Heterogeneity of Hydroxyl and Deuteroxyl Groups on the Surface of TiO2Polymorphs

Potentiometric titration data from pure rutile, anatase, and a commercial fumed titania (Degussa P25) were analyzed in terms of proton binding isotherms from which proton affinity distributions (PADs) of surface sites were obtained. As-received samples, whose thermal and storage history were not systematically controlled, as well as samples subjected to controlled calcination–rehydration–drying treatments were studied. The results indicated the occurrence of a limited number of surface groups on the two polymorphs. The behavior of pure rutile and anatase could be admixed to simulate the acid–base behavior of the commercial sample; on this basis the surface of fumed titania consists largely of anatase-like structures with a small contribution (7%) of rutile-like groups. The region of νODstretching vibrations of isolated -OD groups on extensively dehydroxylated samples was found to correlate with the pK’s determined from PADs. A qualitative assignment of measured pKvalues based on either the original MUSIC model (Hiemstra, T., de Wit, J. C. M., and Van Riemsdijk, W. H.,J. Colloid Interface Sci.133,105 (1989)) or a refined version of it is presented.