N. Hasegawa

Thermodynamic evaluation of water splitting by a cation-excessive (Ni, Mn) ferrite

Water-splitting potential by cation-excessive (Ni, Mn) Ferrite, Ni0.5(1+e)Mn0.50(1+e)Fe1.99(1+e)O4 was evaluated using the standard Gibbs free energy change (ΔG°) for the cation-excessive ferrite formation in different O2 partial pressures. The cation-excessive degree e ranged from 0.026 to 0.16 in pO2 values of 7.9×10−7 to 1.0×10−1. From the relation between e value of (Ni, Mn) ferrite and oxygen partial pressure, equilibrium constant K(th) was determined. Furthermore ΔH°s for O2 releasing and water-splitting reactions with cation-excessive (Ni, Mn) ferrite were evaluated from the vant Hoff plot and compared with that for magnetite-wustite system; where ΔH°s were assumed to be the same values for both (Ni, Mn) ferrite and magnetite–wustite system: +300 kJ for O2 releasing and −35 ∼ −63 kJ for water-splitting. ΔG°s evaluated for water-splitting with cation-excessive (Ni, Mn) ferrite and wustite were −38 kJ and −35 kJ, respectively, at 298K. It suggests that water splitting with cation-excessive (Ni, Mn) ferrite proceed easily compared with that with wustite. ΔS°s for water-splitting are −0.93 kJ K−1 for the former and −0.83 kJ K−1 for the latter. H2 generation rates by reaction with H2O for (Ni, Mn) ferrite were studied at temperatures of 573 K–1073 K. It reached the maximum at 1000 K while it proceeds preferentially below 830 K from thermodynamics.

SOLAR HYDROGEN PRODUCTION BY USING FERRITES

Abstract Hydrogen production by water splitting with Mn(II) ferrite and CaO (or Na2CO3) at 1273 K (or 873 K) was studied. The mixed powder of MnFe2O4 and CaO (CaO/MnFe2O4>3) (or Na2CO3) generates H2 by reaction with H2O at 1273 K (or 873 K). This H2 evolution is caused by the oxidation of Mn(II) ion in the ferrite to the Mn(III) ion. The temperature of 873 K is considerably lower for the solar furnace reaction (O2 releasing step) in the two-step water splitting (1500–2300 K) process. This lower temperature and economical availability of required elements would permit further progress in the direct solar energy absorption/conversion into H2.

Stoichiometric studies of H2 generation reaction for H2O/Zn/Fe3O4 system

Abstract Water splitting with H2O/Zn/Fe3O4 reaction system is suggested for H2 generation in order to utilize concentrated solar heat. This system has the possibility of generating more H2 per 1 mol of Zn than Zn/H2O reaction system at the same temperature, which is noted recently. When a mixture of Zn and Fe3O4 was heated to 873 K and steam was passed through, H2 was obtained in 93.4% of the theoretical yield. The formation of Zn-ferrites with high content of zinc (ZnxFe3−xO4; 0.2⩽x⩽1) and ZnO was determined by XRD and Mossbauer spectroscopy. The yield of ZnO was 26–29%, which shows that the ratio of the competition-reaction (Zn/H2O system) has less effects on the total H2 yield. The stoichiometry and the reaction mechanism of the system are discussed upon the analyses for the solid product.

Synthesis of hydrotalcite with high layer charge for CO2 adsorbent

Abstract Hydrotalcite-like compounds (HT) with 24% to 48% Al 3+ -substitution have been synthesized in the Mg 2+ -Al 3+ -Fe(CN) 6 4- system. Conditioning of the synthesized and air-dried compound with K 4 Fe(CN) 6 4- solution at 80°C was essential to obtain the 80–90% pure ionic Fe(CN) 6 4- form on an equivalent basis. A linear decrease in a o with an increase in the mole ratio of R=Al 3+ /(Mg 2+ +Al 3+ ) was extended to R=0.48. The CO 2 adsorption profiles were dependent upon both the interlayer distance and the Al 3+ -substitution. The expanded space with the large anion Fe(CN) 6 4- can accommodate more effectively CO 2 gas in comparison with the NO 3 − and mixed ionic forms. The optimum space and charge density in the interlayer as a CO 2 adsorption field could be found on the hydrotalcite with the Al 3+ -substitution of 37%. The isosteric heat of CO 2 adsorption was 43.3 kJ mol −1 in the range of adsorption of 20 to 40 cm 3 g −1 at 298 K and 0.1 MPa.

Two-Step Water Splitting Process With Solid Solution of YSZ and Ni-Ferrite for Solar Hydrogen Production (ISEC 2005-76151)

Ni-ferrite (NiFe 2 0 4 ) is a promising reactive ceramics of the ferrite for the solar hydrogen production by a two-step water splitting process using concentrated solar energy. However, it should be pretreated before H 2 -generation reaction by grinding the Ni-ferrite sintered after the O 2 -releasing reaction to make a fine powder If the Ni-ferrite and yttria stabilized zirconia (YSZ) form a solid solution between these oxides (YSZ/NiFe 2 0 4 solid solution =YSZ(Ni,Fe)), it is expected that the two-step water splitting process with the Ni-ferrite system can proceed without treatment of the reduced product because of the high thermal stability of the YSZ/NiFe 2 0 4 solid solution. The YSZ/NiFe 2 0 4 solid solution was prepared by calcination of the mixture of the YSZ balls and NiFe 2 0 4 powder at T=1823 K for 1 h, and its reactivity and thermal stability were examined for the two-step water splitting process. During the ten times repetition of the two-step water splitting reaction (T = 1773 K for 0 2 -releasing, and 1473 K for H 2 -generation) with the YSZ/NiFe 2 0 4 solid solution using infrared imaging furnace, the reactivity for O 2 -releasing and H 2 -generation was kept constant. The molar ratio of the released O 2 gas volume (the average O 2 gas, 1.9 cm 3 /g) and the generated H 2 gas volume (the average H 2 gas, 3.8 cm 3 / g) was nearly 1:2, indicating that the water decomposition process via two steps proceeds. The X-ray diffractometry (XRD) measurement showed that the YSZ(Ni,Fe) keeps the YSZ phase structure during the ten times repetition of the two-step water splitting process. The successive H 2 gas production by the two-step water splitting process was performed (ten times repetition of the two-step water splitting process). From comparative study on the reactivity and the thermal stability for the two-step water splitting reaction among the YSZ/NiFe 2 0 4 solid solution, NiFe 2 0 4 and ZnFe 2 0 4> it is concluded that the YSZ/NiFe 2 0 4 solid solution is superior to the others.

Stoichiometric consideration of steam reforming of methane on Ni/Al2O3 catalyst at 650°C by using a solar furnace simulator

Stoichiometry of the steam reforming of methane (CH4+H2O=CO+3H2) was studied using a steam generator, which can keep the steam content in the reactant gases (methane and steam). The reactor was irradiated using the concentrated Xe-lamp beam (solar simulator) to 650°C. The mole ratio of H2O/CH4 in the reactant gases was kept 1/1, and the Ni/Al2O3 catalyst was used. The deposited C and unreacted H2O amounts were calculated by the mass balance equations with the assumption that inlet gases of CH4+H2O produce outlet gases of CO, CO2, H2, CH4 and H2O, and Carbon. The gaseous contents in the outlet gases from the reactor were estimated from the data of the gas chromatography. Those values and the calculated C and H2O values were close to those of equilibrium condition theoretically evaluated using thermodynamic data at 650°C. The inlet gases controlled at the mole ratio of H2O/CH4=1/1 converted to CO and CO2 (CO/CO2=3/1) stoichiometrically on the Ni/Al2O3 catalyst at 650°C.

Characterization of carbon deposited from carbon dioxide on oxygen-deficient magnetites

Oxygen-deficient magnetite (ODM) powder has been reacted with CO2 at 300 °C; black particles of carbon separated from the ODM were studied by Raman, energy-dispersive X-ray (EDX) and wave-dispersive X-ray (WDX) spectroscopies. The carbon was a mixture of graphite and amorphous carbon in at least two levels of crystallization. Temperature-programmed desorption–mass spectrometry (TPD-MS) showed that a large portion of the carbon deposited on the ODM was released as CO and CO2 gas in a flow of Ar at ca. 520 °C, indicating that the carbon particles formed on the ODM are reactive. X-Ray photoelectron spectra of the carbon-deposited ODM showed no indication of carbide (Fe3C) or α-Fe phase formation.

Development of Reactive Ceramics for Conversion of Concentrated Solar Heat Into Solar Hydrogen With Two-Step Water-Splitting Reaction

The reactive ceramics suitable for the rotary-type solar reactor (proposed by Tokyo Institute of Technology, Tokyo) with two-step water-splitting reaction were developer. It is confirmed that O 2 gas is evolved in the two-step water-splitting reaction with the reactive ceramics vigorously by rapid heating (α-O 2 -releasing reaction). The α-O 2 -releasing reaction is due to the formation of interstitial defect and the conversion of lattice oxygen into O 2 gas at a nonequilibrium state. Reactive ceramics (NiFe 2 O 4 and yttria stabilized zirconia (YSZ)-NiFe 2 O 4 solid solution) can absorb solar thermal energy and convert thermal energy into chemical energy under high O 2 partial pressure atmosphere in the α-O 2 -releasing reaction. Repetitive evolutions of O 2 gas were observed in the two-step water-splitting reaction with YSZ-Fe 3 O 4 solid solution and cerium based metal oxides (CeO 2 ―NiO, CeO 2 ―ZrO 2 , and CeO 2 ―Ta 2 O 5 ) at high O 2 partial pressure. The CeO 2 ―Ta 2 O 5 (Ce: Ta = 90: 10) released a large amount of O 2 gas (3.95 cm 3 /g) in the α-O 2 releasing reaction in the flow of air.

Synthesis of carbon-iron(II) oxide layer on the surface of magnetite and its reactivity with H2O for hydrogen generation

Synthesis of a carbon-iron(II) oxide layer on the surface of magnetite and its reactivity with H2O for hydrogen generation reaction have been studied. X-ray diffractometry and chemical analysis showed that the carbon-bearing magnetite synthesized by the carbon-deposition reaction from CO2 gas with the hydrogen-reduced magnetite, was magnetite with a carbon-iron(II) oxide layer (CIO layer-M; M is stoichiometric magnetite) represented by (Fe3O4)1−δ (Fe3O3)δCτ. The TG-MS spectra also showed the evidence for the formation of the CIO layer on the bulk stoichiometric magnetite. Some amorphous phase was formed in the CIO layer during the activation step (in vacuo at 300 °C for 30 min). This amorphous phase reacted with H2O and evolved H2 gas at 350 °C. This H2 generation reaction resulted in the oxidation of the CIO layer into γ-Fe2O3 component and the release of a part of carbon in the CIO layer as CO2. The X-ray diffractometry and Mössbauer spectroscopy indicated that the solid solution of Fe3O4-γ-Fe2O3 was formed in the solid phase after the H2 generation reaction. This shows the cation movement in the B site of the bulk magnetite of the activated CIO layer-M during the H2 generation reaction. The TG-MS spectra also supported the above estimation.

Reaction mechanism of H2 generation for H2O/Zn/Fe3O4 system

Abstract Several ideas on the reaction mechanism of H2O/Zn/Fe3O4 water splitting system were considered from the aspect of Zn mobility. The vapor deposition of Zn onto the Fe3O4 surface was confirmed by H2 generation reaction with Zn and Fe3O4 set separately. XPS measurements suggested that the surface of Fe3O4 is covered by Zn before the reaction with steam. The rapid reaction process has been supported by this Zn deposition, which greatly enlarges the number of sites of Zn/Fe3O4 pair that is ready to react with steam. In order to keep the Zn from vaporizing off the system, the Zn/Fe3O4 mixture was covered with additional Fe3O4 to capture the Zn vapor. This resulted in improved H2 yield of 99.5%.