ESTGV - DA - Artigo em revista científica, indexada ao WoS/Scopus
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- Solid-state single and triple-quantum MAS NMR studies of ferroelectric Pb(Mg1/3Nb2/3)O3 and a related pyrochlorePublication . Cruz-Lopes, Luísa; Cruz, L.P.; Rocha, J.; De Jesus, J.D.P.; Savariault, J.M.; Galy, J.Pb Mg Nb O PMN, a well-known relaxor ferroelectric material, and a related pyrochlore phase have been studied by single- and triple-quantum 93Nb MAS NMR spectroscopy. The assignment of the NMR resonances has been attempted.
- Synthesis and Characterization of Tin NiobatesPublication . Cruz-Lopes, Luísa; Savariault, J.-M.; Rocha, J.; Jumas, J.-C.; Pedrosa de Jesus, J.D.The synthesis and the structural characterization of tin niobates Sn11x Nb2 O61x , x50.0, 0.5, and 1.0, are reported. The materials have been characterized by bulk chemical analysis, single-crystal and powder X-ray di4raction, and 119Sn Mossbauer and 93Nb and 119Sn solid-state NMR spectroscopies. SnNb2 O6 is a synthetic analog of the rare mineral foordite. Red Sn2 Nb2 O7 crystallizes in a pyrochlore structure with a510.5386 A. 119Sn Mossbauer reveals the presence of both Sn(II) and Sn(IV) in pyrochlore samples. 119Sn MAS NMR spectroscopy supports the presence in synthetic foordite of Sn(II) in eight-fold coordination. The six-coordinated Sn(IV) and eight-coordinated Sn(II) 119Sn MASNMRresonances are not resolved in the Sn2 Nb2 O7 pyrochlore spectrum. 93Nb NMR indicates a fairly distorted local environment for niobium in synthetic foordite, much more so than in the Sn2 Nb2 O7 pyrochlore.
- Pyrochlore-type tin niobatePublication . Cruz-Lopes, Luísa; Savariault, Jean-Michel; Rocha, JoãoA single crystal of Sn(1.59)Nb(1.84)O(6.35) was grown at 1273 K from a mixture of sodium niobate and tin(II) chloride. The structure is of pyrochlore type A(2)B(2)O(7). The tin is partially oxidized to tin(IV) and competes with niobium for the occupation of site B. The stereoactivity of the Sn(2+) lone pair induces displacement of tin towards the O atoms of the tunnel.
- An easy way to Pb(Mg1/3Nb2/3)O3 synthesisPublication . Cruz-Lopes, Luísa; Segadães, Ana M.; Rocha, João; Pedrosa De Jesus, Júlio D.This work describes the very expeditious combustion synthesis of Pb(Mg1/3Nb2/3)O3 perovskite powders from mixtures of Pb(NO3)2, Mg(NO3)2·6H2O and Nb2O5, as cation precursors, and urea as fuel. The powders produced were characterized by XRD, SEM (EDS), and ICP and MAS NMR spectroscopies. Since, with this method, exposure to high temperatures is reduced to a minimum, the as-prepared combustion product shows no PbO loss. The % PMN in the as-prepared combustion powder is sensitive to, and increases with, the rise in the ignition temperature, and gets further improved upon calcination. However, the benefit of calcination, in terms of the reached maximum in the % PMN, becomes more modest when the ignition temperature is increased. Subsequent heat treatments of powder compacts in air (normal processing usually includes some kind of sintering step in order to improve crystallinity and densify the material), showed that over 90% Pb(Mg1/3Nb2/3)O3 can be readily obtained at 870°C.
- Non-stoichiometric mixed lead and tin niobatesPublication . Cruz-Lopes, Luísa; Savariault, J.-M; Morais, C.M; Fernandez, C; Pedrosa de Jesus, J.D; Rocha, JNovel non-stoichiometric mixed lead and tin niobates with the pyrochlore structure have been synthesized via a solid-state route. The materials have been characterized by bulk chemical analysis, single-crystal and powder X-ray diffraction, 207Pb, 119Sn and 93Nb (single and triple quantum) solid-state NMR spectroscopy. The structures of the new non-stoichiometric mixed lead–tin niobate pyrochlore compounds have been Rietveld refined in the cubic system, space group Fd-3m. The steroactivity of the Sn(II) free electron pair leads to the general formula (Sn1.6−xPbx)(Nb2−ySny)O6.6−0.5y, where x (0–1.6) is the Pb(II) and y (0–0.34) is the Sn(IV) contents. 207Pb, 119Sn and (in particular) 93Nb NMR support the isomorphous substitution of tin for lead in the compounds studied. 93Nb triple-quantum magic-angle spinning NMR spectroscopy show that the materials contain, at least, two distinct niobium sites, in a 2:1 population ratio, which may be associated with different sample
- Evaluation of isoprene degradation in the detailed tropospheric chemical mechanism, MCM v3, using environmental chamber dataPublication . Pinho, P.G.; Pio, C.A.; Jenkin, M.E.The isoprene degradation mechanism included in version 3 of the Master Chemical Mechanism (MCM v3) has been evaluated and refined, using the Statewide Air Pollution Research Center (SAPRC) environmental chamber datasets on the photo-oxidation of isoprene and its degradation products, methacrolein (MACR) and methylvinyl ketone (MVK). Prior to this, the MCM v3 butane degradation chemistry was also evaluated using chamber data on the photo-oxidation of butane, and its degradation products, methylethyl ketone (MEK), acetaldehyde (CH3CHO) and formaldehyde (HCHO), in conjunction with an initial evaluation of the chamber-dependent auxiliary mechanisms for the series of relevant chambers. The MCM v3 mechanisms for both isoprene and butane generally performed well and were found to provide an acceptable reaction framework for describing the NOx-photo-oxidation experiments on the above systems, although a number of parameter modifications and refinements were identified which resulted in an improved performance. All these relate to the magnitude of sources of free radicals from organic chemical process, such as carbonyl photolysis rates and the yields of radicals from the reactions of O3 with unsaturated oxygenates, and specific recommendations are made for refinements. In addition to this, it was necessary to include a representation of the reactions of O(3P) with isoprene, MACR and MVK (which were not previously treated in MCM v3), and conclusions are drawn concerning the required extent of free radical formation from these reactions. Throughout the study, the performance of MCM v3 was also compared with that of the SAPRC-99 mechanism, which was developed and optimized in conjunction with the chamber datasets.
- Evaluation of alkene degradation in the detailed tropospheric chemistry mechanism, MCM v3, using environmental chamber dataPublication . Pinho, P. G.; Pio, C. A.; Carter, W. P. L.; Jenkin, M. E.The representation of alkene degradation in version 3 of the Master Chemical Mechanism (MCM v3) has been evaluated, using environmental chamber data on the photo-oxidation of ethene, propene, 1-butene and 1-hexene in the presence of NOx, from up to five chambers at the Statewide Air Pollution Research Center (SAPRC) at the University of California. As part of this evaluation, it was necessary to include a representation of the reactions of the alkenes with O(3P), which are significant under chamber conditions but generally insignificant under atmospheric conditions. The simulations for the ethene and propene systems, in particular, were found to be sensitive to the branching ratios assigned to molecular and free radical forming pathways of the O(3P) reactions, with the extent of radical formation required for proper fitting of the model to the chamber data being substantially lower than the reported consensus. With this constraint, the MCM v3 mechanisms for ethene and propene generally performed well. The sensitivity of the simulations to the parameters applied to a series of other radical sources and sink reactions (radical formation from the alkene ozonolysis reactions and product carbonyl photolysis; radical removal from the reaction of OH with NO2 and β-hydroxynitrate formation) were also considered, and the implications of these results are discussed. Evaluation of the MCM v3 1-butene and 1-hexene degradation mechanisms, using a more limited dataset from only one chamber, was found to be inconclusive. The results of sensitivity studies demonstrate that it is impossible to reconcile the simulated and observed formation of ozone in these systems for ranges of parameter values which can currently be justified on the basis of the literature. As a result of this work, gaps and uncertainties in the kinetic, mechanistic and chamber database are identified and discussed, in relation to both tropospheric chemistry and chemistry important under chamber conditions which may compromise the evaluation procedure, and recommendations are made for future experimental studies. Throughout the study, the performance of the MCM v3 chemistry was also simultaneously compared with that of the corresponding chemistry in the SAPRC-99 mechanism, which was developed and optimized in conjunction with the chamber datasets.
- Evaluation of α- and β-pinene degradation in the detailed tropospheric chemistry mechanism, MCM v3.1, using environmental chamber dataPublication . Pinho, P. G.; Pio, C. A.; Carter, W. P. L.; Jenkin, M. E.The representation of the degradation of the monoterpenes, α- and β-pinene, in version 3.1 of the Master Chemical Mechanism (MCM v3.1) has been evaluated, using environmental chamber data from the Statewide Air Pollution Research Center (SAPRC) at the University of California. As part of this evaluation, a representation of the reactions of the monoterpenes with O(3P) has also been included, these reactions being significant under chamber conditions but generally insignificant under atmospheric conditions. The results demonstrate that MCM v3.1 provides a consistent description of the photo-oxidation of α-pinene/NOX mixtures for a range of initial VOC/NOX, but with the formation rate of ozone and decay rate of α-pinene generally being overestimated. Sensitivity of the system to parameter uncertainties and mechanistic variations proposed in the literature are described. The collective implementation of a number of refinements allows the simulations to be brought into good agreement with the experimental observations for the complete series of experiments, with each of the refinements being consistent with reported parameter uncertainty ranges or mechanistic adjustments. The system is particularly sensitive to the magnitudes of sources and sinks of free radicals. The impacts of several other reported mechanistic variations which potentially influence the first generation product distribution and the ozone formation chain length of the initial oxidation step are also described and assessed. MCM v3.1 is shown to provide a reasonable, but less consistent, description of photo-oxidation of β-pinene/NOX mixtures. The simulated magnitudes of the ozone formation rates and β-pinene removal rates are broadly comparable with the experimental observations, but the mechanism tends towards overestimation of ozone formation at low VOC/NOX and underestimation at high VOC/NOX. Implementation of a number of mechanistic variations reported in the literature does not allow the associated simulations to be brought into good agreement with the observations for the entire VOC/NOX range. The system is particularly sensitive to changes which influence the formation of HCHO (and resultant radical production upon its photolysis), and the impacts of the tested mechanistic variations are usually dominated by this effect. As a result of this work, gaps and uncertainties in the kinetic, mechanistic and chamber database for the monoterpenes are identified and discussed.
- ChemInform Abstract: Pyrochlore-Type Tin Niobate.Publication . Cruz, Luisa Paula; Savariault, Jean-Michel; Rocha, JoaoChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
- Chemical composition of grape stalks of Vitis vinifera L. from red grape pomacesPublication . Prozil, Sónia O.; Evtuguin, Dmitry V.; Cruz-Lopes, LuísaThe chemical composition and the structure of macromolecular components of grape stalks from red grape pomaces have been evaluated. These are composed mainly of cellulose (30.3%), hemicelluloses (21.0%), lignin (17.4%), tannins (15.9%) and proteins (6.1%). Among hemicelluloses the xylan was the most abundant (ca. 12%). The parameters of cellulose unitary cell, average diameter of nanofibrils and the degree of crystallinity (75.4%) were assessed by X-ray scattering analysis. The xylan was partially acetylated glucuronoxylan (DS=0.49) possessing the Xylp:MeGlcpA ratio of 25:1. The lignin of grape stalks was suggested to be of HGS type with H:G:S molar proportion of 3:71:26 as revealed by analysis of nitrobenzene oxidation products. Among alkali soluble condensed tannins procyanidins prevailed over prodelphinidins. The abnormal response of grape stalks to kraft pulping, leading to poorly delignified fibrous material, was attributed to a particular lignin structure and it structural association with other macromolecular components of grape stalks.