Early stages in the TiO2-photocatalyzed degradation of simple phenolic and non-phenolic lignin model compounds

The TiO2-photocatalyzed degradation of phenolic and non-phenolic benzyl alcohols, which can be considered simple lignin model compounds has been studied in aqueous solution under a variety of conditions, with the aim of acquiring fundamental information on the early stages of the degradation process. The role of the electron scavengers O-2, K2S2O8 and N2O on the degradation process as well as the influence of pH have been investigated. A major role in the degradation process is played by (OH)-O-. and O-2(.-), while SO4.- plays a minor role. (OH)-O-. plays a fundamental role in the conversion of non-phenolic substrates into phenolic ones, while O-2(.-) exerts its role mainly by reacting with phenoxyl radicals to give aromatic ring-opening products. Addition of K2S2O8 as electron scavenger allows substantial substrate degradation also in the absence of O-2, while N2O which is an excellent scavenger for the hydrated electron seems to be a very poor electron scavenger under the heterogeneous conditions employed. A general mechanistic scheme which proceeds through the formation of aromatic radical cations via one-electron oxidation of the neutral substrates by TiO2 (h(+)), accounting for all the experimental observations has been proposed. (C) 2004 Elsevier B.V. All rights reserved.