169,764 research outputs found
Efficient and selective oxidation of methyl substituted cycloalkanes by heterogeneous methyltrioxorhenium-hydrogen peroxide systems
No full textPolymer-supported methyltrioxorhenium (MTO) systems are efficient catalysts for the oxidative functionalisation of cyclohexane and cyclopentane, derivatives with H2O2 as oxygen donor. Using poly (4-vinyl)pyridine and poly(4-vinyl)pyridine-N-oxide as MTO supports, cycloalkanol, cycloalkanediol, cycloalkanone and omega-hydroxy methyl ketone derivatives were obtained in different yields depending on the experimental conditions. Interestingly, cycloalkane dimers were selectively recovered in acceptable to good yields when the oxidation was performed with polystyrene-microencapsulated MTO catalyst. The EPR investigation suggests that the homolytic cleavage of the CH3-Re bond with formation of CH3 center dot radicals occurs inside the polystyrene capsule, indicating a possible role of methyl radical in the cycloalkane dimerisation pathway. (c) 2006 Published by Elsevier Ltd
Defect chemistry in ruthenium-supported tin dioxide: a spectromagnetic approach
No full textThe reactivity towards CO and air of SnO2 defects, singly and doubly ionized oxygen vacancies (V(o)/· V(o)/··), bivalent tin centers, was studied in ruthenium-supported tin oxide (Ru/SnO2), and compared with that in pure SnO2. Electron paramagnetic resonance and X-ray photoelectron spectroscopy studies demonstrated that CO treatment produces V(o)/· and V(o)/·· defects in SnO2, some V(o)/· transferring their electrons to Sn4+ centers; instead in Ru/SnO2, some V(o)/· defects transferred their electrons to Ru(n+) centers (n=0,1,2,3) and no one bivalent tin center was observed. When contacted with air, pure SnO2 transferred a part of the electrons of V(o)/· and of bivalent tin centers to O2, instead Ru/SnO2 emptied all V(o)/· defects transferring the electrons from V(o)/· to ruthenium and to O2. The via ruthenium transfer increases the number of electrons exchanged between SnO2 and the surrounding atmosphere and gave a rationale for the higher sensitivity towards CO displayed by transition metal-doped SnO2 with respect to pure SnO2
Inverted opal luminescent Ce-doped silica glasses
No full textInverted opal Ce-doped silica glasses (Ce : Si molar ratio 1 ⋅ 10−3 were prepared by a sol-gel method using opals of latex microspheres as templates. The rare earth is homogeneously dispersed in silica host matrix, as evidenced by the absence of segregated CeO2, instead present in monolithic Ce-doped SG with the same cerium content. This suggests that the nanometric dimensions of bridges and junctions of the host matrix in the inverted opal structures favor the RE distribution avoiding the possible segregation of CeO2
Influence of doping elements on the formation rate of silicon nanowires by silver-assisted chemical etching
No full textMetal-assisted chemical etching (MACE) has gained great interest for the preparation of vertically aligned silicon nanowires (SiNWs); however, the process mechanism has not yet been identified. In this study, the influence of doping elements on the formation rate of SiNWs prepared by silver-assisted chemical etching was investigated. Two n-type silicon substrates, bulk silicon and silicon-on-insulator (SOI) samples, containing different dopant concentrations (from 1011 to 1019 atoms cm− 3) and species (phosphorous and arsenic), were considered. The SiNW formation rates increase with dopant concentration and are influenced by the dopant species. The use of SOI samples allowed highlighting the remarkable loss of the starting device layer in conventional process conditions, never previously observed; such occurrence limits the most achievable SiNW length. The study of the gas evolution both from bulk and SOI samples allowed for the first time the in situ H2 detection as well as to identify one definite overall process reaction
Spectroscopic investigation of the molybdenum active sites on MoVI heterogeneous catalysts for alkene epoxidation
No full textSpectroscopic and spectromagnetic investigations on molybdenum centres in MoO3·H2O and its polymer-supported form have been carried out. Both were tested as catalysts for cyclohexene epoxidation in a heterogeneous phase; both needed activating pretreatment before the reaction. XPS and EPR data show that the metal is mainly present as MoVI. The activation process increases the acidic character of MoVI and its efficiency depends both on the oxidizing power of the activating reactant and on the accessibility of the metal to the reactant. The steric hindrance of MoVI lowers the catalytic activity owing to the replacement of H2O by carboxylic groups of the functionalized resin
Electrically Detected Magnetic Resonance of Donors and Interfacial Defects in Silicon Nanowires
No full textWe report our work on the characterization by electrically detected magnetic resonance (EDMR) measurements of silicon nanowires (SiNWs) produced by different top-down processes. SiNWs were fabricated starting from SOI wafers using standard e-beam lithography and anisotropic wet etching or by metal-assisted chemical etching. Further oxidation was used to reduce the wire cross section. Different EDMR implementations were used to address the electronic wave function of donors (P) and to characterize point defects at the SiNWs/SiO2 interface. The EDMR spectra of as produced SiNWs with high donor concentration ([P] = 1018 cm−3) show a single line related to delocalized electrons. SiNWs produced on substrates with lower donor concentration ([P] < 1016 cm−3) reveal the doublet related to substitutional P in Si, as well as lines related to interfacial defects such as Pb0, Pb, E', and E'-like. The EDMR spectra of samples produced by metal-assisted chemical etching exposed to post production oxidation reveal a disordered and defective interface and the disappearance of the P related signal. Forming gas annealing, on the other hand, reduces the contribution of interfacial defects and allows a better resolution of the P related doublet
Composition/Structure Relationships in Monolithic Borophosphosilicate Glasses Obtained by the Sol-Gel Route
No full textElectron paramagnetic resonance characterization of silica-dispersed copper molybdate obtained by sol-gel and impregnation methods
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Understanding the microstructure of mortars for cultural heritage using X-ray CT and MIP
No full textIn this study, the microstructure of mock-up mortar specimens for a historic environment, composed of different mixtures, was studied using mercury intrusion porosity (MIP) and micro-computed tomography (μCT), highlighting the advantages and drawbacks of both techniques. Po-rosity, sphericity, and pores size distribution were studied, evaluating changes according to mortar composition (aerial and hydraulic binders, quartz sand, and crushed limestone aggregate). The μCT results were rendered using 3D visualization software, which provides complementary information for the interpretation of the data obtained using 3D data-analysis software. Moreover, μCT contrib-utes to the interpretation of MIP results of mortars. On the other hand, MIP showed significant ink-bottle effects in lime and cement mortars samples that should be taken into account when interpret-ing the results. Moreover, the MIP results highlighted how gypsum mortar samples display a porosity distribution that is best studied using this technique. This multi-analytical approach provides important insights into the interpretation of the porosimetric data obtained. This is crucial in the characterization of mortars and provides key information for the study of building materials and cultural heritage conservation
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