Tailoring the selectivity of Ti-based photocatalysts (TiO2 and microporous ETS-10 and ETS-4) by playing with surface morphology and electronic structure

In the first part of this work we report an exhaustive characterization of the bulk and surface properties of ETS-4 and ETS-10 microporous titanosilicates by means of combined N-2 volumetric measurements, SEM, IR, Raman, and UV-Vis techniques. The structure of the surface titanols, derived from literature XRD studies, is shown using a molecular graphic approach. UV-Vis titration experiments using H2O2, and catechol molecules, allowed us to directly measure the total number of available titanols (both in the channels and on the crystal external surface) and the surface titanols, respectively. In the second part of the paper, the ability of ETS-4 and ETS-10 (and of the standard P25) in the photodegradation of phenol (P), 4-chlorophenol (CP), 2,5-dichlorophenol (DCP), 2,4,5-trichlorophenol (TCP), 1,3,5-trihydroxybenzene (THB), and 2,3-dihydroxynaphthalene (DHN) is investigated, using both UV and visible lights, exciting above and below the materials energy gap, respectively. While microporous ETS-4 and ETS-10 exhibit a significant selectivity in the photodegradation of the above-mentioned molecules using both lights, P25 selectivity is observed with visible light only. This means that besides the inverse shape selectivity effect already observed for the microporous materials [Xamena et al. J. Am. Chem. Soc. 2003, 125, 2264], selectivity may be achieved also by selecting the excitation light in accordance with the electronic transition of the adsorbed molecule (determined by a previous systematic UV-Vis study). In such a case the photodegradation may occur if the conduction band of the Ti-based material is opportunely matched with the LUMO level of the adsorbed molecule so that it can receive the electron of the excited adsorbate. The concept of band alignment, well-established in the field of solid-state physics applied to semiconductor heterostructures [Margaritondo, G. Rep. Prog. Phys. 1999, 62, 765; Lamberti, C. Surf. Sci. Rep. 2004, 53, 1], is transferred to the field of photocatalysis for the first time.