JP2000104534A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst for purifying exhaust gas to be excellent in purification of NOx even after especially thermal durability. SOLUTION: This catalyst is a catalyst for purifying exhaust gas to purify NOx under oxygen excessive atmosphere. A catalyst layer contains a precious metal, such at Pt, Pd, and Rh, a composite compound A represented by a formula of LnαAOβ, and a composite compound B represented by a formula BxCoy, the two formulas wherein α is 0<α<1, β indicates an oxygen amount to satisfy a valence of each element, Ln is at least one kind selected from a group consisting of La, Ce, and Nd, and A indicates at least one kind selected from a group consisting of Fe, Co, Ni, and Mn, x is 0<x<1, y is an oxygen amount to satisfy a valence of each element, B is a composite compound B represented by Cs and/or Na, and C indicates A1 and/or Ti. It is desirable that composite compounds A and B are contained in respective independent powder configurations.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in exhaust gas discharged from internal combustion engines such as automobile engines (gasoline and diesel) and boilers. More specifically, the present invention relates to an exhaust gas purifying catalyst for purifying NO.
The present invention relates to an exhaust gas purifying catalyst that focuses on x purification.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for fuel-efficient vehicles due to the problem of depletion of petroleum resources and the problem of global warming. For gasoline vehicles, the development of lean-burn vehicles has attracted attention. In lean-burn vehicles, during lean-burn operation,
Although the exhaust gas atmosphere becomes an oxygen-excess atmosphere (lean) compared to the stoichiometric air-fuel state, when a normal three-way catalyst is used in the lean region, there is a problem that the effect of excessive oxygen makes the NOx purification action insufficient. Was. Therefore, development of a catalyst that can purify NOx even when oxygen becomes excessive has been desired.

Hitherto, various catalysts for purifying NOx in a lean region have been proposed.
No. 60 discloses a catalyst in which Pt and lanthanum are supported on a porous carrier. As typified by this catalyst, a catalyst that absorbs NOx in a lean region and releases NOx during stoichiometry to purify the catalyst is disclosed. Proposed.

[0004]

However, such a P
Even if a t-lanthanum-based catalyst is used, there is a problem that the NOx purification performance becomes insufficient due to, for example, performance degradation after durability.

The present invention has been made in view of such problems of the prior art, and an object of the present invention is to provide an exhaust gas purifying catalyst which is excellent in purifying NOx, especially even after heat endurance. It is in.

[0006]

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems, and as a result, in addition to so-called noble metal components such as Pt, Pd, and Rh, two different types containing a specific element are used. When a catalyst was prepared by mixing the above composite compounds, it was found that the above problems were solved, and the present invention was completed.

That is, the exhaust gas purifying catalyst of the present invention is formed by coating a catalyst layer on a catalyst carrier. In the exhaust gas purifying catalyst for purifying nitrogen oxides in an oxygen-excess atmosphere, the catalyst layer comprises Pt, At least one noble metal selected from the group consisting of Pd and Rh, and the following general formula LnαAOβ (where α is 0 <α <1, β is the amount of oxygen satisfying the valence of each element, and Ln is At least one selected from the group consisting of La, Ce and Nd, A is Fe, Co, Ni and Mn
At least one selected from the group consisting of: ) And the following general formula BxCOy (where x is 0 <x <1, y is the amount of oxygen satisfying the valence of each element, B is Cs and / or Na, C And Al and / or Ti).

[0008] In a preferred embodiment of the exhaust gas purifying catalyst of the present invention, the catalyst layer comprises a composite compound A and a composite compound B.
Are contained in the form of their own powders separated from each other.

Further, another preferred embodiment of the present invention is characterized in that another catalyst layer other than the above-mentioned catalyst layer is further coated, and the other catalyst layer does not contain the composite compounds A and B. .

[0010]

In the present invention, a catalyst layer in which two different composite compounds (composite compound A and composite compound B) each containing a specific element are mixed in addition to noble metal components such as Pt, Rd and Rh is coated. Therefore, a high NOx absorption function can be expressed at the interface between the composite compound A and the composite compound B. Further, since the composite compounds A and B have the above-mentioned composition, deterioration of the catalyst due to heat can be suppressed.

In the catalyst of the present invention, the catalyst layer contains the complex compound A and the complex compound B. The content of the complex layer is such that the powders of the complex compounds A and B each independently formed, that is, the complex compound It is preferable that the powders contain only powders of either A or B and that the composite compounds A and B are not mixed. By realizing such a contained state, the deterioration due to the solid solution of the complex compounds A and B is suppressed, and the durability of the catalyst can be improved.

Further, even when the above-mentioned compounds A and B are in the above-mentioned powder form, they impede HC purification in the three-way catalytic function, so that other catalyst layers not containing the composite compounds A and B are used. The catalyst of the present invention is preferably provided, and by further providing such another catalyst layer, a sufficient three-way catalyst function can be exhibited.

Further, the present catalyst has an air-fuel ratio of 10 to 50.
It is suitable for use in exhaust gas purification of lean burn engine vehicles that repeat the range described above. According to this usage method, NOx is absorbed in a region where the air-fuel ratio is large (lean region), and NOx is purified in a region where the air-fuel ratio is small (rich to stoichiometric), so that high NOx purification performance can be obtained. In this case, it is more preferable that the region with a small air-fuel ratio is 10 to 14.8 and the region with a large air-fuel ratio is 15 to 50.

[0014]

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 exhaust gas purifying catalyst of the present invention comprises a precious metal and a composite compound A on a catalyst carrier.
And a catalyst layer containing the composite compound B. here,
The catalyst carrier is preferably of an integral structure type. As the monolithic carrier, a monolith carrier made of a heat-resistant material is preferable. For example, a ceramic carrier such as cordierite or a metal carrier such as ferrite stainless steel can be used.

As the noble metal, Pt, Pd or R
h and an arbitrary mixture thereof are used, and these noble metal components are blended because the catalyst of the present invention needs to exhibit a function as a three-way catalyst at the time of stoichiometry. Such a noble metal is preferably used by being supported on a porous carrier, and is particularly preferably supported by alumina having a high specific surface area. In this case, the specific surface area of alumina is 5
Activated alumina of about 0 to 300 m ² / g is preferred.

The complex compound A has the following general formula: LnαAOβ (where α is 0 <α <1, β is the amount of oxygen satisfying the valence of each element, and Ln is composed of La, Ce and Nd. At least one selected from the group, A is Fe, Co, Ni and Mn
At least one selected from the group consisting of: Further, the composite compound B is represented by the following general formula BxCOy (where x is 0 <x <1, y is the amount of oxygen satisfying the valence of each element, B is Cs and / or Na, C is Al and / Or Ti).

It is preferable that all of the above-mentioned compound compounds A and B are compounded, but the desired action can be obtained even if a part of them is compounded.

As a method for producing the composite compounds A and B,
For example, a mixture of aqueous solutions of metal salts (nitrate, carbonate, acetate, citrate, hydrochloride, etc.) of each component is prepared, and a porous carrier such as alumina is appropriately mixed therewith, and a precipitant ( Ammonia, ammonium carbonate, etc.), drying and calcining to obtain a mixed oxide.
Powders of the complex compounds A and B described below are also obtained by this method. According to such a method, at least a part of each of the element components is compounded to meet the purpose. However, the method of producing the composite compounds A and B is not necessarily limited to the present production method, and any other method may be used as long as the desired composite compound is formed.

The above-mentioned composite compounds A and B may contain impurities contained in the constituent elements as long as they do not interfere with their functions. For example, among the elements constituting the composite compound A, lanthanum may contain a small amount of cerium, neodymium, samarium, or the like.

As described above, the catalyst of the present invention contains the above-mentioned noble metal, composite compound A and composite compound B as essential components, and may further contain, if necessary, materials used in ordinary three-way catalysts. For example, La, Ce, Zr or the like may be added to alumina as a porous carrier to improve thermal stability.

The content of various components in the catalyst layer of the present catalyst is not particularly limited, but typically,
It is preferable that the amount of the noble metal is 0.1 to 20 g, the compound A is 5 to 100 g, and the compound B is 5 to 100 g per liter of the catalyst. If the various components deviate from the above ranges, a sufficient performance improving effect may not be obtained or a significant effect of increasing the amount may not be obtained. The total amount of these catalyst components supported is typically 10 to 400 g per liter of the catalyst.

As described above, the catalyst of the present invention can be obtained by coating a catalyst layer containing the above essential components on a monolithic carrier, in which at least the composite compounds A and B are contained in powder form. It is preferable that the powders A and B corresponding to the composite compounds A and B do not contain the other. In other words, the powder A containing the complex compound A but not the complex compound B and the powder B containing the complex compound B and not containing the complex compound A are present in the catalyst layer without being combined or complexed with each other. It is desirable to do. However, in this case, powder A and powder B
Needless to say, they may be adjacent to each other.

In the catalyst of the present invention, by controlling the state of the composite oxides A and B in the catalyst layer as described above, deterioration due to the solid solution of the composite compounds A and B can be suppressed. The durability of the present catalyst can be further improved. In such a state, the average particle size of the powder A and the powder B is preferably 1 to 5 μm.
If it is less than 1 μm, the inside of the catalyst layer is blocked and gas diffusion is hindered. If it exceeds 5 μm, the number of catalytically active sites is reduced, and sufficient performance may not be obtained, which is not preferable.

Further, the catalyst of the present invention can be provided with a catalyst layer other than the catalyst layer containing the above essential components, whereby the effect of suppressing the HC purification of the complex compounds A and B can be reduced. Can be mitigated or avoided. Incidentally, such another catalyst layer may be provided not only a single layer but also a plurality of layers,
Further, it may be provided as an upper layer and / or a lower layer of a catalyst layer containing an essential component. However, from the catalyst layer of the present invention, N
When Ox is desorbed, it may be effective to provide a layer for purifying Ox in the upper layer. Therefore, it is preferable to dispose another catalyst layer as the upper layer.

[0025]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, Comparative Examples and Test Examples, but the present invention is not limited to these Examples.

Example 1 An activated alumina powder was impregnated with an aqueous solution of Pd nitrate, dried and calcined in air at 400 ° C. for 1 hour to obtain a Pd-supported alumina powder (powder 1). The Pd concentration of this powder was 2.8% by weight. An activated alumina powder was impregnated with an aqueous solution of dinitrodiammine Pt, dried, and calcined at 400 ° C. for 1 hour in the air to obtain a Pt-supported alumina powder (powder 2). The Pt concentration of this powder was 2.8% by weight. An activated alumina powder was impregnated with an aqueous solution of Rh nitrate, dried and calcined in air at 400 ° C. for 1 hour to obtain a Pd-supported alumina powder (powder 3). The Rh concentration of this powder was 0.7% by weight.

Citric acid was added to a mixture of lanthanum carbonate and cobalt carbonate, dried and calcined at 700 ° C.
(Powder 4) was obtained. This powder had a metal atom ratio of lanthanum / cobalt = 8/10. Add citric acid to the mixture of cesium carbonate and aluminum nitrate,
After drying, firing was performed at 700 ° C. to obtain a powder (powder 5) corresponding to the composite compound B. This powder had a metal atom ratio of cesium / aluminum = 5/10.

180 g of each of the powders 1 to 5 obtained as described above and 900 g of water were charged into a magnetic ball mill and mixed and pulverized to obtain a slurry liquid. This slurry solution was attached to a cordierite-based monolith carrier (1.3 L, 400 cells), excess slurry in the cells was removed by an air stream, dried at 130 ° C., and baked at 400 ° C. for 1 hour, and baked at 250 g. The catalyst of Example 1 having a / L coat layer was obtained.

Example 2 A catalyst of Example 2 was obtained by repeating the same operation as in Example 1 except that Nd was used instead of La in powder 4.

Example 3 A catalyst of Example 3 was obtained by repeating the same operation as in Example 1 except that Mn was used instead of Co in powder 4.

Example 4 A catalyst of Example 4 was obtained by repeating the same operation as in Example 1 except that Na was used instead of Cs in the powder 5.

Example 5 A catalyst of Example 5 was obtained by repeating the same operation as in Example 1 except that Ti was used instead of Al in the powder 5.

Comparative Example 1 A catalyst of Comparative Example 1 was obtained by repeating the same operation as in Example 1 except that activated alumina powder was used instead of powder 4.

Comparative Example 2 The same operation as in Example 1 was repeated, except that activated alumina powder was used instead of powder 5, to obtain a catalyst of Comparative Example 2.

Table 1 shows the catalyst composition in each of the above examples.

[0036]

[Table 1]

(Test Examples) The catalysts of the above Examples and Comparative Examples were evaluated for the following performance. [Endurance method] The catalyst of each of the above examples was attached to the exhaust system of an engine with a displacement of 4400 cc, and the inlet temperature of the pre-stage catalyst was reduced to 7
The temperature was set to 00 ° C. and the operation was performed for 50 hours. [Evaluation method] The catalyst of each example was installed in the exhaust system of an engine with a displacement of 2000 cc, and A / F = 14.6 was applied for 60 seconds →
The operation of A / F = 50 for 10 seconds was repeated. The inlet temperature of the pre-stage catalyst was set to 350 ° C. The total conversion for one cycle of this switching operation was determined. Table 2 shows the obtained results.

[0038]

[Table 2]

[0039]

As described above, according to the present invention, in addition to so-called noble metal components such as Pt, Pd, and Rh, two different types of composite compounds each containing a specific element are mixed to form a catalyst. Therefore, it is possible to provide an exhaust gas purifying catalyst which is excellent in NOx purification even after heat durability.

Continued on the front page F-term (reference) 3G091 AA02 AA06 AA12 AA17 AA18 AB03 AB05 BA07 BA14 BA15 BA19 BA39 FB10 FB11 FB12 GA06 GA18 GB01W GB01X GB02W GB04W GB05W GB06W GB07W GB10W GB10X GB16X GB17X 4D048 AAX AB13 ABAX BA28X BA31X BA36Y BA37X BA38Y BA41X BA42X BA48X BB01 CA01 CC33 CC46 DA03 DA20 EA04 4G069 AA03 BA01B BB06A BC02A BC02B BC06A BC06B BC16A BC16B BC42A BC42B BC43A BC44A BC44B BC50A BC50A BC66ABC50A BC50A BC50A

Claims (6)

[Claims] A catalyst layer coated on a catalyst carrier,
In an exhaust gas purifying catalyst for purifying nitrogen oxides in an oxygen-excess atmosphere, the catalyst layer comprises at least one noble metal selected from the group consisting of Pt, Pd and Rh, and the following general formula LnαAOβ Is 0 <α <1, β is the amount of oxygen satisfying the valence of each element, Ln is at least one selected from the group consisting of La, Ce and Nd, and A is Fe, Co, Ni and Mn.
At least one selected from the group consisting of: ) And the following general formula: BxCOy (where x is 0 <x <1, y is the amount of oxygen satisfying the valence of each element, B is Cs and / or Na, C A composite compound B represented by the following formula (1): Al and / or Ti). 2. The exhaust gas purifying catalyst according to claim 1, wherein the catalyst layer contains the composite compound A and the composite compound B in the form of their own powders separated from each other. 3. The exhaust gas purifying catalyst according to claim 1, wherein the average particle diameter of the powders of the composite compounds A and B is 1 to 5 μm. 4. A catalyst layer other than the above-mentioned catalyst layer is further coated, and the other catalyst layer is composed of the composite compounds A and B.
The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the catalyst does not contain any. 5. The exhaust gas purifying apparatus according to claim 1, wherein the exhaust gas purifying apparatus is used for purifying exhaust gas of a lean burn engine vehicle having an air-fuel ratio repeating a range of 10 to 50. catalyst. 6. An air-fuel ratio of 10 to 14.8, 15 to 50
6. The exhaust gas purifying catalyst according to claim 5, wherein the catalyst is used for purifying exhaust gas of a lean burn engine vehicle which repeats the following range.