Exhaust gas purification materials

Abstract

To provide an exhaust gas purification material in which reduction of OSC performance by high-temperature exposure is suppressed.SOLUTION: An exhaust gas purification material 100 includes: at least one α-alumina particle 10 having a multilayer structure, ACZ oxides 50 containing ceria-zirconia composite oxide and alumina, and noble metal 30. The ACZ oxides 50 are present inside and outside the α-alumina particles 10. The noble metal 30 is carried by the ACZ oxides 50 present outside the α-alumina particles 10 and inside the α-alumina particles 10.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas purification material. [Background technology]

[0002] Three-way catalysts are used in automobile exhaust gas purification devices to purify exhaust gases by simultaneously oxidizing carbon monoxide (CO) and hydrocarbons (HC) and reducing nitrogen oxides (NOx).A widely known three-way catalyst of this type is one in which a layer of a porous carrier is formed on a heat-resistant substrate made of cordierite or the like, and a precious metal such as platinum (Pt) or rhodium (Rh) is supported on the porous carrier.

[0003] Patent Document 1 describes an exhaust gas purification catalyst powder that includes an α-alumina multilayer body having a multilayer structure including a plurality of layers, a precious metal supported between adjacent layers of the plurality of layers, and a ceria-zirconia composite oxide that covers the outer surface of the α-alumina multilayer body, wherein the exhaust gas purification catalyst powder contains 10 wt % or more of the ceria-zirconia composite oxide based on the total weight of the exhaust gas purification catalyst powder, and describes that this exhaust gas purification catalyst powder retains high exhaust gas purification performance even after exposure to high temperatures.

[0004] Automotive exhaust gas purification devices typically perform a particularly high function of purifying harmful components from exhaust gases produced when an internal combustion engine burns a mixture with a stoichiometric air-fuel ratio. However, the actual air-fuel ratio fluctuates between excess fuel and excess oxygen depending on the vehicle's driving conditions. To suppress fluctuations in exhaust gas purification performance due to fluctuations in the air-fuel ratio, materials with oxygen storage capacity (OSC), i.e., OSC materials, are used in exhaust gas purification catalyst devices. OSC materials store oxygen when there is an excess of oxygen and release oxygen when there is an excess of fuel. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-062325 Summary of the Invention [Problem to be solved by the invention]

[0006] It is desirable for an exhaust gas purification device to maintain high OSC performance even after exposure to high temperatures. The present invention provides an exhaust gas purification material in which deterioration of OSC performance due to exposure to high temperatures is suppressed. [Means for solving the problem]

[0007] The present invention can be embodied in the following manner, for example. [Section 1] at least one α-alumina particle having a multilayer structure; an ACZ oxide containing ceria-zirconia composite oxide and alumina; Precious metals and Including, the ACZ oxide is present inside and outside the α-alumina particles; The exhaust gas purification material, wherein the noble metal is supported on the ACZ oxide present outside the α-alumina particles and on the interior of the α-alumina particles. [Section 2] Item 2. The exhaust gas purification material according to Item 1, wherein the mass of the ACZ oxide is within a range of 25 mass % to 75 mass % relative to the total mass of the α-alumina particles and the ACZ oxide. [Section 3] the at least one α-alumina particle is a plurality of α-alumina particles; Item 3. The exhaust gas purification material according to item 1 or 2, wherein the plurality of α-alumina particles are bonded via the ACZ oxide present outside the α-alumina particles. [Section 4] Item 4. The exhaust gas purification material according to any one of Items 1 to 3, wherein the mass of the ACZ oxide is within a range of 50% by mass to 75% by mass relative to the total mass of the α-alumina particles and the ACZ oxide. [Section 5] A substrate; The exhaust gas purification material according to any one of items 1 to 4, which is disposed on the substrate; An exhaust gas purification device having the above structure. [Effects of the Invention]

[0008] The exhaust gas purification material of the present invention is inhibited from decreasing in OSC performance due to exposure to high temperatures. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram conceptually showing an exhaust gas purification material according to an embodiment. [Figure 2] FIG. 2 is a graph showing the amount of decrease in OSC of catalyst pellets of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments will be described with reference to the accompanying drawings. In the drawings referred to in the following description, the dimensional proportions and shapes of the various parts in the drawings may be exaggerated for the sake of convenience and may differ from the actual dimensional proportions and shapes.

[0011] In this application, unless otherwise specified, numerical ranges expressed using the symbol "~" include the numerical values ​​before and after the symbol "~" as the lower and upper limits, respectively. The upper and lower limits of the numerical ranges described in this application can be used alone or in any combination to define a preferred range.

[0012] In this application, unless otherwise specified, "comprising" and "containing" mean that additional components may be included, and encompass "consisting essentially of" and "consisting of." "Consisting essentially of" means that additional components may be included that do not have a substantial adverse effect. "Consisting of" means that the material contains only the listed materials, but does not exclude the inclusion of unavoidable impurities.

[0013] (1) Exhaust gas purification materials As shown in FIG. 1, an exhaust gas purification material 100 according to the embodiment includes at least one α-alumina particle 10 , an ACZ oxide 50 , and a precious metal 30 .

[0014] The α-alumina particles 10 have a multilayer structure including a plurality of layers 12. There are gaps between adjacent layers 12. The size of the gaps, i.e., the distance between adjacent layers 12, may be, for example, 2 nm to 50 nm, from the viewpoints of suppressing grain growth of the precious metal 30 in a high-temperature atmosphere and facilitating the loading of the precious metal 30 between the layers 12. The size of the gaps can be determined by measuring the interlayer distances at five randomly selected locations in an electron micrograph taken using a scanning electron microscope (SEM) in accordance with JIS K 0132:1997 and averaging the measured values. The exhaust gas purification material 100 may include a plurality of α-alumina particles 10.

[0015] The ACZ oxide 50 can function as an OSC material that absorbs and releases oxygen. The ACZ oxide 50 includes a ceria-zirconia composite oxide (CZ composite oxide) and alumina. The CZ composite oxide is a solid solution of ceria (CeO2) and zirconia (ZrO2) mixed at the atomic level. The ACZ oxide 50 may be a mixture of the CZ composite oxide and alumina on the submicron order. The fact that the ACZ oxide is a mixture of the CZ composite oxide and alumina can be confirmed by the presence of an X-ray diffraction peak derived from the solid solution of ceria and zirconia and an X-ray diffraction peak derived from alumina.

[0016] The ACZ oxide 50 is present both inside and outside the α-alumina particles 10. "Present inside the α-alumina particles 10" means that it is present between the layers 12 of the α-alumina particles 10, but the ACZ oxide may also be present inside the layers 12. Because the ACZ oxide 50 contains alumina, it is well bonded to the α-alumina particles 10 and is not easily peeled off. Therefore, even if the exhaust gas purification material 100 is exposed to a high-temperature exhaust gas flow, the α-alumina particles 10 and the ACZ oxide 50 are prevented from peeling off and being swept away by the exhaust gas flow. Therefore, the exhaust gas purification material 100 can exhibit high OSC performance even after being exposed to high temperatures.

[0017] When a plurality of α-alumina particles 10 are present, the plurality of α-alumina particles 10 may be bonded together via ACZ oxide 50 present on the exterior of the α-alumina particles 10 .

[0018] The mass of the ACZ oxide 50 relative to the total mass of the α-alumina particles 10 and the ACZ oxide 50 may be, for example, in the range of 25% by mass to 75% by mass. In this case, deterioration of the OSC performance of the exhaust gas purification material 100 due to exposure to high temperatures is particularly suppressed. When the mass of the ACZ oxide 50 relative to the total mass of the α-alumina particles 10 and the ACZ oxide 50 is in the range of 50% by mass to 75% by mass, the exhaust gas purification material 100 has particularly high OSC performance.

[0019] The noble metal 30 may be any material that can function as a catalyst for the exhaust gas purification reaction, such as Pt, Rh, Pd, Ir, Ru, etc., and is preferably Pt or Pd, and particularly Pd. The amount of the noble metal 30 supported may be determined appropriately so that the exhaust gas purification material 100 has sufficient exhaust gas purification performance and the production cost of the exhaust gas purification material 100 does not become too high, and may be, for example, 0.1 wt % or more, particularly 0.5 to 20 wt %, based on the total weight of the exhaust gas purification material 100.

[0020] The precious metal 30 is supported both on the ACZ oxide 50 present outside the α-alumina particles 10 and inside the α-alumina particles 10. "Supported inside the α-alumina particles 10" means supported between the layers 12 of the α-alumina particles 10. The precious metal 30 may be supported directly on the layers 12 of the α-alumina particles 10, or on the ACZ oxide 50 present inside the α-alumina particles 10. The precious metal 30 may have a particulate form. By supporting the precious metal 30 on the ACZ oxide 50 present outside the α-alumina particles 10 and inside the α-alumina particles 10, the precious metal 30 becomes more sparse than when the precious metal 30 is supported only inside the α-alumina particles 10. This suppresses aggregation of the precious metal 30 at high temperatures, and as a result, suppresses deterioration in OSC performance and exhaust gas purification performance.

[0021] As will be shown in the examples described later, the exhaust gas purification material 100 according to the embodiment shows little deterioration in OSC performance even when exposed to high temperatures.

[0022] The exhaust gas purification material 100 according to the embodiment can be used as it is as an oxidation catalyst, a three-way catalyst, etc. The exhaust gas purification material 100 according to the embodiment can also be used as a NOx storage reduction catalyst by further supporting a NOx storage material such as Ba or K.

[0023] (2) Manufacturing method of exhaust gas purification material The exhaust gas purification material 100 according to the embodiment can be produced, for example, as follows.

[0024] The aluminum hydroxide crystal particles are dried and then calcined at 1000°C or higher, for example, at about 1200°C. This results in the formation of α-alumina particles 10. It is believed that the aluminum hydroxide that had been tightly packed in layers within the aluminum hydroxide crystals shrinks during the calcination process, causing gaps to form between the α-alumina layers. Aluminum hydroxide commercially available from Sumitomo Chemical Co., Ltd. and other companies can be used. The size of the gaps between the layers 12 of the α-alumina particles 10 can be adjusted by, for example, the calcination temperature of the aluminum hydroxide.

[0025] Next, a sol that serves as a precursor of ACZ oxide 50 is prepared. The sol may be a mixed sol containing Al2O3 particles, CeO2 particles, and ZrO2 particles as dispersoid particles. The prepared sol is impregnated into α-alumina particles 10 and dried. After that, firing produces ACZ oxide 50, which is a mixture of CZ composite oxide and alumina. This results in a composite containing α-alumina particles 10 and ACZ oxide 50. In this composite, ACZ oxide 50 is present both inside and outside the α-alumina particles 10. ACZ oxide 50 is porous.

[0026] The obtained composite is impregnated with a precious metal compound solution (i.e., a solution of a compound of a precious metal). The precious metal compound solution penetrates into the ACZ oxide 50 and the α-alumina particles 10 by capillary action. Next, the solvent of the precious metal compound solution is evaporated. As a result, the precious metal 30 is supported on the ACZ oxide 50 outside the α-alumina particles 10 and inside the α-alumina particles 10. As a result, the exhaust gas purification material 100 according to the embodiment is obtained.

[0027] (3) Exhaust gas purification equipment An exhaust gas purification device can be manufactured by disposing the exhaust gas purification material according to the above-described embodiment on a substrate.

[0028] The substrate is not particularly limited, but may be, for example, a monolith substrate having a honeycomb structure. The substrate may be formed from, for example, a ceramic material having high heat resistance, such as cordierite (2MgO·2Al2O3·5SiO2), alumina, zirconia, or silicon carbide, or a metal material made of metal foil, such as stainless steel. From the viewpoint of cost, the substrate is preferably made of cordierite.

[0029] When the substrate is a porous body having a plurality of pores, the exhaust gas purification material may be disposed on an inner surface of the substrate that defines the pores. In other words, in this application, "disposed on the substrate" includes both being disposed on the outer surface of the substrate and being disposed on the inner surface of the substrate.

[0030] The exhaust gas purification material can be disposed on a substrate, for example, as follows. First, a slurry containing the exhaust gas purification material is prepared. The slurry may further contain a binder, additives, etc. The properties of the slurry, such as viscosity and particle size of the solid components, may be adjusted as appropriate. The prepared slurry is applied to a predetermined region of the substrate. For example, the predetermined region of the substrate is immersed in the slurry, and after a predetermined time has passed, the substrate is pulled out of the slurry, thereby applying the slurry to the predetermined region of the substrate. Alternatively, the slurry may be applied to the substrate by pouring it onto the substrate and spreading it by blowing air with a blower. Next, the slurry is dried and fired at a predetermined temperature and for a predetermined time. This results in the exhaust gas purification material being disposed on the substrate.

[0031] The exhaust gas purification device can be applied to various vehicles equipped with an internal combustion engine.

[0032] The present invention is not limited to the above-described embodiment, and various design modifications can be made without departing from the technical scope of the present invention as defined in the claims. [Example]

[0033] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples.

[0034] Example 1 Gibbsite-type aluminum hydroxide powder was calcined at 1200°C for 5 hours in air to produce α-alumina particles with a multilayer structure. The layer spacing of the resulting α-alumina particles was 40 nm.

[0035] A mixed sol containing Al2O3 particles, CeO2 particles, and ZrO2 particles with particle diameters of 10 nm to 30 nm in a solid content ratio of 1:2:2 was prepared. 75 parts by mass of α-alumina particles were impregnated with the mixed sol containing 25 parts by mass of solid content, dried by microwave drying, and fired at 500°C for 2 hours. This resulted in a composite powder of α-alumina particles and ACZ oxide.

[0036] The obtained composite powder was impregnated with an aqueous palladium nitrate solution, dried, and calcined at 500°C for 2 hours. This resulted in a catalyst powder in which Pd was supported on the composite powder. The amount of Pd supported relative to the total mass of the catalyst powder was 2 mass%. The catalyst powder was compacted and then pulverized to produce catalyst pellets with diameters of 0.5 to 1.7 mm.

[0037] Example 2 A pellet catalyst was produced in the same manner as in Example 1, except that a mixed sol containing 50 parts by mass of α-alumina particles and 50 parts by mass of solid content was used.

[0038] Example 3 A pellet catalyst was produced in the same manner as in Example 1, except that a mixed sol containing 25 parts by mass of α-alumina particles and 75 parts by mass of solid content was used.

[0039] Comparative Example 1 A mixed sol containing Al2O3 particles, CeO2 particles, and ZrO2 particles with particle diameters of 10 nm to 30 nm in a solid content ratio of 1:2:2 was prepared. The mixed sol was dried by microwave drying and then calcined at 500°C for 2 hours. This yielded ACZ oxide powder.

[0040] The obtained ACZ oxide powder was impregnated with an aqueous palladium nitrate solution, dried, and calcined at 500°C for 2 hours. This resulted in a catalyst powder in which Pd was supported on the ACZ oxide powder. The amount of Pd supported relative to the total mass of the catalyst powder was 2 mass%. The catalyst powder was compacted and then pulverized to prepare catalyst pellets with diameters of 0.5 to 1.7 mm.

[0041] Comparative Example 2 A pellet catalyst was prepared in the same manner as in Example 1, except that a mixed sol containing CeO2 particles and ZrO2 particles with particle diameters of 10 nm to 30 nm at a solid content ratio of 1:1 and not containing Al2O3 particles was used.

[0042] Comparative Example 3 A pellet catalyst was prepared in the same manner as in Example 2, except that a mixed sol containing CeO2 particles and ZrO2 particles with particle diameters of 10 nm to 30 nm at a solid content ratio of 1:1 and not containing Al2O3 particles was used.

[0043] Comparative Example 4 A pellet catalyst was prepared in the same manner as in Example 3, except that a mixed sol containing CeO2 particles and ZrO2 particles with particle diameters of 10 nm to 30 nm at a solid content ratio of 1:1 and not containing Al2O3 particles was used.

[0044] Comparative Example 5 A pellet catalyst was prepared in the same manner as in Comparative Example 1, except that a mixed sol containing CeO2 particles and ZrO2 particles with particle diameters of 10 nm to 30 nm at a solid content ratio of 1:1 and not containing Al2O3 particles was used.

[0045] Comparative Example 6 α-Alumina particles were prepared in the same manner as in Example 1. 75 parts by mass of the α-alumina particles were impregnated with an aqueous palladium nitrate solution, dried, and fired at 500° C. for 2 hours. As a result, Pd was supported on the α-alumina particles.

[0046] Next, a mixed sol containing Al2O3 particles, CeO2 particles, and ZrO2 particles with particle diameters of 10 nm to 30 nm in a solid content ratio of 1:2:2 was prepared. The mixed sol containing 25 parts by mass of solid content was impregnated into Pd-supported α-alumina particles, which were then dried by microwave drying and calcined at 500°C for 2 hours. This yielded a catalyst powder. The amount of Pd supported relative to the total mass of the catalyst powder was 2 mass%. The catalyst powder was compacted and then pulverized to produce catalyst pellets with diameters of 0.5 to 1.7 mm.

[0047] Comparative Example 7 A pellet catalyst was prepared in the same manner as in Comparative Example 6, except that a mixed sol containing 50 parts by mass of α-alumina particles and 50 parts by mass of solid content was used.

[0048] Comparative Example 8 A pellet catalyst was prepared in the same manner as in Comparative Example 6, except that a mixed sol containing 25 parts by mass of α-alumina particles and 75 parts by mass of solid content was used.

[0049] <Structural evaluation> The catalyst pellets of Example 3 were sliced ​​and subjected to STEM observation and elemental mapping by EDX. Ce and Zr were present inside the α-alumina particles in addition to Al, and Al, Ce, and Zr were also present outside the α-alumina particles, indicating the presence of ACZ oxide inside and outside the α-alumina particles. It was also confirmed that particulate Pd was present in the ACZ oxide outside the α-alumina particles and inside the α-alumina particles.

[0050] <Performance evaluation> 1 g of pellet catalyst from each example and comparative example was weighed and exposed to a mixed gas flow of O2 and N2 (O2 content 1 vol%, flow rate 15 L / min, gas temperature 600°C) for 2 minutes. Next, the flow was switched to a mixed gas flow of CO2 and N2 (CO content 2 vol%, flow rate 15 L / min, gas temperature 600°C), and the amount of CO2 generated for 15 seconds immediately after switching was measured to calculate the amount of O2 stored in the pellet catalyst (initial OSC amount).

[0051] The catalyst pellets were exposed to 1000°C in an electric furnace for 5 hours.

[0052] After the high-temperature exposure, 1 g of the pellet catalyst was exposed to a mixed gas flow of O2 and N2 (O2 content 1 vol%, flow rate 15 L / min, gas temperature 600°C) for 2 minutes. The flow was then switched to a mixed gas flow of CO2 and N2 (CO content 2 vol%, flow rate 15 L / min, gas temperature 600°C), and the amount of CO2 generated was measured for 15 seconds immediately after the switch, and the amount of O2 stored in the pellet catalyst (OSC amount after high-temperature exposure) was calculated.

[0053] The difference between the initial OSC amount and the OSC amount after high-temperature exposure for each of the pellet catalysts in the Examples and Comparative Examples, i.e., the amount of OCS decrease due to high-temperature exposure, was calculated. The results are shown in Figure 2. The pellet catalysts in Examples 1 to 3 had a smaller OSC decrease than the pellet catalysts in Comparative Examples 1 to 8.

[0054] Specifically, the results of Examples 1 to 3 and Comparative Example 1 showed that the use of ACZ oxide in combination with α-alumina resulted in a smaller decrease in OSC than the use of ACZ oxide alone. Furthermore, the results of Examples 1 to 3 and Comparative Examples 1 to 5 showed that the reduction in OSC reduction achieved by combining ACZ oxide with α-alumina was more significant than the reduction in OSC reduction achieved by combining CZ oxide with α-alumina. In Comparative Examples 2 to 4, it is believed that the CZ oxide peeled off from the α-alumina during high-temperature exposure, causing the catalyst pellets to collapse. The subsequent mixed gas flow swept away some of the collapsed pellets, resulting in a decrease in OSC. In contrast, in Examples 1 to 3, the catalyst pellets did not collapse, or only slightly, as in Comparative Examples 2 to 4. This is believed to be due to the alumina contained in the ACZ oxide used in Examples 1 to 3 providing good adhesion to the α-alumina.

[0055] The results of Examples 1 to 3 and Comparative Examples 6 to 8 show that when Pd was loaded onto α-alumina particles after the mixed sol was impregnated, the decrease in OSC was smaller than when the mixed sol was impregnated onto α-alumina particles after Pd was loaded onto them. In the latter case, Pd was unevenly distributed inside the α-alumina particles, whereas in the former case, Pd was distributed both in the ACZ oxide outside the α-alumina particles and inside the α-alumina particles, making it difficult for Pd to aggregate during high-temperature exposure, which is thought to have resulted in suppressed decrease in OSC. [Explanation of symbols]

[0056] 10: α-alumina particles, 12: layer, 30: precious metal, 50: ACZ oxide, 100: exhaust gas purification material

Claims

1. at least one α-alumina particle having a multilayer structure; an ACZ oxide containing ceria-zirconia composite oxide and alumina; Precious metals and Including, the ACZ oxide is present inside and outside the α-alumina particles; the noble metal is supported on the ACZ oxide present outside the α-alumina particles and on the interior of the α-alumina particles; The exhaust gas purification material has a mass of the ACZ oxide in the range of 50 mass % to 75 mass % relative to the total mass of the α-alumina particles and the ACZ oxide.

2. the at least one α-alumina particle is a plurality of α-alumina particles; 2. The exhaust gas purification material according to claim 1, wherein the plurality of α-alumina particles are bonded together via the ACZ oxide present on the exterior of the α-alumina particles.

3. A substrate; The exhaust gas purification material according to claim 1 or 2, which is disposed on the substrate; An exhaust gas purification device having the above structure.

Images