JP2001129404A - Catalyst for purifying exhaust gas and exhaust gas purifying method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for purifying hydrocarbon and a malodor emitting material in the exhaust gas containing excess of oxygen, methane and the malodor emitting material. SOLUTION: The catalyst for decomposing hydrocarbons and the malodor emitting material in the exhaust gas containing excess of oxygen, methane and the malodor emitting material is obtained by depositing palladium on tin oxide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a catalyst for purifying hydrocarbons and odor-causing substances in exhaust gas containing excess oxygen and containing methane and odor-causing substances, and a method for purifying exhaust gas using the catalyst.

[0002] In the present invention, the term "exhaust gas containing excessive oxygen" refers to the gas to be treated (exhaust gas) to be brought into contact with the catalyst of the present invention.
Is a gas containing an oxidizing component such as oxygen or nitrogen oxide in an amount required to completely oxidize a reducing component such as hydrocarbon or carbon monoxide contained therein.

[0003]

2. Description of the Related Art Among hydrocarbons contained in various kinds of exhaust gas such as combustion exhaust gas, the necessity of decomposing / removing higher hydrocarbons which easily cause photochemical smog has been recognized, and necessary countermeasures have been taken. Has been taken.
On the other hand, lower hydrocarbons such as methane and ethane are not harmful to the human body and do not cause photochemical smog, and therefore, it has not been recognized that decomposition / removal is necessary.

[0004] However, it has been pointed out that methane may have an undesirable effect on the global environment, and it has become necessary to reduce it.

[0005] Furthermore, with the recent increase in awareness of the comfort of the living environment, in a house or a place near a house,
It has also been pointed out that it is necessary to further reduce trace amounts of odor-causing substances. That is, some of the odor-causing substances are detected by the sense of smell of the human body, even in a trace amount of about 0.1 ppb or less. Some of the odor-causing substances have not yet been elucidated, but as odor-causing substances (components) in exhaust gas, aldehydes such as formaldehyde and acetaldehyde and carboxylic acids such as acetic acid are considered to be particularly important. I have.

Heretofore, it has been known that a catalyst supporting a platinum group metal such as platinum or palladium exhibits high performance as a catalyst for oxidizing and removing hydrocarbons in exhaust gas. For example,
JP-A-51-106691 discloses an exhaust gas purifying catalyst in which platinum and palladium are supported on an alumina carrier. However, when exhaust gas treatment is performed using these catalysts, a problem is that sufficient methane conversion cannot be obtained because methane has high chemical stability among hydrocarbons. There is.

[0007] Furthermore, in the flue gas, catalytic activity inhibitors such as sulfur oxides are usually present, and it is inevitable that the catalytic activity is significantly deteriorated with time. Petroleum fuels such as kerosene and light oil usually contain sulfur-containing compounds derived from raw materials. Even natural gas fuels containing almost no sulfur compounds, compounds containing sulfur are added as odorants to the city gas supplied in Japan, and these are burned by sulfur oxides due to combustion. Generate

[0008] Lampert et al. Found that when methane oxidation was performed using a palladium catalyst, only 0.1 ppm
In the presence of sulfur dioxide almost completely loses its catalytic activity within a few hours, revealing that the presence of sulfur oxides has a significant effect on methane oxidation activity (Applied Catalysis B: Environmental (Applied Catalysis B: Environmental) vol.14, 211
-233 (19997)).

Also, Yamamoto et al. Report the results of oxidizing and removing hydrocarbons present in city gas fuel exhaust gas using a catalyst in which platinum and palladium are supported on alumina. During which a marked decrease in catalytic activity was observed. (Preliminary proceedings of the 1996 Catalyst Research Presentation Meeting (published on September 13, 1996))
The publication discloses, as a catalyst for oxidizing low concentration hydrocarbons in an oxygen-excess exhaust gas, a catalyst in which palladium is supported at 7 g / l or more and platinum is supported at 3 to 20 g / l via an alumina carrier on a honeycomb substrate. ing. However, even if this catalyst is used, long-term durability is not sufficient, and deterioration of the catalyst activity with time cannot be avoided.

As described above, one of the problems of the prior art relating to the purification of exhaust gas is that a high conversion rate cannot be obtained for methane oxidation, and furthermore, under conditions where sulfur oxides coexist, the reaction time is short. A significant reduction in conversion occurs.

Regarding the removal of aldehyde, which is a major odor-causing substance, Japanese Patent Application Laid-Open No. Hei 7-71341 discloses that after adsorbing an aldehyde onto an adsorbent, the adsorbent is heated, and the desorbed aldehyde is removed using manganese dioxide. A method for catalytic oxidation is disclosed. However, since this method is composed of a plurality of steps, there is a problem that a facility cost and an operating cost are significantly increased as a method for purifying an exhaust gas that requires a large amount of gas treatment.

Further, JP-A-9-206596 and JP-A-9-2857
No. 20 and JP-A-10-192656 disclose an aldehyde removal method as a method for deodorizing combustion exhaust gas.
It is not clear whether these methods are also effective in removing methane. Rather, the composition of the catalyst,
Judging from the method of use and the like, it is unlikely that these methods can remove methane.

Furthermore, the oxidation reaction of hydrocarbons is a reaction that leads to carbon dioxide, which is the final product, via an oxygen-containing organic compound such as aldehyde, which is a reaction intermediate. When used, these reaction intermediates may remain depending on the use conditions, and these may become new odor sources.

[0014]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above circumstances, and has been made in consideration of the above problem. It is an object to provide a purification catalyst and a purification method.

[0015]

Means for Solving the Problems As a result of diligent studies, the inventors have found that a catalyst in which palladium is supported on tin oxide has high resistance to inhibition of activity by sulfur oxides, It was also found that high methane oxidizing ability was stably maintained even in the case of

Further, the inventor has found that a catalyst in which platinum and platinum are supported on tin oxide exhibits high methane oxidizing ability even at a lower temperature.

Furthermore, it has been found that these two types of catalysts exhibit an excellent effect not only on methane but also on oxidative decomposition of a substance causing an odor.

The present invention has been made based on such findings, and provides the following exhaust gas purifying catalyst and exhaust gas purifying method. 1. An exhaust gas purifying catalyst that contains excess oxygen and decomposes hydrocarbons and odor-causing substances in exhaust gas containing methane and odor-causing substances, the catalyst comprising palladium supported on tin oxide. 2. The supported amount of palladium is 1 in weight ratio to tin oxide.
The catalyst of claim 1, which is 〜25%. 3. An exhaust gas purifying catalyst that decomposes hydrocarbons and odor-causing substances in exhaust gas containing excess oxygen and containing methane and odor-causing substances, the catalyst comprising palladium and platinum supported on tin oxide. 4. The supported amount of palladium is 1 in weight ratio to tin oxide.
The catalyst of claim 3 which is 〜25%. 5. The supported amount of platinum is 5 to 5 in weight ratio to palladium.
5. The catalyst according to the above 3 or 4, which is 0%. 6. The catalyst according to any one of the above 1 to 5, wherein the malodor causing substance is an aldehyde. 7. An exhaust gas purification method for decomposing hydrocarbons and an odor-causing substance in exhaust gas containing excess oxygen and containing methane and an odor-causing substance, wherein the method uses a catalyst comprising palladium on tin oxide. 8. The supported amount of palladium is 1 in weight ratio to tin oxide.
The purification method according to the above 7, wherein the purification method is 2525%. 9. An exhaust gas purification method for decomposing hydrocarbons and odor-causing substances in exhaust gas containing excess oxygen and containing methane and odor-causing substances, the method comprising purifying palladium and platinum on tin oxide . 10. The supported amount of palladium is
The exhaust gas purifying method according to the above item 9, wherein the amount is 1 to 25%. 11. The supported amount of platinum is 5 by weight relative to palladium.
The exhaust gas purifying method according to the above item 9 or 10, wherein the amount is up to 50%. 12. 12. The purification method according to any one of the above 7 to 11, wherein the odor-causing substance is an aldehyde.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The catalyst of the present invention is a catalyst comprising tin oxide (SnO ₂ ) carrying palladium and, if necessary, platinum.

The amount of palladium carried is not particularly limited, but is usually about 1 to 25%, more preferably about 2 to 20%, based on the weight of tin oxide. The supported amount of palladium is
When the amount is too small, the catalytic activity is not sufficiently exhibited, while when the amount is too large, the particle size of palladium becomes too large, so that palladium cannot be used effectively as an active metal.

In the catalyst of the present invention, when palladium and platinum are used in combination, the amount of platinum carried is usually about 5 to 50%, more preferably about 10 to 50% by weight relative to palladium. is there. If the amount of supported platinum is too small, the effect of the combined use of both metals is not sufficiently improved, whereas if it is too large, the function of palladium as an active metal may be impaired. . In the catalyst of the present invention, when palladium and platinum are used in combination, the amount of supported palladium is based on the weight of tin oxide.
Usually, it is about 1 to 25%, more preferably about 2 to 20%, and the supported amount of platinum is usually about 5 to 50%, more preferably about 10 to 50% by weight relative to palladium.

The specific surface area of the catalyst of the present invention is not particularly limited as long as the obtained catalyst can be used stably and dispersion of the supported metal can be maintained.
50 m ² / g approximately, and preferably from 1-20 m ² / g approximately.

The catalyst of the present invention may be formed into pellets by adding a binder as necessary, or may be used as a wash coat on a refractory honeycomb, but is preferably used on a refractory honeycomb. Wash coat is used.

The catalyst of the present invention is obtained, for example, by a method of impregnating tin oxide with a solution in which a palladium compound and, if necessary, a platinum compound are dissolved, drying and calcining in an oxidizing atmosphere. .

The tin oxide used is not particularly limited, and tin oxide produced by a known production method or commercially available tin oxide can be used. As a known production method, for example,
A method of firing tin hydroxide, tin oxalate, or the like under an oxidizing atmosphere such as air or oxygen to obtain tin oxide is given. The firing temperature for producing tin oxide is usually about 750 to 1100 ° C, preferably about 850 to 950 ° C. If the firing temperature is too low, stable tin oxide will not be formed, while if it is too high, the surface area of the tin oxide will be extremely reduced, and the dispersibility of the active metal carried will be low. The firing time when producing tin oxide is usually about 0.5 to 20 hours, preferably 1 hour.
About 10 hours.

The palladium compound and the platinum compound used as required are not particularly limited as long as they dissociate the respective metal ions in the solution. For example, nitrates and ammine complexes of these metals can be mentioned. The solvent is preferably an aqueous solution, but may be a mixed solvent to which a water-soluble organic solvent such as acetone or ethanol is added.

Next, the tin oxide impregnated with a predetermined active metal is dried and then fired in an oxidizing atmosphere such as in air. The firing temperature is usually about 450 ° C to 700 ° C, more preferably about 500 ° C to 650 ° C. If the firing temperature is too high, the grain growth of the supported active metal proceeds,
Also, if it is too low, the active metal grains grow during use of the catalyst, so that it is not possible to obtain a catalyst that stably exhibits a high catalytic activity. The firing time is usually about 1 to 20 hours, preferably about 2 to 10 hours.

When the catalyst of the present invention is wash-coated on a refractory honeycomb, even if the catalyst prepared by the above method is slurry-coated and washed, tin oxide or tin hydroxide, a precursor thereof, is previously prepared. Or the like may be wash-coated on a refractory honeycomb, fired if necessary, and then loaded with palladium or platinum according to the above method.

The exhaust gas purifying method of the present invention is characterized by using a catalyst comprising palladium supported on tin oxide.

The amount of catalyst used is determined by the space velocity (G
HSV), usually about 500,000 h ^-1 or less, more preferably about 1,000 to 300,000 h ^-1 . If the amount of the catalyst used is too small, that is, if the GHSV value is too large, an effective purification rate cannot be obtained. On the other hand, when the amount of the catalyst used is too large, that is, when the GHSV value is too small, the purification rate is improved,
In addition to the problem of economy, there is a possibility that a problem that the pressure loss in the catalyst layer becomes large may occur.

The temperature of the catalyst layer in the exhaust gas purifying method of the present invention is usually about 350 to 600 ° C, preferably about 370 to 580 ° C. The catalyst of the present invention has high activity, but if the temperature is too low, the catalyst activity may be reduced, and an effective conversion may not be obtained. On the other hand, if the temperature is too high, the durability of the catalyst may be deteriorated. There is.

The hydrocarbon in the exhaust gas to be purified by the method of the present invention is not particularly limited as long as it is a hydrocarbon in the exhaust gas, and examples thereof include lower saturated hydrocarbons such as methane and ethane. According to the method of the present invention, a high conversion can be obtained even for these lower saturated hydrocarbons, for which it has been difficult to obtain an effective conversion with a conventionally known catalyst. The concentration of hydrocarbons in the exhaust gas is not particularly limited, but is usually 50
About 5000 ppm.

The odor-causing substance in the exhaust gas to be purified by the method of the present invention is not particularly limited as long as it is a odor-causing substance in the exhaust gas. Examples thereof include aldehydes such as formaldehyde and acetaldehyde, and carboxylic acids such as acetic acid. Can be

The oxygen concentration in the exhaust gas is not particularly limited as long as it contains an excessive amount of oxidizing components such as oxygen and nitric oxide. For example, the oxygen concentration in the exhaust gas is about 1% or less on a volume basis. Is too low, sufficient catalytic activity may not be obtained. As described above, when the oxygen concentration in the exhaust gas is too low, or when the temperature of the exhaust gas is high, and the temperature of the catalyst layer may exceed the predetermined temperature range, the temperature of the exhaust gas may fall below a suitable range. The mixed air may be mixed with an appropriate amount of air, and then the mixed air may be brought into contact with the catalyst.

[0035]

The exhaust gas usually contains about 5% to 15% of water vapor. However, according to the method of the present invention, an effective purification performance can be obtained even for an exhaust gas containing water vapor. can get.

The exhaust gas usually contains sulfur oxides, which are known to significantly reduce the catalytic activity, but the catalyst of the present invention has a high resistance to the activity decrease due to the sulfur component. Therefore, the purification performance is kept high.

Further, when the catalyst of the present invention is used, not only the hydrocarbons in the exhaust gas but also the odor-causing substances can be removed at the same time.

[0038]

EXAMPLES Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

Example 1 (Preparation of Pd / tin oxide catalyst) Tin oxalate (SnC ₂ O ₄ ) was calcined in air at 900 ° C. for 6 hours to obtain tin oxide (SnO ₂ ). The specific surface area by the BET method was 2 m ² / g. 25 g of this tin oxide was immersed in 20 ml of an aqueous solution of palladium nitrate containing 1.25 g of palladium. This is evaporated to dryness, and further calcined in air at 550 ° C for 9 hours to obtain 5% Pd
/ Tin oxide catalyst was obtained.

Example 2 (Preparation of Pd-Pt / tin oxide catalyst) The same 25 g of tin oxide as in Example 1 was used and 1.25 g of palladium was used.
Pure water was added to an aqueous solution of palladium nitrate containing 0.25 g of platinum and an aqueous solution of tetraammineplatinum nitrate containing 0.25 g of platinum, mixed, stirred and immersed in a solution of 20 ml. This is evaporated to dryness, and further calcined in air at 550 ° C. for 9 hours to form 5% Pd
A -1% Pt / tin oxide catalyst was obtained.

Comparative Example (Preparation of Pd / Alumina Catalyst) NK-124 alumina manufactured by Sumitomo Chemical Co., Ltd. was calcined in air at 800 ° C. for 2 hours. This 5 g was impregnated with 20 ml of an aqueous palladium nitrate solution containing 0.25 g as palladium at 0 ° C. for 15 hours. This is evaporated to dryness and calcined at 550 ° C for 2 hours in air.
A 5% Pd / alumina catalyst was obtained.

Example 3 (Catalyst evaluation test) The catalysts of Examples 1 and 2 and Comparative Example were tableted and 1 ml was taken, methane 1000 ppm, oxygen 10%, carbon dioxide 6%, steam
A gas having a composition of 9%, 100 ppm formaldehyde, 3 ppm sulfur dioxide, and the balance helium was passed under the condition of GHSV (gas hourly space velocity) of 80,000 h ^{-1 and} the temperature of the catalyst layer was 450 ° C.
, And the change over time of the conversion of methane and formaldehyde was measured. Table 1 shows the results.

The gas composition before and after the reaction layer was measured by a gas chromatograph equipped with a flame ionization detector. Formaldehyde was detected as methane through a methane converter. Here, the conversion rates of methane and formaldehyde are values determined by the following equation.

[0044]

(Equation 1)

[0045]

[Table 1]

The catalysts of Examples 1 and 2 clearly show a more stable methane conversion than the catalyst of Comparative Example 1.
In addition, formaldehyde, which is an odor-causing substance, can be removed at the same time. Therefore, when the catalyst of the present invention is used, the problems of removing methane from the exhaust gas and removing the odor-causing substance are simultaneously achieved, and the catalyst of the present invention also has sufficient durability.

Continued on the front page (72) Inventor Masataka Masuda 4-1-2, Hirano-cho, Chuo-ku, Osaka-shi, Osaka Inside Osaka Gas Co., Ltd. (72) Takashi Nakahira 4-1-1, Hirano-cho, Chuo-ku, Osaka-shi, Osaka No. 2 Inside Osaka Gas Co., Ltd. BC22A BC22B BC72A BC72B BC75A BC75B CA02 CA07 CA10 CA15 CA17 CD02 FC08

Claims (12)

[Claims] 1. A catalyst for purifying exhaust gas which decomposes hydrocarbons and odor-causing substances in exhaust gas containing excess oxygen and containing methane and odor-causing substances, the catalyst comprising palladium supported on tin oxide. 2. The catalyst according to claim 1, wherein the supported amount of palladium is 1 to 25% by weight based on tin oxide. 3. An exhaust gas purifying catalyst for decomposing hydrocarbons and odor-causing substances in exhaust gas containing an excess amount of oxygen and containing methane and odor-causing substances, comprising palladium and platinum supported on tin oxide. catalyst. 4. The catalyst according to claim 3, wherein the supported amount of palladium is 1 to 25% by weight relative to tin oxide. 5. The catalyst according to claim 3, wherein the amount of platinum carried is 5 to 50% by weight relative to palladium. 6. The catalyst according to claim 1, wherein the odor-causing substance is an aldehyde. 7. An exhaust gas purification method for decomposing hydrocarbons and odor causing substances in exhaust gas containing excess oxygen and containing methane and odor causing substances, wherein a catalyst comprising palladium supported on tin oxide is used. Exhaust gas purification method. 8. The purification method according to claim 7, wherein the carried amount of palladium is 1 to 25% by weight relative to tin oxide. 9. A method for purifying exhaust gas which decomposes hydrocarbons and odor-causing substances in exhaust gas containing excess oxygen and containing methane and odor-causing substances, comprising a catalyst comprising palladium and platinum supported on tin oxide. Exhaust gas purification method using 10. The method for purifying exhaust gas according to claim 9, wherein the carried amount of palladium is 1 to 25% by weight relative to tin oxide. 11. The method for purifying exhaust gas according to claim 9, wherein the carried amount of platinum is 5 to 50% by weight relative to palladium. 12. The purification method according to claim 7, wherein the odor-causing substance is an aldehyde.