1,721,376 research outputs found
An organometallic approach for very small maghemite nanoparticles: Synthesis, characterization, and magnetic properties
No full textMaghemite (gamma-Fe2O3) nanoparticles stabilized by long-alkyl-chain amines are synthesized by using an orgonometallic approach. This method consists of the hydrolysis and oxidation of an organometallic precursor, Fe[N(SiMe3)(2)](2), in the presence of amine ligands as stabilizing agent in an organic solvent, namely tetrohydrofuron or toluene. Whatever the experimental conditions, particles with a diameter of 2.8 nm are obtained. The use of high-resolution transmission electron microscopy and wide-angle X-ray scattering, together with Mossbauer spectroscopy and SQuld magnetometry, allows a complete characterization of these particles. Herein, we show that their structure is composed of a well-ordered core surrounded by a more disordered shell. The size of the latter varies from 0.65 to 0.50 nm depending on the experimental conditions and is of prime importance for the understanding of the magnetic properties. We demonstrate that the shorter the alkyl chain length of the amine 1) the better the crystallinity of the particle's core and 2) the stronger the interparticle interactions
Synthesis and characterization of FeRh nanoparticles
No full textFe0:8Rh0:2 nanoparticles with an average size of 1.8 nm have been synthesized by simultaneous decomposition of stoichiometric quantities of Fe[N(Si(CH3)(3))(2)](2) and Rh(C3H5)(3) in mild conditions of temperature and hydrogen pressure, using hexadecylamine as stabilizer. These nanoparticles display a magnetization of 1.13 mu B per iron atom, which is more than two times smaller than the bulk value with the same composition. No surface contamination with iron oxides was detected by Mossbauer spectroscopy. Superconducting quantum interference device and Mossbauer spectroscopy investigations reveal that the origin of this variation may be a core-shell Rh/Fe chemical arrangement
Magnetic induction heating in catalysis: toward adaptive catalytic systems for challenging hydrogenation and hydrodeoxygenation reactions
No full textCatalysis is a fundamental pillar of the chemical industry and is used across the whole value chain, ranging from the production of fuels to fine chemicals, agrochemicals, and pharmaceuticals. In light of diminishing fossil resources, the permanent evolution of catalysts and catalytic processes is crucial to cope with new renewable energy sources and carbon feedstock and to face the inevitable variations in electricity production. In particular, energy efficiency, benignity of reaction conditions, and adaptivity to intermittent energy supply are parameters of ever-increasing importance. In this context, magnetic induction heating has been attracting great attention as it offers the possibility to directly heat magnetic nanocatalysts in a highly localized, rapid, and energy-efficient manner, saving the need to heat the complete reactor environment (catalyst, reactor parts, solvent, etc.). This results in milder overall process temperatures and a lower energy consumption. In addition, the rapid heating and cooling of magnetically heated nanocatalysts is of great interest to address the challenges associated with using fluctuating renewable energy. An up-and-coming field of application is hydrogen activation, and in particular selective hydrogenation and hydrodeoxygenation reactions that are critical transformations for hydrogen storage, conversion of biogenic feedstocks and platform chemicals as well as for novel synthetic pathways to fine chemicals and even pharmaceuticals. Although the selective heating of catalysts by magnetic induction has made significant progress in the past few years, developing versatile catalytic systems capable of reaching high temperatures under alternating current magnetic field (ACMF) while precisely controlling chemical reactions remains highly desirable but challenging. Through this thesis, the author aims to design and develop new magnetically responsive materials that are capable of being activated and heated by ACMF and used as catalysts to perform selective hydrogenation and hydrodeoxygenation of aromatic aldehydes, aromatic ketones, and amides. In particular, two distinct approaches to catalyst design will be explored. The first approach entails functionalizing conventional heterogeneous hydrogenation catalysts (e.g. Cu2Cr2O5 and Pt/Al2O3) with magnetic iron carbide nanoparticles (ICNPs) to make them responsive to magnetic induction heating. With the benefit of the associated features from induction heating, in particular, adaptivity to intermittent power supply for the hydrogenation of aromatic ketones and amides, respectively. The second approach involves the surface modification of ICNPs with small Cu nanoparticles to produce new hydrodeoxygenation catalysts that can be activated by magnetic induction to reduce selectively aromatic aldehydes including biomass-derivatives under mild observable conditions. These projects have the potential to facilitate and broaden the use of magnetic induction in heterogeneous catalysis and lead to promising breakthroughs in materials development, hydrogen activation, and, more broadly, chemical energy conversion
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
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
Bimetallic ruthenium- and rhodium-based nanoparticles immobilized on supported ionic liquid phases (SILPs) for selective hydrogenation and hydrogenolysis reactions
No full textThis thesis deals with the development of bimetallic nanoparticles immobilized on supported ionic liquid phases (SILPs) combining noble metals (i.e. Ru and Rh) with 3d metals (i.e. Fe, Co, Ni) as catalysts for the selective hydrogenation, hydrodeoxygenation and hydrogenolysis of complex aromatic substrates using H2 as a benign reducing agent. Using an organometallic approach, bimetallic RuxM100-x@SILP (M = Co, Ni) catalysts were developed and compared to the previously reported RuxFe100-x@SILP catalytic system in order to investigate the influence of the used 3d metals on the formation of bimetallic nanoparticles on SILP as well as on their reactivity. Applying characterization techniques including TEM, SEM-EDX and STEM-HAADF-EDX allowed for a detailed analysis of the developed materials, evidencing the formation of well-dispersed bimetallic nanoparticles on the SILP support. The catalysts reactivity was investigated using the catalytic hydrogenation of benzylideneacetone as a model reaction. The Ru:M ratio was found to have a great influence on the activity and selectivity of the catalysts, especially with regards to arene and ketone hydrogenation. Interestingly, the use of 3d metals with increasing atomic number showed a consistent trend requiring decreased Ru:M ratios to enable the hydrogenation of the ketone as well as the aromatic ring. Then, the addition of a sulfonic acid-functionalized ionic liquid (IL-SO3H) to the RuxM100-x@SILP produced bifunctional bimetallic catalysts (RuxM100-x@SILP+IL-SO3H) that were applied to the selective hydrodeoxygenation of benzylic and non-benzylic ketones as well as the hydrogenolysis of diphenyl ether, with a special focus on the production of aromatic products. The investigated catalytic systems showed for optimized Ru:M ratios (e.g. Ru50Co50@SILP+IL-SO3H) a good activity and selectivity for the selective hydrodeoxygenation of the benzylic and non-benzylic ketones towards the formation of aromatic alkanes. However, a trade-off between activity and selectivity was observed in dependency of the Ru:M ratio for the hydrogenolysis of diphenyl ether, along with a rapid deactivation of the bifunctional catalysts under the considered reaction conditions. This deactivation of RuxM100-x@SILP+IL-SO3H in the hydrogenolysis of diphenyl ether was explained by the observed agglomeration of the bimetallic nanoparticles under acidic reaction conditions. Subsequently, the hydrogenolysis of substituted diaryl ethers was focused using a bifunctional monometallic Ru@SILP-SO3H catalyst to investigate the influence of different lignin-based substitution patterns on the resulting product distribution. Interestingly, asymmetric diaryl ethers possessing methoxy-functionalities were selectively cleaved adjacent to the unsubstituted ring resulting in the selective production of substituted phenols and cyclohexane. The catalytic system was also successfully used under continuous flow conditions for the hydrogenolysis of 2-methoxy-4-methylphenoxybenzene, highlighting the catalyst’s practicability and stability. Additionally, the influence of water accumulation on the catalytic properties was investigated, and the selective extraction and isolation of phenols from the reaction mixtures was demonstrated. Finally, the synthesis of bimetallic RhxCo100-x@SILP catalysts was developed finding [Rh(allyl)3] and [Co(COD)(Cyclooctadienyl)] as suitable organometallic precursors. The catalysts were used for the selective hydrogenation of benzylideneacetone and a variety of (substituted) heteroaromatics, showing a distinct switch between Rh:Co ratios of 70:30 to 75:25 to yield either partially or fully saturated products. The detailed characterization by advanced spectroscopic techniques, such as STEM-HAADF-EDX and XAS allowed for the structural elucidation of the catalysts showing homogeneously alloyed and monodisperse nanoparticles immobilized on the SILP. In summary, the controlled synthesis of bimetallic Ru- and Rh-based nanoparticles immobilized on SILP was achieved using the versatile and powerful organometallic approach. The well-defined nature of these finely tunable catalytic systems allowed for their precise tailoring to the specific task and gave detailed insights into the structure-reactivity correlations
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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