Equilibrium adsorption of 11-tungstophosphate anion on different supports

The equilibrium adsorption on TiO2, SiO2 and gamma-Al2O3 (Spheralite and Ak zo) of [PW11O39](7-) anion from solution in water was studied at 20 degrees C. The solutions were characterized by UV-visible spectroscopy, both befor e and after the contact with the supports, and the results indicated that t he main species present was the undegraded anion. The solids so obtained we re dried at 70 degrees C and then studied by X-ray diffraction, showing the same diffraction patterns as that of the supports. This may be due to a hi gh dispersion of non-crystalline adsorbed species. By Fourier transform inf rared and nuclear magnetic resonance spectroscopies of SiO2 or TiO2 impregn ated with [PW11O39](7-) anion, it was found that at 70 degrees C the specie s present are this anion without transformation or this anion together with the dimeric species [P2W18O62](6-) and [P2W21O71](6-), respectively. For t he silica-supported sample, any important transformation of the [PW11O39](7 -) anion does not occur up to 425 degrees C, meanwhile calcination until th is temperature of titania-supported sample generates an increase of the [P2 W21O71](6-) species amount. In [PW11O39](7-) on gamma-Al2O3 samples dried a t 70 degrees C, diffuse reflectance and nuclear magnetic resonance spectros copies allowed to observe a partial degradation of the anion only when alum ina Spheralite was used. In the samples prepared on both aluminas, calcinat ion at 425 degrees C leads to total degradation of the present species. The results obtained were compared with those previously found when the same s upports were impregnated with tungstophosphoric acid solution in ethanol-wa ter. (C) 1999 Elsevier Science B.V. All rights reserved.