Photocatalytic conversion of acetate into molecular hydrogen and hydrocarbons over Pt/TiO2: pH dependent formation of Kolbe and Hofer-Moest products

The photocatalytic generation of H-2 and hydrocarbons from aqueous acetic acid has been studied employing TiO2 P25-based photocatalyst particles. The effect of pH on the distribution of reaction products and their formation rates during the photochemical as well as the photoelectrochemical oxidation of acetic acid has been investigated. The photocatalytic formation rates for H-2 and hydrocarbons were found to be higher for Pt-loaded TiO2 than for bare TiO2. The major reaction products resulting from the photocatalytic decomposition of aqueous acetic acid, as determined quantitatively in the gas phase, were found to be H-2, CO2, and CH4. Furthermore, traces of C2H6, C3H8, CO, CH3CHO, HCHO, CH3OH, C2H5OH, and HCOOH were also detected. After 15 h of illumination, the average formation rates of H-2, CO2, CH4, and C2H6 evolved at pH 2 were found to be 22, 65, 35, and 2 mu mol/h, respectively. The ratio of H-2 to hydrocarbons strongly depends on the pH values, i.e., at acidic pH the ratio of H-2 to CH4 formation was found to be 0.6. On the contrary, at neutral and basic pH values negligible amounts of hydrocarbons were formed and H-2 was found to be the main product with formation rates of 12 and 5 mu mol/h at pH 7 and 11, respectively. It is therefore assumed that the hydroxide ion has a significant effect on the reaction pathways. Due to the fact that methanol and ethanol are formed as reaction products, water or hydroxide ions are apparently required for the formation of the major oxidizing agent, that is the hydroxyl radical. Herein, a detailed mechanism for the photocatalytic decomposition of acetic acid at different pH values is presented. (C) 2017 Elsevier Inc. All rights reserved.