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

Abstract

TiO 2 /α-Al 2 O 3 porous membranes were prepared and tested under different stimuli, including UV and simulated solar irradiation, with or without the addition of hydrogen peroxide (H 2 O 2 ), and by using Methylene Blue (MB) as a model organic pollutant to probe the oxidative catalytic activity of the membranes. An ultrathin TiO 2 layer was coated on porous α-Al 2 O 3 substrates by combining a sol−gel process with a spin coating technique. Uniform TiO 2 -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 TiO 2 /α-Al 2 O 3 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 H 2 O 2 in the feed solution increased the efficiency of MB degradation. Furthermore, and for the first time, the ability of a TiO 2 coated membrane to perform catalytic oxidation in the presence of H 2 O 2 was demonstrated in dark conditions. Enhanced membrane performances were obtained under solar light irradiation, expecially when the TiO 2 /α-Al 2 O 3 photocatalytic activity was synergically combined with the H 2 O 2 -assisted oxidative reaction, allowing the complete MB degradation in only 40 min. The excellent performance of these TiO 2 /α-Al 2 O 3 membranes under solar light was mainly ascribed to the absorption and in situ dye-sensitization of the thin TiO 2 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.

Mesoporous TiO2 Thin Films: State of the Art

“…The photocatalyst activity of titania can also be evaluated via photocatalytic oxidation of methylene blue (MB) under UV illumination. MB is often used as model for recalcitrant azodye pollutant, and, being a highly colored organic molecule, its photodecomposition can easily be detected in situ through spectrophotometric methods [65].…”

WO3/Buckypaper Membranes for Advanced Oxidation Processes

“…In all cases WO 3 /BP membranes are able to efficiently degrade the water contaminants with a kinetic constant value of 0.085 ± 0.002, 0.064 ± 0.001, and 0.019 ± 0.001 min −1 , respectively. Such values are of the same order of magnitude or lower than the kinetic constants against the same pollutants, found with WO 3 , TiO 2 , or other catalyst nanoparticles dispersed either in the solutions or casted on carbon nanotubes, flakes of graphene oxide or porous polymer membranes [ 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 ], as no BP bearing photocatalysts, to the knowledge of authors, was ever proposed in literature.…”

Galvanic Replacement-Enabled Synthesis of In(OH)3/Ag/C Nanocomposite as an Effective Photocatalyst for Ultraviolet C Degradation of Methylene Blue

“…Methylene blue is a reference material used in the evaluation of the photocatalytic activity of inorganic materials under UV radiation as described in “ISO 10678:2010Determination of photocatalytic activity of surfaces in an aqueous medium by degradation of methylene blue”, which was last reviewed in 2017 and remains current. For visible-light-driven photocatalysis, methylene blue is also commonly employed to help gauge the photocatalytic performance of photocatalysts, especially when dye-assisted-sensitization could be achieved. − Sensitizing semiconductor/metal nanoparticles with dyes to harvest visible photons represents an excellent way to enhance the photocatalytic activity of a photocatalyst. In water, methylene blue exhibits a strong absorption peak at 664 nm (“red light”) and therefore may be employed as a sensitizer under solar radiation or visible light.…”

“…In water, methylene blue exhibits a strong absorption peak at 664 nm (“red light”) and therefore may be employed as a sensitizer under solar radiation or visible light. In 2017, Cabir et al successfully sensitized TiO 2 nanopowders with Cu-phthalocyanine, which helped enhance the photodegradation of methylene blue under visible light while Mastropietro et al employed in situ sensitization approach to improve the degradation rates of methylene blue over a TiO 2 /α-Al 2 O 3 photocatalyst under solar radiation. , In our system, methylene blue was employed as a model organic pollutant to probe the oxidative catalytic activity of In­(OH) 3 /Ag/C, but it was not acting as a sensitizing agent because methylene blue did not absorb UVC effectively and therefore not able to function as a sensitizer. It would be interesting to test the In­(OH) 3 /Ag/C photocatalyst under both UVC and visible light in the future for the purpose of performance optimization.…”