Tungsten carbide-based electrochemical sensors for hydrogen determination in gas mixtures

An amperometric study of gas-diffusion electrodes (GDE) catalysed by two types of tungsten carbide, WC₍₁₎ and WC₍₂₎, which differ considerably in their specific surface area (0.5 and 6 m² g^–1), was carried out. The H₂–air gas mixture (H₂ 1–4%) measurements show that for this range of hydrogen concentration the hydrogen limiting diffusion current (i _d(H2)) may be attained so that a curve of limiting current density against hydrogen concentration can be obtained. The response and stability of the electrode performance were compared to those of platinum catalysed GDEs. The most promising for use in amperometric hydrogen sensing is the WC₍₁₎ catalyst of small specific surface area. Electrodes catalysed with this catalyst show inferior response time in comparison to electrodes catalysed with the other two catalysts (WC₍₂₎ and Pt) but their overall stability is much better.     

Catalytic Performance of Activated Carbon Supported Tungsten Carbide for Hydrazine Decomposition

Activated carbon (AC) supported tungsten carbide was reported for the catalytic decomposition of hydrazine for the first time. It was found that the WC _x /AC-H catalyst prepared by a carbothermal hydrogen reduction process exhibited excellent catalytic performances both in the microreactor and in a 1 Newton hydrazine microthruster. The XRD, TEM and microcalorimetry results suggest that the formation of well crystallized W₂C phase, the restraining of the carbon deposition, as well as the prohibiting of the methanation should be responsible for the high activity and stability of the WC _x /AC-H catalyst in the hydrazine decomposition reaction.     

Synthesis and characterization of carbon-supported nanoparticles for catalytic applications

Metallic and carburized tungsten nanoparticles have been prepared using WCl₆ as precursor. The synthesis steps lead from tungsten hexachloride to tungsten carbide following the sequence WCl₆ → W⁰ → α-W₂C → WC. These different compounds have been mainly characterized by X-ray diffraction, TEM observations and adsorption measurements. The total reduction of tungsten hexachloride can be completed at T = 973 K. The carburizing of the metallic lattice into carbide WC is observed at 1223 K although incomplete. At this temperature, crystal growth phenomena appear which are not desired for catalysis applications. The synthesis of nanoparticles supported on activated carbon is accompanied by a decrease of the specific surface area of samples, especially during high temperature carburizing.     

Role of carbon deficiency and anodic activation in the electrochemistry of carbide materials

This paper aims at establishing the influence of non-stoichiometry and anodic oxidation on the electrochemical properties of the carbides WC, WC₂C, Cr₃C₂ and Cr₇C₃ through a well-grounded choice of carbide materials for working out each of the problems. Data was compared for compact WC and WC_0.9. The role of anodic activation was investigated chiefly on dispersed WC and WC₂C with good stoichiometry and phase purity obtained from elements (elemental carbides). Measurements on WC synthesized from oxides (oxide WC), Cr₃C₂ and Cr₇C₃ were made chiefly for the purpose of comparison. On the basis of measurements of charging curves, potentiodynamic curves, polarization curves of ionization-evolution of hydrogen, adsorption of ions, X-ray photoelectron spectra as well as an analysis of the corrosion products, a qualitative scheme was proposed for the processes taking place on the surfaces of tungsten carbides at anodic potentials. A method of dc activation of carbide electrodes was suggested. For the first time a systematic study was made of the electrochemical behaviour of chromium carbides and W₂C; the results obtained point to the promising prospects for the use of these materials for electrocatalysis.     

Synthesis of hexagonal phases of WN and W2.25N3 by high‐pressure metathesis reaction

Hexagonal tungsten nitrides were synthesized by the metathesis reaction between WCl₆ and NaN₃ under high pressure and temperature. As well as tungsten mononitride (WN), which is isostructural with hexagonal tungsten carbide (WC), a nitrogen‐rich hexagonal compound was also confirmed in the product. The ratio of nitrogen to tungsten was determined to be 1.34 by the quantitative combustion method. The composition was estimated to be W_2.25N₃ by considering the crystal structure that is best explained by the X‐ray diffraction profile. Volume compression measurements under hydrostatic pressure revealed that the WC‐type WN has a higher bulk modulus (K₀ = 342 ± 1 GPa) than that of hexagonal W_2.25N₃ (K₀ = 291 ± 2 GPa).     

Novel single phase (Ti0.2W0.2Ta0.2Mo0.2V0.2)C0.8 high entropy carbide using ball milling followed by reactive spark plasma sintering

Single phase novel (Ti_0.2W_0.2Ta_0.2Mo_0.2V_0.2)C_0.8 high entropy carbide (HEC) compacts were successfully synthesized by reactive spark plasma sintering of ball milled metal-carbon elemental mixture at temperatures of 1400−1800 °C. X-ray diffraction and element distribution maps indicated single phase carbide formation with lattice parameter ranging from 4.307 Å to 4.312 Å with small amount of TiO₂. X-ray energy dispersive spectroscopy (EDS) mapping showed uniform distribution of the transition metals in the carbide phase. The microhardness, elastic modulus, fracture toughness, electrical resistivity and thermal expansion coefficient (25 °C–600 °C) of the compact sintered at 1800 °C were found to be 25.8 ± 2.8 GPa, 461 ± 36 GPa, 3.7 ± 0.4 MPa.m^1/2, 7 × 10⁻⁴ Ω/m² and 7 × 10^-6 K⁻¹ respectively.     

Carbon-coated tungsten and molybdenum carbides for electrode of electrochemical capacitor

New electrode materials for electrochemical capacitor, tungsten carbide WC and molybdenum carbide Mo₂C coated by porous carbon, were prepared through a simple heat treatment of the mixture of K₂WO₄ and K₂MoO₄, respectively, with hydroxy propyl cellulose. Carbide changed to hydroxide during the 1st charge-discharge cycle in H₂SO₄ aqueous electrolyte, which showed redox reaction in further charge-discharge cycles, in addition to electric double layers of the carbon formed on its surface. The carbon-coated carbide gave a high capacitance in 1 mol L⁻¹ H₂SO₄ electrolyte, as about 350 F cm⁻³ for carbon-coated WC and 550-750 F cm⁻³ for carbon-coated Mo₂C. Coating of carbon inhibits the growth of carbide particles during their formation, of which the small particle size make possible to complete transformation to hydroxides during the 1st charge-discharge cycle, and also disturbs the agglomeration of tungsten and molybdenum hydroxides during charge-discharge cycles, as well as porous carbon coated act as electrode material for electric double layers of electrolyte ions.     

Mechanism studies on CVD of boron carbide from a gas mixture of BCL3, CH4, and H2 in a dual impinging‐jet reactor

Nearly pure boron carbide free from impurities was produced on a tungsten substrate in a dual impinging‐jet chemical vapor deposition reactor from a BCl₃, CH₄, and H₂ mixture. The Fourier Transform Infrared (FTIR) analysis proved the formation of reaction intermediate BHCl₂, which is proposed to occur mainly in the gaseous boundary layer next to the substrate surface. Among a large number of reaction mechanisms proposed only the ones considering the molecular adsorption of boron carbide on the substrate surface gave reasonable fits. In the proposed mechanism dichloroborane is formed in the gas phase only as a by‐product. Boron carbide, on the other hand, is formed through a series of surface reactions involving adsorbed boron trichloride, adsorbed methane and gas phase hydrogen. The simultaneous fit of the experimental rate data to the model expressions gave correlation coefficient values of 0.977 and 0.948, in predicting the B₄C and BHCl₂ formation rates, respectively. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Microstructural Properties of Air Plasma Sprayed Coating Consisting of Tungsten Carbide and Yttria‐Stabilized Zirconia

Thermal Spraying technologies are proven to be capable of producing composite materials and structures. In the present work, an innovative composite coating was produced to achieve high wear and thermal resistant properties in a single‐step process using air plasma spraying (APS) technique. Tungsten carbide has shown high wear resistance and zirconia coatings exhibited excellent tribological and insulation properties. It is speculated that a composite material consisting of zirconia and tungsten carbide exhibits excellent thermomechanical properties. A powder mixture of 50wt% WC‐10wt% Ni (WC‐Ni) and 50wt% ZrO₂‐8wt% Y₂O₃ (YPSZ) was deposited on a low carbon steel substrate using APS technique. Important microstructural properties of WC‐Ni/YPSZ coating such as splat boundaries, pore and grain morphology, microcracks, phase composition, elemental distribution of coatings, and lattice parameters of the crystals were investigated using optical microscopy, scanning electron microscopy (SEM), energy dispersive X‐ray (EDS), and X‐ray diffractometry (XRD). A good adhesion was observed between different phases in tungsten carbide mixed with zirconia coatings. Decarburization process which occurred during APS process resulted in formation of tungsten hemi‐carbide (W₂C) phase in plasma sprayed samples. The calculated crystal size for APS‐deposited coating was smaller than those of feedstock powder.     

Low thermal expansion porous SiC–WC composite ceramics

Low thermal expansion porous SiC–WC composite ceramics were prepared by solid state reaction of Si and WC at 1560 °C, with NH₄HCO₃ as a pore generating agent. Phase composition, thermal expansion, flexural strength, and microstructure of the carbide ceramics were examined. Presence of the SiC, WC and WC_1−X phases were detected in the carbide ceramics. As Si content increased from 2 to 14 wt%, the coefficient of thermal expansion first decreased and then increased, with a minimum of 4.11 × 10⁻⁶ °C at 8 wt% Si, whereas the flexural strength decreased gradually, from 143.9 to 82.7 MPa. Pores of SiC–WC ceramics were less than 2 μm in diameter, because of the stacking interstice of carbide particles and volatilization of silicon. However in the presence of NH₄HCO₃, pores of SiC–WC ceramics were bimodally distributed, the stacking interstice of carbide particles loosened from 1 to 4 μm and pores larger than 5 μm were also formed.     

In vitro and Microbiological Assay of Functionalized Hybrid Nanomaterials To Validate Their Efficacy in Nanotheranostics: A Combined Spectroscopic and Computational Study

Functionalized nanoparticles reveal new frontiers in therapeutics and diagnostics, simultaneously referred to as theranostics. Functionalization of an inorganic nanoparticle (NP) with an organic ligand determines the interaction of the functionalized NPs with various cellular components, leading to the desired therapeutic effect, while diminishing adverse side effects. Apart from the therapeutic effect of the nanoparticles, other physical properties of the organic-inorganic complex (nanohybrid) including fluorescence, X-ray or MRI contrast offer diagnosis of the anomalous target cell. In this study we functionalized Mn₃O₄ NPs with organic citrate (C−Mn₃O₄) and folic acid (FA−Mn₃O₄) ligands and investigated their antimicrobial activities using Staphylococcus hominis as a model bacteria, which can be remediated through their membrane rupture. While high-resolution transmission microscopy (HR-TEM), XRD, DLS, absorbance and fluorescence spectroscopy were used for structural characterisation of the functionalised NPs, zeta potential measurements and temperature-dependent reactive oxygen speices (ROS) generation reveal their drug action. We used high-end density functional theory (DFT) calculations to rationalise the specificity of the drug action of the NPs. Picosecond-resolved FRET studies confirm the enhanced affinity of FA−Mn₃O₄ to the bacteria relative to C−Mn₃O₄, leading to enhanced antimicrobial activity. We have shown that the functionalised nanoparticles offer significant X-ray contrast in in-vitro studies, indicating the FA−Mn₃O₄ NPs to be a potential theranostic agent against bacterial infection.     

Structural and thermal analysis of in situ synthesized C–WC nanocomposites

Nanostructure of carbon encapsulated tungsten carbide (WC@C) has been prepared using reaction between metallic magnesium (Mg), acetone (C₃H₆O) and tungsten trioxide (WO₃) in an autoclave at 600 °C. The resultant powders were characterized by X-ray diffraction (XRD), differential thermal analysis/thermal gravimetric analysis and transmission electron microscopy (TEM). The XRD results showed that the optimization of the reaction time facilitates the reduction as well as carburization of the tungsten source. The apparent activation energy for decarburization of carbide phase was also evaluated from the data of thermal analysis to find the thermal stability of carbide phase. TEM image showed that the synthesized sample consisted of particles with an average size of 35 nm.     

Precursor synthesis and properties of nanodispersed tungsten carbide and nanocomposites WC:nC

Nanoscale tungsten carbide (WC) and WC:nC nanocomposites have been synthesized by the precursor method. The precursor, obtained in the form of a glassy mass by thermal treatment of a mixture of (NH₄)₁₀W₁₂O₄₁∙7H₂O and glycerol, was heated in inert gaseous atmosphere up to 1050–1100 °C. The concentration of chemically active carbon in the precursor and nanocomposites depends on the W/C ratio in the initial mixture. At W/C=1/3 pure tungsten carbide is formed; at W/C>1/3 composites of WC and free carbon (WC:nC) are formed. Heating of the precursor with W/C=1/6 up to 1100 °C in helium atmosphere results in the formation of carbon-encapsulated tungsten carbide nanoparticles. An increase in the precursor-heating rate leads to the formation of chain-like structures. Each chain consists of hexagonal WC grains with unit cell parameters a=2.93 Å and c=2.83 Å. Free carbon in WC:nC composites forms agglomerates of carbon “nano-onions” of spherical or multi-layered tubular shapes.     

Study of the preparation of bulk tungsten carbide catalysts with C2H6/H2 and C2H4/H2 carburizing mixtures

The influence of the preparation procedure of tungsten carbide on the mechanism of carburization is discussed. This work is focused on the reduction and the carburization of tungsten trioxide by a mixture of hydrocarbon and H₂ to form WC. Temperature-programmed reaction spectra obtained with CH₄, C₂H₆ and C₂H₄ have been measured. In presence of the CH₄-H₂ mixture, H₂ is the reducing agent and the hydrocarbon is consumed for the carburization whereas C₂H₆ or C₂H₄ participates in the reduction of the tungsten oxide. The temperatures of reduction and carburization are lower by about 150 K using C₂H₆ or C₂H₄ instead of CH₄. Such a decrease of the temperature of reduction of tungsten oxide is needed to avoid the formation of poorly reducible compounds that can occur during the preparation of supported tungsten carbide. Furthermore, the surface area of the resulting carbide is 25 m²/g with C₂H₆ and C₂H₄ and 10 m²/g with CH₄. During the carburization, the deposit of excess carbon on the WC surface is larger with the C₂ hydrocarbons than with CH₄, but it protects the carbide and can be removed by hydrogen treatment. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of Ethylene Oxide Group in the Anionic–Nonionic Mixed Surfactant System on Microemulsion Phase Behavior

The phase behavior of microemulsions stabilized by a binary anionic–nonionic surfactant mixture of sodium dihexyl sulfosuccinate (SDHS) and C12-14 alcohol ethoxylate (C_{12 − 14}E_j) that contains an ethylene oxide (E_j) group number, j, of either 1, 5, or 9 was investigated for oil remediation. The oil–water interfacial tension (IFT) and optimal salinity of the microemulsion systems with different equivalent alkane carbon numbers (EACN) were examined. The anionic–nonionic surfactant ratio was found to play a pivotal role in the phase transition, IFT, and optimal salinity. The minimum IFT of mixed SDHS − C_{12 − 14}E_j systems were about three times lower than those of neat SDHS systems. A hydrophilic–lipophilic deviation (HLD) empirical model for the mixed anionic–nonionic surfactant system with the characteristic parameter was proposed, as represented in the excess free energy term . The results suggested that the mixed system of SDHS − C_{12 − 14}E₁ was more lipophilic, while SDHS − C_{12 − 14}E₉ was more hydrophilic than the ideal mixture (no excess free energy during the microemulsion formation), and the SDHS − C_{12 − 14}E₅ system was close to the ideal mixture. The findings from this work provide an understanding of how to formulate mixed anionic–nonionic microemulsion systems using the HLD model for oils that possess a wide range of EACN.     

Single step synthesis of tungsten carbide (WC) nanoparticles from scheelite ore

To prepare tungsten carbide (WC) nanoparticles high purity precursors like W, WO₃, WCl₄, WCl₆ and W(CO)₆ are used. Moreover, these precursors are obtained after high temperature and multistage processing of the ore. In this article a single step synthesis method is reported to get tungsten carbide (WC) nanoparticles directly from scheelite ore. The mixture of scheelite, activated charcoal and magnesium was heated for 20 h at 800 °C in an autoclave which led to the direct conversion of scheelite to nanocrystalline WC. The undesired reaction products and impurities (CaO, MgO and SiO₂) were washed firstly with dilute HCl (1:1) and then with base (0.25 M NaOH). The obtained powders were characterized by high resolution scanning electron microscopy (HRSEM), high resolution transmission electron microscopy (HRTEM) and X-ray diffraction.     

Effect of vapor‐grown carbon nanofibers and in situ hydrolyzed silica on the mechanical and shape memory properties of water‐borne epoxy composites

Vapor‐grown carbon nanofiber (VGCNF)/water‐borne epoxy (WEP) and SiO₂/WEP composites were successfully synthesized via freeze drying and hot‐press molding. VGCNFs were mixed directly with a WEP emulsion, while SiO₂ was synthesized by in situ hydrolysis of TEOS solution (3‐triethoxysilylpropylamine (KH550): tetraethoxysilane (TEOS): absolute ethanol = 1:5:20, w/w/w) dispersed in the WEP emulsion. WEP composites were obtained from these mixtures by freeze drying and compressing under a pressure of 10 MPa at 120°C for 2 h. The morphology and mechanical properties of the WEP composites were investigated by transmission electron microscopy, scanning electron microscopy, dynamic mechanical analysis and tensile testing. The shape memory (SM) properties of the WEP composites were evaluated by fold‐deploy SM testing. The effects of filler content and recovery temperature on the SM properties were revealed through systematic variation. The results confirmed that VGCNF and in situ hydrolyzed SiO₂ were homogenously dispersed and incorporated into the WEP matrices. Thus, significant improvements in the mechanical and SM properties of the composites were achieved. POLYM. COMPOS., 36:1712–1720, 2015. © 2014 Society of Plastics Engineers     

Reaction pathway of NiAl/WC nanocomposite synthesized from mechanical activated NiAlWC powder system

In this work, the mechanism of WC formation during mechanical alloying and subsequent annealing of nickel, aluminum, tungsten, and graphite powder mixtures was investigated. X-ray diffraction was used to evaluate phase changes. Microstructural and morphological evaluations of the powders were examined by FESEM and TEM. The XRD results confirmed that phase changes occurred by increasing milling time. After 10?h of ball milling, NiAl and W₂C phases formed and new tungsten carbides were synthesized by increasing of milling time. After 40?h, W was consumed completely and WC, WC_1-x, W₂C carbides along with NiAl were produced. After heat treatment of 40?h milled powder, W₂C and WC_1-x phases disappeared and NiAl/WC nanocomposite was formed. The results confirmed that the WC formation was a gradual reaction controlled by atomic diffusion.     

Changes in phospholipid bilayers caused by sodium dodecyl sulfate/nonionic surfactant mixtures

The interaction of mixtures of sodium dodecyl sulfate (SDS) and oxyethylenated nonylphenol (30 mol of ethylene oxide) [NP(EO)₃₀] with phosphatidylcholine liposomes was investigated. Permeability alterations were detected as a change in 5(6)-carboxyfluorescein (CF) released from the interior of vesicles, and bilayer solubilization was measured as a decrease in the static light scattered by liposome suspensions. Three parameters were described as the effective surfactant/lipid molar ratios (Re) at which the surfactant system: (i) resulted in 50% CF release (Re _50%CF); (ii) saturated the liposomes (Re _SAT); (iii) led to complete solubilization of these structures (Re _SOL). The corresponding surfactant partition coefficients (K _50%CF, K _SAT, and K _SOL) were determined from these parameters. The free surfactant concentrations S _W were lower than the mixed surfactant critical micellar concentration at subsolubilizing levels, whereas they remained similar to these values during saturation and solubilization of bilayers. Although the Re values increased linearly as the mole fraction of the SDS rose (X _SDS), the K parameters showed maximum values at X _SDS 0.6 for K _50%CF and approximately at X _SDS 0.2 for K _SAT and K _SOL, respectively. Thus, the lower the surfactant contribution in the surfactant/lipid system, the higher the X _SDS at which the maximum bilayer/water partitioning of added mixed surfactant systems occurred. As a consequence, the influence of SDS in this partition appears to be more significant at the sublytic level (monomeric effect), whereas the influence of NP(EO)₃₀ seems to be greater during saturation and solubilization of liposomes via formation of mixed micelles.     

Kinetics and mechanism of oxidative coupling of methane over sodium-manganese oxide catalyst

The kinetics of methane oxidative coupling (OCM) was studied using 1 g of Na-Mn₂O₃ catalyst at 1073 to 1123 K, in an integral flow reactor (I.d. = 10 mm), at atmospheric pressure with methane and oxygen partial pressures of 0.27 and 0.13 bar, respectively, so that the ratio of CH₄ to O₂ was 2. The flow rate range was 50 to 200 ml/min. the kinetic data were analyzed by the Rideal-redox type of rate equation assuming the methyl radical and active surface oxygen to be the steady-state intermediates. Oxidation and reduction rate constants (K_ox, K_red) for methane consumption were calculated from experimental catalysis results by computer simulation using the multiple least squares method. The activation energies at rate constants K_ox and K_red for this type of catalyst were reported as 43.26 and 62.2 kcal/mol, respectively.