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排序方式: 共有238条查询结果,搜索用时 15 毫秒
31.
Jacopo La Nasa Marco Zanaboni Daniele Uldanck Ilaria Degano Francesca Modugno Hartmut Kutzke Eva Storevik Tveit Biljana Topalova-Casadiego Maria Perla Colombini 《Analytica chimica acta》2015
Modern oil paints, introduced at the beginning of the 20th century, differ from those classically used in antiquity in their chemical and compositional features. The main ingredients were still traditional drying oils, often used in mixtures with less expensive oils and added with several classes of additives. Consequently, detailed lipid profiling, together with the study of lipid degradation processes, is essential for the knowledge and the conservation of paint materials used in modern and contemporary art. 相似文献
32.
The conversion of molecular nitrogen to ammonia is a key biological and chemical process and represents one of the most challenging topics in chemistry and biology. In Nature the Mo-containing nitrogenase enzymes perform nitrogen 'fixation' via an iron molybdenum cofactor (FeMo-co) under ambient conditions. In contrast, industrially, the Haber-Bosch process reduces molecular nitrogen and hydrogen to ammonia with a heterogeneous iron catalyst under drastic conditions of temperature and pressure. This process accounts for the production of millions of tons of nitrogen compounds used for agricultural and industrial purposes, but the high temperature and pressure required result in a large energy loss, leading to several economic and environmental issues. During the last 40 years many attempts have been made to synthesize simple homogeneous catalysts that can activate dinitrogen under the same mild conditions of the nitrogenase enzymes. Several compounds, almost all containing transition metals, have been shown to bind and activate N? to various degrees. However, to date Mo(N?)(HIPTN)?N with (HIPTN)?N= hexaisopropyl-terphenyl-triamidoamine is the only compound performing this process catalytically. In this review we describe how Density Functional Theory calculations have been of help in elucidating the reaction mechanisms of the inorganic compounds that activate or fix N?. These studies provided important insights that rationalize and complement the experimental findings about the reaction mechanisms of known catalysts, predicting the reactivity of new potential catalysts and helping in tailoring new efficient catalytic compounds. 相似文献
33.
In this paper we develop a new method to prove the existence of minimizers for a class of constrained minimization problems on Hilbert spaces that are invariant under translations. Our method permits to exclude the dichotomy of the minimizing sequences for a large class of functionals. We introduce family of maps, called scaling paths, that permits to show the strong subadditivity inequality. As byproduct the strong convergence of the minimizing sequences (up to translations) is proved. We give an application to the energy functional I associated to the Schrödinger-Poisson equation in R3
iψt+Δψ−(|x|−1?2|ψ|)ψ+|ψ|p−2ψ=0 相似文献
34.
César Ricardo Teixeira Tarley Felipe Nascimento Andrade Fernanda Midori de Oliveira Marcela Zanetti Corazza Luiz Fernando Mendes de Azevedo Mariana Gava Segatelli 《Analytica chimica acta》2011,(2):145
A novel ion imprinted polyvinylimidazole-silica hybrid copolymer (IIHC) was synthesized and used as a selective solid sorbent for Pb2+ ions preconcentration using an on-line solid phase extraction (SPE) system coupled to TS-FF-AAS. The ionic hybrid sorbent was prepared using 1-vinylimidazole and 3-(trimethoxysilyl)propylmethacrylate as monomers, Pb2+ ions as template, tetraethoxysilane as reticulating agent and 2,2′-azobis-isobutyronitrile as initiator. The best on-line SPE conditions concerning sorption behavior, including sample pH (6.46), buffer concentration (9.0 mmol L−1), eluent (HNO3) concentration (0.5 mol L−1) and preconcentration flow rate (4.0 mL min−1), were optimized by means of full factorial design and Doehlert matrix. The analytical curve ranged from 2.5 to 65.0 μg L−1 (r = 0.999) with limit of detection of 0.75 μg L−1; the precision (repeatability) calculated as relative standard deviation (n = 10) was 5.0 and 3.6% for Pb2+ concentration of 10.0 and 60.0 μg L−1, respectively. From on-line breakthrough curve, column capacity was 3.5 mg g−1. Preconcentration factor (PF), consumptive index (CI) and concentration efficiency (CE) were 128.0, 0.16 mL and 25.6 min−1, respectively. The selective performance of the sorbent, based on relative selectivity coefficient, was compared to NIC (non imprinted copolymer) for the binary mixture Pb2+/Cd2+, Pb2+/Cu2+ and Pb2+/Zn2+. The results showed that ion imprinted polyvinylimidazole-silica hybrid polymer had higher selectivity for Pb2+ than NIC at 64.9, 16.0 and 8.8 folds. The developed method was successfully applied for highly sensitive and selective Pb2+ determination in different kinds of water samples, parenteral solutions and urine. Accuracy was also assessed by analyzing certified reference fish protein (DORM-3) and marine sediment (MESS-3 and PACS-2) with satisfactory results. 相似文献
35.
36.
Erlandson R.E. Zanetti L.J. Potemra T.A. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1989,17(2):196-200
Data from the Viking magnetic-field experiment are described which demonstrate the diverse types of magnetic fluctuations which may be observed from a polar-orbiting satellite. The Viking observations provide an opportunity to observe Pc 1 waves at middle-latitudes above the ionosphere and to determine the spectral structure and polarization of the waves. ULF/ELF broadband noise represents a second type of magnetic fluctuation measured by Viking. This type of magnetic fluctuation was observed at high latitudes near the polar cusp and may be useful in the identification of polar-cusp boundaries. Thirdly, electromagnetic ion-cyclotron waves have also been observed in the polar-cusp region. These waves occur only during an unusually high level of magnetic activity and appear toe be generated locally 相似文献
37.
38.
The classical perturbation series is used to follow an invariant curve, of fixed winding number, up to its break-up point. The result obtained for the critical value of the perturbation and the corresponding behaviour of the invariant curve are in complete agreement with the results of Kadanoff and Shenker. 相似文献
39.
Multigram scale, solventless, and diffusion-controlled route to highly monodisperse PbS nanocrystals
Cademartiri L Bertolotti J Sapienza R Wiersma DS von Freymann G Ozin GA 《The journal of physical chemistry. B》2006,110(2):671-673
High-quality PbS nanocrystals were produced in multigram-scale quantities through a solventless, heterogeneous, and relatively green route. The heterogeneous nature of this reaction allows one to limit the diffusion in the system, allowing for unprecedented monodispersity and quality of the product demonstrated by a full-width at half-maximum of the photoluminescence peak (PL fwhm) as low as 52 meV, a Stokes shift as low as 10 meV, and a quantum yield (QY) of 40%. The growth of the nanocrystals is interpreted in the framework of a diffusion-controlled Ostwald growth in conditions of strong supersaturation. 相似文献
40.
Hannah Schreyer Rene Eckert Sarah Immohr Jacopo deBellis Michael Felderhoff Ferdi Schüth 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(33):11384-11387
Supported catalysts are among the most important classes of catalysts. They are typically prepared by wet‐chemical methods, such as impregnation or co‐precipitation. Here we disclose that dry ball milling of macroscopic metal powder in the presence of a support oxide leads in many cases to supported catalysts with particles in the nanometer size range. Various supports, including TiO2, Al2O3, Fe2O3, and Co3O4, and different metals, such as Au, Pt, Ag, Cu, and Ni, were studied, and for each of the supports and the metals, highly dispersed nanoparticles on supports could be prepared. The supported catalysts were tested in CO oxidation, where they showed activities in the same range as conventionally prepared catalysts. The method thus provides a simple and cost‐effective alternative to the conventionally used impregnation methods. 相似文献