JP2001003730A - Exhaust emission control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To purify NOx at good efficiency in a wide temperature region by supplying a NOx reducing agent into exhaust emission flowing into an exhaust emission purifying catalyst containing zeolite and rhodium(Rh) when the temperature of exhaust emission is in the NOx adsorption temperature range of zeolite. SOLUTION: An exhaust emission purifying catalyst contains zeolite in a carrier to carry Rh. Rh has a catalytic action of reducing NOx by a NOx reducing agent. When the exhaust emission temperature is higher than the NOx adsorption temperature, NOx adsorbed to zeolite is desorbed from zeolite. At this time, the temperature is in the range of high catalytic activity of Rh, so that desorbed NOx is reduced on Rh to be purified into N2. Accordingly, NOx adsorption at low temperature is performed by zeolite, and NOx reduction at high temperature is performed by Rh. Accordingly, based on an exhaust emission temperature, it is determined whether the temperature is in the NOx adsorption temperature range of zeolite or not, and only when it is within he range, an NOx reducing agent is supplied, whereby the NOx reducing agent is effectively utilized and exhaust emission can be purified efficiently.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine of an automobile or the like, and more particularly, to an exhaust gas containing excess oxygen, that is, carbon monoxide (CO) contained in the exhaust gas. CO), hydrogen (H
₂ ) and an exhaust gas purifying apparatus capable of efficiently reducing and purifying nitrogen oxides (NOx) in exhaust gas containing oxygen in excess of the amount of oxygen necessary to completely oxidize reducing substances such as hydrocarbons (HC). About.

[0002]

2. Description of the Related Art Conventionally, as a catalyst for purifying exhaust gas from automobiles, a three-way catalyst has been used which purifies by simultaneously oxidizing CO and HC and reducing NOx in exhaust gas at a stoichiometric air-fuel ratio (stoichiometric). Tokiko 56-2729
5). As such a three-way catalyst, for example, a porous carrier layer composed of γ-alumina is formed on a heat-resistant substrate composed of cordierite or the like, and platinum (P) is formed on the porous carrier layer.
t) and those carrying a catalytic noble metal such as rhodium (Rh) are widely known.

On the other hand, in recent years, from the viewpoint of protection of the global environment, carbon dioxide in exhaust gas discharged from internal combustion engines such as automobiles has been a problem, and as a solution to this problem, in an oxygen-excess atmosphere with an increased air-fuel ratio (A / F). Lean-burn engines and lean-burn engines have attracted attention.
In this lean burn engine, the fuel consumption is improved because the fuel efficiency is improved, and the generation of carbon dioxide as the combustion exhaust gas can be suppressed.

On the other hand, a conventional three-way catalyst purifies CO, HC, and NOx in exhaust gas by simultaneously oxidizing and reducing the exhaust gas at a stoichiometric air-fuel ratio. In exhaust gas emitted from lean-burn systems and diesel engines, the oxidizing component is more chemically equivalent than the oxidizable component, so that a reduction reaction is less likely to occur. Is difficult to purify. Therefore, development of a catalyst and a purification system capable of purifying NOx even in an oxygen-excess atmosphere has been desired.

[0005] Japanese Patent Application Laid-Open No. Hei 5-103985 discloses a method of purifying NOx by selectively reducing NOx with hydrocarbons by using a catalyst in which a catalytic noble metal such as Pt is supported on a support made of alumina or the like and supplying hydrocarbons to the catalyst. I have. JP-A-5-317652 uses a NOx storage / reduction catalyst carrying a NOx storage material such as an alkaline earth metal together with a noble metal, and further makes the air-fuel ratio of the mixture supplied to the engine pulsed from the lean side. A NOx occlusion / reduction system has been developed which controls so as to be on the stoichiometric to rich side. In this NOx storage / reduction system, NO in exhaust gas is oxidized and stored in the NOx storage material when the air-fuel ratio is lean, and NOx stored when the air-fuel ratio is stoichiometric to rich is stored on the catalyst. Is reduced and purified.
Therefore, NOx can be efficiently purified even in a lean burn engine.

[0006]

However, it has been difficult to say that the conventional exhaust gas purifying apparatus and method exert the maximum purifying performance of the catalyst used.

According to the method disclosed in Japanese Patent Laid-Open No. Hei 5-103985, NOx is efficiently purified only at around 250 ° C., and the purification temperature range is extremely narrow. Therefore, NOx cannot be efficiently purified in a region where the exhaust gas temperature is relatively low such as when the engine is started or idling, or in a region where the load on the engine is high and the exhaust gas temperature is high. In addition, Pt used here has a high NOx purification activity, but when reducing NOx, as a reduction product thereof,
In addition to N ₂ , there was a problem that a large amount of N ₂ O, a greenhouse gas causing global warming, was generated.

Further, when a NOx storage / reduction type catalyst is used, sulfur oxides caused by sulfur in fuel are reduced to NO.
x It is occluded by the occlusion material to produce sulfate. As a result, N
The Ox storage capacity was remarkably reduced, and the sulfate was difficult to decompose even in a reducing atmosphere, so that it was difficult to recover the NOx storage capacity of the NOx storage agent.

The present invention has been made in view of the above circumstances, and provides an exhaust gas purifying apparatus for maximizing the performance of a catalyst and purifying NOx efficiently in a wide temperature range from low to high temperatures. The purpose is to:

[0010]

According to the study of the present inventors, at a certain temperature, NOx can be reduced by adding a NOx reducing agent even at a relatively low temperature where the catalytic activity is low.
It became clear that NOx and NOx reducing agent were combined and adsorbed on zeolite. Furthermore, at higher temperatures NOx
Was desorbed as a complex when desorbed. Therefore, when Rh which can decompose NOx at this temperature is used as a catalyst, N ₂ can be added without adding a new reducing agent.
Can be reduced to The present invention has been completed based on such findings.

[0011] That is, an exhaust gas purifying apparatus of the present invention for solving the above-mentioned problems comprises an exhaust gas purifying catalyst containing zeolite and Rh, a temperature detecting means for detecting the temperature of the exhaust gas, and NOx contained in the exhaust gas flowing into the exhaust gas purifying catalyst. A NOx reducing agent supply unit for supplying a reducing agent, an adsorption temperature determining unit for determining whether or not the temperature of the exhaust gas is within a NOx adsorption temperature range of the zeolite based on a temperature detection value of the temperature detecting unit, and a NOx reducing agent supplying unit. And a control means for driving the NOx reducing agent supply driving unit to supply the NOx reducing agent when the temperature of the exhaust gas is within the NOx adsorption temperature range by the adsorption temperature determining unit. It is characterized by comprising.

In the exhaust gas purifying apparatus of the present invention, the adsorption temperature judging section of the control means determines the NOx adsorption temperature range of zeolite (hereinafter, simply referred to as "NOx adsorption temperature range") based on the temperature of the exhaust gas detected by the temperature detecting means. It is determined whether the exhaust gas temperature is within the NOx adsorption temperature range and the NOx reducing agent supply unit is driven to supply the NOx reducing agent only when the exhaust gas temperature is within the NOx adsorption temperature range. Thereby, the NOx reducing agent is effectively used, and the NOx in the exhaust gas is efficiently purified.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An exhaust gas purifying apparatus according to the present invention comprises an exhaust gas purifying catalyst containing zeolite and Rh, a temperature detecting means for detecting the temperature of exhaust gas, and a NOx reducing agent contained in exhaust gas flowing into the exhaust gas purifying catalyst. NOx reducing agent supply means for supplying NO, an adsorption temperature determining unit for determining whether or not the temperature of the exhaust gas is within the NOx adsorption temperature range of zeolite based on the temperature detection value of the temperature detecting means, and driving the NOx reducing agent supplying means A NOx reducing agent supply driving unit, and a control unit for driving the NOx reducing agent supply driving unit to supply the NOx reducing agent when the temperature of the exhaust gas is within the NOx adsorption temperature range by the adsorption temperature determining unit. ing.

In this exhaust gas purifying catalyst, a carrier contains at least zeolite, and at least Rh is carried on the carrier. Rh has a catalytic action for a reaction of reducing NOx with a NOx reducing agent. When the exhaust gas temperature rises above the NOx adsorption temperature due to a change in the operation state of the internal combustion engine, NOx adsorbed on the zeolite desorbs from the zeolite. Temperature NOx is desorbed, because the temperature catalytic activity of Rh is high zone, the desorbed NOx is reduced on Rh, it is purified to N _2.

Therefore, NOx adsorption at a low temperature is carried out by the zeolite of the carrier, and N
Ox reduction is possible. It is the NOx that uses Rh
This is because the generation of greenhouse gases such as N ₂ O due to the reduction is smaller than that of Pt or the like, and the temperature range where the NOx reduction activity is high is relatively higher than the NOx adsorption temperature range.

As the zeolite used for the carrier, ordinary ones can be used and are not limited. Among them, mordenite, ZSM-5, USY, zeolite beta and the like are preferable. Zeolite has excellent NOx adsorption ability in a low temperature range, and it is considered that the adsorption of NOx is such that NOx co-adsorbs with zeolite together with the NOx reducing agent.
Therefore, it is desirable that the zeolite has a large adsorption amount of the NOx reducing agent.

The cation in the zeolite is desirably ion-exchanged with H ⁺ (proton) or a transition metal such as Ag, Fe, Co or the like, since the adsorption amount of the NOx reducing agent is large. Those exchanged with an alkali ion such as Na ⁺ are not preferred. The amount of zeolite is 1L
20 g or more is preferable. If less than this, the effect is small. The upper limit is not particularly limited, but is preferably up to 300 g from an economic or physical viewpoint.

The shape of the carrier can be the same as a conventional one such as a pellet or a honeycomb shape, and can also be used by coating a cordierite carrier substrate or a metal carrier substrate. The amount of Rh supported is preferably 0.0001 g to 10 g per liter of the monolith carrier, and more preferably 0.01 g to 10 g.
5 g to 5 g are more preferred. This is because even if the amount of supported Rh is further increased, the activity does not increase, and if the amount of supported Rh is less than this, practically sufficient activity cannot be obtained. A noble metal other than Rh, for example, Pt, palladium, iridium, osmium, etc., may be used if necessary.
It can also be used with.

As a method of supporting such a noble metal such as Rh on a carrier, an acetate, a nitrate, or the like is used to carry a noble metal in a conventional manner such as an impregnation method, a spray method, or a slurry mixing method. be able to.

The temperature detecting means detects the temperature of the exhaust gas.
The exhaust gas temperature detecting method of the temperature detecting means is not limited to the method of directly detecting the exhaust gas temperature, the method of detecting the catalyst bed temperature, the operating condition of the internal combustion engine such as the cooling water temperature, the engine speed, the engine intake air amount, and the like. Can also be specified indirectly from. Among them, it is preferable to directly detect the exhaust gas temperature in terms of accuracy, and examples thereof include an internal combustion engine outlet temperature, a catalyst inlet portion, the inside of the catalyst, and a catalyst outlet portion. It is preferable from the point of accuracy that the detection is performed at the catalyst entrance portion on the side connected to the internal combustion engine or inside the catalyst.

The NOx reducing agent is not particularly limited as long as it can reduce NOx on Rh.
In particular, hydrocarbons, for example, those using the hydrocarbons in the exhaust gas as they are, fuels, hydrocarbons obtained by reforming the fuel with a reforming catalyst, and the like, or a combination thereof are preferable.

As the NOx reducing agent supply means, there are a method of changing the hydrocarbon concentration in the exhaust gas by operating the operating condition of the internal combustion engine and a method of directly supplying the NOx reducing agent to the exhaust gas. Driven by the drive unit.

When the temperature of the exhaust gas detected by the temperature detecting means is within the NOx adsorption temperature range, the control means
It drives x reducing agent supply means. This control means can be composed of, for example, various digital circuits, digital circuits using a microcomputer, or various analog circuits. Further, a storage means for recording various control information can be provided, and this storage means can be formed by a digital circuit using a memory such as a ROM or a RAM, or an analog circuit such as a resistor or a capacitor.

The operation of each part of the control means will be described below.

The adsorption temperature judging section compares the NOx adsorption temperature range with the exhaust gas temperature detected by the temperature detecting means to judge whether or not the exhaust gas temperature is within the NOx adsorption temperature range. When there is, an operation signal is output to the NOx reducing agent supply driving unit. This NOx adsorption temperature range can be stored in advance in a storage unit included in the control unit. For example, conditions such as the NOx adsorption temperature range can be represented by a relational expression depending on the temperature, the NOx concentration and the type of the NOx reducing agent, or a table measured under predetermined conditions in advance, and these are stored in the storage means in advance. .

Since these relational expressions, tables and the like differ depending on the type of the internal combustion engine used and the NOx reducing agent,
When these are changed, it is necessary to measure and store the relationship between them each time.

Here, the NOx adsorption temperature range is defined as
Temperature range where NOx can co-adsorb with the NOx reducing agent
It is an enclosure. Specifically, 100 ° C to 300 ° C is preferable.
When exhaust gas temperature is in this temperature range, NOx return of Rh
Although the original capacity is low, the required amount of NOx reducing agent is present in the exhaust gas.
NOx can be adsorbed on zeolite
it can. The space velocity is 5 to the volume of the catalyst.
00-200000h ^-1, More preferably, 500 to 1
00000h^-1Is preferred. This adsorbed NOx is N
When the temperature exceeds the Ox adsorption temperature range, it is desorbed from zeolite, and R
NO is reduced by h. At the temperature where NOx desorbs
A catalyst component capable of purifying NOx is preferable.
d, Ir, Ru, Os, etc. may be used, but Rh is preferred.
New

The NOx reducing agent supply driving section supplies the NOx reducing agent into the exhaust gas when an operation signal is input from the adsorption temperature determining section. It is preferable that the supply amount of the NOx reducing agent is set such that the NOx reducing agent amount corresponding to the NOx concentration in the exhaust gas is supplied without wasteful consumption of the NOx reducing agent. The NOx reducing agent amount corresponding to the NOx concentration can be stored in the storage means. When the NOx reducing agent is a hydrocarbon, the concentration of the hydrocarbon in terms of CH ₄ is preferably about 1 to 100 times, more preferably about 3 to 20 times the NOx concentration. If the amount is too small, NOx cannot be reliably adsorbed. If the amount is too large, the adsorption efficiency of NOx decreases and fuel efficiency deteriorates.

The exhaust gas purifying apparatus of the present invention is further provided with NOx concentration detecting means for detecting the NOx concentration in the exhaust gas upstream and downstream of the exhaust gas purifying catalyst, and the control means detects the NOx concentration by the NOx concentration detecting means. A NOx concentration ratio judging unit for judging whether or not the NOx concentration ratio on the upstream side and the downstream side of the exhaust gas purifying catalyst is equal to or lower than a predetermined value; and the temperature of the exhaust gas detected by the temperature detecting means reaches the adsorption capacity recovery temperature. And a recovery temperature determining unit for determining whether or not the recovery has been performed. Accordingly, when the NOx reducing agent supply driving unit is being driven and the NOx concentration ratio determining unit determines that the NOx concentration ratio detected by the NOx concentration detecting unit is equal to or less than the predetermined value, the driving of the NOx reducing agent supply driving unit is performed. Pause,
Thereafter, when the exhaust gas temperature reaches the adsorption capacity recovery temperature, the recovery temperature determining unit cancels the suspension of the drive of the NOx reducing agent supply driving unit, and then the exhaust gas temperature falls within the NOx adsorption temperature range by the adsorption temperature determining unit. At a certain time, the NOx reducing agent supply driving unit is driven to supply the NOx reducing agent.

Thus, when the adsorption of NOx to the zeolite is approaching the saturation state, the supply of the NOx reducing agent is temporarily stopped, so that the unnecessary supply of the NOx reducing agent is eliminated and the hydrocarbon in the exhaust gas is eliminated. Etc. can be prevented from increasing.

As the NOx concentration detecting means, a generally used NOx sensor can be used.

The operation of each part of the control means will be described below.

The NOx concentration ratio judging section of the control means judges whether or not the NOx concentration ratio on the upstream side and the downstream side of the exhaust gas purifying catalyst is equal to or lower than a predetermined value under the adsorbable temperature condition. If the ratio is equal to or less than a predetermined value, an adsorption saturation signal is output. This makes it possible to predict the saturation of adsorption of NOx on zeolite.

Here, the NOx concentration ratio in the exhaust gas on the upstream side and the downstream side of the purification catalyst refers to the concentration of the NOx concentration in the exhaust gas upstream of the exhaust gas purification catalyst with respect to the NOx concentration in the exhaust gas downstream of the exhaust gas purification catalyst. (NOx concentration ratio = (upstream NOx concentration) / (downstream NOx concentration)).
The predetermined value can be arbitrarily determined according to the purpose of use. For example, when the NOx emission of the engine to be used is relatively high, this value is set high and the NOx
If the emission is relatively low, set this value low.

It is also possible to determine that the NOx purifying ability of the catalyst has decreased by the differential value of the concentration ratio. That is, when the differential value of the concentration ratio is equal to or smaller than the predetermined value, it can be determined that the adsorption state of NOx to zeolite is approaching the saturation state.

When the adsorption saturation signal is output, NOx is output even if the operation signal is output from the adsorption temperature determination unit.
The driving of the reducing agent supply drive unit is not performed and the operation is temporarily stopped.

The recovery temperature determining section of the control means determines whether or not the exhaust gas temperature has reached the adsorption recovery temperature, and outputs a recovery signal when the exhaust gas temperature has reached the adsorption recovery temperature. However, if the recovery signal is output while the driving of the NOx reducing agent supply driving unit is temporarily stopped, the adsorption saturation signal is reset and the control returns to the normal control.

Here, the adsorption capacity recovery temperature is a temperature range in which NOx adsorbed on zeolite is desorbed and reduced from zeolite or reduced on zeolite. Specifically, 2
The temperature is at least 00 ° C, more preferably at least 250 ° C. If there is no increase to the adsorption recovery temperature after the adsorption reaches saturation, it is also possible to forcibly heat by the engine control, the heater, and the burner. However, even without such a forced heating device, the present invention is effective under most operating conditions.

These judging means can be constituted by, for example, various digital circuits, digital circuits using a microcomputer or the like, or various analog circuits, similarly to the control means.

NOx is stored in the storage means included in the control means.
The adsorption state is stored in the storage means in advance using a relational expression based on the temperature, the NOx concentration, the type of the NOx reducing agent, and the like, or a table measured under predetermined conditions in advance. Since these relational expressions, tables, and the like differ depending on the type of the internal combustion engine used and the NOx reducing agent, it is necessary to measure and store the relationship between them each time they are changed.

The exhaust gas purifying apparatus according to the present invention uses N
Utilizing Ox adsorption capacity, N required for NOx adsorption
This is to eliminate unnecessary use of the Ox reducing agent. When the NOx reducing agent is required in other parts of the exhaust system incorporating the exhaust gas purifying apparatus of the present invention, the control means of the exhaust gas purifying apparatus of the present invention is used, even when the exhaust gas temperature is not within the adsorption temperature range. Alternatively, the NOx reducing agent can be supplied even when the NOx adsorption of the zeolite is saturated.

Instead of ZSM-5 powder, commercially available SiO
Except that _two powders were used, they were prepared in the same manner as the Rh / zeolite catalyst. (Test Example 1) Test Using the Rh / zeolite catalyst prepared by the above method, Table 1 was used.
An exhaust model gas having the composition shown in FIG.
Flow under the condition of 00 / hour and raise the temperature of the upstream gas of the catalyst to 500 ° C.
The temperature was lowered at a rate of 5.8 ° C./min to １００100 ° C. The NOx purification rate at each temperature at this time was determined.

[0043]

[Table 1]

Results The results are shown in FIG. In the NOx purification rate of FIG. 1, a white portion is a portion where NOx is purified by being reduced to N ₂ or N ₂ O, and a hatched portion is a portion where NOx is adsorbed in the catalyst and purified. From this, it was found that under the model gas atmosphere shown in Table 1, when the exhaust gas temperature was in the NOx adsorption temperature range, NOx was adsorbed in the catalyst. In another experiment, it was found that NOx was hardly adsorbed in the absence of hydrocarbons, so it was also found that hydrocarbons were essential for NOx adsorption. In yet another experiment, NOx was found to form a complex with hydrocarbons and co-adsorb. FIG.
After the test of the above, using a gas obtained by removing NO from the model gas
When the concentration of N ₂ , N ₂ O and NOx in the gas downstream of the catalyst was measured when the temperature was raised from 0 ° C. to 500 ° C. at a rate of 5.8 ° C./min, the concentration was about 300 ° C. to 400 ° C. And N
Only ₂ was measured (FIG. 2). That is, it was found that NOx adsorbed at a low temperature is reduced to N2 by Rh in the process of raising the temperature to a high temperature.

As described above, even in an actual exhaust gas atmosphere, by actively supplying hydrocarbons to the exhaust gas within the NOx adsorption temperature range, the NOx adsorption capacity of the catalyst is utilized to improve the NOx purification rate. I was able to. The catalytic activity did not decrease even in the presence of sulfur oxide. (Test Example 2) Test Using a Rh / silica catalyst prepared by the above-described method, a gas having an atmosphere change at the cycle shown in Table 2 was flowed at a space velocity of 30000 / hour, and the temperature of the upstream gas of the catalyst was 500 to 100 ° C.
The temperature was lowered to 5.8 ° C at a rate of 5.8 ° C / min. The NOx purification rate at each temperature at this time was obtained and is shown in FIG. In the case of a silica carrier, NO as shown in FIGS.
Since it has been confirmed that there is no purification by adsorption of x,
The NOx purification rate curve shown in FIG. 3 is based on the fact that NOx has been reduced and purified.

[0046]

[Table 2]

The model gas 1 is a gas that does not change in atmosphere. On the other hand, the model gas 2-4 is a model gas in periodic variations in the concentration of C ₃ H ₆ and O _2, C ₃ deflection width of the concentration of H ₆ 0
６ 6000 ppmC, fluctuates in a cycle P seconds, meaning that the concentration fluctuation width of O ₂ is 10 (initial value) ５ 5% and fluctuates in a cycle P. All of the model gases have time-average concentrations of C ₃ H ₆ and O ₂ of 3000 ppmC and 7.5%, respectively. -Results The results are shown in Fig. 3. From FIG. 3, it is found that the catalyst activity is improved by changing the atmosphere, and the longer the cycle of the change, the more NO
It was found that the x purification rate started from a low temperature, and the maximum value of the NOx purification rate also increased. It was also found that the temperature of the maximum value of the NOx purification rate increases as the period P increases. (Example 1) Example 1 relates to a hydrocarbon supply method for maximizing the low-temperature NOx adsorption ability of zeolite even in actual exhaust gas purification.

FIG. 4 shows the configuration of the exhaust gas purifying apparatus of this embodiment. The exhaust gas purifying apparatus includes a catalyst 2 containing Rh / zeolite prepared by the above-described method and disposed in an exhaust gas passage of an engine 1 and a temperature sensor 3 disposed upstream of the catalyst 2.
And NOx sensors 4 and 5 disposed upstream and downstream of the catalyst 2, a device 6 capable of supplying hydrocarbons to exhaust gas, a temperature sensor 3 and upstream and downstream of the catalyst 2. The electronic control unit 7 receives signals from the NOx sensors 4 and 5 and varies the supply amount of the hydrocarbon supply device 6 in accordance with the values.
The electronic control unit 7 includes A / D converters 70 to 72 for converting an analog signal of the temperature sensor 3 into a digital signal.
CPU receives signals from A / D converters 70 to 72
76, an input port 74 for outputting to the
U76 and NOx that outputs a signal to the hydrocarbon supply device 6
Upon receiving signals from the reducing agent supply driving section 73 and the CPU 76, N
An output port 75 for outputting to the Ox reducing agent supply driving unit 73 is provided.

The ROM 77 stores a table relating to the determination of the exhaust gas temperature and the hydrocarbon supply. Table 3 shows the table. This means that a certain amount (M) of hydrocarbons is supplied in a temperature range where the actual exhaust gas temperature is effective for NOx adsorption, but is not supplied outside the temperature range.

[0050]

[Table 3]

Tr is the actual exhaust gas temperature Tl and Th sent from the temperature sensor 3 are the lower limit and the upper limit of the NOx adsorption temperature range of the catalyst 2, and are obtained in advance by a model gas experiment.
M is the amount of supplying hydrocarbons, and the minimum amount of hydrocarbons capable of sufficiently adsorbing NOx is obtained in advance by a model gas experiment or the like. Further, the ROM 77 stores a table relating to a temporal change in the concentration of NOx and a continuation determination of the supply of hydrocarbons. Table 4 shows the table. According to the table of Table 3, when the supply of hydrocarbons is started, when the NOx concentration ratio between the upstream side of the catalyst 2 and the downstream side is equal to or more than a predetermined value, it is determined that NOx is adsorbed, and the supply of hydrocarbons is continued. However, when the concentration ratio becomes equal to or less than a predetermined value, it means that the NOx adsorption is determined to be in a saturated state and the supply of hydrocarbons is stopped.

[0052]

[Table 4]

Ci (S) is the average value of the NOx concentration on the upstream side of the catalyst 2 detected by the NOx sensor 4 on the upstream side of the catalyst 2 during S seconds. Co (S) is the downstream side of the catalyst 2 detected by the NOx sensor 5 on the downstream side of the catalyst. The average value A of the NOx concentration on the side for S seconds is a boundary value that determines whether or not the adsorption of NOx has reached saturation. The values of S and A are arbitrarily determined. Further, the ROM 77 stores a table relating to the determination of the recovery of the exhaust gas temperature and the NOx adsorption ability. Table 5 shows the table. When it is determined in Table 5 that the adsorption of NOx is saturated, it is necessary to wait for the next recovery of the NOx adsorption ability. Therefore, when the exhaust gas temperature reaches the NOx adsorption ability recovery temperature, it is determined that the NOx adsorption ability of the zeolite has recovered, and this means that the processing of the electronic control unit 7 is started from the beginning.

[0054]

[Table 5]

Tr is the actual exhaust gas temperature Tk sent from the temperature sensor 3, and Tk is the lower limit of the NOx adsorption ability recovery temperature. (See FIG. 2) Hereinafter, the processing contents of the electronic control unit 7 will be described with reference to the flowchart shown in FIG. First, the actual exhaust gas temperature (Tr) is read. From Table 3 stored in the ROM 77, it is determined whether or not Tr is within the NOx adsorption temperature range to determine the supply of hydrocarbons. If it is determined that the supply is to be started, the hydrocarbon supply device 6 is driven through the drive circuit to supply a certain amount (M) of hydrocarbon, but if not, the hydrocarbon supply device 6 is stopped and the process returns to START.

When the supply is started, the NOx concentration on the upstream side and the NOx concentration on the downstream side of the catalyst 2 are read, and the ROM
Judgment of the continuation and stop of the hydrocarbon supply is made by referring to Table 4 stored in 77. The supply is continued while it is determined that the adsorption has not reached saturation, but when it is determined that the adsorption has reached saturation, the hydrocarbon supply device 6 is stopped through the drive circuit. If the hydrocarbon supply is stopped,
Next, Tr is read, and it is checked from Table 5 stored in the ROM 77 whether or not the NOx adsorbing ability has been restored. If the NOx adsorbing ability has been restored, the process returns to START to perform the processing from the beginning. (Example 2) As means for supplying a hydrocarbon, Example 1
Instead of using a device for supplying hydrocarbons to the exhaust gas used in (1), the amount of unburned hydrocarbons contained in the exhaust gas from the engine 1 was changed by controlling the fuel supply amount to the engine 1. Other control methods of the apparatus are the same as those in the first embodiment. (Embodiment 3) FIG. 6 shows the configuration of an exhaust gas purifying apparatus of this embodiment. Since the configuration of the device is the same as that of the first embodiment, the description is omitted here.

Hereinafter, a control method will be described. In ROM77,
A table for determining whether to perform suction or to change the atmosphere is recorded. Table 6 shows the table.

[0058]

[Table 6]

Tr is the actual temperature of the gas entering the catalyst 2 sent from the temperature sensor 3. Tl is the lower limit of the NOx adsorption temperature. M is the amount of supplying hydrocarbons, and is the minimum amount of hydrocarbons that can sufficiently adsorb NOx. T0 and Tn are the lower limit and the upper limit of the temperature at which the activity is improved by the fluctuation of the atmosphere.

Table 4 shown in Example 1 is recorded in R0M77.

The ROM 77 records a determination as to whether or not to proceed to the fluctuation routine. This table is shown in Table 7.

[0062]

[Table 7]

Tr is the actual exhaust gas temperature upstream of the catalyst 2 sent from the temperature sensor 3. T0 and Tn are the lower limit and the upper limit of the temperature at which the activity is improved by the fluctuation of the atmosphere. Further, the ROM 77 stores a table relating to the exhaust gas temperature and the fluctuation cycle of the hydrocarbon supply. Table 8 shows the table. In the temperature range where the actual exhaust gas temperature upstream of the catalyst 2 is improved by periodically fluctuating the hydrocarbon supply, hydrocarbons are supplied at an optimum cycle in accordance with the temperature, but in other temperature ranges, It means that no hydrocarbon is supplied.

[0064]

[Table 8]

Tr is the actual temperature of the gas entering the catalyst 2 sent from the temperature sensor 3. T0 and Tn are determined in advance by the method as in Test Example 2 at the lower and upper limits of the temperature at which the activity is improved due to the change in atmosphere. The division of the temperature range between T0 and Tn is arbitrarily determined. The supply of hydrocarbons is
Fluctuation width M '(initial value)-0, fluctuating in a cycle of Pn seconds Period P
The numerical value of n is P1> P2... Pn-1> Pn. Hereinafter, FIG.
Electronic control unit 7 according to the flowchart shown in FIG.
The details of the processing will be described. First, the actual exhaust gas temperature (Tr) upstream of the catalyst 2 is read. From Table 6 stored in R0M, it is determined whether to perform variation control or control for adsorption. Except for the control for fluctuation and adsorption, the supply of hydrocarbons is stopped and the process returns to START. In the case of the variation control, the process proceeds to the variation routine, and from Table 8 stored in the ROM 77, the optimum cycle corresponding to the temperature is read.
Feed a hydrocarbon 1 with an amplitude of 供給 す る 0. On the other hand, if it is determined that the adsorption is performed, the process proceeds to the adsorption routine, and the supply of a certain amount (M) of hydrocarbons is started. After the supply of the hydrocarbon cable for adsorption is started, the NOx concentration upstream and downstream of the catalyst 2 is read, and the continuation and stop of the hydrocarbon supply are determined by referring to Table 4 stored in the ROM 77. .

The supply is continued while it is determined that the adsorption has not reached saturation. However, when it is determined that the adsorption has reached saturation, the hydrocarbon supply device 6 is stopped through the drive circuit. When the supply of hydrocarbons is stopped, Tr is read and it is determined from Table 7 stored in the ROM 77 whether or not the adsorbed NOX is at a temperature that can be reduced by atmospheric fluctuation. If it is determined that the return is possible, the process proceeds to the fluctuation routine. If not, the supply is stopped. (Results) From Examples 1 and 2, by performing these feedback treatments, the low-temperature NOx adsorption capacity of the Rh / zeolite catalyst is maximized, and NOx is efficiently reduced to N _2.
Could be reduced and purified. Further, since the supply of hydrocarbons is the minimum required, deterioration of fuel efficiency due to the supply of hydrocarbons was also minimized.

Further, from the third embodiment, NOx in exhaust gas can be efficiently reduced to N ₂ even in an oxygen-excess atmosphere by a purification system without maximizing the ability to improve the catalytic activity of Rh due to fluctuations in the gas atmosphere. After purification, exhaust gas purification with less generation of N ₂₀ was enabled (FIG. 8).

[0068]

According to the present invention, the purifying performance of the catalyst used is maximized, and the exhaust gas temperature is relatively low, such as when the engine is started or idling, or when the load on the engine is high and the exhaust gas temperature is high. NOx can be efficiently purified in the region. Further, a decrease in catalytic activity due to the sulfur oxide can be suppressed.

[Brief description of the drawings]

FIG. 1 is a diagram showing the temperature dependence of the NOx purification rate in the process of decreasing the temperature of a catalyst used in Test Example 1 and Examples.

FIG. 2 shows N adsorbed on the catalyst used in Test Example 1 and Examples.
It is a figure regarding the temperature dependence of the desorption behavior in the temperature rise process of Ox.

FIG. 3 is a diagram relating to temperature dependence of a NOx purification rate in a temperature decreasing process when model gases 1 to 4 of a catalyst used in Test Example 2 are used.

FIG. 4 is a block diagram illustrating a configuration of an exhaust gas purifying apparatus according to Embodiments 1 and 2 of the present invention.

FIG. 5 is a view showing a flowchart of a control method of the exhaust gas purifying apparatus according to the first and second embodiments of the present invention.

FIG. 6 is a block diagram illustrating a configuration of an exhaust gas purifying apparatus according to a third embodiment of the present invention.

FIG. 7 is a view illustrating a flowchart of a control method of the exhaust gas purifying apparatus according to the third embodiment of the present invention.

FIG. 8 is a diagram relating to temperature dependence of a NOx purification rate in a temperature decreasing process according to a third embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine 2 ... Catalyst 3 ... Temperature sensor 4 ... Catalyst upstream NOx sensor 5 ... Catalyst downstream NOx sensor 6 ... Hydrocarbon supply apparatus 7 ... Electronic control unit 70-72 ... A / D converter 73 ... NOx reducing agent supply Driving unit 75 ... Output port 76 ... CPU 77 ... ROM

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Minoru Tanabe 41-41, Chuchu, Yokomichi, Nagakute-cho, Aichi-gun, Aichi F-term in Toyota Central R & D Laboratories, Inc. (reference) 3G091 AA02 AB05 AB06 AB09 BA03 BA14 BA15 BA19 BA32 BA39 CA02 CA03 CA18 DA08 DA10 DB05 EA01 EA05 EA16 EA17 EA18 EA33 FA02 FA12 FA13 FB02 FB03 FC05 FC06 FC07 FC08 GA01 GA06 GB01X GB05W GB06W GB07W GB09Y GB10Y HA18 HA36 HA37 HA38 HA39 HA42

Claims (2)

[Claims] An exhaust gas purifying catalyst containing at least zeolite and rhodium; a temperature detecting means for detecting a temperature of the exhaust gas; and a NOx reducing agent supply for supplying a NOx reducing agent to the exhaust gas flowing into the exhaust gas purifying catalyst. Means, an adsorption temperature judging section for judging whether or not the temperature of the exhaust gas is within the NOx adsorption temperature range of the zeolite based on a temperature detection value of the temperature detecting means, and a NOx reducing agent for driving the NOx reducing agent supply means Control means for supplying the NOx reducing agent by driving the NOx reducing agent supply driving unit when the temperature of the exhaust gas is in the NOx adsorption temperature range by the adsorption temperature determining unit. An exhaust gas purifying apparatus comprising: 2. An exhaust gas purifying apparatus according to claim 1, further comprising NOx concentration detecting means for detecting NOx concentration in said exhaust gas upstream and downstream of said exhaust gas purifying catalyst, wherein said control means comprises means for purifying said exhaust gas detected by said NOx concentration detecting means. A NOx concentration ratio judging unit for judging whether the NOx concentration ratio on the upstream side and the downstream side of the catalyst for use is equal to or lower than a predetermined value, and the temperature of the exhaust gas detected by the temperature detecting means reaches an adsorption capacity recovery temperature. A recovery temperature determining unit for determining whether the NOx reducing agent supply driving unit is being driven and the NOx concentration ratio determining unit determines
When the NOx concentration ratio detected by the x concentration detecting means is equal to or less than a predetermined value, the driving of the NOx reducing agent supply driving unit is temporarily stopped, and then the recovery temperature determining unit determines that the exhaust gas temperature is equal to the adsorption capacity recovery temperature. When the exhaust gas temperature is within the NOx adsorption temperature range by the adsorption temperature determination unit, and then the NOx reducing agent supply driving unit is driven. The exhaust gas purifying apparatus according to claim 1, wherein the NOx reducing agent is supplied to the exhaust gas purifying apparatus.