JPH0871371A - Exhaust gas purification method - Google Patents

Abstract

(57) [Abstract] [Purpose] To more efficiently purify NOx in exhaust gas with excess oxygen. [Composition] A catalyst made of Y-type zeolite or mordenite carrying a noble metal is placed in a flow path of exhaust gas in excess of oxygen, and paraffin-based HC is added to the exhaust gas to bring it into contact with the catalyst to be included in the exhaust gas. It is characterized by reducing and purifying the generated NOx. Since Y-type zeolite or mordenite has a relatively large pore size as compared with other zeolites, it is considered to be excellent in adsorbing paraffinic HC having a large molecule, and exhibits high NOx purification performance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification method for purifying exhaust gas discharged from a diesel engine or the like, and more specifically, it oxidizes carbon monoxide (CO) or hydrocarbon (HC) in the exhaust gas. Nitrogen oxides (NOx) in the exhaust gas containing excess oxygen than is necessary to
The present invention relates to a method for efficiently purifying water.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by simultaneously oxidizing CO and HC and reducing NOx has been used as a catalyst for purifying exhaust gas of an automobile.
As such a catalyst, for example, a catalyst in which a supporting layer made of γ-alumina is formed on a heat resistant carrier such as cordierite and a catalytic precious metal such as Pt, Pd, Rh is supported on the supporting layer is widely known. There is.

By the way, the purification performance of such an exhaust gas purification catalyst greatly differs depending on the air-fuel ratio (A / F) of the engine. That is, in the operation on the lean side where the air-fuel ratio is large, that is, the fuel concentration is lean, the amount of oxygen in the exhaust gas is large and the oxidation reaction for purifying CO and HC is active, while the reduction reaction for purifying NOx is Become inactive. On the contrary, when the air-fuel ratio is small, that is, when the engine is operated on the rich side where the fuel concentration is high, the amount of oxygen in the exhaust gas is small and the oxidation reaction becomes inactive but the reduction reaction becomes active.

On the other hand, when driving an automobile, acceleration and deceleration are frequently performed in urban areas, and the air-fuel ratio frequently changes within the range from near stoichiometric (theoretical air-fuel ratio) to the rich state. In order to meet the demand for low fuel consumption in such traveling, it is necessary to operate on the lean side to supply an air-fuel mixture with excess oxygen as much as possible. Therefore, there is a demand for the development of a catalyst that can sufficiently purify NOx during operation on the lean side.

Therefore, JP-A-1-135541 and JP-A-3-232533 disclose an exhaust gas purifying catalyst in which a catalytic precious metal is supported on zeolite.
According to this exhaust gas purification catalyst, the NOx purification performance in the lean atmosphere is excellent and the heat resistance is excellent.
High purification performance can be maintained for a long time.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention In an exhaust gas purifying catalyst in which a catalytic precious metal is supported on zeolite,
Although the mechanism of the NOx reduction reaction has not been clarified, it is considered that NOx reacts with HC contained in the exhaust gas on the catalytic noble metal. However, in the exhaust gas in the lean atmosphere, the amount of HC in the exhaust gas is small, the molecular weight is low, and the polycycle is formed, so that the reducing power is insufficient and further improvement of the NOx purification performance cannot be expected.

The present invention has been made in view of the above circumstances, and an object thereof is to more efficiently purify NOx in exhaust gas in excess of oxygen.

[0008]

Means for Solving the Problems A first method for solving the above problems is described below.
The exhaust gas purifying method of the present invention comprises placing a catalyst composed of Y-type zeolite or mordenite carrying a catalytic noble metal in a flow path of exhaust gas in excess of oxygen, and adding paraffinic HC to the exhaust gas to bring it into contact with the catalyst. Is characterized by reducing and purifying NOx contained in the exhaust gas.

In the exhaust gas purifying method of the second invention, the silica / alumina ratio (SiO ₂ / Al ₂ O ₃ ) is 20 to 25.
A catalyst in which a catalyst noble metal is supported on a carrier of 0 is disposed in a flow path of exhaust gas in excess of oxygen, and paraffin-based HC is added to the exhaust gas to bring the catalyst into contact with the catalyst to remove NOx contained in the exhaust gas. It is characterized by reducing and purifying. In the exhaust gas purification method of the third invention, the average pore diameter is 0.5 to
A catalyst in which a catalytic noble metal is supported on a carrier having a thickness of 1.0 nm is placed in a flow path of exhaust gas in excess of oxygen, and paraffinic HC is added to the exhaust gas to bring it into contact with the catalyst to be contained in the exhaust gas. It is characterized by reducing and purifying NOx.

Further, in the exhaust gas purifying method of the fourth invention, a catalyst in which a catalytic noble metal and at least one metal selected from alkali metals and alkaline earth metals are supported on zeolite is provided in the exhaust gas flow path in excess of oxygen. The NOx contained in the exhaust gas is reduced and purified by adding a reducing substance to the exhaust gas and bringing it into contact with the catalyst.

[0011]

In the exhaust gas purification method of the first aspect of the present invention, paraffinic HC is added to the exhaust gas in excess of oxygen, and this is brought into contact with a catalyst composed of Y-type zeolite or mordenite carrying a catalytic noble metal. Since Y-type zeolite or mordenite has a relatively large pore size compared to other zeolites, it is excellent in adsorbing paraffinic HC having a large molecular size, and paraffinic HC having a chain shape and a high molecular weight and a single ring. When used as a reducing agent, Y-type zeolite or mordenite reforms these HCs into HCs that easily react with NO, resulting in high NO
x It is considered to show purification performance. As shown in Examples described later, even if other olefinic HCs or other zeolites are used, the effect as much as the Y-type zeolite or the combination of mordenite and paraffinic HC is not obtained.

In the exhaust gas purification method of the second invention, silica is used.
/ Alumina ratio (SiO₂/ Al₂O ₃) Is 20-250
A catalyst in which a precious metal catalyst is supported on a carrier is used.
It By setting the silica / alumina ratio within this range,
It is thought that the number of acid points will be optimal, and high NOx purification performance
Is shown. Similar to the first invention, other olefinic HC, etc.
Is not as effective as paraffinic HC.

In the exhaust gas purifying method of the third aspect of the present invention, a catalyst is used in which a catalyst precious metal is supported on a carrier having an average pore size of 0.5 to 1.0 nm. It is considered that by setting the average pore diameter within this range, the adsorbability of paraffinic HC having a large molecule will be excellent, and high NOx purification performance will be exhibited. Similar to the first aspect of the invention, other olefinic HCs and the like are not as effective as the paraffinic HCs.

Further, in the exhaust gas purifying method of the fourth invention, a catalyst in which a catalytic precious metal and at least one metal selected from alkali metals and alkaline earth metals are supported on zeolite is used. At least one metal selected from alkali metals and alkaline earth metals easily absorbs NOx, and zeolite easily adsorbs reducing substances, so that NOx and reducing substances coexist in the vicinity of the catalytic noble metal, and NOx Since the reduction reaction of 1 is likely to occur, the purification performance is improved.

In the case of the fourth aspect of the invention, the reducing substance is not limited to paraffinic HC but olefinic HC and aromatic H.
C etc. can also be used. This is considered to be because the addition of an alkali metal or the like suppresses the oxidation activity of the catalytic noble metal, increases the reaction between HC and NOx (improves the HC utilization efficiency), and improves the NOx purification rate. Further, it has been clarified that the catalytic noble metal and the alkali metal or alkaline earth metal supported on the zeolite exhibit higher NOx purification performance when adsorbed instead of ion-exchanged.

[0016]

【Example】

[Specific Example of the Present Invention] As the catalyst noble metal, platinum (Pt) is used.
Is typically used, but palladium (Pd) or rhodium (Rh) can also be used. The amount of the catalytic noble metal supported is 0.5 to 0.5 with respect to a carrier such as Y-type zeolite.
A range of 10% by weight is desirable.

The proportion of paraffinic HC or reducing substance added to the exhaust gas is 500 to 3000 ppm in terms of carbon.
A range of (ppmC) is suitable. If it is less than this, the effect added is not exhibited, and if it is added more than this, the effect is saturated. In order to adjust the silica / alumina ratio to the above range, silica and alumina may be mixed in the composition ratio, but it is convenient to adjust the dealumination degree of the zeolite.

Alkali metals include Li, Na, K,
A metal selected from Rb and Cs is used. As the alkaline earth metal, a metal selected from Be, Mg, Ca, Sr and Ba is used. Each may be used alone or in combination of two or more kinds. In addition,
The supported amount of this metal is 0.06 to 1 with respect to zeolite.
A range of 0% by weight is desirable. If the amount is less than this, the effect of supporting will not appear, and the effect will be saturated if more than this is supported. [Examples] Hereinafter, examples and comparative examples will be specifically described. (Example 1) <Preparation of catalyst 1> Pure water 9 to 100 parts by weight of Y-type zeolite
A slurry was prepared by mixing 0 parts by weight. Next, length 5
A cordierite honeycomb carrier having a diameter of 0 mm and a diameter of 30 mm is immersed in pure water to blow off excess water, and then immersed in this slurry. Then blow off the excess slurry, 10
After drying at 0 ° C. for 3 hours, baking is performed at 300 ° C. for 1.5 hours. This operation was repeated 3 times, and the mixture was further baked at 500 ° C. for 3 hours to form a Y-type zeolite layer. The Y-type zeolite layer was formed at 120 ± 5 g per 1 L of the honeycomb carrier.

Next, the honeycomb carrier having the Y-type zeolite layer is immersed in a tetravalent platinum ammine aqueous solution for 24 hours, pulled up to blow off the excess solution, and then dried at 250 ° C. for 1 hour, and the exhaust gas purifying catalyst 1 Was prepared. The amount of Pt carried is 2 g per 1 L of the honeycomb carrier. This support is not an ion exchange support, but a simple adsorption support. <Test Example 1> As shown in FIG. 1, the exhaust gas purifying catalyst 1 was attached to an exhaust system of a 2.4 L diesel engine,
The amount of NOx before and after the catalyst was changed from NOx amount before and after the catalyst under the conditions of SV = 60,000 / h and the catalyst bed temperature of 150 to 450 ° C. while adding light oil on the upstream side of the catalyst so that the carbon conversion concentration was 3000 ppmC.
x Purification rate was measured. And maximum N in the entire temperature range
Table 2 shows the Ox purification rate. <Catalyst 2 to 7> Mordenite instead of Y-type zeolite
ZSM-5, Zeolite-β, Ferrierite, SiO
Exhaust gas purifying catalysts 2 to 7 were prepared in the same manner as the catalyst 1 except that ₂ and Al ₂ O ₃ were used, respectively.

Then, the maximum NOx purification rate was measured in the same manner as the catalyst 1, and the results are shown in Table 2. <Test Example 2> Next, the maximum NOx purification rate of six types of catalysts except the catalyst 5 was measured using model gas.
As model gases, hexane (paraffinic HC), hexene (olefinic HC), cyclohexane (cycloparaffinic HC) and benzene (aromatic HC) were added to the base gas shown in Table 1 as carbon concentration 3000 ppmC Was measured under the condition of SV = 40,000 / h. The results are shown in Table 2 and FIG.

[0021]

[Table 1]

[0022]

[Table 2] From Table 2 and FIG. 2, while the catalyst using ordinary zeolite, silica or alumina as a carrier shows excellent NOx purification performance when the added gas species is olefinic HC,
The catalyst using Y-type zeolite or mordenite as a carrier is
Particularly excellent NO when the additive gas type is paraffinic HC
It can be seen that x purification performance is exhibited. Since paraffinic HC occupies about 75% of light oil, it is convenient to use light oil which is a fuel of a diesel engine in practical use. (Example 2) As shown in Table 3, carriers having various silica / alumina ratios were prepared, and 2 g of Pt was added in the same manner as in Example 1.
An exhaust gas-purifying catalyst carrying / L was prepared. Each catalyst was installed in the exhaust system of the 2.4-liter diesel engine shown in Fig. 1, and light oil was added at a carbon conversion concentration of 20 at the upstream side of the catalyst.
SV = 60,000 / while adding so that it becomes 00 ppmC
The NOx purification rate was measured under the conditions of h and catalyst bed temperature of 150 to 450 ° C. The maximum NOx purification rate is shown in Table 3 and FIG.

In the above test, the N ₂ concentration in the exhaust gas discharged from the catalyst was also measured, and the ratio of the converted NOx converted to N ₂ was calculated. The results are shown in Table 3 and FIG.

[0024]

[Table 3] From Table 3 and FIG. 3, the silica / alumina ratio is 20 to 250.
If the maximum NOx purification rate is 25% or more, N ₂
The selectivity is also 50% or more, and it is clear that the NOx purification performance is superior to other catalysts. (Example 3) As shown in Table 4, a carrier made of SiO ₂ -Al ₂ O _{3 having} various pore diameters was prepared, and an exhaust gas purifying catalyst carrying 2 g / L of Pt was prepared in the same manner as in Example 1. Prepared. Each catalyst was installed in the exhaust system of the 2.4-liter diesel engine shown in FIG. 1, and SV = 60,000 / h, catalyst bed temperature was added while adding light oil to the carbon conversion concentration of 2500 ppmC on the upstream side of the catalyst. The NOx purification rate was measured under the condition of 150 to 450 ° C. Table 4 shows the maximum NOx purification rate
And shown in FIG.

[0025]

[Table 4] From Table 4 and FIG. 4, when the average pore diameter is in the range of 0.5 to 1.0 nm, the maximum NOx purification rate is 35% or more,
It is clear that the NOx purification performance is superior to other catalysts. Example 4 A catalyst carrying 2 g / L of Pt was prepared from each of the carriers shown in Tables 5 and 6 in the same manner as the catalyst 1. Then, if necessary, water-soluble salts of various alkali metals or alkaline earth metals shown in Tables 5 and 6 are used, and the metals are impregnated with an aqueous solution and then calcined.
It was carried in the carrying amount shown in.

Each catalyst was installed in the exhaust system of a 2.4-liter diesel engine shown in FIG. 1, and diesel oil or propylene (C ₃ H ₆ ) was added at a carbon conversion concentration of 1100 on the upstream side of the catalyst.
SV = 60,000 / h, while adding so as to be ppmC,
The NOx purification rate was measured under the condition that the catalyst bed temperature was 150 to 450 ° C. The maximum NOx purification rates are shown in Tables 5 and 6. In the above test, N in the exhaust gas discharged from the catalyst
₂ concentration was measured to calculate the percentage conversion to N ₂ of the clarified NOx. The results are shown in Table 5 and Table 6 together.

[0027]

[Table 5]

[0028]

[Table 6] From Tables 5 and 6, the catalyst in which the zeolite is loaded with the alkali metal and / or the alkaline earth metal is N compared with the other catalysts.
It is clear that the Ox purification performance is excellent. It is also found that this catalyst exhibits excellent NOx purification performance with both paraffinic HC and olefinic HC as additive gas species, and that there is no selectivity of additive gas species.

Further, the average pore diameter is 0.5 to 1.0 nm.
It is also clear that the one using Y-type zeolite with a silica / alumina ratio of 20 to 250 (No.30-38, No.53-56) is particularly excellent in NOx purification performance in combination with light oil. Is. (Example 5) Using the catalyst 1 prepared in Example 1, the exhaust system of a 3.7 L diesel engine was mounted as shown in FIG.
The 10/15 mode running was performed while adding various concentrations of 0 ppmC to the exhaust gas.

The NOx purification rate during running is calculated from the NOx concentration before and after the catalyst, the amount of particulates discharged is measured, and the cumulative amount of exhausted after running 1 km is calculated. The results are shown in FIG. . From FIG. 6, the NOx purification rate improves as the amount of light oil added increases, but it is about 3
It can be seen that the NOx purification rate is saturated at an addition amount of about 000 ppmC. On the other hand, the amount of particulates is reduced by the addition of light oil, but after reaching the lowest value at around 2000ppmC, it increases again and returns to the same emission level as when no diesel oil is added at around 4000ppm. The amount of curate discharged is increasing rapidly. Therefore, in order to reduce the emission amount of particulates as compared with the case where no diesel oil is added and to obtain the NOx purification rate above a predetermined level, it is understood that the amount of diesel oil added is preferably 500 to 3000 ppmC.

[0031]

According to the exhaust gas purifying method of the present invention, NOx can be surely purified by adding the paraffinic HC even if the exhaust gas has a lean atmosphere. Therefore, the lean side can be operated with low pollution, and the demand for low fuel consumption in city driving can be met. If a predetermined amount of paraffinic HC is added, not only NOx but also the amount of particulates in the exhaust gas from the diesel engine can be reduced.

If light oil is used as the paraffinic HC, the exhaust gas from the diesel engine can be easily purified, which is extremely practical.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a NOx purification rate measurement device used in an example of the present invention.

FIG. 2 is a diagram showing NOx purification performance in the case of using various hydrocarbons in a catalyst using various carrier materials.

FIG. 3 is a diagram showing a relationship between a silica / alumina ratio and NOx purification performance.

FIG. 4 is a diagram showing a relationship between average pore diameter and NOx purification performance.

5 is a schematic diagram of a measuring device used in Example 5. FIG.

FIG. 6 is a graph showing the relationship between the NOx purification rate and the amount of particulates with respect to the amount of light oil added.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location B01J 29/12 ZAB A 29/22 ZAB A (72) Inventor Tatsushi Mizuno 1 Toyota Town, Toyota City, Aichi Prefecture Address Toyota Motor Co., Ltd. (72) Inventor Takeshi Hirabayashi 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Co., Ltd. (72) Inventor Ryusuke Tsuji 1 of 41, Nagakage-cho, Aichi-gun, Aichi Prefecture Toyota Central Research Institute Co., Ltd. (72) Inventor Miho Hayakawa 1st 41st Yokomichi Nagakute-cho, Aichi-gun, Aichi Prefecture

Claims (4)

[Claims] 1. A catalyst composed of Y-type zeolite or mordenite carrying a catalytic noble metal is arranged in a flow path of an exhaust gas in excess of oxygen, and paraffin hydrocarbon is added to the exhaust gas and brought into contact with the catalyst. The exhaust gas purification method is characterized in that the NOx contained in the exhaust gas is reduced and purified by the above method. 2. A silica / alumina ratio (SiO ₂ / Al ₂
A catalyst in which a catalytic noble metal is supported on a carrier having O ₃ ) of 20 to 250 is arranged in a flow path of an oxygen-excess exhaust gas, and paraffin hydrocarbon is added to the exhaust gas to bring it into contact with the catalyst. Thus, the exhaust gas purification method is characterized in that NOx contained in the exhaust gas is reduced and purified. 3. A catalyst in which a catalyst noble metal is supported on a carrier having an average pore diameter of 0.5 to 1.0 nm is arranged in a flow path of exhaust gas in excess of oxygen, and paraffin hydrocarbon is contained in the exhaust gas. Is added to bring the catalyst into contact with the catalyst to reduce and purify NOx contained in the exhaust gas. 4. A catalyst in which a catalytic noble metal and at least one metal selected from alkali metals and alkaline earth metals are supported on zeolite is arranged in a channel of exhaust gas with excess oxygen, and reduced to the exhaust gas. An exhaust gas purification method, characterized in that NOx contained in the exhaust gas is reduced and purified by adding a volatile substance and bringing it into contact with the catalyst.