JP2010030823A - Compound oxide and catalyst containing the compound oxide for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compound oxide having high exhaust gas cleaning performance. <P>SOLUTION: The compound oxide used as a catalyst contains a mixture of a Ruddlesden-Popper type crystal phase expressed by general formula of Sr<SB>3</SB>T<SB>2-x</SB>M<SB>x</SB>O<SB>7-δ</SB>and a perovskite type crystal phase expressed by general formula of SrT<SB>1-y</SB>M<SB>y</SB>O<SB>3-δ</SB>, wherein x is a number satisfying 0≤x<0.1; y is a number satisfying 0≤y<0.1; δ represents an oxygen defect; T represents at least one kind selected from Fe, Mo, Zr, Ti, V, Cr, Mn and Co; and M represents at least one kind selected from Pd, Ru, Rh, Pt and Ir. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

    The present invention relates to a composite oxide and an exhaust gas purifying catalyst containing the composite oxide.

    In a three-way catalyst for a gasoline engine, a Pd-supported alumina-based catalyst component has been widely used in the past so that exhaust gas can be purified when the temperature immediately after the engine is started is low. However, in the case of Pd-supported alumina, when exposed to high-temperature exhaust gas, Pd agglomeration (sintering) is likely to occur due to the heat of the exhaust gas, and the Pd-supported alumina is likely to deteriorate.

On the other hand, a catalyst material is known in which Pd is solid-solved in the crystal of the perovskite complex oxide in order to suppress the aggregation of Pd. For example, Patent Document 1 includes a general formula of ABPdO ₃ (wherein A always includes a rare earth element that does not vary in valence other than trivalence, and does not include a rare earth element that can take a valence smaller than trivalence. At least one element selected from rare earth elements, and B represents at least one element selected from Co, Pd, transition elements other than rare earth elements, and Al.) Complex oxidation of perovskite structure An exhaust gas-purifying catalyst containing a product is disclosed. In connection with this, a catalyst containing a perovskite complex oxide of LaFe _0.98 Pd _0.02 O _{3 + δ} has also been put into practical use, and Pd is dissolved in crystal grains when the exhaust gas atmosphere is lean. It is said that, in a rich state, it precipitates on the surface of the crystal grains, so that Pd grain growth is suppressed.
JP 2004-41866 A

However, when the perovskite type complex oxide of LaFe _0.98 Pd _0.02 O _{3 + δ} was examined, its heat resistance was relatively high, but it was initially used as a catalyst (not subjected to a long-term thermal history). However, the light-off performance related to HC (hydrocarbon) purification and the NOx purification performance at high temperatures are not so high. To improve the exhaust gas purification performance, a relatively large amount of Rh or Pt is used as the catalyst metal. I knew I had to do it.

    That is, an object of the present invention is to obtain high exhaust gas purification performance even with a small amount of catalytic metal.

    In the present invention, in order to solve such problems, a catalyst metal is combined with a composite oxide in which a Ruddlesden-Popper type crystal phase and a perovskite type crystal phase are mixed.

That is, Table invention, the general formula _{Sr 3 T 2-x M x} O and Ruddlesden-Popper type crystal phase represented by the _7-[delta], the general formula _{SrT 1-y M y O 3} -δ according to claim 1 Perovskite-type crystal phase to be mixed,
In the above general formulas, x is a number satisfying 0 ≦ x <0.1, y is a number satisfying 0 ≦ y <0.1, δ is an oxygen defect component, T is Fe, Mo, Complex oxidation characterized in that it is at least one selected from Zr, Ti, V, Cr, Mn and Co, and M is at least one selected from Pd, Ru, Rh, Pt and Ir. It is a thing.

    An invention according to claim 2 is an exhaust gas purifying catalyst comprising the composite oxide according to claim 1.

The invention according to claim 3 is a composite oxidation in which a Ruddlesden-Popper type crystal phase represented by a general formula Sr ₃ T ₂ O _7-δ and a perovskite type crystal phase represented by a general formula SrTO _3-δ are mixed. (Wherein T is at least one selected from Fe, Mo, Zr, Ti, V, Cr, Mn and Co, and δ is an oxygen defect), Pd, Ru, Rh, An exhaust gas purification catalyst comprising a catalyst component on which at least one catalyst metal M selected from Pt and Ir is supported.

    The composite oxide of each of the above inventions is an oxygen-deficient Ruddlesden- having Sr and at least one metal T selected from Fe, Mo, Zr, Ti, V, Cr, Mn and Co as components. Similarly to the popper type crystal phase, oxygen-deficient perovskite type crystal phase having Sr and at least one metal T selected from Fe, Mo, Zr, Ti, V, Cr, Mn and Co as components. And are common in that they are mixed.

Thus, by using such a complex oxide as a constituent element, according to the present invention, oxidation purification of HC and CO in exhaust gas and reduction purification of NOx proceed efficiently. It is considered that an oxygen defect caused by a change in the valence of the transition metal T becomes an active point for purifying the exhaust gas, and excellent exhaust gas purification performance can be obtained. In particular, when the transition metal T is Fe, it takes an abnormal valence of Fe ⁴⁺ , and oxygen defects caused by the valence change become active sites in each of the Ruddlesden-Popper type crystal phase and the perovskite type crystal phase. High catalytic activity can be obtained. In the case of Co as well, since the anomalous valence of Co ⁴⁺ can be obtained, the same effect as in the case of Fe can be obtained.

    In the inventions according to claims 1 and 2, in each of the Ruddlesden-Popper type crystal phase and the perovskite type crystal phase, at least one metal M selected from Pd, Ru, Rh, Pt and Ir is When the metal T is replaced with a part of the metal T, further excellent exhaust gas purification performance can be obtained by the action of the active point due to the oxygen defect and the metal M (catalyst metal).

    In the case of the invention according to claim 3, oxygen defects (active points) generated in the crystal of the composite oxide due to the valence change of the transition metal T, and Pd, Ru, Rh, Pt supported on the composite oxide And exhaust gas purifying performance can be obtained by the action of at least one catalyst metal M selected from Ir and Ir.

    As described above, according to the present invention, oxygen-deficient Ruddlesden having Sr and at least one metal T selected from Fe, Mo, Zr, Ti, V, Cr, Mn, and Co as components. -Popper type crystal phase, oxygen defect type perovskite type crystal having Sr and at least one metal T selected from Fe, Mo, Zr, Ti, V, Cr, Mn and Co as components In the exhaust gas, a composite oxide in which phases are mixed is used as a constituent element, and the composite oxide is combined with at least one metal M selected from Pd, Ru, Rh, Pt and Ir. Thus, oxidation purification of HC and CO and reduction purification of NOx can be performed efficiently.

    Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that the following description of the preferred embodiment is merely illustrative in nature, and is not intended to limit the present invention, its application, or its use.

<Embodiment 1>
The composite oxide according to this embodiment includes a Ruddlesden-Popper type crystal phase represented by a general formula Sr ₃ T _2−x M _x O _7−δ and a general formula SrT _1−y M _y O _3−δ . Perovskite type crystal phase mixed.

In the general formula Sr ₃ T _2-x M _x O _7-δ of the Ruddlesden-Popper type crystal phase, x is a number satisfying 0 ≦ x <0.1, and δ is an oxygen defect component. T is at least one selected from Fe, Mo, Zr, Ti, V, Cr, Mn, and Co, and M is at least one selected from Pd, Ru, Rh, Pt, and Ir.

x = 0, that is, a Ruddlesden-Popper type crystal phase of Sr ₃ T ₂ O _7-δ that does not contain metal M, but metal M functions to enhance catalytic activity. The ratio x is preferably 0.01 or more. However, when the atomic ratio x is 0.1 or more, the metal M is difficult to dissolve in the composite oxide, and the cost is disadvantageous.

In the general formula SrT _1-y M _y O _3-δ of the perovskite crystal phase, y is a number satisfying 0 ≦ y <0.1, and δ is an oxygen defect component. T is at least one selected from Fe, Mo, Zr, Ti, V, Cr, Mn, and Co, and M is at least one selected from Pd, Ru, Rh, Pt, and Ir.

Although y = 0, that is, a perovskite crystal phase of SrTO _3-δ that does not contain metal M, metal M functions to enhance the catalytic activity, and therefore the atomic ratio x of metal M is 0.01. The above is preferable. However, when the atomic ratio x is 0.1 or more, the metal M is difficult to dissolve in the composite oxide, and the cost is disadvantageous.

    The composite oxide in which the Ruddlesden-Popper type crystal phase and the perovskite type crystal phase are mixed can be produced by a known method such as a coprecipitation method, a citric acid complex method, or an alkoxide method. When the composite oxide is used as an exhaust gas purification catalyst, a catalyst layer containing the composite oxide may be formed on the honeycomb carrier. This composite oxide can further carry a catalytic metal such as Pd, Pt, Rh, etc. by an impregnation method, an evaporation to dryness method, or the like. Further, this composite oxide and other catalyst components (for example, Pt-supported alumina) can be mixed and disposed in the catalyst layer.

<Embodiment 2>
In the composite oxide according to the present embodiment, a Ruddlesden-Popper type crystal phase represented by a general formula Sr ₃ T ₂ O _7-δ and a perovskite type crystal phase represented by a general formula SrTO _3-δ are mixed. . In both general formulas, T is at least one selected from Fe, Mo, Zr, Ti, V, Cr, Mn and Co, and δ is an oxygen defect component. The exhaust gas purifying catalyst according to the present embodiment is a catalyst component in which at least one catalyst metal M selected from Pd, Ru, Rh, Pt and Ir is supported on the mixed phase composite oxide. It is characterized by containing.

    The composite oxide in which the Ruddlesden-Popper type crystal phase and the perovskite type crystal phase are mixed can be produced by a known method such as a coprecipitation method, a citric acid complex method, or an alkoxide method. The catalytic metal M can be supported on the composite oxide by an impregnation method, an evaporation to dryness method, or the like. When this composite oxide is used as an exhaust gas purification catalyst, a catalyst layer containing the composite oxide may be formed on the honeycomb carrier. This composite oxide and other catalyst components (for example, Pt-supported alumina) can be mixed and disposed in the catalyst layer.

<Examples and Comparative Examples>
Examples and comparative examples relating to automobile exhaust gas purification catalysts will be described below.

[Preparation of catalyst]
Example 1
31.7 g of strontium nitrate (anhydrous), 39.6 g of iron (III) nitrate nonahydrate, 4.9 g of Pd nitrate solution (Pd = 4.362% by mass), and 96.1 g of citric acid (anhydrous) in distilled water The mixture was stirred and then heated to remove the solvent to obtain a mixture of Sr, Fe and Pd citrate complexes. This citric acid complex mixture was calcined in the air at a temperature of 400 ° C. for 2 hours and then calcined at a temperature of 1000 ° C. for 6 hours. The obtained fired product was pulverized to obtain a catalyst powder. The composition formula of this catalyst powder is Sr ₃ Fe _1.96 Pd _0.04 O _7-δ .

The catalyst powder was subjected to structural analysis using an XRD (X-ray diffraction) analyzer. FIG. 1 is an XRD chart thereof. The peak of each diffraction angle marked with a white circle in FIG. 1, particularly the peak at a diffraction angle of 42 degrees, appears in Ruddlesden-Popper type crystal Sr ₃ Fe ₂ O ₇ , and the peak of each diffraction angle marked with a black triangle, especially diffraction. It is known that a peak at an angle of 40.5 degrees appears in SrFeO ₃ of a perovskite crystal. On the other hand, the peak indicating Pd and its oxide does not appear in the XRD chart of FIG. Thus, in the case of the above catalyst powder, a small amount of Pd is contained. Since the ionic radius of the Pd is close to the ionic radius of Fe, the Pd is solidified in the composite oxide by substituting part of Fe. It is thought that it is melted.

Accordingly, it can be said that each peak with white circles in FIG. 1 is due to Sr ₃ Fe _2−x Pd _x O _7−δ of Ruddlesden-Popper type crystal in which a small amount of Pd is dissolved, and a black triangle is shown. Each attached peak can be said to be due to SrFe _1-y Pd _y O _3-δ of a perovskite crystal in which a small amount of Pd is dissolved. That is, the catalyst powder is a composite of a mixed phase in which a Ruddlesden-Popper type crystal phase of Sr ₃ Fe _2-x Pd _x O _7-δ and a perovskite type crystal phase of SrFe _1-y Pd _y O _3-δ are mixed. It is an oxide powder.

    In this Example 1, Pd solution is added to the complex oxide raw material to dissolve Pd in the complex oxide crystal. Such a Pd support method is hereinafter referred to as a dope method.

A predetermined amount of each of the catalyst powder and ZrO ₂ binder was weighed, and distilled water was added to prepare a slurry. A cordierite honeycomb carrier (cell wall thickness 4 mil (101.6 × 10 ⁻³ mm), 400 cells per square inch (645.16 mm ² )) was immersed in this, and the amount of Pd supported was 1 L per carrier. Excess slurry was blown off with pressurized air so that the amount became 0.98 g, followed by firing in the atmosphere at a temperature of 500 ° C. for 2 hours. As a result, an exhaust gas purifying catalyst having a catalyst layer containing the catalyst powder formed on the honeycomb carrier was obtained.

-Example 2-
By stirring 31.7 g of strontium nitrate (anhydrous), 40.4 g of iron nitrate (III) nonahydrate, and 96.1 g of citric acid (anhydrous) in distilled water, the solvent is removed by heating. A mixture of citrate complexes of Sr and Fe was obtained. This citric acid complex mixture was calcined in the air at a temperature of 400 ° C. for 2 hours and then calcined at a temperature of 1000 ° C. for 6 hours. The fired product (composite oxide) obtained was pulverized, and a Pd nitrate solution was added to the Pd content to be the same as in Example 1, and Pd was supported on the powder by evaporation to dryness. The catalyst powder which concerns on a present Example was obtained by performing the baking hold | maintained at the temperature of 500 degreeC for 2 hours. In this catalyst powder, Pd is supported on a mixed oxide of a mixed phase in which a Ruddlesden-Popper type crystal phase of Sr ₃ Fe ₂ O _7-δ and a perovskite type crystal phase of SrFeO _3-δ are mixed.

    In this example, after the composite oxide was prepared, Pd was supported thereon, so such a Pd supporting method is hereinafter referred to as a post-supporting method.

Then, a predetermined amount of each of the catalyst powder and the ZrO ₂ binder was weighed, and an exhaust gas purification catalyst in which a catalyst layer containing the catalyst powder was formed on the honeycomb carrier under the same conditions and method as in Example 1. Obtained. The amount of Pd supported is 0.98 g / L as in Example 1.

-Comparative Example 1-
15.5 g of strontium oxide, 8.0 g of iron (III) oxide, and 4.9 g of a Pd nitrate solution (Pd = 4.362% by mass) were placed in distilled water and stirred, and then heated to remove the solvent (evaporated to dryness). Hard). Next, firing was performed in the air at a temperature of 500 ° C. for 2 hours. The obtained fired product was pulverized to obtain a catalyst powder. This catalyst powder is a mixture of SrO supporting Pd and Fe ₂ O ₃ supporting Pd, and SrO: Fe ₂ O ₃ = 3: 1 (molar ratio). In Comparative Example 1, since Pd is supported on each powder of strontium oxide and iron oxide, the Pd supporting method can be called a post-supporting method.

Then, a predetermined amount of each of the catalyst powder and the ZrO ₂ binder was weighed, and an exhaust gas purification catalyst in which a catalyst layer containing the catalyst powder was formed on the honeycomb carrier under the same conditions and method as in Example 1. Obtained. The amount of Pd supported is 0.98 g / L as in Example 1.

-Comparative Example 2-
In place of the strontium nitrate of Example 1, 43.3 g of lanthanum nitrate was used, and the catalyst powder and the exhaust gas purifying catalyst were prepared under the same conditions as in Example 1. The catalyst powder of Comparative Example 2 is a complex oxide powder having a perovskite crystal structure of LaFe _0.98 Pd _0.02 O ₃ . The amount of Pd supported is 0.98 g / L as in Example 1. The Pd support method of Comparative Example 2 is a dope method.

    Table 1 shows the configurations of the catalyst powders of Examples 1 and 2 and Comparative Examples 1 and 2.

[Evaluation of catalyst]
For each of the exhaust gas purifying catalysts of Examples 1 and 2 and Comparative Examples 1 and 2, an aging sample that was aged for 24 hours at a temperature of 1000 ° C. in the atmosphere, and a fresh sample that was not aged. Prepared. Each sample of this example and comparative example was set in a model exhaust gas flow reactor, and the exhaust gas purification performance (light-off temperature T50 and high temperature purification rate C400) was evaluated. The model exhaust gas was A / F = 14.7 ± 0.9. That is, the A / F is forced at an amplitude of ± 0.9 by adding a predetermined amount of fluctuation gas in a pulse form at 1 Hz while constantly flowing the main stream gas of A / F = 14.7. Vibrated. Table 2 shows the gas composition when A / F = 14.7, A / F = 13.8 and A / F = 15.6. The space velocity SV was 60000 / h, and the temperature elevation rate was 30 ° C./min.

    T50 indicates that the concentration of each component (HC, CO and NOx) of the gas detected downstream of the catalyst is reduced to half of the concentration of each component (HC, CO and NOx) of the gas flowing into the catalyst as the model exhaust gas temperature rises. This is the catalyst inlet gas temperature (light-off temperature) at the time when it becomes (that is, when the purification rate reaches 50%) and represents the low-temperature purification performance of the catalyst. C400 is the purification rate of each component (HC, CO and NOx) of the gas when the model exhaust gas temperature at the catalyst inlet is 400 ° C., and represents the high temperature purification performance of the catalyst.

    FIG. 2 shows the light-off temperature T50 of the fresh samples of the above examples and comparative examples, and FIG. 3 shows the high-temperature purification rate C400 of the samples. In both T50 and C400, Examples 1 and 2 are better than Comparative Examples 1 and 2. In Examples 1 and 2 and Comparative Example 1, the amounts of the constituent elements of the catalyst powder are substantially the same, but the exhaust gas purification performance of Examples 1 and 2 is better. Therefore, Ruddlesden- It can be seen that it is preferable to employ a composite oxide in which a popper crystal phase and a perovskite crystal phase are mixed. Further, comparing Example 1 and Example 2, Example 1 is better, but this is an effect of promoting dissociative adsorption of NO by bringing Pd close to oxygen defects by doping. I guess.

    FIG. 4 shows the light-off temperature T50 of the aging samples of the above examples and comparative examples, and FIG. 5 shows the high temperature purification rate C400 of the samples. In this sample after aging, although there is almost no difference between Example 2 and Comparative Example 1 with respect to CO and NOx high temperature purification rate C400 (FIG. 5), HC high temperature purification rate C400 and light off Regarding the temperature T50 (FIG. 4), Examples 1 and 2 show better results than Comparative Examples 1 and 2, and it can be said that the exhaust gas purifying catalysts according to Examples 1 and 2 have high heat resistance. .

It is an XRD chart figure of the catalyst powder of Example 1 of this invention. It is a graph which shows light-off temperature T50 at the time of the fresh of the Example and comparative example of this invention. It is a graph which shows the high temperature purification rate C400 at the time of the fresh of the Example and comparative example of this invention. It is a graph which shows light-off temperature T50 after the aging of the Example and comparative example of this invention. It is a graph which shows the high temperature purification rate C400 after the aging of the Example and comparative example of this invention.

Explanation of symbols

    None

Claims (3)

A Ruddlesden-Popper type crystal phase represented by the general formula Sr ₃ T _2-x M _x O _7-δ and a perovskite type crystal phase represented by the general formula SrT _{1- y My} O _3-δ coexist. ,
In the above general formulas, x is a number satisfying 0 ≦ x <0.1, y is a number satisfying 0 ≦ y <0.1, δ is an oxygen defect component, T is Fe, Mo, Complex oxidation characterized in that it is at least one selected from Zr, Ti, V, Cr, Mn and Co, and M is at least one selected from Pd, Ru, Rh, Pt and Ir. object.     An exhaust gas purifying catalyst comprising the composite oxide according to claim 1. A composite oxide in which a Ruddlesden-Popper type crystal phase represented by a general formula Sr ₃ T ₂ O _7-δ and a perovskite type crystal phase represented by a general formula SrTO _3-δ are mixed (where T is Fe , Mo, Zr, Ti, V, Cr, Mn, and Co, and δ is an oxygen defect component), and is selected from Pd, Ru, Rh, Pt, and Ir. An exhaust gas purification catalyst comprising a catalyst component on which at least one kind of catalytic metal M is supported.

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