Synthesis of tungsten carbides by temperature-programmed reaction with CH4-H-2 mixtures. Influence of the CH4 and hydrogen content in the carburizingmixture

The influence of the composition of a carburizing CH4-H-2 gas mixture on th e process of reduction-carburization over WO3 has been studied. Bulk tungst en carbide synthesis has been carried out from WO3 in different CH4-H-2 mix tures (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 o f the reaction products has been monitored and quantified by gas chromatogr aphy analysis (GCA) and the results have been compared to those obtained fo r a reference sample WC20 (CH4-H-2 = 1/4). The solids have been characteriz ed by elemental analysis, XRD, XPS, and BET surface area measurements. The overall process is complex. Considering first the reduction, both H-2 and C H4 act as oxides reducing agents and are converted respectively into H2O, C O2 and to a less extent CO2. If the reduction steps follow the same sequenc e observed under pure H-2, WO3 --> W20O58 --> WO2 --> W, with the strong di fference that W metal is detected only at the surface to be rapidly carburi zed, the overall reduction process can be accomplished under CH4-H-2 mixtur es 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 tr ansformation. Studies of the carburization suggest that CH4 decomposes on a metallic surface into C (or CH4) species before bulk WO2 reduction followe d by surface carburization, Then carbon diffuses into the bulk of the solid to give first alpha -W2C whose formation occurs rapidly. alpha -W2C transf ormation into WC is slower and seems to be very much influenced by the rati o P-CH4/P-H2 which controls the rate of carbon deposit at the surface of th e solid. The best surface area carbide of 27 m(2).g(-1) consisting of a cor e of alpha -W2C covered with alpha -WC has been obtained by using WO2 as st arting material. (C) 2000 Academic Press.