JP5640521B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

  A NOx occlusion reduction type catalyst that stores NOx contained in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and reduces and purifies the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich An internal combustion engine arranged in an engine exhaust passage is known. In such an internal combustion engine, NOx generated when combustion is performed under a lean air-fuel ratio is stored in the NOx storage reduction catalyst. On the other hand, when the NOx storage capacity of the NOx storage reduction catalyst approaches saturation, the air-fuel ratio of the exhaust gas is temporarily made rich, whereby NOx is released from the NOx storage reduction catalyst and reduced and purified.

By the way, sulfur is contained in the fuel and the lubricating oil, and therefore SOx is contained in the exhaust gas. This SOx is stored in the NOx occlusion reduction type catalyst together with NOx, and reacts with a NOx holding substance such as barium carbonate (BaCO ₃ ) contained in the NOx occlusion reduction type catalyst to form a sulfate (BaSO ₄ ). Since such a sulfate is very stable, it is left as it is without being decomposed only by making the air-fuel ratio of the exhaust gas rich. For this reason, with the passage of time, the amount of sulfate formed in the NOx storage reduction catalyst increases, the amount of NOx that can be stored in the NOx storage reduction catalyst decreases, and consequently the NOx purification performance of the NOx storage reduction catalyst decreases. Will cause.

  In Patent Document 1, NOx is absorbed when the air-fuel ratio of exhaust gas is lean, and NOx absorbent (NOx occlusion reduction type catalyst) that releases the absorbed NOx when the oxygen concentration in the exhaust gas is reduced is engine exhaust. An engine exhaust passage upstream of the NOx absorbent is disposed in the passage and absorbs SOx when the air-fuel ratio of the exhaust gas is lean and releases SOx absorbed when the air-fuel ratio of the exhaust gas becomes rich. The bypass passage for bypassing the NOx absorbent is branched from the engine exhaust passage located between the SOx absorbent and the NOx absorbent, and either the NOx absorbent or the bypass passage is provided at a branch portion of the bypass passage. Describes an exhaust gas purification apparatus for an internal combustion engine in which a switching valve for allowing exhaust gas to flow is arranged. Further, in Patent Document 1, when NOx should be released from the NOx absorbent, the switching valve is held at a position where the exhaust gas flows into the NOx absorbent, and the oxygen concentration in the exhaust gas flowing into the SOx absorbent is reduced, When SOx is to be released from the SOx absorbent, the switch valve is switched to a position where the exhaust gas flows into the bypass passage, and the air / fuel ratio of the exhaust gas flowing into the SOx absorbent is made rich to release SOx from the SOx absorbent. It is described that the SOx released from the SOx absorbent can be prevented from being absorbed by the NOx absorbent.

Japanese Patent No. 2605580

  In an exhaust purification catalyst generally used in an exhaust purification device of an internal combustion engine, for example, a NOx purification catalyst, when a catalytic metal such as a noble metal contained as a catalyst component is exposed to a lean atmosphere containing excess oxygen, the surface of the catalytic metal is There is a problem that the surface of the catalyst metal is covered with oxygen, that is, the surface of the catalyst metal is subjected to so-called oxygen poisoning, and as a result, the NOx purification performance of the NOx purification catalyst is deteriorated. When this NOx purification catalyst is a NOx occlusion reduction type catalyst containing a NOx retention material together with a catalyst component such as a noble metal, sulfur poisoning caused by SOx contained in the exhaust gas is a problem as described above. Become. However, when the NOx purification catalyst is a material that does not contain such a NOx holding substance and is simply formed by supporting only the catalyst metal on the catalyst carrier, the above-described problem of sulfur poisoning may be overcome. Decreasing NOx purification performance due to oxygen poisoning of a catalyst metal is a bigger problem.

  Further, the catalyst metal used in the NOx purification catalyst or the like is more easily oxidized at a low temperature, and therefore the catalyst metal is more susceptible to oxygen poisoning at a low temperature such as when the internal combustion engine is cold started. In recent years, regulations on exhaust gases from automobiles and the like have become stricter. Therefore, there are problems concerning oxygen poisoning of catalyst metals when the catalyst itself is not sufficiently warmed, such as during cold start of an internal combustion engine. It is necessary to deal with it sufficiently.

  Accordingly, an object of the present invention is to provide an exhaust purification device for an internal combustion engine that can suppress a decrease in NOx purification performance due to oxygen poisoning of the NOx purification catalyst with a novel configuration.

The present invention for solving the above problems is as follows.
( 1) a NOx holding material that is disposed in the exhaust passage of the internal combustion engine and can hold at least NOx in the exhaust gas;
A NOx purification catalyst that is disposed in the exhaust passage downstream of the NOx holding material and supports a catalyst metal capable of purifying NOx in exhaust gas on a catalyst carrier, wherein the catalyst metal is a base metal. A purification catalyst;
A bypass passage branching from a branch portion of the exhaust passage located between the NOx holding material and the NOx purification catalyst, and joining at a joining portion of the exhaust passage downstream of the NOx purification catalyst;
A switching valve that is provided in the branch portion and allows exhaust gas to flow into either the NOx purification catalyst or the bypass passage;
With
When the NOx holding material is a NOx occlusion reduction type catalyst further containing a noble metal, and the air-fuel ratio of the exhaust gas flowing into the NOx occlusion reduction type catalyst is lean, the switching valve is located at a position where the exhaust gas flows into the bypass passage. And the exhaust gas flowing into the NOx occlusion reduction catalyst has a rich air-fuel ratio, and the exhaust gas flows into the NOx purification catalyst when NOx is released from the NOx occlusion reduction catalyst. and to switch the switching valve, the internal combustion engine exhaust gas purification apparatus.
( 2) a NOx holding material disposed in the exhaust passage of the internal combustion engine and capable of holding at least NOx in the exhaust gas;
A NOx purification catalyst that is disposed in the exhaust passage downstream of the NOx holding material and supports a catalyst metal capable of purifying NOx in exhaust gas on a catalyst carrier, wherein the catalyst metal is a base metal. A purification catalyst;
A bypass passage branching from a branch portion of the exhaust passage located between the NOx holding material and the NOx purification catalyst, and joining at a joining portion of the exhaust passage downstream of the NOx purification catalyst;
A switching valve that is provided in the branch portion and allows exhaust gas to flow into either the NOx purification catalyst or the bypass passage;
With
The NOx holding material is a NOx adsorbent, and further includes a catalyst that is arranged on the upstream side of the NOx adsorbent or on the downstream side of the merging portion and carries a noble metal on a catalyst carrier, and the NOx purification catalyst has its activity The switch valve is held at a position where the exhaust gas flows into the bypass passage when it has not reached the control temperature, and the temperature of the exhaust gas reaches a temperature at which NOx is released from the NOx adsorbent or the NOx adsorbent the amount of adsorbed NOx exhaust gas at the stage reached saturation was to switch the switching valve to a position that flows into the NOx purification catalyst, exhaust gas purification apparatus for an internal combustion engine.
(3) The exhaust gas purification apparatus for an internal combustion engine according to (1) or (2), wherein the base metal is Fe, Ni, Cu, or Co.

  According to the exhaust gas purification apparatus for an internal combustion engine of the present invention, for example, when a NOx occlusion reduction type catalyst is used as the NOx holding material, the exhaust gas flows into the bypass passage when the air-fuel ratio of the exhaust gas is lean. By switching the switching valve to the position, it is possible to prevent the NOx purification catalyst from being poisoned by such a high concentration of oxygen. When the air-fuel ratio of the exhaust gas is rich, the switching valve is switched to a position where the exhaust gas flows into the NOx purification catalyst, so that NOx is reduced by the upstream NOx storage reduction catalyst and the downstream NOx purification catalyst. Purified. In this way, NOx that could not be purified by the upstream NOx storage reduction catalyst can be reliably reduced and purified by the downstream NOx purification catalyst, so that the NOx is higher than that of the conventional exhaust purification device. It is possible to achieve a purification rate. Further, according to the exhaust gas purification apparatus for an internal combustion engine of the present invention, the position where the exhaust gas flows into the bypass passage when the catalyst metal of the NOx purification catalyst is easily affected by oxygen poisoning, for example, during cold start. By switching the switching valve, the NOx purification catalyst can be prevented from being poisoned by oxygen in the exhaust gas. Then, by switching the switching valve to the NOx purification catalyst side when the temperature of the exhaust gas reaches the temperature at which NOx is released from the NOx holding material, the released NOx can be reduced and purified by the NOx purification catalyst. is there.

It is the figure which showed typically the 1st embodiment of the exhaust gas purification apparatus of this invention. It is a flowchart of the switching valve operation in the 1st embodiment of the exhaust gas purification apparatus of this invention. It is the figure which showed typically the 2nd embodiment of the exhaust gas purification apparatus of this invention. It is a flowchart of the switching valve operation in the 2nd embodiment of the exhaust gas purification apparatus of this invention.

  The exhaust gas purification apparatus for an internal combustion engine according to the present invention is disposed in an exhaust passage of the internal combustion engine, and is disposed in the exhaust passage downstream of the NOx retaining material and the NOx retaining material capable of retaining at least NOx in the exhaust gas. Branched from a branch portion of the exhaust passage located between the NOx purifying catalyst and the NOx holding material and the NOx purifying catalyst, which are supported on a catalyst carrier and capable of purifying a catalytic metal capable of purifying NOx in the exhaust gas. And a bypass passage that joins at a joining portion of the exhaust passage downstream of the NOx purification catalyst, and a branch portion, for allowing exhaust gas to flow into either the NOx purification catalyst or the bypass passage. A switching valve, and when the NOx should be held by the NOx holding material, the switching valve is held at a position where exhaust gas flows into the bypass passage, Exhaust gas when the NOx from Ox holding member is released is characterized in that so as to switch the switching valve to a position that flows into the NOx purification catalyst.

  According to the present invention, the NOx holding material is not particularly limited as long as it can hold at least NOx contained in the exhaust gas. For example, when the air-fuel ratio of the exhaust gas is lean, it stores NOx. , Materials such as NOx occlusion reduction catalyst that releases and reduces the stored NOx when the air-fuel ratio of the exhaust gas becomes rich, and NOx adsorbent that adsorbs NOx in the low temperature range and releases the adsorbed NOx in the high temperature range Can be used.

  The term “NOx holding material” used in the present invention includes any material that can hold NOx as it is or in another form, such as nitrate. To do. In the present invention, “holding” includes “adsorption”, “absorption”, and “occlusion”.

  According to the present invention, as the NOx purification catalyst, any material obtained by supporting a catalyst metal capable of purifying NOx in exhaust gas on a catalyst carrier can be used. Such a material is not particularly limited as long as it can reduce and purify NOx in the exhaust gas. For example, a noble metal such as rhodium (Rh) or a base metal such as Fe, Ni, Cu and Co is used as a catalyst metal. And a material obtained by supporting the catalyst metal on an arbitrary metal oxide generally used as a catalyst carrier of a NOx purification catalyst can be used.

  Base metals such as Fe, Ni, Cu and Co have lower reducing ability than lean metals such as Rh in a lean atmosphere or an atmosphere near the stoichiometric air-fuel ratio (stoichiometric), and therefore the air-fuel ratio of the exhaust gas is lean or stoichiometric. If so, NOx contained in the exhaust gas cannot be sufficiently reduced and purified. Therefore, when these base metals are used as the catalyst metal of the NOx purification catalyst in the exhaust purification apparatus of the present invention, it is preferable to control the air-fuel ratio of the exhaust gas to a rich atmosphere corresponding to each catalyst metal. Such rich atmosphere control may be performed only at the timing of reducing and purifying NOx in the exhaust gas, and it is not always necessary to control the rich atmosphere. Such control may be performed, for example, by injecting fuel into the combustion chamber of the internal combustion engine body, or by attaching a reducing agent supply valve in the upstream exhaust passage of the NOx holding material or the NOx purification catalyst, and You may carry out by supplying the reducing agent which consists of a reducing agent, Preferably hydrocarbon or a fuel from a reducing agent supply valve.

  Hereinafter, with reference to the drawings, preferred embodiments of the exhaust gas purification apparatus for an internal combustion engine of the present invention will be described in more detail. However, these descriptions are only intended to illustrate preferred embodiments of the present invention, and It is not intended that the invention be limited to these specific embodiments.

  FIG. 1 is a diagram schematically showing a first embodiment of the exhaust emission control device of the present invention.

  Referring to FIG. 1, the exhaust side of the internal combustion engine 10 is connected to a casing 13 containing a NOx holding material 12 via an exhaust pipe 11, and an outlet portion of the casing 13 is connected to a NOx purification catalyst 15 via an exhaust pipe 14. Is connected to a casing 16 containing the. A bypass passage 17 is branched from a branch portion on the upstream side of the casing 16, and the bypass passage 17 is connected to an exhaust pipe 18 connected to an outlet portion of the casing 16. Note that a switching valve 19 is provided at the branch portion, and the switching valve 19 is provided with a bypass closed position in which the entire amount of exhaust gas flows into the NOx purification catalyst 15 side by an electronic control unit (ECU) 30 and exhaust gas. The position can be switched to one of the bypass open position where the entire amount flows into the bypass passage 17. In the first embodiment of the present invention, the switching valve 19 is always held in the bypass open position. Further, a reducing agent supply valve 20 for supplying a reducing agent made of, for example, hydrocarbons into the exhaust gas is attached in the exhaust pipe 11. Temperature sensors 21 and 22 for detecting the temperature of the exhaust gas flowing out from the casings 13 and 16 are attached to the exhaust pipes 14 and 18, respectively. The temperatures of these exhaust gases are the NOx holding material 12 and the NOx purification respectively. This is for detecting the temperature of the catalyst 15. Further, an air-fuel ratio sensor 23 is attached to the exhaust pipe 14 as necessary.

  According to the first embodiment of the present invention, the exhaust emission control device of the present invention is preferably used in an internal combustion engine that burns an oxygen-rich lean gas mixture, such as a diesel engine or a lean combustion engine. In this case, the NOx holding material 12 is a so-called NOx storage-reduction catalyst that stores NOx when the air-fuel ratio of the exhaust gas is lean and releases the stored NOx to reduce and purify when the air-fuel ratio of the exhaust gas becomes rich. (Hereinafter referred to as “NSR catalyst”) is preferably used.

Examples of the NSR catalyst 12 include a compound containing a noble metal such as platinum (Pt) and an element such as an alkali metal, an alkaline earth metal, or a rare earth element on a porous carrier such as alumina (Al ₂ O ₃ ), for example. In addition, those carrying carbonates, nitrates, and oxides of these elements can be used.

  In an internal combustion engine such as a diesel engine or a lean combustion engine, an oxygen-excess lean air-fuel mixture is combusted, so that the air-fuel ratio of the exhaust gas flowing into the NOx purification catalyst 15 naturally also becomes an oxygen-excess lean air-fuel ratio. For this reason, the NOx purification catalyst 15 disposed in the exhaust passage of the internal combustion engine is exposed to such exhaust gas having a high oxygen concentration and is subjected to oxygen poisoning, and the NOx purification performance of the NOx purification catalyst 15 is reduced. End up.

  According to the first embodiment of the present invention, in a lean atmosphere with a high oxygen concentration, the switching valve 19 is switched to a position where the exhaust gas flows into the bypass passage 17, whereby the oxygen of the NOx purification catalyst 15 as described above. It is possible to prevent poisoning.

  FIG. 2 is a flowchart of the switching valve operation in the first embodiment of the exhaust gas purification apparatus of the present invention. This switching valve operation is performed as a routine that is executed by interruption every predetermined time set in advance by the electronic control unit (ECU) 30. Further, in the present embodiment, as a method of adapting the operation of the switching valve 19 to the NOx occlusion amount of the NSR catalyst 12, for example, the ECU 30 is previously discharged from the internal combustion engine 10 per unit time according to the operating state of the internal combustion engine 10. A method of storing the NOx amount NOXA to be stored in the form of a map and calculating the NOx amount ΣNOX stored in the NSR catalyst 12 using this map is employed.

  Referring to FIG. 2, first, at step 100, the NOx amount NOXA discharged per unit time is calculated from the above map. Next, at step 101, NOXA is added to the NOx amount ΣNOX stored in the NSR catalyst 12. Next, at step 102, it is determined whether or not the occluded NOx amount ΣNOX exceeds the allowable amount MAX. When ΣNOX> MAX, the routine proceeds to step 103. On the other hand, when the occluded NOx amount ΣNOX is less than or equal to the allowable amount MAX, the routine is terminated without performing the rich process (that is, the NOx release process).

  In step 103, bypass closed position switching control for switching the switching valve 19 to the bypass closed position is performed to prepare for rich processing. Next, at step 104, a rich process is performed in which the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 12 is temporarily made rich from lean to release NOx. Such rich processing may be performed by supplying a reducing agent or fuel from a reducing agent supply valve 20 attached in the upstream side exhaust pipe 11 of the NSR catalyst 12, as shown in FIG. Alternatively, it may be performed by injecting fuel into the combustion chamber of the internal combustion engine body. Next, in step 105, bypass opening position switching control for switching the switching valve 19 to the bypass opening position is performed. Finally, in step 106, the stored NOx amount ΣNOX is cleared to 0 and the routine is ended.

  As described above, according to the first embodiment of the present invention, when the oxygen concentration in the exhaust gas is high, particularly when the air-fuel ratio of the exhaust gas is lean, switching to a position where the exhaust gas flows into the bypass passage 17 is performed. By switching the valve 19, it is possible to reliably prevent the NOx purification catalyst 15 from being poisoned by such a high concentration of oxygen. When the oxygen concentration in the exhaust gas is low, especially when the air-fuel ratio of the exhaust gas is rich, the upstream side NSR catalyst 12 is switched by switching the switching valve 19 to a position where the exhaust gas flows into the NOx purification catalyst 15. NOx released from the catalyst is reduced and purified by the two catalysts of the NSR catalyst 12 and the downstream NOx purification catalyst 15. In this way, NOx that could not be completely purified by the upstream NSR catalyst 12 can be reliably reduced and purified by the downstream NOx purification catalyst 15, so that it can be compared with a conventional exhaust purification device for an internal combustion engine. A high NOx purification rate can be achieved.

  Next, the second embodiment of the exhaust emission control device of the present invention will be specifically described with reference to FIG. FIG. 3 is a diagram schematically showing a second embodiment of the exhaust emission control device of the present invention.

  The second embodiment of the present invention is shown in FIG. 1 except that a so-called three-way catalyst 24 formed by supporting a noble metal such as Pt on a catalyst carrier is disposed in the upstream exhaust passage 11 of the NOx holding material 12. The configuration is almost the same as that of the first embodiment of the present invention. Note that the three-way catalyst 24 may be disposed in the exhaust pipe 18 on the downstream side of the joining portion of the bypass passage 17 and the exhaust pipe 18, for example. However, in order to allow the three-way catalyst 24 to reach its activation temperature more quickly even at a low temperature such as during cold start, the three-way catalyst 24 is disposed on the exhaust side of the internal combustion engine 10 as shown in FIG. It is preferable to arrange in the upstream exhaust passage 11 of the near NOx holding material 12. In the present embodiment, the reducing agent supply valve 20 is not installed in the upstream exhaust pipe 11 of the NOx holding material 12 as in the first embodiment, but in the upstream exhaust pipe 14 of the NOx purification catalyst 15. It is attached. However, the reducing agent supply valve 20 is not necessarily required when the NOx purification catalyst 15 is a material that can reduce and purify NOx in the exhaust gas near the stoichiometric air-fuel ratio (stoichiometric), for example. What is necessary is just to provide suitably according to the material used as 15.

  According to the second embodiment of the present invention, the exhaust emission control device of the present invention can be applied to a gasoline engine controlled mainly near the stoichiometric air-fuel ratio (stoichiometric). In this case, as the NOx holding material 12, it is preferable to use a NOx adsorbent that adsorbs NOx in a low temperature region and releases the adsorbed NOx in a high temperature region.

The NOx adsorbent 12 is not particularly limited as long as it is a material that can adsorb NOx in a low temperature region and release the adsorbed NOx in a high temperature region. For example, a metal such as zirconia (ZrO ₂ ) Oxides, zeolites, etc., or materials carrying noble metals or the like can be used. Examples of such a material include Ag / ZSM-5 and Ag / Al ₂ O ₃ .

  In general, in order for the exhaust purification catalyst to fully exhibit its performance, it is necessary to raise the temperature of the exhaust purification catalyst to a predetermined activation temperature. However, when the internal combustion engine is cold started, the temperature of the exhaust purification catalyst is low, and therefore, harmful components in the exhaust gas discharged from the internal combustion engine main body cannot be sufficiently purified. On the other hand, since the catalytic metal used in the exhaust purification catalyst is more easily oxidized at such a low temperature, there is a problem that the reducing ability for NOx is particularly low among harmful components contained in the exhaust gas.

  According to the second embodiment of the present invention, during normal operation, unburned HC (hydrocarbon) and CO in the exhaust gas are oxidized by the three-way catalyst 24 disposed mainly upstream of the exhaust passage, and NOx is simultaneously reduced. Reduce and purify. On the other hand, at a low temperature such as when the internal combustion engine is cold started, the NOx purification catalyst 15 is poisoned by oxygen in the exhaust gas by switching the switching valve 19 to a position where the exhaust gas flows into the bypass passage 17. Can be prevented. In this case, NOx in the exhaust gas is adsorbed by the NOx adsorbent 12 disposed on the upstream side in the exhaust passage. Then, when the temperature of the exhaust gas reaches the temperature at which NOx is released from the NOx adsorbent 12, the switching valve 19 is switched to a position where the exhaust gas flows into the NOx purification catalyst 15 side, so that the released NOx is reduced to NOx. Reduction purification by the purification catalyst 15 is possible.

  FIG. 4 is a flowchart of the switching valve operation in the second embodiment of the exhaust emission control device of the present invention. This switching valve operation is performed as a routine that is executed by interruption every predetermined time set in advance by the electronic control unit (ECU) 30. Further, in this embodiment, as in the first embodiment, the NOx amount NOXA discharged from the internal combustion engine 10 per unit time according to the operating state of the internal combustion engine 10 is previously stored in the ECU 30 in the form of a map. Then, the NOx amount ΣNOX adsorbed on the NOx adsorbent 12 is calculated using this map. In FIG. 4, the case where a material in which a base metal such as Fe is supported on a catalyst carrier is used as the NOx purification catalyst 15 will be specifically described. As described above, these base metals cannot sufficiently reduce and purify NOx contained in the exhaust gas if the air-fuel ratio of the exhaust gas is lean or stoichiometric. Therefore, in the routine shown in the flowchart of FIG. 4, when NOx in the exhaust gas is reduced and purified by the NOx purification catalyst 15, the reducing agent supply valve attached in the upstream side exhaust pipe 14 of the NOx purification catalyst 15. Fuel is supplied from 20 as a reducing agent.

Referring to FIG. 4, first, at step 200, it is determined whether or not the bed temperature T of the NOx adsorbent 12 detected by the temperature sensor 21 has reached the NOx release temperature T ₀ , and if T ≧ T ₀ . Goes to step 204. In step 204, bypass closed position switching control for switching the switching valve 19 to the bypass closed position is performed. Next, at step 205, fuel is supplied from the reducing agent supply valve 20, and the air-fuel ratio of the exhaust gas flowing into the NOx purification catalyst 15 is made rich (rich process). Next, the NOx released from the NOx adsorbent 12 is reduced and purified by the NOx purification catalyst 15, and the routine ends.

On the other hand, if the bed temperature T of the NOx adsorbent 12 is T <T _{0 in} the previous step 200, the process proceeds to step 201. In step 201, the NOx amount NOXA discharged per unit time is calculated from the map stored in the ECU 30 in the same manner as in the first embodiment described above. Next, in step 202, NOXA is converted into NOx adsorbent. 12 is added to the amount of NOx ΣNOX adsorbed by 12. Next, at step 203, it is determined whether or not the adsorbed NOx amount ΣNOX exceeds the allowable amount MAX. When the adsorbed NOx amount ΣNOX is ΣNOX> MAX, the routine proceeds to step 204, where NOx in the exhaust gas is reduced and purified by the NOx purification catalyst 15 in steps 204 and 205 in the same manner as before, and the routine is ended. On the other hand, when the adsorption NOx amount ΣNOX is equal to or less than the allowable amount MAX, the routine is terminated without performing the rich process.

  As described above, according to the second embodiment of the present invention, the NOx in the exhaust gas is reduced to NOx under a low temperature at which the NOx purification catalyst 15 has not reached its activation temperature, such as during a cold start of the internal combustion engine. Since it is held by the adsorbent 12, NOx can be prevented from being discharged into the atmosphere. Further, while the NOx in the exhaust gas is held by the NOx adsorbent 12, the switching valve 19 is held in the bypass open position so that the NOx purification catalyst 15 is not poisoned by oxygen contained in the exhaust gas. Can be prevented. On the other hand, after the NOx purification catalyst 15 is warmed over time and the activity of the NOx purification catalyst 15 is expressed, the temperature of the exhaust gas reaches the temperature at which NOx is released from the NOx adsorbent 12 or the NOx adsorbent. When the amount of adsorbed NOx 12 reaches saturation, the switching valve 19 is switched to a position where the exhaust gas flows into the NOx purification catalyst 15 side. Then, NOx released from the NOx adsorbent 12 or NOx that has not been adsorbed by the NOx adsorbent 12 because the amount of adsorption has reached saturation can be reduced and purified by the NOx purification catalyst 15.

  According to the second embodiment of the present invention, during normal operation, harmful components in exhaust gas, particularly NOx, are generally reduced and purified by the upstream three-way catalyst 24 and the downstream NOx purification catalyst 15; Under a low temperature such as during cold start, the NOx purification catalyst 15 is prevented from being poisoned by oxygen contained in the exhaust gas by holding the switching valve 19 in the bypass open position, and the NOx adsorbent 12 is used for the atmosphere. Release of NOx into the inside is suppressed. Therefore, according to the second embodiment of the present invention, it is possible to achieve a higher NOx purification rate than the conventional exhaust purification device of an internal combustion engine.

  In the present specification, as an embodiment (second embodiment of the present invention) when the exhaust gas purification apparatus of the present invention is applied to a gasoline engine controlled mainly near the stoichiometric air-fuel ratio, the upstream side of the NOx adsorbent 12 is provided. The configuration using the three-way catalyst 24 and using a material in which a base metal such as Fe is supported on the catalyst carrier as the NOx purification catalyst 15 has been described in detail. However, in addition to such a configuration, for example, the following configuration can be applied as the second embodiment of the present invention.

  Specifically, as a modification of the second embodiment of the present invention, for example, HC is adsorbed in the low temperature region instead of the three-way catalyst on the upstream side of the NOx adsorbent 12, and the adsorbed HC is released in the high temperature region. An HC adsorbent that performs adsorption and / or a CO adsorbent that adsorbs CO in a low temperature region and releases the adsorbed CO in a high temperature region may be disposed. As such HC adsorbent and CO adsorbent, any material known to those skilled in the art can be used. For example, zeolite such as ZSM-5 or a metal such as Cu is supported on the zeolite. Material can be used. In this case, it is preferable to use a three-way catalyst as the NOx purification catalyst 15.

  In the three-way catalyst, a noble metal such as Pt is mainly used as a catalyst metal. However, even in the case of this Pt, under an atmosphere having a high oxygen concentration, oxygen poisoning is caused in the same manner as in the case of a base metal such as Fe, and the catalytic activity is reduced. Therefore, according to the above modification, the switching valve 19 is bypass-opened at a low temperature when the three-way catalyst used as the NOx purification catalyst 15 has not reached its activation temperature, such as when the internal combustion engine is cold started. The three-way catalyst as the NOx purification catalyst 15 can be reliably prevented from being subjected to oxygen poisoning by oxygen contained in the exhaust gas. In this case, unburned HC, CO, and NOx in the exhaust gas are adsorbed by the HC adsorbent, the CO adsorbent, and the NO adsorbent disposed upstream of the exhaust passage. It is not discharged into the atmosphere. On the other hand, after the NOx purification catalyst 15 is warmed over time and its catalytic activity is exhibited, the temperature of the exhaust gas is set to a temperature at which any of the HC, CO, and NOx components is released from each of the adsorbents. The switching valve 19 is switched to a position where the exhaust gas flows into the NOx purification catalyst 15 side at the stage when it reaches or when the adsorption amount of each adsorbent reaches saturation. These components can be purified by the NOx purification catalyst 15, that is, a three-way catalyst.

The results of examining the influence of the NOx purification catalyst on oxygen poisoning in the case where a base metal such as Fe is used as the catalyst metal of the NOx purification catalyst are shown below. Specifically, Fe / Al ₂ O ₃ prepared by supporting Fe on an alumina (Al ₂ O ₃ ) support as a NOx purification catalyst was prepared by a conventionally known method, and this Fe / Al ₂ O ₃ catalyst was used as air. A poisoning treatment with oxygen was performed in a simulated manner by oxidizing at 500 ° C. for 2 hours. On the other hand, the Fe / Al ₂ O ₃ catalyst prepared by a conventionally known method was reduced at 500 ° C. for 1 hour in a hydrogen-containing atmosphere, and the NO purification performance of these two catalysts was evaluated.

First, each NOx purification catalyst is set in a quartz glass tube, and then the temperature of the catalyst bed is changed from room temperature while flowing an evaluation model gas (NO: 1000 ppm, CO: 1000 ppm, N ₂ balance) to the catalyst bed. The temperature was raised to 500 ° C., and the NO purification rate at this temperature was measured. The results are shown in Table 1 below.

From the results shown in Table 1, the Fe / Al ₂ O ₃ catalyst subjected to oxidation treatment, that is, the Fe / Al ₂ O ₃ catalyst subjected to oxygen poisoning, is Fe / Al ₂ O ₃ catalyst not subjected to such oxygen poisoning. It can be seen that the reduction ability of NOx is extremely low compared to the Al ₂ O ₃ catalyst. According to the first and second embodiments of the present invention described above, even in a lean combustion engine or a normal gasoline engine controlled by stoichiometry, the switching valve is appropriately operated according to the atmosphere and temperature of the exhaust gas. By doing so, it is possible to reliably suppress a decrease in NOx purification performance due to oxygen poisoning of the catalyst metal as described above.

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 11, 14, 18 Exhaust pipe 12 NOx holding material 13, 16 Casing 15 NOx purification catalyst 17 Bypass passage 19 Switching valve 20 Reducing agent supply valve 21, 22 Temperature sensor 23 Air-fuel ratio sensor 30 Electronic control unit

Claims (3)

A NOx holding material that is disposed in the exhaust passage of the internal combustion engine and can hold at least NOx in the exhaust gas;
A NOx purification catalyst that is disposed in the exhaust passage downstream of the NOx holding material and supports a catalyst metal capable of purifying NOx in exhaust gas on a catalyst carrier, wherein the catalyst metal is a base metal. A purification catalyst;
A bypass passage branching from a branch portion of the exhaust passage located between the NOx holding material and the NOx purification catalyst, and joining at a joining portion of the exhaust passage downstream of the NOx purification catalyst;
A switching valve that is provided in the branch portion and allows exhaust gas to flow into either the NOx purification catalyst or the bypass passage;
With
When the NOx holding material is a NOx occlusion reduction type catalyst further containing a noble metal, and the air-fuel ratio of the exhaust gas flowing into the NOx occlusion reduction type catalyst is lean, the switching valve is located at a position where the exhaust gas flows into the bypass passage. And the exhaust gas flowing into the NOx occlusion reduction catalyst has a rich air-fuel ratio, and the exhaust gas flows into the NOx purification catalyst when NOx is released from the NOx occlusion reduction catalyst. and to switch the switching valve, the internal combustion engine exhaust gas purification apparatus. A NOx holding material that is disposed in the exhaust passage of the internal combustion engine and can hold at least NOx in the exhaust gas;
A NOx purification catalyst that is disposed in the exhaust passage downstream of the NOx holding material and supports a catalyst metal capable of purifying NOx in exhaust gas on a catalyst carrier, wherein the catalyst metal is a base metal. A purification catalyst;
A bypass passage branching from a branch portion of the exhaust passage located between the NOx holding material and the NOx purification catalyst, and joining at a joining portion of the exhaust passage downstream of the NOx purification catalyst;
A switching valve that is provided in the branch portion and allows exhaust gas to flow into either the NOx purification catalyst or the bypass passage;
With
The NOx holding material is a NOx adsorbent, and further includes a catalyst that is arranged on the upstream side of the NOx adsorbent or on the downstream side of the merging portion and carries a noble metal on a catalyst carrier, and the NOx purification catalyst has its activity The switch valve is held at a position where the exhaust gas flows into the bypass passage when it has not reached the control temperature, and the temperature of the exhaust gas reaches a temperature at which NOx is released from the NOx adsorbent or the NOx adsorbent the amount of adsorbed NOx exhaust gas at the stage reached saturation was to switch the switching valve to a position that flows into the NOx purification catalyst, exhaust gas purification apparatus for an internal combustion engine. The exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 2, wherein the base metal is Fe, Ni, Cu, or Co.

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