Covellite (CuS) is an important mineral sulfide that can be used in many technological applications. It has a simple formula but a complex structure consisting of alternating layers of planar CuS3 triangles and CuS4 tetrahedrons with S-S bonds. Accurate first-principles calculations are performed for covellite structure (CuS), aiming to provide insights about its structural, mechanical and electronic properties and to unveil the nature of its chemical bonding. DFT and DFT+U methods have been used and showed to be sensitive to the correlation treatment (U value). Although it is not possible to extract a universal value of the U, this study indicates that U = 5 eV is an adequate value. The electronic structure analysis shows a significant metallic character due to p(S)-d(Cu) orbital interactions up to Fermi level. The projected density of states indicates that most of the contribution comes from the atomic orbitals in the [001] plane of the covellite, explaining the conductivity anisotropy observed experimentally. Topological analysis of the electron density was performed by means of quantum theory of atoms in molecules (QTAIM). Two different topological charges in Cu and S were calculated, confirming an ionic model with mix-charges. This mineral presents ionic degree of ∼ 32%. On the basis of the QTAIM analysis, the covalent character of S-S bond is confirmed, and the favored cleavage of CuS at the [001] surface might be at the Cu-S bond. The S atoms occupy most of the cell volume, and their contributions dominate the crystal compressibility: κ(S) ≈ κ(CuS).
Electrochemical reactions depend on the electrochemical interface between the electrode surfaces and the electrolytes. To control and advance electrochemical reactions there is a need to develop realistic simulation models of the electrochemical interface to understand the interface from an atomistic point‐of‐view. Here we present a method for obtaining thermodynamic realistic interface structures, a procedure we use to derive specific coverages and to obtain ab initio simulated cyclic voltammograms. As a case study, the method and procedure is applied in a matrix study of three Cu facets in three different electrolytes. The results have been validated by direct comparison to experimental cyclic voltammograms. The alkaline (NaOH) cyclic voltammograms are described by H* and OH*, while in neutral medium (KHCO3) the CO3* species are dominating and in acidic (KCl) the Cl* species prevail. An almost one‐to‐one mapping is observed from simulation to experiments giving an atomistic understanding of the interface structure of the Cu facets. Atomistic understanding of the interface at relevant eletrolyte conditions will further allow realistic modelling of electrochemical reactions of importance for future eletrocatalytic studies.
The mechanism of the initial steps of pyrite (100) surface oxidation was investigated in detail by means of density functional theory/plane-wave calculations. Pyrite oxidation is related to many environmental and technological issues, and its mechanism has not been completely understood. A chemical picture of the pyrite oxidation process in the presence of oxygen and water was proposed in the present investigation. The reaction steps of the oxidation mechanism can be separated into two types. Type I reactions present lower activation energies and are redox processes that involve oxidation of two Fe(II) sites on the surface to form predominantly the Fe(III)−OH − . This species is formed from hydrogen transfer between the adsorbed water to the adsorbed oxygen molecule on the Fe(II) sites. Type II reactions present higher activation energies and lead to the formation of a SO bond through the hydrogen atom transference from a water molecule to the Fe(III)−OH − species, forming Fe(II)−OH 2 . These reactions present higher activation energies. The determinant step of this oxidation mechanism involves the formation of two adsorbed hydroxide species (OH − ) on the surface. The hydroxides in the presence of water from the bulk liquid react to form two water molecules adsorbed on the surface and the first S−O chemical bond. Parallel reactions were investigated explaining the experimental detection of the O 2 − and OOH − species. Furthermore, the proposed mechanism explains the experimental observation that the oxygen present in the sulfate is mostly originated from water instead of an oxygen molecule. The present study strengthens the importance of the water/solid interface to understand the oxidation mechanism of pyrite in the presence of water at a molecular level.
Glycerol electrolysis affords a green and energetically favorable route for the production of value‐added chemicals at the anode and H2 production in parallel at the cathode. Here, a facile method for trapping Pt nanoparticles at oxygen vacancies of molybdenum oxide (MoOx) nanosheets, yielding a high‐performance MoOx/Pt composite electrocatalyst for both the glycerol oxidation reaction (GOR) and the hydrogen evolution reaction (HER) in alkaline electrolytes, is reported. Combined electrochemical experiments and theoretical calculations reveal the important role of MoOx nanosheets for the adsorption of glycerol molecules in GOR and the dissociation of water molecules in HER, as well as the strong electronic interaction with Pt. The MoOx/Pt composite thus significantly enhances the specific mass activity of Pt and the kinetics for both reactions. With MoOx/Pt electrodes serving as both cathode and anode, two‐electrode glycerol electrolysis is achieved at a cell voltage of 0.70 V to reach a current density of 10 mA cm−2, which is 0.90 V less than that required for water electrolysis.
RESUMO -Durante período de 20 anos , 56 pacientes com diagnóstico de mielorradiculopatia esquistossomótica foram internados em três hospitais de Belo Horizonte Minas Gerais. Dados de cada paciente foram coletados retrospectivamente de seus prontuários. Em todos os casos, o diagnóstico foi presumido e baseou-se nos seguintes critérios: 1) quadro de comprometimento medular torácico baixo ou lombossacro (síndrome de cone medular e/ou cauda equina); 2) epidemiologia positiva para esquistossomose; 3) comprovação laboratorial de esquistossomose através de exame parasitológico de fezes ou biópsia retal; e 4) exclusão de outras patologias que pudessem causar quadro semelhante. Vários aspectos clínicos e epidemiológicos foram estudados para determinar o perfil diagnóstico da mielorradiculopatia esquistossomótica nesta amostra e são apresentados neste artigo. Reconhecendo os vários problemas no diagnóstico da mielorradiculopatia esquistossomótica, enfatizamos a importância de se pensar nesta entidade e sugerimos critérios para definição diagnóstica.PALAVRAS-CHAVE: esquistossomose mansoni, radiculopatia, mielite. Clinical profile and criteria for the diagnosis of schistosomotic myeloradiculopathyABSTRACT -During 20 year period 56 patients with a diagnosis of schistosomotic myeloradiculopathy were admitted in three hospitals of Belo Horizonte -Minas Gerais. Data from pacients were collected retrospectively from their medical records. In all cases, the diagnosis was inferred in a presumably way and was based on the following considerations: 1) the finding of low thoracic/upper lumbar neurological symptoms; 2) positive epidemiology for schistosomiasis; 3) demonstration of exposure to schistosomiasis through parasitologic or serologic techniques; and 4) the exclusion of other known causes of transverse myelitis and myeloradiculitis. Several clinical and epidemiological aspects were studied to determine the diagnosis profiles of the schistosomotic myeloradiculopathy in this sample and are presented in this paper. In recognizing the many problems in diagnosis of schistosomotic myeloradiculopathy we emphasize how important is thinking about this entity and criteria to improve diagnostic evaluation are suggested. O comprometimento do sistema nervoso no curso da esquistossomose mansoni vem ganhando, paulatinamente, importância cada vez maior na literatura especializada. As dificuldades no seu reconhecimento provavelmente limitam seu diagnóstico. Com relação ao sistema nervoso, não se conhece a frequência do seu envolvimento. Scrimgeour & Gajdusek (1981) relatam que, na Tanzânia, a esquistossomose foi responsável por 1% dos casos de paraplegia não traumática e que, em 5% de outros casos, estabeleceu-se o seu diagnóstico presuntivo 4 . A neuroesquistossomose pode resultar numa grande variedade de complicações. Seu diagnóstico é difí-
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