JP4179215B2 - Catalyst for exhaust gas purification of internal combustion engine - Google Patents

Description

  The present invention relates to a catalyst for purifying exhaust gas discharged from an internal combustion engine.

  Among exhaust gases emitted from internal combustion engines such as automobiles, CO (carbon monoxide) and HC (hydrocarbon) are generated due to incomplete combustion of fuel, and NOx (nitrogen oxide) is in the air. Generated by high temperature oxidation of nitrogen. A catalyst that simultaneously purifies the three components of CO, HC, and NOx is called a three-way catalyst. Specifically, a precious metal such as Pt, Rh, and Pd is highly dispersed in a metal oxide carrier such as alumina having a large specific surface area. The structure carried on is taken.

Although CO and HC are oxidized and NOx is reduced by the above three-way catalyst, it is important that the stoichiometric ratio of the oxidizing substance and the reducing substance is equivalent for the three-way catalyst to work sufficiently. . That is, it is important that the air / fuel mixture ratio in the internal combustion engine is optimized, and this optimum state is called stoichiometric (theoretical air-fuel ratio). When the stoichiometric ratio of the oxidizing substance and reducing substance in the exhaust gas composition is not equivalent, it becomes a state called lean (excess air with respect to fuel) or rich (excess fuel with respect to air), In this case, it is desirable to correct the stoichiometric ratio to the equivalent by adding an oxidizing substance or a reducing substance to purify the exhaust gas.

  Therefore, in recent exhaust gas purification catalysts in internal combustion engines, a promoter having OSC ability (oxygen storage ability) such as ceria (CeO 2) capable of occluding and releasing oxygen contained in exhaust gas may be used in combination. Many. Oxygen, which is an oxidizing substance, is occluded or released by a cocatalyst having OSC ability to adjust the stoichiometric ratio of the oxidizing substance and the reducing substance to an equivalent amount, thereby purifying the exhaust gas.

  CeO2 as a co-catalyst not only has OSC ability, but also has a strong affinity with noble metal supported on CeO2, and thus exhibits an effect of suppressing sintering (grain growth) of the noble metal. . However, when compared with other metal oxides, CeO2 is coarsened by aggregation in a high-temperature atmosphere of 1000 ° C. or higher, so that the specific surface area is reduced, and the noble metal supported on CeO2 moves along with this aggregation, and Therefore, a technique for improving the heat resistance of CeO2 itself is necessary.

  In order to improve this heat resistance, Patent Document 1 proposes that a noble metal is supported on a composite oxide obtained by firing CeO2 and zirconia (ZrO2) excellent in heat resistance. Patent Document 2 proposes that a noble metal is supported on a mixed oxide obtained by calcination, which is made of CeO2 and ZrO2 having excellent heat resistance.

Japanese Patent Laid-Open No. 10-194742 JP-A-6-279027

In the techniques proposed in Patent Documents 1 and 2, the heat resistance of the particulate carrier itself can be improved because ZrO2 having excellent heat resistance exists in the vicinity of CeO2, but it exists on the surface of the particulate carrier. It is difficult to control the substance only to CeO2. Therefore, a considerable amount of ZrO2 is present on the surface of the particulate carrier, and the noble metal supported on the ZrO2 surface has a weaker affinity than the noble metal supported on the CeO2 surface, so that coarsening due to aggregation is likely to occur. Sintering suppression effect is not sufficient. Therefore, the exhaust gas purification performance at a high temperature is not sufficiently improved.

  Therefore, in the present invention, the effect of improving the heat resistance by ZrO2 and the noble metal supported on CeO2 are supported by supporting the noble metal on the particulate carrier in which the outer core part mainly composed of CeO2 is arranged in the central part mainly composed of ZrO2. An exhaust gas purifying catalyst capable of simultaneously obtaining the sintering suppression effect is provided.

In order to solve the above-mentioned problems, in the present invention, in a catalyst in which a noble metal is supported on a particulate carrier, the particulate carrier comprises a central part mainly composed of zirconia (ZrO ₂ ) and ceria (CeO ₂ ) as a principal ingredient. An exhaust gas purifying catalyst is provided, characterized in that the CeO ₂ content in the particulate carrier is 40 mol% or more and 65 mol% or less.
Here, in the present invention, the particulate carrier is composed of one of an acidic and stable ceria sol and zirconia sol having an isoelectric point on the alkali side, and an alkaline and stable ceria sol and zirconia sol having an isoelectric point on the acid side. The other sol is mixed to obtain a mixed sol, and then, as a first step, the pH of the mixed sol is adjusted to a pH close to the isoelectric point of the zirconia sol constituting the central portion to start aggregation. After that, as a second step, the pH of the mixed sol is readjusted to a pH close to the isoelectric point of the ceria sol constituting the outer skin, and the zirconia sol that has already started agglomeration is agglomerated and precipitated, It may be produced by drying and baking this. When adjusting the pH of the mixed sol, when adjusting to the acidic side, the pH is adjusted to a lower pH exceeding the isoelectric point, and when adjusting to the alkaline, a higher pH exceeding the isoelectric point is adjusted. PH can be adjusted up to.

  Furthermore, the present invention provides an exhaust gas purifying catalyst characterized in that the CeO2 content in the particulate carrier is regulated to 45 mol% or more and 55 mol% or less.

  In the present invention, there is provided an exhaust gas purifying catalyst characterized in that one or more elements selected from alkaline earth metals, Y and rare earth elements are added to the outer skin part.

  In the present invention, there is provided an exhaust gas purifying catalyst characterized in that the element added to the outer skin part is at least one of Y and Nd.

  In the present invention, there is provided an exhaust gas purifying catalyst characterized in that one or more elements selected from alkaline earth metals, Y and rare earth elements are added to the central portion.

  In the present invention, there is provided an exhaust gas purifying catalyst characterized in that the element added to the central portion is Y.

  In the particulate carrier of the present invention, CeO2 sol and ZrO2 sol as raw materials for producing the particulate carrier are obtained by using as small a particle size as possible the CeO2 powder and ZrO2 powder. Keep the particle size of the particles small. The specific surface area is increased by reducing the size of the particulate carrier. In addition, the particle size of the noble metal supported on the particulate carrier is reduced to increase the specific surface area.

  In the particulate carrier comprising the central part mainly composed of ZrO2 and the outer skin part mainly composed of CeO2 according to the present invention, the central part by ZrO2 is defined for the first time by defining the CeO2 content to be 40 mol% or more and 65 mol% or less. Is moderately large, and CeO2 can form a layer with an appropriate thickness with the outer skin covering the entire central portion. As a result, both effects of improving heat resistance by ZrO2 and suppressing sintering of noble metal by CeO2 having strong affinity with noble metal and suppressing aggregation are simultaneously exhibited at a certain level or more.

  Furthermore, by defining the CeO2 content in the particulate carrier to be 45 mol% or more and 55 mol% or less, both the heat resistance improvement due to ZrO2 and the sintering of noble metal due to CeO2 having strong affinity with noble metal and suppressing aggregation can be achieved. It is expressed at a higher level simultaneously.

  By adding one or more elements selected from alkaline earth metals, Y and rare earth elements to CeO2, which is the outer skin, the lattice strain of CeO2 increases, and Ce ions are trivalent to tetravalent and tetravalent to trivalent. It is easy to exchange electrons. When the valence of Ce ions changes, the lattice size of CeO2 changes to a larger or smaller value, so that the oxygen storage / release characteristics are improved and the OSC ability is improved, contributing to the improvement of the performance as a catalyst.

  Here, when the element added to CeO2, which is the outer skin portion, is at least one of Y and Nd, the above phenomenon is more likely to occur.

  On the other hand, the addition of one or more elements selected from alkaline earth metals, Y, and rare earth elements to ZrO2, which is the central part, contributes to the improvement of heat resistance, although the reason is not clear.

  At that time, when the element added to ZrO2 which is the central portion is Y, the above effect is further clarified.

  Increasing the specific surface area of the particulate carrier makes it possible to increase the amount of noble metal that can be supported with the same weight. Also, increasing the specific surface area of the noble metal makes it possible to increase the area that contacts the exhaust gas with the same weight. That is, it is possible to improve the exhaust gas purification performance by combining a smaller amount of the particulate carrier and the noble metal.

  As shown in FIG. 1, the optimum form in the present invention is that the central portion 1 made of ZrO2 in the particulate carrier is covered with CeO2 of the outer skin portion 2 with an appropriate thickness. The noble metal 3 (Pt) is supported on the particulate carrier. Further, the particle size of the particulate carrier is preferably small. It was confirmed that the exhaust gas purifying catalyst in which the fine noble metal 3 is supported on the particulate carrier having this configuration has excellent exhaust gas purifying performance even after a durability test at a high temperature. The same effect can be obtained in a monolith catalyst in which the particulate carrier is coated on a honeycomb substrate.

  The method for producing the particulate carrier of the present invention is based on the premise that the starting material containing the component constituting the central portion and the starting material containing the component constituting the outer skin portion have different isoelectric points, and the timing of aggregation. By shifting the position, a two-layered state of the central portion and the outer skin portion can be obtained. That is, after mixing an acidic and stable sol (the isoelectric point is on the alkali side) and an alkaline and stable sol (the isoelectric point is on the acidic side), the pH is changed from acidic to alkaline, or from alkaline to acidic. Make adjustments in two stages. For this pH adjustment, it is possible to use a general acidic solution and an alkaline solution. Examples of the acidic solution include nitric acid and hydrochloric acid, and examples of the alkaline solution include aqueous ammonia and sodium hydroxide. Ingredients constituting the outer skin part as the second stage after these acidic solutions and alkaline solutions are used to adjust the pH to be close to the isoelectric point of the ingredient constituting the central part as the first stage and start aggregation. The pH is adjusted to a value close to the isoelectric point, and the components that have already started agglomerating are aggregated and settled. The particulate carrier consisting of the central part and the outer skin part is obtained by drying and baking this. At this time, when further adding an element to each part, it is preferable to prepare a sol solution having the same liquidity and stability as the part to be added and previously mixing the two kinds of sols.

  In order to divide the central part and the outer skin part of the particulate carrier more clearly into two layers, when adjusting to the acidic side when adjusting the pH, the pH exceeds the isoelectric point to a lower pH. In the case of adjusting to, it is preferable to add the solution to a higher pH beyond the isoelectric point. In this way, by adding the solution until the isoelectric point of each other is exceeded, the sol containing the component intended for aggregation passes through an electrically neutral region, so that the aggregation occurs reliably, and the remaining Since the sol exists in a more stable region, the purity of the main component constituting the central portion and the outer skin portion can be further increased.

  Next, the noble metal can be supported by dispersing the particulate carrier in distilled water, adding the noble metal solution, stirring and drying and firing. The noble metal supported here is preferably at least one selected from Pt, Pd, Rh, Ir, and Au, more preferably at least one of Pt, Pd, and Rh, and Pt is optimal.

  The exhaust gas-purifying catalyst thus obtained was molded into a 1 mm square pellet, and the 1000 ° C. rich atmosphere / lean atmosphere gas shown in Table 1 as an endurance test was alternately switched every 60 seconds for 5 hours. did. This durability test was performed in common before evaluation in all Examples and Comparative Examples described below.

  To evaluate the catalyst performance, the rich atmosphere / lean atmosphere gas shown in Table 1 is repeatedly flowed at 1 Hz while raising the temperature, and the content of C3H6 contained in the gas after passing through the catalyst is reduced to 50% of the input gas by purification. The temperature was determined (HC-T50). The OSC ability (O2 mol% / CeO2-1 mol) was calculated from the amount of CO2 generated during O2 circulation by alternately switching CO (2%) / O2 (1%) in 60 seconds.

  Furthermore, the specific surface area was determined by the BET 1-point method, and the Pt particle size was determined by the CO pulse adsorption method at −80 ° C.

  The optimum contents of CeO2 and ZrO2 are verified below. The CeO2 particle diameter in the CeO2 sol and the ZrO2 particle diameter in the ZrO2 sol used below are both 100 nm or less, and the particle size of the produced particulate carrier is 6 μm or less. The supported Pt particle diameter was 3 nm or less.

  The composition as the catalyst is Pt (1 wt%) / outer part CeO 2 -center part ZrO 2, and the composition of the particulate carrier is CeO 2: ZrO 2 = 60: 40 (mol%). Evaluation items are specific surface area, Pt particle size, HC-T50, and OSC ability.

  An acidic stable CeO2 sol (isoelectric point pH = 8.5, CeO2 content 15 wt%) and an alkaline stable ZrO2 sol (isoelectric point pH = 3.5, ZrO2 content 10.2 wt%) were mixed with CeO2: ZrO2 = 60. : After mixing to 40 (mol%), HNO3 aqueous solution was added to this mixed sol until pH = 3.0, and then NH3 aqueous solution was added until pH = 11.0. Thereafter, it was dried at 120 ° C. for 24 hours and calcined at 700 ° C. for 5 hours.

  The particulate carrier thus obtained was dispersed in 6 times the weight of distilled water, and a dinitrodiamine platinum aqueous solution (Pt 4.4 wt%) was added so that the Pt was 1 wt% by weight, followed by stirring for 1 hour. , Dried at 120 ° C. for 24 hours, and calcined at 500 ° C. for 2 hours. The catalyst obtained by this production method was molded into 1 mm square pellets and used for evaluation.

  The catalyst composition is Pt (1 wt%) / outer part CeO2 + Y2O3-center part ZrO2 + Y2O3, and the composition of the particulate carrier is CeO2: ZrO2: Y2O3 = 45: 52: 3 (mol%). Evaluation items are specific surface area, Pt particle size, and HC-T50. Y is added as an oxide.

  Acid stable CeO2 sol (isoelectric point pH = 8.5, CeO2 content 15 wt%), alkaline stable ZrO2 sol (isoelectric point pH = 3.5, ZrO2 content 10.2 wt%), Y2O3 sol (Y2O3 content) 15 wt%) was mixed so that CeO 2: ZrO 2: Y 2 O 3 = 45: 52: 3 (mol%), and an aqueous HNO 3 solution was added to this mixed sol until pH = 3.0, and then pH = 11.0 NH3 aqueous solution was added until. Thereafter, it was dried at 120 ° C. for 24 hours and calcined at 700 ° C. for 5 hours.

  The particulate carrier thus obtained was dispersed in 6 times the weight of distilled water, and a dinitrodiamine platinum aqueous solution (Pt 4.4 wt%) was added so that Pt would be 1 wt% by weight, followed by stirring for 1 hour. , Dried at 120 ° C. for 24 hours, and calcined at 500 ° C. for 2 hours. The catalyst obtained by this production method was molded into 1 mm square pellets and used for evaluation.

  The catalyst composition is Pt (1 wt%) / outer part CeO2 + Y2O3-center part ZrO2 + Y2O3, and the composition of the particulate carrier is CeO2: ZrO2: Y2O3 = 50: 47: 3 (mol%). Evaluation items are specific surface area, Pt particle size, and HC-T50. Y is added as an oxide.

  Acid stable CeO2 sol (isoelectric point pH = 8.5, CeO2 content 15 wt%), alkaline stable ZrO2 sol (isoelectric point pH = 3.5, ZrO2 content 10.2 wt%), Y2O3 sol (Y2O3 content) 15 wt%) was mixed so that CeO 2: ZrO 2: Y 2 O 3 = 50: 47: 3 (mol%), and an aqueous HNO 3 solution was added to this mixed sol until pH = 3.0, and then pH = 11.0 NH3 aqueous solution was added until. Thereafter, it was dried at 120 ° C. for 24 hours and calcined at 700 ° C. for 5 hours.

  The particulate carrier thus obtained was dispersed in 6 times the weight of distilled water, and a dinitrodiamine platinum aqueous solution (Pt 4.4 wt%) was added so that Pt would be 1 wt% by weight, followed by stirring for 1 hour. , Dried at 120 ° C. for 24 hours, and calcined at 500 ° C. for 2 hours. The catalyst obtained by this production method was molded into 1 mm square pellets and used for evaluation.

  The catalyst composition is Pt (1 wt%) / outer part CeO2 + Y2O3-center part ZrO2 + Y2O3, and the composition of the particulate support is CeO2: ZrO2: Y2O3 = 55: 42: 3 (mol%). Evaluation items are specific surface area, Pt particle size, and HC-T50. Y is added as an oxide.

  Acid stable CeO2 sol (isoelectric point pH = 8.5, CeO2 content 15 wt%), alkaline stable ZrO2 sol (isoelectric point pH = 3.5, ZrO2 content 10.2 wt%), Y2O3 sol (Y2O3 content) 15 wt%) was mixed so that CeO 2: ZrO 2: Y 2 O 3 = 55: 42: 3 (mol%), and an aqueous HNO 3 solution was added to this mixed sol until pH = 3.0, and then pH = 11.0 NH3 aqueous solution was added until. Thereafter, it was dried at 120 ° C. for 24 hours and calcined at 700 ° C. for 5 hours.

The particulate carrier thus obtained was dispersed in 6 times the weight of distilled water, and a dinitrodiamine platinum aqueous solution (Pt 4.4 wt%) was added so that Pt would be 1 wt% by weight, followed by stirring for 1 hour. , Dried at 120 ° C. for 24 hours, and calcined at 500 ° C. for 2 hours. The catalyst obtained by this production method was molded into a 1 mm square pellet and used for evaluation.
[Comparative Example 1]

  The catalyst structure is Pt (1 wt%) / ZrO2, and the particulate carrier is only ZrO2 powder. Evaluation items are specific surface area, Pt particle size, and HC-T50.

Disperse ZrO2 powder in 6 times the weight of distilled water, add dinitrodiamine platinum aqueous solution (Pt 4.4 wt%) so that Pt is 1 wt% by weight, stir for 1 hour, and then dry at 120 ° C for 24 hours. And calcined at 500 ° C. for 2 hours. The catalyst obtained by this production method was molded into 1 mm square pellets and used for evaluation.
[Comparative Example 2]

  The composition as the catalyst is Pt (1 wt%) / outer part CeO2 + Y2O3-center part ZrO2 + Y2O3, and the composition of the particulate carrier is CeO2: ZrO2: Y2O3 = 25: 72: 3 (mol%). Evaluation items are specific surface area, Pt particle size, and HC-T50. Y is added as an oxide.

  Acid stable CeO2 sol (isoelectric point pH = 8.5, CeO2 content 15 wt%), alkaline stable ZrO2 sol (isoelectric point pH = 3.5, ZrO2 content 10.2 wt%), Y2O3 sol (Y2O3 content) 15 wt%) was mixed so that CeO 2: ZrO 2: Y 2 O 3 = 25: 72: 3 (mol%), and an aqueous HNO 3 solution was added to this mixed sol until pH = 3.0, and then pH = 11.0 NH3 aqueous solution was added until. Thereafter, it was dried at 120 ° C. for 24 hours and calcined at 700 ° C. for 5 hours.

The particulate carrier thus obtained was dispersed in 6 times the weight of distilled water, and a dinitrodiamine platinum aqueous solution (Pt 4.4 wt%) was added so that Pt would be 1 wt% by weight, followed by stirring for 1 hour. , Dried at 120 ° C. for 24 hours, and calcined at 500 ° C. for 2 hours. The catalyst obtained by this production method was molded into 1 mm square pellets and used for evaluation.
[Comparative Example 3]

  The composition as the catalyst is Pt (1 wt%) / outer part CeO2 + Y2O3-center part ZrO2 + Y2O3, and the composition of the particulate carrier is CeO2: ZrO2: Y2O3 = 75: 22: 3 (mol%). Evaluation items are specific surface area, Pt particle size, and HC-T50. Y is added as an oxide.

  Acid stable CeO2 sol (isoelectric point pH = 8.5, CeO2 content 15 wt%), alkaline stable ZrO2 sol (isoelectric point pH = 3.5, ZrO2 content 10.2 wt%), Y2O3 sol (Y2O3 content) 15 wt%) was mixed so that CeO 2: ZrO 2: Y 2 O 3 = 75: 22: 3 (mol%), and an aqueous HNO 3 solution was added to this mixed sol until pH = 3.0, and then pH = 11.0 NH3 aqueous solution was added until. Thereafter, it was dried at 120 ° C. for 24 hours and calcined at 700 ° C. for 5 hours.

The particulate carrier thus obtained was dispersed in 6 times the weight of distilled water, and a dinitrodiamine platinum aqueous solution (Pt 4.4 wt%) was added so that Pt would be 1 wt% by weight, followed by stirring for 1 hour. , Dried at 120 ° C. for 24 hours, and calcined at 500 ° C. for 2 hours. The catalyst obtained by this production method was molded into a 1 mm square pellet and used for evaluation.
[Comparative Example 4]

  The catalyst structure is Pt (1 wt%) / CeO2, and the particulate carrier is only CeO2 powder. Evaluation items are specific surface area, Pt particle size, and HC-T50.

  CeO2 powder was dispersed in 6 times the weight of distilled water, dinitrodiamine platinum aqueous solution (Pt 4.4 wt%) was added so that Pt would be 1 wt% by weight, stirred for 1 hour, and then dried at 120 ° C for 24 hours. And calcined at 500 ° C. for 2 hours. The catalyst obtained by this production method was molded into 1 mm square pellets and used for evaluation.

  The evaluation results for Examples 1 to 4 and Comparative Examples 1 to 4 are shown in Table 2 and FIGS. Table 2 summarizes SSA (specific surface area), Ce particle size, and HC-T50, which is a measure of catalyst performance, according to CeO2 content. FIG. 2 summarizes the relationship between CeO2 content and SSA, FIG. 3 summarizes the relationship between CeO2 content and Pt particle diameter, and FIG. 4 summarizes the relationship between CeO2 content and HC-T50.

  The particulate carrier of Example 1 is composed only of CeO2 and ZrO2, and the particulate carriers of Examples 2 to 4 and Comparative Examples 1 to 4 are each Y2O3 in the outer and central portions composed of CeO2 and ZrO2. Was added and examined. Therefore, since the data of Example 1 and Examples 2 to 4 and Comparative Examples 1 to 4 cannot be simply compared, the results of Example 1 are not described in FIGS.

  From the results shown in Table 2, in Example 1, it can be confirmed that HC-T50 is lower than that of the comparative example even without the addition of Y2O3, but the specific surface area is larger by adding Y2O3, and the Pt particle size is It can be seen that it is smaller and the HC-T50 is lower and improved in all evaluation items. 3 and 4, when the content of CeO2 in the particulate carrier is 40 mol% or more and 65 mol%, both the heat resistance improvement by ZrO2 and the suppression of sintering of the noble metal by CeO2 are simultaneously manifested at a certain level or more. Furthermore, since the OSC ability by CeO2 also contributes, it turns out that the characteristic outstanding as an exhaust gas purification catalyst is acquired. Among these, as can be seen from FIG. 4, in the particulate carrier to which 3 mol% of Y2O3 is added, when the CeO2 content is 45 mol% or more and 55 mol% or less, the HC-T50 is the lowest and is excellent as a catalyst.

  The effect of the additive element is verified below. In addition, the CeO2 particle diameter in the CeO2 sol used below and the ZrO2 particle diameter in the ZrO2 sol are both 100 nm or less, and the particle size of the produced particulate carrier is 5 μm or less. The supported Pt particle diameter was 3 nm or less.

  The catalyst composition is Pt (1 wt%) / outer part CeO2 + Nd2O3 + Y2O3-center part ZrO2 + Y2O3, and the composition of the particulate support is CeO2: ZrO2: Y2O3: Nd2O3 = 58: 38: 2: 2 (mol%). . The survey items are specific surface area, Pt particle size, HC-T50, and OSC ability. Y and Nd are added as oxides.

  Acid stable CeO2 sol (isoelectric point pH = 8.5, CeO2 content 15 wt%), alkaline stable Y2O3-ZrO2 composite sol (Y2O3 content 5 wt%, composite sol content 10.2 wt%), neodymium nitrate, CeO2 : ZrO2: Y2O3: Nd2O3 = 58: 38: 2: 2 (mol%) is mixed, and HNO3 aqueous solution is added to this mixed sol until pH = 3.0, and then pH = 11.0. NH3 aqueous solution was added until. Thereafter, it was dried at 120 ° C. for 24 hours and calcined at 700 ° C. for 5 hours.

  The particulate carrier thus obtained was dispersed in 6 times the weight of distilled water, and a dinitrodiamine platinum aqueous solution (Pt 4.4 wt%) was added so that Pt would be 1 wt% by weight, followed by stirring for 1 hour. , Dried at 120 ° C. for 24 hours, and calcined at 500 ° C. for 2 hours. The catalyst obtained by this production method was molded into 1 mm square pellets and used for evaluation.

  The composition as the catalyst is Pt (1 wt%) / outer part CeO2 + Y2O3-center part CeO2 + ZrO2 + Y2O3, and the composition of the particulate carrier is CeO2: ZrO2: Y2O3 = 58: 38: 4 (mol%). The survey items are specific surface area, Pt particle size, HC-T50, and OSC ability. Y is added as an oxide.

  Acid stable CeO2 sol (isoelectric point pH = 8.5, CeO2 content 15 wt%), alkaline stable Y2O3-ZrO2 composite sol (Y2O3 content 5 wt%, composite sol content 10.2 wt%), Y2O3 sol (Y2O3 included) 15 wt%) was mixed so that CeO 2: ZrO 2: Y 2 O 3 = 58: 38: 4 (mol%), and an aqueous HNO 3 solution was added to the mixed sol until pH = 3.0, and then pH = 11.1. NH 3 aqueous solution was added until zero. Thereafter, it was dried at 120 ° C. for 24 hours and calcined at 700 ° C. for 5 hours.

The particulate carrier thus obtained was dispersed in 6 times the weight of distilled water, and a dinitrodiamine platinum aqueous solution (Pt 4.4 wt%) was added so that Pt would be 1 wt% by weight, followed by stirring for 1 hour. , Dried at 120 ° C. for 24 hours, and calcined at 500 ° C. for 2 hours. The catalyst obtained by this production method was molded into 1 mm square pellets and used for evaluation.
[Comparative Example 5]

  The composition as the catalyst is Pt (1 wt%) / CeO2 + ZrO2 + Nd2O3, and the composition of the particulate carrier is CeO2: ZrO2: Nd2O3 = 58: 38: 4 (mol%). The survey items are specific surface area, Pt particle size, HC-T50, and OSC ability. Nd is added as an oxide.

  Mix cerium nitrate, zirconium oxynitrate, and neodymium nitrate in distilled water so that Ce: Zr: Nd = 58: 38: 4 (mol%), and stir and dissolve. NH3 until pH = 9.0 An aqueous solution was added. Thereafter, it was dried at 120 ° C. for 24 hours and calcined at 700 ° C. for 5 hours.

The particulate carrier thus obtained was dispersed in 6 times the weight of distilled water, and a dinitrodiamine platinum aqueous solution (Pt 4.4 wt%) was added so that Pt would be 1 wt% by weight, followed by stirring for 1 hour. , Dried at 120 ° C. for 24 hours, and calcined at 500 ° C. for 2 hours. The catalyst obtained by this production method was molded into 1 mm square pellets and used for evaluation.
[Comparative Example 6]

  The composition as the catalyst is Pt (1 wt%) / CeO2 + ZrO2 + Y2O3, and the composition of the particulate carrier is CeO2: ZrO2: Y2O3 = 58: 38: 4 (mol%). The survey items are specific surface area, Pt particle size, HC-T50, and OSC ability. Y is added as an oxide.

  Mix cerium nitrate, zirconium oxynitrate, and yttrium nitrate in distilled water so that Ce: Zr: Y = 58: 38: 4 (mol%), and stir and dissolve. NH3 until pH = 9.0 An aqueous solution was added. Thereafter, it was dried at 120 ° C. for 24 hours and calcined at 700 ° C. for 5 hours.

  The particulate carrier thus obtained was dispersed in 6 times the weight of distilled water, and a dinitrodiamine platinum aqueous solution (Pt 4.4 wt%) was added so that Pt would be 1 wt% by weight, followed by stirring for 1 hour. , Dried at 120 ° C. for 24 hours, and calcined at 500 ° C. for 2 hours. The catalyst obtained by this production method was molded into 1 mm square pellets and used for evaluation.

  The results of Example 1, Examples 5 and 6, and Comparative Examples 5 and 6 are summarized below using Table 3 and FIGS. Table 3 shows SSA (specific surface area), Pt particle size, OSC ability and catalytic performance when an element is added to each part of the particulate carrier consisting of the outer skin part and the central part and when an element is added to the composite oxide carrier. HC-T50 which becomes a standard of the above is summarized. FIG. 5 shows the difference in SSA depending on the carrier composition and the additive element, FIG. 6 shows the difference in the Pt particle size due to the carrier structure and the additive element, and FIG. 7 shows the difference in OSC ability due to the carrier composition and the additive element. FIG. 8 summarizes the differences in the HC-T50 depending on the carrier structure and the added elements.

  First, Examples 5 and 6 and Example 1 are summarized. The particulate carrier of Example 6 has a configuration in which Y 2 O 3 is added to the outer skin portion and the central portion of the particulate carrier of Example 1. In Example 6, as compared with Example 1, Pt sintering was further suppressed, the Pt particle diameter was smaller than 10 nm, and the OSC ability was further improved. As a result, the HC-T50 exhibiting catalytic performance was also improved. The temperature is low. In Example 5, Y2O3 is added to the outer skin portion and the central portion of the particulate carrier, and Nd2O3 is further added to the outer skin portion. As a result of adding Nd 2 O 3, Example 5 has a Pt particle size smaller than that of Example 6 and further improved OSC ability, and HC-T50, which shows catalyst performance, has a lower temperature. From the above, it can be seen that in the exhaust gas purifying catalyst carrier, Y2O3 and Nd2O3 are added to the outer skin portion, and Y2O3 is added to the central portion, so that the catalyst performance is superior. Other than Y and Nd, similar effects can be obtained by adding alkaline earth metals such as Mg, Ca, Sr and Ba and rare earth elements such as La, Pr, Sm, Eu and Gd.

  Next, Examples 5 and 6 and Comparative Examples 5 and 6 are summarized. Although the difference cannot be confirmed by the OSC ability, in Examples 5 and 6, since the main component of the outer portion of the particulate carrier is CeO2, Pt sintering is further suppressed, and the Pt particle diameter is as small as 10 nm or less. As a result, the HC-T50, which shows catalyst performance, has a lower temperature, and the carrier of the exhaust gas purification catalyst has a higher temperature characteristic when the structure of the outer skin part and the central part are taken than those in which a plurality of oxides are integrated. Can be seen to be excellent.

It is the schematic which shows the support | carrier shape of this invention, and the loading state of a noble metal. It is a figure which shows the change of the specific surface area by the amount of CeO2. It is a figure which shows the change of the Pt particle diameter by CeO2 amount. It is a figure which shows the catalyst performance (HC-T50) by CeO2 amount. It is a figure which shows the change of the specific surface area by an addition element. It is a figure which shows the change of the Pt particle diameter by an additive element. It is a figure which shows the catalyst performance (OSC capability) by an additional element. It is a figure which shows the catalyst performance (HC-T50) by an additive element.

Explanation of symbols

1 Particulate carrier center part 2 Particulate carrier skin part 3 Precious metal

Claims (8)

In a catalyst in which a noble metal is supported on a particulate carrier, the particulate carrier is composed of a central part mainly composed of zirconia (ZrO ₂ ) and an outer skin part mainly composed of ceria (CeO ₂ ), and the particulate A catalyst for exhaust gas purification of an internal combustion engine, wherein the CeO ₂ content in the carrier is 40 mol% or more and 65 mol% or less. A mixed sol is obtained by mixing one sol of an acidic and stable ceria sol and zirconia sol having an isoelectric point on the alkali side and the other sol of an alkaline and stable ceria sol and zirconia sol having an isoelectric point on the acid side. Then, as a first step, after adjusting the pH of the mixed sol to a pH close to the isoelectric point of the zirconia sol constituting the central portion to start aggregation, the mixed sol is set as the second step. The pH of the ceria sol is readjusted to a pH close to the isoelectric point of the ceria sol constituting the outer skin part, and the ceria sol is already agglomerated, and the ceria sol is agglomerated and settled, dried and calcined. 2. The exhaust gas purifying catalyst for an internal combustion engine according to claim 1, wherein the particulate carrier is manufactured by the method. In adjusting the pH of the mixed sol, when adjusting to the acidic side, the pH is adjusted to a lower pH exceeding the isoelectric point, and when adjusting to the alkaline, the pH is adjusted to a higher pH exceeding the isoelectric point. The catalyst for exhaust gas purification of an internal combustion engine according to claim 2, wherein the particulate carrier is manufactured by adjusting The exhaust gas purification catalyst for an internal combustion engine according to any one of claims 1 to 3, wherein the CeO ₂ content in the particulate carrier is 45 mol% or more and 55 mol% or less. The exhaust gas purification catalyst for an internal combustion engine according to any one of claims 1 to 4, wherein one or more elements selected from alkaline earth metals, Y, and rare earth elements are added to the outer skin portion. . The elements added to the outer skin portion, characterized in that Y and Nd is at least one or more, the exhaust gas purifying catalyst for an internal combustion engine according to claim 5. The exhaust gas purification catalyst for an internal combustion engine according to any one of claims 1 to 6 , wherein at least one element selected from an alkaline earth metal, Y, and a rare earth element is added to the central portion. . Element added to the center, characterized in that it is a Y, an internal combustion engine exhaust gas purifying catalyst according to claim 7.

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