JP2009208039A - Catalyst for removing particulate matter and method for removing particulate matter by using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for removing particulate matter, which has excellent performance of oxidizing particulate matter and satisfactorily high-level performance of removing particulate matter and can be used for satisfactorily removing the particulate matter contained in exhaust gas. <P>SOLUTION: The catalyst for removing particulate matter, which is used for oxidizing/removing the particulate matter contained in the exhaust gas from an internal-combustion engine, is provided with: a carrier consisting of a compound oxide of iron and molybdenum; and at least one noble metal which is deposited on the carrier and selected from the group consisting of silver, gold, palladium, platinum, rhodium, iridium and ruthenium. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a particulate matter purification catalyst and a particulate matter purification method using the same.

  The exhaust gas discharged from the internal combustion engine contains particulate matter (PM) including soot and other carbon particulate matter generated by combustion. Such particulate matter is known as an air pollutant that adversely affects animals and plants. Therefore, various particulate matter purification catalysts have been used to reduce particulate matter from exhaust gas.

As a catalyst that can be used as such a particulate matter purification catalyst, for example, in JP-A-2004-42021 (Patent Document 1), it is stabilized with silver (Ag) and / or cobalt (Co). In addition, a catalyst composed of ceria (CeO ₂ ) is disclosed. In JP-A-10-151348 (Patent Document 2), at least one metal selected from a support made of at least one of cerium oxide and zirconium oxide and copper, iron and manganese supported on the support. And a catalyst component comprising an oxide of the above. Furthermore, in Japanese Patent Application Laid-Open No. 2007-216099 (Patent Document 3), one metal element A selected from the group consisting of Sc, Y, Ho, Er, Tm, Yb, and Lu, and a metal element consisting of Mn. A catalyst made of a composite oxide containing B, a catalyst in which a group VIII element is doped in the crystal as a constituent element of the composite oxide, and the like are disclosed. However, the conventional catalysts as described in Patent Documents 1 to 3 above are not always sufficient in the particulate matter purification performance.
JP 2004-42021 A JP-A-10-151348 JP 2007-216099 A

  The present invention has been made in view of the above-described problems of the prior art, and has excellent particulate matter oxidation performance, sufficiently high particulate matter purification performance, and sufficient particulate matter contained in exhaust gas. It is an object of the present invention to provide a particulate matter purification catalyst that can be purified and a particulate matter purification method using the particulate matter purification catalyst.

  As a result of intensive studies to achieve the above object, the present inventors have found that a support made of a composite oxide of iron and molybdenum, silver, gold, palladium, platinum, rhodium, iridium and By providing at least one kind of noble metal selected from the group consisting of ruthenium, it has excellent oxidation performance of particulate matter, has sufficiently high particulate matter purification performance, and is sufficient for particulate matter contained in exhaust gas. The inventors have found that a particulate matter purifying catalyst that can be purified is obtained, and have completed the present invention.

That is, the particulate matter purification catalyst of the present invention is a particulate matter purification catalyst for oxidizing and purifying particulate matter contained in exhaust gas from an internal combustion engine,
A support comprising a composite oxide of iron and molybdenum, and at least one noble metal selected from the group consisting of silver, gold, palladium, platinum, rhodium, iridium and ruthenium supported on the support. To do.

The composite oxide according to the present invention has a composition formula: FeMo _x O _y (wherein x represents a numerical value in the range of 0.5 to 1.0, and y represents a range of 2.5 to 4.0. A composite oxide represented by the following formula is more preferable, and a composite oxide represented by the composition formula: FeMoO ₄ is particularly preferable.

  In the particulate matter purification catalyst of the present invention, it is preferable that the amount of the noble metal supported is in the range of 1 to 20% by mass with respect to the total amount of the catalyst.

  Further, the particulate matter purification method of the present invention is a method characterized by contacting exhaust gas with the particulate matter purification catalyst of the present invention and oxidizing and purifying the particulate matter contained in the exhaust gas. is there.

The reason why the above object is achieved by the particulate matter purification catalyst and the particulate matter purification method of the present invention is not necessarily clear, but the present inventors speculate as follows. That is, an oxide of molybdenum (for example, MoO ₃ ) is relatively easy to cause a valence change in a temperature rising process under a lean atmosphere from a thermodynamic equilibrium concentration calculation. For this reason, it is presumed that such molybdenum oxide has a relatively high oxygen release performance in the temperature rising process under a lean atmosphere. On the other hand, ceria used as a carrier in a catalyst or the like containing conventional silver and ceria hardly undergoes a change in valence under a lean atmosphere and does not provide sufficient oxygen releasing properties. Therefore, when ceria is used as a carrier, it is presumed that sufficient particulate matter purification performance is not necessarily obtained in the temperature rising process under a lean atmosphere. On the other hand, molybdenum oxide is difficult to use as a catalyst material because it easily sublimes under normal use conditions when used for purification of exhaust gas. Therefore, in the present invention, molybdenum oxide is used as a composite oxide of iron (Fe) and molybdenum (Mo) for the catalyst carrier. By using such a composite oxide, the thermal stability of the molybdenum oxide can be improved, so that the molybdenum oxide can be suitably used as the carrier for the particulate matter purification catalyst. When such a composite oxide support is used as a catalyst, a sufficient amount of oxygen derived from the molybdenum oxide can be released from the support in the temperature rising process under a lean atmosphere. . Further, the oxygen released in this way is sufficiently activated by the noble metal supported on the carrier. Therefore, in the particulate matter purification catalyst of the present invention, the particulate matter existing on or near the surface of the carrier can be efficiently oxidized, and the particulate matter contained in the exhaust gas can be sufficiently purified. It is assumed that it will be possible. In addition, since such a support | carrier consists of complex oxide of iron and molybdenum, according to the catalyst for particulate matter purification of this invention, it is guessed that reduction of an environmental load will also be aimed at in-vehicle.

  ADVANTAGE OF THE INVENTION According to this invention, the particulate matter purification catalyst which is excellent in the oxidation performance of particulate matter, has sufficiently high particulate matter purification performance, and can sufficiently purify particulate matter contained in exhaust gas In addition, it is possible to provide a particulate matter purification method using the particulate matter purification catalyst.

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

First, the particulate matter purification catalyst of the present invention will be described. That is, the particulate matter purification catalyst of the present invention is a particulate matter purification catalyst for oxidizing and purifying particulate matter contained in exhaust gas from an internal combustion engine,
A support comprising a composite oxide of iron and molybdenum, and at least one noble metal selected from the group consisting of silver, gold, palladium, platinum, rhodium, iridium and ruthenium supported on the support. To do.

  Such a carrier is made of a complex oxide of iron and molybdenum. By using such a carrier, it is possible to obtain a sufficient particulate matter purification performance in a temperature rising process under a lean atmosphere and to reduce the environmental load. Such a complex oxide is not particularly limited, and includes various forms such as an amorphous structure and a physical mixture.

As such a complex oxide of iron and molybdenum, composition formula: FeMo _x O _y (wherein x represents a numerical value in the range of 0.5 to 1.0, and y is 2.5 to 4.0). Is more preferable. The composition ratio of molybdenum in such a composition formula, that is, the value of x is more preferably 0.75 to 1.0. If the value of x is less than the lower limit, the effect of molybdenum tends to be difficult to develop. On the other hand, if it exceeds the upper limit, molybdenum tends to sublime. In addition, the composition ratio of oxygen in such a composition formula, that is, the value of y is more preferably 3.25 to 4.0. When the value of y is less than the lower limit, the stability as a composite oxide tends to be poor, and on the other hand, the stability as a composite oxide tends to be poor even when the upper limit is exceeded. Among these composite oxides, a composite oxide represented by the composition formula: FeMoO ₄ is particularly preferable from the viewpoint that higher stability can be obtained under high temperature conditions.

  Moreover, such a complex oxide is preferably a particulate powder because it has a large surface area per unit mass. Although it does not restrict | limit especially as an average particle diameter of such a particulate complex oxide, It is preferable that it is 0.1-100 micrometers, and it is more preferable that it is 1-10 micrometers. When the average particle size is less than the lower limit, the support tends to be easily sintered under high temperature conditions. On the other hand, when the upper limit is exceeded, the specific surface area tends to be small and the catalytic activity tends to decrease.

Further, although not particularly limited as specific surface area of the carrier is preferably 1~200m ² / g, and more preferably 10 to 100 m ² / g. When the specific surface area exceeds the upper limit, the support tends to sinter and the heat resistance of the resulting catalyst tends to be reduced. On the other hand, when the specific surface area is less than the lower limit, the dispersibility of the noble metal tends to decrease. Such a specific surface area can be calculated as a BET specific surface area from the adsorption isotherm using the BET isotherm adsorption equation.

  Moreover, the manufacturing method in particular of such complex oxide is not restrict | limited, The well-known method which can manufacture the said complex oxide can be employ | adopted suitably. Moreover, as such a complex oxide, a commercially available one may be used.

  In the present invention, the carrier is selected from the group consisting of silver (Ag), gold (Au), palladium (Pd), platinum (Pt), rhodium (Rh), iridium (Ir) and ruthenium (Ru). At least one kind of noble metal is supported. Such a noble metal makes it possible to obtain sufficiently high particulate matter purification performance. Moreover, as such a noble metal, it is preferable to use silver, platinum, rhodium, and palladium, and it is especially preferable to use silver and platinum from a viewpoint that the higher oxidation performance with respect to a particulate matter is obtained. These noble metals may be used alone or in combination of two or more.

  The amount of the noble metal supported is not particularly limited, but is preferably in the range of 1 to 20% by mass, based on the total amount of the particulate matter purification catalyst, and in the range of 2 to 10% by mass. It is more preferable. If the amount of the noble metal supported is less than the lower limit, the effect of the noble metal tends to be insufficient. On the other hand, if the amount exceeds the upper limit, a problem such as an increase in cost tends to occur. In addition, the method for supporting such a noble metal on the carrier is not particularly limited, and a known method capable of supporting the noble metal on the carrier can be appropriately employed. For example, a salt of a noble metal (for example, nitrate) A method may be employed in which an aqueous solution containing a complex is impregnated into the carrier, followed by drying and baking.

  Moreover, in such a particulate matter purification catalyst, other components that can be used for the particulate matter purification catalyst may be supported on the carrier within a range not impairing the effects of the present invention. Examples of such other components include alkali metals and alkaline earth metals, and alkali metals are preferably used. By supporting an alkali metal on the carrier, the particulate matter purification performance tends to be improved. In addition, it does not restrict | limit especially as a method to carry | support such other components, A well-known method can be employ | adopted suitably.

  The form of the particulate matter purification catalyst of the present invention is not particularly limited, and may be, for example, a honeycomb-shaped monolith catalyst, a pellet-shaped pellet catalyst, or the like. The base material used in such a form is not particularly limited, and a particulate filter base material (DPF base material), a monolithic base material, a pellet base material, a plate-like base material, and the like are preferably used. it can. Further, the material of such a base material is not particularly limited, but a base material made of a ceramic such as cordierite, silicon carbide, mullite, or a base material made of a metal such as stainless steel including chromium and aluminum is preferably used. be able to. Further, the method for supporting the catalyst on such a substrate is not particularly limited, and a known method can be appropriately employed.

  The particulate matter purification catalyst of the present invention has been described above. Hereinafter, the particulate matter purification method of the present invention will be described. That is, the particulate matter purification method of the present invention is a method characterized by contacting the particulate matter purification catalyst of the present invention with exhaust gas to oxidize and purify the particulate matter contained in the exhaust gas. . In such a particulate matter purification method of the present invention, the method for bringing the particulate matter purification catalyst of the present invention into contact with exhaust gas is not particularly limited. For example, the particulate matter purification method of the present invention is disposed in a gas flow path in an exhaust gas pipe. A particulate matter purification catalyst may be disposed, and the particulate matter purification catalyst may be brought into contact with the exhaust gas by supplying exhaust gas discharged from the internal combustion engine into the exhaust gas pipe. Such a particulate matter purification method of the present invention is a method capable of sufficiently purifying the particulate matter because the particulate matter purification catalyst of the present invention is used.

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

Example 1
A particulate matter purifying catalyst (Ag / FeMoO ₄ catalyst) in which silver (Ag) was supported on a carrier composed of a composite oxide represented by composition formula: FeMoO ₄ was produced. That is, first, 28.5 g of FeMoO ₄ powder (manufactured by High-Purity Chemical Laboratory) and AgNO ₃ (manufactured by Wako Pure Chemical Industries, Ltd.) so that the loading ratio of the noble metal (Ag) with respect to the total amount of the catalyst is 5% by mass. 2.36 g and 200 g of ion-exchanged water were mixed to obtain a mixed solution. Next, the mixture was heated at 80 ° C. for 3 hours to evaporate to dryness to obtain a catalyst precursor. Next, the catalyst precursor is dried in air at 110 ° C. for 12 hours and then calcined in air at 750 ° C. for 5 hours to obtain an Ag / FeMoO ₄ catalyst (particulate matter purification catalyst). Obtained. The Ag / FeMoO ₄ catalyst thus obtained was pulverized by compacting to obtain a pellet-shaped catalyst (particle size: 150 to 250 μm).

(Comparative Example 1)
Except that Fe ₂ O ₃ powder (manufactured by Toda Kogyo) was used as a carrier instead of FeMoO ₄ powder, Ag was supported on Fe ₂ O ₃ by using the same method as used in Example 1. A pellet-shaped catalyst composed of an Ag / Fe ₂ O ₃ catalyst (catalyst for particulate matter purification) was obtained.

(Comparative Example 2)
Except for using CeO ₂ powder (manufactured by Daiichi Rare Element Chemical Co., Ltd.) as a carrier instead of FeMoO ₄ powder, Ag was supported on CeO ₂ by using the same method as used in Example 1. A pellet-shaped catalyst made of Ag / CeO ₂ catalyst (particulate matter purification catalyst) was obtained.

(Comparative Example 3)
A YMnO ₃ catalyst composed of an oxide represented by the composition formula: YMnO ₃ was prepared as described below with reference to the description of JP-A-2007-216092. That is, first, Mn _{(NO 3) 2} (Wako Pure Chemical Industries, Ltd.) and _{18.74g, Y (NO 3) 3} · 6H 2 O ( manufactured by Wako Pure Chemical Industries, Ltd.) and _25.00g, NH ₂ CONH 2 (OR 23.52 g (manufactured by Kojun Pharmaceutical Co., Ltd.) was pulverized and mixed to obtain a reaction mixture. Next, the reaction mixture was reacted by heating in air at 300 ° C. for 3 hours to obtain a catalyst precursor. Next, the catalyst precursor was calcined in air at 800 ° C. for 1 hour to obtain a YMnO ₃ catalyst (particulate matter purification catalyst). The YMnO ₃ catalyst thus obtained was pulverized by compacting to give a pellet catalyst (particle size 150-250 μm).

[Performance evaluation of particulate matter purification catalysts obtained in Example 1 and Comparative Examples 1 to 3]
The particulate matter purification catalyst (pellet catalyst) obtained in Example 1 and Comparative Examples 1 to 3 was tested for particulate matter oxidation activity. That is, first, 0.475 g of a pellet-shaped catalyst and 0.025 g of carbon black (manufactured by Tokai Carbon) as a simulated particulate material were added to a cylindrical sample tube bottle. Next, the cylindrical sample tube bottle was rotated for 6 hours, the pellet catalyst and the carbon black were stirred, and carbon black was adhered to the outer surface of the pellet catalyst to obtain a sample. Next, after filling 0.5 g of the obtained sample into a quartz tube having a diameter of 30 mm and a length of 300 mm, O ₂ (10% by volume) / H ₂ O (10% by volume) / N ₂ from the inlet of the quartz tube. A mixed gas consisting of (80% by volume) was supplied. The flow rate of the mixed gas (entry gas) supplied into the quartz tube was 30 L / min with respect to the sample. The temperature of the mixed gas (entry gas) supplied from the inlet of the quartz tube was raised from 200 ° C. to 20 ° C./min at an initial temperature of 200 ° C. Then, while raising the temperature of the mixed gas to 720 ° C. from 200 ° C., it was measured the change in CO ₂ and CO concentrations in outlet gas discharged from the outlet of the quartz tube. A graph showing the thus relation between the temperature of the gas entering the CO ₂ concentration and CO exiting gas was measured using a pellet catalyst obtained in Example 1 and Comparative Examples 1 to 3, respectively Figure 1 (Example 1), FIG. 2 (Comparative Example 1), FIG. 3 (Comparative Example 2), and FIG. 4 (Comparative Example 3).

Further, the oxidation rate of particulate matter (hereinafter referred to as “PM oxidation rate”) was calculated from the results of measuring the concentration of CO ₂ and CO in the outgas. The results are shown in Table 1. Such PM oxidation rate is expressed by the following formula:
[PM oxidation rate] = ([PM oxidation amount] / [PM addition amount]) × 100
It was obtained by calculating. [PM addition amount] in the above formula indicates the amount of carbon black added when the sample is produced. [PM oxidation amount] in the above formula indicates the amount of particulate matter (carbon black) oxidized while the mixed gas is heated from 200 ° C. to 650 ° C. Such PM oxidation amount is calculated from the result of measuring the concentration of CO ₂ and CO in the output gas, and calculating the amount of carbon oxidized while the mixed gas is heated from 200 ° C. to 650 ° C. Determined by The reason why the upper limit value of the mixed gas temperature when calculating the PM oxidation amount is set to 650 ° C. is as follows. That is, when purifying exhaust gas discharged from a diesel engine, a particulate matter purification catalyst and a NOx occlusion reduction type catalyst are often used in combination. Such NOx occlusion reduction type catalysts tend to gradually deteriorate in performance due to sulfur poisoning during use. In order to regenerate the NOx storage reduction catalyst from such sulfur poisoning, it is common to raise the temperature of the catalyst to about 650 ° C. On the other hand, since the particulate matter purification catalyst is generally used by supporting the catalyst on a substrate such as DPF, the particulate matter (PM) tends to be deposited by use and the pressure loss tends to increase. In order to regenerate the particulate matter purification catalyst from such a state in which the pressure loss has increased, it is common to perform a regeneration process (PM regeneration process) in which the catalyst is heated to 600 ° C. or more to burn PM. Is. When the particulate matter purification catalyst is heated to a temperature of 650 ° C. or higher during such a PM regeneration process, the combined NOx storage reduction catalyst tends to be thermally deteriorated. Accordingly, when the PM regeneration process is actually performed, it is necessary to raise the temperature of the catalyst with the upper limit of about 650 ° C. in order to prevent thermal deterioration of the NOx storage reduction catalyst. Therefore, the upper limit value of the temperature of the mixed gas when calculating the PM oxidation amount is set to 650 ° C. in accordance with the temperature condition in which the particulate matter purification catalyst is generally used.

As is clear from the results shown in Table 1, the particulate matter purification catalyst of the present invention (Example 1) was compared with the particulate matter purification catalyst obtained in Comparative Examples 1 to 3, compared with PM oxidation. The rate was high and it was confirmed to have a sufficiently high particulate matter oxidation activity. From these results, it was found that the particulate matter purification catalyst (Example 1) of the present invention has sufficiently high particulate matter purification performance. Also, as is clear from the results shown in Table 1, from the viewpoint of contribution to the PM oxidation rate, the order of the catalyst is:
Ag / FeMoO ₄ (Example 1)> Ag / CeO ₂ (Comparative Example 2)> Ag / Fe ₂ O ₃ (Comparative Example 1)> YMnO ₃ (Comparative Example 3)
It is. From these results, in the particulate matter purification catalyst of the present invention (Example 1), the carrier (CeO ₂ , Fe) used in the conventional catalyst is used depending on the carrier used (complex oxide of Fe and Mo). ₂ O ₃ and YMnO ₃ etc.) sufficiently high oxygen release performance is obtained, and the oxygen released from the carrier is sufficiently activated by the noble metal supported on the carrier. The present inventors infer that high particulate matter purification performance can be obtained.

  As described above, according to the present invention, the particulate matter is excellent in oxidation performance, has sufficiently high particulate matter purification performance, and can sufficiently purify particulate matter contained in exhaust gas. It is possible to provide a particulate matter purification catalyst and a particulate matter purification method using the particulate matter purification catalyst.

  Therefore, the particulate matter purification catalyst of the present invention is particularly useful as a catalyst for purifying particulate matter contained in exhaust gas from an internal combustion engine such as a diesel engine because of its excellent particulate matter purification performance.

Were measured using the obtained pellets catalyst (catalyst for removing particulate matter) in Example 1 is a graph showing the relationship between the temperature of the gas entering the CO ₂ concentration and CO concentration of the outgoing gas. The resulting catalyst pellets in Comparative Example 1 was measured using the (catalyst for removing particulate matter) is a graph showing the relationship between the temperature of the gas entering the CO ₂ concentration and CO concentration of the outgoing gas. Pellet catalyst obtained in Comparative Example 2 was measured using the (catalyst for removing particulate matter) is a graph showing the relationship between the temperature of the gas entering the CO ₂ concentration and CO concentration of the outgoing gas. Pellet catalyst obtained in Comparative Example 3 were measured using a (catalytic for removing particulate matter) is a graph showing the relationship between the temperature of the gas entering the CO ₂ concentration and CO concentration of the outgoing gas.

Claims (5)

A particulate matter purification catalyst for oxidizing and purifying particulate matter contained in exhaust gas from an internal combustion engine,
A support comprising a composite oxide of iron and molybdenum, and at least one noble metal selected from the group consisting of silver, gold, palladium, platinum, rhodium, iridium and ruthenium supported on the support. To purify particulate matter. The composite oxide has a composition formula: FeMo _x O _y (wherein x represents a numerical value in the range of 0.5 to 1.0, and y represents a numerical value in the range of 2.5 to 4.0). The catalyst for purifying particulate matter according to claim 1, wherein the catalyst is a composite oxide represented by the formula: 3. The particulate matter purification catalyst according to claim 1, wherein the composite oxide is a composite oxide represented by a composition formula: FeMoO ₄ .   4. The particulate matter purifying catalyst according to claim 1, wherein the amount of the noble metal supported is in the range of 1 to 20 mass% with respect to the total amount of the catalyst.   A particulate matter characterized in that exhaust gas is brought into contact with the particulate matter purification catalyst according to any one of claims 1 to 4 to oxidize and purify the particulate matter contained in the exhaust gas. Purification method.

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