JPH0889801A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE: To provide a catalyst for purification of an exhaust gas exhibiting a high NOx absorption rate without using an NOx absorbing material such as barium. CONSTITUTION: A catalyst for purification of an exhaust gas consists of a carrier consisting of Mn-Zr double oxides and a catalyst precious metal incorporated in the carrier. The Mn-Zr double oxides have lattice defects based on oxygen defects and as diffusion of oxygen into the lattice defects is smoothly performed, it is estimated that not only oxygen but also NOx are diffused and adsorbed and as the result, a high NOx absorption rate is exhibited.

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, and more particularly to a catalyst capable of efficiently purifying nitrogen oxides (NOx) even in lean exhaust gas.

[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 made of γ-alumina is formed on a heat resistant carrier such as cordierite and a catalytic precious metal such as Pt, Pd, 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, acceleration and deceleration are frequently performed in urban areas, and the air-fuel ratio frequently changes within the range from near stoichiometric (theoretical air-fuel ratio) 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 an exhaust gas purifying catalyst in which an alkaline earth metal and Pt are carried on a porous carrier such as alumina (Japanese Patent Laid-Open No. 5-317652).
issue). 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.

In the catalyst disclosed in Japanese Unexamined Patent Publication No. 5-317652, for example, barium is supported on a carrier as a single oxide, which reacts with NOx to produce barium nitrate (Ba (N
It is believed to absorb NOx by producing O ₃ ) ₂ ).

[0007]

However, in the exhaust gas, sulfur (S) contained in the fuel is burned to generate S.
Ox is contained, and it is oxidized to SO ₃ by the catalytic metal in the oxygen excess atmosphere. It was also clarified that sulfuric acid is easily converted to sulfuric acid by the water vapor contained in the exhaust gas, which reacts with barium to generate barium sulfite or barium sulfate, which causes the barium to be poisoned and deteriorated. In addition, the porous carrier such as alumina is SOx.
Since sulfur is easily absorbed, there is a problem that the sulfur poisoning is promoted.

When barium becomes a sulfite or a sulfate as described above, NOx can no longer be adsorbed, and as a result, the catalyst has a problem that the NOx purification performance after durability is deteriorated. Therefore, N such as alkaline earth metal and alkali metal such as barium
An exhaust gas purifying catalyst in which Mn or Zr and a catalytic noble metal are supported on an alumina carrier without using an Ox absorbent has been studied, but there is a problem that the amount of NOx absorbed is smaller than that of a NOx absorbent such as barium. It was

The present invention has been made in view of the above circumstances, and an object thereof is to provide an exhaust gas purifying catalyst showing a high NOx absorption rate without using a NOx absorbent such as barium.

[0010]

The exhaust gas purifying catalyst of the present invention for solving the above-mentioned problems is characterized by comprising a carrier composed of Mn-Zr composite oxide and a catalytic noble metal contained in the carrier. And

[0011]

In the exhaust gas purifying catalyst of the present invention, the carrier is Mn.
-It consists of a Zr compound oxide. This Mn-Zr composite oxide has lattice defects due to oxygen defects, has a large specific surface area, and has characteristics that oxygen can be absorbed and released smoothly. Therefore, it has an oxygen storage capacity equal to or higher than that of cerium oxide and absorbs variations in oxygen concentration in the vicinity of stoichiometry, so that stable purification performance with a catalytic noble metal can be obtained.

Oxygen diffuses smoothly into the lattice defects of the Mn-Zr composite oxide, so that N simultaneously with oxygen.
It is assumed that Ox diffuses and is adsorbed, and exhibits a high NOx absorption rate. For example, it is considered that NO in the exhaust gas is oxidized in the lean atmosphere by the catalytic action of the catalytic noble metal to become NO ₂ and diffused and absorbed into the lattice defects together with oxygen. As a result, NOx emission is suppressed even on the lean atmosphere side.

The NOx absorbed in the Mn-Zr composite oxide is separated from the Mn-Zr composite oxide together with oxygen in a stoichiometric or rich atmosphere, and reduced by the reducing substance in the exhaust gas by the catalytic action of the catalytic noble metal. It is considered to be one. Due to the above actions, the exhaust gas purifying catalyst of the present invention exhibits high NOx purifying performance.

[0014]

[Examples of the invention] The Mn / Zr ratio in the Mn-Zr composite oxide constituting the carrier is preferably a molar ratio close to 1/1. For example, if the ratio is biased such as 1/9 or 9/1, the NOx absorption rate tends to decrease. Note that Mn
The specific surface area of the —Zr composite oxide is preferably 100 or more.

With respect to the shape of the carrier, a carrier substrate such as a pellet carrier may be formed from the Mn-Zr complex oxide itself, or a heat-resistant substrate such as cordierite may be coated with the Mn-Zr complex oxide. Can also be formed into a carrier.
Alternatively, the Mn-Zr composite oxide may be mixed with alumina or the like to form a carrier. To form the Mn-Zr composite oxide, known methods such as coprecipitation method and sol-gel method using metal alkoxide can be used.

At least one of platinum (Pt), palladium (Pd) and rhodium (Rh) is used as the catalytic noble metal. The content of platinum or palladium is 10
The range of 0.1 to 20.0 g is desirable with respect to 0 g,
A range of 0.3-10.0 g is particularly preferred. If the content is less than 0.1 g, the NOx purification performance at the initial stage and after endurance will be deteriorated, and if the content exceeds 20.0 g, the effect will be saturated and the catalyst noble metal contained in excess will not be effectively utilized.

The content of rhodium is preferably in the range of 0.001 to 1.0 g, and 0.05 to 100 g per 100 g of the carrier.
A range of 0.5 g is particularly preferred. Content is 0.001g
If it is less, the NOx purification performance at the initial stage and after durability will deteriorate, and if it exceeds 1.0 g, the effect of platinum or palladium will conversely decrease. The content of rhodium is preferably determined relative to the content of platinum or palladium, and is 1/3 or less of the total content of platinum or palladium,
More preferably, it should be 1/5 or less. [Examples] Hereinafter, specific examples will be described. (Example 1) Manganese nitrate aqueous solution (concentration: 0.25 to 1 mo)
l / L) and zirconium oxynitrate aqueous solution (concentration 0.25
˜1 mol / L) in a volume ratio of 1: 1 mixed solution,
While stirring, 0.5 ml of 10% concentration aqueous ammonia was gradually added dropwise.

The deposited precipitate was filtered, washed with distilled water several times, dried, and calcined at 450 ° C. for 6 hours to obtain Mn-Z.
An r complex oxide powder was prepared. As a result of X-ray diffraction analysis, it was confirmed that Zr was solid-solved in MnO _{2 of the} Mn-Zr composite oxide powder. Next, a predetermined amount of Mn-Zr composite oxide powder was dipped in a dinitrodiammine platinum aqueous solution with a predetermined concentration, stirred, evaporated to dryness, and then baked at 250 ° C for 1 hour to support Pt. Then, the obtained powder was pelletized by an ordinary method to prepare an exhaust gas purifying catalyst of Example 1 carrying Pt at 2.0 g / L. (Example 2) In the same manner as in Example 1 except that a Mn-Zr composite oxide powder was prepared using a mixed solution in which the mixing ratio of the manganese nitrate aqueous solution and the zirconium oxynitrate aqueous solution was 1: 9 by volume. , Pt was carried at 2.0 g / L to prepare an exhaust gas purifying catalyst of Example 2. This Mn-Z
As a result of X-ray diffraction analysis, it was confirmed that Zr was solid-solved in MnO ₂ in the r composite oxide powder. (Example 3) In the same manner as in Example 1 except that a Mn-Zr composite oxide powder was prepared using a mixed solution in which the mixing ratio of the manganese nitrate aqueous solution and the zirconium oxynitrate aqueous solution was 9: 1 by volume. , Pt was carried at 2.0 g / L to prepare an exhaust gas purifying catalyst of Example 3. This Mn-Z
As a result of X-ray diffraction analysis, it was confirmed that Zr was solid-solved in MnO ₂ in the r composite oxide powder. (Example 4) A Pt-supported alumina powder was prepared by immersing a predetermined amount of γ-alumina powder in an aqueous solution of dinitrodiammine platinum having a predetermined concentration, stirring, evaporating to dryness, and baking at 250 ° C for 1 hour.

Then, 10 g of the Mn-Zr composite oxide powder prepared in Example 1 and 10 g of Pt-supported alumina powder were physically mixed, and the obtained powder was pelletized by a conventional method.
An exhaust gas purifying catalyst of Example 4 carrying 2.0 g / L of was prepared. (Example 5) Manganese dioxide powder was immersed in an aqueous zirconium oxynitrate solution having a predetermined concentration, stirred, evaporated to dryness, and then dried.
Pt was 2.0% in the same manner as in Example 1 except that the Mn-Zr composite oxide powder was prepared by firing at 50 ° C for 3 hours.
An exhaust gas purifying catalyst of Example 5 carrying g / L was prepared. As a result of X-ray diffraction analysis, it was confirmed that Zr was solid-solved in MnO _{2 of the} Mn-Zr composite oxide powder. (Example 6) Zirconium dioxide powder was immersed in an aqueous solution of manganese nitrate having a predetermined concentration, stirred, evaporated to dryness, and then 450 ° C.
The Pt content was 2.0 g / L in the same manner as in Example 1 except that the Mn-Zr composite oxide powder was prepared by firing for 3 hours.
The supported exhaust gas-purifying catalyst of Example 6 was prepared. As a result of X-ray diffraction analysis, the Mn-Zr composite oxide powder was
It was confirmed that Mn was in solid solution in ZrO ₂ . (Comparative Example 1) A predetermined amount of manganese dioxide powder was immersed in an aqueous dinitrodiammine platinum solution having a predetermined concentration, stirred, evaporated to dryness, and then baked at 250 ° C for 1 hour to obtain Pt-supported MnO _2.
A powder was prepared.

The Pt-supported MnO ₂ powder thus obtained was pelletized by a conventional method to prepare an exhaust gas purifying catalyst of Comparative Example 1 carrying Pt at 2.0 g / L. (Comparative Example 2) A predetermined amount of zirconium dioxide powder was immersed in an aqueous dinitrodiammine platinum solution having a predetermined concentration, stirred, evaporated to dryness, and then baked at 250 ° C for 1 hour to obtain Pt-supported Z.
An rO ₂ powder was prepared.

The Pt-supported ZrO ₂ powder thus obtained was pelletized by a conventional method to prepare an exhaust gas purifying catalyst of Comparative Example 2 in which Pt was supported at 2.0 g / L. (Comparative Example 3) Manganese dioxide powder and zirconium dioxide powder were mixed at a molar ratio of 1: 1 to prepare a mixed powder. Then, a predetermined amount of the mixed powder was immersed in an aqueous dinitrodiammine platinum solution having a predetermined concentration, stirred, evaporated to dryness, and then baked at 250 ° C. for 1 hour to prepare a Pt-supported mixed powder.

The resulting Pt-supported mixed powder was pelletized by a conventional method to prepare an exhaust gas purifying catalyst of Comparative Example 3 in which Pt was supported at 2.0 g / L. (Comparative Example 4) γ-alumina powder was immersed in an aqueous solution of manganese nitrate having a predetermined concentration, stirred, evaporated to dryness, and then baked at 250 ° C for 1 hour. Then, it was immersed in an aqueous dinitrodiammine platinum solution of a predetermined concentration, stirred, evaporated to dryness, and then baked at 650 ° C. for 2 hours to prepare an Mn—Pt-supported alumina powder.

The obtained Mn-Pt-supported alumina powder was pelletized by a conventional method to prepare an exhaust gas purifying catalyst of Comparative Example 4 in which Mn was 0.3 mol / L and Pt was 2.0 g / L. (Comparative Example 5) γ-in a zirconium nitrate aqueous solution having a predetermined concentration
The alumina powder was dipped, stirred, evaporated to dryness, and then baked at 250 ° C. for 1 hour. Then, it was immersed in a dinitrodiammine platinum aqueous solution of a predetermined concentration, stirred, evaporated to dryness and then baked at 650 ° C. for 2 hours to prepare a Zr—Pt-supported alumina powder.

The obtained Zr-Pt-supported alumina powder was pelletized by a conventional method to prepare an exhaust gas purifying catalyst of Comparative Example 5 carrying Zr of 0.3 mol / L and Pt of 2.0 g / L. (Comparative Example 6) γ-alumina powder was immersed in an aqueous solution of manganese nitrate having a predetermined concentration, stirred, evaporated to dryness, and then baked at 250 ° C for 1 hour. Next, it was immersed in an aqueous zirconium nitrate solution having a predetermined concentration, stirred, evaporated to dryness, and then baked at 250 ° C. for 1 hour. Further, it was immersed in an aqueous dinitrodiammine platinum solution having a predetermined concentration, stirred, evaporated to dryness and then baked at 650 ° C. for 2 hours to prepare an Mn—Zr—Pt-supported alumina powder.

The obtained Mn-Zr-Pt-supported alumina powder was pelletized by a conventional method to obtain Mn of 0.3 mol / L,
An exhaust gas purifying catalyst of Comparative Example 6 supporting Zr of 0.3 mol / L and Pt of 2.0 g / L was prepared. As a result of X-ray diffraction, Mn and Zr did not form a composite oxide and were present as individual oxides. (Comparative Example 7) A Pt-supported alumina powder was prepared by immersing a predetermined amount of γ-alumina powder in an aqueous solution of dinitrodiammine platinum having a predetermined concentration, stirring the mixture, evaporating it to dryness, and then firing at 250 ° C for 1 hour.

The Pt-supported alumina powder thus obtained was immersed in an aqueous barium acetate solution having a predetermined concentration, stirred, evaporated to dryness, and calcined at 500 ° C. for 1 hour. The obtained powder was pelletized by a conventional method, and Pt was 2.0 g / L and Ba was 0.3 mol.
An exhaust gas purifying catalyst of Comparative Example 7 carrying / L was prepared. (Comparative Example 8) 2 g of Pt was added to a γ-alumina carrier containing 0.25 mol% of ceria and 0.13 mol% of lanthanum.
The L-supported catalyst was used as the exhaust gas purifying catalyst of Comparative Example 8. (Evaluation of NOx absorption rate) 800p NO in a lean atmosphere
Exhaust gas containing pm was caused to flow through each catalyst for 2 minutes at an inlet gas temperature of 300 ° C., and absorbed N with respect to the total amount of NOx.
The ratio of the amount of Ox was calculated and used as the NOx absorption rate. The respective results are shown in Table 1.

[0027]

[Table 1] (NOx Purifying Performance) Using the exhaust gas purifying catalysts of Example 1 and Comparative Example 8, the respective purifying rates were measured for exhaust gases at various temperatures, and the results are shown in FIG. The NOx purification rate is the NOx temperature characteristic of a full size catalyst evaluated on a bench.
It was measured according to the A / F characteristics. Further, the purification rate when the A / F was changed was measured for the catalyst of Example 1, and the results are shown in FIG.

From FIG. 1, the catalyst of Example 1 has an activity of 20 at a low temperature as compared with the catalyst of Comparative Example 8 which has been conventionally used.
%, And the A / F window width is expanded by about 0.1. Further, from FIG. 2, the HC / CO-NOx cross purification rate of the catalyst of Example 1 was A / F = 14.
It is located near 9 and it is clear that the catalyst moves from the vicinity of A / F = 14.7 of the conventional catalyst to the lean side.

[0029]

In other words, according to the exhaust gas purifying catalyst of the present invention, the window width is expanded and the low temperature activity is improved. This is because the carrier made of Mn-Zr composite oxide is NOx.
It is clear that this is due to the fact that it has a NOx absorption capacity similar to that of an absorber, and it is clear that it is an exhaust gas purification catalyst that has high NOx purification performance without using a NOx absorber.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between temperature and NOx purification rate.

FIG. 2 is a graph showing the relationship between A / F and purification rate.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location B01D 53/36 104 A

Claims (1)

[Claims] 1. An exhaust gas purifying catalyst comprising a carrier made of a Mn-Zr composite oxide and a catalytic noble metal contained in the carrier.