JPH0356140A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To enhance heat resistance by supporting a catalytic metal composed of a solid solution of one or more kind of a component among platinum, plalladium and rhodium and one or more kind of a component among niobium, tantalum and tungsten by an inorg. catalyst carrier. CONSTITUTION:There is no special limit as an inorg. carrier base material but a general one wherein a catalyst support layer composed of a porous inorg. substance such as alumina is formed on the surface of cordierite having a pellet, honeycomb or monolithic shape is used. On this carrier, a metal catalyst constituted of a solid solution of one or more kind of a component among platinum, palladium and rhodium and one or more kind of a component among niobium, tantalum and tungsten is supported. Or, a mixture of platinum or the like and niobium or the like may be supported by the carrier so as to form the solid solution at the time of the use of the catalyst at high temp. By this method, the m.p. of the catalystic metals is made high and particle growth is suppressed and heat resistance is enhanced.

Description

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

[Prior technology] Precious metals such as platinum, palladium, and rhodium are supported on inorganic catalyst carriers such as alumina and cordierite as catalysts for purifying harmful substances such as hydrocarbons, carbon monoxide, and nitrogen oxides contained in automobile exhaust gas. A catalyst is used. This is because noble metals such as platinum, palladium, and rhodium are expensive but have excellent catalytic performance and durability. In addition, an inorganic catalyst support is usually composed of a catalyst support layer in which the surface of a support material such as peresote type or monolith type cordierite is coated with a porous inorganic oxide such as alumina. A technique has been disclosed in which rare earth elements and transition metals are added to stabilize this and improve heat resistance (Japanese Patent Laid-Open No. 57-87839,
4B-18180, 61-3531,
Japanese Patent Publication No. 60-753'7, U.S. Patent No. 30030
20, 3951860, 4170573, etc.). Furthermore, Japanese Patent Laid-Open No. 190644/1989 proposes a technique for improving heat resistance by dispersing catalytically active particles made of noble metal particles and oxide particles of base metals such as magnesium and niobium on an inorganic catalyst carrier. ).

[Problem to be solved by the invention]

Although the above-mentioned precious metals have excellent durability, the usage conditions for autocatalysts are extremely strict, reaching temperatures close to 1000°C during high-speed driving, and under such high-temperature conditions, even these precious metals can become grainy or elongated, causing thermal deterioration. cause 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, they cannot sufficiently suppress the particle length of the noble metal and are not sufficiently effective in improving heat resistance.

In addition, a catalyst in which base metal oxide particles are dispersed on noble metal particles has the risk of deactivating the catalytic activity of the noble metal because it disperses substances with poor catalytic activity on the surface of the noble metal. It cannot be expected to be effective enough to suppress growth.

Therefore, an object of the present invention is to provide an exhaust gas purifying catalyst that acts directly on noble metals to stabilize and improve heat resistance.

[Means for Solving the Problems] In order to achieve the above object, the present invention includes one or more of platinum, palladium and rhodium and one or more of niop, tantalum and tungsten in an inorganic catalyst carrier. To provide a catalyst for purifying exhaust gas, which is characterized in that it supports a catalytic metal formed of a solid solution with a catalytic metal.

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 alumina is formed on the surface of an inorganic carrier base material such as light.

The metal catalyst is a solid solution of one or more of platinum, palladium, and rhodium and one or more of niobium, tantalum, and tungsten. Platinum, etc., and niobium, tantalum, and tungsten are thought to suppress grain length by forming a solid solution, but when producing a catalyst, platinum, etc., and niobium, tantalum, and tungsten are formed as a solid solution. It is not necessary to support them, and they may simply be supported as a mixture so that a solid solution forms when the catalyst is used at high temperatures. The atomic ratio of one or more of niobium, tankard, and tungsten to one or more of platinum, palladium, and rhodium is 0.01 to 0.5.
It is preferable to use an amount in the range of . The amount of two ops etc. is 0
If the amount is less than 0.01, there is no effect of improving heat resistance, and if the amount of niobium is more than 0.5, it will not form a solid solution with platinum or the like, so no further effect of suppressing grain length can be expected.

A conventional method can be used to support the metal catalyst on an inorganic support. The amount of metal catalyst supported is the same as in the conventional case, but usually 0.01 wt% to 5 wt% is supported on the carrier.

[For production]

The melting point of Niobium etc. is Niobium 2470'C, tantalum 299'C.
Platinum (1769°C), palladium (1552°C), rhodium (196°C) at 0°C, tungsten 3400"C
It has a higher melting point than that of 0'C) and forms a solid solution with platinum, etc.

As a result, platinum etc. and niobium etc. form a solid solution on the catalyst of the present invention, and the melting point of the catalyst metal increases.

If the melting point is increased to -m, the grain length is also suppressed, so that the catalyst of the present invention has improved heat resistance compared to conventional metal catalysts made of platinum or the like.

〔Example〕

To a mixed solution of 30 parts by weight of an aqueous solution containing 40 wt% of aluminum occilate and 100 parts by weight of water, 100 parts by weight of activated alumina (surface area 50 to 200 rrf/g) was added 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 lifted up, the excess slurry was blown off with an air flow, dried at 200"C for 1 hour, and then at 600°C for 2 hours. It was incinerated.

Next, this monolithic support was immersed in an aqueous solution containing dinitrodiammine platinum, palladium chloride, and/or rhodium chloride and niobium chloride for 1 hour, pulled out, blown off excess moisture, and dried at 200°C for 1 hour to prepare the catalyst IA. I got it.

By the same operation, by changing the concentration of the metal salt solution and changing the type of metal salt (tantalum pentachloride, tungsten hexachloride), the metal combination catalysts IB~IG, 2A~2 shown in Table 1 were prepared.
C, and 3A to 3C were obtained. In the same manner as above, except that a noble metal solution containing no Nb, Ta, and W was used, a catalyst containing no Nb, Ta, and W was obtained as a comparative example.

Each of the obtained exhaust gas purification catalysts was installed in the exhaust system of a 3L inline 6-cylinder engine, and the catalyst bed temperature was 950°C.
A 50-hour durability test was conducted at an air-fuel ratio of 14.6.

After that, each catalyst was tested using the same engine as in the durability test, at 2000 rpm/-360 mmHg,
The purification rates of IC, Go, and NOX were measured under the conditions of A/F = 14.6 and human gas temperature of 400''C. The results are shown in Table 1.

Furthermore, the catalyst after durability is decomposed and the catalyst support layer is peeled off.
The alumina was dissolved and the noble metal was concentrated by chemical treatment, and X-ray diffraction measurements were performed, and the noble metal particle size was measured from the half-width of the diffraction peak. The results have been added to Table 1.

As is clear from Table 1 below, the particle size of the catalyst to which Nb, Ta, and 1 are added is small, the grain length is suppressed, and as a result, the catalyst performance is excellent.

Similarly, by X-ray diffraction, Nb, Ta, and W are PL, Pd
, it was confirmed that Rh and solid solutions were present. The grain length suppression effect is thought to be due to the formation of this solid solution.

From the above studies, it is clear that the catalyst of the present invention has extremely excellent durability under high temperatures.

[Effects of the Invention] The exhaust gas purifying catalyst of the present invention was able to significantly improve the heat resistance of the metal catalyst by adding niobium, tantalum and/or tungsten to the noble metal catalyst of platinum, palladium and/or rhodium.

Claims (1)

[Claims] 1. A catalyst metal comprising a solid solution of one or more of platinum, palladium and rhodium and one or more of niobium, tantalum and tungsten is supported on an inorganic catalyst carrier. Catalyst for exhaust gas purification.