Photocatalytic oxidation of ethanol: Isotopic labeling and transient reaction

Transient reaction techniques were combined with isotope labeling to study the reaction steps for the room-temperature, photocatalytic oxidation (PCO) of ethanol on TiO2. Carbon-13 labeled ethanol ((CH3CH2OH)-C-13) was adsorbed on the catalyst and photocatalytically oxidized in the absence of gas-phase ethanol. The amounts of species remaining on the surface after PCO were determined by temperature-programmed oxidation. During PCO, only CO2 and H2O formed for low coverages of ethanol, whereas acetaldehyde also desorbed for saturation coverage. Acetaldehyde forms rapidly from ethanol oxidation during PCO. At both low and high ethanol coverages, the alpha-carbon is preferentially oxidized and thus (CO2)-C-13 forms faster than (CO2)-C-12 at short illumination times. At longer times, the rates of (CO2)-C-13 and (CO2)-C-12 formation are nearly identical. The difference in behavior between (CO2)-C-13 and (CO2)-C-12 formation suggests two parallel reactions of ethanol, which may be due to two adsorption sites on TiO2.