JP3532979B2 - Exhaust gas purification catalyst - Google Patents

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 discharged from an internal combustion engine of an automobile or the like, and more particularly, to an exhaust gas containing excess oxygen, that is, carbon monoxide (CO) contained in the exhaust gas. ), Hydrogen (H ₂ ) and hydrocarbons (HC) such as nitrogen oxides (NO _x ) in exhaust gas containing excess oxygen in excess of the amount of oxygen necessary to completely oxidize reducing components. The present invention relates to an exhaust gas purifying catalyst that can be purified.

[0002]

Conventionally, as an exhaust gas purifying catalyst of an automobile, a three-way catalyst for purifying exhaust gas by performing the reduction of CO and HC oxidation and NO _x simultaneously is used. As such a three-way catalyst, for example, a porous carrier layer made of γ-alumina is formed on a heat-resistant substrate made of cordierite or the like, and platinum (Pt), rhodium (Rh), or the like is formed on the porous carrier layer. What carried the catalyst noble metal is widely known. Also known is a three-way catalyst having a low-temperature activity enhanced by using ceria (cerium oxide) having oxygen storage capacity in combination.

On the other hand, in recent years, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem from the viewpoint of protection of the global environment. As a solution, lean combustion in an oxygen-rich atmosphere is performed. Burn is promising. In this lean burn, the use of fuel is reduced to improve fuel efficiency, and the generation of CO ₂ , which is the combustion exhaust gas, can be suppressed.

On the other hand, in the conventional three-way catalyst, when the air-fuel ratio is the stoichiometric air-fuel ratio (stoichiometric), CO, H
The three-way catalyst simultaneously oxidizes and reduces C and NO _x to purify the catalyst. The three-way catalyst exhibits sufficient purification performance for the reduction and removal of NO _x under an oxygen-excess atmosphere of the exhaust gas during lean burn. Absent. Therefore, development of a catalyst and purification system is desired can purify NO _x even in an oxygen rich atmosphere.

Accordingly, the applicant of the present application has proposed an exhaust gas purifying catalyst in which an alkaline earth metal and Pt are first supported on a porous carrier such as alumina (Japanese Patent Laid-Open No. 5-317652), or a lanthanum and Pt as a porous carrier. A supported exhaust gas purifying catalyst (JP-A-5-168860) has been proposed. According to these exhaust gas purifying catalysts, NO _x
There is occluded in the oxide or oxides of lanthanum alkaline earth metal, because it is purified by reacting with reducing components such as HC and CO in the stoichiometric or rich side, purification performance of even NO _x in the lean side Is excellent.

[0006]

In order to store NO _{x in} a NO _x storage material such as an alkaline earth metal or lanthanum, it is necessary that NO or the like be oxidized to nitrate ions. However, since even the lean exhaust gas contains reducing components such as H ₂ , CO, and HC, the oxidation of NO and the like on the exhaust gas purifying catalyst is hindered, and the NO _x storage material is not occluded. There is a problem of being hindered.

The catalytic activity varies depending on the type of catalytic noble metal, and Pt is particularly excellent in NO _x oxidizing activity.
d has the property of being excellent in the oxidation activity of HC and CO. Therefore, in order to enhance ternary activity, Pt and Rh
And Pd are used together. However, when Pt is carried close to Rh or Pd, Rh or Pd is concentrated on the surface of Pt in an oxidizing atmosphere, and the oxidation activity of Pt may be impaired. As a result, the oxidation of NO _x becomes insufficient, and N
There is a problem that it is discharged without being stored in the O _x storage material.

On the other hand, during stoichiometric and rich conditions, the stored NO _x is released from the NO _x storage material, and is reacted with reducing components such as HC and CO on the catalytic noble metal to be reduced to N ₂ and purified. You. Here, NO _x purifying performance in the rich among the catalytic noble metal is Rh is most excellent. Therefore, if Rh is positively used, the N ₂ selectivity on the lean side is improved, but the Pt oxidation activity is impaired as described above.

[0009] Further components having oxygen storage capacity such as ceria is present in the vicinity _the NO _x storage material, oxygen is oxygen consumed in the oxidation of the NO _x to be occluded is reduced to its component in the lean side, NO _x amount of NO _x occluded in the occluding material there is a disadvantage that the emissions of reduced NO _x increases. The present invention has been made for improving the above problems, by a structure in which respective functions, such as catalytic precious metal and _the NO _x storage material is sufficiently fulfilled, N in the exhaust gas
An object of the present invention is to provide an exhaust gas purifying catalyst capable of reducing and purifying O _x more efficiently.

[0010]

Means for Solving the Problems A first method for solving the above problems is described below.
Exhaust gas purifying catalyst of the invention, the NO _x during lean NO _x
Occluded in the storage material, or _the NO _x storage material during the stoichiometric-rich
A catalyst for reducing and purifying the al released NO _x, first
A porous support, a first catalytic noble metal consisting of at least one of platinum and rhodium supported on the first porous carrier, alkali metal and alkaline earth contained in the first porous carrier
And at least one NO _x storage material selected from metal, and more become the first catalyst support layer, a second porous carrier, and a second catalytic noble metal supported on the second porous carrier, more will first catalyst loaded And a second catalyst supporting layer coated on the layer.

An exhaust gas purifying catalyst according to a second aspect of the present invention comprises the second exhaust gas purifying catalyst.
The catalyst support layer is characterized by further containing a component having an oxygen storage ability. Further, the exhaust gas purifying catalyst of the third invention is:
The component having the oxygen storage ability is ceria.

[0012]

[Action]

(1) Lean time In the exhaust gas purifying catalyst of the first invention, the exhaust gas first comes into contact with the upper second catalyst supporting layer, and the second catalytic noble metal catalyzes H ₂ , CO, HC and the like in the presence of oxygen. The reducing components are oxidized and removed.

In the first catalyst supporting layer, NO in exhaust gas
NO _x, such as become nitrate ions are oxidized by the catalytic action of the first catalytic noble metal in the presence of oxygen, and are inserted into _the NO _x storage material present in the first neighborhood catalytic noble metal. Since the exhaust gas reaching the first catalyst supporting layer hardly contains reducing components such as H ₂ , CO, and HC due to contact with the second catalyst supporting layer, the oxidation reaction of NO _x by the reducing component is performed. inhibition of oxidized such as NO in the first catalyst supporting layer is promoted not occur, NO _x in the exhaust gas is occluded efficiently _the NO _x storage material.

If the first catalytic noble metal is Pt, N
Oxidation reaction such as O becomes more active, and N _x
O _x adsorption action is further improved. Rh or P near Pt
Since d does not exist, Rh and Pd are not concentrated on the Pt surface, and Pt exhibits high catalytic activity. (2) At the time of stoichiometric richness In the exhaust gas purifying catalyst of the first invention, the N
NO _x is released from the O _x storage material, and the NO _x is reduced to some extent by the first catalytic noble metal in the presence of the reducing component and comes into contact with the second catalyst supporting layer.

In the second catalyst supporting layer, NO _x is further reduced by the second catalytic noble metal in the presence of a reducing component, and N ₂
It is purified. Here, when at least Rh is supported as the second catalytic noble metal, NO _x is more efficiently reduced by the excellent reduction activity of Rh, and the NO _x purification rate is further improved.

Further, if the second catalyst supporting layer contains a component having an oxygen storage ability as in the second invention, the oxygen stored during the lean operation is released from this component, so that the oxygen is contained in the exhaust gas. The remaining reducing components such as HC and CO used for the reduction of NO _x are oxidized and purified, and the ternary activity is improved. Further, as in the third invention, as a component having an oxygen storage ability, ceria is most preferable because of its excellent heat resistance.

[0017]

【Example】

[Specific Examples of the Invention] The material of the first and second porous carriers is not particularly limited, and may be selected from alumina, silica, silica-alumina, titania and the like. Among them, it is particularly preferable to use alumina having excellent heat resistance and noble metal dispersibility.

As the first catalytic noble metal, one of Pt and Rh is used.
One or more species can be used, with Pt being particularly desirable. The loading amount of any noble metal is preferably 0.2 to 40 g with respect to 100 g of the first porous carrier, and is 1 to 2 g.
0 g is particularly preferred. When converted to 1 liter volume of the entire catalyst, 0.1 to 20 g is preferable, and 0.5 to 1 g.
0 g is particularly preferred.

As the second catalytic noble metal, Pt, Rh, P
One or more of d can be used, and at least R
It is particularly desirable to include h. The desirable loading amount is the same as in the case of the first catalytic noble metal. Even if the supported amount of the catalytic noble metal is further increased, the activity is not improved, and its effective use cannot be achieved. On the other hand, if the supported amount of the catalytic noble metal is smaller than this, practically sufficient activity cannot be obtained.

In order to support the first catalytic noble metal and the second catalytic noble metal on the respective porous carriers, the chloride, nitrate and the like are conventionally used by impregnation, spraying, slurry mixing and the like. Can be carried in the same manner as described above. As the NO _x storage material contained in the first catalyst support layer, an alkali metal
And at least one selected from alkaline earth metals . Examples of the alkali metal include lithium, sodium, potassium, rubidium, cesium, and francium. Further, the alkaline earth metal refers to an element of Group 2A of the periodic table, and examples thereof include barium, beryllium, magnesium, calcium, and strontium.

The content of the NO _x occluding material is preferably in the range of 0.05 to 1.0 mol per 100 g of the first porous carrier. If the content is less than 0.05 mol, the NO _x storage capacity is small and the NO _x purification performance is reduced. Even if the content exceeds 1.0 mol, the NO _x storage capacity is saturated and the emission of HC increases at the same time. Such troubles occur. Examples of the component having the oxygen storage ability include iron, nickel, and ceria. Among them, ceria is typically exemplified as the one having the highest heat resistance. The content of this component is 0.05 to 1.0 with respect to 100 g of the second porous carrier.
Mol, more preferably 0.1 to 0.5 mol. If converted to 1 liter of the volume of the entire catalyst, it is preferably from 0.025 to 0.5 mol, particularly preferably from 0.05 to 0.25 mol. The effect is saturated even if this component is contained more than this amount, and if it is less than this, the practical effect cannot be sufficiently obtained.

The first catalyst supporting layer and the second catalyst supporting layer can be formed by coating on the surface of a monolithic carrier substrate, a metal carrier substrate or a pellet substrate. Also, for example, the base material is
It is also possible to adopt a structure in which a catalyst supporting layer is provided and the surface thereof is covered with a second catalyst supporting layer. Note that the thickness of the second catalyst supporting layer is preferably in the range of 10 to 100 μm. If the thickness is too thick, it is difficult for the exhaust gas to reach the first catalyst support layer, and if it is too thin, the reaction in the second catalyst support layer becomes insufficient, which is not preferable. Further, the thickness of the first catalyst supporting layer is preferably set to 10 μm or more in order to sufficiently perform the reaction. [Examples] Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples. It should be noted that all “parts” described below mean “parts by weight”. (Embodiment 1) FIG. 1 is a sectional view of a main part of an exhaust gas purifying catalyst of Embodiment 1. The exhaust gas purifying catalyst includes a honeycomb-shaped carrier substrate 1, a first catalyst carrier layer 2 coated on the surface of the carrier substrate 1, and a second catalyst carrier layer 3 supported on the surface of the first catalyst carrier layer 2. The first catalyst supporting layer 2 has Pt20
And Rh21 as well as Ba22 as a NO _x storage material, and Pt30 and Pd31 on the second catalyst support layer 3.

Hereinafter, a method for producing the exhaust gas purifying catalyst will be described, and the detailed description of the structure will be substituted. (1) Formation of First Catalyst Support Layer 100 parts of alumina powder, 70 parts of alumina sol (alumina content: 10% by weight), 15 parts of a 40% by weight aqueous solution of aluminum nitrate and 30 parts of water were mixed, and the mixture was thoroughly stirred to obtain a slurry. Was prepared.

Then, the cordierite honeycomb carrier base material 1 was immersed in water, and after blowing off excess water droplets, it was immersed in the slurry. After removing from the slurry, excess slurry is blown off and dried at 80 ° C. for 20 minutes.
The resultant was fired at 1 ° C. for 1 hour to form an alumina coat layer on the surface of the honeycomb carrier substrate. The coating amount of alumina was 50 g per liter of the honeycomb carrier substrate.

Next, the obtained honeycomb carrier was immersed in an aqueous solution of dinitrodiammineplatinum having a predetermined concentration, pulled up, blown off excess droplets, and dried at 250 ° C. to carry Pt20. Then, it was immersed in an aqueous solution of rhodium nitrate having a predetermined concentration, pulled up, blown off extra droplets, and dried at 250 ° C. to carry Rh21. In the coat layer, 2 g of Pt and 0.2 g of Rh are supported on 100 g of alumina of the first catalyst supporting layer.

The honeycomb carrier carrying the catalytic noble metal was immersed in an aqueous solution of barium acetate having a predetermined concentration, pulled up, blown off excess droplets, dried at 250 ° C., and fired at 500 ° C. to carry Ba22. Thus, the first catalyst supporting layer 2 was formed. Ba is alumina 1 of the first catalyst supporting layer.
0.6 mol of metal Ba is supported per 00 g. (2) An aqueous solution of dinitrodiammineplatinum and an aqueous solution of palladium nitrate were mixed with the slurry prepared before the formation of the second catalyst-supporting layer to a predetermined concentration. The honeycomb carrier substrate 1 on which the first catalyst supporting layer 2 is formed is immersed in the slurry, taken out, and then the excess slurry is blown off, dried at 80 ° C for 20 minutes, and dried at 600 ° C.
For 1 hour to form a second catalyst support layer 3 on the surface of the first catalyst support layer 2.

The amount of alumina coated on the second catalyst supporting layer 3 is 50 g per liter of the honeycomb substrate 1, and 100 g of alumina on the second catalyst supporting layer is Pt.
30 of 2 g and Pd31 of 0.2 g are supported. Example 2 The same as Example 1 except that the alumina coat layer was impregnated with an aqueous solution of dinitrodiammine platinum and an aqueous solution of barium acetate to form a first catalyst support layer 2 supporting Pt and Ba without supporting Rh. Thus, the exhaust gas purifying catalyst of Example 2 was prepared. The amounts of Pt and Ba carried are the same as in Example 1. (Example 3) Exhaust gas purification catalyst of Example 3 in the same manner as in Example 1 except that rhodium nitrate was used instead of palladium nitrate to form second catalyst-supporting layer 3 supporting Pt and Rh. Was prepared. The amount of Rh supported was P in Example 1.
Same as d. Example 4 An alumina coat layer was impregnated with an aqueous solution of dinitrodiammine platinum and an aqueous solution of barium acetate to form Pt and Ba.
Was formed in the same manner as in Example 1 except that the first catalyst supporting layer 2 supporting Pt and Rh was formed using rhodium nitrate instead of palladium nitrate. The exhaust gas purifying catalyst of Example 4 was prepared. Pt
The amounts of supported Ba and Ba are the same as in Example 1, and the amounts of supported Rh are the same as those of Pd in Example 1. Example 5 An exhaust gas purifying catalyst of Example 5 was prepared in the same manner as in Example 1 except that the second catalyst supporting layer 3 was formed using a slurry mixed with cerium oxide (ceria) powder. . Ceria is alumina 1 of the second catalyst support layer.
0.6 mol is supported per 00 g. Example 6 Pt and Ba were impregnated in an alumina coat layer with an aqueous solution of dinitrodiammine platinum and an aqueous solution of barium acetate.
Is formed, and rhodium nitrate is used in place of palladium nitrate and Pt, Rh and C are mixed using a slurry in which cerium oxide (ceria) powder is mixed.
The exhaust gas purifying catalyst of Example 6 was prepared in the same manner as in Example 1 except that the second catalyst supporting layer 3 supporting e was formed. The supported amount of Pt is the same as in Example 1, the supported amount of Rh is the same as Pd of Example 1, and the supported amount of ceria is the same as Example 5.

FIG. 2 shows the structure of the exhaust gas purifying catalyst of this embodiment. (Comparative Example 1) An alumina coat layer was formed in the same manner as in Example 1 except that the amount of alumina coating was set to 100 g per liter of the honeycomb carrier base material, which was twice as large as that of Example 1. Pt, Rh, Pd, and Ba were uniformly carried in the same manner as in the formation of the layer, to obtain an exhaust gas purifying catalyst of Comparative Example 1. The supported amounts of Pt, Rh, Pd and Ba were 2 g, 0.1 g and 100 g of alumina, respectively.
g, 0.1 g and 0.3 mol. (Comparative Example 2) An exhaust gas purifying catalyst of Comparative Example 2 was prepared in the same manner as in Comparative Example 1 except that an alumina coat layer was formed using a slurry mixed with ceria powder. The supported amount of ceria is 0.3 mol per 100 g of alumina. (Performance evaluation test) Each exhaust gas purification catalyst (volume 3
5 cc) was placed in a quartz reaction tube, and A / F = 14.5 (stoichiometric) using the two types of model gases shown in Table 1.
To A / F = 22.0 (lean) for a fixed time (2 minutes)
The gas was passed at a flow rate of 25 liter / min while changing every time, and the NO _x purification rate at that time was measured. The incoming gas temperature is 300 ° C. Table 2 shows the results.

[0029]

[Table 1]

[0030]

[Table 2] From a comparison between Example 1 and Comparative Example 1 and a comparison between Example 5 and Comparative Example 2, the example in which each component is separately supported on the first catalyst supporting layer 2 and the second catalyst supporting layer 3 has a higher NO _x purification. It is clear that the rate has improved.

Comparing the examples, Example 2 has a higher NO _x purification rate than Example 1, and Example 4 has a higher NO _x purification rate than Example 3. Therefore, it is understood that it is preferable to support only Pt as the first catalytic noble metal of the first catalyst supporting layer 2. Further, towards the Example 3 than in Example 1 is higher _the NO _x purification rate, towards the Example 2 than in Example 4 is higher _the NO _x purification rate. Therefore, the second catalyst supporting layer 3
It is apparent that the second catalytic noble metal preferably contains Rh.

Further, the embodiment 5 is more NO than the embodiment 1.
_{The x} purification rate is higher, and the sixth embodiment has a higher NO _x purification rate than the fourth embodiment. Therefore, it is clear that it is preferable to further support ceria on the second catalyst support layer 3.

[0033]

Effects of the Invention] That is, according to the exhaust gas purifying catalyst of the first invention, the inhibition of the oxidation reaction of the NO _x by reducing component in the exhaust gas is NO _x not occur are inserted efficiently _the NO _x storage material, The NO _x purification performance is improved. When the first catalytic noble metal is composed only of Pt, the oxidation of NO and the like becomes more active, and the NO _x storage effect is further improved. Also P
Since there is no Pd or Rh near t, Rh on the Pt surface
Pt and Pd are not concentrated, and a high catalytic activity of Pt is exhibited. Therefore, the NO _x purification performance is further improved.

When the second catalytic noble metal is Rh, R
Since h shows a high reduction activity, NO _x is reduced more efficiently, and the purification performance of NO _x is further improved. Further if it contains a component having oxygen storage capacity in the second catalyst support layer, since the component contributes to the oxidation of the reducing components such as HC and CO the remainder used for the reduction of NO _x, the three-way The activity is further improved.

When the component having oxygen storage ability is ceria, heat resistance is improved.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view illustrating a configuration of an exhaust gas purifying catalyst according to an embodiment of the present invention.

FIG. 2 is a schematic sectional view illustrating a configuration of an exhaust gas purifying catalyst according to a sixth embodiment of the present invention.

[Explanation of symbols]

1: support base material 2: first catalyst supporting layer 3:
Second catalyst support layer

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification code FI B01J 23/56 301A (72) Inventor Naoto Miyoshi 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Takeshima Shinichi 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Toshiaki Tanaka 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (56) References JP 62-197148 (JP, JP) A) JP-A-6-246159 (JP, A) JP-A-62-71536 (JP, A) WO 93/008383 (WO, A1) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53/94

Claims (3)

(57) [Claims] 1. A occluding NO _x during lean _the NO _x storage material
And released from NO _x storage material during stoichiometric rich
A catalyst for reducing and purifying the NO _x, a first porous carrier, and a first catalytic noble metal consisting of at least one of platinum and rhodium supported on the first porous carrier, the first porous carrier Alkali metal and Al contained
At least one kind of NO _x storage selected from potassium earth metals
Material, a first catalyst support layer made of a material, a second porous support, and a second support supported on the second porous support.
An exhaust gas purifying catalyst, comprising: a catalytic noble metal; and a second catalyst supporting layer made of a material coated on the first catalyst supporting layer. 2. The exhaust gas purifying catalyst according to claim 1, wherein the second catalyst supporting layer further contains a component having an oxygen storage ability. 3. The exhaust gas purifying catalyst according to claim 2, wherein the component having an oxygen storage ability is ceria.