JP2006104966A - Exhaust emission control device and exhaust emission control method of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain lowering in NOx purifying performance when transferring to stoichiometric (rich), in an internal combustion engine operated in a lean burn state of being leaner in fuel than the stoichiometric air-fuel ratio. <P>SOLUTION: This exhaust emission control device has an exhaust emission control catalyst capable of capturing and reducing NOx in an internal combustion engine exhaust gas passage in which exhaust gas of the lean air-fuel ratio and exhaust gas of the rich or stoichiometric air-fuel ratio flows; and purifies the captured NOx when the air-fuel ratio is stoichiometric or rich by capturing the NOx in exhaust gas to the catalyst when the air-fuel ratio is lean. A means is arranged for dropping the temperature of the exhaust gas flowing in the exhaust emission control catalyst or the temperature of the catalyst; and drops the temperature of the exhaust gas or the temperature of the catalyst before the air-fuel ratio of the exhaust gas flowing in the exhaust emission control catalyst transfers to stoichiometric or rich or in a process of transferring. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification device and an exhaust gas purification method for an internal combustion engine that is operated in a lean burn state in which fuel is leaner than the stoichiometric air-fuel ratio.

  In recent years, a lean burn engine in which the air-fuel ratio is a lean fuel has attracted attention. Here, the air-fuel ratio represents the ratio of air to fuel in the gas. When the air-fuel ratio of the exhaust gas is lean, it is difficult to purify NOx with a three-way catalyst that has been conventionally used for exhaust gas purification of a stoichiometric engine. For this reason, exhaust gas purification catalysts for lean burn engines have been studied. As one of the exhaust gas purification catalysts for lean burn engines, an oxide of at least one NOx storage element selected from alkali metals, alkaline earth metals and rare earth metals and a catalyst noble metal are supported on a porous carrier. In some cases, NOx can be purified even when lean (see, for example, Patent Document 1). As an exhaust gas purification method of a NOx purification catalyst having this type of NOx trapping function, NOx in the exhaust gas is once oxidized and trapped in the catalyst during lean, and if a certain amount of NOx is trapped, the air-fuel ratio is stoichiometric or A method of purifying trapped NOx by switching to rich (hereinafter referred to as stoichiometric (rich)) is known.

  However, the NOx purification catalyst has a temperature range (temperature window) showing good purification performance, and when the temperature of the exhaust gas flowing into the catalyst is out of the temperature range, the NOx purification performance is lowered. As a countermeasure, it has been studied to blow secondary air into the exhaust passage (see, for example, Patent Documents 2 and 3).

JP 11-319564 A (summary) JP 2002-147227 A (summary) JP 2002-235532 A (summary)

  Patent Document 2 describes that secondary air is supplied while keeping exhaust gas in a rich state to facilitate desorption of NOx trapped in the zeolite catalyst. Patent Document 3 describes that during the lean operation, when the catalyst bed temperature is equal to or higher than the predetermined temperature, the secondary air is supplied to lower the exhaust gas temperature to prevent the NOx trapping ability from being lowered during lean operation. ing.

  By the way, the present inventor has found out that the NOx purification rate at the initial stage of stoichiometric (rich) operation becomes low when NOx captured at the time of leaning is next tried to purify by switching the air-fuel ratio to stoichiometric (rich). .

  An object of the present invention is to provide an exhaust gas purifying apparatus and an exhaust gas purifying method capable of suppressing a decrease in the NOx purification rate in the initial stage of transition from lean to stoichiometric (rich).

  The present invention captures NOx in the exhaust gas when the air-fuel ratio is lean and captures the NOx in the exhaust gas into the internal combustion engine exhaust gas flow path into which the exhaust gas with a lean air-fuel ratio and the exhaust gas with a stoichiometric (rich) air-fuel ratio flows. In the exhaust gas purification apparatus having an exhaust gas purification catalyst that reduces and purifies NOx captured when (rich), a temperature decrease that reduces at least one of the temperature of the exhaust gas flowing into the exhaust gas purification catalyst and the temperature of the exhaust gas purification catalyst And a temperature lowering operation is performed by the temperature lowering means when the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst shifts from a lean state to a stoichiometric (rich) state.

  Further, the present invention provides an exhaust gas purification catalyst for capturing NOx in exhaust gas when the air-fuel ratio is lean and reducing and purifying NOx captured when the air-fuel ratio is stoichiometric or rich in the exhaust gas flow path of the internal combustion engine. In the method for purifying exhaust gas, at least one of the temperature of the exhaust gas and the temperature of the exhaust gas purification catalyst is decreased when the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst is shifted from a lean state to a stoichiometric or rich state. It is in doing so.

  According to the present invention, it is possible to reduce the rate at which the NOx purification rate decreases at the initial stage when the air-fuel ratio of exhaust gas shifts from lean to stoichiometric (rich).

The reason why the NOx purification rate decreases in the initial stage when the air-fuel ratio of the exhaust gas shifts from the lean state to the stoichiometric (rich) state is that the catalyst temperature rises due to combustion of the reducing agent during the stoichiometric (rich) transition and is captured when lean It is considered that the NOx that has been released is desorbed from the catalyst. Reducing agents such as H ₂ , CO, and hydrocarbons contained in the stoichiometric (rich) gas reduce NOx trapped on the catalyst or reduce NOx contained in the stoichiometric (rich) gas. Part of it burns on the catalyst. It is presumed that the temperature of the catalyst rises due to this combustion, and NOx trapped by the catalyst at the time of leaning is desorbed from the catalyst when the catalyst temperature rises.

  In the present invention, when shifting from lean to stoichiometric (rich), the temperature of the exhaust gas is lowered in advance, or the temperature of the catalyst is lowered, so that the temperature of the catalyst increases at the beginning of the transition to stoichiometric (rich). It suppresses and makes NOx difficult to desorb.

  The operation of lowering the exhaust gas temperature or lowering the catalyst temperature is preferably performed before or during the transition of the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst to stoichiometric (rich). Thereby, even if the combustion reaction of the reducing agent occurs during stoichiometric (rich), it is possible to suppress an increase in the catalyst temperature, and it is possible to suppress the capture NOx from desorbing from the catalyst. Even if the exhaust gas temperature or the catalyst temperature drops when the air-fuel ratio is lean, the NOx once trapped during lean will remain trapped by the catalyst, and if the catalyst temperature does not exceed the catalyst temperature at which NOx was trapped, it will be trapped. Since the NOx amount is considered to be substantially maintained, the NOx purification performance does not deteriorate.

  The timing for lowering the exhaust gas temperature or the catalyst temperature is preferably before or during the transition to stoichiometric (rich). If the time to lower the temperature is too early, the air-fuel ratio remains lean and the exhaust gas temperature or the catalyst temperature becomes low. In this case, the NO oxidation reaction by the noble metal is difficult to proceed, and NOx is hardly trapped on the catalyst, and the NOx purification performance at the time of lean may be deteriorated. On the other hand, if the timing of lowering the temperature is too late, the catalyst temperature cannot be lowered in accordance with the combustion of the reducing agent, and the NOx purification performance at the time of stoichiometric (rich) is lowered. The temperature is lowered in order to prevent the temperature from rising due to the combustion reaction of the reducing agent when shifting to stoichiometric (rich), so the temperature should be lowered as soon as possible before shifting to stoichiometric (rich). is there. Note that the process of shifting to stoichiometric (rich) means a process from when the exhaust gas air-fuel ratio becomes lean to a predetermined air-fuel ratio stoichiometric (rich). Normally, when the engine is switched from lean to stoichiometric (rich), it takes several seconds to several tens of seconds for the exhaust gas to shift to a predetermined air-fuel ratio. In this process, the temperature can be lowered.

  If the exhaust gas temperature or the catalyst temperature falls to a predetermined temperature, the temperature lowering operation is stopped, or the temperature lowering operation is continued until the stoichiometric (rich) state is finished. Cancel. If the lean state continues even after the temperature has been lowered, the NOx trapped in the low temperature state is likely to be desorbed due to the catalyst temperature rise when shifting to stoichiometric (rich), so the timing to lower the temperature Is important.

  The temperature decrease depends on the composition of the catalyst or exhaust gas, but if the exhaust gas temperature or catalyst temperature before the temperature decrease is 350 ° C or higher, a temperature decrease of about 40 ° C is preferable, and if it is 350 ° C or lower For this, a range of about 20 ° C. is preferable. If the reduction width is small, the effect of suppressing NOx desorption is small, and if the reduction width is too large, the reducing ability of the noble metal on the catalyst is reduced, and the trapped NOx is difficult to be reduced.

  As means for lowering the exhaust gas temperature or the catalyst temperature, it is conceivable to blow air or blow water into the exhaust gas flowing into the exhaust gas purification catalyst. It is also conceivable that EGR (Exhaust Gas Recirculation) is provided in the internal combustion engine so that a part of the engine exhaust gas is circulated again in the engine before or during the transition of the air-fuel ratio of the exhaust gas to stoichiometric (rich). It is done. Here, EGR is an exhaust gas recirculation device that returns a part of the exhaust gas to the intake system and lowers the temperature when the mixture of gasoline and air burns. The means for lowering the catalyst temperature is not limited to the above method. In addition to these, it is possible to install a cooler on the catalyst, reduce the engine speed, reduce the amount of gasoline supplied, and the like. When air is blown in, the amount of air blown so that the air-fuel ratio of the exhaust gas after the transition does not remain lean despite the transition from lean operation to stoichiometric (rich) operation. Should be adjusted.

  The operation of lowering the temperature is desirably performed when the temperature of the lean exhaust gas is 250 ° C. or higher, and not performed when the temperature is lower than 250 ° C. When the temperature of the lean exhaust gas is lower than 250 ° C., if the temperature of the catalyst is lowered by injecting air or water, desorption of trapped NOx at the time of stoichiometric (rich) transition can be suppressed, but catalytic active components such as noble metals The performance of the reduction reaction of trapped NOx due to becomes worse, and the purification performance of trapped NOx decreases.

  The exhaust gas purification catalyst used in the present invention can capture NOx in lean exhaust gas, and can reduce and purify captured NOx when the air-fuel ratio of exhaust gas shifts to stoichiometric (rich). For this reason, the catalyst needs to contain a NOx trapping material, a NOx oxidizing material, and a NOx reducing material. By containing at least one of an alkali metal and an alkaline earth metal as the NOx trapping material, and by containing at least one of Pt, Pd, and Rh as the NOx oxidizing material and the trapping NOx reducing material, at the time of lean and stoichiometric (rich) High NOx purification performance can be obtained. One kind of alkali metal and alkaline earth metal may be used, but two or more kinds are more preferable. This is because the temperature range in which NOx can be captured differs depending on the element. One kind of noble metal may be used, but two or three kinds of precious metals are preferred. Pt is mainly effective in the oxidation reaction of NO in the lean exhaust gas, and Pd and Rh are mainly effective in the reduction reaction of trapped NOx during stoichiometric (rich).

  When Ti is contained as a catalytically active component in addition to the above components, the NOx purification performance at the time of stoichiometric (rich) transition is further improved. This is considered to be due to the effect on the reduction reaction of trapped NOx. The amount of Ti is desirably 0.05 or more and 1 or less in molar ratio with respect to the total amount of alkali metal and alkaline earth metal contained in the catalyst. If the amount is less than 0.05, the effect of Ti is small, and if it is too large, the active points such as the NOx trapping material and noble metal are covered.

  As a method for preparing the catalyst, any of a physical preparation method such as an impregnation method, a kneading method, a coprecipitation method, a sol-gel method, an ion exchange method, and a vapor deposition method, a preparation method using a chemical reaction, and the like can be applied. Further, various compounds such as nitric acid compounds, acetic acid compounds, complex compounds, hydroxides, carbonic acid compounds, organic compounds, or metals and metal oxides can be used as starting materials for the catalyst.

  The catalyst may be composed of only the catalytically active component, but it is desirable to support the catalytically active component on the porous carrier. By supporting the catalytically active component on the porous carrier, the highly active dispersion of the catalytically active component proceeds, and the NOx purification performance at the time of the stoichiometric (rich) transition is improved. The porous carrier can be further supported on a substrate such as cordierite, SiC or stainless steel. In this case, when the loading amount of the porous carrier is 50 g or more and 400 g or less with respect to 1 liter of the base material, the NOx purification performance at the time of the stoichiometric (rich) transition becomes high. When the amount of the porous carrier supported is less than 50 g, the effect of the porous carrier is small. When the amount of the porous carrier exceeds 400 g, the specific surface area of the porous carrier itself decreases, and when the substrate has a honeycomb shape, clogging easily occurs. Become. As the porous carrier, alumina is optimal, but metal oxides or composite oxides such as titania, silica, silica-alumina, zirconia, and magnesia can also be used. Alumina is preferred because of its high heat resistance and the effect of increasing the dispersion of catalytic active components such as NOx trapping materials or noble metals.

  The shape of the catalyst may be arbitrary. In addition to a honeycomb shape obtained by coating a porous carrier carrying a catalytically active component on a substrate having a honeycomb shape, it can be used in any shape such as a pellet shape, a plate shape, a granular shape, and a powder shape. In the case of a honeycomb shape, a cordierite base material is optimal, but when the catalyst temperature becomes high, it is preferable to use a material that does not easily react with the catalytically active component, for example, a metal base material. Further, the carrier itself can be made into a honeycomb and the catalytically active component can be supported thereon.

  The temperature decrease range and the temperature decrease timing are important. In determining these, it is preferable to refer to the NOx concentration of the exhaust gas flowing out from the NOx purification catalyst. For this purpose, a NOx sensor for detecting the NOx concentration of the exhaust gas from which the NOx purification catalyst flows out may be provided. By measuring the amount of NOx with a NOx sensor installed at the subsequent stage of the NOx purification catalyst in the exhaust gas flow path, it is possible to determine the exhaust gas temperature or the catalyst temperature decrease range that minimizes the NOx amount.

  FIG. 5 shows an example in which means for blowing air or water into the exhaust gas is provided as the temperature lowering device 30 on the upstream side of the exhaust gas purification catalyst 12 having the NOx trapping and reducing function in the exhaust gas flow path 20 of the engine 99. When the combustion in the lean atmosphere is stopped and the stoichiometric (rich) atmosphere is obtained, room temperature air or water is blown from the temperature lowering device 30. In the case of air blowing, the temperature of the exhaust gas flowing into the exhaust gas purification catalyst 12 can be lowered due to the temperature difference with the exhaust gas. When water is blown in, the temperature of the exhaust gas flowing into the exhaust gas purification catalyst 30 can be lowered by the heat of vaporization of water.

  FIG. 6 is provided with an EGR (Exhaust Gas Recirculation) 35 and a part of the exhaust gas in the exhaust gas passage 20 is returned to the intake system 40 to lower the temperature when the mixture of gasoline and air burns. The temperature of the exhaust gas flowing into the exhaust gas purification catalyst 12 can be lowered by stopping the lean operation and circulating a part of the exhaust gas of the engine 99 again in the engine immediately before shifting to the stoichiometric (rich) operation. Valves 36 and 37 are provided between the EGR 35 and the exhaust gas flow path and the intake system so that the exhaust gas is circulated only at the time of the stoichiometric (rich) transition.

  FIG. 7 shows a case where the NOx sensor 10 is provided at the subsequent stage of the exhaust gas purification catalyst 12 in the exhaust gas passage 20. Here, the illustration of the temperature lowering device is omitted. If the temperature reduction range of the exhaust gas purification catalyst is large before or during the transition of the air-fuel ratio to stoichiometric (rich), the reduction reaction of the trapped NOx by the noble metal is difficult to proceed and the NOx purification performance is likely to deteriorate. On the other hand, if the temperature decrease range is small, the catalyst temperature rises when shifting to stoichiometric (rich), and the NOx purification performance is likely to be lowered. By measuring the amount of NOx in the exhaust gas contained in the subsequent stage of the exhaust gas purification catalyst 12, the temperature reduction range of the exhaust gas purification catalyst can be determined so that the amount of NOx is minimized. In addition, the timing for lowering the temperature can be optimized.

  FIG. 8 is an overall configuration diagram showing an embodiment of an internal combustion engine equipped with the exhaust gas purifying apparatus of the present invention. In this embodiment, a lean burnable engine 99, an air cleaner 1, an air flow sensor 2, an intake system having a slot valve 3, etc., an oxygen concentration sensor (or A / F sensor) 7, an exhaust gas purification catalyst 12, an exhaust gas purification catalyst inlet An exhaust system and a control unit (ECU: Engine Control Unit) 11 having a temperature sensor 8 for measuring the gas temperature, an air blowing device 9, a NOx sensor 10 and the like are provided. The ECU includes an I / O as an input / output interface, an LSI, an arithmetic processing unit MPU, a storage device RAM and ROM storing a number of control programs, a timer counter, and the like.

  In this embodiment, the intake air to the engine is filtered by the air cleaner 1, then measured by the air flow sensor 2, passed through the slot valve 3, further receives fuel injection from the injector 5, and is supplied to the engine 99 as an air-fuel mixture. . An airflow sensor signal or other sensor signal is input to the ECU 11. The ECU 11 evaluates the operating state of the internal combustion engine and the state of the exhaust gas purification catalyst to determine the operating air-fuel ratio, and controls the injection time of the injector 5 to set the fuel concentration of the air-fuel mixture to a predetermined value. The air-fuel mixture sucked into the cylinder is ignited and burned by a spark plug 6 controlled by a signal from the ECU 11. The combustion exhaust gas is guided to the exhaust system. The exhaust gas introduced to the exhaust system is purified by NOx, HC, CO by the three-way catalyst function of the exhaust gas purification catalyst 12 during stoichiometric operation, and is captured by the NOx trapping function during lean operation, and by the combustion function HC and CO are purified. Based on the determination and control signal of the ECU 11, the NOx purification ability of the exhaust gas purification catalyst is always determined, and when it is in the stage of shifting from lean operation to stoichiometric (rich) operation, air is blown into the air blowing device 9 from the ECU. A command is issued. This suppresses a decrease in NOx purification performance during stoichiometric (rich). Further, by constantly monitoring the NOx concentration in the exhaust gas flowing out from the exhaust gas purification catalyst 12 using the NOx sensor 10, the air blowing amount and time by the air blowing device 9 and the injection amount and time of the injector 5 are optimized and discharged. The amount of NOx can be reduced. By the above operation, it is possible to effectively reduce the NOx emission amount for all internal combustion engines that perform lean operation and stoichiometric (rich) operation.

Hereinafter, experimental results will be described.
[Method for preparing exhaust gas purification catalyst]
Exhaust gas purification catalysts A, B, C and D were prepared by the following method.

Preparation of exhaust gas purification catalyst A: A slurry made of alumina powder and an alumina precursor and made acidic with nitric acid was coated on a cordierite honeycomb (400 cells / inc ² ), dried and fired, and the apparent volume of the honeycomb An alumina-coated honeycomb coated with 1.9 mol of alumina per liter was obtained. This alumina-coated honeycomb was impregnated with a dinitrodiammine Pt nitric acid solution, a dinitrodiammine Pd nitric acid solution, a nitric acid Rh solution, a mixed solution of Na nitrate and acetic acid K, dried at 200 ° C., and then fired at 600 ° C. for 1 hour. As described above, a catalyst containing 190 g of alumina, 12.4 g of Na, 15.6 g of K, 15.6 g of K, 0.139 g of Rh, 2.792 g of Pt, and 1.35 g of Pd per liter of honeycomb. Obtained.

  Preparation of exhaust gas purification catalyst B: In the same manner as exhaust gas purification catalyst A, 190 g of alumina, 8.2 g of Na in terms of element, 10.4 g of K, 0.046 g of Rh in terms of element A catalyst containing 0.93 g of Pt and 0.45 g of Pd was obtained.

Preparation of exhaust gas purification catalyst C: In the same manner as exhaust gas purification catalyst A, 190 g of alumina, 18.2 g of Na, 0.139 g of Rh, and 2.792 g of Pt in terms of elements with respect to 1 liter of honeycomb made of cordierite A catalyst containing Ti and Ti was obtained. TiO ₂ sol was used as the Ti raw material, and the Ti content was 0 to 1.0 in terms of elements (mol) with respect to Na.

Preparation of exhaust gas purification catalyst D: In the exhaust gas purification catalyst A, a catalyst was prepared in which the amount of Al ₂ O ₃ was changed from 50 g to 400 g with respect to 1 liter of cordierite honeycomb.
[Catalyst performance evaluation method]
A stoichiometric model gas that assumes exhaust gas when an automobile engine is operated at a stoichiometric air-fuel ratio by fixing any one of exhaust gas purifying catalysts A, B, C and nominal D having a capacity of 6 cc in a quartz glass reaction tube. And the lean model gas which assumed the exhaust gas when the engine of the car is performing the lean burn operation was circulated alternately. The reaction tube was introduced into an electric furnace so that the gas temperature could be controlled. The composition of the stoichiometric model gas is NOx: 1000 ppm, C ₃ H ₆ : 600 ppm, CO: 0.5%, CO ₂ : 5%, O ₂ : 0.5%, H ₂ : 0.3%, H ₂ O : 10%, N ₂ : balance. The composition of the lean model gas is NOx: 600 ppm, C ₃ H ₆ : 500 ppm, CO: 0.1%, CO ₂ : 10%, O ₂ : 5%, H ₂ O: 10%, N ₂ : balance. .
[Experiment 1: Examination of catalyst temperature reduction effect]
The exhaust gas purification catalyst A is fixed in the reaction tube, the output of the electric furnace is adjusted so that the temperature of the gas flowing through the reaction tube is 500 ° C., and the stoichiometric model gas and the lean model gas are allowed to flow for 3 minutes each. After that, the output of the electric furnace was stopped, and then the stoichiometric model gas was circulated for 3 minutes. This is Example 1.

  On the other hand, with the exhaust gas purification catalyst A fixed in the reaction tube and the gas temperature set to 500 ° C., the stoichiometric model gas, the lean model gas, and the stoichiometric model gas were circulated for 3 minutes each in this order. The power of the electric furnace is not stopped in this experiment. This is referred to as Comparative Example 1.

FIG. 1 shows the relationship between the NOx purification rate obtained by the following formula and the gas circulation time.
NOx purification rate (%) = ((NOx amount flowing into the catalyst) − (NOx amount flowing out from the catalyst)) ÷ (NOx amount flowing into the catalyst) × 100
In the case of Example 1, the gas temperature was reduced to 460 ° C. about 1 minute after the shift to stoichiometry. As shown in FIG. 1, the NOx purification rate after the stoichiometric transition is higher in the first embodiment than in the comparative example 1, and the NOx amount in the exhaust gas after the catalyst circulation can be reduced by reducing the gas temperature immediately before the stoichiometric transition. Became clear.
[Experiment 2: Examination of catalyst temperature reduction effect]
After the exhaust gas purification catalyst B is fixed in the reaction tube and the temperature of the gas flowing through the reaction tube is set to 400 ° C., the stoichiometric model gas and the lean model gas are circulated for 3 minutes in this order, and then the output of the electric furnace Or the electric furnace was opened and stoichiometric model gas was circulated for 3 minutes. This is Example 2.

  On the other hand, with the exhaust gas purification catalyst B fixed in the reaction tube and the gas temperature set at 400 ° C., the stoichiometric model gas, the lean model gas, and the stoichiometric model gas were circulated for 3 minutes each in this order. The power of the electric furnace is not stopped in this experiment. This is referred to as Comparative Example 2.

  In the case of Example 2, the gas temperature was lowered to 320 ° C. to 380 ° C. 1 minute after the transition to stoichiometry. For Example 1 and Comparative Example 2 described above, the NOx purification rate one minute after shifting from lean to stoichiometric was determined by the following equation.

Stooki NOx purification rate (%) = ((NOx amount flowing into the catalyst 1 minute after shifting to stoichiometric) − (NOx amount flowing into the catalyst 1 minute after flowing to stoichiometric)) ÷ (1 minute after shifting to stoichiometric NOx flowing into the catalyst) x 100
FIG. 2 shows the relationship between the gas temperature one minute after the transition to stoichiometry and the NOx purification rate. The NOx purification rate when the gas temperature is 400 ° C. is that of Comparative Example 2. From FIG. 2, the stoichiometric NOx purification rate increases as the gas temperature one minute after the transition to stoichiometry is lower. From this, when the gas temperature is lowered while shifting to the stoichiometry, the NOx of the exhaust gas after the catalyst circulation It became clear that the amount could be reduced.
[Experiment 3: Examination of Ti coexistence effect]
After the exhaust gas purification catalyst C is fixed in the reaction tube and the temperature of the gas flowing through the reaction tube is set to 400 ° C., the stoichiometric model gas and the lean model gas are circulated for 3 minutes in this order, and then the output of the electric furnace The stoichiometric model gas was circulated for 3 minutes. One minute after moving from lean to stoichiometric, the gas temperature had dropped to 380 ° C. FIG. 3 shows the relationship between the stoichiometric NOx purification rate of the exhaust gas purification catalyst C and the Ti amount one minute after shifting from lean to stoichiometric. FIG. 3 shows that the addition of Ti improves the NOx purification rate after 1 minute of stoichiometry. Note that the lean NOx purification rate was substantially constant regardless of the amount of Ti. From the results of FIG. 3, it was found that when the Ti amount is 0.05 or more and 1.0 or less in mol ratio with respect to the alkali amount, the stoichiometric NOx purification rate immediately after the transition to stoichiometry becomes high.
[Experiment 4: Examination of alumina content]
After the exhaust gas purification catalyst D is fixed in the reaction tube and the temperature of the gas flowing through the reaction tube is set to 400 ° C., the stoichiometric gas and the lean model gas are circulated for 3 minutes each in this order, and then the output of the electric furnace The stoichiometric model gas was circulated for 3 minutes. One minute after the transition to stoichiometry, the gas temperature had dropped to 380 ° C. FIG. 4 shows the relationship between the NOx purification rate and the amount of Al ₂ O ₃ after 1 minute from the transition to stoichiometry. From FIG. 4, it was found that the NOx purification performance after 1 minute of stoichiometry improves as the amount of alumina increases. When the amount of alumina was 50 g or more, the stoichiometric NOx purification rate exceeded 20%. Note that the lean NOx purification rate also tended to improve as the amount of alumina increased. From the above, when the amount of alumina is 50 g or more and 400 g or less with respect to 1 liter of honeycomb, the NOx purification rate immediately after the transition to stoichiometry increases, and in addition to the improvement of lean NOx purification activity, It is clear that the amount of NOx is lowered.

  The present invention greatly contributes to the realization of an internal combustion engine with low fuel consumption and excellent exhaust gas purification performance, and has a great industrial applicability.

The characteristic view which shows the relationship between NOx purification rate and gas distribution time. The characteristic view which shows the relationship between NOx purification rate and gas temperature 1 minute after moving to stoiki. The characteristic view which shows the relationship between the NOx purification rate and Ti / Na ratio 1 minute after moving to stoiki. The characteristic view which shows the relationship between the NOx purification rate 1 minute after moving to stoiki and the amount of alumina. 1 is a schematic view of an exhaust gas purification apparatus provided with means for blowing air or water as temperature lowering means upstream of an exhaust gas purification catalyst in an exhaust gas flow path. Schematic of the exhaust gas purification apparatus provided with EGR for returning engine exhaust gas to an intake system. Schematic of the exhaust gas purification apparatus provided with the NOx sensor in the back | latter stage rather than the exhaust gas purification catalyst of an exhaust gas flow path. 1 is a schematic view of an internal combustion engine equipped with an exhaust gas purification apparatus of the present invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 9 ... Air blowing apparatus, 10 ... NOx sensor, 12 ... Exhaust gas purification catalyst, 20 ... Exhaust gas flow path, 30 ... Temperature reduction apparatus, 35 ... EGR, 99 ... Engine.

Claims (13)

  When the air-fuel ratio is lean, NOx in the exhaust gas is captured in the internal combustion engine exhaust gas flow path into which the exhaust gas with lean air-fuel ratio and the exhaust gas with rich or stoichiometric air-fuel ratio flows, and when the air-fuel ratio is stoichiometric or rich In the exhaust gas purification apparatus having an exhaust gas purification catalyst for reducing and purifying the trapped NOx, the exhaust gas purification device comprises a temperature reduction means for reducing at least one of the temperature of the exhaust gas flowing into the exhaust gas purification catalyst and the temperature of the exhaust gas purification catalyst, An exhaust gas purifying apparatus for an internal combustion engine, wherein a temperature lowering operation is performed by the temperature lowering means when the air-fuel ratio shifts from a lean state to a stoichiometric or rich state.   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the temperature lowering means performs an operation of injecting air or water into the exhaust gas flowing into the exhaust gas purification catalyst.   2. An exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the temperature lowering means performs an operation of circulating a part of the exhaust gas of the engine again in the engine.   2. The temperature reduction means according to claim 1, wherein the temperature reduction means starts the temperature reduction operation before or during the transition of the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst from the lean state to the stoichiometric or rich state. An exhaust gas purification apparatus for an internal combustion engine characterized by the above.   2. The exhaust gas purifying catalyst according to claim 1, wherein the exhaust gas purifying catalyst includes at least one selected from alkali metals and alkaline earth metals and at least one selected from Pt, Pd, and Rh as catalytic active components. An exhaust gas purification device for an internal combustion engine.   6. The exhaust gas purifying apparatus for an internal combustion engine according to claim 5, wherein the exhaust gas purifying catalyst further contains Ti as a catalytic active component.   7. The exhaust gas purifying apparatus for an internal combustion engine according to claim 5, wherein the catalytically active component in the exhaust gas purifying catalyst is supported on a surface of a porous carrier, and alumina is contained as the porous carrier.   2. The internal combustion engine according to claim 1, wherein the temperature reduction means has a function of changing a temperature reduction width or a temperature reduction timing in accordance with a NOx concentration of exhaust gas flowing out from the exhaust gas purification catalyst. Exhaust gas purification device.   In a method for purifying exhaust gas by installing an exhaust gas purification catalyst in an exhaust gas flow path of an internal combustion engine that captures NOx in exhaust gas when the air-fuel ratio is lean and reduces and purifies NOx captured when the air-fuel ratio is stoichiometric or rich When the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst shifts from a lean state to a stoichiometric or rich state, at least one of the temperature of the exhaust gas flowing into the exhaust gas purification catalyst and the temperature of the exhaust gas purification catalyst is An exhaust gas purification method for an internal combustion engine, characterized in that the exhaust gas is reduced.   The exhaust gas purification method for an internal combustion engine according to claim 9, wherein the temperature of the exhaust gas is lowered by injecting air or water from the outside to the exhaust gas flowing into the exhaust gas purification catalyst.   The exhaust gas purification method for an internal combustion engine according to claim 9, wherein the temperature of the exhaust gas is lowered by circulating a part of the exhaust gas of the engine again in the engine.   In Claim 9, before or during the transition of the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst from the lean state to the stoichiometric or rich state, at least of the temperature of the exhaust gas and the temperature of the exhaust gas purification catalyst An exhaust gas purification method for an internal combustion engine, wherein one of the two is reduced. 10. The internal combustion engine according to claim 9, wherein the temperature reduction width or timing of the exhaust gas flowing into the exhaust gas purification catalyst is changed according to the NOx amount of the exhaust gas flowing out from the exhaust gas purification catalyst. Exhaust gas purification method.

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