Exhaust gas purification catalyst

Abstract

To provide a catalyst for exhaust purification with improved exhaust purifying performance.SOLUTION: A catalyst includes a Rh-carrying CeZr complex oxide 5 and a Pd-carrying CeZr complex oxide 7. In its fresh state, the CeZr complex oxide carrying Rh has a peak pore size of 20-70 nm based on the BJH method for pore distribution, and a total pore capacity of 0.6-0.8 cm3 / g. In its fresh state, the CeZr complex oxide carrying Pd has a peak pore size of 40-90 nm and a total pore capacity of 0.25-0.5 cm3 / g.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a catalyst for purifying exhaust gases. [Background technology]

[0002] Three-way catalysts are known for purifying exhaust gases from automobiles and other vehicles, simultaneously purifying HC (hydrocarbons), CO, and NOx (nitrogen oxides) near the stoichiometric pressure. In these three-way catalysts, noble metals such as Pt, Pd, and Rh are supported on the surface of support material particles, such as oxygen storage and release materials containing Ce. These three-way catalysts are supported on the cell walls of a honeycomb support via a fine particulate material with binder properties.

[0003] One of the technical challenges of three-way catalysts is improving their low-temperature activity. The problem is that oxygen storage and release materials supporting precious metals have low thermal stability, making it difficult to maintain the catalyst's low-temperature activity over long periods. Exposure of the oxygen storage and release material to high-temperature exhaust gases reduces its specific surface area and pore capacity, and furthermore, the precious metals become embedded in the oxygen storage and release material, reducing the number of active sites in the catalyst, which worsens the low-temperature activity.

[0004] To address these challenges, Patent Document 1 discloses a zirconia-based porous material having a pore size suitable for supporting a catalyst metal, with small variation in pore size, and possessing a sufficient specific surface area even after heat treatment at 1000°C for 12 hours. Specifically, it is a zirconia-based porous material having a peak in the pore size range of 20 to 100 nm in the pore distribution based on the BJH method, a P / W ratio of 0.05 or more when W is the full width at half maximum of the peak and P is the peak height obtained from the measured pore distribution curve, a total pore volume of 0.5 cm³ / g or more, and after heat treatment at 1000°C for 12 hours, having a peak in the pore size range of 20 to 100 nm, a P / W ratio of 0.03 or more, a specific surface area of ​​at least 40 m² / g, and a total pore volume of 0.3 cm³ / g or more. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent No. 5744274 [Summary of the Invention] [Problem to be Solved by the Invention]

[0006] Incidentally, Rh and Pd are typical catalytic metals employed in three-way catalysts, and these two are used in combination. The present inventors have found that, in this combination, even when the pore diameter of the oxygen storage / release material supporting each catalyst metal is similarly controlled, the expected effect is not necessarily obtained. That is, when Rh is supported on the same oxygen storage / release material, excellent low-temperature activity is exhibited, but when Pd is supported, the expected low-temperature activity cannot be obtained; conversely, when Pd is supported, the effect of improving low-temperature activity is high, but when Rh is supported, the effect is low.

[0007] An object of the present invention is to improve the exhaust gas purification performance of a catalyst in the case where Rh and Pd are combined as catalytic metals. [Means for Solving the Problem]

[0008] In order to solve the above problem, the present invention makes the physical properties of the support materials supporting Rh and Pd respectively different from each other.

[0009] The exhaust gas purification catalyst disclosed herein contains a Rh-supported CeZr-based composite oxide and a Pd-supported CeZr-based composite oxide, the CeZr-based composite oxide for supporting Rh, in an unused fresh state after production, has a peak at a pore diameter of 20 nm or more and 70 nm or less in a pore distribution based on the BJH method, and has a total pore volume of 0.6 cm 3 / g or more and 0.8 cm 3 / g or less, the CeZr-based composite oxide for supporting Pd, in an unused fresh state after production, has a peak at a pore diameter of 40 nm or more and 87 nm or less in the pore distribution, and has a total pore volume of 0.25 cm 3 / g or more and 0.48 cm 3 / g. The composite oxide is characterized in that it is less than

[0010] In a preferred embodiment, the CeZr-based composite oxide for supporting Rh is 930℃ for 50 hours after heat treatment, has a peak at a pore diameter of 20 nm or more and 70 nm or less in the pore distribution, and has a specific surface area of 44 m 2 / g or more, and a total pore volume of 0.3 cm 3 / g or more.

[0011] In a preferred embodiment, the CeZr-based composite oxide for supporting Rh has a total pore volume after the heat treatment of 0.3 cm 3 / g or more and 0.5 cm 3 / g or less.

[0012] In a preferred embodiment, after the heat treatment, the CeZr-based composite oxide for supporting Rh has a peak at a pore diameter of 30 nm or more and 65 nm or less in the pore distribution, and has a total pore volume of 0.35 cm 3 / g or more.

[0013] In a preferred embodiment, the CeZr-based composite oxide for supporting Pd is 930℃ for 50 hours after heat treatment, has a peak at a pore diameter of 30 nm or more and 90 nm or less in the pore distribution, and has a specific surface area of 41 m 2 / g or more, and a total pore volume of 0.25 cm 3 / g or more.

[0014] In a preferred embodiment, the CeZr-based composite oxide for supporting Pd has a total pore volume after the heat treatment of 0.25 cm 3 / g or more and 0.3 cm 3 / g.

[0015] In a preferred embodiment, after the heat treatment, the CeZr-based composite oxide for supporting Pd has a peak at a pore diameter of 35 nm or more and 85 nm or less in the pore distribution.

[0016] In a preferred embodiment, the CeZr-based composite oxide for Pd support has a specific surface area of ​​41 m² after the heat treatment. 2 / g or more 48m 2 It is less than / g. [Effects of the Invention]

[0017] According to the present invention, by supporting Rh and Pd on a suitable CeZr-based composite oxide, a high level of exhaust gas purification performance is maintained even after prolonged exposure to high-temperature exhaust gas. [Brief explanation of the drawing]

[0018] [Figure 1] A schematic cross-sectional view showing an example of an exhaust gas purification catalyst according to the present invention. [Figure 2] A graph showing the pore distribution of fresh pore control materials with pore sizes ranging from 10 nm to 30 nm for Rh support. [Figure 3] A graph showing the pore distribution of fresh pore control materials with pore sizes ranging from 40 nm to 60 nm for Rh support. [Figure 4] Graphs showing the pore distribution of fresh composite oxides with 70nm pore control, 80nm pore control, and comparative examples for Rh support. [Figure 5] A graph showing the pore distribution after heat treatment of pore-controlled materials with pore sizes ranging from 10 nm to 30 nm for Rh support. [Figure 6] A graph showing the pore distribution after heat treatment of pore-controlled materials with pore sizes ranging from 40 nm to 60 nm for Rh support. [Figure 7] Graphs showing the pore distribution after heat treatment of 70nm pore control material, 80nm pore control material, and comparative example composite oxide for Rh support. [Figure 8] A graph showing the pore distribution of fresh pore control materials with pore sizes ranging from 40 nm to 60 nm for Pd support. [Figure 9] A graph showing the pore distribution of fresh pore control materials with pore sizes ranging from 70 nm to 90 nm for Pd support. [Figure 10] A graph showing the pore distribution of a 100nm pore control material for Pd support and a comparative example composite oxide in fresh form. [Figure 11] A graph showing the pore distribution after heat treatment of pore-controlled materials with pore sizes ranging from 40 nm to 60 nm for Pd support. [Figure 12] A graph showing the pore distribution after heat treatment of pore-controlled materials with pore sizes ranging from 70 nm to 90 nm for Pd support. [Figure 13] A graph showing the pore distribution after heat treatment of a 100nm pore control material for Pd support and a comparative example composite oxide. [Figure 14] A graph showing the light-off temperatures for HC purification of different Rh-supported CeZr-based composite oxides for various catalysts. [Figure 15] Graph showing the light-off temperatures for HC purification of different Pd-supported CeZr-based composite oxide catalysts. [Modes for carrying out the invention]

[0019] The following descriptions of embodiments for carrying out the present invention will be based on the drawings. The following descriptions of preferred embodiments are essentially illustrative and are not intended to limit the present invention, its applications, or its uses.

[0020] The exhaust gas purification catalyst according to the present invention is suitable for purifying HC, CO, and NOx in exhaust gas emitted from a vehicle engine.

[0021] <Example of catalyst configuration> The exhaust gas purification catalyst shown in Figure 1 comprises an upper catalyst layer 2 and a lower catalyst layer 3 covered by the upper catalyst layer 2, on a carrier 1. The carrier 1 is a honeycomb carrier, and the catalyst layers 2 and 3 are formed in layers on the cell walls of the honeycomb carrier 1. The surface of the upper catalyst layer 2 is exposed to exhaust gas. The upper catalyst layer 2 contains a first Rh catalyst 5 and a second Rh catalyst 6, each having Rh as the catalytic metal, and activated alumina (Al2O3) without supporting catalytic metal. The lower catalyst layer 3 contains a first Pd catalyst 7 and a second Pd catalyst 8, each having Pd as the catalytic metal, and an oxygen storage and release material (CeZrNdLaYOx) without supporting catalytic metal.

[0022] The first Rh catalyst 5 is a catalyst obtained by supporting Rh on a CeZr-based composite oxide (oxygen storage and release material) having oxygen storage and release ability. The second Rh catalyst 6 is a catalyst obtained by supporting Rh on activated alumina. The first Pd catalyst 7 is a catalyst obtained by supporting Pd on a CeZr-based composite oxide (oxygen storage and release material) having oxygen storage and release ability. The second Pd catalyst 8 is a catalyst obtained by supporting Pd on activated alumina.

[0023] That is, the exhaust gas purification catalyst according to the embodiment contains a Rh-supported CeZr-based composite oxide (first Rh catalyst 5) and a Pd-supported CeZr-based composite oxide (first Pd catalyst 7).

[0024] <CeZr-based composite oxide supporting each of Rh and Pd> The CeZr-based composite oxide for supporting Rh, in an unused fresh state after production, has a peak in a pore diameter range of 20 nm or more and 70 nm or less in the pore distribution based on the BJH method, and the total pore volume is 0.6 cm 3 / g or more and 0.8 cm 3 / g or less.

[0025] Although not intended to be limiting, preferably, after heat treatment of the CeZr-based composite oxide for supporting Rh at 800°C to 950°C for 10 hours to 100 hours, more preferably at 930°C for 50 hours, the pore distribution has a peak in a pore diameter of 20 nm to 70 nm, more preferably 30 nm to 65 nm, and the specific surface area is 44 m 2 / g or more, preferably 46 m 2 / g or more and 60 m 2 / g or less, and the total pore volume is 0.3 cm 3 / g or more, preferably 0.3 cm 3 / g or more and 0.5 cm 3 / g or less, more preferably 0.35 cm 3 / g or more and 0.5 cm 3 / g or less.

[0026] The BJH (Barrett-Joyner-Halenda) method is an analysis method for mesopore distribution based on Kelvin's capillary condensation theory, which assumes that mesopores have a cylindrical shape.

[0027] In its fresh, unused state after manufacturing, the Pd-supported CeZr-based composite oxide exhibits a peak in the pore size range of 40 nm to 87 nm in the pore distribution described above, with a total pore volume of 0.25 cm³. 3 / g or more 0.48cm 3 Less than 0.25 cm / g, preferably 0.25 cm 3 / g or more 0.47cm 3 It is less than / g.

[0028] Furthermore, although not intended to be limiting, preferably, the CeZr-based composite oxide for Pd support has a peak in the pore size of 30 nm to 90 nm, preferably 35 nm to 85 nm, in the pore distribution after the above heat treatment, and has a specific surface area of ​​41 m². 2 / g or more, preferably 41m 2 / g or more 48m 2 / g or less, more preferably 41m 2 / g or more 47m 2 The total pore volume is less than or equal to / g and less than or equal to 0.25 cm³. 3 / g or more, preferably 0.25cm 3 / g or more 0.3cm 3 Less than 0.25 cm / g, more preferably 0.25 cm 3 / g or more 0.28cm 3 It is less than / g.

[0029] Fresh CeZr-based composite oxides for Rh-supported and Pd-supported applications can be prepared by the following method.

[0030] After heating a sulfate solution (e.g., aqueous sodium sulfate solution) and a zirconium salt solution (e.g., aqueous zirconium oxychloride solution) to 95°C or higher, the two solutions are mixed to obtain a reaction solution containing basic zirconium sulfate. The mixture contains SO4 2- The reaction is carried out over several hours while maintaining a temperature of 95°C or higher, so that the mass ratio of / ZrO2 is 0.4 to 0.6. The resulting reaction solution is then aged by holding it at a temperature of 95°C or higher for several hours.

[0031] After cooling the aged reaction solution to room temperature, an aqueous cerium nitrate solution and, if necessary, a solution of a rare earth element salt such as an aqueous neodymium nitrate solution, an aqueous lanthanum nitrate solution, or an aqueous yttrium nitrate solution are added thereto and uniformly mixed. An alkaline solution is added to the obtained mixed solution to obtain a hydroxide precipitate. The obtained hydroxide precipitate is filtered and washed with water. The obtained hydroxide is dried and calcined to obtain the target CeZr-based composite oxide.

[0032] <Physical Property Evaluation of CeZr-based Composite Oxide> (Preparation of Evaluation Samples) Using an aqueous sodium sulfate solution as the sulfate solution, an aqueous zirconium oxychloride solution as the zirconium salt solution, and an aqueous cerium nitrate solution, an aqueous neodymium nitrate solution, an aqueous lanthanum nitrate solution, and an aqueous yttrium nitrate solution as the rare earth element salt solutions, each CeZr-based composite oxide for supporting Rh and for supporting Pd was prepared by the above preparation method.

[0033] The composition ratio of the CeZr-based composite oxide for supporting Rh was adjusted to be CeO₂:ZrO₂:Nd₂O₃:La₂O₃:Y₂O₃ = 10:75:5:5:5 (mass%). The composition ratio of the CeZr-based composite oxide for supporting Pd was adjusted to be CeO₂:ZrO₂:Nd₂O₃:La₂O₃:Y₂O₃ = 35:50:8:2:5 (mass%).

[0034] For the CeZr-based composite oxide for supporting Rh, eight types (Production Examples A1 to A8) were prepared, wherein the target values of the pore diameter at which a peak appears in the pore distribution based on the BJH method (also referred to as "peak pore diameter" in the present specification) are 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, and 80 nm. Hereinafter, the eight types of CeZr-based composite oxides for supporting Rh of Production Examples A1 to A8 are referred to by names combining the target pore diameter and the pore control material, such as 10 nm pore control material for supporting Rh and 20 nm pore control material for supporting Rh.

[0035] In addition to the eight types of pore control materials for Rh support described above, a CeZr-based composite oxide for Rh support related to the comparative example was prepared by a general coprecipitation method. Specifically, a coprecipitation was obtained by neutralizing a solution of cerium nitrate hexahydrate, zirconium oxynitrate, neodymium nitrate hexahydrate, lanthanum nitrate, and yttrium nitrate dissolved in deionized water with ammonia water. After washing this coprecipitation with water by centrifugation, it was dried, pulverized, and calcined to obtain a comparative example composite oxide powder for Rh support. Its composition ratio was the same as that of the pore control materials for Rh support described above: CeO2:ZrO2:Nd2O3:La2O3:Y2O3 = 10:75:5:5:5 (mass%).

[0036] For Pd-supported CeZr-based composite oxides, seven types (manufacturing examples B1 to B7) were prepared, with target pore diameters of 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, and 100 nm, where peaks appear in the pore distribution based on the BJH method. Hereinafter, these seven types of Pd-supported CeZr-based composite oxides from manufacturing examples B1 to B7 will be referred to by names that combine the target pore diameter and pore control material, such as 10 nm pore control material and 20 nm pore control material for Pd support.

[0037] In addition, separate from the seven types of pore control materials for Pd support described above, a CeZr-based composite oxide (hereinafter referred to as the "comparative composite oxide for Pd support") was prepared using the general coprecipitation method described above, with a composition ratio for Pd support of CeO2:ZrO2:Nd2O3:La2O3:Y2O3 = 35:50:8:2:5 (mass%).

[0038] (Physical properties of CeZr-based composite oxides for Rh support) Figures 2 to 4 show the pore distribution of each of the eight types of pore-controlling materials for Rh support and comparative example composite oxides for Rh support in their fresh state. The vertical axis of each graph in Figures 2 to 4 represents the log differential pore volume (cm²). 3 / g) This point is the same as in Figures 5 to 13.

[0039] The peak pore diameter, total pore volume, and specific surface area of ​​each of the eight types of pore control materials for Rh support and comparative example composite oxides for Rh support are shown in Table 1.

[0040] [Table 1]

[0041] The peak pore size was determined from the pore distribution based on the BJH method. The total pore volume was determined by the BJH method using nitrogen gas as the adsorption gas. The specific surface area was determined by the BET method.

[0042] Figures 5 to 7 show the pore distribution of each of the eight types of pore-controlling materials for Rh support and comparative example composite oxides for Rh support after heat treatment at 930°C for 50 hours. Table 2 shows the peak pore diameter, total pore volume, and specific surface area of ​​these samples after heat treatment.

[0043] [Table 2]

[0044] The heat treatment described above was carried out as follows: Each fresh sample was held at a temperature of 930°C for 50 hours in an atmosphere in which simulated exhaust gases with a lean air-fuel ratio and simulated exhaust gases with a rich air-fuel ratio flowed alternately. After that, the pore distribution of each sample was measured by the BJH method, and the peak pore diameter, total pore volume, and specific surface area were determined. The compositions of the simulated exhaust gases with lean and rich air-fuel ratios are shown in Tables 3 and 4.

[0045] [Table 3]

[0046] [Table 4]

[0047] (Physical properties of CeZr-based composite oxides for Pd support) Figures 8 to 10 show the pore distribution of each of the seven types of pore-controlling materials for Pd support and the CeZr-based composite oxides for Pd support, in their fresh state.

[0048] Table 5 shows the fresh peak pore diameter, total pore volume, and specific surface area of ​​each of the seven types of pore control materials for Pd support and comparative example composite oxides for Pd support.

[0049] [Table 5]

[0050] Figures 11 to 13 show the pore distribution of each of the seven types of pore-controlling materials for Pd support and the comparative example composite oxides for Pd support after the aforementioned heat treatment at 930°C for 50 hours. Table 6 shows the peak pore diameter, total pore volume, and specific surface area of ​​these samples after the heat treatment.

[0051] [Table 6]

[0052] <Catalyst performance evaluation> (Influence of the physical properties of pore size control material for Rh support) In the two-layer exhaust gas purification catalyst shown in Figure 1, the effect of differences in the physical properties of the Rh-supported CeZr-based composite oxide on the catalytic performance was investigated. The formulations of the upper catalyst layer 2 and the lower catalyst layer 3 are shown in Table 7.

[0053] [Table 7]

[0054] In Table 7, "g / L" represents the amount of component per liter of carrier volume. "YSZ" represents Y-stabilized zirconia containing 3 mol% Y2O3, and "La-Al2O3" represents activated Al2O3 containing 4 mass% La2O3. "Rh / CeZrNdLaYOx" is Rh-supported CeZr-based composite oxide, "Rh / Al2O3" is Rh-supported activated alumina, "Pd / CeZrNdLaYOx" is Pd-supported CeZr-based composite oxide, and "Pd / Al2O3" is Pd-supported activated alumina.

[0055] The binder "Rh-CeZrNdLaYOx" in the upper catalyst layer is Rh-doped CeZrNdLaYOx that also functions as an oxygen storage and release agent. The composition ratio of this CeZrNdLaYOx is CeO2:ZrO2:Nd2O3:La2O3:Y2O3 = 10:75:5:5:5 (mass%), and the amount of Rh doping is 0.1 mass%.

[0056] Rh-doped CeZrNdLaYOx was prepared as follows: A coprecipitate was obtained by neutralizing a solution of cerium nitrate hexahydrate, zirconium oxynitrate, neodymium nitrate hexahydrate, lanthanum nitrate, yttrium nitrate, and rhodium nitrate dissolved in deionized water with ammonia water. This coprecipitate was washed with water by centrifugation, dried in air, pulverized, and then calcined to obtain Rh-doped CeZrNdLaYOx.

[0057] The obtained Rh-doped CeZrNdLaYOx powder was subjected to a reduction treatment at 600°C for 60 minutes in a 1% CO environment. Ion-exchanged water was added to this to form a slurry (25% solid content by mass), which was then placed in a ball mill and ground with 0.5 mm zirconia beads for approximately 3 hours. This yielded a sol in which Rh-doped CeZrNdYOx powder, with a particle size small enough to be used as a binder, was dispersed in a solvent. This procedure reduced the median diameter of the Rh-doped CeZrNdYOx powder to 200 nm or less.

[0058] The oxygen storage and release material "CeZrNdLaYOx" in the lower catalyst layer is a composite oxide CeZrNdLaYOx having a composition ratio of CeO2:ZrO2:Nd2O3:La2O3:Y2O3 = 35:50:2:5:8 (mass%).

[0059] Seven catalyst samples were prepared using the exhaust gas purification catalyst formulations shown in Table 7. For the upper catalyst layer's first Rh catalyst, CeZrNdLaYOx was selected from 20nm, 30nm, 40nm, 50nm, 60nm, and 80nm pore control materials for Rh support, as well as comparative composite oxides for Rh support. For the lower catalyst layer's first Pd catalyst, comparative composite oxides for Pd support were selected in all cases. A ceramic honeycomb support (approximately 100cc capacity) was used as the support 1.

[0060] The preparation method for each catalyst sample is as follows: First, the lower catalyst layer 3 was formed by coating a carrier 1 with a slurry obtained by mixing the catalyst material and binder material of the lower catalyst layer with ion-exchanged water, and then drying and calcining it. Next, the upper catalyst layer 2 was formed by coating the lower catalyst layer 3 with a slurry obtained by mixing the catalyst material and binder material of the upper catalyst layer with ion-exchanged water, and then drying and calcining it.

[0061] Next, each catalyst sample underwent bench aging. This involved attaching each catalyst sample to the engine's exhaust pipe, setting the engine speed and load so that the catalyst temperature reached 900°C, and exposing the catalyst sample to the engine's exhaust gas for 50 hours.

[0062] Subsequently, core samples with a carrier volume of approximately 25 mL (25.4 mm in diameter, 50 mm in length) were cut from each catalyst sample and attached to a fixed-bed flow reactor. Then, evaluation tests were conducted to assess the light-off performance for HC purification.

[0063] Specifically, the simulated exhaust gas temperature flowing into the catalyst sample was gradually increased from 50°C to 500°C at a heating rate of 30°C / min, and then gradually decreased from 500°C to 100°C at a cooling rate of 30°C / min. After this pretreatment, the simulated exhaust gas temperature was gradually increased from 100°C at a heating rate of 30°C / min, and the gas temperature T50 (°C) at the catalyst inlet was measured when the HC purification rate reached 50%. The simulated exhaust gas had an A / F ratio of 14.7. In other words, a mainstream gas with an A / F ratio of 14.7 was continuously flowed. The space velocity SV was 60,000 h. -1 That is the case.

[0064] Figure 14 shows the measurement results of the light-off temperature (T50). Catalysts in which Rh was supported on 20nm to 80nm pore control materials showed a lower light-off temperature than those in which Rh was supported on comparative composite oxides. The light-off temperature was particularly low in the cases where Rh was supported on 30nm to 60nm pore control materials.

[0065] The 30nm to 60nm pore control materials for Rh support have peaks in pore diameters between 20nm and 70nm when fresh, and a total pore volume of 0.6cm³. 3 / g or more 0.8cm 3 The amount is less than / g. Furthermore, these pore-controlled materials have a peak in pore diameter between 20 nm and 70 nm after heat treatment at 930°C for 50 hours, and a specific surface area of ​​44 m². 2 The total pore volume is 0.3 cm³ or more and is greater than / g. 3 It is 1 / g or more.

[0066] According to Figure 14, from the viewpoint of improving light-off characteristics, more preferable pore control materials for Rh support are 30 nm to 50 nm pore control materials, which have a total pore volume of 0.6 cm³ when fresh. 3 / g or more 0.75cm 3 The amount is less than / g. Furthermore, these pore-controlled materials have a specific surface area of ​​46 m² after heat treatment. 2 For 1 / g or more, the total pore volume after heat treatment is 0.34 cm³. 3 / g or more 0.5cm 3 It is less than / g.

[0067] (Influence of the physical properties of the pore-controlling material for Pd support) Eight catalyst samples were prepared for exhaust gas purification catalysts with the formulations shown in Table 7. For the lower catalyst layer's first Pd catalyst, CeZrNdLaYOx was selected from 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, and 100nm pore control materials for Pd support, as well as comparative composite oxides for Pd support. For the upper catalyst layer's first Rh catalyst, a 40nm pore control material for Rh support was used for all samples. A ceramic honeycomb support (approximately 100cc) was used as the support material 1.

[0068] The preparation method for each catalyst sample was the same as that used when investigating the effect of the physical properties of the pore size control material for Rh support, as described earlier. Then, each catalyst sample was subjected to the same bench aging treatment as the previous catalyst samples, and a core sample with a carrier volume of approximately 25 mL was cut from each catalyst sample. This core sample was then attached to a fixed-bed flow reactor, and a light-off performance evaluation test for HC purification was conducted. The test method was the same as that used for the catalyst sample evaluation test described earlier.

[0069] Figure 15 shows the measurement results of the light-off temperature (T50). Catalysts in which Pd is supported on 40 nm to 100 nm pore control materials exhibit a lower light-off temperature than those in which Pd is supported on comparative composite oxides. The light-off temperature is particularly low in the cases where Pd is supported on 60 nm to 90 nm pore control materials.

[0070] The 60nm to 90nm pore control materials for Pd support have a peak in pore diameters between 40nm and 87nm when fresh, and a total pore volume of 0.25cm³. 3 / g or more 0.48cm 3 The amount is less than / g. Furthermore, these pore-controlled materials have a peak in pore diameter between 30nm and 90nm after heat treatment at 930°C for 50 hours, and a specific surface area of ​​41m². 2 The total pore volume is 0.25 cm³ or more, and the total pore volume is 0.25 cm³ or 3 It is 1 / g or more.

[0071] According to Table 6, the 60nm to 90nm pore-controlled materials for Pd support have a total pore volume of 0.25 cm³ after the heat treatment described above. 3 / g or more 0.3cm 3 It is less than / g.

[0072] (Consideration) As described above, in the case of Rh, the light-off characteristics of the catalyst are particularly improved when supported on a pore-controlling material with the above-mentioned physical properties and a pore size of 30 nm to 60 nm, as shown in Tables 1 and 2, Figures 2 to 7 and Figure 14.

[0073] On the other hand, in the case of Pd, as shown in Tables 5 and 6, Figures 8 to 13 and 15, the light-off characteristics of the catalyst are particularly good when supported on a pore-controlling material with the above-mentioned physical properties and a pore size of 60 nm to 90 nm.

[0074] Thus, the physical properties of suitable CeZr-based composite oxides as support materials differ between Rh and Pd in ​​terms of light-off characteristics.

[0075] When Rh and Pd were supported on the comparative example composite oxide described above and subjected to the bench aging treatment, the average particle sizes of Rh and Pd after the treatment were examined. Rh was 102 nm and Pd was 252 nm. Thus, the average particle sizes of Rh and Pd after aging differed significantly. This is because Pd is less resistant to heat and more prone to sintering.

[0076] For exhaust gas purification, it is crucial that the exhaust gas diffuses through the catalyst metal support and comes into contact with the catalyst metal. The catalytic performance results shown in Figure 14 suggest that contact between Rh and exhaust gas occurs effectively when Rh is supported on a 40 nm pore control material. The large total pore capacity of the 40 nm pore control material suggests that the diffusion efficiency of the exhaust gas is high. As shown in Figure 14, the reason why the light-off temperature is lower when Rh is supported on a 30 nm to 60 nm pore control material is that, as shown in Figures 5(C), 6 and Table 2, the log differential pore capacity in the 30 nm to 65 nm pore diameter region remains at 0.05 cm⁻¹ even after heat treatment. 3 The presence of peaks above / g indicates that the exhaust gas has good diffusion contact with Rh.

[0077] Next, the catalytic performance results shown in Figure 15 suggest that when Pd is supported on an 80 nm pore control material, contact between Pd and exhaust gas occurs effectively. In the case of Pd whose particle size increases with aging (average particle size 252 nm), it is thought that small pores with a diameter of, for example, less than approximately 35 nm contribute less to the diffusion contact of exhaust gas with Pd. Furthermore, it is thought that maintaining the structure with aging becomes difficult when the pores become large, for example, exceeding approximately 85 nm in diameter. As shown in Figure 15, the reason why the light-off temperature is low when Pd is supported on a pore control material with a diameter of 60 nm to 90 nm is that, as shown in Figure 11(C), Figure 12, and Table 6, even after heat treatment, the log differential pore volume in the pore diameter region of 35 nm to less than 85 nm is 0.05 cm⁻¹. 3 The presence of pores with a density of 1 / g or more indicates good diffusion contact of the exhaust gas with Pd.

[0078] In the above embodiment, the Rh-supported CeZr composite oxide and the Pd-supported CeZr composite oxide were arranged with the former in the upper catalyst layer and the latter in the lower catalyst layer. However, the former may be arranged in the lower catalyst layer and the latter in the upper catalyst layer, or both may be used in a mixed form.

[0079] Furthermore, the CeZr-based composite oxide is not limited to the composition of the above embodiment, but may be any composite oxide containing Ce and Zr. [Explanation of Symbols]

[0080] 1 Carrier 2 Upper catalyst layer 3 Lower catalyst layer 5. First Rh catalyst (Rh-supported CeZr-based composite oxide) 7. First Pd catalyst (Pd-supported CeZr-based composite oxide)

Claims

1. An exhaust gas purification catalyst containing a Rh-supported CeZr-based composite oxide and a Pd-supported CeZr-based composite oxide, The above-mentioned CeZr-based composite oxide for Rh support, in its fresh state after manufacturing and unused, exhibits a peak in pore diameters between 20 nm and 70 nm in the pore distribution based on the BJH method, with a total pore volume of 0.6 cm³. ３ / g or more 0.8cm ３ / g or less, The CeZr-based composite oxide for Pd support described above, in its fresh, unused state after manufacturing, has a peak in the pore size range of 40 nm to 87 nm in its pore distribution, and a total pore volume of 0.25 cm³. ３ / g or more 0.48cm ３ An exhaust gas purification catalyst characterized by having a value of less than / g.

2. In claim 1, The above-mentioned CeZr-based composite oxide for Rh support, after heat treatment at 930°C for 50 hours, exhibits a peak in the pore size range of 20 nm to 70 nm in the pore distribution, and has a specific surface area of ​​44 m². ２ The total pore volume is 0.3 cm³ or more, and the total pore volume is 0.3 cm³. ３ A catalyst for purifying exhaust gas, characterized by having a concentration of 1 / g or more.

3. In claim 2, The above-mentioned CeZr-based composite oxide for Rh support has a total pore volume of 0.3 cm after the heat treatment described above. ３ / g or more 0.5cm ３ A catalyst for purifying exhaust gas, characterized by having a concentration of less than or equal to / g.

4. In claim 2 or claim 3, The above-mentioned CeZr-based composite oxide for Rh support is characterized in that, after the above-mentioned heat treatment, it has a peak in the pore size of 30 nm to 65 nm in the above-mentioned pore distribution, and the total pore volume is 0.35 cm³ / g or more, making it a catalyst for purifying exhaust gas.

5. In any one of claims 1 to 4, The CeZr-based composite oxide for supporting Pd has a peak at a pore diameter of 30 nm or more and 90 nm or less in the pore distribution after heat treatment at 930° C. for 50 hours, and has a specific surface area of 41 m ２ / g or more, and a total pore volume of 0.25 cm ３ / g or more. An exhaust gas purification catalyst according to claim

6. In claim 5, The CeZr-based composite oxide used for Pd support has a total pore volume of 0.25 cm² after the heat treatment described above. ３ / g or more 0.3cm ３ An exhaust gas purification catalyst characterized by having a value of less than / g.

7. In claim 5 or claim 6, The CeZr-based composite oxide for Pd support described above is an exhaust gas purification catalyst characterized in that, after the heat treatment described above, it has a peak in the pore size distribution between 35 nm and 85 nm.

8. In any one of claims 5 to 7, The CeZr-based composite oxide for Pd support described above has a specific surface area of ​​41 m² after the heat treatment described above. ２ / g or more 48m ２ A catalyst for purifying exhaust gas, characterized by having a concentration of less than or equal to / g.

Images