Masamichi Tsuji

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.

Synthesis of CuAl hydrotalcite like compound and its ion exchange property

Abstract An attempt of synthesis of copper-aluminum hydrotalcite like compound (CuAl-HTLC) was carried out by adding a 2M sodium carbonate solution to a mixture of 0.1M copper nitrate and aluminium nitrate solutions with different molar ratios of Cu ( Cu + Al ) and CO 3 ( Cu + Al ) at 30°C. The CuAl-HTLC obtained has a layered structure which belongs to the monoclinic system with the lattice parameters of a = 15.32 A , b = 2.900 A , c = 5.861 A , and β = 100.327°, with A type of Bravis lattice, and showed anion exchange properties for mono- and divalent anions in a range of pH 6–9.

Decomposition of carbon dioxide to carbon by hydrogen-reduced Ni(II)-bearing ferrite

Hydrogen-activated Ni(II)-bearing ferrite, Ni0.372+Fe0.492+Fe2.093+O4.00, showed a high rate of decomposition of carbon dioxide to carbon at 300°C. This is based on the redox process of the Ni(II)-bearing ferrite with the spinel type of crystal structure. The rates of both activation by hydrogen gas and oxidation in carbon dioxide gas were much improved in the Ni (II)-bearing ferrite. The rate of decomposition was 0.178 mol h−1 for the activated Ni(II)-bearing ferrite and 0.005 92 mol h−1 for the activated magnetite in the batch mode, being 30 times larger. The rate of carbon dioxide decomposition was 16 times higher in the flow system in comparison with that of the activated magnetite.

Cation exchange properties of a layered manganic acid

There has been a great deal of research in developing highly selective Li+ exchangers which can extract Li+ from sea water and other brines because of the increased use of lithium in batteries. In a search for highly selective Li+ exchangers based on MnO2 composition, a new layered manganic acid of the rancieite structure with a (001) ‘d’ spacing of 0.742 nm and a chemical composition of H2Mn4O9 · 3·7 H2O has been synthesized. The cation selectivity order of this phase was found to depend on the pH and the extent of loading in batch experiments. Below pH 4, the selectivity order increased with increasing ionic radius as follows: Li+ < Na+ < K+ < Cs+. Above pH 4, the selectivity was found to be as follows: Na+ < K+ < Cs+ ⪡ Li+. These selectivity results show that the layered manganic acid reported here may be useful for selective separation of Li+ from sea water and assorted brines.

Carbon dioxide decomposition into carbon with the rhodium-bearing magnetite activated by H2-reduction

The CO2 decomposition into carbon with the rhodium-bearing activated magnetite (Rh-AM) was studied in comparison with the activated magnetite (AM). The Rh-AM and the AM were prepared by flowing hydrogen gas through the rhodium-bearing magnetite (Rh-M) and the magnetite (M), respectively. The rate of activation of the Rh-M to the Rh-AM was about three times higher than that of the M to the AM at 300 °C. The reactivity for the CO2 decomposition into carbon with the Rh-AM (70% CO2 was decomposed in 40 min) was higher than that with the AM (30% in 40 min) at 300 °C. The Rh-M was activated to the Rh-AM at a lower temperature of 250 °C, and the Rh-AM decomposed CO2 into carbon at 250 °C. On the other hand, the M was little activated at 250 °C.

Selective exchange of divalent transition metal ions in cryptomelane-type manganic acid with tunnel structure

The ion-exchange selectivity of divalent transition metal ions on cryptomelane-type manganic acid (CMA) with tunnel structure has been studied using the distribution coefficients ( K d ) at a small fractional exchange in nitrate media. All metal ions studied showed linear relationships with a slope of −2 on the log-log plot of K d vs [HNO 3 ] which clearly indicated that the adsorption process is an “ideal” ion-exchange. The selectivity increased in the following order: Pb ≫ Mn > Co > Cu > Hg > Cd > Zn > Ni. The high selectivity of the manganic acid was successfully utilized in the removal of Co 2+ from seawater and tap water.

Thermochemical decomposition of H2O to H2 on cation-excess ferrite

Abstract A cation-excess (Ni, Mn) ferrite Ni0.52Mn0.51Fe2.05O4.0 with a single phase was synthesized by heating the mixture of corresponding metal salts and oxide in 5% O2/75% CO2/20% N2 gas mixture for 18 h at 1100 †C. It could be thermochemically activated to form a cation-excess ferrite Ni0.51(1+e)Mn0.50(1+e)Fe1.99(1+e)O4.0 in N2 gas at elevated temperature: The e value was 0.031 at 1100 †C. The activated (Ni, Mn) ferrite was reacted with water to form H2 gas at 700 †C. The process was reversible and could be carried out repeatedly. The solid was studied by Mossbauer and XRD spectrometries.

Reactivity of oxygen-deficient Mn(II)-bearing ferrites (Mn x Fe3-xO4-δ, O⩽x⩽1, δ>0) toward CO2 decomposition to carbon

The reduction of CO2 to carbon was studied in oxygen-deficient Mn(II)-bearing ferrites (MnxFe3-xO4-δ, O⩽x⩽1, δ>0) at 300 °C. They were prepared by reducing Mn(II)-bearing ferrites with H2 gas at 300°C. The oxygen-deficient Mn(II)-bearing ferrites showed a single phase with a spinel structure having an oxygen deficiency. The decomposition reaction of CO2 to carbon was accompanied by oxidation of the oxygen-deficient Mn(II)-bearing ferrites. The decomposition rate slowed when the Mn(II) content in the Mn(II)-bearing ferrites increased. A Mössbauer study of the phase changes of the solid samples during the H2 reduction and CO2 decomposition indicated the following. Increases in the Mn(II) content lowered the electron conductivity of the Mn(II)-bearing ferrites. Increases in the oxygen deficiency, δ, contributed to an increase in electron conductivity and suggested that electron conduction due to the electron hopping determines the reductivity of CO2 to carbon by the donation of an electron at adsorption sites.

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.