Photooxidation of ethanol and 2-propanol on TiO₂(110): Evidence for methyl radical ejection

The photooxidation of ethanol and 2-propanol was studied under UHV conditions on a single crystal TiO₂(110) surface using a combination of temperature programmed desorption (TPD) and pump-probe laser ionization techniques. Previous studies of these reactions have shown that the first step involves photocatalytic dehydrogenation to either an acetaldehyde or acetone intermediate. In this work, we show that when adsorbed alcohols are irradiated with UV light in the presence of molecular oxygen, methyl radicals are ejected from the surface. Furthermore, it is shown that these radicals possess kinetic energy distributions which are remarkably similar to those measured for the photooxidation of acetaldehyde and acetone. This result suggests that methyl radicals are produced during a second photocatalytic step which involves photooxidation of the aldehyde/ketone intermediates.