An infrared spectroscopic investigation of thin alumina films: measurementof acid sites and surface reactivity

The surface chemical activity of an alumina films grown on Mo(100) by oxida tion of aluminum evaporated onto the surface and oxidized using water is ex amined using Auger, X-ray photoelectron and reflection/absorption infrared spectroscopies. The formation of alumina is confirmed using Auger and X-ray photoelectron spectroscopy from the positions and intensities of the alumi num features and using reflection-absorption infrared spectroscopy from the longitudinal optical modes of the Al-O bonds measured at similar to 935 cm (-1). The presence of surface hydroxyls is monitored by forming films using D2O which are evidenced by a feature at similar to 2700 cm(-1). Ammonia ad sorption on a dehydroxylated surface yields a single peak at 1260 cm(-1) du e to ammonia adsorbed at a surface Lewis site where the principle symmetry axis of ammonia is oriented perpendicularly to the surface plane. Ammonia a lso appears to adsorb at Lewis sites on a hydroxylated surface with a sligh tly different adsorption geometry from that on a dehydroxylated surface. Fi nally, the chemistry of trimethyl aluminum adsorbed on the planar hydroxyla ted alumina surface is compared with that found on high-surface-area gamma- alumina where the spectra and the chemistry found in both regimes is exactl y identical except that the low-frequency methyl bending modes (at 769 and 718 cm(-1)) are not obscured on the thin film by the intense substrate wher eas they are on the high-surface-area support. (C) 1999 Elsevier Science B. V. All rights reserved.