Horikoshi Satoshi

Anionic (DBS), cationic (CTAB, BTDAC, C₁₂-PC), non-ionic (C₁₂E5, N-DHA) and amphoteric (C₁₂-betaine, C₁₂-amidobetaine, C₁₂-HAA) surfactants were photodegraded in TiO₂ semiconductor suspensions under UV irradiation. Dependence of the rate of photodegradation on the chemical structure was investigated based on temporary variation of total organic carbon (TOC) in photodegraded solution. Differences in the photocatalytic effect of other metal-oxide catalysts such as ZnO, W03, TiO₂ (anatase or rutile), TiO₂ (UV-100) and TiO₂ (T-805) were also examined. TiO₂ (anatase) and Zn0 showed superior catalytic activity. TOC decreased with increase in irradiation time, since the surfactants were oxidized with the consequent evolution of CO₂ gas. TOC in solution after sonication without exposure to UV was less than that of the starting solution, indicating that surfactant to be preadsorbed on the TiO₂ surface. The rate of photodegradation of the anionic surfactants was greater than for the nonionic, cationic and amphoteric surfactants. The increase in concentration caused decrease in the photodegradation rate. From the results of laser flash photolysis using colloidal TiO₂ and benzene sulfonate (BS), the surfactant is implicated in the scavenging of the charge carrier, thereby producing either a ground-state BS⁺ cation radicals or ·OH adducts with the benzene ring of BS.

Effects of such n-type semiconductors as TiO₂ (anatase, rutile or surface-modified TiO₂ and Pt-loaded TiO₂), ZnO, WO₃ and MoS₂ on the photodegradation of anionic DBS and cationic BDDAC surfactants, and related model compounds were examined in the present study. The anatase form of TiO₂ showed greater photocatalytic activity than the rutile form. Noble-metal loaded catalysts (e. g., TiO₂/Pt) were less photooxidative than the naked anatase TiO₂ catalyst, possibly due to suppression of electron transfer of O₂ to give O₂, namely photoreduction. The ZnO semiconductor catalyst decomposed DBS more rapidly than anatase Ti0₂. The photocatalytic activity of the other semiconductors examined (WO₃ and MoS₂) was only slight in each case.