JP2011131142A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for cleaning exhaust gas, which has excellent performance of cleaning exhaust gas. <P>SOLUTION: The catalyst for cleaning exhaust gas includes: at least one of a first compound oxide shown by general formula, A<SB>i</SB>O<SB>j</SB>*p(D<SB>2-q-r</SB>E<SB>q</SB>G<SB>r</SB>O<SB>3</SB>) and a second compound oxide shown by general formula, (Al<SB>2-x-y</SB>J<SB>x</SB>L<SB>y</SB>)O<SB>3</SB>; and an alkaline earth metal, which is selected from the group including a single substance of the alkaline earth metal, salt of the alkaline earth metal, an organometallic compound including the alkaline earth metal, a simple oxide of the alkaline earth metal, and a mixture thereof, or a compound of the selected alkaline earth metal. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an exhaust gas purifying catalyst.

In recent years, exhaust gas regulations for automobiles and the like have been strengthened. In order to cope with this, various exhaust gas purification catalysts have been developed for more efficiently purifying hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO _x ), etc. in the exhaust gas. .

  For example, Patent Document 1 discloses that “a rare earth element, an alkaline earth element, and a noble metal are included, and a part of the rare earth element and a part of the alkaline earth element form a complex oxide, and this complex oxide. And a part of the noble metal form a solid solution ”.

  Patent Document 2 states that “the rare earth element, the alkaline earth element, zirconium and a noble metal are included, the atomic ratio of the alkaline earth element to the sum of the rare earth element and zirconium is 10 atomic% or more, Part and part of zirconium form a composite oxide with at least part of an alkaline earth element, and the composite oxide and part of a noble metal form a solid solution. "Catalyst" is described.

  Furthermore, Patent Document 3 states that “a catalyst composition containing a composite oxide, in which the composite oxide is dissolved in an oxide atmosphere and precipitated in a reducing atmosphere with respect to the composite oxide. "Catalyst compositions characterized in that they contain elements (excluding platinum group elements)" are described.

JP 2006-130444 A JP 2006-346587 A International Publication No. 2008/096575 Pamphlet

  However, there is room for further improvement in the exhaust gas purification performance of the exhaust gas purification catalyst. Then, an object of this invention is to provide the catalyst for exhaust gas purification which has the outstanding exhaust gas purification performance.

According to one aspect of the present invention, the first composite oxide represented by the general formula A _i O _j · p (D _{2 -qr} E _q G _r O ₃ ) and the general formula (Al _{2 -xy} J _x L _y ) At least one of the second composite oxide represented by O ₃ , an alkaline earth metal simple substance, an alkaline earth metal salt, an organometallic compound containing an alkaline earth metal, an alkaline earth metal simple oxide, and An alkaline earth metal selected from the group consisting of these mixtures or a compound thereof, and element A is at least one element selected from the group consisting of monovalent elements, divalent elements, and rare earth elements , Element D is Al or a combination of Al and a transition metal element excluding a platinum group element, element E is a transition metal element excluding a platinum group element, element G is a platinum group element, and element J is platinum. Transition metal elements excluding group elements, and element L is a platinum group element. I and j are combinations of the minimum natural numbers satisfying ki = 2j when the valence of the element A in the oxide represented by the general formula A _i O _j is k, and p is a positive number Q satisfies 0 <q <2, r satisfies 0 <r <2, x satisfies 0 <x <2, and y satisfies 0 <y <2. An exhaust gas purifying catalyst is provided.

  According to the present invention, it is possible to provide an exhaust gas purification catalyst having excellent exhaust gas purification performance.

The graph which shows an example of the powder X-ray diffraction profile of the catalyst for exhaust gas purification. The SEM photograph of the catalyst for exhaust gas purification which concerns on an Example. The SEM photograph of the catalyst for exhaust gas purification concerning a comparative example.

  Hereinafter, embodiments of the present invention will be described. Here, the “composite oxide” is not simply a physical mixture of a plurality of oxides, but a thing in which a plurality of oxides form a solid solution. Further, “alkaline earth metal element” includes Be and Mg.

An exhaust gas purifying catalyst according to one embodiment of the present invention includes a first composite oxide represented by a general formula A _i O _j · p (D _{2 -qr} E _q G _r O ₃ ) and a general formula (Al _{2 -xy).} at least one of J _x L _y) O ₃ in the second composite oxide represented, and a alkaline earth metal or a compound thereof. The alkaline earth metal or a compound thereof is composed of a simple substance of alkaline earth metal, a salt of alkaline earth metal, an organic metal compound containing alkaline earth metal, a simple oxide of alkaline earth metal, and a mixture thereof. Selected from the group consisting of

First, the first composite oxide represented by the general formula A _i O _j · p (D _{2 -qr} E _q G _r O ₃ ) will be described. As will be described later, p represents a molar ratio between a portion represented by the general formula A _i O _j and a portion represented by the general formula D _{2 -qr} E _q G _r O ₃ .

This first composite oxide is an XYO-type composite oxide. That is, the composition of the first composite oxide is represented by the general formula _{X l Y m O n (l} , m and n are positive numbers) can also be represented by. Here, the element X and the element Y typically occupy different sites in the unit cell. Element X is element A, and element Y is a combination of elements D, E, and G. That is, in the first composite oxide, typically, a part of the site corresponding to Y in the XYO-type composite oxide is occupied by the element D, the other part is occupied by the element E, and the rest A compound in which the portion is occupied by the element G.

  The element A is at least one element selected from the group consisting of a monovalent element, a divalent element, and a rare earth element. The element A is, for example, at least one element selected from the group consisting of alkali metal elements, alkaline earth metal elements, and rare earth elements. The element A is typically an element other than the platinum group element.

  As the monovalent element, for example, alkali metal elements such as Li, Na, and K are used.

  As the divalent element, for example, alkaline earth metal elements such as Be, Mg, and Ca are used. Alternatively, a divalent transition metal element such as Co, Ni, Cu, and Zn may be used as the divalent element.

  As the rare earth element, for example, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu are used.

  Typically, element A contains an alkaline earth metal element, in particular Mg. For example, the element A is Mg, a combination of Mg and another divalent element, or a combination of Mg and a monovalent element.

  The element D is Al or a combination of Al and a transition metal element. The element D is an element that does not cause a change in valence associated with a change in the composition of the atmosphere or has a very small change in valence. The element D plays a role of maintaining the crystal structure of the first composite oxide. That is, the element D plays a role of suppressing a change in the crystal structure of the first composite oxide accompanying a change in the composition of the atmosphere.

  The transition metal element that can be contained in the element D is an element different from elements E and G described later. The transition metal element is typically an element other than the platinum group element. This transition metal element is, for example, Ti, V, Cu, or Zn. The element D may contain a plurality of transition metal elements. Typically, element D is Al.

  Element E is a transition metal element excluding platinum group elements. The element E tends to cause a change in valence associated with a change in the composition of the atmosphere.

  The element E is dissolved in the first composite oxide. At least a part of the element E is precipitated from the first composite oxide in a reducing atmosphere. The behavior of the element E in the reducing atmosphere and the oxidizing atmosphere will be described in detail later.

  The element E plays a role as an active component that catalyzes the exhaust gas purification reaction. The element E is a transition metal element that can have divalent and trivalent valences, such as Cr, Mn, Fe, Co, and Ni. The element E may contain a plurality of elements. Typically, the element E is Fe.

  The element G is a platinum group element. The element G tends to cause a change in valence accompanying a change in the composition of the atmosphere.

  The element G is dissolved in the first composite oxide. At least a part of the element G is precipitated from the first composite oxide in a reducing atmosphere. The behavior of the element G in the reducing atmosphere and the oxidizing atmosphere will be described in detail later.

  The element G plays a role of promoting the precipitation of the element E in the reducing atmosphere. The element G plays a role as an active component that catalyzes the exhaust gas purification reaction. The element G is, for example, Pd, Pt, or Rh. The element G may contain a plurality of platinum group elements. Typically, the element G is Pd.

The p is a positive number, and the molar ratio between the portion represented by the general formula A _i O _j and the portion represented by the general formula D _2-qr E _q G _r O ₃ in the first composite oxide is expressed as follows. Represents. As will be described later, the crystal phase taken by the first composite oxide changes according to the magnitude of p. p may be a natural number or may not be a natural number.

  p typically satisfies 1 ≦ p ≦ 9. In this case, the first composite oxide is a composite oxide having a spinel crystal phase, a magnetoplumbite crystal phase, an alumina crystal phase, or a mixed phase thereof.

For example, when 1 ≦ p ≦ 3, the first composite oxide is a composite oxide having a spinel crystal phase as a main crystal phase. When 4 ≦ p ≦ 6, the first composite oxide is a composite oxide having a mixed phase of a spinel crystal phase, a magnetoplumbite crystal phase, and an alumina crystal phase as the main crystal phase. In the case of 7 ≦ p ≦ 9, the first composite oxide is a composite oxide having an alumina crystal phase as a main crystal phase.
The first composite oxide may be a mixture of a plurality of composite oxides having different p.

The above i and j are combinations of the minimum natural numbers that satisfy ki = 2j when the valence of the element A in the oxide represented by the general formula A _i O _j is k. For example, when the element A is divalent (when k = 2), (i, j) = (1, 1). Further, when the element A is monovalent (when k = 1), (i, j) = (2, 1). Note that in the portion represented by the general formula A _i O _j , part of oxygen may be deficient. That is, the above equation “ki = 2j” does not have to be satisfied with mathematical rigor.

  The q satisfies 0 <q <2, and the r satisfies 0 <r <2. That is, the first composite oxide includes all of the element D, the element E, and the element G as essential components.

  The q satisfies, for example, 0 <q ≦ 1.2. If q is small, the catalytic activity of element E may not be sufficiently expressed. If q is large, the crystal structure of the first composite oxide may become unstable.

  The r satisfies, for example, 0 <r ≦ 0.5. If r is small, the element E is difficult to precipitate in a reducing atmosphere, and the catalytic activity of the element G may not be fully expressed. If r is large, the crystal structure of the first composite oxide may become unstable.

As the first composite oxide, a composite oxide in which i = j = p = 1 in the above general formula is typically used. The composite oxide is a general formula A compound represented by _{(D 2-qr E q G} r) O 4, has a spinel crystal structure. As the compound, for example, a compound represented by the formula _{MgAl 2-qr Fe q Pd r} ) O 4 and the like.

Examples of the first composite oxide include MgO (Al _1.588 Fe _0.397 Pd _0.015 O ₃ ), that is, MgAl _1.588 Fe _0.397 Pd _0.015 O ₄ , MgO (Al _0.9925 Fe _0.9925 Pd _0.015 O ₃ ), that is, MgAl _0.9925 Fe _0.9925 Pd _0.015 O. ₄ and _MgO.1.1 (Al _1.589 Fe _0.397 Pd _0.014 O ₃ ) and the like.

Note that the first composite oxide may be partially deficient in oxygen. That is, the first composite oxide may have a composition different from the theoretical composition ratio.
Moreover, this catalyst may further contain a simple substance of element G or a compound thereof. By including a simple substance of element G or a compound thereof in the catalyst, for example, the initial performance of the catalyst can be improved.

Next, the general formula _{(Al 2-xy J x L} y) second composite oxide represented by O ₃ will be described.

  Element J is a transition metal element excluding platinum group elements. The element J tends to cause a change in valence due to a change in the composition of the atmosphere.

  Element J is dissolved in the second composite oxide. At least a part of the element J is precipitated from the second composite oxide in a reducing atmosphere. The behavior of the element E in the reducing atmosphere and the oxidizing atmosphere will be described in detail later.

  Element J is an element having outermost electrons in 3d orbitals such as Mn, Fe, Co, and Ni. The element J may include a plurality of elements. Typically, the element J is Fe.

  The element L is a platinum group element. The element L is likely to cause a change in valence accompanying a change in the composition of the atmosphere.

  The element L is dissolved in the second composite oxide. At least a part of the element L is precipitated from the second composite oxide in a reducing atmosphere. The behavior of the element L in the reducing atmosphere and the oxidizing atmosphere will be described in detail later.

  The element L plays a role as an active component that catalyzes the exhaust gas purification reaction. The element L is, for example, Pd, Pt, or Rh. The element L may contain a plurality of platinum group elements. Typically, the element L is Pd.

  The above x satisfies 0 <x <2, and the above y satisfies 0 <y <2. That is, the second composite oxide contains all of Al, element J, and element L as essential components. If x is large, the crystal structure of the second composite oxide may become unstable. If y is large, there is a possibility that sintering of the element L is likely to occur.

The second composite oxide is a compound in which element J and element L are dissolved in alumina Al ₂ O ₃ . The second composite oxide has the same shape as, for example, γ, δ, θ, α, or κ-alumina. The second composite oxide may be partially deficient in oxygen.

  Further, this catalyst may further contain a simple element of element L or a compound thereof. By including the element L alone or a compound thereof in the catalyst, for example, the initial performance of the catalyst can be improved.

  The total amount of element E and element J is, for example, in the range of 0.1% by mass to 14% by mass based on the unit mass of the exhaust gas purifying catalyst (excluding the base material).

  The total amount of element G and element L is, for example, in the range of 0.01% by mass to 10% by mass based on the unit mass of the exhaust gas purifying catalyst (excluding the base material).

  As described above, the alkaline earth metal or a compound thereof includes a simple substance of alkaline earth metal, a salt of alkaline earth metal, an organic metal compound containing alkaline earth metal, a simple oxide of alkaline earth metal, and these Selected from the group consisting of mixtures.

  The alkaline earth metal salt is, for example, an inorganic or organic salt of an alkaline earth metal. Examples of the alkaline earth metal inorganic salts include sulfates, nitrates, phosphates and chlorides thereof. Examples of the alkaline earth metal organic salts include acetates, oxalates and citrates thereof.

  Examples of the organometallic compound containing an alkaline earth metal include an alkaline earth metal alkoxide. Alkali earth metal alkoxides include, for example, their alcoholates and alkoxy alcoholates.

  The alkaline earth metal element constituting these simple substances or compounds is, for example, Mg, Ca, Sr or Ba. Typically, the alkaline earth metal element is Ba.

  This alkaline earth metal element plays a role of suppressing a decrease in catalytic activity of the element G due to alloying of the element E and the element G. Further, the alkaline earth metal element plays a role of suppressing a decrease in catalytic activity of the element L due to alloying of the element J and the element L. The behavior of the alkaline earth metal or its compound in a reducing atmosphere and an oxidizing atmosphere will be described in detail later.

  The alkaline earth metal element is typically dispersed almost uniformly in the exhaust gas purifying catalyst. This facilitates contact between the alkaline earth metal element and the element E and element G and / or the element J and element L. Therefore, in this case, it is possible to more effectively suppress a decrease in catalytic activity due to alloying of element E and element G and / or alloying of element J and element L. Examples of methods that allow such distribution of alkaline earth metal elements will be described later.

  In the exhaust gas-purifying catalyst, each of the first composite oxide, the second composite oxide, and the alkaline earth metal or a compound thereof typically exists in the form of particles. The first particles made of the first composite oxide and / or the second composite oxide typically carry the second particles made of an alkaline earth metal or a compound thereof. The particle size of the first particles is typically larger compared to the particle size of the second particles.

  The present inventors consider that this exhaust gas-purifying catalyst exhibits the following state changes when the composition of the atmosphere is changed under high temperature conditions. Here, “high temperature condition” means, for example, “1000 ° C.”.

  Below, the case where the exhaust gas purifying catalyst contains the first composite oxide will be described as an example. As described above, both the element E and the element G are dissolved in the first composite oxide.

  First, a description will be given of a state change that may occur when the exhaust gas-purifying catalyst is exposed to a low oxygen concentration atmosphere under a high temperature condition, for example, when a large amount of fuel is continuously supplied to the engine. That is, the state change that can occur when this exhaust gas-purifying catalyst is exposed to a high-temperature reducing atmosphere will be described.

  In this case, as described above, at least a part of the element E and at least a part of the element G are precipitated from the first composite oxide. At least a part of the deposited element E forms particles made of the element E alone. Similarly, at least a part of the precipitated element G forms particles composed of the element G alone. These particles function as active components that catalyze the exhaust gas purification reaction. Since these particles generally have a very small particle size, they can exhibit excellent activity.

  Next, the exhaust gas-purifying catalyst is exposed to a high oxygen concentration atmosphere under high temperature conditions. That is, the exhaust gas-purifying catalyst is exposed to a high-temperature oxygen atmosphere. For example, the fuel supply to the engine is stopped. As a result, at least a part of the element E and at least a part of the element G precipitated from the first composite oxide are again dissolved in the first composite oxide.

  Next, when the exhaust gas-purifying catalyst is again exposed to a reducing atmosphere, at least a part of the element E and at least a part of the element G are precipitated from the first composite oxide, as described above. These elements can exhibit excellent catalytic activity as described above.

  As described above, when the atmosphere surrounding the exhaust gas purifying catalyst is alternately changed between the high temperature reducing atmosphere and the high temperature oxidizing atmosphere, the element E from the first composite oxide is changed according to the change in the atmosphere. And precipitation of G and the solid solution of elements E and G in the first composite oxide are repeated. Therefore, this catalyst can exhibit excellent exhaust gas purification performance over a long period of time.

  By the way, the present inventors may form an alloy (hereinafter also referred to as an alloy EG) between a part of the element E precipitated from the first composite oxide and a part of the element G in a high temperature reducing atmosphere. Is heading. The element G forming the alloy EG is less likely to cause solid solution in the first composite oxide as compared with the element G as a simple substance. Therefore, particles made of the alloy EG grow, and as a result, the catalytic activity of the element G may be reduced.

  In addition, the present inventors presume the cause of this alloying as follows. That is, since the elements E and G were dissolved in the first composite oxide, they are likely to precipitate in a state of being close to each other. Therefore, the deposited particles of these elements are easily in contact with each other. Therefore, these particles tend to cause an alloying reaction.

As described above, the exhaust gas-purifying catalyst contains an alkaline earth metal or a compound thereof. In the high-temperature oxidizing atmosphere, at least a part of the alkaline earth metal element contained in the alkaline earth metal or the compound thereof reacts with at least a part of the element E contained in the oxide of the alloy EG, thereby forming a composite. An oxide is formed. For example, when the element E is Fe and the alkaline earth metal element is Ba, these elements typically form a compound represented by BaFe ₁₂ O ₁₉ .

  Complex oxides of element E and alkaline earth metal elements are typically thermodynamically and kinetically stable. That is, the element E forming the composite oxide hardly forms the element G and the alloy EG again.

  Thus, since at least a part of the element E contained in the alloy EG is consumed in the reaction with the alkaline earth metal element, at least a part of the element G forming the alloy EG is a single metal or single element. One metal oxide. The element G as a single metal and the element G contained in the single metal oxide are more easily dissolved in the first composite oxide as compared with the element G forming the alloy EG. That is, at least a part of the element G forming the alloy EG is re-dissolved in the first composite oxide in the oxidizing atmosphere without forming the alloy EG again in the high temperature reducing atmosphere.

  Therefore, when an alkaline earth metal or a compound thereof is used, more elements G are precipitated from the first composite oxide and dissolved in the first composite oxide as compared with the case where this is not used. Can be repeated. That is, in this case, the catalytic activity of the element G can be maintained over a longer period.

  In addition, although the case where the 1st complex oxide was used was demonstrated here, the same effect can be acquired also when the 2nd complex oxide is used. In this case, the present inventors can maintain the catalytic activity of the element L over a long period of time as described above even when the exhaust gas-purifying catalyst contains the second composite oxide. It is considered that the same state change occurs, that is, the element J and the alkaline earth metal element form a composite oxide.

  This exhaust gas-purifying catalyst is produced, for example, by the following method.

<Production of first composite oxide>
First, the compound of element A, the compound of element D, and the compound of element E are dissolved in a solvent such as water. Next, a coprecipitate is obtained by adding a base to the obtained mixed solution. Next, the coprecipitate is dried and fired. In this way, a composite oxide containing element A, element D, and element E is obtained. Although a coprecipitation method is used here, an impregnation method, a citric acid complex method, an alkoxide method, or the like may be used instead.

  Subsequently, the obtained complex oxide is impregnated with a solution of the element G compound. Next, this is dried and fired, and the element G is dissolved in the composite oxide. In this way, the first composite oxide is obtained. Instead of the above impregnation method, a coprecipitation method, a citric acid complex method, an alkoxide method, or the like may be used.

<Production of second composite oxide>
First, the alumina powder is immersed in a solution of a compound containing the element J. Thereafter, this is dried and fired to prepare an alumina powder in which the element J is dissolved. Although the impregnation method is used here, a coprecipitation method, a citric acid complex method, an alkoxide method, or the like may be used instead.

  Next, this powder is immersed in a solution of the element L compound. Thereafter, this is dried and fired to obtain an alumina powder in which element J and element L are dissolved. That is, the second composite oxide is obtained in this way. Instead of the above impregnation method, a coprecipitation method, a citric acid complex method, an alkoxide method, or the like may be used.

<Introduction of alkaline earth metals or their compounds>
The powder of the first composite oxide and / or the second composite oxide is impregnated with an inorganic salt or organic salt solution of an alkaline earth metal element. Next, after removing the solvent by heating, it is fired. In this way, the second particles made of alkaline earth metal or a compound thereof are supported on the first particles made of the first composite oxide and / or the second composite oxide.

  Further, the obtained powder is compression-molded, and the molded product is pulverized as necessary. As described above, a pellet-shaped exhaust gas-purifying catalyst is obtained.

  In addition, you may introduce | transduce alkaline-earth metal or its compound using a coprecipitation method, an alkoxide method, etc. instead of said impregnation method. However, when an alkaline earth metal or a compound thereof is introduced using the above impregnation method, the alkaline earth metal element can be dispersed almost uniformly in the catalyst. Therefore, in this way, as described above, it is possible to more effectively suppress a decrease in catalytic activity due to alloying of element E and element G and / or alloying of element J and element L. Become.

  Moreover, although the case where the exhaust gas purifying catalyst is a pellet catalyst has been described above as an example, the exhaust gas purifying catalyst can take various forms. For example, the exhaust gas purifying catalyst may be a monolith catalyst.

  Examples of the present invention will be described below.

<Example 1: Production of catalyst C1>
107.2 g of magnesium acetate tetrahydrate [(CH ₃ COO) ₂ Mg · 4H ₂ O], 337.6 g of aluminum nitrate nonahydrate [Al (NO ₃ ) ₃ · 9H ₂ O], 40 4 g of iron nitrate nonahydrate [Fe (NO ₃ ) ₃ .9H ₂ O] was dissolved in 2000 mL of ion-exchanged water. That is, a mixed aqueous solution of these compounds was prepared so that the atomic ratio of Mg, Al, and Fe was 1: 1.8: 0.2.

To this mixed aqueous solution, an aqueous solution in which 70 g of NH ₃ was dissolved in 1000 mL of ion exchange water was dropped at room temperature to obtain a coprecipitate. The coprecipitate was dried at 110 ° C. and calcined at 600 ° C. for 3 hours in the air. The obtained powder was pulverized using a mortar, and then baked at 980 ° C. for 5 hours in the air.

The X-ray diffraction spectrum was measured about the powder after baking. As a result, it was found that this powder was composed of a single phase of MgAl _1.8 Fe _0.2 O ₄ having a spinel type crystal structure.

Next, this powder was impregnated with an aqueous solution of palladium nitrate. This impregnation was performed such that the Pd content was 0.5% by mass based on the total mass of the catalyst. Thereafter, it was dried at 110 ° C. for 12 hours. This was then calcined at 800 ° C. for 3 hours in air. In this way, Pd was dissolved in the compound represented by MgAl _1.8 Fe _0.2 O ₄ .

A part of the obtained powder was extracted and immersed in a 10% aqueous hydrogen fluoride solution maintained at room temperature for 12 hours. This condition is a condition in which only the composite oxide is dissolved in the previous powder. Subsequently, this liquid was filtered, and the filtrate was subjected to inductively coupled radio frequency plasma (ICP) spectroscopic analysis. As a result, it was found that 35% of Pd contained in the aqueous palladium nitrate solution was dissolved in the compound represented by MgAl _1.8 Fe _0.2 O ₄ .

Moreover, it was found by fluorescent X-ray analysis (XRF analysis; PANalytical MagicX PRO) that the obtained compound was a compound represented by the formula MgAl _1.798 Fe _0.200 Pd _0.002 O ₄ .

Subsequently, 20 g of the obtained powder was added to a solution in which 2.24 g of barium acetate tetrahydrate [(CH ₃ COO) ₂ Ba · 4H ₂ O] was dissolved in 50 mL of ion-exchanged water. After stirring for 30 minutes, moisture was removed by heating. As a result, Ba was supported on a compound in which Pd was dissolved in MgAl _1.8 Fe _0.2 O ₄ . Thereafter, the obtained powder was fired at 500 ° C. in the air for 1 hour. In the obtained powder, the ratio of Ba to Fe was 25 atomic%.

  The resulting powder was then compression molded. Further, this molded product was pulverized to obtain a pellet-shaped exhaust gas purification catalyst having a particle size of 0.5 mm to 1.0 mm. Hereinafter, this catalyst is referred to as “catalyst C1”.

<Example 2: Production of catalyst C2>
Exhaust gas purification catalyst was produced in the same manner as in Example 1 except that the step of supporting Ba was omitted. Hereinafter, this catalyst is referred to as “catalyst C2”.

<Durability test>
Each of the catalysts C1 and C2 was subjected to an endurance test. Specifically, a first period in which a lean gas in which 5% by volume of oxygen is added to nitrogen gas is circulated for 5 minutes, and a second period in which a rich gas in which 5% by volume of carbon monoxide is added to nitrogen gas are circulated for 5 minutes; The cycle consisting of was repeated for 40 hours under conditions of a bed temperature of 1000 ° C. and a flow rate of 100 mL / min.

<Activity evaluation>
Each of the catalysts C1 and C2 after the durability test was installed in an atmospheric pressure fixed bed flow reactor. Next, the temperature was raised from 100 ° C. to 500 ° C. at a rate of 12 ° C./min while circulating a model gas having a theoretical air-fuel ratio. During this time, THC (total hydrocarbons), CO and NO _x purification rates were continuously measured. Then, THC, for each of the CO and NO _x, was determined 50% purification temperature. The results are shown in Table 1 below.

As shown in Table 1, the catalyst C1, as compared to the catalyst C2, THC, the purification performance of the CO and NO _x were superior.

<X-ray diffraction profile>
A powder X-ray diffraction spectrum was measured for each of the catalysts C1 and C2 after the durability test. The result is shown in FIG.

FIG. 1 is a graph showing an example of a powder X-ray diffraction profile of a catalyst. In FIG. 1, the horizontal axis represents the diffraction angle 2θ, and the vertical axis represents the diffraction intensity.
As shown in FIG. 1, in the catalyst C1, a peak corresponding to a simple substance of Pd was confirmed together with a peak corresponding to an alloy of Pd and Fe. On the other hand, in the catalyst C2, although a peak corresponding to the alloy of Pd and Fe was observed, a peak corresponding to the simple substance of Pd was not confirmed.

<Scanning electron microscope (SEM) observation>
Each of the catalysts C1 and C2 after the durability test was observed using a field emission SEM (FE-SEM). The results are shown in FIGS.

  FIG. 2 is an SEM photograph of the catalyst according to the example. FIG. 3 is an SEM photograph of a catalyst according to a comparative example.

  As shown in FIG. 2, in the catalyst C1, Pd particles having a particle size of 100 nm or less were observed. On the other hand, as shown in FIG. 3, in the catalyst C2, particles made of an alloy of Pd and Fe having a particle size larger than 100 nm were observed.

Claims (10)

The first composite oxide represented by the general formula A _i O _j · p (D _{2 -qr} E _q G _r O ₃ ) and the second composite oxide represented by the general formula (Al _{2 -xy} J _x L _y ) O ₃ At least one of the composite oxides;
Alkaline earth metal selected from the group consisting of simple alkaline earth metals, alkaline earth metal salts, organometallic compounds containing alkaline earth metals, simple alkaline earth metal oxides, and mixtures thereof, or the like A compound,
Element A is at least one element selected from the group consisting of monovalent elements, divalent elements, and rare earth elements, and element D is a combination of Al or Al and a transition metal element excluding a platinum group element. The element E is a transition metal element excluding a platinum group element; the element G is a platinum group element; the element J is a transition metal element excluding the platinum group element; the element L is a platinum group element; j is a combination of the smallest natural numbers satisfying ki = 2j when the valence of the element A in the oxide represented by the general formula A _i O _j is k, p is a positive number, and q Satisfies 0 <q <2, r satisfies 0 <r <2, x satisfies 0 <x <2, and y satisfies 0 <y <2. .   The exhaust gas purifying catalyst according to claim 1, wherein the alkaline earth metal element contained in the alkaline earth metal or a compound thereof is Ba.   The exhaust gas-purifying catalyst according to claim 1 or 2, wherein the element E is Fe.   The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the element G is Pd.   The exhaust gas purifying catalyst according to any one of claims 1 to 4, wherein the element D is Al.   The exhaust gas purifying catalyst according to any one of claims 1 to 5, wherein the element A is Mg.   The exhaust gas purifying catalyst according to any one of claims 1 to 6, wherein i = j = p = 1, and the first composite oxide has a spinel crystal structure. In a high temperature reducing atmosphere, at least a part of the element E and at least a part of the element G are precipitated from the first composite oxide,
In a high temperature oxidizing atmosphere, at least a part of the element E deposited in the high temperature reducing atmosphere and at least a part of the alkaline earth metal element contained in the alkaline earth metal or compound thereof form a third composite oxide. The exhaust gas purifying catalyst according to any one of claims 1 to 7. 9. The element according to claim 8, wherein the element E is Fe, the alkaline earth metal element contained in the alkaline earth metal or a compound thereof is Ba, and the third composite oxide contains BaFe ₁₂ O ₁₉ . Exhaust gas purification catalyst.   The exhaust gas purification device according to any one of claims 1 to 9, wherein the exhaust gas purification device is manufactured by a method including impregnating at least one of the first and second composite oxides with a solution of an alkaline earth metal salt. catalyst.

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