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Reactive working material for use in hydrogen production by decompostion of waterRelated Patent Categories: Chemistry Of Inorganic Compounds, Hydrogen Or Compound Thereof, Elemental Hydrogen, By Reacting Water Or Aqueous Solution With Metal Or Compound Thereof, IronReactive working material for use in hydrogen production by decompostion of water description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20080089834, Reactive working material for use in hydrogen production by decompostion of water. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates to a technique of producing hydrogen by splitting water through the utilization of solar heat, industrial waste heat or the like, and more particularly to a hydrogen production process based on a two-step thermochemical water-splitting cycle and a reactive working material for use in the hydrogen production process. BACKGROUND ART [0002] A hydrogen production process based on a two-step thermochemical water-splitting cycle has been widely known before the filing of this patent application. The hydrogen production process is designed to repeat the following two reaction formulas. MOox.fwdarw.MOred+1/2O.sub.2 First Step MOred+H.sub.2O.fwdarw.MOox+H.sub.2 Second Step [0003] Specifically, this hydrogen production process comprises a first step of reducing a metal oxide MOox to form a reduced metal oxide MOred and produce oxygen through a high-temperature thermal decomposition reaction, and a second step of reacting the reduced metal oxide with water to oxidize the reduced metal oxide to a metal oxide and produce hydrogen. [0004] Typically, magnetite Fe.sub.3O.sub.4, which is known and described as "iron-based oxide" or "ferrite", is used as the metal oxide MOox, i.e., a reactive working material for the hydrogen production process. This iron-based oxide as the reactive working material is reduced to wustite FeO in the first step to release oxygen, and the wustite FeO is reacted with water in the second step to release hydrogen and return to magnetite Fe.sub.3O.sub.4. Then, the reactive working material will be reused. [0005] In the above reaction formulas, the process of releasing oxygen in the first step is generally required to perform under a high-temperature atmosphere of 1800 to 2300.degree. C. In practice, under such a high-temperature atmosphere for the oxygen release reaction, the iron-based oxide is sintered to be deactivated and cause quite strong vaporization. Therefore, it is required to quench the vaporized substance, and this requirement makes it difficult to put the two-step thermochemical water-splitting cycle to practical use. [0006] With a view to solve the problem concerning the reactive working material for use in the hydrogen production process based on the two-step thermochemical water-splitting cycle, the applicant of this patent application previously disclosed a hydrogen production-process based on a two-step thermochemical water-splitting cycle which uses a reactive working material comprising ferrite fine powder and zirconia fine powder supporting the ferrite fine powder, in Japanese Patent Application No. 2003-060101. [0007] This reactive working material is formed such that ferrite fine powder is supported on zirconia fine powder. The zirconia fine powder is hardly sintered even at high temperatures, and the ferrite fine particles supported on the zirconia fine powder is prevented from coming in close contact with other ferrite particles each other so as to suppress grain growth thereof to provide enhanced reactivity and reusability even at a relatively low temperature of 1400.degree. C. or less. [0008] Fe.sub.3O.sub.4 and FeO to be repeatedly formed during the reaction cycle have specific gravities of 5.2 and 5.7, respectively. Thus, due to volumetric changes of these powders during the reaction cycle, the ferrite fine powder will scale off from the zirconia fine powder to spoil the zirconia powder's effect of suppressing grain growth in the ferrite fine powder., Moreover, under a reaction atmosphere repeatedly having a temperature of 1400.degree. C. or more which is greater than a melting point of FeO, the ferrite fine powder will be gradually agglomerated to cause grain growth while repeating melting and solidification, resulting in deterioration of reaction efficiency. DISCLOSURE OF THE INVENTION [0009] In a reactive working material for use in a hydrogen production process based on a two-step thermochemical water-splitting cycle, which comprises ferrite and zirconia supporting the ferrite, it is an object of the present invention to provide an effective means for preventing the ferrite from scaling off the zirconia due to volumetric changes of the ferrite during repeated use. [0010] It is another object of the present invention to provide a means for suppressing growth of FeO grains due to repetition of melting and solidification when used as a reactive working material for a cyclical reaction under a high temperature of 1400.degree. C. or more. [0011] In order to achieve the above objects, the present invention provides a reactive working material for use in a two-step thermochemical water-splitting cycle, which comprises ferrite and zirconia supporting the ferrite, wherein the zirconia supporting ferrite is a cubic zirconia. [0012] As used in this specification, the "ferrite" means an oxide represented by a composition formula of M(II)O.Fe.sub.2O.sub.3, wherein M(II) is a divalent metal, such as Fe, Mn, Co, Mg, Ni, Zn or Cu. The oxide constituting the ferrite may have any configuration. For example, Fe.sub.3O.sub.4 having a spinel crystal structure may be used. The divalent metals, such as Mn, Co or Mg, may be effectively doped as ions by replacing ferrous ion in Fe.sub.3O.sub.4 partially or all. When used in the form of a fine powder, the ferrite is prepared to have a particle size, preferably, of 10 .mu.m or less, more preferably 1 .mu.m or less. [0013] Further, the "cubic zirconia" means a fully-stabilized zirconia or a partially-stabilized zirconia which contains a stabilizer, such as calcia or yttria, and a zirconia including a cubic crystal phase. Preferably, the cubic zirconia contains yttria or calcia in an amount of 2 mol % or more. If the content rate of the stabilizer is less than 2 mol %, the suppression of grain growth in the ferrite will become insufficient. An excessive content rate of the stabilizer causes deterioration in reactivity. Thus, more preferably, an upper limit of the content rate is set at 25 mol % or less. [0014] In the present invention, the reactive working material comprising a ferrite and a zirconia supporting the ferrite may be formed as a ferrite/zirconia composite powder. Alternatively, the reactive working material may be formed as a ferrite-supporting porous zirconia ceramics. In this case, a ferrite fine powder may be coated on, i.e., supported on, a porous structure of a porous zirconia ceramics. [0015] The ferrite/zirconia composite powder may be prepared by the following specific method. [0016] As one example, a method using an aqueous Fe(II) salt solution may be employed. Specifically, an yttria fully-stabilized or partially stabilized zirconia fine powder or a calcia fully-stabilized or partially stabilized zirconia fine powder which has a particle size of 10 .mu.m or less, preferably 1 .mu.m or less, is dispersed in an aqueous Fe(II) salt solution or an aqueous Fe(II) salt solution containing another metal salt dissolved therein as a doping metal [M(II)], and an aqueous alkali hydroxide solution is added to the zirconia fine powder-dispersed aqueous solution to form a Fe (II) hydroxide colloid therein. Then, air is bubbled in the colloid-containing aqueous solution to oxidize the Fe (II) hydroxide colloid. Then, a dissolution-precipitation reaction where the Fe (II) hydroxide colloid is dissolved in the zirconia fine powder-dispersed aqueous solution and then precipitated as a ferrite is promoted to grow Fe.sub.3O.sub.4 or M.sub.xFe.sub.3-xO.sub.4 on the dispersed zirconia fine powder so as to form a ferrite/zirconia composite powder. [0017] The porous zirconia ceramics supporting the ferrite may be prepared by immersing a porous zirconia ceramics including a cubic crystal phase into the above aqueous Fe(II) salt solution or the above aqueous Fe(II) salt solution containing another metal salt dissolved therein as a doping metal [M(II)], drying the pulled-out porous zirconia ceramics body, and subjecting the dried porous zirconia ceramics to a heat treatment. In the same manner, a reactive working material having a ferrite fine powder coating can be prepared using a cubic zirconia having any other configuration. [0018] As another method for preparing the ferrite/zirconia composite powder, a solvent impregnation process may be used. Specifically, a fully-stabilized or partially-stabilized zirconia fine powder is dispersed in an aqueous solution of a metal salt, such as iron nitrate, iron chloride or organic iron, and a salt of the doping metal. The obtained mixture is evaporated and dried, and then the dried mixture is burnt to allow the metal salt on the zirconia to be decomposed to the metal oxide. Then, the metal oxide is heated under a H.sub.2/H.sub.20 mixed gas atmosphere or a CO/CO.sub.2 mixed gas atmosphere at a temperature of 300.degree. C. or more. [0019] The porous zirconia ceramics supporting the ferrite may be prepared by immersing a porous zirconia ceramics including a cubic crystal phase into the above aqueous Fe(II) salt solution or the above aqueous Fe(II) salt solution containing another metal salt dissolved therein as a doping metal [M(II)], drying the pulled-out porous zirconia ceramics body, and subjecting the dried porous zirconia ceramics to a heat treatment. In the same manner, a reactive working material having a ferrite fine powder coating can be prepared using a cubic zirconia having any other configuration. [0020] When a reaction temperature is increased up to 1300 to 1500.degree. C., the ferrite fine powder supported on the zirconia is formed as FeO through release of oxygen therefrom. Subsequently, when the reaction temperature is decreased to 1000.degree. C. and water vapor is introduced, FeO returns to the original Fe.sub.3O.sub.4 through oxidization while decomposing water to generate hydrogen. [0021] In the above process using the fully-stabilized cubic zirconia, during the course of the formation of FeO at the high temperatures, FeO is incorporated into the zirconia as a solid solution to form a cubic zirconia containing iron ions in the zirconia lattice. In this case, even during the course of the oxidization to Fe.sub.3O.sub.4 as well as that of the thermal reduction of Fe.sub.3O.sub.4, it is impossible that FeO particles supported on the zirconia fine power scale off the zirconia support and exist as independent grains since FeO phase is not formed anymore as the reduced iron-based oxide. 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