A model-free approach for simulation of EPR spectra of nitroxide spin probes in liquid-crystalline materials was suggested and used to obtain parameters of molecular orientation and rotational ...mobility. The developed method is based on experimental recording and numerical simulation of the angular dependence of EPR spectra, which is shown to be much more informative in comparison with a single EPR spectrum. Quantitative spectral simulations considering both local orientational ordering and distribution of local directors in the sample were used for discrimination of models of rotational mobility and orientational alignment. The method was applied for detailed quantitative characterization of axial, orthorhombic, and low-symmetry non-orthorhombic molecular orientation distributions. It is shown that the ordinarily used model of rotational diffusion in a mean-field potential is suitable for the description of molecular mobility and orientational ordering only for relatively low sample temperatures and low-mobility probe molecules with large sizes. In cases of high molecular mobility, the more realistic jump mechanism of molecular moves can be approximately described as quasi-librations. For ordered liquid crystals it was found that mostly the order parameters up to rank 12–14 are essential and easily determined. When well-aligned materials are described, the order parameters up to 18th rank or even higher become meaningful. Both molecular and sample biaxiality is analyzed and quantitatively characterized. The local molecular ordering and sample orientational alignment are quantitatively characterized separately.
Bismuth-based compounds are widely used as superconductors, catalysts and materials for optical devices. Their properties, and the possibility of controlling these properties, are determined by their ...electronic structure and the local environment of the Bi-centres. Although X-ray spectroscopy is a powerful method for revealing crystal and electronic structure, the results obtained so far have been limited by the energy resolution of the experimental data. Here, for the first time, we report X-ray absorption near edge structure (XANES) data, recorded in high energy resolution fluorescence detection (HERFD) mode at the Bi LIII and LI edges for a number of bismuth compounds. Experimental data are analysed using ab initio calculations based on a finite difference method (FDMNES) code, for metallic Bi, Bi2O3, BiPO4, Bi4(GeO4)3 and NaBiO3 compounds. It was observed that both the oxidation state and the length of the Bi-ligand bonds determine the exact position of the absorption edge, and this is in good agreement with other studies. Additionally, due to the high energy resolution, spectral features corresponding to strong Bi p-d orbital mixing were resolved. The results obtained here can be used as input for further investigations of the electronic structure of bismuth compounds in different chemical states.
Display omitted
•High energy resolution x-ray absorption spectroscopy for Bi compounds has been performed.•Experimental results are in good agreement with ab initio calculations.•Resolved pre-edge spectral feature indicates strong p-d orbital mixing.•Mixing with ligand orbitals has been also detected.
The effect of pretreatment and Pt content on the catalytic properties as well as mechanistic features of DR were investigated for structured catalysts comprised of Pt supported on CeO
2–ZrO
2 oxide ...doped by Pr. Progressive reduction of cationic Pt species by the reaction feed lowers the activity in CH
4 dry reforming while accelerating the reverse water gas shift reaction catalyzed only by Pt
0, which then decreases progressively the H
2/CO ratio in the effluent. This process is counteracted by the mobility of surface oxygen supplying oxygen atoms to reduced Pt centers thus ensuring their reoxidation and generating in parallel surface oxygen vacancies for the dissociation of CO
2.
A mathematical model and software were developed for numerically studying the transients of the complex catalytic reactions. The processing of experimental data was fulfilled taking into account the importance of cationic forms of Pt, reactivity of carbonate complexes coordinated to these cations and oxygen surface/bulk diffusion. A quantitative evaluation of the density of catalyst's active sites and their coverage by reactive species was accomplished and the rates both of the lattice oxygen diffusion and main stages of the catalytic reaction were estimated.
The formation energies of oxygen vacancies and the migration barriers of oxygen in TiO
2
with a rutile structure have been calculated by the projector augmented-wave method within the electron ...density functional theory. We have derived analytical expressions for the temperature-dependent diffusion coefficients along two crystallographic directions using Landman’s method and calculated the activation energies and the pre-exponential factor. The temperature-dependent diffusion coefficient is shown to be in good agreement with the experiment and with the results obtained previously by the statistical method. The influence of impurities on the diffusion properties of oxygen in the oxide is discussed.
5-aminolevulinic acid (5-ALA) is a natural precursor of protoporphyrin IX (PP IX), which possesses fluorescent properties and is more intensively accumulated in tumor cells than in normal tissue. ...Therefore, the use of 5-ALA in the surgical treatment of intracranial tumors, particularly gliomas, has gained popularity in the last years, whereas its use in other intracranial pathological entities including meningiomas has been reported occasionally. This study describes a series of 28 patients with intracranial meningiomas, who were administered 5-ALA for a better visualization of tumor boundaries. Twelve patients underwent also laser spectroscopic analysis in order to confirm the visual impression of tumor tissue visualization. Bone infiltration was readily demonstrated. In one case, the tumor recurrence could have been prevented by removal of a tumor remnant, which would possibly have been better recognized if spectroscopic analysis had been used. Fluorescent navigation (FN) is a useful method for maximizing the radicality of meningioma surgery, particularly if the tumor infiltrates the bone, the skull base, and/or the surrounding structures.
The influence of simple and transition metal impurities, as well as interstitial impurities (B, C, and N), on the oxygen adsorption on the Ti
5
Si
3
titanium silicide surface has been studied using ...the of projector augmented wave method within the electron density functional theory. It has been shown that titanium-substituting impurities belonging to the latter halves of the 3
d
–5
d
periods cause most significant changes in the adsorption energy. In addition, simple metals and interstitial impurities reduce the oxygen–surface interaction. By analyzing local electron densities of states, charge density difference distribution, charge transfer, and the overlap population of oxygen bonds to nearest-neighbor atoms, we have revealed special aspects of impurity influence on chemical bonding between the titanium silicide surface and oxygen. Factors responsible for an increase/decrease in the adsorption energy of oxygen on the doped surface are discussed. A correlation between the change in adsorption energy and the electronegativity of impurities has been found.
The comparative study of sorption of polar substances acetonitrile and water into powders and membranes (>10 μm thick) of modified Hummers (HGO) and Brodie (BGO) graphite oxides was performed using ...isopiestic method (IM) and differential scanning calorimetry (DSC). Additional sorption data were obtained for pyridine and 1-octanol. Sorption measurements were accompanied by conventional XRD and XPS control. Electron paramagnetic resonance (EPR) was additionally used to characterize ordering of the membranes. The impact on sorption of synthetic procedure (Brodie or Hummers), method of making membranes, chemical nature of the sorbent, and method of sorption was systematically examined. It was demonstrated that variations in synthetic procedures within both Hummers and Brodie methods did not lead to changes in the sorption properties of the corresponding powders. Sorption of acetonitrile and pyridine was reduced by approximately half when switching from powders to membranes at ambient temperature. DSC measurements at a lower temperature gave equal sorption of acetonitrile into HGO powder and membranes. Water has demonstrated unique sorption properties. Equal sorption of water was measured for HGO membranes and powders at T = 298 K and at T = 273 K. It was demonstrated that lowering the orientational alignment of the membranes led to the increase of sorption. In practice this could allow one to tune sorption/swelling and transport properties of the GO membranes directly by adjusting their internal ordering without the use of any composite materials.
The atomic and electronic structures, the elastic moduli, the vibrational frequencies, and the thermodynamic characteristics of the Ti
5
Si
3
titanium silicide are calculated by the projector ...augmented wave method within the density functional theory. The Fermi surface is calculated and the electron velocity on all its four sheets is estimated. An analysis of the spatial dependence of linear compressibility, Young’s modulus, and shear modulus demonstrates their weak anisotropy, and the anisotropy of Poisson’s ratio is more pronounced. The phonon spectrum of the titanium silicide is calculated and its thermodynamic characteristics are estimated. On the whole, the calculation results are in good agreement with the experimental results.
The atomic and electronic structure of the Al
2
O
3
/Ti
5
Si
3
interface has been calculated for the first time by the method of projected augmented waves within the electron density functional ...theory. The values of the adhesion energy are determined depending on the cleavage plane. It is shown that the work of separation obtained for the plane passing through the interplanar distance closest to the interface in titanium silicide is significantly less than that at the interface.
PDF
