Home – Home – Jornals – Journal of Structural Chemistry 2010 number 7

A new technical approach to investigation of heterogeneous catalysts with deposited metallic nanoparticles of the active component is suggested. Small-angle X-ray scattering (SAXS) patterns for powder samples of the support matrices with deposited metal particles (Pt, Pd and others) taken at varying density contrast are shown to provide essential information on the structure and dispersity of the active components and their interaction with the support surface, which allows an integral assessment of the formation of strong chemical binding of deposited metal to the support matrix. The measured structural characteristics are compared with data obtained by other physicochemical methods, in particular, transmission electron microscopy (TEM). The suggested technique is verified using mathematical models and applied to a real heterogeneous catalyst Pt/γ-Al₂O₃. The proposed method of SAXS data analysis makes it possible to reveal the formation of active sites in heterogeneous catalysts and optimize their preparation procedures.

The possibility of controlling the state of platinum deposited on the support surface via minor changes in the catalyst preparation procedure is demonstrated using a series of highly dispersed Pt/γ-Al₂O₃ catalysts with different particle size of the active component. Dispersity, local structure and electronic state of supported platinum were examined by a combination of high resolution transmission electron microscopy and X-ray absorption spectroscopy (EXAFS/XANES). It was shown that various platinum species can be obtained on the surface of the support: bulk or surface Pt(II) or Pt(IV) oxides, mixed metal-oxide structures, bulk particles of metallic platinum, and two-dimensional surface Pt⁰ particles strongly interacting with the support.

The potential of modern solid-state NMR spectroscopy techniques applied to quadrupolar nuclei with half-integer spin is demonstrated. Correlations of NMR parameters with the local environment of nucleus (for 14 nuclei) are presented in the second part of the review.

Precision X-ray diffraction at synchrotron radiation was used to reveal the separation of perovskite-like oxides SrCo_0.8-xFe_0.2Nb_xO_3-z (x = 0.2 and 0.3) into two phases of a similar structure identical to initial perovskite structure, but having different unit cell parameters and supposedly different oxygen deficiency. The structural transformation is accompanied by oxygen outlet from the structure. The study of oxygen atoms intercalation from air into the oxygen-deficient structure showed that the structural changes are reversible: heating to 400°C in air restores the initial state of samples.

Fundamental and methodological aspects of secondary ion mass spectrometry applied to analysis of elemental/phase composition and structure of the surface layers of fine materials are discussed.

The charged state of atoms in layered cation-substituted disulfides CuCr_1-xV_xS₂ (x = 0-0.4) was studied using X-ray photoelectron spectroscopy. The study was performed with polycrystalline powder and ceramic samples of chromium-copper disulfides. CuCr_1-xV_xS₂ samples were shown to comprise differently charged atoms of chromium, copper and vanadium, the charged state of which varies with the concentration of vanadium cations (x).

Polyimide films on BK7 glass, which are used to produce matrices for infrared detectors, have been synthesized. When these systems are investigated by monochromatic zero ellipsometry, the least difference between parameters of the incident light polarization, which are determined from suppression angles of polarizer and analyzer and calculated within a certain model, is obtained via introducing air between mounting glass and the film. Validity of the solution was verified by holding the film in water vapor; then, solution of the inverse problem of ellipsometry indicated the appearance of liquid water between the film and mounting glass. Data of the electron microscopy study were used to suggest a water penetration mechanism and develop a physical model of the system polyimide film/mounting glass.

The structure of trans-[RuNO(NH₃)₄(H₂O)](NO₃)₃ (I) and trans-[RuNO(NH₃)₄(NO₃)](NO₃)₂ (II) was determined by XRD. Crystallographic data are as follows: space group I4₁/a; a = b = 18.280(1) Å, c = 15.129(1) Å, R = 0.0244 (I), and space group Cm, a = 11.5620(3) Å, b = 7.9934(2) Å, c = 7.7864(2) Å, β = 127.124(1)°, R = 0.0139 (II). Interatomic distances for complex particles of fac- and mer-[RuNO(NH₃)₂(NO₃)₃] (III and IV, respectively) were determined by EXAFS.

Reverse micelles of oxyethylated nonylphenol (Triton N-42) were used to synthesize ultradispersed powder of ammonium nitrate with the yield 75% and content 79%. Its composition and morphology were examined by colorimetry, IR Fourier spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). According to XRD, the sample is stable with time, although it is a mixture of three polymorphic forms of crystal ammonium nitrate - one stable and three metastable forms. The agglomerates consists of 20-50 nm nanoparticles; a part of nanoparticles form filaments of length 0.5-1.5 μm and thickness 25-30 nm, as estimated by SEM.

Texture characteristics of the oxidized Technosorb carbon material were investigated. The BET surface area, specific surface area of mesopores, mesopore diameter distribution, and total pore volume were determined using an ASAP 2400 adsorption instrument and a Sorbtometer-M analyzer. Changes in the texture characteristics upon oxidation were monitored.

Results obtained by the authors on the development of holographic methods for investigation of photopolymer materials are presented. The work considers the methods for pulse recording of dynamic transmission gratings and characterization of hologram angular selectivity, the double-beam method of two-photon holographic recording, and heterodyne detection of inhomogeneities in volume holograms.

The initial stages of iron film growth on monocrystalline silicon were studied using a combined method of magneto-optic ellipsometry. The growth of iron silicide nanoclusters on the silicon surface was shown to occur at thickness d < 1.2 nm in non-magnetic phase.

Novel multifunctional hybrid nanocomposites with silver and gold nanoparticles stabilized by original polymer matrix based on poly-1-vinyl-1,2,4-triazole were synthesized and studied using UV and IR spectroscopy, X-ray diffraction analysis and transmission electron microscopy. The obtained nanocomposites comprise silver or gold nanoparticles of spherical and elliptical shape with size 3-20 nm and 1-10 nm, respectively.

The study deals with octogen (HMX) and aluminum, which are among the most widely used components of condensed high-energy systems. The structure, thermal behavior and combustion parameters were determined for the octogen-based monofuels and octogen-aluminum binary systems at different dispersity of components. Irrespective of differences in thermal behavior, monofuels obtained with standard and ultrafine octogen powders were shown to have virtually identical combustion parameters. In the binary systems, replacement of standard microsized aluminum by ultrafine one increases the combustion rate by a factor of 2.5, and completeness of the metal reaction (oxidation) by a factor of 4.

The effect of crystallization conditions on the relaxation properties of ultra-thin surface layers in the melt-grown ultra-high molecular weight polyethylene samples was studied using a novel Nanoluminograph device. The device can record thermoluminescence generated upon heating the sample preactivated by high-frequency low-temperature low-power glow-discharge plasma. The glow curves were analyzed, and activation energy of thermoluminescence for the observed glow maxima was calculated. The effect of crystallization conditions on the formation of a lamellar structure on the surface of ultra-high molecular weight polyethylene was examined. The possibility of structural characterization of disordered interlamellar regions using thermoluminescence data is discussed. The estimated activation energy of thermoluminescence was used to calculate the apparent dimensions of kinetic units of motion in the region of β transition, which are supposed to characterize the cooperativeness in the motion of molecular segments.

Experimental conditions of synchronous thermal analysis (STA) for gaining the reliable values of kinetic parameters and constructing a thermokinetic model of octogen decomposition were determined. A decrease in the samples' heating rate in the range of 20-0.5 K/min was shown to diminish the values of activation energy (E_a) and preexponential factor (A). E_a measured at low heating rates (2-0.5 K/min) is close to the energy of N-NO₂ bond cleavage, which is postulated as the first stage of the decomposition. Results of thermokinetic modeling of the octogen thermal decomposition are presented.

A new approach to synthesis of the ordered germanium nanorod arrays using thermal sputtering on the matrices of porous alumina with the ordered channel arrangement is presented. The synthesized filamentary nanostructures were examined by scanning electron microscopy (SEM), EXAFS and XANES spectroscopy. Data on nanorod geometry in arrays, parameters of the local atomic structure such as interatomic distances and coordination numbers for initial samples and those annealed at 450°C in the argon atmosphere, and data on changes in the electronic states near the absorption K-edge were acquired. A comparison was made with the data of EXAFS studies of a continuous Ge film synthesized on a smooth surface of non-porous Al₂O₃.

A microcapsulated material containing dibenzo-18-crown-6 (DB18C6) or 18-crown-6 (18-C6) as an extractant was synthesized. IR spectroscopy was used to investigate the state of encapsulated crown ether and the mechanism of Sc³⁺ extraction from a sulfate solution, which proceeds via the formation of a "host-guest" complex with inclusion of a metal ion into the macrocycle hole. The electron microscopy examination revealed the effect of solvent addition during the synthesis and the crown ether composition on the morphology of synthesized samples.

Electron and chemical ionization mass spectrometry and MALDI were used to investigate a medicinal preparation, calcium gluconate (CG), and its more active mechanoactivated form (MACG). Admixtures in CG were identified. Mechanoactivation was shown to produce no new compounds. The enhancement of medicinal activity of MACG may be related with its conformational transition.

The phase composition, surface morphology and crystalline structure of carbon-containing silicon layers grown on silicon plates of various orientation by vacuum gas phase epitaxy using different operating conditions are considered. The possibility of phase transition from a Si_1-xC_x solid solution to silicon carbide upon annealing of the structures obtained by low-temperature epitaxy is discussed. The films were examined by the electron, probe and interference optical microscopy, electron diffractometry, and X-ray diffraction methods. The effect of germanium intercalated in a film during its growth on the surface morphology and crystalline structure of carbon-containing silicon layers is examined. Irrespective of the method of germanium insertion in the growing film, a maximum Ge concentration is attained at the interface of silicon and 3C-SiC layers. A comparative study of the surface roughness of 3C-SiC films grown on Si(100) was performed upon variation of the temperature and germanium content in a mixture of gases. The interference optical microscopy was used to investigate the surface morphology of heteroepitaxial structures 3С-SiC/Si in comparison with the surface characteristics of buffer structures based on Si and Ge. The 3C-SiC layers grown on Si(100) and Si(110) were shown to have quite a low surface roughness, which is comparable with the characteristics of Si_1-хGe_х/Si(100) layers and СР(Ge-Si_1-хGe_х)/Si(100) superlattices at the initial roughness of Si underlayers ~1-2 nm.

Principles of the stoichiography and differential dissolution method (DD) providing deep insight into the composition of mixtures containing unknown chemical compounds are considered. Compounds can be identified using their primary feature - the stoichiometry of elemental composition; reference samples of the compounds are not needed. The DD method makes it possible to analyze mixtures of crystalline and/or amorphous phases with constant and/or variable composition in the form of dispersed powders, ceramics, crystals, thin films and nanosize objects. Various aspects of the stoichiography and DD method application to investigation of composition, structure and properties of the functional materials are discussed.

The possibilities of gas chromatography for determination of physicochemical properties of the surface are demonstrated. Changes in the surface properties of poly(1-trimethylsilyl-1-propyne) (PTMSP) with time were investigated. It was found that with the lapse of time the sorption-desorption processes proceed mainly in mesopores due to decreasing the polymer micropore volume. The formation of a chemically more uniform surface strongly increases the symmetry of chromatographic peaks and the column efficiency.

A method for preparation of divinylbenzene-styrene copolymer porous layer on the inner wall of a fused silica capillary was developed. Capillary gas chromatography was used to investigate the sorption capacity of copolymer film toward substances of various classes.

Solid state voltammetry was used for phase analysis of the natural oxide films on A^IIIB^V semiconductors. Such films, both anodic and thermal, were shown to be inhomogeneous mixtures of various phases and their structural modifications. Unstable state of the natural oxide layers on A^IIIB^V semiconductors was caused by their surface oxidation leading to the formation of nonequilibrium set of phases, their amorphous and unstable crystalline forms. A technique for express identification of superconducting phases and their characteristics was developed. All electrochemical characteristics of the detected signal indicate the presence of oxygen in superconducting phases, which is able to accept electrons and thus provide electroneutrality of the molecule.

An optimized and unified technique for CHN analysis of functional materials was suggested to enhance the accuracy of results due to increasing the completeness of sample combustion and improving the resolution of C and N peaks. The obtained contents of carbon, nitrogen and hydrogen agree well with the calculated values and are supported in most cases by structural studies.

Specific features of elemental composition analysis of silicon carbonitride thin films by energy dispersive spectroscopy (EDS) are considered. The films were preliminarily examined by IR spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron (SEM) and atomic force microscopy (AFM), and X-ray diffraction analysis using synchrotron radiation (SR-XRD) to acquire data on their chemical and phase composition, crystalline structure and surface morphology. The effect of film thickness, substrate material and electron beam energy on the results of energy dispersive analysis was investigated.

A set of specially selected classic and automated elemental analysis techniques was used to determine with a required accuracy the composition (C, H, N, Cl, Br, F, Ti, Fe) of transition metal complexes employed in post-metallocene catalytic systems. The element determination was in error by ±0.3-0.5 abs.%. Molecular weights of the synthesized oligomeric ligands were measured within the accuracy of 0.2-2.6 rel.% using the vapor-phase osmometry.

A unified technique was developed for chemical atomic-emission analysis of high-purity germanium(IV), molybdenum(VI), tellurium(IV) and tungsten(VI) oxides based on preliminary concentration of non-volatile impurities by distilling off the matrices after chemical transformations into volatile fluorides by xenon difluoride vapor in an autoclave. The impurity concentrate was analyzed by atomic-emission method with arc and induction discharges. The detection limits of impurities in a gram analytical sample were found to be 10^-6-10^-8 wt.%.

A technique has been developed for analysis of impurities in precursors of the optic ceramics: aluminum isopropoxide, yttrium and neodium oxide nanopowders (weight fraction of Nd ≤5%) by atomic-emission spectroscopy with inductively coupled plasma. In the case of aluminum isopropoxide, the effect of matrix is compensated by applying Bi as an internal standard. The detection limits of impurities were found to be10^-5-10^-6 wt.%.

A method for estimation of the phase composition of electrolytic deposits using voltammetric curves of their electrooxidation was developed. A formula for calculating the potential displacement in the case of selective electrooxidation of electronegative component of an alloy was suggested. If variation in the molar fraction of the alloy component does not affect the potentials of additional anodic peaks, this results in the formation of electrolytic deposit with intermetallic compounds on the electrode. If potential of additional anodic peak changes with variation in the molar fraction of the alloy component, a deposit forming on the electrode is represented by a solid solution. Displacement of the anodic peak potential of electronegative alloy component toward "solid solution" can be used to calculate a molar fraction of the alloy component, i.e., to solve the inverse problem.