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

In this work, we investigated the phase transformation of γ-Al 2 O 3 to θ-Al 2 O 3 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 °C over γ-, θ-, and α-Al 2 O 3, respectively. Ethanol TPD over a γ-Al 2 O 3 sample calcined at 800 °C clearly shows that the surface of the resulting material possesses θ-alumina characteristics, even though only the γ-alumina phase was detected by XRD. These results strongly suggest that the γ-to-θ 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 Al 3+ sites, formed on the (100) facets of the alumina surface, is strongly correlated with the thermal stability of γ-alumina.