Patent Application
20250161919
The present application claims priority from Japanese patent application JP 2023-198392 filed on Nov. 22, 2023, the entire content of which is hereby incorporated by reference into this application. The present application claims priority from Japanese patent application JP 2024-061442 filed on Apr. 5, 2024, the entire content of which is hereby incorporated by reference into this application.
The present disclosure relates to an oxygen storage material.
An exhaust gas emitted from an internal combustion engine of an automobile and the like contains harmful components, such as carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx), and the harmful components are removed by an exhaust gas purification catalyst and then released into the atmosphere. Conventionally, a three-way catalyst that simultaneously oxidizes CO, HC and reduces NOx is used for the exhaust gas purification catalyst. As the three-way catalyst, a porous oxide carrier such as alumina (Al2O3), silica (SiO2), zirconia (ZrO2), and titania (TiO2) on which noble metals such as platinum (Pt), palladium (Pd), and rhodium (Rh) are supported is widely used.
In recent years, an oxygen storage material, which is an inorganic material having an oxygen storage capacity (OSC performance), has been used for the exhaust gas purification catalyst in order to enhance an exhaust gas purification ability of the three-way catalyst in response to variations in oxygen concentration in the exhaust gas. Ceria (CeO2) is known to have an excellent OSC performance and is widely used as an oxygen storage material in the form of ceria-zirconia composite oxide (CeO2—ZrO2).
As an oxygen storage material of such a ceria-zirconia composite oxide, for example, JP 2009-84061 A discloses a ceria-zirconia-based composite oxide containing a composite oxide of ceria and zirconia, in which a pyrochlore phase-type ordered array phase is formed by cerium ions and zirconium ions in the composite oxide, and 50% or more of the pyrochlore phase-type ordered array phase remains after heating in the atmosphere under a temperature condition of 1000° C. for 5 hours as compared with that before heating.
JP 2023-55681 A discloses a melilite-type oxide with an oxygen absorbing/releasing function. The oxide has a melilite-type structure, and has a composition represented by the general formula AxB2-xCD2E7+δ, where A is cerium (Ce), B is one or two or more elements selected from the group consisting of an alkali metal element, an alkaline earth metal element, magnesium (Mg), indium (In), and a rare earth metal element (except for cerium), C and D are one or two or more elements selected from the group consisting of a transition metal element, a group 12 element, a group 13 element, a group 14 element, and a group 15 element, E is oxygen (O), x is 0 or more and 2.0 or less, and δ is −1.0 or more and 1.0 or less. The oxide retains electrical neutrality.
However, in the ceria-zirconia composite oxide in which the pyrochlore phase-type ordered array phase is formed as disclosed in JP 2009-84061 A, the use of zirconium (Zr), which is a rare metal, may increase the cost. Further, in the composite oxide, an activation energy for releasing excess oxygen from the crystalline structure is high, and there is a problem of a high temperature required for oxygen release.
On the other hand, the oxide having the melilite-type structure as disclosed in JP 2023-55681 A is made of calcium, aluminum, and the like, which are abundant elements. The oxide has many spaces in the crystalline structure into which excess oxygen can be incorporated, and the temperature required for oxygen release is low. However, in the oxide, the composition in which excess oxygen is incorporated into all of the spaces is unstable, resulting in a problem of a small oxygen storage amount.
Therefore, the present disclosure provides an inexpensive oxygen storage material having an excellent oxygen release performance at low temperature.
As a result of various examination of means to solve the above-described problems, the inventors have found that a composite oxide having an apatite-type crystalline structure containing Ce and/or europium (Eu) and silicon (Si) and/or phosphorus (P) has a smaller activation energy than Ce2Zr2O7 having a pyrochlore-type crystalline structure containing Ce and Zr useful as a conventional oxygen storage material, and has an excellent oxygen storage property and oxygen release property at low temperature. Thus, the inventors achieved the present disclosure.
That is, the gist of the present disclosure is as follows.
The present disclosure provides an inexpensive oxygen storage material having an excellent oxygen release performance at low temperature.
The following describes some embodiments of the present disclosure in detail.
In the description, features of the present disclosure will be described with reference to the drawings as necessary. In the drawings, dimensions and shapes of respective components are exaggerated for clarification, and actual dimensions and shapes are not accurately illustrated. Accordingly, the technical scope of the present disclosure is not limited to the dimensions or the shapes of respective components illustrated in the drawings. An oxygen storage material of the present disclosure is not limited to the embodiments below, and can be performed in various configurations where changes, improvements, and the like which a person skilled in the art can make are given without departing from the gist of the present disclosure.
The present disclosure relates to an oxygen storage material containing a composite oxide. The composite oxide contains cerium and/or europium and silicon and/or phosphorus. The composite oxide has an apatite-type crystalline structure.
In the present disclosure, a crystalline structure of the composite oxide containing cerium and/or europium and silicon and/or phosphorus can be determined by an X-ray diffraction (XRD) analysis. In the present disclosure, the composite oxide containing cerium and/or europium and silicon and/or phosphorus has an apatite-type crystalline structure.
The basic XRD spectrum of the apatite-type crystalline structure is well known in the art. For example, when the composite oxide is Ce9.33Si6O26, 2θ has peaks at four or more positions selected from the group consisting of 27.3°±0.5°, 28.2°±0.5°, 31.0°±0.5°, and 32.1°±0.5° in an XRD spectrum using copper Kα radiation.
It is a well-known fact in the art that the peak position of the basic XRD spectrum of the apatite-type crystalline structure changes based on the composition of the composite oxide.
The composite oxide containing cerium and/or europium and silicon and/or phosphorus of the present disclosure can also be represented by the following formula:
AαTβXγ
(In the formula, A includes cerium and/or europium; T includes silicon and/or phosphorus; X includes oxygen; α, β, and γ each represent a mole ratio; and α is 9.33 to 10.0, β is 6, and γ is 24 to 30.).
In one embodiment, A includes cerium. In one embodiment, A includes europium. In one embodiment, A includes cerium and europium.
In addition to Ce and/or Eu, A may further include one or more metal elements selected from the group consisting of an alkali metal, an alkaline earth metal, and a rare earth metal other than Ce or Eu. In one embodiment, A further includes, in addition to Ce and/or Eu, one or more metal elements selected from the group consisting of lithium (Li), sodium (Na), potassium (K), calcium (Ca), strontium (Sr), barium (Ba), yttrium (Y), lanthanum (La), praseodymium (Pr), samarium (Sm), terbium (Tb), ytterbium (Yb), and lutetium (Lu). In one embodiment, A further includes, in addition to Ce and/or Eu, one or more elements selected from the group consisting of Ca, Sr, Y, La, Pr, Sm, Yb, and Lu. In one embodiment, when A includes Ce, A further includes one or more elements selected from the group consisting of Ca, Sr, Y, La, Pr, Sm, Eu, Yb, and Lu. In one embodiment, when A includes Eu, A further includes one or more elements selected from the group consisting of Ce, La, and Pr.
The content of Ce in A is usually 20 mol % or more, 78 mol % or more in one embodiment, and 80 mol % or more in one embodiment, relative to the total amount-of-substance (mol) of A. The content of Ce in A may be 100 mol % relative to A (total mol).
The content of Eu in A is usually 20 mol % or more, 78 mol % or more in one embodiment, and 80 mol % or more in one embodiment, relative to the total amount-of-substance (mol) of A. The content of Eu in A may be 100 mol % relative to A (total mol).
The content of Ce and/or Eu in A within the above-described range can ensure a sufficient oxygen storage capacity.
The contents of elements other than Ce or Eu in A are not limited. The contents of elements other than Ce or Eu in A can be determined so as to compensate for charges of the contained Ce and/or Eu, as described in detail below.
T may further include, in addition to Si and/or P, one or more elements selected from the group consisting of a group 13 element, a group 14 element other than Si, and a group 15 element other than P. In one embodiment, T further includes, in addition to Si and/or P, one or more elements selected from the group consisting of boron (B), aluminum (Al), gallium (Ga), and germanium (Ge). In one embodiment, T further includes B in addition to Si and/or P.
The content of Si or P in T is usually 66 mol % or more, and 83 mol % or more in one embodiment, relative to the total amount-of-substance (mol) of T. The content of Si or P in T may be 100 mol % relative to T (total mol).
The content of Si or P in T within the above-described range can ensure a small cation (Si4+ or P5+) located in the center of a TO4 tetrahedron, which is required to form the apatite-type structure.
The contents of elements other than Si or P in T can also be determined so as to compensate for charges of the contained Ce and/or Eu and Si and/or P, as described in detail below.
In the apatite-type crystalline structure of the composite oxide included in the present disclosure, Ce is contained as Ce3+, and Eu is contained as Eu2+. In the crystalline structure, Ce3+ and Eu2+ each have different valences, but occupy the same site. Thus, it is not possible to produce apatite by simply substituting Ce with Eu in a composite oxide. Therefore, when ions present in a composite oxide are changed to ions having a valence different from that of those ions, other ions present separately in the composite oxide, such as ions that do not contribute to the oxygen storage amount, that is, ions with a fixed valence, can also be changed in the same manner to perform charge compensation. For example, when substituting Ce3+ with Eu2+ in the composite oxide, this change in charge of −1 can be compensated for by a change in charge of +1 caused by, for example, a substitution of Ca2+ (Sr2+), which may occupy the same site as Ce and Eu, with La3+, and a substitution of Si4+, which may occupy a site different from that of Ce and Eu, with P5+. For example, when it is desired to determine a difference between the effects of Ce3+ and Eu2+ in the composite oxide, this can be performed by comparing Ce8A2T6O26 (A=Ca or Sr, T=Si) with Eu8A2T6O26 (A=La, T=P).
Further, in the apatite-type crystalline structure of the composite oxide included in the present disclosure, when Ce3+ in the composite oxide Ce8A2T6O26 (A=Ca or Sr, T=Si) is substituted with Eu2+, Eu8A2T6O26 (A=Ce, T=P) can also be produced by charge compensation by changing Ca or Sr of A to Ce and by substituting Si4+ with P5+.
X includes O, and is usually made of O. In X, O may optionally be present in the form of OH.
The content of O in X is usually 92 mol % or more, and 99 mol % or more in one embodiment, relative to the total amount-of-substance (mol) of X. The content of O in X may be 100 mol % relative to X (total mol).
The content of O in X within the above-described range can ensure a sufficient oxygen storage capacity.
In one embodiment, the composite oxide containing cerium and silicon or phosphorus of the present disclosure is represented by the following formula:
(Ce,A′)α(Si,P,T′)βOγ
(In the formula, A′ is one or more metal elements selected from the group consisting of Ca, Sr, Y, La, Pr, Sm, Eu, Yb, and Lu; T′ is B; α, β, and γ each represent a mole ratio; and α is 9.33 to 10, β is 6, and γ is 24 to 30.). Here, the mole ratio of Ce to A′ (Ce:A′) is usually 1:0 to 1:4, and the mole ratio of Si or P to T′ (Si, P:T′) is usually 1:0 to 5:1.
In one embodiment, the composite oxide containing cerium and silicon or phosphorus of the present disclosure is represented by Ce9.33Si6O26, Ce2Ca8P6O26, Ce9.33Si5BO25.5, Ce7.33 Y2Si6O26, Ce7.33La2Si6O26, Ce7.33Pr2Si6O26, Ce7.33Sm2Si6O26, Ce7.33Lu2Si6O26, Ce8Ca2Si6O26, Ce8Sr2Si6O26, Ce8Eu2Si6O26, or Ce8Yb2Si6O26.
In one embodiment, the composite oxide containing europium and phosphorus of the present disclosure is represented by the following formula:
(Eu,A′)α(P)βOγ
(In the formula, A′ is one or more metal elements selected from the group consisting of Ce, La, and Pr; α, β, and γ each represent a mole ratio; and α is 9.33 to 10, β is 6, and γ is 24 to 30.).
In one embodiment, the composite oxide containing europium and phosphorus of the present disclosure is represented by Eu8Ce2P6O26, Eu8La2P6O26, or Eu8La2P6O26.
The oxygen storage material of the present disclosure may further contain a catalyst metal. The catalyst metal is supported on the composite oxide described above. The catalyst metal includes a noble metal. The noble metal includes, but is not limited to, a platinum group noble metal. The platinum group noble metal includes ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt). In one embodiment, the noble metal is one or more selected from the group consisting of Rh, Pt, and Pd. A support amount of the noble metal is similar to that of the conventional exhaust gas purification catalyst and is, but not limited to, 0.01 weight % to 5 weight %, and 0.5 weight % to 2 weight % in one embodiment, relative to the total weight of the oxygen storage material.
The oxygen storage material of the present disclosure is excellent in an oxygen release at low temperature. Therefore, the present disclosure also relates to an exhaust gas purification catalyst and/or a redox catalyst containing the oxygen storage material of the present disclosure.
The exhaust gas purification catalyst and/or the redox catalyst of the present disclosure may contain a carrier material other than the oxygen storage material of the present disclosure. The carrier material other than the oxygen storage material of the present disclosure includes a metal oxide that is porous and has an excellent heat resistance. For example, an aluminum oxide (alumina: Al2O3), a zirconium oxide (zirconia: ZrO2), a silicon oxide (silica: SiO2), a composite oxide containing these metal oxides as a main component, or the like can be used. In the exhaust gas purification catalyst, a supporting method may be a conventional supporting method, such as an adsorption supporting method or a water-absorption supporting method.
The exhaust gas purification catalyst and/or redox catalyst of the present disclosure can exhibit an excellent ordered structure durability and OSC performance over a wide temperature range. The exhaust gas purification catalyst of the present disclosure is usually used in a low temperature range of about 200° C. to 600° C.
Further, by utilizing the property of absorbing and releasing oxygen and the property of having different energy states in the state of adsorbing oxygen and the state of releasing oxygen of the oxygen storage material of the present disclosure, the use of the oxygen storage material of the present disclosure allows storing oxygen (oxygen storage method), enriching oxygen (oxygen enrichment method), removing oxygen (oxygen removal method), and/or heating and/or cooling (heating/cooling method).
The oxygen storage material of the present disclosure can be produced by an ordinary method, such as a solid phase method, a liquid phase method, or an alkoxide method. For example, the oxygen storage material is produced by mixing an aqueous solution of a cerium compound, an europium compound, a silicon compound, a phosphorus compound, and optionally a compound containing an element other than Ce, Eu, Si or P as described above (hereinafter, also referred to as “cerium compound, silicon compound, and the like”) and an aqueous solution of a complexing agent, and drying the mixture to deposit a product containing Ce and/or Eu and Si and/or P, and then firing the product in a reducing atmosphere. The cerium compound, the silicon compound, and the like can also be used as a solution of a non-aqueous solvent, such as alcohol or organic carboxylic acid ester.
As the cerium compound, for example, water-soluble compounds including nitrates such as cerium nitrate and diammonium cerium nitrate, sulfates such as ceric sulfate, and chlorides such as cerium chloride, and alcohol-soluble compounds including alkoxides such as cerium isopropoxide can be used.
As the europium compound, for example, water-soluble compounds including nitrates such as europium nitrate, sulfates such as europium sulfate, and chlorides such as europium chloride, and alcohol-soluble compounds including alkoxides such as europium isopropoxide can be used.
As the silicon compound, alkoxides such as tetraethyl orthosilicate, tetraisopropyl orthosilicate, and the like can be used.
As the phosphorus compound, ammonium dihydrogenphosphate and the like can be used.
As the compound containing elements other than Ce or Si, water-soluble or alcohol-soluble compounds such as nitrate, sulfate, chloride, or alkoxide can be used.
Examples of the complexing agent include, but are not particularly limited to, polycarboxylic acids, amino acids, and the like. Examples of the polycarboxylic acid include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, citric acid, tartaric acid, and the like. In one embodiment, the complexing agent is citric acid. Examples of the amino acid include glycine, alanine, asparagine, aspartic acid, and the like.
If a precipitate is formed by mixing the aqueous solution of the cerium compound, the silicon compound, and the like with the aqueous solution of the complexing agent, drying may be performed after filtering the precipitate.
The drying of the mixed solution of the aqueous solution of the cerium compound, the silicon compound, and the like and the aqueous solution of the complexing agent can be performed usually at 50° C. to 150° C., usually for 5 hours to 48 hours.
The firing of the product containing Ce and/or Eu and Si and/or P can usually be performed by heating and holding the product at 600° C. to 1500° C. for 2 hours to 48 hours in a reducing atmosphere. The reducing atmosphere can be an inert gas atmosphere or a non-oxidizing atmosphere, and can be an atmosphere containing a reducing gas such as H2, CO, and the like in one embodiment. Thus, the apatite-type oxygen storage material is obtained.
While the following describes some Examples regarding the present disclosure, it is not intended to limit the present disclosure to those described in such Examples.
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to La(NO3)3·6H2O (6.59 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Pr(NO3)3·6H2O (6.62 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Nd(NO3)3·6H2O (6.67 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Sm(NO3)3·6H2O (6.76 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Gd(NO3)3·6H2O (6.87 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Tb(NO3)3·6H2O (6.89 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Ce(NO3)3·6H2O (1.36 g, 3.12 mmol), Ca(NO3)3·4H2O (2.95 g, 12.5 mmol), and (NH4)H2PO4 (1.08 g, 9.38 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Ce(NO3)3·6H2O (6.61 g, 15.2 mmol), Si(OEt)4 (1.70 g, 8.15 mmol), and H3BO3 (0.101 g, 1.63 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Ce(NO3)3·6H2O (5.19 g, 12.0 mmol), Y(NO3)3·6H2O (1.24 g, 3.26 mmol), and Si(OEt)4 (2.04 g, 9.78 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Ce(NO3)3·6H2O (5.19 g, 12.0 mmol), La(NO3)3·6H2O (1.41 g, 3.26 mmol), and Si(OEt)4 (2.04 g, 9.78 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Ce(NO3)3·6H2O (5.19 g, 12.0 mmol), Pr(NO3)3·6H2O (1.42 g, 3.26 mmol), and Si(OEt)4 (2.04 g, 9.78 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Ce(NO3)3·6H2O (5.19 g, 12.0 mmol), Sm(NO3)3·6H2O (1.45 g, 3.26 mmol), and Si(OEt)4 (2.04 g, 9.78 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Ce(NO3)3·6H2O (5.19 g, 12.0 mmol), Lu(NO3)3·4H2O (1.41 g, 3.26 mmol), and Si(OEt)4 (2.04 g, 9.78 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Ce(NO3)3·6H2O (5.43 g, 12.5 mmol), Ca(NO3)2·4H2O (0.738 g, 3.12 mmol), and Si(OEt)4 (1.95 g, 9.38 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Ce(NO3)3·6H2O (5.43 g, 12.5 mmol), Sr(NO3)2 (0.661 g, 3.12 mmol), and Si(OEt)4 (1.95 g, 9.38 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Ce(NO3)3·6H2O (5.43 g, 12.5 mmol), Eu(NO3)3·6H2O (1.39 g, 3.12 mmol), and Si(OEt)4 (1.95 g, 9.38 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Ce(NO3)3·6H2O (5.43 g, 12.5 mmol), Yb(NO3)3·5H2O (1.40 g, 3.12 mmol), and Si(OEt)4 (1.95 g, 9.38 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Eu(NO3)3·6H2O (5.58 g, 12.5 mmol), La(NO3)3·6H2O (1.35 g, 3.12 mmol), and (NH4)H2PO4 (1.08 g, 9.38 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Eu(NO3)3·6H2O (5.58 g, 12.5 mmol), Ce(NO3)3·6H2O (1.36 g, 3.12 mmol), and (NH4)H2PO4 (1.08 g, 9.38 mmol).
The product was collected similarly to Example 1, except that Ce(NO3)3·6H2O (6.61 g, 15.2 mmol) and Si(OEt)4 (2.04 g, 9.78 mmol) as raw materials in Example 1 were changed to Eu(NO3)3·6H2O (5.58 g, 12.5 mmol), Pr(NO3)3·6H2O (1.36 g, 3.12 mmol), and (NH4)H2PO4 (1.08 g, 9.38 mmol).
An X-ray diffraction (XRD) measurement was performed on each of the products of Examples 1 to 15 and Comparative Examples 1 to 8. The measurement device and the measurement condition are described below.
The X-ray diffraction diagrams predicted from the crystalline structure were calculated and compared with the X-ray diffraction diagrams of the respective products.
2-3. Confirmation of Oxygen Desorption Temperature under Reducing Atmosphere (Confirmation of Oxygen Storage Ability of Product Containing Ce)
The products of Example 1 and Comparative Examples 1 to 2 were each subjected to a hydrogen temperature-programmed reduction (H2TPR). The measurement device is described below.
The measurement conditions are shown in
The products of Example 1 and Comparative Examples 1 to 2 on which Pd is supported were also subjected to H2TPR. The Pd supporting condition is described below.
Subsequently, for the H2TPR results of the products of Example 1 and Comparative Examples 1 to 2 after Pd supporting, since the large hydrogen consumption peaks at 150° C. to 250° C. are considered to be due to oxygen release of the oxygen storage material, the position of the hydrogen consumption peak was taken as an oxygen release temperature and an oxygen storage amount was determined from the total hydrogen consumption excluding Pd-derived hydrogen consumption.
From
2-4. Confirmation of Weight Change in Oxidizing Atmosphere (Confirmation of Oxygen Storage Ability of Product having Apatite-type Crystalline Structure)
The products of Examples 1 to 15 and Comparative Examples 3 to 8 were oxidized in the atmosphere at 500° C. for 2 hours, and the oxygen storage amount was determined from the weight change (OSC=((weight after oxidation− weight before oxidation)/weight before oxidation)×100).
ore
O26
O26
O26
O26
O26
O26
O26
O26
O26
O26
Ca2Si6O26
Sr2Si6O26
Yb2Si6O26
La2P
O26
Ce2P6O26
Pr2P6O26
indicates data missing or illegible when filed
From
From
It is originally known that apatite containing Ce(Ce3+) does not exhibit the oxygen storage amount expected when all Ce3+ is oxidized to Ce4+, that is, not all Ce3+ contained in the structure is oxidized to Ce4+. On the other hand, it is considered that in apatite containing Eu2+, which is more easily oxidized than Ce3+, Eu2+ is easily oxidized to Eu3+ and more oxygen (O2) can be incorporated into the structure.
2-5. Discussion on Oxygen Storage Mechanism (Relationship between Crystalline Structure and Oxygen Storage Ability)
According to Table 2, although incorporating oxygen into the crystalline structure by oxidation (oxygen storage) increases the weight and increases the number of atoms per unit cell, the volume of the unit cell remains almost unchanged. This is because Ce changes from Ce3+ to Ce4+ by oxidization, and at the same time, O2− is inserted. It is considered that the unit cell expands in the a-axis and b-axis directions by being pushed apart by O2−, but slightly contracts in the c-axis direction since Ce3+ becomes Ce4+, resulting in a very small change in the volume of the unit cell.
2-6. Explanation for Fact that Apatite Containing Eu(Eu2+) is Superior to Apatite Containing Ce(Ce3+) in Oxygen Storage Amount
The apatite of the present disclosure is synthesized in a reducing atmosphere, and Ce is contained as Ce3+ and Eu is contained as Eu2+. Since the standard electrode potentials of ions are 1.61 eV for Ce3+→Ce4++e− and −0.35 eV for Eu2+→Eu3++e−, it is considered that, as an ion, Eu2+ is more easily oxidized and more easily stores oxygen than Ce3+. In the crystalline structure, Ce3+ and Eu2+ occupy the same site, but have different valences, making it difficult to carry out an experiment in which elements other than Ce or Eu and amounts thereof are completely equalized. However, charge compensation can be performed without affecting the oxygen storage amount using ions with a valence that does not change.
For example, while Example 9 (Ce8Ca2Si6O26) and Example 10 (Ce8Sr2Si6O26) are slightly different from Example 13 (Eu8La2P6O26) in elements other than Ce or Eu and amounts thereof, those skilled in the art would recognize that this is for charge compensation, which does not affect the oxygen storage amount, and is the effect of substituting Ce with Eu. Next, the composition, the proportions of Ce and Eu, and the oxygen storage amount of Examples 9 to 10 and 13 are shown in Tables 3 and
O2
O2
+
Ca2Si6O26
Ca2Si6O26+0.9
Sr2Si6O26
Sr2Si6O26+0.9
La2P
O26
La2P
O26+3.0
indicates data missing or illegible when filed
From Table 3 and
All publications, patents and patent applications cited in the present description are herein incorporated by reference as they are.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2023-198392 | Nov 2023 | JP | national |
| 2024-061442 | Apr 2024 | JP | national |
