Reduction of tungsten oxides with carbon. Part 1: Thermal analyses

Abstract

The kinetics and mechanism of the reduction of WO 3 with carbon (in the form of graphite and of lamp black) were studied using isothermal thermogravimetry of small sample masses (< 50 mg) in the temperature range 935 to 1100°C. Two stages were observed in the reduction. The first stage corresponds approximately to the formation of WO 2 and the final product of the reduction was tungsten. The CO CO 2 ratio in the gaseous products had a considerable influence on the reactions occurring. The rate of the first stage of the reduction under isothermal conditions could be described by diffusion models, and is proposed to involve diffusion of CO(g) and CO 2(g) through the pores of the reacting tungsten oxides. The activation energies of the graphite and lamp black systems differed significantly for this first stage of reduction (386 compared to 465 kJ mol −1). These activation energies are high for a diffusion process and may be inflated by changes in the structure of the product and the CO CO 2 equilibrium ratio as the temperature increases. The rate of the second stage of reaction can be described by a first-order rate equation, and it is proposed that the second stage of reaction is limited by the reaction of carbon with carbon dioxide, rather than by the reduction of a tungsten oxide. The measured activation energy of 438 kJ mol −1 is slightly higher than the reported values for the carbon-carbon dioxide reaction (up to 400 kJ mol −1).