JP2000061308A - Catalyst and method for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably decompose nitrogen oxides for a long period of time by a method wherein palladium and platinum are supported by a zirconia sulfate group, and a catalyst for decomposing nitrogen oxides in presence of a methane is used in an atmosphere of excessive oxygen. SOLUTION: This catalyst is obtained by supporting palladium and platinum on a zirconia sulfate group support. A method for supporting palladium and platinum on the zirconia sulfate group support is preferably executed by impregnating the support with an aqueous solution of a nitrate salt an ammine complex, etc., of both metals. A support amount of palladium is about 0.1 to 0.5% in a zirconia sulfate group weight equivalent. Further, a support amount of platinum is about 20 to 100% in a palladium weight equivalent. Then, the zirconia sulfate group wherein palladium and platinum are supported by being impregnated is dried and thereafter burnt. Nitrogen oxides are decomposed under a presence of methane in an atmosphere of excessive oxygen by using this catalyst.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a catalyst for decomposing nitrogen oxides (NO _x ) contained in exhaust gas and having an adverse effect on the environment by using methane in an atmosphere of excess oxygen, and nitrogen using the catalyst. The present invention relates to a method for purifying oxides. In the present invention, the “oxygen excess atmosphere” means that the gas to be treated brought into contact with the catalyst according to the present invention has an amount equal to or more than the amount necessary for completely oxidizing reducing components such as hydrocarbons and carbon monoxide contained therein. It means a gas containing an oxidizing component such as oxygen or nitrogen oxide.

[0002]

2. Description of the Related Art A catalyst for reducing nitrogen oxides in an atmosphere of excess oxygen using hydrocarbon as a reducing agent is disclosed in JP-A-63-100919.
Japanese Patent Laid-Open No. 1-135541 and the like.
However, these known documents do not disclose the use of methane as a hydrocarbon. Methane is present in exhaust gas generated when burning various fuels. Furthermore, since methane is the main component of natural gas-based city gas widely supplied to homes and factories in Japan, if it becomes possible to reduce nitrogen oxides using this,
It is an extremely effective means for reducing nitrogen oxides in an oxidizing atmosphere. Japanese Unexamined Patent Publication (Kokai) No. 5-192582 discloses a method of contacting a combustion exhaust gas with a zeolite catalyst in which cobalt or rhodium is ion-exchanged in the presence of methane to reduce nitrogen oxides in the exhaust gas. However,
The activity of this catalyst is not sufficient, and furthermore, the activity of the catalyst in the coexistence of water vapor necessarily contained in actual combustion exhaust gas is not mentioned at all. That is, steam is well known to bring about a decrease in catalytic activity in the reaction of reducing nitrogen oxides in an oxidizing atmosphere using hydrocarbon as a reducing agent. Decrease and countermeasures against it
Not shown. JP-A-6-254352 discloses that a catalyst in which palladium is supported on a ZSM-5 type zeolite by ion exchange shows activity for reducing and removing nitrogen oxides using methane as a reducing agent. However, there is no disclosure about the activity of the catalyst in the presence of water vapor. Japanese Patent Publication No. 8-500772 discloses a catalyst in which 0.3 to 2% by weight of palladium is supported on MFI type zeolite by ion exchange, using methane as a reducing agent and reducing high nitrogen oxides even in the presence of steam. It is disclosed that it exhibits activity. In addition, Satokawa et al. Reported in the proceedings of the 1996 catalyst research presentation (published on September 13, 1996) that the catalyst in which palladium was ion-exchanged with mordenite was highly effective in reducing nitrogen oxides even in the presence of steam. It is disclosed that it exhibits activity.
However, these catalysts, in the presence of steam,
There is a problem that the activity decreases rapidly. For example, Hoshi et al. (August 1997)
(Published on 25th), the durability of mordenite with palladium ion-exchanged catalyst in the presence of water vapor is reported. According to this report, 50% at the start of the reaction
The removal rate of nitrogen oxides, which was moderate, dropped rapidly to 40%.
After 30 hours it will be 30% and after 70 hours it will be 15%.

Regarding carriers other than zeolite,
Applied catalysis by Resasco and others
Bee: Applied Catalysis B: En
vironmental) Vol. 7, p. 113 (1995), reports the results of reducing nitrogen oxides using methane as a reducing agent, using a catalyst in which palladium is supported on sulfate zirconia. However, according to the graph showing the change with time of the catalytic activity described therein, the activity of this catalyst shows a clear tendency of deterioration within a short time of about 100 minutes even in the absence of water vapor. ing. As revealed above,
Since the conventional catalyst for decomposing nitrogen oxides has a problem that the activity is remarkably deteriorated due to the presence of water vapor, when treating combustion exhaust gas in which water vapor is inevitably present, high denitration is required for a long time. You cannot sustain the rate. Furthermore, in the combustion exhaust gas, there is a trace amount of sulfur oxide derived from a trace amount of organic sulfur content in the fuel,
It is also known that even if the concentration is extremely small, such as about 0.2 ppm, it has a cumulative adverse effect on the catalyst and gradually decreases its activity (for example, Nishisaka et al.
Proceedings of the 1997 Lecture on Catalysis Research Presentation, published on August 25, 1997).

[0004]

Therefore, according to the present invention, exhaust gas purification which can stably purify nitrogen oxides for a long period of time by using methane as a reducing agent even in the presence of activity inhibiting substances such as steam and sulfur oxides. The main purpose is to provide a catalyst for use.

[0005]

Means for Solving the Problems As a result of intensive studies made by the inventor while paying attention to the current state of the art as described above, the catalyst in which platinum is supported on sulphate zirconia together with palladium and methane is used as a reducing agent. As a result, it has been found that when decomposing nitrogen oxides, high nitrogen oxide decomposing activity is maintained for a long period of time even in the presence of activity inhibiting substances such as steam and sulfur oxides. The present invention has been completed based on such new knowledge, and provides the following exhaust gas purifying catalyst and exhaust gas purifying method. 1. A catalyst for purifying exhaust gas, which comprises palladium and platinum supported on sulfate zirconia and decomposes nitrogen oxides in the presence of methane in an oxygen excess atmosphere. 2. The catalyst according to item 1 above, wherein the amount of palladium supported is 0.1 to 0.5% by weight relative to the sulfate group zirconia. 3. The amount of platinum supported is 20 to 10 in weight ratio to palladium.
The catalyst according to the above item 1 or 2, which is 0%. 4. An exhaust gas purification method in which nitrogen oxides are decomposed in the presence of methane in an oxygen excess atmosphere using a catalyst in which palladium and platinum are supported on sulfate radical zirconia. 5. The exhaust gas purifying method as described in the above item 4, wherein the amount of palladium supported is 0.1 to 0.5% by weight ratio to sulfate zirconia. 6. The amount of platinum supported is 20 to 10 in weight ratio to palladium.
6. The exhaust gas purification method according to the above item 4 or 5, which is 0%. 7. An exhaust gas purification method for decomposing nitrogen oxides in exhaust gas containing excess oxygen and containing sulfur oxides in the presence of methane using a catalyst in which palladium and platinum are supported on sulfate radical zirconia. 8. The exhaust gas purifying method as described in 7 above, wherein the amount of palladium supported is 0.1 to 0.5% by weight with respect to sulfate zirconia. 9. The loading amount of platinum is 20 to 10 by weight ratio to palladium.
9. The exhaust gas purification method as described in 7 or 8 above, which is 0%. The catalyst according to the present invention is obtained by supporting palladium and platinum on a sulfate radical zirconia carrier. Sulfate zirconia itself is a known substance (for example, Makoto Hino and Kazushi Arata, "Surface", Vol. 28, No. 7, page 481 (1990);
"Surface", Vol. 34, No. 2, p. 51 (1996), etc.). Sulfate-based zirconia is, for example, after dipping commercially available zirconium hydroxide in dilute sulfuric acid, or by impregnating zirconium hydroxide with an aqueous solution of ammonium sulfate, and then in air at 450 to 650 ° C.
It can be obtained by firing at a temperature of about 500 to 600 ° C. If the firing temperature is too high, a large amount of sulfate radicals may volatilize and disappear, whereas if the firing temperature is too low, the effect of firing becomes insufficient, and unreacted zirconium hydroxide remains in the carrier. Or, the calcined product becomes amorphous and stable sulfate zirconia is not formed. Since a part of the sulfate radicals is volatilized during the firing operation, it is preferable to add an excess amount of sulfate radicals corresponding to the volatile components to the zirconium hydroxide during the above treatment or immersion. Sulfate sulphate in zirconia carrier (S
The content of O ₄ ^2- ) is usually 1 to 5 based on the weight of zirconia.
It is about 20%, more preferably about 3 to 10%.
If the amount of sulfate is too small, the effect of adding sulfate is not sufficiently exhibited, whereas if it is excessive, a stable sulfate zirconia carrier cannot be obtained. The method for supporting palladium and platinum on the sulfate radical zirconia carrier is
There is no particular limitation as long as both components are supported on the carrier in a highly dispersed state, but it is preferably carried out by impregnating the carrier with an aqueous solution of a nitrate of both metals or an ammine complex. The amount of palladium carried is based on the weight of sulfate zirconia,
It is usually about 0.05 to 1%, more preferably about 0.1 to 0.5%, and the amount of platinum supported is 10 based on the weight of palladium.
It is about 200%, more preferably about 20% to 100%. If the amount of palladium supported is too small, the catalytic activity will be low, whereas if it is too large, the catalytic effect will be rather lost due to aggregation. If the amount of platinum supported is too small, the catalytic activity will be low, whereas if it is too high, the nitrogen oxide removal activity will be decreased.

Then, after the sulfate radical zirconia impregnated and supported with palladium and platinum as described above is dried,
Bake. If the calcining temperature is too low, the calcining effect will be insufficient and stable catalytic activity will be difficult to obtain, whereas if it is too high, the aggregation of palladium and platinum will be promoted. Therefore, the firing temperature is usually in the range of about 300 to 600 ° C, and more preferably in the range of about 450 to 550 ° C.

The catalyst of the present invention may be molded into pellets according to a conventional method and used, or may be wash-coated on a refractory honeycomb carrier before use. In either case, a binder can be added if desired.

The exhaust gas purification method according to the present invention is characterized by using the catalyst obtained above. If the amount of the catalyst used is too small, an effective purification rate cannot be obtained, whereas if it is too large, the performance commensurate with the amount of the catalyst used cannot be obtained. GHS
In V), preferably used in an amount of 2,000~200,000h ^-1, it is more preferably in the range of 2,000~60,000h ^-1. Although the catalyst of the present invention has high activity, it may still not exhibit effective purification performance when the temperature of the exhaust gas is too low. On the other hand, if the temperature of the exhaust gas is too high, the durability of the catalyst may be impaired. Therefore, the catalyst of the present invention is preferably used in the range of about 350 to 500 ° C, more preferably 375 to 475 ° C. The nitrogen oxide concentration of the exhaust gas to be purified by the method of the present invention is not particularly limited, but is usually 10 to 5000 vol.p.
in the pm range. The methane concentration during nitrogen oxide decomposition may vary depending on the required denitration rate and other reaction conditions.
In order to obtain a high denitrification rate, nitrogen oxides in exhaust gas are usually used.
It is preferable that the amount is 1 time or more, more preferably about 5 times or more. When the amount of methane contained in the exhaust gas is less than that required for the reduction of nitrogen oxides, methane or a methane-containing gas such as natural gas-based city gas may be added to the exhaust gas in an appropriate amount. There is no particular upper limit to the methane concentration, and the higher the concentration, the higher the denitration rate. However, even if an excessive amount of methane is added to the exhaust gas, the improvement of the nitrogen oxide decomposition rate corresponding to the cost increase accompanying it cannot be achieved, which is economically disadvantageous, and
There is a risk of increasing the amount of residual methane in the gas after treatment. Further, the gas to be processed needs to be in an oxygen excess state in a state where methane as a reducing agent is added, so that the amount of methane to be added has an upper limit determined by the composition of the gas to be processed. The oxygen concentration in the exhaust gas is not particularly limited as long as it contains excess oxygen, but sufficient reaction activity may not be obtained when the oxygen concentration is extremely low, for example, when the volume is 1% or less. There is. If the oxygen concentration in the exhaust gas is too low, or if the temperature of the exhaust gas is high and the temperature of the catalyst may exceed the preferred temperature range, while paying attention so that the exhaust gas temperature does not fall below the suitable range. After mixing an appropriate amount of air, the air-mixed exhaust gas may be brought into contact with the catalyst.

[0009]

When the exhaust gas containing nitrogen oxides is treated with the catalyst of the present invention, it is stable for a long period of time even under the condition that a large amount of water vapor is present when NO _{x is} decomposed and removed using methane as a reducing agent. The catalytic activity is maintained. Further, since the conversion rate of methane is also kept high, the concentration of residual methane in the treated gas can be kept low. Furthermore, even in the coexistence of sulfur oxides, the activity of which has been remarkably reduced in the conventional catalyst, a high catalytic activity is stably maintained.

[0010]

EXAMPLES Examples and comparative examples will be shown below to illustrate the present invention.
The features of the present invention will be described in more detail.
It is not limited to these examples. Example 1Preparation of 0.5% Pd-0.5% Pt / sulfate zirconia catalyst (1) Dissolve 15 g of ammonium sulfate in 150 ml of an aqueous solution of
150 g of zirconium iodide was impregnated for 10 hours. Dry the impregnated body
And calcination at 550 ℃ for 3 hours to remove sulfated zirconia
Obtained. Palladium nitrate solution containing 10% by weight as Pd
Tetraammine containing 1.25 g and 6.27% by weight as Pt
Mix and stir 2 g of platinum nitrate aqueous solution, and then add 20 ml of pure water.
The solution diluted to
25 g of acid radical zirconia was impregnated for 10 hours. Sulfate after impregnation
After drying the zirconia, calcination at 500 ℃ for 9 hours, 0.5
% Pd-0.5% Pt / sulfate radical zirconia catalyst (1) was obtained. Example 2Preparation of 0.25% Pd-0.1% Pt / sulfate zirconia catalyst Hydroxy acid was added to 400 ml of an aqueous solution to dissolve 54 g of ammonium sulfate.
360 g of zirconium oxide was impregnated for 15 hours. Dry the impregnated body
And calcination at 550 ℃ for 6 hours to remove sulfated zirconia
Obtained. As a result of inductively coupled plasma-emission spectroscopy analysis, this sulfur
Acid root zirconia contains 2.2% by weight of sulfate as sulfur.
I was there. Palladium nitrate containing 10% by weight as Pd
Tetraammine containing 0.94 g of solution and 5.8 wt% as Pt
Platinum nitrate aqueous solution (0.65 g) is mixed and stirred, and then pure water is added.
Prepare a solution diluted to 30 ml in advance and add it to the solution obtained above.
The resulting sulfated zirconia (37.5 g) was impregnated for 15 hours. After impregnation
After sulphate zirconia is dried, it is baked at 500 ℃ for 9 hours
To obtain 0.25% Pd-0.1% Pt / sulfate zirconia catalyst
It was Example 3Preparation of 0.5% Pd-0.5% Pt / sulfate zirconia catalyst (2) 2.5 g of palladium nitrate solution containing 10% by weight as Pd and
Tetraammine platinum nitrate containing 5.8 wt% as Pt
4.3 g of the aqueous solution was mixed and stirred, and then diluted with pure water to 50 ml.
Was prepared in advance and obtained in the same manner as in Example 2
50 g of zirconia sulfate was impregnated for 15 hours. Sulfuric acid after impregnation
After drying the root zirconia, calcination at 500 ℃ for 9 hours,
0.5% Pd-0.5% Pt / sulfate zirconia catalyst (2) was obtained. Comparative Example 1Preparation of 0.5% Pd / sulfate zirconia catalyst Palladium nitrate solution containing 10% by weight as Pd 1.25 g
Was diluted to 20 ml with pure water in the same manner as in Example 1.
25 g of the prepared sulfated zirconia was immersed for 10 hours. Impregnation
After drying the latter sulfated zirconia, baked at 500 ℃ for 9 hours
To obtain 0.5% Pd / sulfate zirconia catalyst. Comparative example 2Preparation of 0.5% Pt / sulfate zirconia catalyst Tetraammine platinum nitrate containing 5.8 wt% as Pt
A solution prepared by diluting 3.0 g of the aqueous solution with pure water to 30 ml was prepared as in Example 2.
Dip 35 g of sulfate zirconia prepared in the same way for 15 hours
did. After drying the sulfated zirconia after impregnation, 500 ℃
Baking for 9 hours at 0.5% Pt / sulfate zirconia catalyst
Obtained. Comparative Example 3Preparation of Pd / mordenite catalyst (1) H-type mordenite (manufactured by Tosoh Corporation, silica / alumina)
Ratio 16) 20 g, tetraammine palladium nitrate 0.3 g
And 300 ml of an aqueous solution in which 1 g of ammonium acetate is dissolved.
Ion exchange was carried out at 60 ° C. for 18 hours. Ion exchange H
Form mordenite is separated by filtration and washed, then 110
Dry at 5 ℃ for 5 hours, and then bake in air at 500 ℃ for 9 hours.
Then, a Pd / mordenite catalyst (1) was obtained. Inductively coupled plasma
As a result of the composition analysis by the Mar-emission spectroscopy,
The amount of Pd supported on the converted H-type mordenite was 0.5%. Comparative Example 4Preparation of Pd / mordenite catalyst (2) H type mordenite (manufactured by Tosoh Corporation, silica / alumina)
Ratio 16) 60 g, tetraammine palladium nitrate 0.83 g
And 700 g of ammonium acetate to dissolve 700 ml of aqueous solution.
Ion exchange was carried out at 60 ° C. for 18 hours. Ion exchange H
Form mordenite is separated by filtration and washed, then 110
Dry at 5 ℃ for 5 hours, and then bake in air at 500 ℃ for 9 hours.
Then, a Pd / mordenite catalyst (2) was obtained. Inductively coupled plasma
As a result of the composition analysis by the Mar-emission spectroscopy,
The amount of Pd supported on the converted H-type mordenite was 0.42%. Comparative Example 5Preparation of Pd / ZSM-5 catalyst ZSM-5 Zeolite (Swiss Chemie Wetikon GmbH)
Made of silica / alumina ratio 30) 14 g, tetraammine
Dissolve 0.2 g of palladium nitrate and 2 g of ammonium acetate
Ion exchange for 18 hours at 60 ° C using 300 ml of aqueous solution.
went. ZSM-5 zeolite is separated by filtration, washed and
And then dried at 110 ℃ for 3 hours and 150 ℃ for 2 hours.
Pd / ZSM-5 was obtained by firing in air at 500 ° C. for 9 hours. Induction
As a result of composition analysis by combined plasma-emission spectroscopy,
The amount of Pd supported on Pd / ZSM-5 was 0.6%. Comparative Example 6Preparation of Pd-Pt / mordenite catalyst H type mordenite (manufactured by Tosoh Corporation, silica / alumina)
Ratio 16) Tetraane containing 25 g of 6.3 wt% as Pt
Min platinum nitrate aqueous solution 2 g and tetraammine palladium
Dissolve 0.35 g of nitrate and 1 g of ammonium acetate 300 m
Ion exchange was performed at 60 ° C. for 18 hours using the aqueous solution of 1 l.
The ion-exchanged H-type mordenite obtained is separated by filtration.
, Wash and dry at 110 ° C for 5 hours, then in air 5
Calcination at 00 ℃ for 9 hours to obtain Pd-Pt / mordenite catalyst
It was Of composition analysis by inductively coupled plasma-emission spectroscopy
As a result, the loading amount of Pd was 0.38% and the loading amount of Pt was 0.27%.
It was Example 4Catalyst activity test 1 Obtained in Examples 1, 2 and Comparative Examples 1, 2, 3 and 5
The resulting catalysts were each tablet-molded and then crushed to a particle size of 1
The size was adjusted to ~ 2 mm. Filled with 4 ml of each obtained catalyst.
Nitrogen oxide 150 ppm, methane 200
Consists of 0 ppm, 10% oxygen, 9% steam and balance helium
Simulated exhaust gas with gas hourly space velocity (GHSV) 15,000
h^-1The catalyst layer is circulated at a temperature of 450 ° C to activate the catalyst.
Sex (NO_xConversion rate and CH_FourThe conversion rate) was tested.
Concentration of methane, carbon monoxide and carbon dioxide at the catalyst layer outlet
Is measured by gas chromatography and at the catalyst layer inlet and
Exit NO _xConcentration is chemiluminescent NO_xIt was measured by an analyzer.
The actual combustion exhaust gas is usually 5 to 15% in addition to the above components.
Carbon dioxide, which has an essential effect on reaction activity
It was confirmed separately that it did not reach.

The conversion rate (%) of NO _x and methane indicates a value calculated by the following formula.

(NO _x conversion rate) = 100 × (1-NO _x -out / NO _x -in) (methane conversion rate) = 100 × (CO-out + CO ₂ -out) / (CO-out
+ CO ₂ -out + CH ₄ -out) where NO _x -in is the NO _x concentration at the catalyst layer inlet, NO _x -out is the NO _x concentration at the catalyst layer outlet, and CO-out is the monoxide at the catalyst layer outlet. Carbon concentration, CO ₂ -out is carbon dioxide concentration at the catalyst layer outlet, CH ₄ -out
Represents the methane concentration at the catalyst layer outlet. Of Example 1
NO _x by 0.5% Pd-0.5% Pt / sulfate group zirconia catalyst (1)
The changes with time in the conversion rate and the conversion rate of methane were as shown in FIG. That is, 0.5% Pd-0.5% Pt / sulfuric acid group zirconia catalyst (1) was 52% after 30 hours and 45% after 50 hours.
%, The NOx conversion rate was 35% even after 88 hours, which shows that it has high durability. In addition, 0.25% Pd of Example 2
-0.1% Pt / sulfate zirconia catalyst is 55%, 5% after 30 hours
It showed a NO _x conversion of 45% after 0 hour and 40% after 86 hours. The methane conversion by the catalyst of Example 2 was 50% after 30 hours.
It was 48%, 45% and 42% after the hour and 86 hours, respectively. 2 to 4 are each according to Comparative Example 1.
0.5% Pd / sulfate zirconia catalyst, 0.5% according to Comparative Example 2
4 shows changes with time in the NO _x conversion rate and the methane conversion rate of the Pt / sulfate zirconia catalyst and the Pd / mordenite catalyst (1) according to Comparative Example 3.

With the 0.5% Pd / sulfate zirconia catalyst (Comparative Example 1: FIG. 2) and the 0.5% Pt / sulfate zirconia catalyst (Comparative Example 2: FIG. 3), the activity level was low from the beginning, and even over time. to degrade. The Pd / mordenite catalyst (1) (Comparative Example 3: FIG. 4) initially showed a high activity level, but the activity deteriorated significantly over time, and the NO _x conversion was 44 after 30 hours. %, After 50 hours 22%, after 65 hours
It fell to 14%. The Pd / ZSM-5 catalyst of Comparative Example 5 showed a NO _x conversion rate of 73% and a methane conversion rate of 43% after 3 hours.
58% and 40% after hours and 18 hours after respectively
It dropped to 32% and 30%.

From the above results, it is clear that the catalyst of the present invention has significantly higher durability in the presence of water vapor than known catalysts. Further, the catalyst of the present invention is
It is clear that it has the characteristic that the conversion rate of methane is also kept high. Example 5 Catalyst Activity Test 2 The catalysts of Examples 2 and 3 and Comparative Examples 4 and 6 were used, respectively, and the simulated exhaust gas composition was set to 150 ppm of nitric oxide and methane 2.
A catalytic activity test was conducted in the same manner as in Example 4 except that 000 ppm, 10% oxygen, 9% steam, 3 ppm sulfur dioxide and the balance helium were used. 0.25% Pd-0.1% Pt / according to Example 2
The changes with time of the NO _x conversion rate and the methane conversion rate of the sulfate radical zirconia catalyst were as shown in FIG. Ie 5
61% and 61% NO _x conversion after 0, 80 and 120 hours respectively
And 60%, and the methane conversion after 50, 80 and 120 hours is extremely stable at 54%, 50% and 46%, respectively. Furthermore, surprisingly, even if sulfur oxide (sulfur dioxide) coexists in the gas to be treated, unlike the tendency generally known in the reduction of nitrogen oxides using hydrocarbons as a reducing agent, The activity is hardly adversely affected, and rather the catalytic activity tends to be stable. 0.5% Pd-of Example 3
Even in the case of 0.5% Pt / sulfate zirconia catalyst (2), 30, 50
NO _x conversions after 51 and 80 hours are 51%, 46% and 4 respectively.
4% with a methane conversion of 58 after 30, 50 and 80 hours
%, 51% and 50%, showing stable catalytic activity. The changes with time of the NO _x conversion rate and the methane conversion rate of the Pd / mordenite catalyst (2) according to Comparative Example 4 were as shown in FIG. NO _x conversion and the methane conversion rate from the test started within 30 hours, both become 10% or less, Example 4 was carried out using a substantially Pd / mordenite catalyst having the same composition (1) (Comparative Example 3 catalyst) It can be seen that the activity decrease tendency is remarkable as compared with the results in 1 above, and that the coexistence of sulfur oxides causes a remarkable decrease in activity. When the Pd-Pt / mordenite catalyst of Comparative Example 6 was used, the NO _x conversion was 63%, 30% and 18% after 10, 30 and 50 hours from the start of the test, and the methane conversion was respectively 64%, 25% and 13%. It is clear that even in the case of a catalyst supporting Pd and Pt, when the carrier is different, the catalytic activity is significantly inferior to the catalyst of the present invention.

[Brief description of drawings]

FIG. 1 is a graph showing changes with time of NO _x conversion rate and methane conversion rate of 0.5% Pd-0.5% Pt / sulfate radical zirconia catalyst (1) according to Example 1.

FIG. 2 is a graph showing changes with time of NO _x conversion rate and methane conversion rate of a 0.5% Pd / sulfate root zirconia catalyst according to Comparative Example 1.

FIG. 3 is a graph showing changes with time in NO _x conversion rate and methane conversion rate of a 0.5% Pt / sulfate root zirconia catalyst according to Comparative Example 2.

FIG. 4 N of Pd / mordenite catalyst (1) according to Comparative Example 3
3 is a graph showing changes with time of O _x conversion rate and methane conversion rate.

FIG. 5 is a graph showing changes over time in NO _x conversion rate and methane conversion rate of 0.25% Pd-0.1% Pt / sulfate zirconia catalyst according to Example 2 in the presence of sulfur oxides.

FIG. 6 is a graph showing changes over time in the NO _x conversion rate and the methane conversion rate of the Pd / mordenite catalyst (2) of Comparative Example 4 in the presence of sulfur oxides.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Masataka Masuda             4-1-2 Hirano-cho, Chuo-ku, Osaka-shi, Osaka Prefecture               Within Osaka Gas Co., Ltd. (72) Inventor Takenori Hirano             Kagoshima Prefecture Izumi City Kami-Saba ▼ Fuchi 1385-2 Kyushu             Catalyst Research Co., Ltd.

Claims (9)

[Claims] 1. An exhaust gas purifying catalyst for decomposing nitrogen oxides in the presence of methane in an oxygen excess atmosphere, which comprises palladium and platinum supported on sulfate radical zirconia. 2. The catalyst according to claim 1, wherein the supported amount of palladium is 0.1 to 0.5% by weight relative to the sulfate group zirconia. 3. The catalyst according to claim 1, wherein the amount of platinum supported is 20 to 100% by weight relative to palladium. 4. A method for purifying exhaust gas, which comprises decomposing nitrogen oxides in the presence of methane in an oxygen excess atmosphere using a catalyst comprising palladium and platinum supported on sulfate radical zirconia. 5. The exhaust gas purification method according to claim 4, wherein the amount of palladium supported is 0.1 to 0.5% by weight with respect to sulfate radical zirconia. 6. The exhaust gas purification method according to claim 4, wherein the amount of platinum supported is 20 to 100% by weight relative to palladium. 7. An exhaust gas purification method for decomposing nitrogen oxides in exhaust gas containing excess oxygen and containing sulfur oxides in the presence of methane using a catalyst comprising palladium and platinum supported on sulfate zirconia. . 8. The exhaust gas purifying method according to claim 7, wherein the amount of palladium supported is 0.1 to 0.5% by weight with respect to sulfate radical zirconia. 9. The exhaust gas purifying method according to claim 7, wherein the amount of platinum supported is 20 to 100% by weight relative to palladium.