JP3202676B2 - Adsorbent for automotive exhaust gas purification - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an adsorbent for purifying automobile exhaust gas, which is provided in a catalytic converter or the like.

[0002]

2. Description of the Related Art Catalytic converters used to purify exhaust gas from automobiles and the like require a temperature rise above a predetermined temperature in order for the catalyst to exert a catalytic action. However, if the temperature of the catalyst has not been sufficiently raised, it is necessary to heat the catalyst. Conventionally,
As a proposal for heating such a catalyst, for example, a technique described in Japanese Utility Model Laid-Open No. 63-67609 is known. Japanese Utility Model Application Laid-Open No. 63-67609 discloses a catalytic converter in which an electrically conductive metal monolith catalyst in which a metal carrier is coated with alumina is disposed close to the upstream side of a ceramic main monolith catalyst.

[0003] In addition, harmful components (HC, C
O, NO _x ), especially HC (hydrocarbon) causes photochemical smog (oxidant), so regulations are being tightened, and HC emitted in large quantities at engine start-up
Attempts have been made to purify water using the adsorption action of zeolite. For example, an automobile exhaust gas purifying apparatus in which a purifying catalyst is disposed in an exhaust gas system and an adsorbent such as zeolite or an adsorbent carrying the catalyst is disposed upstream thereof has been proposed (for example, Japanese Patent Application Laid-Open No. 2-75327, Kaihei 2-1
73312 and JP-A-2-135126). Further, JP-A-2-126937 discloses that
An adsorbent in which zeolite is coated on a metal carrier is disclosed.

[0004]

However, the catalytic converter described in Japanese Utility Model Laid-Open No. 63-67609 is
Since it is composed of a metal monolith catalyst and a main monolith catalyst as a preheater, there is a problem that it is difficult to purify HC components in exhaust gas at the time of starting the engine. In addition, a vehicle exhaust gas purifying apparatus in which a purifying catalyst and an adsorbent such as zeolite are disposed upstream of the purifying catalyst (Japanese Patent Laid-Open No. 2-753)
No. 27), even if HC is adsorbed by the adsorbent on the upstream side of the purification catalyst, there is a problem that HC is desorbed from the adsorbent with warm-up, and as a result, a considerable amount of HC passes through the purification catalyst. is there.

[0005] Japanese Patent Application Laid-Open No. 173332/1990 discloses that a catalyst is provided in a main passage and an adsorbent is provided in a bypass passage. Exhaust gas is passed through the bypass passage by a switching means when the engine is started. A technique is disclosed in which the exhaust gas is caused to flow to the catalyst in the main passage by the switching means when the temperature reaches the temperature.However, in this case, waiting until the catalyst in the main passage is completely warmed up requires a complicated mechanism, and the catalyst in the main passage itself becomes complex. The passage of the unpurified components through the catalyst until the water is warm cannot be ignored.

Further, in an automobile exhaust gas purifying apparatus in which an exhaust gas system is provided with a purifying catalyst and an adsorbent carrying the catalyst upstream thereof (JP-A-2-135126), the purifying catalyst uses the heat capacity of the adsorbent itself. There is a problem that the rise of the catalyst is delayed, and furthermore, even if a catalyst component is added to the adsorbent, its capacity is limited, and there is a drawback that sufficient purification cannot be performed. Further, Japanese Patent Laid-Open No. 2-126937 describes only adsorbent, CO, HC, it is not described catalytic converter in the exhaust gas including NO _X. Further, in any of the above-mentioned known examples, the zeolite used for the adsorbent is Y-type or mordenite-type, and has a problem in heat resistance and simultaneously strongly adsorbs moisture in exhaust gas,
Decreases the adsorption capacity of the adsorbent.

[0007]

Accordingly, an object of the present invention is to provide an adsorbent for purifying automobile exhaust gas which has solved the above-mentioned conventional problems. And the purpose is
According to the present invention, it can be achieved by an adsorbent for purifying automobile exhaust gas, comprising a high silica zeolite having a Si / Al ratio of 48 or more and adsorbing HC (hydrocarbon).

In the present invention, it is preferable to carry a catalyst on an adsorbent containing high-silica zeolite, because the adsorbing action of the zeolite and the catalytic action cooperate to improve the purification performance of exhaust gas.

As the zeolite, it is preferable to use a high silica zeolite having a Si / Al ratio of 48 or more because heat resistance is improved and the application conditions of the catalyst are relaxed. Further, catalysts supported on the adsorbent are (a) Pt, Pd, Rh, Ir and
A high silica zeolite having an Si / Al ratio of at least 48 ion-exchanged with at least one noble metal selected from Ru,
(b) It is preferable to use a composition comprising a heat-resistant oxide containing at least one noble metal selected from Pt, Pd, Rh, Ir and Ru.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is an adsorbent for purifying automobile exhaust gas containing high silica zeolite having a Si / Al ratio of 48 or more. Thus, the adsorbent of the present invention has a Si / Al ratio of 48.
Since the high-silica zeolite is contained, the catalytic converter equipped with this adsorbent captures unburned HC in low-temperature exhaust gas using the adsorption function of zeolite at the start of the engine driven by the cell motor. At the same time, when electricity is supplied to the honeycomb heater disposed downstream of the adsorbent, the trapped HC starts to be desorbed, and the main monolith catalyst and / or the catalyst on the heater, which are usually disposed downstream thereof, are heated instantaneously. Therefore, HC is suitably purified by reaction. When a catalyst is supported on the adsorbent,
HC is desorbed and purified by reaction.

Since the engine exhaust gas at the time of starting the engine is on the rich side (oxygen-deficient atmosphere), it is necessary to introduce an oxidizing gas necessary for oxidizing HC and CO, for example, secondary air into the exhaust gas. Hereinafter, for convenience, a catalytic converter provided with the adsorbent of the present invention will be described as an example. In the catalytic converter, a honeycomb heater (or a honeycomb heater carrying a catalyst) 2, a main monolith catalyst 3
Preferred arrangements and configurations of the zeolite adsorbent (or adsorbent carrying a catalyst) 1 are shown in FIGS. 1 (a) to 1 (f).

Among these configurations, FIG. 1A is preferable because the zeolite adsorbent 1 is located at the most upstream side of the exhaust gas flow path of the automobile, so that the adsorption is easiest. In this case, both the honeycomb heater 2 and the zeolite adsorbent 1 can be used with or without a catalyst. In the configuration of the honeycomb heater 2, the zeolite adsorbent 1, and the main monolith catalyst 3 from the upstream side as shown in FIG. 1 (b), the HC adsorbed on the zeolite adsorbent 1 can be desorbed by energizing the heater 2. Therefore, there is an advantage that control is easy. Also in this case, both the honeycomb heater 2 and the zeolite adsorbent 1 can be used regardless of the presence or absence of the catalyst.

Further, as shown in FIGS. 1 (c) to 1 (f), when the main monolith catalyst 3 is installed in front (most upstream side), the catalyst on the heater 2 and the zeolite adsorbent 1 Is less likely to lose its function and is excellent in durability. In this case, in the configuration of FIGS. 1C and 1D, both the zeolite adsorbent 1 and the honeycomb heater 2 disposed in the middle can be used regardless of the presence or absence of the catalyst, but are disposed at the most downstream side. The zeolite adsorbent 1 or the honeycomb heater 2 needs to carry a catalyst. On the other hand, FIG.
(e) When the zeolite adsorbent 1 and the honeycomb heater 2 are arranged between the main monolith catalysts 3 as shown in FIG. 1 (f), regardless of the presence or absence of the catalyst, both the honeycomb heater 2 and the zeolite adsorbent 1 Can be used.

1A to 1F, the honeycomb heater 2 may carry a zeolite adsorbent or an adsorbent / catalyst composition in which a catalyst is carried on a zeolite adsorbent. In this case, the control of adsorption and desorption can be easily performed by energizing the heater. The type of zeolite used as the adsorbent in the present invention is not particularly limited, but may be Y-type, mordenite, or the like, as well as ZSM-5, ZSM-5, commercially available from Mobile and Contec.
8, silicalite (UOP) and the like are preferred. In addition, X-type, Y-type, mordenite and the like obtained by subjecting a zeolite skeleton to dealumination treatment to increase the Si / Al ratio can also be suitably used. In the present invention, it is preferable to use a high silica zeolite having a Si / Al ratio of 48 or more. Si / A
If the l ratio is less than 48, the heat resistance becomes insufficient and the hydrophilicity increases, so that the water in the exhaust gas is strongly adsorbed, which is not preferable.

In the high silica zeolite, the minimum unit of the crystal unit cell is a crystalline aluminosilicate, similar to a well-known ordinary zeolite, and Al ₂ O ₃ and SiO ₂ are continuously bonded to each other by oxygen ions. But Si / Al
The ratio is as high as about 10 or more as compared with 1 to 5 of ordinary zeolites. In the present invention, a high silica zeolite having a Si / Al ratio of 48 or more is preferable as described above, but from the viewpoint of heat resistance, the Si / Al ratio is more preferably 50 or more, and particularly preferably 60 or more. On the other hand, when the Si / Al ratio exceeds 1000, the adsorption capacity itself decreases, and when a catalyst component is added, the number of ion exchange sites decreases, so that only a small amount of noble metal can be ion-exchanged into zeolite, Not preferred. The high silica zeolite is H
The (proton) type is preferred from the viewpoint of heat resistance.

In the present invention, the catalyst supported on the adsorbent mainly composed of zeolite preferably contains a noble metal such as Pt, Pd, and Rh, and further contains a heat-resistant oxide having a high specific surface area. Is preferred in terms of improving the ignition characteristics. Pt, Pd, although noble metals Rh and the like are supported on a zeolite and / or refractory oxide, and the like selective removal capability of heat resistance and NO _X (by-product NH ₃ generation suppression), ion exchange zeolites Is preferably carried.

In view of the above catalyst characteristics and the like, the adsorption / catalyst composition in which a catalyst is supported on an optimal adsorbent in the present invention is (a) at least one selected from Pt, Pd, Rh, Ir and Ru. A high silica zeolite ion-exchanged with one kind of noble metal and having a Si / Al ratio of 48 or more, and (b) Pt, Pd, Rh,
And a heat-resistant oxide containing at least one noble metal selected from Ir and Ru.
Here, the component (a) is composed of high silica zeolite and Pt, Pd
, Rh, Ir and Ru can be obtained by ion exchange with at least one noble metal selected from a suitable aqueous solution. In order to exhibit the desired performance described above, the ion exchange rate of the noble metal is preferably 10 to 85%,
30-85% is more preferred.

The noble metal ion-exchanged with the high-silica zeolite is fixed at the zeolite exchange site, has high dispersibility, effectively retains the catalytic activity, is hardly scattered, and does not aggregate even at a high temperature for a long time. High activity over a long period of time. Further, when an adsorbent / catalyst composition in which a catalyst component is supported on an adsorbent is used, coking of HC generated at the time of desorption can be prevented, and the durability of the adsorbent is improved. This noble metal ion exchanged zeolite is
For example, it is manufactured as follows. 1 high silica zeolite
It is immersed in a solution containing 0 ^-4 to 10 ^-1 mol / l of a cationic metal ion, and is immersed in a solution containing from room temperature to 100 ° C., preferably from 80 to
The noble metal component is ion-exchanged by standing, stirring or refluxing at 90 ° C. for about 2 hours or more.
Repeating the water washing removes non-exchanged noble metals. After ion exchange, it is usually dried at 80 to 150 ° C., and is further dried at 300 to 1000 ° C. under an oxidizing or reducing atmosphere for about 1 hour.
It can be manufactured by firing for 10 to 10 hours.

Further, when an oxide of a rare earth metal such as CeO ₂ or La ₂ O ₃ or an alkaline earth metal is added to zeolite, the ternary purification performance due to the oxygen storage capacity of the rare earth metal is expanded, and its application is diversified. It is preferable because the heat resistance can be improved by adding an alkaline earth metal. On the other hand, as the heat-resistant oxide of the component (b), Al ₂ O ₃ , TiO ₂ ,
ZrO ₂ , SiO ₂ or a composite oxide thereof can be used. In addition, the addition of oxides of rare earth metals and alkaline earth metals such as CeO ₂ and La ₂ O ₃ to these oxides also increases the three-way purification performance as described above, and also improves the heat resistance. preferable. The component (b) is constituted by supporting at least one of the above-mentioned noble metals on a heat-resistant oxide.

The component weight ratio of the component (a) to the component (b) is preferably (a) :( b) = 10 to 85:90:15. (a) If the component is less than 10% by weight, NO _X
Is difficult to obtain (selective generation of by-product NH ₃ ), and if it exceeds 85% by weight, the ignition performance becomes inferior. In the above-mentioned adsorption / catalyst composition, the total amount of the noble metal carried is preferably in the range of 10 to 35 g / ft ³ and 15 to 30 g / ft ^3.
g / ft ³ is more preferred. If the noble metal loading is less than 10 g / ft ³ , there is a problem in ignition characteristics and durability, and 35 g / ft ³
Exceeding the cost increases. As described above, by using a high silica zeolite having a Si / Al ratio of 48 or more, in the present invention, Rh of a noble metal is converted to 5 g / ft by a conventional catalyst for purifying exhaust gas.
^Although it was necessary to carry ³ or more, even with a carrying amount of less than 5 g / ft ³ , the selective purification ability of NO _X to N ₂ can be sufficiently exhibited, and even with a carrying amount of 0 to ₂ g / ft ³ , When used under relatively mild conditions such as a low poisoning substance content at a low temperature, practically sufficient selectivity can be exhibited.

The honeycomb heater used for the catalytic converter has a honeycomb structure made of metal, and the surfaces of the partition walls and pores of the honeycomb structure are made of Al ₂ O ₃ or Cr ₂ O.
Coating with a heat-resistant metal oxide such as ₃ is preferable because heat resistance, oxidation resistance, and corrosion resistance are improved.

The constituent material of the honeycomb structure used for the honeycomb heater is not limited as long as it is made of a material that generates heat when energized, and may be a metal material or a ceramic material, but a metal material is preferable because of its high mechanical strength. . In the case of metal, for example, stainless steel or Fe-Cr-Al,
Fe-Cr, Fe-Al, Fe-Ni, W-Co, Ni
-A material having a composition such as Cr. Among the above, Fe-Cr-Al, Fe-Cr, Fe
-Al is excellent in heat resistance, oxidation resistance and corrosion resistance, and is inexpensive and preferable. Further, in the case of metal, a foil type may be used.

The honeycomb structure may be porous or non-porous, but when a catalyst is supported, the porous honeycomb structure has high adhesion to the catalyst layer and is thermally expanded. This is preferable since the separation of the catalyst due to the difference hardly occurs.

In the present invention, the honeycomb shape of the honeycomb structure covering the adsorbent is not particularly limited, but specifically, for example, 6 to 1500 cells / In ² (0.
It is preferable to form the cell so as to have a cell density in the range of 9 to 233 cells / cm ² ). The thickness of the partition is 50-2.
A range of 000 μm is preferred. Further, as described above, the honeycomb structure may be porous or non-porous, and the porosity thereof is not limited, but it is 0 to 50%, preferably 25 to 50%.
% Is desirable from the viewpoint of strength characteristics, oxidation resistance and corrosion resistance. When a catalyst is supported, the catalyst preferably has a porosity of 5% or more from the viewpoint of adhesion to the catalyst.

In the present invention, the term “honeycomb structure” refers to an integrated structure having a large number of through-holes partitioned by partition walls. For example, the cross-sectional shape (cell shape) of the through-hole is circular, polygonal, corrugated, or the like. Various arbitrary shapes can be used. As the main monolith catalyst used in the catalytic converter, a conventionally known one can be used, but a three-way catalyst is preferable. The zeolite adsorbent may have any structure such as beads, pellets, and honeycomb, but preferably has a honeycomb structure in terms of pressure loss. In this case, the honeycomb structure itself may be composed mainly of zeolite, but the adsorbent mainly composed of zeolite is coated on a ceramic or metal substrate having thermal shock resistance. Is more practically preferable.

[0026]

EXAMPLES Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples. (Examples 1 to 8 and Comparative Examples 1 and 2) Selection of zeolite: commercially available Si / Al shown in Table 1
Mordenite zeolite A and ZSM of H type having different ratios
-5 zeolite B to E, zeolite F and Na type Z in which zeolite A is boiled with hydrochloric acid to increase the Si / Al ratio.
SM-5 zeolite G was used. The alkali metal content of zeolite G was 0.85% by weight, and the alkali metal content of other zeolites was 0.1% by weight or less. Here, the BET specific surface area (m ² / g) measured after heating each zeolite at room temperature at 900, 1000 and 1100 ° C. for 5 hours in an electric furnace, respectively, is shown in Table 1. .

[0027]

[Table 1]

As is clear from Table 1, the heat resistance of zeolite depends on the Si / Al ratio. Generally, the maximum operating temperature when used as an adsorbent or catalyst for automobile exhaust gas is 1000 ° C. Since the high specific surface area is maintained even at 1000 ° C., it is understood that the zeolite used preferably has a Si / Al ratio of 48 or more. Further, using unheated products (powder) of zeolite A and B, the adsorption amount of propane / propylene (1/2) was measured at room temperature in the presence of 10% H ₂ O. Thus, zeolite A was strongly poisoned by water.

Production of honeycomb heater: average particle size 1
0, 20, and 22 μm Fe powder, Fe-Al powder (Al50
wt%), Fe-Cr powder (Cr50wt%) raw material,
Raw materials were blended so as to have a composition of Fe-22Cr-5Al (wt%), and an organic binder (methylcellulose), an antioxidant (oleic acid) and water were added thereto to prepare a clay, and a rib thickness of 4 mil, 400 cells / In ² (cpi
² ) A honeycomb formed of square cells is extruded, dried, fired at 1300 ° C. in an H ₂ atmosphere, and then dried in air.
Heat treatment was performed at 00 ° C. The porosity of the obtained honeycomb structure was 22%, and the average pore diameter was 5 μm.

The outer diameter 90 mmφ obtained by the above method,
As shown in FIG. 2, two electrodes 11 were set on an outer wall 10 of a honeycomb structure having a length of 25 mm. Also, FIG.
As shown in the figure, six slits 12 having a length of 70 mm are provided in the axial direction of the through hole (the slit length at both ends is 50 mm), and the number of cells between the slits 12 is seven (about 10 mm). did. Further, the outer peripheral portion 13 of the slit 12 was filled with a zirconia-based heat-resistant inorganic adhesive to form an insulating portion, thereby producing a honeycomb heater.

Loading of catalyst A on heater: γ-Al ₂ O ₃ .CeO was deposited on the honeycomb heater obtained above.
₂ (70:30 by weight), and then convert Pt and Rh to Pt /
Rh is 35 g / ft ³ supported so as to be a ratio of 5/1, 600 ° C.
The catalyst A was supported on the heater by firing.

Loading of catalyst B on heater: On the other hand, H-ZS ion-exchanged for Pt was placed on a similar honeycomb heater.
M-5 (Si / Al ratio = 48) 50 parts and γ-Al ₂ O _3.
A mixture of 50 parts of CeO ₂ (80:20 by weight) and then converting Pt and Rh to the γ-Al ₂ O ₃ .CeO _2.
The catalyst was impregnated and supported, and finally 35 g / ft ³ of Pt / Rh was provided at a ratio of 19/1, and baked at 600 ° C., thereby coating a catalyst B having a film thickness of 50 μm on the honeycomb heater.

Zeolite adsorbent: A commercially available cordierite honeycomb carrier having a length of 25 mm (manufactured by NGK Insulators, Ltd.)
H-ZSM-5 (Si / Al) was formed on a honeycomb structure consisting of a square cell having a rib thickness of 6 mil and a through-hole number of 400 cells / In ^2.
Ratio = 48) to a film thickness of 50 μm,
To produce a zeolite adsorbent.

Loading of catalyst B on zeolite adsorbent: length 2 in the same manner as for loading catalyst B on the heater.
It was coated and supported on a 5 mm cordierite honeycomb carrier.

Loading of adsorbent on heater: A zeolite adsorbent was coated and supported on a honeycomb heater using the same method as that for preparing a zeolite adsorbent on a cordierite honeycomb carrier.

Main monolith catalyst: A commercially available three-way catalyst (a honeycomb structure having a ceramic support, a rib thickness of 6 mil, and a square cell having 400 cells / In ² through holes) was used.
Using a honeycomb heater, a zeolite adsorbent, and a main monolith catalyst of the types described above, a catalytic converter having these components arranged as shown in Table 2 was evaluated as follows.

That is, in order to confirm the performance at the time of starting the engine, a Bag 1 test was performed in the United States FTP using an automobile having a displacement of 2400 cc. Here, the supply voltage to the heater was 12 V, the energization was started 10 seconds after the engine was started, and the energization was stopped 40 seconds later. During energization, the gas temperature at the center of the heater was controlled to 400 ° C. In addition, 200l / m for 50 seconds after starting the engine
In the secondary air was introduced into the catalytic converter. Table 2 shows the obtained results. On the other hand, for comparison, when the main monolith catalyst alone was used (Comparative Example 1), when the zeolite adsorbent and the main monolith catalyst were used but the honeycomb heater was not used (Comparative Example 2), the same evaluation as above was performed. And the results are shown in Table 2. As is evident from the results in Table 2, according to the catalytic converter using the adsorbent of the present invention, each emission such as HC, CO, and NO can be satisfactorily purified.

[0038]

[Table 2]

[0039]

As described above, according to the adsorbent of the present invention, since it is a high-silica zeolite, it has excellent heat resistance, the application conditions of the catalyst are relaxed, and the adsorbent can be suitably used for purifying automobile exhaust gas. . Further, according to the present invention, the emission effect of zeolite and the effect of energizing and heating the heater greatly improve each emission in exhaust gas, particularly the purification of HC and CO, and significantly reduce the amount of emission to the atmosphere. it can.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a preferred arrangement and configuration of a catalytic converter using the adsorbent of the present invention.

FIG. 2 is an explanatory diagram illustrating an example of a honeycomb heater.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Zeolite adsorbent, 2 ... Honeycomb heater, 3 ... Main monolith catalyst, 10 ... Outer wall, 11 ... Electrode, 12 ... Slit, 13 ... Peripheral part of slit.

Claims (3)

(57) [Claims] An adsorbent for purifying automobile exhaust gas, comprising high silica zeolite having a Si / Al ratio of 48 or more and adsorbing HC (hydrocarbon). 2. An adsorbent for purifying automotive exhaust gas, wherein the adsorbent according to claim 1 carries a catalyst. 3. An adsorbent for purifying automotive exhaust gas, wherein the adsorbent according to claim 1 or 2 is coated on a honeycomb structure having a large number of through holes.