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

Abstract

Controlling the conditions of the oxygen partial pressure and temperature to prepare the WO2.72 (W18O49) via reduction was possible through thermodynamic consideration. WO2.72 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 WO2.72 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 WO2.72 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.