Method for producing reducing agents and gases

Abstract

[Problem] To provide: a reducing agent that has a high efficiency of conversion of carbon dioxide into a valuable carbon substance (i.e., high yield of a valuable carbon substance) and high efficiency of reduction by hydrogen, and that sufficiently cuts down on carbon dioxide, by setting, in a perovskite oxide, the combination of metal elements, the composition ratio thereof, and the like; and a gas production method using such a reducing agent. [Solution] A reducing agent according to the present invention generates a valuable carbon substance by reduction of carbon dioxide, and is easily reduced by hydrogen. This reducing agent has a perovskite type crystal structure represented by the compositional formula ABOx (where x is a real number of 2-4), and contains an oxygen carrier having oxygen ion conductivity. Site A elements include at least one metal element belonging to Groups 1-3 in the periodic table. Site B elements include at least one metal element differing from the site A elements. The relations Aχ<Bχ and 104×[(Bχ-Aχ) / T]<8.31 are satisfied, where Aχ is the electronegativity of the site A elements, Bχ is the electronegativity of the site B elements, and T(K) is the temperature at which carbon dioxide is brought into contact with the reducing agent.

Description

[Technical Field]

[0001] The present invention relates to reducing agents and methods for producing gases, and more particularly to reducing agents usable in chemical looping methods, and methods for producing gases using such reducing agents. [Background technology]

[0002] In recent years, the concentration of carbon dioxide, a type of greenhouse gas, in the atmosphere has continued to rise. This increase in atmospheric carbon dioxide concentration contributes to global warming. Therefore, capturing carbon dioxide released into the atmosphere is crucial, and if the captured carbon dioxide can be converted into valuable substances for reuse, a carbon-recycling society can be realized. Conventionally, a method utilizing the reverse water-gas shift reaction has been known for producing carbon monoxide from carbon dioxide. However, this conventional reverse water-gas shift reaction has a problem in that the conversion efficiency of carbon dioxide to carbon monoxide is low due to the constraints of chemical equilibrium, as the products, carbon monoxide and water, coexist in the system.

[0003] Therefore, to solve the above problem, the conversion (synthesis) of carbon dioxide to carbon monoxide is carried out using the chemical looping method. The chemical looping method, as referred to here, divides the above reverse water-gas shift reaction into two reactions: a reduction reaction by hydrogen and a carbon monoxide production reaction from carbon dioxide, and these reactions are carried out using an oxygen carrier (e.g., metal oxide: MO x This method involves bridging the gap using (see formula below). H2+ MO x → H2O + MO x-1 CO2+ MO x-1 → CO + MO x In addition, in the above formula, MO x-1 This indicates a state in which some or all of the metal oxide has been reduced.

[0004] In this chemical looping method, since water and carbon monoxide, which are substrates for the reverse reaction, are not present during each reaction, it is possible to obtain a higher efficiency in converting carbon dioxide to carbon monoxide than in the chemical equilibrium of the reverse water-gas shift reaction. For example, Patent Document 1 discloses a catalyst composite usable in a chemical looping method, comprising a perovskite oxide of formula ABO3 (where A is an alkaline earth element, a rare earth element, an alkali metal element, a metal element, or a combination thereof, and B is a transition metal element, a metal element, or a combination thereof) and an oxide support having a different formula from the perovskite oxide. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2018 / 222749 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, according to the inventors' research, Patent Document 1 lists La as a specific example of a perovskite oxide. 0.75 Sr 0.25 Only FeO3 was disclosed, and it was found that a large amount of hydrogen is required to reduce this perovskite oxide. Generating hydrogen inevitably consumes a great deal of energy, and since this energy is also produced in the process of generating carbon dioxide, it cannot be said that it has a high carbon dioxide reduction effect. The present invention has been made in view of the above circumstances, and its object is to provide a reducing agent that, in a perovskite oxide, achieves a sufficient reduction of carbon dioxide while having a high efficiency in converting carbon dioxide into carbon valuables (i.e., the yield of carbon valuables) and / or reduction efficiency by hydrogen, by setting the combination of metal elements, their composition ratio, etc., and a method for producing gas using such a reducing agent. [Means for solving the problem]

[0007] Such an object is achieved by the present invention described below. (1) The reducing agent of the present invention is a reducing agent that generates a carbon-based valuable substance by reducing carbon dioxide, The reducing agent has a perovskite-type crystal structure represented by the composition formula: ABO x (x represents a real number from 2 to 4.) and contains an oxygen carrier having oxygen ion conductivity, The A-site element contains at least one metal element belonging to Groups 1 to 3 of the periodic table, The B-site element contains at least one metal element different from the A-site element, When the electronegativity of the A-site element is Aχ, the electronegativity of the B-site element is Bχ, and the temperature at which carbon dioxide and the reducing agent are brought into contact is T (K), Aχ < Bχ and 10 4 ×[(Bχ - Aχ) / T] < 8.31 is satisfied.

[0008] (2) In the reducing agent of the present invention, it is preferable that the electronegativity Aχ, the electronegativity Bχ, and the temperature T (K) satisfy the relationship of 10 4 ×[(Bχ - Aχ) / T] ≤ 8.07. (3) In the reducing agent of the present invention, it is preferable that the electronegativity Aχ, the electronegativity Bχ, and the temperature T (K) satisfy the relationship of 10 4 ×[(Bχ - Aχ) / T] ≤ 7.47. (4) In the reducing agent of the present invention, it is further preferable that the electronegativity Aχ and the electronegativity Bχ further satisfy the relationship that Bχ - Aχ is 0.90 or less. (5) In the reducing agent of the present invention, it is further preferable that the electronegativity Aχ and the electronegativity Bχ further satisfy the relationship that Bχ - Aχ is 0.75 or less.

[0009] (6) In the reducing agent of the present invention, it is preferable that the electronegativity Aχ is 0.93 to 1.3. <http: / / www.wipo.int / standards / XMLSchema / ST96 / ST96-20090320 / (7) In the reducing agent of the present invention, it is preferable that the electronegativity Aχ is 1 to 1.2. (8) In the reducing agent of the present invention, the electronegativity Bχ is preferably 1.40 to 1.88.

[0010] (9) In the reducing agent of the present invention, it is preferable that the A-site element contains at least one of lanthanum (La), calcium (Ca), strontium (Sr), barium (Ba), neodymium (Nd), samarium (Sm), gadolinium (Gd), and praseodymium (Pr). (10) The reducing agent of the present invention is further preferably characterized in that the A-site element is at least one element selected from lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and magnesium (Mg). (11) In the reducing agent of the present invention, it is preferable that the A-site element comprises a first metal element having an electronegativity greater than 1 and a second metal element having an electronegativity of 1 or less, and the molar ratio of the first metal element to the second metal element is 2.5 or less.

[0011] (12) In the reducing agent of the present invention, it is preferable that the B-site element contains at least one of magnesium (Mg), scandium (Sc), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and gallium (Ga). (13) In the reducing agent of the present invention, it is preferable that the B-site element includes at least one metal element having an electronegativity of 1.83 or less. (14) In the reducing agent of the present invention, it is preferable that the B-site element includes at least one metal element having an electronegativity of 1.81 or less. (15) In the reducing agent of the present invention, it is preferable that the amount of oxygen carrier is more than 90 parts by mass per 100 parts by mass of the reducing agent. (16) The reducing agent of the present invention is preferably used to reduce carbon dioxide by contacting it with a raw material gas containing carbon dioxide, thereby producing a product gas containing carbon monoxide as a carbon valuable substance.

[0012] (17) In the reducing agent of the present invention, it is preferable that the reducing agent is reduced by contact with a reducing gas containing hydrogen. (18) In the reducing agent of the present invention, it is preferable that the amount of hydrogen brought into contact with the reducing agent is 0.01 to 50 mmol per 1 g of the reducing agent. (19) In the reducing agent of the present invention, it is preferable that the amount of hydrogen brought into contact with the reducing agent is 1 to 50 mmol per 1 g of the reducing agent. (20) In the reducing agent of the present invention, it is preferable that the amount of carbon dioxide brought into contact with the reducing agent is 0.01 to 50 mmol per 1 g of the reducing agent. (21) In the reducing agent of the present invention, it is preferable that the amount of carbon dioxide brought into contact with the reducing agent is 1 to 50 mmol per 1 g of the reducing agent.

[0013] (22) It is preferable that the reducing agent of the present invention is used in the reduction reaction of carbon dioxide and the reduction reaction of the reducing agent in separate reaction steps. (23) The present invention is a method for producing a gas characterized by bringing the reducing agent of the present invention into contact with a raw material gas containing carbon dioxide, thereby reducing the carbon dioxide and producing a product gas containing carbon monoxide. [Effects of the Invention]

[0014] According to the present invention, carbon valuables can be efficiently produced from carbon dioxide while significantly reducing carbon dioxide emissions. Furthermore, the reducing agent of the present invention can be used, for example, in the chemical looping method. [Brief explanation of the drawing]

[0015] [Figure 1] This is a schematic diagram showing a perovskite-type crystal structure. [Modes for carrying out the invention]

[0016] The present invention's reducing agent and gas production method will be described in detail below based on preferred embodiments. [Reducing agent] The reducing agent of the present invention is used to reduce carbon dioxide by contacting it with a raw material gas containing carbon dioxide, thereby producing a product gas containing carbon monoxide (a carbon valuable substance) (i.e., it is used in the gas production method of the present invention). Furthermore, the reducing agent can be reduced (regenerated) by contacting the oxidized reducing agent with a reducing gas. Preferably, in this process, the raw material gas and the reducing gas are alternately passed through a reaction tube (reaction vessel) filled with the reducing agent of the present invention, thereby converting carbon dioxide to carbon monoxide by the reducing agent and regenerating the oxidized reducing agent by the reducing gas.

[0017] The reducing agent of the present invention contains an oxygen carrier that has oxygen ion conductivity. Here, an oxygen carrier is a compound that can undergo reversible oxygen deficiency. It loses oxygen from itself through reduction, but when it comes into contact with carbon dioxide in this oxygen-deficient state (reduced state), it exhibits the action of removing oxygen from carbon dioxide and reducing it. The oxygen carrier in this invention has the compositional formula: ABO x It has a perovskite-type crystal structure represented by (x represents a real number between 2 and 4) (see Figure 1). Composition formula: ABO x The crystal structure can be of the perovskite type, and x can take any real number in the range of 2 to 4.

[0018] Here, a perovskite-type crystal structure is defined as having the chemical formula ABO x This refers to a structure in which the B-site element, represented as , exists within an octahedral structure with six oxygen elements at its vertices in the crystal structure, and the A-site element exists in the gaps between multiple such octahedra. This octahedral structure, composed of six oxygen elements at its vertices, may or may not be a regular octahedron, and the crystal structure as a whole may be isotropic or anisotropic. More specifically, the perovskite crystal structure may take the form of a cubic, rhombohedral, tetragonal, or orthorhombic crystal system. Depending on the number of oxygen elements contained in the crystal structure, oxygen elements may not be present at at least one vertex of the octahedral structure. Furthermore, oxygen elements that do not belong to the vertices of the octahedral structure may also be present.

[0019] The metal composition and crystal structure in perovskite-type crystal structures can be measured by emission spectroscopy methods such as energy-dispersive X-ray spectroscopy (EDX), inductively coupled plasma emission spectroscopy (ICP), X-ray fluorescence analysis (XRF), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Preferred methods include ICP, XRD, scanning electron microscope-energy-dispersive X-ray spectroscopy (SEM-EDX), and transmission electron microscope-energy-dispersive X-ray spectroscopy (TEM-EDX), and at least one of these methods can identify the perovskite-type reducing agent. More preferably, the above analytical methods can be used in combination, and the obtained analytical results can be combined to analyze or identify the reducing agent of the present invention.

[0020] In the present invention, the oxygen carrier has a perovskite-type crystal structure in which the A-site element contains at least one metallic element belonging to groups 1 to 3 of the periodic table, and the B-site element contains at least one metallic element different from the A-site element. The inventors focused on the electronegativity of the A-site and B-site elements constituting such a perovskite-type crystal structure and conducted extensive research. In this specification, electronegativity refers to Pauling's electronegativity. Furthermore, if each site element consists of two or more metallic elements, the electronegativity of each site element is calculated as the sum (weighted average) of the electronegativity of the metallic elements that constitute it multiplied by the molar ratio of that metallic element within each site element.

[0021] Specifically, the metallic element composition of the A-site element is A1 x1 A2x2 ···An xn (n is a natural number, xn represents the molar ratio, and x1 + x2 + ··· + xn = 1. When the electronegativity of An is χn, the electronegativity Aχ of the A-site element is χ1 × x1 + χ2 × x2 + ··· + χn × xn.) Also, the electronegativity Bχ of the B-site element is the same as the electronegativity Aχ of the A-site element.

[0022] According to the studies of the present inventors, the electronegativity Aχ of the A-site element contributes to the ease of entry and exit of oxygen element (oxygen ions) with respect to the oxygen carrier (perovskite-type crystal structure), and the electronegativity Bχ of the B-site element is estimated to contribute to enhancing the carbon dioxide adsorption property and the activity of carbon dioxide with respect to the oxygen carrier. And in the present invention, the electronegativity Aχ of the A-site element, the electronegativity Bχ of the B-site element, and the temperature T (K) at which carbon dioxide and the reducing agent are brought into contact are set so as to satisfy the relationship of Aχ < Bχ and 10 4 ×[(Bχ - Aχ) / T] < 8.31. In the present invention, it is preferable to set the electronegativity Aχ of the A-site element and the electronegativity Bχ of the B-site element so as to further satisfy the relationship of Aχ < Bχ and Bχ - Aχ being 0.90 or less.

[0023] In the perovskite-type crystal structure, the A-site element and the B-site element can exist in close proximity. In particular, in the present invention, the electronegativity Aχ of the A-site element and the electronegativity Bχ of the B-site element do not deviate too much. For this reason, oxygen element is easily transferred smoothly between the A-site element and the B-site element. As a result, even with a small amount of reducing gas, oxygen element can be extracted from the oxygen carrier in the oxidized state and its regeneration can be stably performed. Thus, by reducing the amount of the reducing gas used, the energy consumption for generating the reducing gas can be reduced, and as a result, the carbon dioxide generated when obtaining energy can be reduced, so the carbon dioxide reduction effect is high.

[0024] Furthermore, conventionally, when predicting the performance of reducing agents with a perovskite crystal structure, it is necessary to calculate the oxygen vacancy generation energy for each manufactured reducing agent (oxygen carrier) using specialized software such as VASP. The oxygen vacancy generation energy is an important parameter for predicting the performance of oxygen carriers, which is used to predict oxygen diffusion. On the other hand, as the number of added metal species increases and the composition formula becomes more complex, the diversity in local environments at the atomic and lattice levels, such as the degrees of freedom for ion arrangement and the location of defect generation, increases infinitely, making the calculation extremely time-consuming and laborious. In contrast, in the present invention, in an oxygen carrier having a perovskite-type crystal structure, the performance of the reducing agent can be predicted to some extent by setting the types of A-site elements and B-site elements, the relationship between the electronegativity Aχ of the A-site element and the electronegativity Bχ of the B-site element, and the relationship between Aχ, Bχ and the temperature at which the reducing agent is brought into contact. Therefore, the calculation of oxygen vacancy generation energy using dedicated software can be omitted, making it extremely simple.

[0025] 10 4 ×[(Bχ-Aχ) / T] should be less than 8.31, but is preferably 8.07 or less, and more preferably 7.47 or less. Also, the above 10 4 ×[(Bχ-Aχ) / T] is preferably 3.50 or higher. 10 4 By setting ×[(Bχ-Aχ) / T] within the above range, the oxygen carrier's ability to absorb and release oxygen, as well as its carbon dioxide adsorption and activation capabilities, can be appropriately utilized in each temperature range, enabling the efficient conversion of carbon dioxide into carbon valuables. Furthermore, the perovskite crystal structure is easily stabilized.

[0026] The Bχ-Aχ ratio should be 0.90 or less, but is preferably 0.75 or less, more preferably 0.65 or less, and even more preferably 0.55 or less. The Bχ-Aχ ratio is preferably 0.3 or more, more preferably 0.35 or more, and even more preferably 0.4 or more. By setting the Bχ-Aχ ratio within the above range, the above effects can be further improved. In addition, the perovskite-type crystal structure is more easily stabilized. Furthermore, oxygen carriers with a perovskite crystal structure are less likely to adsorb impurities and can maintain their ability to extract oxygen from carbon dioxide for extended periods. As a result, the efficiency of conversion from carbon dioxide to carbon monoxide by reducing agents can be increased, while the reduction efficiency by hydrogen-containing reducing gases can also be enhanced.

[0027] The electronegativity Aχ of the A-site element is preferably 0.93 to 1.3, more preferably 1 to 1.2, even more preferably 1.025 to 1.15, and even more preferably 1.05 to 1.1. This allows for smoother movement of oxygen elements in and out of the oxygen carrier. The A-site element may include at least one metallic element belonging to groups 1 to 3 of the periodic table, but it is preferable that it includes at least one of the following: lanthanum (La; 1.1), calcium (Ca; 1.00), strontium (Sr; 0.95), barium (Ba; 0.89), neodymium (Nd; 1.14), samarium (Sm; 1.17), gadolinium (Gd; 1.2), and praseodymium (Pr; 1.13). The numbers after the element symbols represent the Pauling electronegativity. When the A-site element includes these metallic elements, the exchange of oxygen with the oxygen carrier becomes smoother.

[0028] Furthermore, it is preferable that the A-site element includes a first metal element with an electronegativity greater than 1 and a second metal element with an electronegativity of 1 or less. By including two or more metal elements in such a combination in the A-site element, it becomes easier to adjust the degree of oxygen element exchange with the oxygen carrier. The molar ratio of the first metal element to the second metal element contained in the A-site element is preferably 2.5 or less, more preferably 1.5 or less, and even more preferably 1.2 or less. In this case, it becomes easier to adjust the electronegativity Aχ of the A-site element to the above range.

[0029] Examples of the first metallic element include metallic elements belonging to the lanthanides and actinides, with metallic elements belonging to the lanthanides being preferred, and at least one of lanthanum, samarium, neodymium, and gadolinium being preferred, and neodymium and lanthanum being more preferred. On the other hand, examples of secondary metallic elements include metallic elements belonging to alkali metals and alkaline earth metals, with at least one of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, and barium being preferred, and at least one of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, and strontium being more preferred. By selecting these metallic elements, the A-site and B-site elements interact more highly, allowing for a smoother transfer of oxygen between them.

[0030] On the other hand, the electronegativity Bχ of the B-site element is preferably 1.40 to 1.88, more preferably 1.50 to 1.85, and even more preferably 1.55 to 1.75. This makes it possible to further enhance the adsorption capacity of carbon dioxide to oxygen carriers and the activity of carbon dioxide. The B-site element may include at least one metallic element different from the A-site element, but it is preferable to include at least one of the following: magnesium (Mg; 1.31), scandium (Sc; 1.36), chromium (Cr; 1.66), manganese (Mn; 1.55), iron (Fe; 1.83), cobalt (Co; 1.88), nickel (Ni; 1.91), tungsten (W; 2.36), palladium (Pd; 2.20), aluminum (Al; 1.61), indium (In; 1.78), copper (Cu; 1.90), and gallium (Ga; 1.81). When these metallic elements are present in the B-site element, the adsorption capacity of carbon dioxide to oxygen carriers and the reducing activity of carbon dioxide can be further enhanced, thereby increasing the reduction efficiency by a reducing gas containing hydrogen.

[0031] The B-site element preferably contains at least one metallic element with an electronegativity of 1.83 or less, more preferably at least one metallic element with an electronegativity of 1.81 or less, even more preferably at least one metallic element with an electronegativity of 1.7 or less, and even more preferably at least one metallic element with an electronegativity of 1.6 or less. In this case, it becomes easier to adjust the electronegativity Bχ of the B-site element to the above range. Examples of such metallic elements include magnesium, scandium, chromium, manganese, and gallium, with at least one of magnesium, scandium, chromium, and manganese being preferred, and manganese being more preferred. By selecting these metallic elements, the A-site element and the B-site element interact more highly, and the oxygen element is transferred more smoothly between them. Furthermore, when the B-site element contains two or more metallic elements, suitable combinations include iron and manganese, manganese and magnesium, and manganese and cobalt.

[0032] The amount of oxygen carrier is preferably more than 90 parts by mass, more preferably 95 parts by mass or more, and may be 100 parts by mass, per 100 parts by mass of reducing agent. By setting the amount of oxygen carrier 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 while maintaining a sufficient carbon dioxide reduction effect, that is, to further increase the conversion efficiency, and to increase the reduction efficiency by the reducing gas containing hydrogen. One embodiment in which the reducing agent is not composed entirely of oxygen carriers is one in which oxygen carrier particles are bound together with a binder (support). The binder can be any material that is not easily modified depending on the raw material gas and reaction conditions, and is not particularly limited. Specific examples of binders include carbon materials (graphite, graphene, etc.), zeolites, montmorillonite, SiO2, ZrO2, TiO2, V2O5, MgO, Al2O3, or composite oxides containing these.

[0033] The packing density of the reducing agent is preferably 4 g / mL or less, more preferably 0.5 to 3 g / mL, and even more preferably 1 to 2.5 g / mL. If this packing density is too low, the gas passage rate becomes too fast, reducing the contact time between the reducing agent and the raw material gas and the reducing gas. As a result, the efficiency of the reducing agent's conversion of carbon dioxide to carbon monoxide and the efficiency of the reducing agent's regeneration from its oxidized state by the reducing gas tend to decrease. On the other hand, if this packing density is too high, the gas passage rate becomes too slow, making it difficult for the reaction to proceed or requiring a long time to produce the generated gas.

[0034] The pore volume of the reducing agent is 0.1 cm³. 3 It is preferable that the amount be 1 to 30 cm or more. 3 It is more preferable that the value be / g, and the length is 5-20cm. 3It is even more preferable that the pore volume is / g. If this pore volume is too small, the raw material gas and reducing gas will have difficulty penetrating into the interior of the reducing agent. As a result, the contact area between the reducing agent and the raw material gas and reducing gas decreases, and the efficiency of carbon dioxide conversion to carbon monoxide by the reducing agent and the efficiency of regeneration of the oxidized reducing agent by the reducing gas tend to decrease. 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, it tends to decrease in mechanical strength.

[0035] The shape of the reducing agent is not particularly limited, but granular form is preferred, for example. If it is granular, it is easier to adjust the packing density of the reducing agent within the above range. Here, "granular" is a concept that includes powder, particulate, lump, pellet, etc., and its form can be spherical, plate-like, polygonal, crushed, columnar, needle-like, or flaky. 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. A reducing agent having such an average particle size tends to have a packing density within the above range.

[0036] In this specification, the average particle size refers to the average value of the particle sizes of any 200 reducing agents in a single field of view observed with an electron microscope. In this case, "particle size" refers to the maximum distance between two points on the contour line of the reducing agent. If the reducing agent is columnar, the "particle size" refers to the maximum distance between two points on the contour line of its end face. Furthermore, if the reducing agent is, for example, in a lump form and primary particles are aggregated, the average particle size refers to the average particle size of secondary particles. The BET specific surface area of ​​the reducing agent is 1 to 500 m². 2 It is preferable that the amount be / g, and 3 to 450m 2 It is more preferable that the value be / g, and 5 to 400m 2 It is even more preferable that the BET specific surface area is within the above range. Having a BET specific surface area within the above range makes it easier to improve the efficiency of carbon dioxide conversion to carbon monoxide by the reducing agent.

[0037] Furthermore, in the present invention, the strain of lattice defects in the oxygen carrier (perovskite-type crystal structure) can be 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 (around 400°C) to high temperatures (around 1000°C). In other words, the reducing agent of the present invention can efficiently convert carbon dioxide to carbon monoxide over a wide temperature range and can be efficiently reduced by a reducing gas containing hydrogen. The oxygen capacity of the 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 operating temperatures (650°C or higher), and the reducing agent can be said to have extremely high efficiency in converting carbon dioxide to carbon monoxide.

[0038] [Method for producing reducing agents] Next, we will explain the method for producing the reducing agent. There are no particular limitations on the method for producing the reducing agent, but examples include the sol-gel method, coprecipitation method, solid-phase method, and hydrothermal synthesis method. A reducing agent can be manufactured, for example, as follows: First, an aqueous solution is prepared by dissolving a salt of the metal element constituting the reducing agent in water. Next, this aqueous solution is gelled, dried, and then calcined. In other words, the reducing agent of the present invention can be easily and reliably manufactured by the so-called sol-gel method. For preparing the aqueous solution, acidic water may be used, for example, by adjusting its acidity with citric acid, acetic acid, malic acid, tartaric acid, hydrochloric acid, nitric acid, or a mixture thereof.

[0039] Examples of metal element salts include nitrates, sulfates, chlorides, hydroxides, carbonates, or compound thereof, but nitrates are preferred among these. Hydrates may also be used as metal element salts as needed. The gel should preferably be dried at a temperature of 20 to 200°C, more preferably 50 to 150°C, for a period of 0.5 to 20 hours, more preferably 1 to 15 hours. This drying method ensures that the gel is dried uniformly.

[0040] The gel is preferably calcined at a temperature of 300 to 1200°C, more preferably 700 to 1000°C, for a period of 1 to 24 hours, more preferably 1.5 to 20 hours. The gel preferably becomes an oxide upon calcination, but can be easily converted into a reducing agent by calcination under the above conditions. Furthermore, calcination under the above conditions can prevent excessive particle growth of the reducing agent. Until the above firing temperature is reached, the heating rate should be increased at 1 to 20°C / min, preferably 2 to 10°C / min. This promotes the growth of reducing agent particles and also helps to avoid cracking of the crystals (particles).

[0041] [How to use reducing agents] As described above, the reducing agent of the present invention can be used, for example, in the chemical looping method. Furthermore, as described above, the reducing agent of the present invention can be used for the reduction of 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. In addition, another reducing agent (reducing gas) is used in the reduction reaction of the reducing agent.

[0042] Furthermore, 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, it is carried out by dividing it into the reduction reaction of the reducing agent (first process) and the reduction reaction of carbon dioxide (second process), with the reduction reaction of the reducing agent being the reaction shown in formula (A) below, and the reduction reaction of carbon dioxide being the reaction shown in formula (B) below.

[0043] H2 (gas) + Aa B b O x (solid) →H2O(gas)+A a B b O x-n (Solid) (A) CO2 (gas) + A a B b O x-n (solid) →CO (gas) + A a B b O x (B) In formulas (A) and (B), n is usually a value less than 3, preferably between 0.02 and 1.5, more preferably between 0.1 and 1.2, and even more preferably between 0.15 and 1.0. Within this range, the hydrogen utilization rate of the reducing agent can be increased while maintaining the perovskite crystal structure of the oxygen carrier well. In other words, in the reduction reaction of a reducing agent, hydrogen, a type of reducing gas, is oxidized to produce water. Also, in the reduction reaction of carbon dioxide, carbon dioxide is reduced to produce carbon monoxide.

[0044] The reaction temperature in the reduction reaction of the reducing agent can be any temperature at which the reduction reaction can proceed, but it 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 be carried out efficiently. The upper limit of this reaction temperature is preferably 1000°C or less, more preferably 850°C or less, and even more preferably 800°C or less. By setting the upper limit of the reaction temperature within the above range, economic efficiency can be improved.

[0045] Furthermore, during the reduction reaction of the reducing agent, the amount of hydrogen brought into contact with the oxidized reducing agent is preferably 0.01 to 50 mmol per gram of reducing agent. The amount of hydrogen is preferably 0.1 mmol or more, and more preferably 1 mmol or more, per gram of reducing agent. The amount of hydrogen is preferably 35 mmol or less, and more preferably 20 mmol or less, per gram of reducing agent. The amount of hydrogen is also preferably 1 mmol to 50 mmol per gram of reducing agent. For the reasons described above, the reducing agent of the present invention allows for smooth entry and exit of oxygen atoms, resulting in a high hydrogen utilization rate. Therefore, the reducing agent of the present invention can sufficiently reduce (regenerate) with a small amount of hydrogen. Thus, it contributes to reducing the energy required for hydrogen production and, consequently, to reducing the carbon dioxide generated when obtaining energy.

[0046] The specific hydrogen utilization rate of the reducing agent of the present invention is as follows: When 5.2 mmol of hydrogen is brought into contact with 1 g of the reducing agent, La 0.75 Sr 0.25 The hydrogen utilization rate is preferably 1.05 times or more, more preferably 2 times or more, even more preferably 3 times or more, particularly preferably 4 times or more, and most preferably 5 times or more, compared to the hydrogen utilization rate of a reducing agent (oxygen carrier) having a perovskite-type crystal structure represented by FeO3. The upper limit of the hydrogen utilization rate is usually 18 times or less. The hydrogen utilization rate (%) is the ratio of the amount of carbon monoxide produced (in moles) to the amount of hydrogen added (in moles) that came into contact with 1 g of reducing agent, expressed as a percentage.

[0047] 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 efficient reduction reaction of carbon dioxide can be carried out. The upper limit of this reaction temperature is preferably 1000°C or less, more preferably 850°C or less, and even more preferably 800°C or less. Since the reducing agent can reduce carbon dioxide to carbon monoxide with high efficiency even at low temperatures, the carbon dioxide reduction reaction can be set at a relatively low temperature. Furthermore, by setting the upper limit of the reaction temperature within the above range, not only is it easier to utilize waste heat, but economic efficiency can also be further improved.

[0048] Furthermore, when the carbon dioxide reduction reaction occurs, the amount of carbon dioxide brought into contact with the reducing agent is preferably 0.01 to 50 mmol per gram of reducing agent. The amount of carbon dioxide is preferably 0.1 mmol or more, and more preferably 1 mmol or more, per gram of reducing agent. The amount of carbon dioxide is preferably 30 mmol or less, and more preferably 20 mmol or less, per gram of reducing agent. For the reasons described above, the reducing agent of the present invention readily adsorbs carbon dioxide and allows for smooth entry and exit of oxygen atoms. For this reason, the reducing agent of the present invention has a high efficiency in converting carbon dioxide to carbon monoxide (i.e., a large amount of carbon monoxide is produced), and from this viewpoint as well, it contributes to the reduction of carbon dioxide. On the other hand, since the reduction reaction is carried out efficiently by a reducing gas containing hydrogen, the reducing agent can be regenerated with a small amount of hydrogen.

[0049] The specific amount of carbon monoxide produced by the reducing agent of the present invention is when 5.2 mmol of carbon dioxide is brought into contact with 1 g of the reducing agent, La 0.75 Sr 0.25 The amount of carbon monoxide produced by the reducing agent (oxygen carrier) having a perovskite-type crystal structure represented by FeO3 is preferably 1.05 times or more, more preferably 2 times or more, even more preferably 3 times or more, particularly preferably 4 times or more, and most preferably 5 times or more. The upper limit of carbon monoxide production is usually 18 times or less. The amount of carbon monoxide generated in the reducing agent of the present invention is preferably about 0.95 to 3.75 mmol per 1 g of the reducing agent.

[0050] In this invention, the reduced product (carbon-valuable substance) obtained by the reduction reaction of carbon dioxide may be a substance other than carbon monoxide, specifically methane. It is preferable that the reduced product such as carbon monoxide obtained by the above reduction reaction of carbon dioxide be further converted into organic substances by microbial fermentation or the like. Anaerobic fermentation is an example of microbial fermentation. Examples of organic substances obtained include methanol, ethanol, acetic acid, butanol, derivatives thereof, or mixtures thereof, and C5 or higher compounds such as isoprene. Furthermore, 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 petrochemicals. 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. Specifically, when a reducing agent is packed to a height of 40 cm into a stainless steel reaction tube with an inner diameter of 8 mm and a pressure gauge placed in the flow path, and nitrogen gas with a concentration of 100% by volume is passed through at a rate of 30 mL / min, it is preferable that the pressure rise over 10 minutes be 0.03 MPaG or less, and more preferably 0.01 MPaG or less. Reducing agents exhibiting such characteristics can be judged to satisfy the above ranges in terms of packing density and pore volume, and can sufficiently increase the efficiency of carbon dioxide conversion to carbon monoxide.

[0052] The present invention has described the reducing agent and the method for producing the gas above, but the present invention is not limited to these. For example, the reducing agent and gas production method of the present invention may have any other additional configurations compared to the above embodiment, may be replaced with any configuration that performs a similar function, and some configurations may be omitted. [Examples]

[0053] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0054] (Example 1) 1. Manufacturing of reducing agents First, predetermined amounts of lanthanum nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%), magnesium nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%), and iron(III) nitrate nonahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) were weighed out as precursors to the reducing agent.

[0055] Next, 4.32 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 above precursor (metal nitrate salt) was added to the aqueous citric acid solution at room temperature while stirring to prepare an aqueous precursor solution. The molar ratio of La:Mg:Fe in the aqueous precursor solution was set to 0.3:0.7:1. After 30 minutes, 2.09 g of ethylene glycol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) was added to the precursor aqueous solution, and the temperature was raised to 80°C.

[0056] The mixture was continuously stirred while maintaining a temperature of 80°C until a viscous gel formed. The gel was then transferred to a drying oven. The gel was dried at 120°C for 5 hours. The resulting swollen mass of organic and inorganic compounds was pulverized, heated from room temperature to 450°C at a rate of 8°C / min, and then calcined at 450°C for 4 hours. Subsequently, the temperature was further increased to 950°C at a rate of 8°C / min, and then calcined at 950°C for 8 hours. Finally, the calcined mass was mechanically ground into fine particles to obtain the desired reducing agent. The reducing agent was in granular form.

[0057] 2. Identification of the reducing agent The metal composition in the oxygen carrier (metal oxide) was analyzed and identified by ICP emission spectroscopy using argon gas with an AMETEK SPECTRO ARCOS system. The measurement solution was prepared by the following method: 50-100 mg of reducing agent was dissolved in 100 mL of 1% nitric acid or 1% hydrofluoric acid, and the resulting solution was further diluted 10-fold. Analysis of the above measurement solution revealed that the molar ratio of La:Mg:Fe in the oxygen carrier was 0.3:0.7:1.

[0058] Furthermore, crystallographic data were collected by X-ray diffraction (XRD) using a RINT-TTRIII instrument (CuKα radiation, 50kV, 300mA). As a result, the oxygen carrier (metal oxide) containing La, Mg, and Fe had a perovskite-type crystalline structure, and the La:Mg:Fe (molar ratio) was 0.3:0.7:1, similar to the ratio obtained by ICP emission spectrometry. Furthermore, its content was approximately 100 parts by mass per 100 parts by mass of reducing agent.

[0059] (Examples 2 to 54 and Comparative Examples 1 to 12) In Example 51, the reducing agent was manufactured and identified in the same manner as in Example 1, except that the types and ratios of metal elements constituting the reducing agent (oxygen carrier) were changed to those shown in Tables 1 to 3. In Example 51, the reduced agent was manufactured and identified in the same manner as in Example 1, except that the swollen lumps of organic and inorganic compounds produced in Example 1 were pulverized, the temperature was raised from room temperature to 450°C at a rate of 8°C / min, then calcined at 450°C for 4 hours, and then further raised to 750°C at a rate of 8°C / min, and then calcined at 750°C for 8 hours. Each reducing agent was in granular form. Furthermore, each oxygen carrier (metal oxide) had a perovskite-type crystalline structure, and its content was approximately 100 parts by mass per 100 parts by mass of the reducing agent.

[0060] 3. Characterization of reducing agents The properties of the reducing agent were evaluated using a rapid catalyst evaluation system ("Single μ-Reactor Rx-3050SR" manufactured by Frontier Lab Co., Ltd.) which includes a microreactor and a gas chromatograph-mass spectrometer (GC / MS) directly connected to the microreactor, according to the following procedure.

[0061] First, 0.2 g of reducing agent was packed into 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 raised at a heating rate of 40 °C / min and heated for 20 minutes. Next, to activate the oxygen carriers, hydrogen gas (reducing gas) was flowed at a flow rate of 5 mL / min for 20 minutes to carry out the reduction reaction of the oxygen carriers (first process), thereby reducing the oxygen carriers. At this time, the gas discharged from the outlet contained water vapor. Subsequently, for gas exchange, helium gas was flowed at a flow rate of 5 mL / min for 10 minutes, followed by carbon dioxide gas flowing at a flow rate of 5 mL / min for 20 minutes to carry out the carbon dioxide reduction reaction (second process) and reduce the carbon dioxide gas (source gas). At this time, the product gas discharged from the outlet contained carbon monoxide. Next, to characterize the reducing agent, hydrogen gas (reducing gas) was flowed through the microreactor at a flow rate of 5 mL / min or 15 mL / min for 5 minutes to carry out the reduction reaction (first process) of the reducing agent. Therefore, the amount of hydrogen input was 5.2 mmol or approximately 15 mmol per gram of reducing agent. At this time, the gas discharged from the outlet of the microreactor contained water vapor.

[0062] Subsequently, for gas exchange, helium gas was flowed at a flow rate of 20 mL / min for 10 minutes, followed by carbon dioxide gas flowing 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 (source gas). Therefore, the amount of carbon dioxide input was 5.2 mmol per gram of reducing agent. At this time, the product gas discharged from the reactor outlet contained carbon monoxide. Subsequently, helium gas was flowed at a flow rate of 20 mL / min for 10 minutes to exchange the gas. In this test, the microreactor temperature was maintained at one of the following temperatures, as shown in Tables 1-3, when flowing any of the gases: 650°C (923.15K), 800°C (1073.15K), or 850°C (1123.15K), and the tests were conducted under atmospheric pressure conditions.

[0063] The characteristics of the reducing agent in Example 18 and Comparative Example 13 at a hydrogen input amount of 0.78 mmol / g were evaluated using the following procedure. First, 0.514 g of a cylindrical reducing agent, molded to a major axis of 3 mm, was packed into a quartz reaction tube with an inner diameter of 4 mm and a length of 430 mm. Then, while flowing helium gas at a flow rate of 20 mL / min, the temperature was raised at a heating rate of 40 °C / min and heated for 20 minutes. Next, to activate the oxygen carriers, hydrogen gas (reducing gas) was flowed at a flow rate of 5 mL / min for 20 minutes to carry out the reduction reaction of the oxygen carriers (first process), thereby reducing the oxygen carriers. At this time, the gas discharged from the outlet contained water vapor. Subsequently, for gas exchange, helium gas was flowed at a flow rate of 5 mL / min for 10 minutes, followed by carbon dioxide gas flowing at a flow rate of 5 mL / min for 20 minutes to carry out the carbon dioxide reduction reaction (second process) and reduce the carbon dioxide gas (source gas). At this time, the product gas discharged from the outlet contained carbon monoxide.

[0064] Next, to evaluate the characteristics of the reducing agent, hydrogen gas (reducing gas) was flowed through the microreactor at a flow rate of 3 mL / min for 3 minutes to carry out the reduction reaction (first process) of the reducing agent. Therefore, the amount of hydrogen input was 0.78 mmol per gram of reducing agent. At this time, the gas discharged from the outlet of the microreactor contained water vapor. Subsequently, for gas exchange, helium gas was flowed at a flow rate of 20 mL / min for 10 minutes, followed by carbon dioxide gas flowing at a flow rate of 3 mL / min for 3 minutes to carry out the carbon dioxide reduction reaction (second process) and reduce the carbon dioxide gas (source gas). Therefore, the amount of carbon dioxide input was 0.78 mmol per gram of reducing agent. At this time, the product gas discharged from the reactor outlet contained carbon monoxide. Subsequently, helium gas was flowed at a flow rate of 20 mL / min for 10 minutes to exchange the gas. In this test, the microreactor temperature was maintained at 850°C (1123.15K) and the tests were conducted under atmospheric pressure conditions when flowing any of the gases.

[0065] 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: 2.5m, φ (inner diameter): 0.15mm, t: 0mm) Column flow rate: 1.00 mL / min Split ratio: 250 Purge flow rate: 3.0 mL / min

[0066] The hydrogen utilization rate by the reducing agent was calculated using the following formula. Hydrogen utilization rate (%) = Carbon monoxide production (mmol / 1g of reducing agent) ÷ Amount of hydrogen added (mmol / 1g of reducing agent) × 100 The hydrogen utilization rates are shown in Tables 1 to 3 below. In addition, in Tables 1 to 3, the hydrogen utilization rates of the reducing agents for each of Examples 1 to 54 and Comparative Examples 2 to 12 are shown as relative values ​​with those of Comparative Example 1 set to "1". However, the hydrogen utilization rate when the hydrogen input amount to the reducing agent is 0.78 mmol per gram is shown as a relative value with that of Comparative Example 13 set to "1". These results are shown in Tables 1 to 3 below.

[0067] [Table 1]

[0068] [Table 2]

[0069] [Table 3]

[0070] The reducing agents in each example showed high hydrogen utilization rates. Furthermore, the hydrogen utilization rate could be adjusted by changing the type and ratio of metal elements that make up the reducing agent (oxygen carrier). In contrast, the reducing agents in each comparative example had low hydrogen utilization rates. Furthermore, the composition with the highest hydrogen utilization rate differs depending on the reaction temperature, 10 4 The hydrogen utilization rate at each reaction temperature could be adjusted by changing the value of ×[(Bχ-Aχ) / T].

Claims

1. A reducing agent that produces carbon valuables by reducing carbon dioxide, The reducing agent in question has the chemical formula: ABO ｘ It has a perovskite-type crystal structure represented by (x represents a real number from 2 to 4), and contains oxygen carriers that possess oxygen ion conductivity. The A-site element comprises at least one metallic element selected from lanthanum (La), calcium (Ca), strontium (Sr), barium (Ba), neodymium (Nd), samarium (Sm), gadolinium (Gd), and praseodymium (Pr), and at least one metallic element selected from lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and magnesium (Mg). The B-site element includes at least one metallic element different from the A-site element, When the electronegativity of the A-site element is Aχ, the electronegativity of the B-site element is Bχ, and the temperature at which carbon dioxide is brought into contact with the reducing agent is T (K), then Aχ < Bχ and 10 ４ A reducing agent characterized by satisfying the relationship ×[(Bχ - Aχ) / T] < 8.

31.

2. The electronegativity Aχ, the electronegativity Bχ, and the temperature T(K) are 10 ４ The reducing agent according to claim 1, which satisfies the relationship ×[(Bχ - Aχ) / T] ≤ 8.

07.

3. The electronegativity Aχ, the electronegativity Bχ, and the temperature T(K) are 10 ４ The reducing agent according to claim 1, which satisfies the relationship ×[(Bχ - Aχ) / T] ≤ 7.

47.

4. The reducing agent according to any one of claims 1 to 3, wherein the electronegativity Aχ and the electronegativity Bχ further satisfy the relationship that Bχ - Aχ is 0.90 or less.

5. The reducing agent according to any one of claims 1 to 3, characterized in that the electronegativity Aχ and the electronegativity Bχ further satisfy the relationship that Bχ - Aχ is 0.75 or less.

6. The reducing agent according to any one of claims 1 to 5, wherein the electronegativity Aχ is 0.93 to 1.

3.

7. The reducing agent according to any one of claims 1 to 5, wherein the electronegativity Aχ is 1 to 1.

2.

8. The reducing agent according to any one of claims 1 to 7, wherein the electronegativity Bχ is 1.40 to 1.

88.

9. The A-site element comprises a first metal element having an electronegativity greater than 1 and a second metal element having an electronegativity of 1 or less. The reducing agent according to any one of claims 1 to 8, wherein the molar ratio of the first metal element to the second metal element is 2.5 or less.

10. The reducing agent according to any one of claims 1 to 9, wherein the B-site element comprises at least one of magnesium (Mg), scandium (Sc), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and gallium (Ga).

11. The reducing agent according to claim 10, wherein the B-site element comprises at least one metal element having an electronegativity of 1.83 or less.

12. The reducing agent according to claim 10, wherein the B-site element comprises at least one metal element having an electronegativity of 1.81 or less.

13. The reducing agent according to any one of claims 1 to 12, wherein the amount of the oxygen carrier is more than 90 parts by mass per 100 parts by mass of the reducing agent.

14. The reducing agent according to any one of claims 1 to 13, wherein the reducing agent is used to produce a product gas containing carbon monoxide as a carbon valuable by reducing the carbon dioxide by contacting the carbon dioxide-containing raw material gas.

15. The reducing agent according to any one of claims 1 to 14, wherein the reducing agent is reducible by a reducing gas containing hydrogen.

16. The reducing agent according to claim 15, wherein the amount of hydrogen brought into contact with the reducing agent is 0.01 to 50 mmol per 1 g of the reducing agent.

17. The reducing agent according to claim 15, wherein the amount of hydrogen brought into contact with the reducing agent is 1 to 50 mmol per 1 g of the reducing agent.

18. The reducing agent according to any one of claims 14 to 17, wherein the amount of carbon dioxide brought into contact with the reducing agent is 0.01 to 50 mmol per 1 g of the reducing agent.

19. The reducing agent according to any one of claims 14 to 17, wherein the amount of carbon dioxide brought into contact with the reducing agent is 1 to 50 mmol per 1 g of the reducing agent.

20. The reducing agent according to any one of claims 1 to 19, wherein the reducing agent is used in the reduction reaction of carbon dioxide and the reduction reaction of the reducing agent as separate reaction steps.

21. A method for producing a gas, characterized by bringing a reducing agent according to any one of claims 1 to 20 into contact with a raw material gas containing carbon dioxide, thereby reducing the carbon dioxide and producing a product gas containing carbon monoxide.

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