JPH06327945A - Purifying method of exhaust gas - Google Patents

Abstract

PURPOSE:To efficiently purify NOx in the exhaust gas excess in oxygen and to prevent especially the deterioration of NOx purifying efficiency after duration. CONSTITUTION:The exhaust gas excess in oxygen is allowed to contact with the exhaust gas purifying catalyst carrying composite oxide consisting of at least 1 kind of Ba. Ti, Zr, Ce and Hf and at least 1 kind of Pt and Pb on the carrier consisting of a porous body. Because the Ba presents as the composite oxide, the deterioration due to contamination with poison in which the Ba becomes BaSO4 by SO3 is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying exhaust gas, and more specifically, carbon monoxide (CO) and hydrocarbon (HC).
The present invention relates to a method for efficiently purifying nitrogen oxides (NOx) in exhaust gas containing oxygen in excess of the amount required to oxidize hydrogen.

[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 in which a supporting layer is formed from γ-alumina or the like on a refractory carrier such as cordierite and a precious metal catalyst such as Pt, Pd or Rh is supported on the supporting layer is widely known. There is.

By the way, the purification performance of such an exhaust gas purification catalyst greatly differs depending on the air-fuel ratio (A / F) of the engine. That is, on the lean side where the air-fuel ratio is large, that is, where the fuel concentration is lean, the amount of oxygen in the exhaust gas is large, and the oxidation reaction for purifying CO and HC is active, but N
The reduction reaction for purifying Ox becomes inactive. On the contrary, on the rich side where the air-fuel ratio is small, that is, where the fuel concentration is high, the amount of oxygen in the exhaust gas is small, and the oxidation reaction becomes inactive but the reduction reaction becomes active.

On the other hand, when driving an automobile, the vehicle is frequently started and stopped when traveling in urban areas, and the air-fuel ratio frequently changes within the range from near the theoretical value to the over lean 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, even on the lean side, N
Development of a catalyst capable of sufficiently purifying Ox is desired.

[0005]

Therefore, the applicant of the present application is
Previously, a catalyst supporting an alkaline earth metal and Pt was proposed (Japanese Patent Application No. 4-130904). According to this catalyst, NOx is adsorbed on the alkaline earth metal and is reacted with a reducing gas such as HC to be purified, so that the lean side is also excellent in NOx purification performance.

However, even with this catalyst, it became clear that the NOx purification rate drops to around 50% of the initial level after the durability test. Therefore, it is required to further improve the purification rate after endurance before it is put into practical use. 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.

[0007]

An exhaust gas purifying method of the present invention which solves the above-mentioned problems is to purify exhaust gas for simultaneously purifying carbon monoxide, hydrocarbons and nitrogen oxides in the exhaust gas in an oxygen excess atmosphere. A method for purifying exhaust gas, comprising a composite carrier comprising Ba and at least one of Ti, Zr, Ce and Hf and at least one of Pt and Pd supported on a porous carrier. It is characterized in that exhaust gas in excess of oxygen is brought into contact with the catalyst.

Examples of the porous material include alumina, zeolite, zirconia, silica-alumina, silica and the like. The carrier may be formed from these porous bodies themselves, or may be used by coating on a honeycomb body formed of cordierite, heat-resistant metal or the like.

[0009]

In the exhaust gas purification method of the present invention, Ba is Ti,
A catalyst that is supported as at least one of Zr, Ce, and Hf in the form of a composite oxide and at least one of Pt and Pd is used, and exhaust gas in excess of oxygen is contacted and purified. Here, in the catalyst disclosed in Japanese Patent Application No. 4-130904, barium is supported alone, and it reacts with NOx to produce Ba (NO ₃ ) ₂ to generate N.
It is considered to adsorb Ox. However, the exhaust gas contains SO ₂ produced by the combustion of sulfur (S) contained in the fuel, and this is further oxidized by the catalyst metal in the oxygen excess atmosphere to become SO ₃ . Then, again, it is easily converted into sulfuric acid by the water vapor contained in the exhaust gas, reacts with the barium, and barium sulfate (BaSO 4
It was clarified that it suffers from poisoning deterioration generated by ₄ ). When barium becomes sulfate in this way, NO
It is considered that x cannot be adsorbed, and as a result, the catalyst has a reduced NOx purification performance after the durability test.

However, it has been clarified that in the catalyst used in the present invention, since barium exists as a complex oxide, the production of barium sulfate is suppressed. This is presumed to be because barium atoms are present in the complex oxide lattice in a highly dispersed state, so that even if SO ₃ is adsorbed, it is easily desorbed and crystal growth as barium sulfate is suppressed.

On the other hand, since the above-mentioned composite oxide is basic, the NOx adsorption capacity is not impaired, and it exhibits the same performance as barium alone before poisoning deterioration by SO ₂ .

[0012]

EXAMPLES The present invention will be specifically described below with reference to examples. (Example 1) <Preparation of Catalyst> surface area of 100 m ² / g for cerium oxide (CeO ₂₎ powder was impregnated with barium acetate aqueous solution, to form a Ba-Ce composite oxide powder was calcined at 800 ° C. After drying .

Next, 100 parts by weight of activated alumina powder,
30 parts by weight of the composite oxide powder, 10 parts by weight of alumina sol (alumina content 10% by weight), and 40 parts by weight of water are mixed to prepare a slurry, and a slurry is prepared by immersing the cordierite honeycomb carrier in the slurry. The resulting slurry was blown off, dried and then baked at 500 ° C. to form an alumina coat layer carrying a composite oxide. The coating amount is 120 g / liter.

The carrier having the alumina coat layer was dipped in a dinitrodiammineplatinum aqueous solution, excess water droplets were blown off, dried and then calcined at 300 ° C. to obtain a catalyst A. In addition, in the catalyst A, Ba-Ce composite oxide was used as barium in an amount of 0.3.
Mol / liter and 0.3 mol / liter of cerium are carried. Platinum is loaded at 2.0 g / liter. <Evaluation of Purification Performance> The catalyst was installed in the exhaust passage of a vehicle equipped with a lean burn engine (1.6 liters), and the NOx purification rate was measured by traveling in an urban driving mode (10.15 mode). Next, an endurance test was carried out in which the air-fuel ratio (A / F) was 18, and the gas temperature with the catalyst was 600 ° C. for 100 hours, and then the NOx purification rate was measured under the same conditions as above. The respective results are shown in Table 1. (Example 2) 100 parts by weight of activated alumina powder, 10 parts by weight of alumina sol (alumina content of 10% by weight), and 40 parts by weight of water were mixed to prepare a slurry, and a cordierite honeycomb carrier was added to the slurry. After the immersion, the excess slurry was blown off, dried and then baked at 500 ° C. to form an alumina coat layer. The coating amount is 120 g / liter.

Next, the carrier having an alumina coat layer was impregnated with a mixed aqueous solution of cerium acetate and barium acetate, and the excess water droplets were blown off to dry the product, followed by firing at 600 ° C. Further, it was immersed in a dinitrodiammine platinum aqueous solution, excess water droplets were blown off, dried and then calcined at 300 ° C. to obtain a catalyst B. In this catalyst B, a Ba-Ce composite oxide is 0.2 mol / liter as barium and 0.2 mol as cerium.
/ Liter is carried. Platinum is loaded at 2.0 g / liter.

The NOx purification rate of this catalyst B was measured in the same manner as in Example 1, and the results are shown in Table 1. (Example 3) In order to further enhance the oxygen storage effect, which is another great characteristic of the three-way catalyst, a part of Ce in Example 1 was changed to
It was replaced with Nb, which is a valent element. That is, a niobium oxide (Nb ₂ O ₅ ) powder is impregnated with a mixed aqueous solution of barium acetate and cerium acetate, dried and baked at 800 ° C. to form Ba−.
A Ce-Nb composite oxide powder was formed.

Then, a coat layer was formed in the same manner as in Example 1 and platinum was supported thereon to obtain a catalyst C. In this catalyst C, barium was 0.3 mol / liter, cerium was 0.24 mol / liter, and niobium was 0.0
6 mol / liter of Ba-Ce-Nb composite oxide is supported. Platinum is loaded at 2.0 g / liter.

The NOx purification rate of this catalyst C was measured in the same manner as in Example 1, and the results are shown in Table 1. (Comparative Example) A carrier having the same alumina coating layer as in Example 2 was dipped in an aqueous barium acetate solution, blown off excess water droplets, dried, and then baked at 600 ° C. Further, it was immersed in a dinitrodiammine platinum aqueous solution, excess water droplets were blown off, dried and then calcined at 300 ° C. to obtain catalyst D. The catalyst D contained 0.3 mol / liter of barium and 2.0 of platinum.
g / l is supported, and barium is supported in the state of a single oxide.

The NOx purification rate of this catalyst D was measured in the same manner as in Example 1, and the results are shown in Table 1.

[0020]

[Table 1] <Evaluation> From Table 1, it is understood that the catalysts A to C carrying barium as a composite oxide have a much lower degree of reduction in the NOx purification rate after endurance than the catalyst D of the comparative example. On the other hand, when the X-ray diffraction of each catalyst after the durability test was measured, the diffraction line identified by BaSO ₄ was observed in catalyst D, but not observed in catalysts A to C. Therefore, the difference in the NOx purification rate after endurance of each catalyst is B
It is speculated that the presence of aSO ₄ has a great influence.

Further, the catalyst C is NO both in the initial stage and after the endurance.
x purification rate is superior to catalyst A and catalyst B. this is,
It is presumed that a part of Ce ⁴⁺ was changed to Ce ³⁺ by the addition of pentavalent Nb, and the oxygen storage effect was enhanced. In addition,
It should be noted that the catalysts A to D have no difference in purification performance of carbon monoxide (CO) and hydrocarbon (HC), and are all excellent.

[0022]

[Effects of the Invention] That is, according to the exhaust gas purifying method of the present invention, the catalyst used exhibits good NOx purifying performance even after the durability test, and can purify NOx stably and efficiently on the lean side of excess oxygen. it can.

Claims (1)

[Claims] 1. A method for purifying exhaust gas for purifying carbon monoxide, hydrocarbons and nitrogen oxides in the exhaust gas in an oxygen-excess atmosphere at the same time, wherein Ba, Ti, Zr, Exhaust gas purification, characterized by bringing an exhaust gas in excess of oxygen into contact with an exhaust gas purification catalyst that carries at least one of Pt and Pd and a complex oxide composed of at least one of Ce and Hf. Method.