DE112011105502B4 - Use of a redox material for producing hydrogen by thermochemical water splitting and methods for producing hydrogen - Google Patents

Abstract

Use of a redox material comprising a redox metal oxide selected from the group consisting of composite metal oxides of the perovskite type, composite metal oxides of the fluorite type and combinations thereof, and a metal oxide carrier; wherein the redox metal oxide is supported on the metal oxide carrier in a dispersed state for producing hydrogen by thermochemical water splitting.

Description

FIELD OF THE INVENTION

The present invention relates to the use of a redox material for producing hydrogen by thermochemical water splitting and to a method for producing hydrogen.

DESCRIPTION OF THE PRIOR ART

In recent years, numerous proposals have been made to use the clean energy of hydrogen as an energy source. To produce hydrogen, steam reforming using a hydrocarbon fuel is common practice. Furthermore, in recent years, consideration has also been given to obtaining hydrogen from water by splitting water, in particular by thermochemical splitting of water.

• MO (hoher Oxidationszustand) → MO (niedriger Oxidationszustand) + O₂ (endotherme Reaktion)
• MO (niedriger Oxidationszustand) + H₂O → MO (hoher Oxidationszustand) + H₂ (exotherme Reaktion)
• Gesamtreaktion: H₂O → H₂ + 1/2 O₂

"Thermochemical water splitting" is the process of combining chemical reactions to cause the decomposition of water at a temperature lower than that of the direct thermal decomposition of water. In particular, in thermochemical water splitting, for example, oxidation and reduction reactions between metal oxides, which differ in their oxidation states, are used to split water into hydrogen and oxygen in the following way (MO means "metal oxide"):

• MO (high oxidation state) → MO (low oxidation state) + O ₂ (endothermic reaction)
• MO (low oxidation state) + H ₂ O → MO (high oxidation state) + H ₂ (exothermic reaction)
• Overall reaction: H ₂ O → H ₂ + 1/2 O ₂

In such thermochemical water splitting, it becomes particularly important to lower the temperature required for the reaction, especially lowering the temperature required for the reaction of splitting metal oxides of high oxidation state into metal oxide of low oxidation state and oxygen.

In this regard, for example, describes "Reactive ceramics of CeO ₂ -MO _x (M = Mn, Fe, Ni, Cu) for H ₂ generation by two-step water splitting using concentrated solar thermal energy", H. Kaneko et al., Energy, Vol. 32, Issue 5, May 2007, pages 656-663 , CeO _{2 -MO x} (MO _x = MnO, Fe ₂ O ₃ , NiO, CuO) and other composite metal oxides with a fluorite structure can also be used for thermochemical water splitting. In particular, this document describes that when such a composite metal oxide is used, it is possible to reduce a metal oxide with a high oxidation state to a metal oxide with a low oxidation state at the temperature of about 1500 ° C.

Also US 2009/0107044 A1 , WO 2010/131515 A1 and WO 2009/061795 A1 relate to processes for thermochemical water splitting. US 2009/0107044 A1 uses unsupported porous fluorite metal oxide, WO 2010/131515 A1 uses a continuous perovskite-type metal oxide film and WO 2009/061795 A1 uses a spinel-type metal oxide.

It also describes Japanese Unexamined Patent Publication JP 2008-94636 A the use of a heating rate greater than 80 ° C / min to efficiently reduce high oxidation metal oxides to low oxidation metal oxides at a relatively low temperature. In particular, this document describes that by using such a large heating rate, it is possible to efficiently reduce a metal oxide with a high oxidation state to a metal oxide with a low oxidation state at the temperature of about 1500 ° C.

Incidentally, in the field of exhaust gas purification of automobiles, etc., it is known to use alumina, porous silica, etc. as a porous metal oxide carrier for carrying the noble metals or other catalyst components.

For example, includes the exhaust gas purifying catalyst developed by the inventors of Japanese Unexamined Patent Publication JP 2008-12382 A (corresponding to U.S. patent application publication number US 2009/286677 A1 ), a porous silica support comprising silica having an internal pore structure and particles of composite metal oxide of the perovskite type supported in the internal pore structure of the porous silica support. The porous silica support has has a pore distribution having a peak derived from distances between primary silica particles from 3 to 100 nm. Also US 2006/0166816 A1 refers to an exhaust gas purification catalyst.

US 2008/0169449 A1 refers to the reforming of (diesel) fuel using a catalytic membrane in which perovskite-type metal oxide is supported in a dispersed manner.

SUMMARY OF THE INVENTION

The present invention provides the use of an improved redox material for producing hydrogen by thermochemical water splitting which can be used in particular for thermochemical water splitting at a relatively low temperature.

The redox material used for thermochemical water splitting of the present invention comprises a redox metal oxide selected from the group consisting of perovskite-type composite metal oxides, fluorite-type composite metal oxides, and combinations thereof; and a metal oxide support. The redox metal oxide is supported on the metal oxide carrier in a dispersed state. It should be noted that in the present invention, the “internal pore structure” of silicon dioxide means the regularly arranged pores of a molecular level formed by silicon atoms and oxygen atoms that constitute silicon dioxide.

Further, the present invention provides a method for producing hydrogen by splitting water using a redox material for thermochemical splitting of water of the present invention. This method of producing hydrogen by thermochemical water splitting comprises (a) heating a redox material of the present invention comprising a high oxidation state redox metal oxide to remove oxygen from the high oxidation state redox metal oxide, thereby obtaining a redox -Material comprising a low oxidation state redox metal oxide and oxygen; and (b) contacting the redox material comprising the low oxidation state redox metal oxide with water to oxidize the low oxidation state redox metal oxide and reduce the water, thereby obtaining the redox material the high oxidation state redox metal oxide, and hydrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

1 FIG. 13 is a HAADF-STEM image of the redox material obtained in Example 3. FIG.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Redox material for thermochemical water splitting

The redox material for thermochemical water splitting of the present invention comprises a redox metal oxide selected from the group consisting of perovskite-type composite metal oxides, fluorite-type composite metal oxides, and combinations thereof, and a metal oxide carrier. The redox metal oxide is supported on the metal oxide carrier in a dispersed state. It should be noted that the metal oxides that are oxidized and reduced for thermochemical water splitting are referred to as "redox oxides" in the present invention.

In the redox material for thermochemical water splitting of the present invention, perovskite type composite metal oxide or other redox metal oxide is supported on the metal oxide support in a dispersed state. Thereby, compared with the case where the redox metal oxide is present alone, the particle size of the redox metal oxide can be kept small. Such a relatively small particle size unexpectedly enables the oxidation and reduction reactions of the redox metal oxide for thermochemical water splitting at a relatively low temperature, particularly the reduction reaction from a redox metal oxide with a high oxidation state to a redox metal oxide with a low oxidation state.

Without being limited in theory, with a redox metal oxide having a relatively small particle size, the surface energy thereof becomes large. Therefore, when such a redox metal oxide having a high oxidation state is heated, the oxygen easily becomes unstable. It is believed that even at a relatively low temperature, such a redox metal oxide having a high oxidation state is reduced to a redox metal oxide having a low oxidation state.

This redox material of the present invention can be used not only as molded, but also used by coating on a monolithic substrate such as a ceramic honeycomb body.

Metal oxide carrier

As the metal oxide carrier for supporting the redox metal oxide, any metal oxide carrier can be used. However, the metal oxide support is preferably a support which can support the redox metal oxide in a highly dispersed state.

For such a metal oxide support, it is possible to use a porous silica support comprising silicon dioxide having an internal pore structure and to support the redox metal oxide in the internal pore structure of the porous silica support. In this case, the redox metal oxide is anchored in the internal pore structure of the porous silica support, whereby movement and sintering of the redox metal oxide, thereby increasing the particle size of the redox metal oxide, can be suppressed even at a high temperature. In this regard, for example, the peak derived from the internal pore structure of silica may be in the range of 1 to 5 nm in the pore distribution of the porous silica support.

In particular, for such a porous silica support, it is possible to use a porous silica support having a pore distribution where the peak derived from the distances between primary particles of silica is in the range of 3 to 100 nm, particularly 5 to 50 nm .

In this way, if the peak derived from the distances between primary particles of silica is in the above range in the pore distribution of the porous silica support having the internal pore structure, that is, if the porous silica support has relatively small primary particles, it is assumed that the porous silicon dioxide support increases the contact between the redox metal oxide, which is supported in the internal pore structure of the porous silicon dioxide support, and the atmosphere, thereby promoting the oxidation and reduction of the redox oxides.

Such a porous silica support can be obtained, for example, by causing an alkylamine to self-assemble in an aqueous solvent, adding alkoxysilane and an optional base to this solution to use the self-assembled alkylamine as a template and causing the porous to precipitate Silica support precursor around the self-assembled alkylamine, and firing the precipitate.

Therefore, in this method, for example, it is possible to use an aqueous ethanol solution as the aqueous solvent, hexadecylamine as the alkylamine, tetraethoxysilane as the alkoxysilane, and ammonia as the optional base.

The alkylamine and alkoxysilane used in the method for producing a porous silica support can be selected depending on the intended primary particle size, pore distribution, etc. of the porous silica support. For example, by lengthening the alkyl chain of the alkylamine used in the manufacture of the porous silica support, it is possible to obtain a larger pore size of the internal pore structure. In particular, by using cetyl (ie, C ₁₆ H ₃₃ ) trimethylammonium chloride as the alkylamine, it is possible to adjust the pore size of the internal pore structure to about 2.7 nm. By using lauryl (ie, C ₁₂ H ₂₅ ) trimethylammonium chloride, it is possible to adjust the pore size of the internal pore structure to about 2.0 nm. By using tetracosyl (ie, C ₂₄ H ₄₉ ) trimethylammonium chloride, it is possible to adjust the pore size of the internal pore structure to about 4.0 nm.

Redox metal oxide

The redox metal oxide used in the redox material of the present invention is perovskite-type composite metal oxides, fluorite-type composite metal oxides, or combinations thereof.

The redox metal oxide has an average particle size of 20 nm or less, 15 nm or less, 10 nm or less, or 5 nm or less, for example an average particle size of 1.5 nm to 5 nm and can be on the metal oxide support in be supported in a dispersed state.

Further, the amount of the redox metal oxide supported on the metal oxide carrier can be selected in a range in which the particle growth of the redox metal oxide is suppressed and which enables the provision of sufficient thermochemical water splitting performance. Therefore, for example, the amount of the redox metal oxide supported may be 0.01 mol / 100 g or more, or 0.05 mol / 100 g or more, to 100 mol / 100 g or less, 10 mol / 100 g or less, 1 mol / 100 g or less, or 0.5 mol / 100 g or less in terms of a number of moles of transition metal in the redox metal oxide based on the mass of the metal oxide carrier supporting the redox metal oxide.

In particular, the perovskite-type composite metal oxides may be a composite metal oxide of a rare earth and a transition metal. In this case, it is considered that the perovskite-type composite metal oxide functions as a redox metal oxide due to the change in the oxidation number of the transition metal. Specifically, the perovskite-type composite metal oxide may be perovskite-type composite metal oxides of the following formula: A _a B _b O ₃ (where A is a rare earth element, in particular one selected from the group consisting of lanthanum La and cerium Ce, an alkaline earth element selected from strontium Sr, barium Ba, and calcium Ca, or a combination thereof;
B is a transition metal element, in particular one selected from the group consisting of cobalt Co, iron Fe, nickel Ni, chromium Cr, manganese Mn and combinations thereof;
O is oxygen;
a + b = 2; and
a: b = 1.2: 0.8 to 0.8: 1.2, in particular 1.1: 0.9 to 0.9: 1.1).

That is, the perovskite-type composite metal oxides may be, for example, composite metal oxides of the following formulas (x = 0.1 to 0.4): La _a Mn _b O ₃ or La _a Mn _bx Fe _x O ₃

A perovskite-type composite metal oxide which contains, together with a rare earth metal such as lanthanum, a transition metal of manganese, and iron which partially substitutes the manganese, can be efficiently oxidized and reduced at a relatively low temperature, and therefore, is relative to thermochemical water gap characteristics are particularly preferred.

Further, specifically, the fluorite type composite metal oxide may be a composite metal oxide of rare earth and transition metal. In this case, the fluorite-type composite metal oxide is considered to function as a redox metal oxide due to the change in the oxidation number of the transition metal. More specifically, the fluorite-type composite metal oxides can be fluorite-type composite metal oxides of the following formula: A ¹ _a1 A ² _a2 O ₄ (where A ^{1 is} a rare earth element, in particular one selected from the group consisting of lanthanum La and cerium Ce, an alkaline earth element selected from strontium Sr, barium Ba and calcium Ca, or a combination thereof;
A ^{2 is} a transition metal element, in particular one selected from the group consisting of cobalt Co, iron Fe, nickel Ni, chromium Cr, manganese Mn and combinations thereof;
O is oxygen
a1 + a2 = 2; and
a1: a2 = 1.3: 0.7 to 0.7: 1.3, in particular 1.2: 0.8 to 0.8: 1.2, in particular 1.1: 0.9 to 0.9: 1.1).

That is, the fluorite-type composite metal oxide may be, for example, a composite metal oxide represented by the following formulas (where x = 0.1 to 0.4, and δ is the amount of decrease in oxygen due to oxygen defects): Ce _a1 Mn _a2 O ₄ or Ce _a Mn _bx Fe _x O _4-δ

Here, the fluorite type composite metal oxide containing, together with a rare earth metal such as cerium, a transition metal of manganese, and iron partially substituting the manganese, can be efficiently oxidized and reduced at a relatively low temperature, and therefore is particularly preferred with regard to the thermochemical water gap characteristics.

A redox metal oxide can be carried on a metal oxide carrier by impregnating a solution of salts of the metals constituting the redox metal oxide in the metal oxide carrier, and drying and firing the obtained metal oxide carrier. As the salts of the metals constituting the redox metal oxide, salts of inorganic acids such as nitrates and chlorates or salts of organic acids such as an acetate can be used.

Removal and drying of solvent from a saline solution can be accomplished by any method and at any temperature. This can be achieved, for example, by placing a metal oxide support impregnated with the salt solution in an oven at 120 ° C. The metal oxide carrier from which the solvent is removed and thus dried can be baked to obtain the redox material of the present invention. This firing can be carried out at the temperature usually used in forming metal oxides, for example at a temperature of 500 to 1100 ° C.

Note that, regarding such a porous silica support, and a method of supporting a redox metal oxide on such a porous silica support, refer to Japanese Unexamined Patent Publication JP 2008-12382 A (corresponding to U.S. patent application publication number US 2009/286677 A1 ) is taken. The descriptions of this patent document and the other documents cited in this specification are incorporated into this specification by reference.

Process for producing hydrogen of the present invention

In the method for producing hydrogen of the present invention, hydrogen is produced by thermochemical splitting of water using the redox material of the present invention. In particular, the method of the present invention for producing hydrogen by thermochemical water splitting comprises (a) heating a redox material comprising a redox metal oxide having a high oxidation state to remove oxygen from the redox metal oxide having a high oxidation state, and thereby obtaining a Redox material comprising a redox metal oxide having a low oxidation state and oxygen, and (b) contacting the redox material comprising the redox metal oxide having a low oxidation state with water to form the redox metal oxide having a low oxidation state to oxidize and reduce the water, thereby obtaining the redox material comprising the redox metal oxide having a high oxidation state and hydrogen.

Further, in the method for producing hydrogen in the present invention, by using the redox material of the present invention, it is possible to remove oxygen from the redox metal oxide having a high oxidation state at a relatively low temperature. This can be achieved, for example, by heating the redox metal oxide to a temperature of 1300 ° C. or less, 1200 ° C. or less, 1100 ° C. or less, or 1000 ° C. or less. This heating can be carried out in an inert atmosphere, in particular in a nitrogen atmosphere or a noble gas atmosphere such as an argon atmosphere, in order to promote the removal of oxygen.

In the method for producing hydrogen of the present invention, by using the redox material of the present invention, it is possible to react the redox metal oxide having a low oxidation state with water at a relatively low temperature to generate hydrogen. This can be achieved, for example, by heating the redox metal oxide to a temperature of 1100 ° C. or less, 1000 ° C. or less, 900 ° C. or less, or 800 ° C. or less.

In the following, the present invention will be further explained based on examples, but the present invention is not limited thereto.

EXAMPLES

Examples 1 to 5

Formation of porous silica support

• Cetyltrimethylammoniumchlorid wurde in Wasser in einer Menge von 0,5 mol/L gelöst. Die erhaltene wässrige Lösung wurde für 2 Stunden gerührt, um die Selbstorganisation des Cetyltrimethylammoniumchlorids zu bewirken. Als nächstes wurden Tetraethoxysilan und Amoniakwasser zu der selbstorganisierten Cetyltrimethylammoniumchloridlösung gegeben, um die Lösung auf einen pH 9,5 einzustellen.

The porous silicon dioxide of the metal oxide support was formed as follows:

• Cetyltrimethylammonium chloride was dissolved in water in an amount of 0.5 mol / L. The resulting aqueous solution was stirred for 2 hours to cause the cetyltrimethylammonium chloride to self-assemble. Next, tetraethoxysilane and ammonia water were added to the self-assembled cetyltrimethylammonium chloride solution to adjust the solution to pH 9.5.

In this solution, tetraethoxysilane was hydrolyzed for 30 hours to cause silicon dioxide to precipitate around the organized cetyltrimethylammonium chloride and to form secondary particles composed of primary particles with nano-sized pores. Next, a small amount of nitric acid is added to adjust the aqueous solution to pH 7, and the secondary particles were further agglomerated and aged for one hour to obtain a porous silica support precursor.

Thereafter, the obtained porous silica support precursor was washed with an aqueous solution of ethanol, filtered, dried, and calcined at 800 ° C in the air for 2 hours to obtain the porous silica support used in the present invention. Note that the size of the pores derived from the internal pore structure of silica in the obtained silica porous support was about 2.7 nm. Further, the obtained porous silica support had not only pores derived from the internal pore structure of silica, but also pores larger than 10 nm in size derived from the distances between the primary particles of silica.

Redox metal oxide carriers

_{LaMnO 3 of} the perovskite type (example 1), LaMn _0.8 Fe _0.2 O _{3 of} the perovskite type (example 2), CeFeO _{3 of} the perovskite type (example 3), CeMnO _{4 of} the fluorite structure were used as the redox metal oxide -Type (Example 4), and CeMn _0.8 Fe _0.2 O _{4-δ of} the fluorite structure type (Example 5) supported on the porous silicon dioxide carriers. They were supported in a manner wherein the number of moles of the transition metal in the redox metal oxide is 0.12 mol / 100 g carrier and the number of moles of all metals in the redox metal oxide is 0.24 mol / 100 g carrier. Furthermore, the redox metal oxides were absorptively supported on the porous silicon dioxide supports in a manner customary for automobile catalytic converters.

Specifically, for Example 1, about 0.5 mol / L lanthanum nitrate, about 0.5 mol / L manganese nitrate, and about 1.2 mol / L citric acid were added to distilled water as a stabilizing agent to obtain a solution. This solution was saved for 2 hours. Thereafter, the porous silica support was added to this solution in a dry state, and ultrasonic waves were applied thereto while stirring until bubbles were no longer generated from the porous silica support.

The porous silica carrier carrying the absorbed solution was filtered to separate it from the solution, and then dried at 250 ° C and fired at 800 ° C for 2 hours to obtain a porous silica carrier containing redox metal oxide, composed of lanthanum-manganese composite metal oxides of perovskite type. The amounts of lanthanum and manganese supported were 0.12 mol / 100 g-carrier, respectively.

Evaluation of the supported state of the redox metal oxide

A HAADF-STEM image of the redox material of Example 3 obtained by supporting a perovskite-type composite metal oxide on a porous silica support is shown in FIG 1 shown. In the HAADF-STEM picture by 1 the portions corresponding to the internal pore structure of the porous silica support are shown in white. Therefore, it can be understood that the redox metal oxide composed of the perovskite type composite metal oxide was supported in the internal pore structure of the porous silica carrier. Furthermore, from the HAADF-STEM image is from 1 understandable that one Perovskite-type composite metal oxide was supported in the internal pore structure of the porous silica carrier as particles having a particle size of about 2 to 3 nm. Note that HAADF-STEM forms an image by the scattering phenomenon of electron beams caused in relation to the root of the elemental mass.

Evaluation of characteristics of oxygen removal and hydrogen production

The redox materials of Examples 1 to 5 were respectively heated up to 1000 ° C in a nitrogen atmosphere to effect oxygen removal, and then they were heated up to 800 ° C in a steam atmosphere to effect hydrogen generation. The results obtained are shown in Table 1. Note that, in Table 1, the amounts of oxygen removal and hydrogen generation are shown in terms of the mass of redox metal oxides such as perovskite type composite metal oxides (µmol / g redox metal oxide).

Comparative examples 1 and 2

Redox metal oxide composed of Ce _0.9 Fe _0.1 O _1.5 (Comparative Example 1) composite _{metal oxides, and Ce 0.9} Mn _0.1 O ₂ (Comparative Example 2) fluorite type composite metal oxides were obtained by Co -Precipitation received. The obtained redox metal oxides were formed in the form of particles having a particle size of about 2 to 3 nm.

These redox metal oxides were treated in the same manner as in Example 1 for an oxygen removal reaction and a hydrogen generation reaction. However, in Comparative Examples 1 and 2, when the deoxygenation reaction was carried out at 1000 ° C and the hydrogen generation reaction was carried out at 800 ° C, the reactions did not proceed in an observable amount. Therefore, the oxygen removal reaction was carried out at 1400 ° C and the hydrogen generation reaction was carried out at 1000 ° C. The results obtained are shown in Table 1. Table 1. Results of the evaluation Redox metal oxides Oxygen removal Hydrogen production composition shape Temp. (° C) Amount of removed (µmol / g) Temp. (° C) Amount of production (µmol / g) Ex. 1 LaMnO ₃ (perovskite type) Carried on to carrier (0.24 mol / 100 g carrier) 800 100.4 800 156.5 Ex. 2 LaMn _{0, 8 0} Fe ₂ O ₃ (perovskite type) Carried on to carrier (0.24 mol / 100 g carrier) 800 122.2 800 184.7 Ex. 3 CeFeO ₃ (perovskite type) Carried on to carrier (0.24 mol / 100 g carrier) 800 32.3 800 56.5 Ex. 4 CeMnO ₄ (fluorite type) Carried on to carrier (0.24 mol / g carrier) 800 128.0 800 136.7 Ex. 5 CeMn _0.8 Fe _0.2 O _4-δ (fluorite type) Carried on to carrier (0.24 mol / 100 g carrier) 800 105.3 800 165.6 See example 1 Ce _0.9 Fe _0.1 O _1.5 Particle 1400 83.0 1000 35.2 See Ex. 2 Ce _0.9 Mn _0.1 O ₂ (fluorite type) Particle 1400 69.0 1000 52.2

From Table 1, it can be seen that, in comparison with the redox materials of Comparative Examples 1 and 2, the redox materials of Examples 1 to 5 show excellent thermochemical water splitting characteristics even at a low temperature.

Claims (10)

Use of a redox material comprising a redox metal oxide selected from the group consisting of composite metal oxides of the perovskite type, composite metal oxides of the fluorite type and combinations thereof, and a metal oxide carrier; wherein the redox metal oxide is supported on the metal oxide carrier in a dispersed state for producing hydrogen by thermochemical water splitting. Use after Claim 1 wherein the redox metal oxide has an average particle size of 1.5 nm to 20 nm and is supported on the metal oxide carrier in a dispersed state. Use after Claim 1 or 2 wherein the metal oxide support is a porous silica support comprising silicon dioxide having an internal pore structure, and wherein the redox metal oxide is supported in the internal pore structure of the porous silica support. Use after Claim 3 wherein, in the distribution of pores of the porous silica support, a peak derived from the distances between primary particles of silica is in the range of 3 to 100 nm. Use after Claim 4 , wherein the peak derived from the distances between primary particles of silicon dioxide is in the range of 5 to 50 nm. Use after one of the Claims 3 until 5 wherein, in the distribution of pores of the porous silica support, a peak derived from the internal pore structure of silica is in the range of 1 to 5 nm. Use after one of the Claims 1 until 6th wherein the perovskite-type composite metal oxide and / or the fluorite-type composite metal oxide are composite metal oxides of a rare earth and transition metal. A method for producing hydrogen by thermochemical water splitting, which comprises (a) heating a redox material comprising a redox metal oxide having a high oxidation state and a metal oxide carrier to remove oxygen from the redox metal oxide having a high oxidation state, thereby obtaining a redox material which is a redox metal oxide having a low oxidation state and oxygen, wherein the redox metal oxide having a high oxidation state is supported on the metal oxide carrier in a dispersed state, and the redox metal oxide having a high oxidation state is selected from the group consisting of composite metal oxides of the perovskite type, composite metal oxides of the fluorite type, and combinations thereof is selected; and (b) bringing the redox material comprising the redox metal oxide having a low oxidation state into contact with water to oxidize the redox metal oxide having a low oxidation state and reduce the water, thereby obtaining the redox material containing the Includes high oxidation state redox metal oxide and hydrogen. Procedure according to Claim 8 wherein, in the step (a), the redox material is heated to a temperature of 1300 ° C or less to obtain the redox material comprising the redox metal oxide of low oxidation state and oxygen. Procedure according to Claim 8 or 9 wherein, in the step (b), the redox material is reacted with water at a temperature of 1100 ° C or less to obtain the redox material comprising the redox metal oxide of high oxidation state and hydrogen.

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