Acidic and Electrosurface Properties of Binary TiO₂-SiO₂ Xerogels Using EPR of pH-Sensitive Nitroxides

The binary xerogels TiO₂-SiO₂ are widely used as catalysts and their carriers in organic synthesis. Characterization and adjustment of the electrostatic properties of the surface and the local acidity inside the pores, are necessary for the further development of TiO₂-SiO₂ xerogels applications. This research investigates acid-base equilibria in the pores, and the surface electrostatic potential (SEP) of binary TiO₂-SiO₂ xerogels, by the EPR of stable pH-sensitive nitroxide radicals. These radicals are small enough to penetrate directly into the pores, and to be adsorbed onto the surface of the material under study. This makes it possible to obtain valuable information on the acidic and electrosurface properties of the studied system. The highest negative surface electrical charge associated with surface electrical potential (SEP) was equal to -196 ± 6 mV. It was induced by the surface of the sample with a 7% TiO₂ content. The local acidity inside the pores of this sample was found to be higher, by approximately 1.49 pH units, as compared to that in the external bulk solution.