JP2007105632A - Exhaust gas cleaning catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning catalyst capable of suppressing the deterioration of a catalytic performance of Rh even when the Rh excellent in low temperature performance and an oxygen storing/releasing material as a co-catalyst are included. <P>SOLUTION: The exhaust gas cleaning catalyst contains oxygen storing/releasing material particles 1, carrier particles 2 and noble metal fine particles 3 including the Rh in the same layer. The noble metal fine particles 3 include the Rh singly or other noble metal together with the Rh, and are carried on the carrier particles 2 made of metal oxide. Primary particle size of the oxygen storing/releasing material particles 1 is 5 nm to 50 nm and primary particle size of the carrier particles 2 is 5 nm to 50 nm. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification catalyst for purifying exhaust gas discharged from an automobile engine.

  With increasing awareness of environmental conservation, regulations on the amount of exhaust gas from automobiles and the like have been strengthened. For this reason, various studies have been conducted to improve the performance of exhaust gas purification catalysts that purify exhaust gas discharged from automobile engines.

Exhaust gas purification catalysts usually have a particulate metal oxide support surface made of alumina (Al ₂ O ₃ ), etc., supported with fine particles of noble metals such as platinum (Pt), palladium (Pd), and rhodium (Rh) Because of the catalytic action of these noble metal particles, harmful gases such as unburned hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO _x ) contained in the exhaust gas are harmless. It is converted into water and gas.

Among precious metals used in exhaust gas purification catalysts, Rh is an essential component because of its excellent low-temperature purification performance, and is always contained in the exhaust gas purification catalyst. The exhaust gas purification catalyst may contain an oxygen storage / release material as a promoter component. This oxygen storage / release material is, for example, Ce oxide (CeO ₂ ). By including such an oxygen storage / release material in the exhaust gas purification catalyst, the reaction atmosphere ranges from a region where the amount of oxygen and the reducing agent are equal (stoichiometry region) to a region where the amount of oxygen is high (lean region) or a region where the amount of oxygen is rich (rich). Even if it varies, the oxygen storage / release effect of the oxygen storage / release material is exhibited, the atmospheric fluctuation around the noble metal particles is relaxed, the activity of the catalyst is increased, and the performance of the exhaust gas purification catalyst can be improved.

With regard to the exhaust gas purification catalyst containing such Ce oxide, the first catalyst layer formed on the support and the second catalyst layer formed on the first catalyst layer, One catalyst layer contains an oxygen-storing ceria (CeO ₂ ) -zirconia (ZrO ₂ ) composite oxide, and the second catalyst layer contains a low heat-degradable ceria-zirconia composite oxide and the low heat-degradable ceria. -The thing containing the rhodium carry | supported by the zirconia complex oxide is disclosed (refer patent document 1).
Japanese Patent Laid-Open No. 2004-289813

However, the conventional exhaust gas purifying catalyst containing both Rh and an oxygen storage / release material such as CeO ₂ sometimes deteriorates the catalytic performance of Rh. This is considered to be because oxidation of Rh was promoted by contact of Rh with CeO ₂ in the exhaust gas purification catalyst. It is known that Rh undergoes oxidation in an oxidizing atmosphere or at the interface with a material that releases active oxygen, and when it becomes Rh in a higher oxidation state via Rh2O3, the catalytic activity is significantly reduced.

  The decrease in the catalyst performance of Rh is the same in the catalyst disclosed in Patent Document 1, that is, in the second catalyst layer of the catalyst disclosed in Patent Document 1, Rh is a ceria-zirconium composite oxide. Therefore, oxidation is promoted from the interface of Rh in contact with the ceria of the composite oxide, and the catalytic performance of Rh is lowered.

  The exhaust gas purifying catalyst of the present invention comprises noble metal fine particles containing rhodium, carrier particles supporting the noble metal fine particles, and oxygen storage / release material particles that transfer active oxygen in the exhaust gas in the same layer. The gist is that the primary particle diameter of the release material particles is 5 nm to 50 nm, and the primary particle diameter of the carrier particles is 5 nm to 50 nm.

  According to the exhaust gas purification catalyst of the present invention, in the catalyst layer containing Rh, by containing the carrier particles carrying Rh and the oxygen storage / release material so as to suppress high oxidation of Rh, It is possible to obtain an exhaust gas purification catalyst catalyst with improved performance that suppresses the high oxidation of Rh and utilizes the oxygen storage / release capability of the oxygen storage / release material.

  FIG. 1 is a diagram schematically showing an example of an exhaust gas purification catalyst of the present invention. The exhaust gas purifying catalyst shown in the figure includes oxygen storage / release material particles 1, carrier particles 2, and noble metal fine particles 3 containing Rh in the same layer. The noble metal fine particles 3 contain other noble metal alone or together with Rh, and are supported on the carrier particles 2. The primary particle diameter of the oxygen storage / release material particle 1 is 5 nm to 50 nm, and the primary particle diameter of the carrier particle 2 is 5 nm to 50 nm.

  In general, it is widely known that Rh used for a catalyst is remarkably lowered in catalytic activity when it is in a highly oxidized state. Therefore, it is desired that Rh should not be in a highly oxidized state in the atmosphere in the exhaust gas.

On the other hand, the oxygen storage / release material is an additive essential to the catalyst because it can give and receive active oxygen and greatly improve the reaction activity in an atmosphere in exhaust gas that repeats an oxidation-reduction atmosphere. However, the oxygen supply ability of this oxygen storage / release material is not only used for the reaction of supplying active oxygen to CO molecules adsorbed on Rh and quickly purifying it as CO _2, for example, but directly giving active oxygen to Rh, It has been found that Rh is in a highly oxidized state. For this reason, in a catalyst containing both Rh and an oxygen storage / release material, it is desired to improve the activity of the oxygen storage / release material while maintaining the catalytic activity by keeping the electronic state of Rh as reduced as possible. Is done.

  As a result of intensive studies, the inventors have found that the above demand can be realized by controlling the distance in the catalyst between Rh and the oxygen storage / release material. That is, the direct supply of active oxygen of the oxygen storage / release material to Rh resulting in high oxidation of Rh proceeds at the contact interface between Rh and the oxygen storage / release material, whereas the active oxygen used for the catalytic reaction is If the oxygen storage / release material is present at a distance in the range of several to several tens of nm with respect to Rh, specifically a distance in the range of about 5 to 30 nm, the Rh activity should be sufficiently improved. Clarified that is possible.

  Such control of the distance is carried out by supporting Rh on a carrier having a predetermined particle size so that a direct interface between Rh and the oxygen storage / release material is not formed, so that the primary particle size is 5 nm to 50 nm. This can be realized by mixing the oxygen storage / release material in the catalyst. With such a configuration, as shown in FIG. 1, the oxygen storage / release material does not form a direct interface with Rh, but exists with a distance in the range of about 5 to 30 nm from Rh. Is suppressed from becoming a highly oxidized state, and the ability to store and release oxygen can be sufficiently exhibited.

Further, the exhaust gas purifying catalyst of the present invention, the oxygen storage material, since it is a carrier in the same layer, the oxygen storage material adsorbs of H ₂ O in the exhaust gas, H ₂ at high temperature It is considered that sintering of the support by O molecules is suppressed. Thus, Rh sintering can be suppressed even after long-term use.

  It is preferable that the noble metal fine particles containing Rh are supported on a carrier exclusively, and the oxygen storage / release material does not support Rh directly. When Rh is directly supported, as described above, at the interface between the oxygen storage / release material and Rh, active oxygen is supplied to Rh due to the oxygen storage / release ability of this oxygen storage / release material, and the catalyst This is because the electronic state of Rh becomes a highly oxidized state with low activity immediately after preparation. This is confirmed by comparing the binding energies of Rh3d5 orbitals by XPS.

  The primary particle diameter of the oxygen storage / release material particles is in the range of 5 nm to 50 nm. When the primary particle diameter is less than 5 nm, the ratio of contact between Rh and the oxygen storage / release material particles increases when the oxygen storage / release material is mixed, and oxidation of Rh is promoted. On the other hand, if it exceeds 50 nm, the surface area of the oxygen storage / release material is relatively reduced, so that a sufficient oxygen storage / release capability cannot be obtained and the catalyst performance is reduced.

  The particle size of the carrier particles is the primary particle size of 5 nm to 50 nm. By setting the particle size in the above range, the particle size of the oxygen storage / release material is limited to the above range, and the distance between Rh and the oxygen storage / release material can be within the above-described appropriate range. it can. From the viewpoint of thermal aggregation of the carrier particles or the supported noble metal fine particles, the primary particle diameter of the carrier is more preferably 10 to 50 nm. If it is less than 10 nm, thermal aggregation of the support generally tends to proceed, and accordingly, thermal aggregation of Rh also tends to proceed, and the Rh surface area after long-time use of the catalyst is remarkably reduced. If it exceeds 50 nm, the range in which Rh moves due to thermal aggregation increases and repeats aggregation with more Rh present in the surroundings, which also decreases the Rh surface area after prolonged use.

  In a preferred embodiment of the exhaust gas purification catalyst of the present invention, the primary particle diameter of the oxygen storage / release material particles is 5 nm to 20 nm, and the primary particle diameter of the carrier particles is 5 to 30 nm.

  As described above, the primary particle diameter of the oxygen storage / release material particles in the exhaust gas purifying catalyst of the present invention can be kept at an appropriate distance from Rh by being 50 nm or less, more preferably 20 nm or less. It is good to be. By setting it to 20 nm or less, the distance between the oxygen storage / release material and Rh can be made closer in the catalyst layer.

  Further, the primary particle diameter of the carrier particles is more preferably 5 to 30 nm. By making the flow diameter of the carrier particles 30 nm or less, it is possible to further improve the dispersibility of Rh in the catalyst.

As the oxygen storage / release material, a material containing Ce oxide is preferable. An example is cerium oxide (CeO ₂ ). This is because CeO ₂ has the highest active oxygen donating ability and oxygen storage ability as an oxide having oxygen storage / release ability. Further, the present invention is not limited to the case where the CeO _{2 is a} single metal oxide, and it is a more desirable embodiment that it is a composite compound containing CeO ₂ and ZrO ₂ . By forming a composite compound of CeO ₂ —ZrO ₂ , the heat resistance of the oxygen storage / release material is improved, and as a result, a high oxygen storage / release ability can be obtained even after the catalyst has been used for a long time. In addition, in order to stabilize the oxygen storage / release material, it is possible to add one or more components such as La, Nd, Ca, Ti and Y.

In the oxygen storage / release material containing cerium oxide, the cerium content is 1 to 30 in any case where the cerium oxide is contained alone or in the composite oxide in the particles. It is preferable to make it the range of mol%. If the Ce content is less than 1 mol%, sufficient oxygen storage / release capability cannot be obtained. On the other hand, when the Ce content exceeds 30 mol%, Rh particle migration occurs on the carrier supporting the noble metal due to long-term use at high temperature, and when the oxygen storage / release material comes into contact, Rh is more It is because it becomes easy to take a high oxidation state. Although details are unknown, as the mol% of Ce increases, the crystal plane spacing of the oxygen storage / release material expands, and Rh enters between the lattices of the oxygen storage / release material particles, and the activity decreases. This is probably because a so-called solid solution state occurs. When the Ce content is 30 mol% or less, the crystal plane spacing is reduced, so that even when Rh and the oxygen storage / release material are in contact with each other due to heat transfer, the crystals of this oxygen storage / release material The penetration of Rh into the lattice can be suppressed to some extent, and high catalyst performance can be obtained even after long-term use. In addition, when the oxygen storage / release material is a CeO ₂ -ZrO ₂ -based composite compound, with increasing Ce content, heat resistance generally decreases at 30 mol% or more, and after long-term use The surface area tends to decrease. When the Ce content is in the range of 1 to 30 mol%, a sufficient surface area can be maintained even after long-term use.

The support may contain one or more selected from Al ₂ O _3, ZrO ₂ , TiO ₂ and La ₂ O ₃ . The metal oxide supporting the noble metal is preferably excellent in heat resistance and has a large surface area. All of Al ₂ O _3, ZrO ₂ , TiO ₂ and La ₂ O ₃ listed above satisfy these requirements. Among these, it is desirable to use an oxide containing ZrO ₂ as a main component. The reason for this is that ZrO ₂ has no interaction with Rh, so that Rh is dissolved in the support and the activity is reduced, and Rh is in a highly oxidized state due to oxygen supply from the support to Rh. It is because it can prevent. Al ₂ O _{3 and the} like can also be used as a support, but when a metal oxide having many pores such as Al ₂ O ₃ is used as a support, Rh is easily supported inside the pores, and the catalyst The active oxygen supplied from the oxygen storage / release material mixed in the layer may not always be sufficiently used for the reaction. On the other hand, when an oxide containing ZrO ₂ as a main component is used, Rh is supported on the surface of the carrier, and the oxygen supply ability of the oxygen storage / release material existing in the vicinity can be utilized more effectively.

  As for the carrier and the method for producing the noble metal supported on the carrier, a coprecipitation method, a reverse homogeneous precipitation method, a sol-gel method, a reverse micelle method, or the like can be appropriately used.

In order to improve the active oxygen capacity supplied from the oxygen storage / release material, it is important to maintain a high specific surface area even after a long period of use of the catalyst. Preferably, firing is performed at 1000 ° C. for 4 hours in an air stream. After performing, it is desirable to have a specific surface area of 25 m ² / g, more preferably 30 m ² / g or more.

  It is preferable that the carrier has no oxygen storage / release capacity component (for example, Ce or Pr), or is a total of Ce and Pr reduced to 0.3 wt% or less (3 mol% or less in mol%). . In other words, it is preferable that Ce and Pr are not contained as much as possible in the metal oxide supporting the noble metal. The reason for this is that when Ce or Pr is contained, active oxygen is provided to Rh due to the oxygen storage / release ability of Ce and Pr at the interface between Ce and Pr and Rh. This is because the electronic state of Rh becomes a highly oxidized state with low activity immediately after preparation. This can be confirmed by comparing the binding energies of Rh3d5 orbitals by XPS. The presence of Ce and Pr around Rh in a form that does not directly contact Rh in the catalyst makes it possible to exchange active oxygen during the reaction and greatly improve the catalytic activity. However, on the direct interface between Rh and Ce and Pr, the above donation makes the electronic state of Rh highly oxidized, and the catalytic activity of Rh tends to decrease. In the present invention, the total amount of Ce and Pr is 0.3 wt% or less, thereby suppressing the oxidation of Rh, and the Rh in the exhaust gas purification catalyst of the present invention in which the distance between Rh in the catalyst and Ce and Pr is optimized is Thus, it is possible to obtain a state where the catalytic activity is most easily exhibited.

  The noble metal of the exhaust gas purification catalyst of the present invention requires Rh, but may contain other noble metals together with Rh. As a two-layered catalyst, it is needless to say that a catalyst having a noble metal such as Pt supported on the underlayer and a noble metal Rh supported on the surface layer can be used.

  The exhaust gas purification catalyst of the present invention can be produced by mixing a carrier carrying a noble metal and an oxygen storage / release material by a conventionally known method.

  Exhaust gas purification catalysts of Examples 1 to 6 and Comparative Examples 1 to 3 described below were produced. Each exhaust gas purification catalyst has a two-layer structure. In the following description, a layer close to the honeycomb substrate is described as an inner layer, and a layer close to the surface is described as a surface layer.

[Example 1]
(Step 1) Activated alumina (specific surface area 180 m ² / g) was dispersed in a solution of cerium nitrate hexahydrate dissolved in ion-exchanged water, and stirred for about 1 hour with a magnetic stirrer. This was evaporated to dryness, pulverized and sized, and then fired at 400 ° C. for 1 hour in an air stream. The powder was dispersed in a mixed solution of an acidic aqueous solution of dinitrodiamine platinum nitrate (manufactured by Tanaka Kikinzoku: 8.83 wt%) and ion-exchanged water, and stirred for 1 hour with a magnetic stirrer. This was evaporated to dryness, pulverized and sized, and then fired at 400 ° C. for 1 hour in an air stream to obtain cerium-carrying alumina carrying Pt.

(Step 2) Activated alumina (specific surface area 180 m ² / g) was dispersed in an aqueous solution in which a rhodium nitrate solution (Rh: 13.81 wt% made by Tanaka Kikinzoku) and pure water were mixed, and the mixture was stirred with a magnetic stirrer for about 1 hour. This was evaporated to dryness, pulverized and sized, and then fired in an air stream at 400 ° C. for 1 hour to obtain Al ₂ O ₃ carrying Rh.

(Step 3) 363.6g of the catalyst powder obtained by repeating the operation in Step 1, 50.9g of boehmite, 42.0g of 10% nitric acid, 575.3g of ion-exchanged water, 575.3g, put into a magnetic pot and shaken and ground with alumina balls As a result, an inner layer catalyst slurry was obtained. At this time, the particle size of the slurry was 2.8 μm.

(Step 4) 290.9g of the catalyst powder obtained by repeating the operation of Step 2, 72.72g of CeO ₂ powder (90m ² / g), 50.9g of boehmite, 42.0g of 10% nitric acid, 575.3g of ion-exchanged water Then, it was put into a magnetic pot and shaken and pulverized together with alumina balls to obtain a surface layer catalyst slurry. The slurry particle size at this time was 3.5 μm.

(Step 5) The catalyst slurry of Step 3 is put into a cordierite-made honeycomb substrate (900 cells / 2.5 mil), excess slurry is removed with an air stream, dried at 120 ° C, 400 ° C, air stream Baked in. The coating amount was 100 g / L (gram / liter, the same applies hereinafter). Next, the catalyst slurry of Step 4 was applied in the same manner. The coating amount was 100 g / L. As a result, a catalyst of Example 1 was obtained in which the inner layer was coated with 0.3 g / L Pt, 20 g / L CeO ₂ , the surface layer was coated with 0.3 wt% Rh, and 20 g / L CeO ₂ .

[Example 2]
(Step 1) Activated alumina (specific surface area 180 m ² / g) was dispersed in a solution of cerium nitrate hexahydrate dissolved in ion-exchanged water, and stirred for about 1 hour with a magnetic stirrer. This was evaporated to dryness, pulverized and sized, and then fired at 400 ° C. for 1 hour in an air stream. This powder was dispersed in a mixed solution of an aqueous palladium nitrate solution (Tanaka Kikinzoku: 20.52 wt%) and ion-exchanged water, and stirred with a magnetic stirrer for 1 hour. This was evaporated to dryness, pulverized and sized, and then fired in an air stream at 400 ° C. for 1 hour to obtain cerium-carrying alumina carrying Pd.

(Step 2) Zirconium oxide (specific surface area 90m ² / g) prepared by precipitating zirconium oxynitrate aqueous solution with ammonia in an aqueous solution mixed with rhodium nitrate solution (Tanaka Kikinzoku Rh: 13.81wt%) and pure water. The mixture was dispersed and stirred with a magnetic stirrer for about 1 hour. This was evaporated to dryness, pulverized and sized, and then fired in an air stream at 400 ° C. for 1 hour to obtain ZrO ₂ carrying Rh.

(Step 3) 363.6g of the catalyst powder obtained by repeating the operation in Step 1, 50.9g of boehmite, 42.0g of 10% nitric acid, 575.3g of ion-exchanged water, and put into a magnetic pot, shaken and ground with alumina balls As a result, an inner layer catalyst slurry was obtained. At this time, the particle size of the slurry was 2.8 μm.

(Step 4) 236.3 g of the catalyst powder obtained by repeating the operation of Step 2, 127.3 g of CeO ₂ —ZrO ₂ composite oxide (surface area 60 m ² / g), 50.9 g of boehmite, 42.0 g of 10% nitric acid, 575.3 g of ion-exchanged water was put into a magnetic pot and shaken and pulverized with alumina balls to obtain a catalyst slurry for the surface layer. The slurry particle size at this time was 3.5 μm.

(Step 5) The catalyst slurry of Step 3 is put into a cordierite-made honeycomb substrate (900 cells / 2.5 mil), excess slurry is removed with an air stream, dried at 120 ° C, 400 ° C, air stream Baked in. The coating amount was 100 g / L. Next, the catalyst slurry of Step 4 was applied in the same manner. The coating amount was 100 g / L. As a result, a catalyst of Example 2 was obtained in which Pd was 0.3 g / L, CeO ₂ was 20 g / L on the inner layer, Rh was 0.2 wt% on the surface layer, and 35 g / L of CeO ₂ —ZrO ₂ composite oxide was coated.

[Example 3]
(Step 1) Activated alumina (specific surface area 180 m ² / g) was dispersed in a solution of cerium nitrate hexahydrate dissolved in ion-exchanged water, and stirred for about 1 hour with a magnetic stirrer. This was evaporated to dryness, pulverized and sized, and then fired at 400 ° C. for 1 hour in an air stream. The powder was dispersed in a mixed solution of an acidic aqueous solution of dinitrodiamine platinum nitrate (manufactured by Tanaka Kikinzoku: 8.83 wt%) and ion-exchanged water, and stirred for 1 hour with a magnetic stirrer. This was evaporated to dryness, pulverized and sized, and then fired at 400 ° C. for 1 hour in an air stream to obtain cerium-carrying alumina carrying Pt.

(Step 2) ZrO ₂ -La ₂ O ₃ prepared by precipitating a zirconium oxynitrate aqueous solution and a lanthanum nitrate aqueous solution with ammonia in an aqueous solution obtained by mixing a rhodium nitrate solution (Rh: 13.81 wt% made by Tanaka Kikinzoku) and pure water. The composite compound (specific surface area 70 m ² / g) was dispersed and stirred with a magnetic stirrer for about 1 hour. This was evaporated to dryness, pulverized and sized, and then fired in an air stream at 400 ° C. for 1 hour to obtain a ZrO ₂ —La ₂ O ₃ composite compound carrying Rh.

(Step 3) 363.6g of the catalyst powder obtained by repeating the operation in Step 1, 50.9g of boehmite, 42.0g of 10% nitric acid, 575.3g of ion-exchanged water, and put into a magnetic pot, shaken and ground with alumina balls As a result, an inner layer catalyst slurry was obtained. At this time, the particle size of the slurry was 2.8 μm.

(Step 4) 272.7 g of the catalyst powder obtained by repeating the operation of Step 2, 90.9 g of CeO ₂ —ZrO ₂ composite oxide (surface area 55 m ² / g), 50.9 g of boehmite, 42.0 g of 10% nitric acid, 575.3 g of ion-exchanged water was put into a magnetic pot and shaken and pulverized with alumina balls to obtain a catalyst slurry for the surface layer. The slurry particle size at this time was 3.5 μm.

(Step 5) The catalyst slurry of Step 3 is put into a cordierite-made honeycomb substrate (900 cells / 2.5 mil), excess slurry is removed with an air stream, dried at 120 ° C, 400 ° C, air stream Baked in. The coating amount was 100 g / L. Next, the catalyst slurry of Step 4 was applied in the same manner. The coating amount was 100 g / L. As a result, a catalyst of Example 3 in which Pt was 0.2 g / L, CeO ₂ was 20 g / L on the inner layer, Rh was 0.4 wt% on the surface layer, and 25 g / L of CeO ₂ —ZrO ₂ composite oxide was obtained.

[Example 4]
(Step 1) Activated alumina (specific surface area 180 m ² / g) was dispersed in a solution of cerium nitrate hexahydrate dissolved in ion-exchanged water, and stirred for about 1 hour with a magnetic stirrer. This was evaporated to dryness, pulverized and sized, and then fired at 400 ° C. for 1 hour in an air stream. The powder was dispersed in a mixed solution of an acidic aqueous solution of dinitrodiamine platinum nitrate (manufactured by Tanaka Kikinzoku: 8.83 wt%) and ion-exchanged water, and stirred for 1 hour with a magnetic stirrer. This was evaporated to dryness, pulverized and sized, and then fired at 400 ° C. for 1 hour in an air stream to obtain cerium-carrying alumina carrying Pt.

(Step 2) Anatase-type titanium oxide (60m ² / g) is mixed and dispersed in an aqueous solution containing rhodium nitrate solution (Rh: 13.81wt% made by Tanaka Kikinzoku) and pure water. Stir. This was evaporated to dryness, pulverized and sized, and then fired in an air stream at 400 ° C. for 1 hour to obtain TiO ₂ carrying Rh.

(Step 3) 363.6g of the catalyst powder obtained by repeating the operation in Step 1, 50.9g of boehmite, 42.0g of 10% nitric acid, 575.3g of ion-exchanged water, and put into a magnetic pot, shaken and ground with alumina balls As a result, an inner layer catalyst slurry was obtained. At this time, the particle size of the slurry was 2.8 μm.

(Step 4) 309.1 g of the catalyst powder obtained by repeating the operation of Step 2, 54.5 g of CeO ₂ —ZrO ₂ composite oxide (surface area 70 m ² / g), 50.9 g of boehmite, 42.0 g of 10% nitric acid, 575.3 g of ion-exchanged water was put into a magnetic pot and shaken and pulverized with alumina balls to obtain a catalyst slurry for the surface layer. The slurry particle size at this time was 3.0 μm.

(Step 5) The catalyst slurry of Step 3 is put into a cordierite-made honeycomb substrate (900 cells / 2.5 mil), excess slurry is removed with an air stream, dried at 120 ° C, 400 ° C, air stream Baked in. The coating amount was 100 g / L. Next, the catalyst slurry of Step 4 was applied in the same manner. The coating amount was 100 g / L. As a result, a catalyst of Example 4 was obtained, in which the inner layer was coated with 0.3 g / L of Pt, CeO ₂ of 20 g / L, the surface layer was coated with 0.1 wt% of Rh, and the CeO ₂ —ZrO ₂ composite oxide was coated with 15 g / L.

[Example 5]
(Step 1) Activated alumina (specific surface area 180 m ² / g) was dispersed in a solution of cerium nitrate hexahydrate dissolved in ion-exchanged water, and stirred for about 1 hour with a magnetic stirrer. This was evaporated to dryness, pulverized and sized, and then fired at 400 ° C. for 1 hour in an air stream. This powder was dispersed in a mixed solution of an aqueous palladium nitrate solution (Tanaka Kikinzoku: 20.52 wt%) and ion-exchanged water, and stirred with a magnetic stirrer for 1 hour. This was evaporated to dryness, pulverized and sized, and then fired in an air stream at 400 ° C. for 1 hour to obtain cerium-carrying alumina carrying Pd.

(Step 2) Zirconium oxide (specific surface area 90m ² / g) prepared by precipitating zirconium oxynitrate aqueous solution with ammonia in an aqueous solution mixed with rhodium nitrate solution (Tanaka Kikinzoku Rh: 13.81wt%) and pure water. The mixture was dispersed and stirred with a magnetic stirrer for about 1 hour. This was evaporated to dryness, pulverized and sized, and then fired in an air stream at 400 ° C. for 1 hour to obtain ZrO ₂ carrying Rh.

(Step 3) 363.6g of the catalyst powder obtained by repeating the operation in Step 1, 50.9g of boehmite, 42.0g of 10% nitric acid, 575.3g of ion-exchanged water, and put into a magnetic pot, shaken and ground with alumina balls As a result, an inner layer catalyst slurry was obtained. At this time, the particle size of the slurry was 2.8 μm.

(Step 4) 327.2 g of the catalyst powder obtained by repeating the operation of Step 2, 36.4 g of CeO ₂ —ZrO ₂ composite oxide (surface area 45 m ² / g), 50.9 g of boehmite, 42.0 g of 10% nitric acid, 575.3 g of ion-exchanged water was put into a magnetic pot and shaken and pulverized with alumina balls to obtain a catalyst slurry for the surface layer. The slurry particle size at this time was 3.2 μm.

(Step 5) The catalyst slurry of Step 3 is put into a cordierite-made honeycomb substrate (900 cells / 2.5 mil), excess slurry is removed with an air stream, dried at 120 ° C, 400 ° C, air stream Baked in. The coating amount was 100 g / L. Next, the catalyst slurry of Step 4 was applied in the same manner. The coating amount was 100 g / L. As a result, a catalyst of Example 5 in which Pd was 0.3 g / L, CeO ₂ was 20 g / L, Rh was 0.3 wt%, and CeO ₂ —ZrO ₂ composite oxide was coated at 10 g / L on the inner layer was obtained.

[Example 6]
(Step 1) Activated alumina (specific surface area 180 m ² / g) was dispersed in a solution of cerium nitrate hexahydrate dissolved in ion-exchanged water, and stirred for about 1 hour with a magnetic stirrer. This was evaporated to dryness, pulverized and sized, and then fired at 400 ° C. for 1 hour in an air stream. This powder was dispersed in a mixed solution of an aqueous palladium nitrate solution (Tanaka Kikinzoku: 20.52 wt%) and ion-exchanged water, and stirred with a magnetic stirrer for 1 hour. This was evaporated to dryness, pulverized and sized, and then fired in an air stream at 400 ° C. for 1 hour to obtain cerium-carrying alumina carrying Pd.

(Step 2) Zirconium oxide (specific surface area 90m ² / g) prepared by precipitating zirconium oxynitrate aqueous solution with ammonia in an aqueous solution mixed with rhodium nitrate solution (Tanaka Kikinzoku Rh: 13.81wt%) and pure water. The mixture was dispersed and stirred with a magnetic stirrer for about 1 hour. This was evaporated to dryness, pulverized and sized, and then fired in an air stream at 400 ° C. for 1 hour to obtain ZrO ₂ carrying Rh.

(Step 3) 363.6g of the catalyst powder obtained by repeating the operation in Step 1, 50.9g of boehmite, 42.0g of 10% nitric acid, 575.3g of ion-exchanged water, and put into a magnetic pot, shaken and ground with alumina balls As a result, an inner layer catalyst slurry was obtained. At this time, the particle size of the slurry was 2.8 μm.

(Step 4) 236.3 g of the catalyst powder obtained by repeating the operation of Step 2, 127.3 g of CeO ₂ —ZrO ₂ —La ₂ O ₃ complex oxide (surface area 60 m ² / g), 50.9 g of boehmite, 10% Nitric acid (42.0 g) and ion-exchanged water (575.3 g) were added to a magnetic pot, and shaken and pulverized with alumina balls to obtain a catalyst slurry for the surface layer. The slurry particle size at this time was 3.2 μm.

(Step 5) The catalyst slurry of Step 3 is put into a cordierite-made honeycomb substrate (900 cells / 2.5 mil), excess slurry is removed with an air stream, dried at 120 ° C, 400 ° C, air stream Baked in. The coating amount was 100 g / L. Next, the catalyst slurry of Step 4 was applied in the same manner. The coating amount was 100 g / L. As a result, the inner layer was coated with 0.4 g / L of Pd, 20 g / L of CeO ₂ , 0.3 wt% of Rh on the surface layer, and 35 g / L of CeO ₂ —ZrO ₂ —La ₂ O ₃ composite oxide. A catalyst was obtained.

[Comparative Example 1]
(Step 1) Activated alumina (specific surface area 180 m ² / g) was dispersed in a solution of cerium nitrate hexahydrate dissolved in ion-exchanged water, and stirred for about 1 hour with a magnetic stirrer. This was evaporated to dryness, pulverized and sized, and then fired at 400 ° C. for 1 hour in an air stream. The powder was dispersed in a mixed solution of an acidic aqueous solution of dinitrodiamine platinum nitrate (manufactured by Tanaka Kikinzoku: 8.83 wt%) and ion-exchanged water, and stirred for 1 hour with a magnetic stirrer. This was evaporated to dryness, pulverized and sized, and then fired at 400 ° C. for 1 hour in an air stream to obtain cerium-carrying alumina carrying Pt.

(Step 2) Cerium oxide (90m ² / g) was mixed and dispersed in an aqueous solution mixed with rhodium nitrate solution (Rh: 13.81wt% made by Tanaka Kikinzoku) and pure water, and stirred with a magnetic stirrer for about 1 hour. . This was evaporated to dryness, pulverized and sized, and then fired at 400 ° C. for 1 hour in an air stream to obtain CeO ₂ carrying Rh.

(Step 3) 363.6g of the catalyst powder obtained by repeating the operation in Step 1, 50.9g of boehmite, 42.0g of 10% nitric acid, 575.3g of ion-exchanged water, and put into a magnetic pot, shaken and ground with alumina balls As a result, an inner layer catalyst slurry was obtained. At this time, the particle size of the slurry was 2.8 μm.

(Step 4) 363.9g of catalyst powder obtained by repeating the operation of Step 2, 50.9g of boehmite, 42.0g of 10% nitric acid, 575.3g of ion-exchanged water, put into a magnetic pot, shaken and ground with alumina balls Thus, a catalyst slurry for the surface layer was obtained. The slurry particle size at this time was 3.2 μm.

(Step 5) The catalyst slurry of Step 3 is put into a cordierite-made honeycomb substrate (900 cells / 2.5 mil), excess slurry is removed with an air stream, dried at 120 ° C, 400 ° C, air stream Baked in. The coating amount was 100 g / L. Next, the catalyst slurry of Step 4 was applied in the same manner. The coating amount was 100 g / L. As a result, a catalyst of Comparative Example 1 was obtained in which the inner layer was coated with Pt of 0.3 g / L, CeO ₂ of 20 g / L, and the surface layer of Rh of 0.3 wt% (CeO ₂ of 100 g / L).

[Comparative Example 2]
(Step 1) Activated alumina (specific surface area 180m ² / g) is dispersed in a solution of dinitrodiamineplatinic acid acidic aqueous solution (Tanaka Kikinzoku: 8.83wt%) and ion-exchanged water and stirred with a magnetic stirrer for 1 hour. did. This was evaporated to dryness, pulverized and sized, and then calcined at 400 ° C. for 1 hour in an air stream to obtain alumina carrying Pt.

(Step 2) Activated alumina (specific surface area 180 m ² / g) was dispersed in a solution of cerium nitrate hexahydrate dissolved in ion-exchanged water, and stirred for about 1 hour with a magnetic stirrer. This was evaporated to dryness, pulverized and sized, and then fired at 400 ° C. for 1 hour in an air stream. This powder was dispersed in a solution in which a rhodium nitrate solution (Rh: 13.81 wt% made by Tanaka Kikinzoku) and ion-exchanged water was mixed, and stirred for 1 hour with a magnetic stirrer. This was evaporated to dryness, pulverized and sized, and then calcined at 400 ° C. for 1 hour in an air stream to obtain alumina carrying Rh and cerium oxide.

(Step 3) 363.6g of the catalyst powder obtained by repeating the operation in Step 1, 50.9g of boehmite, 42.0g of 10% nitric acid, 575.3g of ion-exchanged water, and put into a magnetic pot, shaken and ground with alumina balls As a result, an inner layer catalyst slurry was obtained. At this time, the particle size of the slurry was 2.8 μm.

(Step 4) 363.9g of catalyst powder obtained by repeating the operation of Step 2, 50.9g of boehmite, 42.0g of 10% nitric acid, 575.3g of ion-exchanged water, put into a magnetic pot, shaken and ground with alumina balls Thus, a catalyst slurry for the surface layer was obtained. The slurry particle size at this time was 3.2 μm.

(Step 5) The catalyst slurry of Step 3 is put into a cordierite-made honeycomb substrate (900 cells / 2.5 mil), excess slurry is removed with an air stream, dried at 120 ° C, 400 ° C, air stream Baked in. The coating amount was 100 g / L. Next, the catalyst slurry of Step 4 was applied in the same manner. The coating amount was 100 g / L. As a result, a catalyst of Comparative Example 2 was obtained in which the inner layer was coated with 0.3 g / L of Pt, the surface layer was coated with 0.3 wt% of Rh, and (CeO ₂ was 8 g / L).

[Comparative Example 3]
(Step 1) Activated alumina (specific surface area 180m ² / g) is dispersed in a solution of dinitrodiamineplatinic acid acidic aqueous solution (Tanaka Kikinzoku: 8.83wt%) and ion-exchanged water and stirred with a magnetic stirrer for 1 hour. did. This was evaporated to dryness, pulverized and sized, and then calcined at 400 ° C. for 1 hour in an air stream to obtain alumina carrying Pt.

(Step 2) Zirconium oxide (specific surface area 90m ² / g) prepared by precipitating zirconium oxynitrate aqueous solution with ammonia in an aqueous solution mixed with rhodium nitrate solution (Tanaka Kikinzoku Rh: 13.81wt%) and pure water. The mixture was dispersed and stirred with a magnetic stirrer for about 1 hour. This was evaporated to dryness, pulverized and sized, and then fired in an air stream at 400 ° C. for 1 hour to obtain ZrO ₂ carrying Rh.

(Step 3) 363.6g of the catalyst powder obtained by repeating the operation in Step 1, 50.9g of boehmite, 42.0g of 10% nitric acid, 575.3g of ion-exchanged water, and put into a magnetic pot, shaken and ground with alumina balls As a result, an inner layer catalyst slurry was obtained. At this time, the particle size of the slurry was 2.8 μm.

(Step 4) 363.9g of catalyst powder obtained by repeating the operation of Step 2, 50.9g of boehmite, 42.0g of 10% nitric acid, 575.3g of ion-exchanged water, put into a magnetic pot, shaken and ground with alumina balls Thus, a catalyst slurry for the surface layer was obtained. The slurry particle size at this time was 3.2 μm.

(Step 5) The catalyst slurry from Step 3 is put into a cordierite-made honeycomb substrate (900 cells / 2.5 mil), excess slurry is removed by air flow, dried at 120 ° C, 400 ° C, air stream Baked in. The coating amount was 100 g / L. Next, the catalyst slurry of Step 4 was applied in the same manner. The coating amount was 100 g / L. As a result, a catalyst of Comparative Example 3 was obtained in which the inner layer was coated with 0.3 g / L of Pt and the surface layer was coated with 0.3 wt% of Rh.

  About each catalyst of Examples 1-6 obtained by the method described above and Comparative Examples 1-3, the catalyst durability test shown below was done, and the performance of the catalyst was evaluated after that.

In the catalyst durability test, unleaded gasoline was used as fuel using a V-6 engine manufactured by Nissan Motor Co., Ltd. In this durability test, a jig capable of adjusting the exhaust gas flow rate to the honeycomb substrate was used for the test piece, and the catalysts of Examples 1 to 6 and Comparative Examples 1 to 3 were used as exhaust gas purification catalysts. The catalyst position was adjusted so that the catalyst inlet temperature was 800 ° C., and the engine was operated for 50 hours. Thereafter, the catalyst after the durability test was cut and a part of the catalyst was cut out to evaluate the catalyst performance with a catalyst capacity of 40 cc. In addition, as conditions for evaluating the performance of the catalyst, each catalyst after the endurance test is incorporated into the simulated exhaust gas circulation device, and the simulated exhaust gas having the composition shown in Table 1 is circulated through the simulated exhaust gas circulation device at a gas flow rate of 40 liters per minute. While raising the catalyst temperature at a rate of 30 ° C./min, the temperature (T50) at which the NO _x purification rate was 50% was examined. The evaluation results are shown in Table 2.

  In Table 2, the coating amount is the coating amount of each layer, and shows the dry coating amount at 150 ° C. after the final baking. In each example, the amount of noble metal supported in each powder is not described, but the amount supported was appropriately adjusted so that the amount of noble metal shown in Table 2 was obtained in each layer coat amount. The composition of the oxygen storage / release material in the surface layer is a molar ratio, and the abundance is the abundance in the surface layer. The primary particle size is a particle size confirmed by TEM observation after the final firing. The primary particle size is not the aggregate size (secondary particle size) in the state where the particles are aggregated, but the arithmetic average value of the particle size of a single smallest unit at a TEM observation magnification of 500,000 times or more. It is a thing. Here, the particle diameter is obtained by setting an intermediate value of values obtained by measuring the long side and the short side of each particle from the TEM image as the particle diameter.

  As is clear from Table 2, Examples 1 to 6 have a lower T50 temperature than Comparative Examples 1 to 3, and exhibit excellent catalytic performance.

In contrast, Comparative Example 1 is the carrier is CeO ₂ that supporting Rh in the surface layer, since Rh on the CeO ₂ are supported, because Rh in the durability test becomes higher oxidation state, carried The T50 temperature is higher than in Examples 1-6. In Comparative Example 2, the carrier carrying Rh on the surface layer is a composite metal oxide of Al ₂ O ₃ and CeO _2, and a part of Rh is carried on CeO ₂ , so the durability test is in progress. Since Rh was in a highly oxidized state, the T50 temperature was higher than in Examples 1-6. In Comparative Example 3, since the carrier supporting Rh was ZrO ₂ , Rh was not in a highly oxidized state during the durability test, but was sintered during the durability test with H ₂ O in the exhaust gas. As a result, sintering of Rh was invited, so the temperature of T50 was higher than in Examples 1-6.

  About the catalyst of Example 1 and the comparative example 2 which were listed above, the surface layer catalyst powder was analyzed by XPS. This composite surface analyzer uses PHI ESCA-5600, the X-ray source is Mg-Kα ray (1253.6 eV), 300 W, the measurement depth is about 4 nm (photoelectron extraction angle 45 °), and the measurement area is The analysis was conducted on a sample that was 2 mm × 0.8 mm and pulverized in a menor mortar and then manually compacted. The analysis result is shown in the graph of FIG.

  From the graph of FIG. 2, it can be seen that the Rh3d5 orbital binding energy is larger in Comparative Example 2 than in Example 1 and shifted to the higher oxidation side (higher energy side), and the oxidation of Rh proceeds. . The binding energy of Example 1 was 308.6 eV, and the binding energy of Comparative Example 2 was 309.8 eV.

The Rh3d5 orbital binding energy is known to be 307.2 eV for metal Rh, 308.8 eV for Rh ₂ O ₃ , and 309.8 eV for high oxidation Rh. By comparison with these values, Comparative Example 2 is almost high. It can be said that it is a state close to oxidation Rh. It is widely known that the catalytic activity is higher in the order of metal Rh> Rh ₂ O ₃ > high oxidation Rh, and in the state of high oxidation Rh, the catalytic activity is significantly reduced. On the other hand, in Example 1, the binding energy is 308.6 eV, a state close to Rh ₂ O ₃ , and the catalyst can be configured with a higher catalytic activity than Comparative Example 2, and as a result The catalyst performance can be greatly improved.

The schematic diagram which shows an example of the exhaust gas purification catalyst of this invention. The graph which shows the result of having analyzed surface layer catalyst powder by XPS.

Explanation of symbols

1 Oxygen storage / release material 2 Carrier 3 Precious metal fine particles

Claims (5)

Noble metal fine particles containing rhodium,
Carrier particles carrying the noble metal fine particles,
Oxygen storage / release material particles that exchange active oxygen in exhaust gas are provided in the same layer, and the primary particle size of the oxygen storage / release material particles is 5 nm to 50 nm, and the primary particle size of the carrier particles is 5 nm. An exhaust gas purification catalyst characterized by having a thickness of ˜50 nm. 2. The exhaust gas purification catalyst according to claim 1, wherein a primary particle diameter of the oxygen storage / release material particles is 5 nm to 20 nm, and a primary particle diameter of the carrier particles is 5 to 30 nm.   The exhaust gas purification catalyst according to claim 1 or 2, wherein the oxygen storage / release material contains Ce oxide, and the content of Ce in the oxygen storage / release material is 1 to 30 mol%. Wherein said carrier, an exhaust gas purification according to any one of claims 1 to 3, characterized in that it comprises one or more selected from ZrO _2, TiO ₂ and _{La 2 O 3, Al 2 O} 3 catalyst.   The exhaust gas purification catalyst according to any one of claims 1 to 4, wherein the carrier is obtained by reducing Ce and Pr to 0.3 wt% or less in total.

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