JP7363215B2 - Exhaust gas purification catalyst and its manufacturing method - Google Patents

Description

The present invention relates to an exhaust gas purifying catalyst and a method for manufacturing the same.

A three-way catalyst is known as a catalyst that purifies exhaust gas emitted from engines of automobiles and the like. For this three-way catalyst, noble metal-supported alumina, in which a noble metal such as Pt as a catalytic metal is supported on γ-alumina as a support material, is widely used.

It is also known to employ zirconium phosphate as a support material in exhaust gas purification catalysts. For example, Patent Document 1 discloses that zirconium phosphate (ZrP ₂ O ₇ ), La-stabilized alumina, and an alumina binder are added to a Rh solution, subjected to a wet pulverization treatment, and then a Rh nitric acid solution is added to obtain an Rh-containing powder. It is described that an exhaust gas purifying catalyst is obtained by applying a slurry to a honeycomb carrier, drying and firing the slurry.

JP2016-26874A

When the precious metal-supported alumina is exposed to high-temperature exhaust gas for a long time, the specific surface area of γ-alumina as a support material decreases and the precious metals agglomerate, resulting in a decrease in the degree of dispersion of the noble metal in the support material and the active sites. Decrease. That is, noble metal supported alumina has high exhaust gas purification performance when fresh, but has low exhaust gas purification performance after durability.

Patent Document 1 describes that Rh-supported ZrP ₂ O ₇ has better NOx purification performance in the lean region after durability than Rh-supported ZrO ₂ , etc. Regarding the purification performance, no significant effect has been obtained.

An object of the present invention is to improve the low-temperature activity and high-temperature purification performance of an exhaust gas purification catalyst.

In order to solve the above problems, the present invention combines γ-alumina and amorphous zirconium phosphate to form a noble metal support material.

Exhaust gas purification catalysts are made by supporting noble metals as catalytic metals on support materials.
The support material is made of γ-alumina and amorphous zirconium phosphate.

Amorphous zirconium phosphate has a large specific surface area, and phosphoric acid is highly dispersed in zirconia to form acid sites. Therefore, the noble metal M can be supported in a relatively stable and highly dispersed manner through the formation of M--O--P bonds. This combination of amorphous zirconium phosphate and γ-alumina with a high specific surface area also increases the dispersibility of the amorphous zirconium phosphate itself. Therefore, the noble metal is stably supported in a highly dispersed state on the support material. Therefore, even after exposure to high-temperature exhaust gas, the agglomeration of precious metals is suppressed in the exhaust gas purification catalyst, and as a result, it exhibits good low-temperature activity and high-temperature purification performance even after durability. In short, the present invention uses γ-alumina with a large specific surface area to highly disperse amorphous zirconium phosphate, and uses acid sites of the amorphous zirconium phosphate to prevent agglomeration of noble metals.

The amorphous zirconium phosphate is supported on the γ-alumina. Therefore, the dispersibility of amorphous zirconium phosphate becomes high due to the high specific surface area of γ-alumina. In other words, strong acid sites are highly dispersed on γ-alumina. As a result, the dispersibility of the noble metal increases and its agglomeration is prevented.

As the above-mentioned noble metal, Pt, Pd, Rh, etc. can be adopted. In particular, when the noble metal is Pt, the stability of Pt increases due to the formation of Pt--O--P bonds, which is advantageous in preventing its agglomeration.

The method for producing an exhaust gas purification catalyst disclosed herein includes a step of mixing a zirconium compound and an aqueous solution containing phosphoric acid and/or a phosphate to form a precipitate of zirconium phosphate;
While stirring the dispersion of the precipitate, adding and mixing alumina gel to the dispersion and evaporating to dryness;
A step of drying and firing the obtained dried product to obtain a support material in which amorphous zirconium phosphate is supported on γ-alumina;
The present invention is characterized by comprising a step of supporting a noble metal on the support material.

As a result, an exhaust gas purifying catalyst is obtained in which a noble metal is supported on a support material in which amorphous zirconium phosphate is supported on γ-alumina.

According to the present invention, the exhaust gas purification catalyst is formed by supporting a noble metal on a support material, and the support material is made of γ-alumina and amorphous zirconium phosphate. Even when exposed to gas, the reduction in specific surface area and the aggregation of precious metals are suppressed, so the reduction of active sites is suppressed, and the effect of exhibiting good low-temperature activity and high-temperature purification performance over a long period of time can be obtained.

FIG. 1 is a conceptual diagram of an exhaust gas purifying catalyst according to an embodiment. FIG. 3 is a graph diagram showing the temperature characteristics of HC purification rate after 780° C. fluctuation durability treatment of Examples and Comparative Examples. FIG. 3 is a graph diagram showing the temperature characteristics of HC purification rate after 880° C. fluctuation durability treatment of Examples and Comparative Examples. The graph figure which shows the _NH3 temperature-programmed desorption curve of an Example and a comparative example. The graph figure which shows the _NH3 adsorption amount of an Example and a comparative example. A graph diagram showing an XRD profile of an example.

EMBODIMENT OF THE INVENTION Hereinafter, the form for implementing this invention is demonstrated based on drawing. The following description of preferred embodiments is merely exemplary in nature and is not intended to limit the invention, its applications, or its uses.

The exhaust gas purification catalyst described herein is suitable for purifying the exhaust gas of an automobile engine, and is disposed in the exhaust passage of the engine. This exhaust gas purification catalyst can constitute a three-way catalyst that purifies the exhaust gas of a gasoline engine, or can constitute an oxidation catalyst that purifies the exhaust gas of a diesel engine. .

<Exhaust gas purification catalyst>
As conceptually shown in FIG. 1, in the exhaust gas purifying catalyst 1, noble metals 3 as catalytic metals are supported on a support material 2 in a dispersed manner. The support material 2 is made of γ-alumina (activated alumina) 4 and porous amorphous zirconium phosphate 5. Amorphous zirconium phosphate 5 is dispersed and supported on γ-alumina 4.

The exhaust gas purifying catalyst 1 can be supported, for example, on the wall surfaces of cells of a honeycomb carrier. In this case, each cell of the honeycomb carrier becomes an exhaust gas flow path, and when exhaust gas passes through this cell, it is purified by the exhaust gas purifying catalyst 1. The exhaust gas purification catalyst 1 can be supported on the wall of the cell by itself with a binder, or it can be mixed with other catalysts and/or co-catalysts such as oxygen storage/release materials to form a single layer on the wall of the cell. It can be supported in a layered manner or in a separate layer with other catalysts and/or co-catalysts.

The support material 2 has a mass ratio of amorphous zirconium phosphate 5 to the support material 2 (mass ratio of each of Al, Zr, and P converted into oxides, in this case, (PO ₄ +ZrO ₂ )/ (PO ₄ +ZrO ₂ + _Al2O3 ) ) is preferably 20% or more and 70% or less, more preferably 20% or more and 55% _or less. In addition, the amorphous zirconium phosphate 5 has a mass ratio of phosphoric acid to zirconia (mass ratio of Zr and P converted into oxides, in this case (PO ₄ /ZrO ₂ )) of 2% to 25%. It is preferable that the content be 2% or more and 10% or less.

As the noble metal 3, Pt, Pd, Rh, etc. can be adopted, and the amount supported on the support material 2 is preferably 0.1% by mass or more and 10% by mass or less of the amount of the support material.

<Production method of exhaust gas purification catalyst>
The above exhaust gas purifying catalyst can be manufactured by the following method.

-Generation of zirconium phosphate precursor-
A zirconium compound and an aqueous solution containing phosphoric acid and/or a phosphate salt are mixed to form a precipitate of a zirconium phosphate precursor. Examples of the zirconium compound include zirconium hydroxide, zirconium nitrate, zirconium acetate, zirconium sulfate, zirconium carbonate, basic zirconium carbonate, basic zirconium sulfate, zirconium oxysulfate, and zirconium oxychloride. Examples of phosphates include sodium phosphate, potassium phosphate, and ammonium phosphate.

For example, zirconium hydroxide is dispersed in ion-exchanged water, an ammonium phosphate aqueous solution is added thereto, and a zirconium phosphate precursor is precipitated by a reaction between the gelled zirconium hydroxide and the ammonium phosphate aqueous solution. A suspension in which the bodies are dispersed can be obtained.

- Evaporation to dryness (generation of support material precursor) -
While stirring the dispersion of the zirconium phosphate precursor precipitate, alumina gel (alumina hydrate) is added to the dispersion, mixed, and evaporated to dryness. As a result, a support material precursor (precursor of amorphous zirconium phosphate-supported alumina) is obtained.

-Drying/Baking-
The dried product (mixture of zirconium phosphate precursor and alumina gel) obtained by the above-mentioned evaporation to dryness is dried, the dried product is pulverized, and the amorphous phosphor is converted into γ-alumina by firing in the air. A support material on which acid zirconium is supported is obtained. Drying can be carried out by maintaining the dried product at a temperature of, for example, 90° C. or higher and 120° C. or lower for a predetermined period of time in the air. Firing can be carried out by maintaining the pulverized material at a temperature of 350° C. or higher and 450° C. or lower for several hours, for example, in an air atmosphere.

- Supporting precious metals -
The support material is impregnated with a precious metal solution. After impregnation, drying and calcination are performed to obtain an exhaust gas purifying catalyst. Drying can be carried out, for example, by holding the material at a temperature of 100° C. or more and 250° C. or less in an air atmosphere for a predetermined period of time, and baking can be carried out, for example, by holding the material at a temperature of 400° C. or more and 600° C. or less in an air atmosphere for several hours. be able to.

<Examples and comparative examples>
Using the above-mentioned manufacturing method, gelled zirconium hydroxide is used as the zirconium source and ammonium phosphate aqueous solution is used as the phosphoric acid source to produce supports with different compositions in which amorphous zirconium phosphate is supported on γ-alumina. Exhaust gas purifying catalysts of Examples 1-8 were prepared by impregnating and supporting Pt as a catalyst metal in each material (Table 1). As Comparative Example 1, an exhaust gas purification catalyst was prepared in which Pt was supported on commercially available γ-alumina (La-stabilized alumina), and as Comparative Example 2, Pt was supported on amorphous zirconium phosphate (without alumina). A catalyst for purifying exhaust gas was prepared (Table 1).

The amorphous zirconium phosphate of Comparative Example 2 employs zirconium hydroxide as a zirconium source and ammonium phosphate as a phosphoric acid source to produce the zirconium phosphate precursor, evaporate to dryness, dry and calcinate. It was created by doing the following. The amount of Pt supported is 3.0% by mass of the amount of support material in both Examples 1-8 and Comparative Examples 1 and 2.

For each of the exhaust gas purifying catalysts of Examples 1-8 and Comparative Examples 1 and 2, the BET specific surface area was measured when fresh (before durability treatment) and after durability treatment. The durability treatment involves holding the exhaust gas purifying catalyst at a temperature of 1000° C. for 3 hours in an O ₂ -containing gas atmosphere (O ₂ ; 2%, H ₂ O; 10%, remainder N ₂ ). The results are shown in Table 1.

Furthermore, for each of the exhaust gas purification catalysts of Examples 1-8 and Comparative Examples 1 and 2, the HC purification performance and Pt dispersion degree after 880° C. fluctuation durability treatment were measured.

The 880°C fluctuation durability treatment was performed using CO-containing gas (CO; 1%, H ₂ O; 10%, CO ₂ ; 13%, balance N ₂ ) and O _2- containing gas (O ₂ ; 2%, H ₂ O; 10%). %, CO ₂ ; 13%, balance N ₂ ) was flowed through the catalyst alternately for 1 minute at 3 minute intervals for 24 hours, and the temperature of the CO-containing gas and O _2- containing gas at that time was set to 880℃. ℃. The 780° C. fluctuation durability treatment described later is performed with the temperature of the CO-containing gas and the O _2- containing gas set to 780° C.

The HC purification performance was determined by creating a test catalyst (diameter 25 mm, length 50 mm) in which the catalyst was supported on a honeycomb carrier, and gradually increasing the temperature of the model exhaust gas flowing into the test catalyst from room temperature. The HC purification rate is determined by detecting the HC concentration of the gas flowing out of the test catalyst. The composition of the model exhaust gas is: HC = iso-pentane; 1500 ppm C, CO; 0.17%, O ₂ ; 9.6%, CO ₂ ; 12.4%, H ₂ O; 10.0%, balance N ₂ It is. The gas flow rate was 26.1 L/min (space velocity SV=about 60,000 h ^-1 ), and the gas temperature increase rate was 30° C./min.

Table 1 shows the HC purification rate (HC-C350) of each test catalyst when the inlet gas temperature was 350°C.

The degree of Pt dispersion is the ratio of the amount of Pt present on the surface of the support material to the amount of Pt supported on the support material, and from the amount of CO adsorption measured by the CO pulse method, the Pt contained in the catalyst is 1:1 with CO. The amount of Pt present on the surface of the support material was calculated assuming that Pt is adsorbed at a rate of .

Looking at the HC purification rate (HC-C350) at a catalyst inlet gas temperature of 350°C after 880°C fluctuation durability treatment in Table 1, Examples 1-8 are better than Comparative Example 1 (Pt/γ-Al ₂ O ₃ ). The prices are also getting higher.

On the other hand, although Comparative Example 2 has a higher (HC-C350) than Examples 1-8, Figure 2 shows the temperature characteristics of the HC purification rate of Examples 3 and 5 and Comparative Examples 1 and 2 (780°C As is clear from FIG. 3 ₍ after 880°C fluctuation durability treatment) and _FIG . 4 mass%, PO ₄ =7 mass%)) is higher than Comparative Example 2 (Pt/PO ₄ -ZrO ₂ ). This is because Examples 3 and 5 have amorphous zirconium phosphate supported on γ-alumina, and the degree of Pt dispersion after the variable durability treatment is greater than that of Comparative Example 2. Examples 1, 2, 4, and 6-8 also have higher low-temperature activity than Comparative Example 2 because the degree of Pt dispersion after the variable durability treatment is larger than that of Comparative Example 2.

Here, looking at the BET specific surface area, in Example 1-8, it is 150 m ² /g or more when fresh, and it is 40 m ² /g or more even after durability treatment. On the other hand, although Comparative Example 2 has a high freshness of 257 m ² /g, it decreases to 4.6 m ² /g after durability treatment. A comparison between the two shows that when amorphous zirconium phosphate is supported on γ-alumina, the decrease in BET specific surface area due to durability treatment is suppressed.

If the mass ratio of (PO ₄ +ZrO ₂ )/ (PO ₄ +ZrO ₂ + Al ₂ O ₃ ) of the support material is 20% or more and 70% or less, the retention rate of the BET specific surface area after durability treatment is high, especially , when the mass ratio is 20% or more and 55% or less, the BET specific surface area retention rate is high. Regarding the Pt dispersion degree after the 880°C fluctuation durability treatment, Examples 1-8 are significantly higher than Comparative Example 2.

In Examples 3 and 6-8, the amount of Al ₂ O ₃ in the support material is fixed and the amount of PO ₄ in the amorphous zirconium phosphate is varied. When the PO ₄ /ZrO ₂ mass ratio of amorphous zirconium phosphate is 2% or more and 25% or less, the BET specific surface area after durability treatment is high. In particular, when the mass ratio is 2% or more and 10% or less, the BET specific surface area after durability treatment is high.

The BET specific surface area of the support material is preferably 150 m ² /g or more when fresh and 40 m ² /g or more after durability treatment.

Further, the Pt dispersion degree in the support material is preferably 90% or more when fresh, and 5.0% or more after 880° C. fluctuation durability treatment.

Figures 4 and 5 show Example 3 (Pt/PO ₄ -Z-Al (PO ₄ =4 mass%)) and Example 5 (Pt/PO ₄ -Z-Al (PO ₄ = 7mass%)) and Comparative Example 2 (Pt/PO ₄ -ZrO ₂ ) after being subjected to 880°C fluctuation durability treatment. FIG. 4 shows the TPD curve, and FIG. 5 shows the ammonia adsorption amount (desorption amount).

The desorption peaks of Examples 3 and 5 and Comparative Example 2 are all around 200°C, and the strengths of the acid sites are approximately the same. However, in Comparative Example 2 (Pt/PO ₄ -ZrO ₂ ) in which amorphous zirconium phosphate was not supported on γ-alumina, PO ₄ was 8% by mass, and in Example 3 (Pt/PO 4 -ZrO 2 ), PO ₄ was 8% by mass. -Z-Al (PO ₄ =4 mass%)) and Example 5 (Pt/PO ₄ -Z-Al (PO ₄ =7 mass%)), the ammonia adsorption amount was 3 compared to the example. Less than 5. That is, the amount of acid sites is small. From this, it can be seen that when amorphous zirconium phosphate is supported on γ-alumina as in the example, the decrease in the amount of acid sites due to durability treatment is suppressed and it is advantageous for maintaining catalyst activity.

FIG. 6 shows XRD of the support material according to Example 3 (PO ₄ -Z-Al (PO ₄ =4 mass%)) and the support material according to Example 5 (PO ₄ -Z-Al (PO ₄ =7 mass%)) It is a profile. The diffraction lines are not clear and are broad. γ-Alumina originally does not show clear diffraction lines, but in PO ₄ -Z-Al (PO ₄ = 4 mass%), peaks specific to γ-Alumina appear (diffraction angles around 46deg and 67deg). It can be seen that amorphous zirconium phosphate is supported on γ-alumina.

1 Exhaust gas purification catalyst 2 Support material 3 Precious metal 4 γ-alumina 5 Amorphous zirconium phosphate

Claims (1)

mixing a zirconium compound and an aqueous solution containing phosphoric acid and/or a phosphate to form a precipitate of a zirconium phosphate precursor;
While stirring the dispersion of the precipitate, adding and mixing alumina gel to the dispersion and evaporating to dryness;
A step of drying and firing the obtained dried product to obtain a support material in which amorphous zirconium phosphate is supported on γ-alumina;
A method for producing an exhaust gas purifying catalyst, comprising the step of supporting a precious metal on the support material.

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