JPH09299761A - Method of purifying nitrogen oxides in exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To steadily and efficiently purify exhaust gas discharged in lean- burning combustion by using a catalyst in which Ti and Pt are deposited as active components on a porous carrier consisting of an inorganic oxide compound other than titania. SOLUTION: The nitrogen oxides in the exhaust gas discharged under a lean-burning combustion condition can be efficient reduced into nitrogen by using hydrocarbons and carbon monoxide as reduction agents. Also the hydrocarbons and carbon monoxide can be oxidized with the oxygen in the exhaust gas. Thus, the hydrocarbons and carbon monoxide can be treated. When the catalyst 3 contains Ti, Pt and Rd in the inorganic carrier, Rd, the catalyst has remarkably high catalyst activity under a stoichiometric operational condition. By mounting the catalyst 3 on an exhaust gas flow path 2 in the back stream of an internal-combustion engine 1, the nitrogen oxides can be considerably suppressed from being exhausted to the outside of the vehicle.

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 efficiently purifying nitrogen oxides from exhaust gas containing nitrogen oxides such as combustion exhaust gas, and also relates to a catalyst for purifying nitrogen oxides.

[0002] The catalyst of the present invention is suitable as a catalyst for purifying exhaust gas discharged from a lean burn engine.

[0003]

2. Description of the Related Art Exhaust gas discharged from an internal combustion engine of an automobile or the like contains nitrogen oxides (NOx). Nitrogen oxides are harmful to the human body and cause the global environment to be destroyed such as acid rain. Therefore, various catalysts for purifying nitrogen oxides in exhaust gas have been studied.

Currently, the exhaust gas purifying catalyst for automobiles, which is generally installed in automobiles, has a fuel-air ratio, that is, an air-fuel ratio (air / fuel weight ratio) is stoichiometric, that is, a theoretical air-fuel ratio (air / fuel = 14.7 weight ratio). ) Has been aimed at the exhaust gas treatment of automobiles set in the vicinity. Since stoichiometric combustion emits hydrocarbons and carbon monoxide in addition to nitrogen oxides, which are also environmental pollutants, the main focus was on the development of a three-way catalyst that could simultaneously process these three substances. . The three-way catalyst is a general term for a catalyst that can simultaneously treat nitrogen oxides, hydrocarbons, and carbon monoxide in exhaust gas. Most of the three-way catalysts are mainly composed of noble metals (rhodium, palladium, platinum) and rare earth metals (cerium).

However, in recent years, from the viewpoint of reducing fuel consumption, a lean-burn (lean-burn) engine that burns at an air-fuel ratio equal to or higher than the stoichiometric air-fuel ratio has been developed. In lean burn, the amount of oxygen in the exhaust gas increases, and conventional three-way catalysts have poor catalytic activity in the presence of oxygen and cannot purify nitrogen oxides efficiently. Therefore, it was necessary to develop an exhaust gas purifying catalyst for a lean burn engine that could replace the three-way catalyst.

As an exhaust gas purifying catalyst for a lean burn engine, barium oxide, lanthanum oxide and platinum are supported on an alumina carrier as a catalyst for storing NOx during lean burn and releasing stored NOx during stoichiometric combustion for reduction. A catalyst (JP-A-5-261287) has been reported.

[0007]

In a catalyst in which barium oxide, lanthanum oxide, and platinum are supported on an alumina carrier, NOx is released and reduced from the catalyst before the NOx reaches storage saturation during lean burn operation on the system. Is required. Although the catalyst works practically well in combination with the system, further improvement in performance has been studied.

An object of the present invention is to provide an exhaust gas treatment method capable of steadily and efficiently purifying exhaust gas discharged in lean burn combustion.

[0009]

The exhaust gas treatment method of the present invention comprises contacting a nitrogen oxide-containing gas stream with a catalyst in the presence of at least one of hydrocarbons and carbon monoxide to convert nitrogen oxides to nitrogen. A method of reduction, wherein the catalyst comprises:
It is characterized in that a porous carrier made of an inorganic oxide other than titania and supporting titanium and platinum or titanium, platinum and rhodium as an active component is used.

The catalyst used in the present invention is prepared by impregnating a carrier with a solution containing an active ingredient and then calcining.

As a result, titanium becomes titania (Ti
It exists in the form of O ₂ ). Platinum and rhodium exist in the form of metals or metal oxides.

The amount of the active ingredient is 0.4 to 1.6% by weight of platinum, 0.05 to 0.30% by weight of rhodium, and 0.4% by weight of titanium with respect to 100% by weight of the inorganic oxide carrier.
It is desirable that the content is up to 1.2% by weight.

The catalyst of the present invention is preferably prepared by first supporting titanium on an inorganic oxide carrier, and then supporting platinum and rhodium simultaneously or sequentially.

In the catalyst of the present invention, as the inorganic oxide carrier, alumina, silica, zirconia, zeolite,
It is preferable to use at least one kind of porous oxide selected from tungsten oxide, but it is particularly preferable to use alumina.

The catalyst of the present invention is used as it is or by coating a substrate such as a cordierite honeycomb with the catalyst having the above-mentioned constitution.

In the catalyst of the present invention, it is considered that the active component functions as follows and can purify exhaust gas steadily. Titanium interacts with alumina as a carrier to adjust the solid acidity of alumina. The carrier having the adjusted solid acidity promotes the activation of hydrocarbons. Platinum and rhodium serve as a reaction site for reducing nitrogen oxides to nitrogen. The activated hydrocarbon reacts with oxygen in nitrogen oxide to generate carbon dioxide.

The catalyst of the present invention has high exhaust gas purification performance under both stoichiometric and lean burn conditions.
Specifically, nitrogen oxides in exhaust gas discharged under these combustion conditions can be efficiently reduced to nitrogen using hydrocarbons and carbon monoxide as reducing agents. Further, hydrocarbons and carbon monoxide can be oxidized by oxygen in exhaust gas. This also makes it possible to treat hydrocarbons and carbon monoxide. When the catalyst contains titanium, platinum and rhodium in the inorganic carrier, it can exhibit extremely high catalytic activity under stoichiometric operating conditions.

By mounting the catalyst of the present invention in the exhaust system of an internal combustion engine, it is possible to significantly suppress the emission of nitrogen oxides outside the vehicle.

The catalyst of the present invention is also effective in treating exhaust gas discharged from diesel engines of diesel automobiles. Since the diesel engine is operated at a high air-fuel ratio with excess oxygen, and the catalyst of the present invention exhibits excellent activity even in the presence of oxygen, even the exhaust gas discharged from the diesel engine can efficiently remove nitrogen oxides. Can be purified.

When the catalyst of the present invention is prepared by the impregnation method, the inorganic oxide carrier is impregnated with a solution containing a titanium compound, dried and calcined, and then impregnated with a solution containing platinum or a platinum and rhodium compound and dried. Baking is desirable. This makes it possible to prepare a catalyst in which titanium is supported on the inorganic oxide carrier and platinum or platinum and rhodium are further supported.

As the metal compound, various compounds such as nitric acid compounds, acetic acid compounds, chlorides, sulfides, carbonic acid compounds and organic compounds can be used.

The catalyst of the present invention has a temperature of 200 ° C. or higher and 500 ° C.
Excellent activity in the following temperature range, especially 200 ° C
It has high activity in the temperature range of ~ 400 ° C. Therefore, the temperature at which the catalyst and the gas stream are brought into contact with each other, so-called reaction gas temperature, should be set within the above temperature range. A catalyst containing a platinum group metal and titanium as an active component is disclosed in JP-A-62-14.
9344, JP 8-24641, JP 6-198178, JP 3-151045, JP 6-327980, JP 6-14251
No. 7, JP-A-4-27429, etc., but the catalyst preparation method is different from that of the present invention. When titanium and platinum group metals are coated in multiple layers by means such as the slurry coating method, only low NOx purification performance is obtained. Also, when titania is used as a carrier, only low NOx purification performance can be obtained.

[0023]

FIG. 1 shows an example in which an exhaust gas purifying catalyst of the present invention is installed in an automobile. In FIG. 1, a catalyst 3 is provided in an exhaust gas passage 2 downstream of the engine 1.

[Example 1] 100 g of boehmite, 0.05 g of crystals of aluminum nitrate nonahydrate and 400 g of water
Was mixed with a ball mill for 3 hours to coat a cordierite honeycomb (400 cells / inc2). The firing temperature was 600 ° C. and the final alumina coating amount was 200 g / l. This alumina coated honeycomb was impregnated with a solution of titanium isopropoxide in ethanol, dried at 200 ° C, and then dried at 700 ° C for 1 hour.
Fired for hours. Then, a dinitrodiammine Pt nitric acid aqueous solution was impregnated, and similarly dried and baked. From the above,
0.9% by weight of titanium to 100% by weight of alumina,
Example catalyst 1 containing 1.6% by weight of platinum was obtained. Further, in the same manner, a catalyst containing titanium and platinum,
Further, by impregnating with an aqueous solution of Rh nitric acid, and similarly drying and firing, 0.9% by weight of titanium, 0.8% by weight of platinum, Rh
Example catalyst 2 containing 0.16% by weight was obtained. Furthermore, Comparative Examples catalysts 1 to 3 in which platinum was replaced with rhodium, palladium, or iridium by the same method were obtained.

The composition of the prepared catalyst is summarized in Table 1.

[0026]

[Table 1]

With respect to the example catalysts 1 and 2 and the comparative example catalysts 1 to 3, nitrogen oxide purification performance experiments were conducted by the following experimental method.

Experimental method 1: (1) Honeycomb catalyst 6 cc (17 mm square × 21 mm length)
In a Pyrex reaction tube.

(2) Put the reaction tube in an annular electric furnace and
Heat to 0 ° C. The temperature measures the honeycomb inlet gas temperature. When the temperature reaches 450 ° C and stabilizes, the distribution of lean burn model exhaust gas (called lean model exhaust gas) is started. NOx in the gas discharged from the reaction tube is measured once every 30 seconds by the chemiluminescence method.

As lean model exhaust gas, NO is reduced to 0.
06 vol%, C ₃ H ₆ 0.04 vol%, CO 0.1 vol%
, CO ₂ 10 vol%, O _{2 5} vol%, steam 10 vol
%, And the balance was nitrogen. The space velocity of the gas is 60,0 for dry gas (not containing water vapor).
00 / h.

(3) When the NOx purification rate becomes stable at 450 ° C, the temperature is lowered by 50 ° C while the lean model exhaust gas is still flowing. When the NOx purification rate becomes stable at 400 ° C, the NOx purification rate at 350 ° C is measured by further lowering the temperature by 50 ° C. Finally, the temperature is lowered to 150 ° C., and the experiment is completed.

Table 2 shows the NOx purification rate in the lean model exhaust gas at each temperature. The NOx purification rate was calculated according to the following formula.

[0033]

[Equation 1]

Comparative catalyst 1 in which Rh, Pd and Ir were used alone among the noble metals (Rh, Pt, Pd and Ir)
3 to N are the same as those of the example catalysts 1 and 2 using Pt.
Ox purification rate is low. Further, the example catalyst 2 is Rh and Pt.
In combination with Pt and Rh alone, high activity at 200 ° C. was observed, which was not observed with Pt and Rh alone.

[0035]

[Table 2]

In the catalysts 1 and 2 of the example, the nitrogen oxide purification performance experiment was conducted by the following experiment method.

Experimental Method 2: (1) As in Experimental Example 1, a catalyst is installed in the reaction tube.

(2) The temperature is raised to 150 ° C.

(3) When the temperature reaches 150 ° C., the flow of gas simulating the exhaust gas composition of stoichiometric combustion (called stoichiometric model exhaust gas) is started, and at the same time, heating by the electric furnace is restarted. The heating rate is 10 ° C / min, and heating is performed until the temperature reaches 400 ° C.

The method of measuring the NOx concentration at the outlet of the reaction tube is the same as the experimental method 1.

As stoichiometric model exhaust gas, NO was 0.1 vol%, C ₃ H ₆ was 0.05 vol%, and CO was 0.6 vol%.
%, O ₂ 0.6 vol%, H ₂ 0.2 vol%, steam 10 vo
A gas containing 1% and the balance being nitrogen was used. The space velocity of the gas in a dry state (not including water vapor) is 60,
000 / h.

(4) When the measured temperature reaches 400 ° C., the gas flow is stopped and the experiment is terminated. Table 3 shows NOx in the stoichiometric model exhaust gas in Example catalysts 1 and 2.
The purification rate was shown.

[0043]

[Table 3]

"Example 2" Table 4 shows the same catalyst preparation method as in Example catalyst 1, except that the inorganic oxide carrier was changed.

[0045]

[Table 4]

The NOx purification rate at a reaction gas temperature of 300 ° C. was obtained by the same method as the experimental method 1 of Example 1. Table 5 shows the results.

[0047]

[Table 5]

[Example 3] As for Example catalyst 1, catalysts having different loadings of platinum and titanium were prepared, and the NOx purification rate at the reaction gas temperature of 300 ° C was determined according to the experimental method 1 of Example 1. It was The results are shown in FIG.

Example 4 For Example catalyst 2, a catalyst was prepared in which the amount of supported Rh was changed when platinum was 0.8% by weight and titanium was 0.9% by weight. According to Method 1, the NOx purification rate at a reaction gas temperature of 200 ° C. was obtained. The results are shown in FIG.

Example 5 An example catalyst 7 was prepared which had the same composition and loading amount of the catalytically active components as the example catalyst 1, but the loading order was different. According to the experimental method 1 of Example 1, the NOx purification rate at the reaction gas temperature of 300 ° C. was obtained.
Table 6 shows the results.

[0051]

[Table 6]

Comparative Example The same cordierite honeycomb as used in Example 1 was coated with titania sol. The firing temperature was 600 ° C. and the firing was performed for 1 hour. The coating amount was 200 g / l. Next, dinitrodiamine platinum nitric acid aqueous solution was impregnated and baked at 700 ° C. for 1 hour to contain 0.9% by weight of platinum based on 100% by weight of TiO ₂ . This is designated as Comparative Example Catalyst 4. Example 1
The NOx purification rate was obtained in accordance with the experimental method 1 of. Table 7 shows the results.

[0053]

[Table 7]

[0054]

According to the present invention, nitrogen oxides can be efficiently purified from exhaust gas containing nitrogen oxides, hydrocarbons and carbon monoxide, or exhaust gas containing oxygen.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of a NOx purification device.

FIG. 2 is a contour map of NOx purification rate at a reaction gas temperature of 300 ° C.

FIG. 3 is a graph of NOx purification rate at a reaction gas temperature of 200 ° C.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Engine, 2 ... Exhaust gas flow path, 3 ... Catalyst.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication location B01D 53/36 102B (72) Inventor Hiroshi Hanaoka 7-1 Omika-cho, Hitachi-shi, Ibaraki Hitachi Ltd., Hitachi Research Laboratory (72) Inventor Toshio Ogawa 7-1, 1-1 Omika-cho, Hitachi City, Ibaraki Prefecture Hitachi Ltd. Hitachi Research Laboratory (72) Inventor Toshio Yamashita 7, 1 Omika-cho, Hitachi City, Ibaraki Prefecture No. 1 Incorporated company Hitachi Ltd. Hitachi Research Laboratory (72) Inventor Shigeru Shodohata 1-1-1 Omika-cho, Hitachi City, Hitachi, Ibaraki Prefecture Incorporated Hitachi Ltd. Hitachi Research Laboratory (72) Toshifumi Hiratsuka, Hitachinaka City, Ibaraki Prefecture Takaba 2520 Incorporated company Hitachi, Ltd., Automotive Equipment Division (72) Inventor Yuichi Kitahara Takata, Hitachinaka City, Ibaraki Prefecture Field No. 2520 Incorporated company Hitachi, Ltd. Automotive equipment division

Claims (8)

[Claims] 1. A method for treating exhaust gas produced when a fuel is burned at a stoichiometric air-fuel ratio or in an oxygen excess of the stoichiometric air-fuel ratio, wherein the exhaust gas containing nitrogen oxides, hydrocarbons and carbon monoxide is contacted with a catalyst. In the method of reducing nitrogen oxides to nitrogen by using at least one of hydrocarbon and carbon monoxide as a reducing agent, the catalyst is impregnated with a solution containing an active ingredient in a porous carrier made of an inorganic oxide other than titania. A method for purifying nitrogen oxides in exhaust gas, characterized in that the exhaust gas is brought into contact with the catalyst by firing and then supporting the carrier on which platinum and titanium are supported as active components. . 2. A method for purifying nitrogen oxides in exhaust gas according to claim 1, wherein the catalyst further contains rhodium as an active component. 3. The method according to claim 1, wherein the catalyst contains 0.4 to 1.6% by weight of platinum and 0.4 to 1.2% by weight of titanium with respect to 100% by weight of the carrier. A method for purifying nitrogen oxides in exhaust gas, comprising: 4. The method according to claim 2, wherein the catalyst contains 0.4 to 1.6% by weight of platinum and 0.05 to 0.3% by weight of rhodium with respect to 100% by weight of the carrier. Titanium to 0.
A method for purifying nitrogen oxides in exhaust gas, comprising 4 to 1.2% by weight. 5. The method according to claim 1 or 2, wherein
A method for purifying nitrogen oxides in exhaust gas, wherein the carrier comprises at least one selected from alumina, silica, zeolite, zirconia and tungsten oxide. 6. The method according to claim 1, wherein the catalyst is prepared by supporting titanium on a carrier and then supporting platinum on the carrier. Purification method. 7. The exhaust gas according to claim 2, wherein the catalyst is prepared by supporting titanium on a carrier and then supporting platinum and rhodium simultaneously or sequentially. Method for purifying nitrogen oxides inside. 8. The method according to claim 1, wherein
The method for purifying nitrogen oxides in exhaust gas, wherein the catalyst contains titanium in the form of a metal oxide and platinum and rhodium in the form of a metal or a metal oxide.