Paper SS+HC+MI-TuA10
Molecular Water Adsorption and Reactions on α-Al₂O₃(0001) and α-Alumina Particles

Tuesday, October 23, 2018, 5:20 pm, Room 203C

Alumina and its interactions with water are important in areas ranging from electronics and catalysis to environmental science. However, a basic understanding of the adsorption and reactions of water on even the simplest alumina surface, the (0001) surface of α-alumina, remains elusive. We have investigated the adsorption and reaction of water on single crystal, α-Al₂O₃(0001) in ultrahigh vacuum, and α-alumina particles in ambient conditions, using temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and other surface science techniques. For a water coverages of 1 and 2 H₂O/(surface Al³⁺) on α-Al­₂O₃(0001), no evidence for the surface hydroxyls expected from dissociative adsorption was observed, while the ν₂ vibration of molecular water was observed. Electron-stimulated desorption of molecular water at low coverages also indicated molecular or mixed (molecular plus dissociative) adsorption. In contrast with the single crystal results, IR spectra of water adsorption on alumina particles indicated the presence of surface hydroxyls that persist even after annealing to high temperatures in oxygen. The results, which are consistent with at most a small amount of water dissociation on the Al-terminated (0001) surface, are difficult to reconcile with calculations suggesting that the barrier to dissociation is small. However, the results are consistent with recent vibrational sum frequency experiments showing that the hydroxylation of the Al-terminated (0001) surface takes many days even at ambient pressures and temperatures.