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Opis: Copper-modified TiO2–SiO2photocatalysts were prepared by sol–gel method based on organic copper,silicon and titanium precursors. Copper concentration varied from 0.1 to 3.0 mol%. A widely appliedmodel reaction of photocatalytic oxidation of terephtalic acid (TPA) in water solution was used in order toevaluate the catalytic activities of elaborated samples. The crystal structures of the titania components ofall tested titania–silica species were studied using XRD analysis. The influence of Cu2+cation incorporationon the crystal structure of titania, as well as the chemical states and the neighbouring structures of coppercations, have been examined by means of Cu K-edge EXAFS and XANES analysis. The experimental datashow that there is a ten times increase in photocatalytic activity when TiO2–SiO2matrix is modified with0.1 mol% of Cu. It can be supposed that an enhancement of photocatalytic activity of low-concentratedcopper-modified titania–silica nanocomposites is probably due to a close attachment of Cu2+cationsto the surfaces of photocatalytically active TiO2nanoparticles. In this case, Cu2+cations may possiblyact as free electron traps reducing the intensity of recombination between opposite free charge carriers(electrons, holes) available at the photocatalyst’s surface.
Ključne besede: Cu-modified TiO2–SiO2photocatalysts, Titanium dioxide, Metal doping, Cu K-edge XANES, EXAFS, Photocatalytic activitya
Objavljeno v RUNG: 17.01.2017; Ogledov: 5253; Prenosov: 0
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Opis: The positive effect of intermetallic ordering of platinum alloy nanoparticles on oxygen reduction reaction (ORR) activity has been well established. What is still missing is an understanding of selective leaching of the less noble metal from the ordered structure and its correlation to longterm ORR performance. Using a combination of kinetic Monte Carlo simulations and advanced characterization techniques, we provide unprecedented insight into dealloying of intermetallic PtCu3 nanoparticles a well-known binary alloy. Comparison of ordered and disordered samples with identical initial compositions and particle size distributions reveals an unexpected correlation: whereas the copper dealloying rates in the ordered and disordered counterparts are almost the same, in the ordered structure Pt atoms are surrounded by 15−30% more Cu atoms throughout all the stages of acid leaching. This more convenient Pt−Cu coordination explains the statistically significant increase of 23−37% in ORR activity of the ordered structure at all stages of alloy degradation.
Ključne besede: ORR activity, fuel cells, platinum alloy, nanoparticle stability, intermetallic ordering, kinetic Monte Carlo, dealloying, in situ ICP-MS
Objavljeno v RUNG: 27.09.2016; Ogledov: 5646; Prenosov: 0
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Opis: Mercury (Hg) – plant – fungal interactions are only poorly studied. Hg speciation and ligand environment in maize roots inoculated with arbuscular mycorrhizal (AM) fungi were investigated in order to better understand the role of AM in Hg soil to root transfer. The maize plants were grown in Hg polluted substrate (50 mg g1 as dissolved HgCl2) and inoculated with AM fungi originating from: a) highly Hg polluted environment of a former Hg smelting site in Idrija, Slovenia, (Glomus sp. – sample AmI), and b) non-polluted environment (commercial AM inoculum Symbivit1 – sample AmC). Hg speciation and ligand environment in maize roots was studied by Hg-L3 XANES and EXAFS with emphasis on XAS methodology – modelling and fitting the XAFS spectra to extract in a reliable way as much information on Hg coordination as possible. The AmI plants developed more arbuscules and less vesicles than the AmC plants, and also accumulated more Hg in the roots. A clear difference in Hg coordination between the AM (AmC & AmI) and the control (ConC & ConI) plants is recognized in Hg L3-edge EXAFS analysis: in the ConC & ConI maize roots 73–80% of Hg is attached between two sulphur atoms at the distance of 2.34 Å. The remaining ligand is nitrogen at 2.04 Å. In AmI & AmC roots another Hg-S attachment encompassing four thiol groups at the S-distance of 2.50 Å are identified, accounting for 21–26%. AM fungi can modify Hg ligand environment in plant roots, thus playing an important role in biogeochemical cycling of Hg in terrestrial ecosystems.
Ključne besede: EXAFS XANES Arbuscular mycorrhiza Phytoremediation Toxicity Hg coordination Ligand environment
Objavljeno v RUNG: 27.09.2016; Ogledov: 6235; Prenosov: 0
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Opis: The development of efficient TiO2-based photocatalysts for water treatment is mainly performed by doping with transition metals or by establishing junctions between different phases, metal–semiconductor or semiconductor–semiconductor. We present, for the first time, the synthesis of Zr- and Mn-modified TiO2 by a redesigned sol–gel technique that allows the formation of heterometallic bridges on the TiO2 surface. Cations of the doping metals are located in the pores of mesoporous anatase and attached to the crystalline TiO2 walls. The presence of the Zr enhances the photoactivity of the TiO2 catalyst. However, the introduction of Mn decreases the photocatalytic efficiency in a nonadditive manner. The inhibition effect was assigned to the side reaction between hydroxyl radicals and Mn ions. The fact that Mn effectively scavenges the hydroxyl radicals and, consequently, inhibits the whole oxidation process is direct proof that hydroxyl radicals are the main reactive species in the photocatalytic oxidative processes on TiO2 surfaces in aqueous media and the process of COH generation is the rate-determining step, which was confirmed using a method based on the decolorization of a commercial dye Bezaktiv Blau in a reaction with Fenton’s reagent as a source of hydroxyl radicals.
Ključne besede: doping, manganese, oxidation, X-ray absorption spectroscopy, zirconium
Objavljeno v RUNG: 21.07.2016; Ogledov: 4240; Prenosov: 0
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Opis: Li-S batteries are considered as one of the most promising candidates for future batteries in applications where high energy density is required [1]. Despite that the general principle of operation is known for a long time [2], the lack of detailed understanding of relevant operation mechanisms has so far prevented their extensive use. A Li-S battery is composed of a lithium metal anode and a sulfur based cathode, separated by a porous separator wetted with electrolyte. During the battery cycle the reduction and oxidation of S to Li2S and back proceeds through a complicated equilibrium mixture of compounds that are typically dissolved in the electrolyte in the form of long and short chain polysulfides. In order to improve our understanding of polysulfide formation and its interactions within electrode, which are essential to achieve the long term cycling stability, development and application of new analytical tools is required. In this work sulfur K-edge resonant X-ray emission (RXES) measurements were performed on the Li-S battery in operando mode. The experiment was performed at the ID26 beamline at ESRF using the Johansson type tender x-ray emission spectrometer [3]. Full K-L RIXS maps were recorded on a set of chemically prepared Li2Sx sample standards characterized by different Li:S stoichiometric ratio, followed by the operando measurements on Li-S battery. Using the spectra recorded on Li2Sx standards two excitation energies were chosen and RXES spectra from the back of the battery cathode were sequentially acquired during one discharge cycle (C20). The relative amounts of each sulfur compound in the cathode during the discharge cycle were determined from the linear combination fit using measured reference standard spectra. Because of resonant excitation conditions the sensitivity for the polysulfide detection was significantly enhanced. Our work sets up S K-edge RIXS spectroscopy as an important analytical tool to study the mechanism of Li-polysulfide formation in the cathode and their interaction with the host matrix and electrolyte.
Ključne besede: RIXS, RXES, Li-S battery, operando, Sulphur K-edge XANES, Lithium polysulphides, Li2S
Objavljeno v RUNG: 28.06.2016; Ogledov: 5697; Prenosov: 0
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