JPS60232253A - Monolithic catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To obtain a monolithic catalyst for purifying exhaust gas wherein the purification capacity and durability of a Pt-Pd-Rh catalyst are enhanced, by suppressing the particle growth of palladium to eliminate the adverse influence of palladium upon platinum or rhodium. CONSTITUTION:The titled catalyst forms a columnar shape and is provided with a large number of cells from the inlet side of exhaust gas toward the outlet thereof, wherein an alumina layer is formed to the inner wall surfaces of said cells and catalytic components are supported by said alumina layer. The aforementioned alumina layer is constituted of a first alumina layer 2 supporting palladium and neodium inside the inner wall surface of each cell and a second alumina layer 3 supporting platinum and rhodium in the surface side thereof.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a monolith catalyst for purifying exhaust gas of an internal combustion engine.

[Prior art]

Catalysts for purifying exhaust gas from internal combustion engines, particularly automobiles, are required to have extremely high performance in terms of durability, purification performance, and the like. Traditionally, monolithic catalysts and granular catalysts have been used as exhaust gas purification catalysts, but recently, monolithic catalysts have the advantage of having a smaller heat capacity, better warm-up performance, and lower back pressure than granular catalysts. As a result, monolithic catalysts are becoming widely adopted. This monolithic catalyst has activated alumina supported on a cordierite monolithic support having an integrally molded structure, and then an active component having a catalytic action (usually a noble metal) supported thereon.

The catalyst components include platinum (Pt), rhodium (Rh
), palladium (Pd), and other noble metals are used.

When a monolithic catalyst supporting these catalyst components is attached to a catalytic converter and exhaust gas is passed through, the harmful substances contained in the exhaust gas such as hydrocarbons (HC), -carbon oxides (Co) and nitrogen oxides are removed. (NOx) is efficiently purified by oxidation or reduction reaction. The three main harmful substances contained in exhaust gas are hydrocarbons, carbon monoxide, and nitrogen oxides, and a catalyst that can purify these three components at once is called a three-way catalyst. There is.

Among the noble metals constituting the three-way catalyst, platinum or palladium is necessary for the oxidation of hydrocarbons and carbon monoxide, and platinum in particular has excellent activity and durability.

On the other hand, rhodium is necessary for the reduction of nitrogen oxides.

Therefore, Pt-Rh catalysts are used as three-way catalysts, but there is an urgent need to reduce the amount used from the viewpoint of resources and prices, and one method is to use the same precious metals but with resources and prices. Studies are underway on a Pt-Pd-Rh based catalyst in which palladium, which has high activity in oxidation reactions, is used in place of platinum.

By the way, Pt, -Pd-Rh based catalysts have the advantage of less deterioration in oxidizing atmospheres and lower purification start temperature compared to Pt-Rh based catalysts, but on the other hand, palladium tends to grow grains in reducing atmospheres. Its use has been limited because it reduces the surface area and also covers the surface of platinum and rhodium, reducing the durability of the catalyst.

Therefore, in order to prevent palladium from having an adverse effect on platinum and rhodium in Pt-Pd-Rh based catalysts, the applicant separated the alumina layer supporting palladium and the alumina layer supporting platinum and rhodium separately. (Japanese Unexamined Patent Publication No. 146441/1982: Japanese Patent Application No. 1983-1983)
No. 27455). According to this proposal, the adverse effect of palladium on platinum and rhodium can be significantly reduced compared to conventional methods, thereby improving the durability and purification performance of the catalyst.

However, although this method can reduce the adverse effect of palladium on platinum and rhodium, there is a problem in that it cannot suppress the grain growth of palladium itself.

[Purpose of the invention]

The present invention has been made to solve the problems of the prior art described above, and an object of the present invention is to suppress the grain growth of palladium and eliminate the negative influence of palladium on platinum and rhodium. The objective is to improve the purification performance of Rh-based catalysts.

[Structure of the invention]

According to the present invention, this object is achieved by the following monolithic catalyst for purifying exhaust gas.

That is, the monolith catalyst for exhaust gas purification of the present invention has a columnar shape, and a large number of cells are provided from the exhaust gas inlet side to the outlet side, and an alumina layer is formed on the inner wall surface of the cell. A monolithic catalyst for exhaust gas purification in which a catalytic component is supported on the monolithic catalyst, wherein the alumina layer is composed of two layers: a first alumina layer on the inside and a second alumina layer on the surface side. Palladium and neodymium are supported on the second alumina layer, and platinum and rhodium are supported on the second alumina layer.

[Action of the invention]

According to the present invention, since neodymium is added, a composite oxide is formed with palladium and neodymium, and therefore, grain growth of palladium is suppressed.

Furthermore, since palladium is supported on an alumina layer separate from platinum and rhodium, the adverse effects of palladium grain growth do not affect platinum and rhodium. As a result, high activity is maintained.

〔Effect of the invention〕

As described above, according to the monolithic catalyst for exhaust gas purification of the present invention, purification performance can be improved, and durability can also be improved.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

1000g of γ-alumina, aluminum nitrate aqueous solution (2
3% by weight) 150g, alumina silua 00g and water 3
00g were mixed and stirred to prepare an alumina slurry. A cordierite monolithic carrier 1 having a volume of 1.7 Il as shown in FIG. It was fired for a period of time to form the inner first alumina layer 2 (FIG. 2). Next, palladium chloride, aqueous solution (0.85 g/l: Pd equivalent) 2j
2 for 2 hours to support palladium. Next, 75 g of neodymium nitrate hexahydrate was dissolved in ion exchange water and 3
00ml of an aqueous solution was prepared using TTI, and the monolithic carrier 1 was immersed in this aqueous solution to absorb it by an impregnation method.
After drying at °C for 1 hour, it was baked at 700 °C for 2 hours. As a result, palladium and neodymium were supported on the first alumina layer 2.

Next, a second alumina layer 3 was formed on the first alumina layer 2 in the same manner as the first alumina layer 2 described above (FIG. 2). Subsequently, this monolithic carrier 1 was mixed with a dinitrodiammine platinum aqueous solution 2Il (0.85g//
2: Pt equivalent), taken out, blown off excess aqueous solution, dried, and fired, then immersed in rhodium chloride aqueous solution (0
, 17 g/Il (Rh equivalent) 2N, taken out, excess aqueous solution blown off, dried, and fired. As a result, platinum and rhodium were supported on the second alumina layer 3.

As described above, a monolithic catalyst a was obtained in which palladium and neodymium were supported on the first alumina layer 2 and platinum and rhodium were supported on the second alumina layer 3.

(Comparative Example 1) A first alumina layer was formed by the same method as in Example, and then immersed in an aqueous palladium chloride solution (0.85 g/z) 2z to support palladium on the first alumina layer. Thereafter, similarly to the example, a second alumina layer was formed on the first alumina layer, and a dinitrodiammine platinum aqueous solution (
0,85g/A) 211 rhodium chloride aqueous solution (0,17
Platinum and rhodium were supported on the second alumina layer by immersion in g//li 2β. As a result, a monolith catalyst was obtained in which palladium was supported on the first alumina layer and platinum and rhodium were supported on the second alumina layer.

(Comparative Example 2) An alumina layer having a thickness corresponding to the total of the first alumina layer and the second alumina layer of the example was formed on a monolithic carrier substantially the same as that of the example. Next, it was immersed in a neodymium nitrate aqueous solution, dried, and fired, and then sequentially immersed in a palladium chloride aqueous solution, a dinitrodiammine platinum aqueous solution, and a rhodium chloride aqueous solution to support palladium, platinum, and rhodium. As a result, a monolithic catalyst C was obtained in which neodymium, palladium, platinum, and rhodium were supported on the alumina layer.

Next, the supported amount of the catalyst used in the above examples and comparative examples was
Shown in the table.

Table 1 Each of the above three monolithic catalysts a, b, and c: 2.81
After a 200-hour durability test was carried out by attaching the device to the exhaust system of an engine, the purification efficiency was measured by changing the exhaust gas inlet temperature to 300°C and 350°C. The results are shown in Table 2.

As is clear from Table 2, it can be seen that the monolithic exhaust gas purifying catalyst of this example has extremely high activity both at low temperatures and at high temperatures.

Although specific embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and includes various embodiments within the scope of the claims. .

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram showing the outline of a monolith carrier used in an example of the present invention, and FIG. 2 is an enlarged view of the main part of FIG. 1. 1-------Monolith carrier 2--m--First alumina layer 0 3-------Second alumina layer 11 FIG.

Claims (1)

[Claims] (11 columns, with a large number of cells provided from the inlet side to the outlet side of the exhaust gas, an alumina layer is formed on the inner wall surface of the cell, and a catalyst component is contained in this alumina layer. The supported monolithic catalyst for exhaust gas purification includes two alumina layers: a first alumina layer on the inside and a second alumina layer on the surface side, and the first alumina layer contains palladium and A monolithic catalyst for purifying exhaust gas, characterized in that neodymium is supported, and a second alumina layer supports platinum and rhodium.