M. Tsuji

SeO32−-selective properties of inorganic materials synthesized by the soft chemical process

Abstract Three groups of inorganic materials were studied with a view to finding adsorbents selective for selenium from aqueous solution in competitive conditions: (A) divalent-cation-substituted bismutite-type compounds (Bi 1− x , M x ) 2 (O 1− x , (OH) x ) 2 CO 3 · n H 2 O, (B) hydrotalcites [M 2+ 1− x N 3+ x (OH) 2 ] +x (CO 3 2− ) x /2 · n H 2 O and (C) amorphous titanic acid TiO 2 · n H 2 O. The first group of materials was synthesized by hydrolyzing mixed solution of M(NO 3 ) 2 and Bi(NO 3 ) 3 with Na 2 CO 3 solution at Na 2 CO 3 /(M+Bi)=20 on a molar basis at room temperature. Other materials were prepared by the soft chemical process according to our previous reports. These materials were identified using powder X-ray diffraction (XRD), FTIR spectra, and TG/DTA. Amorphous titanic acid and bismutite-type compounds showed excellent selectivity for SeO 3 2− . Divalent oxyanions were exchanged with CO 3 2− in the bismutite-type compounds and hydrotalcites through the ion-exchange mechanism. Selenite-selective properties of these materials found in the present study will be useful for its removal from contaminated aqueous effluents.

CO2 decomposition to carbon by ultrafine Ni(II)-bearing ferrite at 300 °C

Abstract Ultrafine Ni(II)-bearing ferrite (UNF) Ni2+0.36Fe2+0.45Fe3+2.19O4.10 was synthesized with a view to investigating materials with the high reactivity toward CO2 → C decomposition. UNF was prepared by coprecipitation of Ni2+, Fe2+ and Fe3+ with NaOH. The electron micrographs of the UNFs showed their size to be 16–29 nm in diameter. The rate of the H2-reduction of UNF was 3 times faster than that of conventional Ni(II)-bearing ferrite (NF) with the size of 100–200 nm prepared by the wet method. The H2-reduced UNF with the spinel structure revealed the much improved reactivity toward CO2 decomposition. The initial rate and amount of CO2 decomposition for the UNFs were 6.5 and 2.5 times greater than those for NF with the same Ni content at 300 °C. Such high reactivity of UNF is attributed to both the level of activation and surface area in comparison with NF.

CO2 decomposition to carbon with ferrite-derived metallic phase at 300 °C

Abstract CO 2 decomposition to carbon was studied on α-Fe and Fe-Ni alloy formed by H 2 -reduction of magnetite and Ni(II)-bearing ferrite at 300 °C. The metallic phases were transformed to ferrite phase by incorporating oxygen from CO 2 . The integrated amount of decomposed CO 2 reached 30 mmol CO 2 g −1 of Fe 3 O 4 by repeating the H 2 reduction and CO 2 decomposition 9 times. The decomposition of CO 2 on the H 2 -reduced magnetite was kept nearly constant at 72–86%, irrespective of the run number. A small portion of the α-Fe was transformed to carbide in the 1st run; however, the formation of carbide scarcely proceeded by repeating these processes on magnetite. The decomposition reactivity of H 2 -reduced Ni(II)-bearing ferrite decreased gradually. The integrated amount of decomposed CO 2 was 20 mmol CO 2 g −1 of Ni 0.39 Fe 2.61 O 4 by repeating the processes 11 times. For Ni(II)-bearing ferrite, the formation of carbide proceeded more effectively. The lower reactivity of H 2 -reduced Ni(II)-bearing ferrite for CO 2 decomposition will be due to the formation of carbide.

Thermochemical oxygen pump with praseodymium oxides using a temperature-swing at 403-873 K

The praseodymium oxide redox system releases and incorporates oxygen into an inert gas. Reductions of Pr6O11 to Pr5O9 and Pr5O9 to Pr9O16 were analyzed while heating a mixture of air with N2 at O2 partial pressures of 2 × 102-6 × 103 Pa from 373 to 833 K. Oxidations of Pr9O16 to Pr5O9 and Pr5O9 to Pr6O11 were analyzed while cooling from 873 to 373 K. The enthalpy changes for oxidation reactions (5Pr9O16 + 12O2→9Pr5O9 and 6Pr5O9 + 12O2→5Pr6O11) were determined to be − 128 ± 26 and − 138 ± 10 kJ, respectively. The oxygen pump with praseodymium oxides incorporated can be operated between air and a gas with Po2 of 4 × 102 Pa by using a temperature-swing method between 403 and 873 K. The minimum Po2 in diluent N2 achieved was 0.1 Pa at 403 K.

Water splitting with the Mn(II)-ferrite–CaO–H2O system at 1273K

Water splitting with Mn(II) ferrites (MnFe2O4, Mn0.5Fe2.5O4) and CaO at 1273K has been studied. The process accompanies a phase change and a calcium-manganese oxide (Cax(Fe,Mn)yOz) is formed. For CaO/MnFe2O4 mole ratios above 3, a single phase of Ca3(Fe2.02/3,Mn0.98/3)3O7.02 with crystal structure of the A3M3O8-type was formed for the MnFe2O4 sample [Mn(II)/Fe=1:2]. Associated evolution of H2 (17.8cm3/g) was observed. The chemical composition of the solid produced shows that the H2 comes from oxidation of Mn(II) to the Mn(III) ion. In the Mn0.5Fe2.5O4 sample [Mn(II)/Fe=1:5], the crystal structures of the solid products were assigned to types of Ca3(Fe,Mn)3O8 and Ca2Fe2O5, the chemical compositions of which could not be determined due to the mixed phase. The H2 volume (21.6cm3/g) was nearly the same as for an MnFe2O4 sample. Thus, the H2 evolution reaction for the Mn0.5Fe2.5O4 sample involves oxidation of both Fe(II) and Mn(II) ions in ferrite. H2 evolution due to oxidation of Mn(II) ions in ferrites was confirmed by the fact that the H2 volume increased in the order NiFe2O4≪Ni0.5Mn0.5Fe2O4

Thermodynamic study of alkali metal ion exchanges on a manganese dioxide with hexagonal structure

Ion-exchange equilibria of alkali metal ions and protons of a manganese dioxide material have been studied thermodynamically. The material was synthesized by pyrolysis from a mixture of manganese carbonate and potassium tert-butoxide with content of 40% on a molar basis, at 530 °C. The chemical composition of the material is MnO2.09 · 0.40H2O, and its crystal structure belongs to a hexagonal system with lattice constants a = 22.14 ± 0.03 A and c = 4.900 ± 0.002 A, which has not been reported in JCPDS. The ion-exchange titration curve showed that the material exhibited a property of weak acid, and the maximum uptakes were 3.67 meq Li/g, 2.59 meq Na/g, 2.54 meq K/g, 2.18 meq Rb/g, and 2.10 meq Cs/g. The plot of the corrected selectivity coefficient vs. fractional exchange (Kiellands plot) was almost straight for Li+H+, Rb+H+, and Cs+H+ exchanges, but curved for Na+H+ and K+H+ exchanges. The selectivity order at infinitesimal exchange is Li < Na < Cs ⪡ K < Rb at 30~60 °C. The pore structure in the material is expected to fit with K+ and Rb+. The negative Gibbs free energy change for K+H+ and Rb+H+ exchanges gDGideal exch0(K) = −22 kJ/mol and ΔGidealexech0 (Rb) = −27 kJ/mol) showed that these exchange reactions were preferable in the present material. The entropy change was large for K+/H+ and Rb+/H+ exchanges (ΔS0hyp exch(K) = 282 J/K · mol and Δ0hyp exch(Rb) = 458 J/K · mol), indicating high selectivities for K+ and Rb+ in the present material. The small ΔS0hyp exch for Cs+/H+ exchange may express the size misfit of the ion with the opening Of the exchange Site.

Methanation reactivity of carbon deposited directly from CO2 on to the oxygen-deficient magnetite

The methanation reactivity of surface carbon deposited from CO2 on the oxygen-deficient magnetite was studied by the isothermal methanation reaction and the temperature programmed surface reaction (TPSR). In the methanation reaction with H2 gas, in a closed system at 150–200‡C, active carbon, which was not observed by the TPSR measurements, was found. About 20% out of the deposited carbon was in the form of atomic carbon and readily converted into CH4 (5 min) above 150‡C with H2, and about 80% at 300‡C. At 350‡C these atomic carbons were transformed into polymerized carbons, which was less reactive for methanation.

CO2-decomposition capacity of H2-reduced ferrites

The rate and degree of CO 2 decomposition with the H 2 -reduced magnetite at 300°C was studied with respect to the reduction degree of the material. The CO 2 decomposition proceeded more effectively on the H 2 -reduced magnetite by a prolonged H 2 -reduction. At a low reduction degree of magnetite, CO 2 was decomposed to carbon with oxygen-deficient magnetite (ODM). On the other hand, at a high reduction degree, the mixed solid phase of ODM and α-Fe decomposed CO 2 to carbon, accompanying transformation of the mixed phase to the magnetite phase. The amount of CO 2 decomposed reached 30 mmol/g of Fe 3 O 4 by repeating the H 2 reduction and CO 2 decomposition 9 times. The relationship between the CO 2 decomposition rate and the reduction degree was studied in detail by the thermogravimetry

Carbon recycling system through methanation of CO2 in flue gas in LNG power plant

A bench scale test at ambient pressure has been performed for carbon recycling system using a ferrite process in LNG power plant. The chemical reaction consists of two steps; (i) adsorptive separation and decomposition of CO 2 to carbon on oxygen-deficient Ni ferrite and (ii) methanation of deposited carbon. CO 2 decomposition and methanation reactivities were much improved when the honeycomb support was used instead of a packed-bed-type reactor. The overall methanation rate of CO 2 in flue gas from LNG power plant could achieve 1,000 Nm 3 h -1 when 12,000 kg Ni ferrite loaded on a honeycomb support was used. The present methanation and carbon recycling system could be extended to other CO 2 sources such as IGCC power plant and depleted natural gas plant.

Coal gasification by the coal/CH4ZnOZnH2O solar energy conversion system

The coal/CH 4 -ZnO-Zn-H 2 O system has been proposed for the 2-step efficient chemical conversion system of the fossil fuels using a solar energy as the process heat and fossil fuel/ZnO redox system. The overall reaction is represented by CH x (coal) + CH 4 + 2H 2 O(g) = 2CO + (4 + x/2)H 2 The complete coal gasification to H 2 /CO gas mixture with the coal/ZnO redox system could be demonstrated with zero CO2 emissions at 1000-1100°C, eliminating the need to separate, while the steam gasification efficiency of coal remained 47% on a carbon basis. Zn-H 2 O reaction was conducted to generate H2 gas successfully. In the overall reaction, the product gas of the H2/CO mole ratio 2 could be obtained and can be directly used for synthesis of methanol and other chemicals.