Molybdophosphoric acid adsorption on titania from ethanol-water solutions

The equilibrium adsorption at 20 degrees C of molybdophosphoric acid solutions, using ethanol-water as solvent, on titania was studied. The molybdenum adsorption isotherm showed a sigmoidal shape; low values of molybdenum adsorbed were observed for final equilibrium concentrations lower than 50 mg Mo/ml, and for higher concentrations, the adsorbed molybdenum amount almost reached a plateau. From this isotherm it could be concluded that the solute-support interaction was not strong. UV-visible and NMR spectra of the solutions before and after the adsorption on titania showed that the species PMo12O403- was present. This species also was observed by DRS in the wet samples and by NMR, FT-IR, and DRS, in the solid samples dried at room temperature and calcined at 255, 310, 365, and 425 degrees C, showing that the thermal stability of molybdophosphoric acid adsorbed on titania is similar to that of the bulk acid. The impregnating solutions and the impregnated solid changed with the time to a bluish color as a consequence of the formation of heteropoly blues which presented Mo6+ partially reduced to Mo5+. The XRD patterns indicated that the species adsorbed onto the support surface are highly dispersed like a noncrystalline form. (C) 1998 Academic Press.