Morphology-dependent phase transformation of γ-Al2O3

Abstract

The phase transformations of platelet- and rod-shaped gamma-Al2O3 were investigated and compared to that of a commercial sample by XRD, BET surface area measurements, transmission electron microscopy (TEM), solid state Al-27-NMR, and ethanol temperature programmed desorption (TPD) after sequential annealing in air up to 1100 degrees C. After annealing at 1100 degrees C, commercial gamma-Al2O3 mostly transformed into alpha-Al2O3 with drastic surface area reduction (from 200 m(2)/g to 25 m(2)/g). Interestingly, platelet- and rod-shaped gamma-Al2O3 which showed exactly the same XRD patterns transformed into different phases upon the high temperature calcinations. Platelet-shaped gamma-Al2O3 transformed into theta-phase while the rod-shaped gamma-Al2O3 transformed into the delta-phase and not to the alpha-polymorph. Both platelet- and rod-shaped aluminas retained significantly higher surface area (similar to 60 m(2)/g) than the commercial one after the same treatment at 1100 degrees C. These results suggest that the phase transformation in gamma-Al2O3 is strongly affected by not only the crystal structure of the starting material, but its morphology as well. Ethanol TPD from platelet- and rod-shaped alumina after 1100 degrees C annealing, showed significantly different desorption profiles which suggest different surface characteristics even though they had almost the same surface areas. These different phase transformations were also supported by solid state Al-27-NMR. After 1100 degrees C annealing commercial alumina showed the presence of mostly octahedral Al3+ ions, but the other two samples displayed even higher number of tetrahedral Al3+ ions than the initial gamma-Al2O3. Morphological changes were also confirmed by TEM. All these results consistently suggest the morphology-dependent phase transformations of gamma-Al2O3 and the improved thermal stability of platelet- and rod-shaped gamma-Al2O3 in comparison to a commercial gamma-Al2O3. (C) 2015 Elsevier B.V. All rights reserved.