Kinetic study on preparation of substoichiometric tungsten oxide WO2.72 via hydrogen reduction process

Controlling the conditions of the oxygen partial pressure and temperature to prepare the WO_2.72 (W₁₈O₄₉) via reduction was possible through thermodynamic consideration. WO_2.72 was synthesized via heating to 1073 K in 5% H₂–95% Ar mixture gas flow from ammonium tungstate which was prepared by hydrothermal process. With the reducing prolonging time, the products were changed from WO_2.72 to WO₂ and then metal W. Thermogravimetric (TG) analysis showed ammonium tungstate decomposed completely to WO₃ at 773 K. Isothermal reductions using TG analysis were carried out at 905 K, 925 K, 945 K and 973 K in 5% H₂–95% Ar mixture gas flow, respectively. The whole reduction from WO₃ to WO_2.72 divided into three parts: initial nucleation and growth stage, final interfacial reaction stage and intermediate stage, was controlled jointly by both mechanisms. Fitting results showed that the initial stage obey the one-dimensional Avrami–Erofeev equation, the apparent activation energy was 132.7?±?1.1 kJ mol^?1 and the pre-exponent factor was 4.82?×?10⁵ min^?1; the final stage expressed by 2-dimensional interfacial reaction, the apparent activation energy was 144.0?±?2.1 kJ mol^?1 and the pre-exponent factor was 3.20?×?10⁵ min^?1.