Porous ceramic structure

Abstract

We propose techniques to allow the reaction to proceed at lower temperatures and techniques to improve the carbon monoxide yield. [Solution] A porous ceramic structure having multiple pores and mainly composed of a proton-conducting oxide has less than 0.4 wt% of a catalyst metal supported on it, and the catalyst metal is at least one of nickel (Ni), copper (Cu), iron (Fe), and ruthenium (Ru).

Description

[Technical Field]

[0001] This disclosure relates to porous ceramic structures. [Background technology]

[0002] Conventionally, the reverse shift reaction shown in equation (1) below is known as a reaction that produces carbon monoxide (CO) from carbon dioxide (CO2) and hydrogen (H2). CO2 + H2 → CO + H2O … (1)

[0003] Considering the composition (equilibrium composition) of the synthesis gas produced by the reverse shift reaction, it is preferable to carry out the reverse shift reaction at a high temperature of 600°C or higher. Therefore, a technology has been proposed to provide a catalyst for the reverse shift reaction that can be used at high temperatures (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2010-194534 [Overview of the project] [Problems that the invention aims to solve]

[0005] According to the technology described in Patent Document 1 above, a catalyst for reverse shift reactions that can be used at high temperatures can be obtained. However, when the catalyst is used at high temperatures of 600°C or higher, the catalyst metal may deteriorate, for example, due to sintering.

[0006] Furthermore, the methane reaction shown in equations (2) and (3) below may occur simultaneously with the reverse shift reaction, potentially reducing the proportion (yield) of carbon monoxide in the final mixed gas. CO + 3H2 → CH4 + H2O … (2) CO2 + 4H2 → CH4 + 2H2O … (3)

[0007] Therefore, there is a need for techniques to carry out reverse shift reactions at lower temperatures and techniques to improve the yield of carbon monoxide. Furthermore, these challenges are not limited to reverse shift reactions; there is a need for techniques to carry out reactions involving proton transfer at lower temperatures and techniques to improve the yield of the target reactant. [Means for solving the problem]

[0008] This disclosure is made to solve at least some of the problems described above and can be implemented in the following forms. (1) According to one embodiment of the present disclosure, a porous ceramic structure having a plurality of pores is provided, the main component being a proton-conducting oxide. The porous ceramic structure has less than 0.4 wt% of a catalytic metal supported on it, the catalytic metal being at least one of nickel (Ni), copper (Cu), iron (Fe), and ruthenium (Ru).

[0009] This type of porous ceramic structure, being primarily composed of proton-conducting oxides, allows for the application of an electric field. Therefore, by applying an electric field to this type of porous ceramic structure to induce a catalytic reaction, the reaction can proceed at a lower temperature compared to when no electric field is applied, thereby suppressing the degradation of the catalytic metal due to high heat.

[0010] Because this form of porous ceramic possesses proton conductivity, it can be used as a catalyst to promote reactions involving proton transfer, thereby accelerating the reaction. Therefore, reactions can be promoted with no catalyst metal, or with only trace amounts of catalyst metal, contributing to lower catalyst costs, longer lifespan, and improved cost-effectiveness.

[0011] Since the supported amount of the catalytic metal is less than 0.4 wt% and is small, for example, when this form of porous ceramic structure is used as a catalyst in the reaction of carbon dioxide and hydrogen, the methanation reaction is suppressed, the selectivity of the reverse shift reaction can be improved, and the yield of carbon monoxide can be improved.

[0012] (2) The porous ceramic structure of the above form, wherein the oxide is doped with a doping element, and the doping element may be at least one of a rare earth element and an alkaline earth metal element. By doing so, the proton conductivity can be improved compared to those without the doping element, so the reaction can be promoted more, and the yield of the target reactant can be improved.

[0013] (3) The porous ceramic structure of the above form, wherein the doping amount of the doping element in the oxide may be 5 mol% or more. By doing so, the yield of carbon monoxide can be further improved.

[0014] (4) The porous ceramic structure of the above form, wherein the doping element may be any one of gadolinium (Gd), lanthanum (La), yttrium (Y), and praseodymium (Pr). Even in this case, the proton conductivity can be improved compared to those without the doping element, so the reaction can be promoted more, and the yield of the target reactant can be improved. In particular, since gadolinium (Gd) and lanthanum (La) have relatively large ionic radii, when used as doping elements, lattice distortion is likely to occur, the proton conductivity is improved, and the reaction involving proton transfer can be promoted more.

[0015] (5) The porous ceramic structure of the above form may have its catalytic performance activated by applying an electric field. By doing so, when the porous ceramic structure is used as a catalyst, since the catalytic performance is activated by applying an electric field, the reaction can be promoted by applying an electric field. Therefore, for example, by applying an electric field, the target reaction can be promoted at a lower temperature than when no electric field is applied.

[0016] (6) The porous ceramic structure of the above form may have the oxide contain cerium (Ce). By doing so, since the porous ceramic structure exhibits high proton conductivity, a more suitable catalyst can be constituted for use with an applied electric field.

[0017] The present disclosure can be realized in various forms other than the above. For example, it can be realized in forms such as a catalyst, a catalyst for the reverse shift reaction, a method for manufacturing a porous ceramic structure, a method for manufacturing a catalyst for the reverse shift reaction, a method for manufacturing carbon monoxide, and the like.

Brief Description of the Drawings

[0018] [Figure 1] It is an explanatory diagram conceptually showing the configuration of the porous ceramic structure. [Figure 2] It is an explanatory diagram conceptually showing a magnified part of the porous ceramic structure. [Figure 3] It is a process diagram showing an example of a method for manufacturing a porous ceramic structure. [Figure 4] It is a diagram showing the specifications of each sample. [Figure 5] It is an explanatory diagram showing the configuration of the evaluation apparatus. [Figure 6] It is a diagram showing the relationship between the presence or absence of a catalytic metal and the yields of methane and carbon monoxide. [Figure 7] It is a diagram showing the relationship between the presence or absence of a catalytic metal and the yields of methane and carbon monoxide. [Figure 8] It is a diagram showing the difference in carbon monoxide yield depending on the doping amount of the doping element. [Figure 9] This figure shows the composition of the main oxide components of each sample. [Figure 10] This figure shows the carbon monoxide yield for each sample (horizontal axis: applied current). [Figure 11] This figure shows the carbon monoxide yield for each sample (horizontal axis: input power). [Figure 12] This figure shows the X-ray diffraction (XRD) patterns of each sample. [Figure 13] This figure shows a magnified portion of the X-ray diffraction pattern. [Figure 14] This figure shows the ionic radii of the rare earth elements contained in each sample. [Figure 15] This figure shows the relationship between ionic radius and lattice constant. [Figure 16] This figure shows the carbon monoxide yields for samples T1 and T2. [Modes for carrying out the invention]

[0019] <Embodiment> A. Composition of porous ceramic structures: Figure 1 is a conceptual diagram illustrating the configuration of a porous ceramic structure 100 as an embodiment of the present disclosure. The porous ceramic structure 100 of this embodiment mainly consists of a proton-conducting oxide and has a plurality of pores 20. The porous ceramic structure 100 may further contain unavoidable trace components, for example, derived from the raw material powder.

[0020] As shown in Figure 1, the porous ceramic structure 100 of this embodiment is a sintered body molded into a rectangular prism shape with a rectangular base. This improves handling compared to powdered or granular porous ceramics. Furthermore, for example, when the porous ceramic structure 100 of this embodiment is used as a catalyst, metal electrodes can be provided at both ends to constitute an electric field applied catalyst. In other embodiments, the shape of the porous ceramic structure may be cylindrical, polygonal prism, spherical, pellet-shaped, honeycomb-shaped, sponge-shaped, etc.

[0021] As shown in the enlarged view in Figure 1, the porous ceramic structure 100 has a ceramic portion 10 and a plurality of pores 20. The plurality of pores 20 are in communication with each other, forming a plurality of connecting holes 22. As shown in Figure 1, the gas supplied to the porous ceramic structure 100 flows through the connecting holes 22. In other embodiments, other fluids such as liquids may be supplied.

[0022] The ceramic portion 10 can be formed from any ceramic whose main component is a proton-conducting oxide. Proton conduction in the ceramic portion 10 may proceed inside the ceramic portion 10 or on the surface of the ceramic portion 10. The oxide that is the main component of the ceramic portion 10 may be a single oxide or a composite oxide.

[0023] The ceramic portion 10 may include, for example, at least one of a metal oxide and a metal phosphate. Suitable ceramics include fluorite-type structured metal oxides, specifically CeO2-based oxides containing cerium and ZrO2-based oxides containing zirconium. Among these, cerium oxide (CeO2) is desirable because it exhibits high proton conductivity, and is known to have higher proton conductivity than, for example, ZrO2-based oxides (e.g., X. Sun, et al., Phys. Chem. Chem. Phys., 2022, 24, 11856, and X. Sun, et al., Applied Surface Science, 2023, 611, 155590). These metal oxides are suitable when the porous ceramic structure 100 of this embodiment is used with an applied electric field, as described later.

[0024] Other metal oxides may be used as the main component of the ceramic part 10, for example, metal oxides with a perovskite structure exhibiting proton conductivity (e.g., BaCeO3-based oxides), metal oxides with a fergusonite-type or schelite-type structure exhibiting proton conductivity (e.g., LaNbO4-based oxides), metal oxides with a pyrochlore structure exhibiting proton conductivity (e.g., La2Zr2O7-based oxides), and metal oxides with a meienite-type structure exhibiting proton conductivity (e.g., Ce 12 Al 14 O 33 Examples of metal oxides that can be used include proton-conducting metal oxides (e.g., brown-miralite type oxides), metal oxides with a proton-conducting fluorite type structure (e.g., La6WO6 type oxides), proton-conducting phosphoric acid compounds (e.g., LaPO4, SnP2O7, and CsH2PO4 systems), and proton-conducting sulfuric acid compounds (e.g., CsHSO4 systems).

[0025] As the metal oxide, for example, a doped material such as GDC (Gadolinia Doped Ceria / Gadolinium Doped Ceria) may be used. The doped element is not particularly limited, but for example, at least one of a rare earth element and an alkaline earth metal element can be used. In this way, the proton conductivity can be improved compared to an undoped material, so the reaction can be further accelerated and the yield of the desired reactant can be improved. Rare earth elements include scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Alkaline earth metal elements include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).

[0026] The rare earth elements used as doping elements are not particularly limited, but it is preferable that they be one of gadolinium (Gd), lanthanum (La), yttrium (Y), and praseodymium (Pr). It is even more preferable that they be either gadolinium (Gd) or lanthanum (La). Because gadolinium (Gd) and lanthanum (La) have relatively large ionic radii, using them as doping elements makes it easier to generate lattice distortion, improves proton conductivity, and can further promote reactions involving proton transfer.

[0027] The amount of doping element in the oxide is not particularly limited, but is preferably 5 mol% or more, and more preferably 10 mol% or more. This improves the yield of the target reactant. Furthermore, the amount of doping element is preferably 40 mol% or less.

[0028] The doping amount (mol%) of doping elements in oxides can be determined using XRF (X-ray fluorescence analysis). A porous ceramic structure 100 is crushed in a mortar and pestle, and the powder is quantitatively analyzed. The output result is then converted to mol%. Similarly, doping elements can be identified using XRF.

[0029] In this embodiment, it is preferable that the catalytic performance of the porous ceramic structure 100 is activated by the application of an electric field. This way, when the porous ceramic structure is used as a catalyst, the catalytic performance is activated by the application of an electric field, and the reaction can be accelerated by applying the electric field. Therefore, for example, by applying an electric field, the desired reaction can be accelerated at a lower temperature than when no electric field is applied. The activation of catalytic performance by applying an electric field can be confirmed by comparing the yield of the desired product with and without the electric field.

[0030] Figure 2 is an explanatory diagram illustrating a conceptual, enlarged view of a part of the porous ceramic structure 100 of this embodiment. Although not shown in Figure 2, the porous ceramic structure 100 has a plurality of pores 20, as shown in Figure 1.

[0031] The porous ceramic structure 100 has less than 0.4 wt% of catalyst metal 110 supported on it. Figure 2 shows an example in which catalyst 110 is supported. Here, "less than 0.4 wt%" includes "0 wt%", and the porous ceramic structure 100 includes those in which no catalyst metal is supported. The amount of catalyst metal 110 supported is preferably 0.1 wt% or less, and more preferably 0 wt% (not supported). According to the porous ceramic structure 100 of this embodiment, the reaction can be promoted even with no catalyst metal or only a trace amount of catalyst metal, thus contributing to the reduction of catalyst cost, extension of lifespan, and improvement of cost benefits.

[0032] The catalyst metal is at least one of nickel (Ni), copper (Cu), iron (Fe), and ruthenium (Ru). These are metals used as catalysts for reverse shift reactions. Therefore, the reverse shift reaction can be promoted by using the porous ceramic structure 100. Since the reverse shift reaction is a reaction involving the transfer of protons, and the porous ceramic structure 100, which is mainly composed of proton-conducting oxides, can function as a catalyst, the reverse shift reaction can be promoted even if the porous ceramic structure 100 does not support the catalyst metal. Because the amount of catalyst metal supported is small, less than 0.4 wt%, for example, when the porous ceramic structure 100 is used as a catalyst in the reaction between carbon dioxide and hydrogen, the methanation reaction can be suppressed, the selectivity of the reverse shift reaction can be improved, and the yield of carbon monoxide can be improved.

[0033] The type and amount of catalyst metal can be determined by ICP-AES analysis (inductively coupled plasma atomic emission spectroscopy). The amount of catalyst metal (wt%) is the amount of catalyst metal (wt) relative to the total amount (wt) of the porous ceramic structure 100.

[0034] Furthermore, the porous ceramic structure 100 can also be used as a catalyst to promote reactions other than reverse shift reactions involving proton transfer. Examples of reactions involving proton transfer include the hydrogenation reaction (reduction reaction) of carbon dioxide, or reactions that produce hydrogen through dehydrogenation.

[0035] Examples of hydrogenation reactions of carbon dioxide include reactions that produce organic substances such as hydrocarbons and alcohols from carbon dioxide. As examples of such reactions, the reaction that produces methanol from carbon dioxide is shown in equation (4) below, and the reaction that produces formic acid from carbon dioxide is shown in equation (5) below. When the oxide present in the porous ceramic structure 100 of this embodiment is an oxide of an alkaline earth metal or an alkali metal, the oxide has a relatively high basicity and a property that makes it easy to adsorb carbon dioxide, which is desirable because it makes it easier to increase the activity of the above-mentioned reactions that use carbon dioxide as a reactant.

[0036] CO2+ 6H + + 6e - → CH3OH + H2O … (4) CO2 + 2H + + 2e - → HCOOH … (5)

[0037] Examples of reactions that produce hydrogen by dehydrogenation reactions include dehydrogenation reactions of hydrocarbons and alcohols. Specifically, examples of reactions that produce hydrogen from hydrocarbons and alcohols by steam reforming reactions or partial oxidation reactions can be given. Below, as an example of such a reaction, the general formula for the steam reforming reaction of hydrocarbons is shown in Equation (6). Also, the general formula for the partial oxidation reaction of hydrocarbons is shown in Equation (7), 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 (8). Further, as an example of a reaction that produces hydrogen from alcohol, the steam reforming reaction of methanol is shown in Equation (9), the steam reforming reaction of ethanol is shown in Equation (10), and the partial oxidation reaction of methanol is shown in Equation (11). All of these reactions involve the transfer of protons.

[0038] C n H m + 2nH2O → (m / 2+2n)H2+ nCO2… (6) C n H m + (n / 2)O2→ nCO + (m / 2)H2… (7) CO + H2O → CO2+H2… (8) CH3OH + H2O → CO2+ 3H2… (9) C2H5OH + 3H2O → 2CO2+ 6H2… (10) CH3OH + 1 / 2O2→ CO2+ 2H2… (11)

[0039] Since the oxide included in the porous ceramic structure 100 of the present embodiment has proton conductivity, proton conduction on the surface of the oxide, for example, becomes prominent by applying an electric field, and it becomes possible to enhance the catalytic activity. As a result, for example, when proceeding with the above-described reaction involving the transfer of protons, the reaction can proceed under relatively mild conditions (relatively low-temperature conditions or relatively low-pressure conditions). As a result, deterioration of the catalytic metal due to high heat can be suppressed. However, the porous ceramic structure 100 may be used without applying an electric field.

[0040] For example, the reverse shift reaction that produces carbon monoxide from carbon dioxide and hydrogen takes place at around 600-700°C when no electric field is applied. In contrast, when the porous ceramic structure 100 of this embodiment is used as a catalyst in the reaction between carbon dioxide and hydrogen, and an electric field is applied to carry out the reaction, carbon monoxide can be produced even at low temperatures of 250°C or below.

[0041] According to the porous ceramic structure 100 of this embodiment, since it has proton conductivity, this form of porous ceramic can be used as a catalyst to promote reactions involving the transfer of protons, thereby accelerating the reaction. Therefore, since the reaction can be promoted with no catalyst metal or with only a trace amount of catalyst metal, it can contribute to reducing the cost of catalysts, extending their lifespan, and improving cost benefits.

[0042] B. Method for manufacturing porous ceramic structures: Figure 3 is a process diagram showing an example of a method for manufacturing a porous ceramic structure 100. In step P102, the raw material powder constituting the ceramic part 10 is mixed with a solvent. For example, GDC(Gd X Ce 1-X GDC(Gd)γ, strontium zirconate (SrZrO3), etc. can be used. As a solvent, for example, ethanol can be used. X Ce 1-X For example, (Gd 0.2 Ce 0.8 O 1.9 ) can be used.

[0043] In step P104, the raw material powder is pulverized using a planetary ball mill at a predetermined rotational speed and for a predetermined time. This process finely pulverizes the raw material powder and mixes it with the solvent to produce a slurry. In this process, the specific surface area can be increased by finely grinding the raw material powder, and the firing temperature can be lowered.

[0044] In step P106, the slurry obtained in step P104 is transferred to a bowl, dried in a water bath at 80°C, and the ethanol is thoroughly evaporated to form a powder. Steps P102 to P106 produce porous ceramic powder (granular material).

[0045] In step P108, the powdered porous ceramics obtained in step P106 are mixed with a binder, a solvent, and a porosity-adjusting material (organic beads). The mixture is then stirred in a mortar until the solvent has completely evaporated, producing granular porous ceramics. For example, Celna SE604 (Chukyo Oil & Fat Co., Ltd.) can be used as the binder, and ethanol can be used as the solvent.

[0046] In step P110, a compacted powder is obtained by pressing it with a single-screw press using a predetermined mold. The shape of the mold used for pressing in step P110 allows the final porous ceramic to be shaped as desired. For example, it can be molded into a prismatic shape (Figure 1), cylindrical shape, honeycomb shape, etc. It may also be molded into a pellet shape.

[0047] In step P111, a CIP (Cold Isostatic Pressing) machine is used to apply kinetic pressure to the compacted powder formed in step P110. For example, it can be pressurized to approximately 147 MPa.

[0048] In step P112, the binder components in the compacted powder are volatilized by heating. The heating temperature should be such that the binder components in the compacted powder volatilize, for example, heating at 200°C to 350°C (in an atmospheric environment).

[0049] In step P114, the molded body obtained in step P112 is fired to obtain a porous ceramic sintered body. In step P114, firing is performed at a temperature (e.g., 400°C to 600°C) that prevents excessive necking while maintaining the shape. If the firing temperature is too high, grain growth will progress, causing the particles to connect with each other and reducing the specific surface area. On the other hand, if the firing temperature is too low, the connections between particles will be poor, making it prone to fracture. In step 114, by performing heat treatment at a temperature that prevents excessive necking while maintaining the shape, the specific surface area and strength can be appropriately adjusted. Methods for measuring the degree of necking include observing the cross-sectional structure of the porous ceramic sintered body with an SEM, or measuring the specific surface area and checking whether the value has decreased.

[0050] In step P116, the catalyst metal is supported on the porous ceramic sintered body. Step P116 is carried out, for example, by the incipient wetness method. Various known methods can be used to support the catalyst metal, such as other impregnation methods, coprecipitation methods, and ion exchange methods. If the porous ceramic structure does not support the catalyst metal, step P116 is not performed. Since step P116 may or may not be performed, it is shown as a dashed line in the figure. [Examples]

[0051] Multiple samples of porous ceramic structures were prepared, and the yields of methane and carbon monoxide were investigated by reacting carbon dioxide and hydrogen under an applied electric field. The samples differed in the composition of the main oxide component and the presence or absence of a catalyst metal. The specifications of the samples will be described later. Each sample was manufactured using the method shown in Figure 3 and formed into a rectangular prism shape with a rectangular base as shown in Figure 1.

[0052] Figure 4 shows the specifications of each sample. Samples 1, 2, 5, and 6 have a composite oxide GDC (Gadolinia Doped Ceria / Gadolinium Doped Ceria) as their main component oxide, with gadolinium (Gd) doping amounts of 10 mol% for samples 1 and 2, 20 mol% for sample 5, and 30 mol% for sample 6. Samples 3 and 4 have a single oxide ceria (CeO2) as their main component oxide. In this example, commercially available GDC is used as the raw material powder, but in step P102 shown in Figure 3, CeO2 may be doped with Gd so that the amount of Gd doping is as described above.

[0053] The doping amount (mol%) was determined using XRF (X-ray fluorescence analysis). Each sample was ground in a mortar and pestle, and the powder was quantitatively analyzed. The output results were converted to mol%. Prior to determining the doping amount, it was confirmed that the doping source was properly dissolved using XRD (X-ray diffraction).

[0054] Samples 1 and 3 have 0.4 wt% nickel (Ni) supported as a catalytic metal, while samples 2, 4, 5, and 6 do not have any catalytic metal supported. The amount of nickel supported was determined by ICP-AES analysis (inductively coupled plasma atomic emission spectroscopy). The amount of nickel supported (wt%) is the amount of nickel (wt) relative to the total amount (wt) of the porous ceramic structure 100. Samples 2, 4, 5, and 6 are examples of the porous ceramic structure 100 according to the above embodiment.

[0055] Figure 5 is an explanatory diagram showing the configuration of the evaluation device 1000. Figure 5 shows how the porous ceramic structure 100 samples are set up. As shown in the figure, each sample is provided with electrodes 150 at both ends. The evaluation device 1000 includes a power supply 200 for applying an electric field to the porous ceramic structure 100, a reaction vessel 300 that houses the porous ceramic structure 100 and has a space for the catalytic reaction to proceed, a furnace 400 that houses the reaction vessel, a raw material gas supply unit 500 that supplies a mixed gas of carbon dioxide and hydrogen as a raw material gas into the reaction vessel 300, and an analyzer 600 that analyzes the mixed gas including the product gas generated by the catalytic reaction. The reaction vessel 300 is a hollow tube and has lids 310 at both ends that seal the internal space. The furnace 400 is configured to allow control of the internal temperature, and the power supply 200 is configured to allow control of the current applied to the porous ceramic structure 100. The analyzer 600 is configured to analyze the composition of the mixed gas discharged from the reaction vessel 300. In this embodiment, a gas chromatograph was used as the analyzer 600.

[0056] The evaluation criteria are as follows: ·Furnace temperature: 250℃ (when no electric field is applied) • Furnace pressure: 1 atmosphere • Raw material gas flow rate: 50 sccm • Raw material gas composition: Hydrogen (H2) / Carbon dioxide (CO2) = 4 • Space velocity (SV) of gas inside the reaction vessel: 3000 h -1 • Setting current: 0, 0.1, 0.5, 1, 2, 3, 4, 5, 10, 15, 20mA

[0057] After each sample is placed in the evaluation apparatus 1000, a reduction treatment is performed using a reducing gas instead of the source gas. In the reduction treatment, nickel oxide (NiO) is reduced to nickel (Ni) with hydrogen without applying an electric field. The reduction treatment conditions are as follows. ·Furnace temperature: 350℃ • Furnace pressure: 1 atmosphere • Reduction gas flow rate: 60 sccm • Composition of reducing gas: Hydrogen (H2) / Argon (Ar) = 1 / 2 Processing time: 30 minutes

[0058] Figure 6 shows the difference in methane yield and carbon monoxide yield depending on the presence or absence of catalyst metal support. In Figure 6, the main oxide component of the porous ceramic structure is GDC. In Figure 6, a catalytic reaction test was performed using samples 1 and 2 under the above evaluation conditions, and the results of analyzing the mixed gas discharged from the reaction vessel 300 with an analyzer 600 are shown. Sample 1 has catalyst metal support, and sample 2 does not. Figure 6(A) is a graph with the applied current (mA) on the horizontal axis and the methane yield (%) on the vertical axis, and Figure 6(B) is a graph with the applied current (mA) on the horizontal axis and the carbon monoxide yield (%) on the vertical axis. Here, the yield of each gas is the amount of each gas relative to the amount of mixed gas discharged from the reaction vessel 300.

[0059] As shown in Figure 6(A), in the porous ceramic structure with nickel supported (Sample 1), applying an electric current improved the methane yield compared to when no current was applied. Furthermore, in Sample 1, the methane yield increased with increasing current up to 3 mA, and up to 10 mA, it was possible to obtain a methane yield similar to that at 2 mA. Even at 15 mA and 20 mA, the methane yield was significantly higher, more than three times higher, compared to when no current was applied. On the other hand, in the porous ceramic structure 100 without nickel supported (Sample 2), no methane was produced even when the applied current was increased. From these results, it was confirmed that methane is not produced without nickel (the catalyst metal).

[0060] As shown in Figure 6(B), in the porous ceramic structure with nickel supported (Sample 1), the carbon monoxide yield did not increase significantly even when the applied current was increased up to 10 mA. At applied currents of 15 mA and 20 mA, the carbon monoxide yield could be increased compared to when no current was applied. The carbon monoxide yield could be increased in proportion to the increase in the applied current. On the other hand, in the porous ceramic structure 100 without nickel supported (Sample 2), the carbon monoxide yield could be increased in proportion to the increase in the applied current. From these results, it was confirmed that carbon monoxide can be produced even without nickel (catalyst metal) supported. Furthermore, the reverse shift reaction shown in equation (1) above proceeds at around 600°C to 700°C when no electric field is applied, but it was confirmed that the reverse shift reaction can proceed even at a low temperature of 250°C (set temperature) by applying an electric field. Furthermore, comparing Sample 1 and Sample 2, Sample 2 yielded a significantly higher carbon monoxide, confirming that the absence of nickel (catalyst metal) improves the selectivity of the reverse shift reaction shown in equation (1) above.

[0061] Figure 7 shows the difference in methane yield and carbon monoxide yield depending on the presence or absence of catalyst metal support. In Figure 7, the main oxide component of the porous ceramic structure is ceria (CeO2). In Figure 7, a catalytic reaction test was performed using samples 3 and 4 under the same evaluation conditions as shown in Figure 6, and the results of analyzing the mixed gas discharged from the reaction vessel 300 with an analyzer 600 are shown. Sample 3 has catalyst metal support, and sample 4 does not. Figure 7(A) is a graph with the applied current (mA) on the horizontal axis and the methane yield (%) on the vertical axis, and Figure 7(B) is a graph with the applied current (mA) on the horizontal axis and the carbon monoxide yield (%) on the vertical axis.

[0062] As shown in Figure 7(A), in the porous ceramic structure with nickel supported (Sample 3), applying an electric current improved the methane yield compared to when no current was applied. Furthermore, in Sample 3, the methane yield increased as the applied current increased. On the other hand, in the porous ceramic structure 100 without nickel supported (Sample 4), no methane was produced even when the applied current was increased.

[0063] As shown in Figure 7(B), both the porous ceramic structure with nickel supported (Sample 3) and the porous ceramic structure 100 without nickel supported (Sample 4) showed a slight increase in carbon monoxide yield as the applied current increased. Furthermore, comparing Sample 3 and Sample 4, Sample 4 had a higher carbon monoxide yield, confirming that the absence of a catalyst metal improved the selectivity of the reverse shift reaction shown in equation (1) above. From the test results shown in Figures 6 and 7, it was confirmed that regardless of whether the main component of the porous ceramic structure is a single oxide or a complex oxide, the absence of a catalyst metal improves the selectivity of the reverse shift reaction and increases the carbon monoxide yield. It should be noted that even when the porous ceramic structure supports a catalyst metal, increasing the applied current can promote carbon monoxide generation, but it is preferable that the amount of catalyst metal supported is less than 0.4 wt%. In methanation, the three reaction processes shown in equations (1) to (3) above can normally occur. However, since samples 2 and 4 do not have a supporting catalyst metal, the hydrogenation reactions in equations (2) and (3) above did not proceed, and the reverse shift reaction in equation (1) selectively occurred, which is thought to have improved the carbon monoxide yield.

[0064] Considering that, without an applied electric field, methane is produced instead of carbon monoxide from carbon dioxide and hydrogen at a low temperature of 250°C, and in light of the test results shown in Figures 6 and 7, it can be said that the selectivity of the generated gas can be changed by varying the electric field application conditions (presence or absence of electric field application, magnitude of applied current), the presence or absence of metal support, and the composition of the oxide.

[0065] Figure 8 shows the difference in carbon monoxide yield depending on the amount of doping element. Figure 8 illustrates samples 2, 4, 5, and 6 shown in Figure 4. None of the samples shown in Figure 8 have a catalyst metal supported. The main oxide component of the porous ceramic structure 100 of sample 4 is ceria (CeO2), and the amount of doping element is 0 mol%. The main oxide component of the porous ceramic structures 100 of samples 2, 5, and 6 is GDC, and the doping amounts of the doping element Gd are 10 mol%, 20 mol%, and 30 mol%, respectively.

[0066] As shown in the figure, samples 2, 5, and 6, in which the main component oxide of the porous ceramic structure 100 is a composite oxide doped with doping elements, were able to improve the carbon monoxide yield compared to sample 4, in which the main component oxide is a single oxide, at doping amounts of 10 mol%, 20 mol%, and 30 mol%.

[0067] As shown in Figure 8, the results confirm that when the main component of the porous ceramic structure 100 is a composite oxide doped with doped elements, the carbon monoxide yield can be improved compared to when the main component is an oxide that is not doped with doped elements.

[0068] Furthermore, since an improvement in carbon monoxide yield was confirmed for all doping amounts of the doping element from 10 mol% to 30 mol%, it can be said that a doping amount of 10 mol% or more is preferable. It should also be noted that doping increases oxygen vacancies, which can improve proton conduction and allow CO2 to be adsorbed onto these oxygen vacancies; therefore, even a doping amount of 5 mol% can be effective. In other words, a doping amount of 5 mol% or more is preferable.

[0069] In Samples 1-6, ceria-based oxides are used as the main oxide component of the porous ceramic structure 100. However, similar effects can be obtained by using proton-conducting oxides other than ceria-based oxides as the main oxide component. Since the reverse shift reaction involves the transfer of protons, the porous ceramic structure 100, which has a proton-conducting oxide as its main component, can promote the reverse shift reaction by functioning as a catalyst. Furthermore, because it is proton-conducting, an electric field can be applied, allowing the reverse shift reaction to be promoted at low temperatures.

[0070] In Samples 1 and 3, nickel is used as the catalytic metal, but the selectivity of the reverse shift reaction and the carbon monoxide yield can also be improved when copper, iron, or ruthenium are used as catalytic metals. This is because copper, iron, and ruthenium are representative metals used in reverse shift reactions, just like nickel. Furthermore, since these metals also have the function of promoting the methanation reaction, when these metals are used, as with nickel, methane production can be suppressed by keeping the supported amount below 0.4 wt%.

[0071] In samples 1, 2, 5, and 6, gadolinium is used as the doping element, but the carbon monoxide yield can be similarly improved by using at least one of other rare earth elements and alkaline earth metal elements as the doping element. Proton conductivity is exhibited by the formation of oxygen vacancies in the crystal. For example, since cerium is a +4 valence element, substituting it with a +3 valence rare earth element or a +2 valence alkaline earth metal element can create oxygen vacancies and increase proton conductivity. As a result, the reverse shift reaction can be promoted.

[0072] Next, several porous ceramic structure samples (Samples S2-S5) with different doping elements of the main oxide component, and sample S1, a porous ceramic structure with ceria (CeO2) as the main component, were prepared, and the carbon monoxide yield was investigated in the same manner as for samples 1-6 above. Each sample was manufactured using the method shown in Figure 3 and formed into a rectangular prism shape with a rectangular base as shown in Figure 1.

[0073] Figure 9 shows the composition of the main oxide components of samples S1 to S5. Sample S1 has ceria (CeO2) as its main oxide component. Samples S2 to S5 are composite oxides in which ceria (CeO2) is doped with a rare earth element at a ratio of 10 mol%. The doping elements are gadolinium (Gd) for sample S2, lanthanum (La) for sample S3, yttrium (Y) for sample S4, and praseodymium (Pr) for sample S5. In other words, the main oxide components of samples S2 to S5 are, respectively, GDC (Gadolinium Doped Ceria), LaDC (Lanthanum Doped Ceria), YDC (Yttrium Doped Ceria), and PrDC (Praseodymium Doped Ceria). In this embodiment, commercially available raw material powders were used for each sample. However, in step P102 shown in Figure 3, each rare earth element may be doped into CeO2 so that the doping amount of rare earth elements is 10 mol%. Samples S1 to S5 do not have a catalyst metal supported.

[0074] Figure 10 shows the carbon monoxide yield for samples S1 to S5 (horizontal axis: applied current). In Figure 10, catalytic reaction tests were performed using samples S1 to S5 under the same evaluation conditions as for samples 1 to 6, and the results of analyzing the mixed gas discharged from the reaction vessel 300 using the analyzer 600 are shown. In Figure 10, the horizontal axis is the applied current (mA), and the vertical axis is the carbon monoxide yield (%). Here, the carbon monoxide yield is the amount of carbon monoxide relative to the amount of mixed gas discharged from the reaction vessel 300.

[0075] As shown in the figure, samples S2 to S5, in which the main oxide component of the porous ceramic structure 100 is a composite oxide doped with rare earth elements, were able to improve the carbon monoxide yield compared to sample S1, in which the main oxide component was a single oxide, regardless of the type of rare earth element. From the results shown in Figure 10, it was confirmed that the carbon monoxide yield can be improved by doping ceria (CeO2) with rare earth elements. Doping ceria (CeO2) with rare earth elements causes the generation of oxygen vacancies and lattice distortion. With the increase in oxygen vacancies, an improvement in carbon dioxide adsorption capacity and an improvement in proton conductivity via oxygen vacancies can be expected. It is thought that the improvement in both of these activated the catalytic reaction, leading to an improvement in the carbon monoxide yield.

[0076] Figure 11 shows the carbon monoxide yield for samples S1 to S5 (horizontal axis: input power). The input power was calculated using the following formula (20). Input power (W) = Applied current (A) × Response voltage (V) ... (20) The voltage between the two electrodes 150 in the evaluation device 1000 shown in Figure 5 was defined as the response voltage. Figure 11 shows the evaluation results from Figure 10, with the horizontal axis converted to input power.

[0077] When using the porous ceramic structure of this disclosure as a catalyst and applying an electric field to carry out the reverse shift reaction shown in equation (1) above, it is desirable to have low input energy from the viewpoint of energy saving. Therefore, when the relationship between input power and carbon monoxide yield was investigated, it was found that sample S2 (doped element: gadolinium (Gd)) and sample S3 (doped element: lanthanum (La)) had higher carbon monoxide yields than sample S4 (doped element: yttrium (Y)) and sample S5 (doped element: praseodymium (Pr)).

[0078] As mentioned above, the reason why doping ceria (CeO2) with gadolinium (Gd) and lanthanum (La) can improve carbon monoxide yield is thought to be as follows:

[0079] Figure 12 shows the X-ray diffraction (XRD) patterns of each sample. Figure 13 shows a magnified view of a portion of the X-ray diffraction pattern. In Figure 13, the peak near 2θ = 28.5° (hereinafter also referred to as the first peak) is shown in the upper panel, and the peak near 2θ = 56.3° (hereinafter also referred to as the second peak) is shown in the lower panel. In Figures 12, 13, and Figure 15 (described later), the main oxide component is indicated along with the sample number.

[0080] The lattice constant of each sample was calculated from the diffraction peaks obtained by X-ray diffraction (XRD) measurements using Bragg's law. The XRD measurements were performed under the following conditions. ·X-ray source: Cu-Kα ray (wavelength λ=1.5406Å) • Scan range (2θ): 20°~80° • Sample form: A porous ceramic structure (sintered body) was crushed using a mortar and then subjected to measurement. The lattice constant was calculated using the peak position of the peak (first peak) on the (111) plane.

[0081] Figure 14 shows the ionic radii of the rare earth elements contained in each sample. Figure 15 shows the relationship between ionic radius and lattice constant. The ionic radii shown in Figure 14 are literature values. In Figure 15, the lattice constant calculated from the X diffraction pattern shown in Figure 12 is shown in correspondence with the ionic radii shown in Figure 14. Also, in Figure 15, cerium (Ce) and praseodymium (Pr) are shown as tetravalent values ​​because they are assumed to exist mainly in the tetravalent ionic state within the crystal lattice.

[0082] As shown in Figure 13, the peak positions of sample S2 doped with gadolinium (Gd) and sample S3 doped with lanthanum (La) are shifted to lower angles from sample S1 (cerium (Ce)). As shown in Figure 14, the peaks of gadolinium ions (Gd 3+ ) and lanthanum ions (La 3+ The ionic radius of the cerium ion (Ce 4+ It is larger than the radius of ). Samples S2 and S3 contain cerium ions (Ce 4+) This is because part of it has been substituted by an element with a larger ionic radius, causing the crystal lattice to expand and the lattice constant to increase.

[0083] As shown in Fig. 15, the lattice constant of sample S5 is approximately the same as that of sample S1. The doping element ion of sample S5 is Pr 4+ which has the same valence as cerium ion (Ce 4+ ) and also has an approximately the same ionic radius (Fig. 14). The valences of the doping elements in samples S2 - 4 are all +3 and the same. Since the ionic radius is S4 < S2 < S3 and the lattice constant is S4 < S2 < S3, when the valences are the same, it can be said that the change in the lattice constant is due to the ionic radius.

[0084] Thus, in samples S2 and S3, rare earth element ions with a valence lower than that of cerium ion (Ce 4+ ) are doped, generating oxygen vacancies. Furthermore, since the ionic radius of the doping element is larger than that of cerium ion (Ce 4+ ), relatively large lattice strain occurs. As a result, in samples S2 and S3, the adsorption of carbon dioxide and proton conductivity are improved, so it is considered that the carbon monoxide yield is further improved compared to samples S4 and S5.

[0085] From the above examples (samples S1 - S5), it was confirmed that by doping rare earth elements into ceria (CeO2), the catalytic performance can be activated and the carbon monoxide yield can be improved. Among them, it was confirmed that by using gadolinium (Gd) and lanthanum (La) as doping elements, the carbon monoxide yield can be further improved. In the above examples, the carbon monoxide yield was evaluated. However, as described above, since the catalytic reaction is activated by the generation of oxygen vacancies and lattice strain, reactions involving proton transfer other than the reverse shift reaction (the above formula (1)) can also be promoted in the same way. Also, when the main component of the porous ceramic structure is an oxide with proton conductivity other than ceria, doping with rare earth elements can activate the catalytic performance.

[0086] Next, sample T1 of the porous ceramic structure according to the above embodiment and sample T2 (comparative example) of a porous ceramic structure mainly composed of alumina (Al2O3) were prepared, and the carbon monoxide yield was investigated. The main oxide component of sample T1 is a composite oxide (GDC) obtained by doping ceria (CeO2) with gadolinium (Gd) at a ratio of 30 mol%. Each sample was manufactured by the method shown in Figure 3 and molded into a rectangular prism shape with a rectangular base as shown in Figure 1. In this example, commercially available raw material powders were used for each sample; the raw material powder for sample T1 was GDC powder, and the raw material powder for sample T2 was alumina powder. Samples T1 and T2 do not have a catalyst metal supported.

[0087] Figure 16 shows the carbon monoxide yields for samples T1 and T2. Here, a catalytic reaction test was performed using sample T1 under the same evaluation conditions as for samples 1-6 (the set current was gradually increased from 0 mA to 20 mA). In addition, a catalytic reaction test was performed using samples T1 and T2 without applying an electric field (set current was 0 mA), by gradually increasing the set temperature (furnace temperature) to 250°C, 350°C, 450°C, and 550°C. Other conditions in the catalytic reaction test with varying set temperatures (furnace pressure, raw material gas flow rate, raw material gas composition, space velocity of gas in the reaction vessel, and reduction treatment conditions) were the same as those for samples 1-6. Figure 16 shows the results of analyzing the mixed gas discharged from the reaction vessel 300 using an analyzer 600.

[0088] In sample T1 (whose main oxide component is GDC), carbon monoxide could be produced by increasing the temperature even without applying an electric field. However, by applying an electric field, carbon monoxide could be produced at a lower temperature than when no electric field was applied. In other words, the catalytic performance of sample T1 was activated by the application of an electric field.

[0089] On the other hand, in sample T2 (whose main oxide component is alumina (Al2O3)), carbon monoxide was not produced even when the temperature was increased. Furthermore, since alumina (Al2O3) is an insulator, it is difficult to apply an electric field to it, and therefore it is difficult to promote the reverse shift reaction, making it difficult to produce carbon monoxide. In other words, when the main component of a porous ceramic structure is alumina (Al2O3), the catalytic performance is not activated.

[0090] The results shown in Figure 16 confirm that by using a porous ceramic structure mainly composed of a proton-conducting oxide and having multiple pores, the reverse shift reaction can be promoted by applying an electric field or raising the temperature, even without supporting a catalytic metal.

[0091] 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.

[0092] This disclosure can also be implemented in the following forms: [Application Example 1] A porous ceramic structure having multiple pores and mainly composed of a proton-conducting oxide, It contains less than 0.4 wt% of catalyst metal. The catalyst metal is characterized by being at least one of nickel (Ni), copper (Cu), iron (Fe), and ruthenium (Ru). Porous ceramic structure. [Application Example 2] The porous ceramic structure described in Application Example 1, The aforementioned oxide is doped with a doped element, The doped element is characterized by being at least one of a rare earth element and an alkaline earth metal element. Porous ceramic structure. [Application Example 3] A porous ceramic structure as described in Application Example 1 or Application Example 2, The amount of the doped element in the oxide is 5 mol% or more, characterized in that Porous ceramic structure. [Application Example 4] A porous ceramic structure described in any one of Application Examples 1 to 3, The doped element is characterized by being one of the following: gadolinium (Gd), lanthanum (La), yttrium (Y), and praseodymium (Pr). Porous ceramic structure. [Application Example 5] A porous ceramic structure described in any one of Application Examples 1 to 4, A characteristic feature is that the catalytic performance is activated when an electric field is applied. Porous ceramic structure. [Application Example 6] A porous ceramic structure described in any one of Application Examples 1 to 5, The oxide is characterized by having cerium (Ce), Porous ceramic structure. [Explanation of Symbols]

[0093] 10…Ceramics Department 20…Pore 22...Communication hole 100…Porous ceramic structure 110... Catalyst 150...electrode 200…Power supply 300… Reaction vessel 310…Lid part 400...Furnace 500... Raw material gas supply unit 600…Analyzer 1000...Evaluation device

Claims

1. A porous ceramic structure having multiple pores and mainly composed of a proton-conducting oxide, It contains less than 0.4 wt% of catalyst metal. The catalyst metal is characterized by being at least one of nickel (Ni), copper (Cu), iron (Fe), and ruthenium (Ru). Porous ceramic structure.

2. A porous ceramic structure according to claim 1, The aforementioned oxide is doped with a doped element, The doped element is characterized by being at least one of a rare earth element and an alkaline earth metal element. Porous ceramic structure.

3. A porous ceramic structure according to claim 2, The amount of the doped element in the oxide is 5 mol% or more, characterized in that Porous ceramic structure.

4. A porous ceramic structure according to claim 2, The doped element is characterized by being one of the following: gadolinium (Gd), lanthanum (La), yttrium (Y), and praseodymium (Pr). Porous ceramic structure.

5. A porous ceramic structure according to claim 1, A characteristic feature is that the catalytic performance is activated when an electric field is applied. Porous ceramic structure.

6. A porous ceramic structure according to any one of claims 1 to 5, The oxide is characterized by having cerium (Ce), Porous ceramic structure.

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