1,721,039 research outputs found
Reduction Properties of (001) Maghemite Surfaces
No full textUsing Density Functional Theory (DFT) calculations we have studied the reduction properties of the (001) maghemite (y - Fe 2 O 3 ) surface, which is a promising material for catalysis applications. We have calculated the formation energies of oxygen vacancies on the surface correlating them with the oxygen atom positions. We have found that the oxygen vacancy formation energy is lower for the oxygen atoms not bonded to the tetrahedral irons in the layer below the surface. Also, the presence of an iron vacancy in the third layer below the surface lowers the oxygen vacancy formation energy. An high oxygen vacancy formation energy correlates with a change in the oxidation state of some iron atoms
Surface reducibility, reactivity, and stability induced by noble metal modifications on the γ -Fe2O3maghemite (001) surfaces
No full textIn these last years large research efforts have been devoted to the synthesis and investigation of reducible metal oxide surfaces modified with metal atoms and nanoparticles for improving their performance in a number of advanced applications. Among reducible metal oxides, iron oxides have the advantage to be made up from two of the most common elements on Earth. In this paper we analyze the structural, electronic, and magnetic consequences of the insertion of isolated noble metal atoms (Cu, Ag, Au) on the γ-Fe2O3 (001) surface. We have considered many different configurations for the single atoms: adsorbed, substitutional to iron atoms, or to oxygen atoms, and, using first principles calculations, we have studied how the presence of the noble metal atoms on the surface influences the surface stability, its reducibility, and, therefore, its catalytic activity, and how these properties depend on the kind of noble metal atom and its position. Our results show that noble metal atoms adsorbed on the surface facilitate the adsorption of CO molecules, and, among them, Cu atoms are those that bind best to the surface also providing the strongest adsorption energy for the CO molecule. At ambient temperature and pressure the noble metal atoms prefer to substitute the iron atoms than to just adsorb on the surface, but for Ag atoms the adsorption and substitutional energies are very close. The surfaces with Ag in place of Fe are the most reducible and reactive for exchange of oxygen atoms. Finally, Au is the best noble metal for oxygen substitution. Our results provide useful insights for the researchers designing and synthesizing new noble metal - iron oxides nanostructures for applications in biology, medicine, catalysis, and chemical analysis
Reduction and Oxidation of Maghemite (001) Surfaces: The Role of Iron Vacancies
Full text linkThe knowledge of surface reduction and oxidation energetics in reducible oxides is essential for the design of improved catalysts for oxidation reactions. This is particularly true for iron oxides, a very attractive material system, because of the availability and biocompatibility of its constituents. In this work, by means of the density functional theory, we have thoroughly studied the ?-Fe2O3(001) maghemite surfaces, taking full account of iron vacancies beyond a mean field approach. Despite the structural similarity with the more studied magnetite Fe3O4 surfaces from which maghemite differs only for the presence of iron vacancies in the octahedral sites and for the absence of reduced Fe2+ cations, the surface properties are quite different. Our investigation shows that the presence of Fe vacancies is responsible for an increase in the surface reducibility. Also, it favors surface oxidation. The main reason is that the Fe vacancies cause a decrease of electronic charge of the surface oxygen atoms, which then become more reactive. We have considered different surface terminations and found that the reduced surfaces are more stable than the simple bulk-truncated ones. The reduction leads to a new reconstruction of the surface, which is the most stable surface termination among those investigated. We have examined the charge transfers and the modifications in the electronic structure caused by the surface reduction
BICM and TCM comparison in 100 Gbps optical coherent links in nonlinear regime
No full textThe popular single-carrier QPSK modulation format is compared with 8PSK BICM and TCM schemes in 100 Gbps optical links in linear and nonlinear regime, with or without inline dispersion compensation. © 2011 Optical Society of America
Methane Activation on Metal-Doped (111) and (100) Ceria Surfaces with Charge-Compensating Oxygen Vacancies
No full textReducing the temperature for methane activation is an important objective that would benefit many technological applications. In this work, we explore the possibility to achieve this goal using single-atom catalysts (SACs) on ceria surfaces. We focus on Ag SACs, which have recently been suggested to be promising catalysts for both H2 and CH4 oxidation. Using first-principles calculations, we investigate methane activation on CeO2(111) and CeO2(100), two frequently exposed surfaces on ceria nanoparticles. The presence of Ag is found to reduce the activation energy for methane dissociation on both surfaces. On Ag-doped CeO2(111), the formation of methanol via the Mars-van Krevelen mechanism has a slightly lower energy barrier than the dissociation to CH3 + H, suggesting that methanol is a likely product of methane activation on this surface. A novel aspect of this work is the focus on stable surface structures where each Ag dopant substituting Ce forms a complex with a charge-compensating surface oxygen vacancy. These complexes are found to play a critical role and accounting for their presence is essential for a proper description of the surface reactivity
Reduced cerium configurations in CeO2/Ag inverse catalysis
No full textInverse catalysts consisting of thin layers of a reducible oxide supported on noble metal surfaces have recently attracted much attention and shown to be competitive with the conventional catalysts, where metal nanoparticles are instead deposited on metal-oxide supports, in terms of their catalytic activity. We present here a computational study based on the density functional theory of the reduction geometries induced on ceria layers by a Ag support and by the creation of oxygen vacancies. We find that electrons are transferred from the Ag support to the ceria layers. These electrons localize at some Cerium sites to form 2D long range ordered patterns of Ce3+ and Ce4+ cations. Depending on their location the oxygen vacancies contribute and modify these patterns. The oxidation state of the cerium atoms affects the catalytic activity of the oxides
Investigation of the structural and electronic differences between silver and copper doped ceria using the density functional theory
No full textUsing the density functional theory (DFT) we have investigated how Ag and Cu atoms, substitutional to Ce, arrange themselves within Ceria (CeO2) and their effect on the ceria lattice, the oxidation states of the metal atoms, and the oxygen vacancy formation energies. Noble metal doped ceria has been proposed in substitution of platinum for a number of catalytic reactions. We have considered single noble metal atoms substituting Ce atoms in the (111) CeO2 surface unit cell, and investigated the thermodynamic stability of few configurations of one, two, and four Ag or Cu atoms in the unit cell. We have found that the noble metal atoms prefer to be located in the cation layer closer to the surface. An interesting result is that Cu causes a strong lattice distortion contrary to Ag. Ag, instead, causes a stronger reduction (oxygen loss) than Cu, since the electrons released in the oxygen vacancy formation are transferred mainly to Ag atoms, whose oxidation state tends towards +1, their preferred one, as found experimentally. In the Cu doped ceria, instead, part of the electrons reduces cerium atoms from +4 to +3 since Cu tends to stay in the +2 oxidation state, its preferred one. When we increase the concentration of Ag and Cu, substituting four Ce atoms, the noble metal atoms prefer to sit closer to each other, forming structural motifs resembling those typical of the Ag2O and CuO oxides
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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