The hydrolysis behavior of vanadium species in aqueous solutions and their adsorption on alumina, silica and titania surfaces

Abstract

Acoustophoresis was applied to study reactions of vanadium species in aqueous solutions both in presence and in absence of counter ions, specifically phosphate-ions. Interaction of hydrolyzed vanadium species, with a defined average valence state and either or not in the presence of phosphate-ions, with silica, alumina and titania has also been examined with the objective to develop a method for the preparation of dispersed supported vanadium-phosphorus oxide catalysts. With titania and alumina, phosphate-ions specifically adsorbed on the surface, resulting in a shift of the isoelectric point, for titania from a pH of 7 to 1.8, and alumina from pH>8 to 5.5. Quantification of the amount of phosphate adsorbed (based on NH 4H 2PO 4) yielded values of 0.035 and 0.2 g/g for titania and alumina, respectively. This corresponds with surface areas of 21.4 and 9.3Å 2 available per phosphate atom, respectively. Interaction of dissolved vanadium-phosphorus oxide precursors with (the surface of) the support material was demonstrated with all support oxides. However, the extent of interaction with silica was small and resulted in formation of vanadium-phosphorus oxide as unsupported bulk material. The large affinity of alumina towards phosphate resulted in a supported, phosphate depleted, vanadium-phosphorus oxide phase. With titania this effect was less pronounced and a well dispersed vanadium-phosphorus oxide phase could be obtained.