Tungsten Compounds Research Articles

Thermodynamic aspects of WO3 tungsten oxide reduction by carbon, silicon, aluminum and titanium

The development and research of new materials for machine parts of the mining and metallurgical complex by the method of surfacing with flux cored wire has a lot of attention nowadays. Flux cored wires are widely used for surfacing of steels with high wear resistance, in which reduced tungsten in the form of ferroalloys, ligatures and metal powder of various degrees of purity are used as fillers. However, due to the scarcity and high cost of tungsten, its rational use is an urgent task. For practical application, the technology of surfacing with tungsten-containing flux cored wire is of interest; using it the maximum extraction of tungsten into the deposited layer is achieved due to reduction processes in the arc. In order to increase the beneficial use of tungsten, the technologies of indirect alloying with tungsten during surfacing under the flux of flux cored wires, in which tungsten oxide is used as a filler on the one hand, and reducing agent – on the other, deserve consideration. It can be expected that during arc discharge, tungsten and (or) chemical compounds of tungsten with reducing agents can be formed during the surfacing process. This paper presents the results of a comparative analysis of the thermodynamic processes of tungsten oxide reduction by carbon, silicon, aluminum and titanium during arc discharge occurring during surfacing with flux cored wires under a layer of flux. The thermodynamic analysis of 41 reactions in standard states showed that the presence of reducing agents (carbon, silicon, aluminum, titanium) in the flux cored wire used for surfacing will contribute to the formation of silicides and tungsten carbides, and, possibly, tungsten itself. It was determined that the best state for the participation of tungsten oxide in reactions in the arc is WO3(g) gaseous state.

(Digital Presentation) The Effects of Heat Treatment on Electroreduction of Tungsten (VI) Oxide

According to the classic scheme of the FFC Cambridge process, oxides of refractory metals are reduced in the solid state. Before electrolysis, oxides are shaped into cylindrical preforms and annealed at high-temperatures (~1000 °С). Therefore, the purpose of this study is to establish the effect of heat treatment of the initial sample of tungsten trioxide on its electrochemical reduction on a solid cathode in a CaCl2–NaCl melt.There are two types of tablets (preforms) were used for research: non-sintered (only compressed) WO3 and annealed at a temperature of 1000 °C. The surface of a pelletized tungsten trioxide sample without sintering is characterized by a uniform distribution of particles, shown in the SEM image (Fig. a). The particles agglomeration is characteristic for the sample WO3 after heat treatment (1000 °С) (Fig. b), which leads to a decrease in the area and an increase in the size of the voids. The unsintered sample has a larger surface area between the particles due to their dispersed state, which facilitates the penetration of the melt compared to the annealed sample.Electrochemical reduction of pressed WO3 samples (d=0.8 cm, h=0.15±0.20 cm, m=0.45–0.50 g) was carried out under galvanostatic conditions (800 °С) with using steel as the cathode and graphite as the anode. It was established that preliminary heat treatment of WO3 does not contribute to its electrochemical reduction. After applying 1.20 A·h of electricity, the porous granules of unsintered WO3 were completely transformed into metallic tungsten (W), which is shown in Fig. c, line 1. However, sintered samples after electrolysis (quantity of electricity is 1.42 A·h) reduced only partially: W – 74.1%; CaWO4 – 25.9% (Fig. c, line. 2).The results show that the reduction occurs more intensively in samples with more micropores, through which mass transfer occurs at the phase interface: tungsten compound to be reduced molten electrolyte mixture. That is why in the unsintered WO3 sample, the electrochemical reduction is faster. It can be assumed that the degree of reduction of WO3 on a solid electrode in CaCl2–NaCl also depends on its crystal structure. The annealed sample (tetragonal structure) is less susceptible to electrochemical reduction compared to the unannealed sample of tungsten trioxide (monoclinic structure). This is due to the fact that tetragonal WO3 (above 740 °C) is resistant to chemical attack, a poor conductor of electricity and has a low surface area. Monoclinic WO3 (from 17 to 330 °C) is more reactive, a better conductor of electricity and has a higher surface area. Figure 1

Prediction of machining performances in powder mixed electro-discharge machining to process skd61 steel by response surface methodology

In electro-discharge machining (EDM) with mixing powder, it is called powder mixed electro-discharge machining (PMEDM), then machining performances- i.e. material removal rate(MRR) and tool wear rate (TWR) has great significance in evaluating the effectiveness and machining accuracy of the machining method. Therefore, in this study, response surface methodology (RSM) was utilized for estimating functions of process variables {comprising peak current (Ip), pulse on time (Ton), and powder concentration (Cp)} for the machining performances for processing SKD61 steel during EDM process with tungsten compound powder. Box-Behnken matrix was utilized for designing and conducting a series of empirical trials. Analysis of variance (ANOVA) was applied to evaluate the adequate of predictive models. The outcomes reveal that the predicted models of MRR and TWR have a high precision with R2 values of MRR and TWR being 99.2% and 99.11%, respectively. The error comparison of the predictive and empirical values for the confirmed experiments is less than 5%, this once again consolidates that the developed models' accuracy. These development models can efficiently prognosticate the desired machining performances of the PMEDM method for processing SKD61 steel

STUDY OF THE INFLUENCE OF PLASMA-ELECTROLYTICE OXIDATION PARAMETERS ON THE FUNCTIONAL PROPERTIES OF HETEROXIDE COATINGS

The results of a study of the influence of the parameters of plasma-electrolytic oxidation in diphosphate electrolyte solutions of metal platforms made of titanium alloys, as the basis for the formation of heterooxide coatings with photocatalytic activity, on the level of their functional properties are presented. It has been established that heterooxide coatings obtained in PEO modes on titanium alloy platforms, in which zinc and tungsten compounds were incorporated as dopants, demonstrate photocatalytic activity in the processes of azo dye degradation under UV irradiation. An increase in the concentration of oxygen-containing compounds of dopants in electrolyte solutions has a positive effect on their incorporation into the composition of the monoxide matrix of the composite, morphology and catalytic properties of the obtained coatings. Based on the results of determining the morphological features of the structure of the coatings, it was proved that, compared with titanium monoxide, as a material for a photocatalytic platform, heterooxide coatings have a more developed globular surface microstructure, which positively affects the level of their functional performance. A symbatic dependence between the content of incorporated dopants and the specific surface area of coatings, as well as their photocatalytic activity, has been proven. It is noted that the change in the phase structure of the surface layers in the process of plasma-electrolytic formation of the TiO/ZnO-WO3 coating on the surface of the carrier metal causes an increase in the mechanical characteristics of the resulting coatings, in particular, the microhardness almost doubles. The obtained results can become the basis for the creation of functional materials for the catalytic neutralization of natural, synthetic and man-made toxicants, which will lead to the solution of a number of environmental problems both during the war crisis and during the post-war reconstruction of the country.

ELECTROCHEMICAL PRODUCTION OF TUNGSTEN: STATUS AND PROSPECTS

An overview of studies of the electroche­mical reduction of tungsten compounds of different composition in various reaction media is presented. It is shown that among the variety of existing scientific and technical methods for obtaining tungsten, there are attractive prospects for the creation and development of a new industrial process that would ensure the direct electrochemical release of oxygen from its oxygen-containing compounds into molten mixtures based on chloride and calcium oxide. This scientific and technical solution is known in the literature as the FFC Cambridge process (FFC process).In contrast to the known methods of electrochemical reduction of tungsten compounds, this process allows the reduction of oxygen-containing tungsten compounds in the solid state and does not depend on the course of acid-base equilibria at the electrode/electrolyte phase separation boundary. The most favorable conditions for the reduction of oxygen-containing tungsten compounds are provided by electrolysis using a liquid gallium cathode in both galvanostatic and potentiostatic modes, and it is advisable to use the initial tungsten compounds in a finely dispersed state. The electrochemical reduction of tungsten trioxide in thee utectic melt of sodium and calcium chlorides occurs through the intermediate stage of calcium tungstate formation, so it is advisable to use CaWO4 instead of WO3 as the starting compound for reduction. Electrochemical reduction on a liquid gallium cathode in a molten eutectic mixture of sodium and calcium chlorides allows obtaining highly dispersed tungsten powder (11–35 nm) of high purity (99.9%) with a degree of extraction of at least 90.0% from both tungsten trioxide and from calcium tungstate. In addition, in this way it is possible to obtain not only pure tungsten, but also metal alloys and composites based on it.

Synthesis, Characterization, and Single-Crystal X-ray Structures of Refractory Metal Compounds as Precursors for the Single-Source Chemical Vapor Deposition of Metal Nitrides.

The chemical vapor deposition of refractory metal nitrides requires volatile precursors and has previously been achieved by using metal complexes containing a variety of imide ligands. Recently, the 1,4-di-tert-butyl-1,3-diazabutadiene (DAD) adduct of bis(tert-butylimide)dichloridemolybdenum(VI) was shown to be an excellent precursor for the single-source CVD of Mo2N thin films. Leveraging the success of this work, we prepared chromium and tungsten compounds with the same framework. Additionally, the framework has been modified slightly to allow the isolation of mono(tert-butylimide)trichloride complexes of vanadium, niobium, tantalum, and molybdenum(V) to extend the search for new vapor-phase precursors. These compounds were all fully characterized using the standard methods of multinuclear magnetic resonance spectroscopy, combustion analysis, and single-crystal X-ray diffraction. Their thermal properties were determined by using thermogravimetric analysis and differential scanning colorimetry to assess their utility as vapor-phase precursors. Finally, preliminary deposition studies were carried out to investigate their potential as single-source CVD precursors.

Synthesis and Volatility Characterization of Mo(II) and W(II) Compounds for Thin Films.

Mo(II) and W(II) compounds, Mo(η3-allyl)(CO)2(Tri-MEDA)Br (1), Mo(η3-allyl)(CO)2(TMEDA)Br (2), W(η3-allyl)(CO)2(Tri-MEDA)Br (3), and W(η3-allyl)(CO)2(TMEDA)Br (4) (Tri-MEDA = N,N,N'-trimethylethylenediamine), were synthesized and characterized. The molecular structures of 1 and 3 were nearly identical with a pseudo-octahedral geometry except for the different Mo and W metal centers. The thermogravimetric analysis of 1 and 3 showed approximately 53 and 64% residues at 550 °C, respectively, which were significantly higher than the values for the expected materials. However, 1 and 3 sublimed at 100 °C under 0.40 Torr and 120 °C under 0.50 Torr, respectively, confirming that they were volatile. For 1 and 3, the temperatures at a vapor pressure of 1 Torr and enthalpies of vaporization (ΔHvap) were 168.78 °C and 143.8 kJ mol-1, and 167.48 °C and 148.5 kJ mol-1, respectively. The tungsten compound (3) exhibited good durability for 5 weeks under a thermal stability test at a sublimation temperature of 120 °C.

Crystal structures and luminescence properties of Li6 MN4:Ce3+ (M = Mo, W)

Abstract The compounds Li6MoN4 and Li6WN4 were prepared from the reactants M (M = Mo, W) and Li3N in a radiofrequency furnace at 1000 °C. The lithonitridometallates crystallize in the tetragonal system with the lattice parameters: a = 6.6844(1), c = 4.9294(1) Å for Li6WN4 based on single-crystal X-ray diffraction data and a = 6.6611(3), c = 4.9338(3) Å for Li6MoN4 taken from powder X-ray diffraction data. Colorless to slightly reddish single-crystals of the tungsten compound were isolated and the crystal structure was refined in the space group P42/nmc (no. 137) with Z = 2 and the powder X-ray data of the molybdenum compound were analyzed by a Rietveld refinement. Both structures belong to the Li6[ZnO4] type published by Hoppe et al. in 1987 (Untenecker H., Hoppe R. Z. Anorg. Allg. Chem. 1987, 551, 147–150) and could be doped with Ce3+ for the first time. The investigated compounds show a reddish color impression upon UV to blue irradiation and exhibit a broad emission band with a maximum at λ max = 693 nm (fwhm 97 nm) for Li6MoN4 and at λ max = 653 nm (fwhm 133 nm) for Li6WN4.

New insights into the surface features of SKD61 steel at heat-treated and non-heat-treated states as processed by powder-mixed EDM

In this paper, the two states of SKD61 steel include heat-treated (HT) and non-heat-treated (NHT), were processed by powder-mixed electrical discharge machining (PMEDM) using tungsten compound powder. Several machined surface properties were investigated by methods such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), 3D Laser Scanning Microscope, and X-ray diffraction (XRD). In both states, their surfaces were changed in chemical composition and content of elements (CCCE), and tungsten carbide phase (W2C) was created. In comparison with the HT state, some surface features of NHT specimens, such as: the percentage of micro-crack acreage per surface (PCAS) is better improved, the penetration of the W element into surfaces and the intensity of the tungsten carbide phase are larger, the distribution of C and W elements on the surface is a more even, and the surface topography has more uniformity and flatness.

Effect of Modifying Additives of Molybdenum and Tungsten Compounds on the Catalytic Properties of MgO-Al2O3 in the Process of Gas-Phase Conversion of Ethanol Into 1-Butanol

Effect of Modifying Additives of Molybdenum and Tungsten Compounds on the Catalytic Properties of MgO-Al2O3 in the Process of Gas-Phase Conversion of Ethanol Into 1-Butanol

THE INTERACTION OF TUNGSTEN (VI) OXIDE AND CALCIUM TUNGSTATE IN THE CaCl2–NaCl EUTECTIC MELT

The interaction of tungsten (VI) oxide and calcium tungstate with a molten eutectic mixture of calcium-sodium chlorides in the temperature range from 600 to 800 °С was investigated by the methods of isothermal saturation, mass spectrometry with inductively coupled plasma (ICP–MS) and X-ray phase analysis. It was noted that the solubility of both tungsten trioxide and calcium tungstate depends to a large extent on temperature. Thus, in the temperature range from 600 to 700 °C, the equilibrium concentration of tungsten increases by an average of 1.7 times, and in the range from 700 to 800 °C, its values increase by 3.9 times.
 Based on the results obtained by inductively coupled plasma mass spectrometry, it was found that the equilibrium content of tungsten (in the calculation of pure metal) in the molten eutectic mixture of sodium and calcium chlorides both in contact with tungsten trioxide and in contact with calcium tungstate at different temperatures within the error of definition has approximately the same values. The obtained results indicate in favor of the fact that the dissolution of tungsten trioxide in the molten mixture of sodium and calcium chlorides proceeds through the stage of formation of calcium tungstate. It is possible that sodium tungstate may be formed in this case, but no direct evidence of this has been found. At temperatures higher than 750 °C, almost all of the loaded tungsten trioxide turns into calcium tungstate. This gives grounds for asserting that in the molten salt phase tungsten is mainly in the form of calcium tungstate.
 The obtained results give reason to consi­der that the interaction of the mentioned oxy­gen-containing compounds of tungsten produces homogeneous salt phases of the ternary system CaCl2–NaCl–CaWO4, the content of which calcium tungstate in the temperature range of 700÷800 °С is satisfactorily described by the dependence ln S = 8.331 – 14057/T.

Facile route for synthesizing WS2/W2C nano hollow flowers and their application for hydrogen evolution reaction

Recently, catalysts based on tungsten disulfide nanoflowers (WS2 NFs) and tungsten carbide (W2C) materials have been intensively investigated for hydrogen evolution reaction (HER). In this study, a facile and scalable route was utilized to synthesize a nano hollow flowers catalyst based on heterostructure of WS2 and W2C (WS2/W2C NHFs). Besides that, other structures of tungsten compounds including WS2–WO3 flower particles (FPs) and pure WS2 NFs were also prepared by the similar route with slight modification, to compare the differences in their structures and HER activities. Materials properties were comprehensively studied via X-ray diffraction analysis, field emission scanning microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. Then, the HER catalytic activities of synthesized materials were also carefully studied. The results indicate that WS2/W2C NHFs exhibits the HER activities much better than that of prepared WS2–WO3 FPs and pure WS2 NFs. The HER performance of WS2/W2C NHFs exhibits a low Tafel slope of about 61 mV.dec−1, an overpotential of − 280 mV at − 10 mA.cm-2, high double-layer capacitance (11.9 mF cm-2) and the superstability in acid media. This improvement is attributed to the synergistic effect of the WS2 and W2C heterostructure as well as the special structure of WS2/W2C NHFs catalyst.

Mechanism of Reductive Fluorination by PTFE-Decomposition Fluorocarbon Gases for WO3

Reductive fluorination, which entails the substitution of O2- from oxide compounds with F- from fluoropolymers, is considered a practical approach for preparing transition-metal oxyfluorides. However, the current understanding of the fundamental reaction paths remains limited due to the analytical complexities posed by high-temperature reactions in glassware. Therefore, to expand this knowledgebase, this study investigates the reaction mechanisms behind the reductive fluorination of WO3 using polytetrafluoroethylene (PTFE) in an Ni reactor. Here, we explore varied reaction conditions (temperature, duration, and F/W ratio) to suppress the formation of carbon byproducts, minimize the dissipation of fluorine-containing tungsten (VI) compounds, and achieve a high fluorine content. The gas-solid reaction paths are analyzed using infrared spectroscopy, which revealed tetrafluoroethylene (C2F4), hexafluoropropene (C3F6), and iso-octafluoroisobutene (i-C4F8) to be the reactive components in the PTFE-decomposition gas during the reactions with WO3 at 500 °C. CO2 and CO are further identified as gaseous byproducts of the reaction evincing that the reaction is prompted by difluorocarbene (:CF2) formed after the cleavage of C═C bonds in i-C4F8, C3F6, and C2F4 upon contact with the WO3 surface. The solid-solid reaction path is established through a reaction between WO3 and WO3-xFx where solid-state diffusion of O2- and F- is discerned at 500 °C.

Tuning the electrocatalytic performances of Ni-MOF with combined tungsten compounds (WO3-x/WS2-x) decorated rGO by interface coupling for efficient water oxidation

The novel design of noble metal-free electrocatalyst with new structure/combination and cost-effectiveness are vital sources for the featuring hydrogen energy sector (mainly hydrogen and oxygen evolution reaction (OER)) in the viewpoint of commercialization of water electrolyzers. Herein, we constructed a new kind of ternary rG-W-N electrocatalyst (reduced graphene oxide - a new formation of heterostructured tungsten compound W-O-S (WO3-x/WS2-x) - Ni-MOF) for enhancing the OER activity. Interestingly, the seeded W-O-S grew from inside of rGO layer with a spherical formation in binary composition. The excellent electrocatalyst for OER was identified by systematically investigating from the binary and ternary electrocatalysts using different concentrations. The homogeneous rG-W-N nanocomposite exhibits remarkable performance for water oxidation, with an ultra-low over potential value of 170 mV at 10 mA cm−2 towards OER performance. In addition, it shows an efficient stability performance after 5000 cycles. The rG-W-N electrocatalyst boosts the sluggish intermediate pathway for OER, which is the primary reason for attaining the lowest over potential. The electronic and synergetic effects in rG-W-N leads to interfacial interaction/connectivity between the ternary materials and large active sites enhance the efficient charge transfer reaction.

(Digital Presentation) Electrochemical Reduction of Oxygen-Containing Tungsten Compounds on a Liquid Gallium Cathode in an Eutectic Melt of Calcium and Sodium Chlorides

This paper presents the results of studies of electrochemical reduction of oxygen-containing tungsten compounds (WO3, CaWO4) in a molten eutectic mixture of calcium and sodium chlorides. In contrast to the known methods of direct electrochemical reduction of refractory metal oxides, it is proposed to reduce the initial oxygen-containing tungsten compounds in powder form on a liquid gallium cathode without resorting to pre-pressing and sintering operations.In the temperature range 600-1000 °C, at which the electrochemical reduction of these compounds takes place, gallium does not evaporate, does not interact with the components of the melt, or with the initial compounds of tungsten or pure tungsten. The liquid gallium cathode not only provides reliable contact with highly dispersed initial tungsten powder compounds and their uniform polarization but also protects the reduced metal from contamination by products of electrolysis and electrolyte. In addition, gallium, due to its lower specific weight than tungsten oxide, protects WO3 from unwanted interactions with calcium chloride, which causes the formation of volatile tungsten oxochlorides and the loss of the initial tungsten trioxide. Due to the large difference in crystallization temperatures of the molten electrolyte (over 500 °C) and the gallium cathode (~ 30 °C), as well as the difference in specific weights of the electrochemical cell components, fine tungsten powder isolated during electrolysis is easily separated from melt and gallium.During the reduction of tungsten trioxide at the beginning of electrolysis on the surface of the gallium cathode, sodium and calcium cations, which are part of the electrolyte mixture, are discharged. In the range of reduction temperatures (700-800 °C) the discharge potentials of sodium and calcium cations are almost the same. As the gallium cathode is saturated with isolated metals (sodium and calcium), tungsten trioxide is reducedWO3 + 3nM → W + 3MnO, where М = Na, Ca,and n = 2 when sodium is involved in the reduction and n = 1 when the reducing agent is calcium. The formed metal oxides (MnO) react with tungsten trioxide, forming a mixture of tungstates of the appropriate metalsWO3 + MnO → MnWO4 Both oxides and tungstates of calcium and sodium have a lower specific weight than gallium, so as they accumulate, they float to the interface: cathode/melt, where at the appropriate values of the electrode potential, also participate in electrode processesMnWO4 + 6е− → W + MnO + 3О2− MnO + 2е− → nM + О2−.Reduced tungsten settles to the bottom of the gallium cathode container, and calcium and sodium contribute to the reduction of tungstates.The results of voltammetric studies of oxidized and unoxidized monocrystalline tungsten, tungsten cathode, on the surface of which is applied a film of WO3, gallium cathode with tungsten starting compounds (WO3, CaWO4) in a molten eutectic mixture of sodium and calcium chlorides indicate in favor of the fact that this process takes place in stages in several stages. This is also supported by the results of electrolysis under potentiostatic conditions at different potentials of the cathode relative to the unpolarized tungsten electrode. The mixture of tungsten and calcium tungstate is the reduction product at potentials lower than the reduction potentials of sodium and calcium tungstates. For example, with a reduction potential of −1.3 V, the reduction products of both tungsten trioxide and calcium tungstate contain no more than 2.0% of tungsten, the rest is calcium tungstate. Reduction at a potential of −2.0 V provides a product with a tungsten content of not less than 20.0%. Finally, the reduction at a potential equal to or greater than −2.4 V at the cathode produces tungsten without the content of extraneous phases. The degree of extraction of tungsten from tungsten trioxide is not less than 65.0%, from calcium tungstate is about 72.8%.The morphology of reduced tungsten was studied by scanning electron microscopy. The resulting tungsten powder has the form of dendritic crystals with a size of several tens of micrometers.

Effect of B4C barrier layer on layer intermixing in nanoscale W/Be multilayers before and after annealing

The effect of an ultrathin B4C barrier layer on the content of beryllides formed as a result of W and Be mixing during deposition in a short-period W/Be multilayers, and, as a consequence, on the reflectivity of the multilayers and their thermal stability is discussed in the paper. All the multilayers were prepared by means of DC magnetron sputtering. Chemical composition analysis and evaluation of thickness extensions were carried out utilizing X-ray photoelectron spectroscopy and transmission electron microscopy, respectively. The results obtained showed that the insertion of the ultrathin B4C barrier layer led to a decrease in the content of beryllides in multilayers due to the formation of tungsten compounds with boron/carbon. In the case of the B4C deposited onto the beryllium layer it was also associated with the additional formation of Be2C. It was suggested that the formation of the borides and carbides is more optically favorable compared to the beryllides in the multilayer without the barrier layer, which causes an increase in reflectivity. Ex situ annealing of the multilayer structures at temperatures up to 300 °C did not provoke additional mixing of the layers, but only increased a surface BeO, which led to decreasing in reflectivity of the multilayers.

Facile synthesis of “WCl3(DME)” and structural and electronic characterizations

AbstractIn contrast to the readily accessed mid‐valent molybdenum precursors, trivalent tungsten compounds as good synthons are rare. The most commonly used trivalent ditungsten species, NaW2Cl7(THF)5, for the synthesis of the triply W − W bonded compounds of the type W2X6(X = bulky anionic ligand) can no longer be prepared because it is obtained from Na/Hg reduction of WCl4, and the use of mercury is banned. In this work, we report the characterization of a useful ditungsten(III) complex W2(μ‐Cl)2Cl4(κ2‐DME)2(2) (DME = 1,2‐dimethoxyethane) to replace NaW2Cl7(THF)5. The efficient reduction of the monomeric W(IV) compound WCl4(DME) (1) with granular Sn in DME gives2in a 97% isolated yield.2is a diamagnetic species based on SQUID magnetometry, and its formulation was established by single‐crystal X‐ray crystallography. The structure of2shows an edge‐sharing bioctahedral (ESBO) conformation with overallCisymmetry and the W − W bond length is 2.6443 (6) Å. Density functional theory (DFT) calculations suggest a σ2π2δ*2ground state configuration corresponding to the W − W single bond.

Replacement of Molybdenum by Tungsten in a Biomimetic Complex Leads to an Increase in Oxygen Atom Transfer Catalytic Activity.

Upon replacement of molybdenum by tungsten in DMSO reductase isolated from the Rhodobacteraceae family, the derived enzyme catalyzes DMSO reduction faster. To better understand this behavior, we synthesized two tungsten(VI) dioxido complexes [WVIO2L2] with pyridine- (PyS) and pyrimidine-2-thiolate (PymS) ligands, isostructural to analogous molybdenum complexes we reported recently. Higher oxygen atom transfer (OAT) catalytic activity was observed with [WO2(PyS)2] compared to the Mo species, independent of whether PMe3 or PPh3 was used as the oxygen acceptor. [WVIO2L2] complexes undergo reduction with an excess of PMe3, yielding the tungsten(IV) oxido species [WOL2(PMe3)2], while with PPh3, no reactions are observed. Although OAT reactions from DMSO to phosphines are known for tungsten complexes, [WOL2(PMe3)2] are the first fully characterized phosphine-stabilized intermediates. By following the reaction of these reduced species with excess DMSO via UV–vis spectroscopy, we observed that tungsten compounds directly react to WVIO2 complexes while the Mo analogues first form μ-oxo Mo(V) dimers [Mo2O3L4]. Density functional theory calculations confirm that the oxygen atom abstraction from WVIO2 is an endergonic process contrasting the respective reaction with molybdenum. Here, we suggest that depending on the sacrificial oxygen acceptor, the tungsten complex may participate in catalysis either via a redox reaction or as an electrophile.

New Low-Temperature Method of Synthesis of Boron Carbide Matrix Ceramics Ultra-Dispersive Powders and their Spark Plasma Sintering

The ultra-dispersive powders of pre-ceramic precursors for boron carbide based composites were obtained by relatively low-temperature (at 200 – 1000 °C) synthesis from liquid charges containing available compounds such as salts and oxides. Boron carbide matrix ceramics were compacted by their reactive spark plasma sintering (SPS) at 1500 – 1700 °C. It is noted that the X-ray diffraction (XRD) peaks corresponding (m)ZrO2 and WC phases presented in the synthetic pre-ceramic precursors disappear after the SPS is conducted at 1500 °C. It is established that the addition of tungsten and cobalt compounds promotes both the low-temperature synthesis of ceramic components and sintering processes of their powders. Energy dispersive X-ray (EDX) analysis showed that the ceramics contain a small amount of Co (0.8 – 2 wt.%). The density of samples of cobalt-containing ceramics B4C–ZrB2–W2B5–Co is higher compared to that of cobalt-free ceramics B4C–ZrB2–W2B5.

Influence of Tungsten Nanopowders on the Kinetics of Cement Stone Strength Gain

The paper presents the results of research of influence of nanopowders of tungsten compounds obtained from hard-alloy waste on structure formation and kinetics of cement stone strength gain. The positive effect of additives on the mechanical and structural characteristics of the cement stone in the concentration range of 1...5 wt.% has been established.