JPH08173811A - Catalyst for purification of exhaust gas and its production - Google Patents

Abstract

PURPOSE: To obtain a catalyst capable of enhancing NOx removing performance in an excess oxygen-contg. atmosphere in which the conventional catalyst has no activity and capable of removing NOx in exhaust gas over the wide temp. region from a low temp. to a high temp. CONSTITUTION: This catalyst is made of a silver carried alumina-based multiple oxide obtd. by carrying silver on an alumina-based multiple oxide and represented by the formula, Aga /Xb Alc Od (where X is at least one kind of element selected from among Fe, Co, Ni, Cu and Zn; (b) and (c) show the atomic ratio between the element; (b) is 0.005-0.5 when (c) is 2.0; (a) is 0.1-10wt.% (expressed in terms of elemental silver); and (d) is the number of oxygen atoms required to satisfy the valence of each component).

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 more particularly to a nitrogen compound (hereinafter,
The present invention relates to an exhaust gas purifying catalyst capable of purifying "NO _x ").

[0002]

2. Description of the Related Art Conventionally, as an exhaust gas purifying catalyst for purifying exhaust gas emitted from an internal combustion engine of an automobile or the like, platinum (Pt), palladium (Pd), rhodium (Rh), etc. are added to alumina, cerium oxide, etc. Carrying noble metal of
A monolith carrier coated with this is used. Since this catalyst mainly focuses on improving the exhaust gas purifying ability in stoichiometry, sufficient performance for removing NO _x could not be obtained in an oxygen excess atmosphere.

On the other hand, many catalysts have been proposed which improve the purification performance of NO _x in an oxygen excess atmosphere. In particular, JP-A-4-298235 and JP-A-5-103
953 and JP-A-5-212248 disclose NO _x purification catalysts using an alumina-based composite oxide, and JP-A-6-71139 and JP-A-6-71175.
In the publication, a NO _x purification catalyst using silver-supported alumina is proposed. The catalyst for NO _x purification using the alumina-based composite oxide and the silver-supported alumina described in these publications uses a porous inorganic oxide (alumina) as a catalyst component.
It is dispersed / supported by impregnation or immersion in N to improve the catalytic performance, and N in exhaust gas in an oxygen excess atmosphere
_Ox is reduced and removed.

[0004]

However, the above conventional alumina-based composite oxide and silver-supported alumina catalyst are
Since the active ingredient is later loaded on alumina by the impregnation method or the dipping method, or only the active ingredient is loaded later by the precipitation method, the specific surface area of the catalyst is reduced, and the active phase is unevenly distributed on the alumina carrier surface. However, the NO _x purification performance was not sufficient under the condition of a large exhaust gas flow rate.

Also, harmful components (HC, C
O, NO _x ), especially NO _x catalyst purification performance is strongly influenced by exhaust gas composition (HC species), temperature and moisture contained in the exhaust gas. _x Purification performance does not appear effectively. Therefore, it has been a major problem to improve the catalytic activity so as to have the NO _x purification performance in a wide temperature range from low temperature to high temperature.

Therefore, the object of the present invention is to improve the NO _x purification performance in an oxygen-rich atmosphere, which was not active in conventional catalysts, and to reduce NO in exhaust gas over a wide temperature range from low temperature to high temperature. An object is to provide an exhaust gas purifying catalyst that can purify _x .

[0007]

Means and Action for Solving the Problems The present inventors have
As a result of research for solving the above problems, at least one transition metal element selected from the group consisting of iron, cobalt, nickel, copper and zinc is used as a catalytically active component at a specific composition ratio and uniformly has an alumina crystal structure. The inventors have found that the catalyst in which silver is supported on the alumina-based composite oxide incorporated therein has sufficient NO _x purification performance in a low temperature range to a high temperature range even in an oxygen excess atmosphere, and has reached the present invention.

The exhaust gas purifying catalyst according to the present invention has the following general formula: Ag _a / X _b Al _c O _d (1) (wherein X is a group consisting of iron, cobalt, nickel, copper and zinc). At least one element selected, b and c represent atomic ratios of each element, b = 0.005 to 0.5 when c = 2.0, and a represents weight% converted to silver element, a = 0.1 to 10% by weight, and d is the number of oxygen atoms required to satisfy the valences of the above components)
The silver-loaded alumina-based composite oxide catalyst represented by

The silver-supported alumina-based composite oxide catalyst according to the present invention represented by the above general formula (1) is composed of alumina (Al
₂ O ₃ ), at least one transition metal element selected from the group consisting of iron, cobalt, nickel, copper and zinc is added at a specific composition ratio to allow silver to be supported on an alumina-based composite oxide, HC compared to silver-supported alumina catalyst
And the catalytic activity for NO _x can be significantly improved.

The optimum composition ratio of the transition metal element to be added to alumina is such that c = 2.0 in the general formula (1) but b =
It is in the range of 0.005 to 0.5. b = 0.0 for c = 2.0
When it is less than 05, the catalytic activity for both HC and NO _x decreases, and when b exceeds 0.5, the NO _x purification performance deteriorates.

It is considered that these transition metal elements enter the alumina crystal structure to form a spinel structure, and the active phase of the metal aluminate is uniformly dispersed on the catalyst surface and bulk.

In the above-mentioned alumina-based composite oxide in which the composition of the transition metal element is limited, silver is converted into an element so that a in the general formula (1) is in the range of 0.1 to 10% by weight. Are carried. Such supported silver is uniformly dispersed on a metal aluminate, to significantly improve the catalyst activity for HC and NO _x. In the above general formula (1), a indicating the amount of silver supported is 0.
If it is less than 1, the effect of adding silver is not exhibited, and conversely, a is 10
When it exceeds, the silver on the surface of the carrier aggregates and the NO _x conversion performance tends to deteriorate.

Regarding the number of oxygen atoms, it is necessary to identify the valences of all elements, but in a multi-component system, the valences of the elements differ depending on the structure and coordination state of the oxide formed, so it is not possible to specify it. Very difficult.

As the raw material compound for preparing the catalyst used in the present invention, nitrates, carbonates, ammonium salts, acetates, halides, oxides and the like of each element can be used in any combination, but especially water-soluble. it is preferred from the viewpoint of improving the catalytic performance for HC and NO _x to use sexual salt.

The method for preparing the catalyst according to the present invention is not limited to a special method, and unless significant uneven distribution of components is involved,
It can be appropriately selected and used from various methods such as known evaporation-drying method, precipitation method, impregnation method, and the like, and in particular, a group consisting of aqueous ammonia, ammonium carbonate, ammonium hydrogen carbonate, ammonium sulfate and ammonium hydrogen sulfate. It is preferable to use a precipitation method in which an aqueous solution of at least one compound selected from the above is added as a precipitant, because the surface area of the catalyst can be sufficiently secured and the supported metal can be uniformly dispersed.

In producing the exhaust gas purifying catalyst according to the present invention, a catalyst raw material containing at least one component selected from the group consisting of iron, cobalt, nickel, copper and zinc and an aluminum component is added to pure water. And stir. At this time, a liquid in which each catalyst raw material is dissolved simultaneously or separately may be added. Then, at least one aqueous solution selected from the group consisting of aqueous ammonia, ammonium carbonate, ammonium hydrogencarbonate, ammonium sulfate and ammonium hydrogensulfate is gradually added to the mixed solution containing the catalyst raw material to adjust the pH of the solution to 7.0 to 9.0. After adjusting the content to be within the range, water is removed, the residue is heat-treated to obtain an alumina-based composite oxide, and silver is added to this to further heat-treat to obtain the catalyst of the present invention.

The exhaust gas purifying catalyst according to the present invention has a fine pore structure and a large specific surface area of the alumina-based composite oxide obtained by the precipitation method, and a uniformly dispersed state of the active phase of the metal aluminate at a low temperature. Plays an important role in the expression of catalytic activity. On the other hand, an alumina-based composite oxide obtained by supporting a transition metal component on alumina by using, for example, an impregnation method without using the above-mentioned precipitation method is more excellent than an alumina-based composite oxide obtained by the precipitation method. The surface area effective for the reaction becomes small due to lack of fine pore structure, and the active phase is ubiquitous on the surface of the carrier and the interaction with silver is not fully exerted, so the catalytic activity and NO _x purification performance after endurance Is reduced.

If the above-mentioned ammonia water or ammonium compound is used as the precipitant used in the above-mentioned precipitation method, the metal element does not remain even if the washing is insufficient, and the ammonium compound (mainly ammonium nitrate etc. after dropping) remains. However, it can be easily decomposed and removed by subsequent firing. On the other hand, when a metal salt such as sodium hydroxide or sodium carbonate is used, metal elements such as sodium remain in the obtained precipitate, and these residual elements adversely affect the catalytic performance, so these are removed. Therefore, a cleaning process is required.

In carrying out the above-mentioned precipitation method, the pH of the solution is adjusted within the range of 7.9 to 9.0, whereby precipitates of various metal salts can be formed. If the pH is lower than 7.0, various elements will not form a precipitate sufficiently, and conversely the pH will be low.
If it is higher than 9.0, some of the precipitated components may be redissolved.

Water can be removed by appropriately selecting from known methods such as filtration and evaporation to dryness.
The first heat treatment for obtaining the alumina-based composite oxide used in the present invention is not particularly limited, but is, for example, 500 to 1000 ° C.
It is preferable to carry out at a temperature in the range of and in the air and / or under air circulation.

The method of adding silver to the alumina-based composite oxide can be appropriately selected from known methods such as an impregnation method and a kneading method, but the impregnation method is particularly preferable. . The heat treatment after the addition of silver is not particularly limited, but it is preferably performed in the air and / or under the air flow at a temperature in the range of 400 to 800 ° C, for example.

The thus obtained exhaust gas purifying catalyst according to the present invention can be effectively used even without a carrier, but it is pulverized and slurried and coated on a catalyst carrier,
It is preferable to use by firing at 400 to 900 ° C. The catalyst carrier can be appropriately selected and used from known catalyst carriers, and examples thereof include a monolith carrier and a metal carrier.

The shape of the catalyst carrier is not particularly limited, but it is usually preferable to use it in a honeycomb shape, and the catalyst powder is applied to various honeycomb-shaped base materials for use.
As the honeycomb material, a cordierite material is generally used, but a honeycomb material made of a metal material can be used, and the catalyst powder itself may be formed into a honeycomb shape. By making the shape of the catalyst honeycomb, the contact area between the catalyst and the exhaust gas becomes large and the pressure loss can be suppressed, which is extremely effective when used as a catalyst for purifying exhaust gas for automobiles.

[0024]

The present invention will be described below with reference to the following examples and comparative examples, but the present invention is not limited thereto. In Examples and Comparative Examples, unless otherwise specified, parts are parts by weight and% are% by weight.

Example 1 0.3 part of copper nitrate and 50 parts of aluminum nitrate were added to 400 parts of pure water and stirred and dissolved. Next, while stirring this solution, 5% ammonia water was gradually added dropwise to adjust the pH of the solution.
Was adjusted to be between 7.0 and 9.0. The precipitate formed is filtered off, dried at 150 ° C for 12 hours and then
It was calcined in air at 0 ° C. for 2 hours. To 50 parts of the alumina-based composite oxide powder thus obtained, 1.6 parts of silver nitrate was added with pure water.
A solution dissolved in 50 parts was added, dried at 150 ° C. for 12 hours, and calcined in air at 600 ° C. for 2 hours to obtain a silver-supported alumina-based composite oxide. After mixing 500 parts of the above silver-supported alumina-based composite oxide powder and 1000 parts of pure water with a ball mill, pulverizing and adhering the resulting slurry to a monolith carrier substrate, firing at 400 ° C for 1 hour, and exhaust gas A purification catalyst was obtained. At this time, the adhesion amount of silver-supported alumina-based composite oxide
It was set to 200 g / L. The composition of components other than oxygen of the obtained catalyst was Ag 2% / Cu _0.02 Al _2.0 .

Example 2 Example 2 was repeated except that 5% aqueous ammonium carbonate solution was used instead of 5% aqueous ammonia. The composition of components other than oxygen of the obtained catalyst was Ag2% / Cu.
It was _0.02 Al _2.0 .

Example 3 Example 3 was repeated except that 5% aqueous ammonium hydrogen carbonate solution was used instead of 5% aqueous ammonia.
The composition of components other than oxygen of the obtained catalyst was Ag2% / C.
It was u _0.02 Al _2.0 .

Example 4 Example 4 was repeated except that a 5% aqueous ammonium sulfate solution was used instead of the 5% aqueous ammonia. The composition of components other than oxygen of the obtained catalyst was Ag 2% / Cu.
It was _0.02 Al _2.0 .

Example 5 Example 5 was repeated except that a 5% ammonium hydrogensulfate aqueous solution was used instead of the 5% ammonia water.
The composition of components other than oxygen of the obtained catalyst was Ag2% / C.
It was u _0.02 Al _2.0 .

Example 6 Example 6 was repeated except that 4.1 parts of silver nitrate was used. The composition of the components other than oxygen of the obtained catalyst was Ag 5%.
/ Cu _0.02 Al _2.0 .

Example 7 Example 1 was repeated except that 1.9 parts of nickel nitrate was used in place of 0.3 part of copper nitrate. The composition of components other than oxygen of the obtained catalyst was Ag 2% / Ni _0.1 Al _2.0 .

Example 8 Example 8 was repeated except that 5.9 parts of zinc nitrate was used instead of 0.3 part of copper nitrate. The composition of components other than oxygen of the obtained catalyst was Ag 2% / Zn _0.3 Al _2.0 .

Example 9 Example 9 was repeated except that 1.0 part of cobalt nitrate was used instead of 0.3 part of copper nitrate. The composition of components other than oxygen of the obtained catalyst was Ag 2% / Co _0.05 Al _2.0 .

Example 10 Except that 1.4 parts of iron nitrate was used instead of 0.3 part of copper nitrate,
It carried out like Example 1. The composition of components other than oxygen of the obtained catalyst was Ag 2% / Fe _0.05 Al _2.0 .

Example 11 Example 11 was repeated except that 0.2 part of nickel nitrate and 0.2 part of cobalt nitrate were used in place of 0.3 part of copper nitrate. The composition of components other than oxygen of the obtained catalyst was Ag2.
% / Ni _0.01 Co _0.01 Al _2.0 .

Example 12 The procedure of Example 1 was repeated except that 0.2 part of nickel nitrate and 0.16 part of copper nitrate were used instead of 0.3 part of copper nitrate.
The composition of components other than oxygen of the obtained catalyst was Ag2% / N
It was i _0.01 Cu _0.01 Al _2.0 .

Example 13 Example 13 was effected in the same manner as in Example 1 except that 0.2 part of nickel nitrate and 0.2 part of zinc nitrate were used instead of 0.3 part of copper nitrate. The composition of the components other than oxygen of the obtained catalyst was Ag 2%
/ Ni _0.01 Zn _0.01 Al _2.0 .

Example 14 The procedure of Example 1 was repeated except that 0.16 parts of copper nitrate and 0.2 parts of cobalt nitrate were used instead of 0.3 part of copper nitrate.
The composition of components other than oxygen of the obtained catalyst was Ag2% / C.
It was u _0.01 Co _0.01 Al _2.0 .

Example 15 The procedure of Example 1 was repeated except that 0.16 parts of copper nitrate and 0.2 parts of cobalt nitrate were used instead of 0.3 part of copper nitrate.
The composition of components other than oxygen of the obtained catalyst was Ag2% / C.
It was u _0.01 Zn _0.01 Al _2.0 .

Comparative Example 1 50 parts of aluminum nitrate was added to 400 parts of pure water and stirred and dissolved. Next, while stirring this solution, 5% ammonia water was gradually added dropwise to adjust the pH of the solution to 7.0 to 9.0. The precipitate formed was filtered off, dried at 150 ° C. for 12 hours and then calcined in air at 800 ° C. for 2 hours. To 50 parts of the alumina powder thus obtained, a solution of 1.6 parts of silver nitrate in 50 parts of pure water was added.
After being dried at 50 ° C. for 12 hours, it was calcined at 600 ° C. for 2 hours in air to obtain silver-supported alumina. After mixing 500 parts of the silver-supported alumina powder and 1000 parts of pure water with a ball mill, the mixture was pulverized, and the resulting slurry was attached to a monolith carrier substrate and calcined at 400 ° C for 1 hour to prepare an exhaust gas purifying catalyst. Obtained. At this time, the adhesion amount of silver-supported alumina was set to 200 g / L. The composition of components other than oxygen of the obtained catalyst was Ag.
It was 2% / Al _2.0 .

Comparative Example 2 The procedure of Example 1 was repeated except that 5% aqueous ammonia was not used. The composition of the components other than oxygen of the obtained catalyst,
It was Ag 2% / Al _2.0 .

Comparative Example 3 The procedure of Example 1 was repeated except that 16.1 parts of copper nitrate was used instead of 0.3 part of copper nitrate. The composition of components other than oxygen of the obtained catalyst was Ag 2% / Cu _1.0 Al _2.0 .

Comparative Example 4 The procedure of Example 1 was repeated except that 19.7 parts of silver nitrate was used instead of 1.6 parts of copper nitrate. The composition of components other than oxygen of the obtained catalyst was Ag 20% / Cu _0.02 Al _2.0 .

Comparative Example 5 The procedure of Example 1 was repeated except that 0.3 part of copper nitrate and 1.6 parts of silver nitrate were not used. The composition of components other than oxygen of the obtained catalyst was Al _2.0 .

Comparative Example 6 4.0 parts of copper nitrate and silver nitrate were added to the alumina prepared in Comparative Example 5.
1.6 parts were impregnated. The composition of the resulting components other than oxygen in the catalyst was _{Ag2% -Cu2% / Cu 1.0 Al} 2.0.

Test Example 1 The catalysts of Examples 1 to 15 and Comparative Examples 1 to 6 were evaluated for catalytic activity under the following evaluation conditions. For activity evaluation,
An automatic evaluation device using model gas simulating the exhaust gas of an automobile was used. The L value used here represents the stoichiometric ratio of the oxidizing gas (NO, O ₂ ) and the reducing gas (CO, C ₃ H ₉ ) and is defined by the following formula.

[Equation 1] Evaluation condition 1 (L = 10.9) Catalyst Monolith type multi-component catalyst Total gas flow rate 40 L / min Catalyst inlet gas temperature 100-600 ° C Temperature rising rate 30 ° C / min Space velocity About 20000 H ^-1 Inlet gas composition Average air-fuel ratio A model gas composition corresponding to 20.0 CO 0.2% C ₃ H ₆ 3000 ppmC NO 200 ppm O ₂ 6.0% CO ₂ 10.0% H ₂ O 10.0% N ₂ balance A / F amplitude None The catalytic activity evaluation value is calculated by the following formula. Decided.

[Equation 2] The obtained catalytic activity evaluation results are shown in Tables 1 and 2. The catalytic activity of the example was higher than that of the comparative example, and the effect of the present invention described later could be confirmed.

[0047]

[Table 1]

[0048]

[Table 2]

[0049]

The exhaust gas purifying catalyst of the present invention is an alumina-based composite oxide containing at least one transition metal element selected from the group consisting of iron, cobalt, nickel, copper and zinc in a specific composition ratio. By using a substance to support silver on the alumina-based composite oxide, it is possible to improve the NO _x purification performance in a low temperature range in an oxygen excess atmosphere, which is not active in conventional silver-supported alumina catalysts, and therefore from low temperatures. NO in exhaust gas over a wide temperature range up to high temperature
_The catalyst performance for _x can be effectively maintained.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01J 23/50 ZAB A 23/60 ZAB A 23/72 ZAB A 23/755 B01D 53/36 102 A 102 B B01J 23/74 321 A

Claims (5)

[Claims] 1. The following general formula: Ag _a / X _b Al _c O _d (wherein X is at least one element selected from the group consisting of iron, cobalt, nickel, copper and zinc, and b and c indicates the atomic ratio of each element, and when c = 2.0, b =
0.005 to 0.5, a represents weight% converted to silver element, a = 0.1 to 10% by weight, and d is the number of oxygen atoms required to satisfy the valences of the above components). An exhaust gas purifying catalyst comprising a silver-supported alumina-based composite oxide represented by supporting silver on the alumina-based composite oxide. 2. The exhaust gas purifying catalyst according to claim 1, wherein the alumina-based composite oxide is an aqueous solution or a water dispersion containing each metal component constituting the oxide, and an aqueous ammonia solution, an ammonium carbonate, Add at least one aqueous solution selected from the group consisting of ammonium hydrogen carbonate, ammonium sulfate and ammonium hydrogen sulfate to adjust the pH of the solution.
A catalyst for exhaust gas purification, which is an alumina-based composite oxide obtained by adjusting the water content in the range of 7.0 to 9.0, removing water, and heat-treating the residue. 3. The exhaust gas purifying catalyst according to claim 1 or 2, wherein the catalyst carrier is provided with a silver-supported alumina-based composite oxide as a coat layer. 4. The exhaust gas purifying catalyst according to claim 3, wherein the catalyst carrier is a honeycomb monolith carrier substrate. 5. In producing the exhaust gas purifying catalyst according to claim 1, a catalyst raw material containing at least one component selected from the group consisting of iron, cobalt, nickel, copper and zinc and an aluminum component is pure. Add to water and stir,
Ammonia water, ammonium carbonate,
At least one aqueous solution selected from the group consisting of ammonium hydrogencarbonate, ammonium sulfate and ammonium hydrogensulfate is gradually added to adjust the pH of the solution to be in the range of 7.0 to 9.0, then water is removed and heat treatment is performed. Then, a method for producing an exhaust gas purifying catalyst, characterized in that silver is added to this and further heat treatment is performed.