An investigation about the activation energies of the reduction transitions of fine dispersed CuWO<inf>4-x</inf>/WO<inf>3-x</inf> oxide powders

Abstract

Non-isothermal reduction of fine dispersed CuWO4-x/WO3-x oxide phases were investigated by thermogravimetry (TG) and differential thermogravimetry (DTG) analyses under hydrogen atmosphere and the activation energies of the corresponding reduction transitions determined by Kissinger-Akaira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) methods. The activation energy of the first reduction stage (CuWO4 - x over(→, I) Cu + WO3 - x) using the mentioned methods were defined as EKAS (stepI) = 34 kJ mol- 1 and EFWO (stepI) = 41 kJ mol- 1, respectively. Also, the "E" values were determined as EKAS (stepII) = 91.8 kJ mol- 1 and EFWO (stepII) = 101 kJ mol- 1 for the second and EKAS (stepIII) = 147.2 kJ mol- 1 and EFWO (stepIII) = 156.3 kJ mol- 1 for the third reduction steps (WO3 - x over(→, II) WO2 over(→, III) W), respectively. The results showed that copper from one side helps (WO2 → W) reduction to start at lowered temperatures, but, from the other side, it enlarges the activation energy of this reaction by about 20 kJ mol-1. This increase explained by the fact that copper acts as a barrier for formation and transportation of volatile WO2(OH)2 compound which is the main agent responsible for reduction of tungsten oxide. © 2009 Elsevier Ltd. All rights reserved.