Thermogravimetric study of the reduction of CuO–WO<inf>3</inf> oxide mixtures in the entire range of molar ratios

Abstract

The oxide mixtures CuO–WO3 of various compositions were synthesized by citrate–gel combustion process, starting with the aqueous solutions of copper nitrate, ammonium tungstate and citric acid. Cu–W nanocomposite powders were produced by reduction of oxide mixtures in hydrogen atmosphere under thermogravimetric control. Characterization of CuO–WO3 mixtures and their reduction products was performed by SEM and XRD methods. The morphology of both oxide and metal particles displayed notable dependence on composition. Copper displayed promoting action during reduction of CuO–WO3 mixtures. The mean reduction temperature shifted monotonously from that of CuO toward that of WO3. The reduction proceeded in three steps, first of which was mainly reduction of CuO and the other two originated mainly of the two-step reduction of WO3. The composition CuO–WO3 at a molar ratio 1:1 was shown to form the compound CuWO4. The reduction of this compound was subjected to a detailed thermokinetic study. For those purposes, model-free expanded Friedman, multiple heating rate Coats–Redfern and Kissinger methods and some model-fitting methods, incorporated in the software Kinetics2015, were used. Among model-fitting models, the nucleation and growth kinetic model enabled the best fit of experimental results for all three reduction stages.