TiO2-loaded zeolites and mesoporous materials in the sonophotocatalytic decomposition of aqueous organic pollutants:: the role of the support

The present work investigates Hombikat titania, TiO2-loaded USY and beta-zeolites, and TiO2-loaded MCM-41 molecular sieve as catalysts for the UV-assisted aqueous sonophotochemical destruction of salicylic acid. We found that the hydrophilicity of the support and catalyst is an important property with determining role for photocatalytic performance. More specifically, the hydrophilicity of p-zeolite does not change even after loading of titania, but it was significantly increased in the case of USY, and significantly decreased in the case MCM-41 after titania loading. However, titania loaded MCM-41 exhibited the highest specific activity per gram of titania. It was also found that the presence of ultrasonic field enhances the rate of photodegradation for the catalysts employed. The enhancement due to ultrasound observed for the supported titania was found to be more significant than that for neat titania for photodestruction of salicylic acid. Our results indicate that for both zeolites and MCM-41 there is a threshold of specific energy that should be provided in the reaction medium. This threshold is about 0.1 kW/1 and operation under this volume will ensure structural stability of these molecular sieves. We employed two types of single-stage oxidation reactions to investigate the enhancement of radical generation by using composite catalysts. The effect of supports on the hydroxyl radicals produced by the titania (tested by HCOOH) was found to be minimal. The cumulative generation of (OH)-O-. and H2O2 species (as tested by Fricke dosimeter solution) was indeed affected by the presence of a support. Fricke dosimetry experiments showed that the titania loaded R-zeolite generated a larger amount of hydroxyl ((OH)-O-.) and peroxide radicals (HO2.) than other zeolites generated. Surprisingly, the same catalyst was inactive for the destruction of salicylic acid. The mass transfer diffusion limitations were found even for the larger pores of MCM-41. (C) 2002 Elsevier Science B.V. All rights reserved.