Role of Fe3+/Fe2+ as TiO2 dopant ions in photocatalytic degradation of carboxylic acids

The photocatalytic degradation of organic carboxylic acids by Fe-doped TiO2 has been studied. Results indicate that Fe is extracted by means of the formation of a [Fe-carboxylic acid](n+) complex by which the organic molecule is degraded. Fe2+ ions that remain in solution after the degradation go back to the catalyst surface due to their reaction with the photogenerated holes in the TiO2. This way the catalyst is reactivated becoming ready for a new degradation process. XPS studies have confirmed these results. Also, it has been observed that the catalyst preparation method affects the photoactivity of the formed complexes. The catalyst with lower Fe content and prepared from Fe(NO3)(3) by the incipient wetness impregnation method degrades the formic acid more readily than the undoped catalyst and the catalysts prepared by other methods. Complexes formed by means of the iron interaction with formic and maleic acids are more photoactive than those formed with acetic or acrylic acids. The slower degradation of these last acids could be related with the photo-Kolbe reaction or other reduction processes. FTIR studies have been conducted with the goal of identifying the intermediates generated from the interaction of the different acids with the catalysts surface. (C) 2002 Elsevier Science B.V. All rights reserved.