JP2008163871A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine capable of performing NOx storage reduction without disturbing the exhaust gas conversion function of a catalyst. <P>SOLUTION: An NOx storage reduction type catalyst 22 and an ozone supply device 30 are arranged in an exhaust gas passage of the internal combustion engine. The NOx storage reduction type catalyst 22 is provided with a cell 24 through which exhaust gas flows. An NOx retention layer 26 and a catalyst layer 27 are coated on an inner surface of the cell 27 in an order of the NOx retention layer 26 and the catalyst layer 27 from the inner surface side of the cell 24. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification device for an internal combustion engine.

  Conventionally, as disclosed in, for example, Japanese Patent Application Laid-Open No. 2002-89246, an exhaust gas purification apparatus having a NOx occlusion reduction type catalyst is known. In the above conventional technique, a substance capable of storing NOx (hereinafter also referred to as “NOx holding substance”) is provided in the exhaust passage of the internal combustion engine together with the catalyst. In such a configuration, NOx in the exhaust gas is occluded by the NOx occlusion reduction type catalyst in a lean atmosphere. In the rich atmosphere, the stored NOx is released and reduced and decomposed.

In order for the above reaction to be carried out smoothly, it is desirable that the catalyst reaches the activation temperature and sufficiently exhibits its activity function. However, when the internal combustion engine is started, the catalyst temperature is low. Therefore, in the conventional exhaust gas purification device, ozone (O ₃ ) is added to the exhaust gas when the internal combustion engine is started in order to cope with this. By adding ozone, NOx in the exhaust gas can be oxidized and the NOx occlusion reaction can be promoted. Thus, according to the above-described conventional technique, even when the catalyst is not in a sufficiently active state, such as when the internal combustion engine is started, NOx occlusion can be promoted to purify the exhaust gas.

JP 2002-89246 A JP-A-5-192535 JP 2005-538295 A JP-A-6-185343 JP-A-10-169434 Japanese Patent No. 3551346

  By the way, the conventional NOx occlusion reduction type catalyst is formed by coating a base material (also called a carrier) with a layer containing a catalyst and a NOx holding substance. In such NOx occlusion reduction type catalysts, the exhaust gas purification ability of the catalyst (the ability to purify NOx, HC, CO) tends to be lower than that of a conventional three-way catalyst having no NOx retention substance. There is a problem that the exhaust gas purification function is hindered.

  The present invention has been made to solve the above-described problems, and an object thereof is to provide an exhaust gas purification device for an internal combustion engine that can perform NOx occlusion reduction without interfering with the exhaust gas purification function of the catalyst. And

In order to achieve the above object, a first invention is an exhaust gas purification apparatus for an internal combustion engine,
A NOx occlusion reduction catalyst provided in the exhaust passage of the internal combustion engine;
Ozone supply means for supplying ozone so as to be mixed with exhaust gas flowing into the NOx storage reduction catalyst,
The NOx occlusion reduction type catalyst includes a cell through which exhaust gas flows, and the inner surface of the cell has a first layer containing a NOx holding material and a content of the NOx holding material containing a noble metal is substantially zero. The second layer allowing NOx permeation is coated so as to be positioned in the order of the first layer and the second layer from the inner surface side of the cell.

The second invention is the first invention, wherein
The content of the NOx holding substance in the second layer is substantially zero.

The third invention is the first or second invention, wherein
The apparatus further comprises ozone supply amount adjusting means for adjusting the ozone supply amount so that the molar ratio of ozone to nitrogen monoxide (NO) in the mixed gas flowing into the NOx storage reduction catalyst becomes larger than 1. .

Moreover, 4th invention is set in 3rd invention,
The ozone supply amount adjusting means adjusts the ozone supply amount so that a molar ratio of ozone (O ₃ ) to nitrogen monoxide (NO) in the mixed gas flowing into the NOx storage reduction catalyst becomes 2 or more. Features.

  According to the first invention, since the NOx retention layer and the catalyst layer individually form layers, the catalyst can exhibit its exhaust gas purification function well. The NOx holding substance is considered to be a catalyst poison for the noble metal element and to be a factor that lowers the exhaust gas purification ability of the catalyst. According to the first invention, the NOx retention layer and the catalyst layer are individually formed in layers, and the NOx occlusion reaction is promoted without relying on the catalyst layer by the ozone supply means. The NOx occlusion reduction can be performed while suppressing the influence and preventing the exhaust gas purification function of the catalyst from being hindered.

  According to the second invention, the influence of the catalyst poison in the first invention can be further effectively suppressed.

According to the third aspect of the invention, NO in the exhaust gas is oxidized into higher-order nitrogen oxides than NO ₂ such as NO ₃ and N ₂ O ₅ (HNO ₃ is also generated if moisture is present). be able to. For this reason, it is possible to increase the amount of higher-order nitrogen oxides than NO ₂ such as NO ₃ or N ₂ O ₅ contained in the exhaust gas flowing into the NOx holding material. As a result, the NOx occlusion reaction can be promoted and the exhaust gas purification ability can be enhanced.

According to the fourth invention, sufficient to oxidize NO to form higher-order nitrogen oxides than NO ₂ such as NO ₃ and N ₂ O ₅ (HNO ₃ is also generated if moisture is present). An amount of ozone can be supplied. As a result, the NOx occlusion reaction can be effectively promoted, and the exhaust gas purification ability can be enhanced.

Embodiment 1 FIG.
[Configuration of Embodiment 1]
FIG. 1 is a diagram for explaining an exhaust gas purification apparatus according to Embodiment 1 of the present invention. As shown in FIG. 1, the exhaust gas purification apparatus of Embodiment 1 has a catalyst device 20 in the exhaust passage 12 of the internal combustion engine 10. The catalyst device 20 houses a NOx occlusion reduction type catalyst 22. According to such a configuration, the exhaust gas that has passed through the exhaust passage 12 flows into the catalyst device 20 and flows into the NOx occlusion reduction type catalyst 22.

The NOx occlusion reduction catalyst 22 includes a layer containing a NOx holding substance (hereinafter also referred to as “NOx holding layer”) and a layer containing a noble metal (hereinafter also referred to as “catalyst layer”) as a base material (support). (Also called). The NOx retention layer has a function of storing NOx in the exhaust gas and releasing it in a high temperature atmosphere or a rich atmosphere. The catalyst layer has a function of causing a reaction between NOx and HC or CO to decompose into N ₂ , H ₂ O, CO ₂ or the like. According to such a configuration, NOx contained in the exhaust gas can be effectively purified.

  Hereinafter, the configuration of the NOx occlusion reduction type catalyst 22 will be further described with reference to FIG. FIG. 2A corresponds to a view of the NOx storage-reduction catalyst 22 taken along the line AA in FIG. 1 as viewed from the left side of the drawing. As shown in FIG. 2, the NOx occlusion reduction type catalyst 22 has a ceramic base material 25 whose outer shape is circular. As shown in the figure, the base material 25 has a honeycomb shape, and a plurality of rectangular cells 24 are present therein. Each of these cells 24 penetrates the base material 25 in the depth direction of the paper in FIG. 2A, and the exhaust gas can flow in that direction.

  FIG. 2B is an enlarged view of one of the cells 24. One cell 24 corresponds to one of the sections partitioned by the base material 25. Inside the cell 24, a NOx retention layer 26 and a catalyst layer 27 are provided. These layers are individually laminated on the surface of the base material 25 sequentially from the base material 25 side. A flow path 28 is formed in the cell 24 on the center side of the catalyst layer 27. The flow path 28 extends in the depth direction of FIG. 2B, and the exhaust gas flows through the flow path 28.

The NOx holding layer 26 is formed by coating the base material 25 with a NOx holding material containing BaCO ₃ . BaCO ₃ functions as a NOx holding substance (also referred to as a NOx storage agent) that stores NOx in the exhaust gas as nitrate (specifically, Ba (NO ₃ ) ₂ ). And the occluded Ba (NO ₃ ) ₂ is actively released mainly as the exhaust gas is richer or the NOx retention substance is higher in temperature.

  The catalyst layer 27 is formed by further coating the NOx retention layer 26 with a catalyst material containing a noble metal such as Pt. A noble metal such as Pt functions as an active point for simultaneously activating the oxidation reaction of CO and HC and the reduction reaction of NOx. Therefore, the catalyst layer 27 functions as a three-way catalyst that simultaneously purifies NOx, CO, and HC. The catalyst layer 27 has a gas permeability that allows NOx to move between the flow path 28 and the NOx retention layer 26.

Moreover, the apparatus of Embodiment 1 is provided with the ozone supply apparatus 30, as shown in FIG. The ozone supply device 30 communicates with the air inlet 34. The ozone supply device 30 can obtain air from the air inlet 34 to generate ozone (O ₃ ) and supply it downstream. In addition, since various techniques are already well-known about the structure of an ozone generator which produces | generates ozone from air, a function, etc., the detailed description is abbreviate | omitted.

  The ozone supply device 30 has an ozone injection port 32 through which gas is injected in the catalyst device 20. The ozone injection port 32 is disposed upstream of the NOx storage reduction catalyst 22 in the catalyst device 20. According to such a configuration, ozone or air can be supplied to the exhaust gas flowing through the exhaust passage 12 by injecting ozone from the ozone injection port 32. Then, the supplied ozone or air and the exhaust gas are mixed, and the mixed gas flows into the NOx occlusion reduction type catalyst 22.

  The exhaust gas purification apparatus of the first embodiment has an ECU (Electronic Control Unit) 50. The ECU 50 is connected to the ozone supply device 30. When the ECU 50 transmits a control signal to the ozone supply device 30, the ozone injection timing and the injection amount are controlled. With such a configuration, ozone can be supplied at a desired timing.

  From the viewpoint of efficiently performing the NOx occlusion reaction described above, it is desirable that the NOx in the exhaust gas exists in a more oxidized state. In the first embodiment, ozone can be appropriately supplied to the exhaust gas using the ozone supply device 30. Thereby, NOx in the exhaust gas can be oxidized in the gas phase reaction, and the exhaust gas can be effectively purified.

  The ECU 50 is connected to various sensors provided in the internal combustion engine 10. As a result, the ECU 50 can acquire information such as the temperature of the internal combustion engine 10, the engine speed Ne, the air-fuel ratio A / F, the load, and the intake air amount.

[Features of Embodiment 1]
(Configuration features)
As described above, the NOx holding substance (BaCO _{3 in the} first embodiment) contained in the NOx holding material has a function of occluding NOx in the exhaust gas. Further, the noble metal contained in the catalyst (in the first embodiment, Pt, Rh, Pd, etc.) functions as an active point when purifying the exhaust gas. Effective use of these functions together is extremely important for efficiently performing NOx occlusion reduction and exhaust gas purification.

  Conventionally, various catalysts in which these NOx holding materials and catalysts are integrated are known. Such a catalyst is disclosed in, for example, Japanese Patent No. 3551346, and is also referred to as “NOx storage reduction catalyst” or “NSR catalyst”. The NOx occlusion reduction catalyst has the feature that the NOx occlusion reaction can be promoted by accelerating the oxidation of NOx by the catalyst, and the exhaust gas can be purified by the catalyst when NOx is released.

  However, when the NOx holding material is integrated with the catalyst in this way, the exhaust gas purification capacity of the catalyst (the ability to purify NOx, HC, and CO) is lower than that of a conventional three-way catalyst that does not contain a NOx holding material. turn into. It is considered that this is because the NOx holding substance becomes a catalyst poison with respect to the catalyst (noble metal element), and the NOx holding substance reduces the activity capacity of the catalyst. From the viewpoint of performing efficient exhaust gas purification, it is preferable to avoid such harmful effects and maximize the ability of the catalyst.

  Therefore, in the exhaust gas purifying apparatus of the first embodiment, when the NOx occlusion reduction type catalyst 22 is configured, the base material 25 is individually coated with the NOx retention layer 26 and the catalyst layer 27, and these are independently separated. It is as a layer. As described above, the reduction in the exhaust gas purification capacity of the catalyst is caused by the NOx retention substance acting as a catalyst poison. According to the first embodiment, since the NOx retention layer 26 and the catalyst layer 27 are independently formed, it is possible to prevent the NOx retention material from becoming a catalyst poison with respect to the catalyst layer 27. Hereinafter, the operation at the time of NOx occlusion and NOx release in the configuration of the first embodiment will be described.

(Operation when storing NOx)
As described above, the NOx storage reduction type catalyst 22 of the first embodiment has the catalyst layer 27. The catalyst layer 27 contains a noble metal such as Pt and has a function of simultaneously purifying NOx, CO, and HC (hereinafter also referred to as “exhaust gas purification function”). However, in order for the catalyst to exhibit its exhaust gas purification function, it is required that the catalyst has reached a sufficient activation temperature. For this reason, when the internal combustion engine 12 is started, particularly during a cold start, the temperature of the NOx storage reduction catalyst 22 is low, and it is difficult to purify NOx contained in the exhaust gas.

  Therefore, in the present embodiment, NOx is occluded in the NOx retention layer 26 in the above situation. For the purpose of promoting this NOx occlusion, in this embodiment, ozone is supplied using the ozone supply device 30 so as to be mixed with the exhaust gas flowing into the NOx occlusion reduction type catalyst 22. By adding ozone, it is possible to oxidize NOx in the exhaust gas and make it easier to occlude.

  The NOx oxidized by ozone reaches the NOx occlusion reduction type catalyst 22 and flows into the flow path 28 in each cell 24. In the process of flowing through the flow path 28, NOx passes through the catalyst layer 27 that has not yet reached the activation temperature, and reaches the NOx retention layer 26. Thereafter, an occlusion reaction occurs in the NOx retention layer 26, and NOx is occluded as nitrate. With such an operation, when the internal combustion engine 12 is started, even if the catalyst layer 27 of the NOx storage reduction catalyst 22 has not yet reached the activation temperature, NOx in the exhaust gas can be stored.

(Operation when NOx is released)
After the internal combustion engine 12 is started, as the NOx occlusion described above is performed, the temperature of the NOx occlusion reduction type catalyst 22 also increases. Therefore, if a sufficient time has elapsed after the internal combustion engine 12 is started, the temperature of the catalyst layer 27 in the NOx storage reduction catalyst 22 reaches the activation temperature. Therefore, in the first embodiment, when the catalyst layer 27 reaches the activation temperature and the exhaust gas purification function can be sufficiently exerted, the ozone supply is stopped and the fuel injection amount of the internal combustion engine 12 is slightly rich. (Slightly rich) control.

  When the ozone supply is stopped, the NOx occlusion reaction is not promoted. Further, in the situation where the NOx storage reduction catalyst 22 is at a high temperature, the NOx retention layer 26 is naturally at a high temperature. The NOx retention layer 26 actively releases the stored NOx as the temperature is higher and the atmosphere is richer. For this reason, the NOx releasing reaction is activated by the above-described control.

When NOx is released from the NOx retention layer 26, this NOx reaches the catalyst layer 27, and is reduced by a reducing agent such as HC contained in the exhaust gas in the catalyst layer 27, and N ₂ , H ₂ O, CO _2, etc. become. In the present embodiment, as described above, the NOx retention layer 26 and the catalyst layer 27 form two independent layers, and the NOx retention material is prevented from becoming a catalyst poison with respect to the catalyst layer 27. . Therefore, in the present embodiment, exhaust gas purification can be performed effectively without hindering the exhaust gas purification function of the catalyst layer 27.

  As described above, according to the present embodiment, since the NOx retention layer 26 and the catalyst layer 27 individually form layers, the NOx retention substance does not become a catalyst poison, and the exhaust gas of the catalyst layer 27 It can be avoided that the purification ability is hindered. According to the first embodiment, the NOx occlusion reaction is promoted without relying on the catalyst using the ozone supply by the ozone supply device 30, so that the exhaust gas purifying function of the catalyst is sufficiently exhibited and the NOx occlusion is performed. Reduction can be performed.

  Further, according to the NOx oxidation method using ozone, NOx can be reliably oxidized by a gas phase reaction without depending on the catalyst even under low temperature conditions such as when the internal combustion engine is started. Further, if water vapor is present, nitric acid is generated and easily reacts with the NOx-retaining substance, so that NOx can be occluded efficiently.

  In the first embodiment described above, the NOx occlusion reduction type catalyst 22 is the “NOx occlusion reduction type catalyst” in the first invention, and the ozone supply device 30 is the “ozone supply means” in the first invention. Each corresponds. In the first embodiment described above, the cell 24 is the “cell” of the first invention, the NOx retention layer 26 is the “first layer” of the first invention, and the catalyst layer 27 is the first cell. This corresponds to the “second layer” of the present invention.

[Specific Processing in First Embodiment]
Hereinafter, a specific process performed by the exhaust gas purifying apparatus according to the first embodiment will be described with reference to FIG. FIG. 3 is a flowchart of a routine executed by the ECU 50 in the first embodiment, which is executed when the internal combustion engine 10 is started and in a low temperature state (for example, at Cold Start).

In the routine shown in FIG. 3, ozone supply is first executed (step S100). Specifically, the ECU 50 transmits a control signal to the ozone supply device 30 so that ozone supply is performed at a predetermined flow rate. And ozone is injected based on this control signal. As a result, NO in the exhaust gas is oxidized to NO ₃ , and the occlusion reaction is efficiently performed in the NOx retention layer 26.

Next, it is determined whether or not an O ₃ supply stop condition is satisfied (step S110). Here, it is determined whether or not a predetermined time, which is obtained in advance through experiments or the like, until the catalyst layer 27 reaches the activation temperature has elapsed. If the stop condition is not satisfied, it is determined that the temperature of the catalyst layer 27 has not yet reached the activation temperature, and the processing from step S100 is performed again.

When the O ₃ stop condition is satisfied, the operation state of the internal combustion engine 10 is controlled so that the air-fuel ratio becomes slightly rich (slight rich) from stoichiometric after the supply of O ₃ is stopped. (Step S130). Thereby, NOx occluded in the NOx retention layer 26 is released. Thereafter, this NOx reaches the catalyst layer 27 and is reduced and purified. Thereafter, the current routine ends.

  According to the above processing, NOx occlusion reduction can be performed while reliably avoiding the NOx retention substance from becoming a catalyst poison and maximizing the exhaust gas purification ability of the catalyst layer 27. In addition, the NOx oxidation method using ozone can reliably oxidize NOx without relying on the catalyst even under low temperature conditions such as when the internal combustion engine is started, and ensure good emission characteristics.

[Experimental result of Embodiment 1]
Hereinafter, the results of experiments performed on the first embodiment of the present invention will be described with reference to FIGS.

(Configuration of measurement system)
FIG. 4 is a diagram showing a measurement system used in this experiment. This measurement system includes a model gas generator 230 and a plurality of gas cylinders 232 in order to generate a model gas imitating the exhaust gas of an internal combustion engine. The model gas generator 230 can create a simulated gas having the following composition by mixing the gas in the gas cylinder 232.
Simulated gas composition: C ₃ H ₆ 1000 ppm,
CO 7000 ppm,
NO 1500ppm,
O ₂ 7000 ppm
CO ₂ 10%,
H ₂ O 3%
Remaining N ₂

  The model gas generator 230 communicates with an electric furnace in which the test body 222 is installed. FIG. 5 is an enlarged view of the test body 222 and the vicinity thereof. As shown in FIG. 5, the test body 222 houses the example sample 224 in a quartz tube. In this experiment, as a comparative example, the example sample 224 is replaced with a later-described comparative example sample, and the same experiment as the example sample 224 is performed.

  The measurement system in FIG. 4 has an oxygen cylinder 240. The oxygen cylinder 240 communicates with the flow rate control units 242, 244 on the downstream side. The flow rate control unit 242 communicates with the ozone generator 246. The ozone generator 246 can generate ozone in response to oxygen supply from the oxygen cylinder 240. The ozone generator 246 communicates with the downstream position of the model gas generator 230 and the upstream position of the test body 222 via the ozone analyzer 248 and the flow rate control unit 250.

On the other hand, the flow rate control unit 244 communicates directly with the ozone analyzer downstream thereof. According to such a configuration, a mixed gas of O ₃ and O ₂ when the power to the ozone generator 246 is turned ON is only O ₂ if the power is turned OFF is supplied to the upstream position of the test body 222 .

In the measurement system of FIG. 4, by using the flow rate control units 242 and 244 and the ozone generator 246 as appropriate, gases having the following two types of compositions can be respectively created. This gas is a gas to be injected into the test body 222, and will be simply referred to as “injected gas” hereinafter.
Injection gas composition: (1) 30000 ppm of O ₃ , balance O ₂
(2) Only O ₂ and the flow rate control unit 250 can supply the injection gas at a desired flow rate.

  Downstream of the test body 222, exhaust gas analyzers 260 and 262 and an ozone analyzer 264 are provided. With these analyzers, the component of the gas flowing out from the test body 222 can be measured.

The measurement equipment used in this experiment is shown below.
Ozone generator 246: Iwasaki Electric OP100W
Ozone analyzer 248 (upstream side): Sugawara Jigyo EG600
Ozone analyzer 264 (downstream): Sugawara Jigyo EG2001B
Exhaust gas analyzers 260 and 262:
HORIBA, Ltd. MEXA9100D (HC, CO, NOx measurement)
HORIBA, Ltd. VAI-510 (measures CO ₂ )

(Sample preparation method)
FIG. 6 is a diagram for explaining the first example sample and the comparative example sample used in this experiment. FIG. 6A shows one cell of the example sample 224 also shown in FIG. FIG. 6B is a diagram showing one cell of the comparative example sample 324. The comparative example sample 324 uses the same honeycomb substrate as that of the example sample 224, but has a coating different from that of the example sample 224.

The example sample 224 shown in FIG. 6A was produced by the following procedure. First, γ-Al ₂ O ₃ is dispersed in ion exchange water, and an aqueous barium acetate solution is added thereto. The water in the mixture was removed by heating, dried at 120 ° C., and pulverized into a powder. This powder was calcined at 500 ° C. for 2 hours. The fired powder was immersed in a solution containing ammonium bicarbonate, dried at 250 ° C., and Al ₂ O ₃ supported on barium (hereinafter also referred to as “barium-supported catalyst”). . The supported amount of barium is 0.2 mol per 120 g of γ-Al ₂ O ₃ .

Next, γ-Al ₂ O ₃ is dispersed in ion-exchanged water, and an aqueous solution containing dinitrodiammine platinum is added thereto to carry Pt, dried, pulverized, and then baked at 450 ° C. for 1 hour to form Al ₂ O ₃ . A platinum-supported material (hereinafter also referred to as “platinum-supported catalyst”) was obtained. The supported amount of platinum is 4 g per 120 g of γ-Al ₂ O ₃ .

Subsequently, a barium-supported catalyst was coated on a cordierite honeycomb 125 of φ30 mm × L50 mm, 4 mil / 400 cpsi, and fired at 450 ° C. for 1 hour. Thereby, a Ba-supported catalyst layer 126 was obtained. The coating amount was such that Al ₂ O ₃ was coated at 60 g / L. Subsequently, a platinum-supported catalyst was further coated on the honeycomb 125 coated with such a coating, followed by firing at 450 ° C. for 1 hour to obtain a Pt-supported catalyst layer 127. The coating amount was such that Al ₂ O ₃ was coated at 60 g / L. Through this process, an example sample 224 having a two-layer coated catalyst was obtained.

As a result, in Example Sample 224, the total Pt loading was 2 g, the Ba loading was 0.1 mol / Al ₂ O ₃ 120 g, and the coating amount was 120 g / L (Al ₂ O ₃ ).

On the other hand, the comparative example sample 324 shown in FIG. 6B was produced by the following procedure. First, γ-Al ₂ O ₃ is dispersed in ion exchange water, and an aqueous barium acetate solution is added thereto. The water in the mixture was removed by heating, dried at 120 ° C., and pulverized into a powder. This powder was calcined at 500 ° C. for 2 hours. The calcined powder was immersed in a solution containing ammonium hydrogen carbonate and dried at 250 ° C. to obtain a barium-supported catalyst.

The barium-supported catalyst was dispersed in ion-exchanged water, an aqueous solution containing dinitrodiammine platinum was added thereto, Pt was supported, dried, pulverized, and calcined at 450 ° C. for 1 hour. Thus, a catalyst for a comparative example coating having a barium loading amount of 0.1 mol per 120 g of γ-Al ₂ O ₃ and a platinum loading amount of ₂ g per 120 g of γ-Al ₂ O ₃ was obtained. Subsequently, a cordierite honeycomb 325 of φ30 mm × L50 mm, 4 mil / 400 cpsi was coated with the comparative coating catalyst prepared as described above, and fired at 450 ° C. for 1 hour. As a result, a PtBa catalyst layer 326 was obtained. The coating amount was such that Al ₂ O ₃ was coated at 120 g / L.

As a result, in the comparative sample 324 as well, the total Pt loading was 2 g, the Ba loading was 0.1 mol / Al ₂ O ₃ 120 g, and the coating amount was 120 g / L (Al ₂ O ₃ ). Therefore, the example sample 224 and the comparative example sample 324 are configured to include the same amount of Pt and Ba.

(Experiment contents)
In the measurement system as described above, the simulated gas having various compositions described above and the injected gas were combined and supplied to the test body 222 under the following conditions. Then, the electric furnace was controlled so that the catalyst temperature increased at the following temperature increase rate, and the concentration of NOx flowing out downstream was investigated.
Temperature: 30 ° C to 500 ° C
Temperature increase rate: 10 ° C / min (constant)
Simulated gas flow rate: 30L / min
Injection gas flow rate: 6 L / min
The injection gas was supplied in the range of 30 ° C. to 300 ° C., and in the range of 300 ° C. to 500 ° C., the injection gas was not supplied and only the simulation gas was circulated.

(Calculation method of purification rate)
FIG. 7 is an image diagram of calculation of the exhaust gas purification rate in this experiment. FIG. 7A is an image of the component amount in the supplied exhaust gas obtained by multiplying the simulated gas concentration by the test time. In this experiment, based on such an image, specifically, the component amount in the exhaust gas supplied within the measurement time was calculated by the equation of the concentration of the component in the simulated gas × simulated gas flow rate × test time.

  FIG. 7B is an image of the downstream exhaust gas component amount obtained by multiplying the concentration of gas flowing out downstream of the test body 222 by the test time. Based on this image, the amount of component flowing out downstream was calculated by the equation of component concentration detected by exhaust gas analyzer × gas flow rate × test time.

  Then, using these calculated values, as shown in FIG. 7C, the amount of component flowing out downstream from the amount of gas supplied in the measurement time (FIG. 7A) (FIG. 7B). ) Was subtracted, and this was divided by the amount of gas supplied within the measurement time (FIG. 7 (a)) and calculated at 100 minutes.

(Experimental result)
FIG. 8 is a diagram for explaining a first experimental result of the above experiment. As shown in the graph of FIG. 8, it can be seen that the use of the example sample 224 shows a higher purification rate for NOx, HC, and CO than the case of the comparative example sample 324.

  From this result, according to the first embodiment, the NOx occlusion reaction is realized while avoiding the influence of the catalyst poison, so that the catalyst exhibits its exhaust gas purification ability to the maximum, and good emission is achieved. It can be seen that characteristics can be obtained. Further, as described above, the amounts of barium and platinum contained in the example sample 224 are the same as those contained in the comparative sample 324. That is, according to the present embodiment, the NOx holding substance and the noble metal can be used efficiently.

[Modification of Embodiment 1]
(First modification)
In the first embodiment, the substrate 25 is coated with the NOx retention layer 26 containing BaCO ₃ . However, the material constituting the NOx retention layer is not limited to the material specified in the above description. Besides BaCO ₃ , for example, as disclosed in Japanese Patent No. 3551346, alkali metals such as Na, K, Cs, and Rb, alkaline earth metals such as Ba, Ca, and Sr, Y, Ce, La, Pr, and the like Rare earth elements can be used as needed.

Therefore, when NOx is occluded as a nitrate in the NOx retention substance, the composition of the nitrate is not limited to Ba (NO ₃ ) ₂ described in the first embodiment. Regarding Ba, ₃ mol of NO ₃ can be occluded with respect to 1 mol of Ba, the feature is that the occlusion amount is large, and the thermal stability is higher than other materials, and it is a NOx holding substance used in exhaust gas purification devices. Is suitable.

Further, the material constituting the catalyst layer 27 is not limited to the materials such as Pt, Rh, and Pb specified in the above description. Various catalyst materials known as noble metal materials constituting a catalyst for purifying exhaust gas can be used in the present invention. Furthermore, as a material for supporting the noble metal or the NOx holding substance, a suitable material such as ceramics or alumina (Al ₂ O ₃ ) can be used as appropriate.

  Similarly, various known materials such as ceramics and metal can be used for the material of the base material. In the first embodiment, the NOx retention layer 26 and the catalyst layer 27 are coated on the base material 25 having the cells 24 partitioned in a lattice pattern. However, the present invention is not limited to this, and various other known base materials can be used in the present invention.

(Second modification)
In the first embodiment, ozone is added to the exhaust gas by the ozone supply device 30. This ozone addition may more preferably be performed in the following manner. It is known that when ozone (O ₃ ) is added to exhaust gas, NOx in the exhaust gas is oxidized by a gas phase reaction. Specifically, NOx reacts with ozone and the following reaction occurs.
NO + O ₃ → NO ₂ + O ₂ [1]
NO ₂ + O ₃ → NO ₃ + O ₂ ... [2]
NO ₂ + NO ₃ → N ₂ O ₅ [3]
In the following description, the reaction formula [1] is also called “first formula”, the reaction formula [2] is called “second formula”, and the reaction formula [3] is also called “third formula”. . In addition, although only the arrow which shows the reaction to the right direction is described in the 3rd type | formula, the reaction to the left direction may also arise.

The NOx occlusion generated in the NOx holding substance is caused by the NOx holding substance storing higher-order nitrogen oxides (or HNO ₃ produced by reaction of these nitrogen oxides with water) generated by oxidation of NOx. Realized. For example, NO ₃ is occluded in the NOx holding material by becoming a nitrate such as Ba (NO ₃ ) ₂ . Therefore, from the viewpoint of performing NOx occlusion reaction efficiently, it is desirable that the NOx in the exhaust gas becomes more NO _{3, N} 2 _{O 5} and other nitrogen oxides of higher order than NO ₂ and the like.

Therefore, in the second modification, ozone is added so that the molar ratio of ozone to NO in the mixed gas is larger than 1, so that the reaction of the second formula occurs. That is, the ratio of mol (O ₃ ) in which the amount of ozone in the mixed gas is converted to mol and mol (NO) in which the amount of nitric oxide in the mixed gas is converted into mol also satisfies the following relational expression. Thus, ozone is added.
mol (O ₃ ) / mol (NO)> 1 ... [4]
In the following description, the equation [4] is also referred to as “fourth equation”.

In the state where the molar ratio of ozone to NO in the mixed gas is 1 or less (mol (O ₃ ) / mol (NO) ≦ 1), NO ₂ is generated by the reaction of the above-mentioned first formula, but the second formula The production of NO ₃ and N ₂ O ₅ by the reaction of the third formula does not reach. In the second modification, the amount of ozone added in consideration of this point is made larger than the amount of NO in the exhaust gas, so NO is oxidized to NO ₃ and N ₂ O ₅ . A sufficient amount of ozone can be supplied (to cause the reactions of formulas 2 and 3). As a result, the amount of higher-order nitrogen oxides in the exhaust gas can be reliably increased, and NOx occlusion can be performed effectively.

  In addition, such a process is performed by the ECU 50 "a process of adjusting the ozone supply amount so that the molar ratio of ozone to nitrogen monoxide (NO) in the mixed gas flowing into the NOx storage reduction catalyst becomes larger than 1". This is realized by executing (ozone supply amount adjustment processing). This process can be performed, for example, before step S100 of the routine of FIG. In addition, the ozone supply amount that satisfies the above-described molar ratio is included in the exhaust gas based on, for example, the operating state of the internal combustion engine 10 (engine speed Ne, air-fuel ratio A / F, load, intake air amount, etc.). It can be determined by estimating the molar amount of NOx and calculating the flow rate of ozone supplied in accordance with the estimated molar amount of NOx.

(Third Modification)
Furthermore, the ozone supply amount may be further increased so that the molar ratio of ozone to nitrogen monoxide in the mixed gas is 2 or more (Mol (O ₃ ) / Mol (NO) ≧ 2). If the molar ratio of ozone (O ₃ ) to nitrogen monoxide (NO) in the mixed gas is greater than 1, ozone is still in the mixed gas even after NO is oxidized to NO ₂ by the above-described first equation. Since it remains, the reactions of the second and third formulas occur to generate NO ₃ and N ₂ O ₅ . However, when the amount of ozone remaining after the reaction of the first formula is very small, the amounts of NO ₃ and N ₂ O ₅ produced by the reactions of the second and third formulas are also reduced.

Therefore, in the third modification, the supply amount of ozone is adjusted so that the molar ratio of ozone and NO in the mixed gas is Mol (O ₃ ) / Mol (NO) ≧ 2. As a result, after the reaction of the first formula, a sufficient amount of ozone that contributes to the reaction during the reactions of the second formula and the third formula remains, and the amount of higher-order nitrogen oxides can be increased reliably. it can. As described above, according to the third modification, it is possible to supply a sufficient amount of ozone to oxidize NO into NO ₃ and N ₂ O _5, and effectively promote the NOx occlusion reaction. can do.

In this process, the ECU 50 adjusts the ozone supply amount so that the molar ratio of ozone (O ₃ ) to nitrogen monoxide (NO) in the mixed gas flowing into the NOx storage reduction catalyst becomes 2 or more. This is realized by executing “processing to perform”. This process can be performed, for example, before step S100 of the routine of FIG.

(Fourth modification)
In Embodiment 1, the ozone supply device 30 was provided outside the catalyst device 20, and ozone was supplied in a configuration in which the ozone injection port 32 was disposed in the catalyst device 20. However, the present invention is not limited to this. Ozone can be added to the exhaust gas using various known ozone generators and ozone generation methods. For example, the configuration may be such that ozone is directly generated in the exhaust passage 12 or the catalyst device 20 by plasma discharge.

  The NOx holding material may not only occlude NOx but also adsorb NOx. That is, the NOx occlusion reduction type catalyst 22 may not only occlude NOx but also adsorb it. For this reason, “holding” in the NOx holding material includes not only the meaning of “occluding” NOx but also the meaning of “adsorbing” NOx.

  Further, the content of the NOx holding substance in the catalyst layer 27 is preferably substantially zero, but the present invention is not limited to this. A configuration in which the NOx retention substance content of the catalyst layer 27 is relatively smaller than the NOx retention substance content of the NOx retention layer 26 may be employed.

It is a figure for demonstrating the structure of the exhaust-gas purification apparatus of the internal combustion engine of Embodiment 1 of this invention. FIG. 3 is a diagram for explaining a configuration of a device according to the first embodiment. 3 is a flowchart executed by the ECU in the first embodiment. 6 is a diagram for explaining an experimental result regarding the first embodiment. FIG. 6 is a diagram for explaining an experimental result regarding the first embodiment. FIG. 6 is a diagram for explaining an experimental result regarding the first embodiment. FIG. 6 is a diagram for explaining an experimental result regarding the first embodiment. FIG. 6 is a diagram for explaining an experimental result regarding the first embodiment. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 12 Exhaust passage 20 Catalytic device 22 NOx occlusion reduction type catalyst 24 Cell 25 Base material 26 NOx retention layer 27 Catalyst layer 28 Flow path 30 Ozone supply device 32 Ozone injection port 34 Air inlet 50 ECU

Claims (4)

A NOx occlusion reduction catalyst provided in the exhaust passage of the internal combustion engine;
Ozone supply means for supplying ozone so as to be mixed with exhaust gas flowing into the NOx storage reduction catalyst,
The NOx occlusion reduction type catalyst includes a cell through which exhaust gas flows, and the inner surface of the cell includes a first layer containing a NOx holding material and a content of the NOx holding material containing a noble metal in the first layer. And a second layer which allows less NOx permeation and is coated so as to be positioned in the order of the first layer and the second layer from the inner surface side of the cell. An exhaust gas purification device for an internal combustion engine.   The exhaust gas purifying device for an internal combustion engine according to claim 1, wherein the content of the NOx retention substance in the second layer is substantially zero.   The apparatus further comprises ozone supply amount adjusting means for adjusting the ozone supply amount so that the molar ratio of ozone to nitrogen monoxide (NO) in the mixed gas flowing into the NOx storage reduction catalyst becomes larger than 1. The exhaust gas purifying device for an internal combustion engine according to claim 1 or 2. The ozone supply amount adjusting means adjusts the ozone supply amount so that a molar ratio of ozone (O ₃ ) to nitrogen monoxide (NO) in the mixed gas flowing into the NOx storage reduction catalyst becomes 2 or more. The exhaust gas purification apparatus for an internal combustion engine according to claim 3,

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