JP4768475B2 - Composite oxide and filter for PM combustion catalyst - Google Patents

Description

  The present invention relates to a composite oxide suitable for a catalyst for burning PM (particulate matter) discharged from a diesel engine such as an automobile, and a PM combustion catalyst and a diesel exhaust gas purification filter using the same.

Nitrogen oxides (NO _x ) and PM are particularly problematic with regard to exhaust gas from diesel engines. Among these, PM is fine particles mainly composed of carbon, and a method for trapping PM by installing a particulate filter (DPF) in an exhaust gas flow path is becoming common as a method for removing the particulate matter. The trapped PM is burned intermittently or continuously, and the DPF is regenerated.

In this DPF regeneration process, a method of burning PM by external heating using an electric heater, a burner or the like, an oxidation catalyst is installed on the engine side of the DPF, NO in the exhaust gas is changed to NO ₂ by the oxidation catalyst, and NO ₂ There is a method of burning PM by the oxidizing power of the. However, electric heaters and burners need to be externally applied, which complicates the system. In addition, regarding the oxidation catalyst, the exhaust gas temperature is not so high that the catalytic activity is sufficiently exhibited, and NO necessary for PM combustion is not contained in the exhaust gas unless under certain operating conditions. There's a problem. Under such circumstances, a catalyst system in which a catalyst is supported on a DPF, the PM combustion temperature is lowered by the catalytic action, and combustion is continuously performed at the exhaust gas temperature is desirable.

  Patent Document 1 discloses a material carrying Pt as a catalyst metal. However, since Pt has a low catalytic action for burning PM at the exhaust gas temperature level, it is considered difficult to continuously burn PM at the fuel exhaust gas temperature. Further, since noble metals are used, an increase in cost is inevitable.

Patent Document 2 discloses a catalyst using various elements not containing a platinum group metal. Many of them contain Ag. A composition that does not contain Ag and has an effect on PM combustion exhibits high combustion activity against PM in the presence of NO ₂ , but does not exhibit sufficient combustion activity in an atmosphere where oxygen is the only oxidant. It was greatly influenced by the combustion atmosphere. Since the composition of the exhaust gas is not constant, it is desirable to have a high combustion activity for PM without being affected by the concentration of gas contained in the exhaust gas.

Japanese Patent Laid-Open No. 11-253757 JP 2004-42021 A

  The present applicant has proposed a PM combustion catalyst containing a composite oxide of Ce and a transition metal element in Japanese Patent Application No. 2005-336408 as a catalyst that does not use a noble metal element, thereby significantly reducing the PM combustion temperature. It has been realized. However, in this technology, no special consideration is given to the heat resistance of the PM combustion catalyst. Considering the case where the catalyst temperature suddenly rises due to the heat generated during PM combustion, Development of a catalytic material having heat resistance that can maintain high catalytic activity is awaited.

  The present invention develops and provides composite oxides that have catalytic activity that can burn PM of diesel engine exhaust gas at low temperatures without containing precious metal elements, and that have heat resistance that can withstand the heat generated during PM combustion. It is to try.

The above object is achieved by a composite oxide for PM combustion catalyst which is composed of Ce and Co and oxygen, and the molar ratio of Ce and Co satisfies the following [a].
[A] When the molar ratio of Ce and Co is Ce: Co = (1-x): x, 0 <x ≦ 0.5 is established.
Also provided is a composite oxide for PM combustion catalyst, which is composed of Ce, Co and one or more elements M and oxygen, and the molar ratio of Ce, Co and element M satisfies the following [b].
[B] When the molar ratio of Ce, Co and element M is Ce: Co: M = (1-xy): x: y, 0 <x ≦ 0.5, 0 <y ≦ 0.4 , X + y <1 holds.
However, the element M is an element selected from an element group consisting of a rare earth element other than Ce (Y is also treated as a rare earth element) and an alkaline earth metal element. For example, Y, La, Pr, Nd, Sm, Gd, Tb , Dy, Ba, and Sr.

These composite oxides mainly have a cerium oxide (CeO ₂ ) structure. In the following composition formula (1), 0 <x ≦ 0.5, 0 ≦ y ≦ 0.4, x + y <1. It is a complex oxide having a composition to satisfy.
_{Ce (1-xy) Co x} M y O δ ...... (1)
Here, δ> 0, and typically δ = 2 or a value close thereto, for example, 1 ≦ δ ≦ 3, or 1.3 ≦ δ ≦ 2.5. In this composite oxide, a peak of a cerium oxide structure is recognized by X-ray diffraction, and in addition to a cerium oxide phase, a cobalt oxide phase may be detected from a microscopic analysis means.

The composite oxide exhibits high catalytic activity when the crystallite diameter Dx of the cerium oxide structure is 20 nm or less. Even after being subjected to heat treatment at 800 ° C. for 2 hours in the air, the crystallite diameter Dx has a property of maintaining 20 nm or less. The composite oxide has a BET specific surface area of 5 to 70 m ² / g. Further, the composite oxide has a property capable of converting NO in the atmospheric gas into NO ₂ in a temperature range of 200 to 500 ° C.

  Such a complex oxide is extremely suitable as a catalyst (PM combustion catalyst) for burning PM in exhaust gas from a diesel engine. A catalyst using this composite oxide as a catalyst material is supported on a DPF made of a porous material such as SiC, cordierite, mullite, alumina, etc., so that the collected PM can be effectively used for the heat of exhaust gas. A filter for purifying diesel exhaust that can be burned off is constructed.

Since the composite oxide of the present invention exhibits high catalytic activity from a relatively low temperature, when it is used as a catalyst substance, the PM combustion temperature can be lowered, and the exhaust gas temperature can be effectively used to burn and remove PM. It is possible to construct a diesel exhaust gas purification filter that can be used. Even when the concentration of NO ₂ in the exhaust gas is low, high catalytic activity can be obtained, so that it is possible to cope with fluctuations in the exhaust gas concentration. Moreover, since the composite oxide of the present invention is excellent in heat resistance, high catalytic activity is maintained even when sudden heat generation due to PM combustion occurs. Furthermore, since no precious metal element is required, it can contribute to the reduction of the material cost of DPF.

  The composite oxide of the present invention contains Ce and Co as essential components, and contains the element M as necessary. This composite oxide is mainly composed of an oxide phase having a form in which Co or further an element M is combined with a cerium oxide structure. A typical composition formula is represented as the above-mentioned formula (1).

A part of the cerium atom of the cerium oxide structure is substituted with a cobalt atom. At this time, the apparent valence change of the cation of the complex oxide mainly composed of cerium atoms occurs, and the lattice distortion due to substitution of elements having different ionic radii causes oxygen in the lattice to move out of the lattice. It is considered that the active oxygen necessary for PM combustion is supplied to the PM from a relatively low temperature range, and the PM combustion temperature is lowered. In the case of the conventional mere cerium oxide (CeO ₂ ) that does not contain Co, it is considered that it has a stable structure in which the release of oxygen in the lattice is unlikely to occur, so that high catalytic activity for PM combustion is obtained. It is difficult.

  According to the study by the inventors, a composite oxide having a cerium oxide structure partially substituted with Co, obtained by the manufacturing method described later, has high catalytic activity for PM and is heated to a high temperature such as 800 ° C. Even if it receives a heat history, the catalytic activity is kept high.

  Even with the composite oxide structure added with the element M, the excellent catalytic activity inherent in the composite oxide having a cerium oxide structure partially substituted with Co can be obtained, and the heat resistance tends to be further improved. It is done. Although the mechanism by which the heat resistance is improved by the element M has not been sufficiently elucidated at present, it is presumed that the addition of the element M has an influence on the suppression of the coarsening of the composite oxide particles due to heat.

  As shown in the above [a], x representing the Co molar ratio is preferably in the range of 0 <x ≦ 0.5. When x = 0, that is, when Co is not present, high catalytic activity cannot be obtained as described above. When x is larger than 0.5, Co is likely to exist as cobalt oxide without being complexed with the cerium oxide structure, so that a mixed phase of the cerium oxide structure and cobalt oxide is easily formed. Then, since the cobalt oxide has a low catalytic activity for PM combustion, it is considered that the presence of cobalt oxide reduces the catalytic activity point of the composite oxide and the catalytic activity is lowered. The range of x is more preferably 0.05 ≦ x ≦ 0.5, and more preferably 0.10 ≦ x ≦ 0.45.

  As shown in [b] above, y representing the molar ratio of the element M is preferably in the range of 0 ≦ y ≦ 0.4. Even if y = 0, that is, the element M does not exist, the composite oxide obtained by the manufacturing method described later has relatively good heat resistance, but if the element M is included so that 0 <y ≦ 0.4. Furthermore, the tendency which heat resistance improves is seen. However, if y exceeds 0.4 and the element M content increases, the coarsening of the composite oxide particles is suppressed, but the necking between the particles tends to occur excessively, and the catalytic point is reduced by reducing the active point of PM combustion. It is thought that a decrease in activity is likely to occur. The range of y is more preferably 0 <y ≦ 0.35, and more preferably 0.01 ≦ y ≦ 0.30.

  In addition, according to the research by the inventors, as a point to be noted regarding the combustion start temperature of PM, the crystallite diameter of the composite oxide which is a catalyst substance can be mentioned, and the smaller the crystallite diameter, the higher the PM combustion activity. I understood it. The combustion activity of PM is thought to involve oxygen in and out of the lattice, that is, oxygen on the surface of the lattice. As the crystallite diameter decreases, the ratio of the number of oxygen atoms on the surface of the crystal lattice to the number of oxygen atoms in the lattice increases. Therefore, it is considered that oxygen enters and exits easily. Since the crystallite diameter (X-ray particle diameter Dx) of the composite oxide of the present invention in which Co is contained in the cerium oxide structure is as small as 20 nm or less, oxygen enters and exits actively even in a relatively low temperature range, and PM It is assumed that the combustion temperature is lowered. The composite oxide of the present invention has a property of maintaining a crystallite diameter Dx of 20 nm or less even when subjected to heat treatment at 800 ° C. for 2 hours in the atmosphere.

In order for active oxygen (hereinafter referred to as “lattice oxygen”) coming out of the lattice to reach the PM without delay, it is necessary that the specific surface area of the catalyst powder is appropriate. The specific surface area depends on the pore diameter and the particle diameter. When the specific surface area is large, the pore diameter may be small. In that case, the smooth passage of lattice oxygen is hindered, and there is a possibility that lattice oxygen is not rapidly supplied to PM. Therefore, it is not necessarily advantageous to increase the specific surface area in order to increase the catalytic activity, and there is an appropriate specific surface area range depending on the type of the catalyst substance. As a result of various studies, in the composite oxide in which a part of the cerium oxide structure is substituted with Co, excellent combustion activity with respect to PM can be obtained when the BET specific surface area is in the range of about 5 to 70 m ² / g. It is still more preferable to set it as the range of 10-40 m < ² > / g.

The composite oxide of the present invention can be produced, for example, by a usual coprecipitation method, an organic complex method, a production method using an amorphous precursor, or the like. Hereinafter, each manufacturing method will be described.
[Co-precipitation method]
In the coprecipitation method, a raw salt solution containing salts of elements constituting the composite oxide in a stoichiometric ratio suitable for producing a composite oxide in which the molar ratio of Ce, Co, or further the element M is as described above. The aqueous solution and the neutralizing agent are mixed and coprecipitated, and the obtained coprecipitate is dried and then heat-treated. Although it does not specifically limit as a salt of each element, For example, organic acid salts, such as inorganic salts, such as a sulfate, nitrate, phosphate, and chloride, acetate, oxalate, etc. can be used. Of these, acetates and nitrates can be preferably used. The raw salt aqueous solution can be prepared by adding the salt of each of the above elements to water so as to achieve the desired stoichiometric ratio and stirring.

  Then, this raw salt aqueous solution and a neutralizing agent are mixed and coprecipitated. Although it does not specifically limit as a neutralizing agent, For example, organic bases, such as inorganic bases, such as ammonia, caustic soda, and caustic potash, a triethylamine, and a pyridine can be used. Moreover, a neutralizing agent is mixed so that the pH of the slurry produced | generated after adding the neutralizing agent may be 6-14. By mixing in this way, it is possible to obtain a coprecipitate of a hydroxide of each element having good crystallinity.

  The obtained coprecipitate is washed with water as necessary. For example, the coprecipitate is dried by, for example, vacuum drying or ventilation drying, and then heat-treated at, for example, 500 to 1200 ° C., preferably 550 to 1000 ° C. The composite oxide can be obtained. At this time, the atmosphere during the heat treatment is not particularly limited as long as the above complex oxide is generated. For example, in air, nitrogen, argon, hydrogen, and an atmosphere in which water vapor is combined with them, preferably in air. In addition, an atmosphere in nitrogen and a combination thereof with water vapor can be used.

[Organic complex method]
In the organic complex method, for example, a salt that forms an organic complex such as citric acid, malic acid, sodium ethylenediaminetetraacetate, and the salt of each of the aforementioned elements is added to water so as to achieve the desired stoichiometric ratio, and stirred. Can be prepared.
The raw material aqueous solution is dried to form an organic complex of each element described above, and then calcined and heat-treated to form a composite oxide having a composition in which the molar ratio of Ce, Co, or further element M is as described above. Obtainable.

As the salt of each element, the same salt as in the coprecipitation method can be used. The raw salt aqueous solution is prepared by mixing the raw salt of each element in the desired stoichiometric ratio and dissolving it in water. It can be prepared by mixing with an aqueous solution of the salt that forms. In addition, it is preferable that the mixture ratio of the salt which forms an organic complex is about 1.2-3 mol with respect to 1 mol of complex oxides obtained.
Thereafter, this raw material solution is dried to obtain the aforementioned organic complex. Drying is not particularly limited as long as it is a temperature at which the organic complex is not decomposed. For example, water is rapidly removed at room temperature to about 150 ° C., preferably at room temperature to 110 ° C. Thereby, the above-mentioned organic complex is obtained.

  The obtained organic complex is heat-treated after calcination. Pre-baking may be performed at 250 ° C. or higher, for example, in a vacuum or an inert gas atmosphere. Thereafter, for example, a heat treatment is performed at 500 to 1000 ° C., preferably 550 to 950 ° C., whereby a composite oxide having a desired composition can be obtained. At this time, the atmosphere during the heat treatment is not particularly limited as long as the above complex oxide is generated. For example, in air, nitrogen, argon, hydrogen, and an atmosphere in which water vapor is combined with them, preferably in air. In addition, an atmosphere in nitrogen and a combination thereof with water vapor can be used.

[Production method using amorphous precursor]
In the production method using an amorphous precursor, a powder containing each of the aforementioned elements in a stoichiometric ratio suitable for producing a composite oxide having a composition in which the molar ratio of Ce, Co, or further the element M is as described above. The amorphous precursor material can be obtained by heat treatment at a low temperature.

  Such an amorphous precursor is prepared by preparing a raw salt aqueous solution containing a salt of each of the above elements in a stoichiometric ratio suitable for forming a composite oxide having a target composition, and containing an alkali carbonate or ammonium ion. It can be obtained by reacting with a precipitating agent such as carbonate at a reaction temperature of 60 ° C. or lower and a pH of 6 or higher to produce a precipitation product, and drying the filtrate.

  More specifically, first, an aqueous solution in which water-soluble mineral salts such as nitrates, sulfates, and chlorides of each element are dissolved so as to have a molar ratio of a target composition is prepared. The upper limit of the ion concentration of the constituent element in the liquid for generating the precipitate is determined by the solubility of the salt used, but it is desirable that the crystalline compound of the constituent element does not precipitate. Usually, the total ion concentration of each of the above elements is 0. The range of about 0.01 to 0.60 mol / L is desirable, but in some cases, it may exceed 0.60 mol / L.

  In order to obtain an amorphous precipitate from this liquid, it is preferable to use a precipitating agent composed of a carbonate containing an alkali carbonate or ammonium ion. Examples of such a precipitating agent include sodium carbonate, sodium hydrogen carbonate, ammonium carbonate, Ammonium hydrogen carbonate or the like can be used, and a base such as sodium hydroxide or ammonia can be added as necessary. Further, after forming a precipitate using sodium hydroxide, ammonia or the like, an amorphous suitable for the precursor material for obtaining the composite oxide of the present invention can be obtained by blowing carbon dioxide gas. When obtaining an amorphous precipitate, the pH of the liquid should be controlled in the range of 6-11. In the region where the pH is less than 6, rare earth elements may not form a precipitate, which is inappropriate. On the other hand, in the region where the pH exceeds 11, in the case of the precipitating agent alone, the generated precipitate may not be sufficiently amorphized and a crystalline precipitate such as a hydroxide may be formed. The reaction temperature is preferably 60 ° C. or lower. When the reaction is started at a temperature exceeding 60 ° C., crystalline compound particles of the constituent elements may be formed, which is not preferable because it prevents the precursor material from becoming amorphous.

  The obtained amorphous precursor is washed with water as necessary, dried by vacuum drying or ventilation drying, and then heat treated at, for example, 500 to 1000 ° C., preferably 500 to 800 ° C. The composite oxide can be obtained. At this time, the atmosphere during the heat treatment is not particularly limited as long as the above complex oxide is generated. For example, in air, nitrogen, argon, hydrogen, and an atmosphere in which water vapor is combined, preferably air. Medium, nitrogen, and an atmosphere that combines them with water vapor can be used.

<Production of catalyst material>
The catalyst materials of each Example and Comparative Example were prepared as follows.
[Examples 1-3]
Cerium nitrate and cobalt nitrate were mixed so that the molar ratio of Ce: Co was as shown in Table 1. Water was added to the mixture so that the total molar concentration of Ce and Co in the liquid was 0.2 mol / L to obtain a raw material solution. While stirring this solution, the temperature of the solution was adjusted to 15 ° C., and when the temperature reached 15 ° C., the pH was adjusted to 8 while adding ammonium carbonate as a precipitant. The resulting precipitate was collected by filtration, washed with water, and dried at 125 ° C. The obtained powder is called precursor powder.

Next, the precursor powder was heat-treated at 600 ° C. for 2 hours in an air atmosphere and fired. The obtained powder is called “baked product”. In order to evaluate heat resistance using a part of the fired product, heat treatment was further performed at 800 ° C. for 2 hours in an air atmosphere. This heat treatment is called “heat-resistant treatment”, and the powder after the heat-treatment is called “800 ° C. heat-treated product”.
In this heat-resistant treatment, 3 g of powder is put in an alumina crucible (capacity 50 cc, outer diameter 54 mm, height 43 mm), the crucible is put in a muffle furnace having an in-furnace temperature of 800 ° C. and held for 2 hours, and then the atmosphere outside the furnace It went by the method of quenching in the inside.

[Examples 4 to 8]
Cerium nitrate, cobalt nitrate, and lanthanum nitrate are mixed so that the molar ratio of Ce: Co: La becomes the value shown in Table 1, and the total molar concentration of Ce, Co, and La in the liquid becomes 0.2 mol / L. Thus, it produced on the conditions similar to Examples 1-3 except having added water and having obtained the raw material solution.

Example 9
Cerium nitrate, cobalt nitrate and neodymium nitrate are mixed so that the molar ratio of Ce: Co: Nd becomes the value shown in Table 1, and the total molar concentration of Ce, Co and Nd in the liquid becomes 0.2 mol / L. Thus, it produced on the conditions similar to Examples 1-3 except having added water and having obtained the raw material solution.

Example 10
Cerium nitrate, cobalt nitrate, and praseodymium nitrate are mixed so that the molar ratio of Ce: Co: Pr becomes the value shown in Table 1, and the total molar concentration of Ce, Co, and Pr in the liquid becomes 0.2 mol / L. Thus, it produced on the conditions similar to Examples 1-3 except having added water and having obtained the raw material solution.

Example 11
Cerium nitrate, cobalt nitrate and barium nitrate are mixed so that the molar ratio of Ce: Co: Ba becomes the value shown in Table 1, and the total molar concentration of Ce, Co and Ba in the liquid becomes 0.2 mol / L. Thus, it produced on the conditions similar to Examples 1-3 except having added water and having obtained the raw material solution.

Example 12
Cerium nitrate, cobalt nitrate, lanthanum nitrate and neodymium nitrate are mixed so that the molar ratio of Ce: Co: La: Nd is the value shown in Table 1, and the total molar concentration of Ce, Co, La and Nd in the liquid is It produced on the conditions similar to Examples 1-3 except having added water so that it might be set to 0.2 mol / L, and having obtained the raw material solution.

[Comparative Example 1]
Commercially available γ-alumina (specific surface area 250 m ² / g) was impregnated with Pt using a dinitrodiamine platinum aqueous solution, and then air-dried at 90 ° C. for 12 hours. The impregnated material obtained for fixing Pt was heat-treated at 500 ° C. for 1 hour in an air atmosphere to obtain Pt-supported alumina. This was subjected to a heat treatment at 600 ° C. × 2 h in the air or 800 ° C. × 2 h in the air to obtain a test catalyst material. A product heat treated at 600 ° C. is called a “baked product”, and a product heat treated at 800 ° C. is called an “800 ° C. heat treated product”. The “800 ° C. heat-treated product” had a Pt content in alumina of 3.42% by mass.

[Comparative Example 2]
It was produced under the same conditions as in Examples 1 to 3, except that water was added so that cobalt nitrate was 0.2 mol / L to obtain a raw material solution.

[Comparative Example 3]
It was produced under the same conditions as in Examples 1 to 3, except that water was added so that cerium nitrate was 0.2 mol / L to obtain a raw material solution.

The following experiment was conducted using the catalyst material obtained as described above.
<< X-ray diffraction measurement >>
X-ray diffraction measurement was performed on the “baked product” obtained in each example. The measurement conditions are as follows.
X-ray diffractometer: RINT-2100 manufactured by Rigaku Corporation
・ Measurement range: 2θ = 10-90 °
-Tube: Co tube-Tube voltage: 40 kV
・ Tube current: 30mA
As a result of the measurement, it was confirmed that all of the catalyst materials obtained in the respective examples were complex oxides having a CeO ₂ structure.
FIG. 1 illustrates an X-ray diffraction pattern for the catalyst material of Example 1.

<< Measurement of crystallite diameter Dx >>
The “baked product” and “800 ° C. heat-treated product” obtained in each Example and Comparative Example 3 were subjected to X-ray diffraction measurement in the vicinity of 2θ satisfying the Bragg condition of the (220) plane of the cerium oxide structure. The diameter Dx was measured. The measurement conditions are as follows.
X-ray diffractometer: RINT-2100 manufactured by Rigaku Corporation
Measurement range: 2θ = 53.5-58.5 °
・ Sampling width: 0.02 °
・ Counting time: 5 sec
・ Number of integration: 5 times ・ Tube: Co tube ・ Tube voltage: 40 kV
・ Tube current: 30mA
The Scherrer equation below was used to calculate the crystallite diameter Dx.
t = 0.9λ / B · cos θ
Where t: crystallite diameter (nm)
λ: wavelength of Co Kα ₁ (nm)
B: Half width of peak (°)
θ: Angle (°) that satisfies the Bragg condition of the (220) plane of CeO ₂
The results are shown in Table 1.

<< Measurement of BET specific surface area >>
The “baked product” and “800 ° C. heat-treated product” obtained in each Example and Comparative Example were pulverized in an agate mortar to form a powder, and then the specific surface area was determined by the BET method. The measurement was carried out using 4 Saab US made by Your Sonics.
The results are shown in Table 1.

<< Evaluation of PM combustion start temperature by powder sample >>
For the “baked product” and “800 ° C. heat-treated product” obtained in each Example and Comparative Example, mixed powder with carbon black was prepared, and the PM combustion start temperature was evaluated by determining the carbon black combustion start temperature. Specifically, it was as follows.

A commercially available carbon black (Mitsubishi Chemical, average particle size 2.09 μm) was used as a simulated PM and weighed so that the mass ratio of the powder of the catalyst material to the carbon black was 6: 1. Mixed for 20 minutes to obtain a mixed powder of carbon black and each powder. The mixed powder was subjected to thermogravimetry (TG), and the combustion temperature of carbon black was determined from the weight loss associated with the combustion of carbon black. The evaluation method uses a TG / DTA apparatus (TG / DTA6300 type, manufactured by Seiko Instruments Inc.), and 20 mg of the mixed powder is heated from the normal temperature to 700 ° C. at a temperature increase rate of 10 ° C./min. went. FIG. 2 schematically shows a weight change curve (TG curve). The carbon black combustion start temperature was defined as the temperature at the point where the tangent line before the weight reduction starts and the tangent line at the point where the weight reduction rate (slope) becomes maximum in the TG curve (see FIG. 2).
The results are shown in Table 1.

  From the experimental results shown in Table 1, the catalyst material of each example greatly reduces the carbon black combustion start temperature as compared with Pt catalyst (Comparative Example 1) and cobalt oxide (Comparative Example 2). I understand. In addition, although the cerium oxide (comparative example 3) was comparatively favorable about the carbon black combustion start temperature in this experiment, evaluation by the below-mentioned granular sample is inferior.

As described above, in the composite oxide of Ce and Co, the apparent valence of the cation of the composite oxide mainly containing cerium is obtained by substituting a part of the cerium atom of the cerium oxide structure with a cobalt atom. It is considered that the lattice distortion due to the change or the substitution of elements with different ionic radii makes it easier for oxygen in the lattice to be released out of the lattice, and this oxygen oxidizing power improves the catalytic activity for PM combustion. It is done. When the BET specific surface area exceeds, for example, 70 m ² / g, or the crystallite diameter Dx exceeds 20 nm, the PM combustion start temperature cannot be expected to be significantly reduced as in the catalyst materials obtained in each example.

  Moreover, the catalyst material of Examples 4-12 which added the element M has the outstanding heat resistance similarly to the catalyst material of Examples 1-3 which does not add the element M. In Table 1, the difference ΔT between the carbon black combustion start temperatures of the “800 ° C. heat-treated product” and the “baked product” is displayed. However, by adding the element M, ΔT tends to be reduced. Is considered to contribute to the improvement of heat resistance.

<< Evaluation of PM combustion start temperature by granular sample >>
The “baked products” obtained in Example 2, Example 4 and Comparative Example 3 were each compression-molded at 100 kg / cm ² with a mold press and then pulverized to obtain particles having a particle diameter of 0.5 to 1.0 mm. A granular sample was prepared. Carbon black was added to this granular sample so that it might become 5 mass%, and these were mixed by rotating in a glass bottle.

The granular sample mixed with carbon black is filled in a flow-type fixed bed, and a simulated diesel engine exhaust gas of “500 ppm NO + 10% O ₂ + remaining N ₂ ” is circulated at a space velocity of SV75000 / h, and the temperature rising rate is 10 ° C. / The CO ₂ , NO, and NO ₂ concentrations discharged from the flow-through fixed bed were continuously measured while raising the temperature from room temperature to 800 ° C. in min. For measurement, Nicolet 4700FT-IR manufactured by NICOLET was used.
FIG. 3 illustrates a curve representing changes in the CO ₂ , NO and NO ₂ concentrations discharged for Example 4. The temperature at which the total amount of CO ₂ emitted becomes 10% of the total amount of CO ₂ emitted from the flow-through fixed bed was determined as the carbon black combustion start temperature T ₁₀ here.
The results are shown in Table 2.

As can be seen from Table 2, the composite oxides of Example 2 and Example 4 had a low carbon black combustion start temperature T _{10 as} viewed from the CO ₂ emission, and exhibited high activity. In this case as well, as in the previous evaluation using a TG curve, the apparent cation valence change of the composite oxide or lattice distortion due to element substitution with different ionic radii occurred, so oxygen in the lattice was released out of the lattice. It is considered that the combustion of carbon black occurred from a low temperature range due to the oxidizing power of oxygen.

In addition, as seen in FIG. 3, the composite oxide obtained by the above-described manufacturing method in which a part of the cerium oxide structure is replaced with Co is NO in the simulated diesel exhaust gas in the temperature range of 200 to 500 ° C. It has a structure that can be converted to ₂ , and it is considered that combustion of carbon black occurs using the oxidizing power of NO ₂ . Since NO ₂ produced by this type of complex oxide becomes NO due to the combustion of carbon black, the emission amount of NO ₂ becomes very low at temperatures of 500 ° C. or higher.
In the case of cerium oxide containing no Co (Comparative Example 3), it is considered that NO → NO ₂ conversion does not occur sufficiently in the temperature range of 200 to 500 ° C., and as a result, carbon black combustion starts from the viewpoint of CO ₂ emission. temperature T ₁₀ was higher than that of example. In the example according to the present invention, high activity is obtained regardless of the atmosphere, and in addition, NO in the exhaust gas can be used for combustion of carbon black.

The X-ray-diffraction pattern about complex oxide (before heat-resistant process) obtained in Example 1. FIG. The figure which showed the TG curve typically. The catalytic material of Example 4, a graph illustrating a change in the discharge CO _2, NO and NO ₂ concentration in the carbon black combustion test using simulated diesel exhaust.

Claims (8)

And Ce and Co, having a cerium oxide structure that consists of oxygen, the molar ratio of Ce and Co meets following [a], a PM combustion catalyst crystallite diameter Dx of the cerium oxide structure is 20nm or less Complex oxide.
[A] When the molar ratio of Ce and Co is Ce: Co = (1-x): x, 0 <x ≦ 0.5 is established. Ce, has a Co and one or more of the following elements M, cerium oxide structure that consists of oxygen, Ce, the molar ratio of Co and the element M is less than the following [b], the crystallite in the cerium oxide structure A composite oxide for PM combustion catalyst having a diameter Dx of 20 nm or less .
[B] When the molar ratio of Ce, Co and element M is Ce: Co: M = (1-xy): x: y, 0 <x ≦ 0.5, 0 <y ≦ 0.4 , X + y <1 holds.
However, the element M is an element selected from the group consisting of rare earth elements other than Ce (Y is also treated as a rare earth element) and alkaline earth metal elements.   The composite oxide for PM combustion catalyst according to claim 2, wherein the element M is an element selected from the group consisting of Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ba, and Sr.   The composite oxide for PM combustion catalyst according to any one of claims 1 to 3, which has a property that a crystallite diameter Dx in a cerium oxide structure is maintained at 20 nm or less after being subjected to a heat treatment at 800 ° C for 2 hours in the air. The composite oxide for PM combustion catalyst according to any one of claims 1 to 4 , which has a BET specific surface area of 5 to 70 m ² / g. The composite oxide for PM combustion catalyst according to any one of claims 1 to 5 , which has a property capable of converting NO in the atmospheric gas into NO ₂ in a temperature range of 200 to 500 ° C. A PM combustion catalyst using the composite oxide according to any one of claims 1 to 6 as a catalyst substance. A diesel exhaust gas purification filter using the catalyst according to claim 7 .

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