JPH04284827A - Exhaust gas purifying apparatus - Google Patents

Abstract

PURPOSE:To provide an exhaust gas purifying apparatus capable of more effectively adsorbing cold HC contained in exhaust gas. CONSTITUTION:In an exhaust gas purifying apparatus wherein an HC adsorbing material 33 is arranged on the upstream side of the exhaust gas purifying catalyst of an exhaust gas system E1, the HC adsorbing material 33 is composed of zeolite heat-treated in an H2S atmosphere.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention efficiently removes HC in the exhaust gas when the exhaust gas temperature is low and when it reaches a high temperature of 300°C or more from the low temperature, such as when starting an engine in a cold state. The present invention relates to an exhaust gas purification device that can purify exhaust gas by adsorbing it well.

0002

2. Description of the Related Art Conventionally, pellet type catalysts or monolith type catalysts have been used as exhaust gas purification devices for purifying exhaust gas from automobiles. When purifying exhaust gas with an exhaust gas purification device, harmful components HC in the exhaust gas,
Among CO and NOx, the performance of catalytic purification of HC is particularly strongly affected by the exhaust gas temperature.

[0003] That is, the purification performance of conventional exhaust gas purifying catalysts is such that, for example, the catalytic function is fully exhibited when the exhaust gas temperature is 300°C or higher. Therefore, the catalyst is sufficient to remove the HC (hereinafter referred to as cold HC) component in the low-temperature exhaust gas when the exhaust gas temperature is low, such as when starting the engine, and during the transition from the low temperature until it reaches a high temperature of 300°C or more. It may be discharged into the atmosphere without being purified.

[0004] In addition, generally, when an engine is started in a cold state, a mixture with a higher concentration than during normal operation is supplied to the engine, and the amount of HC contained in the exhaust gas is greater than when it is warmed up. ing. For this reason, it is desired to purify cold HC particularly efficiently during startup, etc., and measures such as installing an HC trapper upstream of the catalyst to adsorb cold HC are taken in addition to the catalyst described above. An exhaust gas purification device has been proposed. For example, JP-A-2-75
As disclosed in Publication No. 327, an exhaust gas purification device has been proposed in which an HC trapper equipped with zeolite such as Y-type zeolite or mordenite as an HC adsorbent is placed upstream of an exhaust gas purification catalyst in an exhaust gas system. .

[0005]

[Problem to be solved by the invention] However, the above-mentioned Japanese Patent Application Laid-Open No.
As shown in Figure 4 of the same publication, the HC adsorption performance of various zeolites used in the exhaust gas purification device disclosed in Publication No. 75327 is highest at an exhaust gas temperature of 200°C;
It decreases as the temperature rises to °C. For this reason, in the exhaust gas purification device, the adsorption rate of HC in the exhaust gas is poor during the transition period until the exhaust gas temperature reaches a high temperature of 300°C or higher, at which the catalytic function of the exhaust gas purification catalyst is fully demonstrated. cannot be purified.

[0006] The chemical adsorption power of HC in zeolite, which is used as an adsorbent for cold HC in the HC trapper of the conventional exhaust gas purification device, is affected by its acid strength. In other words, in the case of zeolite with a low silica/alumina ratio, although the amount of solid acid is large, it is mostly occupied by weak acid sites that do not have enough adsorption power to adsorb and hold HC as chemical adsorption sites, so the amount of solid acid in the exhaust gas is It has the property of adsorbing a large amount of water, and the adsorption amount of cold HC is small. Conversely, in the case of zeolite with a high silica/alumina ratio, the acid strength is strong but the adsorption point (
Since the amount of acid sites is small, the amount of adsorption is small. Therefore, as a zeolite used as an adsorbent for cold HC, one having an intermediate silica/alumina ratio has been used.

[0007] Therefore, an exhaust gas purification device having an HC adsorbent should be able to adsorb cold HC contained in the exhaust gas more efficiently both when the exhaust gas temperature is low and when transitioning to a high temperature. This is considered an issue. An object of the present invention is to provide an exhaust gas purification device that solves the above problems.

[0008]

[Means for Solving the Problems] The present invention provides an exhaust gas purification device having an HC adsorbent disposed upstream of an exhaust gas purification catalyst in an exhaust gas system equipped with an exhaust gas purification catalyst. , H2S atmosphere.

As the zeolite, conventionally used Y-type zeolite, mordenite, ZSM-5, etc. can be used. The silica/alumina ratio constituting the zeolite can range widely from high to low. The conditions for heat treatment of zeolite are temperature: 300~
Preferably, 500°C, time: 2 to 5 H, H2S flow rate: space velocity number + h-1. After being heat-treated under the above conditions, the solid acid strength of the zeolite is increased and H
The chemical adsorption power of C becomes stronger.

[0010] Specific usage forms of the zeolite include pellet-shaped zeolite, zeolite coated on the surface of a pellet-shaped core material, and zeolite coated on the surface of a cordierite carrier base material and a metal carrier base material. A material coated with zeolite can be used. Furthermore, a three-way catalyst can be used as the exhaust gas purifying catalyst provided downstream of the HC adsorbent. As a three-way catalyst, a catalytic precious metal such as Pt, Rh, or Pd is added as a catalytic component to a catalyst support layer formed on the surface of a pellet-like core material, a cordierite carrier base material, a metal carrier base material, etc. It is preferable to use one that supports

[0011]

[Operation and Effect] According to the exhaust gas purification device of the present invention,
Zeolite heat treated in H2S atmosphere is used as HC adsorbent. The acid strength of this zeolite as an HC adsorbent is increased by the heat treatment,
It increases the HC adsorption power and retention amount, and improves the so-called adsorption function. In other words, the zeolite whose acid strength has been strengthened by heat treatment does not weaken its HC adsorption capacity as the exhaust gas temperature increases (the kinetic energy of HC molecules increases), unlike in the case where the heat treatment is not performed. At the same time, the upper limit temperature at which the previously adsorbed HC is desorbed also increases, so the previously adsorbed HC desorbs until the temperature reaches a high temperature when the catalyst function for exhaust gas purification is fully demonstrated.
will not be desorbed (released). Therefore, if an HC adsorbent is placed and used upstream of the exhaust gas purification catalyst in an exhaust gas system equipped with an exhaust gas purification catalyst, the HC contained in the low-temperature exhaust gas when the engine is started, and the exhaust gas can be purified. Exhaust gas temperature 3 at which the catalytic function of the catalyst is fully demonstrated
HC contained in the exhaust gas during transition until reaching a high temperature of 00° C. or higher can be efficiently adsorbed, and HC desorption can be prevented.

[0012] After the exhaust gas temperature moves to a high temperature,
HC desorbed from the HC adsorbent is oxidized by an exhaust gas purifying catalyst placed downstream of the HC adsorbent and whose catalytic function is sufficiently enhanced by high temperature, converting it into H2O and CO2, and purifying the exhaust gas. Can be done. Therefore, according to the exhaust gas purification device of the present invention, the HC contained in the low-temperature exhaust gas, which has conventionally had poor purification efficiency, and the exhaust gas before the transition to high temperature is efficiently adsorbed by the HC adsorbent. , can be purified.

[0013]

Embodiments (Embodiment 1) Embodiment 1 of the exhaust gas purification apparatus of the present invention will be described below with reference to FIGS. 1 and 2. The exhaust gas purification device of the embodiment is installed in an exhaust gas system E1 that purifies exhaust gas discharged from an engine E and then discharges the exhaust gas into the atmosphere. That is, the exhaust gas system EI includes a first catalytic converter 1, a second catalytic converter 2, and a second catalytic converter, which are arranged at a constant interval from the upstream side to the downstream side of an exhaust gas passage E10 connected to the engine E. The HC trapper 3 is arranged upstream of the HC trapper 2.

The exhaust gas passage E10 includes a bypass passage E11 that connects the upstream and downstream sides of the HC trapper 3.
is provided. A first switching valve 4 is provided at the upstream connection position and the downstream connection position, respectively.
First and second switching valves 42 are arranged. Further, in the exhaust gas passage E10 between the first switching valve 41 and the first catalytic converter 1, the first and second switching valves 41,
A temperature sensor 43 is disposed for detecting the timing of switching 42 based on the level of exhaust gas temperature and operating the switching.
The first catalytic converter 1 includes a cylindrical central portion 100 and a conical cylindrical end portion 10 having a flange-like attachment portion 101 formed on both sides thereof.
A three-way catalyst 13 is housed in the container 10 and fixed by a holder 11 and a wire net 12. This three-way catalyst 13 is made of a commercially available cordierite monolith carrier (capacity: 0.7 liters, number of cells: 400 cells/in2) and Pt (1
．． 5g/l-cat) and Rh (0.3g/l-cat)
t).

The second catalytic converter 2 has a cylindrical central portion 2
00 and a conical cylindrical end portion 202 formed on both sides thereof and having a flange-like attachment portion 201, and this container 20.
A three-way catalyst 23 is housed inside and fixed by a holder 21 and a wire net 22. This three-way catalyst 2
3 is a commercially available cordierite monolith carrier (capacity: 1
．． 3l, number of cells: 400 cells/in2), Pt (1.5g/l-cat) and Rh (0.3g/l-cat) and Rh (0.3g/l-cat) as catalyst components.
3g/l-cat).

The HC trapper 3 has a cylindrical central portion 300.
and a conical cylindrical end portion 302 formed on both sides thereof and having a flange-like attachment portion 301, and an HC adsorbent 33 housed within the container 30 and fixed by a holder 31 and a wire net 32. . This HC adsorbent 33 is an HC adsorbent having enhanced HC adsorption function by heat-treating zeolite supported on a cordierite monolith carrier in an H2S atmosphere.

The HC adsorbent 33 is formed as follows. That is, commercially available Na-H/Y type zeolite (
Silica/alumina molar ratio: 30, Na2O: 0.2w
100 parts of silica sol (20 wt%), 35 parts of silica sol (20 wt%), and 50 to 70 parts of water are mixed and stirred to prepare a wash coating slurry. A commercially available cordierite monolith carrier (capacity: 1.0 liters) was added to the slurry.
, 400 cells/in2) after water absorption treatment, immersion,
After a certain period of time, pull it up and blow off the excess slurry, and
After drying at 0°C, temporary firing was performed at 350°C. After the pre-calcination, the above-mentioned wash coat is further repeated on the surface, resulting in a total of 100g/l-cat of Na-H.
The wash-coated product coated with /Y-type zeolite was calcined at 500°C for 3 hours to obtain an intermediate HC adsorbent before final heat treatment.

Next, the intermediate HC adsorbent was heated to a temperature of 350°C.
By heat treatment at ℃ for 3 hours under H2S flow (1.0 l/min), an HC adsorbent 33 having a coat layer of an HC adsorbent made of zeolite with increased solid acid strength was obtained. . The HC adsorbent 33 manufactured as described above is housed in a container 30 and fixed with a holder 31 and a wire net 32, and the HC trapper 3
is formed.

As described above, this HC trapper 3 is arranged in the exhaust gas passage E10 upstream of the second catalytic converter 2. Next, in the exhaust gas purification apparatus of this embodiment configured as described above, the first switching valve 41 is moved to the A and B positions as shown in FIG. 2 according to the exhaust gas temperature T detected by the temperature sensor 43. The second switching valve 42 is switched to the C and D positions to direct the exhaust gas flow in the S1 direction and the S
By controlling in either of two directions, the HC adsorption action of the HC adsorbent 33, the desorption (release) action of the adsorbed HC, the regeneration of the HC adsorption capacity, the prevention of thermal deterioration, and the desorption of H2S due to high temperatures. Take preventive measures. Then, i) when T≦200° C., the first switching valve 41 is controlled to be in the A position and the second switching valve 42 is in the C position. After passing through the first catalytic converter 1, the exhaust gas discharged from the engine E is introduced into the HC trapper 3 through the exhaust gas passage E10, and the cold HC in the exhaust gas is reduced by about 53% by the HC adsorbent 33. It is adsorbed. Then, the exhaust gas whose cold HC content has decreased is introduced from the HC trapper 3 into the second catalytic converter 2 as indicated by arrow S1, and is purified by the three-way catalyst 23.

ii) When 200°C<T≦300°C,
The first switching valve 41 is in the B position, and the second switching valve 42 is in the D position.
The position is controlled. Then, after passing through the first catalytic converter 1, the exhaust gas discharged from the engine E is introduced into the second catalytic converter 2 from the bypass passage E11 in the direction of the arrow S2. That is, during this control, if exhaust gas having a temperature higher than 200° C. is introduced into the HC trapper 3, the HC adsorbed on the HC adsorbent 33 starts to be desorbed. However, since the three-way catalyst 23 of the second catalytic converter 2 on the downstream side is not yet sufficiently activated, the desorbed HC cannot be completely purified. Therefore, exhaust gas is prevented from flowing into the HC trapper 3 in the temperature range of 200°C to 300°C. This prevents the HC adsorbed on the HC adsorbent 33 from being desorbed, and since the exhaust gas flows through the bypass E11, there is almost no loss of heat, so the three-way catalyst 23 of the second catalytic converter 2 on the downstream side , which has the effect of faster activation because the temperature rises faster.

iii) When 300°C<T≦400°C, the first switching valve 41 is controlled to be in the A position and the second switching valve 42 is in the C position. Then, the exhaust gas discharged from the engine E is purified as it passes through the first catalytic converter 1 in which the three-way catalyst 13 has been sufficiently activated due to the high temperature, and the exhaust gas is purified by the H
The C component is introduced into the HC trapper 3 from the exhaust gas passage E10 in a state containing almost no C component, and until now the HC adsorbent 3
3 is desorbed by high temperature within 400° C., and the HC-containing state is introduced into the second catalytic converter 2 as shown by the arrow S1, and purified by the three-way catalyst 23.

In this case, since the HC that has been adsorbed so far is removed from the HC adsorbent 33 of the HC trapper 3, the HC adsorption performance of the HC adsorbent 33 is regenerated, and the coking of the HC adsorbent 33 Deterioration is prevented. . Also in the second catalytic converter 2 on the downstream side, the three-way catalyst 23 is sufficiently activated by the high temperature, and the exhaust gas containing HC is purified as the HC adsorbent 33 is regenerated.

iv) When 400° C.<T, the first switching valve 41 is controlled to be in the B position and the second switching valve 42 is in the D position. Then, after passing through the first catalytic converter 1, the exhaust gas discharged from the engine E is transferred from the bypass passage E11 to the second catalytic converter 2 in the direction of the arrow S2.
to be introduced. That is, during this control, if high exhaust gas with a temperature of 400°C or higher is introduced into the HC trapper 3, H2S (hydrogen sulfide) adsorbed on the zeolite of the HC adsorbent 33 due to the heat treatment during the manufacturing process will be removed. ) begins to desorb, and to prevent this, and
In order to prevent deterioration of the HC adsorbent 33 due to hydrothermal
A bypass is made to prevent high temperature exhaust gas exceeding 400°C from flowing into the C trapper-3. (Comparative Example) In order to confirm the effect of the HC adsorbent of the above example, an HC adsorbent of Comparative Example 1 and an HC adsorbent of Comparative Example 2 were formed. (Comparative Example 1) The HC adsorbent of Comparative Example 1 is the same as the example except that it is not heat-treated in an H2S atmosphere like the HC adsorbent of the example. (Comparative Example 2) In the HC adsorbent of Comparative Example 2, Na-H/Y type zeolite (silica/alumina molar ratio: 100) was used as the zeolite, and like the HC adsorbent of the example, it was used in an H2S atmosphere. The process is the same as the example except that the heat treatment is not performed.

(Performance Comparison Test) The HC adsorbent of Example, the HC adsorbent of Comparative Example 1, and the adsorbent of Comparative Example 2 were incorporated into the HC trapper 3 of the exhaust gas purification device shown in FIG. 1, and the purification performance was evaluated in an actual vehicle. did. The evaluation is a cold start evaluation (
Start → Idle → Acceleration → Steady running at 60km/h).

The results of this evaluation are shown in Table 1. As is clear from Table 1, in the case of the example, a cold HC reduction rate superior to that of Comparative Examples 1 and 2 can be obtained. This means that H
This is thought to be because C is reliably adsorbed and adsorbed HC is not desorbed.

[0026] As described above, the HC adsorbent in which zeolite heat-treated in an H2S atmosphere is used as the HC adsorbent has an improved HC adsorption ability. Furthermore, the reason why the HC adsorption ability of the HC adsorbent has improved is presumed to be that the solid acid site strength of zeolite etc. has become stronger and the HC adsorption power has become stronger. In addition, in the exhaust gas purification system for alcohol-fueled vehicles, when a performance evaluation test was conducted using an HC adsorbent using zeolite heat-treated in an H2S atmosphere as an HC adsorbent, it was found that HC, including formaldehyde adsorption ability, It was found that the adsorption capacity was improved.

[Brief explanation of the drawing]

FIG. 1 is a partial sectional view showing an example of use of an exhaust gas purification device according to an embodiment.

FIG. 2 is an explanatory diagram showing control states of a first switching valve and a second switching valve in the exhaust gas purification device of the embodiment.

[Explanation of symbols]

1...First catalytic converter 2...Second catalytic converter 3...HC trapper 33...H2 Zeolite heat-treated in an S atmosphere is
HC adsorbent used as C adsorbent

Claims (1)

[Claims] Claim 1: In an exhaust gas purification device having an HC adsorbent disposed upstream of the exhaust gas purification catalyst in an exhaust gas system equipped with an exhaust gas purification catalyst, the HC adsorbent is heat treated in an H2S atmosphere. An exhaust gas purification device characterized by being made of zeolite.