Catalysts for exhaust gas purification

Abstract

According to the technology disclosed in the present invention, there is provided an exhaust gas purification catalyst that can take into account both the suppression of OSC when a new vehicle is used and the maintenance of OSC during its life cycle. The exhaust gas purification catalyst disclosed herein includes a substrate, and a catalyst coating formed on the surface of the substrate, the catalyst coating containing an OSC material having an oxygen storage capacity. The catalyst coating includes an Rh layer having Rh as a catalyst metal as a main body, and a Pd / Pt layer having Pd and / or Pt as a catalyst metal as a main body. Here, at least a portion of the Pd / Pt layer in the catalyst coating contains a cerium oxide-zirconium oxide composite oxide having a specific surface area of ​​40 m 2 / g above 60m 2 / g or less low specific surface area OSC material as the above-mentioned OSC material.

Description

Technical Field

[0001] The present invention relates to an exhaust gas purification catalyst. In particular, it relates to an exhaust gas purification catalyst having a substrate and a catalyst coating formed on the surface of the substrate. In addition, this international application claims priority based on Japanese patent application No. 2020-110654 filed on June 26, 2020, and the entire contents of the application are incorporated into this specification as a reference. Background Art

[0002] To purify HC (hydrocarbons), NO x In order to remove CO and other exhaust gases, a three-way catalyst containing at least one of the noble metal catalysts such as Pt (platinum), Pd (palladium) and Rh (rhodium) is often used. In a typical structure of such a three-way catalyst, a catalyst coating composed of aluminum oxide is formed on the surface of a high heat-resistant ceramic substrate, and one or more of the noble metals Pt, Pd and Rh are carried in the catalyst coating. By using Pt, Pd and Rh together, harmful components in the exhaust gas can be effectively purified at one time.

[0003] In order to effectively purify the components in the exhaust gas using such a three-way catalyst, it is desired that the mixing ratio of air and gasoline supplied to the engine, i.e., the air-fuel ratio (A / F ratio), is near the theoretical air-fuel ratio (theoretical ratio, i.e., 14.7). In the prior art, in order to mitigate the atmosphere fluctuation of the air-fuel ratio at which the catalyst can effectively work, Ce-containing oxides (e.g., cerium oxide-zirconium oxide composite oxides) with oxygen storage capacity (OSC: oxygen storage capacity) are widely used as carriers of the above-mentioned precious metals (e.g., patent documents 1 and 2). Ce-containing oxides can play the role of storing oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas is lean (i.e., the atmosphere on the oxygen-excess side), and releasing the stored oxygen when the air-fuel ratio of the exhaust gas is rich (i.e., the atmosphere on the fuel-excess side). As a result, even when the oxygen concentration in the exhaust gas changes, A / F control is easy, and stable exhaust oxidation-reduction catalyst performance can be obtained, and the exhaust purification performance of the three-way catalyst is improved.

[0004] In recent years, exhaust gas restrictions have been gradually strengthened, and exhaust gas purification catalysts carried by vehicles need to have better catalyst capabilities. In addition, the durable driving distance of vehicles has also been extended, and exhaust gas purification catalysts with higher durability are required. In order to improve the durability of exhaust gas purification catalysts, for example, Patent Documents 1 and 2 disclose a method of arranging a material having OSC (hereinafter referred to as OSC material) at a specific position on a substrate.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2014-97459

[0008] Patent Document 2: Japanese Patent Application Publication No. 2006-181476 Summary of the invention

[0009] Problems to be solved by the invention

[0010] However, as a strategy to strengthen exhaust gas restrictions in recent years, most of them are currently responding by increasing the amount of precious metals and OSC materials used in catalysts. In addition, Europe has introduced real road driving exhaust (RDE: Real Driving Emissions, actual driving emissions) restrictions, and the exhaust gas test method has changed from driving tests on chassis dynamometers to actual tests on public roads. With this situation, in order to deal with the purification of exhaust gas discharged under various operating conditions and from the perspective of extending the durable driving distance of vehicles, there is a tendency to further increase the amount of precious metals and OSC materials used.

[0011] However, the amount of exhaust gas purification catalyst used cannot be increased indefinitely. When OSC is too large, especially after the vehicle is first used, the change in the oxygen content in the exhaust gas is small, and sometimes air-fuel ratio learning and on-board diagnosis (OBD: On-Board Diagnosis, vehicle fault diagnosis system) of oxygen after passing through the exhaust gas purification device become difficult. An exhaust gas purification catalyst with excellent durability that can suppress OSC when a new vehicle is first used and maintain the desired OSC after long-term use is sought.

[0012] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an exhaust gas purification catalyst capable of achieving both suppression of OSC to an appropriate level when mounted on a vehicle and starting use, and maintenance of OSC during the life cycle (long-term use).

[0013] Technical solutions to solve problems

[0014] The inventors of the present invention have found that by setting the specific surface area (herein, the specific surface area measured by the BET method, the same below) of an OSC material composed of a ceria-zirconia composite oxide to a predetermined range smaller than that of a material used in a conventional catalyst for purifying exhaust gas of this type, the OSC amount at the beginning of use of the OSC material (hereinafter referred to as the initial OSC amount) can be effectively controlled to an appropriate level. Moreover, such an OSC material can suppress the rate of decrease in the OSC amount after long-term use (hereinafter referred to as the post-durable OSC amount) compared to conventional OSC materials having a high initial OSC amount.

[0015] That is, the exhaust gas purification catalyst provided by the present invention includes a substrate and a catalyst coating formed on the surface of the substrate. The catalyst coating contains an OSC material having an oxygen storage capacity, and the catalyst coating includes an Rh layer having Rh as a catalyst metal as a main body, and a Pd / Pt layer having Pd and / or Pt as a catalyst metal as a main body. At least a portion of the Pd / Pt layer in the catalyst coating contains a cerium oxide-zirconia composite oxide having a specific surface area of ​​40 m 2 / g above 60m 2 / g or less low specific surface area OSC material as the above-mentioned OSC material.

[0016] In the exhaust gas purification catalyst thus constructed, the specific surface area of ​​the OSC material contained therein is adjusted to the above-specified range, so that the initial OSC amount can be appropriately suppressed, and the decrease in the OSC amount after long-term use, i.e., after durability, can be suppressed, and the OSC maintenance rate (the ratio of the OSC amount after durability under certain conditions expressed as a percentage to the initial OSC amount, the same below) can be increased to be higher than that of the prior art. Therefore, by using the exhaust gas purification catalyst thus constructed, appropriate A / F control can be achieved over a long period of time.

[0017] In a preferred embodiment of the exhaust gas purification catalyst disclosed herein, the catalyst coating includes a lower layer formed on the substrate and an upper layer formed on the lower layer, one of the upper layer and the lower layer constitutes the Rh layer, and the other constitutes the Pd / Pt layer.

[0018] With the exhaust gas purification catalyst having such a configuration, appropriate A / F control can be achieved over a long period of time, and in particular, the desired exhaust gas purification performance of the Pd / Pt layer can be maintained over a long period of time.

[0019] In the above configuration, it is preferred that the lower layer constitutes the Pd / Pt layer.

[0020] With such a configuration, the intended exhaust gas purification performance of the Pd / Pt layer can be particularly appropriately exhibited.

[0021] In a preferred embodiment of the exhaust gas purification catalyst disclosed herein, the catalyst coating layer includes a front portion located on the upstream side of the exhaust gas inflow direction and a rear portion located on the downstream side, the front portion is formed from the end of the exhaust gas inflow side of the substrate toward the exhaust gas outflow side, and the rear portion is formed from the end of the exhaust gas outflow side of the substrate toward the exhaust gas inflow side. The total length of the substrate in the exhaust gas flow direction is set as L w When the length L1 of the front portion is 0.3×L w ≤L1≤0.7×L w, the length L2 of the rear portion is 0.5×L w ≤L2≤0.85×L w , and satisfy (L1+L2)≥1.05×L w The front portion and the rear portion are formed in a partially overlapping manner. Here, the content (g / L) of the low specific surface area OSC material in the front portion relative to the volume (L) of the substrate is 40 g / L or less, and the content (g / L) of the low specific surface area OSC material in the rear portion relative to the volume (L) of the substrate is 1.2 times or more of the content (g / L) of the low specific surface area OSC material in the front portion.

[0022] In the exhaust gas purifying catalyst of this structure, in the relationship between the front part and the rear part divided as described above, the low specific surface area OSC material is arranged more in the rear part. By arranging the OSC material in this way, it is possible to appropriately balance the reduction of the initial OSC amount and the maintenance of the OSC amount after durability.

[0023] In a preferred embodiment of the exhaust gas purification catalyst disclosed herein, the low specific surface area OSC material contains the following metal elements in the following ratios in terms of molar percentage:

[0024]

[0025] In such a configuration, Pr is contained in a ratio of 1 mol % to 10 mol % of the total metal elements. This improves the catalyst activity in a lower temperature range (about 200 to 400° C.).

[0026] In the present specification, “containing 0% or more and 10% or less” means “may not be contained, but when contained, it is 10% or less.” In addition, the content ratio of metal elements other than the above-mentioned metal elements is not limited. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram showing the structure of a substrate of an exhaust gas purification catalyst according to an embodiment.

[0028] Figure 2 It is a schematic diagram showing the structure of a catalyst coating layer according to one embodiment.

[0029] Figure 3 It is a graph showing the relationship between the endurance temperature and the specific surface area of ​​the OSC material.

[0030] Figure 4 It is a graph showing the relationship between the endurance time and the OSC maintenance rate.

[0031] Figure 5It is a graph showing the relationship between the gas temperature at the catalyst inlet and the CO purification rate.

[0032] Figure 6 It is a graph showing the relationship between the amount of the ceria-zirconia composite oxide disposed in the front portion and the amount of OSC.

[0033] Figure 7 It is a graph showing the relationship between the ratio of the ceria-zirconia composite oxide disposed in the front part and the rear part, respectively, and the OSC maintenance rate.

[0034] Figure 8 This is a graph showing the relationship between the amount of the ceria-zirconia composite oxide disposed in the front portion and the warm-up characteristics. DETAILED DESCRIPTION

[0035] Below, the preferred embodiment of the exhaust gas purification catalyst disclosed herein is described with reference to the accompanying drawings. The following embodiments are of course not intended to specifically limit the technology disclosed herein. Among them, matters other than matters specifically mentioned in this specification (such as the composition of the porous carrier, etc.) and matters necessary for the implementation of the present invention (such as general matters such as the configuration of the exhaust gas purification catalyst) can be grasped as design matters for those skilled in the art based on the prior art in the field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the field.

[0036] In addition, when a numerical range is expressed as A to B (wherein A and B are arbitrary numerical values) in the present specification, it means not less than A and not more than B.

[0037] In the following figures, the same symbols are used for parts and locations that play the same role, and repeated descriptions are sometimes omitted or simplified. In addition, the dimensional relationships (length, thickness, width, etc.) in the following figures do not necessarily reflect the actual dimensional relationships, nor do they limit the composition of the exhaust gas purification catalyst at all.

[0038] The exhaust gas purification catalyst disclosed herein includes a substrate and a catalyst coating layer formed on the inner surface of the substrate, and the catalyst coating layer is formed in a single-layer structure or a stacked-layer structure.

[0039] Figure 1 Schematic diagram showing the structure of a substrate of an exhaust gas purifying catalyst according to an embodiment. An exhaust gas purifying catalyst 100 according to an embodiment includes a substrate 10 having regularly arranged cells 12 and rib walls 14 constituting the cells 12 .

[0040] As the above-mentioned substrate 10 constituting the catalyst 100 for exhaust purification, substrates of various raw materials and forms currently used for such purposes can be used. For example, it is preferred to use a honeycomb substrate having a honeycomb structure formed of ceramics such as cordierite, silicon carbide (SiC) or alloys (stainless steel, etc.). As an example, a honeycomb substrate having a cylindrical shape can be cited, in which a through hole (chamber) serving as an exhaust passage is provided in the axial direction of the cylinder, and the exhaust gas can contact the partition wall (rib wall) separating each chamber. In addition, regarding the overall shape of the substrate, an elliptical cylinder or a polygonal cylinder can also be used instead of a cylindrical shape. It should be noted that in this specification, the volume (capacity) of the substrate 10 refers to the loose volume including the internal void volume in addition to the pure volume of the substrate. That is, the volume (capacity) of the substrate 10 includes the catalyst coating formed in the void.

[0041] Figure 2 It is a schematic diagram showing the structure of a catalyst coating layer according to one embodiment.

[0042] The catalyst coating layer 30 may include a lower layer 32 formed on the substrate 10, and an upper layer 34 formed on the lower layer 32. In addition, the catalyst coating layer 30 includes a front portion 36 formed from the end of the exhaust gas inlet side of the substrate 10 toward the exhaust gas outlet side, and a rear portion 38 formed from the end of the exhaust gas outlet side of the substrate 10 toward the exhaust gas inlet side.

[0043] Here, when the catalyst coating 30 includes a lower layer 32, an upper layer 34, a front portion 36 and a rear portion 38, the front portion of the upper layer 34 is referred to as an upper layer front portion, the rear portion of the upper layer 34 is referred to as an upper layer rear portion, the front portion of the lower layer 32 is referred to as a lower layer front portion, and the rear portion of the lower layer 32 is referred to as a lower layer rear portion.

[0044] The catalyst coating layer 30 includes a catalyst metal and an OSC material. The catalyst coating layer 30 includes an Rh layer mainly containing Rh as the catalyst metal, and a Pd / Pt layer mainly containing Pd and / or Pt as the catalyst metal.

[0045] It should be noted that the layers constituting the catalyst coating layer 30 are not limited to the Rh layer and the Pd / Pt layer, and may include layers other than these. Furthermore, the number of layers is not limited to one.

[0046] In the present application, "main body" refers to the component with the largest mass in the catalyst metal constituting the layer possessed by the catalyst coating layer. Generally, the catalyst metal as the main body accounts for 75 mass% or more, preferably 80 mass% or more, more preferably 90 mass% or more, particularly preferably 95 mass% or more, for example, 98 mass% or more, of the catalyst metal contained in the layer constituting the catalyst coating layer, but it is not limited thereto.

[0047] The content of the catalyst metal contained in the catalyst coating is not particularly limited, and is usually 2.5 g / L or more, preferably 3.5 g / L or more, more preferably 4.5 g / L or more, and is usually 10 g / L or less, preferably 8 g / L or less, more preferably 7 g / L or less, relative to 1 L of the substrate capacity.

[0048] The catalyst coating layer may contain noble metals other than Rh, Pd and Pt, such as Ru, Ir, Os, etc., to an extent that the effects of the present invention are not impaired.

[0049] The OSC material disclosed herein may be a material having oxygen storage capacity such as CeO2 or a ceria-zirconia composite oxide, and preferably a material composed of a ceria-zirconia composite oxide. The mixing ratio of cerium oxide in the ceria-zirconia composite oxide is usually 0.1 to 0.9, preferably 0.25 to 0.75, and more preferably about 0.25.

[0050] Here, at least a portion of the Pd / Pt layer is characterized by containing, as an OSC material, a material having a specific surface area of ​​40 to 60 m 2 / g low specific surface area OSC material. Compared with the specific surface area of ​​currently used OSC materials (70~100m 2 / g), this value is controlled to be a lower value.

[0051] In this specification, low specific surface area OSC materials refer to materials with a specific surface area of ​​40 to 60 m 2 / g of OSC materials. The existing OSC materials refer to those with a specific surface area greater than 60m 2 / g of OSC material.

[0052] Low specific surface area OSC materials are produced by sintering materials constituting OSC materials such as ceria-zirconia composite oxides at high temperatures for a certain period of time. For example, by heating the materials constituting OSC materials at 800°C or more for more than one hour, the specific surface area of ​​the OSC material can be adjusted to 40 to 60 m 2 / g. By controlling the specific surface area to a lower value compared to the existing OSC materials, the initial OSC amount can be appropriately suppressed, and the OSC maintenance rate can be further improved compared to the prior art. Here, the firing temperature and firing time can be appropriately set according to the expected initial OSC amount and OSC maintenance rate. Usually, the OSC material can be appropriately prepared by heating the materials constituting the OSC material at 800 to 1200°C for 0.1 to 10 hours, for example, at 900 to 1100°C for 1 to 5 hours.

[0053] The reason why such an effect is obtained can be considered as follows.

[0054] Figure 3 This is a graph showing the relationship between the endurance temperature and the specific surface area of ​​the OSC material. Since the initial specific surface area of ​​the low specific surface area OSC material is lower than that of the existing OSC material, the initial OSC amount is low, but after endurance, the specific surface area of ​​the low specific surface area OSC material reaches the same level as that of the existing OSC material, and the OSC amount after endurance is also the same level.

[0055] For such reasons, compared with the case where the amount of existing OSC material is increased relative to the catalyst coating, the initial OSC amount of the exhaust purification catalyst as a whole can be controlled to be lower by increasing the amount of low specific surface area OSC material contained, and by increasing the amount of OSC material, the durable OSC amount of the exhaust purification catalyst as a whole can be maintained.

[0056] In addition, as described above, since the specific surface area of ​​the low specific surface area OSC material is not easy to decrease, the particle growth of the catalyst metal can be suppressed, thereby suppressing the degradation of the catalyst performance. For example, by including the low specific surface area OSC material in the Pd / Pt layer containing Pd and Pt whose particles grow more easily than Rh, the degradation of the catalyst performance of the Pd / Pt can be effectively suppressed.

[0057] In addition, the OSC material contained in the catalyst coating is not limited to the low specific surface area OSC material disclosed herein, and may also contain an existing OSC material. The catalyst coating may contain only a low specific surface area OSC material, or may contain both a low specific surface area OSC material and an existing OSC material, but is not particularly limited. In addition, the content of the low specific surface area OSC material and the existing OSC material may be changed according to the layer or part of the layer (e.g., the front part and the rear part) constituting the catalyst coating, and a layer not containing a low specific surface area OSC material may also be provided.

[0058] There is no particular restriction on the amount of OSC material (ceria-zirconia composite oxide) contained in the catalyst coating. Relative to 1 L of the substrate capacity, the amount of CeO2 in the ceria-zirconia composite oxide is usually more than 5 g / L, preferably more than 10 g / L, and usually less than 50 g / L, preferably less than 40 g / L.

[0059] In the OSC material, for example, in order to suppress sintering or improve durability, compounds other than the ceria-zirconia composite oxide may be contained to a degree that does not impair performance. Examples of such compounds include La2O3, Y2O3, Pr6O 11 Polycrystals or single crystals of rare earth oxides, alkali metal oxides, alkaline earth metal oxides such as BaO, transition metal species, metal oxides such as Al2O3, alkali metals, alkaline earth metals, rare earth metals, SiO2, etc.

[0060] Among them, as a carrier for supporting the metal catalyst, it is preferred to use a porous metal oxide with excellent heat resistance. Among them, by using Al2O3 with excellent heat resistance, the thermal stability of the catalyst as a whole is improved.

[0061] In a preferred embodiment of the technology disclosed herein, the low specific surface area OSC material is characterized by containing, by mole percentage, 15% to 55% Ce, 40% to 80% Zr, 1% to 10% Pr, 0% to 10% La, and 0% to 10% Y. The above metal elements are, for example, CeO2, ZrO2, Pr6O 11 , La2O3, Y2O3, etc. Preferably, the low specific surface area OSC material contains Pr 2-7%.

[0062] The reason for such an effect can be considered as follows. When HC contained in the exhaust gas is decomposed in the exhaust gas purification catalyst, Pd is prone to absorb hydrogen with heat release at about 200 to 400°C, and the catalyst activity decreases. 11 OSC materials such as La2O3 and Y2O3 can appropriately improve the catalytic activity even within the above temperature range.

[0063] In one embodiment disclosed herein, the catalyst coating layer 30 includes a lower layer 32 formed on the substrate 10, and an upper layer 34 formed on the lower layer 32. One of the upper layer 34 and the lower layer 32 constitutes an Rh layer, and the other constitutes a Pd / Pt layer. At least a portion of the OSC material constituting the Pd / Pt layer contains a low specific surface area OSC. With such a configuration, the initial OSC amount can be appropriately adjusted, and the OSC maintenance rate can be improved.

[0064] In one embodiment, the lower layer is a Pd / Pt layer. With such a structure, the Pd / Pt layer can particularly appropriately exhibit the expected exhaust gas purification performance.

[0065] In one embodiment disclosed herein, the catalyst coating layer 30 includes a front portion 36 located on the upstream side in the exhaust gas inflow direction and a rear portion 38 located on the downstream side.

[0066] The total length of the substrate 10 in the exhaust gas flow direction is represented by L w When the length L1 of the front portion 36 is 0.3×L w Above, and 0.7×L w The length L2 of the rear portion 38 is 0.85×L w Less than 0.5×L w Furthermore, the front portion 36 and the rear portion 38 are configured to partially overlap, and the overlapping length is 0.05×L wAbove, preferably 0.2×L w More preferably 0.4×L w above.

[0067] The content of low specific surface area OSC material constituting the front portion 36 is less than 40 g / L relative to the substrate capacity 1L, and the content of low specific surface area OSC material constituting the rear portion 38 is generally greater than the content of low specific surface area OSC material constituting the front portion 36, preferably greater than 1.2 times, for example greater than 1.5 times.

[0068] The reason for such an effect is that the OSC material in the front part is activated earlier than the OSC material in the rear part. In particular, when the exhaust gas purification catalyst starts to be used, the contribution rate of the OSC material in the front part to the total OSC amount is high. That is, by making the amount of OSC material used in the front part less than that in the rear part, the initial OSC amount can be suppressed to a low value.

[0069] However, from the perspective of warm-up characteristics, that is, the catalyst performance when the temperature of the exhaust gas purification catalyst is not warm, it is not preferred not to use any OSC material in the front portion. The preferred content of the OSC material contained in the front portion is greater than 0 g / L, for example, 1 g / L or more, more preferably 10 g / L or more, and preferably 60 g / L or less, more preferably 40 g / L or less.

[0070] The catalyst coating layer 30 can be formed by a known method. For example, first, a slurry is prepared for each layer constituting the catalyst coating layer 30. The slurry contains the catalyst metal, OSC material and other components contained in each layer. The slurry prepared for each layer constituting the catalyst coating layer is sequentially applied (wash coat) on the substrate, dried and fired, thereby forming each layer.

[0071] The drying conditions of the slurry after shower coating are affected by the shape and size of the substrate or carrier, and are typically about 80 to 300°C (e.g., 100 to 250°C) and about 1 to 10 hours. The firing conditions vary depending on the target specific surface area of ​​the OSC material, and are about 400 to 1200°C and about 0.1 to 10 hours, for example, about 400 to 1000°C and about 1 to 4 hours. Among them, in the process of forming the catalyst coating 30 by shower coating, the slurry may contain a binder in order to make the slurry fit properly with the substrate. As a binder, it is preferred to use, for example, alumina gel, silica gel, etc. The viscosity of the slurry is appropriately adjusted so that the slurry can easily flow into the chamber of the substrate.

[0072] The exhaust gas purification catalyst 100 disclosed herein has an appropriately adjusted initial OSC amount and suppresses the decrease in OSC amount after endurance, thereby further improving the OSC maintenance rate compared with the prior art. The exhaust gas purification catalyst 100 can be used for various purposes, for example, it is suitable for use as a vehicle-mounted exhaust gas purification catalyst.

[0073] Hereinafter, preferred embodiments will be described with reference to Examples, but the present invention is not limited to these Examples.

[0074] <Test base material>

[0075] A cylindrical cordierite honeycomb substrate having a total length of 110 mm and a substrate capacity of 1.3 L was prepared. In subsequent tests, this honeycomb substrate was used as a test substrate.

[0076] <Production of catalyst for exhaust gas purification for testing>

[0077] <Example 1>

[0078] As shown in Table 1, cerium oxide-zirconia composite oxide, Al2O3 powder, Rh nitrate and water were mixed so that the coating amount of the material relative to the test substrate capacity was 50 g / L of cerium oxide-zirconia composite oxide (10 g / L of cerium oxide), 50 g / L of Al2O3, 10 g / L of BaO, and 2 g / L of Pd, and slurry 1-Pd was prepared. In addition, as shown in Table 1, cerium oxide-zirconia composite oxide, Al2O3 powder, BaO powder, Pd nitrate and water were mixed so that the coating amount of the material relative to the test substrate capacity was 25 g / L of cerium oxide-zirconia composite oxide (5 g / L of cerium oxide), 25 g / L of Al2O3, and 0.2 g / L of Rh, and slurry 1-Rh was prepared.

[0079] The slurry 1-Pd was shower-coated from the end of the exhaust gas inlet side of the catalyst coating to the downstream side, and from the end of the exhaust gas outlet side to the upstream side, on 100% of the length of the substrate. After drying at 250°C for 1 hour, it was fired at 900°C for 3 hours to form a lower layer. Next, the slurry 1-Rh was shower-coated on the lower layer formed on the test substrate from the end of the exhaust gas inlet side to the downstream side, and from the end of the exhaust gas outlet side to the upstream side, on 100% of the length of the substrate, and after drying at 250°C for 1 hour, it was fired at 500°C for 1 hour to form an upper layer on the lower layer, and the catalyst example 1 for exhaust gas purification was prepared.

[0080] <Example 2>

[0081] Exhaust Gas Purifying Catalyst Example 2 was prepared in the same manner as in Example 1 except that the calcination temperature of the slurry 1-Pd was set to 500°C.

[0082] <Example 3>

[0083] Mix the materials and water such that the coating amount of the materials relative to the test substrate reaches the coating amount described in Table 1 to prepare Slurry 3-Pd.

[0084] Except for using Slurry 3-Pd instead of Slurry 1-Pd, perform the same operations as in Example 1 to fabricate Catalyst Example 3 for exhaust gas purification.

[0085] <Example 4>

[0086] Mix the materials and water such that the coating amount of the materials relative to the test substrate reaches the coating amount described in Table 2 to prepare Slurry 4-F and Slurry 4-R.

[0087] To form the front part of the lower layer, perform shower coating of Slurry 4-F from the end on the exhaust gas inflow side of the test substrate to 70% of the length of the substrate towards the downstream side, at a rate of coating the amount of cerium-zirconium oxide described in Table 2 per 1 L of coating width. To form the rear part of the lower layer, perform shower coating of Slurry 4-R from the end on the exhaust gas outflow side of the test substrate to 75% of the length of the substrate towards the upstream side, at a rate of coating 50 g / L of cerium-zirconium oxide per 1 L of coating width. After drying at 250 °C for 1 hour, sinter at 900 °C for 3 hours to thereby form a lower layer having a front part and a rear part. And, use Slurry 1-Rh and perform the same operations as in Example 1 to form the upper layer, fabricating Catalyst Example 4 for exhaust gas purification.

[0088] <Examples 5 - 9>

[0089] Mix the materials and water such that the coating amount of the materials relative to the test substrate reaches the coating amount described in Table 2 to prepare Slurries 5-F to 9-F. Thereafter, perform the same operations as in Example 4 to fabricate Catalyst Examples 5 to 9 for exhaust gas purification.

[0090] [Table 1]

[0091] Table 1

[0092]

[0093] *1 CZ(C): Content of cerium-zirconium composite oxide (content of cerium therein)

[0094] [Table 2]

[0095] Table 2

[0096]

[0097] <OSC Evaluation>

[0098] The OSC amount of the exhaust gas purification catalyst is evaluated as follows. First, the exhaust gas purification catalyst is installed in the exhaust system of the engine, and then an O2 sensor is installed downstream of the exhaust gas purification catalyst. Then, the air-fuel ratio of the mixed gas supplied to the engine is periodically switched between rich fuel and lean fuel at predetermined intervals, and the OSC of the exhaust gas purification catalyst is calculated based on the delay of the condition of the O2 sensor.

[0099] <Durability Test 1>

[0100] Endurance tests were conducted on catalysts for exhaust purification, Example 1 and Example 2. Based on the comparison of the OSC amount before and after the endurance test, the OSC maintenance rate was compared between the case where the Pd layer contained a low specific surface area OSC material (Example 1) and the case where the Pd layer contained an existing OSC material (Example 2). Endurance test 1 was conducted as follows: Exhaust purification catalysts Example 1 and Example 2 were respectively set in the exhaust system of a V-type engine with an exhaust volume of 4L, the engine was operated, and the catalyst temperature was maintained at 950°C for 150 hours. The conditions of this endurance test were equivalent to a vehicle driving distance of approximately 200,000 km.

[0101] like Figure 4 As shown, the OSC maintenance rate after 150 hours of durability is 50% or more in the case of containing a low specific surface area OSC material (Example 1), and is 40% or less in the case of containing a conventional OSC material (Example 2). It can be confirmed that the decrease in the OSC maintenance rate can be suppressed by containing a low specific surface area OSC material in the Pd layer.

[0102] <CO purification rate test>

[0103] The CO purification rate test was conducted for the exhaust purification catalyst examples 2 and 3. The CO purification rate was measured using a fixed bed flow reactor. A heat exchanger and a catalyst were placed in the engine exhaust system, the temperature was raised from 100°C to 500°C at 10°C / min, and the CO concentration at the catalyst inlet and catalyst outlet was measured to calculate the CO purification rate.

[0104] like Figure 5 As shown, the exhaust gas purification catalyst example 3 containing oxides of Pr, La, and Y has a higher HC purification rate at about 200 to 400° C. than the exhaust gas purification catalyst example 2 not containing these oxides. The catalyst activity in the above temperature range is improved by adding oxides of Pr, La, and Y.

[0105] <Durability Test 2>

[0106] Durability tests were conducted on Exhaust Purification Catalyst Examples 4 to 9, and the relationship between the content of the cerium oxide-zirconia composite oxide in the front and rear parts of the lower layer (Pd layer), the initial OSC amount, and the OSC maintenance rate was studied based on the comparison of the OSC amount before and after the durability test. Durability Test 2 was conducted as follows: Exhaust Purification Catalyst Examples 4 to 9 were respectively set in the exhaust system of a V-type engine with an exhaust volume of 4L, and the engine was operated and maintained at a catalyst temperature of 950°C for 150 hours.

[0107] like Figure 6 As shown in FIG. 1 , the lower the content of the ceria-zirconia composite oxide in the front portion of the lower layer, the lower the initial OSC can be. Figure 7 As shown, the lower the proportion of low specific surface area OSC material contained in the lower front part, the higher the OSC maintenance rate. When the low specific surface area OSC material contained in the lower front part is less than that contained in the lower rear part, the OSC maintenance rate can reach more than 40%.

[0108] <Warm-up characteristics test>

[0109] A warm-up characteristic test was conducted on Exhaust Gas Purifying Catalyst Examples 4 to 9. The room temperature exhaust gas purifying catalyst was installed in an exhaust system of an engine with an exhaust gas volume of 2L, 500°C exhaust gas was introduced, and the time (T50) until the HC purification rate exceeded 50% was measured.

[0110] like Figure 8 As shown, it can be confirmed that: when the front part of the lower layer does not contain cerium oxide-zirconia composite oxide (Example 4), and when the front part of the lower layer contains more low specific surface area OSC material than the rear part of the lower layer (Example 9), T50 exceeds 26 seconds and the warm-up characteristics are poor.

[0111] The results of the endurance test 2 and the warm-up characteristics test confirmed that the lower the content of the ceria-zirconia composite oxide contained in the lower front portion, the higher the OSC maintenance rate. However, from the viewpoint of warm-up characteristics, it is not preferable to not contain any ceria-zirconia composite oxide.

[0112] Although the specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the scope of the claims includes various modifications and changes of the specific examples described above.

Claims

1. An exhaust gas purification catalyst, which is arranged in an exhaust passage of an internal combustion engine and purifies exhaust gas discharged from the internal combustion engine, wherein: comprising a substrate, and a catalyst coating formed on the surface of the substrate, The catalyst coating contains an OSC material having oxygen storage capacity, and, The catalyst coating layer includes an Rh layer mainly configured with Rh as a catalyst metal, and a Pd / Pt layer mainly configured with Pd and / or Pt as a catalyst metal. in, At least a portion of the Pd / Pt layer in the catalyst coating layer contains a cerium oxide-zirconia composite oxide having a specific surface area of ​​40 m 2 / g above 60m 2 / g or less low specific surface area OSC material, as the OSC material, The catalyst coating layer includes a front portion located on the upstream side of the exhaust gas inflow direction and a rear portion located on the downstream side. The front portion is formed from an end portion of the substrate on the exhaust gas inlet side toward an exhaust gas outlet side, and the rear portion is formed from an end portion of the substrate on the exhaust gas outlet side toward an exhaust gas inlet side. The total length of the substrate in the exhaust gas flow direction is defined as L w When the length L of the front portion 1 0.3×L w ≤L 1 ≤0.7×L w , the length L of the rear portion 2 0.5×L w ≤L 2 ≤0.85×L w , and satisfy (L 1 +L 2 )≥1.05×L w The front portion and the rear portion are formed by partially overlapping each other, wherein the content (g / L) of the low specific surface area OSC material in the front portion relative to the substrate capacity (L) is greater than 0 g / L and less than 40 g / L, and, The content (g / L) of the low specific surface area OSC material in the rear portion relative to the substrate volume (L) is 1.2 times or more of the content (g / L) of the low specific surface area OSC material in the front portion.

2. The exhaust gas purification catalyst according to claim 1, Features: The catalyst coating layer includes a lower layer formed on the substrate and an upper layer formed on the lower layer. One of the upper layer and the lower layer constitutes the Rh layer, and the other constitutes the Pd / Pt layer.

3. The exhaust gas purification catalyst according to claim 2, Features: The lower layer constitutes the Pd / Pt layer.

4. The exhaust gas purification catalyst according to any one of claims 1 to 3, Features: The low specific surface area OSC material contains the following metal elements in the following ratios by mole percentage: Ce 15% or more but less than 55% Zr 40% or more but less than 80% Pr 1% or more but less than 10% La 0% or more but less than 10% Y 0% or more and 10% or less.

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