JP2002253969A - Catalyst for purifying waste gas and method of purifying waste gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a purifying technique of hydrocarbon in a waste gas containing methane and excess oxygen. SOLUTION: The catalyst for purifying the waste gas is for decomposing hydrocarbon contained in the waste gas containing methane and excess oxygen and is formed by supporting palladium on tin oxide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst for purifying hydrocarbons in exhaust gas containing excess oxygen and containing methane, and a method for purifying exhaust gas using the catalyst.

In the present invention, the term “exhaust gas containing excessive oxygen (combustion)” refers to the gas to be treated brought into contact with the catalyst of the present invention.
(Exhaust gas) means a gas containing oxygen and nitrogen oxides and other oxidizing components in an amount greater than that necessary to completely oxidize the reducing components such as hydrocarbons and carbon monoxide contained therein. I do.

[0003]

2. Description of the Related Art It is known that a catalyst carrying 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, even when such a catalyst is used, when methane is a main component of hydrocarbons, such as the combustion exhaust gas of natural gas, a sufficient purification rate is required because methane has high chemical stability. There is a problem that can not be obtained.

[0005] Since the combustion exhaust gas inevitably contains sulfur oxides, which are a factor for reducing the catalytic activity, and steam, which is a reaction inhibitor, the catalytic activity is reduced within a short time. Petroleum fuels such as kerosene and light oil contain sulfur compounds. Further, even natural gas-derived fuels which essentially contain no sulfur compounds, for example, city gas supplied in Japan, also contain a sulfur compound as an odorant. These sulfur compounds produce sulfur oxides by combustion.

For example, Lampert et al. Found that when methane oxidation was carried out using a palladium catalyst, only 0.1 ppm of sulfur dioxide was present, and almost no catalytic activity was lost within a few hours. Which shows that the presence of sulfur oxides has a significant adverse effect on the catalytic activity (Applied Catalysis B: Applied Cataly
sis B: Environmental, vol.14, pp211-223 (1997)).

Further, Yamamoto et al. Reported the results of oxidative removal of hydrocarbons in the exhaust gas of city gas fuel using a catalyst in which platinum and palladium were supported on alumina. A decrease in activity is seen (Preliminary proceedings of the 1996 Catalyst Research Conference (published September 13, 1996).

Further, Japanese Patent Application Laid-Open No. 8-332392 discloses an oxidation catalyst for low-concentration hydrocarbons in an exhaust gas containing excess oxygen, comprising 7 g / l or more of palladium and 3 to 20 g / l on a honeycomb substrate through an alumina carrier. Disclosed are catalysts carrying l platinum.

However, even if this catalyst is used, long-term durability is not sufficient, and deterioration of the activity with time cannot be avoided.

As described above, the major problems of the prior art are that a high removal rate for methane cannot be obtained, and that the removal rate greatly decreases with time under conditions where sulfur oxides coexist. No.

In view of such a situation, Japanese Patent Application Laid-Open No. 2000-33266
The publication discloses palladium or tin oxide carrying palladium and platinum as a catalyst that maintains high methane oxidation activity even in the presence of sulfur oxide.

However, even if this catalyst is used, a large amount of catalyst is required in order to obtain a high methane removal rate particularly in a low temperature range of about 400 ° C. or lower.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and is intended to purify hydrocarbons in exhaust gas containing excess oxygen and containing methane at relatively low temperatures. It is another object of the present invention to provide a catalyst having a high activity and a purification method using the catalyst.

[0014]

Means for Solving the Problems As a result of intensive studies, the inventor has found that, as in the case of Japanese Patent Application Laid-Open No. 2000-33266, when palladium or palladium and platinum are used as active metals, tin oxide is used instead of tin oxide. It has been found that a catalyst having remarkably high methane oxidation activity can be obtained even at a low temperature of about 400 ° C. by using tin oxide subjected to strong oxidation treatment as a carrier. In addition, they have found that this catalyst stably maintains a high methane oxidizing ability even under conditions of combustion exhaust gas in which steam and sulfur oxides coexist.

The present invention has been made based on such findings, and provides the following exhaust gas purifying catalyst and exhaust gas purifying method. 1. A catalyst for purifying hydrocarbons in a combustion exhaust gas containing methane and excess oxygen, wherein palladium is supported on tin oxide that has been subjected to strong oxidation treatment. 2. A catalyst for purifying hydrocarbons in a combustion exhaust gas containing methane and containing excess oxygen, wherein palladium and platinum are supported on strongly oxidized tin oxide. 3. A method for purifying hydrocarbons in a combustion exhaust gas containing methane and containing an excessive amount of oxygen, wherein the method uses a catalyst in which palladium is supported on tin oxide that has been subjected to strong oxidation treatment. 4. A method for purifying hydrocarbons in a combustion exhaust gas containing methane and containing an excessive amount of oxygen, the method comprising using a catalyst in which palladium and platinum are supported on strongly oxidized tin oxide.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The catalyst of the present invention is a catalyst in which palladium or both palladium and platinum are supported on tin oxide (SnO ₂ ) which has been subjected to a strong oxidation treatment.

As the strong oxidation treatment of tin oxide, a known method can be applied. For example, "Makoto Hino, Kazushi Arata, Surface, 28
Volume 481 (1990) '', Makoto Hino, Kazushi Arata, Surface, 34, 51
Page (1996) "and the like.

The following method can be exemplified as a typical strong oxidation treatment. Impregnating tin hydroxide (Sn (OH) ₄ ) with an aqueous solution of ammonium sulfate (the weight ratio of sulfur contained in ammonium sulfate to Sn (OH) ₄ is about 0.5 to 8% (preferably about 3 to 6%)) After holding for about 20 hours (preferably about 3 to 10 hours), evaporate to dryness. Then, in an oxidizing atmosphere such as air, about 400 to 600 ° C. (preferably
By baking at about 500 to 550 ° C. for about 1 to 20 hours (preferably about 3 to 10 hours), tin oxide subjected to strong oxidation treatment is obtained. In addition, it is necessary to perform treatment by contacting with sulfuric acid ions in the state of tin hydroxide.
Even if the above treatment is performed using ordinary tin oxide,
The effect of the present invention cannot be obtained.

The following method can be exemplified as a method for preparing tin hydroxide (Sn (OH) ₄ ). Tin chloride (SnCl ₄ ), tin sulfate (Sn
(SO ₄ ) ₂ )) to an aqueous solution such as ammonia water to adjust the pH to 9
~ 10 or so. The solution is aged at about 20 to 80 ° C. for about 30 minutes to 20 hours while stirring, and then the precipitate is collected by filtration, washed with water as necessary, and then precipitated with tin hydroxide (Sn (OH) ₄ ). Get.

As a method of the strong oxidation treatment, in addition to the above, after tungstic acid (H ₂ WO ₄ ) and tin hydroxide (Sn (OH) ₄ ) are wet-kneaded, the mixture is subjected to a wet kneading process in an oxidizing atmosphere such as air. 9
A method of baking at about 00 ° C. (preferably about 700 to 800 ° C.) for about 1 to 20 hours (preferably about 3 to 10 hours) to obtain tungsten-modified tin oxide can also be exemplified.

The specific surface area of the catalyst of the present invention is not particularly limited, usually 50 to 200 m ² / g, preferably about 50 to 100 m ² / g
Degree, more preferably about 55 to 80 m ² / g. The specific surface area of the strongly oxidized tin oxide used for preparing the catalyst is almost the same as the above range because the specific surface area does not substantially change in the firing step. The value of the specific surface area in this case is a value measured by the BET method.

The amount of palladium carried (including the case where platinum is used together with platinum) is not particularly limited, but is usually about 0.5 to 20%, more preferably about 1 to 5%, based on the weight of the strongly oxidized tin oxide. It is. If the amount of supported palladium is too small, the catalytic activity may not be sufficiently exhibited.On the other hand, if the amount is too large, the particle size of palladium becomes too large and palladium is effectively used as an active metal. There is a risk of getting lost.

The supported amount of platinum is usually about 2 to 100% by weight relative to palladium, and more preferably 5 to 50%.
%. If the amount of supported platinum is too small, the effect of the combination of the two metals may not be sufficiently improved, whereas if it is too large, the function of palladium as an active metal may be impaired. There is.

The catalyst of the present invention may be formed into an arbitrary shape such as a pellet or a honeycomb by adding a binder if necessary. Alternatively, it may be used after being wash-coated on a fire-resistant honeycomb. Preferably, it is used after being wash-coated on a refractory honeycomb.

The catalyst of the present invention can be obtained, for example, by impregnating a strongly oxidized tin oxide with a solution in which a palladium compound and, if necessary, a platinum compound are further dissolved, and baking it in an oxidizing atmosphere. Before baking, drying may be performed, if necessary.

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. Examples of the palladium compound include palladium nitrate, tetraammine palladium nitrate, palladium chloride and the like. Of these, palladium nitrate and tetraammine palladium nitrate are preferred. As a platinum compound,
Examples thereof include tetraammineplatinum nitrate and chloroplatinic acid. Of these, tetraammineplatinum nitrate is preferred. The solvent is preferably water, but may be a mixed solvent to which a water-soluble organic solvent such as ethanol or acetone is added. Alternatively, an acidic aqueous solution such as nitric acid may be used as the solvent.

When palladium and platinum are used together,
Both may be carried sequentially or simultaneously.
Unless it is particularly necessary, it is more economical to carry both at the same time.

Next, the tin oxide that has been subjected to the strong oxidation treatment and impregnated with a predetermined active metal is dried if necessary, and then fired in an oxidizing atmosphere such as in air. The firing temperature is usually about 400 ° C to 600 ° C, more preferably 500 ° C to 5 ° C.
It is around 50 ° C. If the firing temperature is too high, the supported active metal may be extremely agglomerated and impair the activity, while if too low, the stability of the supported active metal may be problematic. 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, the catalyst prepared in advance may be slurry-coated and wash-coated, or tin oxide which has been subjected to a strong oxidation treatment may be washed with a refractory honeycomb. After wash-coating on top, palladium and, if necessary, platinum may be supported.

The exhaust gas purifying method of the present invention is characterized by using a catalyst in which palladium is supported on tin oxide subjected to strong oxidation treatment and, if necessary, platinum is further supported.

The amount of catalyst is determined by the gas hourly space velocity (GHSV).
And usually about 200,000 h ^-1 or less, more preferably 10
It is about 00 to 60000 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 may not 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, but 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. is there.

The temperature of the catalyst layer in the exhaust gas purification method of the present invention is usually about 350 to 600 ° C., preferably about 400 to 500 ° 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, while if the temperature is too high, the durability of the catalyst may be deteriorated. .

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. The concentration of the hydrocarbon in the exhaust gas is not particularly limited, but is usually about 50 to 5000 ppm in terms of methane. When the concentration of hydrocarbon is extremely high, the temperature rises in the catalyst layer together with the rapid reaction, so that the temperature rise in the catalyst layer (difference between the catalyst layer outlet temperature and the inlet temperature) is 150 ° C or less. It is preferable to use them.

The oxygen concentration in the exhaust gas is not particularly limited as long as it contains oxidizing components such as oxygen and nitrogen oxide in excess. The oxygen concentration is preferably about 2% or more on a volume basis and about 5 times or more the oxidation equivalent of a reducing component such as hydrocarbon in the exhaust gas. If the oxygen concentration is very low (eg, about 1% or less by volume), the reaction rate may be too slow. When the oxygen concentration in the exhaust gas is too low as described above, an air-mixed exhaust gas obtained by mixing an appropriate amount of air may be brought into contact with the catalyst while taking care that the temperature of the exhaust gas does not fall below a suitable range.

[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 significantly reduce the catalytic activity, in addition to the water vapor. However, the catalyst of the present invention has high resistance to the decrease in the activity due to the sulfur component. Remains high over time.

According to the catalyst of the present invention, a high catalytic activity can be exhibited even at a low temperature of about 400 ° C.

[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.

[0039] Example 1 (5% Pd / preparation of a sulfated tin oxide (1)) of tin chloride _{(SnCl 4 · 5H 2 O)} 48g was dissolved in pure water 500 ml, 60
Aqueous ammonia was added dropwise while maintaining the temperature at 0 ° C and stirring well.
When the pH reached 9.5, the dropping of the aqueous ammonia was stopped, and stirring was continued at 60 ° C. for 1 hour. This solution was filtered, and the collected solid was washed with pure water, dried and dried with tin hydroxide (Sn (O
H) ₄ ) was obtained. This tin hydroxide (6.2 g) was treated with ammonium sulfate
After immersing for 15 hours in an aqueous solution (30 ml) in which 0.97 g was dissolved, the residue was evaporated to dryness and calcined at 550 ° C. for 4 hours in the air to obtain sulfated tin oxide (1).

This sulfated tin oxide (1) (5.35 g) was used as Pd at 0.1%.
After immersion in 10 ml of an aqueous solution of palladium nitrate containing 27 g for 15 hours, the mixture was evaporated to dryness, and calcined at 500 ° C. for 4 hours in air.
% Pd / sulfated tin oxide (1) was obtained. The BET specific surface area of this catalyst was 58 m ² / g.

Comparative Example 1 (Preparation of 5% Pd / tin oxide) Tin hydroxide obtained in the same manner as in Example 1 was calcined in air at 550 ° C. for 4 hours to obtain tin oxide. 5 g of this tin oxide was immersed in 20 ml of an aqueous palladium nitrate solution containing 0.25 g of Pd for 15 hours, evaporated to dryness, and calcined at 500 ° C. for 4 hours to obtain 5% Pd /
Tin oxide was obtained. The BET surface area of this catalyst was 16 m ² / g.

Comparative Example 2 (Preparation of 5% Pd / Zirconia) 3 g of zirconia (TZ-0, manufactured by Tosoh Corporation) was immersed in 20 ml of an aqueous palladium nitrate solution containing 0.15 g of Pd for 15 hours, and evaporated to dryness. Fired in air at 500 ℃ for 4 hours, 5% Pd /
Zirconia was obtained.

Example 2 (5% Pd-1% Pt / sulfated tin oxide)
Preparation of (1) An aqueous solution of palladium nitrate containing 0.27 g of Pd, 0.088 g of dinitrodiammineplatinum and 0.5 ml of 69% nitric acid were mixed and dissolved, and diluted with pure water to prepare a solution (10 ml). 5.35 g of sulfated tin oxide (1) obtained in the same manner as in Example 1 was added to this solution.
It was immersed for an hour, evaporated to dryness, and calcined in air at 500 ° C. for 4 hours to obtain 5% Pd-1% Pt / sulfated tin oxide (1). B of this catalyst
The ET surface area was 58 m ² / g.

Example 3 (5% Pd-1% Pt / sulfated tin oxide)
(Preparation of (2)) 18.6 g of tin hydroxide obtained in the same manner as in Example 1 was immersed in a 20 ml aqueous solution of 3.1 g of ammonium sulfate for 15 hours, evaporated to dryness, and calcined at 400 ° C. in air for 4 hours. Thus, sulfated tin oxide (2) was obtained.

An aqueous solution of palladium nitrate containing 0.32 g of Pd, 0.105 g of dinitrodiammine platinum and 0.5 ml of 69% nitric acid
Was mixed and dissolved, and diluted with pure water to prepare a solution (10 ml). 6.4 g of sulfated tin oxide (2) was immersed in this solution for 15 hours, evaporated to dryness, and calcined in air at 500 ° C. for 4 hours to obtain 5% Pd-1.
% Pt / sulfated tin oxide (2) was obtained.

Example 4 (5% Pd-1% Pt / sulfated tin oxide)
(Preparation of (3)) 6.2 g of tin hydroxide obtained in the same manner as in Example 1 was immersed in 10 ml of an aqueous solution in which 0.21 g of ammonium sulfate was dissolved for 15 hours, evaporated to dryness, and calcined at 550 ° C. for 4 hours. Thus, sulfated tin oxide (3) was obtained.

An aqueous solution of palladium nitrate containing 0.27 g of Pd, 0.088 g of dinitrodiammine platinum and 0.5 ml of 69% nitric acid
Was mixed and dissolved, and diluted with pure water to prepare a solution (10 ml). 5.4 g of sulfated tin oxide (3) was immersed in this solution for 15 hours, evaporated to dryness, and calcined in air at 500 ° C. for 4 hours to obtain 5% Pd-1.
% Pt / sulfated tin oxide (3) was obtained.

Example 5 (5% Pd-1% Pt / sulfated tin oxide)
(Preparation of (4)) 6.2 g of tin hydroxide obtained in the same manner as in Example 1 was immersed in a 10 ml aqueous solution of 1.55 g of ammonium sulfate for 15 hours, evaporated to dryness, and calcined at 550 ° C. for 4 hours in air. Thus, sulfated tin oxide (4) was obtained.

An aqueous solution of palladium nitrate containing 0.27 g of Pd, 0.09 g of dinitrodiammine platinum and 0.5 ml of 69% nitric acid
Was mixed and dissolved, and diluted with pure water to prepare a solution (10 ml). 5.5 g of sulfated tin oxide (4) was immersed in this solution for 15 hours, evaporated to dryness, and calcined in air at 500 ° C. for 4 hours to form 5% Pd-1.
% Pt / sulfated tin oxide (4) was obtained.

Comparative Example 3 (Preparation of 5% Pd-1% Pt / tin oxide impregnated with sulfuric acid) Tin hydroxide obtained in the same manner as in Example 1 was calcined at 550 ° C. for 4 hours to obtain tin oxide. This 8.1 g was put into 150 ml of 3M sulfuric acid, stirred for 30 minutes, filtered, and calcined in air at 550 ° C. for 4 hours to obtain sulfuric acid-impregnated tin oxide.

The sulfuric acid-impregnated tin oxide is replaced with sulfated tin oxide.
Except that it was used in place of (1), 5% Pd
-1% Pt / sulfuric acid impregnated tin oxide was obtained. BET surface area of this catalyst
Is 37m ^Two/ g.

Example 6 (Catalyst Evaluation Test 1) Each of the catalysts prepared in Examples 1 to 5 and Comparative Examples 1 to 3 was tableted, and 1 ml of each was filled in a reaction tube. A gas composed of 1000 ppm of methane, 10% of oxygen, 10% of water vapor, 8 ppm of sulfur dioxide, and the balance of nitrogen was passed through this reactor under the conditions of GHSV (gas hourly space velocity) of 40,000 h ^-1 and the catalyst layer While maintaining the temperature at 400 ° C., the change with time of the methane conversion was measured. The gas composition before and after the reaction layer was measured by a gas chromatograph equipped with a flame ionization detector. Table 1 shows the methane conversion (%) 1, 5, 10, 30, 36, and 42 hours after the start of the test. Here, the methane conversion is
It is a value obtained by the following equation.

[0053]

(Equation 1)

[0054]

[Table 1]

As is evident from Table 1, the catalysts of the examples were in the presence of sulfur dioxide, which significantly reduced the catalytic activity, and exhibited extremely stable activities even at temperatures as low as 400 ° C.

On the other hand, when mere tin oxide was used as the carrier, the activity was significantly reduced (Comparative Example 1). It is also clear that the same effect as in the present invention cannot be obtained when tin oxide impregnated in sulfuric acid is used as a carrier (Comparative Example 3).

Example 7 (Catalyst Evaluation Test 2) The catalyst of Example 2 was tested under the same conditions as in Example 6 for a longer time. The methane conversion rate after the start of the test changed as shown in the table below.

[0058]

[Table 2]

It is clear that the catalysts of the present invention show stable methane conversion over a long period.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4D048 AA18 AB01 AB07 BA21X BA30X BA31X BA46X BB01 BB02 BC01 4G069 AA03 BA45A BB02A BB02B BB04A BB10A BB10B BC22A BC22B BC60A BC72A BC72B BC75 EA02 CA03 EA02 CA03 EA02

Claims (4)

[Claims] 1. A catalyst for purifying hydrocarbons in a combustion exhaust gas containing methane and containing an excess of oxygen, wherein palladium is carried on strongly oxidized tin oxide. 2. A catalyst for purifying hydrocarbons in a combustion exhaust gas containing methane and containing an excessive amount of oxygen, wherein palladium and platinum are supported on strongly oxidized tin oxide. 3. A method for purifying hydrocarbons in a combustion exhaust gas containing methane and containing an excessive amount of oxygen, the method comprising using a catalyst in which palladium is carried on tin oxide subjected to strong oxidation treatment. 4. A method for purifying hydrocarbons in a combustion exhaust gas containing methane and containing an excessive amount of oxygen, which comprises using a catalyst in which palladium and platinum are supported on tin oxide subjected to strong oxidation treatment.