DEVELOPMENT OF SURFACE-ACIDITY IN WATER-OXIDIZED ALUMINA FILMS - ETHYLENE ADSORPTION AS A FUNCTION OF PRETREATMENT TEMPERATURE

The structure and surface acidity of thin water-oxidized alumina films on Al(110) have been studied as a function of film treatment temperat ure and ethylene adsorption using X-ray photoelectron spectroscopy. Th e OH content of the oxides decreases monotonically with temperature wh ile the crystallinity increases uniformly to form a polycrystalline ga mma-alumina at 500 degrees C. Ethylene adsorbed on the oxides identifi es the presence of acid sites with a maximum spectral intensity for al umina films pretreated between 350 and 400 degrees C. An interpretatio n is proposed in terms of a molecular orbital adsorbate-surface model based on the core level and valence level photoemission data to explai n how chemisorbed molecular ethylene adsorbate may form from a a-compl ex adduct on the acid sites. The results are consistent with the forma tion of a (C2H5)-C-+ carbenium ion.