The effect of MgO-doping (2-10 mol%) on solid-solid interactions and phase
transformation process in MoO3/Al2O3 system were studied using thermogravim
etry and X-ray diffraction (TG, DTG and XRD) techniques. The proportions of
molybdena expressed as weight percent were 12.36, 22.01 and 41.37. The res
ults obtained showed that the MgO-doping promoted the solid-solid interacti
on taking place at 500 degrees C between Al2O3 and MoO3 to produce Al-2(MoO
4)(3). However, MgO interacts readily with MoO3 at temperatures starting fr
om 500 degrees C yielding MgMoO4 which remains stable even when heating at
1000 degrees C, The produced Al-2(MoO4)3 decomposed at temperatures startin
g from 800 degrees C producing alpha-Al2O3 and MoO3, a portion of which sub
limed and the other portion dissolved in alumina matrix forming MoO3-Al2O3
solid solution. MgO-doping decreased slightly the solubility of MoO3 in Al2
O3, The promotion of Al-2(MoO4)3 formation at 500 degrees C and the decreas
e in the solubility of MoO3 in Al2O3 by MgO have been attributed to dissolu
tion of a small portion of MgO in the MoO3 lattice with subsequent increase
in the mobility of Mo6+ ions. The promotion effect of magnesia towards Al-
2(MoO4)3 formation and dissolution of MoO3 in Al2O3 solid are relatively sm
all when compared to the observed effects reported in the case of Li2O-dopi
ng of MoO3/Al2O3 system. The limited effect of MgO-doping has been attribut
ed to a limited solubility of MgO in MoO3/Al2O3 system due to the formation
of MgMoO4. (C) 1999 Elsevier Science B.V. All rights reserved.