Oxygen carrier, method for producing oxygen carrier, and method for producing gas

Abstract

[Problem] To provide: an oxygen carrier having a microstructure that improves gas contact efficiency, increases reactive sites, exhibits high reactivity even under a condition in which oxidation and reduction by a chemical loop method are repeated, for example, and that can suppress reduction of lifespan; a method for producing the oxygen carrier; and a method for producing a gas using the oxygen carrier. [Solution] According to one aspect of the present invention, provided is an oxygen carrier in which oxygen defects are generated through contact with a reducing gas and oxygen defects are eliminated through contact with an oxidizing gas, the oxygen carrier having oxygen ion conductivity and including cerium (Ce), the cumulative pore volume of the oxygen carrier measured by pore size distribution measurement using a mercury intrusion method with a mercury porosimeter being 0.4 cm3 / g or more, the median pore diameter being 2.5 μm or less, and the BET specific surface area being 10 m2 / g or more.

Description

Oxygen carrier, method for producing oxygen carrier, and method for producing gas

[0001] The present invention relates to an oxygen carrier, a method for producing an oxygen carrier, and a method for producing a gas.

[0002] In recent years, the concentration of carbon dioxide, a greenhouse gas, has been increasing in the atmosphere. This increase in atmospheric carbon dioxide concentration contributes to global warming. Therefore, it is important to capture carbon dioxide released into the atmosphere. Furthermore, if the captured carbon dioxide could be converted into valuable carbon resources and reused, a carbon-recycling society could be realized. Conventionally, a method using the reverse water-gas shift reaction (RWSR) has been known as a method for producing carbon monoxide from carbon dioxide. However, this conventional RWSR has a problem in that the conversion efficiency (yield) of carbon dioxide to carbon monoxide is low due to constraints of chemical equilibrium, since the products, carbon monoxide and water, coexist in the system.

[0003] To solve the above problem, the chemical looping method is used to convert (synthesize) carbon dioxide into carbon monoxide. The chemical looping method is a method in which the reverse water gas shift reaction is divided into two reactions: a reduction reaction with hydrogen and a production reaction of carbon monoxide from carbon dioxide. These reactions are carried out using an oxygen carrier (e.g., a metal oxide: MO). ｘ ) is used to bridge the gap (see the formula below). ２ + MO ｘ → H ２ O + MO ｘ－１ CO ２ + MO ｘ－１ → CO + MO ｘ In the above formula, MO ｘ－１ indicates a state in which the metal oxide is partially or completely reduced.

[0004] In this chemical looping method, since water and carbon monoxide, which are the substrates for the reverse reaction, do not coexist during each reaction, it is possible to obtain a higher carbon dioxide to carbon monoxide conversion efficiency than the chemical equilibrium of the reverse water gas shift reaction. On the other hand, since the oxygen carrier repeatedly releases and absorbs oxygen through a reduction reaction with hydrogen and an oxidation reaction with carbon dioxide, there is a problem that the structure changes in the reaction system, which is likely to cause performance degradation and a shortened lifespan. In this chemical looping method, it has been proposed to use a perovskite-type oxide as the oxygen carrier (see Patent Document 1). Patent Document 1 also discloses that silicon oxide (SiO ) is used to maintain the structure of the oxygen carrier. ２ ), cerium oxide (CeO ２ However, because a carrier with low reactivity is used, the amount of oxygen carrier that can be filled per unit volume is reduced, and the reaction efficiency of the entire process is reduced. In addition, since the oxygen carrier does not have a microstructure, the contact efficiency with the gas is low and there are few reaction sites, so it is difficult to say that the reactivity is sufficiently high.

[0005] U.S. Patent No. 1,1691,882

[0006] In view of the above circumstances, the present invention aims to provide an oxygen carrier having a microstructure that improves the contact efficiency with gas, increases the number of reaction sites, and exhibits high reactivity even under conditions where oxidation-reduction is repeated, for example, by a chemical looping method, while suppressing a decrease in its lifespan; a method for producing the same; and a method for producing a gas using such an oxygen carrier.

[0007] According to one aspect of the present invention, there is provided an oxygen carrier which generates oxygen vacancies upon contact with a reducing gas and which eliminates the oxygen vacancies upon contact with an oxidizing gas, the oxygen carrier having oxygen ion conductivity and containing cerium (Ce), and which has a cumulative pore volume of 0.4 cm or less as measured by pore size distribution measurement using a mercury intrusion method with a mercury porosimeter. ３ / g or more, and the median pore diameter is 2.5 μm or less, and the BET specific surface area is 10 m ２ / g or greater.

[0008] According to this embodiment, the reaction efficiency of the oxygen carrier can be increased and the decrease in life can be suppressed.

[0009] 1 is a scanning electron microscope (SEM) image of the oxygen carrier obtained in Example 1. FIG. 1 is an enlarged SEM image of FIG. 1. FIG. 2 is a scanning transmission electron microscope (STEM) image of the oxygen carrier obtained in Example 3. FIG. 3 is a STEM image of the oxygen carrier obtained in Example 3 after three cycles of this test. FIG. 4 is an X-ray diffraction profile (predetermined angle range) obtained by X-ray diffraction measurement for the oxygen carriers obtained in Examples 1 to 3. FIG. 5 is an X-ray diffraction profile (predetermined angle range) obtained by X-ray diffraction measurement for the oxygen carriers obtained in Examples 1 to 3 and Comparative Example 4.

[0010] Hereinafter, the oxygen carrier, the method for producing the oxygen carrier, and the method for producing the gas will be described in detail based on preferred embodiments. [Oxygen Carrier] The oxygen carrier is used when a raw material gas (oxidizing gas) containing carbon dioxide is brought into contact with an oxygen carrier having oxygen deficiencies to reduce the carbon dioxide and produce a product gas containing carbon monoxide (carbon valuables) (i.e., it is used in the gas production method described below). Furthermore, the oxygen carrier oxidized by contact with carbon dioxide can be reduced (regenerated) by contacting it with a reducing gas containing hydrogen (reducing substance). The hydrogen used in this case may not be derived from a fossil fuel. Hydrogen not derived from a fossil fuel is, for example, hydrogen produced by using tungsten trioxide (WO ３ ) or the like to water (H ２ In this case, preferably, the raw material gas and the reducing gas are passed alternately through a reaction tube (reaction vessel) filled with an oxygen carrier, whereby the conversion of carbon dioxide to carbon monoxide by the oxygen carrier and the regeneration of the oxidized oxygen carrier by the reducing gas are carried out.

[0011] The oxygen carrier has oxygen ion conductivity. Here, the oxygen carrier is a compound that can generate reversible oxygen vacancies, which occur upon contact with a reducing gas and disappear upon contact with an oxidizing gas. This oxygen carrier contains cerium (Ce). Oxygen carriers containing cerium are prone to distortion in the crystal lattice, facilitating the transfer of oxygen elements between them and other substances (e.g., carbon dioxide, hydrogen). Therefore, such oxygen carriers can increase the conversion efficiency from carbon dioxide to carbon monoxide (e.g., the amount of carbon monoxide produced) and the conversion efficiency from hydrogen to water.

[0012] When the pore size distribution of this oxygen carrier was measured using a mercury intrusion method with a mercury porosimeter, the cumulative pore volume measured was 0.4 cm ３ / g or more, and the median pore diameter is 2.5 μm or less, and the BET specific surface area is 10 m ２ / g or more. Such an oxygen carrier has a microstructure and can increase the number of reaction sites with the gas it comes into contact with. Therefore, the conversion efficiency of carbon dioxide to carbon monoxide and the conversion efficiency of hydrogen to water can be sufficiently increased. The cumulative pore volume (total pore volume) is 0.4 cm ３ / g or more 2cm ３ / g or less, and ３ / g or more 1.2cm ３ / g or less, and more preferably 0.5 cm ３ / g or more 1.5cm ３ / g or less, and more preferably 0.56 cm ３ / g or more 1.2cm ３ / g or less is particularly preferred. If this pore volume is too small, it becomes difficult for the raw material gas and the reducing gas to penetrate into the inside of the oxygen carrier. As a result, the contact area between the oxygen carrier and the raw material gas and the reducing gas decreases, and the efficiency of converting carbon dioxide to carbon monoxide by the oxygen carrier and the efficiency of regenerating the oxidized oxygen carrier by the reducing gas tend to decrease.

[0013] In addition, the oxygen carrier has a first section pore volume of 0.12 cm in the section with a pore diameter of 100 nm or less.３ / g or more, and ３ / g or more 0.3cm ３ / g or less, and more preferably 0.18 cm ３ / g or more 0.25cm ３ On the other hand, the oxygen carrier has a second section pore volume of 0.35 cm3 in the section having a pore diameter of more than 100 nm. ３ / g or more, and ３ / g or more, and more preferably 0.7 cm ３ It is more preferable that the second section pore volume is 1.5 cm ３ The second section pore volume is, for example, 0.35 cm ３ / g or more 1.5cm ３ / g or less. When the first section pore volume and the second section pore volume (particularly the second section pore volume) are within the above ranges, the raw material gas and the reducing gas can easily penetrate into the inside of the oxygen carrier, and as a result, a sufficient contact area between the oxygen carrier and the raw material gas and the reducing gas can be secured. This makes it possible to further improve the efficiency of converting carbon dioxide to carbon monoxide by the oxygen carrier and the efficiency of regenerating the oxidized oxygen carrier by the reducing gas.

[0014] On the other hand, the median pore diameter is preferably 2.25 μm or less, more preferably 2 μm or less, even more preferably 1.75 μm or less, and particularly preferably 1.5 μm or less. The lower limit of the median pore diameter is usually preferably about 0.05 μm, more preferably about 0.075 μm, even more preferably about 0.1 μm, and particularly preferably about 0.125 μm. The median pore diameter can be, for example, 0.05 μm or more and 2.25 μm or less, 0.075 μm or more and 2 μm or less, 0.1 μm or more and 1.75 μm or less, or 0.125 μm or more and 1.5 μm or less. When the cumulative pore volume and median pore diameter are within the above numerical ranges, the reaction sites with the gas that the oxygen carrier comes into contact with can be further increased while maintaining the mechanical strength and durability of the oxygen carrier. Therefore, the conversion efficiency of carbon dioxide to carbon monoxide and the conversion efficiency of hydrogen to water can be further improved, and a decrease in lifespan can be suppressed. An example of an oxygen carrier having a fine structure is a fibrous aggregate structure (see FIG. 1).

[0015] The BET specific surface area of ​​the oxygen carrier is 12.5 m ２ / g or more, and ２ / g or more, and 17.5m ２ / g or more. It is considered that an oxygen carrier having such a BET specific surface area has a sufficient number of reaction sites with the gas it comes into contact with. Therefore, the conversion efficiency from carbon dioxide to carbon monoxide and the conversion efficiency from hydrogen to water can be further increased. The upper limit of the BET specific surface area of ​​the oxygen carrier is usually 40 m ２ The BET specific surface area of ​​the oxygen carrier is, for example, 12.5 m ２ / g or more 40m ２ The BET specific surface area is a specific surface area measured by a nitrogen gas adsorption method and obtained based on the BET method (preferably the BET multipoint method).

[0016] When X-ray diffraction measurement is performed on the oxygen carrier, in the X-ray diffraction profile, the full width at half maximum of the peak having the maximum intensity among the peaks corresponding to the (110) plane is preferably 0.3° or more in 2θ, more preferably 0.32° or more, and even more preferably 0.35° or more. The upper limit of the full width at half maximum of the peak is not particularly limited, but is usually 0.65° or less. This full width at half maximum can be, for example, 0.3° or more and 0.65° or less. Oxygen carriers with such a large full width at half maximum of the peak tend to have smaller crystallite sizes. In this case, the oxygen carrier has a shorter migration distance of the oxygen element, so that both the reduction reaction of the oxygen carrier by a reducing gas (oxygen deficiency induction reaction) and the reduction reaction of carbon dioxide by the oxygen carrier can be promoted.

[0017] Furthermore, when X-ray diffraction measurement is performed on the oxygen carrier, in the X-ray diffraction profile, the ratio of the sum of the peak intensities of peaks corresponding to the (200) plane among peaks having their peak tops in the 2θ range of 30° to 35° to the sum of the peak intensities of peaks corresponding to the (111) plane among peaks having their peak tops in the 2θ range of 25° to 30° is preferably 27% or less, more preferably 20% to 27%, and even more preferably 22% to 27%. In this case, the crystal structure of the oxygen carrier has many highly reactive (111) planes. Such oxygen carriers have excellent conversion efficiencies of carbon dioxide to carbon monoxide and hydrogen to water.

[0018] Preferably, the oxygen carrier further contains at least one metal element other than cerium, belonging to Groups 3 and 4 of the periodic table, including lanthanides and actinides. In such an oxygen carrier, the inclusion of two or more metal elements, including cerium, makes it easier for distortion to occur in the crystal lattice. This facilitates smooth exchange of oxygen elements with the other substance (e.g., carbon dioxide, hydrogen). As a result, the conversion efficiency from carbon dioxide to carbon monoxide and the conversion efficiency from hydrogen to water can be further improved. Examples of the at least one metal element other than cerium include scandium (Sc), yttrium (Y), lanthanum (La), samarium (Sm), gadolinium (Gd), titanium (Ti), zirconium (Zr), and hafnium (Hf). Among these, the metal element is preferably selected from the group consisting of titanium (Ti), zirconium (Zr), and lanthanum (La). By including these metal elements, the oxygen carrier is more likely to cause distortion in the crystal lattice.

[0019] That is, the oxygen carrier may contain only cerium (Ce), or may contain zirconium (Zr) in addition to cerium (Ce). In this case, the content of cerium (Ce) is preferably 50 mol% to 100 mol%, more preferably 55 mol% to 95 mol%, and even more preferably 60 mol% to 90 mol%, when the total content of cerium (Ce) and zirconium (Zr) is taken as 100 mol%.

[0020] Furthermore, when the oxygen carrier contains zirconium (Zr), when X-ray diffraction measurement is performed on the oxygen carrier in the 2θ ranges of 26° to 36° and 45° to 55°, it is preferable that no peaks are observed in the 2θ ranges of 31.0° to 32.0° and 49.0° to 50.8° in the X-ray diffraction profile. Usually, ZrO is present in the 2θ ranges of 31.0° to 32.0°. ２ A peak corresponding to the (111) plane of the monoclinic crystal was observed, and ZrO was observed in the range of 49.0° to 50.8° in 2θ. ２A peak corresponding to the (022) plane of the monoclinic crystal is observed. Therefore, if no peak is observed in this range, it is considered that the zirconium content in the oxygen carrier is low and cerium and zirconium are uniformly dissolved in solid solution. Such an oxygen carrier has excellent reactivity.

[0021] Here, in this specification, "no peak is observed in the X-ray diffraction profile" means that there is no maximum value in the X-ray diffraction profile where the NET intensity (the intensity obtained by subtracting the background intensity from the peak intensity) exceeds 100 cps within a predetermined angle range. However, when two peaks overlap each other, an approximate curve is created by approximating the curve including the apex of one peak on the side farther from the other peak with a Gaussian function, and the distance between this approximate curve and the apex of the other peak is regarded as the NET intensity of the other peak.

[0022] Furthermore, the oxygen carrier may contain elements other than cerium and the above metal elements. Examples of other elements include niobium (Nb), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu). Among these, the oxygen carrier preferably contains iron. By adding these other elements, the reactivity of the oxygen carrier can be adjusted.

[0023] When the oxygen carrier has a fibrous aggregate structure containing fibrous primary particles (hereinafter also referred to as "fibrous particles") as shown in Figure 1, it is preferable that the short side of the fibrous particles is 100 nm or less, and the aspect ratio, which is the ratio of the long side to the short side, is 5 or more. Such an oxygen carrier tends to have a cumulative pore volume, median pore diameter, and BET specific surface area within the above-mentioned ranges. Furthermore, the short side of the fibrous particles is more preferably 1 nm or more and 1000 nm or less, and even more preferably 5 nm or more and 100 nm or less. Meanwhile, the aspect ratio (long side / short side) of the fibrous particles is more preferably 5 or more and 100 or less, and even more preferably 10 or more and 50 or less.

[0024] When observing multiple 50 μm square regions (excluding overlapping regions) in an SEM pixel (magnification: 50,000 times), the oxygen carrier preferably contains fewer than 20 spherical primary particles (hereinafter also referred to as "spherical particles") with an aspect ratio of 4 or less in each region, more preferably fewer than 5, even more preferably fewer than 3, and particularly preferably 0. In this case, the oxygen carrier is considered to have a low content of metal elements other than cerium (e.g., zirconium), and cerium and the other metal elements are uniformly dissolved in a solid solution. Such an oxygen carrier has superior reactivity. Here, the multiple regions are selected to be separated from each other by 100 μm or more in the SEM pixel. The multiple regions to be observed may be two or more, preferably three to nine, more preferably three to seven, and even more preferably three to five, for example, three.

[0025] The plurality of regions preferably includes at least one region in which the number of spherical particles is less than 10, and more preferably the number of spherical particles is less than 10 in all regions. When spherical particles are present, a clear difference in chemical composition (shading) from that of fibrous particles can be confirmed in an elemental mapping image of the oxygen carrier obtained by scanning electron microscope (SEM)-energy dispersive X-ray spectroscopy (EDX). In the present invention, only particles that show a clear difference in shading from that of fibrous particles and have an aspect ratio of 4 or less as a result of elemental mapping by SEM-EDX are deemed to be spherical particles.

[0026] In addition to the cerium-containing compound, the oxygen carrier may contain (solid-solve) other compounds. Examples of such other compounds include aluminum oxide, magnesium oxide, titanium oxide, and hafnium oxide. Oxygen carriers containing these other compounds can further enhance their heat resistance. In this case, when X-ray diffraction measurement is performed on the oxygen carrier, it is preferable that only one peak is observed in the X-ray diffraction profile in the 2θ range of 47° to 51°. In the X-ray diffraction profile of the oxygen carrier, the peak observed in the 2θ range of 47° to 51° is a peak derived from cerium oxide. However, if the other compound is present uniformly and in small amounts, the peak derived from the other compound will no longer be observed, and only one peak will be observed in the range. Therefore, in this state, it is believed that cerium oxide and the other compound are uniformly distributed (solid-solved). Such an oxygen carrier can fully exhibit excellent reactivity and heat resistance. The full width at half maximum of the only peak observed is preferably 1.25 or less, more preferably 0.3 or more and 1 or less, and even more preferably 0.45 or more and 0.75 or less.

[0027] The loose bulk density of the oxygen carrier is 1.2 g / cm ３ 3g / cm or more ３ It is preferable that the density is 1.4 g / cm or less. ３ 2.5g / cm or more ３ More preferably, it is 1.6 g / cm or less. ３ 2g / cm or more ３It is more preferable that the loose bulk density is less than 0.05. If the loose bulk density is too low, the gas passage speed becomes too fast, and the time for contact between the oxygen carrier and the raw material gas (oxidizing gas) and the reducing gas is reduced. As a result, the amount of carbon monoxide produced by the oxygen carrier is likely to decrease. On the other hand, if the loose bulk density is too high, the gas passage speed becomes too slow, which may make it difficult for the reaction to proceed or may require a long time to produce the product gas. The shape of the oxygen carrier is not particularly limited, but is preferably granular, for example. If it is granular, it is easy to adjust the packing density of the oxygen carrier to within the above range. Here, granular is a concept that includes powder, particle, lump, pellet, etc., and the shape may be any of spherical, plate, polygonal, crushed, columnar, needle, scale, etc.

[0028] The average particle size of the powdered oxygen carrier (including secondary particles) is preferably 1 μm or more and 100 μm or less, more preferably 3 μm or more and 75 μm or less, and even more preferably 5 μm or more and 50 μm or less. An oxygen carrier having such an average particle size can easily adjust its loose bulk density to the above range. As a result, sufficient reaction points for the contacting gust can be secured. The average particle size of the oxygen carrier can be determined by acquiring volumetric particle size distribution data using a laser diffraction / scattering particle size distribution measuring device (e.g., "Partica LA-960" manufactured by Horiba, Ltd.) and processing the data. Measurement is usually performed wet. The powdered oxygen carrier can also be molded and used as a molded body (e.g., pellets). In this case, the maximum length of the projected image of the molded body (e.g., the diameter of the bottom or top surface in the case of a disk-shaped pellet) is preferably 2.5 mm or more and 6 mm or less.

[0029] [Method for producing oxygen carrier] Next, a method for producing the above-mentioned oxygen carrier will be described. The method for producing the oxygen carrier of this embodiment includes a step [1] of producing a compound containing elements and oxygen (O) by precipitation from an aqueous solution containing ions of the elements that constitute the oxygen carrier, a step [2] of recovering the produced compound, a step [3] of drying the recovered compound, and a step [4] of calcining the dried compound to obtain the oxygen carrier.

[0030] Step [1] First, a compound containing the element and oxygen (O) is produced from an aqueous solution containing ions of the element constituting the oxygen carrier by precipitation (coprecipitation when ions of multiple elements are contained). The aqueous solution can be prepared by dissolving salts of the elements constituting the oxygen carrier in water. Examples of the salts of the elements include nitrates, sulfates, chlorides, hydroxides, carbonates, and composites thereof. Among these, nitrates are preferred as the salts of the elements. Furthermore, hydrates of the salts of the elements may be used as needed.

[0031] The aqueous solution may be prepared using acidic water adjusted to an acidic state with, for example, citric acid, acetic acid, malic acid, tartaric acid, hydrochloric acid, nitric acid, or a mixture thereof. Next, a separately prepared alkaline aqueous solution is added dropwise to the aqueous solution. This causes the elements constituting the oxygen carrier and compounds containing oxygen (O) to precipitate. Examples of alkalis that can be used include ammonia, urea, triethylamine, sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, and potassium carbonate. Examples of the compounds include hydroxides, hydroxide oxides, and oxides.

[0032] The aqueous solution is preferably aged. This can promote precipitation and aggregation of the produced compound. Compounds having a fibrous aggregate structure and compounds having a porous structure can then be produced. Adding a long-chain compound to the aqueous solution as an additive facilitates the production of compounds having a fibrous aggregate structure. Examples of long-chain compounds include long-chain carboxylic acids, long-chain sulfuric acids, and long-chain amines. Among these, it is preferable that the long-chain compound contains a long-chain carboxylic acid. The use of a long-chain carboxylic acid as an additive more reliably facilitates the production of compounds having a fibrous aggregate structure. The chain portion of such long-chain compounds contains at least one structure selected from the group consisting of saturated aliphatic structures, unsaturated aliphatic structures, and aromatic structures. The chain portion may be linear or branched.

[0033] The number of carbon atoms in the long-chain compound is not particularly limited, but is preferably 5 to 25, more preferably 8 to 22, and even more preferably 11 to 19. The amount of additive added is preferably 1 to 50 parts by mass, more preferably 3 to 30 parts by mass, and even more preferably 5 to 20 parts by mass, per 100 parts by mass of the salt of the above element. In this case, a compound having a fiber aggregate structure is more easily produced. The temperature (liquid temperature) during aging is not particularly limited, but is preferably 30 to 90°C, more preferably 40 to 80°C, and even more preferably 50 to 70°C. In this case, the product precipitates smoothly and reliably.

[0034] Step [2] Next, the produced compound is recovered. This recovery can be performed by a method such as decantation, filtration, centrifugation, spray drying, etc. If necessary, the recovered compound is washed, for example, with distilled water, etc. Step [3] Next, the recovered compound is dried. This drying can be performed by a method such as natural drying, blow drying, heat drying, freeze drying, etc.

[0035] Step [4] Next, the dried compound is calcined to obtain an oxygen carrier. Examples of the atmosphere in which the compound is calcined (calcination atmosphere) include an air atmosphere (oxidizing atmosphere), a reduced pressure atmosphere, a nitrogen atmosphere (reducing atmosphere), and an inert atmosphere. Among these, the air atmosphere is preferred as the calcination atmosphere. The atmospheric temperature in which the compound is calcined (calcination temperature) is preferably 750°C or higher, more preferably 850°C or higher and 1200°C or lower, and even more preferably 950°C or higher and 1100°C or lower. The time for which the compound is calcined (calcination time) is preferably 1 hour or longer, more preferably 2 hours or higher and 12 hours or lower, and even more preferably 3 hours or higher and 10 hours or lower.

[0036] The compound is preferably converted into an oxide by calcination, but can be easily converted into an oxygen carrier by calcination under the above-mentioned calcination conditions. Furthermore, an oxygen carrier can be obtained that maintains the structure (fibrous aggregate structure) of the compound. Normally, when fine particles are exposed to high temperatures in an atmospheric environment, the primary particles that constitute them tend to aggregate, reducing the non-surface area. In contrast, the oxygen carrier obtained by the above-mentioned method is fibrous, so the primary particles are less likely to aggregate, allowing a high specific surface area to be maintained. The temperature should be increased at a rate of 1°C / min to 20°C / min, preferably 2°C / min to 10°C / min, until the above-mentioned calcination temperature is reached. This promotes particle growth of the oxygen carrier and also prevents crystal (particle) cracking.

[0037] [Method of Using Oxygen Carrier] As described above, the oxygen carrier can be used, for example, in a chemical looping method. Furthermore, as described above, the oxygen carrier can be used in applications in which carbon dioxide is reduced by contact to produce carbon monoxide (carbon valuables). More specifically, a reduction reaction of carbon dioxide and a reduction reaction of the oxygen carrier are preferably carried out, and the oxygen carrier is preferably used so as to circulate between the reduction reaction of carbon dioxide and the reduction reaction of the oxygen carrier. In the reduction reaction of the oxygen carrier, a reducing gas containing another reducing substance is used.

[0038] Furthermore, the oxygen carrier is preferably used in the so-called reverse water gas shift reaction. The reverse water gas shift reaction is a reaction that produces carbon monoxide and water from carbon dioxide and hydrogen. When the chemical looping method is applied, the reverse water gas shift reaction is carried out by dividing it into a reduction reaction of the oxygen carrier (first process) and a reduction reaction of carbon dioxide (second process). The reduction reaction of the oxygen carrier is the reaction shown in formula (A) below, and the reduction reaction of carbon dioxide is the reaction shown in formula (B) below.

[0039] H ２ (Gas) + MO ｘ (Solid) → H ２ O (gas) + MO ｘ－１ (solid) 　 (A) CO ２ (Gas) + MO ｘ－１ (solid) → CO (gas) + MO ｘ (Solid) (B) In formulas (A) and (B), x is usually 2. That is, in the reduction reaction of the oxygen carrier, hydrogen, which is a type of reducing substance, is oxidized to produce water. Also, in the reduction reaction of carbon dioxide, carbon dioxide is reduced to produce carbon monoxide, which is a type of carbon valuable.

[0040] The reaction temperature (contact temperature with the reducing gas) in the reduction reaction of the oxygen carrier may be any temperature at which the reduction reaction can proceed, but is preferably a temperature above 650°C, more preferably 700°C or higher, even more preferably 750°C or higher, and particularly preferably 800°C or higher. Within this temperature range, the reduction reaction of the oxygen carrier can proceed efficiently. The upper limit of this reaction temperature is preferably 1050°C or lower, more preferably 1000°C or lower, even more preferably 950°C or lower, particularly preferably 900°C or lower, and most preferably 850°C or lower. Setting the upper limit of the reaction temperature within the above range can improve economic efficiency. The lower limit and upper limit can be combined in any way to define the reaction temperature range.

[0041] Furthermore, during the reduction reaction of the oxygen carrier, the amount of hydrogen (reducing gas) brought into contact with the oxidized oxygen carrier is preferably 1 mmol to 50 mmol, more preferably 2.5 mmol to 35 mmol, and even more preferably 5 mmol to 20 mmol, per 1 g of oxygen carrier. The oxygen carrier has a high hydrogen utilization rate because oxygen elements can be smoothly transferred in and out. Therefore, the oxygen carrier is sufficiently reduced (regenerated) with a small amount of hydrogen. This reduces the energy required to generate hydrogen, and ultimately contributes to reducing the amount of carbon dioxide generated when obtaining energy. The hydrogen utilization rate (%) is the ratio, expressed as a percentage, of the amount of carbon monoxide produced (number of moles) to the amount of hydrogen input (number of moles) brought into contact with 1 g of oxygen carrier.

[0042] Furthermore, the reaction temperature (contact temperature with the oxidizing gas) in the carbon dioxide reduction reaction is preferably a temperature above 650°C, more preferably 700°C or higher, even more preferably 750°C or higher, and particularly preferably 800°C or higher. Within this temperature range, the carbon dioxide reduction reaction can proceed efficiently. The upper limit of this reaction temperature is preferably 1050°C or lower, more preferably 1000°C or lower, even more preferably 950°C or lower, particularly preferably 900°C or lower, and most preferably 850°C or lower. Since oxygen carriers can efficiently reduce carbon dioxide to carbon monoxide even at low temperatures, the carbon dioxide reduction reaction can be set to a relatively low temperature. Furthermore, setting the upper limit of the reaction temperature within the above range not only facilitates waste heat utilization but also further improves economic efficiency. The lower limit and upper limit can be combined in any way to define the reaction temperature range.

[0043] Furthermore, during the carbon dioxide reduction reaction, the amount of carbon dioxide brought into contact with the oxygen carrier is preferably 1 mmol to 50 mmol per 1 g of the oxygen carrier, more preferably 2.5 mmol to 30 mmol, and even more preferably 5 mmol to 20 mmol. The oxygen carrier allows smooth entry and exit of oxygen elements. Therefore, the oxygen carrier has a high carbon dioxide conversion rate (hence, a large amount of carbon monoxide produced), and from this perspective, also contributes to the reduction of carbon dioxide. On the other hand, since the reduction reaction is efficiently carried out by a reducing gas containing hydrogen, the oxygen carrier can be regenerated with a small amount of hydrogen. The amount of carbon monoxide produced in the oxygen carrier is preferably about 0.3 mmol to 1 mmol per 1 g of the oxygen carrier.

[0044] In this embodiment, the reduction product (carbon valuable product) obtained by the carbon dioxide reduction reaction contains carbon monoxide. However, it may also contain substances other than carbon monoxide, or may be a mixture of carbon monoxide and other substances. Specific examples of other substances include methane. Preferably, the reduction products, such as carbon monoxide, obtained by the carbon dioxide reduction reaction are further converted into organic substances by microbial fermentation or the like. Examples of microbial fermentation include anaerobic fermentation. Examples of the resulting organic substances include methanol, ethanol, acetic acid, butanol, derivatives thereof, mixtures thereof, and C5 or higher compounds such as isoprene. Furthermore, reduction products such as carbon monoxide may be converted into C1 to C20 compounds, including hydrocarbons and alcohols conventionally synthesized by petrochemical methods, using metal oxides or the like. Specific examples of the resulting compounds include methane, ethane, propylene, methanol, ethanol, propanol, acetaldehyde, diethyl ether, acetic acid, butyric acid, diethyl carbonate, and butadiene.

[0045] [Characteristics of the Oxygen Carrier] The oxygen carrier preferably has the following characteristics. That is, when a stainless steel reaction tube having an inner diameter of 8 mm and a pressure gauge disposed in the flow path is filled with the oxygen carrier to a height of 40 cm and nitrogen gas having a concentration of 100% by volume is passed through at a rate of 30 mL / min, the pressure increase over 10 minutes is preferably 0.03 MPaG or less, and more preferably 0.01 MPaG or less. An oxygen carrier exhibiting such characteristics can be determined to have a loose bulk density and pore volume that satisfy the above ranges, and can sufficiently increase the conversion efficiency of carbon dioxide to carbon monoxide and the conversion efficiency of hydrogen to water.

[0046] According to the above-described embodiment, it is possible to provide an oxygen carrier that can be stably used for a long period of time in high-temperature reactions, a method for producing the same, and a method for producing a gas using the oxygen carrier. The oxygen carrier of this embodiment can maintain its microstructure even after high temperatures (it can withstand high-temperature use), and therefore, enables more efficient conversion of carbon dioxide to carbon monoxide (carbon valuables) through a chemical looping reaction at high temperatures. Furthermore, the oxygen carrier may be provided in the following aspects:

[0047] (1) An oxygen carrier that generates oxygen vacancies upon contact with a reducing gas and eliminates the oxygen vacancies upon contact with an oxidizing gas, the oxygen carrier having oxygen ion conductivity and containing cerium (Ce), and having a cumulative pore volume of 0.4 cm or less as measured by pore size distribution measurement using a mercury intrusion method with a mercury porosimeter. ３ / g or more, and the median pore diameter is 2.5 μm or less, and the BET specific surface area is 10 m ２ / g or more.

[0048] (2) In the oxygen carrier described in (1) above, the oxygen carrier has a first section pore volume of 0.12 cm in a section with a pore diameter of 100 nm or less. ３ / g or more, and the second section pore volume in the section with a pore diameter of more than 100 nm is 0.35 cm ３ / g or more.

[0049] (3) The oxygen carrier according to (1) or (2) above, wherein when X-ray diffraction measurement is performed on the oxygen carrier, the full width at half maximum of the peak having the maximum intensity among the peaks corresponding to the (111) plane in the X-ray diffraction profile is 0.3° or more in 2θ.

[0050] (4) In the oxygen carrier described in (1) to (3) above, when X-ray diffraction measurement is performed on the oxygen carrier, in the X-ray diffraction profile, the ratio of the sum of the peak intensities of the peaks corresponding to the (200) plane among the peaks having their peak tops in the 2θ range of 30° to 35° to the sum of the peak intensities of the peaks corresponding to the (111) plane among the peaks having their peak tops in the 2θ range of 25° to 30° is 27% or less.

[0051] (5) The oxygen carrier according to any one of (1) to (4) above, wherein when X-ray diffraction measurement is performed on the oxygen carrier, only one peak is observed in the X-ray diffraction profile in the range of 2θ of 47° to 51°.

[0052] (6) The oxygen carrier according to any one of (1) to (5) above, further comprising at least one metal element other than cerium, belonging to Groups 3 and 4 of the periodic table, including lanthanides and actinides.

[0053] (7) The oxygen carrier according to (6) above, wherein the at least one metal element other than cerium is selected from the group consisting of titanium (Ti), zirconium (Zr), and lanthanum (La).

[0054] (8) The oxygen carrier according to (7) above, which contains zirconium (Zr), and when X-ray diffraction measurement is performed on the oxygen carrier, no peaks are observed in the X-ray diffraction profile in the 2θ range of 31.0° to 32.0° and in the 2θ range of 49.0° to 50.8°.

[0055] (9) The oxygen carrier according to any one of (1) to (8) above, further comprising iron (Fe).

[0056] (10) The oxygen carrier according to any one of (1) to (9) above, having a fiber assembly structure containing fibrous primary particles, the short sides of the primary particles being 100 nm or less, and the aspect ratio, which is the ratio of the long sides to the short sides, being 5 or more.

[0057] (11) An oxygen carrier according to (10) above, wherein when multiple 50 μm square regions are observed in the SEM pixel (magnification: 50,000 times) of the oxygen carrier, the number of spherical primary particles with an aspect ratio of 4 or less present in each of the regions is less than 20.

[0058] (12) The oxygen carrier according to (11) above, wherein the plurality of regions includes at least one region in which the number of the spherical primary particles is less than 10.

[0059] (13) The oxygen carrier according to any one of (1) to (12) above, wherein the contact with the reducing gas is carried out at a temperature higher than 650°C.

[0060] (14) The oxygen carrier according to any one of (1) to (13) above, wherein the contact with the oxidizing gas is carried out at a temperature exceeding 650°C.

[0061] (15) In the oxygen carrier according to any one of (1) to (14), the loose bulk density of the oxygen carrier is 1.2 g / cm ３ 3g / cm or more ３ The following are oxygen carriers:

[0062] (16) A method for producing an oxygen carrier according to any one of (1) to (15) above, comprising the steps of: generating a compound containing the element and oxygen (O) by a precipitation method from an aqueous solution containing ions of the element constituting the oxygen carrier; and calcining the compound to obtain the oxygen carrier, wherein a long-chain compound is added to the aqueous solution as an additive.

[0063] (17) The method for producing an oxygen carrier according to (16) above, wherein the long-chain compound includes a long-chain carboxylic acid.

[0064] (18) The method for producing an oxygen carrier according to (16) or (17) above, wherein the compound is calcined in an air atmosphere.

[0065] (19) The method for producing an oxygen carrier according to any one of (16) to (18) above, wherein the atmospheric temperature for firing the compound is 750° C. or higher.

[0066] (20) The method for producing an oxygen carrier according to any one of (16) to (19) above, wherein the compound is calcined for 1 hour or more.

[0067] (21) A gas production method using the oxygen carrier according to any one of (1) to (15) above, wherein a raw material gas containing carbon dioxide is brought into contact with the oxygen carrier in which oxygen deficiencies have occurred, thereby reducing the carbon dioxide to produce a product gas containing carbon valuables. Of course, the method is not limited to this.

[0068] As described above, various embodiments have been described, but these are presented as examples and do not limit the scope of the invention in any way. The novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. Such embodiments and their modifications are intended to be included within the scope and spirit of the invention, as well as within the scope of the inventions and their equivalents as set forth in the claims.

[0069] For example, the oxygen carrier, the method for producing the oxygen carrier, and the method for producing the gas may have any additional configuration compared to the above embodiments, may be replaced with any configuration that exhibits the same function, or some configurations may be omitted. In the above embodiments, a gas containing hydrogen has been described as a representative reducing gas, but the reducing gas may also use a gas containing at least one selected from hydrocarbons (e.g., methane, ethane, acetylene, etc.) and ammonia as a reducing substance instead of or in addition to hydrogen.

[0070] The present invention will be described in more detail below with reference to examples and comparative examples, but is not limited to these examples. 1. Preparation of oxygen carrier precursors The following compounds were prepared as oxygen carrier precursors: Cerium (III) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 98.0%) Zirconium nitrate dihydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 97.0%) Lanthanum (III) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) Iron (III) nitrate nonahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%)

[0071] 2. Production of Oxygen Carrier (Example 1) First, a predetermined amount of 5.05 g of cerium (III) nitrate hexahydrate was weighed and dissolved in 18 mL of deionized water to prepare a precursor aqueous solution. Separately, 25% aqueous ammonia was added to 12 mL of deionized water to obtain an alkaline aqueous solution with a pH of 10. This alkaline aqueous solution was heated in a water bath set to 60°C or higher but 70°C or lower, after which 0.5 g of lauric acid was added and heated and stirred until the lauric acid was dissolved. This resulted in the preparation of a lauric acid-mixed alkaline aqueous solution. It was confirmed that the temperature of the lauric acid-mixed alkaline aqueous solution was 50°C or higher.

[0072] Next, the precursor aqueous solution was added dropwise to the lauric acid mixed alkali aqueous solution. At this time, an appropriate amount of 25% ammonia aqueous solution was also added dropwise at the same time to maintain the pH at 10. After the precursor aqueous solution was added dropwise, the mixture was heated and stirred (aging) for 6 hours while maintaining the temperature of the water bath at 60°C or higher and 70°C or lower. This resulted in the precipitation of a compound containing cerium and oxygen. The compound was then recovered by filtration and washed with distilled water. The recovered compound was transferred to a drying furnace and heated and dried. The compound was dried at 120°C for 5 hours. The compound was then heated from room temperature to 950°C at a rate of 10°C / min in an air atmosphere, and then calcined at 950°C for 8 hours.

[0073] Finally, the calcined compound was mechanically pulverized to obtain the desired oxygen carrier. The oxygen carrier was granular. As shown in the scanning electron microscope (SEM) images of Figures 1 and 2, this oxygen carrier had a fibrous aggregate structure. Figures 2(b) and 2(c) are SEM images of two different locations on the oxygen carrier of Example 1, taken at a higher magnification than the SEM image of Figure 2(a). The aggregated state of the fibrous particles can be more clearly observed in Figures 2(b) and 2(c).

[0074] (Example 2) An oxygen carrier was produced in the same manner as in Example 1, except that predetermined amounts of cerium (III) nitrate hexahydrate and zirconium nitrate dihydrate were weighed and used. The oxygen carrier was granular. (Example 3) An oxygen carrier was produced in the same manner as in Example 1, except that predetermined amounts of cerium (III) nitrate hexahydrate, zirconium nitrate dihydrate, lanthanum (III) nitrate hexahydrate, and iron (III) nitrate nonahydrate were weighed and used. The oxygen carrier was granular. This oxygen carrier had a fibrous aggregate structure, as shown in the scanning transmission electron microscope (STEM) image of FIG. 3.

[0075] Comparative Example 1 An oxygen carrier was produced in the same manner as in Example 1, except that the addition of lauric acid was omitted. The oxygen carrier was granular. This oxygen carrier did not have a fibrous aggregate structure as shown in the SEM or STEM images of FIGS. 1 to 3. Comparative Example 2 Commercially available cerium oxide particles (manufactured by Aldrich, "Cerium (IV) oxide: 50 nm particle size") were heat-treated at 950°C for 8 hours in an air atmosphere to produce an oxygen carrier. The oxygen carrier was granular. This oxygen carrier did not have a fibrous aggregate structure as shown in the SEM or STEM images of FIGS. 1 to 3.

[0076] (Comparative Example 3) An oxygen carrier was produced in the same manner as in Example 2, except that the addition of lauric acid was omitted. The oxygen carrier was granular. This oxygen carrier did not have a fibrous aggregate structure as shown in the SEM or STEM images of FIGS. 1 to 3. (Comparative Example 4) An oxygen carrier was produced in the same manner as in Comparative Example 3, except that the firing conditions were changed to heating from room temperature to 650°C at a rate of 10°C / min in an air atmosphere, and then maintaining the temperature at 650°C for 6 hours. This oxygen carrier did not have a fibrous aggregate structure as shown in the SEM or STEM images of FIGS. 1 to 3, and fibrous particles and spherical particles were mixed.

[0077] 2. Measurement and Evaluation 2-1. Measurement of Pore Volume and Median Pore Diameter The pore volume and median pore diameter were measured by measuring the pore size distribution using a mercury intrusion method with a mercury porosimeter of the oxygen carrier. The median pore diameter was defined as the pore diameter when the amount of mercury invaded the oxygen carrier was half (50%). Specifically, measurements were performed using a Poremaster manufactured by Quantachrome Corporation, using a sample (powder) of approximately 0.1 g to 1.5 g. The measurement range was a pore diameter of 0.0036 μm to 400 μm, a surface tension of mercury of 480 dyn / cm, a contact angle of 140°, a cell size of 10 mmφ × 3 cm, and a measurement cell volume of 0.5 cc. The pore volume measured in the pore diameter range of 0.0036 μm or more and 400 μm or less was defined as the "cumulative pore volume (total pore volume)", the pore volume measured in the pore diameter range of 0.0036 μm (3.6 nm) or more and 100 nm or less was defined as the "first interval pore volume", and the pore volume measured in the pore diameter range of more than 100 nm and 400 μm or less was defined as the "second interval pore volume".

[0078] 2-2. Measurement of BET Specific Surface Area The BET specific surface area was measured by nitrogen gas adsorption and determined based on the BET multipoint method. Specifically, the BET specific surface area was measured using a specific surface area / pore size analyzer (MicrotracBEL Corp., "BelSorp mini II"). First, approximately 0.2 g to 1.0 g of sample (powder) was placed in the measurement cell and vacuum degassed at 100°C for 3 hours. Then, fully automated gas adsorption measurement was performed using nitrogen gas by the constant volume method. The measurement temperature was 77.35 K (-195.8°C). Measurement items were several points on the adsorption side where P / P0 = 0 to 0.3. Note that P is the pressure inside the measurement cell, and P0 is the saturated vapor pressure. The analysis item was specific surface area by the BET multipoint method (several arbitrary points were used during analysis). The above measurement was repeated twice, and the average value of the measurement results was taken as the BET specific surface area.

[0079] 2-3. X-ray Diffraction Measurement Before X-ray diffraction measurement, an oxygen carrier sample was prepared. First, approximately 100 mg of oxygen carrier was weighed into a mortar and crushed using a pestle. The oxygen carrier was then uniformly filled into the hole in the sample filling section of the sample plate, and the surface of the sample plate and the surface of the oxygen carrier were adjusted to be flush with each other. X-ray diffraction measurements were performed using an X-ray diffractometer (PANanalytical, "AREIS") using the focusing method. A ceramic tube was used as the anticathode, and CuKα characteristic X-rays (Kα1 wavelength (λ) = 1.54056 Å (0.154056 nm), Kα2 wavelength (λ) = 1.54439 Å (0.154439 nm), Kα2 ratio = 0.50000) were used for diffraction. The diffractometer was set to a divergence slit of 1 / 2°, a divergence longitudinal limiting slit of 10 mm, a scattering slit of 2°, and a receiving slit of 0.15 mm. The goniometer radius was 145 mm.

[0080] Thereafter, the prepared oxygen carrier sample was irradiated with X-rays under conditions of a tube voltage of 40 kV and a tube current of 15 mA. The measurement was performed by setting the scan angle of the goniometer to a range of 10° to 90.5°, a scan speed of 3.5° / min, and a measurement step of 0.01. The measurement was performed in air at room temperature. After the measurement was completed, the data obtained without separation of Kα1 and Kα2 was analyzed. Software (High Score, manufactured by PANanalytica) was used to analyze the data. In the X-ray diffraction profile, the full width at half maximum of the peak with the maximum intensity among the peaks corresponding to the (111) plane was determined. Specifically, the value with the highest peak intensity was taken as the apex, and the distance between two points at half the intensity of the maximum peak value at this time was taken as the full width at half maximum. In addition, the ratio (%) of the sum of the peak intensities of all peaks corresponding to the (200) plane among peaks having their peak tops in the 2θ range of 30° to 35° inclusive to the sum of the peak intensities of all peaks corresponding to the (111) plane among peaks having their peak tops in the 2θ range of 25° to 30° inclusive in the X-ray diffraction profile was also calculated.

[0081] Furthermore, it was also confirmed whether peaks were observed in the 2θ ranges of 31.0° to 32.0°, 48.0° to 51.0°, and 49.0° to 50.8°. Figure 5 shows X-ray diffraction profiles (predetermined angle range) obtained by X-ray diffraction measurement for the oxygen carriers obtained in Examples 1 to 3. Figure 6 shows X-ray diffraction profiles (predetermined angle range) obtained by X-ray diffraction measurement for the oxygen carriers obtained in Examples 1 to 3 and Comparative Example 4.

[0082] 2-4. Particle size distribution measurement (wet type) Particle size distribution measurement was performed using a laser diffraction / scattering particle size distribution analyzer (Horiba, Ltd., "Partica LA-960") and a wet flow cell. The flow cell was made of synthetic quartz, and the light source was a semiconductor laser (650 nm) 5 mW and an LED (405 nm) 3 mW, and the detector was a ring-shaped silicon photodiode. The measurement was performed at a temperature of 25°C and a humidity of 20% on a dispersion prepared by dispersing approximately 10 mg of oxygen carrier in approximately 180 mL of purified water (pH 6.8 to 7.2). The measurement was performed at least three times, and the median particle sizes obtained by each volume-based particle size distribution measurement were calculated as the arithmetic mean to obtain the average particle size.

[0083] 2-5. Measurement of Primary Particle Size The size of the primary particles can be calculated by statistically processing images of the oxygen carrier taken with a scanning transmission electron microscope (STEM (HD-2700B Hitachi High-Tech Corporation)). Specifically, first, the oxygen carrier was dispersed in ethanol, applied to a grid, dried, and observed with a scanning transmission electron microscope (STEM) at an acceleration voltage of 200 kV. Three scanning transmission electron microscope (STEM) images (observed images) in which 20 or more primary particles contained in the oxygen carrier were confirmed were arbitrarily selected. Then, 10 primary particles were randomly selected from each STEM image, and the short side (nm) and long side (nm) of the 10 primary particles were measured. The average aspect ratio of the primary particles was calculated from the average value of all the data obtained.

[0084] 2-6. Measurement of loose bulk density First, the oxygen carrier was dried in a dryer at 120°C for 12 hours, then placed in a desiccator and cooled to room temperature. After cooling, the oxygen carrier was passed through a SUS test mesh sieve (mesh size: 100 μm), and the passed oxygen carrier was used as the sample for measurement. Next, the mass of a cylindrical measurement container with an inner diameter of 30 mm and a height of 65 mm was measured in advance. The sample was poured into this measurement container until it overflowed. Note that care was taken not to vibrate the measurement container or compress the sample. Next, the sample protruding from the top surface of the measurement container was leveled off using a leveling plate. The leveling plate was tilted backward from the leveling direction to avoid compressing the sample. The mass of the measurement container was then measured on a balance, and the mass of the measurement container was subtracted to calculate the mass of the sample.

[0085] 2-7. Number of regions with zero spherical particles Using a scanning electron microscope (SEM)-energy dispersive X-ray spectroscopy (EDX) device (Hitachi High-Technologies Corporation, "S-4800"), oxygen carriers were imaged at an acceleration voltage of 1.0 kV to obtain SEM images and element mapping images at a magnification of 50,000 times. Three 50 μm square regions spaced 100 μm or more from each other were observed. Primary particles that showed a clear difference in chemical composition (shading) from fibrous particles and had an aspect ratio of 4 or less were determined to be spherical particles. Regarding whether the shape of the oxygen carrier was spherical or not, only those whose interfaces were clearly visible were counted. Therefore, those whose entire shape was hidden behind another oxygen carrier and could not be seen, or those whose imaged region was interrupted, were not counted because the aspect ratio could not be obtained.

[0086] 2-8. Measurement of carbon monoxide production amount Using a rapid catalyst evaluation system equipped with a fixed-bed flow reactor and a quadrupole mass spectrometer directly connected to the reactor, the carbon monoxide production amount by the oxygen carrier was measured according to the following procedure. Specifically, a quartz reaction tube with an inner diameter of 6 mm was prepared and filled with a granular oxygen carrier so that the filling amount was 500.0 mg.

[0087] Next, while flowing helium gas at a flow rate of 10 mL / min, the temperature was increased to 850 °C at a heating rate of 15 °C / min, and the temperature was stabilized for 5 minutes. Next, to activate the oxygen carrier, hydrogen gas (reducing gas) was flowed at a flow rate of 10 mL / min for 10 minutes to carry out a reduction reaction of the oxygen carrier (first process), thereby reducing the oxygen carrier. At this time, the gas discharged from the outlet contained water vapor. Thereafter, for gas exchange, helium gas was flowed at a flow rate of 10 mL / min for 5 minutes, and then carbon dioxide gas was flowed at a flow rate of 10 mL / min for 10 minutes to carry out a reduction reaction of carbon dioxide (second process), thereby reducing the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the outlet contained carbon monoxide.

[0088] Next, for this test, the following process was performed. First, for gas exchange, helium gas was flowed at a flow rate of 10 mL / min for 5 minutes. Next, hydrogen gas (reducing gas) was flowed at a flow rate of 10 mL / min for 4 minutes to perform a reduction reaction of the oxygen carrier (first process), thereby reducing the oxygen carrier. At this time, the gas discharged from the outlet contained water vapor. Thereafter, for gas exchange, helium gas was flowed at a flow rate of 10 mL / min for 5 minutes, and then carbon dioxide gas was flowed at a flow rate of 10 mL / min for 5 minutes to perform a reduction reaction of carbon dioxide (second process), thereby reducing the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the outlet contained carbon monoxide. The above operations are defined as one cycle.

[0089] In the above process, the temperature of the oxygen carrier was maintained at 850°C and the process was carried out under atmospheric pressure conditions when any gas was flowing. As the initial performance, the carbon dioxide conversion rate (CO ２ Carbon dioxide conversion rate (%) = (detected amount of carbon monoxide per unit second) / (detected amount of carbon monoxide per unit second + detected amount of carbon dioxide per unit second) × 100 Specifically, after the start of flowing carbon dioxide gas (feed gas), during the period in which the carbon dioxide conversion rate was maintained at 80% or higher, an average value of the carbon dioxide conversion rates obtained in unit-second intervals was calculated, and this average value was used as the carbon dioxide conversion rate in the first cycle.

[0090] To evaluate the lifespan, the ratio of the amount of carbon monoxide produced at the 50th cycle to that at the 1st cycle was calculated. Specifically, for each cycle, the amount of carbon monoxide produced per unit mass of oxygen carrier (mmol / g) was calculated during the period in which the carbon dioxide conversion rate remained at 5% or higher after the start of carbon dioxide gas (feed gas) flow. The amount of carbon monoxide produced at the 50th cycle was divided by the amount of carbon monoxide produced at the 1st cycle, and the resulting value was expressed as a percentage. These results are shown in Tables 1 and 2 below.

[0091]

[0092]

[0093] In Examples 1 to 3, no spherical particles were observed in any of the three regions. Furthermore, as shown in Figures 5 and 6, in each of the X-ray diffraction profiles obtained in Examples 1 to 3, two peaks were observed in the measurement range of 2θ from 25° to 35°, while only one peak was observed in the measurement range of 2θ from 47.0° to 51.0°. Furthermore, in Examples 1 to 3, the full widths at half maximum of the peaks observed in the measurement range of 2θ from 47.0° to 51.0° were 0.346°, 0.461°, and 0.709°, respectively.

[0094] In addition, the cumulative pore volumes of Comparative Examples 1 to 3 are not in the preferred range. ２ The conversion rates are the same as those of Examples 1 to 3. ２ The conversion rate was lower than that of the fibrous particles. In Comparative Examples 1 to 3, the constituent components observed by SEM-EDX were the same as or similar to those of the fibrous particles, and no difference in chemical composition from the fibrous particles was confirmed. Furthermore, in the SEM images, no fibrous particles were observed, and many spherical particles were observed. Due to this external shape, the cumulative pore volume did not fall within the preferred range. Accordingly, the CO ２ The conversion rate is the CO ２In Comparative Examples 1 to 3, the full widths at half maximum of the peaks observed in the measurement range of 2θ from 47.0 to 51.0° were 0.307°, 0.346°, and 0.346°, respectively.

[0095] Furthermore, as shown in FIG. 6 , in Comparative Example 4, two overlapping peaks were observed in the measurement range of 2θ from 47.0° to 51.0°: one near 47° and the other near 49°. If these two peaks with maximum values ​​were considered as a single peak, the full width at half maximum of that peak was 1.802°. Therefore, the full width at half maximum of Comparative Example 4 was also larger than the full width at half maximum of each example. This is thought to be due to the non-uniform distribution of each element. Furthermore, the non-uniform distribution of each element reduces the lifespan evaluated by the amount of CO generated.

[0096] Furthermore, the oxygen carrier obtained in Example 3 maintained its fibrous aggregate structure even after being exposed to high temperatures in three cycles of this test, as shown in the STEM image in FIG.

Claims

1. An oxygen carrier that generates oxygen vacancies when it comes into contact with a reducing gas and that eliminates the oxygen vacancies when it comes into contact with an oxidizing gas, the oxygen carrier has oxygen ion conductivity and contains cerium (Ce), and the cumulative pore volume of the oxygen carrier measured by a pore size distribution measurement using a mercury intrusion method with a mercury porosimeter is 0.4 cm ３ / g or more, the median pore diameter is 2.5 μm or less, and the BET specific surface area is 10 m ２ / g or more.

2. The oxygen carrier according to claim 1, wherein the first section pore volume in the section having a pore diameter of 100 nm or less is 0.12 cm ３ / g or more, and the second section pore volume in the section with a pore diameter of more than 100 nm is 0.35 cm ３ / g or more.

3. The oxygen carrier according to claim 1 or 2, wherein when X-ray diffraction measurement is performed on the oxygen carrier, the full width at half maximum of the peak having the maximum intensity among the peaks corresponding to the (111) plane in the X-ray diffraction profile is 0.3° or more in 2θ.

4. An oxygen carrier according to any one of claims 1 to 3, in which, when X-ray diffraction measurement is performed on the oxygen carrier, in an X-ray diffraction profile, the ratio of the sum of the peak intensities of peaks having their peak tops in the 2θ range of 30° to 35° and corresponding to the (200) plane to the sum of the peak intensities of peaks having their peak tops in the 2θ range of 25° to 30° and corresponding to the (111) plane is 27% or less.

5. The oxygen carrier according to any one of claims 1 to 4, wherein when X-ray diffraction measurement is performed on the oxygen carrier, only one peak is observed in the X-ray diffraction profile in the range of 2θ from 47° to 51°.

6. The oxygen carrier according to any one of claims 1 to 5, further comprising at least one metal element other than cerium, which belongs to Groups 3 and 4 of the periodic table, including lanthanides and actinides.

7. The oxygen carrier according to claim 6, wherein the at least one metal element other than cerium is selected from the group consisting of titanium (Ti), zirconium (Zr) and lanthanum (La).

8. An oxygen carrier according to claim 7, which contains zirconium (Zr), and when X-ray diffraction measurement is performed on the oxygen carrier, no peaks are observed in the X-ray diffraction profile in the 2θ range of 31.0° to 32.0° and in the 2θ range of 49.0° to 50.8°.

9. The oxygen carrier according to any one of claims 1 to 8, further comprising iron (Fe).

10. An oxygen carrier according to any one of claims 1 to 9, having a fiber assembly structure containing fibrous primary particles, the short sides of the primary particles being 100 nm or less, and the aspect ratio, which is the ratio of the long side to the short side, being 5 or more.

11. An oxygen carrier as described in claim 10, wherein, when multiple regions of 50 μm square are observed in an SEM pixel (magnification: 50,000 times) of the oxygen carrier, the number of spherical primary particles having an aspect ratio of 4 or less present in each of the regions is less than 20.

12. The oxygen carrier according to claim 11, wherein said plurality of regions includes at least one region in which the number of said spherical primary particles is less than 10.

13. An oxygen carrier according to any one of claims 1 to 12, wherein the contact with the reducing gas is carried out at a temperature above 650°C.

14. An oxygen carrier according to any one of claims 1 to 13, wherein the contact with the oxidizing gas is carried out at a temperature above 650°C.

15. The oxygen carrier according to any one of claims 1 to 14, wherein the loose bulk density of the oxygen carrier is 1.2 g / cm ３ 3g / cm or more ３ The following are oxygen carriers:

16. A method for producing an oxygen carrier according to any one of claims 1 to 15, comprising the steps of: generating a compound containing an element constituting the oxygen carrier and oxygen (O) by a precipitation method from an aqueous solution containing ions of the element; and calcining the compound to obtain the oxygen carrier, wherein a long-chain compound is added as an additive to the aqueous solution.

17. The method for producing an oxygen carrier according to claim 16, wherein the long-chain compound comprises a long-chain carboxylic acid.

18. The method for producing an oxygen carrier according to claim 16 or 17, wherein the atmosphere in which the compound is calcined is an air atmosphere.

19. The method for producing an oxygen carrier according to any one of claims 16 to 18, wherein the atmospheric temperature for firing the compound is 750°C or higher.

20. The method for producing an oxygen carrier according to any one of claims 16 to 19, wherein the time for calcining the compound is 1 hour or longer.

21. A gas production method using the oxygen carrier according to any one of claims 1 to 15, comprising contacting a raw material gas containing carbon dioxide with the oxygen carrier in which oxygen deficiencies have occurred, thereby reducing the carbon dioxide to produce a product gas containing carbon valuables.

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