JPH07136510A - Production of catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To further efficiently purify NOx in exhaust gas with excess oxygen and improve the purifying performance for NOx after used for a long term. CONSTITUTION:The catalyst is produced by successively carrying a rare earth element (e.g. La), noble metal (e.g. Pt) and alkaline earth metal (e.g. Ba) on a porous carrier containing alumina on its surface. Since the alumina and the rare earth element react to modify the surface state of the carrier, the alkaline earth metal can be adsorbed in a high dispersion state so that alkaline earth metal of a small particle size can be carried.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an exhaust gas purifying catalyst. The exhaust gas purifying catalyst obtained by this method contains nitrogen oxides (NOx) in exhaust gas containing oxygen in excess of the amount required to oxidize carbon monoxide (CO) and hydrocarbons (HC). ) Is preferred.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by simultaneously oxidizing CO and HC and reducing NOx has been used as a catalyst for purifying exhaust gas of an automobile.
As such a catalyst, for example, a catalyst is widely known in which a supporting layer made of γ-alumina is formed on a heat resistant carrier such as cordierite, and a precious metal such as Pt, Pd, or Rh is supported on the supporting layer. .

[0003]

However, the purification performance of the above-mentioned conventional exhaust gas purifying catalyst greatly differs depending on the air-fuel ratio of the engine. That is, on the lean side, the amount of oxygen in the exhaust gas increases, and while the oxidation reaction for purifying CO and HC is active, the reduction reaction for purifying NOx becomes inactive. On the other hand, on the rich side, the amount of oxygen in the exhaust gas decreases and the oxidation reaction becomes inactive, but the reduction reaction becomes active.

On the other hand, when the vehicle is traveling in urban areas, the vehicle is frequently started and stopped, so that the air-fuel ratio frequently changes within the range from near the stoichiometric state to the rich state. In order to meet the demand for low fuel consumption in such traveling, it is necessary to operate on the lean side to supply an air-fuel mixture with excess oxygen as much as possible. Therefore, it is desired to develop a catalyst that can sufficiently purify NOx even on the lean side.

Therefore, the applicant of the present application has previously proposed a catalyst carrying an alkaline earth metal oxide and Pt (Japanese Patent Application No. 4-130904). According to this catalyst, NOx
Adsorbs on alkaline earth metals and reacts with reducing gases such as HC to be purified, so that NO
x is excellent in purification performance. In this catalyst, for example, barium as an alkaline earth metal is supported on a carrier as a single oxide, which reacts with NOx to generate barium nitrate (Ba (NO ₃ ) ₂ ) to adsorb NOx. It is believed that.

However, the exhaust gas contains SO ₂ produced by the combustion of sulfur (S) contained in the fuel, which is further oxidized by a noble metal such as Pt in an oxygen excess atmosphere to form SO _3. Becomes Then, it was also clarified that it was easily converted into sulfuric acid by the water vapor contained in the exhaust gas and reacted with barium to generate barium sulfate (BaSO ₄ ), and barium was poisoned and deteriorated. As described above, when barium becomes a sulfate, NOx can no longer be adsorbed, and as a result, the catalyst has a problem that the NOx purification performance after durability is deteriorated. This tendency is particularly large after endurance at high temperature.

The present invention has been made in view of the above circumstances, and an object thereof is to further efficiently purify NOx in exhaust gas with excess oxygen and to improve the purification performance of NOx after endurance. To do.

[0008]

As a result of intensive studies to solve the above problems, the present inventor carried out barium loading on a porous carrier containing alumina on the surface, and then loaded a noble metal. It has been discovered that the catalyst or the catalyst in which barium is supported on the porous carrier supporting the noble metal has a large decrease in NOx purification performance after endurance. Then, the inventors have discovered a method for producing a catalyst that can reduce deterioration of NOx purification performance after endurance, and completed the present invention.

That is, the method for producing an exhaust gas purifying catalyst of the present invention is an exhaust gas purifying method for purifying exhaust gas by simultaneously purifying carbon monoxide, hydrocarbons and nitrogen oxides in the exhaust gas in an oxygen excess atmosphere. A method for producing a catalyst for use in which at least one element selected from rare earth elements is supported on a porous support containing alumina on its surface, followed by supporting a noble metal, and then selecting from an alkaline earth metal. And carrying at least one element of

The rare earth elements include lanthanum (La), cerium (Ce) and neodymium (N) in lanthanoids.
d) etc., scandium (Sc), yttrium (Y) can be adopted.

[0011]

When barium is supported on a porous carrier containing alumina on its surface, or when barium is supported on this porous carrier carrying a noble metal, barium is not dispersed properly and remains in a large particle size. Barium is supported. Barium having a large particle diameter becomes large crystal barium sulfate due to S in the exhaust gas, which makes regeneration difficult and deteriorates the NOx purification performance after endurance.

According to the test results of the present inventor, the relationship shown in FIG. 1 is established between the particle size of barium and the reproducibility (durability) of decomposing barium sulfate. Therefore, the reason why the durability is not sufficient is that the particle size of barium supported on the carrier is large. Further, if the alumina and barium are brought into direct contact with each other in this way, the alumina and barium react with each other at a high temperature to produce BaAl ₂ O _4, and N by Ba is added.
It is considered that the adsorption of Ox is hindered.

On the other hand, in the production method of the present invention, at least one element selected from rare earth elements is first supported on the porous carrier containing alumina on the surface thereof, and thereafter, the noble metal is further supported, and further after this. It carries an alkaline earth metal. In such a case, first, it is considered that the alumina and the rare earth element react with each other, and thereby the surface condition of the carrier is modified. According to the inventors' consideration, this modification of the surface state is considered to be a state in which the rare earth aluminate is produced in a highly dispersed state on the carrier due to the reaction between alumina and the rare earth element. That is, if the rare earth element is La, lanthanum aluminate (AlLaO ₃ ) is produced in a highly dispersed state on the carrier.

Then, it is considered that the surface condition of the carrier is further modified by supporting the noble metal. After this,
It is considered that when an alkaline earth metal such as barium is supported, the surface state is modified so that the alkaline earth metal is adsorbed in a highly dispersed state, and an alkaline earth metal having a small particle size is supported. That is, the rare earth element and then the noble metal carried thereon improve the dispersibility of the alkaline earth metal carried later. By firing during loading, the alkaline earth metal splits into smaller particle sizes. Alkaline earth metal having a small particle size can be converted into a small crystalline alkaline earth metal sulfate even by S in exhaust gas, which facilitates regeneration and improves NOx purification performance after endurance.

Further, in the production method of the present invention, since the rare earth element and the noble metal are interposed between the alumina and the alkaline earth metal, the reaction between the alumina and barium at a high temperature is suppressed, and the NOx by Ba is reduced. It is considered that the adsorption of is not hindered. It is considered that the rare earth element also produces a sulfate by S contained in the fuel. That is,
If the rare earth element is La, lanthanum sulfate (La ₂ (S
O ₄ ) ₃ ), if the rare earth element is Ce, cerium sulfate (Ce (SO ₄ ) ₂ ) or cerium sulfate (Ce ₂ (S ₂
When O ₄ ) ₃ ) and the rare earth element are Nd, neodymium sulfate (Nd ₂ (SO ₄ ) ₃ ) is produced. However, the decomposition temperatures of these rare earth element sulfates are 1150 (° C.) for La ₂ (SO ₄ ) ₃ , 350 (° C.) for Ce (SO ₄ ) ₂ and C
e ₂ (SO ₄ ) ₃ is 920 (° C), Nd ₂ (SO ₄ ) ₃
Is 1176 (° C.), which is lower than that of barium sulfate (BaSO ₄ ) having a decomposition temperature of 1580 (° C.). For this reason,
Rare earth element sulfates are more easily decomposed than alkaline earth metal sulfates such as barium sulfate.

[0016]

EXAMPLES Examples of the present invention will be described below.
In the following examples, "parts" means "parts by weight" unless otherwise specified. (Preparation of catalysts of Example products No. 1 to 11) 100 parts of alumina powder and alumina sol (alumina content rate 10 wt
%) 70 parts and 40 wt% aluminum nitrate aqueous solution 15
And 30 parts of water were mixed to prepare a coating slurry.

After dipping the cordierite honeycomb carrier in the slurry, the excess slurry is blown off. Next, this carrier was impregnated with La, Ce, and Nd as nitrates, dried, and then baked at 650 ° C. for 2 hours to form an alumina coat layer. The coating amount is 120 g per liter of the volume of the honeycomb carrier. The amounts of La, Ce and Nd carried are as shown in Table 1.

[0018]

[Table 1]

The honeycomb carrier having the alumina coat layer was dipped in a dinitrodiammineplatinum aqueous solution to blow off excess water droplets and then baked at 250 ° C. for 1 hour to support Pt. The supported amount of Pt is as shown in Table 1. next,
The Pt-supported honeycomb carrier was immersed in an aqueous solution of barium acetate having a predetermined concentration or each nitrate of magnesium, calcium and strontium prepared so as to have the supported amount shown in Table 1, and the excess water droplets were blown off, and after drying, 500 ° C. In 1
Burned for hours.

Thus, as shown in Table 1, No. 1 as an example product is loaded in the order of rare earth element loading, precious metal loading, and alkaline earth metal loading. 1-11 catalysts were prepared. (Preparation of Catalysts of Comparative Example Products Nos. 1 to 4) As shown in Table 2, the order of carrying rare earth elements, carrying noble metals and carrying alkaline earth metals was changed to prepare No. 1-4 catalysts were prepared. Other conditions are the same as those in the example.

[0021]

[Table 2]

(Evaluation of purification performance) Lean (A / F = 18
Equivalent), rich (A / F = 14 equivalent) model gas 2
Switching every minute, NOx purification rate (%) at the beginning and end
It was evaluated after reproduction. The results are also shown in Tables 1 and 2. However, the durability and regeneration conditions were as follows. Durability conditions: Inlet gas temperature = 600 ° C x 30 minutes, SO ₂ =
Regeneration condition of lean gas of 600 ppm: input gas temperature = 600 ° C. × 10 minutes, rich gas From Tables 1 and 2, from Example No. 1 manufactured in the order of rare earth element loading, precious metal loading, alkaline earth metal loading. 1 to
The catalyst of Comparative Example No. 11 in which the order is changed is the catalyst of No. 11. 1-4
It can be seen that NOx in the exhaust gas in excess of oxygen can be purified more efficiently than the catalyst of No. 1 and the purification performance of NOx after endurance is also improved.

[0023]

As described above in detail, in the production method of the present invention, since the rare earth element, the noble metal and the alkaline earth metal are loaded in this order on the porous carrier containing alumina on the surface,
It is possible to manufacture an exhaust gas purifying catalyst that exhibits good NOx purification performance even after endurance and can stably purify NOx stably on the lean side of excess oxygen.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the particle size of barium as an alkaline earth metal and the reproducibility (durability) of decomposing barium sulfate.

Claims (1)

[Claims] 1. A method for producing an exhaust gas purifying catalyst for purifying exhaust gas by simultaneously purifying carbon monoxide, hydrocarbons and nitrogen oxides in the exhaust gas under an oxygen excess atmosphere, comprising: The porous carrier to be contained contains at least one element selected from rare earth elements, then supports a noble metal, and then supports at least one element selected from alkaline earth metals. Exhaust gas purification catalyst manufacturing method.