Exhaust gas purification catalyst

Abstract

The present invention is an exhaust gas purification catalyst. The purpose of the present invention is to provide an exhaust gas purification catalyst having improved structural stability. This exhaust gas purification catalyst is provided with a base material and a catalyst layer formed on the base material, and is characterized in that the catalyst layer has a two-layer structure comprising a first coating layer formed on the base material and a second coating layer formed on the first coating layer, the first coating layer comprising alumina and an OSC material, and the second coating layer comprising alumina and an OSC material. The average particle diameter of the alumina is 6.0 [mu] m or more, the average particle diameter of the OSC material is 6.0 [mu] m or more, and the OSC material is a ceria-zirconia composite oxide that does not contain alumina.

Description

Technical Field

[0001] The present invention relates to a catalyst for exhaust gas purification. Background Art

[0002] Exhaust gas discharged from an internal combustion engine of an automobile or the like contains harmful gases such as carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons (HC). In order to decompose such harmful gases, a catalyst for exhaust gas purification (so-called three-way catalyst) is used. As the catalyst for exhaust gas purification, a catalyst in which a catalyst layer is coated on a substrate is widely used. In the catalyst layer of the catalyst for exhaust gas purification, in addition to the catalyst metal, an oxygen occlusion / release material (also referred to as an OSC material) having an oxygen occlusion / release ability (also referred to as an oxygen storage capacity (OSC: Oxygen Storage Capacity)) is widely used (for example, Patent Document 1). The oxygen occlusion / release material adjusts the air-fuel ratio (A / F) by occluding and releasing oxygen, and can suppress a decrease in the purification rate associated with a change in the exhaust gas composition.

[0003] On the other hand, under recent exhaust gas regulations, it is required to maintain the purification performance even when driving a longer distance. Therefore, further improvement in the structural stability is required for the catalyst for exhaust gas purification. However, in the catalyst for exhaust gas purification using an OSC material, sometimes during long-term use, the coating peels off from the substrate and the purification performance deteriorates. It is considered that this is caused by the following reasons. Specifically, with the occlusion and release of oxygen, thermal shrinkage of the OSC material occurs, and due to the shrinkage difference from other materials and the substrate, the structural stability decreases. As a result, the strength between the substrate and the coating decreases, and the coating peels off from the substrate.

[0004] Regarding this problem, in Patent Document 2, it is addressed by disposing a catalyst coating on a honeycomb substrate with an anti-peeling coating mainly composed of alumina particles interposed therebetween. However, due to the presence of the anti-peeling coating, the coating thickness increases, and there is a possibility of affecting the output of the engine. Therefore, it is desired to avoid forming a layer having only an anti-peeling effect.

[0005] Prior Art Documents

[0006] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2018-38999

[0007] Patent Document 2: Japanese Patent No. 6820739 Summary of the Invention

[0008] As described above, for the conventional catalyst for exhaust gas purification using an OSC material, improvement in the structural stability is required. Therefore, an object of the present invention is to provide a catalyst for exhaust gas purification with improved structural stability.

[0009] The inventor of the present invention has found that by controlling the average particle sizes of alumina and the OSC material within a specific range in the lower layer of the catalyst layer, the structural stability of the catalyst is improved, thus completing the present invention.

[0010] That is, the gist of the present invention is as follows.

[0011] (1) An exhaust gas purification catalyst is an exhaust gas purification catalyst including a substrate and a catalyst layer formed on the substrate,

[0012] The catalyst layer has a two-layer structure composed of a first coating formed on the substrate and a second coating formed on the first coating,

[0013] The first coating contains alumina and an OSC material,

[0014] The average particle size of the alumina is 6.0 μm or more, and the average particle size of the OSC material is 6.0 μm or more,

[0015] The OSC material is a cerium dioxide-zirconium dioxide-based composite oxide that does not contain alumina.

[0016] (2) The exhaust gas purification catalyst according to (1) above,

[0017] The average particle size of the alumina is 6.0 μm to 12.0 μm, and the average particle size of the OSC material is 6.0 μm to 12.0 μm.

[0018] (3) The exhaust gas purification catalyst according to (1) or (2) above,

[0019] The cerium dioxide-zirconium dioxide-based composite oxide has a fluorite structure and does not contain Pr.

[0020] (4) The exhaust gas purification catalyst according to any one of (1) to (3) above,

[0021] The first coating contains Pd as a catalyst metal.

[0022] According to the present invention, an exhaust gas purification catalyst with improved structural stability can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a graph showing the OSC performance of the catalysts of Examples 1 to 6 and Comparative Example 1 and the peeling rate after the durability test.

[0024] Figure 2 It is a graph showing the OSC performance of the catalysts of Comparative Example 1 and Comparative Example 3 and the peeling rate after the durability test.

[0025] Figure 3 It is a graph showing the OSC performance of the catalysts of Example 3 and Comparative Example 2 and the peeling rate after the durability test. Detailed Description of the Invention

[0026] Hereinafter, preferred embodiments of the present invention will be described in detail.

[0027] The exhaust gas purification catalyst of the present invention (hereinafter also referred to as the catalyst) includes a substrate and a catalyst layer formed on the substrate.

[0028] As the substrate, a honeycomb-shaped material having a plurality of cells can be used. As the material of the substrate, heat-resistant ceramic materials, metal materials such as stainless steel can be cited. As the ceramic material, cordierite (2MgO·2Al2O3·5SiO2), alumina, zirconia, silicon carbide, etc. can be cited.

[0029] The catalyst layer has a two-layer structure composed of a first coating and a second coating.

[0030] The first coating is formed on the substrate. That is, the first coating is in contact with the substrate. The coating width of the first coating is usually 50% to 100% of the total length of the substrate, and in one embodiment, it is 100% of the length.

[0031] The first coating contains alumina (Al2O3) and an OSC material. In the present invention, by controlling the average particle diameters of the alumina and the OSC material in the first coating in contact with the substrate within a specific range, it is possible to suppress the peeling of the coating from the substrate while ensuring the OSC performance.

[0032] The alumina may also be in a form in which an oxide of other metal elements is compounded with Al2O3. However, the oxide of other metal elements is an oxide of metal elements other than cerium (Ce) and zirconium (Zr). As the oxide of other metal elements, for example, La2O3 and Y2O3 can be cited. The content of the oxide of other metal elements is usually 10% by weight or less, preferably 5% by weight or less, and particularly preferably 1% by weight or less.

[0033] In the first coating, the content of alumina is usually 10 g / L to 50 g / L with respect to the substrate volume.

[0034] In the first coating, the average particle size of alumina is 6.0 μm or more, and from the viewpoints of high structural stability and sufficiently low pressure loss, it is preferably 6.0 μm to 12.0 μm, more preferably 8.0 μm to 10.0 μm. The average particle size of alumina in the first coating can be obtained by measuring the particle sizes of alumina particles contained in 50 points of the first coating and calculating the average particle size. This measurement can be carried out, for example, using an Electron Probe Micro Analyzer (EPMA) device.

[0035] The OSC material contained in the first coating is a cerium dioxide (CeO2)-zirconium dioxide (ZrO2) composite oxide that does not contain alumina. Although an OSC material containing alumina has an effect of suppressing coating peeling, the OSC performance deteriorates due to a decrease in the utilization efficiency of cerium dioxide. In the present invention, by controlling the average particle sizes of alumina and the OSC material in the first coating, coating peeling can be suppressed. Therefore, even when using an OSC material that does not contain alumina, high OSC performance can be achieved while suppressing coating peeling. The OSC material may also contain oxides of metal elements other than aluminum (Al), Ce, and Zr. The oxides of such metal elements are not particularly limited, and examples include Nd2O3, La2O3, and Y2O3. In one embodiment, the OSC material does not contain praseodymium (Pr) or an oxide of Pr. From the viewpoint of high OSC performance, the content of zirconium dioxide in the OSC material is usually 30% by weight or more, for example, 40% by weight or more or 50% by weight or more. In one embodiment, the content of zirconium dioxide in the OSC material is 30% by weight to 80% by weight, preferably 40% by weight to 80% by weight.

[0036] In one embodiment, the cerium dioxide-zirconium dioxide composite oxide has a fluorite structure. In one embodiment, the cerium dioxide-zirconium dioxide composite oxide has a fluorite structure and does not contain Pr.

[0037] In the first coating, the content of the OSC material is usually 20 g / L to 100 g / L, preferably 40 g / L to 80 g / L, relative to the substrate volume.

[0038] In the first coating, the average particle size of the OSC material is 6.0 μm or more, and from the viewpoints of high structural stability and sufficiently low pressure loss, it is preferably 6.0 μm to 12.0 μm, more preferably 8.0 μm to 10.0 μm. The average particle size of the OSC material in the first coating can be obtained in the same manner as alumina by measuring the particle sizes of OSC material particles contained in 50 points of the first coating and calculating the average particle size. This measurement can be carried out, for example, using an EPMA device.

[0039] The first coating may also contain a catalyst metal in addition to alumina and the OSC material. The catalyst metal is not particularly limited, and noble metals can be cited. As the catalyst metal, for example, platinum group metals such as ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt) can be used. In one embodiment, the catalyst metal is Pd. In the first coating, the content of the catalyst metal is generally 0.1 g / L to 1.5 g / L relative to the substrate volume.

[0040] The second coating is formed on the first coating. The coating width of the second coating is generally 50% to 100% of the full length of the substrate, and in one embodiment, it is 100% of the length. The second coating, for example, contains a catalyst metal, an OSC material, and a metal oxide other than the OSC material.

[0041] In the second coating, as the catalyst metal, the catalyst metals described above for the first coating can be used. In one embodiment, the catalyst metal is Rh. The catalyst metal can also be used in a form supported by the OSC material or the metal oxide. In the second coating, the content of the catalyst metal is generally 0.01 g / L to 1.0 g / L relative to the substrate volume.

[0042] In the second coating, as the OSC material, for example, cerium dioxide, a composite oxide containing cerium dioxide (e.g., cerium dioxide - zirconium dioxide - based composite oxide) can be cited. The cerium dioxide - zirconium dioxide - based composite oxide may also contain oxides of metal elements other than Ce and Zr. The oxides of such metal elements are not particularly limited, and for example, Nd2O3, La2O3, and Y2O3 can be cited. The content of zirconium dioxide in the OSC material is generally 50% by weight or more, for example, 60% by weight or more or 70% by weight or more. In one embodiment, the content of zirconium dioxide in the OSC material is 50% to 90% by weight.

[0043] In the second coating, the content of the OSC material is generally 20 g / L to 100 g / L relative to the substrate volume.

[0044] In the second coating, as the metal oxide other than the OSC material, for example, alumina, a composite oxide of alumina and an oxide of other metal elements (e.g., zirconium dioxide) can be cited. As the alumina, the alumina described above for the first coating can be used. In the second coating, the total content of the metal oxides is generally 20 g / L to 100 g / L relative to the substrate volume.

[0045] The catalyst of the present invention can be manufactured by coating a slurry containing the components of the coating on a substrate by a method well-known to those skilled in the art. In one embodiment, for example, a first slurry containing the components of the first coating is coated over a specified range from the end face of the substrate, and then dried and fired to form a first coating on the substrate. Next, a second slurry containing the components of the second coating is coated over a specified range from the end face of the substrate on the side opposite to the first slurry, and then dried and fired to form a second coating on the first coating. The first slurry is obtained, for example, by dispersing a solid material containing the components of the first coating in a solvent such as water. The average particle diameters of the alumina and the OSC material of the first coating can be controlled by adjusting the average particle diameter of the solid material containing the components of the first coating. The catalyst of the present invention is obtained, for example, using a first slurry in which the average particle diameter of the solid material is adjusted to 6.0 μm or more (6.0 μm to 12.0 μm in one embodiment). In the present invention, the average particle diameter of the solid material refers to the 50% cumulative particle diameter based on volume (also referred to as the median diameter or D50). The average particle diameter (D50) of the solid material can be measured, for example, by the laser diffraction scattering method. Furthermore, the larger the average particle diameter of the solid material containing the components of the first coating, the larger the average particle diameters of the alumina and the OSC material of the first coating of the obtained catalyst.

[0046] Examples

[0047] Hereinafter, the present invention will be described more specifically using examples. However, the technical scope of the present invention is not limited by these examples.

[0048] <Preparation of Catalyst>

[0049] Raw Materials Used

[0050] Material 1 (Al2O3): La2O3 - complexed Al2O3 (La2O3: 1 wt%)

[0051] Material 2 (CZ): CeO2 - ZrO2 composite oxide (CeO2: 40 wt%; ZrO2: 50 wt%; Nd2O3, La2O3, and Y2O3 are added in trace amounts and high heat resistance is achieved)

[0052] Material 3 (CZ): CeO2 - ZrO2 composite oxide (CeO2: 20 wt%; ZrO2: 70 wt%; Nd2O3, La2O3, and Y2O3 are added in trace amounts and high heat resistance is achieved)

[0053] Material 4 (AZ): Al2O3 - ZrO2 composite oxide (Al2O3: 30 wt%; ZrO2: 60 wt%; Nd2O3, La2O3, and Y2O3 are added in trace amounts and high heat resistance is achieved)

[0054] Material 5 (Ba sulfate): Barium sulfate

[0055] Material 6 (Pd): Palladium nitrate

[0056] Material 7 (Rh): Rhodium nitrate

[0057] Material 8 (ACZ): Al2O3-CeO2-ZrO2 composite oxide (Al2O3: 20 wt%; CeO2: 40 wt%; ZrO2: 30 wt%; Nd2O3, La2O3, and Y2O3 are added in trace amounts and heat resistance improvement has been carried out)

[0058] Substrate: Cordierite honeycomb substrate of 875 cc (600-cell chamber, hexagonal, wall thickness 2 mils (mil))

[0059] Comparative Example 1

[0060] First, while stirring, palladium nitrate (Material 6), Al2O3 (Material 1), CZ (Material 2), barium sulfate (Material 5), and an Al2O3-based binder were added to distilled water to prepare a suspended slurry 1. At this time, the average particle diameter (D50) of the solid materials for slurry 1 was adjusted to 4.5 μm. The slurry 1 was made to flow into the substrate, and the unnecessary part was blown off with a blower, thereby coating the material on the substrate wall surface. At this time, the material was coated in such a way that, with respect to the substrate volume, Material 6 was 0.75 g / L-zone in terms of Pd, Material 1 was 20 g / L-zone, Material 2 was 60 g / L-zone, and Material 5 was 10 g / L-zone. The coating width was adjusted to 100% with respect to the total length of the substrate. Finally, after allowing the moisture to evaporate for 2 hours using a dryer maintained at 120 °C, firing was carried out at 500 °C for 2 hours using an electric furnace to prepare the first coating (lower layer).

[0061] Rh / AZ with Rh supported on AZ was prepared by adding rhodium nitrate (Material 7) and AZ (Material 4) to distilled water while stirring, followed by drying and firing. Rh / AZ, Al2O3 (Material 1), CZ (Material 3), and an Al2O3-based binder were added to distilled water while stirring to prepare a suspended slurry 2. At this time, the average particle diameter (D50) of the solid materials for slurry 2 was adjusted to 4.5 μm. Slurry 2 was made to flow into the substrate coated with slurry 1 from the end face on the opposite side of slurry 1, and the unnecessary parts were blown off with a blower, thereby coating the material on the substrate wall surface. At this time, the materials were coated such that, with respect to the substrate capacity, Material 7 was 0.25 g / L-region in terms of Rh, Material 1 was 30 g / L-region, Material 3 was 60 g / L-region, and Material 4 was 30 g / L-region. The coating width was adjusted to 100% with respect to the total length of the substrate. Finally, after allowing the moisture to evaporate for 2 hours using a dryer maintained at 120 °C, firing was carried out at 500 °C for 2 hours using an electric furnace to prepare the second coating (upper layer).

[0062] In the first coating of the catalyst of Comparative Example 1 obtained, the average particle diameter of alumina (Al2O3) was 4.9 μm, and the average particle diameter of the OSC material (CZ) was 4.3 μm.

[0063] Comparative Example 2

[0064] It was prepared in the same manner as Comparative Example 1 except that Material 2 (CZ) of Slurry 1 was changed to Material 8 (ACZ).

[0065] Comparative Example 3

[0066] It was prepared in the same manner as Comparative Example 1 except that the average particle diameter of the solid materials for Slurry 2 was changed to 8.0 μm.

[0067] Example 1

[0068] It was prepared in the same manner as Comparative Example 1 except that the average particle diameter of the solid materials for Slurry 1 was changed to 6.0 μm. In the first coating of the catalyst of Example 1 obtained, the average particle diameter of alumina (Al2O3) was 6.4 μm, and the average particle diameter of the OSC material (CZ) was 6.0 μm.

[0069] Example 2

[0070] It was prepared in the same manner as Comparative Example 1 except that the average particle diameter of the solid materials for Slurry 1 was changed to 6.5 μm.

[0071] Example 3

[0072] It was prepared in the same manner as Comparative Example 1 except that the average particle diameter of the solid materials for Slurry 1 was changed to 7.0 μm.

[0073] Example 4

[0074] Except that the average particle size of the solid material for Slurry 1 was changed to 8.0 μm, it was prepared in the same manner as in Comparative Example 1. In the first coating of the obtained catalyst of Example 4, the average particle size of alumina (Al2O3) was 8.3 μm, and the average particle size of the OSC material (CZ) was 8.2 μm.

[0075] Example 5

[0076] Except that the average particle size of the solid material for Slurry 1 was changed to 10.0 μm, it was prepared in the same manner as in Comparative Example 1.

[0077] Example 6

[0078] Except that the average particle size of the solid material for Slurry 1 was changed to 12.0 μm, it was prepared in the same manner as in Comparative Example 1. In the first coating of the obtained catalyst of Example 6, the average particle size of alumina (Al2O3) was 12.0 μm, and the average particle size of the OSC material (CZ) was 11.7 μm.

[0079] <Measurement of average particle size>

[0080] The average particle size (D50) of the solid material for Slurry 1 was measured by the laser diffraction / scattering method using a laser diffraction / scattering particle size distribution analyzer (Partica LA-960 manufactured by HORIBA).

[0081] For each catalyst of Example 1, 4, 6 and Comparative Example 1, using an EPMA device (JXA-8530F manufactured by JEOL), 50 points of each material were observed, and the average particle sizes of alumina (Al2O3) and the OSC material (CZ) in the first coating (lower layer) were calculated.

[0082] <Durability test>

[0083] For each of the prepared catalysts, a durability test was carried out using an actual engine. Specifically, the durability test was carried out by installing each catalyst in the exhaust system of a V-type 8-cylinder engine and allowing the exhaust gas of the theoretical air-fuel ratio and lean atmospheres to flow repeatedly at a certain time (ratio of 3:1) for 50 hours at a catalyst bed temperature of 950 °C.

[0084] <Performance evaluation>

[0085] For each of the catalysts that had undergone the durability test, a performance evaluation was carried out using an actual engine. Specifically, each catalyst was installed in the exhaust system of an L-type 4-cylinder engine, and the OSC performance evaluation and peeling evaluation were carried out under the following conditions.

[0086] OSC Performance Evaluation

[0087] Exhaust gas with an air-fuel ratio (A / F) of 14.4 - 15.1 was supplied, and the oxygen absorption and desorption ability during repeated rich-lean cycles in a short period was measured. The larger the value, the more capable it is of absorbing the A / F variation of the gas discharged from the engine, maintaining the atmosphere inside the catalyst near the stoichiometric air-fuel ratio, and sustaining high purification performance.

[0088] Peeling Evaluation

[0089] The peeling evaluation was carried out in the same manner as the "Measurement of Catalyst Coating Peeling Rate" in the examples of Japanese Patent Publication No. 6820739.

[0090] Specifically, each prepared catalyst was cut into a cube of 18 mm × 18 mm × 18 mm and used as the measurement sample. The above measurement sample was placed in a magnetic crucible and heat-treated in air at 1050 °C for 5 hours. The mass of the coating peeled off during the heat treatment and fallen into the crucible was weighed and recorded as "Mass 1". In addition, the mass of the measurement sample after the heat treatment was weighed and recorded as the mass before applying vibration (Mass 2). The measurement sample after the heat treatment was hung on a jig obtained by bending the end of a metal wire and suspended in the cleaning tank of an ultrasonic cleaner, and ultrasonic waves with a frequency of 40 - 45 kHz and a sound pressure of 10 - 12 mV were applied for 10 minutes. The measurement sample after the application of the above ultrasonic waves was recovered, dried at 180 °C for more than 1 hour, and then weighed to investigate the mass after applying vibration (Mass 3). Using the above Mass 1, Mass 2, and Mass 3, the coating peeling rate was calculated by the following mathematical formula (1).

[0091] Coating peeling rate (%) = [{(Mass 1 + Mass 2) - Mass 3} ÷ (Mass 1 + Mass 2)] × 100 (1)

[0092] Table 1 and Figure 1 Examples 1 - 6 and Comparative Example 1 with different average particle sizes of the material of the first coating (lower layer) show the OSC performance and the peeling rate after the durability test of the catalysts. In Figure 1 it, the value of the "average particle size of the OSC material" used the value of the average particle size of the solid material for the first slurry. Furthermore, the larger the average particle size of the material in the slurry, the larger the average particle size of the material in the coating obtained using the slurry.

[0093] Table 1

[0094]

[0095] As shown in Table 1 and Figure 1As shown, by increasing the average particle diameter of the OSC material in the first coating (lower layer), it is possible to suppress the peeling of the coating after the durability test while ensuring the OSC performance. It is considered that by increasing the average particle diameter of the OSC material, the thermal shrinkage of the OSC material can be suppressed, and as a result, the structural stability is improved.

[0096] Figure 2 Show the OSC performance of the catalysts in Comparative Example 1 and Comparative Example 3 with different average particle diameters of the material of the second coating (upper layer) and the peeling rate after the durability test. As shown in Table 1 and Figure 2 As shown, even when controlling the average particle diameter of the material of the upper layer that does not contact the substrate, it is possible to suppress the peeling of the coating after the durability test. However, the effect of suppressing peeling in this case is significantly reduced compared to the case where the average particle diameter of the material of the lower layer is controlled.

[0097] Figure 3 Show the OSC performance of the catalysts in Example 3 and Comparative Example 2 with different materials used in the first coating (lower layer) and different average particle diameters and the peeling rate after the durability test. In the catalyst of Example 3, the average particle diameter of the material of the lower layer is within the scope of the present invention, and the OSC material contained in the lower layer is a CeO2-ZrO2 composite oxide. On the other hand, in the catalyst of Comparative Example 2, the average particle diameter of the material of the lower layer is outside the scope of the present invention, and the OSC material contained in the lower layer is an Al2O3-CeO2-ZrO2 composite oxide. As shown in Table 1 and Figure 3 As shown, compared with the catalyst of Comparative Example 2, the peeling of the coating after the durability test of the catalyst of Example 3 is significantly suppressed, and the OSC performance is high.

Claims

1. An exhaust gas purification catalyst, which is an exhaust gas purification catalyst having a substrate and a catalyst layer formed on the substrate. The catalyst layer has a two-layer structure composed of a first coating formed on the substrate and a second coating formed on the first coating. The first coating contains alumina and an OSC material. The average particle size of the alumina is 6.0 μm or more, and the average particle size of the OSC material is 6.0 μm or more. The OSC material is a cerium-zirconium composite oxide without alumina.

2. The exhaust gas purification catalyst according to claim 1. The average particle size of the alumina is 6.0 μm to 12.0 μm, and the average particle size of the OSC material is 6.0 μm to 12.0 μm.

3. The exhaust gas purification catalyst according to claim 1. The cerium-zirconium composite oxide has a fluorite structure and does not contain Pr.

4. The exhaust gas purification catalyst according to claim 1. The first coating contains Pd as a catalyst metal.

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