Methane combustion catalyst, method for manufacturing the same, and method for purifying combustion exhaust gas

Abstract

The present invention relates to a methane combustion catalyst for combusting methane in combustion exhaust gas containing sulfur oxides, which is formed by loading platinum and iridium on a tin oxide support. When analyzing and measuring this methane combustion catalyst by X-ray photoelectron spectroscopy (XPS), the ratio R of platinum oxide to metallic platinum is calculated based on the presence ratios of metallic platinum (Pt) and platinum oxides (PtO and PtO2) obtained from the platinum 4f spectrum and by the following formula TO is 8.00 or more. In the following formula, R Pt is the presence ratio of metallic platinum (Pt), R Pto is the presence ratio of PtO, R Pto2 is the presence ratio of PtO2. R To =(R Pto +R Pto2 ) / R Pt

Description

Technical Field

[0001] The present invention relates to a catalyst for oxidizing and removing methane in combustion exhaust gas containing sulfur oxides. Specifically, it relates to a catalyst that suppresses catalyst poisoning caused by sulfur oxides and can oxidize methane with higher activity than conventional products. In addition, the present invention relates to a purification method for oxidizing and removing methane in combustion exhaust gas using this catalyst. Background Art

[0002] Exhaust gas generated from engines and boilers fueled by hydrocarbons such as natural gas, city gas, light oil, and kerosene contains, in addition to nitrogen oxides (NOx), sulfur oxides (SO 2 , SO x ), carbon monoxide (CO), odor substances / soot, etc., and also includes unburned hydrocarbons. Since these are all factors of environmental pollution, the exhaust gas is treated with a filter or a purification catalyst and then discharged. In particular, it is reported that the greenhouse effect of methane is more than 20 times that of carbon dioxide, and from the viewpoint of environmental protection, the necessity of removing methane is very high.

[0003] As a purification catalyst for hydrocarbons, a noble metal catalyst in which catalyst particles of a noble metal such as platinum are supported on a carrier such as alumina or zirconia has been used for a long time. However, it is known that such a noble metal catalyst is not very effective for the oxidative removal of methane. In particular, when sulfur oxides are contained in the exhaust gas, there is a problem that the activity is significantly reduced due to catalyst poisoning.

[0004] Against the above background, as a catalyst for oxidizing and removing methane in combustion exhaust gas containing sulfur oxides, the catalysts described in Patent Document 1 and Patent Document 2 are known. These catalysts are constituted by supporting catalyst particles composed of platinum on a tin oxide (SnO 2 ) carrier. In particular, in the catalyst of Patent Document 2, the durability is improved by further supporting iridium.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent No. 4283037 Specification

[0008] Patent Document 2: Japanese Patent No. 4429950 Specification Summary of the Invention

[0009] [Problems to be Solved by the Invention]

[0010] It can be confirmed that the catalysts of the above Patent Documents 1 and 2 (hereinafter, sometimes referred to as Pt / SnO2 Catalyst, Pt-Ir / SnO 2 Catalyst. In addition, these are sometimes collectively referred to as Pt / SnO 2 The catalyst of the Pt / SnO series has an oxidative removal effect on methane in the combustion exhaust gas containing sulfur oxides. 2 The catalyst has the interesting property that the activity is improved by increasing the sulfur oxide concentration in the exhaust gas to 1 ppm or more. 2 The catalyst exhibits effective activity for methane by loading platinum, which has not been recognized to be effective so far, on a tin oxide carrier. In this regard, Pt / SnO 2 The catalyst is a meaningful catalyst. 2 Pt-Ir / SnO obtained by adding iridium to the catalyst 2 In the catalyst, improvement in durability was confirmed.

[0011] However, in the field of catalysts, there is often a need for catalysts that can exert higher activity. Methane combustion catalysts are also required to exert higher activity than the prior art. The present invention is completed under such a background, and its purpose is to provide a Pt / SnO 2 A methane combustion catalyst based on a series of catalysts and having higher activity than these catalysts and a method for producing the same.

[0012] [Solution to solve the problem]

[0013] Even the traditional Pt / SnO 2 Although the catalysts of this series require sulfur oxides for methane combustion, this does not mean that sulfur oxides will not have an adverse effect on the catalyst activity. 2 The active mechanism of the methane combustion activity of the Pt / SnO catalyst was studied, especially the state of the platinum as the catalyst particles. The following conclusions were obtained: 2 In the catalyst system, platinum is only present in the form of oxides (PtO or PtO 2 ) is active for methane combustion. As will be described in detail later, Pt / SnO 2 The catalyst is produced by supporting a platinum complex on a tin oxide carrier and firing the carrier. It is believed that during this process, platinum is oxidized due to the oxidation effect of firing and the interaction with the tin oxide carrier, which is an oxide.

[0014] Moreover, as Pt / SnO 2 The present inventors considered that the catalyst poisoning mechanism caused by sulfur oxides was that the sulfur oxides deteriorated the catalyst particles (platinum oxide).2 The catalyst does not show the usual deterioration tendency caused by the adsorption of poisons. Sulfur oxides are common poisons. If their adsorption hinders the adsorption of combustion components (methane) and catalyst particles and reduces activity, the activity should stop decreasing when the adsorption of poisons reaches equilibrium. However, according to the experimental research of the inventors, Pt / SnO 2 The catalyst of the 200-nm-thick catalyst did not show such a tendency, and even when the amount of sulfur oxide adsorption reached equilibrium at the initial stage of the reaction, the activity continued to decrease, indicating that the catalyst particles (platinum oxide) were deteriorated by sulfur oxides.

[0015] The sulfur oxides investigated by the inventors cause the Pt / SnO 2 The specific mechanism of the decrease in the activity of the catalyst is as follows. SO flying in the vicinity of the catalyst particles (platinum oxide) 2 Sulfur oxides such as NH4O2 are easily oxidized, so they take oxygen from Pt oxide and oxidize to SO 3 The metallized platinum particles deprived of oxygen sinter with the nearby similarly metallized platinum particles to form coarse platinum particles. Such metallized and coarsened platinum particles lack methane combustion activity. Moreover, even if the adsorption of sulfur oxides reaches equilibrium, the metallization of platinum oxides and the sintering of platinum particles will continue, so the activity continues to decrease.

[0016] As mentioned above, the present inventors have investigated Pt / SnO 2 Based on this observation, the mechanism of the activity of the Pt / SnO catalyst described in Patent Document 2 can also be explained. 2 Pt-Ir / SnO obtained by additionally loading iridium on the catalyst 2 The durability of the catalyst is good. It is speculated that in Pt-Ir / SnO 2 In the catalyst, iridium, like platinum, is present as an oxide (IrO 2 ) exists in the form of iridium and iridium oxide, but iridium and iridium oxide have almost no active effect on methane combustion. However, it is believed that iridium oxide has the function of keeping the oxidized state of platinum by supplying oxygen to the platinum metalized by the oxidation of sulfur oxides. Moreover, iridium oxide is metalized by supplying oxygen to the platinum and utilizing the reduction of sulfur oxides, but the iridium after the reduction can accept the oxygen supply from the tin oxide carrier and keep the state of iridium oxide. It is believed that the durability of the methane combustion catalyst is improved by the effect of this iridium oxide.

[0017] Based on the above investigation, in order to improve the Pt / SnO 2Regarding the initial activity and durability assurance of the catalyst, several guidelines were derived. That is, in order to improve the initial activity, it is necessary to increase the amount of platinum in the oxide state, which is the active source. In addition, in order to ensure durability, it is necessary to additionally load iridium. However, in order for iridium to function effectively, it is necessary to finely disperse platinum during the catalyst manufacturing stage and load iridium in the vicinity thereof. Based on these guidelines, the present inventors conducted in-depth research on the optimization of the manufacturing method and composition of the Pt / SnO 2 -based catalyst. Then, as the Pt / SnO 2 -based catalyst in which platinum in an appropriate oxidation state is dispersed, the present invention was conceived.

[0018] The present invention for solving the above problems is a methane combustion catalyst in which platinum and iridium are loaded on a tin oxide carrier and used for combusting methane in combustion exhaust gas containing sulfur oxides, characterized in that when the methane combustion catalyst is measured by X-ray photoelectron spectroscopy (XPS), based on the abundance ratios of metallic platinum (Pt), platinum oxides (PtO and PtO 2 ) obtained from the platinum 4f spectrum and the ratio R TO of platinum oxide to metallic platinum calculated by the following formula is 8.00 or more (16.00 or less).

[0019] [Mathematical formula 1]

[0020] R TO =(R PtO +R PtO2 ) / R Pt

[0021] Here, R Pt is the abundance ratio of metallic platinum (Pt), R Pto is the abundance ratio of PtO, and R Pto2 is the abundance ratio of PtO 2 .

[0022] Hereinafter, the present invention will be described in more detail. Regarding the methane combustion catalyst composed of the Pt / SnO 2 -based catalyst according to the present invention, its basic composition is the same as that of the conventional Pt / SnO 2 -based catalyst (Patent Documents 1 and 2), and it is formed by loading platinum and iridium on a carrier composed of tin oxide. In the following description, each component of the catalyst will be described, and the manufacturing method of the catalyst will be described in detail.

[0023] (A) Composition of the methane combustion catalyst according to the present invention

[0024] As described above, the present invention is different from the conventional Pt / SnO 2The catalyst is characterized by the oxidation state of the supported platinum. Hereinafter, this characteristic will be elucidated, and platinum, iridium, and the carrier constituting the methane combustion catalyst according to the present invention will be described.

[0025] (A-1) Platinum

[0026] As described above, the catalyst according to the present invention is characterized in terms of the oxidation state of platinum. In the present invention, the oxidation state of platinum is determined based on the analysis results of X-ray photoelectron spectroscopy (XPS). XPS is a method capable of analyzing the types of binding states of atoms constituting the analysis object and the proportion (existence ratio) of each binding state, and thus is applied in the present invention. Specifically, based on the platinum 4f (Pt4f) spectrum observed when analyzing the catalyst by XPS, the oxidation state of platinum is determined according to the detection intensity of the binding energy corresponding to each binding state. Here, in the platinum 4f spectrum, the peak of metallic Pt appears in the range of 71.0 eV to 72.0 eV, the peak of PtO appears in the range of 72.8 eV to 73.2 eV, and the peak of PtO 2 appears in the range of 74.6 eV to 75.0 eV. The existence ratios R Pt 、R PtO 、R PtO2 of the respective states of platinum are calculated based on the peak areas of the respective states.

[0027] Moreover, in the present invention, it is required that based on the existence ratios R Pt 、R PtO 、R PtO2 of the respective states of platinum, and according to the ratio R TO of platinum oxide to metallic platinum in the above mathematical formula 1, it is 8.00 or more. The higher the ratio R TO of this platinum oxide, the higher the proportion of platinum atoms in the oxidized state. In a catalyst where R TO is less than 8.000, the oxidation state of platinum is insufficient, and it is greatly affected by sulfur oxides in the exhaust gas, and the activity of methane combustion deteriorates. The value of R TO is considered to be preferably higher, but in practice, it is difficult to oxidize all platinum atoms. Therefore, as the upper limit of R TO , it is preferably set to 16.00.

[0028] In the present invention, in order to increase the ratio R TO of platinum oxide as described above, it is necessary to finely and uniformly disperse platinum atoms in the step of loading platinum onto the tin oxide carrier. In addition, in addition to this, the value of the ratio R TO of platinum oxide is also affected by the operations performed from the loading step to the firing step. The details of the method for setting R TO within the preferred range will be described later.

[0029] It should be noted that in the methane combustion catalyst involved in the present invention, for the existence ratio R of PtO Pto and PtO 2 the respective values of the existence ratio R Pto2 are not particularly limited. However, in the present invention, it is preferable to set the value of R Pto2 to be above the value of R Pto so that R Pto2 / R Pto ≥ 1.

[0030] Regarding the loading amount of platinum in the methane combustion catalyst of the present invention, in terms of metallic platinum conversion, based on the mass of the whole catalyst, it is preferably set to be 2.0% by mass or more and 15% by mass or less. The loading amount of platinum is more preferably 4.0% by mass or more and 15% by mass or less. The above loading amounts of 2.0% by mass or more and 15% by mass or less, 4.0% by mass or more and 15% by mass or less of platinum are taken as the target loading amounts.

[0031] (A-2) Iridium

[0032] In the methane combustion catalyst of the present invention, iridium acts as a promoter for improving durability. Iridium in the methane combustion catalyst is also in an oxide state. Moreover, iridium oxide has the function of maintaining the oxidation state of platinum by supplying oxygen to platinum metallized by sulfur oxides. In the methane combustion catalyst of the present invention, there is no particular limitation on the oxidation state of iridium. According to the research of the present inventors, it can be confirmed that in the case of iridium, most of the loaded iridium is oxidized. As will be described later, for the oxidation of iridium, conditions as strict as those for converting platinum into platinum oxide are not required.

[0033] In terms of metallic iridium conversion, based on the mass of the whole catalyst, the loading amount of iridium in the methane combustion catalyst of the present invention is preferably set to be 0.1% by mass or more and 5.0% by mass or less. The loading amount of iridium is more preferably 0.2% by mass or more and 2.0% by mass or less.

[0034] (A-3) Tin oxide carrier

[0035] The tin oxide carrier is an essential component because it has the original function of maintaining the catalyst particles in a dispersed state and imparts methane combustion activity to platinum (platinum oxide) as the catalyst particles. In addition, as described above, the present inventors have investigated that the tin oxide carrier has the effect of supplying oxygen to iridium. Therefore, it can be considered that the tin oxide carrier is also an important component in maintaining the durability of the catalyst by using iridium oxide.

[0036] The form of the tin oxide support is in a form corresponding to that of the methane combustion catalyst. Here, as the form of the methane combustion catalyst, it can be any one of granular, particulate, pellet, and flake shapes. In such a methane combustion catalyst, the tin oxide support also has the same granular, particulate, pellet, and flake shapes. In such a tin oxide support, the specific surface area is preferably 10 m 2 / g or more and 27 m 2 / g or less, and more preferably set to 11 m 2 / g or more and 15 m 2 / g or less.

[0037] In addition, as the form of the methane combustion catalyst, there are many application examples in which the methane combustion catalyst is supported on a suitable support. As the support, a support in any one of plate, cylinder, sphere, and honeycomb shapes is known. In such a catalyst, tin oxide is coated / coated on the support as a so-called washcoat to form a tin oxide support. In the case of coating the tin oxide support on the support in this way, based on the volume of the support, the suitable amount of the tin oxide support is preferably set to 250 g / L or more and 400 g / L or less. When it is less than 250 g / L, the amount of the support is too small, and the dispersibility of platinum and iridium may be reduced, making it difficult to fully combust methane. In addition, when it exceeds 400 g / L, the amount of the support is too large, forming a region where the processing gas does not come into contact, and in this case, the efficiency of methane combustion is also reduced. The specific surface area of this form of tin oxide support is also preferably set to the same specific surface area as the above-mentioned pellet-shaped and other supports.

[0038] (B) Manufacturing method of the methane combustion catalyst according to the present invention

[0039] Next, the manufacturing method of the catalyst according to the present invention will be described. The methane combustion catalyst according to the present invention is manufactured by loading platinum and iridium on a tin oxide support. In this regard, it is the same as the prior art, and the manufacturing method of the catalyst of the present invention can basically apply traditional methods. Here, as a traditional methane combustion catalyst, the impregnation method is applied. The impregnation method is a well-known catalyst manufacturing method in which a solution of a noble metal salt (noble metal compound) to be loaded is impregnated into a support, and then fired and heat-treated to precipitate the noble metal on the support.

[0040] However, in the present invention, it is clarified that in order to improve Pt / SnO 2Regarding the initial activity of the catalyst, it is necessary to increase the proportion of the amount of platinum that becomes the active source, i.e., platinum oxide. In addition, in order to ensure the durability of the catalyst, it is necessary to finely disperse platinum during the catalyst manufacturing stage and to load iridium close to platinum so as to effectively exert the role of iridium. The present inventors studied a catalyst manufacturing method capable of satisfying these requirements and found that it is necessary to strictly set the impregnation step of the noble metal salt solution in the impregnation method, the drying conditions after impregnation, and the temperature of the firing heat treatment.

[0041] That is, the manufacturing method of the methane combustion catalyst according to the present invention is a manufacturing method of such a methane combustion catalyst, including: a first loading step of impregnating a carrier composed of tin oxide with a platinum salt solution and a first firing step of firing the carrier after the first loading step; and a second loading step of impregnating the carrier after the first firing step with an iridium salt solution and a second firing step of firing the carrier after the second loading step. The first loading step is a step of impregnating the carrier with a platinum salt solution having a platinum content lower than that of the platinum salt solution containing the target loading amount of platinum multiple times, and in each of the multiple impregnations, the target loading amount of platinum is impregnated by drying the impregnated carrier at a temperature of 60°C or higher and 150°C or lower. In addition, the heating temperature of the first firing step is set to 350°C or higher and 500°C or lower.

[0042] As described above, the catalyst manufacturing method according to the present invention is based on the impregnation method, and at the same time, each of the impregnation step of the noble metal salt solution and the firing heat treatment step is improved. Hereinafter, each step of the present invention will be described.

[0043] (B-1) Pre-loading step (optional step)

[0044] As a preparation step of the tin oxide carrier corresponding to the form of the methane combustion catalyst, various forms of tin oxide can be prepared, and a tin oxide slurry can be coated on a support. As the preparation of tin oxide, heat treatment for adjusting the specific surface area can be cited. This heat treatment is preferably carried out by firing powdery, granular, particulate, pill-shaped, or sheet-shaped tin oxide in the atmosphere at a temperature of 450°C or higher and 700°C or lower. In addition, when the methane combustion catalyst is supported on a support such as a honeycomb, the tin oxide powder is slurried and coated on the support. The tin oxide slurry can be prepared by mixing a binder using water or an organic solvent as a dispersion medium in the tin oxide powder after the above heat treatment. The coating of the tin oxide slurry on the support can be carried out by using various known methods such as blowing, spraying, and dipping.

[0045] (B-2) Loading step of platinum and iridium

[0046] The loading of platinum and iridium onto the tin oxide support is accomplished by impregnation with a noble metal salt solution of each noble metal followed by firing heat treatment. In the present invention, the loading of platinum and the loading of iridium are performed separately. This is because, before the loading of iridium, by loading platinum (platinum oxide) in a fine and dispersed state and then loading iridium, a state in which platinum (platinum oxide) is close to iridium easily occurs. In this regard, in the conventional method, co-loading using a mixed solution of platinum and iridium is allowed, but co-loading is not employed in the present invention.

[0047] (B-2-1) Platinum loading step (first loading step) and firing step (first firing step)

[0048] In the present invention, the platinum loading step is an important step for dispersing and loading fine platinum particles and efficiently converting platinum into platinum oxide. As described above, in the present invention, when loading platinum, a platinum salt solution having a lower platinum content than the platinum salt solution containing the target loading amount of platinum is impregnated into the tin oxide support multiple times until the target loading amount is reached (hereinafter, this operation may sometimes be referred to as divided loading). In addition, in the present invention, in each impregnation of the divided loading, it is a requirement to perform a drying treatment under predetermined conditions.

[0049] Examples of the platinum salt solution impregnated into the tin oxide support include, in addition to aqueous platinum nitrate solution, aqueous platinum chloride solution, and aqueous platinum acetate solution: aqueous tetraammineplatinum salt solution, dinitrodiammineplatinum-ammonia aqueous solution, dinitrodiammineplatinum-ethanolamine solution, etc. as platinum complex solutions. Among these platinum salt solutions, dinitrodiammineplatinum-ammonia aqueous solution, dinitrodiammineplatinum-ethanolamine solution, and aqueous platinum nitrate solution are preferred. There is no particular limitation on the method of impregnating the platinum salt solution into the tin oxide support, and it may be any of spraying, dropping, and dipping.

[0050] In the present invention, the reason for the necessity of divided loading for loading platinum is for the refinement and high dispersion of platinum. In the case of impregnating a platinum salt solution having a target loading concentration of platinum at one time, platinum (platinum salt) may be unevenly loaded, resulting in deteriorated dispersibility. By repeatedly impregnating with a dilute platinum salt solution, fine platinum can be loaded in a highly dispersed manner. The combination of this divided loading and the drying treatment described later is necessary for the fine dispersion of platinum and also contributes to the formation of platinum oxide in the subsequent firing step.

[0051] The platinum content of the platinum salt solution impregnated by divided loading in the present invention may vary depending on each impregnation operation, but it is preferred to impregnate a platinum salt solution having an equal platinum content. In addition, as the number of impregnations in the divided loading, although it depends on the target loading amount, it is preferably about 3 to 5 times. Therefore, when the target loading amount is set to M, it is preferred to set the platinum content of the platinum salt solution impregnated in one impregnation operation to M / 3 to M / 5.

[0052] Moreover, in the present invention, a drying treatment is performed for each of the multiple impregnation treatments in the split load. The drying treatment is a treatment for evaporating / removing the water / crystallization water contained in the carrier after impregnation to fix the platinum salt on the carrier. The drying treatment must be performed because, when a high-temperature sintering heat treatment is performed in a state where water remains, platinum may move due to water, thereby reducing dispersibility. An aspect that should be noted in the drying treatment is that water is removed without decomposing the platinum salt adsorbed on the carrier during the impregnation treatment. When the decomposition of the platinum salt occurs in the drying treatment of each impregnation treatment of the split load, the platinum produced by the decomposition is fixed, and the impregnated platinum salt in the next impregnation treatment tends to be adsorbed on the fixed platinum and condensed. Due to the condensation of the platinum salt, the dispersibility of the platinum (platinum oxide) of the catalyst is reduced. Therefore, it is necessary to effectively evaporate / remove water in the drying treatment while heating at a temperature at which the platinum salt does not decompose.

[0053] Specifically, in the drying process, heating is required at a temperature of 60°C to 150°C. When the temperature is less than 60°C, the evaporation of water slows down and the treatment time is too long. In addition, when the temperature exceeds 150°C, decomposition of the platinum salt may occur. In addition, the treatment time of the drying process is adjusted according to the water content of the carrier, but it is preferably heated within the aforementioned temperature range for at least 30 minutes. The upper limit of the drying time does not need to be particularly limited, but considering the manufacturing efficiency, it is preferably set to 120 minutes or less.

[0054] In the present invention, the platinum salt solution is impregnated and dried multiple times until the target loading amount of platinum salt is loaded through the above-mentioned split loading, and then a firing process is performed. The firing process is a process for decomposing the platinum salt to generate platinum oxide as an active source. The heating temperature in the first firing process is 350°C to 500°C. This is because, when it is less than 350°C, the generation of platinum oxide becomes insufficient. Moreover, when it exceeds 500°C, the platinum oxide generated with great difficulty decomposes and becomes metallic platinum. The heating temperature is more preferably 380°C to 480°C. The processing time of the firing process is preferably set to 1 hour to 5 hours. There is no particular limitation on the atmosphere of the firing process, as long as it is an oxidizing atmosphere in the air.

[0055] The firing process and the above-mentioned drying process (drying process after the last impregnation process in the split load) can be carried out in conjunction. That is, in the drying process, it is possible to heat at a temperature of 60°C to 150°C for a certain period of time, and then start heating again, and heat to the above-mentioned firing temperature for firing. In this case, it is possible to heat from the drying temperature to the firing temperature without stopping at the same heating rate. In addition, the heating rate can also be adjusted in the process of heating from the drying temperature to the firing temperature. As a behavior of making the platinum salt after the drying process become platinum oxide, the platinum salt decomposes at 150°C to about 300°C, and becomes an oxide above about 300°C, but when it exceeds 500°C, the oxide decomposes into metallic platinum. Therefore, from the drying temperature to 300°C, the platinum salt is decomposed at a slower heating rate (1 to 3°C / minute), and between 300°C and 500°C, the heating rate is set to a faster heating rate (5 to 10°C / minute), so that platinum oxide can be efficiently and stably generated. However, such stepwise adjustment of the temperature increase rate is not essential, and platinum oxide can be formed by continuously heating from the drying temperature to the firing temperature as described above. The temperature increase rate at this time is preferably about 1 to 3°C / min.

[0056] Through the above platinum firing step (first firing step), platinum oxide is loaded on the carrier. It should be noted that, as is clear from the above, in the present invention, the platinum salt solution is impregnated multiple times, but the firing step is only performed once after the last impregnation, rather than performing the firing step for each impregnation. This is because if the split load is fired after each impregnation, the platinum salt may be adsorbed / bonded to the platinum oxide generated by firing, and when it is fired, it may be sintered to become a coarse oxide.

[0057] (B-3) Iridium Supporting Step (Second Supporting Step) and Firing Step (Second Firing Step)

[0058] After platinum is loaded and fired on the tin oxide carrier, iridium is loaded and fired. Iridium loading is also accomplished by impregnation with an iridium salt solution (second loading step), but unlike platinum loading, it is accomplished by a single impregnation treatment without split loading.

[0059] As the iridium salt solution for impregnation of iridium loading, there can be listed: iridium chloride aqueous solution, iridium nitrate aqueous solution and other iridium halide aqueous solution, hexachloroiridic acid aqueous solution and other iridium halide acid aqueous solution, hexaammine iridium hydroxide solution, etc. The impregnation method of the iridium salt solution is also not particularly limited, and can be any of spraying, dripping, and dipping.

[0060] After the impregnation treatment of the iridium salt solution, drying treatment is carried out in the same manner as for platinum. In the drying treatment of the iridium salt, strict conditions for the decomposition of platinum salts like those for platinum do not need to be considered. However, similar to the case of platinum, a temperature of 60°C or higher and 150°C or lower is preferably used. Additionally, the drying time is preferably 30 minutes or more and 2 hours or less of heating.

[0061] Then, through the firing process (the second firing process), the generation of iridium and iridium oxide precipitated from the iridium salt is completed. According to the present inventor, different from platinum, the firing conditions for iridium do not need to be strictly set. This is because, in the case of iridium, compared with platinum, oxides are more easily generated and the possibility of its decomposition is also small. In the firing of iridium, heating is preferably carried out at 350°C or higher. The atmosphere for the firing process of iridium is not particularly limited, as long as it is an oxidizing atmosphere such as in the air. However, when the firing process of iridium is at an excessively high temperature, decomposition of platinum oxide may occur. Therefore, in the firing process of iridium, it is also preferable to set the upper limit temperature to 500°C. Additionally, the firing time is also preferably set to a time equivalent to the loading of platinum.

[0062] By performing the above-mentioned iridium loading process and firing process, the methane combustion catalyst according to the present invention is manufactured.

[0063] (C) A method for burning methane using the methane combustion catalyst according to the present invention

[0064] The method for burning methane applying the methane combustion catalyst according to the present invention described above is basically the same as the conventional method. The object of the method for burning methane according to the present invention is combustion exhaust gas containing sulfur oxides together with methane. Additionally, in addition to methane and sulfur oxides, other hydrocarbons such as ethane and propane and combustible components such as carbon monoxide, oxygen, oxygen-containing compounds, and nitrogen oxides may also be contained.

[0065] In the combustion of methane in the combustion exhaust gas, the gas to be treated is passed through a combustion device equipped with the methane combustion catalyst according to the present invention and brought into contact with the methane combustion catalyst. Regarding the combustion device, a publicly known device can be applied. For example, a fixed-bed flow-through reaction device or the like can be applied. The usage amount of the catalyst in such a combustion device is usually set by the space velocity (GHSV) of the gas per hour. In the present invention, in order to ensure the combustion rate of methane, the space velocity is preferably set to 80,000 h -1 or less. By reducing the space velocity, the catalyst activity can be improved, so the lower the space velocity, the more preferable. However, considering the catalyst activity, economy, and pressure loss, the space velocity is preferably set to 1,000 h -1 or more.

[0066] Regarding the heating temperature, i.e., the reaction temperature, of the methane combustion catalyst for the purification of combustion exhaust gas, the reaction temperature is set to be not less than 340°C and not more than 500°C. The reaction temperature is more preferably not less than 350°C and not more than 475°C.

[0067] [Effects of the Invention]

[0068] As described above, the present invention relates to a Pt / SnO 2 -based catalyst (Pt-Ir / SnO 2 catalyst), which effectively generates platinum oxide as an active source on a tin oxide support. Compared with the conventional Pt / SnO 2 -based catalyst, the methane combustion activity is improved. According to the present invention, by improving the initial activity of the methane combustion catalyst, combustion exhaust gas can be treated for a longer time compared with the prior art. Description of the Drawings

[0069] Figure 1 is a diagram schematically showing the configuration of a test apparatus for a methane combustion test implemented in the present embodiment.

[0070] Figure 2 is a diagram showing the Pt4f spectrum and the Ir4f spectrum obtained by XPS analysis of the methane combustion catalyst of Example 2 of the first embodiment. Detailed Embodiments

[0071] First Embodiment : Hereinafter, embodiments of the present invention will be described. In the present embodiment, a Pt-Ir / SnO 2 catalyst was manufactured according to the manufacturing process (basic manufacturing process) of the manufacturing method of the methane combustion catalyst according to the present invention. In addition, a Pt-Ir / SnO 2 catalyst was also manufactured by referring to the manufacturing processes (comparative manufacturing processes 1 and 2) of the prior art (Patent Document 2). Then, XPS analysis was performed on each catalyst to measure the proportion (R TO ) of platinum oxide, and a combustion test of a gas containing methane and sulfur oxides was performed to measure / evaluate the methane conversion rate.

[0072] [Basic Manufacturing Process]

[0073] The manufacturing process of the methane combustion catalyst according to the present embodiment is as follows. A commercially available tin oxide powder is fired at 600°C. Then, a commercially available SnO 2 ​​The sol was mixed with the fired tin oxide powder and pulverized to produce a tin oxide slurry. The tin oxide slurry was coated on a commercially available cordierite honeycomb (manufactured by NGK Insulators, Ltd.: φ25.4 mm × 50 mm L) by blowing air to serve as a tin oxide support (specific surface area of tin oxide: 12.09 m 2 / g). At this time, the mass of the tin oxide support was adjusted by adjusting the number of times of blowing air.

[0074] Platinum was supported on the above tin oxide support (honeycomb support) in a divided manner. A diammineplatinum dinitrate-ammonia aqueous solution was used as the platinum salt solution impregnated in the divided support. For the diammineplatinum dinitrate-ammonia aqueous solution, a solution obtained by dissolving diammineplatinum dinitrate in ammonia water and adjusting the pH to 12 was used. In the present embodiment, the number of divided support times was set to 4 times, and the platinum concentration of the platinum salt solution was adjusted for each target loading amount. In each impregnation of the platinum salt solution, the platinum salt solution was blown onto the tin oxide support. After the impregnation of the platinum salt solution, a drying process was carried out. In the drying process, the impregnated tin oxide support was put into a dryer maintained at 110 °C and kept for 30 minutes for drying treatment. This impregnation and drying process of the platinum solution was carried out 4 times.

[0075] Then, the tin oxide support after impregnation and drying of the platinum salt solution was fired. The temperature was raised from the drying temperature (110 °C) in the final drying process of the above loading process at a rate of 1 °C / minute to 275 °C, and held at this temperature for 3 hours. The heating process up to this point took into account the decomposition of the platinum salt. Then, the temperature was raised from 275 °C at a rate of 1 °C / minute until the set firing temperature was reached, and held at this temperature for 3 hours for firing treatment. Thus, a tin oxide support loaded with platinum oxide was manufactured.

[0076] Iridium was supported on the above tin oxide support (platinum-loaded) and fired. Here, an aqueous solution of hexachloroiridic acid was used as the iridium salt solution, and the target loading amount was adjusted according to its iridium concentration. The method of impregnating the iridium salt solution was the same as that of the platinum salt solution, and the entire amount of the solution was impregnated at one time. After the iridium salt impregnation, it was dried at 110 °C for 0.5 hour, and the temperature was raised from this temperature at a rate of 5 °C / minute until the same firing temperature as that of platinum was reached, and held at this temperature for 3 hours for firing treatment. Through the above process, the methane combustion catalyst according to the present embodiment was manufactured.

[0077] In this embodiment, the following catalyst was manufactured according to the above basic process: the loading amount of platinum based on the mass of the whole catalyst was set to 8.0% by mass, and the loading amount of iridium was set to 0.8% by mass. At this time, the number of divided loadings in the platinum loading process was set to 4 times, and each loading / drying was equivalent to a platinum salt solution of 2% by mass, and a firing treatment was performed after platinum loading. In this embodiment, the firing temperature after platinum loading was set to 350 °C (Example 1), 400 °C (Example 2), 450 °C (Example 3), 500 °C (Example 4), 650 °C (Reference Example 1), thereby manufacturing 5 kinds of methane combustion catalysts. It should be noted that the firing temperature after iridium loading was set to the same temperature as the firing temperature after platinum loading.

[0078] [Comparative manufacturing process 1]

[0079] As a comparative example (Comparative Example 1) of the manufacturing process of the methane combustion catalyst according to this embodiment, referring to the prior art (Patent Document 2), the drying process and the firing treatment were respectively performed at temperatures higher than the basic manufacturing process, thereby manufacturing the catalyst. Compared with the basic manufacturing process of this embodiment, divided loading (4 times) was performed, and after heating at 175 °C for 3 hours, heating at 275 °C for 3 hours was performed as the drying treatment after each impregnation. In addition, in the firing process after the final drying treatment, heating was performed at 550 °C for 3 hours. The platinum loading process other than these drying treatments and firing treatments was the same as the basic manufacturing process of this embodiment. In addition, the iridium loading process and the firing process were also the same as the basic manufacturing process. The loading amounts of platinum and iridium were the same as those in Example 1 and the like.

[0080] [Comparative manufacturing process 2]

[0081] As another comparative example (Comparative Example 2) of this embodiment, a catalyst loaded with platinum was manufactured without relying on divided loading. The loading amounts of platinum and iridium were the same as those in Example 1 and the like. In this comparative manufacturing process 2, compared with the basic manufacturing process of this embodiment, instead of divided loading, the platinum salt solution containing the target loading amount of platinum in the total amount was impregnated into the tin oxide carrier by one operation. After the impregnation of the platinum salt solution, drying and firing were performed. In the drying treatment, heating was performed at 110 °C for 1 hour, and in the firing treatment, heating was performed at 450 °C for 3 hours. The heating rate in each treatment was the same as the basic manufacturing process. Then, the iridium loading process and the firing process were performed in the same manner as the basic manufacturing process.

[0082] Determination of the proportion of platinum oxide using XPS

[0083] XPS analysis was performed on the methane combustion catalysts of each example / comparative example manufactured by the above various manufacturing methods, and the ratio (R) of platinum oxide was obtained TO)。When performing XPS analysis, each catalyst was pulverized in an agate mortar to prepare a sample, and XPS analysis was carried out under the following conditions. The XPS analysis was performed by conducting a survey scan and a narrow scan to obtain the Pt4f spectrum and the Ir4f spectrum.

[0084] · Analytical instrument: K-Alpha+ manufactured by Thermo Fisher Scientific

[0085] · Irradiated X-ray: Single crystal spectroscopic AlKα ray

[0086] · X-ray spot diameter: 400 μm

[0087] · Neutralization electron gun: Used

[0088] · Standardization of binding energy: Standardize C-C and C-H to 2884.6 eV

[0089] Figure 2 The Pt4f spectrum and Ir4f spectrum obtained by narrow scan measurement for Example 2 (platinum loading 8.0 mass%, iridium loading 0.8 mass%, firing temperature 400 °C). In the XPS spectrum of platinum, peaks were obtained in the range of binding energy from 71 to 75 eV. In this range, as platinum oxides, the peaks in the range of 72.8 - 73.2 eV were identified as PtO, and the peaks in the range of 74.6 - 75 eV corresponded to PtO 2 . It should be noted that referring to Figure 2 the spectrum of iridium, iridium was in the state of Ir 3+ or Ir 4+ , and no peak of metallic iridium was confirmed. From this, it can be seen that almost all iridium exists in the state of iridium oxide.

[0090] For the calculation of the existence ratio of platinum oxide, the waveform of the obtained XPS distribution was separated, and the peak areas of each state of PtO 2 , PtO, metallic Pt, and iridium (oxide) were measured. In addition, the peak areas of each element of oxygen (O), tin (Sn), and carbon (C) were also measured simultaneously. Then, the peak areas of each component of Pt, Ir, O, Sn, and C were corrected with their respective relative sensitivity coefficients (RSF), and based on the total of the peak areas of each component (100), the existence ratios of PtO 2 , PtO, and metallic Pt were calculated. Further, based on the calculated existence ratios of PtO 2 , PtO, and metallic Pt, the ratio of platinum oxide to metallic platinum (R TO ) was calculated. It should be noted that the analytical software (Avantage-Thermo) was used in the above analysis.

[0091] Evaluation test of methane combustion performance

[0092] Next, using each of the manufactured methane combustion catalysts, a test for performance evaluation was conducted. In this evaluation test, each catalyst was installed in a test apparatus of a fixed-bed type reaction apparatus simulating Figure 1 , and the test gas was passed through to measure the methane conversion rate. The test conditions are as follows.

[0093] · Reaction temperature (catalyst temperature): 400 °C

[0094] · Test gas composition

[0095] CH 4 : 2000 ppm

[0096] CO 2 : 5%

[0097] O 2 : 10%

[0098] H 2 O: 10%

[0099] SO 2 : 1 ppm

[0100] N 2 : Balance

[0101] · Space velocity (GHSV): 80,000 h -1

[0102] Under the above conditions, the test gas was passed through the catalyst, and the composition of the exhaust gas was analyzed at the time point 5 hours after the start of the test to measure the methane conversion rate. For the measurement of the methane conversion rate, the exhaust gas was analyzed using an FID type THC gas analyzer, a non-dispersive infrared analyzer, and a magnetic oxygen analyzer to obtain the CH 4 , CO 2 , O 2 concentrations. Then, based on the measured values, the methane conversion rate was calculated using the following formula.

[0103] [Mathematical formula 2]

[0104]

[0105] The results of the evaluation test of the methane combustion performance conducted in this embodiment and the results of the ratio (R TO ) of platinum oxide analyzed by XPS for each methane combustion catalyst are shown in Table 1.

[0106] [Table 1]

[0107]

[0108] As can be seen from Table 1, the methane conversion rates of the methane combustion catalysts of Examples 1 to 4 exceeded 80%. When compared with the methane combustion catalysts of Comparative Examples 1 and 2, they can be considered to have excellent activity. Then, when observing the ratio (R TO ) of platinum oxide measured by XPS, it can be seen that the methane combustion catalysts of Examples 1 to 4 were all 8.00 or more, and platinum oxide was generated in a high proportion. This can also be confirmed from the Figure 2 Pt4f spectrum. On the other hand, in the catalyst with a high firing temperature (650 °C) in Reference Example 1, the ratio (R TO ) of platinum oxide measured by XPS became low, less than 8.00. Moreover, the methane conversion rate was also lower than that of each example.

[0109] Regarding the methane combustion catalysts of Comparative Examples 1 and 2, the formation of platinum oxide was also fully confirmed, but the ratio was significantly lower than that of Examples 1 to 4. It is considered that the catalyst activity is basically proportional to the platinum loading amount. The platinum loading amounts of the examples and the comparative examples are the same. Therefore, it is considered that the activity is improved by increasing the ratio of platinum oxide.

[0110] Based on the difference in the ratio of platinum oxide, the importance of the drying treatment after impregnation with the platinum salt solution and the temperature management in the firing process in the basic process of this embodiment can be understood. In Comparative Manufacturing Process 1 for manufacturing the catalyst of Comparative Example 1, the drying treatment temperature was high, and the firing temperature was also high. It is considered that due to the high-temperature drying treatment, decomposition of the platinum salt occurred, so the formation of platinum oxide in the subsequent firing was insufficient. In addition, decomposition of platinum oxide occurred due to the high firing temperature. In addition, in Comparative Manufacturing Process 2 for manufacturing the catalyst of Comparative Example 2, a platinum salt solution containing the target loading amount (8.0 mass%) of platinum was loaded at one time without relying on divided loading. Therefore, it is considered that the dispersion of platinum was poor and the formation of platinum oxide was also insufficient.

[0111] Second Embodiment : In this embodiment, methane combustion catalysts with different platinum loading amounts and iridium loading amounts were manufactured and their performance was evaluated. Here, in the same process as the basic manufacturing process of the first embodiment, while changing the platinum concentration of the platinum salt solution and the iridium concentration of the iridium salt solution, the loading amounts of each noble metal were adjusted to manufacture a plurality of methane combustion catalysts. In addition, a methane combustion catalyst for comparison was also manufactured based on Comparative Manufacturing Process 1. Then, the same performance evaluation test as in the first embodiment was carried out. The performance evaluation test was carried out at two reaction temperatures (340 °C, 400 °C). The results are shown in Table 2.

[0112] [Table 2]

[0113]

[0114] As can be seen from Table 2, when the reaction temperature is set at 340 °C, the methane combustion activity of the methane combustion catalysts of the comparative examples is extremely poor. On the other hand, the methane combustion catalysts of the respective examples exhibit methane combustion activity even at 340 °C. When comparing catalysts with the same noble metal loading amount (Example 9 and Comparative Example 4), it is obvious that the methane combustion catalysts of the examples are highly active.

[0115] Furthermore, by raising the reaction temperature to 400 °C, the methane combustion catalysts of the respective examples exhibit remarkable methane combustion activity. In particular, by setting the platinum loading amount to 4 mass% or more, a high activity of 70% or more is exhibited. It should be noted that even the catalyst with a platinum loading amount of 2.1 mass% (Example 5) exhibits the same activity as Comparative Example 4 with a platinum loading amount of 8.0 mass%. Therefore, considering the catalyst cost and according to the required performance, this catalyst can be considered a useful catalyst.

[0116] Third Embodiment : In the present embodiment, the appropriate range of the treatment temperature of the firing process after impregnation and drying of the platinum salt solution was confirmed. In the basic manufacturing process of the first embodiment, the firing temperature after the platinum loading process was set at 350 °C (Example 12), 450 °C (Example 13), 500 °C (Example 14), 550 °C (Reference Example 2), and 600 °C (Reference Example 3), thereby manufacturing catalysts. It should be noted that the temperature of the firing process of iridium is the same as that of the firing process of platinum. In addition, based on Comparative Manufacturing Process 1 (drying temperature 175 °C, firing temperature 550 °C), a methane combustion catalyst for comparison (Comparative Example 5) was also manufactured. Then, the same performance evaluation test as in the first embodiment (reaction temperature 400 °C) was conducted on each catalyst. The results are shown in Table 3.

[0117] [Table 3]

[0118]

[0119] As can be seen from Table 3, the methane combustion catalysts fired at 350 °C (Example 12), 450 °C (Example 13), and 500 °C (Example 14) showed high methane conversion rates of over 90%. On the other hand, it can be seen that the methane conversion rate of the methane combustion catalyst fired at a temperature exceeding 500 °C (550 °C: Reference Example 2, 600 °C: Reference Example 3) tended to decrease. Therefore, based on the results of this embodiment, it was also confirmed that the treatment temperature in the firing process of platinum is preferably 500 °C or lower. However, the activity of the methane combustion catalyst fired at 550 °C in Reference Example 2 was higher than that of the methane combustion catalyst in Comparative Example 5 manufactured at the same firing temperature. It is believed that in the manufacturing process of the catalyst in Comparative Example 5 (Comparative Manufacturing Process 1), the drying temperature after impregnation with the platinum salt solution was high (175 °C), and the difference in the drying temperature caused a difference in the formation of platinum oxide, resulting in poor activity in Comparative Example 5.

[0120] In addition, in this embodiment, the metal dispersion of the methane combustion catalysts of Example 13, Reference Example 2, Reference Example 3, Reference Example 4, and Comparative Example 5 was measured / evaluated by the CO gas adsorption method. In the analysis using the CO gas adsorption method, the sample mass was set to 50 mg, and pretreatment with a He gas flow was performed at 50 °C for 2 hours, and then CO gas adsorption measurement was performed at 50 °C. It should be noted that in this evaluation test, a catalyst (Reference Example 4) obtained by applying the basic manufacturing process and setting the firing temperature in the firing process to a higher temperature (650 °C), and a catalyst (Comparative Example 6) obtained by using a manufacturing method of loading platinum by a single impregnation operation in Comparative Manufacturing Process 2 without relying on divided loading were manufactured and the same evaluation was performed. The measurement results are shown in Table 4.

[0121] [Table 4]

[0122]

[0123] When comparing Example 13 (firing temperature 450 °C), Reference Example 2 (firing temperature 550 °C), Reference Example 3 (firing temperature 600 °C), and Reference Example 4 (firing temperature 650 °C) in Table 4, it can be seen that the metal dispersion decreases as the firing temperature increases. It is believed that in the methane combustion catalyst manufactured by firing at 550 °C to 650 °C, the sintering of platinum particles metallized by the decomposition of platinum oxide makes the particle size coarser, resulting in a decrease in metal dispersion.

[0124] The catalyst of Comparative Example 5 has a higher drying temperature and firing temperature than that of Example 13, so the metal dispersion is poor. In addition, although the firing temperature of the catalyst of Comparative Example 6 is the same as that of Example 13 (450 °C), it is a catalyst manufactured by loading the platinum salt solution at one time instead of by divided loading. It is considered that since the metal salt solution is not dividedly loaded, the dispersion of platinum is poor and the formation of platinum oxide is also insufficient.

[0125] [Industrial Applicability]

[0126] The Pt / SnO 2 -based methane combustion catalyst of the present invention has excellent methane combustion activity compared to the prior art. This is because platinum oxide as an active source is effectively generated during its manufacturing process. According to the present invention, by improving the initial activity, it is possible to treat combustion exhaust gas for a long time while suppressing catalyst poisoning caused by sulfur oxides. The methane combustion catalyst of the present invention can be suitably applied to the purification of various exhaust gases of engines, boilers, power generation systems, etc. that use hydrocarbon fuels such as natural gas and town gas. In addition, 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, which is a methane combustion catalyst obtained by loading a catalyst component composed of platinum and iridium on a tin oxide support and used for combusting methane in combustion exhaust gas containing sulfur oxides, characterized in that When measuring the methane combustion catalyst using X-ray photoelectron spectroscopy (XPS), based on the abundance ratios of metallic platinum (Pt), platinum oxides (PtO and PtO 2 ) obtained from the platinum 4f spectrum and the ratio R of platinum oxide to metallic platinum calculated by the following formula TO is 8.25 or more and 16.00 or less, [Mathematical formula 1] R TO = (R PtO + R PtO2 ) / R Pt Here, R Pt is the existence ratio of platinum (Pt), R Pto is the existence ratio of PtO, R Pto2 is the existence ratio of PtO 2 and R Pt 、R PtO 、R PtO2 are calculated as follows: Based on the Pt 4f spectrum observed when analyzing the catalyst using XPS, calculate R, R, and R respectively according to the peak area of metallic Pt that appears in the range of 71.0 eV to 72.0 eV, the peak area of PtO that appears in the range of 72.8 eV to 73.2 eV, and the peak area of PtO that appears in the range of 74.6 eV to 75.0 eV 2 2 Pt 3 PtO 4 PtO2 .

2. The methane combustion catalyst according to claim 1, wherein the loading amount of platinum based on the mass of the whole catalyst is 2.0% by mass or more and 15.0% by mass or less.

3. The methane combustion catalyst according to claim 1 or claim 2, wherein the loading amount of iridium based on the mass of the whole catalyst is 0.1% by mass or more and 5.0% by mass or less.

4. The methane combustion catalyst according to any one of claims 1 to 3, which is in any one of the shapes of granular, particulate, pill-shaped, or sheet-shaped.

5. The methane combustion catalyst according to any one of claims 1 to 3, which is supported by a support in any one of the shapes of plate-shaped, cylindrical, spherical, or honeycomb-shaped.

6. A method for manufacturing a methane combustion catalyst, which is a method for manufacturing the methane combustion catalyst according to any one of claims 1 to 5, comprising: a first loading step of impregnating a platinum salt solution into a support composed of tin oxide and a first firing step of firing the support after the first loading step; and a second loading step of impregnating an iridium salt solution into the support after the first firing step and a second firing step of firing the support after the second loading step, wherein the first loading step is a step of impregnating the support with a platinum salt solution having a lower platinum content than the platinum salt solution containing the target loading amount of platinum multiple times, and in each of the multiple impregnations, impregnating the target loading amount of platinum by drying the impregnated support at a temperature of 60°C or higher and 150°C or lower, furthermore, setting the heating temperature of the first firing step to 350°C or higher and 500°C or lower.

7. The method for manufacturing a methane combustion catalyst according to claim 6, wherein in the first loading step, a platinum salt solution having an equal platinum content is impregnated multiple times.

8. The method for manufacturing a methane combustion catalyst according to claim 6 or claim 7, wherein setting the heating temperature of the second firing step to 350°C or higher and 500°C or lower.

9. A method for purifying combustion exhaust gas, which is a method for purifying combustion exhaust gas by oxidizing and removing methane in combustion exhaust gas containing sulfur oxides, characterized in that setting the reaction temperature to 340°C or higher and 500°C or lower and bringing the combustion exhaust gas into contact with the methane combustion catalyst according to any one of claims 1 to 5.

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