Anderson Dias

Magnetic amphiphilic composites based on carbon nanotubes and nanofibers grown on an inorganic matrix: effect on water-oil interfaces

Novos compositos magneticos anfifilicos foram preparados pelo crescimento de nanotubos e nanofibras de carbono contendo particulas magneticas atraves de deposicao quimica de vapor (CVD), utilizando etanol como fonte de carbono e lama vermelha (RM, subproduto do processo Bayer de producao de alumina) como suporte e catalisador. Monitoramento da reacao CVD a temperatura programada (TPCVD), difracao de raios X (XRD), espectroscopia Mossbauer, espectroscopia de energia dispersiva (EDS), espectroscopia Raman, termogravimetria (TG/DTA), analise elementar (CHN), determinacao de area superficial (BET), microscopia eletronica de varredura (SEM) e de transmissao (TEM) e medidas magneticas mostraram que etanol reduz ions de ferro na RM para formar fases magneticas, por exemplo Fe 3 O 4 e Fe 0 , e depositos de carbono (5-42 wt.%) na forma de nanotubos e nanofibras. A combinacao de nanoestruturas hidrofobicas de carbono com oxidos hidrofilicos de Al, Si e Ti presentes na lama vermelha produziu materiais anfifilicos com excelente interacao com a interface agua-oleo. Misturas de oleo de soja ou de decalina com agua (completamente imisciveis) foram emulsificadas facilmente na presenca dos compositos anfifilicos. Quando os compositos foram adicionados a uma emulsao agua-biodiesel estavel, as particulas anfifilicas difundiram-se para a interface agua- oleo. As particulas do composito foram atraidas por imas e carregaram com elas as gotas de oleo, levando a completa desemulsificacao e separacao entre biodiesel e agua. New magnetic amphiphilic composites were prepared by the catalytic carbon vapor deposition (CVD) growth of carbon nanotubes and nanofibers using ethanol as carbon source and red mud waste (RM, a by-product of the Bayer process of alumina production) as catalyst and support. Temperature-programmed CVD (TPCVD), analyses by X-ray diffractometry (XRD), Mossbauer spectroscopy, energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy, thermogravimetry (TG/DTA), elemental analysis (CHN), superficial area determination (BET), scanning (SEM) and transmission (TEM) electron microscopies and magnetic measurements showed that ethanol reduces the iron ions in the red mud to form magnetic phases, e.g., Fe 3 O 4 and Fe 0 , and carbon deposits (5-42 wt.%), particularly nanotubes and nanofibers. The combination of the hydrophobic carbon nanostructures with the hydrophilic Al, Si and Ti oxides present in the RM produced amphiphilic materials with excellent interaction with the water-oil interface. Soybean oil or decalin mixtures with water (completely immiscible) were easily emulsified in the presence of the amphiphilic composites. When the composites were added to stable biodiesel-water emulsions, the amphiphilic particles diffused to the interface oil-water. These composite particles were attracted by a magnet, carrying the oil droplets with them and leading to the complete demulsification and separation of the biodiesel from the water.

Ce1−xSmxO1.9−δ nanoparticles obtained by microwave-assisted hydrothermal processing: an efficient application for catalytic oxidation of α-bisabolol

Heterogeneous catalysts based on Sm-doped ceria were employed for the first time in the liquid-phase oxidation of α-bisabolol. Nanometer-sized catalysts were obtained by microwave-hydrothermal synthesis and were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), Raman spectroscopy and N2-physisorption. The influence of Sm doping, temperature and the solvent used on the catalytic behavior was investigated. Conversions up to 84% and a combined selectivity for the products up to 77% were obtained for Ce0.9Sm0.15O1.85−δ catalysts. The reactions were highly selective for the epoxidation products (only bisabolol oxides A and B were obtained) and shown to be strongly dependent on the temperature and solvent employed. Best results were achieved for higher Sm concentrations, which indicate that changes in the textural properties due to doping produced a significantly more active catalyst.

Magnetic composites based on metallic nickel and molybdenum carbide: A potential material for pollutants removal

New magnetic composites based on metallic nickel and molybdenum carbide, Ni/Mo(2)C, have been produced via catalytic chemical vapor deposition from ethanol. Scanning electron microscopy, thermal analysis, Raman spectroscopy and X-ray diffraction studies suggest that the CVD process occurs in a single step. This process involves the reduction of NiMo oxides at different temperatures (700, 800 and 900°C) with catalytic deposition of carbon from ethanol producing molybdenum carbide on Ni surface. In the absence of molybdenum the formation of Ni/C was observed. The magnetic molybdenum carbide was successfully used as pollutants removal by adsorption of sulfur and nitrogen compounds from liquid fuels and model dyes such as methylene blue and indigo carmine. The dibenzothiofene adsorption process over Ni/Mo(2)C reached approximately 20 mg g(-1), notably higher than other materials described in the literature and also removed almost all methylene blue dye. The great advantage of these carbide composites is that they may be easily recovered magnetically and reused.

Microwave and infrared dielectric properties of Sr1−3x/2CexTiO3 (x = 0.154–0.400) incipient ferroelectrics at cryogenic temperatures

Sr1−3x/2CexTiO3 (x = 0.154–0.400) or Sr2+nCe2Ti5+nO15+3n (n ≤ 8) ceramics were prepared by the mixed oxide route. The microwave (MW) dielectric properties of the compounds were investigated in the temperature range from 8 to 295 K. The permittivity increases for decreasing temperatures and saturates below 30 K, following Barretts equation, demonstrating the incipient ferroelectric nature of the investigated materials. The dielectric loss tangent decreases for decreasing temperatures, reaching a minimum at about 80–120 K, and again increases with further cooling due to the rotations of TiO6 octahedra. Infrared-reflectivity data show that the dielectric response of the system is driven by the lowest-frequency polar (soft) mode, particularly at lower temperatures, where the phonons become practically uncoupled. The results help us to understand why Sr1−3x/2CexTiO3 materials present more appropriate dielectric properties for MW tunable applications, compared with pure SrTiO3.

Microstructural evolution of fast-fired nickel-zinc ferrites from hydrothermal nanopowders

Abstract Nanocrystalline nickel–zinc ferrites were produced by hydrothermal processing. The powders were submitted to a rapid sintering method (fast-firing) in a wide range of temperatures (1100–1450°C), times (5–240 min), and heating/cooling rates (100, 250, and 400°C/min). The results showed that different microstructures are obtained under these fast-firing conditions. Their effects on the microstructural evolution are discussed in terms of a qualitative model, which indicated the relative predominance of the densification and grain growth rates.

Gold, palladium and gold–palladium supported on silica catalysts prepared by sol–gel method: synthesis, characterization and catalytic behavior in the ethanol steam reforming

Noble-metal-based catalysts supported on silica (Au/SiO2, Pd/SiO2 and Au–Pd/SiO2) were prepared by the sol–gel method and were evaluated in the steam reforming of ethanol for hydrogen production. The catalysts were characterized by N2 physisorption (BET/BJH methods), X-ray diffraction, temperature programmed reduction analysis, H2 chemisorption, atomic absorption spectrophotometry and Raman spectroscopy. The structural characterization of the Au- and Pd-containing catalysts after calcination showed that the solids are predominantly formed by Au0, Pd0 and PdO species and was observed that the metallic Pd dispersion diminished in the presence of Au0. The results revealed that the catalytic behavior could be influenced by the experimental conditions and the nature of the catalyst employed. The Pd/SiO2 catalyst showed the best performance among the catalysts tested at the highest reaction temperature (600xa0°C) due to the more effective action of the metallic active phase, which covers a greater area in this sample. At this same reaction temperature, the Au–Pd/SiO2 catalyst showed a significant deactivation, probably due to the lower Pd dispersion presented by this catalyst.

Optical phonon characteristics of incommensurate and commensurate modulated phases of Bi3NbO7 ceramics

Raman and infrared-reflectivity spectroscopies were used to investigate the optical phonon modes of the cubic-incommensurate and tetragonal-commensurate phases of Bi3NbO7 ceramic samples. For both structures, the observed features could be well described by applying the factor-group method to average oxygen-deficient fluorite subcells; i.e., it was possible to predict the correct number and the activities of the observed phonon modes. The spectra showed rather broad bands, which are mainly for the cubic-incommensurate phase, probably because of the disorder of the oxygen vacancies and the folding of the Brillouin zone due to the modulation of the atomic positions. This can partially explain the higher quality factor and the potential interest in the tetragonal phase of this material for applications in microwave devices.

Effect of the processing parameters on the crystalline structure of lanthanide orthotantalates

The influence of the synthesis parameters on the crystalline structures of orthotantalate ceramics has been investigated. Powder materials were prepared by the solid-state reaction route. X-ray diffraction and Raman scattering measurements were employed to investigate the crystal structure of the produced materials. In this work, we analyzed three different examples in which the temperature and time were decisive on the final crystal structure of LnTaO 4 compounds besides the lanthanide ionic size. Firstly, the thermal evolution for NdTaO 4 samples showed that mixed crystal phases are formed up to 1100 °C, while well-crystallized M-NdTaO 4 (I2/a) materials are obtained in temperatures higher than 1200 °C. Also, the influence of the synthesis time was investigated for the LaTaO 4 ceramics: it was necessary 14 h to obtain samples in the P2 1 /c structure. Finally, two polymorphs could be obtained for the DyTaO 4 ceramics: P2/a and I2/a space groups were obtained at 1300 °C and 1500 °C, respectively. This study indicated that the temperature, time and lanthanide size are directly correlated with the crystalline arrangement of the orthotantalate materials.

Optical phonon modes and infrared dielectric properties of monoclinic CoWO4 microcrystals

The phonon characteristics of CoWO4 microcrystals with monoclinic Wolframite structure were investigated by far-infrared (IR) and Raman spectroscopies. Near-normal spectra were taken for IR light polarization along the principal b-axis (A u modes) and along several angles within the ac-plane (B u modes). The IR spectra were analyzed with a generalized Drude–Lorentz model, and all predicted polar phonon modes were fully determined, including their symmetries, the dielectric Lorentz parameters and the non-orthogonal phonon polarizations for the B u modes. Anomalous dispersion and negative values for the real and imaginary parts of the off-diagonal components of the dielectric tensor functions were identified and discussed under the light of the varying phonon polarization directions (spread out in the ac-plane). The obtained static and background dielectric tensors gave an average permittivity of 16.1 (at microwave region), refractive indices along the principal dielectric axes of 2.22, 2.33 and 2.44 (at 1 μm), the optical axes, and an estimated value for the biaxial angle of the crystal. Polarized Raman spectra on appropriate scattering configurations revealed the 18 non-polar gerade phonons of CoWO4 crystals, with their correct symmetries attributed.

Infrared dispersion analysis and Raman scattering spectra of taurine single crystals

A comprehensive set of optical vibrational modes of monoclinic taurine crystals was determined by Raman scattering, and infrared reflectivity and transmission spectroscopies. By using appropriate scattering/reflection geometries, the vibrational modes were resolved by polarization and the most relevant modes of the crystal could be assigned. In particular, we were able to review the symmetry of the gerade modes and to resolve ambiguities in the literature. Owing to the non-orthogonal character of Bu modes in monoclinic crystals (lying on the optic axial plane), we carried out a generalized Lorentz dispersion analysis consisting of simultaneous adjust of infrared-reflectivity spectra at various light polarization angles. The Au modes (parallel to the C2-axis) were treated within the classical Lorentz model. The behavior of off-diagonal and diagonal terms of the complex dielectric tensors and the presence of anomalous dispersion were discussed as consequences of the low symmetry of the crystal.