JP2002095967A - Exhaust gas cleaning catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning catalyst, wherein a matrix is inhibited from being deteriorated so as to prevent the catalyst from deteriorating in strengths, e.g. thermal shock resistance. SOLUTION: This catalyst consists of a matrix having a composition containing SiO2 and a coat layer comprising a porous support carrying at least a noble metal and an alkali metal and formed on the surface of the matrix, wherein the matrix has an SiO2 content of 50-65 wt.% based on the entire base. Even when the SiO2 is consumed by reaction with the alkali metal, the composition of the matrix can fall within the optimal range.

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 used in an exhaust system of an automobile engine, and more particularly to an exhaust gas purifying catalyst having improved strength such as thermal shock resistance.

[0002]

2. Description of the Related Art In recent years, global warming due to carbon dioxide has become a problem, and reducing carbon dioxide emissions has become an issue. In automobiles, reduction of the amount of carbon dioxide in exhaust gas has become an issue, and lean burn engines have been developed in which fuel is burned lean in an oxygen-excess atmosphere. According to this lean burn engine, the emission of carbon dioxide can be suppressed by improving fuel efficiency.

[0003] In this lean burn engine,
Burns in an oxygen-excess fuel-lean condition,
Exhaust gas is reduced in a reducing atmosphere by stoichiometric to rich conditions.
NO as atmosphere_x Has been developed to reduce and purify
Has been used. And the best catalyst for this system
NO in fuel lean atmosphere_x And occluded NO _x 
To release NOx in fuel stoichiometric rich atmosphere_x Occlusion material
NO used_x A storage reduction type exhaust gas purification catalyst has been developed
I have.

For example, Japanese Patent Application Laid-Open No. Hei 5-317652 proposes an exhaust gas purifying catalyst in which an alkaline earth metal such as Ba and Pt are supported on a porous oxide carrier such as γ-Al ₂ O ₃ . Also, JP-A-6-31139 discloses that an alkali metal such as K is used.
An exhaust gas purifying catalyst in which Pt is supported on a porous oxide carrier such as γ-Al ₂ O ₃ has been proposed. Further, Japanese Patent Application Laid-Open No. 5-168860 proposes an exhaust gas purifying catalyst in which a rare earth element such as La and Pt are supported on a porous oxide carrier such as γ-Al ₂ O ₃ .

[0005] By using this NO _x storage-and-reduction type catalyst, by controlling the air-fuel ratio so that the fuel stoichiometric-rich side of the fuel lean side in a pulsed manner, the stoichiometric-rich atmosphere exhaust gas from a lean atmosphere pulsed Becomes Therefore, on the lean side, NO _x is stored in the NO _x storage material,
It is released on the stoichiometric to rich side and is purified by reacting with reducing components such as hydrocarbons (HC) and carbon monoxide (CO) contained in the exhaust gas in large amounts. can efficiently purify NO _x even. The HC and CO in the exhaust gas, since it is consumed in the reduction of the NO _x while being oxidized by the noble metal, HC
And CO is also purified efficiently.

[0006]

In an exhaust gas purifying catalyst for an automobile, the catalyst is stably held in an exhaust gas flow passage of the automobile, the contact area between the exhaust gas and the catalyst component is increased, and an increase in pressure loss is suppressed as much as possible. There is a need to. Therefore, conventionally, a honeycomb-shaped substrate formed of MgO / Al ₂ O ₃ / SiO ₂ composite oxide such as cordierite has been used,
A monolith catalyst having a coat layer formed of a porous carrier carrying a catalyst component formed on the surface of the substrate is often used.

[0007] However in _the NO _x storage reduction type monolithic catalyst, one using an alkali metal as _the NO _x storage material, degradation of the substrate occurs during use, thermal shock resistance as compared with other catalysts such as three-way catalyst It was clarified that the strength of such as decreased. Therefore, it is necessary to increase the frequency of replacement, and there is a problem that the cost per unit usage time is high.

The present invention has been made in view of such circumstances, and an object of the present invention is to prevent deterioration in strength such as thermal shock resistance by suppressing deterioration of a substrate.

[0009]

The feature of the exhaust gas purifying catalyst of the present invention that solves the above-mentioned problems is that a base material containing SiO ₂ in its composition and at least a noble metal and an alkali metal formed on the surface of the base material are used. An exhaust gas purifying catalyst comprising a coat layer formed of a supported porous carrier, wherein the content of SiO ₂ in the substrate is set to 50 to 65% by weight of the entire substrate.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present application have conducted intensive studies on the causes of deterioration of a base material, and have found that deterioration occurs in a base material containing SiO ₂ in the composition. That is, MgO ・ Al ₂ O ₃
A NO _x storage reduction type exhaust gas purification catalyst that uses a structure made of SiO ₂ composite oxide as a base material and carries a noble metal and an alkali metal has a higher thermal expansion coefficient than one that does not contain an alkali metal Became clear. When the coefficient of thermal expansion increases, the thermal shock resistance decreases and the strength decreases.

When the distribution of the alkali metal in the catalyst in which the above problem occurred was examined, the alkali metal was also present in the base material. The reaction between the _two components and the alkali metal results in the formation of a glass phase, which consumes SiO _2, causing a preferential reaction from the interface between the coat layer and the substrate, resulting in a change in the composite oxide composition. It was revealed.

Therefore, in the present invention, the content of SiO ₂ in the substrate is set to 50 to 65% by weight of the whole substrate. By increasing the amount of SiO _{2 in} this manner, the composition of the MgO.Al ₂ O ₃ .SiO ₂ composite oxide can be set to an optimum range even when SiO ₂ is consumed in the reaction with the alkali metal. Therefore, a decrease in strength is suppressed, and in some cases, the strength is improved by a reaction.

As a substrate containing SiO ₂ in its composition, MgO
Al ₂ O ₃ · SiO ₂ composite oxide, Si ₃ N _4, but such as SiC is exemplified, heat resistance, particularly excellent MgO · Al ₂ O ₃ · Si cost
O ₂ -based composite oxides are desirable, and cordierite is particularly preferably used.

In the present invention, the SiO ₂ content of the substrate is set to 50 to 65% by weight of the whole substrate. SiO ₂ content is 50% by weight
If the amount is smaller, the coefficient of thermal expansion increases due to the reaction with the alkali metal, resulting in insufficient strength. In addition, the amount of alkali metal carried in the NO _x storage reduction type exhaust gas purifying catalyst is at most 2 mol per liter of the base material.
An upper limit of the SiO ₂ content of 65% by weight is sufficient. If the content is more than 65% by weight, the coefficient of thermal expansion becomes high and the strength becomes insufficient, although the reason is unknown.

The shape of the substrate is not particularly limited as long as it has a large contact area with the exhaust gas, such as a honeycomb shape, a porous shape such as a foam, a net shape, and a pellet shape. A substrate obtained by coating a substrate formed of a metal foil or the like with a powder of the above substrate can also be used as the substrate.

The coating layer comprises a porous carrier, and at least a noble metal and an alkali metal supported on the porous carrier. Al ₂ O ₃ , ZrO ₂ , TiO ₂ , Ce
Various types such as O ₂ can be used. These oxides may be used alone, or two or more kinds may be mixed and used as a plurality of kinds of composite oxides. For example, from the viewpoint of catalytic activity, it is preferable to contain at least Al ₂ O _3. If TiO ₂ is mixed, the sulfur poisoning resistance is improved. If CeO ₂ or CeO ₂ —ZrO ₂ solid solution is mixed, the purification activity is improved by its oxygen storage / release ability.

The noble metal can be selected from Pt, Rh, Pd, Ir, Ru and the like. Of these, one type may be used, or a plurality of types may be supported. The supporting amount is preferably about 0.1 to 20% by weight per liter of the catalyst.

Examples of the alkali metal include K, Na and Cs. The loading amount of the alkali metal is preferably in the range of 0.01 to 1.0 mol per liter of the catalyst. If the amount is less than this range, the NO _x storage ability is low, and if the amount exceeds this range, the probability of reaction with the base material component increases and the strength of the base material becomes insufficient.

In the case of the NO _x storage reduction catalyst, it is preferable that at least one of an alkaline earth metal and a rare earth element is supported in addition to the alkali metal. Examples of the alkaline earth metal include Ba, Be, Ca, Mg, Sr and the like. Examples of rare earth elements include Sc, Y, La, Yb, Nd, and Sm. The alkaline earth metal and the rare earth element are SiO ₂
Is not reacted with, so that it is preferably 0.1 to 2 mol per liter of the catalyst as in the prior art.

[0020] Note that the exhaust gas purifying catalyst of the present invention may be those containing an alkali metal to the coating layer, NO _x storage reduction catalyst, NO _x absorbent catalyst or three-way catalyst optionally, as such an oxidation catalyst Can be used.

[0021]

The present invention will be described below in detail with reference to examples.

Example 1 A mixed powder containing 51% by weight of SiO ₂ powder, 14% by weight of MgO powder, and 35% by weight of Al ₂ O ₃ powder was mixed, and calcined at 1300 to 1400 ° C. to obtain MgO · Al. _{A 2} O ₃ · SiO ₂ composite oxide was prepared. This composite oxide is classified as cordierite. The powder obtained by pulverizing the composite oxide was mixed with water and a binder to form a paste, extruded into a honeycomb shape, and fired at 1400 ° C. to form a honeycomb substrate.

On the other hand, 100 parts by weight of Al ₂ O ₃ powder and TiO ₂ powder
Prepare a slurry in which 100 parts by weight and 30 parts by weight of CeO ₂ -ZrO ₂ solid solution powder were dispersed in distilled water together with alumina sol, dipped the above honeycomb substrate, pulled up, dried and fired to coat the honeycomb substrate surface A layer was formed. The coating layer is formed at 230 g per liter of the honeycomb substrate, and has a thickness of 100 to 200 μm.

Next, the honeycomb substrate having the coating layer is immersed in an aqueous solution of dinitrodiammine platinum having a predetermined concentration, pulled up and blown off excess droplets, and then dried in the air at room temperature to adsorb and carry Pt. did. The supported amount of Pt is the honeycomb substrate 1
2 g per liter.

Further, a predetermined amount of an aqueous solution of barium acetate having a predetermined concentration is impregnated into the coat layer, and after evaporating to dryness, in the air
Baking was carried out at 300 ° C. for 1 hour to carry Ba. The supported amount of Ba is 0.2 mol per liter of the honeycomb substrate. Subsequently, a predetermined amount of an aqueous solution in which a predetermined concentration of potassium acetate is dissolved is impregnated, evaporated and dried, and then calcined at 300 ° C. for one hour in the air to carry K and carry a NO _x storage reduction type exhaust gas purifying catalyst. Was prepared. The amount of K supported per liter of honeycomb substrate
0.2 mol.

Example 2 A honeycomb substrate was prepared in the same manner as in Example 1 except that the content of SiO ₂ in the MgO.Al ₂ O ₃ .SiO ₂ based composite oxide was 54% by weight. It was prepared NO _x storage-and-reduction type exhaust gas purifying catalyst in the same manner as in example 1.

Example 3 A honeycomb substrate was prepared in the same manner as in Example 1 except that the content of SiO ₂ in the MgO.Al ₂ O ₃ .SiO ₂ composite oxide was 58% by weight. It was prepared NO _x storage-and-reduction type exhaust gas purifying catalyst in the same manner as in example 1.

Example 4 A honeycomb substrate was prepared in the same manner as in Example 1 except that the SiO ₂ content in the MgO.Al ₂ O ₃ .SiO ₂ -based composite oxide was changed to 62% by weight. It was prepared NO _x storage-and-reduction type exhaust gas purifying catalyst in the same manner as in example 1.

Comparative Example 1 A honeycomb substrate was prepared in the same manner as in Example 1 except that the content of SiO ₂ in the MgO.Al ₂ O ₃ .SiO ₂ -based composite oxide was 49% by weight. It was prepared NO _x storage-and-reduction type exhaust gas purifying catalyst in the same manner as in example 1.
The composition of the honeycomb substrate is a cordierite composition which has been widely used in the past.

Comparative Example 2 A honeycomb substrate was prepared in the same manner as in Example 1 except that the content of SiO ₂ in the MgO.Al ₂ O ₃ .SiO ₂ based composite oxide was 45% by weight. It was prepared NO _x storage-and-reduction type exhaust gas purifying catalyst in the same manner as in example 1.

Comparative Example 3 A honeycomb substrate was prepared in the same manner as in Example 1 except that the content of SiO ₂ in the MgO.Al ₂ O ₃ .SiO ₂ -based composite oxide was 70% by weight. It was prepared NO _x storage-and-reduction type exhaust gas purifying catalyst in the same manner as in example 1.

<Test / Evaluation> For each of the obtained catalysts, the coefficient of thermal expansion of the honeycomb substrate was measured. FIG. 1 shows the results obtained by plotting the SiO ₂ content of each honeycomb substrate on the horizontal axis and the thermal expansion coefficient on the vertical axis.

FIG. 1 shows that when the SiO ₂ content of the substrate is in the range of 50 to 65% by weight, the coefficient of thermal expansion is less than 2 × 10 ⁻⁶ / ° C. An improved range. However, the catalyst of Comparative Example in which the SiO ₂ content is not in this range has a high thermal expansion coefficient and is insufficient in thermal shock resistance as an exhaust gas purifying catalyst for automobiles.

[0034]

According to the exhaust gas purifying catalyst of the present invention, even when an alkali metal is used as a carrier component, there is no problem that the thermal shock resistance of the base material is lowered, so that the use period until the deterioration is extended. And cost per unit usage time can be reduced.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a SiO ₂ content ratio and a coefficient of thermal expansion.

Continued on the front page F-term (reference) 3G091 AA02 AB06 BA10 BA14 BA39 GA06 GB01X GB02W GB03W GB04W GB04X GB05W GB06W GB07W GB10X GB17X 4D048 AA06 AB02 AB07 BA01X BA03X BA06X BA07X BA08X BA14X BA15X BA19A BAY ABAY ABAY BAY ABAY BAY BAY XA BA02A BA02B BA04B BA06A BA06B BA13A BA13B BA20A BA20B BB02A BB02B BB04A BB04B BB06B BC01A BC02A BC03A BC03B BC06A BC09A BC10A BC11A BC12A BC13A BC13B BC43B BC51B BC69A BC70 BC07ABC08A

Claims (1)

[Claims] An exhaust gas purifying catalyst comprising: a base material containing SiO ₂ in a composition; and a coat layer formed on the surface of the base material and formed of a porous support supporting at least a noble metal and an alkali metal. An exhaust gas purifying catalyst, wherein the content of SiO ₂ in the substrate is 50 to 65% by weight of the entire substrate.