ —La ₂ O ₃ were mixed at the primary particle level. The composite oxide was mixed with water-soluble neodymium nitrate and stirred. After stirring, it was dried and fired at 900 ° C. As a result, a powdery carrier obtained by concentrating Nd ₂ O ₃ on the surface layer of the first particles and the second particles was obtained. This carrier was immersed in an aqueous rhodium nitrate solution and calcined, so that 0.3 g / L of rhodium was supported on a 55 g / L carrier. On the support on which rhodium was supported, 35 g / L La-added Al ₂ O ₃ and 4 g / L Al ₂ O ₃ binder were mixed to prepare slurry (B) for the upper catalyst layer.

The slurry (C) for the upper stage catalyst layer was 6.0 g / L of Pd, 77 g / L of La-added Al ₂ O ₃ , and 35 g / L of CeO ₂ —ZrO ₂ —La ₂ O ₃ —Pr _6. O ₁₁ composite oxide (oxygen storage / release material), 20 g / L BaSO ₄ , and 10 g / L Al ₂ O ₃ binder were mixed to prepare slurry (C) for the upper catalyst layer.

  The above three types of slurries (A), (B), and (C) were coated on the substrate in the same manner as in Study 1. The capacity of the substrate is 875 cc. The lower catalyst layer slurry (A) was coated on the entire surface of the substrate. The coating length of the slurry (B) for the rear upper catalyst layer was 80% with respect to the total length of the substrate. The coating length of the slurry (C) for the upstream catalyst layer was 35% with respect to the total length of the substrate. The coating amount of the lower catalyst layer of the catalyst produced as described above is changed between 80 and 160 g / L by increasing or decreasing the coating amount of the carrier.

  Each manufactured catalyst was subjected to a durability test at a bed temperature of 1000 ° C. for 50 hours under the exhaust gas of a V-type 8-cylinder engine (3UZ-FE). After the durability test, the HC50 purification time and OSC performance were measured. The HC 50% purification temperature was measured in the same manner as in Study 2, and the results are shown in FIG. The OSC performance was measured in the same manner as in Study 4 above, and the results are shown in FIG.

  As can be seen from FIG. 15, when the coating amount of the lower catalyst layer was 105 to 155 g / L or more, the HC50% purification temperature was low, and excellent low temperature purification performance was exhibited. In the case of 90 g / L or less, the HC 50% purification temperature became high and the low temperature purification performance deteriorated. As known from FIG. 16, the OSC performance tended to decrease as the coating amount of the lower catalyst layer decreased. When the coating amount of the lower catalyst layer was 105 to 155 g / L, it was separately confirmed by vehicle evaluation that there was no problem in OSC performance.

  From these results, it can be seen that when the coating amount of the lower catalyst layer is 105 g / L or more, the low-temperature purification characteristics and OSC performance of the lower catalyst layer are excellent. Combined with the result of Study 4 above, if the coating amount of the lower catalyst layer is 155 g / L or less, the pressure loss can be lowered, so that the coating amount of the lower catalyst layer is 105 to 155 g / L. Is good.

<Examination 6: Total-GSA (total geometric surface area) after catalyst coating>
The OSC performance was measured for each catalyst in which the Total-GSA after the catalyst coating was changed.

The slurry (A) of the lower catalyst layer is composed of 0.5 g / L of Pd, 69 to 120 g / L of ZrO ₂ —CeO ₂ —La ₂ O ₃ —Y ₂ O ₃ composite oxide (OSC material), 29 A slurry (A) for the lower catalyst layer was prepared by mixing ˜40 g / L La-added Al ₂ O ₃ , 10 g / L BaSO ₄ , and 3 g / L Al ₂ O ₃ binder.

The slurry (B) for the latter upper catalyst layer is a water-soluble composite oxide in which first particles composed of La-added CeO ₂ —ZrO ₂ and second particles composed of La-added Al ₂ O ₃ are mixed at the primary particle level. The mixed neodymium nitrate was mixed and stirred. After stirring, it was dried and fired at 900 ° C. As a result, Nd ₂ O ₃ was segregated on the surface layer of the first particles and the second particles, and a powdery carrier was obtained. This carrier was immersed in an aqueous rhodium nitrate solution and calcined, and 0.15 g / L of rhodium was supported on a 57 g / L carrier. 29 g / L of La-added Al ₂ O ₃ and 3 g / L of Al ₂ O ₃ binder were mixed on the carrier on which rhodium was supported to prepare slurry (B) for the upper catalyst layer.

The slurry (C) for the upper stage catalyst layer is 1.0 g / L Pd, 25 g / L La-added Al ₂ O ₃ , and 11 g / L CeO ₂ —ZrO ₂ —La ₂ O ₃ —Pr _6. O ₁₁ composite oxide (oxygen storage / release material), 7 g / L BaSO ₄ , and 3 g / L Al ₂ O ₃ binder were mixed to prepare slurry (C) for the upper catalyst layer.

  A substrate having a capacity of (i) 875 cc, (ii) 1100 cc, (iii) 1294 cc was prepared. These base materials are all cylindrical bodies having a diameter of 103 mm, the cross-sectional areas thereof are the same, and the lengths in the longitudinal axis direction are 105 mm, 130 mm, and 150 mm, respectively. Each substrate has 600 regular hexagonal cells. Further, a base material (iv) having the same diameter and length as the 875 cc base material and 750 regular hexagonal cells was prepared.

The above three types of slurries (A), (B), and (C) were coated on the substrates (i) to (iv), respectively. The lower catalyst layer slurry (A) was coated on the entire surface of each substrate. The coating length of the slurry (B) for the upper stage catalyst layer is (i) 84 mm, (ii) 104 mm, (iii) 120 mm, and (iv) 84 mm for each base material. 80% of the length. The coating length of the slurry (C) for the upstream catalyst layer is (i) 37 mm, (ii) 46 mm, (iii) 53 mm, and (iv) 37 mm for each base material. The length is 35%. The coating amount for each substrate is 276 g, which are all the same. Then, the surface area of the catalyst coated on the cells in the cross section perpendicular to the longitudinal direction of the base material is measured with a microscope for 100 cells, and the average value of GSA is included in the base material. Total-GSA was derived by multiplying the number of cells to be measured and the length in the longitudinal direction of the substrate. Total-GSA was (i) 22.4 cm ² / piece, (ii) 25.6 cm ² / piece, (iii) 29.7 cm ² / piece, and (iv) 37.2 cm ² / piece.

  About each manufactured catalyst, the OSC performance in each Ga was measured. Each catalyst was placed in the exhaust system of a 2.4 L engine, and the oxygen concentration in the output gas was continuously measured at an air-fuel ratio of 14.1 to 15.1 at a catalyst bed temperature of 450 ° C. The amount of oxygen released between when the air-fuel ratio is high and when it is low is shown in FIG. 17 as the oxygen storage amount.

As can be seen from FIG. 17, in the low Ga region where Ga is 20 g / s or less, the OSC performance is almost the same regardless of GSA, but in the high Ga region where Ga is 30 g / s or more, Total-GSA The larger the value, the better the OSC performance. In order to obtain high OSC performance even in a high Ga region of 30 g / s or more, Total-GSA is preferably 25 to 36 cm ² / piece. This is considered that the OSC performance is rate-determined by the catalyst coating amount and the catalyst noble metal amount in the low Ga region, whereas the influence of gas diffusibility to the catalyst coating layer is large in the high Ga region.

DESCRIPTION OF SYMBOLS 1 Base material 2 Lower catalyst layer 3 Previous stage upper catalyst layer 4 Rear stage upper catalyst layer 6 Gas passage 10 Catalyst layer 11 Partition

Claims (8)

A base material forming a gas flow path through which exhaust gas flows and a catalyst layer formed on the base material, the catalyst layer comprising: a lower catalyst layer formed on the surface of the base material; and the lower catalyst A front upper catalyst layer covering the upstream side in the gas flow direction on the surface of the layer, and a rear upper layer covering the downstream side in the gas flow direction from the front upper catalyst layer on the surface of the lower catalyst layer An exhaust gas purifying catalyst comprising a catalyst layer,
The lower catalyst layer carries at least one of Pd and Pt;
The latter upper catalyst layer carries Rh,
The previous upper catalyst layer carries Pd,
An exhaust gas purifying catalyst, wherein the carrier supporting Pd in the upstream catalyst layer is a ZrO ₂ composite oxide containing Y ₂ O ₃ . The ZrO ₂ composite oxide, based on the total mass of the ZrO ₂ composite oxide, containing 20-30 wt% of Y ₂ O _3, the catalyst for purification of exhaust gas according to claim 1. The exhaust gas-purifying catalyst according to claim 1 or 2, wherein a bulk density of the ZrO ₂ composite oxide is 0.98 to 1.52 g / ml.   The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the Pd carrying density of the upstream catalyst layer is 4.5 to 12% by mass.   The exhaust gas purifying catalyst according to any one of claims 1 to 4, wherein a ratio of a length of the upstream catalyst layer to a length of the lower catalyst layer is 20 to 40%.   The exhaust gas purifying catalyst according to any one of claims 1 to 5, wherein a length of the rear upper catalyst layer with respect to a length of the lower catalyst layer is 71 to 81%.   The lower catalyst layer contains ceria, the content of the ceria is 21 to 30 g per liter of the base material, and the coating amount of the lower catalyst layer per liter of the base material is 105 to 155 g. The exhaust gas-purifying catalyst according to any one of claims 1 to 6, wherein Geometric surface area of the catalyst after coating is 25~36cm ² / number, the exhaust gas purifying catalyst according to any one of claims 1 to 7.

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