Repositório Institucional da Universidade de Aveiro
Not a member yet
41903 research outputs found
Sort by
Enhanced extraction of caffeine from guarana seeds using aqueous solutions of ionic liquids
No full textThe extraction of caffeine from bioresources using more benign and cost-effective processes is of fundamental relevance towards the finding of alternative (bio) pesticides. Classically, the best attempts to extract caffeine from biomass have resulted in low efficiency and in a large consumption of hazardous organic solvents and/or energy requirements. Here, we report an enhanced and selective extraction of caffeine from guarana (Paullinia cupana, Sapindaceae) seeds using aqueous solutions of ionic liquids. Several ionic liquids composed of imidazolium or pyrrolidinium cations combined with the chloride, acetate and tosylate anions were investigated. Furthermore, the effect of the cation alkyl side chain length and the presence of functionalized groups were also addressed. Additional conditions such as the ionic liquid concentration, the contact time, the solid-liquid ratio and temperature were further optimized by a response surface methodology. Outstanding extraction yields (up to 9 wt% of caffeine per guarana dry weight) were obtained at a moderate temperature and in a short-time. The recyclability and reusability of the ionic liquids were also confirmed. For the first time it is shown that aqueous solutions of ionic liquids are superior alternatives for the solid-liquid extraction of caffeine from biomass samples and, as a result, the development of an ionic-liquid-based process is straightforwardly envisaged
Dielectric and magnetic studies of (NKNLS)(1-x)-(NZFO)(x) multiferroic composites
No full textPolycrystalline samples of perovskite Na0.5K0.5[(0.95)(Li)(0.05)(Sb)(0.05)(Nb)(0.95)O-3] (NKNLS)(1-x)-[Ni0.5Fe2O4] (NZFO)(x) were prepared by the conventional solid-state reaction method. X-ray powder patterns indicated that the NKNLSNZFO phases retain their tetragonal and spinel structure in composites. There are no structural changes observed for both the phases in the composites. SEM figures show rectangular regions of a chain of self-connected NZFO grains distributed in the matrix of NKNLS phase, indicating 3-1-type connectivity. In the same composite, 3-3-type connectivity is also observed. The dielectric transition temperature of x = 0.20 is close to the ferroelectric transition temperature (T-c(FE)) for pure NKNLS. For composites T-c(FE) is observed to decrease with the increase in the mole percentage of NZFO. From magnetic measurement we observed that for each sample both measurements (ZFC and FC) show separate M(T) curves overlapping at the highest temperature. This behavior probably arises from the magnetic domain with different sizes. (C) 2014 Elsevier B.V. All rights reserved
Phase coexistence in Bi1-xPrxFeO3 ceramics
No full textBi1-x Pr (x) FeO3 ceramics across the rhombohedral-orthorhombic phase boundary have been studied by X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. The structural phase transitions in Bi1-x Pr (x) FeO3 driven by doping concentration and temperature are significantly different from those in BiFeO3 compounds doped with other rare-earth elements. The features of the structural transformations have been discussed based on the specific character of the chemical bonds associated with praseodymium ions. The detailed study of the crystal structure evolution clarified the ranges of both single-phase and phase coexistence regions at different temperatures and dopant concentrations. For x = 0.125, compound extraordinary three-phase coexistence state has been observed in a narrow temperature range at about 400 A degrees C. The results explicate driving forces of the structural transitions and elucidate the origin of the remarkable physical properties of BiFeO3-based compounds near the morphotropic phase boundary
Hydrothermal Synthesis of Copper Zirconium Phosphate Hydrate [Cu(OH)(2)Zr(HPO4)(2)center dot 2H(2)O] and an Investigation of its Lubrication Properties in Grease
No full textCopper zirconium phosphate hydrate (Cu(OH)(2)Zr(HPO4)2 center dot 2H(2)O, hereafter referred to as Cu-alpha-ZrP) with high crystallinity was directly synthesized in a NaF-CuO-ZrO-P2O5-H2O system under hydrothermal conditions. The copper ion was confirmed to be an exchangeable cation in the Cu-alpha-ZrP through elemental analysis and a proton ion exchange process. The crystal structure of the Cu-alpha-ZrP was determined ab initio by using X-ray powder diffraction data. In the structure, the CuO6 octahedron would be located in an exchangeable atom position. Moreover, Cu-alpha-ZrP was evaluated as an additive in grease in a four ball test. The maximum nonseizure load (P-B, representing the load-carrying capacity) of the base grease containing Cu-alpha-ZrP was increased from 353 to 1235 N. The excellent load-carrying capacity may be explained by the easier adherence of the material to the worn surface forming a tight protective film
Structure-Properties Relationship in Iron Oxide-Reduced Graphene Oxide Nanostructures for Li-Ion Batteries
No full textNon-aqueous sol-gel routes involving the reaction of metal oxide precursors in organic solvents (e.g., benzyl alcohol) at moderate temperature and pressure, offer advantages such as high purity, high reproducibility and the ability to control the crystal growth without the need of using additional ligands. In this paper, a study carried out on a series of iron oxide/reduced graphene oxide composites is presented to elucidate a structure-properties relationship leading to an improved electrochemical performance of such composites. Moreover, it is demonstrated that the easy production of the composites in a variety of temperature and composition ranges, allows a fine control over the final particles size, density and distribution. The materials obtained are remarkable in terms of the particle's size homogeneity and dispersion onto the reduced graphene oxide surface. Moreover, the synthesis method used to obtain the graphene oxide clearly affects the performances of the final composites through the control of the restacking of the reduced graphene oxide sheets. It is shown that a homogeneous and less defective reduced graphene oxide enables good electrochemical performances even at high current densities (over 500 mAh/g delivered at current densities as high as 1600 mA/g). The electrochemical properties of improved samples reach the best compromise between specific capacity, rate capability and cycle stability reported so far
Sustainable synthesis of a catalytic active one-dimensional lanthanide-organic coordination polymer
No full textRationalization of the synthetic conditions allowed the predictable fast sustainable preparation of [La-2(H(3)nmp)(2)(H2O)(4)]center dot 4.5H(2)O having a 1D coordination polymer. The material exhibits a remarkable chemical stability, can be converted into other layered compounds, and is an excellent catalyst surpassing other related materials
La2NiO4 Ceramic Electrodes for Hydrogen Peroxide Electroreduction
No full textDirect borohydride/peroxide fuel cells (DBPFCs) use sodium borohydride (NaBH4) as an anodic fuel and hydrogen peroxide (H2O2) as the oxidant. Currently, we are focused on finding low-cost, stable, durable, and efficient electrocatalytic materials for H2O2 reduction in DBPFCs. Recently, some perovskite-type oxides were found to have reasonable activity for H2O2 electroreduction. Here, La2NiO4 ceramic disks are prepared by mechanosynthesis and characterized by scanning electron microscopy and X-ray diffraction. Cyclic voltammetry, chronoamperometry and chronopotentiometry are used to investigate La2NiO4 activity for H2O2 reduction in alkaline media, and its possible use as cathodes for DBPFCs
Structural and defect chemistry guidelines for Sr(V,Nb)O-3-based SOFC anode materials
No full textStructural and defect chemistry guidelines were used for Nb-substituted SrVO3-delta materials, designed to meet SOFC anode requirements, with emphasis on redox tolerance, thermochemical compatibility with other SOFC materials, electrical conductivity and adjustable changes in oxygen stoichiometry for their prospective impact on electrocatalytic performance. SrV1-xNbxO3-delta (x = 0-0.30) ceramics were prepared by solid-state synthesis and sintered at 1773 K in a reducing atmosphere. XRD and SEM/EDS showed that under these conditions a single-phase cubic perovskite structure appears up to x approximate to 0.25. Electrical conductivity is metallic-like and nearly p(O-2)-independent. Although substitution by niobium decreases the conductivity, which still exceeds 100 S cm(-1) for x <= 0.20 at temperatures below 1273 K, it also expands the stability domain of the cubic perovskite phase and suppresses partly high thermochemical expansion characteristic of parent SrVO3-delta. The upper p(O-2) limit of phase stability was found to shift from similar to 2 x 10(-15) atm for the undoped material to similar to 2 x 10(-12) atm for x = 0.30, whereas the average thermal expansion coefficient at 773-1223 K decreased from 22.7 x 10(-6) to 13.3 x 10(-6) K-1. SrV1-xNbxO3-delta perovskites undergo oxidative decomposition in air, which causes dimensional and microstructural changes. However, sluggish kinetics of oxidation under inert gas conditions results in nearly reversible behavior in relatively short-term redox cycles between reducing and inert atmospheres. Subtle structural changes and a close correlation with point defect chemistry clarify these sluggish changes and provide guidelines to retain the metastability
Microwave-Assisted Immobilization of Manganese Salen Complexes: Increased Activity and Chemoselectivity in Catalytic Epoxidation
No full textDirectional solidification of ZrO2-BaZrO3 composites with mixed protonic-oxide ionic conductivity
No full textThe synthesis and characterization of directionally solidified Y-doped zirconia-barium zirconate eutectic materials by the laser floating zone technique (LFZ) is presented in the search for novel mixed ionic conductors by protons and oxide ions. Emphasis is placed on the control of the LFZ conditions that promote the solubilization of Y in both eutectic phases, and how the dopant influences the ionic conductivity. Fast growth under constitutional supercooling conditions leads to the development of ZrO2-dendrites above the solidification interface, which are dipped into a finer-interpenetrated network of ZrO2 and BaZrO3. Yttrium-doping of the ZrO2 fluorite-type lattice is favored by the high thermodynamic stability of BaZrO3. However, even low Y-substitution levels in BaZrO3 lead to dominating protonic conduction in the composite under humidified atmospheres below 500 degrees C. At higher temperatures, the proton incorporation is unfavorable, and the total conductivity of the composite is determined by the hole conduction in the Ba(Zr,Y)O-3, and/or oxide-ion conductivity through the (Zr,Y)O-2. (C) 2013 Elsevier B.V. All rights reserved