Preparation and characterisation of TiO_2 (anatase) supported on TiO_2 (rutile) catalysts employed for 4-nitrophenol photodegradation in aqueous medium and comparison with TiO_2 (anatase) supported on Al_2O_3

摘要

In this paper the preparation of two sets of polycrystalline photocatalysts prepared by supporting TiO_2 (anatase) on TiO_2 (rutile) or Al_2O_3 is reported. The powders were prepared by a wet impregnation method using titanium(IV) isopropylate. The impregnation was followed by thermal treatment and this procedure was repeated several times in order to obtain samples with progressively increasing amounts of TiO_2 (anatase). The photocatalytic activity of all of the specimens was tested by carrying out the 4-nitrophenol photodegradation, used as a "probe" reaction, in aqueous medium and in different kinds of batch photoreactors. The solids were characterised by X-ray diffraction (XRD), specific surface area (BET) and porosity determination, scanning electron microscopy observation (SEM) coupled with energy dispersive X-ray analysis (EDX), visible-ultraviolet diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR) monitoring of surface acidity. The characterisation results indicate an increase of TiO_2 anatase phase with the number of impregnations. The porosity and the surface area of all the photocatalysts prepared by supporting TiO_2 (anatase) on TiO_2 (rutile) were always higher than that of the bare support, while those of the samples prepared by supporting TiO_2 (anatase) on Al_2O_3 were lower than that of bare Al_2O_3. The FTIR measurements indicated that Lewis acid sites (Ti~(4+) or both Ti~(4+) and Al~(3+)) were present on the surface of the two sets of supported samples quite resistant to disaggregation when they were mixed in water. Moreover they showed to be easily decantable after the occurrence of the runs. Both sets of TiO_2 (anatase) supported samples resulted photoactive and the photoactivity increased by increasing the content of the anatase phase, although it was less significant than that of a pure home prepared and Degussa P25 TiO_2 samples, tested for the sake of comparison.