Methane combustion catalyst and method for producing same

Abstract

To provide, about a methane combustion catalyst with zirconia as a carrier and with Pt, Pd as catalyst metal, a methane combustion catalyst excellent durability by suppressing sintering of catalyst particles.SOLUTION: The present invention relates to a methane combustion catalyst in which catalyst particles made of at least one of Pt, Pd, or an alloy thereof are carried on a zirconia carrier. In the methane combustion catalyst according to the present invention, the zirconia carrier contains silica. Then, the coefficient of variation (CV value) of a ratio (CSi / CZr) between the Si concentration (CSi) and the Zr concentration (CZr) based on the essential metal concentration when arbitrarily analyzing a plurality of regions for the methane combustion catalyst of the present invention.) is 30% or less. In the present invention, the uniform dispersion of fine silica suppresses sintering of the zirconia carrier, thereby reducing the activity lowering due to sintering of catalyst particles.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a catalyst for oxidizing and removing methane contained in various gases. More specifically, it relates to a methane combustion catalyst having Pt and Pd supported on a zirconia carrier, which has superior durability compared to conventional catalysts. [Background technology]

[0002] Exhaust gases from fuels such as natural gas and city gas contain unburned hydrocarbon components such as methane, in addition to carbon dioxide and nitrogen oxides. All of these can be considered causes of environmental pollution, but until now, carbon dioxide has been the target of emission reductions because it has the greatest greenhouse effect. However, recently, there has been increasing discussion about regulating emissions of methane in addition to carbon dioxide. Although methane has a lower concentration in the atmosphere than carbon dioxide, its greenhouse effect per molecule is greater than that of carbon dioxide. For this reason, methane is considered the second most potent greenhouse gas after carbon dioxide, and a 30% reduction target by 2030 has been set under the agreement reached at the 26th Conference of the Parties to the United Nations Framework Convention on Climate Change (COP26) held at the end of last year.

[0003] Treatment using purification catalysts is a well-known method for reducing pollutants contained in various gases. For the removal of methane, catalysts that oxidize and burn methane and have Pt and / or Pd supported as catalyst particles on inorganic oxide supports such as zirconia (ZrO2) and alumina (Al2O3) are considered effective (Patent Documents 1 to 3). In particular, zirconia supports are known to impart high initial activity to methane combustion catalysts. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-225343 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-91119 [Patent Document 3] Special Publication No. 2019-534777 Summary of the Invention [Problem to be solved by the invention]

[0005] In general, challenges in catalyst technology development include improving initial catalytic activity while ensuring durability so that activity is maintained over the long term. This is also true for methane combustion catalysts, but ensuring durability is particularly important for methane combustion catalysts. Because methane has low reactivity, a relatively high reaction temperature is required for effective combustion. It is known that the cause of catalyst activity decline is sintering due to the movement and aggregation of catalyst particles at high temperatures. Therefore, the trend toward higher reaction temperatures promotes the movement and sintering of catalyst particles, resulting in lower durability of methane combustion catalysts.

[0006] In previous studies of methane combustion catalysts, the initial activity has often been determined by optimizing the inorganic oxides that serve as supports and the materials that make up the catalyst particles. However, durability has not been sufficiently studied, and the main approach has been to increase the amount of catalyst metal (catalyst particles) supported.

[0007] However, increasing the amount of catalytic metal loaded naturally leads to an increase in the cost of methane combustion catalysts. The price of precious metal bullion has been steadily rising in recent years, with the price of Pd in ​​particular experiencing a notable increase. As mentioned above, the reduction of methane emissions is an urgent issue, and demand for methane combustion catalysts is expected to increase, making cost a major problem. Furthermore, the measure of increasing the amount of catalytic metal loaded is intended to compensate for catalyst particles that have been deactivated by sintering, and cannot be considered a measure to directly suppress sintering.

[0008] The present invention was made in light of the above background, and aims to directly address the problem of sintering of catalyst particles at high temperatures in methane combustion catalysts. To this end, the present invention provides a methane combustion catalyst that uses zirconia as a support and Pt and / or Pd as a catalytic metal, and has improved durability, as well as a method for producing the same. [Means for solving the problem]

[0009] To solve the above problems, the present inventors have investigated methane combustion catalysts in which catalyst particles (Pt, Pd) are supported on a zirconia support, and have investigated methods for suppressing activity decline from the perspective of improving the microstructure of the zirconia support. The reason for focusing on zirconia supports is that zirconia supports are particularly effective in initial activity compared to other inorganic oxides such as silica. Through this investigation, the present inventors have concluded that sintering of catalyst particles progresses because sintering also occurs in the zirconia support, and that this is involved in the sintering of catalyst particles.

[0010] Based on this consideration, the mechanism of catalytic activity decline in methane combustion catalysts is explained using Figure 1. Zirconia supports are aggregates of zirconia particles, and catalytic metals are adsorbed and supported on individual zirconia particles (Figure 1(a)). During methane combustion, if the zirconia particles remain intact, even if catalyst particles move and sinter on individual zirconia particles, they are unlikely to move to other zirconia particles, and further sintering is unlikely to occur (Figure 1(b)). However, if zirconia particles also sinter during methane combustion, the range of catalyst particle movement expands, causing further sintering of the catalyst particles (Figure 1(c)). Such further sintering of the catalyst particles further reduces catalytic activity.

[0011] According to the inventors' investigations, the sintering of zirconia particles discussed above is a phenomenon that actually occurs, as will be described later. Therefore, the inventors considered that suppressing the sintering of the zirconia particles, which serve as the carrier, would be effective in directly suppressing the sintering of catalyst particles, and investigated methods to achieve this. As a result, they came up with the idea of ​​placing silica (SiO2), another inorganic oxide, between zirconia particles and using this silica as a spacer to suppress the sintering of zirconia particles (Figure 1(d)).

[0012] However, even if the above-mentioned arrangement of silica spacers is effective, if the dispersion state is uneven, sufficient effect cannot be obtained. This is because sintering of zirconia particles occurs in areas where there are no spacers locally, resulting in sintering of catalyst particles in those areas. The inventors conducted further studies and discovered a method for preparing a carrier for favorably dispersing silica, which serves as a spacer for zirconia particles, and an optimal dispersion state in a methane combustion catalyst, which led to the present invention.

[0013] The present invention, which solves the above-mentioned problems, provides a methane combustion catalyst comprising a zirconia support and catalyst particles made of at least one metal selected from Pt and Pd or an alloy thereof, wherein the zirconia support contains silica, and when a plurality of regions of the methane combustion catalyst are analyzed, the Si concentration (C Si ) and Zr concentration (C Zr ) and the ratio (C Si / C Zr The methane combustion catalyst is characterized in that the coefficient of variation (CV value) of the methane combustion catalyst is 30% or less. The methane combustion catalyst and the method for producing the same according to the present invention will be described in detail below.

[0014] A. Configuration of methane combustion according to the present invention As described above, the methane combustion catalyst according to the present invention is a catalyst in which catalyst particles made of Pt and / or Pd are supported on a zirconia carrier having a predetermined configuration. Each configuration will be explained below.

[0015] (I) Catalyst particles (catalytic metal) The catalyst particles for methane combustion according to the present invention are made of Pt and / or Pd. Pt and Pd are active in the reaction of oxidizing and burning methane in cooperation with the zirconia support. The supported catalyst particles may consist of only Pt or only Pd. Alternatively, both Pt and Pd may be supported, in which case the catalyst particles may be formed by alloying Pt and Pd, or Pt particles and Pd particles may be dispersed and supported. The particle size of the catalyst particles is preferably 3 nm or more and 15 nm or less.

[0016] The amount of catalyst particles made of Pt and / or Pd supported is preferably 3% by mass or more and 20% by mass or less based on the mass of the entire catalyst. If it is 3% by mass or less, the required activity cannot be obtained. Furthermore, if it exceeds 20% by mass, there is no significant improvement in initial activity, which increases the cost of the catalyst. In the present invention, durability can be ensured without increasing the amount of catalyst particles supported, so the amount supported can be set within the above range to obtain the required initial activity. Note that the mass of the entire catalyst, which is the basis for the supported amount, is the sum of the zirconia support and the catalyst metal. When the methane catalyst of the present invention is supported on a support such as a honeycomb, as described below, this support is not included in the catalyst.

[0017] (II) Zirconia support The term "zirconia carrier" refers to a carrier made of an inorganic oxide containing zirconia (ZrO2) as the main component (preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 95% by mass or more, and particularly preferably 99% by mass or more). The zirconia carrier may be a carrier made of zirconia alone and containing no other components than silica, which will be described later. Also applicable as the zirconia carrier is an inorganic oxide carrier in which zirconia is doped with a metal other than a noble metal (platinum, iridium, etc.) or a compound thereof to the extent that the catalytic properties are not affected. Furthermore, the zirconia carrier may contain unavoidable impurities such as Hf in an amount of up to 10% by mass.

[0018] A catalyst support generally has the function of maintaining catalyst particles in an appropriate dispersed state. The zirconia support of the present invention not only has this inherent function of a support, but also has the important function of enabling catalyst particles (Pt, Pd) to exhibit methane combustion activity. While the reason why zirconia is essential as a support is unclear, the use of zirconia clearly enhances methane combustion activity compared to other inorganic oxides.

[0019] The zirconia support of the methane combustion catalyst according to the present invention contains silica (SiO2). The silica is dispersed in the zirconia support and coats part or all of the zirconia particles. As described above, this silica suppresses sintering of the zirconia particles. Silica is used as the inorganic oxide added to the zirconia support to suppress sintering because silica has high heat resistance and is less susceptible to change or deterioration in the high-temperature atmosphere of methane combustion, thereby maintaining its sintering-suppressing effect on the zirconia particles. Furthermore, silica has good compatibility with the zirconia support, and even when mixed with the zirconia support, which is an aggregate of zirconia particles, it can maintain the support without separating the zirconia particles. In other words, silica is an optimal additive because it functions as a spacer to suppress sintering in the zirconia support and can also act as a binder.

[0020] In the methane combustion catalyst according to the present invention, silica is uniformly dispersed in the zirconia carrier. As described above, the reason why the silica needs to be uniformly dispersed is to ensure a uniform effect of suppressing sintering of zirconia particles. In the present invention, the concentration distribution of Si derived from silica is specified as a requirement for the dispersion state of silica. Specifically, the Si concentration (C Si ) and Zr concentration (C Zr ) and the ratio (C Si / C ZrThe coefficient of variation (CV value) of the Si concentration ratio (C) is set to 30% or less. The reason why the standard of the Si concentration is set to the Zr concentration is that the carrier in the present invention is basically composed of zirconia and silica, and Zr derived from zirconia is the main element of the carrier. In addition, the Si concentration ratio (C Si / C Zr ) is measured to confirm the uniformity of silica dispersion. Furthermore, this uniformity is confirmed by the Si concentration ratio (C Si / C Zr The reason for the judgment based on the coefficient of variation (CV value) of the Si concentration ratio (C) is to evaluate the uniformity of the Si concentration without considering the amount of silica added, taking into consideration that the amount of silica added to the carrier (Si concentration) is allowed to have a certain degree of variation, as will be described later. The coefficient of variation CV is calculated by the Si concentration ratio (C Si / C Zr ) are the mean and standard deviation of C av , ρ av When this is done, "CV=(ρ av / C av ) × 100”. Also, the C Si / C Zr The coefficient of variation is more preferably 28% or less.

[0021] The present invention's Si concentration (C Si ) and Zr concentration (C Zr ) is a concentration based on the essential metal concentration standard. The concentration based on the essential metal concentration standard is a concentration when the total concentration of the four elements Pt and / or Pd, Zr, and Si, which are metals that are essential to the methane combustion catalyst of the present invention, is taken as 100%. In the present invention, Si, which is a semimetal, is also considered to be a metal. The Si concentration ratio (C Si / C Zr As a specific embodiment of the measurement of the coefficient of variation of the Si concentration (C), it is preferable to arbitrarily set at least five or more measurement areas in order to statistically evaluate the uniformity of the dispersed state of silica. Si ) and Zr concentration (C Zr) can be measured by various analytical methods such as electron probe microanalysis (EPMA), energy dispersive X-ray analysis (EDX), fluorescent X-ray analysis (FRX), and X-ray photoelectron spectroscopy (XPS), and elemental concentrations are analyzed using a scheme appropriate for each analytical method. In these analytical methods, the magnification of the measurement area is set to between 500 and 5000 times, and the analysis area per location is set to 0.5 μm 2 More than 10μm 2 It is preferable to set the following: When analyzing a methane combustion catalyst formed on a support, the analysis may be performed while the catalyst is supported on the support, or the methane combustion catalyst may be peeled off and collected from the support and then analyzed.

[0022] The methane combustion catalyst according to the present invention has a Si concentration ratio (C Si / C Zr ) must be 30% or less. If it exceeds 30%, the silica will not be dispersed uniformly, making it difficult to effectively suppress sintering of zirconia particles. The smaller this coefficient of variation is, the better (more uniform), but it is extremely difficult to reduce it to zero, so it is preferable to set the lower limit at around 3%.

[0023] Furthermore, the content of silica in the zirconia carrier is related to the Si concentration in the catalyst. The Si concentration of the methane combustion catalyst according to the present invention is preferably 1% by mass or more and 12% by mass or less. Assuming that silica is uniformly dispersed in the zirconia carrier, the size of the silica is proportional to its content. In the catalyst, catalyst particles (Pt, Pd) are also adsorbed and supported on the surface of the silica. Therefore, when the Si concentration is high, that is, when the silica content is high, the proportion of catalyst particles supported on silica increases. Since silica does not have an effective effect on catalyst particles in terms of catalytic activity (initial activity), an increase in the proportion of catalyst particles supported on silica results in a decrease in catalytic activity. Therefore, the Si concentration in the catalyst is preferably 12% by mass or less. On the other hand, if the Si concentration is too low, regions of the zirconia carrier lacking silica will be generated, and the Si concentration ratio (C Si / CZr ) exceeds 30%, which may cause sintering of the zirconia particles. Therefore, the Si concentration is preferably 1 mass% or more. The Si concentration is more preferably 2 mass% or more and 10 mass% or less, and even more preferably 3 mass% or more and 8 mass% or less. The Si concentration is based on the essential metal concentration as above, and its average value is preferably within the above range. In addition, the Si concentration is determined by the above-mentioned Si concentration ratio (C Si / C Zr ) is measured in multiple regions. Therefore, the average value of the Si concentration can be calculated using the results.

[0024] The methane combustion catalyst according to the present invention is constructed by supporting catalytic particles made of Pt and / or Pd on the silica-containing zirconia support described above. The form of the zirconia support is adapted to the form of the methane combustion catalyst. Here, a particularly widely used form of methane combustion catalyst is one in which a zirconia support is fixed to an appropriate support and catalyst particles are supported thereon. Supports in this catalyst form are known to have any of the following shapes: plate, cylindrical, spherical, and honeycomb. Supports are known to be made of metal or ceramic. In such a methane combustion catalyst, silica-containing zirconia particles are made into a slurry (so-called washcoat). The zirconia support can be formed by applying and fixing this slurry to a support.

[0025] The methane combustion catalyst according to the present invention may be in the form of particles, granules, pellets, or tablets. In such a methane combustion catalyst, the zirconia support has the same form as particles, granules, pellets, or tablets.

[0026] B. Physical Properties of the Methane Combustion Catalyst of the Present Invention The methane combustion catalyst according to the present invention uses a zirconia support in which the silica described above is dispersed in an appropriate state, thereby suppressing support sintering even at high temperatures. This sintering suppression effect results in characteristic behavior in the physical properties of the methane combustion catalyst according to the present invention. That is, the methane combustion catalyst according to the present invention is preferably one in which, when heated at a high temperature of 700°C, the rate of increase in the crystallite diameter of the zirconia constituting the zirconia support is 30% or less. The crystallite diameter is the diameter of the crystals constituting the polycrystalline zirconia particles, and is the unit dimension of the largest region that can be regarded as a single crystal. Since the crystallite diameter of zirconia particles increases upon sintering at high temperatures, a preferred embodiment of the present invention is one in which the rate of increase in the crystallite diameter at this time is suppressed to 30% or less.

[0027] The reason why the crystallite size increase rate is determined after heating at 700°C is that this temperature is likely to cause sintering of zirconia particles, taking into account the operating temperature of the methane combustion catalyst. Furthermore, the crystallite size and its increase rate can be measured by X-ray diffraction analysis (XRD) of the methane combustion catalyst at room temperature and after heating at 700°C. Based on the diffraction pattern obtained by XRD, a diffraction peak from any crystal plane is selected, and the half-width of the diffraction peak is measured, and the crystallite size can be calculated based on the Scherrer formula. When measuring the increase rate of the crystallite size, the catalyst is preferably heated for a period of 1 hour to 5 hours.

[0028] Furthermore, suppressing sintering of zirconia particles also affects the behavior of the specific surface area of ​​the methane combustion catalyst according to the present invention. Similar to the increase rate of the crystallite size, the methane combustion catalyst according to the present invention exhibits a decrease rate of the specific surface area of ​​30% or less when heated at 650°C. The specific surface area can be measured by known methods such as a gas adsorption method, such as the BET method. In the methane combustion catalyst according to the present invention, the preferred specific surface area of ​​the zirconia carrier is 60 m 2 / g or more 150m 2 / g or less, more preferably 70m 2 / g or more 110m 2 / g or less. This preferred range of specific surface area does not depend on the form of the catalyst, and the above specific surface area is preferred even when the catalyst is in the form of pellets or washcoated on a support. Based on these preferred specific surface areas, it is preferred that the reduction rate of the specific surface area after heating at 650°C is 30% or less.

[0029] C. Method for producing a methane combustion catalyst according to the present invention Next, a method for producing a catalyst according to the present invention will be described. The methane combustion catalyst according to the present invention is produced using the same basic process as a conventional methane combustion catalyst in which Pt or the like is supported on a zirconia support. That is, a methane combustion catalyst can be produced by preparing a zirconia support and then adsorbing a compound solution of catalytic metals onto the support to support catalyst particles. Preparing the zirconia support involves forming a slurry of zirconia powder in an appropriate solvent, and, if necessary, coating and fixing the slurry on a support. The methane combustion catalyst according to the present invention uses a zirconia support in which silica is uniformly dispersed so as to satisfy the above-mentioned requirements. The method for producing a methane combustion catalyst according to the present invention is characterized by adding and uniformly dispersing silica particles. This characteristic is achieved in the process of preparing the zirconia support. Below, the method for producing a methane combustion catalyst according to the present invention will be described in detail, focusing on a methane combustion catalyst supported on a support.

[0030] (1) Zirconia support preparation process In preparing the zirconia support, zirconia powder is mixed with a dispersion medium such as water to prepare a slurry. In this mixing step, the zirconia powder is typically pulverized and dispersed in the dispersion medium simultaneously using a wet mill such as a ball mill. The mixing and uniform dispersion of silica particles, which is a characteristic of the zirconia support of the methane combustion catalyst according to the present invention, is achieved in this mixing step.

[0031] As described above, the methane combustion catalyst according to the present invention is composed of a zirconia carrier with silica uniformly dispersed therein. One way to achieve this uniform dispersion of silica is to optimize the particle size of the silica particles added in the mixing step and the mixing and crushing conditions using a crushing device.

[0032] The silica particles added during the mixing process must be relatively small, less than 10 nm in diameter. Adding silica particles exceeding 10 nm reduces the effect of suppressing sintering of the zirconia powder. As the silica particle diameter increases, it becomes difficult for the silica to be dispersed over a wide area of ​​the zirconia powder. Even taking this into account, the silica particles are also crushed during the mixing process, making it difficult to disperse large silica particles over a wide area. Furthermore, increasing the amount of silica added to broaden the dispersion range increases the number of catalyst particles supported on the silica, which may result in a decrease in activity. Therefore, it is necessary to achieve uniform dispersion by adding fine silica particles from the carrier preparation stage. The lower limit of the silica particle diameter is approximately 1 nm, taking into account the limits of production feasibility. Commercially available silica sols can be used as silica particles of the above particle diameter that can be used in the present invention. Furthermore, in order to set the Si concentration in the methane combustion catalyst within the above-mentioned preferred range, the amount of silica particles added in this step is preferably 2% by mass or more and 15% by mass or less, and more preferably 3% by mass or more and 12% by mass or less, relative to the zirconia powder.

[0033] To uniformly disperse silica in the zirconia carrier, it is necessary to add silica particles of the above particle size and to thoroughly mix and grind them. The conditions for this mixing and grinding are that the grinding must be carried out until the particle size reduction rate is 50% or more, based on the average particle size of the zirconia powder before and after grinding. The particle size reduction rate is calculated by dividing the average particle size r of the zirconia powder before grinding. s The average particle size of the zirconia powder after grinding is r p When "(r s -r p ) / r s× 100". By such thorough pulverization to reduce the particle size by half or more, the contact area between the silica particles and the zirconia powder is increased, while the silica concentration (Si concentration) relative to the zirconia is made uniform at each location. The particle size reduction rate in pulverization is more preferably 55% or more, and even more preferably 60% or more. The upper limit of the particle size reduction rate is preferably 80%. Even if pulverization is performed to reduce the particle size by more than 80%, no significant improvement in the uniformity of silica in the carrier is observed.

[0034] The particle size of the zirconia powder to be ground as a raw material for the zirconia carrier in the present invention is preferably an average particle size of 3 μm to 8 μm, and the zirconia powder as a raw material preferably has the above average particle size and a particle size distribution in the range of 2 μm to 12 μm.

[0035] In the carrier preparation step, the slurry prepared through the above-mentioned mixing step is applied to a support and fixed by drying, etc., to form a zirconia carrier. The slurry can be applied to the support by various known methods such as air blowing, spraying, dipping, etc.

[0036] After the slurry is applied to the support, it is preferably dried and / or fired to remove the dispersion medium of the slurry and fix the silica-containing zirconia powder to the support. The drying temperature is preferably 80°C or higher and 150°C or lower. The firing temperature is preferably 400°C or higher and 600°C or lower.

[0037] Up to this point, we have explained a method for forming a zirconia carrier on a support, but methane combustion catalysts can also be in the form of pellets, granules, etc. For supports for these forms of methane combustion catalysts, zirconia powder can be separated from the slurry in which silica is uniformly dispersed as described above, and then granulated and molded by an appropriate method into the desired shape, such as pellets. Even when producing catalysts in the form of pellets, it is preferable to use zirconia powder that has been subjected to a wet process first in order to uniformly disperse the silica.

[0038] (2) Catalyst metal (Pt, Pd) loading process Pt and / or Pd are supported on the zirconia support prepared as above by impregnation treatment with a metal salt solution of each metal.

[0039] Examples of Pt metal salt solutions to be impregnated into the zirconia support include platinum nitrate aqueous solution, platinum chloride aqueous solution, platinum acetate aqueous solution, as well as platinum complex solutions such as tetraammine platinum salt aqueous solution, dinitrodiammine platinum-ammonia aqueous solution, dinitrodiammine platinum-ethanolamine solution, dinitrodiammine platinum-nitric acid aqueous solution, etc. Of these platinum salt solutions, dinitrodiammine platinum-nitric acid aqueous solution, dinitrodiammine platinum-ethanolamine solution, and platinum nitrate aqueous solution are preferred.

[0040] Examples of the Pd metal salt solution include a palladium nitrate solution, a palladium chloride solution, and an aqueous solution of dinitrodiamminepalladium-nitric acid. Among these palladium salt solutions, the preferred are a palladium nitrate solution and an aqueous solution of dinitrodiamminepalladium-nitric acid.

[0041] In the case of impregnating the zirconia support with the metal salt solution, when both Pt and Pd are to be supported, the respective metal salt solutions may be simultaneously impregnated into the zirconia support or separately, and the order of impregnation is not limited. There is no particular limitation on the method of impregnation with the metal salt solution, and any of spraying, dropping, and dipping may be used.

[0042] After impregnation with the Pd and / or Pd metal salt solution, the material is dried appropriately and then calcined. The drying temperature is preferably 80°C or higher and 150°C or lower. The calcination temperature is preferably 400°C or higher and 600°C or lower.

[0043] By the above-mentioned calcination treatment, Pt and / or Pd are supported on the zirconia support, and the methane combustion catalyst according to the present invention can be obtained.

[0044] (D) Method for burning methane using the methane combustion catalyst according to the present invention The methane combustion method using the methane combustion catalyst according to the present invention as described above is basically the same as the conventional method. The target of the methane combustion method according to the present invention is a combustion exhaust gas containing methane and sulfur oxides. In addition to methane and sulfur oxides, the combustion exhaust gas may also contain other hydrocarbons such as ethane and propane, as well as combustible components such as carbon monoxide, oxygen, oxygen-containing compounds, and nitrogen oxides.

[0045] In the combustion of methane in various gases, the gas to be treated is passed through a combustion device equipped with the methane combustion catalyst of the present invention and brought into contact with the methane combustion catalyst. Any known combustion device can be used, such as a fixed-bed flow reactor. The amount of catalyst used in such combustion devices is generally determined by the gas hourly space velocity (GHSV). In the present invention, the space velocity is set to 80,000 h in order to ensure the methane combustion efficiency. -1 It is preferable that the space velocity is 1,000 h or less. Since lowering the space velocity improves catalytic activity, the lower the space velocity, the better. However, taking into consideration catalytic activity, economy, and pressure loss, the space velocity should be 1,000 h -1 More than this is preferable.

[0046] The heating temperature of the methane combustion catalyst for purifying various gases, that is, the reaction temperature, is set to 250° C. or higher and 500° C. or lower, and more preferably 300° C. or higher and 450° C. or lower. [Effects of the Invention]

[0047] As described above, the present invention clarifies that one of the causes of activity decline in methane combustion catalysts in which precious metal particles such as Pt are supported on a zirconia support is sintering of the zirconia particles that make up the support. Furthermore, it proposes that silica be dispersed in a suitable state to suppress sintering of the zirconia support. The methane combustion catalyst of the present invention can improve initial activity and ensure durability. [Brief explanation of the drawings]

[0048] [Figure 1]FIG. 1 is a diagram illustrating the mechanism of activity decline in a methane combustion catalyst using a conventional zirconia support and the effect of adding silica in the present invention. [Figure 2] 1 shows XRD diffraction profiles of methane combustion catalysts of Example 1 and Comparative Example 1 of the first embodiment. [Figure 3] 1 is a graph showing the results of a methane combustion test (initial activity, after durability at 650° C.) for Example 1 of the first embodiment and Comparative Example 1. [Figure 4] 10 is a graph showing the results of a methane combustion test (initial activity, after durability at 650° C.) of Comparative Examples 2 and 3 of the first embodiment. [Figure 5] 10 is a graph showing the results of a methane combustion test (initial activity, after durability at 650° C.) of Comparative Examples 4 and 5 of the first embodiment. [Figure 6] TEM images of methane combustion catalysts of Example 1 and Comparative Example 1 of the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0049] First embodiment A preferred embodiment of the present invention will now be described. In this embodiment, a slurry was prepared by adding silica sol to a zirconia carrier, and this slurry was then applied to a support. Pt and Pd were then supported on the support to produce a methane combustion catalyst (Example 1). After confirming the silica dispersion state (Si concentration distribution) in this methane combustion catalyst, a methane combustion test was conducted.

[0050] 100 g of zirconia powder with an average particle size of 3.29 μm was placed in a ball mill together with 300 mL of water and zirconia balls (diameter 1 mm) as grinding media, and then a commercially available silica sol with a particle size of 5 nm was added and stirred and ground. The amount of silica sol added was 4% by mass relative to the zirconia powder. The grinding process involved stirring at a rotation speed of 40 rpm for 10 hours to produce a zirconia support slurry.

[0051] The particle size of the slurry thus produced was measured using a particle size distribution analyzer, and it was confirmed that the specified average particle size was achieved. The average particle size of the zirconia powder after pulverization was 1.26 μm (particle size reduction rate: 61.7%). Regarding the particle size distribution before and after pulverization, the zirconia powder, which was composed of powder with particle sizes of 2 μm to 12 μm, became a particle size of 0.4 μm to 2 μm.

[0052] Next, the zirconia slurry prepared above was applied to a support (metal honeycomb: 18 mmφ×50 mm length) by air blowing. The amount of the slurry applied was 120 g / L based on the support volume. After that, gas drying was performed at 120°C for 10 minutes, and then firing was performed in an electric furnace at 500°C for 60 minutes to produce a zirconia carrier supported on a metal honeycomb.

[0053] Then, Pt and Pd were supported on the zirconia support. The metal salt solution used was a mixed solution of dinitrodiammine platinum (2 g / L) and palladium nitrate (8 g / L). The metal salt solution was impregnated from both ends of the support; it was applied to one end with an air blow and air-dried (30 minutes), and then applied to the other end and dried. The amount of each catalytic metal supported was 1.5 mass% for Pt and 6.2 mass% for Pd relative to the total catalyst (zirconia support and catalytic metal). Finally, the catalyst was calcined at 500°C for 60 minutes to obtain a methane combustion catalyst.

[0054] Comparative Example 1 As a comparative example to the above Example 1, a methane combustion catalyst was produced in which Pt and Pd were supported on a zirconia support without added silica (Comparative Example 1). In the above support preparation process, zirconia sol (ZrO(NO3)2) was added as a binder instead of silica sol when zirconia powder was pulverized using a ball mill, and a support slurry was produced under the same conditions as in Example 1. Thereafter, a methane combustion catalyst was produced by supporting Pt and Pd using the same process as in Example 1.

[0055] Comparative Example 2As a comparative example to Example 1, a zirconia carrier was produced by adding the same silica sol with a particle size of 5 nm as in Example 1 to zirconia powder while changing the pulverization conditions (Comparative Example 2). The pulverization conditions in this comparative example were a rotation speed of 40 rpm and 0.5 hours. The average particle size of the zirconia particles in the slurry produced by pulverization was measured, and the average particle size was 2.59 μm (particle size reduction rate: 21.3%). Furthermore, the particle size distribution after pulverization was composed of zirconia powder with a particle size of 1 μm to 5 μm. Thereafter, Pt and Pd were supported in the same manner as in Example 1 to produce a methane combustion catalyst.

[0056] Comparative Example 3 In this comparative example, silica sol with a particle size of 12 nm was used as the silica added during mixing and grinding of zirconia powder (Comparative Example 3). Except for this, a zirconia support slurry was produced in the same manner as in Example 1. Then, Pt and Pd were supported in the same manner as in Example 1 to produce a methane combustion catalyst.

[0057] Comparative Example 4 In this comparative example, silica sol with a particle size of 200 nm was used as the silica added during mixing and grinding of zirconia powder. Apart from this, a zirconia support slurry was produced under the same grinding conditions as in Example 1. Then, Pt and Pd were supported in the same manner as in Example 1 to produce a methane combustion catalyst.

[0058] Comparative Example 5 In this comparative example, silica sol with a particle size of 400 nm was used as the silica added during mixing and grinding of zirconia powder. Apart from this, a zirconia support slurry was produced under the same grinding conditions as in Example 1. Then, a methane combustion catalyst was produced by supporting Pt and Pd in ​​the same manner as in Example 1.

[0059] Examination of silica dispersion state The methane combustion catalysts of Example 1 and Comparative Examples 1 to 5 prepared above were examined for the dispersion state of silica in the zirconia carrier. Here, nine measurement regions (with an area of ​​1.0 μm) were arbitrarily selected for each methane combustion catalyst. 2 ) was set, and each measurement area was quantitatively analyzed using EPMA.

[0060] In the EPMA analysis, essential elements such as Pt, Pd, Si, and Zr, non-metallic elements such as O derived from zirconia, silica, etc., as well as metal elements such as Fe and Cr were detected. The detection of metal elements such as Fe originates from the metal honeycomb support, and Fe, etc. are not metal elements that constitute the methane combustion catalyst. In the present invention, in order to accurately evaluate the concentration and dispersion state of Si contained in the methane combustion catalyst, the total concentration of the essential elements Pt, Pd, Si, and Zr is used as the standard. In this embodiment, the measured Si concentration and Zr concentration were also converted to concentrations based on the essential metal concentration standard. Then, the Si concentration (C Si ) and Zr concentration (C Zr ) based on the ratio of these (C Si / C Zr ) was calculated. Si / C Zr Calculate the average value and standard deviation of C Si / C Zr As an example of the above results, the analysis results of the methane combustion catalyst of Example 1 are shown in Table 1. The Si concentration of the methane combustion catalyst of Example 1 was 3.4 mass % on average.

[0061] [Table 1]

[0062] Measurement of crystallite size of zirconia particles before and after high-temperature heating The methane combustion catalysts of Example 1 and Comparative Examples 1 to 5 were analyzed by XRD (X-ray source: Cu) immediately after production and after heating at 700°C. X-ray diffraction (kα ray) was performed. Heating to 700°C was performed in an electric furnace in the atmosphere for 2 hours. Figure 2 shows the X-ray diffraction patterns at each stage of the methane combustion catalysts of Example 1 and Comparative Example 1. The crystallite diameter of the zirconia particles was calculated based on these diffraction patterns. The crystallite diameter of the zirconia particles was calculated by measuring the half-width of the peak of the (-104) plane near 2θ = 71.38°, a position near which overlapping peaks are unlikely to occur.

[0063] The methane combustion catalysts of Example 1 and Comparative Examples 1 to 5 were evaluated and measured for the dispersion state of silica (CSi / C Zr ) and the crystallite diameter and increase rate of the zirconia particles immediately after production (Fresh) and after heating at 700°C are shown in Table 2.

[0064] [Table 2]

[0065] From Table 2, it was confirmed that the methane combustion catalyst of Comparative Example 1 without added silica had an increase rate of 63.7% in the crystallite size of zirconia particles due to high-temperature heating, and sintering of zirconia occurred. In contrast, the methane combustion catalyst of Example 1 with added silica had an increase rate of 63.7% in the crystallite size of zirconia particles due to high-temperature heating, and sintering of zirconia occurred. Si / C Zr The coefficient of variation of the silica content was 30% or less, which means that silica was uniformly dispersed in the catalyst. In Example 1, the increase rate of the crystallite size of the zirconia particles was significantly low at 14.8%. This confirmed that adding silica to the zirconia carrier exerted the effect of suppressing sintering by acting as a spacer.

[0066] However, in Comparative Example 2, where silica was added to the zirconia carrier, the mixing and grinding conditions during silica addition were different from those in Example 1, and the particle size reduction rate of the zirconia powder after grinding was less than 50%. Si / C Zr In Comparative Example 2, the rate of increase in the crystallite size increases due to high-temperature heating, and sintering occurs.

[0067] Furthermore, even if the particle size of silica added to the zirconia support exceeds 10 nm, C Si / C Zr The coefficient of variation of the silica dispersion in the catalyst exceeded 30%, resulting in an inhomogeneous dispersion state (Comparative Example 3). Even if the grinding conditions were as appropriate as in Example 1, dispersibility would be poor if the silica particle size was large. As a result, the rate of increase in crystallite size due to high-temperature heating was large. This was also the case when the silica particle size was 200 nm (Comparative Example 4) and 400 nm (Comparative Example 5).

[0068] From the above, it was confirmed that in order to suppress sintering of the zirconia support, it is necessary to suitably increase the particle size reduction rate of the zirconia powder by adjusting the milling conditions in addition to adding silica of an appropriate particle size. It is also believed that the appropriate addition of silica to the zirconia support can suppress sintering of zirconia particles at high temperatures.

[0069] Methane combustion test (evaluation of initial activity and durability) Next, a methane combustion test was carried out to evaluate the performance of the methane combustion catalysts of Example 1 and Comparative Examples 1 to 5. In this evaluation test, the methane combustion catalyst was set in a fixed-bed reactor, and the methane conversion rate was measured by passing a test gas through it. The test conditions were as follows: Test gas composition CH4: 1000 ppm O2: 15% H2O: 10% N2: Balance Pressure: 0.36MPa ·Space velocity (GHSV): 30,000h -1 Reaction temperature (catalyst temperature): 300℃~450℃ Testing time: 24 hours

[0070] The initial activity of the methane combustion catalyst was measured by passing test gas through the catalyst at each reaction temperature, and analyzing the composition of the exhaust gas after one hour to measure the methane conversion rate. The methane concentration in the exhaust gas was analyzed using an FID-type THC gas analyzer. The methane conversion rate was then calculated from the measured values ​​using the following formula:

[0071]

number

[0072] On the other hand, in the durability evaluation test, the methane combustion catalyst after the above initial activity evaluation was set in the test equipment, and then the catalyst was heated to 650°C and the above test gas was passed through for 2 hours to deteriorate the catalyst (650°C durability). Thereafter, the methane conversion rate at each test temperature was measured in the same manner as above.

[0073] The results of the methane combustion test for the methane combustion catalysts of Example 1 and Comparative Examples 1 to 5 manufactured in this embodiment are shown in Figures 3 to 5. Table 3 also shows the 90% conversion temperatures (temperatures at which the methane addition rate becomes 90%) in the initial state (Fresh) and after endurance at 650°C obtained from these test results.

[0074] [Table 3]

[0075] Comparing the methane combustion catalysts of Example 1 and Comparative Example 1, which differ in the presence or absence of added silica, with reference to Figure 3 and Table 3, it can be said that Example 1 (with added silica) and Comparative Example 1 (without added silica) have the same initial activity. However, after endurance testing at 650°C, the methane combustion catalyst of Example 1 exhibits a higher methane addition rate. The 90% conversion temperature was 385°C for Example 1 (with added silica) and 404°C for Comparative Example 1 (without added silica). This confirms that adding silica to a zirconia carrier improves durability.

[0076] Here, the results of examining the state of the methane combustion catalysts of Example 1 and Comparative Example 1 after endurance at 650°C are shown. Figure 6 shows TEM images of each catalyst after endurance at 650°C. These TEM images were taken at the same magnification of catalysts manufactured from the same zirconia powder in their initial state (before endurance at 650°C). Figure 6 confirms that the zirconia particles in the comparative example have become coarse, and that the zirconia particles in Comparative Example 1 have been sintered by high-temperature heating.

[0077] Next, the methane combustion catalysts of Comparative Examples 2 to 5 will be examined. As described above, when the silica is not mixed or crushed sufficiently into the zirconia powder (Comparative Example 2) or when the particle size of the added silica is coarse (Comparative Examples 3 to 5), the C related to the dispersion state of the silica in the zirconia carrier is observed. Si / C Zr The coefficient of variation of the above values ​​exceeds 30%. Referring to Fig. 4 and Table 4, the methane combustion catalysts of Comparative Examples 2 to 5 have no difference in initial activity from Example 1, but the increase in 90% conversion temperature after endurance at 650°C is large. The addition of silica is effective for methane combustion catalysts equipped with a zirconia carrier. However, it has been confirmed that optimizing the dispersion state of silica (optimizing the pulverization conditions and reducing the size of the added silica) is essential to make this effect effective.

[0078] Changes in specific surface area of ​​zirconia support after initial activity and durability at 650°C In the methane combustion test, the specific surface area of ​​the zirconia carrier was measured for the methane combustion catalysts of Example 1 and Comparative Examples 1 to 5 in their initial state (fresh) and in their states after a 650°C durability and combustion test. The specific surface area was measured by the BET method. The results are shown in Table 4.

[0079] [Table 4]

[0080] In Table 4, the catalyst without added silica in Comparative Example 1 had a lower specific surface area after production than the other catalysts (with added silica). Although the same zirconia powder was used as the support for both catalysts, the metal salt solution used to support Pt and Pd was acidic, which likely resulted in the pore blockage of the zirconia powder without added silica. However, considering the initial activity in the combustion test, this change in specific surface area was not significant enough to directly affect catalytic activity, as would be the case with sintering. However, the methane combustion catalyst in Comparative Example 1 showed a significant decrease in specific surface area after endurance testing at 650°C, which is likely due to sintering of the zirconia particles. Meanwhile, in Comparative Examples 2 and 3, the specific surface area after production was comparable to that of Example 1 due to the addition of silica, but the specific surface area significantly decreased after endurance testing at 650°C. Even with the addition of silica, sintering of the zirconia particles is unlikely to be suppressed unless the silica dispersion is optimized.

[0081] Second embodiment In this embodiment, methane combustion catalysts were produced by varying the amount of silica added to the zirconia carrier, and their initial activity and durability were evaluated.

[0082] In the manufacturing process of the methane combustion catalyst of Example 1 of the first embodiment, the same silica sol (particle size 5 nm) as in Example 1 was used when preparing the support slurry. Then, 2 mass % (Example 2), 8 mass % (Example 3), and 15 mass % (Example 4) of silica sol were added to the zirconia powder to prepare the support slurry. Then, Pt and Pd were supported using the same process as in Example 1 to manufacture a methane combustion catalyst. Then, methane combustion tests were performed under the same test conditions as in the first embodiment, including initial activity and after durability testing at 650°C. The evaluation results, 90% conversion temperature for each catalyst, are shown in Table 5. Table 5 also shows the results of Example 1.

[0083] [Table 5]

[0084] Table 5 shows that adding silica (5 nm) to a zirconia carrier basically shifts the 90% conversion temperature to a lower value, and therefore adding an appropriate amount of silica contributes to improving durability. However, when the amount of silica added is large, the initial activity and the 90% conversion temperature after durability testing become slightly higher (Example 4). If the balance between initial activity and durability is considered important, it is considered appropriate to add silica less than 15% to optimize the Si concentration.

[0085] In addition, in Example 2, in which the amount of silica added was 2%, the 90% conversion temperature after the durability test was slightly higher, and the temperature difference from the 90% conversion temperature in the initial state was also relatively large. Si / C Zr The coefficient of variation of the silica content is close to 30%. When the amount of silica added is small, more thorough pulverization is considered necessary to disperse the silica uniformly. It is also considered preferable to add a slightly higher amount of silica than in Example 2 (3% by mass or more). [Industrial Applicability]

[0086] The methane combustion catalyst according to the present invention has improved durability while maintaining initial activity equal to or greater than that of conventional technologies. According to the present invention, it is possible to ensure the durability of a methane combustion catalyst without increasing the amount of catalytic metal supported, thereby contributing to reducing the cost of methane combustion catalysts. The methane combustion catalyst according to the present invention can be used in a wide range of applications, including exhaust gas purification systems and air purification systems that use fuels such as natural gas and city gas, as well as in off-gas sensors and other applications for SOFCs (solid oxide fuel cells) that utilize the heat generated during methane combustion.

Claims

1. A methane combustion catalyst comprising a zirconia support and catalyst particles made of at least one metal selected from the group consisting of Pt and Pd or an alloy thereof, The amount of the catalyst particles supported relative to the total mass of the catalyst is 3 mass% or more and 20 mass% or less, The zirconia support contains silica, When a plurality of regions of the methane combustion catalyst are arbitrarily analyzed, the Si concentration (C Ｓｉ ) and Zr concentration (C Ｚｒ ) and the ratio (C Ｓｉ / C Ｚｒ ) the coefficient of variation (CV value) is 30% or less, The methane combustion catalyst is characterized in that the average value of the Si concentration is 1 mass % or more and 12 mass % or less.

2. 2. The methane combustion catalyst according to claim 1, wherein when heated at 700°C, the crystallite size of the zirconia constituting the zirconia support increases by 30% or less.

3. 3. A method for producing a methane combustion catalyst according to claim 1 or 2, comprising: The method includes a carrier preparation step of mixing zirconia powder and silica particles to produce a zirconia carrier, and a supporting step of supporting catalyst particles on the zirconia carrier, The carrier preparation step is a step of mixing, while pulverizing, silica particles having a particle size of 3 nm to 10 nm and zirconia powder having an average particle size of 3 μm to 8 μm, The pulverization step includes pulverizing the zirconia powder until a particle size reduction rate of the zirconia powder is 50% or more.

Images