Investigation of Rutile, Anatase, and Industrial Titania/Water Solution Interfaces using Potentiometric Titration and Microelectrophoresis

Abstract

The rutile, anatase, and industrial titania (used in catalysis as support and consisting of rutile and anatase)-water solution interfaces were studied using potentiometric titration and microelectrophoretic mobility measurements. Concerning the simple oxides it was found that specific adsorption of the ions of the background electrolyte (NH4NO3) does not take place. The electrical double layer developed in the anatase/aqueous solution and rutile/aqueous solution interfaces is described by the diffuse layer model. On the basis of this model the surface acidity constants, the point of zero charge, and the concentration of the various surface hydroxyls in the pH range 4-8 were calculated using a computer program called SURFEQL. It was confirmed that the surface of rutile is more acidic than the surface of anatase. Concerning industrial titania used in catalysis as support it was demonstrated that its surface has a patchwise structure consisting of rutile (10%) and anatase (90%) regions. The acid-base behavior of this material is identical to a mechanical mixture of rutile and anatase of the same composition. Taking into account its patchwise structure we calculated the variation, with pH, of the concentration of each surface hydroxyl of industrial titania [TirOH+2, TiIOH, TirO-, TiaOH+2, TiaOH, and TiaO-; a, r stand for anatase and rutile, respectively]. These variations allow one to model the deposition of species containing catalytically active ions on the surface of industrial titania.