Cu-decorated Raschig-TiO 2 rings inducing MB repetitive discoloration without release of Cu-ions under solar light

Abstract

A B S T R A C TThe methylene blue (MB) discoloration on innovative photocatalysts made up of Raschig-rings coated by pre-formed TiO 2 nanoparticles decorated with CuOx (RR@TiO 2 -Cu) is presented. The MB-discoloration was drastically accelerated by the subsequent addition of 0.004% Cu. This Cu-content being $3000 times below the TiO 2 present in the RR@TiO 2 samples led to MB-discoloration in shorter times. The Cu-ions released from the RR@TiO 2 -Cu 0.004% were determined 1ppb by inductively coupled plasma massspectrometry (ICP-MS). Cu-percentages >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 TiO 2 absorption into the visible region even when added in sub-microgram quantities (0.004%) to RR@TiO 2 as detected by diffuse reflectance spectroscopy (DRS). The Cu level added to RR@TiO 2 was too low to be detected by X-ray diffraction (XRD). The roughness of the RR@TiO 2 -Cu (0.018%-0.081% Cu) presented Rg-values between 11.8 nm and 23.9 nm. The RR@TiO 2 Cu presented agglomerate sizes of $200 nm for TiO 2 and for the Cu aggregates sizes of $20 nm. The MB discoloration kinetics and reuse of the RR@TiO 2 -Cu suggest the potential as a supported catalyst in environmental cleaning process. A reaction mechanism is suggested taking into consideration the role of the Cu 1+/2+ intra-gap states involving redox catalysis as detected by X-ray photoelectron-spectroscopy (XPS).