JP2005095763A - Exhaust gas purification catalyst manufacturing method and exhaust gas purification catalyst - Google Patents

Abstract

In a catalyst in which at least Pt is supported on a ceria-based support, Pt grain growth is suppressed and durability is improved.
A platinum complex is supported on a carrier containing ceria to form a complex carrier, and the complex carrier is heated in an oxygen-deficient atmosphere to decompose the platinum complex to carry metal platinum.
By decomposing the complex-supported carrier in an oxygen-deficient atmosphere, no Pt oxide is produced, and the Pt complex becomes metal Pt. Therefore, the metal Pt produced is extremely fine and has a uniform particle size, and is supported in a highly dispersed state like the Pt complex.

Description

  The present invention relates to a method for producing an exhaust gas purification catalyst capable of purifying exhaust gas from an automobile and an exhaust gas purification catalyst, and more particularly to an exhaust gas purification catalyst having at least Pt supported on a ceria-based carrier containing ceria and a method for producing the same.

  As a catalyst for purifying exhaust gas from automobiles, for example, a three-way catalyst in which a noble metal such as Pt or Rh is supported on a porous carrier such as alumina is widely used. In order to exhibit high activity as a catalyst, it is desirable to support fine noble metals in a highly dispersed state. Therefore, the mainstream for supporting the noble metal is to impregnate the support with an aqueous solution of a noble metal nitrate or a complex and calcinate it.

  According to the above method, fine noble metals can be supported in a highly dispersed state. However, when used as a catalyst, there is a problem that noble metal easily grows due to high temperature acting in a harsh atmosphere, and the activity decreases due to a decrease in active sites. In particular, Pt has a high oxidation activity and is an essential noble metal for an exhaust gas purification catalyst, but has a drawback of easy grain growth.

  Japanese Patent Publication No. 04-122441 describes a method of preventing the growth of Pt grains accompanying the growth of a carrier crystal in use by growing the carrier crystal in advance. Japanese Patent Laid-Open No. 08-038897 discloses that an alumina carrier is impregnated with a Pt complex and calcined in the atmosphere, and then calcined in a vacuum or a reducing atmosphere at a temperature of 800 ° C. or more to shrink the carrier pores. Describes a method for fixing grain and suppressing grain growth.

  In recent years, ceria-based carriers such as ceria and ceria-zirconia solid solutions have been used. Since the ceria-based carrier has an oxygen absorption / release capability, it can be adjusted to near the stoichiometric range by reducing atmospheric fluctuations, and is thus extremely advantageous as a three-way catalyst. It has been found that oxygen absorption / release capability is further improved by supporting Pt or the like on a ceria-based carrier. For example, JP 09-299797 A describes a catalyst in which Pt is supported on a mixed carrier of alumina and ceria-zirconia composite oxide.

However, the catalyst in which Pt is supported on a ceria-based support has a problem that Pt is likely to grow grains and inferior in durability compared to a catalyst in which Pt is supported on alumina. Therefore, it was considered that the method described in Japanese Patent Application Laid-Open No. 08-038897 was used, and in the case of a ceria-based support, the effect as in the case of an alumina support was not obtained, and the grain growth of Pt was suppressed. It was difficult.
No. 04-122441 JP 08-038897 JP 09-299797

  This invention is made | formed in view of such a situation, and makes it a subject to suppress the grain growth of Pt and to improve durability in the catalyst formed by carrying | supporting at least Pt on a ceria-type support | carrier.

  A feature of the method for producing an exhaust gas purifying catalyst of the present invention that solves the above-mentioned problems is that a complex supporting step of supporting a platinum complex on a carrier containing ceria to form a complex supporting carrier, and heating the complex supporting carrier in an oxygen-deficient atmosphere. A decomposition step of decomposing the platinum complex and supporting metal platinum. The heating temperature in the decomposition step is preferably 400 to 700 ° C. The platinum complex is soluble in an organic solvent, and the complex supporting step is preferably performed by impregnating a carrier with a solution in which the platinum complex is dissolved in the organic solvent and drying the organic solvent.

  The exhaust gas purifying catalyst of the present invention is characterized in that it is an exhaust gas purifying catalyst comprising a carrier containing ceria and at least platinum supported on the carrier, and platinum is supported as metallic platinum having a particle size of 0.5 to 5 nm. There is in being.

  According to the exhaust gas purifying catalyst of the present invention, the grain growth of the supported Pt can be greatly suppressed, and the durability is excellent. Further, according to the production method of the present invention, the exhaust gas purifying catalyst of the present invention can be produced easily and stably.

  According to the study by the present inventors, it has been clarified that the behavior of Pt grain growth differs greatly between a catalyst supporting Pt on a ceria-based support and a catalyst supporting Pt on alumina. For example, for a catalyst in which Pt is supported on a ceria-zirconia solid solution (hereinafter referred to as CZ) and a catalyst in which Pt is supported on alumina, the relationship between the time when heated at 1000 ° C. in the atmosphere and the Pt particle size is shown in FIG. Shown in In the catalyst with Pt supported on alumina, the Pt particle size increases almost linearly with the passage of time, whereas in the catalyst with Pt supported on CZ, the Pt particle size increases greatly at the beginning of heating. There is almost no increase in the Pt particle size.

  Further, FIGS. 2 and 3 show the results of XPS analysis performed immediately after preparation and after heating at 1000 ° C. in the atmosphere for a catalyst supporting Pt on CZ and a catalyst supporting Pt on alumina, respectively. A catalyst with Pt supported on CZ contains a large amount of Pt oxide even after heating at 1000 ° C. in the atmosphere, whereas a catalyst with Pt supported on alumina has a small amount of Pt oxide and a large amount of metal Pt. It has become. In other words, it is considered that a catalyst having Pt supported on CZ has a stronger bonding force between Pt and oxygen than a catalyst having Pt supported on alumina, and the Pt oxide is not easily reduced.

  Further, according to the method described in JP-A-08-038897, a catalyst carrying Pt was prepared by impregnating CZ with Pt complex and calcining in the atmosphere at 500 ° C. for 1 hour, and its XRD chart is shown in FIG. Shown in Although a weak Pt (111) peak is observed in FIG. 4, this peak is not a diffraction peak of all Pt particles, but a part of Pt particles is enlarged and detected by XRD. . In other words, since the Pt complex was decomposed in the atmosphere, a part of the Pt oxide generated by the complex decomposition was vapor-phase grown, and the particle size was considered to have increased.

  The reason why noble metal grain growth occurs is that the larger particles are more energetically stable than the smaller particles, and the number of small particles is reduced by the thermal load, and the larger particles become larger and larger. Therefore, the variation in the Pt particle diameter is also a factor for promoting the grain growth, and it can be said that the narrower the distribution of the Pt particle diameter, the more difficult the grain growth. Therefore, even if the method described in Japanese Patent Application Laid-Open No. 08-038897 is applied, since the vapor phase growth of Pt oxide occurs after the decomposition of the Pt complex, the Pt particle diameter varies, so that means for suppressing grain growth It cannot be.

  That is, in a catalyst in which Pt is supported on CZ, Pt grain growth at the initial stage of durability is dominant, and it is important to suppress Pt grain growth in the process. That is, after the Pt complex is supported on the carrier, it is important to reduce the Pt oxide to metal Pt at the earliest possible stage during firing.

  Therefore, in the production method of the present invention, a complex-supported carrier in which a Pt complex is supported on a carrier containing ceria is heated in an oxygen-deficient atmosphere that does not contain oxygen in an atmosphere necessary to oxidize Pt. A decomposition process for carrying metal Pt by decomposition is performed. Pt oxide is not generated by decomposing the complex-supported carrier in an oxygen-deficient atmosphere, and the Pt complex suddenly becomes metal Pt. Therefore, the produced metal Pt is very fine and has a uniform particle size, and is supported in a highly dispersed state like the Pt complex.

  The complex supporting step is a step of forming a complex supporting carrier by supporting a Pt complex on a carrier containing ceria. Examples of the carrier containing ceria include ceria, CZ, ceria-zirconia-alumina composite oxide, ceria-zirconia-yttria composite oxide, and the like. At least one of these may be used as a simple substance, or at least one selected from these may be used by mixing various oxides such as alumina, zirconia, and titania.

The Pt complexes, it is possible to use a water-soluble complex, such as dinitrodiammineplatinum, cyclo tetrakis (di -μ- acetato - platinum (II)) ([Pt ( C 2 H 3 O 2) 2] 4 ), Bis (acetylacetonato) platinum (Pt (C ₅ H ₇ O ₂ ) ₂ ), bis (2-ethylhexanoic acid) platinum (C ₁₆ H ₃ O ₄ Pt), organic substances such as acetone, chloroform, and dichloromethane It is desirable to use a complex that is soluble in the solvent. When supported using an aqueous solution, the particle size of the supported metal Pt may increase due to aggregation of the complexes when the solvent is dried. If a complex containing an organic group is used, the metallization of Pt proceeds faster due to the C and H contained therein.

  The decomposition step is a step of supporting the metal Pt by heating the complex-supporting carrier in an oxygen-deficient atmosphere to decompose the Pt complex. The oxygen-deficient atmosphere is preferably an oxygen-free atmosphere such as a vacuum atmosphere, an inert gas atmosphere, or a reducing atmosphere, but contains oxygen if the amount is sufficient to burn the organic group of the Pt complex or the remaining organic solvent. It can also be an atmosphere.

  The heating temperature in the decomposition step is preferably 400 to 700 ° C. If it is less than 400 ° C, it takes a long time to decompose the Pt complex, which is not practical. Pt particles are generated.

  In the exhaust gas purifying catalyst of the present invention produced by the production method of the present invention, Pt is highly dispersed and supported as uniform and fine metal Pt. According to the production method of the present invention, the particle size of the supported metal Pt can be made extremely fine, 0.5 to 5 nm. Therefore, Pt grain growth during use is suppressed, and the durability is excellent.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples. In Table 1, the manufacturing method of the catalyst of each Example and each comparative example is shown collectively.

(Example 1)
120 g of CZ (Ce: Zr = 50 mol: 50 mol) was impregnated with a dinitrodiammine platinum aqueous solution equivalent to 2 g of Pt, evaporated to dryness at 100 ° C. with good stirring, and dried at 100 ° C. for 3 hours in a dryer. .

The obtained Pt / CZ powder is placed in a tube furnace and heated at 500 ° C. for 1 hour while circulating 1% H ₂ / N ₂ gas to decompose and reduce dinitrodiammine platinum and to support metal Pt I let you. This was pelletized by a conventional method to prepare the catalyst of this example.

(Comparative Example 1)
The Pt / CZ powder obtained in the same manner as in Example 1 was heated in air at 500 ° C. for 1 hour to decompose dinitrodiammine platinum and to support Pt. In this state, Pt is supported as Pt oxide. This was pelletized by a conventional method to prepare a catalyst of this comparative example.

<Test Example 1>
The catalysts of Example 1 and Comparative Example 1 were placed in apparatuses capable of XRD measurement while heating, and the peak of the Pt (111) plane at room temperature was measured while circulating the atmosphere. The results are shown in FIG. The sample was heated to 1000 ° C. while circulating the air, and the change with time of the peak of the Pt (111) plane was measured. The Pt particle size was calculated from the measured values by Scherrer's equation, and the results are shown in FIG.

  FIG. 5 shows that a Pt (111) diffraction peak is observed in the catalyst of Comparative Example 1, but not in the catalyst of Example 1. In the catalyst of Example 1, Pt that has grown after heating at 500 ° C. for 1 hour. It can be seen that does not exist. That is, in the catalyst of Example 1, the particle size of the Pt particles is small and there is no variation before heating at 1000 ° C.

  In addition, FIG. 6 shows that the Pt particle size of the catalyst of Example 1 is 1/4 or less that of Comparative Example 1 even after heating at 1000 ° C., which is that dinitrodiammine platinum was decomposed at 500 ° C. in a reducing atmosphere. It is clear that this is an effect.

(Example 2)
The Pt / CZ powder obtained in the same manner as in Example 1 was placed in a tubular furnace, heated at 300 ° C. for 1 hour while circulating 1% H ₂ / N ₂ gas, and further 1% H ₂ / N _2. Firing was carried out at 800 ° C. for 1 hour while circulating the gas to decompose and reduce dinitrodiammineplatinum, thereby supporting metal Pt. This was pelletized by a conventional method to prepare the catalyst of this example.

(Comparative Example 2)
The Pt / CZ powder obtained in the same manner as in Example 1 was heated in air at 300 ° C. for 1 hour, and further calcined at 800 ° C. for 1 hour while circulating 1% H ₂ / N ₂ gas, and dinitrodiammine. Platinum was decomposed and Pt was supported. This was pelletized by a conventional method to prepare a catalyst of this comparative example. This comparative example is an application of the method described in JP-A-08-038897.

<Test Example 2>
For the catalysts of Example 2 and Comparative Example 2, the peak of the Pt (111) plane at room temperature was measured in the same manner as in Test Example 1. The results are shown in FIG. The sample was heated to 1000 ° C. while circulating the air, and the change with time of the peak of the Pt (111) plane was measured. The Pt particle size was calculated from the measured values by Scherrer's equation, and the results are shown in FIG.

  From FIG. 8, it is clear that according to the catalyst of the present invention, the Pt grain growth can be further suppressed as compared with the technique described in JP-A-08-038897. Further, from FIG. 7, in the treatment of Comparative Example 2, a slight Pt (111) diffraction peak was already observed before heating at 1000 ° C., and part of the Pt grain growth occurred at this stage. This non-uniformity is considered to affect the difference in grain growth in FIG.

<Test Example 3>
A CZ powder was impregnated with a dinitrodiammine platinum aqueous solution so that Pt was 3 wt%, evaporated to dryness, and then dried at 100 ° C. in the atmosphere. About 45 mg of this catalyst powder, the weight change at the time of temperature rising was measured by the thermogravimetric analysis (TG) under a vacuum, and a result is shown in FIG.

On the other hand, the Pt / CZ powder obtained in the same manner as in Example 1 was placed in a tubular furnace and heated at two levels of 500 ° C. and 800 ° C. for 1 hour while circulating 1% H ₂ / N ₂ gas. The complex was decomposed. Thereafter, the mixture was heated to 1000 ° C. while circulating the air, and the change with time of the peak of the Pt (111) plane was measured. The Pt particle size was calculated from the measured values by Scherrer's equation, and the results are shown in FIG.

  FIG. 9 shows that dinitrodiammine platinum decomposes at about 400 ° C. under vacuum. In addition, as shown in FIG. 10, in the case of decomposition at 800 ° C., the Pt particle size becomes 20 nm or more just by heating at 1000 ° C. for 1 hour. Therefore, it can be seen that the decomposition temperature of the complex is preferably in the range of 400 to 700 ° C.

(Example 3)
120 g of CZ as in Example 1 was impregnated with an acetone solution of bis (2-ethylhexanoic acid) platinum equivalent to 2 g of Pt, and stirred for 1.5 hours at room temperature to evaporate and dry the acetone.

The obtained Pt / CZ powder is placed in a tube furnace and heated at 500 ° C. for 1 hour while circulating a mixed gas of air and N ₂ (Air: N ₂ = 1: 20) to decompose and reduce the complex. Then, metal Pt was supported. This was pelletized by a conventional method to prepare the catalyst of this example.

Example 4
120 g of CZ as in Example 1 was impregnated with an acetone solution of bis (2-ethylhexanoic acid) platinum equivalent to 2 g of Pt, and stirred for 1.5 hours at room temperature to evaporate and dry the acetone.

The obtained Pt / CZ powder was placed in a tubular furnace and heated at 500 ° C. for 1 hour while circulating 1% H ₂ / N ₂ gas to decompose and reduce the complex, thereby supporting metal Pt. . This was pelletized by a conventional method to prepare the catalyst of this example.

<Test Example 4>
For the catalysts of Examples 3 and 4, the Pt (111) surface peak after heating at 1000 ° C. for 2.5 hours while circulating the air in the same manner as in Test Example 1 was measured. The result of calculating the diameter is shown in FIG. 11 together with the results of Example 1 and Comparative Example 1.

  From the comparison between Example 1 and Example 4, it was found that the complex was supported using a solution in which an organic solvent-soluble complex was dissolved in a highly volatile organic solvent. It can be seen that the grain growth can be further suppressed. This is thought to be because the drying rate of the organic solvent is higher than that of water, so that the aggregation of the supported complex is suppressed, and the metallization of Pt proceeds faster due to C and H contained in the complex.

  From the results of Example 3, it can be seen that when a complex containing an organic group is used, Pt grain growth can be suppressed even if the complex is decomposed in an atmosphere containing some oxygen. In an actual production line, it is difficult to mass-produce a catalyst under the condition that there is no oxygen, but if the atmosphere is the same as that used in Example 3, there will be no effect on the atmosphere in the line. Management is easy and mass production is possible.

The exhaust gas purifying catalyst and the method for producing the same of the present invention can be used not only for automobile three-way catalysts, but also for NO _x storage reduction catalysts, hydrogen generation catalysts, and the like.

It is a graph which shows the relationship between time when heating at 1000 degreeC in air | atmosphere, and Pt particle size. 3 is a Pt4f-XPS chart of a catalyst in which Pt is supported on CZ. ₃ is a Pt4d-XPS chart of a catalyst in which Pt is supported on Al ₂ O ₃ . It is an XRD chart immediately after decomposing | disassembling the complex of the catalyst which carry | supported Pt in CZ. It is an XRD chart immediately after decomposing | disassembling the complex of the catalyst of Example 1 and Comparative Example 1. FIG. It is a graph which shows the relationship between the time when the catalyst of Example 1 and the comparative example 1 is heated at 1000 degreeC in air | atmosphere, and Pt particle size. It is an XRD chart immediately after decomposing | disassembling the complex of the catalyst of Example 2 and Comparative Example 2. FIG. It is a graph which shows the relationship between the time when the catalyst of Example 2 and the comparative example 2 is heated at 1000 degreeC in air | atmosphere, and Pt particle size. It is a graph which shows the relationship between the temperature of the catalyst which carry | supported dinitrodiammine platinum in CZ, and the weight loss. It is a graph which shows the relationship between the time when heating at 1000 degreeC in air | atmosphere of the catalyst which carry | supported dinitrodiammine platinum on CZ, and decomposed | disassembled at 500 degreeC or 800 degreeC by reducing atmosphere. 3 is a graph showing the Pt particle size of each catalyst when heated at 1000 ° C. for 2.5 hours in the air.

Claims (4)

A complex supporting step of supporting a platinum complex on a carrier containing ceria to form a complex supporting carrier;
And a decomposition step of decomposing the platinum complex to support metal platinum by heating the complex-supporting carrier in an oxygen-deficient atmosphere.   The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the heating temperature in the decomposition step is 400 to 700 ° C.   The exhaust gas purification according to claim 1, wherein the platinum complex is soluble in an organic solvent, and the complex supporting step is performed by impregnating the carrier with a solution obtained by dissolving the platinum complex in an organic solvent and drying the organic solvent. For producing a catalyst for use. An exhaust gas purifying catalyst comprising a carrier containing ceria and at least platinum carried on the carrier,
An exhaust gas purifying catalyst, wherein the platinum is supported as metallic platinum having a particle size of 0.5 to 5 nm.

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