JP2010094625A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for cleaning an exhaust gas which enhances the cleaning efficiency of NOx at a low temperature of the exhaust gas. <P>SOLUTION: In the catalyst for cleaning an exhaust gas which includes such an occluding agent that NOx in the exhaust gas is occluded when the air-fuel ratio is leaner than a theoretical air-fuel ratio or that the NOx is released when the air-fuel ratio is near or richer than the theoretical air-fuel ratio, to a catalyst layer 2 formed on a carrier 1, and CePr-based composite oxide particles; the CePr-based composite oxide particles are compounded so that Ce, Pr and a catalyst metal may form oxide particles, at least parts of respective oxides of Ce and Pr are solid-dissolved into each other, and the catalyst metal is solid-dissolved into the oxide particles. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

    The present invention relates to an exhaust gas purification catalyst.

    A relatively large amount of NOx (nitrogen oxide) is contained in exhaust gas discharged from a diesel engine or a lean burn gasoline engine. In order to purify this NOx, NOx is occluded in an oxygen-excess atmosphere (when the air-fuel ratio of the exhaust gas is lean), and when the oxygen concentration in the atmosphere decreases (near the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio) A so-called lean NOx trap catalyst is used in which NOx stored in the exhaust gas is released and reduced and purified by a reducing agent component such as HC (hydrocarbon) in the exhaust gas.

    The lean NOx trap catalyst is generally composed of a combination of a catalyst metal such as Pt or Rh, a NOx occlusion material made of alkali metal, alkaline earth metal or rare earth metal, and an oxygen occlusion material made of Ce-containing oxide. Has been. In this case, the oxygen storage material functions to promote the storage of NOx by the NOx storage material.

That is, the oxygen storage material stores oxygen in an oxygen-excess atmosphere, and releases the oxygen as active oxygen when the oxygen concentration in the atmosphere decreases. This active oxygen promotes the oxidation of HC and CO by the catalytic metal, and the catalytic reaction heat increases the temperature of the NOx occlusion material, thereby increasing the NOx occlusion property of the NOx occlusion material. Further, the oxygen storage material has a property of causing an oxygen exchange reaction in which oxygen in the exhaust gas is taken in and exchanged with oxygen in the oxygen storage material due to a change in the valence of Ce ions even in an oxygen-excess atmosphere. The active oxygen released by the oxygen exchange reaction works to convert NO in the exhaust gas into NO ₂ that is easily stored in the NOx storage material.

    Known examples of the oxygen storage material include CeZr-based composite oxides and CePr-based composite oxides. For example, Patent Document 1 discloses a lean NOx catalyst for a spark ignition engine containing a CeZrSr composite oxide. Patent Document 2 discloses a lean NOx trap catalyst containing a CePr composite oxide.

Patent Document 3 discloses at least one surface dope selected from the group consisting of V, Mn, Cu, Mo, W, Pt, Tl, Pb and Bi as a composition for separating oxygen from air. A CePr composite oxide containing a small amount of an agent is disclosed. However, the surface dopant disclosed as an example is Ag, and there is no disclosure about specific examples using other metals such as Pt as the surface dopant. Further, this document does not disclose the oxygen storage / release performance of the CePr composite oxide, and does not disclose the use of the CePr composite oxide for exhaust gas purification.
JP 2004-267843 A JP 2003-245552 A JP 50-73893 A

    By the way, the exhaust gas of a diesel engine or a lean-burn gasoline engine has a low temperature of about 250 ° C. to 400 ° C. in a steady operation state, and a high NOx purification rate is required even in such a low temperature range. It has been. There is also a demand for an improvement in the NOx purification rate when the exhaust gas temperature is lower during the period from engine startup to steady operation. In addition, in a hybrid vehicle equipped with an engine and a motor as a power source, when a diesel engine is used, the engine is frequently started and stopped repeatedly, and the exhaust gas temperature is expected to be 250 ° C. or lower. In addition, when a mixed fuel obtained by adding alcohol to light oil is used as the fuel, the exhaust gas temperature is expected to be further lowered.

    That is, an object of the present invention is to improve the NOx purification performance of the catalyst when the exhaust gas temperature is low.

    In order to solve the above-mentioned problems, the present invention configures an exhaust gas purification catalyst using a CePr-based composite oxide in which a catalytic metal is dissolved.

That is, according to the present invention, when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, NOx in the exhaust gas is occluded in the catalyst layer formed on the carrier, and near the stoichiometric air-fuel ratio or from the stoichiometric air-fuel ratio. In the exhaust gas purifying catalyst containing the NOx storage material that releases the NOx when it is also rich, and the CePr-based composite oxide particles,
The CePr-based composite oxide particles are composed of Ce, Pr, and a catalyst metal so as to form oxide particles, and at least a part of each of the Ce oxide and the Pr oxide is fixed to each other. The catalyst metal is dissolved in the oxide particles.

    Here, since the CePr-based composite oxide particles combine Ce and Pr as metal components, high oxygen storage / release performance can be obtained, and further, the catalyst metal dissolved in the CePr-based composite oxide can be obtained. Oxygen storage and release of particles is promoted, and high oxygen storage and release performance can be obtained even in a low temperature range. The CePr-based composite oxide particles have a function of adsorbing NOx in the exhaust gas.

Therefore, even when the temperature of the exhaust gas is low, in the oxygen-excess atmosphere with a high oxygen concentration (when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio), the CePr-based composite oxide particles are the first. the active oxygen released by oxygen exchange reactions mentioned, by the conversion of NO in the exhaust gas to NO _2, increase the NOx occlusion of the NOx-absorbing material. On the other hand, when the oxygen concentration of the exhaust gas decreases (in the vicinity of the stoichiometric air-fuel ratio or when it becomes richer than the stoichiometric air-fuel ratio), the active oxygen released by the CePr-based composite oxide particles causes HC and CO in the exhaust gas. As the oxidation proceeds, the catalyst temperature, that is, the temperature of the NOx occlusion material becomes higher, and the NOx occlusion / release property becomes higher. Moreover, since the CePr-based composite oxide particles have a function of adsorbing NOx in the exhaust gas, NOx in the exhaust gas is suppressed from being unpurified when the temperature of the exhaust gas is low.

    The NOx storage material can be composed of at least one selected from alkali metals, alkaline earth metals and rare earth metals.

It is preferable that a part of the catalyst metal dissolved in the CePr-based composite oxide particles is dispersedly exposed on the surface of the composite oxide particles. Thus, the oxygen storage performance of the composite oxide particles is increased, HC and CO oxidation in the exhaust gas by the composite oxide particles, and which is advantageous in the oxidation of NO into NO _2.

    The concentration of the catalyst metal in the surface layer region from the surface of the CePr-based composite oxide particles to a depth of 2 nm is preferably 2.5 times or less of the average concentration of the Pt in the entire composite oxide particles. The surface Pt concentration ratio is more preferably less than 2.0 times, further preferably 1.8 times or less, and more preferably 1.5 times or less.

    That is, in the conventional composite oxide particles, most of the catalyst metal is supported on the particle surface as an oxide, and therefore the catalyst metal concentration on the particle surface is high. The catalytic metal supported as such an oxide is bonded to Ce or the like through oxygen in an oxygen-excess atmosphere, but when the oxygen concentration in the atmosphere decreases, the bond is broken and agglomerates / sinters. It is considered easy. Therefore, when such a composite oxide is exposed to high-temperature exhaust gas, the oxygen storage / release performance or the exhaust gas purification performance of the composite oxide is greatly deteriorated due to sintering of the catalyst metal on the particle surface.

    On the other hand, in the present invention, the catalyst metal concentration of the particle surface layer is suppressed to 2.5 times or less of the average concentration of the entire particle as described above, which means that it is supported as an oxide on the particle surface. It means that the amount of catalytic metal is small, that is, most of the catalytic metal is dissolved in the composite oxide particles. Therefore, even if the composite oxide particles are exposed to high-temperature exhaust gas, it is difficult for the catalyst metal to agglomerate and sinter, and even if agglomeration / sintering, the degree of sintering is low. In other words, even if the catalytic metal on the particle surface is sintered, the oxygen storage / release performance or the active point that works effectively for purification of exhaust gas is not greatly reduced. Moreover, due to the solid solution of the catalyst metal, sintering of the composite oxide particles itself is suppressed, so that a wide pore volume is ensured even when exposed to high-temperature exhaust gas, and good gas diffusibility is maintained. Is done.

    The catalyst metal solid-dissolved in the CePr-based composite oxide particles is Pt, and the catalyst layer is impregnated with a Pt solution in addition to Pt solid-dissolved in the CePr-based composite oxide particles. Preferably, supported Pt is provided.

As a result, Pt impregnated and supported on the CePr-based composite oxide particles converts NO in the exhaust gas into NO ₂ that is easily stored in the NOx storage material, and oxidizes and purifies HC and CO in the exhaust gas. Thus, the catalyst temperature, and thus the NOx storage material temperature, is increased to increase the NOx storage performance, which is advantageous in improving the NOx purification performance.

    The ratio of the solid solution Pt amount in the total amount of the solid solution Pt amount dissolved in the CePr-based composite oxide particles and the Pt amount impregnated and supported on the catalyst layer (hereinafter referred to as “dope”). The ratio is preferably 20% by mass or more and 60% by mass or less.

    That is, according to experiments, the NOx purification rate varies depending on the dope ratio, and the NOx purification rate increases when the dope ratio is 20 mass% or more and 60 mass% or less. A more preferable dope ratio is 20 mass% or more and 45 mass% or less.

    The carrier is provided with a catalyst layer containing oxide particles supporting Rh, and a catalyst layer containing the NOx storage material and CePr-based composite oxide particles is disposed below the catalyst layer. Preferably it is.

    As a result, in an oxygen-excess atmosphere where the oxygen concentration of the exhaust gas is high, the lower catalyst layer containing the NOx storage material and the CePr-based composite oxide particles mainly stores NOx and suppresses NOx emission. . When the oxygen concentration of the exhaust gas decreases, Rh supported on the oxide particles in the upper catalyst layer reduces and purifies NOx released from the NOx occlusion material using HC and CO in the exhaust gas as a reducing agent. To work effectively.

    As described above, according to the present invention, in the exhaust gas purification catalyst in which the catalyst layer formed on the support contains the NOx storage material and the CePr-based composite oxide particles, the CePr-based composite oxide particles Since the catalyst metal is in solid solution, the CePr-based composite oxide enhances the NOx occlusion of the NOx occlusion material and adsorbs NOx, so that NOx is not present even when the exhaust gas temperature is low. The exhaust of the purification is suppressed, and the oxidation of HC and CO in the exhaust gas when the oxygen concentration of the exhaust gas is reduced easily proceeds. Therefore, the temperature of the NOx occlusion material is utilized using the heat of the oxidation reaction. And NOx occlusion / release performance can be improved, which is advantageous for improving the NOx purification performance of the exhaust gas purification catalyst.

    Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that the following description of the preferred embodiment is merely illustrative in nature, and is not intended to limit the present invention, its application, or its use.

    FIG. 1 shows the configuration of an exhaust gas purifying catalyst that purifies exhaust gas discharged from a diesel engine or a lean burn gasoline engine, particularly purifies NOx in the exhaust gas. In the figure, 1 is a cell wall of the honeycomb carrier, 2 is a lower catalyst layer formed on the cell wall 1, and 3 is an upper catalyst layer laminated on the lower catalyst layer 2. The honeycomb carrier 1 can be formed of a heat resistant inorganic material such as cordierite.

    Preferably, CePr-based composite oxide particles and activated alumina particles in which Pt as a catalyst metal is dissolved (doped) are arranged in the lower catalyst layer 2, and activated alumina carrying Rh as the catalyst metal in the upper catalyst layer 3. The particles are arranged, and the upper and lower catalyst layers 2 and 3 are impregnated with and supported by Pt, Rh and NOx storage material components. This impregnation support can be performed by impregnating the catalyst layers 2 and 3 with Pt solution, Rh solution and NOx storage material solution, followed by drying and firing. As the NOx occlusion material, an alkali metal, an alkaline earth metal or a rare earth metal can be employed, and an alkaline earth metal such as Ba or Sr is particularly preferable.

In this case, in an oxygen-excess atmosphere, Pt supported on the activated alumina of the lower catalyst layer 2 converts NO in the exhaust gas into NO ₂ that is easily stored in the NOx storage material. The Pt-doped CePr-based composite oxide powder occludes oxygen in an oxygen-excess atmosphere and releases the occluded oxygen as active oxygen when the oxygen concentration in the atmosphere decreases. As a result of the change in the valence of oxygen, oxygen in the exhaust gas is taken in and exchanged with oxygen in the composite oxide particles to cause an oxygen exchange reaction. The active oxygen released by the oxygen exchange reaction is converted into NO in the exhaust gas. Works to convert NO to NO ₂ . When the oxygen concentration of the exhaust gas decreases, Pt supported on the activated alumina oxidizes HC and CO in the exhaust gas by the active oxygen released from the composite oxide powder, and raises the catalyst temperature by the reaction heat. On the other hand, NOx released from the NOx storage material is reduced and purified using HC or CO as a reducing agent. In addition, Rh supported on the activated alumina of the upper catalyst layer 3 works effectively for reducing and purifying NOx using HC and CO as a reducing agent in a state where the oxygen concentration in the atmosphere is reduced.

    An important feature of the present invention resides in that CePr-based composite oxide particles doped with a catalytic metal are used for the exhaust gas purification catalyst. Hereinafter, the characteristics of the exhaust gas purifying catalyst will be described focusing on this point.

<TEM photograph of Pt-doped CePr-based composite oxide particles>
FIG. 2 is a TEM (transmission electron microscope) photograph of the Pt-doped CePr-based composite oxide particles according to the present invention after being heated to a temperature of 1000 ° C. for 24 hours in an air atmosphere. This composite oxide particle is a CePr-based composite oxide particle in which Pt as a catalyst metal is solid-solved so as to be arranged between crystal lattices or atoms, and the composition formula excluding Pt is Ce _0.9 Pr _{0. 1} O ₂ . The Pt doping amount of the composite oxide particles, that is, the Pt concentration (average concentration) of the entire particles is 0.5% by mass. The arrows in the TEM photograph indicate the Pt particles that are dispersed and exposed on the surface of the composite oxide particles, and the diameter of the Pt particles is 3 nm or less. The Pt doping amount is preferably 0.1% by mass or more and 2.0% by mass or less.

<Method for producing Pt-doped CePr-based composite oxide powder>
An acidic solution containing Ce ions, Pr ions, and a hydroxo complex of Pt is prepared. Cerium (III) nitrate hexahydrate as the Ce source, praseodymium nitrate (III) nitrate hexahydrate as the Pr source, hexahydroxoplatinum (IV) acid ethanolamine solution or hexagonal as the Pt source (hydroxo complex) Hydroxoplatinum (IV) acid nitric acid solution can be respectively employed. A predetermined amount of each of these Ce source, Pr source and Pt source and water are mixed to obtain a raw material solution (acidic). FIG. 3 shows the structure of the hexahydroxoplatinum (IV) complex ion.

    A basic solution is added to and mixed with the raw material solution to produce precipitated particles of Ce, Pr and Pt composite hydroxides, which are precursors of the composite oxide particles. In this case, after stirring the raw material solution at room temperature for about 1 hour, an aqueous ammonia having a concentration of, for example, about 7% may be added thereto as a basic solution. Other basic solutions such as aqueous caustic soda can also be employed.

    The solution containing the particle precursor precipitate is centrifuged to remove the supernatant. The water washing and dehydration operation of adding ion-exchanged water to the precipitated dehydrated product from which the supernatant has been removed, stirring, and re-centrifuged (dehydrated) is repeated as many times as necessary. The excess basic solution is removed by the washing / dehydrating operation.

    The precipitate dehydrated product is dried, fired and pulverized. Drying can be performed, for example, by holding at a temperature of about 100 ° C. to 250 ° C. for a predetermined time in an air atmosphere. Moreover, baking can be performed by hold | maintaining at the temperature of about 400 to 600 degreeC for several hours, for example in an atmospheric condition.

    As a result, Ce, Pr, and Pt are combined so as to form oxide particles, and at least a part of each of the Ce oxide and the Pr oxide is in solid solution, and Pt is dissolved in the oxide particles. As a result, composite oxide particles that are partly dispersed and exposed on the surface of the oxide particles can be obtained.

<Pt concentration of various composite oxide particle surface layers>
The composite oxide powders of Examples 1 to 4 and Comparative Example 1 described below were prepared, and it was examined how many times the average concentration of Pt in each surface layer region was Pt.

Example 1
A composite oxide powder according to Example 1 was prepared according to the above-described method for producing a Pt-doped CePr-based composite oxide powder using a hexahydroxoplatinum (IV) acid ethanolamine solution as a Pt source. The Ce / Pr molar ratio was 9/1. The amount of Pt source charged was adjusted so that the Pt concentration of the entire particle was 0.5% by mass. The obtained composite oxide powder is described as “Ethanol Pt-doped Ce / Pr = 9/1” or “Ethanol Pt-doped Ce _0.9 Pr _0.1 O ₂ ” in the drawings or tables.

-Example 2-
A composite oxide powder according to Example 2 was prepared under the same conditions and method as in Example 1 except that the Ce / Pr molar ratio was 1/9. The obtained composite oxide powder is described as “Ethanol Pt-doped Ce / Pr = 1/9” or “Ethanol Pt-doped Ce _0.1 Pr _0.9 O ₂ ” in the drawings or tables.

-Example 3-
A composite oxide powder according to Example 3 was prepared under the same conditions and method as in Example 1 except that a nitric acid solution of dinitrodiamine platinum (II) shown in FIG. . In the case of the nitric acid solution Pt source of dinitrodiamine platinum (II), even if a basic solution such as ammonia is added, the coprecipitation as Pt hydroxide is about 80%. By adjusting the charged amount of the Pt source to 1.25 times the target value, the Pt concentration of the entire particle was adjusted to 0.5% by mass. The obtained composite oxide powder is described as “Pt—P doped Ce / Pr = 9/1” or “Pt—P doped Ce _0.9 Pr _0.1 O ₂ ” in the drawings or tables.

Example 4
A composite oxide powder according to Example 4 was prepared under the same conditions and method as in Example 3 except that the Ce / Pr molar ratio was 1/9. The obtained composite oxide powder is described as “Pt—P doped Ce / Pr = 1/9” or “Pt—P doped Ce _0.1 Pr _0.9 O ₂ ” in the drawings or tables.

-Comparative Example 1-
A CePr composite oxide powder having a Ce / Pr molar ratio of 9/1 and a dinitrodiamine platinum (II) nitric acid solution were mixed and evaporated to dryness to prepare a composite oxide powder according to Comparative Example 1. The amount of Pt supported was 0.5% by mass of the composite oxide powder. The obtained composite oxide powder is described as “Pt—P dry Ce / Pr = 9/1” or “Pt—P dry Ce _0.9 Pr _0.1 O ₂ ” in the drawings or tables.

-Pt concentration measurement-
The Pt concentration in the surface layer region (region from the surface to a depth of 2 nm) of each composite oxide particle of Examples 1 to 4 and Comparative Example 1 was measured by XPS (X-ray photoelectron spectroscopy) analysis, and the surface Pt concentration ratio (surface layer) It is known that the penetration depth of the characteristic X-ray from the surface depends on the intensity of the characteristic X-ray, and this time the intensity of 1000 eV The above “2 nm” was specified from the above X-rays.) The results are shown in FIG.

    Looking at the surface Pt concentration ratio of each composite oxide particle, Examples 1-4 are 2.5 times or less, but in Comparative Example 1, the concentration ratio is larger than 2.5 times. That is, in Examples 1 to 4, the amount of Pt supported as an oxide on the surface of the composite oxide particles is less than that of Comparative Example 1, and most of Pt is dissolved in the composite oxide particles. It can be said. The surface Pt concentration ratio is preferably less than 2.0 times, more preferably 1.8 times or less, and further preferably 1.5 times or less. Further, the surface Pt concentration ratio is preferably larger than 1 time. This is for the purpose of allowing the catalytic metal exposed on the surface of the composite oxide particles to exist as an intermediate for storing and releasing oxygen and to contribute to exhaust gas purification.

<Pore volume, pore diameter, crystallite diameter, specific surface area of various composite oxide particles>
In addition to the composite oxide powders of Examples 1 to 4 and Comparative Example 1, the composite oxide powders of the following Examples 5 to 7 and Comparative Examples 2 and 3 were further prepared. I investigated.

-Example 5
A composite oxide powder according to Example 5 was prepared under the same conditions and method as in Example 1 except that the Ce / Pr molar ratio was 7/3. The obtained composite oxide powder is described as “Ethanol Pt-doped Ce / Pr = 7/3” or “Ethanol Pt-doped Ce _0.7 Pr _0.3 O ₂ ” in the drawings or tables.

-Example 6
A composite oxide powder according to Example 6 was prepared under the same conditions and method as in Example 1 except that the Ce / Pr molar ratio was 5/5. The obtained composite oxide powder is described as “Ethanol Pt-doped Ce / Pr = 5/5” or “Ethanol Pt-doped Ce _0.5 Pr _0.5 O ₂ ” in the drawings or tables.

-Example 7-
A composite oxide powder according to Example 7 was prepared under the same conditions and method as Example 3 (Pt—P dope) except that the Ce / Pr molar ratio was 5/5. The obtained complex oxide powder is described as “Pt—P doped Ce / Pr = 5/5” or “Pt—P doped Ce _0.5 Pr _0.5 O ₂ ” in the drawings or tables.

-Comparative Example 2-
A composite oxide powder according to Comparative Example 2 was prepared under the same conditions and method as Comparative Example 1 (Pt—P dried) except that the Ce / Pr molar ratio was 5/5. The obtained composite oxide powder is described as “Pt—P dry Ce / Pr = 5/5” or “Pt—P dry Ce _0.5 Pr _0.5 O ₂ ” in the drawings or tables.

-Comparative Example 3-
A composite oxide powder according to Comparative Example 3 was prepared under the same conditions and method as Comparative Example 1 (Pt—P dried) except that the Ce / Pr molar ratio was 1/9. The obtained composite oxide powder is described as “Pt—P dry Ce / Pr = 1/9” or “Pt—P dry Ce _0.1 Pr _0.9 O ₂ ” in the drawings or tables.

-Measurement results of pore volume, pore diameter, crystallite diameter, specific surface area-
About each complex oxide of the said Examples 1-7 and Comparative Examples 1-3, after performing the aging which heats to the temperature of 750 degreeC or 1000 degreeC in air | atmosphere for 24 hours, pore volume, pore diameter, crystallite Table 1 shows the measurement results of the diameter and BET specific surface area. In Table 1, “pore diameter” is an average pore diameter, and “crystallite diameter” is an X-ray diffractometer, and Scherrer's formula (crystallite diameter (hkl) = 0.9λ / (β _1/2 (Cos θ), where hkl is the Miller index, λ is the characteristic X-ray wavelength (angstrom), β _1/2 is the half width (radian) of the (hkl) plane, and θ is the X-ray reflection angle. Determined by In addition, “Ethanol Pt-doped Ce / Pr = 1/9” and “Pt-P dried Ce / Pr = 1/9” cause Pr ₆ O ₁₁ phase separation by aging at 1000 ° C. × 24 hours. The crystallite size after aging is an average value of the parent phase oxide and the phase separation oxide.

    The “Ethanol Pt-doped” product is an exception in the case of Ce / Pr = 1/9, but basically, unlike the “Pt-P-doped” product and the “Pt-P dry” product, The pore volume is larger than the pore size, and the BET specific surface area is also larger. This means that the “Ethanol Pt-doped” product has a large number of pores and good gas diffusibility, and therefore has high oxygen storage / release performance. In addition, comparing the “Pt—P dope” product and the “Pt—P dry” product, the “Pt—P dope” product has a smaller pore volume and pore size than the “Pt—P dry” product. On the other hand, the former has a larger BET specific surface area. Therefore, it is considered that the “Pt—P dope” product has a larger number of pores than the “Pt—P dry” product, and the gas diffusibility is good and the oxygen storage / release performance is high.

    In addition, looking at the average crystallite diameter according to Scherrer formula, the “Ethanol Pt-doped” product is smaller than the “Pt-P dry” product, and the grain growth due to aging is not significant, that is, the heat resistance is high. I understand that.

Here, in the complex oxide of “EthanolPt dope” having a Ce / Pr molar ratio of 1/9 or more and 9/1 or less, the crystallite diameter by Scherrer formula after aging at 1000 ° C. × 24 hours is less than 60 nm. it is preferable that the pore volume after 750 ° C. × 24 hours aging is 0.12 cm ³ / g or more, or the average pore size after 750 ° C. × 24 hours aging is less than 35 nm.

<Oxygen storage and release performance>
With respect to the composite oxide powders of Examples 1 to 3, 5 to 7 and Comparative Example 1, and the “EthanolPt-doped CeO ₂ ” powder, the oxygen storage / release amount after aging that is heated to a temperature of 750 ° C. for 24 hours in the air atmosphere. Examined. “Ethanol Pt-doped CeO ₂ ” powder was prepared in the production method of Example 1 with the amount of Pr source being zero, and the Pt concentration was 0.5 mass%. In measuring the amount of oxygen occluded and released, the temperature was increased at a rate of 20 ° C./min while supplying 5% O ₂ gas (residue; He) at a flow rate of 100 mL / min to 0.10 g of the test material. After maintaining at a temperature of 600 ° C. for 20 minutes, a pretreatment (oxygen storage treatment) for cooling to room temperature was performed. Then, while supplying 2% CO gas (residue; He) at a flow rate of 100 mL / min, the temperature was increased at a rate of 10 ° C./min, and the change in the amount of CO ₂ released from the test material depending on the temperature. Was measured. The CO ₂ release amount corresponds to the oxygen release amount of the test material.

The results are shown in FIG. All of the complex oxide powders doped with Pt have a higher oxygen release amount than that of the Pt-supported composite oxide powder, and the oxygen release amount increases in the temperature range of 150 ° C. to 200 ° C. or higher. ing. In addition, when the composite oxide powders doped with Pt are compared, the “EthanolPt-doped” product and the “Pt-P-doped” product with Ce / Pr = 9/1 (Ce _0.9 Pr _0.1 O ₂ ) are used. The former has a larger oxygen release amount than the latter, and its release peak temperature is on the low temperature side. Looking at the “EthanolPt-doped” product and the “Pt-P-doped” product with Ce / Pr = 5/5 (Ce _0.5 Pr _0.5 O ₂ ), the former has a larger oxygen release amount than the latter, And the release peak temperature is on the low temperature side. Further, all of the EthanolPt dope examples release a relatively large amount of oxygen in a wide temperature range from low temperature to high temperature. From the above, the composite oxide powder doped with Pt has high heat resistance and excellent oxygen storage / release performance (particularly, the composite oxide powder of “Ethanol Pt dope” is excellent), and therefore, exhaust gas. It turns out that it is useful for the catalyst for purification | cleaning. In addition, regarding “EthanolPt-doped Ce _0.1 Pr _0.9 O ₂ ”, the peak of the oxygen release amount appears in two places, the low temperature portion and the high temperature portion, because of the phase separation of Pr ₆ O _11. It is thought that there is.

FIG. 7 shows 50 ° C. to 600 ° C. of the “EthanolPt dope” product of the example in which Ce / Pr = 9/1 (Ce _0.9 Pr _0.1 O ₂ ) and the “Pt-P dry” product of the comparative example. Comparison of oxygen release amounts at From the figure, it can be seen that the embodiment has a large oxygen release amount.

FIG. 8 is based on the data of Examples 1, 2, 5, 6 and “Ethanol Pt-doped CeO ₂ ” in FIG. 6, and regarding the “Ethanol Pt-doped” product, the Pr / (Ce + Pr) molar ratio is 50 ° C. to 600 ° C. This shows the effect of oxygen on the amount of oxygen released (total amount). In the figure, “□” is a plot for a “Pt—P dried Ce _0.9 Pr _0.1 O ₂ ” product. According to the figure, it can be seen that the oxygen release amount increases as the Pr ratio increases.

<NOx adsorption / desorption characteristics of Pt-doped CePr-based composite oxide>
For each of the composite oxide powders of Examples 1, 2, 5, 6 and “Ethanol Pt-doped CeO ₂ ” and “Ethanol Pt-doped Pr ₆ O ₁₁ ”, the NOx adsorption / desorption characteristic is NO-TPD (temperature-programmed desorption). It was examined by testing. "EthanolPt doped _Pr 6 _{O 11"} powder is one prepared for zero amount of Ce source in the preparation process of Example 1, Pt concentration of 0.5% by weight.

First, while supplying a reducing gas (H ₂ ; 0.45%, residual He, flow rate: 100 mL / min) to the test powder, the gas temperature was increased from room temperature (25 ° C.) at a rate of 30 ° C./min. And kept at a temperature of 600 ° C. for 10 minutes, and then the gas temperature was returned to room temperature. Then subjected試粉end to the NO-containing gas _{(NO; 4000ppm, O 2;} 3.0%, remaining He, flow rate; 100 mL / min) were fed for 15 minutes at room temperature, then, He gas (flow rate; 100 mL / min ), The gas temperature was increased to 600 ° C. at a rate of 20 ° C./min to desorb NOx from the test powder, and the amount of desorption was measured.

    The results are shown in FIG. From this figure, CePr-based composite oxides have a high NOx adsorption performance when a small amount of Pr is added (when the Pr / (Ce + Pr) molar ratio is 20% or less), and NOx increases as the Pr ratio increases. Although the amount of desorption tends to decrease, it is understood that a relatively high NOx adsorption performance can be obtained even if the Pr ratio increases, not a large decrease.

<Relationship between Pt doping ratio of exhaust gas purification catalyst and NOx purification performance>
In the configuration of the exhaust gas purifying catalyst shown in FIG. 1, (composition except _{Pt; Ce 0.9 Pr 0.1 O 2} ) to the lower catalyst layer 2 Pt doped CePr composite oxide Place powder and activated alumina, Four kinds of test catalysts having different Pt dope ratios in which Rh-supported activated alumina powder is disposed in the upper catalyst layer 3 and the upper and lower catalyst layers 2 and 3 are impregnated with Ba and Sr as Pt, Rh and NOx storage materials. Was prepared.

    The supported amount per liter of the carrier was 135 g / L for the CePr composite oxide powder and the activated alumina powder in the lower catalyst layer 2 and 50 g / L for the activated alumina powder in the upper catalyst layer 3. The amount of Pt supported is such that the total amount of the amount doped in the CePr composite oxide and the amount impregnated and supported on the catalyst layers 2 and 3 is 2.0 g / L. Was 0.1 g / L, the impregnation load of Ba was 30 g / L, and the impregnation load of Sr was 10 g / L. The amount of Rh supported on the upper catalyst layer 3 as Rh-supported activated alumina powder was 0.4 g / L.

    The four types of test catalysts differ in the Pt doping amount of the CePr composite oxide particles, and the doping amounts are 0% by mass, 0.1% by mass, 0.5% by mass, and 1.0% by mass. Therefore, the Pt dope ratio (Pt amount by dope / (Pt amount by dope + impregnated supported Pt amount)) of each of the four types of test catalysts is calculated as 0%, 6.75%, 33.75%, 67.5%. Each Pt-doped CePr composite oxide powder was prepared in the same manner as in Example 1 using a hexahydroxoplatinum (IV) acid ethanolamine solution as the Pt source.

    Thus, the test catalyst was aged for 24 hours in an air atmosphere at 750 ° C., and then the NOx purification performance was examined using a model gas flow reactor and an exhaust gas analyzer. First, the pretreatment gas shown in Table 2 was allowed to flow at a temperature of 600 ° C. for 10 minutes, and then a lean (A / F = 22) model exhaust gas was allowed to flow for 60 seconds to enrich the gas composition (A / F = 14. 5) After switching the model exhaust gas to the model exhaust gas for 60 seconds and repeating the cycle several times, the lean NOx purification rate for 60 seconds from the time when the gas composition was switched from rich to lean and the lean to rich were switched. The rich NOx purification rate for 60 seconds from the time was measured, and the average value was obtained. The composition of the lean model exhaust gas and the rich model exhaust gas is as shown in Table 2, and the space velocity was 35000 / h. Further, the model exhaust gas has three temperatures of 150 ° C., 175 ° C., and 200 ° C.

    The results are shown in FIG. The NOx purification rate is the highest when the Pt doping ratio is 33.75% (Pt doping amount = 0.5 mass%). This is presumably because the oxygen storage / release performance of the CePr composite oxide particles was enhanced by doping Pt. The NOx purification rate when the Pt doping ratio is 6.75% (Pt doping amount 0.1% by mass) is lower than when the Pt doping rate is 0%. Then, it is considered that the effect of improving the oxygen storage / release performance due to the Pt doping is not remarkable, and the effect of the NOx purification performance reduction due to the decrease in the amount of impregnated Pt due to the Pt doping is greater. When the Pt doping ratio increases beyond 33.75%, the NOx purification rate decreases. This is considered to be because the effect of lowering the NOx purification performance due to the decrease in the amount of impregnated Pt was increased.

    From the NOx purification rate measurement result at a temperature of 150 ° C., it can be said that the Pt dope ratio is preferably 20% or more and 60% or less. From the NOx purification rate measurement result at a temperature of 200 ° C., the Pt dope ratio is 20%. It can be said that it is more preferable to make it 45% or less.

<Relation between Pr ratio of CePr composite oxide and NOx purification performance>
A plurality of Pt-doped CePr composite oxide powders in which Pr / (Ce + Pr) mol% is changed with 0.5% by mass of Pt doping, and Pr / (Ce + Pr) with 0.5% by mass of Pt-doping. ) A plurality of Pt-supported CePr composite oxide powders with different mol% were prepared. As in Example 1, each Pt-doped CePr composite oxide powder was prepared by coprecipitation using a hexahydroxoplatinum (IV) acid ethanolamine solution as a Pt source. As in Comparative Example 1, all of the Pt-supported CePr composite oxide powders were prepared by evaporation to dryness using a dinitrodiamine platinum (II) nitric acid solution as a Pt source.

    Then, using each of the Pt-doped CePr composite oxide powder and the Pt-supported CePr composite oxide powder, 50 g / L of Rh-supported activated alumina powder was added to the upper catalyst layer 3 in the same manner as in the previous evaluation of NOx purification performance. (Rh; 0.4 g / L), 135 g / L of Pt-doped CePr composite oxide powder and 135 g / L of active alumina powder are arranged in the lower catalyst layer 2, and Pt is added to both catalyst layers 2 and 3. Each test catalyst was impregnated and supported by 1.325 g / L, Rh 0.1 g / L, Ba 30 g / L, and Sr 10 g / L.

    For each of the above test catalysts, the lean NOx purification rate and the rich NOx purification rate at the model exhaust gas temperatures of 200 ° C., 250 ° C., and 300 ° C. were measured under the same conditions and methods as in the previous evaluation of the NOx purification characteristics. The results are shown in FIG. “CePrPt composite oxide” in the figure means a test catalyst made of Pt-doped CePr composite oxide powder, and “Pt-supported CePr composite oxide” means a test catalyst made of Pt-supported CePr composite oxide powder.

    According to the figure, Examples (black-colored ◆, ■, and ▲) in which Pt-doped CePr composite oxide powder was prepared by a coprecipitation method using a hexahydroxoplatinum (IV) acid ethanolamine solution as a Pt source In addition, the NOx purification rate is higher than that of comparative examples (open ◇, □, and Δ) employing the evaporation to dryness method. Except for the lean NOx purification rate (♦ and ◇) at 200 ° C., all the others are decreasing as the Pr / (Ce + Pr) mol% increases. This is the NOx adsorption shown in FIG. This agrees with the tendency of desorption characteristics, and it can be said that as the Pr / (Ce + Pr) mol% increases, the NOx adsorption amount decreases, and as a result, the NOx purification rate also decreases.

    In addition, although the metal component of the composite oxide of the said Example is Ce, Pr, and Pt, other metal components, such as Zr, can also be mix | blended with this.

It is sectional drawing which shows the structure of the exhaust gas purification catalyst which concerns on the Example of this invention. It is a TEM photograph of complex oxide particles concerning an example of the present invention. It is a figure which shows the structural formula of a hexahydroxo platinum (IV) complex ion. It is a figure which shows the structural formula of dinitrodiamine platinum (II). It is a graph which shows the surface Pt density | concentration ratio of various composite oxide particles. It is a graph which shows the temperature change of the oxygen storage-and-release amount of various composite oxide powder. It is the graph which compared the oxygen release amount of the complex oxide which concerns on the Example of this invention, and the conventional complex oxide. It is a graph which shows the relationship between Pr / (Ce + Pr) molar ratio of the complex oxide which concerns on the Example of this invention, and oxygen release amount. It is a graph which shows the relationship between the (Pr / (Ce + Pr) molar ratio and NOx adsorption / desorption performance of Pt dope CePr complex oxide powder. It is a graph which shows the relationship between the Pt dope ratio with respect to the CePr complex oxide powder of the exhaust gas purification catalyst, and a NOx purification rate. It is a graph which shows the relationship between the (Pr / (Ce + Pr) molar ratio and NOx purification rate of Pt dope CePr complex oxide powder of the exhaust gas purification catalyst.

Explanation of symbols

1 Support 2 Upper catalyst layer 3 Lower catalyst layer

Claims (6)

When the catalyst layer formed on the carrier stores NOx in the exhaust gas when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, and is near the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio In the exhaust gas purifying catalyst containing the NOx storage material for releasing the NOx and CePr-based composite oxide particles,
The CePr-based composite oxide particles are composed of Ce, Pr, and a catalyst metal so as to form oxide particles, and at least a part of each of the Ce oxide and the Pr oxide is fixed to each other. An exhaust gas purifying catalyst, wherein the catalyst metal is dissolved in the oxide particles. In claim 1,
An exhaust gas purifying catalyst, wherein a part of the catalyst metal solid-dissolved in the CePr-based composite oxide particles is dispersed and exposed on the surface of the composite oxide particles. In claim 1 or claim 2,
The concentration of the catalytic metal in the surface layer region from the surface of the CePr-based composite oxide particles to a depth of 2 nm is 2.5 times or less of the average concentration of the catalytic metal in the entire composite oxide particles, Exhaust gas purification catalyst. In any one of Claim 1 thru | or 3,
The catalyst metal solid-dissolved in the CePr-based composite oxide particles is Pt, and the catalyst layer is impregnated with a Pt solution in addition to Pt solid-dissolved in the CePr-based composite oxide particles. An exhaust gas purifying catalyst characterized in that a supported Pt is provided. In claim 4,
The ratio of the solid solution Pt amount in the total amount of the solid solution Pt amount dissolved in the CePr-based composite oxide particles and the Pt amount impregnated and supported in the catalyst layer is 20% by mass or more and 60% by mass. % Or less, an exhaust gas purification catalyst characterized by being less than or equal to%. In any one of Claims 1 thru | or 5,
The carrier is provided with a catalyst layer containing oxide particles supporting Rh, and a catalyst layer containing the NOx storage material and CePr-based composite oxide particles is disposed below the catalyst layer. An exhaust gas purifying catalyst characterized by comprising: