Abstract:
The reduction process of vanadium pentoxide by CO and H2 was studied by means of thermal analysis techniques. X-ray powder diffraction analysis and electron microscopy were also used as complementary techniques. It was found that the reduction of V205 to V203 occurs through the formation of the following intermediate vanadium oxides: V2O5 → V6O13 → VO2 (rutile) → VnO2n-1 (4≤n≤8) → V2O3. The mechanism and kinetics of the reduction process were found to be sensitive to structure and to change with temperature. It is proposed that two mechanisms of solid-gas interaction contribute towards the overall reduction process, with the extent of contribution of each mechanism depending on the grain morphology and reduction temperature. These complex processes that occur cause difficulties with the kinetic description of the reduction process, nevertheless, the Avrami-Erofe'ev model seems to off er a general description of the mechanism of reduction, describing a nucleation and growth process. However, the significance of the parameter n in the Avrami-Erofe'ev equation is still uncertain. Reduction by both CO and H2 seems to start preferentially at defects such as steps, kinks and dislocations in the crystal surface. After coverage of the surface, the reaction interface advances inwards, resembling a contracting volume mechanism. Throughout this study, higher reduction rates were achieved with H2 than with CO. This is contrary to thermodynamic predictions and can probably be ascribed to kinetic favourability of the H2 reduction reaction over the CO reaction. The reduction of ammonium metavanadate (AMV) was also investigated and it was found that the in situ reduction of V205 (formed as decomposition intermediate) leads to higher reduction rates than the direct reduction of V205. This can probably be ascribed to the disorder and oxygen deficiency introduced into the V205 lattice during the decomposition of AMY in the absence of 02. It is possible that the catalytic decomposition of ammonia (evolved during decomposition) on the V205 leads to the formation of an active boundary between V205 and lower oxides such as V307 and V6013 which enhances the reactivity.
