Reducing agent, and gas production method

Abstract

[Problem] To provide: a reducing agent with which it is possible to simplify or omit an operation for separating out valuable carbon, starting with late-stage carbon monoxide; and a gas production method in which such a reducing agent is used. [Solution] According to the present invention, there is provided a reducing agent for reducing carbon dioxide through contact therewith to generate valuable carbon. The reducing agent contains an oxygen carrier provided with oxygen-ion transmission properties. The oxygen carrier includes cerium (Ce) as a primary metal element, and includes a metal element belonging to group 3 of the periodic table as an auxiliary metal element.

Description

Method for producing reducing agent and gas

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

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

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

[0004] In the chemical looping method, because the substrates for the reverse reaction, water and carbon monoxide, do not coexist during each reaction, it is possible to achieve a higher conversion efficiency of carbon dioxide to carbon monoxide than the chemical equilibrium of the reverse water-gas shift reaction. In the chemical looping method, metal oxides that are prone to oxygen deficiency, such as cerium oxide, and mixtures of these, are widely used as metal oxides that mediate the reactions (see Patent Document 1).

[0005] International Publication No. 2020 / 175426

[0006] Patent Document 1 evaluates the performance of metal oxides based on their oxygen storage and release capacity (OSC). This OSC allows the efficiency of carbon dioxide to be converted to carbon monoxide by the metal oxide to be evaluated. According to the inventors' research, while conversion efficiency is important when evaluating metal oxides, when considering simplifying or eliminating the subsequent separation operation of carbon valuables such as carbon monoxide, the ratio of the amount of carbon valuables to the total amount of carbon dioxide and carbon valuables (carbon valuables / carbon dioxide+carbon valuables) in the gas after a predetermined time from contacting carbon dioxide with a reducing agent becomes important. In view of the above circumstances, the present invention provides a reducing agent that can simplify or eliminate the subsequent separation operation of carbon valuables such as carbon monoxide, and a method for producing gas using such a reducing agent.

[0007] According to one aspect of the present invention, there is provided a reducing agent that reduces carbon dioxide by contact to produce carbon values. The reducing agent contains an oxygen carrier having oxygen ion conductivity. The oxygen carrier contains cerium (Ce) as a primary metal element and a metal element belonging to Group 3 of the periodic table as a secondary metal element.

[0008] According to this embodiment, a reducing agent can be obtained that can simplify or omit the separation operation of carbon valuables such as carbon monoxide in the subsequent stage.

[0009]

[0023] Fig. 1 is a schematic diagram illustrating an embodiment of carrying out a chemical looping reaction using the reducing agent of the present invention.

[0024] Fig. 2 is a schematic diagram illustrating an embodiment of carrying out a chemical looping reaction using the reducing agent of the present invention.

[0025] Fig. 3 is a schematic diagram illustrating an embodiment of carrying out a chemical looping reaction using the reducing agent of the present invention.

[0010] The reducing agent and gas production method of the present invention will be described in detail below based on preferred embodiments. [Reducing Agent] The reducing agent of the present invention is used when a feed gas containing carbon dioxide is brought into contact with the reducing agent to reduce carbon dioxide and produce a product gas containing carbon monoxide (a carbon valuable) (i.e., it is used in the gas production method of the present invention). Furthermore, the reducing agent oxidized by contact with carbon dioxide can be reduced (regenerated) by contact with a reducing gas containing hydrogen (a reducing substance). In this case, preferably, the feed gas and the reducing gas are passed alternately through a reaction tube (reaction vessel) filled with the reducing agent of the present invention, whereby the carbon dioxide is converted to carbon monoxide by the reducing agent and the oxidized reducing agent is regenerated by the reducing gas.

[0011] The reducing agent of the present invention contains an oxygen carrier having oxygen ion conductivity. Here, the oxygen carrier refers to a compound that can generate reversible oxygen deficiency, that is, a compound that loses oxygen element by reduction itself, and that, when contacted with carbon dioxide in an oxygen-deficient state (reduced state), exhibits the action of depriving carbon dioxide of oxygen element and reducing it. The oxygen carrier of the present invention contains cerium (Ce) as a main metal element and a metal element belonging to Group 3 of the periodic table as a secondary metal element.

[0012] In such oxygen carriers, the auxiliary metal element is sufficiently solid-dissolved (doped) in the crystal of the main metal element, thereby appropriately distorting the crystal structure of cerium oxide (oxygen carrier). This increases the mobility of oxygen elements (oxygen ions) within the oxygen carrier. Therefore, the oxygen carrier (reducing agent) efficiently releases oxygen elements upon contact with a reducing gas, smoothly inducing oxygen vacancies. This induced oxygen vacancy facilitates the removal of oxygen elements from carbon dioxide, improving the conversion efficiency of carbon dioxide to carbon monoxide (hereinafter also referred to simply as "conversion efficiency"). Furthermore, elements belonging to Group 3 of the periodic table have electrons in d and f orbitals, and these electrons contribute to the formation of bonds with oxygen. The presence of electrons in these orbitals facilitates the removal of oxygen from carbon dioxide, presumably improving the ratio of the amount of carbon monoxide to the total amount of carbon dioxide and carbon monoxide (carbon monoxide / carbon dioxide + carbon monoxide) in the gas after contact with carbon dioxide.

[0013] Therefore, the oxygen carrier of the present invention can sufficiently increase the ratio of the amount of carbon monoxide to the total amount of carbon dioxide and carbon monoxide in the gas after contact with carbon dioxide (hereinafter, also simply referred to as "conversion rate"). Furthermore, since the auxiliary metal element is stably dissolved in the oxygen carrier (cerium oxide), such an oxygen carrier also increases the efficiency of carbon dioxide conversion to carbon monoxide (oxygen capacity). Therefore, according to the present invention, the carbon monoxide separation operation in the subsequent stage can be simplified or omitted. In particular, by selecting a metal element belonging to Group 3 as the auxiliary metal element, the degree of solid solution of the auxiliary metal element and the degree of distortion of the crystal structure tend to be appropriate, thereby further improving the above-mentioned effects.

[0014] The ratio of the molar amount of the main metal element (Ce) to the total molar amount of the metal elements contained in the oxygen carrier is preferably 0.3 to 0.95, more preferably 0.4 to 0.95, even more preferably 0.5 to 0.9, particularly preferably 0.55 to 0.9, and most preferably 0.6 to 0.85. By including Ce as the main metal element in a sufficient amount, the oxygen carrier can maintain the excellent oxygen ion conductivity of cerium oxide. The ratio of the molar amount of the auxiliary metal element to the molar amount of the main metal element (Ce) is preferably 0.01 to 0.99, more preferably 0.1 to 0.5, and even more preferably 0.25 to 0.5. By including the main metal element and the auxiliary metal element in such a ratio (molar ratio), the conversion rate of the oxygen carrier (reducing agent) can be further increased while maintaining the conversion efficiency (oxygen capacity) of the oxygen carrier.

[0015] The auxiliary metal element is a metal element belonging to Group 3, and is preferably yttrium (Y) or a rare earth element, and more preferably at least one of yttrium (Y), lanthanum (La), praseodymium (Pr), samarium (Sm), and gadolinium (Gd). These metal elements are likely to cause distortion in the crystal structure of cerium oxide (oxygen carrier) due to the relationship between their ionic radius and the ionic radius of Ce, and also have electrons in the d orbital and f orbital, and these electrons are easily distorted by the ionic radius of cerium oxide (CeO ２ It is presumed that this contributes to the bond strength of the oxygen element in the oxygen carrier. Therefore, both the conversion rate and conversion efficiency can be further increased. Among these, it is preferable to select La as the auxiliary metal element. Both Ce and La have electrons in the 5d electron orbital, and it is thought that not only is there distortion in the crystal structure due to the difference in their ionic radii, but also that an electronically stable structure is easily formed. The inventors presume that this further increases the mobility of the oxygen element (oxygen ion) in the oxygen carrier.

[0016] The oxygen carrier preferably further contains an auxiliary element other than the auxiliary metal element. By including the auxiliary element, the oxygen carrier can improve the conversion rate and / or conversion efficiency depending on the type of auxiliary element and its combination with other elements. The auxiliary element is preferably at least one element belonging to the fourth and fifth periods of the periodic table. Since the ionic radius of such an auxiliary element is relatively close to that of Ce, it can prevent or suppress the crystal structure of the oxygen carrier from becoming unstable. Specific examples of auxiliary elements include scandium (Sc), chromium (Cr), iron (Fe), manganese (Mn), nickel (Ni), copper (Cu), zirconium (Zr), niobium (Nb), cobalt (Co), zinc (Zn), hafnium (Hf), and indium (In). These auxiliary elements can be used alone or in combination of two or more. Among these, the auxiliary element is preferably one of Fe, Mn, Cu, and Zr.

[0017] The ratio of the molar amount of the auxiliary element to the total molar amount of the main metal element and the auxiliary metal element is preferably 0.7 or less, more preferably 0.5 or less, even more preferably 0.3 or less, and particularly preferably 0.1 or less. The lower limit of this amount ratio (molar ratio) is not particularly limited, but is preferably 0.01 or more, more preferably 0.05 or more. In this case, the effect of the oxygen carrier due to the inclusion of the auxiliary element is fully exhibited. In the gas obtained after contacting carbon dioxide with such an oxygen carrier (reducing agent), the ratio of the amount of carbon monoxide to the total amount of carbon dioxide and carbon monoxide (conversion rate) is preferably 0.85 or more, more preferably 0.9 or more, even more preferably 0.92 or more, and particularly preferably 0.95 or more. If the reducing agent has such a high conversion rate, the carbon monoxide separation operation in the subsequent stage can be more reliably simplified or omitted.

[0018] It is also believed that the crystal structure of the main metal element is stabilized by the presence of the auxiliary metal element in solid solution, thereby improving the heat resistance temperature. As a result, the reducing agent of the present invention can be used at high temperatures exceeding 650°C. Furthermore, when the auxiliary metal element is solid-solved in the crystal of the main metal element, it is believed that the crystal structure is more stable when used at temperatures exceeding 650°C than when used at temperatures below 650°C. At high temperatures exceeding 650°C, the reducing agent is likely to spontaneously return to a thermodynamically stable solid solution structure, and therefore the initial activity is likely to be maintained even when the reducing agent is repeatedly subjected to oxidation-reduction.

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

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

[0021] The packing density of the reducing agent is preferably 4 g / mL or less, more preferably 0.5 g / mL to 3 g / mL or less, and even more preferably 1 g / mL to 2.5 g / mL or less. If this packing density is too low, the gas passage rate becomes too fast, and the contact time between the reducing agent and the raw material gas and the reducing gas is reduced. As a result, the efficiency of converting carbon dioxide to carbon monoxide by the reducing agent and the efficiency of regenerating the oxidized reducing agent by the reducing gas tend to decrease. On the other hand, if this packing density is too high, the gas passage rate becomes too slow, making it difficult for the reaction to proceed and requiring a long time to produce the product gas.

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

[0023] The shape of the reducing agent is not particularly limited, but is preferably granular, for example. Granular reduces the packing density within the above range. Here, granular is a concept that includes powder, particles, lumps, pellets, etc., and the form may be any of spherical, plate-like, polygonal, crushed, columnar, needle-like, and scale-like. The average particle size of the reducing agent is preferably 1 μm or more and 5 mm or less, more preferably 10 μm or more and 1 mm or less, and even more preferably 20 μm or more and 0.5 mm or less. A reducing agent having such an average particle size can easily adjust its packing density within the above range.

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

[0025] Furthermore, in the present invention, because the auxiliary metal element is stably dissolved in the oxygen carrier (cerium oxide), the oxygen capacity of the reducing agent can be maintained at a high level over a wide temperature range from low temperatures (approximately 400°C) to high temperatures (approximately 850°C). In other words, the reducing agent of the present invention can efficiently convert carbon dioxide to carbon monoxide over a wide temperature range, and can be efficiently reduced by a reducing gas containing hydrogen. The oxygen capacity of the reducing agent at 400°C is preferably 0.1 to 40 mass%, and more preferably 0.5 to 30 mass%. If the oxygen capacity of the reducing agent at low temperatures is within the above range, this means that the oxygen capacity is sufficiently high even at temperatures during actual operation (temperatures above 650°C), and it can be said that the reducing agent has an extremely high efficiency of converting carbon dioxide to carbon monoxide.

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

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

[0028] The gel is preferably fired at a temperature of 300°C to 1200°C, more preferably 700°C to 1000°C, for a time of preferably 1 hour to 24 hours, more preferably 1.5 hours to 20 hours. The gel is preferably converted into an oxide by firing, but firing under the above firing conditions can easily convert it into a reducing agent. Furthermore, firing under the above firing conditions can prevent excessive particle growth of the reducing agent. The temperature should be increased at a rate of 1°C / min to 20°C / min, preferably 2°C / min to 10°C / min, until the firing temperature is reached. This promotes particle growth of the reducing agent and also prevents cracking of the crystals (particles).

[0029] The reducing agent can also be produced as follows. First, oxides containing each of the metal elements constituting the reducing agent are mixed and pulverized. For pulverization, for example, a ball mill, bead mill, jet mill, hammer mill, etc. can be used. This pulverization can be performed either dry or wet. If a binder is used, it is mixed with the oxide. In addition to the inorganic binders listed above, organic binders (organic binders) can also be used as the binder. Examples of organic substances include various resins such as olefin resins, acrylic resins, styrene resins, ester resins, ether resins, and vinyl resins, as well as various waxes and fatty acids. In this case, the oxides can be produced using the sol-gel method, coprecipitation method, solid-phase method, hydrothermal synthesis method, etc. Next, the pulverized aggregates are pulverized and then calcined. This firing is preferably carried out at a temperature of 300°C to 1200°C, more preferably 700°C to 1000°C, for a time of preferably 1 hour to 24 hours, more preferably 1.5 hours to 20 hours. In this case, part or all of the organic binder may be lost by firing. The fired mass is then pulverized in the same manner as above to obtain the reducing agent.

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

[0031] The reducing agent of the present invention is preferably used in the so-called reverse water gas shift reaction. The reverse water gas shift reaction is a reaction that produces carbon monoxide and water from carbon dioxide and hydrogen. When the chemical looping method is applied to the reverse water gas shift reaction, the reaction is carried out separately into a reduction reaction of the reducing agent (first process) and a reduction reaction of carbon dioxide (second process). The reduction reaction of the reducing agent is the reaction shown in formula (A) below, and the reduction reaction of carbon dioxide is the reaction shown in formula (B) below.

[0032] H ２ (gas) + nMO ｘ (Solid) → H ２ O (gas) + nMO ｘ－１／ｎ (solid) 　 (A) CO ２ (gas) + nMO ｘ－１／ｎ (solid) → CO (gas) + nMO ｘ (Solid) (B) In formulas (A) and (B), n is a natural number, and x varies depending on the metal element contained in M. For example, when M is cerium (Ce), it is usually 2. That is, in the reduction reaction of the reducing agent, hydrogen, which is a type of reducing substance, is oxidized to produce water. In addition, in the reduction reaction of carbon dioxide, carbon dioxide is reduced to produce carbon monoxide, which is a type of carbon valuable.

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

[0034] Furthermore, during the reduction reaction of the reducing agent, the amount of hydrogen (reducing gas) contacted with the oxidized reducing agent is preferably 0.01 mmol / g or more and 50 mmol / g or less, more preferably 0.1 mmol / g or more and 45 mmol / g or less, even more preferably 0.5 mmol / g or more and 35 mmol / g or less, particularly preferably 1 mmol / g or more and 20 mmol / g or less, and most preferably 2 mmol / g or more and 10 mmol / g or less, per unit mass of the oxygen carrier. The reducing agent of the present invention has a high hydrogen utilization rate because oxygen elements can be smoothly transferred in and out. Therefore, the reducing agent of the present invention is sufficiently reduced (regenerated) with a small amount of hydrogen. This reduces the energy required for hydrogen production and ultimately contributes to reducing carbon dioxide generated during energy production. The hydrogen utilization rate (%) is the ratio, expressed as a percentage, of the amount of carbon monoxide produced (number of moles) to the amount of hydrogen input (number of moles) contacted with 1 g of reducing agent. Furthermore, when the reducing agent is composed of an oxygen carrier alone (single), "per unit mass of the oxygen carrier" can be read as "per unit mass of the reducing agent."

[0035] Furthermore, the reaction temperature (contact temperature of the reducing agent with carbon dioxide) in the carbon dioxide reduction reaction is preferably above 650°C, more preferably 700°C or higher, even more preferably 750°C or higher, and particularly preferably 800°C or higher. Within this temperature range, the carbon dioxide reduction reaction can proceed efficiently. The upper limit of this reaction temperature is preferably 1050°C or lower, more preferably 1000°C or lower, even more preferably 950°C or lower, particularly preferably 900°C or lower, and most preferably 850°C or lower. Since the reducing agent can efficiently reduce carbon dioxide to carbon monoxide even at low temperatures, the carbon dioxide reduction reaction can be set to a relatively low temperature. Furthermore, setting the upper limit of the reaction temperature within the above range not only facilitates waste heat utilization but also further improves economic efficiency. The lower limit and upper limit can be combined in any way to define the reaction temperature range.

[0036] Furthermore, during the reduction reaction of carbon dioxide, the amount of carbon dioxide (raw material gas) contacted with the reducing agent reduced by contact with hydrogen (reducing gas) is preferably 0.01 mmol / g or more and 50 mmol / g or less per unit mass of the oxygen carrier, more preferably 0.1 mmol / g or more and 45 mmol / g or less, even more preferably 0.5 mmol / g or more and 35 mmol / g or less, particularly preferably 1 mmol / g or more and 20 mmol / g or less, and most preferably 2 mmol / g or more and 10 mmol / g or less. The reducing agent of the present invention allows smooth inflow and outflow of oxygen elements. Therefore, the reducing agent of the present invention has a high conversion efficiency of carbon dioxide to carbon monoxide (i.e., a large amount of carbon monoxide is produced), and from this perspective, it also contributes to the reduction of carbon dioxide. On the other hand, since the reduction reaction is efficiently carried out using a reducing gas containing hydrogen, the reducing agent can be regenerated with a small amount of hydrogen. Furthermore, the concentration of carbon dioxide contained in the raw material gas to be contacted with the reducing agent is preferably 50% or more, more preferably 80% or more, even more preferably 85% or more, particularly preferably 90% or more, and most preferably 95% or more. This makes it possible to further simplify or eliminate the operation of separating the converted carbon monoxide. The raw material gas may be carbon dioxide gas with a concentration of 100%.

[0037] In the gas produced at this time, the ratio of the amount of carbon monoxide to the total amount of carbon dioxide and carbon monoxide (conversion rate) is preferably 0.85 or more (more preferably 0.9 or more, even more preferably 0.92 or more, particularly preferably 0.95 or more). In the gas satisfying these conditions, the amount of carbon monoxide per unit mass of the oxygen carrier is preferably 0.01 mmol / g or more, more preferably about 0.1 mmol / g or more and 1 mmol / g or less.

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

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

[0040] [Gas Production Method] The gas production method of the present invention is a method for producing a product gas containing carbon valuables by reducing carbon dioxide by contacting a source gas containing carbon dioxide with a reducing agent. The reducing agent used in the gas production method of the present invention contains an oxygen carrier having oxygen ion conductivity, and this oxygen carrier contains cerium (Ce) as a primary metal element and a metal element belonging to Group 3 of the periodic table as a secondary metal element. The preferred configuration, shape, characteristics, etc. of the reducing agent are the same as those described above.

[0041] In the gas production method of the present invention, when a reducing gas containing hydrogen in an amount of 0.01 mmol / g to 50 mmol / g per unit mass of oxygen carrier is contacted with a reducing agent, and then a raw material gas containing carbon dioxide is contacted, it is preferable to produce a product gas in which the ratio of the amount of carbon values ​​to the total amount of carbon dioxide and carbon values ​​(conversion rate) is 0.85 or more and the amount of carbon values ​​is 0.01 mmol / g or more per unit mass of oxygen carrier. Note that the preferred ranges for the amount of reducing gas contacted with the reducing agent, the conversion rate, the amount of carbon values ​​produced, and the like are also the same as those described above.

[0042] As described above, the present invention provides a reducing agent that can simplify or eliminate the subsequent separation operation of carbon valuables such as carbon monoxide, and a method for producing a gas using the reducing agent. The reducing agent of the present invention can withstand use at high temperatures, and therefore enables more efficient conversion of carbon dioxide to carbon monoxide (a carbon valuable) through a chemical looping reaction at high temperatures.

[0043] Furthermore, it may be provided in the following aspects.

[0044] (1) A reducing agent that reduces carbon dioxide through contact to produce carbon valuables, the reducing agent containing an oxygen carrier having oxygen ion conductivity, the oxygen carrier containing cerium (Ce) as a primary metal element and a metal element belonging to Group 3 of the periodic table as a secondary metal element.

[0045] (2) The reducing agent according to (1) above, wherein the ratio of the molar amount of the main metal element to the total molar amount of the metal elements contained in the oxygen carrier is 0.3 or more and 0.95 or less.

[0046] (3) In the reducing agent according to (1) or (2) above, the auxiliary metal element is at least one of yttrium (Y), lanthanum (La), praseodymium (Pr), samarium (Sm), and gadolinium (Gd).

[0047] (4) The reducing agent according to any one of (1) to (3) above, wherein the oxygen carrier further contains an auxiliary element other than the auxiliary metal element.

[0048] (5) The reducing agent according to (4) above, wherein the ratio of the molar amount of the auxiliary element to the total molar amount of the main metal element and the auxiliary metal element is 0.7 or less.

[0049] (6) The reducing agent according to (4) or (5) above, wherein the auxiliary element is at least one element belonging to the fourth and fifth periods of the periodic table.

[0050] (7) The reducing agent according to any one of (4) to (6) above, wherein the auxiliary element is at least one of iron (Fe), manganese (Mn), copper (Cu), and zirconium (Zr).

[0051] (8) The reducing agent according to any one of (1) to (7) above, wherein the ratio of the molar amount of the auxiliary metal element to the molar amount of the main metal element is 0.01 or more and 0.99 or less.

[0052] (9) The reducing agent according to any one of (1) to (8) above, further comprising a binder that binds the oxygen carrier.

[0053] (10) The reducing agent according to (9) above, wherein the proportion of the binder contained in the reducing agent is 1 part by mass or more and 60 parts by mass or less per 100 parts by mass of the reducing agent.

[0054] (11) In the reducing agent according to (9) or (10) above, the binder is an oxide containing at least one of magnesium (Mg), titanium (Ti), zirconium (Zr), aluminum (Al), and silicon (Si).

[0055] (12) The reducing agent according to any one of (1) to (11), wherein the carbon valuable material includes carbon monoxide.

[0056] (13) The reducing agent according to any one of (1) to (12) above, wherein the contact with the carbon dioxide is carried out at a temperature exceeding 650°C.

[0057] (14) The reducing agent according to any one of (1) to (13) above, wherein the reducing agent oxidized by contact with the carbon dioxide is reduced by contact with a reducing gas containing hydrogen.

[0058] (15) The reducing agent according to (14) above, wherein the contact with the hydrogen-containing reducing gas is carried out at a temperature exceeding 650°C.

[0059] (16) The reducing agent according to (14) or (15) above, wherein the amount of the hydrogen-containing reducing gas to be brought into contact with the reducing agent is 0.01 mmol / g or more and 50 mmol / g or less per unit mass of the oxygen carrier.

[0060] (17) The reducing agent according to any one of (1) to (16) above, wherein, in a gas obtained after contacting the reducing agent with the carbon dioxide, the ratio of the amount of the carbon valuable to the total amount of the carbon dioxide and the carbon valuable is 0.85 or more.

[0061] (18) The reducing agent according to (16) or (17) above, wherein, in a gas obtained after contacting the carbon dioxide with the reducing agent reduced by contact with the hydrogen-containing reducing gas, the ratio of the amount of the carbon valuables to the total amount of the carbon dioxide and the carbon valuables is 0.85 or more, and the amount of the carbon valuables is 0.01 mmol / g or more per unit mass of the oxygen carrier.

[0062] (19) A gas production method for producing a product gas containing carbon valuables by reducing a raw material gas containing carbon dioxide by contacting the raw material gas with a reducing agent, wherein the reducing agent contains an oxygen carrier having oxygen ion conductivity, and the oxygen carrier contains cerium (Ce) as a primary metal element and a metal element belonging to Group 3 of the periodic table as a secondary metal element.

[0063] (20) In the gas production method described in (19) above, when a reducing gas containing hydrogen in an amount of 0.01 mmol / g or more and 50 mmol / g or less per unit mass of the oxygen carrier is contacted with the reducing agent and then a raw material gas containing carbon dioxide is contacted, the product gas produced has a ratio of the amount of the carbon values ​​to the total amount of the carbon dioxide and the carbon values ​​of 0.85 or more and an amount of the carbon values ​​of 0.01 mmol / g or more per unit mass of the oxygen carrier. Of course, this is not limited to the above.

[0064] As described above, various embodiments of the present invention have been described, but these are presented as examples and do not limit the scope of the invention in any way. The novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. Such embodiments and their modifications are intended to be included within the scope and spirit of the invention, as well as within the scope of the inventions and their equivalents as set forth in the claims.

[0065] For example, the reducing agent and gas manufacturing method of the present invention may have any other additional components compared to the above-mentioned embodiments, may be replaced with any components that exhibit similar functions, or may omit some components. In the above-mentioned embodiments, a gas containing hydrogen has been described as a representative reducing gas, but the reducing gas may also use a gas containing at least one selected from hydrocarbons (e.g., methane, ethane, acetylene, etc.) and ammonia as a reducing substance instead of or in addition to hydrogen.

[0066] Furthermore, as the main metal element, at least one of iron (Fe), manganese (Mn), copper (Cu), zirconium (Zr), strontium (Sr), barium (Ba), and calcium (Ca) can be used in place of or in combination with cerium (Ce). In this case, an element different from the main metal element can be selected and used as the auxiliary element.

[0067] In particular, by using the reducing agent of the present invention in a chemical looping reaction, the carbon monoxide separation operation can be simplified or omitted. This chemical looping reaction can be carried out in the manners shown in Figures 1 to 3. Figures 1 to 3 are schematic diagrams illustrating the manner in which a chemical looping reaction is carried out using the reducing agent of the present invention. In the manner shown in Figure 1, the reaction is carried out by sequentially supplying a reducing gas and a raw material gas to a single reactor 1 containing a reducing agent 2.

[0068] 2, a reducing gas is supplied to one of two reactors 1a, 1b containing a reducing agent 2, and a raw material gas is supplied to the other, and the gas to be supplied (reducing gas or raw material gas) is switched over time. Furthermore, in the embodiment shown in Fig. 3, a reducing gas is supplied to three of a plurality of reactors 1a to 1d (four in the illustrated configuration) containing a reducing agent 2, and a raw material gas is supplied to the remaining reactor, and the gas to be supplied (reducing gas or raw material gas) is switched over time.

[0069] Specifically, in the first turn shown in FIG. 3(I), the source gas (CO ２ ) is supplied to the reactor 1a, and the raw material gas (CO) that has passed through it is discharged. On the other hand, reducing gas (H ２ ) is fed to reactor 1b, and then the reducing gas (remaining H ２ ) is supplied to the reactor 1c. After that, the reducing gas (remaining H ２ ) is supplied to the reactor 1d, and the reducing gas (H ２ O) is discharged.

[0070] Next, in the second turn shown in FIG. 3(II), the raw material gas (CO ２ ) was fed and passed through the reactors 1c, 1d, and 1a in this order, while reducing gas (H ２ ) is continuously supplied and passed through the reactor 1c. Next, in the third turn shown in FIG. 3(III), the raw material gas (CO ２ ) is fed and passed through the reactors 1d, 1a, and 1b in this order, while reducing gas (H ２Next, in the fourth turn shown in FIG. 3(IV), the raw material gas (CO ２ ) is fed and passed through the reactors 1a, 1b, and 1c in this order, while reducing gas (H ２ ) are continuously fed and passed through.

[0071] In this embodiment, a series of operations from the first turn to the fourth turn is regarded as one cycle, and by repeating multiple cycles, CO ２ For example, CO ２ From the conversion efficiency of H ２ When using a reducing agent 2 with low reduction efficiency of the oxidized reducing agent 2 by (reducing substance), if the reducing gas is passed through one reactor only once, the H that was not used for the reduction of the oxidized reducing agent 2 will be used. ２ (Remaining H ２ In contrast, in this embodiment, the reducing gas is passed through three reactors in succession, in other words, it can be passed through one reactor three times. ２ This can prevent the reduction gas from being wasted.

[0072] The number of reactors through which the reducing gas is continuously passed is not limited to three, and may be two, four, or more. The raw material gas may be continuously passed through two or more reactors. In this case, by appropriately controlling the connection order of the multiple reactors or the supply order of the reducing gas and raw material gas, the reducing gas can be continuously passed through one reactor two or more times. This increases the apparent amount of reducing gas (hydrogen) per unit mass of the oxygen carrier, thereby efficiently reducing (regenerating) the oxidized reducing agent 2.

[0073] 3, the amount of the reducing gas (hydrogen) supplied per unit mass of the oxygen carrier only appears to increase. Therefore, in this specification, the amount of the reducing gas per unit mass of the oxygen carrier that is actually brought into contact with the reducing agent 2 is defined based on the unit mass of the oxygen carrier that can react with the raw material gas.

[0074] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. 1. Preparation of reducing agent precursors The following compounds were prepared as reducing agent precursors (sources of elements constituting the reducing agent). (Main metal element sources) Cerium (III) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 98.0%) Sodium tungstate (VI) dihydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)

[0075] (Auxiliary metal elements) Lanthanum (III) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) Gadolinium nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) Samarium nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) Yttrium (III) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: approximately 100%)

[0076] Praseodymium (III) nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) Cesium nitrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%)

[0077] (Auxiliary elements) Iron (III) nitrate nonahydrate (Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) Manganese (II) nitrate hexahydrate (Fujifilm Wako Pure Chemical Industries, Ltd., purity: 98.0%) Copper (II) nitrate trihydrate (Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.0%) Zirconium nitrate dihydrate (Fujifilm Wako Pure Chemical Industries, Ltd., purity: 97.0%)

[0078] (Binder) Titanium (IV) chloride solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., containing 16.0 to 17.0% titanium and 29.0 to 33.0% chlorine), or 30% titanium (IV) sulfate solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) Aluminum (III) nitrate nonahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) Aluminum oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)

[0079] 2. Preparation of Reducing Agent (Example 1) First, predetermined amounts of cerium (III) nitrate hexahydrate, lanthanum (III) nitrate hexahydrate, and iron (III) nitrate nonahydrate were weighed out. Next, 6.06 g of citric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.5%) was weighed out and dissolved in 96 mL of deionized water to obtain a citric acid aqueous solution. The precursor (metal nitrate salt) was then added to the citric acid aqueous solution at room temperature while stirring to prepare a precursor aqueous solution. The molar ratio of Ce:La:Fe in the precursor aqueous solution was 0.7:0.3:0.05. After 30 minutes, 2.15 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.

[0080] The temperature was maintained at 80°C with continuous stirring until a viscous gel was formed. The gel was then transferred to a drying oven. The gel was dried at 120°C for 5 hours. The resulting swollen mass of organic and inorganic compounds was pulverized and heated from room temperature to 450°C at a rate of 8°C / min in an air atmosphere, followed by calcination at 450°C for 4 hours. The temperature was then further increased to 950°C at a rate of 8°C / min, followed by calcination at 950°C for 8 hours. Finally, the calcined mass was mechanically pulverized to obtain the target reducing agent composed solely of the oxygen carrier. The reducing agent was in granular form. For ease of loading into the reactor or to satisfy the pressure increase requirements described in the "Properties of the Reducing Agent" section, the reducing agent was optionally compressed into tablets for testing.

[0081] (Examples 2 to 27 and Comparative Examples 1 and 2) By changing the type and amount of the precursor of the reducing agent or binder used, reducing agents composed of an oxygen carrier alone or an oxygen carrier and a binder were produced in the same manner as in Example 1. The reducing agents were in granular form.

[0082] Example 28 First, an oxygen carrier was produced so that the molar ratio of Ce:La:Fe was 0.9:0.1:0.01 in the same manner as in Example 1. Next, 2.90 g of this was weighed out and mixed with 0.10 g of aluminum oxide, and then pulverized in a mortar for 10 minutes to obtain a reducing agent composed of an oxygen carrier and a binder.

[0083] Comparative Example 3 First, sodium tungstate (VI) dihydrate, lanthanum (III) nitrate hexahydrate, and manganese (II) nitrate hexahydrate were prepared as raw materials. Next, these raw materials were dissolved in distilled water to prepare a 1 mol / L raw material aqueous solution (liquid A). The molar ratio of La, W, and Mn in liquid A was adjusted to 1:1:1. Furthermore, sodium carbonate was dissolved in distilled water to prepare a 1 mol / L sodium carbonate aqueous solution (liquid B).

[0084] Next, liquids A and B were simultaneously dropped into a container containing 20 mL of distilled water at 70°C to prepare a mixed solution. The dropping rate of liquid A 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 dropping rate of liquid B was also adjusted so that the pH was 7.0. Thereafter, the mixed solution was aged for 3 hours while continuing to stir at a speed of 50 rpm or higher while maintaining the temperature at 70°C. After aging, the precipitate was collected by filtration and thoroughly washed with water.

[0085] The recovered precipitate was then dried in a dryer at 120°C for 12 hours, and then heated from room temperature to 450°C at a rate of 8°C / min in an air atmosphere, followed by calcination at 450°C for 4 hours. The temperature was then further increased to 950°C at a rate of 8°C / min, followed by calcination at 950°C for 8 hours. Finally, the calcined mass was mechanically pulverized to obtain the target reducing agent composed solely of the oxygen carrier. The reducing agent was in granular form. To facilitate filling into the reaction tube or to satisfy the pressure rise requirement described in the section [Properties of the Reducing Agent], the reducing agent was tableted as needed and used in the tests.

[0086] 3. Identification of the Reducing Agent The metal composition in the reducing agent (oxygen carrier) was analyzed and identified by ICP atomic emission spectroscopy using argon gas using a SPECTRO ARCOS manufactured by AMETEK Corporation. The measurement solution was prepared as follows: 50 to 100 mg of the reducing agent was dissolved in 100 mL of 1% nitric acid or 1% hydrofluoric acid, and the resulting solution was further diluted 10 times. The analysis results of the above measurement solution (molar ratio of metal elements) are shown in Table 1.

[0087] 4. Measurement of Conversion Rate Using a rapid catalyst evaluation system equipped with a fixed-bed flow reactor and a gas chromatograph mass spectrometer (GC / MS) directly connected to the reactor, the amount of carbon monoxide produced by the reducing agent was measured according to the following procedure. Specifically, a quartz reaction tube with an inner diameter of 4 mm was prepared and filled with a powdered reducing agent or a cylindrical reducing agent molded to a major axis of 3 mm so that the filling amount was 200 mg to 1100 mg.

[0088] Next, while flowing helium gas at a flow rate of 5 mL / min, the temperature was increased to 850 °C at a rate of 15 °C / min, and the temperature was stabilized for 5 minutes. Next, to activate the oxygen carrier, hydrogen gas (reducing gas) was flowed at a flow rate of 5 mL / min for 20 minutes to carry out a reduction reaction of the oxygen carrier (first process), thereby reducing the oxygen carrier. At this time, the gas discharged from the outlet contained water vapor. Thereafter, for gas exchange, helium gas was flowed at a flow rate of 5 mL / min for 10 minutes, and then carbon dioxide gas was flowed at a flow rate of 5 mL / min for 20 minutes to carry out a reduction reaction of carbon dioxide (second process), thereby reducing the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the outlet contained carbon monoxide.

[0089] Next, for this test, the following process was performed. First, helium gas was flowed at a flow rate of 5 mL / min for 10 minutes for gas exchange. Next, hydrogen gas (reducing gas) was flowed at a flow rate of 3 mL / min for 4 to 16 minutes, or at a flow rate of 5 mL / min for 5 minutes to carry out a reduction reaction of the reducing agent (first process), thereby reducing the reducing agent. At this time, the gas discharged from the outlet contained water vapor.

[0090] After that, for gas exchange, helium gas was flowed at a flow rate of 5 mL / min for 5 minutes, and then carbon dioxide gas was flowed at a flow rate of 3 mL / min for 4 to 16 minutes or at a flow rate of 5 mL / min for 5 minutes to carry out a carbon dioxide reduction reaction (second process) to reduce the carbon dioxide gas (raw material gas). At this time, the product gas discharged from the outlet contained carbon monoxide. In the above processes, the temperature of the reducing agent was maintained at 800°C or 850°C when flowing any of the gases, and the processes were carried out under atmospheric pressure conditions.

[0091] The conversion rate was calculated from the time when the amount of carbon monoxide produced per unit second reached 0.001 mmol or more after the start of flowing carbon dioxide gas (feed gas) until the conversion rate value calculated according to the following formula based on the detected amounts of carbon monoxide and carbon dioxide calibrated from the detection signal intensity obtained by GC / MS, described below, fell below 0.80. The average value of the values ​​calculated during this period was defined as the conversion rate. Note that the region where the amount of carbon monoxide produced was 0.001 mmol or less was not used in calculating the conversion rate. Conversion rate = (detected amount of carbon monoxide per unit second) ÷ (detected amount of carbon monoxide per unit second + detected amount of carbon dioxide per unit second)

[0092] The amount of carbon monoxide in the region where the conversion rate calculated by the above formula did not fall below 0.80 was detected by GC / MS and defined as the amount of carbon monoxide produced. The amount of carbon monoxide produced was evaluated according to the following criteria: [Evaluation criteria] ⊚: 0.1 mmol / g or more ◯: 0.01 mmol / g or more and less than 0.1 mmol / g △: Less than 0.01 mmol / g

[0093] (GC / MS analysis conditions) For the GC / MS analysis, a "QP-2020" manufactured by Shimadzu Corporation was used. The conditions for the GC section were as follows: Column temperature: 200°C Injection temperature: 200°C Detector temperature: 250°C Column: EGA tube (L: 2.5 m, φ (inner diameter): 0.15 mm, t: 0 mm) Column flow rate: 0.55 mL / min Split ratio: 400 Purge flow rate: 5.0 mL / min

[0094] The MS section (detection section) was used after being calibrated under the following conditions: A mass flow controller calibrated with carbon dioxide, with a discharge full scale of 10 mL / min or 50 mL / min, a flow rate accuracy of ±1.0%, and a reproducibility accuracy of ±0.2%, was used to plot the detection signal intensity when carbon dioxide was flowed at 1, 2, and 3 mL / min, and the calibration curve, when approximated by a straight line passing through the origin using the least squares method, was found to be within R ２ The detection signal strength was adjusted so as to satisfy the relationship of >0.98. Based on this, the amount of carbon dioxide detected relative to the detection signal strength was calculated. Carbon monoxide was also calibrated in the same manner using the following procedure. The detection signal strengths when carbon monoxide was flowed at 1, 2, 3, and 5 mL / min using the mass flow controller were plotted, and the calibration curve, when approximated by a straight line passing through the origin using the least squares method, was found to be R ２ The detection signal strength was adjusted so that the relationship between the detection signal strength and the carbon monoxide concentration was satisfied. Based on this, the detection amount of carbon monoxide was calculated based on the detection signal strength. It was assumed that the carbon dioxide, carbon monoxide, and other gases were fed from the mass flow controller to the MS section via the GC section set under the above conditions, and that the settings of the mass flow controller were changed using a conversion factor when carbon monoxide was fed. The conversion factor values ​​for carbon dioxide and carbon monoxide, respectively, were 0.74 and 1.00.

[0095] The results are shown in Tables 1 and 2. In Tables 1 and 2, the amount of contact gas and the amount of carbon monoxide produced (amount of CO produced) are each shown as values ​​per unit mass of the reducing agent (oxygen carrier).

[0096] The metal elements shown in the binder column are contained in the reducing agent as oxides. The proportion of the reducing agent contained in the reducing agent is shown outside the parentheses as the molar ratio of the metal element to Ce, and inside the parentheses as the parts by mass of the binder relative to 100 parts by mass of the total mass of the reducing agent.

[0097] The reducing agents of each Example had a higher conversion rate value than the reducing agents of each Comparative Example. It was also found that the conversion rate could be adjusted by changing the type and / or amount of the auxiliary metal element and auxiliary element. Note that the reducing agent obtained in Comparative Example 3 did not have a region where the conversion rate per unit second was 0.80 or more, which was sufficient to calculate the average conversion rate.

[0098] 1: Reactor 1a to 1d: Reactors 2: Reducing agent

Claims

1. A reducing agent that reduces carbon dioxide upon contact to produce carbon values, Contains an oxygen carrier having oxygen ion conductivity, The oxygen carrier is a reducing agent containing cerium (Ce) as a main metal element and a metal element belonging to Group 3 of the periodic table as a secondary metal element.

2. The reducing agent according to claim 1, A reducing agent, wherein the ratio of the molar amount of the main metal element to the total molar amount of metal elements contained in the oxygen carrier is 0.3 or more and 0.95 or less.

3. The reducing agent according to claim 1, The reducing agent, wherein the secondary metal element is at least one of yttrium (Y), lanthanum (La), praseodymium (Pr), samarium (Sm), and gadolinium (Gd).

4. The reducing agent according to claim 1, The oxygen carrier further contains an auxiliary element other than the secondary metal element.

5. The reducing agent according to claim 4, A reducing agent in which the ratio of the molar amount of the auxiliary element to the total molar amount of the main metal element and the sub-metal element is 0.7 or less.

6. The reducing agent according to claim 4, A reducing agent, wherein the auxiliary element is at least one of elements belonging to the fourth and fifth periods of the periodic table.

7. The reducing agent according to claim 4, The reducing agent, wherein the auxiliary element is at least one of iron (Fe), manganese (Mn), copper (Cu), and zirconium (Zr).

8. The reducing agent according to claim 1, A reducing agent, wherein the ratio of the molar amount of the minor metal element to the molar amount of the major metal element is 0.01 or more and 0.99 or less.

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

10. The reducing agent according to claim 9, A reducing agent, wherein the ratio of the binder contained in the reducing agent is 1 part by mass or more and 60 parts by mass or less per 100 parts by mass of the reducing agent.

11. The reducing agent according to claim 9, The binder is an oxide containing at least one of magnesium (Mg), titanium (Ti), zirconium (Zr), aluminum (Al), and silicon (Si).

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

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

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

15. The reducing agent according to claim 14, The reducing agent wherein the contact with the reducing gas comprising hydrogen occurs at a temperature above 650°C.

16. The reducing agent according to claim 14, A reducing agent, wherein the amount of the reducing gas containing hydrogen that is brought into contact with the reducing agent is 0.01 mmol / g or more and 50 mmol / g or less per unit mass of the oxygen carrier.

17. The reducing agent according to claim 1, A reducing agent, wherein in a gas obtained after contacting the carbon dioxide with the reducing agent, the ratio of the amount of the carbon valuable to the total amount of the carbon dioxide and the carbon valuable is 0.85 or more.

18. 17. The reducing agent according to claim 16, A reducing agent, wherein in a gas obtained after contacting the carbon dioxide with the reducing agent reduced by contact with the hydrogen-containing reducing gas, the ratio of the amount of the carbon values ​​to the total amount of the carbon dioxide and the carbon values ​​is 0.85 or more, and the amount of the carbon values ​​is 0.01 mmol / g or more per unit mass of the oxygen carrier.

19. A gas production method for producing a product gas containing carbon valuables by reducing a raw material gas containing carbon dioxide by contacting the raw material gas with a reducing agent, the method comprising: the reducing agent contains an oxygen carrier having oxygen ion conductivity; The method for producing a gas, wherein the oxygen carrier contains cerium (Ce) as a main metal element and a metal element belonging to Group 3 of the periodic table as a secondary metal element.

20. 20. The method for producing a gas according to claim 19, a reducing gas containing hydrogen in an amount of 0.01 mmol / g or more and 50 mmol / g or less per unit mass of the oxygen carrier is contacted with the reducing agent, and then a raw material gas containing carbon dioxide is contacted with the reducing agent, whereby the ratio of the amount of the carbon values ​​to the total amount of the carbon dioxide and the carbon values ​​is 0.85 or more and the amount of the carbon values ​​is 0.01 mmol / g or more per unit mass of the oxygen carrier.