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WO2024203959A1 - Reducing agent, and method for producing gas - Google Patents

Reducing agent, and method for producing gas Download PDF

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Publication number
WO2024203959A1
WO2024203959A1 PCT/JP2024/011495 JP2024011495W WO2024203959A1 WO 2024203959 A1 WO2024203959 A1 WO 2024203959A1 JP 2024011495 W JP2024011495 W JP 2024011495W WO 2024203959 A1 WO2024203959 A1 WO 2024203959A1
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Prior art keywords
reducing agent
carbon dioxide
oxygen carrier
gas
ray diffraction
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PCT/JP2024/011495
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French (fr)
Japanese (ja)
Inventor
健一 仲尾
昂嗣 滝沢
理沙 秦
錦良 張
照央 ▲高▼橋
里絵 大西
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積水化学工業株式会社
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Publication of WO2024203959A1 publication Critical patent/WO2024203959A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/10—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of rare earths
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
    • B01J23/76—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
    • B01J23/83—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36 with rare earths or actinides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/20—Carbon compounds
    • B01J27/22—Carbides
    • B01J27/224—Silicon carbide
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/40—Carbon monoxide
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G25/00—Compounds of zirconium
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G49/00—Compounds of iron

Definitions

  • the present invention relates to a reducing agent and a method for producing a gas, and more specifically, to a reducing agent that can be used, for example, in a chemical looping method, and a method for producing a gas using such a reducing agent.
  • the chemical looping method refers to 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, and these reactions are bridged by an oxygen carrier (e.g., metal oxide: MO x ) (see the following formula).
  • MO x-1 represents a state in which the metal oxide is partially or completely reduced.
  • the present invention provides a reducing agent that can be stably used for a long period of time in a reaction at high temperature, and a method for producing a gas using such a reducing agent.
  • a reducing agent for reducing carbon dioxide to produce carbon values contains an oxygen carrier having oxygen ion conductivity, which contains at least two metal elements belonging to Group 3 or Group 4 of the periodic table, including lanthanides and actinides.
  • the oxygen carrier has an electrical conductivity of 1 ⁇ 10 ⁇ 7 S/cm or more in a pure oxygen atmosphere at a temperature of 700° C., and when X-ray diffraction measurement is performed on the reducing agent in a measurement range of 10° to 90.5°, the full width at half maximum of all peaks observed in the X-ray diffraction profile is less than 1.5°.
  • a reducing agent that can be stably used for a long period of time in reactions at high temperatures can be obtained.
  • FIG. 1(a) is an example of an image obtained by STEM-EDS measurement of a reducing agent in which an oxygen carrier and a sub-compound exist independently
  • FIG. 1(b) is a graph showing a line profile of the mapping of each element on the white line in FIG. 1(a).
  • FIG. 2(a) is an example of an image obtained by STEM-EDS measurement of a reducing agent in which an oxygen carrier and a sub-compound are dissolved
  • FIG. 2(b) is a graph showing a line profile of the mapping of each element on the white line in FIG. 2(a).
  • FIG. 3 is a graph showing an example of the change in electrical conductivity due to the change in oxygen partial pressure.
  • FIG. 4 is an X-ray diffraction profile of the reducing agent obtained in Example 2.
  • FIG. 5 is an X-ray diffraction profile of the reducing agent obtained in Example 4.
  • FIG. 6 is an X-ray diffraction profile of the reducing agent obtained in Example 5.
  • FIG. 7 is an X-ray diffraction profile of the reducing agent obtained in Example 6.
  • FIG. 8 is an X-ray diffraction profile of the reducing agent obtained in Comparative Example 2.
  • the reducing agent of the present embodiment is used when producing a product gas containing carbon monoxide (carbon valuable) by reducing carbon dioxide by contacting a raw material gas containing carbon dioxide with the reducing agent (by making the reducing agent act on the raw material gas) (i.e., it is used in the gas production method of the present embodiment). Furthermore, the reducing agent oxidized by contact with carbon dioxide can be reduced (regenerated) by contacting with a reducing gas containing hydrogen (reducing material).
  • the raw material gas and the reducing gas are alternately passed through a reaction tube (reaction vessel) filled with the reducing agent of this embodiment, whereby carbon dioxide is converted to carbon monoxide by the reducing agent, and the reducing agent in an oxidized state is regenerated by the reducing gas.
  • the reducing agent of the present embodiment contains an oxygen carrier having oxygen ion conductivity.
  • the oxygen carrier is a compound that can cause reversible oxygen deficiency, and loses oxygen element from itself through reduction. When the compound comes into contact with carbon dioxide in an oxygen-deficient state (reduced state), it acts to remove the oxygen element from the carbon dioxide and reduce it.
  • the oxygen carrier in this embodiment contains at least two kinds of metal elements belonging to Group 3 or Group 4 of the periodic table, including lanthanides and actinides. In such an oxygen carrier, the crystal lattice is distorted by containing two or more kinds of metal elements, and oxygen element is smoothly transferred between the oxygen carrier and the other substance (e.g., carbon dioxide, hydrogen).
  • the conversion efficiency from carbon dioxide to carbon monoxide also referred to as carbon dioxide conversion rate
  • the conversion efficiency from hydrogen to water also referred to as hydrogen conversion rate
  • the carbon dioxide conversion rate and the hydrogenation rate can be determined by the methods described in the examples.
  • the oxygen carrier has an electrical conductivity of 1 ⁇ 10 ⁇ 7 S/cm or more in a pure oxygen atmosphere at a temperature of 700° C.
  • a reducing agent containing an oxygen carrier having such electrical conductivity can sufficiently increase the migration speed of oxygen ions in the oxygen carrier, so that oxygen elements are efficiently released by contact with a reducing gas, oxygen vacancies are smoothly induced, and the induced oxygen vacancies facilitate the deprivation of oxygen elements from carbon dioxide.
  • the electrical conductivity in a pure oxygen atmosphere at a temperature of 700° C. may be 1 ⁇ 10 ⁇ 1 S/cm or less, or may be 1 ⁇ 10 ⁇ 2 S/cm or less.
  • under a pure oxygen atmosphere refers to a condition in which the oxygen concentration is 99% by volume or more.
  • the full width at half maximum of all peaks observed in the X-ray diffraction profile is less than 1.5°.
  • the full width at half maximum of all peaks may be less than 1.5°, but is preferably 1.25° or less, and more preferably 1° or less.
  • the full width at half maximum of the peak is preferably 0.1° or more, and more preferably 0.5° or more.
  • the reducing agent to be measured by X-ray diffraction can be accurately measured if it has a mass of about 50 mg.
  • the timing for measuring the electrical conductivity and the X-ray diffraction was both before the reducing agent was used in the reaction with the gas.
  • the reducing agent of the present embodiment preferably further contains an element belonging to Groups 6 to 14 of the periodic table.
  • the element belonging to Groups 6 to 14 of the periodic table may be, for example, indium (In), tin (Sn), technetium (Tc), silver (Ag), or the like, or may be an element exemplified below.
  • the elements belonging to Groups 6 to 14 of the periodic table more preferably include elements belonging to Groups 8 to 12 of the periodic table, and more preferably include iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), gallium (Ga), and silicon (Si). These elements may be used alone or in combination of two or more.
  • the electrical conductivity of the oxygen carrier in a pure oxygen atmosphere at a temperature of 700° C.
  • ⁇ B/ ⁇ A is ⁇ A [S/cm] and the electrical conductivity in an argon gas atmosphere containing 1% by volume of carbon monoxide is ⁇ B [S/cm], it is preferable that ⁇ B/ ⁇ A is less than 85.
  • ⁇ B/ ⁇ A may be less than 85, but is preferably 80 or less, and more preferably 60 or less.
  • the ratio ⁇ B/ ⁇ A may be 10 or more, or 20 or more.
  • the reducing agent of the present embodiment preferably contains an element belonging to Groups 6 to 14 of the periodic table as a sub-compound containing the element.
  • the secondary compounds include oxides, nitrides, oxynitrides, carbides, etc.
  • the secondary compounds include NiO, Fe 2 O 3 , SiC, etc.
  • the oxygen carrier and the sub-compound are observed individually, as shown in FIG. 1, and it is preferable that the sub-compound is in the form of particles and in contact with the oxygen carrier.
  • This makes it possible to sufficiently increase the reactivity between the oxygen element and the hydrogen element in the crystal lattice of the oxygen carrier. As a result, it is possible to further improve the carbon dioxide conversion rate and the hydrogen conversion rate of the reducing agent.
  • the oxygen carrier and the sub-compound are in a solid solution, they cannot be observed individually, as shown in FIG.
  • the peaks derived from the oxygen carrier and the peaks derived from the sub-compounds observed in the X-ray diffraction profile are preferably shifted by less than 0.3° in 2 ⁇ with respect to the peaks observed in the X-ray diffraction profile when X-ray diffraction measurement is performed on the oxygen carrier alone and the sub-compound alone, respectively.
  • the amount of this shift is more preferably 0.25° or less, even more preferably 0.2° or less, and particularly preferably 0.15° or less.
  • the amount of shift may be 0.05° or more, or may be 0.1° or more.
  • the average particle size of the sub-compound particles is preferably 1 nm to 10 ⁇ m, more preferably 3 nm to 8 ⁇ m, and even more preferably 5 nm to 6 ⁇ m. This increases the area of the interface formed between the sub-compound particles and the oxygen carrier, allowing the hydrogen element activated by the sub-compound to move more smoothly through the oxygen carrier. This allows the reactivity between the oxygen element and the hydrogen element in the crystal lattice of the oxygen carrier to be sufficiently increased.
  • the average particle size of the particles can be determined by acquiring volume-based particle size distribution data using a laser diffraction/scattering type particle size distribution measuring device (e.g., "Partica LA-960" manufactured by Horiba, Ltd.) and processing the data. The measurement is usually performed in a wet manner.
  • the crystal structure of the oxygen carrier is stabilized and the heat resistance temperature is increased by sufficiently dissolving each metal element in the oxygen carrier.
  • the reducing agent of the present embodiment can be used even at high temperatures exceeding 650° C.
  • the metal elements constituting the oxygen carrier are alloyed and tend to form a stable crystal structure, so that the initial activity is likely to be maintained even if the oxidation-reduction of the reducing agent is repeated.
  • the packing density of the reducing agent is preferably 4 g/mL or less, more preferably 0.5 g/mL to 3 g/mL or less, and even more preferably 1 g/mL to 2.5 g/mL or less. If the packing density is too low, the gas passage speed becomes too fast, and the time during which the reducing agent is in contact with the raw material gas and the reducing gas decreases. As a result, the carbon dioxide conversion rate and hydrogen conversion rate by the reducing agent tend to decrease. On the other hand, if the packing density is too high, the gas passage speed becomes too slow, making it difficult for the reaction to proceed and requiring a long time to produce the product gas.
  • the pore volume of the reducing agent is preferably 0.1 cm 3 /g or more, more preferably 1 cm 3 /g or more and 30 cm 3 /g or less, and even more preferably 5 cm 3 /g or more and 20 cm 3 /g or less. If the pore volume is too small, it becomes difficult for the raw material gas and the reducing gas to penetrate into the inside of the reducing agent. As a result, the contact area between the reducing agent and the raw material gas and the reducing gas decreases, and the carbon dioxide conversion rate and the hydrogen conversion rate by the reducing agent tend to decrease. On the other hand, even if the pore volume is increased beyond the upper limit, no further increase in effect can be expected, and depending on the type of reducing agent, the mechanical strength tends to decrease.
  • the shape of the reducing agent is not particularly limited, but is preferably, for example, granular, since the packing density of the reducing agent can be easily adjusted to the above range if the reducing agent is granular.
  • granular is a concept including powder, particle, lump, pellet, etc., and the form may be any of spherical, plate-like, polygonal, crushed, columnar, needle-like, and scale-like.
  • the average particle size of the reducing agent is preferably 1 ⁇ m or more and 5 mm or less, more preferably 10 ⁇ m or more and 1 mm or less, and even more preferably 20 ⁇ m or more and 0.5 mm or less. If the reducing agent has such an average particle size, it is easy to adjust the packing density to the above range.
  • the average particle size means the average particle size of 200 arbitrary reducing agents in one visual field observed by an electron microscope.
  • the "particle size” means the maximum length of the distance between two points on the contour line of the reducing agent.
  • the maximum length of the distance between two points on the contour line of the end face is taken as the "particle size”.
  • the average particle size means the average particle size of the secondary particles.
  • the BET specific surface area of the reducing agent is preferably 1 m2 /g or more and 500 m2 /g or less, more preferably 3 m2 /g or more and 450 m2 /g or less, and even more preferably 5 m2 /g or more and 400 m2 /g or less.
  • the BET specific surface area can be determined by a low-temperature gas adsorption method using a dynamic constant pressure method in accordance with the BET method (preferably the BET multipoint method).
  • the oxygen capacity of the reducing agent can be maintained at a high level over a wide range from low temperature (about 400° C.) to high temperature (about 850° C.)
  • the reducing agent of this embodiment can efficiently convert carbon dioxide to carbon monoxide over a wide temperature range, and can be efficiently reduced by a reducing gas containing hydrogen.
  • the oxygen capacity of the oxygen carrier contained in the reducing agent at 400° C. is preferably 0.1% by mass or more and 40% by mass or less, and more preferably 0.5% by mass or more and 30% by mass or less.
  • the oxygen capacity of the oxygen carrier contained in the reducing agent at low temperatures is within the above range, it means that the oxygen capacity is sufficiently high even at temperatures during actual operation (temperatures exceeding 650° C.), and it can be said that the reducing agent has extremely high carbon dioxide conversion rates and hydrogen conversion rates.
  • the method for producing the reducing agent is not particularly limited, but examples thereof include the sol-gel method, coprecipitation method, solid phase method, and hydrothermal synthesis method.
  • the reducing agent can be produced as follows. First, a salt of a metal element constituting the reducing agent is dissolved in water to prepare an aqueous solution. Next, this aqueous solution is gelled, and then dried and baked. That is, the reducing agent of this embodiment can be produced easily and reliably by the so-called sol-gel method.
  • the aqueous solution may be prepared by using, for example, acidic water adjusted to be acidic with citric acid, acetic acid, malic acid, tartaric acid, hydrochloric acid, nitric acid, or a mixture thereof.
  • Examples of the salt of a metal element include nitrates, sulfates, chlorides, hydroxides, carbonates, and compounds thereof, among which nitrates are preferred. Furthermore, hydrates of the salt of a metal element may be used as needed.
  • the gel is dried at a temperature of preferably 20° C. to 200° C., more preferably 50° C. to 150° C., for a time of preferably 0.5 hours to 20 hours, more preferably 1 hour to 15 hours. By drying in this manner, the gel can be dried uniformly.
  • the gel is preferably fired at a temperature of 300° C. to 1200° C., more preferably 700° C. to 1000° C., for a time of 1 hour to 24 hours, more preferably 1.5 hours to 20 hours.
  • the gel is preferably converted into an oxide by firing, but can be easily converted into a reducing agent by firing under the above firing conditions. In addition, firing under the above firing conditions can prevent excessive particle growth of the reducing agent. Until the above-mentioned firing temperature is reached, the temperature is increased at a rate of 1° C./min to 20° C./min, preferably 2° C./min to 10° C./min, which can promote the growth of the reducing agent particles and prevent the cracking of the crystals (particles).
  • the reducing agent can also be produced as follows. First, oxides containing the respective metal elements constituting the reducing agent are mixed and pulverized. For pulverization, for example, a ball mill, a bead mill, a jet mill, a hammer mill, a rod mill, etc. can be used. In addition, this pulverization may be performed by either a dry method or a wet method. Next, the crushed aggregate is crushed and then calcined at a temperature of preferably 300° C. to 1200° C., more preferably 700° C. to 1000° C., for a time of preferably 1 hour to 24 hours, more preferably 1.5 hours to 20 hours.
  • the reducing agent containing the secondary compound may be produced by adding the secondary compound to the aqueous solution or by mixing the secondary compound with the oxide, with the former being preferred.
  • the reducing agent of the present embodiment can be used in, for example, a chemical looping method as described above.
  • the reducing agent of the present embodiment can be used in an application for reducing carbon dioxide through contact to generate carbon monoxide (carbon value) as described above. More specifically, it is preferable to carry out a reduction reaction of carbon dioxide and a reduction reaction of a reducing agent, and it is preferable to use the reducing agent so as to circulate between the reduction reaction of carbon dioxide and the reduction reaction of the reducing agent. Note that in the reduction reaction of the reducing agent, a reducing gas containing other reducing substances is used.
  • the reducing agent of this embodiment 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.
  • the reverse water gas shift reaction is carried out separately into a reduction reaction of the reducing agent (first process) and a reduction reaction of carbon dioxide (second process), with the reduction reaction of the reducing agent being the reaction shown in the following formula (A) and the reduction reaction of carbon dioxide being the reaction shown in the following formula (B).
  • x is usually 2. That is, in the reduction reaction of the reducing agent, hydrogen, which is a type of reducing substance, is oxidized to produce water, and in the reduction reaction of carbon dioxide, carbon dioxide is reduced to produce carbon monoxide, which is a type of carbon valuable.
  • the reaction temperature in the reduction reaction of the reducing agent may be any temperature at which the reduction reaction can proceed, but is preferably a temperature higher than 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 reducing agent can proceed efficiently.
  • the upper limit of the reaction temperature is preferably 1050° C. or less, more preferably 1000° C. or less, even more preferably 950° C. or less, particularly preferably 900° C. or less, and most preferably 850° C. or less. By setting the upper limit of the reaction temperature in the above range, it is possible to improve economic efficiency.
  • the lower limit and the upper limit can be combined in any manner to define the range of the reaction temperature.
  • the amount of hydrogen (reducing gas) to be brought into contact with the reducing agent in an oxidized state is preferably 1 mmol to 50 mmol per 1 g of the reducing agent, more preferably 2.5 mmol to 35 mmol, and even more preferably 5 mmol to 20 mmol.
  • the reducing agent of this embodiment has a high hydrogen utilization rate because oxygen elements can be smoothly introduced and removed. Therefore, the reducing agent of this embodiment is sufficiently reduced (regenerated) with a small amount of hydrogen. This contributes to reducing the energy required for hydrogen production, and thus also to reducing carbon dioxide generated when obtaining energy.
  • the hydrogen utilization rate (%) is a value expressed as a percentage of the amount (number of moles) of carbon monoxide produced relative to the amount (number of moles) of hydrogen introduced into contact with 1 g of reducing agent.
  • the reaction temperature in the reduction reaction of carbon dioxide is preferably a temperature exceeding 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 carbon dioxide can proceed efficiently.
  • the upper limit of the reaction temperature is preferably 1050° C. or less, more preferably 1000° C. or less, even more preferably 950° C. or less, particularly preferably 900° C. or less, and most preferably 850° C. or less.
  • the reducing agent can perform the reduction reaction of carbon dioxide to carbon monoxide with high efficiency even at low temperatures, so that the reduction reaction of carbon dioxide can be set at a relatively low temperature. Furthermore, by setting the upper limit of the reaction temperature in the above range, not only can waste heat be easily utilized, but also further economic efficiency can be improved.
  • the lower limit and the upper limit can be combined in any manner to define the range of the reaction temperature.
  • the amount of carbon dioxide brought into contact with the reducing agent is preferably 1 mmol to 50 mmol per 1 g of the reducing agent, more preferably 2.5 mmol to 30 mmol, and even more preferably 5 mmol to 20 mmol.
  • the reducing agent of this embodiment allows smooth inflow and outflow of oxygen elements. Therefore, the reducing agent of this embodiment has a high carbon dioxide conversion rate (hence, a large amount of carbon monoxide is produced), and from this viewpoint, it also contributes to the reduction of carbon dioxide.
  • the reducing agent can be regenerated with a small amount of hydrogen.
  • the amount of carbon monoxide produced in the reducing agent of this embodiment is preferably about 0.3 mmol or more and 1 mmol or less per 1 g of the reducing agent.
  • the reduction product (carbon valuable product) obtained by the reduction reaction of carbon dioxide contains carbon monoxide, but may contain other substances other than carbon monoxide, or may be a mixture of carbon monoxide and other substances.
  • a specific example of the other substances is methane.
  • the reduction products such as carbon monoxide obtained by the reduction reaction of carbon dioxide are further converted into organic substances by microbial fermentation or the like. Examples of microbial fermentation include anaerobic fermentation. Examples of the obtained organic substances include methanol, ethanol, acetic acid, butanol, derivatives thereof, or mixtures thereof, and compounds of C5 or more such as isoprene.
  • the reduced products such as carbon monoxide may be converted by metal oxides, etc., into C1 to C20 compounds including hydrocarbons and alcohols that are conventionally synthesized by petrochemistry.
  • Specific compounds that can be obtained include methane, ethane, propylene, methanol, ethanol, propanol, acetaldehyde, diethyl ether, acetic acid, butyric acid, diethyl carbonate, butadiene, etc.
  • the reducing agent of the present embodiment preferably has the following properties. That is, when a reducing agent is filled to a height of 40 cm in a stainless steel reaction tube having an inner diameter of 8 mm and a pressure gauge disposed in the flow path, and nitrogen gas having a concentration of 100% by volume is passed through at 30 mL/min, the pressure increase over 10 minutes is preferably 0.03 MPaG or less, and more preferably 0.01 MPaG or less.
  • a reducing agent exhibiting such characteristics can be determined to have a packing density and a pore volume that satisfy the above ranges, and can sufficiently increase the carbon dioxide conversion rate and the hydrogen conversion rate.
  • a reducing agent that can be stably used for a long period of time in a reaction at high temperature, and a method for producing a gas using this reducing agent.
  • the reducing agent of the present embodiment can withstand use at high temperatures, and therefore enables more efficient conversion of carbon dioxide to carbon monoxide (carbon value) through a chemical looping reaction at high temperatures. Furthermore, it may be provided in the following aspects:
  • a reducing agent that reduces carbon dioxide to produce carbon values comprising an oxygen carrier having oxygen ion conductivity, the oxygen carrier containing at least two metal elements belonging to Group 3 or Group 4 of the periodic table, including lanthanides and actinides, the oxygen carrier having an electrical conductivity of 1 x 10-7 S/cm or more in a pure oxygen atmosphere at a temperature of 700°C, and when X-ray diffraction measurement is performed on the reducing agent in a measurement range of 10° to 90.5°, the full width at half maximum of all peaks observed in the X-ray diffraction profile is less than 1.5°.
  • the reducing agent described in (1) above further contains an element belonging to Groups 6 to 14 of the periodic table, and the oxygen carrier has an electrical conductivity of ⁇ A [S/cm] in a pure oxygen atmosphere at a temperature of 700°C and an electrical conductivity of ⁇ B [S/cm] in an argon gas atmosphere containing 1% by volume of carbon monoxide, where ⁇ B/ ⁇ A is less than 85.
  • the reducing agent according to (2) above further contains an auxiliary compound containing an element belonging to Groups 6 to 14 of the periodic table, and when the reducing agent is subjected to a scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDS) measurement, the oxygen carrier and the auxiliary compound are observed separately, and the auxiliary compound is in the form of particles and in contact with the oxygen carrier.
  • STEM-EDS scanning transmission electron microscope-energy dispersive X-ray spectroscopy
  • the reducing agent contains at least two metal elements belonging to Group 3 or Group 4 of the periodic table, including lanthanides and actinides, and contains an oxygen carrier having oxygen ion conductivity, the oxygen carrier having an electric conductivity of 1 x 10-7 S/cm or more in a pure oxygen atmosphere at a
  • containing a metal element does not only mean that a single metal element is contained, but also that two or more metal elements are in an alloy state, or that the metal element is in a compound state.
  • the compound includes various states such as oxides, nitrides, chlorides, sulfides, and salts including nitrates.
  • the reducing agent and gas producing method of the present invention may have any other additional configuration compared to the above-mentioned embodiment, may be replaced with any configuration that exhibits a similar function, or some configurations may be omitted.
  • a gas containing hydrogen has been described as a representative reducing gas.
  • the reducing gas may also be 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.
  • Example 2 Production of reducing agent (Example 1) First, predetermined amounts of cerium (III) nitrate hexahydrate and zirconium nitrate dihydrate were each weighed out. Next, 6.06 g of citric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) was weighed and dissolved in 96 mL of deionized water to obtain an aqueous citric acid solution. The precursor (metal nitrate salt) was then added to the aqueous citric acid solution at room temperature while stirring to prepare an aqueous precursor solution. The molar ratio of Ce:Zr in the aqueous precursor solution was 0.9:0.1.
  • ethylene glycol manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%
  • the temperature was maintained at 80° C. with continuous stirring until a viscous gel was formed, after which the gel was transferred to a drying oven.
  • the gel was dried at 120° C. for 5 hours.
  • the resulting swollen mass of organic and inorganic compounds was pulverized, and the temperature was raised from room temperature to 450° C. at a rate of 8° C./min in an air atmosphere, and then calcined at 450° C. for 4 hours. Thereafter, the temperature was further raised to 950° C. at a rate of 8° C./min, and then calcined at 950° C. for 8 hours.
  • the fired mass was mechanically pulverized to obtain the target reducing agent composed of the oxygen carrier alone.
  • the reducing agent was in granular form. In order to facilitate filling into the reaction tube or to satisfy the requirement of pressure rise described in the section on [Properties of the Reducing Agent], the reducing agent was compressed into tablets for use in the tests, as necessary.
  • Example 2 to 6 and Comparative Examples 1 and 2 A reducing agent composed of an oxygen carrier alone or an oxygen carrier and a sub-compound was produced in the same manner as in Example 1, except for changing the type and amount of the oxygen carrier precursor used and/or adding a sub-compound.
  • the sub-compound was added to the precursor aqueous solution in the form of particles.
  • the resulting reducing agent was granular.
  • the diffractometer was set with a divergence slit at 1/2°, a divergence longitudinal limiting slit at 10 mm, a scattering slit at 2°, and a receiving slit at 0.15 mm.
  • the goniometer radius was 145 mm.
  • the prepared reducing agent 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 scanning angle of the goniometer to a range of 10° to 90.5°, the scanning speed to 3.5°/min, and the measurement step to 0.01. The measurement was performed in air at room temperature.
  • the data obtained without separation of K ⁇ 1 and K ⁇ 2 was analyzed.
  • the data was analyzed using software (High Score, manufactured by PANanalytical). If it is difficult to distinguish the baseline, the software may be used to correct it.
  • the highest intensity of the diffraction peak was taken as the apex, and the distance between the two points at half the intensity of the maximum peak value was taken as the full width at half maximum.
  • STEM-EDS Measurement The reducing agent was ultrasonically dispersed in ethanol, dropped onto a grid, and then dried. Observation and EDS analysis were performed using STEM (mapping ⁇ line profiling). For the line profile, a line was drawn so as to intersect with the interface between the oxygen carrier and the sub-compound observed in the element mapping of the STEM-EDS measurement, and the count number of each element at a certain point on the line was plotted.
  • STEM-EDS: EDAX (TEAM) ⁇ Parameters Acceleration voltage: 200 kV
  • EDS mapping resolution 256 x 200 Dwell: 200
  • Particle size distribution measurement (wet method) The particle size distribution was measured using a laser diffraction/scattering type particle size distribution measuring device ("Partica LA-960" manufactured by Horiba, Ltd.) in a wet flow cell.
  • the flow cell was made of synthetic quartz, the light source was a semiconductor laser (650 nm) of 5 mW and an LED (405 nm) of 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 obtained by dispersing about 10 mg of the reducing agent in about 180 mL of purified water (pH 6.8 to 7.2).
  • the measurement was carried out at least three times, and the median particle diameters obtained by each particle size distribution measurement were averaged to obtain the average particle diameter.
  • the electrical conductivity measured in the pure oxygen atmosphere was designated as " ⁇ A", and the electrical conductivity measured in the carbon monoxide-containing atmosphere was designated as " ⁇ B".
  • Data was analyzed using software (Microsoft's "Excel").
  • the reducing agent consisting only of an oxygen carrier was used as is for the electrical conductivity measurement.
  • an oxygen carrier was separately produced in the same manner as described above without adding the secondary compound to the precursor aqueous solution, and this oxygen carrier was used for the electrical conductivity measurement.
  • the conversion rate was calculated from the time when the amount of carbon monoxide generated per unit second became 0 mmol or more after the start of flowing carbon dioxide gas (feed gas) until the value of the following formula calculated based on the signal detected by a quadrupole mass spectrometer fell below 80. The average value calculated during this period was defined as the carbon dioxide conversion rate. Note that the region where the amount of carbon monoxide generated was 0 mmol or less was not used in calculating the conversion rate.
  • the amount of carbon monoxide produced in each cycle was measured when the carbon monoxide concentration detected by NDIR (non-dispersive infrared) reached 2.0% or more after carbon dioxide gas (raw material gas) was started to flow.
  • the carbon monoxide concentration detected by NDIR was recorded every 5.0 seconds, and the amount of carbon monoxide produced was calculated from the carbon monoxide concentration recorded during this period by quadrature by division. Note that the area where the carbon monoxide concentration was less than 2.0% was not used in the calculation of the amount of carbon monoxide produced.
  • the full width at half maximum of all peaks observed in the X-ray diffraction profile of the reducing agent obtained in each Example is less than 1.5°.
  • the full width at half maximum of at least one peak observed in the X-ray diffraction profile of the reducing agent obtained in each Comparative Example is 1.5° or more.
  • the X-ray diffraction profiles of the reducing agents obtained in Examples 2, 4, 5, and 6 and Comparative Example 2 are shown in FIGS. 4 to 8, respectively.

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Abstract

[Problem] To provide: a reducing agent which can be used stably for a long period in reactions at high temperatures; and a method for producing a gas using the reducing agent. [Solution] According to one aspect of the present invention, there is provided a reducing agent with which carbon dioxide is reduced to produce a valuable carbon substance. The reducing agent comprises an oxygen carrier having oxygen ion conductivity and containing at least two metal elements selected from metal elements belonging to Group 3 or Group 4 on the periodic table including lanthanoid elements and actinoid elements. The oxygen carrier has electroconductivity of 1×10-7 S/cm or more at a temperature of 700°C under a pure oxygen atmosphere. When the reducing agent is subjected to an X-ray diffraction measurement in the measurement range of 10° to 90.5° inclusive, the full width at half maximum of all of peaks observed in an X-ray diffraction profile is less than 1.5°.

Description

還元剤およびガスの製造方法Method for producing reducing agent and gas
　本発明は、還元剤およびガスの製造方法に関し、より詳しくは、例えば、ケミカルルーピング法に利用可能な還元剤、およびかかる還元剤を使用したガスの製造方法に関する。 The present invention relates to a reducing agent and a method for producing a gas, and more specifically, to a reducing agent that can be used, for example, in a chemical looping method, and a method for producing a gas using such a reducing agent.
　近年、温室効果ガスの一種である二酸化炭素は、その大気中の濃度が上昇を続けている。大気中の二酸化炭素の濃度の上昇は、地球温暖化を助長する。したがって、大気中に放出される二酸化炭素を回収することは重要であり、さらに回収した二酸化炭素を炭素有価物に変換して再利用できれば、炭素循環社会を実現することができる。
　従来、二酸化炭素から一酸化炭素を製造する方法として、逆水性ガスシフト反応を利用した方法が知られている。しかしながら、この従来の逆水性ガスシフト反応は、生成物である一酸化炭素と水とが系内に共存するため、化学平衡の制約により二酸化炭素の一酸化炭素への変換効率（収率）が低くなるという点で問題があった。 In recent years, the concentration of carbon dioxide, a type of greenhouse gas, in the atmosphere has been increasing. The increase in the concentration of carbon dioxide in the atmosphere contributes to global warming. Therefore, it is important to capture carbon dioxide emitted into the atmosphere, and if the captured carbon dioxide can be converted into carbon valuables and reused, a carbon circulation society can be realized.
Conventionally, a method utilizing the reverse water-gas shift reaction has been known as a method for producing carbon monoxide from carbon dioxide. However, this conventional reverse water-gas shift reaction has a problem in that the conversion efficiency (yield) of carbon dioxide to carbon monoxide is low due to the constraint of chemical equilibrium because the products, carbon monoxide and water, coexist in the system.
　そこで、上記問題を解決するため、ケミカルルーピング法を利用して二酸化炭素から一酸化炭素の変換（合成）が行われる。ここで言うケミカルルーピング法とは、上記逆水性ガスシフト反応を、水素による還元反応と、二酸化炭素からの一酸化炭素の生成反応との２つの反応に分割し、これらの反応を酸素キャリア（例えば、金属酸化物：ＭＯｘ）によって橋渡しさせるという方法である（下記式参照）。
　　Ｈ２　＋　ＭＯｘ　→　Ｈ２Ｏ　＋　ＭＯｘ－１
　　ＣＯ２　＋　ＭＯｘ－１　→　ＣＯ　＋　ＭＯｘ
　なお、上記式中、ＭＯｘ－１は、金属酸化物の一部または全部が還元された状態を示す。 In order to solve the above problems, carbon dioxide is converted (synthesized) into carbon monoxide using a chemical looping method. The chemical looping method refers to 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, and these reactions are bridged by an oxygen carrier (e.g., metal oxide: MO x ) (see the following formula).
H 2 + MO x → H 2 O + MO x-1
CO 2 + MO x-1 → CO + MO x
In the above formula, MO x-1 represents a state in which the metal oxide is partially or completely reduced.
　かかるケミカルルーピング法では、それぞれの反応時には、逆反応の基質である水および一酸化炭素が共存しないため、逆水性ガスシフト反応の化学平衡よりも高い二酸化炭素の一酸化炭素への転換効率を得られる可能性がある。
　このケミカルルーピング法では、従来、酸素キャリアに形成された酸素欠損を介して、二酸化炭素を一酸化炭素へ変換する反応を６５０℃以下の温度条件で行っている（特許文献１参照）。 In such a chemical looping method, since water and carbon monoxide, which are substrates for the reverse reaction, do not coexist during each reaction, it is possible to obtain a higher conversion efficiency of carbon dioxide to carbon monoxide than the chemical equilibrium of the reverse water-gas shift reaction.
In the chemical looping method, a reaction for converting carbon dioxide into carbon monoxide via oxygen vacancies formed in an oxygen carrier is conventionally carried out at a temperature of 650° C. or less (see Patent Document 1).
Ｉｎｄ．Ｅｎｇ．Ｃｈｅｍ．Ｒｅｓ．２０１３，５２，８４１６－８４２６Ind. Eng. Chem. Res. 2013, 52, 8416-8426
　本発明者らの検討によれば、水素により酸素キャリアの結晶構造中に酸素欠損を生じさせる反応は吸熱反応であるため、この酸素欠損を効率よく生成させるためには、上記反応をより高温で行うのがよいことが判明した。しかしながら、これまで、酸素欠損を生じさせる反応の高温領域（６５０℃を上回る温度）での転化率および安定性に関する検討は不十分であった。
　本発明では上記事情に鑑み、高温での反応において、長期にわたって安定的に使用可能な還元剤、およびかかる還元剤を使用したガスの製造方法を提供することとした。 According to the study by the present inventors, since the reaction of generating oxygen vacancies in the crystal structure of the oxygen carrier by hydrogen is an endothermic reaction, it was found that in order to generate the oxygen vacancies efficiently, it is better to carry out the reaction at a higher temperature. However, the conversion rate and stability of the reaction of generating oxygen vacancies at high temperatures (temperatures above 650° C.) have not been sufficiently studied.
In view of the above circumstances, the present invention provides a reducing agent that can be stably used for a long period of time in a reaction at high temperature, and a method for producing a gas using such a reducing agent.
　本発明の一態様によれば、二酸化炭素を還元して炭素有価物を生成する還元剤が提供される。この還元剤は、ランタノイドおよびアクチノイドを含む周期表の第３族、もしくは第４族に属する金属元素のうちの少なくとも２種を含む酸素イオン伝導性を備える酸素キャリアを含有する。酸素キャリアは、７００℃の温度において、純酸素雰囲気下で１×１０－７Ｓ／ｃｍ以上の電気伝導率を有し、かつ当該還元剤に対して測定範囲１０°以上９０．５°以下の範囲におけるＸ線回折測定を行ったとき、Ｘ線回折プロファイルにおいて観察される全てのピークの半値全幅が１．５°未満である。 According to one aspect of the present invention, there is provided a reducing agent for reducing carbon dioxide to produce carbon values. The reducing agent contains an oxygen carrier having oxygen ion conductivity, which contains at least two metal elements belonging to Group 3 or Group 4 of the periodic table, including lanthanides and actinides. The oxygen carrier has an electrical conductivity of 1×10 −7 S/cm or more in a pure oxygen atmosphere at a temperature of 700° C., and when X-ray diffraction measurement is performed on the reducing agent in a measurement range of 10° to 90.5°, the full width at half maximum of all peaks observed in the X-ray diffraction profile is less than 1.5°.
　かかる態様によれば、高温での反応において、長期にわたって安定的に使用可能な還元剤を得ることができる。 According to this embodiment, a reducing agent that can be stably used for a long period of time in reactions at high temperatures can be obtained.
図１（ａ）は、酸素キャリアと副化合物とが独立して存在する還元剤のＳＴＥＭ－ＥＤＳ測定により得られる画像の一例であり、図１（ｂ）は、図１（ａ）中の白線線上における各元素のマッピングをラインプロファイル化したグラフである。FIG. 1(a) is an example of an image obtained by STEM-EDS measurement of a reducing agent in which an oxygen carrier and a sub-compound exist independently, and FIG. 1(b) is a graph showing a line profile of the mapping of each element on the white line in FIG. 1(a). 図２（ａ）は、酸素キャリアと副化合物とが固溶している還元剤のＳＴＥＭ－ＥＤＳ測定により得られる画像の一例であり、図２（ｂ）は、図２（ａ）中の白線線上における各元素のマッピングをラインプロファイル化したグラフである。FIG. 2(a) is an example of an image obtained by STEM-EDS measurement of a reducing agent in which an oxygen carrier and a sub-compound are dissolved, and FIG. 2(b) is a graph showing a line profile of the mapping of each element on the white line in FIG. 2(a). 図３は、酸素分圧の変化による電気伝導率の変化の一例を示すグラフである。FIG. 3 is a graph showing an example of the change in electrical conductivity due to the change in oxygen partial pressure. 図４は、実施例２で得られた還元剤のＸ線回折プロファイルである。FIG. 4 is an X-ray diffraction profile of the reducing agent obtained in Example 2. 図５は、実施例４で得られた還元剤のＸ線回折プロファイルである。FIG. 5 is an X-ray diffraction profile of the reducing agent obtained in Example 4. 図６は、実施例５で得られた還元剤のＸ線回折プロファイルである。FIG. 6 is an X-ray diffraction profile of the reducing agent obtained in Example 5. 図７は、実施例６で得られた還元剤のＸ線回折プロファイルである。FIG. 7 is an X-ray diffraction profile of the reducing agent obtained in Example 6. 図８は、比較例２で得られた還元剤のＸ線回折プロファイルである。FIG. 8 is an X-ray diffraction profile of the reducing agent obtained in Comparative Example 2.
　以下、本発明の還元剤およびガスの製造方法について、好適実施形態に基づいて詳細に説明する。
　［還元剤］
　本実施形態の還元剤は、二酸化炭素を含む原料ガスを還元剤と接触させること（原料ガスに還元剤を作用させること）により、二酸化炭素を還元して一酸化炭素（炭素有価物）を含む生成ガスを製造する際に使用される（すなわち、本実施形態のガスの製造方法に使用される）。また、二酸化炭素との接触により酸化された還元剤は、水素（還元物質）を含む還元ガスと接触させることにより還元（再生）され得る。
　この際、好ましくは、本実施形態の還元剤を充填した反応管（反応容器）内に、原料ガスおよび還元ガスを交互に通過させることにより、還元剤による二酸化炭素の一酸化炭素への変換と、還元ガスによる酸化状態の還元剤の再生とが行われる。 Hereinafter, the reducing agent and gas production method of the present invention will be described in detail based on preferred embodiments.
[Reducing Agent]
The reducing agent of the present embodiment is used when producing a product gas containing carbon monoxide (carbon valuable) by reducing carbon dioxide by contacting a raw material gas containing carbon dioxide with the reducing agent (by making the reducing agent act on the raw material gas) (i.e., it is used in the gas production method of the present embodiment). Furthermore, the reducing agent oxidized by contact with carbon dioxide can be reduced (regenerated) by contacting with a reducing gas containing hydrogen (reducing material).
In this case, preferably, the raw material gas and the reducing gas are alternately passed through a reaction tube (reaction vessel) filled with the reducing agent of this embodiment, whereby carbon dioxide is converted to carbon monoxide by the reducing agent, and the reducing agent in an oxidized state is regenerated by the reducing gas.
　本実施形態の還元剤は、酸素イオン伝導性を備える酸素キャリアを含有する。
　ここで、酸素キャリアとは、可逆的な酸素欠損を生じ得る化合物であり、それ自体から還元により酸素元素が欠損するが、酸素元素が欠損した状態（還元状態）で、二酸化炭素と接触すると、二酸化炭素から酸素元素を奪い取って還元する作用を示す化合物のことを言う。
　本実施形態における酸素キャリアは、ランタノイドおよびアクチノイドを含む周期表の第３族、もしくは第４族に属する金属元素のうちの少なくとも２種を含んでいる。かかる酸素キャリアでは、２種以上の金属元素を含むことにより結晶格子に歪みが生じており、相手物質（例えば、二酸化炭素、水素）との間での酸素元素の授受が円滑になされる。このため、二酸化炭素からの一酸化炭素への変換効率（または、二酸化炭素転化率とも記載する。）、および水素からの水への変換効率（または、水素転化率とも記載する。）を高めることができる。
　なお、二酸化転化率および水素添加率は、それぞれ実施例に記載の方法により求めることができる。 The reducing agent of the present embodiment contains an oxygen carrier having oxygen ion conductivity.
Here, the oxygen carrier is a compound that can cause reversible oxygen deficiency, and loses oxygen element from itself through reduction. When the compound comes into contact with carbon dioxide in an oxygen-deficient state (reduced state), it acts to remove the oxygen element from the carbon dioxide and reduce it.
The oxygen carrier in this embodiment contains at least two kinds of metal elements belonging to Group 3 or Group 4 of the periodic table, including lanthanides and actinides. In such an oxygen carrier, the crystal lattice is distorted by containing two or more kinds of metal elements, and oxygen element is smoothly transferred between the oxygen carrier and the other substance (e.g., carbon dioxide, hydrogen). Therefore, the conversion efficiency from carbon dioxide to carbon monoxide (also referred to as carbon dioxide conversion rate) and the conversion efficiency from hydrogen to water (also referred to as hydrogen conversion rate) can be increased.
The carbon dioxide conversion rate and the hydrogenation rate can be determined by the methods described in the examples.
　酸素キャリアは、７００℃の温度において、純酸素雰囲気下で１×１０－７Ｓ／ｃｍ以上の電気伝導率を有している。かかる電気電導度を有する酸素キャリアを含有する還元剤であれば、酸素キャリア内での酸素イオンの移動速度を十分に高め得るため、還元ガスとの接触により酸素元素が効率よく離脱して、酸素欠損が円滑に誘発されるとともに、この誘発された酸素欠損により、二酸化炭素から酸素元素を奪い取り易くなる。７００℃の温度における純酸素雰囲気下での電気伝導率は、１×１０－７Ｓ／ｃｍ以上であればよいが、１×１０－６Ｓ／ｃｍ以上であることが好ましく、１×１０－５Ｓ／ｃｍ以上であることがより好ましく、１×１０－４Ｓ／ｃｍ以上であることがさらに好ましい。なお、７００℃の温度における純酸素雰囲気下での電気伝導率は、１×１０－１Ｓ／ｃｍ以下であってもよく、１×１０－２Ｓ／ｃｍ以下であってもよい。なお、本明細書において、純酸素雰囲気下とは、酸素濃度が９９体積％以上の条件を意図するものである。 The oxygen carrier has an electrical conductivity of 1×10 −7 S/cm or more in a pure oxygen atmosphere at a temperature of 700° C. A reducing agent containing an oxygen carrier having such electrical conductivity can sufficiently increase the migration speed of oxygen ions in the oxygen carrier, so that oxygen elements are efficiently released by contact with a reducing gas, oxygen vacancies are smoothly induced, and the induced oxygen vacancies facilitate the deprivation of oxygen elements from carbon dioxide. The electrical conductivity in a pure oxygen atmosphere at a temperature of 700° C. may be 1×10 −7 S/cm or more, but is preferably 1×10 −6 S/cm or more, more preferably 1×10 −5 S/cm or more, and even more preferably 1×10 −4 S/cm or more. The electrical conductivity in a pure oxygen atmosphere at a temperature of 700° C. may be 1×10 −1 S/cm or less, or may be 1×10 −2 S/cm or less. In this specification, the term "under a pure oxygen atmosphere" refers to a condition in which the oxygen concentration is 99% by volume or more.
　また、本実施形態の還元剤は、この還元剤に対して測定範囲１０°以上９０．５°以下の範囲におけるＸ線回折測定を行ったとき、Ｘ線回折プロファイルにおいて観察される全てのピークの半値全幅が１．５°未満である。この場合、酸素キャリアにおいて、結晶格子が十分に成長していることを示し、結晶としての安定性が高いことを意味する。したがって、かかる還元剤は、長期間使用しても、その性能を維持することができる。全てのピークの半値全幅は、１．５°未満であればよいが、１．２５°以下であることが好ましく、１°以下であることがより好ましい。なお、ピークの半値全幅は、０．１°以上であることが好ましく、０．５°以上であることがより好ましい。また、前記Ｘ線回折測定される還元剤は５０ｍｇ程度の質量があれば精度よく測定が可能である。
　なお、電気伝導率の測定およびＸ線回折測定のタイミングは、いずれも還元剤をガスとの反応に使用する前である。 In addition, when the reducing agent of this embodiment is subjected to X-ray diffraction measurement in a measurement range of 10° to 90.5°, the full width at half maximum of all peaks observed in the X-ray diffraction profile is less than 1.5°. In this case, it is shown that the crystal lattice is sufficiently grown in the oxygen carrier, and the stability as a crystal is high. Therefore, such a reducing agent can maintain its performance even if used for a long period of time. The full width at half maximum of all peaks may be less than 1.5°, but is preferably 1.25° or less, and more preferably 1° or less. The full width at half maximum of the peak is preferably 0.1° or more, and more preferably 0.5° or more. In addition, the reducing agent to be measured by X-ray diffraction can be accurately measured if it has a mass of about 50 mg.
The timing for measuring the electrical conductivity and the X-ray diffraction was both before the reducing agent was used in the reaction with the gas.
　本実施形態の還元剤は、さらに、周期表の第６族～第１４族に属する元素を含むことが好ましい。周期表の第６族～第１４族に含まれる元素は、例えば、インジウム（Ｉｎ）、錫（Ｓｎ）、テクネチウム（Ｔｃ）、銀（Ａｇ）等であってもよいし、後述で例示する元素であってもよい。
　前記周期表の第６族～第１４族に属する元素は、周期表の第８族～第１２族に属する元素を含むことがより好ましく、鉄（Ｆｅ）、ニッケル（Ｎｉ）、銅（Ｃｕ）、亜鉛（Ｚｎ）、ガリウム（Ｇａ）、ケイ素（Ｓｉ）を含むことがさらに好ましい。これらの元素は、１種を単独で使用してもよく、２種以上を併用してもよい。また、酸素キャリアは、７００℃の温度において、純酸素雰囲気下での電気伝導率をσＡ［Ｓ／ｃｍ］とし、一酸化炭素を１容量％で含有するアルゴンガス雰囲気下での電気伝導率をσＢ［Ｓ／ｃｍ］としたとき、σＢ／σＡは８５未満であることが好ましい。この場合、図３に示すように、異なる酸素分圧の雰囲気下での酸素キャリアの電気伝導率の差が小さいことを示し、酸素キャリアの反応性変化が小さいことを意味する。したがって、かかる酸素キャリアを含有する還元剤は、長期間使用しても、安定的に存在し、その性能を維持することができる。
　σＢ／σＡは８５未満であればよいが、８０以下であることが好ましく、６０以下であることがより好ましい。なお、σＢ／σＡは１０以上であってもよく、２０以上であってもよい。 The reducing agent of the present embodiment preferably further contains an element belonging to Groups 6 to 14 of the periodic table. The element belonging to Groups 6 to 14 of the periodic table may be, for example, indium (In), tin (Sn), technetium (Tc), silver (Ag), or the like, or may be an element exemplified below.
The elements belonging to Groups 6 to 14 of the periodic table more preferably include elements belonging to Groups 8 to 12 of the periodic table, and more preferably include iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), gallium (Ga), and silicon (Si). These elements may be used alone or in combination of two or more. In addition, when the electrical conductivity of the oxygen carrier in a pure oxygen atmosphere at a temperature of 700° C. is σA [S/cm] and the electrical conductivity in an argon gas atmosphere containing 1% by volume of carbon monoxide is σB [S/cm], it is preferable that σB/σA is less than 85. In this case, as shown in FIG. 3, the difference in electrical conductivity of the oxygen carrier under atmospheres of different oxygen partial pressures is small, which means that the reactivity change of the oxygen carrier is small. Therefore, a reducing agent containing such an oxygen carrier can be stably present and maintain its performance even when used for a long period of time.
The ratio σB/σA may be less than 85, but is preferably 80 or less, and more preferably 60 or less. The ratio σB/σA may be 10 or more, or 20 or more.
　本実施形態の還元剤は、周期表の第６族～第１４族に属する元素を、これを含む副化合物として含有することが好ましい。
　副化合物としては、例えば、酸化物、窒化物、酸窒化物、炭化物、等が挙げられる。副化合物の一例としては、例えば、ＮｉＯ、Ｆｅ２Ｏ３、ＳｉＣ等が挙げられる。これらの副化合物を使用することにより、還元剤の二酸化炭素転化率および水素転化率の維持時間を十分に長くすることができる。これらの中でも、副化合物としては、酸化物が好ましい。酸化物は、酸素キャリアとの親和性が高いため、還元剤全体としての機械的強度を高めることができる。また、酸化物中の酸素元素の一部を酸素イオンとして利用することができるため、還元剤の二酸化炭素転化率および水素転化率の維持効果に優れる。 The reducing agent of the present embodiment preferably contains an element belonging to Groups 6 to 14 of the periodic table as a sub-compound containing the element.
Examples of the secondary compounds include oxides, nitrides, oxynitrides, carbides, etc. Examples of the secondary compounds include NiO, Fe 2 O 3 , SiC, etc. By using these secondary compounds, the time for which the carbon dioxide conversion rate and hydrogen conversion rate of the reducing agent are maintained can be sufficiently extended. Among these, oxides are preferable as the secondary compounds. Since oxides have a high affinity with oxygen carriers, the mechanical strength of the reducing agent as a whole can be increased. In addition, since a part of the oxygen element in the oxide can be used as oxygen ions, the effect of maintaining the carbon dioxide conversion rate and hydrogen conversion rate of the reducing agent is excellent.
　この還元剤に対して走査透過型電子顕微鏡－エネルギー分散型Ｘ線分光（ＳＴＥＭ－ＥＤＳ）測定を行ったとき、図１に示すように、酸素キャリアと副化合物とが個別に観察され、副化合物は、粒子の形態で酸素キャリアに接していることが好ましい。これにより、副化合物によって活性化された水素元素が酸素キャリア内で円滑に移動するスピルオーバー効果が発揮される。このため、酸素キャリアの結晶格子内の酸素元素と水素元素との反応性を十分に高めることができる。その結果、還元剤の二酸化炭素転化率および水素転化率をより向上させることができる。
　なお、酸素キャリアと副化合物とが固溶している場合、図２に示すように、個別に観察することができない。 When the reducing agent is subjected to a scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDS) measurement, the oxygen carrier and the sub-compound are observed individually, as shown in FIG. 1, and it is preferable that the sub-compound is in the form of particles and in contact with the oxygen carrier. This produces a spillover effect in which the hydrogen element activated by the sub-compound moves smoothly within the oxygen carrier. This makes it possible to sufficiently increase the reactivity between the oxygen element and the hydrogen element in the crystal lattice of the oxygen carrier. As a result, it is possible to further improve the carbon dioxide conversion rate and the hydrogen conversion rate of the reducing agent.
When the oxygen carrier and the sub-compound are in a solid solution, they cannot be observed individually, as shown in FIG.
　また、この還元剤に対してＸ線回折測定を行ったとき、Ｘ線回折プロファイルにおいて観察される酸素キャリアに由来するピークおよび副化合物に由来するピークは、それぞれ酸素キャリア単独および副化合物単独に対してＸ線回折測定を行ったときのＸ線回折プロファイルにおいて観察されるピークに対して、２θで０．３°未満シフトしていることが好ましい。これは、酸素キャリアと副化合物との固溶が抑制され、それらの粒子が個別に（独立して）存在していることを意味している。このため、かかる還元剤では、酸素キャリアの作用と副化合物の作用とがバランスよく発揮される。このシフト量は、０．２５°以下であることがより好ましく、０．２°以下であることがさらに好ましく、０．１５°以下であることが特に好ましい。なお、シフト量は、０．０５°以上であってもよく、０．１°以上であってもよい。 Furthermore, when X-ray diffraction measurement is performed on this reducing agent, the peaks derived from the oxygen carrier and the peaks derived from the sub-compounds observed in the X-ray diffraction profile are preferably shifted by less than 0.3° in 2θ with respect to the peaks observed in the X-ray diffraction profile when X-ray diffraction measurement is performed on the oxygen carrier alone and the sub-compound alone, respectively. This means that the solid solution of the oxygen carrier and the sub-compound is suppressed, and the particles of these exist individually (independently). Therefore, in such a reducing agent, the action of the oxygen carrier and the action of the sub-compound are exerted in a well-balanced manner. The amount of this shift is more preferably 0.25° or less, even more preferably 0.2° or less, and particularly preferably 0.15° or less. The amount of shift may be 0.05° or more, or may be 0.1° or more.
　また、副化合物の粒子の平均粒径は、１ｎｍ以上１０μｍ以下であることが好ましく、３ｎｍ以上８μｍ以下であるであることがより好ましく、５ｎｍ以上６μｍ以下であることがさらに好ましい。これにより、副化合物の粒子と酸素キャリアとの間に形成される界面の面積が増え、副化合物によって活性化された水素元素が酸素キャリアにより円滑に移動することができる。このため、酸素キャリアの結晶格子内の酸素元素と水素元素との反応性を十分に高めることができる。
　なお、粒子の平均粒径は、レーザ回折／散乱式粒子径分布測定装置（例えば、株式会社堀場製作所製、「Ｐａｒｔｉｃａ　ＬＡ－９６０」）により体積基準の粒子径分布のデータを取得し、そのデータを処理することにより求めることができる。測定は、通常、湿式で行われる。 The average particle size of the sub-compound particles is preferably 1 nm to 10 μm, more preferably 3 nm to 8 μm, and even more preferably 5 nm to 6 μm. This increases the area of the interface formed between the sub-compound particles and the oxygen carrier, allowing the hydrogen element activated by the sub-compound to move more smoothly through the oxygen carrier. This allows the reactivity between the oxygen element and the hydrogen element in the crystal lattice of the oxygen carrier to be sufficiently increased.
The average particle size of the particles can be determined by acquiring volume-based particle size distribution data using a laser diffraction/scattering type particle size distribution measuring device (e.g., "Partica LA-960" manufactured by Horiba, Ltd.) and processing the data. The measurement is usually performed in a wet manner.
　上述したように、酸素キャリアでは、各金属元素が十分に固溶することにより、その結晶構造が安定化して耐熱温度も高まっていると考えられる。その結果、本実施形態の還元剤は、６５０℃を上回る高温においても使用することができる。
　さらに、６５０℃を上回る高温では、酸素キャリアを構成する金属元素が合金化し、安定した結晶構造をとり易いため、還元剤の酸化還元を繰り返しても初期活性を維持し易い。 As described above, it is considered that the crystal structure of the oxygen carrier is stabilized and the heat resistance temperature is increased by sufficiently dissolving each metal element in the oxygen carrier. As a result, the reducing agent of the present embodiment can be used even at high temperatures exceeding 650° C.
Furthermore, at high temperatures exceeding 650° C., the metal elements constituting the oxygen carrier are alloyed and tend to form a stable crystal structure, so that the initial activity is likely to be maintained even if the oxidation-reduction of the reducing agent is repeated.
　還元剤の充填密度は、４ｇ／ｍＬ以下であることが好ましく、０．５ｇ／ｍＬ以上３ｇ／ｍＬ以下であることがより好ましく、１ｇ／ｍＬ以上２．５ｇ／ｍＬ以下であることがさらに好ましい。この充填密度が低過ぎると、ガスの通過速度が速くなり過ぎ、還元剤と原料ガスおよび還元ガスとが接触する時間が減少する。その結果、還元剤による二酸化炭素転化率および水素転化率が低下し易い。一方、この充填密度が高過ぎると、ガスの通過速度が遅くなり過ぎ、反応が進行し難くなったり、生成ガスを製造するのに長時間を要するようになったりする。 The packing density of the reducing agent is preferably 4 g/mL or less, more preferably 0.5 g/mL to 3 g/mL or less, and even more preferably 1 g/mL to 2.5 g/mL or less. If the packing density is too low, the gas passage speed becomes too fast, and the time during which the reducing agent is in contact with the raw material gas and the reducing gas decreases. As a result, the carbon dioxide conversion rate and hydrogen conversion rate by the reducing agent tend to decrease. On the other hand, if the packing density is too high, the gas passage speed becomes too slow, making it difficult for the reaction to proceed and requiring a long time to produce the product gas.
　還元剤の細孔容積は、０．１ｃｍ３／ｇ以上であることが好ましく、１ｃｍ３／ｇ以上３０ｃｍ３／ｇ以下であることがより好ましく、５ｃｍ３／ｇ以上２０ｃｍ３／ｇ以下であることがさらに好ましい。この細孔容積が小さ過ぎると、原料ガスおよび還元ガスが還元剤の内部にまで入り難くなる。その結果、還元剤と原料ガスおよび還元ガスとの接触面積が減少し、還元剤による二酸化炭素転化率および水素転化率が低下し易い。一方、この細孔容積の上限値を超えて大きくしても、それ以上の効果の増大が期待できず、還元剤の種類によっては機械的強度が低下する傾向を示す。 The pore volume of the reducing agent is preferably 0.1 cm 3 /g or more, more preferably 1 cm 3 /g or more and 30 cm 3 /g or less, and even more preferably 5 cm 3 /g or more and 20 cm 3 /g or less. If the pore volume is too small, it becomes difficult for the raw material gas and the reducing gas to penetrate into the inside of the reducing agent. As a result, the contact area between the reducing agent and the raw material gas and the reducing gas decreases, and the carbon dioxide conversion rate and the hydrogen conversion rate by the reducing agent tend to decrease. On the other hand, even if the pore volume is increased beyond the upper limit, no further increase in effect can be expected, and depending on the type of reducing agent, the mechanical strength tends to decrease.
　還元剤の形状としては、特に限定されないが、例えば、粒状が好ましい。粒状であれば、還元剤の充填密度を上記範囲に調整し易い。
　ここで、粒状とは、粉末状、粒子状、塊状、ペレット状等を含む概念であり、その形態も球状、板状、多角状、破砕状、柱状、針状、鱗片状等のいずれでもよい。
　還元剤の平均粒径は、１μｍ以上５ｍｍ以下であることが好ましく、１０μｍ以上１ｍｍ以下であることがより好ましく、２０μｍ以上０．５ｍｍ以下であることがさらに好ましい。かかる平均粒径を有する還元剤であれば、その充填密度を上記範囲に調整し易い。 The shape of the reducing agent is not particularly limited, but is preferably, for example, granular, since the packing density of the reducing agent can be easily adjusted to the above range if the reducing agent is granular.
Here, granular is a concept including powder, particle, lump, pellet, etc., and the form may be any of spherical, plate-like, polygonal, crushed, columnar, needle-like, and scale-like.
The average particle size of the reducing agent is preferably 1 μm or more and 5 mm or less, more preferably 10 μm or more and 1 mm or less, and even more preferably 20 μm or more and 0.5 mm or less. If the reducing agent has such an average particle size, it is easy to adjust the packing density to the above range.
　なお、本明細書において、平均粒径とは、電子顕微鏡で観察される一視野中の任意の２００個の還元剤の粒径の平均値を意味する。この際、「粒径」とは、還元剤の輪郭線上の２点間の距離のうち最大の長さを意味する。なお、還元剤が柱状である場合、その端面の輪郭線上の２点間の距離のうち最大の長さを「粒径」とする。また、平均粒径は、例えば、塊状等であり、一次粒子が凝集している場合には、二次粒子の平均粒径を意味する。
　還元剤のＢＥＴ比表面積は、１ｍ２／ｇ以上５００ｍ２／ｇ以下であることが好ましく、３ｍ２／ｇ以上４５０ｍ２／ｇ以下であることがより好ましく、５ｍ２／ｇ以上４００ｍ２／ｇ以下であることがさらに好ましい。ＢＥＴ比表面積が上記範囲内であることで、還元剤の二酸化炭素転化率および水素転化率を向上させ易くなる。
　なお、ＢＥＴ比表面積は、ＢＥＴ法（好ましくはＢＥＴ多点法）に従って、動的定圧法による低温ガス吸着法により求めることができる。 In this specification, the average particle size means the average particle size of 200 arbitrary reducing agents in one visual field observed by an electron microscope. In this case, the "particle size" means the maximum length of the distance between two points on the contour line of the reducing agent. In addition, when the reducing agent is columnar, the maximum length of the distance between two points on the contour line of the end face is taken as the "particle size". In addition, when the reducing agent is, for example, clump-like and the primary particles are aggregated, the average particle size means the average particle size of the secondary particles.
The BET specific surface area of the reducing agent is preferably 1 m2 /g or more and 500 m2 /g or less, more preferably 3 m2 /g or more and 450 m2 /g or less, and even more preferably 5 m2 /g or more and 400 m2 /g or less. When the BET specific surface area is within the above range, it becomes easier to improve the carbon dioxide conversion rate and hydrogen conversion rate of the reducing agent.
The BET specific surface area can be determined by a low-temperature gas adsorption method using a dynamic constant pressure method in accordance with the BET method (preferably the BET multipoint method).
　また、本実施形態では、酸素キャリアにおいて各金属元素が安定的に固溶されているため、低温（４００℃程度）～高温（８５０℃程度）の広い範囲において、還元剤の酸素容量を高い状態に維持することができる。すなわち、本実施形態の還元剤は、広い温度範囲で二酸化炭素を一酸化炭素へ効率よく変換することができ、水素を含む還元ガスによって効率的に還元されることができる。
　還元剤に含まれる酸素キャリアの４００℃における酸素容量は、０．１質量％以上４０質量％以下であることが好ましく、０．５質量％以上３０質量％以下であることがより好ましい。還元剤に含まれる酸素キャリアの低温における酸素容量が上記範囲であれば、実稼働時の温度（６５０℃を上回る温度）においても酸素容量が十分に高いことを意味しており、二酸化炭素転化率および水素転化率が極めて高い還元剤であると言える。 In addition, in this embodiment, since each metal element is stably dissolved in the oxygen carrier, the oxygen capacity of the reducing agent can be maintained at a high level over a wide range from low temperature (about 400° C.) to high temperature (about 850° C.) In other words, the reducing agent of this embodiment can efficiently convert carbon dioxide to carbon monoxide over a wide temperature range, and can be efficiently reduced by a reducing gas containing hydrogen.
The oxygen capacity of the oxygen carrier contained in the reducing agent at 400° C. is preferably 0.1% by mass or more and 40% by mass or less, and more preferably 0.5% by mass or more and 30% by mass or less. If the oxygen capacity of the oxygen carrier contained in the reducing agent at low temperatures is within the above range, it means that the oxygen capacity is sufficiently high even at temperatures during actual operation (temperatures exceeding 650° C.), and it can be said that the reducing agent has extremely high carbon dioxide conversion rates and hydrogen conversion rates.
　［還元剤の製造方法］
　次に、還元剤の製造方法について説明する。
　還元剤の製造方法としては、特に限定されないが、例えば、ゾル－ゲル法、共沈法、固相法、水熱合成法等が挙げられる。
　還元剤は、一例として、例えば、次のようにして製造することができる。まず、還元剤を構成する金属元素の塩を水に溶解して水溶液を調製する。次いで、この水溶液をゲル化した後、乾燥および焼成する。すなわち、本実施形態の還元剤は、いわゆるゾル－ゲル法により、容易かつ確実に製造することができる。
　なお、水溶液の調整には、例えば、クエン酸、酢酸、リンゴ酸、酒石酸、塩酸、硝酸またはこれらの混合物等で酸性に調整した酸性水を用いてもよい。 [Method of producing reducing agent]
Next, a method for producing the reducing agent will be described.
The method for producing the reducing agent is not particularly limited, but examples thereof include the sol-gel method, coprecipitation method, solid phase method, and hydrothermal synthesis method.
As an example, the reducing agent can be produced as follows. First, a salt of a metal element constituting the reducing agent is dissolved in water to prepare an aqueous solution. Next, this aqueous solution is gelled, and then dried and baked. That is, the reducing agent of this embodiment can be produced easily and reliably by the so-called sol-gel method.
The aqueous solution may be prepared by using, for example, acidic water adjusted to be acidic with citric acid, acetic acid, malic acid, tartaric acid, hydrochloric acid, nitric acid, or a mixture thereof.
　金属元素の塩としては、例えば、硝酸塩、硫酸塩、塩化物、水酸化物、炭酸塩またはこれらの複合物等が挙げられるが、これらの中でも硝酸塩であることが好ましい。また、金属元素の塩には、必要に応じて、水和物を使用してもよい。
　ゲルの乾燥は、好ましくは２０℃以上２００℃以下、より好ましくは５０℃以上１５０℃以下の温度で、好ましくは０．５時間以上２０時間以下、より好ましくは１時間以上１５時間以下の時間で行うとよい。このように乾燥することで、ゲルを均一に乾燥させることができる。 Examples of the salt of a metal element include nitrates, sulfates, chlorides, hydroxides, carbonates, and compounds thereof, among which nitrates are preferred. Furthermore, hydrates of the salt of a metal element may be used as needed.
The gel is dried at a temperature of preferably 20° C. to 200° C., more preferably 50° C. to 150° C., for a time of preferably 0.5 hours to 20 hours, more preferably 1 hour to 15 hours. By drying in this manner, the gel can be dried uniformly.
　ゲルの焼成は、好ましくは３００℃以上１２００℃以下、より好ましくは７００℃以上１０００℃以下での温度で、好ましくは１時間以上２４時間以下、より好ましくは１．５時間以上２０時間以下の時間で行うとよい。ゲルは、焼成により、好ましくは酸化物となるが、上記焼成条件での焼成により還元剤に容易に変換され得る。また、上記焼成条件で焼成すれば、還元剤の過度の粒子成長を防ぐこともできる。
　上記焼成温度に到達するまでは、昇温速度１℃／分以上２０℃／分以下、好ましくは昇温速度２℃／分以上１０℃／分以下で昇温するとよい。これにより、還元剤の粒子の成長を促進させるとともに、結晶（粒子）の割れを回避することもできる。 The gel is preferably fired at a temperature of 300° C. to 1200° C., more preferably 700° C. to 1000° C., for a time of 1 hour to 24 hours, more preferably 1.5 hours to 20 hours. The gel is preferably converted into an oxide by firing, but can be easily converted into a reducing agent by firing under the above firing conditions. In addition, firing under the above firing conditions can prevent excessive particle growth of the reducing agent.
Until the above-mentioned firing temperature is reached, the temperature is increased at a rate of 1° C./min to 20° C./min, preferably 2° C./min to 10° C./min, which can promote the growth of the reducing agent particles and prevent the cracking of the crystals (particles).
　また、還元剤は、次のようにして製造することもできる。
　まず、還元剤を構成する金属元素のそれぞれを含む酸化物を混合しつつ粉砕する。粉砕には、例えば、ボールミル、ビーズミル、ジェットミル、ハンマーミル、ロッドミル等を使用することができる。また、この粉砕は、乾式または湿式のいずれで行ってもよい。
　次に、粉砕後の塊状物を粉砕した後、焼成する。この焼成は、好ましくは３００℃以上１２００℃以下、より好ましくは７００℃以上１０００℃以下での温度で、好ましくは１時間以上２４時間以下、より好ましくは１．５時間以上２０時間以下の時間で行うとよい。
　その後、焼成した塊状物を上記と同様に粉砕することにより、還元剤を得ることができる。
　なお、副化合物を含有する還元剤は、副化合物を上記水溶液に添加することにより製造するようにしてもよく、副化合物を上記酸化物に混合することにより製造するようにしてもよいが、前者が好ましい。 The reducing agent can also be produced as follows.
First, oxides containing the respective metal elements constituting the reducing agent are mixed and pulverized. For pulverization, for example, a ball mill, a bead mill, a jet mill, a hammer mill, a rod mill, etc. can be used. In addition, this pulverization may be performed by either a dry method or a wet method.
Next, the crushed aggregate is crushed and then calcined at a temperature of preferably 300° C. to 1200° C., more preferably 700° C. to 1000° C., for a time of preferably 1 hour to 24 hours, more preferably 1.5 hours to 20 hours.
Thereafter, the fired mass is pulverized in the same manner as above to obtain the reducing agent.
The reducing agent containing the secondary compound may be produced by adding the secondary compound to the aqueous solution or by mixing the secondary compound with the oxide, with the former being preferred.
　［還元剤の使用方法］
　本実施形態の還元剤は、上述したように、例えば、ケミカルルーピング法で利用することができる。また、本実施形態の還元剤は、上述したように、接触により二酸化炭素を還元して一酸化炭素（炭素有価物）を生成する用途に使用することができる。
　より具体的には、二酸化炭素の還元反応と、還元剤の還元反応とを行うとよく、還元剤は、二酸化炭素の還元反応と還元剤の還元反応との間で循環するように使用することが好ましい。なお、還元剤の還元反応では、他の還元物質を含む還元ガスを使用する。 [How to use the reducing agent]
The reducing agent of the present embodiment can be used in, for example, a chemical looping method as described above. In addition, the reducing agent of the present embodiment can be used in an application for reducing carbon dioxide through contact to generate carbon monoxide (carbon value) as described above.
More specifically, it is preferable to carry out a reduction reaction of carbon dioxide and a reduction reaction of a reducing agent, and it is preferable to use the reducing agent so as to circulate between the reduction reaction of carbon dioxide and the reduction reaction of the reducing agent. Note that in the reduction reaction of the reducing agent, a reducing gas containing other reducing substances is used.
　また、本実施形態の還元剤は、いわゆる逆水性ガスシフト反応に使用することが好ましい。逆水性ガスシフト反応とは、二酸化炭素と水素とから、一酸化炭素と水とを生成する反応である。逆水性ガスシフト反応は、ケミカルルーピング法を適用する場合、還元剤の還元反応（第１プロセス）と二酸化炭素の還元反応（第２プロセス）とに分割して行われ、還元剤の還元反応が下記式（Ａ）で示す反応となり、二酸化炭素の還元反応が下記式（Ｂ）で示す反応となる。 The reducing agent of this embodiment 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 separately into a reduction reaction of the reducing agent (first process) and a reduction reaction of carbon dioxide (second process), with the reduction reaction of the reducing agent being the reaction shown in the following formula (A) and the reduction reaction of carbon dioxide being the reaction shown in the following formula (B).
　Ｈ２（ガス）　＋　ＭＯｘ（固体）　→　Ｈ２Ｏ（ガス）　＋　ＭＯｘ－１（固体）　　（Ａ）
　ＣＯ２（ガス）　＋　ＭＯｘ－１（固体）　→　ＣＯ（ガス）　＋　ＭＯｘ（固体）　（Ｂ）
　なお、式（Ａ）および（Ｂ）において、ｘは、通常２である。
　すなわち、還元剤の還元反応では、還元物質の一種である水素が酸化されて水が生成される。また、二酸化炭素の還元反応では、二酸化炭素が還元されて炭素有価物の一種である一酸化炭素が生成される。 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 the formulas (A) and (B), x is usually 2.
That is, in the reduction reaction of the reducing agent, hydrogen, which is a type of reducing substance, is oxidized to produce water, and in the reduction reaction of carbon dioxide, carbon dioxide is reduced to produce carbon monoxide, which is a type of carbon valuable.
　還元剤の還元反応における反応温度（還元剤の還元ガスとの接触温度）は、還元反応が進行できる温度であればよいが、６５０℃を上回る温度であることが好ましく、７００℃以上であることがより好ましく、７５０℃以上であることがさらに好ましく、８００℃以上であることが特に好ましい。かかる温度範囲で、効率的な還元剤の還元反応を進行させることができる。
　この反応温度の上限は、１０５０℃以下であることが好ましく、１０００℃以下であることがより好ましく、９５０℃以下であることがさらに好ましく、９００℃以下であることが特に好ましく、８５０℃以下であることが最も好ましい。反応温度の上限を上記範囲に設定することにより、経済性の向上を図ることができる。
　なお、下限値と上限値とは、任意に組み合わせて、反応温度の範囲を規定することができる。 The reaction temperature in the reduction reaction of the reducing agent (contact temperature of the reducing agent with the reducing gas) may be any temperature at which the reduction reaction can proceed, but is preferably a temperature higher than 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 reducing agent can proceed efficiently.
The upper limit of the reaction temperature is preferably 1050° C. or less, more preferably 1000° C. or less, even more preferably 950° C. or less, particularly preferably 900° C. or less, and most preferably 850° C. or less. By setting the upper limit of the reaction temperature in the above range, it is possible to improve economic efficiency.
The lower limit and the upper limit can be combined in any manner to define the range of the reaction temperature.
　また、還元剤の還元反応の際、酸化状態の還元剤に接触させる水素（還元ガス）の量は、還元剤１ｇに対して１ｍｍｏｌ以上５０ｍｍｏｌ以下であることが好ましく、２．５ｍｍｏｌ以上３５ｍｍｏｌ以下であることがより好ましく、５ｍｍｏｌ以上２０ｍｍｏｌ以下であることがさらに好ましい。本実施形態の還元剤は、酸素元素の出入りが円滑になされるため、水素利用率が高い。したがって、本実施形態の還元剤は、少量の水素で十分に還元（再生）される。よって、水素の生成に必要なエネルギーを減少させること、ひいてはエネルギーを得る際に発生する二酸化炭素の削減にも寄与する。
　なお、水素利用率（％）は、還元剤１ｇに接触させた水素投入量（モル数）に対する生成した一酸化炭素の量（モル数）の比率を１００分率で表した値である。 In addition, during the reduction reaction of the reducing agent, the amount of hydrogen (reducing gas) to be brought into contact with the reducing agent in an oxidized state is preferably 1 mmol to 50 mmol per 1 g of the reducing agent, more preferably 2.5 mmol to 35 mmol, and even more preferably 5 mmol to 20 mmol. The reducing agent of this embodiment has a high hydrogen utilization rate because oxygen elements can be smoothly introduced and removed. Therefore, the reducing agent of this embodiment is sufficiently reduced (regenerated) with a small amount of hydrogen. This contributes to reducing the energy required for hydrogen production, and thus also to reducing carbon dioxide generated when obtaining energy.
The hydrogen utilization rate (%) is a value expressed as a percentage of the amount (number of moles) of carbon monoxide produced relative to the amount (number of moles) of hydrogen introduced into contact with 1 g of reducing agent.
　また、二酸化炭素の還元反応における反応温度（還元剤の二酸化炭素との接触温度）は、６５０℃を上回る温度であることが好ましく、７００℃以上であることがより好ましく、７５０℃以上であることがさらに好ましく、８００℃以上であることが特に好ましい。かかる温度範囲で、効率的な二酸化炭素の還元反応を進行させることができる。
　この反応温度の上限は、１０５０℃以下であることが好ましく、１０００℃以下であることがより好ましく、９５０℃以下であることがさらに好ましく、９００℃以下であることが特に好ましく、８５０℃以下であることが最も好ましい。還元剤は、低温下でも高い効率で二酸化炭素の一酸化炭素への還元反応を行うことができるので、二酸化炭素の還元反応を比較的低温に設定することができる。また、反応温度の上限を上記範囲に設定することにより、廃熱活用が容易になるばかりでなく、更なる経済性の向上を図ることができる。
　なお、下限値と上限値とは、任意に組み合わせて、反応温度の範囲を規定することができる。 Furthermore, the reaction temperature in the reduction reaction of carbon dioxide (contact temperature of the reducing agent with carbon dioxide) is preferably a temperature exceeding 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 carbon dioxide can proceed efficiently.
The upper limit of the reaction temperature is preferably 1050° C. or less, more preferably 1000° C. or less, even more preferably 950° C. or less, particularly preferably 900° C. or less, and most preferably 850° C. or less. The reducing agent can perform the reduction reaction of carbon dioxide to carbon monoxide with high efficiency even at low temperatures, so that the reduction reaction of carbon dioxide can be set at a relatively low temperature. Furthermore, by setting the upper limit of the reaction temperature in the above range, not only can waste heat be easily utilized, but also further economic efficiency can be improved.
The lower limit and the upper limit can be combined in any manner to define the range of the reaction temperature.
　また、二酸化炭素の還元反応の際、還元剤に接触させる二酸化炭素の量は、還元剤１ｇに対して１ｍｍｏｌ以上５０ｍｍｏｌ以下であることが好ましく、２．５ｍｍｏｌ以上３０ｍｍｏｌ以下であることがより好ましく、５ｍｍｏｌ以上２０ｍｍｏｌ以下であることがさらに好ましい。本実施形態の還元剤は、酸素元素の出入りが円滑になされる。このため、本実施形態の還元剤は、二酸化炭素転化率が高く（したがって、一酸化炭素の生成量が多く）、かかる観点からも、二酸化炭素の削減に寄与する。一方、水素を含む還元ガスによって効率的に還元反応が行われるため、少ない水素量により還元剤を再生することができる。
　本実施形態の還元剤における上記一酸化炭素の生成量は、還元剤１ｇに対して０．３ｍｍｏｌ以上１ｍｍｏｌ以下程度であることが好ましい。 In addition, during the reduction reaction of carbon dioxide, the amount of carbon dioxide brought into contact with the reducing agent is preferably 1 mmol to 50 mmol per 1 g of the reducing agent, more preferably 2.5 mmol to 30 mmol, and even more preferably 5 mmol to 20 mmol. The reducing agent of this embodiment allows smooth inflow and outflow of oxygen elements. Therefore, the reducing agent of this embodiment has a high carbon dioxide conversion rate (hence, a large amount of carbon monoxide is produced), and from this viewpoint, it also contributes to the reduction of carbon dioxide. On the other hand, since the reduction reaction is efficiently carried out by the reducing gas containing hydrogen, the reducing agent can be regenerated with a small amount of hydrogen.
The amount of carbon monoxide produced in the reducing agent of this embodiment is preferably about 0.3 mmol or more and 1 mmol or less per 1 g of the reducing agent.
　なお、本実施形態では、二酸化炭素の還元反応で得られる還元物（炭素有価物）は、一酸化炭素を含むが、一酸化炭素以外の他の物質を含んでもよく、一酸化炭素と他の物質との混合物であってもよい。他の物質の具体例としては、例えば、メタンが挙げられる。上記二酸化炭素の還元反応で得られた一酸化炭素等の還元物は、さらに微生物発酵等により有機物質等に変換されることが好ましい。微生物発酵としては、嫌気性発酵が挙げられる。得られる有機物質としては、メタノール、エタノール、酢酸、ブタノール、これらの誘導体、またはこれらの混合物、イソプレン等のＣ５以上の化合物等が挙げられる。
　さらに、一酸化炭素等の還元物は、金属酸化物等により、従来石油化学により合成される炭化水素、アルコールを含むＣ１からＣ２０までの化合物に変換されてもよい。得られる具体的な化合物としては、メタン、エタン、プロピレン、メタノール、エタノール、プロパノール、アセトアルデヒド、ジエチルエーテル、酢酸、酪酸、炭酸ジエチル、ブタジエン等が挙げられる。 In this embodiment, the reduction product (carbon valuable product) obtained by the reduction reaction of carbon dioxide contains carbon monoxide, but may contain other substances other than carbon monoxide, or may be a mixture of carbon monoxide and other substances. A specific example of the other substances is methane. It is preferable that the reduction products such as carbon monoxide obtained by the reduction reaction of carbon dioxide are further converted into organic substances by microbial fermentation or the like. Examples of microbial fermentation include anaerobic fermentation. Examples of the obtained organic substances include methanol, ethanol, acetic acid, butanol, derivatives thereof, or mixtures thereof, and compounds of C5 or more such as isoprene.
Furthermore, the reduced products such as carbon monoxide may be converted by metal oxides, etc., into C1 to C20 compounds including hydrocarbons and alcohols that are conventionally synthesized by petrochemistry. Specific compounds that can be obtained include methane, ethane, propylene, methanol, ethanol, propanol, acetaldehyde, diethyl ether, acetic acid, butyric acid, diethyl carbonate, butadiene, etc.
　［還元剤の特性］
　本実施形態の還元剤は、次のような特性を有することが好ましい。
　すなわち、流路内に圧力計を配置した内径８ｍｍのステンレス鋼製の反応管内に、還元剤を４０ｃｍの高さで充填し、濃度１００体積％の窒素ガスを３０ｍＬ／分で通過させたとき、１０分間での圧力上昇が０．０３ＭＰａＧ以下であることが好ましく、０．０１ＭＰａＧ以下であることがより好ましい。
　かかる特性を示す還元剤は、充填密度および細孔容積が上記範囲を満たすと判断することができ、二酸化炭素転化率および水素転化率を十分に高めることができる。 [Characteristics of reducing agents]
The reducing agent of the present embodiment preferably has the following properties.
That is, when a reducing agent is filled to a height of 40 cm in a stainless steel reaction tube having an inner diameter of 8 mm and a pressure gauge disposed in the flow path, and nitrogen gas having a concentration of 100% by volume is passed through at 30 mL/min, the pressure increase over 10 minutes is preferably 0.03 MPaG or less, and more preferably 0.01 MPaG or less.
A reducing agent exhibiting such characteristics can be determined to have a packing density and a pore volume that satisfy the above ranges, and can sufficiently increase the carbon dioxide conversion rate and the hydrogen conversion rate.
　以上のような実施形態によれば、高温での反応において、長期にわたって安定的に使用可能な還元剤、およびこの還元剤を使用したガスの製造方法を提供することができる。
　本実施形態の還元剤は、高温での使用に耐え得るため、高温でのケミカルルーピング反応により、二酸化炭素から一酸化炭素（炭素有価物）へのより効率のよい変換が可能となる。
　さらに、次に記載の各態様で提供されてもよい。 According to the embodiment as described above, it is possible to provide a reducing agent that can be stably used for a long period of time in a reaction at high temperature, and a method for producing a gas using this reducing agent.
The reducing agent of the present embodiment can withstand use at high temperatures, and therefore enables more efficient conversion of carbon dioxide to carbon monoxide (carbon value) through a chemical looping reaction at high temperatures.
Furthermore, it may be provided in the following aspects:
（１）二酸化炭素を還元して炭素有価物を生成する還元剤であって、ランタノイドおよびアクチノイドを含む周期表の第３族、もしくは第４族に属する金属元素のうちの少なくとも２種を含む酸素イオン伝導性を備える酸素キャリアを含有し、前記酸素キャリアは、７００℃の温度において、純酸素雰囲気下で１×１０－７Ｓ／ｃｍ以上の電気伝導率を有し、かつ当該還元剤に対して測定範囲１０°以上９０．５°以下の範囲におけるＸ線回折測定を行ったとき、Ｘ線回折プロファイルにおいて観察される全てのピークの半値全幅が１．５°未満である、還元剤。 (1) A reducing agent that reduces carbon dioxide to produce carbon values, the reducing agent comprising an oxygen carrier having oxygen ion conductivity, the oxygen carrier containing at least two metal elements belonging to Group 3 or Group 4 of the periodic table, including lanthanides and actinides, the oxygen carrier having an electrical conductivity of 1 x 10-7 S/cm or more in a pure oxygen atmosphere at a temperature of 700°C, and when X-ray diffraction measurement is performed on the reducing agent in a measurement range of 10° to 90.5°, the full width at half maximum of all peaks observed in the X-ray diffraction profile is less than 1.5°.
（２）上記（１）に記載の還元剤において、さらに、周期表の第６族～第１４族に属する元素を含み、前記酸素キャリアは、７００℃の温度において、純酸素雰囲気下での電気伝導率をσＡ［Ｓ／ｃｍ］とし、一酸化炭素を１容量％で含有するアルゴンガス雰囲気下での電気伝導率をσＢ［Ｓ／ｃｍ］としたとき、σＢ／σＡが８５未満である、還元剤。 (2) The reducing agent described in (1) above further contains an element belonging to Groups 6 to 14 of the periodic table, and the oxygen carrier has an electrical conductivity of σA [S/cm] in a pure oxygen atmosphere at a temperature of 700°C and an electrical conductivity of σB [S/cm] in an argon gas atmosphere containing 1% by volume of carbon monoxide, where σB/σA is less than 85.
（３）上記（２）に記載の還元剤において、さらに、周期表の第６族～第１４族に属する元素を含む副化合物を含有し、当該還元剤に対して走査透過型電子顕微鏡－エネルギー分散型Ｘ線分光（ＳＴＥＭ－ＥＤＳ）測定を行ったとき、前記酸素キャリアと前記副化合物とが個別に観察され、かつ前記副化合物は、粒子の形態で前記酸素キャリアと接している、還元剤。 (3) The reducing agent according to (2) above further contains an auxiliary compound containing an element belonging to Groups 6 to 14 of the periodic table, and when the reducing agent is subjected to a scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDS) measurement, the oxygen carrier and the auxiliary compound are observed separately, and the auxiliary compound is in the form of particles and in contact with the oxygen carrier.
（４）上記（３）に記載の還元剤において、当該還元剤に対してＸ線回折測定を行ったとき、Ｘ線回折プロファイルにおいて観察される前記酸素キャリアに由来するピークおよび前記副化合物に由来するピークは、それぞれ前記酸素キャリア単独および前記副化合物単独に対してＸ線回折測定を行ったときのＸ線回折プロファイルにおいて観察されるピークに対して、２θで０．３°未満シフトしている、還元剤。 (4) The reducing agent according to (3) above, in which, when X-ray diffraction measurement is performed on the reducing agent, the peaks derived from the oxygen carrier and the peaks derived from the minor compound observed in the X-ray diffraction profile are shifted by less than 0.3° in 2θ relative to the peaks observed in the X-ray diffraction profile when X-ray diffraction measurement is performed on the oxygen carrier alone and the minor compound alone, respectively.
（５）上記（３）又は（４）に記載の還元剤において、前記副化合物の粒子の平均粒径は、１ｎｍ以上１０μｍ以下である、還元剤。 (5) The reducing agent according to (3) or (4) above, wherein the average particle size of the sub-compound particles is 1 nm or more and 10 μm or less.
（６）上記（１）～（５）のいずれか１つに記載の還元剤において、前記二酸化炭素との反応は、６５０℃を上回る温度で行われる、還元剤。 (6) A reducing agent according to any one of (1) to (5) above, wherein the reaction with carbon dioxide is carried out at a temperature exceeding 650°C.
（７）上記（１）～（６）のいずれか１つに記載の還元剤において、前記二酸化炭素により酸化された前記還元剤は、水素を含む還元ガスにより還元される、還元剤。 (7) The reducing agent according to any one of (1) to (6) above, wherein the reducing agent oxidized by the carbon dioxide is reduced by a reducing gas containing hydrogen.
（８）上記（７）に記載の還元剤において、前記水素を含む還元ガスとの反応は、６５０℃を上回る温度で行われる、還元剤。 (8) The reducing agent described in (7) above, wherein the reaction with the hydrogen-containing reducing gas is carried out at a temperature exceeding 650°C.
（９）二酸化炭素を含む原料ガスに還元剤を作用させることにより、前記二酸化炭素を還元して炭素有価物を含む生成ガスを製造するガスの製造方法であって、前記還元剤は、ランタノイドおよびアクチノイドを含む周期表の第３族、もしくは第４族に属する金属元素のうちの少なくとも２種を含み、酸素イオン伝導性を備える酸素キャリアを含有し、前記酸素キャリアは、７００℃の温度において、純酸素雰囲気下で１×１０－７Ｓ／ｃｍ以上の電気伝導率を有し、かつ当該還元剤に対して測定範囲１０°以上９０．５°以下の範囲におけるＸ線回折測定を行ったとき、Ｘ線回折プロファイルにおいて観察される全てのピークの半値全幅が１．５°未満である、ガスの製造方法。
　もちろん、この限りではない。例えば、金属元素を含むとは、単体の金属元素として単に含むだけでなく、２種以上の金属元素が合金状態になっていてもよいし、金属元素が化合物状態になっていてもよい。ここで、化合物とは、酸化物や窒化物、塩化物や硫化物や硝酸塩を含む塩等、種々の状態を含むものである。 (9) A gas production method for producing a product gas containing carbon values by reacting a reducing agent with a raw material gas containing carbon dioxide to reduce the carbon dioxide, wherein the reducing agent contains at least two metal elements belonging to Group 3 or Group 4 of the periodic table, including lanthanides and actinides, and contains an oxygen carrier having oxygen ion conductivity, the oxygen carrier having an electric conductivity of 1 x 10-7 S/cm or more in a pure oxygen atmosphere at a temperature of 700°C, and when X-ray diffraction measurement is performed on the reducing agent in a measurement range of 10° to 90.5°, the full width at half maximum of all peaks observed in the X-ray diffraction profile is less than 1.5°.
Of course, this is not the only possible meaning. For example, containing a metal element does not only mean that a single metal element is contained, but also that two or more metal elements are in an alloy state, or that the metal element is in a compound state. Here, the compound includes various states such as oxides, nitrides, chlorides, sulfides, and salts including nitrates.
　既述のとおり、本発明に係る種々の実施形態を説明したが、これらは、例として提示したものであり、発明の範囲を何ら限定するものではない。当該新規な実施形態は、その他の様々な形態で実施されることが可能であり、発明の要旨を逸脱しない範囲で、種々の省略、置き換え、変更を行うことができる。当該実施形態やその変形は、発明の範囲や要旨に含まれるとともに、特許請求の範囲に記載された発明とその均等の範囲に含まれるものである。 As mentioned above, various embodiments of the present invention 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 gist of the invention. The embodiments and their variations are included within the scope and gist of the invention, and are included in the scope of the invention and its equivalents as set forth in the claims.
　例えば、本発明の還元剤およびガスの製造方法は、上記実施形態に対して、他の任意の追加の構成を有していてもよく、同様の機能を発揮する任意の構成と置換されていてよく、一部の構成が省略されていてもよい。
　上記実施形態では、還元ガスとして水素を含むガスを代表に説明したが、還元ガスには、還元物質として、水素に代えてまたは水素に加えて、炭化水素（例えば、メタン、エタン、アセチレン等）およびアンモニアから選択される少なくとも１種を含むガスを使用することもできる。 For example, the reducing agent and gas producing method of the present invention may have any other additional configuration compared to the above-mentioned embodiment, may be replaced with any configuration that exhibits a similar function, or some configurations may be omitted.
In the above embodiment, a gas containing hydrogen has been described as a representative reducing gas. However, the reducing gas may also be 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.
　以下に、実施例および比較例を挙げて、本発明をさらに具体的に説明するが、本発明は、これらの実施例に限定されるものではない。
　１．還元剤の材料の準備
　還元剤の材料として、以下の化合物を準備した。
　（酸素キャリアの前駆体）
　　・硝酸セリウム（ＩＩＩ）六水和物（富士フイルム和光純薬工業株式会社製、純度：９８．０％）
　　・硝酸ジルコニウム二水和物（富士フイルム和光純薬工業株式会社製、純度：９７．０％）
　　・硝酸サマリウム六水和物（富士フイルム和光純薬工業株式会社製、純度：９９．５％）
　　・硝酸銅（ＩＩ）三水和物（富士フイルム和光純薬工業株式会社製、純度：９９．０％）
　　・硝酸鉄（ＩＩＩ）九水和物（富士フイルム和光純薬工業株式会社製、純度：９９．９％）
　　・硝酸ランタン（ＩＩＩ）六水和物（富士フイルム和光純薬工業株式会社製、純度：９９．９％）
　（副化合物）
　　・炭化ケイ素（株式会社高純度化学研究所製、純度：９９．０％）
　　・酸化ニッケル（ＩＩ）（富士フイルム和光純薬工業株式会社製、純度：９９．９％） The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.
1. Preparation of reducing agent materials The following compounds were prepared as reducing agent materials.
(Oxygen Carrier Precursor)
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%)
Samarium nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%)
Copper (II) nitrate trihydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.0%)
Iron (III) nitrate nonahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%)
Lanthanum (III) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%)
(Minor Compound)
Silicon carbide (manufactured by High Purity Chemical Laboratory Co., Ltd., purity: 99.0%)
Nickel (II) oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%)
　２．還元剤の製造
　（実施例１）
　まず、硝酸セリウム（ＩＩＩ）六水和物と、硝酸ジルコニウム二水和物とを、それぞれ所定量を計量した。
　次いで、６．０６ｇのクエン酸（富士フイルム和光純薬工業株式会社製、純度：９９．５％）を計量し、９６ｍＬの脱イオン水に溶解してクエン酸水溶液を得た。その後、上記前駆体（硝酸金属塩）を、攪拌しつつクエン酸水溶液に室温で添加して、前駆体水溶液を調製した。なお、前駆体水溶液中におけるＣｅ：Ｚｒ（モル比）を、０．９：０．１とした。
　３０分経過後、２．１５ｇのエチレングリコール（富士フイルム和光純薬工業株式会社製、純度：９９．５％）を前駆体水溶液に添加し、温度を８０℃に上昇させた。 2. Production of reducing agent (Example 1)
First, predetermined amounts of cerium (III) nitrate hexahydrate and zirconium nitrate dihydrate were each weighed out.
Next, 6.06 g of citric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) was weighed and dissolved in 96 mL of deionized water to obtain an aqueous citric acid solution. The precursor (metal nitrate salt) was then added to the aqueous citric acid solution at room temperature while stirring to prepare an aqueous precursor solution. The molar ratio of Ce:Zr in the aqueous precursor solution was 0.9:0.1.
After 30 minutes had elapsed, 2.15 g of ethylene glycol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) was added to the aqueous precursor solution, and the temperature was raised to 80°C.
　粘性のゲルが形成されるまで、連続して撹拌しつつ、８０℃の温度を維持した。その後、ゲルを乾燥炉へ移動させた。
　ゲルの乾燥は、１２０℃、５時間で行った。
　生成された有機および無機化合物の膨潤した塊状物を粉砕し、大気雰囲気下で、室温から４５０℃まで８℃／分の速度で昇温した後、４５０℃で４時間焼成した。その後、さらに９５０℃まで８℃／分の速度で昇温した後、９５０℃で８時間焼成した。
　最後に、焼成した塊状物を機械的に細かく粉砕して、目的とする酸素キャリア単独で構成される還元剤を得た。なお、還元剤は粒状であった。
　反応管への充填を容易にするため、または［還元剤の特性］の項で記載した圧力上昇の要件を満たすため、必要に応じて還元剤を打錠したものを試験に用いた。 The temperature was maintained at 80° C. with continuous stirring until a viscous gel was formed, after which the gel was transferred to a drying oven.
The gel was dried at 120° C. for 5 hours.
The resulting swollen mass of organic and inorganic compounds was pulverized, and the temperature was raised from room temperature to 450° C. at a rate of 8° C./min in an air atmosphere, and then calcined at 450° C. for 4 hours. Thereafter, the temperature was further raised to 950° C. at a rate of 8° C./min, and then calcined at 950° C. for 8 hours.
Finally, the fired mass was mechanically pulverized to obtain the target reducing agent composed of the oxygen carrier alone. The reducing agent was in granular form.
In order to facilitate filling into the reaction tube or to satisfy the requirement of pressure rise described in the section on [Properties of the Reducing Agent], the reducing agent was compressed into tablets for use in the tests, as necessary.
　（実施例２～６および比較例１～２）
　使用する酸素キャリアの前駆体の種類および量の変更、および／または副化合物の追加を行うこと以外は、実施例１と同様にして、酸素キャリア単独または酸素キャリアと副化合物とで構成される還元剤を製造した。なお、副化合物は、粒子の状態で前駆体水溶液に添加した。また、得られた還元剤は粒状であった。 (Examples 2 to 6 and Comparative Examples 1 and 2)
A reducing agent composed of an oxygen carrier alone or an oxygen carrier and a sub-compound was produced in the same manner as in Example 1, except for changing the type and amount of the oxygen carrier precursor used and/or adding a sub-compound. The sub-compound was added to the precursor aqueous solution in the form of particles. The resulting reducing agent was granular.
　２．測定および評価
　２－１．Ｘ線回折測定
　Ｘ線回折測定を行う前に、還元剤のサンプル調製を行った。
　まず、１００ｍｇ程度の還元剤を乳鉢に計り取り、乳棒を用いて磨り潰した。その後、還元剤を試料板の試料充填部の穴に均一に充填し、試料板の表面と還元剤による表面が同一面になるように調整した。
　Ｘ線回折測定には、Ｘ線回折装置（ＰＡＮａｎａｌｙｔｉｃａｌ製、「Ａｅｒｉｓ」）を使用し、集中法により測定を実施した。対陰極には、セラミック管球を用い、ＣｕＫαの特性Ｘ線（Ｋα１の波長（λ）＝１．５４０５６Å（０．１５４０５６ｎｍ）、Ｋα２の波長（λ）＝１．５４４３９Å（０．１５４４３９ｎｍ）、Ｋα２の比率＝０．５００００）を回折に使用した。
　回折計は、発散スリットを１／２°、発散縦制限スリットを１０ｍｍ、散乱スリットを２°、受光スリット０．１５ｍｍに設定した。ゴニオメーター半径は１４５ｍｍであった。 2. Measurement and Evaluation 2-1. X-ray Diffraction Measurement Before performing X-ray diffraction measurement, a sample of the reducing agent was prepared.
First, about 100 mg of the reducing agent was weighed out in a mortar and ground with a pestle. After that, the reducing agent was evenly 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 reducing agent were adjusted to be flush with each other.
For the X-ray diffraction measurement, an X-ray diffractometer (PANanalytical's "Aeris") was used, and the measurement was performed by the focusing method. A ceramic tube was used as the anticathode, and characteristic X-rays of CuKα (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 with a divergence slit at 1/2°, a divergence longitudinal limiting slit at 10 mm, a scattering slit at 2°, and a receiving slit at 0.15 mm. The goniometer radius was 145 mm.
　その後、管電圧４０ｋＶ、管電流を１５ｍＡの条件で、調製した還元剤のサンプルに対してＸ線を照射した。なお、ゴニオメーターの走査角度を１０°以上９０．５°以下の範囲に設定し、走査速度３．５°／分、測定ステップ０．０１に設定して測定を行った。測定は、大気中、室温で行った。測定終了後、Ｋα１およびＫα２の分離処理を経ずに得られたデータの解析を行った。
　データの解析には、ソフトウェア（ＰＡＮａｎａｌｙｔｉｃａｌ製、Ｈｉｇｈ　Ｓｃｏｒｅ）を用いた。なお、この際、ベースラインの判別が難しい場合は、前記ソフトウェアを用いて補正してもよい。回折ピークの強度が最も高い値を頂点とし、このときの最大ピーク値の１／２強度の２点間の間隔を半値全幅とした。 Thereafter, the prepared reducing agent 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 scanning angle of the goniometer to a range of 10° to 90.5°, the scanning speed to 3.5°/min, and the measurement step to 0.01. The measurement was performed in air at room temperature. After the measurement, the data obtained without separation of Kα1 and Kα2 was analyzed.
The data was analyzed using software (High Score, manufactured by PANanalytical). If it is difficult to distinguish the baseline, the software may be used to correct it. The highest intensity of the diffraction peak was taken as the apex, and the distance between the two points at half the intensity of the maximum peak value was taken as the full width at half maximum.
　２－２．ＳＴＥＭ－ＥＤＳ測定
　還元剤をエタノール超音波分散の上、グリッドに滴下し、乾燥後、ＳＴＥＭにて観察およびＥＤＳ分析を実施（マッピング→ラインプロファイル化）した。ラインプロファイルは、ＳＴＥＭ－ＥＤＳ測定の元素マッピングで観測された酸素キャリアと副化合物との界面と交わるように線を引きそのライン上のある点における各元素のカウント数をプロットした。
　・本測定で使用したＳＴＥＭ－ＥＤＳ装置のメーカーおよび型番
　　ＳＴＥＭ：日立（ＨＤ－２７００）　ＳＴＥＭ－ＥＤＳ：ＥＤＡＸ（ＴＥＡＭ）
　・パラメーター
　　加速電圧：２００ｋＶ
　　ＥＤＳマッピング分解能：２５６×２００　Ｄｗｅｌｌ：２００ 2-2. STEM-EDS Measurement The reducing agent was ultrasonically dispersed in ethanol, dropped onto a grid, and then dried. Observation and EDS analysis were performed using STEM (mapping → line profiling). For the line profile, a line was drawn so as to intersect with the interface between the oxygen carrier and the sub-compound observed in the element mapping of the STEM-EDS measurement, and the count number of each element at a certain point on the line was plotted.
・Manufacturer and model number of the STEM-EDS device used in this measurement: STEM: Hitachi (HD-2700) STEM-EDS: EDAX (TEAM)
・Parameters Acceleration voltage: 200 kV
EDS mapping resolution: 256 x 200 Dwell: 200
　２－３．粒度分布測定（湿式）
　粒度分布測定には、レーザ回折／散乱式粒子径分布測定装置（堀場製作所製、「Ｐａｒｔｉｃａ　ＬＡ－９６０」）を使用し、湿式フローセルにて実施した。
　フローセルは合成石英製、光源には半導体レーザ（６５０ｎｍ）５ｍＷおよびＬＥＤ（４０５ｎｍ）３ｍＷ、検出器にはリング状シリコンフォトダイオードを使用した。
　なお、測定は、温度２５℃、湿度２０％において、約１０ｍｇの還元剤を約１８０ｍＬの精製水（ｐＨ６．８以上７．２以下）に分散させた分散液に対して行った。
　測定は、少なくとも３回行い、各粒度分布測定によって得られた粒子径の中央値を相加平均させ平均粒径とした。 2-3. Particle size distribution measurement (wet method)
The particle size distribution was measured using a laser diffraction/scattering type particle size distribution measuring device ("Partica LA-960" manufactured by Horiba, Ltd.) in a wet flow cell.
The flow cell was made of synthetic quartz, the light source was a semiconductor laser (650 nm) of 5 mW and an LED (405 nm) of 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 obtained by dispersing about 10 mg of the reducing agent in about 180 mL of purified water (pH 6.8 to 7.2).
The measurement was carried out at least three times, and the median particle diameters obtained by each particle size distribution measurement were averaged to obtain the average particle diameter.
　２－４．酸素キャリアの電気伝導率測定
　電気伝導率を測定する前にサンプルの調整を行った。酸素キャリアを１３００℃で焼成することで焼結体を作製し、その焼結体を直方体に切り出し、Ｐｔペーストで白金電極と接続した。
　電気伝導率測定は、Ａ＆Ｄ７４６１Ｐデジモル、北斗電工ガルバノスタットＨＡ１０１を使用し、直流４端子伝導率を測定した。なお、測定温度は７００℃とし、純酸素雰囲気（Ｏ２高純度（９９．９体積％）ガス雰囲気）下、および一酸化炭素含有雰囲気（一酸化炭素を１容量％で含有するアルゴンガス雰囲気）下において行った。なお、純酸素雰囲気下で測定される電気伝導率を「σＡ」とし、一酸化炭素含有雰囲気下で測定される電気伝導率を「σＢ」とした。
　データの解析には、ソフトウェア（Ｍｉｃｒｏｓｏｆｔ製、「Ｅｘｃｅｌ」）を用いた。
　なお、副化合物を含まない還元剤の場合、酸素キャリアにみからなる還元剤をそのまま電気伝導率測定に使用し、一方、副化合物を含む還元剤の場合、前駆体水溶液に副化合物を添加することなく、上記と同様にして、別途、酸素キャリアを製造し、この酸素キャリアを電気伝導率測定に使用した。 2-4. Measurement of electrical conductivity of oxygen carrier Before measuring the electrical conductivity, the sample was adjusted. The oxygen carrier was sintered at 1300°C to produce a sintered body, which was then cut into a rectangular parallelepiped and connected to a platinum electrode with Pt paste.
The electrical conductivity was measured using an A&D 7461P digital mole and a Hokuto Denko galvanostat HA101, and the DC four-terminal conductivity was measured. The measurement temperature was 700°C, and the measurements were performed in a pure oxygen atmosphere ( high purity (99.9% by volume) O2 gas atmosphere) and in a carbon monoxide-containing atmosphere (argon gas atmosphere containing 1% by volume of carbon monoxide). The electrical conductivity measured in the pure oxygen atmosphere was designated as "σA", and the electrical conductivity measured in the carbon monoxide-containing atmosphere was designated as "σB".
Data was analyzed using software (Microsoft's "Excel").
In the case of a reducing agent that did not contain a secondary compound, the reducing agent consisting only of an oxygen carrier was used as is for the electrical conductivity measurement. On the other hand, in the case of a reducing agent that contained a secondary compound, an oxygen carrier was separately produced in the same manner as described above without adding the secondary compound to the precursor aqueous solution, and this oxygen carrier was used for the electrical conductivity measurement.
　２－５．二酸化炭素転化率測定および水素転化率測定
　固定床流通式反応装置と、反応装置に直結する四重極型質量分析計とを備える迅速触媒評価システムを用いて、以下の手順により還元剤による一酸化炭素の生成量を測定した。
　具体的には、内径６ｍｍの石英反応管を用意し、粉末状の還元剤を充填量が５００．０ｍｇになるように充填した。 2-5. Measurement of Carbon Dioxide Conversion Rate and Hydrogen Conversion Rate Measurement The amount of carbon monoxide produced by the reducing agent was measured using a rapid catalyst evaluation system equipped with a fixed-bed flow-type reactor and a quadrupole mass spectrometer directly connected to the reactor, according to the following procedure.
Specifically, a quartz reaction tube having an inner diameter of 6 mm was prepared, and a powdered reducing agent was filled in the reaction tube so that the filling amount was 500.0 mg.
　次いで、５ｍＬ／分の流量でヘリウムガスを流しつつ、１５℃／分の昇温速度で８５０℃に昇温させ、温度安定化のため同じ温度で５分間加熱した。次に、還元剤を賦活化するために、水素ガス（還元ガス）を流量５ｍＬ／分で２０分間流して還元剤の還元反応（第１プロセス）を実施して、還元剤を還元した。このとき、排出口から排出されるガスには、水蒸気が含まれていた。
　その後、ガス交換のために、ヘリウムガスを流量５ｍＬ／分で１０分間流した後、二酸化炭素ガスを流量５ｍＬ／分で２０分間流して、二酸化炭素の還元反応（第２プロセス）を実施して、二酸化炭素ガス（原料ガス）を還元した。このとき、排出口から排出される生成ガスには、一酸化炭素が含まれていた。 Next, while flowing helium gas at a flow rate of 5 mL/min, the temperature was raised to 850°C at a heating rate of 15°C/min, and heated at the same temperature for 5 minutes to stabilize the temperature. Next, in order to activate the reducing agent, hydrogen gas (reducing gas) was flowed at a flow rate of 5 mL/min for 20 minutes to carry out a reduction reaction (first process) of the reducing agent, thereby reducing the reducing agent. At this time, the gas discharged from the exhaust port contained water vapor.
Thereafter, for gas exchange, helium gas was flowed at a flow rate of 5 mL/min for 10 minutes, and then carbon dioxide gas was flowed at a flow rate of 5 mL/min for 20 minutes to carry out a reduction reaction of carbon dioxide (second process) and reduce the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the exhaust port contained carbon monoxide.
　次に、本試験のため、以下のプロセスを行った。
　まず、ガス交換のために、ヘリウムガスを流量５ｍＬ／分で１０分間流した。
　次に、水素ガス（還元ガス）を流量５ｍＬ／分で７分間流して還元剤の還元反応（第１プロセス）を実施して、還元剤を還元した。このとき、排出口から排出されるガスには、水蒸気が含まれていた。 Next, for this test, the following process was carried out.
First, for gas exchange, helium gas was flowed at a flow rate of 5 mL/min for 10 minutes.
Next, hydrogen gas (reducing gas) was flowed at a flow rate of 5 mL/min for 7 minutes to carry out a reduction reaction (first process) of the reducing agent, thereby reducing the reducing agent. At this time, the gas discharged from the exhaust port contained water vapor.
　その後、ガス交換のために、ヘリウムガスを流量５ｍＬ／分で５分間流した後、二酸化炭素ガスを流量５ｍＬ／分で７分間流して、二酸化炭素の還元反応（第２プロセス）を実施して、二酸化炭素ガス（原料ガス）を還元した。このとき、排出口から排出される生成ガスには、一酸化炭素が含まれていた。
　なお、以上のプロセスでは、いずれのガスを流す際にも、還元剤の温度を８５０℃に維持するとともに、大気圧条件で行った。 Thereafter, for gas exchange, helium gas was flowed at a flow rate of 5 mL/min for 5 minutes, and then carbon dioxide gas was flowed at a flow rate of 5 mL/min for 7 minutes to carry out a reduction reaction of carbon dioxide (second process) to reduce the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the exhaust port contained carbon monoxide.
In the above process, the temperature of the reducing agent was maintained at 850° C. and the process was carried out under atmospheric pressure conditions when any of the gases was flowed.
　転化率は、二酸化炭素ガス（原料ガス）を流通させ始めた後に、単位秒あたりの一酸化炭素の生成量が０ｍｍｏｌ以上となってから、四重極型質量分析計で検出される信号をもとに算出した次式の値が８０を下回るまでの期間計算した。この期間計算された値の平均値を二酸化炭素転化率として規定した。なお、一酸化炭素の生成量が０ｍｍｏｌ以下の領域は、転化率の計算に使用しないこととした。
　二酸化炭素転化率［％］＝（単位秒あたりの一酸化炭素の検出量）÷（単位秒あたりの一酸化炭素の検出量＋単位秒あたりの二酸化炭素の検出量）×１００
　水素転化率［％］＝（二酸化炭素転化率８０％以上の範囲における一酸化炭素の検出量［ｍｍｏｌ］）÷（水素流量［ｍＬ／ｍｉｎ］×水素の流通時間［ｍｉｎ］÷標準状態における気体１ｍｏｌの体積［ｍＬ／ｍｍｏｌ］）×１００ The conversion rate was calculated from the time when the amount of carbon monoxide generated per unit second became 0 mmol or more after the start of flowing carbon dioxide gas (feed gas) until the value of the following formula calculated based on the signal detected by a quadrupole mass spectrometer fell below 80. The average value calculated during this period was defined as the carbon dioxide conversion rate. Note that the region where the amount of carbon monoxide generated was 0 mmol or less was not used in calculating the conversion rate.
Carbon dioxide conversion rate [%] = (amount of carbon monoxide detected per unit second) ÷ (amount of carbon monoxide detected per unit second + amount of carbon dioxide detected per unit second) × 100
Hydrogen conversion rate [%] = (detected amount of carbon monoxide [mmol] in the range of carbon dioxide conversion rate of 80% or more) ÷ (hydrogen flow rate [mL/min] × hydrogen flow time [min] ÷ volume of 1 mol of gas under standard conditions [mL/mmol]) × 100
　２－６．二酸化炭素転化量相対値測定
　固定床流通式反応装置と、反応装置に直結するとＮＤＩＲ（ＳＩＣＫ製　ＧＭＳ８１０）を備える触媒評価システムを用いて、以下の手順により還元剤による一酸化炭素の生成量を測定した。
　具体的には、内径７ｍｍの石英反応管を用意し、長径３ｍｍに成型した円柱状の還元剤を充填量が９ｇ以上１６ｇ以下（充填高さ１２ｃｍ以上１５ｃｍ以下）になるように充填した。次いで、１００ｍＬ／分の流量で窒素ガスを流しつつ、１５℃／分の昇温速度で８５０℃に昇温させ、温度安定化のため同じ温度で５分間加熱した。 2-6. Measurement of relative value of carbon dioxide conversion amount Using a catalyst evaluation system equipped with a fixed-bed flow-type reactor and an NDIR (GMS810 manufactured by SICK) directly connected to the reactor, the amount of carbon monoxide produced by the reducing agent was measured by the following procedure.
Specifically, a quartz reaction tube with an inner diameter of 7 mm was prepared and filled with a cylindrical reducing agent molded to a major axis of 3 mm so that the filling amount was 9 g to 16 g (filling height: 12 cm to 15 cm). Next, while flowing nitrogen gas at a flow rate of 100 mL/min, the temperature was increased to 850° C. at a heating rate of 15° C./min, and the same temperature was heated for 5 minutes to stabilize the temperature.
　２－６－１．寿命サイクル
　次に、本試験のため、以下のプロセスを行った。
　まず、還元剤を賦活化するために、水素ガス（還元ガス）を流量１００ｍＬ／分で４分間流して還元剤の還元反応（第１プロセス）を実施して、還元剤を還元した。このとき、排出口から排出されるガスには、水蒸気が含まれていた。
　その後、二酸化炭素ガスを流量１００ｍＬ／分で４分間流して、二酸化炭素の還元反応（第２プロセス）を実施して、二酸化炭素ガス（原料ガス）を還元した。このとき、排出口から排出される生成ガスには、一酸化炭素が含まれていた。
　なお、以上のプロセスでは、いずれのガスを流す際にも、還元剤の温度を８５０℃に維持するとともに、大気圧条件で行った 2-6-1. Life cycle Next, the following process was carried out for this test.
First, in order to activate the reducing agent, hydrogen gas (reducing gas) was flowed at a flow rate of 100 mL/min for 4 minutes to carry out a reduction reaction (first process) of the reducing agent, thereby reducing the reducing agent. At this time, the gas discharged from the exhaust port contained water vapor.
Thereafter, carbon dioxide gas was flowed at a flow rate of 100 mL/min for 4 minutes to carry out a reduction reaction of carbon dioxide (second process) to reduce the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the exhaust port contained carbon monoxide.
In the above process, the temperature of the reducing agent was maintained at 850° C. during the flow of any gas, and the process was carried out under atmospheric pressure.
　以上のプロセスを１サイクルとし、１０００サイクルごとに、次の項「２－６－２．測定サイクル」で説明する方法に従って、１０００サイクルごとの一酸化炭素生成量を算出した。 The above process was considered as one cycle, and the amount of carbon monoxide produced for every 1000 cycles was calculated according to the method described in the next section, "2-6-2. Measurement cycle."
　２－６－２．測定サイクル
　まず、水素ガス（還元ガス）を流量１００ｍＬ／分で４分間流して還元剤の還元反応（第１プロセス）を実施して、還元剤を還元した。このとき、排出口から排出されるガスには、水蒸気が含まれていた。
　その後、ガス交換のために、窒素ガスを流量１００ｍＬ／分で５分間流した後、二酸化炭素ガスを流量１００ｍＬ／分で４分間流して、二酸化炭素の還元反応（第２プロセス）を実施して、二酸化炭素ガス（原料ガス）を還元した。このとき、排出口から排出される生成ガスには、一酸化炭素が含まれていた。
　なお、以上のプロセスでは、いずれのガスを流す際にも、還元剤の温度を８５０℃に維持するとともに、大気圧条件で行った。 2-6-2. Measurement cycle First, hydrogen gas (reducing gas) was flowed at a flow rate of 100 mL/min for 4 minutes to carry out the reduction reaction of the reducing agent (first process), thereby reducing the reducing agent. At this time, the gas discharged from the exhaust port contained water vapor.
Thereafter, for gas exchange, nitrogen gas was passed at a flow rate of 100 mL/min for 5 minutes, and then carbon dioxide gas was passed at a flow rate of 100 mL/min for 4 minutes to carry out a reduction reaction of carbon dioxide (second process) and reduce the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the exhaust port contained carbon monoxide.
In the above process, the temperature of the reducing agent was maintained at 850° C. and the process was carried out under atmospheric pressure conditions when any of the gases was flowed.
　各サイクルにおける一酸化炭素生成量は、二酸化炭素ガス（原料ガス）を流通させ始めた後、ＮＤＩＲ（非分散型赤外線）で検出された一酸化炭素の濃度が２．０％以上となってから測量を開始した。ＮＤＩＲで検出される一酸化炭素濃度を５．０秒ごとに記録し、この期間に記録された一酸化炭素濃度から区分求積法により一酸化炭素生成量を算出した。なお、一酸化炭素の濃度が２．０％未満の領域は、一酸化炭素生成量の計算に使用しないこととした
　また、二酸化炭素転化量相対値については、次式に基づいて算出した。
　二酸化炭素転化量相対値＝（各サイクルにおける一酸化炭素生成量）÷（１サイクル目における一酸化炭素生成量）×１００ The amount of carbon monoxide produced in each cycle was measured when the carbon monoxide concentration detected by NDIR (non-dispersive infrared) reached 2.0% or more after carbon dioxide gas (raw material gas) was started to flow. The carbon monoxide concentration detected by NDIR was recorded every 5.0 seconds, and the amount of carbon monoxide produced was calculated from the carbon monoxide concentration recorded during this period by quadrature by division. Note that the area where the carbon monoxide concentration was less than 2.0% was not used in the calculation of the amount of carbon monoxide produced. The relative value of the carbon dioxide conversion amount was calculated based on the following formula.
Relative value of carbon dioxide conversion amount=(amount of carbon monoxide produced in each cycle)÷(amount of carbon monoxide produced in the first cycle)×100
　これらの結果を、以下の表１に示す。なお、評価の欄の◎は最も好ましいことを示し、〇は好ましいこと示し、×は好ましくなかったことを示している。
Figure JPOXMLDOC01-appb-T000001
 The results are shown in the following Table 1. In the evaluation column, ⊚ indicates most preferable, ◯ indicates preferable, and × indicates unpreferable.
Figure JPOXMLDOC01-appb-T000001
　なお、各実施例で得られた還元剤のＸ線回折プロファイルにおいて観察される全てのピークの半値全幅は、１．５°未満である。これに対して、各比較例で得られた還元剤のＸ線回折プロファイルにおいて観察される少なくとも１本のピークの半値全幅は、１．５°以上である。
　実施例２、４、５、６及び比較例２で得られた還元剤のＸ線回折プロファイルを、それぞれ図４～図８に示す。
　また、各実施例で得られた還元剤のＸ線回折プロファイルにおいて観察される酸素キャリアに由来するピークおよび副化合物に由来するピークは、それぞれ酸素キャリア単独および副化合物単独に対してＸ線回折測定を行ったときのＸ線回折プロファイルにおいて観察されるピークに対して、２θで０．３°未満シフトしている。
　さらに、実施例４及び６で得られた還元剤に対してＳＴＥＭ－ＥＤＳ測定を行うと、図１に示す画像と同様の画像が得られる。 The full width at half maximum of all peaks observed in the X-ray diffraction profile of the reducing agent obtained in each Example is less than 1.5°. In contrast, the full width at half maximum of at least one peak observed in the X-ray diffraction profile of the reducing agent obtained in each Comparative Example is 1.5° or more.
The X-ray diffraction profiles of the reducing agents obtained in Examples 2, 4, 5, and 6 and Comparative Example 2 are shown in FIGS. 4 to 8, respectively.
In addition, the peaks derived from the oxygen carrier and the peaks derived from the minor compounds observed in the X-ray diffraction profiles of the reducing agents obtained in each Example are shifted by less than 0.3° in 2θ relative to the peaks observed in the X-ray diffraction profiles when X-ray diffraction measurements are performed on the oxygen carrier alone and the minor compounds alone, respectively.
Furthermore, when the reducing agents obtained in Examples 4 and 6 are subjected to STEM-EDS measurement, images similar to those shown in FIG. 1 are obtained.

Claims (9)

  1. 　二酸化炭素を還元して炭素有価物を生成する還元剤であって、
    　ランタノイドおよびアクチノイドを含む周期表の第３族、もしくは第４族に属する金属元素のうちの少なくとも２種を含む酸素イオン伝導性を備える酸素キャリアを含有し、
    　前記酸素キャリアは、７００℃の温度において、純酸素雰囲気下で１×１０－７Ｓ／ｃｍ以上の電気伝導率を有し、かつ当該還元剤に対して測定範囲１０°以上９０．５°以下の範囲におけるＸ線回折測定を行ったとき、Ｘ線回折プロファイルにおいて観察される全てのピークの半値全幅が１．５°未満である、還元剤。     A reducing agent that reduces carbon dioxide to produce carbon values,
    An oxygen carrier having oxygen ion conductivity, which contains at least two metal elements belonging to Group 3 or Group 4 of the periodic table, including lanthanides and actinides;
    The oxygen carrier has an electrical conductivity of 1 x 10 -7 S/cm or more in a pure oxygen atmosphere at a temperature of 700°C, and when the reducing agent is subjected to X-ray diffraction measurement in a measurement range of 10° to 90.5°, the full width at half maximum of all peaks observed in the X-ray diffraction profile is less than 1.5°.
  2. 　請求項１に記載の還元剤において、
    　さらに、周期表の第６族～第１４族に属する元素を含み、
    　前記酸素キャリアは、７００℃の温度において、純酸素雰囲気下での電気伝導率をσＡ［Ｓ／ｃｍ］とし、一酸化炭素を１容量％で含有するアルゴンガス雰囲気下での電気伝導率をσＢ［Ｓ／ｃｍ］としたとき、σＢ／σＡが８５未満である、還元剤。     The reducing agent according to claim 1 ,
    Further, the present invention includes an element belonging to Groups 6 to 14 of the periodic table,
    The oxygen carrier is a reducing agent in which, at a temperature of 700°C, when the electrical conductivity in a pure oxygen atmosphere is σA [S/cm] and the electrical conductivity in an argon gas atmosphere containing 1 volume % of carbon monoxide is σB [S/cm], the ratio σB/σA is less than 85.
  3. 　請求項２に記載の還元剤において、
    　さらに、周期表の第６族～第１４族に属する元素を含む副化合物を含有し、
    　当該還元剤に対して走査透過型電子顕微鏡－エネルギー分散型Ｘ線分光（ＳＴＥＭ－ＥＤＳ）測定を行ったとき、前記酸素キャリアと前記副化合物とが個別に観察され、かつ前記副化合物は、粒子の形態で前記酸素キャリアと接している、還元剤。     The reducing agent according to claim 2,
    Further, it contains a sub-compound containing an element belonging to Groups 6 to 14 of the periodic table,
    A reducing agent, wherein when the reducing agent is subjected to a scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDS) measurement, the oxygen carrier and the sub-compound are observed separately, and the sub-compound is in the form of particles and in contact with the oxygen carrier.
  4. 　請求項３に記載の還元剤において、
    　当該還元剤に対してＸ線回折測定を行ったとき、Ｘ線回折プロファイルにおいて観察される前記酸素キャリアに由来するピークおよび前記副化合物に由来するピークは、それぞれ前記酸素キャリア単独および前記副化合物単独に対してＸ線回折測定を行ったときのＸ線回折プロファイルにおいて観察されるピークに対して、２θで０．３°未満シフトしている、還元剤。     The reducing agent according to claim 3,
    A reducing agent, wherein, when X-ray diffraction measurement is performed on the reducing agent, a peak derived from the oxygen carrier and a peak derived from the minor compound observed in the X-ray diffraction profile are shifted by less than 0.3° in 2θ relative to the peaks observed in the X-ray diffraction profiles when X-ray diffraction measurement is performed on the oxygen carrier alone and the minor compound alone, respectively.
  5. 　請求項３又は請求項４に記載の還元剤において、
    　前記副化合物の粒子の平均粒径は、１ｎｍ以上１０μｍ以下である、還元剤。     The reducing agent according to claim 3 or 4,
    A reducing agent, wherein the average particle size of the sub-compound particles is 1 nm or more and 10 μm or less.
  6. 　請求項１～請求項５のいずれか１項に記載の還元剤において、
    　前記二酸化炭素との反応は、６５０℃を上回る温度で行われる、還元剤。     The reducing agent according to any one of claims 1 to 5,
    The reaction with carbon dioxide is carried out at a temperature above 650° C.
  7. 　請求項１～請求項６のいずれか１項に記載の還元剤において、
    　前記二酸化炭素により酸化された前記還元剤は、水素を含む還元ガスにより還元される、還元剤。     The reducing agent according to any one of claims 1 to 6,
    The reducing agent oxidized by the carbon dioxide is reduced by a reducing gas containing hydrogen.
  8. 　請求項７に記載の還元剤において、
    　前記水素を含む還元ガスとの反応は、６５０℃を上回る温度で行われる、還元剤。     The reducing agent according to claim 7,
    The reaction with the hydrogen-containing reducing gas is carried out at a temperature above 650° C.
  9. 　二酸化炭素を含む原料ガスに還元剤を作用させることにより、前記二酸化炭素を還元して炭素有価物を含む生成ガスを製造するガスの製造方法であって、
    　前記還元剤は、ランタノイドおよびアクチノイドを含む周期表の第３族、もしくは第４族に属する金属元素のうちの少なくとも２種を含み、酸素イオン伝導性を備える酸素キャリアを含有し、
    　前記酸素キャリアは、７００℃の温度において、純酸素雰囲気下で１×１０－７Ｓ／ｃｍ以上の電気伝導率を有し、かつ当該還元剤に対して測定範囲１０°以上９０．５°以下の範囲におけるＸ線回折測定を行ったとき、Ｘ線回折プロファイルにおいて観察される全てのピークの半値全幅が１．５°未満である、ガスの製造方法。     A gas production method for producing a product gas containing carbon valuables by reducing carbon dioxide by reacting a reducing agent with a raw material gas containing carbon dioxide, the method comprising the steps of:
    the reducing agent contains at least two metal elements belonging to Group 3 or Group 4 of the periodic table, including lanthanides and actinides, and contains an oxygen carrier having oxygen ion conductivity;
    The oxygen carrier has an electrical conductivity of 1 x 10 -7 S/cm or more in a pure oxygen atmosphere at a temperature of 700°C, and when X-ray diffraction measurement is performed on the reducing agent in a measurement range of 10° to 90.5°, the full width at half maximum of all peaks observed in the X-ray diffraction profile is less than 1.5°.
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