JP2024099150A - Exhaust gas purification catalyst - Google Patents

Abstract

To provide an exhaust gas purification catalyst in which provided is means for achieving both emission control performance and low-temperature activity of a non-regulated substance.SOLUTION: Provided is an exhaust gas purification catalyst having a substrate, a first catalyst layer including rhodium, arranged on a top face of the substrate, and a second catalyst layer including palladium, arranged on the top face of the first catalyst layer. The first catalyst layer is arranged in an area ranging from 80 to 100% with respect to the full length of the substrate from a downstream side end face of the exhaust gas purification catalyst, on the second catalyst layer, the palladium in a range from 1 to 4 g is supported by an area ranging from 20 to 50% with respect to the full length of the substrate from an upstream side end face of the exhaust gas purification catalyst, and the palladium included on the second catalyst layer, with an amount of 80% or more, is supported by a composite oxide (ACZ) of alumina, ceria and zirconia.SELECTED DRAWING: Figure 1

Description

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

Catalysts for purifying exhaust gas from automobiles, etc. oxidize the hydrocarbons (HC) and carbon monoxide (CO) contained in the exhaust gas discharged from the engine to water and carbon dioxide, and reduce nitrogen oxides (NOx) to nitrogen. Such catalytically active exhaust gas purification catalysts usually use precious metal-supported catalysts, which are made by coating a heat-resistant substrate with a catalyst layer containing particles of catalytic precious metals such as palladium (Pd), rhodium (Rh) and platinum (Pt). The catalyst layer of a precious metal-supported catalyst usually contains, in addition to the catalytic precious metal particles described above, an OSC material with oxygen storage capacity (hereinafter also referred to as "OSC") as a carrier or promoter. The OSC material supports the exhaust gas purification reaction by the catalytic precious metal by storing and releasing oxygen.

For example, Patent Document 1 describes an exhaust gas purification catalyst having a first catalytic layer on the exhaust gas upstream side and a second catalytic layer on the exhaust gas downstream side on a substrate, the first and second catalytic layers containing a NOx retention substance, each having a lower layer containing Rh and an upper layer containing Pd and Pt, the first catalytic layer having a higher Pd concentration than the second catalytic layer, and the second catalytic layer having a higher Pt concentration than the first catalytic layer.

Patent document 2 describes an exhaust gas purification catalyst comprising a substrate, a catalyst coating layer consisting of a porous carrier formed on the surface of the substrate, and a precious metal catalyst supported on the porous carrier of the catalyst coating layer, in which the catalyst coating layer is formed in a laminated structure having upper and lower layers with the lower layer being closer to the substrate surface and the upper layer being relatively farther away, the upper layer comprising Rh particles as a precious metal catalyst, the lower layer comprising Pd particles as a precious metal catalyst, and the lower layer comprising a carrier composed of an _ACZ composite oxide consisting of alumina ( _Al2O3 ), ceria ( _CeO2 ) and zirconia ( _ZrO2 ) as a porous carrier supporting the Pd particles.

JP 2009-101252 A JP 2012-187518 A

Emission regulations for exhaust gas from automobiles and other sources are becoming stricter every year. In particular, Euro 7, the next European exhaust gas regulation, is scheduled to add unregulated substances such as _N2O , _NH3 , and _CH4 that were not previously regulated as new regulated substances, and also to introduce a -10°C regulation. For this reason, catalysts for purifying exhaust gas from automobiles and other sources are required to have both improved emission suppression performance for these unregulated substances and improved low-temperature activity.

Therefore, the present invention aims to provide a means for achieving both low-temperature activity and the ability to suppress emissions of unregulated substances in an exhaust gas purification catalyst.

The present inventors have studied various means for solving the above problems. The present inventors have found that in an exhaust gas purification catalyst in which a first catalytic layer containing rhodium and a second catalytic layer containing palladium are laminated on a substrate, the palladium contained in the second catalytic layer is supported on a composite oxide of alumina, ceria, and zirconia (ACZ), and a predetermined amount of palladium is supported in a region in a predetermined range from the upstream end face of the exhaust gas purification catalyst in the second catalytic layer, thereby improving the performance of suppressing the emission of unregulated substances such as N ₂ O and improving low-temperature activity. The present inventors have completed the present invention based on the above findings.

That is, the present invention includes the following aspects and embodiments.
(Embodiment 1)
A substrate;
a first catalyst layer comprising rhodium disposed on an upper surface of the substrate;
a second catalyst layer containing palladium disposed on an upper surface of the first catalyst layer;
An exhaust gas purifying catalyst comprising:
the first catalyst layer is disposed in a region ranging from a downstream end face of the exhaust gas purification catalyst to 80 to 100% of the entire length of the substrate,
In the second catalyst layer, palladium is supported in an amount of 1 to 4 g in a region ranging from 20 to 50% of the entire length of the substrate from an upstream end face of the exhaust gas purifying catalyst,
The palladium contained in the second catalytic layer is supported on a composite oxide of alumina, ceria and zirconia (ACZ) in an amount of 80% or more.
The exhaust gas purifying catalyst.
(Embodiment 2)
2. The exhaust gas purifying catalyst according to embodiment 1, wherein the second catalyst layer is disposed in an area ranging from an upstream end face of the exhaust gas purifying catalyst to 20 to 50% of the entire length of the substrate.
(Embodiment 3)
The exhaust gas purifying catalyst according to embodiment 1 or 2, wherein the second catalyst layer is disposed in a region ranging from an upstream end face of the exhaust gas purifying catalyst to 30 to 40% of the total length of the substrate, and palladium is supported in the region in an amount ranging from 1 to 2 g.

The present invention makes it possible to provide a means for achieving both low-temperature activity and the ability to suppress emissions of unregulated substances in exhaust gas purification catalysts.

1 is a cross-sectional view illustrating an embodiment of an exhaust gas purification catalyst according to one aspect of the present invention. FIG. 2 is a cross-sectional view showing another embodiment of the exhaust gas purification catalyst according to one aspect of the present invention. 1 is a graph showing the results of a low-temperature activity evaluation test and a nitrogen oxide emission evaluation test for exhaust gas purification catalysts of Comparative Example and Examples. In the figure, the left vertical axis indicates the temperature (°C) at which the purification rate of nitrogen oxides ( _NOx ) in exhaust gas reaches 50% in the low-temperature activity evaluation test. The right vertical axis indicates the average _N2O emission amount (ppm) in the _N2O emission evaluation test. Data shown by filled bars corresponds to the left vertical axis, and data shown by open circles corresponds to the right vertical axis.

A preferred embodiment of the present invention is described in detail below.

One aspect of the present invention relates to an exhaust gas purification catalyst. The exhaust gas purification catalyst of this aspect has a substrate, a first catalyst layer containing rhodium (Rh) arranged on the upper surface of the substrate, and a second catalyst layer containing palladium (Pd) arranged on the upper surface of the first catalyst layer. By arranging Pd in the second catalyst layer, which is the uppermost surface, the exhaust gas purification catalyst of this aspect can exhibit high low-temperature activity.

In the exhaust gas purification catalyst of this embodiment, the substrate may be in any form, such as a honeycomb, pellet, or particle, and is preferably a monolith substrate in the form of a honeycomb. In addition, the substrate preferably contains a heat-resistant inorganic material such as cordierite or a metal. By using a substrate having the above characteristics, a high exhaust gas purification ability can be achieved even under high temperature conditions.

In the exhaust gas purification catalyst of this embodiment, the catalytic precious metal Rh contained in the first catalytic layer may be supported on any carrier. Examples of carriers for Rh include alumina, a composite oxide of ceria and zirconia (ACZ), a composite oxide of alumina and zirconia (AZ), a composite oxide of ceria and zirconia (CZ), and alumina.

The first catalyst layer may contain any trace components in addition to Rh and _ACZ . Examples of the trace components include _{La2O3 , Y2O3 , Nd2O3} , and _BaSO4 . By further containing the support material and the trace components exemplified above, the catalytic activity and heat resistance of the first catalyst layer can be improved.

In the exhaust gas purification catalyst of this embodiment, the first catalyst layer may be disposed in an area ranging from the downstream end face of the exhaust gas purification catalyst to 80 to 100% of the total length of the substrate. In each embodiment of the present invention, the "upstream end face of the exhaust gas purification catalyst" means the upstream end face in the flow direction when exhaust gas is caused to flow through the exhaust gas purification catalyst, and the "downstream end face of the exhaust gas purification catalyst" means the downstream end face in the flow direction when exhaust gas is caused to flow through the exhaust gas purification catalyst. By disposing the first catalyst layer in an area within the above range, the exhaust gas purification catalyst of this embodiment can exhibit high exhaust gas purification ability.

In the exhaust gas purification catalyst of this embodiment, the Pd contained in the second catalyst layer is usually supported on ACZ. Pd can also catalyze a reaction that reduces NOx to produce N ₂ O. Therefore, when Pd is used as a catalytic precious metal in the exhaust gas purification catalyst, the amount of emission of unregulated substances such as N ₂ O may increase. In contrast, the exhaust gas purification catalyst in which Pd is supported on ACZ can suppress the amount of emission of unregulated substances such as N ₂ O. Therefore, since the Pd contained in the second catalyst layer is supported on ACZ, the exhaust gas purification catalyst of this embodiment can not only exhibit high low-temperature activity, but also exhibit a high emission suppression effect against unregulated substances.

The second catalyst layer may contain any carrier material and trace components in addition to Pd and ACZ. Examples of the carrier material include AZ, CZ _, and alumina. Examples _of the trace components include _La2O3 , _Y2O3 , _Nd2O3 , and _BaSO4 . By further containing the carrier material and trace components exemplified above, the catalytic activity _and heat resistance of the second catalyst layer can be improved.

The Pd contained in the second catalyst layer is usually supported on ACZ in an amount of 80% or more, preferably in the range of 85 to 100%. If the amount supported on ACZ is less than 100%, Pd may be supported on the other support materials listed above in addition to ACZ.

In the second catalyst layer, a predetermined amount of Pd is supported in an area ranging from 20 to 50%, preferably 20 to 40%, and more preferably 30 to 40% of the total length of the substrate from the upstream end face of the exhaust gas purification catalyst. The amount of Pd supported in this area is usually in the range of 1 to 4 g, and preferably in the range of 1 to 2 g. In this case, Pd may not be substantially supported in the area of the second catalyst layer other than the above range, and additional Pd may be supported. It is preferable that Pd is not substantially supported in the area of the second catalyst layer other than the above range. By supporting Pd in the amount within the above range in the area, the exhaust gas purification catalyst of this embodiment can exhibit high low-temperature activity.

In the exhaust gas purification catalyst of this embodiment, the second catalyst layer is preferably arranged in an area ranging from 20 to 50% of the total length of the substrate from the upstream end face of the exhaust gas purification catalyst, more preferably in an area ranging from 20 to 40%, and even more preferably in an area ranging from 30 to 40%. By arranging the second catalyst layer in an area within the above range, the Pd contained in the second catalyst layer can be more efficiently brought into contact with the exhaust gas. By arranging in this manner, the exhaust gas purification catalyst of this embodiment can exhibit higher low-temperature activity.

Particularly preferably, in the exhaust gas purification catalyst of this embodiment, the second catalyst layer is disposed in an area ranging from 30 to 40% of the total length of the substrate from the upstream end face of the exhaust gas purification catalyst, and Pd is supported in an amount ranging from 1 to 2 g in this area. By having the above characteristics, the exhaust gas purification catalyst of this embodiment can not only exhibit high low-temperature activity, but also exhibit a high emission suppression effect for unregulated substances.

In the present embodiment of the exhaust gas purification catalyst, the type and content of each material or component contained in the first catalyst layer and the second catalyst layer are not limited, but can be determined, for example, by dissolving each catalyst layer using an acid or the like, and then subjecting the components in the resulting solution to inductively coupled plasma (ICP) emission spectrometry.

In the present embodiment of the exhaust gas purification catalyst, the low-temperature activity can be evaluated, for example, by the following procedure, although this is not limited thereto. The exhaust gas purification catalyst is attached to the exhaust system of an engine, and exhaust gas with a specified air-fuel ratio is supplied to measure the temperature rise characteristics. The temperature is measured at the point when the purification rate of nitrogen oxides (NOx) in the exhaust gas reaches 50%. In this test, the lower the temperature, the higher the low-temperature activity of the exhaust gas purification catalyst can be evaluated.

In the exhaust gas purification catalyst of this embodiment, the performance of suppressing the emission of unregulated substances can be evaluated, for example, but not limited to, by the following procedure. The exhaust gas purification catalyst is attached to the exhaust system of an engine, exhaust gas with a predetermined air-fuel ratio is supplied, and the amount of emission of unregulated substances (e.g., _N2O ) is measured. The average amount of emission of unregulated substances within a predetermined time from the start of exhaust gas supply is calculated. In this test, the lower the amount of emission of unregulated substances, the higher the performance of the exhaust gas purification catalyst in suppressing the emission of unregulated substances can be evaluated.

A cross-sectional view showing one embodiment of the exhaust gas purification catalyst of this aspect is shown in FIG. 1. As shown in FIG. 1, the exhaust gas purification catalyst 100 of one embodiment has a substrate 10, a first catalyst layer 11 arranged on the upper surface of the substrate 10, and a second catalyst layer 12 arranged on the upper surface of the first catalyst layer 11, where the first catalyst layer 11 contains rhodium 13 and the second catalyst layer 12 contains palladium 14. The arrows in the figure indicate the flow direction when exhaust gas is caused to flow through the exhaust gas purification catalyst 100. In the second catalyst layer 12, palladium 14 is supported in the range of 1 to 4 g in an area ranging from 20 to 50% of the entire length of the substrate from the upstream end face of the exhaust gas purification catalyst 100.

2 shows a cross-sectional view of another embodiment of the exhaust gas purification catalyst of this aspect. As shown in FIG. 2, the exhaust gas purification catalyst 200 of one embodiment has a substrate 10, a first catalyst layer 11 arranged on the upper surface of the substrate 10, and a second catalyst layer 12 arranged on the upper surface of the first catalyst layer 11, the first catalyst layer 11 containing rhodium 13, and the second catalyst layer 12 containing palladium 14. The arrows in the figure indicate the flow direction when exhaust gas is caused to flow through the exhaust gas purification catalyst 200. The second catalyst layer 12 containing palladium 14 is arranged in an area ranging from the upstream end face of the exhaust gas purification catalyst 200 to 20 to 50% of the total length of the substrate. By arranging in this way, the exhaust gas purification catalyst of this embodiment can exhibit higher low-temperature activity.

As described above, the exhaust gas purification catalyst of this embodiment not only exhibits high low-temperature activity, but also exhibits a high emission suppression effect for unregulated substances. Therefore, by applying the exhaust gas purification catalyst of this embodiment to automobiles, etc., it is possible to provide automobiles, etc. that can comply with strict exhaust gas emission regulations.

The present invention will be described in more detail below using examples. However, the technical scope of the present invention is not limited to these examples.

<I: Materials>
(1) Material 1 ( _{Al2O3 support} material)
_{La2O3 composites} containing 1 _to 10 wt% _{La2O3 were} used _.
(2) Material 2 (ACZ carrier material)
_{An Al2O3-CeO2-ZrO2 composite oxide was used, which contains 30 mass% CeO2 and has trace amounts of La2O3 and Y2O3 added thereto to improve heat} resistance.
(3) Material 3 (AZ support material)
An Al ₂ O ₃ —ZrO ₂ composite oxide was used, which contains 50 to 80 mass % ZrO ₂ and has small amounts of Nd ₂ O ₃ , La ₂ O ₃ and Y ₂ O ₃ added thereto to improve heat resistance.
(4) Material 4 (CZ support material)
A CeO ₂ -ZrO ₂ composite oxide was used, which contains 30 mass % CeO ₂ and has trace amounts of La ₂ O ₃ and Y ₂ O ₃ added thereto to improve heat resistance.
(5) Material 5 (Pd material)
Palladium nitrate was used.
(6) Material 6 (Rh material)
Rhodium nitrate was used.
(7) Material 7 (Ba material)
Barium sulfate was used.
(8) Substrate
A 875cc (600 cell hexagonal, 2 mil wall thickness) cordierite honeycomb substrate was used.

<II: Preparation of exhaust gas purification catalyst>
[II-1: Comparative Example 1]
First, the Rh material (material 6) and the AZ carrier material (material 3) were added to distilled water while stirring, and the mixture was dried and sintered to prepare the Rh/AZ material. The Rh/AZ material, ACZ carrier material (material 2), Al ₂ O ₃ carrier material (material 1), and Al ₂ O ₃ binder were added to distilled water while stirring, and the suspended slurry 1 was prepared. Next, the prepared slurry 1 was poured onto the substrate, and unnecessary material was blown away with a blower to coat the material on the substrate wall. At that time, the coating material was prepared so that the coating layer was 0.3 g/L-region as Rh for material 6, 30 g/L-region for material 1, 60 g/L-region for material 2, and 30 g/L-region for material 3, relative to the substrate volume. The coating width was adjusted to be 100% of the region from the downstream end face to the entire length of the substrate. Finally, the substrate coated with the slurry 1 was placed in a dryer maintained at 120°C for 2 hours to evaporate the moisture in the coating material, and then the substrate was calcined in an electric furnace at 500°C for 2 hours to prepare a first catalyst layer containing Rh.

Similarly, Pd material (material 5) and CZ support material (material 4) were added to distilled water while stirring, and then dried and sintered to prepare Pd/CZ material. Pd/CZ material, Al ₂ O 3 support material (material 1), Ba material (material 7), and Al ₂ O ₃ binder were added to _distilled water while stirring to prepare suspended slurry 2. This was poured into the substrate on which the first catalyst layer was prepared from the end face opposite to the end face on which slurry 1 was applied, and unnecessary material was blown away with a blower to coat the material on the substrate wall. At that time, the coating material was prepared so that the coating layer was 4.0 g/L-region as Pd for material 5, 25 g/L-region for material 1, 75 g/L-region for material 4, and 13 g/L-region for material 7 relative to the substrate volume. The coating width was adjusted to be 40% of the substrate's total length from the upstream end face. Finally, the substrate coated with Slurry 2 was placed in a dryer maintained at 120°C for 2 hours to evaporate the water in the coating material, and then the substrate was calcined in an electric furnace at 500°C for 2 hours to prepare a second catalyst layer containing Pd. In the second catalyst layer containing Pd, 1.4 g of Pd was supported in an area ranging from the upstream end face to 40% of the total length of the substrate.

[II-2: Comparative Example 2]
The exhaust gas purifying catalyst of Comparative Example 2 was prepared in the same manner as in Comparative Example 1, except that the procedure was changed so that the Pd-containing catalyst layer was the first catalyst layer and the Rh-containing catalyst layer was the second catalyst layer. In the first Pd-containing catalyst layer, 1.4 g of Pd was supported in an area ranging from the upstream end face to 40% of the entire length of the substrate.

[II-3: Example 1]
The exhaust gas purifying catalyst of Example 1 was prepared in the same manner as in Comparative Example 1, except that the CZ support material (material 4) used in the preparation of the second catalyst layer was changed to the ACZ support material (material 2). In the second catalyst layer containing Pd, 1.4 g of Pd was supported in an area ranging from the upstream end face to 40% of the total length of the substrate.

[II-4: Comparative Example 3]
In the procedure of Example 1, the coating amount of material 5 was changed to 2.0 g/L-region as Pd relative to the substrate volume, and the coating width of the second catalyst layer was changed to 80% of the entire length of the substrate from the upstream end face, but the same procedure was used to prepare a catalyst for purifying exhaust gas in Comparative Example 3. In the second catalyst layer containing Pd, 1.4 g of Pd is supported in a region ranging from the upstream end face to 80% of the entire length of the substrate. That is, 0.7 g of Pd is supported in a region ranging from the upstream end face to 40% of the entire length of the substrate.

The configurations of the exhaust gas purification catalysts for Comparative Examples 1 to 3 and Example 1 are shown in Table 1.

<III: Evaluation of exhaust gas purification catalysts>
[III-1: Durability test]
A durability test was carried out on the prepared exhaust gas purification catalysts of the Comparative Example and the Example using an actual engine. Specifically, the exhaust gas purification catalysts of the Comparative Example and the Example were respectively mounted in the exhaust system of a V8 engine, and exhaust gases in stoichiometric and lean atmospheres were repeatedly passed through the catalyst bed at a catalyst bed temperature of 950°C for 50 hours for a fixed period of time (ratio of 3:1).

[III-2: Low-temperature activity evaluation test]
The exhaust gas purification catalysts of the comparative example and the example, which had been subjected to the durability test according to the procedure of III-1, were respectively attached to the exhaust system of an L-type 4-cylinder engine, and exhaust gas with an air-fuel ratio (A/F) of 14.4 was supplied to measure the temperature rise characteristics up to Ga=30 g/s and 500°C. The temperature at the point when the purification rate of nitrogen oxides (NO _x ) in the exhaust gas reached 50% was measured. In this test, the lower the temperature, the higher the low-temperature activity of the exhaust gas purification catalyst can be evaluated.

[III-3: _N2O emission evaluation test]
The exhaust gas purification catalysts of the comparative example and the example, which had been subjected to durability testing according to procedure III-1, were each installed in the exhaust system of an L-type 4-cylinder engine, and exhaust gas with an air-fuel ratio (A/F) of 14.4 was supplied to measure the _N2O emissions at Ga = 30 g/s and 550°C. The average _N2O emissions over 180 seconds from the start of exhaust gas supply were calculated. In this test, the lower the _N2O emissions, the higher the _N2O emission suppression performance of the exhaust gas purification catalyst.

[III-4: Evaluation test results for exhaust gas purification catalysts]
The results of the low-temperature activity evaluation test and the nitrogen oxide emission evaluation test of the exhaust gas purification catalysts of the comparative example and the example are shown in Figure 3. In the figure, the left vertical axis indicates the temperature (°C) at which the purification rate of nitrogen oxides ( _NOx ) in the exhaust gas reached 50% in the low-temperature activity evaluation test. The right vertical axis indicates the average _N2O emission amount (ppm) in the _N2O emission evaluation test. Data indicated by filled bars corresponds to the left vertical axis, and data indicated by open circles corresponds to the right vertical axis.

As shown in FIG. 3, by disposing Pd in the second catalyst layer of the exhaust gas purification catalyst, the low-temperature activity can be improved, but the _N2O emission amount is high (Comparative Example 1). On the other hand, by disposing Pd in the first catalyst layer of the exhaust gas purification catalyst, the _N2O emission suppression effect can be improved, but the low-temperature activity is reduced (Comparative Example 2). In contrast, it was revealed that by disposing Pd in the second catalyst layer of the exhaust gas purification catalyst and supporting Pd on ACZ, the low-temperature activity can be improved while improving the _N2O emission suppression effect (Example 1). In the configuration of the exhaust gas purification catalyst of Example 1, when the coating width of the second catalyst layer was doubled and the amount of Pd supported on the upstream end face of the second catalyst layer was reduced to half, almost the same _N2O emission suppression effect was exhibited, but the low-temperature activity was reduced (Comparative Example 3).

The present invention is not limited to the above-described embodiments, but includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those having all of the configurations described. In addition, it is possible to add, delete, and/or replace part of the configuration of each embodiment with other configurations.

100, 200... catalyst for exhaust gas purification, 10... substrate, 11... first catalyst layer, 12... second catalyst layer, 13... rhodium, 14... palladium

Claims (3)

A substrate;
a first catalyst layer comprising rhodium disposed on an upper surface of the substrate;
a second catalyst layer containing palladium disposed on an upper surface of the first catalyst layer;
An exhaust gas purifying catalyst comprising:
the first catalyst layer is disposed in a region ranging from a downstream end face of the exhaust gas purification catalyst to 80 to 100% of the entire length of the substrate,
In the second catalyst layer, palladium is supported in an amount of 1 to 4 g in a region ranging from 20 to 50% of the entire length of the substrate from an upstream end face of the exhaust gas purifying catalyst,
The palladium contained in the second catalytic layer is supported on a composite oxide of alumina, ceria and zirconia (ACZ) in an amount of 80% or more.
The exhaust gas purifying catalyst. The exhaust gas purification catalyst according to claim 1, wherein the second catalyst layer is disposed in an area ranging from the upstream end face of the exhaust gas purification catalyst to 20 to 50% of the total length of the substrate. The exhaust gas purification catalyst according to claim 1, wherein the second catalyst layer is disposed in an area ranging from 30 to 40% of the total length of the substrate from the upstream end face of the exhaust gas purification catalyst, and palladium is supported in an amount ranging from 1 to 2 g in said area.

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