TiO2, SnO2 and ZnO catalysts supported on mesoporous SBA-15 versus unsupported nanopowders in photocatalytic degradation of methylene blue

Semiconductor metal oxides (TiO2, ZnO or SnO2) deposited on a highly ordered mesoporous silica material (called SBA-15 in the literature) were compared for the first time as photocatalysts in the elimination of cationic methylene blue (MB) dye from neutral aqueous solutions by adsorption and/or photodegradation under ultraviolet (UV) irradiation. Prepared catalysts were characterized by N-2 physisorption, X-ray diffraction, UV-visible diffuse reflectance spectroscopy, ammonia temperature-programmed desorption, isoelectric point determination, scanning and transmission electron microscopy. Characterization results showed that the mesoporous structure of the SBA-15 was preserved in the catalysts. However, the dispersion of metal oxides in the catalysts was very different. Zinc oxide was well-dispersed in the ZnO/SBA-15 catalyst, whereas TiO2 and SnO2 crystalline phases were detected in the TiO2/SBA-15 and SnO2/SBA-15 samples, being SnO2 oxide the worst-dispersed. All SBA-15-supported catalysts showed high capacity for MB adsorption, especially the ZnO/SBA-15 one, and had a significantly higher (2-5 times) photocatalytic activity than the corresponding commercial TiO2, ZnO and SnO2 nanopowders. The TiO2/SBA-15 and ZnO/SBA-15 catalysts resulted in a higher degree of mineralization of MB dye (63%) than titania Degussa P25 (39%) in the photocatalytic degradation of MB after the adsorption step. This was attributed to the higher specific surface area of the SBA-15-supported catalysts, their higher adsorption capacity and better dispersion of the supported metal oxides in comparison with titania P25. In addition, deposition of ZnO on the SBA-15 surface prevented the photocorrosion of ZnO that is an important advantage compared to pure ZnO nanoparticles.