Equilibrium adsorption of 11-tungstophosphate anion on different supports

The equilibrium adsorption on TiO2, SiO2and g-Al2O3 Spheralite and Akzo.ofwPW11O39x7y anion from solution in water was studied at 208C. The solutions were characterized by UV–visible spectroscopy, both before and after the contact with the supports, and the results indicated that the main species present was the undegraded anion. The solids so obtained were dried at 708C and then studied by X-ray diffraction, showing the same diffraction patterns as that of the supports. This may be due to a high dispersion of non-crystalline adsorbed species. By Fourier transform infrared and nuclear magnetic resonance spectroscopies of SiO2 or TiO2 impregnated withwPW11O39x7y anion, it was found that at 708C the species present are this anion without transformation or this anion together with the dimeric specieswP2W18O62x6y andwP2W21O71x6y, respectively. For the silica-supported sample, any important transformation of thewPW11O39x7y anion does not occur up to 4258C, meanwhile calcination until this temperature of titania-supported sample generates an increase of thewP2W21O71x6y species amount. InwPW11O39x7y on g-Al2O3 samples dried at 708C, diffuse reflectance and nuclear magnetic resonance spectroscopies allowed to observe a partial degradation of the anion only when alumina Spheralite was used. In the samples prepared on both aluminas, calcination at 4258C leads to total degradation of the present species. The results obtained were compared with those previously found when the same supports were impregnated with tungstophosphoric acid solution in ethanol–water. q1999 Elsevier Science B.V. All rights reserved