Exhaust gas purifying catalyst

Abstract

To provide an exhaust gas purifying catalyst, in particular a three-way catalyst, with improved catalytic activity.SOLUTION: The exhaust gas purifying catalyst of the present disclosure comprises rhodium particles, zirconium dioxide particles doped with cations having an oxidation number higher than that of zirconium, and carrier particles, where the rhodium particles are supported on the zirconium dioxide particles, and (i) the carrier particles are porous carrier particles, and the rhodium particles and the zirconium dioxide particles are supported in pores of the porous carrier particles, or (ii) the carrier particles are composed of a plurality of primary particles, and the rhodium particles and the zirconium dioxide particles are dispersed inside the carrier particles.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a catalyst for exhaust gas purification.

Background Art

[0002] Patent Document 1 discloses a catalyst for exhaust gas purification containing a noble metal; alumina support particles; and ZrO2 semiconductor particles supported on the surface of the alumina support particles.

[0003] Patent Document 2 discloses an exhaust gas purification catalyst containing at least one or more noble metals selected from the group of Pt, Pd, and Rh, and a compound of at least one or more metal elements selected from the group of Al, Ce, La, Zr, Co, Mn, Fe, Mg, Ba, and Ti, which is a composite compound substantially uniformly dispersed in at least one or more oxides selected from the group of Al2O3, ZrO2, and CeO2, and the exhaust gas purification catalyst is characterized in that a part of the surface area of the noble metal is covered by the composite compound and the noble metal is supported on the composite compound.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] There is a demand for improving the catalytic activity of a catalyst for exhaust gas purification, particularly a three-way catalyst.

Means for Solving the Problems

[0006] The present inventors have found that the above problems can be achieved by the following means: 《Aspect 1》 Rhodium particles, zirconium dioxide particles doped with a cation having an oxidation number higher than that of zirconium, and support particles and having, the rhodium particles are supported on the zirconium dioxide particles, and (i) the support particles are porous support particles, and the rhodium particles and the zirconium dioxide particles are supported in the pores of the porous support particles, or (ii) the support particles are composed of a plurality of primary particles, and the rhodium particles and the zirconium dioxide particles are dispersed inside the support particles, an exhaust gas purification catalyst. <<Aspect 2>> The exhaust gas purification catalyst according to Aspect 1, satisfying the above (i). <<Aspect 3>> The exhaust gas purification catalyst according to Aspect 1, satisfying the above (ii). <<Aspect 4>> The exhaust gas purification catalyst according to any one of Aspects 1 to 3, wherein the ratio of the mass of the zirconium dioxide particles to the total mass of the zirconium dioxide particles and the porous support particles is 1.0 to 20.0% by mass. <<Aspect 5>> The exhaust gas purification catalyst according to any one of Aspects 1 to 4, wherein the cation is niobium. <<Aspect 6>> The exhaust gas purification catalyst according to Aspect 5, wherein the content of niobium in the zirconium dioxide particles is 1.0 to 7.0% by mass. <<Aspect 7>> The exhaust gas purification catalyst according to any one of Aspects 1 to 6, wherein the support particles are aluminum oxide.

Advantages of the Invention

[0007] According to the present disclosure, an exhaust gas purification catalyst with improved catalytic activity, particularly the catalytic activity of a three-way catalyst, can be provided.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out 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 gist of the disclosure.

[0010] 1. Catalyst for Purifying Exhaust Gas The exhaust gas purification catalyst of the present disclosure has rhodium particles, zirconium dioxide particles doped with cations having an oxidation number higher than that of zirconium, and porous carrier particles. The rhodium particles are supported on the zirconium dioxide particles. The rhodium particles and the zirconium dioxide particles are supported in the pores of the porous carrier particles. Instead of the above, the exhaust gas purification catalyst of the present disclosure can have rhodium particles, zirconium dioxide particles doped with cations having an oxidation number higher than that of zirconium, and carrier particles. The rhodium particles are supported on the zirconium dioxide particles, and the rhodium particles and the zirconium dioxide particles can be dispersed inside the carrier particles. Here, the carrier particles are secondary particles in which the primary particles constituting the carrier particles are aggregated, and it can also be said that the rhodium particles and the zirconium dioxide particles are arranged between the primary particles.

[0011] The exhaust gas purification catalyst of the present disclosure may be a three-way catalyst.

[0012] Generally, rhodium as a catalyst metal exhibits particularly high catalytic activity with respect to NOx reduction compared to palladium and platinum. By improving the catalytic activity of rhodium, it is expected to reduce the amount of rhodium used.

[0013] In this regard, the rate-determining step for the reduction of NOx in the exhaust gas is the CO2 generation pathway. That is, the reaction between the O atom adsorbed on the rhodium particles and CO is rate-determining. Therefore, if the adsorption energy of the O atom adsorbed on the rhodium particles can be reduced, the NOx reduction performance of the exhaust gas purification catalyst can be improved.

[0014] In the exhaust gas purification catalyst of the present disclosure, rhodium particles are supported on zirconium dioxide particles. And the zirconium dioxide particles are doped with a cation having a higher oxidation number than zirconium, and thus are electron-rich. This electron flows into the rhodium particles and reduces the adsorption energy of the O atom. Therefore, the exhaust gas purification catalyst of the present disclosure has high NOx reduction performance.

[0015] Also, zirconium dioxide particles have particularly high adsorption energy among metal oxides and are likely to adsorb rhodium, for example, rhodium oxide (RhO2). Among them, zirconium dioxide particles doped with a cation, particularly niobium, have particularly high adsorption energy compared to zirconium dioxide particles not doped with a cation. Thereby, the evaporation of rhodium is suppressed and high heat resistance is achieved.

[0016] In addition, in the exhaust gas purification catalyst of the present disclosure, the zirconium dioxide particles are supported within the pores of the porous carrier particles, thereby suppressing the aggregation of the zirconium dioxide particles with each other, and thus the heat resistance is further improved. The rhodium particles are also supported on the zirconium dioxide particles and are supported within the pores of the porous carrier particles. Or, in the exhaust gas purification catalyst of the present disclosure, the zirconium dioxide particles are dispersed within the carrier particles, thereby suppressing the aggregation of the zirconium dioxide particles with each other, and thus the heat resistance is further improved. The rhodium particles are also supported on the zirconium dioxide particles and are supported within the carrier particles.

[0017] That is, the exhaust gas purification catalyst of the present disclosure achieves both high NOx reduction performance and high heat resistance.

[0018] The specific surface area of the exhaust gas purification catalyst of the present disclosure after the durability test can be 90 m 2 / g or less. Here, the durability test is performed by placing the exhaust gas purification catalyst in a flow-through durability furnace and alternately flowing a reducing gas (CO) and an oxidizing gas (O2) repeatedly for 5 hours at 1000°C.

[0019] The specific surface area of the exhaust gas purification catalyst of the present disclosure after the durability test is 90 m 2 / g or less, 89.9 m 2 / g or less, 89.8 m 2 / g or less, or 89.7 m 2 / g or less, and may be 88.0 m 2 / g or more, 88.1 m 2 / g or more, 88.2 m 2 / g or more, or 88.3 m 2 / g or more.

[0020] The NOx 50% purification temperature after the durability test of the exhaust gas purification catalyst of the present disclosure can be 250°C or lower. Here, the NOx 50% purification temperature is measured as follows. The exhaust gas purification catalyst is placed in a flow-through reactor, heated to 500°C at a heating rate of 50°C / min in an evaluation model gas, held at this temperature for 10 minutes, and then cooled to 100°C. Thereafter, it is heated at a heating rate of 20°C / min, and the temperature reached when the purification rate of NOx in the gas becomes 50% is calculated as the NOx 50% purification temperature. The composition of the evaluation model gas is CO (0.65% by volume), CO2 (10.00% by volume), C3H6 (0.10% by volume), NO (0.15% by volume), O2 (0.70% by volume), H2 (3.00% by volume), and N2 (the balance).

[0021] The NOx 50% purification temperature after the durability test of the exhaust gas purification catalyst of the present disclosure may be 250°C or lower, 249°C or lower, 248°C or lower, 247°C or lower, or 246°C or lower, and may be 245°C or higher, 246°C or higher, 247°C or higher, or 248°C or higher.

[0022] Figure 1 is a schematic diagram of an exhaust gas purification catalyst according to one embodiment of the present disclosure.

[0023] As shown in Figure 1, an exhaust gas purification catalyst 1 according to one embodiment of the present disclosure includes rhodium particles 10, zirconium dioxide particles 20 doped with a cation having an oxidation number higher than that of zirconium, and porous support particles 30. The rhodium particles 10 are supported on the zirconium dioxide particles 20. The rhodium particles 10 and the zirconium dioxide particles 20 are supported in the pores of the porous support particles 30.

[0024] Note that Figure 1 is not intended to limit the exhaust gas purification catalyst of the present disclosure.

[0025] Figure 2 is a schematic diagram of an exhaust gas purification catalyst according to another embodiment of the present disclosure.

[0026] As shown in FIG. 2, the exhaust gas purification catalyst 2 according to another embodiment of the present disclosure includes rhodium particles 10, zirconium dioxide particles 20 doped with cations having an oxidation number higher than that of zirconium, and carrier particles. The carrier particles are secondary particles in which primary particles 40 constituting the carrier particles are aggregated. The rhodium particles 10 are supported on the zirconium dioxide particles 20. The rhodium particles 10 and the zirconium dioxide particles 20 are dispersed inside the carrier particles.

[0027] Note that FIG. 2 is not intended to limit the exhaust gas purification catalyst of the present disclosure.

[0028] 1-1. Rhodium Particles In the exhaust gas purification catalyst of the present disclosure, the rhodium particles are supported on the zirconium dioxide particles.

[0029] The size and shape of the rhodium particles may be any size and shape used as a catalyst metal of the exhaust gas purification catalyst.

[0030] More specifically, the rhodium particles may have a median diameter (D50) of 0.1 to 10.0 nm. The median diameter (D50) of the rhodium particles may be 0.1 nm or more, 1.0 nm or more, 2.0 nm or more, or 2.5 nm or more, and may be 10.0 nm or less, 5.0 nm or less, 3.0 nm or less, or 2.5 nm or less.

[0031] The median diameter (D50) can be measured, for example, by measuring the particle size distribution using a laser diffraction particle size distribution measuring device (SALD-2300) manufactured by Shimadzu Corporation, and measuring it as the particle diameter at 50% cumulative frequency.

[0032] The amount of rhodium particles supported on zirconium dioxide particles may be, for example, 0.1 to 5.0% by mass based on the whole exhaust gas purification catalyst. The amount of rhodium particles supported on zirconium dioxide particles may be 0.1% by mass or more, 0.2% by mass or more, 0.3% by mass or more, or 0.4% by mass or more, and may be 5.0% by mass or less, 2.5% by mass or less, 1.0% by mass or less, or 0.5% by mass or less.

[0033] When the amount of rhodium particles supported on zirconium dioxide particles is 0.1% by mass or more, there are more rhodium particles contributing to NO X purification, so higher NO X purification performance can be achieved. On the other hand, when the supported amount is 5.0% by mass or less, the amount of rhodium, which is an expensive noble metal, can be reduced, and the cost performance is excellent.

[0034] 1-2. Zirconium Dioxide Particles The zirconium dioxide particles contained in the exhaust gas purification catalyst of the present disclosure are doped with cations having an oxidation number higher than that of zirconium. Further, the zirconium dioxide particles can be tetragonal.

[0035] The ratio of the mass of the zirconium dioxide particles to the total mass of the zirconium dioxide particles and the porous carrier particles can be 1.0 to 20.0% by mass. This ratio may be 1.0% by mass or more, 5.0% by mass or more, 10.0% by mass or more, or 15.0% by mass or more, and may be 20.0% by mass or less, 15.0% by mass or less, 10.0% by mass or less, or 5.0% by mass or less. Note that the mass of the zirconium dioxide particles is the value obtained by combining both the zirconium dioxide part and the cation part in the particles.

[0036] The ratio of the mass of zirconium dioxide particles to the total mass of zirconium dioxide particles and porous carrier particles being 1.0% by mass or more can significantly increase the number of zirconium dioxide particles that can act on rhodium particles. On the other hand, when the ratio is 20.0% by mass or less, the dispersibility of zirconium dioxide particles in the pores of the porous carrier is particularly good.

[0037] Examples of the cation include niobium.

[0038] The content rate of niobium in the zirconium dioxide particles, that is, the mass% of the niobium part with respect to the total of the zirconium dioxide part and the niobium part in the niobium-doped zirconium dioxide particles, can be 1.0 to 7.0% by mass. The content rate of niobium may be 1.0% by mass or more, 1.5% by mass or more, 2.0% by mass or more, or 5.0% by mass or more, and may be 7.0% by mass or less, 6.0% by mass or less, 5.0% by mass or less, or 3.0% by mass or less.

[0039] When the content rate of niobium is 1.0% by mass or more, the adsorption energy of the zirconium dioxide particles is particularly large, and it is easy to suppress the transpiration of rhodium particles. On the other hand, when the content rate of niobium is 7.0% by mass or less, the electron transfer to the rhodium particles is particularly good, and the 50% NOx purification temperature can be particularly reduced.

[0040] The crystallite diameter of the zirconium dioxide particles is preferably 6.0 to 8.0 nm. The crystallite diameter 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.

[0041] When the crystallite diameter of the zirconium dioxide particles is 6.0 nm or more, the effect of suppressing sintering of the catalyst metal particles and the like 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.

[0042] The secondary particle diameter (D50) of the zirconium dioxide particles is preferably 40 nm or less. 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, and may be more than 0 nm, 5 nm or more, 10 nm or more, or 15 nm or more.

[0043] When the secondary particle diameter (D50) of the zirconium dioxide particles is such a size, the dispersibility of the zirconium dioxide particles in the pores of the porous carrier can be further enhanced.

[0044] 1-3. Carrier Particles The carrier that the exhaust gas purification catalyst of the present disclosure has can be porous carrier particles used for the exhaust gas purification catalyst. Alternatively, the carrier, the carrier particles can be secondary particles in which the primary particles 40 constituting the carrier particles are aggregated. The carrier may be, for example, a metal oxide, more specifically, a metal oxide containing Al, still more specifically, aluminum oxide (Al2O3), or a composite oxide containing Al and Zr, more specifically, an Al2O3-ZrO2 composite oxide.

[0045] The average primary particle diameter (D50) of the porous carrier particles may be, for example, 1 to 1000 μm.

[0046] The average primary particle diameter (D50) of the porous carrier particles 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.

[0047] The average primary particle diameter is the number average value calculated from the equivalent circle diameters obtained by observing at least 200 primary particles of porous carrier particles using a scanning electron microscope (SEM) and taking the equivalent circle diameter when a perfect circle equal in area is defined as an equal-area circle.

[0048] The pore diameter of the pores in the porous carrier particles is not particularly limited as long as it is large enough to allow rhodium particles and zirconium dioxide particles to be supported in the pores. For example, the pore diameter may be 10 nm or more, 50 nm or more, or 100 nm or more, and may be 1000 nm or less, 500 nm or less, or 200 nm or less.

[0049] 2. Method for manufacturing the exhaust gas purification catalyst The method for manufacturing the exhaust gas purification catalyst of the present disclosure is not particularly limited, and for example, the following three methods can be mentioned.

[0050] 2-1. First manufacturing method The first manufacturing method for manufacturing the exhaust gas purification catalyst of the present disclosure involves dispersing porous carrier particles, a zirconium dioxide source, and a cation source in an acidic dispersion medium, followed by drying and firing to obtain porous carrier particles with cation-doped zirconium dioxide particles supported in the pores, and then supporting rhodium particles on the zirconium dioxide particles in the pores of the porous carrier particles, in this order. The rhodium particles are supported in the pores of the porous carrier particles and on the zirconium dioxide particles.

[0051] Here, as the zirconium dioxide source, for example, zirconium oxynitrate can be used. As the cation source, for example, niobium oxalate can be used. As the porous carrier particles, for example, aluminum dioxide, that is, Al2O3 can be used, and more specifically, La2O3-composited Al2O3 (La2O3: 1% by mass) can be used. Further, as the acidic dispersion medium, for example, citric acid can be used.

[0052] Drying can be carried out by evaporating and drying the acidic dispersion medium to obtain a precipitate, and then drying in a constant temperature furnace at 100°C to 200°C, preferably 100°C to 150°C, more preferably 100°C to 120°C for 1 to 36 hours, preferably 10 to 24 hours.

[0053] Firing can be carried out, for example, by atmospheric firing at 600°C to 1000°C, preferably 700°C to 900°C, more preferably 700°C to 800°C for 1 to 10 hours, preferably 2 to 5 hours, more preferably 3 to 4 hours.

[0054] After firing, a reduction treatment, for example, a reduction treatment at 800°C for 2 hours in a 3% hydrogen atmosphere, can be carried out.

[0055] The pH of the dispersion medium can be, for example, 1.0 or more, 1.5 or more, or 2.0 or more, and can be 5.0 or less, 4.5 or less, or 4.0 or less. The pH is particularly preferably 2.5 to 3.5.

[0056] The method for supporting rhodium particles on zirconium dioxide particles in the pores of the porous carrier particles is not particularly limited, but a dispersion medium in which the porous carrier particles having zirconium dioxide particles supported in the pores are dispersed, such as water, more specifically distilled water, a rhodium source, such as rhodium nitrate, is added and stirred, and then drying and firing can be carried out. This drying and firing can be carried out under the same conditions as the drying and firing for obtaining the porous carrier particles having zirconium dioxide particles doped with cations in the pores.

[0057] 2-2. Second manufacturing method The second manufacturing method for producing the exhaust gas purification catalyst of the present disclosure includes dispersing porous carrier particles in an organic solvent to obtain a first dispersion liquid, mixing a zirconium dioxide source and a cation source in the first dispersion liquid, boiling and stirring the first dispersion liquid, removing the organic solvent to obtain a powder, drying and firing the powder to obtain porous carrier particles having cation-doped zirconium dioxide particles supported in the pores, and supporting rhodium particles on the zirconium dioxide particles in the pores of the porous carrier particles, in this order. The rhodium particles are supported in the pores of the porous carrier particles and on the zirconium dioxide particles.

[0058] Here, as the zirconium dioxide source, for example, zirconium butoxide can be used. As the cation source, for example, niobium butoxide can be used. As the porous carrier particles, for example, aluminum dioxide, that is, Al2O3 can be used, and more specifically, La2O3 composite Al2O3 (La2O3: 1% by mass) can be used. Further, as the organic solvent, for example, hexane can be used.

[0059] In the second manufacturing method, the methods of drying and firing and supporting the rhodium particles on the zirconium dioxide particles in the pores of the porous carrier particles may adopt the same methods as those described in the first manufacturing method.

[0060] 2-3. Third manufacturing method The third manufacturing method for manufacturing the exhaust gas purification catalyst of the present disclosure is to disperse a zirconium dioxide source such as zirconium oxynitrate and a cation source such as niobium oxalate in a solvent such as water to obtain a first dispersion liquid. For example, by adjusting the pH of the first dispersion liquid to a base using ammonia, a precipitate is obtained. To the first dispersion liquid, raw materials for primary particles constituting carrier particles, such as aluminum nitrate and lanthanum nitrate, are added and mixed. Then, the solvent is removed from the first dispersion liquid, for example, by a centrifuge, and dried and calcined to obtain a first powder. The first powder and a rhodium source are dispersed in a solvent such as water, and then dried and calcined to obtain a second powder. The second powder and raw materials for primary particles constituting carrier particles, such as an Al2O3-CeO2-ZrO2 composite oxide (Al2O3: 30% by mass / CeO2: 20% by mass / ZrO2: 44% by mass / Nd2O3: 2% by mass / La2O3: 2% by mass / Y2O3: 2% by mass), a CZ composite oxide (CeO2: 51.5% by mass / ZrO2: 45.5% by mass / Pr6O 11 : mass%), and optionally a binder such as an Al2O3-based binder are suspended to obtain a slurry. The slurry can be pressure-molded and pulverized by cold isostatic pressing (CIP) or the like in this order.

[0061] 3. Exhaust gas purification method The exhaust gas purification method of the present disclosure includes bringing exhaust gas into contact with the exhaust gas purification catalyst of the present disclosure. The method of bringing exhaust gas into contact with the exhaust gas purification catalyst is not particularly limited.

[0062] Here, the exhaust gas can contain, for example, NOx, CO, and HC.

Examples

[0063] 4. Preparation of samples 4-1. Example 1 While stirring, porous support particles (La2O3 - composite Al2O3 (La2O3: 1 mass%)) were added to hexane, and bubbling with N2 was carried out for 10 minutes to obtain a dispersion. To this dispersion, zirconium butoxide and niobium butoxide were added, and the temperature was raised to the boiling point of hexane, and stirring was continued for 2 hours in the boiling state. Then, hexane was removed by a centrifuge, dried at 120 °C for one day and night, and further calcined in air at 800 °C for 3 hours to obtain a first powder. This powder is a powder of porous support particles in which zirconium dioxide particles doped with niobium as a cation are supported in the pores.

[0064] While stirring, rhodium nitrate and this powder were dispersed in distilled water to obtain a dispersion. By drying and calcining this dispersion, a second powder was obtained. In this powder, rhodium particles are supported in the pores of the porous support particles and on the zirconium dioxide particles.

[0065] While stirring, the second powder, Al2O3 - CeO2 - ZrO2 composite oxide, CeO2, and an Al2O3 - based binder were added to distilled water to prepare a suspended slurry.

[0066] The suspended slurry was dried, then pressure - molded at a pressure of 1 ton by cold isostatic pressing (CIP), and then sieved while pulverizing to obtain a sample of Example 1.

[0067] Here, in the sample of Example 1, the ratios of niobium, zirconium dioxide particles, and porous support particles to the total of niobium, zirconium dioxide particles, and porous support particles were, in order, niobium: 0.2 mass%, zirconium dioxide particles: 9.8 mass%, and porous support particles: 90.0 mass%.

[0068] 4 - 2. Example 2 A sample of Example 2 was obtained in the same manner as in Example 1, except that the amounts of zirconium butoxide and niobium butoxide were adjusted so that the ratios of niobium, zirconium dioxide particles, and porous carrier particles were 0.5% by mass of niobium, 9.5% by mass of zirconium dioxide particles, and 90.0% by mass of porous carrier particles, respectively, in that order.

[0069] 4-3. Example 3 While stirring, zirconium oxynitrate and niobium oxalate were added to distilled water. Then, the pH was adjusted to a base using ammonia to form a precipitate. To the solution, a solution in which aluminum nitrate and lanthanum nitrate were previously mixed was further added. Then, the solvent was removed by a centrifuge and dried at 120 °C for one day and night, and further calcined at 800 °C for 3 hours in air to obtain the target powder.

[0070] Next, while stirring, the powder adjusted with rhodium nitrate was added to distilled water, dried, and calcined to prepare a rhodium-supported powder. Then, while stirring, the rhodium-supported powder, an Al2O3-CeO2-ZrO2 composite oxide (Al2O3: 30% by mass / CeO2: 20% by mass / ZrO2: 44% by mass / Nd2O3: 2% by mass / La2O3: 2% by mass / Y2O3: 2% by mass), a CZ composite oxide (CeO2: 51.5% by mass / ZrO2: 45.5% by mass / Pr6O 11 : mass%), and an Al2O3-based binder were added to prepare a suspended slurry. Then, it was pressure-molded at a pressure of 1 ton by cold isostatic pressing (CIP), and then sieved while pulverizing to obtain a sample of Example 3.

[0071] 4-4. Comparative Example 1 A sample of Comparative Example 1 was obtained in the same manner as in Example 1, except that niobium butoxide was not used.

[0072] Here, in the sample of Comparative Example 1, the ratios of zirconium dioxide particles and porous carrier particles to the total of zirconium dioxide particles and porous carrier particles were 10.0% by mass of zirconium dioxide particles and 90.0% by mass of porous carrier particles, respectively, in that order.

[0073] The sample of Comparative Example 1 does not have niobium.

[0074] 4-5. Comparative Example 2 While stirring, rhodium nitrate and porous carrier particles were added to distilled water, and after drying and firing, a powder in which rhodium particles were supported on the porous carrier particles was obtained. This powder, an Al2O3-CeO2-ZrO2 composite oxide, CeO2, and an Al2O3-based binder were added to obtain a suspended slurry. The rest was carried out in the same manner as in Example 1 to obtain a sample of Comparative Example 2.

[0075] The sample of Comparative Example 2 has neither niobium nor zirconium dioxide particles.

[0076] 4-6. Comparative Example 3 A powder was obtained by adding niobium oxalate, zirconium oxynitrate, and porous carrier particles to distilled water and drying and firing them. A sample of Comparative Example 3 was obtained in the same manner as in Comparative Example 2, except that this powder was used instead of the porous carrier particles.

[0077] Here, in the sample of Comparative Example 3, the ratios of niobium, zirconium dioxide particles, and porous carrier particles to the total of niobium, zirconium dioxide particles, and porous carrier particles were, in order, niobium: 0.5% by mass, zirconium dioxide particles: 9.5% by mass, and porous carrier particles: 90.0% by mass.

[0078] Also, in the sample of Comparative Example 3, niobium is not doped into the zirconium dioxide particles.

[0079] 4-7. Comparative Example 4 While stirring, zirconium butoxide and niobium butoxide were added to hexane, and the temperature was raised to the boiling point of hexane, and stirring was continued for 2 hours in the boiling state. Then, hexane was removed with a centrifuge, dried at 120°C for one day and night, and further fired in air at 800°C for 3 hours to obtain a first powder. This powder is a powder of zirconium dioxide particles doped with niobium.

[0080] The rest was treated in the same manner as in Example 1 to obtain a sample of Comparative Example 4. Here, in the sample of Comparative Example 4, the ratios of niobium and zirconium dioxide particles to the total of niobium and zirconium dioxide particles were, in order, niobium: 5% by mass and zirconium dioxide particles: 95% by mass.

[0081] Comparative Example 4 does not have porous carrier particles.

[0082] 4-8. Comparative Example 5 While stirring, the first powder used in Comparative Example 4, rhodium nitrate, and porous carrier particles were added to distilled water, and after drying and firing, a powder in which rhodium particles were supported on the first powder and porous carrier particles used in Comparative Example 4 was obtained. This powder, an Al2O3-CeO2-ZrO2 composite oxide, CeO2, and an Al2O3-based binder were added to obtain a suspended slurry. The rest was treated in the same manner as in Example 1 to obtain a sample of Comparative Example 5.

[0083] Here, in the sample of Comparative Example 5, the ratios of niobium, zirconium dioxide particles, and porous carrier particles to the total of niobium, zirconium dioxide particles, and porous carrier particles were, in order, niobium: 0.2% by mass, zirconium dioxide particles: 9.8% by mass, and porous carrier particles: 90.0% by mass.

[0084] In Comparative Example 5, the zirconium dioxide particles were not substantially dispersed in the pores of the porous carrier particles.

[0085] 4-9. Comparative Example 6 A sample of Comparative Example 6 was obtained in the same manner as in Example 3, except that niobium oxalate was not used.

[0086] 5. Test 5-1. X-ray Diffraction Test To identify the crystal structure of zirconium dioxide particles in each sample, an X-ray diffraction test was carried out using an X-ray diffractometer. CuKα rays were excited for the measurement, and the scan speed was 0.05° / min, and the range of 10 - 90° was measured.

[0087] The results are shown in Table 1.

[0088] 5-2. Durability Test The samples of each example were respectively placed in a flow-through durability furnace, and a durability test was carried out by alternately repeating the flow of reducing gas (CO) and oxidizing gas (O2) at 1000°C for 5 hours. The flow rate and flow time of the reducing gas and oxidizing gas were respectively 20 L / min and flowed every 10 min.

[0089] The results are shown in Table 1.

[0090] 5-3. Exhaust Gas Purification Test After carrying out the above-mentioned "2-2. Durability Test", 2 g of each sample was placed in a flow-through reactor, heated to 500°C at a heating rate of 50°C / min in the evaluation model gas, held at this temperature for 10 minutes, and then cooled to 100°C. Next, it was heated at a heating rate of 20°C / min, and the NOx purification performance during heating was measured. Specifically, the temperature at which 50% of the NOx in the gas was purified was calculated.

[0091] The composition of the evaluation model gas is CO (0.65% by volume), CO2 (10.00% by volume), C3H6 (0.10% by volume), NO (0.15% by volume), O2 (0.70% by volume), H2 (3.00% by volume), and N2 (the remainder).

[0092] The results are shown in Table 1, and Figures 3 and 4.

[0093] 6. Results The manufacturing conditions and test results of each example are summarized in Table 1 below. Also, the results of the exhaust gas purification test are shown in Figures 3 and 4.

[0094]

Table 1

[0095] 3-1. X-ray Diffraction Test As shown in Table 1, zirconium dioxide particles undergo a phase transition from a monoclinic structure to a tetragonal structure by doping with niobium. Therefore, the presence or absence of nobium doping can be confirmed by examining the crystal structure of zirconium dioxide particles.

[0096] As shown in Table 1, in Examples 1, 2, and 3, and Comparative Examples 4, 5, and 7, zirconium dioxide was tetragonal. In contrast, in Comparative Examples 1 and 3, zirconium dioxide was monoclinic. In Comparative Example 2, zirconium dioxide was not used.

[0097] 3-2. Durability Test As shown in Table 1, in Examples 1 and 2 where zirconium dioxide was doped with niobium particles and the zirconium dioxide particles were supported within the pores of the porous carrier particles, the specific surface areas after the durability test were 89.6 m 2 / g and 88.3 m 2 / g, respectively. In contrast, in Comparative Example 4 without porous carrier particles, aggregation of zirconium dioxide particles was not suppressed, and the specific surface area after the durability test was 2.2 m 2 / g, a significantly lower value.

[0098] In Comparative Examples 1 and 3, since the zirconium dioxide particles were supported within the pores of the porous carrier particles in the same manner as in Examples 1 and 2, the specific surface areas after the durability test were equivalent to those of Examples 1 and 2. In Comparative Example 2, since there were no zirconium dioxide particles, the specific surface area within the pores of the porous carrier particles was larger than that of Examples 1 and 2, and therefore the specific surface area after the durability test was larger than that of Examples 1 and 2. In Comparative Example 5, since there were no zirconium dioxide particles within the pores of the porous carrier particles, the specific surface area after the durability test was the sum of that of the porous carrier particles and the zirconium dioxide particles existing outside the pores of the porous carrier particles. Since the specific surface area before the durability test was large, the specific surface area after the durability test was also large.

[0099] In Example 3 where zirconium dioxide was doped with niobium particles and the zirconium dioxide particles were dispersed inside the carrier particles, the specific surface area after the durability test was 22.6 m 2 / g. On the other hand, in Comparative Example 6 where niobium was not doped, the specific surface area after the durability test was 25.5 m 2 / g.

[0100] 3-3. Exhaust gas purification test As shown in FIG. 3 and Table 1, in Examples 1 and 2 where zirconium dioxide was doped with niobium and the zirconium dioxide particles were supported in the pores of the porous carrier particles, in the exhaust gas purification test after the durability test, the NOx 50% purification temperatures (°C) were 245.8 °C and 248.5 °C respectively, and the NOx purification rate reached 50% at a lower temperature than other examples.

[0101] Also, as shown in FIG. 4 and Table 1, in Example 3 where zirconium dioxide was doped with niobium particles and the zirconium dioxide particles were dispersed inside the carrier particles, in the exhaust gas purification test after the durability test, the NOx 50% purification temperature (°C) was 240.4 °C, and the NOx purification rate reached 50% at a particularly lower temperature than other examples.

Description of symbols

[0102] 1 and 2 Catalysts for exhaust gas purification 10 Rhodium particles 20 Zirconium dioxide particles 30 Porous carrier particles 40 Primary particles

Claims

1. rhodium particles, zirconium dioxide particles doped with a cation having an oxidation number higher than that of zirconium, and support particles and having wherein the rhodium particles are supported on the zirconium dioxide particles, and either (i) the support particles are porous support particles, and the rhodium particles and the zirconium dioxide particles are supported within the pores of the porous support particles, or (ii) the support particles are composed of a plurality of primary particles, and the rhodium particles and the zirconium dioxide particles are dispersed within the support particles an exhaust gas purification catalyst.

2. The exhaust gas purification catalyst according to claim 1, satisfying (i).

3. The exhaust gas purification catalyst according to claim 1, satisfying (ii).

4. The exhaust gas purification catalyst according to any one of claims 1 to 3, wherein the ratio of the mass of the zirconium dioxide particles to the total mass of the zirconium dioxide particles and the porous support particles is 1.0 to 20.0% by mass.

5. The exhaust gas purification catalyst according to any one of claims 1 to 3, wherein the cation is niobium, and the content of niobium in the zirconium dioxide particles is 1.0 to 7.0% by mass.

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