JP2007098251A - Catalyst carrier and exhaust gas purifying catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst carrier suitable for carrying rhodium to use it and an exhaust gas purifying catalyst which carries rhodium. <P>SOLUTION: The catalyst carrier comprises zirconia particles 11 covered with an oxide 12 of a metal having α value of 0.30 or lower determined from Equation (1) α = absolute value of (ionic radius of metal A - ionic radius of rhodium)/ionic radius of rhodium wherein the ion diameters of the metal and rhodium are those of the metal and rhodium in a state of trivalent ions and a six-coordinate bond. The exhaust gas purifying catalyst comprises rhodium 13 carried by the catalyst carrier. The exhaust gas purifying catalyst includes a catalyst keeping rhodium 13 carried by particles of an oxide of a metal selected from the group consisting of iron, gallium, scandium, indium, lutetium, ytterbium, thulium, erbium, yttrium, and holmium. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a catalyst carrier and a rhodium-supported exhaust gas purification catalyst.

The exhaust gas from an internal combustion engine such as an automobile engine contains nitrogen oxides (NO _x ), carbon monoxide (CO), hydrocarbons (HC), etc., and these substances oxidize CO and HC. At the same time, it can be purified by an exhaust gas purifying catalyst capable of reducing NO _x. A typical example of such an exhaust gas purification catalyst is a three-way catalyst in which a noble metal such as platinum (Pt), rhodium (Rh), palladium (Pd) is supported on a porous metal oxide carrier such as γ-alumina. Etc. are known.

Although this metal oxide support can be made of various materials, conventionally, alumina (Al ₂ O ₃ ) has been generally used to obtain a high surface area. In recent years, however, various other materials such as ceria (CeO ₂ ), zirconia (ZrO ₂ ), titanium (TiO ₂ ), etc., have been combined with alumina in order to promote exhaust gas purification utilizing the chemical nature of the support. It has also been proposed to use it in combination or not.

For example, rhodium which is a noble metal is known to have favorable NO _x purification performance. However, conventionally used alumina supports generally have many oxygen vacancies. Therefore, in a catalyst in which rhodium is supported on such an alumina support, rhodium is dissolved in oxygen vacancies in alumina in a high-temperature oxidizing atmosphere of 900 ° C. or higher. However, there is a problem that the exhaust gas purification performance of rhodium is significantly lowered. In order to solve this problem, it is known to use a catalyst powder in which rhodium is supported on zirconia (Patent Document 1, etc.). The zirconia used here is not easily dissolved in rhodium during the use of the catalyst, and therefore, deterioration of the catalyst using rhodium can be suppressed.

  In order to improve the heat resistance of such a catalyst in which rhodium is supported on zirconia, Patent Document 1 discloses that a carrier in which at least one element selected from lanthanoid elements is added to zirconia, rhodium supported on the carrier, An exhaust gas purifying catalyst is provided. Further, in the cited document 2, similarly, in a catalyst in which rhodium is supported on zirconia, the zirconia support is formed into hollow particles made of zirconia to which at least one additive element selected from alkaline earth metals and rare earth elements is added. Furthermore, it is proposed to further improve the heat resistance.

In Patent Document 3, a layer containing zirconia as a main component and containing at least one of an alkaline earth metal and a rare earth element is coated with core particles such as alumina to obtain a catalyst carrier, and this catalyst carrier stores NO _x. An exhaust gas purifying catalyst is obtained by supporting a material and a noble metal. According to this cited document 3, such an exhaust gas purifying catalyst is such that a carrier such as alumina easily undergoes a solid phase reaction with an alkali metal or the like that is a NO _x storage material in a high temperature exhaust gas atmosphere, and It is possible to prevent the NO _x storage material from sintering.

JP 2000-15101 A Japanese Patent Laid-Open No. 2002-282692 JP 2004-275919 A JP 2000-70717 A

  As described above, when rhodium is used in the exhaust gas purification catalyst, the problem of solid solution of rhodium in the carrier is solved by using zirconia as the carrier. However, the fact that rhodium does not form a solid solution in the carrier zirconia means that the interaction between rhodium and the carrier is small, so that rhodium easily moves on the surface of the carrier, and thus rhodium tends to grow during use. It means having.

  Therefore, the present invention provides a catalyst carrier and a rhodium-supported exhaust gas purification catalyst that solve these problems.

（金属Ａ及びロジウムのイオン半径は、３価イオン及び６配位結合の状態での値）。 The catalyst support of the present invention is a catalyst support having zirconia particles and a metal oxide covering the zirconia particles, wherein the metal A constituting the metal oxide has a trivalent oxidation number, and A catalyst carrier in which the value of α obtained by the following formula (1) for metal A is 0.30 or less:

(The ionic radii of metal A and rhodium are values in the state of trivalent ions and hexacoordinate bonds).

  Thus, a metal oxide having the same trivalent oxidation number as rhodium and having a small α value obtained by the formula (1), that is, a relatively small difference in ionic radius from rhodium ions, is rhodium. Has a relatively large affinity. Accordingly, the catalyst carrier of the present invention obtained by coating the surface of the zirconia particles with such a metal oxide, when the exhaust gas purification catalyst is obtained by loading rhodium, the carrier and the rhodium supported on the carrier. Some degree of interaction can be generated between them, thereby suppressing rhodium sintering during use of the catalyst. In the exhaust gas purifying catalyst of the present invention thus obtained, the metal oxide layer is relatively thin, so that the solid solution of rhodium in the support is suppressed and / or the high heat resistance of the zirconia particles is increased. It can also be used.

  Here, the metal is selected from the group consisting of iron, gallium, scandium, aluminum, indium, lutetium, ytterbium, thulium, erbium, yttrium and holmium, particularly iron, gallium, scandium, indium, ytterbium and yttrium. A metal selected from the group can be used.

  That is, in the catalyst carrier of the present invention, the affinity for rhodium is provided by the metal oxide coated on the surface of the zirconia particles. However, if this affinity is too large, the exhaust gas purification performance may be lowered due to the solid solution of the metal oxide and rhodium. Therefore, the metal which coats the surface of the zirconia particles with an oxide of a metal selected from the group consisting of scandium and indium having an α value of 0.06 to 0.18 obtained by the above formula (1) More preferably, it is selected as an oxide.

  As described above, the value used as the ionic radius in the formula (1) is a value in the state of trivalent ions and hexacoordinate bonds. For reference, specific values for some metals are shown in Table 1 below.

尚、ジルコニウムは４価の酸化数を有し、基本的に３価の酸化数を有さない。

Zirconium has a tetravalent oxidation number and basically does not have a trivalent oxidation number.

  The metal oxide used for coating the zirconia support in the present invention can be used in an amount that prevents sintering of the supported rhodium and does not excessively dissolve this rhodium. The metal element can be used in an amount of 1 to 20 mol%, particularly 5 to 15 mol%, more particularly 8 to 12 mol% with respect to zirconium.

  The exhaust gas purifying catalyst of the present invention comprises rhodium supported on the catalyst carrier of the present invention.

  As described above, in the exhaust gas purifying catalyst of the present invention, the rhodium supported can be prevented from sintering and solid solution in the carrier.

  The other exhaust gas purifying catalyst of the present invention is selected from the group consisting of iron, gallium, scandium, indium, lutetium, ytterbium, thulium, erbium, yttrium, and holmium, particularly selected from the group consisting of scandium and yttrium. Rhodium is supported on metal oxide particles.

  According to the exhaust gas purifying catalyst of the present invention, rhodium sintering can be suppressed by the affinity between these metal oxides and rhodium.

  The catalyst carrier and exhaust gas purification catalyst of the present invention will be described with reference to FIGS. Here, both (a) and (b) of FIG. 1 are cross-sectional views of the exhaust gas purifying catalyst of the present invention.

  As shown in FIGS. 1 (a) and 1 (b), the catalyst support of the present invention comprises zirconia particles 11 and an oxide of a metal oxide such as scandium, yttrium or iron covering the zirconia particles. It has a layer 12. The exhaust gas purifying catalyst of the present invention comprises rhodium 13 supported on this catalyst carrier.

  Here, the boundary between the zirconia particles 11 and the outer layer 12 of the metal oxide such as scandium is not necessarily clear, and may appear as a portion where the composition gradually changes. Further, even if the metal oxide such as scandium covers the entire zirconia particles 11 as the continuous layer 12 as shown in FIG. 1A, the oxide is discontinuous as shown in FIG. 1B. The zirconia particles 11 may be only partially coated as the thick layer 12 '.

  FIG. 2 is an enlarged cross-sectional view of the exhaust gas purification catalyst of the present invention, in which zirconia particles 11, a metal oxide layer 12 covering the zirconia particles 11, and rhodium supported on the metal oxide. Particle 13 is shown. As shown here, when a metal oxide having a relatively high affinity for rhodium is coated as a relatively thin layer 12 on zirconia particles 11 having a low affinity for rhodium, the oxide of this metal However, even if rhodium is dissolved, it is considered that this solid solution is prevented from becoming excessive. Here, in FIG. 2, the region in which rhodium is dissolved is shown as 12a.

  The catalyst carrier and exhaust gas purification catalyst of the present invention can be used not only by molding itself but also by mixing with other carrier particles such as alumina, and also coated on a monolithic carrier such as a ceramic honeycomb. It can also be used.

  The catalyst carrier of the present invention can be produced by any method. For example, it provides zirconia particles, and the zirconia particles are obtained from the above-mentioned formula (1) and a metal oxide having a value of α of 0.30 or less. Can be produced by a method comprising coating by:

  The exhaust gas purification catalyst of the present invention can be obtained by supporting rhodium on zirconia particles coated with the metal oxide obtained as described above.

  Hereinafter, each step of the catalyst carrier and the catalyst production method of the present invention will be described in more detail.

(A) Provision of zirconia particles Commercially available zirconia particles can be used as the zirconia particles for the catalyst carrier of the present invention. The zirconia particles also provide a zirconium salt solution containing a zirconium salt, such as zirconium nitrate, from which a precipitate containing zirconium hydroxide is precipitated, and the resulting precipitate is optionally washed. , And drying and baking. Here, when it is preferable that the zirconia particles are stabilized by containing an alkali metal, an alkaline earth metal or a rare earth metal, a zirconium salt solution containing a zirconium salt is added to another zirconium salt solution such as yttrium nitrate. Such a zirconia particle can be obtained by adding a metal salt.

  For precipitation of the hydroxide from the zirconium salt solution, an alkaline solution such as aqueous ammonia or sodium hydroxide can be used. In addition, for drying and firing the precipitate, general conditions for producing the metal oxide can be used. For example, firing can be performed at a temperature of 400 ° C to 600 ° C.

  As described above, the heat resistance of the zirconia particles used in the present invention may be improved by adding an element selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals, particularly yttrium. In this regard, the addition rate of the metal added to the zirconia particles may be, for example, 0.1 to 20 mol%, in particular 1 to 10 mol%, based on the number of moles of zirconium atoms in the support.

(B) Coating of zirconia particles with metal oxide The coating of the zirconia particles for the catalyst support of the present invention with a metal oxide can be performed by any method, for example, constituting a metal oxide intended to be coated. This can be achieved by dispersing the zirconia particles provided as described above in a solution of a metal salt such as a nitrate solution, depositing the metal hydroxide or the like on the zirconia particles, and drying and firing. .

  The precipitation performed here, and the drying and baking of the precipitate can be performed in the same manner as in the case of providing zirconia particles.

(C) Supporting rhodium on zirconia particles coated with metal oxide Rhodium can be supported by any method. For example, zirconia particles coated with metal oxide are dispersed in a rhodium salt-containing solution. This dispersion can be stirred and filtered, and the resulting particles can be dried and calcined. The amount of rhodium supported on the metal oxide particles may be, for example, 0.01 to 5 mass%, particularly 0.1 to 2 mass% with respect to the metal oxide particles.

  Another exhaust gas purification catalyst of the present invention will be described with reference to FIG. Here, FIG. 3 is a sectional view of the exhaust gas purifying catalyst of the present invention.

  As shown in FIG. 3, the exhaust gas purifying catalyst of the present invention comprises rhodium 33 supported on oxide particles 32 such as scandium, yttrium and iron.

  This exhaust gas purifying catalyst of the present invention can be used not only by molding itself but also by mixing with other carrier particles such as alumina, or by coating a monolithic carrier such as a ceramic honeycomb. You can also.

  The exhaust gas purifying catalyst of the present invention can be produced by any method, and is obtained by a method including providing particles of oxides of metals such as scandium, yttrium, and iron, and supporting rhodium on the particles of the metal oxides. be able to.

  Below, each process of the manufacturing method of this exhaust gas purification catalyst of this invention is demonstrated more concretely.

(A) Provision of metal oxide particles such as scandium, yttrium, and iron Commercially available metal oxide particles can be used as these particles for the exhaust gas purifying catalyst of the present invention. These particles also provide a salt solution containing a salt such as scandium nitrate, from which a precipitate containing a hydroxide of these metals is precipitated, and the resulting precipitate is optionally added. It can also be obtained by washing and drying and baking.

  For precipitation of the hydroxide from the salt solution, an alkaline solution such as aqueous ammonia or sodium hydroxide can be used. In addition, for drying and firing the precipitate, general conditions for producing the metal oxide can be used. For example, firing can be performed at a temperature of 400 ° C to 600 ° C.

(B) Loading of rhodium on metal oxide particles such as scandium, yttrium and iron Rhodium can be loaded by any method. For example, the metal oxide particles are dispersed in a rhodium salt-containing solution. This dispersion can be stirred and filtered, and the resulting particles can be dried and calcined. The amount of rhodium supported on the metal oxide particles may be, for example, 0.01 to 5 mass%, particularly 0.1 to 2 mass% with respect to the metal oxide particles.

  EXAMPLES Hereinafter, although this invention is further demonstrated based on an Example, this invention is not limited to these.

[Example 1]
Zirconium-yttrium salt solution A is obtained by dissolving 100 g of zirconium oxynitrate dihydrate in 250 ml of ion-exchange water and yttrium nitrate. It was. Moreover, 150 ml of aqueous ammonia (28%) was added to 250 ml of ion-exchanged water to obtain an ammonia solution. The zirconium-yttrium salt solution was added to the ammonia solution thus obtained to obtain a precipitate.

  The resulting precipitate was washed and filtered five times with ion exchange water at 80 ° C. The washed precipitate was dried overnight at 120 ° C. and calcined at 500 ° C. for 2 hours to obtain yttrium-stabilized zirconia particles.

  Iron nitrate 9-hydrate is dissolved in 200 ml of ion-exchanged water in such an amount that iron becomes 10 mol% with respect to zirconium of the zirconia particles to obtain an iron nitrate solution, and the yttrium stable solution obtained above is obtained in this iron nitrate solution. 20 g of zirconia particles were added to obtain a zirconia particle dispersion. In addition, sodium carbonate having a molar ratio of 1.1 times the nitrate ion of iron nitrate / 9 hydrate used here was dissolved in 100 ml of ion-exchanged water to obtain a sodium carbonate solution. The sodium carbonate solution thus obtained was added dropwise to the zirconia particle dispersion.

  Thereafter, the precipitate in the zirconia particle dispersion to which the sodium carbonate solution was added was washed and filtered with ion exchange water at 80 ° C. five times. The washed precipitate was dried at 120 ° C. overnight and calcined at 500 ° C. for 2 hours to obtain yttrium-stabilized zirconia particles coated with iron oxide.

  The yttrium-stabilized zirconia particles coated with iron oxide are dispersed in 100 ml of ion-exchanged water, and rhodium nitrate is added in such an amount that the supported amount of rhodium is 0.5% by mass with respect to the support, and stirred for 2 hours. Filtration and drying, followed by calcination at 500 ° C. for 2 hours, gave the exhaust gas purification catalyst of Example 1.

[Example 2]
An exhaust gas purification catalyst of Example 2 was obtained in the same manner as Example 1 except that gallium nitrate.9hydrate was used instead of iron nitrate.9hydrate.

Example 3
An exhaust gas purification catalyst of Example 3 was obtained in the same manner as Example 1 except that scandium nitrate.9hydrate was used instead of iron nitrate.9hydrate.

Example 4
An exhaust gas purification catalyst of Example 4 was obtained in the same manner as Example 1 except that indium nitrate trihydrate was used instead of iron nitrate 9 hydrate.

Example 5
Exhaust gas purification catalyst of Example 5 was obtained in the same manner as Example 1 except that ytterbium nitrate nonahydrate was used instead of iron nitrate nonahydrate.

Example 6
An exhaust gas purification catalyst of Example 6 was obtained in the same manner as in Example 1 except that yttrium nitrate hexahydrate was used instead of iron nitrate.9 hydrate.

Example 7
A predetermined amount of scandium nitrate nonahydrate was dissolved in 200 ml of ion-exchanged water to obtain a scandium salt solution. In addition, sodium carbonate solution having a molar ratio of 1.1 times the nitrate ion of scandium nitrate nonahydrate used here was dissolved in 100 ml of ion-exchanged water to obtain a sodium carbonate solution. The sodium carbonate solution thus obtained was added dropwise to the above scandium salt solution to obtain a precipitate.

  The resulting precipitate was washed and filtered five times with ion exchange water at 80 ° C. The washed precipitate was dried at 120 ° C. overnight and calcined at 500 ° C. for 2 hours to obtain scandium oxide particles.

  The scandium oxide particles are dispersed in 100 ml of ion-exchanged water, and rhodium nitrate is added in such an amount that the supported amount of rhodium is 0.5% by mass with respect to the support, stirred for 2 hours, filtered and dried, and then The exhaust gas purification catalyst of Example 7 was obtained by calcination at 500 ° C. for 2 hours.

Example 8
Exhaust gas purification catalyst of Example 8 was obtained in the same manner as Example 7 except that yttrium nitrate hexahydrate was used instead of scandium nitrate.9 hydrate.

[Comparative Example 1]
An exhaust gas purification catalyst of Comparative Example 1 was obtained in the same manner as in Example 1 except that neodymium nitrate pentahydrate was used instead of iron nitrate.9 hydrate.

[Comparative Example 2]
An exhaust gas purification catalyst of Comparative Example 2 was obtained in the same manner as in Example 1 except that lanthanum nitrate hexahydrate was used instead of iron nitrate.9 hydrate.

[Comparative Example 3]
An exhaust gas purification catalyst of Comparative Example 3 was obtained in the same manner as in Example 1 except that the yttrium-stabilized zirconia particles were not coated with iron oxide. That is, the exhaust gas purification catalyst of Comparative Example 3 was obtained by directly supporting rhodium on the yttrium-stabilized zirconia particles obtained in Example 1.

[Comparative Example 4]
An exhaust gas purification catalyst of Comparative Example 4 was obtained in the same manner as in Example 7 except that aluminum nitrate · 9 hydrate was used instead of scandium nitrate · 9 hydrate.

〔Evaluation methods〕
In order to evaluate the catalysts of Examples and Comparative Examples, these catalysts were formed into 1 mm pellets and used.

Endurance conditions The rich gas and the lean gas having the composition shown in Table 2 below were alternately switched for 1 minute at 800 ° C. for 5 hours, and durability was performed on the catalyst 4g of the example and the comparative example. Here, rich gas and lean gas were supplied at a flow rate of 6 liters / minute.

Rhodium particle diameter The rhodium particle diameters of the catalysts of Examples and Comparative Examples in which durability was performed were measured. This measurement was performed by the CO pulse method. As pretreatment for measurement by the CO pulse method, oxidation treatment was performed at 400 ° C. for 15 minutes, and then reduction treatment was performed at 400 ° C. for 15 minutes. Here, the CO / Rh adsorption ratio was set to 1. The results are shown in FIG.

  As can be understood from FIG. 4, compared with Comparative Examples 1 to 3, particularly Comparative Example 3 using rhodium particles not having a metal oxide coating, Examples 1 to 8 and Comparative Example 4 are relatively A small rhodium particle size is maintained. In particular, the ion radius of trivalent ions and hexacoordinate bonds is close to rhodium (the value of α in formula (1) is small) iron (α = 0.025), gallium (α = 0.056), scandium ( In Examples 1 to 4 and Comparative Example 4 using α = 0.099), indium (α = 0.168), and aluminum (α = 0.161), a significantly smaller rhodium particle size was maintained. ing. This indicates that it is preferable for preventing rhodium sintering that the ionic radius of trivalent ions and hexacoordinate bonds is close to rhodium (the value of α in formula (1) is small). In particular, in Comparative Example 4 using alumina particles, it is shown that rhodium migration and sintering by the rhodium partial solid solution with respect to alumina is prevented.

  In addition, Examples 3 and 6 using carrier particles obtained by coating stabilized zirconia with scandium oxide and yttrium oxide, respectively, are compared with Examples 7 and 8 using carrier particles consisting only of scandium oxide and yttrium oxide, respectively. Even so, a relatively small rhodium particle diameter is maintained due to the heat resistance of zirconia.

Catalyst low-temperature activity Each of 1.5 g of Examples and Comparative Examples subjected to durability was evaluated. In this evaluation, while the evaluation gas having the composition shown in Table 3 below is supplied at a flow rate of 6 liters / minute, the catalyst temperature is gradually decreased from 500 ° C. to 100 ° C., and the purification rate of hydrocarbon components is 50. The temperature (HC-T50) was obtained when it decreased to%. This relatively low temperature means that exhaust gas purification is achieved even at a relatively low temperature, that is, the catalyst has a relatively good low-temperature activity. The results are shown in FIG.

  As understood from FIG. 5, compared with Comparative Examples 1 to 4, Examples 1 to 8 show a relatively low HC-T50 temperature, that is, a relatively good catalyst low-temperature activity. In particular, in Examples 3 and 4 using scandium (α = 0.099) and indium (α = 0.168) in which the value of α in the formula (1) is in the range of 0.06 to 0.18, A significantly smaller HC-T50 is shown. Here, in Comparative Example 4, although the rhodium particle diameter is small, a relatively large HC-T50 temperature is shown. This suggests that the catalytic activity of rhodium is reduced due to the solid solution of rhodium in the oxygen defect portion of alumina.

  In addition, Examples 3 and 6 using carrier particles obtained by coating stabilized zirconia with scandium oxide and yttrium oxide, respectively, are compared with Examples 7 and 8 using carrier particles consisting only of scandium oxide and yttrium oxide, respectively. Even so, it has a relatively low HC-T50 temperature.

It is sectional drawing showing the exhaust gas purification catalyst of this invention. It is sectional drawing showing a part of exhaust gas purification catalyst of this invention of FIG. It is sectional drawing showing the other exhaust gas purification catalyst of this invention. It is a figure which shows the evaluation result of the rhodium particle diameter about the catalyst of an Example and a comparative example. It is a figure which shows the evaluation result of the exhaust gas purification performance about the catalyst of an Example and a comparative example.

Explanation of symbols

11 Zirconium particles 12, 12 'Coating of oxides such as scandium 13 Rhodium particles 32 Particles of oxides such as scandium

Claims (7)

A catalyst support having zirconia particles and a metal oxide covering the zirconia particles,
A catalyst carrier in which the metal A constituting the metal oxide has a trivalent oxidation number, and the value of α obtained by the following formula (1) for the metal A is 0.30 or less:

(The ionic radii of metal A and rhodium are values in the state of trivalent ions and hexacoordinate bonds).   The catalyst carrier according to claim 1, wherein the metal A is selected from the group consisting of iron, gallium, scandium, aluminum, indium, lutetium, ytterbium, thulium, erbium, yttrium and holmium.   The catalyst carrier according to claim 2, wherein the metal A is selected from the group consisting of iron, gallium, scandium, indium, ytterbium and yttrium.   The catalyst carrier according to claim 3, wherein the metal A is selected from the group consisting of scandium and indium.   The catalyst carrier according to any one of claims 1 to 4, wherein the metal A is 1 to 20 mol% with respect to zirconium of the zirconia particles.   An exhaust gas purification catalyst comprising rhodium supported on the catalyst carrier according to any one of claims 1 to 5.   An exhaust gas purifying catalyst comprising rhodium supported on oxide particles of a metal selected from the group consisting of iron, gallium, scandium, indium, lutetium, ytterbium, thulium, erbium, yttrium and holmium.

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