JP2006130463A - Purification filter, production method for the same, and exhaust gas purification apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a purification filter having an improved collection efficiency and suppressed decrease of pressure loss, a production method therefor, and an exhaust gas purification apparatus. <P>SOLUTION: The purification filter comprises a whisker formed on a raw material substrate made of an alloy or a ceramic containing Mn, Al, Cr, In, Ag, Ga, Sn, Cu, Sc, Ge, Ti, Si, and the like. The raw material substrate is a porous material having an average fine pore diameter larger than the average diameter of the whisker. The raw material substrate is composed by filling a carrier with a powdery granule. The purification filter is produced by adjusting the contents of the elements contained in the raw material substrate and controlling the thickness and length of the whisker to be formed by heating in an inert gas atmosphere in the presence of a slight amount of oxygen. The exhaust gas purification apparatus comprises two or more layers of purification filters; a purification filter comprising a whisker formed on a raw material substrate and having a longer average length and a thicker thickness on the upstream side and a purification filter comprising a whisker formed on a raw material substrate and having a shorter average length and a thinner thickness on the downstream side. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a purification filter, a manufacturing method thereof, and an exhaust gas purification device, and more particularly to a purification filter using fine whiskers, a manufacturing method thereof, and an exhaust gas purification device.

DPF (Diesel Particulate Filter) is an exhaust gas purification filter mainly used to remove particulate matter (DEP) contained in exhaust gas from diesel vehicles. For example, cordierite, silicon carbide, silica-based filter DPFs using fibers, Fe—Cr—Al alloys, SUS, and the like are known.
The DPF using these materials has the following merits and demerits. That is, the cordierite honeycomb has a high collection efficiency, but has a low heat resistance and is easily melted and damaged. The silicon carbide honeycomb has high collection efficiency and high heat resistance, but is easily damaged by thermal expansion and is expensive. The silica-based fiber wrapping has high collection efficiency, but has low heat resistance, high cost, and a small collection area. The cordierite foam has a large collection area, but has a low collection efficiency, a low heat resistance, and is difficult to regenerate. The Fe—Cr—Al metal foam has a large collection area, but has low collection efficiency, low heat resistance, and is difficult to regenerate. The SUS mesh is excellent in earthquake resistance, but has low collection efficiency, is difficult to regenerate, and is expensive.

The particulate matter (DEP) is composed of solid carbon particles, hydrocarbons, water, acid, base, alcohol, CO, CO ₂ , NOx, SOx, and other organic substances, and as shown in FIG. It has a particle size distribution in the range of 10 nm. Moreover, as can be seen from the mass distribution, there are peaks in the ranges of 0.01 to 0.1 μm, 0.1 to 1 μm, and 1 to 10 μm, respectively.
Therefore, in order to remove DEP, it is important how to remove particles of all these sizes efficiently.

From such a background, it has been proposed that a DPF with a catalyst composed of a ceramic honeycomb structure is composed of a plurality of types having different average pore diameters of cell partition walls, and those having a large average pore and a small one are arranged in series (for example, Patent Document 1).
However, in this DPF, in order to reduce the pressure loss, the size of the pore is about 10 μm even if it is small. For this reason, it is difficult to capture fine particles of 100 nm or less. Moreover, since ceramics are used, there are problems such as damage due to earthquake resistance and thermal shock. Furthermore, in order to increase the capture efficiency, the filter body needs to have a certain size.
JP 2003-225540 A

In addition, a filter that combines a metal mesh and a porous catalyst layer, specifically, a porous metal composite composed of a metal mesh composed of a conductive metal wire and a sintered layer of a metal powder. A DPF having a combined function of the heater function has been proposed (see, for example, Patent Document 2).
However, since this DPF has a wire mesh structure, the mesh holes are very large. Further, if the sintered layer of the metal powder is thickened, the hole is reduced, but the pressure loss is increased. For this reason, there is a problem that it is difficult to achieve both capture efficiency and low pressure loss.
JP 2003-97253 A

Furthermore, a ceramic honeycomb DPF in which the average value of the pore diameter and the standard deviation thereof are controlled has been proposed (see, for example, Patent Document 3). This DPF is made of porous cordierite having an average particle diameter of 10 μm, and can trap particulate matter (DEP) of about 0.1 μm to 1 μm.
WO2002 / 026351

On the other hand, the present inventors have proposed a manufacturing method of whiskers having different sizes (Japanese Patent Application No. 2004-319018). That is, it has been found that when the composition of manganese (Mn) contained in the raw material base for growing whiskers is changed and heat treatment is performed in an inert gas, the size of the obtained whiskers changes.
Here, the “whisker” means an elongated bearded crystal and is generally used as a reinforcing material for plastics and ceramics. In addition, since the surface area of the substrate on which whiskers are formed increases, it can be used as a catalyst carrier.

Therefore, the present inventors have compared the DPF using such whiskers with a conventionally known DPF at the same size, and found that the former exhibits extremely excellent characteristics.
That is, the DPF using the whisker has a large specific surface area and can carry the catalyst with good dispersibility (reducing the amount of catalyst used) compared to the honeycomb filter which is currently used frequently, and the impact and thermal stress due to the elasticity of the whisker. It has a merit such that it is resistant to heat and the base material can be made of metal so that heat conduction can be enhanced. Also, compared to thin non-woven filters such as SiC, whiskers become uneven, making it easier for particles to capture, whisker size can be controlled without worrying about strength, and smaller whiskers can remove smaller particles There are advantages such as being possible.

  The present invention has been made in view of the problems and new knowledge of the prior art, and the object of the present invention is to provide a purification filter that improves the capture efficiency and suppresses the decrease in pressure loss, and its A manufacturing method and an exhaust gas purification device are provided.

  As a result of intensive studies to solve the above problems, the present inventors have found that the above problems can be solved by fine whiskers grown on a raw material substrate containing a predetermined element, and to complete the present invention. It came.

  According to the present invention, since the whisker having a fine and increased surface area is used, the pressure loss is small and it is possible to supplement particles up to 1/10 or less of the supplemental limit of conventional products. In addition, a filter having the same supplementary efficiency as the conventional product can be reduced in size and weight (cost reduction).

  Hereinafter, the purification filter of the present invention will be described in detail. In the claims and the specification, “%” represents a mass percentage unless otherwise specified.

As described above, the purification filter of the present invention is formed by forming whiskers on a raw material substrate. Further, this raw material base is made of manganese (Mn), aluminum (Al), chromium (Cr), indium (In), silver (Ag), gallium (Ga), tin (Sn), copper (Cu), scandium (Sc). ), Germanium (Ge), titanium (Ti) or silicon (Si), and an alloy containing any combination of these elements, or a ceramic containing at least one of these elements.
Such elements form whiskers on the surface of the raw material substrate, whereby a purification filter having a large specific surface area can be obtained. In addition, since one end of the whisker is open, there is little pressure loss. Furthermore, particles having a size of 10 μm or less are captured by the sieving effect, and the particles are also captured by collision capture by the unevenness of the whisker, so that the capture efficiency is improved.

Here, the collision capture exhibited by the purification filter of the present invention will be described.
As shown in FIG. 2 (left figure), a filter that uses only the sieving effect by pores can not only capture particles corresponding to the size of the pores, but also once the particles are captured, The pressure loss increases due to clogging.
On the other hand, since the purification filter of the present invention uses whiskers, in addition to the sieving effect, when the flowing particles collide with the whiskers, they have the effect of being caught by being caught by the whiskers (collision capture). That is, as shown in FIG. 2 (right figure), since the particles can be captured three-dimensionally without actually reducing the pore diameter to the size of the captured particles, it is possible to capture up to 1/10 or less of the conventional particles, The pressure loss problem is improved.
FIG. 3 is a graph showing the relationship between the particle size of PM particles that can be collected by collision capture by a purification filter filled with whiskers having a thickness of 1 μm and the collection rate.

Moreover, the reduction of the pressure loss by using the purification filter of the present invention will be described.
For example, as shown in FIG. 4 (left figure), when whiskers (cones) having a thickness of 2 μm and a length of 20 μm are densely formed on a granular material having a diameter of 10 μm used as a raw material substrate, a space for a sphere having a diameter of 50 μm. The occupation ratio is 3%.
In addition, as shown in FIG. 4 (right figure), when whiskers (cones) having a thickness of 20 nm and a length of 1 μm are densely formed on a granular material having a diameter of 5 μm used as a raw material substrate, a space for a sphere having a diameter of 7 μm is provided. The occupation ratio is 47%.
Since the purification filter with high porosity can be configured by the space secured in the gap between the whiskers in this way, the pressure loss can be greatly reduced.

As the raw material substrate, a porous body can be used. In this case, it is preferable that the average pore diameter of the porous body is larger than the average diameter of the whiskers. Accordingly, whiskers are easily formed in the porous pores, and the average pore diameter becomes smaller, so that smaller particles can be captured than before the whisker formation.
Examples of the porous body include an alloy mesh and a porous metal (foamed metal) as shown in FIG. 5, and when forming whiskers on these, sieve to particles finer than the pore diameter (about 1 μm). The effect can be exhibited, and even finer particles (˜0.01 μm) can be removed by the collision capturing effect.

Moreover, a granular material can also be used as said raw material base | substrate. In this case, it is preferable to fill them in a carrier to obtain a purification filter. At this time, whiskers formed in the granular material are entangled with each other, and a porous shape in which an appropriate porosity is secured can be configured. As a result, a purification filter with improved trapping efficiency and low pressure loss, high strength (seismic resistance) and excellent thermal shock resistance can be obtained. For example, when a whisker having a diameter of 1 μm and a length of 10 μm is formed on half of the surface area of the granular material, the surface area is 8 times or more compared to the case where no whisker is provided. In addition, a filter having the same effect as a conventional filter can be reduced in size and weight (cost reduction).
As the above-mentioned granular material, typically, a powder of Ni-Mn alloy, Inconel (Ni-Cr-Fe alloy), stainless steel (Fe-Cr-Ni alloy, Fe-Cr-Al alloy, etc.), etc. In addition, these alloys including chips, columns, short fibers, and the like are also included. Moreover, when manufacturing a purification filter, as shown in FIG. 6, you may fill a support | carrier after the annealing process (heating process) of a granular material (left figure), or a granular material is made into a porous metal etc. Annealing may be performed after filling (right figure). As the carrier, the above-mentioned porous body or metal carrier can be used. In addition, the granular material with which it fills in a support | carrier may be fixed, and does not need to be fixed.

Next, the purification filter manufacturing method of the present invention will be described in detail.
In the production method of the present invention, first, the content of the elements constituting the whisker among the elements contained in the alloy or ceramic as the raw material substrate is adjusted. Next, the raw material substrate is placed in a reaction furnace and subjected to heat treatment in an inert atmosphere and in the presence of a trace amount of oxygen, whereby a whisker having a controlled length and thickness can be formed on the surface of the raw material substrate.
For example, when the nickel powder metal containing manganese powder is used as the raw material base, the average thickness is changed by changing the content ratio of Mn powder in the range of 10 to 80% under the same heat treatment conditions as shown in FIG. Can be arbitrarily designed in the range of 10 nm to 5 μm and an average length of 2 to 50 μm.
As described above, in the present invention, a purification filter having a size suitable for the size of the target particle can be manufactured by appropriately controlling the size of the whisker.

Here, at present, the principle of the growth of such whiskers is not clear, but the elements constituting the whiskers have a high vapor pressure and are easy to volatilize in the raw material substrate, and are the same as other components in the raw material substrate. It can be inferred that there is an oxide having a lower vapor pressure at temperature (however, in the case of a metal having a plurality of oxidation numbers, it is not necessary to satisfy this condition for all oxidation numbers). That is, first, a state is formed in which the vapor of the whisker constituent element or the vapor of its oxide exists around the surface of the raw material substrate. Next, a nucleus serving as a base point of the whisker is formed on the surface of the raw material substrate. Furthermore, whiskers grow with this nucleus as the tip. At this time, there is an optimum range for the temperature of the raw material substrate surface and the flow rate of the inert gas, and it is considered that the formation density is reduced regardless of whether the range is larger or smaller. For example, when the surface of the raw material substrate containing manganese and indium is 1000 ° C., the vapor pressure of manganese is about 3 × 10 ⁻² Torr (about 4 Pa) and the vapor pressure of manganese oxide (MnO) is about 3 × 10 ^−7. It is considered that the vapor pressure of Torr and indium is 2 × 10 ⁻² Torr and the vapor pressure of indium oxide is 7 × 10 ⁻⁸ Torr.

The raw material substrate can be used in the form of an alloy or ceramic in a porous shape, mesh shape (woven fabric / nonwoven fabric), powdery particle shape, etc., but it is particularly preferable to heat-treat after forming into a filter structure. In this case, the trapping efficiency is improved and the pressure loss can be easily controlled as compared with the case where the carrier is filled with the whisker-forming substrate.
The “filter structure” means that the raw material substrate is made porous, or the granular material that is the raw material substrate is supported in advance in the pores of the porous body as a carrier (regardless of the properties as the raw material substrate). For example, filling.

Further, as the inert atmosphere, for example, argon, nitrogen, helium, neon, krypton, xenon and radon, or a gas in which these are arbitrarily combined can be used.
Furthermore, in the heat treatment, it is necessary that a small amount of oxygen be present together with these inert gases. This is because whiskers formed from the raw material base grow as oxides. However, the oxygen is sufficient to remain in the atmosphere before the inert gas is fed into the reactor (about 1 to 1000 ppm).
Furthermore, the firing temperature during the heat treatment also depends on the reactor used, the size and shape of the sample, and the like. For example, when the volume of the reaction furnace is 3 L, an inert gas can be supplied at 0.1 to 5 L / min and baked at 900 to 1100 ° C. for 30 to 1000 minutes.

Next, the exhaust gas purification apparatus of the present invention will be described in detail.
The exhaust gas purification apparatus of the present invention is configured using two or more layers of the above-described purification filter. Further, a purification filter having a large average length and thickness of the whisker is disposed on the upstream side with respect to the flow direction of the exhaust gas, and a purification filter having a small average length and thickness is disposed on the downstream side.
As a result, large particles are captured on the upstream side and small particles are captured on the downstream side, so that the exhaust gas can be purified with high efficiency. Further, in the purification filter, the larger the whisker size ratio with respect to the raw material substrate, the higher the porosity, so that it is possible to prevent a decrease in pressure loss due to accumulation of trapped particles. Furthermore, the lifetime until reproduction is extended.
In the purification filter 1 layer of the present invention, even when the whisker constituent elements contained in the raw material base are biased to have whisker partially different in size, the same as in the exhaust gas purification device of the present invention. Needless to say, it has an effect.

Here, a typical manufacturing method of the exhaust gas purification apparatus of the present invention will be described below.
As shown in FIG. 8, as a first example, two porous metal plates (raw material bases) containing whisker constituent elements are used in the pores, and the whisker constituent elements are contained upstream of the exhaust gas passage. Those having a large amount and those having a small content of the whisker constituent element are disposed on the downstream side. As a second example, a porous metal substrate having a large content of whisker constituent elements on the outer side (site where the exhaust gas contacts first) and a porous metal substrate having a small content on the inner side are processed into cylindrical shapes. Housing. As a third example, a single porous metal substrate is formed into a cylindrical shape, containing a large amount of whisker constituent elements on the outside and containing a small amount of whisker constituent elements on the inside (controlling the composition during powder loading or metal formation) ).
An exhaust gas purification device can be obtained by annealing the substrates of the first to third examples in an inert gas and then installing each of them in the exhaust gas flow path.

  As shown in FIG. 9, as a fourth example, a porous metal substrate containing whisker constituent elements is formed in a honeycomb shape in which plugs are alternately plugged. At this time, the exhaust gas flow path in which the downstream side is sealed with respect to the direction of the exhaust gas flow path (having an opening on the upstream side) increases the content of whisker constituent elements, and the upstream side is sealed (downstream side) This content is reduced in the exhaust gas flow path (having an opening at the top). The substrate is annealed in an inert gas, and then installed in the exhaust gas flow path to obtain an exhaust gas purification device.

  Furthermore, as shown in FIG. 10, as a fifth example, whisker-forming particles are filled in a carrier installed in the exhaust gas flow path. At this time, particles with large whiskers can be filled upstream with respect to the exhaust gas flow direction, and particles with small whiskers can be disposed downstream (left figure). Further, a honeycomb-shaped carrier in which the upstream side and the downstream side are alternately plugged is prepared, and among these, the exhaust gas passage having an opening on the upstream side is filled with particles having large whiskers, and the downstream side is filled with particles. The exhaust gas flow path having an opening can be filled with particles having small whiskers (right figure).

In the exhaust gas purification apparatus of the present invention, it is preferable that the purification filter is a dust collecting electrode and a discharge electrode made of a conductive material is disposed in the vicinity thereof.
In this case, when a DC voltage is applied between the two electrodes, the exhaust gas particles are charged by the discharge electrode and are attracted to the dust collecting electrode, so that the trapping efficiency is further improved. Further, the direct-current voltage varies depending on the size of the purification filter, but can be set to about 1 k to 10 kV, for example.

Furthermore, in the exhaust gas purification apparatus of the present invention, it is preferable to carry an oxidation catalyst on the surface of the whisker.
By carrying the oxidation catalyst, the regeneration process can be performed efficiently, so that the life of the exhaust gas purification device can be extended. Further, since there are fine irregularities on the surface, the catalyst is easily supported, and the amount supported is small. Examples of the oxidation catalyst include palladium, rhodium, platinum, gold, iron, cerium, vanadium, nickel, cobalt, tungsten, and the like.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, this invention is not limited to these Examples.

Example 1
A disk-type porous metal container is filled with Ni-Mn alloy powder having an average particle size of 10 μm (upstream side: 80% Mn, downstream side: 50% Mn), and this is heated at 1000 ° C./h in an Ar gas flow. A heat treatment was carried out at 1000 ° C. for 2 hours to obtain a purification filter equipped with whiskers.
Specifically, as shown in FIG. 11, whiskers (with a porosity of about 95%) having an average thickness of 1 to 5 μm and an average length of 50 μm are formed on the upstream side, and the average thickness is 100 on the downstream side. Whiskers (having a porosity of about 50%) having a length of ˜1000 nm and an average length of 10 μm were formed.

(Comparative Example 1)
It is obtained by adjusting kaolin, talc, silica, and alumina powder having a particle size of 10 μm so as to have the same external shape (size) as in Example 1, and in a mass ratio, SiO ₂ : 50%, Al ₂ O ₃ : A cordierite raw material powder containing 35% and MgO: 15% was molded into a disk shape together with a pore former and fired at 1400 ° C. to obtain a ceramic filter having a porosity of 50%.

<Evaluation measurement>
The filters obtained in Example 1 and Comparative Example 1 were installed in the exhaust gas flow path, and the gas particle size distribution on the exhaust side when diesel exhaust gas was introduced at 1 m / s was measured. The result is shown in FIG.

From FIG. 11, the filter obtained in the example can capture particles up to 0.01 μm with high efficiency by capturing particles by the sieve function due to the formation of whiskers and capturing effect by particle collision. Recognize.
On the other hand, it can be seen that the filter obtained in Comparative Example 1 has a small trapping effect due to particle collision and has a large pore diameter, so that it can capture only about 0.1 μm even if it is small.

Although the present invention has been described in detail with some preferred embodiments, the present invention is not limited to these embodiments, and various modifications can be made within the scope of the gist of the present invention.
For example, Inconel can be used as the raw material substrate.

It is a graph which shows the particle size distribution of a particulate matter (DEP). It is a schematic diagram showing an example of collision capture. It is a graph which shows the relationship between the particle diameter of PM particle which can be collected by collision capture, and a collection rate. It is the schematic which shows the space occupation rate by the denseness degree of a whisker. It is the schematic which shows an example of a porous body. It is the schematic which shows an example of the whisker formed by the annealing process of a granular material. It is a photograph which shows an example of the content ratio of a whisker constituent element, and the obtained whisker. It is the schematic which shows an example of an exhaust-gas purification apparatus. It is the schematic which shows the other example of an exhaust-gas purification apparatus. It is the schematic which shows the further another example of an exhaust-gas purification apparatus. 2 is a photograph showing whiskers formed in Example 1. FIG. It is a gas particle size distribution before DPF passage, and a particle size distribution figure after a filter passage (an example and a comparative example).

Claims (9)

A purification filter formed by forming whiskers on a raw material substrate,
An alloy containing at least one element selected from the group consisting of manganese, aluminum, chromium, indium, silver, gallium, tin, copper, scandium, germanium, titanium and silicon, or at least one of these elements A purification filter characterized by being ceramics containing   The purification filter according to claim 1, wherein the raw material base is a porous body, and an average pore diameter of the porous body is larger than an average diameter of the whisker.   2. The purification filter according to claim 1, wherein the raw material substrate is a granular material and is filled in a carrier. In manufacturing the purification filter according to any one of claims 1 to 3,
Adjusting the content of at least one element contained in the raw material substrate, and heat-treating the raw material substrate in an inert atmosphere and in the presence of a trace amount of oxygen to control the length and thickness of the whisker to be formed. A purification filter manufacturing method characterized by the above.   The purification filter manufacturing method according to claim 4, wherein the raw material substrate is heat-treated after being formed into a filter structure.   6. The method for producing a purification filter according to claim 4, wherein the raw material substrate is a granular material, and heat treatment is performed after the granular material is supported on a porous body. An exhaust gas purification apparatus configured using two or more layers of the purification filter according to any one of claims 1 to 3,
A purification filter having a large average length and thickness of whiskers formed on the raw material substrate is disposed upstream of the exhaust gas flow direction, and a purification filter having a small average length and thickness is disposed downstream. An exhaust gas purifying device comprising:   The purification filter is a dust collecting electrode, a discharge electrode made of a conductive material is disposed in the vicinity thereof, and exhaust gas particles are sucked and captured by the dust collecting electrode by applying a DC voltage between the two electrodes. The exhaust gas purification apparatus according to claim 7.   The exhaust gas purifying device according to claim 7 or 8, wherein an oxidation catalyst is supported on a surface of the whisker.

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