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

Abstract

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 ∼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 ≤1ppb by inductively coupled plasma mass-spectrometry (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 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 11.8nm and 23.9nm. The RR@TiO2Cu presented agglomerate sizes of ∼200nm for TiO2 and for the Cu aggregates sizes of ∼20nm. 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).