Laponite immobilized TiO2 catalysts for photocatalytic degradation of phenols

Abstract Laponite immobilized titania catalysts were prepared by a pillaring process and by hydrothermal synthesis (HT) applying different titania sources such as TiCl4 and TiOSO4. Textural investigations (XRD, TEM, N2 physisorption) evidenced that by the pillaring procedure a high specific surface area (˜450 m2/g) mesoporous composite with 5-6 nm sized anatase nanoparticles were formed retaining the morphology of parent laponite structure. In contrast, by hydrothermal treatment with titanium oxysulfate the initial laponite structure was destroyed and a more opened nanoporous silica/titania material was formed with bigger, about 14 nm anatase particles. FT-IR spectroscopic investigations revealed the different acidic character of titania/laponite composite samples showing stronger Lewis and weak Bronsted acid sites on both catalysts. However, acidic centers in titania pillared laponite stem from Ti–O–Si bonds, whereas in HT sample from the separated, ionic, surface sulfate species on titania. Catalytic activity of titania/laponite composites were tested in photo-oxidation of model 10–5 M phenol and 2,4,6-trichlorophenol (TCP) water solutions. Catalytic tests were carried out in a home constructed batch-type photo-reactor with oxygen bubbling, and applying commercial low pressure Hg lamps emitting UV-light at 254 nm and 361 nm. Catalytic results showed that utilization of titania/laponite catalysts enhanced the photo-oxidation activity. Hydrothermally prepared sample showed much better catalytic performance than titanium chloride pillared one, probably due to the bigger titania particles and the more opened mesoporous structure of titania/laponite HT, and moreover to the peculiar surface acidic properties of sulfated titania species. Separation of catalysts from reaction media even in tap water was much easier than that of commercial titania, i.e. by self-settling.