Crystal structure and properties of the Na1−x Ru2O4 phase

Sodium ruthenium(III,IV) oxide Na_1−x Ru₂O₄ was synthesized by the solid state reaction of Na₂CO₃ and RuO₂ in inert atmosphere and characterized by X-ray powder diffraction, electron diffraction, and high-resolution transmission electron microscopy. The compound crystallizes in the CaFe₂O₄-type structure (space group Pnma, Z = 4, a = 9.2641(7) Å, b = 2.8249(3) Å, c = 11.1496(7) Å). Double rutile-like chains of the RuO₆ octahedra form a three-dimensional framework, whose tunnels contain sodium cations. The structure contains two crystallographically independent sites of ruthenium atoms randomly occupied by the Ru^III and Ru^IV cations. The superstructure with the doubled b parameter found for one of the samples under study using electron diffraction is caused, probably, by ordering of the Ru cations in the rutile-like chains. The Na_{1− x} Ru₂O₄ compound exhibits temperature-independent paramagnetism with χ₀ = 1.9·10⁻⁴ cm³ (mole of Ru⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1655–1660, October, 2006.     

Synthesis and characterization of Y2Ti2O7 and ErxY2−xTi2O7 nanofibers

Journal of Sol-Gel Science and Technology - 1-D nanostructures are deemed to be important building blocks for future optical and optoelectronic nanodevices. We have used electrospinning methods to...     

Improving the Catalytic Performance of the Hydrogen Evolution Reaction of α-MoB2 via Rational Doping by Transition Metal Elements

Abundant transition metal borides are emerging as promising electrochemical hydrogen evolution reaction (HER) catalysts which have a potential to substitute noble metals. Those containing graphene-like (flat) boron layers, such as α-MoB2, are particularly promising and their performance can be further enhanced via doping by the second metal. In order to understand intrinsic effect of doping and rationalize selection of dopants, we employ density functional theory (DFT) calculations to study substitutional doping of α-MoB₂ by transition metals as a route towards systematic improvement of intrinsic catalytic activity towards HER. We calculated thermodynamic stability of various transition metal elements to select metals which form a stable ternary phase with α-MoB₂. We inspected surface stability of dopants and assessed catalytic activity of doped surface through hydrogen binding free energy at various hydrogen coverages. We calculated the reaction barriers and pathways for the Tafel step of HER for the most promising dopants. The results highlight iron as the best dopant, simultaneously lowering the reaction barrier of the Tafel step while having suitable thermodynamic stability within MoB₂ lattice.     

Effect of ZnO on the interfacial bonding between Na2O–B2O3–SiO2 vitrified bond and diamond

Diamond composites were prepared by sintering diamond grains with low melting Na₂O–B₂O₃–SiO₂ vitrified bonds in air. The influence of ZnO on the wettability and flowing ability of Na₂O–B₂O₃–SiO₂ vitrified bonds was characterized by wetting angle, the interfacial bonding states between diamond grains and the vitrified bonds were observed by scanning electron microscope (SEM), and the micro-scale bonding mechanism in the interfaces was investigated by means of energy-dispersive spectrometer (EDS), Fourier transform infrared (FTIR) spectrometer and X-ray photoelectron spectroscopy (XPS). The experimental results showed that ZnO facilitated the dissociation of boron/silicon–oxygen polyhedra and the formation of larger amount of non-bridging oxygen in the glass network, which resulted in the increase of the vitrified bonds' wettability and the formation of –CO, –O–H and –C–H bonds on the surface of diamond grains. B and Si diffused from the vitrified bonds to the interface, and C–C, C–O, CO and C–B bond formed on the surface of sintered diamond grains during sintering process, by which the interfacial bonding between diamond grains and the vitrified bonds was strengthened.     

Spectroscopic investigation and magnetic properties of Na2Pb1−xCuxP2O7 glasses

Phosphate glasses have several technological interests due to their specific physical properties such as high thermal expansion coefficient, high refractive indice and low melting temperature, that make them suitable for use as conductors, ionic conductors, semiconductors and biomedical materials. The phosphate glasses, in particular the pyrophosphate forms, are not widely studied. In this work we have elaborated the Na₂Pb_1−xCu_xP₂O₇ glasses, with a large range of composition (0 ≤ x ≤ 1), by conventional melting method. Thermal parameters of the glasses were determined using the differential scanning calorimetry. The structure of the glasses was investigated by IR spectroscopy. The local environment of paramagnetic ions Cu²⁺ was analyzed by EPR and magnetic measurements. It was showed that the network structure of the glasses was drastically influenced by the copper content.     

Synthesis of sol–gel Na2Zr x Ti6−x O13 (0≤x≤1) materials and their performance in photocatalytic degradation of organic dyes

In this work we present the synthesis and characterization of four compositions of the solid solution with the general formula Na₂Zr _x Ti_6−x O₁₃ (0≤x≤1) which was prepared by a sol-gel method. The main goal of this work is to evaluate the influence of the incorporation of different amounts of zirconium into the binary phase Na₂Ti₆O₁₃ on two properties: the textural surface and E _g values. This titanate phase crystallizes in a monoclinic unit cell and is built into an octahedral TiO₆ framework forming a rectangular tunnel structure. Additionally, we have compared their photocatalytic performances in degradation of organic dyes under visible light. The heat-treated sol-gel samples were characterized by X-ray powder diffraction, N₂ physisorption, thermal analysis, UV and FT-IR spectroscopy. According to the X-ray powder diffraction results, the new ternary tunnel compound Na₂ZrTi₅O₁₃ was obtained as a single phase at 800°C. The cell parameters for the two end-member phases of that solid solution were refined to confirm that Zr ion was incorporated into the structural framework. This ternary compound had an E _g value of about 2.9 eV. The activity of all heat-treated Na₂ZrTi₅O₁₃ samples was tested in the photocatalytic degradation of methylene blue and Rhodamine B under visible light. The Na₂ZrTi₅O₁₃ calcined at 400°C showed the best performance with 95% of photodegradation of methylene blue and a half time t _1/2 of 15 min.     

Controlled Growth of SrxBa1−xNb2O6 Hopper- and Cube-Shaped Nanostructures by Hydrothermal Synthesis

Controlling the shape and size of nanostructured materials has been a topic of interest in the field of material science for decades. In this work, the ferroelectric material Sr_xBa_1−xNb₂O₆ (x=0.32–0.82, SBN) was prepared by hydrothermal synthesis, and the morphology is controllably changed from cube-shaped to hollow-ended structures based on a fundamental understanding of the precursor chemistry. Synchrotron X-ray total scattering and PDF analysis was used to reveal the structure of the Nb-acid precursor, showing Lindqvist-like motifs. The changing growth mechanism, from layer-by-layer growth forming cubes to hopper-growth giving hollow-ended structures, is attributed to differences in supersaturation. Transmission electron microscopy revealed an inhomogeneous composition along the length of the hollow-ended particles, which is explained by preferential formation of the high entropy composition, SBN33, at the initial stages of particle nucleation and growth.     

Defect structures in the brannerite-type vanadates: VIII. Synthesis of the Mg1−yLiyV2−yMoyO6 and Mg1−x−yØxLiyV2−2x−yMo2x+yO6 solid solutions: Effect of strengthening the crystal lattice by substitution of monovalent ion for bivalent ion

It is shown that MgV₂O₆ and LiVMoO₆, both of the brannerite-type structure, exhibit miscibility in the entire composition range resulting in the formation of MgLi = Mg_1−yLi_yV_2−yMo_yO₆ solid solutions. For y > 0.5 significant capacity of the MgLi matrix to the excess Mo⁶⁺ cations compensated by cation vacancies Ø appears and MgLiØ = Mg_1−x−yØ_xLi_yV_2−2x−yMo_2x+yO₆ solutions become stable. Pronounced negative deviations from Vegard's law are simultaneously observed for MgLi solid solutions. An unusual phenomenon is thus observed: monovalent cation (Li⁺) substituted for bivalent cation (Mg²⁺) strengthens the brannerite-type lattice and increases its toleration to cation vacancies. A similar effect has recently been observed for ZnLi and ZnLiØ solid solutions (K. Mocała and J. Ziółkowski, J. Solid State Chem. 71, 426 (1987)); the effect is absent, however, in the case of MnLi and MnLiØ (J. Ziółkowski, K. Krupa, and K. Mocała, J. Solid State Chem. 48, 376 (1983)). Some speculations concerning this effect and some predictions are offered. X-ray data are listed for MgLi solid solutions of various compositions.     

Synergistic Effect Between Sb2O3 Nanoparticles–Trichloromelamine and Carbon Nanotube on the Flame Retardancy and Thermal Stability of the Cellulose Acetate

Sb₂O₃ nanoparticles were synthesized via a simple surfactant-free sonochemical reaction. Multi-walled carbon nanotubes (MWCNT) have been successfully functionalized with amino groups. Cellulose acetate (CA) as a polymeric matrix was choosing in this work. In order to improve the thermal stability and flame retardancy of the CA, nanoparticles, aminated-MWCNT and trichloromelamine (TCM) were added to the polymeric matrix. The nanocomposites were characterized by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy and UL-94 analysis. Flame retardancy of the nanocomposite was improved as a result of synergistic effect between Sb₂O₃ and TCM. Result show that thermal decomposition of the nanocomposites was shifted towards higher temperatures.     

Phase-field Determination of NaSICON Materials in the Quaternary System Na2O−P2O5−SiO2−ZrO2: The Series Na3Zr3–xSi2PxO11.5+x/2

Two types of solid electrolytes have reached technological relevance in the field of sodium batteries: ß/ß”-aluminas and NaSICON-type materials. Today, significant attention is paid to room-temperature stationary electricity storage technologies and all-solid-state Na batteries used in combination with these solid electrolytes are an emerging research field besides sodium-ion batteries. In comparison, NaSICON materials can be processed at lower sintering temperatures than the ß/ß”-aluminas and have a similarly attractive ionic conductivity. Since Na₂O−SiO₂−ZrO₂−P₂O₅ ceramics offer wider compositional variability, the series Na₃Zr_3–xSi₂P_xO_11.5+x/2 with seven compositions (0≤x≤3) was selected from the quasi-quaternary phase diagram in order to identify the predominant stability region of NaSICON within this series and to explore the full potential of such materials, including the original NaSICON composition of Na₃Zr₂Si₂PO_l2 as a reference. Several characterization techniques were used for the purpose of better understanding the relationships between processing and properties of the ceramics. X-ray diffraction analysis revealed that the phase region of NaSICON materials is larger than expected. Moreover, new ceramic NaSICON materials were discovered in the system crystallizing with a monoclinic NaSICON structure (space group C2/c). Impedance spectroscopy was utilized to investigate the ionic conductivity, giving clear evidence for a dependence on crystal symmetry. The monoclinic NaSICON structure showed the highest ionic conductivity with an optimum ionic conductivity of 1.22×10⁻³ at 25 °C for the composition Na₃Zr₂Si₂PO₁₂. As the degree of P⁵⁺ content increases, the total ionic conductivity is initially enhanced until x=1 and then decreases again. Simultaneously, the increasing amount of phosphorus leads a decrease in the sintering temperatures for all samples, which was confirmed by dilatometry measurements. The thermal and microstructural properties of the prepared samples are also evaluated and discussed.     

Observation of Transition-Metal–Boron Triple Bonds in IrB2O− and ReB2O−

Multiple bonds between boron and transition metals are known in many borylene (:BR) complexes via metal d_π→BR back-donation, despite the electron deficiency of boron. An electron-precise metal–boron triple bond was first observed in BiB₂O⁻ [Bi≡B−B≡O]⁻ in which both boron atoms can be viewed as sp-hybridized and the [B−BO]⁻ fragment is isoelectronic to a carbyne (CR). To search for the first electron-precise transition-metal-boron triple-bond species, we have produced IrB₂O⁻ and ReB₂O⁻ and investigated them by photoelectron spectroscopy and quantum-chemical calculations. The results allow to elucidate the structures and bonding in the two clusters. We find IrB₂O⁻ has a closed-shell bent structure (C_s, ¹A′) with BO⁻ coordinated to an Ir≡B unit, (⁻OB)Ir≡B, whereas ReB₂O⁻ is linear (C_∞v, ³Σ⁻) with an electron-precise Re≡B triple bond, [Re≡B−B≡O]⁻. The results suggest the intriguing possibility of synthesizing compounds with electron-precise M≡B triple bonds analogous to classical carbyne systems.     

The Effect of Platinum on Stability of the B2O3/TiO2-ZrO2 Catalyst for Beckmann Rearrangement of Cyclohexanone Oxime

It has been well illustrated that the rapid catalyst deactivation with time is the most serious limitation of vapor phase approach to the production of ε-caprolactam from cyclohexanone oxime (CHO) (Scheme 1), and is a common problem with all catalyst typ…     

Striking Size and Doping Effects of Ti−Si−O Clusters on Methane Conversion Reactions

The size and doping effects in methane activation by Ti−Si−O clusters have been explored by using a combination of gas-phase experiments and quantum chemical calculations. All [Ti_mSi_nO_2(m+n)]^.+ (m+n=2, 3, 8, 10, 12, 14) clusters can extract a hydrogen from methane. The associated energies and structures have been revealed in detail. Moreover, the doping and size effects have been discussed involving generalized Kohn-Sham energy decomposition analysis, natural population analysis, Wiberg bond indexes (WBI), molecular polarity index (MPI) and ionization potential (IP). It suggested that Ti−Si−O clusters with a low Ti : Si ratio is beneficial to adsorbing methane and inclination to the hydrogen atom transfer (HAT) process, while the clusters with a high Ti : Si ratio favors the generation of a terminal oxygen radical and results in high reactivity and turnover frequency. On the other hand, a cluster size of m+n=12 is recommended considering both the ionization potential and the turnover frequency of the reaction. Hopefully, these finding will be instructive for the design of high-performance Ti−Si−O catalyst toward methane conversion.     

Effect of gamma-irradiation on surface and catalytic properties of CuO−ZnO/Al2O3 system

A copper, zinc and aluminium mixed oxides sample having the nominal composition 0.25 CuO/0.03 ZnO/Al₂O₃ was prepared by impregnating Al(OH)₃ with copper and zinc nitrate solutions, drying at 100 °C then heating in air at 600 °C. The obtained solid was exposed to different doses of -rays (20–160 Mrad). The surface characteristics namelyS _BETV_P andr of different treated adsorbents were determined from N₂ adsorption isothems measured at –196 °C. The catalytic activity of various irradiated solids was determined by following up the kinetics of CO-oxidation by O₂ at 150–200 °C. The results showed that the doses up to 80 Mrad resulted in no significant change in theS _BET but increased slightly theV _P (20%) of the treated adsorbents. The irradiation at 160 Mrad caused an increase of 20% in theS _BET of the irradiated solid sample. The catalytic activity increased progressively by increasing the dose, a dose of 160 Mrad brought about an increase of 140% in the catalyst's activity. The apparent activation energy of the catalytic reaction decreased monotonically by increasing the absorbed dose of -rays which was attributed to a parallel induced decrease in the value of pre-exponential term of the Arrhenius equation. The observed increase in the catalytic activity due to -irradiation has been interpreted as a result of increasing the concentration of catalytically-active sites contributing in chemisorption and catalysis of CO-oxidation via a possible fragmentation of CuO crystallites.     

Formation of crystalline Na2V6O16·3H2O ribbons into belts and rings

Single-crystalline nanobelts and nanorings of Na(2)V(6)O(16)·3H(2)O structures have been facilely synthesized through a direct hydrothermal reaction between NaVO(3) and H(3)PO(4), without the addition of any harmful solvents or surfactants. The analytical techniques of scanning electron microscopy, transmission electron microscopy (TEM), powder X-ray diffraction, thermogravimetric analysis, energy-dispersive X-ray spectroscopy, Fourier transform infrared, high-resolution TEM, and selected-area electron diffraction have been used to characterize the morphology, composition, and structure of the synthesized products. The Na(2)V(6)O(16)·3H(2)O nanobelts are up to several hundreds of micrometers in length and 100-300 nm in thickness, and for nanorings, the diameters are 4.5-6.5 μm. H(3)PO(4) plays a key role in maintaining the pH of the solution as well as producing PO(4)(3-) ions in solution. The chemical reactions and a possible growth mechanism involved in the formation of Na(2)V(6)O(16)·3H(2)O nanobelts and nanorings are briefly discussed.