Masahiro Tabata

CO2 decomposition with oxygen-deficient Mn(II) ferrite

An oxygen-deficient Mn(II) ferrite (Mn0.97Fe2.02O3.92) was synthesized and its reactivity to reduce CO2 gas into carbon was studied at 300°C. The oxygen-deficient Mn(II) ferrite was obtained by flowing H2 gas through Mn(II) ferrite with a nearly stoichiometric composition of Mn0.97Fe2.02O4.00 at 300° C. The lattice constant of the oxygen-deficient Mn(II) ferrite (0.8505nm) is larger than that of the Mn(II) ferrite with a nearly stoichiometric composition (0.8498nm). The chemical composition of the Mn(II) ferrite changed from Mn0.97Fe2.02O4.00 to Mn0.97Fe2.02O3.92 during the H2 reduction process, indicating that the oxygen is deficient in the spinel structure of the Mn(II) ferrite. This was confirmed by Mössbauer spectroscopy and X-ray diffractometry. The efficiency of CO2 decomposition into carbon at 300°C with the oxygen-deficient Mn(II) ferrite was much lower by about 105 than that of oxygen-deficient magnetite. This is considered to be due to the difference in electron conductivity between Mn(II) ferrite and magnetite, which determines the reductivity for CO2 into carbon by donation of an electron at the adsorption site.

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.

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.

Decomposition of CO2 and CO into carbon with active wüstite prepared from Zn(II)-bearing ferrite

CO2 decomposition reaction into carbon was studied at 300 °C using the H2-reduced Zn(II)-bearing ferrite which consisted of the Zn(II) oxide and the active wüstite. The H2-reduced Zn(II)-bearing ferrite was prepared from Zn(II)-bearing ferrite by the reduction with H2 gas at 300 °C. The wüstite (FeδO) in the H2-reduced Zn(II)-bearing ferrite had a higher δ value (δ=0.97, active wüstite) than those of the normal wüstites (0.90570 °C). The decomposition reaction of CO2 proceeds in two steps: (1) CO2 reduction to CO, and (2) CO decomposition into carbon. In the initial stage, the reduction of CO2 into CO takes place, accompanying both the oxidation of the active wüstite to the slightly oxidized wüstite, and the transformation of active wüstite and Zn(II) oxide into the Zn(II)-bearing ferrite. After the reaction of the initial stage attains equilibrium of an apparent state of rest, the adsorbed CO is decomposed into carbon, associated with the transformation of the slightly oxidized wüstite and the Zn(II) oxide into the Zn(II)-bearing ferrite.

Methanation of CO2 with H2-reduced magnetite

The methanation reaction of CO2 was studied with H2-reduced magnetite. A high conversion ratio of about 0.9 (in 30 min of the reaction time) with a selectivity of nearly 100% was obtained at 300°C and at 0.1 MPa for H2-reduced magnetite which had been prepared by passing H2 gas for 1–5 h at 300°C. From the results of X-ray diffractometry and Mössbauer spectroscopy, and from chemical analysis of the deposited carbon, H2-reduced magnetite is considered to decompose adsorbed CO2 into carbon, and to incorporate the oxygen of the CO2 into the spinel-type structure of the magnetite, associated with oxidation of the Fe2+ ion into Fe3+ ion in the magnetite. The high conversion ratio in the methanation reaction is considered to come from a higher reactivity of the elementary carbon deposited on the surface of the H2-reduced magnetite.

Adsorption of CO2 on oxygen-deficient magnetite : adsorption enthalpy and adsorption isotherm

The adsorption isotherm and the enthalpy of adsorption of CO2 on oxides of oxygen-deficient magnetite have been studied by adsorption techniques. Oxygen-deficient magnetite was prepared by flowing H2 gas through magnetite powder at 300 °C. Adsorption of CO2 onto oxygen-deficient magnetite was studied in the temperature range 150–300 °C. We found that the adsorption can be expressed by the Langmuir dissociative isotherm for three fragments: one carbon atom and two oxygen ions. Deposition of carbon after the adsorption reaction suggests that reduction of surface carbon seems to be involved in the adsorption reaction. The high reactivity for the reduction of CO2 to carbon is considered to come from such a reactive site where an electron is readily donated to the carbon of the CO2 molecule and the oxygen in the CO2 molecule is readily incorporated into a lattice point in the form of O2–. Electron hopping between the Fe2+ and Fe3+ ions in the spinel structure of the magnetite would facilitate the donation of an electron at the adsorption site. The distorted spinel structure of the surface of the H2-reduced magnetite, where the oxygen site is defected, would facilitate the incorporation of the oxygen of CO2.

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.

CO2 decomposition with mangano-wüstite

Mn(II)-ferrite (Mn0.97Fe2.02O4.00) prepared by the wet method was reduced in a hydrogen at 300°C to form highly reactive mangano-wüstite ((Fe0.67, Mn0.32)O) for CO2 decomposition. Approximately 23% CO2 injected (3.40 mmol) was decomposed to CO by the mangano-wüstite (3.22 g) in the initial stage of the reaction in a batch system at 400°C. 88% CO was further decomposed to carbon. Approximately 58% CO2 injected was reversibly adsorbed on the surface and the remaining 12% was unchanged after 200 h reaction. The mangano-wüstite was concurrently transformed to Mn(II)-bearing ferrite (Mn0.23Fe2.77O4.00) and manganeserich mangano-wüstite ((Fe0.60, Mn0.40)O). The higher CO2 decomposition capacity for this mangano-wüstite than that for oxygen-deficient Mn(II)-ferrite is discussed in detail, based on electron hopping and movement of ions in the bulk.

High‐vacancy‐content ferrite with fine particles

The high‐vacancy‐content ferrites represented by x(MFe2O4)⋅y(Fe3O4)⋅z(γ‐Fe2O3), where x+y+z=1 (z≳0.50), were obtained in the clear and strongly alkaline solutions of Fe(III) and M(II) tartrate [M(II)=Zn(II), Ni(II), and Cd(II)] or dextrose at 100 °C. The vacancies were replaced with the bivalent metal ions in the reaction solutions, and the replaced number increased with an increase in the concentration of the bivalent metal ions. The ferrite particle size was dependent on the bivalent metal species and the content (x,y). The Fe(II) ions enhanced the crystal‐growth rate. The particle size of the magnetites (x=0) increased from 100 to 800 A with an increase in the Fe(II) ion content (y=0.10–0.35). The high‐vacancy‐content magnetite was transferred from superparamagnetic to ferrimagnetic particles as the size increased. The Zn(II), Ni(II), and Cd(II) ions did not enhance the growth rate so much as compared to the Fe(II) ions. The particle sizes were less than 200 A, and most of the particles were superparam...

Enhanced conversion of CO2 with a mixed system of metal oxide and carbon

An endothermic chemical system composed of pulverized metal oxide and carbon has been investigated with a view to utilizing waste heat and carbonaceous compounds effectively. Oxides of Cu and Mn and also MMn2O4 (M = Li or Mg) were used. In an N2-gas stream, the metal oxide was reduced by reaction with carbon which was concurrently oxidized to generate CO and CO2, i.e. MOx + δC → MOx-δ + δCO. The process may be referred to as an activation step of the metal oxide phase. The activated metal oxide may be used for the effective reduction of CO2 to CO by the reverse reaction MOx−δ + δCO2 → MOx + δCO. LiMn2O4 as starting material showed the most rapid rates for both the activation and reduction steps at 700°C. The material was reduced to the Li-containing MnO phase in the activation step, which was also reactive for CO formation from CO2. These processes involving the Li+-containing MnO can be repeated until carbon is completely used up to produce CO at 700°C.