JP3376872B2 - Method for producing NOx catalyst - Google Patents

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 a NO _x purifying catalyst, and more particularly to a method for producing an exhaust gas purifying catalyst having excellent NO _x decomposing characteristics, which comprises a step of etching an alloy having a quasi crystalline phase.

[0002]

2. Description of the Related Art Conventionally, noble metals such as Pt and Pd have been used alone or in combination as catalyst components of exhaust gas purifying catalysts for automobiles and the like, and are usually supported on a catalyst carrier. . Regarding the catalyst in which Pt is supported on Al ₂ O ₃ , for example, the following is disclosed in Chemistry Seminar II, published by Maruzen Co., Ltd. In engines, etc., CO, NO, CH are discharged depending on operating conditions (air / fuel ratio).
Since the ratios of the three components are different, it is important to make these three components harmless in the most efficient manner. For NO _x , in order to make harmless to N ₂ and CO ₂ by utilizing the reaction between NO and CO in the exhaust gas, Al ₂ O ₃ is loaded with a noble metal such as Pt at a high temperature of 300 to 400 ° C. In the region where the amount of air is small and the amount of hydrocarbons is relatively high using the above catalyst, NO _X is rather hydrogenated and denitrated. In this reaction, the NO _X decomposition characteristic is drastically reduced when the amount of oxygen increases. It is described that a catalyst in which Pt or the like is supported on Al ₂ O ₃ is used as a catalyst for positively utilizing this denitration.

Recently, nitrogen oxide purification characteristics in an atmosphere of excess oxygen in diesel engine exhaust have become important. Furthermore, the conventional catalyst in which Pt is supported on Al ₂ O ₃ has a low purification rate and a narrow temperature range (window) in which purification functions. Is expected to become difficult in the future. Therefore, it is necessary to develop a new catalyst that has a higher purification rate than the conventional catalyst and a wider temperature range (window) in which purification functions. On the other hand, in the conventional method using a crystalline material, the dramatic improvement in performance is approaching the limit. Therefore, it is necessary to fundamentally change the characteristics by tracing back to the crystal structure which is the basis of the substance. However, most of the current catalyst development is limited to the crystalline region. Thus, the catalytic properties by quasicrystalline alloy, consider the degradation properties of the NO _X to develop excellent exhaust gas purifying catalyst of this property has been desired.

[0004]

The object of the present invention is NO.
_The object of the present invention is to study the application of a quasi-crystal alloy to a decomposition catalyst for _X and to provide a method for producing a NO _X catalyst having excellent catalytic properties. Another object of the present invention is to investigate a method of projecting a noble metal element on the surface of the alloy, and change the structure itself to N.
It is to provide a method for producing an _Ox catalyst. Furthermore, object of the present invention, the NO decomposition catalyst to achieve optimization of the manufacturing process for applying the quasi-crystalline alloy of _X, temperature range clean than conventional catalysts to function (window) is widely NO _X
It is to provide a method for producing a catalyst.

[0005]

The above-mentioned object is, as a general formula, Al _{100- (ab)} X _a Mn _b (where X is Pd or Pt, 18 at% ≤ a ≤ 22 at%, 8 at% ≤ b ≤12at%)
Has a composition represented in, and tissue is accomplished by the manufacturing method of the NO _X catalyst, characterized in that projecting the Pd or Pt on a surface of the alloy by etching an alloy consisting of quasicrystals. In addition, the above-mentioned object is, as a general formula, Al _{100- (ab)} X _a Mn _b (where X is Pd.
Or Pt, 18at% ≤ a ≤ 22at%, 8at% ≤ b ≤ 12at
Has a composition represented by%), and a step of pulverizing the alloy structure is composed of quasicrystals, a step of etching the ground alloy projected surface Pd or Pt of the alloy, which is the etching alumina as a quasi-crystalline alloy and metal oxide, titania, a step of pulverizing simultaneously mixing one or more ceria also achieved by a manufacturing method of the NO _X catalyst, characterized in that it consists of.

Further, the above-mentioned object has the general formula A
l _{100- (ab)} X _a Mn _b , where X is Pd or P
t, 18 at% ≤ a ≤ 22 at%, 8 at% ≤ b ≤ 12 at%) and has a composition of quasi-crystal structure and one or more of alumina, titania, and ceria as metal oxides. N comprising a step of simultaneously pulverizing and a step of etching the alloy in the pulverized mixture to project Pd or Pt on the surface of the alloy.
It is also achieved by the method for producing an _Ox catalyst. In addition, the above-mentioned purpose is to further improve the final process in any step of the above-mentioned method.
As degree also achieved by the manufacturing method of the NO _X catalyst and performing a heat treatment to stabilize the _{Al 100- (ab) X a Mn} b alloys. Incidentally, the quasicrystalline phase of the present invention,
As a unique phase generated in the alloy by rapid solidification,
What is reported. (See, for example, The Japan Institute of Metals, Vol. 25, No. 2 (1986), P99.) This quasicrystal, unlike crystalline, has no regularity in the long range, and has five-fold symmetry (Fig. 3)). That is, by changing the basic structure of the material, excellent characteristics that were not available in the past have been imparted. According to the above-mentioned document, it has been confirmed by a high resolution electron microscope that this quasicrystal has a uniform phase composed of an icosahedron.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, first, an Al-Pd (Pt) -Mn quasi-crystalline phase having a specific composition range, in which pure metal (pure Al, pure Pd, pure Mn) is arc-melted as a raw material. As a result, a quasicrystalline mother alloy is obtained as a button ingot. Unlike the conventional quenching method, this method is a simpler, more cost-effective method, and has relatively high manufacturing efficiency.

[0008] So far, as a research on phase transformation, A
It is reported that the alloy is an l-Pd-Mn-based alloy and has an icosahedral structure (for example, Mater. Trans. JIM., 32 (19).
91), page 1089). The present invention, as a starting material for an exhaust gas purifying catalyst, further coarsely pulverizes a button ingot having a quasi-crystalline phase, and adds a surface polishing step by etching in a dilute hydrochloric acid solution. The feature is that they are projected as sufficiently fine Pd (Pt) particles. After such a step, as a final step of the catalyst carrier, Al ₂ O ₃ or the like is mixed and added to form pellets. At this stage, heat treatment for stabilizing the quasicrystal is performed.

By quasi-crystallizing, the deviation from the crystal is increased, the activity as a catalyst is increased, and also various types of polyhedrons on the surface obtained by the quasi-crystal are generated, resulting in a large kinetic energy in the atomic state. From the above, it is considered that the instability can be imparted in the atomic order, the increase in activity as a catalyst is inherited, and the effect is expressed.

As described above, the catalyst of the present invention is a catalyst having excellent NO _x decomposition characteristics in both the purification temperature range and the purification rate as compared with the conventional catalyst. The heat treatment of the present invention is carried out within a range in which diffusion sufficiently occurs and is stabilized at that temperature, but it is important to avoid a range in which alloy particles are coarsened and catalytic activity is deteriorated. In the present invention, Pd or Pt as a catalytically active species acts as a constituent species of the metal compound and has a structure in which they are uniformly projected and dispersed on the surface, so that the catalytic activity is improved.

The method for producing the catalyst of the present invention is shown in FIG. 1, and the steps thereof will be described. First, pure Pd in Cu hearth 5
2, arc from electrode 1 to pure Mn3, pure Al4,
The mother alloy is arc-melted (step (a)), after melting, the molten metal is cast into a button ingot, the button ingot is roughly crushed (b), and then etched with dilute hydrochloric acid (c), and Pd is formed on the surface. Alternatively, Pt is made to protrude, and this is pulverized (d) by a ball mill, and further pelletized to manufacture. The melting step may be a conventional melting method such as high frequency melting. Since it contains an element that is relatively easy to form an oxide, it is desirable to dissolve it in a vacuum or an inert atmosphere.

Although the heat treatment temperature and time of the present invention are not limited, the growth of metal particles is suppressed, and by this heat treatment, a part of the metal elements Al and Mn becomes an oxide. Al and Mn that do not become an oxide diffuse in the oxide to form fine particles. The fine particles thus formed are firmly bonded to some of the oxides and are evenly dispersed. It is considered that the catalyst activity is improved because the fine particles and the oxide are firmly bonded. Further, the production process of the catalyst of the present invention includes a process of mixing and pulverizing one or more of alumina, titania, and ceria as a single oxide or a composite oxide, and is defined in the production process of the second invention and the third invention. As described above, the same effect can be obtained regardless of whether the mixing / grinding step is performed before or after the etching step. In addition, 18 at% of this alloy ≤ a ≤
The compositional definition of 22 at% and 8 at% ≦ b ≦ 12 at% is because it is difficult to stably obtain a quasicrystalline alloy outside this compositional range. Hereinafter, the present invention will be described in more detail based on examples.

[0013]

EXAMPLE A sample of this example was prepared according to FIG. FIG. 1 shows a schematic view of the sample manufacturing process. Al ₇₀ Pd ₂₀ Mn
Approximately 2 by melting and solidifying ₁₀ (at%) with an arc melting device.
0 g of quasicrystalline button ingot was obtained. The X-ray diffraction pattern of the obtained quasicrystal is shown in FIG. It can be seen from the diffraction lines in this figure that the sample consists of a quasi-crystalline phase. Next, the obtained quasicrystalline button ingot was coarsely crushed by a rotor speed mill or the like. After the coarse pulverization, the coarse pulverized material was etched with a 1% dilute hydrochloric acid aqueous solution. The X-ray diffraction pattern after etching is shown in FIG. It can be seen from this figure that the Pd particle size is about 10 nm by passing through the non-equilibrium quasicrystalline phase.
It can be seen that the particle size is extremely fine, which cannot be obtained by the usual melting and solidification method.

The obtained powder and Al ₂ O ₃ were mixed at a weight ratio of 1: 1. The mixed powder was milled again in a planetary ball mill using isopropanol in a wet process at a rotation speed of 700 rpm for 24 hours to be finely divided. The mixed fine powder obtained was dried and degassed in a vacuum. Then at CIP
A load of 1 ton was applied to produce a green compact, which was then crushed to produce pellets with a particle size of approximately 1 to 2 mm. The pellets were heated to 600 ° C at a heating rate of 20 K / sec and stabilized.

5 g of pellets were added to NO: 500 ppm, C
₃ H ₆ : 670 ppm, CO ₂ : 10%, O ₂ : 6.5
%, H ₂ O: 9%, N ₂ : balance, gas flow rate: 6.6
The activity of NO _X decomposition was evaluated at 1 / min, temperature rising rate: 20 K / sec, and measurement temperature: room temperature to 600 ° C. The results of the material of the present invention are shown in FIG. As a comparative example, the conventional catalyst P
The results of the catalyst in which t is supported on Al ₂ O ₃ are shown in FIG. The conventional catalyst was produced with a Pt amount of 0.068 g. The conventional catalyst is Pt (NO ₂ ) ₂ (NH ₃ ) ₂ (4.6 × 10 ⁻³ m
γ / Al ₂ O ₃ powder was added to an aqueous solution containing ol / l)), and the mixture was stirred for 3 hours and then dried in the atmosphere at 120 ° C. for 24 hours.

From FIGS. 4 and 5, Al ₇₀ Pd ₂₀ Mn ₁₀ (at%)
It was revealed that the material of the present invention using the quasi-crystalline alloy as a starting material exhibits high activity over the comparative example of the conventional material both in the purification temperature range (cleaning end temperature-purification start temperature) and the purification rate. For other compositions, the purification temperature range (purification end temperature-purification start temperature) and the purification rate were determined according to the present invention material example Al ₇₀ Pd ₁₈ Mn ₁₂ (at
%), Al ₇₀ Pd ₂₂ Mn ₈ (at%) and the results of the catalyst supported on Comparative Example Pt which is a conventional catalyst and Al ₂ O ₃ are shown in Table 1.

[0017]

[Table 1]

As a result, it has been revealed that particularly in the composition range of the material of the present invention represented by the above general formula, higher activity than that of the conventional material is exhibited. It was also found that a quasicrystalline phase cannot be obtained outside the above composition range.

[0019]

According to the present invention, since a quasicrystalline alloy is used as a starting material, the characteristics of the quasicrystalline alloy are inherited, the activation as a catalyst is excellent, and the operating temperature range (window) is expanded. Yes, more N than conventional NO _x purification catalyst
O _X decomposition characteristics are improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a manufacturing process of a catalyst according to the present invention,
It is a figure which shows (a) arc melting, (b) coarse crushing process, (c) etching process, and (d) pulverization process.

FIG. 2 is a view showing an X-ray diffraction pattern according to the present invention, showing a result after (a) coarse pulverization and (b) etching.

FIG. 3 is a photograph of a scanning electron microscope image replacing the figure of the metal structure of the quasicrystalline alloy according to the present invention.

FIG. 4 is a diagram showing a result of activity evaluation of exhaust gas purification of the catalyst according to the present invention.

FIG. 5 is a diagram showing a result of an exhaust gas purification activity evaluation of a conventional catalyst.

[Explanation of symbols]

1 ... Electrode 2 ... Pure Pd 3 ... Pure Mn 4 ... Pure Al 5 ... Cu hearth 6 ... Rotor speed mill 7. Dilute hydrochloric acid 8 ... Coarsely ground sample 9 ... Ball mill

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) B01J 21/00-38/74 CA (STN) JISC file (JOIS)

Claims (4)

(57) [Claims] 1. As a general formula, Al _{100- (ab)} X _a Mn
_b , (where X is Pd or Pt, 18at% ≤ a ≤ 22at
%, NO _X catalyst, characterized in that projecting the Pd or Pt on the surface of the alloy by having a composition represented by 8at% ≦ b ≦ 12at%) , and etching the alloy structure is composed of quasicrystals Manufacturing method. 2. As a general formula, Al _{100- (ab)} X _a Mn
_b , (where X is Pd or Pt, 18at% ≤ a ≤ 22at
%, 8 at% ≤ b ≤ 12 at%), and a step of crushing an alloy having a quasi-crystalline structure and etching the crushed alloy with Pd or Pt on the surface of the alloy.
Process and the alumina as etched quasicrystalline alloy and metal oxide, titania, a manufacturing method of the NO _X catalyst, wherein the step of milling at the same time a mixture of one or more ceria, in that it consists of projecting the. 3. As a general formula, Al _{100- (ab)} X _a Mn
_b , (where X is Pd or Pt, 18at% ≤ a ≤ 22at
%, 8 at% ≦ b ≦ 12 at%), and a step of mixing an alloy having a composition of quasicrystal and one or more of alumina, titania, and ceria as metal oxides and simultaneously crushing, It has been a step of projecting the Pd or Pt to the alloy in the mixture on the surface of the etched alloy, a manufacturing method of the NO _X catalyst, characterized in that it consists of. 4. The process according to claim 1 , 2 or 3, further
As a final _{step, Al 100- (ab) X a} Mn b alloy production process of the NO _X catalyst and performing a heat treatment to stabilize the.