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. WO3−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 WOxCy or β-W2C between 800–870 K. Finally, a second carburisation occurs at temperatures between 870–920 K to produce α-WC.