JPH11262663A - Exhaust gas cleaning catalyst and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high heat resistant catalyst capable of more suppressing sintering of Pt. SOLUTION: This catalyst is constituted of both a particulate carrier 1 consisting of oxide and a Pt complex oxide layer 2 formed on the surface of the carrier 1. In this case, since a contact area of the Pt complex oxide with the carrier 1 becomes larger, compared to the case in which the particulate Pt complex oxide is mixed with the particular carrier 1, a suppressing work of sintering of the Pt complex oxide due to the carrier is realized to the utmost.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst and a method for producing the same, and more particularly to an exhaust gas purifying catalyst and a method for producing the same, the purification performance of which is prevented from being deteriorated even in a high-temperature lean atmosphere.

[0002]

2. Description of the Related Art Conventionally, an exhaust gas purifying catalyst disposed in an exhaust system of an automobile or the like includes platinum (Pt) as a catalyst component.
Noble metals such as rhodium (Rh) and palladium (Pd) are used, and these noble metals are used by being supported on an oxide carrier such as alumina (Al ₂ O ₃ ). Among them, Pt has more resources than Rh and exhibits higher catalytic activity than Pd, so Pt is mainly used as an exhaust gas purifying catalyst for automobiles.
Is used.

[0003] However, Pt is oxidized in a high-temperature lean atmosphere, and has a problem that the surface area is reduced by sintering and the catalytic activity is significantly reduced. In addition, since the exhaust gas temperature tends to become higher due to the recent improvement in engine performance, increase in high-speed running, and stricter exhaust gas regulations, there is a strong demand for the development of means for suppressing Pt sintering.

[0004] The applicant of the present invention has previously disclosed Japanese Patent Application Laid-Open No. 62-27715.
No. 0 proposes a catalyst using a perovskite-type composite oxide of Pt and a lanthanoid element or an alkaline earth metal. According to this catalyst, the thermal deterioration and alloying of Pt are suppressed as compared with the conventional Pt / Al ₂ O ₃ catalyst, so that the durability is greatly improved. However, the temperature of exhaust gas in recent years has become extremely high, and even in the catalyst disclosed in Japanese Patent Application Laid-Open No. 62-277150, the perovskite-type composite oxide starts to thermally decompose in a region exceeding 900 ° C. Therefore, in order to cope with a further increase in the exhaust gas temperature in the future, it is necessary to suppress sintering of Pt even in a region exceeding 1000 ° C.

[0005] The inventors of the present invention have conducted intensive studies and disclosed in Japanese Patent Application Laid-Open No. 10-358 a Pt containing one or more elements selected from the group consisting of Pt and an alkaline earth element or a Group 3A element in the periodic table. A catalyst comprising a mixture of a composite oxide powder and γ-Al ₂ O ₃ powder has been proposed. According to this catalyst, Pt is incorporated into the crystal of the composite oxide and stabilized, so that high heat resistance of 1000 ° C. or higher can be achieved.

[0006]

However, Japanese Patent Laid-Open No. Hei 10-358
It has been clarified that even with the catalyst disclosed in Japanese Patent Application Laid-Open No. H11-264, a certain degree of sintering occurs in the Pt composite oxide in a high-temperature lean atmosphere, and it is required to further suppress Pt sintering. The present invention has been made in view of such circumstances, and an object of the present invention is to provide a highly heat-resistant catalyst that can further suppress sintering of Pt.

[0007]

The feature of the exhaust gas purifying catalyst of the present invention which solves the above-mentioned problems is that it comprises an oxide carrier and a Pt composite oxide layer formed on the surface of the oxide carrier. . The feature of the method for producing an exhaust gas purifying catalyst of the present invention, which is optimal for producing the above catalyst, is that an oxide carrier is mixed with a solution in which an alkoxide containing Pt is dissolved, and the alkoxide is hydrolyzed and then calcined. The object is to form a Pt composite oxide layer on the surface of an oxide carrier.

[0008]

DETAILED DESCRIPTION OF THE INVENTION According to the study of the present inventors, sintering of a Pt composite oxide in a high-temperature oxidizing atmosphere is as follows.
Largest when the Pt composite oxide alone is used, sintering decreases when mixed with the oxide carrier, but is still large.Sintering occurs when a thin Pt composite oxide layer is formed on the surface of the oxide carrier. Was found to be most suppressed. The present invention has been made based on this finding.

That is, in the exhaust gas purifying catalyst of the present invention,
A Pt composite oxide layer is formed on the surface of the oxide carrier. It is thought that the contact area between the Pt composite oxide and the carrier is larger than when the particulate Pt composite oxide and the oxide carrier are mixed, thereby suppressing sintering of the Pt composite oxide. Can be As the oxide carrier, one selected from alumina, silica, titania, zirconia, ceria, silica-alumina, and the like, or a mixture of a plurality of types can be used. However, these oxides may react with the alkaline earth metal in the Pt composite oxide, changing the structure of the Pt composite oxide and decreasing the catalytic activity. Therefore, it is desirable to use a composite oxide containing an alkaline earth metal for the oxide carrier itself. This allows the oxide carrier and Pt
Since the reaction with the composite oxide is suppressed, the durability of the catalyst is further improved.

As such a composite oxide carrier, it is preferable to use a composite oxide comprising alumina, silica, titania, zirconia, ceria, silica-alumina, etc. and an alkaline earth metal or a lanthanoid element. The preferred composition ratio is such that when M represents Al, Si, Ti, Zr, Ce, etc., and X represents an alkaline earth metal or a lanthanoid element, the molar ratio is in the range of X: M = 1: 0.3 to 1:20. Are preferred, and X:
The range of M = 1: 0.5 to 1: 4 is particularly preferred.

The higher the specific surface area of the oxide carrier, the better.
However, it is important that the specific surface area does not change before and after endurance.
It is. 15m if the specific surface area does not change before and after endurance^Two/ g
Sufficient effects can be obtained even with this degree. Pt composite oxide
Examples of the Pt composite oxide constituting the layer include the following chemicals
A composite oxide represented by the formula is exemplified. (1) X_FourPtO₆(X = Ca, Sr, Ba, Mg) (2) X'Pt_TwoO₇(X '= Sc, La, Pr) In this case, Sc, La, Pr
It is to make an icroa system. (3) SrX''PtO₆(X '' = Co, Ni, Cu) These are Sr_FourPtO₆of
It is obtained by substituting one Sr. (4) Ba_TwoZPtO₆(Z = Pr, Ce) (5) Ba₈Y_ThreePt_FourO_17.5 In addition, Cu, Ni, CO, Fe
Those containing any of them can be preferably used. This
By combining base metals such as
Is further improved. For example, if X is Ba and further contains Cu
For example, Ba_FourCuPt_TwoO₉, Ba_TwoY_TwoCuPtO₈, Ba_TwoY_TwoCu_Two
PtO_Ten, Ba_ThreeY_TwoCu_TwoPtO_Ten, Ba_1.3Sr_1.7Y_TwoCu _TwoPtO_Ten, Ba_TwoHo_Two
CuPtO₈, Ba_TwoHo_TwoCu_TwoPtO_Ten, Ba_TwoEr_TwoCuPtO₈, Ba_ThreeEr_TwoCu_TwoPtO
_TenAnd the like.

The thickness of the Pt composite oxide layer is preferably set to 0.05 μm or less, particularly preferably of the order of primary particles. An increase in the thickness of the Pt composite oxide layer is not preferable because sintering occurs between the secondary particles in the Pt composite oxide layer. In the exhaust gas purifying catalyst of the present invention, since the specific surface area of the catalyst is proportional to the number of Pt ions present on the surface, the larger the specific surface area, the better. In order to increase the specific surface area, it is desirable to use a fine particulate oxide carrier having a specific surface area of 15 m ² / g or more and form a thin Pt composite oxide layer on the surface thereof as described above. An alkoxide method (sol-gel method), a gas phase decomposition method, or the like can be used to form a thin Pt composite oxide layer as described above.

In the production method of the present invention for stably producing the exhaust gas purifying catalyst of the present invention, first, an oxide carrier is mixed with a solution in which an alkoxide containing Pt is dissolved, and the alkoxide is hydrolyzed and then calcined. By doing so, a Pt composite oxide layer is formed on the surface of the oxide carrier. That is, in the production method of the present invention, the Pt composite oxide layer is formed by the alkoxide method (sol-gel method). In the alkoxide method, an alkoxide is dissolved in a solvent such as alcohol to form a solution, and an oxide precursor that is a solid hydroxide is precipitated by hydrolysis and aging, and then the precipitated oxide precursor is calcined by calcination. Form an object. Therefore, in the production method of the present invention, the hydroxide formed by hydrolysis and aging is precipitated on the surface of the coexisting oxide support, and by firing it, the Pt composite oxide is formed into a thin film on the surface of the particulate support. It is formed as a Pt composite oxide layer in the shape of a circle.

As the oxide carrier used in the production method of the present invention, the same oxide carrier as used in the exhaust gas purifying catalyst of the present invention can be used. That is, alumina, silica, titania, zirconia, ceria, silica
One selected from alumina and the like, or a mixture of a plurality of types can be used. However, these oxides may react with the alkaline earth metal in the Pt composite oxide, changing the structure of the Pt composite oxide and decreasing the catalytic activity. Therefore, it is desirable to use a composite oxide containing an alkaline earth metal for the oxide carrier itself. Thereby, the reaction between the support and the Pt composite oxide is suppressed, so that the durability of the catalyst is further improved.

As such a composite oxide carrier, it is preferable to use a composite oxide comprising alumina, silica, titania, zirconia, ceria, silica-alumina, etc. and an alkaline earth metal or a lanthanoid element. The preferred composition ratio is such that when M represents Al, Si, Ti, Zr, Ce, etc., and X represents an alkaline earth metal or a lanthanoid element, the molar ratio is in the range of X: M = 1: 0.3 to 1:20. Are preferred, and X:
The range of M = 1: 0.5 to 1: 4 is particularly preferred.

The higher the specific surface area of the oxide carrier, the better, but it is important that the specific surface area does not change before and after endurance. 15m ² / g if the specific surface area does not change before and after endurance
Sufficient effects can be obtained even with this degree. As the solvent for dissolving the alkoxide, various alcohols can be used alone or as a mixture of two or more, and it is preferable to use a mixed solvent of ether and alcohol. Thereby, precipitation of Pt acetylacetonate during hydrolysis is suppressed, and a homogeneous gel is obtained. Various kinds and mixing ratios of ether and alcohol can be selected depending on the kind and amount of alkoxide used.

The obtained catalyst of the present invention is pelletized by a conventional method and can be put to practical use as a pellet catalyst. Further, it can be applied to a honeycomb support substrate made of cordierite or metal, and used as a monolith catalyst. The oxidation catalyst, three-way catalyst, can be used lean-burn catalysts, diesel catalysts, for applications such as _the NO _x storage-reduction catalyst.

[0018]

The present invention will be specifically described below with reference to examples and comparative examples. Example 1 75 g of 2-propanol and 25 g of 2-methoxymethanol were mixed, and Sr (OC ₃ H ₇ ) ₂ was added to the mixed solvent.
1.187 g and 0.756 g of Pt (C ₅ H ₇ O ₂ ) ₂ were charged, and 7 g
The mixture was stirred at 0 ° C. for 12 hours to prepare an alkoxide solution.

On the other hand, 75 g of 2-propanol and 25 g of 2-methoxymethanol were mixed, and commercially available M
28.77 g of gAl ₂ O ₄ (specific surface area 40 m ² / g) was added and stirred,
The mixture was heated to 70 ° C. to prepare a carrier dispersion. Next, the alkoxide solution at 70 ° C was added to the carrier dispersion at 70 ° C, and the mixture was stirred at reflux at 70 ° C for 1 hour. There Ni (CH ₃ COO) ₂
· 4H ₂ O was 478g added, and further stirred for 1 hour to the suspension in the holding to reflux in 70 ° C..

Further, while the suspension was stirred, 0.208 g of deionized water was gradually added thereto, and the mixture was stirred at reflux at 70 ° C. for 5 hours to carry out hydrolysis and ripening. The resulting suspension degreased after further N ₂ in 300 ° C. at 100 ° C. at aspirator
And baked at 500 ° C. for 3 hours to obtain a catalyst powder of this example. This catalyst has Sr ₃ NiPtO _{6 on the} surface of MgAl ₂ O ₄ support.
It is considered that a Pt composite oxide layer represented by the following formula is formed, and the carried amount of Pt is 1.25% by weight.

Example 2 75 g of 2-propanol and 2-
Mix 25 g of methoxymethanol, and add Sr
1.582 g of (OC ₃ H ₇ ) ₂ and 0.756 g of Pt (C ₅ H ₇ O ₂ ) ₂ were added, and the mixture was stirred at 70 ° C. for 12 hours under reflux to prepare an alkoxide solution. On the other hand, 75 g of 2-propanol and 25 g of 2-methoxymethanol were mixed, and commercially available MgA was added to the mixed solvent.
28.77 g of l ₂ O ₄ (specific surface area 40 m ² / g) was added and stirred.
C. to prepare a carrier dispersion.

Further, while the suspension was stirred, 0.208 g of deionized water was gradually added thereto, and the mixture was stirred at reflux at 70 ° C. for 5 hours to effect hydrolysis and ripening. The resulting precipitate was degreased at 100 ° C. Further in N ₂ 300 ° C. at aspirator
And baked at 500 ° C. for 3 hours to obtain a catalyst powder of this example. This catalyst is considered to have a structure in which a Pt composite oxide layer represented by Sr ₄ PtO ₆ is formed on the surface of the MgAl ₂ O ₄ support, and the carried amount of Pt is 1.25% by weight.

Comparative Example 1 313 g of dinitrodiammine P
50 g of γ-Al ₂ O in an aqueous solution (containing 0.2% by weight as Pt)
₃ Powder (specific surface area: 180 m ² / g) was added, water was evaporated on a water bath with stirring, dried at 120 ° C. all day and night, and calcined in air at 500 ° C. for 1 hour to obtain a catalyst powder of Comparative Example 1. Obtained.

It is considered that the catalyst of Comparative Example 1 has a high dispersion of Pt even inside the pores of the γ-Al ₂ O ₃ particles. Comparative Example 2 1500 g of 2-propanol and 500 g of 2-methoxymethanol were mixed, and Sr (OC ₃ H ₇ ) ₂ was mixed in the mixed solvent.
Was added and 15.12 g of Pt (C ₅ H ₇ O ₂ ) ₂ was added, and the mixture was stirred at 70 ° C. for 12 hours under reflux. Ni (CH ₃ COO) ₂・ 4H ₂ O
9.56 g was added, and the mixture was stirred at reflux at 70 ° C. for 1 hour to prepare an alkoxide solution.

While stirring the alkoxide solution, 4.16 g of deionized water was gradually added, and the mixture was stirred at reflux at 70 ° C. for 5 hours to carry out hydrolysis and ripening. Then, the obtained precipitate is degreased at 100 ° C. with an aspirator, and further N ₂
Degreasing at 300 ℃, baking for 3 hours at 500 ℃, Sr ₃ NiPt
A Pt composite oxide powder represented by O ₆ was prepared. Then this
Commercially available MgAl ₂ O ₄ (specific surface area 40 m ² /
g) was added and mixed well in a mortar to obtain a catalyst powder of Comparative Example 2. This catalyst is composed of MgAl ₂ O ₄ support and Sr ₃ NiPtO ₆
Is uniformly mixed with the Pt composite oxide represented by the formula, and the amount of Pt carried is 1.25% by weight.

(Comparative Example 3) 1500 g of 2-propanol and 2
500 g of methoxymethanol, 23.74 g of Sr (OC ₃ H ₇ ) ₂ and 15.12 g of Pt (C ₅ H ₇ O ₂ ) ₂ were put into the mixed solvent, and the mixture was refluxed at 70 ° C. for 12 hours. The alkoxide solution was prepared by stirring. While stirring the alkoxide solution, 4.16 g of deionized water was gradually added, and the mixture was stirred at reflux at 70 ° C. for 5 hours to carry out hydrolysis and ripening. Then, the obtained precipitate is degreased at 100 ° C. with an aspirator, and further N ₂
Degreasing at 300 ° C, baking at 500 ° C for 3 hours, Sr ₄ PtO ₆
A Pt composite oxide powder represented by was prepared.

Next, commercially available MgA was added to the Pt composite oxide powder.
28.77 g of l ₂ O ₄ (specific surface area 40 m ² / g) was added and mixed well in a mortar to obtain a catalyst powder of Comparative Example 2. This catalyst is MgA
The l ₂ O ₄ carrier and the Pt composite oxide represented by Sr ₄ PtO ₆ are uniformly mixed, and the amount of Pt carried is 1.25% by weight. (Comparative Example 4) 1500 g of 2-propanol and 500 g of 2-methoxymethanol were mixed, and Sr (OC ₃ H ₇ ) ₂ was added to the mixed solvent.
Was added and 15.12 g of Pt (C ₅ H ₇ O ₂ ) ₂ was added, and the mixture was stirred at 70 ° C. for 12 hours under reflux. Ni (CH ₃ COO) ₂・ 4H ₂ O
9.56 g was added, and the mixture was stirred at reflux at 70 ° C. for 1 hour to prepare an alkoxide solution.

While stirring the alkoxide solution, 4.16 g of deionized water was gradually added, and the mixture was stirred at reflux at 70 ° C. for 5 hours to effect hydrolysis and ripening. Then, the obtained precipitate is degreased at 100 ° C. with an aspirator, and further N ₂
Degreasing at 300 ℃, baking for 3 hours at 500 ℃, Sr ₃ NiPt
A Pt composite oxide powder represented by O ₆ was prepared and used as a catalyst powder of Comparative Example 4. This catalyst is composed only of a Pt composite oxide represented by Sr ₃ NiPtO ₆ , and the amount of Pt carried is 1.25% by weight.

(Evaluation Test) With respect to the catalysts of Examples 1, 2, 2, 3 and 4, the particles were observed with a scanning electron microscope, and the elements on the surface were analyzed by EDX. went. Table 1 shows the EDX analysis.
The conditions were as follows.

[0030]

[Table 1] Although the catalysts of Examples 1 and 2 were uniform, two types of particles were observed in the catalysts of Comparative Examples 2 and 3.

FIG. 3 shows the result of EDX analysis of the catalyst of Example 2. In the catalyst of Example 2, Sr and Pt were detected on all surfaces as shown in FIG. In contrast, in the catalyst of Comparative Example 3, Sr and Pt were not detected at all on the surface as shown in FIG. Further, from the surface of the portion of the catalyst of Comparative Example 3 which was quantitatively overwhelmingly small, Sr and Pt were detected at a high concentration as shown in FIG.
This is considered to be the structure of Sr ₄ PtO ₆ . Exactly the same phenomenon was observed for the catalysts of Example 1 and Comparative Example 2.

In the catalyst of Comparative Example 2, MgAl ₂ O ₄ support and Sr ₃ Ni
It is apparent that the catalyst is uniformly mixed with the particulate Pt composite oxide represented by PtO ₆ , and the above analysis shows that the catalyst of Example 1 has a particulate form as shown in FIG. of
A Pt composite oxide layer 2 represented by Sr ₃ NiPtO ₆ was formed on the entire surface of the MgAl ₂ O ₄ support 1, and the MgAl ₂ O ₄
It is clear that the Pt composite oxide layer 2 represented by Sr ₄ PtO ₆ is formed on the entire surface of the support 1. Here, in FIG. 1, the MgAl ₂ O ₄ carrier 1 is schematically illustrated in a spherical shape,
The surface may be irregular if it is particulate.

Next, Examples 1, 2 and Comparative Examples 1 to
Each of the catalysts No. 3 was pressurized by a normal temperature isostatic press (CIP) and then pulverized to form pellets of 1.0 to 1.7 mm.
Each of the obtained pellet catalysts was placed in a durability test apparatus, and a durability model gas (A / F = 16 equivalent to SO ₂
) At 1000 ° C for 10 hours.

After the endurance treatment, 2.0 g of each pellet catalyst was placed in a normal-pressure flow reactor, and an evaluation model gas (equivalent to stoichiometry) shown in Table 2 was supplied at a flow rate of 5 L / min. Is 500 ℃, 450 ℃, 400 ℃, 350 ℃,
The purification rates of the C ₃ H ₆ component and the NO component in the steady state at 300 ° C. and 250 ° C. were measured, respectively. The definition of the purification rate is expressed by the following equation. Purification rate = 100 x {(concentration in incoming gas-concentration in outgoing gas) / concentration in incoming gas}

[0035]

[Table 2] From the obtained results, the relationship between the catalyst bed temperature and the purification rate was plotted, and the temperature at which the purification rate was 50% was determined. Table 3 shows the results. In Table 3, HC-T ₅₀ means the 50% purification temperature of the C ₃ H ₆ component, and NO-T ₅₀ means the 50% purification temperature of the NO component.

[0036]

[Table 3] The same durability treatment as described above was performed on the catalyst powder of Comparative Example 4, and the specific surface area before and after the durability treatment was measured. As a result, the specific surface area was 40 m ² / g before the durability treatment, but the specific surface area was reduced to about 1 m ² / g after the durability treatment,
Sintering had occurred. Therefore, as shown in FIG. 2, in the catalysts of Comparative Examples 2 and 3 in which only the Pt composite oxide powder and the particulate carrier powder were mixed, sintering occurred in the Pt composite oxide in the durability test. it is conceivable that.

As apparent from Table 3, Example 1
Comparison between Comparative Example 2 and Comparative Example 2 shows that the catalyst of Example 1 is lower in both the 50% purification temperature of the C ₃ H ₆ component and the 50% purification temperature of the NO component than Comparative Example 2, and is superior in the purification performance after the durability treatment. ing.
Similarly, comparing Example 2 with Comparative Example 3, the catalyst of Example 2 is superior to Comparative Example 3 in purification performance after the durability treatment. Furthermore, the catalysts of Example 1 and Example 2 are superior to the catalyst of Comparative Example 1 in purification performance after the durability treatment.

That is, the catalysts of Examples 1 and 2 produced by the production method of the present invention had higher purification than the catalysts of Comparative Examples 2 and 3 in which the Pt composite oxide powder and the carrier powder were mixed. Performance, which is shown in Examples 1 and 2
This catalyst is considered to have an effect of suppressing sintering of the Pt composite oxide as compared with the catalysts of Comparative Examples 2 and 3.

[0039]

According to the exhaust gas purifying catalyst of the present invention, sintering of Pt can be further suppressed, high purification performance is exhibited even after endurance treatment at 1000 ° C., and heat resistance is extremely excellent. Further, according to the production method of the present invention, the above-mentioned catalyst having excellent heat resistance can be produced stably and reliably.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing the structure of an exhaust gas purifying catalyst according to one embodiment of the present invention.

FIG. 2 is an explanatory view showing a sintering mechanism in a conventional exhaust gas purifying catalyst.

FIG. 3 is an ED showing the results of elemental analysis of the catalyst surface of Example 2.
It is an X chart figure.

FIG. 4 is an EDX chart showing the results of elemental analysis of the surface occupying most of the catalyst of Comparative Example 3.

5 is an EDX chart showing the results of elemental analysis of the surface of an overwhelmingly small portion of the catalyst of Comparative Example 3. FIG.

[Explanation of symbols]

1: MgAl ₂ O ₄ carrier (particulate carrier) 2: Pt composite oxide layer

Claims (2)

[Claims] 1. An exhaust gas purifying catalyst comprising: an oxide carrier; and a platinum composite oxide layer formed on the surface of the oxide carrier. 2. A platinum composite oxide layer is formed on the surface of the oxide carrier by mixing an oxide carrier with a solution in which an alkoxide containing platinum is dissolved, hydrolyzing the alkoxide, and firing the mixture. For producing exhaust gas purifying catalysts.