Synthesis of Tungsten Carbides by Temperature-Programmed Reaction with CH4–H2 Mixtures. Influence of the CH4 and Hydrogen Content in the Carburizing Mixture

Abstract

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.