Ž . Journal of Molecular Catalysis A: Chemical 155 2000 121–129 www.elsevier.comrlocatermolcata Intramolecularly selective decomposition of surfactant molecules on photocatalytic oxidative degradation over TiO 2 photocatalyst Michitaka Ohtaki ) , Hirofumi Sato, Hiroyuki Fujii, Koichi Eguchi Department of Molecular and Material Sciences, Graduate School of Engineering Sciences, Kyushu UniÕersity, 6-1 Kasugakouen, Kasuga, Fukuoka 816-8580, Japan Abstract Investigation on oxidative mineralization of various surfactants over TiO photocatalyst reveals an anomalously retarded 2 degradation of the trimethylammonium moiety in cationic surfactants. Model reactions using tetraalkylammonium chlorides as substrates confirm that the stepwise behavior of the mineralization time courses is ascribed to intramolecularly selective decomposition caused by a considerably slow degradation rate of the methyl groups directly bound to the quaternary nitrogen. A strong inhibition effect of bromide or iodide anions preventing the complete mineralization is also revealed. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Semiconductor photocatalysis; Photoassisted mineralization; Photodegradation; Photodecomposition; Alkyltrimethylammonium surfactant 1. Introduction Serious environmental concerns have arisen for water pollution caused by organic surfac- tants widely used for household and industrial purposes, because biodegradation of surfactants is generally very slow and sometimes ineffec- w x tive 1–3 . Since depollution and bactericidal capabilities of TiO semiconductor photocata- 2 lysts have been attracting keen interest and have begun to be commercialized in our daily lives wx 4, degradation of toxic surfactants through light-driven oxidation by semiconductor photo- ) Corresponding author. Tel.: q 81-92-583-7465; fax: q 81-92- 573-0342; e-mail: ohtaki@mm.kyushu-u.ac.jp catalysis is expected to be promising as an inexpensive and environment-friendly alterna- wx tive for water purification 5 . The strong oxi- dizing power of TiO photocatalysts for degrad- 2 ing organic substances is mainly attributed to Ž POH and POOH radicals formed from water or y . OH and dissolved oxygen, respectively, by the electron-hole pair generated by photoexcita- tion of valence electrons across the band gap of w x TiO as shown in Fig. 1 5,6 . 2 Mineralization is conversion of carbon atoms in organic compounds into inorganic carbon species, which is usually CO . Photomineraliza- 2 tion of organic substances by semiconductor photocatalysis has been investigated for a num- w x ber of well-known and potential pollutants 5–8 . 1381-1169r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S1381-1169 99 00325-8