Microwave heating element, catalyst, and method for manufacturing a microwave heating element

Abstract

The goal is to enhance the reactivity of the reaction gas by minimizing pressure loss during the flow of the reaction gas while ensuring the efficiency of the reaction site and heating the reaction site. [Solution] The microwave heating element comprises a porous ceramic body that generates heat upon microwave irradiation. The porous ceramic body has a first peak with the largest peak value and a second peak smaller than the first peak in the differential pore volume distribution when the pore size distribution is measured by the mercury intrusion method. One of the first and second peaks is located in the range of pore diameters between 5 μm and 100 μm, and the other of the first and second peaks is located in the range of pore diameters of 1 μm or less.

Description

[Technical Field]

[0001] This disclosure relates to a microwave heating element, a catalyst comprising a microwave heating element, and a method for manufacturing a microwave heating element. [Background technology]

[0002] Conventionally, in apparatuses where chemical reactions proceed using components in a gas as reactants, such as reactors equipped with catalysts, various techniques have been proposed to improve the performance of the reactor by increasing the reactivity of the gas. To increase the reactivity of the gas, it is considered effective to, for example, reduce the pressure loss in the reactor to improve gas flowability and to ensure a more sufficient reaction site within the reactor. To achieve these objectives, conventionally, for example, in reactors equipped with catalysts, a porous material has been used as a catalyst support to reduce pressure loss during gas flow and to ensure a sufficient reaction site. Specifically, by using a porous ceramic structure such as alumina, which has a relatively large specific surface area, as a catalyst support, pressure loss has been reduced, as has the uniform dispersion of the catalyst supported on the catalyst support and the securing of a sufficient reaction site (see, for example, Patent Document 1).

[0003] Furthermore, it is considered important to sufficiently heat the reaction site in order to increase the reactivity of the gas. As a configuration to increase the activity that allows the chemical reaction to proceed by heating the reaction site, for example, a configuration has been proposed in which a dielectric heating element is placed in the catalyst layer and dielectric heating is performed using microwave irradiation without external heating (see, for example, Patent Document 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2010-58041 [Patent Document 2] Japanese Patent Publication No. 2007-84389 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, in general, porous materials tend to have a smaller specific surface area as pressure loss is reduced, so further improvement of gas reactivity was desired. Also, in a reactor equipped with a porous material through which gas flows, when the reaction field is heated to increase the reactivity of the gas, reducing the pressure loss of the porous material increases the space within the porous material, which can lead to a problem where the heating efficiency for the gas passing through the porous material decreases. Furthermore, for example, when a reaction layer is provided that contains a material that absorbs microwaves and generates heat, as described in Patent Document 2, and such a reaction layer is formed on a carrier made of alumina porous material as described in Patent Document 1, even if the reaction layer generates heat due to microwave irradiation, the heating of the reaction field may be insufficient due to heat dissipation by the porous carrier. Therefore, there was a need for a technology that could suppress pressure loss when the reaction gas flows, secure the reaction field, ensure the efficiency of heating the reaction field, and further increase the reactivity of the gas. [Means for solving the problem]

[0006] This disclosure can be implemented in the following forms: [1] According to one embodiment of the present disclosure, a microwave heating element is provided. The microwave heating element comprises a ceramic porous body that generates heat upon microwave irradiation, wherein the ceramic porous body has a first peak with the largest peak value and a second peak smaller than the first peak in the differential pore volume distribution when the pore size distribution is measured by the mercury intrusion method, and one of the first and second peaks is located in the range of pore diameters of 5 μm to 100 μm, and the other of the first and second peaks is located in the range of pore diameters of 1 μm or less. With this type of microwave heating element, when a gas is supplied to the microwave heating element and a chemical reaction using the components in the gas as reactants proceeds on the microwave heating element, internal heating is performed by microwave irradiation, thereby maintaining a high level of activity in the chemical reaction. Furthermore, by having pores with a peak pore size in the range of 5 μm to 100 μm, pressure loss is suppressed and gas flow is ensured, while the heating efficiency of the space on the porous ceramic body where the chemical reaction proceeds can be ensured. In addition, by having pores with a peak pore size in the range of 1 μm or less, the specific surface area of ​​the porous ceramic body can be increased, thereby increasing the area available as the reaction site for the chemical reaction, and further increasing the heating efficiency of the space on the porous ceramic body where the chemical reaction proceeds can be enhanced. As a result, the reactivity of the chemical reaction using the gas can be further enhanced in the microwave heating element. [2] In the microwave heating element of the above form, the second peak may be the second largest peak in the differential pore volume distribution. With such a configuration, the effect of ensuring the efficiency of heating the reaction site and the reaction site while suppressing pressure loss when the reaction gas flows can be further enhanced. [3] In the microwave heating element of the above form, the differential pore volume distribution may exhibit a bimodal distribution. With such a configuration, the effect of ensuring the efficiency of the reaction site and heating the reaction site while suppressing pressure loss when the reaction gas flows can be further enhanced. [4] In the microwave heating element of the above form, the porous ceramic body may be made of an iron-containing oxide. With such a configuration, by using a magnetic loss material with excellent heating performance due to microwave irradiation as the constituent material of the microwave heating element, the reactivity of the chemical reaction proceeding on the microwave heating element can be further enhanced. [5] In the microwave heating element of the above form, the iron-containing oxide may be a spinel-type ferrite. With such a configuration, the reactivity of the chemical reaction proceeding on the microwave heating element by microwave irradiation can be further enhanced because spinel-type ferrite has particularly excellent magnetic properties. [6] In the microwave heating element of the above form, the spinel-type ferrite is (Ni 1-x Zn x It may also be represented as Fe2O4 (where 0≦x≦0.7). With this configuration, the firing temperature of the porous ceramic body can be lowered by using a material in which zinc (Zn) is added to nickel ferrite. [7] The microwave heating element of the above form may be formed in particulate form with an average absolute maximum length of 1 mm or more. With such a configuration, it becomes easier to form pores of a desired diameter within the microwave heating element particles and to ensure gas flow within the microwave heating element particles. [8] The microwave heating element in the above form may be formed in the shape of spherical particles. With such a configuration, the fluidity of the microwave heating element particles is improved, and it becomes easier to fill the reactor with microwave heating element particles. [9]According to another embodiment of the present disclosure, a catalyst is provided, comprising a microwave heating element as described in any one of [1] to [8], and a catalyst supported on the ceramic porous body constituting the microwave heating element. This type of catalyst allows for the suppression of pressure loss during gas flow in the microwave heating element of the catalyst, while simultaneously securing a reaction site and ensuring efficient heating of the reaction site. As a result, the reactivity of chemical reactions using the gas carried out by the catalyst can be enhanced.

[10] In the catalyst of the above form, the catalyst may contain at least one of nickel (Ni) and zeolite. With such a configuration, the catalyst can be used effectively to carry out, for example, hydrocarbon decomposition reactions or exhaust gas decomposition reactions.

[11] Another embodiment of the present disclosure provides a method for manufacturing a microwave heating element. This method for manufacturing a microwave heating element involves granulating a wet powder containing raw material ceramic powder, which is a powder of the constituent material of the ceramic porous body, using a rolling granulator to form the microwave heating element described in [8], wherein the rolling granulator comprises a cylindrical drum, a disk disposed to close the opening at the bottom of the drum, and a first drive device for rotating the disk. According to this method for manufacturing microwave heating elements, within the rolling granulator, as the disk rotates, the wet powder containing the raw ceramic powder rises high along the inner surface of the cylindrical drum due to the rotational motion caused by the disk's rotation and centrifugal force, and then falls and flows like an avalanche. This flow of the wet powder in the direction of the disk's rotation and vertically along the inner surface of the cylindrical drum makes it possible to granulate spherical, particulate microwave heating elements.

[12] In the method for manufacturing a microwave heating element according to the above embodiment, the rolling granulation apparatus may further include a second drive device that rotates the drum. With such a configuration, the fluidity of the wet powder inside the cylindrical drum can be further increased, thereby improving the granulation efficiency. This disclosure can be implemented in various forms other than those described above, for example, in the form of a method for manufacturing a microwave heating element, a method for manufacturing a catalyst, a fossil fuel reformer equipped with a catalyst, an exhaust gas purification device equipped with a catalyst, and so on. [Brief explanation of the drawing]

[0007] [Figure 1] An explanatory diagram showing an example of the differential pore volume distribution for a porous ceramic material. [Figure 2] An explanatory diagram showing an example of smoothing the measurement results of the differential pore volume distribution. [Figure 3] A flowchart illustrating the manufacturing method of a microwave heating element. [Figure 4] A cross-sectional view showing the schematic configuration of a rolling granulation machine. [Figure 5]Flowchart showing a method for manufacturing a microwave heating element. [Figure 6] Explanatory diagram showing measurement results of differential pore volume distribution for a ceramic porous body. [Figure 7] Explanatory diagram showing the state of temperature change during microwave irradiation.

Embodiments for Carrying Out the Invention

[0008] A. Configuration of the microwave heating element: The microwave heating element as an embodiment of the present disclosure is used to advance a chemical reaction using components in a gas as reactants, and includes a ceramic porous body that generates heat by microwave irradiation. The microwave heating characteristics in the microwave heating element are due to at least one of the three losses of Joule loss, dielectric loss, and magnetic loss, and the ceramic porous body is formed of a material with relatively high microwave heating characteristics. As a constituent material of the ceramic porous body of the present embodiment, for example, an iron-containing oxide that is a magnetic loss material can be used, and among them, it is preferable to use a spinel-type ferrite having excellent magnetic properties. As the spinel-type ferrite, for example, (Ni 1-x Zn x )Fe2O4 (where 0 ≦ x ≦ 0.7) represented Ni-Zn-based ferrite can be preferably used. Alternatively, as a constituent material of the ceramic porous body of the present embodiment, for example, a perovskite-type oxide such as BaTiO3 that is a dielectric loss material may be used.

[0009] When the pore size distribution of the ceramic porous body of the present embodiment is measured by the mercury intrusion method, in the differential pore volume distribution, it has a first peak with the largest peak value (the value at the peak top) and a second peak smaller than the first peak. And, one of the peaks of the first peak and the second peak is present in the range where the pore diameter is 5 μm or more and 100 μm or less, and the other peak of the first peak and the second peak is present in the range where the pore diameter is 1 μm or less. Among the first peak and the second peak, the pores corresponding to the peak with a larger pore diameter, that is, the peak present in the range where the pore diameter is 5 μm or more and 100 μm or less, are also called "large-diameter pores". Also, the pores corresponding to the peak with a smaller pore diameter, that is, the peak present in the range where the pore diameter is 1 μm or less, are also called "small-diameter pores".

[0010] FIG. 1 is an explanatory diagram showing an example of the differential pore volume distribution when the pore size distribution of the ceramic porous body constituting the microwave heating element of the present embodiment is measured by the mercury intrusion method. In FIG. 1, the first peak with the largest peak value is shown as peak (α), and the second peak is shown as peak (β). In FIG. 1, peak (α) is present at a position where the pore diameter is about 20 μm, that is, in the range of 5 μm or more and 100 μm or less. Also, peak (β) is present at a position where the pore diameter is about 0.25 μm, that is, in the range of 1 μm or less. As described above, in the differential pore volume distribution shown in FIG. 1, peak (α) corresponds to the large-diameter pores, and peak (β) corresponds to the small-diameter pores. However, it is also possible that the first peak with the largest peak value corresponds to the small-diameter pores and the second peak corresponds to the large-diameter pores.

[0011] Here, when the pore size distribution of the porous ceramic body of this embodiment is measured by the mercury intrusion method, it is desirable that the differential pore volume distribution shows a bimodal distribution (two-peaked distribution), as shown in Figure 1. That is, it is desirable that the differential pore volume distribution described above has only a first peak and a second peak as peaks. Furthermore, the number of peaks in the differential pore volume distribution may be 3 or more, provided that the influence on the effects described later due to having a first peak and a second peak is within an acceptable range, but it is desirable that it be 4 or less. However, even if the number of peaks in the differential pore volume distribution is 3 or more, it is desirable that the combination of the peak corresponding to a large diameter pore and the peak corresponding to a small diameter pore be the first peak with the largest peak value and the peak with the second largest peak value.

[0012] In such porous ceramic bodies, large-diameter pores can be easily formed by adding resin beads as a pore-forming agent to the material during the manufacturing of the porous ceramic body, while small-diameter pores can be easily formed by adjusting the firing conditions during the manufacturing of the porous ceramic body. When forming small-diameter pores by adjusting the firing conditions during the manufacturing of the porous ceramic body, the pore diameter of the small-diameter pores can be made smaller by making the firing conditions more stringent (higher firing temperature or longer firing time). However, if the firing temperature is increased and the firing conditions are made stringent, the peaks of small-diameter pores can disappear relatively easily as sintering progresses. Therefore, when measuring the pore diameter distribution by the mercury intrusion method, it is desirable that the peaks corresponding to small-diameter pores in the differential pore volume distribution are in the range of pore diameters of 0.07 μm or more, and from the viewpoint of enhancing the effect of creating small pores, it is even more desirable that they be in the range of pore diameters of 0.1 μm or more. A preferred manufacturing method for obtaining the porous ceramic body of this embodiment will be described later.

[0013] Even if the pores formed in the porous ceramic material are classified into two types, large-diameter pores and small-diameter pores, if fluctuations or noise are observed in the measured values ​​of the differential pore volume distribution described above, smoothing may be performed to facilitate the identification of the first and second peaks in the differential pore volume distribution. Smoothing can be performed, for example, by the adjacent average method, the Savitzky-Golay method, the percentile filter method, or the FFT filter method.

[0014] Figure 2 is an explanatory diagram showing an example of smoothing the measurement results of the differential pore volume distribution. In Figure 2, the same data as the differential pore volume distribution shown in Figure 1 is used as the data before processing. The result after smoothing is shown using the 50th percentile filter method, which collects only those with a pore diameter of 50% or less and averages them. As shown in Figure 2, when the differential pore volume distribution shows a bimodal distribution, the positions of the first and second peaks are hardly changed even after performing the smoothing process described above, making it easier to identify the first and second peaks.

[0015] The microwave heating element of this embodiment, which is composed of a porous ceramic material as described above, is preferably formed in particulate form. The average absolute maximum length of the microwave heating element particles constituting the microwave heating element is preferably 0.5 mm or more, more preferably 1 mm or more, and even more preferably 2 mm or more. This makes it easy to form pores of a desired diameter within the microwave heating element particles and to ensure gas flow within the microwave heating element particles. The average absolute maximum length of the microwave heating element particles can be 15 mm or less, or 10 mm or less. Here, "absolute maximum length of microwave heating element particles" refers to the maximum distance between any two points on the outer circumference of the projection image of the microwave heating element particles. The average absolute maximum length of the microwave heating element particles can be determined by capturing an SEM image of the microwave heating element and using the image analysis software WinRoof (manufactured by Mitani Corporation) on the obtained image. Specifically, the method involves taking 10 measurements with a SEM so that the entire microwave heating element particle fits within a 30x magnification SEM image of the microwave heating element. Then, using the image analysis software WinRoof (manufactured by Mitani Corporation), the absolute maximum length of the microwave heating element particle is determined for each captured image, and the average value is calculated.

[0016] Furthermore, the particle shape of the microwave heating element in this embodiment is preferably spherical. By making the microwave heating element particles spherical, the fluidity of the microwave heating element particles is improved, and the filling of the microwave heating element particles into the reactor becomes easier. The degree of sphericity of the microwave heating element particles can be evaluated, for example, by sphericity. Sphericity is expressed as the ratio of the short axis to the long axis of the particle and can be measured, for example, by dynamic image analysis. The sphericity of the microwave heating element particles is preferably 0.80 or higher, and more preferably 0.85 or higher. However, the shape of the microwave heating element particles can also be other than spherical, for example, they can be pellet-shaped or donut-shaped.

[0017] B. Method for manufacturing a microwave heating element: Figure 3 is a flowchart illustrating an example of a method for manufacturing a microwave heating element according to this embodiment. Figure 3 shows an example of a method for manufacturing a microwave heating element that exhibits a bimodal distribution having the first and second peaks described above in the differential pore volume distribution when the pore size distribution is measured by the mercury intrusion method.

[0018] When manufacturing a microwave heating element, first, raw ceramic powder and pore-forming material are prepared (step T100). Here, the raw ceramic powder is the powder of the constituent material of the ceramic porous body of the microwave heating element, and powders of magnetic loss materials and dielectric loss materials as described above can be used. The pore-forming material is a particle that burns away during firing and is used to form the large-diameter pores described above. The pore-forming material only needs to be formed from a material that disappears during firing, and can be, for example, particles derived from natural products such as starch, rice husks, nut shells, walnut shells, corn cobs, apricot kernels, and peach kernels, or particles (resin beads) composed of organic compounds such as polyethylene glycol, melamine, acrylic, polyethylene, polypropylene, and polymethyl methacrylate. Depending on the type of raw ceramic and firing conditions, the ceramic porous body will shrink due to sintering, so the particle size of the pore-forming material should be appropriately set to be slightly larger than the size of the large-diameter pores to be formed.

[0019] After step T100, a slurry is prepared by adding a solvent such as water or ethanol and a binder to the raw material ceramic powder and pore-forming material (step T110). The binder used can be any binder that can be removed in a later step by decomposition, etc., and for example, organic binders such as acrylic binders or polyethylene oxide binders can be used. After that, the slurry is formed into granules to produce a molded body (step T120). In step T120, the molding method is not particularly limited as long as it can be molded into granules. For example, well-known molding methods such as press molding, crushing granulation, stirring granulation, fluid bed granulation, and rolling granulation can be used. The rolling granulation method will be explained in more detail later.

[0020] After step T120, the resulting granular molded body is fired to complete the microwave heating element (step T130). In step T130, the firing conditions are set so that small-diameter pores are formed within the porous ceramic body as a result of the firing. During firing, necking occurs between the powder particles constituting the raw ceramic powder, and the gaps between the powder particles gradually become smaller. Therefore, by adjusting the firing conditions, especially the firing temperature, the size of the gaps between the powder particles can be controlled to form small-diameter pores of a desired size. Since sinterability differs depending on the ceramic material, the firing conditions in step T130 should be appropriately set according to the type of raw ceramic powder used so that small-diameter pores of a desired size are formed after firing. For example, the firing temperature should be appropriately set within a temperature range of approximately 1000°C to 1300°C. During the firing in step T130 described above, the pore-forming material, such as resin beads, is further burned away, and large-diameter pores with a pore size corresponding to the particle size of the resin beads are formed.

[0021] As previously mentioned, the constituent material of the porous ceramic is (Ni 1-x Zn x Spinel-type ferrite represented by Fe2O4 (where 0 ≤ x ≤ 0.7) can be suitably used. As described above, using a material obtained by adding zinc (Zn) to nickel ferrite, which is a spinel-type ferrite, is desirable because it allows for a lower firing temperature of the porous ceramic body.

[0022] As a method for forming materials containing raw ceramic powder, the previously described rolling granulation method can be suitably used. The rolling granulation method is a method for producing relatively spherical particles by subjecting a wet powder containing raw ceramic powder to rolling motion. The rolling granulation method will be explained further below.

[0023] Figure 4 is a cross-sectional view showing the schematic configuration of a rolling granulator 10 used to manufacture the microwave heating element of this embodiment. The rolling granulator 10 comprises a cylindrical drum 12, a disk 14 positioned to close the opening at the bottom of the drum 12, and a disk rotation motor 16 that rotates the disk 14. The disk rotation motor 16 is also called the first drive unit 16. The rolling granulator 10 has a granulation tank 20 formed as a space surrounded by the inner circumferential surface of the drum 12 and the disk 14. When manufacturing the microwave heating element of this embodiment, the disk 14 is rotated by the first drive unit 16 with the wet powder WP containing the raw material ceramic powder placed in the granulation tank 20. As a result, the wet powder WP rises high along the inner circumferential surface 13 of the drum, which is the inner wall of the cylindrical drum, due to the rotational motion of the disk 14 and centrifugal force, and then falls and flows like an avalanche. In this way, the wet powder WP moves vertically on the inner wall surface of the drum 12 in the direction of rotation of the disc 14, causing vortex flow, which makes it possible to granulate spherical particles.

[0024] In this case, the drum 12 may be fixed, but as shown in Figure 4, it is desirable to further provide a drum rotation motor 18 in the rolling granulator 10 to rotate the drum. The drum rotation motor 18 is also called a second drive device 18. By using the second drive device 18 to rotate the drum 12 in the opposite direction to the rotation of the disk 14, the flow resistance increases and the vortex flow is strengthened, which reduces the adhesion of nucleus particles, raw ceramic powder, and pore-forming material to the inner surface of the drum 12 and improves granulation efficiency. As such a rolling granulator 10, for example, the ECX40Ps manufactured by Chipton Co., Ltd. can be suitably used.

[0025] Figure 5 is a flowchart showing an example of a method for manufacturing a microwave heating element according to this embodiment using the rolling granulation method. Step T210 in Figure 5 includes steps T100 and T200 in Figure 3, and steps T210 and T220 in Figure 5 correspond to step T120 in Figure 3. When manufacturing a microwave heating element, first, nuclei for granulation are introduced into the rolling granulator 10, and the disk 14 and preferably the drum 12 are rotated (step T200). The nuclei for granulation can be appropriately selected according to the particle size of the microwave heating element to be produced. For example, when producing particles with an average absolute maximum length of about 1 mm to 2 mm, nuclei with an average absolute maximum length of about 0.1 mm to 0.3 mm can be used. When producing particles with an average absolute maximum length exceeding 2 mm, larger nuclei, for example, nuclei with an average absolute maximum length of 1 mm or more, can be used. The material of the nuclei used may be the same as the raw material ceramic powder, or it may be a different material from the raw material ceramic powder, such as clay minerals.

[0026] Subsequently, the rolling granulator 10 is alternately fed with a mixed powder of raw ceramic powder, pore-forming material, and binder, and mist-like water (step T210). Although not shown in Figure 4, the rolling granulator 10 is equipped with a water supply pipe with nozzles for spraying water into the granulation tank 20. Therefore, by rotating the disc 14 while spraying water onto the mixed powder of raw ceramic powder, pore-forming material, and binder via this water supply pipe, granulation of the wet powder WP containing the raw ceramic powder can be performed. The binder used in the rolling granulation method only needs to have the effect of adhering the raw ceramic powder and pore-forming material to nuclei, and for example, starch, polyvinyl alcohol, acrylic, cellulose, etc. can be used.

[0027] In the granulation tank 20 of the rolling granulator 10, the wet powder WP is flowed while the mixed powder and water are alternately added repeatedly. This process causes the raw ceramic powder and pore-forming material to adhere to the nuclei, and granulation proceeds. This granulation process is continued until the particles reach the desired particle size (process T220). In process T220, if the rotation speed of the disc 14 is insufficient, the nuclei in the granulation tank 20 will not flow, and granulation cannot be completed. Conversely, if the rotation speed of the disc 14 is excessive, the wet powder WP will flow and granulation will proceed, but crushing due to centrifugal force may occur. Therefore, in processes T200 to T220, it is necessary to appropriately control the rotation speed of the disc 14 so that granulation proceeds properly. For example, when using the ECX40Ps manufactured by Chipton Co., Ltd. as the rolling granulator 10, the rotation speed of the disc 14 should be adjusted within the range of 150 to 300 rpm.

[0028] After step T220, the resulting granular molded body is fired to complete the microwave heating element (step T230). Step T230 is the same as step T130 in Figure 3. By appropriately adjusting the firing conditions, small-diameter pores with the desired pore size are formed, and the pore-forming material disappears, forming large-diameter pores corresponding to the particle size of the pore-forming material. Granulation by rolling granulation makes it possible to easily produce microwave heating element particles with a higher degree of sphericity.

[0029] In step T200, particles that have been pre-granulated by the same operation as in steps T200 to T220 may be used as nuclei to be introduced into the apparatus. With this configuration, microwave heating elements can be granulated using nuclei with a relatively high degree of sphericity, making it easier to increase the degree of sphericity of the resulting microwave heating elements when granulating larger particles as microwave heating elements. When granulating nuclei by the rolling granulation method as described above, it is desirable to use raw material ceramic powder, which is a constituent material of the microwave heating element, as the material used to granulate the nuclei.

[0030] Furthermore, if a ceramic porous body having large-diameter pores and small-diameter pores can be obtained, a microwave heating element may be manufactured by a method different from the manufacturing method described above using Figures 3 and 5. For example, a microwave heating element may be manufactured by mixing a pore-forming material for forming large-diameter pores and a pore-forming material for forming small-diameter pores with raw ceramic powder, and then producing a slurry, molding it, and firing it.

[0031] C. Catalyst: By supporting a catalyst on a porous ceramic material constituting the microwave heating element of this embodiment, a catalyst can be produced to promote a desired reaction. The catalyst supported on the microwave heating element is not particularly limited and can be appropriately selected according to the desired chemical reaction using components in the gas as reactants. For example, metal catalysts and oxide catalysts can be used. As the catalyst metal, for example, it may be a noble metal such as platinum (Pt), gold (Au), silver (Ag), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), or osmium (Os), or it may be a base metal such as manganese (Mn), cobalt (Co), nickel (Ni), iron (Fe), copper (Cu), or zinc (Zn). As the oxide catalyst, for example, various metal oxide catalysts, composite oxide catalysts such as perovskite-type oxide catalysts, or zeolites can be used.

[0032] For example, a catalyst containing at least one of nickel (Ni) and zeolite can be suitably used as a catalyst for the hydrocarbon decomposition reaction and exhaust gas decomposition reaction described later. Furthermore, when nickel ferrite is used as the microwave heating element that serves as the support, it is desirable to use nickel (Ni) or iron (Fe), which are elements contained in the microwave heating element, as the catalyst metal. This is because reducing and precipitating the above metals from the microwave heating element allows for easy support of the catalyst metal on the microwave heating element. Moreover, even when the catalyst metal and the microwave heating element react, the occurrence of large compositional deviations in the microwave heating element is suppressed, thereby reducing the impact on the heating characteristics of the microwave heating element. Furthermore, when zeolite is used as the catalyst, it is desirable because zeolite is the same oxide as the microwave heating element, and the reaction between the microwave heating element and the catalyst can be suppressed over a wide temperature range, including relatively high temperatures.

[0033] There are no particular restrictions on the method for supporting a catalyst on a microwave heating element, and various known methods for supporting catalysts on a support can be employed. When a metal catalyst is used as the catalyst, for example, an impregnation method can be employed, in which the microwave heating element is immersed in a solution containing a catalyst metal salt, and then subjected to calcination and reduction treatment to disperse and support the catalyst metal on the microwave heating element. In this case, it is desirable to use negative pressure conditions when immersing the microwave heating element in the solution containing the catalyst metal salt, as this allows the solution to easily penetrate into the pores (large-diameter and small-diameter pores) formed inside the microwave heating element, and makes it easy to support the catalyst metal on the surface of the pores formed inside the microwave heating element. In addition to the impregnation method described above, various methods for supporting catalyst metal on a support can be used, such as coprecipitation, ion exchange, arc discharge, sputtering, ion plating, vacuum deposition, and plating treatment. Furthermore, when an oxide catalyst is supported on a microwave heating element, for example, when a composite oxide catalyst is used as the oxide catalyst, methods such as the solid-phase reaction method, coprecipitation method, Pechini method, citrate complex method, and sol-gel method can be used.

[0034] The catalyst described above, which includes the microwave heating element of this embodiment as a catalyst support, can be used, for example, to accelerate the reforming reaction of fossil fuels, the decomposition reaction of hydrocarbons, or the decomposition reaction of exhaust gas containing volatile organic compounds (VOCs), etc.

[0035] Examples of the reforming reactions of fossil fuels include reactions that produce hydrogen from hydrocarbons or alcohols through steam reforming reactions or partial oxidation reactions. Below, as an example of such a reaction, the general formula for the steam reforming reaction of hydrocarbons is shown in Equation (1). Also, the general formula for the partial oxidation reaction of hydrocarbons is shown in Equation (2), and the shift reaction that produces carbon dioxide and hydrogen from carbon monoxide and steam generated in the partial oxidation reaction is shown in Equation (3). Also, as an example of a reaction that produces hydrogen from alcohol, the steam reforming reaction of methanol is shown in Equation (4), the steam reforming reaction of ethanol is shown in Equation (5), and the partial oxidation reaction of methanol is shown in Equation (6). Also, as an example of the decomposition reaction of hydrocarbons, the decomposition reaction of methane is shown in Equation (7), and as an example of the decomposition reaction of volatile organic compounds (VOCs), the decomposition reaction of toluene is shown in Equation (8). All of these reactions involve the transfer of protons and electrons.

[0036] C n H m + 2nH2O → (m / 2+2n)H2+ nCO2… (1) C n H m + (n / 2)O2→ nCO + (m / 2)H2… (2) CO + H2O → CO2+H2… (3) CH3OH + H2O → CO2+ 3H2… (4) C2H5OH + 3H2O → 2CO2+ 6H2… (5) CH3OH + 1 / 2O2→ CO2+ 2H2… (6) CH4→ C + 2H2… (7) C7H8+ 9O2→ 7CO2+ 4H2O … (8)

[0037] The microwave heating element of this embodiment, configured as described above, comprises a porous ceramic body that generates heat upon microwave irradiation. The porous ceramic body has a first peak with the largest peak value and a second peak smaller than the first peak in the differential pore volume distribution when the pore size distribution is measured by the mercury intrusion method. One of the two peaks is located in the range of pore size between 5 μm and 100 μm, and the other peak is located in the range of pore size of 1 μm or less. Therefore, when supplying gas to such a microwave heating element and carrying out a chemical reaction on the microwave heating element using the components in the gas as reactants, the microwave heating element can be heated by irradiating it with microwaves, thereby efficiently raising the temperature of the reaction site where the chemical reaction is carried out, and in particular, the activity of endothermic reactions can be increased.

[0038] In other words, since internal heating occurs by irradiating with microwaves, which heats the microwave heating element itself as the reaction proceeds on its surface, heating can be performed with higher energy efficiency compared to external heating using heaters, etc. Furthermore, if a porous material that does not generate internal heat is used, even if external heating is performed, or even if a catalyst layer on which internal heating proceeds is provided on the porous material, the temperature rise of the reaction site may be insufficient due to heat dissipation (heat absorption) by the porous material. In contrast, in the microwave heating element of this embodiment, since the porous material itself generates heat through internal heating, the temperature rise is not suppressed due to heat dissipation by the porous material, and it becomes possible to maintain a high level of activity in the proceeding chemical reaction.

[0039] Furthermore, the porous ceramic material constituting the microwave heating element of this embodiment has a first peak with the largest peak value and a second peak smaller than the first peak in the differential pore volume distribution when the pore size distribution is measured by the mercury intrusion method, and the microwave heating element has the large-diameter pores and small-diameter pores described above. In this way, by having large-diameter pores, it is possible to suppress the pressure loss when gas is supplied to the microwave heating element and ensure gas flow compared to the case where only smaller pores are present. This reduction in pressure loss is sufficiently achieved by having a pore diameter of 5 μm or more at the peak corresponding to the large-diameter pore in the differential pore volume distribution. In addition, by having a pore diameter of 100 μm or less at the peak corresponding to the large-diameter pore in the differential pore volume distribution, it is possible to ensure the heating efficiency of the space on the porous ceramic material where the chemical reaction proceeds, and to maintain a high level of activity of the proceeding chemical reaction. Furthermore, as described above, having small-diameter pores increases the specific surface area of ​​the porous ceramic material compared to having only larger pores, thereby increasing the area available as a reaction site within the porous ceramic material, and further improving the heating efficiency of the space on the porous ceramic material where the chemical reaction proceeds.

[0040] In the microwave heating element of this embodiment, when comparing the pore diameter of the first peak and the pore diameter of the second peak in the differential pore volume distribution when the pore diameter distribution of the ceramic porous material is measured by the mercury intrusion method, it is preferable that the pore diameter of the peak corresponding to the large diameter pore is 10 times or more than the pore diameter of the peak corresponding to the small diameter pore. By doing so, the effects of having both large and small diameter pores can be obtained more fully. Specifically, for example, by having relatively small diameter pores, the effect of maintaining the strength and structure of the ceramic porous material can be enhanced, while by having relatively large diameter pores, the effect of reducing pressure loss and improving fluid flow within the pores can be enhanced. Here, the ratio of the pore diameter of the peak corresponding to the large diameter pore to the pore diameter of the peak corresponding to the small diameter pore is obtained by dividing the value of the pore diameter at the peak top corresponding to the large diameter pore by the value of the pore diameter at the peak top corresponding to the small diameter pore. In the example shown in Figure 2, the first peak (peak(α)) corresponding to a large pore is located at a pore diameter of approximately 20 μm, while the second peak (peak(β)) corresponding to a small pore is located at a pore diameter of approximately 0.25 μm. Thus, the ratio of the pore diameter of the peak corresponding to a large pore to the pore diameter of the peak corresponding to a small pore is approximately 80 times, which is more than 10 times.

[0041] Furthermore, in the microwave heating element of this embodiment, the differential pore volume distribution obtained by measuring the pore size distribution of the ceramic porous material by the mercury intrusion method includes a first peak and a second peak corresponding to the large-diameter and small-diameter pores described above. Therefore, it becomes possible to more easily control the temperature when heating the ceramic porous material by microwave irradiation. Specifically, by using the microwave heating element of this embodiment, it becomes possible to stop heating the ceramic porous material near the Curie temperature of the ceramic material constituting the ceramic porous material when heating it by microwave irradiation.

[0042] Here, ceramic porous materials composed of the magnetic loss materials and dielectric loss materials described above generally heat up over time when irradiated with microwaves. Therefore, in order to control the temperature, complicated operations are required, such as measuring the temperature of the above-mentioned material during microwave irradiation and switching the microwave irradiation on and off, making it relatively difficult to achieve sufficiently accurate temperature control. For example, ferromagnetic materials, which are one of the materials for ceramic porous materials, are known to become paramagnetic when they reach the Curie temperature, and it is known that microwave heating due to magnetic loss disappears above the Curie temperature. However, even if a ceramic porous material is formed using a ferromagnetic material, it usually continues to heat up above the Curie temperature when microwave irradiation is continued. This is presumed to be because the magnitude of losses other than magnetic loss, such as Joule loss, becomes relatively large around the Curie temperature, causing heating to continue above the Curie temperature. In contrast, in the microwave heating element of this embodiment, in which the differential pore volume distribution shows the bimodal distribution described above, heating stops around the Curie temperature when microwave irradiation is performed. Therefore, excessive heating above the Curie temperature is suppressed, and more accurate temperature control becomes possible with simpler control. One possible reason for this is that by using a porous material that exhibits the bimodal distribution of differential pore volume distribution described above as the microwave heating element, not only magnetic loss but also Joule loss and dielectric loss become sufficiently small near the Curie temperature. The Curie temperature of the ceramic material constituting the porous ceramic body is determined by the composition of the ceramic material.

[0043] D. Other embodiments: In the embodiments described above, the catalyst is supported on a porous ceramic body that constitutes the microwave heating element, but different configurations are also possible. For example, instead of supporting a catalyst on the porous ceramic body, a catalyst-free thermal decomposition reaction (a reaction that can proceed under temperature conditions raised by heating, without lowering the reaction temperature using a catalyst) may be carried out on the porous ceramic body as a chemical reaction using components in the gas. For example, it can be used for the decomposition reaction of volatile organic compounds (VOCs) as described above. In this case as well, by having large-diameter and small-diameter pores in the porous ceramic body, the same effects as when used as a catalyst support can be achieved, namely, the effect of suppressing pressure loss during gas flow in the microwave heating element while securing the reaction site and ensuring the efficiency of heating the reaction site. [Examples]

[0044] Using a ceramic material that generates heat upon microwave irradiation, a microwave heating element having the large-diameter pores and small-diameter pores described above was fabricated as the microwave heating element of the example. Furthermore, a microwave heating element having only pores corresponding to the large-diameter pores was fabricated as the microwave heating element of the comparative example.

[0045] <Fabrication of microwave heating elements> [Preparation of Samples for Examples] The microwave heating element of the example was fabricated according to the manufacturing method shown in Figure 3. In step T100, nickel ferrite (NiFe2O4) powder was prepared as the raw material ceramic powder. In addition, resin beads with a particle size of 30-40 μm were prepared as the pore-forming material. In step T130, the molded body formed into spherical particles was fired at a firing temperature of 1200°C. As a result, the microwave heating element of the example, with an average absolute maximum length of 5 mm as described above, was obtained.

[0046] [Preparation of comparative sample] A porous ceramic body was fabricated under the same conditions as the sample in the example, except that the firing temperature in step T130 was set to 1300°C, to obtain the comparative example microwave heating element. This resulted in the comparative example microwave heating element having an average absolute maximum length of 5 mm, as described above.

[0047] <Measurement of pore size distribution> [Method for measuring pore size distribution] The pore size distribution was measured for each sample in the examples and comparative examples using the mercury intrusion method.

[0048] Figure 6 is an explanatory diagram showing the results of measuring the pore size distribution for the microwave heating element of the example and the microwave heating element of the comparative example. The pore size distribution measured for the microwave heating element of the example shown in Figure 6 is the same as the pore size distribution shown in Figure 1. As shown in Figure 6, the sample of the example had a bimodal distribution, with a peak at a position of approximately 20 μm corresponding to a large pore and a peak at a position of approximately 0.25 μm corresponding to a small pore. In contrast, the sample of the comparative example had only a peak at a position of approximately 25 μm corresponding to a large pore, and no peak corresponding to a small pore was observed. This is understood to be because, in the comparative example sample, the firing temperature was set higher than that of the sample of the example, resulting in more sintering and the acquisition of a porous ceramic body in which the small pores disappeared.

[0049] <Temperature measurement during microwave irradiation> [Method for measuring temperature during microwave irradiation] Each sample in the examples and comparative examples was irradiated with a microwave at 2.45 GHz in multimode using a microwave oscillator, and the heat generated on the sample surface was measured using a radiation thermometer. The microwave output was set to 100-500 W. Each sample was placed in a flow-through reaction tube with a flow rate of 50 cm³. 3 The evaluation was performed under an Ar gas flow rate of / min.

[0050] Figure 7 is an explanatory diagram showing the temperature change when microwave irradiation is performed with a microwave output of 500W as an example. In Figure 7, the horizontal axis represents the elapsed time from the start of microwave irradiation, and the vertical axis represents the sample temperature. As shown in Figure 7, in all samples, the sample temperature rose rapidly with the passage of time for a while after the start of microwave irradiation. However, in the example sample, after a while from the start of microwave irradiation, specifically when it reached near the Curie temperature of the ceramic material constituting the sample, the heat generation stopped, the temperature rise was suppressed, and the sample temperature was maintained near the Curie temperature. In contrast, in the comparative example sample, after a while from the start of microwave irradiation, the degree of temperature rise was suppressed at near the Curie temperature, but then the temperature rose rapidly again.

[0051] Regarding microwave irradiation of the sample in the example shown in Figure 7, when the microwave output was increased from 100W, the exothermic temperature increased (the heating rate increased) as the microwave output was increased. However, even when microwave irradiation was performed at an output above a certain level, the observed exothermic temperature reached remained the same. Therefore, it is considered that in the sample in the example, the temperature rise stopped because the heat generation due to magnetic loss ceased once the Curie temperature was reached. Furthermore, the fact that the exothermic temperature reached in the sample in the example was the Curie temperature was confirmed by calculating a correction factor from the difference between the value measured with a fluorescence optical fiber thermometer capable of measuring up to 400°C (internal temperature of the sample) and the value measured with a radiation thermometer (surface temperature of the sample), and then correcting the data measured with the radiation thermometer using the correction factor (data not shown).

[0052] As shown in Figure 7, in the comparative example sample having only large-diameter pores, the temperature rose above the Curie temperature when microwave irradiation was continued. In contrast, in the example sample having both large-diameter and small-diameter pores, the temperature rise stopped near the Curie temperature even when microwave irradiation was continued. These results demonstrate that by using a microwave heating element with both large-diameter and small-diameter pores, it is possible to suppress the microwave heating element from rising to an undesirable temperature exceeding the Curie temperature without requiring special temperature control, thereby simplifying the temperature control of the microwave heating element.

[0053] This disclosure is not limited to the embodiments described above, and can be implemented in various configurations without departing from its spirit. For example, the technical features in the embodiments corresponding to the technical features in each form described in the summary of the invention can be replaced or combined as appropriate in order to solve some or all of the above-mentioned problems, or to achieve some or all of the above-mentioned effects. Furthermore, if a technical feature is not described as essential in this specification, it can be deleted as appropriate.

[0054] This disclosure can also be implemented in the following forms: [Application Example 1] A microwave heating element, Equipped with a ceramic porous body that generates heat upon microwave irradiation, The aforementioned porous ceramic material, in terms of the differential pore volume distribution when the pore size distribution is measured by the mercury intrusion method, It has a first peak with the largest peak value and a second peak that is smaller than the first peak, One of the first and second peaks is located in the range of pore diameters between 5 μm and 100 μm. The other of the two peaks, the first peak and the second peak, is characterized by being located in a range where the pore diameter is 1 μm or less. Microwave heating element. [Application Example 2] The microwave heating element described in Application Example 1, The second peak is characterized by being the second largest peak in the differential pore volume distribution. Microwave heating element. [Application Example 3] A microwave heating element as described in Application Example 1 or 2, The difference pore volume distribution is characterized by exhibiting a bimodal distribution. Microwave heating element. [Application Example 4] A microwave heating element described in any one of the application examples 1 to 3, The aforementioned porous ceramic body is characterized by being composed of an iron-containing oxide. Microwave heating element. [Application Example 5] The microwave heating element described in Application Example 4, The iron-containing oxide is characterized by being a spinel-type ferrite. Microwave heating element. [Application Example 6] The microwave heating element described in Application Example 5, The aforementioned spinel-type ferrite is (Ni 1-x Zn x It is characterized by being represented as Fe2O4 (where 0≦x≦0.7). Microwave heating element. [Application Example 7] A microwave heating element described in any one of the application examples 1 to 6, It is characterized by being formed in particulate form with an average absolute maximum length of 1 mm or more. Microwave heating element. [Application Example 8] The microwave heating element described in Application Example 7, Characterized by being formed in the form of spherical particles. Microwave heating element. [Application Example 9] A microwave heating element described in any one of the application examples 1 to 8, A catalyst supported on the ceramic porous body constituting the microwave heating element, A catalyst characterized by comprising the following features. [Application Example 10] The catalyst described in Application Example 9, The catalyst is characterized by containing at least one of nickel (Ni) and zeolite. Catalyst body. [Application Example 11] A method for manufacturing a microwave heating element, A wet powder containing raw ceramic powder, which is the constituent material of the ceramic porous body, is granulated using a rolling granulator to form a microwave heating element according to any one of Application Examples 1 to 10. The rolling granulation apparatus is characterized by using an apparatus comprising a cylindrical drum, a disk positioned to close the opening at the bottom of the drum, and a first drive device for rotating the disk. A method for manufacturing microwave heating elements. [Application Example 12] A method for manufacturing a microwave heating element as described in Application Example 11, The rolling granulation apparatus is characterized by using an apparatus that further includes a second drive device for rotating the drum. A method for manufacturing microwave heating elements. [Explanation of Symbols]

[0055] 10…Rolling granulation device 12… Drums 13…Inner surface of the drum 14…Disk 16…First drive unit 18…Second drive unit 20…Granulation tank

Claims

1. A microwave heating element, Equipped with a ceramic porous body that generates heat upon microwave irradiation, The aforementioned porous ceramic material, in terms of the differential pore volume distribution when the pore size distribution is measured by the mercury intrusion method, It has a first peak with the largest peak value and a second peak that is smaller than the first peak, One of the first and second peaks is located in the range of pore diameters between 5 μm and 100 μm. The other of the first and second peaks is characterized in that it exists in a range where the pore diameter is 1 μm or less. Microwave heating element.

2. A microwave heating element according to claim 1, The second peak is characterized by being the second largest peak in the differential pore volume distribution. Microwave heating element.

3. A microwave heating element according to claim 1, The difference pore volume distribution is characterized by exhibiting a bimodal distribution. Microwave heating element.

4. A microwave heating element according to claim 1, The aforementioned porous ceramic body is characterized by being composed of an iron-containing oxide. Microwave heating element.

5. A microwave heating element according to claim 4, The iron-containing oxide is characterized by being a spinel-type ferrite. Microwave heating element.

6. A microwave heating element according to claim 5, The spinel-type ferrite is (Ni １－ｘ Zn ｘ ) Fe ２ O ４ (However, it is characterized by being expressed as 0 ≤ x ≤ 0.7) Microwave heating element.

7. A microwave heating element according to claim 1, It is characterized by being formed in a particulate form with an average absolute maximum length of 1 mm or more. Microwave heating element.

8. A microwave heating element according to claim 7, Characterized by being formed in the form of spherical particles. Microwave heating element.

9. A microwave heating element according to any one of claims 1 to 8, A catalyst supported on the ceramic porous body constituting the microwave heating element, A catalyst characterized by comprising the following features.

10. The catalyst according to claim 9, The catalyst is characterized by containing at least one of nickel (Ni) and zeolite. Catalyst body.

11. A method for manufacturing a microwave heating element, A wet powder containing raw ceramic powder, which is the constituent material of the ceramic porous body, is granulated using a rolling granulator to form the microwave heating element described in claim 8. The rolling granulation apparatus is characterized by using an apparatus comprising a cylindrical drum, a disk positioned to close the opening at the bottom of the drum, and a first drive device for rotating the disk. A method for manufacturing microwave heating elements.

12. A method for manufacturing a microwave heating element according to claim 11, The rolling granulation apparatus is characterized by using an apparatus that further includes a second drive device for rotating the drum. A method for manufacturing microwave heating elements.

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