Exhaust gas purification catalyst

Abstract

To provide an exhaust gas purification catalyst having improved structural stability.SOLUTION: There is provided an exhaust gas purification catalyst having a base material and a catalyst layer formed on the base material, where the catalyst layer has a two-layer structure consisting of a first coat layer formed on the base material and a second coat layer formed on the first coat layer, the first coat layer containing alumina and an OSC material, the average particle diameter of the alumina being 6.0 μm or more, the average particle diameter of the OSC material being 6.0 μm or more and the OSC material being a ceria-zirconia-based composite oxide containing no alumina.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a catalyst for purifying exhaust gas.

Background Art

[0002] Exhaust gas discharged from internal combustion engines such as automobiles 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 purifying exhaust gas (so-called three-way catalyst) is used. As the catalyst for purifying exhaust gas, a catalyst layer coated on a substrate is widely used. In the catalyst layer of the catalyst for purifying exhaust gas, in addition to the catalyst metal, an oxygen storage and release material (also called OSC material) having an oxygen storage and release ability (also called Oxygen Storage Capacity (OSC)) is widely used (for example, Patent Document 1). The oxygen storage and release material can suppress a decrease in the purification rate due to fluctuations in the exhaust gas composition by storing and releasing oxygen and adjusting the air-fuel ratio (A / F).

[0003] On the other hand, recent exhaust gas regulations require maintaining the purification performance even when traveling longer distances. For this reason, further improvement in the structural stability is required for the catalyst for purifying exhaust gas. However, in the catalyst for purifying exhaust gas using an OSC material, during long-term use, the coating layer may peel off from the substrate and the purification performance may be impaired. This is considered to be due to the following reasons. Specifically, thermal contraction of the OSC material occurs with oxygen storage and release, and the structural stability decreases due to the difference in shrinkage with other materials and the substrate. As a result, the strength between the substrate and the coating layer decreases, and the coating layer peels off from the substrate.

[0004] In response to this problem, Patent Document 2 addresses it by disposing a catalyst coating layer on a honeycomb substrate via a peeling prevention coating layer mainly composed of alumina particles. However, the presence of the peeling prevention coating layer increases the coating thickness, which may affect the engine output. Therefore, it is desirable to avoid forming a layer having only the role of preventing peeling.

Prior Art Documents

[0005] [Patent Document 1] JP 2018-38999 A [Patent Document 2] Patent No. 6820739 Summary of the Invention [Problem to be solved by the invention]

[0006] As described above, conventional exhaust gas purification catalysts using OSC materials are required to have improved structural stability. Therefore, an object of the present invention is to provide an exhaust gas purification catalyst having improved structural stability. [Means for solving the problem]

[0007] The present inventors have found that the structural stability of the catalyst can be improved by controlling the average particle diameters of alumina and OSC material in a specific range in the lower layer of the catalyst layer, and have completed the present invention.

[0008] That is, the gist of the present invention is as follows. (1) A catalyst for purifying exhaust gas comprising a substrate and a catalyst layer formed on the substrate, the catalyst layer has a two-layer structure including a first coating layer formed on the substrate and a second coating layer formed on the first coating layer, the first coating layer 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 ceria-zirconia-based composite oxide that does not contain alumina. Catalyst for purifying exhaust gas. (2) The exhaust gas purifying catalyst according to (1) above, wherein the alumina has an average particle size of 6.0 μm to 12.0 μm, and the OSC material has an average particle size of 6.0 μm to 12.0 μm. (3) The exhaust gas purification catalyst according to (1) or (2) above, wherein the ceria-zirconia-based composite oxide has a fluorite structure and does not contain Pr. (4) The exhaust gas purification catalyst according to any one of (1) to (3) above, wherein the first coat layer contains Pd as a catalyst metal.

Advantages of the Invention

[0009] According to the present invention, it becomes possible to provide an exhaust gas purification catalyst with improved structural stability.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0011] Hereinafter, preferred embodiments of the present invention will be described in detail. The exhaust gas purification catalyst of the present invention (hereinafter also referred to as a catalyst) includes a substrate and a catalyst layer formed on the substrate.

[0012] As the substrate, a honeycomb-shaped material having a large number of cells can be used. Examples of the material of the substrate include heat-resistant ceramic materials and metal materials such as stainless steel. Examples of the ceramic material include cordierite (2MgO·2Al2O3·5SiO2), alumina, zirconia, and silicon carbide.

[0013] The catalyst layer has a two-layer structure composed of a first coat layer and a second coat layer.

[0014] The first coat layer is formed on the substrate. That is, the first coat layer is in contact with the substrate. The coating width of the first coat layer is usually 50% to 100% of the total length of the substrate, and in one embodiment, it is 100% of the length.

[0015] The first coat layer contains alumina (Al2O3) and an OSC material. In the present invention, by controlling the average particle sizes of the alumina and the OSC material in the first coat layer in contact with the substrate within a specific range, it is possible to suppress the peeling of the coat layer from the substrate while ensuring the OSC performance.

[0016] The alumina may be in a form in which oxides of other metal elements are complexed with Al2O3. However, the oxides of other metal elements are oxides of metal elements other than cerium (Ce) and zirconium (Zr). Examples of the oxides of other metal elements include La2O3 and Y2O3. The content of the oxides 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.

[0017] In the first coat layer, the content of alumina is usually 10 g / L to 50 g / L with respect to the volume of the substrate.

[0018] In the first coat layer, the average particle size of the 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, and more preferably 8.0 μm to 10.0 μm. The average particle size of the alumina in the first coat layer can be obtained by measuring the particle sizes of 50 alumina particles contained in the first coat layer and calculating the average particle size. This measurement can be carried out, for example, using an Electron Probe Micro Analyzer (EPMA) device.

[0019] The OSC material contained in the first coating layer is a ceria (CeO2)-zirconia (ZrO2)-based composite oxide that does not contain alumina. Although the OSC material containing alumina has an effect of suppressing the peeling of the coating layer, the OSC performance deteriorates due to the decrease in the utilization efficiency of ceria. In the present invention, by controlling the average particle sizes of the alumina and the OSC material in the first coating layer, the peeling of the coating layer can be suppressed. Therefore, even when using an OSC material that does not contain alumina, high OSC performance can be achieved while suppressing the peeling of the coating layer. The OSC material may contain oxides of metal elements other than aluminum (Al), Ce, and Zr. Such oxides of 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 zirconia 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 zirconia in the OSC material is 30% to 80% by weight, preferably 40% to 80% by weight.

[0020] In one embodiment, the ceria-zirconia-based composite oxide has a fluorite structure. In one embodiment, the ceria-zirconia-based composite oxide has a fluorite structure and does not contain Pr.

[0021] In the first coating layer, the content of the OSC material is usually 20 g / L to 100 g / L, preferably 40 g / L to 80 g / L, based on the volume of the substrate.

[0022] In the first coating layer, 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 layer can be determined in the same manner as for alumina, by measuring the particle sizes of 50 OSC material particles contained in the first coating layer and calculating the average particle size. This measurement can be carried out using, for example, an EPMA apparatus.

[0023] In addition to alumina and the OSC material, the first coat layer may contain a catalytic metal. The catalytic metal is not particularly limited and examples thereof include noble metals. As the catalytic 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 catalytic metal is Pd. In the first coat layer, the content of the catalytic metal is usually 0.1 g / L to 1.5 g / L with respect to the volume of the substrate.

[0024] The second coat layer is formed on the first coat layer. The coating width of the second coat layer is usually 50% to 100% of the total length of the substrate, and in one embodiment, it is 100% of the length. The second coat layer contains, for example, a catalytic metal, an OSC material, and a metal oxide other than the OSC material.

[0025] In the second coat layer, as the catalytic metal, the above-mentioned catalytic metal for the first coat layer can be used. In one embodiment, the catalytic metal is Rh. The catalytic metal may be used in a form supported on the OSC material or the metal oxide. In the second coat layer, the content of the catalytic metal is usually 0.01 g / L to 1.0 g / L with respect to the volume of the substrate.

[0026] In the second coat layer, examples of the OSC material include ceria and composite oxides containing ceria (for example, ceria-zirconia-based composite oxides). The ceria-zirconia-based composite oxide may contain oxides of metal elements other than Ce and Zr. Examples of such oxides of metal elements are not particularly limited and include, for example, Nd2O3, La2O3, and Y2O3. The content of zirconia in the OSC material is usually 50% by weight or more, for example, 60% by weight or more, or 70% by weight or more. In one embodiment, the content of zirconia in the OSC material is 50% to 90% by weight.

[0027] In the second coat layer, the content of the OSC material is usually 20 g / L to 100 g / L with respect to the volume of the substrate.

[0028] In the second coat layer, examples of metal oxides other than the OSC material include alumina and composite oxides of alumina and oxides of other metal elements (e.g., zirconia). As the alumina, those described above for the first coat layer can be used. In the second coat layer, the total content of the metal oxide is usually 20 g / L to 100 g / L with respect to the substrate volume.

[0029] The catalyst of the present invention can be produced by coating a slurry containing the components of the coat layer on a substrate by a method known to those skilled in the art. In one embodiment, for example, a first slurry containing the components of the first coat layer is coated over a predetermined range from the end face of the substrate, and then dried and fired to form a first coat layer on the substrate. Next, a second slurry containing the components of the second coat layer is coated over a predetermined 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 coat layer on the first coat layer. The first slurry can be obtained, for example, by dispersing a solid material containing the components of the first coat layer in a solvent such as water. The average particle diameters of the alumina and the OSC material in the first coat layer can be controlled by adjusting the average particle diameter of the solid material containing the components of the first coat layer. The catalyst of the present invention can be 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 (in one embodiment, 6.0 μm to 12.0 μm). In the present invention, the average particle diameter of the solid material refers to the volume-based 50% cumulative particle diameter (also called 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. Note that, as the average particle diameter of the solid material containing the components of the first coat layer increases, the average particle diameters of the alumina and the OSC material in the first coat layer of the obtained catalyst also increase.

Examples

[0030] Hereinafter, the present invention will be described more specifically using examples. However, the technical scope of the present invention is not limited to these examples.

[0031] <Preparation of Catalyst> Raw Materials Used Material 1 (Al2O3): La2O3 - composite Al2O3 (La2O3: 1 wt%) Material 2 (CZ): CeO2 - ZrO2 composite oxide (CeO2: 40 wt%; ZrO2: 50 wt%; trace amounts of Nd2O3, La2O3 and Y2O3 added, heat - resistant treated) Material 3 (CZ): CeO2 - ZrO2 composite oxide (CeO2: 20 wt%; ZrO2: 70 wt%; trace amounts of Nd2O3, La2O3, Y2O3 added, heat - resistant treated) Material 4 (AZ): Al2O3 - ZrO2 composite oxide (Al2O3: 30 wt%; ZrO2: 60 wt%; trace amounts of Nd2O3, La2O3 and Y2O3 added, heat - resistant treated) Material 5 (Barium sulfate): Barium sulfate Material 6 (Pd): Palladium nitrate Material 7 (Rh): Rhodium nitrate Material 8 (ACZ): Al2O3 - CeO2 - ZrO2 composite oxide (Al2O3: 20 wt%; CeO2: 40 wt%; ZrO2: 30 wt%; trace amounts of Nd2O3, La2O3 and Y2O3 added, heat - resistant treated) Substrate: 875 cc (600 - cell hexagonal wall thickness 2 mil) cordierite honeycomb substrate

[0032] Comparative Example 1 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 size (D50) of the solid materials for slurry 1 was adjusted to 4.5 μm. Slurry 1 was poured onto the substrate, and the unnecessary components were blown off with a blower to coat the substrate wall with the materials. At that time, the coating materials were coated such that for the substrate volume, Material 6 was 0.75 g / L-zone as 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% of the total length of the substrate. Finally, after removing moisture with a dryer maintained at 120°C for 2 hours, firing was carried out at 500°C for 2 hours in an electric furnace to prepare the first coat layer (lower layer).

[0033] While stirring, rhodium nitrate (Material 7) and AZ (Material 4) were added to distilled water, dried, and fired to prepare Rh / AZ in which Rh was supported on AZ. While stirring, Rh / AZ, Al2O3 (Material 1), CZ (Material 3), and an Al2O3-based binder were added to distilled water to prepare a suspended slurry 2. At this time, the average particle size (D50) of the solid materials for slurry 2 was adjusted to 4.5 μm. Slurry 2 was poured onto the substrate coated with slurry 1 from the end face opposite to that of slurry 1, and the unnecessary components were blown off with a blower to coat the substrate wall with the materials. At that time, the coating materials were coated such that for the substrate volume, Material 7 was 0.25 g / L-zone as Rh, Material 1 was 30 g / L-zone, Material 3 was 60 g / L-zone, and Material 4 was 30 g / L-zone. The coating width was adjusted to 100% of the total length of the substrate. Finally, after removing moisture with a dryer maintained at 120°C for 2 hours, firing was carried out at 500°C for 2 hours in an electric furnace to prepare the second coat layer (upper layer).

[0034] In the first coat layer of the catalyst of Comparative Example 1 obtained, the average particle size of alumina (Al2O3) was 4.9 μm, and the average particle size of the OSC material (CZ) was 4.3 μm.

[0035] Comparative Example 2 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).

[0036] Comparative Example 3 It was prepared in the same manner as Comparative Example 1, except that the average particle size of the solid material for Slurry 2 was changed to 8.0 μm.

[0037] Example 1 It was prepared in the same manner as Comparative Example 1, except that the average particle size of the solid material for Slurry 1 was changed to 6.0 μm. In the first coat layer of the catalyst of Example 1 obtained, the average particle size of alumina (Al2O3) was 6.4 μm, and the average particle size of the OSC material (CZ) was 6.0 μm.

[0038] Example 2 It was prepared in the same manner as Comparative Example 1, except that the average particle size of the solid material for Slurry 1 was changed to 6.5 μm.

[0039] Example 3 It was prepared in the same manner as Comparative Example 1, except that the average particle size of the solid material for Slurry 1 was changed to 7.0 μm.

[0040] Example 4 It was prepared in the same manner as Comparative Example 1, except that the average particle size of the solid material for Slurry 1 was changed to 8.0 μm. In the first coat layer of the catalyst of Example 4 obtained, 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.

[0041] Example 5 It was prepared in the same manner as Comparative Example 1, except that the average particle size of the solid material for Slurry 1 was changed to 10.0 μm.

[0042] Example 6 It was prepared in the same manner as Comparative Example 1, except that the average particle size of the solid material for Slurry 1 was changed to 12.0 μm. In the first coat layer of the catalyst of Example 6 obtained, 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.

[0043] <Measurement of average particle size> The average particle size (D50) of the solid material for Slurry 1 was measured by a laser diffraction scattering method using a laser diffraction / scattering type particle size distribution measuring device (HORIBA Partica LA-960).

[0044] For each of the catalysts of Examples 1, 4, and 6 and Comparative Example 1, the average particle size of the alumina (Al2O3) and OSC material (CZ) of the first coating layer (lower layer) was measured at 50 points for each material using an EPMA device (JEOL JXA-8530F) and the average particle size was calculated.

[0045] <Durability test> A durability test was carried out for each catalyst prepared using an actual engine. Specifically, the durability test was carried out by attaching each catalyst to the exhaust system of a V8 engine and repeatedly flowing exhaust gases in stoichiometric and lean atmospheres for a fixed period of time (ratio of 3:1) for 50 hours at a catalyst bed temperature of 950°C.

[0046] <Performance evaluation> For each catalyst that underwent durability testing, 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 OSC performance evaluation and peeling evaluation were carried out under the following conditions.

[0047] OSC performance evaluation Exhaust gas with an air-fuel ratio (A / F) of 14.4-15.1 was supplied and the oxygen absorption and release capacity was measured when the test was repeated in short cycles. The higher the value, the better the A / F fluctuations of the gas emitted from the engine are absorbed, the more the atmosphere inside the catalyst can be kept close to stoichiometric, and the higher the purification performance can be maintained.

[0048] Peeling evaluation Peeling evaluation was carried out in the same manner as in "Measurement of catalyst coating layer peeling rate" in the examples of Japanese Patent No. 6820739.

[0049] Specifically, each prepared catalyst cut into a cube of 18 mm × 18 mm × 18 mm was used as a measurement sample. The above measurement sample was placed in a magnetic crucible and heat-treated at 1050 °C for 5 hours in air. The mass of the coating layer peeled off and fallen into the crucible during the heat treatment was weighed and recorded as "Mass 1". Also, the mass of the measurement sample after the heat treatment was weighed and recorded as the mass before vibration application (Mass 2). The measurement sample after the heat treatment was hooked on a jig with the tip of the wire bent and suspended in the cleaning layer 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 above ultrasonic wave application was recovered, dried at 180 °C for 1 hour or more, and then weighed to examine the mass after vibration application (Mass 3). Using the above Mass 1, Mass 2, and Mass 3, the coating layer peeling rate was calculated by the following formula (1). Coating layer peeling rate (%) = [{(Mass 1 + Mass 2) - Mass 3} ÷ (Mass 1 + Mass 2)] × 100 (1)

[0050] Table 1 and Figure 1 show the OSC performance of the catalysts of Examples 1 - 6 and Comparative Example 1 and the peeling rate after the durability test, where the average particle size of the material of the first coating layer (lower layer) is different. In Figure 1, the value of "average particle size of the OSC material" uses the value of the average particle size of the solid material for the first slurry. Note that the larger the average particle size of the material in the slurry, the larger the average particle size of the material in the coating layer obtained using the slurry.

[0051]

Table 1

[0052] As shown in Table 1 and Figure 1, by increasing the average particle size of the OSC material in the first coating layer (lower layer), it was possible to suppress the peeling of the coating layer after the durability test while ensuring the OSC performance. This is considered to be because by increasing the average particle size of the OSC material, the thermal shrinkage of the OSC material can be suppressed, and as a result, the structural stability is improved.

[0053] Fig. 2 shows the OSC performance of the catalysts of Comparative Example 1 and Comparative Example 3 and the peeling rate after the durability test, where the average particle size of the material of the second coat layer (upper layer) is different. As shown in Table 1 and Fig. 2, even when the average particle size of the material of the upper layer not in contact with the substrate was controlled, peeling of the coat layer after the durability test could be suppressed. However, the peeling suppression effect in this case was significantly smaller compared to the case where the average particle size of the material of the lower layer was controlled.

[0054] Fig. 3 shows the OSC performance of the catalysts of Example 3 and Comparative Example 2 and the peeling rate after the durability test, where the materials used and the average particle sizes of the first coat layer (lower layer) are different. In the catalyst of Example 3, the average particle size 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 size 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 Fig. 3, the catalyst of Example 3 had significantly suppressed peeling of the coat layer after the durability test and had high OSC performance compared to the catalyst of Comparative Example 2.

Claims

1. A catalyst for purifying exhaust gas comprising a substrate and a catalyst layer formed on the substrate, the catalyst layer has a two-layer structure including a first coating layer formed on the substrate and a second coating layer formed on the first coating layer, the first coating layer includes 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 ceria-zirconia-based composite oxide that does not contain alumina. Catalyst for purifying exhaust gas.

2. 2. The exhaust gas purifying catalyst according to claim 1, wherein the alumina has an average particle size of 6.0 μm to 12.0 μm, and the OSC material has an average particle size of 6.0 μm to 12.0 μm.

3. 2. The exhaust gas purifying catalyst according to claim 1, wherein the ceria-zirconia based composite oxide has a fluorite structure and does not contain Pr.

4. 2. The exhaust gas purifying catalyst according to claim 1, wherein the first coating layer contains Pd as a catalytic metal.

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