CHEMISTRY OF SURFACE TUNGSTEN SPECIES ON WO3/AL2O3 COMPOSITE OXIDES UNDER AQUEOUS CONDITIONS

The method of potentiometric titration in conjunction with an improved procedure for smoothing the experimental data and deconvoluting the proton binding isotherms was used to characterize the proton binding processes at the oxide/solution interface for Al2O3, WO3 and a series of WO3/Al2O3 composite oxides. A similar procedure was applied to reference tungsten compounds (BaWO4, ammonium metatungstate) for which the buffering capacity due to proton consumption in hydrolysis equilibria was characterized. The results showed that the reactivity towards protons of WO3/Al2O3 composites with low WO3 loading is much closer to that of the alumina support, while for high WO3 loading it is better described by the aqueous chemistry of tungstate/isopolytungstate solutions rather than of crystalline WO3. At surface densities close or above monolayer, supported tungsten species showed a definite tendency to form polymeric surface aggregates in solutions with pH < 6 and more extensively below pH 4. The consequences of these phenomena on the equilibrium adsorption of cationic activators (Ni, Co) on preformed WO3/Al2O3 supports are discussed.