Time filter

Source Type

São João del Rei, Brazil

Ferreira A.R.,Federal University of Juiz de fora | Martins M.J.F.,Federal University of Juiz de fora | Konstantinova E.,IFSudeste MG | Capaz R.B.,Federal University of Rio de Janeiro | And 3 more authors.
Journal of Solid State Chemistry | Year: 2011

We selected two important γalumina models proposed in literature, a spinel-like one and a nonspinel one, to perform a theoretical comparison. Using ab initio calculations, the models were compared regarding their thermodynamic stability, lattice vibrational modes, and bulk electronic properties. The spinel-like model is thermodynamically more stable by 4.55 kcal/mol per formula unit on average from 0 to 1000 K. The main difference between the models is in their simulated infrared spectra, with the spinel-like model showing the best agreement with experimental data. Analysis of the electronic density of states and charge transfer between atoms reveal the similarity on the electronic structure of the two models, despite some minor differences. © 2011 Elsevier Inc. All rights reserved.

Ferreira A.R.,Federal University of Juiz de fora | Kucukbenli E.,Ecole Polytechnique Federale de Lausanne | De Gironcoli S.,International School for Advanced Studies | De Gironcoli S.,CNR Institute of Materials | And 4 more authors.
Chemical Physics | Year: 2013

The activation of highly catalytic γ-alumina surfaces by thermal treatment and the description of the related chemical processes at atomic scale is a topical issue. According to a recent study [J. Am. Chem. Soc. 134 (2012) 14430], the enhanced reactivity of y-alumina has been associated to tri-coordinated aluminum sites which supposedly are exposed exclusively on the (110) surfaces of this oxide. In this work, we explore this possibility by modeling the (1 00) and (110) terminations using Krokidis et al. [J. Phys. Chem. B 105 (2001)5121] bulk structure and performing an extensive search of the most stable hydrated surface models at conditions consistent with experiment. Among the 156 structures analyzed, we identify several "metastable" models for the (110) surface with a considerable probability of containing the Aim centers at OH coverages of 9.0 and 6.0 OH/nm2. We then test the reactivity of these sites through their Lewis acidity by simulating the CO adsorbtion on the surface and our results confirm the high reactivity of Alm centers. Based on the Gibbs free energy of the explored structures, we carry on a thermodynamical analysis at varying hydroxylation degrees and pretreatment temperatures and simulate the experimental volcano-type behavior reported in [J. Am. Chem. Soc. 134 (2012) 14430] and predict the optimum pretreatment temperature as 700 °C, in very good agreement with experimental findings. We further use infrared and solid state MAS NMR spectroscopies and reproduce the 1H MAS NMR spectra under high vacuum conditions (10~5 Torr). The strong resemblance of spectra to the experimental ones in the literature [J. Phys. Chem. C 116 (2012) 834] validate further the structural models we have generated in this study. © 2013 Published by Elsevier B.V.

Pereira M.H.R.,Federal University of Minas Gerais | De Souza C.L.,IFSudeste MG | Padua F.L.C.,Federal University of Minas Gerais | Silva G.D.,Federal University of Minas Gerais | And 2 more authors.
Multimedia Tools and Applications | Year: 2014

This paper presents a novel multimedia information system, called SAPTE, for supporting the discourse analysis and information retrieval of television programs from their corresponding video recordings. Unlike most common systems, SAPTE uses both content independent and dependent metadata, which are determined by the application of discourse analysis techniques as well as image and audio analysis methods. The proposed system was developed in partnership with the free-to-air Brazilian TV channel Rede Minas in an attempt to provide TV researchers with computational tools to assist their studies about this media universe. The system is based on the Matterhorn framework for managing video libraries, combining: (1) discourse analysis techniques for describing and indexing the videos, by considering aspects, such as, definitions of the subject of analysis, the nature of the speaker and the corpus of data resulting from the discourse; (2) a state of the art decoder software for large vocabulary continuous speech recognition, called Julius; (3) image and frequency domain techniques to compute visual signatures for the video recordings, containing color, shape and texture information; and (4) hashing and k-d tree methods for data indexing. The capabilities of SAPTE were successfully validated, as demonstrated by our experimental results, indicating that SAPTE is a promising computational tool for TV researchers. © 2014 Springer Science+Business Media New York

Discover hidden collaborations