- A tale of two metals: New cerium iron borocarbide intermetallics grown from rare-earth/transition metal eutectic fluxes
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R33Fe14-xAlx+yB25-yC 34 (R = La or Ce; x ≥ 0.9; y ≥ 0.2) and R33Fe 13-xAlxB18C34 (R = Ce or Pr; x a = 14.617(1) A, Z = 2, R1 = 0.0155 for Ce33Fe13.1Al1.1B24.8C34, and a = 14.246(8) A, Z = 2, R1 = 0.0142 for Ce 33Fe13B18C34). Their structures can be described as body-centered cubic arrays of large Fe13 or Fe 14 clusters which are capped by borocarbide chains and surrounded by rare earth cations. The magnetic behavior of the cerium-containing analogs is complicated by the possibility of two valence states for cerium and possible presence of magnetic moments on the iron sites. Temperature-dependent magnetic susceptibility measurements and Moessbauer data show that the boron-centered Fe14 clusters in Ce33Fe14-xAl x+yB25-yC34 are not magnetic. X-ray photoelectron spectroscopy data indicate that the cerium is trivalent at room temperature, but the temperature dependence of the resistivity and the magnetic susceptibility data suggest Ce3+/4+ valence fluctuation beginning at 120 K. Bond length analysis and XPS studies of Ce33Fe 13B18C34 indicate the cerium in this phase is tetravalent, and the observed magnetic ordering at TC = 180 K is due to magnetic moments on the Fe13 clusters.
- Tucker, Patricia C.,Nyffeler, Jason,Chen, Banghao,Ozarowski, Andrew,Stillwell, Ryan,Latturner, Susan E.
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- A novel one-step synthesis of Ce/Mn/Fe mixed metal oxide nanocomposites for oxidative removal of hydrogen sulfide at room temperature
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In this study, CeO2/Fe2O3, CeO2/Mn2O3, and CeO2/Mn2O3/Fe2O3nanocomposites were synthesized by the calcination of molten salt solutions. The microscopic images confirmed polyhedral nanocrystals of 10-20 nm size, clustered to form nanospheres. The elemental mapping confirmed the uniform distribution of transition metal oxides in the CeO2matrix. The X-ray diffraction analysis confirmed the phase purity of metal oxides in nanocomposites. The surface area of nanocomposites was in the range of 16-21 m2g?1. X-ray photoelectron spectroscopy confirmed 25-28% of Ce3+ions in the CeO2of nanocomposites. These nanocomposites were tested for the removal of hydrogen sulfide gas at room temperature. The maximum adsorption capacity of 28.3 mg g?1was recorded for CeO2/Mn2O3/Fe2O3with 500 ppm of H2S gas and 0.2 L min?1of flow rate. The adsorption mechanism probed by X-ray photoelectron spectroscopy showed the presence of sulfate as the only species formed from the oxidation of H2S, which was further confirmed by ion chromatography. Thus, the study reports room-temperature oxidation of H2S over mixed metal composites, which were synthesized by a novel one-step approach.
- Bae, Jiyeol,Gupta, Nishesh Kumar,Kim, Kwang Soo
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- Genesis of iron carbides and their role in the synthesis of hydrocarbons from synthesis gas
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A series of iron-based Fischer-Tropsch catalysts, either pure or promoted with Ce or Mn, were subjected to different activation treatments with H2, CO, or H2/CO. The surface species formed after different treatments were characterized by temperature-programmed surface reaction with hydrogen (TPSR-H2) and temperature-programmed surface desorption with argon (TPD-Ar). After activation and temperature-programmed treatments, the samples were passivated and characterized by X-ray diffraction and Raman spectroscopy. After each activation treatment, the catalysts were tested in the Fischer-Tropsch synthesis, and selected postreaction samples were characterized by Moessbauer spectroscopy. After CO and syngas treatment, cementite (θ-Fe3C) and Haegg (χ-Fe2.5C) carbides were formed, respectively. Different surface carbonaceous species were stabilized over these carbides. Cementite species are less active in the Fischer-Tropsch synthesis; however, under the Fischer-Tropsch reaction environment, they can evolve into the Haegg carbide, over which the more active carbonaceous intermediate species are formed. Modification of the catalyst composition by Mn and Ce is effective only when the samples are activated under CO, accelerating the stabilization of active carbonaceous intermediates.
- Herranz, Tirma,Rojas, Sergio,Perez-Alonso, Francisco J.,Ojeda, Manuel,Terreros, Pilar,Fierro, Jose Luis G.
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- Investigation of phase equilibria in the Ce-Co-Fe system during solidification
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Phase transformations in the Ce-Co-Fe ternary system have been studied using DTA, X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). No ternary compounds have been identified in this system. Liquidus and solidus projections and a Scheil diagram for solidification were constructed. At the solidus temperatures two continuous solid solutions form: (1) between isostoichiometric compounds Ce2Fe17 (Zn17Th2-type structure, hR57-R-3m) and Ce17Co2 (Th2Ni17-type structure, hP38-P63/mmc) and (2) between the Laves phases CeFe2 and CeCo2 (MgCu2-type structure, cF24-Fd-3m). Among the binary system-based phases, CeCo3 has the widest homogeneity region and dissolves up to 58 at% Fe at the solidus temperature. The homogeneity regions of the remaining phases are smaller. Five three-phase fields in the solidus surface result from four transition type and one eutectic invariant four-phase equilibria.
- Fartushna,Mardani,Khvan,Donkor,Cheverikin,Kondratiev,Dinsdale
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- Large spontaneous volume magnetostriction and pressure effects on the magnetic properties of amorphous Ce-Fe alloys
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The thermal expansion anomaly, the effects of pressure and hydrogenation on the magnetization and the Curie temperature have been investigated in order to discuss the magnetovolume effects in the amorphous CexFe100-x alloys prepared by high-rate DC sputtering. These amorphous alloys have a large spontaneous volume magnetostriction which increases with decreasing x. The pressure effects on the magnetization, the spin freezing temperature and the Curie temperature are significant. The magnetization M varies linearly with the square of temperature T2 in the ferromagnetic state. The hydrogenation increases markedly both the Curie temperature and the magnetization of the amorphous CeFe2 alloy. However, no huge coercive force related to the random magnetic anisotropy has been confirmed at 4.2 K against the expectation based on a valence change of the Ce ion. For the amorphous CeFe2 alloy, the compressibility estimated from the increases in the Curie temperature and the Fe-Fe interatomic distance by hydrogenation, does not coincide with the experimental value obtained by Brillouin scattering. Therefore, the expansion due to hydrogenation is not regarded to be caused by a so-called negative hydrostatic pressure effect.
- Fukamichi,Aoki,Masumoto,Goto,Murayama,Mori
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