Preparation and photocatalytic behavior of Zn/A1/W(Mn) mixed oxides via polyoxometalates intercalated layered double hydroxides

Zn/Al/W(Mn) mixed oxides were prepared via calcination of the precursors of monovacant and monosubstituted Keggin type polyoxometalate (POM) intercalated layered double hydroxides (LDHs) such as Zn2Al(OH)(6)-[SiW11O39](0.12) . 0.2H(2)O and Zn2Al(OH)(6)[SiW11O39Mn(H2O)](0.16) . 0.3H(2)O at 600-700 degreesC. The physicochemical properties of the products were characterized by the methods of powder X-ray diffraction, elemental analysis, scanning electron microscopy, UV diffusion reflectance spectroscopy, infrared spectroscopy, thermogravimetric analysis, electron spin resonance and N-2 adsorption-desorption measurements. Phase changes during the process of calcination of the POM containing LDH precursors were studied, indicating that the precursors decomposed at 400 degreesC and formed ZnO- and Al2O3-like phases together with an X-ray amorphous phase, and then yielded a mixture of well-crystallized phase of ZnAl2O4 formal spinel and ZnWO4 inverse spinel at 600-700 degreesC. As for the thermally decomposed products, the changes of BET surface areas and pore sizes are related to their crystallinity and structures. Compared with the POM-LDH precursors, the mixed oxides thus obtained exhibited higher photocatalytic activity to degrade aqueous organochlorine pesticides hexachlorocyclohexane. (C) 2002 Elsevier Science Inc. All rights reserved.