Reducing agent and method for producing gas

Abstract

To provide a reducing agent that has high conversion efficiency of carbon dioxide to carbon monoxide, and can be used, for example, in a chemical looping method, and a method for producing a gas using the reducing agent.SOLUTION: The reducing agent of the present invention is used in the production of a product gas containing carbon monoxide by bringing a raw material gas containing carbon dioxide into contact with the reducing agent to reduce the carbon dioxide, the reducing agent comprising an oxygen carrier having oxygen ionic conductivity and a basic oxide supported on the oxygen carrier. The basic oxide preferably contains at least one selected from the group consisting of lithium (Li), sodium (Na), potassium (K), magnesium (Mg), manganese (Mn), cobalt (Co), strontium (Sr), and rubidium (Rb).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 reaction to generate carbon monoxide from carbon dioxide. 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, oxygen carriers such as cerium oxide and iron oxide, which have oxygen ion conductivity, are widely used as metal oxides that mediate the reaction. Furthermore, Non-Patent Document 1 describes that the oxygen transfer rate of a mixed oxygen carrier can be accelerated by using a mixed oxygen carrier of nickel oxide and iron oxide supported on a carrier such as Al2O3 and further supporting magnesium oxide, which is a basic oxide. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] ASC Omega 2018,3,4378-4383 Summary of the Invention [Problem to be solved by the invention]

[0006] However, according to the investigations of the present inventors, the mixed oxygen carrier described in Non-Patent Document 1 is not intended for use in a chemical looping reaction aimed at converting carbon dioxide to carbon monoxide. Therefore, this mixed oxygen carrier does not have sufficient reactivity when adsorbing carbon dioxide or sufficient interaction with basic oxides, and therefore, even when used to convert carbon dioxide to carbon monoxide, the conversion efficiency is low. 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]

[0007] 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, an oxygen carrier having oxygen ion conductivity; and a basic oxide supported on the oxygen carrier.

[0008] (2) In the reducing agent of the present invention, the amount of the basic oxide is preferably 60 parts by mass or less per 100 parts by mass of the reducing agent. (3) In the reducing agent of the present invention, it is preferable that the basic oxide contains at least one selected from the group consisting of lithium (Li), sodium (Na), potassium (K), magnesium (Mg), manganese (Mn), cobalt (Co), strontium (Sr), and rubidium (Rb).

[0009] (4) In the reducing agent of the present invention, it is preferable that the oxygen carrier contains at least one selected from the group consisting of vanadium (V), iron (Fe), titanium (Ti), molybdenum (Mo), yttrium (Y), chromium (Cr), lanthanum (La), nickel (Ni), copper (Cu), tungsten (W), niobium (Nb), and cerium (Ce). (5) The reducing agent of the present invention further comprises Ce supported on the oxygen carrier. 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).

[0010] (6) 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. (7) 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).

[0011] (8) 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. (9) In the reducing agent of the present invention, the oxidized reducing agent is preferably reduced by contacting with a reducing gas containing hydrogen.

[0012] (10) 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. (11) 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]

[0013] 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. DETAILED DESCRIPTION OF THE INVENTION

[0014] 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.

[0015] The reducing agent of the present invention comprises an oxygen carrier and a basic oxide supported on the 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. On the other hand, basic oxides are compounds that have the property of adsorbing carbon dioxide. In this way, by supporting a basic oxide on the oxygen carrier, carbon dioxide can be efficiently converted (reduced) to carbon monoxide even at relatively low temperatures, and the activity of the oxygen carrier is well maintained even after repeated use, thereby extending its life as a reducing agent.

[0016] The reason for this effect is unclear, but is presumed to be as follows: The basic oxide adsorbs carbon dioxide, which is stored on the surface of the oxygen carrier. The stored carbon dioxide diffuses to the surface of the oxygen carrier due to the so-called surface migration phenomenon, which is presumed to improve the efficiency of carbon dioxide conversion to carbon monoxide. In particular, in chemical looping reactions, a gas containing a large amount of carbon dioxide as a reactant is brought into contact with a reducing agent, so the adsorption effect of carbon dioxide on the oxygen carrier has a significant impact on the reaction efficiency. Here, the basic oxide may be present on the surface of the oxygen carrier in the form of particles, or in the form of a layer that covers at least a portion of the surface of the oxygen carrier. Furthermore, when a reducing agent is repeatedly used in a chemical looping process, for example, the oxygen carrier repeatedly undergoes deformation due to expansion and contraction, but it is thought that the degree of deformation is alleviated by the basic oxide, thereby suppressing destruction of the oxygen carrier. This is presumably the reason why the activity of the reducing agent is well maintained even after repeated use.

[0017] [[Oxygen Carrier]] The oxygen carrier is not particularly limited as long as it is a compound that can generate reversible oxygen deficiency, but examples thereof include compounds having 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.

[0018] 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 into a compound represented by 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. As mentioned above, the oxygen carrier is MO yM1O 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 of the oxygen carrier 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:

[0019] Examples of the metal element M1 contained in the oxygen carrier 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), iron (Fe), titanium (Ti), molybdenum (Mo), yttrium (Y), chromium (Cr), lanthanum (La), 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. By using an oxygen carrier containing such a metal element M1, the conversion efficiency of carbon dioxide to carbon monoxide is further increased, and the reaction can be easily carried out even at a relatively low temperature. The oxygen carrier may be amorphous or crystalline, and the crystal formed by the oxygen carrier may have any structure.

[0020] [[Basic oxides]] A basic oxide is an oxide of a metal element (metal oxide) that reacts with water to produce a base or with an acid to produce a salt. Among them, a metal oxide that has the property of adsorbing carbon dioxide is preferable. Note that a basic oxide is a compound different from an oxygen carrier. The basic oxide may adsorb carbon dioxide chemically or physically. Here, a basic oxide that chemically adsorbs carbon dioxide refers to a metal oxide that reacts with carbon dioxide and captures carbon dioxide in its molecules, while a basic oxide that physically adsorbs carbon dioxide refers to a metal oxide that captures carbon dioxide by van der Waals forces or the like without going through a chemical reaction. Among these, the basic oxide is preferably a metal oxide that chemically adsorbs carbon dioxide.

[0021] Examples of metal elements contained in the basic oxide include metal elements belonging to Groups 1, 2, 7, and 9 of the periodic table. The metal element is preferably at least one selected from the group consisting of lithium (Li), sodium (Na), potassium (K), magnesium (Mg), manganese (Mn), cobalt (Co), strontium (Sr), and rubidium (Rb), and more preferably at least one selected from the group consisting of lithium, magnesium, manganese, and strontium. These basic oxides are preferred because of their excellent carbon dioxide adsorption ability. The reaction of the basic oxide to adsorb carbon dioxide is an exothermic reaction, and therefore, heat is generated by the basic oxide adsorbing carbon dioxide, which facilitates the reduction reaction of carbon dioxide to carbon monoxide by the oxygen carrier.

[0022] The reducing agent is preferably used in a carbon dioxide reduction reaction that requires a reaction temperature equal to or higher than the temperature at which the equilibrium reaction begins (hereinafter also referred to as the "equilibrium reaction beginning temperature"). The equilibrium reaction beginning temperature refers to the temperature at which an adsorption reaction, which is an equilibrium reaction, begins when a basic oxide reacts with and adsorbs carbon dioxide. For example, if the basic oxide is magnesium oxide (MgO), the equilibrium reaction beginning temperature is approximately 250°C under 1 atmosphere. If the reaction temperature is higher than the equilibrium reaction initiation temperature, carbon dioxide is more easily supplied to the oxygen carrier due to the surface migration phenomenon, and the conversion (reduction) of carbon dioxide to carbon monoxide is more efficiently carried out. For information on the equilibrium reaction of basic oxides, see Energy & Fuels 2007, 21, 426-434, "Screening of CO2 Adsorbing Materials for Zero Emission Power Generation Systems," etc.

[0023] Furthermore, the oxygen carrier supports a basic oxide, which improves the activity of the reducing agent and promotes its stabilization. This is thought to be due to the basic oxide's enhanced carbon dioxide adsorption capacity. For example, when a metal element (alkali metal element or alkaline earth metal element) is added (doped) to an oxygen carrier, a large amount of the oxide of the metal element (basic oxide) is present on the surface of the oxygen carrier. Here, the decomposition of carbon dioxide on the oxygen carrier can be described as a three-step cation redox mechanism consisting of carbon dioxide activation (cleavage of the O-C-O bond and generation of surface oxygen), diffusion of the surface oxygen, and generation of a metal-oxygen-metal bond.

[0024] All these steps are driven by redox reactions and therefore strongly depend on the electronic properties of the surface of the oxygen carrier. When the oxygen carrier incorporates a small amount of metal elements in its interior and on its surface, the surface migration rate of oxygen elements in the interior and on its surface increases. The inventors believe that the addition of a metal element reduces the work function of the surface of the oxygen carrier, and this effect is induced by the surface ionization of the basic oxide on the surface of the oxygen carrier, forming a diffuse surface dipole and imparting a change in electron density to the oxygen carrier. Thus, the reduced work function of the oxygen carrier surface promotes the strong adsorption of carbon dioxide on its surface as well as the conversion of carbon dioxide to carbon monoxide.

[0025] On the other hand, the addition of an excessive amount of metal element tends to inhibit the reduction of carbon dioxide (i.e., conversion to carbon monoxide) for the following four reasons. 1. Steric effect caused by the blocking of the reduction-active sites on the surface of the oxygen carrier by the added metal ions 2. The electronic effect of added metal ions adversely affects not only the desorption of water and carbon monoxide but also the adsorption of hydrogen and carbon dioxide. 3. High basicity of the oxygen carrier surface, which reduces the reaction selectivity from carbon dioxide to carbon monoxide and increases the reaction selectivity to C3, C4, and C5 hydrocarbons. 4. Formation of phase-separated metal oxides by promoting aggregation of active metal elements (Fe in the case of spinel) in the oxygen carrier.

[0026] In the present invention, it is preferable to select a basic oxide that exhibits the above-mentioned properties and use it in an amount within an appropriate range in the reducing agent. Therefore, it goes without saying that the basic oxides that can be used in the present invention are not limited to the basic oxides exemplified above.

[0027] For the reasons mentioned above, the amount of the basic oxide is preferably 60 parts by mass or less, more preferably 50 parts by mass or less, even more preferably 35 parts by mass or less, particularly preferably 20 parts by mass or less, and most preferably 1 to 10 parts by mass, relative to 100 parts by mass of the reducing agent. By setting the quantitative relationship between the oxygen carrier and the basic oxide within the above range, the amount of carbon dioxide adsorbed by the basic oxide can be made appropriate, and the conversion of carbon dioxide to carbon monoxide by the oxygen carrier can be more easily promoted.

[0028] [[Complex metal oxides]] In the reducing agent of the present invention, the oxygen carrier preferably further supports a composite metal oxide that interacts with the oxygen carrier to more efficiently migrate and retain oxygen vacancies (defects) generated when the oxygen carrier loses oxygen elements. Furthermore, the composite metal oxide preferably does not itself participate in the reaction, or participates only partially, and also functions as a structural maintainer for the reducing agent. This can further increase the conversion efficiency. The composite metal oxide has the general formula: Ce 1-x (M) x O y In a composite metal oxide, the constituent metal elements are combined and arranged close to each other, which allows for efficient interaction with the oxygen carrier, resulting in an excellent effect of migrating oxygen vacancies in the oxygen carrier. In contrast, in a mixture of simple metal oxides, the particles are in contact only near the surface, and the effect of migrating oxygen vacancies may not be enhanced to the expected level. Note that the composite metal oxide is a different compound from the oxygen carrier and the basic oxide.

[0029] 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 (M3+ ) is preferably a metal element having an ionic radius of 54 pm or more and less than 101 pm.

[0030] 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 oxygen carrier is strengthened to compensate for the distortion, thereby facilitating smooth migration of oxygen vacancies in the oxygen carrier. Furthermore, simple metal oxides or mixtures thereof tend to easily adsorb impurities contained in the source gas or reducing gas, resulting in low stability, whereas composite metal oxides are less likely to adsorb impurities and can maintain the ability to migrate oxygen vacancies for a long period of time, thereby increasing the efficiency of carbon dioxide conversion to carbon monoxide by the reducing agent.

[0031] Here, x is a positive real number, preferably 0.2 to 0.8, and more preferably 0.3 to 0.7, which tends to make the degree of distortion of lattice defects in the composite metal oxide 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.

[0032] 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, but in either case, 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. This prevents the difference between the ionic radius of Ce and the ionic radius of the metal element M from being too large, making it possible to prevent the metal element M from being separated from the composite metal oxide while generating appropriate distortion in the lattice defects of the composite metal oxide.

[0033] 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), and more preferably samarium. By using any of the above metals as the metal element M, the migration of oxygen vacancies can be sufficiently accelerated, significantly increasing the efficiency of carbon dioxide conversion to carbon monoxide by the reducing agent.

[0034] 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 In 3+ is 80pm, coordination number 6 Nd 3+ is 98.3 pm, and Sc with coordination number 6 3+ is 74.5pm.

[0035] When the oxygen carrier supports a composite metal oxide, 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 setting the quantitative relationship between the oxygen carrier and the composite metal oxide within the above range, the conversion of carbon dioxide to carbon monoxide by the oxygen carrier can be promoted, and the migration of oxygen vacancies in the oxygen carrier by the composite metal oxide can be promoted. Therefore, the efficiency of carbon dioxide conversion to carbon monoxide can be further increased by the reducing agent.

[0036] 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.

[0037] The pore volume of the reducing agent is 0.4 cm 3 / g or more, and 1 to 30 cm 3 / g, and 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 this pore volume is increased beyond the upper limit, no further increase in effect can be expected, and depending on the type of reducing agent, mechanical strength may decrease.

[0038] 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.

[0039] 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.

[0040] Furthermore, in the present invention, the composite metal oxide can promote the migration of oxygen vacancies in the oxygen carrier, 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.

[0041] [Method of producing reducing agent] Next, a method for producing the reducing agent will be described. A reducing agent comprising an oxygen carrier supported with a basic oxide (and, if necessary, a composite metal oxide) can be produced as follows. First, an oxygen carrier, a basic oxide, and, if necessary, salts of metal elements constituting 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 present invention 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.

[0042] 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.

[0043] 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 an oxygen carrier, a basic oxide, or a composite metal oxide. 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).

[0044] In this case, if the amount of the salt of the metal element that constitutes the oxygen carrier is made sufficiently larger than the amount of the salt of the metal element that constitutes the basic oxide and the composite metal oxide, the oxygen carrier will serve as a core, and the basic oxide and the composite metal oxide will be supported on this core. In this case, it is thought that the basic oxide and the composite metal oxide will coat the periphery of the oxygen carrier (core portion) or be scattered in granular form to form a surface layer (shell portion).

[0045] [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.

[0046] 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.

[0047] H2 (gas) + MO y (solid) → H2O (gas) + MO y-n (Solid) (A) CO2 (gas) + MO y-n (solid) → CO (gas) + MO y (Solid) (B) In the formulas (A) and (B), M1, y, and n are the same as above. That is, in the reduction reaction of a reducing agent, hydrogen, which is a type of reducing gas, is oxidized to produce water, and in the reduction reaction of carbon dioxide, carbon dioxide is reduced to produce carbon monoxide.

[0048] 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.

[0049] 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. When the reducing agent contains a basic oxide, the reaction temperature is preferably set to a temperature equal to or higher than the equilibrium reaction initiation temperature of the basic oxide, as described above. 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.

[0050] 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.

[0051] [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.

[0052] 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]

[0053] 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) 1. Preparation of reducing agent First, as precursors of the reducing agent, 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%), 4.20 g of cerium(III) nitrate hexahydrate (manufactured by Sigma-Aldrich Co., Ltd., purity: 99.0%), and 0.06 g of magnesium(II) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) were weighed out.

[0054] 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.

[0055] 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.

[0056] 2. Identification of Reducing Agents 2-1.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).

[0057] 2-2.X-ray diffraction (XRD) Crystallographic data were collected on a RINT-TTRIII instrument (CuKα radiation, 50 kV, 300 mA). The as-prepared reducing agents were Fe2O3 (33.158, 35.618, 40.858, 49.488, 54.098), MgO (29.24, 42.86, 51.64, 62.38, 78.26) and Ce. 0.5 Zr 0.5 O y It showed diffraction peaks corresponding to the (29.108, 33.708, 48.508, 57.508) phase.

[0058] The diffraction pattern of Fe2O3 is clearly identified, while Ce 0.5 Zr 0.5 O y (y = 1 to 2) can be clearly distinguished from the absence of the CeO2 (28.558) peak. 0.5 Zr 0.5 O y The diffraction peaks of closely overlapped with other 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.

[0059] Furthermore, no distinct diffraction peaks of ZrO2 were observed. From the above, the obtained reducing agent is composed of an oxygen carrier represented by Fe2O3 and Ce supported on this oxygen carrier. 0.5 Zr 0.5 O y It was found that the composition contained a composite metal oxide represented by the formula (I) and MgO (basic oxide). The total amount of Fe, Ce, and Zr was 50 parts by mass relative to 100 parts by mass of the reducing agent (the same applies to the following Examples A2 to A5). The amount of MgO was 1 part by mass relative to 100 parts by mass of the reducing agent (that is, the amount of MgO contained in the reducing agent was 1% by mass).

[0060] (Example A2) A reducing agent was produced in the same manner as in Example A1, except that the amount of MgO contained in the reducing agent was 5 mass %. The reducing agent was in granular form. (Example A3) A reducing agent was produced in the same manner as in Example A1, except that the amount of MgO contained in the reducing agent was 10 mass %. The reducing agent was in granular form.

[0061] (Example A4) A reducing agent was produced in the same manner as in Example A2, except that the basic oxide was changed from MgO to Li2ZrO4. The reducing agent was in granular form. (Example A5) A reducing agent was produced in the same manner as in Example A3, except that the basic oxide was changed from MgO to Li2ZrO4. The reducing agent was in granular form. (Comparative Example A1) A reducing agent was prepared in the same manner as in Example A1, except that the basic oxide was omitted. The reducing agent was in granular form.

[0062] (Examples B1 to B3 and Comparative Example B1) The composite metal oxide is omitted, and the oxygen carrier is changed from Fe2O3 to MgFe x Al 2-x Reducing agents were produced in the same manner as in Examples A1 to A3 and Comparative Example A1, except that the reducing agent was changed to O4. Each reducing agent was in granular form. (Examples B4 to B8) Reducing agents were produced in the same manner as in Example B1, except that the type of basic oxide and the amount in the reducing agent were changed as shown in Table 1. Each reducing agent was in granular form.

[0063] (Example C1 and Comparative Example C1) The reducing agents were produced in the same manner as in Example A1 and Comparative Example A1, except that the composite metal oxide was omitted and the oxygen carrier was changed from Fe2O3 to CeO2. Each reducing agent was in granular form. (Example C2) A reducing agent was produced in the same manner as in Example C1, except that the basic oxide was changed from MgO to Li 2 O. The reducing agent was in granular form.

[0064] (Example D1 and Comparative Example D1) The reducing agents were produced in the same manner as in Example C2 and Comparative Example C1, except that the composite metal oxide was omitted and the oxygen carrier was changed from CeO2 to CeZrO2. Each reducing agent was in granular form.

[0065] Example E1 A reducing agent was produced in the same manner as in Example A1, except that the composite metal oxide was omitted and the basic oxide contained in the reducing agent was changed from MgO to SrO, and the amount thereof was changed to 50 mass %. The reducing agent was in granular form.

[0066] Example E2 A reducing agent was produced in the same manner as in Example E1, except that cobalt nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.0%) and rubidium nitrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were used instead of strontium nitrate, and the amounts of Cr2O3 and SrO (basic oxides) contained in the reducing agent were each 1 mass %. The reducing agent was in granular form.

[0067] Example E3 First, 8.08 g of iron (III) nitrate nonahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) and 1.44 g of manganese nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.0%) were weighed out as precursors of the reducing agent. Next, 10.51 g of citric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) was weighed and dissolved in 60 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.

[0068] After 30 minutes had passed, 3.41 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 temperature was maintained at 80°C with continuous stirring until a viscous gel was formed, after which the gel was transferred to a drying oven. The gel was dried at 120°C for 5 hours. The resulting swollen mass of organic and inorganic compounds was crushed and calcined at 450°C for 4 hours at a rate of 5°C / min, followed by further calcination at 950°C for 8 hours at a rate of 5°C / min. Finally, the fired mass was mechanically pulverized to obtain the target reducing agent, which was in granular form.

[0069] (Comparative Example E1) Iron (III) oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 95.0%) was crushed with a pestle for 10 minutes and then baked at 700°C for 3 hours to produce a reducing agent. The reducing agent was in granular form.

[0070] Example F1 First, chromium (III) nitrate nonahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 98.5%) and manganese nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.0%) were prepared as precursors of the reducing agent. Next, this precursor was dissolved in distilled water to prepare a 1 mol / L raw material aqueous solution (liquid A). Also, sodium carbonate was dissolved in distilled water to prepare a 1 mol / L aqueous sodium carbonate solution (liquid B).

[0071] Next, liquids A and B were simultaneously added dropwise to a container containing 20 mL of distilled water at 70°C to form a mixed solution. The rate at which liquid A was added was 10 mL / min, and the mixed solution was stirred to prevent the resulting precipitate from agglomerating. The temperature of the mixed solution was maintained at 70°C, and the rate of dropping Solution B was also adjusted so that the pH would be 7.0. Thereafter, the mixture was aged for 3 hours while continuing to stir while maintaining the temperature at 70° C. After aging was completed, the precipitate was recovered by filtration and thoroughly washed with water. The collected precipitate was then dried in a dryer at 120°C for 12 hours, and then calcined at 700°C for 3 hours to obtain a reducing agent. The reducing agent was in granular form. The amount of MnO2 (basic oxide) contained in the reducing agent was adjusted to 50 mass%.

[0072] (Comparative Example F1) A reducing agent was prepared in the same manner as in Example F1, except that the basic oxide was omitted. The reducing agent was in granular form.

[0073] 3. Characterization of reducing agents (conversion efficiency) 3-1. Characterization of reducing agents A to D 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.

[0074] 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.

[0075] 3-2. Characterization of reducing agents in the E-F series 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, to evaluate the characteristics of the oxygen carrier in a microreactor, the following process was performed to activate the reducing agent. A quartz reaction tube with an inner diameter of 3 mm and a length of 78 mm was filled with 0.2 g of the oxygen carrier. 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 heated for 20 minutes.

[0076] Next, hydrogen gas (reducing gas) was passed through the reaction tube of the microreactor at a flow rate of 5 mL / min for 5 minutes to carry out the reduction reaction of the oxygen carrier (first process), thereby reducing the reducing agent. At this time, the gas discharged from the outlet of the microreactor contained water vapor. After that, for gas exchange, helium gas was passed through at a flow rate of 20 mL / min for 5 minutes, and then carbon dioxide gas was passed through at a flow rate of 5 mL / min for 5 minutes to carry out the carbon dioxide reduction reaction (second process) and reduce the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the outlet of the microreactor contained carbon monoxide. Thereafter, for gas exchange, helium gas was flowed at a flow rate of 20 mL / min for 5 minutes.

[0077] Next, for this test, the following process was carried out. Hydrogen gas (reducing gas) was flowed into the reaction tube of the microreactor at a flow rate of 15 mL / min for 5 minutes to carry out the reduction reaction of the reducing agent (first process) and reduce the oxygen carrier. At this time, the gas discharged from the outlet of the microreactor contained water vapor. After that, for gas exchange, helium gas was passed through at a flow rate of 20 mL / min for 5 minutes, and then carbon dioxide gas was passed through at a flow rate of 5 mL / min for 5 minutes to carry out the carbon dioxide reduction reaction (second process) and reduce the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the outlet of the microreactor contained carbon monoxide. In the above process, the temperature of the reducing agent was maintained at 650° C. when any gas was flowed, and the operation was carried out under atmospheric pressure conditions.

[0078] 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.

[0079] In Table 1, the X of the reducing agent in each of Examples A1 to A5 is 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 ) as the X of the reducing agent in each of Examples B1 to B8 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 ) as the X of the reducing agent in each of Examples C1 and C2 COThe value of X of the reducing agent of Comparative Example C1 CO The relative value when the value of is "1" (X CO,r ) as the reducing agent X of Example D1 CO The value of X of the reducing agent of Comparative Example D1 CO The relative value when the value of is "1" (X CO,r ) In addition, X of the reducing agent in each of Examples E1 to E3 CO The value of X of the reducing agent of Comparative Example E1 CO The relative value when the value of is "1" (X CO,r ) as the reducing agent X of Example F1 CO The value of X of the reducing agent of Comparative Example F1 CO The relative value when the value of is "1" (X CO,r )

[0080] 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

[0081] The results are shown in Tables 1 and 2 below. [Table 1]

[0082] [Table 2]

[0083] 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 basic oxide, 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. CO The value of X at 650°C was about 1 time that of Comparative Example A1, about 1.15 times that of Comparative Example C1, and about 1.39 times that of Comparative Example D1. CO The value of Comparative Example E1 was about 2.16 times that of Comparative Example F1.

Claims

[Claim 1] A reducing agent used in producing a product gas containing carbon monoxide by contacting a raw material gas containing carbon dioxide with the raw material gas to reduce the carbon dioxide, an oxygen carrier having oxygen ion conductivity; and a basic oxide supported on the oxygen carrier.

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