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排序方式: 共有284条查询结果,搜索用时 15 毫秒
1.
Ray Leslie Withers Carlos Otero-Diaz Adrian Gómez-Herrero Albert Prodan Lasse Norén 《Journal of solid state chemistry》2005,178(10):3159-3168
Ternary derivatives of 1T-TaS2 have been synthesized and the variation in the highly structured diffuse intensity distributions characteristic of such materials carefully monitored to investigate the effect that such substitution has upon the band structures and Fermi surfaces (FSs) of the materials. Removal of d electrons via the replacement of Ta ions with lower valent transition metal ions leads to a systematic increase in the radii of the characteristic structured diffuse intensity distribution. Extended Hückel tight binding calculations of the FSs of the doped samples are carried out and used to predict possible nesting wave-vectors. The results are in reasonably good agreement with the radii of the experimentally observed diffuse intensity distributions. 相似文献
2.
3.
The suitability of a new technology single-monochromator diode array spectroradiometer for UV-radiation safety measurements, in particular for sunbed measurements, was evaluated. The linearity, cosine response, temperature response, wavelength scale, stray-light and slit function of the spectroradiometer were determined and their effects on the measurement accuracy evaluated. The main error sources were stray-light and nonideal cosine response, for which correction methods are presented. Without correction, the stray-light may reduce the accuracy of the measurement excessively, particularly in the UV-B range. The expanded uncertainty of the corrected UV measurements is estimated to be 14%, which is confirmed with the comparative measurements carried out with a well-characterized double-monochromator spectroradiometer. The measurement accuracy is sufficient for sunbed measurements, provided that all corrections described above have been done and the user of the instrument has a good understanding of the instrument's operating principles and potential error sources. If these requirements are met, the tested spectroradiometer improves and facilitates market surveillance field measurements of sunbeds. 相似文献
4.
Luzgin MV Stepanov AG Arzumanov SS Rogov VA Parmon VN Wang W Hunger M Freude D 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,12(2):457-465
By using 13C MAS NMR spectroscopy (MAS = magic angle spinning), the conversion of selectively 13C-labeled n-butane on zeolite H-ZSM-5 at 430-470 K has been demonstrated to proceed through two pathways: 1) scrambling of the selective 13C-label in the n-butane molecule, and 2) oligomerization-cracking and conjunct polymerization. The latter processes (2) produce isobutane and propane simultaneously with alkyl-substituted cyclopentenyl cations and condensed aromatic compounds. In situ 13C MAS NMR and complementary ex situ GC-MS data provided evidence for a monomolecular mechanism of the 13C-label scrambling, whereas both isobutane and propane are formed through intermolecular pathways. According to 13C MAS NMR kinetic measurements, both pathways proceed with nearly the same activation energies (E(a) = 75 kJ mol(-1) for the scrambling and 71 kJ mol(-1) for isobutane and propane formation). This can be rationalized by considering the intermolecular hydride transfer between a primarily initiated carbenium ion and n-butane as being the rate-determining stage of the n-butane conversion on zeolite H-ZSM-5. 相似文献
5.
By means of model calculations it could be shown for an irreversible surface reaction of 1st order that the determination of the activation energy of the desorption of the reactant or, respectively, of the surface reaction is possible by application of the method of variation of the heating rate to the desorption curve of the reactant, according to circumstances whether the ratio of the activation energy of the surface reaction and of the desorption of the reactant is greater or smaller than one.The possibilities of the kinetic evaluation are applied to the isomerization of cyclopropane on a NaX-zeolite catalyst. The resulting heat of adsorption of cyclopropane and the activation energy of the reaction agree well with the values of literature obtained by isothermal measurements in a pulse reactor.
Zusammenfassung Mit Modellrechnungen für eine irreversible Oberflächenreaktion 1. Ordnung konnte gezeigt werden, daß durch Anwendung der Methode der Variation der Heizgeschwindigkeit auf die Desorptionskurve des Ausgangsstoffes die näherungsweise Bestimmung der Aktivierungsenergie der Desorption des Ausgangsstoffes bzw. der Oberflächenreaktion möglich ist, je nachdem ob das Verhältnis der Aktivierungsenergien für die Oberflächenreaktion und der Desorption des Ausgangsstoffes größer oder kleiner als eins ist.Die Möglichkeiten der kinetischen Auswertung werden auf die isomerisierung von Cyclopropan an einem NaX-Zeoliten angewendet. Die erhaltene Adsorptionswärme für Cyclopropan und die Aktivierungsenergie für die Reaktion stimmen gut mit Literatur-werten von isothermen Messungen im Impulsreaktor überein.相似文献
6.
Wang W Buchholz A Ivanova II Weitkamp J Hunger M 《Chemical communications (Cambridge, England)》2003,(20):2600-2601
A general method for the synthesis of quaternary ammonium cations in acidic zeolites by a direct reaction of tertiary amines and alcohols is described. 相似文献
7.
The composition range and (composite modulated) structure of compounds within the wide range non-stoichiometric LaSb2Snx, 0.1?x?0.75, solid solution is carefully investigated via a combined electron diffraction, XRD and electron probe microanalysis study. Evidence for metastability of the LaSb2Snx phase at the low x composition end of the solid solution is presented. Direct evidence is found for a reasonably (although by no means perfectly) well ordered Sn sub-structure which is, in general, mutually incommensurable with respect to a very well ordered underlying LaSb2 sub-structure along both a and c directions. The overall (3+2)-d superspace group symmetry is given along with a discussion of the consequences as regards the arrangement of the Sn atoms. The Sn sub-structure c-axis cell dimension shows very little variation with composition x providing direct experimental evidence of the importance of Sn-Sn metallic bonding (along one-dimensional [001] Sn strings) for the stability of the phase. 相似文献
8.
Jiao J Kanellopoulos J Wang W Ray SS Foerster H Freude D Hunger M 《Physical chemistry chemical physics : PCCP》2005,7(17):3221-3226
27Al spin-echo, high-speed MAS (nu(rot) = 30 kHz), and MQMAS NMR spectroscopy in magnetic fields of B0 = 9.4, 14.1, and 17.6 T were applied for the study of aluminum species at framework and extra-framework positions in non-hydrated zeolites Y. Non-hydrated gamma-Al2O3 and non-hydrated aluminum-exchanged zeolite Y (Al,Na-Y) and zeolite H,Na-Y were utilized as reference materials. The solid-state 27Al NMR spectra of steamed zeolite deH,Na-Y/81.5 were found to consist of four signals. The broad low-field signal is caused by a superposition of the signals of framework aluminum atoms in the vicinity of bridging hydroxyl protons and framework aluminum atoms compensated in their negative charge by aluminum cations (delta(iso) = 70 +/- 10 ppm, C(QCC) = 15.0 +/- 1.0 MHz). The second signal is due to a superposition of the signals of framework aluminum atoms compensated by sodium cations and tetrahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 65 +/- 5 ppm, C(QCC) = 8.0 +/- 0.5 MHz). The residual two signals were attributed to aluminum cations (delta(iso) = 35 +/- 5 ppm, C(QCC) = 7.5 +/- 0.5 MHz) and octahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 10 +/- 5 ppm, C(QCC) = 5.0 +/- 0.5 MHz). By chemical analysis and evaluating the relative solid-state 27Al NMR intensities of the different signals of aluminum species occurring in zeolite deH,Na-Y/81.5 in the non-hydrated state, the aluminum distribution in this material was determined. 相似文献
9.
Pekka Juusola Pentti Minkkinen Lasse Leino Jarmo K. Laihia 《Monatshefte für Chemie / Chemical Monthly》2007,138(10):951-965
Summary. (E)- and (Z)-Urocanic acids are endogenous chemicals in the normal mammalian skin. The first and the second thermodynamic dissociation
constants (pK
a1 and pK
a2) of urocanic acid isomers were determined using UV spectrophotometry in aqueous solutions. The values with standard deviation
(pK
a1 = 3.43 ± 0.12 and pK
a2 = 5.80 ± 0.04) and (pK
a1 = 2.7 ± 0.3 and pK
a2 = 6.65 ± 0.04) were obtained to (E)- and (Z)-urocanic acids, respectively. The second dissociations were studied also by potentiometric titration in aqueous sodium chloride
solutions up to the isotonic salt concentration (0.154 mol dm−3), and the second thermodynamic dissociation constants as well as activity parameters for both isomers were determined at
temperature 25°C and for (E)-urocanic acid also at 37°C. The obtained pK
a2 values were close to those found by UV spectrophotometry. The equations for the calculation of the second stoichiometric
dissociation constants of urocanic acid isomers on molality and molarity scale in aqueous sodium chloride solutions were derived.
The obtained pK
a1 and pK
a2 values for (Z)-urocanic acid appear to be essentially lower than some previously reported values in literature.
An erratum to this article is available at . 相似文献
10.
Juuso Sammaljärvi Antero Lindberg Mikko Voutilainen Jussi Ikonen Marja Siitari-Kauppi Petteri Pitkänen Lasse Koskinen 《Journal of Radioanalytical and Nuclear Chemistry》2017,314(3):1557-1575
The purpose of this work was to support the long-term experiments for investigation of rock matrix retention properties at the spent nuclear fuel repository site ONKALO, in Olkiluoto, Western Finland. Towards this end, rock samples were characterised in terms of porosity connected to mineralogy, with emphasis on pore sizes, and fillings. Both filled and unfilled pores were found with unfilled pores being majority in the pegmatitic granite veins of the mostly veined gneiss samples. It was found that the fillings were often connected to local alteration and the alteration caused variation in the range of pore sizes. 相似文献