JP2017023999A - Catalyst for exhaust purification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To propose a catalyst for exhaust purification using an apatite compound, capable of effectively inhibiting the sintering of metal catalyst particles.SOLUTION: The present invention provides a catalyst comprising an apatite compound represented by formula (1): A(B'O)B"X(1) (A is at least one metal element selected from rare earth, Sr, Ba and Ca; B' is Si or P, or both of them; B" is a metal element; X is O or OH, or both of them; when a valence of B" is M and a valence of X is N, -2≤M×a+N×b<0; 0<a≤2; 0<b≤2)SELECTED DRAWING: None

Description

  The present invention relates to an exhaust gas purification catalyst that can be used for purifying exhaust gas discharged from an internal combustion engine, and more particularly to an exhaust gas purification catalyst containing an apatite compound.

  The exhaust gas of internal combustion engines such as automobiles that use gasoline as fuel contains harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). It is necessary to purify the components using an oxidation-reduction reaction and exhaust the components. For example, hydrocarbons (HC) are oxidized and converted to water and carbon dioxide, carbon monoxide (CO) is oxidized and converted to carbon dioxide, and nitrogen oxides (NOx) are reduced and converted to nitrogen for purification. There is a need to.

  As a catalyst for treating exhaust gas from such an internal combustion engine (hereinafter referred to as “exhaust gas purification catalyst”), a three-way catalyst (Three way catalysts: TWC) capable of oxidizing and reducing CO, HC and NOx. Is used.

  As this type of three-way catalyst, for example, a noble metal is supported on a refractory oxide porous body such as an alumina porous body having a high surface area, and this is made of a base material such as a refractory ceramic or a metal honeycomb structure. It is known that it is supported on a monolith type substrate or supported on refractory particles.

  At this time, since the bonding force between the noble metal as the catalytic active component and the substrate is not so strong, it is difficult to ensure a sufficient loading amount even if the noble metal is directly supported on the substrate. Therefore, in order to support a sufficient amount of the catalytically active component on the surface of the substrate, a noble metal is supported on a catalyst carrier having a high specific surface area.

  Conventionally, porous bodies made of refractory inorganic oxides such as silica, alumina and titania compounds are known as catalyst carriers. In recent years, a catalyst carrier made of an apatite-type composite oxide has attracted attention as a catalyst carrier that is excellent in heat resistance and can prevent sintering of the supported metal catalyst particles.

As a catalyst carrier made of an apatite-type composite oxide, for example, Patent Document 1 (Japanese Patent Laid-Open No. 7-24323) discloses a general formula M ₁₀ · (ZO ₄ ) ₆ · X ₂ (a part or all of M is a periodic table. One or more transition metals selected from Group 1B and / or Group 8 of the above, preferably one or more transition metals selected from copper, cobalt, nickel and / or iron, and A catalyst carrier comprising an apatite compound represented by the formula (1) containing 0.5 to 10 wt% of these transition metals, Z represents a 3 to 7 valent cation and X represents a 1 to 3 valent anion. Yes.

Patent Document 2 (Japanese Patent Application Laid-Open No. 2007-144212) discloses (La _a-) as a catalyst that achieves an exhaust gas purification effect even in a relatively low temperature state and achieves a purification performance as a three-way catalyst even in a high temperature range. _{xM x} ) (Si _6-y N _y ) O _27-z and a noble metal component that is solid solution or supported on the composite oxide, has high low-temperature activity, and An exhaust gas purification catalyst that is excellent in heat resistance and can obtain stable exhaust gas purification performance is disclosed.

Patent Document 3 (JP 2011-16124), the general formula _{(A a-w-x M} w M 'x) (Si 6-y N y) O 27-z ( wherein, A is La and Cation of at least one element of Pr, M is a cation of at least one element of Ba, Ca and Sr, M ′ is at least one of Nd, Y, Al, Pr, Ce, Sr, Li and Ca N is a cation of at least one element of Fe, Cu and Al, 6 ≦ a ≦ 10, 0 <w <5, 0 ≦ x <5, 0 <w + x ≦ 5, 0 ≦ y ≦ 3, 0 ≦ z ≦ 3, A ≠ M ′, and when A is a La cation, x ≠ 0) and a solid solution or supported by the composite oxide An exhaust gas purifying catalyst comprising a precious metal component is disclosed.

In Patent Document 4 (Japanese Patent Laid-Open No. 2013-6170), it is represented by the general formula A ₁₀ (PO ₄ ) ₆ (OH) ₂ (wherein A is at least one of Sr, Ba and Ca). An exhaust gas purifying catalyst in which a noble metal component is supported on an apatite compound is disclosed.

Japanese Patent Laid-Open No. 7-24323 JP 2007-144212 A JP 2011-16124 A JP 2013-6170 A

Exhaust gas purification catalyst using apatite compound disclosed in Patent Document 4, that is, general formula A ₁₀ (PO ₄ ) ₆ (OH) ₂ (wherein A is at least one of Sr, Ba and Ca) The exhaust gas purifying catalyst in which a noble metal component is supported on an apatite compound represented by the formula (1) is included in exhaust gas in an internal combustion engine that supplies and burns fuel under periodic rich or lean conditions. It has an excellent feature that it can exhibit high purification performance from low temperature to high temperature for THC and NOx. On the other hand, however, the apatite compound has a problem that when it is used in an environment such as an automobile, the supported metal catalyst particles are easily sintered and deteriorated.

  Accordingly, an object of the present invention relates to an exhaust gas purification catalyst using a predetermined apatite compound, and is to propose a new exhaust gas purification catalyst that hardly deteriorates even when used in an environment such as an automobile.

The present invention relates to formula (1): A ₁₀ (B′O ₄ ) ₆ B ″ _a X _b (wherein A is one selected from the group consisting of rare earth, Sr, Ba and Ca) Or two or more metal elements, B ′ is Si or P, or both, B ″ is a metal element, X is O or OH, or both, and the valence of B ″ is M , When the valence of X is N, it satisfies the relational expression of −2 ≦ M × a + N × b <0, and 0 <a ≦ 2 and 0 <b ≦ 2.) The present invention proposes an exhaust gas purifying catalyst characterized by containing the above.

  The exhaust gas purifying catalyst proposed by the present invention is compared with a catalyst of a type in which a catalytically active species formed by supporting a catalytically active metal on a predetermined apatite compound disclosed in Patent Document 4 is supported on the surface, Even if the apatite compound is repeatedly used in a use environment such as an automobile, if the apatite compound is cooled after being heated at a high temperature, ions of the metal element B ″ (referred to as “B” ions ”) are expressed by the above formula (1). By exhibiting the property of precipitating from the hexagonal channel present in the apatite compound represented, or being dissolved, it is excellent in that the deterioration of the exhaust gas purification performance can be effectively suppressed.

  That is, in the exhaust gas purification catalyst proposed by the present invention, at least in the initial state immediately after production (normal temperature and normal pressure state), the metal element B ″ ion that functions as a catalytically active species is present in the hexagonal channel of the apatite compound, In an environment where the external atmosphere changes such as in an automobile, for example, in an environment where high temperature heating and rapid cooling are repeated, the metal element B ″ ions are repeatedly dissolved and surface precipitated. Thus, since the metal element B ″ ions repeatedly enter and exit the hexagonal channel present in the apatite compound represented by the above formula (1), compared to the type in which the catalytically active species is supported on the surface, The frequency with which the metal element B ″ is placed in an environment where sintering is easy to occur is reduced, and as a result, deterioration of the exhaust gas purification performance can be suppressed.

It is known that sintering of catalytically active species such as Pd is suppressed by solid solution and precipitation in the perovskite oxide structure represented by the general formula ABO ₃ . However, since the solid solution / precipitation of the catalytically active species into the perovskite oxide structure occurs at the B site forming the structural skeleton, the catalytically active species that occupied the B site of the perovskite oxide structure is deposited. The crystal structure was unstable because a part of the structural skeleton collapsed. In contrast, in the exhaust gas purifying catalyst proposed by the present invention, as described above, the metal element B ″ ion that functions as a catalytically active species exists in the apatite compound represented by the above formula (1). When the high temperature heating and cooling are repeated in the channel and the metal element B ″ ions enter and exit the hexagonal channel, the structural skeleton does not collapse and is crystallographically stable. Are better.

It is a measurement result of the high temperature X-ray diffraction of the sample obtained in Example 1, and is an XRD pattern measured at each temperature when it is cooled to 1100 ° C. and then cooled.

  Next, the present invention will be described based on an embodiment. However, the present invention is not limited to the embodiment described below.

<This apatite compound>
The apatite compound (referred to as “the present apatite compound”) according to an example of the embodiment of the present invention has the formula (1): A ₁₀ (B′O ₄ ) ₆ B ″ _a X _b (wherein A is One or more metal elements selected from the group consisting of rare earth, Sr, Ba and Ca, B ′ is Si and / or P, B ″ is a metal element, and X is O or OH, or both. When the valence of B ″ is M and the valence of X is N, the relational expression −2 ≦ M × a + N × b <0 is satisfied, and 0 <a ≦ 2 And 0 <b ≦ 2.).

In the apatite compound represented by the formula (1), a and b are in the above-described relationship. From the viewpoint of charge balance, the metal element B ″ ion is dissolved in the apatite compound, and at least the initial state immediately after the production. Under normal temperature and normal pressure conditions, all B ″ ions are present in the apatite compound represented by the above formula (1), that is, in the hexagonal channel existing in the apatite compound having the crystal structure represented by the above formula (1). And is distinguished from the carried state.
Whether the metal element B ″ ion is present in the hexagonal channel present in the apatite compound represented by the above formula (1) by confirming the a-axis lattice constant obtained from powder X-ray diffraction. Alternatively, it can be distinguished whether it is carried.

In the formula (1), “A” represents an element belonging to a rare earth (ie, La, Sc, Y, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). Any one or more metal elements selected from the group consisting of Sr, Ba and Ca may be used. These metal elements exist in an ionic state in the apatite compound.
In the examples described later, examples in which Ca is used as A are shown. However, in the apatite compound, A ions exist between (B′O ₄ ) units, and the size of the unit cell of the crystal is large. Therefore, if the element has a large ionic radius, such as Ca, the unit cell becomes larger and the channel voids can be considered to be expanded accordingly. It can be considered that the same effect can be obtained.

In the formula (1), “B ′” may be Si, P, or both. These elements exist in an ionic state in the apatite compound.
In the example described later, an example in which P is used as B ′ is shown. However, since (PO ₄ ) ions and (SiO ₄ ) ions have the same ion radius, instead of P or Even if Si is used together with P, the same effect as P can be obtained.

In the formula (1), “B ″” may be a metal element, particularly a metal element that functions as a catalytically active species. For example, one or more metal elements selected from the group consisting of Cu, Co, Mn, Fe and Ni can be mentioned. These metal elements exist in an ionic state in the apatite compound.
In Examples described later, since the sintering suppressing effect could be confirmed even with Cu that is easy to sinter among catalytic active species, other catalytic active species such as Co, Mn, Fe, Ni, etc. As for Cu, it is considered that the same sintering suppression effect as Cu can be obtained.

  In the formula (1), “X” may be O, OH, or both. These exist in the state of oxide ions or hydroxide ions in the apatite compound.

In Formula (1), when the valence of B ″ is M and the valence of X is N, the relational expression −2 ≦ M × a + N × b <0 is satisfied, and 0 <a ≦ 2 and 0 < In particular, from the viewpoint of improving exhaust gas purification performance, a is preferably 0.5 ≦ a ≦ 2, and more preferably 1 ≦ a ≦ 2.
In addition, satisfying the relational expression of −2 ≦ M × a + N × b <0 indicates that the charge balance for the apatite compound is satisfied. Therefore, if it is an apatite compound, this relational expression should be satisfied, and if it deviates from this relational expression, there is a possibility that a different phase or foreign matter other than the apatite compound is generated.

For example, as a preferred example of the present apatite compound, Ca ₁₀ (B′O ₄ ) ₆ Cu _a O _b (wherein B ′ is Si or P or both, 0 <a ≦ 2, 0 <b ≦ 2)).

(Shape / color)
The apatite compound may be in the form of particles, needles, rods, or other shapes.
Further, for example, when Cu is used as the metal element B ″, when the catalytically active species formed by supporting copper on the apatite compound is supported on the surface, the powder color is substantially black, The apatite compound has a characteristic of exhibiting a color such as red, purple, or blue depending on the type of “A”, and can be distinguished from the type in which the catalytically active species is supported on the surface in terms of color.

(Functional features)
In the present apatite compound, the metal element B ″ ions are present in the hexagonal channel existing in the apatite compound represented by the above formula (1) at least under normal temperature and normal pressure conditions in the initial state immediately after production.

  At this time, whether or not the metal element B ″ ion is present in the hexagonal channel present in the apatite compound represented by the above formula (1) is determined from the a-axis lattice constant obtained from powder X-ray diffraction. For example, when Cu is used as B ″, the a-axis lattice constant of the present apatite compound obtained at normal temperature and normal pressure in the initial state immediately after production is set to “L ′”, and Cu ions are contained in the hexagonal channel. If the a-axis lattice constant determined in the normal temperature and normal pressure state of the apatite compound not containing Cu, in other words the Cu-free apatite compound, is “L”, if the metal element Cu ion is present in the hexagonal channel, Depending on the content, the relational expression L ′ ≦ L + 0.01 nm is satisfied. Therefore, if L ′ satisfies this relational expression, it can be confirmed that the metal element Cu ions are present in the hexagonal channel existing in the apatite compound represented by the above formula (1).

In addition, B ″ ions in the apatite compound enter and exit the hexagonal channel due to changes in the external atmosphere. That is, when high-temperature heating and cooling are repeated, the metal element B ″ ions repeatedly enter and exit the hexagonal channel. It will be.
More specifically, when the apatite compound is heated in an air atmosphere, the metal element B ″ ions are precipitated out of the hexagonal channel existing in the apatite compound represented by the above formula (1). When the metal element B ″ ions are rapidly cooled after being heated to around 1100 ° C., the metal element B ″ ions are present in the hexagonal channel. On the other hand, when heated to about 1100 ° C. and then gradually cooled (also referred to as “slow cooling”), the metal element B ″ ions behave out of the hexagonal channel.

  Therefore, for example, when the apatite compound is heated at 1100 ° C. in the air atmosphere and then cooled (rapidly cooled) to room temperature at a temperature decrease rate of, for example, 100 ° C./min, B ″ ions in the apatite compound are represented by the above formula (1). While showing the state existing in the hexagonal channel present in the represented apatite compound, after heating the apatite compound at 1100 ° C. in an air atmosphere, for example, gradually cooling to room temperature at a temperature decrease rate of 10 ° C./min, The B ″ ions will be deposited outside the hexagonal channel.

At this time, as described above, for example, when Cu is used as B ″, the a-axis lattice constant of the apatite compound obtained in the initial state immediately after production at normal temperature and normal pressure is “L ′”, and the Cu ion is When the a-axis lattice constant of the apatite compound that does not exist in the hexagonal channel, in other words, the apatite compound not containing Cu is “L”, the apatite compound is heated at 1100 ° C. in an air atmosphere. Then, when the temperature is gradually cooled to room temperature at a rate of 10 ° C./min, the Cu ions are present outside the hexagonal channel. In this case, the a-axis lattice constant L ₁ approaches L, and L ₁ = L ± 0.0002 nm.
On the other hand, when the apatite compound is heated at 1100 ° C. in an air atmosphere and then rapidly cooled to room temperature at a rate of 100 ° C./min, Cu ions in the apatite compound exist in the hexagonal channel. The lattice constant L ₂ approaches L ′, and L ₂ = L ′ ± 0.0002 nm.

  It should be noted that “high temperature heating” and “rapid cooling” in the present invention are different depending on the metal element A and metal element B ”used, oxygen partial pressure, etc. When Cu is used as the metal element B ″, heating at 500 ° C. or more is called high-temperature heating, and lowering at 50 ° C./min or more is called rapid cooling.

As described above, in the apatite compound, the metal element B ″ ion that functions as a catalytically active species exists in the apatite compound represented by the above formula (1) under normal temperature and normal pressure conditions in the initial state immediately after production. However, when heated to about 500 to 900 ° C. (exhaust gas temperature range during steady running), the metal element B ″ ions are deposited outside the hexagonal channel, so that the catalytic activity is increased. It will increase. When the metal element B ″ ion returns to the inside of the hexagonal channel when the metal element B ″ ion is rapidly cooled after being heated to a higher temperature region and reaching about 1100 ° C., it is placed in an environment where the metal element B ″ ion is easy to sinter. Infrequently, deterioration of exhaust gas purification performance can be suppressed.
Further, in a severe use environment such as in an automobile, that is, a use environment in which high temperature heating and cooling are repeated, the metal element B ″ ion repeatedly enters and exits the hexagonal channel, so that a catalytically active metal is supported on the apatite compound. Compared with the type in which the catalytically active species is supported on the surface, the metal element B ″ is less likely to be placed in an environment that is easy to sinter, and the deterioration of exhaust gas purification performance can be suppressed. Yes.

(Manufacturing method)
An example of a method for producing the apatite compound will be described. However, the manufacturing method demonstrated here is an example to the last, and the manufacturing method of this apatite compound is not limited.

First, an aqueous solution I of a compound containing rare earth, Sr, Ba, or Ca or two or more kinds of these metal ions is prepared.
Under the present circumstances, as a compound containing rare earth, Sr, Ba, Ca, or these 2 or more types, each acetate can be used, for example. When Sr or Ba acetate is used, it is preferable to add an acid such as nitric acid to dissolve it.
On the other hand, an aqueous solution II is prepared by dissolving at least one of a silicic acid compound or a phosphoric acid compound in water.
Examples of the silicate compound include silicates such as silica sol. Moreover, as a phosphoric acid compound, potassium dihydrogen phosphate, sodium dihydrogen phosphate, etc. can be used, for example.

Next, the aqueous solution I and the alkaline aqueous solution are added little by little to the aqueous solution II at the same time, and a precipitate is generated while keeping the mixed aqueous solution alkaline.
At this time, the pH of the mixed aqueous solution is preferably maintained at 13 or more.
As the alkaline aqueous solution to be dropped, for example, an aqueous solution of about 1 to 2 mol / L such as sodium hydroxide or potassium hydroxide can be used.

  The precipitate thus obtained is aged at 40 to 90 ° C. for about 12 to 24 hours, washed with water, filtered, and dried at 30 to 120 ° C. to obtain an apatite compound precursor.

Next, the apatite compound is impregnated in a B ″ solution containing a metal element B ″ that functions as a catalytically active species, and evaporated to dryness to obtain a dried product.
At this time, examples of the B ″ solution containing the metal element B ″ include a nitrate solution and an acetate solution.

Further, the dried product is preferably 900 to 1200 ° C. in an air atmosphere or a nitrogen atmosphere, and particularly preferably 1000 to 1200 ° C. from the viewpoint of dissolving all the metal element B ″ in the channel, and after quenching for 1 to 5 hours, it is rapidly cooled. The apatite compound can be obtained by treatment.
At this time, as the rapid cooling treatment, it is preferable to cool at a cooling rate of 50 to 300 ° C./min, and it is more preferable to cool at a cooling rate of 50 ° C./min or more or 100 ° C./min or less.

<Application>
The apatite compound can be used as it is as a three-way catalyst.

Further, in the present apatite compound, since the amount of the metal element B ″ that can be present in the hexagonal channel is limited, a catalytically active species such as the metal element B ″ is further supported to improve the catalyst performance. You can also.
In this case, examples of the catalytically active species to be supported include Ag, Pt, Pd, and Rh in addition to Cu, Co, Mn, Fe, and Ni mentioned as the metal element B ″.
Moreover, a well-known method can be employ | adopted as the method of making this apatite compound carry | support catalyst active species. For example, the present apatite compound in a powder or slurry state is immersed in a B ″ solution (basic or acidic solution) containing a metal element B ″ or mixed with a B ″ solution containing a metal element B ″ to form a metal element B ″ may be adsorbed on the apatite compound, heated to evaporate to dryness, and then fired. However, the method is not limited to this method.
At this time, the additional supported amount of the catalytically active species is not particularly limited. For example, it is preferably about 0.1 to 20% by mass based on the mass of the apatite compound.

The apatite compound can also be used by mixing with other catalysts, that is, catalyst particles having a structure in which a catalytic component is supported on a carrier component.
In this case, examples of the carrier component include a porous body composed of a compound selected from alumina, silica, silica-alumina, alumino-silicates, alumina-zirconia, alumina-chromia, alumina-ceria, and titania. Can do.
Examples of the catalytically active species include palladium, platinum, rhodium, gold, silver, ruthenium, iridium, nickel, manganese, iron, cerium, cobalt, copper, osmium, and strontium.

<This catalyst>
The apatite compound may be used as a catalyst (referred to as “the present catalyst component”) by mixing with an OSC material and other components, if necessary, and forming into an appropriate shape such as a pellet. It can also be used as an exhaust gas purifying catalyst (referred to as “the present catalyst”) in a form supported on a base material made of ceramics or a metal material.

  Examples of the OSC material, that is, a promoter (OSC material) having oxygen storage capacity (OSC) include cerium oxide, zirconium oxide, ceria-zirconia composite oxide, and the like.

In addition to the above components, the present catalyst component can contain other components such as a stabilizer.
Examples of the stabilizer include alkaline earth metals and alkali metals. Of these, one or more metals selected from the group consisting of magnesium, barium, boron, thorium, hafnium, silicon, calcium, and strontium can be selected. Among these, barium is preferable from the viewpoint that the temperature at which PdOx is reduced is highest, that is, it is difficult to reduce.

Further, the catalyst component may contain a known additive component such as a binder component.
As the binder component, an inorganic binder such as an aqueous solution such as alumina sol can be used.

(Base material)
Examples of the material of the substrate include refractory materials such as ceramics and metal materials.
Examples of the material for the ceramic substrate include refractory ceramic materials such as cordierite, cordierite-alpha alumina, silicon nitride, zircon mullite, alumina-silica magnesia, zircon silicate, sillimanite, magnesium silicate, zircon, Examples include petalite, alpha alumina, and aluminosilicates.

The material of the metal substrate can include refractory metals such as other suitable corrosion resistant alloys based on stainless steel or iron.
Examples of the shape of the substrate include a honeycomb shape, a pellet shape, and a spherical shape.

(Production method of this catalyst)
As an example for producing the present catalyst, the present apatite compound is mixed with a catalytically active species, an OSC material, a stabilizer, a binder component, etc., if necessary, and water is added and mixed and stirred to form a slurry. Examples thereof include a method in which the obtained slurry is applied to a substrate such as a ceramic honeycomb body and fired to form a catalyst layer on the surface of the substrate.
The slurry is applied to a substrate such as a ceramic honeycomb body to form a catalyst carrier layer, which is then immersed in a solution of catalytically active species to adsorb the catalytically active species to the catalyst carrier layer. The method of baking and forming a catalyst layer on the substrate surface can also be mentioned.
In addition, the method for manufacturing this catalyst can employ | adopt all the well-known methods, and is not limited to the said example.

  In any of the production methods, the catalyst layer may be a single layer or a multilayer of two or more layers.

(Three-way catalyst application)
The present catalyst can be used as a three-way catalyst used for purifying exhaust gas discharged from an internal combustion engine such as an automobile. That is, hydrocarbon (HC) is oxidized and converted into water and carbon dioxide, carbon monoxide (CO) is oxidized and converted into carbon dioxide, and nitrogen oxide (NOx) is reduced and converted into nitrogen. Can be purified.

  Thus, when this apatite compound is used as a three-way catalyst mounted on an automobile, for example, the following occurs. That is, when the accelerator is depressed and the engine is rotated at a high speed, the catalyst temperature reaches about 500 ° C. in steady running, and the catalytically active species starts to precipitate from the hexagonal channel, thereby exhibiting exhaust gas purification performance. Further, when the accelerator is stepped on and the catalyst temperature rises to about 1100 ° C. and then rapidly cooled by applying a brake, the catalytically active species will return into the channel again. Depending on the actual use situation of such an automobile, since the catalytically active species always enters and exits the hexagonal channel without precipitating on the surface, sintering of the catalytically active species can be suppressed.

<Explanation of words>
In the present specification, when expressed as “x to y” (x and y are arbitrary numbers), unless otherwise specified, “preferably greater than x” or “preferably y” with the meaning of “x to y”. It also includes the meaning of “smaller”.
Further, when expressed as “more than x” (x is an arbitrary number) or “y or less” (y is an arbitrary number), it is “preferably greater than x” or “preferably less than y”. Includes intentions.

  Hereinafter, the present invention will be described in more detail based on examples and comparative examples.

<Example 1>
First, calcium acetate weighed so as to have a predetermined ratio of the target compound was dissolved in pure water to obtain a transparent aqueous solution I. Next, the above transparent aqueous solution I and 2 mol / L potassium hydroxide aqueous solution were simultaneously added dropwise to a transparent aqueous solution II obtained by dissolving potassium dihydrogen phosphate weighed to a predetermined ratio in pure water. In this precipitation operation, the pH of the aqueous solution was maintained at 13 or higher to obtain a precipitate. The obtained precipitate was aged at 60 ° C. for 24 hours, then washed with water, filtered, and dried at 60 ° C. for 12 hours to obtain an intermediate substance Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ .

Next, an aqueous solution of copper nitrate was impregnated with Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ so as to be Ca ₁₀ (PO ₄ ) ₆ Cu _0.6 O _1.3, and then evaporated to dryness to dry. Solid material was obtained. Further, the dried product was calcined at 1100 ° C. and then rapidly cooled to obtain an apatite compound, that is, an exhaust gas purifying catalyst (sample).
The color of the catalyst powder was reddish brown.

<Comparative Example 1>
Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ obtained as an intermediate substance in Example 1 was impregnated with an aqueous copper nitrate solution so that the Cu loading amount was 3.7 wt% (corresponding to the Cu solid solution amount in Example 1). Then, it was evaporated to dryness and baked at 600 ° C. to obtain an exhaust gas purifying catalyst (sample). In addition, the color of the catalyst powder was black.

<High temperature X-ray diffraction>
The exhaust gas purification catalyst (sample) obtained in Example 1 was measured for high temperature X-ray diffraction, and the solid solution and precipitation behavior of Cu was evaluated (FIG. 1). The measurement conditions are as follows: air temperature is increased / decreased at 20 ° C./min between each temperature, held for 5 min after reaching the measurement temperature, and the diffraction line measurement range is 10-60 ° at a scan rate of 10 ° / min. did.

From FIG. 1, when the exhaust gas purification catalyst (sample) obtained in Example 1 was heated, CuO deposition was confirmed at an ultimate temperature of 700 ° C. or 900 ° C. From the results of thermal analysis conducted separately, an increase in weight was observed at 500 to 900 ° C., so it is estimated that CuO precipitation is accompanied by oxidation of Cu ions, and CuO is considered to precipitate at this temperature range. Can do.
Further, when the temperature reached by heating reaches 1100 ° C., the CuO diffraction line disappears, and when it is gradually cooled thereafter, the CuO diffraction line appears again. Therefore, when it is gradually cooled after heating at high temperature, Cu is precipitated again from within the hexagonal channel. I understood that.

<Examination of solid solution / precipitation behavior of catalytically active species>
Each exhaust gas purifying catalyst (sample) obtained in Example 1 and Comparative Example 1 was subjected to powder X-ray diffraction at room temperature and normal pressure to analyze a diffraction pattern, and a lattice constant in the a-axis direction was calculated ("Table 1" Example 1 "" Comparative Example 1 "). The intermediate material obtained in Example 1 was similarly subjected to powder X-ray diffraction at room temperature and normal pressure, and the diffraction pattern was analyzed to calculate the lattice constant in the a-axis direction (“Intermediate material” in Table 1).
Further, each exhaust gas purification catalyst (sample) obtained in Example 1 was heated at 1100 ° C. in an air atmosphere, and then cooled to room temperature at a temperature decrease rate of 10 ° C./min. The pattern was analyzed, and the lattice constant in the a-axis direction was calculated (“Example 1 + heated slow cooling” in Table 1).
Further, each exhaust gas purification catalyst (sample) obtained in Example 1 was heated at 1100 ° C. in an air atmosphere, and then cooled to room temperature at a temperature decrease rate of 100 ° C./min. The pattern was analyzed, and the lattice constant in the a-axis direction was calculated (“Example 1 + Heating rapid cooling” in Table 1).
In each diffraction pattern, the lattice constant in the a-axis direction was calculated from the strongest line of the apatite compound.

From the above examples and the results of tests conducted by the inventors so far, the exhaust gas purifying catalyst proposed by the present invention is a metal element that functions as a catalytically active species at least under normal temperature and normal pressure conditions in the initial state immediately after production. It was confirmed that B ″ ions were present in the hexagonal channel of the apatite compound.
Next, when heating was performed from the initial state, it was found that the metal element B ″ ions were precipitated from the hexagonal channel in the vicinity of 500 to 900 ° C., considering the result of the thermal analysis performed separately.
After further heating to near 1100 ° C., for example, when rapidly cooled to room temperature at a rate of 100 ° C./min, the metal element B ″ ions remain in the hexagonal channel of the apatite compound, while after heating to near 1100 ° C. For example, when it was gradually cooled to room temperature at a temperature lowering rate of 10 to 20 ° C./min, it was found that the metal element B ″ ions behave out of the hexagonal channel.

  As described above, in the exhaust gas purifying catalyst proposed by the present invention, in the initial state, B ″ ions exist in the hexagonal channel, but the hexagonal channel enters and exits due to changes in the external atmosphere. In a severe use environment, that is, a use environment in which high-temperature heating and rapid cooling are repeated, the metal element B ″ ion repeats solid solution and surface precipitation. In other words, since it repeatedly enters and exits the hexagonal channel, the metal element B ″ is less frequently placed in the sintering generation environment than the type in which the catalytically active species is supported on the surface, and therefore the exhaust gas purification performance is deteriorated. Can be suppressed.

<Exhaust gas purification performance test>
Each exhaust gas purifying catalyst (sample) obtained in Example 1 and Comparative Example 1 was pelletized and pulverized, and each was sieved to collect a portion of 355 to 600 μm. Thereafter, 0.2 g of each was put into a reactor of a fixed bed flow reactor (detector: VMS-1000F manufactured by Shimadzu Corporation and PG-240 manufactured by Horiba Manufacturing Co., Ltd.), and model gas having the composition shown in Table 2 was added. The exhaust gas purification performance under a temperature condition of 100 to 700 ° C. was evaluated with respect to each of NO and HC (first purification ability) by circulating at 1.4 L / min.
Then, after heating and holding at 700 ° C. for 10 minutes in the atmosphere, the temperature was lowered to room temperature at 50 ° C./min, and a model gas having the composition shown in Table 2 was further circulated at 1.4 L / min in the same manner. The exhaust gas purification performance under a temperature condition of 700 ° C. was evaluated for each of NO and HC (second exhaust purification ability).
As described above, the purification capacity maintenance rate (second purification capacity / first purification capacity) at 500 ° C. when the purification performance evaluation was repeated twice is shown in Table 3 for NO and HC, respectively.

(Discussion)
It can be seen that the deterioration of the exhaust gas purification performance can also be effectively suppressed in an environment where high-temperature heating and rapid cooling, which are actual usage environments of automobiles, are repeated. The above test confirmed the purification capacity maintenance rate at 500 ° C. at which the exhaust gas flow rate increased, and it was shown that the exhaust gas purification performance of Example 1 is maintained more than that of Comparative Example 1.

<Examples 2-3>
In Example 1, an aqueous copper nitrate solution was impregnated so that Ca ₁₀ (PO ₄ ) ₆ Cu _a O _b (a = 0.2 to 1.0, b = 1.1 to 1.5) shown in Table 4 was obtained. Exhaust gas purifying catalyst (sample) was obtained in the same manner as in Example 1 except that. The color of any catalyst powder was reddish brown, and the color density was improved according to the Cu density.

<C ₃ H ₆ -O ₂ reaction (Light-off test)>
As a pretreatment for the C ₃ H ₆ oxidation activity evaluation test, a gas of 1.5% O ₂ / He (600 ° C.) was used for 0.1 g of each exhaust gas purifying catalyst (sample) of Examples 1 to 3. Gas pre-treatment was carried out at a gas flow rate of 500 cm ³ / min for 10 min.

Next, the exhaust gas purification catalysts (samples) of Examples 1 to 3 were evaluated for purification performance with simulated exhaust gas using a fixed bed flow type reactor. That is, 0.1 g of each exhaust gas purification catalyst (sample) was set in the reaction tube, and quartz wool was packed before and after each exhaust gas purification catalyst (sample).
Then, after the pretreatment, a simulated exhaust gas having a composition of C ₃ H ₆ 1500 ppm, O ₂ 9000 ppm and the balance He is introduced into the reaction tube at a total flow rate of 500 cm ³ / min, and 10 ° C. from 100 ° C. to 600 ° C. The temperature was continuously raised at / min, and the exhaust gas at the outlet of the reaction tube was analyzed using a quadrupole mass spectrometer to determine the component composition in the reaction gas. The results are shown in Table 4. In addition, the purification rate of HC showed the value in the measurement temperature of 500 degreeC.

(Discussion)
From the above examples and the results of tests conducted by the inventors so far, 0.5 ≦ a ≦ 2 with respect to the apatite compound represented by the formula (1): A ₁₀ (B′O ₄ ) ₆ B ″ _a X _b It is preferable that 1 ≦ a ≦ 2 is more preferable.

In the apatite compound represented by the formula (1): A ₁₀ (B′O ₄ ) ₆ B ″ _{a Xb} , the above examples are cases where A is Ca, but the A ion in the apatite compound is In order to determine the size of the unit cell of the crystal that exists between the (B′O ₄ ) units, an element having a large ionic radius such as Ca has a large unit cell, and the channel voids correspondingly. Therefore, even if rare earth, Sr, Ba, or Ca is used instead of Ca, it can be considered that the same effect as in the embodiment can be obtained.
In the above embodiment, P is used as B ′. However, since both (PO ₄ ) ions and (SiO ₄ ) ions have the same size, Si is used instead of or together with P. Even in this case, the same effect as in the embodiment can be expected.

Further, in the above-described example, the sintering suppression effect was examined using Cu which is easily sintered among catalytic active species as B ″. Other catalytic active species such as Co, Mn, Fe and Ni Even if considering the valence, ion radius, etc., it is possible to enter and exit the hexagonal channel of the apatite compound as well as Cu, and the sintering suppressing effect can be obtained even with Cu that is easy to sinter. Even when a catalytically active species is used, it can be inferred that a sintering suppression effect similar to or higher than Cu can be obtained.

Claims (12)

Formula (1): A ₁₀ (B′O ₄ ) ₆ B ″ _a X _b (In Formula (1), A is one or more selected from the group consisting of rare earth, Sr, Ba, and Ca) B ′ is Si or P, or both, B ″ is a metal element, X is O or OH, or both. The valence of B ″ is the valence of M or X. When the number is N, it satisfies the relational expression of −2 ≦ M × a + N × b <0, and 0 <a ≦ 2 and 0 <b ≦ 2. An exhaust gas purifying catalyst characterized by.   2. The exhaust gas purifying catalyst according to claim 1, wherein B ″ in the formula (1) is one or more metal elements selected from the group consisting of Cu, Co, Mn, Fe and Ni.   The catalyst for exhaust gas purification according to claim 1 or 2, wherein B "ions in the apatite compound are present in a hexagonal channel existing in the apatite compound represented by the formula (1).   The B ″ ion in the apatite compound enters and exits a hexagonal channel existing in the apatite compound represented by the above formula (1) due to a change in the external atmosphere. The catalyst for exhaust gas purification as described.   5. The exhaust according to claim 4, wherein B ″ ions in the apatite compound are precipitated from a hexagonal channel existing in the apatite compound represented by the formula (1) when heated in an air atmosphere. Gas purification catalyst.   When the exhaust gas purifying catalyst is heated at 1100 ° C. in an air atmosphere and then cooled to room temperature at a temperature decreasing rate of 100 ° C./min, the B ″ ion in the apatite compound is converted to the apatite compound represented by the above formula (1). The exhaust gas purifying catalyst according to claim 4, wherein the exhaust gas purifying catalyst is present in a hexagonal channel existing therein. Ca ₁₀ (B′O ₄ ) ₆ Cu _a O _b (wherein B ′ is Si or P, or both, 0 <a ≦ 2 and 0 <b ≦ 2). An exhaust gas purifying catalyst comprising an apatite compound.   The exhaust gas purifying catalyst according to claim 7, wherein Cu ions in the apatite compound exist in a hexagonal channel existing in the apatite compound represented by the formula (1).   The exhaust gas according to claim 7 or 8, wherein Cu ions in the apatite compound enter and exit the hexagonal channel existing in the apatite compound represented by the formula (1) by a change in an external atmosphere. Purification catalyst.   The exhaust gas according to claim 9, wherein Cu ions in the apatite compound are precipitated from a hexagonal channel existing in the apatite compound represented by the formula (1) when heated in an air atmosphere. Purification catalyst.   When the exhaust gas purifying catalyst is heated at 1100 ° C. in an air atmosphere and then cooled to room temperature at a temperature decreasing rate of 100 ° C./min, Cu ions in the apatite compound are contained in the apatite compound represented by the above formula (1). The exhaust gas purifying catalyst according to claim 9, wherein the exhaust gas purifying catalyst is present in a hexagonal channel existing in the exhaust gas.   The exhaust gas purifying catalyst according to any one of claims 1 to 11, further comprising a catalytically active species supported on the apatite compound.

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