Synthesis of tungsten carbides by temperature-programmed reaction with CH4-H2 mixtures.: Influence of the CH4 and hydrogen content in the carburizing mixture

The influence of the composition of a carburizing CH4-H-2 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-H-2 mixtures (CH4-H-2 = 1/1-3/1; CH4-N-2 = 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-H-2 = 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 H-2 and CH4 act as oxides reducing agents and are converted respectively into H2O, CO2 and to a less extent CO2. If the reduction steps follow the same sequence observed under pure H-2, 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-H-2 mixtures at temperatures all the lower than P-CH4/P-H2 increases. Prereduction of WO3 into bulk WO2 allows an easier reduction in practically pure CH4 (95% (v/v) CH4-H-2) as reduction with CH4 increases the rate of the WO3 --> W transformation. Studies of the carburization suggest that CH4 decomposes on a metallic surface into C (or CH4) species before bulk WO2 reduction followed by surface carburization, Then carbon diffuses into the bulk of the solid to give first alpha -W2C whose formation occurs rapidly. alpha -W2C transformation into WC is slower and seems to be very much influenced by the ratio P-CH4/P-H2 which controls the rate of carbon deposit at the surface of the solid. The best surface area carbide of 27 m(2).g(-1) consisting of a core of alpha -W2C covered with alpha -WC has been obtained by using WO2 as starting material. (C) 2000 Academic Press.