Increasing Complexity Approach to the Fundamental Surface and Interface Chemistry on SOFC Anode Materials

In this Account, we demonstrate an increasing complexity approach to gain insight into the principal aspects of the surface and interface chemistry and catalysis of solid oxide fuel cell (SOFC) anode and electrolyte materials based on selected oxide, intermetallic, and metal–oxide systems at different levels of material complexity, as well as into the fundamental microkinetic reaction steps and intermediates at catalytically active surface and interface sites. To dismantle the complexity, we highlight our deconstructing step-by-step approach, which allows one to deduce synergistic properties of complex composite materials from the individual surface catalytic properties of the single constituents, representing the lowest complexity level: pure oxides and pure metallic materials. Upon mixing and doping the latter, directly leading to formation of intermetallic compounds/alloys in the case of metals and oxygen ion conductors/mixed ionic and electronic conductors for oxides, a second complexity level is reached. Finally, the introduction of an (inter)metall(ic)–(mixed) oxide interface leads to the third complexity level. A shell-like model featuring three levels of complexity with the unveiled surface and interface chemistry at its core evolves. As the shift to increased complexity decreases the number of different materials, the interconnections between the studied materials become more convoluted, but the resulting picture of surface chemistry becomes clearer. The materials featured in our investigations are all either already used technologically important or prospective components of SOFCs (such as yttria-stabilized zirconia, perovskites, or Ni–Cu alloys) or their basic constituents (e.g., ZrO2), or they are formed by reactions of other compounds (for instance, pyrochlores are thought to be formed at the YSZ/perovskite phase boundary). We elaborate three representative case studies based on ZrO2, Y2O3, and Y-doped ZrO2 in detail from all three complexity levels. By interconnection of results, we are able to derive common principles of the influence of surface and interface chemistry on the catalytic operation of SOFC anode materials. In situ measurements of the reactivity of water and carbon surface species on ZrO2- and Y2O3-based materials represent levels 1 and 2. The highest degree of complexity at level 3 is exemplified by combined surface science and catalytic studies of metal–oxide systems, oxidatively derived from intermetallic Cu–Zr and Pd–Zr compounds and featuring a large number of phases and interfaces. We show that only by appreciating insight into the basic building blocks of the catalyst materials at lower levels, a full understanding of the catalytic operation of the most complex materials at the highest level is possible

The pervasive presence of oxygen in ZrC

Based on the recent interest in oxy-carbide materials in catalysis, we employ a thin film model concept to highlight that variation of key reaction parameters in the reactive magnetron sputtering of zirconium carbide films (sputtering power, template temperature or reactive plasma environment) under realistic preparation and application conditions often results in zirconium oxy-carbide films of varying stoichiometry. The composition of the films grown on silicon wafers and in vacuo - cleaved NaCl (001) single crystal facets was confirmed by depth profiling X-ray photoelectron spectroscopy and electron microscopy analysis. A correlation between methane-to-argon ratio, excess carbon and template temperature with elemental composition emphasizes the exclusive presence of oxygen-containing zirconium carbides. To generalize the approach, we also show that embedding of highly ordered Cu particles with uniform sizes in zirconium oxy-carbide matrices yields well-defined metal / oxy-carbide interfaces. As the presence of an oxy-carbide and its reactivity has been inextricably linked to enhanced activity and selectivity in a variety of processes, including hydrogenation, oxidation or reduction reactions, our model thin film approach provides the necessary well-defined catalysts to derive mechanistic details and to study the decomposition/re-carburization cycles of oxy-carbides. We have exemplified the concept for zirconium oxy-carbide, but deliberate extension to similar systems is easily possible

X-ray Absorption Near-Edge Structure (XANES) at the O K-Edge of Bulk Co₃O₄: Experimental and Theoretical Studies

We combine theoretical and experimental X-ray absorption near-edge spectroscopy (XANES) to probe the local environment around cationic sites of bulk spinel cobalt tetraoxide (Co3O4). Specifically, we analyse the oxygen K-edge spectrum. We find an excellent agreement between our calculated spectra based on the density functional theory and the projector augmented wave method, previous calculations as well as with the experiment. The oxygen K-edge spectrum shows a strong pre-edge peak which can be ascribed to dipole transitions from O 1s to O 2p states hybridized with the unoccu- pied 3d states of cobalt atoms. Also, since Co3O4 contains two types of Co atoms, i.e., Co3+ and Co2+, we find that contribution of Co2+ ions to the pre-edge peak is solely due to single spin-polarized t2g orbitals (dxz, dyz, and dxy) while that of Co3+ ions is due to spin-up and spin-down polarized eg orbitals (dx2−y2 and dz2 ). Furthermore, we deduce the magnetic moments on the Co3+ and Co2+ to be zero and 3.00 μB respectively. This is consistent with an earlier experimental study which found that the magnetic structure of Co3O4 consists of antiferromagnetically ordered Co2+ spins, each of which is surrounded by four nearest neighbours of oppositely directed spins

The Crystallographic and Electronic Phase Diagrams of Yttria-Stabilized Zirconia Model Electrolytes

Yttria-stabilized zirconia model electrolyte systems with four different compositions are analyzed regarding their crystallographic and electronic structure. By investigating the unit cell height, obtained from electron diffraction patterns, it is shown that a phase transformation between the tetragonal and cubic polymorphs takes place between 8 and 9.3 mol%. Furthermore, the direct band gaps are shown to exhibit the same behavior as the lattice parameter, featuring a discontinuity at the phase transition. By measuring the emitted Čerenkov radiation, an electronic transition that is smaller than the band gaps is found, suggesting that localized defect states are present within the band gap, which is in agreement with UV photoelectron spectra.(VLID)2852382Accepted versio

Recuperação de área degradada com sistema agroflorestal no Vale do Rio Doce, Minas Gerais.

O Vale do Rio Doce, MG, apresenta um histórico de ocupação e uso do solo que favorece a degradação ambiental, em que predominam pastagens sob o uso constante de queimadas. Os sistemas agroflorestais têm se mostrado eficientes na recuperação de áreas degradadas. Neste estudo foram avaliados os efeitos de um sistema agroflorestal na recuperação do solo em área degradada por pastagem na comunidade de Ilha Funda,Município de Periquito, Minas Gerais. A implantação do sistema se deu em 1994 e está sendo conduzido segundo os princípios agroecológicos, potencializando a regeneração natural e a sucessão de espécies. Em 1998, foram coletadas amostras de solo na área em recuperação e em duas áreas adjacentes: uma área degradada, que se encontrava em condições semelhantes às da área em recuperação no início do processo, e outra ocupada por pastagem. Foram determinados atributos químicos do solo e realizada a caracterização da matéria orgânica. O solo da área em recuperação com sistema agroflorestal mostrou-se em melhores condições do que o solo sob pastagem e o da área degradada, apresentando maior dinâmica do carbono orgânico e maior disponibilidade de nutrientes. Embora o teor de carbono orgânico total apresentado pelo solo sob pastagem tenha sido maior que nas demais condições avaliadas, o solo do sistema agroflorestal já está se igualando ao da pastagem no acúmulo das formas mais estáveis de carbono e apresentando maior dinâmica das frações orgânicas menos estáveis. Este estudo comprovou a eficiência dos sistemas agroflorestais, conduzidos segundo os princípios agroecológicos, na recuperação de áreas degradadas

Diagnosis of Hepatozoon canis in young dogs by cytology and PCR

<p>Abstract</p> <p>Background</p> <p><it>Hepatozoon canis </it>is a widespread tick-borne protozoan affecting dogs. The diagnosis of <it>H. canis </it>infection is usually performed by cytology of blood or buffy coat smears, but this method may not be sensitive. Our study aimed to evaluate the best method to achieve a parasitological diagnosis of <it>H. canis </it>infection in a population of receptive young dogs, previously negative by cytology and exposed to tick infestation for one summer season.</p> <p>Results</p> <p>A total of 73 mongrel dogs and ten beagles younger than 18 months of age, living in an animal shelter in southern Italy where dogs are highly infested by <it>Rhipicephalus sanguineus</it>, were included in this study. In March-April 2009 and in October 2009, blood and bone marrow were sampled from each dog. Blood, buffy coat and bone marrow were examined by cytology only (at the first sampling) and also by PCR for <it>H. canis </it>(second sampling). In March-April 2009, only one dog was positive for <it>H. canis </it>by cytological examination, whereas in October 2009 (after the summer season), the overall incidence of <it>H. canis </it>infection by cytological examinations was 43.9%. Molecular tests carried out on samples taken in October 2009 showed a considerably higher number of dogs positive by PCR (from 27.7% up to 51.2% on skin and buffy coat tissues, respectively), with an overall positivity of 57.8%. All animals, but one, which were positive by cytology were also PCR-positive. PCR on blood or buffy coat detected the highest number of <it>H. canis</it>-positive dogs displaying a sensitivity of 85.7% for both tissues that increased up to 98% when used in parallel. Twenty-six (74.8%) out of the 28 <it>H. canis</it>-positive dogs presented hematological abnormalities, eosinophilia being the commonest alteration observed.</p> <p>Conclusions</p> <p>The results suggest that PCR on buffy coat and blood is the best diagnostic assay for detecting <it>H. canis </it>infection in dogs, although when PCR is not available, cytology on buffy coat should be preferred to blood smear evaluation. This study has also demonstrated that <it>H. canis </it>infection can spread among young dogs infested by <it>R. sanguineus </it>and be present in the majority of the exposed population within 6 months.</p

The Chemical Evolution of the La0.6Sr0.4CoO3−δ Surface Under SOFC Operating Conditions and Its Implications for Electrochemical Oxygen Exchange Activity

© The Author(s) 2018Owing to its extraordinary high activity for catalysing the oxygen exchange reaction, strontium doped LaCoO3 (LSC) is one of the most promising materials for solid oxide fuel cell (SOFC) cathodes. However, under SOFC operating conditions this material suffers from performance degradation. This loss of electrochemical activity has been extensively studied in the past and an accumulation of strontium at the LSC surface has been shown to be responsible for most of the degradation effects. The present study sheds further light onto LSC surface changes also occurring under SOFC operating conditions. In-situ near ambient pressure X-ray photoelectron spectroscopy measurements were conducted at temperatures between 400 and 790 °C. Simultaneously, electrochemical impedance measurements were performed to characterise the catalytic activity of the LSC electrode surface for O2 reduction. This combination allowed a correlation of the loss in electro-catalytic activity with the appearance of an additional La-containing Sr-oxide species at the LSC surface. This additional Sr-oxide species preferentially covers electrochemically active Co sites at the surface, and thus very effectively decreases the oxygen exchange performance of LSC. Formation of precipitates, in contrast, was found to play a less important role for the electrochemical degradation of LSC.Fonds zur Förderung der wissenschaftlichen Forschung (FWF)212921411