Cu-decorated Raschig-TiO2 rings inducing MB repetitive discoloration without release of Cu-ions under solar light

The methylene blue (MB) discoloration on innovative photocatalysts made up of Raschig-rings coated by pre-formed TiO2 nanoparticles decorated with CuOx (RR@TiO2-Cu) is presented. The MB-discoloration was drastically accelerated by the subsequent addition of 0.004% Cu. This Cu-content being similar to 3000 times below the TiO2 present in the RR@TiO2 samples led to MB-discoloration in shorter times. The Cu-ions released from the RR@TiO2-Cu 0.004% were determined 0.018% decreased the MB-discoloration kinetics due to the Cu acting as recombination sites for the photogenerated charges. The total organic carbon (TOC) reduction concomitant with the MB-discoloration is reported as well as the effect of the initial MB-concentration and pH on the discoloration kinetics. The Cu-addition shifted significantly the TiO2 absorption into the visible region even when added in sub-microgram quantities (0.004%) to RR@TiO2 as detected by diffuse reflectance spectroscopy (DRS). The Cu level added to RR@TiO2 was too low to be detected by X-ray diffraction (XRD). The roughness of the RR@TiO2-Cu (0.018%-0.081% Cu) presented Rg-values between 11.8 nm and 23.9 nm. The RR@TiO2Cu presented agglomerate sizes of similar to 200 nm for TiO2 and for the Cu aggregates sizes of similar to 20 nm. The MB discoloration kinetics and reuse of the RR@TiO2-Cu suggest the potential as a supported catalyst in environmental cleaning process. A reaction mechanism is suggested taking into consideration the role of the Cu1+/2+ intra-gap states involving redox catalysis as detected by X-ray photoelectron-spectroscopy (XPS). (C) 2016 Elsevier Ltd. All rights reserved.