JPH1190235A - Catalyst for purifying exhaust gas from diesel engine and purification method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To display a high activity even at a low temperature for NOx removal and maintain the high activity even after the endurable use at a high temperature by making a composition of a chemical compound containing at least, one kind of element such as boron, potassium or tin, and platinum and a fire- resistant inorganic oxide containing zeolite. SOLUTION: This catalyst for purifying an exhaust gas comprises (a), (b) and (c). (a) is a chemical compound containing at least, one kind of element such as boron, potassium or tin. This chemical compound is preferably boron oxide, potassium sulfate, potassium pyrophosphate or tin oxide. (b) is platinum alone but may carry rhodium beside platinum. (c) is the carrier of the catalyst which can be made of zeolite alone and also can be made of a mixture of zeolite and a fire-resistant inorganic oxide other than zeolite. The fire-resistant inorganic oxide to be used is preferably active alumina, titania or zirconia.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a catalyst for purifying exhaust gas of a diesel engine. More specifically, the present invention relates to a catalyst for removing harmful substances in diesel engine exhaust gas, and particularly to a catalyst for removing nitrogen oxides (NOx) at low temperatures.
The present invention relates to a catalyst having high activity for removal and maintaining high activity even after endurance at high temperatures.

[0002]

2. Description of the Related Art Exhaust gas from diesel engines contains harmful substances for environmental protection, such as nitrogen oxides (NOx), hydrocarbons (HC), carbon monoxide (CO), and particulate matter harmful to the human body. Have been. In particular, since NOx causes photochemical smog and acid rain, it is urgently required to prevent the emission of NOx from the viewpoint of environmental conservation. In order to remove these harmful substances, exhaust gas is brought into contact with a catalyst to decompose harmful substances and purify the exhaust gas. In recent years, the proposed catalyst has a considerably high removal performance for HC, CO, particulate matter, etc., but the exhaust gas of a diesel engine is in an oxygen-excess atmosphere, and the removal of NOx is in the range of practical use. Not reached.

[0003] As a catalyst for removing NOx in an oxygen-excess atmosphere, a catalyst in which copper is supported on a porous carrier such as alumina, silica, zeolite or the like (JP-A-63-1)
No. 00919), a zeolite catalyst ion-exchanged with a noble metal (JP-A-1-135541), a catalyst in which copper or a noble metal is supported on a support made of titanium oxide and zirconium oxide (JP-A-3-221144), and the like have been proposed. I have. In addition, the present applicant has previously prepared a catalyst in which iridium is supported on at least one carrier selected from metal carbides and metal nitrides (Japanese Patent Application Laid-Open No. 6-31173) and a catalyst in which iridium and an alkaline earth metal are supported. Catalyst (Japanese Unexamined Patent Publication No.
No. 3,188,4) has been found to enhance the selective reduction of NOx by hydrocarbons even in an excess oxygen atmosphere.

On the other hand, in recent years, diesel vehicles tend to combine a direct fuel injection engine with a turbo intercooler, thereby improving fuel efficiency and shifting the exhaust gas temperature to a lower temperature side. Therefore, catalysts have been required to have high activity at low temperatures.
Further, since the exhaust gas temperature becomes 600 ° C. or higher during high-load operation, a catalyst that maintains its activity even after being exposed to such high-temperature exhaust gas has been required.

[0005]

However, the conventional catalysts described above, for example, a catalyst in which copper is supported on a porous carrier such as alumina, silica and zeolite, and a zeolite catalyst ion-exchanged with a noble metal include 650-500. At a high temperature of 700 ° C., there is a problem that irreversible deactivation occurs in a few hours due to the water vapor contained in the exhaust gas, making it impractical. In addition, copper and precious metals are used on a carrier made of titanium oxide and zirconium oxide. The supported catalyst also has a problem in that the hydrocarbon serving as the NOx reducing agent reacts preferentially with the oxygen present in excess due to the strong oxidation catalytic activity of the noble metal, and the NOx reduction reaction cannot be enhanced. Further, a catalyst in which iridium is supported on a carrier made of at least one selected from metal carbides and metal nitrides, or a catalyst in which iridium and an alkaline earth metal are supported, are activated when exposed to high temperatures in exhaust gas having a high oxygen concentration. However, it was not always sufficient.

An object of the present invention is to provide a catalyst which removes HC, CO, and particulate matter in exhaust gas of a diesel engine and has a high activity at a low temperature for NOx removal and maintains a high activity even after endurance at a high temperature. It is in.

[0007]

Means for Solving the Problems The inventors of the present invention provide a refractory inorganic oxide containing at least zeolite carrying platinum, and further containing at least one element of boron, potassium or tin. The present inventors have found that the object can be achieved by a catalyst to which a compound is added, and have completed the present invention. In addition, they have found that by further supporting rhodium on the catalyst, the NOx removal activity after high-temperature durability is further improved.

That is, the present invention relates to a refractory inorganic oxide containing (a) a compound containing at least one element of boron, potassium or tin, (b) platinum, and (c) at least zeolite. The present invention relates to a catalyst for purifying exhaust gas of a diesel engine, which is composed of a product. The present invention also relates to a method for purifying exhaust gas of a diesel engine using the catalyst.

As the compound containing at least one element of boron, potassium or tin, which is the component (a) of the present invention, boron oxide, potassium sulfate, potassium pyrophosphate or tin oxide is preferable. The platinum of the present invention (b) may be platinum alone, but may contain other elements or compounds other than platinum. For example, it may contain a noble metal element other than platinum. More preferably, a component composed of platinum and rhodium containing rhodium is used. The refractory inorganic oxide containing at least zeolite, which is the component (c) of the present invention, includes:
Although zeolite can be used alone, it may contain a refractory inorganic oxide other than zeolite, for example, γ-alumina.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a refractory inorganic oxide containing at least zeolite carrying platinum and further adding a compound containing at least one element of boron, potassium or tin. The compound comprising at least one element of boron, potassium, or tin according to the present invention includes various compounds containing these elements. However, at least one of boron oxide (B ₂ O ₃ ), potassium sulfate (K ₂ SO ₄ ), potassium pyrophosphate (K ₄ P ₂ O ₇ ), and tin oxide (SnO ₂ ), or two or more of them can be used. The use of more than one species is preferred. By containing these compounds, the catalyst of the present invention can maintain high activity even after high-temperature durability.

In the compound of the present invention containing at least one element of boron, potassium or tin, one of these compounds may be used alone, or a mixture of two or more compounds may be used. May be used. The loading amount of these is preferably 0.1 to 40 g, more preferably 0.5 to 20 g, per liter of the completed catalyst. 0.1 g carrying weight
If the amount is less, the activity after high-temperature durability is low, and if it is more than 40 g, the active sites of the zeolite are reduced.

The platinum (Pt) in the catalyst of the present invention is NOx
As well as hydrocarbons (HC), carbon monoxide (C)
O), it is effective to simultaneously decompose and remove particulate matter and the like. That is, the catalyst of the present invention is an effective catalyst for removing HC, CO, particulate matter, and the like, and has a higher activity for removing NOx than a conventional catalyst. The supported amount of platinum may be any amount as long as the required catalyst activity is obtained, but is preferably 0.01 to 5.0 g, more preferably 0.05 to 2.0 g, per liter of the completed catalyst. .

In the present invention, by supporting rhodium (Rh) in addition to platinum, NOx
The removal activity can be further improved. The loading weight of rhodium is preferably 0.1 to 1 liter of finished catalyst.
0.01 to 2.0 g, and more preferably 0.01 to 2.0 g.
1.0 g. When the supported weight is less than 0.01 g, the effect of supporting rhodium cannot be obtained, and when the supported weight is more than 2.0 g, the surface of platinum is easily covered and the catalytic activity is reduced.

As the carrier of the catalyst of the present invention, zeolite can be used alone, or zeolite and a refractory inorganic oxide other than zeolite can be used in combination. Various zeolites can be used as the zeolite of the present invention, but zeolites represented by the following general formula are preferred.

XM ₂ / n · Al ₂ O ₃ · ySiO ₂ (where M represents an n-valent metal, n represents the valence of the metal M, and x and y represent the molar ratio of each component. ), And is crystalline and has a three-dimensional network structure. The pore diameter in the crystal structure varies depending on the values of x and y, and there are many types. Preferred zeolites used in the present invention are not particularly limited,
The SiO ₂ / Al ₂ O ₃ molar ratio is preferably from 20 to 300,
20-100 is more preferable. The BET specific surface area is preferably from 200 to 600 m ² / g, and from 250 to 4 m ² / g.
00 m ² / g is more preferred.

Examples of the refractory inorganic oxides other than the zeolite of the present invention include activated aluminas such as γ-alumina, δ-alumina, η-alumina, θ-alumina, α-alumina, silica, titania, zirconia and the like. A composite oxide or a mixture thereof is mentioned. Among these refractory inorganic oxides, activated alumina, titania and zirconia are preferably used, and activated alumina is more preferably used. BET specific surface area of these refractory inorganic oxide is preferably 5~400m ² / g, 20~250m ² /
g is more preferred.

The amount of zeolite supported is not particularly limited, but is preferably 25 to 300 per liter of the finished catalyst.
g, more preferably 50 to 250 g. The amount of the refractory inorganic oxide other than zeolite to be carried is not particularly limited, but is preferably from 2 to 150 g, more preferably from 5 to 100 g, per liter of the finished catalyst.

The catalyst of the present invention is preferably used by being supported on a support made of a refractory material. As the support made of a refractory material used to support the catalyst of the present invention, a refractory material made of a refractory metal oxide or a refractory metal is preferable, and the shape of the support made of the refractory material may be various. The shape may be, for example, a honeycomb shape, a three-dimensional net shape, or a foam. As these refractory metal oxides, cordierite, mullite, α-alumina, sillimanite, magnesium silicate, zirconia, pentalite, spodumene,
Aluminosilicates and the like can be mentioned. Examples of the refractory metal include a refractory iron-based alloy, a refractory nickel-based alloy, and a refractory chromium-based alloy. Among these supports, a honeycomb-shaped support made of cordierite is particularly preferably used.

Next, a method for producing a catalyst will be described.
The method for producing the catalyst of the present invention is not particularly limited, and a conventionally known method is applied. For example, a compound containing at least one element of boron, potassium, or tin or a starting material thereof, preferably boron oxide, or each starting material of potassium pyrophosphate, potassium sulfate, or
A method of mixing and pulverizing at least one kind of tin oxide, a starting material of platinum, a carrier powder, and pure water with a ball mill, coating the obtained slurry on a support made of a refractory material, drying and firing. Is mentioned. Alternatively, first, an aqueous slurry of a carrier powder is obtained, coated on a support made of a refractory material, dried and calcined, and then an aqueous slurry of a supported component such as a platinum starting material is coated on the support. , Drying and baking, or alternatively, coating the support with a carrier powder and one or more aqueous slurries of boron oxide and potassium pyrophosphate, potassium sulfate and tin oxide, drying and baking, A method of impregnating the support with an aqueous solution of a starting material of platinum, and further, coating the support with a slurry of the carrier powder and the starting material of platinum, and then boron oxide, the respective starting materials of potassium pyrophosphate, potassium sulfate, Various methods are applied, such as coating one or more aqueous slurries of tin oxide.

Boric acid (H ₃ BO ₃ ) is usually used as a starting material for boron oxide. In addition, dipotassium phosphate (K ₂ HPO ₄ ) is usually used as a starting material for potassium pyrophosphate. The starting material of platinum is not particularly limited, and conventionally known materials such as chloroplatinic acid, dinitrodiaminoplatinum, potassium chloroplatinate, sodium chloroplatinate, and amine hydroxide platinate can be used. When rhodium is contained, a starting material of rhodium can be added to any of the above-mentioned slurries in the preparation method, but a preparation method in which the starting material of platinum is added simultaneously with the starting material of platinum is preferable. There are no particular restrictions on the starting material of rhodium, for example, rhodium nitrate,
Rhodium chloride, rhodium acetate and the like can be used.

If necessary, a co-catalyst can be further added to the above-mentioned catalyst components. As a co-catalyst,
Examples include oxides such as cerium, lanthanum, zirconium, neodymium, promethium, and samarium, mixtures of these oxides, and composite oxides of these elements. Further, a binder may be added to the slurry of the present catalyst. Examples of the binder include oxides such as alumina, silica, and titania. Next, the support coated with the catalyst is dried and then calcined. Drying is usually 100 to 15
Perform at 0 ° C. for 10-30 minutes. Firing is 300-60 in air
0 ° C., preferably at 400 to 550 ° C. for 20 minutes to 2 hours,
Preferably, heating is performed for 20 to 40 minutes.

The present invention provides a method for purifying exhaust gas of a diesel engine, which comprises using the catalyst of the present invention. The method for purifying exhaust gas of a diesel engine according to the present invention includes the steps of: installing the catalyst of the present invention in an exhaust gas flow path of a diesel engine so that the exhaust gas contacts the catalyst;
The method comprises decomposing and removing CO and particulate matter, and decomposing and removing NOx using HC and SOF (organic matter soluble in an organic solvent contained in the particulate matter) in the exhaust gas as a reducing agent. In the method for purifying exhaust gas of a diesel engine of the present invention, the catalyst of the present invention can be used alone, but if necessary, the exhaust gas can be purified in combination with a purifying device using another catalyst. . The gas space velocity of the exhaust gas in contact with the catalyst is not particularly limited.
20,000-200,000 / hr is preferable, and 20,
000-150,000 / hr is more preferable. The catalyst inlet temperature of the exhaust gas for the reduction of NOx is preferably 100 to 500C, more preferably 150 to 350C.

HC and S in exhaust gas serving as NOx reducing agents
If sufficient NOx removal cannot be obtained due to the lack of OF, the exhaust gas before contact with the catalyst contains hydrocarbons (H
A reducing agent such as C) can be added. Examples of the reducing agent to be added include light oil, kerosene, and the like, and it is advantageous to use light oil, which is the fuel of a diesel engine, in consideration of the cost of the adding device. In this case, the light oil may be added in a liquid form, but is preferably added in a spray form.

[0024]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

Example 1 (1) Commercially available zeolite powder having a BET specific surface area of 400 m ² / g (molar ratio of SiO ₂ / Al ₂ O ₃ = 50) 10
00 g was placed in a mixer, and while stirring, 300 ml of an aqueous solution of amine hydroxide hydroplatinate containing 10 g of platinum was added dropwise little by little to uniformly disperse. Next, 100 ml of a 20% by weight acetic acid aqueous solution was added dropwise little by little, and the mixture was uniformly dispersed to prepare a zeolite powder containing platinum.

(2) The platinum-containing zeolite powder obtained in (1) is 950 g by dry weight, and boric acid 89
g, 25 ml of 90% by weight aqueous acetic acid solution and 8 deionized water
00g was put in a mill and mixed and pulverized to obtain a catalyst slurry. The crushing time is 90% or more of the particle diameter in the slurry.
The thickness was set to 0 μm or less. The slurry was immersed in a core piece having a diameter of 1 inch and a length of 5 inches penetrated from a commercially available cordierite honeycomb (400 cells / in ² ) and pulled up. Then, excess slurry was removed by an air flow. Next, it was dried at 50 ° C. for 30 minutes, and further baked in air at 500 ° C. for 30 minutes to obtain a catalyst-coated honeycomb (A-
1) was obtained. The catalyst-coated honeycomb (A-1) contained 100 g of zeolite, 5 g of boron oxide, and 1 g of platinum per liter of the completed catalyst.

Example 2 In Example 1 (1), 800 g of the same zeolite powder as in Example 1 and 200 g of a commercially available γ-alumina powder having a BET specific surface area of 150 m ² / g were used instead of 1000 g of the zeolite powder. In the same manner as in (1) and (2) of Example 1 except that
-2) was obtained. The catalyst-coated honeycomb (A-2) contained 80 g of zeolite, 20 g of γ-alumina, 5 g of boron oxide, and 1 g of platinum per liter of the completed catalyst.

Example 3 In Example 2 (2), 909 g of a dry weight of 950 g of a mixed powder of zeolite containing platinum and γ-alumina was used.
Then, a catalyst-coated honeycomb (A-3) was obtained in the same manner as in Example 2, except that 89 g of boric acid was changed to 161 g. This catalyst-coated honeycomb (A-3) is
It contained 80 g of zeolite, 20 g of γ-alumina, 10 g of boron oxide and 1 g of platinum.

Example 4 The procedure of Example 3 was repeated, except that the dry weight of the mixed powder of platinum-containing zeolite and γ-alumina was 950 g, and 971 g of potassium mixed sulfate was used instead of 161 g of boric acid. And catalyst-coated honeycomb (A-4)
I got This catalyst-coated honeycomb (A-4) contained 80 g of zeolite, 20 g of γ-alumina, 3 g of potassium sulfate and 1 g of platinum per liter of the completed catalyst.

Example 5 A catalyst-coated honeycomb (A-5) was obtained in the same manner as in Example 2 (2) except that 53 g of dipotassium phosphate was used instead of 89 g of boric acid. This catalyst-coated honeycomb (A-5) contained 80 g of zeolite, 20 g of γ-alumina, 5 g of potassium pyrophosphate and 1 g of platinum per liter of the finished catalyst.

Example 6 A catalyst-coated honeycomb (A-6) was obtained in the same manner as in Example 2 (2) except that 50 g of tin oxide was used instead of 89 g of boric acid. This catalyst-coated honeycomb (A-6) contained 80 g of zeolite, 20 g of γ-alumina, 5 g of tin oxide and 1 g of platinum per liter of the completed catalyst.

Example 7 In Example 1 (1), 300 ml of an aqueous solution of amine hydroxide platinate containing 10 g of platinum was added dropwise in small amounts, and the mixture was uniformly dispersed. Then, 30 ml of an aqueous solution of rhodium nitrate containing 1 g of metal rhodium was added in small amounts. Each of the catalyst-coated honeycombs (A-A) was dropped in the same manner as in Example 1 except that the catalysts were uniformly dispersed.
7) was obtained. This catalyst-coated honeycomb (A-7) contained 100 g of zeolite, 5 g of boron oxide, 1 g of platinum, and 0.1 g of rhodium per liter of the completed catalyst.

Comparative Example l In Example 1, BET was used instead of the zeolite powder.
A catalyst-coated honeycomb (B-1) was obtained in the same manner as (1) and (2) of Example 1 except that a commercially available γ-alumina powder having a specific surface area of 150 m ² / g was used. This catalyst-coated honeycomb (B-1) had 100 g of γ-alumina, 5 g of boron oxide and 1 g of platinum per liter of the completed catalyst.
g.

COMPARATIVE EXAMPLE 2 In Comparative Example 1, BE instead of γ-alumina powder was used.
A catalyst-coated honeycomb (B) was prepared in the same manner as in Comparative Example 1 except that a commercially available titania powder having a T specific surface area of 80 m ² / g was used.
-2) was obtained. The catalyst-coated honeycomb (B-2) contained 100 g of titania, 5 g of boron oxide, and 1 g of platinum per liter of the completed catalyst.

Comparative Example 3 A catalyst-coated honeycomb (B-3) was obtained in the same manner as in Example 2 ( ₂ ) except that 50 g of cerium oxide (CeO ₂ ) was used instead of 89 g of boric acid. Was. This catalyst-coated honeycomb (B-3) contained 80 g of zeolite, 20 g of γ-alumina, 5 g of cerium oxide and 1 g of platinum per liter of the completed catalyst.

Performance Evaluation Example 1 (Initial Activity) Using the catalyst-coated honeycombs of Examples 1 to 7 and Comparative Examples 1 to 3, NOx, particulate matter (PM), SOF, HC, CO It was evaluated purification of SO _2, the initial activity of the conversion. While the exhaust gas of the diesel engine was flowed at a gas space velocity of 100,000 / hr, the catalyst-coated honeycomb inlet temperature was maintained at 250 ° C., and the concentrations of the above components were measured at the catalyst-coated honeycomb inlet and outlet sides. Purification and conversion were determined. Table 1 shows the results.

[0037]

[Table 1]

Performance Evaluation Example 2 (Activity after Endurance) With respect to the catalyst-coated honeycombs of Examples 1 to 7 and Comparative Examples 1 to 3, while maintaining the center temperature of the catalyst-coated honeycomb at 700 ° C., water vapor and SOx were reduced. A 24-hour endurance test was performed while flowing the diesel engine exhaust gas at a gas space velocity of 100,000 / hr. For the catalyst-coated honeycomb after the durability test, the respective purification rates and conversion rates were determined under the same conditions as in Performance Evaluation Example 1. Table 2 shows the results.

[0039]

[Table 2]

From Tables 1 and 2, the catalyst of the present invention (A-
Nos. 1 to A-7) have a high activity for removing NOx at a low temperature, show a small reduction in the purification rate even after endurance at a high temperature, and maintain a high activity. Further, the catalyst of the present invention further containing rhodium (A-7)
It can be seen that the decrease in the purification rate after high-temperature durability was further reduced. Furthermore, particulate matter, SOF, HC, C
It was shown that it also has excellent activity against O and SO ₂ .

[0041]

Industrial Applicability The catalyst of the present invention can effectively remove particulate matter, hydrocarbons and carbon monoxide in diesel engine exhaust gas, has high activity at low temperatures for NOx removal, and has high activity even after high-temperature durability. It has an excellent effect of maintaining high activity.

Claims (9)

[Claims] (1) at least one of the compounds containing at least one element of boron, potassium or tin;
A catalyst for purifying exhaust gas of a diesel engine, comprising a seed, (b) platinum, and (c) a refractory inorganic oxide containing at least zeolite. (A) at least one of the compounds containing at least one element of boron, potassium or tin is at least one of boron oxide, potassium sulfate, potassium pyrophosphate or tin oxide; The catalyst for purifying exhaust gas of a diesel engine according to claim 1. 3. The diesel engine exhaust gas purifying catalyst according to claim 1, wherein (b) the platinum further contains rhodium in addition to platinum. 4. The method according to claim 1, wherein (c) the refractory inorganic oxide containing at least zeolite further contains at least one of activated alumina, titania and / or zirconia in addition to zeolite. The diesel engine exhaust gas purifying catalyst according to any one of claims 1 to 3. 5. The catalyst for purifying exhaust gas of a diesel engine according to claim 4, wherein the refractory inorganic oxide is zeolite and activated alumina. 6. The weight of at least one compound containing at least one element of the component (a) boron, potassium or tin is 0.1 to 1 per liter of the finished catalyst.
The catalyst for purifying exhaust gas of a diesel engine according to any one of claims 1 to 5, which weighs 40 g. 7. The weight of the platinum of the component (b) is the finished catalyst 1
7. The method according to claim 1, wherein the weight of the rhodium is 0.01 to 2.0 g per liter of the completed catalyst when the rhodium further contains rhodium. The catalyst for purifying exhaust gas of a diesel engine according to the above. 8. The support according to claim 1, wherein the support is made of a refractory material.
8. A catalyst for purifying exhaust gas of a diesel engine, which carries the catalyst for purifying exhaust gas of a diesel engine according to any one of claims 1 to 7. 9. A method for purifying exhaust gas of a diesel engine, comprising using the catalyst for purifying exhaust gas of a diesel engine according to claim 1. Description: