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Study of the water-gas-shift reaction over Pd@CeO2/Al2O3 core-shell catalysts
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An alumina-supported, Pd@CeO2, core−shell catalyst having 1 wt % Pd and 9 wt % ceria was characterized for the water-gas-shift (WGS) reaction. Although the catalyst initially exhibited similar WGS rates to that of a conventional Pd/ceria catalyst at 623 K in 25 Torr each of CO and H2O, the Pd@CeO2 catalyst deactivated severely over the period of 1 h. The WGS activity of the Pd@CeO2 could be completely restored by mild oxidation, and oxygen-titration measurements showed that the ceria shell in the Pd@CeO2 catalyst was significantly reduced after being used for the WGS reaction. These observations are in sharp contrast to those found with a conventional Pd/ceria catalyst, for which the ceria remains almost fully oxidized under WGS conditions. CO adsorption measurements, using FTIR at room temperature and CO uptakes at 195 K, indicated that Pd in the oxidized Pd@CeO2 catalyst was accessible to CO, but adsorption was completely suppressed on the reduced catalyst. A model is presented to explain the results, which assumes that cracks and fissures in the oxidized ceria shell allow access to the Pd core but that reduction blocks access, either due to changes in the density of the ceria, which closes the fissures, or to coverage of the metal surface with ceria
A Versatile Route to Core-Shell Catalysts: Synthesis of Dispersible M@Oxide (M = Pd, Pt; Oxide = TiO2, ZrO2) Nanostructures by Self-Assembly
No full textA method, based on self assembly, for preparing core–shell nanostructures that are dispersible in organic solvents is demonstrated for Pd and Pt cores with CeO2, TiO2, and ZrO2 shells. Transmission electron microscopy (TEM) of these nanostructures confirmed the formation of distinct metal cores, approximately 2 nm in diameter, surrounded by amorphous oxide shells. Functional catalysts were prepared by dispersing the nanostructures onto an Al2O3 support; and vibrational spectra of adsorbed CO, together with adsorption uptakes, were used to demonstrate the accessibility of the metal core to CO and the porous nature of the oxide shell. Measurements of water-gas-shift (WGS) rates demonstrated that these catalysts exhibit activities similar to that of conventional supported catalysts despite having lower metal dispersions. Pd-based CeO2 and TiO2 core–shell catalysts exhibit significant transient deactivation, which is probably caused by a decrease in the exposed metal surface area due to the ease of reduction of the shells. Alternatively, Pt-based analogous core–shell catalysts do not exhibit such a transient decrease. Both Pd- and Pt-based ZrO2 core–shell catalysts deactivate at a significantly lower rate due to the less reducible nature of the ZrO2 shell
Synthesis of dispersible Pd@CeO2 nanostructures by self-assembly
No full textA methodology is described for the preparation of Pd@CeO2 core−shell nanostructures that are easily dispersible in common organic solvents. The method involves the synthesis of Pd nanoparticles protected by a monolayer of 11-mercaptoundecanoic acid (MUA). The carboxylic groups on the nanoparticle surfaces are used to direct the self-assembly of a cerium(IV) alkoxide around the metal particles, followed by the controlled hydrolysis to form CeO2. The characterization of the nanostructures by means of different techniques, in particular by electron microscopy, allowed us to demonstrate the nature of core−shell systems, with CeO2 nanocrystals forming a shell around the MUA-protected Pd core. Finally, an example of the use of these nanostructures as flexible precursors for the preparation of heterogeneous catalysts is reported by investigating the reactivity of Pd@CeO2/Al2O3 nanocomposites toward CO oxidation, water−gas shift (WGS), and methanol steam reforming reactions
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Highly Active and Thermally Stable Core-Shell Catalysts for Solid Oxide Fuel Cells
No full textThe effect of catalyst nanostructure on the performance of solid oxide fuel cell (SOFC) anodes prepared by infiltration of an electronic conductor (45 wt % La(0.8)Sr(0.2)Cr(0.5)Mn(0.5)O(3), LSCM) and a catalyst (1 wt % Pd and 9 wt % CeO(2)) into porous yttria-stabilized zirconia (YSZ) scaffolds was examined. When Pd and CeO(2) were added by classical infiltration with nitrate salts, the initial electrode impedance in 97% H(2)-3% H(2)O at 973 K was similar to 0.1 Omega cm(2); however, the impedance was found to increase significantly with time at 973 K and with heating to 1173 K. SEM images showed that the loss of performance coincided with a large increase in the size of the Pd crystallites. When Pd@CeO(2) dispersible core-shell structures obtained through self-assembly were infiltrated into the anode and used as the catalytic component, the initial performance was excellent and the activity was remarkably stable with time at 973 K and upon heating to 1173 K. The improved stability is shown to be the result of greatly suppressed particle-size growth for Pd@CeO(2) within the electrode structure. This study highlights the potential use of core-shell materials as stable structures in various fields of materials science and heterogeneous catalysi
Variations on the Author
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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