Reaction medium and method for producing hydrogen

Abstract

To provide a reaction medium having high reaction activity and capable of producing hydrogen efficiently, and to provide a method for producing hydrogen efficiently.SOLUTION: The reaction medium of the present invention is a reaction medium used in a method for producing hydrogen by pyrolysis of water, and is characterized by containing a complex metal oxide of Fe, Mg and Ni. The complex metal oxide is preferably represented by Fe0.33Mg0.33 Ni0.33Ox. The method for producing hydrogen of the present invention has: a first step of thermally reducing a reaction medium according to claim 1; and a second step of bringing the thermally reduced reaction medium into contact with water to oxidize the reaction medium and generate hydrogen.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a reaction medium and a method for producing hydrogen. [Background technology]

[0002] A two-stage water pyrolysis cycle using cerium oxide (CeO2, ceria) as a reaction medium is considered promising as a method for producing hydrogen by water pyrolysis using heat obtained by concentrating solar light (see, for example, Non-Patent Documents 1 and 2).

[0003] This two-stage water thermal decomposition cycle involves a thermal reduction step in which some of the oxygen atoms that make up CeO2 are extracted by heating to obtain a non-stoichiometric oxide, and a step in which the non-stoichiometric oxide is reacted with water vapor to obtain hydrogen (water decomposition reaction).

[0004] In the two-stage water splitting cycle using cerium oxide (CeO2), from a thermodynamic point of view, the thermal reduction process was carried out at temperatures above 1500°C, and the water splitting reaction process was carried out at temperatures below 1000°C.

[0005] Regarding the cost of hydrogen production using such a two-stage water pyrolysis cycle, there are problems in that the heat loss is large and the reactor materials are expensive when the processing temperature in the thermal reduction step is 1500°C or higher. Furthermore, in order to obtain a high temperature of 1500°C by concentrating sunlight, a higher concentration is required in the sunlight concentrating system, which requires an increase in the number of reflectors installed and a need to increase the concentration with secondary and tertiary reflectors, resulting in high energy loss and high costs.

[0006] Regarding the energy loss of the solar reactor, in the conventional two-stage water thermal decomposition cycle using cerium oxide, the temperature difference between the thermal reduction step and the water decomposition reaction step is more than 500°C. There was a problem in that the heat loss was large when the temperature of the reactor was swung with such a large temperature difference.

[0007] Regarding the reaction medium, a research group at Pohang University of Science and Technology (POSTECH) in South Korea developed a composite metal oxide Fe 0.25 Co 0.25 Ni 0.25 Mg 0.25 O x It has been reported that in (MNCFO), by increasing the entropy of the rock-salt wüstite produced in the thermal reduction process, it is possible to achieve a reaction activity exceeding that of cerium oxide (CeO2) in a two-stage water thermal decomposition cycle, and to lower the temperature of the thermal reduction process to 1,300°C (see, for example, Non-Patent Document 3).

[0008] However, even with such composite metal oxides, it is difficult to say that the reaction activity of the reaction medium and the energy efficiency in hydrogen production are sufficiently high, and there has been a demand for further improvements in the reaction activity of the reaction medium and the efficiency in hydrogen production. [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] N. Gokon, T. Suda, T. Kodama “Thermochemical reactivity of 5-15 mol% Fe, Co, Ni, Mn-doped cerium oxides in two-step water-splitting cycle for solar hydrogen production”, Thermochimica Acta, 617 (2015) 179-190. [Non-Patent Document 2] N. Gokon, T. Suda, T. Kodama “Oxygen and hydrogen productivities and repeatable reactivity of 30-mol%-Fe, Co-, Ni-, Mn-doped CeO2-δ for thermochemical two-step water splitting cycle”, Energy, 90 (2015) 1280-1289. [Non-Patent Document 3] Shang Zhai et. al., Energy Environ. Sci., 11, 2018, pp.2172-2178. Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to provide a reaction medium having high reaction activity and capable of efficiently producing hydrogen, and to provide a hydrogen production method capable of efficiently producing hydrogen. [Means for solving the problem]

[0011] These objects can be achieved by the present invention described below in (1) to (6). (1) A reaction medium used in a method for producing hydrogen by thermal decomposition of water, comprising: A reaction medium comprising a composite metal oxide of Fe, Mg and Ni.

[0012] (2) The composite metal oxide is Fe 0.33 Mg 0.33 Ni 0.33 O x The reaction medium according to (1) above,

[0013] (3) a first step of thermally reducing the reaction medium described in (1) or (2); and a second step of contacting the thermally reduced reaction medium with water to oxidize the reaction medium and generate hydrogen.

[0014] (4) The method for producing hydrogen according to (3) above, wherein the reaction temperature in the first step is 1200°C or higher and 1400°C or lower.

[0015] (5) The method for producing hydrogen according to (3) or (4) above, wherein the reaction temperature in the second step is 1000°C or higher and 1200°C or lower.

[0016] (6) The method for producing hydrogen according to any one of (3) to (5), further comprising repeatedly carrying out a series of steps including the first step and the second step. Effect of the Invention

[0017] According to the present invention, it is possible to provide a reaction medium having high reaction activity and capable of efficiently producing hydrogen, and to provide a hydrogen production method capable of efficiently producing hydrogen. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic diagram showing a preferred embodiment of a hydrogen production device. [Diagram 2] FIG. 2 is a diagram illustrating a method for producing a reaction medium in an embodiment. [Diagram 3] FIG. 2 shows the TGA weight change when the reaction medium of Example 1 was subjected to a two-stage hydrothermal decomposition cycle. [Figure 4] FIG. 2 shows the amounts of hydrogen and oxygen produced and the H2 / O2 molar ratio in each cycle for the reaction medium of Example 1. [Diagram 5] FIG. 2 is a diagram showing the average amounts of hydrogen and oxygen produced per cycle and the H2 / O2 molar ratio for the reaction media of Example 1 and Comparative Examples 1 to 5. [Figure 6] FIG. 1 is a diagram showing the amounts of hydrogen and oxygen produced per cycle and the H2 / O2 molar ratio when the thermal reduction temperature is changed for the reaction medium of Example 1. [Figure 7] FIG. 1 is a diagram showing a schematic configuration of a differential scanning calorimeter. [Figure 8]FIG. 2 is a diagram showing the results of differential scanning calorimetry for the reaction media of Example 1, Comparative Example 3, and Comparative Example 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Preferred embodiments of the present invention will now be described. [1] Reaction medium First, the reaction medium of the present invention will be described.

[0020] The reaction medium according to the present invention is used in a method for producing hydrogen by thermal decomposition of water, and is characterized in that it contains a composite metal oxide of Fe, Mg and Ni.

[0021] In the present invention, by reducing the number of constituent metal elements compared to the conventional four-component rock salt-spinel type high entropy oxide, a composite metal oxide of Fe, Co, Ni and Mg (MNCFO), the content of Fe ions that directly contribute to the redox reaction in the reaction medium is increased, thereby improving hydrogen production capacity.

[0022] Specifically, in the present invention, by removing Co from the Fe, Co, Ni, and Mg that constitute conventional composite metal oxides (MNCFO) and forming a composite metal oxide of Fe, Mg, and Ni, it is possible to increase the Fe ion content while still obtaining a sufficient high entropy effect.

[0023] By reducing the number of constituent metal elements of the composite metal oxide from four to three, it is possible to reduce the number of synthesis raw materials, specifically, for example, the types of nitrates of the constituent metals, as shown in the examples described later, and also to simplify the synthesis reaction.

[0024] In particular, Co is expensive because it is a rare metal, and because its production areas are concentrated in politically unstable regions, it is difficult to ensure a stable supply. By excluding Co from the conventional four-component complex metal oxide (MNCFO), it is possible to reduce raw material costs and enable stable production of the reaction medium.

[0025] Moreover, MgO and NiO have relatively high melting points. For example, the melting point of MgO is 2852°C, and the melting point of NiO is 1984°C. Among the constituent metal elements of the conventional four-component composite metal oxide (MNCFO), Mg and Ni are not excluded, and by including them in the three-component composite metal oxide of the present invention, the composite metal oxide of the present invention has a high melting point, and it is expected to have an effect of suppressing melting and sintering of the composite metal oxide during high-temperature treatment.

[0026] By producing hydrogen using such a reaction medium, the oxidation-reduction reaction proceeds favorably, and hydrogen can be produced more efficiently.

[0027] The composite metal oxide constituting the reaction medium preferably has a melting point of 1500° C. or higher. If the melting point of the complex metal oxide is lower than the processing temperature (e.g., 1400°C) in the thermal reduction step (first step) in the hydrogen production method described below, the complex metal oxide particles will melt due to the temperature during thermal reduction, and further, sintering accompanying the thermal reduction will significantly reduce the surface area, greatly reducing the reaction activity of the reaction medium.

[0028] When the melting point of the composite metal oxide constituting the reaction medium is sufficiently high as described above, it is possible to suppress a decrease in the reaction activity of the reaction medium in the thermal reduction step.

[0029] The composite metal oxide of the present embodiment is not limited in composition as long as it contains Fe, Mg, and Ni, but it is preferable that the composite metal oxide contains each metal in an equimolar amount. 0.33 Mg 0.33 Ni 0.33 O x It is preferable that the ionic liquid is represented by the following formula: This makes it possible to make the above-mentioned effects of the present invention more pronounced.

[0030] In the following description of this embodiment, the composite metal oxide constituting the reaction medium is Fe 0.33 Mg 0.33 Ni0.33 O x This section will mainly explain the case where

[0031] The reaction medium of the present embodiment has a higher reaction activity than conventional reaction media, and therefore, even if the treatment temperature of the thermal reduction step in the two-stage water pyrolysis cycle described below is set to a lower temperature than conventional ones, the target reaction can be sufficiently promoted. This makes it possible to improve the energy efficiency of hydrogen production.

[0032] For example, as shown in the examples described below, the reaction medium (Fe 0.33 Mg 0.33 Ni 0.33 O x ) was used to test the reaction activity in a cycle of thermal reduction temperature: 1400°C - water decomposition temperature: 1000°C. 0.25 Co 0.25 Ni 0.25 Mg 0.25 O x This is about 10.5 times the amount of hydrogen produced when the reaction was carried out in a cycle of thermal reduction temperature: 1400°C - water decomposition temperature: 1000°C using conventional cerium oxide (CeO2).

[0033] In the reaction medium of this embodiment, the treatment temperature in the thermal reduction step can be made lower than in the past, and the temperature difference in the two-stage water pyrolysis cycle can be made smaller, thereby improving the energy efficiency of hydrogen production.

[0034] In particular, the reaction medium (Fe 0.33 Mg 0.33 Ni 0.33 O x By producing hydrogen using this catalyst, the redox reaction proceeds smoothly, hydrogen and oxygen are produced with good cyclical efficiency, and the H2 / O2 molar ratio is close to the stoichiometric ratio of 2, so the reaction proceeds stably at a stoichiometric ratio.

[0035] In this way, by using the reaction medium of this embodiment in a method for producing hydrogen by thermal decomposition of water, the amount and efficiency of hydrogen production can be significantly improved.

[0036] The volume-based average particle size of the reaction medium is preferably 10 μm or more and 300 μm or less. This not only improves the fluidity and ease of handling of the reaction medium, but also allows appropriate gaps to be formed between particles when the reaction medium is stored in a container, allowing for more optimal flow of gases (reaction gas, carrier gas, product gas) in the first step (thermal reduction step) and second step (water decomposition step) of the hydrogen production method described below.

[0037] The average particle size of the reaction medium can be determined, for example, by adding a sample to methanol, dispersing the sample for 3 minutes using an ultrasonic disperser, and measuring the resulting dispersion using a Coulter Counter particle size distribution analyzer (TA-II model, manufactured by COULTER ELECTRONICS INS) with an aperture of 50 μm.

[0038] The reaction medium of this embodiment may have at least a part of its surface made of the above material (composite metal oxide of Fe, Mg, and Ni), and may have a structure in which the surface of a base (support) made of a material other than the above material (composite metal oxide of Fe, Mg, and Ni) is coated with the above material (composite metal oxide of Fe, Mg, and Ni). In this case, for example, zirconia can be used as the base (support).

[0039] [2] Hydrogen production methods Next, the method for producing hydrogen according to the present invention will be described.

[0040] The method for producing hydrogen of the present invention includes a first step (thermal reduction step) of thermally reducing the above-mentioned reaction medium, and a second step (water decomposition step) of contacting the thermally reduced reaction medium with water to oxidize the reaction medium and generate hydrogen.

[0041] In the present invention, hydrogen is produced using a reaction medium in which the hydrogen production capacity (reaction activity) is improved by increasing the Fe ion content in a composite metal oxide, thereby allowing the oxidation-reduction reaction to proceed favorably and enabling hydrogen to be produced efficiently.

[0042] In the hydrogen production method of the present invention, the heat used in the reaction may be of any type and does not have to be heat obtained by concentrating sunlight. However, the following description will focus on the case where the heat is obtained by concentrating sunlight.

[0043] In the present invention, each of the first step and the second step may be carried out at least once, but it is preferable to repeatedly carry out a series of treatments including the first step and the second step.

[0044] This makes it possible to increase the overall amount of water (steam) processed, further improving hydrogen productivity, and since the effect of reducing the temperature difference between the two processes is repeated, the effects described above are more pronounced.

[0045] In the present invention, a pre-treatment step may be included in the first step, an intermediate treatment step may be included between the first step and the second step, or a post-treatment step may be included in the second step. The pre-treatment step and the post-treatment step may be included in the series of treatments, or may not be included in the series of treatments.

[0046] Furthermore, when the above-mentioned series of treatments is repeated, the treatment conditions may be changed in at least one step in each cycle.

[0047] [2-1] First step In the first step (thermal reduction step), the reaction medium is thermally reduced. The reaction medium used contains the above-mentioned composite metal oxide.

[0048] This makes it possible to improve the reaction activity in the first and second steps, and to improve the energy efficiency of hydrogen production and the productivity of hydrogen.

[0049] Specifically, the reaction activity can be improved by suitably progressing the reduction reaction in the first step and the oxidation reaction in the second step.

[0050] The reaction in this step can be represented by the following formula (1). Fe 0.33 Mg 0.33 Ni 0.33 O x (spinel structure) → Fe 0.33 Mg 0.33 Ni 0.33 O x-σ (rock salt structure) +σ / 2O2 (1)

[0051] The reaction temperature (T1) in this step is preferably 1200°C or higher and 1400°C or lower, more preferably 1300°C or higher and 1400°C or lower, and even more preferably 1350°C or higher and 1400°C or lower.

[0052] This makes it possible to increase the reaction rate in this step and to further improve the energy efficiency of the entire hydrogen production method of the present invention.

[0053] In particular, since the above-mentioned reaction medium of the present invention has a higher reaction activity than conventional reaction media, by using the reaction medium of the present invention, the temperature of the thermal reduction process can be lowered below the temperature in the conventional thermal reduction process, for example, 1500°C.

[0054] This makes it possible to reduce the temperature difference between the thermal reduction step (first step) and the water decomposition step (second step), resulting in better energy efficiency.

[0055] More specifically, if the temperature difference between the two processes is large, for example, the energy loss during temperature cycling in a reactor increases, and energy efficiency decreases. However, in the present invention, the temperature difference between the two processes can be suitably reduced, so that such problems do not occur and the energy efficiency in hydrogen production can be made particularly excellent.

[0056] By lowering the processing temperature in the thermal reduction process, heat loss due to re-radiation, heat conduction, and convection is reduced, and cheaper materials can be used as the constituent materials of the reactor. In addition, by lowering the processing temperature, the reaction can be carried out at a lower concentration in the solar concentrating system compared to the conventional high temperatures (over 1500°C), which reduces the number of reflectors installed and makes secondary and tertiary reflectors unnecessary. This increases energy efficiency and reduces costs. As a result, the present invention can greatly increase the amount of hydrogen produced and the energy efficiency.

[0057] Furthermore, by reducing the temperature difference between the two steps, the time required to adjust the temperature to an appropriate level in each step can be shortened, thereby improving hydrogen productivity.

[0058] In addition, because the temperature difference between the two processes is small, the burden on the equipment (reactor) used to produce hydrogen can be reduced. As a result, hydrogen can be produced stably over a long period of time, and the lifespan of the equipment used to produce hydrogen can be extended and maintenance can be simplified.

[0059] In addition, since high resistance to shocks caused by repeated large temperature changes is not required, the range of materials from which the reactor can be constructed can be expanded, and even if the device configuration is relatively simple rather than special (for example, a configuration in which the two processes are carried out in different parts of different reactors), the energy efficiency during hydrogen production and hydrogen productivity can be made sufficiently excellent, thereby making it possible to reduce the manufacturing costs of the hydrogen production device. It can also contribute to reducing the running costs of the hydrogen production device.

[0060] In addition, by carrying out the thermal reduction reaction at a lower temperature than in the conventional two-stage water pyrolysis cycle using sunlight, hydrogen can be suitably produced by the two-stage water pyrolysis cycle even in regions other than the sunbelt (regions with a relatively small amount of sunlight) that were previously unsuitable for implementing the two-stage water pyrolysis cycle. In other words, the regions where hydrogen can be suitably produced can be expanded compared to the conventional case.

[0061] In addition, by performing the thermal reduction reaction at a lower temperature than in the conventional two-stage water pyrolysis cycle using sunlight, the time required to raise the temperature of the reaction medium to a suitable temperature can be shortened, further improving hydrogen productivity. In addition, the burden on the device (reactor) used for hydrogen production can be reduced.

[0062] In addition, when the treatment temperature in this step changes with time, the maximum treatment temperature in this step can be used as T1.

[0063] The treatment time in this step (when the above series of treatments is repeated, the treatment time in one first step) is preferably from 20 minutes to 180 minutes, more preferably from 20 minutes to 120 minutes, and even more preferably from 20 minutes to 30 minutes.

[0064] This allows the thermal reduction reaction of the reaction medium to proceed sufficiently, and hydrogen can be produced in a short period of time, thereby further improving hydrogen productivity.

[0065] The treatment time in this step can be the time during which no gas containing water vapor (gas with a water vapor partial pressure of 10 kPa or more) is supplied and the temperature of the reaction medium is 1100°C or higher.

[0066] This step is usually carried out in an atmosphere with a low partial pressure of oxygen gas. Specifically, the partial pressure of oxygen gas in the atmosphere in which this step is carried out is preferably 1 kPa or less, and more preferably 0.1 kPa or less.

[0067] This allows the reduction reaction of the reaction medium to proceed smoothly, and also effectively prevents the reaction medium that has been reduced once from being unintentionally re-oxidized in this step.

[0068] This step is usually carried out while supplying an inert gas such as argon or nitrogen gas into a vessel (reactor) containing a reaction medium.

[0069] This allows the oxygen generated by the reduction reaction of the reaction medium to be efficiently discharged from the system containing the reaction medium, and more effectively prevents the reaction medium that has been reduced by the oxygen from being unintentionally oxidized. In addition, when the reaction medium is in the form of particles, the reaction medium can be efficiently flowed in the vessel that contains the reaction medium, and the reaction efficiency in this step can be further improved.

[0070] This step may be carried out in a vacuum environment or a reduced pressure environment (for example, an environment of 10 kPa or less).

[0071] [2-2] The second step In the second step (water decomposition step), the reaction medium thermally reduced in the first step is brought into contact with water to oxidize the reaction medium and generate hydrogen.

[0072] The reaction in this step can be represented by the following formula (2). Fe 0.33 Mg 0.33 Ni 0.33 O x-σ (Rock salt structure) + δH2O → Fe 0.33 Mg 0.33 Ni 0.33 O x (Spinel structure) + δH2 (2) (In the formula, 1≦x≦2 and 0<δ≦x.)

[0073] The reaction temperature (T2) in this step is preferably 800°C or higher and 1200°C or lower, more preferably 900°C or higher and 1200°C or lower, and even more preferably 1000°C or higher and 1200°C or lower.

[0074] This makes it possible to sufficiently increase the reaction rate in this step while further improving the energy efficiency of the entire hydrogen production method of the present invention.

[0075] In addition, when the treatment temperature in this step changes with time, the maximum treatment temperature in this step can be used as T2.

[0076] The treatment time in this step (when the above series of treatments is repeated, the treatment time in one second step) is preferably from 10 minutes to 150 minutes, more preferably from 10 minutes to 100 minutes, and even more preferably from 10 minutes to 80 minutes.

[0077] This allows the water decomposition reaction (hydrogen production and oxidation reaction of the reaction medium) to proceed sufficiently, and also enables hydrogen to be produced in a short period of time, thereby further improving hydrogen productivity.

[0078] The treatment time in this step can be the time during which the gas containing water vapor is being supplied and the temperature of the reaction medium is 950° C. or higher.

[0079] This step is usually carried out while supplying water vapor into a vessel (reactor) containing the reaction medium. This allows the reaction between the thermally reduced reaction medium and water to proceed more stably, and the efficiency of hydrogen generation can be improved. Also, the generated hydrogen can be easily recovered. Furthermore, when the reaction medium is in a particulate form, the reaction medium can be efficiently flowed in a vessel containing the reaction medium, and the reaction efficiency in this step can be further improved.

[0080] The water vapor partial pressure in the atmosphere during this step is preferably 1 kPa or more and 1 MPa or less, and more preferably 10 kPa or more and 100 kPa or less.

[0081] This allows the target reaction to proceed more suitably, and hydrogen productivity can be further improved.

[0082] In addition, this step is preferably carried out while supplying a gas (carrier gas) that is inert to the reaction in this step, such as an inert gas or nitrogen gas, together with water vapor, into a vessel (reactor) that contains the reaction medium.

[0083] This allows the reaction between the thermally reduced reaction medium and water to proceed more stably, improving the efficiency of hydrogen generation and making it easier to collect the generated hydrogen.

[0084] In this process, when the partial pressure of water vapor in the gas supplied into the vessel (reactor) containing the reaction medium is P1 [Pa] and the partial pressure of the carrier gas is P2 [Pa], it is preferable to satisfy the relationship 0.2≦P1 / P2≦0.999, and it is more preferable to satisfy the relationship 0.3≦P1 / P2≦0.995.

[0085] This allows the target reaction to proceed more suitably, and hydrogen productivity can be further improved.

[0086] This step is usually carried out in an atmosphere with a low partial pressure of oxygen gas. Specifically, the partial pressure of oxygen gas in the atmosphere in which this step is carried out is preferably 1 kPa or less, and more preferably 0.1 kPa or less.

[0087] This effectively prevents the thermally reduced reaction medium from being oxidized by an oxidation reaction (oxidation reaction with oxygen gas) that does not contribute to the generation of hydrogen, and allows the reaction between the thermally reduced reaction medium and water to proceed more smoothly.

[0088] [3] Manufacturing equipment Hereinafter, a hydrogen production device that can be used in the hydrogen production method of the present invention will be described. FIG. 1 is a schematic diagram showing a preferred embodiment of a hydrogen production device.

[0089] The hydrogen production device 100 comprises a ground reflector (sunlight concentrating means) 20 installed on the ground called a heliostat, a tower reflector (sunlight concentrating means) 30 installed in a tower (not shown), and a reactor (container) 10 into which the light reflected by the tower reflector 30 enters and the reaction of the aforementioned reaction medium takes place.

[0090] In the illustrated hydrogen production device 100, a ground reflector 20 and a tower reflector 30 constitute a beam-down type light collecting system.

[0091] This beam-down type light collecting system is configured to collect sunlight S and irradiate the central portion of the upper surface of the fluidized bed 2 contained in the reactor 10.

[0092] The reactor 10 is made of a heat-resistant material capable of withstanding the temperatures in the first and second steps described above. Examples of the reactor 10 include those made of stainless steel alloys and Inconel alloys. The reactor 10 contains a fluidized bed 2 containing the reaction medium described above.

[0093] In addition, at the bottom of the reactor 10, a gas introduction plate 3 is provided which can retain the reaction medium particles constituting the fluidized bed 2 within the reactor 10 and introduces gases led from gas introduction means 4 and 5 into the inside of the reactor 10.

[0094] The gas introduction plate 3 is provided with a large number of minute openings. The gas introduction means (low oxygen partial pressure gas introduction means) 4 is a flow path (pipe) for introducing the gas used in the first step into the reactor 10, and the gas introduction means (water vapor-containing gas introduction means) 5 is a flow path (pipe) for introducing the gas used in the second step into the reactor 10.

[0095] A window 6 having optical transparency and heat resistance is provided at the top of the reactor 10 so as to transmit sunlight S. Examples of materials for the window 6 include quartz.

[0096] At the sides of the upper portion of the reactor 10, outlets 8 and 9 for taking out the gas that has passed through the fluidized bed 2 are provided.

[0097] More specifically, the reactor 10 is provided with an outlet (oxygen-containing gas outlet) 8 for removing gas that has passed through the fluidized bed 2 in the first step (gas containing oxygen gas produced by the reaction), and an outlet (hydrogen-containing gas outlet) 9 for removing gas that has passed through the fluidized bed 2 in the second step (gas containing hydrogen gas produced by the reaction).

[0098] The outlets 8 and 9 are each provided with a shutter (not shown). This makes it possible to separate and recover the gas that has passed through the fluidized bed 2 in the first step and the gas that has passed through the fluidized bed 2 in the second step.

[0099] More specifically, in the first step, outlet 8 is opened and outlet 9 is closed, so that gas containing oxygen gas produced by the reaction is recovered from outlet 8, and in the second step, outlet 8 is closed and outlet 9 is opened, so that gas containing hydrogen gas produced by the reaction is recovered from outlet 9.

[0100] This makes it possible to effectively prevent the gas containing oxygen gas and the gas containing hydrogen gas from being unintentionally mixed together in the recovered gas.

[0101] Hereinafter, hydrogen production using the hydrogen production device 100 will be described. First, before starting the first step described above, a low oxygen partial pressure gas introduced from a gas introduction means (low oxygen partial pressure gas introduction means) 4 is introduced into the reactor 10 containing the reaction medium through the gas introduction plate 3, and the fluidized bed 2 is fluidized to form an internal circulation flow.

[0102] Next, while a low oxygen partial pressure gas is flowed into the reactor 10, sunlight S concentrated by the sunlight collecting means 20, 30 is irradiated through the window 6 onto the inside of the reaction medium (fluidized bed 2) in the reactor 10, thereby heating the fluidized bed 2 to the desired temperature and allowing the reaction in the first step to proceed.

[0103] Next, the amount of sunlight S (amount per unit time) input to the reactor 10 is reduced to lower the temperature of the reaction medium (fluidized bed 2) to a desired temperature, and the gas supplied to the inside of the reactor 10 is switched from the low oxygen partial pressure gas to a water vapor-containing gas. That is, the supply of gas from the gas introduction means 4 is interrupted, and a water vapor-containing gas is supplied to the inside of the reactor 10 from the gas introduction means 5 via the gas introduction plate 3. This allows the second step to proceed.

[0104] Thereafter, a series of processes including the first and second steps is repeated a desired number of times. When the first step is carried out again after the second step, the amount of sunlight S (amount per unit time) input into the reactor 10 is increased to raise the temperature of the reaction medium (fluidized bed 2) to a desired temperature, and the gas supplied to the inside of the reactor 10 is switched from the water vapor-containing gas to the low oxygen partial pressure gas. That is, the supply of gas from the gas introduction means 5 is interrupted, and the low oxygen partial pressure gas is supplied from the gas introduction means 4 to the inside of the reactor 10 via the gas introduction plate 3.

[0105] As described above, by performing the first step and the second step in the same container (particularly, performing them in the same location of the same container), the configuration of the hydrogen production apparatus can be simplified, the manufacturing costs of the hydrogen production apparatus can be reduced, and installation of the hydrogen production apparatus can be made easier.

[0106] In addition, as described above, by carrying out the first step and the second step while circulating the fluidized bed 2 containing the powdered reaction medium within the reaction vessel, it is possible to further improve the productivity of hydrogen.

[0107] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these.

[0108] For example, the hydrogen production method of the present invention is not limited to being carried out using the hydrogen production device described above, and may be carried out using a hydrogen production device of another configuration. More specifically, in the above-mentioned embodiment, the first step and the second step are mainly performed in the same part of the hydrogen production device (reactor), but for example, the first step and the second step may be performed in different parts of the hydrogen production device (reactor).

[0109] In the above-mentioned embodiment, the reaction medium is mainly described as being particulate (powdered), but the reaction medium may be of any shape, and may be, for example, a molded body made of a porous body (for example, a foam). This allows the reaction medium to be suitably contacted with the gas in the first and second steps, and allows the target reaction to proceed suitably.

[0110] In addition, in the above-described embodiment, the case where the reaction is carried out using heat from sunlight has been mainly described. However, in the present invention, heat from sources other than sunlight may be used as at least a portion of the heat used in the reaction. EXAMPLES

[0111] The present invention will be described in detail below based on specific examples, but the present invention is not limited thereto.

[0112] In the following explanation, treatments for which no particular temperature conditions are indicated were performed at room temperature (23°C) and a relative humidity of 50%. In addition, for various measurement conditions, values ​​for which no particular temperature conditions are indicated are those at room temperature (23°C) and a relative humidity of 50%.

[0113] [4] Preparation of reaction medium The reaction medium was prepared as follows. FIG. 2 is a diagram illustrating a method for producing a reaction medium in an embodiment.

[0114] Example 1 First, equimolar amounts of iron nitrate (Fe(NO3)3·9H2O), magnesium nitrate (Mg(NO3)2·6H2O), and nickel nitrate (Ni(NO3)2·6H2O) were weighed and mixed to obtain a mixture.

[0115] Next, 100 parts by mass of the above mixture was dissolved in 400 parts by mass of ion-exchanged water to obtain an aqueous solution.

[0116] To the above aqueous solution, ethylenediaminetetraacetic acid (EDTA) was added in an amount of 60% of the total molar amount of metal ions, and then citric acid was added in an amount of 75% of the total molar amount of metal ions. The pH was then adjusted to 11 by adding an aqueous ammonium hydroxide solution while stirring with a stirrer rotating at 300 rpm.

[0117] Thereafter, while continuing stirring at 300 rpm, this aqueous solution was heated at 200° C. When the aqueous solution gelled, stirring was stopped. The heating time at 200° C. was about 5 hours.

[0118] The gelled aqueous solution was dried overnight at 300°C, which resulted in the gel becoming almost solid. The solidified gel was heated at 300° C. for 1 hour in a box furnace.

[0119] The mixture was then sintered in air using an electric furnace. The temperature was increased at a rate of 10°C / min up to 800°C, then increased at a rate of 5°C / min from 800°C to 1000°C, and the mixture was held at 1000°C for 1 hour.

[0120] Then, the temperature was decreased to 800°C at a rate of 5°C / min, and then the specimen was allowed to cool naturally. 0.33 Mg 0.33 Ni0.33 O x A reaction medium consisting of was obtained.

[0121] XRD measurement revealed that the obtained Fe 0.33 Mg 0.33 Ni 0.33 O x was a single phase with a spinel structure.

[0122] Comparative Example 1 The procedure of Example 1 was repeated except that iron nitrate, cobalt nitrate (Co(NO3)2·6H2O), and magnesium nitrate were used as the metal nitrates. 0.33 Co 0.33 Mg 0.33 O x A reaction medium consisting of was obtained.

[0123] XRD measurement revealed that the obtained Fe 0.33 Co 0.33 Mg 0.33 O x was a single phase with a spinel structure.

[0124] Comparative Example 2 The same procedure as in Example 1 was repeated except that iron nitrate, cobalt nitrate, and nickel nitrate were used as the metal nitrates. 0.33 Co 0.33 Ni 0.33 O x A reaction medium consisting of was obtained.

[0125] XRD measurement revealed that the obtained Fe 0.33 Co 0.33 Ni 0.33 O x was a single phase with a spinel structure.

[0126] Comparative Example 3 The same procedure as in Example 1 was repeated except that iron nitrate, cobalt nitrate, nickel nitrate, and magnesium nitrate were used as the metal nitrates. 0.25 Co 0.25 Ni 0.25 Mg 0.25 O xA reaction medium composed of (MNCFO) was obtained.

[0127] XRD measurement revealed that Fe 0.25 Co 0.25 Ni 0.25 Mg 0.25 O x It was a complex phase consisting of rock salt and spinel structures.

[0128] Comparative Example 4 First, a given amount of Ce(NO3)3·6H2O and MnSO4·5H2O were added to water and stirred at room temperature to obtain a solution.

[0129] Next, while stirring this aqueous solution, an aqueous sodium hydroxide solution was added dropwise to this aqueous solution to adjust the pH to 8.5, which resulted in the formation of a precipitate.

[0130] The precipitate was then collected by centrifugation, washed with purified water and acetone, and dried at room temperature for 24 hours.

[0131] After that, a calcination treatment was performed at 900°C for 2 hours to obtain a reaction medium composed of Mn-doped CeO2.

[0132] The reaction medium thus obtained had a substitution rate of 15 mol % of Mn for Ce constituting CeO2.

[0133] Comparative Example 5 A reaction medium was prepared in the same manner as in Comparative Example 4, except that MnSO4·5H2O was not used.

[0134] The reaction medium thus obtained consisted of CeO2 (not doped with any metal other than Ce).

[0135] Comparative Example 6 Commercially available Fe3O4 was prepared and used as the reaction medium.

[0136] [5] Hydrogen production Using the reaction media produced in each of the examples and comparative examples, hydrogen was produced as follows. A thermobalance (Rigaku, TG8120) was used to produce hydrogen.

[0137] First, a predetermined amount (33 mg to 38 mg) of the reaction medium obtained above was prepared and filled into a platinum cup with a diameter of 5 mm and a depth of 5 mm, which was then placed in a heating furnace, and the atmosphere in the electric furnace was replaced with nitrogen gas. Note that, since the CeO2 in Comparative Example 5 had low reactivity and little weight change, the sample amount for Comparative Example 5 was set to 66 mg.

[0138] Next, while nitrogen gas was flowed into the electric furnace at 300 mL / min, the temperature was raised to 1000°C at a heating rate of 30°C / min, and then raised to the thermal reduction temperature (1400°C) at a heating rate of 20°C / min, and held at 1400°C for 10 minutes. At this time, the pressure inside the electric furnace was adjusted to 100 kPa. The nitrogen gas used at this time had a purity of 99.9% or more, and the partial pressure of water vapor and the partial pressure of oxygen gas in the electric furnace were both 0.1 kPa or less.

[0139] After that, the temperature in the electric furnace was cooled to 1000°C at a rate of 20°C / min. When the water decomposition temperature (1000°C) was reached, the gas flowing into the electric furnace was switched to a mixed gas of water vapor and nitrogen gas. The flow rate of nitrogen gas in the mixed gas was 50 mL / min. At this time, the pressure in the electric furnace was adjusted to 100 kPa. At this time, the partial pressure of nitrogen gas in the electric furnace was 70 kPa, the partial pressure of water vapor was 30 kPa, and the partial pressure of oxygen gas was 0.1 kPa or less.

[0140] The above-mentioned heat treatment under a nitrogen gas atmosphere (particularly, heat treatment at 1100°C or higher) is the first step in which the thermal reduction reaction of the reaction medium is carried out, and the heat treatment under an atmosphere of a mixed gas of water vapor and nitrogen gas is the second step in which the water decomposition reaction is carried out.

[0141] After the second step was performed for 60 minutes, the gas flowing into the electric furnace was switched to the same nitrogen gas as in the first step (thermal reduction step) and the temperature was raised to the thermal reduction temperature (1400°C), and the same treatment as above was repeated, and the first step and the second step were each performed four times in total. In other words, the set of the first step and the second step was repeated four cycles.

[0142] The amounts of hydrogen and oxygen produced in the thermal reduction and water decomposition processes were calculated from the weight changes of the samples. The reaction activity (amount of hydrogen produced) was evaluated in four consecutive cycles.

[0143] In the above measurements, PtPAN was used as the blank sample, and about 33 mg of Pt plate was used as the reference sample. However, as described above, in Comparative Example 5 (CeO2), the sample amount was 66 mg, so the blank sample amount was also 66 mg.

[0144] [6] Results and Discussion [6-1] Comparison of reaction activity in two-stage water splitting cycles (1) For the reaction media of the examples and comparative examples, the reaction activity (amount of hydrogen produced) in a two-stage water pyrolysis cycle of thermal reduction temperature: 1400°C-water decomposition temperature: 1000°C was examined using thermogravimetric analysis (TGA).

[0145] FIG. 3 shows the TGA weight change when the reaction medium of Example 1 was subjected to a two-stage hydrothermal decomposition cycle.

[0146] In the thermal reduction process, the weight loss occurs because the lattice oxygen is released as oxygen molecules, while in the subsequent water decomposition process, the weight loss of the reduced Fe 0.33 Mg 0.33 Ni 0.33 O x is oxidized by water vapor, in other words, hydrogen is produced, resulting in an increase in weight.

[0147] As described above, in the present invention, it was found that the target reaction proceeds favorably in the first and second steps in the two-stage water thermal decomposition cycle.

[0148] From this weight change, the amounts of hydrogen and oxygen produced were calculated for cycles 2 to 4. The ratio of produced hydrogen to produced oxygen (H2 / O2 molar ratio) was also calculated.

[0149] FIG. 4 is a diagram showing the amounts of hydrogen and oxygen produced and the H2 / O2 molar ratio in each cycle for the reaction medium of Example 1.

[0150] In the first cycle, the unreacted sample is reduced in the thermal reduction step, resulting in a larger amount of oxygen being generated than in a normal cycle reaction, and therefore this was excluded from the data.

[0151] From FIG. 4, it was suggested that in the reaction medium of Example 1, hydrogen and oxygen were produced with good cyclicity, the H2 / O2 molar ratio was close to the stoichiometric ratio of 2, and the reaction proceeded approximately stoichiometrically.

[0152] Fig. 5 is a diagram showing the average amounts of hydrogen and oxygen produced and the H2 / O2 molar ratio per cycle for the reaction media of Example 1 and Comparative Examples 1 to 5. Note that the amounts of hydrogen and oxygen produced and the H2 / O2 molar ratio shown in Fig. 5 are average values ​​for the second to fourth cycles.

[0153] As a result, it was found that the present invention exhibits high reaction activity, and the target reaction proceeds smoothly in the first and second steps.

[0154] In particular, the average hydrogen production amount per cycle of the reaction medium of Example 1, 835 μmol / g, is about 1.47 times that of the MNCFO of Comparative Example 3, 570 μmol / g, and high reaction activity is obtained. The hydrogen production amount of Example 1 was about 2.2 times that of Comparative Example 4 (Mn-doped CeO2) and about 11.3 times that of Comparative Example 5 (CeO2).

[0155] In addition, the average H2 / O2 molar ratio of the reaction medium in Example 1 was approximately 1.82, which is closer to the stoichiometric ratio than the other reaction media, and it can be seen that the reaction proceeds more stably at the stoichiometric ratio.

[0156] As described above, in the present invention, it was found that by excluding Co from MNCFO and increasing the Fe content, hydrogen and oxygen can be generated with good cyclic properties, and a high amount of hydrogen can be produced.

[0157] In Comparative Example 1, in which Ni was omitted from MNCFO, and Comparative Example 2, in which Mg was omitted, a sufficient amount of hydrogen was not produced.

[0158] From the above, it can be seen that by using the reaction medium of the present invention, a high amount of hydrogen can be produced even if the processing temperature in the thermal reduction step is reduced from the conventional high temperature of over 1500° C. to 1400° C. For these reasons, the present invention can reduce the temperature difference of the temperature swing while at the same time lowering the cycle temperature.

[0159] [6-2] Comparison of reaction activity in two-stage water splitting cycles (2) For the reaction medium of Example 1, the reaction activity (amount of hydrogen produced) was examined by changing the thermal reduction temperature in the two-stage water pyrolysis cycle using thermogravimetric analysis (TGA).

[0160] The reaction medium of Example 1 (Fe 0.33 Mg 0.33 Ni 0.33 O x ) was used to produce hydrogen in the same manner as above, except that the treatment temperatures in the first step of the two-step water pyrolysis cycle were 1300°C and 1200°C.

[0161] Fig. 6 is a diagram showing the amounts of hydrogen and oxygen produced and the H2 / O2 molar ratio per cycle when the thermal reduction temperature is changed for the reaction medium of Example 1. The amounts of hydrogen and oxygen produced and the H2 / O2 molar ratio shown in Fig. 6 are average values ​​for the second to fourth cycles.

[0162] As a result, in the present invention, even at a thermal reduction temperature of 1300°C (water decomposition temperature of 1000°C), Fe 0.25 Co 0.25 Ni 0.25 Mg 0.25 O x It was confirmed that the average amount of hydrogen generated was higher than the thermal reduction temperature of (MNCFO), 1400°C (water decomposition temperature, 1000°C).

[0163] [6-3] Melting point The melting points of the reaction media of Example 1, Comparative Example 3 and Comparative Example 6 were examined using a differential scanning calorimeter (DSC) device. FIG. 7 is a diagram showing a schematic configuration of a differential scanning calorimetry (DSC) device.

[0164] The conditions for measuring the heat flow difference are as follows: Pt cup: diameter 5mm, depth 2.5mm Sample: Approximately 2 mg Reference sample: Al3O3 approx. 2mg Temperature: Room temperature to 1500[℃] Atmosphere: N2 gas Holding time: 0 [minutes] Heating rate: 20°C / min N2 gas flow rate: 50 [mL / min]

[0165] FIG. 8 is a diagram showing the results of differential scanning calorimetry for the reaction media of Example 1, Comparative Example 3, and Comparative Example 6.

[0166] In Comparative Example 6, when ordinary Fe3O4 was heated under a nitrogen atmosphere, an endothermic peak appeared around 1230°C due to the melting of FeO produced by the reduction of Fe3O4 → FeO. However, in the reaction medium of Example 1, no endothermic peak was observed.

[0167] These results show that the reaction medium of Example 1 does not melt at a temperature lower than 1500° C. In other words, the melting point of the reaction medium of Example 1 is 1500° C. or higher. [Industrial Applicability]

[0168] The reaction medium of the present invention is used in a method for producing hydrogen by thermal decomposition of water, and is characterized in that it contains a composite metal oxide of Fe, Mg, and Ni, and therefore can provide a reaction medium with high reaction activity.

[0169] The hydrogen production method of the present invention includes a first step of thermally reducing the above-mentioned reaction medium, and a second step of contacting the thermally reduced reaction medium with water to oxidize the reaction medium and generate hydrogen. Therefore, it is possible to provide a hydrogen production method capable of efficiently producing hydrogen.

[0170] Therefore, the reaction medium of the present invention and the method for producing hydrogen of the present invention have industrial applicability. [Explanation of symbols]

[0171] 100...Hydrogen production equipment 10...Reactor (container) 2...Fluidized bed 3...Gas inlet plate 4...Gas introduction means (low oxygen partial pressure gas introduction means) 5...Gas introduction means (water vapor-containing gas introduction means) 6. Window 8...Outlet (oxygen-containing gas outlet) 9...Outlet (hydrogen-containing gas outlet) 20…Ground reflector (sunlight condensing means) 30...Tower reflector (sunlight collecting means) S…Solar light

Claims

1. A reaction medium for use in a method for producing hydrogen by thermal decomposition of water, comprising: A reaction medium comprising a composite metal oxide of Fe, Mg and Ni.

2. The composite metal oxide is Fe ０．３３ Mg ０．３３ Ni ０．３３ O ｘ The reaction medium according to claim 1, wherein

3. A first step of thermally reducing the reaction medium of claim 1; and a second step of contacting the thermally reduced reaction medium with water to oxidize the reaction medium and generate hydrogen.

4. 4. The method for producing hydrogen according to claim 3, wherein the reaction temperature in the first step is 1200° C. or higher and 1400° C. or lower.

5. 4. The method for producing hydrogen according to claim 3, wherein the reaction temperature in the second step is 1000° C. or higher and 1200° C. or lower.

6. The method for producing hydrogen according to claim 3 , wherein a series of processes including the first step and the second step is repeatedly performed.

Images