Hydroxylation of metal-supported sheet-like silica films

Adsorption of water on a metal-supported sheet-like silica film was studied by infrared reflection absorption spectroscopy (IRAS) and temperature- programmed desorption (TPD). As expected, the silica surface is essentially hydrophobic. Hydroxo species, primarily in the form of isolated silanols (Si-OH), were observed only upon water condensation at low temperatures and subsequent heating above 200 K. The amounts of silanol species account for less than a few percent of the surface Si atoms, and they are found to be thermally stable up to 900 K. Isotopic experiments showed that hydroxyls form almost exclusively from the adsorbed water molecules and do not undergo scrambling with the lattice oxygen atoms upon heating. Steps within the silica sheet, due to a terraced topography and/or the presence of "holes", are proposed as the active sites for hydroxylation. The acidic properties of silanol species were studied with CO and NH₃ as probe molecules. In the case of ammonia, an H-D exchange reaction was observed between OD species and NH ₃, and the same reaction was found to occur for OD(OH) and H ₂O(D₂O), respectively. The results are compared with those reported in the literature for amorphous silica.