The objective of this work was to study two-step sintering as a means of controlling the microstructure of coarse Al2O3 matrix composites containing submicrometric and nanometric inclusions of ZrO2 ranging from 0-30 wt. % by weight based on commercially available powders and evaluate its hydrothermal degradation as function of a water vapour pressure and its mechanical properties. The results showed that two-step sintering allowed a more efficient microstructural control than single-step sintering, resulting in good mechanical properties. The highest flexural strength was achieved for ZTA samples sintered in two-stage sintering conditions TSS2 with T1 = 1560 degrees C for 3 h, T2 = 1460 degrees C for 8 h. The studied composites showed good resistance to hydrothermal degradation compared to composites sintered in single step sintering conditions.

The effects of the ZrO2 content and the particle size of ZrO2 powders on the microstructure, phase composition, physical and mechanical properties, and the abrasion wear resistance of advanced Al2O3 ceramics and zirconiatoughened alumina (ZTA) composites containing 0 to 30 mass% yttria-stabilised zirconia (YSZ) were investigated. The composite with a ZTA content of 30 mass% of ZrO2 exhibited the greatest resistance to abrasion wear. alpha-Al2O3 reflex broadening (hid = 113) as a result of the microstresses in the Al2O3 crystal lattice during sandblasting decreased with increasing ZrO2 amount, where the ZrO2 particles located along the grain boundaries of Al2O3, hindering their growth and deformation. The use of nanodispersed ZrO2 powder produced by the plasma chemical technique led to a 1.5-fold increase in wear resistance in the resultant ZTA ceramic.

The nature of the active sites for palladium supported on nitrogen doped carbon nanofibers (Pd/N-CNFs) catalysts for selective hydrogenation of acetylene to ethylene was examined. Palladium concentrations of 0.05-0.6 wt% were deposited on carbon nanofibers doped by nitrogen atoms (N-CNFs) and characterized by transmission electron microscopy (TEM), CO chemisorption, X-ray photoelectron spectroscopy (XPS), Extended X-ray Absorption Fine Structure (EXAFS) and quantum-chemical calculations. The data indicates that the Pd initially forms highly-dispersed particles. However, decreasing of the Pd concentration on the carbon nanofibers doped by nitrogen atoms below 0.15 wt% led to stabilization of the metal in the atomic state. Porphyrin-like defects with four nitrogen atoms are formed on the surface of the CNFs and strongly interact with palladium atoms, and thus can be sites for stabilization of atomic Pd. The catalytic activity and selectivity of Pd/N-CNFs catalysts depend on the ratio of these two palladium states.

The paper considers the system of nitrogen-doped carbon nanofibers (N-CNFs) with palladium atoms deposited on their surfaces. The concentration of deposited palladium varied in the interval of 0.05-0.6 wt.%. The state of palladium was studied with the methods of quantum chemistry, electron microscopy, CO adsorption, and EXAFS. Carbon structures that contain porphyrin-like defects with four nitrogen atoms in the graphene layer interact strongly with palladium atoms and therefore can be stabilization centers of atomic palladium.

The application of nitrous oxide as an alternative oxidant provides new opportunities for selective oxidation of olefins. Here, we studied for the first time the thermal oxidation of isobutene with N2O in the liquid phase. The study revealed that the oxidation proceeds via 1,3-dipolar cycloaddition of N2O to the CC bond by two routes forming unstable 4,5-dihydro-[1,2,3]-oxadiazole intermediates. The main route (the contribution of 91%) includes the addition of the N2O oxygen to the second carbon atom in olefin. In this case, the oxadiazole decomposes with the CC bond cleavage yielding acetone, methylene (:CH2), and N2. The methylene then readily reacts with isobutene and benzene (solvent). The minor route involves the addition of the N2O oxygen to the first carbon atom and the oxadiazole decomposition with a hydrogen shift leading to isobutanal and N2.

Reactions of anion-radical a-oxygen (Fe-III-O center dot-)(alpha) with propylene and 1-butene on sodium-modified FeZSM-5 zeolites were studied in the temperature range from -60 to 25 degrees C. Products were extracted from the zeolite surface and identified. It was found that main reaction pathway was the epoxides formation. Selectivity for epoxides at -60 degrees C was 59-64%. Other products were formed as a result of secondary transformations of epoxides on the zeolite surface. According to IR spectroscopy, the oxidation of propylene over the entire temperature range and 1-butene at -60 degrees C were not accompanied by the formation of (Fe-III-OH)(alpha) groups, in distinction to methane oxidation. This testifies that hydrogen abstraction does not occur. In case of 1-butene reaction with alpha-oxygen at 25 degrees C, hydrogen abstraction occurred but was insignificant, ca 7%. According to DFT calculation ferraoxetane intermediate formation is preferable over hydrogen abstraction. Following decomposition of this intermediate leads to the propylene oxide (PO) formation. The results may be relevant to the low selectivity problem of the silver catalyst in propylene epoxidation and raise doubts about the presently accepted mechanism explaining an adverse effect of allylic hydrogen. (c) 2017 Elsevier B.V. All rights reserved.

The structure of a cobalt-boron catalyst obtained by reduction of cobalt chloride in an aqueous solution of sodium borohydride at ambient temperature has been investigated by TEM HR, XPS, EXAFS and Xray diffraction method. It was found that nanoparticles of the forming catalyst consisted of a core and a shell. The oxygen-containing compounds (cobalt hydroxide and cobalt borates) mainly reside in the shell. In spite of the amorphous state of the catalyst the EXAFS and X-ray diffraction analysis indicated the presence of ferromagnetic clusters with the shortest Co-Co distance of about 2.5 angstrom and coordination number of 2.7 which are regularly distributed within the core with average separations of 5.8 angstrom. The presence in the catalyst of the Co-B bonds, a short Co-Co distance and the absence of long Co-Co distances (4.3 and 4.8 angstrom) as well as the presence of boron bound to hydrogen allows suggestion to be made that stabilization of the cobalt tetramers occurs due to the presence of a boron-hydrogen structure formed by BH4- ions upon the reduction of cobalt chloride. To substantiate the above model of the active phase of the cobalt-boron catalyst the OFT method with a PBE functional in the plane-wave basis has been employed which showed that the ferromagnetic tetramers may exist within a boron-hydrogen matrix isolated from the surrounding medium by an oxygen-containing shell. (C) 2017 Elsevier B.V. All rights reserved.

The publication considers the modifying effect of refractory zirconium carbide in determining elements by hydride generation atomic absorption spectrometry. It is shown that the nature of binding of atoms of the analytes consists in the ability of the ZrC surface to strong adsorption. For As, Cd, Pb, Sb, Se, Te elements the adsorption energies of atoms on the ZrC(100) surface are calculated by the density functional theory. It is found that the necessary condition for adsorption is the preliminary cleaning of the ZrC surface from chemisorbed hydrogen. Correlation is demonstrated between the pyrolysis temperature that is maximum achievable for the analyte in the specimen with the adsorption energy of its atomic form on the ZrC(100) surface.

Using infrared (IR) spectroscopy and density functional theory (DFT) calculations, interaction of CO with the strongest known pure Brønsted carborane superacids, H(CHB11Hal11) (Hal = F, Cl), was studied. CO readily interacted at room temperature with H(CHB11F11) acid, forming a mixture of bulk salts of formyl and isoformyl cations, which were in equilibrium An(-)H(+)CO COH(+)An(-). The bonding of CO to the surface Brønsted centers of the weaker acid, H(CHB11Cl11), resulted in breaking of the bridged H-bonds of the acid polymers without proton transfer (PT) to CO. The binding occurred via the C atom (blue shift ΔνCO up to +155-167 cm(-1), without PT) or via O atom (red shift ΔνCO up to -110 cm(-1), without PT) always simultaneously, regardless of whether H(+) is transferred to CO. IR spectra of all species were interpreted by B3LYP/cc-pVQZ calculations of the simple models, which adequately mimic the ability of carborane acids to form LH(+)CO, LH(+)CO, COH(+)L, and COH(+)L compounds (L = bases). The CO bond in all compounds was triple. Acidic strength of the Brønsted centers of commonly used acid catalysts, even so-called superacidic catalysts, is not sufficient for the formation of the compounds studied.

The possibility of co-crystallization of polyfluoroaromatic meta-diamines with 18-crown-6 in the 2:1 ratio was tested using a large group of substrates with various frameworks (benzene, naphthalene, and pyridine) and substituents (H, Cl, CF3). Co-crystals of 2:1 stoichiometry were obtained from hexafluoro2,7-naphthylenediamine, 4-trifluoromethyltrifluoro- and 2-trifluoromethy1-4,6-difluro-1,3-phenyle nediamines, 4-chlorodifluoro-2,6-diaminopyridine. According to X-ray crystallographic data, associates of CF3-containing polyfluorophenylenediamines are 2D assemblies connected via synthon C-ar-N-H ...O-cr.Polyfluorinated 2,7-diaminonaphthalene and 2,6-diamino-4-chloropyridine form 1D assemblies, in which the pi-stacking of the naphthylenediamine molecules and the hydrogen bond C-ar-N-H ... N-py, respectively, are the additional structure-supporting interactions. The co-crystallization of 18-crown-6 with less electron withdrawing arylenediamines gives only associates of stoichiometry 1:1 irrespective of the components ratio and the solvent nature. To rationalize the different co-crystallization behavior of the arylenediamines, the co-crystal structures were studied using a special version of the DFT, which accounts for the van der Waals interactions. The calculated difference between the specific sublimation enthalpies for the 1:1 and 2:1 associates was suggested to be a measure of the preferable co crystallization ratio: the both types of associates are formed when vertical bar Delta Delta H-subl vertical bar is smaller than 15 J g(-1), a higher value indicates a significant energy preference for one of the structures. The experimental melting enthalpy values (DSC data) for the 1:1 and 2:1 co-crystals based on the same diamine are quite similar. (C) 2016 Elsevier B.V. All rights reserved.

Liquid phase oxidation of olefins with nitrous oxide is a promising synthetic route to ketones. The effect of cis/trans isomerism on the reactivity of olefins towards N2O and on the reaction mechanism was studied for the first time using 3-heptene oxidation as an example. Our experimental study revealed that cis- and trans-isomers of 3-heptene have similar reactivity and yield the same set of products. However, the cis/trans isomerism of the olefin has a pronounced effect on the reaction route involving the cleavage of the initial CC bond and, accordingly, on the products ratio. The yield of ketones is lower for the trans-isomer due to higher contribution of the cleavage route.

Different radical forms of oxygen (O−, O 2 − and O 3 − ) on the surface of nanocrystalline MgO are well known. It was earlier demonstrated that EPR-silent species with properties very similar to those of O− radicals exist on the surface of magnesium oxide in addition to the O− radicals observed by EPR. In this study we characterized the reactivity of these two types of O− radicals in reaction with ethylene. It was demonstrated that this reaction yields different products for observable and unobservable O− radicals. Conventional \({\text{O}}_{{3{\text{C}}}}^{ - }\) radicals generated by MgO illumination with UV light at room temperature in the presence of oxygen initiate hydrogen atom abstraction from ethylene to form secondary radicals H2C=C− with hyperfine splitting A 1 = 59 G, A 2 = 6 G. The \({\text{O}}_{{4{\text{C}}}}^{ - }\) radicals not observed directly by EPR were synthesized by MgO illumination in the presence of oxygen at 163 K followed by evacuation at 203 K. They were shown to react with ethylene to form an addition product with two groups of two equivalent protons with isotropic hyperfine constants A 1 = 38 G and A 2 = 23 G. Such radicals were obtained for the first time by reaction of O− radicals with ethylene on the MgO surface. Their concentration was approximately equal to the concentration of [\({\text{O}}_{{4{\text{C}}}}^{ - }\)·O2] complexes observed by EPR before the oxygen desorption. The structures of both radicals were simulated by DFT, and a good match between the experimental and computational results was obtained.

The energies of chemisorption of As, Se, Pb, and Cd atoms by the metal surface of iron are calculated by the quantum chemistry method. The possibility of analytical application of an iron-containing chemical modifier (CM) on the basis of activated carbon for ETAAS determination of volatile elements is suggested. By the methods of thermodynamic modeling, thermal analysis, and ETAAS, the preparation procedure of iron-containing CM is justified on the basis of activated carbon, which ensures the formation of the metallic phase Fe-0 and the course of low-temperature thermal stabilization of analytes. Using the developed chemical modifier, the contents of As, Cd, and Pb are determined in the suspensions of the standard sample of seaweeds. The results are in satisfactory agreement with the certified values.

Inhomogeneous surface charging could lead to a distortion of X-ray photoelectron (XP) spectra, which complicates the spectra analysis and sometimes results in an incorrect interpretation of elements chemical states of the sample. The charging effects might be especially strong in the case of XPS application for the characterization of heterogeneous catalysts, which are usually based on the dielectric or semiconductor materials with complex morphology. In this paper, we propose an algorithm to restore XP spectra when distortion is caused by inhomogeneous and/or non-constant surface charging effects. A photoelectron line of a reference element can be used to eliminate the distortions from experimental spectra of other elements by an iterative deconvolution procedure. The successful application of the algorithm for the restoration of a Pd3d line shape using a reference Sn3d(5/2) line was demonstrated for the Pd/SnO2 and Pd/CeO2-SnO2 catalysts. (C) 2015 Elsevier B.V. All rights reserved.

A recently discovered surface reaction of ethylene oxide (EO) with ethylene yielding butyraldehyde (BA) was studied on ZSM-5 zeolites with different contents of Al, Na, and Fe in the temperature range of −25 to 100 °C. Products formed on a zeolite as a result of EO and ethylene transformations were extracted from the surface and identified using gas chromatography, gas chromatography–mass spectrometry, and nuclear magnetic resonance. In addition to BA, the other major reaction products are acetaldehyde and dioxane. Product distribution strongly depends on the composition of the zeolite samples and experimental conditions. Varying of the Al content in the zeolites as well as their temperature pretreatment showed that a key role in the formation of BA play dehydroxylated Al-contained sites. Possible transformations of EO and ethylene on the zeolite were analyzed by quantum-chemical calculations.

The energies of the chemisorption of As, Se, Pb, and Cd atoms by palladium and cobalt metal surfaces were calculated by a quantum chemistry method. The analytical applicability of a cobalt-containing chemical modifier based on activated carbon to the determination of the highly volatile elements by electrothermal atomic absorption spectrometry (ETAAS) was hypothesized. A procedure for the preparation of the cobalt-containing chemical modifier based on activated carbon was substantiated using thermodynamic simulation, thermal analysis, and ETAAS. This procedure ensured the formation of a Co0 metal phase and the occurrence of thermal stabilizing interactions with analytes at the stage of drying. The concentrations of arsenic and lead in the suspensions of a standard reference sample of marine algae were determined with the use of the developed chemical modifier based on activated carbon. The experimental results were consistent with the certified values.

Методом квантовой химии рассчитаны энергии хемосорбции атомов As, Se, Pb, Cd металлическими поверхностями палладия и кобальта. Сделано предположение о возможности аналитического применения кобальтсодержащего химического модификатора (ХМ) на основе активного угля для определения легколетучих элементов методом электротермической атомно-абсорбционной спектрометрии (ЭТААС). Методом термодинамического моделирования, термического анализа и ЭТААС обоснована методика подготовки кобальтсодержащего ХМ на основе активного угля, которая обеспечивает формирование металлической фазы Co0 и протекание термостабилизирующих взаимодействий с аналитами на стадии высушивания. С применением разработанного химического модификатора на основе активного угля определено содержание мышьяка и свинца в суспензиях стандартного образца морских водорослей. Полученные результаты удовлетворительно совпадают с аттестованными значениями.

Hydrogen abstraction reactions of the ferryl [FeO]2+ moiety are investigated theoretically using a simple molecular model (OH)2FeO by DFT methods. Obtained thermochemical and structural data are compared with high-level electron-correlated methods: CCSD(T), BD and MCQDPT2. Two bonding patterns are observed: triple oxo-iron bond Fe+≡O+ and single FeO bond with radical oxygen. Predicted thermochemistry and reactivity with H2 and CH4 molecules for the later one fits well with available experimental data on the N2O+FeZSM5 catalytic system.