Photocatalytic Efficiency of Zeolite-Based TiO2 Composites for Reduction of Cu( II): Kinetic Models

This study evaluates the photoactivity of novel zeolite-based TiO2 composites modified with metal particles for the photoreduction of Cu2+, depending on the zeolite and metal particles (Ag or Fe) content and to establish the photoreduction mechanism. The composites have been prepared by incorporating zeolite particles in the sols during the sol-gel preparation of the TiO2, followed by impregnation of the gels in solutions containing metal ions. The zeolite-based TiO2 composites have been characterized using X-ray diffraction, Raman and FTIR spectroscopy techniques and TEM microscopy. The role of zeolite particles in the photoreduction process was demonstrated by using different kinetic models. The apparent constant rate for photoreduction increases in direct proportion with the content of zeolite and is higher for the composites modified with Fe than for those modified with Ag nanoparticles. The results demonstrate that composites have a higher photocatalytic efficiency than TiO2. The experiments in this study reveal that the migration of Cu2+ in solution occurs rapidly and that the diffusion through the film on the composite's surface and, subsequently, the photoreduction process are the rate limiting steps. The role of the zeolite in photoreduction is to diminish the recombination rate of the charge carriers.