Synthesis,morphology, and activity of La<Subscript>1–x</Subscript>Ag<Subscript>x</Subscript>MnO<Subscript>3 ± y</Subscript> catalysts

La_1–xAg_xMnO_{3 ± y} (x = 0-0.3) mixed oxides have been synthesized by the pyrolysis of polymer–salt compositions using different organic compounds and different salt: organic compound ratios. The correlation between the reaction medium temperature during pyrolysis, the composition of the resulting oxide, and synthesis conditions has been investigated. The effect of these conditions on the character of the pyrolysis process, on the phase composition and microstructure of the resulting oxide particles and metallic silver, and on their mutual distribution is reported. The catalytic properties of the synthesized oxides in methane and soot oxidation are considered, and a correlation is established between the catalytic activity of the oxides and the synthesis conditions.     

A theoretical investigation on the structures and stabilities of C<Subscript>60</Subscript>X<Subscript>18</Subscript> and C<Subscript>70</Subscript>X<Subscript>10</Subscript> (X = H,F, Cl,and Br)

A density functional theory study was performed on fullerene derivatives C₆₀X₁₈ and C₇₀X₁₀ (X = H, F, Cl, and Br). The calculated results show that the lowest energy isomers are IPR-satisfying for C₆₀X₁₈ (X = H, F, Cl, and Br). It is found that the addition patterns of X (X = Cl and Br) are different from those of X (X = H and F) for C₆₀, demonstrating that the stability of fullerene derivatives is partly attributed to the steric repulsion and electronegativity of added atoms. However, the lowest energy isomers are IPR-violating for C₇₀X₁₀ (X = H, F, and Cl), suggesting that many more fullerene derivatives may violate the isolated pentagon rule.     

Molecular geometries,electronic properties,and vibrational spectroscopic studies of endohedral metallofullerenes TM@C<Subscript>24</Subscript> and TM@C<Subscript>24</Subscript>H<Subscript>12</Subscript> (TM = Cr,Mo, and W)

Molecular geometries, electronic properties, and vibrational spectroscopies of TM@C₂₄ and TM@C₂₄H₁₂ (TM = Cr, Mo, and W) in their different spin configurations have been systematically investigated with the hybrid DFT-(U)B3PW91 functional. The results show that the TM atoms bind over the pentagon ring inside C₂₄ cage, and they move gradually toward the center of C₂₄ cage along with the increasing atomic radii. The most stable Mo@C₂₄H₁₂ and W@C₂₄H₁₂ are in their spin-triplet states. The analyses of dissociation energy and energy gap reveal that TM@C₂₄ (TM = Cr, Mo, and W) and Cr@C₂₄H₁₂ are not only thermodynamically stable, but also considerably stable kinetically. Meanwhile, natural population analyses tell us that the two cages act as electron acceptors, and the transferred charge from the W atom to C₂₄ cage is the largest in the endohedral metallofullerenes. Additionally, the vibrational frequencies and active infrared intensities may be used as evidence to characterize these unknown species.     

Study of optical,dimensional, and catalytic properties of nanodispersed CdS–Na<Subscript>2</Subscript>SiO<Subscript>3</Subscript> powders

Composite powders of the CdS–Na₂SiO₃(SiO₂) type were synthesized. It was found that varying the amount of Na₂SiO₃ introduced in the synthesis stage affects both the texture characteristics of the powders obtained and their optical and photocatalytic properties. All the powders absorb electromagnetic radiation in the visible spectral range (~420–620 nm). It was shown for the example of photodestruction of Nile Blue and Rhodamine C dyes that the powders exhibit good photocatalytic properties under exposure to visible light (λ ≥ 410 nm). One of important advantages of the composites used as photocatalysts is that the transfer of cadmium ions under photoirradiation, characteristic of CdS particles, does not occur.     

Liquid-phase synthesis and physicochemical properties of xerogels,nanopowders and thin films of the CeO<Subscript>2</Subscript>–Y<Subscript>2</Subscript>O<Subscript>3</Subscript> system

Low-agglomerated xerogels, ultrafine oxide powders with particle sizes of 12–20 nm, and uniform thin films with particle sizes of 8–14 nm are prepared in the CeO₂–Y₂O₃ system using liquid-phase low-temperature methods, namely via coprecipitation of hydroxides and cocrystallization of salts, sol—gel technology. A comparative characterization of the prepared xerogels and nanopowders is performed using a set of physicochemical analytical methods. A dependence of phase composition, microstructure, and particle size on synthetic parameters is elucidated.     

Phase equilibria and the thermodynamic properties of saturated solid solutions of BiTeI,Bi<Subscript>2</Subscript>TeI,and Bi<Subscript>4</Subscript>TeI<Subscript>1.25</Subscript> compounds of the AgI–Bi–Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>–BiTeI system

The phase equilibria of the Ag–Bi–Te–I system in the part AgI–Bi–Bi₂Te₃–BiTeI is studied in the interval of 500–540 K by means of physicochemical analysis. Thermodynamic properties of phases are determined via EMF. Potential-forming processes occur in electrochemical cells (ECCs) of the C|Ag|glass Ag₃GeS₃I|D|C structure (where C denotes inert (graphite) electrodes; Ag, D denotes ECC electrodes; D denotes four-phase alloys of the AgI–Bi–Bi₂Te₃–BiTeI system; and Ag₃GeS₃I glass is the selective Ag⁺ conducting membrane). Linear dependences of the EMFs of cells Е(Т) in the interval of 505–535 K are used to calculate the values of the thermodynamic functions of BiTeI, Bi₂TeI, and Bi₄TeI_1.25 phases saturated over silver.     

The character of interactions of CO,H<Subscript>2</Subscript>, and CH<Subscript>3</Subscript>OH with the surface of γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Cu-Zn oxide catalyst

The thermal desorption of CO, H₂, and CH₃OH from the surface of Katalco-58 industrial catalyst for the synthesis of methanol and γ-Al₂O₃ was studied. Weak interaction of the gases with the surface of samples was observed over the temperature range 75–400°C. The desorption of the gases obeyed the second-order Wigner-Polyani equation. The desorption energies of the gases were calculated. The mechanism of dimethyl ether synthesis was studied.     

Sol–Gel Synthesis of Na<Subscript>2</Subscript>O-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript> Glasses and their Characterization by NMR,SNMS, and AFM

Sodium aluminophosphate samples with composition 43.8Na₂O12.5Al₂O₃43.8P₂O₅ were prepared by the sol–gel route using different precursors and working in different pH ranges from pH < 1 up to pH > 10. The structures of the gels and of the corresponding glasses were investigated by solid state NMR and compared to that of a glass with the same composition prepared by a traditional melting process. In addition to bulk materials, thin films were deposited by dip coating on silica glasses. Applying secondary neutral mass spectrometry (SNMS), the expected elements and residual carbon were identified. The surfaces of the coatings and fracture surfaces of bulk material were investigated using atomic force microscopy (AFM). Solid state NMR revealed that samples prepared via a lactate route exhibited local Al and P environments closest to that of the melt-prepared glass, with the highest extent of Al-O-P connectivity.     

Thermal and BF<Subscript>3</Subscript>⋅Et<Subscript>2</Subscript>O-catalyzed cleavage of 1-hydrohexafluoroisobutenyloxytrimethylsilane. Perfluoromethacrolein,its isomers,dimers, and homopolymers

During vacuum pyrolysis in a quartz or steel tube, 1-hydrohexafluoroisobutenyloxytrimethylsilane (1) eliminates fluorotrimethylsilane to give perfluoromethacrolein, which was detected by NMR. Similar pyrolysis of silane 1 over KF results in E- and Z--hydrotetrafluoromethacryloyl fluorides. At 20 °C, perfluoromethacrolein undergoes homopolymerisation and/or cyclodimerization to yield 4,4-difluoro-2-pentafluoroisopropenyl-5-trifluoromethyl-4H-1,3-dioxin, which rearranges into E-7,7,7-trifluoro-2,6-bis(trifluoromethyl)-4-oxahepta-2,5-dienoyl fluoride under the action of BF₃Et₂O. The same fluoride is also formed, together with fluorotrimethylsilane, directly from silane 1 under the action of BF₃Et₂O.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1629–1635, August, 2004.Deceased     

Preparation specifics and properties of AMn<Subscript>3</Subscript>V<Subscript>4</Subscript>O<Subscript>12</Subscript> (А = Ca,Ce, and Sm) high-pressure phases

Perovskite-like phases AMn₃V₄O₁₂ (A = Ca, Ce, and Sm) were prepared under borothermic conditions (p = 7.0–9.0 GPa, T = 700–1100°C). Their X-ray diffraction structure (space group Im \(\bar 3\), Z = 2) was determined, and unit cell parameters were calculated: for CaMn₃V₄O₁₂: а = 7.40824(3) Å, for SmMn₃V₄O₁₂: а = 7.45280(8) Å, and for CeMn₃V₄O₁₂: а = 7.46965(4) Å. The temperature-dependent electrical resistivity (10–300 K) and magnetic susceptibility (2–300 K) of the prepared phases were studied.     

The effect of Me<Subscript>4</Subscript>NBr,Et<Subscript>4</Subscript>NBr,Bu<Subscript>4</Subscript>NBr,and (EtOH)<Subscript>3</Subscript>EtNBr on the Temperature of Maximum Density of Water

The densities of aqueous solutions of Me₄NBr, Et₄NBr, Bu₄NBr, and Et(OH)₃EtNBr were measured in the concentration range 0.002 to 0.05 mol⋅kg⁻¹. The temperature of the determinations ranged from 275.15 to 279.15 K in 0.5 K steps, and the uncertainty of the densities was around ±1×10⁻⁶ g⋅cm⁻³. Eleven concentrations were used for each of the salts. It was found that all the solutes follow Despretz’ law. The absolute value of the Despretz’s constants increases with increasing number of carbon atoms in the cation, except for Et(OH)₃EtNBr which has the highest value. The ionic contributions to the Despretz’s constants were calculated. The volumetric data obtained allows the calculation proposed by Kalgud and Pokale. The effective ionic radii were calculated using a semi-empirical equation, as proposed previously by several workers. The nonlinearity of the plot of the ionic Despretz constants versus effective ionic radius is confirmed.     

Structure of bismuth oxoborate Bi<Subscript>4</Subscript>B<Subscript>2</Subscript>O<Subscript>9</Subscript> at 20, 200, and 450°C

Single crystals of bismuth oxoborate Bi₄B₂O₉ have been grown by slowly cooling the melt of a stoichiometric Bi₂O₃ + H₃BO₃ mixture. The structure of the borate (monoclinic space group P2₁/c, a = 11.107 Å, b = 6.629 Å, c = 11.044 Å, β = 91.04°, Z = 4) has been studied at 20, 200, and 450°C. The structure is described not only in terms of full BiO₆ ^? and BiO₇ polyhedra but also in terms of truncated BiO₃ ^? and BiO₄ ^? polyhedra and BO₃ triangles, as well as oxo-centered OBi₃ triangles and OBi₄ tetrahedra. It is shown that both the B-O and Bi-O bond lengths are practically unaffected by temperature. Only the angles between polyhedra change with temperature, being responsible for the strong anisotropy of Bi₄B₂O₆ thermal expansion, which was measured by high-temperature powder X-ray diffraction: α₁₁ = 20, α₂₂ = 15, α₃₃ = 6 × 10^?6 °C^?1, and μ = (c, α₃₃) = ?19°.     

Phase equilibria,water dissolution,and peculiarities of charge transfer in Ca-doped La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7–α</Subscript>

A series of oxides La_{2 - x} Ca _x Zr₂O_7–α (x = 0.00, 0.05, 0.10, 0.15, 0.20) is synthesized. It is found that in samples with the calcium content x = 0.15, 0.20, the second phase Ca_0.9La_0.2Zr_0.9O₃ is present in the fraction increasing with the increase in x. The solubility limit of calcium to form solid solutions based on La₂Zr₂O₇ corresponds to x = 0.1. By high-temperature gravimetry, the proton concentration in La_1.95Са_0.05Zr₂O_7–α is obtained as a function of temperature in the interval of 300–950°С in Н₂О–О₂ atmosphere. According to temperature programmed desorption studies, in the temperature range of 400–900°С at least two types of OH defects with different binding energies are present in the oxide lattice. The temperature dependences of conductivity are obtained for La_1.95Са_0.05Zr₂O_7–α in dry and humid air atmosphere in the temperature range of 350–800°С by the method of impedance spectroscopy. The electrolyte conductivity in humid air is shown to substantially exceed the corresponding values in dry air, which can be associated with manifestation of protonic conductivity in humid atmosphere. The dependences of oxide conductivity on the oxygen content in the gas phase are determined. The conductivity is divided into its ionic and hole components.     

Structure,microstructure and physicochemical properties of BaW<Subscript>1−x</Subscript>Nb<Subscript>x</Subscript>O<Subscript>4−δ</Subscript> materials

Nb-doped BaWO₄ with the assumed formula BaW_1?xNb_xO_4?δ (x = 0, 0.005, 0.01, 0.02 and 0.05) were prepared by solid-state reaction method. Crystal structure and phase composition were determined by X-ray diffraction method. Scanning electron microscopy (SEM) coupled with energy-dispersive spectrometry (EDS) was used to describe microstructure and chemical composition of synthesised materials. It was found that solubility limit of niobium in the BaWO₄ structure is the range 0.5–1 mol%, as formation of second phase—Ba₅Nb₄O₁₅—was observed for samples with higher dopant content. For evaluation of the chemical stability of synthesized materials, the comparative CO₂/H₂O exposure test was performed. Samples were exposed to carbon dioxide- and water vapour-rich atmosphere (7% CO₂ in air, 100% RH) at 298 K for 700 h. During this exposition, the chemical reactions between the samples and the surrounding gaseous atmosphere resulting in formation of barium hydroxide and/or barium carbonate can process. Thermogravimetry (TG) method was used for chemical stability evaluation. The comparison of samples before and after the CO₂/H₂O exposure test was performed. To support the interpretation of TG results, the analysis of gaseous products evolved during thermal treatment of the samples was done using mass spectrometer. The effect of dopant on the BaWO₄ chemical stability improvement was observed. In order to determine the electrical properties of obtained materials, the DC resistance measurements in synthetic air atmosphere were taken. It was shown that niobium doping and the presence of second phase—Ba₅Nb₄O₁₅—leads to an increase in the total conductivity of synthesised materials.     

Thermal,XRD, and magnetization studies on ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript> and NiAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinels,synthesized by citrate precursor method and annealed at 450 and 650 °C

Two aluminate spinel materials (ZnAl₂O₄ and NiAl₂O₄) were synthesized by the citrate precursor method. The citrate precursors consisting of coprecipitated citrates of Zn²⁺ or Ni²⁺ and aluminum were first subjected to thermal analysis (TG-DSC) for determining the optimum temperature for annealing. Two step decomposition was observed incorporating dehydration and formation of the aluminate. The second step gives an endo peak (−2937 J/g) at 356 °C in the DSC curve of the coprecipitated nickel(II) citrate–aluminum citrate gel in O₂ atmosphere. Kinetic/mechanistic analysis of the TG data has also been carried out and values of E _a, ΔS ^#, ΔG ^#, and A were approximated. On the basis of the findings, 450 °C has been chosen for annealing of the gels. Annealing has also been done at 650 °C for 1 h in muffle furnace in an attempt to obtain nanometric particles of aluminates (MAl₂O₄) {M = Ni, Zn} and to find out their magnetic properties which could render them useful for chemical sensing applications, etc. The TG-DSC curves of various powders which were obtained on annealing at the two temperatures did exhibit thermal instability when carried out in N₂ atmosphere. NiAl₂O₄ and ZnAl₂O₄ spinels (particle size 17 and 34 nm, respectively) are obtained in pure crystalline phase at 650 °C. ZnAl₂O₄ prepared this way shows coercivity values of 470 and 58.37 G and NiAl₂O₄, 107 and 23.24 G when annealed at 450 and 650 °C, respectively. ZnAl₂O₄ prepared by a polymer precursor method and annealed at 1000 °C, has earlier been reported to have coercivity value of 469 G. Thus, the citrate precursor method is good for the synthesis of ZnAl₂O₄, producing single phase nanocrystalline powder of high quality and crystallinity. The value of magnetization was found to be small in the present case for the NiAl₂O₄ spinel obtained at 450 °C.     

Balance,uniformity, and asymmetry of the structure of volborthite Cu<Subscript>3</Subscript>(OH)<Subscript>2</Subscript>(V<Subscript>2</Subscript>O<Subscript>7</Subscript>)·2H<Subscript>2</Subscript>O

Four structural models of volborthite Cu₃(OH)₂(V₂O₇)·2H₂O (a = 10.646(2) Å, b = 5.867(1) Å, c = 14.432(2) Å, β = 95.19(1)°, V = 897.7(5) ų, Z = 4, R/R _w = 0.038/0.046) calculated in the space groups determined from the systematic absences are compared. Based on the structure balance and the similarity of constituting polyhedra, values of the R factor, and isotropic thermal parameters, the space group Ia is found to be preferable, which is the only possible asymmetric and uniform variant. Hydrogen atoms of OH-groups, oxygen atoms and, partially, hydrogen atoms of water are localized.     

Tree of phases of the quinary reciprocal system Li,K || F,Br, MoO<Subscript>4</Subscript>, WO<Subscript>4</Subscript> and study of the stable tetrahedron LiF–KBr–Li<Subscript>2</Subscript>MoO<Subscript>4</Subscript>–Li<Subscript>2</Subscript>WO<Subscript>4</Subscript>

The quinary reciprocal system Li, K || F, Br, MoO₄, WO₄ was partitioned into simplexes using graph theory by writing an adjacency matrix and solving a logical expression. A tree of phases of the system was constructed. The tree of phases has a linear structure and consists of four stable partitioning tetrahedra, two stable pentatopes, and three stable hexatopes. In the quinary reciprocal system Li, K || F, Br, MoO₄, WO₄, crystallizing phases were predicted. The stable tetrahedron LiF–KBr–Li₂MoO₄–Li₂WO₄ of the quinary reciprocal system Li, K || F, Br, MoO₄, WO₄ was studied by differential thermal analysis and X-ray powder diffraction. There are no invariant equilibrium points in the tetrahedron. Continuous series of solid solutions Li₂Mo_xW_1–xO₄ do not decompose.     

Preparation,characterization and catalytic studies of V<Subscript>2</Subscript>O<Subscript>5</Subscript>–SiO<Subscript>2</Subscript> xerogel composite

In this work, we report the synthesis, characterization and catalytic properties of a vanadium oxide–silicon oxide composite xerogel prepared by a soft chemistry approach. In order to obtain such material, we submitted a vanadium pentoxide gel previously synthesized via protonation of metavanadate species to an “in situ” progressive polycondensation into silica gel. The material has been characterized by X-ray diffraction, infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy. Further, the catalytic activity of this material was evaluated for the epoxidation of styrene and cyclooctene using iodosylbenzene, hydrogen peroxide and m-chloroperbenzoic acid as the oxidizing agent.     

Study of the formation temperature of mixed LaREO<Subscript>3</Subscript> (RE ≡ Dy,Ho, Er,Tm, Yb,Lu) and NdGdO<Subscript>3</Subscript> oxides

Mixed LaREO₃ (RE ≡ Dy, Ho, Er, Tm, Yb, Lu) and NdGdO₃ oxides were prepared by thermal decomposition of the corresponding co-precipitated mixed oxalates. The decomposition of La/RE and Nd/Gd oxalates was studied by means of differential thermal analysis and thermogravimetric (DTA-TG) measurements; in particular the last step, consisting of the loss of a CO₂ molecule from the corresponding oxycarbonate, has been thoroughly investigated, as it is particularly interesting for the study of the formation temperature of mixed rare earth oxides. After the release of CO₂, the oxides crystallize in a distorted perovskitic cell or one of the structures typical of rare earth sesquioxides, depending on the cationic size difference and on the average cationic radius. The mixed rare earth oxycarbonate decomposition has been studied in comparison to the decomposition of single rare earth oxycarbonates. A trend of the mixed oxides formation temperature as a function both of the average cationic size and of the cationic sizes difference has been observed and compared to the behaviour of single rare earth oxides.