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J. Phys. Chem. 1996, 100, 15886-15889
Photocatalytic Oxidation of Ethanol: Isotopic Labeling and Transient Reaction Darrin S. Muggli, Sheldon A. Larson,† and John L. Falconer* Department of Chemical Engineering, UniVersity of Colorado, Boulder, Colorado 80309-0424 ReceiVed: February 27, 1996; In Final Form: June 10, 1996X
Transient reaction techniques were combined with isotope labeling to study the reaction steps for the roomtemperature, photocatalytic oxidation (PCO) of ethanol on TiO2. Carbon-13 labeled ethanol (CH313CH2OH) 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 R-carbon is preferentially oxidized and thus 13CO2 forms faster than 12CO2 at short illumination times. At longer times, the rates of 13CO2 and 12CO2 formation are nearly identical. The difference in behavior between 13CO2 and 12CO2 formation suggests two parallel reactions of ethanol, which may be due to two adsorption sites on TiO2.
Introduction Heterogeneous photocatalytic oxidation (PCO) has great potential for completely oxidizing organic pollutants in dilute waste streams. A wide range of organics can be destroyed photocatalytically1-11 at room temperature on TiO2 catalysts in the presence of UV or near-UV illumination, but the reaction mechanisms are poorly understood. The catalyst absorbs UV light, which excites electrons from the valence to the conduction band and creates a hole in the valence band of the semiconductor catalyst. The resulting electron/hole pairs can migrate to the catalyst surface and initiate redox reactions with organic pollutants. Ethanol was used in this study because it is a common solvent in spraying and coating applications and a major component of bakery and brewery emissions. Industrial gas emissions often contain contaminants in low concentrations (
