JPH10357A - Nitrogen oxide containing exhaust gas purification catalyst and exhaust gas purification method - Google Patents

Abstract

(57) [Problem] To provide a catalyst capable of purifying nitrogen oxides with high efficiency and an exhaust gas treatment method. Kind Code: A1 A rare-earth metal, strontium, silica, a noble metal and magnesium are supported as active components on a porous material carrier, and a sulfur oxide is used by using a catalyst in which the alkaline-earth metal and silica are amorphous. And the nitrogen oxides in the exhaust gas containing oxygen are purified using at least one of the hydrocarbons and carbon monoxide contained in the exhaust gas as a reducing agent. [Effect] It is possible to purify NOx while suppressing poisoning by SOx.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention contains nitrogen oxides such as flue gas discharged from an internal combustion engine such as an automobile engine or flue gas discharged from 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, 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 emitted from a lean burn automobile 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.

At present, the exhaust gas purifying catalyst for automobiles (three-way catalyst) generally installed in automobiles has a stoichiometric air-fuel ratio (air / fuel weight ratio), that is, a stoichiometric air-fuel ratio (air / fuel =). (14.7 weight ratio). This is because the three-way catalyst can simultaneously treat hydrocarbons and carbon monoxide in addition to nitrogen oxides only for stoichiometric combustion exhaust gas. 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 and 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 has become necessary to develop an exhaust gas purifying catalyst (lean NOx catalyst) for a lean burn engine instead of a three-way catalyst.

As an exhaust gas purifying catalyst for a lean burn engine, a catalyst in which barium oxide, lanthanum oxide, and platinum are supported on an alumina carrier of a type that stores NOx during lean burn and releases and reduces stored NOx during stoichiometric combustion. (JP-A-5-261287) and many others have been reported.

[0007]

An important technical problem of lean NOx catalysts is catalyst poisoning by SO ₂ . In general,
Catalyst poisoning by sulfur oxides is caused by oxidation of SO ₂ to SO ₃
Tend to be strengthened by Therefore, in lean burn exhaust gas in which oxygen is excessively present, SO ₃ of SO ₂
The lean NOx catalyst uses S
Susceptible to Ox poisoning. Therefore, the S-resistance of the lean NOx catalyst
Attention has also been paid to the enhancement of the Ox property, for example, the addition of a component that promotes the low-temperature decomposition of sulfate (Japanese Patent Application Laid-Open No. 6-142458).
Have been reported.

It is an object of the present invention to provide a novel exhaust gas purifying catalyst and an exhaust gas treatment method capable of efficiently purifying exhaust gas discharged in lean burn combustion while suppressing poisoning by sulfur oxides.

[0009]

SUMMARY OF THE INVENTION The present invention relates to a method for purifying exhaust gas using a catalyst for purifying nitrogen oxides in a sulfur-containing gas stream. The method comprises the steps of: A catalyst having an alkaline earth metal and silicon as active components, wherein the alkaline earth metal and silicon are amorphous. The catalyst of the present invention may contain a crystalline alkali metal and silicon.

The silicon and the alkaline earth metal are desirably supported by 7 to 30% by weight of the alkaline earth metal and 0.6 to 5% by weight of silicon in terms of silica based on 100 parts by weight of the porous carrier.

The catalyst of the present invention can further contain a rare earth metal and a noble metal as active components in the porous carrier, whereby the nitrogen oxide purification rate can be further improved.

The alkaline earth metal is preferably at least one of strontium, barium and calcium and magnesium, the noble metal is preferably platinum and rhodium, and the rare earth metal is preferably at least one of cerium and lanthanum. As a catalyst preparation method, a rare earth metal is first supported, then an alkaline earth metal other than magnesium is supported, silica is further supported, platinum and rhodium are supported as noble metals, and finally magnesium is supported. desirable.

The catalyst of the present invention comprises 5 to 20% by weight of a rare earth metal, 7 to 30% by weight of an alkaline earth metal, 0.5 to 3% by weight of platinum, and 0.5 to 3% by weight of rhodium, based on 100 parts by weight of a porous carrier. Is 0.05 to 0.3% by weight, and magnesium is 0.5 to 0.3% by weight.
It is desirable that 2.0% by weight be supported.

In the catalyst of the present invention, as the porous carrier, a porous oxide such as silica or zirconia can be used, but alumina is most preferable.

[0015] In the catalyst of the present invention, each active component is considered to function as follows. Taking strontium as an example of the alkaline earth metal, strontium is a NOx adsorption field, but strontium sulfate is generated by steam and SO ₃ , and the NOx adsorption capacity is reduced.
In order to suppress the formation of strontium sulfate, it is preferable to suppress the adsorption of water molecules to strontium. By dispersing a large number of water-repellent silica particles around the strontium particles, generation of strontium sulfate is suppressed. Furthermore, it is thought that the water repellency effect is further enhanced by the silica atoms being adjacent to the strontium atoms. When strontium and silica are mixed randomly at the atomic level, they become amorphous and cannot be measured with powder X-rays. Such a state is desirable.

[0016] Platinum and rhodium provide a reaction site for reducing nitrogen oxides to nitrogen.

Magnesium is highly dispersed on platinum and rhodium particles, thereby suppressing aggregation of platinum and rhodium particles due to heat.

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. Thereby, hydrocarbons and carbon monoxide can also be treated.

SOx is also adsorbed on the noble metal but is not permanently poisoned. Therefore, the catalyst is heated to a temperature of 500 ° C. to 800 ° C. using a reducing atmosphere, ie, a combustion exhaust gas having a stoichiometric air-fuel ratio or less, to reactivate the noble metal. Can be achieved. When the catalyst activity is reduced, it is preferable to reactivate the catalyst in this way and to perform the exhaust gas treatment again.

By mounting the catalyst of the present invention in the exhaust system of an internal combustion engine, the emission of nitrogen oxides to the outside of the vehicle can be significantly suppressed.

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 nitrogen oxides. can do.

As a method for preparing the catalyst, any of an impregnation method, a kneading method, a coprecipitation method, and a sol-gel method can be applied.

When a catalyst containing cerium, silica, strontium, magnesium, platinum and rhodium is prepared by an impregnation method, a porous carrier is impregnated with a solution containing a cerium compound, dried and calcined, and then silica and a precursor thereof. It is preferable to impregnate with a solution containing a strontium compound and then dry and fire, then impregnate a solution containing platinum or a platinum and rhodium compound, dry and fire, and finally impregnate a solution containing a magnesium compound and then dry and fire.

As the metal compound, a nitric acid compound,
Various compounds such as acetic acid compounds, chlorides, sulfides, carbonate compounds, and organic compounds can be used.

The catalyst of the present invention has a temperature of 200.degree.
Excellent activity in the following temperature range, especially 200 ° C
It has high activity in the temperature range of ~ 400 ° C. Therefore, it is effective to set the temperature at which the catalyst is brought into contact with the gas stream, the so-called reaction gas temperature, within the above temperature range. Further, even if oxygen and sulfur oxides having a stoichiometric air-fuel ratio or more coexist in the exhaust gas, it is possible to maintain a high NOx purification ability.

[0026]

FIG. 1 shows an example in which an exhaust gas purifying apparatus 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 Alumina was impregnated with a Ce nitrate solution, dried at 200 ° C., and fired at 600 ° C. for 1 hour. Subsequently, an aqueous solution in which Sr nitrate and silica sol were mixed was impregnated, and dried and fired in the same manner. Thus, with respect to 100 wt% of alumina, CE18 wt%, SR15 wt%, SiO ₂ 4% by weight, of platinum 1.6 wt%, Rh0.15
A catalyst powder containing 1.5% by weight of magnesium and 1.5% by weight of magnesium was obtained. A slurry obtained by adding alumina sol and aluminum nitrate to the catalyst powder and stirring and mixing the resulting mixture was coated on a cordierite honeycomb (400 cells / inc ² ). The firing temperature is about 450 ° C and the final coating amount is 20
Example catalyst 1 was obtained at 0 g / l.

In the same manner, the rare earth metal is Ce, the noble metal is Rh, Pt, and the alkaline earth metal is Sr, Ba, Ca,
Example catalysts 2 to 6 which were Mg and supported SiO ₂ were obtained. In addition, Comparative Example Catalysts 1 to 6 were prepared by the same catalyst preparation method as Example Catalyst 1, but not carrying SiO ₂ .

Table 1 summarizes the compositions of the prepared catalysts. In addition, the loading order in Table 1 is such that after loading the first component,
This indicates that the second component is loaded, and then the third component and the fourth component are loaded sequentially. The supported amount is indicated before the supported metal type.

[0030]

[Table 1]

(Experimental Example 1) Nitrogen oxide purifying performance experiments were conducted on the catalysts of Example 1 to 6 and Comparative Catalysts 1 to 6 by the following experimental method.

Experimental method: (1) A 6 cc honeycomb catalyst (17 mm square × 21 mm length) is filled in 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 300 ° C. and stabilizes, the circulation of stoichiometric combustion model exhaust gas (referred to as stoichiometric model exhaust gas) is started. After three minutes from the circulation, the stoichiometric model exhaust gas is stopped, and the distribution of the lean burn model exhaust gas (referred to as lean model exhaust gas) is started. NOx in the gas discharged from the reaction tube is measured by a chemiluminescence method. The NOx purification performance at this time is defined as the initial performance.

(3) The reaction tube filled with the honeycomb catalyst used in the above (2) is placed in a ring-shaped electric furnace and heated to 300 ° C. The temperature measures the honeycomb inlet gas temperature. When the temperature reaches 300 ° C. and becomes stable, the flow of SO ₂ -containing stoichiometric combustion model exhaust gas (referred to as poisoning gas) is started. The SO ₂ poisoning is terminated by flowing poisoned gas for 5 hours. The SO ₂ with a honeycomb catalyst after poisoning by the same tests as (2), to obtain a SO ₂ NOx purifying performance after poisoning.

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.
%, The O ₂ 0.6 vol%, the H ₂ 0.2 vol%, steam 1
A gas containing 0 vol% and the balance nitrogen was used. In addition, as a lean model exhaust gas, NO is 0.06 vol.
%, 0.04vol% of C ₃ H _6, 0.1vol% of CO, CO ₂
Contains 10 vol%, O2 ₅ vol%, water vapor 10 vol%,
A gas consisting of nitrogen as the balance was used. Further, as the poisoning gas, NO was 0.1 vol%, C ₃ H ₆ was 0.05 vol%,
0.6 vol% of CO, 0.6 vol% of O _2, and 0.02 of SO ₂
A gas containing 05 vol%, 10 vol% of water vapor, and the balance nitrogen was used. The space velocity of the three gases is
The drying gas (not including water vapor) was adjusted to 60,000 / h.

Table 2 shows the stoichiometric and lean model exhaust gas flow of the honeycomb catalyst at the initial stage and after SO ₂ poisoning.
The NOx purification rate after one minute was shown. The NOx purification rate was calculated according to the following equation.

[0037]

(Equation 1)

The decrease in lean NOx purification rate due to SO ₂ poisoning was calculated according to the following equation.

[0039]

(Equation 2)

The reduction rate of lean NOx purification rate due to SO ₂ poisoning was improved to -5% to -15% by supporting SiO ₂ .

[0041]

[Table 2]

[Example 2] As a modified catalyst of Example catalyst 1, comparative catalysts 7 to 7 were prepared in the same manner as in Example 1.
9 was obtained. Table 3 shows comparative catalysts.

[0043]

[Table 3]

The NOx purification rate was obtained in the same manner as in Experimental Example 1. Table 4 shows the results. The example catalyst 1 was the most excellent in both the NOx purification rate and the reduction rate.

[0045]

[Table 4]

Example 3 Example catalysts 7 and 8 were obtained by the same catalyst preparation method as in Example 1 except that the rare earth metal of Example catalyst 1 was changed. Table 5 shows example catalysts.

[0047]

[Table 5]

The NOx purification rate was obtained in the same manner as in Experimental Example 1. Table 6 shows the results.

[0049]

[Table 6]

Example 4 Example catalyst 1 and comparative example catalyst 1
, The sulfuric acid concentration after SO ₂ poisoning treatment in the same manner as in Experimental Example 1 was determined by FT-IR. The sulfuric acid concentration of Example Catalyst 1 was about 1/3 of Comparative Example Catalyst 1.

"Example 5" Example catalyst 1 and comparative example catalyst 1
The NO adsorption amount before and after the SO ₂ poisoning treatment was compared in the same manner as in Experimental Example 1. In the measurement, the catalyst after the reduction treatment was kept at 50 ° C., and the amount of NO adsorbed on the catalyst was obtained by introducing a pulse of NO. Table 7 shows S per unit weight of catalyst.
The relative ratio of the amount of adsorbed NO after poisoning when the amount of adsorbed NO before O ₂ poisoning is set to 100 is shown. In the example catalyst 1, the decrease in the NO adsorption point required for NOx purification is suppressed as compared with the comparative example catalyst 1.

[0052]

[Table 7]

"Example 6"
The NOx purification rate when the amount of O ₂ carried was changed was measured. The catalyst preparation method was the same as in Example Catalyst 1, and the experimental method was the same as Experimental Example 1. The results are shown in Table 8. By supporting SiO ₂ , the initial NOx purification rate is improved. Further, by setting the carried amount of SiO ₂ to 0.6 wt% to 5 wt%, the NOx purification rate after SO ₂ poisoning can be made 60%.

[0054]

[Table 8]

Example 7 In Example 1, the amount of cerium supported was 0 to 4 with respect to 100 parts by weight of alumina.
Catalysts varied in the range of 0 parts by weight were prepared. Experimental example 1
The NOx purification rate before the SO ₂ treatment was measured in the same manner as described above. The results are shown in FIG. 5 to 20% by weight of rare earth metal
By doing so, a high NOx purification rate can be obtained.

Example 8 In Example 1, the amount of strontium to be supported was 100 parts by weight of alumina.
Catalysts varied from 0 to 40 parts by weight were prepared. The NOx purification rate before SO ₂ treatment was measured in the same manner as in Experimental Example 1. The results are shown in FIG. The amount of strontium supported is 7
By setting it to 率 30% by weight, a high NOx purification rate can be obtained.

Example 9 A catalyst was prepared in the same manner as in Example Catalyst 1 except that the amount of the noble metal carried was changed in the range of 0 to 4% by weight based on 100 parts by weight of alumina. The NOx purification rate before SO ₂ treatment was measured in the same manner as in Experimental Example 1. FIG. 4 shows the results. By setting the supported amount of platinum to 0.1 to 2.8% by weight and the supported amount of rhodium to 0.1 to 0.3% by weight, a high N is obtained.
An Ox purification rate is obtained.

[Example 10] In Example catalyst 1, the amount of magnesium supported was changed with respect to 100 parts by weight of alumina.
Catalysts varied in the range of 0-2.5% by weight were prepared.
The NOx purification rate before SO ₂ treatment was measured in the same manner as in Experimental Example 1. FIG. 5 shows the results. A high NOx purification rate can be obtained by setting the supported amount of magnesium to 0.5 to 2.0% by weight.

Example 11 The catalyst of Example 1 and the catalyst of Comparative Example 1 were calcined at 800 ° C. or 900 ° C. for 5 hours. Table 9 shows the results. Even if the calcination temperature is increased, the SO ₂ degradation of the catalyst of Example 1 is low.

[0060]

[Table 9]

"Example 12"
The powder X-ray diffraction measurement of Sr, SiO ₂ , and Ce supported catalysts not supporting g, Rh, and Pt was performed. The results are shown in FIG.
There was no peak other than Al ₂ O ₃ , and Sr and SiO ₂ were supported as amorphous. It is considered that a composite oxide of SiO ₂ and Sr was formed. On the other hand, Sr, Ce
As a result of powder X-ray diffraction measurement of the supported catalyst, a peak attributed to SrCO ₃ was confirmed.

Example 13 The catalyst of Example 1 was prepared in the same manner as in Example 1, except that the SO ₂ treatment time was 5 hours. Here, the NOx purification rate after 1 minute of lean after 10 hours of SO ₂ treatment was measured. Next, a stoichiometric treatment was performed in which the stoichiometric model exhaust gas described in Experimental Example 1 was circulated for 30 minutes. The temperature at that time was 500 ° C. After cooling the temperature to 300 ° C,
The lean model exhaust gas was circulated, and the NOx purification rate after one minute was measured. The results are shown in the table. The catalyst performance was restored by the stoichiometric treatment.

[0063]

[Table 10]

Example 14 The catalyst of Example 1 had a honeycomb volume of 1.7 liters, and was subjected to a 10-15 mode actual vehicle running test to measure the total NOx emissions, total hydrocarbon emissions, and total CO emissions. Did. The test device was a gasoline vehicle having an engine with a displacement of 1800 cc installed on a chassis dynamometer, and one honeycomb was installed under the floor of the vehicle and in an exhaust pipe flow path. Test conditions are 15
After running in the mode, a hot start for a 10-15 mode test was performed.

As a result of the test, the total NOx emission was 0.068 g / km against the domestic regulation value of 0.25 g / km, and the total hydrocarbon emission was 0.025 g / km (the domestic regulation value 0.25 g / k).
m), and the total CO emission was 0.016 g / km (2.1 g / km of the domestic regulation value), which sufficiently satisfied the domestic regulation value.

The catalyst 25 of Example 1 having a honeycomb volume of 1.7 liters was subjected to SO ₂ poisoning according to the method of (3) of Experimental Example 1, and then subjected to the 10-15 mode test described above.

As a result of the test, the total NOx emission was 0.010
g / km, total hydrocarbon emissions are 0.049 g / km (the national regulation level is 0.25 g / km), and total CO emissions are 0.030 g / km.
km (domestic regulation value of 2.1 g / km), which sufficiently satisfied the domestic regulation value even after SO ₂ poisoning.

Example 15 The catalyst of Example 1 had a honeycomb volume of 1.7 liters, and one catalyst 1 was installed under the floor of a gasoline vehicle having an engine with a displacement of 1800 cc, in the exhaust pipe flow path. Exhaust gas containing 20 ppm SO ₂ is 10,000
After traveling 0 km, the NOx purification rate was measured.
The steady value of the Ox purification rate was 50%.

[0069]

According to the present invention, nitrogen oxides can be efficiently purified from exhaust gas containing oxygen, and the catalyst has resistance to sulfur, which is a catalyst poisoning substance contained in a trace amount in the exhaust gas. You can have.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of an automobile equipped with a catalyst of the present invention.

FIG. 2 is a graph showing the relationship between the amount of rare earth metal carried and the NOx purification rate.

FIG. 3 is a graph showing the relationship between the amount of strontium carried and the NOx purification rate.

FIG. 4 is a contour diagram showing the relationship between the amount of noble metal carried and the NOx purification rate.

FIG. 5 is a contour diagram showing a relationship between a magnesium carrying amount and a NOx purification rate.

FIG. 6 is a view showing an XRD measurement result of a catalyst.

[Explanation of symbols]

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

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[Procedure amendment]

[Submission date] July 3, 1996

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0009

[Correction method] Change

[Correction contents]

[0009]

SUMMARY OF THE INVENTION The present invention relates to a method for purifying exhaust gas using a catalyst for purifying nitrogen oxides in a sulfur-containing gas stream. The method comprises the steps of: A catalyst having an alkaline earth metal and silicon as active components, wherein the alkaline earth metal and silicon are amorphous. The catalyst of the present invention may contain a crystalline alkaline earth metal and silicon.

Continued on the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical indication B01J 23/66 B01D 53/36 102H 102B 102C (72) Inventor Hiroshi Hanaoka 7-chome, Omikacho, Hitachi City, Ibaraki Prefecture 1 Hitachi, Ltd. Hitachi Research Laboratory (72) Inventor Toshio Ogawa 7-1-1, Omikacho, Hitachi City, Ibaraki Prefecture Hitachi, Ltd. Hitachi Research Laboratory (72) Inventor Hisao Yamashita Omikamachi, Hitachi City, Ibaraki Prefecture 7-1-1, Hitachi, Ltd., Hitachi Research Laboratory, Hitachi, Ltd. (72) Inventor Shigeru Azumahata 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture, Ltd. Hitachi, Ltd., Hitachi Research Laboratory, Ltd. Hitachi, Ltd. Automotive Equipment Division, Hitachi, Ltd. Automotive Equipment Division (72) Inventor Toshifumi Hiratsuka, Hitachi, Ibaraki Pref.

Claims (10)

[Claims] 1. A catalyst for purifying nitrogen oxides in flue gas using at least one of carbon monoxide and hydrocarbons as a reducing agent, comprising an alkaline earth metal and silicon as active components on a porous carrier. A catalyst for purifying exhaust gas containing nitrogen oxides, wherein the alkaline earth metal and silicon are amorphous. 2. The catalyst according to claim 1, further comprising a rare earth metal and a noble metal as active components. 3. A method for purifying nitrogen oxides in flue gas using at least one of carbon monoxide and hydrocarbon as a reducing agent, said method comprising the steps of: providing an alkaline earth metal and silicon on a porous carrier; A method for purifying exhaust gas, comprising using a catalyst in which a class metal and silicon are amorphous. 4. The method according to claim 3, wherein said catalyst further comprises a rare earth metal and a noble metal as active components. 5. The catalyst according to claim 1, wherein
A nitrogen oxide-containing exhaust gas containing at least one of strontium, barium, and calcium as the alkaline earth metal, containing platinum and rhodium as the noble metal, and containing at least one of cerium and lanthanum as the rare earth metal. Purification catalyst. 6. The catalyst according to claim 1, wherein
A catalyst for purifying exhaust gas containing nitrogen oxides, wherein the porous carrier is alumina. 7. The catalyst according to claim 1, which contains 7 to 30% by weight of alkaline earth metal and 0.6 to 5% by weight of silicon in terms of silica based on 100 parts by weight of the porous carrier. A catalyst for purifying exhaust gas containing nitrogen oxides. 8. The catalyst according to claim 2, wherein 5 to 20% by weight of a rare earth metal, 7 to 30% by weight of an alkaline earth metal, and silicon in terms of silica, based on 100 parts by weight of the porous carrier. Nitrogen oxide containing 0.6 to 5% by weight, 0.5 to 3% by weight of platinum, 0.05 to 0.3% by weight of rhodium, and 0.5 to 2.0% by weight of magnesium. Purification catalyst for contained exhaust gas. 9. The method for preparing a catalyst according to claim 1, wherein the catalyst is supported on a porous carrier by using a mixed solution of an acidic solution of an alkaline earth metal and an acidic silica sol. For preparing a purification catalyst for waste gas containing exhaust gas. 10. The exhaust gas purifying method according to claim 3 or 4, wherein the catalyst poisoned by the sulfur oxide and having reduced activity uses a combustion exhaust gas having a stoichiometric air-fuel ratio or lower and has an exhaust gas temperature of 500 ° C to 800 ° C. A method for purifying exhaust gas, wherein the method is reactivated by operating at a temperature of ° C. and then subjected to an exhaust gas purification process again.