Structural and surface approach to the enhanced photocatalytic activity of sulfated TiO2 photocatalyst

TiO2 materials prepared by sol–gel method and then impregnated with sulfuric acid and further calcined at different temperatures show high photon efficiencies for the photocatalytic degradation of phenol under UV-illumination. Best photocatalyst was obtained after calcination around 700 8C, giving specific activities (i.e. per m2) significantly higher than those exhibited by similarly prepared non-sulfated TiO2 or by pure Degussa P25. Structural analysis of these new materials by XRD, TG–DTG and Raman spectroscopy shows that once calcined at 700 8C the material was a well-crystallized, high surface area and sulfate-free 90% anatase. Surface characterization in this work by XPS, LEIS and 1H MAS-NMR confirms a complete loss of the sulfate and OH-groups, and a low XPS O/Ti-atomic ratio with the O(1s) peak shifted to higher binding energies (1.7 versus 2 _ 0.1 and 530.4 eV versus 529.8 eV, respectively, against the reference materials). This indicates the existence of oxygen vacancies, which give a broad band at 400–600 nm in the reflectance spectra. However, LEIS spectra show an O/Ti composition at the topmost exposed atomic surface layer similar to that of TiO2 reference materials. Adsorption microcalorimetry of pyridine gives a profile of acid sites quite similar to those found for reference anatase samples, what discards the presence of superacid sites as the origin of its enhanced UV-photoefficiency. A mechanism is proposed, on the basis of earlier results in the literature for acidic TiO2 surfaces, to explain the nature of these materials. We also try to correlate the contribution of the oxygen vacancies within the anatase sub-surface layers to the high photon UV-efficiency of the system and, likely, to an enhanced vis-photoactivity of these materials.