JP2013215718A - Catalyst and method for oxidizing and removing methane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a catalyst that exerts high methane decomposition capability even at low temperature (especially at 400°C or lower, and further 375°C or lower) in a method for oxidizing and removing methane in a processed gas including methane and excessive oxygen; and an oxidation removing method of methane in the processed gas using the catalyst.SOLUTION: A catalyst is to remove methane in a processed gas including methane and excessive oxygen, the catalyst comprises supporting platinum, iridium and tantalum to at least one carrier chosen from a group consisting of a zirconium oxide carrier, a tin oxide carrier, and a titanium oxide carrier, and a method oxidizes and removes the methane in the processed gas including the methane and excessive oxygen. The method comprises contacting the processed gas to the catalyst at a temperature of 300-600°C.

Description

  The present invention relates to a catalyst for oxidation removal of methane in a gas to be treated containing methane and excess oxygen, and a method for removing oxidation.

  In this specification, “containing excess oxygen” means that the gas to be treated which is brought into contact with the catalyst of the present invention is required to completely oxidize reducing components such as hydrocarbon and carbon monoxide contained therein. It means that an oxidative component such as oxygen and nitrogen oxide is contained in an excessive amount.

  As a hydrocarbon oxidation removal catalyst, it is known that a catalyst supporting a platinum group metal such as platinum or palladium exhibits high performance. For example, an exhaust gas purifying catalyst having platinum and palladium supported on an alumina carrier is disclosed (see Patent Document 1). However, even when such a catalyst is used, when the main component of the hydrocarbons contained is methane, such as methane fermentation gas or natural gas combustion exhaust gas, methane has high chemical stability. There is a problem that sufficient methane removal is not achieved.

  Furthermore, when the gas to be treated is combustion exhaust gas, reaction inhibitory substances such as sulfur oxides (SOx) derived from sulfur compounds contained in the fuel are inevitably contained. It is unavoidable that the catalyst activity significantly decreases with time due to the precipitation of the reaction inhibiting substance.

  For example, Lampert et al. (Lampert et al.) Show that when methane oxidation is carried out using a palladium catalyst, only 0.1 ppm of sulfur dioxide is present and its catalytic activity is almost lost within a few hours. It has been clarified that the presence of sulfur oxide has a significant adverse effect on the catalytic activity (see Non-Patent Document 1).

  As a catalyst for oxidation of low-concentration hydrocarbons contained in exhaust gas in which an excessive amount of oxygen is present, a catalyst in which 7 g / l or more of palladium and 3 to 20 g / l of platinum are supported on a honeycomb substrate via an alumina carrier is also available. It is disclosed (see Patent Document 2). However, even when this catalyst is used, durability over a long period is not sufficient, and deterioration of the catalyst activity over time is inevitable under the condition where sulfur oxides coexist.

  In recent years, the global warming problem has been strongly recognized, and it is regarded as a problem that a large amount of gas containing dilute (about 0.1 to 1%) methane, such as coal mine ventilation gas, is diffused. The economic processing is also an issue. Here too, there is a need for a highly active catalyst that can be treated at the lowest possible temperature when removing methane by catalytic oxidation. In addition to methane, coal mine ventilation gases may contain sulfur compounds derived from coal components (hydrogen sulfide, mercaptans, sulfur dioxide), etc., and hydrogen sulfide, mercaptans, etc. are oxidized on the catalyst and oxidized by sulfur. Since it changes into a product, the activity decreases due to poisoning as in the case of combustion exhaust gas.

  Therefore, the big problem of the prior art is that a high removal rate cannot be obtained with respect to methane, and further, the removal rate is greatly lowered under the condition where a sulfur compound coexists.

  In view of such circumstances, it has been disclosed that a catalyst in which palladium or palladium and platinum are supported on a zirconium oxide support continues to maintain high methane oxidation activity even in the presence of sulfur oxide (see Patent Document 3). ). However, since this catalyst has a low methane oxidation activity particularly in a low temperature range of about 400 ° C. or less, a large amount of catalyst is required to ensure sufficient performance at a low temperature.

  In addition, an unburned hydrocarbon oxidation catalyst in exhaust gas in which platinum and palladium are supported on a titanium oxide support has been proposed (see Patent Document 4), but this catalyst is also particularly in a low temperature range of about 400 ° C. or lower. Methane oxidation activity is not enough.

  While it was theorized that palladium is effective for the oxidation of methane (see Non-Patent Document 2 and Non-Patent Document 3), it does not contain palladium and only platinum is supported on a support made of tin oxide. There is also a document showing that the catalyst is active in removing methane from combustion exhaust gas by oxidation (see Patent Document 5). However, this catalyst does not have sufficient methane removal performance at 400 ° C. or lower, and requires a large amount of expensive platinum.

A catalyst for purifying hydrocarbons in combustion exhaust gas containing methane and containing excess oxygen, comprising zirconium oxide supporting at least one selected from the group consisting of platinum, palladium, rhodium and ruthenium and iridium. It is also disclosed that a catalyst having a specific surface area of ² to 60 m ² / g continues to maintain high methane oxidation activity even at a low temperature of about 400 ° C. in the presence of sulfur oxide (patent document) 6). However, this catalyst has a practical problem in that it requires a relatively large amount of iridium, which is a very rare noble metal.

  Also proposed is a catalyst for oxidizing and removing methane in combustion exhaust gas containing sulfur oxide in a low temperature region, in which iridium is supported as a promoter on a catalyst in which platinum is supported on tin oxide (see Patent Document 7). However, this catalyst does not have sufficient methane removal performance below 400 ° C.

In addition, in order to decompose and remove NOx components contained in the combustion exhaust gas of gas fuel, a porous carrier made of one or more of alumina, zirconium oxide, and titanium oxide is used with one or more of iridium, platinum, and rhodium. A catalyst for removing NOx carrying a plurality of species has been proposed (see Patent Document 8 ). However, this document only shows NOx removal performance, and does not teach any removal rate of hydrocarbons. What is the possibility of oxidative decomposition of the most difficult-to-decompose methane among hydrocarbons? I did not suggest.

A method for producing an exhaust gas purifying catalyst in which at least one of platinum and rhodium and at least one of iridium and ruthenium are supported on an inorganic support such as activated alumina by a specific method using citric acid. Is disclosed (see Patent Document 9 ). According to this document, iridium and / or ruthenium forms a solid solution having a high melting point with platinum and / or rhodium, and thus the heat resistance of the obtained catalyst is improved. However, this document only shows that the NOx conversion rate of the obtained catalyst has been improved, and does not teach any oxidative decomposition of methane, which is particularly difficult to decompose among the hydrocarbons contained in the exhaust gas. .

There has been proposed a lean burn engine exhaust gas denitration catalyst in which iridium is supported on various supports such as alumina, silica, zirconium oxide, and titanium oxide (see Patent Document 10 ). However, this document also does not show recognition that methane is particularly difficult to decompose among various hydrocarbons present in the exhaust gas. Therefore, it has not been clarified at all about how methane can be efficiently oxidized and decomposed.

Japanese Patent Laid-Open No. 51-106691 JP-A-8-332392 JP-A-11-319559 JP 2000-254500 A Japanese Patent Laid-Open No. 2004-351236 International Publication WO2002 / 040152 JP 2006-272079 A Japanese Patent Laid-Open No. 3-293035 Japanese Patent Laid-Open No. 3-98644 Japanese Unexamined Patent Publication No. 7-80315

Applied Catalysis B: Environmental, Vol. 14, 1997, p.211-223 Industrial and Engineering Chemistry, Vol. 53, 1961, p.809-812 Industrial and Engineering Chemistry Product Research and Development, Vol. 19, 1980, p.293-298

  An object of the present invention is to provide a catalyst that exhibits high methane resolution even at a low temperature (particularly 400 ° C. or lower, further 375 ° C. or lower) in oxidizing and removing methane in a gas to be treated containing methane and excess oxygen, An object of the present invention is to provide a method for oxidizing and removing methane in a gas to be treated using a catalyst.

The present invention provides a catalyst for oxidizing and removing methane in a gas to be treated and a method for oxidizing and removing methane in the gas to be treated as described below.
Item 1. A catalyst for oxidizing and removing methane in a gas to be treated containing methane and excess oxygen,
A catalyst comprising platinum, iridium and tantalum supported on at least one carrier selected from the group consisting of a zirconium oxide carrier, a tin oxide carrier and a titanium oxide carrier.
Item 2. Item 2. The catalyst according to Item 1, wherein the support comprises a zirconium oxide support and / or a titanium oxide support.
Item 3. 3. A method for oxidizing and removing methane in a gas to be treated containing methane and excess oxygen, wherein the gas to be treated is brought into contact with the catalyst according to item 1 or 2 at a temperature of 300 to 600 ° C.

  Hereinafter, the present invention will be described in detail.

  The catalyst of the present invention is a catalyst for oxidative removal of methane in a gas to be treated, and at least one selected from the group consisting of zirconium oxide, tin oxide and titanium oxide as a carrier, platinum as a catalyst active component, It is characterized by carrying iridium and tantalum.

  As the support, any of zirconium oxide, tin oxide and titanium oxide may be used, or these may be used in combination. However, the use of a support containing zirconium oxide and / or titanium oxide, particularly titanium oxide, increases the catalytic activity. While improving further, stability of activity over time is also ensured, which is preferable.

  At least one surface area selected from the group consisting of zirconium oxide, tin oxide and titanium oxide as the support keeps the catalytically active component more highly dispersed and further enhances the thermal stability of the support during use of the catalyst. Furthermore, it is preferable to adjust appropriately in order to make it difficult to sinter the support itself.

The specific surface area of zirconium oxide (referred to herein as the specific surface area by the BET method) is usually about ^{2 to} 90 m ² / g, preferably about 5 to 30 m ² / g. The crystal form of zirconium oxide is preferably a monoclinic crystal, but may contain tetragonal crystals or cubic crystals of 25% or less on a mass basis. A known method such as X-ray diffraction measurement can be applied to the measurement of the crystal phase content ratio. As such zirconium oxide, commercially available zirconium oxide for catalyst carriers can be used as it is. Commercially available zirconium oxide can also be used after being fired at 500 to 800 ° C. in an oxidizing gas atmosphere such as air or a gas obtained by appropriately mixing oxygen and an inert gas such as nitrogen. Thus, by firing in advance, it is possible to further suppress the sintering of the zirconium oxide support during use as a catalyst. Thereby, it can suppress more that a carrying | support metal sinters in the form wound by sintering of a zirconium oxide support | carrier, or is coat | covered with a zirconium oxide support | carrier.

The specific surface area of tin oxide is usually about ^{2 to} 90 m ² / g, preferably about 5 to 30 m ² / g. The crystal form of tin oxide is preferably a rutile type, but may contain other crystal phases of 10% or less based on mass. A known method such as X-ray diffraction measurement can be applied to the measurement of the crystal phase content ratio. As such tin oxide, commercially available high surface area tin oxide can be used as it is. Commercially available tin oxide can also be used after being calcined at 500 to 800 ° C. in an oxidizing gas atmosphere such as air or a gas obtained by appropriately mixing oxygen and an inert gas such as nitrogen. Thus, by firing beforehand, sintering of the tin oxide carrier during use as a catalyst can be further suppressed. Thereby, it can suppress more that a carrying | support metal sinters in the form wound by sintering of a tin oxide support | carrier, or is coat | covered with a tin oxide support | carrier.

The specific surface area of titanium oxide is usually about ^{2 to} 150 m ² / g, preferably about 30 to 90 m ² / g. The crystal form of titanium oxide is preferably anatase type, but may contain 25% or less of rutile type titanium oxide on a mass basis. A known method such as X-ray diffraction measurement can be applied to the measurement of the crystal phase content ratio. Such a titanium oxide may be a commercially available titanium oxide for a catalyst carrier, or a commercially available titanium oxide in an oxidizing atmosphere such as a gas in which air or oxygen and an inert gas such as nitrogen are appropriately mixed. It can also be prepared by a method such as baking at 500 ° C. In this way, sintering in advance can further suppress sintering of the titanium oxide carrier during use as a catalyst. Thereby, it can suppress more that a carrying | support metal sinters in the form wound by sintering of a titanium oxide support | carrier, or is coat | covered with a titanium oxide support | carrier.

  The catalyst carrier may contain trace amounts of components other than zirconium oxide such as alumina and silica, tin oxide and titanium oxide in order to improve adhesion to a support such as cordierite and sinterability. These components should not exceed 2% by weight.

  The amount of platinum, iridium, and tantalum supported on at least one selected from the group consisting of zirconium oxide, tin oxide, and titanium oxide as a carrier makes it possible to adjust the particle size so that the catalytically active component can be used more effectively. From the viewpoint of increasing the catalytic activity, it is preferable to adjust appropriately.

  The amount of platinum supported is preferably about 0.5 to 20%, more preferably about 0.5 to 5% by mass ratio to the support. The amount of iridium supported is preferably about 0.5 to 20%, more preferably about 0.5 to 5%, by mass ratio to the carrier. The ratio of the supported amount of platinum and iridium is preferably about 0.3 to 5 and more preferably about 1 to 2 in terms of the mass ratio of Ir / Pt. The amount of tantalum supported is preferably about 0.01 to 10%, more preferably about 0.05 to 2% by mass ratio to the carrier. The ratio of the supported amount of platinum and tantalum is preferably about 0.01 to 2 and more preferably about 0.03 to 0.5 in terms of the mass ratio of Ta / Pt.

  The catalyst of the present invention is, for example, impregnated with a mixed aqueous solution of a platinum compound, an iridium compound and a tantalum compound in at least one selected from the group consisting of the above-mentioned zirconium oxide, tin oxide and titanium oxide, dried and calcined. Is obtained.

  The impregnation operation may be performed using an aqueous solution prepared by dissolving a water-soluble compound such as a chloro complex, an ammine complex, nitrate, or chloride in pure water, ethanol or the like, or an acetylacetonate complex, pentaethoxy metal Alternatively, an organic solvent solution in which an organic metal compound such as acetone is dissolved in an organic solvent such as acetone or hexane may be used.

  Water-soluble compounds include chloroplatinic acid (hexachloroplatinic acid), tetraammineplatinum nitrate, dinitrodiammineplatinum, hexahydroxoplatinic acid, chloroiridium acid (hexachloroiridium acid), hexaammineiridium nitrate, iridium nitrate, tantalum (III) chloride And tantalum chloride (V). If the solubility is low and the desired concentration cannot be obtained by dissolving in pure water, dilute nitric acid, dilute hydrochloric acid, or aqueous ammonia may be added to increase the solubility.

  Examples of the organometallic compound include tris (acetylacetonato) iridium, bis (acetylacetonato) platinum, pentaethoxytantalum and the like.

  The loading of platinum, iridium, and tantalum may be performed sequentially as necessary, and in that case, between the respective loadings, the steps such as evaporation to dryness or drying, baking, etc. are sandwiched as necessary. Good. In this case, it is preferable that tantalum is first supported on at least one carrier selected from the group consisting of zirconium oxide, tin oxide and titanium oxide, then dried or fired, and then platinum and iridium are further supported. Since tantalum chloride has a high vapor pressure at a relatively low temperature, at least one carrier selected from the group consisting of zirconium oxide, tin oxide and titanium oxide and tantalum chloride (V) are physically used. Tantalum can be supported on these carriers also by a method of mixing and firing at a temperature of about 300 ° C. to 400 ° C. in a nitrogen stream or dry air. Since this method can be dry-processed, the process is simple and may be economically advantageous. However, since a part of tantalum is volatilized in the baking process, the amount of tantalum supported may be lower than the charged value.

  The impregnation time is not particularly limited as long as a predetermined loading amount is ensured, but is usually about 10 minutes to 100 hours, preferably about 1 to 50 hours, more preferably about 3 to 20 hours. In the case where platinum, iridium and tantalum are supported on platinum and iridium after supporting tantalum on at least one carrier selected from the group consisting of zirconium oxide, tin oxide and titanium oxide, The impregnation time is preferably about 10 minutes to 100 hours, more preferably about 1 to 50 hours, and further preferably about 3 to 20 hours. Moreover, the impregnation time of the platinum compound and the iridium compound is preferably about 10 minutes to 100 hours, more preferably about 1 to 50 hours, and further preferably about 3 to 20 hours.

  Next, it is preferable to calcine after at least one selected from the group consisting of zirconium oxide, tin oxide and titanium oxide carrying a predetermined metal component, if necessary, by evaporation to dryness or drying.

  Firing may be performed under air circulation. Or you may carry out in distribution | circulation of oxidizing gas, such as gas which mixed air or inert gas, such as oxygen, and nitrogen suitably.

  The calcination temperature increases the activity without advancing the grain growth of the supported metal and prevents the metal particles supported during the use of the catalyst from being enlarged by sufficient calcination. From the viewpoint of obtaining stable activity, it is preferable to adjust appropriately. Therefore, in order to stably obtain high catalytic activity, the calcination temperature is preferably about 450 to 650 ° C, more preferably about 500 to 600 ° C.

  The firing time is not particularly limited, but is usually about 1 to 50 hours, preferably about 1.5 to 20 hours.

  The catalyst of the present invention may be used after being formed into an arbitrary shape such as a pellet shape or a honeycomb shape, or may be used after being wash-coated on a refractory honeycomb. Preferably, the refractory honeycomb is wash coated.

  When wash-coating on the fire-resistant honeycomb, the catalyst prepared by the above method may be slurry-coated and washed, or at least one selected from the group consisting of zirconium oxide, tin oxide and titanium oxide in advance. After the seed is washcoated on the refractory honeycomb, the active ingredient may be supported according to the impregnation technique described above. In either case, a binder can be added as necessary. As a preferred example, a binder (for example, zirconium oxide sol, tin oxide sol or titanium oxide sol), an appropriate amount of water, and a thickener as necessary are added to at least one carrier selected from the group consisting of zirconium oxide, tin oxide and titanium oxide. Add to prepare a slurry, coat this on a refractory honeycomb, dry and fire at 650-750 ° C. to form a carrier layer on the refractory honeycomb, platinum, iridium and tantalum And a method of impregnating and supporting the above.

The specific surface area of the catalyst of the present invention is usually about ^{2 to} 150 m ² / g, preferably about ² to 90 m ² / g, more preferably about 5 to 50 m ² / g, still more preferably 5 to 30m ² / g. By making the specific surface area of the catalyst within this range, it is possible to prevent the progress of sintering of the support during use and to maintain the durability of the catalyst more, and to increase the dispersion of the active metal and to increase the activity. it can.

  The methane oxidation removal method of the present invention is to be treated with gas containing methane and excess oxygen, such as combustion exhaust gas, coal mine ventilation gas, and gas released from various chemical processes. The gas to be treated may contain flammable components such as lower hydrocarbons such as ethane and propane, carbon monoxide, and oxygen-containing compounds in addition to methane. Since these are easily decomposable as compared with methane, they can be easily oxidized and removed simultaneously with methane by the method of the present invention.

  The concentration of the combustible component in the gas to be treated is not particularly limited, but it is preferably adjusted appropriately from the viewpoint of maintaining the durability of the catalyst without causing an extreme temperature rise in the catalyst layer. Specifically, the concentration of the combustible component is preferably 1% by volume or less in terms of methane, and more preferably 5,000 ppm or less by volume ratio.

  The method for oxidizing and removing methane of the present invention is characterized by using the catalyst obtained as described above.

In order to obtain a more effective removal rate, it is preferable to use an amount of the catalyst that is 500,000 h ⁻¹ or less in space velocity per gas hour (GHSV). On the other hand, the lower the gas hourly space velocity (GHSV), the greater the amount of catalyst, so the purification rate improves, but in order not to increase the cost excessively and to further reduce the pressure loss in the catalyst layer, the lower limit of GHSV may preferably be about 1,000 h ^-1, and more preferably about 5,000h ^-1.

  The oxygen concentration in the gas to be treated is not particularly limited as long as it contains excess oxygen, but it is about 2% or more (more preferably about 5% or more) on a volume basis, and oxidation of reducing components composed of hydrocarbons or the like. It is preferred that there be about 5 times or more (more preferably about 10 times or more) of oxygen equivalent.

  When the oxygen concentration in the gas to be treated is extremely low, the reaction rate may be lowered. Therefore, a required amount of air, exhaust gas containing excess oxygen, or the like may be mixed in advance.

  The catalyst for oxidative removal of methane of the present invention has a high activity, but it is preferable to adjust appropriately in order to obtain a desired methane removal rate by further increasing the activity and to further improve the durability of the catalyst. .

  The temperature of the catalyst layer is usually about 300 to 600 ° C, preferably about 300 to 450 ° C, more preferably about 325 to 400 ° C, and further preferably about 325 to 375 ° C. In the present invention, when zirconium oxide is used as the support, the catalytic activity can be remarkably improved at a low temperature of 375 ° C. or lower, particularly 350 ° C. or lower. Further, when tin oxide is used as the carrier, the catalytic activity can be remarkably improved not only at a low temperature of 400 ° C. or lower but also at a high temperature of 400 ° C. or higher.

  Further, when the concentration of hydrocarbons in the gas to be treated is extremely high, a rapid reaction occurs in the catalyst layer, which may adversely affect the durability of the catalyst. It is preferable to use it under the condition of 200 ° C. or less, preferably about 100 ° C. or less.

  In the case of coal mine ventilation gas, about 1 to 3% by volume, and the combustion exhaust gas contains about 5 to 15% by volume of water vapor. Effective methane oxidation removal is achieved.

  The combustion exhaust gas usually contains sulfur oxides that significantly reduce the catalyst activity. However, the catalyst of the present invention exhibits a particularly high resistance to the activity reduction caused by sulfur oxides, so that 0.1% by volume. Even when about 30 ppm of sulfur oxide is contained, the methane conversion is not substantially affected.

  According to the present invention, it becomes possible to stably oxidize and remove methane in the gas to be treated. Therefore, combustion exhaust gas such as methane fermentation gas and natural gas city gas, coal mine ventilation gas, various process gases, etc. By treating the gas containing methane and sulfur compounds with the method of the present invention, the methane contained in the gas can be oxidized and removed, and the reaction heat can be recovered and used effectively as energy, as well as improving the global environment. Also contributes.

  Since the catalyst of the present invention exhibits extremely excellent resistance to activity inhibition by water vapor and sulfur compounds, high methane oxidation can be achieved even in exhaust gas containing a large amount of water vapor and sulfur oxide like combustion exhaust gas. Demonstrate activity.

  In addition, the catalyst of the present invention exhibits high activity even at low temperatures (particularly 400 ° C. or lower, more preferably 375 ° C. or lower), so that the amount of expensive noble metal used can be reduced and the economy is excellent.

  EXAMPLES Hereinafter, although an Example is shown and this invention is demonstrated in detail, this invention is not limited to these Examples.

Comparative Example 1 (Preparation of 2% Pt-3% Ir / zirconium oxide catalyst)
Zirconium oxide (“N-PC” manufactured by Nippon Electric Works, specific surface area 18 m ² / g) was calcined in air at 700 ° C. for 4 hours to obtain calcined zirconium oxide (specific surface area 9 m ² / g).

Mix the hexachloroplatinic acid (H ₂ PtCl ₆ ) aqueous solution (0.69 g) containing 15.3 mass% as Pt and the dilute hydrochloric acid solution (1.84 g) of hexachloroiridate (H ₂ IrCl ₆ ) containing 8.6 mass% as Ir. , Diluted with pure water (20 ml). This solution was impregnated with 5 g of the calcined zirconium oxide. After evaporating to dryness and drying at 60 ° C., it was calcined in air at 500 ° C. for 2 hours to obtain a 2% Pt-3% Ir / zirconium oxide catalyst. The specific surface area of this catalyst was 8 m ² / g.

Example 1 (Preparation of 2% Pt-3% Ir-0.5% Ta / zirconium oxide catalyst)
While stirring a solution of 0.0525 g of tantalum chloride (TaCl ₅ ) in 100 ml of ethanol, 5 g of the calcined zirconium oxide was added, and this solution was impregnated with the calcined zirconium oxide. The mixture was evaporated to dryness with stirring, dried at 60 ° C., and calcined in air at 500 ° C. for 2 hours to obtain 0.5% Ta / zirconium oxide.

Mix the hexachloroplatinic acid (H ₂ PtCl ₆ ) aqueous solution (0.69 g) containing 15.3 mass% as Pt and the dilute hydrochloric acid solution (1.84 g) of hexachloroiridate (H ₂ IrCl ₆ ) containing 8.7 mass% as Ir. , Diluted with pure water (10 ml). This solution was impregnated with 0.5% Ta / zirconium oxide. After evaporating to dryness and drying at 60 ° C., it was calcined in air at 500 ° C. for 2 hours to obtain a 2% Pt-3% Ir-0.5% Ta / zirconium oxide catalyst. The specific surface area of this catalyst was 7 m ² / g.

Example 2 (Preparation of 2% Pt-3% Ir-1% Ta / zirconium oxide catalyst)
2% Pt-3 in the same manner as in Example 1 except that the amount of Ta supported was 1% by mass with respect to zirconium oxide (that is, 0.105 g of tantalum chloride was used to increase the amount of Ta supported). % Ir-1% Ta / zirconium oxide catalyst was obtained.

Comparative Example 2 (Preparation of 3% Ir-1% Ta / zirconium oxide catalyst)
A 3% Ir-1% Ta / zirconium oxide catalyst was obtained in the same manner as in Example 2 except that Pt was not supported (that is, the hexachloroplatinic acid aqueous solution was not used).

Comparative Example 3 (Preparation of 2% Pt-1% Ta / zirconium oxide catalyst)
A 2% Pt-1% Ta / zirconium oxide catalyst was obtained in the same manner as in Example 2 except that Ir was not supported (that is, a dilute hydrochloric acid solution of hexachloroiridium acid was not used).

Comparative Example 4 (Preparation of 2% Pt-3% Ir / tin oxide catalyst)
Tin oxide (“SL” manufactured by Nippon Kagaku Sangyo Co., Ltd., specific surface area 31 m ² / g) was calcined in air at 700 ° C. for 4 hours to obtain calcined tin oxide (specific surface area 7 m ² / g).

0.69 g of hexachloroplatinic acid (H ₂ PtCl ₆ ) aqueous solution containing 15.3 mass% as Pt and 1.84 g of dilute hydrochloric acid solution of hexachloroiridium acid (H ₂ IrCl ₆ ) containing 8.6 mass% as Ir are mixed with pure water ( 10 ml). This solution was impregnated with 5 g of the calcined tin oxide. After evaporating to dryness and drying at 60 ° C., it was calcined in air at 500 ° C. for 2 hours to obtain a 2% Pt-3% Ir / tin oxide catalyst. The specific surface area of this catalyst was 6 m ² / g.

Example 3 (Preparation of 2% Pt-3% Ir-0.5% Ta / tin oxide catalyst)
While stirring a solution of 0.0525 g of tantalum chloride (TaCl ₅ ) in 100 ml of ethanol, 5 g of the calcined tin oxide was added, and this solution was impregnated with calcined zirconium oxide. The mixture was evaporated to dryness while stirring, dried at 60 ° C., and calcined in air at 500 ° C. for 2 hours to obtain 0.5% Ta / tin oxide.

Mix 1.22 g of hexachloroplatinic acid (H ₂ PtCl ₆ ) aqueous solution containing 8.7% by mass as Pt and 1.85 g of dilute hydrochloric acid solution of hexachloroiridate (H ₂ IrCl ₆ ) containing 8.6% by mass as Ir. 20 ml). This solution was impregnated with the calcined tin oxide. After evaporating to dryness and drying at 60 ° C., it was calcined in air at 500 ° C. for 2 hours to obtain a 2% Pt-3% Ir / tin oxide catalyst. The specific surface area of this catalyst was 6 m ² / g.

Example 4 (Preparation of 2% Pt-3% Ir-1% Ta / tin oxide catalyst)
2% Pt-3 in the same manner as in Example 3 except that the amount of Ta supported was 1% by mass with respect to tin oxide (that is, 0.105 g of tantalum chloride was used to increase the amount of Ta supported). % Ir-1% Ta / tin oxide catalyst was obtained.

[Activity evaluation test 1]
Each of the catalysts prepared in Examples 1 to 4 and Comparative Examples 1 to 4 was tableted and then crushed to have a particle size of 1 to 2 mm. A quartz reaction tube (outer diameter: 16 mm, inner diameter: 14 mm) was charged with 1.5 ml of each catalyst (2.0 to 2.1 g for a catalyst using zirconium oxide as a carrier and 2.7 to 2.8 g for a catalyst using tin oxide as a carrier). Next, a gas having a composition consisting of 1,000 ppm of methane, 10% oxygen, 10% water vapor (both based on volume) and the balance nitrogen is circulated through the reaction tube at a flow rate of 2 liters / minute (volume in the standard state). The reaction tube was heated in a furnace and the methane conversion was measured at catalyst layer temperatures of 350, 375, 400 and 425 ° C. The catalyst layer temperature was measured with a thermocouple placed in a protective tube (outer diameter 4 mm, inner diameter 2 mm) in the layer. The gas composition before and after the reaction layer was measured by a gas chromatograph having a flame ionization detector and a thermal conductivity detector.

The measurement results of the methane conversion rate (%) are shown in Tables 1 and 2. Here, the methane conversion is a value obtained by the following equation.
CH ₄ conversion (%) = 100 × (1-CH ₄ -OUT / CH ₄ -in)
In the formula, “CH ₄ -OUT” indicates the methane concentration at the catalyst layer outlet, and “CH ₄ -in” indicates the methane concentration at the catalyst layer inlet.

  Following the above activity evaluation, while maintaining the catalyst layer temperature at 425 ° C., the reaction was continued by adding 3 ppm of sulfur dioxide to the reaction gas, and at each time point after 20 and 45 hours, the catalyst layer temperature was 350 ° C. The methane conversion at 375 ° C, 400 ° C and 425 ° C was measured in the same way. The results are shown in Tables 1 and 2.

  The catalyst of Comparative Example 1 showed a methane conversion of 52% at the initial reaction temperature of 350 ° C. On the other hand, in the catalysts of Examples 1 and 2, higher methane conversion rates of 64 and 57% were obtained, respectively, and it was clear that low temperature activity was greatly improved by supporting tantalum in addition to platinum and iridium. is there. Even after 45 hours, the catalyst of Comparative Example 1 had a methane conversion of 61% at 350 ° C., whereas the catalysts of Examples 1 and 2 were 64 and 67%, respectively. The catalyst maintained higher activity.

  The catalyst of Comparative Example 2 is a catalyst containing only iridium and tantalum and no platinum, but the methane conversion rate at 350 ° C. in the initial stage of the reaction is only 5%. The catalyst of Comparative Example 3 is a catalyst containing only platinum and tantalum and containing no iridium, but the methane conversion at 37 ° C. in the initial stage of the reaction is 37%, which is not comparable to the catalysts of Examples 1 and 2. As is clear from the results of Comparative Examples 2 and 3, the coexistence of three components of platinum, iridium and tantalum is essential to obtain high activity, and a combination of iridium and tantalum alone or only platinum and tantalum is sufficient. You cannot gain activity.

  A catalyst in which platinum and iridium were supported on tin oxide (Comparative Example 4) also showed a certain activity in methane oxidation at a low temperature, and showed a methane conversion rate of 23% at 350 ° C. in the initial stage of the reaction. On the other hand, when 0.5% (Example 3) to 1% (Example 4) of tantalum was added, the methane conversion was greatly improved to 35% and 33%, respectively. It is clear that low temperature activity is greatly improved by supporting tantalum in addition to platinum and iridium. Even after 45 hours, the catalyst of Comparative Example 4 had a methane conversion of 40% at 350 ° C., whereas the catalysts of Examples 3 and 4 were 58 and 61%, respectively. The catalyst maintained higher activity.

Comparative Example 5 (Preparation of 2% Pt-3% Ir / zirconium oxide catalyst)
Zirconium oxide (“N-PC” manufactured by NIPPON Denko Corporation, specific surface area 28 m ² / g) was calcined in air at 700 ° C. for 6 hours to obtain calcined zirconium oxide (specific surface area 17 m ² / g).

Mix the hexachloroplatinic acid (H ₂ PtCl ₆ ) aqueous solution (4.3 g) containing 16.3% by mass as Pt and the dilute hydrochloric acid solution (12.2 g) of hexachloroiridate (H ₂ IrCl ₆ ) containing 8.7% by mass as Ir. , Diluted with pure water (20 ml). This solution was impregnated with 35 g of the calcined zirconium oxide. After evaporating to dryness and drying at 120 ° C., it was calcined in air at 550 ° C. for 6 hours to obtain a 2% Pt-3% Ir / zirconium oxide catalyst. The specific surface area of this catalyst was 17 m ² / g.

Example 5 (Preparation of 2% Pt-3% Ir-0.34% Ta / zirconium oxide catalyst)
Zirconium oxide (“N-PC” manufactured by NIPPON Denko Corporation, specific surface area 28 m ² / g) was calcined in air at 700 ° C. for 6 hours to obtain calcined zirconium oxide (specific surface area 17 m ² / g). 50 g of this calcined zirconium oxide and 1.0 g of tantalum chloride (TaCl ₅ ) were mixed and calcined at 350 ° C. for 1 hour in air. This Ta / zirconium oxide contained 0.34% by mass of Ta (by fluorescent X-ray analysis), and the BET specific surface area was 17 m ² / g. A hexachloroplatinic acid (H ₂ PtCl ₆ ) aqueous solution (3.68 g) containing 16.3% by mass as Pt and a dilute hydrochloric acid solution (10.47 g) of hexachloroiridate (H ₂ IrCl ₆ ) containing 8.6% by mass as Ir were mixed. , Diluted with pure water (12 ml). This solution was impregnated with 30 g of the above Ta / zirconium oxide. After evaporating to dryness and drying at 120 ° C., it was calcined in air at 550 ° C. for 6 hours to obtain a 2% Pt-3% Ir-0.34% Ta / zirconium oxide catalyst. The specific surface area of this catalyst was 16 m ² / g.

Example 6 (Preparation of 2% Pt-3% Ir-0.18% Ta / zirconium oxide catalyst)
50 g of zirconium oxide (“N-PC” manufactured by Nippon Electric Works, specific surface area 28 m ² / g) and 1.0 g of tantalum chloride (TaCl ₅ ) were mixed and baked at 350 ° C. for 1 hour in air. This Ta / zirconium oxide contained 0.18% by mass of Ta (by fluorescent X-ray analysis), and the BET specific surface area was 19 m ² / g. Mix the hexachloroplatinic acid (H ₂ PtCl ₆ ) aqueous solution (3.07 g) containing 16.3% by mass as Pt and the dilute hydrochloric acid solution (8.72 g) of hexachloroiridate (H ₂ IrCl ₆ ) containing 8.6% by mass as Ir. , Diluted with pure water (10 ml). This solution was impregnated with 25 g of the above Ta / zirconium oxide. After evaporating to dryness and drying at 120 ° C., it was calcined at 550 ° C. for 6 hours in air to obtain a 2% Pt-3% Ir-0.18% Ta / zirconium oxide catalyst. The specific surface area of this catalyst was 18 m ² / g.

[Activity evaluation test 2]
Each of the catalysts prepared in Examples 5 to 6 and Comparative Example 5 was tableted and then crushed to have a particle size of 1 to 2 mm. 1.45 g of each catalyst was packed in a quartz reaction tube (outer diameter 17.5 mm, inner diameter 14 mm). Next, a gas having a composition consisting of 1,000 ppm of methane, 10% oxygen, 10% water vapor (both based on volume) and the balance nitrogen is circulated through the reaction tube at a flow rate of 2 liters / minute (volume in the standard state) to form a catalyst. Methane conversion was measured at bed temperatures of 350 ° C, 400 ° C, 425 ° C and 450 ° C. The gas composition before and after the reaction layer was measured by a gas chromatograph having a flame ionization detector and a thermal conductivity detector. Following the measurement of the initial activity, the reaction was continued by adding 3 ppm of sulfur dioxide to the reaction gas while keeping the catalyst layer temperature at 450 ° C., which is more likely to deteriorate than the activity evaluation test 1, and after 20 and 60 hours, respectively. At that time, the methane conversion rate at the catalyst layer temperatures of 350 ° C., 400 ° C., 425 ° C. and 450 ° C. was measured in the same manner.

  In any of the catalysts of Examples 5 to 6 and Comparative Example 5, the production of carbon dioxide corresponding to the decrease in the methane concentration was confirmed, and the methane was completely oxidized on the catalyst. Table 3 shows the measurement results of methane conversion (%).

  The methane conversion in the catalyst of Comparative Example 5 was initially 30% at 350 ° C. and 86% at 400 ° C., but there was a slight decrease over time, and after 60 hours, 29% at 350 ° C. It was 80% at 400 ° C.

  In contrast, the catalyst of Example 5 showed a high methane conversion of 46% at the initial 350 ° C. and 91% at 400 ° C., and even after 60 hours, 45% at 350 ° C. and 87% at 400 ° C. The methane conversion was shown. The catalyst of Example 5 shows a higher initial activity in methane oxidation than the catalyst of Comparative Example 5, and its superiority is maintained even after the deterioration of the activity over time. The catalyst of Example 6 also showed high methane oxidation activity as in Example 5.

Comparative Example 6 (Preparation of 2% Pt-3% Ir / titanium oxide catalyst)
Titanium oxide (“MC-50” manufactured by Ishihara Sangyo Co., Ltd., specific surface area 62 m ² / g) was calcined in air at 700 ° C. for 6 hours to obtain calcined titanium oxide (specific surface area 32 m ² / g). A 2% Pt-3% Ir / titanium oxide catalyst was obtained in the same manner as Comparative Example 5 except that this calcined titanium oxide was used instead of calcined zirconium oxide. The specific surface area of this catalyst was 32 m ² / g.

Example 7 (Preparation of 2% Pt-3% Ir-0.08% Ta / titanium oxide catalyst)
28.3 g of titanium oxide (“MC-90” manufactured by Ishihara Sangyo Co., Ltd., specific surface area 82 m ² / g) and 1.0 g of tantalum chloride (TaCl ₅ ) were mixed and baked at 350 ° C. for 1 hour. This Ta / titanium oxide contained 0.08% by mass of Ta (by fluorescent X-ray analysis), and the BET specific surface area was 39 m ² / g. Mix the hexachloroplatinic acid (H ₂ PtCl ₆ ) aqueous solution (2.45 g) containing 16.3% by mass as Pt and the dilute hydrochloric acid solution (6.98 g) of hexachloroiridate (H ₂ IrCl ₆ ) containing 8.6% by mass as Ir. , Diluted with pure water (15 ml). This solution was impregnated with 20 g of the above Ta / titanium oxide. After evaporating to dryness and drying at 120 ° C., the mixture was calcined in air at 550 ° C. for 6 hours to obtain a 2% Pt-3% Ir-0.08% Ta / titanium oxide catalyst. The specific surface area of this catalyst was 35 m ² / g.

[Activity evaluation test 3]
Each of the catalysts prepared in Example 7 and Comparative Example 6 was tableted and then crushed to have a particle size of 1 to 2 mm. 1.45 g (about 2.5 ml) of each catalyst was packed in a quartz reaction tube (inner diameter 14 mm). Next, a gas having a composition consisting of 1,000 ppm of methane, 10% oxygen, 10% water vapor (both based on volume) and the balance nitrogen is circulated through the reaction tube at a flow rate of 2 liters / minute (volume in the standard state) to form a catalyst. Methane conversion was measured at bed temperatures of 350 ° C, 375 ° C, 400 ° C and 450 ° C. The gas composition before and after the reaction layer was measured by a gas chromatograph having a flame ionization detector and a thermal conductivity detector. Following the measurement of the initial activity, the reaction was continued by adding 3 ppm of sulfur dioxide to the reaction gas while keeping the catalyst layer temperature at 450 ° C., and the catalyst layer temperature was 350 ° C. at 20 and 60 hours later, respectively. The methane conversion at 375 ° C, 400 ° C and 450 ° C was measured in the same manner.

  In any of the catalysts of Example 7 and Comparative Example 6, production of carbon dioxide corresponding to the decrease in methane concentration was confirmed, and methane was completely oxidized on the catalyst. Table 4 shows the measurement results of methane conversion (%).

  The methane conversion rate in the catalyst of Comparative Example 6 was initially 66% at 350 ° C. and 89% at 375 ° C., but there was a slight decrease over time, and after 60 hours, 48% at 350 ° C. It became 77% at 375 ° C.

  In contrast, the catalyst of Example 7 showed high methane conversion of 76% at the initial 350 ° C. and 94% at 375 ° C., and after 60 hours, 61% at 350 ° C. and 86% at 375 ° C. The methane conversion was shown. The catalyst of Example 7 exhibits high methane oxidation activity and superior stability over time of the activity as compared with the catalyst of Comparative Example 6.

Claims (3)

A catalyst for oxidizing and removing methane in a gas to be treated containing methane and excess oxygen,
A catalyst comprising platinum, iridium and tantalum supported on at least one carrier selected from the group consisting of a zirconium oxide carrier, a tin oxide carrier and a titanium oxide carrier. The catalyst according to claim 1, wherein the support comprises a zirconium oxide support and / or a titanium oxide support. A method for oxidizing and removing methane in a gas to be treated containing methane and excess oxygen, wherein the gas to be treated is brought into contact with the catalyst according to claim 1 or 2 at a temperature of 300 to 600 ° C.