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

Abstract

Abstract Non-isothermal reduction of fine dispersed CuWO 4− x /WO 3− 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 ( CuWO 4 - x → I Cu + WO 3 - x ) using the mentioned methods were defined as E KAS ( stepI ) = 34 kJ mol - 1 and E FWO ( stepI ) = 41 kJ mol - 1 , respectively. Also, the “ E ” values were determined as E KAS ( stepII ) = 91.8 kJ mol - 1 and E FWO ( stepII ) = 101 kJ mol - 1 for the second and E KAS ( stepIII ) = 147.2 kJ mol - 1 and E FWO ( stepIII ) = 156.3 kJ mol - 1 for the third reduction steps ( WO 3 - x → II WO 2 → III W ) , respectively. The results showed that copper from one side helps ( WO 2 → 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 WO 2 (OH) 2 compound which is the main agent responsible for reduction of tungsten oxide.