JPH0398644A - Preparation of catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To obtain a catalyst for purifying exhaust gas having heat resistance by adding a specific noble metal to an inorg. catalyst carrier. CONSTITUTION:An inorg. catalyst carrier is immersed in a solution mixture of platinum and/or rhodium, iridium and/or ruthenium and citric acid. Next, said carrier is dried, calcined and reduced to support the above-mentioned noble metals on the inorg. catalyst carrier. The catalyst for purifying exhaust gas thus obtained has heat resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing an exhaust gas purifying catalyst, and particularly to a method for producing an exhaust gas purifying catalyst in which the heat resistance of the metal catalyst itself is improved.

[Conventional technology]

Catalysts that purify harmful substances such as hydrocarbons, carbon monoxide, and nitrogen oxides contained in automobile exhaust gas are made by supporting precious metals such as platinum, palladium, and rhodium on inorganic catalyst carriers such as alumina and cordierite. It is used. This is because noble metals such as platinum, palladium, and rhodium are expensive but have excellent catalytic performance and durability. One of platinum, palladium, and rhodium is used as the main catalyst, and ruthenium, rhodium,
A catalyst supporting one or more of palladium, osmium, and iridium as a promoter component has been disclosed as having excellent low-temperature activity (Japanese Patent Laid-Open No. 103484/1984).
.

In addition, an inorganic support is usually composed of a catalyst support layer in which a porous inorganic oxide such as alumina is coated on the surface of a carrier base material such as pellet type or monolith type cordierite. A technique has been disclosed in which rare earth elements and transition metals are added to alumina to stabilize it and improve heat resistance (Japanese Patent Application Laid-open Nos. 57-87839, 4B-18180, and 61-3531). No. 60-7537, U.S. Patent No. 3003020
No., No. 3951860, No. 4170573, etc.).

[Problems to be Solved by the Invention] Although the above-mentioned precious metals have excellent durability, the usage conditions for autocatalysts are extremely severe, reaching nearly 1000°C during high-speed driving, and under such high-temperature conditions, these precious metals degrade. However, it causes grain lengthening and thermal deterioration. Therefore, there is still a demand for improvement in the heat resistance of catalysts.

The above-mentioned technique of adding rare earth metals or transition metals to the catalyst supporting layer was proposed for the purpose of improving the heat resistance of the catalyst in response to such demands. However, since these stabilize the catalyst carrier and do not act directly on the noble metal, grain growth of the noble metal cannot be sufficiently suppressed, and the effect of improving heat resistance is not sufficient.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a method for producing an exhaust gas purifying catalyst that directly acts on noble metals to stabilize them and improve heat resistance.

[Means to solve the problem]

In order to achieve the above object, the present invention immerses an inorganic catalyst carrier in a mixed solution of platinum and/or rhodium, iridium and/or ruthenium, and citric acid, and after drying,
Provided is a method for producing an exhaust gas purifying catalyst, which comprises calcination and reduction.

There are no particular restrictions on the inorganic catalyst carrier used, and it may be any conventionally used carrier, but pellet-type, honeycomb-shaped, or foam-shaped monolithic carriers are typical. Preferably, a catalyst support layer made of a porous inorganic material such as activated alumina is formed on the surface of a N\t-free carrier base material such as light.

The catalytic metals include one or both of Pt and Rh, and Ru.
, Ir, or both, and the ratio of the supported amounts of Ru and Ir to the supported amounts of Pt and Rh is 0.01 to 0.01 in terms of atomic ratio.
It is set to 0.5. In the present invention, a mixed solution of Rt and/or Rh, Ru and/or Ir, and citric acid is used to support these metals on the catalyst carrier. PL supported by using a mixed solution with citric acid
, Rh and Ru, Ir have the effect of promoting solid solution. The concentration of citric acid depends on the Pt, Rh. Ru,
It is preferable to use it in an amount equivalent to the concentration of r r. This is because citric acid is thought to complex pt and the like, so an amount of citric acid suitable for complexing the metal used is used.

As the solutions of Pt, Rh, Ru, and Ir, known solutions such as nitrates of these metals are used.

After the inorganic catalyst carrier is immersed in the mixed solution and dried, it is subjected to calcining and reduction treatment. Calcination is in the air at 200-5
The reduction is carried out at 00°C for about 1 to 5 hours, for example, 1 to 5 hours.
This can be carried out by processing in a 00% hydrogen gas atmosphere at 300 to 800°C for about 1 to 5 hours.

[For production]

The melting points (°C) of Ru and Ir are higher than the melting points (°C) of Pt and Rh, as shown below, and a solid solution is formed between them.

Pt Rh Ru rr1769
1960 2250 2243PL, Rh is R
Forming a solid solution with u and Lr increases the melting point and has the effect of suppressing grain growth. Also, Ir, Ru
is NO. It is known that it is a catalytic metal that is excellent in reduction reactions, HC reforming reactions, etc., but oxides (RuOz, I
Pt, which is stable even in the form of an oxide, has been developed in the present invention.
By forming a solid solution with Rh, it does not volatilize even under oxidizing conditions, and as a result, I
r, Ru can be used. Therefore, the catalytic performance of these elements can also be expected.

In the present invention, citric acid and PL, Rh and Ru.
By using a mixed solution of Ir, Pt, Rh and R
Solid solution with u and Ir is promoted, and the above effects are achieved. Even if citric acid is not used, Pt, Rh, Ru, and Ir may partially form a solid solution, but this is not sufficient to suppress grain growth and prevent volatilization of r2u and Ir.

[Example] 100 parts by weight of activated alumina was added to a mixed solution of 30 parts by weight of an aqueous solution containing 40 wt% of aluminum ξ nitrate and 100 parts by weight of water with stirring, and the mixture was thoroughly stirred to prepare a slurry. A honeycomb-shaped monolithic catalyst carrier base material made of cordierite was immersed in this slurry for 1 minute, then pulled up, and the excess slurry was blown off with an air flow.
After drying at 0°C for 1 hour, it was fired at 600°C for 2 hours.

Next, this monolithic carrier was immersed in an aqueous solution containing dinitrodiammine platinum, ruthenium nitrate, and an equivalent amount of citric acid, and then pulled out and the excess liquid was blown off.
Vacuum-dried at °C for 12 hours using a rotary evaporator, calcined at 300 °C for 2 hours, and heated at 600 °C for 3 hours.
Reduction treatment was performed in a z100% air flow to obtain catalyst 1a.

Further, a catalyst 1b in which ruthenium was replaced with iridium was obtained by the same operation.

Hokumu Tomodo An alumina-coated carrier prepared as in Example 1 was immersed in an aqueous solution containing dinitrodiammine platinum and ruthenium nitrate to absorb it, pulled out, blown off the excess liquid, and heated at 100'C for 1 hour. After drying, reduction treatment was performed at 600°C for 3 hours in a Hzloo% air flow to obtain a catalyst IC. Further, a catalyst 1d in which ruthenium was replaced with iridium was obtained by the same operation.

Uragai Group 1 Perform the same operation as in Example 1, replacing dinitrodiammine platinum with rhodium nitrate. Catalysts 2a and 2b were obtained. Comparison Aim 1 Catalysts 2c and 2d were obtained in the same manner as in Comparative Example except that dinitrodiammine platinum was replaced with rhodium nitrate.

The same operation was carried out by replacing the solution of Main Example 1 with a mixed solution of dinitrodiane≧platinum and rhodium nitrate to prepare catalysts 3a and 3.
'b got.

Northern Comparison Plate 1 The same operation was carried out except that the solution of Comparative Example 1 was replaced with a mixed solution of dinitrodiane platinum and rhodium nitrate, and catalyst 3c.
I got 3d.

Dinitrodiammine platinum, ruthenium nitrate,
The same operation was performed by replacing the aqueous solution containing citric acid in an equivalent amount with dinitrodiammine platinum, ruthenium nitrate, iridium nitrate, and an aqueous solution containing citric acid in an equivalent amount thereto to obtain catalyst 4a.

Catalyst 4b was obtained by performing the same operation except for replacing the solution of Comparative Example 1 with a mixed solution of dinitrodiammine platinum, ruthenium nitrate, and iridium nitrate.

Each of the exhaust gas purification catalysts obtained in the above Examples and Comparative Examples was added to a model gas simulating automobile exhaust gas at 9%
A durability test was conducted by exposing the sample to 00'C*5 hours. Using a model gas equivalent to stoichiometry, No. The temperature when the amount reached 50% was measured. Furthermore, after the durability test, the catalyst is decomposed, the catalyst support layer is stripped off, and the alkali
The precious metals were concentrated and the X#A diffraction was measured.

The noble metal particle size was measured from the half width of the diffraction peak. The lattice constant was also measured. The results are shown in Table 1.

Since a catalyst containing only Rh is not practical, durability tests were not conducted.

As is clear from the table, in the case of using citric acid, the particle size of the catalyst is small, grain growth is suppressed, and as a result, the catalyst performance is excellent. At the same time, it was confirmed by X-ray diffraction that the use of citric acid promoted the solid solution of Ir, Ru and Pt, Rb. Moreover, the effect of suppressing grain length can be considered to be due to the promotion of solid solution formation.

〔Effect of the invention〕

According to the present invention, by using citric acid, Pt,
It is possible to promote the solid solution of Rh with Ru and Ir, thereby suppressing the grain size of the catalyst and preventing the volatilization of Ru and Ir, thereby improving the durability and performance of catalysts for purifying automobile exhaust gas. can.

Claims (1)

[Claims] 1. Production of a catalyst for exhaust gas purification, characterized in that an inorganic catalyst carrier is immersed in a mixed solution of platinum and/or rhodium, iridium and/or ruthenium, and citric acid, dried, then calcined and reduced. Method.