1,720,973 research outputs found
Alkyl chain grafting on silica-zirconia mixed oxides: preparation and characterization
No full textSilica–zirconia mixed oxide substrates were prepared and grafted with C18 alkyl chains using noctadecyltrichlorosilane.
Si : Zr molar ratios in the mixed oxide samples before and after alkyl chain
modification were determined by X-ray photoelectron spectroscopy (XPS). Nitrogen sorption
measurements provided the porous features of these materials. By means of solid-state 29Si nuclear
magnetic resonance (NMR) the amounts of the silanol groups present in the as-prepared, humidified
and grafted mixed oxide systems as well as the degree of cross-linking of the silanes were obtained. 13C
NMR spectroscopy was used to examine the conformational order of the n-octadecyl chains grafted on
the silica–zirconia mixed oxide substrates. Similar information was available from variable temperature
Fourier transform infrared (FTIR) investigations. Scanning electron microscopy (SEM) measurements
and energy dispersive X-ray (EDX) analyses yielded the morphological features of the solid substrates
and information about the distribution of the alkyl chains on the oxide surfaces. The annealing
temperature for the preparation of the mixed oxides and the elemental composition of the samples, i.e.
relative amount of the oxide components, were found to determine the surface properties and in turn
the alkyl chain assembly on the mixed oxide surfaces. FTIR and solid-state 13C NMR data gave
evidence for an unusual high conformational order of the grafted C18 alkyl chains in all samples,
irrespective of the low surface coverage, which exceeds comparable C18 and C30 grafted silica and metal
oxide systems. This finding was attributed to the formation of island structures with aggregated
octadecyl chains of high conformational order due to strong intermolecular interactions. Differences in
the conformational order of the various samples most likely arise from vertical polymerization which
varies with the actual sample
Understanding Oxygen Release from Nanoporous Perovskite Oxides and Its Effect on the Catalytic Oxidation of CH4and CO
No full textThe design of nanoporous perovskite oxides is considered an efficient strategy to develop performing, sustainable catalysts for the conversion of methane. The dependency of nanoporosity on the oxygen defect chemistry and the catalytic activity of perovskite oxides toward CH4 and CO oxidation was studied here. A novel colloidal synthesis route for nanoporous, high-temperature stable SrTi0.65Fe0.35O3-δ with specific surface areas (SSA) ranging from 45 to 80 m2/g and pore sizes from 10 to 100 nm was developed. High-temperature investigations by in situ synchrotron X-ray diffraction (XRD) and TG-MS combined with H2-TPR and Mössbauer spectroscopy showed that the porosity improved the release of surface oxygen and the oxygen diffusion, whereas the release of lattice oxygen depended more on the state of the iron species and strain effects in the materials. Regarding catalysis, light-off tests showed that low-temperature CO oxidation significantly benefitted from the enhancement of the SSA, whereas high-temperature CH4 oxidation is influenced more by the dioxygen release. During isothermal long-term catalysis tests, however, the continuous oxygen release from large SSA materials promoted both CO and CH4 conversion. Hence, if SSA maximization turned out to efficiently improve low-temperature and long-term catalysis applications, the role of both reducible metal center concentration and crystal structure cannot be completely ignored, as they also contribute to the perovskite oxygen release properties
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
Combined use of XAFS, XRD and TEM to unravel the microstructural evolution of nanostructured ZrO2-SiO2 binary oxides: from nanometers down to molecular domain
No full textFunctional Nanostructured Perovskite Oxides from Radical Polymer Precursors
No full textIn the present work, nanostructured perovskite oxides with improved reactivity, tunable morphology, and different forms (powder, thin films) were prepared using acrylic molecules such as acrylamide, acrylic acid, and methacrylic acid as novel chelating agents in a straightforward fashion. The approach, developed for LaCoO3, was also applied to oxides of the type LaMO3 (M = Fe, Ni), SrTiO3, and solid solutions thereof. The polymer-to-oxide evolution followed by XRD and IR showed merely a minimal amount of carbonate residuals even at temperatures as low as 600 degrees C. The different cross-linking degree of the polymeric compounds influenced the material crystallization leading to oxides with different grain sizes at the same calcination temperature. Among the prepared perovskites, acrylamide-derived LaCoO3 exhibited the highest oxygen surface reactivity as demonstrated by XPS and TPD measurements. As a result, the materials showed enhanced catalytic performance, leading to complete oxidation of CO at approximately 200 degrees C, which was almost 100 degrees C lower than for citric-acid-based samples. Finally, by exploiting the UV photopolymerization of the acrylic group, homogeneous, crystalline perovskite thin films of optical quality were successfully prepared through a straightforward spin-coating approach. The findings of this work demonstrate that this novel synthesis route is a better alternative to state-of-the-art citrate-based methods for the preparation of prospective catalysis, sensing, and energy conversion materials of high purity, activity, and tunable form
Combined use of XAFS, XRD and TEM to unravel the microstructural evolution of nanostructured binary oxides: from nanometers down to molecular domain
No full textIn this paper, the detailed study of the microstructural evolution under annealing of zirconium-based inorganic–organic hybrid materials to give silica–zirconia mixed oxides was addressed by X-ray absorption fine structure (XAFS) spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The silica materials embedding different amounts of ZrO2 nanoparticles were prepared by copolymerisation of the organically modified oxozirconium cluster (Zr4O2(OMc)12 (OMc = methacrylate)) with methacryloxypropyltrimethoxysilane (MAPTMS). By the free radical copolymerisation of the oxoclusters bearing 12 methacrylate groups with the methacrylate-functionalised siloxanes, a stable anchoring of the clusters to the silica network was achieved. The thermal treatment of these hybrids at high (≥500 °C) temperatures yielded the SiO2–ZrO2 mixed oxides. The microstructural evolution upon heating was studied at increasing temperatures, namely 500, 600, 700, 900, 1000 and 1300 °C. Furthermore, different samples characterised by different Zr : Si atomic ratios and annealed at 1000 °C were comparatively analysed to study the effect of the composition on the evolution of the hybrids to give the mixed oxides. In a third experiment, samples characterised by the same composition were annealed at the same temperature by using either a conventional muffle or a microwave oven in order to evidence whether the different processing could also affect the microstructural features of the final oxide materials. Through XRD and XAFS it was demonstrated that at temperatures above 800 °C, crystallisation of tetragonal zirconia occurs in the samples of high zirconium concentration treated in muffle, whereas amorphous oxide materials form upon annealing in microwave oven. The presence of zirconia nanoclusters having an average size of 5–10 nm was evidenced by TEM
Silica-zirconia mixed oxide samples from silica-based hybrid materials: influence of preparation procedure and composition on the active sites
No full textIn this work, the interaction of amorphous silica–zirconia mixed oxide samples obtained from inorganic–
organic silica-based hybrid materials with pyridine and CO2 was studied to investigate their acid/base
character. Several silica–zirconia mixed oxide powders characterized by different [Zr/Si] atomic ratios
were prepared and treated at increasing temperatures both in a conventional muffle and with microwave
technology. The powder samples were characterized with Diffuse Reflectance Infrared Fourier Transform
(DRIFT) and X-ray Photoelectron (XP) spectroscopies. The surface acidic and basic active sites were investigated
(with DRIFT spectroscopy) by chemisorbing probe molecules (pyridine, carbon dioxide). The
obtained results revealed the presence of both Lewis and Brønsted acidic sites on the amorphous silica–
zirconia mixed oxide powder surfaces. Several acidic sites characterized by different strength were
observed; the acidic sites distribution is markedly influenced by the sample composition and by the heat
treatment: more numerous acidic sites form on the surface of the samples treated with microwaves with
respect to the muffle treated ones; the increment of the temperature and the decrease of the zirconium
content cause a significant decrement of the acidic sites. No basic sites were revealed
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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