JP2017154108A - Catalyst for exhaust purification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst for exhaust purification having improved NOx occlusion performance and NOx reduction performance.SOLUTION: A catalyst for exhaust purification comprises a composite metal fine particle, and an NOx occlusion material, and the composite metal fine particle comprises Rh and Co.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an exhaust gas purification catalyst.

  Exhaust gas discharged from an internal combustion engine for automobiles, for example, an internal combustion engine such as a gasoline engine or a diesel engine, contains harmful components such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). ) Etc.

  For this reason, in general, an exhaust gas purification device for decomposing and removing these harmful components is provided in the internal combustion engine, and these harmful components are substantially reduced by the exhaust gas purification catalyst installed in the exhaust gas purification device. Has been detoxified. As an example of such an exhaust gas purification catalyst, a NOx storage reduction catalyst is known.

In general, the NOx reduction efficiency of a NOx storage reduction catalyst is based on the ability to oxidize and store NO in exhaust gas to NO ₂ in a lean atmosphere and to reduce this NO ₂ to nitrogen (N ₂ ) in a stoichiometric atmosphere and a rich atmosphere. And the ability to do.

  Platinum group elements, particularly Pt, Pd, Rh, and the like are known as examples of constituent elements of the NOx storage reduction catalyst exhibiting these capabilities. However, platinum group elements are relatively expensive rare metals. Therefore, other elements that replace the platinum group elements have been studied.

Non-patent document 1 discloses a method for producing a Co-containing NOx occlusion reduction catalyst in which hexachloroplatinum (IV) acid hexahydrate, barium nitrate, cobalt nitrate, and rhodium (III) chloride trihydrate precursor are dissolved in distilled water. Preparing a solution, impregnating the solution into a support and calcining to prepare a calcined product, and reducing the calcined product to prepare a Co-containing NOx storage reduction catalyst. In this Non-Patent Document 1, Co present in the form of Co ₃ O ₄ in the Co-containing NOx occlusion reduction catalyst produced by the above method provides high oxidation by providing an oxidation site that oxidizes NO to NO _2. And is described as providing a larger contact area for NO ₂ effluent to the NOx storage site of Ba.

Rohh Vijay, Christopher M. et al. Snivly, Jochen Lauterbach, 2006, Performance of Co-containing NOx storage and reduction catalytics as a function of cycling conditions. Journal of Catalysis, Volume 243, Issue 2, 25 October 2006, Pages 368-375.

  An object of the present invention is to provide an exhaust gas purification catalyst having improved NOx storage performance and NOx reduction performance.

  The present inventors have found that the above problem can be solved by the following means.

<1> An exhaust gas purification catalyst containing fine composite metal particles and a NOx storage material,
The composite metal fine particles include Rh and Co.
Exhaust gas purification catalyst.

  According to the present invention, it is possible to provide an exhaust gas purification catalyst with improved NOx storage performance and NOx reduction performance.

FIG. 1 is a schematic view of a conventional exhaust gas purification catalyst produced by an impregnation method. 2A, 2B, and 2C are schematic views of a liquid phase reduction method for producing an embodiment of the exhaust gas purification catalyst of the present invention. FIGS. 3A to 3E are STEM images of the exhaust gas purification catalysts of Examples 1 to 5, respectively. 4A is an STEM image of the exhaust gas purification catalyst of Example 3, and FIGS. 4B to 4D are STEM-EDX mappings of FIG. 4A regarding barium, Rh, and Co, respectively. FIG. 4 (e) shows the EDX analysis result in the area of the frame line in FIG. 4 (a). Fig.5 (a) is a STEM image of the exhaust gas purification catalyst of the comparative example 3, FIG.5 (b) and (c) respectively show the EDX analysis result in the area of the frame 1 and 2 of Fig.5 (a). Show. FIG. 6 shows X-ray diffraction patterns of exhaust gas purifying catalysts of Examples 1 to 5; and diffraction peak positions of Rh ₂ O ₃ , Rh, Co, CoO, Co ₃ O ₄ , and BaCO ₃ . FIG. 7 shows the relationship between Co / Rh (mol ratio) and NOx occlusion amount (μmol / g) for the samples of Examples 1 to 5 and Comparative Examples 1 to 4. FIG. 8 shows the relationship between Co / Rh (mol ratio) and NOx reduction efficiency (%) for the samples of Examples 1 to 5 and Comparative Examples 1 to 4.

  Hereinafter, embodiments of the present invention will be described in detail. Note that the present invention is not limited to the following embodiments, and can be implemented with various modifications within the scope of the gist of the present invention.

<Exhaust gas purification catalyst>
<Conventional exhaust gas purification catalyst, especially NOx storage reduction catalyst>
As shown in Non-Patent Document 1, a NOx occlusion reduction catalyst including Rh and Co and a NOx occlusion material is known. Since this NOx occlusion reduction catalyst has a particularly high ability to oxidize Co, particularly Co ₃ O ₄ contained therein, it is possible to oxidize NO in exhaust gas to NO ₂ in a lean atmosphere and efficiently occlude it. In addition, since the NOx reduction ability by Rh is particularly high, it is possible to reduce the stored NO ₂ to N ₂ in a stoichiometric atmosphere and a rich atmosphere.

However, in general, when the distance between Co, particularly Co-containing fine particles, and the closest Rh, particularly Rh-containing fine particles, is long, it is generated in the vicinity of the Co-containing fine particles and is stored in the NOx storage material. There is a possibility that the reduced NO ₂ is not reduced by the Rh-containing fine particles.

  The present inventors have intensively studied a catalyst capable of efficiently storing NOx and efficiently reducing the NOx, and have come up with the present invention. The invention is shown below.

<Exhaust gas purification catalyst of the present invention>
The exhaust gas purifying catalyst of the present invention includes composite metal fine particles and a NOx storage material, and the composite metal fine particles include Rh and Co.

In the exhaust gas purifying catalyst of the present invention, since Rh and Co are compounded in the composite metal fine particles, Rh and Co are close to each other. Therefore, Co oxidizes NO to NO ₂ in a lean atmosphere, and this is occluded by the NOx occlusion material. In a rich atmosphere, Rh can quickly reduce the occluded NOx.

  As a result, according to the present invention, it is possible to provide an exhaust gas purification catalyst having improved NOx storage performance and NOx reduction performance.

  Below, the several component which comprises the exhaust gas purification catalyst of this invention is demonstrated.

(Composite metal fine particles)
The exhaust gas purifying catalyst of the present invention contains composite metal fine particles, and the composite metal fine particles contain Rh and Co.

  In the present invention, “composite metal fine particles” mean fine particles having at least two kinds of metal elements and / or oxides thereof in one particle. Therefore, “a composite metal fine particle containing Co and Rh” as an example of the composite metal fine particle means a fine particle having Co and Rh and / or an oxide thereof in one particle.

  In the exhaust gas purifying catalyst of the present invention, the metal fine particles are analyzed by a scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDX) method, whereby composite metal fine particles containing Rh and Co are present. Can be confirmed.

  When the particle diameter of the composite metal fine particles is sufficiently small, the specific surface area is increased, the number of Rh active points and the number of Co active points are increased, and the NOx purification ability of the exhaust gas purification catalyst may be improved. is there.

  Moreover, when the particle diameter of the composite metal fine particles is moderately large, there is a possibility that the NOx purification ability of the exhaust gas purification catalyst can be sufficiently exhibited.

  Therefore, an example of the average particle diameter of the plurality of composite metal fine particles is not particularly limited, but may be an average particle diameter of more than 0 nm, 1 nm or more, or 2 nm or more. The average particle diameter of the plurality of composite metal fine particles is not particularly limited, but may be an average particle diameter of 100 nm or less, 70 nm or less, 40 nm or less, 10 nm or less, 7 nm or less, 5 nm or less, 4 nm or less, or 3 nm or less. .

  In the present invention, the “average particle diameter” means a circle equivalent diameter (Heywood diameter) of 10 or more particles randomly selected using means such as a scanning transmission electron microscope (STEM) unless otherwise specified. ) Is the arithmetic average value of those measured values.

  When the average ratio of Co to the total of Rh and Co in the composite metal fine particles is sufficiently large, the oxidation ability by Co is sufficiently exhibited. Moreover, when this ratio is not too large, it is possible to secure a sufficient number of Rh active points.

  In particular, as an example of the average ratio of Co to the total of Rh and Co in a plurality of fine composite metal particles, more than 0 atomic%, 1 atomic% or more, 2 atomic% or more, and 5 atomic% or more, and / or 100 atomic% And an average ratio of 90 atomic% or less, 70 atomic% or less, 50 atomic% or less, and 30 atomic% or less.

(NOx storage material)
The exhaust gas purification catalyst of the present invention optionally contains a NOx storage material.

  The NOx storage material is not particularly limited, but may be a basic material. Examples of NOx storage materials include alkali metals and salts thereof such as potassium (K) and potassium acetate; alkaline earth metals and salts thereof such as barium (Ba) and barium acetate; and combinations thereof. be able to.

(Powder carrier)
The exhaust gas purifying catalyst of the present invention optionally contains a powder carrier. In this case, the composite metal fine particles and / or the NOx storage material are supported on the powder carrier.

  The powder carrier on which the composite metal fine particles are supported is not particularly limited, and may be any oxide generally used as a powder carrier in the technical field of exhaust gas purification catalysts.

Examples of such a powder carrier include silica (SiO ₂ ), magnesia (MgO), zirconia (ZrO ₂ ), ceria (CeO ₂ ), alumina (Al ₂ O ₃ ), titania (TiO ₂ ), and their Examples thereof include solid solutions and combinations thereof.

  It should be understood that the exhaust gas purification ability of the exhaust gas purification catalyst of the present invention, particularly the NOx purification ability, may be improved depending on the types of powder carriers, combinations thereof, and the like.

<Others>
Regarding the exhaust gas purification catalyst of the present invention, the following method for producing an exhaust gas purification catalyst of the present invention can be referred to.

<Method for producing exhaust gas purification catalyst>
<Conventional method for producing exhaust gas purification catalyst, especially impregnation method>
In general, nano-sized metal fine particles have an electron energy structure different from that of the bulk due to the quantum size effect, and thus exhibit electrical and optical characteristics depending on the particle size. Furthermore, it is expected that nano-sized metal fine particles having a very large specific surface area will work as a highly active catalyst.

  Regarding the method for producing such nano-sized fine metal particles, the method includes preparing a mixed solution containing a salt of each metal element, and impregnating the mixed solution into a powder carrier and calcining to prepare an exhaust gas purification catalyst. Methods, so-called impregnation methods are known. However, generally, fine particles in which Rh and Co are combined cannot be produced by an impregnation method.

  This is not intended to be bound by any particular logic, but Rh-containing fine particles and Co-containing fine particles are caused by factors such as a difference in redox potential between metal ions and repulsion between ions. It is thought that it precipitates separately.

In this case, the distance between the Co-containing fine particles and the closest Rh-containing fine particles is long. Therefore, in the exhaust gas purification catalyst manufactured by the impregnation method, NO ₂ generated in the vicinity of the Co-containing fine particles and stored in the NOx storage material may not be reduced by the Rh-containing fine particles.

  FIG. 1 is a schematic view of a conventional exhaust gas purification catalyst manufactured by an impregnation method. In the conventional exhaust gas purification catalyst 1, the Rh-containing fine particles 2 and the Co-containing fine particles 3 are separately deposited, and the distance between them is long. Note that a NOx occlusion material 4 is present immediately beside the Rh-containing fine particles 2 and the Co-containing fine particles 3.

<Production method of exhaust gas purification catalyst of the present invention: liquid phase reduction method>
The method for producing an exhaust gas purification catalyst of the present invention includes a step of producing composite metal fine particles by heating and refluxing a solution containing Rh ions, Co ions, a protective agent, and a reducing agent.

  In contrast to the above impregnation method, fine particles in which Rh and Co are complexed can be produced by a liquid phase reduction method. This is not intended to be bound by any particular logic, but since the hydroxide containing Rh ions and Co ions is protected by the protective agent, the aggregation of a plurality of hydroxides is moderate. This is considered to be because the hydroxides in this state are collectively reduced by the reducing agent.

  Examples of the time for heating and refluxing the solution containing Rh ions, Co ions, a protective agent, and a reducing agent are not particularly limited, but are 0.5 hours or more, 1 hour or more, 1.5 hours or more, and Mention may be made of 3 hours or more and / or 48 hours or less, 24 hours or less, 12 hours or less and 6 hours or less.

  In addition, the method may further optionally include a step of supporting the composite metal fine particles on the powder carrier during or after the step of generating the composite metal fine particles.

  The order and method of supporting the composite metal fine particles on the powder carrier may be any order and method. The order and method of supporting the composite metal fine particles on the powder carrier is, for example, heating and refluxing a solution containing Rh ions, Co ions, a protective agent, and a reducing agent, and then adding the powder carrier to this solution and stirring. The order and method of supporting the composite metal fine particles on the powder carrier may be used. Thereby, the composite metal fine particles can be efficiently supported on the powder carrier.

  2A, 2B, and 2C are schematic views of a liquid phase reduction method for producing an embodiment of the exhaust gas purification catalyst of the present invention. FIG. 2A shows a state before mixing Rh ions 101, Co ions 102, a protective agent 103, a reducing agent 104, and a solvent (not shown); FIG. 2B shows a mixed solution in which these are mixed. And FIG. 2 (c) shows the exhaust gas purification catalyst of the present invention produced by drying and / or calcining the hydroxide 105. One embodiment 100 is shown.

  In this embodiment 100 of the exhaust gas purifying catalyst of the present invention, Rh and / or its oxide 106, and Co and / or its oxide 107 are combined together in the composite metal fine particle 110, so that Rh and Co is close. Note that the NOx occlusion material 108 exists immediately next to the composite metal fine particles 110, and these are supported on the carrier 109.

<Rh ion and Co ion>
Rh ions and Co ions are contained in a solution containing a protective agent and a reducing agent.

  Although it does not specifically limit as a raw material of Rh ion, For example, the salt of Rh, Rh halide, etc., and these combinations can be mentioned. As raw materials for Rh ions, inorganic salts of Rh, such as nitrates, phosphates, and sulfates; organic salts of Rh, such as oxalates and acetates; halides of Rh, such as fluorides , Chlorides, bromides, and iodides; and combinations thereof.

  Regarding the Co ion raw material, the above description of the Rh ion raw material can be referred to.

  The concentration of Rh ions and Co ions is not particularly limited. As an example of the concentration of Rh ions and Co ions, the total ion concentration is preferably in the range of 0.01M to 0.20M.

  Although it does not specifically limit as an example of the molar ratio of Rh ion and Co ion, It may correlate with the molar ratio of Rh and Co in the target composite metal fine particle, for example, the range of 99: 1 to 1:99 is sufficient. Mention may be made of a molar ratio, a molar ratio in the range of 80:20 to 20:80, and a molar ratio in the range of 60:40 to 40:60.

  The molar ratio of Rh ions and Co ions is not particularly limited as long as the composite metal fine particles of the above-described exhaust gas purification catalyst of the present invention can be produced. These molar ratios may correlate with the average ratio of Co to the total of Rh and Co in the composite metal fine particles of the exhaust gas purification catalyst of the present invention. In this case, these molar ratios may be determined in consideration of a reduction scale of those ions, for example, a redox potential and easiness of solid solution of each element.

<Reducing agent>
The reducing agent is contained in a solution containing Rh ions, Co ions, and a protective agent. The reducing agent can be used to reduce Rh ions and Co ions to produce composite metal fine particles.

The example of a reducing agent will not be specifically limited if the hydroxide containing Rh and Co can be reduced. An example of a reducing agent may be sodium borohydride (NaBH ₄ ).

  The amount of the reducing agent is not particularly limited, but the molar amount in the range of 1 to 100000 times, the molar amount in the range of 1 to 50000 times, or 1 to 10000 times the total molar amount of Rh and Co. A mol amount in the range of may be sufficient.

<Protective agent>
The protective agent is contained in a solution containing Rh ions, Co ions, and a reducing agent.

  The protective agent can suppress excessive aggregation of a plurality of hydroxides and can appropriately disperse the hydroxide in the solution. Therefore, the protective agent serves to moderately disperse a plurality of substantially uniform nano-sized hydroxides. As a result, an exhaust gas purification catalyst in which composite metal fine particles containing Rh and Co are dispersed can be realized.

  Examples of the protective agent include, but are not limited to, polyvinylpyrrolidone (PVP), polyvinylpyrrolidone K25 (PVP-K25), polyethyleneimine, and the like, and combinations thereof. Among these, PVPK25 is preferable from the viewpoint of high solubility.

  The concentration of the protective agent is not particularly limited as long as aggregation of a plurality of hydroxides can be suppressed. For example, the molar amount in the range of 1 to 1000 times the total molar amount of Rh and Co, The concentration may be a mol amount in the range of 1 to 500 times, or a mol amount in the range of 1 to 100 times. Here, when the protective agent is a polymer such as PVPK25, the molar amount of the protective agent means the molar amount of the monomer unit.

<solvent>
The solvent is optionally contained in a solution containing Rh ions, Co ions, a protective agent, and a reducing agent.

  Examples of the solvent are not particularly limited as long as the solvent is not reduced by the above reducing agent. Examples of solvents may be glycols such as tetraethylene glycol.

<Others>
Regarding the method for producing the exhaust gas purifying catalyst of the present invention, reference can be made to the above description of the exhaust gas purifying catalyst of the present invention.

  The present invention will be described in more detail with reference to the following examples, but it goes without saying that the scope of the present invention is not limited by these examples.

  << Example 1 (liquid phase reduction method) >>

<Process A>
In a 200 mL beaker, 120 mL of tetraethylene glycol was added and heated and stirred at 80 ° C., and 22.5 g (202.5 mmol in terms of monomer) of PVPK25 was added thereto and completely dissolved. To this solution was added 420 mg (1.69 mmol) of cobalt (II) acetate tetrahydrate dissolved in 3 mL of distilled water.

<Process B>
Next, 11.2 g (5.06 mmol) of an Rh chloride solution (Rh concentration 4.64 wt%) dissolved in 40 mL of tetraethylene glycol was added to this solution, and this was stirred at room temperature for 10 minutes under a nitrogen atmosphere. .

<Process C>
While heating the above beaker containing the solution in an oil bath, 1.02 g (27.0 mmol) of sodium borohydride dissolved in 40 mL of tetraethylene glycol was gradually added to the beaker. The oil bath temperature was heated to 160 ° C. and stirring was continued for 1 hour. The solution was cooled to room temperature, acetone was added to the beaker containing the solution, and the total amount of the solution was reduced to 2 L (diluted 10 times). This solution was centrifuged with a centrifuge (3000 rpm × 10 minutes) to precipitate the product, and the supernatant of the solution was discarded. Further, about 20 ml of acetone was added to the solution in the centrifuge along the wall surface, and the supernatant of the solution was discarded, thereby obtaining a slurry.

<Process D>
The obtained slurry was dispersed in 50 mL of ethanol, divided into half, 26.0 g of γ-alumina carrier was added, and the solvent was evaporated by heating to obtain a solid. Further, the solid was dried at 120 ° C. for 2 hours, pulverized, and calcined at 500 ° C. for 3 hours, thereby obtaining a catalyst precursor.

<Process E>
Distilled water (100 mL) was placed in a 300 mL beaker, and further 5.2 g of barium acetate was added to the beaker and stirred at room temperature, whereby barium acetate was completely dissolved. Further, 26.0 g of the catalyst precursor obtained above was added to this beaker and heated to 120 ° C., and the dispersion medium was evaporated to obtain a solid. This solid was further dried at 120 ° C. overnight. This solid was pulverized in a mortar to form a uniform powder, and baked in an electric furnace at 500 ° C. for 2 hours to obtain an exhaust gas purification catalyst.

Example 2 (Liquid Phase Reduction Method)
Exhaust gas purification catalyst of Example 2 was obtained in the same manner as Example 1 except for the following changes:
-In step A, the amount of cobalt (II) acetate tetrahydrate is 841 mg (3.38 mmol);
The amount of Rh chloride solution in step B is 7.49 g (3.38 mmol);
The amount of γ-alumina support in step D is 17.4 g each;
In step E, the amount of barium acetate is 3.4 g and the amount of catalyst precursor is 16.6 g.

Example 3 (Liquid Phase Reduction Method)
Except for the following changes, the exhaust gas purification catalyst of Example 3 was obtained in the same manner as in Example 1:
The amount of cobalt (II) acetate tetrahydrate in step A is 1260 mg (5.06 mmol);
The amount of Rh chloride solution is 3.75 g (1.69 mmol) in step B;
The amount of γ-alumina support in step D is 8.7 g each;
In step E, the amount of barium acetate is 1.5 g and the amount of catalyst precursor is 8.1 g.

Example 4 (Liquid Phase Reduction Method)
Except for the following changes, the exhaust gas purification catalyst of Example 4 was obtained in the same manner as in Example 1:
-The amount of cobalt (II) acetate tetrahydrate in step A is 1510 mg (6.08 mmol);
The amount of Rh chloride solution is 1.50 g (0.68 mmol) in step B;
The amount of γ-alumina support in step D is 3.5 g each;
In step E the amount of barium acetate is 0.7 g and the amount of catalyst precursor is 3.3 g;
-Operation of process AE is performed twice.

Example 5 (Liquid Phase Reduction Method)
Exhaust gas purification catalyst of Example 5 was obtained in the same manner as Example 1 except for the following changes:
The amount of cobalt (II) acetate tetrahydrate is 1630 mg (6.53 mmol) in step A;
The amount of Rh chloride solution is 0.49 g (0.22 mmol) in Step B;
The amount of γ-alumina support in step D is 1.2 g each;
In step E the amount of barium acetate is 0.2 g and the amount of catalyst precursor is 1.0 g;
-Operation of process AE is performed 6 times.

<< Comparative Example 1 (impregnation method) >>
<Process A>
Cobalt (II) acetate tetrahydrate (210 mg, 0.85 mmol) and 100 mL of distilled water were mixed to obtain a solution.

<Process B>
To the above solution, 5.6 g (2.53 mmol) of Rh chloride solution (Rh concentration 4.64 wt%) was added.

<Process C>
Omitted (no operation is performed).

<Process D>
To the above solution (Step B), 26.0 g of γ-alumina carrier was added and the solvent was evaporated by heating, whereby a solid was obtained. The solid was dried at 120 ° C. for 2 hours, ground and calcined at 500 ° C. for 3 hours, thereby obtaining a catalyst precursor.

<Process E>
Distilled water (100 mL) was placed in a 300 mL beaker, and further 5.2 g of barium acetate was added to the beaker and stirred at room temperature, whereby barium acetate was completely dissolved. Further, 26.0 g of the catalyst precursor obtained above was added to this beaker and heated to 120 ° C., and the dispersion medium was evaporated to obtain a solid. This solid was further dried at 120 ° C. overnight. This solid was pulverized in a mortar to form a uniform powder, and baked in an electric furnace at 500 ° C. for 2 hours to obtain an exhaust gas purification catalyst.

<< Comparative Example 2 (impregnation method) >>
Except for the following changes, the exhaust gas purification catalyst of Comparative Example 2 was obtained in the same manner as Comparative Example 1:
The amount of cobalt (II) acetate tetrahydrate is 420 mg (1.69 mmol) in step A;
The amount of Rh chloride solution is 3.75 g (1.69 mmol) in step B;
The amount of γ-alumina support is 17.4 g in step D;
In step E, the amount of barium acetate is 3.4 g and the amount of catalyst precursor is 17.0 g.

<< Comparative Example 3 (impregnation method) >>
Except for the following changes, the exhaust gas purification catalyst of Comparative Example 3 was obtained in the same manner as Comparative Example 1:
-In step A, the amount of cobalt (II) acetate tetrahydrate is 630 mg (2.53 mmol);
The amount of Rh chloride solution is 1.88 g (0.85 mmol) in step B;
-In step D, the amount of γ-alumina support is set to 8.7 g;
In step E, the amount of barium acetate is 1.5 and the amount of catalyst precursor is 8.1 g.

<< Comparative Example 4 (impregnation method) >>
Except for the following changes, the exhaust gas purification catalyst of Comparative Example 4 was obtained in the same manner as Comparative Example 1:
-The amount of cobalt acetate (II) tetrahydrate in step A is 750 mg (3.04 mmol);
-In step B, the amount of the Rh chloride solution is 0.75 g (0.34 mmol);
In step D, the amount of γ-alumina support is 3.5 g;
In step E, the amount of barium acetate is 0.7 and the amount of catalyst precursor is 3.3 g.

  Regarding the exhaust gas purification catalysts of Examples 1 to 5 and Comparative Examples 1 to 4, the manufacturing method, main materials of Steps A to E, and the molar ratio of Rh: Co in the composite metal fine particles are shown in Table 1 below. .

  From Table 1, the molar ratio of Co to Rh in the exhaust gas purification catalyst is small in the order of Examples 1, 2, 3, 4, and 5, that is, Example 1 is the smallest and Example 5 is the largest. Please note that. Further, from Table 1, the molar ratio of Co to Rh in the exhaust gas purification catalyst is smaller in the order of Comparative Examples 1, 2, 3, and 4, that is, Comparative Example 1 is the smallest and Comparative Example 4 is the largest. Please keep in mind.

<Evaluation>
Regarding the exhaust gas purification catalysts of Examples 1 to 5 and Comparative Example 3, evaluation by a scanning transmission electron microscope with an energy dispersive X-ray analyzer (STEM-EDX), evaluation by powder X-ray diffraction (XRD), and NOx purification The characteristics were evaluated.

<Evaluation by STEM-EDX>
The exhaust gas purifying catalysts of Examples 1 to 5 and Comparative Example 3 were mixed with ethanol in appropriate amounts, the mixed solution was dropped onto a Cu grid and dried, and the Cu grid was set to an acceleration voltage of 200 kV. (Measured with JEM-1000 manufactured by JEOL Ltd.) The results of Examples 1 to 5 are shown in FIGS. 3 (a) to (e), respectively; the detailed results of Example 3 are shown in FIGS. 4 (a) to (e): and the detailed results of Comparative Example 3 are shown. Are shown in FIGS. 5 (a) to 5 (c).

  FIGS. 3A to 3E are STEM images of the exhaust gas purification catalysts of Examples 1 to 5, respectively. 3A to 3E show that the particle size of the fine particles increases as the molar ratio of Co to Rh increases in the exhaust gas purification catalyst. Further, EDX analysis was performed on the fine particles in FIGS. 3A to 3E, and it was confirmed that Rh and Co were contained in the particles in FIGS. 3A to 3E. .

  4A is an STEM image of the exhaust gas purification catalyst of Example 3, and FIGS. 4B to 4D are STEM-EDX mappings of FIG. 4A regarding barium, Rh, and Co, respectively. FIG. 4 (e) shows the EDX analysis result in the area of the frame line in FIG. 4 (a).

  The position of the fine particles in FIG. 4A, the position of Rh in FIG. 4C, and the position of Co in FIG. 4D are substantially the same. From these figures, Rh and Co are close to each other. I understand that FIG. 4 (e) shows that Rh and Co exist in the area of the frame line in FIG. 4 (a), and that Rh and Co are also present in the vicinity. This is presumably because the exhaust gas purification catalyst of Example 3 was produced by the liquid phase reduction method.

  FIG. 5A is an STEM image of the exhaust gas purification catalyst of Comparative Example 3, and FIGS. 5B and 5C are the results of EDX analysis in the areas of the frame lines 1 and 2 in FIG. 5A, respectively. Is shown. From FIGS. 5A and 5B, only Rh exists in the area of the frame line 1, and from FIGS. 5A and 5C, only Co exists in the area of the frame line 2. I understand that This is presumably because the exhaust gas purification catalyst of Comparative Example 3 was produced by the impregnation method.

<Evaluation by XRD>
The exhaust gas purifying catalysts of Examples 1 to 5 were measured by an X-ray diffraction (XRD) method. This measurement was performed with RINT2000RS manufactured by Rigaku Corporation. The X-ray source is CuKα (λ = 1.5418 nm), 10 to 90 deg. Of step width 0.02 deg. The tube voltage was 50 kV and the tube current was 300 mA. The diffraction peak position was compared with the known data in the JCPDS data file. The results are shown in FIG.

FIG. 6 is a diagram showing X-ray diffraction patterns of exhaust gas purifying catalysts of Examples 1 to 5; and diffraction peak positions of Rh ₂ O ₃ , Rh, Co, CoO, Co ₃ O ₄ , and BaCO ₃ . (1) to (5) in FIG. 6 correspond to the X-ray diffraction patterns of the exhaust gas purification catalysts of Examples 1 to 5, respectively. Further, the diffraction peak positions of Rh ₂ O ₃ , Rh, Co, CoO, Co ₃ O ₄ , and BaCO ₃ in FIG. 6 correspond to known data in the JCPDS data file.

From FIG. 6, it can be seen that the Co ₃ O ₄ peak increases as the molar ratio of Co to Rh increases in the exhaust gas purification catalyst, so that Co exists as Co ₃ O ₄ .

<Evaluation of NOx purification characteristics>
The NOx purification storage amount and NOx reduction efficiency of the exhaust gas purification catalysts of Examples 1 to 5 and Comparative Examples 1 to 4 were evaluated. In the evaluation, a gas flow type catalyst evaluation apparatus was used. Samples of the exhaust gas purification catalyst of each example are placed in the apparatus, and each gas at a predetermined temperature is measured with an FT-IR analyzer and a magnetic pressure analyzer (SESAM-HL and Bex manufactured by Best Sokki Co., Ltd.). The conversion behavior was analyzed. The results are shown in FIGS.

  The mass of the above sample was 3 g. Further, under the condition of 500 ° C., one cycle in which the lean gas was 60 seconds and the rich gas was 5 seconds was repeated 5 cycles. The NOx occlusion amount during the lean period and the NOx reduction efficiency during the rich period were each an average value of 2 to 5 cycles. The composition of lean gas and rich gas is shown in Table 2 below.

  FIG. 7 shows the relationship between Co / Rh (mol ratio) and NOx occlusion amount (μmol / g) for the samples of Examples 1 to 5 and Comparative Examples 1 to 4. FIG. 7 shows that the NOx occlusion amounts of the samples of Examples 1 to 5 are larger than those of the samples of Comparative Examples 1 to 4. This is presumably because the composite metal fine particles containing Rh and Co are dispersed in the samples of Examples 1 to 5.

FIG. 8 shows the relationship between Co / Rh (mol ratio) and NOx reduction efficiency (%) for the samples of Examples 1 to 5 and Comparative Examples 1 to 4. FIG. 8 shows that the NOx reduction efficiency of the samples of Examples 1 to 5 is higher than that of the samples of Comparative Examples 1 to 4 manufactured by the impregnation method. This is because Rh and Co are close to each other in the composite metal fine particles, whereby in a lean atmosphere, Co oxidizes NO to NO ₂ , and this is occluded with a NOx occlusion material, and in a rich atmosphere, It is considered that Rh rapidly reduced the stored NOx.

  Although the preferred embodiment of the present invention has been described in detail, it is possible to change the manufacturer, grade, quality, etc. of the apparatus, equipment, chemicals, etc. used in the present invention without departing from the scope of the claims. Those skilled in the art understand that.

DESCRIPTION OF SYMBOLS 1 Conventional exhaust gas purification catalyst 2 Rh containing fine particle 3 Co containing fine particle 4 NOx occlusion material 100 One Embodiment of the exhaust gas purification catalyst of this invention 101 Rh ion 102 Co ion 103 Protective agent 104 Reducing agent
105 Hydroxides 106 Rh and / or their oxides 107 Co and / or their oxides 108 NOx storage materials 109 Supports 110 Composite metal fine particles

Claims (1)

An exhaust gas purification catalyst comprising composite metal fine particles and a NOx storage material,
The composite metal fine particles include Rh and Co;
Exhaust gas purification catalyst.