XXVIII. Rotating Ring-Disk Electrode Studies of Photo-oxidation of Acetate and Iodide at n-TiO~

The photo-Kolbe reaction (electro-oxidation of acetate) on TiO2 in aqueous solution has been studied by means of a rotating ring-disk electrode (RRDE) with a TiO2 disk and Pt ring. The oxidation of acetate competes very efficiently with water for photogenerated holes on the illuminated n-TiO2 disk as shown by a decrease in the collection efficiency for oxygen reduction. No oxidation product of acetate was detected on the ring. Competitive oxidation of iodide with acetate and water was also investigated. Equations for determining relative rate constants for competitive reactions are derived and the applicability of the RRDE in studying such systems discussed. Since the ini t ial studies of the photo-oxidation of water at n type TiO2 (ruti le) electrodes by Fuj i sh ima and Honda (1), investigations of this and many other reactions on this mater ia l have been reported. For example, the oxidation of many other substantces (e.g., iodide, bromide, and cyanide ions) can be carried out at n-TiO2 in competit ion with water oxidation (2, 3). Of par t icular recent interest has been the photo-Kolbe reaction in which a carboxylic acid is photo-oxidized (4, 5). For example, previous studies from this laboratory have established that in aqueous solutions conta in ing acetate ( A c ) / a c e t i c acid (HAc) mixtures i rradiat ion of an n-TiO2 electrode leads almost exclusively to the oxidation of acetate CHACO2+ h + "-> 89 + CO2 [I] (where h + represents a hole photogenerated at the TiO2 surface). Photocatalytic decomposition of acetate (predominantly to methane and COs) has also been carried out by irradiation of suspensions of platinized TiO2 (6, 7). While total product yields and current-potential (i-E) curves have yielded information about the overall course of the reaction (5) and electron spin resonance spin trapping experiments have established the intermediacy of methyl radicals in the reaction (8), studies of the relative rates of acetate vs. water oxidation have not been reported. We describe here an investigation of acetate oxidation at a TiO2-disk Pt-ring rotating ring-disk electrode (RRDE). In this research, products generated by photo-oxidation at the n-TiO2 disk are swept to the Pt ring where they are detected. Relative values of the disk current (iD) and the ring current (JR) are then employed to obtain information on reaction rates. Previous photoelectrochemical studies employing the RRDE have proven very useful in elucidating reaction paths, relative reaction rates, and semiconductor stability (2, 9-13). In work closely related to that reported here, a TiO2' disk RRDE was employed to study photo-oxidation of halide ions and reducing agents (2, 9), Experimental Chemicals.--Sodium sulfate, sodium acetate, glacial acetic acid (Fisher Scientific Company) , and sodium iodide (Mallinckrodt Chemical Works) were of reagent grade. Tr iply distilled water was used as a solvent. Apparatus and procedure.--A Tascussel Electronique Bipotentiostat, Model Bipad 2, was used for all experiments. A Wavetek funct ion generator provided a * Electrochemical Society Active Member.