Using a Surface-Sensitive Chemical Probe and a Bulk Structure Technique to Monitor the gamma- to theta-Al2O3 Phase Transformation

In this work, we investigated the phase transformation of gamma-Al2O3 to theta-Al2O3 by ethanol TPD and XRD. Ethanol TPD showed remarkable sensitivity toward the surface structures of the aluminas studied. Maximum desorption rates for the primary product of ethanol adsorption, ethylene, were observed at 225, 245, and 320 degrees C over gamma-, theta-, and alpha-Al2O3, respectively. Ethanol TPD over a gamma-Al2O3 sample calcined at 800 degrees C clearly shows that the surface of the resulting material possesses theta-alumina characteristics, even though only the gamma-alumina phase was detected by XRD. These results strongly suggest that the gamma-to-theta phase transformation of alumina initiates at oxide particle surfaces. The results obtained are also consistent with our previous finding that the presence of penta-coordinated Al3+ sites, formed on the (100) facets of the alumina surface, is strongly correlated with the thermal stability of gamma-aluminaopen141