Solid-solid interaction in the pure and Li 2O-doped MoO 3Al 2O 3 system

Abstract

The effects of Li 2O doping (1.5 and 6 mol%) on solid-solid interactions and the phase transformation process in the MoO 3Al 2O 3 system were investigated using thermogravimetry, differential thermal analysis and X-ray diffraction (TG, DTA and XRD) techniques. The proportions of molybdena expressed at wt% MoO 3 were 22 and 41.4. The results obtained revealed that MoO 3 interacted with Al 2O 3 at temperatures starting from 500°C, forming orthorhombic Al 2(MoO 4) 3; doping with lithium oxide was found to promote the reaction. The complete transformation of molybdenum trioxide into aluminum molybdate required heating of the mixed oxides at 700°C. The molybdate produced decomposed at temperatures above 800°C yielding α-Al 2O 3 and MoO 3. The Li 2O doping enhanced the crystallization of α-alumina and retarded the thermal decomposition of Al 2(MoO 4) 3. The MoO 3 produced partly sublimed and the remaining portion dissolved in the alumina matrix forming an MoO 3Al 2O 3 solid solution. Li 2O doping increased the amount of MoO 3 sublimed at 900–1100°C to an extent proportional to the concentration of the dopant. In other words, Li 2O treatment decreased the solubility of MoO 3 in Al 2O 3. The promoting effect of Li 2O on Al 2(MoO 4) 3 formation at 500°C was attributed to dissolution of a small portion of Li 2O in the MoO 3 lattice, with subsequent increase in the mobility of Mo 6+ ions. The possible increase in the mobility of these ions might also account for the observed decrease in the solubility of MoO 3 in Al 2O 3.