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 temperature and ethylene adsorption using X-ray photoelectron spectroscopy. The OH content of the oxides decreases monotonically with temperature while the crystallinity increases uniformly to form a polycrystalline [gamma]-alumina at 500[degree]C. Ethylene adsorbed on the oxides identifies the presence of acid sites with a maximum spectral intensity for alumina films pretreated between 350 and 400[degree]C. An interpretation is proposed in terms of a molecular orbital adsorbate-surface model based on the core level and valence level photoemission data to explain how chemisorbed molecular ethylene adsorbate may form from a [pi]-complex adduct on the acid sites. The results are consistent with the formation of a [sup +]C[sub 2]H[sub 5] carbenium ion. 31 refs., 10 figs., 3 tabs.