US20110107751A1

US20110107751A1 - Exhaust gas purification catalyst

Info

Publication number: US20110107751A1
Application number: US12/912,254
Authority: US
Inventors: Masaaki Akamine; Hiroshi Yamada; Masahiko Shigetsu; Akihide Takami
Original assignee: Mazda Motor Corp
Current assignee: Mazda Motor Corp
Priority date: 2009-11-10
Filing date: 2010-10-26
Publication date: 2011-05-12
Also published as: EP2319609B1; JP2011101840A; EP2319609A1; CN102049254B; JP5428773B2; CN102049254A

Abstract

In an exhaust gas purification catalyst in which Rh-doped CeZr-based mixed oxide powder and precious-metal (at least one of Pt and Pd)-supporting heat-resistant powder are included in a catalyst layer 2 provided on a support 1, Rh-doped CeZr-based mixed oxide particles either contain at least one of Pr, La, and Y, or are complexed with Al₂O₃, and support none of Pt and Pd, and heat-resistant particles supporting the precious metal are one of at least one of activated Al₂O₃particles containing La, BaSO₄particles, and complex particles made of CeZr-based mixed oxide and Al₂O₃.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2009-257280 filed on Nov. 10, 2009, the disclosure of which including the specification, the drawings, and the claims is hereby incorporated by reference in its entirety.

BACKGROUND

The present disclosure relates to exhaust gas purification catalysts.
Catalysts for purification of HC (hydrocarbon), CO, and NOx (nitrogen oxides) in engine exhaust gas are required of having high purification efficiencies in the wide temperature ranges from about 200° C. to about 1100° C. Thus, rare metals such as Pt, Pd, and Rh are used as catalytic metals, and are contained in catalyst layers on a support while being supported on heat-resistant oxide particles such as activated alumina, zirconium oxide, or Ce-based oxide having an oxygen storage/release capacity. However, it is known that when a catalyst is exposed to high-temperature exhaust gas, a catalytic metal gradually agglomerates to have its surface area reduced, and as a result, performance of the catalyst degrades. For this reason, a catalyst layer contains a relatively large amount of a catalytic metal in expectation of this agglomeration.
On the other hand, some attempts have been made to prevent agglomeration of a catalytic metal. For example, Japanese Patent Publications Nos. 2006-35043 and 2008-62156 propose that CeZr-based mixed oxide (composite oxide) is doped with Rh and, in addition, some Rh particles are partially exposed at the surface of this mixed oxide. This Rh-doped CeZr-based mixed oxide reduces agglomeration of Rh, and also increases the oxygen storage/release amount and the oxygen storage/release speed of the CeZr-based mixed oxide. These functions are advantageous in solving problems inherent in automobiles, i.e., Rh-doped CeZr-based mixed oxide is capable of quickly returning an atmosphere around the catalyst to a near-stoichiometric atmosphere suitable for purification of exhaust gas, even with a variation in the exhaust gas air-fuel ratio (A/F).
In addition, in exhaust gas purification catalysts, Pt or Pd principally utilizing an oxidation catalytic activity and Rh principally utilizing a reduction catalytic activity are combined together. For example, as such an exhaust gas purification catalyst, a bimetal catalyst made of a combination of two catalytic metals, i.e., a combination of Pt and Rh or a combination of Pd and Rh, and a trimetal catalyst, i.e., a combination of Pt, Pd, and Rh, are known. In Japanese Patent Publications Nos. 2006-35043 and 2008-62156 mentioned above, Pt and Pd are supported on activated alumina.

SUMMARY

In Rh-doped CeZr mixed oxide powder as a catalytic component, an increase in the Ce content increases the oxygen storage/release amount but might reduce the thermal resistance, whereas an increase in the Zr content increases the thermal resistance but might reduce the oxygen storage/release amount. Precious-metal-supporting heat-resistant powder supporting Pt or Pd needs to have its purification performance enhanced in consideration of a relationship with the Rh-doped CeZr mixed oxide powder.
In view of such a requirement, the present disclosure provides a bimetal catalyst or a trimetal catalyst showing high purification performance in a long period even under situations of being exposed to high-temperature exhaust gas.
An exhaust gas purification catalyst according to the present disclosure includes a support and a catalyst layer provided on the support. In this catalyst, the catalyst layer includes Rh-doped CeZr-based mixed oxide powder in which Rh is dissolved in CeZr-based mixed oxide particles containing Ce and Zr, and also includes precious-metal-supporting heat-resistant powder in which a precious metal of at least one of Pt and Pd is supported on heat-resistant particles. The CeZr-based mixed oxide particles in which Rh is dissolved either contain at least a material selected from the group consisting of Pr, La, and Y, or are complexed with Al₂O₃, and support none of Pt and Pd. The heat-resistant particles supporting the precious metal are at least one type of particles selected from the group consisting of activated Al₂O₃particles containing La, BaSO₄particles, and complex particles made of CeZr-based mixed oxide and Al₂O₃.
In this exhaust gas purification catalyst, CeZr-based mixed oxide in the Rh-doped CeZr-based mixed oxide powder contains at least one of Pr, La, and Y, or is complexed with Al₂O₃, and thus, shows an enhanced oxygen storage/release capacity and a high thermal resistance. Specifically, in the case of containing at least one of Pr, La, and Y, this rare earth metal dissolved in CeZr mixed oxide can increase the thermal resistance and cause distortion of crystal of the mixed oxide, thereby enhancing the oxygen storage/release capacity. As the rare earth metal, Y is the most preferable, and is followed by La and Pr in this order. In a complex of CeZr-based mixed oxide and Al₂O₃, Al₂O₃serves as steric hindrance, thereby reducing sintering of CeZr-based mixed oxide primary particles (i.e., reducing a decrease in the oxygen storage/release capacity). In addition, dissolving of Rh in Al₂O₃(i.e., degradation of the oxygen storage/release capacity or catalyst performance) is also reduced.
Further, since the heat-resistant particles supporting a precious metal of at least one of Pt and Pd are at least one of La-containing activated Al₂O₃(La-containing Al₂O₃) particles, BaSO₄particles, and complex particles of CeZr-based mixed oxide and Al₂O₃, high catalyst performance and high thermal resistance can be obtained. In this case, as the heat-resistant particles, La-containing Al₂O₃particles are the most preferable, and are followed by complex particles of CeZr-based mixed oxide and Al₂O₃and BaSO₄particles in this order. In the case of La-containing Al₂O₃particles, these particles have high thermal resistance and a large number of pores, and thus have a large surface area. Accordingly, La-containing Al₂O₃particles can support Pt or Pd with high dispersivity, thereby reducing sintering of Pt or Pd supported on these particles. The complex particles of CeZr-based mixed oxide and Al₂O₃are formed by agglomeration of CeZr-based mixed oxide primary particles and Al₂O₃primary particles. Accordingly, the Al₂O₃primary particles can reduce sintering of CeZr-based mixed oxide primary particles, and can maintain a large specific surface area even after a long-term use. On the other hand, in the case of the BaSO₄particles, although the BaSO₄particles do not have a specific surface area as large as activated Al₂O₃, but do not substantially show a decrease in the specific surface area even after exposure to high-temperature exhaust gas, and thus, are very stable as a support for Pt and Pd. In addition, the poisoning with P, Zn, or S mixed into exhaust gas from engine oil (i.e., degradation of the catalyst) can be reduced.
Accordingly, the present disclosure can provide an exhaust gas purification catalyst showing high purification performance for a long period even after exposure to high-temperature exhaust gas.
In a preferred embodiment, the catalyst layer includes a layer containing the Rh-doped CeZr-based mixed oxide powder and a layer containing the precious-metal-supporting heat-resistant powder, and the layer containing the Rh-doped CeZr-based mixed oxide powder is located above the layer containing the precious-metal-supporting heat-resistant powder. Specifically, although a precious metal (i.e., Pt or Pd) in the precious-metal-supporting heat-resistant powder easily causes sintering as compared to Rh, the precious-metal-supporting heat-resistant powder is provided in the lower layer, and thus, is advantageous in reducing sintering of Pt or Pd.
In another preferred embodiment, exhaust gas purification catalyst includes, as the precious-metal-supporting heat-resistant powder, Pt-supporting heat-resistant powder in which Pt is supported on the heat-resistant particles, and Pd-supporting heat-resistant powder in which Pd is supported on the heat-resistant particles, the catalyst layer includes a layer containing the Rh-doped CeZr-based mixed oxide powder and a layer containing the Pd-supporting heat-resistant powder, the layer containing the Rh-doped CeZr-based mixed oxide powder is located above the layer containing the Pd-supporting heat-resistant powder, and the Pt-supporting heat-resistant powder is included in at least one of the layer containing the Rh-doped CeZr-based mixed oxide powder and the layer containing the Pd-supporting heat-resistant powder.
Accordingly, the three types of catalytic metals, i.e., Pt, Pd, and Rh, efficiently contribute to exhaust gas purification, thereby enhancing purification performance of the exhaust gas purification catalyst. In this case, Pt-supporting heat-resistant powder is preferably included in the layer containing the Rh-doped CeZr-based mixed oxide powder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a structure of a catalyst layer of an exhaust gas purification catalyst according to a first embodiment of the present disclosure.

FIG. 2 is a cross-sectional view illustrating a structure of a catalyst layer of an exhaust gas purification catalyst according to a second embodiment of the present disclosure.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described hereinafter with reference to the drawings. Note that the following description of the preferred embodiments is merely illustrative in nature, and is not intended to limit the scope, applications, and use of the present disclosure.

First Embodiment

In an engine exhaust gas purification catalyst illustrated in FIG. 1, reference numeral 1 denotes a honeycomb support, and a catalyst layer 2 is formed on a cell wall surface 1 a of the honeycomb support 1. This catalyst layer 2 contains a mixture of Rh-doped CeZr-based mixed oxide powder having an oxygen storage/release capacity and precious-metal-supporting heat-resistant powder.
In the Rh-doped CeZr-based mixed oxide powder, Rh is dissolved in CeZr-based mixed oxide particles containing Ce and Zr. The CeZr-based mixed oxide particles contain at least one of Pr, La, and Y, or are complexed with Al₂O₃. The CeZr-based mixed oxide particles (i.e., secondary particles) complexed with Al₂O₃are formed by agglomeration of primary particles of CeZr-based mixed oxide and primary particles of Al₂O₃.
In the precious-metal-supporting heat-resistant powder, a precious metal of at least one of Pt and Pd is supported on heat-resistant oxide particles. Heat-resistant particles thereof are at least one of activated Al₂O₃particles containing La (i.e., La-containing Al₂O₃), BaSO₄particles, and complex particles of CeZr-based mixed oxide and Al₂O₃(i.e., CeZrAl).

EXAMPLE AND COMPARATIVE EXAMPLES

Rh-Doped CeZr-Based Mixed Oxide Powder

As the Rh-doped CeZr-based mixed oxide powder, powders of Rh—CeZrPr in which Rh was dissolved in CeZrPr mixed oxide particles, Rh—CeZrLa in which Rh was dissolved in CeZrLa mixed oxide particles, Rh—CeZrY in which Rh was dissolved in CeZrY mixed oxide particles, and Rh—CeZrAl as a complex of Al₂O₃and CeZr mixed oxide in which Rh was dissolved, were prepared. None of these powders contained Nd.
Each of the Rh—CeZrPr, Rh—CeZrLa, and Rh—CeZrY powders was prepared by coprecipitation. Specifically, as an example, the Rh—CeZrPr powder was prepared in the following manner. First, aqueous ammonia was added to a solution containing nitrates of Ce, Zr, Pr, and Rh with the solution being stirred, and the resultant mixture was neutralized. Then, the obtained coprecipitate was rinsed with water, was dried for one day and night at 150° C. in an atmospheric environment, was pulverized, and then was calcined by being held at 500° C. for two hours. In this manner, the Rh—CeZrPr powder was obtained. At least part of Rh used for doping was exposed at the surface of the mixed oxide particles. In each Rh-doped CeZr-based mixed oxide, the composition except Rh was CeO₂:ZrO₂:(Pr₂O₃or La₂O₃or Y₂O₃)=45:45:10 (% by mass), and the amount of Rh used for doping was 0.1% by mass.
The Rh—CeZrAl powder was prepared in the following manner. First, aqueous ammonia was added to an aluminum nitrate solution with the solution being stirred, thereby obtaining a precipitate of aluminium hydroxide as a precursor of alumina particles. Then, an aqueous ammonia solution was added to the solution from which the above precipitate was obtained. Thereafter, solutions of nitrates of Ce, Zr, and Rh were further added to, and mixed with, the solution, thereby obtaining coprecipitates of hydroxides of Ce, Zr, and Rh and the aluminium hydroxide. This mixture of precipitates was rinsed with water, was dried for one day and night at 150° C. in an atmospheric environment, was pulverized, and then was calcined by being held at 500° C. for two hours. In this manner, the Rh—CeZrAl powder as an agglomeration of primary particles of Rh-doped CeZr mixed oxide containing Ce and Zr, doped with Rh, and having a particle surface at which part of Rh used for doping was exposed, and primary particles of alumina, was obtained. The composition except Rh was CeO₂:ZrO₂:Al₂O₃=25:25:50 (% by mass), and the amount of Rh used for doping was 0.1% by mass.
Rh-Supporting CeZr-Based Mixed Oxide Powder
Powders of Rh/CeZrPr in which Rh was supported on CeZrPr mixed oxide particles by evaporation to dryness, Rh/CeZrLa in which Rh was supported on CeZrLa mixed oxide particles by evaporation to dryness, Rh/CeZrY in which Rh was supported on CeZrY mixed oxide particles by evaporation to dryness, and Rh/CeZrAl in which Rh was supported on CeZrAl mixed oxide as a complex of CeZr mixed oxide and Al₂O₃by evaporation to dryness, were prepared.
The powders of CeZrPr mixed oxide, CeZrLa mixed oxide, CeZrY mixed oxide, and CeZrAl mixed oxide were prepared in the same manner that for the powders of Rh—CeZrPr, Rh—CeZrLa, Rh—CeZrY, and Rh—CeZrAl described above, except that Rh nitrate was not added.
The composition of each of the CeZrPr mixed oxide powder, the CeZrLa mixed oxide powder, the CeZrY mixed oxide powder, and the CeZrAl mixed oxide powder was CeO₂:ZrO₂:(Pr₂O₃, La₂O₃, or Y₂O₃)=45:45:10 (% by mass), and the amount of Rh supported on each powder was 0.1% by mass.
Precious-Metal-Supporting Heat-Resistant Powder
As the precious-metal-supporting heat-resistant powder, La-containing Al₂O₃supporting Pd, BaSO₄supporting Pd, CeZrAl supporting Pd, La-containing Al₂O₃supporting Pt, BaSO₄supporting Pt, and CeZrAl supporting Pt, were prepared. CeZrAl was mixed oxide particles formed by agglomeration of primary particles of CeZr mixed oxide (not doped with Rh) described above, and primary particles of alumina, and had a composition of CeO₂:ZrO₂:Al₂O₃=25:25:50 (% by mass). La-containing Al₂O₃contained 4%, by mass, of La₂O₃. The amount of supported Pt or Pd was adjusted such that the amount of Pt or Pd supported on the honeycomb support was 1.4 g/L when the amount of the precious-metal-supporting heat-resistant powder supported on the honeycomb support was 70 g/L.
No-Precious-Metal-Supporting Heat-Resistant Powder
Powders of La-containing Al₂O₃, BaSO₄, and CeZrAl mixed oxide each of which supported no precious metal were prepared.
Preparation of Catalysts According to Example
The Rh-doped CeZr-based mixed oxide powder (which is one of Rh—CeZrPr, Rh—CeZrLa, Rh—CeZrY, and Rh—CeZrAl) and the precious-metal-supporting heat-resistant powder (which is one of Pd-supporting La-containing Al₂O₃, Pd-supporting BaSO₄, Pd-supporting CeZrAl, Pt-supporting La-containing Al₂O₃, Pt-supporting BaSO₄, and Pt-supporting CeZrAl) were mixed together as appropriate, and the honeycomb support was coated with this mixture, thereby preparing various types of catalysts according to Example having different compositions of catalyst layers. With respect to the amount of a material supported on 1 L of the honeycomb support, the Rh-doped CeZr-based mixed oxide powder was 100 g/L, the precious-metal-supporting heat-resistant powder was 70 g/L, and Pd or Pt was 1.4 g/L. The coating with catalyst powder such as the Rh-doped CeZr-based mixed oxide powder was performed by adding a binder and water to the catalyst powder to change the catalyst powder into slurry (the same hereinafter).
As the honeycomb support, a support (having a volume of 25 ml) having a cell wall thickness of 3.5 mil (i.e., 8.89×10⁻²mm), including 600 cells per square inch (i.e., 645.16 mm²), made of cordierite, and having a cylindrical shape with a diameter of 25.4 mm and a length of 50 mm, was used. This holds true for other examples and comparative examples.

Preparation of Catalysts According to Comparative Example 1

Various types of catalysts according to Comparative Example 1 having different compositions of catalyst layers were prepared in the same manner as that for the catalysts of Example, except that the Rh-doped CeZr-based mixed oxide powder was replaced by Rh-supporting CeZr-based mixed oxide powder (which is one of Rh/CeZrPr, Rh/CeZrLa, Rh/CeZrY, and Rh/CeZrAl). The amount of the supported Rh-supporting CeZr-based mixed oxide powder was 100 g/L, the amount of the supported precious-metal-supporting heat-resistant powder was 70 g/L, and amount of supported Pd or Pt was 1.4 g/L.

Preparation of Catalysts According to Comparative Example 2

In Comparative Example 2, Rh-supporting CeZr-based mixed oxide powder (which is one of Rh/CeZrPr, Rh/CeZrLa, Rh/CeZrY, and Rh/CeZrAl) and no-precious-metal-supporting heat-resistant powder (which is one of La-containing Al₂O₃, BaSO₄, and CeZrAl mixed oxide) were mixed together as appropriate, and the honeycomb support was coated with this mixture. Then, this coating layer was impregnated with a Pd solution or a Pt solution, and was dried and calcined, thereby preparing various types of catalysts according to Comparative Example 2 having different compositions of the catalyst layers. The amount of the supported Rh-supporting CeZr-based mixed oxide powder was 100 g/L, and the amount of the supported no-precious-metal-supporting heat-resistant powder was 70 g/L, and the amount of Pd or Pt supported by impregnation was 1.4 g/L.
The catalysts of Comparative Example 2 differ from those of Example in that the Rh-doped CeZr-based mixed oxide powder was replaced by the Rh-supporting CeZr-based mixed oxide powder, the precious-metal-supporting heat-resistant powder was replaced by the heat-resistant powder supporting no precious metal, and a precious metal was supported by impregnation (i.e., Pd or Pt was supported by impregnation on the Rh-supporting CeZr-based mixed oxide powder and the heat-resistant powder).
Evaluation of Exhaust Gas Purification Performance
The catalysts of Example and Comparative Examples were aged by keeping these catalysts at 1000° C. for 24 hours in an atmospheric environment. Then, these catalysts were placed in a model gas flow reactor, and light-off temperatures T50 concerning purification of HC, CO, and NOx were measured using model gas for evaluation. The light-off temperature T50 is a gas temperature at a catalyst entrance when purification efficiency reaches 50% by gradually increasing the temperature of model gas flowing in the catalyst from room temperature. The evaluation model gas had an A/F ratio of 14.7±0.9. Specifically, a mainstream gas with an A/F ratio of 14.7 was allowed to constantly flow, and a predetermined amount of gas for changing the A/F ratio was added in pulses at a rate of 1 Hz, so that the A/F ratio was forcedly oscillated within the range of ±0.9. The space velocity SV was set at 60000/h⁻¹, and the rate of temperature increase was set at 30° C./min.
Table 1 shows results of Example, Table 2 shows results of Comparative Example 1, and Table 3 shows results of Comparative Example 2. In Tables 1-3, “Al₂O₃” means “Al₂O₃” and “BaSO₄” means “BaSO₄,” and the same holds for other tables.

TABLE 1

First Embodiment		T50
Single-layer	Rh-doped CeZr-based	(° C.)

Structure	Mixed Oxide Powder	Heat-resistant Powder	HC	CO	NOx	Precious Metal

Mixture of Rh-doped	Rh—CeZrPr	La-containing Al2O3	268	260	263	Pd-supporting
CeZr-based Mixed		BaSO4	293	284	289
Oxide Powder and		CeZrAl Mixed Oxide	281	272	274
Pd-supporting	Rh—CeZrLa	La-containing Al2O3	267	259	261
Heat-resistant		BaSO4	289	278	281
Powder		CeZrAl Mixed Oxide	276	269	272
	Rh—CeZrY	La-containing Al2O3	265	257	257
		BaSO4	288	279	281
		CeZrAl Mixed Oxide	275	267	266
	Rh—CeZrAl	La-containing Al2O3	264	257	258
		BaSO4	285	277	279
		CeZrAl Mixed Oxide	273	267	267
Mixture of	Rh—CeZrPr	La-containing Al2O3	276	268	267	Pt-supporting
Rh-supporting		BaSO4	297	288	288
CeZr-based		CeZrAl Mixed Oxide	285	278	276
Mixed Oxide	Rh—CeZrLa	La-containing Al2O3	273	265	266
Powder and		BaSO4	293	285	285
Pt-supporting		CeZrAl Mixed Oxide	282	276	275
Heat-resistant	Rh—CeZrY	La-containing Al2O3	272	265	265
Particles		BaSO4	293	285	286
		CeZrAl Mixed Oxide	282	276	276
	Rh—CeZrAl	La-containing Al2O3	271	263	262
		BaSO4	292	283	283
		CeZrAl Mixed Oxide	281	274	273

TABLE 2

Comparative	Rh-supporting		T50
Example 1	CeZr-based	Heat-resistant	(° C.)

Single-layer Structure	Mixed Oxide Powder	Powder	HC	CO	NOx	Precious Metal

Mixture of	Rh-supporting CeZrPr	La-containing	284	277	277	Pd-supporting
Rh-supporting		Al2O3
CeZr-based		BaSO4	305	297	298
Mixed Oxide Powder		CeZrAl Mixed	293	287	286
and Pd-supporting		Oxide
Heat-resistant	Rh-supporting CeZrLa	La-containing	281	274	276
Particles		Al2O3
		BaSO4	301	294	295
		CeZrAl Mixed	290	285	285
		Oxide
	Rh-supporting CeZrY	La-containing	280	274	275
		Al2O3
		BaSO4	301	294	296
		CeZrAl Mixed	290	285	286
		Oxide
	Rh-supporting CeZrAl	La-containing	280	272	271
		Al2O3
		BaSO4	305	296	297
		CeZrAl Mixed	293	284	282
		Oxide
Mixture of	Rh-supporting CeZrPr	La-containing	292	284	284	Pt-supporting
Rh-supporting		Al2O3
CeZr-based		BaSO4	312	304	303
Mixed Oxide Powder		CeZrAl Mixed	301	295	293
and Pt-supporting		Oxide
Heat-resistant	Rh-supporting CeZrLa	La-containing	289	281	283
Particles		Al2O3
		BaSO4	314	305	309
		CeZrAl Mixed	302	293	294
		Oxide
	Rh-supporting CeZrY	La-containing	288	281	282
		Al2O3
		BaSO4	309	301	303
		CeZrAl Mixed	298	292	293
		Oxide
	Rh-supporting CeZrAl	La-containing	286	280	277
		Al2O3
		BaSO4	306	300	296
		CeZrAl Mixed	295	291	286
		Oxide

TABLE 3

Comparative	Rh-supporting		T50
Example 2	CeZr-based	Heat-resistant	(° C.)

Single-layer Structure	Mixed Oxide Powder	Powder	HC	CO	NOx	Precious Metal

Pd-impregnated	Rh-supporting CeZrPr	La-containing	291	283	281	Pd-impregnated
Mixture Layer of		Al2O3
Rh-supporting		BaSO4	314	305	305
CeZr-based		CeZrAl Mixed	301	293	290
Mixed Oxide Powder		Oxide
and No-precious-	Rh-supporting CeZrLa	La-containing	288	281	279
metal-supporting		Al2O3
Heat-resistant		BaSO4	309	301	300
Powder		CeZrAl Mixed	297	291	288
		Oxide
	Rh-supporting CeZrY	La-containing	286	277	278
		Al2O3
		BaSO4	308	296	298
		CeZrAl Mixed	295	287	289
		Oxide
	Rh-supporting CeZrAl	La-containing	286	277	278
		Al2O3
		BaSO4	307	297	299
		CeZrAl Mixed	296	288	289
		Oxide
Pt-impregnated	Rh-supporting CeZrPr	La-containing	295	289	284	Pt-impregnated
Mixture Layer of		Al2O3
Rh-supporting		BaSO4	317	308	304
CeZr-based		CeZrAl Mixed	304	299	295
Mixed Oxide Powder		Oxide
and No-precious-	Rh-supporting CeZrLa	La-containing	293	286	283
metal-supporting		Al2O3
Heat-resistant		BaSO4	314	306	304
Powder		CeZrAl Mixed	303	297	294
		Oxide
	Rh-supporting CeZrY	La-containing	292	286	284
		Al2O3
		BaSO4	312	306	303
		CeZrAl Mixed	301	297	293
		Oxide
	Rh-supporting CeZrAl	La-containing	291	284	283
		Al2O3
		BaSO4	312	304	304
		CeZrAl Mixed	300	294	292
		Oxide

Tables 1 and 2 show that in purification of each of HC, CO, and NOx, if the types of CeZr-based mixed oxide powder and precious-metal-supporting heat-resistant powder are the same, the temperature T50 in the first embodiment using Rh-doped CeZr-based mixed oxide is lower than that in Comparative Example 1 using Rh-supporting CeZr-based mixed oxide. Tables 2 and 3 show that the temperature T50 in Comparative Example 1 is lower than that in Comparative Example 2 (i.e., catalysts in which a mixture layer of Rh-supporting CeZr-based mixed oxide powder and heat-resistant powder supporting no precious metal is impregnated with Pd or Pd). Accordingly, it is preferable that Pd or Pt is supported on heat-resistant powder beforehand and Pd or Pt is not supported on oxide powder supporting Rh.
Regarding the influence of the type of CeZr-based mixed oxide in the Rh-doped CeZr-based mixed oxide powder on the temperature T50 in the first embodiment (see, Table 1), the temperature T50 of Rh—CeZrAl is basically the lowest, and is followed by the those of Rh—CeZrY, Rh—CeZrLa, and Rh—CeZrPr in this order (i.e., the temperature T50 of Rh—CeZrY is the second lowest). However, with respect to purification of NOx in the case where Pd is supported on heat-resistant powder, the temperature T50 of Rh—CeZrY is exceptionally lower than that of Rh—CeZrAl.
Regarding the influence of the type of heat-resistant particles in the precious-metal-supporting heat-resistant powder, the temperature T50 in the case of using La-containing Al₂O₃is the lowest, and is followed by those of CeZrAl and BaSO₄in this order (i.e., the temperature T50 of CeZrAl is the second lowest). Comparison of precious metal supported on heat-resistant particles shows that the temperature T50 in the case of supporting Pd is lower than that in the case of supporting Pt.

Second Embodiment

FIG. 2 illustrates a structure of a catalyst layer of an engine exhaust gas purification catalyst according to this embodiment. Unlike the catalyst layer 2 of the first embodiment, a catalyst layer 2 formed on a cell wall surface 1 a of a honeycomb support 1 according to the second embodiment has a double-layer structure including an upper layer 2 a containing Rh-doped CeZr-based mixed oxide powder and a lower layer 2 b containing precious-metal-supporting heat-resistant powder.

Example

Preparation of Catalysts According to Example

Various types of catalysts according to Example having different compositions were prepared by combining various types of Rh-doped CeZr-based mixed oxide powder and precious-metal-supporting heat-resistant powder described above as appropriate. These catalysts were prepared in the same manner as in the first embodiment, except that precious-metal-supporting heat-resistant powder was first supported on the honeycomb support to form the lower layer 2 b and then Rh-doped CeZr-based mixed oxide powder was supported on the honeycomb support to form the upper layer 2 a. With respect to the amount of a material supported on 1 L of the honeycomb support, the Rh-doped CeZr-based mixed oxide powder was 100 g/L, the precious-metal-supporting heat-resistant powder was 70 g/L, and the amount of Pd or Pt was 1.4 g/L.
Evaluation of Exhaust Gas Purification Performance
The above-described catalysts of Example were aged under the same conditions as those in the first embodiment, and light-off temperatures T50 concerning purification of HC, CO, and NOx were measured. Table 4 shows results.

TABLE 4

Second Embodiment		T50
Double-layer	Rh-doped CeZr-based	(° C.)

Structure	Mixed Oxide Powder	Heat-resistant Powder	HC	CO	NOx	Precious Metal

Upper Layer: Rh	Rh—CeZrPr	La-containing Al2O3	259	252	253	Pd-supporting
Lower Layer: Pd		BaSO4	282	274	277
		CeZrAl Mixed Oxide	269	262	262
	Rh—CeZrLa	La-containing Al2O3	256	250	251
		BaSO4	277	270	272
		CeZrAl Mixed Oxide	265	260	260
	Rh—CeZrY	La-containing Al2O3	254	246	250
		BaSO4	276	265	270
		CeZrAl Mixed Oxide	263	256	261
	Rh—CeZrAl	La-containing Al2O3	253	247	248
		BaSO4	273	267	267
		CeZrAl Mixed Oxide	262	258	257
Upper Layer: Rh	Rh—CeZrPr	La-containing Al2O3	266	258	259	Pt-supporting
Lower Layer: Pt		BaSO4	286	278	278
		CeZrAl Mixed Oxide	275	269	268
	Rh—CeZrLa	La-containing Al2O3	263	255	258
		BaSO4	288	279	284
		CeZrAl Mixed Oxide	276	267	269
	Rh—CeZrY	La-containing Al2O3	262	255	257
		BaSO4	283	275	278
		CeZrAl Mixed Oxide	272	266	268
	Rh—CeZrAl	La-containing Al2O3	261	253	254
		BaSO4	282	273	275
		CeZrAl Mixed Oxide	270	263	263

Comparison with the catalysts of Example of the first embodiment (see, Table 1) shows that in purification of each of HC, CO, and NOx, if the types of Rh-doped CeZr-based mixed oxide powder and precious-metal-supporting heat-resistant powder are the same, the temperature T50 in the second embodiment is lower than that in the first embodiment. This is considered to be because the precious-metal-supporting heat-resistant powder was provided in the lower layer 2 b in the second embodiment, and thus, sintering of Pt or Pd supported on this powder was reduced by the upper layer 2 a. The influence of the type of Rh-doped CeZr-based mixed oxide powder on the temperature T50, the influence of the type of CeZr-based mixed oxide powder in the Rh-doped CeZe-based mixed oxide powder on the temperature T50, the influence of the type of heat-resistant particles in the precious-metal-supporting heat-resistant powder on the temperature T50, and the influence of the type of precious metal supported on the heat-resistant particles show similar tendencies as those in the first embodiment. However, purification of CO in the case of supporting Pd on the heat-resistant powder, the temperature T50 of Rh—CeZrY is exceptionally lower than that of Rh—CeZrAl.

Third Embodiment

In a third embodiment of the present disclosure, in an engine exhaust gas purification catalyst including a catalyst layer 2 with a double-layer structure as illustrated in FIG. 2, one of an upper layer and a lower layer is a mixture layer of two types of catalyst powder and the other of the upper layer and the lower layer is a single layer of a single type of catalyst powder (including a binder, however). This structure may be implemented in two ways. In a first case, the upper layer 2 a is a mixture layer of Rh-doped CeZr-based mixed oxide powder and Pt-supporting heat-resistant powder, and the lower layer 2 b is a single layer of Pd-supporting heat-resistant powder. In a second case, the upper layer 2 a is a single layer of Rh-doped CeZr-based mixed oxide powder, and the lower layer 2 b is a mixture layer of Pd-supporting heat-resistant powder and Pt-supporting heat-resistant powder.

Example and Comparative Example

Preparation of Catalysts According to Example

Various types of catalysts according to Example having different compositions of catalyst layers 2 were prepared by combining various types of Rh-doped CeZr-based mixed oxide powder and precious-metal-supporting heat-resistant powder described above as appropriate. These catalysts were prepared in the same manner as in the second embodiment, except that in the first case, Pd-supporting heat-resistant powder was first supported on a honeycomb support to form the lower layer 2 b and then a mixture of Rh-doped CeZr-based mixed oxide powder and Pt-supporting heat-resistant powder was supported on the honeycomb support, and in the second case, a mixture of Pd-supporting heat-resistant powder and Pt-supporting powder was first supported on the honeycomb support to form the lower layer 2 b and then Rh-doped CeZr-based mixed oxide powder was supported on the honeycomb support to form the upper layer 2 a.
In each of the first and second cases, with respect to the amount of a material supported on 1 L of the honeycomb support, the Rh-doped CeZr-based mixed oxide powder was 100 g/L, each of the Pd-supporting heat-resistant powder and the Pt-supporting heat-resistant powder was 35 g/L (i.e., 70 g/L in total), and each of Pd and Pt was 0.7 g/L (i.e., 1.4 g/L in total).

Preparation of Catalysts According to Comparative Example 3

The Rh-supporting CeZr-based mixed oxide powder and the Pd-supporting heat-resistant powder described above were combined together as appropriate, and were mixed with heat-resistant powder supporting no precious metal (i.e., the same heat-resistant powder as Pd-supporting heat-resistant powder), and a honeycomb support was coated with the resultant mixture. Then, this coating layer was impregnated with a Pt solution, and was dried and calcined, thereby preparing catalysts according to Comparative Example 3 having different compositions of catalyst layers. With respect to the amount of a material supported on 1 L of the honeycomb support, the Rh-supporting CeZr-based mixed oxide powder was 100 g/L, the Pd-supporting heat-resistant powder was 35 g/L, the no-precious-metal-supporting heat-resistant powder was 35 g/L, and each of Pd supported on heat-resistant powder and Pt supported by impregnation was 0.7 g/L (i.e., 1.4 g/L in total).
Evaluation of Exhaust Gas Purification Performance
The above-described catalysts of Example and Comparative Example were aged under the same conditions as those in the first embodiment, and light-off temperatures T50 concerning purification of HC, CO, and NOx were measured. Table 5 shows results of Example, and Table 6 shows results of Comparative Example 3.

TABLE 5

Third Embodiment		T50
Double-layer	Rh-doped CeZr-based	(° C.)

Structure	Mixed Oxide Powder	Heat-resistant Powder	HC	CO	NOx	Precious Metal

Upper Layer: Rh + Pt	Rh—CeZrPr	La-containing Al2O3	254	247	245	Pd-supporting,
Lower Layer: Pd		BaSO4	276	266	265	Pt-supporting
(First Case)		CeZrAl Mixed Oxide	263	257	256
	Rh—CeZrLa	La-containing Al2O3	252	244	244
		BaSO4	273	264	265
		CeZrAl Mixed Oxide	262	255	255
	Rh—CeZrY	La-containing Al2O3	251	244	245
		BaSO4	271	264	264
		CeZrAl Mixed Oxide	260	255	254
	Rh—CeZrAl	La-containing Al2O3	251	242	243
		BaSO4	276	266	269
		CeZrAl Mixed Oxide	264	254	254
Upper Layer: Rh	Rh—CeZrPr	La-containing Al2O3	257	250	249	Pd-supporting,
Lower Layer: Pd + Pt		BaSO4	280	272	273	Pt-supporting
(Second Case)		CeZrAl Mixed Oxide	267	260	258
	Rh—CeZrLa	La-containing Al2O3	254	248	247
		BaSO4	275	268	268
		CeZrAl Mixed Oxide	263	258	256
	Rh—CeZrY	La-containing Al2O3	252	244	246
		BaSO4	274	263	266
		CeZrAl Mixed Oxide	261	254	257
	Rh—CeZrAl	La-containing Al2O3	252	245	246
		BaSO4	273	265	267
		CeZrAl Mixed Oxide	261	255	255

TABLE 6

Comparative	Rh-supporting		T50
Example 3	CeZr-based	Heat-resistant	(° C.)

Single-layer Structure	Mixed Oxide Powder	Powder	HC	CO	NOx	Precious Metal

Pt-impregnated	Rh-supporting CeZrPr	La-containing	294	288	287	Pd-supporting
Mixture Layer of		Al2O3				Pt-impregnated
Rh-supporting		BaSO4	317	310	311
CeZr-based		CeZrAl Mixed	304	298	296
Mixed Oxide Powder		Oxide
and Pd-supporting	Rh-supporting CeZrLa	La-containing	291	286	285
Heat-resistant		Al2O3
particles		BaSO4	312	306	306
		CeZrAl Mixed	300	296	294
		Oxide
	Rh-supporting CeZrY	La-containing	289	282	284
		Al2O3
		BaSO4	311	301	304
		CeZrAl Mixed	298	292	295
		Oxide
	Rh-supporting CeZrAl	La-containing	288	283	282
		Al2O3
		BaSO4	308	303	301
		CeZrAl Mixed	297	294	291
		Oxide

Comparison with the catalysts of Example of the second embodiment (see, Table 4) shows that in purification of each of HC, CO, and NOx, if the types of the Rh-doped CeZr-based mixed oxide powder and precious-metal-supporting heat-resistant powder are the same in a first case and a second case, the temperature T50 is lower than that in the second embodiment. This is because of the use of three types of precious metals (i.e., Ru h, Pd, and Pt).
Comparison of the first case (where the upper layer is Rh+Pt, and the lower layer is Pd) and the second case (where the upper layer is Rh, and the lower layer is Pd+Pt) shows that the temperature in the first case is generally lower than that in the second case. The influence of the type of CeZr-based mixed oxide powder in the Rh-doped CeZr-based mixed oxide powder on the temperature T50 and the influence of the type of heat-resistant particles in the precious-metal-supporting heat-resistant powder on the temperature T50 show similar tendencies as those in the first embodiment. However, the temperature T50 does not significantly differ between Rh—CeZrAl and Rh—CeZrY, and thus, in some cases of using some types of heat-resistant powder, the temperature T50 in the second case is lower than the temperature T50 in the first case.
In Comparative Example 3 (see, Table 6), three types of precious metals (i.e., Rh, Pd, and Pt) were used as in the third embodiment. However, if the types of the CeZr-based mixed oxide powder and precious-metal-supporting heat-resistant powder are the same, the temperature T50 in Comparative Example 3 is lower than the temperature T50 not only in the third embodiment but also in the first embodiment.
In the first through third embodiments, precious-metal-supporting heat-resistant powder supports one of Pt and Pd as a precious metal, but may support both Pt and Pd.
In addition, Rh—CeZrAl may be a complex of Al₂O₃and CeZr-based mixed oxide in which Rh is dissolved and which contains at least one of Pr, La, and Y.

Claims

1. An exhaust gas purification catalyst, comprising a support and a catalyst layer provided on the support, wherein

the catalyst layer includes Rh-doped CeZr-based mixed oxide powder in which Rh is dissolved in CeZr-based mixed oxide particles containing Ce and Zr, and also includes precious-metal-supporting heat-resistant powder in which a precious metal of at least one of Pt and Pd is supported on heat-resistant particles,

the CeZr-based mixed oxide particles in which Rh is dissolved either contain at least a material selected from the group consisting of Pr, La, and Y, or are complexed with Al₂O₃, and support none of Pt and Pd, and

the heat-resistant particles supporting the precious metal are at least one type of particles selected from the group consisting of activated Al₂O₃particles containing La, BaSO₄particles, and complex particles made of CeZr-based mixed oxide and Al₂O₃.

2. The exhaust gas purification catalyst of claim 1, wherein the catalyst layer includes a layer containing the Rh-doped CeZr-based mixed oxide powder and a layer containing the precious-metal-supporting heat-resistant powder, and

the layer containing the Rh-doped CeZr-based mixed oxide powder is located above the layer containing the precious-metal-supporting heat-resistant powder.

3. The exhaust gas purification catalyst of claim 1, including, as the precious-metal-supporting heat-resistant powder, Pt-supporting heat-resistant powder in which Pt is supported on the heat-resistant particles, and Pd-supporting heat-resistant powder in which Pd is supported on the heat-resistant particles,

the catalyst layer includes a layer containing the Rh-doped CeZr-based mixed oxide powder and a layer containing the Pd-supporting heat-resistant powder,

the layer containing the Rh-doped CeZr-based mixed oxide powder is located above the layer containing the Pd-supporting heat-resistant powder, and

the Pt-supporting heat-resistant powder is included in at least one of the layer containing the Rh-doped CeZr-based mixed oxide powder and the layer containing the Pd-supporting heat-resistant powder.