Reductant and method for producing gas

Abstract

To provide a reductant having high conversion efficiency of carbon dioxide to carbon monoxide, the reductant being utilizable, for example, in a chemical looping method, and to provide a method for producing gas using the reductant.SOLUTION: A reductant of the present invention is used when a raw material gas containing carbon dioxide is brought into contact with the reductant to reduce the carbon dioxide and produce a product gas containing carbon monoxide, and includes a composite metal oxide represented by Ce1-x(M)xOy (wherein M is a metal element whose ionic radius is smaller than an ionic radius of Ce having the same valence and the same coordination number; x represents a positive real number; and y represents a real number from 1 to 4).SELECTED DRAWING: None

Description

[Technical Field]

[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. [Background technology]

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

[0003] To solve the above problem, the chemical looping method is used to convert (synthesize) carbon dioxide into carbon monoxide. The chemical looping method is a method in which the reverse water gas shift reaction is divided into two reactions: a reduction reaction with hydrogen and a production reaction of carbon monoxide from carbon dioxide, and these reactions are carried out using a metal oxide (MO x ) to bridge the gap (see formula below). H2+ MO x → HO + MO x-1 CO2+ 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, water and carbon monoxide, which are the substrates for the reverse reaction, do not coexist during each reaction, and therefore it is possible to achieve a higher carbon dioxide to carbon monoxide conversion efficiency than the chemical equilibrium of the reverse water-gas shift reaction. In this chemical looping method, metal oxides that mediate the reaction include cerium oxide, which has oxygen ion conductivity, and iron oxide, which is highly reactive with carbon dioxide. For example, Patent Document 1 describes a method using cerium oxide containing zirconium having reversible oxygen deficiency, and Non-Patent Document 1 describes a method using a mixed metal oxide of cerium oxide containing zirconium and iron oxide. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5858926 [Non-patent literature]

[0006] [Non-Patent Document 1] Journal of CO2 Utilization 17 (2017) 60-68 Summary of the Invention [Problem to be solved by the invention]

[0007] However, Patent Document 1 only examines zirconium-doped cerium oxide, and does not investigate structural changes in cerium oxide doped with other metals or the accompanying changes in the energy required to generate oxygen vacancies, and therefore the conversion efficiency from carbon dioxide to carbon monoxide is still insufficient.

[0008] Furthermore, Non-Patent Document 1 does not adequately examine the method for preparing the mixed metal oxide of iron oxide and cerium oxide containing zirconium, or the mixing ratio thereof. This results in uneven distribution and uneven abundance of the metal oxides in the mixed metal oxide. As a result, the interaction between the metal oxides, which is necessary for the transfer of oxygen ions and electrons, is weakened, and the conversion efficiency of carbon dioxide to carbon monoxide is not improved.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a reducing agent that has a high conversion efficiency of carbon dioxide to carbon monoxide and that can be used in, for example, a chemical looping method, and a method for producing a gas using such a reducing agent. [Means for solving the problem]

[0010] Such an object can be achieved by the present invention described below. (1) The reducing agent of the present invention is a reducing agent used in producing a product gas containing carbon monoxide by contacting the carbon dioxide with a raw material gas containing carbon dioxide and reducing the carbon dioxide, Ce 1-x (M) x O y (wherein M is a metal element whose ionic radius is smaller than the ionic radius of Ce having the same valence and coordination number, x is a positive real number, and y is a real number from 1 to 4).

[0011] (2) In the reducing agent of the present invention, the difference between the ionic radius of the Ce and the ionic radius of the metal element M is preferably greater than 0 pm and less than 47 pm when the valence is 3 and the coordination number is 6, and greater than 0 pm and less than 24 pm when the valence is 4 and the coordination number is 8. (3) In the reducing agent of the present invention, the composite metal oxide preferably has oxygen ion conductivity and functions as an oxygen carrier.

[0012] (4) In the reducing agent of the present invention, the metal element M is preferably at least one selected from the group consisting of samarium (Sm), hafnium (Hf), yttrium (Y), gadolinium (Gd), niobium (Nb), praseodymium (Pr), titanium (Ti), neodymium (Nd), europium (Eu), silver (Ag), gold (Au), and scandium (Sc).

[0013] (5) In the reducing agent of the present invention, the x is preferably 0.05 to 0.4. (6) In the reducing agent of the present invention, the amount of the composite metal oxide is preferably 60 parts by mass or more per 100 parts by mass of the reducing agent.

[0014] (7) The reducing agent of the present invention preferably further comprises a carrier for supporting the composite metal oxide, the carrier having oxygen ion conductivity and functioning as an oxygen carrier. (8) In the reducing agent of the present invention, the metal element M is preferably at least one selected from the group consisting of samarium (Sm), zirconium (Zr), and hafnium (Hf).

[0015] (9) In the reducing agent of the present invention, the support preferably contains at least one selected from the group consisting of vanadium (V), manganese (Mn), iron (Fe), titanium (Ti), molybdenum (Mo), yttrium (Y), chromium (Cr), lanthanum (La), cobalt (Co), nickel (Ni), copper (Cu), tungsten (W), niobium (Nb), and cerium (Ce).

[0016] (10) In the reducing agent of the present invention, the amount of the composite metal oxide is preferably 50 parts by mass or less per 100 parts by mass of the reducing agent. (11) The reducing agent of the present invention is preferably used to produce a product gas containing carbon monoxide by bringing it into contact with a raw material gas containing carbon dioxide to reduce the carbon dioxide.

[0017] (12) In the reducing agent of the present invention, the oxidized reducing agent is preferably reduced by contacting with a reducing gas containing hydrogen. (13) The reducing agent of the present invention is preferably used in separate reaction steps for the reduction of carbon dioxide and the reduction of the oxidized reducing agent.

[0018] (14) The gas production method of the present invention is characterized in that the reducing agent of the present invention is brought into contact with a raw material gas containing carbon dioxide to reduce the carbon dioxide and produce a product gas containing carbon monoxide. [Effects of the Invention]

[0019] According to the present invention, a product gas containing carbon monoxide can be efficiently produced from a raw material gas containing carbon dioxide. Furthermore, the reducing agent of the present invention can be used, for example, in a chemical looping process. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a photograph showing the results of STEM-EDS analysis of the reducing agent obtained in Example B2. DETAILED DESCRIPTION OF THE INVENTION

[0021] 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 to produce a product gas containing carbon monoxide by reducing carbon dioxide through contact with a raw material gas containing carbon dioxide (i.e., used in the gas production method of the present invention). In addition, the reducing agent can be reduced (regenerated) by contacting a reducing gas with the oxidized reducing agent. In this case, preferably, the raw material gas and the reducing gas are passed alternately through a reaction tube (reaction vessel) filled with the reducing agent of the present invention, whereby conversion of carbon dioxide to carbon monoxide by the reducing agent and regeneration of the oxidized reducing agent by the reducing gas are carried out.

[0022] First Embodiment The reducing agent of the first embodiment contains a composite metal oxide that functions as an oxygen carrier. Here, an oxygen carrier is a compound that has oxygen ion conductivity and can generate reversible oxygen deficiency. The oxygen carrier itself loses oxygen element through reduction, but when it 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 composite metal oxide in the present invention is represented by the general formula: Ce 1-x (M) x O y This general formula is: Ce 1-x (M) x O y In the formula, M is a metal element that can be trivalent or tetravalent, and its ionic radius is smaller than that of Ce having the same valence and coordination number.

[0023] In such a composite metal oxide, the difference between the ionic radius of Ce and the ionic radius of the metal element M is preferably more than 0 pm and not more than 47 pm when the valence is 3 and the coordination number is 6, and more than 0 pm and not more than 24 pm when the valence is 4 and the coordination number is 8. For example, when the coordination number of each metal element is 8, Ce +4 The ionic radius of the metal element M is 97 pm, so the metal element M is its tetravalent ion (M +4 ) is preferably a metal element having an ionic radius of 73 pm or more and less than 97 pm. In addition, when the coordination number of each metal element is 6, Ce +3 The ionic radius of is 101 pm, so the metal element M is its trivalent ion (M +3 ) is preferably a metal element having an ionic radius of 54 pm or more and less than 101 pm.

[0024] General formula: Ce 1-x (M) x O y It is believed that when the composite metal oxide represented by the formula (I) contains a metal element M that satisfies the above conditions, oxygen vacancies are generated more efficiently and distortion occurs in the lattice vacancies, which makes it easier to remove oxygen elements from carbon dioxide. Furthermore, simple metal oxides or mixtures thereof tend to adsorb impurities contained in the raw material gas or reducing gas, resulting in low stability. In contrast, complex metal oxides are less likely to adsorb impurities and can maintain their ability to remove oxygen from carbon dioxide for a long period of time. As a result, the conversion efficiency of carbon dioxide to carbon monoxide using a reducing agent can be increased.

[0025] Here, x is a positive real number, preferably 0.05 to 0.4, more preferably 0.1 to 0.25, and even more preferably 0.1 to 0.19, in which case oxygen vacancies are more likely to occur in the composite metal oxide, and the degree of distortion of the lattice vacancies tends to become appropriate. Furthermore, y is a real number of 1 to 4, preferably a real number of 1 to 3, and more preferably a real number of 1 or 2. In this case, the stability of the composite metal oxide is likely to be improved.

[0026] The difference between the ionic radius of Ce and the ionic radius of the metal element M is preferably more than 0 pm and not more than 47 pm when each metal element has a valence of 3 and a coordination number of 6, and more than 0 pm and not more than 24 pm when each metal element has a valence of 4 and a coordination number of 8. In either case, however, it is more preferably more than 0 pm and not more than 20 pm, even more preferably more than 0 pm and not more than 15 pm, and particularly preferably more than 1 pm and not more than 8 pm. It is believed that by setting the content within the above range, it is possible to prevent the metal element M from being separated from the composite metal oxide, while generating an appropriate amount of distortion in the lattice defects of the composite metal oxide.

[0027] Examples of the metal element M include samarium (Sm), zirconium (Zr), hafnium (Hf), yttrium (Y), gadolinium (Gd), niobium (Nb), praseodymium (Pr), titanium (Ti), neodymium (Nd), europium (Eu), silver (Ag), gold (Au), scandium (Sc), etc. Among these, the metal element M is preferably at least one selected from the group consisting of samarium (Sm), hafnium (Hf), yttrium (Y), gadolinium (Gd), niobium (Nb), praseodymium (Pr), titanium (Ti), neodymium (Nd), europium (Eu), silver (Ag), gold (Au), and scandium (Sc), and more preferably samarium, zirconium, and hafnium. If the above metals are used as the metal element M, lattice defects with sufficient strain are generated, and the efficiency of conversion of carbon dioxide to carbon monoxide by the reducing agent can be significantly increased.

[0028] The ionic radius of each metal ion is Zr +4 is 84pm, Hf with coordination number 8 +4 is 83pm, coordination number 8 is Nb +4 is 79pm, Ti with coordination number 8 +4 is 74pm, coordination number 6 is Sm +3 is 95.8 pm, and the coordination number is 6. +3 is 90pm, coordination number 6 Gd +3 is 93.8 pm, coordination number 6 Nd +3 is 98.3pm, and the coordination number is 6. 3+ is 99pm, and Eu with a coordination number of 6 3+ is 94.7 pm, coordination number 6 Ag 3+ is 75pm, and Au with a coordination number of 6 3+ is 85pm, and Sc with coordination number 6 3+ is 74.5pm.

[0029] The amount of the composite metal oxide is preferably 60 parts by mass or more, more preferably 80 parts by mass or more, and even more preferably 90 parts by mass or more, per 100 parts by mass of the reducing agent. By setting the amount of the composite metal oxide contained in the reducing agent within the above range, it is possible to promote the conversion of carbon dioxide to carbon monoxide by the reducing agent, i.e., to further increase the conversion efficiency.

[0030] In an embodiment where the reducing agent is not entirely composed of a composite metal oxide, fine particles of the composite metal oxide are bound with a binder (carrier). The binder is not particularly limited as long as it is not easily modified by the raw material gas, reaction conditions, etc. Specific examples of binders include carbon materials (graphite, graphene, etc.), zeolite, montmorillonite, SiO2, ZrO2, TiO2, VO5, MgO, Al2O3, and composite oxides containing these.

[0031] The packing density of the reducing agent is preferably 1.1 g / mL or less, more preferably 0.4 to 1 g / mL, and even more preferably 0.5 to 0.9 g / mL. If the packing density is too low, the gas passage rate becomes too fast, and the time during which the reducing agent is in contact with 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 the 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.

[0032] The pore volume of the reducing agent is 0.4 cm 3 / g or more, and 1 to 30 cm 3 / g, and more preferably 5 to 20 cm 3 / g 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, which tends to reduce 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. 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.

[0033] The shape of the reducing agent is not particularly limited, but is preferably granular, for example, since the packing density of the reducing agent can be easily adjusted to fall within the above range if the reducing agent is granular. Here, granular is a concept that includes powder, particles, lumps, pellets, etc., and the shape may be any of spherical, plate-like, polygonal, crushed, columnar, needle-like, scale-like, etc. The average particle size of the reducing agent is preferably 1 μm to 5 mm, more preferably 10 μm to 1 mm, and even more preferably 20 μm to 0.5 mm. If the reducing agent has such an average particle size, its packing density is likely to fall within the above range.

[0034] In this specification, the term "average particle size" refers to the average particle size of 200 randomly selected reducing agents in one field of view observed under an electron microscope. In this context, "particle size" refers to the maximum distance between two points on the contour of the reducing agent. When the reducing agent is columnar, the maximum distance between two points on the contour of its end face is taken as the "particle size." Furthermore, when the reducing agent is, for example, in a blocky form where 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 to 500 m 2 / g, and 3 to 450m 2 / g is more preferable, and 5 to 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.

[0035] Furthermore, in the first embodiment, the strain of lattice defects in the composite metal oxide can be made sufficiently large, so that 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 650°C). In other words, the reducing agent can efficiently convert carbon dioxide to carbon monoxide over a wide temperature range. The oxygen capacity of the reducing agent at 400° C. is preferably 1 to 40% by mass, and more preferably 2 to 30% by mass. If the oxygen capacity of 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 (approximately 650° C.), and it can be said that the reducing agent has an extremely high efficiency of converting carbon dioxide to carbon monoxide.

[0036] [Method for producing reducing agent according to the first embodiment] Next, a method for producing the reducing agent of the first embodiment will be described. The method for producing the reducing agent is not particularly limited, but examples thereof include the sol-gel method, co-precipitation method, solid phase method, and hydrothermal synthesis method. For 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 fired. 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 by using acidic water prepared with, for example, citric acid, acetic acid, malic acid, tartaric acid, hydrochloric acid, nitric acid, or a mixture thereof.

[0037] Examples of salts of metal elements include nitrates, sulfates, chlorides, hydroxides, carbonates, and composites thereof, among which nitrates are preferred. Hydrates of the salts of metal elements may also be used as needed. The gel is dried preferably at a temperature of 20 to 200° C., more preferably 50 to 150° C., for a time of preferably 0.5 to 20 hours, more preferably 1 to 15 hours. By drying in this manner, the gel can be dried uniformly.

[0038] The gel is preferably fired at a temperature of 300 to 1200°C, more preferably 350 to 800°C, for a time of preferably 1 to 24 hours, more preferably 1.5 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 also prevent excessive particle growth of the reducing agent. The temperature may be increased at a rate of 1 to 20°C / min, preferably 2 to 10°C / min, until the above-mentioned firing temperature is reached. This promotes the growth of particles of the reducing agent and also prevents cracking of the crystals (particles).

[0039] Second Embodiment The reducing agent of the second embodiment includes a support that functions as an oxygen carrier and a composite metal oxide supported on the support. As described in the first embodiment, an oxygen carrier is a compound that has oxygen ion conductivity and can generate reversible oxygen deficiency. The oxygen carrier itself loses oxygen element through reduction, and when it comes into contact with carbon dioxide in an oxygen element-deficient state (reduced state), it acts to remove the oxygen element from the carbon dioxide and reduce it.

[0040] On the other hand, the composite metal oxide of the second embodiment preferably has the function of more efficiently migrating and retaining oxygen vacancies (defects) generated when oxygen elements are removed from the support by interacting with the support, which functions as an oxygen carrier.Furthermore, since the composite metal oxide itself does not participate in the reaction, or participates only partially, it also preferably functions as a structural maintainer of the reducing agent. Thus, the support that functions as an oxygen carrier has a sufficiently high conversion efficiency of carbon dioxide to carbon monoxide by itself, but by further supporting a composite metal oxide, the migration rate of oxygen vacancies in the support can be improved, thereby further increasing the conversion efficiency.

[0041] [[Carrier]] The compound constituting the carrier (oxygen carrier) is not particularly limited as long as it is a compound that can generate reversible oxygen deficiency, but examples thereof include compounds represented by the general formula: MO y It is preferable that the metal oxide is a metal oxide represented by the following formula: The above general formula: MO y In the formula, M1 represents at least one metal element belonging to groups 2 to 13 of the periodic table, and y represents a positive real number. Here, y is preferably 0.5 to 6, more preferably 1 to 5, and even more preferably 1 to 4. General formula: M1O y The compound represented by the general formula: MO is obtained by reduction with a reducing gas. y-n It is possible to produce compounds represented by the general formula: MO y-n In the formula, M1 and y are each a group represented by the general formula: y where n is a positive real number. Note that n is usually a value smaller than y, and is preferably 0.05 to 5, more preferably 0.1 to 3, and even more preferably 0.15 to 2.

[0042] General formula: M1O y is reduced by contact with a reducing gas to give a compound of the general formula: MO y-n It is converted to the general formula: MO y-n is oxidized by contact with a feed gas containing carbon dioxide to form a compound represented by the general formula: MO y This allows the reducing agent to be recycled for the reduction reaction of carbon dioxide and the reduction reaction of the reducing agent. The compound constituting the carrier is, as described above, MO y M1O y-n and M1O y-n M1O y The compound is not particularly limited as long as it can be oxidized to the above. If the compound can be oxidized and reduced in this way, it can be used in a system involving a carbon dioxide reduction reaction, such as a chemical looping method. The metal element M1 contained in the compound constituting the support is an element having multiple oxidation states. Here, the "metal element having multiple oxidation states" refers to, for example, Fe in the case of iron (Fe). +2 , Fe +3 It means a metal element that can have multiple valences, such as:

[0043] Examples of the metal element M1 contained in the compound constituting the support include metal elements belonging to groups 3 to 12 of the periodic table. The metal element M1 is preferably at least one selected from the group consisting of vanadium (V), manganese (Mn), iron (Fe), titanium (Ti), molybdenum (Mo), yttrium (Y), chromium (Cr), lanthanum (La), cobalt (Co), nickel (Ni), copper (Cu), tungsten (W), niobium (Nb), and cerium (Ce). Among these, cerium, chromium, iron, and the like are preferred as the metal element M1. Using a support composed of a compound containing such a metal element M1 further increases the efficiency of carbon dioxide conversion to carbon monoxide and facilitates the reaction even at relatively low temperatures. The compound constituting the carrier may be amorphous or crystalline, and the crystal may have any structure.

[0044] [[Complex metal oxides]] In the second embodiment, the composite metal oxide is a compound capable of migrating oxygen vacancies in the support, and has the general formula: Ce 1-x (M) x O y A composite metal oxide is a compound represented by the formula: In composite metal oxides, the constituent metal elements are combined and arranged close to each other, so that they interact efficiently with the support, resulting in an excellent effect of oxygen vacancy migration within the support. In contrast, in a mixture of simple metal oxides, the particles only come into contact with each other near the surface, making it difficult to enhance the interaction between the metal elements, and therefore the effect of oxygen vacancy migration cannot be enhanced. Note that composite metal oxides are compounds different from the compounds that make up the support.

[0045] The above general formula: Ce 1-x (M) x O y In the formula, M is a metal element that can be trivalent or tetravalent, and its ionic radius is smaller than that of Ce having the same valence and coordination number. In such a composite metal oxide, the difference between the ionic radius of Ce and the ionic radius of the metal element M is preferably more than 0 pm and not more than 47 pm when the valence is 3 and the coordination number is 6, and more than 0 pm and not more than 24 pm when the valence is 4 and the coordination number is 8. For example, when the coordination number of each metal element is 8, Ce +4 The ionic radius of the metal element M is 97 pm, so the metal element M is its tetravalent ion (M +4 ) is preferably a metal element having an ionic radius of 73 pm or more and less than 97 pm. In addition, when the coordination number of each metal element is 6, Ce +3 The ionic radius of is 101 pm, so the metal element M is its trivalent ion (M +3 ) is preferably a metal element having an ionic radius of 54 pm or more and less than 101 pm.

[0046] General formula: Ce 1-x (M) x O y When the composite metal oxide represented by the formula (I) contains a metal element M that satisfies the above conditions, distortion occurs in the lattice defects, and the interaction force with the support is strengthened to compensate for the distortion, thereby facilitating smooth migration of oxygen defects within the support. Furthermore, simple metal oxides or mixtures thereof tend to adsorb impurities contained in the source gas or reducing gas, resulting in low stability. In contrast, complex metal oxides are less likely to adsorb impurities and can maintain the ability to migrate oxygen vacancies for a long period of time. As a result, the conversion efficiency of carbon dioxide to carbon monoxide by the reducing agent can be increased.

[0047] Here, x is a positive real number, preferably 0.1 to 0.8, and more preferably 0.3 to 0.7, which tends to provide an appropriate degree of distortion of lattice defects in the composite metal oxide. Furthermore, y is a real number of 1 to 4, preferably a real number of 1 to 3, and more preferably a real number of 1 or 2. In this case, the stability of the composite metal oxide is likely to be improved.

[0048] The difference between the ionic radius of Ce and the ionic radius of the metal element M is preferably more than 0 pm and not more than 47 pm when each metal element has a valence of 3 and a coordination number of 6, and more than 0 pm and not more than 24 pm when each metal element has a valence of 4 and a coordination number of 8. In either case, however, the difference is more preferably more than 1 pm and not more than 20 pm, and even more preferably more than 5 pm and not more than 15 pm. It is believed that by setting the content within the above range, it is possible to prevent the metal element M from being separated from the composite metal oxide, while generating an appropriate amount of distortion in the lattice defects of the composite metal oxide.

[0049] Examples of the metal element M include samarium (Sm), zirconium (Zr), hafnium (Hf), yttrium (Y), gadolinium (Gd), niobium (Nb), praseodymium (Pr), lanthanum (La), titanium (Ti), indium (In), neodymium (Nd), and scandium (Sc). Among these, the metal element M is preferably at least one selected from the group consisting of samarium (Sm), zirconium (Zr), and hafnium (Hf), with samarium, zirconium, and hafnium being more preferred. Using any of the above metals as the metal element M can sufficiently accelerate the migration of oxygen vacancies, significantly increasing the efficiency of carbon dioxide conversion to carbon monoxide by the reducing agent.

[0050] The ionic radius of each metal ion is Zr +4 is 84pm, Hf with coordination number 8 +4 is 83pm, coordination number 8 is Nb +4 is 79pm, Ti with coordination number 8 +4 is 74pm, coordination number 6 is Sm +3 is 95.8 pm, and the coordination number is 6. +3 is 90pm, coordination number 6 Gd +3 is 93.8 pm, and the coordination number is 6. 3+ is 99pm, coordination number 6 In3+ is 80pm, coordination number 6 Nd +3 is 98.3 pm, and Sc with coordination number 6 3+ is 74.5pm.

[0051] The amount of the composite metal oxide is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, even more preferably 30 parts by mass or less, and particularly preferably 10 to 25 parts by mass, per 100 parts by mass of the reducing agent. By maintaining the quantitative relationship between the support and the composite metal oxide within the above range, the conversion of carbon dioxide to carbon monoxide by the support can be promoted, and the migration of oxygen vacancies in the support by the composite metal oxide can be promoted. This can further increase the efficiency of carbon dioxide conversion to carbon monoxide by the reducing agent. Note that if the amount of the composite metal oxide is too large (in other words, if the amount of the support is too small), the time during which the high carbon dioxide conversion efficiency to carbon monoxide can be maintained tends to be shortened.

[0052] The packing density of the reducing agent and the pore volume of the reducing agent are the same as those in the first embodiment. The shape of the reducing agent, the average particle size of the reducing agent, and the BET specific surface area of ​​the reducing agent, including their definitions and preferred ranges, are the same as those in the first embodiment.

[0053] In addition, in the second embodiment, the composite metal oxide can promote the migration of oxygen vacancies in the carrier, so 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 650°C). In other words, the reducing agent can efficiently convert carbon dioxide to carbon monoxide over a wide temperature range. The oxygen capacity of the reducing agent at 400° C. is preferably 1 to 40% by mass, and more preferably 2 to 30% by mass. If the oxygen capacity of 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 (approximately 650° C.), and it can be said that the reducing agent has an extremely high efficiency of converting carbon dioxide to carbon monoxide.

[0054] [Method for producing reducing agent according to the second embodiment] Next, a method for producing a reducing agent according to a second embodiment will be described. The reducing agent having a composite metal oxide supported on a carrier can be produced as follows. First, salts of the metal elements that constitute the carrier and the composite metal oxide are dissolved in acidic water to prepare an aqueous solution. Next, this aqueous solution is gelled, dried, and calcined. That is, the reducing agent of the second embodiment can be produced easily and reliably by the so-called sol-gel method. Examples of acids used to prepare acidic water include citric acid, acetic acid, malic acid, tartaric acid, hydrochloric acid, nitric acid, and mixtures thereof.

[0055] Examples of salts of metal elements include nitrates, sulfates, chlorides, hydroxides, carbonates, and composites thereof, among which nitrates are preferred. Hydrates of the salts of metal elements may also be used as needed. The gel is dried preferably at a temperature of 20 to 200° C., more preferably 50 to 150° C., for a time of preferably 0.5 to 20 hours, more preferably 1 to 15 hours. By drying in this manner, the gel can be dried uniformly.

[0056] The gel is preferably calcined at a temperature of 300 to 1200°C, more preferably 350 to 800°C, for a time of preferably 1 to 24 hours, more preferably 1.5 to 20 hours. The gel is preferably converted into an oxide by calcination, and calcination under the above-mentioned conditions can easily convert it into a support and a composite metal oxide. Furthermore, calcination under the above-mentioned conditions can also prevent excessive particle growth of the reducing agent. The temperature may be increased at a rate of 1 to 20°C / min, preferably 2 to 10°C / min, until the above-mentioned firing temperature is reached. This promotes the growth of particles of the reducing agent and also prevents cracking of the crystals (particles).

[0057] In this case, if the amount of salt of the metal element that constitutes the support is made sufficiently larger than the amount of salt of the metal element that constitutes the composite metal oxide, the support will serve as the core and the composite metal oxide will be formed, with the composite metal oxide being supported on this core. In this case, it is thought that the composite metal oxide will coat the periphery of the support (core portion) or be scattered in granular form to form a surface layer (shell portion).

[0058] [How to use the reducing agent] As described above, the reducing agent of the present invention can be used in, for example, a chemical looping method. Also, as described above, the reducing agent of the present invention can be used for reducing carbon dioxide. 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 that it circulates 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, another reducing agent (reducing gas) is used.

[0059] 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 the following formula (A1) [first embodiment] or the following formula (A2) [second embodiment], and the reduction reaction of carbon dioxide is the reaction shown in the following formula (B1) [first embodiment] or the following formula (B2) [second embodiment].

[0060] H2 (gas) + Ce 1-x (M) x O y (solid) →H2O (gas) + Ce 1-x (M) x O y-n (Solid) (A1) CO2 (gas) + Ce 1-x (M) x O y-n (solid) →CO (gas) + Ce 1-x(M) x O y (B1) In the formulas (A1) and (B1), n ​​is usually a value smaller than 2, preferably 0.05 to 1.7, more preferably 0.1 to 1.5, and even more preferably 0.15 to 1.3.

[0061] H2 (gas) + MO y (solid) → HO (gas) + MO y-n (Solid) (A2) CO2 (gas) + MO y-n (solid) → CO(gas) + MO x (Solid) (B2) In the formulae (A2) and (B2), M, y, and n are the same as above. That is, in the reduction reaction of the reducing agent shown by the above formula (A1) or formula (A2), hydrogen, which is a type of reducing gas, is oxidized to produce water, while in the reduction reaction of carbon dioxide shown by the above formula (B1) or formula (B2), carbon dioxide is reduced to produce carbon monoxide.

[0062] 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 300° C. or higher, more preferably 400° C. or higher, even more preferably 500° C. or higher, and particularly preferably 550° 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 850° C. or less, more preferably 750° C. or less, and even more preferably 700° C. or less. By setting the upper limit of the reaction temperature within the above range, economic efficiency can be improved.

[0063] Furthermore, the reaction temperature in the reduction reaction of carbon dioxide is preferably 300° C. or higher, more preferably 350° C. or higher, and even more preferably 400° 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 1000°C or lower, more preferably 850°C or lower, even more preferably 700°C or lower, and particularly preferably 650°C or lower. The reducing agent can reduce carbon dioxide to carbon monoxide with high efficiency even at low temperatures, so the temperature for 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 makes it easier to utilize waste heat, but also allows for further improvement in economic efficiency.

[0064] In the present invention, the reduction product obtained by the reduction reaction of carbon dioxide may be a substance other than carbon monoxide, specifically methane. The reduction product, such as carbon monoxide, obtained by the reduction reaction of carbon dioxide is preferably further converted into an organic substance or the like by microbial fermentation or the like. Examples of microbial fermentation include anaerobic fermentation. Examples of the organic substance obtained 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 or the like into C1 to C20 compounds, including hydrocarbons and alcohols that are conventionally synthesized by petrochemical methods. Specific compounds that can be obtained include methane, ethane, propylene, methanol, ethanol, propanol, acetaldehyde, diethyl ether, acetic acid, butyric acid, diethyl carbonate, and butadiene.

[0065] [Characteristics of reducing agents] The reducing agent of the present invention 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 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 that exhibits such properties can be determined to have a packing density and pore volume that satisfy the above ranges, and can sufficiently increase the efficiency of converting carbon dioxide to carbon monoxide.

[0066] Although the reducing agent and gas production method of the present invention have been described above, the present invention is not limited to these. For example, the method for producing a reducing agent and gas of the present invention may have any other additional configuration compared to the above embodiment, may be replaced with any configuration that exhibits a similar function, or may omit some of the configurations. [Example]

[0067] 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. (Example A1) A1. Manufacturing of reducing agents First, 1.94 g of samarium (III) nitrate hexahydrate (manufactured by Fujifilm Corporation, purity: 99.9%) and 12.61 g of cerium (III) nitrate hexahydrate (manufactured by Sigma-Aldrich Co., Ltd., purity: 99.0%) were dissolved in 100 mL of distilled water to obtain an aqueous solution. Next, this aqueous solution was slowly added dropwise to 200 mL of a solution (80° C.) containing 10.29 g of sodium carbonate while being vigorously stirred.

[0068] The precipitate was then filtered from the solution and washed repeatedly with distilled water until the filtrate reached pH 7 (neutral). Next, to increase the BET specific surface area of ​​the reducing agent and improve the dispersibility of samarium (dopant), the precipitate was immersed in 200 mL of n-butanol with stirring and heated at 80°C to remove the n-butanol. By suspending the precipitate in n-butanol, the particle size can be controlled and uniformly dispersed in the reducing agent for cerium and samarium. The collected material was then further dried in an oven at 120°C for 10 hours, and then calcined in a muffle furnace at 500°C for 6 hours. Finally, the fired mass was mechanically pulverized to obtain the target reducing agent, which was in granular form.

[0069] A2. Identification of reducing agents A2-1.Specific surface area The specific surface area of ​​the reducing agent was measured by nitrogen adsorption / desorption using a Brunauer-Emmett-Teller (BET) analyzer (BELSORP Mini2).

[0070] A2-2.X-ray diffraction (XRD) Crystallographic data were collected on a RINT-TTRIII instrument (CuKα radiation, 50 kV, 300 mA). As a result, the resulting reducing agent was Ce 0.85 Sm 0.15 O y (y=1 to 2 real numbers). That is, Ce 0.85 Sm 0.15 O y The amount of (composite metal oxide) was approximately 100 parts by mass relative to 100 parts by mass of the reducing agent.

[0071] (Example A2) A reducing agent was produced in the same manner as in Example A1, except that the amount of samarium (Sm) contained in the composite metal oxide was 10 mol %. The reducing agent was in granular form.

[0072] (Example A3) A reducing agent was produced in the same manner as in Example A2, except that the metal element contained in the composite metal oxide was changed from samarium (Sm) to gadolinium (Gd). Gadolinium chloride hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) was used as the precursor of the composite metal oxide. The reducing agent was in granular form.

[0073] (Example A4) Ce 0.8 Gd 0.2 O y Composite metal oxide particles (manufactured by American Elements, "CEO-GD20-01") represented by (y = real number between 1 and 2) were used as the reducing agent. 0.8 Gd 0.2 O yThe amount of (composite metal oxide) was approximately 100 parts by mass relative to 100 parts by mass of the reducing agent.

[0074] (Example A5) A reducing agent was produced in the same manner as in Example A2, except that the metal element contained in the composite metal oxide was changed from samarium (Sm) to yttrium (Y). Note that yttrium nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.999%) was used as the precursor of the composite metal oxide. The reducing agent was in granular form.

[0075] (Example A6) A reducing agent was produced in the same manner as in Example A2, except that the metal element contained in the composite metal oxide was changed from samarium (Sm) to praseodymium (Pr). Note that praseodymium nitrate n-hydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) was used as the precursor of the composite metal oxide. The reducing agent was in granular form.

[0076] (Example A7) A reducing agent was produced in the same manner as in Example A2, except that the metal element contained in the composite metal oxide was changed from samarium (Sm) to hafnium (Hf). Hafnium chloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) was used as the precursor of the composite metal oxide. The reducing agent was in granular form.

[0077] (Comparative Example A1) A reducing agent was produced in the same manner as in Example A1, except that samarium (Sm) was not contained in the composite metal oxide. The reducing agent was in granular form.

[0078] (Comparative example A2) A reducing agent was produced in the same manner as in Example A2, except that the metal element contained in the composite metal oxide was changed from samarium (Sm) to aluminum (Al). Alumina (III) nitrate nonahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) was used as the precursor of the composite metal oxide. The reducing agent was in granular form.

[0079] (Comparative example A3) A reducing agent was produced in the same manner as in Example A2, except that the metal element contained in the composite metal oxide was changed from samarium (Sm) to lanthanum (La). Lanthanum nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) was used as the precursor of the composite metal oxide. The reducing agent was in granular form.

[0080] (Comparative example A4) A reducing agent was produced in the same manner as in Example A2, except that the metal element contained in the composite metal oxide was changed from samarium (Sm) to vanadium (V). Vanadium (III) chloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 95.0%) was used as the precursor of the composite metal oxide. The reducing agent was in granular form.

[0081] A3. Characterization of reducing agents (conversion efficiency) The characteristics of the reducing agent were evaluated using a rapid catalyst evaluation system (Frontier Labs, Inc., "Single μ-Reactor Rx-3050SR") equipped with a microreactor and a gas chromatograph mass spectrometer (GC / MS) directly connected to the microreactor, according to the following procedure. First, 0.2 g of the reducing agent was placed in a quartz reaction tube with an inner diameter of 3 mm and a length of 78 mm. Then, while flowing helium gas at a flow rate of 20 mL / min, the temperature was increased at a rate of 40 °C / min and the tube was heated for 20 minutes. Next, hydrogen gas (reducing gas) was passed through the microreactor at a flow rate of 30 mL / min for 20 minutes to carry out the reduction reaction of the reducing agent (first process). At this time, the gas discharged from the outlet of the microreactor contained water vapor.

[0082] After that, helium gas was passed through the reactor at a flow rate of 20 mL / min for 10 minutes for gas exchange, and then carbon dioxide gas was passed through the reactor at a flow rate of 3 mL / min for 20 minutes to carry out the carbon dioxide reduction reaction (second process), reducing the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the reactor outlet contained carbon monoxide. Thereafter, for gas exchange, helium gas was passed through at a flow rate of 20 mL / min for 10 minutes. In this test, the temperature of the microreactor was maintained at 550° C. when any gas was flowed, and the test was carried out under atmospheric pressure conditions.

[0083] The efficiency of conversion of carbon dioxide to carbon monoxide by the reducing agent was calculated using the following formula: The conversion efficiency is the average conversion efficiency for one minute after starting the flow of carbon dioxide gas into the quartz reaction tube. X CO (%)=n CO,out / (n CO2,in ) x 100 In the above formula, n is the mole fraction of carbon dioxide or carbon monoxide in the feed gas or product gas. Also, the selectivity of the reaction is 100%, so the total n CO2 =n CO2,in +n CO,out is. In Table 1, the X of the reducing agent in each of Examples A1 to A7 and Comparative Examples A2 to A4 is shown. CO The value of X of the reducing agent of Comparative Example A1 CO The relative value when the value of is "1" (X CO,r )

[0084] The measurement conditions for the gas chromatograph mass spectrometer are as follows: Column temperature: 200℃ Injection temperature: 200℃ Detector temperature: 250℃ Column: EGA tube (L: 2.5 m, φ (inner diameter): 0.15 mm, t: 0 mm) Column flow rate: 1.00mL / min Split ratio: 250 Purge flow rate: 3.0mL / min

[0085] The results are shown in Table 1 below. [Table 1]

[0086] The reducing agents of each Example had a high conversion efficiency of carbon dioxide to carbon monoxide. Furthermore, by changing the type and amount of the metal element M (doped metal element) constituting the reducing agent, the conversion efficiency of carbon dioxide to carbon monoxide could be adjusted. In contrast, the reducing agents of the comparative examples had low conversion efficiencies of carbon dioxide to carbon monoxide.

[0087] Example B1 B1. Production of reducing agents First, 36.12 g of iron(III) nitrate nonahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%), 2.58 g of zirconium(II) oxynitrate dihydrate (manufactured by Kishida Chemical Co., Ltd., purity: 99.0%), and 4.20 g of cerium(III) nitrate hexahydrate (manufactured by Sigma-Aldrich Co., Ltd., purity: 99.0%) were weighed out as precursors of the reducing agent.

[0088] Next, 27.42 g of citric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) was weighed and dissolved in 200 mL of deionized water to obtain an aqueous citric acid solution. Then, the precursor (metal nitrate salt) was added to the aqueous citric acid solution at 65°C while stirring. After 30 minutes had passed, 3.78 g of ethylene glycol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) was added to the aqueous citric acid solution, and the temperature was raised to 80° C. The molar ratio of metal:citric acid:ethylene glycol was 1:1.2:0.48. 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.

[0089] The gel was dried at 100°C for 20 hours. The resulting swollen mass of organic and inorganic compounds was crushed and calcined at 750°C for 4 hours at a rate of 3°C / min. Finally, the fired mass was mechanically pulverized to obtain the target reducing agent, which was in granular form.

[0090] B2. Identification of reducing agents B2-1. Specific surface area The specific surface area of ​​the reducing agent was measured by nitrogen adsorption / desorption using a Brunauer-Emmett-Teller (BET) analyzer (BELSORP Mini2).

[0091] B2-2.X-ray diffraction (XRD) Crystallographic data were collected on a RINT-TTRIII instrument (CuKα radiation, 50 kV, 300 mA). The XRD data of the as-prepared reducing agent showed Fe2O3 (33.158, 35.618, 40.858, 49.488, 54.098) and Ce 0.5 Zr 0.5 O y Diffraction peaks corresponding to the (29.108, 33.708, 48.508, 57.508) phase were identified.

[0092] The diffraction peaks of Fe2O3 are clearly identified, while the Ce 0.5 Zr 0.5 O y (y = real number between 1 and 2) can be clearly distinguished by the absence of the diffraction peak of CeO2 (28.558). However, it closely overlapped with other diffraction peaks of CeO2 (33.088, 47.478, 56.338). Therefore, the relative intensity ratio calculation of the XRD data was used to further clarify the absence of CeO2 phase.The XRD results indicated the presence of Ce in solid solution with Ce and Zr.

[0093] Furthermore, no distinct diffraction peaks of ZrO2 were observed. From the above, the obtained reducing agent is composed of a carrier represented by Fe2O3 and Ce supported on this carrier. 0.5 Zr 0.5 O y It was found that the composition contains a composite metal oxide represented by the formula: In addition, Ce 0.5 Zr 0.5 O y The amount of (composite metal oxide) was 22.33 parts by mass relative to 100 parts by mass of the reducing agent (that is, the amount of the composite metal oxide contained in the reducing agent was 22.33% by mass). The amount of Fe (metal element M1) was 50 parts by mass relative to 100 parts by mass of the reducing agent (that is, the amount of metal element M1 contained in the reducing agent was 50% by mass).

[0094] (Example B2) A reducing agent was produced in the same manner as in Example B1, except that the amount of zirconium (Zr) contained in the composite metal oxide was 10 mol %. The reducing agent was in granular form. (Example B3) A reducing agent was produced in the same manner as in Example B1, except that the metal element contained in the composite metal oxide was changed from zirconium (Zr) to samarium (Sm). The reducing agent was in granular form.

[0095] (Example B4) A reducing agent was produced in the same manner as in Example B1, except that the metal element contained in the composite metal oxide was changed from zirconium (Zr) to hafnium (Hf). The reducing agent was in granular form. (Example B5) A reducing agent was produced in the same manner as in Example B1, except that the amount of iron (Fe) contained in the reducing agent was changed from 50 mass % to 40 mass %. The reducing agent was in granular form.

[0096] (Example B6) A reducing agent was produced in the same manner as in Example B1, except that the amount of iron (Fe) contained in the reducing agent was changed from 50 mass % to 30 mass %. The reducing agent was in granular form. (Example B7) A reducing agent was produced in the same manner as in Example B1, except that the amount of iron (Fe) contained in the reducing agent was changed from 50 mass % to 20 mass %. The reducing agent was in granular form.

[0097] (Example B8) A reducing agent was produced in the same manner as in Example B1, except that the amount of iron (Fe) contained in the reducing agent was changed from 50 mass % to 10 mass %. The reducing agent was in granular form. (Example B9) A reducing agent was produced in the same manner as in Example B1, except that the iron (Fe) contained in the carrier was changed to cerium (Ce). The carrier was made of Ce2O3. The reducing agent was in granular form.

[0098] (Comparative Example B1) A reducing agent was produced in the same manner as in Example B1, except that the composite metal oxide was omitted. The reducing agent was in granular form. (Comparative example B2) A reducing agent was produced in the same manner as in Example B9, except that the composite metal oxide was omitted. The reducing agent was in granular form.

[0099] B3. Characterization of reducing agents (conversion efficiency) The characteristics of the reducing agent were evaluated using a rapid catalyst evaluation system (Frontier Labs, Inc., "Single μ-Reactor Rx-3050SR") equipped with a microreactor and a gas chromatograph mass spectrometer (GC / MS) directly connected to the microreactor, according to the following procedure. First, 0.2 g of the reducing agent was placed in a quartz reaction tube with an inner diameter of 3 mm and a length of 78 mm. Then, while flowing helium gas at a flow rate of 20 mL / min, the temperature was increased to 650 °C at a rate of 40 °C / min and heated for 20 minutes. Next, hydrogen gas (reducing gas) was passed through the microreactor at a flow rate of 30 mL / min for 20 minutes to carry out the reduction reaction of the reducing agent (first process). At this time, the gas discharged from the reactor outlet contained water vapor.

[0100] After that, helium gas was passed through the reactor at a flow rate of 20 mL / min for 10 minutes for gas exchange, and then carbon dioxide gas was passed through the reactor at a flow rate of 3 mL / min for 20 minutes to carry out the carbon dioxide reduction reaction (second process), reducing the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the reactor outlet contained carbon monoxide. Thereafter, for gas exchange, helium gas was passed through at a flow rate of 20 mL / min for 10 minutes. In this test, the temperature of the microreactor was maintained at 650°C or 550°C and the test was carried out under atmospheric pressure conditions.

[0101] The efficiency of conversion of carbon dioxide to carbon monoxide by the reducing agent was calculated using the following formula: The conversion efficiency is the average conversion efficiency for one minute after starting the flow of carbon dioxide gas into the quartz reaction tube. X CO (%)=n CO,out / (n CO2,in ) x 100 In the above formula, n is the mole fraction of carbon dioxide or carbon monoxide in the feed gas or product gas. Also, the selectivity of the reaction is 100%, so the total n CO2 =n CO2,in +n CO,out is. In Table 2, the X of the reducing agent in each of Examples B1 to B8 is CO The value of X of the reducing agent of Comparative Example B1 CO The relative value when the value of is "1" (X CO,r ) and as the reducing agent X of Example B9 CO The value of X of the reducing agent of Comparative Example B2 CO The relative value when the value of is "1" (X CO,r )

[0102] The measurement conditions for the gas chromatograph mass spectrometer are as follows: Column temperature: 200℃ Injection temperature: 200℃ Detector temperature: 250℃ Column: EGA tube (L: 2.5 m, φ (inner diameter): 0.15 mm, t: 0 mm) Column flow rate: 1.00mL / min Split ratio: 250 Purge flow rate: 3.0mL / min

[0103] These results are shown in Table 2 below. [Table 2]

[0104] The reducing agents of each example had a high conversion efficiency of carbon dioxide to carbon monoxide. Furthermore, the conversion efficiency of carbon dioxide to carbon monoxide could be adjusted by changing the type of metal element M (doped metal element) constituting the composite metal oxide and the amount of metal element M1 constituting the oxygen carrier. In contrast, the reducing agents of the comparative examples had low conversion efficiencies of carbon dioxide to carbon monoxide. CO The value of X at 650 ° C. in Comparative Example B1 CO This was approximately 1.78 times the value of

[0105] B3. Confirmation of the distribution of each element in the reducing agent The reducing agent obtained in Example B2 was subjected to mapping measurement by STEM-EDS analysis using a transmission electron microscope (HD2700). The results are shown in Figure 1. It was confirmed that the distribution of each element was good (highly dispersible) compared with the distribution of each element shown in Figure 2 of Non-Patent Document 1. It was assumed that this difference in distribution would result in an improvement in the efficiency of converting carbon dioxide to carbon monoxide using the reducing agent of the present invention.

Claims

1. A reducing agent used in producing a product gas containing carbon monoxide by reducing carbon dioxide by contacting it with a source gas containing carbon dioxide, wherein the BET specific surface area of ​​the reducing agent is 1 to 500 m² / g. The reducing agent comprises a carrier and a composite metal oxide containing Ce. The carrier comprises at least one metallic element belonging to groups 2 to 13 of the periodic table. The aforementioned composite oxide is Ce １－ｘ (M) ｘ O ｙ A reducing agent characterized by being expressed as follows: (wherein M is a metal element whose ionic radius is smaller than that of Ce with the same valence and coordination number, and which can be trivalent or tetravalent, x is a positive real number, and y is a real number from 1 to 4.)

2. The reducing agent according to claim 1, wherein the oxygen capacity of the reducing agent at 400°C is 1 to 40% by mass.

3. The reducing agent according to claim 1 or 2, wherein the coordination number of the metal M contained in the composite metal oxide is 6 to 8.

4. The reducing agent according to any one of claims 1 to 3, wherein the reducing agent contains iron.

5. The reducing agent according to any one of claims 1 to 4, wherein the packing density of the reducing agent is 0.5 g / mL or more.

6. The reducing agent according to any one of claims 1 to 5, wherein the pore volume of the reducing agent is 0.4 cm³ / g or more.

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