Methane combustion catalyst and method for purifying combustion exhaust gas containing sulfur oxides

Abstract

Regarding a Pt / SnO₂-based methane combustion catalyst, a highly active and durable one is provided without additional loading of iridium or setting of an overcoat layer. 【Solution means】The present invention relates to a methane combustion catalyst for burning methane in combustion exhaust gas containing sulfur oxides, in which platinum and / or platinum oxide is supported on a tin oxide-based carrier containing tin oxide as an essential component. In the methane combustion catalyst according to the present invention, the above tin oxide-based carrier is composed of antimony-doped tin oxide (ATO). At this time, the doping amount of antimony in the tin oxide-based carrier is preferably 0.1% by mass or more and 5.0% by mass or less. In the present invention, antimony doped in tin oxide improves the oxygen supply ability of tin oxide to platinum or the like. Thereby, improvement in the activity and durability of the methane combustion catalyst is achieved.

Description

Technical Field

[0001] The present invention relates to a methane combustion catalyst for oxidatively removing methane in combustion exhaust gas containing sulfur oxides. Specifically, the present invention relates to a methane combustion catalyst that suppresses catalyst poisoning by sulfur oxides, has high activity and high durability, and is low-cost compared to the prior art. The present invention also relates to a method for purifying combustion exhaust gas for oxidatively removing methane using the catalyst.

Background Art

[0002] Exhaust gas generated from engines, boilers, etc. that use hydrocarbons such as natural gas, city gas, light oil, and kerosene as fuels contains nitrogen oxides (NOx), sulfur oxides (SO2, SOx), carbon monoxide (CO), odor substances, dust, etc., in addition to unburned hydrocarbons. Since these are all factors causing environmental pollution, the exhaust gas is treated with a filter or a purification catalyst before being discharged. In particular, methane has been reported to have a greenhouse effect more than 20 times that of carbon dioxide, and the need for its removal has become extremely high from the perspective of environmental protection.

[0003] As a catalyst for oxidatively removing methane in combustion exhaust gas containing sulfur oxides as described above, a methane combustion catalyst in which platinum is supported on a tin oxide carrier (SnO2 carrier) made of tin oxide is known (Patent Document 1). The reason why a tin oxide carrier is applied to the methane combustion catalyst is that alumina carriers, zirconia carriers, etc., which are inorganic oxides that have been used as catalysts for hydrocarbon purification and the like for a long time, are not effective for oxidatively removing methane. In particular, for the treatment of exhaust gas containing sulfur oxides, the activity of a catalyst using an alumina carrier or the like decreases significantly due to catalyst poisoning.

[0004] Improvements have also been made to methane combustion catalysts that use tin oxide supports, and in particular, the effectiveness of a methane combustion catalyst that supports iridium in addition to platinum has been reported (Patent Document 2). The additional support of iridium improves the durability of this methane combustion catalyst against catalyst poisoning by sulfur oxides. In the following explanation, methane combustion catalysts that support platinum or the like on a tin oxide support may be referred to as Pt / SnO2 catalysts or Pt-Ir / SnO2 catalysts. These may also be collectively referred to as Pt / SnO2-based catalysts.

[0005] The applicant of the present application has disclosed a methane combustion catalyst that further improves the activity and durability of the existing Pt / SnO2-based catalyst (Patent Documents 3 and 4). In making this improvement, the applicant of the present application has studied the activation mechanism of the Pt / SnO2-based catalyst for methane combustion. According to this study, the methane combustion activity of the Pt / SnO2-based catalyst requires that platinum is in the form of an oxide (PtO or PtO2).

[0006] They also speculate that the cause of catalyst poisoning by sulfur oxides in Pt / SnO2 catalysts is the alteration of catalyst particles by sulfur oxides. Specifically, sulfur oxides such as SO2 that fly near platinum oxide catalyst particles reduce the platinum oxide, which is easily oxidized, and remove oxygen, becoming SO3 and other compounds that are released. Platinum particles that are reduced and metallized by sulfur oxides such as SO2 sinter with the surrounding metallized platinum particles to form coarse platinum particles with low activity. Catalyst poisoning in Pt / SnO2 catalysts is caused by the above-mentioned metallization of platinum oxides and sintering of platinum particles, and as long as these continue, the catalytic activity will continue to decrease.

[0007] The above mechanism of catalyst poisoning is supported by the fact that additional loading of iridium in the Pt / SnO2-based catalyst improves durability. According to the applicant of the present application, in the Pt-Ir / SnO2 catalyst, iridium exists as an oxide (IrO2) like platinum, and this iridium oxide has a function of maintaining the oxidation state of platinum by supplying oxygen to the metallized platinum. And, the iridium oxide is metallized by reduction due to oxygen supply to platinum and reduction by sulfur oxides, but it is possible to maintain the state of iridium oxide by receiving oxygen supply from the tin oxide support. Such a cycle of reduction (oxygen supply to platinum) and oxidation (oxygen acceptance from the tin oxide support) of iridium oxide causes a cycle of reduction and oxidation in platinum oxide, and by avoiding the state of metallic platinum that is prone to sintering, it contributes to maintaining activity and improving durability.

[0008] Based on the above considerations, the applicant of the present application has disclosed two types of methane combustion catalysts composed of Pt / SnO2-based catalysts that can improve methane combustion activity and durability (Patent Documents 3 and 4). In the first methane combustion catalyst by the applicant of the present application, considering that the methane combustion activity in the Pt / SnO2-based catalyst is exerted due to the presence of platinum oxide, while optimizing the manufacturing process (the process of loading platinum), the ratio of platinum oxide on the support is increased, and the above-described reduction-oxidation cycle is utilized more effectively (Patent Document 3). Further, in the second methane combustion catalyst by the applicant of the present application, a region mainly composed of tin oxide without platinum is formed as an overcoat layer on the surface of the Pt-Ir / SnO2-based catalyst, and it is supplied to platinum oxide from this overcoat layer, and the above-described reduction-oxidation cycle is utilized more effectively (Patent Document 4).

[0009] These methane combustion catalysts by the applicant of the present application optimize the amount and state of platinum oxide on the tin oxide support while applying an appropriate oxygen supply source (iridium, overcoat layer), thereby efficiently promoting the cycle of reduction and oxidation of platinum oxide and improving activity and durability.

Prior Art Documents

Patent Documents

[0010] Patent Document 1 Specification of Japanese Patent No. 4283037 Patent Document 2 Specification of Japanese Patent No. 4429950 Patent Document 3 Specification of Japanese Patent No. 6883135 Patent Document 4 Specification of Japanese Patent No. 7038269 Summary of the Invention Problems to be Solved by the Invention

[0011] The Pt / SnO₂-based methane combustion catalysts of Patent Documents 3 and 4 by the applicant of the present application have all been confirmed to be highly active compared with conventional Pt / SnO₂-based catalysts, and can achieve a lower reaction temperature and improved durability. Therefore, it can be said that it is an extremely good methane combustion catalyst in terms of performance, but there are also points to be improved. That is the improvement requirement in terms of manufacturing cost. That is, not limited to the methane combustion catalysts of Patent Documents 3 and 4, but in conventional Pt / SnO₂-based catalysts, considering the above-described mechanism of catalyst poisoning, additional loading of iridium is considered essential for improving durability. Since iridium is a noble metal like platinum, it is a factor in increasing the cost of the methane combustion catalyst.

[0012] Also, regarding the methane combustion catalyst of Patent Document 4, the formation of the overcoat layer adds one step to the manufacturing process of the methane combustion catalyst. Such an increase in the number of steps is reflected in the manufacturing efficiency and cost of the catalyst. Furthermore, regarding platinum (platinum oxide), which is an active species for methane combustion, there is also a demand to reduce the loading amount as much as possible while maintaining the activity in order to reduce the cost.

[0013] The present invention has been made under the above background, and with respect to the above-described Pt / SnO₂-based methane combustion catalyst, an object thereof is to provide a catalyst that is low-cost, highly active, and has excellent durability compared to the prior art. Means for Solving the Problems

[0014] In the conventional Pt / SnO₂-based methane combustion catalyst, the additionally supported iridium or overcoat layer is an oxygen source for promoting the cycle progress of the reduction and oxidation of platinum oxide. While considering the poisoning mechanism of the above-mentioned Pt / SnO₂-based methane combustion catalyst, the present inventors decided to seek an oxygen source alternative to iridium or the overcoat layer in the carrier. The carrier has a configuration that is most closely related to platinum (platinum oxide), which is a catalyst component. And in the consideration of the applicant of the present application above, the tin oxide carrier also acts as an oxygen source and acts on the reduction and oxidation cycle of iridium (iridium oxide). The present inventors considered that by increasing the mobility of oxygen contained in tin oxide, it can function as a more efficient oxygen source and promote the above cycle in platinum (platinum oxide).

[0015] The promotion of the reduction-oxidation cycle of platinum (platinum oxide) gives freedom in setting the abundance ratio of platinum oxide. If the reduction-oxidation cycle of platinum (platinum oxide) is promoted more than before, even if the abundance ratio of metallic platinum is large, it will quickly become an oxide in the atmosphere of the methane combustion reaction and sintering is less likely to occur. If the reduction and oxidation cycles of platinum-platinum oxide proceed optimally, it is not necessary to increase the ratio of platinum oxide as in the prior art, and it is expected that the required methane combustion activity can be exhibited even if the supported amount of platinum is reduced. This reduction in the supported amount of platinum also contributes to a decrease in the catalyst cost.

[0016] As a result of intensive studies based on the above considerations, the present inventors found that as a method for enhancing the oxygen supply ability of the carrier, it is preferable to apply a tin oxide-based carrier containing other components while using tin oxide as an essential component as in the prior art. Specifically, the present inventors found that it is effective to use a tin oxide-based carrier composed of antimony-doped tin oxide (ATO) in which antimony (Sb) is doped into tin oxide as a catalyst carrier, and thus arrived at the present invention.

[0017] That is, the present invention for solving the above problems is a methane combustion catalyst for burning methane in combustion exhaust gas containing sulfur oxides, which comprises a tin oxide-based carrier containing tin oxide as an essential component, on which platinum and / or platinum oxide is supported, and the tin oxide-based carrier is composed of antimony-doped tin oxide.

[0018] Hereinafter, the present invention will be described in more detail. The methane combustion catalyst, which is a Pt / SnO2-based catalyst according to the present invention, has, as its basic configuration, antimony-doped tin oxide as a tin oxide-based carrier, and platinum (platinum oxide), which is a catalyst component, is supported thereon. In the following description, each component of the catalyst will be described, and the manufacturing method of the catalyst will be described in detail. In the present specification, a carrier containing tin oxide as an essential component is referred to as a tin oxide-based carrier. Further, among the tin oxide-based carriers, a carrier composed only of tin oxide is referred to as a tin oxide carrier.

[0019] (A) Configuration of the methane combustion catalyst according to the present invention (A-1) Catalyst carrier (antimony-doped tin oxide) As described above, the methane combustion catalyst according to the present invention is characterized in that antimony-doped tin oxide, which is antimony-doped tin oxide, is applied as the tin oxide-based carrier. By doping antimony into tin oxide having a rutile-type crystal structure, the mobility (ease of movement) of interstitial oxygen increases. As a result, the efficiency of oxygen supply from the carrier increases, and the rate of the reduction-oxidation cycle of platinum oxide during the methane oxidation reaction can be increased. Such high-efficiency oxygen supply can eliminate the need for oxygen supply by iridium or an overcoat layer. Further, according to the present invention, high-efficiency methane combustion activity can be exhibited with respect to a relatively small supported amount of platinum. This contributes to cost reduction of the methane combustion catalyst in combination with eliminating iridium loading or the like.

[0020] Antimony-doped tin oxide has a state in which antimony atoms, which are dopants, are incorporated into tin oxide (SnO2) crystals and the antimony atoms are substituted for Sn sites. In the present invention, antimony is adopted as the dopant element for the SnO2 carrier because antimony is particularly effective in increasing the oxygen supply efficiency described above and is considered to be effective in improving both the initial activity and durability of the catalyst. Note that the valence of the antimony doped into tin oxide is not limited. Antimony has a valence of trivalent (Sb 3+ ), tetravalent (Sb 4+ ), or pentavalent (Sb 5+ ), and any valence of antimony may be doped, or a plurality of antimony with different valences may be doped.

[0021] And the above effects in the present invention are exhibited by antimony-doped tin oxide, and the above effects are not exhibited in a simple mixture of tin oxide and an antimony compound (such as antimony oxide). The increase in oxygen supply efficiency is exhibited in the tin oxide-based carrier because of the change in its crystal structure. And the above effects are not exhibited in a mixture that does not affect the crystal structure of tin oxide. Therefore, although the mixture of tin oxide and an antimony compound becomes a tin oxide-based carrier in terms of literal interpretation, it does not become the carrier of the present invention.

[0022] That the carrier applied in the catalyst of the present invention is a tin oxide-based carrier and is antimony-doped tin oxide can be confirmed by performing both compositional analysis and structural analysis on the entire carrier or catalyst. That is, in compositional analysis such as ICP (inductively coupled plasma optical emission spectrometry), antimony is detected together with tin from the doped tin oxide. On the other hand, in structural analysis such as XRD (X-ray diffraction), only the peaks derived from tin oxide are confirmed in the diffraction pattern of the doped tin oxide, and no peaks derived from antimony compounds are observed. However, a peak shift of the peaks derived from tin oxide may be observed in the doped tin oxide. Thus, when compositional analysis and structural analysis are performed on the entire carrier or catalyst, the presence of antimony is confirmed, but when the presence of antimony compounds is not confirmed, it is confirmed that the carrier is antimony-doped tin oxide. In addition, in a mixture of tin oxide, peaks derived from the mixed antimony compound appear in the XRD diffraction pattern, so the presence of antimony is detected by both compositional analysis and structural analysis.

[0023] The doping amount of antimony in the antimony-doped tin oxide as the carrier is preferably 0.1% by mass or more and 5.0% by mass or less in terms of antimony relative to the mass of the tin oxide-based carrier. In a tin oxide-based carrier with a doping amount of less than 0.1% by mass, the above-described effects are insufficient. And antimony doped into tin oxide has no effect of improving the catalytic activity, while tin oxide has an effect of imparting methane combustion activity to the catalyst component. Therefore, when the doping amount of antimony becomes excessive, there is a risk of activity reduction, so the doping amount is preferably 5.0% by mass or less. The doping amount of the dopant element can be measured by the above-described compositional analysis (ICP, etc.). Also, as described above, since the present invention defines the doping amount for the tin oxide-based carrier, when analyzing the methane combustion catalyst to measure and calculate the doping amount, it is preferable to exclude the mass of the constituent components other than tin oxide and antimony (such as platinum (platinum oxide) and the constituent materials of the honeycomb support) and calculate the doping amount.

[0024] In addition, the above-mentioned doping amount is preferably within the above range as the average value of the entire carrier. Although the method for producing the doped tin oxide will be described later, the carrier of the present invention can be composed of a single antimony-doped tin oxide having an equal doping amount. Further, it may be formed by mixing antimony-doped tin oxides having different doping amounts.

[0025] The form of the tin oxide-based carrier composed of the antimony-doped tin oxide of the present invention is adopted according to the form of the methane combustion catalyst. Here, as the form of the methane combustion catalyst, it can be any of granular, particulate, pellet-shaped, and tablet-shaped. In such a methane combustion catalyst, the tin oxide-based carrier also has the same granular, particulate, pellet-shaped, and tablet-shaped forms. In such a tin oxide-based carrier, the specific surface area is 10 m 2 / g or more and 60 m 2 / g or less, more preferably 20 m 2 / g or more and 40 m 2 / g or less.

[0026] In addition, as the form of the gas-phase purification catalyst such as the methane combustion catalyst, there are many application examples in a state where the methane combustion catalyst is supported on an appropriate support. As the support at this time, a support having any of plate shape, cylindrical shape, spherical shape, and honeycomb shape is known. In such a catalyst, the tin oxide-based carrier is applied and coated on the support as a so-called wash coat to form the tin oxide-based carrier.

[0027] When the tin oxide-based carrier is applied to the support in this form, the preferable amount of the tin oxide-based carrier is preferably 250 g / L or more and 400 g / L or less based on the volume of the support. If the amount of the carrier is less than 250 g / L, the dispersibility of platinum may decrease and sufficient methane combustion may be difficult. On the other hand, if the amount of the carrier exceeds 400 g / L, a region where the processing gas is not in contact is formed, and in this case as well, the efficiency of methane combustion decreases. The specific surface area of the tin oxide-based carrier in such a form is also preferably the same as the specific surface area of the above-mentioned pellet-shaped carrier or the like.

[0028] (A-2) Catalyst component (platinum and / or platinum oxide) The catalyst component of the methane combustion catalyst according to the present invention is platinum and / or platinum oxide, and the requirement of using these as essential components is the same as that of conventional methane combustion catalysts. In the present invention, the supported amount of platinum and / or platinum oxide as the catalyst component is preferably 1.0% by mass or more and 10.0% by mass or less in terms of metallic platinum based on the whole catalyst. In the present invention, by applying antimony-doped tin oxide as the carrier, efficient utilization of platinum (oxide) can be achieved, and desired methane combustion activity can be obtained even when the platinum supported amount is relatively low. The supported amount of platinum is more preferably 1.0% by mass or more and 6.0% by mass or less.

[0029] Also, the catalyst component supported on the tin oxide-based carrier is platinum and / or platinum oxide, and it is not necessary that all of the catalyst components are metallic platinum or platinum oxide. In the manufacturing process of the methane combustion catalyst according to the present invention, after impregnating and supporting a platinum compound on the carrier, heat treatment such as drying and firing is performed, so at least a part of the supported platinum can become platinum oxide. Therefore, it is not necessary to limit the catalyst component on the tin oxide-based carrier to either platinum or platinum oxide.

[0030] Regarding the ratio of platinum oxide (metallic platinum) in the catalyst component, as described above, in the prior art (Patent Document 3), considering the behavior of platinum (platinum oxide) during methane combustion, it is considered preferable to increase the ratio of platinum oxide on the carrier. However, in the present invention, it is not necessary to increase the ratio of platinum oxide. In the present invention, applying a tin oxide-based carrier with enhanced oxygen supply source ability, metallic platinum on the carrier is rapidly converted into platinum oxide during the methane combustion reaction. Then, by the efficient progress of the oxidation-reduction cycle by the tin oxide-based carrier, activity and durability can be ensured.

[0031] Rather, in the methane combustion catalyst according to the present invention, it can be said that it is preferable that the ratio of metallic platinum in the catalyst component is higher in the state before methane combustion. This is because, originally, the catalytic activity for the methane combustion reaction should be considered to be exhibited in the state of metallic platinum. And by increasing the ratio of metallic platinum, it is considered that methane combustion activity can be ensured with a small amount of platinum.

[0032] When increasing the ratio of metallic platinum on the tin oxide-based carrier in the present invention, the ratio can be determined in the same manner as the determination method in the above prior art (Patent Document 3). In the above prior art, the platinum oxidation state is based on the analysis results by X-ray photoelectron spectroscopy (XPS), and the abundance ratios R of platinum in the states of metallic platinum and platinum oxides (divalent, tetravalent) Pt , R PtO , R PtO2 are used to specify the ratio of total platinum oxides (R TO ) (R TO =(R PtO +R PtO2 ) / R Pt ). And in the prior art, it is considered that the higher the ratio of platinum oxides, the higher the ratio of platinum atoms in the oxidized state.

[0033] Also in the methane combustion catalyst according to the present invention, based on the analysis results by XPS, the abundance ratios R of metallic platinum and platinum oxides Pt , R PtO、 R PtO2 (all in units of %) can be obtained. And in the present invention, a catalyst with a higher abundance ratio R of metallic platinum Pt is considered to be a suitable catalyst. Specifically, it is a methane combustion catalyst preferably having R Pt of 70% or more.

[0034] The abundance ratio R of metallic platinum PtIt can be specified from the detection intensity at the binding energy corresponding to each binding state based on the platinum 4f (Pt4f) spectrum observed when the catalyst is analyzed by XPS. At this time, in the platinum 4f spectrum by XPS, the peak of metallic Pt appears within the range of 71.0 eV to 72.0 eV, the peak of PtO appears within the range of 72.8 eV to 73.2 eV, and the peak of PtO2 appears within the range of 74.6 eV to 75.0 eV. The abundance ratio R Pt of, R PtO、 R PtO2 is calculated from the peak area of each state.

[0035] Incidentally, there is no particular need to specify the upper limit of the abundance ratio R Pt of metallic platinum. However, in the manufacturing process of the methane combustion catalyst, various heat treatment processes are required after supporting metallic platinum, and it is difficult to avoid the formation of platinum oxides by these processes. Also, the presence of a certain amount of platinum oxide is considered to lead to the suppression of the sintering of metallic platinum. Considering these, the upper limit of the abundance ratio R Pt of metallic platinum is preferably set to 90%.

[0036] As described above, the catalyst component of the methane combustion catalyst of the present invention is platinum and / or platinum oxide. And in the present invention, iridium, which has been an essentially essential catalyst component in conventional methane combustion catalysts, is unnecessary. This is because antimony-doped tin oxide or the like acts as an oxygen supply source instead of iridium, which is an oxygen supply source in the prior art. Furthermore, an additional structure such as an overcoat layer is also unnecessary. However, additional loading of iridium and formation of an overcoat layer are not prohibited for the methane combustion catalyst according to the present invention.

[0037] (B) Manufacturing method of the methane combustion catalyst according to the present invention Next, a method for manufacturing a methane combustion catalyst according to the present invention will be described. The production of the methane combustion catalyst according to the present invention can basically apply the conventional impregnation method for noble metal catalysts, and the catalyst can be produced by supporting platinum on a tin oxide-based carrier composed of antimony-doped tin oxide. The impregnation method is a method of depositing a noble metal by impregnating a solution of a noble metal salt (noble metal compound) to be supported on a carrier and then performing a firing heat treatment. Hereinafter, the manufacturing process of the methane combustion catalyst of the present invention by the impregnation method will be described.

[0038] (B-1) Pre-step for platinum loading (preparation of doped tin oxide) The methane combustion catalyst according to the present invention uses antimony-doped tin oxide as a carrier. As methods for manufacturing antimony-doped tin oxide, there are several known methods, and well-known methods include the coprecipitation method and the firing method.

[0039] For example, in the method for manufacturing antimony-doped tin oxide by the coprecipitation method, a tin compound and an antimony compound are used as raw materials, and tin hydroxide and antimony hydroxide are coprecipitated from a mixed solution thereof, and the coprecipitated hydroxide is fired to obtain antimony-doped tin oxide. As the tin compound and antimony compound used as raw materials, chlorides (tin chloride (SnCl4), antimony chloride (SbCl3, SbCl4, SbCl5)) are often used. In addition, water is generally used as the solvent for the mixed solution of these compounds, and a hydroxide is coprecipitated by allowing an alkaline solution (sodium hydroxide solution, potassium hydroxide solution, sodium carbonate solution, potassium carbonate solution, etc.) to act on the mixed aqueous solution. Incidentally, the doping amount of antimony can be adjusted by the mixing ratio of the tin compound and the antimony compound when producing the above-mentioned mixed solution.

[0040] In the method for manufacturing antimony-doped tin oxide by the firing method, tin oxide (SnO2) and antimony oxide (Sb2O3, SbO2, Sb2O5) are mixed and fired at a high temperature to obtain antimony-doped tin oxide. The doping amount of antimony can be adjusted by the mixing ratio at the time of mixing the oxides.

[0041] The antimony-doped tin oxide used in the present invention can be obtained by any of the above methods. Also, commercially available antimony-doped tin oxides are relatively easily available, and any of them may be used.

[0042] As a manufacturing process of the methane combustion catalyst, platinum may be supported using the antimony-doped tin oxide manufactured and prepared above as a carrier as it is. Also, when supporting the methane combustion catalyst on a support such as a honeycomb, it is preferable to prepare a carrier slurry by slurrying the powder of antimony-doped tin oxide before supporting platinum and apply this to the support. The carrier slurry can be prepared by mixing a binder using water or an organic solvent as a dispersion medium with antimony-doped tin oxide. For applying the carrier slurry to the support, various known methods such as air blowing, spraying, dipping, etc. can be applied.

[0043] (B-2) Platinum supporting step The supporting of platinum on the tin oxide-based carrier is carried out through impregnation with a platinum salt solution and firing heat treatment. As the platinum salt solution to be impregnated into the carrier, in addition to an aqueous solution of platinum nitrate, an aqueous solution of platinum chloride, an aqueous solution of platinum acetate, there are also an aqueous solution of tetraammineplatinum salt which is a platinum complex solution, a dinitrodiammineplatinum-nitric acid aqueous solution, a dinitrodiammineplatinum-ammonia aqueous solution, a dinitrodiammineplatinum-ethanolamine solution, etc. There is no particular limitation on the method of impregnating the platinum salt solution into the tin oxide-based carrier, and any of spraying, dropping, dipping may be used. The supported amount of platinum in the methane combustion catalyst can be adjusted by the platinum concentration of the platinum salt solution and the amount of liquid to be impregnated.

[0044] After impregnation with the platinum salt solution, after appropriately drying, a firing step is carried out. The firing step is a step of decomposing the platinum salt to deposit platinum which becomes the active source of methane combustion. The firing temperature in the firing step is preferably 350°C or higher and 550°C or lower. If it is less than 350°C, the generation of platinum becomes insufficient. And if it exceeds 550°C, there is a possibility of platinum aggregation. The treatment time in the firing step is preferably 1 hour or more and 5 hours or less. The atmosphere in the firing step is not particularly limited as long as it is an oxidative atmosphere such as air.

[0045] Through the above firing process, the methane combustion catalyst according to the present invention is manufactured. In the prior art (Patent Document 3), as a means for increasing the ratio R of platinum oxide in the catalyst, in order to obtain a desired platinum loading amount, application of split loading by repeating the impregnation process and drying of a platinum salt solution, and strict control of the drying temperature are carried out. However, such treatment is unnecessary for the methane combustion catalyst according to the present invention. In the methane combustion catalyst according to the present invention, by using antimony-doped tin oxide as a carrier that acts as an oxygen supply source, the ratio R of platinum oxide can be made appropriate. TO In addition, in the present invention, since the platinum loading amount can be reduced by optimizing the tin oxide-based carrier, the required amount of platinum can be supported without relying on split loading. TO

[0046] (C) Method for combusting methane using the methane combustion catalyst according to the present invention The method for combusting methane using the methane combustion catalyst according to the present invention described above is basically the same as the conventional method. The target in the method for combusting methane according to the present invention is combustion exhaust gas containing sulfur oxides together with methane. Further, in addition to methane and sulfur oxides, other combustible components such as other hydrocarbons such as ethane and propane, carbon monoxide, oxygen, oxygen-containing compounds, and nitrogen oxides may be included.

[0047] In the combustion of methane in the combustion exhaust gas, the gas to be treated is passed through a combustion apparatus equipped with the methane combustion catalyst according to the present invention to bring it into contact with the methane combustion catalyst. As the combustion apparatus, a known one can be applied. For example, a fixed-bed flow-type reaction apparatus or the like can be applied. The amount of catalyst used in such a combustion apparatus is generally set by the space velocity per unit time of gas (GHSV). In the present invention, the space velocity is preferably 100,000 h-1 or less in order to ensure the combustion rate of methane. By lowering the space velocity, the catalyst activity is improved, so the lower the space velocity, the more preferable. However, considering the catalyst activity, economy, and pressure loss, the space velocity is preferably 1,000 h-1 or more. -1 -1

[0048] The heating temperature of the methane combustion catalyst for purifying combustion exhaust gas, that is, the reaction temperature, is set to be 300°C or higher and 500°C or lower. The reaction temperature is more preferably 350°C or higher and 475°C or lower.

Advantages of the Invention

[0049] As described above, in the Pt / SnO2-based methane combustion catalyst according to the present invention, by applying a tin oxide-based carrier with a high oxygen supply ability as a carrier, the cycle of reduction (metallization) and oxidation (oxidation) of platinum oxide, which is the active source, can be promoted, and the activity decrease due to sintering of metallized platinum can be suppressed. In the prior art, the cycle was promoted by additional loading of iridium or formation of an overcoat layer, but these are not required in the present invention. Thereby, the present invention can provide a methane combustion catalyst with high activity and high durability while achieving cost reduction.

Brief Description of the Drawings

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Mode for Carrying Out the Invention

[0051] First Embodiment : A preferred embodiment of the present invention will be described. In this embodiment, a methane combustion catalyst in which platinum is supported on a tin oxide-based carrier doped with antimony (antimony pentoxide (Sb2O5)) was produced. Then, by evaluating the methane combustion activity, the effect of doping antimony in the tin oxide-based carrier was confirmed.

[0052] [Preparation of Antimony-Doped Tin Oxide Powder (Tin Oxide-Based Carrier)] An aqueous solution of tin chloride (amount of tin chloride: 234 g) and an aqueous solution of antimony chloride (amount of antimony chloride: 21.6 g) were added to 5 L of water. While maintaining this mixed solution at 80°C, an aqueous solution of sodium hydroxide was added so that the pH became 6 to 7, and the reaction was carried out for 1 hour in this state to form a coprecipitate of tin oxide hydrate and antimony oxide hydrate. Then, this coprecipitate was filtered, recovered, and washed. Thereafter, the coprecipitate was calcined at 550°C for 3 hours. The calcined product was crushed and pulverized to obtain antimony-doped tin oxide.

[0053] The antimony content of the antimony-doped tin oxide produced above was analyzed by ICP (Agilent 5800 manufactured by Agilent Technologies Co., Ltd.) (selected wavelength: Sb = 217.582 nm). As a result, the doping amount of antimony in the antimony-doped tin oxide was 0.97 mass%.

[0054] [Platinum Loading Step] Next, the antimony-doped tin oxide powder produced above was dispersed in water and mixed and pulverized with a ball mill to produce a carrier slurry. This carrier slurry was applied by air blowing onto a commercially available cordierite honeycomb (manufactured by NGK Insulators, Ltd.: φ30 mm × 12.5 mm L, 400 cells). Then, it was fired at 600 °C in the atmosphere to obtain a honeycomb-shaped carrier (carrier specific surface area: 12 m 2 / g). The coating amount of the antimony-doped tin oxide powder on the honeycomb at this time is 300 g / L.

[0055] The above honeycomb-shaped carrier was impregnated with a dinitrodiammineplatinum-nitric acid aqueous solution as a platinum salt solution by air blowing. In this embodiment, the platinum loading amount was 3.23 mass% in terms of metallic platinum (excluding the mass of the honeycomb support).

[0056] After impregnation with the platinum salt solution, a firing treatment was performed through a drying process. In the drying process, the honeycomb-shaped carrier after solution impregnation was put into a dryer maintained at 110 °C and held for 30 minutes. Then, the dried honeycomb-shaped carrier was fired. The firing conditions were firing at 450 °C for 1 hour in the atmosphere. The methane combustion catalyst of this embodiment was obtained through the above steps (Example 1). When ICP analysis was performed on the produced methane combustion catalyst and the antimony content in the tin oxide-based carrier was calculated, it was 0.97 mass%, which was the same as that during carrier production.

[0057] Comparative Example 1, Comparative Example 2 : To compare with the methane combustion catalyst of Example 1, a tin oxide carrier made of SnO2 powder not doped with antimony was prepared as a carrier (Comparative Example 1). Also, a carrier made of a mixed powder of SnO2 powder and Sb2O5 powder was produced (Comparative Example 2). Then, methane combustion catalysts were produced from the above tin oxide carrier and mixed powder carrier. Note that the SnO2 powder of Comparative Example 1 used commercially available SnO2 powder. Also, the mixed powder of Comparative Example 2 used a physical mixture of commercially available SnO2 powder and commercially available Sb2O5 powder with a mass ratio of SnO2:Sb2O5 = 99:1 milled.

[0058] Regarding the methane combustion catalysts of Comparative Examples 1 and 2, carrier slurries were produced in the same manner as in Example 1, coated on cordierite honeycombs, and fired to obtain honeycomb-shaped carriers. Further, after impregnating with a dinitrodiammineplatinum-ammonia aqueous solution under the same conditions as in Example 1, it was dried and fired to obtain a methane combustion catalyst. The platinum loading was also 3.23% by mass, the same as in Example 1.

[0059] [XRD Analysis] XRD analysis was performed on the methane combustion catalyst produced in this embodiment to confirm the state of antimony in the tin oxide-based carrier of the methane combustion catalyst of Example 1. For the XRD analysis, UltimaIV manufactured by Rigaku Corporation was used as the analyzer, and the X-ray source was Cukα ray, and the measurement was performed in the range of 2θ = 20° to 90°.

[0060] Figure 1 shows the XRD diffraction patterns of the methane combustion catalysts of Example 1, Comparative Example 1, and Comparative Example 2. The diffraction patterns of the methane combustion catalyst of Example 1 with antimony-doped tin oxide powder as the carrier and the methane combustion catalyst of Comparative Example 1 with tin oxide powder without antimony doping as the carrier are substantially the same, and diffraction peaks of SnO2 are observed at around 2θ = 26.5° and around 34°. On the other hand, for the methane combustion catalyst of Comparative Example 2 with a mixed powder of SnO2 powder and Sb2O5 powder as the carrier, the basic diffraction pattern is the same as that of Example 1 and Comparative Example 1, but in addition to the diffraction peak of SnO2, a diffraction peak derived from Sb2O5 is observed at around 2θ = 29°. From the results of this XRD analysis and the above-mentioned ICP analysis results, it can be confirmed that the carrier of the methane combustion catalyst of Example 1 is a tin oxide-based carrier composed of antimony-doped tin oxide in which antimony is solid-solved in tin oxide.

[0061] Evaluation Test of Methane Combustion Performance Tests were conducted to evaluate the methane combustion performance using each of the methane combustion catalysts manufactured above. In this evaluation test, each catalyst was set in a test apparatus simulating the fixed-bed reactor shown in Fig. 2, and the test gas was passed through to measure the methane conversion rate. The evaluation test of this embodiment is a constant-temperature test in which the inlet gas temperature, which is the reaction temperature (catalyst temperature), is set to a constant temperature, and the conversion rate at that reaction temperature is evaluated. The test conditions were as follows. · Reaction temperature: 400 °C · Test gas composition CH4: 2000 ppm CO2: 5% O2: 10% H2O: 10% SO2: 3 ppm N2: Balance · Space velocity (GHSV): 100,000 h -1 · Test time: 24 hours

[0062] In the evaluation test, the test gas was passed through the catalyst under the above conditions, and the composition of the exhaust gas was analyzed at 5-hour intervals from the start of the test to measure the methane conversion rate. The measurement of the methane conversion rate was performed by analyzing the CH4, CO2, and O2 concentrations in the exhaust gas using an FID-type THC gas analyzer, a non-dispersive infrared analyzer, and a magnetic oxygen meter. Then, the methane conversion rate was calculated from the measured values using the following formula.

[0063]

Equation

[0064] As a result of this evaluation test, a graph showing the change over time in the methane conversion rate of each methane combustion catalyst is shown in FIG. 3. Comparing the catalyst with antimony-doped tin oxide as the carrier in Example 1 and the catalyst in Comparative Example 1 with tin oxide without antimony doping as the carrier, the methane conversion rate at the start of the test is about 10% higher in Example 1. And, in the methane combustion catalyst of Comparative Example 1, a tendency for the methane conversion rate to decrease is observed as the test time elapses, but in the methane combustion catalyst of Example 1, no significant decrease in the methane conversion rate is observed. As a result, a large difference in the methane conversion rate occurred between Example 1 and Comparative Example 1 at the stage after 24 hours had elapsed. From these results, it can be seen that doping antimony into tin oxide improves the initial methane combustion activity and maintains the activity, clearly improving the durability.

[0065] Also, when comparing Example 1 and Comparative Example 2, it can be seen that the antimony in the tin oxide-based carrier needs to be in a doped (solid solution) state as the existence form of antimony. It can be seen that the desired performance is not exhibited in the state where tin oxide and antimony oxide in Comparative Example 2 are simply mixed. The methane combustion catalyst using the mixed powder of Comparative Example 2 as the carrier has higher initial activity and durability than Comparative Example 1, but the difference is slight. And, there is a large difference from the methane combustion in Example 1.

[0066] Second Embodiment : In this embodiment, for a methane combustion catalyst with antimony-doped tin oxide as the carrier, a plurality of catalysts with different doping amounts of antimony in the tin oxide-based carrier were produced. And, the activity evaluation of those methane combustion catalysts was carried out. Also, the state of platinum (the ratio of metallic platinum) etc. of the methane combustion catalyst produced by each tin oxide-based carrier was examined.

[0067] [Preparation of Tin Oxide-Based Carrier] The antimony-doped tin oxide-based carrier was produced by the coprecipitation method as in the first embodiment. In this embodiment, the mixing ratio of tin chloride and antimony chloride during the production of antimony-doped tin oxide was changed to produce a plurality of types of antimony-doped tin oxide powders with different antimony doping amounts (these are referred to as Example 2, Example 3, Example 5, and Reference Example 1). Also, the antimony-doped tin oxide-based carrier of Example 1 of the first embodiment was examined. Furthermore, an antimony-doped tin oxide powder obtained by uniformly mixing the antimony-doped tin oxide powder of Example 5 and the antimony-doped tin oxide powder of Example 1 (first embodiment) of the antimony-doped tin oxide powders prepared in this embodiment in an equal amount by ball milling was also examined (this is referred to as Example 4). The antimony doping amounts of the antimony-doped tin oxides of Example 1 to Example 5 and Reference Example 1 are as follows.

[0068] [Table 1]

[0069] [Production of Methane Combustion Catalyst] Using the antimony-doped tin oxide powders of the above Examples 1 to 5 and Reference Example 1, a carrier slurry was produced in the same manner as in the first embodiment. Then, the carrier slurry was coated on the same cordierite honeycomb as in the first embodiment and fired to obtain a honeycomb-shaped carrier. Then, in the same manner as in the first embodiment, the honeycomb-shaped carrier was impregnated with a dinitrodiammineplatinum-nitrate aqueous solution, dried, and fired to obtain a methane combustion catalyst. In this embodiment, the platinum loading amount was 3.23% by mass in terms of platinum.

[0070] When ICP analysis was performed on the methane combustion catalyst produced above in the same manner as in the first embodiment, there was almost no change in the antimony doping amount in the carrier (antimony-doped tin oxide) in each methane combustion catalyst.

[0071] Evaluation Test of Methane Combustion Performance Evaluation tests of the methane combustion performance were conducted on the methane combustion catalysts of Examples 1 to 5 and Reference Example 1 manufactured in this embodiment. The evaluation apparatus for this evaluation test is the same as that of the first embodiment. In this embodiment, in addition to the isothermal test similar to the first embodiment, two types of tests, an isothermal test and a temperature-rising test in which the methane conversion rate was measured while raising the reaction temperature, were conducted. The reaction conditions and test methods in the isothermal test are the same as those of the first embodiment (only the test time was set to 50 hours).

[0072] In the temperature-rising test, the inlet gas temperature was raised from 300 °C to 500 °C at a rate of 10 °C / min, and the methane conversion rate was measured. The test gas composition and space velocity in the temperature-rising test are the same as those of the first embodiment. The measurement of the methane conversion rate in the temperature-rising test was calculated by continuously analyzing the composition of the exhaust gas. The temperature-rising test was conducted on two types of catalysts: a fresh catalyst immediately after production and a post-durability catalyst after heating the catalyst in air at 500 °C for 24 hours.

[0073] As a result of the methane combustion test in this embodiment, the results of the isothermal test are shown in FIG. 4, and the results of the temperature-rising test are shown in FIG. 5.

[0074] Referring to the results of the isothermal test in FIG. 4, in the methane combustion catalysts of Example 1 (antimony doping amount: 0.97% by mass), Example 2 (antimony doping amount: 1.30% by mass), and Example 3 (antimony doping amount: 1.93% by mass), the change in methane conversion rate over time was good. In particular, in the methane combustion catalysts of Example 2 and Example 3, a methane conversion rate of 90% or more was maintained even after 50 hours of the test time. On the other hand, in the methane combustion catalyst of Reference Example 1 (antimony doping amount: 7.55% by mass), the initial methane conversion rate was about 80%, but the methane conversion rate decreased with an increase in the test time, showing a relatively fast deterioration rate. Also, good results were obtained in the methane combustion catalyst of Example 4 (antimony doping amount: 2.57% by mass) in which antimony-doped tin oxide powder was mixed.

[0075] Next, referring to the results of the temperature increase test in Fig. 5, the fresh catalysts of each example showed an increase in methane conversion rate with the increase in reaction temperature, and there was no significant difference in methane conversion rate. However, when the reaction temperature was relatively low (350°C to 400°C), the methane combustion catalysts of Example 2 (antimony doping amount: 1.30% by mass), Example 3 (antimony doping amount: 1.93% by mass), and Example 4 (antimony doping amount: 2.57% by mass) exhibited excellent methane conversion rates.

[0076] Then, looking at the change in methane conversion rate of the post - durability catalyst heated to 500°C, the catalyst of Reference Example 1 (antimony doping amount: 7.55% by mass) showed a large drop in methane conversion rate at a reaction temperature of 350°C or higher. Considering this result together with the results of the above isothermal test, it can be seen that it is preferable to set the doping of antimony into tin oxide to 5% by mass or less.

[0077] Measurement of the Ratio of Metallic Platinum by XPS XPS analysis was performed on the methane combustion catalyst applying the tin oxide - based support manufactured in this embodiment to determine the ratio (R Pt ) of metallic platinum. For the XPS analysis, a part of the catalyst was sampled and pulverized in an agate mortar to prepare a sample, and the XPS analysis was performed under the following conditions. The XPS analysis was carried out with a survey scan and a narrow scan, and the Pt4f spectrum was measured. · Analyzer: QuanteraII manufactured by ULVAC - PHI, Inc. · X - ray source: Monochromatic Al (1486.6 eV) · Detection area: 100 μmφ · Detection depth: Approximately 4 - 5 nm (take - out angle 45°)

[0078] Figure 6 shows a Pt4f spectrum measured by a narrow scan of Example 2 (antimony doping amount: 1.30 mass%) as an example of XPS analysis results. In the XPS spectrum of platinum, peak tops are obtained in the range of binding energies of 71 to 75 eV. In this range, the peak in the range of 71.0 to 72.0 eV is identified as metallic Pt. Also, the peak in the range of 72.8 to 73.2 eV is identified as PtO, and the peak in the range of 74.6 to 75 eV corresponds to PtO2.

[0079] The calculation of the abundance ratio (R Pt ) of metallic platinum was performed by performing waveform separation processing on the obtained XPS profile and measuring the peak areas of the states of metallic Pt, PtO2, and PtO. Also, the peak areas of each element of oxygen (O), tin (Sn), and carbon (C) were measured simultaneously. Then, the peak areas of each component of Pt, O, Sn, and C were corrected with their respective relative sensitivity factors (RSF), and the abundance ratio (%) of metallic platinum was calculated with the sum of the peak areas of each component as the reference (100).

[0080] The above XPS analysis was performed on the methane combustion catalysts of Example 1 (antimony doping amount: 0.97 mass%), Example 2 (antimony doping amount: 1.30 mass%), and Reference Example 1 (antimony doping amount: 7.55 mass%), and the methane combustion catalyst using undoped tin oxide as a carrier in Comparative Example 1. Also, it was performed on both the fresh catalyst immediately after production and the post-durability catalyst heat-treated at 500 °C for 24 hours. Table 2 shows the measurement results of the ratios (%) of metallic Pt and Pt oxides (PtO, PtO2) in each methane combustion catalyst.

[0081]

Table 2

[0082] From Table 2, the methane combustion catalysts of Example 1, Example 2, and Reference Example 1 have an abundance ratio (R Pt) can be seen to be increasing. And for the catalysts of Example 1 and Example 2, even in the case of the durable catalyst after heat treatment, the existing ratio of metallic platinum is maintained at a high value. Regarding the durable catalyst, based on the results of the above-mentioned methane combustion test (Figure 5), the relationship between the existing ratio of metallic platinum and the methane conversion rate is shown in Figure 7. From this figure, there is a correlation between the existing ratio of metallic platinum and the methane conversion rate, and an increase in the methane conversion rate can be seen with an increase in the ratio of metallic platinum. The reason for the increase in the ratio of metallic platinum due to the application of the antimony-doped tin oxide-based carrier is not clear, but it is considered to be due to an improvement in the balance between the oxidation rate and the reduction rate of platinum. From the results of Figure 7, it is considered that there is a possibility of obtaining methane combustion activity while reducing the platinum loading rate by applying antimony-doped tin oxide.

[0083] Gas Adsorption Analysis (CO, N 2 ) Examination of Catalyst Properties Specific surface area analysis by the N2 gas adsorption method and the CO gas adsorption method was performed on the methane combustion catalyst produced in this embodiment. The specific surface area by CO gas corresponds to the specific surface area of metallic platinum, which is the active species on the carrier, and the specific surface area of N2 gas is related to the specific surface area including the pores of the tin oxide-based carrier. The specific surface area analysis was performed on the methane combustion catalysts of Example 1 (antimony doping amount: 0.97 mass%), Example 2 (antimony doping amount: 1.30 mass%), Example 4 (antimony doping amount: 2.57 mass%), and Reference Example 1 (antimony doping amount: 7.55 mass%), and the methane combustion catalyst of Comparative Example 1 (without antimony doping). Also, for all of them, the analysis was performed on both the fresh catalyst and the catalyst after durability (heat treatment at 500 °C for 24 hours).

[0084] For the analysis by the N2 gas adsorption method, the BET specific surface area was determined by the BET multipoint method using Belsorp mini manufactured by MicrotracBEL as the analyzer. For the measurement by the N2 gas adsorption method, the sample mass was set to 100 mg, and after performing pretreatment by N2 gas flow at 300 °C for 2 hours, the measurement was carried out.

[0085] In the analysis by the CO gas adsorption method, analysis was performed by the pulse method using Belmetal3 manufactured by MicrotracBEL. In the CO gas adsorption method, the sample mass was set to 50 mg, and after pretreatment with He gas flow at 50 °C for 2 hours, CO gas adsorption measurement was performed at 50 °C.

[0086]

Table 3

[0087] Referring to Table 3, from the results of the N2 gas adsorption analysis, the specific surface area change of the methane combustion catalyst of each example is small before and after heat treatment. The antimony-doped tin oxide-based support is considered to have good heat resistance itself. In addition, in the CO gas adsorption analysis, the amount of active sites (metallic platinum) in the fresh catalyst and the post-durability catalyst can be estimated from the CO adsorption amount. However, the methane combustion catalysts of each example have a larger CO adsorption amount than the catalyst without antimony doping of Comparative Example 1 at any stage.

[0088] Third Embodiment : In this embodiment, the methane combustion catalyst of Example 2 of the second embodiment (antimony doping amount: 1.30% by mass) was compared with a methane combustion catalyst (Pt-Ir / SnO2 catalyst) carrying platinum and iridium described in the prior art (Patent Document 4) and further having an overcoat layer.

[0089] The conventional Pt-Ir / SnO2 catalyst was manufactured based on the description in Patent Document 4. After firing the same tin oxide powder as in the first embodiment at 600 °C, a carrier slurry was manufactured in the same manner as in the first embodiment. Further, the carrier slurry was applied to the cordierite honeycomb in the same manner to obtain a honeycomb-shaped carrier.

[0090] In the conventional methane combustion catalyst, split loading is applied when loading platinum. Here, while using a platinum dinitrodiammine-ammonia aqueous solution, the number of split loadings was set to 4 times. And a drying process was carried out at 110°C each time after impregnation with the platinum salt solution. After the 4th split loading, the final drying process was carried out, and the temperature was raised from the drying temperature to 275°C for a firing treatment for 3 hours. In this conventional example, the platinum loading amount was set to 9.69 mass% (30 g / L). That is, the conventional example had a platinum loading amount three times that of the platinum loading amount (3.23 mass% (10 g / L)) in the second embodiment (Example 2).

[0091] Then, iridium was loaded onto the platinum-loaded tin oxide-based support and fired. Here, an aqueous solution of hexachloroiridic acid was used as the iridium salt solution, and the entire prepared solution was impregnated at once. After impregnation with the iridium salt, it was dried at 110°C and subjected to a firing treatment at 450°C for 3 hours.

[0092] Furthermore, the same tin oxide slurry as that used in the production of the support was applied to the catalyst produced above by air blowing to form an overcoat layer. In this conventional example, 150 g / L of tin oxide was applied based on the volume of the support to form an overcoat layer (thickness 30 μm to 60 μm). Through the above steps, a conventional methane combustion catalyst (Pt-Ir / SnO2 catalyst) was produced.

[0093] Comparison of Methane Combustion Activity : For the methane combustion catalysts of Example 2 (antimony doping amount: 1.30 mass%) and the conventional example (no antimony doping, with iridium loading and an overcoat layer), a methane combustion test was conducted to compare the initial activity and durability. The same methane combustion test apparatus as that in the first embodiment was used. The evaluation test was the same isothermal test (reaction temperature 400°C) as in the first embodiment, and the reaction conditions and test method were the same as those in the first embodiment. The test time was set to 100 hours, and the methane conversion rates of Example 2 and the conventional example were measured at regular intervals. The results of this test are shown in Figure 8.

[0094] Referring to Fig. 8, it can be seen that the methane combustion catalyst of Example 2 has an initial activity about 10% higher than that of the conventional example. Also, regarding durability, Example 2 shows a methane conversion rate higher than that of the conventional example throughout from the start to the end of the test (after 100 hours). Thus, the methane combustion catalyst applying the antimony-doped tin oxide-based carrier of Example 2 exceeds the prior art in terms of performance, and in addition, improvement in cost aspect can also be seen. That is, in the conventional example, the platinum loading amount is three times that of Example 2 and additional iridium is also loaded. Further, in the conventional example, an overcoat layer is also set. Since all these differences affect the catalyst cost, it can be said that the methane combustion catalyst of Example 2 is also significantly superior to the prior art in terms of cost.

Industrial Applicability

[0095] The methane combustion catalyst according to the present invention applies antimony-doped tin oxide as a carrier. This tin oxide-based carrier acts as an effective oxygen supply source for platinum (platinum oxide) which is a catalyst particle. In the conventional methane combustion catalyst, additionally loaded iridium and the overcoat layer are responsible for the action of the oxygen supply source, but the present invention does not require these. According to the present invention, methane combustion activity equivalent to or higher than that of the prior art can be obtained at low cost. The methane combustion catalyst according to the present invention can be suitably applied to the purification of various exhaust gases of engines, boilers, power generation systems, etc. using hydrocarbon fuels such as natural gas and city gas. Further, the present invention is also useful for power generation systems such as cogeneration systems and gas heat pumps (GHPs).

Claims

1. A methane combustion catalyst for burning methane in a combustion exhaust gas containing sulfur oxides, comprising a tin oxide-based support containing tin oxide as an essential component and platinum and / or platinum oxide supported thereon, The tin oxide-based support is made of antimony-doped tin oxide, The doping amount of antimony in the tin oxide-based support is 0.97% by mass or more and 5.0% by mass or less based on the mass of the tin oxide-based support, The methane combustion catalyst is characterized in that, when the methane combustion catalyst is measured by X-ray photoelectron spectroscopy (XPS), the abundance ratio R Pt of metallic platinum obtained from a platinum 4f spectrum is 70% or more.

2. 2. The methane combustion catalyst according to claim 1, wherein the amount of platinum and / or platinum oxide supported on the catalyst as a whole is 1.0% by mass or more and 10.0% by mass or less in terms of metallic platinum.

3. 3. The methane combustion catalyst according to claim 1, which is in the form of any one of particles, granules, pellets and tablets.

4. 3. The methane combustion catalyst according to claim 1, which is supported on a support having any one of a plate shape, a cylindrical shape, a spherical shape and a honeycomb shape.

5. A method for purifying combustion exhaust gas by oxidizing and removing methane from combustion exhaust gas containing sulfur oxides, comprising: A method for purifying a combustion exhaust gas, comprising contacting the combustion exhaust gas with the methane combustion catalyst according to claim 1 or 2 at a reaction temperature of 300° C. to 500° C.

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