The effect of surface OH-population on the photocatalytic activity of rare earth-doped P25-TiO2 in methylene blue degradation

Commercial TiO2 (P25, from Degussa) was modified with variable amounts of La, Ce, Y, Pr, Sm (generally rare earth (RE)), by thermal treatment of physical mixtures of TiO2 and the nitrates of the various RE. Doping of P25 with RE, combined with calcination at 600 or 800 °C, yields materials with surface areas ranging from ∼10 to 50 m2/g, and an anatase to rutile phase ratio ranging from ∼0.03 to 0.7, as determined by evaluation of XRD data. After calcination at 600 °C, unpromoted P25 yields the highest activity in methylene blue degradation, while RE addition decreases the activity. After pretreatment of P25 at 800 °C, RE modified catalysts perform better than unpromoted P-25, La being the preferred RE. By evaluation of the DRIFT spectra of the various catalysts, a correlation between the number of a specific anatase Ti–OH group, yielding an IR absorption at 3635 cm−1, and the methylene blue degradation rate was determined. This suggests that this OH-group is an important precursor for the reactive site in aqueous phase methylene blue degradation, and a dominant factor in controlling performance of P-25 in this reaction.