Metal oxide, oxygen storage material, oxygen adsorption / desorption device, oxygen adsorption / desorption method, oxygen concentrator, and oxygen concentration method

Abstract

An oxygen-deficient perovskite-type metal oxide represented by formula (I).　(I): (Ba4-x1Ca1-y1Fe3-z1M1x1+y1+z1)O9.5+δ1a (in formula (I), M1 represents at least one element selected from the group consisting of an alkali metal, an alkaline earth metal other than Ba or Ca, a rare earth metal, and a transition metal other than Fe, -2.0≤x1≤2.0, 0≤y1≤0.5, -1.5≤z1≤1.5, x1+y1+z1≥0, and -1.5≤δ1a≤4.5.)

Description

[Technical Field]

[0001] The present invention relates to a metal oxide, an oxygen storage material, an oxygen adsorption / desorption device, an oxygen adsorption / desorption method, an oxygen concentrating device, and an oxygen concentrating method. This application claims priority from Japanese Patent Application No. 2020-147524, filed in Japan on September 2, 2020, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, oxygen storage materials that reversibly absorb and release oxygen in response to temperature changes have attracted attention in fields related to energy production and environmental protection, and are expected to be used in oxygen gas concentrators and oxygen gas production systems, among others, that utilize this oxygen adsorption and desorption ability. 7+δ (Patent Documents 1 to 4), CaAlMnO 5+β Various metal oxides are known, such as those disclosed in Patent Documents 5 and 6. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-012619 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-193815 [Patent Document 3] Japanese Patent Application Publication No. 2018-008871 [Patent Document 4] Japanese Patent Application Publication No. 2019-043833 [Patent Document 5] Japanese Patent Application Laid-Open No. 2013-255911 [Patent Document 6] Japanese Patent Application Laid-Open No. 2014-012619 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the metal oxides described in Patent Documents 1 to 4 contain expensive metal elements as constituent elements, so the cost of raw materials is a barrier to practical application. On the other hand, the metal oxides described in Patent Documents 5 and 6 can use only inexpensive metal elements as metal elements, but their activation temperature tends to be high, which poses a practical problem when considering the heat resistance of equipment such as oxygen gas concentrators. Furthermore, even if the problem of heat resistance of the equipment can be solved, the applicable processes are limited when using exhaust heat to heat the material.

[0005] An object of the present invention is to provide a metal oxide that is capable of absorbing and desorbing oxygen in a relatively low temperature range without containing expensive metal elements, and that is excellent in oxygen absorbing and desorbing rate and maximum oxygen adsorption amount. [Means for solving the problem]

[0006] As a result of extensive investigations, the present inventors have found that an oxygen-deficient perovskite-type metal oxide containing Fe as a metallic element has the property of undergoing a phase transition from an oxygen-releasing phase to an oxygen-excessive phase upon oxygen adsorption, and a phase transition from the oxygen-excessive phase to the oxygen-releasing phase upon oxygen desorption, thereby solving the above-mentioned problems and achieving the present invention.

[0007] [1] An oxygen-deficient perovskite metal oxide represented by the following formula (I): (Ba 4-x1 Ca 1-y1 Fe 3-z1 M1 x1+y1+z1 )O 9.5+δ1a (I) (In the above formula (I), M1 represents one or more elements selected from the group consisting of alkali metals, alkaline earth metals other than Ba and Ca, rare earth metals, and transition metals other than Fe; x1, y1, z1, and δ1a satisfy the following formula. -2.0≦x1≦2.0 0≦y1≦0.5 -1.5≦z1≦1.5 -1.5≦δ1a≦4.5 x1+y1+z1≧0) [2] An oxygen-deficient perovskite metal oxide represented by the following formula (II): (Ba 2-x2 Y 1-y2 Fe 1-z2 M2 x2+y2+z2 )O 5+δ2a (II) (In the above formula (II), M2 represents one or more elements selected from the group consisting of alkali metals, alkaline earth metals other than Ba, rare earth metals other than Y, and transition metals other than Fe; x2, y2, z2, and δ2a satisfy the following formula. -1.0≦x2≦1.0 -0.5≦y2≦0.5 -0.5≦z2≦0.5 -0.5≦δ2a≦1.5 x2+y2+z2≧0) [3] An oxygen-deficient perovskite metal oxide represented by the following formula (III): (RE 1-x3 Ca 2-y3 Fe 3-z3 M3 x3+y3+z3 )O 8+δ3a (III) (In the above formula (III), RE represents one or more rare earth elements selected from the group consisting of Nd, Sm, and Eu; M3 represents one or more elements selected from the group consisting of alkali metals, alkaline earth metals other than Ca, rare earth metals other than RE, and transition metals other than Fe; x3, y3, z3, and δ3a satisfy the following formula. -0.5≦x3≦0.5 -1.0≦y3≦1.0 -1.5≦z3≦1.5 -1.5≦δ3a≦2.8 x3+y3+z3≧0) [4] An oxygen storage material comprising the metal oxide according to any one of [1] to [3]. [5] An oxygen adsorption / desorption device comprising the metal oxide according to any one of [1] to [3]. [6] An oxygen concentrator comprising the metal oxide according to any one of [1] to [3]. [7] An oxygen adsorption / desorption method comprising: an oxygen adsorption step of contacting the metal oxide according to any one of [1] to [3] with an oxygen-containing gas at a temperature not higher than the temperature at which oxygen adsorption occurs, thereby adsorbing oxygen onto the metal oxide; and an oxygen desorption step of heating the metal oxide that has adsorbed oxygen in the oxygen adsorption step at a temperature not lower than the temperature at which oxygen desorption occurs but not higher than 700°C, thereby desorbing oxygen from the metal oxide. [8] An oxygen adsorption / desorption method comprising: an oxygen adsorption step of contacting the metal oxide according to any one of [1] to [3] with an oxygen-containing gas having an oxygen partial pressure of more than 0 kPa and not more than 100 kPa, thereby adsorbing oxygen onto the metal oxide; and an oxygen desorption step of placing the metal oxide having oxygen adsorbed in the oxygen adsorption step under an atmosphere having an oxygen partial pressure lower than that of the oxygen-containing gas, thereby desorbing oxygen from the metal oxide. [9] An oxygen concentration method, comprising an oxygen recovery step of recovering oxygen desorbed from the metal oxide by the oxygen adsorption / desorption method according to [7] or [8]. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a metal oxide that is capable of absorbing and desorbing oxygen in a relatively low temperature range without containing an expensive metal element, and that is excellent in oxygen absorbing and desorbing rate and maximum oxygen adsorption amount. This metal oxide can be used as an oxygen storage material, and can also be used in oxygen adsorption / desorption devices and oxygen concentrators. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 1 in response to ambient temperature swings under an air stream. [Figure 2]FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 1 in response to ambient temperature swings under an oxygen stream. [Figure 3] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 1 at 290° C. under oxygen partial pressure swing. [Figure 4] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 1 at 300° C. under oxygen partial pressure swing. [Figure 5] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 1 at 310° C. under oxygen partial pressure swing. [Figure 6] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 1 at 320° C. under oxygen partial pressure swing. [Figure 7] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 1 at 330° C. under oxygen partial pressure swing. [Figure 8] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 1 at 340° C. under oxygen partial pressure swing. [Figure 9] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 1 at 350° C. under oxygen partial pressure swing. [Figure 10] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 1 at 360° C. under oxygen partial pressure swing. [Figure 11] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 1 at 370° C. under oxygen partial pressure swing. [Figure 12] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 1 at 380° C. under oxygen partial pressure swing. [Figure 13] FIG. 1 is a diagram showing the results of powder X-ray diffraction measurement of the metal oxide obtained in Example 1. [Figure 14] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 2 at 250° C. under oxygen partial pressure swing. [Figure 15] FIG. 1 shows the results of powder X-ray diffraction measurement of the metal oxide obtained in Example 2. [Figure 16]FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 3-1 in response to ambient temperature swings under air flow. [Figure 17] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 3-2 in response to ambient temperature swings under air flow. [Figure 18] FIG. 10 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Example 3-3 in response to ambient temperature swings under air flow. [Figure 19] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Comparative Example 3-1 in response to ambient temperature swings under air flow. [Figure 20] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Comparative Example 3-2 in response to ambient temperature swings under air flow. [Figure 21] FIG. 10 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Comparative Example 3-3 in response to ambient temperature swings under air flow. [Figure 22] FIG. 10 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Comparative Example 3-4 in response to ambient temperature swings under air flow. [Figure 23] FIG. 1 is a graph showing the oxygen adsorption / desorption behavior of the metal oxide obtained in Comparative Example 3-5 in response to ambient temperature swings under air flow. [Figure 24] FIG. 1 shows the results of powder X-ray diffraction measurements of the metal oxides obtained in Examples 3-1 to 3-3 and Comparative Examples 3-1 to 3-5. DETAILED DESCRIPTION OF THE INVENTION

[0010] The following describes in detail the embodiments of the present invention, but these descriptions are examples (typical examples) of the embodiments of the present invention, and the present invention is not limited to these contents as long as it does not deviate from the gist of the invention.

[0011] The metal oxides according to the first to third embodiments of the present invention are metal oxides that undergo a phase transition from an oxygen-releasing phase to an oxygen-excess phase upon oxygen adsorption, and from the oxygen-excess phase to the oxygen-releasing phase upon oxygen desorption (release), and contain Fe. These metal oxides are oxygen-deficient perovskite-type metal oxides, and both the cation sites and the anion sites are ordered in both the oxygen-releasing phase and the oxygen-excess phase. These metal oxides may further contain one or more elements selected from rare earth metals, transition metal elements, alkali metal elements, and alkaline earth metal elements. Furthermore, these metal oxides preferably have an average valence of Fe greater than trivalent in the oxygen-excess phase.

[0012] Here, "cation sites and anion sites are ordered in both the oxygen-releasing phase and the oxygen-excess phase" means that the positions of all cations and anions that constitute the crystal can be clearly defined. The positions of cations and anions can be identified by XRD measurement.

[0013] Because both the cation and anion sites are ordered in both the oxygen-releasing and oxygen-excess phases, the material exhibits a clear, reversible phase transition induced by temperature and / or atmosphere, making it possible to utilize this phase transition for applications such as oxygen adsorption / desorption and oxygen concentration.

[0014] Furthermore, in the phase transition from the oxygen-releasing phase to the oxygen-excess phase of the metal oxide according to this embodiment, it is preferable that the cation-oxygen ion bonding relationship in the oxygen-releasing phase is maintained while the adsorbed oxygen ions form bonds with the cations, and that some or all of the anion sites are occupied by the adsorbed oxygen ions, increasing the coordination number of the metal. Therefore, it is preferable that the oxygen-releasing phase and the oxygen-storing phase are in different ordered states, and that the space group changes. Here, "the space group changes" means that the number of peaks with an intensity of 10% or more of the main peak intensity in the XRD pattern changes between the two states.

[0015] <1. Metal oxide according to the first embodiment of the present invention> The metal oxide according to the first embodiment of the present invention is an oxygen-deficient perovskite metal oxide represented by the following formula (I). (Ba 4-x1 Ca 1-y1 Fe 3-z1 M1 x1+y1+z1 )O 9.5+δ1a (I) In the above formula (I), M1 represents one or more elements selected from the group consisting of alkali metals, alkaline earth metals other than Ba and Ca, rare earth metals, and transition metals other than Fe; x1, y1, z1, and δ1a satisfy the following formula. -2.0≦x1≦2.0 0≦y1≦0.5 -1.5≦z1≦1.5 -1.5≦δ1a≦4.5 x1+y1+z1≧0

[0016] In formula (I), δ1a represents the non-stoichiometric amount of oxygen, and is not particularly limited as long as it satisfies the range of −1.5≦δ1a≦4.5, and it changes continuously depending on the external environment such as the atmosphere and temperature. M1 is a substitution element for a site selected from the Fe site, the Ba site, and the Ca site, and can be contained as appropriate within a range that does not impair the effects of the present invention. Furthermore, the metal oxide of formula (I) may contain other elements, for example, M1 may contain other elements such as inevitable impurity elements, as long as the effects of the present invention are not impaired. Furthermore, a metal element selected from Ba, Ca, and Fe may be substituted at other sites.

[0017] From the viewpoint of suppressing the generation of impurity phases, x1 is preferably −1.0≦x1≦1.5, more preferably −0.5≦x1≦0.8, even more preferably 0≦x1≦0.8, and particularly preferably 0≦x1≦0.4. From the viewpoint of suppressing the generation of impurity phases, y1 is preferably 0≦y1≦0.4, more preferably 0≦y1≦0.2, and even more preferably 0≦y1≦0.1. From the viewpoint of suppressing the generation of impurity phases, z1 is preferably −1.0≦z1≦1.0, more preferably −0.5≦z1≦0.6, even more preferably 0≦z1≦0.6, and particularly preferably 0≦z1≦0.3. From the viewpoint of suppressing the generation of impurity phases, δ1a is preferably 0≦δ1a≦1.5.

[0018] The metal oxide according to this embodiment is, for example, Ba4CaFe3O 9.5+δ1 The sites occupied by Fe, Ba, and Ca may be substituted with other elements within a range that does not impair the effects of the present invention, particularly within a range that allows the order of the cation sites and anion sites to be maintained. This Ba4CaFe3O 9.5+δ1 is an oxygen-deficient perovskite-type metal oxide. In the composition formula, δ1 represents the non-stoichiometric amount of oxygen, and is not particularly limited as long as it satisfies the range of 0≦δ1≦1.5, and it changes continuously depending on the external environment such as the atmosphere and temperature.

[0019] <2. Metal oxide according to a second embodiment of the present invention> The metal oxide according to the second embodiment of the present invention is an oxygen-deficient perovskite metal oxide represented by the following formula (II). (Ba 2-x2 Y 1-y2 Fe 1-z2 M2 x2+y2+z2 )O 5+δ2a (II) In the above formula (II), M2 represents one or more elements selected from the group consisting of alkali metals, alkaline earth metals other than Ba, rare earth metals other than Y, and transition metals other than Fe; x2, y2, z2, and δ2a satisfy the following formula. -1.0≦x2≦1.0 -0.5≦y2≦0.5 -0.5≦z2≦0.5 -0.5≦δ2a≦1.5 x2+y2+z2≧0

[0020] In formula (II), δ2a represents the non-stoichiometric amount of oxygen, and is not particularly limited as long as it satisfies the range of −0.5≦δ2a≦1.5, and changes continuously depending on the external environment such as the atmosphere and temperature. M2 is a substitution element at a site selected from the Fe site, the Ba site, and the Y site, and can be contained as appropriate within a range that does not impair the effects of the present invention. Furthermore, the metal oxide of formula (II) may contain other elements, for example, M2 may contain other elements such as inevitable impurity elements, as long as the effects of the present invention are not impaired. Furthermore, a metal element selected from Ba, Y, and Fe may be substituted at other sites.

[0021] From the viewpoint of suppressing the generation of impurity phases, x2 is preferably −0.6≦x2≦0.8, more preferably −0.4≦x2≦0.4, even more preferably 0≦x2≦0.4, and particularly preferably 0≦x2≦0.2. From the viewpoint of suppressing the formation of impurity phases, y2 is preferably −0.4≦y2≦0.4, more preferably −0.2≦y2≦0.2, even more preferably 0≦y2≦0.2, and particularly preferably 0≦y2≦0.1. From the viewpoint of suppressing the formation of impurity phases, z2 is preferably −0.4≦z2≦0.4, more preferably −0.2≦z2≦0.2, even more preferably 0≦z2≦0.2, and particularly preferably 0≦z2≦0.1. From the viewpoint of suppressing the generation of impurity phases, δ2a is preferably 0≦δ2a≦0.5.

[0022] The metal oxide according to this embodiment is, for example, BaYFeO 5+δ2 The sites occupied by Fe, Ba, and Y may be substituted with other elements as long as the effects of the present invention are not impaired, particularly as long as the order of the cation sites and anion sites can be maintained. This Ba2YFeO 5+δ2 is an oxygen-deficient perovskite-type metal oxide. In the composition formula, δ2 represents the non-stoichiometric amount of oxygen, and is not particularly limited as long as it satisfies the range of 0≦δ2≦0.5, and it changes continuously depending on the external environment such as the atmosphere and temperature.

[0023] <3. Metal oxide according to the third embodiment of the present invention> The metal oxide according to the third embodiment of the present invention is an oxygen-deficient perovskite metal oxide represented by the following formula (III). (RE 1-x3 Ca 2-y3 Fe 3-z3 M3 x3+y3+z3 )O 8+δ3a (III) In the above formula (III), RE represents one or more rare earth elements selected from the group consisting of Nd, Sm, and Eu; M3 represents one or more elements selected from the group consisting of alkali metals, alkaline earth metals other than Ca, rare earth metals other than RE, and transition metals other than Fe; x3, y3, z3, and δ3a satisfy the following formula. -0.5≦x3≦0.5 -1.0≦y3≦1.0 -1.5≦z3≦1.5 -1.5≦δ3a≦2.8 x3+y3+z3≧0

[0024] In formula (III), δ3a represents the non-stoichiometric amount of oxygen, and is not particularly limited as long as it satisfies the range of −1.5≦δ3a≦2.5, and changes continuously depending on the external environment such as the atmosphere and temperature. M3 is a substitution element for a site selected from the Fe site, the RE site, and the Ca site, and can be contained as appropriate within a range that does not impair the effects of the present invention. Furthermore, the metal oxide of formula (III) may contain other elements, for example, M3 may contain other elements such as inevitable impurity elements, as long as the effects of the present invention are not impaired. Furthermore, a metal element selected from RE, Ca, and Fe may be substituted at other sites.

[0025] From the viewpoint of the cost of the element or the oxygen absorbing / releasing performance, RE is preferably Sm. From the viewpoint of suppressing the generation of impurity phases, x3 is preferably −0.4≦x3≦0.4, more preferably −0.2≦x3≦0.2, even more preferably 0≦x3≦0.2, and particularly preferably 0≦x3≦0.1. From the viewpoint of suppressing the formation of impurity phases, y3 is preferably −0.6≦y2≦0.8, more preferably −0.4≦y2≦0.4, even more preferably 0≦y3≦0.4, and particularly preferably 0≦y3≦0.2. From the viewpoint of suppressing the formation of impurity phases, z3 is preferably −1.0≦z3≦1.0, more preferably −0.5≦z3≦0.6, even more preferably 0≦z3≦0.6, and particularly preferably 0≦z3≦0.3. From the viewpoint of suppressing the formation of impurity phases, δ3a is preferably −1.0≦δ3a≦2.5, more preferably −0.5≦δ3a≦2.0, and even more preferably 0≦δ3a≦1.5.

[0026] The metal oxide according to this embodiment is, for example, RECa2Fe3O 8+δ3 The sites occupied by Fe, Ca, and RE may be substituted with other elements within a range that does not impair the effects of the present invention, particularly within a range that allows the order of the cation sites and anion sites to be maintained.

[0027] RECa2Fe3O 8+δ3 is an oxygen-deficient perovskite-type metal oxide. In the composition formula, RE represents one or more rare earth elements selected from the group consisting of Nd, Sm, and Eu, and is preferably Sm from the viewpoint of the cost of the element or oxygen absorption / release performance. δ3 represents the oxygen non-stoichiometry, and is not particularly limited as long as it satisfies the range of 0≦δ3≦1.5, and it changes continuously depending on the external environment such as the atmosphere and temperature.

[0028] The metal oxides according to the first to third embodiments may contain a dopant, or may not contain a dopant, within a range that does not impair the effects of the present invention, particularly within a range that maintains the order of the cation sites and the anion sites. Examples of dopants include transition metal elements, from the viewpoint of improving the oxygen adsorption / desorption rate and controlling the oxygen adsorption / desorption temperature. Among these, the dopant is preferably one or more elements selected from the group consisting of Ru, Rh, Pd, Ag, Re, Ir, Pt, and Au.

[0029] In the first to third embodiments, the oxygen adsorption / desorption rate and the oxygen adsorption / desorption temperature can be controlled by adjusting the content of the dopant. From the viewpoint of maintaining the order of the cation sites and the anion sites, the content of the dopant is preferably 5 mol % or less, and more preferably 2.5 mol % or less, of the total amount of cations including the dopant.

[0030] In the metal oxides according to the first to third embodiments, the average valence of Fe in the oxygen-excess phase is preferably more than 3, more preferably 3.1 or more, and is preferably 5.0 or less, more preferably 4.0 or less. By setting the valence transition region of Fe within the above range, even oxides of inexpensive metals such as those mentioned above can be made capable of oxygen adsorption and desorption at relatively low temperatures.

[0031] <4. Metal oxide according to the fourth embodiment of the present invention> A metal oxide according to a fourth embodiment of the present invention contains Mn, and both the cation sites and the anion sites are ordered in both the oxygen-releasing phase and the oxygen-excess phase, and undergoes a phase transition from the oxygen-releasing phase to the oxygen-excess phase upon oxygen adsorption, and from the oxygen-excess phase to the oxygen-releasing phase upon oxygen desorption (release). In this metal oxide, the average valence of the Mn is preferably greater than 2 in the oxygen-excess phase.

[0032] In the metal oxide according to the fourth embodiment of the present invention, "the cation sites and anion sites are ordered in both the oxygen-releasing phase and the oxygen-excess phase" is defined in the same way as in the first to third embodiments of the present invention, and the influence on the clear reversible phase transition induced by temperature and / or atmosphere is also the same. Furthermore, the behavior of this Mn-containing metal oxide during the phase transition from the oxygen-releasing phase to the oxygen-excess phase is also the same as in the metal oxides according to the first to third embodiments of the present invention.

[0033] The composition of the Mn-containing metal oxide according to the fourth embodiment of the present invention can be represented by, for example, the following formula (IV). (Ba 2-x4 Mn 1-y4 Ge 2-z4 M4 x4+y4+z4 )O 7+δ4a (IV) In the above formula (IV), M4: one or more elements selected from the group consisting of alkali metals, alkaline earth metals other than Ba, rare earth metals, and transition metals other than Mn 0≦x4≦1.0 0≦y4≦0.5 0≦z4≦1.0 -1.5≦δ4a≦1.0 Represents.

[0034] In formula (IV), δ4a represents the non-stoichiometric amount of oxygen, and is not particularly limited as long as it satisfies the range of −1.5≦δ4a≦1.0, and it changes continuously depending on the external environment such as the atmosphere and temperature. M4 is a substitution element for a site selected from the Ba site, the Mn site, and the Ge site, and can be contained as appropriate within a range that does not impair the effects of the present invention. Furthermore, the metal oxide of formula (IV) may contain other elements, for example, M4 may contain inevitable impurity elements, as long as the oxygen adsorption / desorption performance is not impaired. Furthermore, a metal element selected from Ba, Mn, and Ge may be substituted at other sites.

[0035] From the viewpoint of suppressing the generation of impurity phases, x4 is preferably 0≦x4≦0.4, and more preferably 0≦x4≦0.2. From the viewpoint of suppressing the generation of impurity phases, y4 is preferably 0≦y4≦0.2, and more preferably 0≦y4≦0.1. From the viewpoint of suppressing the generation of impurity phases, z4 is preferably 0≦z4≦0.4, and more preferably 0≦z4≦0.2. From the viewpoint of suppressing the generation of impurity phases, δ4a is preferably −0.3≦δ4a≦0.2.

[0036] Examples of the metal oxide represented by formula (IV) include Ba2MnGe2O 7+δ4 The sites occupied by Ba, Mn, and Ge may be substituted with other elements within a range that does not impair the oxygen adsorption / desorption performance, particularly within a range that maintains the order of the cation sites and anion sites.

[0037] Ba2MnGe2O 7+δ4 In the composition formula, δ4 represents the non-stoichiometric amount of oxygen, and is not particularly limited as long as it satisfies the range of 0≦δ4≦0.5, and it changes continuously depending on the external environment such as the atmosphere and temperature.

[0038] In the metal oxide according to this embodiment, the average valence of Mn in an oxygen-excess phase is preferably greater than 2, more preferably 2.1 or more, and is preferably 4.0 or less, more preferably 3.0 or less. By setting the valence transition region of Mn within the above range, even oxides of inexpensive metals such as those mentioned above can be made capable of oxygen adsorption and desorption.

[0039] <5. Metal oxide according to a fifth embodiment of the present invention> A metal oxide according to a fifth embodiment of the present invention is a metal oxide that undergoes a phase transition from an oxygen-releasing phase to an oxygen-excess phase upon oxygen adsorption, and a phase transition from the oxygen-excess phase to the oxygen-releasing phase upon oxygen desorption (release), in which both the cation sites and the anion sites are ordered in both the oxygen-releasing phase and the oxygen-excess phase, and contains a transition metal element and an element selected from an alkali metal element and an alkaline earth metal element.

[0040] In the fifth embodiment of the present invention, "cation sites and anion sites are ordered in both the oxygen-releasing phase and the oxygen-excess phase" is defined in the same way as in the first to third embodiments of the present invention, and the influence on the clear reversible phase transition induced by temperature and / or atmosphere is also the same. Furthermore, the behavior of the metal oxide according to the fifth embodiment of the present invention in the phase transition from the oxygen-releasing phase to the oxygen-excess phase is also the same as in the metal oxides according to the first to third embodiments of the present invention.

[0041] A metal oxide according to a fifth embodiment of the present invention contains a transition metal element such as Fe and an element selected from alkali metal elements and alkaline earth metal elements having a larger ionic radius than the transition metal. This configuration enables oxygen adsorption / desorption induced by temperature changes and changes in the oxygen partial pressure of the atmosphere. It is presumed that the alkali metal element and / or alkaline earth metal element having a larger ionic radius forms a crystal skeleton, facilitating the diffusion of oxygen ions.

[0042] The metal oxide according to the fifth embodiment of the present invention is the above-mentioned Ba4CaFe3O 9.5+δ1 , BaYFeO 5+δ2 , SmCa2Fe3O 8+δ3 etc.

[0043] <6. Morphology of Metal Oxides> <6-1. Average primary particle size> The average primary particle size of the metal oxides according to the first to fifth embodiments of the present invention is not particularly limited. From the viewpoint of the oxygen adsorption / desorption rate (oxygen adsorption / release rate), the volume-based average primary particle size is preferably 100 μm or less, more preferably 10 μm or less, and particularly preferably 5 μm or less. On the other hand, since a decrease in particle size increases the specific surface area and improves the oxygen adsorption / desorption rate, it is not necessary to set a lower limit for the average primary particle size, but from the viewpoint of handleability, it is usually 1 nm or more.

[0044] The average primary particle diameter is measured by observation using a scanning electron microscope (SEM). Specifically, in an SEM photograph taken at a magnification such that particles can be seen, for example, 5,000 to 100,000 times, the longest intercepts of a horizontal line formed by the left and right boundaries of the primary particles are determined for any 50 primary particles, and the average value is calculated.

[0045] <6-2.Specific surface area> The specific surface area of the metal oxides according to the first to fifth embodiments of the present invention is not particularly limited. 2 / g or more, and 1m 2 / g or more is more preferable, and 10m 2 On the other hand, since the oxygen adsorption / desorption rate improves as the specific surface area increases, it is not necessary to set an upper limit on the specific surface area, but it is usually 200 m 2 / g or less.

[0046] The specific surface area can be measured by the BET method, for example, using a Tristar II 3000 manufactured by Micromeritics Corp. Specifically, the metal oxide is dried under reduced pressure at 150°C for 1 hour, and then measured by the BET multipoint method (5 points in the relative pressure range of 0.05 to 0.30) using nitrogen gas adsorption.

[0047] <6-3. Shape> The metal oxides according to the first to fifth embodiments of the present invention may be granulated or formed into an appropriate shape in order to improve their ease of handling, and in consideration of the strength when packed into an apparatus and ease of gas flow.

[0048] 7. Evaluation of Metal Oxides <7-1. Composition analysis of metal oxides> The composition of the metal oxide can be analyzed by measuring the content of each element in the metal oxide according to the first to fifth embodiments of the present invention by the following method.

[0049] <7-1-1. Analysis of Metal Elements> The measurement method for determining the composition of metal elements in the metal oxides according to the first to fifth embodiments of the present invention is not particularly limited, but can be, for example, measured by inductively coupled plasma (ICP) optical emission spectroscopy (ICP analysis). As the plasma optical emission spectroscopy analyzer, for example, an ICP-AES "JY46P model" manufactured by JOBIN YVON can be used.

[0050] <7-1-2. Analysis of oxygen element> The measurement method for determining the amount of oxygen element in the metal oxides according to the first to fifth embodiments of the present invention is not particularly limited, but for example, the measurement can be performed by iodometric titration.

[0051] <7-2. Powder X-ray diffraction measurement> The crystal structure of the metal oxides according to the first to fifth embodiments of the present invention can be evaluated by powder X-ray diffraction (XRD) measurement. The measurement method is not particularly limited, but for example, X'Pert Pro MPD (manufactured by PANalytical) can be used under the following conditions. Source: CuKα Measurement range: 2θ angles from 5° to 90° Measurement interval: 0.02° Voltage: 45kV Current: 30mA

[0052] <7-3. SEM Observation and SEM-EDX> As described above, the average particle diameter of the metal oxides according to the first to fifth embodiments of the present invention can be evaluated using an SEM. Furthermore, when the metal oxides according to the first to fifth embodiments of the present invention contain a substitution element or a dopant, the distribution state of the dopant in the metal oxide can be evaluated by using SEM-EDX. There are no particular limitations on the measurement conditions for SEM-EDX, but for example, the measurement can be performed by using an EMAX X-act (manufactured by HORIBA) as an apparatus and mapping the contained elements at a voltage of 15 kV.

[0053] <7-4.Thermogravimetric analysis (TG)> By using a TG device, it is possible to observe the oxygen adsorption behavior of the first to fifth metal oxides of the present invention, and to measure the oxygen adsorption amount and phase transition temperature.

[0054] The TG measurement conditions are not particularly limited. For example, oxygen adsorption / desorption behavior due to temperature swing can be observed using a Thermo Plus 8120 (manufactured by Rigaku) as a TG device by increasing the temperature from room temperature (25°C) to 800°C at a rate of 5°C / min under an air flow (oxygen partial pressure 20.9 kPa, flow rate 400 mL / min), and then decreasing the temperature from 800°C to room temperature at a rate of 5°C / min, while measuring the change in weight during this period.

[0055] Furthermore, for example, the oxygen adsorption / desorption behavior due to oxygen partial pressure swing can be observed by using a Thermo Plus 8120 (manufactured by Rigaku) as a TG apparatus, raising the temperature to a specific temperature under a nitrogen stream (oxygen partial pressure of 0.002 kPa or less, flow rate of 400 mL / min), switching the flowing gas to air (oxygen partial pressure of 20.9 kPa, flow rate of 400 mL / min), and then switching the flowing gas to nitrogen (oxygen partial pressure of 0.002 kPa or less, flow rate of 400 mL / min), and measuring the weight change during this process.

[0056] <7-5. Measurement of average valence> The average Fe valence of a metal oxide can be calculated from the weight change during oxygen absorption measured by TG. The molar amount of oxygen atoms adsorbed during the phase transition from the oxygen-releasing phase to the oxygen-excess phase is determined from the weight increase associated with oxygen adsorption during TG measurement. Assuming that a valence transition (valence increase) of Fe occurs to maintain electrical neutrality in response to this increase in oxygen atoms (anions), the average Fe valence in the oxygen-excess phase can be calculated.

[0057] The average Fe valence of a metal oxide can also be determined by measuring the oxygen content in the metal oxide by cerium titration. An automatic potentiometric titrator (AT-710, Kyoto Electronics Manufacturing Co., Ltd.) can be used as a titrator. More specifically, iron(II) chloride tetrahydrate and 3 mol / L hydrochloric acid are added to the metal oxide, and the mixture is heated and stirred for at least 5 minutes to dissolve the metal oxide. This procedure allows any tetravalent iron ions present in the sample to react with the divalent iron ions in the iron chloride and be reduced to trivalent iron ions. The divalent iron ions from the metal oxide or excess iron(II) chloride tetrahydrate are oxidized with an aqueous cerium(IV) sulfate solution, and the amount of cerium ions required for the oxidation of the iron ions is titrated. The average iron valence can be calculated from the titration result. The aqueous cerium(IV) sulfate solution is prepared by dissolving cerium(IV) sulfate tetrahydrate in pure water, and ammonium iron(II) sulfate hexahydrate can be used for standardization.

[0058] The average valence of Mn in the metal oxide can be measured by iodometric titration using, for example, an automatic potentiometric titrator (AT-710, manufactured by Kyoto Electronics Manufacturing Co., Ltd.).

[0059] More specifically, pure water, potassium iodide, and 3 mol / L hydrochloric acid are added to the metal oxide, and the mixture is heated and stirred for at least 5 minutes to dissolve the metal oxide. The iodine liberated by the reaction is then titrated using sodium thiosulfate to determine the amount of sodium iodide used in the reduction of Mn, and the valence of Mn is calculated from the amount of Mn element.

[0060] 8. Methods for producing metal oxides according to first to fifth embodiments of the present invention The method for producing the metal oxides according to the first to fifth embodiments of the present invention is not particularly limited, and they can be produced according to a known method for producing metal oxides, such as the method described in JP 2011-121829 A.

[0061] Starting materials, compounds of metals contained in metal oxides such as iron-containing compounds, manganese-containing compounds, alkali metal-containing compounds, alkaline earth metal-containing compounds, and substitution element-containing compounds, as well as dopant element-containing compounds as needed, are dissolved in pure water to prepare a nitrate aqueous solution (aqueous solution A). Separately, citric acid is dissolved in pure water to prepare a citric acid aqueous solution (aqueous solution B).

[0062] Next, aqueous solution B is added to aqueous solution A under stirring, and the resulting mixed solution is heated (first firing) to obtain a metal citrate complex as a precursor. The resulting precursor is heated (second firing) to burn off the organic matter and obtain a powder. The obtained powder is pulverized and then heated at a higher temperature (tertiary firing) to obtain the metal oxides according to the first to fifth embodiments of the present invention.

[0063] When the method of dissolving raw materials in a solution is employed as described above, examples of the metal compounds and dopant element-containing compounds include chlorides, oxides, nitrates, sulfates, oxalates, salicylates, acetates, carbonates, hydroxides, etc., and from the viewpoint of controlling the composition of the product, chlorides, nitrates, sulfates, and acetates are preferred, with nitrates being particularly preferred. This is because when the starting materials are these salts, the by-products accompanying the production of metal oxides are gases or water containing nitrogen, oxygen, chlorine, carbon, sulfur, etc., and can be easily removed from the system.

[0064] In addition to the method of calcining a precursor synthesized by the solution method as described above, it is also possible to synthesize the precursor by a solid phase method in which starting materials are mixed with powders of compounds of metals contained in metal oxides, such as iron-containing compounds, manganese-containing compounds, alkali metal-containing compounds, alkaline earth metal-containing compounds, and substitution element-containing compounds, and, if necessary, dopant element-containing compounds, followed by calcination. Generally, when synthesizing by the solid phase method, bias in the composition distribution of each element is more likely to occur in the precursor stage and the stage after calcination than in the solution method, so it is important to control the particle size of the raw material powder and to mix the powder uniformly. For example, it is effective to use raw material powder with small particle size and little aggregation, or to mix using a ball mill, etc.

[0065] When employing a calcination method such as the above-described manufacturing method, the calcination temperature and calcination time are not particularly limited, but from the viewpoint of reducing impurity phases, it is preferable that the calcination temperature and calcination time be such that the target phase is formed as a main phase or a single phase. This is because the target phase being a main phase or a single phase is thought to facilitate ordering of the cation sites and anion sites in the metal oxide in both the oxygen-releasing phase and the oxygen-excess phase, and also because the maximum oxygen adsorption amount per unit weight of the metal oxide increases. Whether the target phase is a main phase or a single phase is determined by XRD measurement. On the other hand, from the viewpoint of suppressing a decrease in the specific surface area due to sintering and reducing side reactions with the crucible, treatment at a temperature 400° C. lower than the melting point of the target phase and lower than the melting point is preferred.

[0066] When a calcination method is employed as in the above-described production method, the calcination atmosphere is not particularly limited and can be appropriately selected depending on the target metal oxide. The final calcination atmosphere may be, for example, an oxygen-inert gas mixed atmosphere or an inert gas atmosphere.

[0067] <9. Uses of Metal Oxides> The metal oxides according to the first to fifth embodiments of the present invention can be used in fields related to energy production and environmental protection, and can be utilized, for example, as oxygen storage materials, catalysts for concentrating oxygen gas, cathode materials for fuel cells, oxygen adsorption / desorption devices, oxygen concentrating devices, etc. An oxygen adsorption / desorption device and an oxygen adsorption / desorption method, as well as an oxygen concentrating device and an oxygen concentrating method, using the metal oxides according to any one of the first to fifth embodiments of the present invention will be described in detail below.

[0068] <9-1. Oxygen adsorption / desorption device and oxygen adsorption / desorption method> An oxygen adsorption / desorption device according to a sixth embodiment of the present invention is an oxygen adsorption / desorption device including the metal oxide according to any one of the first to fifth embodiments of the present invention. Also, an oxygen adsorption / desorption method according to a seventh embodiment of the present invention is an oxygen adsorption / desorption method using the metal oxide according to any one of the first to fifth embodiments of the present invention.

[0069] An example of an oxygen adsorption / desorption method according to a seventh embodiment of the present invention is a method (temperature swing adsorption method) including an oxygen adsorption step in which an oxygen-containing gas is brought into contact with a metal oxide at a temperature below the temperature at which oxygen adsorption occurs, thereby adsorbing oxygen onto the metal oxide, and an oxygen desorption step in which the metal oxide that has adsorbed oxygen in the oxygen adsorption step is desorbed from the metal oxide at a temperature above the temperature at which oxygen desorption (release) occurs. The oxygen-containing gas used here is not particularly limited, but examples include air and oxygen. The oxygen adsorption temperature and oxygen desorption temperature are selected based on the phase transition temperature of the metal oxide used. The phase transition temperature varies depending on the composition of the metal oxide, the oxygen concentration in the oxygen-containing gas, and the like, but the preferred ranges for the oxygen adsorption temperature and oxygen desorption temperature are specifically as follows: The oxygen adsorption temperature is preferably 50°C or higher, more preferably 100°C or higher, even more preferably 150°C or higher, and particularly preferably 200°C or higher, within a range lower than the oxygen desorption temperature. From the viewpoints of lowering the operating temperature and extending the life of the device, the oxygen desorption temperature is preferably 700°C or lower, more preferably 600°C or lower, and even more preferably 500°C or lower, within a range higher than the oxygen adsorption temperature. Furthermore, in order to shorten the time required to lower the temperature of the metal oxide to the oxygen adsorption temperature and the time required to raise the temperature to the oxygen desorption temperature, and to reduce the load required for heat exchange, the smaller the temperature difference between the oxygen adsorption temperature and the oxygen desorption temperature, the better. Specifically, the temperature difference is preferably 300°C or lower, more preferably 200°C or lower, and even more preferably 100°C or lower.

[0070] Furthermore, oxygen adsorption / desorption methods using metal oxides according to a seventh embodiment of the present invention include not only methods utilizing temperature fluctuations as described above, but also methods utilizing oxygen adsorption / desorption due to fluctuations in oxygen partial pressure (pressure swing adsorption). For example, a method including an oxygen adsorption step in which oxygen is adsorbed onto the metal oxide by contacting the metal oxide with an oxygen-containing gas such as air at 200°C or higher and 700°C or lower, where the oxygen partial pressure is greater than 0 kPa and less than 100 kPa, and an oxygen desorption step in which oxygen is desorbed from the metal oxide that has adsorbed oxygen in the oxygen adsorption step under an atmosphere where the oxygen partial pressure is less than 100 kPa and lower than that of the oxygen-containing gas. In this case, from the viewpoint of the amount of oxygen adsorption, the oxygen partial pressure in the oxygen adsorption step is preferably 5 kPa or higher, more preferably 10 kPa or higher, and particularly preferably 15 kPa or higher. The oxygen partial pressure in the oxygen desorption step is usually 0 kPa or higher, and from the viewpoint of the amount of oxygen desorption, the larger the difference from the oxygen partial pressure during adsorption, the better.

[0071] <9-2. Oxygen concentrator and oxygen concentration method> An oxygen concentrator according to an eighth embodiment of the present invention is an oxygen concentrator equipped with the metal oxide according to any one of the first to fifth embodiments of the present invention. An oxygen concentrating method according to a ninth embodiment of the present invention is an oxygen concentrating method using the metal oxide according to any one of the first to fifth embodiments of the present invention, and more specifically, is an oxygen concentrating method including an oxygen recovery step of recovering oxygen desorbed from the metal oxide by the oxygen adsorption / desorption method according to the seventh embodiment of the present invention.

[0072] The oxygen concentration method according to the ninth embodiment of the present invention includes a method for concentrating oxygen (temperature swing adsorption method) by the following steps: an oxygen adsorption step in which an oxygen-containing gas is brought into contact with a metal oxide at a temperature below the temperature at which oxygen adsorption occurs, thereby adsorbing oxygen onto the metal oxide; an oxygen desorption step in which the metal oxide that has adsorbed oxygen in the oxygen adsorption step is desorbed (released) from the metal oxide at a temperature above the temperature at which oxygen adsorption occurs; and an oxygen recovery step in which the oxygen desorbed from the metal oxide in the oxygen desorption step is selectively recovered. The oxygen-containing gas used here is not particularly limited, but examples include air and oxygen. The oxygen adsorption temperature and oxygen desorption temperature are selected based on the phase transition temperature of the metal oxide used. The phase transition temperature varies depending on the composition of the metal oxide, the oxygen concentration in the oxygen-containing gas, etc., but the preferred ranges for the oxygen adsorption temperature and oxygen desorption temperature are specifically as follows. The oxygen adsorption temperature is preferably 50°C or higher, more preferably 100°C or higher, even more preferably 150°C or higher, and particularly preferably 200°C or higher, within a range lower than the oxygen desorption temperature. From the viewpoint of lowering the operating temperature and extending the life of the device, the oxygen desorption temperature is in a range higher than the oxygen adsorption temperature, and is preferably 700°C or lower, more preferably 600°C or lower, and even more preferably 500°C or lower.

[0073] Furthermore, the oxygen concentration method using a metal oxide according to the ninth embodiment of the present invention may include not only the method using temperature fluctuations as described above, but also a method using oxygen adsorption / desorption by fluctuations in oxygen partial pressure (pressure swing adsorption method), for example, a method including an oxygen adsorption step in which oxygen is adsorbed onto the metal oxide by contacting the metal oxide with an oxygen-containing gas such as air at 200°C to 700°C in an oxygen partial pressure range of more than 0 kPa to 100 kPa, an oxygen desorption step in which oxygen is desorbed from the metal oxide that has adsorbed oxygen in the oxygen adsorption step in an atmosphere where the oxygen partial pressure is less than 100 kPa and lower than that of the oxygen-containing gas, and an oxygen recovery step in which the oxygen desorbed from the metal oxide in the oxygen desorption step is selectively recovered. In this case, from the viewpoint of the amount of oxygen adsorption, the oxygen partial pressure in the oxygen adsorption step is preferably 5 kPa or more, more preferably 10 kPa or more, and particularly preferably 15 kPa or more. The oxygen partial pressure in the oxygen desorption step is usually 0 kPa or higher, and from the viewpoint of the amount of oxygen desorbed, the greater the difference from the oxygen partial pressure during adsorption, the more preferable. [Example]

[0074] The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. The compositions of the metal oxides obtained in the examples and comparative examples were determined from the ratios of the raw materials used.

[0075] [Example 1] Starting materials, barium nitrate (Ba(NO3)2), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), and iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O) were weighed in a molar ratio of Ba:Ca:Fe = 4:1:3 and dissolved in pure water to prepare a nitrate solution (aqueous solution A). Citric acid monohydrate was weighed in an amount five times the total metal molar amount and dissolved in pure water to prepare a citric acid solution (aqueous solution B). The entire volume of aqueous solution B was added to aqueous solution A under stirring, and stirring was continued. The resulting mixed solution was heat-treated at 120°C to obtain a metal citrate complex gel. The resulting gel was then heated (primary calcination) at 450°C in air for 1 hour to burn off the organic matter and obtain a powder. The obtained powder was crushed with a mortar and pestle, then transferred to an alumina crucible and fired in a muffle furnace under air atmosphere at 800°C for 5 hours (secondary firing). Subsequently, it was fired in an airtight electric furnace under a nitrogen atmosphere at 1000°C for 5 hours (tertiary firing). After that, it was cooled to room temperature while maintaining the nitrogen atmosphere, and the treated material was removed from the electric furnace and crushed in a mortar and pestle to obtain a metal oxide (Ba4CaFe3O 9.5 ) was obtained.

[0076] <Observation of oxygen adsorption / desorption behavior due to ambient temperature swing> The metal oxide (BaCaFeO) obtained in Example 1 9.5 The oxygen adsorption and desorption behavior of ) under ambient temperature swings was observed by thermogravimetric analysis (TG) using a Thermo Plus 8120 (Rigaku). Specifically, the metal oxide obtained in Example 1 was first refreshed at 800°C in a nitrogen atmosphere to remove oxygen, moisture, and other adsorbed substances. Next, approximately 30 mg of the metal oxide was heated from room temperature (25°C) to 800°C at a heating rate of 5°C / min under an air flow (oxygen partial pressure 20.9 kPa, flow rate 400 mL / min) or an oxygen flow (oxygen partial pressure 101 kPa, flow rate 400 mL / min), and then cooled from 800°C to 100°C at a heating rate of 5°C / min, and the weight change during this period was measured. Graphs obtained by TG measurement under air and oxygen flow are shown in Figures 1 and 2, respectively. The maximum oxygen adsorption capacity of the metal oxide determined by TG measurement was 2.2% by mass.

[0077] Figures 1 and 2 confirm that the weight increased during the temperature rise due to oxygen adsorption. However, as the temperature continued to rise, it was confirmed that the weight eventually began to decrease. This is because a phase transition from the oxygen-excess phase to the oxygen-releasing phase occurred as the sample temperature increased, and the adsorbed oxygen was released. During the temperature decrease from 800°C, a sudden weight increase was again observed from a certain temperature, confirming that oxygen adsorption occurred due to the phase transition from the oxygen-releasing phase to the oxygen-excess phase. It was also shown that the metal oxide obtained in Example 1 had a large maximum oxygen adsorption capacity.

[0078] 1 and 2 show that the obtained metal oxide undergoes a reversible phase transition at low temperatures around 400°C. Metal oxides that undergo a phase transition at such low temperatures not only reduce the energy required for oxygen adsorption and desorption, but also enable oxygen adsorption and desorption below the heat-resistant temperature of the column (e.g., stainless steel pipe) packed with the metal oxide when used in general oxygen adsorption / desorption devices, oxygen concentrators, etc., making them highly practical as oxygen storage materials.

[0079] <Observation of oxygen adsorption / desorption behavior due to oxygen partial pressure swing> The metal oxide (BaCaFeO) obtained in Example 1 9.5 The oxygen adsorption and desorption behavior of the cellulose acetate sintered body due to the oxygen partial pressure swing was observed by thermogravimetric analysis (TG) using a Thermoplus2 TG-8120 (manufactured by Rigaku Co., Ltd.). Specifically, the metal oxide obtained in Example 1 was first refreshed at 800°C in a nitrogen atmosphere to remove oxygen and moisture adsorbed on the metal oxide. The metal oxide was then heated to 290°C, 300°C, 310°C, 320°C, 330°C, 340°C, 350°C, 360°C, 370°C, or 380°C under a nitrogen stream (oxygen partial pressure of 0.002 kPa or less, flow rate of 400 mL / min). Once the temperature stabilized, the flow gas was switched to air (oxygen partial pressure of 20.9 kPa, flow rate of 400 mL / min) and maintained at this temperature for 120 minutes. Subsequently, the flow gas was switched to nitrogen (oxygen partial pressure of 0.002 kPa or less, flow rate of 400 mL / min). The weight change of the metal oxide during this period was measured, and the results are shown in Figures 3 to 12.

[0080] 3 to 12 show that oxygen adsorption and desorption are possible even with an oxygen partial pressure swing close to the actual operating conditions of the oxygen adsorption / desorption device and oxygen concentrator, and that the oxygen adsorption / desorption rate is also fast. Furthermore, at all temperatures, a steep weight increase was observed after the atmospheric gas was switched from nitrogen to air, and it was confirmed that oxygen absorption was completed within two minutes at all temperatures. Conversely, after the atmospheric gas was switched from air to nitrogen, a weight loss occurred, and within about five minutes, the weight returned to the weight before the atmospheric gas was switched from nitrogen to air due to the release of oxygen. Furthermore, the lower the ambient temperature is within the temperature range of 290°C to 380°C, the higher the weight gain rate (i.e., oxygen adsorption rate) and oxygen adsorption amount when the ambient gas is switched from nitrogen to air, and the lower the weight loss rate (i.e., oxygen desorption rate) and oxygen release amount when the ambient gas is switched from air to nitrogen. On the other hand, the higher the ambient temperature is, the opposite tendency is observed. This indicates that the equilibrium between the oxygen-releasing phase and the oxygen-excess phase changes in this temperature range.

[0081] <Powder X-ray diffraction (XRD) measurement> For the metal oxides in the state of the oxygen release phase immediately after manufacturing, the metal oxides in the state of the oxygen excess phase by heating up in the above TG measurement, and the metal oxides subjected to the refresh treatment at 800 °C in a nitrogen atmosphere after the TG measurement, powder XRD measurement was carried out. The powder XRD measurement conditions are as follows. The obtained XRD pattern is shown in Fig. 13. In Fig. 13, for reference, the XRD patterns of Ba4CaFe3O 9.5 、Ba4CaFe3O 10.75 、and BaFeO3 are shown together.

[0082] (Powder XRD measurement conditions) Apparatus: X’Pert Pro MPD (manufactured by PANalytical) X-ray source: CuKα Measurement range: Angle where 2θ is from 5° to 90° Measurement interval: 0.02° Voltage: 45 kV Current: 30 mA

[0083] From Fig. 13, the XRD pattern of the metal oxide immediately after manufacturing had peaks of the XRD pattern of Ba4CaFe3O 9.5 split. That is, it was suggested that the crystal structure of the metal oxide (oxygen release phase) immediately after manufacturing had lower symmetry and was distorted compared to the crystal structure of Ba4CaFe3O 9.5 . The XRD pattern of the metal oxide in the state of the oxygen excess phase showed only peaks attributed to Ba4CaFe3O 10.75 , and no other phases were confirmed. Furthermore, it was confirmed that the XRD pattern of the metal oxide after the refresh treatment was the same pattern as that of the metal oxide immediately after manufacturing. From the above points, the metal oxide obtained in the example was Ba4CaFe3O 9.5 with a distorted crystal structure, and it was shown that reversible phase transition and accompanying oxygen adsorption and desorption occurred.

[0084] (Measurement of average Fe valence) The average Fe valence of the metal oxide in the oxygen-releasing phase immediately after production and the metal oxide in the oxygen-excess phase produced by heating in the TG measurement was measured by the following method. First, the molar amount of oxygen atoms adsorbed during the phase transition from the oxygen-releasing phase to the oxygen-rich phase was calculated from the weight gain associated with oxygen adsorption during TG measurements. Then, in metal oxides, the average valence of Fe in the oxygen-rich phase was calculated, assuming that a valence transition (valence increase) of Fe occurs to maintain electroneutrality in response to the increase in oxygen anions associated with oxygen adsorption. As mentioned above, the metal oxide immediately after production is Ba4CaFe3O 9.5 Since the valence of the alkaline earth elements Ba and Ca is divalent, the average valence of Fe was calculated to be +3.0 in the oxygen-releasing phase. On the other hand, as shown in Figure 1, the maximum oxygen adsorption amount in air was 2.2 mass%, so the oxygen-excess phase was Ba4CaFe3O 10.75 The average valence of Fe at that time was +3.8.

[0085] [Example 2] Starting materials, barium nitrate (Ba(NO3)2), yttrium nitrate hexahydrate (Y(NO3)3·6H2O), and iron nitrate nonahydrate (Fe(NO3)3·9H2O) were weighed in a molar ratio of Ba:Y:Fe = 2:1:1 and dissolved in pure water to prepare a nitrate solution (aqueous solution A). Citric acid monohydrate was weighed in an amount five times the total metal molar amount and dissolved in pure water to prepare a citric acid solution (aqueous solution B). The entire volume of aqueous solution B was added to aqueous solution A under stirring, and stirring was continued. The resulting mixed solution was heat-treated at 120°C to obtain a metal citrate complex gel. The resulting gel was then heated (primary calcination) at 450°C in air for 1 hour to burn off the organic matter and obtain a powder. The resulting powder was crushed using a mortar and pestle, then transferred to an alumina crucible and fired in an airtight electric furnace in a nitrogen atmosphere at 1200°C for 5 hours (secondary firing).Then, while maintaining the nitrogen atmosphere, the mixture was cooled to room temperature, removed from the electric furnace, and crushed using a mortar and pestle to obtain the metal oxide (Ba2YFeO5).

[0086] <Observation of oxygen adsorption / desorption behavior due to oxygen partial pressure swing> The oxygen adsorption / desorption behavior of the metal oxide (Ba2YFeO5) obtained in Example 2 due to oxygen partial pressure swing was observed by thermogravimetric analysis (TG) measurement using Thermoplus2 TG-8120 (manufactured by Rigaku Corporation). Specifically, the metal oxide obtained in Example 2 was first refreshed at 600°C in a nitrogen atmosphere to remove oxygen, moisture, and other adsorbed substances. The metal oxide was then heated to 250°C under a nitrogen stream (oxygen partial pressure of 0.002 kPa or less, flow rate of 400 mL / min). Once the temperature stabilized, the flowing gas was switched to oxygen (nitrogen partial pressure of 0.002 kPa or less, flow rate of 400 mL / min) and maintained at this temperature for 60 minutes. Subsequently, the flowing gas was switched to nitrogen (oxygen partial pressure of 0.002 kPa or less, flow rate of 400 mL / min). The weight change of the metal oxide during this period was measured, and the results are shown in Figure 14. The maximum oxygen adsorption capacity of the metal oxide, as determined by TG measurement, was 0.748% by mass.

[0087] FIG. 14 shows that oxygen adsorption and desorption are possible even with an oxygen partial pressure swing close to the actual operating conditions of the oxygen adsorption / desorption device and oxygen concentrator, and that the oxygen adsorption / desorption rate is also fast. Furthermore, even under the low-temperature condition of 250°C, a steep weight increase was observed after the atmospheric gas was switched from nitrogen to air, confirming the occurrence of oxygen storage. Furthermore, after the atmospheric gas was switched from air to nitrogen, a weight loss occurred, and it was confirmed that the weight returned to the weight before the atmospheric gas was switched from nitrogen to air due to the release of oxygen. These results demonstrate that the metal oxide obtained in Example 2 can function as an oxygen storage material even under the low-temperature condition of 250°C.

[0088] <Powder X-ray diffraction (XRD) measurement> The metal oxide (BaYFeO) in the oxygen-releasing phase immediately after production was refreshed at 600°C in a nitrogen atmosphere, and then subjected to powder XRD measurement. The powder XRD measurement conditions were the same as those in Example 1. The obtained XRD pattern is shown in Figure 15.

[0089] All of the peaks in the XRD pattern of the metal oxide immediately after production were attributed to Ba2YFeO5.

[0090] <Measurement of average Fe valence> The average Fe valence in the metal oxide in the state of the oxygen release phase immediately after production and the metal oxide in the state of the oxygen excess phase by heating during the above TG measurement was measured by the same method as in Example 1. As a result of the measurement, the average Fe valence was +3.0 in the state of the oxygen release phase and +3.5 in the state of the oxygen excess phase.

[0091] [Example 3-1] As starting materials, samarium nitrate hexahydrate (Sm(NO3)3·6H2O), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) and iron nitrate nonahydrate (Fe(NO3)3·9H2O) were weighed so as to have a molar ratio of Sm:Ca:Fe = 1:2:3, dissolved in pure water, and an aqueous nitrate solution was prepared (aqueous solution A). Five times the molar amount of the total metal amount of citric acid monohydrate was weighed and dissolved in pure water to prepare an aqueous citric acid solution (aqueous solution B). The entire amount of aqueous solution B was added to aqueous solution A under stirring, and then stirring was continued. The obtained mixed solution was heat-treated at 120 °C to obtain a metal citrate complex gel. The obtained gel was heated at 450 °C for 1 hour in an air atmosphere (primary firing) to burn the organic matter and obtain a powder. The obtained powder was pulverized with a mortar and pestle, transferred to an alumina crucible, and fired at 1200 °C for 5 hours in an air atmosphere in a highly airtight electric furnace (secondary firing). Then, while maintaining the atmosphere as air, it was cooled to room temperature, the processed product was taken out of the electric furnace and pulverized with a mortar and pestle to obtain a metal oxide (SmCa2Fe3O8).

[0092] [Example 3-2] As starting materials, europium nitrate hexahydrate (Eu(NO3)3·6H2O), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), and iron nitrate nonahydrate (Fe(NO3)3·9H2O) were weighed out to a molar ratio of Eu:Ca:Fe = 1:2:3 and dissolved in pure water to prepare a nitrate aqueous solution (aqueous solution A). Except for this, the metal oxide (EuCa2Fe3O8) was obtained in the same manner as in Example 3-1.

[0093] [Example 3-3] As starting materials, neodymium nitrate hexahydrate (Nd(NO3)3·6H2O), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), and iron nitrate nonahydrate (Fe(NO3)3·9H2O) were weighed out to a molar ratio of Nd:Ca:Fe = 1:2:3 and dissolved in pure water to prepare a nitrate aqueous solution (aqueous solution A). Except for this, the metal oxide (NdCa2Fe3O8) was obtained in the same manner as in Example 3-1.

[0094] [Comparative Example 3-1] As starting materials, ytterbium nitrate n-hydrate (Yb(NO3)3·nH2O, n: 3 to 5), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), and iron nitrate nonahydrate (Fe(NO3)3·9H2O) were weighed out to a molar ratio of Yb:Ca:Fe = 1:2:3 and dissolved in pure water to prepare a nitrate aqueous solution (aqueous solution A). Except for this, a metal oxide (YbCa2Fe3O8) was obtained in the same manner as in Example 3-1.

[0095] [Comparative Example 3-2] As starting materials, yttrium nitrate hexahydrate (Y(NO3)3·6H2O), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), and iron nitrate nonahydrate (Fe(NO3)3·9H2O) were weighed out to a molar ratio of Y:Ca:Fe = 1:2:3 and dissolved in pure water to prepare a nitrate aqueous solution (aqueous solution A). Except for this, the metal oxide (YCa2Fe3O8) was obtained in the same manner as in Example 3-1.

[0096] [Comparative Example 3-3] As starting materials, dysprosium nitrate hexahydrate (Dy(NO3)3·6H2O), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), and iron nitrate nonahydrate (Fe(NO3)3·9H2O) were weighed out to a molar ratio of Dy:Ca:Fe = 1:2:3 and dissolved in pure water to prepare a nitrate aqueous solution (aqueous solution A). Except for this, the metal oxide (DyCa2Fe3O8) was obtained in the same manner as in Example 3-1.

[0097] [Comparative Example 3-4] As starting materials, gadolinium nitrate hexahydrate (Gd(NO3)3·6H2O), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), and iron nitrate nonahydrate (Fe(NO3)3·9H2O) were weighed out to a molar ratio of Gd:Ca:Fe = 1:2:3 and dissolved in pure water to prepare a nitrate aqueous solution (aqueous solution A). Except for this, the metal oxide (GdCa2Fe3O8) was obtained in the same manner as in Example 3-1.

[0098] [Comparative Example 3-5] As starting materials, lanthanum nitrate hexahydrate (La(NO3)3·6H2O), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), and iron nitrate nonahydrate (Fe(NO3)3·9H2O) were weighed out to a molar ratio of La:Ca:Fe = 1:2:3 and dissolved in pure water to prepare a nitrate aqueous solution (aqueous solution A). Except for this, the metal oxide (LaCa2Fe3O8) was obtained in the same manner as in Example 3-1.

[0099] <Observation of oxygen adsorption / desorption behavior due to ambient temperature swing> The oxygen adsorption / desorption behavior of the metal oxides obtained in Examples 3-1 to 3-3 and Comparative Examples 3-1 to 3-5 due to ambient temperature swings was observed by thermogravimetric analysis (TG) using a Thermo Plus 8120 (manufactured by Rigaku). Specifically, the metal oxides obtained in Examples 3-1 to 3-3 and Comparative Examples 3-1 to 3-5 were first refreshed at 800°C in a nitrogen atmosphere to remove oxygen, moisture, and other adsorbed substances from the metal oxides. Next, approximately 30 mg of metal oxide was heated from room temperature (25°C) to 800°C at a heating rate of 5°C / min in an air stream (oxygen partial pressure 20.9 kPa, flow rate 400 mL / min), and then cooled from 800°C to 100°C at a heating rate of 5°C / min, during which the weight change was measured. Graphs obtained by TG measurements in air and oxygen streams are shown in Figures 16 to 23, respectively.

[0100] 16 to 18, it was confirmed that the metal oxides obtained in Examples 3-1 to 3-3 experienced weight gain due to oxygen adsorption during the temperature rise process. On the other hand, it was confirmed that as the temperature continued to rise, the weight eventually began to decrease. This is because a phase transition from the oxygen-excess phase to the oxygen-releasing phase occurred as the sample temperature rose, and the adsorbed oxygen was released. During the temperature drop process from 800°C, a sudden weight increase was again confirmed from a specific temperature, confirming that oxygen adsorption occurred due to the phase transition from the oxygen-releasing phase to the oxygen-excess phase. Furthermore, the TG measurement results revealed that the metal oxides obtained in Examples 3-1 to 3-3 all exhibited a large maximum oxygen adsorption amount of 2.5 mass %.

[0101] Furthermore, Figures 16 to 18 show that the metal oxides obtained in Examples 3-1 to 3-3 undergo a reversible phase transition at low temperatures of 500°C or less, and, like the metal oxide obtained in Example 1, are highly practical as oxygen storage materials.

[0102] On the other hand, Figures 19 to 23 show that the metal oxides of Comparative Examples 3-1 to 3-5, in which the rare earth elements in the metal oxides of Examples 3-1 to 3-3 were replaced with Yb, Y, Dy, Gd, or La, hardly experienced any oxygen adsorption / desorption due to temperature changes, and no reversible phase transition occurred.

[0103] <Powder X-ray diffraction (XRD) measurement> The metal oxide in the state of the oxygen release phase immediately after production was subjected to a refresh treatment at 800 °C in a nitrogen atmosphere, and then powder XRD measurement was performed. The powder XRD measurement conditions are the same as those of the powder XRD in Example 1. The obtained XRD pattern is shown in Fig. 24. For reference, the XRD patterns of YbFeO3, NdCa2Fe3O8, LaCa2Fe3O8, and Ca2Fe2O5 are also shown in Fig. 24.

[0104] When RE is La, Nd, Sm, and Eu, all diffraction patterns were attributed to the Grenier phase. On the other hand, when RE is Yb, Y, Dy, and Gd, the peaks of the perovskite structure represented by REFeO3 were also confirmed, and it was confirmed that impurity phases were mixed. From the above results, it is suggested that when the element has an ionic radius of RE less than or equal to Eu, the Grenier phase is likely to be formed as a single phase, while when an element with an ionic radius larger than Eu is used, the crystal structure of the Grenier phase cannot be maintained and a heterogeneous phase is formed.

[0105] <Measurement of average valence of Fe> The average valence of Fe in the metal oxide in the state of the oxygen release phase immediately after production and the metal oxide in the state of the oxygen excess phase by heating in the above TG measurement was measured by the same method as in Example 1. As a result of the measurement, the average valence of Fe in the metal oxide of Example 3-1 was +3.0 in the state of the oxygen release phase and +3.6 in the state of the oxygen excess phase; the average valence of Fe in the metal oxide of Example 3-2 was +3.0 in the state of the oxygen release phase and +3.6 in the state of the oxygen excess phase; the average valence of Fe in the metal oxide of Example 3-3 was +3.0 in the state of the oxygen release phase and +3.5 in the state of the oxygen excess phase.

Claims

1. An oxygen-deficient perovskite metal oxide represented by the following formula (I): (BL ４－ｘ１ Ca １－ｙ１ Fe ３－ｚ１ M1 ｘ１＋ｙ１＋ｚ１ )O ９．５＋δ１ａ (I) (In the above formula (I), M1 represents one or more elements selected from the group consisting of alkali metals, alkaline earth metals other than Ba and Ca, rare earth metals, and transition metals other than Fe; x1, y1, z1, and δ1a satisfy the following formula. −2.0≦x1≦2.0 0≦y1≦0.5 −1.5≦z1≦1.5 −1.5≦δ1a≦4.5 x1+y1+z1≧0)

2. An oxygen-deficient perovskite metal oxide represented by the following formula (II): <h2 style=";text-align:left;direction:ltr">((Ba<h2 style=";text-align:left;direction:ltr"> ２－ｘ２ <h2 style=";text-align:left;direction:ltr"> Y<h2 style=";text-align:left;direction:ltr"> １－ｙ２ <h2 style=";text-align:left;direction:ltr"> Fe<h2 style=";text-align:left;direction:ltr"> １－ｚ２ <h2 style=";text-align:left;direction:ltr"> 12<h2 style=";text-align:left;direction:ltr"> ｘ２＋ｙ２＋ｚ２ <h2 style=";text-align:left;direction:ltr"> )O<h2 style=";text-align:left;direction:ltr"> ５＋δ２ａ <h2 style=";text-align:left;direction:ltr"> ((II) (In the above formula (II), M2 represents one or more elements selected from the group consisting of alkali metals, alkaline earth metals other than Ba, rare earth metals other than Y, and transition metals other than Fe; x2, y2, z2, and δ2a satisfy the following formula. -1.0≦x2≦1.0 −0.5≦y2≦0.5 −0.5≦z2≦0.5 −0.5≦δ2a≦1.5 x2+y2+z2≧0)

3. An oxygen-deficient perovskite metal oxide represented by the following formula (III): (RE １－ｘ３ Ca ２－ｙ３ Fe ３－ｚ３ M3 ｘ３＋ｙ３＋ｚ３ )O ８＋δ３ａ (III) (In the above formula (III), RE represents one or more rare earth elements selected from the group consisting of Nd, Sm, and Eu; M3 represents one or more elements selected from the group consisting of alkali metals, alkaline earth metals other than Ca, rare earth metals other than RE, and transition metals other than Fe; x3, y3, z3, and δ3a satisfy the following formula. −0.5≦x3≦0.5 −1.0≦y3≦1.0 −1.5≦z3≦1.5 -1.5≦δ3a≦2.8 x3+y3+z3≧0)

4. An oxygen storage material comprising the metal oxide according to any one of claims 1 to 3.

5. An oxygen adsorption / desorption device comprising the metal oxide according to any one of claims 1 to 3.

6. An oxygen concentrator comprising the metal oxide according to any one of claims 1 to 3.

7. 4. An oxygen adsorption / desorption method comprising: an oxygen adsorption step of contacting the metal oxide according to claim 1 with an oxygen-containing gas at a temperature equal to or lower than the temperature at which oxygen adsorption occurs, thereby adsorbing oxygen onto the metal oxide; and an oxygen desorption step of heating the metal oxide that has adsorbed oxygen in the oxygen adsorption step at a temperature equal to or higher than the temperature at which oxygen desorption occurs and equal to or lower than 700°C, thereby desorbing oxygen from the metal oxide.

8. 4. An oxygen adsorption / desorption method comprising: an oxygen adsorption step of contacting the metal oxide according to claim 1 with an oxygen-containing gas having an oxygen partial pressure of more than 0 kPa and not more than 100 kPa, thereby adsorbing oxygen onto the metal oxide; and an oxygen desorption step of placing the metal oxide that has adsorbed oxygen in the oxygen adsorption step under an atmosphere having an oxygen partial pressure lower than that of the oxygen-containing gas, thereby desorbing oxygen from the metal oxide.

9. An oxygen concentration method, comprising an oxygen recovery step of recovering oxygen desorbed from the metal oxide by the oxygen adsorption / desorption method according to claim 7 or 8.

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