JPH0368451A - Production of catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE:To enhance the exhaust gas purifying ability of a catalyst by coating a refractory carrier with a slurry contg. powder of a perovskite type double oxide and refractory powder supporting a noble metal and by drying and calcining the carrier to form the catalyst. CONSTITUTION:Citric acid is added to an aq. soln. contg. compds. of the constituent elements of a perovskite type double oxide to prepare a uniform aq. soln. This aq. soln. is evaporated to dryness and calcined to form powder of the perovskite type double oxide. An aq. slurry contg. the oxide powder and refractory powder (other than the oxide powder) supporting a noble metal is prepd. and a refractory carrier is coated with the slurry, dried and calcined to produce a catalyst for purification of exhaust gas. The pref. amt. (mol) of the citric acid added is about 1.0-1.2 times the amt. of the perovskite type double oxide.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing an exhaust purification catalyst, and more particularly, to a method for manufacturing an exhaust purification catalyst having excellent purification performance using a perovskite type composite oxide. It is something.

[Conventional technology]

Perovskite-type composite oxides composed of rare earth elements, other transition elements, alkaline earth elements, etc. have high heat resistance, are inexpensive, and have catalytic activity against harmful components in automobile exhaust. Therefore, its use as an exhaust purification catalyst has been proposed.

For example, in JP-A-59-87046, the general formula La 'Sr TCol-X Me) (oS (Me is F
e.

An element selected from Mn, Cr, V, Ti, O<x1
) An exhaust purification catalyst made of a perovskite-type composite oxide represented by the following is disclosed.

JP-A No. 60-82138 describes a supported catalyst using a perovskite-type composite oxide represented by the general formula %V, an element selected from Ti, O<X<1) as a catalyst component. discloses an exhaust purification catalyst that is supported by thermal spraying.

Furthermore, in order to further improve the exhaust purification performance, an exhaust purification catalyst in which a noble metal such as palladium, platinum, or rhodium is supported on a perovskite type composite oxide has also been proposed.

For example, Japanese Patent Application Laid-Open No. 62-282642 discloses that an exhaust purification catalyst containing at least Pd as a catalyst component
An exhaust purification catalyst is disclosed in which the Pd is supported as a perovskite-type composite oxide or an equivalent to the composite oxide.

JP-A No. 63-502950 discloses that the following general formula is applied to the surface of the monolith carrier base material:
Sm.

Bu, Sc, Bi, Pd, Ca,
One metal selected from the group consisting of Sr and Ba, B is Fe, Zn, Sm.

Mg, Co, Ni, Ti, Nb, V, Cu and Mn
At least one metal selected from the group consisting of B
/ indicates at least one metal selected from the group consisting of Pt, Rh, Pd, Ru, and Ir) and the following general formula: % formula % (in the formula, C is a rare earth Metal CI, Sr or Ba.

At least one selected beloskite from the group consisting of perovskite-type complex oxides represented by (D represents Ti or ■)! Supporting a coating layer consisting of composite oxide powder, activated alumina and rare earth metal oxide powder, and supporting at least one metal consisting of platinum, rhodium and palladium or an oxide thereof, which are catalytically active components. A catalyst for exhaust purification is disclosed.

[Problem to be solved by the invention]

JP-A No. 59-87046 and JP-A No. 60-8213
Publication No. 8, Publication No. 62-282642 and Publication No. 63-3
All of the exhaust purification catalysts disclosed in JP 02950 are intended to be used at temperatures of 800° C. or lower. Regarding exhaust purification catalysts used in automobiles, 9
Although continuous use may occur in a high temperature range of 00° C. or higher, the exhaust purifying catalysts used in the prior art have poor durability under such conditions, and purification performance tends to deteriorate.

That is, in general, perovskite type composite oxides have 900
Sintering occurs at temperatures above 0.degree. C., reducing the effective surface area and reducing the catalytic activity. In addition, when perovskite-type composite oxides are supported on a refractory carrier made of alumina, titania, etc., or coexisted with these refractory powders, a solid phase reaction occurs with the carrier or powder at 900°C or higher, forming a perovskite. The mold structure could not be maintained and the catalytic activity decreased significantly. Furthermore, a catalyst made only of perovskite-type composite oxides has poor NOx purification performance and cannot be put to practical use as a three-way catalyst. Therefore, it is necessary to devise a method for producing perovskite-type composite oxides to obtain products that are less prone to the above-mentioned defects, and/or
Alternatively, it was hoped that the perovskite-type composite oxide could be used as a three-way catalyst by devising a way to use it, but it was not possible to obtain such an exhaust purification catalyst using conventional technology. .

The present invention is intended to solve the above-mentioned problems with the prior art, and its purpose is to have a perovskite type that has sufficient purification performance even in the high temperature range of 900 degrees Celsius or higher and is steeped in durability. An object of the present invention is to provide a manufacturing method that can easily obtain an exhaust purification catalyst containing a composite oxide.

[Means to solve the problem]

That is, in the first aspect of the present invention, citric acid is added to an aqueous solution containing compounds of each element constituting a perovskite-type composite oxide to form a uniform aqueous solution, and the aqueous solution is evaporated to dryness and then fired to form a perovskite. A type composite oxide powder is prepared, and then a slurry containing the powder and a refractory powder supporting a noble metal (excluding the perovskite type composite oxide powder) is prepared, and then the slurry is transferred to a refractory carrier. This is a method for producing an exhaust purification catalyst, characterized in that the catalyst is coated on top, dried, and then fired.

The second invention provides a perovskite-type composite oxide by adding citric acid to an aqueous solution containing compounds of each element constituting the perovskite-type composite oxide to form a uniform aqueous solution, evaporating the aqueous solution to dryness, and then firing it. An oxide powder is produced by YA, and then a slurry containing the powder and a refractory powder (excluding the perovskite composite oxide powder) is prepared, and then the slurry is coated on a refractory carrier, dried, and then fired to form the oxide powder. This is a method for producing an exhaust gas purification catalyst, which comprises forming a mixed powder layer on a refractory carrier, and then supporting a noble metal on the mixed powder layer.

The third aspect of the present invention is to add citric acid to an aqueous solution containing compounds of each element constituting a perovskite-type composite oxide to obtain a homogeneous aqueous solution, and convert the aqueous solution into a refractory powder carrying a noble metal (perovskite-type composite oxide). (excluding powder), dried and fired to form a perovskite-type composite oxide layer on at least the surface and to prepare a refractory powder supporting a noble metal vI4, and then prepare a slurry containing the refractory powder. The method for producing an exhaust purification catalyst is characterized in that the slurry is then applied onto a refractory carrier, dried and then fired.

The fourth aspect of the present invention is to add citric acid to an aqueous solution containing compounds of each element constituting a perovskite-type composite oxide to form a uniform aqueous solution, and to transform the aqueous solution into a refractory powder (excluding the perovskite-type composite oxide powder). ), dried and fired to form a perovskite-type composite oxide layer on at least the surface, a refractory powder is prepared, a slurry containing the refractory powder is prepared, and then the streaks are impregnated with a refractory carrier. This is a method for producing a catalyst for exhaust purification, characterized in that the refractory powder layer is formed on the refractory carrier by coating, drying and firing, and then the noble metal is supported on the refractory powder layer.

In the present invention, the perovskite type composite oxide is +J
The biggest feature is that citric acid is added during production.

The addition ratio of citric acid is preferably 1.0 to 1.2 times in molar ratio to the obtained perovskite type composite oxide. If the addition ratio is less than the above lower limit, a perovskite-type composite oxide with a sufficiently uniform composition cannot be obtained. Furthermore, if the addition ratio exceeds the above upper limit, problems such as difficulty in firing may occur.

As the compound of each element constituting the perovskite type composite oxide, water-soluble salts such as nitrates are preferable, but
Any solution that becomes a homogeneous solution when citric acid is added can be used.

The perovskite-type composite oxide (including noble metals) has the following general formula: Lnl-xAxBl-y Cy03 (wherein, Ln represents a rare earth element, A represents an alkaline earth element, and B represents a transition other than Ln and C. It is preferable that C represents one or more kinds of elements, C represents one or more kinds of noble metals, and 0<X<1.0<y<1.

Refractory powders include alumina, silica, and general formula AB'O.
,,LnAl! 03 or Ln, Boo, (wherein, A
represents an alkaline earth element, B' is Ti, Zr or H
f and Ln represents one or more rare earth elements).

Examples of noble metals include Pd, Rh, Pt,
Examples include Rn and Ir, and one or two of these
It is better to carry more than one seed.

Mixing ratio of perovskite type composite oxide and refractory powder,
Conditions such as the amount of noble metal supported, slurry concentration, and firing conditions are selected as appropriate.

The refractory carrier may be, for example, a ceramic carrier such as cordierite or a refractory metal carrier such as stainless steel. Its size and shape are selected appropriately. Examples of the shape include pellets and monoliths.

A monolithic refractory carrier is convenient in practice.

[For production]

When preparing a perovskite-type composite oxide, citric acid is added to the aqueous solution of the raw material compounds, so the aqueous solution is not acidic and each raw material compound is stabilized. Therefore, it is possible to prevent problems such as contamination of carbonates due to the absorption of carbon dioxide in the air, and it is possible to obtain a perovskite-type composite having a uniform composition and in the form of fine particles.

〔Example〕

The present invention will be explained in further detail in the following Examples and Comparative Examples. However, the present invention is not limited to the following examples.

<Basic operation 1> (1) Preparation of perovskite-type composite oxide (1) Add citric acid to an aqueous solution of water-soluble salts (nitrates, etc.) of each constituent element and dissolve.

(2) Evaporate and dry the above mixed liquid using an evaporator,
Then, it is vacuum dried.

(Calcinate in air at 31,800°C to produce perovskite-type composite oxide powder.

Regarding step (1), citric acid may be added to a solution obtained by hydrolyzing the alkoxide compound of each constituent element and uniformly dissolved.

L Supporting noble metal on weather-resistant powder (supporting noble metal may be carried out in the step I below) (1) Impregnate a refractory powder such as alumina, 5rZrO, etc. with a noble metal solution.

(2) The powder is dried and optionally fired to support the noble metal.

(1) Mix each of the powders prepared in 1 and 2 with water and a binder such as zirconia sol to obtain a solution with a viscosity of 200 to 300 CpS.
slurry of vf4E! Sur.

(2) The slurry is poured into a refractory carrier (for example, a honeycomb carrier made of cordierite ceramics or a heat-resistant metal), the remaining slurry is blown off with an air stream, and the slurry is dried and fired.

(If no noble metal is supported on the refractory powder in 31■,
Next, the refractory carrier is immersed in a noble metal solution to support the noble metal on the mixed powder layer on its surface.

<Basic operation 2> 1. V of perovskite type composite oxide on refractory powder
4 (1) refractory powder is impregnated with the solution of 1-(1) of Basic Operation 1.

(2) Basic operation 1, 1-(21 and opposite).

(3) Basic operations 1-1 (same as 31).

L Supporting noble metal on refractory powder (supporting noble metal may be carried out in the step I below) (1) Basic operation 1 II-(same as 11).

(2) Same as II-(2) of Basic Operation 1.

! , application to refractory carriers (1) Basic operations 1 summer - (same as 11).

(2) Basic operation 10I- (same as 21).

(3) Basic operations 1-1 (same as 31).

Example 1 (1) La (N03), 6 H, 0590ti
, 5r(NOx)z21 g, Fe(Nos)3”
9H20242g, Co (N03)13H,011
Dissolve ag and Pd(NO3)z 2.39 in pure water 1/2.

(2) Citric acid (C6H1107/N20) 504
Dissolve g in 1/2 part of pure water.

(31. Mix the solution of (1) and the solution of (2) and stir with a stirrer.

(4) Pour the mixed solution into the evaporator and
Evaporate to dryness in a water bath at °C.

(5) Transfer the solid matter to a vacuum dryer and vacuum dry at about 70°C for 12 hours.

(6) Calcinate this in the air at 300°C for 3 hours.

(7) The calcined product is further crushed and mixed using a grinder.

(8) This was calcined in the air at SOO℃ for 10 hours to obtain Lao, S Sro, IFeo, with an average particle size of 5 μm.
A powder having a composition ratio of g coo, 39 p ('0.0103) was obtained.

(9) Commercially available 5rZrO, powder (surface area 1F3rr?/
y, average particle size 7 μm) and 0.5 weight! -% palladium nitrate solution.

Od The above powder was dried in the air at 110°C for 10 hours, and then calcined at 600°C for 3 hours to obtain 5rZr03 powder containing 115% more Pd by weight.

0°C The powders obtained in (8) and 00 above are mixed at a weight ratio of 1:1, and mixed and pulverized using a Lika l/1 machine.

(2) A slurry is prepared by mixing 100 parts by weight of the mixed powder obtained in αη, 70 parts by weight of zirconia sol having a solid content of 10 parts by weight, and 100 parts by weight of water.

The slurry of (6) was poured into O3 commercially available cordierite honeycomb carrier (400 cells/inch2), the excess slurry was blown off with an air stream, and the slurry was dried at 300°C for 2 hours.
One catalyst of Example 1 was obtained by calcining at 650° C. for 8 hours.

Example 2 (1) Mixed solution 1-(3) similar to Example 1 was mixed with S rZ
Impregnated with rO3 powder, 1-? -(57--A perovskite-type composite oxide layer having the same composition ratio as in Example 1 was formed on the 5rZr03 powder in the same steps as in (8).

(2) (Add ammonia water to a 15 weight aqueous solution of palladium nitrate to adjust the pH to 8-10Kv4.

(3) La (115ral Feall
COQ! 1lPd (5rZr with 10103 layers formed)
03 powder was applied.

(4) A honeycomb carrier was impregnated with the solution of (2), and 0.5 weight of Pd was supported on the coating layer.

(5) This honeycomb carrier was dried at 300'C for 2 hours and calcined at 650°C for 8 hours to obtain two catalysts of Example 2.

Example 3 A perovskite-type composite oxide powder having the following composition ratio was prepared in the same manner as in Example 1.

Noble metal salts Pd(NO3)2, Rh(No,)3, P
t(NH,).

(NO2)z and RuC/ were used.

B: Lao, 1lsro, I Co0,3 F'e
0,6 pao, o8Rh0. . , o.

C: La0. gsr6. I Coo, 3 Fe@4P
do, osPto, ozO3D: La0.9srO
,1co,),3Feo,6Pdo,osRuo,o
z(JsE: Ijao,9ceO,l Co(,,
3Feo, 6Pdo, i0s Also, activated alumina powder containing 4% by weight of La2O3 and 30% by weight of CeO2 was used as the refractory powder. Then, precious metals were supported as shown below.

E': Q, 2% by weight Pt C': 0.02% by weight R, h p'': Q, 5% by weight Pd, 0.2% by weight RhE': 0.
2% by weight of Pt, α02% by weight of Rh
- E' powders were used in combination and coated on a honeycomb carrier in the same manner as in Example 1 to obtain catalysts 1B to 1E of Example 2.

Example 4 Example 3 in which a layer made of the same perovskite-type composite oxide as in Example 3 was formed on the surface by the same operation as in Example 2.
Alumina powder similar to that described above (however, no noble metal was supported) was applied to a honeycomb carrier. Next, catalysts 2B to 2E of Example 4 were prepared by impregnating a honeycomb carrier with each precious metal aqueous solution so that the amount of noble metal supported was the same as in Example 3.

Comparative Example 1 Perovskite-type composite oxide powder Lao,e SrO,I containing no noble metal was added to the same honeycomb carrier as in Example 1.
Catalyst 1a coated with Feo, 6 CoO, 4Q, and La, O of Example 3 on the same honeycomb carrier as in Example 1.
s, CeO2-containing alumina was applied, Pd was applied to Q, 5
A catalyst 1b was prepared in which 0.02 weight of Rh was supported.

Comparative Example 2 Solution 11! having the same composition as Example 1-(11) was neutralized by adding 10 more weight of aqueous ammonia to obtain a coprecipitate, which was then washed with water, filtered with r, and dried at 800°C. A perovskite-type composite oxide was obtained by firing for 10FE8.Catalyst 1C was obtained in the same manner as in Example 1.

Performance Comparison Test 1> Catalysts 1A to 1E and 2A to 2E of Examples 1 to 4 and catalysts 1a to 1C of Comparative Examples 1 to 2 were packed in a converter,
A catalyst that has endured by switching between rich gas and lean gas of the following composition for 5 minutes and repeating the cycle of 30 minutes at 900°C and 30 minutes at 750°C 50 times is then heated at 2 second intervals using the same gas. A temperature increase test was conducted while switching the gas.

Rich gas Lean gas Co 4.7 High 0.7 High 020.
65% 4.65φ No (112 many 0.12 many H, 0,25
% CL23%C,H,1116%
C116%Co, 10% 10% F (,03% 3%) The results are shown in Table 1 below as the temperature at which each component of HC, Co, and NOX exhibits a purification rate of 50%.

Table 1 Installed in the exhaust system of an engine at a temperature (°C) that indicates a purification rate of 50%, catalyst gas temperature 850°C, bed temperature 9
After a 200-hour durability test at 00°C, the purification rate of HClC0 and NOx was measured using the same engine at a catalyst gas temperature of 400°C. The results are shown in Table 2 below.

Table 2 Purification rate of HC, Co and NOx (multiple) <Performance comparison test 2> 1.71 with 2 catalysts and 1a specification! gold capacity catalyst! 2/<Performance Comparison Test 3> The W composite oxides of Example 1 and Comparative Example 2 were heat-treated at 800°C for 10 hours in air, and then subjected to X-ray diffraction. The uniformity was evaluated by measuring the surface area. The results are shown in Table 3 below.

Table 3: Uniformity evaluation results A large umbrella value is evidence of the formation of a fine-grained perovskite phase.

As shown in Table 1 above, it can be seen that the catalyst of the example has a lower temperature at which a purification rate of 50% is achieved than the catalyst of the comparative example. Furthermore, as is clear from Table 2 above, the catalysts of the Examples have higher purification rates for HC, CO, and NOx than the catalysts of the Comparative Examples. The reason for these is that, as shown in Table 3, in the method of the present invention, citric acid is used during the preparation of perovskite type composite oxides, so that fine particles with a uniform composition can be obtained.

Furthermore, in the method of the present invention, a perovskite-type composite oxide and a refractory powder are used in combination, and one or both of them support a precious metal, so that it can be used in both rich and lean atmospheres. Shows excellent purification performance. This will be explained with reference to FIG.

FIG. 1 shows changes in properties of various exhaust purification catalysts due to changes in atmosphere when Pd is used as the noble metal. Figures 1 (a) to (C) are examples of catalysts in which alumina is used as the refractory powder and Pd is supported on the alumina. New (a)
Pd, which was dispersed at the time, causes sintering on the rich side and redistributes on the lean side. Therefore, this catalyst has high activity on the lean side, but low activity on the rich side.

Figures 1(d) to (0) are examples of catalysts in which Pd is supported on perovskite-type composite oxide powder.When new (d), part of the Pd is precipitated, and the rest is perovskite-type composite oxide powder. On the rich side, the perovskite-type composite oxide powder and precipitated Pd cause sintering, which causes a decrease in activity.However, solid solution Pd
precipitates, which causes an increase in activity. Further, on the lean side, the perovskite type composite oxide powder undergoes sintering, and the precipitated solid solution Pd becomes solid solution again, which causes a decrease in activity. However, the precipitated Pd that had been sintered is redispersed, which causes an increase in activity. Overall, a catalyst in which Pd is supported on perovskite-type composite oxide powder tends to have higher activity on the rich side. Figure 1 (9) to (i) are examples of catalysts obtained by the method of the present invention using a combination of perovskite type composite oxide powder and other refractory powders and supporting Pd on these. corresponds to Since citric acid is used in the method of the present invention, the perovskite type composite oxide obtained is uniform and in the form of fine particles. Also, since it is used in combination with a suitable refractory powder, sintering does not occur on both the rich side and the lean side.

Furthermore, the precipitated Pd on the rich side and the redispersed Pd on the lean side
Since each of d improves the activity, it becomes highly active on both the rich side and the lean side.

[Effects of the Invention] Since the method for producing an exhaust purification catalyst of the present invention has the above-described configuration, the catalyst obtained by the method of the present invention has various effects as described below.

The perovskite type composite oxide has a uniform composition and is stable, and is in the form of fine particles and has a large surface area, thus improving the durability and activity of the catalyst.

Further, when the exhaust atmosphere is rich, the noble metal-containing perovskite composite oxide powder decomposes and precipitates highly dispersed noble metals, becoming highly active.

On the lean side, the noble metal is dissolved in the perovskite complex oxide powder. By repeating this process, grain growth of the perovskite composite oxide powder and the noble metal contained therein can be prevented. Furthermore, the precious metal supported on the refractory powder is sintered on the Sochi side, redispersed on the Riin side,
This contributes to improving the activity on the cylinder side. Therefore,
The exhaust gas purification catalyst obtained by the method of the present invention is highly active on both the rich side and the lean side, and has excellent durability. In particular, when Pd is used as the noble metal, it is possible to take advantage of the characteristic that Pd is highly dispersed compared to PdO on the lean side, which has a practical advantage.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing changes in properties of various exhaust purification catalysts due to changes in atmosphere when Pd is used as the noble metal.

Claims (4)

[Claims] (1) Citric acid is added to an aqueous solution containing compounds of each element constituting the perovskite-type composite oxide to form a uniform aqueous solution, and the aqueous solution is evaporated to dryness and then fired to produce a perovskite-type composite oxide powder. A slurry containing the powder and a refractory powder supporting a noble metal (excluding perovskite composite oxide powder) is then prepared, and the slurry is then applied onto a refractory carrier, dried, and then fired. A method for producing an exhaust purification catalyst characterized by: (2) Citric acid is added to an aqueous solution containing compounds of each element constituting the perovskite-type composite oxide to form a uniform aqueous solution, and the aqueous solution is evaporated to dryness and then fired to form a perovskite-type composite oxide powder. Then, a slurry containing the powder and a refractory powder (excluding the perovskite composite oxide powder) is prepared, and the slurry is applied onto a refractory carrier, dried, and then fired to coat the refractory powder onto the refractory carrier. A method for producing an exhaust purification catalyst, comprising forming a mixed powder layer and then supporting a noble metal on the mixed powder layer. (3) Add citric acid to an aqueous solution containing compounds of each element constituting the perovskite-type composite oxide to form a uniform aqueous solution, and convert the aqueous solution into a refractory powder (excluding perovskite-type composite oxide powder) carrying a noble metal. ), dried and fired to form a perovskite-type composite oxide layer on at least the surface and to prepare a refractory powder supporting a noble metal, then prepare a slurry containing the refractory powder, and then prepare a slurry containing the refractory powder. A method for producing an exhaust purification catalyst, which comprises coating the catalyst on a refractory carrier, drying it, and then firing it. (4) adding citric acid to an aqueous solution containing compounds of each element constituting the perovskite-type composite oxide to obtain a uniform aqueous solution; impregnating the aqueous solution into a refractory powder (excluding the perovskite-type composite oxide powder); A refractory powder is prepared by drying and firing to form a perovskite-type composite oxide layer on at least the surface, and then a slurry containing the refractory powder is prepared, and then the slurry is applied onto a refractory carrier and dried and then fired. to form the refractory powder layer on the refractory carrier;
A method for producing an exhaust purification catalyst, which comprises then supporting a noble metal on the refractory powder layer.