An investigation about the activation energies of the reduction transitions of fine dispersed CuWO4−x/WO3−x oxide powders

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→ICu+WO3-x) using the mentioned methods were defined as EKAS(stepI)=34kJmol-1 and EFWO(stepI)=41kJmol-1, respectively. Also, the “E” values were determined as EKAS(stepII)=91.8kJmol-1 and EFWO(stepII)=101kJmol-1 for the second and EKAS(stepIII)=147.2kJmol-1 and EFWO(stepIII)=156.3kJmol-1 for the third reduction steps (WO3-x→IIWO2→IIIW), 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.