Photocatalytic degradation of caffeine: Developing solutions for emerging pollutants

The photocatalytic degradation of the psychoactive substance caffeine was studied using composites prepared with multi-walled carbon nanotubes and three different TiO2 materials: one synthesized by a modified sol–gel method and two others obtained from Evonik and Sigma–Aldrich. These materials were characterized by several techniques (e.g., DRIFT, XRD, N2 adsorption–desorption isotherms, TEM, SEM). The tested materials increased the caffeine degradation rate and the oxygenated groups created by acid treatment on the surface of the carbon nanotubes were crucial for the photocatalytic activity of all prepared composites. In addition, the photocatalytic activity of TiO2 from Sigma–Aldrich markedly increased with the addition of functionalized carbon nanotubes, which seems to be related with the larger TiO2 crystallite sizes and the better contact of these TiO2 particles with carbon nanotubes. Selective trapping of photogenerated holes and radicals by EDTA and tert-butanol shows that photogenerated holes are crucial on the photodegradation pathway but free radicals produced by photoexcited electrons do also participate in the mechanism and seem to be responsible for the higher activity observed for composites prepared with functionalized carbon nanotubes and TiO2 from Sigma–Aldrich in comparison to TiO2 alone. Films prepared with these composites revealed higher photocatalytic activity than films of TiO2 from Evonik.