The effect of water on the acidity of TiO2 and sulfated titania

Temperature-programmed desorptions (TPD) of isopropylamine (IPA), NH3, and pyridine were compared with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of pyridine to determine the effect of H2O on the acidities of three titania catalysts. Brønsted acidity increased in the following order: Degussa P-25 TiO2 < TiO2 synthesized in our laboratory (p-TiO2) < sulfated titania (SO42−/TiO2). Although the traditional interpretation of pyridine infrared spectra showed an apparent increase in Brønsted acidity upon treating SO42−/TiO2 with H2O, IPA TPD spectra showed that H2O displaced IPA from approximately one-third to one-half of the Brønsted sites. Similarly, H2O treatment prior to TPD displaced significant amounts of both NH3 and pyridine. The primary effect of H2O is displacement of strongly adsorbed basic probe molecules from Brønsted sites, rather than the conversion of Lewis sites to Brønsted sites.