Catalytic activity and capacitance of tungsten carbide as a function of its dispersity and surface state

The catalytic activity and total capacitance¹ of tungsten carbide (WC) have been studied on various samples without free carbon content. It was found that the true cd in hydrongen oxidation is about the same for all carbides, independently of the total volume content of carbon, starting materials for carbide preparation, temperature, and other technological parameters of carburization, as well as of the particle size (specific surface) of tungsten carbide. It was also shown that in the anodic polarization of tungsten carbide in H₂SO₄ in the presence of H₂, no true activation, ie no increase in the specific activity, takes place, whereas the observed current rise is due to an increase in the catalyst surface.A close dependence of the total çapacitance on the particle sizes of carbide, in the range of the largest ones (up to smooth WC) up to 0.1 μ dia (S ≈ 5m²/g) has been observed. A further decrease of the particle size involved almost no change in capacitance.     

Enhanced one-step purification of C2H4 from C2H2/C2H4/C2H6 mixtures by fluorinated Zr-MOF

The extraction of C₂H₄ from C₂H₆/C₂H₄/C₂H₂ mixtures is of great significance in the chemical industry for C₂H₄ production but the process remains challenging due to the similarity of these C₂ hydrocarbon species in their molecular size and physical properties. Here, we report the fluorination of a stable Zr-MOF, UiO-66, to fine-tune the pore dimensions and pore functionality. In particular, UiO-66-CF₃ shows notably preferential adsorption of C₂H₆ and C₂H₂ over C₂H₄, with C₂H₂/C₂H₄ and C₂H₆/C₂H₄ selectivities of 1.4 and 1.9, respectively. Theoretical calculations provide insight into the binding sites of UiO-66-CF₃ for C₂ hydrocarbon adsorption. Breakthrough experiments further confirmed the capability of the material for purification of C₂H₄ from C₂H₂/C₂H₄/C₂H₆ ternary mixtures, evidenced by the high purity C₂H₄ (99.9%+) obtained directly from outlet gas.     

Surface characterization and catalytic properties of supported tungsten and platinum-tungsten carbide and oxycarbide

Bulk tungsten carbide has been prepared from ammonium paratungstate. Oxidic oxygen in relatively small amount is always present on the WC surface regardless of the method of carburization. Tungsten(VI) oxide in Al₂(WO₄)₃ can be reduced directly and in apparently one single step to the elemental W(0) state by Ar⁺ bombardment. Under H₂ the reduction starts at 720 K. In 12 and 28% W03 supported on -Al₂O₃ and in Al₂(WO₄)₃, almost 90% of the tungsten could be converted to the carbide form. Extensive hydrogenolysis products for n-hexane reactions on freshly prepared WC were observed while selectivity to isomerization associated with decrease in activity occurs upon exposure of the WC to oxygen at 620 K. Supported tungsten carbide(s) and oxycarbide(s) on -Al₂O₃ have comparable catalytic behaviour to those obtained on the bulk systems. The presence of Pt in these supported systems did not improve the catalytic performances and even did not show the catalytic properties of Pt. This was attributed to the severe conditions of catalyst preparations. Although the catalytic activity of tungsten carbide(s) obtained from the carburization of Al₂(WO₄)₃ is very low, the selectivity suggests the presence of tungsten carbide plus some WO, species.     

The electrochemical reduction of CO2 to CH4 and C2H4 at Cu/Nafion electrodes (solid polymer electrolyte structures)

The construction of copper/Nafion electrodes (solid polymer electrolyte structures) by an electroless plating method is described. These electrodes were used for the gas phase electrochemical reduction of CO₂ to hydrocarbon products, including CH₄ and C₂H₄. The faradaic efficiencies of the electrodes under ambient conditions with a counter solution of 1 mM H₂SO₄ at a potential of –2.00 V vs. SCE reached a steady-state value of about 20% after 30 min of electrolysis. This corresponded to a rate of total hydrocarbon production of approximately 9.8×10^–7 mole h^–1 cm^–2. Increasing the potential of the electrode to more negative potentials, or increasing the proton concentration of the counter solution, caused a decrease in the faradaic efficiencies due to a relative increase in the rate of proton reduction vs. that of CO₂ reduction. If the proton concentration of the counter solution was decreased to an alkaline pH, hydrocarbon production quickly ceased because of proton starvation.     

活性炭载碳化钨复合材料的制备及其对对硝基苯酚的电催化性能

以浓度为10%的偏钨酸铵溶液为前驱体、CH4/H2为还原碳化气氛,采用表面修饰技术和还原碳化技术制备了碳化钨(WC)/活性炭(C)复合材料。通过FT-IR、XRD、SEM等对制备的WC/C材料进行表征,结果表明,酸处理后的活性炭外表面大幅度增加了羟基和羰基,WC/C复合材料是由WC、W2C和C三相组成,碳化钨均匀地分散于活性炭表面,粒径约50～100nm。采用循环伏安法研究了碱性介质中WC/C-PME对对硝基苯酚的电还原行为,结果表明,该材料对对硝基苯酚的电催化活性优于WC和C,且WC/C材料在对硝基苯酚电还原过程中保持良好的化学稳定性。     

Separation of C6H6 and C6H12 from H2 using ionic liquid/PVDF composite membrane

Ionic liquid/polyvinylidene fluoride composite membrane was successfully prepared by impregnation method and used for the separation on organic chemical hydride process. The separation factors of C₆H₆/H₂ and C₆H₁₂/H₂ in the ternary mixture system were 7500 and 300, respectively. The ionic liquid membrane showed an excellent possibility as a technology of H₂ purification in the organic chemical hydride process by removing aromatic hydrocarbon and cycloalkane simultaneously from the ternary system. © 2015 American Institute of Chemical Engineers AIChE J, 62: 624–628, 2016     

α-硝基萘在介孔结构碳化钨催化剂上的电化学还原行为

以喷雾干燥处理的偏钨酸铵为前驱体，分别采用CH₄/H₂和CO/CO₂为还原碳化气氛，利用固定床气固反应法制备了两种具有介孔结构的碳化钨（WC）粉体.以WC粉末作为电催化材料制成了碳化钨粉末微电极（WC-PME），并采用循环伏安和线性扫描等方法研究了酸性溶液中两种介孔结构WC对α-硝基萘（NP）电还原过程中的电催化行为.结果表明，以CH₄/H₂为还原碳化气氛所制备的WC粉末，其制成的粉末微电极对NP具有更良好的电催化活性，这主要与该WC粉末的结构形貌及其制备后的处理工艺有关，同时该WC-PME具有良好的化学稳定性.     

Boosting C2H6/C2H4 separation in scalable metal-organic frameworks through pore engineering

The development of ethane (C₂H₆)-selective adsorbents for ethylene (C₂H₄) purification, although challenging, is of prime industrial importance. Pillared-layer metal-organic frameworks (MOFs) possess facilely tunable pore structure and functionality, which means they have excellent potential for high-performance C₂H₆/C₂H₄ separation applications. Herein, we report a family of isostructural pillared-layer MOFs with various metal centers M and co-ligands L, M₂(D-cam)₄L₂ (denoted M-cam-L; M = Cu, Co, Ni; L = pyz, apyz, dabco), with a variety of pore surface properties. All of the M-cam-L materials exhibit preferential adsorption for C₂H₆ over C₂H₄. In particular, Ni-cam-pyz exhibits the highest C₂H₆ capture capacity (68.75 cm³ g⁻¹ at 1 bar and 298 K), Cu-cam-dabco possesses the greatest C₂H₆/C₂H₄ adsorption selectivity (2.3), and the lowest isosteric heat of adsorption is demonstrated for Cu-cam-pyz (20.1 kJ mol⁻¹). Dynamic column breakthrough experiments also confirmed the excellent separation performance of M-cam-pyz and M-cam-dabco materials. The synthesis route of the M-cam-L materials is easily scaled-up under laboratory conditions, and hence this class of MOFs is promising for practical C₂H₄ purification.     

Reaction Sintering of HfC/W Cermets with High Strength and Toughness

Hafnium carbide/tungsten (HfC/W) cermets were prepared by an in situ reaction sintering process, using hafnium oxide (HfO₂) and tungsten carbide (WC) as the raw materials. The reaction path, densification behavior, microstructure development, and mechanical properties of the cermets were comprehensively investigated. It was found that WC decomposed to tungsten semicarbide (W₂C) and tungsten (W) in sequence, and meanwhile HfC was formed by carbothermal reduction between HfO₂ and as‐released carbon from the dissociation of WC. The solid solution formation between HfC and W during sintering was also studied. The obtained cermets (>98% TD) have a Vickers' hardness of 8.16 GPa, a fracture toughness of 14.45 MPa m^1/2, and a high flexural strength of 1211 MPa.     

Structural study of carbon nanomaterials prepared by chlorination of tungsten carbide and bis(cyclopentadienyl)tungsten dichloride

Hollow carbon nanobags have been obtained by the chlorination of bis(cyclopentadienyl)tungsten dichloride (W(C₅H₅)₂Cl₂) at 400 and 900 °C. Transmission electron microscopy images indicate an incipient graphitisation at higher reaction temperatures and an increase in the average dimensions of the particles. When using tungsten carbide (WC) as precursor, carbide-derived carbon has been observed at 900 °C, whereas at lower temperatures core-shell-like structures have been found as intermediate reaction steps. In both type of materials, electron energy-loss spectroscopy shows a very similar sp² carbon bonding content (∼94%). Textural studies show Type 1 adsorption isotherms with surface areas of 1250 and 1320 m²/g for WC and W(C₅H₅)₂Cl₂ respectively at the higher temperature treatment.     

[CoI2(C6H4N3CH2CO2C4H2SCO2CH2C6H4N3)], a coordination polymer containing the thiophene-2,5-di(carboxylatomethylenebenzotriazole) bridging ligand: synthesis,structure, redox properties

The title complex was obtained by reacting CoI₂ with thiophene-2,5-di(carboxylatomethylenebenzotriazole) in dichloromethane. The single-crystal X-ray structure analysis of the green material reveals a polymeric chain of CoI₂ units, tetrahedrally coordinated and connected by the 3-nitrogen atoms of the two benzotriazole groups of the multifunctional ligand. The paramagnetic cobalt(II) polymer can be oxidised by hexachloroethane to give the corresponding cationic cobalt(III) complex [CoI₂(C₆H₄N₃CH₂CO₂C₄H₂SCO₂CH₂C₆H₄N₃)(H₂O)₂]⁺ which is diamagnetic and precipitates as the green–yellow chloride salt.     

The multi-structure of oxidized-reduced tungsten carbide surface(s)

The XPS of bulk tungsten carbide, partially oxidized WC surfaces at 373 and 573 K as well as tungsten trioxide have been reported. Bulk WC has been prepared from WO₃ as a starting material in a mixture of CH₄ (20%) and H₂ (80%) at 1150 K for 4 h, while partially oxidized WC surfaces were prepared by oxygen chemisorption on a clean WC surface at 200 K, then the temperatures were raised to 373 and 573 K respectively. The XPS of a freshly prepared WC reveals the presence of a small amount of WO₃ on the surface and a slightly higher concentration in the bulk. The oxygen-exposed fresh WC surfaces and surfaces treated at temperatures higher than 373 K show the presence of WO₃ in a considerable quantity depending on the length and the treatment temperature. Ar⁺ bombardment of this partially oxidized surface reduces WO₃ to WO₂ and W(0), while WC is partially reduced to W(0). Isomerization reactions of alkanes on oxygen-exposed WC surface occurs in reality on a composite surface structure containing WC, WO₃, WO₂ and elemental W(0).     

Permeation of H2, N2, CH4, C2H6, and C3H8 through asymmetric polyetherimide hollow‐fiber membranes

Permeation properties of pure H₂, N₂, CH₄, C₂H₆, and C₃H₈ through asymmetric polyetherimide (PEI) hollow‐fiber membranes were studied as a function of pressure and temperature. The PEI asymmetric hollow‐fiber membrane was spun from a N‐methyl‐2‐pyrrolidone/ethanol solvent system via a dry‐wet phase‐inversion method, with water as the external coagulant and 50 wt % ethanol in water as the internal coagulant. The prepared asymmetric membrane exhibited sufficiently high selectivity (H₂/N₂ selectivity >50 at 25°C). H₂ permeation through the PEI hollow fiber was dominated by the solution‐diffusion mechanism in the nonporous part. For CH₄ and N₂, the transport mechanism for gas permeation was a combination of Knudsen flow and viscous flow in the porous part and solution diffusion in the nonporous part. In our analysis, operating pressure had little effect on the permeation of H₂, CH₄, and N₂. For C₂H₆ and C₃H₈, however, capillary condensation may have occurred at higher pressures, resulting in an increase in gas permeability. As far as the effect of operating temperature was concerned, H₂ permeability increased greatly with increasing temperature. Meanwhile, a slight permeability increment with increasing temperature was noted for N₂ and CH₄, whereas the permeability of C₂H₆ and C₃H₈ decreased with increasing temperature. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 698–702, 2002     

Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives

In this work, cemented tungsten carbide nano-particles were prepared by a chemical method called acrylamide gel. In this process, first, a xerogel containing tungsten and cobalt oxide particles was synthesized. Then, it was carburized by a hydrogen reduction heating process.Acrylamide and N, N-methylene-bis-acrylamide monomers were used as an in-situ carbon. Ammonium meta tungstate (NH₄)₆H₂W₁₂O₄₀·xH₂O, and cobalt nitrate Co(NO₃)₂·6H₂O salts were used as the precursor.Both reduction and carburization reactions were carried out simultaneously and the formation of the intermediate phases of W₂C, Co₃W₃C, and Co₆W₆C led to decrease in the activation barrier.Transactions of reduction and carburization processes were studied by X-ray diffraction analysis at various temperatures. Accordingly, tungsten carbide phase formation was completed at 1100 °C. The formation of W–C and V–C bonds was verified by Raman spectroscopy. SEM images showed the average nano particle size of 50 nm.     

Synthesis and characterization of [Pd{PhP(C6H4-2-SCH2Cl)(C6H4-2-S)}(Ph2PCH2CH2PPh2)]Cl: An example of hemilability and dichloromethane activation

The stoichiometric reaction of [Pd{PhP(C₆H₄-2-S)₂}(PPh₃)] (1) with Ph₂CH₂CH₂Ph₂ in dichloromethane (CH₂Cl₂) as solvent, affords complex [Pd{PhP(C₆H₄-2-SCH₂Cl)(C₆H₄-2- S)}(Ph₂PCH₂CH₂PPh₂)]Cl (2). This specie, being the result of the nucleophilic reaction of a de-ligated thiolate moiety from the fragment [Pd{PhP(C₆H₄-2-S)₂}] with dichloromethane (chloromethylation).     

XPS and IR analysis of thin barrier films polymerized from C2H4/CHF3 ECR-plasmas

Thin fluorocarbon polymer films are prepared on PE-foils in low-pressure electron cyclotron resonance plasmas using ethylene (C₂H₄) and trifluoromethane (CHF₃) as monomers. The thin fluorinated hydrocarbon layers strongly reduces the permeability of polyethylene to alkanes. For example, the permeation of toluene was decreased by a factor of about 100 by a single, thin fluorocarbon layer. A further reduction of the permeation down to a factor of 1600 can be obtained by a multilayer coating. X-ray photoelectron spectroscopy and Fourier transform IR spectroscopy are used to characterize the plasma polymerized films. It is shown that the addition of CHF₃ to a C₂H₄ plasma leads to an increase of CF₃—, CF₂—, and CF— groups and to a decrease of CH₃— and CH₂— groups in the film. The chemical composition of the polymer layers and their toluene permeabilities are discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 717–722, 1997     

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.     

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.     

Electrochemical promotion of catalytic reactions with Pt/C (or Pt/Ru/C)//PBI catalysts

The paper is an overview of the results of the investigation on electrochemical promotion of three catalytic reactions: methane oxidation with oxygen, NO reduction with hydrogen at 135 °C and Fischer–Tropsch synthesis (FTS) at 170 °C in the [CH₄/O₂(or NO/H₂ or CO/H₂)/Ar//Pt (or Pt/Ru)//PBI(H₃PO₄)/H₂, Ar] fuel cell. It has been shown that the partial methane oxidation to C₂H₂ and the C₂ selectivity were electrochemically promoted by the negative catalyst polarization. This was also the case in NO reduction with hydrogen for low NO and H₂ partial pressures. In both cases the catalytic reactions have been promoted by the electrochemically produced hydrogen. It has been found that the NO reduction with hydrogen on the Pt/PBI strongly depends on NO and hydrogen partial pressures in the working gas mixture. At higher NO and H₂ partial pressures the catalysis is promoted by the electrochemical pumping of H⁺ from the catalyst, i.e. at positive polarization. FTS demonstrated the highest methane production rate (11% of CO conversion) at zero fuel cell voltage.