1,720,957 research outputs found
A new catalyst for propane ammoxidation: The Sn/V/Sb mixed oxide
No full textThe effect of the preparation and calcination method of a Sn/V/Sb mixed oxide on its catalytic performance are analyzed with respect to ammoxidation of propane. Different samples have been prepared with coprecipitation technique (by dissolving the starting materials with ethanol, iso-butanol or water), or by solid state reaction between oxide and tin hydroxide. The sample prepared by ethanol shows the best catalytic performance: this solvent makes the partial alkoxides quite stable in solution and brings about a better coprecipitation with a more intimate mixture of tin, antimony and vanadium. The thermal transformation of the precursor of this sample has been followed during the calcination, both in air and in nitrogen with several characterization techniques. The thermal treatment in air at 700°C leads to the best catalytic performance, that is, good activity and high selectivity. This calcination procedure leads to a homogenous mixed oxide, with numerous well-crystallized microfields of tin oxide, incorporating antimony and vanadium ions in limited concentrations and an excess of poorly crystallized antimony oxide
Propane ammoxidation over Sn/V/Sb mixed oxide: Preparation method and calcination effects
No full textThe aim of this work is to analyse the reactivity of a tin, vanadium, antimony mixed oxide in the ammoxidation of propane to acrylonitrile with respect to the preparation and calcination method. ©Societá Italiana di Fisica
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
Propane ammoxidation to acrylonitrile over a tin-based mixed-oxide catalyst
No full textThe catalytic properties of a Sn/V/Sb mixed oxide for propane ammoxidation to acrylonitrile are studied in this paper. First, preparing the sample with a coprecipitation method from an ethanolic phase, the amounts of antimony and vanadium were changed in order to optimize the relative atomic ratio of the ternary catalyst. The results obtained indicate that vanadium is responsible for the paraffin activation and antimony for the insertion of nitrogen in the molecule tit can be considered as an acrylonitrile selectivity modulator). Other samples, with the optimized atomic ratio, were also prepared using a similar coprecipitation technique (but by dissolving the starting materials with iso-butanol or water instead of ethanol), or by solid state reaction between oxide and tin hydroxide, The sample prepared using ethanol shows the best catalytic performance. This solvent makes the partial alkoxides quite stable in solution and brings about better coprecipitation with a more intimate mixture of tin, antimony and vanadium. Finally, the thermal transformation of the precursor of this sample was followed during calcination, both in air and in nitrogen with several characterization techniques. The thermal treatment in air at 700 degrees C leads to the best catalytic performance, i.e., goad activity and high selectivity. (C) 1998 Elsevier Science B.V. All rights reserved
Mechanism of ammoxidation of propane on a Sb/V/O system
No full textThe mechanism of the direct ammoxidation of propane is investigated on a Sb/V mixed oxide catalyst (atomic ratio 5/1) in order to understand the sequence of the intermediates and the role of ammonia in the formation of acrylonitrile. The sample was characterized by different techniques (BET, FTIR, XRD and XPS) and tested in different conditions (oxidation and ammoxidation of propane and propene). It is proposed that propane is first converted to propene and hence to ''oxidation products'' (which could be acrolein or an intermediate absorbed an the catalyst surface, as allyl radical). In the absence of ammonia these ''oxidation products'' are converted to carbon oxide. But in its presence they are converted into acrylonitrile, which is appreciably more stable and undergoes minor degrees of oxidative degradation
A new ternary mixed oxide catalyst for ammoxidation of propane: Sn/V/Sb
No full textThe catalytic properties of a Sn/V/Sb mixed oxide for propane ammoxidation to acrylonitrile are studied in this paper. In particular the antimony and vanadium amounts were changed in order to optimize the relative atomic ratio. The ternary sample with the best catalytic performance was compared with the analogous binary samples (Sn/Sb and Sb/V). The results obtained indicate that vanadium is responsible for the paraffin activation, nevertheless much vanadium produces a lot of carbon oxides. Antimony is responsible for the insertion of nitrogen in the molecule (it can be considered as an acrylonitrile selectivity modulator); nevertheless, too much antimony deactivates the catalyst. Tin does not act only as a dispersive matrix for the active sites, but leads to the formation of a polyfunctional catalyst, increasing the rate of acrylonitrile formation from the intermediate propene. From a structural point of view, this catalytic system can be described as a homogeneous system, containing microfields of rutile type oxide (SnO2), promoted with antimony and vanadium in substitutional solid solution, dispersed in an excess of amorphous antimony oxide
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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