JPS63240946A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To enhance resistance to poisoning properties and low-temp. activity by carrying Pd or the like on the inner layer of alumina layers coated on the surface of a mineral carrier and carrying Ti and Pt, etc., on the outer layer thereof to form a catalyst for purifying exhaust gas. CONSTITUTION:The alumina layer consisting of at least two layers is laminated on the surface of the base material of a mineral carrier and also Pd or Pd and Rh are carried on the inner layer of the alumina layer and oxide of at least one kind of metal selected from among Ti, Te, Si and Zr, and Pt or Pt and Rh are carried on the outer layer of the alumina layer to form a catalyst for purifying exhaust gas. In this catalyst, Pd liable to poisoning can be effectively utilized as a catalytic component and furthermore activity at a time of low temp. can be enhanced and also the cost of this catalyst can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) This invention simultaneously removes nitrogen oxides (NO, The present invention relates to a catalyst for purifying exhaust gas that is efficient, can be purified at lower temperatures, and is resistant to poisonous substances such as lead (Pb) and phosphorus (P).

(Prior art) Conventional catalysts for purifying this type of exhaust gas include platinum (Pt).
, rhodium 0? h) It has a structure in which the surface of an alumina layer supporting a catalytic noble metal such as palladium (Pd) is coated with a porous alumina layer. In this regard, for example, Japanese Patent Application Laid-Open No. 61-74650,
Since lead compounds are likely to be generated on the gas inlet side of a monolithic catalyst for exhaust gas purification, IW catalysts have been disclosed in which the surface of the catalyst layer on the gas inlet side of the carrier is coated with an IW layer made of activated alumina, for example. .

(Problems to be Solved by the Invention) However, in such conventional exhaust gas purification catalysts, catalysts are used to trap poisonous substances such as lead (Ilb) and phosphorus (P) in fuel oil. Because the structure was such that a porous alumina layer was coated on the surface side of the precious metal-supported alumina layer, even the alumina layer supporting platinum and rhodium, which is less susceptible to poisoning by lead and phosphorus, was covered, and the alumina layer with a thickness of 11° was coated. Since the catalyst purification rate is low and the coating layer is only alumina, it is difficult to remove lead, phosphorus, etc.
There was a problem that one signal was low.

(Means for Solving the Problems) This invention aims to replace only palladium, which is susceptible to poisoning by lead and phosphorus, or an alumina layer mainly supporting palladium and rhodium with titanium (Ti), tellurium (Te), and silicon (Si).
This is based on the finding that the above problems can be solved by coating with an alumina layer containing at least one metal oxide selected from the group consisting of ) and zirconium (Zr).

Therefore, in the exhaust gas purification catalyst of the present invention, the alumina layer coated on the surface of the inorganic carrier base material is composed of at least two layers, the inner layer supports palladium or palladium and rhodium, and the outer layer supports the alumina layer. It is characterized by supporting an oxide of at least one metal selected from the group consisting of titanium, tellurium, silicon, and zirconium, and platinum or platinum and rhodium.

In a preferred embodiment of the catalyst of the present invention, palladium or palladium and rhodium is supported on the activated alumina layer of the inner layer, and a group consisting of titanium, tellurium, silicon, and zirconium is supported on the first activated alumina layer of the outer layer. The inner layer is coated with an oxide of at least one metal selected from the following, and is further provided with a second activated alumina layer on which platinum or platinum and rhodium are supported.

(Function) Like platinum and rhodium, palladium has high activity as a three-way catalyst and is inexpensive. However, palladium is more susceptible to poisoning by lead in fuel than platinum and rhodium, and its activity is significantly reduced in the presence of lead. Therefore, in order to use palladium as a noble metal in a catalyst, it is necessary to provide a lead trapping layer. Generally, an alumina layer is used as a lead trapping layer, but alumina alone does not have sufficient trapping ability.

Looking at the reactivity of lead, it very easily forms compounds with oxides of metals such as Ti, Te, Si, and Zr. For example, TiO□ combines with lead to become PbTi0+. Therefore, the lead trapping layer is preferably an alumina layer containing an oxide of at least one metal selected from the group consisting of Ti, Te, Si, and Zr.

In this case, the total mixed star of the oxide of at least one metal selected from the group consisting of Ti, Te, Si, and Zr is 1 to 50% by weight, preferably 5 to 40% by weight in terms of metal, based on the alumina. It is better to This means that when 1% by weight of the metal oxide is ungrooved, there is little effect of improving the poisoning resistance even if the metal oxide is added, and even if the amount is more than 50% by weight, there is almost no effect of improving the poisoning resistance.

Moreover, the thickness of this trap layer is 10 μm to 50 μm.
It is better to set it to m. This is because if the diameter is less than 10 μm, the lead trapping effect is insufficient and the effect of improving poisoning resistance is reduced.
If the thickness is greater than 0 μm, the activity at low temperatures will decrease more than the effect of improving poisoning resistance.

Further, although palladium is superior to platinum in oxidizing CO and unsaturated hydrocarbons, its ability to oxidize saturated hydrocarbons is inferior, so it is necessary to use both palladium and platinum in the catalyst. However, if platinum and palladium coexist in the coating layer, they will form an alloy at high temperatures and lose their activity. Therefore, it is necessary to separate the coat layer carrying platinum and the coat layer carrying palladium. Further, since Rh is an essential component for NOX conversion and also has the function of suppressing sintering of palladium and platinum, it is desirable to support it in both coat layers.

By the way, the conventional playing card layer was provided on the alumina layer containing the catalytic noble metal component, that is, on the outermost surface of the coating layer, in order to prevent the catalytic noble metal component such as palladium, platinum, or rhodium from being poisoned by lead or the like.

However, the catalyst produced by this method has a problem of low activity at low temperatures. This is because platinum and rhodium, which are relatively resistant to poisoning such as lead, are covered with an alumina trap layer, so this trap layer becomes rate-limiting for diffusion and steals the temperature of the exhaust gas, making it less active at low temperatures. is low. Therefore, the trap layer must cover only the alumina layer mainly containing palladium, and the alumina layer mainly containing platinum must be provided on the surface side of the playing card layer.

From the above, the layer on the surface side of the alumina layer containing at least one metal oxide selected from the group consisting of Ti, Te, Si, and Zr is an alumina layer containing mainly platinum or platinum and rhodium. By using mainly palladium or an alumina layer containing palladium and rhodium as the inner layer, the poisoning resistance and low-temperature activity of the catalyst can be improved.

(Experimental Examples) The present invention will be described below with reference to Examples, Comparative Examples, and Test Examples.

An activated alumina support was impregnated with a cerium nitrate aqueous solution, dried, and then fired at 600°C in an air stream for 1.5 hours to obtain an activated alumina support containing 3.0% by weight of cerium based on alumina in terms of metal. I got it. Next, nitric acid acidic boehmite sol (
10% by weight HN in a 10% by weight suspension of boehmite alumina
Sol obtained by converting O3) 1240g
, 503 g of the above activated alumina carrier, 2 cerium oxide powder
Slurry A was obtained by putting 58 g into a ball mill bot and grinding for 8 hours.

Further, 168 g of the activated alumina carrier, 413 g of nitric acidic boehmite sol, and 123 g of titanium oxide powder were placed in a ball mill pot and ground for 8 hours to obtain slurry B.

Cordierite integrated carrier (400 cells, 1.71
) was coated with slurry A and dried at 650°.
C. for 2 hours in a stream of air to obtain 170 g/piece of coating layer. Using a palladium chloride solution and a rhodium chloride solution on this catalyst carrier, the impregnation method was used to add 0.95 g of palladium per catalyst and 0.0 g of rhodium per catalyst.
．． After supporting 10 g, it was fired at 600°C for 2 hours in a firing gas stream. The catalyst carrier was then coated with slurry B and then calcined at 650'C in an air stream for 2 hours to obtain a coating layer of 57 g/piece. Next, this catalyst carrier was coated with slurry A, and after drying, 65
It was baked at 0° C. for 2 hours in a stream of air, yielding 170 g/piece of coating layer. Using a chloroplatinic acid solution and a rhodium oxide solution, platinum was added to the catalyst carrier by an impregnation method.
0.95g per piece, 0.1 rhodium per catalyst
After supporting 0 g, it was fired at 600°C for 2 hours in a firing gas stream to form exhaust gas purifying catalyst A of this example.

In Example 1, titanium oxide powder 123 of slurry B
Catalyst B was formed in the same manner except that g was changed to 91 g of tellurium oxide (TeO□) powder.

Tip: In main example 1, titanium oxide powder 123 of slurry B
Catalyst C was formed in the same manner except that g was changed to 100 g of zirconium oxide (ZrOz) powder.

In disaster construction example 1, titanium oxide powder 123 of slurry B
A catalyst was formed in the same manner except that 162 g of silicon oxide (SiO□) powder was used instead of 162 g of silicon oxide (SiO□) powder.

In Example 1, titanium oxide powder 123 of slurry B
Catalyst 1 was formed in the same manner except that g was removed.

Comparison 1 Catalyst 2 was prepared in the same manner as in Example 1, except that slurry B was not coated and only the alumina layer containing platinum and rhodium and the alumina layer containing palladium and rhodium were used.
was formed.

In Comparative Example 1, palladium 0.95 g/piece,
The alumina layer supporting 0.10 g of rhodium was then coated on the catalyst carrier with slurry A, and then calcined in an air stream at 650°C for 2 hours to obtain a coating layer of 170 g/layer. This catalyst carrier was loaded with 0.95 g/piece of platinum and 0.10 g/piece of rhodium in the same manner as in Example 1, and then calcined at 600"C for 2 hours in a combustion gas stream. Next, a slurry was added to the catalyst carrier. After coating B, it was baked at 650'C in an air stream for 2 hours to give 57g/
A catalyst 3 was obtained by obtaining two coating layers.

, Comparison ①Catalyst 4 was formed in the same manner as in Example 1 except that the positions of supporting palladium and platinum were reversed.

After carrying out a durability test under the following conditions for Catalysts A-D obtained from Fukkasa Kai Examples 1 to 4 and Catalysts 1 to 4 obtained in Comparative Examples 1 to 4,
A performance evaluation test was conducted and the results are shown in Table 1.

Durability test specifications (1) Exhaust gas temperature 750°C (2) Space velocity (SV) Approx. 70,000 Hr-' (3) Endurance time 100 hours (4) Engine displacement 2.200cc (5) Fuel Leaded gasoline (Pb 12mg/1ls
G) (6) Endurance low emissions air-fuel ratio (A/F)
= 14.6■ Whisper Wipe Aijo Yu (1) Space velocity (SV) 38.80011r
” '(2) Engine displacement 2,0OOc
c (3) Fuel Unleaded gasoline (4) Large emission A/F = 14.6 (5) Concentration in exhaust gas HC: 2,600ppmCo: 6,00
0ppm No: 1,500ppm O□: 7,000ppm For evaluation, the catalyst inlet temperature was gradually raised from 200°C to 400″C (10°C/90sec), and the conversion rate was 50%.
The inlet temperature when becomes T. I closed it and compared it.

Table 1 (Effects of the Invention) As explained above, according to the present invention, only the alumina layer supporting palladium or palladium and rhodium, which is susceptible to poisoning by lead and phosphorus, is Ti,
It has a structure in which it is coated with an alumina layer containing an oxide of at least one metal selected from the group consisting of Te, Si, and Zr, and its surface is coated with platinum, which is relatively resistant to poisoning, or an alumina layer carrying platinum and rhodium. Because of that,
Palladium, which is susceptible to poisoning, can be effectively used as a catalyst component, and furthermore, the activity at low temperatures can be improved, and the cost of the catalyst can also be reduced.

Claims (1)

[Claims] 1. The alumina layer coated on the surface of the inorganic carrier base material is composed of at least two layers, the inner layer carries palladium or palladium and rhodium, and the outer layer consists of titanium, tellurium, silicon, and zirconium. A catalyst for exhaust gas purification, characterized in that it supports an oxide of at least one metal selected from the group consisting of platinum or platinum and rhodium.