Porous ceramic structure

Abstract

This porous ceramic structure, which is mainly composed of an oxide having proton conductivity and has a plurality of pores, carries less than 0.4 wt% of a catalyst metal. The catalyst metal is at least one of nickel (Ni), copper (Cu), iron (Fe), and ruthenium (Ru).

Description

Porous ceramic structure

[0001] The present disclosure relates to a porous ceramic structure.

[0002] Conventionally, as a reaction for producing carbon monoxide (CO) from carbon dioxide (CO ２ 2) and hydrogen (H ２ 2), the reverse water-gas shift reaction represented by the following formula (1) is known. CO ２ + H ２ 2 → CO + H ２ 2O … (1)

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

[0004] Japanese Patent Application Laid-Open No. 2010-194534

[0005] According to the technique described in Patent Document 1 above, a catalyst for the reverse water-gas shift reaction that can be used at a high temperature can be obtained. However, when using the catalyst at a high temperature of 600 °C or higher, the catalyst metal may deteriorate, for example, due to sintering.

[0006] Also, simultaneously with the reverse water-gas shift reaction, the methanation reactions represented by the following formulas (2) and (3) occur, and there is a risk that the proportion (yield) of carbon monoxide in the finally obtained mixed gas will decrease. CO + 3H ２ 2 → CH ４ 4 + H ２ 2O … (2) CO ２ 2 + 4H ２ 2 → CH ４ 4 + 2H ２ 2O … (3)

[0007] Therefore, a technique for causing the reverse water-gas shift reaction to proceed at a lower temperature and a technique for improving the yield of carbon monoxide are desired. Note that such problems are not limited to the reverse water-gas shift reaction, and a technique for causing a reaction involving proton transfer to proceed at a lower temperature and a technique for improving the yield of the target reactant are desired.

[0008] This disclosure has been made to solve at least some of the above-mentioned problems and can be realized in the following forms: (1) According to one embodiment of this disclosure, a porous ceramic structure having a plurality of pores and mainly composed of a proton-conducting oxide is provided. This porous ceramic structure has less than 0.4 wt% of a catalytic metal supported on it, and the catalytic metal is 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] Because the amount of catalyst metal supported is small, less than 0.4 wt%, when this form of porous ceramic structure is used as a catalyst in, for example, the reaction between carbon dioxide and hydrogen, the methane reaction can be suppressed, the selectivity of the reverse shift reaction can be improved, and the yield of carbon monoxide can be increased.

[0012] (2) A 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. In this way, the proton conductivity can be improved compared to the undoped material, thereby promoting the reaction and improving the yield of the desired reactant.

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

[0014] (4) A porous ceramic structure of the above form, wherein the oxide may contain cerium (Ce). In this case, the porous ceramic structure exhibits high proton conductivity, making it possible to construct a catalyst that is more suitable for use when an electric field is applied.

[0015] This disclosure can be implemented in various forms other than those described above, for example, in the form of a catalyst, a catalyst for a reverse shift reaction, a method for manufacturing a porous ceramic structure, a method for manufacturing a catalyst for a reverse shift reaction, a method for manufacturing carbon monoxide, and so on.

[0016] This is a conceptual diagram illustrating the structure of a porous ceramic structure. This is a conceptual diagram showing a magnified portion of a porous ceramic structure. This is a process diagram showing an example of a method for manufacturing a porous ceramic structure. This is a diagram showing the specifications of each sample. This is a diagram illustrating the configuration of the evaluation apparatus. This is a diagram showing the relationship between the presence or absence of a catalyst metal and the yields of methane and carbon monoxide. This is a diagram showing the relationship between the presence or absence of a catalyst metal and the yields of methane and carbon monoxide. This is a diagram showing the difference in carbon monoxide yield depending on the amount of doping element.

[0017] <Embodiments> A. Structure of the porous ceramic structure: Figure 1 is a conceptual diagram illustrating the structure of the 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, for example, unavoidable trace components derived from the raw material powder.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] The ceramic portion 10 may include, for example, at least one of a metal oxide and a metal phosphate. The ceramic may be a fluorite-type structured metal oxide, specifically a cerium-containing oxide such as CeO ２ System oxides and zirconium-containing oxides such as ZrO ２ Various oxides can be suitably used. In particular, cerium oxide (CeO) ２ ) exhibits high proton conductivity, for example, ZrO ２These metal oxides are known to have higher proton conductivity than conventional 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), and are therefore desirable. These metal oxides are suitable when the porous ceramic structure 100 of this embodiment is used with an electric field applied, as will be described later.

[0022] Other metal oxides may be used as the main component of the ceramic part 10, for example, metal oxides with a perovskite structure that exhibit proton conductivity (e.g., BaCeO ３ Metal oxides exhibiting proton conductivity, such as fergusonite-type or scheite-type structures (e.g., LaNbO2). ４ (e.g., La) ２ Zr ２ O ７ (e.g., Ce) １２ Al １４ O ３３ (e.g., Ba) ２ In ２ O ５ (e.g., La) ６ WO ６ (e.g., LaPO2 oxides), phosphate compounds exhibiting proton conductivity (e.g., LaPO2 ４ Systems and SnP ２ O ７ System and CsH ２ PO ４ (e.g., CsHSO4) ４ Systems such as the following can be used.

[0023] 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 rare earth elements and alkaline earth metal elements 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.

[0024] 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.

[0025] 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.

[0026] Figure 2 is an enlarged, conceptual diagram illustrating a portion 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.

[0027] 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 the 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] CO ２ + 6H ＋ + 6e － → CH ３ OH + H ２ O... (4) CO ２ + 2H ＋ + 2e － → HCOOH… (5)

[0033] Examples of reactions that produce hydrogen through dehydrogenation include the dehydrogenation of hydrocarbons and alcohols. Specifically, examples include reactions that produce hydrogen from hydrocarbons and alcohols through steam reforming and 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 (6). 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 water vapor produced in the partial oxidation reaction is shown in equation (8). Furthermore, as examples of reactions that produce hydrogen from alcohols, 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.

[0034] C ｎ H ｍ + 2nH ２ O → (m / 2+2n)H ２ + nCO ２ … (6) C ｎ H ｍ + (n / 2)O ２ → nCO + (m / 2)H ２ … (7) CO + H ２ O → CO ２ +H ２ … (8) CH ３ OH + H ２ O → CO ２ + 3H ２ … (9) C ２ H ５ OH + 3H ２ O → 2CO ２ + 6H ２… (10) CH ３ OH + 1 / 2O ２ → CO ２ + 2H ２ ... (11)

[0035] Since the oxide in the porous ceramic structure 100 of this embodiment is proton conductive, applying an electric field can significantly increase proton conduction, for example, on the surface of the oxide, thereby enhancing catalytic activity. This makes it possible to carry out reactions involving the transfer of protons, for example, under relatively mild conditions (relatively low temperature conditions or relatively low pressure conditions). As a result, degradation of the catalyst metal due to high heat can be suppressed. However, the porous ceramic structure 100 may also be used without applying an electric field.

[0036] For example, the reverse shift reaction that produces carbon monoxide from carbon dioxide and hydrogen takes place at around 600 to 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.

[0037] 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.

[0038] B. Method for manufacturing a porous ceramic structure: 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 O γ ), strontium zirconate (SrZrO ３) etc. can be used. For example, ethanol can be used as a solvent. GDC (Gd Ｘ Ce １－Ｘ O γ For example, (Gd ０．２ Ce ０．８ O １．９ ) can be used.

[0039] 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 step finely pulverizes the raw material powder and mixes it with the solvent to produce a slurry. In this step, the specific surface area can be increased by finely grinding the raw material powder, and the firing temperature can be lowered.

[0040] In step P106, the slurry obtained in step P104 is transferred to a bowl and dried in a water bath at 80°C to allow the ethanol to evaporate completely and turn it into a powder. Steps P102 to P106 produce porous ceramics in powder (granular) form.

[0041] 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.

[0042] In step P110, a compacted powder is obtained by pressing it with a single-screw press using a predetermined mold. Depending on the shape of the mold used for pressing in step P110, the porous ceramics obtained can be made into a desired shape. For example, it can be molded into a prismatic shape (Figure 1), a cylindrical shape, a honeycomb shape, etc. It may also be molded into a pellet shape.

[0043] In step P111, a CIP (Cold Isostatic Pressing) machine is used to apply kinetic pressure to the compact formed in step P110. For example, the pressure can be increased to approximately 147 MPa.

[0044] 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, 200°C to 350°C (in an atmospheric environment).

[0045] 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 (for example, 400°C to 600°C) that does not cause excessive necking and can maintain 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 and the material will be prone to fracture. In step 114, by performing heat treatment at a temperature that does not cause excessive necking and can maintain 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.

[0046] 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.

[0047] 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 are described later. Each sample was prepared using the manufacturing method shown in Figure 3.

[0048] 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 oxide component, and the amount of gadolinium (Gd) doped as the doping element is 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 (CeO) as their main oxide component. ２ ) In this embodiment, commercially available GDC is used as the raw material powder, but in step P102 shown in Figure 3, the amount of Gd doping is set as described above, CeO ２ You can dope it with Gd.

[0049] 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).

[0050] 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.

[0051] 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 analytical apparatus 600 is configured to analyze the composition of the mixed gas discharged from the reaction vessel 300. In this embodiment, gas chromatography was used as the analytical apparatus 600.

[0052] The evaluation conditions are as follows: • Furnace temperature: 250°C (when no electric field is applied) • Furnace pressure: 1 atmosphere • Raw material gas flow rate: 50 sccm • Raw material gas composition: Hydrogen (H) ２ ) / Carbon dioxide (CO2) ２ ) = 4 • Space velocity (SV) of gas in the reaction vessel: 3000 h －１ • Setting current: 0, 0.1, 0.5, 1, 2, 3, 4, 5, 10, 15, 20 mA

[0053] After each sample is placed in the evaluation device 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°C • Furnace pressure: 1 atm • Reducing gas flow rate: 60 sccm • Reducing gas composition: Hydrogen (H) ２ ) / Argon (Ar) = 1 / 2 • Processing time: 30 minutes

[0054] 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.

[0055] 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 with an applied current of 2 mA. Even with applied currents of 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 (a catalytic metal).

[0056] 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 much higher carbon monoxide yield, confirming that the absence of nickel (catalyst metal) improves the selectivity of the reverse shift reaction shown in equation (1) above.

[0057] Figure 7 shows the difference in methane yield and carbon monoxide yield with and without catalyst metal support. Figure 7 shows that the main component oxide of the porous ceramic structure is ceria (CeO ２ Figure 7 shows the results of a catalytic reaction test using samples 3 and 4 under the same evaluation conditions as shown in Figure 6, and the analysis of the mixed gas discharged from the reaction vessel 300 using an analyzer 600. Sample 3 has a metal-supported catalyst, 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.

[0058] 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.

[0059] 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 catalyst metal supported, the hydrogenation reactions in equations (2) and (3) did not proceed, and the reverse shift reaction in equation (1) selectively occurred, which is thought to have improved the carbon monoxide yield.

[0060] 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.

[0061] 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. All samples shown in Figure 8 do not have a catalyst metal supported. The main oxide component of the porous ceramic structure 100 in sample 2 is ceria (CeO2). ２ ) and the doping amount of the doping element is 0 mol%. The main oxide component of the porous ceramic structure 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.

[0062] 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, respectively.

[0063] 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.

[0064] Furthermore, since an improvement in carbon monoxide yield was confirmed for all doping amounts from 10 mol% to 30 mol%, it can be said that a doping amount of 10 mol% or more is preferable. Note that doping increases oxygen vacancies, improving proton conduction and allowing CO to enter these oxygen vacancies. ２ Since adsorption is also a possibility, it can be said that even a doping amount of 5 mol% is effective. In other words, a doping amount of 5 mol% or more is preferable.

[0065] In Samples 1 to 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.

[0066] In Samples 1 and 3, nickel was used as the catalytic metal. However, using copper, iron, or ruthenium as the catalytic metal can also improve the selectivity of the reverse shift reaction and increase the carbon monoxide yield. This is because copper, iron, and ruthenium are representative metals used in reverse shift reactions, similar to nickel. Furthermore, since these metals also have the function of promoting the methanation reaction, methane production can be suppressed by keeping the supported amount below 0.4 wt%, just as with nickel.

[0067] 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.

[0068] 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.

[0069] This disclosure can also be realized in the following forms: [Example 1] A porous ceramic structure having a plurality of pores and mainly composed of a proton-conducting oxide, wherein less than 0.4 wt% of a catalyst metal is supported, and the catalyst metal is at least one of nickel (Ni), copper (Cu), iron (Fe), and ruthenium (Ru). [Example 2] The porous ceramic structure according to Example 1, wherein the oxide is doped with a doping element, and the doping element is at least one of a rare earth element and an alkaline earth metal element. [Example 3] The porous ceramic according to Example 2, wherein the doping amount of the doping element in the oxide is 5 mol% or more. [Example 4] A catalyst structure, the porous ceramic structure according to any one of Examples 1 to 3, wherein the oxide is cerium (Ce).

[0070] 10...Ceramic section 20...Pores 22...Connecting holes 100...Porous ceramic structure 110...Catalyst 150...Electrode 200...Power supply 300...Reaction vessel 310...Lid 400...Furnace 500...Raw material gas supply section 600...Analytical device 1000...Evaluation device

Claims

1. A porous ceramic structure having a plurality of pores and mainly composed of a proton-conducting oxide, wherein less than 0.4 wt% of a catalyst metal is supported, and the catalyst metal is at least one of nickel (Ni), copper (Cu), iron (Fe), and ruthenium (Ru).

2. A porous ceramic structure according to claim 1, wherein the oxide is doped with a doping element, and the doping element is at least one of a rare earth element and an alkaline earth metal element.

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

4. A porous ceramic structure according to any one of claims 1 to 3, wherein the oxide is characterized in that it comprises cerium (Ce).

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