JP2007136380A - Apparatus for cleaning exhaust gas from internal-combustion engine and method for manufacturing exhaust gas cleaning catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning catalyst to be hardly degraded thermally. <P>SOLUTION: The method for manufacturing the exhaust gas cleaning catalyst comprises the steps of: coating a base material, which is formed from ceramics or metals, with slurry containing a metal oxide particle; and firing the slurry-coated base material 23a to form a carrier 23b on the base material. When the exhaust gas cleaning catalyst is manufactured, the metal oxide particle is fired at such high temperature that the metal oxide constituting the carrier is hardly sintered even when exhaust gas flows into the manufactured exhaust gas cleaning catalyst. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust purification device for an internal combustion engine and a method for manufacturing an exhaust purification catalyst.

The exhaust gas of an internal combustion engine contains hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO _x ), etc., and many internal combustion engines have engine exhaust for purifying these components. An exhaust purification catalyst is provided in the passage. As such an exhaust purification catalyst, for example, a three-way catalyst in which a noble metal such as platinum (Pt), rhodium (Rh) and palladium (Pd) is supported on a porous metal oxide carrier such as γ-alumina is known. It has been.

  Since such an exhaust purification catalyst is used in various engine operating conditions, the exhaust purification catalyst is exposed to conditions in which fluctuations in the components, temperature, and flow rate of the exhaust gas are remarkably large. For this reason, it is known that the exhaust purification catalyst deteriorates due to various factors. As a cause of such deterioration, for example, thermal deterioration due to a high temperature of the exhaust purification catalyst, deterioration due to poisoning of lead, phosphorus, sulfur, etc. contained in the exhaust gas can be mentioned. On the other hand, the exhaust purification catalyst is required to have such a high durability that the exhaust purification performance can be ensured over a long period from manufacture to disposal of the vehicle equipped with the internal combustion engine.

  Under such circumstances, development of an exhaust purification catalyst that is highly durable and that maintains a high exhaust gas purification capability is underway. For example, in Patent Document 1, a carrier layer is formed of a first catalyst layer and a second catalyst layer, and the first catalyst layer supports a platinum component on porous alumina that is small in thermal degradation and relatively dispersible. An oxygen-storing ceria-zirconia composite oxide having a high oxygen storage capacity is provided so as to be close to each other, and in the second catalyst layer, the rhodium component is supported on a ceria-zirconia composite oxide having a small thermal degradation and the thermal degradation is small. Porous alumina is provided in the vicinity. By adopting such a structure for the carrier layer, thermal degradation of the carrier can be suppressed to prevent a decrease in catalyst activity, and stable exhaust purification performance can be ensured.

Japanese Patent Laid-Open No. 2004-289813 Japanese Patent Laid-Open No. 3-154635 JP-A-6-137141 Japanese Patent Laid-Open No. 10-148119 JP 2002-336700 A

  By the way, many of the three-way catalysts as described above use γ-alumina as a carrier. This is because the specific surface area of γ-alumina is the largest among the alumina, so that the exhaust gas purification performance can be enhanced by dispersing and supporting noble metals and the like. However, γ-alumina undergoes phase transition to θ-alumina or α-alumina at high temperatures, and the noble metal supported on the alumina causes sintering (grain growth) along with such phase transition, Dispersibility of noble metals and the like is impaired, and so-called thermal degradation of the three-way catalyst occurs.

  On the other hand, if θ-alumina or α-alumina is used from the beginning as the carrier of the three-way catalyst, the phase transition of alumina hardly occurs even when the temperature of the exhaust purification catalyst becomes high. However, even in such a case, sintering of alumina occurs when the exhaust purification catalyst reaches a high temperature, and accordingly, noble metal supported on alumina causes sintering, and so-called thermal degradation of the three-way catalyst occurs. It will be. Thus, in an exhaust purification catalyst in which a noble metal or the like is supported on a porous metal oxide support, the noble metal or the like undergoes sintering due to phase transition or sintering of the metal oxide, and as a result, the heat of the exhaust purification catalyst. Deterioration will occur.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an exhaust purification catalyst that is less susceptible to thermal degradation and an exhaust purification device that can efficiently purify exhaust gas using an exhaust purification catalyst that is less susceptible to thermal degradation. There is.

In order to solve the above-mentioned problems, in the first invention, a slurry containing metal oxide particles is coated on a substrate formed of ceramics or metal, and then the substrate coated with the slurry is fired. In the method for producing an exhaust purification catalyst, wherein the carrier is formed on the base material, the metal oxide particles may be used in the production process of the exhaust purification catalyst even if exhaust gas flows into the completed exhaust purification catalyst. Is fired at such a high temperature that hardly sinters the metal oxide.
According to the first invention, after the exhaust purification catalyst is manufactured, even if the exhaust gas flows into the exhaust purification catalyst, the metal oxide constituting the carrier is hardly sintered. For this reason, even when the exhaust purification catalyst becomes high temperature, sintering of the noble metal supported on the carrier is suppressed.

  In a second invention, in the first invention, a noble metal is supported on the carrier, and the metal oxide particles are fired at a high temperature before the noble metal is supported on the metal oxide particles.

In order to solve the above-described problem, in the third invention, an engine exhaust passage branched into a first branch exhaust passage and a second branch exhaust passage, a first exhaust purification catalyst disposed in the first branch exhaust passage, A second exhaust purification catalyst disposed in the second branch exhaust passage, and a switching means for selectively switching the flow path of the exhaust gas between the first exhaust purification catalyst and the second exhaust purification catalyst, The first exhaust purification catalyst is calcined at a certain temperature or higher in the production process, and the second exhaust purification catalyst is calcined at a temperature lower than the constant temperature in the production process. If the temperature of the second exhaust purification catalyst is higher than the limit temperature or the temperature is expected to be higher than the limit temperature when the exhaust gas is flowing through the exhaust purification catalyst, the switching means can be used to change the exhaust gas flow path. Switch to exhaust gas first And so as to flow into the gas purification catalyst, exhaust gas purification apparatus is provided for an internal combustion engine.
According to the third aspect of the invention, the first exhaust purification catalyst is calcined at a certain temperature or higher in the production process, so that the heat resistance is high but the oxygen storage capacity is low, and the second exhaust purification catalyst is produced in the production process. However, the heat treatment is low, but the oxygen storage capacity is high. In the third aspect of the invention, when the temperature of the second exhaust purification catalyst is equal to or higher than the limit temperature, or when it is expected to become higher than the limit temperature, the exhaust gas is caused to flow into the first exhaust purification catalyst having high heat resistance. It is said. Thereby, it is possible to efficiently purify high-temperature exhaust gas and the like by the first exhaust purification catalyst having high heat resistance while suppressing thermal deterioration of the second exhaust purification catalyst having high storage capacity.
The term “above a certain temperature” means a temperature higher than that in the case of producing an ordinary three-way catalyst or the like, and more specifically, for example, after carrying out a calcination process, the carrier is exposed to a high-temperature exhaust gas. However, the temperature at which the metal oxide constituting the support hardly sinters or undergoes phase transition, the temperature higher than the calcination temperature in the calcination process performed in the production process of a normal three-way catalyst, etc., or exhaust When the purification catalyst is disposed in the exhaust passage, it means a temperature that is about the maximum value that the exhaust purification catalyst can normally take. Further, the “limit temperature” is a temperature at which the carrier of the second exhaust purification catalyst starts thermal degradation when the temperature of the second exhaust purification catalyst becomes higher than that, that is, the carrier of the second exhaust purification catalyst. It means the temperature at which the noble metal starts sintering.

According to the first and second inventions, since noble metal or the like supported on the carrier is suppressed from sintering even when the exhaust purification catalyst becomes high temperature, thermal deterioration of the exhaust purification catalyst is suppressed.
According to the third aspect of the present invention, when the temperature of the second exhaust purification catalyst is equal to or higher than the limit temperature, or when it is expected to become higher than the limit temperature, the exhaust gas is caused to flow into the first exhaust purification catalyst having high heat resistance. Thus, exhaust gas can be efficiently purified using an exhaust purification catalyst that is less susceptible to thermal degradation.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a view showing an entire internal combustion engine equipped with an exhaust purification catalyst and an exhaust purification device of the present invention.

  Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a piston that reciprocates in the cylinder block 2, 4 is a cylinder head fixed on the cylinder block 2, and 5 is a piston 3 and a cylinder head 4. A combustion chamber formed therebetween, 6 is an intake valve, 7 is an intake port, 8 is an exhaust valve, and 9 is an exhaust port. As shown in FIG. 1, a spark plug 10 is arranged at the center of the inner wall surface of the cylinder head 4, and a fuel injection valve 11 is arranged around the inner wall surface of the cylinder head 4. A cavity 12 extending from the lower side of the fuel injection valve 11 to the lower side of the spark plug 10 is formed on the top surface of the piston 3.

  The intake port 7 of each cylinder is connected to a surge tank 14 via a corresponding intake branch pipe 13, and the surge tank 14 is connected to an air cleaner (not shown) via an intake duct 15 and an air flow meter 16. A throttle valve 18 driven by a throttle driving step motor 17 is disposed in the intake duct 15. On the other hand, the exhaust port 9 of each cylinder is connected to an exhaust manifold 19, and the exhaust manifold 19 branches into a first branch pipe 21 and a second branch pipe 22 at a branch portion 20. The first branch pipe 21 is connected to a catalytic converter 24 incorporating a high heat resistant exhaust purification catalyst 23, and the second branch pipe 22 is connected to a catalytic converter 26 containing a high storage capacity exhaust purification catalyst 25. A switching valve 27 that is driven by a switching valve driving step motor (not shown) is provided at a branch portion of the exhaust manifold, and a high heat resistant exhaust purification catalyst is provided to the exhaust gas exhausted from the engine body 1. 23 and the high storage capacity exhaust purification catalyst 25 can be switched.

  The electronic control unit 31 is composed of a digital computer and includes a RAM (random access memory) 33, a ROM (read only memory) 34, a CPU (microprocessor) 35, an input port 36 and An output port 37 is provided. The air flow meter 16 generates an output voltage proportional to the amount of intake air, and this output voltage is input to the input port 36 via the corresponding AD converter 38. An exhaust temperature sensor 40 for detecting the exhaust temperature is attached to the exhaust pipe on the exhaust downstream side of the high heat resistance exhaust purification catalyst 23, and an output signal of the exhaust temperature sensor 40 is input via a corresponding AD converter 38. Input to port 36.

  A load sensor 42 that generates an output voltage proportional to the amount of depression of the accelerator pedal 41 is connected to the accelerator pedal 41, and the output voltage of the load sensor 42 is input to the input port 36 via the corresponding AD converter 38. . For example, the crank angle sensor 43 generates an output pulse every time the crankshaft rotates 15 degrees, and the output pulse is input to the input port 36. The CPU 35 calculates the engine speed from the output pulse of the crank angle sensor 43. On the other hand, the output port 37 is connected to the spark plug 10, the fuel injection valve 11, the throttle driving step motor 17 and the switching valve driving step motor via the corresponding drive circuit 39.

  Next, the high heat resistant exhaust purification catalyst 23 of the present invention will be described. FIG. 2 is a partial cross-sectional view of the high heat resistant exhaust purification catalyst 23 cut perpendicularly to the axial direction. As shown in FIG. 2, the high heat resistance exhaust purification catalyst 23 has a honeycomb base material 23a formed of ceramic or metal such as cordierite, and a carrier 23b arranged in layers on the honeycomb base material 23a. . In the present embodiment, the carrier 23b has θ-alumina and ceria / zirconia and supports a noble metal. In this embodiment, the composition ratio of θ-alumina and ceria zirconia is set to 1: 0 (not completely zero but almost zero) to 1: 1.

  By the way, many three-way catalysts have γ-alumina or δ-alumina and ceria zirconia as a carrier for supporting a noble metal or the like. Thus, the reason why γ-alumina or δ-alumina and ceria zirconia are used in many three-way catalysts will be briefly described.

  γ-alumina or δ-alumina is a metal oxide having a relatively large specific surface area. When γ-alumina or δ-alumina is loaded with a noble metal such as platinum, the dispersibility of the noble metal is relatively high. Thus, when the dispersibility of the noble metal is high, a large number of noble metal particles having a small particle diameter are scattered and arranged, and the surface of the noble metal contributing to the reaction is more exposed. Therefore, by using γ-alumina or δ-alumina as the carrier, the exhaust gas purification ability can be enhanced by the three-way catalyst.

  On the other hand, ceria has the ability to occlude and release oxygen when oxygen is present in a large amount in the atmosphere around ceria, and releases oxygen that is occluded when oxygen is almost absent. For this reason, when oxygen is present in the exhaust gas flowing into the exhaust purification catalyst equipped with the carrier having ceria, oxygen in the exhaust gas is occluded in the ceria, and when there is almost no oxygen in the inflowing exhaust gas, The stored oxygen is released. Such oxygen storage and release capability oxidizes HC and CO in the inflowing exhaust gas by releasing the oxygen stored in the ceria into the exhaust gas even when there is almost no oxygen in the inflowing exhaust gas. The purification window with the three-way catalyst can be widened.

  Furthermore, by adding zirconia to ceria, that is, by forming a composite oxide of ceria and zirconia, the ability to store and release oxygen is higher than that of ceria alone. In particular, the ceria / zirconia composite oxide releases oxygen from a lower temperature than the ceria alone. Further, by using a ceria-zirconia composite oxide, sintering of ceria is suppressed and heat resistance is improved.

However, a three-way catalyst having γ-alumina or δ-alumina and ceria / zirconia as a support causes thermal degradation at a high temperature, and the catalytic ability of the noble metal supported on the support decreases. For example, taking alumina as an example, γ-alumina or δ-alumina having a relatively high specific surface area undergoes a phase transition to θ-alumina or α-alumina having a relatively low specific surface area when exposed to a high temperature. When such an alumina phase transition occurs, sintering of the noble metal supported on the alumina tends to occur. When the sintering of the noble metal occurs, the dispersibility of the noble metal is impaired. As a result, the catalytic ability of the noble metal is lowered, and the oxidation reaction and reduction of HC, CO and NO _{x in} the exhaust gas flowing into the three-way catalyst. The reaction cannot be promoted. Furthermore, when alumina is exposed to high temperatures, the alumina contracts, that is, alumina sintering occurs, and with such alumina sintering, the precious metal supported on the alumina is also sintered. As a result, the catalytic ability of the noble metal is lowered.

  Oxygen occlusion by ceria is performed by directly storing oxygen in the inflowing exhaust gas into ceria, but oxygen stored in ceria is released through a noble metal adjacent to ceria. Therefore, when the dispersibility of the noble metal is impaired due to sintering of the noble metal, oxygen stored in ceria becomes difficult to be released, and thus the ability to store and release oxygen by ceria also decreases.

  FIG. 3 is a diagram showing the maximum oxygen storage amount before and after a high-temperature endurance test (a test in which exhaust gas flows into the exhaust purification catalyst while maintaining the exhaust purification catalyst at a high temperature for a certain period of time). The square marks in the figure indicate the maximum oxygen storage amount for an exhaust purification catalyst similar to the conventional three-way catalyst. Here, the maximum oxygen storage amount means the maximum value of the amount of oxygen that can be stored by the exhaust purification catalyst, particularly ceria supported by the exhaust purification catalyst, and the lower the value, the oxygen storage / release capacity by ceria. Means low. As can be seen from FIG. 3, the conventional three-way catalyst has a large maximum oxygen storage amount before the high-temperature durability test, and thus has a high oxygen storage / release capability. However, after the high-temperature endurance test, that is, after the three-way catalyst has been exposed to a high temperature for a long time, the maximum oxygen storage amount of the three-way catalyst is greatly reduced, and the oxygen storage / release capacity is greatly reduced. I understand.

  Therefore, in the high heat resistance exhaust purification catalyst 23 of the present invention, the carrier having θ-alumina and ceria / zirconia is exposed to a high temperature in advance before the use of the exhaust purification catalyst, that is, during the manufacture of the exhaust purification catalyst. Yes. In this case, alumina or the like constituting the carrier of the high heat resistance exhaust purification catalyst 23 does not undergo phase transition or sintering as described above even when the high heat resistance exhaust purification catalyst 23 becomes hot during use in an internal combustion engine. It won't happen. That is, when the carrier is exposed to a high temperature before the start of use of the high heat resistance exhaust purification catalyst 23, alumina or the like constituting the carrier undergoes phase transition or sintering, and then the temperature of the carrier becomes a similar high temperature. However, alumina or the like constituting the carrier does not cause phase transition or sintering.

  Further, in the high heat resistance exhaust purification catalyst 23 of the present invention, a carrier having θ-alumina and ceria / zirconia is exposed to a high temperature in advance before the start of use of the exhaust purification catalyst, and then the noble metal is supported on the carrier. . That is, as described above, when the support is exposed to a high temperature before the start of use of the exhaust purification catalyst, alumina or the like constituting the support causes phase transition or sintering. Therefore, at this time, if the noble metal is already supported on the carrier, the noble metal also causes sintering, and therefore the exhaust gas purification ability by the noble metal is reduced. On the other hand, like the high heat resistance exhaust purification catalyst 23 of the present invention, the carrier is preliminarily exposed to a high temperature before the start of use and then the noble metal is supported on the carrier, so that the phase transition of alumina or the like constituting the carrier is reduced. Sintering of precious metal accompanying sintering is prevented.

  Further, as the sintering of the noble metal is suppressed, the decrease in oxygen storage / release capability due to ceria is also suppressed. The circles in FIG. 3 indicate the maximum oxygen storage amount for the highly heat-resistant exhaust purification catalyst 23 of the present invention configured as described above. As can be seen from FIG. 3, the maximum heat storage capacity of the highly heat-resistant exhaust purification catalyst 23 of the present invention is almost the same before and after the high temperature durability test. As a result, the oxygen storage / release capability of the high heat resistance exhaust purification catalyst 23 of the present invention is lower than the oxygen storage / release capability of the conventional three-way catalyst before the high temperature endurance test. The oxygen storage / release capability of the three-way catalyst is higher than the oxygen storage / release capability of the heat-resistant exhaust purification catalyst 23.

  The high heat resistance exhaust purification catalyst 23 of the present invention as described above is manufactured as follows. First, a honeycomb substrate 23a formed from ceramics or metal such as cordierite is manufactured. Specifically, in the case of a ceramic honeycomb substrate, for example, a honeycomb substrate 23a is manufactured by extruding and firing a honeycomb having a thin cell plate from a ceramic raw material such as cordierite. In the case of a metal honeycomb substrate, for example, the honeycomb substrate 23a is manufactured by alternately arranging thin flat plates and corrugated plates to form a winding corrugation shape.

  Next, the carrier 23b is formed on the honeycomb substrate 23a. In the case of producing an ordinary three-way catalyst, etc., in forming the carrier, first, the alumina powder is impregnated with an aqueous solution containing a noble metal salt, dried, and then calcined, whereby alumina carrying the noble metal is supported. Obtain a powder. In this case, the firing temperature is about 400 ° C., and the firing time is about 1 hour. Similarly, a ceria / zirconia powder is impregnated with an aqueous solution containing a noble metal salt, dried, and then fired to obtain a ceria / zirconia powder carrying a noble metal. Thereafter, the noble metal-supported alumina powder and the noble metal-supported ceria / zirconia powder thus obtained are mixed, and the mixed powder and alumina sol are added to ion-exchanged water to form a slurry, and this slurry is applied to the honeycomb substrate. Coat. Then, the honeycomb base material coated with the slurry is dried at about 100 ° C. to 120 ° C. for about 2 hours and fired at about 400 ° C. to 500 ° C. for about 1 hour to complete the three-way catalyst. To do.

  On the other hand, when producing the high heat resistance exhaust purification catalyst 23, before the alumina powder or ceria / zirconia powder is impregnated with the aqueous solution containing the noble metal salt, the alumina powder or ceria / zirconia powder is calcined. Is called. The firing temperature at this time is a temperature at which the alumina constituting the carrier hardly undergoes sintering or phase transition even when the carrier 23b completed after the firing process is exposed to high-temperature exhaust gas, or the honeycomb Exhaust purification when the temperature is usually higher than necessary for firing the slurry coated on the substrate (for example, 400 ° C. to 500 ° C. as described above) or when an exhaust purification catalyst is disposed in the exhaust passage. This is the maximum temperature that the catalyst can normally take, and is about 1000 ° C. in this embodiment.

  Thereafter, as in the case of producing the above-described ordinary three-way catalyst, the alumina powder that has undergone the calcining process is impregnated with an aqueous solution containing a noble metal salt, dried, and then calcined, thereby supporting the noble metal. An alumina powder is obtained, and a ceria / zirconia powder supporting a noble metal is obtained in the same manner. Then, a slurry containing the noble metal-supported alumina powder and the noble metal-supported ceria / zirconia powder thus obtained is formed, and this slurry is coated on the honeycomb substrate. Thereafter, the honeycomb substrate coated with the slurry is dried and fired to complete the highly heat-resistant exhaust purification catalyst 23.

  In the above embodiment, the carrier 23b has θ-alumina and ceria / zirconia. However, the support 23b is not limited thereto, and a metal oxide such as ceria, zirconia, silica, or alumina, or a composite in which these metal oxides are mixed. It may be a metal oxide. However, whatever metal oxide or composite metal oxide is used, the metal oxide powder or composite metal oxide powder is used before impregnating the powder with an aqueous solution containing a noble metal salt or these powders. Before the honeycomb substrate is coated with the slurry containing, it is fired at a high temperature as described above. The noble metal supported on the carrier 23b is one or more of noble metals such as ruthenium, rhodium, palladium, osmium, iridium and platinum.

Furthermore, in addition to the noble metal, the carrier 23b may carry various substances used for conventional exhaust purification catalysts. For example, when the air-fuel ratio of the inflowing exhaust gas to the exhaust purification catalyst is lean, NO _X in the exhaust gas is occluded, and when the air-fuel ratio of the inflowing exhaust gas is almost stoichiometric or rich, the occluded NO _x is released. NO _X storage agent (for example, at least one selected from alkali metals such as potassium (K) and sodium (Na), alkaline earth metals such as barium (Ba), and rare earths such as lanthanum (La)) Etc. may be carried.

  Incidentally, as can be seen from FIG. 3, the oxygen storage / release capability of the high heat resistance exhaust purification catalyst 23 of the present invention is relatively low, and therefore the exhaust heat purification performance of the high heat resistance exhaust purification catalyst 23 is not sufficient. On the other hand, as can be seen from FIG. 3, the conventional three-way catalyst exhibits a high oxygen storage / release capability unless exposed to high-temperature exhaust gas and thermally deteriorated, and thus its exhaust purification performance is high. For this reason, when the temperature of the exhaust gas is low or the exhaust purification catalyst does not reach a very high temperature even if the exhaust gas discharged from the engine body 1 flows directly into the exhaust purification catalyst, the conventional three-way catalyst is used. When exhaust gas is allowed to flow into such an exhaust purification catalyst, the exhaust gas can be better purified, and thermal deterioration of the exhaust purification catalyst is also suppressed.

  Therefore, in the present invention, as shown in FIG. 1, the above-described high heat resistance exhaust purification catalyst 23 and the high storage capacity exhaust purification catalyst 25 configured in the same manner as the conventional three-way catalyst are arranged in parallel, and the engine The exhaust gas discharged from the main body basically flows into the high storage capacity exhaust purification catalyst 25, and when the temperature of the high storage capacity exhaust purification catalyst 25 becomes higher than a predetermined limit temperature or a predetermined limit temperature. The exhaust gas is allowed to flow into the high heat-resistant exhaust purification catalyst 23 only when it is expected to be higher than that.

  More specifically, as described above, the switching valve 27 has a heat-resistant side inflow position (in FIG. 1) through which the exhaust gas discharged from the engine body flows into the high heat-resistant exhaust purification catalyst 23 via the first branch pipe 21. ) And an occlusion side inflow position where exhaust gas discharged from the engine body flows into the high occlusion ability exhaust purification catalyst 25 via the second branch pipe 22 (a position indicated by a broken line in FIG. 1). ). The switching valve 27 is normally in the storage side inflow position, and the exhaust gas discharged from the engine body is caused to flow into the high storage capacity exhaust purification catalyst 25. In such a state, when the temperature of the high storage capacity exhaust purification catalyst 25 is higher than a predetermined limit temperature or when it is expected to be higher than the predetermined limit temperature, the switching position of the switching valve 27 is changed. Switch from the occlusion side inflow position to the heat-resistant side inflow position. As a result, the exhaust gas discharged from the engine body 1 is allowed to flow into the high heat resistant exhaust purification catalyst 23.

  Here, the predetermined limit temperature is a temperature at which the carrier 25b of the high storage capacity exhaust purification catalyst 25 starts thermal degradation when the temperature of the high storage capacity exhaust purification catalyst 25 becomes higher, that is, the carrier 25 is supported by the support 25. It means the temperature at which the noble metal starts sintering, for example 600 ° C. The case where the temperature is expected to be higher than the predetermined limit temperature is, for example, the case where the temperature of the high storage capacity exhaust purification catalyst 25 is lower than the limit temperature but relatively high, and is discharged from the engine body 1. The exhaust gas exhausted from the engine body 1 is expected to cause an exothermic reaction on the high storage capacity exhaust purification catalyst 25, or the like. . Specifically, for example, when the temperature of the high storage capacity exhaust purification catalyst 25 is relatively high and the accelerator pedal 41 is stepped on rapidly, the fuel injection amount injected from the fuel injection valve 10 is rapidly increased. In some cases. In such a case, it is expected that the temperature of the exhaust gas suddenly rises as the fuel injection amount increases, and thus the temperature of the high storage capacity exhaust purification catalyst 25 is expected to rise sharply.

  By controlling the switching position of the switching valve 27 in this way, the temperature of the high storage capacity exhaust purification catalyst 25 can be suppressed to a limit temperature or less, and thus the thermal deterioration of the high storage capacity exhaust purification catalyst 25 can be suppressed. it can. On the other hand, when the temperature of the high storage capacity exhaust purification catalyst 25 is higher or higher than the limit temperature, the exhaust gas discharged from the engine body 1 is caused to flow into the high heat resistant exhaust purification catalyst 23, thereby exhausting. The exhaust gas can be purified while suppressing the deterioration of the purification catalyst.

  In the present embodiment, an exhaust purification catalyst having an oxygen storage / release capability higher than that of a conventional ordinary three-way catalyst is used as the high storage capacity exhaust purification catalyst 25. That is, the carrier of the high storage capacity exhaust purification catalyst 25 includes alumina and ceria / zirconia, but all the precious metals are supported on the ceria / zirconia. Accordingly, a large amount of noble metal is disposed adjacent to the ceria, and oxygen stored in the ceria is easily released.

  In the manufacture of such a high storage capacity exhaust purification catalyst 25, a honeycomb substrate is first formed as described above. Further, an aqueous solution containing a noble metal salt is impregnated into a ceria / zirconia powder, dried, and then fired to form platinum-supported ceria / zirconia particles. Thereafter, the platinum-supported ceria / zirconia particles and alumina powder are mixed, and the mixed powder and alumina sol are added to ion-exchanged water to form a slurry. The slurry substrate is coated on a honeycomb substrate, dried, and then fired at about 500 ° C., whereby the high storage capacity exhaust purification catalyst 25 is completed.

  FIG. 4 is a flowchart of a control routine for switching control of an inflow destination of exhaust gas discharged from the engine body. The illustrated control routine is executed by interruption at regular time intervals.

  First, in step 101, the temperature Tc of the high storage capacity exhaust purification catalyst 25 is detected based on the output of the exhaust temperature sensor 40 and the like. Next, at step 102, it is determined whether or not the switching flag Xcat is zero. The switching flag Xcat is a flag that is set to 0 when the switching position of the switching valve 27 is in the occlusion side inflow position, and is set to 1 when the switching position of the switching valve 27 is in the heat resistant side inflow position. If it is determined in step 102 that the switching flag Xcat = 0, that is, if the switching position of the switching valve 27 is at the storage side inflow position, the routine proceeds to step 103. In step 103, it is determined whether or not the temperature Tc of the high storage capacity exhaust purification catalyst 25 detected in step 101 is higher than a predetermined limit temperature Tca. When it is determined that the temperature Tc of the high storage capacity exhaust purification catalyst 25 is equal to or lower than the limit temperature, the control routine is terminated, and the switching position of the switching valve 27 remains at the storage side inflow position.

  On the other hand, when the temperature of the high storage capacity exhaust purification catalyst 25 rises and it is determined in step 103 that the temperature is higher than the limit temperature Tca, the routine proceeds to step 104. In step 104, the switching position of the switching valve 27 is set to the heat-resistant side inflow position, and the exhaust gas exhausted from the engine body 1 is allowed to flow into the high heat-resistant exhaust purification catalyst 23. Xcat is set to 1.

  In the subsequent control routine, it is determined in step 102 that the switching flag Xcat is not 0, that is, it is determined that the switching position of the switching valve 27 is not in the occlusion side inflow position, and the routine proceeds to step 106. In step 106, whether or not the temperature Tc of the high storage capacity exhaust purification catalyst 25 detected in step 101 is equal to or lower than a temperature Tca ′ (hereinafter referred to as “switching temperature”) that is slightly lower than a predetermined limit temperature Tca. Is determined. When it is determined that the temperature Tc of the high storage capacity exhaust purification catalyst 25 is higher than the switching temperature, the control routine is terminated, and the switching position of the switching valve 27 remains at the heat-resistant side inflow position.

  On the other hand, if the temperature of the high storage capacity exhaust purification catalyst 25 decreases and it is determined in step 106 that the temperature is equal to or lower than the switching temperature Tca ', the routine proceeds to step 107. In step 107, the switching position of the switching valve 27 is set to the storage side inflow position, and the exhaust gas discharged from the engine body 1 is allowed to flow into the high storage capacity exhaust purification catalyst 25. Next, in step 108, the switching flag Xcat is set to 0.

  In step 101, the temperature Tc of the high storage capacity exhaust purification catalyst 25 is detected based on the output of the exhaust temperature sensor 40, etc., but other operating parameters of the internal combustion engine (for example, detected by the crank angle sensor 43). The temperature Tc of the high storage capacity exhaust purification catalyst 25 may be estimated based on at least one of the engine speed and the engine load detected by the load sensor 42. Further, as described above, the predicted temperature of the high storage capacity exhaust purification catalyst 25 may be used instead of the temperature Tc of the high storage capacity exhaust purification catalyst 25. In this case, the expected temperature of the high storage capacity exhaust purification catalyst 25 is estimated based on at least one of operating parameters of the internal combustion engine (for example, engine speed, engine load, output of the exhaust temperature sensor 40, etc.). I'm damned.

  In the above embodiment, the switching valve 27 is switched based only on the temperature or the expected temperature of the high storage capacity exhaust purification catalyst 25 based on the output of the exhaust temperature sensor 40 or the like. The switching valve 27 may be switched based on the air-fuel ratio of the exhaust gas discharged. That is, the exhaust purification catalyst is most susceptible to deterioration when the temperature of the exhaust purification catalyst is high and the air-fuel ratio of the inflowing exhaust gas is lean. Therefore, the switching position of the switching valve 27 is heat-resistant when the temperature or the predicted temperature of the high storage capacity exhaust purification catalyst 25 is higher than the limit temperature and the air-fuel ratio of the exhaust gas discharged from the engine body 1 is lean. In other cases, the switching position of the switching valve 27 may be the storage side inflow position.

  Next, the exhaust purification catalyst and the exhaust purification device of the second embodiment of the present invention will be described. The configuration of the exhaust purification device of the second embodiment is basically the same as that of the exhaust purification device of the first embodiment, but the configuration of the high heat resistance exhaust purification catalyst and the carrier of the high storage capacity exhaust purification catalyst of the second embodiment. However, these structures are different from those of the first embodiment.

  FIG. 5 is a partial cross-sectional view of the high heat resistance exhaust purification catalyst 50 of the second embodiment cut perpendicularly to the axial direction. As shown in FIG. 5, the high heat resistance exhaust purification catalyst 50 includes a honeycomb substrate 50a formed of ceramic or metal such as cordierite, and a carrier 50b arranged in layers on the honeycomb substrate 50a. . In the present embodiment, the carrier 50b has a two-layer structure, and an inner carrier layer 50c disposed directly on the honeycomb substrate 50a, and an outer carrier layer 50d directly disposed on the inner carrier layer 50c. Have

  As in the first embodiment, the inner carrier layer 50c has θ-alumina and ceria / zirconia and supports a noble metal. In the present embodiment, for example, the composition ratio of θ-alumina and ceria zirconia is set to 1: 0 (not completely zero but almost zero) to 1: 1. On the other hand, the outer carrier layer 50d has θ-alumina and supports rhodium in this embodiment.

In general, when rhodium and other noble metals, especially platinum, are supported on the same carrier layer, rhodium and other noble metals are partially alloyed and the air-fuel ratio of the inflowing exhaust gas is slightly richer than the stoichiometric air-fuel ratio. Sometimes the NO _x purification performance decreases. In the present embodiment, only rhodium is supported on the outer carrier layer 50d and platinum is not supported, so that rhodium in the outer carrier layer 50d is not alloyed by platinum, so that the air-fuel ratio of the inflowing exhaust gas is higher than the stoichiometric air-fuel ratio. Even when it is slightly rich, the NO _x purification performance is prevented from being lowered. In the above embodiment, the outer support layer 50d is mainly formed of θ-alumina, but is not limited to θ-alumina, and metal oxides such as ceria, zirconia, silica, or alumina, or these metal oxides A mixed metal oxide may be used.

  In the present embodiment, the inner carrier layer 50c of the high heat resistance exhaust purification catalyst 50 is formed by the same method as the carrier 23b of the high heat resistance exhaust purification catalyst 23 of the first embodiment. Then, a rhodium nitrate aqueous solution is impregnated into a θ-alumina powder, dried, and then fired at about 400 ° C. to produce a rhodium-supported θ-alumina powder. Then, rhodium-supported θ-alumina powder and alumina sol are added to ion-exchanged water to form a slurry, and this slurry is coated on the inner carrier layer 50c. Thereafter, this is dried and baked at about 400 ° C., thereby completing the highly heat-resistant exhaust purification catalyst 50 of the present embodiment.

  Further, in the present embodiment, the carrier for the high storage capacity exhaust purification catalyst has a two-layer structure like the carrier for the high heat resistance exhaust purification catalyst 50. In this embodiment, the inner support layer of the high storage capacity exhaust purification catalyst has θ-alumina and ceria / zirconia, and all the noble metals are supported by ceria / zirconia, as in the first embodiment. On the other hand, in this embodiment, the outer carrier layer has θ-alumina and carries rhodium.

It is a figure which shows the whole internal combustion engine carrying the exhaust gas purification catalyst and exhaust gas purification apparatus of this invention. It is a fragmentary sectional view of the high heat resistance exhaust purification catalyst of the present invention. It is a figure which shows the maximum oxygen storage amount before and behind a high temperature endurance test. 3 is a flowchart of a control routine for switching control of an inflow destination of exhaust gas discharged from an engine body. It is a fragmentary sectional view of the high heat resistance exhaust purification catalyst of 2nd embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 19 Exhaust manifold 20 Branch part 21 1st branch pipe 22 2nd branch pipe 23 High heat resistance exhaust purification catalyst 24 Casing 25 High storage capacity exhaust purification catalyst 26 Casing 27 Switching valve

Claims (3)

An exhaust purification catalyst that coats a slurry containing metal oxide particles on a substrate formed of ceramics or metal, and then forms a carrier on the substrate by firing the substrate coated with the slurry. In the manufacturing method of
The metal oxide particles are fired at a high temperature so that the metal oxide constituting the carrier is hardly sintered even when exhaust gas flows into the completed exhaust purification catalyst in the manufacturing process of the exhaust purification catalyst. A method for producing an exhaust purification catalyst.   The method for producing an exhaust purification catalyst according to claim 1, wherein a noble metal is supported on the carrier, and the metal oxide particles are calcined at a high temperature before the noble metal is supported on the metal oxide particles. An engine exhaust passage branched into a first branch exhaust passage and a second branch exhaust passage, a first exhaust purification catalyst disposed in the first branch exhaust passage, and a second exhaust disposed in the second branch exhaust passage A purifying catalyst, and switching means for selectively switching the flow path of the exhaust gas between the first exhaust purification catalyst and the second exhaust purification catalyst,
The first exhaust purification catalyst is subjected to calcination processing at a certain temperature or higher in the production process, and the second exhaust purification catalyst is subjected to calcination processing at a temperature lower than the constant temperature in the production process,
When the exhaust gas is flowing through the second exhaust purification catalyst, if the temperature of the second exhaust purification catalyst becomes higher than the limit temperature or if it is expected to become higher than the limit temperature, the exhaust gas flows through the switching means. An exhaust purification device for an internal combustion engine, wherein the exhaust gas is circulated through the first exhaust purification catalyst by switching the path.

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