Adsorption of molybdate monomers and polymers on titania with a multisite approach

Adsorption of polymers on mineral surfaces is of great interest. A representative important system is the molybdenum supported titania, used in catalysis. The well-known chemistry of molybdate polymers in aqueous solutions allows a detailed study of the contribution of these polymers to the Mo adsorption on titanium oxide surface. A large range of Mo concentrations was covered: from low levels where only Mo monomers are present to high levels where extensive polymerization takes place. Data from different techniques, like potentiometric titrations, proton-ion titrations, and adsorption edges, were used for the description of the interface "molybdate solution/titania surface". All the data could be modeled very well using the recently introduced CD-MUSIC approach. The charge of the surface complexes is spatially distributed in the interface. MoO42- monomers adsorb over the entire pH range forming inner sphere complexes, which are characterized by two types of structure, i.e., bidentate and monodentate. Below pH 5.5 and high total molybdate concentrations the heptamolybdate polyanion Mo7O23(OH)(5-) is also adsorbed, forming an outer sphere complex. Due to its relatively large size, it covers a significant number of surface groups.