JP2010234309A - Exhaust gas purifying catalyst and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas purifying catalyst having preferable desulfurization of an outermost layer especially most subjected to sulfur poisoning which has been a conventional drawback of a catalyst having a multi-layer structure such as a two-layer structure, and having stable NO<SB>x</SB>purification performance under a changeable lean/rich atmosphere, and to provide a method for manufacturing the same. <P>SOLUTION: The exhaust gas purifying catalyst includes a first catalyst layer containing rhodium, and a second catalyst layer containing platinum on a substrate, in this order from the substrate. At least the second catalyst layer includes at least one of first metal oxides containing a first metal element selected from the group consisting of alkali metals and alkali earth metals, and at least one of second metal oxides containing a second metal element selected from the group of rare earth metals, which are supported on a support. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to an exhaust gas purification catalyst suitable for purifying gas such as NO _x contained in exhaust gas and a method for producing the same.

Conventionally, various catalysts have been proposed as exhaust gas purifying catalysts for purifying nitrogen oxides (NO _x ) contained in exhaust gas from internal combustion engines such as automobiles. As such an exhaust gas purification catalyst, various base materials such as cordierite and metal carry a metal oxide such as alumina, zirconia and ceria and a noble metal such as platinum (Pt), rhodium (Rh) and palladium (Pd). Those that have been used are generally used.

In recent years, as a purification catalyst structure, not only a single layer structure in which these metal components are present in a single layer but also a purification catalyst having a layer structure of two or more layers has been proposed, and in particular, Pt and Rh. from the decrease significantly _the NO _x purification performance decreases the catalytic activity and a solid solution in the coexistence, a catalyst having a catalytic coating layer of the upper and lower layers which was included in different layers of Pt and Rh have been proposed. In a lean atmosphere, it is desirable that the lower catalyst coat layer close to the base material contains Rh and the upper catalyst coat layer contains Pt.

  As an example of a catalyst having a single layer structure for exhaust gas purification, an exhaust gas purification catalyst in which a metal oxide such as alumina or rare earth metal oxide is dispersed and a metal oxide is dispersed has been proposed.

Further, oxide composites capable of storing and / or reducing NO _x are also known as materials for exhaust gas purification catalysts. As such an oxide composite, for example, there is an oxide composite of barium and cerium. As a catalyst having a single layer structure, for example, an NO _x storage reduction catalyst including a carrier supporting barium and cerium is disclosed. (For example, refer to Patent Document 1).

  Further, there is a disclosure relating to an exhaust gas purification catalyst having a single layer structure in which an oxide composite is supported on a Pt-supported catalyst (see, for example, Patent Document 2).

Japanese Patent Laid-Open No. 2005-21878 JP 2008-137886 A

However, the catalyst comprising a carrier carrying a barium and cerium as described above, degradation of barium is insufficient as _the NO _x purification performance is low performance of the precious metal required but _the NO _x storage can be suppressed. Further, barium And the oxide complex of cerium cannot be supported on the carrier in a fine state. Further, an exhaust gas purifying catalyst in which an oxide complex is supported on a Pt-supported catalyst exhibits high NO _x storage performance in a lean atmosphere, but is not necessarily sufficient when including the point of NO _x reduction performance in a rich atmosphere. Purification performance cannot be obtained.

  In addition, in response to recent environmental load reduction, further improvement in purification capability is desired not only for lean burn engines, but also for diesel engines and hybrid vehicles that are easily exposed to lean environments due to engine stoppage during travel.

The present invention has been made in view of the above, and has been a defect of a multilayer catalyst having a two-layer structure or the like, and particularly has a good sulfur desorption property on the outermost layer that is most susceptible to sulfur poisoning, and is also lean. An object of the present invention is to provide an exhaust gas purifying catalyst having a stable NO _x purification performance under a rich fluctuation atmosphere and a method for producing the same, and to achieve the object.

The present invention, when the layer structure overlaid with the upper layer of the lower layer and the platinum-containing rhodium-containing from the substrate side as a catalyst layer for _the NO _x purification at a lean / rich change atmosphere, at least the upper layer is easily in contact with the exhaust gas When two or more kinds of metal oxides in a specific range are dispersed and present on the side (preferably uniformly in the form of small particles), sulfur desorption, which has been regarded as a disadvantage of conventional two-layer coated purification catalysts, for example. enhanced effect of improving ability, in addition to the noble metal activity can be maintained stably high, it can be maintained high for _the NO _x storage ability in a lean atmosphere, yet lower purification performance ratio spread in thickness to the total thickness of the catalyst layer The knowledge that it contributes to ensuring stability was obtained, and it was achieved based on this knowledge.

  In order to achieve the above object, an exhaust gas purifying catalyst according to the first invention comprises a first catalyst layer containing rhodium (Rh) and a second catalyst containing platinum (Pt) in this order from the substrate side. At least one first metal oxide containing a first metal element selected from the group consisting of alkali metals and alkaline earth metals, and a group of rare earth metals It is configured to include at least one second metal oxide containing a second metal element selected from a carrier supported on a carrier.

In the first invention, Rh is disposed in the lower layer on the base material side, Pt is disposed in the upper layer, and an alkali metal and / or alkaline earth metal metal oxide and a rare earth metal metal oxide are formed on at least the upper layer side. (Preferably with a small particle size and uniformly dispersed) to suppress the formation of coarse sulfate when sulfur in the exhaust gas reacts with barium (Ba) or the like, and the formed sulfate Since it is easy to decompose, the sulfur desorption ability, which has been regarded as a drawback of the purification catalyst with a multilayer structure such as the two-layer coat type, has been greatly improved, and the precious metal activity can be stably maintained at a high level. Thus, the NO _x storage performance can be stably maintained. In addition, since the NO _x purification ability including the high temperature region is improved, the allowable range of the lower layer thickness that can maintain the NO _x purification ability high in the ratio of the lower layer thickness to the total thickness of the catalyst layer is expanded, The variation in purification performance caused by the influence of the thickness of the catalyst layer containing Rh is reduced.

  In the first invention, it is preferable that the metal oxide particles supported in the second catalyst layer are contained in an average particle size of 30 nm or less. When the particle size is a small particle size of 30 nm or less and dispersed in the layer (preferably uniformly), the coarsening of the sulfur salt produced by the reaction with sulfur in the exhaust gas is further alleviated. It can be easily decomposed, and coarsening can be avoided when grains grow due to durability deterioration of precious metals. Furthermore, this particle size is desirably 0.5 to 10 nm for the same reason.

  The exhaust gas purifying catalyst according to the first aspect of the present invention is the strength ratio of the first metal element (first metal) obtained from TEM-EDX analysis at least 20 measurement points of 10 nm square on the surface of the carrier in the second catalyst layer. The intensity ratio of the first metal element to the sum of the peak area of the element and the intensity ratio of the second metal element (peak area of the second metal element) [[peak area of the first metal element] / [first metal] The standard deviation of the distribution of the peak area of the element and the peak area of the second metal element]] is preferably 10% or less.

In the first invention, the so standard deviation is 10% or less, which is supported in a state where the first metal element and second metal element are uniformly mixed and dispersed, high _the NO _x storage in a lean atmosphere It becomes possible to show performance.

  Moreover, it is preferable that the 2nd catalyst layer of the exhaust gas purification catalyst of 1st invention contains palladium further. By further including Pd, higher catalytic activity can be obtained.

  As the base material constituting the exhaust gas purification catalyst of the first invention, a monolith base material can be suitably used.

  According to a second aspect of the present invention, there is provided a method for producing an exhaust gas purifying catalyst, comprising: applying a slurry containing rhodium on a substrate; calcining to form a first catalyst layer; and forming the first catalyst layer on the first catalyst layer. A platinum-containing layer forming step for forming a platinum-containing layer containing platinum, a first metal compound containing a first metal element selected from the group consisting of alkali metals and alkaline earth metals, and a group of rare earth metals A solution for preparing an oxide-supporting aqueous solution containing the first metal element and the second metal element by mixing a second metal compound containing the second metal element, a compound having a multidentate ligand, and ammonia. The preparation step and the prepared aqueous solution for supporting an oxide are brought into contact with at least the platinum-containing layer and then fired, and at least the first metal element and the second metal element containing the first metal element in the platinum-containing layer Second metal oxidation containing By carrying bets, and a second catalyst layer forming step of forming a second catalyst layer containing platinum, which is constituted by providing a.

In the second invention, after forming a multilayer structure including at least two layers in which Rh is disposed on the lower layer on the substrate and Pt is disposed on the upper layer, the second catalyst layer including at least Pt is used as the aqueous solution for supporting oxide. It is made to contact by making it impregnated, etc., and a state in which the oxide composite precursor is entirely adhered to the layer is formed and fired, so that the oxide is previously formed and supported as before. Instead of forming a metal oxide which becomes an occlusion site after supporting a metal component, oxide particles of a plurality of kinds of metal elements including the first metal element and the second metal element (including the oxide complex) Can be supported in a uniform dispersed state in the layer by uniformly mixing the first metal element and the second metal element with a small particle size. As a result, the sulfur desorption performance of the second catalyst layer that mainly does not contain Rh, which has been regarded as a defect of the two-layer coated purification catalyst such as the two-layer coating type, has been greatly improved, and the precious metal activity is stable. Therefore, it is possible to obtain an exhaust gas purification catalyst that can achieve both high NO _x storage performance in a lean atmosphere and high NO _x reduction performance in a rich atmosphere. In addition, the obtained exhaust gas purifying catalyst, is excellent in _the NO _x purification performance, including high temperature region, among the thickness of the first catalyst layer of lower containing Rh mainly of percentage of the total thickness of the catalyst layer, tolerance of the NO _x purifying capability of the high can be maintained thickness becomes拡Ku, variations of the purification performance caused by the effect of the thickness of the catalyst layer mainly containing the Rh can be reduced.

  In the second invention, as the compound having a polydentate ligand, a compound having a carboxyl group is a bulky and finer oxide particle (including an oxide complex). It can be used suitably.

The first metal compound is preferably a salt of the first metal element, and the second metal compound is preferably a salt of the second metal element. These salts show basicity and can obtain a high NO _x storage capacity when an oxide is formed.

In the platinum-containing layer forming step of the second invention, a slurry containing a metal element oxide is applied on the first catalyst layer to provide a metal oxide layer as an upper layer, and at least the metal oxide layer is formed into a platinum-containing solution. And then firing to form a platinum-containing layer, and in the subsequent second catalyst layer forming step, the first metal oxide and the second metal oxide are supported on the metal element oxide. Can do. As a result, Pt can be uniformly supported, and the NO _x storage capacity is improved.

According to the present invention, it has been a disadvantage of a multilayer catalyst such as a two-layer structure, and the sulfur desorption property of the outermost layer that is most susceptible to sulfur poisoning is good and stable under a lean / rich fluctuation atmosphere. it is possible to provide an exhaust gas purifying catalyst and a manufacturing method thereof were _the NO _x purification performance.

It is a conceptual diagram which shows notionally the support structure of the exhaust gas purification catalyst of this invention with which the 1st metal oxide and the 2nd metal oxide were carry | supported by the support | carrier. It is the schematic which shows the cross-section of the exhaust gas purification catalyst of this invention comprised by the two-layer structure. It is a graph which shows the sulfur desorption amount at the time of a sulfur desorption test. Is a graph showing _the NO _x purification rate at 300 ° C. of the exhaust gas purifying catalyst after the sulfur desorption test. Is a graph showing _the NO _x purification rate at 500 ° C. of the exhaust gas purifying catalyst after the sulfur desorption test. Is a graph showing in comparison the relation between _the NO _x purification rate and the lower coating ratio at 400 ° C. Sulfur each exhaust gas purifying catalyst after desorption test with conventional exhaust gas purifying catalyst. It is a graph which shows the result of the XRD analysis of the exhaust gas purification catalyst of Example 5 and Comparative Examples 4-5. It is a TEM photograph which shows the dispersion | distribution state of Ba and Ce in the exhaust gas purification catalyst of Example 5 which performed the durability test.

Hereinafter, the exhaust gas purification catalyst of the present invention and the production method thereof will be described in detail.
The exhaust gas purifying catalyst according to the first invention has at least a first catalyst layer containing rhodium and a second catalyst layer containing platinum in order from the substrate side on the substrate, and at least the second catalyst layer. And at least one first metal oxide containing a first metal element selected from the group consisting of alkali metals and alkaline earth metals, and a second metal containing a second metal element selected from the group of rare earth metals At least one kind of oxide is provided.

-First catalyst layer-
The first catalyst layer contains at least rhodium (Rh). The first catalyst layer further contains a first metal oxide containing a first metal element selected from the group consisting of alkali metals and alkaline earth metals, and a second metal element selected from the group of rare earth metals. The second metal oxide can be configured, and if necessary, it can be configured using other metal components.

Rh is preferably contained in the form of finer particles, and more preferably Rh is supported on a carrier. Rh can be contained using particles such as a powder of Rh-supported metal oxide such as Rh-supported zirconia. In this case, for example, powdery Rh-supported zirconia can be obtained by mixing zirconium dioxide (ZrO ₂ ) and a rhodium nitrate solution in a medium such as water, stirring, drying, and firing.

  The particle diameter of Rh contained in the particle state is preferably in the range of 1 to 20 nm, and more preferably in the range of 1 to 10 nm. When the particle size is 1 nm or more, it is difficult to deactivate due to oxidation, and when it is 20 nm or less, most of the supported Rh can promote the reaction and the catalytic activity is improved.

Rh is a main noble metal in the first catalyst layer, has a high NO _x purification ability, and contributes to imparting sulfur detachability when sulfur is poisoned. The content concentration of Rh in the first catalyst layer is preferably in the range of 0.01 to 10 g / L (liters; the same applies hereinafter), more preferably in the range of 0.05 to 5 g / L in terms of maintaining the catalytic activity. preferable.

The first catalyst layer may include a first metal oxide of an alkali metal and / or an alkaline earth metal (first metal element) and a second metal oxide of a rare earth metal (second metal element). it can. The first catalyst layer preferably contains the first metal oxide and / or the second metal oxide from the viewpoint of enhancing the NO _x purification ability.
The first metal element, the second metal element, and their oxides will be described later.

  The first catalyst layer includes, for example, particles containing Rh in a medium such as water (such as Rh-supported metal particles) and, if necessary, an oxide of aluminum, zirconium, alkaline earth metal, or rare earth metal (such as ceria). Other components are added, and a slurry is prepared by stirring and the like, and the obtained slurry is applied and immersed on a desired base material to form a layer, followed by firing.

As thickness of a 1st catalyst layer, 15-130 micrometers is preferable and 70-120 micrometers is more preferable. When the thickness of the first catalyst layer is within the above range, Rh having a high NO _x purification performance is disposed in a highly dispersed manner in the coat layer, and thus has a high sulfur desorption property and a NO _x purification performance. Is advantageous.

-Second catalyst layer-
The second catalyst layer comprises platinum (Pt) and a second metal element selected from the group of first metal oxides and rare earth metals containing a first metal element selected from the group consisting of alkali metals and alkaline earth metals. It contains the 2nd metal oxide to contain, and can be comprised using another metal component etc. as needed.

Pt is a main noble metal in the second catalyst layer. The content concentration in the second catalyst layer of Pt, high catalytic activity, in terms of suppressing NO _x emissions under lean steady atmosphere is preferably in the range of 0.05 to 10 g / L, 0.1-5 g The range of / L is more preferable.

  The second catalyst layer may contain other noble metals such as palladium (Pd), osmium (Os), iridium (Ir), and gold (Au) in addition to Pt as a noble metal, to obtain higher catalytic activity. Therefore, palladium is preferable.

Examples of the alkali metal include lithium, sodium, potassium, rubidium, and cesium. Examples of the alkaline earth metal include barium, magnesium, calcium, and strontium. Among these, potassium, rubidium, cesium, and barium are preferable, and barium is more preferable from the viewpoint of having high basicity and exhibiting high NO _x storage performance when an oxide is formed.

The concentration of the first metal oxide containing the first metal element in the first catalyst layer is preferably in the range of 0.02 to 2.0 mol / L, preferably 0.1 to 1.2 mol / L. A range is more preferred. Concentration of the first metal oxide, if it is 0.02 mol / L or more, can be secured sufficient amount of storage material in _the NO _x storage, it has a higher _the NO _x storage performance. Moreover, since it is not reducing the specific surface area of a support | carrier as it is 2.0 mol / L or less, sufficient occlusion performance is securable.

  Examples of the rare earth metal include scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, and the like. Among these, yttrium, lanthanum, and cerium are preferable and cerium is more preferable from the viewpoint of having a relatively high specific surface area when the oxide is formed.

  The concentration of the second metal oxide containing the second metal element in the second catalyst layer is preferably in the range of 0.02 to 2.0 mol / L, more preferably in the range of 0.1 to 1.2 mol / L. preferable. When the content concentration of the second metal oxide is 0.02 mol / L or more, the concentration is sufficient, the production efficiency is good, and the metal oxide can be formed uniformly. In addition, when the concentration is 2.0 mol / L or less, the concentration does not become too high, and the reaction proceeds uniformly during firing, so that the metal oxide can be formed uniformly.

The first metal element of the first metal oxide and the combination mode between the second metallic element of the second metal oxide is not particularly limited and may be suitably selected in terms of _the NO _x storage performance, NO _x From the viewpoint of enhancing the occlusion performance, a combination of barium (Ba) and cerium (Ce) is preferable.

The first metal element and the second metal element each form an oxide and exist as single particles, or a composite of, for example, a Ba compound and CeO ₂ containing the first metal element and the second metal element The form which exists and forms oxide composites, such as, may be sufficient.

The oxide particles (including the oxide composite) of the first metal element and the second metal element in the present invention are finer in order to exhibit sulfur desorption performance while maintaining high NO _x purification performance. It is preferable that the particles are uniformly and highly dispersedly supported on the surface of a porous oxide material such as alumina in the form of particles. From this point, as the particle size of the first metal oxide and the second metal oxide supported in the catalyst layer, or the oxide composite containing the first metal element and the second metal element, the average particle The diameter is preferably 30 nm or less. Furthermore, the average particle diameter is preferably in the range of 0.5 to 10 nm, more preferably in the range of 0.5 to 5 nm. When the particle size is 0.5 nm or more, a more uniform oxide is obtained, and when it is 30 nm or less, preferably 10 nm or less, a large number of surface sites effective for NO _x and SO _x storage are secured. be able to.
The particle size is measured by observation with X-ray diffraction (XRD) or transmission electron microscope (TEM).

  In the present invention, it is not necessary for all the introduced oxides to be supported by being refined, and as shown in FIG. 1, the first metal element (example: Ba) and the second metal element (example: Ce) ) Oxide particles can be made fine and supported on the surface of a carrier such as alumina.

In the first metal oxide and the second metal oxide included in the second catalyst layer and optionally the first catalyst layer, the amount of the second metal element is 0.2 to 0.2 in terms of mole relative to the amount of the first metal element. It is preferably 5 times. When this ratio is within the above range, the ratio of oxide particles formed is increased, and a higher NO _x storage capacity is obtained.

  The second catalyst layer is formed by a method capable of supporting the first metal oxide and the second metal oxide or the oxide complex containing the first metal element and the second metal element on a support formed in advance. Can do. For example, a metal oxide and other components as necessary are mixed, and a slurry is prepared by stirring and the like, and the obtained slurry is applied and immersed on the first catalyst layer described above to form a layer. After applying and firing to form a carrier layer, the first metal oxide and the second metal oxide or the oxide composite containing the first metal element and the second metal element is supported. Can do. Of these, a method of impregnating a layer constituting a carrier with an aqueous solution for supporting an oxide described later and baking it is preferable.

  The second catalyst layer preferably has a form in which the first metal oxide and the second metal oxide are supported on a basic support. The basic carrier can carry the oxide particles in a more highly dispersed state. Examples of such a basic carrier include a carrier containing an oxide of one or more metal elements selected from the group consisting of aluminum, zirconium, alkaline earth metals, and rare earth metals. The basic carrier described in paragraph Nos. [0055] to [0061] of JP-A-137886 can be mentioned as a preferable example.

Of these carriers, zirconia (ZrO ₂ ), alumina (AlO ₃ ), titania (TiO ₂ ), ceria (CeO ₂ ) and a carrier containing these composite oxides are preferable, and more preferably, titania and zirconia. A support containing a composite oxide, a composite oxide of zirconia and ceria.

  The shape of the carrier is not particularly limited, but is preferably in the form of a powder in that the specific surface area is improved and higher catalytic activity can be obtained. When the carrier is in powder form, the particle size (secondary particle size) of the carrier is not particularly limited and is preferably 1 to 100 μm. When the secondary particle diameter is 1 μm or more, the cost for refining the carrier can be suppressed, and the handling thereof is easy, and when it is 100 μm or less, a stable layer can be formed on the substrate.

The specific surface area of the support, preferably at least ^{20m 2 / g, 50~300m 2 /} g is more preferable. When the specific surface area is 20 m ² / g or more, an appropriate amount of the first metal oxide and the second metal oxide can be supported so that stable NO _x purification performance can be exhibited in a lean atmosphere. Further, the specific surface area is less than 300 meters 2 / ^g, it is possible to suppress a decrease in specific surface area due to thermal degradation.
The specific surface area is calculated as the BET specific surface area from the adsorption isotherm using the BET isotherm adsorption formula.

  The carrier can be produced by appropriately adopting known methods. For example, when preparing a support containing two kinds of metal elements, it contains a salt (for example, nitrate) of two or more kinds of metal elements selected from the group consisting of aluminum, zirconium, alkaline earth metals, and rare earth metals. Using an aqueous solution, a coprecipitate of the metal element is formed in the presence of ammonia, and the obtained coprecipitate is filtered, washed, dried, and further baked to obtain a support containing an oxide of the metal element. The method of obtaining is mentioned.

The thickness of the second catalyst layer is preferably 50 to 120 μm, and more preferably 15 to 65 μm. When the thickness of first catalytic layer is within the range, the purification performance of the can secure high _the NO _x purification performance in a lean atmosphere, and because the reaction gas until the second catalyst layer can be sufficiently diffused, due to Rh NO _x and HC It is advantageous in that it can sufficiently secure the sulfur desorption performance when rich.

For example, as shown in FIG. 2, the exhaust gas purification catalyst of the present invention has a two-layer structure in which a first catalyst layer 12 and a second catalyst layer 13 are laminated on a base material 11. The ratio of the thickness B of the first catalyst layer 12 to the total thickness A of the thickness and the thickness of the second catalyst layer 13 (%; the ratio R of the thickness of the first catalyst layer) is preferably selected within a wide range.
Ratio R = thickness B of the first catalyst layer / total thickness A × 100 of the first catalyst layer and the second catalyst layer
In the exhaust gas purifying catalyst of the present invention, the ratio R is, in terms of obtaining good sulfur leaving and NO _x purification performance is preferably in the range of 10% to 95%. The ratio R is more preferably in the range of 50% to 90% from the viewpoint of obtaining better sulfur desorption and NO _x purification ability. When the range of the ratio R is 10% or more and 95% or less, it is possible to stably maintain good sulfur desorption and to obtain stable NO _x purification performance in a lean atmosphere. As a result, performance fluctuations related to sulfur desorption and purification efficiency due to layer thickness change can be suppressed and stabilized, and the selectivity of the layer configuration that can stably maintain NO _x purification performance is expanded, including, for example, Rh It is also possible to reduce the ratio of the first catalyst layer. In the present invention, the relationship (C / B) between the thickness B of the first catalyst layer and the thickness C of the second catalyst layer is preferably 120/15 to 5/130, more preferably 65/70 to 15/120. preferable.

  The exhaust gas purifying catalyst of the present invention is preferably in a state where the first metal oxide and the second metal oxide are mixed with good dispersibility and are uniformly supported, and in particular, the first metal and the second metal are contained. The intensity ratio of the first metal element to the sum of the intensity ratio of the first metal element and the intensity ratio of the second metal element obtained from the TEM-EDX analysis at 20 or more measurement points of 10 nm square on the surface of the carrier to be supported It is preferable that the standard deviation of the distribution is 10% or less. When the standard deviation is 10% or less, the first metal element and the second metal element are in a sufficiently uniform mixed and dispersed state, and the suppression effect on sulfur poisoning is enhanced, and the regeneration treatment is performed after sulfur poisoning. When applied, the detachability of the sulfur component, particularly the detachment rate, is improved. This standard deviation is more preferably 9% or less.

For TEM-EDX analysis, a TEM-EDX apparatus equipped with a conventionally known energy dispersive X-ray spectrometer (EDX analyzer) on a conventionally known transmission electron microscope (TEM) can be used. In the TEM-EDX analysis, first, using a TEM-EDX apparatus, in an arbitrary region on the surface of the carrier supporting the first metal and the second metal, an energy dispersive fluorescent X within a measurement point of 10 nm square. Determine the line spectrum. Next, from the obtained energy dispersive fluorescent X-ray spectrum, the peak area derived from the first metal element and the peak area derived from the second metal element are obtained, and the peak area of the first metal element and The ratio (peak area ratio) of the peak area of the first metal element to the sum of the peak areas of the two metal elements is obtained. This peak area ratio is calculated from the following equation. A standard deviation is calculated | required as follows from the obtained peak area ratio.
Peak area ratio (%) = ([peak area of first metal element] / [sum of peak area of first metal element and peak area of second metal element]) × 100
Here, the “peak” means that the intensity from the baseline of the spectrum to the peak top is 1 cts or more. For example, when the metal element is barium, this peak appears at a position of 4.5 KeV (BaLα line), and when the metal element is cerium, this peak appears at a position of 4.8 KeV (CeLα line).

  The standard deviation of the distribution of the peak area ratio is based on the peak area ratio obtained by performing TEM-EDX analysis at any 20 or more measurement points to obtain the peak area ratio at each measurement point. Thus, it is obtained by calculating the standard deviation (%) of the distribution of the peak area ratio.

  As a reliable method of setting the standard deviation of the distribution of the peak area ratio to 10% or less, as will be described later, an oxide supporting aqueous solution containing a desired metal component is used, and this is fired to form the first metal oxide. The method of forming the product and the second metal oxide is preferred. In this aqueous solution for supporting an oxide, a first metal compound containing a first metal element and a second metal compound containing a second metal element are held in a stable state (preferably at a high concentration). Therefore, the oxide particles (including the oxide composite) obtained by firing the oxide-supporting aqueous solution are the ones in which the first metal element and the second metal element are uniformly mixed and refined. It becomes.

When the exhaust gas purifying catalyst of the present invention is, for example, oxide particles of barium (first metal element) and cerium (second metal element), NO _x in the exhaust gas is reduced when it is brought into contact with the exhaust gas of an automobile or the like. It can be occluded and removed as Ba (NO ₃ ) ₂ to purify the exhaust gas, and is excellent in sulfur desorption at the time of sulfur poisoning. Therefore, by supporting the oxide particles on the second catalyst layer that mainly does not contain at least Rh, high NO _x storage performance can be exhibited over a long period of time.

-Base material-
There is no restriction | limiting in particular as a base material, Although it selects suitably according to the use etc. of a purification catalyst, A DPF base material, a monolithic base material, a pellet-shaped base material, a plate-shaped base material, etc. are suitable. The material of the base material is not particularly limited, and for example, 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 preferable. . Among these, a monolith substrate is preferable.

  As the monolith substrate, those used in conventional exhaust gas purification catalysts can be used, for example, ceramic monolith substrates such as cordierite monolith substrates, refractory metal monolith carriers, such as aluminum-containing ferrite monoliths. Examples include carriers.

<Method for producing exhaust gas purification catalyst>
According to a second aspect of the present invention, there is provided a method for producing an exhaust gas purifying catalyst, comprising: applying a slurry containing rhodium on a substrate; calcining to form a first catalyst layer; and forming the first catalyst layer on the first catalyst layer. A platinum-containing layer forming step for forming a platinum-containing layer containing platinum, a first metal compound containing a first metal element selected from the group consisting of alkali metals and alkaline earth metals, and a group of rare earth metals A solution for preparing an oxide-supporting aqueous solution containing the first metal element and the second metal element by mixing a second metal compound containing the second metal element, a compound having a multidentate ligand, and ammonia. The preparation step and the prepared aqueous solution for supporting an oxide are brought into contact with at least the platinum-containing layer and then fired, and at least the first metal element and the second metal element containing the first metal element in the platinum-containing layer Second metal oxidation containing By carrying bets, and a second catalyst layer forming step of forming a second catalyst layer containing platinum, which is constituted by providing a. The manufacturing method of the exhaust gas purifying catalyst of the present invention may further have other configurations as necessary.

In the second invention, the aqueous solution for supporting an oxide to be used can hold various metal ions in a state of being close to each other by a polydentate ligand (for example, citric acid). The oxide particles (including oxide composites) are formed by oxidative decomposition by firing, but have better sulfur desorption and lean atmosphere than conventional multi-layer catalysts such as a two-layer structure. The stable NO _x purification performance is obtained because the oxide particles of the first metal element and the second metal element are mixed with good dispersibility and fixed in a highly uniform state at least in the second catalyst layer. It is estimated that

-First catalyst layer forming step-
In the first catalyst layer forming step, a slurry containing rhodium (Rh) is applied on a base material and fired to form a first catalyst layer.

For example, the slurry uses particles such as Rh-supported metal oxide powder such as Rh-supported zirconia in a medium such as water, and an oxide of aluminum, zirconium, alkaline earth metal, or rare earth metal (if necessary) For example, it can be prepared by adding other components such as alumina and stirring. Specifically, for example, powdery Rh-supported zirconia can be obtained by mixing zirconium dioxide (ZrO ₂ ) and a rhodium nitrate solution in a medium such as water, stirring, drying, and firing.

  The obtained slurry is applied, dipped, jetted, etc. on a desired substrate to form a layered coating film and fired. The firing conditions are not particularly limited, but as the temperature range, for example, heating in the temperature range of 300 to 600 ° C. is preferable, and heating in the temperature range of 300 to 500 ° C. is more preferable. Further, the heating time varies depending on the heating temperature and cannot be generally specified, but is preferably about 1 to 3 hours.

  The concentration of Rh in the slurry is preferably in the range of 0.01 to 10 g / L, more preferably in the range of 0.1 to 5 g / L.

-Platinum-containing layer formation process-
In the platinum-containing layer forming step, a platinum-containing layer containing platinum (Pt) is formed on the first catalyst layer formed in the first catalyst layer forming step.

The platinum-containing layer is formed, for example, by applying a slurry containing particles such as Pt-supported metal oxide powder in a medium such as water onto the first catalyst layer in the same manner as the first catalyst layer. A method for firing, or a slurry containing a metal oxide is prepared, and this slurry is applied in the form of a film on the first catalyst layer to form an upper layer serving as a carrier, and then the upper layer is contacted with a platinum-containing solution ( For example, it can be formed by a method of supporting Pt by immersing and applying) and baking. The upper layer can carry Pt by firing and can be a carrier on which the first metal oxide and the second metal oxide are carried in the second catalyst layer forming step described later.
Further, the slurry may contain other noble metals such as palladium (Pd), osmium (Os), iridium (Ir), gold (Au) in addition to Pt as a noble metal, thereby obtaining higher catalytic activity. It is preferable that palladium is further included at the point.

  As said platinum containing solution, dinitrodiamine platinum nitric acid solution etc. can be used, for example.

In the latter case, examples of the metal oxide contained in the slurry include metal element oxides described in paragraph numbers [0055] to [0061] of JP-A-2008-137886, for example, Examples thereof include oxides of metals selected from aluminum, zirconium, titanium, silicon, iron, manganese, tungsten, alkaline earth metals (magnesium, calcium, etc.), and rare earth metals. Among them, the metal element oxide contained in the slurry is preferably zirconia (ZrO ₂ ), alumina (AlO ₃ ), titania (TiO ₂ ), ceria (CeO ₂ ), or a composite oxide thereof, more preferably titania. And a composite oxide of zirconia and a composite oxide of zirconia and ceria.

-Solution preparation process-
The solution preparation step includes a first metal compound containing a first metal element selected from the group consisting of alkali metals and alkaline earth metals, a second metal compound containing a second metal element selected from the group of rare earth metals, A compound having a bidentate ligand and ammonia are mixed to prepare an aqueous solution for supporting an oxide containing the first metal element and the second metal element.
The details of the alkali metal, alkaline earth metal, and rare earth metal are as described above in the section of the second catalyst layer of the exhaust gas purifying catalyst according to the first invention.

Examples of the first metal compound include alkali metal or alkaline earth metal hydroxides, acetates, carbonates, nitrates, and the like. Specifically, a compound containing barium (barium nitrate, barium acetate, etc.) is preferable from the viewpoint of having high basicity and capable of exhibiting high NO _x storage ability when an oxide is formed. In addition, a 1st metal compound can be used individually by 1 type or in mixture of 2 or more types.

  Examples of the second metal compound include rare earth metal hydroxides, acetates, carbonates, nitrates, and the like. Specifically, a compound containing cerium (cerium acetate, cerium nitrate, etc.) is preferable, and cerium acetate is more preferable, because it has a relatively high specific surface area when an oxide is formed. In addition, a 2nd metal compound can be used individually by 1 type or in mixture of 2 or more types.

  The compound having a polydentate ligand refers to a compound that can coordinate with two or more coordination groups. Examples of such compounds include polyvalent carboxylic acids such as citric acid and oxalic acid, diols such as glycol and pinacol, diamines such as ethylenediamine, and esters having two carbonyl groups such as ethyl acetoacetate. Can be mentioned. Among these, citric acid, oxalic acid, succinic acid, maleic acid, malic acid, adipic acid, tartaric acid, malonic acid, fumaric acid, aconitic acid, glutaric acid, ethylenediaminetetraacetic acid, lactic acid, glycolic acid, glyceric acid, salicylic acid, Mevalonic acid, ethylenediamine, ethyl acetoacetate, malonic acid ester, glycol, and pinacol are preferable, and among them, citric acid, lactic acid, malic acid, tartaric acid, glycolic acid, and salicylic acid are more preferable in that it is a carboxylic acid having a hydroxy group. Citric acid is particularly preferred from the viewpoint of producing a bulky and more refined oxide composite. In addition, the compound which has a polydentate ligand can be used individually by 1 type or in mixture of 2 or more types.

  In addition, when the first metal compound, the second metal compound, the compound having a multidentate ligand, and ammonia are mixed to prepare an aqueous solution for supporting an oxide, a solvent can be used. As the solvent, water (preferably pure water such as ion exchange water and distilled water) is suitable.

  The content of the first metal compound in the oxide-supporting aqueous solution is preferably 0.05 mol / L or more, and more preferably 0.2 mol / L to 2.0 mol / L. Further, the content of the second metal compound in the oxide-supporting aqueous solution is preferably 0.05 mol / L or more, and more preferably 0.2 mol / L to 1.0 mol / L. When the content of the first metal compound and the second metal compound is 0.05 mol / L or more, the concentration of the first metal compound and the second metal compound is appropriate in terms of production efficiency. When the content is 2.0 mol / L or less, the concentrations of the first metal compound and the second metal compound are appropriate, the reaction proceeds uniformly during firing, and an oxide composite can be produced uniformly.

The content of the second metal element in the second metal compound is preferably 0.2 to 5 times in terms of mole relative to the content of the first metal element in the first metal compound. . If the content of the first and second metal element is within the range, it is possible to further enhance the _the NO _x storage capacity of the oxide composite. When the content of the second metal element in the second metal compound is within the above range relative to the content of the first metal element in the first metal compound, the single phase of the first metal compound or the second metal compound is The ratio of the obtained oxide composite can be maintained without increasing too much.

  The content of the compound having a polydentate ligand in the oxide-supporting aqueous solution is preferably 0.15 mol / L or more, and more preferably 0.7 mol / L to 2.0 mol / L. When the content of the compound having a multidentate ligand is 0.15 mol / L or more, an appropriate amount of the multidentate ligand is present with respect to the first metal compound and the second metal compound, The first metal compound and the second metal compound can be held in close proximity. Further, when the content is 2.0 mol / L or less, the carrying efficiency becomes good.

  Further, the content of the compound having a polydentate ligand is 2 times or more (more preferably 2.5 to 10 times) in terms of mole relative to the content of the metal element contained in the second metal compound. It is preferable that A complex with the second metal compound in an aqueous solution when the content of the compound having a multidentate ligand is 2 times or more in terms of mole relative to the content of the metal element contained in the second metal compound Can be formed satisfactorily, the first metal compound and the second metal compound can be held in proximity to each other, and an oxide composite can be produced favorably.

The content of ammonia in the oxide-supporting aqueous solution is preferably 0.4 mol / L or more, and more preferably 2.0 mol / L to 10 mol / L. Generation | occurrence | production of precipitation in the aqueous solution obtained as content of ammonia is 0.4 mol / L or more can be avoided. In addition, when the content is 10 mol / L or less, the supporting efficiency is improved.
Moreover, it is preferable that ammonia content is 2 times or more (more preferably 2.8 to 5 times) in terms of mole with respect to the content of the compound having a polydentate ligand. When the ammonia content is at least twice as much as the molar amount of the compound having a polydentate ligand, the first metal compound and the second metal compound can be contained in a stable state and precipitated. Occurrence is suppressed.

  The method for preparing the aqueous solution for supporting oxide is not particularly limited as long as it is a method capable of dissolving the first metal compound, the second metal compound, the compound having a polydentate ligand, and ammonia in water. As a preferred method for preparing the aqueous solution for supporting an oxide, for example, a compound having a polydentate ligand is dissolved in water, a second metal compound is added to form a precipitate, and then ammonia is added. There is a method in which the precipitate is dissolved again, and then the first metal compound is further added to the solution and stirred to form a precipitate and then dissolved again to obtain an aqueous solution for supporting an oxide. Alternatively, an oxide supporting aqueous solution may be prepared using an ammonium salt (for example, ammonium citrate) obtained by reacting a compound having a polydentate ligand with ammonia in advance.

-Second catalyst layer forming step-
In the second catalyst layer forming step, the prepared aqueous solution for supporting an oxide is brought into contact with at least the second catalyst layer and then baked, and at least the first metal oxide and the second metal oxide are formed on the platinum-containing layer. To form a second catalyst layer containing platinum.

  As a method of bringing the aqueous solution for supporting an oxide into contact with the platinum-containing layer, a method of immersing the platinum-containing layer in an aqueous solution for supporting an oxide, a method of applying an aqueous solution for supporting an oxide on the platinum-containing layer, an oxide on the platinum-containing layer For example, a method of ejecting the supporting aqueous solution can be applied.

The firing conditions are not particularly limited, but as the temperature range, for example, heating in the temperature range of 300 to 600 ° C. is preferable, and heating in the temperature range of 300 to 500 ° C. is more preferable. Further, the heating time varies depending on the heating temperature and cannot be generally specified, but is preferably about 1 to 3 hours.
By firing the oxide-supporting aqueous solution under the firing conditions, the platinum-containing layer can support the oxide composite containing the first metal and the second metal.

  Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the present invention is not limited to the following examples as long as the gist thereof is not exceeded. Unless otherwise specified, “part” and “%” are based on mass.

(Preparation of catalyst component)
-1. Preparation of Rh-supported zirconia-
A rhodium nitrate solution was added to the zirconia support, stirred for 1 hour, dried, and calcined to prepare rhodium (Rh) -supported zirconia powder. The amount of Rh supported was 1% by mass.

-2. Preparation of aqueous oxide complex precursor solution I-
As a multidentate ligand, 0.9 mol of citric acid was dissolved in 165 mol of ion-exchanged water, and 170 ml of 28% aqueous ammonia was added to prepare an aqueous ammonium citrate solution. To this solution, 0.2 mol of barium acetate and 0.2 mol of cerium acetate were added and stirred to prepare Ba / Ce oxide complex precursor aqueous solution (oxide supporting aqueous solution) I.

Example 1
50 parts of rhodium (Rh) -supported zirconia is mixed with 100 parts of ion exchange water and 5 parts of alumina sol to form a slurry, which is washed on a cordierite monolith substrate (6 mil, cell density 400 cells / square inch, 2.0 L). It was coated, dried and fired to form a lower catalyst coat layer (first catalyst layer). At this time, the amount of Rh supported on the lower catalyst coat layer was 0.5 g / L (liter).

Thereafter, 75 parts of alumina, 75 parts of zirconia titania composite oxide (commercially available ZT powder), 300 parts of ion-exchanged water, and 5 parts of alumina sol were mixed to form a slurry, and this slurry was placed on a cordierite monolith substrate. Wash coating was performed so as to be laminated on the lower catalyst coat layer, and drying and firing were performed to form an upper catalyst coat layer (second catalyst layer).
The zirconia titania composite oxide (ZT powder) is prepared by mixing a titanium tetrachloride aqueous solution and a zirconium oxynitrate aqueous solution, and further adding a hydrogen peroxide solution and a nonionic surfactant, and then stirring the aqueous solution uniformly. The solution prepared by the coprecipitation method by adding ammonia water can be used. The method for producing the zirconia titania composite oxide is not particularly limited, and for example, a known method such as the production method described in JP-A-2000-327329 can be appropriately employed.

  Next, the lower catalyst coat layer and the upper catalyst coat layer are immersed in a 4% dinitrodiamine platinum solution together with the base material, and platinum (Pt) is adsorbed and supported on the catalyst coat layer, followed by drying and firing. Thus, a Pt / Rh-supported two-layer coated catalyst was obtained. At this time, the amount of Pt supported by the upper catalyst coat layer was 2 g / L.

  The ratio of the thickness of the lower catalyst coat layer to the total thickness of the lower catalyst coat layer and the upper catalyst coat layer (lower coat ratio R) was 25%.

Next, the obtained Pt / Rh-supported two-layer coated catalyst is immersed in the oxide composite precursor aqueous solution I prepared as described above, pulled up and blown off the excess aqueous solution, and further dried and fired. Then, oxide composite particles (NO _x storage material) containing Ba and Ce were supported. Here, the solution concentration of the oxide complex precursor aqueous solution I was adjusted so that the supported amount was Ba / Ce = 0.2 [mol / L] /0.2 [mol / L], respectively.
The exhaust gas purification catalyst of the present invention was produced as described above.

(Example 2)
50 parts of rhodium (Rh) -supported zirconia, 25 parts of alumina, and 25 parts of zirconia titania composite oxide are mixed with 200 parts of ion-exchanged water and 5 parts of alumina sol to form a slurry, which is a cordierite monolith substrate (6 mil, The cell density was 400 cells / square inch, 2.0 L), and drying and firing were performed to form a lower catalyst coat layer (first catalyst layer). At this time, the amount of Rh supported on the lower catalyst coat layer was 0.5 g / L.

  Thereafter, 50 parts of alumina, 50 parts of zirconia titania composite oxide (commercially available ZT powder), 200 parts of ion-exchanged water, and 5 parts of alumina sol are mixed to form a slurry, and this slurry is placed on a cordierite monolith substrate. Wash coating was performed so as to be laminated on the lower catalyst coat layer, and drying and firing were performed to form an upper catalyst coat layer (second catalyst layer).

  Next, the lower catalyst coat layer and the upper catalyst coat layer are immersed in a 4% dinitrodiamine platinum solution together with the base material, and platinum (Pt) is adsorbed and supported on the catalyst coat layer, followed by drying and firing. Thus, a Pt / Rh-supported two-layer coated catalyst was obtained. At this time, the supported amount of Pt was 2 g / L.

  The ratio of the thickness of the lower catalyst coat layer to the total thickness of the lower catalyst coat layer and the upper catalyst coat layer (lower coat ratio R) was 50%.

Next, the obtained Pt / Rh-supported two-layer coated catalyst was dipped in the oxide composite precursor aqueous solution I prepared in Example 1, pulled up to blow off the excess aqueous solution, and further dried and fired. Then, oxide composite particles (NO _x storage material) containing Ba and Ce were supported. Here, the solution concentration of the oxide complex precursor aqueous solution I was adjusted so that the supported amount was Ba / Ce = 0.2 [mol / L] /0.2 [mol / L], respectively.
The exhaust gas purification catalyst of the present invention was produced as described above.

Example 3
50 parts of rhodium (Rh) -supported zirconia, 50 parts of alumina, and 50 parts of zirconia titania composite oxide are mixed with 300 parts of ion-exchanged water and 5 parts of alumina sol to form a slurry, which is a cordierite monolith substrate (6 mil, The cell density was 400 cells / square inch, 2.0 L), and drying and firing were performed to form a lower catalyst coat layer (first catalyst layer). At this time, the amount of Rh supported on the lower catalyst coat layer was 0.5 g / L.

  Thereafter, 25 parts of alumina, 25 parts of zirconia titania composite oxide (commercially available ZT powder), 100 parts of ion-exchanged water, and 5 parts of alumina sol were mixed to form a slurry, and this slurry was placed on the lower side on the cordierite monolith substrate. Wash coating was performed so as to be laminated on the catalyst coat layer, followed by drying and firing to form an upper catalyst coat layer (second catalyst layer).

  Next, the lower catalyst coat layer and the upper catalyst coat layer are immersed in a 4% dinitrodiamine platinum solution together with the base material, and platinum (Pt) is adsorbed and supported on the catalyst coat layer, followed by drying and firing. Thus, a Pt / Rh-supported two-layer coated catalyst was obtained. At this time, the amount of Pt supported by the upper catalyst coat layer was 2 g / L.

  The ratio of the thickness of the lower catalyst coat layer to the total thickness of the lower catalyst coat layer and the upper catalyst coat layer (lower coat ratio R) was 75%.

Next, the obtained Pt / Rh-supported two-layer coated catalyst was dipped in the oxide composite precursor aqueous solution I prepared in Example 1, pulled up to blow off the excess aqueous solution, and further dried and fired. Then, oxide composite particles (NO _x storage material) containing Ba and Ce were supported. Here, the solution concentration of the oxide complex precursor aqueous solution I was adjusted so that the supported amount was Ba / Ce = 0.2 [mol / L] /0.2 [mol / L], respectively.
The exhaust gas purification catalyst of the present invention was produced as described above.

Example 4
50 parts of rhodium (Rh) -supported zirconia, 65 parts of alumina, and 65 parts of zirconia titania composite oxide are mixed with 360 parts of ion-exchanged water and 5 parts of alumina sol to form a slurry, which is a cordierite monolith substrate (6 mil, The cell density was 400 cells / square inch, 2.0 L), and drying and firing were performed to form a lower catalyst coat layer (first catalyst layer). At this time, the amount of Rh supported on the lower catalyst coat layer was 0.5 g / L.

  Thereafter, 10 parts of alumina, 10 parts of zirconia titania composite oxide (commercially available ZT powder), 40 parts of ion-exchanged water, and 5 parts of alumina sol are mixed to form a slurry, which is the lower side on the cordierite monolith substrate. Wash coating was performed so as to be laminated on the catalyst coat layer, followed by drying and firing to form an upper catalyst coat layer (second catalyst layer).

  Next, the lower catalyst coat layer and the upper catalyst coat layer are immersed in a 4% dinitrodiamine platinum solution together with the base material, and platinum (Pt) is adsorbed and supported on the catalyst coat layer, followed by drying and firing. Thus, a Pt / Rh-supported two-layer coated catalyst was obtained. At this time, the amount of Pt supported by the upper catalyst coat layer was 2 g / L.

  The ratio of the thickness of the lower catalyst coat layer to the total thickness of the lower catalyst coat layer and the upper catalyst coat layer (lower coat ratio R) was 90%.

Next, the obtained Pt / Rh-supported two-layer coated catalyst was dipped in the oxide composite precursor aqueous solution I prepared in Example 1, pulled up to blow off the excess aqueous solution, and further dried and fired. Then, oxide composite particles (NO _x storage material) containing Ba and Ce were supported. Here, the solution concentration of the oxide complex precursor aqueous solution I was adjusted so that the supported amount was Ba / Ce = 0.2 [mol / L] /0.2 [mol / L], respectively.
The exhaust gas purification catalyst of the present invention was produced as described above.

(Comparative Example 1)
50 parts of rhodium (Rh) -supported zirconia, 75 parts of alumina, and 75 parts of zirconia titania composite oxide (commercially available ZT powder) are mixed with 400 parts of ion-exchanged water and 10 parts of alumina sol to form a slurry, which is a cordierite monolith. A substrate (6 mil, cell density 400 cells / square inch, 2.0 L) was wash-coated, dried and fired to form a catalyst coat layer. At this time, the amount of Rh supported on the catalyst coat layer was 0.5 g / L.

Next, the catalyst coat layer is dipped in a 4% dinitrodiamine platinum solution together with the base material, and platinum (Pt) is adsorbed and supported on the catalyst coat layer, followed by drying and firing, to obtain a single layer Pt / Rh. A supported coat catalyst was obtained. At this time, the amount of Pt supported was 2 g / L.
Subsequently, the Pt / Rh-supported coated catalyst was immersed in an aqueous barium acetate solution, pulled up to blow off excess aqueous solution, and then dried and fired to support a Ba compound (NO _x storage material). Here, the concentration of the barium acetate aqueous solution was adjusted so that the supported amount was Ba = 0.2 [mol / L].
A comparative exhaust gas purification catalyst was produced as described above.

(Comparative Example 2)
A comparative exhaust gas purification catalyst was produced in the same manner as in Comparative Example 1, except that the barium acetate aqueous solution was replaced with the oxide complex precursor aqueous solution I prepared in Example 1 in Comparative Example 1. The solution concentration of the oxide complex precursor aqueous solution I was adjusted so that the supported amount was Ba / Ce = 0.2 [mol / L] /0.2 [mol / L], respectively.

(Comparative Example 3)
In Example 2, a comparative exhaust gas purification catalyst was produced in the same manner as in Example 2, except that the oxide complex precursor aqueous solution I was replaced with a barium acetate aqueous solution. The concentration of the barium acetate aqueous solution was adjusted so that the supported amount was Ba = 0.2 [mol / L].

(Test / Evaluation 1)
The following measurements and evaluations were performed on the exhaust gas purification catalysts obtained in Examples 1 to 4 and Comparative Examples 1 to 3. The evaluation results are shown in the following Tables 1 and 2 and FIGS.

-1. Sulfur poisoning and desorption test
Each of the obtained exhaust gas purification catalysts was installed in an exhaust system of a lean burn engine, and an endurance test was conducted by alternately repeating the operation in a lean and rich atmosphere at a catalyst temperature of 750 ° C. for 200 hours. After that, high-concentration sulfur-containing fuel is burned, and a durability test is performed by alternately repeating the operation in a lean and rich atmosphere at a catalyst temperature of 400 ° C. for a predetermined time, and a fixed amount of sulfur is adsorbed to each purification catalyst. Poisoned.

-2. Catalyst purification performance
After the durability test, the poisoned catalyst was richly operated at 650 ° C. for 10 minutes, and the amount of sulfur desorption at this time was measured. The results are shown in FIG.
In addition, in order to evaluate the NO _x purification performance of each catalyst after lean desorption test during lean running, lean operation was performed for 60 seconds using the same exhaust system, and then rich combustion (rich spike) for 3 seconds was introduced. However, the NO _x purification rate at this time was measured. The measurement was performed for each catalyst temperature of 300 ° C, 400 ° C, and 500 ° C. The results are shown in Table 1 and FIGS.

-3. Standard deviation
Each of the obtained exhaust gas purification catalysts was installed in an exhaust system of a lean burn engine, and an endurance test was performed by alternately repeating the operation in a lean and rich atmosphere at a catalyst temperature of 750 ° C. for 50 hours. After the endurance test, the upper catalyst coat layer is scraped off, and the peak area ratio (%) is obtained as follows using a TEM-EDX apparatus, and measurement of 20 points or more of 10 nm squares on the surface of the carrier carrying Ba and Ce is carried out. In terms of points, the standard deviation of the distribution of peak area ratios was determined.
The standard deviation indicates that Ba (first metal element) and Ce (second metal element) are dispersed and mixed as the value is smaller. When the standard deviation is 10% or less, the uniformity is higher. Indicates the state. The results are shown in Table 2 below.

<TEM-EDX analysis>
(I) Apparatus: JEM-2010FSE [manufactured by JEOL Ltd .; TEM-EDX apparatus equipped with a transmission electron microscope (TEM) and an energy dispersive X-ray spectrometer (EDX analyzer)]
(Ii) Measurement method: Obtained and obtained an energy dispersive X-ray fluorescence spectrum within a 10 nm square measurement point in an arbitrary region on the surface of a carrier supporting Ba and Ce using a TEM-EDX apparatus. From the energy dispersive fluorescent X-ray spectrum, the area of the peak derived from Ba (BaLα line = 4.5 KeV) and the area of the peak derived from Ce (CeLα line = 4.8 KeV) are obtained. The ratio of the peak area of Ba to the sum of the peak area of Ce (peak area ratio;%) was determined.
Peak area ratio (%) = ([Ba peak area] / [Ba peak area and Ce peak area]) × 100

-4. Average particle size
The upper catalyst coat layer of each exhaust gas purification catalyst was scraped off, and the average particle diameter of the supported oxide particles was determined by XRD (X-ray diffraction) measurement. The results are shown in Table 2 below.

As shown in FIG. 3 and Table 1 and FIGS. 4 to 5, in Comparative Example, Comparative Example 2 has higher sulfur desorption performance and NO _x purification rate than Comparative Example 1, and Ba and Ce are finely composited. it is understood that _the NO _x purification performance is improved by. Further, in the purification catalyst of the two-layer structure of Comparative Example 3, the sulfur separation performance is low, is _the NO _x purification performance at hence a high temperature range (500 ° C.) is reduced.
On the other hand, in Examples 1-4, as shown in FIG. 3, the sulfur desorption performance is greatly improved as compared with the comparative example, and as shown in Table 1 and FIGS. not not been able to improve _the NO _x purification performance over a high temperature range.

Moreover, in Examples 1-4, as shown in the said Table 2, the standard deviation was 10% or less, and the purification catalyst of a highly uniform state was obtained.
Further, FIG. 6 shows the result of comparing the NO _x purification rate between the purification catalyst of the example and the conventional two-layered purification catalyst (lower coating ratio R = 50% is Comparative Example 3). As shown in FIG. 6, in the embodiment, NO _x purification rate is greatly improved, had high purification performance in particular regions below the coating ratio R is small.

Of the plots marked with ♦ in FIG. 6, the lower coat ratio R = 75% and 90% are plotted in the same manner as in Comparative Example 3 except that the oxide complex precursor aqueous solution I is acetic acid. The NO _x purification rate when the barium aqueous solution is replaced is shown, and the lower coat ratio R = 20% and 35% plots are obtained by replacing the oxide complex precursor aqueous solution I with the barium acetate aqueous solution in Example 1 and applying the lower coat ratio R = 20%, indicating the _the NO _x purification rate when was 35%.

(Example 5)
50 parts of rhodium (Rh) -supported zirconia, 25 parts of alumina, 10 parts of ceria zirconia composite oxide (commercially available CZ powder), and 25 parts of zirconia titania composite oxide were mixed with 220 parts of ion-exchanged water and 5 parts of alumina sol. The slurry is made into a slurry, and this is wash-coated on a cordierite monolith substrate (6 mil, cell density 400 cells / square inch, 2.0 L), dried and fired to form a lower catalyst coat layer (first catalyst layer). Formed. At this time, the amount of Rh supported on the lower catalyst coat layer was 0.5 g / L.

Thereafter, 50 parts of alumina, 10 parts of ceria zirconia composite oxide (commercially available CZ powder), 50 parts of zirconia titania composite oxide, 220 parts of ion-exchanged water, and 5 parts of alumina sol were mixed to form a slurry. The slurry was wash coated so as to be laminated on the lower catalyst coat layer on the cordierite monolith substrate, dried and fired to form an upper catalyst coat layer (second catalyst layer).
The ceria zirconia composite oxide (CZ powder) may be prepared by coprecipitation by adding ammonia water to a mixture of zirconium compound and cerium compound. Further, the production method of the ceria zirconia composite oxide is not particularly limited, and for example, a known method such as the production method described in JP-A-2003-275580 can be appropriately employed.

  Next, the lower catalyst coat layer and the upper catalyst coat layer are immersed in a 4% dinitrodiamine platinum solution together with a substrate, and platinum (Pt) is mainly adsorbed and supported on the upper catalyst coat layer, followed by drying. Calcination was performed to obtain a Pt / Rh-supported two-layer coated catalyst. At this time, the Pt carrying amount of the upper catalyst coat layer is 2 g / L, and the ratio of the thickness of the lower catalyst coat layer to the total thickness of the lower catalyst coat layer and the upper catalyst coat layer (lower coat ratio R) is 50. %Met.

Next, the obtained Pt / Rh-supported two-layer coated catalyst was dipped in the oxide composite precursor aqueous solution I prepared in Example 1, pulled up to blow off the excess aqueous solution, and further dried and fired. Then, a BaCe oxide composite (NO _x storage material) was supported. Here, the solution concentration of the oxide complex precursor aqueous solution I was adjusted so that the supported amount was Ba / Ce = 0.2 [mol / L] /0.2 [mol / L], respectively.
The exhaust gas purification catalyst of the present invention was produced as described above.

(Comparative Example 4)
A comparative exhaust gas purification catalyst was produced in the same manner as in Example 5 except that the oxide complex precursor aqueous solution I was replaced with a barium acetate aqueous solution in Example 5. The concentration of the barium acetate aqueous solution was adjusted so that the supported amount was Ba = 0.2 [mol / L].

(Comparative Example 5)
A comparative exhaust gas purification catalyst was produced in the same manner as in Example 5 except that the oxide complex precursor aqueous solution I was replaced with two kinds of barium acetate aqueous solution and cerium acetate aqueous solution in Example 5. The concentration of the barium acetate aqueous solution was adjusted so that the supported amount was Ba = 0.2 [mol / L].

(Test / Evaluation 2)
For each of the exhaust gas purification catalysts obtained in Example 5 and Comparative Examples 4 to 5, the standard deviation of the distribution of the metal element was determined by the TEM-EDX analysis in the same manner as in “-3. Measurement was performed at portions other than the zirconia composite oxide, and the peak intensity of the metal element by X-ray diffraction was measured by the same method as in “-4. Average particle diameter −”. The results are shown in Table 3 below and FIG.

  Furthermore, as shown in FIG. 7, in Example 5, compared with Comparative Examples 4 to 5, the peak intensity of the metal element is generally small, the diffraction peak of barium carbonate is also broadened, and the particle diameter of barium Can be seen to be fine. Further, the dispersion state of Ba and Ce in the exhaust gas purifying catalyst of Example 5 after performing the durability test at a catalyst temperature of 750 ° C. for 50 hours is shown in the TEM photograph of FIG.

DESCRIPTION OF SYMBOLS 11 ... Base material 12 ... 1st catalyst layer 13 ... 2nd catalyst layer

Claims (11)

A first catalyst layer containing rhodium and a second catalyst layer containing platinum in order from the substrate side on the substrate,
At least the second catalyst layer includes a second metal element selected from the group consisting of a first metal oxide and a rare earth metal selected from the group consisting of a first metal element selected from the group consisting of alkali metals and alkaline earth metals. An exhaust gas purification catalyst comprising a metal oxide supported on a carrier.   2. The exhaust gas purifying catalyst according to claim 1, wherein the particle size of the metal oxide supported in the second catalyst layer is 30 nm or less in terms of an average particle size.   The exhaust gas purifying catalyst according to claim 2, wherein the particle size is 0.5 to 10 nm in terms of average particle diameter.   The first metal element with respect to the sum of the intensity ratio of the first metal element and the intensity ratio of the second metal element obtained from TEM-EDX analysis at 20 or more measurement points of 10 nm square on the surface of the carrier. The exhaust gas purification catalyst according to any one of claims 1 to 3, wherein the standard deviation of the distribution of the intensity ratio is 10% or less.   The exhaust gas purification catalyst according to any one of claims 1 to 4, wherein the second catalyst layer further contains palladium.   The exhaust gas purification catalyst according to any one of claims 1 to 5, wherein the base material is a monolith base material. A first catalyst layer forming step of applying a slurry containing rhodium on a substrate and firing to form a first catalyst layer;
A platinum-containing layer forming step of forming a platinum-containing layer containing platinum on the first catalyst layer;
A first metal compound containing a first metal element selected from the group consisting of alkali metals and alkaline earth metals, a second metal compound containing a second metal element selected from the group of rare earth metals, and a multidentate ligand A solution preparing step of preparing an oxide-supporting aqueous solution containing the first metal element and the second metal element by mixing the compound having ammonia and ammonia;
The prepared aqueous solution for supporting an oxide is brought into contact with at least the platinum-containing layer and then baked, and at least a second metal element containing a first metal oxide and a second metal element containing the first metal element in the platinum-containing layer. A second catalyst layer forming step of forming a second catalyst layer containing platinum by supporting a metal oxide;
A method for producing an exhaust gas purifying catalyst.   The method for producing an exhaust gas purification catalyst according to claim 7, wherein the compound having a polydentate ligand is a carboxylic acid.   The method for producing an exhaust gas purification catalyst according to claim 7 or 8, wherein the first metal compound is a salt of a first metal element.   The method for producing an exhaust gas purification catalyst according to any one of claims 7 to 9, wherein the second metal compound is a salt of a second metal element. In the platinum-containing layer forming step, a slurry containing a metal element oxide is provided on the first catalyst layer to provide an upper layer, and at least the upper layer is brought into contact with the platinum-containing solution and then baked to contain platinum. Forming a layer,
The said 2nd catalyst layer formation process carries | supports the said 1st metal oxide and the said 2nd metal oxide on the oxide of the said metal element, The any one of Claims 7-10 characterized by the above-mentioned. The manufacturing method of the exhaust gas purification catalyst as described.