WO2013021506A1

WO2013021506A1 - Redox material for thermochemical water decomposition and method for producing hydrogen

Info

Publication number: WO2013021506A1
Application number: PCT/JP2011/068404
Authority: WO
Inventors: 竹島　伸一
Original assignee: トヨタ自動車株式会社
Priority date: 2011-08-05
Filing date: 2011-08-05
Publication date: 2013-02-14
Also published as: JPWO2013021506A1; JP5459322B2; CN103038158A; CN103038158B; DE112011105502B4; DE112011105502T5

Abstract

The present invention provides: an improved redox material which can be used for thermochemical water decomposition; and a method for producing hydrogen using this redox material. A redox material for thermochemical water decomposition of the present invention comprises a redox metal oxide, which is selected from the group consisting of perovskite composite metal oxides, fluorite composite metal oxides and combinations of those oxides, and a metal oxide carrier. The redox metal oxide is dispersed in the metal oxide carrier, thereby being supported by the metal oxide carrier. In a method for producing hydrogen of the present invention, water is decomposed into hydrogen and oxygen by means of oxidation and reduction of the redox material of the present invention.

Description

Redox material for thermochemical water splitting and hydrogen production method

The present invention relates to a redox material for thermochemical water splitting.
<Related technologies>

Recently, many proposals have been made to use hydrogen, which is clean energy, as an energy source. For the production of hydrogen, steam reforming using a hydrocarbon fuel is generally performed. In recent years, it has also been considered to obtain hydrogen from water by water decomposition, particularly thermochemical water decomposition.

The thermochemical water splitting method is a method in which water is split at a lower temperature than in the case of direct water splitting by combining chemical reactions. Specifically, for example, in the thermochemical water splitting method, water is decomposed into hydrogen and oxygen using a redox reaction between metal oxides having different oxidation states as follows (MO is a 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)
Total reaction H ₂ O → H ₂ + 1 / 2O ₂

In such a thermochemical water splitting method, the temperature required for the reaction, in particular, the temperature required for the reaction of decomposing the highly oxidized metal oxide into the low oxidized metal oxide and oxygen is reduced. Has become an important issue.

In this regard, for example, “Reactive ceramics of CeO ₂ -MO _x (M = Mn, Fe, Ni, Cu) for H ₂ generation by two-water splitting using concentrated thermal energy.” Kaneko et al., Energy, Volume 32, Issue 5, May 2007, pp. In 656-663, a composite metal oxide having a fluorite structure such as CeO ₂ —MO _x (MO _x = MnO, Fe ₂ O ₃ , NiO, CuO) is used favorably for the thermochemical water splitting method Yes. Specifically, in this document, when such a composite metal oxide is used, reduction from a highly oxidized metal oxide to a low oxidized metal oxide can be performed at a temperature of about 1500 ° C.

Japanese Patent Application Laid-Open No. 2008-94636 can efficiently reduce a highly oxidized metal oxide to a low oxidized metal oxide at a relatively low temperature by using a heating rate higher than 80 ° C./min. It is said. Specifically, in this document, it is said that the reduction from a highly oxidized metal oxide to a low oxidized metal oxide can be efficiently performed at a temperature around 1500 ° C. by using such a large heating rate. .

In the field of exhaust gas purification for automobiles and the like, it is known to use alumina, porous silica or the like as a porous metal oxide carrier that supports a catalyst component such as a noble metal.

For example, the exhaust gas purifying catalyst proposed in Japanese Patent Application Laid-Open No. 2008-12382 (corresponding to US Patent Application Publication US2009 / 286777A1) by the present inventor is a porous silica carrier composed of silica having an internal pore structure, and porous silica The perovskite-type composite metal oxide particles supported in the internal pore structure of the support, and in the pore distribution of the porous silica support, a peak due to the gap between the primary particles of silica is 3 to It is in the range of 100 nm.

In the present invention, an improved redox material for thermochemical water splitting that can be used for thermochemical water splitting, particularly improved thermochemical water that can be used for thermochemical water splitting at relatively low temperatures. A redox material for decomposition is provided.

The redox material for thermochemical water splitting of the present invention has 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. In addition, the redox metal oxide is dispersed and supported on the metal oxide support. In the present invention, “internal pore structure” of silica means regularly arranged molecular level pores formed by silicon atoms and oxygen atoms constituting silica.

Also, the present invention provides a method for generating hydrogen by decomposing water using the redox material for thermochemical water splitting of the present invention. This method of producing hydrogen by thermochemical water splitting comprises (a) heating the redox material of the present invention having a highly oxidized redox metal oxide to desorb oxygen from the highly oxidized redox metal oxide. A redox material having a low oxidation state redox metal oxide, and oxygen, and (b) contacting the redox material having a low oxidation state redox metal oxide with water to reduce the low oxidation state redox Oxidizing the metal oxide and reducing water, thereby obtaining a redox material having a highly oxidized redox metal oxide, and hydrogen.

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

[Redox materials for thermochemical water splitting]
The redox material for thermochemical water splitting of the present invention has 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. In addition, the redox metal oxide is dispersed and supported on the metal oxide support. In the present invention, a metal oxide that is oxidized and reduced for thermochemical water splitting is referred to as “redox oxide”.

In the redox material for thermochemical decomposition of the present invention, a redox metal oxide such as a perovskite-type composite metal oxide is dispersed and supported on a metal oxide support, whereby the redox metal oxide exists alone. In comparison, the particle size of the redox metal oxide can be kept small. Unexpectedly, such a relatively small particle size can be obtained at a relatively low temperature at redox metal oxide redox reactions for thermochemical water splitting, especially from high oxidation state redox metal oxides to low oxidation state. Enables reduction reaction to redox metal oxides.

Although not limited to theory, in a redox metal oxide having a relatively small particle size, the surface energy is large, whereby oxygen is easily destabilized when heating a highly oxidized redox metal oxide, Therefore, it is considered that reduction to a redox metal oxide in a low oxidation state is performed even at a relatively low temperature.

Such a redox material of the present invention can be used not only by molding itself but also by coating a monolith substrate, for example, a ceramic honeycomb.

(Metal oxide support)
Any metal oxide support can be used as the metal oxide support for supporting the redox metal oxide. However, the metal oxide support is preferably a support that enables highly dispersed support of the redox metal oxide.

As such a metal oxide support, a porous silica support made of silica having an internal pore structure can be used, and a redox metal oxide can be supported in the internal pore structure of the porous silica support. In this case, the redox metal oxide is fixed in the internal pore structure of the porous silica carrier, and the redox metal oxide moves and sinters at a high temperature state, thereby suppressing the particle size from increasing. it can. In this regard, for example, the peak due to the internal pore structure of silica may be in the range of 1-5 nm in the pore distribution of the porous silica support.

In particular, as such a porous silica carrier, a porous silica carrier having a peak due to a gap between primary particles of silica in a pore distribution in the range of 3 to 100 nm, particularly 5 to 50 nm can be used.

Thus, in the pore distribution of the porous silica support having an internal pore structure, the peak due to the gap between the primary particles of silica is in the above range, that is, the porous silica support is relatively small in primary. Having particles is thought to increase the contact between the atmosphere and the redox metal oxide supported in the internal pore structure of the porous silica support, thereby promoting redox oxide redox. .

Such a porous silica support is prepared by, for example, self-aligning an alkylamine in an aqueous solvent, adding an alkoxysilane and an optional base to the solution, and using the self-aligned alkylamine as a template. Can be obtained by precipitating a porous silica carrier precursor and firing it.

Thus, for example, this method can use an aqueous ethanol solution as the aqueous solvent, hexadecylamine as the alkylamine, tetraethoxysilane as the alkoxysilane, and ammonia as an optional base.

The alkylamine and alkoxysilane used in the method for producing a porous silica support can be selected according to the intended primary particle diameter, pore distribution, etc. of the porous silica support. For example, if the length of the alkyl chain of the alkylamine used in the production of the porous silica support is increased, the pore diameter of the internal pore structure can be increased.

Specifically, when cetyl (ie, C ₁₆ H ₃₃ ) trimethylammonium chloride is used as the alkylamine, the pore size of the internal pore structure can be reduced to about 2.7 nm, and lauryl (ie, C ₁₂ H ₂₅ ). When trimethylammonium chloride is used, the pore diameter of the internal pore structure can be about 2.0 nm, and when tetracosyl (ie, C ₂₄ H ₄₉ ) trimethylammonium chloride is used, the pore diameter of the internal pore structure is about 4 nm. .0 nm.

(Redox metal oxide)
The redox metal oxide used in the redox material of the present invention is a perovskite type composite metal oxide, a fluorite type composite metal oxide, or a combination thereof.

The redox metal oxide may be dispersed and supported on the metal oxide support with 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.

Further, the amount of the redox metal oxide supported on the metal oxide support can be selected within a range in which the grain growth of the redox metal oxide can be suppressed and sufficient performance relating to thermochemical water splitting can be provided. Therefore, for example, the loading amount of the redox metal oxide is such that the number of moles of the transition metal in the redox metal oxide is 0.01 mol / g or more, or 0.05 mol / g or more with respect to the mass of the metal oxide support. And 100 mol / g or less, 10 mol / g or less, or 1 mol / g or less, or 0.5 mol / g or less.

Specifically, the perovskite complex metal oxide may be a complex 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 by changing the oxidation number of the transition metal. More specifically, the perovskite complex metal oxide may be a perovskite complex metal oxide represented by the following formula:
A _a B _b O ₃
(A is selected from the group consisting of rare earth elements, particularly lanthanum La, strontium Sr, cerium Ce, barium Ba, calcium Ca, and combinations thereof;
B is selected from the group consisting of transition metal elements, particularly 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 to 1.2, especially 1.1: 0.9 to 0.9: 1.1).

That is, for example, the perovskite-type composite metal oxide may be a composite metal oxide represented by the following formula (x = 0.1 to 0.4):
La _a Mn _b O ₃ ; or
_{_{_{La a Mn b-x Fe x}}} O 3

Here, along with rare earths such as lanthanum, perovskite-type complex metal oxides containing manganese as a transition metal and part of this manganese substituted with iron are effectively reduced and reduced at relatively low temperatures. And is therefore particularly preferred with respect to thermochemical water splitting properties.

Specifically, the fluorite-type composite metal oxide may be a composite metal oxide of a rare earth and a transition metal. In this case, it is considered that the fluorite-type composite metal oxide functions as a redox metal oxide by changing the oxidation number of the transition metal. More specifically, the fluorite-type composite metal oxide may be a fluorite-type composite metal oxide represented by the following formula:
A ¹ _a1 A ² _a2 O ₄
^{(A 1} is a rare earth element, particularly, lanthanum La, strontium Sr, cerium Ce, barium Ba, calcium Ca, and is selected from the group consisting of;
A ² is selected from the group consisting of transition metal elements, particularly 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, especially 1.2: 0.8 to 0.8: 1.2, more particularly 1.1: 0.9 to 0.9: 1.1).

That is, for example, the fluorite-type composite metal oxide may be a composite metal oxide represented by the following formula (x = 0.1 to 0.4, and δ is a decrease in oxygen due to oxygen defects. Min):
Ce _a1 Mn _a2 O _4; or _{_{_{Ce a Mn b-x Fe x}}} O 4-δ

Here, along with rare earth such as cerium, the fluorinated complex metal oxide that contains manganese as a transition metal and in which a part of this manganese is replaced by iron is effectively reduced and reduced at a relatively low temperature. And is therefore particularly preferred with respect to thermochemical water splitting properties.

The support of the redox metal oxide on the metal oxide support is achieved by impregnating the metal oxide support with a solution of a metal salt constituting the redox metal oxide, and drying and calcining the obtained metal oxide support. it can. Examples of the metal salt constituting the redox metal oxide include inorganic acid salts such as nitrates and hydrochlorides, and organic acid salts such as acetates.

The removal of the solvent from the salt solution and drying can be performed by any method and at any temperature. This can be accomplished, for example, by placing a metal oxide support impregnated with a salt solution in an oven at 120 ° C. The redox material of the present invention can be obtained by firing the metal oxide support from which the solvent has been removed and dried in this manner. This calcination can be performed at a temperature generally used in the synthesis of a metal oxide, for example, a temperature of 500 to 1100 ° C.

Regarding the porous silica carrier as described above and the loading of the redox metal oxide on such a porous silica carrier, refer to the description of JP-A-2008-12382 (corresponding to US Patent Application Publication US2009 / 286777A1). can do. The description of this document and other documents cited herein are hereby incorporated herein by reference.

[Method for producing hydrogen of the present invention]
In the hydrogen production method of the present invention, hydrogen is produced by thermochemical water splitting using the redox material of the present invention. Specifically, in the method of the present invention for producing hydrogen by thermochemical water splitting, (a) a redox material having a highly oxidized redox metal oxide is heated to produce oxygen from the highly oxidized redox metal oxide. To obtain a redox material having a low oxidation state redox metal oxide and oxygen, and (b) contacting the redox material of the present invention having a low oxidation state redox metal oxide with water. Oxidizing the low oxidation state redox metal oxide and reducing water, thereby obtaining a redox material having a high oxidation state redox metal oxide and hydrogen.

Further, in the hydrogen production method of the present invention, by using the redox material of the present invention, desorption of oxygen from the highly oxidized redox metal oxide can be achieved at a relatively low temperature. For example, the redox metal oxide can be achieved at a temperature of 1300 ° C. or lower, 1200 ° C. or lower, 1100 ° C. or lower, or 1000 ° C. or lower. Here, this heating can be performed in an inert atmosphere, particularly a rare gas atmosphere such as a nitrogen atmosphere or an argon atmosphere, to promote oxygen desorption.

In the hydrogen production method of the present invention, by using the redox material of the present invention, hydrogen can be generated by reacting a redox metal oxide in a low oxidation state with water at a relatively low temperature, for example, Redox metal oxides can be achieved at temperatures of 1100 ° C. or lower, 1000 ° C. or lower, 900 ° C. or lower, or 800 ° C. or lower.

Hereinafter, the present invention will be further described based on examples, but the present invention is not limited thereto.

[Examples 1 to 5]
(Synthesis of porous silica support)
The synthesis of porous silica as a metal oxide support was performed as follows.

Cetyltrimethylammonium chloride was dissolved in water at 0.5 mol / L. The resulting aqueous solution was stirred for 2 hours to self-align cetyltrimethylammonium chloride. Next, tetraethoxysilane and aqueous ammonia were added to a solution in which cetyltrimethylammonium chloride was self-aligned to bring the pH of the solution to 9.5.

In this solution, tetraethoxysilane was hydrolyzed for 30 hours, and silica was deposited around the arranged cetyltrimethylammonium chloride to form secondary particles composed of primary particles having nano-sized pores. Next, a small amount of nitric acid was added to the aqueous solution to adjust the pH to 7, and the secondary particles were further aggregated and aged for 1 hour to obtain a porous silica carrier precursor.

Thereafter, the obtained porous silica carrier precursor was washed with ethanol water, filtered, dried, and calcined in air at 800 ° C. for 2 hours to obtain a porous silica carrier used in the present invention. In addition, the diameter of the pore resulting from the internal pore structure of the silica in the obtained porous silica support | carrier was about 2.7 nm. Further, the obtained porous silica support had not only pores resulting from the internal pore structure of silica but also pores slightly over 10 nm resulting from the gaps between the primary particles of silica.

(Supporting redox metal oxide)
As the redox metal oxide, a perovskite type composition of LaMnO ₃ (Example 1), LaMn _0.8 Fe _0.2 O ₃ (Example 2), and CeFeO ₃ (Example 3), and CeMnO ₄ (Example) 4) and CeMn _0.8 Fe _0.2 O _4-δ (Example 5) were supported on a porous silica support. Here, the loading is such that the number of moles of transition metal in the redox metal oxide is 0.12 mol / 100 g-support, and the number of moles of all metals in the redox metal oxide is 0.24 mol / 100 g-support. It was done like that. In addition, the loading of the redox metal oxide on the porous silica support was performed by a water absorption supporting method generally used in automobile catalysts.

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 as a stabilizer were added to distilled water. In addition, a solution was obtained and this solution was stored for 2 hours. Thereafter, the porous silica carrier in a dry state was added to this solution, and the mixture was stirred until no bubbles were generated from the porous silica carrier while providing ultrasonic waves.

The water-absorbed porous silica support is separated from the solution by filtration, dried at 250 ° C., and calcined at 800 ° C. for 2 hours to carry the perovskite type lanthanum-manganese composite metal oxide as a redox metal oxide. A porous silica support was obtained. Here, the supported amounts of lanthanum and manganese were 0.12 mol / 100 g-support, respectively.

(Evaluation of loading state of redox metal oxide)
FIG. 1 shows a HAADF-STEM image of the redox material of Example 3 obtained by supporting a perovskite-type composite metal oxide on a porous silica carrier. In the HAADF-STEM image of FIG. 1, the portion corresponding to the internal pore structure of the porous silica support is shown in white, and therefore the perovskite-type composite metal oxide as the redox metal oxide is shown in the internal fine structure of the porous silica support. It is understood that it is carried within the pore structure. Further, from the HAADF-STEM image in FIG. 1, it is understood that the perovskite type composite metal oxide is supported in the internal pore structure of the porous silica support as particles having a size of about 2 to 3 nm. The Note that HAADF-STEM forms an image by a phenomenon in which an electron beam is scattered in proportion to the square of the element mass.

(Characteristic evaluation of oxygen desorption and hydrogen generation)
Each of the redox materials of Examples 1 to 5 was heated to 1000 ° C. in a nitrogen atmosphere to desorb oxygen, and then heated to 800 ° C. in a steam atmosphere to generate hydrogen. The obtained results are shown in Table 1. In Table 1, the oxygen desorption amount and the hydrogen generation amount are amounts (μmol / g-redox metal oxide) with respect to the mass of the redox metal oxide such as a perovskite-type composite metal oxide.

[Comparative Examples 1 and 2]
As a redox metal oxide, a composite metal oxide having a composition of Ce _0.9 Fe _0.1 O _1.5 (Comparative Example 1) and a composition of Ce _0.9 Mn _0.1 O ₂ (Comparative Example 1) A fluorite-type composite metal oxide was obtained by a coprecipitation method. The obtained redox metal oxide had a particle shape of about 2 to 3 nm.

These redox metal oxides were subjected to an oxygen desorption reaction and a hydrogen generation reaction in the same manner as in Example 1. However, in Comparative Examples 1 and 2, when the oxygen desorption reaction was performed at 1000 ° C. and the hydrogen generation reaction was performed at 800 ° C., the reaction did not proceed to an observable level. And the hydrogen production reaction was carried out at 1000 ° C. The obtained results are shown in Table 1.

From Table 1, it is understood that the redox materials of Examples 1 to 5 exhibit superior thermochemical water splitting properties even at low temperatures compared to the redox materials of Comparative Examples 1 and 2.

Claims

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, wherein the redox metal oxide is the metal oxide A redox material for thermochemical water splitting, dispersed and supported on a material carrier.
The redox material according to claim 1, wherein the redox metal oxide has an average particle diameter of 20 nm or less and is dispersed and supported on the metal oxide support.
The metal oxide support is a porous silica support made of silica having an internal pore structure, and the redox metal oxide is supported in the internal pore structure of the porous silica support. Or the redox material of 2.
The redox material according to any one of claims 1 to 3, wherein in the pore distribution of the porous silica support, a peak due to a gap between primary particles of silica is in a range of 3 to 100 nm.
The redox material according to any one of claims 1 to 4, wherein the peak due to a gap between silica primary particles is in a range of 5 to 50 nm.
The redox material according to any one of claims 1 to 5, wherein in the pore distribution of the porous silica support, a peak due to an internal pore structure of silica is in a range of 1 to 5 nm.
The redox material according to any one of claims 1 to 6, wherein the perovskite-type composite metal oxide and / or fluorite-type composite metal oxide is a composite metal oxide of a rare earth and a transition metal.
(A) heating the redox material according to any one of claims 1 to 7 having the highly oxidized redox metal oxide to desorb oxygen from the highly oxidized redox metal oxide; Obtaining the redox material having the redox metal oxide in a low oxidation state and oxygen, and (b) bringing the redox material having the redox metal oxide in a low oxidation state into contact with water, thereby reducing the low oxidation state Oxidizing the redox metal oxide and reducing water, thereby obtaining the redox material having the redox metal oxide in a highly oxidized state, and hydrogen,
A process for producing hydrogen by thermochemical water splitting.
The method according to claim 8, wherein, in the step (a), the redox material is heated to a temperature of 1300 ° C or lower to obtain the redox material having the redox metal oxide in a low oxidation state and oxygen.
The said redox material is made to react with water at the temperature of 1100 degrees C or less in the said process (b), The said redox material which has the said redox metal oxide of a highly oxidized state, and hydrogen are obtained. the method of.