Carlos A. P. Leite

Catalysts for DMFC: relation between morphology and electrochemical performance

Abstract In this work Pt–Ru/C alloys were prepared by chemical reduction using various modifications of the formic acid method with the objective of improving the performance. The supported alloys were characterized by XRD, backscattering and TEM analyses, and the active areas were determined by CO adsorption. The materials showed similar particle sizes and different morphological characteristics. The performance of the supported alloys for methanol electrooxidation was evaluated by linear sweep voltammetry experiments and curves of cell potential vs. current density in a single direct methanol fuel cell. It is shown that the difference in the morphology of the catalysts in nano- and microscale is the main effect in the different performances.

Preparation and characterization of ethanol-treated silk fibroin dense membranes for biomaterials application using waste silk fibers as raw material.

The possibility of producing valued devices from low cost natural resources is a subject of broad interest. The present study explores the preparation and characterization of silk fibroin dense membranes using waste silk fibers from textile processing. Morphology, crystallinity, thermal resistance and cytotoxicity of membranes as well as the changes on the secondary structure of silk fibroin were analyzed after undergoing treatment with ethanol. Membranes presented amorphous patterns as determined via X-ray diffraction. The secondary structure of silk fibroin on dense membranes was either random coil (silk I) or beta-sheet (silk II), before and after ethanol treatment, respectively. The sterilized membranes presented no cytotoxicity to endothelial cells during in vitro assays. This fact stresses the material potential to be used in the fabrication of biomaterials, as coatings of cardiovascular devices and as membranes for wound dressing or drug delivery systems.

Easy polymer latex self-assembly and colloidal crystal formation: the case of poly[styrene-co-(2-hydroxyethyl methacrylate)]

Abstract Poly[styrene-co-(2-hydroxyethyl methacrylate)] core and shell latex particles easily undergo self-ordering, as evidenced by the iridescence of dispersions and dry solids. Microscopic (scanning electron microscopy) observation of the dry latex and fracture surfaces reveals the coexistence of different crystallographic arrangements. Many domains are observed, in which particle positions are correlated for tens of crystallographic planes. Isotherms of latex monolayers were also obtained, showing a strong interparticle repulsion. The ease of particle self-assembly in this latex is assigned to two factors: (i) the existence of strong repulsive interparticle interactions, which allows particle ordering at long distances, while particle diffusion throughout the dispersion is still possible; (ii) the hydrophilic surface layer, which allows for strong capillary adhesion during the whole drying process, according to Nagayamas model (Nature, 361 (1993) 26).

Porphyrins entrapped in an alumina matrix

Novel hybrid organic–inorganic catalysts constituted by iron(III) or manganese(III) 5,10,15,20-tetrakis(pentafluorophenyl) porphyrin entrapped in an alumina amorphous matrix have been prepared. The hybrid materials were obtained by a non-hydrolytic sol–gel route, through the condensation of aluminium chloride with diisopropyl ether in the presence of metalloporphyrin. The presence of the metalloporphyrin entrapped in the alumina matrix is confirmed by ultraviolet–visible spectroscopy and electron spectroscopic imaging. The material was also analysed by infrared spectroscopy, selected area diffraction, scanning electron microscopy, thermogravimetric analysis and differential thermal analysis, and its surface area was determined. Comparison between the leaching of metalloporphyrin from non-hydrolytic materials and adsorbed metalloporphyrin on commercial neutral alumina confirms that in the non-hydrolytic materials the metalloporphyrin is entrapped and not just adsorbed on the alumina surface. The use of a conventional hydrolytic sol–gel process leads to the complete leaching of the metalloporphyrin from the matrix, underlining the importance of the non-hydrolytic alumina gel process in the matrix preparation. The prepared alumina matrix materials are amorphous, even after heat treatment up 270 °C. The new catalysts prepared were tested for their ability to catalyse the epoxidation of (Z)-cyclooctene using iodosylbenzene as oxygen donor, giving high yields in the epoxidation, similar to those obtained using the metalloporphyrin in solution or supported on a silica matrix.

Iron Insertion and Hematite Segregation on Fe-Doped TiO2 Nanoparticles Obtained from Sol–Gel and Hydrothermal Methods

Iron-doped TiO(2) (Fe:TiO(2)) nanoparticles were synthesized by the sol-gel method (with Fe/Ti molar ratio corresponding to 1, 3, and 5%), followed by hydrothermal treatment, drying, and annealing. A similar methodology was used to synthesize TiO(2) and α-Fe(2)O(3) nanoparticles. For comparison, a mixture hematite/titania, with Fe/Ti = 4% was also investigated. Characterization of the samples using Rietveld refinement of X-ray diffraction data revealed that TiO(2) consisted of 82% anatase and 18% brookite; for Fe:TiO(2), brookite increased to 30% and hematite was also identified (0.5, 1.0, and 1.2 wt % for samples prepared with 1, 3, and 5% of Fe/Ti). For hematite/titania mixture, Fe/Ti was estimated as 4.4%, indicating the Rietveld method reliability for estimation of phase composition. Because the band gap energy, estimated as 3.2 eV for TiO(2), gradually ranged from 3.0 to 2.7 eV with increasing Fe content at Fe:TiO(2), it can be assumed that a Fe fraction was also inserted as dopant in the TiO(2) lattice. Extended X-ray absorption fine structure spectra obtained for the Ti K-edge and Fe K-edge indicated that absorbing Fe occupied a Ti site in the TiO(2) lattice, but hematite features were not observed. Hematite particles also could not be identified in the images obtained by transmission electron microscopy, in spite of iron identification by elemental mapping, suggesting that hematite can be segregated at the grain boundaries of Fe:TiO(2).

Synthesis of fluorinated metalloporphyrinosilica imprinted with templates through sol-gel process

Abstract We present the synthesis of a hybrid material containing fluorinated iron porphyrins through hydrolysis and polycondensation of iron porphyrin bearing a trifluorosilyl function with tetraethoxysilane in the presence of nitrogen bases acting as template molecules. The presence of metalloporphyrin Soret band is detected in the ultraviolet–visible absorption spectra of all metalloporphyrinosilica-templates. Thermogravimetric analysis and electron paramagnetic resonance of the materials also confirm the presence of metalloporphyrin in a silica network. Electron spectroscopy imaging was that of a non-crystalline microstructure. The iron and silicon distribution are homogeneous for the elements in all particle sites. The iron porphyrinosilicas-template were active as catalysts for cyclooctene epoxidation using iodozylbenzene as oxygen donor. In general, the epoxidation yield is larger for iron porphyrinosilicas-template than for manganese porphyrinosilicas-template. The manganese porphyrinosilicas-template had a smaller activity due to their manganese oxidation state. The largest catalytic yield were obtained with the iron porphyrinosilica-pyridine (85%).

Core-and-shell nature of Stöber silica particles

Two different samples of monodisperse Stober silica particles were examined in the analytical transmission electron microscope, using different imaging modes: bright-field, dark-field, energy-loss and elemental distribution maps. The particles (effective diameters = 141 and 36 nm) are formed by domains of variable O/Si ratio, which is consistent with a variable degree of hydration, and they coexist with particles with a high O/Si ratio measuring a few nanometers only, which appear dispersed in the picture background. Bright-field and energy-loss images of the larger particles show a core-and-shell morphology, and the shells have a higher amount of high-O/Si domains as well as contaminating carbon compounds. On the other hand, the smaller particles (effective diameter = 36 nm) are also formed by distinct domains, but their morphology is neither spherical or core-and-shell. The mechanisms for particle formation presented in the literature are discussed, considering the present findings.

Synthesis and characterization of magnetic mesoporous particles.

Magnetic mesoporous particles were synthesized and their magnetic and structural properties are reported. The synthesis procedure consists of four steps: (i) preparation of magnetite colloidal nanoparticles; (ii) growth of a silica layer; (iii) development of the mesoporous structure and (iv) template removal. Two different methods for the template removal were studied and their effectiveness was discussed. Magnetization and Mössbauer spectroscopy measurements showed superparamagnetic behavior for the particles at room temperature. X-ray diffraction and nitrogen adsorption measurements showed a mesoporous MCM-41 structure with 2.48nm pore diameter and 1023m(2)/g total area.

Molecular mapping by low-energy-loss energy-filtered transmission electron microscopy imaging.

Structure-function relationships in supramolecular systems depend on the spatial distribution of molecules, ions, and particles within complex arrays. Imaging the spatial distribution of molecular components within nanostructured solids is the objective of many recent techniques, and a powerful tool is electron spectroscopy imaging in the transmission electron microscope (ESI-TEM) in the low-energy-loss range, 0-80 eV. This technique was applied to particulate and thin film samples of dielectric polymers and inorganic compounds, providing excellent distinction between areas occupied by various macromolecules and particles. Domains differentiated by small changes in molecular composition and minor differences in elemental contents are clearly shown. Slight changes in the molecules produce intensity variations in molecular spectra that are in turn expressed in sets of low-energy-loss images, using the standard energy-filtered transmission electron microscopy (EFTEM) procedures. The molecular map resolution is in the nanometer range and very close to the bright-field resolution achieved for the same sample, in the same instrument. Moreover, contrast is excellent, even though sample exposure to the electron beam is minimal.

Synthesis of manganese porphyrinosilica imprinted with templates using the sol-gel process

Abstract The optimized conditions for the preparation of a new manganese porphyrinosilica-template material are reported. The manganese porphyrinosilica-template was prepared by the sol–gel process, by the reaction of –SO 2 Cl groups present in the phenyl rings of MnTDC(SO 2 Cl)PPCl with 3-aminopropyltriethoxysilane. The reaction produces a precursor porphyrinopropylsilyl species, which were then polymerized with tetraethoxysilane. The presence of manganese porphyrin on xerogel is confirmed by ultraviolet visible absorption spectroscopy and thermogravimetric analysis (TGA). The prepared materials have surface areas between 19 and 674 m 2 g −1 . Electron spectroscopy imaging of the materials show that manganese distribution in the xerogel is uniform. Both manganese(III) porphyrinosilica-template and a similar iron(III) porphyrinosilica-template can catalyze the epoxidation of cyclooctene using iodozylbenzene as oxygen donor. The metalloporphyrinosilica-template presents catalytic activity similar to that of metaloporphyrin in solution.