JP2009275559A - Electric heating type catalyst device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve high purifying performance even if a large quantity of exhaust gas at a low temperature has entered. <P>SOLUTION: A catalyst coat layer 15 is formed of an upstream coat 16 and a downstream coat 17, and the downstream coat 17 has higher carrying density which is a carrying amount of noble metal per unit bulk volume of a honeycomb substrate 14 than that of the upstream coat 16. Since an exhaust gas heated by the upstream coat 16 of the energized and heated honeycomb substrate 14 enters the downstream coat 17, the activity of the noble metal is sufficiently exhibited. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electrically heated catalyst device useful for an exhaust system of a hybrid vehicle driven by at least one of an electric motor and an internal combustion engine.

As exhaust gas purification catalysts for automobiles, various exhaust gas purification catalysts such as three-way catalysts, oxidation catalysts, NO _x selective reduction catalysts, NO _x storage reduction catalysts are used. These catalysts include a cordierite or metal honeycomb-shaped carrier substrate, a porous oxide carrier such as alumina carrying a noble metal such as Pt, and, if necessary, an alkaline NO _x storage material. A coating layer is formed.

The exhaust gas-purifying catalyst described above contains noble metals such as Pt, Rh, and Pd as essential components. However, these noble metals do not exhibit a catalytic action at a low temperature and have a characteristic that they are not activated unless the temperature is higher than a predetermined temperature such as 300 ° C. or higher. For this reason, when the exhaust gas temperature is low such as at the time of start-up, there is a problem that no oxidation-reduction reaction occurs and harmful substances such as HC and NO _x are discharged.

  Therefore, it has been proposed to use a metal honeycomb body as a carrier substrate, and arrange a preheater in front of it, or energize the metal honeycomb body itself to generate heat and raise the temperature to the activation temperature of the noble metal. . However, in the metal honeycomb body, the electric resistance value for heat generation is not sufficient, and the output of the battery is small, so the heat generation amount is not sufficient.

  Thus, for example, Japanese Patent Laid-Open No. 06-099079 proposes an energization heat generating type carrier in which an energization heat generating metal wire is disposed between a pair of electrodes. According to this proposal, since the function of the honeycomb body is replaced by many metal wires, a sufficient amount of heat generation can be ensured by energization through the pair of electrodes. Therefore, according to the exhaust gas purifying catalyst in which the catalyst coating layer is formed on the energization heat generating carrier, high purification performance is exhibited even in a low temperature range.

  Japanese Patent Application Laid-Open No. 05-220406 discloses that a strip of metal flat foil and corrugated foil is wound around a central electrode and stored in an outer cylinder, and the wave height or wavelength of the corrugated foil is partially set. Thus, there has been proposed an energization heat-generating carrier in which at least one of the outer peripheral portion and the central portion has a larger calorific value per unit volume than their intermediate portion.

  By the way, in a hybrid vehicle driven by at least one of an electric motor and an internal combustion engine, traveling by only the electric motor is possible. Therefore, if only the electric motor is driven when starting from a parking lot at night, the vehicle can start very quietly and noise can be prevented.

  However, when the internal combustion engine travels continuously with the electric motor alone, the exhaust gas temperature is low and it is difficult to purify with the exhaust gas purifying catalyst because the internal combustion engine is not warmed. Thus, for example, Japanese Patent Application Laid-Open No. 07-071236 describes that an electrically heated catalyst device is disposed in the exhaust system of a hybrid vehicle.

According to the invention described in the publication, the electric heating type catalyst device can be energized and heated during traveling by only the electric motor. Therefore, when switching to running with the internal combustion engine, the electrically heated catalyst device is already sufficiently heated and the noble metal of the catalyst has reached the activation temperature, so that harmful components in the exhaust gas can be purified.
Japanese Patent Laid-Open No. 06-099079 Japanese Patent Laid-Open No. 05-220406 Japanese Unexamined Patent Publication No. 07-071236

  However, in a hybrid vehicle, when only the electric motor is driven at the time of starting and the internal combustion engine is driven after traveling to a certain extent only with the electric motor, the internal combustion engine suddenly operates at a high load because the number of rotations of the wheels is increased. It becomes a state. As a result, a large amount of exhaust gas is generated and flows into the exhaust gas purifying catalyst.

  For this reason, even if an electrically heated catalyst device is used and heated in advance, a large amount of low-temperature exhaust gas flows, so that there is a problem that the catalyst temperature is lowered and sufficient purification performance cannot be obtained.

  This invention is made | formed in view of such a situation, and makes it the subject which should be solved to enable high purification performance to be expressed even when a large amount of low-temperature exhaust gas flows.

  The feature of the electrically heated catalyst device of the present invention that solves the above-described problems is that a carrier base material that can be electrically heated, an oxide carrier formed in an exhaust gas passage of the carrier base material, and at least a noble metal carried on the oxide carrier. And a heating means for heating the carrier substrate by energizing the carrier substrate, the catalyst coat layer comprising an upstream coat portion formed on the exhaust gas upstream side of the carrier substrate, and an upstream The downstream coating portion is formed on the exhaust gas downstream side of the coating portion, and the supporting density, which is the amount of the precious metal supported per unit volume of the carrier substrate, is higher in the downstream coating portion than in the upstream coating portion.

  When low temperature exhaust gas flows from the internal combustion engine into the electrically heated catalyst device while the electrically heated catalyst device is pre-heated by energization, the exhaust gas is heated by heat conduction in the upstream coat portion of the electrically heated catalyst device and is downstream. It flows into the coat part.

  Therefore, in the electrically heated catalyst device of the present invention, the catalyst coat layer is formed so that the loading density of the noble metal is higher in the downstream coat portion than in the upstream coat portion. Therefore, since the exhaust gas whose temperature has risen by passing through the upstream coat part flows into the downstream coat part, which is a high-density carrying part of the noble metal, the activity of the noble metal is fully exhibited, and harmful components in the exhaust gas are efficiently purified. can do.

  That is, according to the electrically heated catalyst device of the present invention, even when the internal combustion engine in a cold state suddenly becomes in a high load operation state and a large amount of low-temperature exhaust gas is generated, it is supported at a high density on the downstream coat portion. Precious metals can greatly reduce the emission of harmful substances. Moreover, since the carrying density of the noble metal in the upstream coat portion can be sufficiently lowered, the amount of the noble metal used can be greatly reduced as compared with the conventional case. Furthermore, since it becomes possible to make heating temperature lower than before, the electric power for heating an electrically heated catalyst apparatus can also be reduced, and a fuel consumption improves.

  The electrically heated catalyst device of the present invention includes a carrier substrate on which a catalyst coat layer is formed, and heating means for electrically heating the carrier substrate. The carrier substrate has an exhaust gas passage, and has a shape such as a honeycomb shape or a foam shape, and can be formed from an electrically heatable material. Examples of the carrier substrate formed from an electrically heatable material include a metal honeycomb body formed from a metal foil such as Fe—Cr—Al, and a monolith honeycomb body formed from SiC.

  A catalyst coat layer is formed in the exhaust gas passage of the carrier substrate. This catalyst coat layer comprises an oxide carrier and at least a noble metal supported on the oxide carrier. Examples of the oxide carrier include alumina, ceria, zirconia, titania, silica, etc., and a single product selected from these, a mixture of a plurality of types selected from these, or a composite oxide consisting of a plurality of types selected from these are used. be able to.

  The noble metal is selected from Pt, Pd, Rh, Ir, Ag, and the like, and can be selected and used in various ways according to the application of the catalyst. Depending on the application of the catalyst, other catalyst metals such as alkali metals, alkaline earth metals, and transition metals may be used in combination.

  The greatest feature of the present invention is that a catalyst coat layer is composed of an upstream coat portion formed on the exhaust gas upstream side of the carrier substrate and a downstream coat portion formed on the exhaust gas downstream side of the upstream coat portion, and supports the noble metal. The density is higher in the downstream coat part than in the upstream coat part. Here, the “supporting density” means a support amount per unit volume of the support substrate, and generally means a support amount per liter of the support substrate.

  The type, composition, or coat amount of the oxide carrier may be the same or different between the upstream coat portion and the downstream coat portion. Since the support density of the noble metal is high in the downstream coat portion, the coat amount in the downstream coat portion may be increased in order to increase the heat capacity. If it does in this way, the temperature fall in a downstream coat part will be controlled, and the activity of the noble metal in a downstream coat part will improve.

  The carrying density of the noble metal is uniformly carried in the upstream coat part and the downstream coat part, and the carrying density can be made different between the upstream coat part and the downstream coat part. Further, the carrying density may be inclined from small to large from the exhaust gas inflow side end surface to the exhaust gas outflow side end surface. In this case, the inclination may be smoothly inclined or may be inclined stepwise. It is also possible to carry a noble metal only on the downstream coat part without carrying the noble metal on the upstream coat part.

  When the noble metal is Pt or Pd, the loading density of the noble metal in the downstream coat part is desirably 2.5 g or more per liter of the bulk volume of the carrier substrate.

  The optimum range of the noble metal loading density of the catalyst as a whole is determined in advance according to the application of the catalyst. For example, in the case of a three-way catalyst, it is preferable that the loading density of Pt or Pd is 0.3 to 10 g and the loading density of Rh is 0.05 to 0.6 g per liter of the bulk volume of the support substrate. Therefore, even if the carrying density of the noble metal is made different between the upstream coat portion and the downstream coat portion, it is desirable to make it within this range as a whole (as an average value).

  In addition, when making a difference in carrying density between the upstream coat portion and the downstream coat portion, the downstream coat portion is preferably formed in the range of 30 to 70% of the total length of the catalyst coat layer, and should be around 50%. Is desirable. If the formation range of the downstream coat portion is out of this range, the purification performance when low-temperature exhaust gas flows may deteriorate.

  In order to form a catalyst coating layer consisting of an upstream coating portion and a downstream coating portion on a carrier substrate, a slurry containing oxide carrier powder as a main component is washed on the exhaust gas passage wall surface of the carrier substrate, dried and fired. There is a method in which a coat layer is formed and then a noble metal solution is used to adsorb and carry a noble metal on the coat layer. In this case, the upstream coat portion and the downstream coat portion can be formed by separately coating the coat layer using noble metal solutions having different concentrations.

  Alternatively, a catalyst powder in which a noble metal is supported in advance on an oxide carrier powder can be prepared, and a catalyst coat layer can be formed using a slurry containing the catalyst powder. In this case, the upstream coat portion and the downstream coat portion can be formed using catalyst powders having different amounts of noble metal supported.

  As a heating means for heating the carrier base material by energizing the carrier base material, a means for supplying a battery power by connecting a pair of electrodes to the center and the outer periphery of the carrier base material can be used. Since the hybrid vehicle is equipped with a high-power battery, the carrier substrate can be rapidly heated by supplying the electric power.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.

Example 1
FIG. 1 shows a cross-sectional view of an electrically heated catalyst device according to this embodiment. In this electrically heated catalyst device, a three-way catalyst 1 having a metal honeycomb substrate (bulk volume 0.7 liter, total length 100 mm) is housed in a metal outer cylinder 2, and the three-way catalyst 1 has an axial center. Is arranged with the central electrode 3 therethrough. The outer cylinder 2 also serves as a ground electrode, and the center electrode 3 is drawn out of the outer cylinder 2 in a state of being electrically insulated from the outer cylinder 2 by an insulator 30.

  As shown in an enlarged view in FIG. 2, the three-way catalyst 1 includes a flat foil 10 made of steel (Fe—Cr—Al), an insulating foil 11 and a corrugated foil 12 made of steel (Fe—Cr—Al). Is formed by winding a band-shaped body in which contact portions are bonded to each other, and a honeycomb base material 14 having a large number of honeycomb passages 13, a flat plate 12 defining the honeycomb passages 13, an insulating foil 11, and a corrugated foil 12. And a catalyst coat layer 15 formed on the surface.

  As shown in an enlarged view in FIG. 1, the catalyst coat layer 15 includes an upstream coat portion 16 formed on the exhaust gas upstream side of the honeycomb substrate 14 and a downstream coat portion 17 formed on the exhaust gas downstream side of the upstream coat portion 16. Both consist of an oxide carrier such as activated alumina powder and a noble metal. The upstream coat portion 16 and the downstream coat portion 17 have different noble metal loading densities. Hereinafter, the manufacturing method of this electric heating type catalyst device will be described, and the detailed description of the configuration will be substituted.

  First, an elongated flat foil 10 made of steel and having a width of 100 mm and a corrugated foil 12 formed by corrugating the flat foil 10 are prepared, and the contact portions of both are laser-welded or brazed. Join by attaching. Further, an insulating foil 11 is laminated on the surface of the flat foil 10. The belt-like body formed in this way is wound up in a spiral shape around the center electrode 3 and wound to a predetermined outer diameter. This was inserted into the outer cylinder 2 and joined to the outer cylinder 2 to obtain a honeycomb substrate 14. Subsequently, one end of the center electrode 3 was pulled out to the outside of the outer cylinder 2 through the insulator 30 and used as an anode, and the ground electrode 20 was connected to the outer cylinder 2.

On the other hand, 100 parts by mass of Pd / Al ₂ O ₃ catalyst powder in which 2% by mass of Pd was previously supported on activated alumina powder, and Rh / CeO ₂ — in which 0.5% by mass of Rh was previously supported on CeO ₂ —ZrO ₂ composite oxide powder. 40 parts by mass of ZrO ₂ catalyst powder, alumina sol as a binder and water were mixed to prepare an upstream slurry.

  This upstream slurry was wash-coated from the exhaust gas inflow side end face of the honeycomb substrate 14 to a range of 50% of the total length and dried at 250 ° C. for 1 hour.

Next, 100 parts by mass of Pd / Al ₂ O ₃ catalyst powder in which 4% by mass of Pd is previously supported on activated alumina powder, and Rh / CeO ₂ − in which 1.0% by mass of Rh is previously supported on CeO ₂ —ZrO ₂ composite oxide powder. A slurry for downstream portion was prepared by mixing 40 parts by mass of ZrO ₂ catalyst powder, alumina sol as a binder and water.

  This downstream slurry was wash-coated from the exhaust gas outlet side end face of the honeycomb substrate 14 to a range of 50% of the total length and dried at 250 ° C. for 1 hour. Then, the whole was fired at 500 ° C., and the upstream coat portion 16 and the downstream coat portion 17 were formed to produce the electrically heated catalyst device of this example.

  110 g of the upstream coat portion 16 and 135 g of the downstream coat portion 17 were formed per liter of the bulk volume of the honeycomb substrate 14. The loading density of Pd and Rh in the upstream coat portion 16 per liter of the bulk volume of the honeycomb base material 14 is 1 g and 0.1 g, respectively, and the Pd and Rh in the downstream coat portion 17 per liter of the bulk volume of the honeycomb base material 14 The loading density is 3 g and 0.3 g, respectively.

  The average supported densities of Pd and Rh in the entire catalyst coat layer 15 are 2 g and 0.2 g, respectively, which are the same as Comparative Example 2 described later.

(Example 2)
An upstream portion slurry and a downstream portion slurry were prepared in the same manner as in Example 1 except that the amount of each catalyst powder was adjusted. Then, using the honeycomb substrate 14 formed in the same manner as in Example 1, using these slurries, an upstream coat portion is formed in a range of 30 mm from the inflow side end surface, and a downstream coat portion is formed in a range of 70 mm from the outflow side end surface. To form a catalyst coat layer.

  110 g of the upstream coat part and 130 g of the downstream coat part were formed per liter of the bulk volume of the honeycomb substrate 14. The loading density of Pd and Rh in the upstream coat part per liter of the honeycomb substrate 14 is 0.8 g and 0.08 g, respectively. The loading density is 2.5 g and 0.25 g, respectively.

  The average supported densities of Pd and Rh in the entire catalyst coat layer 14 are 2 g and 0.2 g, respectively, which are the same as Comparative Example 2 described later.

(Example 3)
An upstream portion slurry and a downstream portion slurry were prepared in the same manner as in Example 1 except that the amount of each catalyst powder was adjusted. Then, using the honeycomb base material 14 formed in the same manner as in Example 1, using these slurries, an upstream coat portion is formed in a range of 70 mm from the inflow side end surface, and a downstream coat portion is formed in a range of 30 mm from the outflow side end surface. To form a catalyst coat layer.

 110 g of the upstream coat portion 16 and 150 g of the downstream coat portion 17 were formed per liter of the bulk volume of the honeycomb substrate 14. The loading density of Pd and Rh in the upstream coat part per liter of the honeycomb substrate 14 is 0.5 g and 0.05 g, respectively, and Pd and Rh in the downstream coat part per liter of the honeycomb substrate 14 are 1 The loading density is 5.5 g and 0.55 g, respectively.

  The average supported densities of Pd and Rh in the entire catalyst coat layer 14 are 2 g and 0.2 g, respectively, which are the same as Comparative Example 2 described later.

Example 4
Instead of Pd / Al ₂ O ₃ catalyst powder, Pt / Al ₂ O ₃ catalyst powder in which ₂ % by mass of Pt was supported on activated alumina powder in advance was used as the upstream slurry, and 4% by mass of Pt in the activated alumina powder in advance. Each slurry was prepared in the same manner as in Example 1 except that the supported Pt / Al ₂ O ₃ catalyst powder was used as the downstream slurry. Then, using the honeycomb base material 14 formed in the same manner as in Example 1, using these slurries, an upstream coat portion is formed in a range of 50 mm from the inflow side end surface, and a downstream coat portion is formed in a range of 50 mm from the outflow side end surface. To form a catalyst coat layer.

  110 g of the upstream coat part and 135 g of the downstream coat part were formed per liter of the bulk volume of the honeycomb substrate 14. The supporting density of Pt and Rh in the upstream coat part per liter of the honeycomb substrate 14 is 1 g and 0.1 g, respectively. The density is 3 g and 0.3 g, respectively.

  The average supported densities of Pt and Rh in the entire catalyst coat layer 14 are 2 g and 0.2 g, respectively, which are the same as Comparative Example 3 described later.

(Comparative Example 1)
The same as Example 1 except that the compositions of the upstream coat portion 16 and the downstream coat portion 17 were reversed. That is, the loading density of Pd and Rh in the upstream coat portion 16 per liter of the honeycomb substrate 14 is 3 g and 0.3 g, respectively, and the Pd and Rd in the downstream coat portion 17 per liter of the honeycomb substrate 14 are 1 The loading density of Rh is 1 g and 0.1 g, respectively.

  The average supported densities of Pd and Rh in the entire catalyst coat layer 15 are 2 g and 0.2 g, respectively, which are the same as Comparative Example 2 described later.

(Comparative Example 2)
A catalyst coat layer was uniformly formed on the entire length of the honeycomb substrate 14 formed in the same manner as in Example 1, using the same upstream slurry as in Example 1. The catalyst coat layer is formed in an amount of 120 g per liter of the bulk volume of the honeycomb base material 14, and the supported densities of Pd and Rh per liter of the bulk volume of the honeycomb base material 14 are 2 g and 0.2 g, respectively.

(Comparative Example 3)
A catalyst coat layer was uniformly formed on the entire length of the honeycomb substrate 14 formed in the same manner as in Example 1 using the same slurry for the upstream portion as in Example 4. The catalyst coat layer is formed in an amount of 120 g per liter of the bulk volume of the honeycomb base material 14, and the supported densities of Pt and Rh per liter of the bulk volume of the honeycomb base material 14 are 2 g and 0.2 g, respectively.

<Test and evaluation>
The electrically heated catalyst device of each example and each comparative example was attached to the exhaust system of a gasoline engine with a displacement of 2 liters, and exhaust gas in a stoichiometric atmosphere burned at A / F = 14.6 was exhausted at 900 ° C for 100 hours. An endurance test was conducted.

Each catalyst device after the durability test was attached to the same exhaust system as that in the durability test, and a voltage of 25 V was applied between the center electrode 3 and the ground electrode 20 and heated to 400 ° C. In this state, the gasoline engine was cold-started at a room temperature of 25 ° C., and the purification rates of HC, CO, and NO _x were measured when exhaust gas immediately after the start flowed to each catalyst device for 1 minute. The results are shown in Table 1. Table 1 also shows the configuration of the electrically heated catalyst device of each example and each comparative example.

  From Table 1, it can be seen that the catalytic devices of Examples 1 to 3 have a higher purification rate than the catalytic device of Comparative Example 2, and the catalytic device of Example 4 has a higher purification rate than the catalytic device of Comparative Example 3. . That is, it is clear that the purification performance when low-temperature exhaust gas flows is improved by increasing the support density of the noble metal in the downstream coat part higher than the support density of the noble metal in the upstream coat part. Further, since the catalytic device of Comparative Example 1 is inferior in purification performance to the catalytic device of Comparative Example 2, it can also be seen that increasing the noble metal loading density on the downstream side higher than the upstream side is counterproductive.

  Further, when the results of Examples 1 to 3 are compared, the catalyst device of Example 1 shows the highest purification rate, so that the formation ratio of the upstream coat layer and the downstream coat layer has an optimum value in the vicinity of 1: 1. It is suggested.

The electrically heated catalyst device of the present invention is used not only for the three-way catalyst exemplified in the examples, but also for a catalyst using a noble metal as a catalyst metal, such as an oxidation catalyst, a NO _x selective reduction catalyst, and a NO _x storage reduction catalyst. Is possible.

It is sectional drawing of the electrically heated catalyst apparatus of one Example of this invention. It is a principal part expanded sectional view of the electrically heated catalyst apparatus of one Example of this invention.

Explanation of symbols

1: Three-way catalyst 2: Outer cylinder 3: Center electrode
10: Flat foil 11: Insulating foil 12: Corrugated foil
13: Honeycomb passage 14: Honeycomb substrate 15: Catalyst coating layer
16: Upstream court 17: Downstream court

Claims (4)

An electrically heatable carrier substrate;
A catalyst coat layer formed in an exhaust gas passage of the carrier substrate and comprising an oxide carrier and at least a noble metal supported on the oxide carrier;
Heating means for heating the carrier substrate by energizing the carrier substrate,
The catalyst coat layer is composed of an upstream coat portion formed on the exhaust gas upstream side of the carrier substrate and a downstream coat portion formed on the exhaust gas downstream side of the upstream coat portion, and per unit bulk volume of the carrier substrate. The electrically heated catalyst device is characterized in that the loading density, which is the loading amount of the noble metal, is higher in the downstream coating portion than in the upstream coating portion.   2. The electrically heated catalyst device according to claim 1, wherein the downstream coat portion is formed in a range of 30 to 70% of a total length of the catalyst coat layer.   3. The electrically heated catalyst according to claim 1, wherein when the noble metal is Pt or Pd, the loading density of the noble metal in the downstream coat portion is 2.5 g or more per liter of the bulk volume of the carrier substrate. apparatus.   The electrically heated catalyst device according to any one of claims 1 to 3, which is used for an exhaust system of a hybrid vehicle driven by at least one of an electric motor and an internal combustion engine.

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