JPH1024218A - Purification of hydrocarbon in exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently purify a hydrocarbon in an exhaust gas by using a catalyst having a double oxide of La and Al as a carrier and carrying a specific quantity of one of Ce, Rh, Pt, Pd as an active component in a process for oxidizing the hydrocarbon in the waste combustion gas stream to carbon dioxide. SOLUTION: In a pre-catalyst for purifying the hydrocarbon composed of a fuel unburned portion and a partially burnt component in an automobile provided with a exhaust gas treating device for passing the exhaust gas through the pre-catalyst 3 at the time of the start up of an engine 1 and passing the exhaust gas through a main catalyst 4 after the start up, the double oxide of La and Al is used as the carrier, one of Ce, Rh, Pt, and Pd is carried on the carrier as the active component and 5-30wt.% Ce per 100wt.% carrier is particularly used. The double oxide of La and Al is constituted so as to contain 5-30wt.%, preferably 5mol% La as the mixed ratio of La to Al, the balance Al and 100mol% sum of La and Al.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a combustion exhaust gas emitted from an internal combustion engine such as an automobile engine, a combustion exhaust gas emitted from a consumer product such as a cooking appliance or a boiler of a factory or a thermal power plant. The present invention relates to a method for purifying combustion exhaust gas and a catalyst used therefor, and more particularly to a method for purifying hydrocarbons composed of unburned fuel and partial combustion components discharged immediately after the start of an internal combustion engine, and a catalyst used therefor.

[0002] The method for purifying catalysts and hydrocarbons according to the present invention comprises:
It is suitable for purifying hydrocarbons in exhaust gas discharged from an automobile engine.

[0003]

2. Description of the Related Art Exhaust gas emitted from an internal combustion engine of an automobile or the like contains air pollutants such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx). Therefore, various catalysts for purifying exhaust gas have been studied.

At present, exhaust gas purifying catalysts for automobiles (referred to as main catalysts) generally installed in automobiles oxidize CO and HC by a three-way reaction at an exhaust gas temperature higher than a predetermined temperature.
These components in the exhaust gas have been rendered harmless by reducing NOx. However, at the conventional catalyst position, when the engine is started, heat diffusion occurs in the exhaust gas passage, so that it takes time to raise the exhaust gas temperature from a low temperature to a predetermined temperature. Further, in general, a catalyst for automobiles has a cordierite honeycomb coated with a catalyst material. However, since cordierite also has a predetermined heat capacity, the catalyst layer cannot be rapidly heated. Therefore, the main catalyst cannot function effectively during that time. In order to purify hydrocarbons when the engine is started, a method of providing another hydrocarbon purifying catalyst (referred to as a pre-catalyst) on the engine side from the position of the conventional main catalyst has been studied. According to this method, the combustion exhaust gas discharged from the engine reaches the pre-catalyst without being cooled too much in the exhaust gas passage, so that the hydrocarbon at the time of starting the engine can be purified earlier than the main catalyst.

[0005] As a hydrocarbon purification catalyst, a catalyst in which platinum and rhodium are supported on a composite oxide of La and aluminum (Japanese Patent Publication No. 8-24844) has been proposed.

[0006]

In the method using the pre-catalyst, the ability to purify hydrocarbons even at low temperature exhaust gas at the time of engine start and the high temperature exhaust gas flow after the engine start, The challenge is to have high thermal durability.

The composite oxide of La and aluminum is 950
It maintains a high specific surface even in a temperature range of ° C., and is suitable as a carrier for a precatalyst. However, if Ce is contained in an amount of 1% by weight or more, the heat resistance of the carrier decreases, so that the active metal must be limited to platinum and rhodium.

An object of the present invention is to provide a catalyst having better heat resistance than a conventional catalyst in which platinum and rhodium are supported on a composite oxide of La and aluminum, and a method for purifying exhaust gas using the catalyst.

[0009]

According to the present invention, La and Al are used.
As a carrier, an alkaline earth metal as an active ingredient, Ce as a rare earth metal, and R as a noble metal.
h, Pt, and Pd, and the carrier 1
The above technical problem was solved by setting Ce to 5 to 30% by weight with respect to 00% by weight. Mg is suitable for the alkaline earth metal in the present invention.

The technique described in Japanese Patent Publication No. 8-24844 can be applied to the composite oxide of La and Al in the present invention. L
The composite oxide of a and aluminum has La as a mixture ratio of La to Al of 1 to 20 mol%, and the balance is Al.
And Al in total of 100 mol%, preferably La 5 mol%
It is desirable to be configured so that The composite oxide of La and Al forms a β structure at a high temperature and has a high specific surface area (12
(20 m ² / g or more when baked at 00 ° C. for 2 hours), so that the carrier can have heat resistance. As a method for preparing the carrier, a coprecipitation method, a sol-gel method, or the like can be used.

In the invention described in Japanese Patent Publication No. H8-24844, when Ce is mixed into a composite oxide of La and aluminum in an amount of 1% by weight or more, the crystal growth is promoted and the heat resistance of the carrier is lowered. In the present invention, Ce is not mixed at the time of preparing the composite oxide of La and Al, the structure is stabilized by a calcination treatment, and then Ce is supported, thereby lowering the heat resistance due to the mixing of Ce. Was avoided. A feature of the present invention resides in that 5 to 30% by weight of Ce is supported on a composite oxide of La and aluminum.

The firing temperature of the composite oxide of La and Al for stabilizing the structure is preferably from 700 ° C. to 1200 ° C.

The amount of the active ingredient to be supported when alkaline earth metal, rare earth metal is Ce, and noble metal is at least one of Rh, Pt, and Pd,
0.5% to 2% by weight of Mg and 5% to 5% by weight of Ce
30% by weight, 0.05 to 0.3% by weight of Rh, and 0.3% by weight of Pt.
Preferably, the content is 5 to 2.5% by weight and Pd is 0.5 to 2.5% by weight. At this loading, the catalyst of the present invention is 95%
It has high hydrocarbon purification activity even in a high temperature range of about 0 ° C.

The active metal supporting sequence of the catalyst of the present invention is such that the rare earth metal is first supported on the composite oxide carrier, the noble metal is supported on the rare earth metal supporting carrier, and finally, the alkaline earth metal is supported. desirable. The state of the supported metal is such that a rare earth metal is an oxide, a noble metal is an oxide or a metal, and an alkaline earth metal is an oxide or a carbonate compound. The supporting method on the carrier includes an impregnation direction, a kneading method, and the like. Further, as a starting material of the supported metal, a nitric acid compound, a chloride, a sulfuric acid compound, an acetic acid compound, a carbonate compound, an organic compound, or the like can be used.

In the catalyst of the present invention, it is considered that each component can purify exhaust gas constantly by the following functions.

Since Ce can store oxygen,
Even if the oxygen concentration in the exhaust gas fluctuates due to the air-fuel ratio fluctuation when the engine is started, it is possible to supply oxygen necessary for oxidizing hydrocarbons to the catalyst without shortage.

The noble metal serves as a reaction field for producing carbon dioxide from hydrocarbons and oxygen. Mg interacts with the noble metal to atomize and highly disperse the noble metal, thereby preventing sintering of the noble metal at high temperatures. Therefore, the noble metal which becomes a reaction field between the hydrocarbon and oxygen by Mg loading is highly activated, and the heat durability is improved.

Furthermore, by using a composite carrier of La and Al, a wide carrier specific surface area can be obtained even in a high temperature region of 900 ° C. or higher, so that high heat resistance of the catalyst can be maintained.

The catalyst of the present invention has high performance in purifying hydrocarbons in exhaust gas under both stoichiometric and lean burn conditions. Specifically, nitrogen oxides and hydrocarbons in the exhaust gas discharged under these combustion conditions can be efficiently oxidized to carbon dioxide. The catalyst is La-A
When the carrier is made of a carrier in which Ce, Pt, Pd, and Mg are supported on an l ₂ O ₃ carrier, extremely high hydrocarbon purification activity and heat resistance can be exhibited.

By mounting the catalyst of the present invention as a pre-catalyst in the exhaust system of an internal combustion engine, it is possible to significantly suppress the emission of nitrogen oxides to the outside of the vehicle. Specifically, the following aspects are possible.

(1) An automobile having a front catalyst provided in an engine exhaust gas flow path and a main catalyst provided in a downstream side thereof, and having an exhaust gas treatment device for sequentially passing exhaust gas to these is provided.

(2) An exhaust gas treatment device having a front catalyst provided in an engine exhaust gas flow path and a main catalyst provided in a downstream side thereof, wherein the exhaust gas flows through the front catalyst when the engine is started, and thereafter the exhaust gas flows through the main catalyst. Vehicle.

As the main catalyst, a conventional three-way catalyst or a catalyst compatible with lean burn can be used.

Further, by combining the catalyst of the present invention with an existing system provided with a hydrocarbon purifying catalyst and a main catalyst, the effect of purifying hydrocarbons can be enhanced. For example, using the catalyst of the present invention as a pre-catalyst, an existing hydrocarbon purification catalyst as a middle catalyst, and a main catalyst as a rear catalyst, exhaust gas is circulated in order from the front catalyst to the middle catalyst and the rear catalyst. There is a method of flowing exhaust gas through the middle catalyst, and then flowing exhaust gas through the rear catalyst.

FIG. 1 shows an example in which the exhaust gas purifying apparatus of the present invention is installed in an automobile. In FIG. 1, an engine 1
The pre-catalyst 3 in the exhaust gas flow path 2 downstream of the
Is installed.

FIG. 2 shows an exhaust gas passage 2 downstream of the engine 1.
1 shows an example of a device in which a front catalyst 3 is provided, and a main catalyst 4 is provided in a subsequent stage, and a bypass device 5 for bypassing the front catalyst is provided.

[0027] When the lean burn catalyst is used as the main catalyst, this device can prevent hydrocarbons required for NOx purification from being purified by the front catalyst.

The present invention does not preclude placing the precatalyst according to the method of the present invention in front of the aforementioned prior art precatalyst (closer to the engine), and various modified embodiments are possible.

FIG. 3 shows an example in which the exhaust gas purifying apparatus of the present invention is installed in an automobile. In FIG. 3, the engine 1
A pre-catalyst 3 is provided in the exhaust gas flow path 2 downstream of the fuel cell, a conventional HC purification catalyst 6 is provided in the downstream, and a main catalyst 4 is further provided in the downstream.

FIG. 4 shows an exhaust gas passage 2 downstream of the engine 1.
The pre-catalyst 3, the HC purification catalyst 6 of the prior art in the downstream, and the main catalyst 4 in the downstream are also provided.
An example of a device having a bypass device 5 for bypassing a C purification catalyst is shown.

The catalyst of the present invention is also effective for treating exhaust gas discharged from a diesel engine of a diesel vehicle. A diesel engine is operated at a high air-fuel ratio with an excess of oxygen, and the catalyst of the present invention exhibits excellent activity even in the presence of oxygen, so that even the exhaust gas discharged from a diesel engine can efficiently purify hydrocarbons. be able to.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION

"Example 1" A composite oxide of La and Al was obtained by a coprecipitation method in which a mixed solution of La nitrate and Al nitrate was added dropwise by dropping ammonia water so that the La / Al molar ratio was 5/95.

A composite oxide of La and Al fired at 700 ° C. (L
(a / Al molar ratio = 5/95) The powder was impregnated with a Ce nitrate solution by a kneading method, then dried at 200 ° C and fired at 700 ° C for 1 hour. Subsequently, after impregnating a mixed solution of a Pd nitrate solution and a dinitrodiammine Pt nitric acid solution by a kneading method,
C. and fired at 700.degree. C. for 1 hour. Finally, a Mg nitrate solution was similarly impregnated, dried and fired. As described above, based on 100% by weight of alumina, 23% by weight of Ce, 1.6% by weight of Pd, 1.6% by weight of Pt, and 2% by weight of Mg
Was contained. 400 g of water is added to 100 g of the catalyst powder.
g, nitric acid encounter alumina sol (alumina solid content 20%) 1
The slurry obtained by mixing 00 g with a ball mill for 3 hours was coated on a cordierite honeycomb (400 cells / inc ² ). The firing temperature was 700 ° C. and the final catalyst coating amount was 200 g / l.

By the above method, a honeycomb-shaped example catalyst 1 was obtained.

Further, in the same manner, Ce is supported, and a mixed solution of an aqueous Rh nitrate solution and a dinitrodiammine Pt nitric acid solution is impregnated by a kneading method, and dried and calcined in the same manner. Was coated on the honeycomb to obtain Example Catalyst 2.

Table 1 summarizes the compositions of the prepared catalysts.

[0037]

[Table 1]

(Experimental Example 1) The catalysts of Examples 1 and 2 were evaluated for their ability to purify hydrocarbons by the following experimental method.

Experimental method: (1) 6 cc of a honeycomb catalyst (17 mm square × 21 mm length) is charged into a Pyrex reaction tube.

(2) Put the reaction tube in an annular electric furnace,
Heat to 0 ° C. The temperature measures the honeycomb inlet gas temperature. When the temperature reaches 150 ° C. and becomes stable, the circulation of the stoichiometric model exhaust gas (referred to as stoichiometric model exhaust gas) is started. 10 ℃ at the same time as the stoichiometric model exhaust gas distribution
/ Minute, and finally heated to 500 ° C. At this time, the hydrocarbons in the gas discharged from the reaction tube are measured once every 30 seconds by the FID method, and the initial performance of the catalyst is examined.

(3) After completion of the experiment of (2), the honeycomb catalyst is calcined in air at 900 ° C. for 5 hours, and then the hydrocarbon purification performance after the heat treatment is examined according to the experimental procedures of (1) and (2). .

As the stoichiometric model exhaust gas, NO was 0.1 vol%, C ₃ H ₆ was 0.05 vol%, and CO was 0.6 vol.
%, 0.6 vol% of O ₂ , 0.2 vol% of H ₂ , 10 vol% of steam, and the balance nitrogen.
The space velocity of the gas in a dry state (not including water vapor) is
30,000 / h.

Table 2 shows the hydrocarbon (HC) purification rates in the stoichiometric model exhaust gas at various temperatures at the initial stage and after the calcination treatment. The HC purification rate was calculated according to the following equation.

[0044]

(Equation 1)

[0045]

[Table 2]

In the frame: HC purification rate (%) "Example 2" La ₂ O ₃ —Al ₂ O ₃ (La / Al molar ratio: 5)
/ 95) A slurry obtained by mixing 100 g of the composite oxide, 400 g of water, and 100 g of alumina sol (20% alumina solid content) with nitric acid in a ball mill for 3 hours was coated on a cordierite honeycomb (400 cells / inc ² ). The firing temperature is 700 ° C and the final composite oxide coating amount is 150
g / l.

The active ingredient loading amount is 100% by weight of the composite oxide.
On the other hand, Ce was supported by 12% by weight, Pt by 1.6% by weight and Pd by 1.6% by weight, and Mg by 1% by weight, respectively, by the impregnation method. All firing temperatures are 700 ° C. Example catalyst 3 carrying these active components was obtained.

Example 3 The catalyst purification rate of the catalyst 3 was determined in the same manner as in Experimental Example 1 of Example 1 at the initial stage (calcination at 700 ° C.) and after the calcination treatment at 900 ° C. for 5 hours. Table 3 shows the results.

[0049]

[Table 3]

In the frame: HC purification rate (%) "Example 3" The catalyst of Example 1 was subjected to a calcining treatment at 900 ° C. for 5 hours while changing the loading ratio of Mg, Ce, Pt, and Pd.
The HC purification performance at 00 ° C. was measured. The experimental method is the same as the experimental example 1 of the first embodiment. FIG. 5 shows the performance when the loading ratio of Mg and Ce was changed. FIG. 6 shows Pt,
The performance when the loading rate of Pd is changed is shown.

Example 4 The HC purification performance when the Rh loading rate of the example catalyst 2 was changed was examined. The carrier is Al ₂ O ₃
Mg is 1% by weight based on 100 parts by weight of the carrier.
t was 1.6% by weight and Ce was 23% by weight. The experimental method is the same as the experimental example 1 of the first embodiment. FIG.
The HC purification performance at 300 ° C. of the catalyst after the calcination treatment at 5 ° C. for 5 hours was shown.

Example 5 Example Catalyst 4 was prepared by setting the honeycomb volume of Example Catalyst 3 to 1.0 L and calcining at 950 ° C. for 50 hours. With the catalyst arrangement shown in FIG. 1, the HC purification rate was evaluated using an actual vehicle engine. Example catalyst 4 is a pre-catalyst,
A three-way catalyst was used as a main catalyst. The main components of the three-way catalyst are Rh,
Pt, Ce, Al ₂ O ₃ . The engine was operated in the LA-4 mode on a 4-cycle, 2.2-liter engine. LA of Example Catalyst 4
-4 Average HC purification rate for 120 seconds from mode start is 40
%Met. Further, the hydrocarbon emission amount in the entire LA-4 mode is 0.03 g / mile, which is the ULEV regulation value (0.04 g / mile).
04g / mile).

Example 6 Example Catalyst 5 was prepared by setting the honeycomb volume of Example Catalyst 3 to 0.12 L and calcining at 950 ° C. for 50 hours. With the catalyst arrangement shown in FIG. 3, the HC purification rate was evaluated using an actual vehicle engine. Example catalyst 5 was a pre-catalyst, an HC purification catalyst according to the prior art was installed downstream of example catalyst 5, and a three-way catalyst was installed downstream of the HC catalyst as a main catalyst. The main components of the three-way catalyst are Rh, Pt, C
e, Al ₂ O ₃ . The engine was operated in the LA-4 mode on a 4-cycle, 2.2-liter engine. The amount of hydrocarbon emission in the entire LA-4 mode is 0.05 g in the case where the example catalyst 5 is not provided.
/ Mile. However, when the catalyst of Example 5 is present, the weight becomes 0.03 g / mile, which is the ULEV regulation value (0.04 g / mile).
g / mile).

"Comparative Example 1" This is a catalyst preparation method similar to that of Example Catalyst 1, except that alumina is used as a carrier, Ce is loaded, and an alkaline earth metal Sr is loaded using a Sr nitrate solution. Pt was loaded. Comparative catalyst 1 was obtained in which Ce was 23% by weight, Sr was 18% by weight, Pt was 1.6% by weight, and Rh was 0.3% by weight based on 100% by weight of the carrier.
In the same manner as in Experimental Example 1, hydrocarbon purification rates were determined at the initial stage and after the calcination treatment at 950 ° C. for 5 hours. Table 4 shows the results. 9
The HC purification rate after the 50 ° C. heat treatment was significantly reduced.

[0055]

[Table 4]

[0056]

As is clear from the examples, according to the present invention, hydrocarbons contained in exhaust gas can be efficiently purified.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of a vehicle equipped with a hydrocarbon purification device.

FIG. 2 is a conceptual diagram of a vehicle equipped with a hydrocarbon purification device.

FIG. 3 is a conceptual diagram of a vehicle equipped with a hydrocarbon purification device.

FIG. 4 is a conceptual diagram of a vehicle equipped with a hydrocarbon purification device.

FIG. 5 is a correlation diagram between the Mg and Ce carrying rates and the hydrocarbon purification rates at a reaction gas temperature of 300 ° C. of the catalyst after heat treatment.

FIG. 6 is a correlation diagram between Pt and Pd carrying rates and a hydrocarbon purification rate at a reaction gas temperature of 300 ° C. of a catalyst after heat treatment.

FIG. 7 is a correlation diagram between the Rh carrying rate and the hydrocarbon purification rate of the catalyst after heat treatment at a reaction gas temperature of 300 ° C.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine, 2 ... Exhaust gas flow path, 3 ... Front catalyst, 4 ... Main catalyst, 5 ... Bypass device, 6 ... HC purification catalyst.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Shigeru Azuhata 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture Inside Hitachi Research Laboratory, Hitachi, Ltd. Hitachi, Ltd.Automotive Equipment Division (72) Inventor Toshio MANAKA 2520 Address, Ojitakata, Hitachinaka City, Ibaraki Prefecture Hitachi, Ltd.Automotive Equipment Division (72) Inventor Seiji Asano Address Co., Ltd.Automotive Equipment Division, Hitachi, Ltd.

Claims (4)

[Claims] 1. An exhaust gas purification method for oxidizing hydrocarbons in a combustion exhaust gas stream to carbon dioxide by a catalytic reaction, wherein the catalyst comprises a composite oxide of La and Al as a carrier, and the carrier comprises an alkaline earth as an active ingredient. Ce and Rare earth metals
A method for purifying hydrocarbon-containing exhaust gas, wherein at least one of Rh, Pt, and Pd is supported as a noble metal, and Ce is 5 to 30% by weight based on 100% by weight of the carrier. 2. A composite oxide of La and Al is used as a carrier, and the carrier is loaded with an alkaline earth metal as an active ingredient, Ce as a rare earth metal, and at least one of Rh, Pt, and Pd as a noble metal. 5 to 10% by weight of Ce
A catalyst for purifying exhaust gas containing hydrocarbons, wherein the catalyst content is 30% by weight. 3. The method according to claim 1, wherein Mg is used as an active ingredient in an amount of 0.5 to 2 parts by weight based on 100 parts by weight of the carrier.
Wt%, Ce is 5 to 30 wt%, and Rh is 0.05 to 0.3.
Wt%, Pt is 0.5 to 2.5 wt%, and Pd is 0.5 to 2.5 wt%.
A method for purifying a hydrocarbon-containing exhaust gas, wherein 5% by weight is supported. 4. The method according to claim 1, wherein La and Al are used.
Wherein the composition ratio of La is 1 to 20 mol% and the total of La and Al is 100 mol%.