JPH1076163A - Catalyst for purification of exhaust gas and method for purifying exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a catalyst for purification of exhaust gas having a improved durability as compared with a conventional zeolite catalyst and having a superior NOx removing performance in a lean atmosphere even in an early stage and after use at a high temp. over a long period of time and to provide a method for purifying exhaust gas. SOLUTION: This catalyst for purification of exhaust gas contains platinum and silicon carbide and/or silicon nitride ceramics as catalytic components. The silicon carbide and/or silicon nitride contains Y, La, Nd, Sm, etc. Exhaust gas contg. NOx and a larger amt. of oxygen than the stoichiometric amt. required to perfectly oxidize reducible components including hydrocarbons is brought into contact with this catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst and an exhaust gas purifying method, and more particularly, to an atmosphere of an excess oxygen exhaust gas discharged from an internal combustion engine such as a diesel engine or a lean burn engine. Lean atmosphere)
TECHNICAL FIELD The present invention relates to an exhaust gas purifying catalyst and an exhaust gas purifying method which can efficiently purify nitrogen oxides (NOx) in the exhaust gas and have excellent NOx purifying performance even after high-temperature durability.

[0002]

2. Description of the Related Art Conventionally, as a catalyst for purifying exhaust gas discharged from an internal combustion engine of an automobile or the like, noble metals such as platinum (Pt), palladium (Pd) and rhodium (Rh) are supported on activated alumina or cerium oxide. A monolithic carrier coated with a monolithic carrier is widely used. Since this catalyst mainly focuses on improving the exhaust gas purification performance near the stoichiometric air, there is a problem that the NOx purification action in a lean atmosphere becomes insufficient.

On the other hand, NOx in a lean atmosphere
Various catalysts for improving the purification performance and various NOx purification methods have been proposed. For example, catalysts using copper-zeolite are disclosed in JP-A-1-131345 and JP-A-4-404.
No. 5, gazette. These exhaust gas purifying catalysts mainly composed of copper-zeolite can not only reduce and remove NOx under a lean atmosphere, but also efficiently purify NOx, carbon monoxide (CO) and hydrocarbons (HC). be able to.

On the other hand, Japanese Patent Application Laid-Open No. Hei 2-233146 discloses that by using silicon nitride (Si ₃ N ₄ ) or silicon carbide (SiC) as a binder together with an organometallic compound of silicon, rearrangement of zeolite particles and A catalyst for purifying exhaust gas, which suppresses densification and is highly active even at high temperatures, is disclosed.

[0005]

However, in the above-mentioned catalyst containing copper-zeolite as a main component, of the harmful components (HC, CO, NOx) in the exhaust gas, the catalytic purification ability of NOx, in particular, is low. , Because of high dependence on exhaust gas composition (HC species and HC concentration) and temperature, the HC concentration (HC / N
Unless the temperature is higher than 350 ° C., the purification of NOx is insufficient. Further, such a copper-zeolite catalyst can be used at a high temperature of 600 ° C. or more containing water (steam) to change from an oxygen-excess atmosphere (lean atmosphere) to an oxygen-deficient atmosphere (rich atmosphere)
When exposed to exhaust gas that fluctuates for a long time, copper, which is an active component supported in the zeolite in an ionic state, escapes from the support site in the zeolite and moves and sinters, so that the purification performance increases over time. There has been a problem that the temperature has deteriorated and cannot withstand long-term use. Therefore, improvement of catalytic activity (NOx conversion activity) from a low temperature range of 300 ° C. or lower, NOx purification efficiency at low HC concentration, and high-temperature durability have been major issues.

The catalyst disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2-233146 is intended to suppress generation of cracks and densification of zeolite particles by adding ceramics such as Si ₃ N ₄ or SiC to the catalyst. However, no stabilization and improvement of durability of the noble metal itself, which is a catalytically active species, have not been achieved, and there has been a problem that the NOx purification performance under a lean atmosphere is not sufficient. The present invention has been made in view of such problems of the prior art, and an object thereof is to improve the durability over the conventional zeolite-based catalyst, and to provide a lean catalyst even at the initial stage and after high-temperature durability. An object of the present invention is to provide an exhaust gas purifying catalyst and an exhaust gas purifying method having excellent NOx purifying performance under an atmosphere.

[0007]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above problems, and as a result, have found that the above problems can be solved by using a ceramic component containing a specific rare earth element. The present invention has been completed.

That is, the exhaust gas purifying catalyst according to the first aspect of the present invention is a monolithic catalyst having a catalyst component-supporting layer and capable of effectively purifying nitrogen oxides in a lean atmosphere. It contains platinum and a ceramic component composed of silicon carbide and / or silicon nitride, wherein the ceramic component is yttrium, lanthanum, neodymium,
And at least one rare earth element selected from the group consisting of samarium and samarium. Further, the exhaust gas purifying catalyst according to claim 2 improves the stability of the noble metal particles after the durability of the catalyst according to claim 1 and sufficiently expresses the catalytic performance, and the rare earth element with respect to the ceramic component is improved. The content of the element is 1 to 10 mol% in terms of metal. Furthermore, the exhaust gas purifying catalyst according to claim 3 is a catalyst in which the initial and post-durability performance of the exhaust gas purifying catalyst according to claim 1 or 2 is effectively developed, and peeling of the catalyst coat layer is prevented. , Activated alumina,
At least one member selected from the group consisting of boehmite alumina and alumina sol is contained in an amount of 0.1 to 20% by weight in terms of alumina based on the total weight of the catalyst component supporting layer.

Further, the exhaust gas purifying method of the present invention comprises:
In purifying an exhaust gas containing a hydrocarbon-containing reducing component, oxygen in excess of a stoichiometric amount necessary and sufficient to completely oxidize the reducing component, and nitrogen oxides, claim 1. 3. The exhaust gas purifying catalyst according to any one of the items 3) is brought into contact with the exhaust gas.

[0010]

The exhaust gas purifying catalyst of the present invention contains a ceramic component composed of silicon carbide and / or silicon nitride having good heat resistance. Therefore, even after high-temperature durability, it is possible to suppress grain growth of the supported noble metal particles, so that performance degradation after durability can be reduced, and as a result, stable purification performance can be maintained for a long period of time.

The oxidation resistance of the catalyst can be improved by the rare earth element such as yttrium contained in the ceramic component, and the stability of the supported noble metal particles can be maintained even at a high temperature. Further, since the noble metal and the basic element are present in the vicinity, the reducing components HC and C
It is also possible to improve the reaction selectivity between O and the like and NOx. This effect is achieved by setting the content of the rare earth element in the ceramic component to 1 to 10 mol%.
It will be noticeable.

The catalyst of the present invention can contain activated alumina, boehmite alumina, etc., and the content thereof is 0.1% in terms of alumina per total weight of the catalyst component supporting layer (coat layer). By controlling the content to 重量 20% by weight, in particular, coating properties and adhesion to the carrier can be improved, and stable purification performance can be realized over a long period of time.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the exhaust gas purifying catalyst of the present invention will be described in detail. As described above, the catalyst of the present invention contains platinum and a ceramic component containing a rare earth element as catalyst components. Here, the content of platinum, which is a noble metal component contained in the present catalyst, is preferably 0.1 to 15 g per liter of the catalyst. If the amount is less than 0.1 g, the low-temperature activity and purification performance are not sufficiently exhibited, and 15 g
The addition of more than this is not preferable because the catalytic activity is saturated and is not economically effective.

In addition, the ceramic component is a heat-resistant material that sufficiently exerts the catalytic activity of the noble metal, which is an active species, suppresses the growth of the substrate and the noble metal after high-temperature durability, and maintains the dispersibility. The catalyst preferably has excellent properties. In the present catalyst, silicon carbide or silicon nitride and a mixture thereof are used. Further, the rare earth element serves to improve the oxidation resistance of the ceramic component and maintain the stability of the noble metal particles after high-temperature durability. Use a mixture of The content of the rare earth element in the ceramic component is 1 to 10 in terms of metal.
If it is less than 1 mol%, the low-temperature activity and purification performance after durability are not sufficiently exhibited, and if it exceeds 10 mol%, the purification performance may be saturated or deteriorated. Absent.

In addition, the present catalyst may contain activated alumina, boehmite alumina or alumina sol and any mixture thereof in addition to the above-mentioned catalyst components. Coating properties and adhesion can be improved. In addition,
The content of these aluminas is preferably 0.1 to 20% by weight based on the total weight of the coat layer in terms of alumina. 0.
If the amount is less than 1% by weight, the coating property may be deteriorated. If the amount is more than 20% by weight, the effect of improving the coating property and the adhesion to the carrier is not preferable.

Hereinafter, a method for producing the present catalyst will be described. First, as a method of supporting platinum on the above-mentioned ceramic component such as Si ₃ N ₄ , for example, an appropriate method can be selected from known methods such as an impregnation method and a kneading method. Here, as a raw material compound of platinum, any compound can be used as long as it is water-soluble, such as dinitrodiamine salt, chloride and nitrate.

The removal of water can be appropriately selected from known methods such as a filtration method and an evaporation to dryness method.
The first heat treatment for obtaining the platinum-supported Si ₃ N ₄ or the like used in the present invention is not particularly limited. In order to support the added platinum with good dispersibility, for example, in air and / or air It is preferable to perform calcination at a relatively low temperature of 400 to 800 ° C. under circulation.

Next, a rare earth element such as yttrium is
The method of supporting on i ₃ N ₄ or the like can be appropriately selected from known methods such as an impregnation method and a kneading method. As the raw material compound of the rare earth element, any compound can be used as long as it is water-soluble such as carbonate, nitrate and acetate.

The removal of water can be appropriately selected from known methods such as a filtration method and an evaporation to dryness method.
Further, the first heat treatment for obtaining a rare earth element-supported ceramic such as yttrium-supported Si ₃ N ₄ used in the present invention is not particularly limited, but in order to support the added element with good dispersibility and stabilize the surface. For example, it is preferable to perform calcination at a temperature of 600 to 1300 ° C. in air and / or in the flow of air.

Note that ceramic components _such as Si 3 _N 4
It can be added simply without carrying the noble metal.
The order in which platinum and a rare earth element are supported on these ceramic components is not particularly limited. For example, with platinum
The co-impregnation method of simultaneously impregnating the raw material salt of the rare earth element, or after first supporting platinum on the rare earth element, impregnating the rare earth element raw salt, or impregnating and supporting the rare earth element raw salt first, A sequential impregnation method such as supporting platinum is used.

The exhaust gas purifying catalyst of the present invention can be effectively used without a carrier. However, the above-mentioned various components obtained by pulverizing the above-mentioned components into a slurry are coated on a catalyst carrier to obtain a catalyst. It is preferable to use by firing at ~ 900 ° C. As the catalyst carrier, for example, a known catalyst carrier such as a monolith carrier or a metal carrier made of a refractory material can be appropriately selected and used.

The shape of the catalyst support is not particularly limited, but it is usually preferable to use a honeycomb shape. As the honeycomb material, cordierite-based materials such as ceramics are generally used in many cases. It is also possible to use a honeycomb material made of a metal material such as. Further, the catalyst component powder itself may be formed into a honeycomb shape. By making the shape of the catalyst into a honeycomb shape, the contact area between the catalyst and the exhaust gas is increased and the pressure loss can be suppressed, so that it is extremely effective when used as an exhaust gas purifying catalyst for automobiles.

Accordingly, the catalyst of the present invention is typically prepared by mixing the rare earth element-supported Si ₃ N ₄ and / or SiC, platinum, and, if necessary, alumina powder or alumina sol, and wet-mixing the mixture. Then, the slurry is attached to the catalyst carrier and calcined at a temperature of 400 to 650 ° C. in the air and / or in the flow of air.

In this case, the amount of the catalyst component coat layer to be attached to the catalyst carrier is preferably 50 to 400 g per liter of the catalyst in total of the entire catalyst components. The larger the number of catalyst component coat layers, the better in terms of catalytic activity and catalyst life.However, if the coat layer is too thick, the reaction gas will be poorly diffused inside the coat layer and will not be able to come into sufficient contact with the catalyst. The effect may be saturated, and the gas passage resistance may increase. For this reason,
The amount of the coating layer is 50 to 400 g per liter of the catalyst.
Is preferred.

[0025]

The present invention will be described below in more detail with reference to examples and comparative examples. (Example 1) An SiC powder was impregnated with an yttrium nitrate aqueous solution or sprayed while stirring at a high speed, dried at 150 ° C for 24 hours, calcined at 400 ° C for 1 hour, and then at 600 ° C for 1 hour to obtain a yttrium-added SiC powder. (Powder A) was obtained.
The Y content of this powder A was 6.9% by weight (3% by mole). The obtained yttrium-added SiC powder A is impregnated with an aqueous solution of dinitrodiamineplatinum or sprayed with high-speed stirring, dried at 150 ° C. for 24 hours, baked at 400 ° C. for 1 hour, and subjected to Pt-supported Y-SiC powder (powder B). ) Got. The Pt concentration of this powder B was 0.78% by weight.

203 g of the powder B, 32 g of boehmite alumina (containing 22.4 g as alumina), and 265 g of an aqueous nitric acid solution were charged into a magnetic ball mill, mixed and pulverized to obtain a slurry. This slurry was attached to a cordierite-based monolithic carrier (120 cc, 400 cells / square inch), and excess slurry in the cells was removed with an air stream. The slurry attached to the carrier was dried, and then dried at 400 ° C. for 1 hour. Fired. This operation is performed twice, and the coat weight 200
g / L-supported catalyst was obtained. Pt loading is 40 g / c
f (1.41 g / L).

Example 2 Instead of SiC powder, Si ₃ N ₄
The same operation as in Example 1 was repeated, except that was used, to obtain an exhaust gas purifying catalyst of this example.

Example 3 The same operation as in Example 1 was repeated, except that La was added instead of Y, to obtain an exhaust gas purifying catalyst of this example.

Example 4 The same operation as in Example 1 was repeated, except that Nd was added instead of Y, to obtain an exhaust gas purifying catalyst of this example.

Example 5 The same operation as in Example 1 was repeated, except that Sm was added instead of Y, to obtain an exhaust gas purifying catalyst of this example.

Example 6 The amount of Y added to SiC powder was 5
The same operation as in Example 1 was repeated, except that the mol% was set, to obtain an exhaust gas purifying catalyst of this example.

Example 7 The same operation as in Example 4 was repeated, except that the amount of Nd added to the SiC powder was 10 mol%, to obtain an exhaust gas purifying catalyst of this example.

Example 8 The same operation as in Example 1 was repeated except that activated alumina was used in place of boehmite alumina so as to be 10% by weight of the total coat layer amount, and the exhaust gas purifying method of this example was repeated. A catalyst was obtained.

Example 9 Alumina sol was used instead of boehmite alumina, and the amount of alumina was 1% of the total coat layer amount.
The same operation as in Example 1 was repeated except that the amount was 0% by weight, to obtain an exhaust gas purifying catalyst of this example.

(Comparative Example 1) A Pt-supported Y-SiC powder B was not used, but 203 g of a Pt-supported alumina powder (Pt concentration 0.78% by weight) was used, and no SiC was used in the catalyst slurry. The same operation as in Example 1 was repeated to obtain an exhaust gas purifying catalyst of this example.

Comparative Example 2 The same operation as in Example 1 was repeated except that Y was not added to the SiC powder to obtain an exhaust gas purifying catalyst of this example.

Comparative Example 3 The same operation as in Example 2 was repeated except that Y was not added to the Si ₃ N ₄ powder to obtain an exhaust gas purifying catalyst of this example.

Comparative Example 4 The same operation as in Example 1 was repeated, except that the amount of Y added to SiC was 15 mol%, to obtain an exhaust gas purifying catalyst of this example.

Table 1 shows the amounts of noble metals, base materials and alumina in the exhaust gas purifying catalysts obtained in Examples 1 to 9 and Comparative Examples 1 to 4.

[0040]

[Table 1]

With respect to the exhaust gas purifying catalysts of Examples 1 to 9 and Comparative Examples 1 to 4, the catalytic activity was evaluated under the following conditions. For the activity evaluation, an automatic evaluation device using a model gas simulating the exhaust gas of an automobile was used. (Durability conditions) Engine displacement 4400cc Fuel unleaded gasoline Catalyst inlet gas temperature 750 ° C Durability time 30 hours Inlet gas composition CO 0.5 ± 0.1% O ₂ 0.5 ± 0.1% HC About 1100ppm NO 1300ppm CO ₂ 15%

(Evaluation conditions: Purification performance) Total gas flow rate 40 L / min Catalyst inlet temperature 350 ° C. Holding time 30 minutes Space velocity About 20,000 / hour (catalyst capacity 120 cc) Inlet gas composition Average air-fuel ratio equivalent to 18.0 Model gas composition CO 0.2% C ₃ H ₆ 5000 ppm C NO 500 ppm O ₂ 6.0% CO ₂ 10.0% H ₂ O 10.0% N ₂ balance A / F amplitude None

The purification performance of each exhaust gas purification catalyst at the initial stage and after the endurance is determined by the following equation (1). The activity was evaluated under the above-mentioned evaluation conditions, and expressed as an average conversion (%) of NOx. The results are shown in Table 2.

[0044]

(Equation 1)

[0045]

[Table 2]

From Table 2, it is clear that the example has a higher catalytic activity than the comparative example, and the effect of the present invention could be confirmed.

[0047]

As described above, according to the present invention, since the ceramic component containing a specific rare earth element is used, the durability is improved as compared with the conventional zeolite-based catalyst, and the initial and high-temperature durability are improved. Even later, it is possible to provide an exhaust gas purifying catalyst and an exhaust gas purifying method having excellent NOx purifying performance under a lean atmosphere.

Claims (4)

[Claims] An integrated catalyst having a catalyst component-supporting layer and capable of effectively purifying nitrogen oxides in a lean atmosphere, wherein platinum and a ceramic component comprising silicon carbide and / or silicon nitride are used as catalyst components. Wherein the ceramic component contains at least one rare earth element selected from the group consisting of yttrium, lanthanum, neodymium, and samarium. 2. The exhaust gas purifying catalyst according to claim 1, wherein the content of the rare earth element relative to the ceramic component is 1 to 10 mol% in terms of metal. 3. The catalyst component-containing layer further contains at least one member selected from the group consisting of activated alumina, boehmite alumina and alumina sol in an amount of 0.1 to 20% by weight in terms of alumina based on the total weight of the catalyst component-supporting layer. The exhaust gas purifying catalyst according to claim 1 or 2, wherein: 4. A method for purifying an exhaust gas containing a reducing component containing hydrocarbons, oxygen in excess of a stoichiometric amount necessary and sufficient to completely oxidize the reducing component, and nitrogen oxides. An exhaust gas purification method comprising: contacting the exhaust gas purification catalyst according to any one of claims 1 to 3 with the exhaust gas.