JPH03186346A - Catalyst for purifying exhaust gas and catalyst structure - Google Patents

Abstract

PURPOSE:To obtain a catalyst enhanced in both of the oxidizing capacity of HC and CO and the reducing capacity of NOx and not lowered in its activity over a long time even at high temp. by using composite oxide having a perovskite crystal structure having a specific composition. CONSTITUTION:A catalyst for purifying exhaust gas is obtained using composite oxide having a perovskite crystal structure and represented by general formula A1-xA'xB1-yB'yO3 wherein 0<=x<=0.6, 0<=y<1, A is a rare earth element other than Ce, A' is Mg or Ca, B is Cr or Cu and B' is Mn or Fe). The catalyst thus obtained is stable in both of a high temp. reductive atmosphere and a high temp. oxidative atmosphere and enhanced in both of the oxidizing capacity of HC and CO and the reducing capacity of NOx and not lowered in its activity even when held at high temp. for a long time.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention oxidizes hydrocarbons (He) and -carbon oxides (Co) in gases discharged from internal combustion engines such as automobile engines, combustion equipment, etc. and nitrogen oxide (N
The present invention relates to a catalyst and a catalyst structure that efficiently purify such gas by reducing o).

[Prior Art] Gas discharged from internal combustion engines such as automobile engines contains HC and Co5No, and platinum (Pt), palladium (Pd),
A combination of rhodium (Rh) and the like is used.

Generally, these precious metal catalysts are prepared by using aluminum oxide (
/V 203) is applied and supported on it.

However, such precious metal catalysts have problems in terms of cost and resource conservation, and when used at high temperatures of 900°C or higher for long periods of time, the precious metals may sinter, and compared to aluminum oxide, which is a wash coat. There is also the problem that the surface area decreases and the catalyst activity deteriorates.

On the other hand, composite oxides having a perovskite structure are seen as promising as catalysts for gas purification, especially for La and 8Sr. 4
Catalysts with a composition of COO3 have been replaced with catalysts that have the same activity as noble metal catalysts in the oxidation of IC and CO, but their stability in an atmosphere with a high concentration of reducing gases such as IC%CO is poor. It has the disadvantage that the NO reduction and purification ability is low, and the reduction and purification ability of NO is hardly seen.

[Problems to be Solved by the Invention] In view of the above points, the present invention is stable in both high-temperature reducing atmospheres and high-temperature oxidizing atmospheres, and also has the oxidizing ability of IC and Go as well as the reducing ability of No. It is an object of the present invention to provide a gas purifying catalyst whose activity does not decrease even if it is kept at high temperature for a long time.

[Means for Solving the Problems] The present invention provides (1) a complex oxide having a perovskite crystal structure, which has the general formula (I): C, where X and y each satisfy 0<x≦0. 6 satisfies 0≦y<1, A is at least one element among rare earth elements (excluding Ce), A' is Mg5CaxS'', B
At least is element selected from the group consisting of a and Ce, B41Cr, Cus Nbs Has Tcs
Ru.

At least one element selected from the group consisting of Rh, Ag, Pt and ^U, Bo is Hn%Fes C05
(2) on a ceramic carrier or a heat-resistant metal carrier; At least one selected from basic metal oxides and perovskite type composite oxides
A gas purification catalyst structure is provided, characterized in that a wash coat containing 20% by volume or more of a species oxide is attached, and the gas purification catalyst described in item (1) above is supported thereon.

[Operations and Examples] The perovskite-type composite oxide catalyst represented by the general formula (1) is stable in both high-temperature reducing atmospheres and high-temperature oxidizing atmospheres, and has excellent oxidation ability of He%CO and NO. It has a high reducing ability and is characterized by its catalytic activity not decreasing even if it is kept at high temperature for a long time.

In the perovskite-type composite oxide, in order to realize a perovskite-type crystal structure, S is added to the A site.
c, Y and lanthanide elements (excluding Ce)
At least one element selected from the group consisting of rare earth elements, preferably at least one element selected from the group consisting of Y, La%P, Nd%Ss%Gd%Dy, and Er is used.

The perovskite type composite oxide is a good three-way catalyst (H
oxidation of CSCO, reduction of NO), the B site has one S electron in the outermost shell in the ground state of the atom.
Individual elements (excluding elements in group 1 of the periodic law),
That is, Crs Cu5Nb%No%Tc, Ru%I
? h% At least one element selected from the group consisting of Ag, Pt and Au, preferably C, Cu, Nb
, No, and at least one element selected from the group consisting of Er.

In addition, in order to improve the catalytic activity of the perovskite-type composite oxide by controlling the valence, the A' site is
At least IN elements selected from the group consisting of 5 Cas Sr, Ba and Ce are used.

Furthermore, by using at least one element selected from the group consisting of Mn, Fe, Go, Ni, and N at the B° site of the perovskite-type composite oxide, it can be used in a reducing atmosphere and/or at high temperatures. It has the excellent effect of maintaining the perovskite structure even when held for a long time and suppressing deterioration of catalyst activity.

A% In order to achieve the above effects of the elements at the A°, B, and B' sites, X and y should each be 0<x≦0.8 0≦y<1, more preferably 0.055 x≦0.4 05y It is necessary to satisfy ≦0.5.

In the present invention, a particularly preferable composite oxide from the viewpoint of catalytic activity, high-temperature durability, etc. is LB Ce.
Crys, La1-xCaxCrys, -x x Nd Sr CrO3, Dy l-x MgxC
r03--x x Nd, xSrxCrl-, Coy03sLad-, Ce
xCrl-, Ni, 03,' r 14M g x
C' l□Fey Os s N d i -x S
r x Cu 03, Lad-xCexCul-, Mn
yo3, Y BaCu M 0
3,5s1-x Bax NbO5,1-x x
l-y y "1-x MgxNbt-, Nl, Oi 1Lal-
xSrxMol-y Mn, 03%GdGd1-8B
ax, −, Fe, Os, Hr, −xBaxRul−,
Fe, Os, S II □Ce x Ru 1-y
Examples include N 1 y Os. In the above, X and y satisfy 0.05≦X≦0,4 and 0≦y≦0.5, respectively.

The perovskite type composite oxide can be prepared by a conventional method such as a powder mixing method or a coprecipitation method. For example, when using the powder mixing method, oxide powders of each component of the perovskite complex oxide are blended in a predetermined stoichiometric ratio, and then wet-pulverized and mixed in alcohol using a ball mill, attrition mill, etc. After drying, calcination is performed at 800-1000"C. Wet pulverization and calcination are repeated until the perovskite single phase is confirmed by X-ray diffraction test, and further pulverization is performed to form a perovskite-type composite oxide with a high specific surface area. Take the powder.

When using the coprecipitation method, nitrates of each component of the perovskite complex oxide are mixed in a predetermined stoichiometric ratio and dissolved in pure water. A mixed aqueous solution of ammonium carbonate and aqueous ammonia is used as the pH adjustment liquid, and this is added dropwise to the nitrate aqueous solution and stirred.

The pH is adjusted to neutral or basic, and the resulting coprecipitate is dried and then calcined at 600 to 800°C to obtain a single-phase perovskite-type composite oxide.

When preparing the perovskite-type composite oxide, the catalytic activity is further improved by setting the specific surface area in powder form to 5.0d7g or more.

The perovskite-type composite oxide catalyst prepared as described above is usually used by being supported on a carrier.

For applications such as automobile exhaust gas purification, the catalytic activity can be improved by attaching an aluminum oxide washcoat to a ceramic carrier or heat-resistant metal carrier, which generally has a honeycomb-shaped cross section, and supporting the catalyst thereon. It has been known.

However, it has been discovered that the perovskite-type composite oxide catalyst of the present invention may react with the aluminum oxide of the washcoat at high temperatures to form secondary products, resulting in a decrease in catalytic activity. As a result of further research based on this, we found that lanthanum oxide (
Basic oxides such as La203), magnesium oxide (Mg0), yttrium oxide (Y2O2), or L
It has been found that when a perovskite type composite oxide such as a#03.5rCe03 is used, the activity of the perovskite type composite oxide catalyst is not reduced, but rather the catalytic activity is further promoted.

The basic oxide and perovskite type composite oxide may be used alone or in combination of 2Fli or more. The specific oxide may be used in combination with other refractory materials, such as oxides such as AJ20s, Cr2O5, COO2% zeolite, or carbides such as SIC, but the total amount of the specific oxide may be washed coated. Among them, it is preferably 20% by volume or more.

Although the carrier is not particularly limited, examples of the ceramic carrier include cordierite (2MgO.2Al2
Os・58102), Mullite (3AJ203・
2S102) or the like, and stainless steel or the like is used as the heat-resistant metal carrier. The shape of the carrier is preferably a monolith type such as a honeycomb for purifying automobile exhaust gas, but it may also be in the form of a mesh, a porous body, a pellet, or the like.

The method of attaching the wash coat onto the carrier, the method of supporting the perovskite complex oxide catalyst onto the wash coat, etc. are not particularly limited, and any conventional method may be used.

The gas purification catalyst of the present invention is particularly advantageously used as a three-way catalyst for purifying automobile exhaust gas, but it can also be used for purifying exhaust gas from various combustion devices such as other internal combustion engines, thermal power generators, and kerosene stoves. Ru.

Next, the present invention will be explained with reference to Examples and Comparative Examples.

Example 1 [Preparation of Lao, sCeo, 2 Crux]
La203130.3g (0.4 mol), Ce02
B4.4g (0.2 mol) and Cr20376.0
g (0.5 mol) were weighed out, and pulverized and mixed for 72 hours in alcohol using a bot mill consisting of aluminum oxide balls and a bot. The obtained slurry was dried, the alcohol was splashed off, it was loosened in a mortar, and then fired in an electric furnace at 850° C. for 5 hours. The obtained powder is subjected to the wet grinding and drying, 8
After repeated firing at 50°C, it was confirmed by X-ray diffraction analysis that a single-phase perovskite-type composite oxide had been produced.

The thus obtained perovskite type composite oxide powder was wet-milled in a bot mill to obtain a powdered catalyst having a high specific surface area.

Further, the powdered catalyst was supported on a wash coat shown in Table 1 which was adhered to cordierite no.1 nicum to obtain a catalyst structure.

Examples 2 to 7 and Comparative Examples A to C Composite oxide catalysts having the compositions shown in Table 1 were prepared in the same manner as in Example 1, and catalyst structures were obtained in the same manner as in Example 1.

The specific surface area of the catalyst obtained above was measured. Measurement of specific surface area is a one-point bet using nitrogen as an adsorption gas.
It was done by law. The results are shown in Table 1.

[Left below] No. 1 table Further, specific tests were conducted on the catalyst obtained above.

(1) Catalytic Activity Measurement Catalytic activity was measured for the catalyst structure. The automobile exhaust model gas with the composition shown in Table 2 was
V) 35,000 hr' was introduced into a reaction tube filled with a catalyst, and the gas purification rate at each temperature was measured.

Table 2 HC concentration was measured using a flame ion analyzer (FID), CO concentration was measured using a non-dispersive infrared absorption analyzer (NDIR), NOx concentration was measured using a chemiluminescence analyzer (CLD), and 02 concentration was measured using a magnetic pressure analyzer. Then, the purification rate at each temperature was determined.

('2J High-temperature durability test) After the catalyst was held at 900° C. for 5 hours in the automobile exhaust model gas atmosphere, activity was measured again to evaluate high-temperature durability.

The results of the test (1) are shown in Figures 1 to 6, and the results of the test (2) are shown in Figures 1 to 6.
The results are shown in Figure 7. In FIG. 7, "after durability" represents the purification rate after the high temperature durability test.

[Effect of the invention] The perovskite type composite oxide catalyst of the present invention has 11c,
It has high Co oxidizing ability and No 2 reducing ability, and its activity does not decrease even if it is kept at high temperature for a long time, making it useful as a catalyst for purifying automobile exhaust gas.

[Brief explanation of drawings]

Figures 1 to 6 are graphs showing purification rates of the exhaust gas purification catalyst of the present invention and conventional products (Figures 1 to 2 are IIC purification rates, Figures 3 to 4 are Co purification rates, and graphs 5 to 6 The figure shows the NO purification rate), and Figure 7 shows the NO purification rate before and after the high temperature durability test.
It is a graph showing a purification rate. Fang 1 Figure temperature (℃) Figure 2 00 200 300 400 500 Temperature (℃) 00 00 Figure 3 00 aη Temperature scale 00500 degrees (C) 00 00 Figure 4 Day (1) Elevation (℃) Figure 5 Temperature (℃) Figure 6 Example 4 Temperature (℃)

Claims (1)

[Claims] 1 A composite oxide having a perovskite crystal structure, which has the general formula (I): A_1_-_xA'_xB_1_-_yB'_yO_(
I) (where x and y each satisfy 0<x≦0.6 0≦y<1, A is at least one element among rare earth elements (excluding Ce), A' is Mg , Ca, Sr, B
At least one element selected from the group consisting of a and Ce, B is Cr, Cu, Nb, Mo, Tc, Ru, Rh
, at least one element selected from the group consisting of Ag, Pt and Au; B' represents at least one element selected from the group consisting of Mn, Fe, Co, Ni and M); A gas purification catalyst characterized by being made of an oxide. 2 In general formula (I), x and y are each 0
．． 05≦x≦0.4 0≦y≦0.5, A is Y, La, Pr, Nd, Sm, Gd, D
The gas purification according to claim 1, wherein B is at least one element selected from the group consisting of y and Er, and B is at least one element selected from the group consisting of Cr, Cu, Hb, Mo, and Ru. Catalyst for use. 3. A wash coat containing at least 20% by volume of at least one oxide selected from basic metal oxides and perovskite-type composite oxides is adhered to a ceramic carrier or a heat-resistant metal carrier, and the wash coat according to claim 1 is further applied to the ceramic carrier or the heat-resistant metal carrier. A gas purification catalyst structure characterized by supporting a gas purification catalyst.