Molecular Water Adsorption and Reactions on alpha-Al2O3(0001) and alpha-Alumina Particles

The adsorption and reaction of water on single crystal alpha-Al2O3(0001) in ultrahigh vacuum, and alpha-alumina particles in ambient conditions, were investigated using temperature-programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), diffuse reflectance infrared Fourier transform spectra (DRIFTS), and other surface science techniques. For a water coverage of 1 H2O/(surface Al3+) on alpha-Al2O3(0001), no evidence for the surface hydroxyls expected from dissociative adsorption was observed in the infrared spectra, while the nu(2) vibration of molecular water was observed. Electron-stimulated desorption of molecular water at low coverages also indicated molecular or mixed (molecular plus dissociative) adsorption. Analysis of Kr TPD spectra showed that the water films wet the alumina substrate and suggested that the films were initially growing layer-by-layer. In contrast with the single crystal results, DRIFTS of water adsorption on alumina particles indicated the presence of surface hydroxyls that persist even after annealing to high temperatures in oxygen. For water on alpha-Al2O3(0001) at coverages less than 0.3 H2O/(surface Al3+), water desorbed over a broad temperature range extending from similar to 250 to 700 K. For larger coverages, water desorption occurred at temperatures between similar to 160 and 250 K, consistent with desorption of molecular water. 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.