Photocatalytic degradation of butanoic acid.: Influence of its ionisation state on the degradation pathway:: comparison with O3/UV process

In an attempt to improve our understanding on the basic mechanisms of the degradation of aqueous organic pollutants by TiO2-based photocatalysis, butanoic acid was selected, especially because it does not react with O-2(.-) and it has several hydrogen atoms able to be abstracted by OH. radicals. This study has been based on the identification and the quantification of the intermediate products of butanoic acid either by TiO2 photocatalysis (pH=3.6 and 6.9) or by a homogeneous process (O-3/UV, pH=3.6) which forms OH. radicals. In the latter case, acetic and butanedioic acids are the two main intermediates detected and, in a lesser extent, 3- and 2-oxobutanoic, oxalic and formic acids. TiO2-based photocatalysis also yielded acetic acid, but an abstraction of one hydrogen atom, preferentially in beta position, and a decarboxylation reaction also occurred giving 35% of 5-oxobutanoic acid and 10% of propanoic acid, after 10 min. The surface of titania and the holes (h(+)) are suggested to explain the differences observed between photocatalysis and O-3/UV processes. TiO2-based photocatalysis at neutral pH showed an increase in formic and butenoic acids formation as well as that of 2-oxobutanoic and oxalic acids which were not observed at acidic pHs. These results with the comparison of the kinetics at both pH have confirmed the participation of h(+) species and the role of the adsorption step in photocatalytic processes. The role of surface hydrogen atom was suggested by taking into account the decrease in the formation of propane and the formation of butenoic acid at pH=6.9. A mechanistic route is proposed for the photocatalytic degradation of butanoic acid. (C) 2000 Elsevier Science S.A. All rights reserved.