Navegando por Autor "Dias, Anderson"

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Adsorption of organic and inorganic arsenic from aqueous solutions using MgAl-LDH with incorporated nitroprusside.
(2020) Borges, Gabriella Alexandre; Ferreira, Gabriel Max Dias; Siqueira, Kisla Prislen Félix; Dias, Anderson; Najera Navarro, Keirom Osmany; Silva, Sílvia Juliana Barros e; Rodrigues, Guilherme Dias; Mageste, Aparecida Barbosa
An evaluation was made of the use of MgAl-LDH with incorporated nitroprusside as an adsorbent to remove inorganic arsenic (As(III) and As(V)) and organic arsenic (DMA) from aqueous matrices. The material was synthesized by the co-precipitation method at constant pH and was characterized by Raman spectroscopy, infrared spectroscopy, thermogravimetry, X-ray diffraction, and high-resolution transmission electron microscopy, before and after use in the adsorption process. The effects on adsorption of contact time, initial metalloid concentration, and pH were investigated. For an initial concentration of 10 mg L1 and pH 2.00, the MgAl-LDH with incorporated nitroprusside was only able to adsorb the DMA and As (V) species, with removal percentages of 25.10 and 103.8%, respectively. At pH 6.02 and 12.00, only the inorganic species were adsorbed, with removal percentages of 22.93% and 60.07%, respectively, for As(III), and 89.81% and 71.64%, respectively, for As(V). Application of the Langmuir and Freundlich isotherm models indicated that the features of the adsorption process depended on the pH of the medium and the arsenic species. The results showed that the use of MgAl-LDH with incorporated nitroprusside has potential for the development of techniques for the speciation of arsenic species.
Carbon nanostructures-modified expanded vermiculites produced by chemical vapor deposition from ethanol.
(2011) Purceno, Aluir Dias; Barrioni, Breno Rocha; Dias, Anderson; Costa, Geraldo Magela da; Lago, Rochel Montero; Moura, Flávia Cristina Camilo
In this work, chemical vapor deposition using ethanol and FeMo catalysts at 600, 700, 800 and 900 °C was used to modify the surface of expanded vermiculite (EV). Scanning electron microscopy, Raman spectroscopy, X-ray diffraction, Mössbauer spectra, N2-BET surface areas, and carbon elemental analyses suggested that the FeMo catalyst promoted 2–3% growth of carbon in different forms, mainly nanofibers, on the EV surface. The amount of 2–3% carbon produced by ethanol/CVD process on EV produced a 500% increase in the absorption of soybean, diesel and engine oil, with a concomitant decrease on water absorption. These results were discussed in terms of a hydrophobization of the EV surface by the carbon structures and a “sponge-like” effect due to the entangled nanofibers structure, as it was observed by microscopy, and an increase of N2-BET surface area from 3 m2 g−1 for EV up to 21 m2 g–1 for the ethanol CVD-treated EV.
Catalytic carbon deposition-oxidation over Ni, Fe and Co catalysts : a new indirect route to store and transport gas hydrocarbon fuels.
(2013) Oliveira, Patrícia E. F.; Ribeiro, Leandro Passos; Rosmaninho, Marcelo Gonçalves; Ardisson, José Domingos; Dias, Anderson; Oliveira, Luiz Carlos Alves de; Lago, Rochel Montero
In this work, a new two-step route to store and transport associated natural gas, promoted by Ni, Fe and Co supported catalyst was presented. Initially, CH4 is converted into carbon deposits (M/C composite), being Fe catalyst the most active catalyst. In Step 2, M/C composite reacts with H2O producing H2, CO and CH4. TPO experiments showed that efficiency and selectivity of oxidation depends on the metal. Ni catalyst produced mainly H2 and CO, while Fe system was more selective to convert carbon into CH4. The formation of C2 and C3 compounds suggests the presence of a Fischer Tropsch like process.
Ce1−xSmxO1.9−δ nanoparticles obtained by microwave assisted hydrothermal processing : an efficient application for catalytic oxidation of α-bisabolol.
(2014) Silva, Anderson Gabriel Marques da; Rodrigues, Thenner Silva; Dias, Anderson; Fajardo, Humberto Vieira; Gonçalves, Rosana de Fátima; Godinho Júnior, Mário; Robles Azocar, Patrícia Alejandra
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.
Comment on ‘‘prediction of lattice constant in cubic perovskites".
(2007) Moreira, Roberto Luiz; Dias, Anderson
In a recent work by Jiang et al. [Prediction of lattice constant in cubic perovskites, J. Phys. Chem. Solids 67 (2006) 1531–1536], the interrelationship between lattice constant, ionic radii and tolerance factor of cubic perovskites has been established and an empirical equation was obtained. However, the assumption of incorrect ionic coordination led to an incorrect mathematical expression even though the average relative errors between predicted and observed lattice constants of 132 materials were below 1%. Here, corrected coefficients for that empirical expression are obtained, which would likely be useful for investigation of general perovskite materials.
Crystal structure and phonon modes of ilmenite-type NaBiO3 investigated by Raman and infrared spectroscopies.
(2010) Dias, Anderson; Moreira, Roberto Luiz
NaBiO3 is an ilmenite-type compound presenting a trigonal structure. In this work, we have performed optical spectroscopic investigations using Raman scattering and infrared reflectivity for this material. By using group-theory calculations, it was possible to determine the number of optically activemodes. Fitting procedures besides Kramers–Kronig (KK) procedures lead to a consistent collection of phononmodes. The Raman spectrumshowed nine active first-ordermodes, while the infrared one revealed eight polar phonons, in good agreement with the theoretical predictions. The results allowed us to confirm the R3 (#148) space group for this compound, and to establish a set of active phonons not yet reported in the literature.
Crystal structure of fluorite-related Ln3SbO7 (Ln=La–Dy) ceramics studied by synchrotron X-ray diffraction and Raman scattering.
(2013) Siqueira, Kisla Prislen Félix; Borges, Raquel Moreira; Granado, Eduardo; Moreira, Leandro Malard; Paula, Ana Maria de; Moreira, Roberto Luiz; Bittar, Eduardo Matzenbacher; Dias, Anderson
Ln3SbO7 (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb and Dy) ceramics were synthesized by solid-state reaction in optimized conditions of temperature and time to yield single-phase ceramics. The crystal structures of the obtained ceramics were investigated by synchrotron X-ray diffraction, second harmonic generation (SHG) and Raman scattering. All samples exhibited fluorite-type orthorhombic structures with different oxygen arrangements as a function of the ionic radius of the lanthanide metal. For ceramics with the largest ionic radii (La–Nd), the ceramics crystallized into the Cmcm space group, while the ceramics with intermediate and smallest ionic radii (Sm–Dy) exhibited a different crystal structure belonging to the same space group, described under the Ccmm setting. The results from SHG and Raman scattering confirmed these settings and ruled out any possibility for the non-centrosymmetric C2221 space group describing the structure of the small ionic radii ceramics, solving a recent controversy in the literature. Besides, the Raman modes for all samples are reported for the first time, showing characteristic features for each group of samples.
Crystal structures and phonon modes of Ba(Ca1/2W1/2)O3, Ba(Ca1/2Mo1/2)O3 and Ba(Sr1/2W1/2)O3 complex perovskites investigated by Raman scattering.
(2010) Dias, Anderson; Siqueira, Kisla Prislen Félix; Moreira, Roberto Luiz
This work investigates the crystal structures and phonon modes of Ba(Ca1/2W1/2)O3, Ba(Ca1/2Mo1/2)O3 and Ba(Sr1/2W1/2)O3 perovskites by Raman spectroscopy. The samples were produced by conventional solid-state processing at 1200 ◦C. X-ray diffraction showed that single-phase homogeneous materials were produced, which are cubic or pseudo-cubic in symmetry. The existing controversies in the literature for these complex perovskiteswere investigated by comparing experimental Raman data with group-theory analysis. Ceramicswith Ca andWorMowere found to be cubic, space group Fm3m. For thesematerials, four Raman-active bands were observed and the fitting parameters showed that the Ba(Ca1/2Mo1/2)O3 ceramic presents bands at lower wavenumbers if compared with the Ba(Ca1/2W1/2)O3 sample. For the Ba(Sr1/2W1/2)O3 material, two hypotheses were investigated for monoclinic or triclinic structures. The experimental results showed 12 Raman-active modes for this ceramic, which is in perfect agreement with the theoretical predictions for a monoclinic (I2/m) structure.
Disorder-induced symmetry lowering in Ba(Y1/2Nb1/2)O3 ceramics probed by Raman spectroscopy.
(2008) Dias, Anderson; Sá, Rodrigo Gomes; Moreira, Roberto Luiz
This work investigates the evolution of the crystal structure of microwave-hydrothermal synthesized Ba(Y1/2Nb1/2)O3 powders as a function of firing temperature by Raman spectroscopy. The samples were produced at 200 °C and fired at temperatures ranging from 600 to 1600 °C. Raman spectra were obtained at room temperature for all samples and the results showed that materials fired at 1600 °C exhibited tetragonal (I4/m or C5 4h) structure, whereas those fired at lower temperatures exhibited the triclinic (P1 or C1 i ) structure. The results were compared with those observed for ceramics obtained by conventional solid-state methods. It is believed that the lowering of the symmetry verified in materials fired below 1600 °C is a consequence of the local disorder of Y+3 and Nb+5 ions in octahedral sites.
Effect of Sn on methane decomposition over Fe supported catalysts to produce carbon.
(2011) Freitas, Patrícia Elizabeth de; Ribeiro, Leandro Passos; Rosmaninho, Marcelo Gonçalves; Ardisson, José Domingos; Dias, Anderson; Lago, Rochel Montero
In this work, alumina-supported Sn containing Fe catalysts were investigated in CVD reactions (Chemical Vapor Deposition) using methane for carbon production. The catalysts were prepared with 10 wt.% of Fe (as Fe2O3) and 3, 6 and 12 wt.% of Sn (as SnO2) supported on Al2O3 named hereon Fe10Sn3A, Fe5Sn6A and Fe10Sn12A, respectively. These catalysts were characterized by SEM, TPCVD, TPR, TG, Raman,XRDand 57Fe and 119Sn Mössbauer spectroscopy.Methane reacts with Fe10A catalyst (without Sn) in the temperature range 680–900◦C to produce mainly Fe0, Fe3 C and 20 wt.% of carbon deposition. TPR and TPCVD clearly showed that Sn strongly hinders the CH4 reaction over Fe catalyst. 57Fe Mössbauer suggested that in the presence of Sn the reduction of Fe+3 by methane becomes very difficult. 119Sn Mössbauer showed Sn+4 species strongly interact with metallic iron after CVD, producing iron-tin phases such as Fe3SnC and FeSn2. This interaction Sn–Fe increases the CVD temperatures and decreases the carbon yield leading to the production of more organized forms of carbon such as carbon nanotubes, nanofibers and graphite.
Effect of the processing parameters on the crystalline structure of lanthanide orthotantalates.
(2014) Siqueira, Kisla Prislen Félix; Dias, Anderson
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 LnTaO4 compounds besides the lanthanide ionic size. Firstly, the thermal evolution for NdTaO4 samples showed that mixed crystal phases are formed up to 1100 °C, while well-crystallized M-NdTaO4 (I2/a) materials are obtained in temperatures higher than 1200 °C. Also, the influence of the synthesis time was investigated for the LaTaO4 ceramics: it was necessary 14 h to obtain samples in the P21/c structure. Finally, two polymorphs could be obtained for the DyTaO4 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.
Electrocatalytic performance of different cobalt molybdate structures for water oxidation in alkaline media.
(2018) Rodriguez, Mariandry del Valle Rodriguez; Stolzemburg, Matheus Cata Preta; Bruziquesi, Carlos Giovani Oliveira; Silva, Adilson Cândido da; Abreu, Cíntia Grossi de; Siqueira, Kisla Prislen Félix; Oliveira, Luiz Carlos Alves de; Pires, Maíra dos Santos; Lacerda, Lívia Clara Tavares; Ramalho, Teodorico de Castro; Dias, Anderson; Pereira, Márcio César
Cobalt molybdates with different crystalline structures, i.e., α, β, and hydrated (H)-CoMoO4, were synthesized, and their electrocatalytic activities were thoroughly examined for catalyzing the oxygen evolution reaction (OER) in alkaline media. The material characteristics were associated with the electrocatalytic properties by evaluating the CoMoO4 crystal structures (XRD and Raman), morphologies (TEM), and electrochemical features (electrochemically active surface area, roughness factor, electrochemical impedance, Tafel analysis, and controlled-current electrolysis). These combined findings revealed that the electrocatalytic performance is greatly influenced by the crystalline structures of CoMoO4, following the order α-CoMoO4 > H-CoMoO4 > β-CoMoO4. The H-CoMoO4 catalysts crystallized in the triclinic space group, P[1 with combining macron] (#2), with Z = 4. On the other hand, the α- and β-CoMoO4 catalysts exhibited a monoclinic structure, C2/m (#12), with Z = 8. In the OER experiments, α-CoMoO4 showed an overpotential of 0.43 ± 0.05 V compared to the 0.51 ± 0.05 V and 0.56 ± 0.04 V exhibited by the H-CoMoO4 and β-CoMoO4 catalysts, respectively, to achieve 10 mA cm−2. All CoMoO4 structures displayed stability for at least 6 h at a controlled current density of 10 mA cm−2. Finally, computational simulations indicate that the coexistence of Co and Mo ions in edge-shared octahedral sites of α-CoMoO4 may favor the interaction between the O atom of the water molecule and the metal adsorption sites due to its surface being electronically less dense than β- and H-CoMoO4 surfaces, thus resulting in its higher performance for OER.
Estudo fotoeletroquímico do CuBi2O4 modificado com cobalto para produção de hidrogênio via clivagem da água.
(2018) Bruziquesi, Carlos Giovani Oliveira; Silva, Adilson Cândido da; Pereira, Márcio César; Silva, Adilson Cândido da; Afonso, André Santiago; Dias, Anderson
Denominado de kusachiite, o mineral de p-CuBi2O4 (CBO) possui um band-gap óptico entre 1,5-1,8eV e uma densidade de fotocorrente teórica máxima de 19,7- 29,0 mA.cm-2, além das energias das bandas de valência e condução favoráveis à oxidação da água e consequente produção de hidrogênio. Todavia, a sua baixa densidade de fotocorrente e fotocorrosão (típica em óxidos) o impossibilita de ser empregado como fotocatodo para produção de energia. Dentre as técnicas de deposição utilizadas, a de spray pyrolysis mostrou-se a melhor para a obtenção de filmes mecanicamente estáveis e uniformes (qualidades essas imprescindíveis para se alcançar reprodutibilidade). Sintetizado in situ sobre o substrato de vidro FTO utilizando spray pyrolysis, variando condições como: solvente, tempo de calcinação, temperatura e espessura da camada depositada, fora alcançada uma densidade de fotocorrente superior à literatura (−2.5 mA.cm-2 a 0.6 V vs RHE em H2O2) 1. No entanto, a problemática da fotocorrosão persistiu, sendo necessário um estudo mais detalhado a respeito do transporte de cargas minoritárias à interface semicondutor-eletrólito. Este trabalho apresenta uma melhora na eficiência em correntes catódicas e instabilidade fotoquímica quando empregada a dopagem de Co2+. Nesses filmes preparados utilizando 3% de Co, não fora observada a fotorredução do material quando estudados em 0,5 M de Na2SO4 (pH~7), sob fonte de Xe (A.M 1,5G) a um potencial de -0,6V vs. Ag/AgCl, atingindo em média, uma densidade de fotocorrente igual a -2,5 mA.cm-2. Valor esse superior ao encontrado em outros trabalhos. Pode-se inferir que a dopagem do material utilizando Co mostrou-se uma alternativa para evitar a fotorredução. Espera-se que este trabalho contribua para o melhoramento da estabilidade do p-CuBi2O4 para aplicação em células fotoeletroquímicas.
Estudos fenomenológicos associados à aplicação de silicato de sódio em aglomeração a frio de finos de minério de ferro.
(2015) Dutra, Flávio de Castro; Dias, Anderson
Este trabalho apresenta o desenvolvimento de um novo produto para uso na sinterização, denominado minipelota, com diâmetro entre 3 e 8 mm, proveniente da aglomeração de finos de minério de ferro e silicato de sódio líquido, produzida através de pelotamento em discos e endurecida em temperaturas de 100 a 550ºC. Investigou-se a influência da temperatura e da atmosfera de secagem sobre a resistência mecânica da minipelota e o uso de nanomateriais de carbono como alternativas tecnológicas capazes de maximizar as propriedades mecânicas da minipelota, através da formação de nanocompósitos de alto desempenho. A minipelota foi submetida à secagem em atmosferas constituídas de ar atmosférico e/ou CO2. Durante o processo de secagem, observou-se a influência da temperatura e da atmosfera sobre a resistência à compressão da minipelota após exposição à água, assim como na morfologia do silicato de sódio, evidenciando uma dependência entre elas. Do ponto de vista microestrutural, observou-se o crescimento de filamentos aciculares de carbonato de sódio mono-hidratado sobre os filmes de silicato de sódio durante a secagem, em temperaturas entre 200ºC e 300ºC. Em atmosferas ricas em CO2, observou-se uma formação mais intensa dos filamentos aciculares. Esses filamentos são solúveis em água, formam-se na presença de CO2 e influenciam a qualidade final do filme de silicato de sódio formado após secagem. O uso de CO2 durante a secagem do silicato de sódio mostrou-se eficiente para a redução do teor de sódio do filme de silicato (via formação de carbonato de sódio). A menor presença de íons sódio na estrutura do silicato contribuiu para a formação de filmes com maior grau de polimerização e menor solubilidade. O uso de nanomateriais de carbono associados ao silicato de sódio promoveu ganhos da ordem de 285% sobre a resistência a compressão da minipelota. A dispersão dos nanomateriais ao silicato de sódio via ultrassom, potencializada pelo tempo de repouso dos nanomateriais no silicato de sódio foi fundamental para promover a formação de nanocompósitos de alta resistência mecânica. Esse novo produto apresentou propriedades físicas, químicas e metalúrgicas adequadas para transporte e manuseio, além de excelente desempenho nas sinterizações de minério de ferro.
Experimental evaluation of the activity and selectivity of pure MnWO4 and doped with rare earth ions in the CO2 photoreduction process.
(2022) Peixoto, Julia Caroline; Nogueira, André Esteves; Dias, Anderson; Torres, Juliana Arriel; Cruz, Jean Castro da; Oliveira, Cauê Ribeiro de; Siqueira, Kisla Prislen Félix
The reduction of CO2 to produce useful carbon compounds offers a way to manage the emissions from various industrial sectors. However, selecting efficient systems remain a crucial issue, especially considering the low selectivity of the photocatalysts traditionally used in CO2 photoreduction processes. Here we demonstrate that the modification of MnWO4 with rare earth ions (Europium and Terbium) synthesized by the coprecipitation method can tune the activity and selectivity of the products formed in the process of CO2 photoreduction under UV irradiation. The CO2 photoreduction assays indicated that the photocatalysts presented good activity, resulting in acetate, acetone, acetic acid, ethanol, methane, and methanol products. Doping provided selectivity near 100% for CO2 conversion to methanol and increased the capacity of C2 (e.g., ethanol) production, opening the way for a better understanding of the activity and selectivity of tungstates in the CO2 photoreduction process.
Facile preparation of carbon coated magnetic Fe3O4 particles by a combined reduction/CVD process.
(2011) Tristão, Juliana Cristina; Oliveira, Aline A. S.; Ardisson, José Domingos; Dias, Anderson; Lago, Rochel Montero
In this work, we report a simple method for the preparation of magnetic carbon coated Fe3O4 particles by a single step combined reduction of Fe2O3 together with a Chemical Vapor Deposition process using methane. The temperature programmed reactionmonitored by Mo¨ ssbauer, X-ray Diffraction and Raman analyses showed that Fe2O3 is directly reduced by methane at temperatures between 600 and 900 8C to produce mainly Fe3O4 particles coated with up to 4 wt% of amorphous carbon. Thesemagneticmaterials can be separated into two fractions by simple dispersion in water, i.e., a settled material composed of large magnetic particles and a suspended material composed of nanoparticles with an average size of 100–200 nm as revealed by Scanning Electron Microscopy and High-resolution Transmission Electron Microscopy. Different uses for these materials, e.g., adsorbents, catalyst supports, rapid coagulation systems, are proposed.
Feasible and clean solid-phase synthesis of LiNbO3 by microwave- induced combustion and its application as catalyst for low- temperature aniline oxidation.
(2018) Carreño, Neftalí Lenin Villarreal; Deon, Vinícius Gonçalves; Silva, Ricardo M.; Santana, Luiza R.; Pereira, Rodrigo M.; Orlandi, Marcelo Ornaghi; Ventura, Wellington Martins; Dias, Anderson; Taylor, Jason Guy; Fajardo, Humberto Vieira; Mesko, Márcia Foster
In this work, a feasible, fast, clean and efficient microwave-induced combustion method for direct synthesis of LiNbO3 in solid phase was developed. X-ray powder diffraction studies showed that quasi-pure Li−Nb−O phases, such as LiNbO3 and Li3NbO4, or mixtures of LiNbO3, Li3 NbO4 and LiNb3O8, could be successfully synthesized. The resulting powders were efficiently applied as catalysts under ambient conditions in the oxidation process of aniline using hydrogen peroxide as oxidant. The proposed method was performed in a commercial system using high-pressure quartz vessels, which allowed safe control of the reactions−that usually occurs in less than 1 min. The results showed that the reaction conditions as well as the structural and morphological characteristics of the catalyst influenced the aniline oxidation process. Therefore, the present method for the preparation of LiNbO3 described herein, displayed many advantages when compared to conventional combustion methods, such as the physical characteristics of the obtained compound. Moreover, this new approach is considerably faster, safer and cleaner than other traditional procedures described in literature for LiNbO3 synthesis. This new microwave-induced combustion method is less time-consuming, saves energy, as well as affording the stoichiometric formation of inorganic particles.
Gold, palladium and gold–palladium supported on silica catalysts prepared by sol–gel method : synthesis, characterization and catalytic behavior in the ethanol steam reforming.
(2013) Silva, Anderson Gabriel Marques da; Robles Azocar, Patrícia Alejandra; Dias, Anderson; Fajardo, Humberto Vieira; Lovón, Adriana Siviero Pagani; Quintana, Juan José Lovón; Valença, Gustavo Paim
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 (600 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.
High-temperature antiferroelectric and ferroelectric phase transitions in phase pure LaTaO4.
(2017) Abreu, Yuslín Gonzále; Siqueira, Kisla Prislen Félix; Matinaga, Franklin Massami; Moreira, Roberto Luiz; Dias, Anderson
Phase-pure LaTaO4 ceramics was prepared by solid-state reaction. Dielectric spectroscopic data as well as differential scanning calorimetric experiments showed the existence of a sequence two high-temperature firstorder structural phase transitions. The first phase transition occurs above 160 °C (on heating), from the monoclinic P21/c space group at room temperature to the polar orthorhombic Cmc21 group, exhibiting a very large thermal hysteresis probably linked to the reconstructive nature of the structural transition. The second transition occurs around 225 °C to the orthorhombic Cmcm space group, with a small thermal hysteresis. Vibrational Raman spectroscopic analyses confirmed these two sequential phase transitions, as well as the thermal hysteresis observed for both first-order transitions in repeated heating and cooling cycles. The existence of a strong monoclinic distortion at room temperature could be related to the presence of defects (oxygen vacancies) in LaTaO4 ceramics, after sintering. Dielectric spectroscopy showed a strong influence of the electric conductivity on the dielectric response with activation energy of dc component of conductivity (0.62 eV) compatible with the presence of oxygen vacancies. Far-infrared data confirmed that the extra modes observed in the Raman spectra are forbidden bands, which were activated by defects into the structure.
Hydrothermal synthesis and polarized micro-Raman spectroscopy of copper molybdates.
(2018) Martins, Guilherme Mendes; Coelho, Pâmela de Oliveira; Moreira, Roberto Luiz; Dias, Anderson
Conventional and microwave-assisted hydrothermal methods were used to synthesize Cu3(MoO4)2(OH)2 ceramics. Different experimental conditions of temperature, time and precipitation rate were employed to explore the growth mechanisms, structural and morphological properties of the obtained copper molybdates. The use of microwave-assisted reactors at 150 °C/10–30 min produced a mixture of copper molybdate hydrates, while only Cu3(MoO4)2(OH)2 was obtained by using microwave processing at longer times (above 120 min) or conventional hydrothermal reactors (110–250 °C/24 h). Experiments conducted after fast and slow coprecipitation rates resulted in different crystallographic phases and morphologies. Flower-like and rod-like micrometer-sized ceramics were produced with high anisotropy and single-crystalline nature. A better understanding on the growth mechanisms, as well as on the structural and morphological characteristics of copper molybdates was attained. Based upon the relative enhancement of Raman bands in parallel or in perpendicular configuration, a symmetry assignment of 34 (17Ag + 17Bg) external modes for Cu3(MoO4)2(OH)2 ceramics (42 bands are expected) was possible. Besides, five (of six possible) internal OH-stretching modes were identified in the spectral region above 3200 cm−1.