Reducing agent, and gas production method

Abstract

[Problem] To provide a reducing agent which can be used in a reaction at a high temperature, and a gas production method in which the reducing agent is used. [Solution] According to one aspect of the present invention, provided is a reducing agent that reduces carbon dioxide through contact to produce a valuable carbon substance. The reducing agent is composed of cerium (Ce) and a metal oxide containing a transition element other than cerium (Ce), and contains an oxygen carrier having oxygen ion conductivity. When the oxygen carrier is subjected to an X-ray diffraction measurement, a peak position of at least one diffraction peak corresponding to the (220) plane in the X-ray diffraction profile is shifted relative to a peak position of the diffraction peak corresponding to the (220) plane of a cerium oxide (CeO2).

Description

Method for producing reducing agent and gas

[0001] The present invention relates to a reducing agent and a method for producing a gas, and more particularly to a reducing agent that can be used in, for example, a chemical looping method, and a method for producing a gas using such a reducing agent.

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

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

[0004] In the chemical looping method, because the substrates for the reverse reaction, water and carbon monoxide, 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. Conventionally, in the chemical looping method, the reaction of converting carbon dioxide to carbon monoxide via oxygen vacancies formed in the oxygen carrier is carried out at a temperature of 650°C or less (see Patent Document 1).

[0005] Ind. Eng. Chem. Res. 2013, 52, 8416-8426

[0006] According to the study of the present inventors, since the reaction that generates oxygen vacancies in the crystal structure of the oxygen carrier by hydrogen is an endothermic reaction, it was found that in order to efficiently generate the oxygen vacancies, it is better to carry out the reaction at a higher temperature. However, up to now, there has been insufficient research into the high temperature range (temperatures above 650°C) of the reaction that generates oxygen vacancies. In view of the above circumstances, the present invention provides a reducing agent that can be used in the reaction at high temperatures, and a method for producing a gas using such a reducing agent.

[0007] According to one aspect of the present invention, there is provided a reducing agent that reduces carbon dioxide by contact to produce carbon valuables. This reducing agent contains an oxygen carrier that is composed of a metal oxide containing cerium (Ce) and a transition element other than cerium (Ce) and has oxygen ion conductivity. When X-ray diffraction measurement of the oxygen carrier is performed, the peak position of at least one diffraction peak corresponding to the (220) plane in the X-ray diffraction profile is located at a position corresponding to the cerium oxide (CeO ２ ) is shifted relative to the peak position of the diffraction peak corresponding to the (220) plane.

[0008] According to this embodiment, a reducing agent that can be used in reactions at high temperatures can be obtained.

[0009] The reducing agent and gas production method of the present invention will be described in detail below based on preferred embodiments. [Reducing Agent] The reducing agent of the present invention is used when a feed gas containing carbon dioxide is brought into contact with the reducing agent to reduce carbon dioxide and produce a product gas containing carbon monoxide (a carbon valuable) (i.e., it is used in the gas production method of the present invention). Furthermore, the reducing agent oxidized by contact with carbon dioxide can be reduced (regenerated) by contact with a reducing gas containing hydrogen (a reducing substance). In this case, preferably, the feed gas and the reducing gas are passed alternately through a reaction tube (reaction vessel) filled with the reducing agent of the present invention, whereby the carbon dioxide is converted to carbon monoxide by the reducing agent and the oxidized reducing agent is regenerated by the reducing gas.

[0010] The reducing agent of the present invention contains an oxygen carrier having oxygen ion conductivity. Here, the oxygen carrier refers to a compound that can generate reversible oxygen deficiency, that is, a compound that loses oxygen element by reduction itself, and that, when contacted with carbon dioxide in an oxygen-deficient state (reduced state), exhibits the action of depriving carbon dioxide of oxygen element and reducing it. The oxygen carrier of the present invention is composed of a metal oxide containing cerium (Ce) and a transition element other than cerium (Ce). In such an oxygen carrier, the synergistic effect of Ce and the transition element promotes the production of carbon monoxide from carbon dioxide.

[0011] In the present invention, when X-ray diffraction measurement of the oxygen carrier is performed, the peak position of at least one diffraction peak corresponding to the (220) plane in the X-ray diffraction profile is determined to be the same as that of cerium oxide (CeO ２ ) is shifted relative to the peak position of a diffraction peak corresponding to the (220) plane of cerium oxide. That is, when X-ray diffraction measurements are performed on the oxygen carrier and cerium oxide under the same conditions, the peak position of at least one diffraction peak corresponding to the (220) plane of the oxygen carrier in the X-ray diffraction profile is shifted relative to the peak position of a diffraction peak corresponding to the (220) plane of cerium oxide. The timing of the X-ray diffraction measurement is before the reducing agent is used in a reaction with a gas, i.e., immediately after the production of the reducing agent.

[0012] Thus, when a peak shift occurs in a specific diffraction peak corresponding to the (220) plane in an X-ray diffraction profile, this indicates that transition elements other than Ce are sufficiently dissolved in the cerium oxide crystal. By sufficiently dissolving transition elements other than Ce in the cerium oxide crystal, the crystal structure of the cerium oxide (oxygen carrier) can be appropriately distorted. This increases the mobility of oxygen elements (oxygen ions) within the oxygen carrier. Therefore, it is believed that the oxygen carrier (reducing agent) efficiently releases oxygen elements upon contact with a reducing gas, smoothly inducing oxygen vacancies, and the induced oxygen vacancies facilitate the removal of oxygen elements from carbon dioxide.

[0013] It is also believed that the crystalline structure of cerium oxide is stabilized by the presence of a transition element other than Ce in solid solution, thereby improving the heat resistance temperature. As a result, the reducing agent of the present invention can be used at high temperatures exceeding 650°C. Furthermore, when a transition element other than Ce is solid-solved in cerium oxide crystal, it is believed that the crystalline structure is more stable when used at temperatures above 650°C than when used at temperatures below 650°C. At high temperatures above 650°C, Ce alloys with transition metals and tends to form a stable crystalline structure, making it easier to maintain the initial activity even when the reducing agent is repeatedly redox-treated.

[0014] The diffraction peak corresponding to the (220) plane of the oxygen carrier is due to cerium oxide (CeO ２ The diffraction peak corresponding to the (220) plane of the oxygen carrier may be shifted either to a higher angle or a lower angle. Whether the diffraction peak corresponding to the (220) plane of the oxygen carrier is shifted to a higher angle or a lower angle, or the amount of the shift, is thought to be affected by, for example, the conditions for producing the reducing agent (particularly, the grinding time, the calcination temperature, and the calcination time). ２The shift amount of the diffraction peak corresponding to the (220) plane of the oxygen carrier relative to the diffraction peak corresponding to the (220) plane of the oxygen carrier is preferably 0.1° or more and less than 0.6° in 2θ. The lower limit of the peak shift amount may be 0.15° or more, 0.2° or more, 0.25° or more, 0.3° or more, 0.35° or more, or 0.4° or more in 2θ. On the other hand, the upper limit of the peak shift amount may be 0.55° or less in 2θ. The lower and upper limits can be arbitrarily combined to define a preferred range of the peak shift amount. By setting the peak shift amount within the above range, it can be considered that transition elements other than Ce are more sufficiently and reliably solid-dissolved in the cerium oxide crystal. Note that if the peak shift amount is too large, the fluorite structure, which is the crystalline structure of cerium oxide, may not be maintained.

[0015] In the X-ray diffraction profile, the specific diffraction peak corresponding to the (220) plane of the oxygen carrier preferably exists in the range of 2θ of 44° to 52°, more preferably in the range of 2θ of 46° to 50°, even more preferably in the range of 2θ of 47° to 49°, and particularly preferably in the range of 47.6° to 48°. This specific diffraction peak corresponds to the diffraction peak corresponding to the (220) plane of the cerium oxide crystal in which the transition element is dissolved. The diffraction peak (particularly the specific diffraction peak) corresponding to the (220) plane of the oxygen carrier preferably has a full width at half maximum of 0.3 or more, more preferably 0.35 or more, and even more preferably 0.4 or more. The full width at half maximum is preferably 0.7 or less, more preferably 0.65 or less. This means that the increase in the crystallite size of the oxygen carrier is suppressed, in other words, the crystallite size of the oxygen carrier is maintained relatively small. In an oxygen carrier with a relatively small crystallite size, the distance traveled by oxygen elements is shortened, and therefore both the reduction reaction of the reducing agent by the reducing gas (reaction inducing oxygen deficiency) and the reduction reaction of carbon dioxide by the reducing agent can be promoted.

[0016] The specific value of the crystallite size of the oxygen carrier is preferably 320 Å or less, more preferably 300 Å or less, even more preferably 280 Å or less, particularly preferably 270 Å or less, and most preferably 260 Å or less. An oxygen carrier with such a small crystallite size is preferable because the migration distance of oxygen elements is shorter. The lower limit of the crystallite size is not particularly limited, but is preferably 210 Å or more, more preferably 220 Å or more, even more preferably 230 Å or more, and particularly preferably 250 Å or more. Reducing agents containing oxygen carriers with crystallite sizes below the above lower limit tend to be difficult to manufacture. The lower limit and upper limit can be arbitrarily combined to define a preferred range of the crystallite size of the oxygen carrier. According to the studies of the present inventors, it has been found that, in addition to the absolute values ​​of the full width at half maximum and crystallite size, there is a high correlation between the ratio of the peak intensity to the full width at half maximum (peak intensity / full width at half maximum) of the diffraction peak (particularly a specific diffraction peak) corresponding to the (220) plane of the oxygen carrier and the reactivity of the reducing agent. The specific value of this peak intensity / full width at half maximum is preferably 6.2 or less, more preferably 6 or less, even more preferably 5.5 or 5 or less, particularly preferably 4.5 or less, and most preferably 4 or less. The lower limit of the peak intensity / full width at half maximum is preferably 2.5 or more, more preferably 3 or more. If the value of the peak intensity / full width at half maximum is less than the above lower limit, the crystallinity of the oxygen carrier may be too low, making it difficult for oxygen elements to migrate. Here, the specific diffraction peak is a peak having a maximum intensity observed in the X-ray diffraction profile in the range of 2θ of 45° to 49°. For this peak, the ratio of the peak intensity to the full width at half maximum (peak intensity / full width at half maximum) is defined.

[0017] The oxygen carrier contains a transition element other than Ce. By including an additional transition element in the oxygen carrier, its crystal structure can be appropriately distorted. As a result, oxygen elements can enter and leave the oxygen carrier more smoothly. The ratio (molar ratio) of the molar amount of cerium to the total molar amount of metal elements contained in the oxygen carrier (metal oxide) is not particularly limited, but is preferably 0.6 or more, more preferably 0.65 or more, and even more preferably 0.7 or more. The upper limit of the molar ratio is usually 0.98 or less.

[0018] The transition element is preferably at least one of elements belonging to the fourth and fifth periods of the periodic table. Since the ionic radius of such elements is relatively close to that of Ce, it is possible to prevent or suppress instability of the oxygen carrier crystal due to the difference in ionic radius. Specific examples of transition elements include scandium (Sc), vanadium (V), chromium (Cr), iron (Fe), manganese (Mn), nickel (Ni), cobalt (Co), copper (Cu), zinc (Zn), zirconium (Zr), niobium (Nb), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), hafnium (Hf), tungsten (W), and titanium (Ti). These transition elements can be used alone or in combination of two or more. Among these, the transition element is preferably one of Fe, Mn, and Zr, and more preferably Fe and / or Zr. Since these transition elements have an ionic radius smaller than that of cerium, they can distort the crystal structure of the oxygen carrier more appropriately, thereby further improving the above-mentioned effect. Note that the oxygen carrier can also contain elements other than the transition elements in addition to the above-mentioned transition elements.

[0019] The reducing agent of the present invention may be composed of an oxygen carrier alone, or may contain an oxygen carrier and a binder that binds the oxygen carrier. An example of the latter embodiment is one in which oxygen carrier particles are bound by a binder (carrier). In this case, the shape retention of the reducing agent can be further improved, and the specific surface area of ​​the reducing agent can be easily adjusted. When the reducing agent contains a binder, the proportion of the binder is preferably 60 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 40 parts by mass or less, per 100 parts by mass of the reducing agent. Furthermore, the proportion of the binder may be 1 part by mass or more, 2 parts by mass or more, 3 parts by mass or more, 4 parts by mass or more, 5 parts by mass or more, 6 parts by mass or more, 7 parts by mass or more, 8 parts by mass or more, 9 parts by mass or more, or 10 parts by mass or more, per 100 parts by mass of the reducing agent. The lower and upper limits can be arbitrarily combined to define the range of the binder proportion. By setting the amount of binder contained in the reducing agent within the above range, it is possible to further improve the efficiency of converting carbon dioxide to carbon monoxide by the reducing agent while maintaining high shape retention of the reducing agent.

[0020] The binder is not particularly limited as long as it is a compound that is not easily denatured in response to contact with the raw material gas or reaction conditions, and examples thereof include inorganic materials such as oxides, nitrides, oxynitrides, and carbides, and carbon materials (graphite, graphene, etc.). Among these, the binder is preferably an oxide, more preferably an oxide containing at least one of magnesium (Mg), titanium (Ti), zirconium (Zr), aluminum (Al), and silicon (Si), and even more preferably aluminum oxide. These oxides are preferred because they have high thermal stability and can easily stably bind oxygen carrier microparticles.

[0021] 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 this packing density is too low, the gas passage rate becomes too fast, and the contact time between the reducing agent and the raw material gas and the reducing gas is reduced. As a result, the efficiency of converting carbon dioxide to carbon monoxide by the reducing agent and the efficiency of regenerating the oxidized reducing agent by the reducing gas tend to decrease. On the other hand, if this packing density is too high, the gas passage rate becomes too slow, making it difficult for the reaction to proceed and requiring a long time to produce the product gas.

[0022] The pore volume of the reducing agent is 0.1 cm ３ / g or more, and ３ / g or more 30cm ３ / g or less, and more preferably 5 cm ３ / g or more 20cm ３ / g or less is even more preferable. If this pore volume is too small, it becomes difficult for the raw material gas and reducing gas to penetrate deep into the reducing agent. As a result, the contact area between the reducing agent and the raw material gas and reducing gas decreases, and the efficiency of converting carbon dioxide to carbon monoxide by the reducing agent and the efficiency of regenerating the oxidized reducing agent by the reducing gas tend to decrease. On the other hand, even if the pore volume is increased beyond this upper limit, no further increase in effect can be expected, and depending on the type of reducing agent, mechanical strength tends to decrease.

[0023] The shape of the reducing agent is not particularly limited, but is preferably granular, for example. Granular reduces the packing density within the above range. Here, granular is a concept that includes powder, particles, lumps, pellets, 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. A reducing agent having such an average particle size can easily adjust its packing density within the above range.

[0024] In this specification, the term "average particle size" refers to the average value of the particle sizes of 200 randomly selected reducing agents in one field of view observed under an electron microscope. In this case, "particle size" refers to the maximum length of the distance between two points on the contour line of the reducing agent. When the reducing agent is columnar, the maximum length of the distance between two points on the contour line of its end face is taken as the "particle size." Furthermore, when the reducing agent is, for example, in a block shape, and primary particles are aggregated, the average particle size refers to the average particle size of secondary particles. The BET specific surface area of ​​the reducing agent is 1 m ２ / g or more 500m ２ / g or less, and ２ / g or more 450m ２ / g or less is more preferable, and 5m ２ / g or more 400m ２ When the BET specific surface area is within the above range, it becomes easier to improve the efficiency of converting carbon dioxide to carbon monoxide by the reducing agent.

[0025] Furthermore, in the present invention, because a transition element other than Ce is stably dissolved in the oxygen carrier (cerium oxide), the oxygen capacity of the reducing agent can be maintained at a high level over a wide temperature range from low temperatures (approximately 400°C) to high temperatures (approximately 850°C). In other words, the reducing agent of the present invention 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 to 40 mass%, and more preferably 0.5 to 30 mass%. If the oxygen capacity of the oxygen carrier contained in the reducing agent at low temperatures is within the above range, this means that the oxygen capacity is sufficiently high even at temperatures during actual operation (temperatures above 650°C), and it can be said that the reducing agent has an extremely high efficiency of converting carbon dioxide to carbon monoxide.

[0026] [Method for Producing Reducing Agent] Next, a method for producing a reducing agent will be described. The method for producing a reducing agent is not particularly limited, but examples thereof include a sol-gel method, a coprecipitation method, a solid-phase method, and a hydrothermal synthesis method. For example, the reducing agent can be produced as follows: First, a salt of a metal element constituting the reducing agent (which may contain a salt of an element constituting the inorganic binder when the inorganic binder is used) is dissolved in water to prepare an aqueous solution. Next, this aqueous solution is gelled, dried, and baked. That is, the reducing agent of the present invention can be produced easily and reliably by the so-called sol-gel method. The aqueous solution may be prepared using acidic water adjusted to an acidic state with, for example, citric acid, acetic acid, malic acid, tartaric acid, hydrochloric acid, nitric acid, or a mixture thereof.

[0027] Examples of salts of metal elements include nitrates, sulfates, chlorides, hydroxides, carbonates, and composites thereof, with nitrates being preferred. Furthermore, hydrates of metal element salts may be used as needed. The gel is dried at a temperature of preferably 20°C or higher and 200°C or lower, more preferably 50°C or higher and 150°C or lower, for a time of preferably 0.5 hours or higher and 20 hours or lower, more preferably 1 hour or higher and 15 hours or lower. By drying in this manner, the gel can be dried uniformly.

[0028] 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 preferably 1 hour to 24 hours, more preferably 1.5 hours to 20 hours. The gel is preferably converted into an oxide by firing, but firing under the above firing conditions can easily convert it into a reducing agent. Furthermore, firing under the above firing conditions can prevent excessive particle growth of the reducing agent. The temperature should be increased at a rate of 1°C / min to 20°C / min, preferably 2°C / min to 10°C / min, until the firing temperature is reached. This promotes particle growth of the reducing agent and also prevents cracking of the crystals (particles).

[0029] The reducing agent can also be produced as follows. First, oxides containing each of the metal elements constituting the reducing agent are mixed and pulverized. For pulverization, for example, a ball mill, bead mill, jet mill, hammer mill, etc. can be used. This pulverization can be performed either dry or wet. If a binder is used, it is mixed with the oxide. In addition to the inorganic binders listed above, organic binders (organic binders) can also be used as the binder. Examples of organic substances include various resins such as olefin resins, acrylic resins, styrene resins, ester resins, ether resins, and vinyl resins, as well as various waxes and fatty acids. In this case, the oxides can be produced using the sol-gel method, coprecipitation method, solid-phase method, hydrothermal synthesis method, etc. Next, the pulverized aggregates are pulverized and then calcined. This firing is preferably carried out at a temperature of 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. In this case, part or all of the organic binder may be lost by firing. The fired mass is then pulverized in the same manner as above to obtain the reducing agent.

[0030] [Method of Using the Reducing Agent] As described above, the reducing agent of the present invention can be used, for example, in a chemical looping method. Furthermore, as described above, the reducing agent of the present invention can be used in applications in which carbon dioxide is reduced by contact to produce carbon monoxide (carbon valuables). More specifically, a reduction reaction of carbon dioxide and a reduction reaction of the reducing agent are preferably carried out, and the reducing agent is preferably used so as to circulate between the reduction reaction of carbon dioxide and the reduction reaction of the reducing agent. In the reduction reaction of the reducing agent, a reducing gas containing another reducing substance is used.

[0031] The reducing agent of the present invention 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 to the reverse water gas shift reaction, the reaction is carried out separately into a reduction reaction of the reducing agent (first process) and a reduction reaction of carbon dioxide (second process). The reduction reaction of the reducing agent is the reaction shown in formula (A) below, and the reduction reaction of carbon dioxide is the reaction shown in formula (B) below.

[0032] H ２ (Gas) + MO ｘ (Solid) → H ２ O (gas) + MO ｘ－１ (solid) 　 (A) CO ２ (Gas) + MO ｘ－１ (solid) → CO (gas) + MO ｘ (Solid) (B) In formulas (A) and (B), x is usually 2. That is, in the reduction reaction of the reducing agent, hydrogen, which is a type of reducing substance, is oxidized to produce water. Also, in the reduction reaction of carbon dioxide, carbon dioxide is reduced to produce carbon monoxide, which is a type of carbon valuable.

[0033] 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 above 650°C, more preferably 700°C or higher, even more preferably 750°C or higher, and particularly preferably 800°C or higher. Within this temperature range, the reduction reaction of the reducing agent can proceed efficiently. The upper limit of this reaction temperature is preferably 1050°C or lower, more preferably 1000°C or lower, even more preferably 950°C or lower, particularly preferably 900°C or lower, and most preferably 850°C or lower. Setting the upper limit of the reaction temperature within the above range can improve economic efficiency. The lower limit and upper limit can be combined in any way to define the reaction temperature range.

[0034] Furthermore, during the reduction reaction of the reducing agent, the amount of hydrogen (reducing gas) contacted with the oxidized reducing agent is preferably 1 mmol to 50 mmol, more preferably 2.5 mmol to 35 mmol, and even more preferably 5 mmol to 20 mmol, per 1 g of reducing agent. The reducing agent of the present invention has a high hydrogen utilization rate because oxygen element can be smoothly introduced and removed. Therefore, the reducing agent of the present invention is sufficiently reduced (regenerated) with a small amount of hydrogen. This reduces the energy required to generate hydrogen, and ultimately contributes to reducing carbon dioxide generated during energy generation. The hydrogen utilization rate (%) is the ratio, expressed as a percentage, of the amount of carbon monoxide produced (number of moles) to the amount of hydrogen input (number of moles) contacted with 1 g of reducing agent.

[0035] Furthermore, during the reduction reaction of the reducing agent, contact with a reducing gas containing hydrogen causes a reversible oxygen deficiency in the reducing agent at a predetermined rate. This specific rate is preferably 1.6% or more, more preferably 2% or more, and even more preferably 2.5% or more, relative to the mass of the oxygen carrier contained in the reducing agent. By generating oxygen deficiencies at such a rate, the conversion of carbon dioxide to carbon monoxide can be further promoted. The upper limit of the oxygen deficiency is not particularly limited, but is usually about 10%. The oxygen deficiency can be determined by the method described in the Examples below.

[0036] Furthermore, the reaction temperature (contact temperature of the reducing agent with carbon dioxide) in the carbon dioxide reduction reaction is preferably above 650°C, more preferably 700°C or higher, even more preferably 750°C or higher, and particularly preferably 800°C or higher. Within this temperature range, the carbon dioxide reduction reaction can proceed efficiently. The upper limit of this reaction temperature is preferably 1050°C or lower, more preferably 1000°C or lower, even more preferably 950°C or lower, particularly preferably 900°C or lower, and most preferably 850°C or lower. Since the reducing agent can efficiently reduce carbon dioxide to carbon monoxide even at low temperatures, the carbon dioxide reduction reaction can be set to a relatively low temperature. Furthermore, setting the upper limit of the reaction temperature within the above range not only facilitates waste heat utilization but also further improves economic efficiency. The lower limit and upper limit can be combined in any way to define the reaction temperature range.

[0037] Furthermore, 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, more preferably 2.5 mmol to 30 mmol, and even more preferably 5 mmol to 20 mmol, per 1 g of reducing agent. The reducing agent of the present invention allows smooth inflow and outflow of oxygen elements. Therefore, the reducing agent of the present invention has a high efficiency of converting carbon dioxide to carbon monoxide (i.e., a large amount of carbon monoxide produced), and from this perspective, also contributes to the reduction of carbon dioxide. On the other hand, since the reduction reaction is efficiently carried out using a reducing gas containing hydrogen, the reducing agent can be regenerated with a small amount of hydrogen. The amount of carbon monoxide produced by the reducing agent of the present invention is preferably about 0.3 mmol to 1 mmol per 1 g of reducing agent.

[0038] In the present invention, the reduction product (carbon valuable product) obtained by the reduction reaction of carbon dioxide contains carbon monoxide, but may also contain substances other than carbon monoxide, or may be a mixture of carbon monoxide and other substances. Specific examples of other substances include methane. Preferably, the reduction products, such as carbon monoxide, obtained by the reduction reaction of carbon dioxide are further converted into organic substances, etc., by microbial fermentation or the like. Examples of microbial fermentation include anaerobic fermentation. Examples of the resulting organic substances include methanol, ethanol, acetic acid, butanol, derivatives thereof, mixtures thereof, and C5 or higher compounds such as isoprene. Furthermore, reduction products such as carbon monoxide may be converted into C1 to C20 compounds, including hydrocarbons and alcohols conventionally synthesized by petrochemical methods, using metal oxides or the like. Specific examples of the resulting compounds include methane, ethane, propylene, methanol, ethanol, propanol, acetaldehyde, diethyl ether, acetic acid, butyric acid, diethyl carbonate, and butadiene.

[0039] [Characteristics of the Reducing Agent] The reducing agent of the present invention preferably has the following characteristics. That is, when a stainless steel reaction tube having an inner diameter of 8 mm and a pressure gauge disposed in the flow path is filled with the reducing agent to a height of 40 cm and nitrogen gas having a concentration of 100% by volume is passed through at a rate of 30 mL / min, the pressure increase over 10 minutes is preferably 0.03 MPaG or less, and more preferably 0.01 MPaG or less. A reducing agent exhibiting such characteristics can be determined to have a packing density and pore volume that satisfy the above ranges, and can sufficiently increase the efficiency of conversion of carbon dioxide to carbon monoxide.

[0040] As described above, the present invention provides a reducing agent that can be used in reactions at high temperatures, and a method for producing a gas using this reducing agent. Because the reducing agent of the present invention can withstand high temperatures, it enables more efficient conversion of carbon dioxide to carbon monoxide (a carbon valuable) through a chemical looping reaction at high temperatures.

[0041] Furthermore, it may be provided in the following aspects.

[0042] (1) A reducing agent that reduces carbon dioxide by contact to generate carbon valuables, the reducing agent comprising an oxygen carrier that is composed of cerium (Ce) and a metal oxide containing a transition element other than cerium (Ce) and has oxygen ion conductivity, and when X-ray diffraction measurement of the oxygen carrier is performed, the peak position of at least one diffraction peak corresponding to the (220) plane in the X-ray diffraction profile is cerium oxide (CeO ２ ) the diffraction peak corresponding to the (220) plane of the reducing agent is shifted relative to the peak position.

[0043] (2) In the reducing agent described in (1) above, the cerium oxide (CeO ２ a shift amount of a diffraction peak corresponding to the (220) plane of the oxygen carrier relative to a diffraction peak corresponding to the (220) plane of the oxygen carrier is 0.1° or more and less than 0.6° in 2θ.

[0044] (3) The reducing agent according to (1) or (2) above, wherein the diffraction peak corresponding to the (220) plane of the oxygen carrier has a full width at half maximum of 0.3 or more.

[0045] (4) The reducing agent according to any one of (1) to (3) above, wherein the oxygen carrier has a crystallite size of 320 Å or less.

[0046] (5) The reducing agent according to any one of (1) to (4) above, wherein the ratio of the peak intensity to the full width at half maximum of the diffraction peak corresponding to the (220) plane of the oxygen carrier is 6.2 or less.

[0047] (6) The reducing agent according to any one of (1) to (5) above, wherein the transition element is at least one element belonging to the fourth and fifth periods of the periodic table.

[0048] (7) The reducing agent according to any one of (1) to (6), wherein the carbon valuable material includes carbon monoxide.

[0049] (8) The reducing agent according to any one of (1) to (7) above, wherein the contact with the carbon dioxide is carried out at a temperature exceeding 650°C.

[0050] (9) The reducing agent according to any one of (1) to (8) above, wherein the reducing agent oxidized by contact with the carbon dioxide is reduced by contact with a reducing gas containing hydrogen.

[0051] (10) The reducing agent according to (9) above, wherein the contact with the hydrogen-containing reducing gas is carried out at a temperature exceeding 650°C.

[0052] (11) The reducing agent according to (9) or (10) above, which generates a reversible oxygen deficiency of 0.45% or more relative to the mass of the oxygen carrier contained in the reducing agent when brought into contact with the reducing gas containing hydrogen.

[0053] (12) The reducing agent according to any one of (1) to (11) above, further comprising a binder that binds the oxygen carrier.

[0054] (13) The reducing agent according to (12) above, wherein the ratio of the binder contained in the reducing agent is 1 part by mass or more and 60 parts by mass or less per 100 parts by mass of the reducing agent.

[0055] (14) A gas production method for producing a product gas containing carbon valuables by bringing a raw material gas containing carbon dioxide into contact with a reducing agent to reduce the carbon dioxide, wherein the reducing agent contains an oxygen carrier that is made of a metal oxide containing cerium (Ce) and a transition element other than cerium (Ce) and has oxygen ion conductivity, and when X-ray diffraction measurement is performed on the oxygen carrier, the peak position of at least one diffraction peak corresponding to a (220) plane in an X-ray diffraction profile is determined to be cerium oxide (CeO ２ ) the diffraction peak corresponding to the (220) plane is shifted relative to the peak position of the diffraction peak corresponding to the (220) plane. Of course, this is not limited to this.

[0056] As described 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 spirit of the invention. Such embodiments and their modifications are intended to be included within the scope and spirit of the invention, as well as within the scope of the inventions and their equivalents as set forth in the claims.

[0057] For example, the reducing agent and gas manufacturing method of the present invention may have any other additional components compared to the above-mentioned embodiments, may be replaced with any components that exhibit similar functions, or may omit some components. In the above-mentioned embodiments, a gas containing hydrogen has been described as a representative reducing gas, but the reducing gas may also use a gas containing at least one selected from hydrocarbons (e.g., methane, ethane, acetylene, etc.) and ammonia as a reducing substance instead of or in addition to hydrogen.

[0058] EXAMPLES The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0059] 1. Production of Reducing Agent (Example 1) First, predetermined amounts of cerium oxide (manufactured by Kojundo Chemical Co., Ltd.), manganese (IV) oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and iron (III) oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were weighed out. Next, the weighed particles of each oxide, 100 mL of ion-exchanged water, and 4 mm diameter zirconia beads were placed in a zirconia container. After that, these were pulverized using zirconia beads at a rotation speed of 400 rpm for 24 hours and then dried at 120°C. The molar ratio of cerium oxide, manganese oxide, and iron oxide was 0.94:0.04:0.02.

[0060] The resulting oxide mass was pulverized and heated from room temperature to 450°C at a rate of 8°C / min in an air atmosphere, followed by firing at 450°C for 4 hours. The temperature was then further raised to 950°C at a rate of 8°C / min, followed by firing at 950°C for 8 hours. Finally, the fired mass was mechanically pulverized into fine particles. This resulted in the desired reducing agent composed solely of the oxygen carrier. The reducing agent was in granular form.

[0061] Example 2 A reducing agent was produced in the same manner as in Example 1, except that the pulverization time was changed from 24 hours to 20 hours.

[0062] Example 3 First, as precursors of the reducing agent, predetermined amounts of 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%), iron (III) nitrate nonahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%), and copper (II) nitrate trihydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.0%) were weighed out.

[0063] 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 a citric acid aqueous solution. The precursor (metal nitrate salt) was then added to the citric acid aqueous solution at room temperature while stirring to prepare a precursor aqueous solution. The molar ratio of Ce:Zr+Sm+Fe+Cu in the precursor aqueous solution was 0.70:0.30. After 30 minutes, 2.4 molar equivalents of ethylene glycol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) relative to the total metal salts was added to the precursor aqueous solution, and the temperature was raised to 80°C.

[0064] The temperature was maintained at 80°C with continuous stirring until a viscous gel was formed. The gel was then 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 heated from room temperature to 450°C at a rate of 8°C / min in an air atmosphere, followed by calcination at 450°C for 4 hours. The temperature was then further increased to 950°C at a rate of 8°C / min, followed by calcination at 950°C for 8 hours. Finally, the calcined mass was mechanically pulverized into fine particles. This resulted in the desired reducing agent composed solely of the oxygen carrier. The reducing agent was in granular form.

[0065] Example 4 A reducing agent was produced in the same manner as in Example 3, except that the type of reducing agent precursor was changed. As the reducing agent precursor, predetermined amounts of cerium (III) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 98.0%), zirconium nitrate dihydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 97.0%), lanthanum (III) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%), 30% titanium (IV) sulfate solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and iron (III) nitrate nonahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) were weighed out. The molar ratio of Ce:Zr+La+Ti+Fe in the precursor aqueous solution was 0.64:0.36.

[0066] Example 5 A reducing agent was produced in the same manner as in Example 3, except that the type of precursor of the reducing agent was changed. As the precursor of the reducing agent, predetermined amounts of 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%), and nickel (II) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) were each weighed out. The molar ratio of Ce:Zr:Ni in the precursor aqueous solution was 0.90:0.08:0.02.

[0067] Example 6 A reducing agent was produced in the same manner as in Example 3, except that the type of precursor of the reducing agent was changed. As the precursor of the reducing agent, predetermined amounts of 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%), and ammonium niobium oxalate (Sigma-Aldrich, purity: 99.99%) were weighed out. The molar ratio of Ce:Zr:Nb in the precursor aqueous solution was 0.90:0.08:0.02.

[0068] Example 7 A reducing agent was produced in the same manner as in Example 3, except that the type of precursor of the reducing agent was changed. As the precursor of the reducing agent, predetermined amounts of cerium (III) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 98.0%), 30% titanium (IV) sulfate solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and lanthanum (III) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) were weighed out. The molar ratio of Ce:Ti:La in the precursor aqueous solution was 0.58:0.25:0.17.

[0069] (Example 8) Aluminum oxide (Wako special grade, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was weighed as a binder and mixed with the oxygen carrier obtained in Example 3 in a mortar for 10 minutes. The mixture was then heated from room temperature to 950°C at a rate of 8°C / min in an air atmosphere, and then fired at 950°C for 8 hours. A reducing agent was thus obtained. The ratio of aluminum oxide contained in the reducing agent was 10 parts by mass relative to 100 parts by mass of the reducing agent.

[0070] Comparative Example 1 The reducing agents were obtained by compressing cerium oxide alone. The packing density of the reducing agents obtained in Examples 1 to 3 was 2 g / mL or more and 2.5 g / mL or less.

[0071] 2. Identification of the Reducing Agent The metal composition in the reducing agent (metal oxide) was analyzed and identified by ICP atomic emission spectroscopy using argon gas using a SPECTRO ARCOS manufactured by AMETEK Corporation. The measurement solution was prepared as follows: 50 to 100 mg of the reducing agent was dissolved in 100 mL of 1% nitric acid or 1% hydrofluoric acid, and the resulting solution was further diluted 10 times. The analysis results of the above measurement solution (molar ratio of metal elements) are shown in Table 1.

[0072] 3. Measurement of particle size distribution The particle size distribution of the reducing agent was measured using a particle size distribution measuring device (Horiba, Ltd., "LA-960S"). Note that water was used as the solvent, and the measurement was carried out under conditions where the transmittance was 70% or more. As a result, the volume-based 50% diameter of the reducing agent was 50 μm or less in all cases.

[0073] 4. X-ray Diffraction Measurements Prior to X-ray diffraction measurements, a reducing agent sample was prepared. First, approximately 100 mg of reducing agent was weighed into a mortar and ground using a pestle. The reducing agent was then uniformly filled into the sample-filling hole 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. X-ray diffraction measurements were performed using an X-ray diffractometer (Bruker Corporation, "D8 DISCOVER") using the focusing method. A pure copper tube was used as the anticathode, and CuKα characteristic X-rays (Kα1 wavelength (λ) = 1.54056 Å (0.154056 nm), Kα2 wavelength (λ) = 1.54439 Å (0.154439 nm), Kα2 ratio = 0.50000) were used for diffraction. The diffractometer was set to a divergence slit of 1 / 2°, a divergence longitudinal limiting slit of 10 mm, a scattering slit of 2°, and a receiving slit of 0.15 mm. The goniometer radius was 169.3 mm.

[0074] The prepared reducing agent sample was then irradiated with X-rays at a tube voltage of 40 kV and a tube current of 40 mA. The goniometer scan angle was set to a range of 5.5 to 100.5°, with a scan rate of 3.5° / min and a measurement step of 0.01. Measurements were performed in air at room temperature. After the measurements were completed, the data obtained without separation of Kα1 and Kα2 was analyzed. Software (DIFFRAC.EVA, manufactured by Bruker) was used for data analysis. The background was removed, and the highest intensity value of the diffraction peak corresponding to the (220) plane was taken as the apex, and the angle of this apex was taken as the peak position. The distance between the two points at half the intensity of this maximum peak value was then taken as the full width at half maximum. For the diffraction peak corresponding to the (220) plane, the peak intensity (unit: cps) after background removal was used to calculate the ratio of the peak intensity to the full width at half maximum.

[0075] [Method for measuring crystallite size] The crystallite size was determined by the Scherrer formula represented by the following formula (1s) based on the diffraction peak corresponding to the (220) plane in the X-ray diffraction profile measured using CuKα characteristic X-rays (Kα1 wavelength (λ) = 1.54056 Å (0.154056 nm), Kα2 wavelength (λ) = 1.54439 Å (0.154439 nm), Kα2 ratio = 0.50000). D = K λ / β cos θ (1s) β = B ｏｂｓ -b [wherein D is the crystallite size (Å), λ is the measurement X-ray wavelength (Å), β is the broadening of the diffraction line width due to the crystallite size, θ is the diffraction angle, K is the Scherrer constant, B ｏｂｓ is the measured half-width, and b is the line width broadening due to the instrument. Here, the calculation was performed assuming K=0.890 and b=0.050.]

[0076] 5. Carbon Monoxide Production Amount (CO Production Amount) Using a rapid catalyst evaluation system equipped with a fixed-bed flow reactor and a gas chromatograph mass spectrometer (GC / MS) directly connected to the reactor, the carbon monoxide production amount by the reducing agent was measured according to the following procedure. Specifically, a quartz reaction tube with an inner diameter of 4 mm was prepared, and cylindrical reducing agent molded to a major diameter of 3 mm was packed into the reaction tube so that the stack height was 30 mm.

[0077] Thereafter, while flowing helium gas at a flow rate of 5 mL / min, the temperature was increased to 850 °C at a rate of 15 °C / min, and the mixture was heated at the same temperature for 5 minutes to stabilize the temperature. Next, to activate the oxygen carrier, hydrogen gas (reducing gas) was flowed at a flow rate of 5 mL / min for 20 minutes to carry out a reduction reaction of the oxygen carrier (first process), thereby reducing the oxygen carrier. At this time, the gas discharged from the outlet contained water vapor. Then, 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), thereby reducing the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the outlet contained carbon monoxide.

[0078] Next, for this test, the following process was performed. 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 3 mL / min for 16 minutes to perform a reduction reaction of the reducing agent (first process), thereby reducing the reducing agent. At this time, the gas discharged from the outlet contained water vapor. Then, for gas exchange, helium gas was flowed at a flow rate of 3 mL / min for 5 minutes, and then carbon dioxide gas was flowed at a flow rate of 3 mL / min for 16 minutes to perform a reduction reaction of carbon dioxide (second process), thereby reducing the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the outlet contained carbon monoxide. Note that in all of the above processes, the temperature of the reducing agent was maintained at 850°C and the processes were performed under atmospheric pressure conditions when flowing any of the gases.

[0079] The amount of carbon monoxide produced was defined as the value obtained by dividing the total amount of carbon monoxide produced from the time when the amount of carbon monoxide produced per unit second after carbon dioxide gas (raw material gas) was made to flow, from 0.001 mmol or more until the amount became 0.001 mmol or less, by the weight (g) of the reducing agent.

[0080] Gas chromatograph mass spectrometry (GC / MS analysis) was performed using a "QP-2020" manufactured by Shimadzu Corporation. The conditions for the GC section were as follows: Column temperature: 200°C Injection temperature: 200°C Detector temperature: 250°C Column: EGA tube (L: 2.5 m, φ (inner diameter): 0.15 mm, t: 0 mm) Column flow rate: 0.55 mL / min Split ratio: 400 Purge flow rate: 5.0 mL / min

[0081] The MS section (detection section) was used after being calibrated under the following conditions: A mass flow controller calibrated with carbon dioxide, with a discharge full scale of 10 mL / min or 50 mL / min, a flow rate accuracy of ±1.0%, and a reproducibility accuracy of ±0.2%, was used to plot the detection signal intensity when carbon dioxide was flowed at 1, 2, and 3 mL / min, and the calibration curve, when approximated by a straight line passing through the origin using the least squares method, was found to be within R ２ The detection signal strength was adjusted so as to satisfy the relationship of >0.98. Based on this, the detection amount (amount of carbon dioxide produced per unit second) relative to the detection signal strength of carbon dioxide was calculated. Carbon monoxide was also calibrated in the same manner using the following procedure. The detection signal strengths when carbon monoxide was flowed at 1, 2, 3, and 5 mL / min using the mass flow controller were plotted, and the calibration curve, when approximated by a straight line passing through the origin using the least squares method, was found to be R ２ The detection signal strength was adjusted to satisfy both the relationship of 0.74 and 1.00, and the relationship of 0.74 > 0.98 was satisfied. Based on this, the detection amount (amount of carbon monoxide produced per unit second) relative to the carbon monoxide detection signal strength was calculated. Note that the carbon dioxide, carbon monoxide, and other gases were assumed to be fed from the mass flow controller to the MS section via the GC section set under the above conditions, and the settings of the mass flow controller were changed using a conversion factor when carbon monoxide was fed, with the conversion factors set to 0.74 and 1.00, respectively.

[0082] The peak position of the diffraction peak corresponding to the (220) plane, its full width at half maximum, crystallite size, peak intensity / full width at half maximum, and amount of carbon monoxide produced are shown in Table 1 below. The reducing agents of each Example produced a larger amount of carbon monoxide than the reducing agent of Comparative Example 1, which was cerium oxide alone.

[0083] 5. Amount of Oxygen Deficiency The amount of oxygen deficiency of the reducing agents obtained in Example 1 and Comparative Example 1 was measured using thermogravimetry (TG). The difference between the mass of the reducing agent reduced by hydrogen and the mass of the reducing agent increased by carbon dioxide was divided by the mass of the oxygen carrier contained in the reducing agent, and the result was multiplied by 100 to determine the amount of oxygen deficiency (%). Specifically, 100 mg of reducing agent was first filled into a sample container. Next, the sample container was heated to 850°C or 650°C at a rate of 10°C / min while flowing helium.

[0084] Thereafter, while maintaining the temperature at 850°C or 650°C, a mixed gas of hydrogen and helium (10% by volume hydrogen) was flowed at 100 mL / min for 10 minutes to reduce the reducing agent. After 10 minutes, helium gas was flowed at 100 mL / min for 30 minutes for gas exchange. The weight of the reducing agent at this time was designated "A". Next, a mixed gas of carbon dioxide and helium (10% by volume carbon dioxide) was flowed at 100 mL / min for 10 minutes to oxidize the reducing agent. After 10 minutes, helium gas was flowed at 100 mL / min for 30 minutes for gas exchange. The weight of the reducing agent at this time was designated "B". The amount of oxygen deficiency (%) was calculated by (B - A / mass of oxygen carrier contained in the reducing agent) x 100.

[0085] The amount of oxygen deficiency is shown in Table 2 below.

[0086] The oxygen carrier contained in the reducing agent of each example had a larger amount of oxygen vacancy than the cerium oxide alone that constitutes the reducing agent of Comparative Example 1. This result is in good agreement with the result that the conversion efficiency of carbon dioxide to carbon monoxide is high.

Claims

1. A reducing agent that reduces carbon dioxide upon contact to produce carbon values, An oxygen carrier is included, which is composed of a metal oxide containing cerium (Ce) and a transition element other than cerium (Ce) and has oxygen ion conductivity; When the oxygen carrier is subjected to X-ray diffraction measurement, the peak position of at least one diffraction peak corresponding to the (220) plane in the X-ray diffraction profile is determined to be cerium oxide (CeO ２ ) the diffraction peak corresponding to the (220) plane of the reducing agent is shifted relative to the peak position.

2. The reducing agent according to claim 1, The cerium oxide (CeO ２ a shift amount of a diffraction peak corresponding to the (220) plane of the oxygen carrier relative to a diffraction peak corresponding to the (220) plane of the oxygen carrier is 0.1° or more and less than 0.6° in 2θ.

3. The reducing agent according to claim 1, A reducing agent, wherein the diffraction peak corresponding to the (220) plane of the oxygen carrier has a full width at half maximum of 0.3 or more.

4. The reducing agent according to claim 1, The oxygen carrier is a reducing agent having a crystallite size of 320 Å or less.

5. The reducing agent according to claim 1, A reducing agent, wherein the ratio of peak intensity to full width at half maximum of a diffraction peak corresponding to the (220) plane of the oxygen carrier is 6.2 or less.

6. The reducing agent according to claim 1, The reducing agent, wherein the transition element is at least one of elements belonging to the fourth and fifth periods of the periodic table.

7. The reducing agent according to claim 1, The carbon value comprises carbon monoxide, a reducing agent.

8. The reducing agent according to claim 1, The reducing agent wherein said contact with carbon dioxide occurs at a temperature above 650°C.

9. The reducing agent according to claim 1, The reducing agent, oxidized by contact with carbon dioxide, is reduced by contact with a reducing gas containing hydrogen.

10. The reducing agent according to claim 9, The reducing agent wherein the contact with the reducing gas comprising hydrogen occurs at a temperature above 650°C.

11. The reducing agent according to claim 9 A reducing agent that generates a reversible oxygen deficiency of 0.45% or more relative to the mass of the oxygen carrier contained in the reducing agent when brought into contact with the reducing gas containing hydrogen.

12. The reducing agent according to claim 1, The reducing agent further comprises a binder that binds the oxygen carrier.

13. The reducing agent according to claim 12, A reducing agent, wherein the ratio of the binder contained in the reducing agent is 1 part by mass or more and 60 parts by mass or less per 100 parts by mass of the reducing agent.

14. A gas production method for producing a product gas containing carbon valuables by reducing a raw material gas containing carbon dioxide by contacting the raw material gas with a reducing agent, the method comprising: the reducing agent is composed of a metal oxide containing cerium (Ce) and a transition element other than cerium (Ce), and contains an oxygen carrier having oxygen ion conductivity; When the oxygen carrier is subjected to X-ray diffraction measurement, the peak position of at least one diffraction peak corresponding to the (220) plane in the X-ray diffraction profile is determined to be cerium oxide (CeO ２ ) the diffraction peak corresponding to the (220) plane is shifted relative to the peak position of the diffraction peak corresponding to the (220) plane of the gas.