JP5825947B2 - Methane oxidation removal catalyst and methane oxidation removal method - Google Patents

Description

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

  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 oxidizing component such as oxygen or nitrogen oxide is contained in an excessive amount or more.

  As a catalyst for removing hydrocarbons by oxidation, 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 oxidizing 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 a situation, it is 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 oxygen excessively, 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.

  Also proposed is a catalyst for oxidizing and removing methane in combustion exhaust gas containing methane, sulfur oxides and excess oxygen, wherein platinum and iridium are supported on a titanium oxide carrier (see Patent Document 8). Yes. This catalyst exhibits high methane oxidation activity even at a low temperature, and its activity decrease due to the coexistence of sulfur compounds is small, but further performance improvement is required. For this reason, addition of antimony and niobium (see Patent Document 9) has also been proposed, but there are difficulties in handling antimony and niobium, and problems remain in practice. Titanium oxide-based catalysts also have problems with durability at high temperatures.

  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 10). 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 11). According to this document, iridium and / or ruthenium form a solid solution having a high melting point with platinum and / or rhodium, so that 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 12). 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 JP 2008-246473 A JP 2009-262131 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 in oxidation removal of methane in a gas to be treated containing methane and excess oxygen, and oxidation of methane in the gas to be treated using this catalyst. It is to provide a removal method.

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, the catalyst comprising platinum, iridium and rhenium supported on a zirconium oxide support.
Item 2. 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 at a temperature of 300 to 450 ° C.

  Hereinafter, the present invention will be described in detail.

  The catalyst of the present invention is a catalyst for oxidizing and removing methane in a gas to be treated, and is characterized in that rhenium is supported on zirconium oxide as a carrier together with platinum and iridium as catalytic active components.

  When the surface area of zirconium oxide as a support is too small, the catalytically active component cannot be kept highly dispersed. On the other hand, when the surface area is too large, the thermal stability of the zirconium oxide is not sufficient, and the sintering of the zirconium oxide itself may proceed during use of the catalyst.

The specific surface area of zirconium oxide (referred to herein as the specific surface area by BET method) is usually about ^{2 to} 90 m ² / g, preferably about 10 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. Such zirconium oxide may be prepared from a commercially available zirconium oxide for a catalyst support as it is, or may be prepared by firing at 500 ° C. to 800 ° C. in an oxidizing atmosphere such as air.

  The catalyst carrier may contain a small amount of components other than zirconium oxide such as alumina and silica in order to improve adhesion to a support such as cordierite and sinterability, but these components are based on mass. It is desirable not to exceed 2%.

  When the amount of platinum, iridium and rhenium supported on zirconium oxide is too small, the catalytic activity is low, whereas when it is too large, the particle size becomes large, and the supported catalytically active components are effectively utilized. Disappear.

  The supported amount of platinum is preferably about 0.5 to 20% by mass ratio with respect to zirconium oxide, and more preferably about 0.5 to 5%. The supported amount of iridium is preferably about 0.5 to 20% by mass ratio with respect to zirconium oxide, and more preferably about 0.5 to 5%. 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 rhenium supported is preferably about 0.1 to 10%, more preferably about 0.2 to 2%, by mass ratio to zirconium oxide. The ratio of the supported amount of platinum and rhenium is preferably about 0.1 to 2 and more preferably about 0.2 to 1 by mass ratio of Re / Pt.

  The catalyst of the present invention can be obtained, for example, by impregnating the above-mentioned zirconium oxide with a mixed aqueous solution of a platinum compound, an iridium compound and a rhenium compound, drying, and firing.

  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, or a nitrate in pure water, or an organic metal compound such as an acetylacetonato complex may be used as an organic material such as acetone. You may carry out using the organic-solvent solution melt | dissolved in the solvent.

  Examples of water-soluble compounds include chloroplatinic acid (hexachloroplatinic acid), tetraammineplatinum nitrate, dinitrodiammineplatinum, hexahydroxoplatinic acid, chloroiridium acid (hexachloroiridium acid), hexaammineiridium nitrate, iridium nitrate, rhenium chloride (V) And ammonium perrhenate. 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 and bis (acetylacetonato) platinum.

  The loading of platinum, iridium, and rhenium may be performed sequentially as necessary. In that case, a drying or firing step may be interposed between the respective loadings as necessary. In this case, it is preferable that rhenium is first supported on a zirconium oxide support, followed by drying or firing, and then further supporting platinum and iridium.

  The impregnation time is not particularly limited as long as a predetermined loading amount is ensured, but is usually about 1 to 50 hours, preferably about 3 to 20 hours.

  Next, the zirconium oxide supporting a predetermined metal component is evaporated and dried or dried as necessary, and then fired.

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

  If the firing temperature is too high, grain growth of the supported metal proceeds and high activity cannot be obtained. On the other hand, if it is too low, the calcination is not performed sufficiently, so that the metal particles supported during the use of the catalyst may become coarse and stable activity may not be obtained. 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 3 to 20 hours.

  The catalyst of the present invention may be used after being molded into an arbitrary shape such as pellets or honeycombs, or may be used by wash coating on a refractory honeycomb. Preferably, the refractory honeycomb is wash coated.

  When wash-coating on a fire-resistant honeycomb, the catalyst prepared by the above method may be slurry-coated and washed, or after zirconium oxide is washed on the fire-resistant honeycomb in advance, the impregnation described above may be performed. The active ingredient may be supported according to the technique. In either case, a binder can be added as necessary. As a preferred example, a slurry is prepared by adding a binder (for example, zirconium oxide sol), an appropriate amount of water and, if necessary, a thickening agent to a zirconium oxide support, and this is coated on a refractory honeycomb and dried. A method of forming a zirconium oxide layer on a refractory honeycomb by firing at 650 to 750 ° C. and impregnating and supporting platinum, iridium, and rhenium is exemplified.

The specific surface area of the catalyst of the present invention is usually about ^{2 to} 90 m ² / g, preferably about 10 to 30 m ² / g. If the specific surface area of the catalyst exceeds 90 m ² / g, the durability of the catalyst may be lowered due to the progress of sintering of the support during use. On the other hand, if it is less than 2 m ² / g, there is a possibility that the active metal dispersion is lowered and sufficient activity cannot be obtained.

  The object of treatment by the method for oxidizing and removing methane of the present invention is a 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 flammable component in the gas to be treated is not particularly limited, but if it is too high, an extreme temperature rise may occur in the catalyst layer, which may adversely affect the durability of the catalyst. The volume percentage is preferably not more than vol%, and more preferably not more than 5,000 ppm by volume.

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

When the amount of the catalyst used is too small, an effective removal rate cannot be obtained. Therefore, it is preferable to use an amount that gives a space velocity per gas hour (GHSV) of 500,000 h ⁻¹ or less. On the other hand, the lower the gas hourly space velocity (GHSV), the greater the amount of catalyst, so the purification rate improves, but if GHSV is too low, it is economically disadvantageous and pressure loss in the catalyst layer Becomes larger. Therefore, 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 oxygen excessively, but it is about 2% or more (more preferably about 5% or more) by volume, and oxidation of reducing components such as hydrocarbons. 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 high activity. However, if the treatment temperature is too low, the activity is lowered and a desired methane removal rate cannot be obtained. On the other hand, when the treatment temperature is too high, the durability of the catalyst may deteriorate.

  The temperature of the catalyst layer is usually about 300 to 600 ° C, preferably about 300 to 450 ° C.

  In addition, when the concentration of hydrocarbons in the gas to be treated is remarkably high, a rapid reaction occurs in the catalyst layer, which adversely affects the durability of the catalyst. Therefore, the temperature rise in the catalyst layer is usually about 200 ° C. or less. It is preferable to use it under conditions of 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.

  Further, since the catalyst of the present invention exhibits high activity even at a low temperature, the amount of expensive noble metal used can be reduced and the economy is excellent.

4 is a graph showing changes over time in the methane conversion rate at 400 ° C. of the catalysts according to Examples 1 and 8 and Comparative Examples 1 and 3. FIG.

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

Example 1 (Preparation of 2% Pt-2% Ir-1% Re / 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).

35 g hydrochloric acid (1.5 g) was diluted with pure water (10 ml), and rhenium chloride (containing 51 mass% as ReCl ₅ , Re) was dissolved in 1.0 g to obtain a rhenium chloride solution (Re 4.1 mass%). . This chlororhenic acid solution (5.4 g), hexachloroplatinic acid (H ₂ PtCl ₆ ) aqueous solution (2.7 g) containing 16.3% by mass as Pt, and hexachloroiridium acid (H ₂ IrCl ₆ ) containing 8.7% by mass as Ir A dilute hydrochloric acid solution (5.1 g) was mixed and diluted with pure water (8 ml). This solution was impregnated with 22 g of the calcined zirconium oxide. After evaporating to dryness, drying at 120 ° C., and calcining in air at 550 ° C. for 6 hours, a 2% Pt-2% Ir-1% Re / zirconium oxide catalyst was obtained. The specific surface area of this catalyst was 17 m ² / g.

Example 2 (Preparation of 2% Pt-3% Ir-1% Re / zirconium oxide catalyst)
A mixture of rhenic acid chloride solution (8.1 g), hexachloroplatinic acid aqueous solution (4.1 g) and hexachloroiridium acid solution (11.6 g) was diluted with pure water (20 ml) and impregnated with 33 g of the calcined zirconium oxide. In the same manner as in Example 1, a 2% Pt-3% Ir-1% Re / zirconium oxide catalyst was obtained. The specific surface area of this catalyst was 17 m ² / g.

Example 3 (Preparation of 2% Pt-3% Ir-0.5% Re / zirconium oxide catalyst)
In addition to impregnating the above-mentioned calcined zirconium oxide 28g, mixed with rhenic chloride solution (3.4g), hexachloroplatinic acid aqueous solution (3.4g) and hexachloroiridate solution (9.8g), diluted with pure water (12ml) In the same manner as in Example 1, a 2% Pt-3% Ir-0.5% Re / zirconium oxide catalyst was obtained. The specific surface area of this catalyst was 17 m ² / g.

Comparative Example 1 (Preparation of 2% Pt-2% Ir / zirconium oxide catalyst)
A mixture of hexachloroplatinic acid aqueous solution (4.3 g) and hexachloroiridium acid solution (8.1 g) was diluted with pure water (20 ml) and impregnated with 35 g of the calcined zirconium oxide as in Example 1. 2% Pt-2% Ir / zirconium oxide catalyst was obtained. The specific surface area of this catalyst was 17 m ² / g.

Comparative Example 2 (Preparation of 2% Pt-3% Ir / zirconium oxide catalyst)
A mixture of hexachloroplatinic acid aqueous solution (4.3 g) and hexachloroiridium acid solution (12.2 g) was diluted with pure water (20 ml) and impregnated with 35 g of the calcined zirconium oxide as in Example 1. 2% Pt-3% Ir / zirconium oxide catalyst was obtained. The specific surface area of this catalyst was 17 m ² / g.

Comparative Example 3 (Preparation of 2% Pt-4% Ir / zirconium oxide catalyst)
A mixture of hexachloroplatinic acid aqueous solution (4.3 g) and hexachloroiridium acid solution (16.3 g) was diluted with pure water (20 ml) and impregnated with 35 g of the calcined zirconium oxide as in Example 1. 2% Pt-4% Ir / zirconium oxide catalyst was obtained. The specific surface area of this catalyst was 17 m ² / g.

Comparative Example 4 (Preparation of 2% Pt-0.5% Re / zirconium oxide catalyst)
A mixture of rhenic acid chloride solution (4.3 g) and hexachloroplatinic acid aqueous solution (4.3 g) was diluted with pure water (20 ml) and impregnated with 35 g of the calcined zirconium oxide as in Example 1. 2% Pt-0.5% Re / zirconium oxide catalyst was obtained. The specific surface area of this catalyst was 17 m ² / g.

Comparative Example 5 (Preparation of 2% Pt-3% Re / zirconium oxide catalyst)
A mixture of rhenic acid chloride solution (17.7 g) and hexachloroplatinic acid aqueous solution (2.87 g) was diluted with pure water (7 ml), and impregnated with 23.4 g of calcined zirconium oxide as in Example 1. Thus, a 2% Pt-3% Re / zirconium oxide catalyst was obtained. The specific surface area of this catalyst was 17 m ² / g.

Example 4 (Preparation of 2% Pt-2% Ir-0.5% Re / zirconium oxide catalyst)
1.0 g of rhenium chloride (containing 51% by mass as ReCl ₅ , Re) was added to 5 g of 35% hydrochloric acid, and diluted with pure water (3 ml) with stirring to obtain a rhenic chloride solution (Re 5.67% by mass). . This chlororhenic acid solution (2.2 g), hexachloroplatinic acid (H ₂ PtCl ₆ ) aqueous solution (3.07 g) containing 16.3% by mass as Pt and hexachloroiridate (H ₂ IrCl ₆ ) containing 8.7% by mass as Ir A dilute hydrochloric acid solution (5.8 g) was mixed and diluted with pure water (10 ml). This solution was impregnated with 25 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-2% Ir-0.5% Re / zirconium oxide catalyst. The specific surface area of this catalyst was 17 m ² / g.

Example 5 (Preparation of 2% Pt-2% Ir-2% Re / zirconium oxide catalyst)
Chlorine rhenic acid solution (6.8g), hexachloroplatinic acid aqueous solution (2.37g), hexachloroiridate solution (4.49g) were mixed, diluted with pure water (10ml), and impregnated with 19.3g of the above-mentioned calcined zirconium oxide Otherwise, in the same manner as in Example 4, a 2% Pt-2% Ir-2% Re / zirconium oxide catalyst was obtained. The specific surface area of this catalyst was 17 m ² / g.

Example 6 (Preparation of 2% Pt-3% Ir-1% Re / zirconium oxide catalyst)
0.72 g of ammonium perrhenate (NH ₄ ReO ₄ ) was dissolved in 45 g of pure water. This solution was impregnated in 50 g of zirconium oxide (“N-PC” manufactured by Nippon Electric Works, specific surface area 28 m ² / g). After evaporating to dryness and drying at 120 ° C., it was calcined in air at 700 ° C. for 6 hours to obtain 1% Re / zirconium oxide. Mix the hexachloroplatinic acid (H ₂ PtCl ₆ ) aqueous solution (3.68 g) containing 16.3% by mass as Pt and the dilute hydrochloric acid solution (10.5 g) of hexachloroiridate (H ₂ IrCl ₆ ) containing 8.7% by mass as Ir. , Diluted with pure water (15 ml). This solution was impregnated with 30 g of the above 1% Re / 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-1% Re / zirconium oxide catalyst. The specific surface area of this catalyst was 21 m ² / g.

Example 7 (Preparation of 2% Pt-2% Ir-2% Re / zirconium oxide catalyst)
2.3 g of ammonium perrhenate (NH ₄ ReO ₄ ) was dissolved in 80 g of pure water. Zirconium oxide (“N-PC” manufactured by NIPPON DENKO Co., Ltd., specific surface area 28 m ² / g) was calcined in air at 700 ° C. for 6 hours to obtain 80 g of calcined zirconium oxide (specific surface area 17 m ² / g). Impregnated. After evaporating to dryness and drying at 120 ° C., it was calcined in air at 500 ° C. for 6 hours to obtain 2% Re / zirconium oxide. Mix the hexachloroplatinic acid (H ₂ PtCl ₆ ) aqueous solution (3.68 g) containing 16.3% by mass as Pt and the dilute hydrochloric acid solution (7.0 g) of hexachloroiridate (H ₂ IrCl ₆ ) containing 8.7% by mass as Ir. , Diluted with pure water (18 ml). This solution was impregnated with 30 g of the above 2% Re / 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-2% Ir-2% Re / zirconium oxide catalyst. The specific surface area of this catalyst was 17 m ² / g.

Example 8 (Preparation of 2% Pt-3% Ir-2% Re / zirconium oxide catalyst)
Except that the aqueous hexachloroplatinic acid solution (3.68 g) and the hexachloroiridium acid solution (10.5 g) were mixed, diluted with pure water (15 ml), and impregnated with 30 g of the above 2% Re / zirconium oxide. Similarly, a 2% Pt-3% Ir-2% Re / zirconium oxide catalyst was obtained. The specific surface area of this catalyst was 17 m ² / g.

  Examples 6 to 8 are examples in which a catalyst was prepared by a sequential supporting method in which Re is first supported on a zirconium oxide support and then Ir and Pt are supported.

Comparative Example 6 (Preparation of 2% Pt-3% Pd / zirconium oxide catalyst)
Diluted nitric acid solution (6.0 g) of hexahydroxoplatinic acid (H ₂ Pt (OH) ₆ ) containing 10% by mass as Pt and dilute palladium nitrate (Pd (NO ₃ ) ₂ ) containing 23.1% by mass as Pd A nitric acid solution (3.9 g) was mixed and diluted with pure water (20 ml). A 2% Pt-3% Pd / zirconium oxide catalyst was obtained in the same manner as in Example 1 except that 30 g of calcined zirconium oxide was impregnated with this solution. The specific surface area of this catalyst was 17 m ² / g.

Comparative Example 7 (Preparation of 2% Pt-3% Pd-1% Re / zirconium oxide catalyst)
A dilute nitric acid solution (7.0 g) of hexahydroxoplatinic acid (H ₂ Pt (OH) ₆ ) containing 10% by mass as Pt and a dilute palladium nitrate (Pd (NO ₃ ) ₂ ) containing 23.1% by mass as Pd A nitric acid solution (4.6 g) was mixed and diluted with pure water (15 ml) in which 0.504 g of ammonium perrhenate (NH ₄ ReO ₄ ) was dissolved. A 2% Pt-3% Pd-1% Re / zirconium oxide catalyst was obtained in the same manner as in Example 1 except that 35 g of the calcined zirconium oxide was impregnated with this solution. The specific surface area of this catalyst was 17 m ² / g.

[Activity evaluation test 1]
Each of the catalysts prepared in Examples 1 to 8 and Comparative Examples 1 to 7 was tableted and then crushed to have a particle size of 1 to 2 mm. 1.45 g (about 1.5 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 (volume basis) 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, 400 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.

Table 1 shows the measurement results of methane conversion (%). 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.

The catalysts of Examples 1-8 showed 45-56% methane conversion at 350 ° C. This is much higher than 25-30% of the catalysts of Comparative Examples 1-3, and it is clear that low temperature activity is greatly improved by loading rhenium in addition to platinum and iridium. Further, as is clear from Comparative Examples 4 and 5, sufficient methane oxidation activity cannot be obtained with a combination of only platinum and rhenium containing no iridium. At this time, as in Comparative Examples 6 and 7, when palladium is used instead of iridium, sufficient catalytic activity cannot be obtained even if platinum and rhenium are used. In particular, when rhenium is used, the catalytic activity is lowered.
Following the above activity evaluation, while maintaining the catalyst layer temperature at 450 ° C., 3 ppm of sulfur dioxide was added to the reaction gas to continue the reaction. At each time point after 20 and 60 hours, the catalyst layer temperature was 350 ° C. The methane conversion at 400 ° C. and 450 ° C. was measured in the same manner. The results are shown in Table 2. Under any catalyst / reaction condition, the production of carbon dioxide corresponding to the decrease in methane concentration was confirmed, and methane was completely oxidized on the catalyst.

  The catalysts of Examples 1-8 showed a methane conversion of 34-44% at 350 ° C and 81-87% at 400 ° C at 20 hours. At 60 hours, the methane conversion was 34 to 45% at 350 ° C and 82 to 88% at 400 ° C. It can be seen that in the first 20 hours, the activity decreased due to poisoning due to sulfur content, but after that, the activity was extremely stable.

  In contrast, the catalysts of Comparative Examples 1 to 3 showed a methane conversion of 23 to 38% at 350 ° C. and 66 to 84% at 400 ° C. at 20 hours, and 19 to 34 at 350 ° C. at 60 hours. %, Methane conversion of 64-83% at 400 ° C. There is also a slight decrease in activity between 20 and 60 hours.

  Comparing the methane conversion after 20 hours and 60 hours, the activity levels of the catalyst of Example 1 and the catalyst of Comparative Example 3 are quite close. This indicates that 2 wt% of Ir can be substituted with a Re loading amount of 1 wt%, and is practically significant from the viewpoint of reducing the amount of rare metal used.

  The catalyst of Comparative Example 6 is a Pd—Pt / zirconium oxide catalyst disclosed in Patent Document 3. The catalyst of Comparative Example 7 is obtained by further adding Re to this catalyst. All the catalysts are less active than the catalyst of the present invention. In addition, the addition of Re significantly suppresses the initial activity of the Pd-Pt / zirconium oxide catalyst, contrary to the case of the present invention, and greatly improves the activity even after 20 to 60 hours. Is not seen. This result suggests that the activity improvement by the addition of Re is not observed in any catalyst, and in order to obtain high activity, the combination of three components of Pt-Ir-Re is essential. Yes.

[Activity evaluation test 2]
Each of the catalysts prepared in Examples 1 and 8 and Comparative Examples 1 and 3 was tableted and then crushed to have a particle size of 1 to 2 mm. 1.45 g (about 1.5 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% of oxygen, 10% of water vapor, 3 ppm of sulfur dioxide (both based on volume) and the balance nitrogen is fed into the reaction tube at a flow rate of 2 liters / minute (volume in the standard state). The change in methane conversion over time at a catalyst layer temperature of 400 ° C. was measured. 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.

  FIG. 1 shows the change over time in the methane conversion rate. It is clear that the catalyst of the example shows a higher and more stable methane conversion than the catalyst of the comparative example. The amount of decrease in methane conversion over 100 hours from 40 hours after the start of the test where the initial large activity fluctuations settle down to 140 hours was 1.7% and 3.3% for Examples 1 and 8 and Comparative Examples 1 and 3, respectively. 5.4% and 6.5%, indicating that the activity of the catalyst of the present invention is more stable.

[Activity evaluation test 3]
Each of the catalysts of Example 2 and Comparative Examples 2 and 3 was tableted and then crushed to have a particle size of 1 to 2 mm. 1.45 g (1.5 ml) of each catalyst was packed in a quartz reaction tube (inner diameter 14 mm). Next, a gas composed of 1,000 ppm methane, 20% oxygen, 3% water vapor (both based on volume) and the balance nitrogen is simulated at a flow rate of 2 liters / minute (volume in the standard state) simulating coal mine ventilation gas. The methane conversion at the catalyst layer temperatures of 300 ° C, 350 ° C and 400 ° C was measured (initial conversion). 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. Subsequently, the reaction was continued by adding 3 ppm of sulfur dioxide to the reaction gas at a catalyst layer temperature of 550 ° C., and methane at catalyst layer temperatures of 300 ° C., 350 ° C., and 400 ° C. at 20 and 60 hours later, respectively. Conversion was measured similarly.

  Table 3 shows the measurement results of methane conversion (%).

  It is clear that the activity of the catalyst of Example 2 is stable even when exposed to a relatively high temperature of 550 ° C., and maintains a high activity even when compared with the catalysts of Comparative Examples 2 and 3 that do not contain rhenium. is there.

Claims (2)

A catalyst for oxidizing and removing methane in a gas to be treated containing methane and excess oxygen, comprising platinum, iridium and rhenium supported on a zirconium oxide support ,
The amount of platinum supported is 0.5 to 20% by mass ratio to zirconium oxide,
The supported amount of iridium is 0.5 to 20% by mass ratio with respect to zirconium oxide,
The ratio of the supported amount of platinum and the iridium is 0.3 to 5 in terms of a mass ratio of Ir / Pt,
The amount of rhenium supported is 0.1 to 10% by mass ratio to zirconium oxide,
Platinum and 0.1-2 der Ru catalyst ratio of supporting amount of the rhenium in a weight ratio of Re / Pt.   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 at a temperature of 300 to 450 ° C.

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