Exhaust gas purification catalyst

Abstract

To provide a catalyst for purifying an exhaust gas having improved exhaust gas purification performance.SOLUTION: There is provided a catalyst for purifying an exhaust gas which has a porous carrier, catalyst metal particles supported in the pores of the porous carrier and zirconium dioxide particles supported in the pores of the porous carrier, wherein the zirconium dioxide particles are uniformly dispersed and supported in the pores of the porous carrier and are monoclinic crystals or mixed crystals of monoclinic crystals and tetragonal crystals and here, that the zirconium dioxide particles are uniformly dispersed means that the ratio of the presence ratio of zirconium in the surface region from the surface of the catalyst for purifying an exhaust gas to a depth of 1.5 μm to the presence ratio of zirconium in the region inside from the surface region of the catalyst for purifying an exhaust gas is 95 to 105 mol% when the catalyst for purifying an exhaust gas is measured using an electron beam microanalyzer.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to a catalyst for purifying exhaust gases. [Background technology]

[0002] Patent Document 1 discloses an exhaust gas purification catalyst containing at least one noble metal selected from the group consisting of Pt, Pd, and Rh, and a composite compound in which a compound of at least one metal element selected from the group consisting of Al, Ce, La, Zr, Co, Mn, Fe, Mg, Ba, and Ti is dispersed substantially uniformly in at least one oxide selected from the group consisting of Al2O3, ZrO2, and CeO2, and characterized in that the noble metal is supported on the composite compound in a state in which part of the surface area of the noble metal is covered with the composite compound. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-198594 Summary of the Invention [Problem to be solved by the invention]

[0004] There is a demand for improved exhaust gas purification performance.

[0005] An object of the present disclosure is to provide an exhaust gas purification catalyst with improved exhaust gas purification performance. [Means for solving the problem]

[0006] The present inventors have found that the above object can be achieved by the following means: <<Aspect 1>> A catalyst for purifying exhaust gases, comprising a porous carrier, catalytic metal particles supported in pores of the porous carrier, and zirconium dioxide particles supported in the pores of the porous carrier, the zirconium dioxide particles are uniformly dispersed and supported in the pores of the porous support, and are monoclinic crystals or a mixed crystal of monoclinic and tetragonal crystals; Here, being supported in a uniformly dispersed state means that, when the catalyst for purifying exhaust gases is measured with an electron probe microanalyzer, the ratio of the abundance ratio of zirconium in a surface region of the catalyst for purifying exhaust gases to a depth of 1.5 μm from the surface to the abundance ratio of zirconium in a region of the catalyst for purifying exhaust gases further inside than the surface region is 95 to 105 mol %. Catalyst for purifying exhaust gas. <<Aspect 2>> 2. The exhaust gas purifying catalyst according to claim 1, wherein the zirconium dioxide particles are monoclinic. Aspect 3 3. The exhaust gas purifying catalyst according to aspect 1 or 2, wherein the ratio of the mass of the zirconium dioxide particles to the mass of the porous carrier is 0.1 to 5.0% by mass. Aspect 4 4. The exhaust gas purifying catalyst according to any one of Aspects 1 to 3, wherein the zirconium dioxide particles have a crystallite diameter of 6.0 to 8.0 nm. Aspect 5 5. The exhaust gas purifying catalyst according to any one of Aspects 1 to 4, wherein the zirconium dioxide particles have a secondary particle diameter (D50) of 40 nm or less. Aspect 6 6. The exhaust gas purifying catalyst according to any one of Aspects 1 to 5, wherein the catalytic metal particles are Rh particles. Aspect 7 7. The exhaust gas purifying catalyst according to any one of aspects 1 to 6, wherein the ratio of the mass of the catalytic metal particles to the mass of the porous support is 0.5 to 2.0 mass %. Aspect 8 A catalyst for purifying exhaust gases according to any one of Aspects 1 to 7, wherein the catalytic metal particles have a primary particle diameter (D50) of 1.0 to 9.0 nm. Aspect 9 A catalyst for purifying exhaust gases according to any one of aspects 1 to 8, wherein the porous support is a composite oxide containing Al and Zr. Aspect 10 The initial specific surface area of the porous carrier is 45 to 115 m 2 10. The exhaust gas purifying catalyst according to any one of aspects 1 to 9, wherein the saturation energy of the catalyst is 0.1 wt % or more. [Effects of the Invention]

[0007] According to the present disclosure, it is possible to provide an exhaust gas purification catalyst with improved exhaust gas purification performance. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic diagram of a catalyst particle for purifying exhaust gas according to one embodiment of the present disclosure. [Figure 2] 1A is a scanning electron microscope (SEM) image of the exhaust gas purifying catalyst particle of Comparative Example 2, and FIG. 1B is an image obtained by cutting out the surface region and the region inside the surface region in the same image. [Figure 3] 1 shows Zr mapping images of exhaust gas purification catalyst particles of Example 1(a) and Comparative Example 2(b) by an electron probe microanalyzer (EMPA). [Figure 4] 1 is a graph showing the temperatures at which exhaust gas purifying catalyst particles of Examples 1 to 6 and Comparative Examples 1 to 6 reach a 50% purification rate of NOx. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the present disclosure.

[0010] <Exhaust gas purification catalyst> The exhaust gas purification catalyst of the present disclosure comprises a porous support, catalytic metal particles supported in the pores of the porous support, and zirconium dioxide (ZrO2) particles supported in the pores of the porous support. The zirconium dioxide particles are supported in a uniformly dispersed state within the pores of the porous support. The zirconium dioxide particles are monoclinic or a mixed crystal of monoclinic and tetragonal crystals.

[0011] Here, being uniformly dispersed and supported means that, when the exhaust gas purifying catalyst is measured with an electron probe microanalyzer, the ratio of the abundance ratio of zirconium in a surface region 11 from the surface of the exhaust gas purifying catalyst 1 to a depth d=1.5 μm, to the abundance ratio of zirconium in a region 12 inside the surface region 11 of the exhaust gas purifying catalyst 1, is 95 to 105 mol %, as shown in FIG.

[0012] Although not limited by the theory, the principle behind the improvement in exhaust gas purification performance of the exhaust gas purification catalyst of the present disclosure is thought to be as follows.

[0013] Generally, catalytic metal particles contained in exhaust gas purification catalysts tend to dissolve and atomize in the carrier during the repeated acidic and reducing atmospheres at high temperatures during the exhaust gas purification process, diffuse into the gas phase, and migrate to other carriers or honeycomb substrates on which the exhaust gas purification catalyst is placed, where they become deactivated.

[0014] In the exhaust gas purification catalyst of the present disclosure, by bringing zirconium dioxide particles, which are monoclinic or a mixed crystal of monoclinic and tetragonal, close to catalytic metal particles, it is possible to suppress the diffusion of the catalytic metal particles into the gas phase due to solid solution and atomization in the support, and to improve catalytic activity.

[0015] Specifically, zirconium dioxide particles, which are monoclinic or a mixed crystal of monoclinic and tetragonal crystals, have a higher surface energy than supports commonly used in exhaust gas purification catalysts, such as Al2O3, CeO2, and composite oxide particles containing Al, Ce, and Zr. Furthermore, zirconium dioxide particles lack lattice matching with catalytic metal oxides, such as rhodium oxide. Therefore, it is believed that zirconium dioxide particles can inhibit the formation of solid solutions and atomization of catalytic metal particles during the purification of exhaust gases by exhaust gas purification catalysts.

[0016] Furthermore, zirconium dioxide particles that are monoclinic or a mixed crystal of monoclinic and tetragonal crystals can reduce catalytic metals, particularly Rh, at lower temperatures than cubic or tetragonal zirconium dioxide.

[0017] On the other hand, zirconium dioxide particles, which are monoclinic or a mixed crystal of monoclinic and tetragonal, do not have high heat resistance and may aggregate and grow in a high-temperature atmosphere. In this regard, in the exhaust gas purification catalyst of the present disclosure, the zirconium dioxide particles are uniformly supported in the pores of the support, so aggregation of the zirconium dioxide particles is unlikely to occur.

[0018] As a result, the exhaust gas purification catalyst of the present disclosure has improved exhaust gas purification performance.

[0019] <Porous carrier> The porous support included in the exhaust gas purification catalyst of the present disclosure is not particularly limited as long as it is a porous support used in exhaust gas purification catalysts, and may be, for example, a metal oxide, more specifically a metal oxide containing Al, even more specifically AlO, or a composite oxide containing Al and Zr, more specifically an AlO-ZrO composite oxide.

[0020] The porous carrier may be, for example, in the form of particles. When the porous carrier is in the form of particles, the average primary particle diameter (D50) may be, for example, 1 to 1000 μm.

[0021] The average primary particle diameter (D50) of the porous support may be 1 μm or more, 10 μm or more, 50 μm or more, or 100 μm or more, and may be 1000 μm or less, 500 μm or less, 200 μm or less, or 100 μm or less.

[0022] The average primary particle diameter is a number average value calculated by observing at least 200 primary particles of a porous carrier using a scanning electron microscope (SEM), determining the equivalent circle diameter when a perfect circle with the same area is taken as an equal-area circle, and then calculating the equivalent circle diameter.

[0023] The initial specific surface area of the porous carrier is 45 to 115 m 2 / g is preferred.

[0024] The initial specific surface area of the porous carrier is 45m 2 / g or more, 50m 2 / g or more, 60m 2 / g or more, or 70m 2 / g or more, and 2 / g or less, 110m 2 / g or less, 100m 2 / g or less, or 90m 2 / g or less.

[0025] Here, the initial specific surface area of the porous support is the specific surface area of the porous support of the exhaust gas purification catalyst of the present disclosure before it is used as a product. The specific surface area can be determined, for example, by a gas adsorption method.

[0026] The pore size of the porous support is not particularly limited as long as it is large enough to support the catalytic metal particles and zirconium dioxide particles in the pores. The pore size may be, for example, 10 nm or more, 50 nm or more, or 100 nm or more, or 1000 nm or less, 500 nm or less, or 200 nm or less.

[0027] <Catalytic metal particles> The catalytic metal particles are metal particles having catalytic activity capable of purifying exhaust gases, such as CO, HC, and NOx. Such metal particles include noble metals, more specifically Rh, Pt, and Pd. The catalytic metal particles may be Rh in particular.

[0028] The ratio of the mass of the catalytic metal particles to the mass of the porous support may be 0.5 to 2.0% by mass, and may be 0.5% by mass or more, 0.6% by mass or more, 0.7% by mass or more, or 0.8% by mass or more, and may be 2.0% by mass or less, 1.5% by mass or less, 1.3% by mass or less, or 1.0% by mass or less.

[0029] The primary particle diameter (D50) of the catalytic metal particles may be 1.0 to 9.0 nm. This primary particle diameter (D50) may be 1.0 nm or more, 2.0 nm or more, 3.0 nm or more, or 4.0 nm or more, and may be 9.0 nm or less, 8.0 nm or less, 7.0 nm or less, or 6.0 nm or less.

[0030] In the pores of the porous support, the catalytic metal particles may be in contact with the zirconium dioxide particles.

[0031] <Zirconium dioxide particles> The zirconium dioxide particles contained in the exhaust gas purifying catalyst of the present disclosure are monoclinic or a mixed crystal of monoclinic and tetragonal, with monoclinic being particularly preferred.

[0032] The mass ratio of the zirconium dioxide particles to the mass of the porous support is preferably 0.1 to 5.0 mass%. When the mass ratio of the zirconium dioxide particles to the mass of the porous support is 0.1 mass% or more, the number of zirconium dioxide particles that can act on the catalytic metal particles can be significantly increased. On the other hand, when the mass ratio of the zirconium dioxide particles to the mass of the porous support is 5.0 mass% or less, the dispersibility of the zirconium dioxide particles in the pores of the porous support is particularly good.

[0033] The ratio of the mass of the zirconium dioxide particles to the mass of the porous support may be 0.1 mass% or more, 0.5 mass% or more, 1.0 mass% or more, or 1.5 mass% or more, and may be 5.0 mass% or less, 4.0 mass% or less, 3.0 mass% or less, or 2.0 mass% or less.

[0034] The crystallite diameter of the zirconium dioxide particles is preferably 6.0 to 8.0 nm.

[0035] When the crystallite diameter of the zirconium dioxide particles is 6.0 nm or more, the effect of suppressing sintering of the catalytic metal particles is particularly good. On the other hand, when the crystallite diameter of the zirconium dioxide particles is 8.0 nm or less, the heat resistance of the zirconium dioxide particles is particularly good, and aggregation of the zirconium dioxide particles due to heating is particularly suppressed.

[0036] The crystallite size of the zirconium dioxide particles may be 6.0 nm or more, 6.2 nm or more, 6.4 nm or more, or 6.8 nm or more, and may be 8.0 nm or less, 7.8 nm or less, 7.6 nm or less, or 7.4 nm or less.

[0037] The secondary particle diameter (D50) of the zirconium dioxide particles is preferably 40 nm or less. When the secondary particle diameter (D50) of the zirconium dioxide particles is within this range, the dispersibility of the zirconium dioxide particles in the pores of the porous support can be further improved.

[0038] The secondary particle diameter (D50) of the zirconium dioxide particles may be 40 nm or less, 35 nm or less, 30 nm or less, or 25 nm or less, or may be greater than 0 nm, 5 nm or more, 10 nm or more, or 15 nm or more.

[0039] <<Method for manufacturing exhaust gas purification catalyst>> The manufacturing method of the present disclosure is a method for manufacturing the exhaust gas purifying catalyst of the present disclosure.

[0040] The manufacturing method of the present disclosure comprises dispersing a porous support and zirconium dioxide particles in an acidic dispersion medium, followed by drying and calcination to support the zirconium dioxide particles in the pores of the porous support, and supporting catalytic metal particles on the porous support, in this order. The zirconium dioxide particles are monoclinic or a mixed crystal of monoclinic and tetragonal crystals.

[0041] In the production method of the present disclosure, by making the dispersion medium acidic, aggregation of zirconium dioxide particles in the dispersion liquid is suppressed, and the zirconium dioxide particles can be supported in the pores of the porous support while adjusting the particle size of the secondary particles so that they do not become too large, thereby further improving the dispersibility of the zirconium dioxide particles in the pores of the porous support.

[0042] The pH of the dispersion medium may be, for example, 1.0 or more, 1.5 or more, or 2.0 or more, and may be 5.0 or less, 4.5 or less, or 4.0 or less. The pH is particularly preferably 2.5 to 3.5.

[0043] The method for supporting catalytic metal particles in the pores of the porous support is not particularly limited. However, the catalytic metal can be supported in the pores of the porous support by adding the catalytic metal to a dispersion medium in which a porous support having zirconium dioxide supported in the pores is dispersed, stirring the mixture, and then drying and calcining the mixture.

[0044] The porous support, catalytic metal particles, and zirconium dioxide particles are as described above in the section "<Catalyst for purifying exhaust gas>". [Example]

[0045] Examples 1 to 6 and Comparative Examples 1 to 6 <Preparation of exhaust gas purification catalyst> (Comparative Example 1) A dispersion liquid in which Al2O3-ZrO2 powder as a porous support was dispersed in water was mixed with a dispersion liquid in which Rh particles (average primary particle diameter (D59) = 2 nm) were dispersed, and the mixture was stirred for 1 hour.

[0046] The mixture was then heated on a hot stirrer to evaporate the water, obtaining a precipitate, which was then dried at 120°C for 24 hours and then calcined in air at 500°C to obtain a catalyst for purifying exhaust gases of Comparative Example 1.

[0047] Example 1 Nitric acid was added to a dispersion of Al2O3-ZrO2 powder as a porous support in water to adjust the pH to 3. This dispersion was mixed with a ZrO2 particle dispersion (a mixed crystal of monoclinic and tetragonal crystals, secondary particle diameter (D50) of the ZrO2 particles in the solution = 15 nm) with a pH of 3, and the mixture was stirred for 1 hour. The amount of ZrO2 particles in the mixture was 4.0 mass% relative to the Al2O3-ZrO2 powder.

[0048] The mixture was then heated on a hot stirrer to evaporate the water, yielding a precipitate, which was then dried overnight at 120°C and then calcined in air at 500°C to obtain Al2O3-ZrO2 powder with ZrO2 particles supported in the pores.

[0049] Next, using the Al2O3-ZrO2 powder with ZrO2 particles supported in the pores, Rh particles were supported in the pores of the Al2O3-ZrO2 powder in the same manner as in Comparative Example 1, thereby obtaining the exhaust gas purification catalyst of Example 1.

[0050] Examples 2 to 4 The exhaust gas purifying catalysts of Examples 2 to 4 were obtained in the same manner as in Example 1, except that in the step of obtaining Al2O3-ZrO2 powder in which ZrO2 particles are supported in the pores, the amounts of ZrO2 particles in the mixed liquid were set to 0.5 mass%, 1.0 mass%, and 2.0 mass%, respectively, relative to the Al2O3-ZrO2 powder.

[0051] Example 5 An exhaust gas purifying catalyst of Example 5 was obtained in the same manner as in Example 1, except that monoclinic ZrO2 particles (secondary particle diameter (D50) of ZrO2 particles in liquid = 38 nm) were used.

[0052] Example 6 An exhaust gas purifying catalyst of Example 6 was obtained in the same manner as in Example 5, except that Al2O3 powder was used as the porous support.

[0053] (Comparative Example 2) An exhaust gas purifying catalyst of Comparative Example 2 was obtained in the same manner as in Example 1, except that in the step of obtaining Al2O3-ZrO2 powder in which ZrO2 particles are supported in the pores, ammonia was added to each of the ZrO2 particle dispersion and the dispersion in which Al2O3-ZrO2 powder was dispersed in water to adjust the pH of both to 7, and then the mixture was mixed and stirred for 1 hour. Note that, by adjusting the pH of the ZrO2 particle dispersion to 7, the ZrO2 particles in the dispersion aggregated to have a secondary particle diameter (D50) of 60 nm.

[0054] (Comparative Example 3) To a dispersion of zirconium in water, an equal molar amount of citric acid was added and thoroughly stirred to dissolve the zirconium. Zirconium oxynitrate dihydrate was then added to obtain an aqueous zirconium nitrate solution. The resulting aqueous zirconium nitrate solution was then added dropwise to tetraethylammonium hydroxide (10% aqueous solution) to obtain a zirconium hydroxide solution (ZrO2 secondary particle diameter (D50) = 10 nm).

[0055] Except for using this zirconium hydroxide solution instead of the ZrO2 particle dispersion, an exhaust gas purifying catalyst of Comparative Example 3 was obtained in the same manner as in Example 1. In the exhaust gas purifying catalyst of Comparative Example 3, the zirconium dioxide supported in the pores of the porous support was amorphous.

[0056] Comparative Example 4 The exhaust gas purification catalyst of Comparative Example 4 was obtained in the same manner as in Example 1, except that a ZrO dispersion liquid in which tetragonal ZrO particles were dispersed (secondary particle diameter (D50) of the ZrO particles in the liquid = 6 nm) was used in the process of obtaining Al2O3-ZrO2 powder in which ZrO2 particles were supported in the pores.

[0057] (Comparative Example 5) The exhaust gas purifying catalyst of Comparative Example 5 was obtained in the same manner as in Example 1, except that in the process of obtaining Al2O3-ZrO2 powder in which ZrO2 particles are supported in the pores, the amount of ZrO2 particles in the mixed liquid was 8.0 mass% relative to the Al2O3-ZrO2 powder.

[0058] (Comparative Example 6) An exhaust gas purifying catalyst of Comparative Example 6 was obtained in the same manner as in Comparative Example 1, except that the porous carrier was AlO3.

[0059] <Electron probe microanalyzer analysis> For the exhaust gas purification catalysts of Comparative Examples 2 to 5 and Examples 1 to 6, the distribution of zirconium dioxide particles in the pores of the porous support was measured using an electron probe microanalyzer (EPMA) (Shimadzu EPMA-8050G, beam current conditions: 15 kV, 50 nA). For the measurement, first, as shown in FIG. 2, the obtained image was divided into a surface region extending from the surface of the porous support to a depth of 1.5 μm and an area inside the surface region. Then, the EPMA-detected amounts (counts) of Zr and Al contained in the porous support per unit area in each region were integrated to calculate the Zr / Al ratio. Specifically, the calculation was performed using the following formula:

[0060] Uniformity (%) = (Zr / Al in the surface region) / (Zr / Al in the region deeper than the surface region) x 100

[0061] A homogeneity (%) of 95 to 105 mol% was determined to be homogeneous, and any other cases were determined to be non-homogeneous.

[0062] FIG. 3 shows the Zr mapping results for Example 1 (FIG. 3(a)) and Comparative Example 2 (FIG. 3(b)). The exhaust gas purifying catalyst of Example 1 had a uniformity of 97 mol%, and the amount of zirconium dioxide particles was almost the same in the surface region and the inner region, and they were uniformly supported. Similarly, in Comparative Examples 3 and 4 and Examples 2 to 6, the zirconium dioxide particles were uniformly supported in the pores of the porous support.

[0063] In contrast, the exhaust gas purifying catalyst of Comparative Example 2 had a uniformity of 108 mol%, which confirmed that the zirconium dioxide particles were supported in large amounts on the surface region, i.e., non-uniformly. Also in Comparative Example 5, the zirconium dioxide particles were supported non-uniformly within the pores of the porous support.

[0064] In Comparative Example 2, the zirconium dioxide particles were supported on the porous support in an agglomerated state in the liquid, and therefore the zirconium dioxide particles were not introduced deep into the pores of the porous support, which is thought to be why the amount supported in the surface region was greater than that in the inner region.

[0065] In addition, in Comparative Example 5, in the process of obtaining an Al2O3-ZrO2 powder in which ZrO2 particles are supported in the pores, the amount of zirconium dioxide particles in the mixed liquid was 8 mass%, which was an excessive amount, and therefore the amount of zirconium dioxide particles filled in the pores of the porous support was too large, and it is thought that the amount supported in the surface region was higher than in the inner region.

[0066] <Evaluation of exhaust gas purification performance> The catalysts of each example and each comparative example were evaluated for their exhaust gas purification performance (three-way purification catalyst performance).

[0067] In order to simulate the catalyst of the actual coating layer, the powders of each example and each comparative example were mixed with Al2O3, Al2O3-CeO2-ZrO2, and CeO2-ZrO2 powders, and the mixture was pressurized and molded at a pressure of 1 ton using a cold isostatic press (CIP), and then crushed and sieved to obtain pellet catalysts.

[0068] Next, 2 g of this catalyst pellet was placed in a flow reactor and heated to 500°C at a heating rate of 50°C / min in a model gas for evaluation, and after maintaining this temperature for 10 minutes, the temperature was lowered to 100°C. Next, the catalyst was heated at a heating rate of 20°C / min, and the three-way purification catalyst performance during heating was measured, and the temperature at which 50% purification rate of NOx in the gas was reached was calculated.

[0069] The composition of the model gas for evaluation was 1600 ppm NO, 6100 ppm O, 10000 ppm CO, 5000 ppm CO, 30000 H2O, and the remainder N2. The gas flow rate was 20 L / min.

[0070] <result> The production conditions of the exhaust gas purification catalyst of each example and the evaluation results of the exhaust gas purification performance are shown in FIG.

[0071] [Table 1]

[0072] 4 and Table 1, increasing the zirconium dioxide content from 0.5% by mass to 8% by mass (Examples 1 to 4 and Comparative Example 5) is thought to have increased the proximity between Rh and zirconium dioxide particles, suppressing Rh sintering and leading to a decrease in the temperature (°C) at which 50% NOx purification rate is achieved. However, when the zirconium dioxide content was 8.0% by mass as in Comparative Example 5, the uniformity of the zirconium dioxide particles in the pores of the porous support deteriorated, which is thought to have reduced the heat resistance of the zirconium dioxide particles and prevented a sufficient effect of suppressing Rh degradation.

[0073] Furthermore, as in Example 5, when the crystal structure of the zirconium dioxide particles was monoclinic, the temperature (° C.) at which 50% NOx purification rate was achieved could be lowered more than when the crystal structure was mixed crystal.

[0074] Furthermore, as shown in Examples 5 and 6, the temperature (°C) at which 50% NOx purification rate was achieved was lowered more when an Al2O3-ZrO2 composite oxide was used as the porous support than when Al2O3 was used (Example 6). [Explanation of symbols]

[0075] 1. Exhaust gas purification catalyst 11 Surface area 12 Area inside the surface area

Claims

1. A catalyst for purifying exhaust gases, comprising a porous carrier, catalytic metal particles supported in pores of the porous carrier, and zirconium dioxide particles supported in the pores of the porous carrier, the zirconium dioxide particles are uniformly dispersed and supported in the pores of the porous support, and are monoclinic crystals or a mixed crystal of monoclinic and tetragonal crystals; Here, being supported in a uniformly dispersed state means that, when the exhaust gas purifying catalyst is measured with an electron probe microanalyzer, the ratio of the abundance ratio of zirconium in a surface region of the exhaust gas purifying catalyst down to a depth of 1.5 μm from the surface to the abundance ratio of zirconium in a region of the exhaust gas purifying catalyst further inside than the surface region is 95 to 105 mol %. Catalyst for purifying exhaust gas.

2. 2. The exhaust gas purifying catalyst according to claim 1, wherein the zirconium dioxide particles are monoclinic.

3. 3. The exhaust gas purifying catalyst according to claim 1, wherein a ratio of the mass of said zirconium dioxide particles to the mass of said porous carrier is 0.1 to 5.0 mass %.

4. 3. The exhaust gas purifying catalyst according to claim 1, wherein the zirconium dioxide particles have a crystallite diameter of 6.0 to 8.0 nm.

5. 3. The exhaust gas purifying catalyst according to claim 1, wherein the catalytic metal particles are Rh particles.

6. 3. The exhaust gas purifying catalyst according to claim 1, wherein the ratio of the mass of said catalytic metal particles to the mass of said porous carrier is 0.5 to 2.0 mass %.

7. 3. The exhaust gas purifying catalyst according to claim 1, wherein the porous support is a composite oxide containing Al and Zr.

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