Patent Application
20250235852
The present application claims priority from Japanese patent application JP 2024-002479 filed on Jan. 11, 2024, the entire content of which is hereby incorporated by reference into this application.
The present disclosure relates to an exhaust gas purification catalyst.
An exhaust gas discharged from an internal combustion engine of an automobile and the like contains harmful gas of carbon monoxide (CO), nitrogen oxide (NOx), unburned hydrocarbon (HC), and the like. To decompose such a harmful gas, an exhaust gas purification catalyst (what is called a three-way catalyst) is used. As the exhaust gas purification catalyst, one including a substrate coated with a catalyst layer is widely used. For the catalyst layer of the exhaust gas purification catalyst, in addition to catalyst metals, an oxygen absorption/release material (also referred to as an OSC material) having an oxygen absorption/release capacity (also referred to as an Oxygen Storage Capacity (OSC)) is widely used (for example, JP 2018-38999 A). The oxygen absorption/release material absorbs and releases oxygen, and adjusts an air-fuel ratio (A/F). Thus, reduction in purification rate due to fluctuation of exhaust gas composition can be suppressed.
On the other hand, under the recent emissions regulations, it is desired to maintain the purification performance even in traveling a longer distance. Therefore, the exhaust gas purification catalyst is required to have further improved structural stability. However, in the exhaust gas purification catalyst using the OSC material, the coat layer peels off from the substrate during the use over a long period of time, and the purification performance is impaired in some cases. This is considered to be caused by the following reasons. Specifically, the OSC material thermally shrinks due to the oxygen absorption/release, and the difference in shrinkage from other materials and the substrate reduces the structural stability. Consequently, the strength between the substrate and the coat layer is reduced, and the coat layer peels off from the substrate.
Japanese Patent No. 6820739 deals with this problem by providing a catalyst coat layer on a honeycomb substrate via an anti-peeling coat layer containing alumina particles as a main component. However, the presence of the anti-peeling coat layer increases the coat thickness, possibly affecting the engine output. Therefore, it is desired to avoid forming a layer having only a function of anti-peeling.
As described above, the conventional exhaust gas purification catalyst using the OSC material is required to have the improved structural stability. Accordingly, the present disclosure provides an exhaust gas purification catalyst improved in structural stability.
The inventors found that the structural stability of a catalyst is improved by controlling average particle diameters of alumina and an OSC material to specific ranges in a lower layer of a catalyst layer, thus completing the present disclosure.
That is, the gist of the present disclosure is as follows.
The present disclosure can provide the exhaust gas purification catalyst improved in structural stability.
The following describes embodiments of the present disclosure in detail.
An exhaust gas purification catalyst (hereinafter also referred to as a catalyst) of the present disclosure comprises a substrate and a catalyst layer disposed on the substrate.
As the substrate, a honeycomb-shaped material with multiple cells can be used. Examples of the material of the substrate include a ceramic material having heat resistance and a metal material, such as stainless steel. Examples of the ceramic material include cordierite (2MgO·2Al2O3·5SiO2), alumina, zirconia, and silicon carbide.
The catalyst layer has a two-layer structure including a first coat layer and a second coat layer.
The first coat layer is formed on the substrate. That is, the first coat layer is in contact with the substrate. The first coat layer has a coat width that is ordinarily a length of 50% to 100%, and a length of 100% in one embodiment with respect to the whole length of the substrate.
The first coat layer contains alumina (Al2O3) and an OSC material. In the present disclosure, by controlling average particle diameters of alumina and the OSC material in the first coat layer in contact with the substrate to specific ranges, the coat layer can be suppressed to peel off from the substrate while the OSC performance is ensured.
Alumina may have a form in which an oxide of another metallic element is combined with Al2O3. However, the oxide of the other metallic element is an oxide of a metallic element other than cerium (Ce) or zirconium (Zr). Examples of the oxide of the other metallic element include La2O3 and Y2O3. The content of the oxide of the other metallic element is ordinarily 10% by weight or less, 5% by weight or less in some embodiments, and 1% by weight or less in some embodiments.
In the first coat layer, the content of alumina is ordinarily 10 g/L to 50 g/L with respect to the substrate volume.
In the first coat layer, alumina has the average particle diameter of 6.0 μm or more, and from the aspect of high structural stability and sufficiently low pressure loss, 6.0 μm to 12.0 μm in some embodiments, and 8.0 μm to 10.0 μm in some embodiments. The average particle diameter of alumina in the first coat layer can be obtained by measuring particle diameters of 50 alumina particles contained in the first coat layer and calculating the average particle diameter. This measurement can be performed, for example, using Electron Probe Micro Analyzer (EPMA).
The OSC material contained in the first coat layer is a ceria (CeO2)-zirconia (ZrO2)-based composite oxide without alumina. While the OSC material containing alumina provides an effect of suppressing peeling off of the coat layer, the OSC performance is reduced due to a reduced ceria usage efficiency. In the present disclosure, by controlling the average particle diameters of alumina and the OSC material in the first coat layer, peeling off of the coat layer can be suppressed. Therefore, even when the OSC material without alumina is used, the high OSC performance can be achieved while suppressing peeling off of the coat layer. The OSC material may contain an oxide of a metallic element other than aluminum (Al), Ce, or Zr. Such an oxide of the metallic element is not especially limited, and examples thereof include Nd2O3, La2O3, and Y2O3. In one embodiment, the OSC material does not contain an oxide of praseodymium (Pr) or Pr. The content of zirconia in the OSC material is, from the aspect of high OSC performance, ordinarily 30% by weight or more, and 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% by weight to 80% by weight, and 40% by weight to 80% by weight in some embodiments.
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.
In the first coat layer, the content of the OSC material is ordinarily 20 g/L to 100 g/L, and 40 g/L to 80 g/L in some embodiments with respect to the substrate volume.
In the first coat layer, the OSC material has the average particle diameter of 6.0 μm or more, and from the aspect of high structural stability and sufficiently low pressure loss, 6.0 μm to 12.0 μm in some embodiments, and 8.0 μm to 10.0 μm in some embodiments. The average particle diameter of the OSC material in the first coat layer can be obtained by, similarly to alumina, measuring particle diameters of 50 OSC material particles contained in the first coat layer and calculating the average particle diameter. This measurement can be performed, for example, using EPMA device.
The first coat layer may contain a catalyst metal in addition to alumina and the OSC material. The catalyst metal is not especially limited, and examples thereof include a noble metal. As the catalyst metal, for example, a platinum group metal, 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 coat layer, the content of the catalyst metal is ordinarily 0.1 g/L to 1.5 g/L with respect to the substrate volume.
The second coat layer is formed on the first coat layer. The second coat layer has a coat width that is ordinarily a length of 50% to 100%, and a length of 100% in one embodiment with respect to the whole length of the substrate. The second coat layer contains, for example, a catalyst metal, an OSC material, and a metal oxide other than the OSC material.
In the second coat layer, as the catalyst metal, the catalyst metals described for the first coat layer can be used. In one embodiment, the catalyst metal is Rh. The catalyst 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 catalyst metal is ordinarily 0.01 g/L to 1.0 g/L with respect to the substrate volume.
In the second coat layer, examples of the OSC material include ceria and a composite oxide containing ceria (for example, ceria-zirconia-based composite oxide). The ceria-zirconia-based composite oxide may contain an oxide of a metallic element other than Ce or Zr. Such an oxide of the metallic element is not especially limited, and examples thereof include Nd2O3, La2O3, and Y2O3. The content of zirconia in the OSC material is ordinarily 50% by weight or more, and 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% by weight to 90% by weight.
In the second coat layer, the content of the OSC material is ordinarily 20 g/L to 100 g/L with respect to the substrate volume.
In the second coat layer, examples of the metal oxide other than the OSC material include alumina and a composite oxide of alumina and an oxide of another metallic element (for example, zirconia). As alumina, those described for the first coat layer can be used. In the second coat layer, the total content of the metal oxide is ordinarily 20 g/L to 100 g/L with respect to the substrate volume.
The catalyst of the present disclosure can be produced by coating a substrate with a slurry containing the components of the coat layer by a method publicly known to those skilled in the art. In one embodiment, for example, a substrate is coated with a first slurry containing components of a first coat layer in a predetermined range from a substrate end surface, and then dried and fired, thus forming the first coat layer on the substrate. Subsequently, the substrate is coated with a second slurry containing components of a second coat layer in a predetermined range from a substrate end surface in the opposite side of the first slurry, and then dried and fired, thus forming the second coat layer on the first coat layer. The first slurry is obtained by, for example, dispersing a solid material containing the components of the first coat layer in a solvent, such as water. The average particle diameters of alumina and the OSC material in the first coat layer can be controlled by adjusting an average particle diameter of the solid material containing the components of the first coat layer. The catalyst of the present disclosure is obtained using, for example, the 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 disclosure, the average particle diameter of the solid material means a 50% cumulative particle diameter (also referred to as a median diameter or D50) based on the volume. The average particle diameter (D50) of the solid material can be measured by, for example, a laser diffraction scattering method. The more the average particle diameter of the solid material containing the components of the first coat layer increases, the more the average particle diameters of alumina and the OSC material in the first coat layer of the obtained catalyst increase.
The following further specifically describes the present disclosure using examples. However, the technical scope of the present disclosure is not limited to these examples.
First, palladium nitrate (Material 6), Al2O3(Material 1), CZ (Material 2), barium sulfate (Material 5), and an Al2O3-based binder were introduced into distilled water while being stirred, thus preparing a suspended slurry 1. At this time, an average particle diameter (D50) of a solid material for the slurry 1 was adjusted to 4.5 μm. The slurry 1 was casted into the substrate and the excess was blown off with a blower, thus coating a substrate wall surface with the materials. At this time, for the coating materials, with respect to the substrate volume, the coating was performed with Material 6 as Pd of 0.75 g/L-zone, Material 1 of 20 g/L-zone, Material 2 of 60 g/L-zone, and Material 5 of 10 g/L-zone. The coat width was adjusted to 100% with respect to the whole length of the substrate. At last, after the water content was reduced for two hours with a dryer kept at 120° C., two hours of firing was performed with an electric furnace kept at 500° C., thus preparing the first coat layer (lower layer).
Rhodium nitrate (Material 7) and AZ (Material 4) were introduced into distilled water while being stirred, and dried and fired, thus preparing Rh/AZ in which Rh is supported on AZ. Rh/AZ, Al2O3(Material 1), CZ (Material 3), and an Al2O3-based binder were introduced into distilled water while being stirred, thus preparing a suspended slurry 2. At this time, an average particle diameter (D50) of a solid material for the slurry 2 was adjusted to 4.5 μm. The slurry 2 was casted into the substrate over which the slurry 1 was applied from an end surface in the opposite side of the slurry 1, and the excess was blown off with a blower, thus coating the substrate wall surface with the materials. At this time, for the coating materials, with respect to the substrate volume, the coating was performed with Material 7 as Rh of 0.25 g/L-zone, Material 1 of 30 g/L-zone, Material 3 of 60 g/L-zone, and Material 4 of 30 g/L-zone. The coat width was adjusted to 100% with respect to the whole length of the substrate. At last, after the water content was reduced for two hours with a dryer kept at 120° C., two hours of firing was performed with an electric furnace kept at 500° C., thus preparing the second coat layer (upper layer).
In the first coat layer of the obtained catalyst of Comparative Example 1, 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.
The preparation was performed similarly to Comparative Example 1 except that Material 2 (CZ) of the slurry 1 was changed to Material 8 (ACZ).
The preparation was performed similarly to Comparative Example 1 except that the average particle diameter of the solid material for the slurry 2 was changed to 8.0 μm.
The preparation was performed similarly to Comparative Example 1 except that the average particle diameter of the solid material for the slurry 1 was changed to 6.0 μm. In the first coat layer of the obtained catalyst of Example 1, 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.
The preparation was performed similarly to Comparative Example 1 except that the average particle diameter of the solid material for the slurry 1 was changed to 6.5 μm.
The preparation was performed similarly to Comparative Example 1 except that the average particle diameter of the solid material for the slurry 1 was changed to 7.0 μm.
The preparation was performed similarly to Comparative Example 1 except that the average particle diameter of the solid material for the slurry 1 was changed to 8.0 μm. In the first coat layer of the obtained catalyst of Example 4, the average particle diameter of alumina (Al2O3) was 8.3 μm, and the average particle diameter of the OSC material (CZ) was 8.2 μm.
The preparation was performed similarly to Comparative Example 1 except that the average particle diameter of the solid material for the slurry 1 was changed to 10.0 μm.
The preparation was performed similarly to Comparative Example 1 except that the average particle diameter of the solid material for the slurry 1 was changed to 12.0 μm. In the first coat layer of the obtained catalyst of Example 6, the average particle diameter of alumina (Al2O3) was 12.0 μm, and the average particle diameter of the OSC material (CZ) was 11.7 μm.
The average particle diameter (D50) of the solid material for the slurry 1 was measured by a laser diffraction scattering method using a laser scattering particle size distribution analyzer (manufactured by HORIBA, Partica LA-960).
For each of the catalysts of Examples 1, 4, 6 and Comparative Example 1, particle diameters of alumina (Al2O3) and the OSC material (CZ) of the first coat layer (lower layer) were measured at 50 points for each material using EPMA device (manufactured by JEOL, JXA-8530F), and the average particle diameters were calculated.
The durability test was performed for each of the prepared catalysts using an actual engine. Specifically, the durability test was performed as follows. The catalysts were each installed to an exhaust system of a V-type eight-cylinder engine, exhaust gases in respective stoichiometric and lean atmospheres were repeatedly flowed for a certain period of time (a ratio of 3:1) at a catalyst bed temperature of 950° C. for 50 hours.
For each of the catalysts subjected to the durability test, the performance was evaluated using an actual engine. Specifically, the catalysts were each installed to an exhaust system of a L-type four-cylinder engine, and the OSC performance evaluation and the peeling evaluation were performed under the following conditions.
The exhaust gas having air-fuel ratio (A/F) of from 14.4 to 15.1 was supplied, and the oxygen absorption/release capacity when the rich and the lean are repeated in a short period was measured. It is indicated that the larger the value is, the more an A/F variation of the gas discharged from the engine can be absorbed, thus an atmosphere inside the catalyst can be held around a stoichiometric state and a high purification performance can be maintained.
Similarly to “Measurement of Catalyst Coat Layer Peeling Rate” in the example of Japanese Patent No. 6820739, the peeling evaluation was performed.
Specifically, the prepared catalysts were each cut in a cube of 18 mm×18 mm×18 mm, and used as measurement samples. The measurement sample was put in a magnetic crucible, and a heating process was performed in air at 1050° C. for five hours. The mass of the coat layer peeled off and fallen into the crucible during the heating process was weighed and recorded as “Mass 1.” The mass of the measurement sample after the heating process was weighed, and recorded as a mass before applying vibration (Mass 2). The measurement sample after the heating process was hooked onto a jig with a bent tip of a wire and suspended in a cleaning tank of an ultrasonic cleaner, and an ultrasonic sound wave having a frequency of 40 to 45 kHz and a sound pressure of 10 to 12 mV was applied for 10 minutes. The measurement sample after the ultrasonic sound wave application was collected, dried at 180° C. for one hour or more, and then weighed, and a mass after the vibration application (Mass 3) was examined. Using the Mass 1, Mass 2, and Mass 3 described above, the coat layer peeling rate was calculated with a formula (1) below.
coat layer peeling rate (%)=[((Mass 1+Mass 2)−Mass 3)/(Mass 1+Mass 2)]×100(1)
Table 1 and
As illustrated in Table 1 and
All publications, patents, and patent applications cited herein are incorporated herein by reference in their entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2024-002479 | Jan 2024 | JP | national |
