Repositório Institucional da Universidade de Aveiro
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Vertically aligned N-doped CNTs growth using Taguchi experimental design
No full textThe Taguchi method with a parameter design L-9 orthogonal array was implemented for optimizing the nitrogen incorporation in the structure of vertically aligned N-doped CNTs grown by thermal chemical deposition (TCVD). The maximization of the I-D/I-G ratio of the Raman spectra was selected as the target value. As a result, the optimal deposition configuration was NH3 = 90 sccm, growth temperature = 825 degrees C and catalyst pretreatment time of 2 min, the first parameter having the main effect on nitrogen incorporation. A confirmation experiment with these values was performed, ratifying the predicted I-D/I-G ratio of 1.42. Scanning electron microscopy (SEM) characterization revealed a uniform completely vertically aligned array of multiwalled CNTs which individually exhibit a bamboo-like structure, consisting of periodically curved graphitic layers, as depicted by high resolution transmission electron microscopy (HRTEM). The X-ray photoelectron spectroscopy (XPS) results indicated a 2.00 at.% of N incorporation in the CNTs in pyridine-like and graphite-like, as the predominant species. (C) 2015 Elsevier B.V. All rights reserved
Effect of Artificial Aging and Surface Treatment on Bond Strengths to Dental Zirconia
No full textThe objective of this project was to study the influence of artificial aging and surface treatment on the microtensile bond strengths (mu TBS) between zirconia and a phosphate monomer-based self-adhesive cement. Thirty zirconia disks (IPS e.max ZirCAD, Ivoclar Vivadent) were randomly assigned to two aging regimens: AR, used as received, which served as a control, and AG, artificial aging to simulate low-temperature degradation. Subsequently, the disks of each aging regimen were assigned to three surface treatments: NT, no surface treatment; CO, surface silicatization with CoJet sand (3M ESPE); and ZP, zirconia surface treated with Z-Prime Plus (Bisco Inc). Thirty discs were made of Filtek Z250 (3M ESPE) composite resin and luted to the zirconia discs using RelyX Unicem (3M ESPE). The specimens were sectioned with a diamond blade in X and Y directions to obtain bonded beams with a cross-section of 1.0 +/- 0.2 mm. The beams were tested in tensile mode in a universal testing machine at a speed of 0.5 mm/min to measure mu TBS. Selected beams were selected for fractographic analysis under the SEM. Statistical analysis was carried out with two-way analysis of variance and Dunnett T3 post hoc test at a significance level of 95%. The mean mu TBS for the three AR subgroups (ARNT, AR-CO, and AR-ZP) were significantly higher than those of the corresponding AG groups (p<0.0001). Both AR-CO and AR-ZP resulted in statistically significant higher mean bond strengths than the group AR-NT (p<0.006 and p<0.0001, respectively). Both AG-CO and AG-ZP resulted in statistically significant higher mean bond strengths than the group AG-NT (both at p<0.0001). Overall, AG decreased mean mu TBS. Under the SEM, mixed failures showed residual cement attached to the zirconia side of the beams. CO resulted in a characteristic roughness of the zirconia surface. AR-ZP was the only group for which the amount of residual cement occupied at least 50% of the interface in mixed failures
Novel data and a group contribution method for the prediction of the speed of sound and isentropic compressibility of pure fatty acids methyl and ethyl esters
No full textSpeed of sound of fatty acid esters are relevant for the formulation of biodiesel fuels but data on this property are scarce on literature. In this work speeds of sound of three saturated fatty acid methyl esters (caprate, myristate, palmitate) two unsaturated (oleate, linoleate) as well as two fatty acid ethyl esters (caprate, myristate) were measured using a pulse echo technique operating at 3 MHz. The measurements were carried out at atmospheric pressure in the temperature range 283.15-373.15 K. Additional density measurements were performed in order to estimate the isentropic compressibility and the molecular compressibility. From these data, a group contribution method was developed to predict the molecular compressibility and speed of sound of both methyl and ethyl esters with an uncertainty of circa 0.1%. (C) 2012 Elsevier Ltd. All rights reserved
Multifunctional micro-and nanosized metalorganic frameworks assembled from bisphosphonates and lanthanides
No full textPhase-pure and highly crystalline [Ln(Hpmd)(H2O)] materials [where Ln(3)+ Eu-3+ (1), Gd-3+ (2) and Tb-3+ (3); H(4)pmd 1,4-phenylenebis(methylene) diphosphonic acid] were prepared by using three distinct approaches: (i) conventional hydrothermal synthesis (180 degrees C, 3 days); (ii) microwave-assisted heating (50 W irradiation power, 40 degrees C, 5 seconds; nano-sized aggregates with sizes ranging between 100 and 150 nm); and (iii) ultrasound-assisted synthesis which, for highly diluted reactive mixtures (Ln(3)+ : H4pmd : H2O of 1 : 1 : 7200), permitted the preparation of isolated nano-crystals at ambient temperature with 5 minutes of ultrasonic irradiation. Compounds were structurally characterized by powder X-ray diffraction (Rietveld refinement and variable-temperature studies), thermogravimetry, vibrational spectroscopy, elemental analysis and electron microscopy (SEM andEDS). The magnetic behavior of 1 and 2 was investigated between ambient temperature and ca. 2 K revealing that the Ln(3)+ cations act as isolated centers. A strategy to calculate the vibrational spectra of MOF structures and based on the known embedded-clusters approach is proposed and applied to [ Ln(Hpmd)(H2O)]. This allowed the identification, along with deuteration of the materials, of the vibrational modes of the confined water molecule in the structure. Compound 1 was tested in the methanolysis of styrene oxide at 55 degrees C: it is shown that microcrystalline 1 (1-m) does not possess significant catalytic activity; on the other hand, the nano-sized counterpart (1-n) exhibits relatively high catalytic activity and excellent selectivity to 2-methoxy-2-phenylethanol (100% yield within 48 h of reaction time). Photoluminescence studies both at ambient and low temperatures showed, on the one hand, that bulk materials are composed of a single lanthanide site and, on the other hand, that the organic linker is a suitable sensitizerof Tb-3+ (absolute quantum yield of ca. 14% for 3). It is further demonstrated that the coordinated water molecule has a pivotal role in the quenching of the photoluminescence of Eu-3+ in 1: deuteration of the material results in a ca. 3.4 times improvement of the decay time (e. g., at 300 K the lifetime improves from 0.58 +/- 0.01 ms to 1.98 +/-0.01 ms)
Asymmetric Heterogeneous Catalysis by Nanoporous Materials Using Privileged Ligands as Chiral Building Blocks
No full textAsymmetric catalysis is unique in the sense that a minute quantity of a chiral catalyst is sufficient to produce large amounts of the desired chiral product [1]. With the increased demand for chiral products a number of efficient asymmetric homogeneous catalysts based on transition metal complexes with versatile chiral privileged ligands have been developed that work for a wide range of organic reactions. Their application in industry has been however hindered by their high cost and the product metal contamination issue. The immobilization of asymmetric homogeneous catalysts on porous supports has been a quite explored strategy to make them recyclable and economical. More recently, the construction of innovative hybrid porous materials such as MOFs and PMOs using a modular approach starting from conveniently di-derived chiral privileged ligands has been a reliable strategy to prepare efficient asymmetric heterogeneous catalysts with higher metal loadings than the post-synthetic techniques. The privileged ligands that have been used as MOFs or PMOs chiral building blocks are the BI-NOL/BINAP, salen and bis(oxazoline). Herein focus will be given to the bis-functionalized chiral privileged ligands which have been used to synthesize this innovative nanoporous materials and the outcome of their application as asymmetric heterogeneous catalysts in organic transformations
Heavy metal and PCB spatial distribution pattern in sediments within an urban catchment-contribution of historical pollution sources
No full textThis work analyzes polychlorinated biphenyl (PCB) and heavy metal contamination in fluvial sediments and soils in an urban catchment, according to the geo-accumulation index and to soil and sediment quality guidelines. The catchment is located in Coimbra, Portugal, being affected by frequent flooding, and its main stream is a tributary to one of the major rivers in Portugal (Mondego). Given the presence of industrial activities over time, some inputs of pollutants are expected, but so far, the legacy of historic pollution in this catchment has not yet been investigated. Twenty-five samples were collected from nine sampling sites at the depths of 0-20, 20-40 and 40-60 cm (to provide a historic perspective) along longitudinal profiles (streamlines) and in soils downstream of pollution sources. These samples were analyzed for six heavy metals (Cu, Cr, Pb, Cd, Zn, Ni), organic carbon, pH and ten PCBs (IUPAC numbers 28, 30, 52, 101, 138, 153, 166, 180, 204, 209). Total PCB concentrations ranged 0.47-5.3 ng g(-1) dry weight (dw), and levels increase from the bottom to the top layers, suggesting an increased input over the last 100 years. PCB congener distribution shows the dominance of hexachlorobiphenyls, especially PCB138, suggesting the existence of local sources. PCB levels did not exceed sediment quality levels, placing sediments/soil under class 1 (not contaminated) or class 2 (trace contamination) with respect to PCB. All six metals exceeded the lowest effect level for sediment quality criteria, and three (Cd, Pb and Zn) largely exceeded the clean levels for dredged materials, placing sediments in class 5 (heavily contaminated). Sampling site S1 presented the highest concentrations of Zn, Pb and Cd, and historic vehicle traffic was identified as the most likely source, given the vertical and horizontal profiles. High levels of Pb, Cd and Zn were found in fluvial sediments at some locations of the Loreto catchment, likely from historic traffic sources. This urban area is frequently affected by flooding events and is currently being subject to urban redevelopment. During these events/actions, historic pollutants in the sediments might surface and be redistributed, impacting the downstream ecosystem of the major Mondego River or increasing the risk of exposure of the urban population
Truncated tetragonal bipyramidal anatase nanocrystals formed without use of capping agents from the supercritical drying of a TiO2 sol
No full textTitanium dioxide (TiO2) nanoparticles are extremely attractive materials for numerous applications, especially in the anatase form. We have made these shaped, <10 nm anatase nanoparticles (NPs) via the super-critical (SC) drying of a titania sol, made by a \"green\" aqueous sol-gel nanosynthesis route. The SC drying was carried out in alcohol at 255-260 degrees C, and no further heating or processing of the NPs was required. The true phase composition (crystalline and amorphous phases) and the microstructure of the NPs was thoroughly characterised by the advanced X-ray methods, such as Rietveld-reference intensity ratio (RIR) and the whole powder pattern modelling (WPPM) technique, and HR-TEM analysis. Furthermore, the NPs were also characterised by Raman, FT-IR and optical spectroscopy. These anatase NPs showed themselves to exhibit a truncated tetragonal bipyramidal shape, exposing the {101} (side) and {001} (top) faces. They had a euhedral crystal habit, with sharply defined and easily recognised faces, and were very homogeneous and monodisperse in both shape and size. The photocatalytic activity (PCA) of the samples was assessed in gas-solid phase by monitoring the degradation of nitrogen oxides (NOx), a major atmospheric pollutant. Results showed that the particular shape of these anatase NPs played a key role in their photocatalytic behaviour. In fact, these truncated tetragonal bipyramidal nanocrystals exhibited an enhanced photocatalytic activity, double that of spherical anatase NPs of a similar size reported previously by the authors. This was attributed to the exposure of mainly the {101} and, to a lesser extent, {001} crystal faces, which are more reactive under photocatalysis for redox reactions
Use of Organomolybdenum Compounds for Promoted Hydrolysis of Phosphoester Bonds in Aqueous Media
No full textThe dissolution of the indenyl (Ind) complex [{(eta(5)-Ind)Mo(CO)(2)(mu-Cl)}(2)] (1) in N,N'-dimethylformamide (DMF) gives the ring-slipped complex [(eta(3)-Ind)Mo(CO)(2)Cl(DMF)(2)] (2). The aerial oxidation of 2 leads to the formation of the dinuclear oxomolybdenum(V) chloride [Mo2O2(DMF)(4)(mu-O)(2)Cl-2] (3). The structures of 2 and 3 DMF have been determined by single-crystal X-ray diffraction. Compounds 1 and 3 were examined as promoters of phosphoester bond hydrolysis in para-nitrophenylphosphate (pNPP), which was used as a model substrate. The reactions were performed in aqueous solution at 55 degrees C and followed by H-1 NMR spectroscopy. For assays performed with 30-100 mol-% of 1 or 3 relative to pNPP, both compounds promote the production of para-nitrophenol (pNPh) from pNPP. Compound 3 is especially active in promoting the hydrolytic cleavage of the phosphoester bond (t(1/2) < 80 min)
A High-Temperature Molecular Ferroelectric Zn/Dy Complex Exhibiting Single-Ion-Magnet Behavior and Lanthanide Luminescence
No full textMultifunctional molecular ferroelectrics are exciting materials synthesized using molecular chemistry concepts, which may combine a spontaneous electrical polarization, switched upon applying an electric field, with another physical property. A high-temperature ferroelectric material is presented that is based on a chiral Zn2+/Dy3+ complex exhibiting Dy3+ luminescence, optical activity, and magnetism. We investigate the correlations between the electric polarization and the crystal structure as well as between the low-temperature magnetic slow relaxation and the optical properties
Novel porous fly ash-containing geopolymers for pH buffering applications
No full textIn this work novel, porous biomass fly ash-containing geopolymers with high pH buffer capacity and tailored alkalis leaching were produced by using hydrogen peroxide as blowing agent. Results demonstrate that the geopolymers leaching rate can be controlled by the hydrogen peroxide content and by the solids-to-liquid ratio during geopolymers preparation. Total leaching of hydroxyl ions from the porous geopolymers ranged between 0.0104 and 0.0180 mol/(dm(3) g). Higher solids-to-liquid ratios promoted a gentler alkalis leaching from the geopolymers, while higher porosities shifted the initial pH towards higher values without compromising their buffering ability. The excellent ability to control pH over time shown by these novel geopolymers demonstrates their potential as pH regulators in applications where high buffer capacity is required. Furthermore the reuse of biomass fly ash in these innovative materials reduces the environmental footprint associated with this waste disposal. (C) 2016 Elsevier Ltd. All rights reserved