R. C. Mercader

Ftir and mössbauer investigation of a substituted palygorskite: Silicate with a channel structure

Crystal data and spectroscopic studies of a silicate with a channel structure were realized. The substituted palygorskite from Las Quijadas (San Luis), Argentina, was studied by X-ray SBET, FTIR and Mossbauer spectroscopy. X-ray results clearly show that the investigated compound crystallizes in a monoclinic structure with space group C2m (C2h3, No. 12), and Z = 2. It is aluminium-rich and dioctahedral. Unit-cell parameters were determined using X-ray diffraction: a = 12.7 A, b = 17.9 A, c = 5.2 A, β = 91.86 °. The Mossbauer results prove that the very small amount of Fe(III) ions substituting Mg in the sample are located at two structural sites in the lattice. Through the FTIR spectrum analysis a more reliable assignment was performed with respect to tetrahedra, octahedra, hydroxyl groups and water molecules.

Kinetic study of the disproportionation of tin monoxide

Abstract The disproportionation reaction kinetics of tin monoxide at 723 K was studied by Mossbauer spectroscopy. The kinetic parameters of the reaction were obtained by objective data processing of the Mossbauer spectra, treating the data matrix with the Single Value Decomposition procedure that, in addition, yields the spectrum of the intermediate oxide. The procedure was applied on the assumptions that the f-factors of Sn(II) and Sn(IV) atoms belonging to the intermediate oxide are equal to those of Sn(II) and Sn(IV) atoms of SnO and SnO2, respectively, that the stoichiometry of the intermediate oxide is Sn3O4, and that the disproportionation proceeds by a set of two successive first order reactions described by the sequence SnO→intermediate oxide→SnO2. The kinetic rate constants obtained for the first and second reactions were k1=2.47×10−4 s−1 and k2=1.13×10−5 s−1, respectively. The time-profile of the mole fraction of each component in partially reacted samples shows that at 723 K, the intermediate accumulates in the time bracket between ca. 100–1000 min.

Influence of the Support on the Activity and Selectivity of High dispersion Fe Catalysts in the Fischer-Tropsch Reaction

In order to study the influence of the support on high dispersion catalysts used for the CO hydrogenation reaction, two catalysts, Fe/SiO{sub 2} and Fe/Al{sub 2}O{sub 3}, were prepared by the dry impregnation method. Selective chemisorption of CO, volumetric oxidation, and Moessbauer spectroscopy were used to determine the Fe species present as well as the metallic crystal size, the degree of dispersion, and the reduction percentage. The presence of small Fe{sup 0} crystallites with high dispersion was determined in both catalysts. Reaction rates were measured in a differential reactor and significant differences, about one order of magnitude less for the Al{sub 2}O{sub 3} than for the SiO{sub 2} supported catalysts, were found in the methane turnover frequencies. They are attributed to the interaction between the metal and the supports. The selectivity differences is also discussed in connection with distinct surface properties.

Synthesis of Fe-FeAl2O4-Al2O3 by high-energy ball milling of Al-Fe3O4 mixtures

Physicochemical and structural changes induced by mechanical activation of Al–Fe3O4 mixtures are studied. After 37 min, the system undergoes a self-sustained reaction with formation of α-Fe, FeAl2O4 and α-Al2O3. The evolution of the composition follows a three-step reaction. The solid solution spinel Fe[Al2−xFex]O4, (0.13<x<0.29), is obtained through a mechanochemical-thermal route.

Study of intermediate oxides in SnO thermal decomposition

Two series of room temperature X-ray and Mossbauer spectroscopy (MS) measurements were performed on powder SnO samples after 1 h annealing in air and Ar at temperatures between 473 and 1073 K in 100 K steps. Complementary measurements on one SnO sample treated in vacuum in this sequence were performed after the thermal treatment at 773 K. In the thermal evolution between SnO and SnO2 the results show the presence of an intermediate oxide not only in the Ar and vacuum annealed samples but also in those annealed in air. The Mossbauer parameters of the 2+ and 4+ states for the intermediate oxide were determined. The results were consistent with a reaction rate dependent on the oxygen partial pressure.

Effect of ion doping on CuO magnetism

We have investigated the magnetism of 0.5 at. % Fe3+-doped CuO samples by Mossbauer spectroscopy and magnetization studies. The Curie–Weiss-type behavior observed for low-field-magnetization measurements at temperatures higher than 220 K is interpreted as an uncompensation of moments enhanced by the doping. The results are compared with 1.2 at. % Zn2+-doped CuO magnetization results and previous ones for samples doped with divalent ions. Trivalent ion doping produces an increase in the susceptibility with a peak at TN. The magnetic coupling of Fe2+ ions to the magnetic lattice is found to be about four times stronger than that of Fe3+.

Crystal structure, raman and 57Fe mo¨ssbauer spectra of the FeII complex of iso-orotic acid

Abstract The crystal structure of [Fe(H 2 isoor) 2 (H 2 O) 2 ]·2H 2 O has been solved by single-crystal X-ray diffractometry. Thermogravimetric analysis shows simultaneous loss of all water molecules between 120 and 266°C, followed immediately by a rapid degradation of the compound. A comparative analysis of the Raman spectra of the complex with that of free-iso-orotic acid allowed a straightforward assignment of the vibrations of the ligand groups involved in coordination. The 57 Fe Mo¨ssbauer spectrum of the complex confirms the presence of a high-spin Fe II ion and the distortion of the octahedral Fe II environment.

Formation of carbides by electro-discharge machining of alpha iron

Abstract The surface of pure iron sheets have been submitted to spark-planing erosion using kerosene as dielectric under a wide range of machining conditions: discharge duration times between 2 and 3072 μs, and spark currents between 3.3 and 25 A. The carbides formed by the electro-discharge machining process have been identified by X-ray diffraction and Mossbauer spectroscopy. Two different regimes of carbide formation were established: for sparks of energy below ≈0.5 J only e -carbide, austenite and martensite were observed; for sparks of higher energy mainly cementite, austenite and traces of martensite, Fe 7 C 3 , or Fe 5 C 2 were detected.

Vibrational and -Mössbauer spectra of some mixed cation diphosphates of the type MIIFe2III(P2O7)2

The infrared (IR), Raman and 57Fe-Mossbauer spectra of SrFe2(P2O7)2, BaFe2(P2O7)2 and PbFe2(P2O7)2 were recorded and discussed on the basis of the structural peculiarities of the compounds.

Assessment of natural radioactivity levels and their relationship with soil characteristics in undisturbed soils of the northeast of Buenos Aires province, Argentina

Surface and depth profile concentrations (down to 50 cm) of ²³²Th chain, ²²⁶Ra, and ⁴⁰K radionuclides were determined in undisturbed coastal and inland soils of La Plata city region, Argentina, through their gamma-ray activity using a high-purity Ge detector spectrometer. These results were compared with superficial activities determined in soils from the surroundings of the Centro Atómico Ezeiza (Ezeiza Atomic Center) located in Ezeiza, Buenos Aires Province, Argentina. The hyperfine and magnetic Fe phases properties of soil profiles were characterized by Mössbauer spectroscopy, magnetic hysteresis loops and AC magnetic susceptibility. No dependence of the activity of the ²³²Th natural chain on depth was found, whereas variations for ²²⁶Ra and ⁴⁰K activities were observed. Positive correlations, determined by the Pearson correlation coefficients, were established between ⁴⁰K, ²²⁶Ra and ²³²Th activity concentrations for the whole set of soil samples. The annual external equivalent dose for adults was similar for La Plata and Ezeiza regions, with average values of 0.08 ± 0.01 mSv and 0.06 ± 0.02 mSv, respectively. The thermal dependence of the AC magnetic susceptibility revealed the existence of magnetite and hematite. The Mössbauer spectra of all soils were made up of signals associated with α-Fe₂O₃, a paramagnetic relaxation component, and Fe³⁺ and Fe²⁺ doublets. In addition, the spectra of inland soils revealed the presence of Fe₃O₄. A negative correlation was found between the activity concentrations and the α-Fe₂O₃ and Fe₃O₄ relative fractions, whereas a positive correlation was found between the Fe³⁺ relative fraction and the ⁴⁰K activity.