Solid-solid interactions in the MoO 3-Al 2O 3, system

Abstract

Molybdenum/aluminium mixed oxide solids were prepared by thermal decomposition of aluminium hydroxide pretreated with different proportions of ammonium molybdate. The proportions of molybdenum expressed as wt.%MoO 3 were 12.4, 22 and 41.4. The prepared mixed solid specimens were calcined in air at 500–1000°C and the solid-solid interactions between MoO 3 and Al 2O 3 were investigated using DTA, TG, DTG and XRD techniques. The results obtained revealed that ammonium molybdate and aluminium hydroxide decomposed readily at 250 and 320° C yielding MoO 3 and an amorphous alumina respectively. The produced oxides underwent solid-solid interaction at 500–700°C, forming well crystallized orthorhombic Al 2(MoO 4) 3 phase. This compound remained thermally stable until about 850 ° C, then decomposed above this temperature giving very well crystallized α-Al 2O 3 (corrundum) and MoO 3. The latter oxide partly volatilized and the remaining portion dissolved in the alumina matrix forming MoO 3-Al 2O 3 solid solution. The amounts of MoO 3 dissolved in Al 2O 3 at 900 or 1000°C are 9.4, 12.2 and 16.4 wt.% for the solids containing 12.4, 22 and 41.4 wt. % MoO 3 respectively. The presence of MoO 3 greatly enhanced the crystallization of alumina into the kappa and alpha phases upon heating at 700 and 900° C respectively. This effect was attributed to the formation of some kind of MoO 3-Al 2O 3 solid solutions. Crystalline MoO 3, when mixed with Al 2O 3 and heated to 500°C, loses its crystallinity due to its dispersion on the alumina surface. The monolayer dispersion capacity of Al 2O 3 for MoO 3 was found to be 0.116 g per 100 m 2 of the alumina surface.