1,720,963 research outputs found
NITROGEN, SULPHUR AND PLATINUM FUNCTIONAL MESOPOROUS CARBONS: SYNTHESIS, CHARACTERIZATION AND PERFORMANCE TOWARD OXYGEN REDUCTION REACTION
Full text linkProton Exchange Membrane (PEM) Fuel Cells are a promising technology for the clean energy production, especially in the automotive field. Actually, the main commercial catalysts employed in this system are based on Pt Nanoparticles supported on high surface area Carbon. The main issues associated to PEM Fuel Cells deal with the sluggish kinetic of oxygen reduction (ORR) at Platinum based electrode, with the low stability of both the carbon support and the metal phase, that tend respectively to oxidize and dissolve or diffuse and with the high cost due to rare and expensive Pt. In fact, nowadays high costs and low durability are the two factors that make PEM fuel cells still not competitive with internal combustion engine. For these reasons, research now focuses on obtaining more stable material with higher performances toward ORR. Two strategies are possible to improve catalyst for oxygen reduction. The first one deals with the enhancing of Pt activity modifying its electronic properties by alloying Pt with other transition metal (ligand effect) or by reducing the Pt-Pt distance (geometric effect). In both cases a Pt d-band shift occurs, which is responsible for the modification of adsorption and desorption energies of all species involved in ORR, and has as a direct consequence a modulation in the electrochemical activity. The second strategy deals with with the utilization of supports more stable respect to corrosion, like graphene, carbon nanotubes or mesoporous carbons. Furthermore, doping of carbon support with heteroatoms like N or S, can help to stabilize the metallic nanoparticles. In fact, doping creates homogeneous and narrow dispersion of small metallic nanoparticles, strongly bound to the surface of carbon support and with a higher resistance to agglomeration. Furthermore, doping has as well an influence on the electronic structure of the Pt catalyst, resulting in a modulation of its electrochemical activity. Doping is not beneficial only in noble-metal catalyst, but may also modify properties of the carbon support in which heteroatoms are present. Wettability, electrical conductivity and electrochemical activity are generally boosted when heteroatoms are inserted in carbonaceous substrates such as carbon blacks (CBs).
The goal of this Ph.D. project consists in synthesizing nitrogen and sulphur doped and co-doped Mesoporous Carbons. MCs are highly porous materials, which offer high surface area (>1000 m2/g), pore diameter in the range between 2 and 50 nm, controlled pore structure, good mechanical and thermal stability and good mass transport. The syntheses of differently doped MCs is conducted via hard template, a technique which allow to obtain materials with a well-defined structure, originating directly from the used template. The effect of template, pyrolysis temperature and carbon precursor on the final properties of synthesized MCs is evaluated. SEM, TEM, elemental analysis, XPS, BET and TGA are employed to characterize morphology, chemical composition and functional groups, surface are, pore dimension and thermal stability of MCs, respectively. Electrochemical performance toward oxygen reduction are evaluated via cyclic voltammetry in steady state conditions, with rotating disk electrode and with rotating ring disk electrode. Electrochemical stability tests are conducted to highlight stability of differently doped MCs and the modification occurring to functional groups.
The synthesized MCs are modified with Pt nanoparticles. An exhaustive deposition investigation is conducted via both chemical reduction and solid state reduction of several Pt salts, in order to find the conditions which allow to have the best Pt distribution on the whole surface of the carbon support. The catalysts made of Pt NPs on doped MCs are characterized by TEM, ICP and TGA techniques. Cyclic Voltammetry in steady conditions and with Rotating Disk Electrode are employed for the determination of electrochemical surface area (ECSA) and catalytic activity toward ORR, respectively, and compared to a commercial Pt/C catalyst. Electrochemical stability tests are conducted in order to assess both Pt degradation and C corrosion. In particular, the comparison of ECSA evolution between doped and reference catalyst determines if there is an enhancement of bounding which can mitigate Ostwald ripening. Finally, Pt on nitrogen and sulphur doped MC are incorporated in a membrane electrode assembly, in order to evaluate their performance and stability during operation as cathode material in a PEM Fuel Cell. Beyond catalytic properties, oxygen transport resistance was evaluated, knowing that all these properties are strongly influenced by the more severe condition of PEMFC environment
Nitrogen and sulfur doped mesoporous carbon cathodes for water treatment
No full textIn this paper, nitrogen and sulfur doped or co-doped mesoporous carbons (N-MC, S-MC and N,S-MC) were prepared
according to a hard template approach and employed for the in situ production of H2O2. N-MC and to a
lesser extent S-MC showed catalytic activity towards oxygen reduction reaction with high selectivity, up to
80%, for the production of H2O2. The possible application of doped MCs for the in situ generation of H2O2 in
water treatments was confirmed by the degradation of methyl orange, which is a benchmark for degradation
of pollutants, in potential controlled electrolysis in an undivided electrochemical cell, resulting in the complete
degradation of the organic dy
Electrochemical activation of carbon–halogen bonds: Electrocatalysis at silver/copper nanoparticles
No full textElectrochemical Activation of Carbon-Halogen Bonds: Electrocatalysis at Palladium-Copper Nanoparticles
No full textPalladized Cu nanoparticles (Pd/Cu NPs) supported on glassy carbon (GC) were synthetized by electrolytic deposition of Cu on GC, followed by galvanic displacement of Cu with Pd. The electrodes were characterized by XPS and EDX, which show that, although the composition of the NPs depends on the deposition time of Pd, Cu is always the main component, whereas Pd is preferentially limited to the uppermost layer of the nanoparticle surface. The reductive cleavage of a series of organic chlorides was investigated at Cu, Pd, Cu NPs and Pd/Cu NPs both in DMF and DMF/H2O (3/1 v/v). A strong dependence of the electrocatalytic properties of the electrodes on the mechanism of dissociative electron transfer (DET) was observed in pure DMF; moderate electrocatalysis was found for the reduction of all alkyl halides undergoing concerted DET, whereas no appreciable catalysis was observed for the reduction of aromatic chlorides, which undergo stepwise DET. However, addition of H2O remarkably enhances the catalytic activity of the metals. In DMF/H2O, all investigated compounds are catalytically reduced at Cu, Cu NPs and Pd/Cu NPs with good to excellent catalytic effects
Metal-support interaction in platinum and palladium nanoparticles loaded on nitrogen-doped mesoporous carbon for oxygen reduction reaction
No full text
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
Nitrogen and Sulfur Doped Mesoporous Carbon as Metal-Free Electrocatalysts for the in Situ Production of Hydrogen Peroxide
No full textMesoporous carbons (MCs) are highly porous materials, which offer high surface area and higher electrochemical performance than traditional carbon materials. Doped carbons are particularly interesting materials due to their possible use as metal-free ORR catalysts. In this paper, nitrogen and sulfur doped or co-doped MCs were prepared according to a hard template approach consisting in pyrolysis of powders obtained by liquid impregnation of mesoporous silica with different heterocyclic condensed aromatic precursors. The synthetized MCs show round shaped particles with mesoporosity of 3-4 nm diameter, a BET surface area higher than 850 m2/g and nitrogen and sulfur contents ranging between 3-8 % and 4-14%, respectively. Final doping has been demonstrated by core level photoemission spectra. The effect of the pyrolysis temperature on the physico-chemical properties of the resulting MCs has been investigated as well as the role of the dopant heteroatoms on their catalytic performances towards oxygen reduction reaction (ORR). Electrochemical tests show that both the oxygen-, sulfur- and nitrogen-containing groups can induce an electrocatalytic activity of MCs for ORR. The catalytic activity shows a linear dependence from the nitrogen content and a prevalent 2 electron reduction process leading to the formation of hydrogen peroxide, both in acid and alkaline solution
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