Photocatalytic bleaching of aqueous malachite green solutions by UV-A and blue-light-illuminated TiO2 spherical nanoparticles modified with tungstophosphoric acid

J. RENGIFO-HERRERA; M. BLANCO; L. PIZZIO

APPLIED CATALYSIS B-ENVIRONMENTAL

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2011 vol. 110 p. 126 - 132

0926-3373

Visible light-responsive spherical TiO2 nano-particles were prepared by sol-gel method adding urea as pore forming agent and tungstophosphoric acid (TPA) in different ratios (20-30%) and annealing at 500 ºC for 2 h. Visible light absorption is probably due to WOx formation coming from the partial degradation of TPA and evidenced by XPS spectra.  Besides RMN 31P and XPS results showed evidence about the existence of unaltered TPA within and TiO2 nano-particles and on its surface after of annealing step.  Adding of TPA not only affected the optical properties of materials but produced also a strong decreasing of their pH of zero point charge (pHpzc). The photocatalytic activity of synthesized powders was tested using malachite green-oxalate (MG) as chemical target. Results obtained under UV-A irradiation revealed that MG aqueous solutions were efficiently bleached through oxidative process of N-de-methylation using the TiO2 powder containing 30% of TPA (TiO2-TPA-30%); this activity was higher than powders containing 20% (TiO2-TPA-20%) and 0% (TiO2-TPA-0%) of TPA and even than Degussa P-25, probably due to unaltered TPA within or in surface TiO2 nano-particles may act as an efficient trap of photo-induced e-CB. On the other hand, when blue light irradiation was used, TiO2-TPA-30% powder revealed also a highest photocatalytic bleaching and also as the UV light results, MG was oxidized through N-de-methylation processes. Finally, blue light experiments carried out under N2 atmospheres showed that MG aqueous solutions were not bleached by TiO2 powders containing TPA since probably photo-induced e-CB trapped on TPA sites should not be efficiently transferred to MG molecules.