The reaction of ethanol with an aluminum oxide surface studied by inelastic electron tunneling spectroscopy

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Abstract
Inelastic electron tunnelingspectroscopy (IETS) has been employed to study the vibrational structure of ethanol reacting with a clean alumina surface as a function of temperature from 295 to 575 K. The spectra for the surface species up to approximately 450 K correspond to infrared spectra of aluminum ethoxide, Al(OC2H5)3. Between 450 and 475 K the structure changes from an ethoxide to an acetate, the most obvious spectral change being the appearance of a new peak at approximately 1585 cm−1 (197 meV) which is characteristic of the asymmetric OCO stretching mode of various inorganic acetates. No further changes are observed above 475 K. Above 370 K, there is also a significant increase in the total concentration of adsorbed species. This is due to a partial dehydration or dehydroxylation of the surface, thus freeing active sites which had previously been blocked by water or hydroxyl groups formed during the initial stages of ethanol adsorption. This dehydration or dehydroxylation of the surface is simultaneously accompanied by a transformation of the bulk aluminum oxide to aluminum hydroxide. While IETS has been applied successfully in the past to study a wide variety of adsorbate–adsorbent systems, here tunnelingspectroscopy is demonstrated explicitly to be effective in monitoring quantitatively the structural changes occurring in a reacting system at low pressures as a function of temperature.