JP2007307485A - Nox storage reduction type catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an NOx storage reduction type catalyst having a sufficiently high NOx storage capability in high temperature regions such as 400°C or higher even if an amount of noble metals supported is small, in particular, even in case of using neither platinum nor palladium. <P>SOLUTION: The NOx storage reduction type catalyst is provided with hematite of a particle diameter of 3 nm-20 μm, an alkali metal, rhodium, and an oxide carrier comprising a metal oxide hard to form a complex oxide with iron. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a NO _x storage reduction catalyst, and more particularly to a NO _x storage reduction type exhaust gas purification catalyst exhibiting a high NO _x purification rate particularly in a high temperature range.

In recent years, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem from the viewpoint of protecting the global environment. Therefore, in order to reduce CO ₂ which is a greenhouse gas, so-called lean burn in which lean combustion is performed in an oxygen-excess atmosphere has been put into practical use. In this lean burn, the amount of fuel used can be reduced, and the amount of CO ₂ discharged as exhaust gas can be reduced.

Then, for example, in JP-A 5-317625 (Patent Document 1), for purifying exhaust gas of the NO _x storage reduction of the _the NO _x storage material and the noble metal is supported on a porous carrier such as alumina, such as alkaline earth metal A catalyst is disclosed. According to such a catalyst, by controlling the air-fuel ratio from the lean side so that it becomes stoichiometric or rich from the lean side (rich spike), NO _x is occluded by the NO _x storage material on the lean side, Since it is purified by reacting with reducing components such as HC and CO on the stoichiometric or rich side, NO _x can be efficiently purified even in lean burn.

However, the temperature of exhaust gas discharged from automobiles in recent years has been high against the background of high output of a direct-injection gasoline engine or an increase in high-speed driving. In such a high temperature range of 400 ° C. or higher, patents _the NO _x storage material is disadvantageously inactivated causing the carrier and the solid phase reaction in _the NO _x storage-reduction type catalyst as described in the literature 1.

Therefore, in JP-A-2002-79094 (Patent Document 2), a high-temperature NO _x storage reduction catalyst is obtained by using a composite oxide having a magnesia-alumina spinel structure with high temperature stability as a carrier. ing.

However, in the high temperature region of 400 ° C. or higher, formation reaction of _{NO 2 (NO + 1 / 2O} 2 → NO 2) equilibrium of and closer to the big NO side, NO ₂ is will be decomposed into readily NO, NO Before being held in the _x- occlusion material, it comes out again in the gas phase as NO. Therefore, in order to prevent this, it is necessary to form as many reaction active points as possible from NO to NO ₂ on the catalyst. In the NO _x storage reduction catalyst as described in Patent Document 2, Pt is used as the reaction active point. It was necessary to use a relatively large amount of expensive and rare noble metals such as Pd and Pd.

On the other hand, in Japanese Patent Laid-Open No. 10-338 (Patent Document 3), as a countermeasure against poisoning by sulfur in the fuel composition, a porous carrier such as titania, silica, alumina or the like carrying Fe ₂ O ₃ is coated. Although a catalyst is disclosed, Fe ₂ O ₃ was added as an adsorbent for SO _x , and it was necessary to use a relatively large amount of noble metal.
JP-A-5-317625 JP 2002-79094 A Japanese Patent Laid-Open No. 10-338

The present invention has been made in view of the above-described problems of the prior art. The amount of noble metal supported is small, and even when platinum or palladium is not used, the NO is sufficiently high in a high temperature range of 400 ° C. or higher. An object of the present invention is to provide a NO _x storage reduction catalyst having _x storage capacity.

As a result of intensive studies to achieve the above object, the present inventors have found that Fe, which is a base metal catalyst, has an oxidation activity at a high temperature, but easily deactivates by a solid-phase reaction with a support such as alumina. Despite the recognition of the fact that it cannot be used in place of noble metals, hematite with a particle size of 3 nm to 20 μm is formed from an oxide carrier made of a metal oxide that is difficult to form a complex oxide with iron. When used in combination, and by using alkali metal as the NO _x storage material and rhodium as the noble metal, surprisingly, a sufficiently high NO _x storage capacity can be achieved even in the case of a small amount of noble metal supported. As a result, the present invention has been completed.

That is, the NO _x storage reduction catalyst of the present invention is
Hematite with a particle size of 3 nm to 20 μm,
With alkali metals,
Rhodium,
An oxide carrier made of a metal oxide that is difficult to form a complex oxide with iron;
It is characterized by providing.

  The metal oxide which is difficult to form a composite oxide with iron according to the present invention is 750 in the atmosphere in contact with a compound containing 0.01 to 1.0 mol of iron with respect to 200 g of the metal oxide. It is preferably a metal oxide capable of allowing hematite having a particle size of 3 nm to 20 μm to be present after heat treatment at 5 ° C. for 5 hours, and particularly preferably a composite oxide having a magnesia-alumina spinel structure.

In addition, the NO _x storage reduction catalyst of the present invention has a small amount of noble metal supported, and in particular, even when platinum or palladium is not used, a sufficiently high NO _x storage capacity is exhibited in a high temperature region. And palladium is preferably substantially not contained, and the supported amount of rhodium is preferably 0.01 to 2.0 g per liter of the catalyst.

The presence of hematite having a particle size of 3 nm to 20 μm according to the present invention can be confirmed by the following method. That is, in general, when the particle size is obtained by X-ray diffraction analysis, the following “Scherrer equation” is used.
d = (0.9λ) / (Bcosθ _B )
(In the formula, d is the particle size, λ is the X-ray characteristic value used, B is the peak half width, and θ _B is the peak angle.)
For example, when using K _alpha of Cu as an X-ray source, by the half-width is observed peak narrower than 2.8 ° in 33.152 ° observed in hematite, the presence of the particle size 3nm or more hematite Is confirmed.

According to the present invention, there is provided a NO _x occlusion reduction type catalyst having a sufficiently high NO _x occlusion ability in a high temperature range of 400 ° C. or higher even when the amount of noble metal supported is small and platinum or palladium is not particularly used. It becomes possible to do.

  Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof.

The NO _x storage reduction catalyst of the present invention is
Hematite with a particle size of 3 nm to 20 μm,
With alkali metals,
Rhodium,
An oxide carrier made of a metal oxide that is difficult to form a complex oxide with iron;
It is characterized by providing.

The hematite used in the present invention is a kind of iron oxide represented by Fe ₂ O ₃ , and its particle size needs to be 3 nm to 20 μm, and more preferably 5 nm to 1000 nm. The mechanism of expression of the activity is not necessarily clear, but when the particle size of hematite is less than 3 nm, the interaction between Fe ₂ O ₃ and the carrier is too strong, so that NO oxidation is suppressed. On the other hand, when the particle size exceeds 20 μm, oxidized NO ₃ The movement to the occlusion material is less likely to occur.

In addition, the content of Fe contained in the NO _x storage reduction catalyst of the present invention is preferably 0.01 to 1.0 mol per liter of catalyst (200 g of support), and 0.05 to 0.5 mol. More preferably. If the Fe content is less than the lower limit, the number of active sites sufficient to oxidize NO to NO ₃ and send it to the storage material tends to be insufficient. On the other hand, when the upper limit is exceeded, a solid solution reaction with the carrier occurs. The specific surface area tends to decrease.

  In addition, as an oxide carrier made of a metal oxide that is difficult to form a complex oxide with iron used in the present invention, it was in contact with a compound containing 0.01 to 1.0 mol of iron with respect to 200 g of the metal oxide. The metal oxide is preferably a metal oxide capable of allowing hematite having a particle size of 3 nm to 20 μm to exist after heat treatment at 750 ° C. in the atmosphere for 5 hours. Particularly preferred.

Such a composite oxide having a magnesia-alumina spinel structure is a composite oxide having an MgAl ₂ O ₄ structure, and the composite oxide itself has high heat resistance and is relatively easy to obtain having a high specific surface area. Is.

  The lattice constant of the {800} plane of the composite oxide having such a magnesia-alumina spinel structure is a theoretical value of 8.083 ± 0.02Å, and the peak of the strongest line of magnesia detected by the X-ray diffraction method is magnesia. It is desirable that it is 1/20 or less of the strongest line of alumina spinel and the composition ratio of magnesium (Mg) and aluminum (Al) is Mg / 2Al = 1 ± 0.05 (molar ratio).

Further, the specific surface area of the carrier according to the present invention is preferably 50 m ² / g or more, more preferably 80 m ² / g or more, and further preferably 100 m ² / g or more. The hematite as the specific surface area is large, to improve the dispersibility of the NOx-absorbing material and a noble metal, they tend to exhibit higher _the NO _x storage ability in a high temperature region.

  Such a composite oxide having a magnesia-alumina spinel structure can be produced, for example, using the production method described in JP-A No. 2000-128527. That is, magnesium hydroxide and aluminum hydroxide are mixed so as to have an atomic ratio of Al: Mg = 2: 1, and mechanically mixed and pulverized to obtain a mixed composition of hydroxide including a composite hydroxide, By heating it, a composite oxide having a magnesia-alumina spinel structure is obtained. At the time of mixing and pulverization, mechanically sufficient energy is given to the mixture to convert at least a part of both hydroxides into composite hydroxides, and the particle diameter of 80% by weight or more becomes 100 nm or less. Until it is sufficiently refined. And the support | carrier consisting of the complex oxide which has the said magnesia alumina spinel structure is obtained by heat-processing the obtained mixed composition at the temperature of 1100 degrees C or less. Such a carrier can also be produced by a sol-gel method or a coprecipitation method.

Note that it is not necessary that all of the oxide support contained in the NO _x storage-reduction catalyst of the present invention is made of the metal oxide that is difficult to form a composite oxide with iron, but is contained in the catalyst. Among the oxide carriers, the content of the metal oxide that is difficult to form a composite oxide with iron is preferably 20 wt% or more, and more preferably 80 wt% or more. If the content of the metal oxide that is difficult to form a complex oxide with iron is less than the lower limit, iron loses its active site due to a solid solution reaction with the support, so that the NO _x storage ability in a high temperature region tends to decrease. is there. Examples of metal oxides other than metal oxides that are difficult to form complex oxides with iron as described above and can be contained in the NO _x storage reduction catalyst of the present invention include zirconia, alumina, titania, ceria, silica, Or these complex oxides are mentioned.

Further, the NO _x storage-reduction catalyst of the present invention may be constituted by supporting the oxide carrier on another substrate described later, in which case the amount of the oxide carrier supported is The amount is preferably 50 to 300 g per liter of catalyst, more preferably 200 to 300 g. If the supported amount of the oxide carrier is less than the lower limit, the contact probability with the reaction gas decreases, so that sufficient occlusion performance tends not to be obtained.On the other hand, if the upper limit is exceeded, the coat amount is too large. There is a tendency that the back pressure of the entire material increases and the engine output decreases.

In the NO _x storage reduction catalyst of the present invention, an alkali metal is used as the NO _x storage material. Examples of such alkali metals include Li, Na, K, and Cs. Among them, K is particularly preferable because it has a high alkalinity and does not scatter even at high temperatures. In the NO _x storage-and-reduction type catalyst of the present invention, in the case of using the alkali metal as well as NO _x alkaline earth metal or a rare earth element, known as storage component, the basic low Therefore, the effect of the present invention that the NO _x storage capacity is high in the high temperature region is not achieved.

The supported amount of alkali metal in the NO _x storage-reduction catalyst of the present invention is preferably 0.1 to 2.0 mol, more preferably 0.3 to 1.0 mol per liter of the catalyst. When the amount of alkali metal supported is less than the lower limit, the NO _x storage capacity tends to be insufficient. On the other hand, when the upper limit is exceeded, iron, which is an active site, is covered or pores of the support are blocked. Activity tends to decrease due to causes such as reduction.

When K is used as the alkali metal, it is preferable to carry 0.3 mol or more, more preferably 0.6 mol or more per liter of the catalyst. When the loading amount of K is less than 0.3 mol / L, the NO _x storage amount tends to decrease. The upper limit of the amount of K supported is not particularly limited, but the NO _x storage capacity tends to be almost saturated at about 1 mol / L, so it is preferable to limit the amount to 1 mol / L.

In the NO _x storage reduction catalyst of the present invention, rhodium is used as a noble metal. As described above, the NO _x storage-reduction catalyst of the present invention has a small amount of noble metal supported, and exhibits a sufficiently high NO _x storage capacity in a high temperature region even when platinum or palladium is not used. It is preferable that platinum and palladium are not substantially contained.

Further, the supported amount of rhodium is preferably 0.01 to 2.0 g, more preferably 0.1 to 1.0 g, per liter of the catalyst. If the amount of rhodium supported is less than the lower limit, the reduction of occluded NO _x tends to be insufficient, whereas if it exceeds the upper limit, the superiority of the present invention tends to decrease economically.

The form of the NO _x storage-reduction catalyst of the present invention is not particularly limited, and may be a honeycomb-shaped monolith catalyst, a pellet-shaped pellet catalyst, or the like. The substrate used here is not particularly limited, and is appropriately selected depending on the use of the obtained catalyst, etc., but a DPF substrate, a monolith substrate, a pellet substrate, a plate substrate, etc. are suitably employed. Is done. Also, the material of such a base material is not particularly limited, but a base material made of a ceramic such as cordierite, silicon carbide, mullite, or a base material made of a metal such as stainless steel including chromium and aluminum is suitably employed. Is done.

Furthermore, the method for producing the NO _x storage reduction catalyst of the present invention is not particularly limited. For example, in the case of producing a monolith catalyst, the above-described present invention is applied to a honeycomb-shaped substrate formed of cordierite or the like. A method of sequentially or collectively forming a coat layer composed of the various components according to the above is appropriately employed. In the method of forming a coat layer on the base material using the powder after the alkali metal and / or rhodium is previously supported on the above-mentioned oxide carrier powder, the above-mentioned oxide carrier powder is used for the base material. A method of supporting an alkali metal and / or rhodium after forming the coating layer may be used.

Further, the method for incorporating the hematite into the NO _x storage reduction catalyst of the present invention is not particularly limited, and examples thereof include iron salts (for example, iron nitrate.9 hydrate, iron chloride (II), iron bromide ( II), an aqueous solution containing iron oxalate dihydrate, iron acetate (II), iron citrate) is brought into contact with the oxide carrier, dried, and further calcined. Further, hematite powder may be mixed with the oxide carrier powder as the whole or a part of the hematite.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

(Preliminary Example 1)
200 g of MgAl ₂ O ₄ spinel powder having a specific surface area of 100 m ² / g prepared by a common coprecipitation method is impregnated with a predetermined amount of an aqueous iron nitrate nonahydrate solution and calcined in air at 300 ° C. for 3 hours. And a heat treatment at 750 ° C. for 5 hours in air was performed to obtain the sample of Preparatory Example 1. The amount of Fe supported was 0.12 mol / L.

(Preliminary comparative example 1)
A sample of Preliminary Comparative Example 1 was obtained in the same manner as Preparative Example 1 except that a commercially available alumina powder (MI 386, manufactured by WR Grace Co.) was used instead of the MgAl ₂ O ₄ spinel powder.

(Preliminary comparative example 2)
A sample of Preliminary Comparative Example 2 was obtained in the same manner as Preparative Example 1 except that a commercially available magnesia powder (500 A manufactured by Ube Materials Co., Ltd.) was used instead of the MgAl ₂ O ₄ spinel powder.

(Preliminary comparative example 3)
A sample of Preliminary Comparative Example 3 was obtained in the same manner as Preparative Example 1 except that a commercially available zirconia powder (RC100, manufactured by 1st Rare Element Chemical Industry Co., Ltd.) was used instead of the MgAl ₂ O ₄ spinel powder.

<XRD measurement>
The samples obtained in Preparatory Example 1 and Preliminary Comparative Examples 1 to 3 were measured for diffraction lines of 30 ° to 35 ° using an X-ray diffractometer (RINT-2200, manufactured by Rigaku Corporation). The obtained results are shown in FIG.

  From the results shown in FIG. 1, a thin peak was observed at 33.152 ° observed in hematite in the sample obtained in Preparative Example 1, and the presence of hematite having an average particle diameter of 44.6 nm was confirmed. In the samples obtained in Preliminary Comparative Examples 1 to 3, the peak was not recognized.

<FE-TEM measurement>
For the samples obtained in Preparatory Example 1 and Preliminary Comparative Example 1, FE-TEM photographs were taken using a transmission electron microscope (manufactured by Hitachi, Ltd., HF-2000). The obtained results are shown in FIG. 2 (Preliminary Example 1) and FIG. 3 (Preliminary Comparative Example 1).

  In FIG. 2, the elemental composition (atom%) in the dark portion 1 and the relatively thin portion 2 indicated by white circles is shown below.

1 2
Mg 35.51 33.46
Al 33.80 62.4
Fe 30.68 4.08
From this result, it was confirmed that in the sample obtained in Preparatory Example 1, Fe ₂ O ₃ was present in a portion having a high contrast. In FIG. 2, Fe ₂ O ₃ particles having a _high contrast were confirmed in the portions indicated by black circles 3 to 7.

  On the other hand, in FIG. 3, the elemental composition (atom%) in the part shown by the black circles 1-4 is shown below.

1 2 3 4
Al 97.19 97.64 98.42 98.21
Fe 2.81 2.36 1.58 1.79
From this result, it was confirmed that in the sample obtained in Preliminary Comparative Example 1, there was no place where Fe ₂ O ₃ was concentrated and present in a wide and uniform manner regardless of the contrast. .

Accordingly, the MgAl ₂ O ₄ spinel powder corresponds to an oxide carrier made of a metal oxide that is difficult to form a composite oxide with iron according to the present invention, while the alumina powder, magnesia powder, and zirconia powder are the iron support according to the present invention. It was confirmed that it does not correspond to an oxide carrier made of a metal oxide which is difficult to form a complex oxide.

Example 1
A mixed powder of MgAl ₂ O ₄ spinel powder and commercially available ZrO ₂ powder (RC100, manufactured by 1st Rare Element Chemical Industry Co., Ltd.) on which Rh is supported so that the amount of Rh supported is 0.5 g / L is determined by a conventional method. It is made into a slurry so that the supported amount of MgAl ₂ O ₄ spinel powder is 200 g / L and the supported amount of ZrO ₂ powder is 50 g / L on a cordierite monolith substrate (diameter 30 mm, length 50 mm, volume 35 cc). A coat layer was formed. Next, the coating layer was impregnated with a predetermined amount of an aqueous iron nitrate nonahydrate solution and calcined at 300 ° C. for 3 hours in air. Further, a predetermined amount of potassium acetate aqueous solution was impregnated in the coat layer, and calcination was performed in air at 300 ° C. for 3 hours. The amount of Fe supported was 0.12 mol / L, and the amount of K supported was 0.6 mol / L. Further, the sample of Example 1 was subjected to heat treatment at 750 ° C. for 5 hours while switching between rich and lean at a flow rate of 10 L / min using a rich model gas and a lean model gas shown in Table 1 at a rate of 10 seconds / 110 seconds. Got.

  When XRD measurement was performed on the sample obtained in Example 1 in the same manner as described above, a thin peak was observed around 33.152 °, and the presence of hematite with a particle size of 3 nm to 20 μm was confirmed.

(Example 2)
MgAl ₂ O ₄ spinel powder, commercially available ZrO ₂ powder (RC100, manufactured by 1 Rare Elemental Chemical Co., Ltd.) on which Rh is supported so that the amount of Rh supported is 0.5 g / L, and Fe ₂ O ₃ powder (Made by Toda Kogyo Co., Ltd., 100ED, average particle size: 0.1 μm) was mixed with a conventional method and slurried with a monolith base material (diameter 30 mm, length 50 mm, volume 35 cc) made of cordierite with MgAl ₂ O ₄ The coat layer was formed so that the supported amount of spinel powder was 200 g / L, the supported amount of ZrO ₂ powder was 50 g / L, and the supported amount of Fe ₂ O ₃ powder was 0.12 mol / L as Fe. Next, a predetermined amount of potassium acetate aqueous solution was impregnated into the coat layer, and calcination was performed in air at 300 ° C. for 3 hours. The amount of K supported was 0.6 mol / L. Further, the sample of Example 2 was subjected to heat treatment at 750 ° C. for 5 hours while switching between rich and lean at a flow rate of 10 L / min using a rich model gas and a lean model gas shown in Table 1 at a rate of 10 seconds / 110 seconds. Got.

  When XRD measurement was performed on the sample obtained in Example 2 in the same manner as described above, the presence of hematite having an average particle size of 0.1 μm was confirmed.

(Example 3)
MgAl ₂ O ₄ spinel on which Rh is supported so that the amount of Rh supported is 0.5 g / L instead of commercially available ZrO ₂ powder on which Rh is supported so that the amount of Rh is 0.5 g / L A sample of Example 3 was obtained in the same manner as Example 1 except that powder was used.

  When XRD measurement was performed on the sample obtained in Example 3 in the same manner as described above, a thin peak was observed around 33.152 °, and the presence of hematite with a particle size of 3 nm to 20 μm was confirmed.

Example 4
Slurry MgAl ₂ O ₄ spinel powder by a regular method and coat it on a cordierite monolith substrate (diameter 30 mm, length 50 mm, volume 35 cc) so that the supported amount of MgAl ₂ O ₄ spinel powder is 200 g / L A layer was formed. Next, the coating layer was impregnated with a predetermined amount of an aqueous rhodium nitrate solution and calcined at 300 ° C. for 3 hours in air. Subsequently, the coating layer was impregnated with a predetermined amount of an aqueous iron nitrate nonahydrate solution, and calcined at 300 ° C. for 3 hours in air. Further, a predetermined amount of potassium acetate aqueous solution was impregnated in the coat layer, and calcination was performed in air at 300 ° C. for 3 hours. The supported amount of Rh was 0.5 g / L, the supported amount of Fe was 0.12 mol / L, and the supported amount of K was 0.6 mol / L. Further, the sample of Example 4 was subjected to heat treatment at 750 ° C. for 5 hours while switching between rich and lean at a flow rate of 10 L / min using a rich model gas and a lean model gas shown in Table 1 at a rate of 10 seconds / 110 seconds. Got.

  When XRD measurement was performed on the sample obtained in Example 4 in the same manner as described above, a thin peak was observed around 33.152 °, and the presence of hematite with a particle size of 3 nm to 20 μm was confirmed.

(Example 5)
A sample of Example 5 was obtained in the same manner as in Example 1 except that the amount of K to be supported was 0.3 mol / L.

(Example 6)
A sample of Example 6 was obtained in the same manner as in Example 1 except that the amount of K to be supported was 0.15 mol / L.

(Comparative Example 1)
A sample of Comparative Example 1 was obtained in the same manner as Example 1 except that a commercially available alumina powder (MI386 manufactured by WR Grace Co.) was used instead of the MgAl ₂ O ₄ spinel powder. When XRD measurement was performed on the sample obtained in Comparative Example 1 in the same manner as described above, no peak was observed near 33.152 °, and the presence of hematite was not confirmed.

(Comparative Example 2)
A sample of Comparative Example 2 was obtained in the same manner as in Example 1 except that a commercially available magnesia powder (500 A manufactured by Ube Materials Co., Ltd.) was used instead of the MgAl ₂ O ₄ spinel powder. When the XRD measurement was performed on the sample obtained in Comparative Example 2 in the same manner as described above, no peak was observed near 33.152 °, and the presence of hematite was not confirmed.

(Comparative Example 3)
A sample of Comparative Example 3 was obtained in the same manner as in Example 1 except that a commercially available zirconia powder (RC100, manufactured by 1st Rare Element Chemical Industry Co., Ltd.) was used instead of the MgAl ₂ O ₄ spinel powder. When the XRD measurement was performed on the sample obtained in Comparative Example 3 in the same manner as described above, no peak was observed near 33.152 °, and the presence of hematite was not confirmed.

(Comparative Example 4)
A sample of Comparative Example 4 was obtained in the same manner as in Example 1 except that instead of impregnating and supporting Fe, Pt was supported using a dinitrodiaminoplatinum aqueous solution so that the supported amount was 2 g / L.

(Comparative Example 5)
A sample of Comparative Example 5 was obtained in the same manner as Comparative Example 1 except that instead of impregnating and supporting Fe, Pt was supported using a dinitrodiaminoplatinum aqueous solution so that the supported amount was 2 g / L.

(Comparative Example 6)
A sample of Comparative Example 6 was obtained in the same manner as in Example 1 except that instead of impregnating and supporting K, Ba was supported using an aqueous barium acetate solution so that the supported amount was 0.4 mol / L.

(Comparative Example 7)
A sample of Comparative Example 7 was obtained in the same manner as in Example 1 except that, instead of impregnating and supporting K, Ba was supported using an aqueous barium acetate solution so that the supported amount was 0.2 mol / L.

<Evaluation of 95% NO _x storage amount>
The samples obtained in Examples 1 to 4 and Comparative Examples 1 to 7 was placed in each evaluation device, with a rich model gas shown in Table 2, enters the gas temperature is 600 ° C., SV is 51400H ^-1 conditions After the rich state was continued for 5 minutes, the gas was switched to the lean model gas shown in Table 2, the NO _x concentration in the outgas was calculated, and the 95% NO _x occlusion amount at 600 ° C. was determined as follows. .

That is, the NO _x concentration in the output gas gradually increases as shown schematically in FIG. 4 immediately after switching from the rich state to the lean state (NO _x concentration after passing through the catalyst). The total amount of NO _x flowing into the catalyst from the time immediately after switching from the rich state to the lean state until the elapse of time t ₁ is represented by the area of A + B shown in FIG. 4, and the amount of NO _x occluded in the catalyst is A It is represented by the area of the part. Therefore, the time t ₁ when the area of the A portion becomes 95% or more of the area of A + B is measured, and the NO _x amount corresponding to the area of the A portion is calculated to obtain a 600 ° C. 95% NO _x occlusion amount. did. Table 3 shows the measurement results of the samples obtained in Examples 1 to 4 and Comparative Examples 1 to 7.

As is apparent from the results shown in Table 3, in the catalyst of the present invention in which MgAl ₂ O ₄ spinel powder was used as a carrier and Fe was impregnated or added as a powder to support hematite with a particle size of 3 nm to 20 μm, It was confirmed to show a very high 95% NO _x storage amount.

Also, determine the 95% NO _x storage amount in the and 600 ° C. in the same manner for the samples obtained in Examples 1, 5 and 6, Figure 5 shows the results obtained. From the results shown in FIG. 5, it was confirmed that the supported amount of K is preferably more than 0.3 mol / L.

As described above, according to the present invention, NO _x having a sufficiently high NO _x storage capacity in a high temperature region of 400 ° C. or higher is obtained even when the amount of noble metal supported is small and platinum or palladium is not particularly used. It is possible to provide an occlusion reduction catalyst.

Therefore, the NO _x storage reduction catalyst of the present invention is very useful as an NO _x storage reduction type exhaust gas purification catalyst exhibiting a high NO _x purification rate particularly in a high temperature range.

It is a graph which shows the X-ray-diffraction profile of the sample obtained by the preliminary Example 1 and preliminary comparative examples 1-3. 2 is an FE-TEM photograph of a sample obtained in Preparatory Example 1. 2 is an FE-TEM photograph of a sample obtained in Preliminary Comparative Example 1. The concentration of NO _x output in the gas immediately after switching from the rich state to the lean state is a graph showing schematically. 4 is a graph showing the 600 ° C. 95% NO _x occlusion amount in the samples obtained in Examples 1, 5, and 6. FIG.

Claims (4)

Hematite with a particle size of 3 nm to 20 μm,
With alkali metals,
Rhodium,
An oxide carrier made of a metal oxide that is difficult to form a complex oxide with iron;
NO _x storage-and-reduction type catalyst, characterized in that it comprises a. The metal oxide which is difficult to form a complex oxide with iron was heat-treated at 750 ° C. for 5 hours in the atmosphere in contact with a compound containing 0.01 to 1.0 mol of iron with respect to 200 g of the metal oxide. NO _x storage-and-reduction type catalyst according to claim 1, characterized in that a metal oxide that may be present hematite particle size 3nm~20μm later. 3. The NO _x storage reduction catalyst according to claim 1, wherein the metal oxide which is difficult to form a composite oxide with iron is a composite oxide having a magnesia-alumina spinel structure. It contains substantially no platinum and palladium, and the supported amount of rhodium is 0.01 to 2.0 g per liter of the catalyst. The NO _x storage-reduction catalyst as described.