Exhaust Gas Purification Catalyst

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 Al.sub.2O.sub.3, and support none of Pt and Pd, and heat-resistant particles supporting the precious metal are one of at least one of activated Al.sub.2O.sub.3 particles containing La, BaSO.sub.4 particles, and complex particles made of CeZr-based mixed oxide and Al.sub.2O.sub.3.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] 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

[0002] The present disclosure relates to exhaust gas purification catalysts.

[0003] 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.degree. C. to about 1100.degree. 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.

[0004] 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).

[0005] 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

[0006] 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.

[0007] 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.

[0008] 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 Al.sub.2O.sub.3, 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 Al.sub.2O.sub.3 particles containing La, BaSO.sub.4 particles, and complex particles made of CeZr-based mixed oxide and Al.sub.2O.sub.3.

[0009] 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 Al.sub.2O.sub.3, 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 Al.sub.2O.sub.3, Al.sub.2O.sub.3 serves 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 Al.sub.2O.sub.3 (i.e., degradation of the oxygen storage/release capacity or catalyst performance) is also reduced.

[0010] 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 Al.sub.2O.sub.3 (La-containing Al.sub.2O.sub.3) particles, BaSO.sub.4 particles, and complex particles of CeZr-based mixed oxide and Al.sub.2O.sub.3, high catalyst performance and high thermal resistance can be obtained. In this case, as the heat-resistant particles, La-containing Al.sub.2O.sub.3 particles are the most preferable, and are followed by complex particles of CeZr-based mixed oxide and Al.sub.2O.sub.3 and BaSO.sub.4 particles in this order. In the case of La-containing Al.sub.2O.sub.3 particles, these particles have high thermal resistance and a large number of pores, and thus have a large surface area. Accordingly, La-containing Al.sub.2O.sub.3 particles 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 Al.sub.2O.sub.3 are formed by agglomeration of CeZr-based mixed oxide primary particles and Al.sub.2O.sub.3 primary particles. Accordingly, the Al.sub.2O.sub.3 primary 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 BaSO.sub.4 particles, although the BaSO.sub.4 particles do not have a specific surface area as large as activated Al.sub.2O.sub.3, 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 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

[0015] 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.

[0016] 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

[0017] 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

[0018] 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 1a 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.

[0019] 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 Al.sub.2O.sub.3. The CeZr-based mixed oxide particles (i.e., secondary particles) complexed with Al.sub.2O.sub.3 are formed by agglomeration of primary particles of CeZr-based mixed oxide and primary particles of Al.sub.2O.sub.3.

[0020] 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 Al.sub.2O.sub.3 particles containing La (i.e., La-containing Al.sub.2O.sub.3), BaSO.sub.4 particles, and complex particles of CeZr-based mixed oxide and Al.sub.2O.sub.3 (i.e., CeZrAl).

EXAMPLE AND COMPARATIVE EXAMPLES

Rh-Doped CeZr-Based Mixed Oxide Powder

[0021] 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 Al.sub.2O.sub.3 and CeZr mixed oxide in which Rh was dissolved, were prepared. None of these powders contained Nd.

[0022] 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.degree. C. in an atmospheric environment, was pulverized, and then was calcined by being held at 500.degree. 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 CeO.sub.2:ZrO.sub.2:(Pr.sub.2O.sub.3 or La.sub.2O.sub.3 or Y.sub.2O.sub.3)=45:45:10 (% by mass), and the amount of Rh used for doping was 0.1% by mass.

[0023] 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.degree. C. in an atmospheric environment, was pulverized, and then was calcined by being held at 500.degree. 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 CeO.sub.2:ZrO.sub.2:Al.sub.2O.sub.3=25:25:50 (% by mass), and the amount of Rh used for doping was 0.1% by mass.

[0024] Rh-Supporting CeZr-Based Mixed Oxide Powder

[0025] 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 Al.sub.2O.sub.3 by evaporation to dryness, were prepared.

[0026] 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.

[0027] 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 CeO.sub.2:ZrO.sub.2:(Pr.sub.2O.sub.3, La.sub.2O.sub.3, or Y.sub.2O.sub.3)=45:45:10 (% by mass), and the amount of Rh supported on each powder was 0.1% by mass.

[0028] Precious-Metal-Supporting Heat-Resistant Powder

[0029] As the precious-metal-supporting heat-resistant powder, La-containing Al.sub.2O.sub.3 supporting Pd, BaSO.sub.4 supporting Pd, CeZrAl supporting Pd, La-containing Al.sub.2O.sub.3 supporting Pt, BaSO.sub.4 supporting 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 CeO.sub.2:ZrO.sub.2:Al.sub.2O.sub.3=25:25:50 (% by mass). La-containing Al.sub.2O.sub.3 contained 4%, by mass, of La.sub.2O.sub.3. 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.

[0030] No-Precious-Metal-Supporting Heat-Resistant Powder

[0031] Powders of La-containing Al.sub.2O.sub.3, BaSO.sub.4, and CeZrAl mixed oxide each of which supported no precious metal were prepared.

[0032] Preparation of Catalysts According to Example

[0033] 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 Al.sub.2O.sub.3, Pd-supporting BaSO.sub.4, Pd-supporting CeZrAl, Pt-supporting La-containing Al.sub.2O.sub.3, Pt-supporting BaSO.sub.4, 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).

[0034] 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.times.10.sup.-2 mm), including 600 cells per square inch (i.e., 645.16 mm.sup.2), 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

[0035] 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

[0036] 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 Al.sub.2O.sub.3, BaSO.sub.4, 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.

[0037] 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).

[0038] Evaluation of Exhaust Gas Purification Performance

[0039] The catalysts of Example and Comparative Examples were aged by keeping these catalysts at 1000.degree. 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.sup.-1, and the rate of temperature increase was set at 30.degree. C./min.

[0040] 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, "Al.sub.2O.sub.3" means "Al.sub.2O.sub.3" and "BaSO.sub.4" means "BaSO.sub.4," and the same holds for other tables.

TABLE-US-00001 TABLE 1 First Embodiment T50 Single-layer Rh-doped CeZr-based (.degree. 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-US-00002 TABLE 2 Comparative Rh-supporting T50 Example 1 CeZr-based Heat-resistant (.degree. 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-US-00003 TABLE 3 Comparative Rh-supporting T50 Example 2 CeZr-based Heat-resistant (.degree. 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

[0041] 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.

[0042] 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.

[0043] 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 Al.sub.2O.sub.3 is the lowest, and is followed by those of CeZrAl and BaSO.sub.4 in 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

[0044] 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 1a of a honeycomb support 1 according to the second embodiment has a double-layer structure including an upper layer 2a containing Rh-doped CeZr-based mixed oxide powder and a lower layer 2b containing precious-metal-supporting heat-resistant powder.

Example

Preparation of Catalysts According to Example

[0045] 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 2b and then Rh-doped CeZr-based mixed oxide powder was supported on the honeycomb support to form the upper layer 2a. 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.

[0046] Evaluation of Exhaust Gas Purification Performance

[0047] 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-US-00004 TABLE 4 Second Embodiment T50 Double-layer Rh-doped CeZr-based (.degree. 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

[0048] 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 2b in the second embodiment, and thus, sintering of Pt or Pd supported on this powder was reduced by the upper layer 2a. 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

[0049] 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 2a is a mixture layer of Rh-doped CeZr-based mixed oxide powder and Pt-supporting heat-resistant powder, and the lower layer 2b is a single layer of Pd-supporting heat-resistant powder. In a second case, the upper layer 2a is a single layer of Rh-doped CeZr-based mixed oxide powder, and the lower layer 2b 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

[0050] 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 2b 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 2b and then Rh-doped CeZr-based mixed oxide powder was supported on the honeycomb support to form the upper layer 2a.

[0051] 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

[0052] 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).

[0053] Evaluation of Exhaust Gas Purification Performance

[0054] 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-US-00005 TABLE 5 Third Embodiment T50 Double-layer Rh-doped CeZr-based (.degree. 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-US-00006 TABLE 6 Comparative Rh-supporting T50 Example 3 CeZr-based Heat-resistant (.degree. 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

[0055] 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).

[0056] 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.

[0057] 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.

[0058] 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.

[0059] In addition, Rh--CeZrAl may be a complex of Al.sub.2O.sub.3 and 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 Al.sub.2O.sub.3, 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 Al.sub.2O.sub.3 particles containing La, BaSO.sub.4 particles, and complex particles made of CeZr-based mixed oxide and Al.sub.2O.sub.3.

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.