Study on preparation of high surface area tungsten carbides and phase transition during the carburisation

Abstract

A detailed study of step-by-step carburisation of WO₃ using 20%CH₄/H₂ and 10%C₂H₆/H₂ under various conditions is described. The catalyst materials have been characterised using thermogravimetric analysis and differential scanning calorimetry (TGA-DTC), temperature programmed reaction (TPR), surface area measurement using the Brunauer–Emmett–Teller (BET) method, X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and elemental X-ray micro-analysis (EDX). The structures of the product carbides were found to be functions of the conditions of synthesis. The use of C₂H₆/H₂ gave the highest surface area materials. During the early steps in the carburisation process at lower temperatures, disorder intergrowth occurs and non-stoichiometric crystallographic shear tungsten oxide (i.e. WO_3−x), and then WO₂ are formed. Three steps are identified during the conversion of WO₃ to WC using 10% C₂H₆/H₂. First WO₂ is formed by the reduction with hydrogen at temperatures of 670–720 K. This is carburised to WO_xC_y or β-W₂C between 800–870 K. Finally, a second carburisation occurs at temperatures between 870–920 K to produce α-WC.