The influence of the composition of a carburizing CH4–H2 gas mixture on the process of reduction–carburization over WO3 has been studied. Bulk tungsten carbide synthesis has been carried out from WO3 in different CH4–H2 mixtures (CH4–H2=1/1–3/1; CH4–N2=1/1; pure CH4) at atmospheric pressure by temperature-programmed reduction–carburization (TPRC). The composition of the reaction products has been monitored and quantified by gas chromatography analysis (GCA) and the results have been compared to those obtained for a reference sample WC20 (CH4–H2=1/4). The solids have been characterized by elemental analysis, XRD, XPS, and BET surface area measurements. The overall process is complex. Considering first the reduction, both H2 and CH4 act as oxides reducing agents and are converted respectively into H2O, CO, and to a less extent CO2. If the reduction steps follow the same sequence observed under pure H2, WO3→W20O58→WO2→W, with the strong difference that W metal is detected only at the surface to be rapidly carburized, the overall reduction process can be accomplished under CH4–H2 mixtures at temperatures all the lower than PCH4/PH2 increases. Prereduction of WO3 into bulk WO2 allows an easier reduction in practically pure CH4 (95% (v/v) CH4–H2) as reduction with CH4 increases the rate of the WO2→W transformation. Studies of the carburization suggest that CH4 decomposes on a metallic surface into C (or CHx) species before bulk WO2 reduction followed by surface carburization. Then carbon diffuses into the bulk of the solid to give first α-W2C whose formation occurs rapidly. α-W2C transformation into WC is slower and seems to be very much influenced by the ratio PCH4/PH2 which controls the rate of carbon deposit at the surface of the solid. The best surface area carbide of 27 m2.g−1 consisting of a core of α-W2C covered with α-WC has been obtained by using WO2 as starting material.

Controlled decomposition studies of the bis(cyclopentadienyl)ditungsten(tetracarbonyl)-dimethylacetylenedicarboxylate (Cp2W2(CO)4(dmad)) under flowing hydrogen show that it totally decomposes at 600 °C for 2 h to give a pure bulk W2C as revealed by X-ray diffraction. Excess oxygen and carbon at the surface are detected by X-ray photoelectron spectroscopy. An in situ temperature-programmed X-ray diffraction experiment performed on Cp2W2(CO)4 (dmad) shows the detection temperature of W2C to be 600 °C, the sample being amorphous or microcrystalline below that temperature. Based on previous results obtained for the decomposition of Cp2Mo2(CO)4(dmad) on the one hand, and thermogravimetric and chromatographic analyses performed on Cp2W2(CO)4 (dmad), on the other, a decomposition scheme of the latter under hydrogen has been proposed. © 1998 Chapman & Hall

The synthesis of bulk tungsten carbides by carburization of W metal or of WO3with mixtures of CH4in hydrogen at various pressures has been studied in temperature programmed experiments. The resulting solids have been characterized by elemental analysis, X-ray diffraction, XPS analysis, and specific surface area measurements. The carburization occurs in two distinct steps: W2C is formed in the first step taking place at about 650°C at atmospheric pressure with a 20% CH4–H2mixture, while the formation of WC occurs only at higher temperatures. During carburization some free carbon is deposited, the importance of which is very much dependent on CH4partial pressure and on the temperature of carburization. It has also been shown that direct carburization of WO3by CH4–H2does not take place, but that the carburization occurs via the reduction of WO3to W metal. The rate of reduction of WO3and that of carburization of W metal are very much dependent on, respectively, hydrogen partial pressure and CH4partial pressure. The extent of reduction of WO3into W metal required for carburization which takes place also depends on CH4partial pressure, indicating a competition between carburization of W metal at the surface and diffusion of W metal into the bulk of the solid.