JP5202034B2 - Method for producing exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to a method for producing an exhaust gas purifying catalyst.

  As an exhaust gas purification catalyst for treating automobile exhaust gas, a three-way catalyst in which a noble metal such as platinum is supported on a carrier such as ceria or alumina is widely used. In this three-way catalyst, the noble metal plays a role of promoting the reduction reaction of nitrogen oxides and the oxidation reaction of carbon monoxide and hydrocarbons. In addition, the support plays a role of suppressing the sintering of the noble metal by increasing the specific surface area of the noble metal and dissipating heat generated by the reaction.

  In this three-way catalyst, a composite oxide containing an alkaline earth metal element may be used as a support for the purpose of improving its performance. For example, it is known that a three-way catalyst containing a solid solution of this composite oxide and a noble metal exhibits excellent exhaust gas purification performance over a long period of time. Therefore, in the production of this catalyst, it is important to efficiently produce the above complex oxide.

A plurality of methods for obtaining such a composite oxide are known, and a simple method is an evaporation to dryness method. In this method, raw material particles supporting a noble metal and an alkaline earth metal salt are mixed in a dispersion medium, and then dried and fired to obtain the composite oxide. For example, in Patent Document 1, an exhaust gas purifying catalyst containing the above complex oxide is manufactured by drying and firing a slurry containing rare earth oxide particles supporting platinum and an alkaline earth metal salt. It is described.
JP 2006-130444 A

  In the manufacturing method described above, it is desirable that the composite oxide as the target product is obtained at a high production ratio. However, the present inventors have found that according to this production method, a relatively large amount of by-products are easily generated in addition to the target composite oxide.

  Then, this invention aims at suppressing the production | generation of the by-product in the manufacturing process of the exhaust gas purification catalyst, and improving the production | generation ratio of the complex oxide which is a target product.

  The present inventors performed detailed verification on the product obtained by the method described in Patent Document 1. As a result, it became clear that this product contained a large amount of alkaline earth metal not supported on the raw material particles as a by-product. It was also found that the complex oxide as the target product has problems such as the content of alkaline earth metal being not constant.

  The present inventors believe that the above problems are caused by the fact that alkaline earth metals have a low affinity with the raw material particles supporting the noble metal, and the alkaline earth metals are difficult to be retained on the raw material particles. It was. In order to solve this problem, the present inventors have intensively studied additives to be added at the time of preparation and have completed the present invention.

That is, according to one aspect of the present invention, the material contains raw material particles containing a metal or a non-metallic element, a noble metal supported on the raw material particles, an alkaline earth metal salt, and a cross- linking substance. Is stirring a slurry which is a water-soluble polymer containing at least one selected from gelatin and agar , drying and firing the stirred slurry, and the metal or non-metal element supporting the noble metal and the alkali There is provided a method for producing an exhaust gas purifying catalyst characterized by comprising producing a complex oxide with an earth metal.

  Without being bound by theory, by adding the above-mentioned cross-linking substance, that is, a substance capable of forming a cross-linking structure, as an additive, a more homogeneous composite oxide can be obtained in a higher yield on the following principle. It is thought that it is obtained by.

  Alkaline earth metal penetrates into the crosslinked structure relatively uniformly during or after the formation of the crosslinked structure of the crosslinked substance. In this way, a crosslinked structure having an alkaline earth metal uniformly held therein is formed.

  Each of the raw material particles supporting the noble metal is coated with the above-mentioned crosslinked structure. That is, the alkaline earth metal is held on the raw material particles through this crosslinked structure.

  When this is fired, the crosslinked structure is burned and removed, and most of the alkaline earth metal is supported on the raw material particles, and they react with each other to form a composite oxide. As described above, a homogeneous composite oxide can be obtained.

  In this method, the size of the gap formed in the crosslinked structure can be adjusted by adjusting the concentration of the crosslinking substance in the slurry. When the concentration of the cross-linking substance is lowered, the gap is increased and the alkaline earth metal is easily detached from the cross-linked structure. When the concentration of the cross-linking substance is increased, the proportion of the cross-linked structure that does not participate in the coating of the raw material particles increases. Therefore, by adjusting the concentration of the cross-linking substance, the ratio of the alkaline earth metal supported on the raw material particles can be controlled.

  According to the present invention, it is possible to suppress the production of by-products in the manufacturing process of the exhaust gas purifying catalyst, and to improve the production ratio of the composite oxide that is the target product.

Hereinafter, embodiments of the present invention will be described.
An exhaust gas purifying catalyst that can be produced by the method according to one embodiment of the present invention includes a carrier containing a complex oxide of an alkaline earth metal element and a metal or non-metal element different from the alkaline earth metal element, and a noble metal supported on the carrier. Including. In addition, at least a part of the noble metal may form a solid solution with the complex oxide.

  As the alkaline earth metal element, for example, barium, strontium, calcium or magnesium can be used. A plurality of alkaline earth metal elements can also be used.

  As the metal element, for example, rare earth elements such as cerium, lanthanum, praseodymium and neodymium, transition metal elements such as titanium and zirconium, typical metal elements such as aluminum, or two or more thereof can be used. As the nonmetallic element, for example, silicon can be used. The composite oxide may contain both a metallic element and a nonmetallic element.

  As the noble metal, for example, platinum group elements such as platinum, palladium, and rhodium can be used. A plurality of precious metals can also be used.

  In this embodiment, the exhaust gas-purifying catalyst is produced, for example, by the following method.

  First, a slurry containing raw material particles containing the metal or nonmetallic element, the noble metal supported thereon, an alkaline earth metal salt, and a crosslinking substance such as gelatin is prepared. At this time, there are no particular restrictions on the order, timing and method of adding the raw material particles, alkaline earth metal salt and cross-linking substance. For example, after the raw material particles, the alkaline earth metal salt, and the dispersion medium are mixed, the cross-linking substance may be further added. Alternatively, the raw material particles, the alkaline earth metal salt, the cross-linking substance, and the dispersion medium may be mixed at a time.

  Subsequently, the slurry is stirred. Further, the stirred slurry is heated to remove the liquid. In this manner, the alkaline earth metal element is supported on the raw material particles supporting the noble metal.

  There is no particular limitation on the method of supporting the alkaline earth metal element on the raw material particles. For example, a method of impregnating raw material particles carrying a noble metal with an alkaline earth metal salt solution, a method using coprecipitation, a method using an alkoxide of an alkaline earth metal, or the like may be used. However, in these methods, it is relatively difficult to uniformly carry the alkaline earth metal element. If the alkaline earth metal is supported non-uniformly, more by-products may be generated.

  Thereafter, the raw material particles supporting the noble metal and the alkaline earth metal element are fired in an oxidizing atmosphere. As a result, the previously added cross-linking substance is eliminated, and a composite oxide of the alkaline earth metal element and the metal or nonmetal element contained in the raw material particles is generated. In addition, you may produce the solid solution of the said complex oxide and a noble metal by this baking process.

  Further, the powder after firing is compression molded, and the molded product is pulverized as necessary. As described above, a pellet-shaped exhaust gas-purifying catalyst is obtained.

  Thus, in the above method, a crosslinking substance is contained in the slurry before firing. When the slurry does not contain a cross-linking substance, a by-product containing an alkaline earth metal is easily generated in addition to the composite oxide that is the target product. That is, in this case, the composite oxide cannot be produced at a high production ratio, and the performance of the exhaust gas purifying catalyst may be insufficient. On the other hand, when a cross-linking substance is added before firing, the production of by-products can be suppressed, and the performance improvement of the exhaust gas purifying catalyst can be achieved.

As the raw material particles, typically, oxide particles of the above-mentioned metal or non-metal element are used. As the oxide particles, for example, ceria (CeO ₂ ), titania (TiO ₂ ), zirconia (ZrO ₂ ), alumina (Al ₂ O ₃ ), or silica (SiO ₂ ) particles can be used. A plurality of raw material particles may be used.

  As the alkaline earth metal salt, for example, acetate, carbonate, nitrate or sulfate of the above alkaline earth metal element can be used. A plurality of alkaline earth metal salts may be used.

  As the cross-linking substance, for example, water-soluble polymers such as gelatin, agar, polyvinyl alcohol (PVA), and polyacrylamide can be used. However, polyacrylamide contains acrylamide that is harmful to the human body and the environment, or may generate such a monomer in the process of producing an exhaust gas purification catalyst. Therefore, it is preferable to use gelatin, agar, PVA, or the like that is harmless to the human body and the environment as the water-soluble polymer.

  In addition, agar needs to be melted at 85 ° C. or higher, and may cause particle aggregation and / or increase in slurry viscosity due to evaporation of moisture during melting. Since PVA can react with an alkaline earth metal, there is a possibility that the production ratio of the composite oxide is lowered. Therefore, it is particularly preferable to use gelatin as the water-soluble polymer.

  As the cross-linking substance, for example, a water-soluble polyol such as glycerin can be used. However, when glycerin is added in a large amount, the specific surface area of the final product may be reduced.

  The addition amount of the cross-linking substance is, for example, in the range of about 0.5 wt% to about 2.5 wt%, typically about 1.25 wt% to about 2.0 wt, based on the weight of the dispersion solvent. Within the mass% range. When the addition amount of the cross-linking substance is small, the cross-linked structure is not sufficiently formed, and the production ratio of the composite oxide may not be sufficiently improved. Moreover, if the addition amount of the crosslinking substance is increased, the time required for completely burning the crosslinking substance may be increased. Alternatively, the cross-linking material may not disappear completely upon firing and remain in the final product. In this case, the specific surface area of the composite oxide may be reduced.

  In the above-described method, the firing temperature and firing time are not particularly limited as long as the crosslinking material can be removed.

  The case where the exhaust gas purifying catalyst is a pellet catalyst has been described above as an example, but the exhaust gas purifying catalyst can take various forms. For example, the exhaust gas purifying catalyst may be a monolith catalyst.

  Examples of the present invention will be described below, but the present invention is not limited thereto.

<Example 1: Preparation of powder P1>
First, 100 g of pure water, 100 g of noble metal-supported powder, and 8.2 g of an alkaline earth metal salt were mixed. Next, 2.5 g of gelatin was added as a cross-linking substance. This was then dried at 60 ° C. for 5 minutes. Then, this was baked at 1000 degreeC for 3 hours, and the powder was obtained (the amount of gelatin additions 1.2 mass%). Hereinafter, this powder is referred to as “powder P1”.

<Example 2: Preparation of powder P2>
A powder was obtained in the same manner as in Example 1 except that 1.3 g of gelatin was used instead of 2.5 g of gelatin as a cross-linking substance (gelatin addition amount 0.62% by mass). Hereinafter, this powder is referred to as “powder P2”.

<Example 3: Preparation of powder P3>
A powder was obtained in the same manner as in Example 1 except that 4.0 g of gelatin was used instead of 2.5 g of gelatin as the cross-linking substance (gelatin addition amount 1.9% by mass). Hereinafter, this powder is referred to as “powder P3”.

<Example 4: Preparation of powder P4>
A powder was obtained in the same manner as in Example 1 except that 5.0 g of gelatin was used instead of 2.5 g of gelatin as a cross-linking substance (amount of gelatin added: 2.4% by mass). Hereinafter, this powder is referred to as “powder P4”.

<Example 5: Preparation of powder P5>
A powder was obtained in the same manner as in Example 1 except that 10 g of gelatin was used instead of 2.5 g of gelatin as a cross-linking substance (gelatin addition amount 4.8% by mass). Hereinafter, this powder is referred to as “powder P5”.

<Example 6: Preparation of powder P6>
A powder was obtained in the same manner as in Example 1 except that 2.5 g of agar was used instead of 2.5 g of gelatin as a cross-linking substance (agar addition amount: 1.2% by mass). Hereinafter, this powder is referred to as “powder P6”.

<Example 7: Preparation of powder P7>
A powder was obtained in the same manner as in Example 1 except that 2.5 g of polyvinyl alcohol (PVA) was used instead of 2.5 g of gelatin as a cross-linking substance (PVA addition amount: 1.2% by mass) . Hereinafter, this powder is referred to as “powder P7”.

<Example 8: Preparation of powder P8>
A powder was obtained in the same manner as in Example 1 except that 2.5 g of glycerin was used instead of 2.5 g of gelatin as the cross-linking substance (glycerin addition amount 1.2% by mass). Hereinafter, this powder is referred to as “powder P8”.

<Example 9: Preparation of powder P9 (comparative example)>
A powder was obtained in the same manner as in Example 1 except that gelatin was not used (crosslinking substance addition amount: 0% by mass). Hereinafter, this powder is referred to as “powder P9”.

<Evaluation of physical properties of powders P1 to P9>
Next, the physical properties of these powders P1 to P9 were examined in detail by the following method.
First, X-ray diffraction (XRD) spectra were measured for the powders P1 and P9. The result is shown in FIG.

  FIG. 1 is a diagram showing an example of a change in an X-ray diffraction spectrum depending on whether or not a cross-linking substance is added. In this figure, the horizontal axis indicates the diffraction angle 2θ, and the vertical axis indicates the X-ray diffraction intensity.

  As shown in FIG. 1, with respect to the powder P9, a diffraction peak was observed in the range of 2θ = 22 ° to 25 ° indicating the presence of the alkaline earth metal salt. On the other hand, no diffraction peak was observed in this range for powder P1. Therefore, it was clarified that alkaline earth metal salt remains in the powder P9, but almost no alkaline earth metal salt remains in the powder P1. That is, it was found that the addition of gelatin suppressed the formation of alkaline earth metal aggregates as a by-product.

  Next, for the powders P1 and P6 to P9, the production ratio of the composite oxide as the target product was examined from the XRD spectrum. The result is shown in FIG.

  FIG. 2 is a bar graph showing the composite oxide formation ratio for each of the powders P1 and P6 to P9.

  As shown in FIG. 2, it was found that the powders P1 and P6 to P8 contained a larger amount of complex oxide than the powder P9. That is, it has been clarified that a powder having a higher composite oxide production ratio can be obtained by adding gelatin, agar, PVA or glycerin. It was also found that the addition of gelatin gave a powder with a particularly high composite oxide production ratio.

  Next, for the powders P1 to P5 and P9, the production ratio and specific surface area of the composite oxide were examined. Further, the specific surface area of the powder P8 was examined. The results are shown in FIG.

  In FIG. 3, the horizontal axis indicates the amount of gelatin or glycerin added, and the vertical axis indicates the composite oxide production ratio and specific surface area.

As shown in FIG. 3, the production ratio of the composite oxide increased as the amount of gelatin added was increased, and a particularly high production ratio could be achieved when the amount of gelatin added was about 1.2% by weight or more. However, when the amount of gelatin added was about 2.4% by mass or more, the production ratio of the composite oxide did not increase even when the amount of gelatin added was increased, and showed a tendency to saturate. On the other hand, the specific surface area was increased by adding gelatin when the amount of gelatin added was about 1.9% by mass or less, but when the amount of gelatin added was about 2.4% by mass or more, gelatin was not added. Reduced by addition. Moreover, it turned out that the specific surface area of the powder P8 is smaller compared with the powder P9.
The invention described in the original claims is appended below.
[1] Stirring a slurry containing raw material particles containing a metal or a non-metallic element, a noble metal supported on the raw material particles, an alkaline earth metal salt, and a cross-linking substance, and the stirred slurry And producing a composite oxide of the metal or non-metal element supporting the noble metal and the alkaline earth metal. A method for producing an exhaust gas purifying catalyst, comprising:
[2] The method for producing an exhaust gas purifying catalyst as described in [1], wherein the cross-linking substance is a water-soluble polymer.

The figure which shows an example of the change of the X-ray-diffraction spectrum by the presence or absence of addition of a crosslinking substance. The graph which shows an example of the relationship between the kind of crosslinking substance to add, and the production | generation ratio of complex oxide. The graph which shows an example of the relationship between the addition amount of a crosslinking substance, the production | generation ratio of a complex oxide, and a specific surface area.

Claims (2)

Containing raw material particles containing a metal or a non-metallic element; a noble metal supported on the raw material particles; a salt of an alkaline earth metal; and a cross-linking substance , wherein the cross-linking substance is at least one selected from gelatin and agar Stirring the slurry which is a water-soluble polymer containing
The catalyst for exhaust gas purification comprising drying and firing the stirred slurry to produce a composite oxide of the metal or non-metal element supporting the noble metal and the alkaline earth metal Manufacturing method.   2. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the slurry has an addition amount of the cross-linking substance of 0.5 mass% or more based on the mass of the dispersion solvent.

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