JPH044043A - Heat-resistant catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To obtain a catalyst having high oxygen storage performance which can remove hydrocarbon, CO, and NOX at high conversion rate by preparing a multiple oxide carrier by simultaneous precipitation of mixed metal in a liquid phase and depositing noble metal on this carrier. CONSTITUTION:Aluminum salt is dissolved in water, to which cerium salt and zirconium salt are added and sufficiently stirred, and then aq. ammonia soln. is added to the mixture to obtain precipitate. This precipitate is baked to obtain a multiple oxide, which can be used as a carrier having Xwt.% (CeO2.ZrO2).Al2 O3 composition. (X is 5-40 and the weight ratio of CeO2 to Zr2 ranges from 70:30 to 20:80.) Then noble metal such as platinum, rhodium, etc., is made to deposit on this carrier. The obtd. catalyst is a ternary catalyst, having high oxygen storage property even after enduring high temp., and has high conversion rate.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a heat-resistant catalyst for exhaust gas purification, which is used to purify exhaust gas, particularly high-temperature exhaust gas discharged from internal combustion engines such as automobiles. It is something.

(Prior art) Conventional catalysts for exhaust gas purification generally consist of a catalytic metal with high catalytic activity (mainly noble metals such as platinum, palladium, and rhodium), a carrier for supporting the metal, and a group that serves as a support for the carrier. It is known that it is made of wood.

These carriers are required to have a large specific surface area and a small decrease in specific surface area even after durability at high temperatures; - alumina, silica, zirconia, titania, etc. are often used for boat fishing.

Regarding catalysts for purifying automobile exhaust gas, a technique is known in which the exhaust gas purifying performance is improved by incorporating rare earth elements and transition metals other than the above-mentioned oxides into the carrier.

For example, a technique using cerium is disclosed in Japanese Patent Laid-Open No. 157347/1983. This utilizes the oxygen storage effect of cerium contained in the carrier, and due to the effect of the reaction shown in equation (1) below,
The purpose is to improve the performance of purifying exhaust gas components such as IC and CO1NOx by relaxing the oxidizing atmosphere and reducing atmosphere.

Ce0z-” CeO,-, +χ/2 oz ---
-(1) (Problem to be solved by the invention) However, in such exhaust gas purifying catalysts, when exposed to high temperatures exceeding 800°C, sintering of the catalyst metal and reduction of the specific surface area of the catalyst carrier occur. In addition to the cause of deterioration, the exhaust gas purification performance is significantly decreased due to a decrease in the oxygen storage capacity of cerium. Therefore, the present invention overcomes the problems of the prior art,
IC with high oxygen storage capacity even after durability at high temperatures
, C01No, and the like at a high conversion rate.

(Means for Solving the Problems) In order to achieve the above object, the present inventor conducted extensive research on improving the heat resistance of ceria's oxygen storage ability, and found that after dissolving aluminum salt in water, cerium salt A mixture of aluminum, cerium, and zirconium hydroxides obtained by adding and dissolving a zirconium salt, stirring thoroughly, and then adding an ammonia aqueous solution to a composite oxide obtained by firing at 500°C or higher, It was discovered that a catalyst supporting a catalytic metal has a high oxygen release ability even after being calcined at a high temperature of 850°C, and based on this knowledge, the present invention was achieved.

That is, the heat-resistant catalyst for exhaust gas purification of the present invention is obtained by the method described above and has the following compositional formula % formula % (in the formula, X weight % is Aft (with respect to the total weight of h, Ce0z, and Zr0t) Exhaust gas made by supporting at least one noble metal selected from the group consisting of platinum, rhodium and palladium on a catalyst carrier made of a composite oxide represented by (representing the ratio of the total weight of CeO□ and ZrO2) It is a heat-resistant catalyst for purification.

Regarding the metal salts used in the present invention, any of aluminum, zirconium, and cerium can be used as long as they are water-soluble and cause hydroxide precipitation in a basic solution. For example, nitrates, Examples include sulfate and acetate.

Regarding the mixing ratio of aluminum, cerium, and zirconium, in the final composite oxide, the total weight of cerium oxide and zirconium oxide is in the range of 5 to 40% by weight of the total weight including aluminum oxide, that is, X in the above composition formula. Mix so that the weight percent X is in the range of 5 to 40. If the total content of cerium oxide and zirconium oxide is less than 5% by weight, sufficient oxygen storage capacity cannot be obtained,
If the amount is more than 40% by weight, the crystal growth of cerium oxide and zirconium oxide becomes significant after high-temperature durability, and the specific surface area decreases greatly, resulting in a decrease in exhaust gas purification performance when used as a catalyst.

Regarding the mixing ratio of cerium oxide and zirconium oxide, the weight ratio of cerium oxide and zirconium oxide is 30.
It is preferable to mix in the range of: 70 to 20: 80.

If the content of zirconium oxide is less than 30% by weight, the effect of improving the durability of oxygen storage capacity by containing zirconium oxide cannot be expected, and the crystal growth of zirconium oxide becomes significant after high-temperature durability when the content exceeds 80% by weight. Since the surface area decreases greatly, when used as a catalyst, the exhaust gas purification performance decreases.

When dissolving a metal salt, it is necessary to dissolve it in a sufficient amount of water. The mixing temperature is not particularly limited;
Although room temperature is sufficient, heating may be necessary to increase the solubility of the metal salt. Regarding the mixing time, it is preferable to carry out the mixing for at least 1 hour or longer in order to ensure that the metals contained are sufficiently dispersed and are easily formed into a composite when fired.

Regarding the addition of ammonia aqueous solution, 3 to 7 mol/l is added in order to prevent precipitation caused by high concentration of base and severe precipitation of hydroxide.
It is preferable to use one with a concentration of about 100%. As the ammonia aqueous solution is added, the pH increases, but when the pH reaches 8 to 9, the dropping of the ammonia aqueous solution is stopped and the precipitate is aged.

There are no particular restrictions regarding filtration, and any filtration method can be selected.

The present invention is characterized by processing at 500°C or higher; however, before processing at 500°C or higher,
Pretreatment may be performed at the following temperatures. There are no particular restrictions on the temperature and atmosphere in the pretreatment.

CeO□-ZrO□-A l obtained as above
An exhaust gas purification catalyst in which at least one noble metal selected from the group consisting of platinum, rhodium, and palladium is supported on a zo3 composite oxide exhibits high purification performance even after durability at high temperatures.

(Effects of the Invention) In the present invention, since the method of simultaneously precipitating the metals mixed in the liquid phase is used, cerium can be dispersed in the form of fine particles in the precipitated state.

Further, when forming a composite oxide by firing, aluminum oxide and zirconium oxide are also produced simultaneously with cerium oxide, so cerium is incorporated into aluminum or zirconium while being dispersed in the form of fine particles. As a result, we were able to synthesize a material with high oxygen storage capacity and high specific surface area even after thermal endurance.

That is, by supporting a catalytic metal on this composite oxide, it is possible to synthesize a catalyst that has high activity even after high-temperature durability, and it also greatly contributes to catalyst durability.

(Examples) The present invention will be described in more detail below based on Examples and Comparative Examples.

Ohba ■ Soil In a 5N plastic container, aluminum nitrate nonahydrate (Aj! (NO:+)+・98zO) 600.
After adding 0 g and 2.000 g of water and stirring at room temperature for 30 minutes, a cerium nitrate aqueous solution (Ce concentration 18.7% by weight) was added.
) 41.6g and zirconium nitrate aqueous solution (Zr concentration 1
8.5% by weight) was added and stirring was continued for 120 minutes. Then, add a 5 mol/l ammonia aqueous solution until the pH is 9.
The mixture was added until the mixture was dissolved, and the hydroxide was precipitated by suction filtration. This precipitate was dried in an oven at 150°C for 15 hours, and then calcined for 2 hours at 500°C in a stream of air to prepare a composite oxide.

The composite oxide prepared above was impregnated with platinum using a dinitrodiammine platinum solution so that the supported amount was 1.0% by weight, and dried in an oven at 150°C for 3 hours.
A platinum-supported composite oxide was prepared by firing at 400''C for 2 hours in an air stream.

Furthermore, this platinum-supported composite oxide was heated at 850°C to
Catalyst 1 was obtained by calcination in an air stream for an hour.

When the crystalline state of catalyst 1 was confirmed by XRD, it was found that ceria, zirconia, and alumina were composite.

The oxygen release behavior of Catalyst 1 was investigated from the amount of hydrogen consumed by temperature programmed reduction (TPR). The obtained TPR curve is shown in FIG.

A platinum-supported composite oxide was prepared in the same manner as in Example 1, except that the platinum-supported concentration was 1.5% by weight.

A 1.0% by weight rhodium-supported composite oxide was prepared in the same manner as in Example 1, except that a rhodium nitrate solution was used instead of the dinitrodiammine platinum solution.

875 g of this platinum-supported composite oxide, 585 g of cerium oxide
Activated alumina 20 whose main component is g, r-alumina
0g, nitric acid acidic boehmite sol (boehmite alumina 1
150 g of the sol obtained by adding 10 wt % HNO3 to a 0 wt % suspension was put into a ball mill and ground for 8 hours to obtain a slurry. The obtained slurry was transferred to a monolithic carrier base material (1.3L 400 cells).
After coating and drying, it was baked at 400° C. for 2 hours in an air atmosphere. The coating amount at this time was set to 210 g/piece. Furthermore, the above rhodium-supported composite oxide LOOOg
, 180 g of nitric acidic boehmite sol, and 530 g of activated alumina powder whose main component is r-alumina were placed in a ball mill pot, and ground for 8 hours. The resulting slurry was applied in an amount of 50 g.
After coating and drying, the catalyst was calcined at 400° C. for 2 hours in an air atmosphere to prepare catalyst 2.

1 The amount of aluminum nitrate nonahydrate in Example 1 was 67
Catalyst 3 was prepared in the same manner as in Example 2, except that the amount of the cerium nitrate aqueous solution was 30.3 g, and the amount of the zirconium nitrate aqueous solution was 12.2 g.

The amount of aluminum nitrate nonahydrate in Example 1 was 44
4.0 g, the amount of cerium nitrate aqueous solution 104.0 g,
Catalyst 4 was prepared in the same manner as in Example 2 except that the amount of the zirconium nitrate aqueous solution was changed to 65.0 g.

First, 500 g of water was placed in a 21-volume plastic container, 76.4 g of an aqueous cerium nitrate solution and 130.0 g of an aqueous zirconium nitrate solution were added, and stirring was continued. 30 minutes later T
-Activated alumina powder 150 whose main component is alumina
g was added thereto, stirred for 60 minutes, dried and fired in the same manner as in Example 1 to prepare a composite oxide.

The composite oxide prepared by the above impregnation method was supported with 1.0% by weight of platinum in the same manner as in Example 1, then dried and fired, and the catalyst was subjected to durability in air at 850°C for 4 hours. was designated as catalyst A.

The TPR curve obtained for catalyst A is shown in FIG.

Northern Comparison I A platinum-supported composite oxide was prepared in the same manner as in Comparative Example 1 except that the platinum-supported concentration was 1.5% by weight.

A 1.0% by weight rhodium-supported composite oxide was prepared in the same manner as in Comparative Example 1, except that a rhodium nitrate solution was used instead of the dinitrodiammine platinum solution.

Catalyst B was prepared in the same manner as in Example 2, except that the above platinum-supported composite oxide and rhodium-supported composite oxide were used.

Hokkaien 1 The amount of cerium nitrate aqueous solution in Comparative Example 1 was 51.1
Catalyst C was prepared in the same manner as in Comparative Example 2, except that the amount of zirconium nitrate aqueous solution was changed to 20.0 g.

1330 g of a 1.5% by weight palladium loading composite oxide prepared using a dinitrodiammine palladium solution instead of a dinitrodiammine platinum solution, and activated alumina 2
Catalyst 5 was prepared in the same manner as in Example 2, except that 30 g of nitric acid boehmite sol and 150 g of nitric acid acidic boehmite sol were used.

A 1.5% by weight palladium supported compound was prepared in the same manner as in Comparative Example 2, except that a dinitrodiammine palladium solution was used in place of the Kitakkiyoto dinitrodiammine platinum solution.

Example 5 except that the above palladium-supported composite oxide was used
A catalyst was prepared in the same manner as above.

TPR measurement conditions for catalysts 1 and A are shown below.

Sample amount: 300 ■ Gas flow rate: 100d/+win Temperature increase rate: 10'' (:/min Measurement temperature: room temperature to 850°C Catalysts 2 to 5 and catalysts B, C, and D were durable under the following conditions After conducting the test, a performance evaluation test was conducted.The results are shown in Table 1.

Durability catalyst Integrated noble metal catalyst Catalyst outlet gas temperature 850°C Space velocity Approximately 70,000 Hr-' Durability time
100 hour engine displacement 2200 cc fuel
Unleaded gasoline durable log gas atmosphere CO0.4~0.6%0□0
．． 5±0.1% NO1001000 pp 2500ppm COz 14.9±0.1% Standing IJb11 vehicle Cedric Displacement 2000 cc (10% H
2/^r)

[Brief explanation of the drawing]

FIG. 1 is a curve diagram showing the relationship between the amount of oxygen released and the temperature in the temperature-promoted conversion method (TPR) of Catalyst 1 of Example 1 and Catalyst A of Comparative Example 1.

Claims (1)

[Claims] 1. In an exhaust gas purification catalyst having a catalyst support layer and a catalyst support metal, the catalyst support layer contains aluminum oxide, cerium oxide, and zirconium oxide obtained by a coprecipitation method, and these oxides have the following composition formula: X weight% (CeO_2・ZrO_2)・Al_2O_3
(X weight% in the formula is Al_2O_3, CeO_2 and Zr
It shows the ratio of the total weight of CeO_2 and ZrO_2 to the total weight of O_2, where X is 5 to 40, and further CeO_
2 and ZrO_2 in a weight ratio of 70:30 to 20:80), and consists of CeO_2, ZrO_2, Al_2O_
Exhaust gas characterized by having excellent oxygen storage properties after durability by forming three of the above three into a composite oxide, and in which the catalytic metal is at least one noble metal selected from the group consisting of platinum, palladium, and rhodium. Heat-resistant catalyst for purification.