Bifunctional catalyst for separating and reducing carbon dioxide, method for producing same, and process using said bifunctional catalyst

Abstract

To provide a bifunctional catalyst which is capable of efficiently occluding and reducing carbon dioxide from a mixed gas that contains at least carbon dioxide and oxygen.SOLUTION: This bifunctional catalyst is composed of at least three components including (A) at least one component that serves as an oxide carrier and is selected from among aluminum oxide, titanium oxide, and zirconium oxide, (B) at least one component that is selected from among alkali metals, and (C) at least one component that is selected from among nickel or iron, wherein the B component and the C component are supported on the surface of a carrier which is the A component, the mass of the C component to the A component is 2.0 wt.% to 9.0 wt.% inclusive, the mass of the B component is from 1.0 times to 3.6 times the mass of the C component, and the bifunctional catalyst is characterized by being prepared by supporting the B component on the A component prior to the C component.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to the separation and recovery of carbon dioxide contained in mixed gases and its effective use, and in particular to a bifunctional catalyst for separating and reducing carbon dioxide contained in mixed gases, a method for producing the same, and a process using the bifunctional catalyst. [Background technology]

[0002] Carbon dioxide is a greenhouse gas and is considered one of the causes of global warming. Japan has also set a goal of reducing carbon dioxide emissions to zero overall by 2050. For this reason, technologies to separate and capture carbon dioxide contained in combustion exhaust gases before it is released into the atmosphere, or to capture carbon dioxide already in the atmosphere, are important. Furthermore, in order to realize a sustainable society, the possibility of treating captured carbon dioxide as a resource and converting it into other useful compounds such as methane is being considered.

[0003] In such conversion processes, the process of separating and capturing carbon dioxide and the process of converting carbon dioxide into useful compounds are required to fulfill different roles, and therefore typically require different catalysts and equipment. For example, Patent Document 1 discloses a methane production method in which exhaust gas is supplied to an adsorption column filled with a solid carbon dioxide adsorbent to adsorb and separate carbon dioxide, the adsorption column is then heated and hydrogen is supplied to desorb the carbon dioxide from the adsorbent, and the carbon dioxide discharged from the desorption step is reacted with a gas containing hydrogen on a Rh / Mn-based methanation catalyst to produce methane.

[0004] On the other hand, in order to save energy and make the entire process more compact, a process has been considered in which carbon dioxide is separated from a mixed gas containing carbon dioxide and the conversion reaction is completed in the same equipment. In such a process, a bifunctional catalyst is used. In the description of this invention, a catalyst that can separate carbon dioxide from a mixed gas containing carbon dioxide and also promote the conversion reaction of carbon dioxide to other useful compounds such as methane is defined as a bifunctional catalyst.

[0005] Such dual-function catalysts capable of separating carbon dioxide from a mixed gas containing at least carbon dioxide and oxygen and reducing it as is are already known (Patent Documents 2 and 3, Non-Patent Documents 1 to 7). For example, Patent Document 2 discloses a carbon dioxide storage-reduction catalyst in which ruthenium (Ru) as a catalyst used in a methanation reaction and calcium oxide (CaO) as a carbon dioxide storage material are supported on a carrier such as alumina. Furthermore, Patent Document 3 discloses a carbon dioxide storage-reduction catalyst in which an alkali metal as a carbon dioxide storage material and ruthenium (Ru) as a carbon dioxide reducing material are supported on a carrier such as alumina, as a carbon dioxide storage-reduction catalyst that exhibits excellent carbon dioxide storage performance and methanation catalytic activity even at low temperatures. All of these catalysts use ruthenium as the carbon dioxide reducing material.

[0006] Furthermore, for example, Non-Patent Document 1 reports a bifunctional catalyst in which 5 wt% ruthenium and 10 wt% calcium oxide are immobilized on aluminum oxide. It is disclosed that by using a reactor packed with this catalyst, carbon dioxide can be absorbed at 320°C in a supply process of a mixed gas of 10 vol% carbon dioxide, 18 vol% oxygen, and 72% nitrogen, and then the supply gas can be switched to hydrogen, thereby reducing the absorbed carbon dioxide and synthesizing methane. However, this method also requires rare and expensive precious metals such as ruthenium and palladium, which leads to the risk of resource depletion and greater costs.

[0007] On the other hand, to avoid the above problems, research and development has also been conducted based on cheaper and more abundant metals. For example, Patent Document 4 discloses a catalyst for methanation reactions, which involve the reaction of carbon dioxide with hydrogen to produce methanation. The catalyst is supported on an alumina support, with nickel as the active metal and a basic oxide as the promoter. Examples of basic oxides used include lanthanum oxide, yttrium oxide, and praseodymium oxide. To increase the gas treatment rate, a catalyst has been developed in which nickel and a basic oxide are dispersed on an alumina support. However, no alkali metals or alkaline earth metals are used as catalytic components, and the methane production rate is measured using a mixture of hydrogen, carbon dioxide, and nitrogen as the treatment gas.

[0008] Patent Document 5 also discloses a catalyst comprising a carrier and a catalytic metal supported on the carrier, the carrier being a metal oxide containing at least one metal element selected from cerium, zirconium, yttrium, aluminum, silicon, and magnesium, and the catalytic metal containing at least one metal selected from nickel, ruthenium, rhodium, potassium, calcium, sodium, and iridium. Specifically, the catalyst comprises a powder of catalytic particles (Ni / CeO2) containing nickel supported on cerium (IV) oxide adhered to the surface of an aluminum molded product. The document also discloses that, when oxygen is present in exhaust gas, catalytic metals such as nickel and ruthenium combine with oxygen to form metal oxides, which makes the catalyst susceptible to deactivation.

[0009] Meanwhile, Patent Document 6 discloses a dual-function catalyst that combines the functions of both occluding and reducing carbon dioxide, in which 40 wt% nickel oxide, 18 wt% magnesium oxide, and 18 wt% calcium oxide are immobilized on aluminum oxide. It discloses that by using a reactor packed with this catalyst, carbon dioxide can be occluded from a mixed gas of 10 vol% carbon dioxide, 5 vol% oxygen, and 85 vol% helium at a temperature of 320°C, and then the supply gas can be switched to hydrogen, thereby reducing the occluded carbon dioxide and synthesizing methane.

[0010] Patent Document 7 discloses a methanation catalyst comprising an oxide support, at least one carbon dioxide storage component selected from the group consisting of alkali metal compounds and alkaline earth metal compounds, and at least one carbon dioxide reduction component selected from the group consisting of ruthenium, nickel, and cobalt. In the methanation catalyst disclosed in this document, Si is supported on the oxide support as an essential component. It is stated that by supporting a predetermined amount of Si on the oxide support relative to the carbon dioxide reduction component, the ability to produce methane from carbon dioxide is improved. Furthermore, the examples in this document only use "(Ru+Si+CaO)-supported alumina powder" and "(Ru+Si+CaO)-supported titania powder," and do not disclose examples using alkali metals or nickel. There is no mention of the difference in effectiveness between alkali metals and alkaline earth metals, the difference in effectiveness between ruthenium and nickel, or the problems associated with using nickel.

[0011] Patent Document 8 discloses a method for capturing carbon dioxide and converting it into synthetic natural gas or the like, and discloses a dual-function material in which the catalyst portion for converting it into synthetic natural gas or the like is ruthenium, nickel, platinum, rhodium, copper, cobalt, a Group VIII transition metal or an oxide thereof, and the carbon dioxide adsorbent portion is an alkaline earth metal oxide of calcium, magnesium, strontium or barium. No examples using alkali metals are disclosed.

[0012] However, when using base metals such as nickel and iron, there is a concern that they may be deactivated by contact with oxygen. For example, Non-Patent Document 2 reports Ni-K / ZrO2 and Ni-La / ZrO2 as dual-function catalysts that can occlude and reduce carbon dioxide, but the carbon dioxide occlusion process is carried out in the absence of oxygen. Non-Patent Document 3 also reports Ni / CaO as a dual-function catalyst that can occlude and reduce carbon dioxide, but the carbon dioxide occlusion process is also carried out in the absence of oxygen.

[0013] Additionally, Non-Patent Document 4 states that when a carbon dioxide separation and conversion reaction was carried out using 10%Ni, 6.1% "Na2O" / Al2O3 at a temperature of 320°C, no methane was produced, and Non-Patent Document 5 also clearly states that when exposed to oxygen, 10%Ni, 6.1% "Na2O" / Al2O3 is unable to produce methane at a temperature of 320°C. However, because bifunctional catalysts are expected to be used in processes for separating and converting carbon dioxide present in exhaust gases or the atmosphere, the mixed gas to be treated usually contains a large amount of oxygen, and therefore resistance to this oxygen is extremely important.

[0014] In response to this background, efforts have been made to develop bifunctional catalytic reaction systems that utilize nickel as the primary component and exhibit activity even in the presence of oxygen. For example, Non-Patent Document 6 discloses the results of a Ni(10)-Ca(30) / Al2O3 catalyst functioning as a bifunctional catalyst to reduce carbon dioxide at a temperature of 450 °C. It is speculated that at the high temperature of 450 °C, the nickel was once oxidized and re-reduced by contact with hydrogen, thereby exerting its catalytic function. On the other hand, Non-Patent Document 7 reports that when Na-Ni / Al2O3, K-Ni / Al2O3, or Ba-Ni / Al2O3 was used at a temperature of 350 °C, the initial catalytic activity decreased in the presence of oxygen compared to the absence of oxygen, and the activity continued to decrease after several cycles.

[0015] Furthermore, as a system using a bifunctional catalyst, Patent Document 9 discloses a methane production system equipped with a reaction tube having a bifunctional catalyst, a hydrogen supply unit that supplies hydrogen, and a control unit that controls the amount of hydrogen supplied. This methane production system produces methane, which can be used as fuel, from carbon dioxide contained in the combustion gas of a factory. The bifunctional catalyst used in this system consists of a metal oxide that absorbs and releases carbon dioxide and a methanation catalyst that produces methane from hydrogen and carbon dioxide. The metal oxide is composed of at least one of lithium oxide, magnesium oxide, calcium oxide, strontium oxide, barium oxide, and lead oxide, and the methanation catalyst uses a complex containing ruthenium. There is no mention of nickel.

[0016] Patent Document 10 also discloses a system using a dual-function catalyst. The carbon dioxide capture device includes a combustor that takes in air and generates combustion gas, a hydrogen supply source that supplies hydrogen, and a carbon dioxide capture / reducer that contains a material that captures carbon dioxide contained in the combustion gas and converts the captured carbon dioxide into fuel using the hydrogen. A mixed gas of hydrogen and methane released from the carbon dioxide capture / reducer is supplied to the combustor, and a control unit controls the opening and closing of the hydrogen / methane gas supply valve and the valve that supplies air to the combustor. This control is said to enable efficient capture of carbon dioxide from the air and conversion into fuel. The carbon dioxide capture / reducer uses a dual-function catalyst, with calcium oxide as the metal oxide with carbon dioxide storage capacity and ruthenium (Ru) as the metal with methanation catalytic capacity. However, no examples are disclosed regarding the amount of carbon dioxide stored or the amount converted to methane. Furthermore, in addition to calcium oxide, potassium oxide and magnesium oxide are listed as metal oxides having carbon oxide storage capacity, and in addition to ruthenium (Ru), examples of metals having methanation catalytic capacity include nickel (Ni), platinum (Pt), palladium (Pd), rhodium (Rd), cobalt (Co), iron (Fe), and manganese (Mn). However, there is no mention of differences in the amount of carbon dioxide storage or methane production rate when these metals are used, or of differences in their effects. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] Patent Publication No. 2021-35909 [Patent Document 2] Japanese Patent Application Publication No. 2020-110769 [Patent Document 3] Japanese Patent Publication No. 2023-157643 [Patent Document 4] Japanese Patent Publication No. 2022-94211 [Patent Document 5] Japanese Patent Application Publication No. 2020-33280 [License 6] Special Announcement No. 2020-39995 [License 7] Special Announcement No. 2019-188353 [License 8] International Publication No. 2016 / 007825 パンフレット [License 9] Special Announcement No. 2020-100597 [License 10] Special Announcement No. 2022-16120 [Non-licensed literature]

[0018] [Non-licensed Document 1] Melis S. Duyar et al, Dual function materials for CO2 capture and conversion using renewable H2, Applied Catalysis B: Environmental 168 169 (2015) 370-376 [Non-licensed Document 2] Lingjun Hu et al, Continuous CO2 capture and reduction in one process:CO2 methanation over unpromoted and promoted Ni / ZrO2, Journal ofCO2 Utilization Volume 25, May 2018, Pages 323-329 [Non-licensed Document 3] Seong Bin Jo et al,CO2 green technologies inCO2 capture and direct utilization processes: methanation, reverse water gas shift, and dry reforming of methane, Sustainable Energy Fuels, 2020, 4, 5543-5549 [Non-licensed Document 4] Martha A. Arellano-Trevino et al, Bimetallic catalysts forCO2 capture and hydrogenation at simulated flue gas conditions, Chemical Engineering JournaL 375 (2019) 121953 [Non-Patent Document 5] Martha A. Arellano-Trevino et al, Catalysts and adsorbents forCO2 capture and conversion with dual function materials: Limitations of Ni-containing DFMs for flue gas applications, Journal of CO2 Utilization 31 (2019) 143-151 [Non-patent document 6] Lingcong Li et al, ContinuousCO2 capture and methanation over Ni-Ca / Al2O3 dual functional materials, RSC Adv., 2023, 13, 2213-2219 [Non-Patent Document 7] Enrique Garcia-Bordeje et al, Dual functional materials based on Ni and different alkaline metals on alumina for the cyclic stepwiseCO2 capture and methanation, Chemical Engineering Journal 472 (2023) 144953 Summary of the Invention [Problem to be solved by the invention]

[0019] Although Non-Patent Document 7 clearly states that the catalyst is deactivated by contact with oxygen during the carbon dioxide absorption process, it does not completely lose its catalytic function. Based on this information, the inventors hypothesized that the oxidized nickel component is partially re-reduced when it comes into contact with hydrogen, even at low temperatures below 400°C. In particular, while nickel has a lower reducibility than precious metals such as ruthenium and palladium, the nickel components present on the outermost surface or in relatively shallow portions of the pores of the catalyst particles are more likely to be reduced because they come into contact with supplied hydrogen more frequently than the nickel components present within the pores. However, in the known bifunctional catalysts Na-Ni / Al2O3, K-Ni / Al2O3, and Ba-Ni / Al2O3 (Non-Patent Document 7), a metal precursor solution is infiltrated into the pores of the oxide support, and the metal components are then immobilized by a subsequent drying and calcination process. This results in a higher proportion of nickel components present inside the pores, which are less susceptible to reduction.

[0020] Additionally, the bifunctional catalyst used in Patent Document 6 contains 40 wt% nickel oxide, which is a significantly higher content of nickel than other bifunctional catalysts. Other bifunctional catalysts often contain 10–15 wt% nickel. Therefore, the absolute amount of nickel exposed on the surface appears to be higher than that of an average bifunctional catalyst. Furthermore, the oxygen concentration during the carbon dioxide absorption process is 5 vol%, less than one-third the atmospheric concentration, which may slow the oxidation of the nickel component. Under these conditions, a carbon dioxide separation and conversion reaction was performed, and the results of the third cycle were disclosed. However, the carbon dioxide conversion rate per unit time and unit mass of carbon dioxide-reduced metal component was 0.054 mg-carbon dioxide / h / g-Ni, and it is unclear whether catalyst degradation progresses with each cycle. In the field of catalytic chemistry, it is known that the higher the amount of immobilized metal, the more likely the metal species are to aggregate, resulting in a decrease in catalytic activity commensurate with the metal amount. Furthermore, among the components that make up a typical bifunctional catalyst, nickel is more expensive than sodium, potassium, magnesium, calcium, aluminum, etc. Furthermore, while nickel is an abundant resource compared to precious metals such as ruthenium, it is also used for hardening and as part of electronic components, so it is desirable to reduce the amount of nickel used.

[0021] Patent Document 7 discloses a methanation catalyst comprising an oxide support, at least one carbon dioxide storage component selected from the group consisting of alkali metal compounds and alkaline earth metal compounds, and at least one carbon dioxide reduction component selected from the group consisting of ruthenium, nickel, and cobalt. However, the examples are composed only of catalysts in which ruthenium is used as the active metal, and therefore the reactivity when nickel and cobalt are used as the active metal is unknown.

[0022] From this publicly known information, it is presumed that, unlike precious metals such as ruthenium, base metals such as nickel and iron are oxidized by contact with oxygen, and therefore the catalytic activity decreases or continues to decrease with each cycle due to the low reduction efficiency by contact with hydrogen in the low temperature range of 400°C or less.

[0023] In light of the above, the objective of this study is to provide a dual-function catalyst that can efficiently re-reduce base metal components even at low temperatures such as 400°C or less, and can efficiently promote separation and conversion reactions even in carbon dioxide-containing gases that coexist with high-concentration oxygen of 20 vol%, which is close to the atmospheric composition. This catalyst uses base metal as a catalytic component, and can maintain high activity even when the base metal content is low. [Means for solving the problem]

[0024] The present inventors have conducted extensive research into base metal component-containing bifunctional catalysts. As a result, they have discovered that in a bifunctional catalyst comprising at least three components, including (A) at least one component selected from aluminum oxide, titanium oxide, or zirconium oxide as an oxide support, (B) at least one component selected from alkali metals, and (C) at least one component selected from nickel or iron, by specifying the ratio of (C) at least one component selected from nickel or iron to the oxide support, specifying the ratio of (B) at least one component selected from alkali metals to the at least one component selected from nickel or iron, and supporting component B on component A before component C, a high carbon dioxide storage / reduction efficiency per carbon dioxide-reducing metal component can be achieved in a carbon dioxide separation / conversion reaction using a carbon dioxide-containing gas coexisting with oxygen, thereby completing the present invention.

[0025] The first invention resides in a bifunctional catalyst comprising at least three components including (A) at least one component selected from aluminum oxide, titanium oxide, or zirconium oxide as an oxide support, (B) at least one component selected from alkali metals, and (C) at least one component selected from nickel or iron, wherein the components B and C are supported on the surface of the support of the component A, wherein the mass of the component C relative to the mass of the component A is 2.0 wt% to 9.0 wt%, the mass of the component B is 1.0 to 3.6 times the mass of the component C, and the bifunctional catalyst is prepared by supporting the component B on the component A prior to supporting the component C.

[0026] A second invention resides in the bifunctional catalyst according to the first invention, characterized in that the mass of component C relative to component A is 2.5 wt % or more and 8.0 wt % or less.

[0027] A third invention resides in the bifunctional catalyst according to the first or second invention, characterized in that the mass of component B is 1.6 to 3.6 times the mass of component C.

[0028] A fourth invention resides in the bifunctional catalyst according to the first or second invention, characterized in that the bifunctional catalyst is prepared by immobilizing component C on component A using a metal precursor solution containing component C in a volume that is at least twice the pore volume of the oxide support that is component A.

[0029] A fifth invention resides in the dual function catalyst according to the first or second invention, characterized in that the amount of carbon dioxide converted per unit time and unit mass of carbon dioxide-reducing metal component (mass of component C) in a carbon dioxide absorption-reduction reaction using a mixed gas containing at least carbon dioxide and oxygen is 0.95 mg-CO2 / h / mg-Metal(C) or more.

[0030] A sixth invention resides in the bifunctional catalyst according to the first or second invention, characterized in that it further contains 1.0 wt % or less of a ruthenium component relative to the A component.

[0031] A seventh invention resides in the bifunctional catalyst according to the first or second invention, characterized in that the B component is sodium or potassium.

[0032] An eighth aspect of the present invention resides in the bifunctional catalyst according to the first or second aspect of the present invention, characterized in that the amount of conversion of the occluded carbon dioxide is 1.5 wt% or more based on the mass of the catalyst.

[0033] A ninth aspect of the present invention resides in a method for producing a bifunctional catalyst comprising at least three components including (A) at least one component selected from aluminum oxide, titanium oxide, or zirconium oxide as an oxide support, (B) at least one component selected from alkali metals, and (C) at least one component selected from nickel or iron, wherein components B and C are supported on the surface of component A, the support, characterized in that the mass of component C relative to component A is 2.0 wt% to 9.0 wt%, the mass of component B is 1.0 to 3.6 times the mass of component C, and the bifunctional catalyst is prepared by supporting component B on component A prior to supporting component C.

[0034] The tenth invention resides in a method for producing a bifunctional catalyst according to the ninth invention, characterized in that the mass of component C relative to component A is 2.5 wt% or more and 8 wt% or less.

[0035] An eleventh invention resides in the method for producing a bifunctional catalyst according to the ninth or tenth invention, characterized in that the mass of component B is 1.6 to 3.6 times the mass of component C.

[0036] A twelfth invention resides in a method for producing a bifunctional catalyst according to the ninth or tenth invention, characterized in that the bifunctional catalyst is prepared by using a metal precursor solution containing component C in a volume that is at least twice the pore volume of the oxide support as component A, and immobilizing component C on component A.

[0037] A thirteenth aspect of the present invention resides in the method for producing a bifunctional catalyst according to the ninth or tenth aspect of the present invention, characterized in that component C further contains a ruthenium component in an amount of 1.0 wt% or less relative to component A.

[0038] A fourteenth aspect of the present invention resides in the method for producing a bifunctional catalyst according to the ninth or tenth aspect of the present invention, characterized in that the component B is sodium or potassium.

[0039] A fifteenth invention resides in the method for producing a bifunctional catalyst according to the ninth or tenth invention, characterized in that the step of immobilizing component C on an oxide support (component A) using a metal precursor solution containing component C is carried out under reduced pressure conditions.

[0040] A sixteenth invention resides in a carbon dioxide occlusion / reduction process, characterized by using a reactor that houses the bifunctional catalyst according to the first or second invention, introducing a mixed gas containing at least carbon dioxide and oxygen into the reactor that houses the bifunctional catalyst to occlude the carbon dioxide, and then introducing hydrogen gas into the reactor that houses the bifunctional catalyst to reduce the carbon dioxide.

[0041] A seventeenth aspect of the present invention resides in a carbon dioxide occlusion / reduction process, characterized in that a reactor containing the bifunctional catalyst according to the first or second aspect of the present invention is used, and the carbon dioxide conversion rate per unit time and unit mass of carbon dioxide-reducing metal component is 0.95 mg-CO2 / h / mg-Metal(C) or more.

[0042] In the present invention, the term "dual-function catalyst" means a catalyst that can promote both the function of separating carbon dioxide from a mixed gas containing carbon dioxide and the function of converting the separated carbon dioxide into other useful compounds such as methane.

[0043] In addition, in the description of units of the present invention, "mg-Metal (C)" represents "the mass (mg) of the carbon dioxide-reduced metal component," that is, "the mass (mg) of immobilized nickel or iron." [Effects of the Invention]

[0044] According to the present invention, there is provided a bifunctional catalyst capable of efficiently separating carbon dioxide from a mixed gas containing at least carbon dioxide and oxygen and reducing the separated carbon dioxide, and capable of achieving a high carbon dioxide to methane conversion rate even when the amount of a carbon dioxide-reducing metal component used in the bifunctional catalyst is reduced.

[0045] Furthermore, according to the present invention, it is possible to provide a process for efficiently separating carbon dioxide from a mixed gas containing at least carbon dioxide and oxygen, using the above-mentioned bifunctional catalyst, and reducing the separated carbon dioxide to convert it into a useful compound. [Brief explanation of the drawings]

[0046] [Figure 1] 1 is a graph showing the amount of carbon dioxide converted per unit time and unit carbon dioxide-reduced metal component mass (component C mass) in a carbon dioxide storage and reduction process when using a bifunctional catalyst in which the order in which a nickel component and a potassium component are immobilized on aluminum oxide is changed. [Figure 2] 1 is a graph showing the amount of carbon dioxide converted per unit time and unit carbon dioxide-reduced metal component mass (component C mass) in a carbon dioxide storage and reduction process when using a bifunctional catalyst for each amount of nickel immobilized on the bifunctional catalyst. [Figure 3] 1 is a graph showing the amount of carbon dioxide converted per unit time and unit carbon dioxide-reduced metal component mass (component C mass) in a carbon dioxide storage and reduction process when a bifunctional catalyst is used for each mass ratio of the potassium component and nickel component fixed to the bifunctional catalyst. DETAILED DESCRIPTION OF THE INVENTION

[0047] Hereinafter, an embodiment of the present invention will be described.

[0048] First, the components contained in the bifunctional catalyst of the present invention will be described.

[0049] The bifunctional catalyst of the present invention comprises at least three components including (A) at least one component selected from the oxide support aluminum oxide, titanium oxide, or zirconium oxide, (B) at least one component selected from alkali metals, and (C) at least one component selected from nickel or iron.

[0050] In this catalyst, the nickel and iron components (C) serve to reduce carbon dioxide, while the alkali metal component (B) serves to selectively separate carbon dioxide from the mixed gas. The use of the bifunctional catalyst of the present invention can alleviate problems associated with conventional bifunctional catalysts, such as a small carbon dioxide storage capacity, a high proportion of unreduced and stored carbon dioxide, a slow reduction rate of stored carbon dioxide, desorption of unreacted carbon dioxide, and catalyst deactivation due to reaction with oxygen. Furthermore, the addition of a small amount of ruthenium as a fourth component can further improve the carbon dioxide storage / reduction efficiency.

[0051] [Component (A)] Component (A) is an oxide support, and is at least one component selected from aluminum oxide, titanium oxide, and zirconium oxide. The oxide support is porous and can support each component stably and highly dispersedly. There are no particular restrictions on the specific surface area of ​​component (A), aluminum oxide, titanium oxide, or zirconium oxide, but it is preferred that the specific surface area be 50 m 2 / g or more is preferable, and 150m 2 If the specific surface area of ​​the oxide support is smaller than the specified range, the efficiency of contact with gas components such as carbon dioxide and hydrogen decreases, impairing the efficiency of carbon dioxide storage and reduction.

[0052] The specific surface area of ​​aluminum oxide, titanium oxide, and zirconium oxide can be measured by the BET adsorption method or the like.

[0053] [(B) Component] The component (B) is at least one selected from alkali metals. The amount of the alkali metal component (B) immobilized is preferably 5 wt% to 18 wt%, more preferably 8 wt% to 15 wt%, and particularly preferably 8 wt% to 13 wt%, based on the oxide support (A). If the component (B) is less than 5 wt%, the amount of carbon dioxide absorbed will decrease, while if immobilized in excess of 18 wt%, the amount and speed of carbon dioxide reduction may be impaired.

[0054] Examples of alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). Of these, sodium (Na) and potassium (K) are preferred because they further improve carbon dioxide absorption performance. The alkali metal compounds used are those that are highly soluble in water. Examples include chlorides, hydroxides, carbonates, nitrates, formates, and acetates.

[0055] [(C) component] The (C) component is at least one component selected from nickel and iron. The amount of the nickel component or iron component immobilized is preferably 2.0 wt% to 9.0 wt%, more preferably 2.5 wt% to 8.0 wt%, relative to the (A) oxide support. If the (C) component is less than 2.0 wt%, the amount and speed of carbon dioxide reduction will decrease, and if it exceeds 9.0 wt%, the amount of carbon dioxide storage may be impaired. In particular, by setting the amount of the nickel component or iron component immobilized to 9.0 wt% or less, high carbon dioxide storage and reduction efficiency can be achieved in the carbon dioxide separation and conversion reaction.

[0056] Examples of nickel and iron compounds include chlorides, carbonates, nitrates, acetates, and metal carbonyls.

[0057] Regarding the ratio of the immobilized amounts of nickel and iron components in component (C) to alkali metal components in component (B), the immobilized amount of alkali metal components in component (B) is preferably 1.0 to 3.6 times, more preferably 1.6 times or more, of the immobilized amount of nickel, iron, or their combined amount in component (C). 2.6 times or less is particularly preferred. If component (B) is less than 1.0 times or more than 3.6 times that of component (C), the balance between the storable and reducible amounts of carbon dioxide will be unbalanced, and performance commensurate with the total immobilized amount of metal will not be achieved.

[0058] [Ruthenium component] Examples of ruthenium components include ruthenium salts such as ruthenium chloride, ruthenium nitrate, ruthenium nitrosyl nitrate, and ruthenium carbonyl. The fixed amount of ruthenium component is preferably 1.0 wt% or less relative to component A. Since ruthenium is expensive and a rare metal, it is preferable to use as little of it as possible. In the composition of the present invention, the effect is sufficiently exhibited by adding 1.0 wt% or less relative to component A.

[0059] The amount of each component immobilized can be measured by inductively coupled plasma (ICP) analysis or energy dispersive X-ray analysis (EDX).

[0060] [Manufacturing method] Next, the method for producing the bifunctional catalyst of the present invention will be described.

[0061] The bifunctional catalyst of the present invention can be produced, for example, by the following procedure. First, (A) an alkali metal compound (B) is attached to an oxide support, followed by drying and calcination to immobilize it. Next, (C) a solution containing a nickel compound, an iron compound, or both is attached to the oxide support on which the alkali metal compound is immobilized, followed by drying and calcination to immobilize the nickel component, the iron compound, or both. Through these steps, the bifunctional catalyst of the present invention, in which each component is immobilized on the oxide support, is obtained. In this procedure, it is important that component (B) is attached to the oxide support (A) before component (C), followed by drying and calcination to immobilize it.

[0062] Furthermore, a bifunctional catalyst with an immobilized ruthenium component can be obtained, for example, in accordance with the above-mentioned production method. The ruthenium component may be immobilized at any stage, such as by immobilizing an alkali metal compound (B) on an oxide support (A) and then applying a solution containing the ruthenium component together with the component (C) and then drying and calcining the resulting mixture; by immobilizing the component (C) and then applying a solution containing the ruthenium component and then drying and calcining the resulting mixture; or by immobilizing the ruthenium component and then immobilizing the component (C).

[0063] In the manufacturing procedure for the bifunctional catalyst of the present invention, component (B), an alkali metal compound, is first immobilized on oxide support (A). Then, component (C) is immobilized. In this process, a metal precursor solution with a volume at least twice the pore volume of the oxide support is used. This process leaves the solution that does not completely penetrate into the pores remaining near the catalyst surface. This increases the proportion of metal components immobilized near the outermost surface of the catalyst in the subsequent drying and calcination process. Furthermore, since the alkali components are preferentially immobilized deep in the pores, it is believed that the nickel and iron components immobilized later are distributed on the outermost surface of the catalyst and in relatively shallow areas within the pores.

[0064] By the above preparation method, even if the immobilized amounts of each metal component, particularly the nickel component and iron component, are small, they can be distributed in a large proportion near the outermost surface of the catalyst, and a catalyst can be obtained that can be efficiently re-reduced even in a low temperature range.

[0065] The drying temperature for the oxide support having the components attached thereto is not particularly limited, but is preferably, for example, 40 to 200° C., more preferably 50 to 150° C. The drying time is also not particularly limited, but is, for example, preferably 1 to 48 hours, more preferably 1 to 16 hours.

[0066] The calcination temperature after drying of the oxide support to which each component is attached is not particularly limited, but is preferably, for example, 200 to 1000° C., more preferably 300 to 800° C. The calcination time is also not particularly limited, but is, for example, preferably 1 to 24 hours, more preferably 1 to 8 hours.

[0067] [process] The carbon dioxide storage-reduction process of the present invention is a technique for reducing carbon dioxide by occluding carbon dioxide in the bifunctional catalyst of the present invention and then contacting the catalyst with hydrogen gas. In particular, the carbon dioxide storage-reduction process of the present invention is effective for treating carbon dioxide present in a mixed gas containing a reaction inhibitor such as oxygen, because the bifunctional catalyst of the present invention can selectively store carbon dioxide. That is, the bifunctional catalyst of the present invention is contacted with a mixed gas containing carbon dioxide and a reaction inhibitor such as oxygen to occlude carbon dioxide in the bifunctional catalyst, and then hydrogen gas is contacted with the bifunctional catalyst to reduce the occluded carbon dioxide.

[0068] In the process of the present invention, the bifunctional catalyst of the present invention is packed into a column, either as a powder or in the form of a molded body, and the bifunctional catalyst packed in the column is brought into contact with carbon dioxide and hydrogen gas to produce methane. The material of the packed column does not matter as long as it can withstand high temperatures and is inert to the reaction of occluding and converting carbon dioxide. Two or more columns can be arranged in parallel and alternately supplied with a carbon dioxide-containing mixed gas and hydrogen gas, allowing the carbon dioxide separation and conversion process to proceed steadily. If the bifunctional catalyst is a powder, it may be in a fluidized bed or moving bed format, and if it is in a molded body, it may be in a fixed bed or moving bed format.

[0069] The carbon dioxide-containing gas to be treated is not particularly limited, but examples thereof include combustion exhaust gases emitted from various plants and the atmosphere. The exhaust gas or the atmosphere can be passed through the column as is, but it may also be passed through the column after being subjected to pretreatment such as concentrating the carbon dioxide, removing catalyst-poisoning components, preheating or cooling the gas, or dehydrating it.

[0070] The carbon dioxide concentration in the mixed gas is not particularly limited, but from the viewpoint of improving the carbon dioxide absorption efficiency, it is preferably 30 vol% or less, more preferably 0.1 vol% to 20 vol% or less, and particularly preferably 0.5 vol% to 10 vol% or less. Furthermore, when the mixed gas contains oxygen, from the viewpoint of suppressing catalyst deterioration due to oxidation, the oxygen concentration is preferably 20 vol% or less, more preferably 10 vol% or less, and particularly preferably 5 vol% or less.

[0071] The hydrogen gas is not particularly limited, but pure hydrogen gas or a hydrogen-containing gas is used. For example, a hydrogen-containing gas obtained from water electrolysis, steam reforming of petroleum resources, coal gasification, etc. can be used. The other gas contained in the hydrogen-containing gas is not particularly limited as long as it does not inhibit the reduction reaction of carbon dioxide, but an inert gas such as helium gas, nitrogen gas, or argon gas is preferred. The hydrogen concentration in the hydrogen-containing gas is preferably 5 vol% or more, more preferably 10 vol% or more, from the viewpoint of shortening the time required for the reduction step.

[0072] The bifunctional catalyst of the present invention is a catalyst that can promote both the function of separating carbon dioxide from a mixed gas containing carbon dioxide and the function of converting the separated carbon dioxide into other useful compounds such as methane, and a reducing gas is used in the process of converting carbon dioxide into useful compounds. In the process of the present invention, a process is provided that assumes conversion to methane using hydrogen gas as the reducing gas, but the useful compound is not limited to methane, and the reducing gas is not limited to hydrogen gas. For example, the reducing gas can also be carbon monoxide gas, or hydrocarbon gases such as methane, propane, and butane. In the carbon dioxide conversion process, for example, using methane gas as the reducing gas enables conversion to carbon monoxide, a useful compound.

[0073] The optimum temperature for carrying out the carbon dioxide storage reduction process of the present invention varies depending on the type of column, but from the viewpoint of improving the carbon dioxide storage efficiency, the set heating temperature of the catalyst layer during hydrogen gas flow is preferably 200°C or higher and 500°C or lower, and more preferably 250°C or higher and 400°C or lower. The dual function catalyst of the present invention can efficiently perform a regeneration cycle of the base metal catalyst even at low temperatures such as 400°C or lower, and can efficiently separate and convert carbon dioxide from a carbon dioxide-containing gas that also contains oxygen.

[0074] The flow rate of the carbon dioxide-containing gas is not particularly limited, and the optimum conditions vary depending on the type of column, but in a fixed bed flow system, the space velocity is preferably 100,000 / h or less.

[0075] After packing the bifunctional catalyst of the present invention into a column, it is preferable to first subject it to a reduction treatment. Because the bifunctional catalyst of the present invention undergoes a reduction reaction during the reaction, it is not necessary to subject it to a reduction treatment, but it is preferable to subject it to a reduction treatment as a pretreatment before use. The reduction temperature is preferably, for example, between 300°C and 350°C, and a mixed gas consisting of 20 vol% hydrogen and 80 vol% nitrogen is passed through the catalyst at a flow rate of 20 mL / min for 30 minutes. However, the reduction is not particularly limited to these conditions.

[0076] After use, the bifunctional catalyst of the present invention is preferably subjected to a passivation treatment, for example, by passing a mixed gas consisting of 1 vol% oxygen and 99 vol% nitrogen at a flow rate of 20 mL / min at a temperature of 0°C to 350°C for 30 minutes. However, the passivation treatment is not particularly limited to these conditions.

[0077] The present invention will be explained in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0078] Example 1 Potassium nitrate was dissolved in ion-exchanged water, and aluminum oxide ("PURALOXSBa200" manufactured by SASOL) was added and homogenized. The solvent was then removed by evaporation. The volume of the ion-exchanged water used was at least twice the pore volume of the aluminum oxide. The remaining solid was collected, dried at 90°C for 1 hour, and then calcined at 600°C for 2 hours to obtain aluminum oxide with immobilized potassium components. The mass of the potassium component, calculated as potassium oxide, was adjusted to be 10 wt% of the aluminum oxide.

[0079] Next, nickel nitrate was dissolved in ion-exchanged water, and aluminum oxide with immobilized potassium was added to the solution, after which the solvent was removed by evaporation. The volume of the ion-exchanged water used was at least twice the pore volume of the aluminum oxide. The remaining solid was collected, dried at 90°C for 1 hour, and then calcined at 500°C for 2 hours to obtain aluminum oxide with immobilized nickel and potassium components. The mass of the nickel component was adjusted to be 5 wt% of the aluminum oxide.

[0080] Example 2 Aluminum oxide having nickel and potassium components immobilized thereon was obtained in the same manner as in Example 1, except that the mass of the nickel component relative to the aluminum oxide was adjusted to 3 wt %.

[0081] Example 3 Aluminum oxide having immobilized nickel and potassium components was obtained in the same manner as in Example 1, except that the mass of the nickel component was adjusted to 8 wt % relative to the mass of the aluminum oxide.

[0082] Example 4 Aluminum oxide having immobilized nickel and potassium components was obtained in the same manner as in Example 1, except that the mass of the potassium component was adjusted to 5 wt % in terms of potassium oxide relative to the aluminum oxide.

[0083] Example 5 Aluminum oxide having immobilized nickel and potassium components was obtained in the same manner as in Example 1, except that the mass of the potassium component was adjusted to 8 wt % in terms of potassium oxide relative to the aluminum oxide.

[0084] Example 6 Aluminum oxide having immobilized nickel and potassium components was obtained in the same manner as in Example 1, except that the mass of the potassium component was adjusted to 13 wt % in terms of potassium oxide relative to the aluminum oxide.

[0085] Example 7 Aluminum oxide having immobilized nickel and potassium components was obtained in the same manner as in Example 1, except that the mass of the potassium component was adjusted to 15 wt % in terms of potassium oxide relative to the aluminum oxide.

[0086] Example 8 Aluminum oxide having immobilized nickel and potassium components was obtained in the same manner as in Example 1, except that the mass of the potassium component was adjusted to 18 wt % in terms of potassium oxide relative to the aluminum oxide.

[0087] Example 9 Aluminum oxide having a nickel component and a sodium component immobilized thereon was obtained in the same manner as in Example 1, except that sodium nitrate was used and sodium was immobilized instead of potassium.

[0088] Example 10 The ruthenium nitrosyl nitrate solution was dropped into ion-exchanged water, and the potassium-immobilized aluminum oxide obtained by the method described in Example 1 was added thereto and homogenized. The solvent was then removed by evaporation. The ion-exchanged water used had a volume at least twice the pore volume of the aluminum oxide. The remaining solid was collected, dried at 90°C for 1 hour, and then calcined at 500°C for 2 hours to obtain aluminum oxide with ruthenium and potassium components immobilized thereon. Subsequently, the nickel component was immobilized by the method described in Example 1 to obtain aluminum oxide with nickel, ruthenium, and potassium components immobilized thereon. The mass of the ruthenium component was adjusted to 0.1 wt% of the aluminum oxide.

[0089] Example 11 Titanium oxide having immobilized nickel and potassium components was obtained in the same manner as in Example 1, except that titanium oxide (reference catalyst "JRC-TIO-16") was used instead of aluminum oxide.

[0090] Example 12 Zirconium oxide having immobilized nickel and potassium components was obtained in the same manner as in Example 1, except that zirconium oxide (reference catalyst "JRC-ZRO-7") was used instead of aluminum oxide.

[0091] Example 13 Aluminum oxide having iron and potassium components immobilized thereon was obtained in the same manner as in Example 1, except that iron nitrate was used and iron was immobilized instead of nickel.

[0092] (Comparative Example 1) Aluminum oxide having nickel and potassium components immobilized thereon was obtained in the same manner as in Example 1, except that the nickel component was immobilized before the potassium component.

[0093] (Comparative Example 2) Aluminum oxide having nickel and potassium components immobilized thereon was obtained in the same manner as in Example 1, except that the nickel component was immobilized together with the potassium component.

[0094] (Comparative Example 3) Aluminum oxide having only potassium immobilized thereon was obtained in the same manner as in Example 1, except that nickel was not immobilized thereon.

[0095] Comparative Example 4 Aluminum oxide having nickel and potassium components immobilized thereon was obtained in the same manner as in Example 1, except that the mass of the nickel component relative to the aluminum oxide was adjusted to 1 wt %.

[0096] (Comparative Example 5) Aluminum oxide having nickel and potassium components immobilized thereon was obtained in the same manner as in Example 1, except that the mass of the nickel component relative to the aluminum oxide was adjusted to 10 wt %.

[0097] (Comparative Example 6) Aluminum oxide having nickel and potassium components immobilized thereon was obtained in the same manner as in Example 1, except that the mass of the nickel component relative to the aluminum oxide was adjusted to 15 wt %.

[0098] (Comparative Example 7) Aluminum oxide having only the nickel component immobilized thereon was obtained in the same manner as in Example 1, except that potassium was not immobilized thereon.

[0099] (Comparative Example 8) Aluminum oxide having immobilized nickel and potassium components was obtained in the same manner as in Example 1, except that the mass of the nickel component relative to the aluminum oxide was adjusted to 10 wt % and the mass of the potassium component calculated as potassium oxide was adjusted to 20 wt %.

[0100] (Comparative Example 9) Aluminum oxide having a nickel component and a magnesium component immobilized thereon was obtained in the same manner as in Example 1, except that magnesium nitrate was used and magnesium was immobilized instead of potassium.

[0101] (Comparative Example 10) Aluminum oxide having a nickel component and a calcium component immobilized thereon was obtained in the same manner as in Example 1, except that calcium nitrate was used and calcium was immobilized instead of potassium.

[0102] (Comparative Example 11) Silicon dioxide having immobilized nickel and potassium components was obtained in the same manner as in Example 1, except that silicon dioxide (reference catalyst "JRC-SIO-16" ground in an agate mortar for 10 minutes) was used instead of aluminum oxide.

[0103] (Comparative Example 12) Cerium oxide having immobilized nickel and potassium components was obtained in the same manner as in Example 1, except that cerium oxide (reference catalyst "JRC-CEO-2") was used instead of aluminum oxide.

[0104] Next, the carbon dioxide storage-reduction process using the bifunctional catalyst of the present invention will be described.

[0105] The carbon dioxide absorption and conversion process was carried out according to the following procedure. 100 mg of the bifunctional catalysts obtained in Examples 1 to 13 and Comparative Examples 1 to 12 was packed into a quartz tube with an inner diameter of 4 mm. First, a mixed gas consisting of 20 vol% hydrogen and 80 vol% nitrogen was passed through the quartz tube at a flow rate of 20 mL / min, and the catalyst was subjected to reduction pretreatment at 300 to 350°C for 30 minutes. Next, as a carbon dioxide absorption step, a mixed gas consisting of 1 vol% carbon dioxide, 20 vol% oxygen, and 79 vol% nitrogen was passed through the tube at a flow rate of 100 mL / min for 5 minutes. Furthermore, as a carbon dioxide reduction step, a mixed gas consisting of 20 vol% hydrogen and 80 vol% nitrogen was passed through the tube at a flow rate of 20 mL / min for 5 minutes. Subsequently, the carbon dioxide absorption step and the carbon dioxide reduction step were repeated six or more times, and the outlet gas components were analyzed by IR. Analysis of the outlet gas components revealed that it was mostly methane gas, with some carbon monoxide present. The carbon dioxide conversion amount relative to the catalyst mass and the carbon dioxide conversion efficiency were calculated using the following formulas from the average values ​​of the methane and carbon monoxide production amounts in the third to sixth cycles.

[0106]

number

[0107]

number

[0108] The time required for one cycle was 10 minutes, since the carbon dioxide absorption step was 5 minutes and the carbon dioxide reduction step was also 5 minutes.

[0109] The bifunctional catalysts prepared in Example 1 and Comparative Examples 1 to 3 and 7 were used to carry out the carbon dioxide storage and conversion process experiments, and the carbon dioxide conversion amount per nickel mass was determined. The results are shown in Table 1.

[0110] [Table 1]

[0111] As is clear from a comparison of Example 1 with Comparative Example 3 and Comparative Example 7 in Table 1, it can be seen that carbon dioxide is hardly converted in this process unless nickel (component C) or potassium (component B) is immobilized. Furthermore, based on the results shown in Table 1, a comparison of Example 1, Comparative Example 1, and Comparative Example 2 is shown in FIG. 1. As is clear from a comparison of the results of Example 1 with Comparative Examples 1 and 2 in FIG. 1 and Table 1, it was confirmed that immobilizing the nickel component (component C) after the potassium component (component B) can reduce carbon dioxide more efficiently than when the nickel component (component C) is immobilized before the potassium component (component B) or when the nickel component (component C) and the potassium component (component B) are immobilized simultaneously. This is thought to be because the presence of a larger amount of nickel component near the outermost surface of the catalyst allows for highly efficient re-reduction of the nickel component during hydrogen gas supply.

[0112] The bifunctional catalysts prepared in Examples 1 to 3 and Comparative Examples 3 to 6 and 8 were used to carry out the carbon dioxide storage and conversion process experiments, and the carbon dioxide conversion amount per mass of nickel was determined. The results are shown in Table 2.

[0113] [Table 2]

[0114] Based on the results shown in Table 2, the carbon dioxide conversion rate was plotted against the amount of immobilized nickel for the bifunctional catalysts obtained in Examples 1 to 3 and Comparative Examples 3 to 6 and 8. The results are shown in Figure 2. As shown in Figure 2, it was confirmed that when the amount of immobilized nickel component was 1 wt%, the carbon dioxide conversion rate significantly decreased. Furthermore, when the amount of immobilized nickel component was 10 wt% or more, there was also a significant tendency for the carbon dioxide conversion rate per mass of nickel to decrease. These results confirmed that immobilizing 2.0 wt% or more and 9.0 wt% or less of the nickel component was effective in efficiently absorbing and reducing carbon dioxide.

[0115] Experiments on the carbon dioxide storage and conversion process were carried out using the bifunctional catalysts prepared in Examples 1 to 8 and Comparative Examples 4 to 7, and the results are summarized in Table 3 in terms of the mass ratio of the alkali metal component to the mass of the nickel component. The results are shown in Figure 3.

[0116] [Table 3]

[0117] As is clear from Table 3 and Figure 3, when an alkali metal component is immobilized on a bifunctional catalyst in an amount between 1.0 and 3.6 times the mass of the nickel component, a bifunctional catalyst capable of promoting the reaction with a high efficiency of 0.95 mg-CO2 / h / mg-Ni or more in terms of carbon dioxide conversion per Ni catalyst was obtained.

[0118] A comparison of the results for Example 1 and Comparative Example 8 shown in Table 2 shows that in a bifunctional catalyst in which the mass of the alkali metal component was immobilized 2.0 times the mass of the nickel component, when the immobilized amount of the nickel component was 10 wt%, the carbon dioxide conversion efficiency decreased, and the amount of carbon dioxide converted per mass of nickel decreased to about one-third, compared to when the immobilized amount was 5.0 wt%. These results show that even if the ratio of the mass of the alkali metal component to the mass of the nickel component is appropriate, when the immobilized amount of metal is high, catalytic activity commensurate with this is not achieved.

[0119] [Table 4]

[0120] Based on the results shown in Table 4, a comparison of the results for Example 1, Example 9, Comparative Example 9, and Comparative Example 10 revealed that the bifunctional catalyst exhibited a higher carbon dioxide conversion rate when alkali metal components such as sodium and potassium were immobilized compared to when alkaline earth metal components such as magnesium and calcium were immobilized.

[0121] [Table 5]

[0122] Comparison of the results for Example 1 and Example 10 shown in Table 5 confirmed that the effect of improving the efficiency of carbon dioxide reduction was achieved by immobilizing a small amount of ruthenium component, together with a nickel component and a potassium component, on aluminum oxide.

[0123] [Table 6]

[0124] A comparison of the results for Example 1, Example 11, Experimental Example 12, Comparative Example 11, and Comparative Example 12 shown in Table 6 confirms that a bifunctional catalyst in which a nickel component and a potassium component are immobilized on aluminum oxide, titanium oxide, or zirconium oxide can reduce carbon dioxide more efficiently than a bifunctional catalyst immobilized on silicon dioxide or cerium oxide.

[0125] [Table 7]

[0126] The results for Example 13 shown in Table 7 confirmed that carbon dioxide can be efficiently reduced by immobilizing a cheaper iron component instead of a nickel component. Among base metals, iron is more abundant in both reserves and production than nickel, and is available at a lower cost. It was also confirmed that iron can be used as a carbon dioxide reduction metal in a bifunctional catalyst. [Industrial Applicability]

[0127] As described above, according to the present invention, it is possible to obtain a dual-function catalyst that can efficiently separate carbon dioxide from a mixed gas containing at least carbon dioxide and oxygen and reduce the separated carbon dioxide. Furthermore, by using the carbon dioxide occlusion / reduction process of the present invention, it is possible to efficiently separate carbon dioxide from a mixed gas containing at least carbon dioxide and oxygen and reduce the separated carbon dioxide. Therefore, this is useful as a technology for separating and capturing carbon dioxide before it is released into the atmosphere, or for separating and capturing carbon dioxide present in the atmosphere, and further converting it into useful compounds.

Claims

1. A dual function catalyst comprising at least three components including (A) at least one component selected from aluminum oxide, titanium oxide, and zirconium oxide as an oxide support, (B) at least one component selected from alkali metals, and (C) at least one component selected from nickel and iron, wherein the components B and C are supported on the surface of the support of the component A, A dual-function catalyst characterized in that the mass of component C relative to component A is 2.0 wt% or more and 9.0 wt% or less, the mass of component B is 1.0 times or more and 3.6 times or less the mass of component C, and the dual-function catalyst is prepared by supporting component B on component A prior to supporting component C.

2. 2. The bifunctional catalyst according to claim 1, wherein the mass ratio of component C to component A is 2.5 wt % or more and 8.0 wt % or less.

3. 3. The bifunctional catalyst according to claim 1, wherein the mass of component B is 1.6 to 3.6 times the mass of component C.

4. 3. The bifunctional catalyst according to claim 1 or 2, characterized in that it is prepared by immobilizing component C on component A using a metal precursor solution containing component C in a volume that is at least twice the pore volume of the oxide support that is component A.

5. 3. The dual-function catalyst according to claim 1, wherein the amount of carbon dioxide converted per unit time and unit mass of carbon dioxide-reducing metal component (mass of component C) in a carbon dioxide absorption-reduction reaction using a mixed gas containing at least carbon dioxide and oxygen is 0.95 mg-CO ２ / h / mg-Metal(C) or more.

6. 3. The bifunctional catalyst according to claim 1, wherein component C further contains a ruthenium component in an amount of 1.0 wt % or less relative to component A.

7. 3. The bifunctional catalyst according to claim 1 or 2, wherein the component B is sodium or potassium.

8. 3. The bifunctional catalyst according to claim 1, wherein the amount of carbon dioxide converted is 1.5 wt % or more based on the mass of the catalyst.

9. A method for producing a bifunctional catalyst comprising at least three components including (A) at least one component selected from aluminum oxide, titanium oxide, and zirconium oxide as an oxide support, (B) at least one component selected from alkali metals, and (C) at least one component selected from nickel and iron, wherein the components B and C are supported on the surface of the support of the component A, A method for producing a dual-function catalyst, characterized in that the mass of component C relative to component A is 2.0 wt% or more and 9.0 wt% or less, the mass of component B is 1.0 times or more and 3.6 times or less the mass of component C, and the catalyst is prepared by supporting component B on component A prior to supporting component C.

10. 10. The method for producing a dual function catalyst according to claim 9, wherein the mass ratio of component C to component A is 2.5 wt % or more and 8.0 wt % or less.

11. 11. The method for producing a dual-function catalyst according to claim 9 or 10, wherein the mass of component B is 1.6 to 3.6 times the mass of component C.

12. 11. The method for producing a bifunctional catalyst according to claim 9 or 10, wherein the bifunctional catalyst is prepared by using a metal precursor solution containing component C in a volume that is at least twice the pore volume of the oxide support as component A, and immobilizing component C on component A.

13. 11. The method for producing a dual function catalyst according to claim 9 or 10, wherein the component C further contains a ruthenium component in an amount of 1.0 wt % or less relative to the component A.

14. 11. The method for producing a bifunctional catalyst according to claim 9 or 10, wherein the component B is sodium or potassium.

15. 11. The method for producing a bifunctional catalyst according to claim 9 or 10, wherein the step of immobilizing component C on the oxide support (component A) using a metal precursor solution containing component C is carried out under reduced pressure conditions.

16. 3. A carbon dioxide storage-reduction process, comprising: using a reactor containing the bifunctional catalyst according to claim 1 or 2; introducing a mixed gas containing at least carbon dioxide and oxygen into the reactor containing the bifunctional catalyst to occlude the carbon dioxide; and then introducing hydrogen gas into the reactor containing the bifunctional catalyst to reduce the carbon dioxide.

17. A reactor containing the bifunctional catalyst according to claim 1 or 2 is used, and the carbon dioxide conversion amount per unit time and unit mass of the carbon dioxide-reducing metal component is 0.95 mg-CO ２ / h / mg-Metal(C) or more.

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