Effect of MgO-doping on solid–solid interactions in MoO 3/Al 2O 3 system

Abstract

The effect of MgO-doping (2–10 mol%) on solid–solid interactions and phase transformation process in MoO 3/Al 2O 3 system were studied using thermogravimetry 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 results obtained showed that the MgO-doping promoted the solid–solid interaction taking place at 500°C between Al 2O 3 and MoO 3 to produce Al 2(MoO 4) 3. However, MgO interacts readily with MoO 3 at temperatures starting from 500°C yielding MgMoO 4 which remains stable even when heating at 1000°C. The produced Al 2(MoO 4) 3 decomposed at temperatures starting from 800°C producing α-Al 2O 3 and MoO 3, a portion of which sublimed and the other portion dissolved in alumina matrix forming MoO 3–Al 2O 3 solid solution. MgO-doping decreased slightly the solubility of MoO 3 in Al 2O 3. The promotion of Al 2(MoO 4) 3 formation at 500°C and the decrease in the solubility of MoO 3 in Al 2O 3 by MgO have been attributed to dissolution of a small portion of MgO in the MoO 3 lattice with subsequent increase in the mobility of Mo 6+ ions. The promotion effect of magnesia towards Al 2(MoO 4) 3 formation and dissolution of MoO 3 in Al 2O 3 solid are relatively small when compared to the observed effects reported in the case of Li 2O-doping of MoO 3/Al 2O 3 system. The limited effect of MgO-doping has been attributed to a limited solubility of MgO in MoO 3/Al 2O 3 system due to the formation of MgMoO 4.