WO2010016641A1 - Hydrogen production method from water by thermochemical cycles using germanium oxide - Google Patents

Hydrogen production method from water by thermochemical cycles using germanium oxide Download PDF

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WO2010016641A1
WO2010016641A1 PCT/KR2008/006407 KR2008006407W WO2010016641A1 WO 2010016641 A1 WO2010016641 A1 WO 2010016641A1 KR 2008006407 W KR2008006407 W KR 2008006407W WO 2010016641 A1 WO2010016641 A1 WO 2010016641A1
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geo
water
hydrogen
reaction formula
thermochemical
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French (fr)
Inventor
Kyoung-Soo Kang
Chu-Sik Park
Chang-Hee Kim
Ki-Kwang Bae
Won-Chul Cho
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Korea Institute of Energy Research KIER
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Korea Institute of Energy Research KIER
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Priority claimed from KR1020080106287A external-priority patent/KR101001873B1/en
Application filed by Korea Institute of Energy Research KIER filed Critical Korea Institute of Energy Research KIER
Priority to US12/990,726 priority Critical patent/US8470292B2/en
Priority to AU2008360451A priority patent/AU2008360451B2/en
Publication of WO2010016641A1 publication Critical patent/WO2010016641A1/en
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B13/00—Oxygen; Ozone; Oxides or hydroxides in general
    • C01B13/02—Preparation of oxygen
    • C01B13/0203—Preparation of oxygen from inorganic compounds
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen; Reversible storage of hydrogen
    • C01B3/02—Production of hydrogen; Production of gaseous mixtures containing hydrogen
    • C01B3/06—Production of hydrogen; Production of gaseous mixtures containing hydrogen by reaction of inorganic compounds containing electro-positively bound hydrogen with inorganic reducing agents
    • C01B3/061—Production of hydrogen; Production of gaseous mixtures containing hydrogen by reaction of inorganic compounds containing electro-positively bound hydrogen with inorganic reducing agents by reaction of water with metal oxides
    • C01B3/063—Cyclic methods
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/36—Hydrogen production from non-carbon containing sources, e.g. by water electrolysis

Definitions

  • the present invention relates to a hydrogen production method from water by using germanium oxide, more precisely a hydrogen production method from water by thermochemical cycles using germanium oxide.
  • Hydrogen energy is the most promising candidate for alternative energy because it uses water as a raw material which is most abundant on earth, it does not produce any pollutant during hydrogen combustion, suggesting that hydrogen energy is very clean energy, and it is also functioning as a storage medium for energy.
  • the representative methods for producing hydrogen from water are biological method, photochemical method, electrolysis, direct thermolysis and thermochemical degradation. Electrolysis, one of the conventional techniques, is in practical use, but others are still under study. To decompose water directly to produce hydrogen, high temperature of at least 4000 K is required. So, direct decomposition of water is in fact very difficult. Therefore, water decomposition is tried stepwise, precisely a method is designed to contain chemical reactions induced stepwise at comparatively low temperatures to decompose water, which is a close cycle. That is the hydrogen production method by using thermochemical cycle.
  • Thermochemical cycle absorbs heat to convert it into hydrogen and oxygen chemically. This method produces hydrogen from water by multi-step reactions including oxidation and reduction of a metal oxide using heat.
  • Themochemical cycle can be classified into pure thermochemical cycle and combined thermochemical cycle.
  • the combined thermochemical cycle is developed to supplement and improve the pure thermochemical cycle, for which electrolysis or photochemical method is introduced.
  • thermochemical cycle is classified as 2 step cycle, 3 step cycle, 4 step cycle, etc, and up to 8 step cycle has been notified so far.
  • thermochemical cycles including 2 step thermochemical cycle using a metal oxide such as Fe, Mn, Zn, Co, Sn and WO 3 , ZnO/Zn, Fe 3 O 4 ZFeO, CeO 2 /Ce 2 O 3 , and SnO 2 ZSn as a material for thermochemical process.
  • a metal oxide such as Fe, Mn, Zn, Co, Sn and WO 3 , ZnO/Zn, Fe 3 O 4 ZFeO, CeO 2 /Ce 2 O 3 , and SnO 2 ZSn
  • a transition metal such as Ni and Co or a refractory such as alumina can be mixed or alloyed, particularly in those cycles of ZnO/Zn thermochemical cycle, Fe 3 O 4 /FeO thermochemical cycle, CeO 2 ZCe 2 O 3 thermochemical cycle and SnO 2 ZSn thermochemical cycle.
  • thermochemical cycle accompanies non- stoichiometric reaction that is the reduction of some of oxygen in M (Co or Ni, or Ni and Mn) -ferrite, resulting in low hydrogen production with the yield of up to 20 cc/g.
  • thermochemical cycle that facilitates reduction at a low temperature to mass-produce hydrogen from water is required.
  • the present invention is characterized by producing hydrogen from water by thermochemical reaction using germanium oxide .
  • the hydrogen production method of the present invention is characterized by producing hydrogen by multi-step thermochemical cycle using germanium oxide as a redox pair.
  • thermochemical cycle of the hydrogen production method of the present invention characteristically produces hydrogen by GeO 2 ZGeO redox pair, Ge ⁇ 2 /Ge redox pair generated by spontaneous decomposition (disproportion) of GeO
  • GeO generated from the decomposition of GeO 2 is apt to be decomposed spontaneously into 1/2Ge and 1/2GeO 2 again during cooling process as shown in the following reaction formula 3 because of chemical instability. At this time, water is decomposed by the following reaction formula 4 to produce hydrogen.
  • Reduction of GeO 2 is performed at 1000 - 1700 °C in the presence of inert gas under the pressure of 1 atm - 0.001 atm.
  • the reduced GeO according to reaction formula 1, decomposes water (vapor) provided in the step of reaction formula 2 to produce hydrogen and GeO itself is oxidized into GeO 2 .
  • the reaction of reaction formula 2 is performed at 200 - 800°C .
  • GeO obtained in the step of reaction formula 1 is a very unstable material, which is easily decomposed into thermodynamically stable Ge and GeO 2 .
  • reaction of reaction formula 1 when the reaction of reaction formula 1 is completed, GeO is cooled down, during which GeO is decomposed spontaneously into Ge and Ge ⁇ 2 , leading not to the cycle of reaction formula 2 but to the cycles of reaction formulas 3 and 4 to produce hydrogen.
  • Some of GeO generated in the step of reaction formula 1 proceeds to the step of reaction formula 3, so simultaneously with the water decomposition in the step of reaction formula 2, hydrogen can be produced by the water decomposition in the step of reaction formula 4.
  • the reaction of reaction formula 4 is performed at 200 - 800 ° C.
  • thermochemical water decomposition using an oxide of Fe, Ce, Sn or Zn requires a high reduction temperature, making the construction of composition of a reactor difficult. In addition, efficiency is also very low because of heat loss caused by such high temperature .
  • the hydrogen production method of the present invention produces hydrogen by thermochemical cycle using germanium oxide, in which the thermochemical cycle is a low temperature reaction, no other materials than water is consumed and instead they are only circulated, hydrogen production efficiency is high and physical properties of germanium oxide are not damaged by the thermochemical cycle.
  • Figure 1 is a graph illustrating the reaction Gibbs ' free energy changes of GeO 2 decomposition according to the temperature changes
  • Figure 2 is a graph illustrating the changes of equilibrium composition of 1 kmol of GeO 2 in the presence of 1 kmol of inert gas under the pressure of 1 atm according to the temperature changes
  • Figure 3 is a graph illustrating the changes of equilibrium composition of 1 kmol of GeO 2 in the presence of 1 kmol of inert gas under the pressure of 0.001 atm according to the temperature changes
  • Figure 4 is a graph illustrating the reaction Gibbs 1 free energy changes over temperatures when the reduced GeO and water are reacted
  • Figure 5 is a graph illustrating the weight changes and oxygen generation due to GeO 2 decomposition over temperatures measured by thermogravimetry analyzer-mass spectrometer
  • Figure 6 is a photograph illustrating that the condensed and filtered particles from GeO 2 decomposition at 1460 ° C, examined under electron microscope,
  • Figure 7 is a graph illustrating that X-ray diffraction peaks of the condensed and filtered particles from GeO 2 decomposition at 1460°C,
  • Figure 8 is a graph illustrating the hydrogen production
  • Figure 9 is a graph illustrating the result of X-ray diffraction performed after the reaction of water and the product of GeO 2 obtained by the processes of decomposition at 1460°C, cooling and filtering thereof in a fixed-bed reactor.
  • Metal oxides can be reduced by thermochemical decomposition at a high temperature range where Gibb's free energy is negative value. By this reaction, solar energy can be converted into chemical energy as the reduced form of metal oxides.
  • the reduced metal oxide can be used for decomposition of water to produce hydrogen.
  • the hydrogen production method of the present invention characteristically produces hydrogen by reducing germanium oxide by thermochemical decomposition and by reacting the reduced germanium oxide with water (vapor) .
  • the hydrogen production method of the present invention using multi-step thermochemical cycle produces hydrogen by using GeC> 2 /GeO redox pair, GeO 2 ZGe redox pair or the combination thereof.
  • the step of reaction formula 1 is the reduction step of germanium oxide by thermochemical decomposition.
  • Figure 1 illustrates the reaction Gibbs ' free energy changes of GeO 2 decomposition (reduction) according to the temperature changes .
  • germanium oxide (GeO 2 ) used in this invention demonstrates the reaction Gibbs ' free energy change close to 0 at 1500 "C under normal pressure, indicating that spontaneous thermal decomposition can be induced. Even if the reaction Gibb's free energy change is a positive value close to 0, decomposition can be continued as long as the reaction product is constantly eliminated by inert gas (argon, helium) .
  • inert gas argon, helium
  • high temperature gas furnace, concentrated solar heat, nuclear reactor, and waste heat of blast furnace can be used.
  • Figure 2 is a graph illustrating the changes of equilibrium composition of 1 kmol of GeO 2 in the presence of 1 kmol of inert gas under the pressure of 1 atm according to the temperature changes.
  • Spontaneous decomposition of germanium oxide (GeO 2 ) is induced fast at 1500 ° C and when the temperature reaches 1700°C, all germanium oxides (GeO 2 ) are decomposed into GeO.
  • GeO is stable even at 1700 "C or higher.
  • Figure 1 and Figure 2 illustrate the changes of equilibrium composition of germanium oxide over the temperatures in the presence of inert gas under the pressure of 1 atm.
  • Figure 3 illustrates the changes of equilibrium composition of 1 kmol of GeO 2 over the temperatures in the presence of 1 kmol of inert gas under the pressure of 0.001 atm.
  • germanium oxide begins to be decomposed spontaneously at 1000 ° C and when the temperature reaches 1200 ° C, all germanium oxides (GeO 2 ) are decomposed into
  • thermochemical reduction (decomposition) of GeO 2 presented in reaction formula 1 is preferably performed in the presence of inert gas, at
  • thermochemical reduction of GeO 2 at 1000 ° C - 1500 "C under the pressure of 0.1 atm - 0.001 atm.
  • the gas phase GeO (g) obtained by thermochemical reduction (decomposition) according to reaction formula 1 is preferably recovered by fast quenching and the recovered GeO (s) is reacted with water to produce hydrogen as shown in reaction formula 2.
  • the mixture of Ge and GeO 2 generated by spontaneous decomposition (disproportion) of GeO can also be recovered along with GeO.
  • the recovered mixture (GeO, Ge, GeO 2 ) is reacted with water to produce hydrogen according to reaction formula 2 and reaction formula 4.
  • reaction formula 4 When all the GeO obtained in the step of reaction formula 1 are decomposed and the mixture of Ge and GeO 2 is recovered by fast quenching, the recovered mixture (Ge, GeO 2 ) is reacted with water to produce hydrogen according to reaction formula 4.
  • Figure 4 illustrates the reaction Gibb's free energy values over the temperatures when the reduced GeO and water are reacted.
  • reaction Gibb's free energy is negative value at the temperature of at lest 200°C, suggesting that the reaction is spontaneous .
  • Reaction of the reduced GeO or Ge generated from GeO and water (vapor) presented in reaction formula 2 or reaction formula 4 is preferably performed at 200 "C - 800 ° C where spontaneous decomposition is induced.
  • reaction speed is so slow that hydrogen production rate is decreased.
  • reaction is induced at higher than 800 ° C, vaporization of GeO is so strong that construction of a production reactor is difficult.
  • Hydrogen can be produced from water by the said multi- step thermochemical cycles. And, satisfactory conversion rate of chemical reaction and production yield of hydrogen can be accomplished by the thermochemical cycles composed of spontaneous reaction at the said conditions (reaction temperature and pressure) .
  • FIG. 6 is a SEM photograph showing the collected product. As shown in Figure 6, the round shaped product (30-60 nm in diameter) was obtained. As shown in Figure 7, the obtained product was confirmed by X-ray diffraction as the mixture of Ge and GeO 2 .
  • Example 3 0.11 g of the collected product on the filter in Example 2 was loaded in a fixed-bed reactor, and then temperature was raised to room temperature to 750°C at the speed of 20 ° C/min.
  • the temperature was maintained at 750 "C for 30 minutes, and then raised again at the speed of 20°C/min to 800 ° C. The temperature was maintained at 800°C for 1 hour.
  • Ar gas was supplied at the flow speed of 18 cc/min until the temperature was raised to 150 "C from room temperature. When the temperature of the reactor reached 150 ° C, Ar gas was saturated with 70 "C water vapor, which was provided into the reactor at the speed of 18 cc/min.
  • the reaction product was cooled down to eliminate moisture, followed by sampling at 2 minutes interval. Hydrogen concentration was measured by gas chromatography (Agilent 7890, TCD) . The amount of generated hydrogen was calculated by hydrogen concentration, gas flow rate and sampling time.
  • the red line in Figure 8 indicates the changes of temperature over the time and the black line indicates hydrogen amount.
  • hydrogen began to be generated from the temperature of 300 ° C . And, when the temperature reached 600 ° C , hydrogen was generated massively. The total amount of hydrogen generated was 28 cc.
  • X-ray diffraction was performed with the sample after water decomposition. As a result, as shown in Figure 9, GeO 2 comprising the two structures of tetragonal and hexagonal was obtained.

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Abstract

The present invention relates to a hydrogen production method from water by using germanium oxide, more precisely a hydrogen and oxygen production method from water by thermochemical cycles using germanium oxide. The method of the present invention facilitates the production of hydrogen by multi-step thermochemical cycle using germanium oxide, so that it is characterized by that the thermochemical cycle is low temperature reaction and only water is consumed and other materials are not consumed but circulated.

Description

[DESCRIPTION]
[invention Title]
HYDROGEN PRODUCTION METHOD FROM WATER BY THERMOCHEMICAL CYCLES USING GERMANIUM OXIDE
[Technical Field]
The present invention relates to a hydrogen production method from water by using germanium oxide, more precisely a hydrogen production method from water by thermochemical cycles using germanium oxide.
[Background Art]
As fossil fuel such as petroleum, coal and natural gas is being exhausted and problems caused by the fossil fuel such as environmental contamination and global warming get serious, it is urgently requested to develop clean energy source to replace the fossil fuel.
As an effort to develop alternative energy for fossil fuel, studies have been undergoing to convert natural energy such as solar heat, wind force and tidal power directly into electric power and to produce hydrogen energy from natural resources such as water.
Hydrogen energy is the most promising candidate for alternative energy because it uses water as a raw material which is most abundant on earth, it does not produce any pollutant during hydrogen combustion, suggesting that hydrogen energy is very clean energy, and it is also functioning as a storage medium for energy. The representative methods for producing hydrogen from water are biological method, photochemical method, electrolysis, direct thermolysis and thermochemical degradation. Electrolysis, one of the conventional techniques, is in practical use, but others are still under study. To decompose water directly to produce hydrogen, high temperature of at least 4000 K is required. So, direct decomposition of water is in fact very difficult. Therefore, water decomposition is tried stepwise, precisely a method is designed to contain chemical reactions induced stepwise at comparatively low temperatures to decompose water, which is a close cycle. That is the hydrogen production method by using thermochemical cycle.
Thermochemical cycle absorbs heat to convert it into hydrogen and oxygen chemically. This method produces hydrogen from water by multi-step reactions including oxidation and reduction of a metal oxide using heat.
Themochemical cycle can be classified into pure thermochemical cycle and combined thermochemical cycle. The combined thermochemical cycle is developed to supplement and improve the pure thermochemical cycle, for which electrolysis or photochemical method is introduced.
According to the chemical reaction step, thermochemical cycle is classified as 2 step cycle, 3 step cycle, 4 step cycle, etc, and up to 8 step cycle has been notified so far.
Up to date, approximately 300 thermochemical cycles have been reported including 2 step thermochemical cycle using a metal oxide such as Fe, Mn, Zn, Co, Sn and WO3, ZnO/Zn, Fe3O4ZFeO, CeO2/Ce2O3, and SnO2ZSn as a material for thermochemical process.
To increase reactivity and thermal stability at high temperature, a transition metal such as Ni and Co or a refractory such as alumina can be mixed or alloyed, particularly in those cycles of ZnO/Zn thermochemical cycle, Fe3O4/FeO thermochemical cycle, CeO2ZCe2O3 thermochemical cycle and SnO2ZSn thermochemical cycle.
(ZnOZZn cycle)
ZnO → Zn +1Z2O2 2000 °C
Zn + H2O → ZnO + H2 400 "C (Fe3O4ZFeO cycle)
Fe3O4 → 3FeO + 1Z2O2 2000 °C
3FeO + H2O → Fe3O4 + H2 400 °C
(CeO2ZCe2O3 cycle)
2CeO2 → Ce2O3 + 1Z2O2 2000 "C 3Ce2O3 + H2O → 2CeO2 + H2 400 °C (SnO2/Sn cycle)
1/2SnO2 → 1/2Sn + 1/2O2 2600 °C 1/2Sn + H2O → 1/2SnO2 + H2 400 °C The above cycles require high reduction temperature of at least 2000°C. So, construction of composition for a reactor is difficult. The temperature of at least 2000 "C can be obtained by concentrating solar heat, however, it is very difficult to use that heat efficiently because of reflection. There was a report on 2 step thermochemical cycle in relation to hydrogen production by water decomposition after thermal reduction at 1000-1400°C using Co-ferrite, Ni-Mn- ferrite and Ni-ferrite. However, the thermochemical cycle accompanies non- stoichiometric reaction that is the reduction of some of oxygen in M (Co or Ni, or Ni and Mn) -ferrite, resulting in low hydrogen production with the yield of up to 20 cc/g.
Therefore, thermochemical cycle that facilitates reduction at a low temperature to mass-produce hydrogen from water is required.
[Disclosure] [Technical Problem]
It is an object of the present invention to provide a hydrogen mass -production method from water by low temperature thermochemical cycle using a specific metal oxide that is highly reactive and stable in thermochemical cycle.
[Technical Solution]
The present invention is characterized by producing hydrogen from water by thermochemical reaction using germanium oxide .
Precisely, the hydrogen production method of the present invention is characterized by producing hydrogen by multi-step thermochemical cycle using germanium oxide as a redox pair.
More precisely, the thermochemical cycle of the hydrogen production method of the present invention characteristically produces hydrogen by GeO2ZGeO redox pair, Geθ2/Ge redox pair generated by spontaneous decomposition (disproportion) of GeO
(GeO → 1/2Ge + 1/2GeO2) of Ge02/Ge0 redox pair or combination thereof .
According to the production method of the present invention, hydrogen is produced by the thermochemical cycles of the following reaction formulas 1 and 2.
(Reaction Formula 1)
GeO2 → GeO + 1/2O2
(Reaction Formula 2)
GeO + H2O (g) → GeO2 + H2 In reaction formula 1, GeO generated from the decomposition of GeO2 is apt to be decomposed spontaneously into 1/2Ge and 1/2GeO2 again during cooling process as shown in the following reaction formula 3 because of chemical instability. At this time, water is decomposed by the following reaction formula 4 to produce hydrogen.
(Reaction Formula 1)
GeO2 → GeO + 1/2O2
(Reaction Formula 3) GeO → 1/2Ge + 1/2GeO2
(Reaction Formula 4)
1/2Ge + 1/2GeO2 + H2O → GeO2 + H2
Reduction of GeO2, as shown in reaction formula 1, is performed at 1000 - 1700 °C in the presence of inert gas under the pressure of 1 atm - 0.001 atm. The reduced GeO, according to reaction formula 1, decomposes water (vapor) provided in the step of reaction formula 2 to produce hydrogen and GeO itself is oxidized into GeO2. At this time, the reaction of reaction formula 2 is performed at 200 - 800°C . As explained hereinbefore, GeO obtained in the step of reaction formula 1 is a very unstable material, which is easily decomposed into thermodynamically stable Ge and GeO2.
Accordingly, when the reaction of reaction formula 1 is completed, GeO is cooled down, during which GeO is decomposed spontaneously into Ge and Geθ2, leading not to the cycle of reaction formula 2 but to the cycles of reaction formulas 3 and 4 to produce hydrogen. Some of GeO generated in the step of reaction formula 1 proceeds to the step of reaction formula 3, so simultaneously with the water decomposition in the step of reaction formula 2, hydrogen can be produced by the water decomposition in the step of reaction formula 4. At this time, the reaction of reaction formula 4 is performed at 200 - 800°C.
[Advantageous Effects]
The conventional hydrogen production method by thermochemical water decomposition using an oxide of Fe, Ce, Sn or Zn requires a high reduction temperature, making the construction of composition of a reactor difficult. In addition, efficiency is also very low because of heat loss caused by such high temperature .
The hydrogen production method of the present invention produces hydrogen by thermochemical cycle using germanium oxide, in which the thermochemical cycle is a low temperature reaction, no other materials than water is consumed and instead they are only circulated, hydrogen production efficiency is high and physical properties of germanium oxide are not damaged by the thermochemical cycle. [Description of Drawings]
The above and other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:
Figure 1 is a graph illustrating the reaction Gibbs ' free energy changes of GeO2 decomposition according to the temperature changes, Figure 2 is a graph illustrating the changes of equilibrium composition of 1 kmol of GeO2 in the presence of 1 kmol of inert gas under the pressure of 1 atm according to the temperature changes,
Figure 3 is a graph illustrating the changes of equilibrium composition of 1 kmol of GeO2 in the presence of 1 kmol of inert gas under the pressure of 0.001 atm according to the temperature changes,
Figure 4 is a graph illustrating the reaction Gibbs1 free energy changes over temperatures when the reduced GeO and water are reacted,
Figure 5 is a graph illustrating the weight changes and oxygen generation due to GeO2 decomposition over temperatures measured by thermogravimetry analyzer-mass spectrometer,
Figure 6 is a photograph illustrating that the condensed and filtered particles from GeO2 decomposition at 1460°C, examined under electron microscope,
Figure 7 is a graph illustrating that X-ray diffraction peaks of the condensed and filtered particles from GeO2 decomposition at 1460°C,
Figure 8 is a graph illustrating the hydrogen production
(black line) over the temperatures (red line) according to the reaction of the product generated by a series of decomposition of GeO2 at 1460°C; cooling; and filtering thereof and water in a fixed-bed reactor,
Figure 9 is a graph illustrating the result of X-ray diffraction performed after the reaction of water and the product of GeO2 obtained by the processes of decomposition at 1460°C, cooling and filtering thereof in a fixed-bed reactor.
[Best Mode]
The hydrogen production method of the present invention is described in more detail hereinafter with the attached figures. The following figures are only examples provided to those in the art to deliver the idea of the present invention to lead them to full understand. Therefore, the present invention is not limited thereto and can be embodied by other forms. The same reference numbers in this description indicate the same constituents. Technical terms and scientific terms used in this invention are understood as general meanings that can be understood by those in the art, unless stated otherwise. So, explanations on general functions and compositions that might mislead them off the idea of the present invention are omitted herein.
Metal oxides can be reduced by thermochemical decomposition at a high temperature range where Gibb's free energy is negative value. By this reaction, solar energy can be converted into chemical energy as the reduced form of metal oxides. The reduced metal oxide can be used for decomposition of water to produce hydrogen.
The hydrogen production method of the present invention characteristically produces hydrogen by reducing germanium oxide by thermochemical decomposition and by reacting the reduced germanium oxide with water (vapor) .
Precisely, the hydrogen production method of the present invention using multi-step thermochemical cycle produces hydrogen by using GeC>2/GeO redox pair, GeO2ZGe redox pair or the combination thereof.
When GeO2/GeO redox pair is used, hydrogen is produced by thermochemical cycles of the following reaction formulas 1 and 2.
(Reaction Formula 1) GeO2 → GeO + 1/2O2
(Reaction Formula 2)
GeO + H2O (g) → GeO2 + H2
When Geθ2/Ge redox pair is used, hydrogen is produced by thermochemical cycles of the following reaction formulas 1, 3 and 4.
(Reaction Formula 1)
GeO2 → GeO + 1/2O2
(Reaction Formula 3) GeO → 1/2Ge + 1/2GeO2
(Reaction Formula 4)
1/2Ge + 1/2GeO2 + H2O → GeO2 + H2
The step of reaction formula 1 is the reduction step of germanium oxide by thermochemical decomposition. Figure 1 illustrates the reaction Gibbs ' free energy changes of GeO2 decomposition (reduction) according to the temperature changes .
As shown in Figure 1, germanium oxide (GeO2) used in this invention demonstrates the reaction Gibbs ' free energy change close to 0 at 1500 "C under normal pressure, indicating that spontaneous thermal decomposition can be induced. Even if the reaction Gibb's free energy change is a positive value close to 0, decomposition can be continued as long as the reaction product is constantly eliminated by inert gas (argon, helium) . As a source of heat for the thermal decomposition, high temperature gas furnace, concentrated solar heat, nuclear reactor, and waste heat of blast furnace can be used.
Figure 2 is a graph illustrating the changes of equilibrium composition of 1 kmol of GeO2 in the presence of 1 kmol of inert gas under the pressure of 1 atm according to the temperature changes. Spontaneous decomposition of germanium oxide (GeO2) is induced fast at 1500 °C and when the temperature reaches 1700°C, all germanium oxides (GeO2) are decomposed into GeO. GeO is stable even at 1700 "C or higher.
Figure 1 and Figure 2 illustrate the changes of equilibrium composition of germanium oxide over the temperatures in the presence of inert gas under the pressure of 1 atm. Figure 3 illustrates the changes of equilibrium composition of 1 kmol of GeO2 over the temperatures in the presence of 1 kmol of inert gas under the pressure of 0.001 atm.
As shown in Figure 3 , in the presence of inert gas under the pressure of 0.001 atm, germanium oxide begins to be decomposed spontaneously at 1000 °C and when the temperature reaches 1200°C, all germanium oxides (GeO2) are decomposed into
GeO.
As shown in Figure 1 - Figure 3 , the thermochemical reduction (decomposition) of GeO2 presented in reaction formula 1 is preferably performed in the presence of inert gas, at
1000 °C - 1700 "C, under the pressure of 1 atm - 0.001 atm.
Considering the low temperature thermochemical process and thermal efficiency, it is more preferred to perform the thermochemical reduction of GeO2 at 1000 °C - 1500 "C under the pressure of 0.1 atm - 0.001 atm.
The gas phase GeO (g) obtained by thermochemical reduction (decomposition) according to reaction formula 1 is preferably recovered by fast quenching and the recovered GeO (s) is reacted with water to produce hydrogen as shown in reaction formula 2.
In the step of fast quenching, the mixture of Ge and GeO2 generated by spontaneous decomposition (disproportion) of GeO can also be recovered along with GeO. At this time, the recovered mixture (GeO, Ge, GeO2) is reacted with water to produce hydrogen according to reaction formula 2 and reaction formula 4.
When all the GeO obtained in the step of reaction formula 1 are decomposed and the mixture of Ge and GeO2 is recovered by fast quenching, the recovered mixture (Ge, GeO2) is reacted with water to produce hydrogen according to reaction formula 4.
Figure 4 illustrates the reaction Gibb's free energy values over the temperatures when the reduced GeO and water are reacted. When the reduced GeO is reacted with water, reaction Gibb's free energy is negative value at the temperature of at lest 200°C, suggesting that the reaction is spontaneous .
Reaction of the reduced GeO or Ge generated from GeO and water (vapor) presented in reaction formula 2 or reaction formula 4 is preferably performed at 200 "C - 800 °C where spontaneous decomposition is induced. When the reaction is induced at lower than 200°C, reaction speed is so slow that hydrogen production rate is decreased. In the meantime, when the reaction is induced at higher than 800°C, vaporization of GeO is so strong that construction of a production reactor is difficult.
Hydrogen can be produced from water by the said multi- step thermochemical cycles. And, satisfactory conversion rate of chemical reaction and production yield of hydrogen can be accomplished by the thermochemical cycles composed of spontaneous reaction at the said conditions (reaction temperature and pressure) .
(Example 1)
43 mg of GeO2 powder (99.98%, Aldrich) was put in a 100 ul platinum crucible, which was installed in thermogravimetry analyzer (SETARAM, SETSYS Evolution, TGA_DSC) connected with mass -spectroscopy. Ar gas was provided to the thermogravimetry analyzer (TGA) at 200 cc/min. The temperature was raised from room temperature to 1480°C at the heating rate of 3°C/min. The temperature was maintained at 1480°C for 2 hours, during which weight changes and oxygen generation by GeO2 decomposition were observed. As shown in Figure 5, GeO2 began to be decomposed at around 1400°C and when the temperature reached 1480°C, decomposition was completed and oxygen generation was confirmed.
(Example 2)
0.145 g of GeO2 powder (99.98%, Aldrich) was put in a 10mm x 10 mm x 5 mm platinum crucible, which was fixed in a reactor composed of quartz tube of 20 mm in outer diameter. To cool down the product generated from the reactor, cooling coil through which cooling water was flowing was quipped to the outlet of the reactor. To collect the decomposed product, sintered metal filter having the pore size of 15 um was installed in the inlet of the reactor. Ar gas was provided to the reactor with a flow rate of 100 cc/min. Temperature of the reactor was regulated by Mo-Si heater using B-type thermocouple and temperature controller. The temperature was raised from room temperature to 1460°C for 50 minutes and then maintained for 2 hours, during which GeO2 was decomposed. Upon completion of the reaction, the temperature of the reactor was cooled down to room temperature and the product collected on the filter was analyzed by electron microscope and X-ray diffraction. Figure 6 is a SEM photograph showing the collected product. As shown in Figure 6, the round shaped product (30-60 nm in diameter) was obtained. As shown in Figure 7, the obtained product was confirmed by X-ray diffraction as the mixture of Ge and GeO2.
(Example 3) 0.11 g of the collected product on the filter in Example 2 was loaded in a fixed-bed reactor, and then temperature was raised to room temperature to 750°C at the speed of 20°C/min.
The temperature was maintained at 750 "C for 30 minutes, and then raised again at the speed of 20°C/min to 800°C. The temperature was maintained at 800°C for 1 hour. In the inside of the reactor, Ar gas was supplied at the flow speed of 18 cc/min until the temperature was raised to 150 "C from room temperature. When the temperature of the reactor reached 150 °C, Ar gas was saturated with 70 "C water vapor, which was provided into the reactor at the speed of 18 cc/min. The reaction product was cooled down to eliminate moisture, followed by sampling at 2 minutes interval. Hydrogen concentration was measured by gas chromatography (Agilent 7890, TCD) . The amount of generated hydrogen was calculated by hydrogen concentration, gas flow rate and sampling time. The red line in Figure 8 indicates the changes of temperature over the time and the black line indicates hydrogen amount. As shown in Figure 8, hydrogen began to be generated from the temperature of 300 °C . And, when the temperature reached 600 °C , hydrogen was generated massively. The total amount of hydrogen generated was 28 cc. X-ray diffraction was performed with the sample after water decomposition. As a result, as shown in Figure 9, GeO2 comprising the two structures of tetragonal and hexagonal was obtained.
Those skilled in the art will appreciate that the conceptions and specific embodiments disclosed in the foregoing description may be readily utilized as a basis for modifying or designing other embodiments for carrying out the same purposes of the present invention. Those skilled in the art will also appreciate that such equivalent embodiments do not depart from the spirit and scope of the invention as set forth in the appended claims.

Claims

[CLAIMS] [Claim l]
A hydrogen production method from water by thermochemical cycles, which is characterized by producing hydrogen by thermochemical cycles using germanium oxide according to reaction formulas 1 and 2. (Reaction Formula 1) GeO2 → GeO + 1/2O2 (Reaction Formula 2) GeO + H2O (g) → GeO2 + H2
[Claim 2]
A hydrogen production method from water by thermochemical cycles, which is characterized by producing hydrogen by thermochemical cycles using germanium oxide according to reaction formulas 1, 3 and 4. (Reaction Formula 1) GeO2 → GeO + 1/2O2 (Reaction Formula 3) GeO → 1/2Ge + 1/2GeO2 (Reaction Formula 4) 1/2Ge + 1/2GeO2 + H2O → GeO2 + H2
[Claim 3] The hydrogen production method from water by thermochemical cycles according to claim 1 or claim 2, wherein the step of reaction formula 1 is performed in the presence of inert gas, at 1000°C - 1700°C, under the pressure of 1 atm - 0.001 atm.
[Claim 4]
The hydrogen production method from water by thermochemical cycles according to claim 1, wherein the step of reaction formula 2 is performed at 200°C - 800°C .
[Claim 5]
The hydrogen production method from water by thermochemical cycles according to claim 2, wherein the step of reaction formula 4 is performed at 200°C - 800°C .
[Claim 6]
The hydrogen production method from water by thermochemical cycles according to claim 5, wherein the reaction presented in reaction formula 3 is spontaneous decomposition of GeO generated in the step of reaction formula
1 during cooling.
PCT/KR2008/006407 2008-08-06 2008-10-30 Hydrogen production method from water by thermochemical cycles using germanium oxide Ceased WO2010016641A1 (en)

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