Síntese de catalisadores do tipo LaNixFe1-xO3 como precursores catalíticos para reação oxidação parcial do metano

Nickel-bases catalysts have been used in several reform reactions, such as in the partial oxidation of methane to obtain H2 or syngas (H2 + CO). High levels of conversion are usually obtained using this family of catalysts, however, their deactivation resulting from carbon deposition still remains a challenge. Different approaches have been tested aiming at minimizing this difficulty, including the production of perovskites and related structures using modern synthesis methods capable of producing low cost materials with controlled microstructural characteristics at industrial scale. To establish grounds for comparison, in the present study LaNixFe1-xO3 (x=0, 0.3 or 0.7) perovskites were prepared following the Pechini method and by microwave assisted self-combustion. All samples were sub sequently calcined at 900 °C to obtain the target phase. The resulting ceramic powders were characterized by thermogravimetric analysis, infrared spectroscopy, X ray diffraction, specific area and temperature programmed reduction tests. Calcined samples were also used in the partial oxidation reaction of methane to evaluate the level of conversion, selectivity and carbon deposition. The results showed that the calcined samples were crystalline and the target phase was formed regardless of the synthesis method. According to results obtained by Rietveld refinement, we observed the formation of 70.0% of LaNi0.3Fe0.7O3 and 30.0% of La2O3 for samples LN3F7-900- P, LN3F7-900-M and 41,6% of LaNi0.7Fe0.3O3, 30.7% of La2NiO4 and 27.7% of La2O3 for samples LN7F3-900-P and LN7F3-900-M.Temperature-programmed profiles of the LaNiO3 sample revealed the presence of a peak around 510 °C, whereas the LaFeO3 sample depicted a peak above 1000°C. The highest l evel of methane conversion was obtained for LaNiO3 synthesized by the Pechini method. Overall, catalysts prepared by the Pechini method depicted better conversion levels compared to those produced by microwave assisted self-combustion

Síntese de catalisadores do tipo LaNixFe1-xO3 como precursores catalíticos para reação oxidação parcial do metano

Nickel-bases catalysts have been used in several reform reactions, such as in the partial oxidation of methane to obtain H2 or syngas (H2 + CO). High levels of conversion are usually obtained using this family of catalysts, however, their deactivation resulting from carbon deposition still remains a challenge. Different approaches have been tested aiming at minimizing this difficulty, including the production of perovskites and related structures using modern synthesis methods capable of producing low cost materials with controlled microstructural characteristics at industrial scale. To establish grounds for comparison, in the present study LaNixFe1-xO3 (x=0, 0.3 or 0.7) perovskites were prepared following the Pechini method and by microwave assisted self-combustion. All samples were sub sequently calcined at 900 °C to obtain the target phase. The resulting ceramic powders were characterized by thermogravimetric analysis, infrared spectroscopy, X ray diffraction, specific area and temperature programmed reduction tests. Calcined samples were also used in the partial oxidation reaction of methane to evaluate the level of conversion, selectivity and carbon deposition. The results showed that the calcined samples were crystalline and the target phase was formed regardless of the synthesis method. According to results obtained by Rietveld refinement, we observed the formation of 70.0% of LaNi0.3Fe0.7O3 and 30.0% of La2O3 for samples LN3F7-900- P, LN3F7-900-M and 41,6% of LaNi0.7Fe0.3O3, 30.7% of La2NiO4 and 27.7% of La2O3 for samples LN7F3-900-P and LN7F3-900-M.Temperature-programmed profiles of the LaNiO3 sample revealed the presence of a peak around 510 °C, whereas the LaFeO3 sample depicted a peak above 1000°C. The highest l evel of methane conversion was obtained for LaNiO3 synthesized by the Pechini method. Overall, catalysts prepared by the Pechini method depicted better conversion levels compared to those produced by microwave assisted self-combustion

Cristallochemical characterization of synthetic Zn-substituted maghemites (g-Fe2-xZn xO3)

Maghemite (g-Fe2O3) is the most usually found ferrimagnetic oxide in red basalt-derived soils. The variable degrees of ionic substitution of Fe3+ for different metals (e.g. Ti4+, Al3+, Mg2+, Zn2+, and Mn2+) and non-metals in the maghemite structure influence some cristallochemical features of this iron oxide. In this study, synthetic Zn-substituted maghemites were prepared by co-precipitation in alkaline aqueous media of FeSO4.7H2O with increasing amounts of ZnSO4.7H2O to obtain the following sequence of Fe3+ for Zn2+ substitutions: 0.0, 0.025, 0.05, 0.10, 0.15, 0.20, and 0.30 mol mol-1. The objective of this work was to evaluate the cristallochemical alterations of synthetic Zn-substituted maghemites. The dark black synthetic precipitated material was heated to 250 °C during 4 h forming a brownish maghemite that was characterized by chemical analysis as well as X ray diffraction (XRD), specific surface area and mass-specific magnetic susceptibility. The isomorphic substitution levels observed were of 0.0013, 0.0297, 0.0590, 0.1145, 0.1764, 0.2292 and 0.3404 mol mol-1, with the formation of a series of maghemites from Fe2Zn0O3 to Fe(1.49)Zn(0.770)O3 . The increase in Fe3+ for Zn2+ substitution, [Zn mol mol-1] increased the dimension a0 of the cubic unit cells of the studied maghemites according to the regression equation: a0 = 0.8343 + 0.02591Zn (R² = 0.98). On the other hand, the mean crystallite dimension and mass-specific magnetic susceptibility of the studied maghemites decreased with increasing isomorphic substitution.

Síntese e caracterização de perovskitas LaNi(1-x)Co xO3 como precursores de catalisadores para a conversão do metano a gás de síntese pela reforma com CO2

LaNiO3 perovskite was modified by partial substitution of nickel by cobalt in order to increase the stability and resistance to carbon deposition during the methane CO2 reforming. The results showed that a suitable combination of precipitation and calcination steps resulted in oxides with the desired structure and with important properties for application in heterogeneous catalysis. The partial substitution of Ni by Co resulted in lower rates of conversion of both the reactants, but the catalyst stability was highly increased. The LaNi0.3Co0.7O3 catalyst, calcined at 800 ºC, was the most active under the reaction conditions.

Electronic and magnetic properties of LaV1-xO3 /

A series of LaVi^xOs compounds (x=0.00, 0.02, 0.04, 0.06, 0.08) were prepeired using the standard solid reaction. The samples were chareicterized by X-ray diffraction (XRD), fourprobe resistivity, smd magnetic susceptibility studies. Powder X-ray diffraction analysis indicated the formation of a single-phase sample with a orthorhombic structure which was first found in GdFeOs (space group Pnma) . The Unit Cell program was used for calculating lattice peirameters from XFID data. The XRD spectnim could be indexed on a cubic lattice with Og = 2ap ~ (7.8578 to 7.9414 A). The lattice parameter was observed to increase as the Vanadium vacancy increased. Four-probe resistivity measurements exhibited semiconductor behavior for all sajnples from room temperature down to 19K. The resistivity of samples increased with increasing Vanadium vacancy. The resistivity of samples demonstrated activated conduction with an activation energy of approximately 0.2 eV. The activation energy increased with increasing lattice parameter. Field cool magnetic susceptibility measurements were performed with field of 500 G from 300 K to 5 K. These measurements indicated the presence of an antiferromagnetic transition at about 140 K. The data was fitted above Neel temperature to Ciurie-Weiss law yielding a negative parameignetic Curie temperature. This implies that antiferromagnetic ordering is present.

Preparation of SrMgx-Ru1-xO3 thin films by pulsed laser deposition /

SrMg^Rui-iOa thin films were made by using pulsed laser deposition on SrTiOa (100) substrates in either O2 or Ar atmosphere. The thin films were characterized by x-ray diffraction, energy dispersive x-ray microanalysis, dc resistivity measurement, and dc magnetization measurement. The effect of Mg doping was observed. As soon as the amount of Mg increased in SrMg-cRui-iOa thin films, the magnetization decreased, and the resistivity increased. It had little effect on the Curie temperature (transition temperature). The magnetization states of SrMgiRui-iOa thin films, for x < 0.15, are similar to SrRuOs films. X-ray diffraction results for SrMga-Rui-iOa thin films made in oxygen showed that the films are epitaxial. The thin films could not be well made in Ar atmosphere during laser ablation as there was no clear peak of SrMg^Rui-iOa in x-ray diffraction results. Substrate temperatures had an effect on the resistivity of the films. The residual resistivity ratios were increased by increasing substrate temperature. It was observed that the thickness of thin films are another factor for film quality: Thin films were epitaxial, but thicker films were not epitaxial.

Caracterização morfológica e microestrutural da perovskita BaCexPr1-xO3 sintetizada a partir do método de complexação combinando EDTA/Citrato

In this study, binary perovskite (BaCexO3) were doped with praseodymium (Pr) to obtainment of the ternary material BaCexPr1-xO3. This material was synthesized by the complexation method combining EDTA/Citrate with the stoichiometric ratio of the element Praseodymium ranging from x = 0.1 to x = 0.9 in order to determine the influence of this rare earth element on the morphology and microstructure of the final powder. At first the material was synthesized based on the route proposed by literature (Santos, 2010), and then characterized by SEM and XRD, besides being refined by the Rietveld method. In the material that had lowest residual parameter, S, and lowest average size of crystal, pH variation of synthesis solution was made in order to identify the influence of this parameter on the morphology and microscopy of the final powder. The results show that addition of praseodymium did not directly influence the crystallographic and lattice parameters, keeping even the same orthorhombic structure of the binary material BaCexO3, according to Yamanaka et al (2003). Material type BaCe0,2Pr0,8O3 had lowest residual parameter (S=1.4) and lowest average size of crystallite (26.4 nm), being used as reference in the pH variation of synthesis solution for 9, 7, 5 and 3, respectively. Variation of this parameter showed that when the synthesis solution pH was decreased to below 11, there was an increase in the average size of crystals, for pH 9, about 58.3%, for pH 7 (30.3 %), for pH 2 (2.3%) and for pH 3 (42%), indicating that the value initially used and quoted by Santos (2010) was the most coherent

Spin reversal and magnetization jumps in ErMexMn1-xO3 perovskites (Me = Ni, Co)

The erbium-based manganite ErMnO3 has been partially substituted at the manganese site by the transition-metal elements Ni and Co. The perovskite orthorhombic structure is found from x(Ni) = 0.2-0.5 in the nickel-based solid solution ErNixMn1-xO3, while it can be extended up to x(Co) = 0.7 in the case of cobalt, provided that the synthesis is performed under oxygenation conditions to favor the presence of Co3+. Presence of different magnetic entities (i.e., Er3+, Ni2+, Co2+, Co3+, Mn3+, and Mn4+) leads to quite unusual magnetic properties, characterized by the coexistence of antiferromagnetic and ferromagnetic interactions. In ErNixMn1-xO3, a critical concentration x(crit)(Ni) = 1/3 separates two regimes: spin-canted AF interactions predominate at x < x(crit), while the ferromagnetic behavior is enhanced for x > x(crit). Spin reversal phenomena are present both in the nickel- and cobalt-based compounds. A phenomenological model based on two interacting sublattices, coupled by an antiferromagnetic exchange interaction, explains the inversion of the overall magnetic moment at low temperatures. In this model, the ferromagnetic transition-metal lattice, which orders at T-c, creates a strong local field at the erbium site, polarizing the Er moments in a direction opposite to the applied field. At low temperatures, when the contribution of the paramagnetic erbium sublattice, which varies as T-1, gets larger than the ferromagnetic contribution, the total magnetic moment changes its sign, leading to an overall ferrimagnetic state. The half-substituted compound ErCo0.50Mn0.50O3 was studied in detail, since the magnetization loops present two well-identified anomalies: an intersection of the magnetization branches at low fields, and magnetization jumps at high fields. The influence of the oxidizing conditions was studied in other compositions close to the 50/50 = Mn/Co substitution rate. These anomalies are clearly connected to the spin inversion phenomena and to the simultaneous presence of Co2+ and Co3+ magnetic moments. Dynamical aspects should be considered to well identify the high-field anomaly, since it depends on the magnetic field sweep rate. (C) 2006 Elsevier B.V. All rights reserved.

Síntese e caracterização de compostos HoMn1-x(Ni,Co)xO3

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Desenvolvimento de material BaCexPr1-xO3 (0 ≤ X ≤ 1) com estrutura perovskita para oxidação catalítica do CO a CO2 em reator de leito fixo

Barium Cerate (BaCeO3) is perovskite type structure of ABO3, wherein A and B are metal cations. These materials, or doped, have been studied by having characteristics that make them promising for the application in fuel cells solid oxide, hydrogen and oxygen permeation, as catalysts, etc .. However, as the ceramic materials mixed conductivity have been produced by different synthesis methods, some conditions directly influence the final properties, one of the most important doping Site B, which may have direct influence on the crystallite size, which in turn directly influences their catalytic activity. In this study, perovskite-type (BaCexO3) had cerium gradually replaced by praseodymium to obtain ternary type materials BaCexPr1-xO3 and BaPrO3 binaries. These materials were synthesized by EDTA/Citrate complexing method and the material characterized via XRD, SEM and BET for the identification of their structure, morphology and surface area. Moreover were performed on all materials, catalytic test in a fixed bed reactor for the identification of that person responsible for complete conversion of CO to CO2 at low operating temperature, which step can be used as the subsequent production of synthesis gas (CO + H2) from methane oxidation. In the present work the crystalline phase having the orthorhombic structure was obtained for all compositions, with a morphology consisting of agglomerated particles being more pronounced with increasing praseodymium in the crystal structure. The average crystal size was between 100 nm and 142,2 nm. The surface areas were 2,62 m²g-1 for the BaCeO3 composition, 3,03 m²g-1 to BaCe0,5Pr0,5O3 composition and 2,37 m²g-1 to BaPrO3 composition. Regarding the catalytic tests, we can conclude that the optimal flow reactor operation was 50 ml / min and the composition regarding the maximum rate of conversion to the lowest temperature was BaCeO3 to 400° C. Meanwhile, there was found that the partially replaced by praseodymium, cerium, there was a decrease in the catalytic activity of the material.

Structural and magnetic characterization of maghemites prepared from Al-substituted magnetites

Synthetic aluminum-substituted maghemites were characterized by total chemical analysis, powder X-ray diffraction (XRD), Mössbauer spectroscopy (ME), and vibrating sample magnetometry (VSM). The aim was to determine the structural, magnetic, and hyperfine properties of γ-Fe2-xAl xO3 as the Al concentration is varied. The XRD results of the synthetic products were indexed exclusively as maghemite. Increasing Al for Fe substitution decreased the mean crystalline dimension and shifted all diffraction peaks to higher º2θ angles. The a0 dimension of the cubic unit cell decreased with increasing Al according to the equation a o = 0.8385 - 3.63 x 10-5 Al (R²= 0.94). Most Mössbauer spectra were composed of one sextet, but at the highest substitution rate of 142.5 mmol mol-1 Al, both a doublet and sextet were obtained at 300 K. All hyperfine parameters from the sub-spectra were consistent with high-spin Fe3+ (0.2 a 0.7 mms-1) and suggested a strong superparamagnetic component associated with the doublet. The magnetic hyperfine field of the sextets decreased with the amount of Al-substitution [Bhf (T) = 49.751 - 0.1202Al; R² = 0.94] while the linewidth increased linearly. The saturation magnetization also decreased with increasing isomorphous substitution.

Characterization of BaTi1-xZrxO3 thin films obtained by a soft chemical spin-coating technique

Single-phase perovskite structure BaZrxTi1-xO3 (BZT) (0.05less than or equal toxless than or equal to0.25) thin films were deposited on Pt-Ti-SiO2-Si substrates by the spin-coating technique. The structural modifications in the thin films were studied using x-ray diffraction and micro-Raman scattering techniques. Lattice parameters calculated from x-ray data indicate an increase in lattice (a axis) with the increasing content of zirconium in these films. Such Zr substitution also result in variations of the phonon mode wave numbers, especially those of lower wave numbers, for BaZrxTi1-xO3 thin films, corroborate to the structural change caused by the zirconium doping. on the other hand, Raman modes persist above structural phase transition, although all optical modes should be Raman inactive in the cubic phase. The origin of these modes must be interpreted as a function of a local breakdown of the cubic symmetry, which could be a result of some kind of disorder. The BZT thin films exhibited a satisfactory dielectric constant close to 181-138, and low dielectric loss tan delta<0.03 at the frequency of 1 kHz. The leakage current density of the BZT thin films was studied at elevated temperatures and the data obey the Schottky emission model. Through this analysis the Schottky barrier height values 0.68, 1.39, and 1.24 eV were estimated to the BZT5, BZT15, and BZT25 thin films, respectively. (C) 2004 American Institute of Physics.

Preparo e caracterização de compósitos polímero/cerâmica com potencial aplicação como sensor multifuncional

Pós-graduação em Química - IQ

New features of the morphotropic phase boundary in the PbZrO 3-PbTiO3 system: A Raman spectroscopy study

In this work we present a Raman study of PbZr1-xTi xO3 ceramics with composition values close to the morphotropic phase boundary region. The analysis of the Raman spectra leads to the determination of the monoclinic phase extension at different temperatures in a very good agreement with those determined by diffraction techniques. Therefore the obtained results show that Raman spectroscopy is a powerful and suitable technique to study structural phase transitions in PbZr 1-xTixO3. © 2002 Taylor & Francis.

Development of Solid Oxide Fuel Cell Electrodes with High Conductivity and Enhanced Redox Stability

The intent of this study was the development of new ceramic SOFC anode materials which possess electrical conductivity as well as redox stability.