JPH1099686A - Exhaust gas purification catalyst, method for producing the same, and exhaust gas purification method - Google Patents

Abstract

[PROBLEMS] To purify exhaust gas having low temperature, high flow rate and high oxygen concentration, such as exhaust gas of a direct injection gasoline engine. In a catalyst in which Pt is supported on γ-alumina, the ratio of Pt present as a simple substance is higher than that of Pt.
t should be greater than the proportion present as an oxide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst for purifying exhaust gas, a method for producing the same, and a method for purifying exhaust gas. This invention is particularly suitable for purifying exhaust gas having a high oxygen concentration.
This invention is particularly suitable for purifying exhaust gas having low temperature, high flow rate, and high oxygen concentration, such as the exhaust gas of a direct injection diesel engine.

[0002]

2. Description of the Related Art Hydrocarbon (hereinafter referred to as HC), CO (carbon monoxide) and nitrogen oxide (hereinafter referred to as NO) in exhaust gas.
x) as a catalyst for purifying Pt on activated alumina
A material carrying Pd and Rh is generally known (see Japanese Patent Application Laid-Open No. 62-68542). That is, the technique proposed in this publication is that a catalyst layer containing Pd and Rh is provided on the front side of the carrier in the exhaust flow direction,
By arranging the catalyst layer containing Pt and Rh on the rear side, the catalyst performance is improved by Pd having excellent low-temperature activity and Pt having excellent high-temperature activity while increasing the toxicity of the catalyst. It is what we are trying to figure out.

[0003]

However, simply combining Pt and Pd as active species, for example, requires
At a low temperature of 0 to 140 ° C. and a high oxygen concentration, HC, CO and NOx in the exhaust gas cannot be sufficiently purified. When the SV (space velocity) is high, the tendency becomes remarkable. That is, in the case of the exhaust gas of the direct injection type diesel engine operated at A / F = 40 to 100, the temperature becomes lower than that of the gasoline engine operated at λ = 1 by about 200 ° C. Even when the exhaust gas is directly connected to the exhaust manifold, the exhaust gas temperature at the catalyst inlet is only about 130 ° C. during the idling operation, and the conventional catalyst cannot purify such low-temperature exhaust gas. In addition, the exhaust gas of the direct injection gasoline engine has a problem that it is difficult to oxidatively decompose because the chemically stable aroma component and paraffin are increased.

[0004]

The present inventor has proposed a conventional Pt.
As a result of research on a catalyst in which Pt was supported on a base material, the amount of Pt formed on the base material by forming an oxide film was large in this catalyst. Have been found to improve the low-temperature activity, and have completed the present invention.

That is, the invention of this application relates to an exhaust gas purifying catalyst having Pt as an active species.
Is characterized in that the ratio of Pt present as a simple substance without an oxide film is higher than the ratio of Pt forming an oxide film and existing as an oxide, and its low-temperature activity is excellent.
The reason is considered as follows.

When Pt as a catalytically active species is present in the form of an oxide, O (oxygen) combined with the Pt is activated by heating, or oxygen in the exhaust gas becomes Pt.
It is considered that the catalyst is adsorbed and activated by an oxide or the like, and oxidation of HC or the like proceeds, but there is a limit to the improvement of the activity at a low temperature. On the other hand, when Pt exists as a simple substance, it is considered that oxygen in the exhaust gas is dissociated and adsorbed to the simple Pt, and this contributes to the oxidation of HC and CO in the exhaust gas. Is more easily dissociated and adsorbed than Pt oxide, and the adsorbed O has a weaker bond with Pt unlike O of Pt oxide, and thus is easily desorbed. Therefore, it is considered that the catalyst of the present invention, in which Pt is used alone, that is, the surface of Pt exists in a metal state without an oxide film in many cases, has excellent low-temperature activity.

The reason why Pt is used as an active species in the catalyst is that Pt is excellent in low-temperature activity. When Pt is combined with Pd, Ht in exhaust gas is reduced.
C can be adsorbed on Pd and oxidized by Pt, which is further advantageous for improving the low-temperature activity of the catalyst.

Another invention of this application is a method for producing an exhaust gas purifying catalyst in which Pt as a single substance is present in a high proportion as described above, wherein a Pt-containing Pt solution is adhered to a base material and calcined. After supporting Pt on the base material by Pt, a heat treatment of heating to a temperature of 200 ° C. to 500 ° C. in a non-oxidizing atmosphere is performed, so that a part of the Pt oxide film supported on the base material is Pt. Is reduced to Pt alone such that the proportion of the element present as an element is higher than the percentage of the element present as an oxide.

In this production method, the proportion of Pt having an oxide film formed on the surface by the above-mentioned baking increases, but the Pt oxide film is heated to a temperature of 200 ° C. or more in a non-oxidizing atmosphere thereafter. Is thermally decomposed to generate Pt alone. The non-oxidizing atmosphere includes a reducing atmosphere, and may be a vacuum atmosphere or an inert gas atmosphere. The heating temperature is set to 200 ° C. or higher because such a temperature is required to decompose the Pt oxide. Even if the heating temperature is higher than 500 ° C., Pt alone can be generated, but the heating equipment becomes expensive, which is not preferable. Note that the heat treatment temperature is preferably set to 300 ° C. to 500 ° C.

As the gas for forming the reducing atmosphere, HC or CO can be used. The heat treatment in the non-oxidizing atmosphere is preferably performed for 5 minutes to 60 minutes in order to increase the proportion of Pt alone. 5
It is difficult to obtain the expected effect in less than a minute. Although it is possible to perform the heat treatment for 60 minutes or more, it is not preferable in terms of productivity because the effect cannot be increased.

As the Pt solution, dinitrodiamineplatinum (II) nitric acid acidic solution, hexaammineplatinum (IV) tetrachloride solution, tetraammineplatinum (II) dichloride solution, chloroplatinic acid and the like can be used. In the case of a platinum (IV) tetrachloride solution, a tetraammineplatinum (II) dichloride solution, or in the case of chloroplatinic acid, it is presumed that chlorine poisons the active site and tends to lower the activity.

When Pt and Pd are supported on the base material, the above-mentioned Pt solution may be used, and the Pd-containing Pd solution may be dinitrodiamine palladium (II). A nitric acid solution, a tetraamminepalladium (II) dichloride solution, a palladium chloride solution, a palladium nitrate solution, etc. can be used. By mixing these two solutions and attaching them to the base material,
Most suitable for improving the low temperature activity of the catalyst.

That is, when a hexaammineplatinum (IV) tetrachloride solution or a tetraammineplatinum (II) dichloride solution is combined with a dinitrodiaminepalladium (II) nitric acid acidic solution, However, when combined with a tetraamminepalladium (II) dichloride solution, precipitation occurs when the solutions are mixed, and there is regret that the dispersibility of Pt and Pd is reduced. This is because of the problem of poison. However, the combination of the above-mentioned dinitrodiamineplatinum (II) nitric acid acidic solution and palladium nitrate showed good results with respect to the low-temperature activity of the catalyst.

Still another aspect of the present invention is to provide an exhaust gas purifying catalyst as described above, wherein NOx as an exhaust gas is brought into contact with an exhaust gas purifying catalyst as described above in an oxidizing atmosphere having an oxygen concentration of 3% or more. And an exhaust gas purifying method characterized by oxidizing nitrogen oxides and reductively decomposing the nitrogen oxides.

Here, the reducing agent may be HC or CO. As the reducing agent is oxidized, reductive decomposition of the NOx is caused, and an oxidizing atmosphere having an oxygen concentration of 3% or more. However, a high NOx purification rate can be obtained at a low temperature of about 100 ° C. to 140 ° C.

[0016]

Therefore, according to the present invention, in the exhaust gas purifying catalyst having Pt as an active species, the ratio of Pt as a simple substance is larger than the ratio of Pt as an oxide. The activity of Pt as an oxidation catalyst is increased, and the low-temperature activity can be improved.

In manufacturing an exhaust gas purifying catalyst having a high proportion of Pt as a single substance as described above,
Since a Pt solution containing Pt is adhered to a base material and baked, the Pt is supported on the base material, and then heat treatment is performed at a temperature of 200 ° C. to 500 ° C. in a non-oxidizing atmosphere. A part of the Pt oxide supported on the base material is adjusted so that the ratio of Pt as a simple substance is higher than the ratio of Pt as an oxide.
It can be reduced to a single substance.

Further, according to the exhaust gas purifying method, NOx as the exhaust gas is brought into contact with the exhaust gas purifying catalyst together with the reducing agent in an oxidizing atmosphere having an oxygen concentration of 3% or more. By utilizing the oxidation of the reducing agent, reductive decomposition of NOx is caused, and NOx can be efficiently decomposed and purified even in a low-temperature and high-oxygen-concentration atmosphere.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The catalyst of the present invention is supported on a honeycomb carrier, and this carrier is disposed in an exhaust passage near an exhaust manifold of a direct injection gasoline engine. In a direct injection gasoline engine, the air-fuel ratio is increased from λ = 1 to A / F = 40.
It can be adjusted from about 10 to about 10.

<Preparation of Catalyst> -Example- 1 hydrated alumina was added to γ-alumina as activated alumina.
A slurry is prepared by adding 0 wt%, wash-coated on a honeycomb carrier made of cordierite,
The firing was performed at 00 ° C. for 2 hours. Then, a predetermined amount of the dinitrodiamineplatinum (II) nitric acid acidic solution (an amount at which Pt becomes 3.6 wt% of γ-alumina) and a predetermined amount of the dinitrodiaminepalladium (II) nitric acid acidic solution (Pd is 2% of γ-alumina) .2 wt%), impregnated into the above-mentioned wash coat layer of the honeycomb carrier, dried at 150 ° C. for 1 hour, and then fired at 500 ° C. for 2 hours in the air to obtain a honeycomb. A catalyst was obtained.

The following reducing gas (A / A
(Corresponding to F = 14.5) at SV = 110,000 / h, and a heat treatment at 400 ° C. for 1 hour to obtain a target exhaust gas purifying catalyst.

HC: 1700 ppmC, CO: 0.8
%, NO; 0.1%, H ₂ ; 0.27%, O ₂ ; 0.5
%, CO ₂ ; 13.7%, balance N ₂

Comparative Example 1 A honeycomb catalyst prepared in the same manner as in Example (without heat treatment in a reducing gas) was used as a catalyst of Comparative Example 1.

Comparative Example 2 A honeycomb catalyst prepared in the same manner as in the example was subjected to a heat treatment at 400 ° C. for one hour in the air instead of a heat treatment in a reducing gas, thereby obtaining an objective comparative catalyst. The catalyst of Example 2 was obtained.

<Evaluation of Catalyst> -Low Temperature Activity- For each of the catalysts of the above Examples and Comparative Examples, the light-off performance, that is, T-50 relating to HC purification (exhaust gas temperature at which a purification rate of 50% is obtained ( Temperature at the catalyst inlet))
Was evaluated by a rig test. The composition of the test gas is as follows and simulates the exhaust gas of a direct injection gasoline engine (corresponding to A / F = 40).

HC: 4000 ppm C, CO; 0.14
%, NO; 250 ppm, O ₂ ; 17%, CO ₂ ;
%, H ₂ ; 0.07%, balance N ₂ test gas was passed through the catalyst at SV = 110,000 / h.

The test results are shown in FIG. According to the figure, the temperature of T-50 is about 15 ° C. lower than that of Comparative Example 1 in the catalyst of the example subjected to the heat treatment in the reducing atmosphere. On the other hand, the catalyst of Comparative Example 2 subjected to the heat treatment in the atmosphere has a lower light-off performance than the untreated Comparative Example 1.

-Regarding NOx purification rate-Further, when the NOx purification rates of the respective catalysts of the above-mentioned Examples and Comparative Examples were measured by a rig test under the same conditions as in the evaluation of the low-temperature activity, the results shown in FIG. 2 were obtained. Was. The exhaust gas temperature immediately before the catalyst in this test is 130 ° C.
According to the figure, the NOx purification rate of the catalyst of the example is much higher than that of each catalyst of the comparative example.

-Presence of Pt- Then, the catalyst of the above example and the catalyst of Comparative Example 1 were
By XPS (X-ray photoelectron spectroscopy), the ratio of the presence of Pt as a simple substance and the ratio of the presence of Pt as an oxide (PtO) were analyzed. The results are shown in FIG.

According to the figure, in the comparative example 1 which has not been processed, Pt
Is present as an oxide more than it exists alone, but in the example, 80% is Pt alone. Therefore, the catalysts of the examples exhibited the excellent light-off performance and the high NOx purification rate described above because of the Pt
It is recognized that the ratio of Pt as a simple substance increased, and the activity of the Pt as a catalyst increased. In the case of Comparative Example 2 which was subjected to a heat treatment in the air, Comparative Example 1
It is considered that the ratio of the Pt oxide is further increased than that.

-About heat treatment temperature-Regarding the heat treatment of the catalyst of the present invention, the treatment time is 1 hour,
The treatment temperature was changed, and the light-off performance of the obtained catalyst was evaluated under the same conditions as in the previous case. The results are shown in FIG. According to the figure, as the heat treatment temperature increases, T
The temperature of −50 decreases, but when the temperature exceeds 300 ° C., T−
It can be seen that the temperature decrease of 50 can hardly be expected.

-Heat Treatment Time- With respect to the heat treatment, the treatment temperature was set to 400 ° C., the treatment time was changed, and the light-off performance of the obtained catalyst was evaluated under the same conditions as in the previous case. The results are shown in FIG. According to the figure, it can be seen that when the heat treatment time is 5 minutes or longer, the temperature of T-50 decreases.

Regarding Pt Raw Material Regarding the catalyst of the present invention, the effect of the type of the Pt raw material on the light-off performance (T-50) was examined. That is, as a Pd raw material, dinitrodiamine palladium (II) “Pd (N
O ₂ ) ₂ (NH ₃ ) ₂ ”acidic nitric acid solution was combined with each of the following Pt raw materials to prepare a catalyst according to the method described in the previous example.
-50 was measured. The heat treatment conditions are the same as in the previous embodiment.

Dinitrodiamine platinum (II) [Pt (N
O ₂ ) ₂ (NH ₃ ) ₂ ] nitric acid acidic solution hexaammineplatinum (IV) tetrachloride [Pt (NH
₃ ) ₆ ] Cl ₄ solution tetraammineplatinum (II) dichloride [Pt (NH ₃ )
₄ ] Cl ₂ solution PtCl ₄ chloroplatinate

The results are shown in FIG. From the figure, P
It can be seen that light-off performance is best when a dinitrodiamine palladium (II) nitric acid acidic solution is used as a raw material and a dinitrodiamine platinum (II) nitric acid acidic solution is used as a Pt raw material. Hexammine platinum
In the case of (IV) tetrachloride solution or tetraammineplatinum (II) dichloride solution, precipitation occurred during the solution mixing. For this reason, it is presumed that the dispersibility of Pt and Pd on γ-alumina was reduced and chlorine poisoning was also caused, and T-50 was considered to have increased. In the case of chloroplatinic acid, no precipitation occurred, but due to chlorine poisoning, T
It is considered that -50 is higher.

Regarding Pd Raw Material Regarding the catalyst of the present invention, the effect of the type of Pd raw material on the light-off performance (T-50) was examined. That is, a dinitrodiamine platinum (II) nitric acid acidic solution was used as a Pt raw material, and the following Pt raw materials were combined with each other to prepare a catalyst by the method described in the previous example. 50 were measured. The heat treatment conditions are the same as in the previous embodiment.

Dinitrodiamine palladium (II) [Pd
(NO ₂ ) ₂ (NH ₃ ) ₂ ] acidic nitric acid solution tetraamminepalladium (II) dichloride [Pd (N
H ₃ ) ₄ ] Cl ₂ solution Palladium chloride PdCl ₂ solution Palladium nitrate Pd (NO ₃ ) ₂ solution

The results are shown in FIG. From the figure, it can be seen that a tetraamminepalladium (II) dichloride solution or a palladium chloride solution is disadvantageous for improving the low-temperature activity of the catalyst. It is considered that the former is due to precipitation during mixing of the solution, and the latter is due to chlorine poisoning. On the other hand, in the example using palladium nitrate, T-50 is low, which indicates that it is advantageous for improving the low-temperature activity of the catalyst.

<About the ratio between Pt and Pd> Pt and Pd
and the other conditions were the same as in Example 1 above.
The catalyst was prepared according to the method, and the influence of the ratio of Pt to Pd on the light-off performance and NOx purification rate of the catalyst was examined under the same conditions and method as in the previous rig test. P
The supported amount of t is 5 g per liter of the catalyst carrier. No heat treatment was performed on the catalyst. The results are shown in FIG.

According to the figure, Pd / Pt = (10/5)
0) to (45/50) to improve the light-off performance and the NOx purification rate, Pd / Pt = 3 /
It turns out that the vicinity of 5 is the best.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of a rig test for evaluating low-temperature activities of Examples and Comparative Examples.

FIG. 2 is a graph showing the results of a rig test relating to the NOx purification rates of Examples and Comparative Examples.

FIG. 3 is a graph showing the ratio of Pt present as a simple substance and the ratio present as an oxide in Example and Comparative Example 1.

FIG. 4 is a graph showing the effect of the heat treatment temperature on the low-temperature activity of the catalyst.

FIG. 5 is a graph showing the effect of the heat treatment time on the low-temperature activity of the catalyst.

FIG. 6 is a graph showing the effect of the type of Pt raw material on the low-temperature activity of a catalyst.

FIG. 7 is a graph showing the effect of the type of Pd raw material on the low-temperature activity of the catalyst.

FIG. 8 is a graph showing the effect of the loading ratio of Pt and Pd on catalyst performance.

[Explanation of symbols]

 None.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Makoto Kyogoku 3-1, Fuchi-machi, Shinchu, Aki-gun, Hiroshima Mazda Co., Ltd. (72) Inventor Kosuke Sumita 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd. Muchida Co., Ltd. (72) Inventor Yoko Kawakami 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Pref.

Claims (7)

[Claims] 1. An exhaust gas purifying catalyst comprising Pt as an active species, wherein said Pt is present as a simple substance at a higher rate than as an oxide. 2. The exhaust gas purifying catalyst according to claim 1, further comprising Pd as an active species in addition to Pt. 3. A method for producing an exhaust gas purifying catalyst having Pt as an active species, wherein the Pt-containing Pt solution is adhered to a base material and calcined, so that the Pt is supported on the base material. By performing a heat treatment of heating to a temperature of 200 ° C. to 500 ° C. in a non-oxidizing atmosphere, a part of the Pt oxide film supported on the base material is converted into an oxide in a proportion in which Pt exists as a simple substance. Pt so that it is greater than
A method for producing an exhaust gas purifying catalyst, wherein the catalyst is reduced to a single substance. 4. The method for producing an exhaust gas purifying catalyst according to claim 3, wherein the Pt solution is a dinitrodiamineplatinum (II) nitric acid acidic solution. 5. The method for producing an exhaust gas purifying catalyst according to claim 4, wherein the dinitrodiamine platinum (II) nitric acid acidic solution and the dinitrodiamine palladium (II) nitric acid acidic solution are mixed to form a base material. A method for producing an exhaust gas purifying catalyst, wherein the catalyst is attached. 6. The exhaust gas purifying catalyst according to claim 3, wherein the non-oxidizing atmosphere is a reducing atmosphere containing hydrocarbons and CO. Recipe. 7. A nitrogen oxide as an exhaust gas together with a reducing agent in an oxidizing atmosphere having an oxygen concentration of 3% or more.
An exhaust gas purification method, comprising: contacting the exhaust gas purification catalyst according to claim 1 to oxidize the reducing agent and reductively decomposing the nitrogen oxide.