Photooxidation Pathway of Sulforhodamine-B. Dependence on the Adsorption Mode on TiO₂ Exposed to Visible Light Radiation

The temporal course of the photooxidation of sulforhodamine-B (SRB) in aqueous media illuminated by visible wavelengths in the presence of TiO2 has been examined to determine the nature of the intermediate species produced and to explore the operative reaction pathway(s). Two pathways are described to account for the differences in the final photooxidation products whose nature depends on the different modes of adsorption of the dye on the metal-oxide mediator. In the SRB/TiO2 system, when SRB is adsorbed on the positively charged TiO2 particle surface through a sulfonate group cleavage of the SRB chromophore structure predominates and N-de-ethylation occurs only to a slight extent with the major photooxidation products being diethylamine and carbon dioxide. In the presence of the anionic dodecylbenzenesulfonate surfactant DBS, when SRB is near the negatively charged DBS/TiO2 interface through the positive diethylamine group N-de-ethylation occurs preferentially before destruction of the structure with the major products being acetaldehyde and carbon dioxide.