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排序方式: 共有210条查询结果,搜索用时 46 毫秒
21.
Lützenkirchen J 《Journal of colloid and interface science》2005,290(2):489-497
Surface titrations of minerals in aqueous electrolyte solutions are used as building blocks for surface complexation modelling. However, these potentiometric data may contain less model relevant information than previously and presently assumed. In the literature, derivative analyses have been applied to experimental surface charge versus pH curves and four or more pK values were extracted for goethite or aluminium oxide. Derivative analysis of specific surface charge versus pH curves calculated for various published model variants for goethite shows that not more than the net-zero proton surface charge condition can be extracted from computer generated data. Generated data can be produced in density and precision superior to experimental data, but yield only relatively little output from such derivative analysis compared to what has previously been extracted from derivatives of experimental data. For the generated goethite data and for all model variants only the point of zero could be extracted. For the various goethite model variants tested a nearly symmetrical peak appeared at the point of zero charge in the derivative curve. A different pattern could be obtained for generic models, for which two sites with unequal sites densities and different pK values were assumed. Variation of these parameters could result in derivatives of the charging curves with two maxima or one maximum and a broad tailing. In the literature, curves with features nearly identical to these generated curves have been interpreted by up to four pK values (i.e., four different sites within a 1-pK model). It is concluded that the interpretation of the generated data is in all cases hampered by the overwhelming electrostatic contributions to the free energy of proton ad/desorption. In no case except for the one-site 1-pK model was it possible to extract the input pK value(s) from the derivatives. Plausible explanations for the discrepancy between generated data and published experimental data are discussed. 相似文献
22.
Naringenin, a flavanone compound highly enriched in grapefruits, has been identified as a possible inhibitor of cell proliferation; and thus has the potential to act as an antitumorigenic agent. In this study, the binding of naringenin to bovine serum albumin (BSA) was studied at the physiological conditions (pH=7.40) by fluorescence and UV-vis spectroscopy. Naringenin strongly quenches the intrinsic fluorescence of BSA, and a decrease in the fluorescence quenching constant was observed together with an increase in temperature, which indicates that the fluorescence quenching of BSA by naringenin is a result of the formation of naringenin-BSA complex. Binding parameters calculating from Stern-Volmer method and Scatchard method showed that naringenin bind to BSA with the binding affinities of the order 104 L mol−1. Thermodynamic parameters such as ΔG, ΔH and ΔS, were calculated at different temperatures, showing that electrostatic interactions were mostly responsible for the binding of naringenin to BSA. Site marker competitive displacement experiments demonstrating that naringenin bind with high affinity to site I (subdomain IIA) of BSA. Furthermore, the effect of metal ions to naringenin-BSA system was studied, and the specific binding distance r (3.30 nm) between donor (Trp-212) and acceptor (naringenin) was obtained according to fluorescence resonance energy transfer (FRET). 相似文献
23.
用第一性原理研究放射性同位素铀在针铁矿(α-FeOOH)中的占位情况, 分别考虑铀原子替代针铁矿中的铁的替位缺陷和铀的多种八面体和多种四面体间隙缺陷. 计算发现了三个最稳定的缺陷构型, 它们分别对应于一个铀替位缺陷(S) 及其中的一个铀的八面体(O)和四面体(T)间隙缺陷, 其形成能分别为-13.49, -3.86, -1.60 eV. 也研究了两个相邻的铀原子在针铁矿中的占位情况, 发现双铀原子很容易掺入到相邻的SS或OS位, 它们的形成能分别为-27.392和-16.214 eV, 结合能分别为-0.417和1.131 eV. 表明双原子铀在针铁矿中会以SS形式发生偏聚而较难以OS形式偏聚.
关键词:
铀
针铁矿
占位
第一性原理 相似文献
24.
T. Malcherek C. M. Domeneghetti V. Tazzoli E. K. H. Salje U. Bismayer 《Phase Transitions》2013,86(1):119-131
The results of a high temperature single crystal X-ray diffraction study of synthetic titanite within the stability field of the A2/a (C2/c) paraphase (T > 500 K) are reported. The structure has been refined using a conventional model and one in which the Ca atom is disordered over two positions. A break in thermal expansion near T c = 825 K correlates with an effective volume contraction of the Ti octahedron. When refined with split Ca position a reorientation of the Ca displacement vector at T c is found, resulting in a more symmetrical structural arrangement of the disordered Ca cations with respect to the surrounding Ti cations. In the conventional model this reorientation is seen as a break in the thermal elongation of the shortest Ti-Ca distance. The observed temperature of the isosymmetrical structural instability is in agreement with previous observations based on Hard Mode IR and Raman spectroscopic measurements. A possible mechanism causing the observed structural changes and similarities to the thermal behaviour of the iso-structural malayaite, CaSnSiO5, are discussed. 相似文献
25.
考虑将双模纠缠相干光场的两模场同时分别注入两个腔中,初态处于W态的三体纠缠二能级原子中的两个分别在这两个腔内,并且都与光场发生共振相互作用,经腔QED演化之后,对纠缠相干光场进行光子探测和对纠缠原子进行选择性测量,通过操纵相互作用的时间和光场的参数可控制W态中处于腔外的第三个原子的非经典效应,如粒子数布居差的崩塌一回复现象和偶极压缩现象,从而实现了更强地远程控制原子的非经典特性. 相似文献
26.
《Acta Crystallographica. Section C, Structural Chemistry》2018,74(3):295-299
A new Cu3Au‐type ternary phase (τ phase) is found in the AuPd‐rich part of the Au–In–Pd system. It has a broad homogeneity range based on extensive (Pd,Au) and (In,Au) replacement, with the composition varying between Au17.7In25.3Pd57.0 and Au50.8In16.2Pd33.0. The occupancies of the crystallographic positions were studied by single‐crystal X‐ray diffraction for three samples of different composition. The sites with mm symmetry are occupied by atoms with a smaller scattering power than the atoms located on 4/mmm sites. Two extreme structure models were refined. Within the first, the occupation type changes from (Au,In,Pd)3(Pd,In) to (Au,Pd)3(In,Pd,Au) with an increase in the Au gross content. For the second model, the occupation type (Au,In,Pd)3(Pd,Au) remains essentially unchanged for all Au concentrations. Although the diffraction data do not allow the choice of one of these models, the latter model, where Au substitutes In on 4/mmm sites, seems to be preferable, since it agrees with the fact that the homogeneity range of the τ phase is inclined to the Au corner and provides the same occupation type for all the studied samples of different compositions. 相似文献
27.
A DFT study on site selectivity in successive 1,3-dipolar cycloadditions of meso-tetraarylporphyrins with azomethine ylide and N-methylnitrone has been carried out. The calculation of the thermodynamic stability of both ylide and nitrone-derived adducts reveals that bacteriochlorins are more stable and have stronger aromatic character than isobacteriochlorins. Calculations of whole reaction pathways show that cycloadditions of azomethine ylide on porphyrin and its derived chlorin are irreversible and hence kinetically controlled. Solvent influence on the site selectivity of this reaction has also been considered, and appears to be decisive in controlling the site selectivity. In contrast, cycloadditions of nitrone over porphyrin and chlorin are clearly reversible, pointing to a thermodynamic control of these reactions. 相似文献
28.
Longbin Li Xiannong Tang Senhe Huang Dr. Chenbao Lu Prof. Dirk Lützenkirchen-Hecht Prof. Kai Yuan Prof. Xiaodong Zhuang Prof. Yiwang Chen 《Angewandte Chemie (International ed. in English)》2023,62(22):e202301642
Iron phthalocyanine-based polymers (PFePc) are attractive noble-metal-free candidates for catalyzing oxygen reduction reaction (ORR). However, the low site-exposure degree and poor electrical conductivity of bulk PFePc restricted their practical applications. Herein, laminar PFePc nanosheets covalently and longitudinally linked to graphene (3D-G-PFePc) was prepared. Such structural engineering qualifies 3D-G-PFePc with high site utilization and rapid mass transfer. Thence, 3D-G-PFePc demonstrates efficient ORR performance with a high specific activity of 69.31 μA cm−2, a high mass activity of 81.88 A g−1, and a high turnover frequency of 0.93 e s−1 site−1 at 0.90 V vs. reversible hydrogen electrode in O2-saturated 0.1 M KOH, outperforming the lamellar PFePc wrapped graphene counterpart. Systematic electrochemical analyses integrating variable-frequency square wave voltammetry and in situ scanning electrochemical microscopy further underline the rapid kinetics of 3D-G-PFePc towards ORR. 相似文献
29.
30.
Ternary compounds M6Ni16Si7 (M=Ti, Zr, Hf) have been investigated by X-ray powder/single crystal and neutron powder diffraction. Compounds with Zr and Hf crystallize in the ordered Th6Mn23 type (Mg6Cu16Si7-type, space group ), whereas Ti6Ni16.7Si7 contains an additional Ni atom partially occupying the 24e site (M2 site, x=0.4637,0,0; occ.=0.119) inside a Ti octahedron; Ti atoms occupy a split position. Ti6Ni16.7Si7 represents a new variant of the filled Th6Mn23 type structure. Ab initio calculations confirm the structural difference: additional Ni atoms favour the 24e site for Ti6Ni16.7Si7, however, for the Zr and Hf-based compounds the unoccupied site renders an energetically lower ground state. Enthalpies of formation of Ti6Ni17Si7, Zr6Ni16Si7, and Hf6Ni16Si7 were calculated to be −68.65, −74.78, and −78.59 kJ/(mol of atoms), respectively. 相似文献