JP2003053187A - Catalyst for cleaning exhaust gas and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide suppress grain growth by suppressing the migration of carried and supported noble metal particles even when a high temperature of >=800 deg.C acts for a long time under a lean atmosphere of oxygen excess. SOLUTION: The noble metal 3 is carried and supported near the surface of a carrier 1 consisting of silica in an embedded state. After the noble metal is carried and supported on the silica carrier, the silica carrier is impregnated with an alkoxy silane, is hydrolyzed and is fired, by which a silica layer is formed and the noble metal is coated. The noble metal particles are coated with the silica layer 2 and the silica layer 2 is integrally bonded to the silica carrier 1 and therefore the migration of the noble metal particles is suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst for purifying exhaust gas from an automobile engine and the like and a method for producing the same, and more particularly to an exhaust gas purifying catalyst having excellent durability and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by simultaneously oxidizing CO and HC and reducing NO _x has been used as an exhaust gas purifying catalyst for automobiles. As such a three-way catalyst, a porous carrier layer made of γ-alumina is formed on a heat resistant substrate made of cordierite, and platinum (Pt), rhodium (Rh), etc. are formed on the porous carrier layer. Those carrying a noble metal are widely known.

By the way, in recent years, the exhaust gas purifying catalyst tends to be installed immediately below the manifold close to the engine, and the exhaust gas temperature becomes high during high-speed traveling. Therefore, the exhaust gas purifying catalyst is often exposed to high temperatures. Has become. However, the conventional exhaust gas-purifying catalyst has a problem that the catalytic performance is deteriorated because the thermal deterioration of γ-alumina proceeds due to the high-temperature exhaust gas, and the catalyst active points are reduced due to the particle growth of the precious metal.

Therefore, for example, Japanese Patent Application Laid-Open No. 4-122441 discloses a method for producing an exhaust gas purifying catalyst in which a noble metal is supported by using alumina which has been heat treated in advance. According to this manufacturing method, since the alumina has already been heat-treated, the resulting exhaust gas-purifying catalyst undergoes almost no thermal deterioration even when exposed to high-temperature exhaust gas, and can prevent grain growth of noble metals. .

In recent years, lean-burn engines, which supply an air-fuel mixture in excess of oxygen, have become the mainstream in order to suppress carbon dioxide emissions. However, in an exhaust gas purifying catalyst produced by the production method disclosed in the above publication, noble metal grain growth is remarkable and catalytic performance is deteriorated when a high temperature of 800 ° C. or higher acts in a lean atmosphere with excess oxygen. There was a problem.

For example, Pt supported on the surface of alumina becomes PtO _{2 in} an atmosphere where oxygen coexists at high temperature, and diffusion and aggregation are promoted by gas phase transfer. Therefore, in a lean atmosphere or stoichiometric atmosphere with excess oxygen, when exposed to high temperatures, grain growth occurs in Pt and the surface area decreases, resulting in a large decrease in catalyst performance.

Therefore, the applicant of the present application has proposed a manufacturing method in which a carrier carrying a noble metal is heat-treated at 800 ° C. or higher in a non-oxidizing atmosphere, as disclosed in JP-A-8-338897. According to this manufacturing method, since the porous carrier is sintered and the pores are shrunk, the supported precious metal is tightly surrounded by the porous carrier. Therefore, since the movement of the noble metal is regulated by the porous carrier even when a high temperature acts in the lean atmosphere, the grain growth of the noble metal can be suppressed.

[0008]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
Even with the exhaust gas-purifying catalyst produced by the production method described in Japanese Patent Publication No. 97, it has been clarified that grain growth occurs in the noble metal when a high temperature exceeding 800 ° C. acts for a long time in a lean atmosphere with excess oxygen. . It is considered that this is because the noble metal particles supported outside the pores are not physically and chemically fixed to the carrier and can move freely.

The present invention has been made in view of the above circumstances, and suppresses the movement of the noble metal particles carried even when a high temperature of 800 ° C. or higher acts for a long time in a lean atmosphere with excess oxygen. By doing so, it is intended to provide an exhaust gas-purifying catalyst that can suppress grain growth.

[0010]

A feature of the exhaust gas purifying catalyst of the present invention for solving the above problems is that it comprises a carrier made of silica and a noble metal supported in a state of being embedded near the surface of the carrier. It is in.

Further, the feature of the method for producing an exhaust gas purifying catalyst of the present invention that can easily and surely produce the above exhaust gas purifying catalyst is that silica powder is impregnated with a solution in which a noble metal compound is dissolved, dried and calcined to obtain silica powder. It consists of a carrying step of preparing a carrying powder carrying a noble metal, and a coating step of impregnating the carrying powder with alkoxysilane, and then hydrolyzing and then drying and firing to form a silica layer on the surface of the carrying powder. .

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the exhaust gas purifying catalyst of the present invention,
The noble metal is supported in a state of being embedded in the vicinity of the surface of the carrier, and the noble metal is entirely covered with silica, at least a part of its surface is covered with silica, or a silica wall is formed around the noble metal particles. Has been done. Therefore, the movement of the noble metal particles is suppressed even at high temperature, and the noble metal particles are suppressed from coagulating and growing. Therefore 800 in a lean atmosphere with excess oxygen
Even if a high temperature exceeding 0 ° C. acts for a long time, the precious metal maintains a highly dispersed state and has many active sites, so that the exhaust gas-purifying catalyst of the present invention exhibits high activity.

Silica is used as the carrier. With other carriers, it is difficult to carry the noble metal in a state of being embedded, or the activity of the noble metal is reduced, so that it is difficult to use with the current technology. The specific surface area of this silica carrier is preferably 30 m ² / g or more. If the specific surface area is smaller than this, the amount of precious metal supported becomes small and it is not practical as an exhaust gas purifying catalyst for automobiles.

As the noble metal, those used in conventional exhaust gas purifying catalysts such as Pt, Rh, Pd, Ir and Ru can be used. In particular, the present invention is effective in the case of Pt which has a high catalytic activity but is prone to grain growth. The amount of the noble metal supported is 0.1% by weight or more, preferably 0.5 to 20% by weight, based on the silica carrier. If the supported amount is less than this range, the activity as an exhaust gas purifying catalyst is too low to be practical, and if the supported amount is more than this range, the activity is saturated and the cost rises.

The precious metal is 10 to 50 nm from the surface of the silica carrier.
It is desirable to be embedded and carried in the range of. In other words, the thickness of the silica layer formed by the manufacturing method of the present invention is 10
It is desirable to set it to -50 nm. If it is buried deeper than this range, the activity decreases, and if it is shallower than this range, noble metal particles may move and grow at high temperatures.

In order to support the noble metal embedded in the silica carrier, it is easiest and surely to carry out by the production method of the present invention described later, but in some cases, it can also be carried out by the ion implantation method or the like. .

The exhaust gas-purifying catalyst of the present invention can be used as it is as an oxidation catalyst, a three-way catalyst, etc., and if it carries a NO _x storage material such as Ba or K, it will serve as a NO _x storage reduction catalyst. Can be used. Although silica is essential as a carrier for supporting a noble metal, other oxide carriers such as alumina, zirconia, ceria and titania can be used in combination with silica as a carrier.

In the method for producing a catalyst for purifying exhaust gas according to the present invention, first, in a supporting step, a solution of a noble metal compound dissolved in silica powder is impregnated and dried / calcined to prepare a supported powder having noble metal supported on silica powder. To do. In this case, the noble metal compound is preferably a noble metal ammine salt capable of adsorbing and supporting the noble metal in a highly dispersed state, but it can be impregnated and supported by using a noble metal nitrate or the like. Further, as the solvent, water, alcohol, etc. may be used as long as it can dissolve the noble metal compound. The firing conditions are 1 to 250 to 500 ° C, where the movement of the supported precious metal particles is suppressed.
It is desirable that the time is about 2 hours.

In the coating step, the carrier powder is impregnated with a stock solution of alkoxysilane or a solution in which alkoxysilane is dissolved, and then hydrolyzed and then dried and fired. As the alkoxysilane, methoxysilane, ethoxysilane, propoxysilane, butoxysilane and the like can be used. The number of alkoxyl groups in one molecule of alkoxysilane is not particularly limited, but it is preferable to use tetraalkoxysilane in order to efficiently generate silica.

The alkoxysilane can be used as it is, but it is preferable to use it as an alcohol solution of methanol, ethanol, isopropanol or the like in order to prevent a rapid reaction. The concentration is not particularly limited, and if the concentration is low, the coating process may be repeated several times to form a silica layer having a desired thickness.

Hydrolysis of the alkoxysilane impregnated in the carrier powder can be carried out by impregnating it with water. Also, steam may be sprayed. For efficient hydrolysis, it is desirable to carry out after the solvent of the impregnated alkoxysilane solution has evaporated.

Alcohol is desorbed from the alkoxysilane by hydrolysis and is dried and calcined to form a silica layer on the surface of the carrier powder. As a result, at least a part of the noble metal particles supported on the silica carrier is covered with the silica layer, and since the silica layer is integrally bonded to the silica carrier, the noble metal particles are embedded between the silica layer and the carrier. It becomes a state and it becomes difficult to move. As a result, even if a high temperature exceeding 800 ° C. acts in a lean atmosphere, grain growth of noble metal particles can be suppressed.

The firing condition in the coating step is preferably such that the noble metal particles of the supported powder do not move, and it is desirable to heat the powder at 250 to 500 ° C. for 1 to 2 hours.

[0024]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples.

(Comparative Example) 60 g of silica powder having a specific surface area of 130 m ² / g was brought into contact with an aqueous solution of tetraammineplatinum hydrate having a concentration of 1.9% by weight, and tetraammineplatinum was adsorbed and supported, followed by washing with water, and then in the atmosphere. It was dried at 120 ° C for 2 hours and calcined at 400 ° C for 2 hours. The loading amount of Pt is 1.6% by weight. The obtained supporting powder is compacted to give a thickness of 0.5 mm to 1.7 mm.
mm pellet catalyst was used.

(Example) 10 g of pellet catalyst of the above comparative example
On the other hand, 100 wt% concentration of tetramethoxysilane was completely impregnated into the pellets with a dropper (the color changed from white to gray) and dried at room temperature for 1 hour to eliminate the wet color. Water was evenly impregnated into the pellets with a dropper. And after drying in air at 120 ℃ for 2 hours,
The pellet catalyst of the example was prepared by calcining at 400 ° C. for 2 hours.

FIG. 1 shows a TEM photograph of the catalyst of this example. FIG. 1 is an observation of the pellet particles from an oblique direction, and a part of the pellet particles is shown. The white part is silica and the black dot is Pt.
Is 100 nm. The spherical portion around each Pt is considered to be the portion where silica surrounds Pt and rises. That is, as shown in the schematic cross-sectional view of FIG. 2, this catalyst comprises a silica carrier 1 and a silica layer 2 formed by hydrolysis and firing of tetramethoxysilane.
And Pt embedded between the silica carrier 1 and the silica layer 2.
It is considered to be composed of particles 3. In addition, it is considered that a part of the surface of the Pt particle 3 is exposed from the silica layer 2.

<Test / Evaluation> The pellet catalysts of Examples and Comparative Examples were subjected to a durability test of heating them at 800 ° C. for 5 hours in the atmosphere. Then, each pellet catalyst before and after the durability test was placed in an evaluation device, and under a model gas flow in a stoichiometric atmosphere shown in Table 1, at a space velocity SV = 400,000 / h,
After pretreatment at 500 ° C for 20 minutes, the temperature was continuously raised at 20 ° C / min, and the HC, CO and NO _x purification rates during that period were continuously measured. From these results, 50% purification temperatures of HC, CO and NO _x were calculated, and the results are shown in FIG.

[0029]

[Table 1]

Each pellet catalyst before and after the durability test was analyzed by the CO pulse method to measure the particle size of Pt carried. The results are shown in FIG. 4 as the initial stage and after the endurance test.

From FIG. 3, the activity of the catalyst of the comparative example is greatly reduced after the durability test, whereas the activity of the catalyst of the example is hardly reduced after the durability test. And Figure 4
From the results, it can be seen that the catalyst of Comparative Example has a large Pt particle growth after the durability test, whereas the Catalyst of the Example maintains a fine particle size of Pt even after the durability test.

That is, it was recognized that the activity of the catalyst of the comparative example was lowered by the grain growth of Pt, and that of the catalyst of the example was Pt.
It is considered that the decrease in the activity was suppressed because the grain growth of was suppressed. It is clear that this is an effect of forming the silica layer with tetramethoxysilane.

[0033]

[Effects of the Invention] That is, according to the exhaust gas purifying catalyst of the present invention, noble metal grain growth is prevented even when a high temperature of about 800 ° C acts in a lean atmosphere with excess oxygen, and it is extremely excellent in heat resistance and durability. . According to the production method of the present invention, such an exhaust gas purifying catalyst can be produced easily and reliably and stably.

[Brief description of drawings]

FIG. 1 is a TEM photograph showing a particle structure of a catalyst of an example.

FIG. 2 is a schematic sectional view of a catalyst of an example.

FIG. 3 is a graph showing the catalysts of Examples and Comparative Examples at the initial stage and after the durability test.
It is a graph which shows% purification temperature.

FIG. 4 shows Pt of the catalysts of Examples and Comparative Examples at the initial stage and after the durability test.
It is a graph which shows a particle size.

[Explanation of symbols]

1: Silica carrier 2: Silica layer 3: Pt particles

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) F01N 3/28 301 B01D 53/36 C 104A F term (reference) 3G091 AA02 AB02 AB03 BA01 BA39 GB05W GB06W GB07W GB10X GB17X 4D048 AA06 AA13 AA18 AB05 BA06X BA30X BA31Y BA32Y BA33Y BA41X BB01 4G069 AA03 AA08 BA02A BA02B BA21C BB02A BB02B BC69A BC70A BC71A BC72A BC74A BC75ABC75B BD05C BE06C CA08FB02EA02

Claims (2)

[Claims] 1. An exhaust gas-purifying catalyst comprising a carrier made of silica and a noble metal supported in a state of being embedded in the vicinity of the surface of the carrier. 2. A supporting step of impregnating a solution in which a noble metal compound is dissolved in silica powder, drying and firing to prepare a supporting powder in which the noble metal is supported on the silica powder, and impregnating the supporting powder with alkoxysilane, A method for producing an exhaust gas-purifying catalyst, which comprises a coating step of forming a silica layer on the surface of the supported powder by hydrolyzing and then drying and firing.