Multistimuli Activation of TiO2/α-Alumina Membranes for Degradation of Methylene Blue

TiO₂/α-Al₂O₃ porous membranes were prepared and tested under different stimuli, including UV and simulated solar irradiation, with or without the addition of hydrogen peroxide (H₂O₂), and by using Methylene Blue (MB) as a model organic pollutant to probe the oxidative catalytic activity of the membranes. An ultrathin TiO₂ layer was coated on porous α-Al₂O₃ substrates by combining a sol–gel process with a spin coating technique. Uniform TiO₂-coated alumina surfaces were obtained, which resulted in an increased pollutant adsorption on the semiconductor surface, a crucial requirement for achieving enhanced catalytic performances. The photocatalytic activity of these functionalized membranes was tested in a photocatalytic membrane reactor (PMR), by monitoring the photodegradation of MB in water. Under UV irradiation, 80% of MB degradation was achieved in 4 h. Concomitantly, the residual pollutant is completely retained by the membrane in the feed solution, and a pollutant-free permeate can be recovered. Noteworthily, the TiO₂/α-Al₂O₃ membranes displayed self-cleaning properties allowing their reuse in successive catalytic runs without reduction of their photocatalytic activity. Under irradiation (UV or solar light), the addition of H₂O₂ in the feed solution increased the efficiency of MB degradation. Furthermore, and for the first time, the ability of a TiO₂ coated membrane to perform catalytic oxidation in the presence of H₂O₂ was demonstrated in dark conditions. Enhanced membrane performances were obtained under solar light irradiation, expecially when the TiO₂/α-Al₂O₃ photocatalytic activity was synergically combined with the H₂O₂-assisted oxidative reaction, allowing the complete MB degradation in only 40 min. The excellent performance of these TiO₂/α-Al₂O₃ membranes under solar light was mainly ascribed to the absorption and in situ dye-sensitization of the thin TiO₂ layer, allowing the visible photon harvesting, as well as to the occurrence of lattice disorder and defects. These findings demonstrate that these catalytic membranes possess a great potential for the sunlight-driven degradation of organic compounds, thus meeting the requirements for future environmental applications.