High Surface Area Tungsten Carbide Research Articles

High Surface Area Tungsten Carbides: Synthesis, Characterization and Catalytic Activity towards the Hydrogen Evolution Reaction in Phosphoric Acid at Elevated Temperatures

High Surface Area Tungsten Carbides: Synthesis, Characterization and Catalytic Activity towards the Hydrogen Evolution Reaction in Phosphoric Acid at Elevated Temperatures

High Surface Area Tungsten Carbide with Novel Architecture and High Electrochemical Stability

Tungsten carbide microspheres comprising an outer shell and a compact kernel have been synthesized by a simple hydrothermal method and exhibit very high surface area promoting a high dispersion of platinum nanoparticles. Tungsten bronze formation in surface of these WC microspheres was clearly identified by combination of electrochemical characterization and XPS measurements. The presence of a tungsten bronze layer at the surface of a WC core leads to a higher electrochemically active surface area (EAS) stability compared to that of the usual Pt/C electrocatalysts used for PEMFC.

High Surface Area Tungsten Carbide with Novel Architecture and High Electrochemical Stability

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High Surface Area Tungsten Carbide and Its Evaluation as an Electrocatalyst Support

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High surface area tungsten carbide microspheres as effective Pt catalyst support for oxygen reduction reaction

In the present work, the preparation of high surface area (256m2g−1) tungsten carbide microspheres (TCMSs) by the aid of a simple hydrothermal method is realized and the performance of the Pt electrocatalyst supported on the as-prepared TCMSs towards the oxygen reduction reaction (ORR) is investigated. The SEM micrographs indicated that both the synthesized carbon microspheres (CMSs) and TCMSs showed perfect microsphere structure and uniform size. The EDX measurements confirmed that when the C/W mass ratio is ∼2.5/1, tungsten and carbon coexist in the microspheres. Moreover, from the XRD results, it can be found that both W2C and WC are detected and W2C exists as the main phase. It was found that the Pt particles are uniformly dispersed on the supports, while the corresponding average particle size is ∼3.7, 4.1 and 4.3nm for Pt/C, Pt/CMSs and Pt/TCMSs, respectively. It was also found that in terms of ORR onset potential and mass activity, the Pt/TCMSs catalyst exhibits superior performance to that of Pt/CMSs and Pt/C, enhancing the ORR catalytic activity by more than 200%. The above behavior could be attributed to its higher electrochemical surface area (ESA), as well as to the synergistic effect between Pt and tungsten carbides.

High surface area synthesis, electrochemical activity, and stability of tungsten carbide supported Pt during oxygen reduction in proton exchange membrane fuel cells

The oxidation of carbon catalyst supports to carbon dioxide gas leads to degradation in catalyst performance over time in proton exchange membrane fuel cells (PEMFCs). The electrochemical stability of Pt supported on tungsten carbide has been evaluated on a carbon-based gas diffusion layer (GDL) at 80 °C and compared to that of HiSpec 4000™ Pt/Vulcan XC-72R in 0.5 M H 2SO 4. Due to other electrochemical processes occurring on the GDL, detailed studies were also performed on a gold mesh substrate. The oxygen reduction reaction (ORR) activity was measured both before and after accelerated oxidation cycles between +0.6 V and +1.8 V vs. RHE. Tafel plots show that the ORR activity remained high even after accelerated oxidation tests for Pt/tungsten carbide, while the ORR activity was extremely poor after accelerated oxidation tests for HiSpec 4000™. In order to make high surface area tungsten carbide, three synthesis routes were investigated. Magnetron sputtering of tungsten on carbon was found to be the most promising route, but needs further optimization.

Synthesis and characterization of high-surface area tungsten carbides and application to electrocatalytic hydrogen oxidation

In analogy with W 2 N and WO 3 , WS 2 and WP were found to undergo the CH 4 carburization to produce α-WC. α-WC obtained from WS 2 and W 2 N exhibited a high specific surface area compared with other routes. The introduction of a small amount of Pt into the WCs prepared in these ways contributed to a remarkable activity enhancement for the hydrogen electro-oxidation reaction. Tungsten carbides (WC) with different crystalline phases prepared from WO 3 as the starting material showed electrocatalytic activity in the order of α-WC > β-WC 1− x > β-W 2 C for the hydrogen oxidation reaction. α-WC obtained by methane carburizing of tungsten nitride (W 2 N), which was prepared from nitridation of WO 3 , produced much higher surface area than that from the direct carburization of WO 3 . We first found capable of carburize tungsten sulfide (WS 2 ) and tungsten phosphide (WP) to produce α-WC though they require severe carburization reaction. In particular, α-WC obtained from WS 2 exhibited a high specific surface area as well as α-WC by way of W 2 N. The XPS analysis revealed that the surface properties of α-WC are significantly dependent on the preparative methods. A large amount of graphite carbon was deposited on the surface of α-WC from direct carburization, whereas, there was far less on α-WC by way of W 2 N and WS 2 .The electrocatalytic activities of WCs obtained in these ways were greatly improved. The binary catalysts of WC promoted with Pt were evaluated in comparison with the current Pt catalyst. The introduction of a small amount of Pt onto the WC contributed to a remarkable anodic current enhancement. It turned out that the specific activity (activity based on the surface area of Pt metal) was higher than that of the commercial catalyst. It is concluded that the interaction Pt with α-WC enhance the utilization of Pt for the hydrogen electro-oxidation reaction.

Study on preparation of high surface area tungsten carbides and phase transition during the carburisation

A detailed study of step-by-step carburisation of WO3 using 20%CH4/H2 and 10%C2H6/H2 under various conditions is described. The catalyst materials have been characterised using thermogravimetric analysis and differential scanning calorimetry (TGA-DTC), temperature programmed reaction (TPR), surface area measurement using the Brunauer–Emmett–Teller (BET) method, X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and elemental X-ray micro-analysis (EDX). The structures of the product carbides were found to be functions of the conditions of synthesis. The use of C2H6/H2 gave the highest surface area materials. During the early steps in the carburisation process at lower temperatures, disorder intergrowth occurs and non-stoichiometric crystallographic shear tungsten oxide (i.e. WO3−x), and then WO2 are formed. Three steps are identified during the conversion of WO3 to WC using 10% C2H6/H2. First WO2 is formed by the reduction with hydrogen at temperatures of 670–720 K. This is carburised to WOxCy or β-W2C between 800–870 K. Finally, a second carburisation occurs at temperatures between 870–920 K to produce α-WC.

Reactions of neopentane, methylcyclohexane, and 3,3-dimethylpentane on tungsten carbides: The effect of surface oxygen on reaction pathways

High surface area tungsten carbides with WC and β-W 2C structure were prepared by direct carburization of W0 3 in CH 4H 2 mixtures. Their surfaces appear devoid of excess polymeric carbon and adsorb between 0.2 and 0.4 monolayers of CO and H. These materials are very active in neopentane hydrogenolysis. Chemisorbed oxygen inhibits hydrogenolysis reactions and leads to the appearance of isopentane among the reaction products. Neopentane isomerization to isopentane occurs only on Pt, Ir, and An surfaces. Thus, oxygen-exposed tungsten carbides catalyze reactions characteristic of noble metal catalysts. 3,3-Dimethylpentane isomerizes much faster than neopentane on oxygen-exposed carbides; the isomer distribution suggests that isomerization proceeds via a methyl shift mechanism rather than through the C 5-ring hydrogenolysis pathways characteristic of highly dispersed Pt. The apparent involvement of 3,3-dimethyl-l-pentene reactive intermediates is consistent with carbenium-type methyl shift pathways. Secondary carbon atoms, capable of forming stable carbenium ions, are present in 3,3-dimethylpentane but not in neopentane; they account for the high 3,3-dimethylpentane isomerization rate and selectivity on oxygen-exposed tungsten carbide powders. Both dehydrogenation and isomerization reactions of methylcyclohexane occur on these carbide powders. These results suggest the presence of a bifunctional surface that catalyzes dehydrogenation and carbenium ion reactions typically occurring on reforming catalysts.

XPS study of tungsten carbide

AbstractThe surface composition and electronic structure of tungsten carbides prepared by direct synthesis and carburization of white tungstic acid were studied with X‐ray photoelectron spectroscopy. The experimental results show that WO3 and free carbon are present on the surface of the carbides. For the high surface area tungsten carbide prepared from white tungstic acid a large extent of oxygen insertion into the crystal lattice was observed. Measurements of the binding energies of the W4f7 2, C1S and O18 core levels indicate that charge transfer is from the W to the C and O atoms, forming W5d‐C3p and W5d‐O2p bonding. The catalytic behavior of the tungsten carbides in hydrocarbon conversion reactions is mentioned and compared with Pt catalysts.

The relation of surface structure to the electrocatalytic activity of tungsten carbide

The electrocatalytic activities of high surface area tungsten carbides for H 2 oxidation in acid electrolytes were studied and found to be dependent on the methods of preparation of the carbides. Auger electron spectroscopy and X-ray photoemission spectroscopy were used to determine how the surface compositions and chemical states of surface atoms varied with methods of preparation. Bulk crystallographic structures were determined using X-ray diffraction. Low temperature (700 °C) carburization of tungstic acid was found to deposit or cause the accumulation of an inert carbon layer over the tungsten carbide. This phenomenon could be minimized by mixing the tungstic acid with 10 w o NH 4Cl. The most active carbide was slightly carbon deficient at both the surface and in the bulk, with the carbon deficiency probably replaced by oxygen. This active surface was most easily prepared by carburizing amorphous, white, tungstic acid hydrate. Oxygen substitution for carbon probably occurs during an intermediate state of carbon dissolution in the reduced tungsten metal and is aided by the defect structure in the tungstic acid. The increased activity of the oxygen substituted carbide is due to a reduced interaction of the surface with the electrolyte, resulting from covalent tungsten-oxygen bonding. The absolute activity of active tungsten carbide for the oxidation of pure H 2 at 25 °C is four orders of magnitude lower than that for Pt.